EMC-COMPATIBLE SHIELDING magnetic materials for shielding - practical examples - device design. 9783658331894, 3658331895

Table of contents :
Preface to the Third Edition
Preface to the Second Edition
Preface to the First Edition
Symbols and Abbreviations
Contents
Part I: Basics
Chapter 1: Introduction
1.1 EMC Act Standardization
Chapter 2: Volume Materials
2.1 Introduction
2.2 Microscopic and Macroscopic Properties of Spinel Ferrites
2.3 Models of Classical Field Theory (Maxwell) Compared to Landau-Lifschitz Theory
2.3.1 Classical Field Theory According to Maxwell
2.4 Considerations on RF Losses in Ferrite Compounds and Ferrite Layers
2.4.1 Ferromagnetic Resonance (FMR) Inversion Ratio in Ferrite Volume Materials
2.4.2 Discussion of Errors
2.5 Dielectric Measurements on Magnetic Materials
2.5.1 Selection of the Measuring Method
2.5.2 Resonator Measurement Method
2.5.3 Description of the Measuring Station
2.5.4 Performing the Measurement
2.6 Relaxation in Ferrite Volume Materials
2.6.1 Spinels Also Have Relaxation Mechanisms
2.6.2 Experimental Considerations
2.7 Texture in Ferrite Volume Materials
2.7.1 Creation of a Texture
2.7.2 Theoretical Considerations on the Relationship Between Texture and RF Loss
2.8 Degree of Filling of Ferrite in Volume Material
2.9 Field Adjustment of the Volume Material
2.9.1 Schelkunov Formula
2.9.2 Error Consideration of the Measuring Arrangement
2.9.3 Summary
2.10 The NiZn-ferrite Spinel System
2.10.1 Material Analyses of Ferrite Compounds
2.10.2 Grain Size Distribution, Change of Grain Size
2.10.3 Modelling Objective of the Layer Synthesis
2.10.4 Type of Spin Wave Modes as a Function of Layer Thickness
2.10.5 Summary
Chapter 3: Nanomaterials
3.1 Layer Analysis, Anisotropy Constant and Grain Size of Ferrimagnetic Layers
3.2 Spin Wave Losses in Ferrite Layers
3.2.1 Spin Wave Excitation
3.3 Influence of the Anisotropy Constants on the RF Loss of the NiZn Ferrite Layer
3.3.1 Theoretical Considerations on Spin Wave Loss
3.3.2 Experimental Considerations on Spin Wave Loss
3.4 Special layer Analysis, Anisotropy Constant and Grain Size of Ferrimagnetic Layers
3.4.1 Absorption Loss of the Ferritic Layer
3.5 Eddy Current Effects in Metallic Magnetic Films: Snoek´s Limit for layer Systems/Single Layer
3.5.1 Schelkunoff Equation
3.6 Maximum Frequency Attenuation Tests, RF Material Evaluation
3.7 Relaxation Effects of Magnetic Materials in the kHz Range
3.7.1 Bloch-Wall Relaxations
3.7.2 Bloch-Wall Eddy Current Relaxations: kHz Effects
3.8 LF Losses
3.8.1 Material Analysis
3.9 Deposition of Ultra-Thin Hematite Layers
3.10 Magnetic Spectroscopic Analysis
3.10.1 Analysis of the Sample Layers
3.10.2 Spin Wave Spectrum Characterisation Definitions
3.11 Waveguide Measuring Station
3.11.1 Results
3.12 X-Ray diffraction Analysis
3.13 RF Analysis by 20,000 MHz
3.14 Ratio of the Granule Size to the Layer Thickness of an Fe-Nanolayer
3.15 Multilayer Systems
3.16 Kittel Frequency
3.17 Wolmann Frequency
3.18 Snoek Frequency
3.19 Radar Effects
3.20 Magnetic Nanoparticles
3.20.1 Theoretical Considerations
3.20.2 Experimental Considerations
Part II: Practical Examples
Chapter 4: Shielding Using Nanomaterials
4.1 Measurement of the Complex Permeability of Nanolayers with One Permeameter
4.1.1 Permeability Measurements
Chapter 5: LF Shielding
Chapter 6: Double Shielding
6.1 Double Shielding
Chapter 7: Polymer Housings
7.1 Previous Material Results
7.2 Housing Results
7.3 Summary
Chapter 8: Shielding Example: Inner Lining of a 2.4 GHz Low-Noise Amplifier Housing to Suppress Higher Modes
Chapter 9: Metal Housing with Magnetic Materials
9.1 Attenuation of Cavity Resonances by Means of Absorbing Magnetic Laminates
9.2 Cavity Resonances
9.3 Coated Housing
9.4 Absorbent Material as Insert
9.5 Ferrite-Containing Thick Films for New EMC Metal Housings
9.5.1 Slurry Layers
9.5.2 Slurry Layers with Conductive Coating
9.5.3 Layers of Ferrite Filler
9.6 Ferrite Volume Housing
9.6.1 Ferrite Volume Housings for New EMC-Resistant Automotive Sensor Housings
9.6.2 Principle of Comparative Shielding Effectiveness Measurement
9.7 Results of the Shielding Effectiveness Measurements
Chapter 10: PCB Shielding
10.1 Technical Setup of the Test Structures/New EMC PCBs
10.2 Electromagnetic Interference Emission (EMC) with Old and New PCB
10.3 Evaluation
10.4 Summary
10.4.1 Experimental Results and Applications
Chapter 11: Shielding Effectiveness on Layers for Cables
11.1 Measurement with Stripline
11.2 Application: Ribbon Cable
11.3 Summary
Chapter 12: Textile Shielding Material
12.1 Summary/Outlook
Chapter 13: Shielding Effectiveness of a Wire Mesh
Part III: Novel Future Ferrites: Hexagonal Volume Materials
Chapter 14: Basic Problem of Today´s EMC Ferrite Interference Suppression Materials
14.1 Introduction
14.2 Theoretical Considerations
14.3 Experimental Investigation
14.4 Summary: Novel Hexaferrites of the Future
Chapter 15: Appendix: Shielding Formulas
15.1 Basic Law of Electromagnetic Shielding According to Schelkunov
15.2 Shielding against Magnetostatic Fields
15.3 Shielding Against Electrostatic Fields
15.4 Shielding Against Quasi-Static Magnetic Fields
15.5 Shielding Against Alternating Magnetic Fields (Skin Effect)
15.6 Extended Shielding Law According to Schwab
15.7 Absorption Loss
15.8 Multiple Reflection Attenuation
15.9 Shielding Effectiveness as a Function of Surface Conductivity
15.10 Extended Shielding Effectiveness Law According to Perumalraj and Dasaradan [58] Considering Real Apertures of Wire Const...
15.11 Shielding of Holes and Apertures
15.12 Near-Field Reflection Loss R on a Plane Plate
15.13 Law of Shielding Effectiveness Considering the Waveguide Effect and Apertures According to Tee Tang [61], Near Fields, F...
15.14 Cut-off Frequency of Length and Depth of Rectangular Structures (Waveguide Like)
15.15 Corner Effect
15.16 Transient Shielding Effectiveness
15.17 Shielding Rules
References
Further Reading
Index

Citation preview

Frank Gräbner

EMC-Compatible Shielding Magnetic Materials for Shielding - Practical Examples - Device Design

EMC-Compatible Shielding

Frank Gräbner

EMC-Compatible Shielding Magnetic Materials for Shielding - Practical Examples - Device Design

Frank Gräbner Nordhausen, Germany

ISBN 978-3-658-33188-7 ISBN 978-3-658-33189-4 https://doi.org/10.1007/978-3-658-33189-4

(eBook)

This book is a translation of the original German edition „EMV-gerechte Schirmung“ by Gräbner, Frank, published by Springer Fachmedien Wiesbaden GmbH in 2016. The translation was done with the help of artificial intelligence (machine translation by the service DeepL.com). A subsequent human revision was done primarily in terms of content, so that the book will read stylistically differently from a conventional translation. Springer Nature works continuously to further the development of tools for the production of books and on the related technologies to support the authors. # Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Fachmedien Wiesbaden GmbH part of Springer Nature. The registered company address is: Abraham-Lincoln-Str. 46, 65189 Wiesbaden, Germany

Preface to the Third Edition

The current developments in the field of EMC and shielding using magnetic materials are the main reason for the third edition of this book. On the one hand, the EMC standards world is in a state of upheaval. This means that more and more standards take into account the higher source frequencies and push the upper frequency limit above 3 GHz. In contrast to these technical conditions and the existing ferrite interference suppression materials (which do not correspond to these new frequency ranges), there are hardly any new EMC magnetic materials available on the market. New EMC ferrites of hexagonal crystal systems take these new requirements into account. These hexagonal materials are presented in the chapter “EMC future ferrites—hexagonal volume materials.” Further shielding rules are also listed in the appendix of the book. As in the first and second editions of the book, the editorial office Technik of Springer Vieweg Verlag—represented by Mrs. Broßler and Mr. Dapper—and the design office Fromm—represented by Mrs. Fromm—have strongly influenced the development of this book and supported the author very much in its elaboration. We would like to express our sincere thanks for this. Nordhausen, Germany October 2015

Frank Gräbner

v

Preface to the Second Edition

The motivation for the second edition is the increasing demand of experts for shielding with the use of magnetic materials. The reason for this is the updates of the generic standards for interference emission and radio interference field strength in industrial/residential areas. The upper frequency limit of 1000 MHz no longer applies. Depending on the internal frequencies of the interferers, an emission frequency limit of more than 6000 MHz can be assumed. This regulation announces a need for new shielding concepts, which are presented in this book by means of practical examples. Moreover, a new chapter “Shielding formulas” in the appendix adds an important contribution to the book. This collection of formulas is intended to introduce the “quick reader” to the basics of the various shielding effects in a condensed form. The author would especially like to thank the media design office Fromm from Selters/ Taunus for helping with the book. Nordhausen, Germany October 2012

Frank Gräbner

vii

Preface to the First Edition

The main aim of this book is to show how newly developed materials can improve shielding. The engineer of the next decades will be given the opportunity to use the developed method of a novel EMC shielding philosophy by using absorber materials, consisting of volume materials or nanomaterials, to facilitate the EMC work by simple shielding rules. The physical principles of these materials are not new, but have been discussed from a different angle. In times of very fast introduction of new technologies and seemingly unlimited technical possibilities, the developer of devices and systems is under pressure to understand the complex EMC coupling paths and to suppress a device very quickly. This book is written for these “long-suffering” experts and it should make your work a little easier. Many of the tasks and solutions have been developed by evaluating practical experiments and research projects. This is due to many research groups, such as the Institute IMG Nordhausen, the Competence Centre BRUNEL IMG GmbH, and Hörmann IMG GmbH (Mr. Hungsberg, Mr. Kallmeyer, Mr. Hildenbrandt, and Mr. Hesse) in Nordhausen, the Ilmenau University of Technology (Mr. Prof. Dr. Dr. Knedlik, Dr. Teichert from the Department of Materials in Electrical Engineering), the University of Telecommunications Leipzig, the former HITK Hermsdorf (Ms. Pawlowski), and the colleagues from the TITK Rudolstadt (Mr. Pflug and Mr. Dr. Schrödner). Nordhausen, Germany March 2011

Frank Gräbner

ix

Symbols and Abbreviations

Latin Letters ! A, A A A a ! B, B c ! D D di d dsi d ! E E ! ! ! e x, e y, e z f ! H, H ΔH ! H z0 ! H0 ! Ha I ! J ! k Ku m ! M ! M0 M0

Vector, RMS value Especially surface Tensor Grating constant Magnetic flux density, effective value Speed of Light Electric flux density Particle size, crystallite Thickness of the material Penetration depth Layer thickness Network level distance Electric field strength Energy Unit vectors Frequency Magnetic field strength, effective value Half width of the FMR Average magnetic field strength component in the z-direction Static pre-magnetizing field strength Anisotropic field strength Electric current Current density Wave number,k ¼ k 0  jk 00 Anisotropy constant, total Ground Vector of magnetization Saturation magnetization Magnetic constant xi

xii

S ! S S11, S12, S22, S21 T t U U V x Z Z Z0

Symbols and Abbreviations

Surface Spin vector Complex scattering matrix elements Temperature Time Real function Scalar (any) Volume Degree of inversion Sheet resistance Impedance Field characteristic impedance of air

Greek Letters α Damping constant of relaxation β Phase constant γ Damping constant electromagnetic field γ0 Gyrotropy constant γ0 Inversion constant γa Propagation constant Δ General difference ε Permittivity ε0 Dielectric constant εr Relative permittivity μ Permeability μr Relative permeability κ Electrical conductance λ Wavelength μB Drill Magneton ρ Surface charge density τ Relaxation time constant χ Magnetic susceptibility ω0 Natural frequency of the recession movement of the magnetization vector ωm Natural frequency of the saturation magnetization vector Abbreviations AC Alternating current CISPR International Committee for Radio Electronics CRAM Currentless Radiation Absorption Material DC Direct current EMC English abbreviation for EMV EMI Electromagnetic emission

Symbols and Abbreviations

EMS EMC EN ESD FFT FMR HCP RF IEC MOM PFC RAM TLM UMTS VDE

Electromagnetic immunity Electromagnetic compatibility European Norm Electrostatic discharge Fast Fourier Transformation Ferromagnetic resonance Horizontal coupling plate Radio frequency International standard Moment matrix method Power factor correction Radiation Absorption Material Transmission line matrix method Universal Mobile Telecommunications System Association of German Electrical Engineers

xiii

Contents

Part I

Basics

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 EMC Act Standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 4

2

Volume Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Microscopic and Macroscopic Properties of Spinel Ferrites . . . . . . . 2.3 Models of Classical Field Theory (Maxwell) Compared to Landau–Lifschitz Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Classical Field Theory According to Maxwell . . . . . . . . . . 2.4 Considerations on RF Losses in Ferrite Compounds and Ferrite Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Ferromagnetic Resonance (FMR) Inversion Ratio in Ferrite Volume Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Discussion of Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Dielectric Measurements on Magnetic Materials . . . . . . . . . . . . . . . 2.5.1 Selection of the Measuring Method . . . . . . . . . . . . . . . . . 2.5.2 Resonator Measurement Method . . . . . . . . . . . . . . . . . . . 2.5.3 Description of the Measuring Station . . . . . . . . . . . . . . . . 2.5.4 Performing the Measurement . . . . . . . . . . . . . . . . . . . . . . 2.6 Relaxation in Ferrite Volume Materials . . . . . . . . . . . . . . . . . . . . . 2.6.1 Spinels Also Have Relaxation Mechanisms . . . . . . . . . . . . 2.6.2 Experimental Considerations . . . . . . . . . . . . . . . . . . . . . . 2.7 Texture in Ferrite Volume Materials . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Creation of a Texture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.2 Theoretical Considerations on the Relationship Between Texture and RF Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Degree of Filling of Ferrite in Volume Material . . . . . . . . . . . . . . . 2.9 Field Adjustment of the Volume Material . . . . . . . . . . . . . . . . . . .

5 5 8 14 14 16 16 21 24 24 24 24 26 27 27 29 30 32 33 33 35

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Contents

2.9.1 Schelkunov Formula . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.2 Error Consideration of the Measuring Arrangement . . . . . 2.9.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The NiZn-ferrite Spinel System . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.1 Material Analyses of Ferrite Compounds . . . . . . . . . . . . 2.10.2 Grain Size Distribution, Change of Grain Size . . . . . . . . . 2.10.3 Modelling Objective of the Layer Synthesis . . . . . . . . . . 2.10.4 Type of Spin Wave Modes as a Function of Layer Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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37 38 39 40 42 42 44

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44 47

Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Layer Analysis, Anisotropy Constant and Grain Size of Ferrimagnetic Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Spin Wave Losses in Ferrite Layers . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Spin Wave Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Influence of the Anisotropy Constants on the RF Loss of the NiZn Ferrite Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Theoretical Considerations on Spin Wave Loss . . . . . . . . 3.3.2 Experimental Considerations on Spin Wave Loss . . . . . . 3.4 Special layer Analysis, Anisotropy Constant and Grain Size of Ferrimagnetic Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Absorption Loss of the Ferritic Layer . . . . . . . . . . . . . . . 3.5 Eddy Current Effects in Metallic Magnetic Films: Snoek’s Limit for layer Systems/Single Layer . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Schelkunoff Equation . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Maximum Frequency Attenuation Tests, RF Material Evaluation . 3.7 Relaxation Effects of Magnetic Materials in the kHz Range . . . . . 3.7.1 Bloch-Wall Relaxations . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2 Bloch-Wall Eddy Current Relaxations: kHz Effects . . . . . 3.8 LF Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8.1 Material Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Deposition of Ultra-Thin Hematite Layers . . . . . . . . . . . . . . . . . . 3.10 Magnetic Spectroscopic Analysis . . . . . . . . . . . . . . . . . . . . . . . . 3.10.1 Analysis of the Sample Layers . . . . . . . . . . . . . . . . . . . . 3.10.2 Spin Wave Spectrum Characterisation Definitions . . . . . . 3.11 Waveguide Measuring Station . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12 X-Ray diffraction Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13 RF Analysis by 20,000 MHz . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Ratio of the Granule Size to the Layer Thickness of an Fe-Nanolayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

49

. . .

49 50 54

. . .

55 55 58

. .

58 58

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60 64 65 68 68 68 70 70 71 72 73 75 75 77 77 78

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79

2.10

3

Contents

3.15 3.16 3.17 3.18 3.19 3.20

xvii

. . . . . . . .

84 84 86 87 88 89 90 93

Shielding Using Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Measurement of the Complex Permeability of Nanolayers with One Permeameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Permeability Measurements . . . . . . . . . . . . . . . . . . . . . . .

97 97 100

5

LF Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101

6

Double Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Double Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 106

7

Polymer Housings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Previous Material Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Housing Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

107 110 110 111

8

Shielding Example: Inner Lining of a 2.4 GHz Low-Noise Amplifier Housing to Suppress Higher Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . .

115

Part II 4

9

Multilayer Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kittel Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wolmann Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Snoek Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radar Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.20.1 Theoretical Considerations . . . . . . . . . . . . . . . . . . . . . . . 3.20.2 Experimental Considerations . . . . . . . . . . . . . . . . . . . . . Practical Examples

Metal Housing with Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . 9.1 Attenuation of Cavity Resonances by Means of Absorbing Magnetic Laminates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Cavity Resonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Coated Housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Absorbent Material as Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Ferrite-Containing Thick Films for New EMC Metal Housings . . . 9.5.1 Slurry Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5.2 Slurry Layers with Conductive Coating . . . . . . . . . . . . . 9.5.3 Layers of Ferrite Filler . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Ferrite Volume Housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6.1 Ferrite Volume Housings for New EMC-Resistant Automotive Sensor Housings . . . . . . . . . . . . . . . . . . . . . 9.6.2 Principle of Comparative Shielding Effectiveness Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7 Results of the Shielding Effectiveness Measurements . . . . . . . . . .

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119

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124 124 125 125 128 129 130 130 131

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132

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137 137

xviii

Contents

PCB Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Technical Setup of the Test Structures/New EMC PCBs . . . . . . . . . 10.2 Electromagnetic Interference Emission (EMC) with Old and New PCB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1 Experimental Results and Applications . . . . . . . . . . . . . . .

149 149

11

Shielding Effectiveness on Layers for Cables . . . . . . . . . . . . . . . . . . . . . 11.1 Measurement with Stripline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Application: Ribbon Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

165 165 167 167

12

Textile Shielding Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Summary/Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

171 171

13

Shielding Effectiveness of a Wire Mesh . . . . . . . . . . . . . . . . . . . . . . . . .

175

10

Part III 14

15

149 157 157 163

Novel Future Ferrites: Hexagonal Volume Materials

Basic Problem of Today’s EMC Ferrite Interference Suppression Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Theoretical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3 Experimental Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4 Summary: Novel Hexaferrites of the Future . . . . . . . . . . . . . . . . .

. . . . .

