Advanced materials for electromagnetic shielding : fundamentals, properties, and applications 9781119128618, 1119128617, 9781119128649, 9781119128632

Table of contents :
Content: List of Contributors xv1 EMI Shielding Fundamentals 1 M. K. Aswathi, Ajay V. Rane , A. R. Ajitha, Sabu Thomas, and Maciej Jaroszewski1.1 Fundamentals of EMI Shielding Theory 11.2 Materials for EMI Shielding 31.3 Mechanism of EM Shielding Materials 3References 82 EM Noise and Its Impact on Human Health and Safety 11 Halina Anio?czyk2.1 Introduction 112.2 Impact of Non?ionizing EMFs on Humans 132.3 Overview of Most Common Sources of EMFs in the Occupational and Residential Modern Human Environment 162.4 Protection Against EMFs in European and International Law 192.5 Assessment of the Level of EMFs in the Workplaces 222.6 Assessment of EMF Levels in Inhabited Area 252.7 Assessment of the Level of EMF From Hi?tech Equipment for Personal Use 262.8 Needs and Possibilities of Shielding to Reduce the Exposure to EMF 292.9 Summary 30References 313 Electromagnetic Field Sensors 35 Vishnu Priya Murali, Jickson Joseph, and Kostya (Ken) Ostrikov3.1 Introduction 363.2 How are EMFs Produced? 363.3 Electromagnetic Field Measurements 383.4 Conclusion 56References 564 Shielding Efficiency Measuring Methods and Systems 61 Saju Daniel and Sabu Thomas4.1 Introduction 624.2 Calculation of Electromagnetic Shielding Effectiveness 654.3 Effect of Various Parameters on Electromagnetic Shielding Effectiveness 694.4 Types of EMI Shielding Effectiveness Tests 714.5 Shielding Effectiveness Measurement Methods and Systems 734.6 Transfer Impedance of Coaxial Cable 844.6.1 Measurement of Transfer Impedance of Coaxial Cable 844.7 Measurement of Transfer Impedance of Conductive Gasket 854.8 Summary 86References 865 Electrical Characterization of Shielding Materials 89 B. J. Madhu5.1 Introduction 895.2 Basics of Electrostatics 895.4 Electric Fields in Materials 985.4.1 Dielectrics 985.4.2 Polarization 985.5 Dielectric Properties 101References 1056 Magnetic Field Shielding 109 Qiang Zhang6.1 Introduction 1096.2 Theories of Magnetic Field Shielding 1106.3 Standard Shielding Materials 1166.4 Multilayer Ferromagnetic Matrix Composite Materials 1216.5 Sandwich Composite/Structure Shielding System 1346.5.1 Fe/Fe?Al Alloy/Fe Sandwich Composite 1356.6 Summary 143References 1447 Recent Progress in Electromagnetic Absorbing Materials 147 Raghvendra Kumar Mishra, Aastha Dutta, Priyanka Mishra, and Sabu Thomas7.1 Introduction 1477.2 Core- Shell Structured Electromagnetic Absorbing Materials 1517.3 CNM? Based Electromagnetic Absorbing Material 1537.4 Graphene Based Polymer Composites for EMI Shielding 1587.5 Challenges and Prospect 160References 1608 Flexible and Transparent EMI Shielding Materials 167 Bishakha Ray, Saurabh Parmar, and Suwarna Datar8.1 Introduction 1678.2 Theory of Transparent EMI Shielding 1688.3 Transparent Thin Films for EMI Shielding 1698.4 Nanocarbon Based Flexible, Transparent EMI Shielding Materials 1708.5 Conducting Polymer?Based Flexible, Transparent EMI Shielding Materials 1728.6 Nanowire Based Flexible, Transparent EMI Shielding Materials 1728.7 Conclusions 174References 1759 Polymer?Based EMI Shielding Materials 177 Chong Min Koo, Faisal Shahzad, Pradip Kumar, Seunggun Yu, Seung Hwan Lee, and Jun Pyo Hong9.1 Introduction 1789.2 Types of Polymer Matrixes 1819.3 Polymer Composites for EMI Shielding Applications 1849.4 Structured Polymer Composites for EMI Shielding 2039.4.1 Foamed Structures 2049.5 Future Perspectives 212References 21310 Textile Based Shielding Materials 219 Julija Baltusnikait??Guzaitien? and Sandra Varnait?? uravliova10.1 Introduction 21910.2 Materials for Production of EMI Textiles 22010.3 Development Trends of Textile Based Shielding Materials 22410.4 Methods of Shielding Effectiveness Measurement 22810.5 Conclusions 232References 23311 Graphene and CNT Based EMI Shielding Materials 241 M.D. Teli and Sanket P. Valia11.1 Introduction to Graphene and Carbon Nanotubes 24111.2 Brief Outline of Synthesis of EMI Shielding Materials 24211.3 General Characteristic of EMI Shielding Materials 24511.4 EMI Shielding Properties of EMI Shielding Materials 24611.5 Overview of Structure and EMI Shielding Property Relationship and Their Applications 25111.6 Future Scope of Research and Application of these Materials 25611.7 Conclusions 257References 25712 Nanocomposites Based EMI Shielding Materials 263 Hossein Yahyaei and Mohsen Mohseni12.1 Nanomaterials and Nanocomposite Materials 26312.2 EMI Shielding Materials 26412.3 Electromagnetic Wave and EMI Shielding Mechanism 26512.4 Carbonous EMI Shielding Nanocomposites 26612.5 Other EMI Shielding Nanocomposites 280References 28413 Silver Nanowires as Shielding Materials 289 Feng Xu, Wenfeng Shen, Wei Xu, Jia Li, and Weijie Song13.1 Introduction 28913.2 Scalable Synthesis of AgNWs 29013.3 Fabrication of Shielding Materials Based on Silver Nanowire/Polymer Conductive Composites 29413.4 Properties of Shielding Materials Based on Silver Nanowire/Polymer Conductive Composites 29513.5 Conclusion 301References 30214 Advanced Carbon Based Foam Materials for EMI Shielding 305 A. R. Ajitha, Anu Surendran, M. K. Aswathi, V. G. Geethamma, and Sabu Thomas14.1 Introduction 30514.2 Carbon Hybrid Materials for EMI Shielding 30614.3 Conclusion 322References 32215 Electromagnetic Interference Shielding Materials for Aerospace Application: A State of the Art 327 Raghvendra Kumar Mishra, Martin George Thomas, Jiji Abraham, Kuruvilla Joseph, and Sabu Thomas15.1 Introduction 32715.2 Radiation in Space Environment 32815.3 Electromagnetic Radiated Field 33015.4 Electromagnetic Interference in Aerospace 33215.5 Electromagnetic Interference Shielding Mechanism for Various Materials 33615.6 Requirement of Shielding Materials for Aerospace 33715.7 Types of Shielding Materials for Aerospace 33815.8 Conclusion 354References 35516 Metamaterials as Shielding Materials 367 Yogesh S. Choudhary and N. Gomathi16.1 Introduction 36716.2 The Need for EMI Shielding 37016.3 Why Metamaterials for Shielding Applications? 37116.4 Metamaterials for Electromagnetic Shielding 37116.5 Design and Fabrication of Metamaterials 37816.6 Other Applications 38516.7 Challenges in Metamaterials 38616.8 Summary 386References 38717 Double Percolating EMI Shielding Materials Based on Polymer Blend Nanocomposites 393 P. Mohammed Arif, Jemy James, Jiji Abraham, K. Nandakkumar, and Sabu Thomas17.1 Introduction 39417.2 Concept of Double Percolation 39417.3 Carbon Black and Carbon Nanofiber Based Composites 39517.4 Carbon Nanotube Based Nanocomposites 40017.5 Hybrid Filler Based Nanocomposites 40517.6 Conclusion 407References 40718 Mechanical Performance Characterization of EMI Shielding Materials Using Optical Experimental Techniques 409 Wenfeng Hao, Can Tang, and Jianguo Zhu18.1 Introduction 40918.2 Characterizing the In?Plane Mechanical Performance of EMI Shielding Materials 41018.2.1 Digital Image Correlation (DIC) 41018.3 Characterizing Out?of?Plane Mechanical Performance of EMI Shielding Materials 41418.4 Characterizing the Fracture and Fatigue Performance of EMI Shielding Materials 41518.5 ConcludingRemarks 420Acknowledgments 420References 420Index 425

Citation preview

Advanced Materials for Electromagnetic Shielding

Advanced Materials for Electromagnetic Shielding Fundamentals, Properties, and Applications

Edited by Maciej Jaroszewski

Faculty of Electrical Engineering, Wrocław University of Science and Technology Wrocław, Poland

Sabu Thomas

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University and School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Ajay V. Rane

Current affiliation: Composite Research Group, Department of Mechanical Engineering Durban University of Technology, Durban, South Africa and International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University, Kottayam, Kerala, India

This edition first published 2019 © 2019 John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. The right of Maciej Jaroszewski, Sabu Thomas, and Ajay V. Rane to be identified as the editor(s) of this work has been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA Editorial Office 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print‐on‐demand. Some content that appears in standard print versions of this book may not be available in other formats. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/ or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging‐in‐Publication Data Names: Jaroszewski, Maciej Wladyslaw, 1966– editor. | Thomas, Sabu, editor. | Rane, Ajay V., 1987– editor. Title: Advanced materials for electromagnetic shielding : fundamentals, properties, and applications /   edited by Maciej Wladyslaw Jaroszewski, Wrocław University of Science and Technology,   Faculty of Electrical Engineering, Wrocław, Poland, Sabu Thomas, Mahatma Gandhi University,   Priyadarshini Hills, Kottayam, Kerala, Ajay V. Rane, Composite Research Group, Durban University   of Technology, South Africa and Mahatma Gandhi University, Priyadarshini Hills, Kottayam, Kerala. Description: First edition. | Hoboken, NJ, USA : Wiley, 2019. | Includes bibliographical references and index. | Identifiers: LCCN 2018022528 (print) | LCCN 2018028981 (ebook) | ISBN 9781119128649 (Adobe PDF) |   ISBN 9781119128632 (ePub) | ISBN 9781119128618 (hardcover) Subjects: LCSH: Shielding (Electricity) Classification: LCC TK7867.8 (ebook) | LCC TK7867.8 .A38 2019 (print) | DDC 621.381–dc23 LC record available at https://lccn.loc.gov/2018022528 Cover design by Wiley Cover image: © iStock.com/olegback Set in 10/12pt Warnock by SPi Global, Pondicherry, India Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

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Contents List of Contributors

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EMI Shielding Fundamentals  1 M. K. Aswathi, Ajay V. Rane , A. R. Ajitha, Sabu Thomas, and Maciej Jaroszewski

1.1 ­Fundamentals of EMI Shielding Theory  1 1.2 ­Materials for EMI Shielding  3 1.3 ­Mechanism of EM Shielding Materials  3 ­References  8 2

EM Noise and Its Impact on Human Health and Safety  11 Halina Aniołczyk

2.1 ­Introduction  11 2.2 ­Impact of Non‐ionizing EMFs on Humans  13 2.3 ­Overview of Most Common Sources of EMFs in the Occupational and Residential Modern Human Environment  16 2.4 ­Protection Against EMFs in European and International Law  19 2.5 ­Assessment of the Level of EMFs in the Workplaces  22 2.6 ­Assessment of EMF Levels in Inhabited Area  25 2.7 ­Assessment of the Level of EMF From Hi‐tech Equipment for Personal Use  26 2.8 ­Needs and Possibilities of Shielding to Reduce the Exposure to EMF  29 2.9 ­Summary  30 ­References  31 3

Electromagnetic Field Sensors  35 Vishnu Priya Murali, Jickson Joseph, and Kostya (Ken) Ostrikov

3.1 ­Introduction  36 3.2 ­How are EMFs Produced?  36 3.2.1 Natural Sources  37 3.2.2 Man‐Made Sources  37 3.2.2.1 Low‐Frequency EMF Sources  37 3.2.2.2 High‐Frequency EMF Sources  38 3.3 ­Electromagnetic Field Measurements  38 3.3.1 Magnetic Field Measurement Techniques  38 3.3.1.1 Induction Based Sensors  39 3.3.1.2 Fluxgate Sensors  42

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3.3.1.3 SQUID Magnetometer  44 3.3.1.4 Hall Probes  46 3.3.1.5 Magneto‐Resistors 47 3.3.1.6 Scalar Magnetometers  49 3.3.2 Electric Field Measurements  51 3.3.2.1 Electric Field Probes  51 3.3.2.2 Electron Drift Instrument  53 3.3.3 Power Density Measurements  55 3.4 ­Conclusion  56 ­References  56 4

Shielding Efficiency Measuring Methods and Systems  61 Saju Daniel and Sabu Thomas

4.1 ­Introduction  62 4.1.1 Mechanism of Shielding  62 4.1.2 Shielding Effectiveness  62 4.1.2.1 Absorption Loss  64 4.1.2.2 Reflection Loss  64 4.1.2.3 Multiple Reflection Correction Factor  65 4.2 ­Calculation of Electromagnetic Shielding Effectiveness  65 4.2.1 Calculation of SE of a Material by Using Plane Wave Theory  65 4.2.2 Calculation of SE of a Metal Foil  66 4.2.3 Calculation of SE for Near Field Shielding  67 4.2.4 Calculation of SE for Shielding a Low‐Frequency Magnetic Field Source  67 4.2.5 Calculation of Shielding Effectiveness from Scattering Parameters  67 4.3 ­Effect of Various Parameters on Electromagnetic Shielding Effectiveness  69 4.4 ­Types of EMI Shielding Effectiveness Tests  71 4.4.1 Open Field or Free Space Test  71 4.4.2 Shielded Box Test  71 4.4.3 Coaxial Transmission Line Test  72 4.4.4 Shielded Room Test  73 4.5 ­Shielding Effectiveness Measurement Methods and Systems  73 4.5.1 Test Methods Using Plaque Measurements  74 4.5.1.1 Testing Methods Based on MIL‐STD‐285T  74 4.5.1.2 Modified Radiation Method for Shielding Effectiveness Testing Based on MIL‐G83528  75 4.5.1.3 Dual Mode Stirred Chamber  76 4.5.1.4 Apertured Transverse Electromagnetic (TEM) Cell in a Reverberating Chamber  77 4.5.1.5 Dual TEM Cell Method  77 4.5.1.6 Split TEM Cell  78 4.5.1.7 ASTM ES‐7 Dual Chamber Test Fixture  78 4.5.1.8 ASTM ES‐7 Coaxial Transmission Line  78 4.5.1.9 ASTM D 4935 Circular Coaxial Transmission Line Holder  79 4.5.1.10 Enclosure Measurement Techniques  81 4.5.1.11 Injection Molded Enclosure Test Method  81 4.5.1.12 IEEE‐STD‐299 81 4.5.2 Free Space Methods  82 4.5.2.1 Free‐Space Measurement Techniques in the Frequency Domain  82

Contents

4.5.2.2 Free‐Space Measurement Techniques in the Time Domain  83 4.6 ­Transfer Impedance of Coaxial Cable  84 4.6.1 Measurement of Transfer Impedance of Coaxial Cable  84 4.7 ­Measurement of Transfer Impedance of Conductive Gasket  85 4.8 ­Summary  86 ­References  86 5

Electrical Characterization of Shielding Materials  89 B. J. Madhu

5.1 ­Introduction  89 5.2 ­Basics of Electrostatics  89 5.2.1 Electrostatic Field  89 5.2.2 Electrical Potential Energy  91 5.2.3 Electric Potential and Electric Field Strength  92 5.3 ­Electrical Conductivity  93 5.3.1 Current and Current Density  93 5.3.2 Resistivity  94 5.3.3 DC Conductivity  95 5.3.4 AC Conductivity  97 5.4 ­Electric Fields in Materials  98 5.4.1 Dielectrics  98 5.4.2 Polarization  98 5.5 ­Dielectric Properties  101 5.5.1 Static Dielectric Constant  101 5.5.2 Complex Dielectric Constant and Dielectric Losses  102 5.6 ­Electromagnetic Interference Shielding Materials  103 5.6.1 Electromagnetic Interference Shielding  103 5.6.2 Conductive Shielding Materials  104 5.6.3 Dielectric Shielding Materials  105 ­References  105 6

Magnetic Field Shielding  109 Qiang Zhang

6.1 ­Introduction  109 6.2 ­Theories of Magnetic Field Shielding  110 6.2.1 Magnetic Field  110 6.2.2 Magnetic Circuit and Magnetic Reluctance  111 6.2.3 Shielding of Magnetic Field  112 6.2.3.1 Shielding of High Frequency Magnetic Field  112 6.2.3.2 Shielding of Low Frequency or Static Magnetic Field  112 6.2.4 Design of Multilayer Shielding  114 6.2.4.1 Case (a)  115 6.2.4.2 Case (b)  115 6.2.5 Design of Magnetic Shielding Room  116 6.3 ­Standard Shielding Materials  116 6.3.1 Basic Magnetic Parameters  116 6.3.2 Metallic and Ferromagnetic Materials  118 6.3.3 Ferrite Materials  119 6.3.4 Superconducting Materials  120

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6.3.5 Amorphous and Nanocrystalline Alloys  120 6.4 ­Multilayer Ferromagnetic Matrix Composite Materials  121 6.4.1 Fe–Ni Alloy/Fe/Fe–Ni Alloy Multilayer Composite  122 6.4.1.1 Fabrication 122 6.4.1.2 Microstructure Characterization  122 6.4.1.3 Geomagnetic Shielding Property: Experiment and Calculation  122 6.4.1.4 Shielding Mechanism  128 6.4.2 Fe–Al Alloy/Fe/Fe–Al Alloy Multilayer Composite  128 6.4.2.1 Fabrication 128 6.4.2.2 Microstructure Characterization  129 6.4.2.3 Geomagnetic Shielding Property: Experiment and Calculation  130 6.4.2.4 Shielding Mechanism  134 6.5 ­Sandwich Composite/Structure Shielding System  134 6.5.1 Fe/Fe‐Al Alloy/Fe Sandwich Composite  135 6.5.1.1 Fabrication 135 6.5.1.2 Microstructure Characterization  135 6.5.1.3 Magnetic Shielding Property  136 6.5.1.4 Shielding Mechanism  140 6.5.2 Composite/Polyester/Composite Sandwich Structure  141 6.5.2.1 Fabrication 141 6.5.2.2 Geomagnetic Shielding Property  141 6.5.2.3 Shielding Mechanism  143 6.6 ­Summary  143 ­References  144 7

Recent Progress in Electromagnetic Absorbing Materials  147 Raghvendra Kumar Mishra, Aastha Dutta, Priyanka Mishra, and Sabu Thomas

7.1 ­Introduction  147 7.1.1 Electromagnetic Wave Absorbing Materials  149 7.2 ­Core–Shell Structured Electromagnetic Absorbing Materials  151 7.3 ­CNM‐Based Electromagnetic Absorbing Material  153 7.3.1 Carbon Nanotubes/Polymer Nanocomposites for Electromagnetic Shielding  154 7.3.2 Carbon Nanofiber Based EMI Shielding Materials  156 7.4 ­Graphene Based Polymer Composites for EMI Shielding  158 7.5 ­Challenges and Prospect  160 ­References  160 8

Flexible and Transparent EMI Shielding Materials  167 Bishakha Ray, Saurabh Parmar, and Suwarna Datar

8.1 ­Introduction  167 8.2 ­Theory of Transparent EMI Shielding  168 8.3 ­Transparent Thin Films for EMI Shielding  169 8.4 ­Nanocarbon Based Flexible, Transparent EMI Shielding Materials  170 8.5 ­Conducting Polymer‐Based Flexible, Transparent EMI Shielding Materials  172 8.6 ­Nanowire Based Flexible, Transparent EMI Shielding Materials  172 8.7 ­Conclusions  174 ­References  175

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Polymer‐Based EMI Shielding Materials  177 Chong Min Koo, Faisal Shahzad, Pradip Kumar, Seunggun Yu, Seung Hwan Lee, and Jun Pyo Hong

9.1 ­Introduction  178 9.1.1 Need for Polymer‐Based EMI Shielding Materials  178 9.1.2 Factors Effecting EMI SE  180 9.2 ­Types of Polymer Matrixes  181 9.2.1 Insulating Polymers  181 9.2.2 Intrinsically Conducting Polymers  181 9.3 ­Polymer Composites for EMI Shielding Applications  184 9.3.1 Carbon Based Filler Materials  184 9.3.1.1 Graphite 184 9.3.1.2 Carbon Fiber  186 9.3.1.3 Carbon Nanotube  186 9.3.1.4 Carbon Black  188 9.3.1.5 Graphene 189 9.3.2 Magnetic Fillers  190 9.3.2.1 Magnetic Fillers and Carbon Materials in Insulating Polymer Matrix  190 9.3.2.2 Magnetic Fillers with Carbon Materials in Conducting Polymer Matrix  192 9.3.2.3 All‐Magnetic Fillers in Insulating Polymer Matrix  194 9.3.2.4 All‐Magnetic Fillers in Conducting Polymer Matrix  198 9.3.3 Metal‐Based Filler Materials  199 9.4 ­Structured Polymer Composites for EMI Shielding  203 9.4.1 Foamed Structures  204 9.4.2 Sandwiched Structures  209 9.4.3 Segregated Structures  210 9.5 ­Future Perspectives  212 ­References  213 10

Textile Based Shielding Materials  219 Julija Baltušnikaitė‐Guzaitienė and Sandra Varnaitė‐Žuravliova

10.1 ­Introduction  219 10.2 ­Materials for Production of EMI Textiles  220 10.2.1 Polymers in EMI Textiles  221 10.2.2 Conductive Coatings  222 10.2.3 Compounding with Conductive Fillers  222 10.2.4 Inherently (Intrinsically) Conductive Polymers (ICP)  223 10.3 ­Development Trends of Textile Based Shielding Materials  224 10.3.1 Shielding Materials Based on Conductive Fillers  224 10.3.2 Shielding Materials Based on Fabric Formation Technology  226 10.3.3 Shielding Materials Based on Fabric Surface Modification  226 10.4 ­Methods of Shielding Effectiveness Measurement  228 10.4.1 Coaxial Transmission Line Method  228 10.4.2 Shielded Box Method  229 10.4.3 Shielded Room Method  230 10.4.4 Open Field or Free Space Method  230 10.4.5 Waveguide Method  232 10.5 ­Conclusions  232 ­References  233

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Graphene and CNT Based EMI Shielding Materials  241 M.D. Teli and Sanket P. Valia

11.1 ­Introduction to Graphene and Carbon Nanotubes  241 11.1.1 Introduction to Graphene Based Materials  241 11.1.2 Introduction to CNT Based Materials  241 11.2 ­Brief Outline of Synthesis of EMI Shielding Materials  242 11.2.1 Brief Outline of Synthesis of Graphene Based Materials  242 11.2.2 Brief Outline of Synthesis of CNT Based Materials  244 11.2.2.1 Electric Arc Discharge  244 11.2.2.2 Laser Ablation  245 11.2.2.3 Chemical Vapor Deposition  245 11.3 ­General Characteristic of EMI Shielding Materials  245 11.3.1 General Characteristic of Graphene Based Materials  245 11.3.2 General Characteristic of CNT Based Materials  246 11.4 ­EMI Shielding Properties of EMI Shielding Materials  246 11.4.1 EMI Shielding Properties of Graphene Based Materials  246 11.4.2 EMI Shielding Properties of CNT Based Materials  250 11.5 ­Overview of Structure and EMI Shielding Property Relationship and Their Applications  251 11.5.1 Structure and EMI Shielding Property Relationship of Graphene Based Materials  251 11.5.2 Structure and EMI Shielding Property Relationship of CNT Based Materials  254 11.6 ­Future Scope of Research and Application of these Materials  256 11.7 ­Conclusions  257 ­References  257 12

Nanocomposites Based EMI Shielding Materials  263 Hossein Yahyaei and Mohsen Mohseni

12.1 ­Nanomaterials and Nanocomposite Materials  263 12.2 EMI Shielding Materials  264 12.3 ­Electromagnetic Wave and EMI Shielding Mechanism  265 12.4 ­Carbonous EMI Shielding Nanocomposites  266 12.4.1 ­Graphene  266 12.4.1.1 Graphene Synthesis  267 12.4.1.2 Case Studies  267 12.4.2 ­Carbon Nanotubes  272 12.4.2.1 Single‐Walled Carbon Nanotubes  273 12.4.2.2 Carbon Nanotube Properties  274 12.4.2.3 Case Studies  274 12.4.3 ­Carbon Nanofibers  277 12.4.3.1 Vapor Grown CNFs  277 12.4.3.2 Electrospun CNFs (ECNFs)  278 12.4.3.3 Case Studies  278 12.5 ­Other EMI Shielding Nanocomposites  280 12.5.1 Mechanical Properties  281 12.5.2 Corrosion Resistance  281 12.5.3 Electrical Conductivity  281 12.5.4 Synthesis of Metal Nanoparticles  282 12.5.5 Case Studies  282 ­References  284

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Silver Nanowires as Shielding Materials  289 Feng Xu, Wenfeng Shen, Wei Xu, Jia Li, and Weijie Song

13.1 ­Introduction  289 13.2 ­Scalable Synthesis of AgNWs  290 13.3 ­Fabrication of Shielding Materials Based on Silver Nanowire/Polymer Conductive Composites  294 13.4 ­Properties of Shielding Materials Based on Silver Nanowire/Polymer Conductive Composites  295 13.4.1 Morphological Properties  295 13.4.2 Electrical Properties  297 13.4.3 EMI Properties  298 13.5 ­Conclusion  301 ­References  302 14

Advanced Carbon Based Foam Materials for EMI Shielding  305 A. R. Ajitha, Anu Surendran, M. K. Aswathi, V. G. Geethamma, and Sabu Thomas

14.1 ­Introduction  305 14.2 ­Carbon Hybrid Materials for EMI Shielding  306 14.2.1 Carbon Foam (CF)  306 14.2.2 Graphene Foam  309 14.2.3 Carbon–Carbon Composites  315 14.2.4 Carbon Aerogels  316 14.2.5 Colloidal Graphite  320 14.3 ­Conclusion  322 ­References  322 15

Electromagnetic Interference Shielding Materials for Aerospace Application: A State of the Art  327 Raghvendra Kumar Mishra, Martin George Thomas, Jiji Abraham, Kuruvilla Joseph, and Sabu Thomas

15.1 ­Introduction  327 15.2 ­Radiation in Space Environment  328 15.3 ­Electromagnetic Radiated Field  330 15.3.1 Low Intensity Radiated Field  331 15.3.2 High Intensity Radiated Field  332 15.4 ­Electromagnetic Interference in Aerospace  332 15.4.1 Classification of Electromagnetic Interference  333 15.4.2 Effect of Electromagnetic Shielding  333 15.5 ­Electromagnetic Interference Shielding Mechanism for Various Materials  336 15.6 ­Requirement of Shielding Materials for Aerospace  337 15.7 ­Types of Shielding Materials for Aerospace  338 15.7.1 Metals Enclosure Based EMI Shielding Materials  338 15.7.1.1 Design Consideration of Metallic Enclosure for EMI Shielding  342 15.7.2 Porous Structure for EMI Shielding Materials  344 15.7.3 Polymer Composites for EMI Shielding  346 15.7.3.1 Metal Coated Polymer for EMI Shielding  346 15.7.3.2 Conducting Polymer Based Materials for EMI Shielding  348 15.7.3.3 Carbonanotube Based Composites for EMI Shielding  349 15.7.3.4 Graphene Based Composites for EMI Shielding  352

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15.8 ­Conclusion  354 ­References  355 16

Metamaterials as Shielding Materials  367 Yogesh S. Choudhary and N. Gomathi

16.1 ­Introduction  367 16.2 ­The Need for EMI Shielding  370 16.3 ­Why Metamaterials for Shielding Applications?  371 16.4 ­Metamaterials for Electromagnetic Shielding  371 16.4.1 Microwave Shielding  373 16.4.2 Optical and Near IR Shielding  375 16.4.3 Frequency Selective Shielding  376 16.5 ­Design and Fabrication of Metamaterials  378 16.5.1 Designing Metamaterials  378 16.5.2 Fabrication of Metamaterials  383 16.6 ­Other Applications  385 16.6.1 Superlensing  385 16.6.2 Antennas  385 16.7 ­Challenges in Metamaterials  386 16.8 ­Summary  386 ­References  387 17

