THE DIELECTRIC BEHAVIOR OF SIMPLE GLASSES

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THE PENNSYLVANIA STATE COLLEGE THE GRADUATE SCHOOL THE DEPARTMENT OF MINERAL TECHNOLOGY DIVISION OF CERAMICS

THE DIELECTRIC BEHAVIOR OF SIMPLE GLASSES A THESIS BY ROBERT REX SHIVELY, Jr.

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

FEBRUARY 1950

APPROVED: 1/ " u 1

HEAD OF THE DEPARTMENT CHIEF OF THE DIVISION '

//

Acknowledgment

G-rateful acknowledgment is made of the guidance given by Dr. Ing. W. A. Weyl, Head of the Department of Mineral Technology. The author acknowledges the suggestions concerning the work and cell design given by Dr. J. B. Whitehead, Professor Emeritus, Johns Hopkins University.

Mr. E. W.

Lindsay of the Westinghouse Research Laboratories contri­ buted valuable suggestions for the experimental procedure. The author appreciates the help given in the laboratory by Mr. Webster Capps and the criticisms of the presentation by Dr. E. C. Henry, Dr. F. R. Matson and Mr. N. R. Thielke of the Division of Ceramics.

INDEX CHAPTER

CONTENTS

PAGE

Acknowledgment 1

Preface--------------------------------------------------I.

Introduction---------------------------------A.

The Constitution

B.

Limitations of the X-ray Theory of Glass

3

Structure-------------------------C.

3

of Glass---------------------

15

The Electrical Characteristics of Solid Dielectrics 2k

---------------------

31

The Theory of Perfect Dielectrics

2.

The Behavior of Real Dielectrics Under D C Stress-—

3.

k. D.

--

1.

The Behavior of Dielectrics Under Alter­ nating Electrical Stress----------------

40

Power Loss in Glasses---------------

kj

Previous Studies of the Relationship Between the Dielectric Properties of Glass and its Chemical Composition and Constitution-------

II. Experimental Procedure A.

Measurement of Dielectric Constants and Power Factors------

55

B.

Preparation of Sample------

60

C.

Discussion of Accuracy and Reproducibility—

6l

D.

Scope of the Experimental Work---------------

63

ii CHAPTER III.

CONTENTS

PAGE

Results A.

Presentation---------

67

B.

General' Experiments---------------------- -

67

C.

Role of Univalent Cations-----------------

63

D.

69

E.

Substitution of Various Cationsfor Mg+*— U.4Substitution of Cations for Si ----------

70

F.

Stratified Metal-Glass Dielectric---------

J1

IV.

Discussion--------------------------------------

73

V.

Summary-----------------------------------------

32

Appendix I:

The Calculated Results of the Dielectric and Density Measurements

References Cited

Index to Figures Fig. N o . 1.

Title

Following Page

Zachariasen1s Picture of the Structure of a Glass------------------------------ —

6

2.

Radial Distribution Curve of Silica Glass-

2>

3.

The Structure of Sodium Silicate Glass

M-.

The Fields of Glass Formation in Mixed Alkali Silicates---------------------------

5a. b. 6.

Electric Force Normal to a Conductor------

26

Charge and Discharge Currents in a Glass 33

The Self-Reversing Discharge Currents of a Glass Condenser--------------

S.

16

Gauss * Theorem

Condenser as a Function of Time----------7.

11

35

The Current-Voltage Relationship in Ideal and Real Condensers Under Alternating Electrical Stress-------------------------

9.

Polarization in Sodium Borosilicate Glasses------------------------------------

10.

12.

52

Polarization in Sodium Borosilicate Glasses------------------------------------

11.

^l

52

Results of Russian Dielectric Experiments on Borate Glasses---------

53

The Measuring Cell and Equipment----------

57

ii Fig. No. 13.

Title

Following Page

The Dielectric Loss of the G-lass IS Na20-l^-R0-66Si02 as a Function of the Polarizability of the Divalent Cation----------------------------------

1U-.

67

Dielectric Loss of the G-lass, l6Na20lJ4-RO-66SiC>2 as a Function of the Ionic Radius of the Divalent Cation---------

15.

Dielectric Loss in Sodium Silicate G-lasses of Increasing Alkali Content

16

.

,

69

Dielectric Loss in Sodium Magnesium Zinc Silicate Glasses------------------------

22

66

Dielectric Loss in Sodium Silver Magnesium Alumino Metaphosphate Glasses----------

21

66

Dielectric Loss in Sodium Lithium Mag­ nesium Alumino Metaphosphate Glasses

20

66

Dielectric Loss in Sodium Potassium Magnesium Alumino Metaphosphate Glasses

19

66

Dielectric Loss in Sodium Lithium Silicate G-lasses---------------------------------

16

66

Dielectric Loss in Potassium Lithium Calcium Silicate G-lasses---------------

17.

66

70

Dielectric Loss in Sodium Magnesium Copper Silicate Glasses----------------

70

lii Fig. No. 23 .

Title

Following Page

Dielectric Loss in Sodium Magnesium Iron Silicate G-lasses--------------------

2U-.

Dielectric Loss in Sodium Magnesium Cobalt Silicate G-lasses------------------

25.

70

Dielectric Loss in Sodium Titanium (Zirconium) Silicate Glasses

22>.

70

Dielectric Loss in Sodium Magnesium Copper Alumino Metaphosphate Glasses

27.

70

Dielectric Loss in Sodium Magnesium Nickel Silicate G-lasses------------------

26.

70

--------

70

Dielectric Loss in Sodium Aluminum Sil­ icate Glasses In Which (NaAlJC^ was Sub­ stituted for SIOp------------------------

29.

71

Dielectric Loss In Sodium Aluminum Sil­ icate Glasses in Which A10n was Subl.p stituted for 3102------------------------

30.

71

Dielectric Loss in Sodium Antimony Silicate Glasses in Which Sb0o c- was Substituted for 3102----------------------------------

71

Preface G-lasses are unique among solid inorganic materials in that they are isotropic and may be made with widely varying compositions and almost equally variable physical and chemi­ cal properties.

The broad range of properties obtainable in

glasses has led to their use in many diverse fields ranging from that of simple containers to that of complex electrical applications.

Like research in general, work in glass may

be divided into two categories, fundamental and applied.

As

is usually true, glass technology has so far surpassed the ability of the scientist to offer fundamental explanations for many of the phenomena which are known and controlled by 1/ the technologist.” Many physical and chemical data on glasses have been gathered by competent workers during the past century, but only comparatively recently has light been thrown on the salient characteristic of the vitreous state, its constitution.

The picture of the constitution of glass

is at present poorly developed compared to that of crystalline substances, and is subject to constant extension and refine­ ment.

However, the current concept of glass constitution

which has been developed over the past twenty years has en­ abled the scientist for the first time to interpret and corre­ late much of the available data on glass.

In addition, even

in its present imperfect form, the theory of the constitution of glass serves a more important purpose in that It provides the investigator with a framework around which to design and evaluate experiments in a contlnuative manner.

2 A direct result of this fundamental approach is evident in the increasing tendency to design and synthesize glasses tailored to fit certain requirements.

The gap between the

scientist and the technologist has been narrowed and future advance in the field of glass can be expected to come more from scientific endeavor. This work comprises an attempt to extend the present picture of glass constitution.

Of the several types of ex­

periment which can be used to pursue such a project, one of the most promising and least explored avenues of attack is that based on the behavior of glasses in electrical fields of various frequencies.

In the following sections of intro­

ductory discussion will be considered the present concept of the constitution of glass, its manifest imperfections and omissions, methods applicable to its refinement and, finally, the dielectric behavior of glasses in detail.

I.

Introduction A.

The Constitution of G-lass:

2/ Before the relatively recent work of Zachariasen 3a/ and of Warren et al, nothing was known about the atomic structure of glass.

Previous attempts to develop a theory

of the constitution of glass were based on assumptions de­ rived from the physical and chemical behavior of glass. Most of these theories were unsatisfactory even in light of the information available when they were proposed.

V thought glasses were supercooled liquids G. Tammann— because of their tendency to crystallize under proper condi5/ tions of temperature and time. Day and Allen demonstrated that glasses do not behave like supercooled liquids whose physical properties are defined by temperature, pressure and chemical composition.

Glasses show very definite hysteresis

effects which are a function of their thermal history.

Meas­

urements made since have shown that the "memory effect" in glasses is pronounced for electrical, optical and mechanical history as well. An early and persistent theory, first proposed by 6/ Lebedeff7 ia that which pictures glass as a mechanical mixture of minute crystallites.

This view, supported by Randall^on

the basis of X-ray diffraction patterns of vitreous silica &/ (193^), by Valenkov and Poray Koshitz~(l937) and by an occas­ ional current publication was rejected by Zachariasen and proven unsatisfactory by Warren.

The mention of the above older theories is Justified because they still appear in text books and scientific papers by authors in the fields of chemistry and physics.

Most of

the other hypotheses have been completely discarded.

Morey 9/ gives an excellent review of most of the older theories.”* The modern theory of the structure of simple glasses was first proposed by Zachariasen who applied the principles of crystal chemistry and X-ray data to the structure of vitreous oxides.

In a brilliant piece of reasoning, Zachariasen began

by rejecting the claim of Randall, Rooksby and Cooper that X-ray patterns of vitreous silica indicated that silica glass o is composed of cristobalite crystallites l^A In size with lattice constants 6.6$ higher than those for larger cristoba­ lite crystals for the following reasons:

First, densities

calculated on the assumption that the crystallite theory was correct were not concordant with observed densities; second, the thermal behavior of silica glass did not show the sudden volume changes expected from a crystallite structure; and third, the theory did not satisfactorily account for the X-ray diffraction patterns of vitreous silica.

If the crystallite

theory were correct, one would expect that on heating, the crystallites would recrystallize and grow and the X-ray halo pattern of vitreous silica would slowly sharpen to the pattern of cristobalite.

Experiment has shown that there is an abrupt

sharpening corresponding to sudden crystal growth or devitri­ fication.

Reasoning from the fact that the mechanical strength and rigidity of glasses compares favorably with crystalline ma­ terials over a large temperature ra-nge, Zachariasen concluded that the binding forces in glass must be comparable with those in crystals, that the atoms or ions in glass must have well defined equilibrium positions, and that the units of the glass structure must form an extended three dimensional network. Xray patterns of vitreous silica indicated that the arrangement is not periodic like that of crystals but not entirely random since the interatomic distances cannot sink below a minimum value.

Zachariasen showed that in such an arrangement all in-

ternuclear distances would not have the same probability. This would explain the X-ray diffraction halos characteristic of glasses and liquids.

He concluded from the above that glasses

are made up of an Infinitely large unit cell with no two atoms structurally equivalent.

This was in agreement with the iso­

tropic character of glass since the structure would be the same in all directions.

It also accounted for the physical be­

havior of glass on heating. Inequivalent,

Since all atoms are structurally

the energy necessary to detach one from the net­

work can have as many values as there are atoms in the network. G-lasses break down slowly rather than abruptly in the fashion of crystals containing a large number of structurally equiva­ lent atoms attached to the crystal network by forces of nearly identical strength.