Appendix: Shielding Formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1 Basic Law of Electromagnetic Shielding According to Schelkunov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Shielding against Magnetostatic Fields . . . . . . . . . . . . . . . . . . . . . 15.3 Shielding Against Electrostatic Fields . . . . . . . . . . . . . . . . . . . . . . 15.4 Shielding Against Quasi-Static Magnetic Fields . . . . . . . . . . . . . . . 15.5 Shielding Against Alternating Magnetic Fields (Skin Effect) . . . . . . 15.6 Extended Shielding Law According to Schwab . . . . . . . . . . . . . . . 15.7 Absorption Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.8 Multiple Reflection Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . 15.9 Shielding Effectiveness as a Function of Surface Conductivity . . . . 15.10 Extended Shielding Effectiveness Law According to Perumalraj and Dasaradan [58] Considering Real Apertures of Wire Constructions of Real Shields with Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.11 Shielding of Holes and Apertures . . . . . . . . . . . . . . . . . . . . . . . . . 15.12 Near-Field Reflection Loss R on a Plane Plate . . . . . . . . . . . . . . . .

179 180 181 184 186 187 188 189 189 189 190 191 191 192 192

192 193 194

Contents

15.13 15.14 15.15 15.16 15.17

xix

Law of Shielding Effectiveness Considering the Waveguide Effect and Apertures According to Tee Tang [61], Near Fields, Far Fields . . . . Cut-off Frequency of Length and Depth of Rectangular Structures (Waveguide Like) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corner Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transient Shielding Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . Shielding Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

195 197 198 198 198

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FurtherReading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

201 203

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

205

Part I Basics

1

Introduction

This book is aimed at engineers, scientists, students, researchers and practical professionals. Electromagnetic compatibility (EMC) has been developing since its beginnings in the 1950s to 1960s as a result of the pulse problems in automation/control engineering. EMC reached a great upswing and a high with the German EMC Act (EMCA) in 1996. Since then, it has been generally known to the industry that devices in an electromagnetic environment must operate without failure and interference-free. Developers in various branches of industry are intensively engaged in the interference suppression or EMC hardening of electrical devices/systems. The understanding of the couplings in an assembly/device and the resulting EMC phenomenon is demonstrated by simple shielding examples using RF materials. This book, in a condensed presentation of the basics of the materials and the solutions of the application of these special materials for shielding, should enable the experts to approach the problem of interference phenomena at a high scientific and technical level. It is intended to provide suggestions in which way materials can be used, from which the possibilities for interference suppression can be derived. Because there is no such thing as an “ideal shielding material as a solution for all problems”. Therefore, knowledge of the interaction of a special material with the EMC fields is important and is presented in the book in a highly topical way. The effect of the materials for shielding is explained to the reader by means of examples and is shown in very concentrated form in shielding rules. The expert should be able to deal with the EMC phenomena with the help of the examples presented and to provide solutions for special shielding by penetrating the effects himself. This book avoids too extensive theoretical explanations, but the most important basics are mentioned.

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4

1.1

1

Introduction

EMC Act Standardization

Modern devices for communication, navigation and data transmission, such as cell phones, GPS receivers, etc., operate in the frequency range under 6 GHz. However, the conducted signal transport within the devices is accompanied by the emission of an electromagnetic wave, so that the exposure to electromagnetic radiation is becoming increasingly important. The devices are therefore subject to the legal regulations for EMC. While the “Law on the Electromagnetic Compatibility of Devices (EMCA)” of 18 September 1998 was an important guideline for controlling the functionality of devices and was then revised several times until 2001, there is a need to comply with and control the guarantee of personal protection in electromagnetic fields (DIN VDE 0848 Part 2). The European standards defined here are EN 50081 (1 + 2) of 1992 and 1993 on EMC, generic standard for emitted interference, and EN 50082 (1 + 2) of 1997 and 1995 on EMC, generic standard for interference immunity (i.e. susceptibility to interference by EMC). As the number of electrical devices and in particular mobile telecommunication (cell phones) and navigation devices is constantly growing and these are subject to constant technical development, it is necessary to make them more reliable in operation. In order not to influence existing frequencies, this requires the use of new higher frequency ranges (currently cell phones use 0.9 and 1.8 GHz, Universal Mobile Telecommunication System (UMTS) is transmitted at 2.4 GHz). One aim must therefore be to achieve optimum shielding of devices that emit such radiation (except for the transmitting antenna, which radiates directionally). However, the use of these high frequencies poses new problems compared to lower frequency devices (100 μm. The ferrimagnetic layers, still to be discussed, are classified as CRC (currentless radiofrequency coating) materials. They form a special group as non-volume material. One difference between the RAM and CRAM is the mathematical description. The RAMs have a mathematically continuous space-time consideration. Current flows through the material, even if the conductivity is frequency dependent. The conductive layers are

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2 Volume Materials

Fig. 2.1 Diagram according to Mikhailowski for the classification of RF materials. (Source: [1])

difficult to observe, especially the nanoscale conductive layers. The graphite-containing foam absorbers (cones or laminates) are easier to observe. With this type of RAM, it is practically “only” necessary to consider conductivity. CRAM, also called spin materials, are more difficult to consider. Since a discrete mathematical space-time observation is required, it is no longer possible to use simple continuous models [1]. One has to consider the difficult conditions of the discrete lattice models of ferrite crystals, for example. An extensive material-physical consideration of the CRAM—as shown in Sect. 2.2—is therefore inevitable. The main subject of the present work is therefore the description of the interrelation between microscopic/macroscopic material properties—RF behaviour. Purely continuous observations as with graphite absorbers are not helpful. A word about the difference between ferrimagnetic volume materials and ferrimagnetic layers. The consideration of ferrimagnetic layers is a contribution to basic research. Many articles on ferrimagnetic layers with a layer thickness of 1000 MHz is discussed in the outlook (Sect. 2.4). In the theoretical modelling, the continuous model of Landau and Lifschitz (LL) for the description of the discrete conditions in the volume material was dealt with. In the layer modelling, the LL model with damping term was applied. The novel scientific approach of this work is the introduction of material parameters such as magnetic moment, grain size, anisotropy into the theoretical model and the subsequent analysis of the RF conditions in the material. This approach was also used for the extremely complex layer modelling. Important for the modelling of the materials is the structural composition of the ferrite materials themselves. Without knowledge of the material properties of the ferrites, the RF

2.1

Introduction

7

material cannot be developed. Simpler interrelationships without more in-depth material considerations, as in the case of graphite absorbers, cannot be used. The aim of the development of a safe passive electromagnetic protection system to achieve electromagnetic “immunity” is to increase the shielding effectiveness and ensure a high level of functional reliability of electronic systems. The problem in today’s metal enclosures is the greatly reduced shielding effectiveness of subracks from 500 MHz and the existing internal reflections and resonances of electromagnetic radiation when an internal electromagnetic source is present. If a sensitive component/assembly is located in a resonance point, it can be influenced. The approach to a solution for such a protection system consists of developing composite material systems with distinct RF-absorbing properties, which can be used instead of or in combination with metallization or metal shielding, which has the considerable disadvantage of internal housing reflections and field elevations. With regard to their properties, these materials must be able to form layers and adhere to metallic and non-metallic substrates, they must have high permeability and high dielectricity, and they must be combinable or mixable with plastics used for the manufacture of housings. Effective RF absorption or loss must be achieved even with layer thicknesses below 1 mm (ideal < 0.1 mm). With their special electrical/magnetic properties, the new materials to be developed should help to master the ever increasing demands on electronics that are insensitive to interference in the information society of the twenty-first century. The aim of the work is to achieve a high degree of reliability in information processing electronics by a new type of housing design. New materials, which should have special electrical/magnetic RF properties, have the task of replacing the simple metal housing of information electronics by a material composite consisting of metal/RF-absorbing thin material or a polymer-absorber-solid mixture. Thus, materials with special properties that are not yet available are derived from the target, such as: • • • • • •

high RF damping high ε00- and μ00-values low thickness special mechanical values: low hardness, drillable the smallest possible change in electromagnetic properties under voltage stress aggregate state: solid, liquid, or as a laminate can be applied/adhered.

8

2.2

2 Volume Materials

Microscopic and Macroscopic Properties of Spinel Ferrites

The knowledge of the crystal structure of microwave ferrites is of great importance, since the absorption effects also have their origins in atomic or crystalline structural properties. With physical models, starting from the material fundamentals, the absorption effects of RF energy, the conversion effects and the resulting energy forms (wall movement of the domains, quantized spin waves, relaxation effects, resonance effects, dynamic rotational movements, etc.) can be described. Ferrites are materials with a high resulting magnetic moment [5]. This manifests itself in the presence of a difference torque or a resulting spin in the material [3]. The ferrimagnetic materials are very diverse and exist in a wide variety of structures. The most important types of ferrite are listed in Table 2.1. In this chapter, the structure of the ferrites shall be presented as simple as possible. The ferrites are divided into the following main groups: • • • • • •

Spinels Grenade Magnetoplumbite Y-type ferrites with hexagonal structure W-type ferrites with hexagonal structure Orthoferrite A description of the exact structure of the ferrites mentioned would go beyond the scope of this work with knowledge of the basic relationships, so only the most important

Table 2.1 Application and properties of the most important ferrite groups Crystal Cubic unit cell

Structure type

Cubically complicated Hexagon unit cell

3þ ðMeÞ2þ 3 ðMeÞ5 O4

Hexagonally symmetrical Hexagonal, consisting of three spinel structures Orthoferrite

3þ Me2þ x Me3x O4

Representative Manganese, zinc, ferrite Nickel, zinc, ferrite Rare earths

Frequency range 1 MHz– 1 GHz

Technical application Ultrashort wave, EMC

1.5–3.5 GHz

Communications engineering Microwave technology Field-controlled device Microwave m.

3þ ðMeÞ2þ 1 ðMeÞ12 O19

Sr-ferrites

1–25 GHz

Sequence of T and S, spinels Sequence of M, Y and S, spinels

Ba2Me2Fe12O22

500 MHz

Ba2Me2Fe24O41

> 1 GHz

No technical application

2.2

Microscopic and Macroscopic Properties of Spinel Ferrites

9

Fig. 2.2 Simplified spinel structure after Philippow

properties are given in Table 2.1 [4, 5]. The most important group for the frequency range of interest, the spinel group, is dealt with in greater detail. This work deals exclusively with the ferrite group of spinels. For a better assessment of the ferrite single crystals and their properties, the following observations on NiZn ferrite are listed. The NiZn ferrite crystallizing in the partially inverse spinel structure presents itself in the following manner. A simplified representation of the spinel structure [6] is shown in Fig. 2.2. Spinels of the general formula A2BO4 consist of a cubically dense spherical packing of the oxygen atoms [9], in which half of the octahedral gaps and 1/8 of the tetrahedral gaps are occupied by the metal atoms. In case the tetrahedral gaps are occupied exclusively by the divalent A atoms, one speaks of normal spinel. If the divalent A atoms occupy the octahedral gaps and half of the trivalent B atoms occupy the tetrahedral gaps, it is called inverse spinel. For the technically interesting ferrites, however, intermediate spinels with transitions between the normal and the inverse distribution are usually present. The following general chemical formula applies to the NiZn ferrites of interest here:
3þ 2þ tet
3þ 2þ oct Fe1x Znx Fe1þx Ni1x O4 The spinel properties listed apply to the single crystal. The real conditions in the polycrystal differ greatly from the material properties of the single crystals. The RF properties of the single crystals also differ from those of the polycrystals. There are contradictions when comparing the mathematical-physical models [6, 13, 14] to the theoretical resonance behaviour of ferrites as a function of the various constants K1, K2, M, H0, τ, M0, the angle of irradiation of the RF field in textured ferrites [16] and the experimental results of the RF losses of polycrystals and ferrite composites [1, 2, 5].

10

2 Volume Materials

Fig. 2.3 Structure of a polycrystal without existing texture

Contradiction 1 According to Kupizka [10], there are differences between the theoretical absorption of χ 00(H ) and practical measurement. The theoretical curve χ 00(H ) shows strongly exaggerated peaks. The measured χ 00(H ) curves do not have these peaks and look very flattened. Contradiction 2 According to [4], it can also be stated that the theoretical resonant frequencies in real polycrystalline samples rarely correspond to the theoretically calculated values. The real sintered ferrites [11] have now been manufactured as polycrystals. How is a polycrystal of a ferrite constructed? According to Blumenauer [12], a polycrystalline sample is structured as shown in Fig. 2.3. In real crystals, foreign inclusions or, in case of insufficient sintering temperatures  (T < 1200 C), foreign phases are also present. In non-textured samples, all grains are randomly distributed and arranged in the polycrystal with different grain sizes (minimum size up to 10 nm [11]). The general literature [8] does not assume a grain size but a grain size distribution in which one grain size has the highest proportion. The grain size and grain size distribution is discussed in more detail in the next chapter. Figure 2.4 shows the sectional view of a MnZn ferrite polycrystal. Now the contradictions will be discussed with the help of the properties of polycrystalline ferrite. On the Contradictions The question posed can now be answered after considering an example orientation of the grains in a polycrystal. The total loss permeability per frequency in a polycrystal is an average value of all partial permeabilities μ00i of crystallites. The losses of all differently oriented and differently distributed grains and also the associated different resonance loss characteristics must be superposed.

2.2

Microscopic and Macroscopic Properties of Spinel Ferrites

11

Fig. 2.4 Microstructure MnZn-ferrite. (Source: HITK Hermsdorf)

It has been illustrated that a sum of partial permeabilities is effective, which does not produce such a clear resonance curve in the mean value as in the ratios of the single crystal or as assumed in the previous theoretical considerations. The mean value curve of this sum of the partial losses of the individual grains in the polycrystal, which are regarded as “ferrite single crystals”, gives two statements compared to a “ferrite single crystal” loss curve or to the theoretical preliminary consideration: • Broadening of the resonance character of the permeability (cause is the averaging of the absorption peaks) • Shift of the resonance frequency. If, in addition to the explained mechanisms of averaging the partial loss processes with the result of broadening the resonance curve, a magnetic thinning in a polymer-ferrite material is added, hardly any resonance behaviour can be assumed in a permeability curve that increases linearly with frequency. However, the resonance display characteristic of the losses is also present in the polycrystal. In a polycrystal, the dipolar interactions of the crystallites must be taken into account, as well as holes, pores [11] and cracks in the volume. In real materials, there is a shift of the

12

2 Volume Materials

resonance field Hi. The internal resonant field is normally calculated from the quantities K1 (first order anisotropy constant) and M. Now V (volume of the total sample) and v (volume of the pores) are added. Since real material sizes have an influence on the size of the internal field, it can be concluded that, via the gyrotropy constant γ and in conjunction with the “normal” resonant field strength Hres, the shift of the resonance frequency to Okamura can be calculated. Thus, the contradictions 1 and 2 could be discussed about the structure of the polycrystal. Based on the discussions on the structure of polycrystals, the average complex permeability component μ00Polyc ðH Þ in polycrystals is to be set. Based on the conditions in the polycrystal [12], the Gaussian distribution [8, 18] of the grain orientation [16] is assumed. μ00Polyc ðH Þ ¼

KERF μ00(H ) i

1 1 X 00 μ ðH ÞK ERF 0:08π i¼0

ð2:1Þ

Probability constant Average complex permeability of each grain i Average

In conclusion, regarding the RF loss in the polycrystal, the probability of grain orientations and the mean value of all grain losses play an important role in the total losses. Besides the consideration of the real case of polycrystal, the analysis of a possible texture [29] is important. For Ba-ferrites and Co2Z, the textured materials have the higher complex permeability losses compared to the non-textured materials μ00 [9, 10]. This fact, which applies to sintered ferrites and also to ferrite composite materials, is also to be investigated for ferrite compounds or for ferrite compound foils. The explanation of the relationship between resonance absorption and texture phenomena [10] is given in Chap. 2 and Sect. 2.7. The real conditions in the polycrystal differ greatly from the material properties of the single crystals. The RF properties of the single crystals are therefore also different from those of the multicrystals. Based on the microscopic and macroscopic material properties described in the last chapter, the RF interactions listed in the next chapter must be explained. The ferrites are strongly magnetic. Now, based on the material properties, the RF losses are to be examined more closely. The knowledge of the crystal structure of microwave ferrites is of great importance since the absorption effects also have their origins in atomic, molecular and crystalline structural properties.

2.2

Microscopic and Macroscopic Properties of Spinel Ferrites

13

Using the microscopic and macroscopic material sizes. M γ d a Ps T K1 K

Magnetization resulting from the sublattice positions of the anions of the sublattices of the various ferrite crystals (garnet, spinel, . . ., hexagonal structures) Degree of inversion, change the lattice positions Average grain size Lattice spacing Grain size distribution and orientation Relaxation time constant Anisotropy constant Texture constant ...

An influence on the RF loss according to the following magnetic dynamics model in the following chapter. The RF interactions in ferritic volume materials are very diverse. In this chapter, the theories of domain loss types (relaxation, resonance) and pinning losses are not considered. We ideally assume a single-domain behaviour. Likewise, mechanical effects such as the conversion of the piezomagnetic phenomenon into elastic stress are not discussed further. With physical models, starting from the material basics, the absorption effects of RF energy, the conversion effects and the resulting energy forms (wall movement of the domains, quantized spin waves, relaxation effects, resonance effects, dynamic rotational movements) can be described. After knowledge of the physical models, measurable parameter equations are established, which allow a general description of the phenomena. Based on these macroscopic equations, ideally a term like absorption can now be explained. In our case, RF-absorption as an overall phenomenon cannot be described yet, which can be explained by the multitude of different processes. Also a description of the clear causes (e.g. low field losses) is often not given in the literature. This work is intended to be an aid in the search for the universal term absorption. Taking absorption into account, thermodynamic problems with microwave ferrite polymers are to be discussed. This is a new application of ferrite materials. A first presentation of the energy conversion of RF energy in volume ferrite is given in the simplified Table 2.2. The RF interactions in layers/layer systems are also to be considered representatively: In Table 2.2, it can be seen that many effects play a role in the volume material of the RF ferrite. Some effects are considered individually theoretically and experimentally in this work. Unfortunately, the effects mix and merge smoothly in the frequency domain. It is hardly possible to influence only one RF effect with only one material size change. Only the most important spin wave loss types are mentioned. However, the aim of the considerations must be to assess the effectiveness of the influence of the individual microscopic and macroscopic material properties on the total loss, which in Table 2.2 is made up of the total partial losses.

14

2 Volume Materials

Table 2.2 Different types of loss according to the frequency ranges

2.3

RF effect in ferrite Bloch wall losses LF relaxation losses NFMR Natural spin waves Spin waves FMR Spin waves relax. Exchange loss

Frequency range 1000 MHz is absolutely necessary. In this work, (a) an absorbing layer for the novel PCBs and (b) a novel multilayer PCB with absorbing layers for the EMC range should be developed.

2.10.3 Modelling Objective of the Layer Synthesis In the following considerations, a short introduction to the effects and synthesis of spin waves in thin ferritic films will be given. In particular, the EMC boundary conditions of dynamic and static field exposure are discussed. The theoretical approach of the NSWR is used. Solid-state physical approaches are only mentioned in very simple basics.

2.10.4 Type of Spin Wave Modes as a Function of Layer Thickness It can be clearly seen that in the lower frequency range from 1 GHz, the surface modes can be synthesized. These play an important role in EMC cases. Thin-film effects in magnetic layers seem to have mystical causes. These interesting physical effects have been under increasing consideration by science and industry for 40 years. For example, the storage industry relied on the use of layered materials in the 1950s with magnetic tape technology. Later processes up to the 1990s reduced the layer thickness of the magnetic materials down to the μm/nm range and changed the storage method. The result was an increased storage density. In the years after 2000, work will continue with smaller wavelengths and higher recording frequencies. The layer thickness of the magnetic layers will be reduced down to the monolayer range. This will allow not only fastest memory access, but also extremely high storage densities. This tendency towards thinnest layer thicknesses is astonishing because the smaller the layer thicknesses and thus the material volume, the smaller the apparent storage capacity. According to this, different effects work in the layer than in a volume. For the nm-ferrite layers considered here, at first glance only zero absorption can be predicted due to a material volume of almost zero.