Double Percolating EMI Shielding Materials Based on Polymer Blend Nanocomposites  393 P. Mohammed Arif, Jemy James, Jiji Abraham, K. Nandakkumar, and Sabu Thomas

17.1 ­Introduction  394 17.2 ­Concept of Double Percolation  394 17.3 ­Carbon Black and Carbon Nanofiber Based Composites  395 17.3.1 Carbon Black Based Composites  395 17.3.2 Carbon Nanofibers  400 17.4 ­Carbon Nanotube Based Nanocomposites  400 17.5 ­Hybrid Filler Based Nanocomposites  405 17.6 ­Conclusion  407 ­References  407 18

Mechanical Performance Characterization of EMI Shielding Materials Using Optical Experimental Techniques  409 Wenfeng Hao, Can Tang, and Jianguo Zhu

18.1 ­Introduction  409 18.2 ­Characterizing the In‐Plane Mechanical Performance of EMI Shielding Materials  410 18.2.1 Digital Image Correlation (DIC)  410 18.2.2 Moiré Interfere  411 18.2.3 Photoelastic Method  413 18.3 ­Characterizing Out‐of‐Plane Mechanical Performance of EMI Shielding Materials  414 18.4 ­Characterizing the Fracture and Fatigue Performance of EMI Shielding Materials  415

Contents

18.4.1 Caustics 415 18.4.2 Coherent Gradient Sensing (CGS)  416 18.4.3 Digital Gradient Sensing (DGS)  418 18.5 ­Concluding Remarks  420 Acknowledgments  420­ References  420 Index  425

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List of Contributors Jiji Abraham

Yogesh S. Choudhary

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India

Department of Chemistry Indian Institute of Space Science and Technology Thiruvananthapuram, Kerala India

A. R. Ajitha

Saju Daniel

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India Halina Aniołczyk

Nofer Institute of Occupational Medicine Lodz Poland P. Mohammed Arif

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India M. K. Aswathi

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India Julija Baltušnikaitė‐Guzaitienė

Center for Physical Sciences and Technology Textile Institute Kaunas Lithuania

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India St. Xavier’s College Vaikom Mahatma Gandhi University Kottayam, Kerala India Suwarna Datar

Department of Applied Physics Defence Institute of Advanced Technology Deemed University Girinagar, Pune 411021 India Aastha Dutta

Department of Plastic and Polymer Engineering, G.S. Mandal’s Maharashtra Institute of Technology Aurangabad, Maharashtra India V. G. Geethamma

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India

xvi

List of Contributors

N. Gomathi

Kuruvilla Joseph

Department of Chemistry, Indian Institute of Space Science and Technology Thiruvananthapuram, Kerala India

Indian Institute of Space Science and Technology Thiruvananthapuram, Kerala  India

Wenfeng Hao

Chong Min Koo

Faculty of Civil Engineering and Mechanics Jiangsu University Zhenjiang, Jiangsu P. R. China Jun Pyo Hong

Center for Materials Architecturing Korea Institute of Science and Technology Seoul Republic of Korea Jemy James

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India School of pure and Applied Physics Mahatma Gandhi University Kottayam, Kerala India FRE CNRS 3744, IRDL University of Southern Brittany Lorient France Maciej Jaroszewski

Faculty of Electrical Engineering Wrocław University of Science and Technology Wrocław Poland Jickson Joseph

Center for Materials Architecturing Korea Institute of Science and Technology Seoul Republic of Korea Nanomaterials Science and Engineering University of Science and Technology Daejeon Republic of Korea KU‐KIST Graduate School of Converging Science and Technology, Korea University Seoul Republic of Korea Pradip Kumar

Center for Materials Architecturing Korea Institute of Science and Technology Seoul Republic of Korea Jia Li

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences Ningbo P. R. China Seung Hwan Lee

Center for Materials Architecturing Korea Institute of Science and Technology Seoul Republic of Korea

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology Brisbane, Queensland 4000 Australia

B. J. Madhu

CSIRO−QUT Joint Sustainable Processes and Devices Laboratory, Commonwealth Scientific and Industrial Research Organisation P. O. Box 218, Lindfield New South Wales 2070 Australia

Raghvendra Kumar Mishra

Department of Post Graduate Studies in Physics, Government Science College Chitradurga 577 501, Karnataka India International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India

List of Contributors

Priyanka Mishra

Ajay V. Rane

Deen Dayal Upadhyay Gorakhpur University Gorakhpur, Uttar Pradesh India Mohsen Mohseni

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology Tehran Iran

Composite Research Group Department of Mechanical Engineering Durban University of Technology Durban South Africa

Vishnu Priya Murali

Bishakha Ray

Department of Biomedical Engineering University of Memphis Memphis, Tennessee USA K. Nandakkumar

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India School of Pure and Applied Physics Mahatma Gandhi University Kottayam, Kerala India Kostya (Ken) Ostrikov

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology Brisbane, Queensland 4000 Australia CSIRO−QUT Joint Sustainable Processes and Devices Laboratory, Commonwealth Scientific and Industrial Research Organisation P. O. Box 218, Lindfield New South Wales 2070 Australia Saurabh Parmar

Department of Applied Physics Defence Institute of Advanced Technology Deemed University Girinagar, Pune 411021 India

Department of Applied Physics Defence Institute of Advanced Technology Deemed University Girinagar, Pune 411021 India Faisal Shahzad

National Center for Nanotechnology Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS) Islamabad Pakistan Center for Materials Architecturing Korea Institute of Science and Technology Seoul Republic of Korea Nanomaterials Science and Engineering University of Science and Technology Daejeon Republic of Korea Wenfeng Shen

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences Ningbo P. R. China Weijie Song

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences Ningbo P. R. China

xvii

xviii

List of Contributors

Anu Surendran

Sandra Varnaitė‐Žuravliova

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India

Center for Physical Sciences and Technology Textile Institute Kaunas Lithuania

Can Tang

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences Ningbo P. R. China

Faculty of Civil Engineering and Mechanics Jiangsu University Zhenjiang, Jiangsu P. R. China M. D. Teli

Department of Fibres and Textile Processing Technology, Institute of Chemical Technology, Matunga (E) Mumbai, Maharashtra India Martin George Thomas

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India Sabu Thomas

International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala India School of Chemical Sciences Mahatma Gandhi University Kottayam, Kerala India Sanket P. Valia

Department of Fibres and Textile Processing Technology, Institute of Chemical Technology, Matunga (E) Mumbai, Maharashtra India

Feng Xu

Wei Xu

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences Ningbo P. R. China Hossein Yahyaei

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology Tehran Iran Seunggun Yu

Center for Materials Architecturing Korea Institute of Science and Technology Seoul Republic of Korea Qiang Zhang

Department of Materials Science Key Laboratory of Advanced StructureFunction Integrated Materials and Green Manufacturing Technology School of Materials Science and Engineering Harbin Institute of Technology Harbin P. R. China Jianguo Zhu

Faculty of Civil Engineering and Mechanics Jiangsu University Zhenjiang, Jiangsu P. R. China

1

1 EMI Shielding Fundamentals M. K. Aswathi1, Ajay V. Rane1,2, A. R. Ajitha1, Sabu Thomas1,3, and Maciej Jaroszewski 4 1 

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India  Composite Research Group, Department of Mechanical Engineering, Durban University of Technology, Durban, South Africa 3  School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India 4  Faculty of Electrical Engineering, Wrocław University of Science and Technology, Wrocław, Poland 2

1.1 ­Fundamentals of EMI Shielding Theory Electromagnetic shielding is process of reducing the dispersion of electromagnetic waves into a desired space by hindering the waves with a shield made of conductive material. The effective performance of electrical instruments or the working of electrical instruments is interrupted, degraded, obstructed, or limited due to the electromagnetic interference (EMI). In a material the main mechanisms for EMI attenuation are reflection, absorption, and multiple reflection [1, 2]. Reflection is the primary mechanism of EMI shielding. For reflection the material must possess mobile charge carriers such as electrons or holes that interact with the electromagnetic radiation. Metals are the most common material for EMI shielding and the available free electrons in metals interact with the electromagnetic waves [3]. If the material is highly conductive the shielding against EM (electromagnetic) waves will occur through the reflection mechanism. However, conductivity is not a condition for EMI shielding but it does enhance the reflection mechanism of an EMI shielding material. The secondary mechanism for EMI shielding is absorption, which requires the existence of electric or magnetic dipoles to interact with the electromagnetic radiation. It changes with the thickness of the material. Materials that have a high dielectric constant provide electric dipoles and materials with high magnetic permeability provide magnetic dipoles for the EMI shielding by absorption [1]. The third mechanism is multiple reflections, which is the reflections at different surfaces or at the interface of the material. Materials that have large specific internal surfaces or ­composites with fillers show a multiple reflection mechanism. Generally, multiple‐reflection decreases the total shielding value if the material is thinner than the skin depth and the value can be neglected if the material has a higher thickness than the skin depth. At higher ­frequencies electromagnetic radiation penetrates only to the near surface region of the ­electrical conductor. This is known as the skin effect. The intensity of penetration of an ­electromagnetic wave decreases exponentially with increasing depth of the conductor [4]. Advanced Materials for Electromagnetic Shielding: Fundamentals, Properties, and Applications, First Edition. Edited by Maciej Jaroszewski, Sabu Thomas, and Ajay V. Rane. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

2

1  EMI Shielding Fundamentals

The skin depth is the depth of the conductor at which the intensity of the incident field drops in to 1/e of the incident value and is denoted by δ [5]:

1

Skin depth,

f



Here f is the frequency, μ is the magnetic permeability, and σ the electrical conductivity in Ω−1 m−1. Skin depth is not directly proportional to frequency, magnetic permeability, and conductivity, i.e. skin depth decreases with increase in frequency, magnetic permeability, or electrical conductivity. Owing to this skin effect, a material that contains a conductive filler with a small unit size of filler is more effective for shielding than a filler with a large unit size. The complete cross section of the filler unit can be used only when the unit size of the filler is less than or comparable with the skin depth. Shielding effectiveness, which is expressed in dB, is the sum of reflection loss, absorption loss, and multiple reflections [6]. When electromagnetic waves strike the surface of an object they undergo reflection, multiple reflection, absorption, and transmission as shown in Figure 1.1. To be a shield against the EM wave, the material should reflect or absorb the electromagnetic wave. Factors determining shielding effectiveness (SE) are classified in Figure 1.2. Incident waves Reflection

Absorption

Multiple reflection Transmission

Figure 1.1  Schematic representation showing mechanism of electromagnetic shielding.

Factors determining SE

Frequency of incident electromagnetic field

Shield thickness

Conductivity, permeability, permittivity of shield material

Source to shield distance

Type of electromagnetic field source

Shielding degradation

Figure 1.2  Factors determining shielding effectiveness [7]. Source: Adapted from Gooch 2007.

1.3  Mechanism of EM Shielding Materials

1.2 ­Materials for EMI Shielding Owing to the increasing use of electronic equipment the shielding of other instruments and of human beings from electromagnetic waves is a very serious issue in the present scenario, which is detailed in Chapter  2. The EM waves harmfully affect both the device performance and human beings. Nowadays, a reduction in the use of electronic equipment is not always practical. What we are able to do is to reduce the penetration of EM waves produced from electronic instruments. To decrease the penetration we must use a shield or block the EM waves from the desired surface. Metals are commonly used for EMI shielding application in the form of thin sheets or sheathing in automotive applications. But, metal is expensive, prone to corrosion, heavy, and the cost of manufacturing processes is also very high, which makes them an undesired choice for electronic application. Conductive polymer nanocomposites have attracted a great deal of academic and industrial interest by considering the cost‐effectiveness, easy processability, and their possible applications in many areas including EMI shielding. Polymer nanocomposites based on CNTs (carbon nanotubes), carbon black (CB), graphene, metal nanoparticles, carbon fibers, foams, and magnetic nanoparticles show good shielding capacity against EM waves. Several groups have studied and reported the EMI shielding effectiveness of different materials and mechanisms behind the EMI shielding ability of those materials. Characterization and requirement for EMI shielding materials are mentioned in the classification chart in Figure  1.3. Chapters 7–14, 16, and 17 describe these materials in detail in accordance to EM shielding.

Characterization and requirements

Shielding effectiveness measurement

Electrical conductivity

Thermal conductivity

Permeability

Permittivity

Surface finish

Contact interface compatibility

Formability

Manufacturability

On field evaluation

Mechanical properties

Figure 1.3  Characterization and requirements for EMI shielding materials [8]. Source: Adapted from Tong 2009.

1.3 ­Mechanism of EM Shielding Materials Carbon nanotubes, a 1D nanostructure, are rolled up sheets of graphene, made up of a ­hexagonal lattice of sp2 hybridized carbon atoms. Depending upon the number of graphene sheets used to form the cylindrical shape the carbon nanotubes are of different types, namely single walled, double walled, multiwalled. Carbon fibers (CFs) come under 1D carbon nano‐ allotropes, having interlocked sheets of graphene. Carbon black is a good filler to enhance the EMI shielding effectiveness of a material; it is produced by the thermal decomposition of hydrocarbons. Carbon black has graphite layers different from that of amorphous carbon.

3

4

1  EMI Shielding Fundamentals

Every carbon atom in the graphite layer forms three covalent bonds with neighboring carbon atoms and the free p‐orbits from each carbon atom overlap to from delocalized π electrons. The presence of these freely moving π electrons make carbon black a good conducting material. Carbon fiber paper (CFP) and nickel coated carbon fiber paper (NCFP) reinforced epoxy composites show EMI shielding efficiencies of 30 and 35 dB, respectively, in the frequency range 3.22–4.9 GHz for 0.5 mm thick sheets at 8 wt% fiber content. This is due to the increased conductivity shown by the nanocomposites; in addition, both absorption loss and reflection loss contribute to the total EMI shielding but the major contribution is from reflection. The material shows higher electrical conductivity due to the presence of mobile charge carriers. These charge carriers interact with the EM waves, which induces reflection as the major mechanism for the shielding [9]. Carbon black reinforced cement composites show a good EMI shielding value due to the presence of freely moving π‐bond electrons. The shielding effectiveness increased with increase in carbon black content because of the conductive network path and through the reflection mechanism [10]. Carbon black reinforced polyaniline/poloxalene composites can be used as a lightweight EMI shielding material with a shielding effectiveness of 19.2–19.9 dB at 10 wt% CB. The good EMI shielding value obtained is due to the formation of a network between carbon black and the blend system. The interconnected network contributes to the shielding value obtained through reflection as the shielding mechanism [11]. Carbon micro coils (CMCs) are another material used as a filler in making EMI shielding material. The polyurethane composites with CMCs show an increased EMI shielding value that depends on the layer thickness of the material. Hence the mechanism is based on absorption [12]. In the case of multiwalled carbon nanotube (MWCNT)/polypropylene (PP) composites the contribution of absorption loss to the total EMI shielding is higher than reflection so the major mechanism is absorption and reflection is the secondary shielding mechanism [13]. Here, multiple‐reflection is excluded from the discussion because it lowers the overall EMI SE. Figures 1.4a–c show power balance graphs for different MWCNT/PP composites with various amounts of MWCNT and different plate thickness. The percentage of power blocked by reflection is increased with increase in MWCNT content in all three cases (Figures 1.4a–c), but in the case of 0.34 and 1 mm plates the percentage of absorption initially increases with increasing MWCNT content and then decreases. In the third case (2.8 mm thick plates) the contribution by absorption increases linearly with increasing MWCNT content [13]. The EMI shielding effectiveness shown by samples of MWCNT, CNF, and high structure carbon black (HS‐CB) nanoparticles with acrylonitrile–butadiene–styrene (ABS) polymer [14] showed that whatever the nanofiller type the reflection loss was always less than the absorption loss (Figure 1.5a and b). The contribution of absorption loss to total EMI SE is 75%. When the shielding by absorption exceeds 10 dB most of the re‐reflected wave will be absorbed within the shield itself and so multiple reflections were ignored. The electromagnetic interference shielding effectiveness of lightweight graphene/polystyrene composite [15] is shown in Figure 1.6a and b. The graphs show that the contribution of reflection loss is negligible over the entire frequency range. The composite has a porous structure. This means that power is dissipated as heat rather than reflected back from the composite’s surface, which clearly describes why absorption is the primary mechanism and the ­secondary mechanism is reflection for such conductive porous composites in the X‐band f­ requency region. The EMI shielding mechanism of PTT/MWCNT composites was studied by resolving the total EMI SE into absorption and reflection loss. Figure 1.7 shows the effect of MWCNT

1.3  Mechanism of EM Shielding Materials

(a) 1.0 0.9

Incident Absorbed Reflected Transmitted

0.8 Power (mW)

Figure 1.4  Power balance graph for MWCNT/PP nanocomposite in the X‐ band frequency range of plate thickness (a) 0.34, (b) 1, and (c) 2.8 mm [13]. Reproduced with permission of Elsevier.

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

2

4 CNT vol.%

(b)

6

8

6

8

1.0 Incident Absorbed Reflected Transmitted

0.9

Power (mW)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

2

4 CNT vol.%

(c) 1.0 0.9

Incident Absorbed Reflected Transmitted

0.8 Power (mW)

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

1

2

3 CNT vol.%

4

5

6

content on absorption and reflection. The graphs shows that with increasing amounts of MWCNT both SE A and SER increased, but the rate of increase of SE A was higher compared to that of SER . At 0.24 vol.% of MWCNT the absorption contribution was 16% but with 4.76 vol.% of MWCNT the absorption contribution increased to 73%. These results show

5

1  EMI Shielding Fundamentals

(a)

40

Figure 1.5  Shielding mechanisms: (a) absorption loss, reflection loss, and total shielding as function of CNF content; (b) power balance as function of CNF content [14]. Reproduced with permission of Elsevier.

Reflection loss Absorption loss EMI total

35 30 SE (dB)

25 20 15 10 5 0 0

(b)

4

6

10 8 CNFwt%

12

14

16

12

14

16

Absorbed power Transmitted power

0.8 Power (mW)

2

Reflected power

1.0

0.6 0.4 0.2 0.0 0

2

4

6

8

10

CNFwt%

(b)

35

35

30

30

25

25

20

SEtotal

15

SER

SEA

EMI SE (dB)

(a)

EMI SE (dB)

6

5

5 9

10

11

Frequency (GHz)

12

SER

15 10

8

SEA

20

10

0

SEtotal

0

8

9

10

11

12

Frequency (GHz)

Figure 1.6  Comparison of SEtotal, SEA, and SER for GPS045 (a) and GPS027 (b) in the 8.2–12.4 GHz range [15]. Reproduced with permission of the Royal Society of Chemistry.

1.3  Mechanism of EM Shielding Materials

Figure 1.7  Graph showing contribution of reflection and absorption loss to total EMI SE in PTT/MWCNT composites [16]. Reproduced with permission of Springer.

0

SEA & SER (dB)

–3 –6 –9 –12 –15

Reflection loss at 8.2 GHz Absorption loss at 8.2 GHz

–18 –1

0

1

2

3

4

5

6

MWCNT (vol.%)

that for PTT/MWCNT composites the primary shielding mechanism is absorption rather than reflection in the observed frequency range [16]. The composite fabricated by dip‐coating process using silver nanowire (AgNW)‐coated c­ ellulose papers shows a reflection dominant EMI shielding mechanism. Figure 1.8 shows that there was a rapid increase in the reflectance, R, at three dip‐coating cycles, which means the dominant shielding mechanism changed from absorbance to reflectance around the number of cycles [17]. An EMI shielding investigation of PET fabric/PPy composite showed that absorption as well as reflection contributes to the total EMI shielding of the composite and that with increasing electrical conductivity the EMI shielding through reflection increased. Figure 1.9 shows that the reflection dominated absorption by considering the total EMI shielding. As shown in Figure  1.9, with decreasing specific volume resistivity shielding effectiveness by reflection increased, and shielding effectiveness by absorption decreased. The increase in reflection mechanism is due to a smaller skin depth of the composite [18]. Graphene is a single sheet of carbon nanostructure in which the carbon atoms are in sp2 hybridization. Graphene is a 2D carbon nanostructure. Graphite is the next member of the 70

1.0

60 0.8 50

R, A, T

0.6

40

0.4

Electrical conductivity in in-plane direction Reflectivity (R) Absorptivity (A) Transmissivity (T)

0.2

0.0

0

10

30 40 20 Dip-coating cycle (count)

30 20 10 0 50

Apparent electrical conductivity (S cm–1)

Figure 1.8  Graph showing contribution of reflection, absorption, and transmittance to total EMI shielding of AgNW/cellulose papers and their dependence on electrical conductivity at 1.0 GHz [17]. Reproduced with permission of the American Chemical Society.

7

1  EMI Shielding Fundamentals

SE A R

35 30

1.0 0.8 0.6

25 0.4

20

0.2

15 10 0.0

0.5

1.0

1.5 2.0 2.5 Specific volume resistivity (ohm cm)

3.0

Reflectance or absorbance

40

EMI SE (dB)

8

Figure 1.9  Graph showing contribution of absorbance, reflectance, and total EMI SE of PET fabric/PPy composites with various specific volume resistivities [18]. Reproduced with permission of Elsevier.

0.0

graphene family, made by the stacking of graphene monolayers. These layers interact through Van der Waals forces of attraction. Graphene nanosheets consist of a monolayer or a few monolayers of graphene and act as an EMI shielding material. These carbon forms are made from sp2 hybridized carbon atoms, with the edges or deformation sites showing the presence of some sp3 hybridized carbon atoms. Graphene oxide (GO), another two‐dimensional material coming under the graphene family, is formed by the introduction of covalent CO bonds in graphene. These graphene forms also contain delocalized π bond electrons  –  the presence of these freely moving electrons make them conducting. Graphene nano‐platelets with polyaniline and poly(3,4‐ethylenedioxythiophene) (PEDOT)/ poly(styrene sulfonate) (PSS) with different ratios give paint like layers and act as an EMI shielding material. The contribution of absorption and reflection to the total EMI shielding value depends on the graphene/polyaniline ratios [19]. Graphene nanoplatelets (GNPs) in the insulating polymer matrix ultrahigh molecular weight polyethylene (UHMWPE) form a conductive network, and with 15 wt% filler the material shows 99.95% EMI shielding attenuation. The presence of a conductive path and the calculation using the power balance point out that the material absorbs more radiation than it reflects [20]. Another EMI shielding material obtained from PEDOT coated MWCNT and polyurethane matrix shows EMI shielding effectiveness through the absorption mechanism [21]. This chapter focuses the fundamentals of EMI shielding (reflection, absorption, and ­multiple reflections). Details on materials used for EMI shielding are given in further c­ hapters of this book. The materials used for EMI shielding are fabricated in the form of an enclosure, i.e. a shielding enclosure. A shielding enclosure is a box or housing or cover providing isolation to the EMI emitter or receiver. This specialized cover is fabricated by considering the requirements for a particular EMI application. The materials covered in this book are fabricated and form part of a shielding enclosure. General principles for designing an enclosure should be followed; the current book deals only with materials for EMI shielding and with advancements in material sciences related to EMI shielding.

­References 1 Hu, Q. and Kim, M. (2008). Electromagnetic interference shielding properties of CO2 activated

carbon black filled polymer coating materials. Carbon Lett. 9: 298–302.

2 Khan, D., Arora, M., Wahab, M.A., and Saini, P. (2014). Permittivity and electromagnetic interference

shielding investigations of activated charcoal loaded acrylic coating compositions. J. Polym. 1–8.

­ ­Reference

3 Jagatheesan, K., Ramasamy, A., Das, A., and Basu, A. (2014). Electromagnetic shielding

4 5 6

7 8 9

10

11 12 13 14 15

16

17

18 19

20 21

behaviour of conductive filler composites and conductive fabrics – a review. Indian J. Fibre Textile Res. 39: 329–342. Lee, B.O. et al. (2002). Influence of aspect ratio and skin effect on EMI shielding of coating materials fabricated with carbon nanofiber / PVDF. J. Mater. Sci. 37: 1839–1843. Chung, D.D.L. (2001). Electromagnetic interference shielding effectiveness of carbon materials. Carbon 39: 279–285. Jose, G. and Padeep, P.V. (2014). Electromagnetic shielding effectiveness and mechanical characteristics of polypropylene based CFRP. Int. J. Theor. Appl. Res. Mech. Eng. 3: 47–53. Gooch, J.W. and Deher, J.K. (2007). Electromagnetic Shielding and Corrosion Protection for Aerospace Vehicles. Springer. Tong, X.C. (2009). Advanced Materials and Design for Electromagnetic Shielding Interference Shielding. CRC Press. Wei, C. et al. (2014). Electromagnetic interference shielding properties of electroless nickel‐ coated carbon fiber paper reinforced epoxy composites. J. Wuhan Univ. Technol. ‐ Mater. Sci. Ed. 29: 1165–1169. doi: 10.1007/s11595‐014‐1060‐y. Huang, S., Chen, G., Luo, Q., and Xu, Y. (2011). Electromagnetic shielding effectiveness of carbon black ‐carbon fiber cement based materials. Adv. Mater. Res. 168–170: 1438–1442. doi: 10.4028/www.scientific.net/AMR.168‐170.1438. Kausar, A. (2016). Electromagnetic interference shielding of polyaniline / poloxalene / carbon black composite. Int. J. Mater. Chem. 6: 6–11. Kang, G. and Kim, S. (2014). Electromagnetic wave shielding effectiveness based on carbon microcoil‐polyurethane composites. J. Nanomater. doi: 10.1155/2014/727024. Al‐saleh, M.H. and Sundararaj, U. (2009). Electromagnetic interference shielding mechanisms of CNT / polymer composites. Carbon N. Y. 47: 1738–1746. Al‐saleh, M.H. (2013). EMI shielding effectiveness of carbon based nanostructured polymeric materials: a comparative study. Carbon N. Y. 60: 146–156. Yan, D.‐X., Ren, P.‐G., Pang, H. et al. (2012). Efficient electromagnetic interference shielding of lightweight graphene / polystyrene composite. J. Mater. Chem. 18772–18774. doi: 10.1039/ c2jm32692b. Gupta, A. and Choudhary, V. (2011). Electrical conductivity and shielding effectiveness of poly(trimethylene terephthalate)/multiwalled carbon nanotube composites. J. Mater. Sci. 46: 6416–6423. Lee, T., Lee, S., and Jeong, Y.G. (2016). Highly effective electromagnetic interference shielding materials based on silver nanowire / cellulose papers. ACS Appl. Mater. Interfaces 8: 13123–13132. doi: 10.1021/acsami.6b02218. Kim, M.S. et al. (2002). PET fabric/polypyrrole composite with high electrical conductivity for EMI shielding. Synth. Met. 126: 233–239. Drakakis, E., Kymakis, E., Tzagkarakis, G. et al. (2017). Applied surface science a study of the electromagnetic shielding mechanisms in the GHz frequency range of graphene based composite layers. Appl. Surf. Sci. 398: 15–18. Al‐saleh, M.H. (2016). Electrical and electromagnetic interference shielding characteristics of GNP / UHMWPE composites. J. Phys. D Appl. Phys. doi: 10.1088/0022‐3727/49/19/195302. Online, V.A., Dhawan, R., Singh, B.P., and Dhawan, S.K. (2015). RSC Adv. doi: 10.1039/ C5RA14105B.