Zachariasen points out that glasses are

not compounds in the stochiometric sense as are crystals. Electroneutrality of the structure is the only restriction on the composition of a glass which may contain any number of different hinds of atoms. Reviewing the work of Golds chmidt7~who attempted to re­ late the cation-anion radius ratios of oxides to their abil­ ity to form glasses, Zachariasen demonstrated that the radius ratio is not necessarily a criterion for glass formation. The problem is more complicated and, in brief, depends upon the ability of an oxide to form an extended, random, three dimen­ sional network whose energy is the crystalline forme.

not much higher than that of

For silica, Zachariasen decided, this

can be accomplished only by the random linkage through the oxygen corners of SiO^ tetrahedra, the standard structural units of crystalline silicates.

Figure 1 is a two-dimen­

sional representation of the structure of silica glass com­ pared to crystalline cristobalite as deduced by Zachariasen. This diagram is largely self-explanatory, but one point should be emphasized.

Pursuant to Pauling1s third rule

12/

for ionic structures, the SiOij. tetrahedra in vitreous silica share oxygen corners as do those in crystalline silicates, but in vitreous silica the Si— 0— Si bond angle of the shared corners varies somewhat in contrast to its regularity in crystalline silicates.

FIGURE 1 - Zachariasen*s Picture of the Structure of a Glass, Top: The Regular Structure of the Crystal, Bottom: The Corresponding Glass Structure*

7 In addition, Zachariasen summarized some empirioal con­ ditions which must he met by a simple oxide if it is to exist in a vitreous structure whose energy is not too high compared with that of the crystalline forms: 1.

An oxygen atom must not be linked to more than two cations.

2.

The number of oxygen anions surrounding the cations must be small (i.e. tetrahedra and triangles).

3.

Oxygen polyhedra must share corners, not edges or faces.

(Pauling*s third rule)

At least three corners of each oxygen polyhedron must be shared to form a three-dimensional network. Zachariasen's theory was shortly afterward confirmed by the work of B. E, Warren, who arrived at the same conclusions concerning the structure of silica glass as those of Zachar­ iasen.

Using a general X-ray technique applicable to any ma­ il/ terial developed by Zernike and Prins and first used by

Debye and Menke~^o study the structure of liquid mercury, Warren was able to show that Zachariasenfs picture of silica glass is correct.

Glasses and liquids have nearly random

structures, but differ in one respect.

In glasses, because

of their high rigidity, the neighbors of an ion remain the same while in liquids, with their relatively low viscosity, the neighbors and neighborhood of any ion are continually changing.

g Briefly, the method used by Warren involves the follow­ ing operations: 1.

A photograph of X-ray diffraction halos of the vit­ reous sample is taken in the usual way.

2.

The photographs are microphotometered to produce a curve of the intensity of scattering (arbitrary units) v.s. the scattering angle; i.e. I vs. sin &

3.

The intensity of scattering is put on an absolute^basis (eu/SiOg) by comparing the experimental curve at large sin 0_

values with that for independent

scattering by \ h e atoms. The experimental curve is corrected for absorbtion, polarization and Compton modified scattering. 5.

The absolute, corrected curve is then converted into a radial distribution curve by a complicated and la­ borious Fourier analysis involving forty or more sep­ arate integrations.

The use of a harmonic analyzer

circumvented this difficulty. The radial distribution curve mentioned in step 5 is shown in Figure 2.

Since there are two kinds of atoms in the

glass, Si and 0, this curve is really a superimposition of two distribution curves, that of neighboring atoms around a silicon and that of atoms around an oxygen.

For a monatomic liquid

like mercury there would be only one curve.

J4000

8000 0-0

0-0

8000

4000

2000

r inA 10

Fig.

2.0

SO

2 — R a d ia l d is t r ib u tio n c u rv e fo r v itre o u s o b ta in e d fro m X - r a y d iffr a c tio n p a tte rn .

S i0 2,

The interpretation of the radial distribution curve for vitreous silica is made by comparing the interatomic dis­ tances indicated by the peak spaclngs along the curve with the various known interatomic distances in crystalline sil­ ica and silicates.

The peaks correspond to the interatomic

distances indicated on the plot by vertical lines whose length is proportional to the area expected under the peak from consideration of the coordination numbers in the crys­ talline forms of silica.

The areas under the peaks indi­

cate the coordination number at the peak distance.

The max­

imum point of the first peak represents the Si - 0 distance and is 1.62A0 , the same as in crystalline silica and sili­ cates.

The area under this peak when calculated from X-ray

data indicates a coordination number of k- within the limits of experimental error. The above two experimental facts showed that siliconoxygen tetrahedra were the network-building units in glasses as they are in crystalline silicates and silica.

On the

basis of this assumption, Warren decided that the 0-0 dis­ tance should be 2 .65A0 and the coordination number of oxy­ gens around each oxygen, six.

The occurrence of a "hidden

peak" at the right distance and of about the right area can­ not be coincidental.

If the two bonds of an oxygen to a

silicon were at 1^0°, the Si - Si distance should be 2xl.62A°, which corresponds to the experimental peak at 3.2A0 .

The

10 orientation of one tetrahedron with its neighbors about the direction of the Si-O-Si link between them can be practically random.

The experimental X-ray results then are ratlonal-

lzable by picturing silica glass as a random network of SiO^ tetrahedra with each oxygen connected to two silicons through bonds which are almost diametrically opposite.

This

is substantially the picture deduced by Zachariasen (Figure De Warren neatly disposed of the crystallite theory pre­ viously mentioned.

Assuming that the crystallite theory is

correct, since there is no formal reason for rejecting it as an interpretation of the X-ray pattern of vitreous silica, he showed by calculations using Bragg*s particle size equa­ tion

X

L - .^9 that the average particle size of the crysB~C b i T " Q tallites would be only 7*7A • Since the unit cell of cris­ tobalite is about 7A°, the crystallites would be about 1

unit cell in size.

In a unit of matter of this size, regu­

lar repetition which is the main feature of the crystalline state is impossible and discussion of crystals of one unit cell is meaningless.

The possibility of 0 = Si = 0 mole-

* cules in glass was ruled out by Warren and Biscoefc*“ since

such an arrangement would demand that the coordination number be 2 and that the area under the first peak be half what it is.

The existence of SiOg molecules in crystalline silicates

is also unknown.

15/

These authors found also that a glass

11 structure is continuous like that of a liquid rather than discontinuous like that of silica gel which contains gaps 3c/ and voids of large atomic dimens ions ,jc:— Silica gel is anal­ ogous to a pail of pebbles and shows the intense X-ray scat­ tering at small angles characteristic of materials made up of discrete particles, while vitreous silica is analogous to a pail of water and its X-ray pattern approaches zero scattering at small angles, Warren and his coworkers used the technique described 3d/ for vitreous silica to examine simple sodium eilicate‘&~and borate glasses.

Figure J 1b a two-dimensional schematic

representation of the structure of soda-silica glass.

Here,

the SiOij. tetrahedra are not all linked together through their oxygen corners.

The Na20 added supplies extra oxygen

which makes the 0 Si ratio greater than 2.

Thus, it is not

possible to have all oxygens bound to two silicons.

The

sodium ions exist in the statistically distributed holes in the structure where they are surrounded on the average by six oxygens. Biscoe^^xamined the structure of simple three component soda-lime-silica glasses.

He found them quite similar to

soda-silica glasses in that the framework of the structure is made up of SiOif. tetrahedra with some of the oxygens bonded to only one silicon.

The divalent cations occupy the holes

in the structure like alkali ions where they are surrounded

Sc

O 0

Na

FIGURE 3 “ The Structure of Sodium Silicate Glass

(Warren and Loring)

by an average of 7 single

(to silicon) bonded oxygens.

The

structure of vitreous BgO-^ is much the same as that of vit­ reous silica except that the boron is surrounded by three oxygens in a triangle with each oxygen bonded to two borons. Borate glasses containing soda show a coordination number of oxygen about boron of between three and four.

It is known

from crystalline borates such as (danburite Oa B2Si20g)that BOty. tetrahedra as well as B0^ triangles exist and one can as­ sume that in sodium borate glasses, the oxygen supplied by the soda allows some of the BO^ triangles to become B0 j| tetra3 e/ hedra.42— This effect is in agreement with the observed maxima and minima in physical properties noted on the addition of 17/ Na20 to glasses containing B20^. Evans— gives a diagramma­ tic summary of the possible ways in which B0^ triangles can be linked at their corners.

At least several and probably all

of these possible linkages are found in vitreous BgO^. Studies of phosphate and. beryllium fluoride glasses have shown their structure to be similar to that of silicate 31/ glasses. The structural polyhedra of the former are P0[|_ tetrahedra like those found in crystalline phosphates (P-0 o 1.55A.) One important difference exists between phosphate and silicate glasses.

The number of oxygens always exceeds

twice the number of phosphorus cations so that for all compo­ sitions there are some single bonded oxygens at the tetra­ hedral corners.

Beryllium fluoride is well known as a "model"

13 of SiOg and It is not surprising that beryllium fluoride glasses contain BeF^ tetrahedra as the main building unit analogous to SiO^ tetrahedra in silicate glasses. Up to this point in the discussion, no case has been made for the ionic nature of glasses.

Indeed to the X-ray

worker, the question is of little moment.

However, abund­

ant evidence exists to support the ionic view and should be correlated here. 1.

It has been shown that the structure of a glass

is quite similar to the ionic material from which it is derived. 2.

The mechanical properties of glasses are strictly

comparable to those of ionic materials. 3.

The thermal properties of glasses and ionic ma­

terials are similar; that is, they both have relatively low coefficients of expansion and fairly high melting or softening points. 4.

13/ Abundant electrical evidence— is available which

shows that glasses are ionic.

They are good insulators at

low temperature but become electrolytic conductors at ele­ vated temperature where their conductivity has been shown by experiment to obey Faraday*s Law, Q, = FN. Our present approximate concept of the structure of simple glasses based on the experimental evidence furnished

u by X-ray diffraction studies and electrical measurements can be summarized as follows: G-lasses are random, continuous, three-dimensional ar­ rays of ions and ionic units.

The ionic polyhedra (SiO^,

BOij., PO^, BeFij. tetrahedra, and BO-^, AsO^ triangles) made up of a small central cation of high field strength surrounded by either three or four anions, are linked together at their anion corners to form the random three-dimensional network. In most simple glasses, all anion corners of the polyhedra are linked to two central cations to preserve electroneu­ trality.

The addition of alkali or alkaline earth oxides (or

fluorides) furnishes extra anions which enter the structure by breaking the corner links of the polyhedra producing 2 anions each linked to the cation of only one polyhedron. The continuous structure which results when all corners of the polyhedra are linked as they are in simple oxide glasses is broken and holes of large atomic dimensions result.

The

single and divalent cations occupy these holes (which are randomly distributed through the network).

In the holes,

these cations distribute their bonding force to a fairly large number of anions attached to the structural polyhedra. This number varies with the auxiliary cation but is usually in the range of 6 - 12.

15 B.

Limitations of the X-ray Theory of G-laes Structure. From the structure of glass derived from X-ray meas­

urements, one would assume that the maintenance of a vit­ reous structure is only dependent on the number of single bonded anions on the structural polyhedra and at most of the field strength of the auxiliary cations.