2.10

The NiZn-ferrite Spinel System

45

Nanomaterials such as layers have fractions of more than 50% of interfaces and grain boundaries in the total volume of the nanocrystallite. In a thin layer, therefore, more interfacial properties than volume properties act. Thin-film effects in magnetic layers seem to have almost mystical causes. These interesting physical effects have been under increasing consideration by science and industry for 40 years. With regard to volume atoms, a thin film or nanomaterial has interface atoms/surface atoms on both sides of the interfaces that have asymmetrical bonding relationships with respect to the volume atom [26]. Nanostructured materials have fractions of more than 50% of interfaces and grain boundaries in the total volume of the nanocrystallite. These interfacial properties thus have a greater effect than the volume properties of the nm crystal. This type of symmetry and interaction refers to an increase in the magnitude of the interface effects (e.g. surface properties of a sample). With regard to the anisotropy of a layer, the following interesting effect can arise. In the volume material, the magnetic anisotropy is perpendicular to the plane of the layer. In the layer state, the interface effects outweigh the volume effects and the anisotropy “folds” in the direction of the surface parallel (out of plane). In addition to anisotropy, energy, for example interfacial energy, is an important variable in the discussion of the difference between volume material and thin-film material. Furthermore, the magnetic anisotropy field strength increases due to the stronger effect of the interfacial energy as the layer thickness decreases (from a critical layer thickness). The following observations are to deal with the different effects of dynamic electromagnetic fields on hexagonal monocrystalline or polycrystalline magnetic layers in EMC cases (small fields, no neglect of field strength components). The question of the different mechanisms of field absorption should be answered. Possible indications for the use, advantages and disadvantages of single crystal layers/ polycrystalline layers in electromagnetic compatibility from 1000 MHz must be presented. These layers with absorptive properties are particularly interesting for electromagnetic compatibility in the information technology sector since the field modes capable of propagation occur in frequency ranges from 1000 to 2000 MHz. This is the effective range of spin wave materials (ferrite layers, nanowires, magnetostrictive layers, magneto-optical devices). In the literature, some authors have described the different effects of polycrystals/single crystals with the dynamic/static magnetic fields. Single crystals are often deposited using the pulse laser deposition method [31]. The single crystal and the ferrite polycrystal are described as follows [32]:

46

2 Volume Materials –> Hi Θ ε

–> HN –> H0

–> M

Fig. 2.28 Display of the demagnetization field of a hexagonal ferrite crystal

Resonance frequency single crystal :

Resonance frequency polycrystal :

Under consideration of pore volume and crystal orientation:    f r ¼ γH i f r ¼ γ H i þ 4M 0 k þ R2 k 2 þ 4Rk þ 2 Another approximation of the pore influence is shown [12]: In [33], reference is made to the special rod structure of a hexaferrite (Fig. 2.28). In the polycrystal, a demagnetizing field with Gauss averaging acts on each individual crystallite: H cos ðθÞ NI sin ðεÞ ¼ H n ¼ H 0 tan ðΘÞ, sin ðεÞ ¼ H tan ðΘÞ=2πM Hi ¼

I Polarization, N ¼ 2π

H n ¼ H 0 tan ðΘÞ Likewise, the problem of polycrystals can be addressed as follows. Intergranular exchange coupling at grain boundaries, the smaller the grains, the greater the exchange coupling: J ex tot ¼ J ex þ J ex grain

ð2:21Þ

J ex grain 1=D H AWW ¼ J ex tot =M 0 Thus, it was possible to clearly refer to some literature references of the state of the art in science and technology. In the following, some details are summarized and the way of the own theoretical preliminary considerations are described in detail. The internal magnetic field is equal to the sum of the magnetic field effects (pores, anisotropy, demagnetization field strength, Gaussian distribution of the crystallites, grain

2.10

The NiZn-ferrite Spinel System

47

size). According to LL, the modelling strategy has to incorporate the microscopic and macroscopic material properties into the system of equations. The comparison polycrystal (holes, grain size, Gaussian distribution of the direction of orientation) with single crystal (ideal crystal without holes, one direction of the hexagonal structure) is to be realized. The theoretical total internal magnetic field strength of the polycrystal of a hexagonal ferrite layer is described as follows: H i ¼ H n þ H 0 þ h þ H an þ H pore þ H grain þ HJ ex For the single crystal applies: H i ¼ H n þ H 0 þ h þ H an þ HJ ex

2.10.5 Summary The following effects in the sense of the assessment of the different interaction of the dynamic RF field with single crystal layers to polycrystalline layers of hexaferrite in the area of electromagnetic compatibility are to be stated as calculation results: Due to the sum of the half-value widths of the magnetization of the polycrystal, a higher RF loss is to be expected in this crystal system. In the single crystal, a higher resonance frequency can be observed. This is most likely caused by the greater internal, effective magnetic field strength in the single crystal. The gyrotropy constant can thus be used to justify this higher resonance frequency. A defined RF absorption per application frequency can be designed with the single crystal. This behaviour of the greater magnetization in the single crystal can be explained by the effect of the greater exchange field strength in the single crystal [36]. The resonant frequency in the single crystal is higher. This is a clear advantage in the electromagnetic compatibility of the information technology sector. Likewise, the resonance can be calculated more precisely since the blurring effect of the crystallites in the polycrystal is missing.

3

Nanomaterials

3.1

Layer Analysis, Anisotropy Constant and Grain Size of Ferrimagnetic Layers

The aim of this work is the deposition of thin ferritic layers as absorber material for EMC applications [43]. Initially, the work concentrates on NiZn ferrite layers. Fundamental investigations on the suitability of ferrite layers were realized. The measurements of the magnetic properties yielded values of the coercivity HC from 0.95 to 1.28 kOe. These extremely high values of coercivity indicate a pronounced anisotropy of the layers. The anisotropy constant of the layer in the unannealed case is K1 ¼  8.1  10+6 erg/ cm3. This value indicates that the anisotropy is too high for a soft ferrite. This is an indication of the special properties of the nanocrystalline material compared to the bulk material. A SEM micrograph still shows the morphology of the layer relatively “roughly” (Fig. 3.1). Figure 3.2 shows the dependence of the ferritic NiZn ferrite layers on the annealing temperature occurring after the sputtering process. The coercive field strength of a magnetic layer is an exemplary indication of the layer anisotropy and is therefore interesting to look at. It can be seen that the coercivity does not increase with the annealing temperature. Even if the mismatch does not ensure optimal field penetration and absorption is lost, the compromise is realized. Sintered RF ferrite plates have a reflection attenuation of up to 25–35 dB (TDK/EUPEN company publications). The reflection attenuation of the ferrite compound with 4-mm thickness shown in Fig. 3.3 shows lower effectiveness of maximum 12 dB at 900 MHz due to the mismatch.

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Nanomaterials

Fig. 3.1 SEM micrograph of the NiZn ferrite layer with 200-nm layer thickness; the bright areas represent the crystalline ferrite areas. (Source: Analysis by CiS Erfurt)

3.2

Spin Wave Losses in Ferrite Layers

The RF ratios in volume and coating materials are fundamentally very different [36, 37, 38]. While in bulk materials the individual spin of the ferromagnet interacts with the respective lattice of the ferro or ferric crystal and the neighbouring spin, the bond in a thin magnetic layer is characterized only by the relationship of the spins to each other (the nearest neighbours). This is called the statistical near-order. Under dynamic conditions [4], spin waves occur in thin layers. Therefore, the question must now be answered: What is a spin wave? The spin wave is excited by a quasi-particle, the ferrimagnon. Each spin wave is associated with the “annihilation” of a quantum of energy [10]. Kittel [31] distinguished between two types of spin waves: the surface spin wave, which is caused by the anisotropy at the surface of a layer, and the transverse and normal-band spin wave inside a layer. While the surface spin wave modes [37] are “clamped—pinning” at the surface of the layer, only odd-numbered multiples of the spin wave (nπ/L, L—layer thickness) can be excited in a layer. Theorem 1 Only a disturbed system can excite spin waves in magnetic layer systems.

3.2

Spin Wave Losses in Ferrite Layers

51

Fig. 3.2 Dependence of the coercivity (field parallel to the layer) of a NiZn ferrite layer on the temperature T

Fig. 3.3 Reflection loss of the MnZn ferrite compound

52

3 Spin system in the layer

Nanomaterials

No spin wave top view

z

y

S1 1

S2 =

2 =

homogeneous excitation in homogeneous coating material

S3 x 3 homogeneous coordinated motion of the spins = no wave

Fig. 3.4 Insufficient excitation of a homogeneous layer does not cause a propagating spin wave motion

Theorem 2 The disturbance can lie in the excitation (inhomogeneous energy influence, e.g. thermal, mechanical, electromagnetic) of spin waves and/or in the material structure of the layer with homogeneous excitation. A pictorial representation of the case when theorems 1 and 2 are not fulfilled and no spin wave propagation occurs can be seen in Fig. 3.4. The inhomogeneous layers under homogeneous energy influence according to the theorem 2 are shown as an example in Fig. 3.5. The state of the inhomogeneous layer with inhomogeneous energy influence is not shown. In Fig. 3.5, a propagating spin wave is shown. Another possibility of excitation of spin waves is the excitation of surface spin waves SSWR, where a homogeneous layer is exposed to a homogeneous energy field. The inhomogeneous excitation is achieved by pinching, also called “pinning” of the spins, at both ends (beginning, end) of the layer. This excitation is shown in Fig. 3.6. Theorems 1 and 2 are not restricted by this type of spin wave mechanism. Theorem 3 Mixed forms of the spin wave excitations shown in the images and listed are possible. The multitude of solutions of the inhomogeneous Maxwell’s equations or the Landau– Lifschitz equation system with Gilbert approach in the perturbed spin wave structures under consideration lead to a number of spin wave modes. This variety of solutions is discussed in detail in the theoretical considerations of the spin wave losses. Theorem 4 Zero-field spin wave excitation is assumed in the present case. Thus, the presence of a large static magnetic field strength is not necessarily necessary for the existence of spin waves [31]. SSWR is highly dependent on the layer thickness [39]. In, Fig. 3.7, Kummer [19] shows the property of multiplicity by the dispersion representation of the spin wave.

3.2

Spin Wave Losses in Ferrite Layers

53

Spin system in the layer

SWR spin wave top view

z

y

S1 ω1

S2 ≠

ω2

S3 ≠

x

ω3

homogeneous excitation in inhomogeneous coating material

inhomogeneous coordinated motion of the spins = electromagnetic wave

Fig. 3.5 Sufficient excitation of an inhomogeneous layer causes a propagating spin wave motion Spin system in the layer

SSWR spin wave top view

z

y

-> S1

-> S2

-> S3

ω1 ≠

ω2 =

ω3 ≠

Length or thickness of a layer homogeneous excitation in homogeneous coating material

-> S4 x ω4 inhomogeneous Coordinated movement of the spins through pinning

Fig. 3.6 Sufficient excitation of a homogeneous layer with two pinned spins with a homogeneous energy influence causes a propagating spin wave motion

Fig. 3.7 Dispersion relationship for spin waves

The literature gives an output frequency of 233 MHz for the occurrence of spin waves in Yttrium iron garnet (YIG) materials at 4πMs ¼ 250 Oe, and Borovik [38] f ¼ 100 MHz in (cubic crystal form) metals. According to [37], “usual” higher-frequency spin waves occur in ferrite films from 1 GHz at higher static magnetic field strengths.

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Fig. 3.8 Excitation modes of spin waves

SWR is caused by the spin–spin interaction [2] in the layer. The SSWR is caused by the spin–exchange interaction.

3.2.1

Spin Wave Excitation

Spin waves can be excited in the way shown in Fig. 3.8. In the following, we will only deal with the direct excitation. Since in the case of an absorbing layer the electromagnetic field sources are not known, the only way remains in an ahomogeneous material design (indirect excitation is too costly). The aim of spin wave excitation in disturbed layers is the RF absorption loss in the material due to a complicated layer or layer sequence design (additional dipole effect). Disturbed ferrite layers can be • • • • • • • • • •

Ferrite layers with a high proportion of secondary ferrite phases Ferrite layer sequences, multilayer [40] Discontinuities in the layer Pores Transition state crystalline/amorphous Voltage in the grid Texture Rough surfaces, generation of scattering effects [25]. Crystal defect generation through surface treatment such as etching Direct application of mechanical stress results in a change in resonance.

3.3

Influence of the Anisotropy Constants on the RF Loss of the NiZn Ferrite Layer

55

In the literature, many disturbed layer structures are explained by the strong increase of the exchange integral. In domain theory, exceeding the critical particle size is the cause of non-homogeneous magnetization processes [40]. The influence of the crystal anisotropy of a layer shall be discussed theoretically for the case of the missing strong static external magnetic field strength by a new model. In the observations of the Dutchman Huijbregste [41], the RF loss μ00m,n increases with anisotropy constant increase K of the layer. Section 3.3.1 deals in particular with NSWR (natural spin wave resonance). Effects such as multilayer films and layer structures are not dealt with in this work. Since these effects have a significant influence on the RF behaviour of a magnetic layer, this topic will be the subject of future research.

3.3

Influence of the Anisotropy Constants on the RF Loss of the NiZn Ferrite Layer

3.3.1

Theoretical Considerations on Spin Wave Loss

Infinitely spread layer material is assumed. The modelling model of spin wave loss is often the Landau–Lifschitz equation with damping term [43]. According to Huijbregste–Sietsma [41], a system of equations with two time derivatives is used as the dynamic output vector equation. This approach allows difficult complex systems of equations to be considered in advance of the modelling. New to the conventional dynamic layer observations is the non-neglecting of component terms compared to a possibly (non-existent) static magnetic field strength. We assume the following still simple system of equations (Eq. 3.1): !
! !  ∂M α ¼ M  H eff  ðl=γ 0 Þ γ0M ∂t

!

M γ M0 α !

H eff

!

∂M M ∂t !

!

Magnetisation vector yrotropy constant aturation magnetization, static Damping constant effective magnetic field strength vector (internal and external effects)

ð3:1Þ

56

3 !

!

Nanomaterials

!

M ¼ M 0 þ mejωt !

m

dynamic magnetization in the material

!

!

!

H eff ¼ H 0 þ h ejωt þ

2K 1 ! e μ0 M 0 z

!

static magnetic field strength

!

dynamic part of the magnetic field strength (harmonic approach)

H0

h ejωt 2K 1 ! μ M0  e z

ð3:2Þ

magnetic anisotropy field strength of the layer (K1 anisotropy constant).

0

In addition to these equations, the following conditions should also apply. (In the literature, the approach H0 < hx, hy, hz is constantly accepted, which simplifies the calculation of the μ tensor very much. We do not take this approach, because in our case of the incidence of the EMC RF field strength is an existing external static field H0  hx, hy, hz). mz ¼ M 0 ;

H 0 ¼ hz γM 0 ¼ ωm γH 0 ¼ ω0

In the case of the condition of the ratios, H0  hz, hx, hy corresponds largely to the NSWR condition. According to Eq. (3.3) and the listed conditions, the dynamic Landau–Lifschitz equation with damping approach for spin waves in layers can be set up and the vectorial system of equations can be derived quite easily. 0

0 0 11 jωmx
 C B! B CC ! ! 1B B jωmy C ¼ μ0 M  H eff þ α BM  B jωmy CC @ A @ @ AA γ ωm jωmz jωmz jωmx

1

ð3:3Þ

m00 ðK 1 , ωÞ ¼ m00z ðK 1 , ωÞ þ m00x ðK 1 , ωÞ þ m00y ðK 1 , ωÞ This is not the description for the material property μ of the layer. However, due to the very difficult and large complex expressions of the layer model quantities, the imaginary part of the total magnetization vector is taken as an indication of an RF loss of the thin layer and its dependence on the anisotropy constant K1.

3.3

Influence of the Anisotropy Constants on the RF Loss of the NiZn Ferrite Layer

57

Fig. 3.9 Modelling of the spin wave loss SWR as a dependence of the imaginary part of the dynamic magnetisation vector Im{m} ¼ m00 on the anisotropy constant K1 of the ferrimagnetic layer

Figure 3.9 shows the modelled magnitude of the total loss of the m00 dynamic magnetisation vector as a maple CAD representation of the equations as a function of K1 of the magnetic layer. Figure 3.9 shows the result of modelling of the complex magnetization vector as an indication of the RF loss of the layer. Two statements can be stated as the result of the theoretical considerations: Theorem 5 With increasing anisotropy, an increasing RF loss is to be expected. Theorem 6 There is an optimal range of anisotropy; outside this range, the RF loss falls. This anisotropy constant, which is optimal for the target function, could be determined theoretically by extreme value analysis of holomorphic functions or could be determined experimentally. In the context of this work, the second way is taken. The next chapters will show to what extent the theoretical approach, the theoretical modelling, and the resulting reference in the layer design were useful. Rule 1 for the Material Design of the Magnetic Layer For a high RF loss, optimal layer anisotropy should be aimed at to realize a high spin wave loss in the material.

58

3.3.2

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Nanomaterials

Experimental Considerations on Spin Wave Loss

When measuring the RF effects, completely new ways of measuring the RF losses in thin layers as opposed to the losses in bulk materials must be taken. A waveguide measuring station for recording the material parameters is based on sample thicknesses from 5 mm. This measuring station is not adjusted for thin magnetic layers.

3.4

Special layer Analysis, Anisotropy Constant and Grain Size of Ferrimagnetic Layers

Extreme post-annealing is not necessary in order to increase Hc. After rule 1 for the material design of a magnetic layer, a second rule can now be established. Rule 2 for the Material Design of a Magnetic Layer A post-annealing in the sense of an increase of Hc is not an absolutely necessary condition. If the post-annealing is low, the proportion of crystalline regions is small and therefore the modelling requirement of the disturbed structures is present. In the post-annealed layers, there are more crystalline areas. Many crystallites mean a larger “order” and could therefore cause a smaller spin wave loss than a few. The literature [33] assumes grain sizes of 800 nm for NiZn ferrite coatings. For annealed NiZn ferrite layers, the grain size of 500 nm was determined from the half-width of the X-ray diffraction peaks. The existing layers are nanocrystalline. We have thus created a nanocrystalline to microcrystalline material. For the unannealed samples, a grain size of 60 nm is determined. With the volume materials, a growing RF loss was observed with increasing grain size. This finding is also evident in the thin layers. In conclusion, it can be said about the layer characterization that the nanocrystalline material properties are shown by the distinct anisotropy of the layer. A natural spin wave loss [35] was also generated with a multilayer structure, but the layer cannot be used for RF visualization because of its low heat generation on the surface. Applications of the synthesized layer are conceivable in communications engineering (filters), converters (convolution analysis component from communications engineering, consisting of a Ba-ferrite layer), and in EMC housing technology.

3.4.1

Absorption Loss of the Ferritic Layer

The considerations on RF attenuation of the ferritic layer do not include material analysis methods. Since these attenuation analyses are not yet common in RF technology and are closely related to the layer design and the material analysis of the thin layers, the observations on absorption attenuation will be discussed in this chapter.

3.4

Special layer Analysis, Anisotropy Constant and Grain Size of . . .

59

The test setup is very important for RF analysis and thus in the broadest sense also for the material design process. The stripline measuring station should continue to be used as the basic test setup. The evaluation of the RF attenuation of the layer is more complex to discuss in connection with the test setup. An important parameter is the field adaptation of a layer to be measured. In principle, it can be used to explain the necessity of the field adjustment of the layer. Moreover, the surface resistivity of layer has to be adapted to the characteristic wave impedance of air. The test setup and the discussion about the course, propagation, and attenuation of electromagnetic radiation is of fundamental importance for the assessment of the absorption of RF energy in the nanostructured material. From a formal point of view, after considering the RF attenuation of a material by means of the conduction theory and the S-parameter description, reflection and transmission are in any case the basis of the discussion. However, with reference to test setup I and II for estimating the effectiveness of the thin film on field attenuation, a characteristic statement can be made about the formula description. The following dependency of the absorption constant applies to test setup I from the simultaneous adaptation to the characteristic wave impedance adaptation of the test setup: a¼r a r

ð3:4Þ

Absorption constant in absolute values (0 . . . 1) Reflection loss in absolute values (0 . . . 1).