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2 EM Noise and Its Impact on Human Health and Safety Halina Aniołczyk Nofer Institute of Occupational Medicine, Lodz, Poland

2.1 ­Introduction The latest approach to ensure a high level of protection of contemporary humanity as well as electrical and electronic equipment against undesirable emission of electromagnetic fields (EMFs) consists of two aspects: technical standardization of products included in those devices (equipment and installations), which must meet the requirements of the electromagnetic compatibility (EMC), and the bio‐hygienic standards that specify the admissible limits for the EMF emissions to ensure the safety and protection of human health. This is particularly important because in recent years the lifestyle and work of people in many countries have significantly changed as a result of the development of modern telecommunication and data communication systems, especially wireless, which is clearly manifested by the expansion of mobile phone systems, development of computer technology, and the rapid spread of the use of wireless internet. At the same time, progress in medicine has enabled individuals with dysfunctional organs to make use of pacemakers, defibrillators, implants, and inner ear prostheses, neurostimulators of retina (active implants) or artificial joints, surgical clips, stents (passive implants), etc. Such people are more exposed to health hazards due to the impact of the EMF from various devices and systems. The Earth’s natural electromagnetic environment is characterized by EMFs related with the phenomena occurring in the atmosphere, and radiation in the vicinity of the surface of the Earth from the Sun and Space (mainly from the center of our Galaxy). Please note the difference between terrestrial natural constant electric and magnetic fields on one hand, and the slow‐ and fast‐varying EMFs on the other. Modern man in the place of life and work is exposed both to the ubiquitous EMFs of natural origin and to EMFs artificially created through conscious global activities of introducing more modern technologies that change the environment. Each piece of electrical and electronic equipment is likely to be a source of electromagnetic disturbances that occur as conducted interference as well as radiated emissions (conducted, in the frequency range up to 30 MHz, and radiated in the frequency range above 30 MHz) that are emitted to the surrounding environment. Operation of the equipment can disturb the operation of other devices, thus becoming a hazard not only to normal functioning of that equipment but also, indirectly, to humans. Some of that equipment is designed specifically for emission of an EMF (for example, all broadcasting equip­ ment). The spectrum of EMFs and radiation covers a very wide frequency range. Figure 2.1 shows the general spectrum classified according to bandwidth. This classification is of significant Advanced Materials for Electromagnetic Shielding: Fundamentals, Properties, and Applications, First Edition. Edited by Maciej Jaroszewski, Sabu Thomas, and Ajay V. Rane. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

Microwave

1014 IR

Radiowave

Low frequency Long wavelength Low quantum energy

1016 UV

1018 X-ray

Radioactive sources

1012

X-ray image

Mobile phone

1010

108

LF, MF, HF, VHF, UHF

ELF, VLF

Visible light

106

Radar radio links radio astronomy

104

Radio and television broadcast

HF welder short diathermy ISM

Industry

102

Wi-Fi wireless network microwave oven microwave diathermy ISM

0

AC power 50 Hz overhead power line

2  EM Noise and Its Impact on Human Health and Safety

Static E, H fields

12

1020 Gamma ray

1022 [Hz] Cosmic ray

Optical High frequency Short wavelength High quantum energy

Non-ionizing radiation Low induces currents Non-thermal effects

High induces currents Thermal effects

Ionizing radiation Electronic excitation Photochemical effects

Broken bonds, DNA damage, cancer mutation, birth defects

Figure 2.1  Spectrum of electromagnetic field and radiation.

importance, because the mechanisms according to which EMF affects living organisms, including humans, depend significantly on the frequency. The relevant literature in this field makes a clear distinction between the frequency bands, which are assigned different names (industrial frequencies, radio waves, microwaves, etc.) and introduces a specific terminology to describe the parameters characterizing those fields, measuring the intensity levels of their prevalence in areas accessible to humans, especially in the area of work, and testing the effects of the exposure. In this study, attention has been focused on the field of non‐ionizing radiation, which has a frequency below 8 × 1014 Hz, that does not cause ionization of the medium that it penetrates (for example, the quantum energy for EMFs with a frequency below 300 GHz is less than 1.25 × 10−3 eV). Currently, the radio‐frequency EMFs most frequently used in practice includes frequencies ranging between 3 kHz and 300 GHz and their regulation and use is governed by the International Telecommunication Union (ITU) [1]. These EMFs are used mainly in sectors such as telecommunication (including broadcasting), wireless communication, radar, radio navigation, meteorology, and global positioning systems (GPS); in industry, mainly for induc­ tive and capacitive heating; in medicine: surgery, physical therapy, diagnostics (EPR spectrom­ eters, magnetic resonance imaging (MRI) systems), for treatment of cancer; and in science. In Europe, the problems of protection of devices and humans from undesirable or excessive emissions to the working environment is governed by Directives: on EMC, 2014/30/EU [2, 3] and protection, 2013/35/EU [4] and uniform requirements apply to the member countries of the European Union (EU). In the field of protection of the general population against EMFs recommendations from the International Commission on Non‐Ionizing Radiation Protection (ICNIRP) 1998 are used [5] and in EU countries the applicable regulation is the Recommendation 1999/519/EU on the limitation of public exposure to EMFs (0 Hz to 300 GHz) 1999 [6]. Implementation of the Council recommendation is not compulsory and thus the individual countries implement their own protective policies [7]. Compliance with the requirements ­contained in the above documents enforces a variety of preventive measures, including organi­ zational and technological ones. Screening of equipment, workstations, rooms that require particular protection and even whole buildings or separate areas (architectural shielding) ­represents one of several intensively developing technological solutions. This chapter discusses

2.2  Impact of Non‐ionizing EMFs on Humans

the basic impacts of EMFs on human health and safety, taking into account the preamble to the recommendation. This work discusses the basic effects of EMFs on human health with refer­ ence to the basis for limiting exposure set forth in ICNIRP 1998, the ICNIRP 2009 update, and taking into account recent literature data [8, 9]. It presents the major and also the most com­ mon devices and installations that are sources of EMF emissions, together with the assessment of the level of exposure to occupational and residential hazards experienced by the members of contemporary society. The rules governing the protection of humans from EMFs recom­ mended for use at the international level, in the countries of the European Union, or at the national level (for example, in Poland) differ in the values of exposure limits. The differences mainly result from the adopted philosophy (biological effects or health effects, remote, or short‐term effects). The process of developing international ICNIRP regulations based on the concept of specific absorption rate (SAR) as a measure of exposure to EMFs and the national provisions using Polish regulations as an example to highlight their original, zonal concept of gradation of the level of risk attributable to EMF exposure in the work environment have been briefly outlined. Exposure to EMFs should be monitored and in cases were the allowable limits are exceeded appropriate measures should be undertaken: prevention, such as the monitoring of the health of workers; organizational, such as proper location of EMF sources relative to the workplaces, rotation of workers employed at workplaces with severe EMF exposure, and per­ sonal protective equipment as well as monitoring of the levels of EMF intensity; personnel training; technological, such as shielding of EMF sources and workplaces. The issues discussed in this chapter mainly include problems relating to the frequency range of 30 kHz to 100 GHz and emission occurring at undesirable or excessive intensity level is considered here as electro­ magnetic disturbance (EM noise) subject to the protective provisions. The devices used in the army and in air or naval navigation, which are dominated by the use of radar and the produc­ tion of pulsed EMFs, are not discussed here.

2.2 ­Impact of Non‐ionizing EMFs on Humans The possibility of a detrimental effect on the health and safety of persons exposed to EMFs and the establishment of threshold values for EMF interaction with living organisms in the widest possible spectrum invariably has aroused the interest of researchers since the 1950s (Schwan, Gordon, Johnson et al.) [8–10]. This is due to the problems discussed below. First of all, it is advisable to know the effects of EMF on the human body and then correlate those effects that have already been detected and documented with their causes, mainly the conditions of exposure, such as operating frequency range of the field, its intensity and the effective time of the exposure. It is not easy in a situation where non‐ionizing EMF spectrum covers the ­frequency range from 0 Hz (static electric and magnetic fields) up to 300 GHz (slow‐ and fast‐ changing EMF). So far, it has been established that: ●●

●●

●●

Electrostatic fields interact with living organisms by inducing an electrical charge on the surface of the body, producing current flow inside the body, inducing dipole and orienting permanent dipoles in accordance with the direction of the field. Static magnetic fields interact with the internal electric currents of the organism, changing the orientation of the structures according to their magnetic properties and affecting spin states of electrons. Alternating electric, magnetic, and EMF – their direct impact on living organisms depends primarily on the frequency of the field and mainly includes: the phenomenon of “feedback” from electric fields and magnetic low frequencies and absorption of energy in the tissues,

13

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2  EM Noise and Its Impact on Human Health and Safety

which turns into other types of energy, e.g. heat, especially with fields of the higher frequency range. Overall, health effects resulting from exposure to EMFs include the stimulation of the sense organs, nervous system, and muscles (non‐thermal effects), and those resulting from the heating of body tissues (thermal effects). Such effects may be classified into direct effects associated with the impact of the EMF on the human body (which may be either thermal or non‐thermal) and indirect effects caused by the EMF interacting with items remaining in the field, such as medical devices (medical equipment, whose work can be disturbed) and active devices (pacemakers, defibrillators) and passive implants, etc. whose performance may become impaired. The high frequency EMFs (>100 kHz) have been divided into four frequency ranges characterized by specific EMF energy absorption characteristics in the human body: ●●

●●

●●

●●

In the frequency range 100 kHz to 20 MHz, energy absorption in the trunk increases with frequency and may reach a significant value in the limbs. In the range 20–300 MHz, the absorption of energy in the whole body may be considerable due to the so‐called geometric resonance (association between the size of the body or its anatomical parts and the wavelength of the incident EMF), i.e. 450 MHz for the head and 150 MHz for the forearm. In the range 300 MHz up to approx. 3 GHz, there may occur significant local, non‐uniform absorption and the hot‐spots‐in‐the‐head phenomenon. In the range above 10 GHz, absorption occurs on the surface of the body [11, 12].

The human body, from the point of view of its interaction with EMFs, is a medium with irregular shapes, varying sizes, and a multi‐layer structure of the tissue showing different dispersive dielectric properties depending on the frequency. Thus, the amount and distribution of EMF energy absorbed by the human body depends on the electrical characteristics of the tissue, the geometry, the frequency, and polarization of the incident EMF. In‐depth theoretical and experimental studies initiated by Johnson and Guy in the 1970s [10] on the distribution and size of the absorbed energy of EMFs in living organisms taking into account the exposure conditions and the characteristics of the incident EMF wave have resulted in the implementation of the concept of SAR as a measure of exposure to EMF. The values of SAR, both local and whole‐body, depend on a number of environmental parameters, including the distance between the source and the object, the frequency, polarization, grounding, and the EMF reflection [10–12]. The concept of SAR was introduced in 1991 in the IEEE C95.1‐1991 [13] and is now accepted as a dosimetric value, which is a measure of the energy absorbed per unit mass of tissue, expressed in W kg–1. Since then, the SAR values have become one of the main criteria in determining the limits of exposure to EMF. In the international ICNIRP guidelines, the SAR value represents the criterion of basic restrictions [5]. In particular: ●●

●●

●●

For frequencies below 1 kHz, the fundamental criterion for the magnetic field is the density of the current induced in the human body, the threshold value of which required for stimulation of the central nervous system (CNS) or evoking of magnetophosphenes (i.e. flashes of light that are seen when one is subjected to a 20 Hz magnetic field and 50 mV m−1 electric field [14]) is 100 mA m−2. In the frequency range 100 kHz to 10 MHz, the criteria include both the value of current density, such as to avoid changes in the nervous system, and the value of SAR, such as to avoid heat stress. In the frequency range 10 MHz to 10 GHz, the criterion is the SAR value independently for the whole body and local exposure, separately for the head, the trunk, and the limbs.

2.2  Impact of Non‐ionizing EMFs on Humans ●●

●●

●●

For frequencies above 10 GHz, the penetration depth of the EMF into the tissues is low and so the SAR is not the best parameter for estimating the energy absorbed (absorption phenomenon occurs mainly on the surface of the body). Thus a more correct dosimetric quantity here is the value of the power density of the incident field. For the frequency range, with assumed dominance of the thermal effect, the experimentally determined threshold concentration SAR averaged over the entire body was 4 W kg−1, which, under moderate environmental conditions, after 30 minutes human exposure in the resting pos­ ture results in a temperature rise of not more than 1 °C [5]. The impact of EMF on humans and the consequent risk to health should be distinguished from terms such as interaction, biological effect, or the perception of hazard. Recent international rules in the EU are based on well‐documented health effects resulting from current (low frequency) and thermal (high frequency) influences for short‐term exposure. The effects of long‐term exposure are not sufficiently scientifically documented. At the same time, it is suggested that if the absorbed energy does not cause an increase in temperature above the level that the human body itself may compensate by thermoregulatory physiological processes, the observed effects, classified as non‐thermal effects, according to the current state of knowledge, are considered to be a weak biological agent. A review of the world literature by Szmigielski (2007) on clinical research (medical and epidemiological studies on people exposed in the workplace to EMF) shows no evidence of a specific disease attributable to that exposure [15].The influence of high‐frequency EMF is likely, but only for cases of long‐term exposure (>10 years), and with intensities of at least the order of a few W m−2, and a daily dose above 30–40 W m−2 h−1 [16]. However, there may be all sorts of disorders and functional changes of physiological systems (CNS, the vegetative regulation, cardio­ vascular, and so on). These may include non‐specific morbidity symptoms (NSMS) such as headaches, fatigue, insomnia, distraction, etc., symptoms of vegetative neuroses, increased number and frequency of functional changes, such as cardiovascular symptoms developing normally with age and also slightly increased risk of developing certain types of cancer [17]. The current state of knowledge points to the existence of limited evidence of increased risk of certain cancers, including leukemia or brain tumors in a population, as described in the publications of the International Agency for Research on Cancer (IARC) based on literature reports on the effects of neoplasia as a result of chronic ­exposure to EMFs with low levels of intensity that: Classify radio frequency electromagnetic fields (RF EMF) as group 2B, which includes f­ actors probably carcinogenic to humans [18]; classify electric and magnetic static extremely low frequency (ELF) fields as the same group 2B of substances possibly carcinogenic to humans [19].

Consequently, some countries have introduced the Principle of Precautionary Approach [20] (for example, Switzerland, Italy) [21, 22], which is a more restrictive recommendation compared to the EU recommendation [6] primarily as far as the protection of the general population is concerned, to national regulations. Our current knowledge on the health effects of long‐term exposure to EMFs is limited. This is mainly due to the lack of relationship between the results of epidemiological studies on populations and experimental animals as well as the lack of a full explanation of mechanisms of EMF effects at that level of the intensity and duration of exposure. Sensational reports on adverse health effects of human exposure to EMFs often result from the application of unfair study protocols or unfair analyses of the study results. This last statement is important from the point of view of protecting the environment and people who are subject to a chronic but poorly controlled exposure to EMFs and confirms that further studies are required [7, 23].

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2  EM Noise and Its Impact on Human Health and Safety

2.3 ­Overview of Most Common Sources of EMFs in the Occupational and Residential Modern Human Environment The EMFs produced artificially as a result of human activity occurring both in the work­ place and in the residential municipal environment may adversely affect human health. The development of technological civilization, especially the electrification (overhead high voltage power lines) and the dissemination of telecommunications and wireless communications (radio and TV broadcasting stations, radar, radio navigation, base stations, mobile phones) mean that most of the world’s population now live in an electromagnetic environment with much higher EMF strengths than existed 100 years ago. Excessive, higher‐than‐natural ­levels of intensity in the biosphere have become especially evident in low‐frequency areas of non‐ionizing radiation. Currently, this area is expanding to the scope of frequencies above 1 GHz, used primarily in wireless communications and the Internet. Radio lines are already operated at frequencies of 60–90 GHz. Modern science does not fully explain the effects of EMFs on living organisms, particularly in such high‐frequency ranges. A huge amount of equipment and installations producing EMFs, hereafter referred to as sources of EMFs, are characterized by great diversity, which means that discussion of the risks they offer to human health and safety is difficult. Therefore, available data from studies ­p erformed at the Institute of Occupational Medicine (NIOM) in Lodz under the leadership of the author of this study are used here, relating to those EMF sources that are most ­distinctive in terms of technical parameters, high EMF intensities occurring in their ­v icinity, ubiquitous prevalence, and employees’ access to areas where the values of the intensities are close to the maximum admissible intensity (MAI) specified by Polish regulations [24–29]. In addition to power supply lines operated in Europe at 50 Hz and in the USA at 60 Hz, the most widely used are radiofrequency (3 kHz to 300 GHz) EMFs. Particularly intensive sources of EMF are used in sectors such as: ●● ●●

●●

●●

●●

●●

Broadcasting – radio and television broadcasting stations (RTVCS); communication, including wireless – line‐of‐sight and satellite radio lines, base stations and mobile stations of mobile radiocommunication (dispatcher radiotelephone networks), mobile phone base stations, public security systems, wireless computer networks (WLAN, WiMax); radiolocation – radar, radio navigation, and meteorological stations (using pulse‐modulated electromagnetic radiation), GPS; industry  –  mainly induction heating (furnaces for hardening) and capacitive heating (dielectric welding machines, dielectric dryers); medicine – surgery (electrosurgical diathermy), physical therapy (short‐wave HF) and micro‐ wave (MF) diathermy, diagnostics (EPR spectrometers, MRI equipment), health care (hyperthermia kits and accelerators used in oncology); science: radio astronomy (space research), laboratories at universities, scientific research institutes, industrial laboratories etc. develop and apply new systems, experimental proto­ types of new devices, testing, and checking of a wide range of aspects – from EMF generation through EMF propagation in various physical media to the effects, including biological (research on plants and animals).

The most common equipment and installations generating EMF in the occupational and residential environment are summarized in Table 2.1. This summary refers to Poland, but may serve as a database to illustrate the problems typical for most countries, not just in Europe.

2.3  Overview of Most Common Sources of EMFs in the Occupational and Residential Modern Human Environment

Table 2.1  Most common equipment and installations generating 0 Hz–300 GHz EMF. Frequency range 1

Device/system 2

Application 3

0 Hz (static magnetic field)

MRI

Medicine, healthcare: diagnostics

0 Hz (static magnetic field)

NMR spectroscopy

Science: academic and industrial laboratories, absorption spectroscopy technique used in biology, biochemistry, chemistry

50 Hz

Overhead power lines, transformers, switchboards

Power engineering: generation and transmission of electricity

135.6 kHz; 440 kHz

Furnaces for induction heating and hardening

Industry: metallurgy

440 kHz; 1760 kHz

Electrosurgery apparatus

Medicine, healthcare: surgery, gynecology, dermatology, ophthalmology

225 kHz–13.87 MHz

AM broadcasting long‐, medium‐, short‐wave

Broadcasting: radio broadcasting stations

27.12 MHz

Short‐wave diathermy (SF)

Medicine, healthcare: physiotherapy

27.12 MHz

Dielectric welders, HF dryers

Industry: electroheat, welding of plastics, timber factory, automobile factory

87.5–108 MHz

VHF FM broadcasting

174–230 MHz

Digital radio, DAB+

Broadcasting: radio broadcasting stations Broadcasting: digital radio broadcasting stations

470–862 MHz

Digital TV, DVB‐T

Broadcasting: TV broadcasting

380–430 MHz

TETRA – terrestrial system trunked radio

Transmission and radiocommunication: civil and military services (including national and public security, emergency fire‐ fighting, medical emergency, and customs system)

120 kHz–868 MHz

EAS – electronic anti‐ theft system

Transmission and radiocommunication: identification of objects, their protection from theft in retail sales, magazines, etc.

125 kHz–5.8 GHz

Radiofrequency identification of objects (RFID)

Transmission and radiocommunication: identification of objects and monitoring of their distribution in retail sales and magazines

420–450 MHz

Mobile communications CDMA

Transmission and radiocommunication: mobile phone base station networks

800–2600 MHz

LTE – high speed Internet long‐ term evolution; mobile phone GSM, UMTS

Transmission and radiocommunication: communication (talking on GSM, WCDMA system, data transmission, multimedia transmission) (Continued )

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2  EM Noise and Its Impact on Human Health and Safety

Table 2.1  (Continued) Frequency range 1

Device/system 2

Application 3

2400 MHz i 5500 MHz

WiFi

Transmission and radiocommunication: data transmission

2450 MHz

Microwave diathermia (MF)

Heath care: physiotherapy

915 MHz, 2450 MHz

Microwave dryers, industrial microwave lines

Industry: processing of plastics, drying

2450 MHz

Bluetooth, Wi‐Fi

Transmission

2450 MHz

Microwave oven (professional and household product)

Gastronomy: restaurants, bars, catering facilities, heating, defrosting of foods

6–150 GHz

Radio links

Transmission: communication network, cellular wireless network

Currently, broadcasting is dominated by VHF FM broadcasting systems, where antennas are installed not only on free‐standing towers but also on the roof of tall public buildings and also on the roof of residential buildings. In radio communications, the dominant systems include GSM900, GSM1800, and UMTS900 and UMTS2100 mobile phone base stations, in which not only antennas but also containers with transceiver units are deployed like the VHF FM stations. To ensure their continuous operation, dedicated teams of workers are employed to install or remove, maintain, and repair the antennas. In health care, the dominant types of such devices include apparatus for electrosurgery and short‐wave (SW) diathermy. These devices are installed indoors, in operation theaters and physical therapy rooms, not always correctly, so there are frequent problems with interference with other equipment due to undesired EMF emissions. Very stringent requirements apply to the installations of MRI, which must be installed in a specially shielded rooms to protect the MRI equipment against EMFs coming in from the environment. In industry, dominant EMF sources include induction furnaces for steel hardening, capaci­ tive welding machines, and high‐frequency (HF) dryers. They are a source of strong EMF due to improper shielding of the main sources of EMFs, such as inductors and electrodes, and also the HF electric current leads. Personnel working in close vicinity to such equipment include workers who make, install, test, operate, maintain, repair, and dismantle it; those workers are subject to varying degrees of risk of exposure to EMFs. In the municipal environment, all people are exposed to EMFs at different frequency ranges, but the extent of that exposure varies depending on the industrialization of the area and popu­ lation density. The most important problem is to meet the MAI standards developed by the bodies of researchers, including biologists, physicists, doctors, and engineers, and keep the EMF inten­ sity within the ranges considered to be acceptable for the safety and protection of human health. What are the intensities of EMFs in the real working and living conditions of modern man in the jungle of so many devices and installations producing the EMFs with respect to the MAI provisions? For risk analysis of EMFs, equipment and installations were selected that are found in each country, and EMFs produced by that equipment and installations were assessed

2.4  Protection Against EMFs in European and International Law

with reference to ICNIRP 1998 recommendations and regulations developed and currently valid in Poland; please note that the requirements of the Polish regulations are more stringent than the recommendations of ICNIRP, Directive 2013/35/EU, and Council Recommendation 1999/519/EU.

2.4 ­Protection Against EMFs in European and International Law Health, as defined by the World Health Organization (WHO), is a state of complete physical, mental, and social well‐being, and not merely the absence of disease or physical disability. The problem of protecting people against excessive or unwanted EMF emissions prevailing in their residential and occupational environments is regulated by the relevant law. These regu­ lations are of various importance: the standards, recommendations, directives, decrees, or ordinances. According to the WHO the most important regulations include ICNIRP 1998, 2010 and 2014 [5, 30, 31]. For the frequency range in which the thermal effect was assumed to be dominant, a whole body SAR threshold value of 4 W kg−1 at which the temperature increase is within 1 °C was experimentally determined. Thus, for occupational exposure, the above value with the safety factor of 10, i.e. SAR  =  0.4 W kg−1, has been adopted as a criterion of basic restriction for whole‐body exposure. Basic restrictions in the ICNIRP Guidelines are given, depending on the frequency EMF as: limits of density of the current induced in the head and trunk, SAR averaged over the whole body, the local SAR in the head and trunk, and separate local values of SAR in the limbs. Basic criteria according to ICNIRP 1998 for occupational exposure to EMF are presented in Table 2.2. The specified values of SAR are not directly measurable. Table 2.2  Basic restriction on current density and SAR for RF EMF (100 kHz–10 GHz).a–g

Exposure characteristics

Occupational exposure

Current density for head and trunk Frequency range f (mA m−2) (rms)a,b,c,d

Whole‐body Localized SAR average SAR (head and trunk) Localized SAR (W kg−1)e (W kg−1)e,f,g (limbs) (W kg−1)e,f,g

100 kHz–10 MHz

0.4

10

20

10 MHz–10 GHz

General public 100 kHz–10 MHz exposure 10 MHz–10 GHz a

f/100 —

0.4

10

20

f/500

0.08

2

4

—

0.08

2

4

  f is the frequency in hertz.   Because of electrical inhomogeneity of the body, current densities should be averaged over a cross‐section of 1 cm2 perpendicular to the current direction. c   For frequencies up to 100 kHz, peak current density values can be obtained by multiplying the rms value by √2 (~1.414). For pulses of duration tp the equivalent frequency to apply in the basic restrictions should be calculated as f = 1/(2tp). d   For frequencies up to 100 kHz and for pulsed magnetic fields, the maximum current density associated with the pulses can be calculated from the rise/fall times and the maximum rate of change of magnetic flux density. The induced current density can then be compared with the appropriate basic restriction. e  All SAR values are to be averaged over any six‐minute period. f  Localized SAR averaging mass is any 10 g of contiguous tissue; the maximum SAR so obtained should be the value used for the estimation of exposure. g  For pulses of duration tp the equivalent frequency to apply in the basic restrictions should be calculated as f = 1/(2tp). Additionally, for pulsed exposures in the frequency range 0.3–10 GHz and for localized exposure of the head, in order to limit or avoid auditory effects caused by thermoelastic expansion, an additional basic restriction is recommended. This is that the SA should not exceed 10 mJ kg−1 for workers and 2 mJ kg−1 for the general public, averaged over 10 g tissue. b

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2  EM Noise and Its Impact on Human Health and Safety

To enable an assessment of exposure, so‐called reference levels were defined, i.e.­values of the electric component of the field, of the magnetic component, magnetic induction, and power density for the equivalent plane wave. The reference levels were determined using the appropriate mathematical models and extrapolating the results of laboratory tests to give the so‐called resonance curve. For the range above 10 GHz, a limit for power density of 50 W m−2 was specified. This value is averaged over any 68/f1.05‐minute period (where f is the frequency in GHz); in other frequency ranges (up to 10 GHz) the averaging time is six minutes for each of any exposure period. Exposure limits for the population are five times lower than those accepted for occupational exposure. ICNIRP recommendations have been adopted by many countries worldwide, including Europe. In the EU, work has been undertaken on the unification of values limiting the exposure to EMFs in the workplace. In 1989, Directive 89/391/EEC was published on projects aimed at improving health and safety in the work environment [32]. It is general in its character and introduces the obligation for employers to ensure the protection of workers against excessive exposure to harmful physical agents, including EMF. In 2013, the detailed Directive 2013/35/ EU was published on the minimum requirements to control risk to the health and safety of workers exposed to EMF; those requirements were to have been implemented for use in all EU member states not later than 1 July 2016 [4]. To limit public exposure to EMF, on 12 July 1999 the European Council recommended ­exposure limits by an official act [6], which was based on the recommendations of ICNIRP [5]. On a global scale, individual countries implement more stringent limits compared to the ICNIRP guidelines and EU recommendations, particularly with regard to exposure of the ­population, using the Principle of Precautionary Approach [20]. Table  2.3 shows as an example the diversity of exposure limits for the 1800 MHz frequency [33]. Poland is one of those European countries that has one of the oldest traditions in the imple­ mentation of protection from EMFs in the workplace [34]. The first limits on the exposure to EMF were introduced for use in 1961 for the microwave range (frequency range >300 MHz) and they were an adaptation of regulations in the former USSR. They were based on the results Table 2.3  EMF radiation regulations for mobile towers (1800 MHz) valid in individual countries. Country

Power density (W m−2)

ICNIRP and EU recommendation

9.0

Australia

9.0

Austria

0.001

Belgium in Flanders or per site in Brussels

0.047

Bulgaria, Russia

0.1

Germany (precaution recommendation only)

0.09

Hungary, Poland, French (Paris)

0.1

Italy (in sensitive areas)

0.1

Luxembourg (in sensitive areas)

0.024

New Zealand

0.5

Switzerland

0.095

2.4  Protection Against EMFs in European and International Law

of research carried out in the 1950s at the Moscow Institute of the Academy of Medical Sciences under the direction of Gordon (1966) [9]. The concept of protection against the non‐thermal effects, based on clinical and medical studies of workers exposed to EMFs, was used as a ­criterion for the protection of human health. The Soviet regulations of 1956 specified 0.1 W m−2 per work shift and 10 W m−2 for temporary (short‐term) exposure limited to five minutes per day as the highest allowable power density in the frequency range 300 MHz to 100 GHz. Somewhat earlier, in 1953, limits of exposure to EMFs were implemented in the United States of America, based on the work by Schwan [8]. The concept of the adopted exposure limits was based on the energy balance for a man who performs light work, and was intended to protect people against thermal effects. The highest allowable power density in the frequency range 10 MHz to 100 GHz was assumed to be 100 W m−2 for each period of six minutes of exposure at any time. Since 1972, Poland has introduced successively its own limit values of occupational exposure to EMF. The philosophy behind the Polish exposure limits was based on the original concept of protection zones: hazardous, dangerous, and intermediate. The area outside the protection zones has been assumed to be the safe zone. The criterion of non‐thermal effects has been adopted as the limit for the safe zone, while the criterion of thermal effects with the appropriate safety factor has been adopted as the limit value for the hazardous zone. Thus, for the hazardous zone, limits on the time spent in the zone were introduced, based on the energy load of the human body exposed to the EMF, which was a measure of exposure, while a total ban on staying in the dangerous zone was introduced. Limits of exposure for the general popu­ lation were introduced for the first time in 1980. Limits of exposure to EMFs in the workplace over the whole frequency range of 0 Hz to 300 GHz were regulated in 2002 [35] and in 2003 for the general population [36]. Figure 2.2 shows a graphical comparison of exposure limits under ICNIRP 1998 and those currently in force in Poland. There are minor differences between the requirements quoted above in the process of deter­ mining the actual values of intensity of the electric (E, V m−1) and magnetic (H, A m−1) fields acting on the worker: averaging (over time) of the measured effective value; maximum acting on the employee (according to ICNIRP); and maximum instantaneous effective value acting in a location corresponding to the axis of a worker body (according to the Polish requirements). 100000 Occupational, Poland General public, Poland Occupational, ICNIRP General public, ICNIRP

E [V m–1]

10000

1000

100

10

1

1 kHz 1

101

102

103

1 MHz 104

105

106

1 GHz 107

108

109

1010 1011 1012

f [Hz]

Figure 2.2  Comparison of exposure limits set by ICNIRP 1998 and those currently in force in Poland (2002).