Much experi­

mental evidence is available which demonstrates that this assumption is untenable and that the structural information available from X-ray studies is incomplete and constitutes only a first approximation.

Not only the absolute number,

but also the number of different kinds of auxiliary cations and their electronic structure, are known to affect the glass structure profoundly.

Glassmakers have known for centuries

that one way to improve a glass in almost any direction is to increase the complexity of its composition.

Nearly all

commercial compositions are quite complex and their complexity is usually directly proportional to the number of different attributes desired in them. Meltability is one of the most frequently sought char­ acteristics in a glass, and one of the ways to achieve it is by using mixed alkalies. It is known from classic experi19/ ments— on the freezing point depression of glass thermome­ ters that alkalies affect the glass structure even at low tem­ perature.

A thermometer tube of glass containing sodium and

potassium ions shows a permanent expansion when heated to 100°C

and then cooled to zero.

The bore volume of the thermometer

is enlarged and thus the meniscus of the mercury in the ther­ mometer stands below the zero mark when the thermometer is returned to zero.

The heating and cooling cycle can be re­

peated with still further increment in volume.

Thermometer

glasses containing exclusively sodium ions or potassium ions do not show this effect.

W. Weyl

20/

showed that the mixed

alkali effect in thermometers can be compared to the devel­ opment of a superstructure in metals.

Apparently the sodium

and potassium ions in mixed alkali glasses have preferred positions and are not scattered in a random statistical fash­ ion through the holes in the silicate structure.

The ob­

served increase in volume is probably a result of the shift of the alkali ions toward their preferred positions. In a series of experiments, Weyl and Bastress showed that it is possible to extend the field of glass formation considerably by using a mixture of alkalies.

Using a stand­

ardized quenching technique, they found that the area of glass formation between two single alkali end member glasses showed large positive deviations from that enclosed by a straight line connecting the limiting points of glass formation for the end members. sults.

Figure ^ shows some of the most extreme re­

The large increase in the field of glass formation in­

dicates that the alkali cations exert a profound effect on the glass structure at higher temperatures.

One would expect from

FIGURE U - The Fields o.f Glass Formation in Mixed Alkali Silicates (Weyl and Bastress)

17 the formal X-ray picture of the structure of glass, which postulates the auxiliary cations in statistical distribution in the holes of the structure, that the locus of the limits of glass formation for the mixed alkali glasses would be a straight line between the limits of the end members since the field strength would be the only difference between the two alkalies.

The situation is obviously more complicated, 22/

Hassan-Ali Sheybany"; a student of A, Dietz el, made an extensive study of the role of alkalies in the structure of simple silicate glasses.

Of special interest are the re­

sults which he obtained from his experiments with mixed al­ kali silicates.

He found a distinct density maximum around

a 1:1 ratio of mixed alkalies in all cases up to a total R^O content of 35

chemical resistance of glasses con­

taining mixed alkalies was much improved over single alkali glasses of the same total alkali content.

However, the co­

efficient of expansion was a linear function of the total alkali content.

Sheybany concluded that the properties which

go through a maximum are due either to the alteration in­ troduced in the conditions of polarization as a consequence of the reciprocal action of the electric fields of two alka­ lies, or by the purely mechanical effect of packing the al­ kali ions of different sizes.

The linear coefficient of ex­

pansion was explained on the basis that the thermal expansion of a glass is a function of the magnitude of oscillations of

13 the ions around their mean positions in the structure.

The

oscillations are governed directly by the field Intensity of the vibrating ions and thus the coefficient of expansion was linear between the single alkali end member glasses. j£/ G-. Gehlhoff and M. Thomas reported that the resistivity of simple mixed alkali silicate glasses goes through a maxi­ mum at a certain sodium—potassium ratio. 2V B. I. Markin found that the electrical conductivity series of mixed alkali and argento-alkali borate glasses dis­ played a sharp minimum.

In the mixed alkali borates, this

minimum appears at a 1:1 alkali ratio while in the argentoalkali series, the minimum appears at anAg to Na or K ratio of lilt. In the past,

it has been customary to divide the cations

of the glass structure into three groups: 1.

Network formers or the cations of the structural

polyhedra, Si^+ , B^+ , P^+ , etc. 2.

Network modifiers monovalent and divalent accessory

cations, Na+ , K+ , Ca++, Sr+ + , Ba++, Pb+4’, etc. 3*

Intermediates, A1

+1

, Be

+2

.

This classification is purely arbitrary and Is based primarily 25/ on field strength considerations. Fajans and Kreidl"”Tiave shown that the use of field strength values as an acceptable basis for the classification of cations in the glass structure is valid only for noble-gas type cations.

Even the noble-gas

19 type cations overlap the classifications and their role In the glass structure is governed by the range in which their field strength lies, rather than by its absolute value. The divalent alkaline earth cations are generally considered to 2+ be network modifiers in silicate glasses, but Be whose Il+ field strength approaches that of Si is definitely in the HintermediateH class.

In support of this point, Fajans and

Kreidl cite the example of Beg SiO^ versus Si SiO^ where molar +2 refraction calculations show that 2Be when substituted for Sl^+is much stronger than SI^+ in tightening the electronic structure of oxygen anions. In silicate glasses, the theoretical upper limit of glass formation is reached at the orthosilicate ratio, where there are sufficient oxygen ions introduced into the structure by the auxiliary cations to make the silicon to oxygen ratio 1:4-.

At this ratio, all of the oxygen anions are singly ij. bonded to silicon cations, with the result that the SiO tetrahedra which make up the structure are not interlinked and the integrity of the vitreous structure is destroyed. In silicate glasses containing only noble-gas type auxiliary cations, glass formation stops far below the orthosilicate ratio, but the presence of major amounts of non-noble gas type cations makes glass formation possible up to and, in some cases, beyond the orthosilicate ratio.

One is forced to the

conclusion that non-noble type auxiliary cations are capable

20 of replacing silicon cations to some degree in the glass structure, in spite of the fact that their field strength is far below that of silicon ion. Perhaps the most startling deviation from the normal modifier role of the divalent cations in glasses is shown by the non-noble gas type cations such as Pb Lead ion, Pb

xp

xp

and Z n .

, which is of the same charge and about the °

°

same size as strontium ion (I.32A vs. I.27A) is character­ ized by a high degree of deformability in the field of other +2 ions* The outer electronic structure of Pb consists of p two 6S electrons in excess of a closed shell of 12. The p directional probability of the outer two 6S electrons of +2 Pb is easily shifted with the result that the ion behaves chemically as if it had a strongly ionic or P b+U^ side and a 0 ^ 6/ metallic side of high electron density like Pb .— Fajans and Kreidl cite the Pb-0 distances in PbO, where Pb

+2

is

coordinated with 2 oxygens at the corners of a tetragonal prism in confirmation of this view.

The lead ion is much

closer to one quartet of oxygen ions on its ionic side (2.Jo 2.) than to the other quartet on its metallic side (1J-.27&).

The 6S2 electrons of the divalent lead ion are

displaced away from the near oxygen ions by the repulsive effect of the electrons around the oxygen ions.

The driving

force for the close approach of the lead ion to one quartet of oxygens is the energy of deformation which can be gained

20 of replacing silicon cations to structure, in

some degree in the glass

spite of the fact thattheir field strength

is far below that of silicon ion. Perhaps the most startling deviation from the normal modifier role of the divalent cations in glasses is shown by the non-noble gas type cations such as Pb+2 and Zn+2. +2 Lead ion, Pb , which is of the same charge and about the /

0

0

same size as strontium ion (1.32A vs. I.27A) is character­ ized by a high degree of deformability in the field of other ions.

The outer electronic structure of Pb c consists of

p two 63 electrons in excess of a closed shell of 1S>. The O directional probability of the outer two 63 electrons of +2 Pb is easily shifted with the result that the ion behaves chemically as if it had a strongly ionic or Pb+^ side and a 0 26/ metallic side of high electron density like Pb .— Fajans +2 and Kreidl cite the Pb-0 distances in PbO, where Pb is coordinated with 6 oxygens at the corners of a tetragonal prism in confirmation of this view.

The lead ion is much

closer to one quartet of oxygen ions on its ionic side (2 .30X) than to the other quartet on its metallic side (IJ-.27S.).

The 6S2 electrons of the divalent lead ion are

displaced away from the near oxygen ions by the repulsive effect of the electrons around the oxygen ions.

The driving

force for the close approach of the lead ion to one quartet of oxygens is the energy of deformation which can be gained

21 by the mutual

polarization between the lead ion and the near

oxygen ions.

The electron density toward the far oxygens

is

high and thus the lead ion exerts weak metallic forces toward them giving the larger interionic distance.

The effects of

this type of dlsproportionatlon of lead ions in glass are known.

Heavy

lead optical glasses with a lead content in ex­

cess of the orthosilicate ratio have been made for many years. Molar refraction studies of lead silicate glasses made by S, SI/ P, Varma have shown that on the addition of lead oxide to an alkali silicate glass, the attendant increase in the ap­ parent molar refraction of the oxygen ions is much greater than is observed on the addition of the comparable

(size,

charge) noble-type cations, Sr, Ba, 2+ From the behavior of Pb in PbO, one can conclude that the stability of lead glasses is due to the deformabillty of lead ion in glasses where it is able to exert strong ionic 2— P forces to certain 0 ions. The outer 6S electrons will be pushed away into regions of lower electron density. 2+ 2+ Mg and Zn have the same charge and nearly the same 2+ 2+ ionic radii, but Mg has an outer electronic octet while Zn has IS outer electrons.

Glass formation in zinc silicates

takes place up to at least the orthosilicate ratio

(66 .6$ M %

ZnO) while the upper limit of glass formation in magnesium silicates is about 140 M$ MgO. The increase in glass forming 2+ 2+ power of Zn over Mg is explainable when one considers the

22 cleformability of Zn++in the field of oxygen anions.

Data

from crystal chemistry shows that in wlllemite, ZngSiOj^, / ° the Zn-0 distance is even smaller than in ZnO (1.92A vs. 1.94-A and 2.04&).

This decrease in interionic distances can

be accounted for by a mutual deformation of Zn

2+

and 0

2—

.

In Mg2SiO^, the Mg-0 distance is nearly the same (2.09 vs. o 2.10A), as in MgO. This means that the electric field 2+ 2+ around Zn is less symmetrical than that around Mg and that in ZngSiO^ the SiOj^ group is no longer an individual group.

This ability to deform in the field of anions can be

thought of as making the Zn

cation a more effective binder

between SiO^ tetrahedra in glass than Mg

.

A more correct general statement of the behavior of nonnoble gas type cations in glass is that, since they can de­ form to meet the requirements of a large range of positions between the tetrahedra, they help to keep the total energy of the structure low. The formal picture of the constitution of glass as di­ vined from X-ray measurements gives no clue to the explana­ tion (or even to the existence) of the mixed alkali phenom­ ena and the specialized role of non-noble type cations in glass.

In order to investigate these effects and their struc­

tural implications, some method other than x-ray analysis must be used.

The response of the glass structure to electro­

magnetic oscillations of longer wave-length is most promising.

27a/

Molar refraction studies by Kordes, Varma and others using visible light waves have been productive in showing the ef­ fects of various constituents on the electronic structure of glasses.

However, the information to be gained by observing

the effect of the glass structure on light waves is limited by their high frequency.