This equation only applies to materials with an electrical conductivity of zero. The reflection loss as a change of the S11 parameter of the stripline arrangement, with and without layer, is to be included in the RF evaluation. For the test setup, Eq. (3.4) is to be used for RF evaluation and the S-parameters S11 and S21 are to be inserted into this formula. Test setup I was chosen because this evaluation method only allows you to record one component (reflection with and without layer) for analysis. The problem of accurate and error-free reflection attenuation arose. A merely changing resonance distribution is to be discussed. This deviation from the real low reflection attenuation resulting from the physics of conduction theory regarding the stripline was averaged out by averaging the resonance shift. With this resonance shift averaging method, the equation could be used as the defined RF evaluation in terms of absorption attenuation. The higher RF loss in the multilayer sample compared to a single-layer sample was clearly determined experimentally. This effect is caused by the spin wave coupling of the individual ferrite layers to each other. In the layer thickness plot with the spin wave propagation, it can be said that the frequency of the spin wave in the layer increases with length. The higher the static magnetic

60

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Nanomaterials

field strength H0 according to the Kittel formula, the higher the resonance frequency according to Eq. (3.5): fr ¼

γH A 2π

ð3:5Þ

Thus, it can be seen that metallic and electrically conductive ferrimagnetic layers have a possible band of spin wave frequencies. Of particular interest in the necessary spin wave loss modelling for each technical application is the influence of the sample geometry on the RF loss in the layer. The literature provides the following findings. It is possible [39] to solve the Landau–Lifschitz equation and the Maxwell system with all boundary and initial value conditions. The result of the observation is difficult to interpret, depending on the spin wave modes and sample geometry the magnetic spectroscopic reflexes have to be determined. It is not possible to define a dependence of the half-width of the spin wave dispersion on the thickness of the layer [40]. In the previous considerations, the dependence of the layer thickness of a multilayer system has been investigated. The considerations and rough physical estimations of this chapter show that the dependence of the dynamic damping of the spin system on the layer thickness of the magnetic layer can only be calculated by a complicated solid state consideration of the dynamic damping of the spin system. In the present spin wave case, an attempt is to be made with relatively simple means to answer the question of a clear RF loss dependence of the sample geometry by means of the damped Landau–Lifschitz system for the EMC case. The dependence of the damping of the spin wave dispersion is analytically discussed by means of the natural spin wave resonance and the consideration of the EMC field conditions as a function of the geometry of the magnetic layer.

3.5

Eddy Current Effects in Metallic Magnetic Films: Snoek’s Limit for layer Systems/Single Layer

In addition to the properties of the real crystal (polycrystal, first and second order defects), the real material properties in magnetic layers include the following layer effects already considered: • Difference spin wave propagation in single crystal/polycrystal • Demagnetization effects—size dependence on the sample • Domain behaviour. Important to mention is the Snoek’s theory besides the Kittel frequency. Snoek gives an upper frequency limit where a magnetic layer shows the “expected” frequency behaviour. From the Snoek’s limit on, a magnetic and conductive layer hardly works.

3.5

Eddy Current Effects in Metallic Magnetic Films: Snoek’s Limit for . . .

61

The cause of the Snoek’s law is the fact that a resonance can also occur in magnetic materials that are not completely saturated. Edge dipole fields are effective, which are included in the resulting magnetic field strength. Snoek’s Law sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   Hk þ Ny  Nz M0 ðμr  1Þ f r ¼ γ 4πM 0 ðmaximum resonant frequencyÞ H k þ ðN x  N z ÞM 0 0

ð3:6Þ

For most ferrites, the Snoek constant is S ¼ 2–5 GHz. Ferromagnetic metals have higher saturation magnetization than, for example iron with 2πM0 ¼ 2.15 T and therefore a resonance frequency of 40 GHz. Roughly speaking, the Kittel frequency can be specified as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffi γ M0HA fr ¼ 2π

ð3:7Þ

The complex conductivity enters into the Wolmann cut-off frequency. The eddy current effect represents: The higher the conductivity, the lower the effective cut-off frequency until a magnetic layer is effective. These frequencies can be estimated very roughly according to Kittel (Eq. 3.7), but the exact spin wave dispersion behaviour can be calculated using the Landau–Lifschitz system and can be dependent on the microstructure sizes M, K, γ, Tc the unit cell of the ferrite. The now considered basics of spin wave dispersion in the EMC case are therefore only valid for ferromagnetic layers. If, in addition to the magnetic behaviour, an electrically conductive behaviour can also be observed in the layer, further considerations are necessary regarding the frequency limit of the effectiveness of the penetration of the dynamic electromagnetic field into the layer according to Snoek. There is an upper frequency effect limit depending on the conductivity of the layer. This limit is described as Snoek limit. The effect of eddy current propagation and penetration depth is used to describe the maximum frequency limit. This frequency limit is based on a complex conductivity and a maximum penetration depth of the electromagnetic field after the eddy current effect. The cut-off frequency is described by the formula in Eq. (3.8) as Wolmann frequency: f g,eddy ¼

μa d ρ

¼M0/Ha Layer thickness Specific resistance.

4ρ μa μd2

ð3:8Þ

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Nanomaterials

Fig. 3.10 Representation of the theoretical frequency limit for magnetic conductive layers

Figure 3.10 shows the Snoek straight line resulting from Eq. (3.8). Figure 3.10 shows the Snoek straight line as a dependence of the maximum cut-off frequency of the magnetic electrically conductive layers on the layer thickness and the conductivity. The almost linear proportionality can be seen in the straight line indicated. This means that a decreasing cut-off frequency can be expected as the sheet resistance decreases. Rule for the Layer Design For electrically conductive coatings, the sheet resistance must be increased in order to increase the application frequency and cut-off frequency. In addition to the magnetic moments (Tables 3.1 and 3.2), statements on magnetic anisotropy are also very important for the desired EMC properties. First measurements of the anisotropy constants are summarized in Table 3.3. Figure 3.11 shows a typical hysteresis curve based on the sample (TU Ilmenau, FG Glas/Keramik). Due to a coercive field strength of approximately 0.1 kOe (100 A/cm), the samples lie between hard and soft magnetic materials. The measurements of absorption losses in the electromagnetic alternating field in the frequency range from 40 to 3800 MHz were performed in a stripline arrangement. The maximum measured RF loss was 2 dB. An improvement in the absorption was achieved by superimposing several thin NiZn ferrite layers (up to four layers). On the one hand, the effect of increasing the effective layer thickness plays a role, but on the other hand, the exchange interaction between the layers also plays a role. The second effect predominates, since increasing the thickness of the NiZn ferrite layer from 200 to 800 nm did not result in any noticeable improvement in RF loss. These results

3.5

Eddy Current Effects in Metallic Magnetic Films: Snoek’s Limit for . . .

63

Table 3.1 Estimated magnetic moments Torque values Sample no. 128 126 111 117 112

pAr [mbar] 5  103 5  103 7  103 10  103 17  103

Hc [kOe] 0.09 0.08 0.09 0.13 0.03

ms [Gcm3] 0.00220 0.00166 0.00317 0.00377 0.00285

mr [Gcm3] 0.00009 0.00005 0.00012 0.00022 0.00009

m(H ) surface [kOeGcm] 0.00052 0.00096 0.00084 0.001 0.00043

Source: TU Ilmenau, FG Glas, Keramik  THeater ¼ 500 C;samples 126, 111, 117, and 112 t ¼ 20 min, sample 128 t ¼ 40 min Table 3.2 Estimated magnetization Magnetization Sample no. 128 126 111 117 112

pAr [mbar] 5  103 5  103 7  103 10  103 17  103

Hc [kOe] 0.09 0.08 0.09 0.13 0.03

Ms [G] 438.2 451.1 159.8 399.8 199.2

Mr [G] 17.9 13.6 6.0 23.3 6.3

M(H ) surface [kOeG] 103.6 260.9 42.3 190.9 30.0

Source: TU Ilmenau, FG Glas, Keramik  THeater ¼ 500 C; samples 126, 111, 117, and 112 t ¼ 20 min, sample 128 t ¼ 40 min Table 3.3 Anisotropy constants Anisotropy measurement T Sample pAr [mbar] [min] no. 126 5  103 20 128 5  103 40

THeater [ C] 500 500

Hk eff [kOe] 3.4375 3.9836

Keff [erg/cm3] 548,682.7 800,351.2

K1 [erg/cm3] 91,625.9 214,160.3

K2 [erg/cm3] 15,767.6 14,519.1

Source: TU Ilmenau, FG Glas, Keramik

lead to the conclusion that sufficient RF loss for a technical application of the layers can only be achieved via multilayer systems with thin non-magnetic intermediate layers. The thin non-magnetic intermediate layers cause a very strong exchange interaction between the magnetic layers, which leads to additional absorption losses in the RF field. In addition, further research activities in the field of thin magnetic layers for EMC applications should also investigate multilayer systems with different magnetic layers. For the measurement of the RF effects, completely new ways of measuring the RF losses in thin layers compared to those in bulk materials should be taken. The waveguide measuring station for recording the material parameters assumes sample thicknesses of 5 mm. This measuring station is not adjusted for thin magnetic layers.

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Fig. 3.11 Typical hysteresis curve NiZn ferrite layer. (Source: Measurement TU Ilmenau, FG Glas/ Keramik)

In the following, a stripline measuring station for the measurement of the RF conditions in the material is presented. In addition to reflection attenuation, multiple reflection plays a role at attenuations of 1. The size of the loss increase by adapting the ratios to the layer design rule is small. As a layer design rule, the theoretical result can be considered and should be a direction for practical layer deposition and RF material analysis. Answer To achieve increased RF absorption, the granule size of a granular magnetic Fe layer should be at least twice the thickness of the layer. Figures 3.32 and 3.33 show the scanning electron microscope images. These were made with a SEM of type ULTRA 55 (Carl Zeiss Oberkochen). The following pictures show that the granular areas, the smallest dimensions of which are found at approximately 10-nm lateral extension, have a tendency to grow together as the layers continue to grow and thus form larger granules. For the metrological evaluation of the nanolayer, its reflection attenuation was determined. In each case, a sample with 1.2 μm and a sample with 0.9-μm layer thickness was measured in a PC7 measuring cell.

3.14

Ratio of the Granule Size to the Layer Thickness of an Fe-Nanolayer

83

Fig. 3.32 SEM image of a rectangular foil, layer thickness 1.2 μm, granule size maximum 50 nm, hematite layer on aluminium substrate. (Source: IOM Leipzig)

Fig. 3.33 SEM image of a triangular foil, layer thickness 0.9 μm, granule size maximum 40 nm, hematite layer on aluminium substrate. (Source: IOM Leipzig)

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Worksheet 0.4

Reflection loss in dB

Layer thickness 1.2 mm, granule size 50nm Layer thickness 0.9 mm, granule size 40nm

0.3

0.2

0.1

0 0

2

4

6

8

10

12

14

20 16 18 Frequency in GHz

Fig. 3.34 Reflection attenuation of the nanolayers. (Source: Measurement FH Telekom Leipzig)

Figure 3.34 shows the reflection attenuation course of the considered nanolayers. The graph of the dependence of the absorption on the granule size shown in Fig. 3.34 shows the tendency of the correct theoretical preliminary consideration of the material design thesis. Further measurements, which offer experimentally different and better information than the reflection loss, are currently being realized.

3.15

Multilayer Systems

The materials to be investigated are a special coating Magnetite Nano in different layer thicknesses (multilayer ¼ eight-fold layer) (Fig. 3.35). Reflection loss 2000 . . . 3800 MHz: With 3.5 dB attenuation, the spin wave loss starts to have an effect in this frequency range (Fig. 3.36). Transmission attenuation 5.85 . . . 8.2 GHz: A very good attenuation of 7 dB could be measured from 5 GHz. The reason is probably the high scattering loss in the 8 layers and the interfaces to the conductive layers (Fig. 3.37).

3.16

Kittel Frequency

A ferromagnetic layer with a saturation magnetization of 2 T is given. It has a layer thickness of 120 μm, an anisotropic field strength of 0.05 T, and a specific resistance of 0.250 Ω m.

3.16

Kittel Frequency

85

Fig. 3.35 Structure of a multilayer

What is the resonant frequency of the EMC layer? Can the layer be used as an EMC-supporting layer for interference frequencies of 4 GHz? The resonance frequency formula according to Kittel is pffiffiffiffiffiffiffiffiffiffiffiffiffi γ M0HA 2π pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1:76  1011 Hz=T 2  0:05  T2 fr ¼ 2π f r ¼ 8:8 GHz

fr ¼

The resonance frequency of the layer is 8.8 GHz and it can be used for interference frequencies up to 4 GHz.

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Fig. 3.36 Reflection loss of a multilayer film in the frequency range 2–3.8 GHz

Fig. 3.37 Transmission loss of a multilayer film in the frequency range 2–3.8 GHz

3.17

Wolmann Frequency

According to Wolmann, the cut-off frequency indicates which frequency is possible at all for which conductivity of a layer. Basics according to the skin effect play a role. Using the layer data of the task of the Kittel frequency, the question has to be answered to what extent the layer can work up to 14 GHz under consideration of the Wolmann effect.

3.18

Snoek Frequency

87

f g,eddy ¼

4ρ πμ0 μa d2

4  0:250 Ω m π12  107 ðVs=AmÞ40ð120 Ω mÞ2 ¼ 460 GHz

f g,eddy ¼ f g,eddy

This layer can be used up to well over 14 GHz.

3.18

Snoek Frequency

The Snoek frequency (law for very thin films—demagnetization) describes the uppermost resonant frequency. According to Rozanov/Walser (1998), the following applies for very thin layers:  3N 2H a ðμs  1Þ f 2r ¼ ðγ4πM 0 Þ2 1  z þ 4π 4πM 0

ð3:17Þ

μs ¼ 1 þ 4πM 0 =H a d  L : N z  πd=L d L μs

Layer thickness Length of the film Static permeability.

According to Snoek, what is the upper resonance frequency for a magnetic layer of the anisotropic field strength of 1 T and a saturation magnetisation of 0.1 T? The layer thickness is 50 μm and the length L is 3 mm.  3N 2H a ðμs  1Þ f 2r ¼ ðγ4πM 0 Þ2 1  z þ 4π 4πM 0 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi u 3N z 2H a u 2 uðγ4πM 0 Þ 1  þ t 4π 4πM 0 fr ¼ ðμs  1Þ f r  310GHz According to Snoek, the upper resonant frequency of around 310 GHz applies to the given magnetic layer with the magnetic data and the layer geometries.

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3.19

Nanomaterials

Radar Effects

Radar Effects and Scattering of Microwave Radiation in Thin Films What influence does the crystallite size in the nm range have when radar radiation in the frequency range of 10 GHz impinges on a magnetite crystal? For radar materials starting in the X- or Ku-band (new designation H-, I-, J-band), further effects have to be considered. In addition to the EMC effects discussed so far, the radar effects of linear and non-linear extinction must be considered integrally. Extinction can be explained as follows: Extinction ðattenuation of the radar radiationÞ ¼ Scattering þ absorption ðdielectric, magnetic hardly consideredÞ Total extinction cross section σ e ¼ σ s + σ a ω σa ¼ c

Z





ε

dV He 00 H

V

in analogy σ a—geometrical cross section through magnetic field effects is considered: ω σa ¼ c

Z





μ

dV He 00 H

V

σe σa Ω

Scattering cross section eometric cross section Surface.

Nanocrystallites on glass substrate (Marshall-Pallmer crystallite size distribution is neglected) are considered. The radar effect at f ¼ 10 GHz is described with Ralay scattering. μ0r ¼ 30 (static), μ00r ¼ 3 . The magnetite crystallites are simplified as materials (μ distribution in sphere homogeneous and isotropic) in spherical form. Diameter d ¼ 20 nm. What power will theoretically be measured at the receiver if the transmitter power is 10 dBm, the distances from transmitter to material 1 km and material to receiver are 0.5 km, and the antenna gains as ratios of 0.1 each? K¼ and

ε1 εþ2

3.20

Magnetic Nanoparticles

89

σa ¼

9kVμ00 j μ þ 2j 2

The spreading cross sections and power are calculated: P2 ¼ P1

λ2 G1 ð16π Þ2 R4

σ e ¼ 3:08  1023 m2 P2 ¼ 21  1037 dBm at a transmission power P1 ¼ 10 dBm. The extinction of the emitted power at and in the radar material is about 0 dBm. It can therefore be concluded that the radar scattering effects at 10 GHz with a crystallite size of 20-nm spherical shape do not cause any scattering loss.

3.20

Magnetic Nanoparticles

The aim of this work is to develop new materials for the substantial improvement of EMC by producing a composite of embedded nanoferrite powders in textile materials [44]. State of the art of shielding materials are mainly reflective surfaces, which in many applications do not lead to sufficient elimination of electromagnetic fields and/or even increase their effect. By combining nanopowders with one or more suitable ferroelectric materials in a polymer matrix in the fibre or in a coating, synergy effects of the two groups of materials can be expected through additional absorption mechanisms. Besides the composition of the ferroelectric, the dependence of the shielding attenuation and the dielectric/magnetic losses on influencing variables such as grain size, size of the ferroelectric domains, textures and Curie temperatures are to be investigated. Existing studies [4] show that a powder form of the nanoparticles has a larger line width of the FMR compared to agglomerated powders. This fact suggests that the nanoparticles in powder form have better spin–spin coupling of the nanoparticles including RF loss than spatially concentrated and more separated particles. In their experimental and theoretical work, Nogues and Sort [6] show the ambiguous effect of nanoparticles in relation to the magnetic effect. It is assumed that due to the surface roughness, high voids, and a high surface anisotropy, a large FMR linewidth (measure of absorption) occurs, but an “arbitrary” change of the ferrimagnetic state to an antiferromagnetic state can occur. Superparamagnetism is also observed below a critical particle thickness. This does not indicate a large magnetic effect. Accordingly, the absorption effect of a smaller particle size of magnetic nanoparticles from a particle diameter of fr Nanolayer

Fig. 3.40 The TEM analysis of magnetic nanoparticles ferrite, scale 50 nm. (Source: IOLITEC GmbH, http://www.iolitec.de)

3.20.2 Experimental Considerations Microscopic Examinations Nanopowders are special powders with grain sizes at 1 μm. Figure 3.40 shows the morphology of a nanopowder material. Using TEM (transmission electron microscopy), the surface shape of magnetic nanopowders was analysed. Grain size distribution is clearly visible in the range below 50 nm. A clear inhomogeneous distribution of the particles can also be seen.

Part II Practical Examples

4

Shielding Using Nanomaterials

For radar materials starting in the X- or Ku-band (new designation H-, I-, J-band), further effects have to be considered. In addition to the EMC effects discussed so far, the radar effects of linear and non-linear extinction must be considered integrally. Extinction can be explained as follows: Extinction (attenuation of the radar radiation) ¼ Scattering absorption +(dielectric, magnetic hardly considered). During material development, it is necessary to determine as many parameters as possible on the sample series using the various available measuring stations in order to ensure a comprehensive assessment. This is necessary to determine the direction of further experiments and material combinations. In addition to the requirement to maintain the chemical/physical properties of the starting materials as far as possible, the achievement of a total shielding effectiveness of at least 60 dB is considered realistic (see Figs. 4.1 and 4.2).

4.1

Measurement of the Complex Permeability of Nanolayers with One Permeameter

Figure 4.3 shows the basic structure of a magnetometer. The structure or form plays a subordinate role at the beginning of the observation [14]. The measuring cell consists of RF-compatible, copper-coated conductor track materials made of epoxy, ceramic or glass reinforced with PTFE. These circuit boards should have low dielectric losses at high frequencies. The measuring cell also requires a characteristic impedance of Z0 ¼ 50 Ω, since the network analyser also works with this [14].