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2  EM Noise and Its Impact on Human Health and Safety

Table 2.4  Permissible levels of electromagnetic fields in the environment under the Polish regulations. Frequency range (f)

Electric field (E)

Magnetic field (H)

Power density (S)

Notes

0 Hz

10 kV m−1

2500 kA m−1

—

—

From 0 Hz to 0.5 Hz

—

—

—

From 0.5 Hz to 50 Hz

10 kV m

—

—

—

For building land

—

—

—

—

—

—

50 Hz From 0.05 kHz to 1 kHz From 0.001 MHz to 3 MHz From 3 to 300 MHz From 300 MHz to 300 GHz

−1

1 kV m−1 —

2500 kA m−1 −1

60 A m

60 A m−1 a

3/f   A m

20 V m−1

3 A m−1

7 V m

—

−1

7 V m−1

−1

—

0.1 W m−2

—

a

 f – As indicated in the frequency range column.

Polish regulations are currently being modified because of their necessary harmonization with Directive 2013/35/EU. Work on the harmonization of national legislation with the Directive has continued in the Interdepartmental Committee on the Maximum Admissible Concentrations and Intensities for Agents Harmful to Health in the Working Environment (Expert Group on Electromagnetic Fields) and is due to be completed by the end of June 2016. Table 2.4 shows the limits for public exposure to EMFs in the Polish regulations.

2.5 ­Assessment of the Level of EMFs in the Workplaces An analysis of environmental conditions of workers occupationally exposed to EMFs based on the results of tests and measurements of RF EMF for more than 450 selected pieces of equip­ ment representative of various sectors of the economy in which they are most often used and the values obtained indicated that the intensities adopted for civil protection had been exceeded [29]. Elevated values were recorded for almost 280 devices in radiocommunication, more than 100 devices in health care, 60 devices in industry, and 8 devices in science. Analyzed parameters characterizing the source of EMF were such as the intensity of the elec­ tric field (E) in the vicinity of the device and at employee work space, dimensions of protection zones, and how many times the intensity at employee space is higher than the MAI. The MAI value corresponds to the value separating the intermediate zone from the dangerous zone, where, according to Polish regulations, workers can stay during an eight‐hour working day. A similar analysis was performed according to the ICNIRP 1998, criteria assuming that EMF was present at the employee space for at least six minutes. The results of the analysis of levels of intensity of the electrical component of the EMFs for selected RF devices used in various sectors of the Polish economy are presented graphically in Figure 2.3. In the broadcasting sector, transmitters and antennas of 50 stations (30 VHF FM radio ­stations, producing EMF in the frequency range 87.5–108 MHz, and 20 television stations operating 48 television channels in the frequency range 470–862 MHz) were analyzed in detail. The highest measured values of E field strength recorded in the vicinity of 8 VHF FM radio antennas installed on masts erected on the roofs of buildings ranged between 2.5 and 83.0 V m−1 (Me: 5.8 V m−1) at the place of maintenance works, accessible from the roofs of the building, superstructures, or platforms on the poles (with the exception of work done in the immediate

Electric field strength [V m–1]

2.5  Assessment of the Level of EMFs in the Workplaces 1000

100

10

1 A

B

C

D

- From the device

- ICNIRP '1998

- At workplace

- MAI, POLAND

E

F Device

G

A - Electrosurgery units B - Short - wave diathermy C - Microwave diathermy D - HF press E - HF dielectric welders

H F GHI J K-

I

J

K

Microwave industry units Nonprofesional microwave oven Radio broadcast antenna Land mobile vehicle radiotelephones Land mobile portable radiotelephones Radio relay antenna

Figure 2.3  Highest levels of electric field intensity for workplaces at selected common RF EMF sources used in Poland.

vicinity of antennas, at the height of their installation). The ICNIRP was higher by a factor of 1.4–4.2 relative to the MAI. In radio and wire communication, the results of the measurements for 18 radio line (RL) objects were analyzed. In an accessible area of five antennas, the RL meas­ ured values were 6.1–44.7 V m−1 (Me: 10.6 V m−1). The quotient exceeds the MAI of 2.2 only in relation to work on platforms near the antenna and do not include activities at the height of the antennas themselves (in the direction of the axis of the main lobe). The ICNIRP limit value was not exceeded. Land mobile radiocommunication uses vehicle and portable radiotelephones. The detailed analysis covered 11 transportable stations installed in vehicles, and three portable phones used by the security services. Radiotelephone stations were operated at 44 MHz and 144–174 MHz, transmitter power: 25 watts for stations operating in the higher band, 10 W for those operating in the lower frequency band, and 1–5 W for portable phones. The highest values of measured E values inside the 11 vehicles with installed transportable units were 5–76 V m−1 (Me: 23 V m−1) in the immediate vicinity of the installation and 4.3–36.0 V m−1 (Me: 7.5 V m−1) in places occu­ pied by the driver and the worker next to the driver. The ICNIRP limits were not exceeded. The intensity of E field at the driver–radio operator position was up to 1.8‐fold MAI. Highest meas­ ured E field intensities in the vicinity of the portable phone antenna were 54–74 V m−1 (Me: 60 V m−1), in the absence of the operator. ICNIRP quotient was up to 1.2‐fold and MAI quotient was over two‐fold. Analysis of the results of tests and measurements of EMFs for extreme work­ ing conditions during the assembly, disassembly, and maintenance operations performed in the vicinity of active radio and television broadcasting antenna systems and sector antennas of mobile phone systems has showed that the highest levels of EMF intensity occur in the immedi­ ate vicinity of antennas, accessible to the staff engaged in the assembly, disassembly, and main­ tenance of antennas, particularly in the multi‐program type objects of RTVCS of high power, in which the antenna systems are installed on high, free‐standing masts. The magnitude of the levels of field intensity E in the case of work on one of the tall masts of a multi‐program installa­ tion (center) can be explained by the following information. Simply walking within VHF FM (power of the transmitters reduced to 16 kW) and TV (power of transmitters 80 kW) antenna

23

24

2  EM Noise and Its Impact on Human Health and Safety

systems results in the exposure of a worker to a field E with very high levels of intensity: on the platforms within the system of VHF FM antennas the electric field intensity ranged from 152 to 180 V m−1 at transmitter power reduced by 50% and was over 180 V m−1 at the power reduced by 25%. Even on the terrace under the TV antenna (frequency range of 535.25 and 615.25 MHz), the field E intensity ranged from 61 to 148 V m−1. The MAI quotient was exceeded 9‐ and 7.4‐ fold while the ICNIRP value was exceeded 2.5–3‐ and 2‐fold. With this type of work, excessive exposure is likely to result from the effective time during which the employee remains in the EMF. Permissible times of exposure to those high field E intensities are six and nine minutes per shift, respectively. When working on masts supporting the 11 sector antennas of mobile phone base stations (BTS) operated in the 900 and 1800 MHz ranges, the measured values were as high as up to 74 V m−1. The ICNIRP values were not exceeded while the MAI was exceeded up to 3.7‐fold. In health care (surgical wards of hospitals and physiotherapy rooms) 105 medical instruments  –  45 electrosurgical, 54 shortwave diathermy, and six microwave diathermy units – were analyzed in detail. The highest measured field E intensities in the close vicinity of electrosurgical apparatus were 23–1000 V m−1 (Me: 180 V m−1), while at the position of the oper­ ator, surgeon, the intensities were 8.5–400 V m−1 (Me: 105 V m−1). The ICNIRP limit was not exceeded, while the MAI excess quotient (EQ) was 4.0. The highest values of field E intensity measured around the shortwave diathermy apparatus ranged from 10 to 1000 V m−1 (Me: 156 V m−1), and at the position of the physiotherapist the corresponding values were 3–220 V m−1 (Me: 19 V m−1). In 35% of cases, protection zones were provided outside the diathermy treat­ ment cabins or rooms. The ICNIRP excess quotient was up to 3.6, while the MAI EQ was up to 11.0. The highest values of field E intensity measured around the microwave diathermy appara­ tus ranged from 194 to 213 V m−1 and at the position of the physiotherapist the intensities ranged between 58 and 160 V m−1 (Me: 122 V m−1). The protection zone encompassed the adjacent treatment cabins and the corridor. The ICNIRP EQ was 1.2 and MAI EQ ranged from 3 to 8. In industry, 60 units were analyzed in detail, including 29 high frequency (HF) presses (with the capacitive type heating) used in wood, paper, and textile industries to remove water or other fluids from absorbent products, and in the automotive industry for drying sound insulation ele­ ments, and 16 dielectric welders, in which a HF generator was used for welding thermoplastic film products, and 15 non‐industrial microwave ovens, which are often used in catering. The highest measured values of field E intensities adjacent the HF presses were 10–200 V m−1 (Me: 45 V m−1) and at the positon of the operator the intensities were 4.3–56.0 V m−1 (Me: 18 V m−1). In 25% of cases, protection zones were provided beyond the separated part of the hall housing the HF presses. The zones of the particular devices overlapped and thus included zones of other devices that were not a source of strong EMF. The ICNIRP limit was not exceeded, while the MAI EQ was 1.2–2.8. Highest field E intensities measured adjacent to dielectric welders were 90–850 V m−1 (Me: 400 V m−1) and at the position of the operator the intensities were 23–240 V m−1 (Me: 70 V m−1). It was found that the zones of individual devices overlapped and covered the zones of other devices that were not a source of strong EMF. ICNIRP and MAI EQs were 3.9 and 1.1–12.0, respectively. We analyzed the results of measurements in the environ­ ment of non‐professional microwave ovens commonly used in catering services. The highest field E intensities measured around microwave ovens were 4–60 V m−1 (Me: 12 V m−1). ICNIRP and MAI EQs were 1 and 3, respectively. In conclusion, the analysis of the results of tests and measurements on EMF RF devices for selected representative sectors of the economy in which they were most often used allowed the evaluation of the current situation regarding the expo­ sure of workers to the fields in the work environment. The results of our analysis confirmed the high values of the intensity of EMF RF prevailing in the real conditions of the working environ­ ment for professional groups such as physiotherapists, dielectric welding operators, and staff involved in repair/maintenance of the antennas in radio broadcasting/communication systems

2.6  Assessment of EMF Levels in Inhabited Area

(including radio, television, mobile phone base stations). Finnish research conducted at 50 enterprises with 230 participants showed that for 16% of them the limit exposure to EMFs was exceeded, while approx. 30–50% of EMF intensity measurement results at workplaces showed exceeded limit for the electric and magnetic field levels and the corresponding numbers for measurements performed in close vicinity of equipment were as high as 70–80% [37]. There are a number of devices for which work is associated with a high risk resulting from exposure to the strongest field and its control is poor or even non‐existent. These include work on assembling, disassembling, and maintenance of antennas in mobile phone base stations, the use of radiotelephones worn for longer periods during working shifts. A neglected problem is the working conditions in the catering sector using microwave ovens, where several microwave ovens are installed in small rooms and the admissible limits of the occupational exposure are exceeded, while the people working there are not regarded as workers adversely affected by EMFs. The results of the analysis of the levels of undesirable exposure to the EMF in the working environment demonstrate the necessity of EMF reduction both in the vicinity of the equipment itself and at workplaces. In the relevant literature, reports are accessible on health effects of exposure to EMFs. For example, in some of the reports on operators of dielectric welders and HF presses, whose ­exposure to EMF reaches high intensity values, frequently reported symptoms included eye irri­ tation or corpus vitreum abnormalities of varying severity, and paresthesia (numbness in the fingertips) that were significantly correlated with exposure to EMF [37, 38]. A significantly lower heart rate (in 24‐h recording) or more numerous episodes of bradycardia as compared to the control group [39] were also recorded. Sińczuk‐Walczak [40] in a study on 57 female operators of RF welders points out that the changes in bioelectrical activity of the brain may be regarded as an indicator of the effects of chronic exposure to EMFs. Wilèn et al. noted changes in neu­ rovegetative regulation [41]. In physiotherapists, on the other hand, EMFs from SF and MF diathermy were seen to adversely affect reproduction [42].

2.6 ­Assessment of EMF Levels in Inhabited Area In recent years, of great interest is the problem of the ever‐increasing number of devices and instal­ lations in the vicinity of residential buildings, schools, and kindergartens, or simply the place of life, study, and leisure of modern man. To assess possible effects of prolonged exposures to low levels of EMF in epidemiological studies it is necessary to determine the exposure to RF EMFs. A resi­ dent of a big city is affected by EMFs produced primarily by numerous outdoor installations, such as broadcasting stations – radio, television, and mobile phone base stations, with the latter tending to dominate, as demonstrated by EMF spectrum analysis. An example analysis of the EMF RF spectrum from the broadcasting facilities and mobile communication base stations at the selected location in the center of a medium‐sized city in Poland are presented graphically in Figure 2.4 [43]. The content of an EMF spectrum is one of the parameters studied in the environment of human life due to EMF specificity being significantly dependent on the frequency range of the field. Determinations of EMF levels confined to the area of a big Polish city were conducted during 2012–2014 by the Nofer Institute of Occupational Medicine (NIOM) in Lodz [43]. Determinations of EMF levels in the external environment for the purpose our research are defined as defined as determinations of EMF background levels. For example, Figure 2.4 shows the RF EMF spectrum registered in the center of a city of over 700 000 people, in which are installed 21 radio stations broadcasting in the VHF FM range in six locations, four broadcast television stations in two locations, and nearly 1200 mobile phone base stations in more than 400 locations.

25

2  EM Noise and Its Impact on Human Health and Safety

0.1

UMTS 2100

GSM 1800

GSM 900

Television DVB-T

1

UKF FM Broadcasting DAB+ Broadcasting

Frequency: 2994.50 MHz Marker value: 5.590 mV m–1

E-Field [V m–1]

26

0.01

0.001

0.0001

500 Isotropic result

1000

1500

2000

2500

3000

Frequency [MHz]

Figure 2.4  EMF RF spectrum in the center of a city of over 700 000 people in Poland.

The analysis of the spectrum revealed highest levels of the electric field for broadcasting radio station VHF FM, television broadcasting station, and mobile communication base sta­ tions\operating in the frequency ranges 900, 1800, and 2100 MHz, respectively. Extended research on the background level of EMF RF in the city in 22 selected locations representative of the city’s architecture resulted in the following data: the highest instantaneous values of the field intensity at a height of 2 m above the ground level were found in the area of the city center, up to 2.2 V m−1 (Me: 0.74 V m−1), in the central area and at the border of large housing estates, up to 1.4 V m−1 (Me: 0.6 V m−1), and within housing estates and residential areas, up to 1.0 V m−1 (Me: 0.44 V m−1), as illustrated in Figure 2.5. The daily monitoring performed in the central part of a large housing estate did not result in significant information about daily variation of the electric field as a function of time, between 6  :  00 a.m. and 10  :  00 p.m. In contrast, the results of measurements as a function of height of buildings (up to 15 m above ground.) indicate a higher field strength of 2.5–3.8 times compared to the results obtained in the measurement at the low level (i.e. at a height of 2 m above ground). The limit value specified in ICNIRP 1998 and in Recommendation 1999/519/EU, i.e. from 28 to 61 V m−1, or in the Polish regulations (2003), i.e. 7 V m−1, was not exceeded at any measure­ ment point within the studied frequency range.

2.7 ­Assessment of the Level of EMF From Hi‐tech Equipment for Personal Use The use of equipment for new technologies, mainly mobile phones and wireless Internet, i.e. devices such as Wi‐Fi routers, indicates elevated EMF levels compared to the background of the field at the point of use, i.e. indoor areas of residential buildings.

2.7  Assessment of the Level of EMF From Hi‐tech Equipment for Personal Use

2.5 2.20

Max.

E [V m–1]

2

Min.

1.40

1.5

1

1.40

1.00

0.87 0.74

0.5 0.30 City

Mean Median

0.68

0.61

0.60

0.60

0.30 Centres of large housing estates

0.46 0.44

0.25 Peripheries of large housing estates

0.16 Residential areas

Figure 2.5  Evaluation of RF EMF level in an area of the city, with a population of 700 000, depending on the character of the area.

Mobile telephones belong to the most frequently used personal devices. In their surroundings they produce an EMF that affects not only users but also nearby bystanders. The aim of the investigations and EMF measurements in the vicinity of phones was to identify the electric field levels with regard to various working modes: Twelve sets of digital enhanced cordless telecommunications (DECT) cordless phones (12 base units, and 15 handsets), 21 mobile tel­ ephones from different manufacturers, and 16 smartphones in various applications (including multimedia) under the conditions of daily use in living rooms were measured. Measurements were performed using the point method at predetermined distances of 0.05–1 m from the devices without the presence of users. In the vicinity of DECT cordless phone handsets, the electric field strength was in the range 0.26–2.30 V m−1 and 0.18–0.26 V m−1 at distances of 0.05 and 1 m, respectively. In the surroundings of DECT cordless telephone base units, the values of EMF were from 1.78–5.44 V m−1 to 0.19–0.41 V m−1 at distances of 0.05 and 1 m, respectively. In the vicinity of mobile phones working in GSM mode with voice transmission, the electric field intensity ranged from 2.34–9.14 to 0.18–0.47 V m−1 at distances of 0.05 and 1 m, respec­ tively, while in the vicinity of mobile phones working in Wideband Code Division Multiple Access (WCDMA) mode the electric field intensity ranged from 0.22–1.83 to 0.18–0.20 V m−1 at distances of 0.05 and 1 m, respectively [44]. A Wi‐Fi router is a device used, among others, in private homes and apartments in order to access the Internet. It emits permanently weak and short 2.4 or 5 GHz radio signals when receiving or sending data. Wi‐Fi is currently being used to build wide area network (WAN) services that enable users of a portable device compatible with Wi‐Fi to access Wi‐Fi networks. This has been made possible owing to the provision of hot spots throughout the busiest parts of the city. The value of field E intensity in the vicinity of the studied 20 models of various types and designs of Wi‐Fi routers while watching a film was a maximum of 2.6 V m−1 at a distance of 0.05 m from the antenna. For models provided with 3G/4G or the ability to connect an external 3G/4G modem, the maximum EMF does not exceed 3.9 V m−1. When downloading files from

27

2  EM Noise and Its Impact on Human Health and Safety Max. 10

Min.

9.15

9

Mean Median

8 E [V m–1]

28

7

6.12

6

5.44

5 4 3

2.74

2.30

1 0.26 Dect handset

2.18

2.50

2 1.14 1.13

2.05

1.78 0.17 Dect base station

0.94

Mobile phone

0.24

1.34

Router

Figure 2.6  Results of measurements of the electric field in the vicinity of hi‐tech equipment for personal use.

a remote server, the maximum field E intensity in the immediate vicinity of the transmitting antenna did not exceed 5.0 V m−1. In places where the user (sofa, chair, computer stand) was located, usually at a distance of more than 1 m from the router antenna, the field intensity decreased to 0.2–0.5 V m−1 and were comparable to the magnitude of the background electro­ magnetic field measured in the studied homes [45]. A summary of the results determinations of field E values from the most popular devices for personal use is shown in Figure 2.6. In conclusion, the mean values of the electric field strength for each group of devices, mobile phones, and DECT wireless phone sets do not exceed the reference value of 7 V m−1 adopted as the limit for general public exposure according to the conservative Polish regulations. To sum up, technological progress has resulted in the implementation of numerous elec­ tronic devices into the work and residential environments. Wireless Wi‐Fi networks belong to the most common examples of modern technology. As cable connections are not necessary, the equipment cooperating with them is mobile, and the cost is low, wireless networks have become very popular in the home environment. Wi‐Fi networks are also popular in the workplace and in places of public entertainment, i.e. almost everywhere. Using modern technology devices connected to Wi‐Fi networks, such as smartphones, tablets, laptops, netbooks, or television, the users continue to be affected by weak EMF produced by these devices. These fields are vis­ ibly changing, however, the natural electromagnetic environment in which people reside per­ manently. Some studies on electromagnetic environment relate to the possible effects of EMFs of slightly elevated values, acting on a regular basis, on human health [17]. The WHO recom­ mends taking measures to reduce exposure to EMFs. Well‐known rules are the Precautionary Principle, Prudent Avoidance or As Low As Reasonably Achievable (ALARA) recommended for implementation by various international organizations and committees [20]. On the other hand, despite the existence of labor laws regulating, among others, the level of occupational exposure to EMFs, some jobs are not subject to adequate control, which may arise through ignorance of employers about the potential risk of exposure to EMF. Thus, we are faced now with the old and the new problems of identification of the hazards and their elimination in order to protect human beings from excessive or undesirable exposure to EMF. Solving those problems will hopefully ensure a harmonious development of new technologies and ensure

2.8  Needs and Possibilities of Shielding to Reduce the Exposure to EMF

that they may be safely used to the benefit of the modern societies. The emphasis on the ­compliance of the owners of EMF sources to the relevant regulations is becoming stronger; in trying to achieve compliance the owners will have to take all reasonable organizational and technical steps to reduce excessive or unwanted exposure to EMFs to keep it least below the admissible limits. One way to protect against EMFs involves shielding of EMF sources and places where people are present, including architectural shielding.

2.8 ­Needs and Possibilities of Shielding to Reduce the Exposure to EMF Shielding is one of the most effective methods of reducing EMF emissions to the environment and protects both against interference with electrical and electronic equipment as well as from the risks to human health. Depending on the needs, screens with varying screening effective­ ness (also known as the attenuation of the screen) are used. Screens with high attenuation should completely eliminate EMFs from such places as banks, data centers, industrial automa­ tion facilities, intensive care units in hospitals, laboratories using MRI, and airports as specified in the requirements of the EMC Directives [2, 3]. While the screening methods and technolo­ gies of producing barrier materials for EMC screening are well known and continue to be perfected, architectural shielding to protect the environment continues to be at the stage of searching for suitable materials that in any way reduce the intensity level of the EMFs in places where people live and work. Sometimes, complementary shielding may be used with success instead of full shielding. In such instances, the shielding effectiveness (SE) need not be as high as for EMC, where for example the SE of a steel plate is above 120 dB in the frequency range 850–950 MHz and the screens made of aluminum plate having a thickness over 0.15 mm ensure attenuation greater than 40 dB at frequencies of 0.1–1000 MHz [46]. Recent work is underway, also in Poland, to produce barrier materials based on textile prod­ ucts. In 2009–2013, the Textile Research Institute in Lodz conducted research within the Project POiG.01.03.01‐00‐006/08 “New‐generation barrier materials used to protect man against the harmful effects of the working and living environment,” co‐financed from the funds of the European Regional Development Fund on developing new barrier materials using inno­ vative technologies. Flexible and lightweight EMF barrier materials based on such products increasingly find practical use as wallpaper for lining the walls of premises or in the form of screens or curtains to reduce or eliminate such fields whose intensity levels are undesirable in a given location. Textile screens are also significantly less expensive than screens made of wire mesh or solid metal. Analysis of the levels of exposure in the work environment in Poland shows that an estimated value of barrier material SE ranging from 23 to 44 dB for devices and between 13 and 31 dB for workplaces is sufficient to protect against EMFs in the frequency range 0.3–3000 MHz [47]. After comparing SE values for the new barrier (shielding) materials developed by the Textile Research Institute in Lodz [48, 49] with the above data it becomes evident that they may be successfully used in practice as a means of protection from excessive and unwanted exposures to EMFs. Besides architectural shielding in the form of wallpaper on interior walls or partitions, screens, etc. it is also possible to use EMF screening in the form of tents, screens, or curtains made of the barrier materials. According to the requirements for textile products’ shielding specified in the Taiwanese standard FTTS‐FA‐003, 2005 [50], for professional applications (in special workplaces for special applications – in medicine or ­electronics) a shielding effectiveness >60 dB is judged as excellent and >40 dB as good, while for common applications (protective clothing, cover devices for household use) shielding effectiveness

29

30

2  EM Noise and Its Impact on Human Health and Safety

Before providing the shields - EMF device - Intermediate zone - Danger zone - Hazard zone

After providing the shields Shielding materials - Layer of WOM E 2001 - Layer of IG-NS - Wall

Figure 2.7  Example of architectural shielding with the use of textile shielding materials in a physiotherapy studio equipped with a microwave diathermy unit.

>30 dB is already deemed as excellent. Figure 2.7 shows an example of the use of architectural shielding in a rehabilitation room in the form of partition walls [48].

2.9 ­Summary Rich material is available concerning the prevalence of EMFs in the real work environment and EMF levels that affect not only the employees but also bystanders who could potentially stay in the area of elevated EMF intensities. The collected data make it possible to identify those EMF‐ producing technologies and groups of people occupationally and non‐occupationally exposed to the fields that, due to the intensity levels of EMF exposure, are most important. Our research results on the work environment are confirmed by the results of a review of the literature, which shows that even highly developed countries of Western Europe experience problems with keeping within the recommended maximum admissible exposure limits (despite their limit values being higher) with respect to such devices as induction furnaces, short wave (HF) and microwave (MF) equipment for diathermy in physiotherapy, industrial furnaces/kilns and drying ovens, HF welding units (especially those used for welding plastics), and capacitive dry­ ing used in the paper, furniture, automotive, and textile industries. This constitutes a major problems in the area of working conditions safety. Excessive or unwanted exposure to EMFs should be eliminated. Methods, both organizational and technological, that enable achieving that objective are already available. The use of architectural shielding made of textile barrier materials produced by modern technology, such as sputtering of metal nanocomposites, seems to be very promising.