Phenomena which involve the oscil­

lation or displacement of the ions in the glass structure are not detectable through molar refraction measurements, since they indicate only the condition of the extranuclear struc­ ture of the ions which make up the structure.

For example,

S. P. Varma found no significant deviation from additivity in the molar refraction of oxygen in a series of glasses con­ taining mixed alkalies.

The relaxation time of effects which

involve the movement of larger units in the structure

(atoms

or ions) is too long to permit their detection where the fre­ quency of oscillation is so high. In this dissertation,

the behavior of glasses in elec­

tromagnetic fields of radio frequency and below will be ex­ plored as a source of information concerning the effect of various glass constituents on the structure.

2.K

C.

The Electrical Characteristics of Solid Dielectrics. 1.

Theory of Perfect Dielectrics. 29/ Benjamin Franklin was the first to recognize the

importance of the dielectric medium to the properties of the 50/ Leyden jar (17^-3). Cavendish extended Franklin's work by a series of experiments in which he, in effect, measured the relative capacities of condensers containing different di­ electrics and compared the values with the capacity of a standard air condenser.

The results were actually a series

of values of the dielectric constant for these materials. Cavendish did not appreciate the difference between con­ ductors and insulators and did not emphasize that the cap­ acities of his condensers containing various dielectric media were fundamentally related to the media. 51/ Faraday,,‘s~ rin his detailed studies on the exact equality between induced and inducing charges on concentric conductors with and without conductors and dielectrics be­ tween them, made the first detailed examination of the prop­ erties of various dielectrics.

During the course of these

experiments, Faraday recognized the importance of the medium between the conductors to the phenomenon of electrostatic Induction.

He crystallized his ideas with the discovery and

enunciation of specific inductive capacity, a fundamental property of dielectric media.

Cavendish had found that when

certain materials were introduced between the plates of a

25 condenser, the charges on the plates were much higher than when the medium between the plates was air.

Faraday examined

this phenomenon and defined specific inductive capacity as the ratio of the charge appearing on one plate of a condenser when a dielectric material was between the plates to the charge appearing on the plate of the same condenser filled with air at the same potential difference.

For many mater­

ials, this ratio is constant over a wide range of potential gradient and is usually considered to be a definite physical constant of the dielectric.

(This constancy of the specific

inductive capacity of dielectrics is not observed in alter­ nating electrical fields where K varies as an inverse func­ tion of the frequency.) The capacitance of a condenser is usually defined as the charge on one plate per unit potential gradient, or C=^. Using this relation, Gauss’ theorem and Faraday’s concept of tubes of force, it is possible to derive the equation relat­ ing the capacitance of a parallel plate condenser to its thickness, area and the dielectric constant of the material between its plates# From Coulomb’s law of the forces acting between point charges,

one can write the force at distance r due to a

point charge, q. f =

*L KT*

where K is the dielectric constant of the medium.

26 If one considers any closed surface surrounding charge q, the normal component outward (N) of the force due to q on a small area of the surface (ds) around any point (P) on the imaginary surface will be:

(Figure 5a )

If 1

Nds = £__ cos 9 ds where 9 is the angle formed by N Krs

with the line qP and

KNds = K n q

This is G-auss1 theorem, which, in words, says that the norm­ al induction outward, KN, taken over any closed surface con­ taining charge q is ^TTq.

If there are multiple charges

within the surface: KNds The proper selection of the G-auss surface combined with Faraday!s familiar concepts of lines of force, tubes of force and electrostatic induction (= KF) allows one to derive sev­ eral relations.

First, if one considers as a Gauss surface

the cylindrical portion of a Faraday tube (in air) formed by its sides and two right sections,

and Sg, on which the

electrical forces are F^ and F^j respectively, the surface integral of the normal component of the electrical force over the surface will be Fg S2 -

S^.

There is no force out­

ward along the sides of the tube because the sides are bound­ ed by lines of force. the tube.

By definition, there is no charge in

Then //kNds = 0 and F^

= F^ Sp for any two

right sections of a tube of force in air.

Fig 5a G a u s s ' T h e o r e m

Fig 5fc> E l e c t r i c F o r c e to a C o n d u c t o r

Normal

27 In Figure 5b,,

is a conductor carrying charge q per

square centimeter.

For a cylindrical Gauss surface whose

sides as above areparallel to the lines of force with one end in the conductor and the other slightly outside the conductor, but close to the surface, the total charge with­ in the cylinder will be qS where S is the cross-sectional area of the cylinder.

The normal component of induction

outward over the sides and the end of the cylinder within the conductor is zero since the sides are parallel to the lines of force and there is no force within the body of a conductor.

Over the remaining outside end section of the

cylinder, the integral of the normal induction is the same as that over the surface of the conductor

and one can

write:

rr«KNds = KFS = % q S and F = Wq ~TT The electric force between large parallel plates where the lines of force may be considered perpendicular to the plates is F = j4-7rq. The potential difference between the plates is “Y " V^-Vg = Fd = ^TTqd where d is the distance between the plates, Then, since C = Q, by definition, 0 = K for parallel plates v vox per square centimeter and for plates of area A: C = KA

4-ttcl

2g Various modifications of tliis equation depending on the units chosen are used extensively to calculate the dielectric con­ stant or capacitance in dielectric experiments. One further point should be brought out in any discus­ sion of the properties of ideal dielectrics.

If one considers

a condenser filled with air (a) and then with a dielectric (d) of specific inductive capacity K, the following is apparent: By definition:

Ca = Qa

and Cd = Q,d

75

VS

and K = Cd = QdVa T3a c^aVa If Va = Vb, Qd = KQja. and if Qa = Qd

Vd = Va 7T

These relations indicate that on changing the dielectric med­ ium in a condenser from air to a dielectric of specific in­ ductive capacity K, the condenser will store a certain charge Q, at only 1/K the potential gradient required with air.

If

the potential difference is maintained constant, the charge on the plates will be increased to KQ. The ability of dielectrics to store charge is usually explained on the basis of the Mpolarlzation theory" of dielec­ trics.

This theory pictures the dielectric as consisting of

structural units which have a positive and a negative portion 32/ which orient in an electrical field. (Helmholz I&7 0 ) The total polarization is the result of three components, orien­ tation polarization, atomic polarization and electronic polar­ ization.

Pt = Po + Pa + Pe.

29 Orientation polarization consists of the orientation of permanent dipoles in liquids and organic materials in response to an imposed electrical field.

Permanent dipoles

are molecules in which the center of positive and negative charge do not coincide.

The high dielectric constant of

water is attributable to the orientation polarization of the water molecules which are permanent dipoles*

In solid

ionic materials such as crystals or glasses, orientation polarization can be neglected since the structure is too rig­ id to permit the orientation of permanent dipoles even if present.

This leaves atomic

(or ionic) and electronic polar­

ization as the interrelated components of the total polar­ ization. In ionic materials, the ions themselves respond to an electric field, because of their net positive or negative charge.

The cations are attracted to the cathode and the

anions to the anode.

This atomic response is manifested by

its contribution to the total polarization as measured by the dielectric constant.

Vitreous silica as a dielectric

gives a capacity which is about 75$ greater than one would expect from the electronic response alone. Electronic polarization is common to all dielectrics and consists of the attraction of the negatively charged electrons and the positively charged nuclei of ions by the anode and the cathode respectively.

This polarization is

30 practically Instantaneous ana. results in an induced dipole; i.e., the electrons of an ion are displaced toward the anode and the positively charged nucleus is displaced toward the cathode. As a first approximation, the atomic and electronic components of the total polarization may be considered in­ dependently, but it should be emphasized that they are always interdependent to some degree according to the structure of the dielectric and to the wave length when alternating elec­ trical stresses are imposed.

For example, B. Szigeti^has

shown that for wave lengths in the infrared region, Pa and Pe reinforce each other for transverse waves, but have op­ posite signs for longitudinal waves in ionic crystals.

Be­

cause of the difference in ranges of relaxation times of ionic and electronic polarization, it is possible to examine the magnitude of these two components in ionic materials by using alternating electrical fields of widely different fre­ quencies.

The electrons respond almost instantaneously and

for practical purposes one can measure their contribution to the total polarization by measuring the refractive index with the sodium line in the visible spectrum.

However, relaxation

time of ionic polarization is too high for it to contribute to the polarization at frequencies within the range of visible light except insofar as it affects the electronic polarization to a secondary degree.

At radio frequencies, the electrons

31 and ions both, respond and one measures the total polarization. The subtraction of the electronic contribution from the total polarization gives the ionic contribution to the total.

This

has been done for glasses using the Maxwell relation and the Clausius-Mosotti Equation, and will be discussed later. 2.

The Behavior of Real Dielectrics Under DC Stress. One might summarize the behavior of a condenser con­

taining perfect ionic dielectric medium as follows: (1)

The condenser should charge almost instantaneous­ ly (i.e., within the time necessary for electron­ ic and ionic polarization) when connected to a battery or other potential source.

(2)

When the source is disconnected and the condenser is short-circuited, it should discharge in a like manner.

(3)

The charging current should quickly fall to zero, indicating no conductivity.

Real condensers do

little more than approach this behavior (except those which contain gases or vacuum as the di­ electric medium). It was early recognized that nearly all solid dielectrics possess electrical conductivity in some degree.

Most solid

dielectrics do not take up their full charge until some time after the voltage has been applied and also require time for complete discharge on short circuit.

Dielectric "absorption"

or "viscosity" are terms used to describe this behavior. For

32 glasses at room temperature the times are of the order of several minutes.

The deviations from ideal behavior have

also led to the use of the term “anomalousw to describe dielectrics whose properties are not in accord with the simple theory of perfect dielectrics. Dielectric absorption in glass was discovered in the earliest experiments with the Leyden jar (17*1-6) which was 29/ remarkable for its display of residual charge. Franklin— 31/ W (17*1-2), and later Faraday and Maxwell, were familiar with the phenomenon and Maxwell made the first attempt to explain dielectric absorption.

Using Poissonrs equation,AxV+ *kfl/>=0,

and the equation for the continuity of current flow (in me­ tals), Maxwell demonstrated mathematically that in a homo­ geneous dielectric, no residual charge can appear and, con­ versely, that a material which shows dielectric absorption could consist of parts which have different values of K and specific conductivity, even though it appears to be perfectly homogeneous.

Maxwell offered his theory only as one which

could correlate -the experimental facts and cautioned his readers: "It by no means follows that every substance which exhibits this phenomenon (dielectric absorption) is so com­ posed, for it may indicate a new kind of electric polarization of which a homogeneous substance may be capable, and this in some cases may perhaps resemble electro-chemical polarization much more than dielectric polarization.11

33 Fortunately for this discussion, one of the first workers to examine anomalous dielectrics carefully used glasses as the subject of his experiments.

Hopkinson, 2 5a/ student of

William Thompson and Maxwell, performed an investigation of the properties of glass condensers, the accuracy and ingen­ uity of which has made his work a classic in the field of dielectric research.

He examined the time dependence of

charge and discharge currents for glass condensers as shown in Figure 6.

The charge and discharge currents show a rapid

decrease within a very short time.