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Fig. 4.1 Waveguide measuring station with 5.85 . . . 20 GHz shielding effectiveness

Fig. 4.2 A 7/16-mm coaxial measuring cell with 40 MHz . . . 3.8 GHz transmission/reflection

Fig. 4.3 Magnetometer with coaxial cable or directly on the SMA adapter

4.1

Measurement of the Complex Permeability of Nanolayers with One Permeameter

99

This characteristic impedance is determined by the correct ratio of trace width/conductor distance to the outer copper shield and the dielectric constant of εr ¼ 3.38 or 2.5 of the trace material. The so-called conductor of the measuring cell is called a strip conductor in our case. Strip lines are special circuit boards in which the carrier material and the arrangement of the conductor tracks are incorporated into the circuit as electrically effective components (for example, inductors or capacitors). Therefore, the electrical properties of the materials used (for example the permittivity number εr of the carrier material) as well as the dimensions of the conductor path and its tolerances are of particular importance. Strip conductors are used, as in our case, in high-frequency technology. A defined inductance per unit of length dL/dl is generated by a defined conductor width and a defined thickness d and a specified permittivity number εr sets a defined capacitance per unit of length dC/dl. Figure 4.3 shows a typical strip conductor. The rear side of the PCB is metallized throughout, and therefore, the actual stripline is located on the front side. In our case, the PCB is coated with copper as mentioned above. This also means that the tolerances for parallel shift between the front and rear sides are greater, which saves production and testing costs. In addition to the simple strip conductor, there is also the triplate strip conductor. The strip conductor is located in the middle of the dielectric, and the upper and lower sides of the PCB are metallized throughout and are connected to ground. The measuring fork also has such a strip conductor. Since the electric field acts to both sides, the strip line can be narrower. The electric field between inner and outer conductor runs only in the dielectric and is symmetrical. Therefore, a triplate stripline is easier to calculate than a stripline with one-sided dielectric and ground, where the electric field runs partly in the dielectric and partly in the air. Furthermore, the measuring cell consists of a coaxial cable with an SMA socket or only one SMA socket, which must be soldered to the actual measuring fork. The transition from coaxial to stripline must be soldered very carefully and accurately, otherwise too many resonances will occur at this point. Between the coaxial line and the permeameter measuring cell, a copper stabilizing ring should be placed, which forms a firm connection by soldering between the ground of the coaxial line (coaxial outer shielding) and the ground of the coil (copper shielding). The same can be done with the SMA socket, except that no coaxial cable and stabilizing ring are interposed. After completion of the measuring fork, it will be calibrated with the help of a network analyser. The network analyzer with the designation Advantest R3765BH displayed the reflection parameters (S11) in a range from 40 MHz to 3.8 GHz on the monitor.

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4.1.1

4

Shielding Using Nanomaterials

Permeability Measurements

A magnetic nanosample was inserted into the fork. For reproducible measurement, the magnetic effect and the adaptation to 50Ω was controlled via the Smith diagram. The stripline was adjusted using a microwave CAD program (see Figs. 4.4 and 4.5).

Fig. 4.4 Final appearance of the permeameter

Fig. 4.5 Measurement of the complex permeability according to Dr. Seemann. (Source: Karlsruhe Research Centre)

5

LF Shielding

The test setup in Fig. 5.1 has a total dynamic range of 50 dB. This high sensitivity was realized by two preamplifiers. The material SPN11 is measured for comparison EMISONIX (4-mm thick with 80 Ma% ferrite in rubber) (Fig. 5.2). As a high absorption material used for attenuation (with high layer thickness), it is compared with the thin Fe layer (Fig. 5.3). The comparison shows that the attenuation is similarly high. Shielding Rule 1 With magnetic nanolayers, such as iron, shielding attenuation of up to 10 dB can be realized in the LF range up to 30 MHz. Shielding Rule 2 With magnetic laminates, such as iron oxide in rubber, shielding attenuation of up to 25 dB can be realized in the LF range up to 30 MHz. The Fe layer shows a magnetic shielding effectiveness of about 5–10 dB.

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LF Shielding

Fig. 5.1 Measurement procedure for measuring the magnetic shielding effectiveness according to MIL (150 kHz . . . 30 MHz)

Fig. 5.2 Magnetic shielding effectiveness of the reference material EMISONIX (4-mm thick)

5

LF Shielding

Fig. 5.3 Magnetic shielding effectiveness of the nanomaterial 50-nm FE on 30-μm PTFE

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6

Double Shielding

A distinction is made between near-field and far-field shield attenuation. The shielding attenuation also differs according to the type of field: electrostatic shielding, magnetostatic shielding, alternating electric field shielding, alternating magnetic field shielding and electromagnetic wave field attenuation [46]. A shielding wall consists of two interfaces. The electromagnetic wave field is reflected at interface 1 (outer surface of a shielding wall). The radiation passing through this interface is partially absorbed and reflected at interface 2. The reflected part of the wave is now absorbed again and partly reflected at the interface 1. Thus, only a part of the source radiation enters the interior of the housing and is further reflected. There is a double shield [12]. The double shield consists of two partial shields. Is the thickness d1 of the inner shield to be increased rather than the thickness of the outer shield d2 in the sense of an increased magnetic shield factor or should the thickness of the outer shield be increased primarily? Before the double shielding is discussed, some terms are explained using the example of a single shielding. From the Maxwell equations, the wave equation for the H-field component can be represented for a simplified case: The equations now satisfy the condition for describing the magnetic shielding factorS ¼ Hi/Ha. S¼

1 cosh ðk dÞ þ μkAU sinh ðk r Þ

ð6:1Þ

r

This complex description of the approach to a shield calculation of a single shield is important for understanding the calculation of multiple shields according to Schwab [14].

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Fig. 6.1 Magnetic shielding factor of a double shield depending on the thickness of the partial shields. Black colour ¼ low shielding factor, grey colour ¼ medium shielding factor, light colour ¼ high shielding factor, d1 ¼ inner shield thickness, d2 ¼ outer shield thickness

6.1

Double Shielding

According to the presented calculations and the presentation, considerations for multiple shields must be made, especially for double shields. According to Fig. 6.1, the magnetic shielding factor increases more with thickness d1 than with thickness d2. In order to achieve a high magnetic shielding effect, it is more effective to increase the inner shield thickness with a double shield. Shielding Rule 3 Higher shielding values can be achieved with double shielding than with single shielding. Shielding Rule 4 In order to achieve a high magnetic shielding effect, it is more effective to increase the inner shield thickness with a double shield.

7

Polymer Housings

By combining conductive polymers with one or more suitable ferroelectric and ferromagnetic materials, synergy effects of the two groups of materials can be expected through additional absorption mechanisms. When choosing the ferroelectric, it should be noted that the greatest dielectric loss is to be expected near the Curie temperature. Suitable mixtures of ferroelectrics with different Curie temperatures must therefore be investigated. In addition to the composition of the ferroelectric, the dependence of shielding effectiveness and dielectric losses on such influencing variables as grain size, size of the ferroelectric domains, textures, etc. must be investigated. Furthermore, the interactions between the conductive particles in the polymer and the ferroelectric particles regarding the absorption of high-frequency electromagnetic fields are to be investigated theoretically and experimentally from the point of view of material physics. The possibilities of further synergy effects through ferromagnetic materials are to be considered in the work. The rapid increase in the number of electronic components makes it necessary, especially in safety-relevant areas, to shield them against external influences by high-frequency electromagnetic fields and on the other hand to prevent the components from radiating unhindered electromagnetic fields. In addition to metal housings, housings made of various polymers are used in electronics due to their material properties. Plastics generally show a completely insufficient shielding effect. Significant improvements can only be achieved by a suitable modification of the plastics. The modification of plastics by adding suitable fillers or by coating them with suitable materials allows a significant increase in shielding effectiveness. The shielding effectiveness includes the attenuation losses reflection, multiple reflection and absorption. The reflection is mainly determined by the electrical conductivity of the material. The increase of the conductivity of plastics is, on the one hand, state of the art and, on the other, still the subject of intensive research. The current state of the art is

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7 Polymer Housings

• The application of conductive paints on the surface of plastics or • The coating of plastics with metal layers by PVD processes or electroplating The increase of the conductivity of thermoplastics by adding conductive fillers has been known for a long time, but is still the subject of intensive research. This research concerns the selection of suitable fillers, questions of lowering the percolation threshold, as well as production-related problems of injection moulding, extrusion, etc. Among other things, suitable fillers include [50]: • Carbon modifications (fibres, powder, nanocrystalline powder) – Graphite – Conductive soot – Anthracite – Metallic coated graphite – Nanotubes • Metals (fibres, flakes, powders, nanocrystalline powders) – Steel – Copper – Aluminium – Metallic coated metal • Semiconductor (powder, nanocrystalline powder) – Semiconducting oxides (tin oxide, etc.). With increasing concentration of the electrically conductive fillers, the conductivity of the filled polymer increases only slightly at first. In a subsequent very narrow range of the filling degree, the so-called percolation threshold, the conductivity changes by many powers of ten. This strong change in conductivity is due to the formation of current paths by an increasing number of contacting filler particles. Above the percolation threshold, only a small change in conductivity is observed. These effects are comprehensively described within the framework of the so-called percolation theory. To ensure sufficient stability and reproducibility of conductivity, the polymers used in practice have so far been adjusted in the supercolative range. Kalkner and colleagues [5] intensively investigated the possibilities of realizing a stable and reproducible electrical conductivity in the percolation threshold range. The solution to achieve the desired goal consists in broadening the percolation threshold. From an application engineering point of view, the focus of the work was on different mixtures of soot and anthracite coal dust. A completely new way of producing conductive polymers was demonstrated by a research group at Huaqiao University (Huaqiao Universität) in China [6]. They produced a conductive polymer/graphite nanocomposite material by intercalation polymerization. An electrical conductivity of 10 2 S/cm is achieved with a graphite content of 3 Ma%. With the classic polymer/graphite composites, significantly higher graphite concentrations are required for this conductivity.

7

Polymer Housings

109

Investigations on steel fibre filled polymers are among other things the subject of his own work. In addition to the filled electrically conductive polymers, intrinsically conductive polymers are the subject of intensive research. In addition to the development of new intrinsically conductive polymers, the application of these polymers in the field of EMC is becoming increasingly important. Representatives of this type of polymers are polyacetylene, polydiacetylene, polyparaphenylene, polypyroll, polythiophene, etc. At present, however, these polymers are not yet used in practical applications due to their enormously high costs. In addition to the attenuation losses through reflection, in which the electrical conductivity plays a prominent role, the absorption of electromagnetic waves plays a very important role. In the development of shielding materials, however, not enough attention has been paid to these mechanisms for increasing shielding effectiveness. From the point of view of material physics, two loss mechanisms play an essential role: • Magnetic loss mechanisms • Dielectric loss mechanisms. While the magnetic loss mechanisms have already been investigated with regard to increasing the shielding effectiveness by using ferromagnetic powders as fillers, materials with high dielectric losses (e.g. ferroelectrics) have so far received almost no attention. Since ferromagnetic powders as polymer fillers have been the subject of intensive research, this problem will be dealt with in more detail in the next section. References to the use of dielectric loss mechanisms can only be found in the work of Wenderoth [14]. In this work, he points out that the polymers themselves give rise to dielectric losses. However, only polar polymers, which also include intrinsically conductive polymers, show noticeable dielectric losses. The dielectric losses are based on the polarization of the polymers in the electromagnetic alternating field. The polarization is composed of two parts: • Displacement polarization • Orientation polarization. Orientation polarization is particularly responsible for the dielectric losses. However, sufficient shielding effectiveness cannot be achieved by the use of polar polymers alone. Wenderoth therefore proposed adding a ferroelectric to the polymer as filler. Triglycine sulphate and semicarbazide hydrochloride were used. By adding the ferroelectric, a significant increase in shielding effectiveness was achieved. However, these two ferroelectrics are unsuitable for practical use because they decompose at the typical temperature of the injection moulding process. The Curie temperature of suitable ferroelectrics must be in the range of the operating temperature of the composite material. In the Curie temperature TC range, a phase transformation occurs in which the

110

7 Polymer Housings

spontaneous polarization of the ferroelectrics is lost and the material becomes paraelectric. Particularly high dielectric losses are to be expected in this temperature range. A further improvement in shielding effectiveness was achieved by combining a conductive filler and a ferroelectric filler. This leads to synergy effects, which still require comprehensive explanation of material physics [15]. The theoretical starting point for the description of shielding effectiveness is the theory according to Schelkunov. The shielding effectiveness includes the attenuation losses from external level to internal level by considering the partial effects reflection, multiple reflection and absorption. In a conductive material, the attenuation losses due to absorption are essentially due to the formation of eddy currents. In the case of additional filling with a suitable ferroelectric, the electrical hysteresis losses must also be taken into account. Both Wenderoth’s previous investigations and initial own work showed that the measured losses cannot be attributed to these mechanisms alone, further loss mechanisms must be clarified. Among other things, capacitor effects and the so-called Maxwell–Wagner–Sillars polarization [16, 17] come into consideration.

7.1

Previous Material Results

The production and characterisation of new materials, technological processes and housing configurations made of plastic with electromagnetic shielding properties were the subject of further considerations. Based on different theoretical approaches, the main focus of the work was in the field of conductive plastics, taking into account different mechanisms for the absorption of highfrequency electromagnetic waves. Within the scope of this work, first approaches for the application of ferroelectric materials in this polymer composite were developed. The investigations showed that an increase of the RF losses is possible. Figure 7.1 shows the influence of a ferroelectric on the shielding effectiveness of conductive and non-conductive polymers. The synergy effects between conductive polymer and ferroelectric are clearly visible. Figure 7.2 shows the temperature dependence of the shielding effectiveness of a polymer with a ferroelectric.

7.2

Housing Results

In the radar range of 12–18 GHZ, the polymer packaging with the 60% magnetic powder in the ABS has the best shielding effects (Fig. 7.3). Figure 7.4 shows a new housing. Shielding effectiveness values of 1–20 GHz. Shielding Rule 5 Mixing of ferrites, iron, Cu or titanates in polymer housings increases the shielding effectiveness by 30–40 dB. Shielding Rule 6 Mixing of ferrites or titanates in polymer housings minimizes the resonances.

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7 Polymer Housings

Fig. 7.2 Temperature dependence of the shielding effectiveness of polymer/ferroelectric

7.3

Summary

Fig. 7.3 Automotive housing material measurement up to 20 GHz, transmission loss

Fig. 7.4 Image of a new type of automotive housing

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Fig. 7.5 Housing values of shielding effectiveness < 2 GHz

7 Polymer Housings

8

Shielding Example: Inner Lining of a 2.4 GHz Low-Noise Amplifier Housing to Suppress Higher Modes Dr. Barbara Friedmann, Noretec GmbH

Metal housings of amplifier circuits can, depending on the geometry and design of the circuit, favour the emergence of higher modes inside the housing. This can cause undesirable natural vibrations (oscillations) in the amplifier at higher frequencies and thus considerably worsen the noise characteristics in the useful frequency range. The remedy for such a problem is to use interference radiation absorbers inside the metal housing. These damp the multiple reflection of the energy radiated by the circuit, so that the interference radiation can no longer have a negative influence on the circuit itself. Figure 8.1 shows a low-noise amplifier for the frequency range 2.4–2.5 GHz, i.e. for WLAN applications. During the development of the amplifier, it became apparent that the inside of the housing had to be lined with a thin absorber layer to improve the signal-tonoise ratio. This was caused by multiple reflections from higher modes, especially from the third harmonic at 7.2 GHz. A thin-layer absorber from noretec GmbH & Co KG was used as an interference radiation absorber. This thin-film absorber (type 6-3-1-01) has the following specifications: • Thickness: 1.66 mm; material: flexible polyurethane • Centre frequency for maximum absorption: fC ¼ 7.65 GHz • 10-dB bandwidth: 1.86 GHz 15-dB bandwidth: 1.11 GHz 20-dB bandwidth: 0.44 GHz • Manufacturer: noretec GmbH & Co KG. Figure 8.2 shows the result of the absorption measurement on the thin-film absorber 6-31-01. The measurement was carried out with an X-band horn antenna and the Rohde & Schwarz Network Analyzer ZVL at vertical incidence. For the measurement, the absorber was terminated at the bottom with a metal plate. The curve shows the pronounced

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Shielding Example: Inner Lining of a 2.4 GHz Low-Noise Amplifier . . .

Fig. 8.1 2.4 GHz low-noise amplifier, housing lined inside with an interference radiation absorber from Noretec GmbH & Co. KG to suppress higher modes (type 6-3-1-01, thickness 1.66 mm)

absorption maximum at f ¼ 7.65 GHz. Here, absorption of more than 20 dB is achieved, i.e. 99% of the irradiated power is absorbed and only 1% is reflected. The absorber is terminated at the bottom with a metal plate. The reflectivity of the interference radiation absorber 6-3-1-01 was also determined with the aid of a simulation program. Here, the reflection at a 1-layer absorber, which is sealed with metal on the underside, is calculated for the vertical wave incidence. The material parameters ε, tanδε, μ and tanδμ as well as the layer thickness d of the absorber layer determine the reflectivity of the overall structure. Figure 8.2 shows the measured and calculated curve in direct comparison. As this example shows, a thin-layer interference radiation absorber can effectively absorb the radiated power in an amplifier housing. The small thickness of the absorber is achieved by using finely distributed ferrous pigments. This gives the material a relative magnetic permeability of μr > 1 and a loss angle of tanδμ > 0. In the case of the type 6-3-101 presented here, the magnetic permeability in the investigated frequency range is μr ¼ 1.32 and the magnetic loss angle tanδμ ¼ 0.212.

Shielding Example: Inner Lining of a 2.4 GHz Low-Noise Amplifier . . .

117

Fig. 8.2 Comparison of the measured reflectivity on thin-film absorber 6-3-1-01 (light grey curve) with a simulation (dark grey curve). The reflectivity S11 (given in dB) is plotted as a function of frequency f (given in GHz)

9

Metal Housing with Magnetic Materials

New housing materials will be modelled, synthesised and analysed. The aim of these new housings is to improve shielding effectiveness and smooth the internal RF field strengths if electronics with RF sources are present in the housing systems. Electronics inside enclosures should work more reliably if field-absorbing layers and volume materials dampen resonances in the frequency range from 30 to 2000 MHz in enclosures. Since this new approach to ferrimagnetic layers with regard to reflection attenuation is new, theoretical models must be drawn up as a first approximation. Based on the microstructural and structural properties of soft ferrites and ferrite polymers, the RF properties are to be developed with the aim of increasing the shielding effectiveness properties of housings. Technological issues of polymer ceramics and layer structure are to be addressed in the initial stages. The most important volume effects of the conversion of electromagnetic energy in the ferrite material are to be investigated. First indications for a good stratification effect of the conversion of electromagnetic interference energy (EMC effect) in ferritic layers are to be modelled, synthesised and applied. Further research in the field of RF effects in magnetic layers is to be prepared. The main goal of the work is the construction of new housings/optional housing components consisting of the newly developed RF ferrite materials. The usability for EMC is to be tested. Starting point of the considerations for the use of the new material is the approach of a property of each closed and also partly open metal body existing by the regularity of the inner case resonances for each field mode even on real test objects with several drawers. The possibility of using the developed EMC material should continue to be tested. It is important to answer the question to what extent the electromagnetic wave influenced by an excitation of any RF sources (generally sum of monochromatic electromagnetic waves, but

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Fig. 9.1 Housing with source field strength probe

also linear narrow-band waves) of complex resonance impedance can be attenuated in a real housing. The background of this very difficult field theoretical consideration is a practical EMC problem. Electronics, if placed in an open/closed metal housing, are exposed to a greater field exposure than if the same RF source radiation is propagated in free space. Thus, the metal housing itself becomes an “EMC load” due to its physical property of interfacial reflection (interfacial shielding model [42]), its electrical conductivity and resonance indication property. It is the basic approach of the project to investigate the ability of RF-absorbing material to reproduce these physical properties for the EMC of electronics in terms of a reduced inherent interference. A metal power supply with metal housing with and without RF material was used as test object. In contrast to conventional operation, the probe was inserted into a slot on the load part of the power supply unit as an RF source. At the other end in only one slot of the power supply, the rod probe was placed as receiver. The RF material in the sense of a housing option was inserted (a) in only one slot and (b) in all slots. As a preliminary consideration, it could be assumed that it was sufficient to insert the RF material in only one slot, since the field strength is attenuated in the vicinity of the receiver probe. The internal resonances were measured with and without material. The overall setup is shown in Figs. 9.1, 9.2, 9.3, 9.4, 9.5 and 9.6. The following internal field strength measurements (magnetic field) were performed to illustrate the approach of developing new housing materials with the effect of smoothing internal field resonances. The following statements are to be made with subsequent discussion of the differently acting RF effects: Statement 1 The RF attenuation effect is low with only one piece of material in only one slot.

9

Metal Housing with Magnetic Materials

Fig. 9.2 Housing with receiving field strength probe

Fig. 9.3 Internal housing resonance with only one piece of RF material in one slot

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Metal Housing with Magnetic Materials

Fig. 9.4 Internal housing resonance without piece of RF material in a slot

Fig. 9.5 Internal housing resonance with five pieces of RF material in all slots

Metal Housing with Magnetic Materials

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Fig. 9.6 Internal housing resonance without pieces of HF material in all slots

Statement 2 A very significant RF attenuation effect occurs in the total attenuation of all slots with RF material. This is due to the overall internal resonance behaviour of all slots in a housing. This effect is positive for the EMC of power supplies and metal housings. Shielding rule 7 Partial use of absorbing materials also dampens the dips in shielding effectiveness (resonances). Shielding rule 8 The more partial materials are used, the greater the shielding effect. The newly developed material (consideration of a selected number of RF effects in the ferrite) acts on the electronics in terms of passive and non-active EMC protection in open and closed metal housings.