­  References

­References 1 International Telecommunication Union (ITU), ITR‐R Recommendation V.431: Nomenclature

of the frequency and wavelength bands used in telecommunications. Geneva. Available from: http://www.itu.int/rec/R‐REC‐V.431/en. Accessed: October 9, 2013. 2 Directive 2014/30/EU of the European Parliament and of the Council of 26 February 2014 on the harmonisation of the laws of the Member States relating to electromagnetic compatibility (recast) (Text with EEA relevance), Off. J. Eur. Union, L 96/79, 29 March 2014. 3 Directive 2014/35/EU of the European Parliament and of the Council of 26 February 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of electrical equipment designed for use within certain voltage limits. Off. J. Eur. Union, L 96/357, 29.3.2014. 4 Directive 2013/35/EU of the European Parliament and of the Council of 26 June 2013 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) (20th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC) and repealing Directive 2004/40/EC, Off. J. Eur. Union, L 179/1, 29 June 2013. 5 International Commission on Non‐Ionizing Radiation Protection (ICNIRP) (1998). Guidelines for limiting exposure to time – varing electric, magnetic and electromagnetic fields (up to 300 GHz). Health Phys. 74 (4): 494–522. 6 Council of the European Union Recommendation of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0Hz to 300 GHz), 1999/519/EC. Off. J. Eur. Union, L 199/59‐61, 1999. 7 Vecchia, P., Matthes, R., Ziegelberger, G. et al. (eds.) (2009). Exposure to High Frequency Electromagnetic Fields, Biological Effects and Health Consequences (100 kHz to 300 GHz). International Commission on Non‐Ionizing Radiation Protection (ICNIRP) ICNIRP 16/2009. 8 Schwan, H.P. (1992). Early history of bioelectromagnetics. Bioelectromagnetics 13: 453–467. 9 Gordon, Z.V. (1966). Voprosy gigieny truda i biologiceskogo deistvija elektromagnitnych polei sverhvysokich castot. Moskwa. Russian: Medicina. 10 Johnson, C.C. and Guy, A.W. (1972). Nonionizing electromagnetic wave effects in biological material and systems. Proc. IEEE 60 (6): 692–718. 11 Durney, C.H., Johnson, C.C., Barber, W., et al. 1978. Radiofrequency Radiation Dosimetry Handbook (Second Edition), Brooks Air Force Base, USAF School of Aerospace Medicine, Aerospace Medical Division, Report SAM‐TR‐78‐22, Texas, May 1978. 12 Durney, C.H., Massoudi, H., Iskander, M.F. 1985. Radiation Dosimetry Handbook, Brooks Air Force Base, TX: U.S. Air Force School of Aerospace, Medical Division, Report SAM‐TR‐85‐73, vol. 74,4 April 1985. 13 Institute of Electrical and Electronics Engineers (IEEE), 1992. C95.1‐1991. IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz. Recognized as an American National Standard (ANSI), Published IEEE, Inc., New York, NY 10017, USA, 28.04.1992. 14 Reilly, P.J. (1998). Applied Bioelectricity. From Electrical Stimulation to Electropathology. New York: Springer‐Verlag. 15 Sobiczewska, E. and Szmigielski, S. (2007). Health effects of occupational exposure to electro­ magnetic fields in view of studies performed in Poland and abroad. Med. Pr. 58 (1): 1–5. Polish. 16 Szmigielski, S., Kubacki, R., and Ciołek, Z. (2000). Application of dosimetry in military epidemiological studies. In: Radiofrequency Radiation Dosimetry and Its Relationship to the Biological Effects to Electromagnetic Fields, NATO Science Series 3, High Technology, vol. 32 (ed. J.B. Klauenberg and D. Miklavčič), 459–472. Dordrecht, Boston, London: Kluwer Academy Publishers.

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17 Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR).2009. Health

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Effects of Exposure to EMF, Opinion adopted at the 28th plenary on 19 January 2009. Brussels. Available from: http://ec.europa.eu/health/ph_risk/committees 04_scenihr/docs/ scenihr_o_022.pdf. International Agency for Research on Cancer (IARC) (2013). Radiation, Part 2: Non‐Ionizing Radiofrequency Electromagnetic Fields, Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 102. Lyon, France: IARC. International Agency for Research on Cancer (IARC) (2002). Non‐Ionizing Radiation, Part 1: Static and Extremely Low‐Frequency (ELF) Electric and Magnetic Fields, Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 80. Lyon: IARC. World Health Organization (WHO) 2000. Electromagnetic Fields and Public Health. Cautionary Polices, March 2000. Available from: http://www.who.int/docstore/peh‐emf/ publications/facts_press/EMF‐Precaution.htm. The Swiss Federal Council, Ordinance relating to Protection from Non‐Ionizing Radiation (ONIR) of 23 December 1999 (as of 1 February 2000). Decree of the Prime Minister July 8, 2003 (Gazzetta Ufficiale della Repubblica Italiana n. 200 of 29‐8‐2003). Zmyślony, M. (2007). Biological mechanism and health effects o EMF in view of requirements of reports on the impact of various installations on the environment. Med. Pr. 58 (1): 27–36. Polish. Aniołczyk, H. and Zmyślony, M. (1996). Collection of occupational EMF exposure data in Poland. Concept of the structure and functioning. Int. J. Occup. Med. Environ. Health 9 (1): 29–35. Aniołczyk, H. (1981). Measurements and hygienic evaluation of electromagnetic fields in the environment of diathermy, welders and induction heaters. Med. Pr. 32 (2): 119–128. Polish. Aniołczyk, H. (ed.) (2000). Electromagnetic Fields. Sources, Effects, Protection. Lodz. Polish: Nofer Institute of Occupational Medicine in Lodz (NIOM). Aniołczyk, H., Mariańska, M., and Mamrot, P. (2011). Optimization of methods for measurement and assessment of occupational exposure to electromagnetic fields in physiotherapy (SW Diathermy). Med. Pr. 62 (5): 499–515. Polish. Aniołczyk, H., Mamrot, P., and Mariańska, M. (2012). Analysis of methods for measurement and assessment of occupational exposure to electromagnetic fields in dielectric heating. Med. Pr. 63 (3): 329–344. Polish. Aniołczyk, H., Mariańska, M., and Mamrot, P. (2015). Assessment of occupational exposure to radio frequency electromagnetic fields. Med. Pr. 66 (2): 199–212. Polish. International Commission on Non‐Ionizing Radiation Protection (ICNIRP) (2010, 818). On the guidelines for limiting exposure to time‐varying electric and magnetic fields (1 Hz – 100 kHz). Health Phys. 99 (6): 818–836. International Commission on Non‐Ionizing Radiation Protection (ICNIRP) (2014). Guidelines for limiting exposure to electric field induces by movement of the human body in a static magnetic field and by time varying magnetic fields below 1 Hz. Health Phys. 106 (3): 418–425. Council Directive 89/391/EEC of 12 June 1989 on the introduction of measures to entourage improvements in the safety and health of workers at work. Off. J. Eur. Union, L 183/1‐8, 1989. Stam, R. 2011. Comparison of international policies on electromagnetic fields (power frequency and radiofrequency fields), National Institute for public Health and the Environment, the Netherlands. Available from: ec.europa.en/health/electromagnetic_fields/ docs/emf_comparison_policies_en.pdf. Accessed: May 15, 2016. Aniołczyk, H. (2008). Supervision of state system of protection against 0 Hz – 300 GHz electromagnetic fields exposure in Poland. In: Electromagnetic Field, Health and Environment.

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Proceedings of EHE’07 (ed. A. Krawczyk, R. Kubacki, S. Wiak and C.L. Antunes), 27–31. Amsterdam; Berlin; Oxford; Tokyo; Washington: IOS Press. Ordinance of the Minister of Labour and Social Politics from 29 Nov. 2002 concerning MAC/ MAI of agents harmful to health in the working environment. J. Law No 217, pos. 1833, 2002. Ordinance of the Minister of Environment from 30 Oct. 2003 concerning admissible EMF levels in environment and control methods of following these levels. J. Law. No 192, pos.1883, 2003. Kolmodin‐Hedman, B., Mild, K.H., Hagberg, M. et al. (1988). Health problems among operators of plastic welding machines and exposure to radio frequency electromagnetic fields. Int. Arch. Occup. Environ. Health 60: 243–247. Bini, M., Checcucci, A., Ignesti, A. et al. (1986). Exposure of workers to intense RF electric fields that leak from plastik sealers. J. Microw. Power 21 (1): 33–40. Wilèn, J., Hörnsten, R., Sandström, M. et al. (2004). Electromagnetic field exposure and health among RF plastic sealer operators. Bioelectromagnetics 25: 5–15. Sińczuk‐Walczak, H. and Iżycki, J. (1981). Assessment of neurological condition and EEG tests in workers exposed to 27–30 MHz electromagnetic fields. Med. Pr. 32 (3): 227–231. Polish. Wilèn, J., Wikulund, U., Hörnsten, R., and Sandström, M. (2007). Changes in heart rate variability among RF plastic sealer operators. Bioelectromagnetics 28 (1): 76–79. Shah, S.G. and Farrow, A. (2014). Systematic literature review of adverse reproductive outcomes associated with physiotherapists, occupational exposures to non‐ionising radiation. J.Occup. Health. 56 (5): 323–331. Available from: https://www.jstage.jst.go.jp/article/joh/ advpub/0/advpub_13‐0196‐RA/_pdf. Aniołczyk H, Mariańska M, Mamrot P. 2013. Electromagnetic fields in modern human environment – an example of the city of Łódź. Telecommunication Review + Telecommunication News, Tele‐Radio‐Electronics, Information Technology, 86 11. Polish. Mamrot, P., Mariańska, M., Aniołczyk, H., and Politański, P. (2015). Electromagnetic fields in the vicinity of DECT cordless telephones and mobile phones. Med. Pr. 66 (6): 803–814. Polish. Mamrot P, Mariańska M. 2015. Electromagnetic fields around Wi‐Fi routers. Telecommunication Review + Telecommunication News, Tele‐Radio‐Electronics, Information Technology, 86 no 4. Polish. Hemming, L.H. (1991). Architectural Electromagnetic Shielding Handbook. New York: IEEE Press Inc. Aniołczyk H. 2009. Criteria for screening efficiency of barrier materials in the contemporary human protection, Telecommunication Review, 11, 1987‐1990. Polish. Aniołczyk, H., Koprowska, J., Mamrot, P., and Lichawska, J. (2004). Application of electrically conductive textiles as electromagnetic shields in physiotherapy. Fibres Textil. East. Eur. 12 (4): 47–50. Mamrot P, Aniołczyk H, Mariańska M, Koprowska J, Filipowska B. 2012. Municipal environmental protection against electromagnetic field using textile barrier materials, Telecommunication Review + Telecommunication News, Tele‐Radio‐Electronics, Information Technology, 8‐9, 859‐864. Polish. Standard FTTS‐FA‐003 Version 2, 2005. Specified Requirements of Electromagnetic Shielding Textiles, Taiwan, Revise Date: Mar/03/2005.

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3 Electromagnetic Field Sensors Vishnu Priya Murali1*, Jickson Joseph2,3*, and Kostya (Ken) Ostrikov 2,3 1

 Department of Biomedical Engineering, University of Memphis, Memphis, Tennessee, USA  School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland, 4000, Australia 3  CSIRO−QUT Joint Sustainable Processes and Devices Laboratory, Commonwealth Scientific and Industrial Research Organisation, P.O. Box 218, Lindfield, New South Wales, 2070, Australia 2

Abbreviations AC Alternating current AM Amplitude modulation AMR Anisotropic magneto‐resistor DC Direct current EMF Electromagnetic field emf Electro‐motive force FM Frequency modulation GMR Giant magnetoresistance effect HTS High temperature SQUID LTS Low temperature SQUID MR Magneto‐resistor PM Phase modulation RCP Rogowski–Chattock potentiometer RF Radio frequency RMS Root mean square SA Specific absorption SAR Specific absorbance rate SIS Superconductor–insulator–superconductor SNS Superconductor–normal–superconductor SQUID Superconducting quantum interference device SSM Scanning SQUID microscope TEM Transient electromagnetics

* Authors contributed equally towards the chapter. Advanced Materials for Electromagnetic Shielding: Fundamentals, Properties, and Applications, First Edition. Edited by Maciej Jaroszewski, Sabu Thomas, and Ajay V. Rane. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

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3.1 ­Introduction The field of electromagnetism is an ever growing one. It has found numerous applications in a wide variety of research and industries. Increased use of electromagnetic fields (EMFs) has led to an increased need to develop improved EMF testing facilities, techniques, and equipment. In the past decade there seems to be an increased interest in assessing the potential hazard caused by the nonionizing electromagnetic emissions on the living matter [1]. The health outcomes of cellular phones has become a public issue due to the steadfast growth of this segment throughout the world since the opening of commercial service in 1983 [2]. There was a drastic increase in the number of users to 10 000 by 1991, in the United States of America alone. This is expected to go up to 6.1 billion worldwide by 2020 [3]. Even though cellular phones transmit very low power, which is 0.6 W with 850 MHz, they are placed very close to the user’s head. These facts increased the need to measure the EMFs in free space as well as material media. All the EMF sensing devices have the common requirements of operating with high frequency and at broader bandwidth. The device should be preferentially be small but nevertheless highly accurate. Two important aspects that should be kept in mind while establishing standards for EMF measurements are: (i) generation of standard or reference EMFs (EMFs complying with some regulations) and (ii) using transfer probes to perform rigorous EM measurements [4]. To choose the best method to measure the EMF near the field, one must first identify the quantities that most accurately characterize the field. These quantities can then be subjected to measurement. It is important to measure the strength of the electric field (E) and magnetic field (H) near the antenna, which then enables one to find the current or charge density along the length of the antenna. These values can be used to determine the radiation pattern and the input impedance of the antenna. The antenna’s radiation pattern in the far field can be calculated by performing some complex calculations on the electric field (E), magnetic field (H), and power density (S) of the near field. The E, H, and S measurements are sufficient while investigating shielding, absorbing, and EMF attenuating materials. However, in biomedical investigations or in investigations regarding protection against unwanted exposure to EMF, the above‐mentioned three parameters will not be sufficient. More precise parameters such as specific absorption rate (SAR) and specific absorption (SA) need to be evaluated. SAR and SA are the parameters used to measure the energy absorbed in a mass unit. SAR and SA can be determined by measuring the temperature rise caused by EM energy absorption. Nowadays measuring the current induced in a body by EMF is also being used to determine SAR and SA. Most of the protection standards measure the root‐mean‐square (RMS) value, which in turn determines the amount of absorbed energy. For the non‐thermal approach, the amplitudes of the field components, the type of modulation (AM, FM, PM), their spatial positioning, temporal variations, and frequency of the carrier wave and modulating spectra are extremely important. In this chapter we will describe the methods used to measure the EMFs. As mentioned above, EMF measurements are made possible by measuring either the electric field or the magnetic field. Therefore, this chapter will explain the different methods employed to measure the electric and magnetic field by using Hall sensor, induction coil sensor, SQUID sensors, proton precession, optical pumping, electric probe sensors, and electron drift techniques.

3.2 ­How are EMFs Produced? An EMF is usually produced when charged particles are set into motion. It is generated when there is a difference in the voltages, the higher the difference the greater the resultant field. It is well known that any charged particle has an electric field around it and that a moving charge

3.2 ­How are EMFs Produced?

particle tends to produce a magnetic field. In other words, an EMF is usually produced when a changing electric field produces a magnetic field or a changing magnetic field produces an electric field. Often an electric field or a magnetic field are considered to be the source of an EMF. An EMF can be produced by natural as well as man‐made phenomenon. We are almost always surrounded by EMFs formed from these sources. 3.2.1  Natural Sources The EMFs from natural sources are produced by the de‐excitation of molecules, atoms, or nuclei in a random manner. Each photon has a different orientation and phase angle, as they are  not triggered simultaneously. Since these photons oscillate in different planes, they are “non‐polarized.” The major natural sources of EMF include the Earth’s magnetic field, thunderstorms, and lightning activity. The Earth has a natural magnetic field that causes the needle of a compass to orient in a North–South direction. Often, electric fields are produced by the build‐up of electrical charges in the atmosphere before or after a thunderstorm. Mainly, lightning activity in the Earth’s ionosphere associated with global thunder activity and local thunderstorms produces EMF. At some distance from the source, the EMF produced by the lightning activity travels as a plane wave with respect to the vector that is horizontally directed to the magnetic field. This field has two components, the global thunder activity and the local thunderstorms. The EMF produced from global thunder activity is more stable than the one produced by local thunderstorms. Local thunderstorms produce EMFs that are more intermittent and appear as separate pulses when measured with a known sensor. 3.2.2  Man‐Made Sources Apart from the natural sources, numerous man‐made sources generate EMF. EMF from man‐ made sources has drastically increased in the recent years due to our increased dependence on electricity. Every power socket has a low‐frequency EMF associated with it and the information transmitted using a television antenna, radio station, or mobile phone generates a high‐frequency EMF associated with it. A lot of equipment in hospitals used for imaging, diagnostics, therapeutics, etc. is also major sources of EMF. Most EMFs from man‐made sources, unlike the natural sources, are polarized. They are produced by electromagnetic oscillation circuits that cause the electrons to move back and forth in an electric circuit. Since these oscillations take place mostly in linear directions, the field thus produced is polarized. The EMFs induce coherent forced‐oscillations in their surrounding medium. These fields affect biological objects and may be potentially hazardous. Despite this, most biomedical imaging techniques use the EM waves due their ability to penetrate into the human body [5]. 3.2.2.1  Low‐Frequency EMF Sources

The frequency range of low‐frequency EMF varies from 1 to 300 Hz. Earth’s geomagnetic field and many man‐made sources produce low‐frequency EMFs. The main sources of man‐ made EMFs are power lines and electronic appliances like vacuum cleaners, hair dryers, ovens, etc. The Earth’s magnetic field is about 60 μT near the poles and 30 μT near the equator. Man‐made EMF sources have a strength of about 17.44–164.75 μT when measured at a distance of 5 cm away from the source. These intensities decrease with increasing distance from the source.

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3.2.2.2  High‐Frequency EMF Sources

Radiofrequency waves mostly produce high‐frequency EMFs. These have a frequency range of around 100 kHz to 300 GHz. They are present all around us in the form of mobile phone signals, radio station signals, TV signals, etc. Even the Sun, Earth, and other black body radiators also produce these radiofrequency EMFs. Some other man‐made EMF sources include cordless phones, utility smart meters, remote control toys, wireless networks, radar, and baby monitors.

3.3 ­Electromagnetic Field Measurements EMFs consist of both electric field and magnetic field perpendicular to each other and propagate in the direction perpendicular to their oscillation. EMF parameters can be determined by either measuring the magnetic or electric field magnitudes associated with it. Consequently, here we discuss the different techniques used to measure magnetic and electric field and also the prevailing methods employed to find out the total power density of the EM wave. 3.3.1  Magnetic Field Measurement Techniques For many years, magnetic sensors have played a crucial role in a large number of applications. For example, non‐contact switching along with magnetic sensors has enabled humankind to fly aircrafts and space exploration vehicles with utmost accuracy and precision. These sensors have also considerably increased the production accuracy and quality of the engine and brake shaft operations in the field of automobile industry. Out of the several ways to measure magnetic fields the majority of them use the very familiar connection between magnetic and electric fields. The vast majority of the magnetic sensors determine the magnetic field intensity by measuring the flux density associated with the field [6]. Due to the linear relationship between flux density B and magnetic field strength H (B = μ0H), B, and H have similar values when measured in air and thus flux density is often expressed in terms of Tesla [7]. Magnetic field measurement devices are categorized based on the way they measure the field i.e. whether they measure the total field or the vector components. Here we explain and compare different types of devices like search coil magnetometer, fluxgate sensors, magneto resistive, Hall effect sensors, SQUID, nuclear precession, and optically pumped sensors used for measuring the magnetic field (Figure 3.1). Magnetometers can further be classified into vector Figure 3.1  Comparison of different sensors based on the detectable field range [6, 8]. Source: Adapted from Lenz 1990 and Herrera‐May 2009.

Magnetoresistive Hall effect Squid Nuclear precession Optically pumped Fluxgate Search coil 10–15 10–12 10–9 10–6 10–3 100 Detectable field (tesla)

103

106

3.3  Electromagnetic Field Measurements

magnetometer and scalar magnetometer. Scalar magnetometers measure only the magnitude of the measure and whereas the vector magnetometers measure its vector components as well. 3.3.1.1  Induction Based Sensors

Induction coil sensors are one of the oldest and most well‐known types of magnetic sensors. Induction based sensors consider the scalar as well as the vector component of the magnetic field and are thus among the vector magnetometer sensors. These sensors are also known as search coil sensors, pickup coil sensors, or magnetic antennae. The working principle of the induction based sensors is based on Faraday’s law of induction:

V

n

d dt

dB dt

o nA

dH (3.1) dt

where φ is the magnetic flux passing through a coil with an area A and with number of turns n. This law states that an emf (V) proportional to the rate of change of the flux is generated between the leads of a coiled conductor when the magnetic flux associated with it changes. The flux through the coil can be changed in several ways, like placing it in a magnetic field that varies with time, rotating it in a uniform magnetic field, or by moving it through a non‐uniform magnetic field [9]. Usually a rod of ferromagnetic material having a high magnetic permeability is placed in the coil to improve the sensitivity of the sensor (the permeability of a material is a measure of the extent to which it modifies the magnetic flux in the material). The operating principle of these sensors is simple and straight‐forward. However, their practical implementation requires technical details known only to specialists. For instance, it is well known that a change in the flux density (B) produces an output voltage (V) proportional to its rate of change (dB/dt), which in turn requires integration of the output signal. However, there are other useful methods by which to obtain results proportional to the flux density, B. As compared to Hall, magnetoresistive, or flux‐gate type sensors, induction sensors can be easily manufactured by the user. The materials used are easily accessible and the manufacturing method is quite straight‐forward. Thus, investigations regarding magnetic field can be performed using simple, cheap, yet accurate, induction coil sensors [10]. 3.3.1.1.1  Variation of Sensitivity between Ferromagnetic Coil and Air Coil

Miniaturization and low sensitivity are the major drawbacks of air coil sensors. In an attempt to overcome this, ferromagnetic core sensors are used, where the core concentrates the flux inside the coil itself. The voltage for a ferromagnetic core induction sensor can be written as:

V

o

r

n A

dH (3.2) dt

The relative permeability (μr) of modern soft magnetic materials is higher than 105, which can lead to higher sensor sensitivity. However, the resultant permeability of the core (μc) is lower than that of the actual material permeability. The main reason behind this lowering is the demagnetizing effect, defined by the demagnetizing factor Nd (a factor that depends on the geometry of the core):

c

1 Hd

r r

1

(3.3)

The resultant permeability of the core (μr) depends more on the demagnetizing factor Nd (which follows an inverse relation 1/N) when materials with larger permeability μr is used. Therefore, for such materials, the geometry of the core plays a crucial role in determining the

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sensitivity of the sensor. The value of Nd, the demagnetizing factor for an ellipsoidal core, depends on the length of the core (lc) and diameter of the core (Dc) and can be approximated using:



N

Dc2 lc2

ln

2lc 1 Dc

(3.4)

From the above equation it can be seen that the Nd value can be reduced by increasing the length of the core and decreasing its diameter, which in turn increases the resultant permeability μc of the core. Using a soft magnetic material as the core reduces the linearity of the system. Even the best ferromagnetic material introduces some amount of nonlinearity to the transfer function of the sensor, which depends on the frequency, temperature, flux density, etc. The resolution of the sensor also decreases due to additional magnetic noises like the Barkhausen noise [11]. Moreover, the magnetic field under investigation is also altered by the ferromagnetic core with important consequences. In 2003 Harland et al. reported such a sensor, with an amorphous core, with high sensitivity [12]. 3.3.1.1.2  Moving Coil Sensors

Since the induction coil sensor is sensitive only to changing magnetic field, a DC magnetic field can be measured by moving the coil. For instance, the DC magnetic field can be measured with good accuracy by rotating the coil using quartz stabilized speed rotations. The flux varies (an important condition for Faraday’s law) when the sensor area varies as A(t) = A cos(ωt) and the voltage induced is given as [13]:

V

t (3.5)

Bx n A sin

Figure 3.2 shows the schematic of moving coil type DC magnetic field sensor. The sensor can be moved in many other ways, the most common being vibration. Groszkowski was among the first to apply such an idea. In his work, vibrations were introduced in the coil by connecting a rotating eccentric wheel to it. Groszkowski also demonstrated the moving coil magnetometer in 1937 [14].

H

V

Figure 3.2  Moving coil sensor for DC magnetic field [7]. Source: Reproduced with permission of Taylor & Francis.

3.3  Electromagnetic Field Measurements

The moving coil method is not commonly used since moving parts are usually avoided in measuring instruments to avoid friction. The most commonly used sensors to measure DC magnetic field are Hall sensors and Fluxgate sensors. 3.3.1.1.3  Rogowski Sensors

The Rogowski sensor was first described by Rogowski and Steinhaus in 1912 [15]. It is a special type of helical coil sensor that is wound uniformly around a long non‐magnetic circular or rectangular strip. It is usually flexible. In some cases, this coil is used for measuring the ­magnetic field strength and is known as a Rogowski–Chattock potentiometer (RCP). This coil sensor works on the principle described by Chattock in 1887 [16]. In the Rogowski coil, the induced voltage is the output signal. Here the operating principle is Ampere’s law instead of the Faraday’s law. Consider a coil of length “l” inserted into a magnetic field. Figure 3.3 shows the Rogowski sensor coil. The total output voltage equals the sum of the voltages induced in each turn, where the turns are connected in series: V

n



d

d dt dl

o

n d A l dx

B A

H dl cos

(3.6)

From the above equation it is clear that the number of turns per unit length (n/l) and the cross sectional area of the coil, A, influence the output of the coil. An accurately designed Rogowski coil when inserted in a magnetic field between any two points A and B produces the same value of the output signal independent of the geometry of the coil between the two points. The output signal of the RCP coil is assumed to be proportional to the magnetic field strength between points A and B and is given as:

V

o

d n A H lAB (3.7) dx l

Here A and B are the points on the potentiometer circuit the Rogowski coil is connected to. The difference of the magnetic potentials H*l can be determined using the RCP coil. It is not convenient to directly measure H (for fixed value of length lAB) using the coil since the output signal is very small and it has to be integrated. In the compensation method, a feedback circuit is connected to the current exciting the correction coils and the feedback is nothing but the output signal of the RCP coil. Due to this negative feedback, all the magnetic field components in the yoke and the air gaps are compensated, i.e. the output signal is equal to zero:

H lAB n I

0 (3.8)

Therefore, since lAB and n are known, the magnetizing current I obtained can be used directly to determine the magnetic field strength H. Figure 3.3  Typical Rogowski coil [7]. Source: Reproduced with permission of Taylor & Francis.

Filter

C Vout

R

Integrator circuit

41

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3  Electromagnetic Field Sensors

Flexible Rogowski coils marked an advancement in this field. They can be wound around the insulation without field shielding. The perpendicular virtual loop due to the winding will be recompensed by the conductor inside the cable, which functions as the return loop. The magnetic field will be detected by the uncompensated windings in the coil. The fabrication and the external integrator circuit description were reported by Abdi‐Jalebi et al. in 2007 with an error coefficient of 1.5% [17]. To improve the resonant frequency the number of turns should be reduced and this in effect causes the lower sensitivity. At high frequency, errors will be induced in the integrator circuit due to the interference. Ward et al. reported an alternative for the integrator circuit in order to overcome this demerit [18]. Rogowski coils are used for transient current measurements by measuring the magnetic field associated with it. A Rogowski coil with improved temperature stability and precision was reported by Kojovic in 2002 by using PCB (printed circuit board) technology [19]. 3.3.1.2  Fluxgate Sensors

Fluxgate sensors are used to measure the magnitude and frequency of a DC or low‐frequency AC magnetic field in the range 10−10–10−4 T. This type of sensor was introduced by Aschenbrenner et  al. in 1936 [20]. These solid‐state devices do not have any moving parts. The  excitation current (Iex) produces an excitation field that is used to periodically saturate the sensor core, which is made of soft magnetic material [21–23]. This causes a change in the core permeability and modulation in the DC flux produced by the DC magnetic field Bo [24]. At the second harmonics of the excitation frequency, a voltage Vind proportional to the measured field intensity is induced in the sensing coil (pick‐up coil). Magnetic modulators, magnetic amplifiers, and DC transformers all use the same principle, the only difference being in the measured variable. In all these cases a DC electrical current through the primary coil is measured instead of the voltage. The basic sensor configuration is sketched in Figure 3.4. An AC current, Iex(t), excites the excitation winding of the sensor, such that the core permeability μ(t) is modulated with twice the excitation frequency. B0 is the measured DC magnetic field and B(t) the corresponding field in the sensor core. Vind is the voltage induced in the pick‐up (excitation) winding with N turns. The analytical description based on Faraday’s law is given by:

V

d dt

d NA

o

dt

t H t (3.9)

where V is the voltage induced in the measuring coil having N turns, φ is the magnetic flux in this coil, φ = BA as the air flux is neglected; H is the magnetic field in the sensor core, A is the core cross‐sectional area, and μ(t) is the sensor core relative permeability. Several types of fluxgate arrangements are available. Recent advances in fluxgate sensors have targeted improvement in stability. Here stability stands for the energy reduction in dissipation detection circuit, which is transferred from excitation circuit and vice versa [25]. According to the energy aspects a steady state is attained Figure 3.4  Operation principle of fluxgate magnetometer [21]. Source: Reproduced with permission of Elsevier.