For example, 97$

charge

is taken up in 0.59 x 10-*!- sec. by a flint glass condenser. However, the currents do not quickly fall to zero, but continue to approach zero asymptotically for days or even months. These small currents following the initial surge of charge or dis­ charge are called "residual currents". Hopkinson also studied the relation between discharging potentials of glass condensers and previous charging history. He observed that if he applied a potential to a glass condenser and then reversed the sign of the potential for a short time the condenser showed a residual charge after being shortcircuited to remove the pure geometric charge.

He also found

that the decay of this charge was accompanied by reversals in polarity which were connected with the sign and order of the previous charging potentials.

Hopkinson summarized his exper­

iments in a statement (due to Lord Kelvin), "The charges come

DISCHARGE CURRENT

figure

6

CHARGING CURRENT

Charge and Discharge Currents in a Glass Condenser as a Function of Time (Guyer)

3^ out of the glass in inverse order in which they go in".

He

went on to propose the principle of "superposition" for an­ omalous dielectrics.

Hopkinson, with Maxwell, thought that

an anomalous dielectric is composed of strata with different K and conductivity values leading to different rates of ap­ proaching polarization and decay.

In 15>77>

^is second

35b/

publication, Hopkinson*1

developed the Boltzmann theory of

elastic residual effects for the electrical case in which he likened the polarization of a dielectric to the angular twist of a torsion thread under a succession of couples of various magnitudes and direction.

Each couple in the mechanical case

was supposed to cause an initial and a sustained, gradual yield with time (i.e. increase in angular displacement). The decay of this displacement appears in subsequent twisting couples.

Hopkinson obtained the following formula: ,

D(t) = K tjqj-

E (t) + / E

Jo

(t-u) $ U'doL

The actual displacement, D, in a dielectric at time (t) is made up of two components, one of which is the normal dis­ placement determined by the dielectric constant (K) of the medium and by the instantaneous potential E(t).

The second

component is the sum of all residual effects of the previous values of E as a function of elapsed time, u.

Hopkinson

found that his equation was only approximately correct when he put it to the test of experimental verification.

However,

he carefully promulgated the law of superposition as follows:

35 "It seems safe to infer that the effects on a dielectric of past and present electrical forces are superposable."

The

use of the elastic analogy to describe the anomalous prop­ erties of dielectrics has been widespread since Hopkinson*s 36/ time, and is encountered in modern publications.— The law of superposition has been checked repeatedly by able workers 37/ and there is no serious doubt of its validity. (Curie, 35/ Von Schweidler ) 29/ The work of E. M. Guyer on the "self-reversing charge" of glass condensers is most illuminating and has been used Ho/ recently by W. A. Weyl to show that the view-point of Max­ well and Hopkinson is still quite acceptable when considered in light of the modern concept of glass constitution.

Figure

7 shows the discharge current obtained by Guyer from a borosilicate glass condenser which was charged by a +225 volt battery for JO seconds, short-circuited and immediately charg­ ed with -112 v for 10 seconds.

The discharge current meas­

ured after the second battery was removed consisted of two parts.

A rapidly decaying discharge current due to the sec­

ond charge (-112 v 10 sec) was first observed followed by a sign reversal, and after about 16 sec., a discharge current due to the first charge (225 v 30 sec).

In another experi­

ment with a heavy lead glass condenser, Guyer was able to produce a discharge current flowing in opposition to the charg­ ing potential on the condenser.

He first charged the condenser

of a Glass 75 SECONDS

90

I 2 V for 10 Sec.

CONDENSER CHARGED

CHARGE REVERSAL

CO ND EN SE R

DISCHARGE REVERSAL

DISCHARGED

20

Discharge

60

7 - The Self-Reversing Condenser (Guyer)

VOLTAGE

45

FIGURE

APPLIED

Currents

4- 2 2 5 V for 30 Sec.

36 with + 225 v 30 sec. and then reduced the charging potential to + 112 v.

The condenser discharge potential was high

enough to charge the +112 v battery as indicated by a current flowing into the battery for a few seconds.

This second re­

sult indicates the degree of reversibility of polarization when conductivity is low due to the low alkali content in the heavy lead glass.

If the conductivity were low enough,

it should be possible to allow a glass to cool under a po­ tential gradient to form an electret

as those discus­

sed in the excellent review by Gutman. Computation of the effective ionization of sodium in glass from low temperature conductivity measurements has shown that only a part of the sodium ions in a glass are cur18>c/ rent carriers even when new cations are supplied at the anode. This fact indicates that the forces which hold the sodium ions in the ’’holes" in the glass structure are not the same for all sodium ions.

From the randomness of the glass structure, one

would conclude that the forces which hold the alkali ions follow a normal distribution curve.

Depending on the binding

forces and the configuration of the holes in the structure the sodium ions in a glass can respond to an electrical field in several ways. 1.

All sodium ions can contribute to atomic polariza­

tion by shifting their average position toward the cathode. This polarization takes place instantaneously and comprises

37 the contribution of the sodium ions to the initial high charging current of a glass condenser.

The shift of the

positively charged sodium ion toward the cathode in its structural hole which is bounded by negatively charged oxy­ gen ions really results in an induced dipole with the posi­ tive part (Na+) toward the cathode and the negative part p

(0

) toward the anode. 2.

Some sodium ions occupy "holes" which are close to

other "holes" in the cathodic direction.

Depending on the

duration of the electric potential applied and the thermal motion, some of these sodium ions are able to migrate to the adjoining holes toward the cathode.

This is a time-consuming

polarization, whose rate is determined by the height of the energy barrier between the first and second holes and by the thermal vibration level of the structure.

The energy bar­

riers between the hole positions in the structure must vary along a normal distribution curve so that the rate of migra­ tion from and return to normal position must be different for the various sodium ions in the network.

This type of

polarization also results in the formation of a dipole con­ sisting of a negative space charge (toward the anode) result­ ing from the removal of a sodium ion from the first hole and a positive charge due to the cation in the new position (to­ ward the cathode).

3* The anomalous charge and discharge currents of a glass condenser can be formally pictured as a manifestation of the production, or disorientation, respectively, of a number of dipoles having different moments and different rates of formation or decay.

On charging, this anomalous current

gradually falls to the value of the conduction current. When an equilibrium number of such dipoles is formed, such a description, from a structural viewpoint, is equivalent for glass to Hopklnson's statement that a dielectric is composed of a number of components characterized by different rates of approach to complete polarization and decay. 3.

Last, a certain number of sodium ions can find their

way through the structure, thus becoming current carriers and giving the conduction current mentioned above. The structural changes which occur in the glass in Guyer's first experiment with superposition have been described by Weyl approximately as follows: 1.

(Figure 7 )

The application of +225 v for 30 seconds causes a

very small number (A) of Na+ to migrate through the glass structure (conductance current).

All Na+ions are displaced

toward the cathode (ionic polarization).

Depending on the

time, temperature and the height of the energy barrier, a certain number (B) succeeds in reaching new positions (holes) toward the cathode producing a space charge-cation dipole as mentioned above.

39 2.

Short circuiting the condenser for an instant stops

the conduction current and releases the electronic and ionic polarization.

The sodium ions (B) which have crossed poten­

tial barriers do not have time to return in appreciable num­ bers. 3.

The -112v charge is applied for 10 seconds.

This

speeds the return of those sodium ions B which crossed poten­ tial barriers under the +225v field.

However, only a part

(b) of the sodium ions B can return in 10 sec.

The rest

(B-b) remain in their displaced positions in opposition to the -112v field.

Another group of sodium ions (C) having low

potential barriers in the direction of the -112v field can re­ spond to it and occupy new positions in the opposite direc­ tion. ij-.

Removal of the battery (-112v) immediately stops the

flow of current.

Most of the sodium ions (b) have returned

to their original Interstitial positions.

Those which have

crossed high potential barriers in response to the +225 volt field (B-b) are displaced in one direction and C are displaced in the other direction due to the -112v field. When the condenser is allowed to discharge, the first current observed is due to the ionic and migratory polariza­ tion from the -112v charge, whose decay is rapid (low acti­ vation energy) and dominates the picture.

Gradually, this cur­

rent falls off and approaches an equilibrium with the decay of

4o the migratory polarization due to (B-b) Na+a.isplaoed by the 225v charge for 30 sec. at which time the total current goes through zero.

The current then flows in the opposite direc­

tion, due to the more slowly decaying polarization of 4.

(B-b)Na .

This structural Interpretation accounts for the

dependence of discharge currents and potentials of glass con­ densers on their charging history. An analysis of this type, which depends on knowledge of the glass structure, would have been impossible before the development of the random network picture of glass consti­ tution by Zachariasen and Warren, et al.

In turn, a struc­

tural analysis of the dielectric behavior of glass provides at least a tentative extension of our knowledge of glass structure which can be tested by correlation with information from measurements of other "structure sensitive" properties. The lack of structural information on dielectric materials frustrated efforts on the part of workers following Hopkinson and Maxwell to offer any more satisfactory explanation of the behavior of anomalous dielectrics. Wagner,

W

k2/

The work of Pellat,—

W

and Von Schweldler^^as confined to the mathematical

extension of the earlier theory and contributed nothing toward an explanation. 3.

The Behavior of Dielectrics Under Alternating Electrical Stress. Two sources of dielectric loss, conductivity and dielectric

absorption, were discussed in the last section covering the

ij.1 behavior of real dielectrics when subjected to a d.c. poten­ tial.

Under alternating electrical stress, a third type of W loss was first observed by Siemens (12>6*I-). This loss is intimately connected with dielectric absorption and greatly exceeds losses due to conductivity.

The oscillatory motion

given to the components of the dielectric is manifested as heat and in some instances, this "Siemens Heat" is suffi­ cient to melt dielectrics in which a large amount of elec­ trical energy is converted to heat. A condenser containing a perfect dielectric (no absorp­ tion, no conductivity) would consume no power when subjected to an alternating potential difference.

The charging current

would be exactly ninety electrical degrees ahead of the po­ tential.

Oases are the only dielectric media which give con­

densers that approach this behavior.

All solid dielectrics

are conductors and thus even if free of dielectric absorption, they consume some energy.

The following electrical vector

and wave diagrams (Figure £>) show the current voltage rela­ tionships for. an ideal condenser and a real condenser with conductivity and absorption.

In addition, a summary of the

terms used to describe the behavior of real dielectrics is given. From Figure 8>, it is apparent that for condensers con­ taining real solid dielectrics, the charging current does not lead the potential by one-fourth of a period.

A condenser

Ic

o

=

Charging

Current

Ia =

Absorption

"

Ix =

Conduction

11

Ir =

Resultant

"

E

=

Potential

"

A

=

D i e l e c t r i c Loss Angle

e

=

D i e l e c t r i c Loss Phase Angle

o

'90° *

*

E

I d e a l Conde ns er

Real

Condenser

+

t2

t,

Ir "

^ R

t :wf:si r> it. c

!

t

;• j

r. a

i

r

r t?

:i! ;•

c

.

Dielectric Definitions

Dielectric Constant - (K)

Property of material which

determines the electrostatic energy stored per unit volume per unit potential gradient. Dielectric Phase Angle - (0 )

Angular difference in

phase between the sinusoidal voltage applied to the dielectric and the component of the resulting current which has the same frequency as the applied emf Dielectric Loss Angle -

(A)

Difference between 90

electrical degrees and the dielectric phase angle

G

.