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9.1

9

Metal Housing with Magnetic Materials

Attenuation of Cavity Resonances by Means of Absorbing Magnetic Laminates

The shielding effect of EMC enclosures for electromagnetic waves is based on the reflection of the waves. With the help of electrically conductive seals, housings can be manufactured, which have a high shielding effect. This shielding effect is already so high that thin absorbing layers inside do not contribute to a noticeable increase of the shielding effectiveness. However, the shielding effectiveness of the housing should not be the only criterion for the use of the absorbing layers. Internal EMC, i.e. the influence of the individual electronic modules on each other, must also be taken into account. Paradoxically, in an EMC enclosure, a greater degree of interaction between the components is to be expected, since the interference energy cannot escape to the outside. It is also conceivable that with the same attenuation of the filters in the supply lines, more interference energy is conducted to the outside, because more interference energy can be coupled into the cables inside. The overall system then shows lower values in an interference emission measurement. An improvement of the EMC behaviour of these devices is conceivable with lining by absorbing materials. The comparison of the absorbing effect of different materials is done by means of cavity resonance measurement. Even if no pronounced resonances occur later in the housing filled with electronics, they can be used for qualification in the empty housing.

9.2

Cavity Resonances

The excitation of the cavity resonances is possible both with electric dipole antennas and with magnetic frame antennas. The excitation of the different modes depends on the location of the transmitting and receiving antenna. For these measurements, frame antennas (Fig. 9.7) were used, which were mounted at the edge, because at this position at least the first modes have maxima of the magnetic field. Another advantage of these antennas is that they do not have to be connected to the housing. Fig. 9.7 Arrangement of the antennas in the housing

9.4

Absorbent Material as Insert

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The cavity resonances λR are calculated according to [1]. λR ¼

c0 2 ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  n2 p2ffi fR 2 m þ þ c a b

ð9:1Þ

Here, c0 is the speed of light, a, b and c are the dimensions of the housing. For a ¼ 43 cm, b ¼ 13 cm and c ¼ 23 cm, the resonance frequencies are given in Table 9.1. With an empty housing, the first three resonance points at 735, 945 and 1185 MHz can be seen very clearly (Fig. 9.8), which almost correspond to the calculation. The resonance at 1095 MHz is due to the measurement setup. This can be seen in the figure, because this resonance point was also present when the transmission was measured without the housing. Figure 9.8 shows the cavity resonances with an empty housing without material. Without material, resonances are to be expected, which result in a reduction of the shielding effectiveness. The most important goal of increasing the shielding effectiveness is to reduce resonances and increase the shielding effectiveness by using absorbing magnetic materials. Figure 9.9 shows the transmission loss without housing and material. The statement of this figure is a comparative value.

9.3

Coated Housing

The coating of the housings leads to a significant damping of the resonance points. The effect was more visible with the thicker slip layer. However, this layer is too porous, i.e. it crumbles very easily, and is therefore not suitable for use in housings. Figure 9.10 shows a clear increase in the shielding effectiveness of the housing with absorber material compared to the dark curve of the housing without material. Also, a clear reduction of the resonances can be seen. This figure proves the correctness of the assumption that the shielding effectiveness can be increased by using an absorbing magnetic material. The use of a slip layer is not relevant in practice, as a slip layer is a manual coating.

9.4

Absorbent Material as Insert

Another conceivable alternative is the use of absorbing materials as slide-in units in the housings. This was simulated with the following test setup, where the absorbing materials were fixed. A damping of the cavity resonances can also be determined (Figs. 9.11 and 9.12).

m n p fR [MHz]

1 0 1 740

2 0 1 955

1 1 0 1205

0 1 1 1325

1 0 2 1350

1 1 1 1371

2 1 1 1498

0 1 2 1741

Table 9.1 Resonant frequencies of a housing as a function of the resonance modes m, n, p 1 1 2 1776

2 1 2 1876

3 1 2 2032

2 1 3 2376

0 2 1 2398

1 2 1 2423

126 9 Metal Housing with Magnetic Materials

9.4

Absorbent Material as Insert

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Fig. 9.8 Cavity resonances with empty housing

Fig. 9.9 Transmission of the measurement setup (without housing)

However, a shift of the resonance points also occurs. A typical EMC phenomenon, i.e. the effect of the samples, is largely determined by the position of the slots. In unfavourable cases, a deterioration of the EMC behaviour of the entire system could also occur. The insertion of the absorbing materials causes an attenuation of the cavity resonances. This will usually lead to an improvement when measuring complete electronic systems. Rack-mount technology could also help to solve EMC problems if there is free space in the housing.

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Metal Housing with Magnetic Materials

Fig. 9.10 Cavity resonances with coated housings

Fig. 9.11 Compound foil as insert

9.5

Ferrite-Containing Thick Films for New EMC Metal Housings

The scientific-technological objective concerns the production and testing of RF-absorbing thick films. Technologically simple coating processes were chosen for this purpose. The aim was to use these coatings to meet the highest demands in terms of environmental compatibility and recyclability. The coatings are completely free of pollutants. No pollutants are used during the coating process. A possibility was found to produce completely recyclable layers, i.e. recovered material can be used for the same purpose with relatively little effort. This complies with the requirements of the German Recycling and Waste Management Act (KrW/AbfG § 4).

9.5

Ferrite-Containing Thick Films for New EMC Metal Housings

129

Fig. 9.12 Cavity resonances with insertion of absorbent material

In order to achieve the above-mentioned goals, the development and testing of bindercontaining materials (slurry and filler) containing ferrite powder was carried out. These materials can be used for coating using simple technologies such as dipping, slip casting, spraying and squeegee, whereby a layer is first formed that still contains solvent. The hardening of the layer takes place when the solvent evaporates. The following requirements for the coating properties can be derived from the application requirements of the RF-absorbing coatings: • Sufficient strength and substrate adhesion to allow mechanical processing of coated housing parts • The highest possible volume fraction of the ferrite material in order to achieve a high RF absorption • Coating thickness adjustable within wide limits to ensure flexible adaptation to the requirements of the respective application The following design variants of ferrite-containing thick films were produced and tested:

9.5.1

Slurry Layers

These are layers with a ferrite content up to 30 Ma-%. These were produced from a slurry, which was applied to the substrates used by dipping.

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9

Metal Housing with Magnetic Materials

The slurry contains an organic binder system, which in addition to its binding function contributes to the dispersion of the ferrite particles, and components to adjust the thixotropic behaviour of the slurry. Furthermore, a solvent component is included, with which the flowability of the slip is adjusted. Ferrite powder Zn0.23Mn0.69Fe2.08O4 with an average particle diameter of 1 μm was used for the slip. Ferrite slurry layers were produced in a thickness range between approximately 50 and 500 μm. A clear RF effectiveness of the ferrite slip layers could only be determined (by means of a stripline) in the layer thickness range from 200 to 500 μm. With thinner layers, a (slight) RF effectiveness can only be determined at very high frequencies (>2 GHz).

9.5.2

Slurry Layers with Conductive Coating

Some of the ferrite slip layers were additionally coated with a metallically conductive coating in order to be able to compare the behaviour of HF-absorbing layers with and without electrical conductivity. A patented process for electroless metal deposition was used to apply the conductive layers. With this process, an internal metallization of plastic housings is also possible with comparatively little effort, whereby a ferrite layer can also be applied to the metallization layer. The surface resistance of the Cu layers can be adjusted in a wide range, from under 10 mΩ/W up to several hundred Ω/W. The metallization layers applied on ferrite slip layers in the course of this work have a thickness up to approximately 20 μm. The surface resistance set for these layers was in the range from 1 to 50 Ω/W. Ferrite slip layers, to which an electrically conductive layer (Cu) has been applied by means of chemical deposition, exhibit increased damping of the resonances of the stripline compared to layers of equal thickness without electrical conductivity.

9.5.3

Layers of Ferrite Filler

As a consequence of the results obtained by means of slurry layers, the aim was to increase the thickness of the layer while at the same time drastically increasing the ferrite content. Both objectives were achieved with the help of a ferrite-containing filler with a significantly increased ferrite content of 85–93 Ma-% compared to the ferrite slip. With this filler, layers with a thickness up to about 5 mm can be produced. Ferrite powder Zn0.23Mn0.69Fe2.08O4 in the particle size range of 10–100 μm was used for the filler. A binder system with water as a solvent was chosen, with which the above-mentioned goals of environmental compatibility and recyclability can be met. The processing of this material into layers was carried out by means of doctor blades. Coatings with a binder content of 15 Ma-% have been shown to have sufficient mechanical layer strength and adhesive strength on aluminium substrates for mechanical processing. The binder system used can potentially be adapted to increased requirements

9.6

Ferrite Volume Housing

131

regarding mechanical strength of the layer, adhesive strength and climate resistance by appropriate chemical modifications. A Co-doped ferrite powder Co0.02Zn0.17Ni0.67Fe2.09O4 was used for experiments that served to investigate the influence of the alignment of the ferrite particles in the magnetic field. Due to the presence of the magnetic field of a permanent magnet (NdFeB) during curing, the alignment of layers made of this material took place. The alignment of the ferrite particles along the field lines of the magnet was visually visible. A significant difference in RF loss between these layers and non-aligned layers of equal thickness could not be detected. The described ferrite filler (with ferrite powder Zn0.23Mn0.69Fe2.08O4) was used for the internal coating of table housings. The thickness of the coating was 2–3 mm. Attenuation measurements in the reactive near field were carried out on a partially coated enclosure (one side wall completely and enclosure bottom approx. 1/2 coated). The comparison with an uncoated cabinet showed an increase in shielding effectiveness of around 10 dB in the lower frequency range with the partially coated housing. The significant attenuation of cavity resonances was observed on a completely coated housing compared to an uncoated housing. In order to be able to carry out measurements that provide a high degree of certainty in the assessment of the RF absorption effect of the layers made of ferrite filler, a spherical plastic housing (outer diameter: 26 cm; inner diameter: 25.5 cm), consisting of two hemispheres with flanged joints, was produced and coated. The thickness of the ferrite layer on the spherical surfaces was in the range between 3 and 4 mm. The damping measurements on this housing were carried out using a ball probe. The ferrite layer has a shielding effectiveness in the range from 30 to 1000 MHz, which increases with frequency. The maximum shielding effectiveness is 9 dB.

9.6

Ferrite Volume Housing

By means of the statements from the modelling of the volume and thin film systems, the basic direction in the material design is given. The target property of the new housings with realized polymer or the metal housings with thin ferrite coating (NiZn ferrite or MnZn ferrite) is an increase of the reflection attenuation of the material in the frequency range from 20 to 2000 MHz and a resulting improvement of the shielding effectiveness property. The aim is to create a new generation of EMC enclosures.

132

9.6.1

9

Metal Housing with Magnetic Materials

Ferrite Volume Housings for New EMC-Resistant Automotive Sensor Housings

Automotive angle sensors are subject to the strict requirements of automobile manufacturers. Particularly with regard to EMC requirements, the demands on electronic systems in automobiles are becoming ever higher. In order to meet these requirements, measures must be taken to keep the influence of interfering electromagnetic fields away from the respective electronic components (Fig. 9.13). Until now, a conductive plastic made of PA66 with 40% carbon fibres has been used for the sensor housings. In order to determine the characteristic values of the various developed material samples, test samples were produced. For the first preliminary investigations, plates (Fig. 9.14) and waveguides (Fig. 9.15) were used. After an improvement in the EMC properties of the newly developed ferrite–polymer compound could be demonstrated, the next step was to adapt the other plastic properties. For this purpose, housing parts close to the sensors were injection moulded from a ferrite– polyamide compound (Fig. 9.16). Measurements have confirmed the improved EMC properties of this plastic. Since the polymer used to bind the ferrite powder has no influence on the EMC properties according to the measurement results, the other properties of the plastic (mechanical, thermal, chemical, etc.) can be adjusted within a certain range by selecting the polymer accordingly. A strategy has been defined for the assessment of RF properties after field modelling and material structure analysis. The stripline method with an adapted arrangement was selected as the measurement method for the RF characterization of small samples. The reflection loss is determined by the S-parameter measurement. The evaluation of the shielding effectiveness d of the complete polymer or metal housings is realized according to the VG standard (defence standard). In contrast to the polymer–carbon fibre housings, the polymer–ferrite housing has a higher magnetic absorption loss. x-axis: y-axis:

Frequency Shielding effectiveness, ref. at 90%

It has been possible to improve the EMC properties of the polymer–carbon fibre material. By using a polymer–ferrite material with RF-absorbing properties (increased μ00-permeability loss), the shielding effectiveness of the polymer material could be improved. Thus, the prerequisites for an automotive sensor housing (Fig. 9.16) with better EMC behaviour for the electronics inside the sensor were created (see Figs. 9.17, 9.18, 9.19 and 9.20).

9.6

Ferrite Volume Housing

133

Fig. 9.13 Angle of rotation sensor with conductive plastic housing made of PA66 with 40% CF

Fig. 9.14 Plate (t ¼ 5 mm)

134

9

Metal Housing with Magnetic Materials

Fig. 9.15 Waveguide (t ¼ 5 mm)

Fig. 9.16 Sensor housing part made of ferrite–polymer compound. (Source: [47])

9.6

Ferrite Volume Housing

135

Fig. 9.17 600 . . . 1000 MHz, ref. 45 dB. Upper curve: shielding effectiveness of the polymer– carbon fibre housing, lower curve: shielding effectiveness of the polymer–ferrite housing

Fig. 9.18 600 . . . 1000 MHz, ref. 0 dB. Difference in shielding effectiveness (improvement of ferrite compared to carbon fibre approx. 3–5 dB)

Shielding rule 9 The material polymer with ferrite mixture has a higher shielding effectiveness than a polymer–carbon fibre mixture.

136

9

Metal Housing with Magnetic Materials

Fig. 9.19 80 . . . 600 MHz, ref. 40 dB. Upper curve: shielding effectiveness of the polymer–carbon fibre housing, lower curve: shielding effectiveness of the polymer–ferrite housing

Fig. 9.20 Measurement setup and novel polymer–ferrite housing

The object of measurement were metal subrack housings in the following variants, as shown in Fig. 9.21a–d. The measurements on the housings were performed as comparative measurements between an original housing and housings of the respective lining variant in accordance with standard VG 95373 T15.

9.7

Results of the Shielding Effectiveness Measurements

137

Fig. 9.21 (a) Housing with 1- or 1.5-mm layer (absorber powder in lacquer as carrier); (b) original housing as reference; (c) housing with 3-mm EMISONIX; (d) housing with nanolayer on copper foil

9.6.2

Principle of Comparative Shielding Effectiveness Measurement

The results of the shielding effectiveness measurements on the housings are shown in Figs. 9.22, 9.23 and 9.24.

9.7

Results of the Shielding Effectiveness Measurements

For the equipment variants with 1.5-mm layer or 3-mm EMISONIX, shielding effectiveness improvements up to approximately 10 dB are possible. When equipped with EMISONIX, the magnetic shielding effectiveness is improved by more than 10 dB, especially in the lower frequency range, and the internal resonances of housing and the associated shielding effectiveness are minimized. The equipment with absorbing nanolayer does not bring any advantages in the considered frequency range that justify the effort. The otherwise proven effect in the GHz range

138

9

Metal Housing with Magnetic Materials

Fig. 9.22 Measurement setup of the shielding effectiveness measurement according to VG 95373 T15

Fig. 9.23 Shielding effectiveness of 30 MHz . . . 1 GHz

would have to be investigated for the housings—then also on non-conductive carriers in order to rule out distortion by the copper foil (Fig. 9.25). Figure 9.26 shows the transmission loss of a non-conductive absorber layer. In the given frequency range, the absorption of this nanolayer is greater than that of a coating. Figure 9.27 shows the shielding effectiveness of a piece of EMISONIX laminate without metal foil and aluminium adhesive. It is noticeable that in the lower area up to 400 MHz, the effect is comparatively low (lower curve). Good values are achieved from 800 MHz and very good values from 2000 MHz. In the middle and upper curve, there is

9.7

Results of the Shielding Effectiveness Measurements

139

Fig. 9.24 Magnetic shielding of 10 kHz . . . 30 MHz

Fig. 9.25 Transmission loss of a magnetic nanolayer

Fig. 9.26 Transmission loss of a thick film

also good shielding effectiveness from 400 MHz and also otherwise a higher shielding effectiveness than the laminate alone. This can be explained by the fact that the metal adhesive tape makes a large contribution to the shielding effectiveness.

140

9

Metal Housing with Magnetic Materials

Fig. 9.27 Material characteristics: EMISONIX

Figure 9.28 shows the effect of an absorbing nanolayer in the frequency range 1 GHz. According to this, the new EMC textile can be used for applications in information technology, communication technology, industrial safety textiles in the medical sector and other higher frequency areas.

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171

172

12

Textile Shielding Material

Fig. 12.1 Transmission (ref.: 0 dB) of an RF textile with embedded particles; the embedding was carried out using a coating technique, particle size 40 μm Textile single layer with nanoparticles

0

–1.5

–3.0

–4.5 6500

7500 f / MHz

Fig. 12.2 Transmission (ref.: 0 dB) of a textile with dispersed magnetic nanoparticles Fe2O3 (grain size D50 ¼ 9 nm), powder filling degree 10 Ma-% in the textile

The nanopowder dispersed in the textile could not prove any particular advantages over the micropowder in this frequency range. The target parameters in Tables 12.1 and 12.2 should be aimed at. Textile (single-layer, multi-layer, Table 12.1) Due to the small number of breakouts, the material itself has a higher RF attenuation than an application prototype—clothing.

12.1

Summary/Outlook

173 Textile single layer with micropowder

0

–2

–4

–6

–8

–10

6500

7500 f / MHz

Fig. 12.3 Transmission (ref.: 0 dB) of a textile with dispersed magnetic nanoparticles MnZn ferrite (grain size D50 ¼ 50 μm), powder filling degree 10 Ma-% in the textile Textile single layer with micropowder

0

–1.5

–3.0

–4.5 6500

7500 f / MHz

Fig. 12.4 Transmission (ref.: 0 dB) of a textile with dispersed magnetic nanoparticles MnZn ferrite (grain size D50 ¼ 50 μm), powder filling degree 30 Ma-% in the textile

Garment (pullover, Table 12.2) An even greater EMC effect is assumed to occur with multilayer textiles. In the future, prototypes of large-area textile mats or capes/pullovers will be developed and produced. Special attention will be paid to the protection against possible toxic effects of the nanopowders.

174

12

Textile Shielding Material

Fig. 12.5 Microscopic image of a new type of EMC textile

Table 12.1 Technical target data of a single-layer textile piece Target parameters Shielding effectiveness Reflection attenuation (absorption) Diagonal tensile load Abrasion resistance according to DIN EN 12947–1 Industrial safety, introduction Occupational safety experiments (genotoxic, cytotoxic)

Value 50 dB, 30–2000 MHz 5–15 dB (note: 20 dB attenuation corresponds to a %attenuation of 90%) 2–9 GHz is therefore important for future electronics [48–50], because today’s EMC suppression ferrites are not designed for PC technology with CPU frequencies from 2 to 3 GHz, for example. Figure 14.2 shows the application frequency of known EMC interference suppression materials. New types of EMC interference suppression materials for the years 2013–2020 serve as a supplement to an existing metal or metal layer, which lead to multiple reflections of RF radiation. Or they are available as sintered materials such as SMD ferrites/folding ferrites. The reflections can cause local field strength increases within electronic devices and lead to interference. The disadvantages of existing EMC materials in 2013 will be improved with newly developed EMC interference suppression materials (no metals) and these materials operate in the frequency ranges that will play an increasingly important role from 2013 onwards. This can be seen in Fig. 14.3. The background to this is to obtain a drastic attenuation provided by the magnetic EMC interference suppression materials in the form of absorption of the multiple reflection and thus reduce it, distribute the heat evenly and thus render it harmless for the operation of the equipment.