Vind Iex(t) μ(t)

B(t) N

B0

3.3  Electromagnetic Field Measurements

when the energy transferred to the detector circuit is equal to the energy dissipated by it. The dissipation in a magnetic core is negligible as the equivalent resistance of a magnetic core constitutes less than 5% of the total. This implies that the fluxgate will be stable if the detection circuit resistance is larger than a particular value known as the “critical resistance.” 3.3.1.2.1  Ring‐Core Sensors

A ring core sensor consists of a pickup coil that is a straight solenoid with the core in its center and an excitation coil wound toroidally around it. Ring‐core sensors can be considered as a balanced double sensor. Two half‐cores make up the magnetic circuit. A thin tape of soft magnetic material is wound several times and made into the core. This type of geometry is advantageous for the low‐noise sensors, despite low sensitivity due to large demagnetization. These sensors rotate the core with respect to the sensing coil and thereby maintain a fine balance in the core symmetry. 3.3.1.2.2  Multi‐Axis Fluxgate Magnetometers

Two‐axial fluxgate magnetometers are commonly used in compasses. A dual‐axis sensor ­consists of a ring‐core with a double cross‐shaped pickup coil. Its short‐time angular accuracy when used as a compass is around five minutes of arc, with a long‐term precision of 0.1°. Such compasses need to be pivoted. To avoid this, nowadays three‐axial fluxgate magnetometers are used. These magnetometers are used along with an inclinometer and the azimuth is determined using pitch, roll, and three components of magnetic field. Despite these aforementioned demerits fluxgate compasses are superior as they can provide an accuracy of 0.1° over wide temperature ranges [21]. Three‐Axial Compensation System  In three‐axial magnetometers, three single axis sensors with three orthogonal rectangular or circular Helmholtz coils (or more complex coils) are used [26]. Such a system with three coils of identical center points has found many applications in the field of space technology (rockets and satellites) [27]. Each coil has nine sections that resemble the ideal spherical coil and generate a uniform magnetic field. In order to have low‐noise ­operation or long‐term stability, the sensors are kept in a very low field. This is achieved by placing the three orthogonal sensors in the center of a three‐dimensional feedback coil [28]. This ensures that the cross field effect does not cause any significant errors in the system. Since the feedback coil system is used to define the measuring axes, an EMF can be easily determined and kept very stable [29]. Individually Compensated Sensors  Despite problems due to the cross‐field effect, individually compensated sensors are popular due to their simplicity and low price. To avoid crosstalk, the individual sensors are mounted symmetrically and placed at a maximum distance from each other. Wherever possible, they are excited by the same generator by connecting their excitation windings in series [28, 29]. The Swedish satellite Astrid‐2 had this type of magnetometer with closely mounted fluxgates having 17 mm ring cores made of amorphous alloy [30]. 3.3.1.2.3  Micro‐Fluxgate Magnetometers

Magnetic noise increases with the reduction in the size of the sensors, which makes realization of micro sensors a complicated task. Small sensors are used in a wide range of applications like sensor arrays, magnetic ink reading, etc. In this type of magnetometers, a thin film of amorphous materials or permalloy is sputtered or electrodeposited [22, 31]. This type of sensors is made through the layer by layer deposition of each component. Cores are made from amorphous material or permalloy. Ghatak et  al. in 1992 reported such a magnetometer with an

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3  Electromagnetic Field Sensors Ferromagnetic film (top) Excitation coil

Excitation coil

Pickup coil

Pickup coil

Figure 3.5  Schematic of a micro‐fluxgate sensor with flat coils [22]. Source: Reproduced with permission of Elsevier.

Ferromagnetic film (bottom)

Silicon substrate

amorphous metal core. This integrated fluxgate sensor did not have the wound coils, which makes it small [32]. Later, in 2000, Kej´ı k et al. built a compact 2D planar fluxgate sensor with a similar core that had lower magnetic noise [33]. Figure  3.5 shows the typical configuration of  planar micro‐fluxgate sensors. In recent times several advancements have taken place in further reducing the noise levels by using different core materials [34]. Normally, micro‐planar fluxgate sensors use two or a few more layers of coils. Heimfarth et al. in 2015 reported single core sensor of NiFe permalloy, electroplated through photoresist, which achieved noise levels down to 40‐nT rms in the 01–10 Hz range [35]. 3.3.1.3  SQUID Magnetometer

The SQUID magnetometer works according to the Josephson effect, which is based on the quantization of magnetic flux and the tunneling effect by a weak link. Normally in a closed superconductive ring, internal flux can be produced due to an external flux. But according to the Meissner effect, Is, a superconducting current flows through a thin boundary area called the London penetration depth. This acts as a screen making the material perfectly diamagnetic and thereby resulting in the shielding of external flux, ϕext. As a result, the material should undergo transition from a superconducting state to a normal state, in order to change the flux trapped inside the material, ϕint. As a solution to this, Josephson in 1962 proposed a tunnel junction known as the Josephson junction. This junction provides the external field penetration without interrupting the superconducting circuit. This proposal was experimentally proved by Anderson and Rowell in 1963 [36]. Zimmerman et al. designed the first SQUID sensors with RF technology in 1967 [37–39]. In 1970 the same group reported the improvised RF SQUID sensor. In this model, to generate the weak link a niobium screw was used. Such screws were later avoided in most applications as they are prone to shock and physical disturbances [40]. Later, DC SQUIDs were introduced, which became more common because of their ease of construction and low susceptibility to shakes. DC SQUIDS were introduced by Jaklevic et al. in 1967 [7]. In this model two superconductors are separated by a thin insulating layer. A superconducting current that flows between them is affected by the presence of a magnetic field. This forms the basis of the SQUID magnetometer. The superconducting material used is tin. With further advancements in technology, very thin layer SQUIDs termed low temperature SQUIDs (LT SQUIDs) were fabricated using photolithographic techniques. Such SQUIDs were developed by Wikswo et al. [41] and by Cantor et al. Cantor et al. used niobium metal (as the superconductor) and aluminum oxide layer for setting up the Josephson junction [42]. SQUID based magnetometers are presently the most sensitive devices for magnetic field measurement. SQUID based devices can make measurements in cases where no other methodologies can be used. Some of these devices have sensitivities lower than 1 fT/√Hz. Due to their superconducting nature SQUID devices have a flat frequency and phase response from DC to GHz frequencies [43].

3.3  Electromagnetic Field Measurements

Figure 3.6  Layout of a typical SQUID magnetometer.

I

1 nm V

Al2O3

Nb

The SQUID consists of two superconductors separated by a 1 nm thick insulating layer. Niobium or lead alloy with 10% gold or indium are the most commonly used superconductor materials and aluminum oxide, magnesium oxide, etc. are commonly used as the insulating layer (Figure 3.6). At very low temperatures of about 4.2 K, there is a superconductor current flow across the junction with 0 V. This current is called the critical current (Ic). This current is a periodic function of the magnetic flux in the junction. The Ic value will be maximum for flux values equal to nϕ0 and minimum when flux values are equal to (n + ½)ϕ0, where ϕ0 is one flux quantum (2fW) [44]. The period between the maximum and minimum values is called one flux quantum and the phenomenon is commonly termed the “DC Josephson effect.” It is only one among the numerous “Josephson effects” [45]. Advances in the field of material science engineering have enhanced the performance of SQUIDs using advanced SQUID sensor materials. Most recently the superconducting material has undergone several changes. The weak links for Josephson’s effect is set up either through superconductor–insulator–superconductor (SIS) or superconductor–normal–superconductor (SNS). In the former the junction is fashioned with an insulator like oxides, whereas the later uses some normal metal layers for the same purpose. The low temperature SQUID (LTS) devices utilize tunnel junctions whereas high temperature SQUID (HTS) devices make use of the grain boundary, bi‐crystal, or SNS to craft the junction. Lead, mercury, niobium, and Ni‐Ti, Nb3Sn alloys are used to craft LTS devices. Ba0.6K0.4BiO3, La1.85Sr0.5CuO4, MgB2, YBa2Cu3O7−δ [46], Bi2Sr2Ca2Cu3O10, Tl2Bi2Ca2Cu3O10 [47], and HgBa2Ca2Cu3O8+δ [48] are the reported superconductors used to assemble the HTS devices [49–51]. Magnetometers based on SQUIDs do not measure the absolute value of the magnetic field. They usually measure the change in the magnetic field from some arbitrary field value. SQUID magnetometers are most extensively used in the field of biomedical instrumentation. Because of their high sensitivities, SQUID magnetometers and gradiometers are useful in measuring weak magnetic fields like those generated by the human body, large multichannel systems for the measurement of the brain or heart (4‐D neuroimaging) [52], imaging of small samples in animal studies, systems for biological susceptometry, etc. [53, 54]. A SQUID is also commonly used in paleomagnetics and magnetotellurics [55]. In paleomagnetics a SQUID is used for measuring any remnant magnetism in rocks, and in magnetotellurics

45

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3  Electromagnetic Field Sensors

SQUIDs measure the Earth’s resistivity. Foley et al. developed high‐Tc SQUIDs for mineral exploration, which are used in LANDTEM (a portable exploration system) based on TEM [56, 57]. One another imaging tool is the scanning SQUID microscope (SSM), used for imaging the magnetic field related to the surfaces of samples. High sensitivity and bandwidth are the advantages of this sensor but the requirement of a cooled SQUID sensor and modest spatial resolution are its major disadvantages. According to Wikswo Jr. et al., SQUIDs can be used in a wide range of applications like magnetic imaging of short circuits in integrated circuits, corrosion currents in aluminum, and trapped flux in superconductors [58]. 3.3.1.4  Hall Probes

Hall effect devices are among the oldest, most familiar and most widely used sensor for ­measuring extremely high magnetic fields (>1 T). They are the most common high‐field vector sensor used in gauss meters. Hall probes are based on the Hall Effect discovered by Edwin Hall in 1897. This effect occurs as a consequence of the Lorentz force law. This law states that “a moving charge q, when acted upon by a magnetic induction field B will experience a force F that is at right angles to the field vector and the velocity vector v of the charge.” According to this law, a potential difference will be generated across the electric conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current (Figure 3.7). This can be expressed with the following equation:

F

q E v B (3.10)

In the absence of a magnetic field, the electrons and holes are represented by their mobility μp and density N. They are considered to be moving in straight lines between the supplying electrodes as an electric current with a velocity vp and current density J:

vp J

pE

q

(3.11)

p NE

(3.12)

When a magnetic field is present, the electrons or holes are deflected in the direction perpendicular to the magnetic field vector B. To counterbalance this effect, a second component of the electric field, EH, is produced, which is expressed as:

EH

v B

p

E B (3.13)

The current direction will be deflected by an angle θH, which is known as Hall angle:

tan

H

EH (3.14) E

B

Figure 3.7  Effect of magnetic field lines on current through Hall sensors [59]. Source: Reproduced with permission of Taylor & Francis.

3.3  Electromagnetic Field Measurements

By considering (3.13), the Hall electric field can be defined as:

EH

1 J B qN

RH J B (3.15)

where RH represents the Hall coefficient. The output voltage is measured along the width (w), across the two plates of the sensor electrode, and is used to measure the field:

VH

p wE x By

RH wJB (3.16)

On substituting J with I/wt, where “I” is the current, “w” the width and “t” thickness of the sensor, the transfer function that represents the Hall sensor will be obtained as: w (3.17) VH VB H l Therefore, for a given bias current I, the Hall sensor measures the magnetic field, B, which is perpendicular to the field plate. The physical dimensions (like width and thickness) and material properties of the sensor (described as the Hall coefficient RH) play an important role in determining the transfer function of the sensor. When comes to the choice of sensor material, it should fulfill two basic criteria for achieving better sensitivity: (i) low carrier concentration and (ii) high carrier mobility. These criteria are met by semiconductors, which have led to intense advancement in this type of sensors since the growth of semiconductor technology in mid‐nineteenth century [60]. The first report using Hall devices as magnetic sensors came in 1948 by Pearson et al. soon after the discovery of high carrier mobility semiconductors [61]. There are some early reviews on the development by Weiss, Wieder, Kuhurt, and Lippmann, etc. [62–64]. Integration of Hall device into a bipolar silicon integrated circuit was first proposed by Bosch in 1968 [65]. InSb and InAs are semiconductors that have a high mobility but a small energy band gap, which means higher carrier concentration, making these semiconductors unsuitable for Hall sensor application above room temperature. The n‐type semiconductors are preferred over ­p‐type as electron mobility is larger compared to hole mobility, which results in high carrier mobility. Silicon and GaAs were the most commonly used materials as they were more compatible with the prevailing technology of the time (microelectronics technology) [60]. Modern advancements have been summarized by J. Heremans [66] in 1993 and later by Schott et al. in 1998 [67]. Silicon Hall probes are most commonly used in position indicators, speed, and rotation ­sensors, non‐contact switches, etc., in industries, and in automobiles – in ABS, speedometer, control systems, electric windows, etc. Hall sensors can also be found in household appliances, laptops, etc. [67]. 3.3.1.5 Magneto‐Resistors

In magnetoresistors (MRs), a magnetic field applied to a ferromagnetic material causes a change in its resistivity. This effect is known as the magnetoresistance effect. The magnitude of the magnetic field and the direction of the current used to measure resistivity determine the change in resistivity of the material. MR sensors are linear magnetic field transducers. They are based either on the ferromagnetic material’s intrinsic magnetoresistance or heterostructures of ferromagnetic/non‐magnetic materials. Sensors that are based on the spontaneous resistance anisotropy in three‐dimensional ferromagnetic alloys depend on the intrinsic magnetoresistance of the ferromagnetic material. This is also called anisotropic magnetoresistance (AMR). Giant magnetoresistance multilayers, spin valve, and tunneling magnetoresistance devices are a few examples of MR sensors, which

47

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3  Electromagnetic Field Sensors

Current (I)

Permalloy θ = 45° Magnetization

(M)

Figure 3.8  An MR element during fabrication [59]. Source: Reproduced with permission of Taylor & Francis.

No applied field Easy axis (I)

Permalloy

(M)

θ

Happlied

utilize ferromagnetic/non‐magnetic heterostructures instead of alloys in AMRs. Currently MR sensors are mostly used for data storage [68]. Due to their ability to detect very weak magnetic fields (in the range of nT) at room temperatures, MR sensors find many applications in addition to recording or data storage. Nowadays, single molecule recognition processes use magnetoresistive chips for biomolecular recognition [68]. The giant magnetoresistance effect (GMR) is used in spin valves, which were introduced first in 1991 [69]. These sensors were designed, tested, and a prototype presented in 1992, 1993, and 1994, respectively [70–72]. In this effect, when the magnetic layers magnetize in parallel directions there is a weak scattering of spin‐up conduction electrons and strong scattering experienced by spin‐down electrons. When this spin‐up channel shorts the current, a low‐ resistance state is created. Upon antiparallel magnetization, the spin‐up and spin‐down electrons are alternately scattered, strongly and weakly, which leads to a high‐resistance state [71]. The construction of AMR sensors involves deposition of long thin film segments of permalloy (permalloy is a soft magnetic alloy, made of nickel and iron, having very high magnetic permeability). To establish an easy axis of magnetization, a magnetic field is applied along the length of the film during the deposition process. Apart from length, the shape of the film also helps establish an easy axis. The permalloy films are connected together in series to form the MR. Thin strips of a highly conductive material like gold are deposited on the permalloy film by forcing a current over the film at a 45° angle to the easy axis. A thin layer of cobalt is deposited over the resistors, which creates a bias field that controls the level of magnetization. The cobalt is magnetized parallel to the easy axis of the permalloy. Applying a magnetic field (H)a at right angles to the magnetization vector, causes the vector to rotate and thereby change the magnetoresistance (Figure 3.8) [73]. A typical GMR sensor unit constitute of a triple layered sandwich structure. In this assembly two ferromagnetic layers are separated by a conductive layer. In 1991 Parkin et al. reported such a system with cobalt layers as the ferromagnetic layers and a copper layer as the separating conductor [69, 74]. In 1992 the same team reported another sensor unit with nickel/iron alloy as the magnetic material with copper sandwiched in between the layers [75]. A gauss meter or magnetometer using a typical AMR sensor consists of four AMRs connected in a Wheatstone bridge (Figure 3.9). The polarity of the transfer function of resistors A and B is opposite to that of B and C, which is done by rotating the current shunt by 90°. This causes the output voltage signal for a given field to increase by a factor of four over a single resistor.

3.3  Electromagnetic Field Measurements

Figure 3.9  Magnetoresistance sensor assembly [76]. Source: Reproduced with permission of Elsevier.

Applied field

Vb

B

R

R

–Δ

+Δ

R

R

A

Bias field

–Δ R

R

C

+Δ

R

R

Vout D

Magnetoresistive sensor

3.3.1.6  Scalar Magnetometers 3.3.1.6.1  Proton Precession Magnetometer

Proton precession magnetometers use the magnetic properties of the atomic nucleus. A ­spinning nucleus with an angular momentum and magnetic moment is made to move with precision in a magnetic field. A torque is induced on the nucleus when a magnetic field is applied to it. This torque then makes the nucleus move about the direction of the field with precision, due to its angular momentum. This rotation of charges establishes a magnetic moment in the nuclei, thereby making them behave as elementary magnets. However, unlike the compass needle, they do not align along the magnetic field. They instead tend to rotate in the selected directions with a resonance frequency fL. This frequency is called the Larmor frequency, fL, which depends on or is proportional to the magnetic field, B0: fL B0 (3.18) 2 where γ is gyromagnetic ratio (unit = rad*s−1*T−1). For a charged body rotating about the symmetrical axis, the gyromagnetic ratio can be expressed as follows: q (3.19) 2m where q is the charge and m is the mass of the body [77]. A block diagram of a proton precession magnetometer is shown in Figure 3.10. The sensor is a container filled with a hydrocarbon that is rich in free hydrogen nuclei. Benzene is commonly used for this purpose. The nuclei are polarized by a solenoid wrapped around the sensor. The magnetic field generated to cause the polarization is around 10 mT. The solenoid is also used to detect the precision produced by the magnetic field. Before the application of a polarizing field, the net magnetization is zero as the magnetic moment of the nuclei is randomly oriented. When a polarizing field is applied, all the nuclei will precess about the field. The precession axis and the applied field are either parallel or antiparallel to each other. According to quantum mechanics, the antiparallel state has lower energy than the parallel state. The hydrocarbon in the sensor

49

3  Electromagnetic Field Sensors

Figure 3.10  Block diagram of proton precision magnetometer [78, 79]. Source: Adapted from Alldredge 1960 and Stuart 1972.

Cylinder with hydrocarbon 2 1

Polarizing field

50

Integrator circuit Current source

Applied field

remains un‐magnetized when there is no thermal agitation (which causes collisions between atoms). When collisions occur, the nuclei in the parallel‐precession axis switch to the anti‐parallel state by losing energy. After a while, the number of nuclei with magnetic moment pointing in the direction of the magnetic field will be larger than the number of magnetic moments pointing away from it. This will make the hydrocarbon fluid reach equilibrium magnetization [80–82]. The field is removed once the equilibrium magnetization is achieved. The nuclei are then allowed to precess in the magnetic field till they become randomly aligned again. This excitation– relaxation might take up to several seconds. When the magnetic field is removed, there is a decaying nuclear precession signal at the output of the coil, which is connected to an amplifier. The magnetic moment of the protons tends to align in the direction of the external magnetic field. The signal is then subjected to amplification and filtration. The period of the Larmor frequency is measured, averaged, scaled, and displayed as a digital output with the units of magnetic field [78]. 3.3.1.6.2  Optically Pumped Magnetometer

The Zeeman effect and the optical pumping effect form the basis of optically pumped magnetometers. The Zeeman effect is the splitting of a spectral line into several components in the presence of a magnetic field. It tends to be significant in alkaline vapor. Bell and Bloom introduced this method of optical detection of magnetic resonance in 1957 [83]. The angular momentum from resonant polarized radiation can be transferred to an atom by delivering a circularly polarized light beam of an appropriate wavelength. This is the principle of optical pumping. But only those electrons that have a spin corresponding to the direction of polarization can be excited to a higher energy level due to the selection rule. Thus only those electrons that are present in the G1 state move to the H level, in the first step as shown in Figure 3.11. In the next step, the electrons will decay to both ground states. Only two electrons can be excited under the influence of light. After a number of steps, only those electrons that are present in the forbidden level remain, since they cannot be excited. This is detected as the transparency of the vapor as no electron is available for absorption [85–87]. Thus, initially the vapor is opaque when the pumping starts. With subsequent steps, the number of electrons available for pumping decreases and the vapor becomes transparent. This pumping action will finally stop and the vapor becomes completely transparent. The electrons in the H state can be made available for pumping by de‐exciting them to the G1 level. This is achieved by applying a small RF magnetic field with a Larmor frequency at right

3.3  Electromagnetic Field Measurements Opaque

Transparent

H

F

EM

/ RF G2 G1 Pumping

Opaque

Figure 3.11  Mechanism of optical pumping [84]. Source: Reproduced with permission of Cambridge University Press.

angles to the magnetic field being measured. In optically pumped magnetometers this is used as a positive feedback rearrangement to develop an oscillator with the Larmor frequency. This frequency is then measured, processed, and displayed in magnetic field units [84]. 3.3.2  Electric Field Measurements The most commonly used means by which to measure the electric field and also EMF are ­electric field probes and electron drift instruments. 3.3.2.1  Electric Field Probes

An electric field in air and in other material media with a wide range of permeability is commonly measured using electric field (E‐field) probes. The E‐field probes can be used to measure the amount of exposure in animals exposed to nonionizing radiation, electric fields in air for electromagnetic compatibility, or for radiation safety purposes. Electric field strengths ranging from 1 to 1000 V m–1 (rms) can be measured using these probes. A dipole antenna with a RF detector mounted across the gap, which separates the arms of the dipole, a nondestructive transmission system, and an output device comprise the electric field probe, which can be assembled in different ways. Diode detectors can be used to make very broadband devices (0.2 MHz to 26 GHz) by using resistive loads or electrically short dipoles [1]. For high peak‐power modulated fields, thermocouple detectors are commonly used to provide true time‐averaged data. A wide‐band unperturbed data link between the dipole and detector and a remote readout can be established by using optical fibers and a suitably modulated light source [88]. 3.3.2.1.1  Basic Principles of Operation

Most electric field sensing probes follow the construction scheme shown in Figure 3.12. The basic elements of the probe include a dipole antenna, a nonlinear detector, optional lumped element shaping and filtering networks, a non‐perturbing transmission line, and monitoring instrumentation [89]. The probe operates in a fairly simple manner. An oscillating voltage is produced across the detector terminals by the antennae when a continuous‐wave incident field having a frequency ω is detected. The detector produces a signal with a DC component proportional to the square of the amplitude of the incident field, due to its nonlinear characteristics. The probe filters this signal and transmits the DC component to the monitoring instrumentation via the transmission lines. Thereby a signal proportional to the square of the amplitude of the incident field is measured [90–92].

51

52

3  Electromagnetic Field Sensors

Z

Antenna networks

Detector

Filter circuit

Transmission circuit

Output decoder

Dipole

Figure 3.12  Schematic of electric field probes [89]. Source: Reproduced with permission of IEEE.

Generally, it is difficult to have a uniform electric field incident along the axis of the dipole antennae (z axis). The antennae are usually made physically and electrically short in order to provide spatial resolution. The variation of the incident electric field along the dipole’s length determines the spatial resolution. The resolution can be increased by decreasing the length of the dipole as compared to the distance over which the gradient of the electric field is significant. 3.3.2.1.2  Antenna Networks

The types of antenna networks used for measuring the electric field are [93]: ●● ●● ●● ●● ●●

Electrically short dipole with a capacitive load Electrically short dipole with diode detector Resistively loaded dipole with diode detector Resonant half wave dipole with a diode detector Tuned dipole with narrow band receiver

In an electrically short dipole with a capacitive load antenna, there is almost purely capacitive input impedance. When the dipole is capacitively loaded, frequency independent response characteristics can be achieved. An electrically short dipole with dipole detector is used when the transmission of RF signal from the antennae is not required. These instruments have shunt diode loads. A RF filter transmission line is used to convey the rectified signal to the metering unit. Diode detectors provide isolation between antenna networks and output instrumentation. In electrically short dipole antennas a flat response in the high‐frequency range is constrained due to the resonant frequency of the dipole. The development of suitable antennas in this range requires a dipole with a length of more than 1 mm, which in turn reduces the sensitivity of the system. This limitation is overcome by altering the dipole by loading resistance to it. These types of antennae networks are called resistively loaded dipole with diode detector [94]. 3.3.2.1.3 Detectors

Commonly used detectors in electric field probes are diode detectors and thermo‐couple detectors. Diode detectors are surface barrier diodes like Schottky diodes, which are used for detector and mixer applications as they do not have any significant delay effects as in the case of junction diodes (due to the charge storage). The diode used in diode detectors is of the Mounted Beam‐Lead package type. In this type one or more beam lead Schottky diodes having coplanar leads are embedded into a ceramic substrate. The advantages of using diode detectors in the electric field probe include flat frequency response, mechanical ruggedness, very low inductance, and use of high resistance DC

3.3  Electromagnetic Field Measurements

transmission line, with sufficient isolation between the readout instrumentation and antenna. This device is convenient as it has minimum detector parasitic inductance thereby providing a wide RF/microwave frequency range. Schottky barrier diodes and junction diodes differ in their type and number of charge carriers. In Schottky diodes only one type of charge carrier is present, unlike the junction diodes where there are two types. In an n‐type Schottky diode, the flow of electrons from n‐type semiconductor to the metal results in the forward current, whereas in the p‐type the flow of holes from the p‐type semiconductor causes the current flow. A contact potential between the semiconductor and the metal results in the diode action. At the time of fabrication, when the metal is brought in contact with an n‐type semiconductor, the electrons will flow from the semiconductor to the metal, thereby making the semiconductor negatively charged with respect to the metal. The region of the semiconductor next to the metal has positively charged donor atoms. As these diodes do not have minority carriers, there is no delay effect. This characteristic has been extensively used in developing EM field detectors, where the diodes have to change their conductance states at a frequency equal to the frequency of the varying field [95–97]. A Schottky diode equivalent circuit is shown in Figure 3.13. This is divided into low‐ and high‐frequency regions. These detecting diodes are designed in such a way that very low RF is also converted into proportional DC output. Since it is operated with a small DC bias the impedance will be high, which in turn requires low capacitance to provide high sensitivity. In a thermocouple detector, the hot junction of the thermocouple along with resistance (Rt, in series) is placed between the terminals of a dipole. The resistor is usually a ~100 nm thin film made of metal. When a RF current flows through the resistor, there is some power dissipation, P(t), which increases the temperature of the hot junction, TH. The temperature difference between the hot junction and cold junction, Tc, gives rise to a thermoelectric voltage Vt, which is proportional to the time‐average power dissipated in the resistor. Figure  3.14 shows the equivalent circuit diagram of the thermocouple. Here, Vt = α(TH − Tc), where α represents the Seebeck coefficient of particular materials used in the thermocouple [93, 100]. 3.3.2.2  Electron Drift Instrument

Space technology has seen extensive use of the electron drift instrument. It measures the in‐ situ electric and magnetic fields using a weak beam of electrons. The instrument is designed in such a way that when the electrons are emitted in a particular direction they return to the source after one or more gyrations. This drift depends mainly on the electric field and to some extent on the gradient of the magnetic field [101]. Figure 3.13  Equivalent circuit diagram of Schottky diode detector [95, 98]. Source: Adapted from Sharma 2003 and Tran 2007.

Rv High frequency

CJ

Rs

Low frequency

RJ

– Vd +

53

54

3  Electromagnetic Field Sensors

Rt0

High frequency

Low frequency

Rt

Ct

Figure 3.14  Equivalent circuit of the thermocouple diode [99]. Source: Reproduced with permission of IEEE.