Dielectric Dissipation Factor - ( D ) tan of the di­ electric loss angle

A

or cotangent of dielectric phase

angle. Dielectric Power Factor of a material, is the cosine of dielectric phase angle 6 angle A . Dielectric Loss Factor-KD

or sine

of dielectric loss

whose loss Is due to conductivity alone would have a capacity Independent of the frequency and a power factor (Cos 9) vary­ ing inversely with the frequency.

Such a condenser is unknown

and even condensers containing the best solid dielectrics show power factors muoh higher than those due to conductivity, and capacities much higher than the geometric values.

The power

factor decreases much less rapidly than 1 and the capacity 7 decreases slowly with increasing frequency. In some cases, the power factor goes through a maximum with frequency. Of the many theories that have been advanced to account for alternating potential losses in dielectrics, the most sat­ isfactory are those which are based on dielectric absorption.

i5/(lg97),

Hopkins on

W

^7/

Hess— (1895) and Beaulard— (189 *0, were able

to show that, if dielectric absorption means the continued flow of current under a continued potential difference, the behavior of dielectrics under alternating stress could be explained at least qualitatively.

This current flow must give rise to an

extra lag of the charging current and this is sufficient to ac­ count for an energy component of the charging current in excess of that due to conductivity. H.

A. Rowland (1897)> made a rigorous, unpublished ex­

tension of Maxwell^ theory of absorption to Include the alM / ternatlng potential case, and F. W. Grover (1911) published an analysis in which he checked the published results of Row­ land closely.

The reader is referred to J. P. Whitehead for a

lucid review of the various extensions of Maxwell’s theory to account for power loss.

It should be emphasized that these

extensions, like Maxwell's original work, are mathematical treatments of a hypothetical, two-layer dielectric that fit measurements in a semi-quantitative way at best, and contain no mention of the atomistic phenomena underlying dielectric absorption which gives rise to the power loss and high capa­ city of glass and other anomalous dielectrics in an alterna­ ting field. 4-.

Power Loss in Glasses. Prom the discussion in the preceeding section con­

cerning the structural changes underlying absorption in glas­ ses in a direct current field, one can extrapolate qualitativ­ ely to the case of losses in glass in an alternating field. In the superposition experiments of Guyer and Hopkinson, the glass condensers were subjected to an alternating field of very low frequency.

A glass in a higher frequency field will respond

in the same manner except that those movements of the ions which are characterized by high relaxation times will not have time to take place, and cannot contribute to the power loss or increase the capacity.

As the decrease in ionic response is

aggravated by increasing frequency, the power loss and the capacity of a glass condenser should decrease with increasing frequency. glasses.

This has been observed in many measurements on The rapid increase of capacity and power loss with

the addition of sodium ions to a simple sodium silicate glass (Table I, Appendix I) indicates that for radio frequencies and below, the alkali ions are chiefly responsible for the deviation of glasses from perfect dielectric behavior in an alternating field.

The behavior of the sodium ions in a

glass condenser under alternating stress can be reasonably outlined as follows: 1.

All sodium ions join in the general electronic and

ionic polarization.

The electronic response is instantan­

eous and contributes to the total polarization at all fre­ quencies up to that of visible light.

The relaxation time

of the pure ionic polarization of the sodium ions is nearly zero and probably is short enough to allow the ions to respond at frequencies up to the infra red. 2.

Depending on the frequency, a certain number of

sodium ions will be able to move across energy barriers dur­ ing each half cycle of the charging potential wave.

The num­

ber of sodium ions which respond at any given frequency and potential will be a function of the normal distribution of energy barriers which the sodium ions must cross to form in­ duced dipoles of the cation (Na+ )- negative hole type.

This

time-consuming polarization would give rise to half-cycle ab­ sorption currents in the same way as they are produced in the direct current case.

There is a fundamental difference, how­

ever, between the behavior of a glass condenser under a direct

ij-5 potential and under an alternating one.

On discharge after

being subjected to a direct potential, the absorption current is quantitatively recovered.

On field reversal in an alter­

nating field, the half cycle absorption current is not en­ tirely recovered as electrical energy since it is known that part is converted to heat.

The conversion of electrical en­

ergy in a glass to heat on field reversal is probably due to the fact that the return of the sodium ions which have crossed potential barriers is accelerated by the field of opposite sign.

The kinetic energy (current) supplied by the

reversed field to the sodium ions is not totally recoverable in the electrical system because part is converted into heat by collision with other ions in the direction of the reversed potential.

The number of ions which can behave in the above

manner varies Inversely as a function of the frequency and thus for glasses one would expect from this discussion that power loss would decrease with increasing frequency as is ob­ served. The nearly parallel decrease of capacity with increas­ ing frequency observed in glasses can in first approximation be attributed to the same processes which govern the depend­ ence of power loss on frequency.

A measuring instrument is

concerned only with the external circuit of a condenser and is unable to differentiate between the various current compo­ nents which make up the total current.

That is, polarization

46 current, absorption current and conductivity current are all the same to a measuring instrument.

Thus, when the polariza­

tion, conductivity and loss currents are high, an instrument records the fact that on each charging cycle, the condenser stores more electrical energy than it actually does and thus gives a high "apparent11 capacity reading. This fact was rec49/ ognlzed by Hoch Who has shown mathematically that the energy loss in a condenser per unit volume is: P = 2 tt fm ,v3 k a irz Where A is the area of the plates, d the distance between the plates, f is frequency, m a unit constant, K the dielectric constant and a delta the dielectric loss angle in radians. That the above theoretical discussion of power loss in glasses is too mechanistic and not completely accurate is man­ ifest from the fact vitreous silica has a very low but appre50/ ciable power factor. Barth jrias shown that rotation of oxygen ions about the Si-Si axis can take place in crystalline sil­ ica.

This rotation, if it takes place in vitreous silica,

must involve an activation energy at low temperature.

In an

alternating electrical field, some of this energy could be supplied by the electrical field.

However, it is interesting

to note that the capacity of vitreous silica changes very little with frequency (of the order of 2 .7^ for a frequency change from 250 to JxLcJops. which is well within the range of experimental error since the measurements by Jaeger were

H-7 made by several different methods depending on the frequency). It is also doubtful whether anyone has ever made dielectric measurements on a silica sample completely free of sodium ions.

MS D.

Previous Studies of the Relationship Between the Dielectrlo Properties of Glass and its Chemical Composition and Constitution. A survey of the literature on the dielectric proper-

ties of glasses produces very little fundamental information on the relationship between dielectric behavior of glasses and their constitution.

Many investigators have siezed upon

glasses as subjects for dielectric study because of their is­ otropy and the wide range of dielectric constants and power factors which may be obtained by altering their composition. Electrical and ceramic engineers interested in devel­ oping new insulating materials and theoretical physicists in­ terested in observing the degree of conformity of glass di­ electrics to the various dielectric theories are responsible for most of the published measurements on glass dielectrics. Practically no effort was made to relate dielectric behavior to constitution.

The obtention of fundamental information

from most of the work has been compromised by the use of un­ known compositions, or ones too complex to interpret.

Com­

positional changes which introduce so many constitutional variables that any structural evaluation of the results is impossible have been common. results obtained as follows: 1.

One might summarize the general 51/

The dielectric properties of glass are a function of

the frequency of the applied field.

The poiirer factor, P,

11-9 decreases with frequency according to the relation P = Bfn where f is the frequency and B and n are constants.

The ap­

parent dielectric constant increases with decreasing frequency. At low temperature ( gO°C), the dielectric constant is not particularly sensitive to frequency under a constant potential. 2.

Both the apparent dielectric constant and the power

factor Increase sharply with temperature beyond the range 8>0 ° to 100°0.

Some workers have observed a gradual decrease in

power factor with temperature at temperatures as low as -2>0°C. 3.

There is considerable information concerning the ef­

fect of non-structural compositional changes on the dielectric properties of glass. The work of Navias and G-reen^^s informative.

The authors

examined the dielectric constant and power loss at ultra high frequencies (3000 and 10,000 megacycles) where dielectric ab­ sorption is very small and power loss is primarily due to ionic oscillation.

They found that alkali ions increase the

dielectric constant and power loss in direct proportion to the alkali

present.

Divalent cations (alkaline earth and lead)

contributed to a lesser degree than the alkali ions to power loss.

Glasses containing several alkalis or divalent cations

showed decreased power loss.

High lead or barium glasses had

high power losses due to the large degree of electronic and atomic oscillation. of alkalis.

These losses were reduced on the addition

The opposite was also noted when the power loss

50 of alkali glasses was lowered by the addition of RO.

These

observations led Navias and Green to the conclusion that in all cases the dielectric loss of glasses in ultra high fre­ quency fields was lowered by increasing the complexity of the RgO and RO (auxiliary cation) content.

Alumina was found to

increase dielectric loss when substituted for silica. Thurnauer and Badger

easured the effect of composition

on power loss at somewhat lower frequencies (100,500 1500 and 5000 kc).

They made small additions (.03 gram atom) of var­

ious elements in the form of their oxides to a base glass of the composition l A N a 2o - 0.9 CaO - 6.0 Si02 and measured the power factor at each of the above frequencies.

They found

that the addition of sodium oxide increased the power factor strongly as did alumina and zirconia to a lesser degree. Other additions, the oxides of Si, V, Zn, B, Mg, Ti, Co, Ca, Ce, Ni, Mn, Pe, Ba, Pb, Li and K, lowered the power factor from that of the base glass in increasing amounts from S102 to K20.

Although these authors did not make their composi­

tional changes in a manner which could be interpreted struc­ turally, one point should be noted.

From the formal theory

of glass constitution, one would expect that high field strength structural cations like Si

would tighten the struc­

ture and decrease the oscillations (power loss) of the ions. The series above indicates that neither Si^+ nor efficient as Ba

or K

containing glasses.

is as

in lowering the power loss of sodium

51 Humphrys and Morgan

5V

measured the refractive indieles

(n^), the dielectric constants at 1525.1 he and the densities of two series of sodium borosilicate glasses at 25°C.

For

isotropic materials which do not exhibit orientation polar­ ization, the dielectric constant measured at radio frequency is roughly equal to the square of the refractive index measo ured with visible light, or K = n^*". (Maxwell Relation) At radio frequencies, one measures the total polarization, Pt, (electronic plus atomic) and at light frequencies, one meas­ ures only the electronic response (Pe), because the relaxa­ tion time of the atomic response is too long.

The dielectric

constant is related to the total polarizability of a medium by the Clausius Mosottl equation, K-l = V37TN K+T" where N represents the number of particles per unit volume, that is; N = Na 1 = Na . d 7m m where Na is Avagadro*s number, Vm the molecular volume, d the density and M the molecular weight, then: K-l M = iJ-77*Na = Pt = Pe+Pa "PF2 c[ J where Pt is the total molar polarization and Pa the molar atomic response. Combining the Maxwell relation with the Clausius-Mosotti equation, one obtains the expression of Lorentz-Lorenz for the molar electronic polarization, Pe, or the molar refraction:

52 M

n2 - 1

3

n2‘~+ 2

= V3

tt* N a

= Pe

By determining Pt and subtracting Pe, Humphrys and Morgan were able to determine Pa and to separate the electronic and atomic polarization.