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179

180

14

Basic Problem of Today’s EMC Ferrite Interference Suppression Materials

Fig. 14.1 EMC interference field strength of a modern EUT, interference frequencies at 2.2, 2.6 and around 5 GHz. (Source: Measurement of the IMST in Kamp Lintfort)

14.1

Introduction

The increasing operation of electronic high-frequency device leads to an increasing RF exposure, which has to be taken into account in the development of novel devices and information technology equipment. This can be achieved by using high-frequency absorbing materials. This can increase the operational safety and reduce the risk of conscious or unconscious interference (EMC) of necessary components. These RF reflections on metallic surfaces can be reduced by using plate material with the appropriate effect [49]. As a result, the radio interference field strength on modern information equipment (PC with CPU 3 GHz, industrial PC with 2 GHz, GPS, tablet, . . .) will be at harmonics of 3 GHz, at frequencies of >6 to 9 GHz. In these frequency ranges, the existing ferrite interference suppression materials are not present. The final frequencies of the best sintered ferrites (e.g. from Würth) are at 1–2 GHz. The problem is that for the new EMC problems of information technology, according to the standards of EMC, there are no EMC materials for absorption available on the market. These hexagonal ferrite materials serve as a supplement to an existing metal or metal layer, which leads to multiple reflections of RF radiation [51]. The reflections can cause local field strength increases within electronic devices and lead to interference.

14.2

Theoretical Considerations

181

Fig. 14.2 Insufficient frequency bandwidths of 2013 existing EMC materials for interference >2 GHz (CPU, higher frequency crystals, internal oscillators), resonant frequency ¼ 500 MHz

An existing EMC soft ferrite material acts as soft ferrite even up to the highest frequency ranges such as around 100 GHz, but only with 10 dB absorption, since the resonant frequency of the soft ferrite is not sufficient for the future frequencies.

14.2

Theoretical Considerations

The theoretical considerations should show the microstructure property dependence of the new ferrite EMC components. The structure image (Fig. 14.4) shows the simplified crystal structure of a new type of hexaferrite EMC material. Examples of this hexagonal structure are W-type, Y-type and Z-type ferrites. The anisotropic structure is clearly visible. There is a preferred crystallographic axis [52]. For ferrites of this crystal type, the anisotropy constant K1 is much larger than for soft ferrites.

182

14

Basic Problem of Today’s EMC Ferrite Interference Suppression Materials Effectiveness of existing EMC ferrite and future EMC ferrite

120

Effectiveness in %

100 80 60 40 20 0 0

1000

2000

3000

4000

5000

6000

f / MHz

Fig. 14.3 Presentation of the effectiveness of conventional EMC materials and future EMC materials (yellow line—effectiveness of current EMC interference suppression materials in 2013, blue line—EMC interferers of the years 2014–2017, red line—EMC interferers of the years 2017–2020, green line—effectiveness of future EMC materials of the years 2013–2020)

Fig. 14.4 Crystal structure of a hexagonal ferrite—structure of EMC future ferrites. (Source: [55])

Representatives of these future ferrites are barium ferrites, strontium ferrites and, for example, cobalt ferrites with the different mixed forms and stoichiometries [53]. The quantities M (magnetic moment, M0 saturation magnetisation), Heff (internal and external magnetic field strengths), μ (permeability ¼ tensor), X (magnetic susceptibility) and B (magnetic flux density) are complex, time-, frequency- and location-dependent quantities. The Landau–Lifschitz Eq. (14.1) with damping term describes the absorption behaviour of ferrites in a very simplified way, for example: ! !
! !  ∂M α ! ∂M ¼ M  H eff  M M0 ∂t ∂t

! ð14:1Þ

After complex forming steps, the system of equations in Eqs. (14.2)–(14.4) applies:

14.2

Theoretical Considerations

183

jωM x ¼ 2ω0 M y þ 2ωm H y

ð14:2Þ

jωM y ¼ 2ω0 M x  2ωm H x

ð14:3Þ

  jωM z ¼ γ M y H y  M y H x

ð14:4Þ

Using: !

$!

B ¼ μH

γ ω

ð14:5Þ

Gyrotropy constant Angular frequency.

The absorption of novel hexaferrite materials as a function of the degree of substitution and different hexaferrites with different structures and the resulting resonant frequencies ω0 can be described as follows. If the lattice of Co–Ti-substituted barium–Sr hexagonal ferrites is distorted in such a way that a stronger anisotropy is produced, the anisotropy frequency and thus the frequency of application of the EMC ferrites increase [52]. The absorption dependence of a novel ferrite compound on the degree of filling, resonance frequency and particle shape of the ferrite crystal can be seen in the Maxwell– Garnett formula [54]. Maxwell–Garnett mixing law according to [54]: X eff X incl ¼p 1 þ n X eff 1 þ n X incl p Xeff Xincl n

ð14:6Þ

Volume fraction of ferrite in compound Effective magnetic susceptibility of the compound Magnetic susceptibility of ferrite Form factor of the ferrite.

As a result of the theoretical considerations, the following theses on new EMC suppression ferrites to be synthesized for the future can be identified: 1. Degree of substitution/resonance frequency: A high resonance frequency of the ferrite is necessary for good EMC interference suppression properties. For this purpose, a degree of substitution in the crystal should be aimed for in the hexaferrite of the future, which causes a high anisotropy of the crystal. This is only possible with hexaferrites compared to today’s soft ferrites.

184

14

Basic Problem of Today’s EMC Ferrite Interference Suppression Materials

Fig. 14.5 Transmission loss in the waveguide, barium ferrite polymer film without MnZn ferrite, effect ¼ 6 dB. (Source: Innovent Jena)

Fig. 14.6 Transmission loss in the waveguide, barium ferrite polymer film with MnZn ferrite, effect up to 12 dB. (Source: Innovent Jena)

2. Volume fraction: In a mixture of substances, for example a compound, the highest possible volume fraction should be ferrite.

14.3

Experimental Investigation

In the experimental investigations, the RF losses in non-electrically conductive polymer ferrite films were determined with transmission loss measurements on a waveguide. The transmission loss measurements were performed from 5.8 to 8.2 GHz. Figures 14.5, 14.6 and 14.7 show the attenuation of the novel hexaferrite EMC materials. A barium ferrite polymer film (Fig. 14.5) as a new type of EMC interference suppression material for the higher frequencies has an RF loss of around 6 dB. Thickness of the material was only 1 mm. From the red reference line, the measured value can be read downwards into the minus dB range. The evaluation of the experiments shows that all types of films with mixed-in novel EMC hexaferrite have reproducible good attenuation values in the high-frequency ranges from 5000 to 8000 MHz. The optimal mixture was a material with hexaferrite powder mixed with MnZn ferrite. This material showed a good effect of 12 dB at a film thickness of 1 mm.

14.3

Experimental Investigation

185 Transmission loss

0

T / dB

–5

–10

–15

6000

7000

f / MHz

8000

Fig. 14.7 Transmission loss in the waveguide (curve with rectangles—barium ferrite polymer film with MnZn ferrite, curve with circles—barium ferrite polymer film without MnZn ferrite, reference line 0 dB) S11 loss 15 10 5

Loss [dB]

0 1.00E+09

1.00E+08

–5

02101_02.CSV 02101_03.CSV

–10 –15 –20 –25 –30

frequency [Hz]

Fig. 14.8 S11 loss attenuation of the samples from Fig. 14.7. (Source: Würth Electronic measurement)

Measurements up to 20 GHz even showed damping effects of very good result of up to 20 dB. The results of the S11 and S12 measurements are shown in Figs. 14.8 and 14.9.

186

14

Basic Problem of Today’s EMC Ferrite Interference Suppression Materials S12 loss

0

1.00E+09

1.00E+08

1.00E+10

–2 –4

Loss [dB]

–6 –8 02101_02.CSV

–10

02101_03.CSV

–12 –14 –16 –18 –20

frequency [Hz]

Fig. 14.9 S12 loss attenuation of samples from Fig. 14.7. (Source: Würth Electronic measurement)

14.4

Summary: Novel Hexaferrites of the Future

In this article, the necessity of the use of new EMC ferrites has been proven, because on the market, the existing soft magnetic ferrites—because of the crystal structure—do not have the possibility to reach the EMC-interferers of the future in frequency ranges >3000 MHz, that novel EMC interference hexaferrites have to be used. With theoretical considerations, it was proven that a special degree of substitution is important (the result should be a high anisotropy and thus a high frequency of use) and that a high degree of filling of hexagonal ferrites is necessary for this. First experimental investigations showed good attenuations of a very thin film of 1-mm thickness with 12 dB when using a novel EMC hexaferrite. The comparative sample of the ferrite mixture of barium hexaferrite and MnZn ferrite is interesting. A higher RF loss can be observed even in the interesting frequency range from 5000 to 8000 MHz of this ferrite mixture compared to the pure hexagonal ferrite. This gives a very wide range of users and applications: PC technology, DSP, Gbit switch technology, WLAN radio applications, industrial PC technology, etc. It is urgently necessary to find the transition from the soft magnetic EMC ferrites, which are no longer effective in all areas of technology today, to the new EMC hexaferrite presented.

Appendix: Shielding Formulas

15

S¼AþRþM

ð15:1Þ


 2 d as ¼ 20lg 1 þ μr 3 ri

ð15:2Þ


 2 d as ¼ 20lg 1 þ εr 3 ri

ð15:3Þ

    1 2   as ¼ 20 cosh ðkd Þ þ K þ sinh ðkd Þ 3 K

ð15:4Þ

References: [56, 63]. Spherical shield

Reference: [62]. Spherical shield

Reference: [62]. Spherical shield

Reference: [57].  d as ¼ 20lg eδ

ð15:5Þ

with: # Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2021 F. Gräbner, EMC-Compatible Shielding, https://doi.org/10.1007/978-3-658-33189-4_15

187

188

15 Appendix: Shielding Formulas

1 δ ¼ pffiffiffiffiffiffiffiffiffiffi πf μk and: μ ¼ μ0 μr Reference: [62]. Real shielding of a technical structure with discontinuities S ¼ A þ R þ B þ K1 þ K2 þ K3

ð15:6Þ

Reference: [57]. A½dB ¼ 131:4d

pffiffiffiffiffiffiffiffiffiffiffiffi f μr σ r

ð15:7Þ

Reference: [57]. If A < 10 dB, then the multiple reflection attenuation gets a higher significance. B½dB ¼ 20lg j 1  e2d

pffiffiffiffiffiffiffi

μπf σ 2jd

pffiffiffiffiffiffiffi

e

μπf σ

j

ð15:8Þ

Reference: [57]. This shielding effectiveness formula applies to thin layers. The surface conductivity is determined with area measurements. S½dB ¼ 20lgð1 þ 0:5dZ 0 σ Þ

ð15:9Þ

Reference: [59].

15.1

Basic Law of Electromagnetic Shielding According to Schelkunov S¼AþRþM

S A R M

Total shielding effectiveness of a material Attenuation by absorption of the electromagnetic wave Attenuation by reflection of the electromagnetic wave on the material Attenuation through multiple reflection in the material.

ð15:1Þ

15.4

Shielding Against Quasi-Static Magnetic Fields

189

References: [56, 63].

15.2

Shielding against Magnetostatic Fields

Spherical shield
 2 d as ¼ 20lg 1 þ μr 3 ri as d ri

ð15:2Þ

Shielding effectiveness Shield thickness of the spherical shield Inner radius. Reference: [62]. Figure 15.1 shows a very high shielding effectiveness.

15.3

Shielding Against Electrostatic Fields

Spherical shield
 2 d as ¼ 20lg 1 þ εr 3 ri as d ri

ð15:3Þ

Shielding effectiveness Shield thickness of the spherical shield Inner radius. Reference: [62].

15.4

Shielding Against Quasi-Static Magnetic Fields

Spherical shield     1 2   as ¼ 20 cosh ðkd Þ þ K þ sinh ðkd Þ 3 K

ð15:4Þ

190

15 Appendix: Shielding Formulas

Fig. 15.1 Example of outstanding shielding effectiveness with a zinc coating

as d k

Shielding effectiveness Shield thickness of the spherical shield Wave number. Reference: [57].

15.5

Shielding Against Alternating Magnetic Fields (Skin Effect)  d as ¼ 20lg eδ

with: 1 δ ¼ pffiffiffiffiffiffiffiffiffiffi πf μk and:

ð15:5Þ

15.7

Absorption Loss

191

μ ¼ μ0 μr as d δ k f

Shielding effectiveness Shield thickness Penetration depth Electrical conductance Frequency. Reference: [62].

15.6

Extended Shielding Law According to Schwab

Real shielding of a technical structure with discontinuities S ¼ A þ R þ B þ K1 þ K2 þ K3 S A R K1 K2 K3

ð15:6Þ

Total shielding effectiveness of a material Damping of a single opening Correction term for multiple reflections Correction term for the number of openings Low-frequency correction Correction term for radiation couplings between the openings. Reference: [57].

15.7

Absorption Loss A½dB ¼ 131:4d

A f d μr σr

Absorption loss Frequency Thickness of the material Relative permeability Relative electrical conductivity. Reference: [57].

pffiffiffiffiffiffiffiffiffiffiffiffi f μr σ r

ð15:7Þ

192

15 Appendix: Shielding Formulas

15.8

Multiple Reflection Attenuation

If A < 10 dB, then the multiple reflection attenuation gets a higher significance. B½dB ¼ 20lg j 1  e2d B f d μr σr

pffiffiffiffiffiffiffi

μπf σ 2jd

pffiffiffiffiffiffiffi

e

μπf σ

j

ð15:8Þ

Multiple reflection attenuation Frequency Thickness of the material Relative permeability Relative electrical conductivity. Reference: [57].

15.9

Shielding Effectiveness as a Function of Surface Conductivity

This shielding effectiveness formula applies to thin layers. The surface conductivity is determined with area measurements. S½dB ¼ 20lgð1 þ 0:5dZ 0 σ Þ S d σ Z0

ð15:9Þ

Shielding effectiveness Thickness of the material Surface conductivity ¼377 Ω. Reference: [59].

15.10 Extended Shielding Effectiveness Law According to Perumalraj and Dasaradan [58] Considering Real Apertures of Wire Constructions of Real Shields with Holes The extended shielding effectiveness law according to Perumalraj and Dasaradan [58] refers strictly speaking only to highly conductive textiles, but can also be extended to general highly conductive technical complex surfaces.

15.11

Shielding of Holes and Apertures

S ¼ A þ R þ B þ K1 þ K2 þ K3 A d w R

f B

K1

a n K2

p K3

193

ð15:10Þ

Attenuation caused by partial discontinuities in dB  A ¼ 27:3 wd dB Depth of an opening in cm Width of an opening in cm, parallel to the E-field vector Reflection attenuation term of an opening as a function of the impedance of the incident wave and the area of the opening R ¼ 20 lg ((1 + 4K2)/4K ) K ¼ j6.69  105fW for flat waves and rectangular openings Frequency in MHz Multiple reflection   Þ2 A=10 B ¼ 20lg 1  ððK1 2 10 Kþ1Þ A < 15 dB Correction term for a large number of openings Applies to a large distance RF source from the rectangular opening. K1 ¼  10 lg (an) in dB Area for each opening in cm2 Number of holes per cm2 Damping caused by the skin depth If the respective skin depth comes close to the diameter of the wire/textile, this effect is effective.   2 K 2 ¼ 20lg 1 þ 35p3 in dB Diameter of a wire mesh or conductive textile fabric/skin depth Damping caused by coupling between the holes/openings of a surface The coupling between periodic holes and an opening in the surface is considered. The condition for this correction term is that the depth of the periodic holes is small compared to the opening in the surface.   A K 3 ¼ 20lg coth 8:686 in dB Reference: [58].

15.11 Shielding of Holes and Apertures (a) Diameter of a hole d > thickness of the material t

194

15 Appendix: Shielding Formulas




λ Single hole : S ¼ 20lg 2d

t d n






 λ Multiple hole : S ¼ 20lg  10lgðnÞ 2d

ð15:11Þ

Thickness of the material Diameter of the hole Number of holes.

(b) Diameter of a hole/slot d > thickness of the material t. Applies according to waveguide principle A ¼ 27:3ðt=wÞ w A

ð15:12Þ

Length of a slot Absorption loss. Reference: [60].

15.12 Near-Field Reflection Loss R on a Plane Plate (a) E-fields R ¼ 20lgð4500=ðr f Z ÞÞ r Z

ð15:13Þ

Distance, r < λ/6 Impedance of the shielding material.

(b) H-fields R ¼ 2rf =Z r Z

Distance, r < λ/6 Impedance of the shielding material. Reference: [61].

ð15:14Þ

15.13

Law of Shielding Effectiveness Considering the Waveguide Effect and . . .

195

15.13 Law of Shielding Effectiveness Considering the Waveguide Effect and Apertures According to Tee Tang [61], Near Fields, Far Fields (a) E-near-field/far-field reflection 0

1

B120  π λ C C R½dB ¼ 20 log 10 B @ 4Z s  2π r A |ffl{zffl} |fflfflffl{zfflfflffl} Far field

ð15:15Þ

Near field

(b) H-near-field/far-field reflection 0

1

B120  π 2πrλ C C R½dB ¼ 20 log 10 B @ 4Z s  λ A |fflfflffl{zfflfflffl} |ffl{zffl} Far field

ð15:16Þ

Near field

In Fig. 15.2, the aperture effect is visible, and in Fig. 15.3, an absorption effect for waveguides with apertures. At real apertures on honeycomb filters with N (number of holes), l (diameter of the honeycomb holes) and t (depth of the honeycomb holes, t  3  l) S½dB ¼ A þ R þ Rr S[dB] A R Rr

Shielding effectiveness Absorption damping Reflection loss Multiple reflection.

A½dB ¼ 131:4t Far field Near field E Near field H

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð f μr σ r Þ

R½dB ¼ 168  10lg½ f ðμr =σ r Þ R½dB ¼ 20lg½4500=ðrfZ S Þ h i R½dB ¼ 20lg 2rf ½MHz =Z S Rr ½dB ¼ 20lg½1  exp ðf2t=δgÞ

δ

Skin depth.

ð15:17Þ

196

15 Appendix: Shielding Formulas

Shielding Effectiveness, dB

200

150

ion

Hi-Z E-field reflection

rpt

o bs

A

100 far-field reflection 50 Low-Z E-field reflection 0

1 MHz

1 kHz

1 GHz

10 GHz

Fig. 15.2 Near-field and far-field reflection. Shielding effectiveness under consideration of the waveguide/aperture effect. (Source: According to Tee Tang)

Shielding Effectiveness, dB

200

150 Hi-Z E-field reflection

ion

rpt

o bs

A

100 Waveguide effect

50 Low-Z E-field reflection 0

1 kHz

1 MHz

1 GHz 10 GHz

Fig. 15.3 Shielding effectiveness considering the waveguide theory and the apertures. (Source: According to Tee Tang)

Apertures S½dB ¼ 20lgðλ=2lÞ  10lgðN Þ þ 27:3t=l Reference: [61].

15.14

Cut-off Frequency of Length and Depth of Rectangular Structures (Waveguide Like) 197

g = gap (aperture dimension) d = depth (distance that fields have to travel)

Shielding effectiveness (SE) in dB 80 d=18” g=6”

60

d=12” g=6”

40

20

0 0.05

d=6” g=2”

d=4” g=2”

d=6” g=6”

0.1

0.2

0.5

d=2” g=2” 1

2

GHz 5

Fig. 15.4 Dependence of the drop in shielding effectiveness on the width and depth of slots in housings. (Source: According to Radu [66])

15.14 Cut-off Frequency of Length and Depth of Rectangular Structures (Waveguide Like) A cut-off frequency is a limiting frequency from which the shielding effectiveness drops sharply to zero. In waveguide geometries (metal housings with slots and entries), this cut-off frequency depends on the width of the slots and the depth of the entry [66]. The drop in shielding effectiveness as a function of the width of the slot and the depth of the insertion is shown in Fig. 15.4. According to Radu [66]:

fc SE g d

Cut-off frequency Shielding effectiveness Width of the slots Depth of the insertion.

f c ½GHz ¼ 5:9=g½inch

ð15:18Þ

SE½dB ¼ 27:2d=g

ð15:19Þ

198

15 Appendix: Shielding Formulas

15.15 Corner Effect In the vicinity of peaks outside of housings, there is a field strength increase of the RF field [65]. On the other hand, inside of housings near the corners, there is a minimum of field strength and also of shielding effectiveness.