– Vt +

In spacecraft, the electrons are emitted from small guns mounted on them. Once emitted in a certain direction, the electrons tend to return to the source detector on the spacecraft after one or more gyrations. While undergoing these gyrations, the electron beam detects the electric field at some distance away from the spacecraft, essentially outside its influence. Some advanced systems have two electron guns, which can be aimed in any direction, over more than a hemisphere. The electrons return to their source detector by a servo loop. Triangulation of the two emission directions is used to calculate the electron drift (Figure 3.15). Electric field and magnetic field gradients can be determined separately by comparing the drifts of the electrons emitted in different directions [102]. 3.3.2.2.1  Principle of Operation [102, 103]

The basic operating principle of the electron drift instrument involves the injection of test electrons and the registration of their gyro center displacements after a couple of gyrations in the magnetic field, B. The displacements, d, and drift velocity, VD are related by:

d VD N Tg (3.20)

where Tg denotes the gyro‐period and N represents the number of such periods before the electron is captured. If the drift depends only on an electric field, E┴, and is transverse to B, then:

d

E B N Tg (3.21) B2

Notably, after completing one gyration, all the electrons from a common source S, while perpendicular to the magnetic field, are focused at a point that is at a distance d from the source. The distance d is called the drift step. The electrons at the focus are detected by placing a detector at that spot. As we need to measure only d, an electron source in not required at S. An electron gun located arbitrarily can be used as the source of electron beam as long as the beam is directed towards S. In cases where two guns are used, while measuring the direction of emission that will return a beam towards the detector gives the displacement d and drift velocity VD. In principle, this triangulation problem can be done continuously and with high resolution. When the guns are placed in locations other than S, the electrons will not focus at D and the travel times will be different from the gyro time, Tg. If the beam is directed towards S, the travel time will be longer

3.3  Electromagnetic Field Measurements

Figure 3.15  Step triangulation concept.

Vd B X

Beam1

E

Detector D Beam 2

Gun1

Beam2

Beam 1

d

Gun2

S

and when the beam moves away from S the travel time reduces. The angle (in radians) between the outgoing and the returning beam is: 2



vD (3.22) v

3.3.2.2.2  Drift Velocity Measurement from Time of Flight

The time of flight of the electrons can be determined with appropriate resolution by appropriate pulse‐coding. But, for this, the magnetic field should be sufficiently stable over the electron gyration period. The electrons from the two beams, while returning to the detectors, travel different distances. The difference in their flight time is given by:

T

Tto Taw

2Tg

vD v

d (3.23) v

where Tto and Taw are the flight times for the beam electrons aimed towards and away from the target, respectively. Therefore, VD can be determined by measuring Tto and Taw. 3.3.2.2.3  Measurement of B

The mean of the travel times can be used to obtain the gyration periods: Tto Taw Tg 2 which can be used to obtain the magnetic field strength, B by:

Tg

2 m eB

(3.24)

(3.25)

The values for Tg typically range from 0.1 to 10 ms. Magnetic field strengths can be determined with very high accuracy by measuring the time of flight. 3.3.3  Power Density Measurements Knowing the electric (E) and magnetic (H) field strength makes it possible to determine the power density by using the Poynting vector, S. We are mostly interested in the average vector, Sa, which quantifies the power flow from a source and is expressed as:

Sa

1 Re E H * (3.26) 2

55

56

3  Electromagnetic Field Sensors

In the far‐field, the EMF acts a transverse field with a shape of the TEM wave. In the TEM wave or the transverse electromagnetic wave, all the electric and magnetic field lines are normal to the direction of propagation. In this case, the relation between the field components is given as:

E Z

H

(3.27)

E Z

H

(3.28)

The sum of electric power densities, SE, and magnetic power densities, SH, gives us the total power density, which is expressed as:

S

SE SH

2SE

2SH (3.29)

Measuring the power dentistry in the far field can be made with certainty using this concept. However, in the near field, the relation between E and H field is not known. Thus, evaluating the power density based on the measurement of one component will have a large error. In the near field, one component may dominate over the other and calculating the power density based on this would lead to an overestimate, and vice versa. Considering the elementary electric dipole, it is quite clear that for R → 0:

lim E R

0

H

(3.30)

Therefore, to measure the power density in the near field the following methods are considered: 1) Power density measurement as the arithmetic bean of SE and SH measurement. 2) Power density measurement as geometric mean of the SE and SH measurement [104].

3.4 ­Conclusion Commonly used EMF intensity and power measurements have been presented in this chapter, which includes both magnetic field sensors and electric field sensors. The calculations of EMF from the obtained values of electric or magnetic field are also mentioned.

­References 1 Kanda, M. and Driver, L.D. (1987). An isotropic electric‐field probe with tapered resistive

dipoles for broad‐band use, 100 kHz to 18 GHz. IEEE Transactions on Microwave Theory and Techniques 35 (2): 124–130. Gajšek, P., Ravazzani, P., Wiart, J. et al. (2015). Electromagnetic field exposure assessment in 2 Europe radiofrequency fields (10 MHz–6 GHz). Journal of Exposure Science and Environmental Epidemiology 25 (1): 37–44. Bren, S.P.A. (1996). Historical introduction to EMF health effects. IEEE Engineering in Medicine 3 and Biology Magazine 15 (4): 24–30.

­  References

4 Nahman, N.S., Kanda, M., Larsen, E.B., and Crawford, M.L. (1985). Methodology for standard

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61

4 Shielding Efficiency Measuring Methods and Systems Saju Daniel1,2 and Sabu Thomas1,3 1 

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India  St. Xavier’s College Vaikom, Mahatma Gandhi University, Kottayam, Kerala, India 3  School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India 2

Abbreviations c Velocity of light dB Decibel E Electric field intensity EM wave Electromagnetic wave EMI SE Electromagnetic interference shielding effectiveness EMI Electromagnetic interference f Frequency G Relative conductivity referred to copper H Plane wave field intensity M Magnetic field intensity S parameters Scattering parameters SE Shielding effectiveness SEA Shielding effectiveness by absorption SEM Shielding effectiveness by multiple reflections SER Shielding effectiveness by reflection t Thickness TEM cell Transverse electromagnetic cell VNA Vector network analyzer Zw Wave impedance δ Skin depth η Intrinsic impedance λ Wavelength μ Permeability σ Electrical conductivity ω Angular frequency

Advanced Materials for Electromagnetic Shielding: Fundamentals, Properties, and Applications, First Edition. Edited by Maciej Jaroszewski, Sabu Thomas, and Ajay V. Rane. © 2019 John Wiley & Sons, Inc. Published 2019 by John Wiley & Sons, Inc.

62

4  Shielding Efficiency Measuring Methods and Systems

4.1 ­Introduction Electromagnetic interference (EMI) is an upshot of terrific escalation of electronics and telecommunication in the modern society. All electronic devices emanate magnetic and electrical energy, and this energy unintentionally integrates with another device and generates electromagnetic interference. The EMI among electronic appliances or instruments may have direct detrimental effects on the performance of extremely sensitive precision electronic equipment as well as on human health. Due to possible hazards of EMI, the use of electromagnetic (EM) wave receiving or emitting electronic gadgets is banned inside sensitive zones. Therefore, methodical strategies and suitable countermeasures are essential to preclude or repress EMI so as to ensure uninterrupted performance of appliances that are susceptible to interference and to protect the health of human beings. To fulfill this requirement, researchers have been concentrating on the development of new materials for electromagnetic shielding and their shielding properties characterization. Shielding effectiveness is a key parameter that often determines the scope of application of a given material as an electromagnetic shield. The most appropriate method and instrument need to be selected to obtain reliable measurement results for shielding effectiveness (SE). This chapter mainly contributes most commonly used standard measurement methods and systems for determining the EMI shielding effectiveness. 4.1.1  Mechanism of Shielding Meticulous knowledge about the mechanism of shielding is the focal criteria to ascertain the SE of a material effectively. The primary mechanism of shielding is reflection and it is due to the impedance mismatch between air and the sample and the material used for shielding by reflection necessitate mobile charge carriers; that is, the shield should have electrical conductivity. Accordingly, metals are the most common shielding material that uses primarily the reflection mechanism for shielding along with a minor absorption component. However, metals exhibit problems like poor wear or scratch resistance, corrosion susceptibility, high density, difficult processing, and high cost. The secondary shielding mechanism is absorption. It occurs because of the energy dissipation while an electromagnetic wave interacts with the material, for which a shield material should have electrical or magnetic dipoles along with finite electrical conductivity. For such purpose, materials with a high dielectric constant like ZnO, SiO2, TiO2, and BaTiO3, or high magnetic permeability, for example, carbonyl iron, Ni, Co, or Fe metals, γ‐Fe2O3, or Fe3O4, are used. However, these materials or their composites pose problems like low permittivity or permeability at gigahertz frequencies, weight penalties, narrow‐band action, and processing difficulties. Other than reflection and absorption, another mechanism of shielding is multiple reflections, which refer to the reflections at various surfaces or interfaces in the shield. Such multiple reflections are a consequence of the scattering effect of inhomogeneity within the materials. This mechanism requires the presence of a large interfacial area and porous structure. An example of a shield with a large surface area is a conducting composite material containing filler, foamed composites, and honeycomb structures [1, 2]. 4.1.2  Shielding Effectiveness The implementation of an EMI shield is decided by measuring its SE. The EMI SE of a material is defined as the enfeeblement of propagating electromagnetic waves produced by the shielding materials. Shielding effectiveness can be specified in the terms of reduction in magnetic field, electric field, or plane‐wave strength caused by shielding. SE is normally expressed in decibels

4.1 Introduction

(dB) as a function of the logarithm of the ratio of the incident and transmitted electric (E), magnetic (H), or plane‐wave field intensities (F) [3]: SE 20 log E0 /E1 SE 20 log H 0 /H1

SE 20 log F0 /F1



SE can also be expressed as a function of the logarithm of the ratio of incident power to the transmitted power: SE = 10 log(Pi/Pt), where Pi is the incident power and Pt is the transmitted power [4]. In addition, SE = A1 – A2where A1 is the source attenuator setting, in decibels, for a measurable output of a specified detector in the absence of the material, and A2 is the source attenuator setting for the same output of the detector in the presence of the material [5]. If the receiver readout is in units of voltage, the following equation is used: SE = 20 log (V1/V2) where V1 and V2 are the respective voltage levels in the presence and absence of a material [4]. Higher values of SE in decibels indicate that a small amount of energy passes through the shield and most of the energy is absorbed or reflected by the shielding material. According to these equations, the SE will have a negative value if less power is received with the material present than when it is absent. Any shielding material attenuates the electromagnetic radiation through three mechanisms: reflection of the wave from the front face of the shield, absorption of the wave as it passes through the shield, and multiple reflections of the waves at various interfaces; the conceptual mechanism for determining SE is shown in Figure 4.1. Therefore, the total SE is the sum of contributions from shielding by reflection (SER), absorption (SEA), and multiple reflections (SEM), and is given by the equation:

SE total

SE R

Incident wave (E0,H0)

SE A

SE M Shield Aborption (A)

Air

Reflection losses (R) Transmitted wave (Et,Ht) Internal reflection t

Figure 4.1  Conceptual mechanism for determining SE.

63

64

4  Shielding Efficiency Measuring Methods and Systems

When SEA is greater than 10 dB, SEM can be neglected and SEtotal will be as follows [6]:

SE total

SE R

SE A



4.1.2.1  Absorption Loss

When an electromagnetic wave passes through a medium its amplitude decreases exponentially. This decay or absorption loss occurs because current induced in the medium produces ohmic losses and heating of the material; E1 and H1 can be expressed as E1 = E0e−t/δ and H1 = H0e−t/δ. For a shielding material, the skin depth (δ) is the distance up to which the intensity of the electromagnetic wave decreases to 1/e of its original strength. The skin depth is related to frequency, relative permeability, and total conductivity as: 1 f





Absorption losses are a function of the physical characteristics of the shield and are independent of the type of source field. Therefore, the absorption loss is the same for all three waves and is given by the expression:

SE A

3.338 10

3

t

fG

where SEA is the absorption or penetration loss expressed in dB and t is the thickness of the shield in mils. The above equation shows that the absorption loss increases with increase in thickness of the shield. In terms of skin depth, SEA can be expressed as [3, 7]: SEA = 8.7

t

.

4.1.2.2  Reflection Loss

The reflection loss depends entirely on the discrepancy between the intrinsic impedance of the shield and the free space and is independent of the thickness of the shield. The equations for the three principal fields are given by the expressions:







G

RE

353.6 10 log

RH

20 log10

Rp

108.2 10 log10

f

3

0.462 r1 Gf

r12 0.136r1

G 106 f

fG

0.354





where RE, RH, and RP are the reflection terms for the electric, magnetic, and plane wave fields expressed in dB; G is the relative conductivity referred to copper, f is the frequency in Hz, μ is the relative permeability referred to free space, and r1 is the distance from the source to the shield in inches [3, 7].

4.2  Calculation of Electromagnetic Shielding Effectiveness

4.1.2.3  Multiple Reflection Correction Factor

In the case of a thin shield, the first boundary re‐reflects the reflected wave from the second boundary and directs it to the second boundary to be reflected again and again so that multiple reflections take place. If the shield is thicker than δ, the conductive material absorbs the reflected wave from the internal surface, and thus multiple‐reflection can be ignored. The factor SEM can be mathematically positive or negative (in practice it is always negative) and becomes insignificant when the absorption loss SEA is >10 dB. It is usually only important when metals are thin and at low frequencies, i.e. below approximately 20 kHz. The multiple reflection correction factor (SEM) can be expressed as:



SE M

K 1

20 log 1

K 1

2

10

2

A

10

e

i 227 A



where A is the absorption loss; K is given by:

K

ZS / ZH

1.3

/ fr 2

where ZS is the shield impedance and ZH is the impedance of the incident magnetic field. When ZH 90

40 ≧ Electromagnetic shielding effectiveness >30

Moderate

90 ≧ Electromagnetic shielding >80

30 ≧ Electromagnetic shielding effectiveness >20

Fair

80 ≧ Electromagnetic shielding >70

where Ei and Et are incident and transmitted electric fields and Hi and Ht are incident and t­ ransmitted magnetic fields respectively.

15.6 ­Requirement of Shielding Materials for Aerospace Metal foams with gas‐filled pores, or sheet metals, Faraday cages, metal coated carbon fibers, conducting spray, intrinsically conductive polymers, and conducting filler based polymer com­ posites are all recognized as EMI shielding materials. While this may seem academic, the reader is reminded that many forms of shield enclosure are available and a variation of shielding effec­ tiveness (SE) are described. Classes of a shielding according to their shielding effectiveness and application are mentioned in Tables  15.5 and 15.6. Table  15.5 shows the quality of shielding according to the SE for professional use. Table 15.6 shows the quality of shielding according to the SE for general purpose. Figure 15.5 represents the shielding effect on equipment: (a) the shield encloses the radiated emission produced by the devices, which do not interference with electronic devices outside of the enclosure; (b) electronics devices inside the shield are not interfered with by radiation from outside the enclosure. However, when using the aluminium materials for EMI shielding, its galvanized corrosion and oxidation characteristics are the main drawback.

337

338

15  Electromagnetic Interference Shielding Materials for Aerospace Application

Table 15.6  Shielding effectiveness for casual wear, office uniform, maternity dress, apron, consumptive electronic products and communication related products, etc. General materials shielding effectiveness (dB)

Performance

Electromagnetic shielding (%)

>30 dB

Excellent

Electromagnetic shielding >99.9

30 ≧ Electromagnetic shielding effectiveness >20

Very Good

99.9 ≧ Electromagnetic shielding >99.0

20 ≧ Electromagnetic shielding effectiveness >10

Good

99.0 ≧ Electromagnetic shielding >90

10 ≧ Electromagnetic shielding effectiveness >7

Moderate

90 ≧ Electromagnetic shielding >80

10 ≧ Electromagnetic shielding effectiveness >7

Fair

80 ≧ Electromagnetic shielding >70

(a)

(b)

EM shield

EM shield

Antenna

Antenna

Figure 15.5  (a) Shield enclosure contains radiated emission; (b) shield enclosure excludes radiated emission.

15.7 ­Types of Shielding Materials for Aerospace A wide variety of metallic enclosures have been reported for EMI shielding in recent decades. Therefore, it is easy to provide a list of examples of what has now become known as “metal based electromagnetic shielding materials.” We now make a few remarks that will hopefully place the area of metal based electromagnetic shielding into perspective for a better understanding of the topic. 15.7.1  Metals Enclosure Based EMI Shielding Materials Metals must be considered for their better electrical conductivity and magnetic properties. Fundamentally, metal conductors are used to ground high‐speed equipment and electrical devices to protect them from dissipated heat and static charges [83]. The electrical conductiv­ ity and magnetic properties of many materials are shown in Tables 15.7 and 15.8. There are two ways to obtain the effective shielding. One is active shielding and the other is passive shielding at low frequency electromagnetic fields. Active shielding refers to the incident field and produces an opposing electromagnetic field that cancels the incident magnetic flux. This approach is used for electronic field sensors and generators, while passive shielding employs rigid electrical steels, which have a high magnetic permeability, such as ferromagnetic materials mu‐metal and nickel‐iron alloys, and conductive materials including iron, steel, aluminium, and specialty electrical steels are used for this

15.7  Types of Shielding Materials for Aerospace

Table 15.7  Electrical conductivity of various metals [84]. Material

Resistivity (ρ) (Ω.m) at 20 °C

Conductivity (σ) (S m−1) at 20 °C

Silver

1.59 × 10−8

6.30 × 107

Copper

1.68 × 10

−8

Gold

2.44 × 10−8

Aluminium

2.82 × 10

−8

Beryllium

4.00 × 10−8

Rhodium

4.49 × 10

−8

Magnesium

4.66 × 10−8

Molybdenum

5.22 × 10

–8

Iridium

5.28 × 10−8

Tungsten

5.49 × 10

−8

Zinc

5.94 × 10−8

Cobalt

6.25 × 10

−8

Cadmium

6.84 × 10−8

Nickel (electrolytic)

6.84 × 10

−8

Ruthenium

7.59 × 10−8

Lithium

8.54 × 10

−8

Iron

9.58 × 10−8

Platinum

1.06 × 10

−7

Palladium

1.08 × 10−7

Tin

1.15 × 10

−7

Selenium

1.19 × 10−7

Tantalum

1.24 × 10

−7

Niobium

1.3 × 10−7

Steel (cast)

1.61 × 10

−7

Chromium

1.96 × 10−7

Lead

2.05 × 10

−7

Vanadium

2.61 × 10−7

Uranium

2.87 × 10

−7

Antimony (semiconductor)

3.92 × 10−7

Zirconium

4.10 × 10

−7

Titanium

5.56 × 10−7

Mercury

9.58 × 10

Germanium (semiconductor) Silicon (semiconductor)

−7

4.6 × 10−1

6.40 × 102

5.98 × 107 4.52 × 107 3.5 × 107

2.50 × 107 2.23 × 107 2.15 × 107 1.91 × 107 1.89 × 107 1.82 × 107 1.68 × 107 1.60 × 107 1.46 × 107 1.46 × 107 1.31 × 107 1.17 × 107 1.04 × 107 9.44 × 106 9.28 × 106 8.7 × 106

8.35 × 106 8.06 × 106 7.66 × 106 6.21 × 106 5.10 × 106 4.87 × 106 3.83 × 106 3.48 × 106 2.55 × 106 2.44 × 106 1.79 × 106 1.04 × 106 2.17

1.56 × 10−3

shielding strategy (Table 15.9). The combination of good permeability and better conductive materials has improved the electromagnetic shielding performance. According to shielding studies, which are based on reflection, lightweight magnesium ­enclosures offer good shielding performance over a wide range frequency spectrum. However, die cast enclosures of magnesium and aluminium show the equivalent SE shielding based on

339

340

15  Electromagnetic Interference Shielding Materials for Aerospace Application

Table 15.8  Relative permeability data for various materials. Materials

Relative permeability

References

Metals

1 000 000

[85–88]

Iron (99.95% pure Fe annealed in H)

200 000

[85–88]

Mu‐metal

20 000, 50 000

[85–88]

Cobalt‐iron

18 000

[85–88]

Permalloy

8000

[85–88]

Iron (99.8% pure)

5000

[85–88]

Ferritic stainless steel (annealed)

1000–1800

[85–88]

Martensitic stainless steel (annealed)

750–950

[85–88]

Carbon steel

100

[85–88]

Nickel

100–600

[85–88]

Martensitic stainless steel (hardened)

40–95

[85–88]

Neodymium magnet

1.05

[89]

Silver

1

[90]

Copper

1

[90]

Gold

1

[90]

Aluminium

1

[90]

Brass

1

[90]

Bronze

1

[90]

Tin

1

[90]

Lead

1

[90]

Wood

1.00000043

[91]

Superconductors

0

absorption. Die cast magnesium alloy enclosures for EMI shielding has many advantages over both plastic and alternative metal housings [92–94]. A magnetic material such as steel shows better magnetic field shielding at low frequencies than a good electrical conductor such as aluminium or copper. At high frequencies, good conductors offer better magnetic shielding. For non‐magnetic material, the SE increases with the frequency. For a magnetic material, the SE may be reduced due to the loss of the permeability with frequency. Lead is most useful economical shielding material. However, it can affect human health and the environment. There are so many ways to offer electromagnetic shielding by metal and its metal alloy, which include a single layer of metal (single shield:), multilayer shields (multimedia laminated shield), double isolated conducting metal sheets separated by insulating dry plywood (isolated double shield), and a shield consisting of a number of circular or square apertures separated by a ­particular spacing between them (perforated shield) [95]. In recent years, electromagnetic (EM) wave absorbing materials have aroused much ­attention because of more and more civil and military applications in EMI shielding and radar cross‐­section (RCS) reduction in the gigahertz (GHz) range, for example cellular sys­ tems, ­ computers, wireless communications, satellite communications, and so on (Table  15.10). Traditional microwave absorbing materials have been widely investigated;

15.7  Types of Shielding Materials for Aerospace

Table 15.9  Properties and application of metals for EMI shielding. Metal

Properties

Application in EMI shielding

Iron

Tremendous plasticity, toughness, and weld ability, great pressure processing, low strength, high iron oxide corrosion

Iron wire mesh and enclosure, at 10 GHz up to 60 dB

Steel

High tensile strength and low cost, low corrosion, galvanized steels are used to prevent corrosion, higher density

Low‐frequency RF shielding, low carbon steel shows better DC and low‐frequency shielding (due to high permeability and saturation point)

Aluminium

High corrosion resistance, lightweight, high electrical conductivity, low impact resistance

Excellent shielding effectiveness of up to 100 dB

Copper, brass

Good electrical and thermal conductivity, suffer from the galvanic corrosion near the contact area; nonferrous characteristics; high cost, low tensile strength, poor resistance to abrasion and common acids, better than steels

Radio frequency interference shielding, marine applications, not good for lightning shielding and cosmic radiations shielding 10 GHz, >60 dB

Copper‐ iron alloy

Excellent electromagnetic properties, electrically conducting, cost, and processing

Tremendous electromagnetic shielding performance

Magnesium

Reflection attenuation of the incident electromagnetic wave, thermal conductivity properties, shock absorber

Superior electromagnetic interference (EMI) shielding

Nickel filaments

Superior magnetic permeability and oxidation resistance properties, better than copper

Excellent shielding effectiveness

Lead (Pb)

Malleable, soft, and corrosion resistance, high density, lighter, health and environmental concern

X‐ray shielding and gamma‐rays shielding, radiation protection

Tin (Sn)

Moderate oxidation compared with copper, galvanized steel, steel, aluminium, low oxidation and high conductivity

Electromagnetic shielding material in the corrosive environment, better shielding performance

Table 15.10  Electromagnetic wave abortion ability of various structured metal, alloys, and their composites.

Materials

Optimal frequency (Hz)

Minimum RL value (dB)

Thickness (mm)

Reference

Ni nanofiber composites

1.3 GHz

−35.4

8.4

[96]

Ni/SnO2 microspheres

14.7

−18.6

7.0

[97]

Ni chains

9.6

−25.29

2.0

[98]

Ni nanowires

10

−8.5

3.0

[99]

Core–shell Ni/SnO2

9.8

−42.8

3.0

[97]

Ni/Sn6O4(OH)4 nanoshells

13.2 GHz

−32.4

5.0

[100]

Flower‐like Ni structures

3.6 GHz

−10

2.0

[101]

Hollow nickel spheres

13.4 GHz

−27.2

1.4

[102]

Chain‐like CoNi

17.5 GHz

−34.33

1.0

[103]

Ordered mesoporous carbon–silica/FeNi nanocomposites

11.1 GHz

−45.6

3.0

[104]

341

342

15  Electromagnetic Interference Shielding Materials for Aerospace Application

however, the high  specific g­ravity of such materials severely restrains their potential ­applications. Thus, microwave absorbing materials with relatively light weight, thin thick­ ness, structurally sound and flexible, and with strong absorption in a wide band range are in high demand nowadays. 15.7.1.1  Design Consideration of Metallic Enclosure for EMI Shielding

1) Wire meshes are more useful for absorption of the electromagnetic waves, because they offer a low weight per unit area as compared to metallic sheets. The high physical f­ lexibility of wire meshes is more suitable for retrofit shielding applications [105]. Shielding effective­ ness of a wire mesh screen depends on many parameters such as frequency of operation, dimensions of the screen mesh, and on the angle of incidence of plane wave [106]. Figure 15.6 shows a schematic structure of (a) a metal wire mesh, (b) an iron wire mesh, (c)  an ­aluminium wire  mesh, and (d) copper wire mesh. Table  15.11 shows the electro­ magnetic effectiveness of various metals and alloys. 2) A Faraday shield is made with a conducting material or a mesh of conducting material. Such a shield is designated as a faraday cage; faraday cage enclosures can block out external static electric fields. Faraday cages are named after the English scientist Michael Faraday. Faraday discovered this cage in 1836. A Faraday cage can be used to protect electronic equipment from lightning strikes and other discharges. The shielding ­p erformance of a Faraday cage depends on the generation of the electrical charges within the cage.

(b)

(a)

as 2rw

(c)

(d)

Figure 15.6  Schematic structure of meshes: (a) metal wire meshes, (b) iron wire mesh, (c) aluminium wire mesh, and (d) copper wire mesh [95]. Source: Adapted from Rai and Yadav 2014.