Since the molecular weight of a glass

has no meaning these authors used the specific form of the Clausius-Mosottl and Lorenz-Lorentz relations by substitu­ ting 1 for M to obtain the specific polarization per gram.

cT

a

Polarization calculated in this manner has the dimensions of volume;

that is, c.c. per gram of glass.

The first series of glasses (A) (Figure 9) were obtained by adding

to a sodium silicate glass (Na20-4-.69 SiC^).

The specific electronic,

atomic and total polarization are

plotted against the o/B+S-j_ ratio.

The addition of BgO^ to

the sodium silicate decreases the number of oxygens per net­ work forming cation from 2.2 to 1 .5 . Humphrys and Morgan made a second series in w h i c h they added SiOg to a Na2B20ij_ glass. ratio does not change.

(B) of glasses The O/B+S-^

Specific polarizations are plotted as

a function of the mol fraction of sodium borate.

Figure 10

shows that electronic polarization is rather insensitive to sodium ions, but that the atomic polarization increases rapid­ ly on the addition of sodium ions.

Because of the excess of

extranuclear electrons over those balanced by the positive nu­ clear charge, the molar electronic polarizabilities of anions are much larger than those of cations and one can assume

that

the electronic polarization of a glass is due almost entirely

SPECIFIC

POLARIZATION

cc/gm

/

0.28-SERIES

A

0.27-0.26-0. 25--

2.2

2.0 RATIO

O / B + Si

SERIES

A

SPECIFIC

POLARIZATION

cc/gm

0.15 -0.1 4 -0.1 30.1 2

--

0.1 0

--

0.09--

2.2

2.1

2.0

1.9 R AT 10

1.7 1.8 0 / B + Si

1.6

1.5

FIGURE 9 - Polarization i n Sodium Borosilicate Glasses. Top: Total Polarization. Bottom: Electronic and Atomic Polarization (Humphrys and Morgan)

0.28T E o> 0.270.26SERIES

B

b 0.25< 0.24-

0.23-

UJ 0 .22 -

Si 0,

D.l

0.2

MOL

FRACTION

0.3

0.4

0.5

0.6

Naa O . B ftO s

O. I6t 0.15 SERIES B

o 0.14-

5 0 .10LU 0.09-

SiO

0

0.I

0.3 0.2 MOL FRACTION

0.4 0.5 Na^O.B^Oj

0.6

FIGURE 10 - Polarization in Sodium Borosilicate Glasses. Tops Total Polarization Bottom:Electronic and Atomic Polarization (Humphrys and Morgan)

53 (Alpha of 0

to anionic response. Br

— .020 Na

V

= .019)

= 2.76, Si^”+ =

.039,

Humphrys a n d Morgan used this f a c t

with the polarization and density information to draw co n ­ clusions concerning the role of oxygen a nd "boron ions in the structures of their glasses. A series of dielectric loss experiments on simple a l ­ kali borate glasses performed by Russian authors is of spe­ cial pertinence to this dissertation.

Skanavi a n d Marty-

55/

ushov"— found a distinct minimum in the dielectric dissipa­ tion factor

(tanA)

ratio of alkalis.

in mixed alkali borates around a 1:1 They found that the minimum in t a n A

became more pronounced with increasing total alkali content (Figure 11 a.b.c.).

At low total alkali content,

the dis­

sipation factor was rather indifferent to a ten-fold change of frequency. tent,

(Figure 11 c) At higher total alkali con­

the minimum part of the tan

A

curve was not markedly

affected by the same frequency change.

However,

tan A

was higher for the lower frequency toward the single alkali end member glasses.

(Figure 11 d)

An examination of the

effect of humidity on the dissipation factor of mixed alkali borates showed that the tan

A

curve was only slightly d i s ­

placed upward by a change from dry air to midity.

relative h u ­

(Figure lie) An evaluation of the effect of tempera­

ture on the t a n A curve indicated the general increase to be expected wi t h increasing temperature.(Figure Ilf)

Figure

FIG.

II

25

6

4

20

2

4L

0 16

Na

12

8

4

(a)

0 u

I Q-

32

24

12 8

32

64

96

64

Na

32

(f)

0

Li

550

540 -

530

w 520 510 64

48

32

16

0

Na

32

N/a

48 80

64 64

(d)

80 48

96 32

112

60

128

L, 50

ca

40

30

20

(h)

10

0

Ba

40

20

o

Figure 11 - Results of Russian Dielectric Experiments on Borate G-laeee s

Note:

All ordinates are tangent delta (10 3 ) unless otherwise indicated.

11a.

Dependence of tangent delta on the composition of the alkali content of the glass, 1:6 (Na+LiJ-lOOBgO^. Wave length, 132 meters.

lib.

Same as 11a for glass, J2 (Na+Li)-100B20^.

11c.

Same as 11a for glassy 6^ (Na+LiJ-lOOB^O^ at two wave lengths, 132 and 10.6 M.

lid.

Same as 11a for glass, 126 (Na+Li)-lOOB2&2 at two wave lengths, 132 and 10.6 M.

lie.

Effect of humidity on tangent delta of the glass, 60 (Na+K)-100B20^ at I32 M.

Ilf.

Effect of temperature on tangent delta of the glass, 126 (Na+Li)-100B20^ at 132 M.

llg.

Dependence of the devitrification temperature on the composition of the alkali content of the glass, 6^ (Na+Li)-lOOB^O^ at 132 M.

llh.

Dependence of tangent delta on the composition of the divalent cation content of the glass, 5° (Ca+Ba)-100B20

Figure 11 - Results of Russian Dielectric Experiments on Borate Glasses

Note:

All ordinates are tangent delta (10 3 ) unless otherwise indicated.

11a.

Dependence of tangent delta on the composition of the alkali content of the glass, 16 (Na+Li)-100B20^. Wave length, 132 meters,

lib.

Same as 11a for glass,

(Ka+LiJ-lOOBgO^.

11c.

Same as 11a for glass, 6^- (Na+Li-)-100 BgO^ at two

wave

lengths, 132 and 10.6 M. lid.

Same as 11a for glass, 120 (Na+LiJ-lOOBpC^ at two wave lengths, 132 and 10.6 M.

lie.

Effect of humidity on tangent delta of the glass, 60 (Na+K)-100B20j.at I32 M.

Ilf.

Effect of temperature on tangent delta of the glass, 120 (Na+Li)-100B20^ at 132 M.

llg.

Dependence of the devitrification temperature on the composition of the alkali content of the glass, 6^ (Na+Li)-l00B_0_ at 132 M.

llh.

Dependence of tangent delta on the composition of the divalent cation content of the glass, 5° (Ca+BaJ-lOOBgO

5^ 11 g shows the dependence of the devitrification temperature on the composition of a series of mixed alkali borates. Val*ter, Gladkikh and Martyushov^in a similar inves­ tigation found a slight minimum in tan A in a series of mixed calcium lithium borates.

Mixed calcium barium borates

showed a non-linear relationship between the dissipation factor and composition, but no minimum.

(Figure 11 h)

The patent literature on glasses with low power factors is of interest.

Several facts regarding the composition of

commercial glasses of low power factor are available in the 57/ claims of a series of patents issued to Armistead (Corning Glass Works).

The glasses are all characterized by a con­

siderable content of lead or barium oxide.

Three of the

compositions cited are free of alkali, while in one which contains a small amount, the alkali is divided among sodium, potassium and lithium.

Boric oxide and fluorine are found

in several of the compositions presumable to enhance the melting of the low alkali glasses. crease the chemical durability.

Alumina is used to in­

These glasses are interest­

ing because they were designed as substitutes for mica, whose dielectric loss is very low.

55 Experimental Procedure. A.

Measurement of Dielectric Constants and Power Factors" ! “ The dielectric constant of a material is defined

as the ratio of the capacity of a condenser filled with the material to its capacity filled with air or evacuated. The dielectric constant of air is very nearly one (1.000003), so that for practical purposes, the air capacity, Ca, is equal to the evacuated capacity, Cv, and K = Cx = Cx. There Ua W are many methods which can he used to determine these cap­ acities, depending on the frequency used and the electric stress desired.

Most measurements are made with instru­

ments which allow the capacity of a test condenser to he compared with the capacity of an accurately variable air condenser of one type or the other by substitution. The *4" Schering Bridge is a very popular instrument and when shielded properly is capable of great accuracy.

The other

type of instrument in common use is known as a MQ H meter*. Both types of instruments were used in this investigation, the former for measurements at 60 cycles and the latter at 600KC.

The power factor was measured with these instruments

coincident to the capacitance determinations. The capacity, C, of a parallel plate condenser is a function of the area, A, of the plates, the distance, d, + General Radio Schering Bridge 671-A * Bo on ton Radio Q,-meter 160A

56 between them and the dielectric constant of the dielectric medium between them according to the equation, C = KA

.

t^rfcr It is thus necessary to know accurately the area of and the distance between the condenser plates very accurately and to avoid all capacitance due to lines of force which do not pass through the volume bounded by the plates.

Various tech­

niques can be used to avoid edge capacitances and control 56/ stray capacitances. ^ The A.S.T.M. discusses standard tech­ niques for measurements.

In general, measurement techniques

become more involved as the frequency and electric stress in­ crease.

For example, measurements at high electric stress or

at frequencies above three megacycles must be performed using guard rings on very carefully machined samples.

At still 52/ higher frequencies, more involved methods are employed.— This dissertation is an attempt to show the value of dielectric measurements as a tool to extend our knowledge of the constitution of glass.

It was desired to obtain reason­

ably accurate dielectric measurements on a large number of simple glasses of systematically varied composition, even though they were often not very amenable to sample fabrica­ tion because of high coefficients of expansion, inhomogen­ ie ties, etc.

The instruments available were not designed for

the use of guard rings, with the result that some other tech­ nique was necessary to control edge and stray capacitances. J. B. Whitehead, on the basis of an extensive background of

57 dielectric measurements, suggested a measuring cell design which was adopted with modifications.

The cell was necessary

to control the stray capacitance from the stressed electrode to its surroundings.

At first, an attempt was made to make

measurements with the cell using secondary standards, vit­ reous silica and mycalex, and samples which were accurately ground to be of the same dimensions as the standards.

The

edge and stray capacitances would then be the same for the secondary standards and samples.

The Blanchard Machine Com­

pany kindly consented to grind about two dozen of these sam­ ples for the initial experiments*

The accuracy of this pro­

cedure was high (Appendix II, Table A, PbO^ vs. PbOg), but the time consumed in transit and the high sample mortality rate during grinding made the adoption of some other tech­ nique mandatory.

E. W. Lindsay, of the Westlnghouse Research

Laboratories suggested the technique which was used for the remainder of the measurements*

This method depended on the

use of painted-on electrodes on samples of the same diameter as the measuring electrodes.

Then, the edge and stray cap­

acitance could be controlled as described below: The cell which was used shielded the high electrode circuit from its point of contact with the instruments to the point of contact with the sample. equipment are shown in Figure 12.

The cell and measuring

The high, or stressed,

electrode of the cell was attached to the roof of the grounded brass case by a column of polystyrene.

The low,

FIGURE 12 - Top: The Measuring Cell Bottom: The Measuring Cell and Instruments

53 or grounded, electrode was on a telescoping spring-loaded mount which could be lowered to receive the sample.