15.16 Transient Shielding Effectiveness In addition to the sinusoidal harmonics of the radio interference field strength, it is also possible that the interference sources have a pulse character. The pulse fields should also be shielded to prevent interference with electrical/electronic systems. According to Herlemann and Koch [64], the transient shielding effectiveness is to be calculated as follows: 0

1 R1 2 S ω dω  AdB SE ¼ 10 log 10 @R 1 0 2 Es H 2s 2 S ω þ dω 2 2 0 E H 2

u

S Es Eu Hs Hu SE

ð15:20Þ

u

Spectral density distribution Electric field strength with shield Electric field strength without shield Magnetic field strength with shield Magnetic field strength without shield Transient total shielding effectiveness. Figure 15.5 shows a magnetic and electrical shielding of a pulse excitation source.

15.17 Shielding Rules Shielding rule 1 With magnetic nanolayers such as iron, shielding effectiveness of up to 10 dB can be realized in the LF range of up to 30 MHz. Shielding rule 2 With magnetic laminates such as iron oxide in rubber, shielding effectiveness of up to 25 dB can be realized in the LF range of up to 30 MHz.

15.17

Shielding Rules

199

Fig. 15.5 Magnetic and electrical transient shielding. (Source: According to Koch/Herlemann [64])

Shielding rule 3 Greater shielding effectiveness can be achieved with double shielding than with single shielding. Shielding rule 4 In order to achieve a high magnetic shielding effect, it is more effective to increase the inner shield thickness with a double shield. Shielding rule 5 Mixing of ferrites, iron, Cu or titanates in polymer housings increases the shielding effectiveness by 30–40 dB. Shielding rule 6 Mixing of ferrites or titanates in polymer housings minimizes the resonances. Shielding rule 7 Partial use of absorbing materials also dampens the dips in shielding effectiveness (resonances). Shielding rule 8 The more partial materials are used, the greater the shielding effect. Shielding rule 9 The material polymer with ferrite mixture has a higher shielding effectiveness than housing of a polymer–carbon fibre mixture.

200

15 Appendix: Shielding Formulas

Shielding rule 10 A metal housing with a laminate inner lining with rubber ferrite has very good damping from 1 to 2 GHz. Shielding rule 11 If absorbing PCB interlayers, such as ferrite, with a μm thickness are used, the attenuation is increased. Shielding rule 12 If a metal outer surface of a cable is coated with a ferrite layer, the shielding effectiveness is increased. Shielding rule 13 If a conductive or ferrite-coated textile is used in several layers, the attenuation is increased. Shielding rule 14 If the mesh size of a metal mesh braid is reduced, the shielding effectiveness is increased. Shielding rule 15 If high-energy pulsed magnetic fields are shielded with, for example, highly permeable materials, the shielding effectiveness is reduced because the outer layers of the shield are saturated. Shielding rule 16 The wider the slot and the shallower the depth of the slot in housings according to Eq. 15.20 in Sect. 15.16, the lower the cut-off frequency and the lower the shielding effectiveness. When designing housings, a slot should therefore be as narrow as possible and the depth of the insertion should be as great as possible. Shielding rule 17 Inside metal housings, there are shielding dips near the corners. At these locations, there is a minimum of shielding effectiveness and there should be no sensitive components and assemblies inside housings near the corners. Shielding rule 18 For transient shielding effectiveness, the basic rules of shielding such as thickest possible shielding material, use of electrically conductive shielding material for electrical interference field strengths, use of highly permeable shielding material for magnetic interference field strengths and the geometric rules such as few openings and slots in housings must be observed.

References

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48. Ötzgür, D. et al.: submitted to Journal of Material Science Materials in Electronics 2009 „Microwave Ferrites“, part 1 49. Sing, C.: Hysteresis analysis of Co-Ti substituted M-Type Ba-Sr hexagonal ferrite. Materials Letters 63 (2009) 1921–1924 50. Dishovsky, N.: Rubber Based Composites Witch Active Behavior To Microwaves. Journal of the University of Chemical Technology and Metallurgy, 44, 2, 2009 51. Hiroyasu Ota: Broadband Microwave Absorber Using M-type hexagonal Ferrites, IEEE 1999, 0-7803-5057-X/99 52. Sing, C.: Electromagnetic Properties of CoZr Substituted BaSr Ferrite Parafin Wax Composite for EMI/EMC Applications. 978-1-4244-6051-9/11/2011 IEEE 53. Rozanov, I.; Koledintseva, M.: INTECH 2012, https://doi.org/10.5772/48769 54. Rapp, U.: Hexagonale Ferrite-Strukturen, 2012, http://www.ulrich-rapp.de 55. http://www.img-nordhausen.de 56. Gräbner, F.: EMV gerechte Schirmung. Springer Vieweg, 2012 57. Schwab, A.; Kürner, W.: Elektromagnetische Verträglichkeit. Springer Verlag, 2007 ISBN 978-3-540-42004-0 58. Perumalraj, R.; Dasaradan, B.: Electromagnetic Shielding Effectiveness of Doupled CopperCotton Yarn Woven Materials, Fibres & Textiles in Eastern Europe 2010, Vol. 18, No. 3(80), pp. 74–80 59. Kim Seong Hun; Soon Ho Jang; Sung Weon Byun; Jin Young Lee: Electrical Properties and EMI Shielding Characteristics of Polypyrrole Nylon 6 Composite Fabrics. Journal of Applied Polymer Science, Vol. 87, 1969–1974 (2003) Wiley Periodicals Inc. 60. Shielding Effectiveness, Telephonics 2003–2005, Rev 3 61. Tee Tang: EMC Lecture. Shielding, 2012, University of Technology Queensland, Australia 62. Lienig, F.; Löbl, G.; Dunsing, S.: Geräteentwicklung für Elektrotechniker. Skript-Vorlesung, TU Dresden, 2004 63. Hagotech GmbH Abschirmtechnik, Siemensweg 3, 31603 Diepenau, 2012 64. Herlemann, H.; Koch, M.: Measurement of the transient shielding effectivness of enclosures using UWB pulses inside an open TEM waveguide. Adv. Radio Sci., 2007, S. 75–79 65. Nagel, M.: Erzeugung hochfrequenter Hochspannung zur Untersuchung des dielektrischen Verhaltens von Isolierstoffen. Dissertation Universität Karlsruhe, 2008 66. Radu, S.: Engineering Aspects of electromagnetic Shielding, SUN Microsystems, 18.12.2009

Further Reading 67. Butera, Y.; Zhou, K.: Standing Spin Waves in granular Fe-SiO2 thin Films. Journal of Applied Physics, Vol. 87, No. 8, P. 5672 68. Carta, St.; Casola, G.: A Structural and Magnetic Investigation of the Inversion Degree in Ferrite Nanocrystals MFe2O4 (M ¼ Mn, Co, Ni), http://pubs.acs.org/doi/pdf/10.1021/jp901077c 69. Dötsch, K.; Bodenber, J.: Abschlussbericht D1 des SFB der Technischen Universität Darmstadt, 2002 70. Gräbner, F.; Knedlik, Ch.: Change of inversion degree with Nickel-Zinc Ferrite. Journal of Magnetism and Magnetic Materials, Dec 1999 71. Gräbner, F.; Teichert, G.: Absorption Experiments of NiZn Ferrite Films for EMC Applications. Proceedings of the 9th International Conference on Spin Electronics. Moscow, 13.–15. Nov. 2000 72. Gräbner, F.; Teichert, G.: Erzeugung, Analyse und HF-Verlust von texturierten ferritischen Werkstoffen, Bewertung der Qualität der Textur mittels einer theoretischen

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Orientierungsverteilungsfunktion. Kompendium EMV-Kongress 2000, VDE Verlag, Berlin, Offenbach 2000 73. Hemming, F.: Architectural Electromagnetic Shielding Handbook. IEEE Press 1992, ISBN 0-87942-287-4 74. Kimel, A. V.: Proceedings of the XVII International School-Workshop „Novel Magnetic Materials for Microelectronics“, Moscow 2000, p. 299 75. Kodali, G.: Engineering EMC-Principles. IEEE Press 1996, Chap. 9, ISBN 0-7803-1117-5 76. Kronacher, G.: Scaling Laws for Large Shields in Quasi Stationary Magnetic Fields. The Bell System Technical Journal, December 1967 77. Leferink, F.: Shielding in Practice. University of Twente, Hengelo, 2012, Hagotech GmbH Abschirmtechnik, Siemensweg 3, 31603 Diepenau, 2012 78. Levy, M.; Izuhara, T.: Michigan Tech Report MRS Spring „Slicing and Bonding of SingleCrystal Ferroelectric and Magnetic Oxide Films“ 79. Montrose, I. M.: Printed Circuit Board Design Technics for EMC Compliance, IEEE Press 80. Nakamura, T: Control of high frequency permeability in polycristalline (Ba, Co) Z-Type hexagonale ferrite, in: Journal of Magnetic Materials and Magnetism 257 (2003) 158–164 81. Singh, Y.; Koledintseva, M.: Hysteresis analysis of Co-Ti substituted M type Ba-Sr hexagona ferrites, in: Material Letters 63 (2009) 1921–1924 82. The Bell System, in: Technical Journal, December 1967, S. 2332–2339 83. Lehner, G.: Elektromagnetische Feldtheorie. Springer Verlag (1990)

Index

A

D

Absorbance Absorption constant, 38, 39, 59 Absorption cross section Absorption damping, 37, 195 Additional coupling, 164 Alternating electrical shielding, 198 Alternating magnetic field screening, 33 Anisotropy, xi, 6, 12, 13, 16, 17, 29–31, 39, 45, 46, 49, 50, 55–60, 62, 63, 75, 181, 183, 186 Anisotropy constant 1st order, 12 Antenna gains, 88 ASTM measuring cell, 75

Degree of inversion, xii, 13, 18–23 Degree of substitution, 183, 186 Demagnetization, 16, 46, 80, 87, 91, 92 Demagnetization effects, 60, 68 Density ferrite, 34 polymer, 34 Dielectric layers, 150, 153 Dielectric measurements, 24–27 Dipole effect, 29, 54 Displacement polarization, 109 Domain grid, 8, 13, 42, 55, 68, 69, 89, 107 Domains, 8, 13, 42, 55, 60, 68, 69, 89, 107 Doped spinels, 40 Dots, 91 Double shielding, 105, 106, 199

B Bloch wall, 14, 30, 69, 70 Bloch-wall eddy current relaxations, 68–70 Bloch-wall movement, 69, 70 Bloch-wall relaxations, 68

C Cavity resonances, 124–125, 127–129, 131 Characteristic impedance, xii, 35, 36, 97, 99 Coating materials, 50 Coaxial conductor samples, 36 Conducting polymers Corner effect, 198 Crystallite size, 88, 89 Crystal phase, 77, 78 Currentless radioabsorbing material (CRAM), xii, 5, 6 Currentless radiofrequency coating (CRC), 5 Currentless radiofrequency material (CRM), 5 Cut-off frequency, 61, 62, 86, 197, 200

E Eddy current propagation, 61 Electromagnetic compatibility of devices (EMCA), 3, 4 Electromagnetic wave field damping Electrostatic shield, 189 EMC textile, 171, 174

F Far-field shielding, 195, 196 FeCoBSi layer, 73, 76 Ferrite layers, 16–23, 45, 47, 49, 50, 52, 54, 55, 59, 66, 68, 130, 131, 149, 157, 169, 200 Ferrites of type W with hexagonal structure, 8, 181 of type Y with hexagonal structure, 8, 181 Ferrite volume materials, 16–20, 30, 31, 33

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206 Ferroelectric materials, 89, 110 Ferroelectrics, 89, 107, 109–112 Ferromagnetic materials, 107 Ferromagnetic resonance (FMR), xi, xiii, 14, 16– 20, 27, 28, 80, 89, 90, 92 Field adjustment, 35–40, 59 Field resonances, 120 Filling degree, 108, 171–173

G Gauss averaging, 46 Geometric cross section, 88 Gilbert approach, 52 Grain boundaries, 39, 45, 46, 72 Grain size average, 13, 30, 42, 43 the FMR, 28, 29 Granules, 81, 82 Granule size, 79–84 Grenade, 8 Gyrotropy constant, xii, 12, 47, 55, 183

Index L Laminates, 6, 7, 43, 44, 101, 124, 138, 139, 141, 144, 147, 157, 162, 198, 200 Landau–Lifschitz approach, 80 Landau–Lifschitz equation, 52, 55, 56, 60, 182 Landau–Lifschitz system, 60, 61, 81 Landau–Lifschitz theory, 14–16 Layer anisotropy, 49, 57, 72 Layers, 4–7, 13, 14, 26, 27, 38, 39, 43–45, 47, 49– 65, 68–73, 75–87, 91, 101, 108, 115, 116, 119, 124, 125, 128–131, 137, 138, 142, 143, 149, 150, 157, 162–169, 171, 179, 180, 188, 192, 200 Layer thicknesses, xi, 5–7, 44–47, 50, 52, 59–62, 68, 73, 78–84, 87, 91, 101, 116, 130, 163– 165, 168, 175 LF frequency resonance, 69 Line width of the FMR, 29, 89, 90 Loss of permeability, 81 Loss of resonance, 10, 35, 164

M H Half-width, xi, 58, 60 Hexagonal ferrites, 46, 47, 180, 182, 183, 186 HF absorption, 130 HF loss HF textile Holes, 11, 47, 192–195 Hysteresis curve, 62, 64

I Ideal crystal, 47 Inhomogeneous layers, 52, 53 Inner field strength measurements, 120 Insertion, 43, 92, 127, 129, 157, 197, 200 Interfaces, 45, 84, 105 Interfacial effects, 45, 120 Interference emission, vii, 124, 149, 150, 153, 157, 162 Inverse spinel, 9, 18, 19 Isotropic, 14, 36, 80–82, 88 Isotropic ferrites, 17, 27

K Kittel formula, 60 Kittel frequency, 60, 61, 84–86

Magnetic field strengths, xi, 16, 17, 47, 52, 53, 55, 56, 60, 61, 80, 182, 198 Magnetic flux density, xi, 80, 182 Magnetisation vector, 55, 57 Magnetization, xi, xii, 13, 17, 18, 27–31, 47, 55– 57, 63, 68–70, 79, 81 Magnetoplumbine, 8 Magnetostatic shield, 189 Maxwell–Garnett mixing law, 183 Maxwell’s equations, 52 Maxwell theory, 14, 16 μ measuring fork, 75 Medium complex permeability component Metal housings, 4, 5, 7, 107, 115, 120, 123–141, 145, 146, 197, 200 Metal subrack, 136, 140 Microscopic and macroscopic material sizes, 13 Microstructure, 11, 23, 39, 61, 70–72, 181 Microwave ferrites, 8, 12, 13 Microwave radiation, 88, 89 MnZn ferrite polycrystal, 10 Monocrystal, 45 Morphology, 49, 70, 90, 93, 167 Multilayer layer Multiple reflections, 4, 37, 38, 64, 107, 110, 115, 179, 180, 188, 191–193, 195

Index

207

N

S

Nanocrystallites, 45, 88 Nanomaterials, ix, 45, 49–93, 97, 99, 100, 103 Nanoparticles, 28, 89–93, 172, 173 Natural ferromagnetic resonance (NFMR), 14, 16, 17, 80 Near-field damping NiZn ferrite layers, 49–51, 55–58, 62, 64 NiZn ferrites, 9, 18–20, 23, 30, 32, 33, 39–47, 58, 67, 131

Saturation magnetization, xi, xii, 17, 40, 41, 55, 61, 84 Scattering, xii, 36, 38, 54, 65, 88, 89, 97 Scattering cross sections, 88 Scattering loss, 84, 89 Schelkunoff equation, 65 Schelkunov formula, 38 Shield attenuation measurement Shield concept, vii, 43, 157 Shielding effectiveness transient, 198, 200 Single-domain Stoner–Wohlfahrt behaviour, 91 Snoek frequency, 87 Snoek law, 60–65, 87 Snoek straight, 62 Specific resistance, 61, 84, 175 Spherical shape, 89 Spinels, 8–13, 17–20, 22, 27, 28, 30, 33, 40, 42–47, 78, 163 Spin–spin interaction, 6, 54 Spin waves, 6, 8, 13, 14, 27–29, 44, 45, 50, 52–56, 59–61, 75, 81, 91, 140, 157, 163, 164 diagram, 71 losses, 13, 50, 52, 54–58, 60, 84 modes, 44–47, 52, 60, 77, 91 propagation, 52, 59, 60, 74, 163 spectrum, 72, 73, 75 Sputtering process, 49, 71, 167 Sputtering system, 72, 73 Stoner and Wohlfahrt, 91 Stripline, 37–39, 43, 59, 62, 64–67, 99, 100, 130, 132, 150, 157, 165–169 Superparamagnetism, 75, 77, 89, 91 Surface anisotropy, 71, 89, 91 Surface roughness, 89 Surfaces, xi, xii, 4, 20, 33, 35, 37, 39, 44, 45, 50, 52, 54, 58, 59, 63, 65, 71, 75, 77, 81, 88–90, 93, 105, 108, 130, 131, 163, 169, 175, 180, 188, 192, 193, 200 Surface spin shafts, 50, 52 Surface spin wave modes, 50

O Orientation distribution, 31, 33 Orientation polarization, 109 Orthoferrite, 8

P Partial permeabilities, 10, 11 Particle diameter, 89, 90, 92, 130 Particle size distribution, 55, 89, 91, 130, 172 Particle thickness, 89 Percent by mass, 34 Permeameter, 97, 99, 100 Polycrystalline, 10, 31, 36, 45, 47 Polycrystals, 9–12, 31, 42, 45–47, 60, 68 Polymer–carbon fibre, 132, 135, 199 Porosity, 29

R Radar effects, 88, 89, 97 Radar radiation, 88, 97 Radiation absorption material (RAM), xii, xiii, 5, 6 Reflection loss, 37, 38, 51, 59, 64, 65, 78, 84, 86, 132, 168, 194, 195 Reflections, 6, 7, 35, 49, 59, 64, 65, 75, 82, 84, 98, 99, 107, 109, 110, 116, 119, 120, 124, 131, 149, 153, 157, 163, 171, 174, 179, 180, 188, 193, 195, 196 Relaxation in ferrite volume materials, 27–30 REM Resonance behaviour, 9, 11, 14–16, 123 Resonance points, 7, 43, 65, 67, 125, 127, 153, 157 Resonance quality, 65, 66 Resonant frequencies, 10, 14, 26, 27, 47, 65, 85, 87, 92, 93, 126, 181, 183 Resonator measuring method, 24 Response Ribbon cables, 167

T Tensor 2nd stage, 80 Texture, 10, 12, 13, 30–33, 54, 89, 107 Thicknesses, xi, 7, 26, 35, 37, 49, 58, 60, 62, 63, 69, 82, 91, 99, 105, 106, 115, 116, 129–131, 140, 141, 162, 163, 184, 186, 189–194, 199, 200

208 Thin-film absorber, 115, 117 Total internal resonance behaviour, 123 Total shielding losses, 37, 64 Total spin wave spectrum, 75 Transmission, xiii, 4, 35, 38, 59, 65, 75, 84, 98, 125, 127, 157, 163, 172, 173 electron microscopy (TEM), 93 loss, 75–79, 86, 113, 125, 138, 139, 163, 164, 171, 184, 185 loss constant, 38 power, 89 Triaxial measurement methods, 165

Index W Waveguide measuring station, 20, 58, 63, 75, 77, 98 Waveguides, 20–22, 24, 29, 33, 36, 37, 75, 78, 79, 132, 134, 184, 185, 194–197 Weight share, 34 Wire netting, 175 Wolmann cut-off frequency, 61 Wolmann effect, 86 Wolmann frequency, 61, 86, 87

X Ultra-high voltage printed circuit boards, 43

X-ray diffraction, 33, 58, 77–78 X-ray diffractogram, 40 X-ray diffractometric analysis

V

Y

U

Volume filling factor, 34 Volume materials, v, ix, 5–47, 58, 119, 140, 141

Yttrium iron garnet (YIG) materials, 53