15.7  Types of Shielding Materials for Aerospace

Table 15.11  Electromagnetic effectiveness of various metal and alloys. Materials

Frequency (Hz)

Shielding effectiveness (dB)

References

2024 Al alloy

900 MHz

∼67 (2 mm thickness)

[107]

2024 Al alloy

1500 MHz

∼52 (2 mm thickness)

[107]

2024 Al/cenospheres composite

900 MHz

∼80 (2 mm thickness)

[107]

2024 Al/cenospheres composite

1500 MHz

∼80 (2 mm thickness)

[107]

Magnesium

900 MHz

51 (2 mm thickness)

[108]

Magnesium

1500 MHz

52 (2 mm thickness)

[108]

AZ31 Mg alloy

900 MHz

64 (2 mm thickness)

[108]

AZ31 Mg alloy

1500 MHz

55 (2 mm thickness)

[108]

Magnesium alloy

30–1500 MHz

88–95 dB (2 mm thickness)

[109]

P (11.1 wt%) + Ni (88.9 wt%) + Co (0 wt%)

1 GHz

67.2 (15.1 μm thickness)

[110]

P (11.1 wt%) + Ni (88.9 wt%) + Co (0 wt%)

100 kHz

67.1 (15.1 μm thickness)

[110]

P (12.4 wt%) + Ni (82.9 wt%) + Co (4.7 wt%)

1 GHz

72.7 (18.7 μm thickness)

[110]

P (12.4 wt%) + Ni (82.9 wt%) + Co (4.7 wt%)

100 kHz

68.5 (18.7 μm thickness)

[110]

P (12.2 wt%) + Ni (79.5 wt%) + Co (8.3 wt%)

1 GHz

88.7 (25.8 μm thickness)

[110]

P (12.2 wt%) + Ni (79.5 wt%) + Co (8.3 wt%)

100 kHz

72.4 (25.8 μm thickness)

[110]

P (6.9 wt%) + Ni (59.1 wt%) + Co (34.0 wt%)

1 GHz

106.5 (19.8 μm thickness)

[110]

P (6.9 wt%) + Ni (59.1 wt%) + Co (34.0 wt%)

100 kHz

91.6 (19.8 μm thickness)

[110]

P (6.6 wt%) + Ni (57.5 wt%) + Co (35.9 wt%)

1 GHz

111.5 (18.1 μm thickness)

[110]

P (6.6 wt%) + Ni (57.5 wt%) + Co (35.9 wt%)

100 kHz

100.1 (18.1 μm thickness)

[110]

P (6.0 wt%) + Ni (50.9 wt%) + Co (43.1 wt%)

1 GHz

112.8 (15.5 μm thickness)

[110]

P (6.0 wt%) + Ni (50.9 wt%) + Co (43.1 wt%)

100 kHz

110.6 (15.5 μm thickness)

[110]

150 °C × 15 h aged ZK60 alloy

900 MHz

∼74 (2 mm thickness)

[111]

150 °C × 15 h aged ZK60 alloy

1500 MHz

∼72 (2 mm thickness)

[111]

Mg–5.18Zn–1.29Y–0.98Zr (wt %)

900 MHz

∼89 (2 mm thickness)

[108]

Mg–5.18Zn–1.29Y–0.98Zr (wt %)

1500 MHz

∼76 (2 mm thickness)

[108]

EHD jet‐printed Ag metal‐mesh layer

8–12 GHz

20

[112]

BiFeO3

8–12 GHz

11

[113]

Ta/Al/Ta‐grid electrode

6 GHz

24 (20 and 50 nm Al thickness)

[114]

Ta/Al/Ta‐grid electrode

6 GHz

30 (100 nm Al thickness)

[114]

Ta/Al/Ta‐grid electrode

6 GHz

40 (125 nm Al thickness)

[114]

Ta/Al/Ta‐grid electrode

6 GHz

48 (150 nm Al thickness)

[114]

Ta/Al/Ta‐grid electrode

1 GHz

38–60 dB (1 μm Al thickness)

[114]

ZnO films

1 GHz

0.9 (100 nm thickness of film)

[115]

ZnO:F (1 at.%) films

1 GHz

2.6 (100 nm thickness of film)

[115]

ZnO:Al (2 at.%) films

1 GHz

2.6 (100 nm thickness of film)

[115] (Continued)

343

344

15  Electromagnetic Interference Shielding Materials for Aerospace Application

Table 15.11  (Continued) Materials

Frequency (Hz)

Shielding effectiveness (dB)

References

ZnO:Al (2 at.%) films

1 GHz

2.1 (100 nm thickness of film)

[115]

ZnO:Al (2 at.%) films

1 GHz

9.7 (300 nm thickness of film)

[115]

ZnO:Al (2 at.%) films

1 GHz

13.1 (300 nm thickness of film)

[115]

Mg(94.72%)–Zn(4.61 wt%)– Cu(0 wt%)–Zr(0.67 wt%) alloys

900 MHz

85 (2 mm thickness)

[108]

Mg(94.72%)–Zn(4.61 wt%)– Cu(0 wt%)–Zr(0.67 wt%) alloys

1500

66 (2 mm thickness)

[108]

Mg(92.28 wt%)–Zn(4.63 wt%)– Cu(0.37 wt %)–Zr(0.71 wt%) alloys

900 MHz

91 (2 mm thickness)

[108]

Mg(92.28 wt%)–Zn(4.563 wt%)– Cu(0.37 wt%)–Zr(0.71 wt%) alloys

1500

66 (2 mm thickness)

[108]

Mg(93.58 wt%)–Zn(5 wt%)– Cu(1 wt%)–Zr(0.7 wt%) alloys

900 MHz

97 (2 mm thickness)

[108]

Mg(92.28 wt%)–Zn(4.58 wt%)– Cu(2.32 wt%)–Zr(0.82 wt%) alloys

1500 MHz

77 (2 mm thickness)

[108]

Mg(92.84 wt%)–Zn(4.74 wt%)– Cu(1.61 wt%)–Zr(0.81 wt%) alloys

900 MHz

99 (2 mm thickness)

[108]

Mg(92.84 wt%)–Zn(4.74 wt%)– Cu(1.61 wt%)–Zr(0.81 wt%) alloys

1500 MHz

77 (2 mm thickness)

[108]

Mg(92.28 wt%)–Zn(4.58 wt%)– Cu(2.32 wt%)–Zr(0.82 wt%) alloys

900 MHz

103 (2 mm thickness)

[108]

Mg(92.28 wt%)–Zn(4.58 wt%)– Cu(2.32 wt%)–Zr(0.82 wt%) alloys

1500 MHz

84 (2 mm thickness)

[108]

15.7.2  Porous Structure for EMI Shielding Materials In recent years, modern strong, lightweight stainless steel, high‐speed steel, and aluminium foam have been used in the field of military and transportation applications, owing to their low density and high‐energy absorption ability with high mechanical properties, to shield against X‐rays, gamma rays, and neutrons [116–119]. The production of metal foam is started in early 1948 [120]. The metal foam was based on aluminium using the gasification of mercury in molten aluminium. In 1956, further developed aluminium foam technology was used for the production of aluminium foam on an industrial level [121]. Porous metals were classified in two categories [122]: 1) Closed‐cell porous metals 2) Open‐cell porous metals. Both types of porous metals have their own application area, for example open‐cell porous ­ etals are utilized as noise absorption materials, filters, and biomaterials, etc. Open‐cell porous m metals are also used as heat radiators in planes, high‐speed vehicles, due to their large specific sur­ face area, thermal conducting (due to a high conducting solid porous worked as the matrix) [123].

15.7  Types of Shielding Materials for Aerospace

Metal foams

Closed cell foams

ML route

Open cell sponges

PM route

Polymer sponge

Alporas (Foamtech) Alusion (Cymat)

Forminal (Fraunhofer) Alulight (SAS)

Investment casting (ERG) M-pore (Mayser)

Aluhab (Aluinvent)

AFS (Pohltec)

Coating (Alantum)

Placeholder

Exxentis Castfoam (Alveotec)

Recemat

Figure 15.7  Schematic grouping of the commercially most relevant production methods for metal foams and sponges and some representative trade names and companies [127]. SAS, Slovak Academy of Sciences; AFS, aluminium foam sandwich; ML, melt; PM, powder metallurgical. Source: Reproduced with permission of Elsevier.

Closed‐cell porous metals are frequently employed as structural materials, for example as impact energy absorbers, electromagnetic shields, etc. [102, 124, 125]. Subsequently, a lot of methods were used for producing metal foams. They include a casting foaming (e.g. aluminium foam) process or solid forming process [126]. Several other methods are also available by which to produce metallic foams (Figure  15.7) [128–130]. Especially, metallic sponges cannot be foamed by a direct method [131]. In addition, manufacturing of open‐cell and closed‐cell sponges can be divided further [122, 131]: 1) Polymeric sponge structure as a pattern or carrier, 2) Placeholder applied (metallic sponges), 3) ML routes, 4) PM routes. Moreover, metal foams are heterogeneous cellular structures composed of metal and gas (usually air) and are distinguished through a very low apparent density. They are utilized in several industrial applications according to their cell topology (open or closed cells), material (aluminium, copper, titanium, etc.), apparent density, and pore size. Metal foams can also show specific physical properties. Therefore, the most specific foams are used for specific engineering applications, ranging from mechanical to automotive, gas and fluid filtration, thermal management, and many others [132–134]. One possible application area that has been little investigated so far is electromagnetic shielding, where the cellular structures of the foams offer good electromagnetic (EM) shielding properties (SE) (Table 15.12). Furthermore, the mechanical properties of the metal foams are described in the terms of specific domain and application [132–134]. Currently, low‐cost metallic foams are manufactured for a broad

345

346

15  Electromagnetic Interference Shielding Materials for Aerospace Application

Table 15.12  Electromagnetic effectiveness of various metal foams. Materials

Frequency

Shielding effectiveness (dB)

References

Al‐SiC foams

1.44–5 GHz

160 (highest porosity)

[135]

Aluminium foam sandwich

100–500 MHz

−118 (15 mm aluminium foam sandwich)

[136]

Aluminium sheet

100–500 MHz

−57 (14 mm Al foam)

[136]

Aluminium foams

130–1800 MHz

25–75

[137]

Cu–Ni–CNT open‐cell foam

8–12 GHz

54.6 (pores per inch (PPI) = 110 and a 1.5 mm thickness)

[138]

range applications such as in the field of sandwich panels, mechanical damping, energy absorption, acoustic absorption, and electromagnetic shielding. 15.7.3  Polymer Composites for EMI Shielding This section aims to cover the topic of polymer composites for EMI shielding and, also, to describe and understand an experimentally accessible route to various kinds of polymer ­composites for EMI shielding. A few initial remarks will hopefully place the area of polymer composites into a perspective from which the reader might understand the problems to be faced in approaching the polymer composites. Wood is a natural composite of cellulose fibers in a matrix of lignin. Initially, synthetic composite materials were the combination of straw and mud to form bricks for building structure [139]. Composites are produced from two or more different constituent materials. The term constituent refers to the form of matrix and ­reinforcement. Composites are always classified according to the matrix, reinforcement mate­ rial, and reinforcement morphology, for example metal matrix, ceramic matrix, polymer matrix composites, or fiber, particulate, etc. reinforced composites. Here, we concentrate on polymer composites [140]. Various kinds of polymers composites are available throughout the market depending upon the starting raw materials. The most common are polyolefin, polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, polypropylene, PEEK, etc. [141]. Recently, a num­ bers of investigation have been carried out on polymer composites to produce EMI shielding materials for practical application. These composites showed outstanding performance for aerospace, defense, electronics, construction, and medical markets. 15.7.3.1  Metal Coated Polymer for EMI Shielding

An interesting EMI shielding material can be produced using fabrics and metal conductors. These metal coated polymer materials have gained much attention for a wide range of appli­ cations, in the automotive industry, aerospace industries, computer industries, microelec­ tronics, and food packaging, to provide EMI shielding properties. There are various types of materials and metal conductors, such as carbon, polyaniline, polypyrrole, polypropylene, polyester ­fabrics, glass fiber, textile fabric, acrylate rubber copper, nickel, silver, and metals alloys, and various techniques such as electroless plating, plasma treatment, foil laminates and tapes, ion plating, vacuum metallization, flame spraying, arc spraying, cathode sputter­ ing, conductive paints, and lacquers, electroplating [142–146]. This is a good point at which to introduce i­norganic fillers and nanofillers for EMI shielding materials. All the above‐ mentioned coating techniques are secondary methods, meaning that once samples are pre­ pared a coating is added to provide a sample surface that is suitable for a specific purpose. From the preceding discussion, clearly, metal coated polymers or fabrics (Figure  15.8)

15.7  Types of Shielding Materials for Aerospace

Figure 15.8  Schematic diagram for conducting coating. Source: Reproduced with permission of Elsevier.

Coating rs be Fi

d ate co l ta rs Me fibe

Table 15.13  Electromagnetic absorption properties of various metal and alloy coated fabrics. Reflection loss (dB)

Materials

Frequency

References

Electroless Cu‐plated layer on poly(ethylene terephthalate) by NH3 plasma treatment + SnCl2, PdCl2, and HCl solution

0.1–1 GHz

35

[155]

Electroless Cu‐plated layer on poly(ethylene terephthalate) by NH3 plasma treatment + PdCl2 solution

0.1–1 GHz

∼45

[155]

Electroless Cu‐plated layer on poly(ethylene terephthalate) by O2 plasma treatment

0.1–1 GHz

30

[155]

Electroless Cu‐plated layer on poly(ethylene terephthalate) by Ar plasma treatment

0.1–1 GHz

30

[155]

Electroless Ni 21 at.%–Co 51 at.%–P 28 at.%‐coated SiC powder

9 GHz

24

[156]

Electroless Ni 75 at.%–Co 19 at.%–P 5.4 at.%‐coated SiC powder

7 GHz

23

[156]

Electroless Ni 68 at.%–Co 27 at.%–P 5.7 at.%‐coated SiC powder

6 GHz

33

[156]

Electroless Ni 68 at.%–Co 27 at.%–P 5.7 at.% ‐coated SiC powder

7 GHz

28

[156]

Electroless Ni 86 at.%–Co 7.1 at.%–P 6.7 at.% ‐coated SiC powder

8 GHz

24

[156]

c­ ontribute design flexibility, good formability, mechanical properties, coherent metal deposi­ tion, excellent ­conductivity EM discharge, EMI protection and RF interference protection, light weight, and thermal expansion matching [147–150]. The perception and enhancement of the EMI SE is defined in the terms of adhesion between metal and polymer at the inter­ faces. Note that the coating adhesion and quality usually depend on various parameters such as coating rate, ­surface morphologies, ­surface resistance, and pH of the solution (especially in the case of electroless plating). However, the surface activity and surface adhesion of the poly­ mer ­substrate with the metals is quite low, so mechanical treatment, chemical modification (­functionalization), plasma surface treatment, etc. are used to improve the surface adhesion between the polymer and coating without altering the polymer bulk properties. It is a fact that  homogeneous metal deposition, excellent c­ onductivity, good SE, and applicability to complex‐shaped materials can be achieved by this technique. For a controlled deposition of metal on the polymer substrate, plasma treatment is a preferable method for generation of the required hydrophilic groups on the polymer substrate surface, which lead to the formation of controlled disposition. Thus, by using the low‐temperature reactive gas plasma treatment it is possible to enhance the surface free energy and reactivity without changing the bulk proper­ ties of the polymer substrate [151–155]. To do this, oxygen, argon, and ammonia gases, etc. are used to treat the polymer substrate with a low‐­temperature reactive gas plasma. The EMI shielding behavior of metal treated polymer substrates were analyzed in several studies. Hence, the effects of metal coating on the electromagnetic absorption and EMI‐SE are given in Tables 15.13 and 15.14.

347

348

15  Electromagnetic Interference Shielding Materials for Aerospace Application

Table 15.14  Electromagnetic shielding effectiveness of various metal and alloy coated fabrics.

Materials

Frequency

Shielding effectiveness (dB)

Electroless copper coated PET fabrics by using hypophosphite as reducing agent (weight of copper coating: 20 g m−2) solution

100 MHz–20 GHz

∼38

[157]

Electroless copper coated PET fabrics by using hypophosphite as reducing agent (weight of copper coating: 20 g m−2)

100 MHz–20 GHz

65

[157]

Electroless copper coated PET fabrics by using hypophosphite as reducing agent (weight of copper coating: 20 g m−2)

100 MHz–20 GHz

85

[157]

Electroless copper plating on polyester fabric in solution of SnCl2 + HCl + AgNO3 + NaOH + NH4OH +  HCHO + CuSO4·5H2O + NaKC4H4O6·4H2O

2–18 GHz

40

[158]

References

15.7.3.2  Conducting Polymer Based Materials for EMI Shielding

Before turning to EMI shielding results in various conducting polymer, describing the importance of the conducting polymer and in particular their shielding related characteris­ tics, we will discuss the effect of electromagnetic fields on humans and devices. First, note that the pervasive presence of radio frequency (RF) and microwave in devices provides a lot of advantages and generates many problems; the problems arise from the interference and health hazards caused by radio frequencies and microwaves [159]. EM shields are needed only to isolate a space from outside electromagnetic waves. This because unwanted EM waves disturb the operational performance of a room, an apparatus, a circuit, devices, etc. Therefore, architectural EM wave shielding has been used to prevent ESD and EM wave  interference [160]. Formally, most polymers are not able to provide a barrier for ­electromagnetic radiation and ESD. Here we introduce conductive polymer coating and composites to describe the charac­ teristics that make them electrically conductive and able to withstand harsh environmental ­conditions [161]. The metallic property of molecularly doped polyacetylene was first pro­ posed in 1977. Interest in this conducting polymer is mainly due to the diverse properties such as reflection and absorption of electromagnetic radiation in the microwave frequency range of 30 MHz to 30 GHz, dissipation of electrostatic charge, and various technological applications such as active electrode materials in energy storage, optoelectronic devices, and display devices [162–164]. Presently, conducting polymers are employed in textiles to produce EM shielding cloths to protect humans from EM waves. These polymers are also an alternative to metals, where metal was once used to obtain suitable shielding nowadays these conductive polymers are being utilized. In addition, such polymers can show shield­ ing due to both absorption and reflection, whereas metal based shielding is due only to the reflection mechanism. Consequently, most textile fabrics have been fabricated from intrin­ sically conducting polymers (ICPs), including polypyrrole (PPy) and polyaniline coated fabrics with natural or synthetic fibers [165, 166]. From the previous discussion, it should be clear that EMI shielding by electromagnetic wave absorption is better than by reflec­ tion. Note that the metals or materials coated with metals possess excellent EMI shielding

15.7  Types of Shielding Materials for Aerospace

values. In the case of a metal, the metal skin depth is small compared to that of conductive polymers, and electromagnetic radiation at high f­requencies can penetrate the surface region of a metal conductor. Therefore, most of the electromagnetic effectiveness is obtained through surface reflection. Conducting polymers offer shielding by absorption and reflection by increasing the skin depth (penetration depth); this is mainly due to the presence of excellent electrical conductivities and the value of dielectric constants within conducting polymers. However, reflection loss is suppressed with increasing frequency, and absorption loss increases with the shield thickness and increasing frequency. The elec­ trical properties of electrically conducting polymers vary from an insulator to a metallic region; in fact the full range of semiconductors and metallic behavior of a conducting poly­ mer are related to л‐bands of delocalized molecular orbital and the degree of band occupy­ ing. The delocalized л‐bonds enable the polymer to be conductive. In conclusion, conducting polymers are novel polymers in terms of EMI shielding. Table 15.15 shows the EMI shielding behavior of various conducting polymer coated fabrics. The grafting of con­ ducting polymer onto fabric substrate and coating thickness are the important aspects for achieving suitable electromagnetic shielding [174–176]. Various methods such as polym­ erization of conducting polymer on insulating surfaces, electrochemical treatment, and chemical or electrochemical oxidation are used to graft the conducting polymer on the fabrics substrate [177, 178]. From the various previous studies, sulfosalicylic acid, benzene sulfonic acid, and p‐toluene sulfonic acid, nine aromatic sulfonic acids, are used to provide the grafting of conducting polymer on fabrics substrate [171]. 15.7.3.3  Carbonanotube Based Composites for EMI Shielding

As already discussed in many articles, various conducting filler‐based composites can be used as reliable EMI shielding materials for distinct applications. Electrical conductivity, dielectric characteristic, distribution, and dispersion of various fillers in the matrix are the key points for producing excellent shielding composites. The conducting filler based EMI shielding compos­ ites have shown better performance than other substitute materials. Carbon nanotubes (CNTs), graphene, dielectric (BaTiO3, TiO2, etc.), or magnetic (γ‐Fe2O3, Fe3O4, BaFe12O19, etc.) based composites possess great design flexibility, excellent compatibility, and requisite electro­ magnetic shielding properties with different polymer matrices (e.g. thermoplastic, rubber, and thermosetting). These composites are used to attenuate the EM waves by electric/magnetic dipoles and free electrons within the fillers. With increasing concentration of these fillers, EMI shielding can be enhanced. Overall EMI shielding in CPCs is based on the above‐mentioned reflection (primary), absorption (secondary), and multiple‐reflection mechanisms. Shielding by refection and absorption are the primary and secondary mechanisms; multiple‐reflection cannot be estimated alone as it depends on both the primary and secondary mechanisms. Note that the electrical conductivity makes materials more suitable for electrical and electromag­ netic shielding applications [166, 179]. In metals, high electrical conductivity (free charge car­ rier) takes a part in reflecting the EM wave; however, high reflection, corrosive nature, and heavy weight restricted their use. Consequently, more attention has been gained by polymer nanocomposites; nanocomposites contain nanofillers and polymer matrix, nanofillers possess a high surface area, novel electrical, thermal, dielectric, magnetic, and/or mechanical proper­ ties [180, 181]. Currently, a number of organic and inorganic nanofillers – such as carbon black, graphite, CNTs, and graphene and dielectric (TiO2, ZnO, SiO2, BaTiO3) or magnetic (ferrites) materials – are being utilized to enhance the SE of polymer composite systems [182–187]. This section will focus on the recent progress in polymer nanocomposites based on carbon ­nanotubes in the field of EMI shielding applications.

349

350

15  Electromagnetic Interference Shielding Materials for Aerospace Application

Table 15.15  Electromagnetic shielding effectiveness of various conducting polymer based fabrics. Materials

Frequency

Shielding effectiveness (dB)

References

PPy/PET woven fabric

1.5 GHz

13.8 (Volume resistivity 2.85 Ohm‐cm)

[167]

PPy/PET woven fabric

0.1 GHz

12.8 (Volume resistivity 2.85 Ohm‐cm)

[167]

PPy/PET woven fabric

1.5 GHz

20.5 (Volume resistivity 2.0 Ohm‐cm)

[167]

PPy/PET woven fabric

0.1 GHz

19.1 (Volume resistivity 2.0 Ohm‐cm)

[167]

PPy/PET woven fabric

1.5 GHz

27.8 (Volume resistivity 0.75 Ohm‐cm)

[167]

PPy/PET woven fabric

0.1 GHz

25.7 (Volume resistivity 0.75 Ohm‐cm)

[167]

PPy/PET woven fabric

1.5 GHz

36.6 (Volume resistivity 0.20 Ohm‐cm)

[167]

PPy/PET woven fabric

0.1 GHz

36.1 (Volume resistivity 0.20 Ohm‐cm)

[167]

Metalized coated fabric

800 MHz

67 (Reflection loss = 61.02 dB and Absorption loss = 2.47 dB), (4% absorption)

[168]

Polypyrrole (PPy) coated fabric

800 MHz

22.41 (Reflection loss = 16.40 dB and Absorption loss = 2.48 dB), (15% absorption)

[168]

Thermoplastic acrylonitrile‐butadiene–styrene (98% ABS) + Polyaniline (2% PANI) composites

101 GHz

5.91

[168]

Thermoplastic acrylonitrile‐butadiene–styrene (60% ABS) + Polyaniline (40% PANI) Composites

101 GHz

45.61

[168]

Thermoplastic acrylonitrile‐butadiene–styrene (50% ABS) + Polyaniline (50% PANI) Composites

101 GHz

>60

[168]

polyaniline‐coated fabrics

100–1000 MHz

30–40 (98% Absorption)

[169]

15.7  Types of Shielding Materials for Aerospace

Table 15.15  (Continued) Materials

Frequency

Shielding effectiveness (dB)

References

Polyaniline‐Nylon fabrics

0.05–5 MHz

37–11

[170]

Polyaniline (PANI)‐Nylon fabrics

10 MHz–1 GHz

7–1

[170]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by using aromatic sulfonic acids (dopant)

0.01 MHz

49

[171]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by using aromatic sulfonic acids (dopant)

1000 MHz

7

[171]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by using p‐toluene sulfonic acid (dopant)

1 MHz

17 (0.15 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by using p‐toluene sulfonic acid (dopant)

  1 MHz

47 (3 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by camphor‐10‐sulfonic acid (dopant)

1 MHz

15 (0.15 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by camphor‐10‐sulfonic acid (dopant)

1 MHz

27 (3 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by using p‐toluene sulfonic acid (dopant)

1 GHz

21 (0.15 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by using p‐toluene sulfonic acid (dopant)

  1 GHz

54 (3 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by camphor‐10‐sulfonic acid (dopant)

1 GHz

10 (0.15 mm thickness)

[172, 173]

PAn‐grafted E‐glass fabrics E‐Glass Fabric by camphor‐10‐sulfonic acid (dopant)

1 GHz

31 (3 mm thickness)

[172, 173]

Figure 15.9  Schematic representation of single‐walled carbon nanotubes and multiwalled carbon nanotubes.

In 1991, Iijima published the results of his attempts to fabricate carbon nanotubes (CNTs) as an allotrope of carbon [182]. The important fact was that the CNT structure appeared to be precisely graphene sheets folded up in the pattern of single or multiple concentric cylinders (Figure 15.9). Here we introduce the properties and characteristics of carbon nanotubes in order to describe the advantage of carbon nanotubes based polymer composites. From the previous reports, it could be clear that carbon nanotubes can show ballistic conductivity, ­ultimate high strength, excellent thermal properties, high aspect ratio, a 3D conducting ­network (which produces tunneling of electrons in a polymer matrix; however, it depends on

351

352

15  Electromagnetic Interference Shielding Materials for Aerospace Application

the morphology and kind of CNT), and low percolation in any kind of polymer matrix. However, the percolation threshold is not only related to the CNTs but is also related to the nature of the matrix. Based on the particular characteristics of the CNTs we can select carbon nanotubes as suitable fillers for each matrix and electromagnetic shielding application. For example, the percolation concentration of conventional fillers such as carbon‐black metal ­particles is usually higher than for CNTs polymer composites; a low CNT concentration is sufficient to achieve the desired mechanical, electrical, thermal and EMI shielding properties when using CNT based polymer composites (polymer nanocomposites). If, though, a greater quantity of CNTs (above a certain level) is embedded within a polymer matrix, the final prop­ erties of composites could be suppressed via CNT agglomeration and cluster formation in the matrix. The dispersion and agglomeration nature of CNTs in the matrix is one of the major drawbacks with CNT composites. The agglomeration is caused by the entanglement nature of CNTs, inherent electrical charge, van der Waals force of attraction between carbon nano­ tubes, and the challenge of CNT’ dispersion is raised from the ultrahigh surface area of CNTs [188]. However, the recent progress reported by scientists shows that CNT polymer compos­ ites can be used as shielding materials in a wide range of areas including aerospace, military, automobile, electronics, etc. (Table 15.16). For CNT polymer composites it has been shown by experiment that the SE can be obtained through absorption and reflection mechanisms and also the multiple reflection mechanism. However, the SE is decreased by the multiple reflections between the internal and external surface of multi‐walled carbon nanotubes MWCNTs. The multiple‐reflection mechanism’s SE can be tuned by the shield thickness. Hence, the SE of CNT based polymer nanocomposites is related to various factors such as CNT concentration, CNT orientation, aspect ratio of CNTs, processing condition, disper­ sion, and distribution. The distribution of CNTs may be controlled by surface fictionalization, but some cases the surface function ­disturbs the π–π conjugation, which reduces the conduc­ tivity [191, 193, 201–210]. 15.7.3.4  Graphene Based Composites for EMI Shielding

The modern application of EMI shielding polymer nanocomposites explains the advantage of a single/multilayered 2D graphene sheets. It is now understood that the exfoliation of graph­ ite along the c‐axis is described as a graphene sheet; the exfoliation can be conducted through mechanical, chemical, or thermal treatment. A graphene sheet has sp2 bonded carbon atoms, honeycomb crystal lattice, higher surface area, excellent ballistic conductivity, thermal prop­ erties, and is less expensive than CNTs. Along with the skin effect, a significant surface area is preferable for shielding; therefore, graphene is also an important material for EMI shielding due to its high conductivity and specific surface area. With various forms of graphene, we are now position to manipulate the morphologies and electrical, mechanical, and thermal prop­ erties of polymer composites for versatile applications [211, 212]. Recently, graphene‐based composites have been employed for various purposes, including mechanical, thermal, gas barrier, electrical, and flame retardant. However, the physical properties of graphene‐polymer composites are a function of graphene dispersion, distribution, concentration, interfacial adhesion between graphene, and polymer matrix. Sometimes, the edge of the graphene sheet is grafted with a suitable group to make it more compatible with an organic polymer. However, such functionalization may decrease the conductivity of a graphene sheet, and so in most cases an amine is used as the functional group. Researchers have pointed out that the combi­ nation of graphene sheet and metal nanoparticle based composites can be used as a good EM wave absorber [213]. Specifically, graphene‐based composites can also be employed as an alternative to CNTs in the radio frequency range. In addition, the orientation and surface functionalization of graphene can enhance the EMI SE of a polymer matrix (Table 15.17).

15.7  Types of Shielding Materials for Aerospace

Table 15.16  Electromagnetic shielding effectiveness of various carbon nanotube polymer nanocomposites. Materials

Frequency

Shielding effectiveness

References

Carbon nanotube (CNT) macro‐films

8.0–12.6 GHz

61–67 (0.004 thickness) (reflection1 and absorption2)

[189]

MWCNT‐fused silica composites

8–12 GHz

30–33 (2 mm thickness) (absorption1 and reflection2)

[190]

PU/SWCNT composites

8.2–12.4

16–19 (2 mm thickness)

[191]

Carbon black (7.5 vol.% CB)/PP composite

8.2–12.4 GHz

–18 (1.0 mm thickness) (absorption1 and reflection2)

[192]

PP/MWCNT (7.5 vol.%) composite

8.2–12.4 GHz

−35 dB (1.0 mm thickness) (absorption1 and reflection2)

[192]

Polystyrene (PS) + 0.5 wt% MWNTs foam composite

8.2–12.4 GHz

2.84 (reflection)

[193]

Polystyrene (PS) + 0.5 wt% carbon nanofiber foam composite

8.2–12.4 GHz

0.41 (aspect ratio and conductivity of CNF