This

arrangement eliminated all variation in the stray capaci­ tance between the high electrode and its lower potential surroundings because the position of the high electrode was fixed.

The only remaining variable which had to be con­

trolled was the edge capacitance which was a function of the electrode spacing.

The edge capacitance was the same at a

given electrode spacing with the sample in or out since the sample had the same diameter as the electrodes, and the edge lines of force were always in air.

The electrodes were

accurately spaced at various distances and capacitance read­ ings were taken.

By subtracting the capacitance of the air

volume between the plates, it was possible to draw a cali­ bration curve of cell capacity versus electrode spacing. The values from this curve were subtracted from the meas­ ured capacitance to give the capacitance of the sample in the measurements of the glass samples of various thicknesses. The use of the calibration curve was possible since all of the capacitances due to the measuring cell and its connec­ tions were in parallel with the sample.

Capacitances in

parallel are directly additive, so that the subtraction of the air capacitance between the electrodes at various spacings gave total cell capacitances, which could be subtracted from the measured capacity to give the sample capacity.

With

59 this technique, it was possible to use a sample that had to conform with only two dimensional requirements.

The

sides had to be vertical and the diameter the same as that of the electrodes.

Prom a determination of the volume of

the sample, one could calculate an average thickness to be used in the dielectric calculations.

The use of a painted-

on conducting silver coating effectively made the surfaces of a sample its electrodes.

An average thickness could be

used because dielectric constant is a function of the vol­ ume of the dielectric medium and shape requirements neces­ sary only to control edge capacitance. Because power factor determinations were desired, some care was exercised in the choice of the insulation of the shielded cable which connected the cell to the instruments and of the column material connecting the stressed electrode to the cell roof.

The power factor measured is a function

of the power factors of the various dielectrics in parallel, i.e., the cable, the polystyrene column and the sample.

For

simplicity*s sake, it was desirable to avoid an appreciable power factor in the measuring apparatus.

Polyethylene in­

sulated coaxial cable was recommended by E. W. Lindsay.

An

electrode supporting column of polystyrene was satisfactory. The power factor of the measuring appendage was found to be negligible when measurements were made with an air gap between the electrodes.

6o B.

Preparation of Sample. The sample glasses were melted in small refractory

crucibles.

The intentional changes in composition were so

great that the small amounts of alumina introduced during melting were not important.

The dielectric properties are

not particularly sensitive to very small amounts of alumina. The melting was done in ’’globar" furnaces.

Several times

during melting, the glasses were stirred to enhance homogenity and to aid in removing gas bubbles.

The glasses

were fined as much as practical and pressed into disc-shaped carbon molds.

This gave a glass disc with straight vertical

sides and with a diameter of 2 inches, the same as that of the electrodes in the measuring cell. Since the dielectric properties of glass are a function £ 1/ of their heat treatment , care was taken to melt and anneal the glasses to be compared in groups so that their thermal histories were comparable. The discs were ground by hand on a lap to remove the flash and sharpen the edges.

The thickness of the glass

samples was kept between l/$H and j/l6u.

The volume and den­

sity of the sample were determined simultaneously by an Archimedes density measurement, which was corrected for water density.

From the volume an average thickness to be used in

the dielectric equations was calculated.

A very low resis­

tance organically suspended silver coating* was carefully applied to the faces of the sample. * Dupont

61 The dielectric properties of glass are affected by the amount of water adsorbed by the glass.

With simple glasses

of high hygroscopicity, it is especially important to con­ trol the amount of moisture present. uring all samples over P2°5*

^

This was done by meas­

cel1 (Figure 12) had a

tight door and a false bottom under which a shallow petri dish of phosphoric anhydride was inserted during measure­ ment.

The samples were removed from their storage dessi-

cator and quickly transferred to the cell.

They were al­

lowed to dehydrate in the cell over the PpO^for twenty minutes.

Power factor measurements with the Schering bridge

showed that the edges of the sample were dry in this length of time because the power factor, which was very sensitive to moisture, reached a stable value.

The smooth dielectric

loss curves for each series of glasses also excluded surface effects due to moisture. K£

One can expect large changes in

due to changes in hydration of the sample.

Never was any

large deviation from continuity observed. C.

Discussion of Accuracy and Reproducibility. Because of the variations in equipment and techniques in

different laboratories, it is practically impossible to es­ tablish the absolute accuracy of dielectric measurements. For the purpose of this thesis, reproducibility was necessary but a high degree of absolute accuracy was not.

However, every

reasonable effort was made to obtain the maximum absolute ac­ curacy with the instruments available.

62 Both instruments were checked against two radio-type condensers previously measured on a very precise, shielded Schering Bridge and found to be accurate to 1$ in their capacitance readings.

The instruments and the measuring

cell were checked against secondary standards, vitreous silica and "Mycalex", and the results showed that the total error in the measuring equipment was of the order of 2$. This error is reasonable in light of the fact that deter­ minations on the "Mycalex" secondary standard at the Westinghouse Laboratories where no expense or labor has been spared on equipment gave dielectric constants of 7*3^ an& 7.Ml-, depending on the method used. Absolute accuracy in power factor determinations is very difficult to obtain.

Power factor is very sensitive to the

electric stress, the frequency and to any variation in meth­ ods or equipment.

The power factor measurements, by Westing-

house, on the "Mycalex" mentioned, above varied by 12$ at the same frequency for two different methods of measurement. How­ ever, power factor measurements of acceptable reproducibility were made by the author by careful maintenance of a standard measuring procedure.

Several checks were made on the reprodu­

cibility of the whole experimental procedure.

The following

table gives a comparison of two glasses of the same composi­ tion which were compounded, melted, annealed, fabricated and measured separately: The cumulative error in K & w a s "$.1% of

63 the larger reading for the higher frequency and \.6% for the lower frequency. Table I Check on Cumulative Error in Dielectric Measurements

Composition Density

k

60 cps .P-b^sln

: kA:

K

60OKC Ek,Bln

KA

Na20 . 3Si02 2.^19

15.9

.to 5

6.63 7.19

.0263

.193

Na 0 . 3Sio

16.2

,kl6

6.95 7.26

.0257

.137

2.kl3

This error is not serious when compared with the magni­ tude of the variations in K A caused by the intentional compo­ sitional changes made through a series of glasses.

The choice

of this particular pair of check glasses as an indicator of the reproducibility obtainable with the technique used in this work was based on the fact that the uncertainty of dielectric measurements increases with the dielectric loss of the sample. This particular glass composition showed one of the highest dielectric losses measured and the KAreproducibility can be regarded as about the lowest obtained. D.

Scope of the Experimental Work. The paucity of published dielectric measurements on simple

glasses led to a series of general experiments which were nec­ essary to obtain enough information to attack the problem of the relationship between the structure of glasses and their dielectric properties with any directness.

It was desirable

to ascertain the effect of the various univalent and divalent

6^ cations on the dielectric loss of simple silicates.

This was

done with a series of eleven glasses of the general composi­ tion 13r 20 - iM- RO — 63 SiOg* 13 Na20

These glasses are listed below:

-1^MgO - 63 Si02

H

ill- CaO

" "

"

Ik- SrO

" "

11

1J4- BaO

M u

"

14- ZnO

" "

"

1^1- CdO

“ 11

"

111- PbO(a)

"

M

Ik- PbO(b)

" "

13 K2o

-1*1-CaO

'»

'» "

13(L1K)20

" "

»

"

13 Li20

" "

"

"

The results from this series indicated that itwould be desirable to examine in detail the effect of alkalis and the effect of more polarizable divalent cations on dielectric loss. The effect of Increasing alkali content was investigated in a series of five sodium silicate glasses whose composition was varied from Na20 - kr Si02 to Na20 . 3 Si02 .

The effect of

mixtures of alkalis on the dielectric loss of silicate glasses was determined in two series of silicate glasses of the general compositions: and

20 RgO - 12 CaO - 63 Si02 . R20 -

3 Si02

R = K - Li R = Na - Li

65 Two series of mixed alkali magnesium alumino phosphates of the general composition R - Mg A1 (PO^)^ where R was Na JC and Na - Li were made to determine the effect of alkali mix­ tures on the dielectric loss of phosphate glasses. In order to examine in detail the effect of various di­ valent cations on the dielectric loss of silicate glasses, five series of glasses of the general formula Na20 - RO - 5*3102 were made in which various divalent cations were substituted 4"*t* for Mg. These were Zn , Cu , Fe , Co , and N1 . A series of phosphate glasses of the general composition Na R A1 (PO^)g in which Cu

J.J.

was substituted for Mg

was measured.

Five series of glasses were prepared to examine the ef­ fect on dielectric loss of the substitution of various ions for silicon ion in a sodium trisilicate glass.

The series had

the following substitutions for Si^+ : TI^+ , Zr^+ , (NaAl uV,

Sb 5 \ Two other series of glasses were examined.

The first was

a phosphate glass of the general composition, R - Mg - Al(P0^)g, in which Ag

was substituted for Na , which was measured to

ascertain if there is a marked change in dielectric loss when a non-noble gas type univalent ion is substituted for a noble gas type univalent cation.

The phosphate was used because the

solubility of silver is very low in silicate glasses. Two sodium borosilicate glasses were measured.

The first

was a base glass and the second was the base glass which was

66 gray with metallic platinum.

These glasses were made in an

attempt to construct a stratified dielectric by introducing metal particles into glass. The detailed compositions of the glasses used are to be found in Appendix I. molar basis.

All substitutions were made on a

67 III.

Results A.

Presentation. The calculated numerical results of the dielectric

and density measurements are given in Appendix

I,

These re­

sults are presented in graphical form to avoid the tedious examination of numerical tables which would distract atten­ tion from the discussion of the measurements and the conclu­ sions to be drawn from them.

K/\, the energy loss per unit

volume of a dielectric, was calculated for the graphs and tables.

This was done to conform with the data presentation

of Littleton and Morey whose book, HThe Electrical Properties of Glass", is the standard reference on the dielectric prop­ erties of glass.

There is also an important theoretical reas­

on for the use of K/las described on page U-6. B.

General Experiments. In the general experiments major substitutions were made

to obtain as large effects as possible.

A sharp decrease in

the dielectric loss, K , was observed as the RO cation was successively Mg++, Ca++, Sr++ and Ba** in the experimental glass, 12 Na20 - 1^ RO - S^SiOg.

Figure 13.

The decreasing

dielectric loss of these glasses is a function of the increas­ ing polarizabilities of the divalent cations.

A comparison of

the noble gas type divalent cations, Mg

, and Sr

, Ca

or

Ba++ with their non-noble gas type counterparts in size and charge, Zn++, Cd++ and Pb++ showed that the dielectric loss of

Dielectric Loss per Uni t Volume , K A

0.2 r 600 Mg + +

Ca + +•

Ba -(-+

0

0.5

1.0

Pol ar i z a b i l i t y

Di el ectri c Loss per U m ’ Vol ume, KA

Kc

of t h e

1.5 Divalent

2.0

2. 5

C a t i o n in A r b i t r a r y U n i t s

Mg t +

Ca + + Sr + +

0.5

0

Polarizability

FI G.

1.0

0.5

13

THE AS

of

1.5

ZO

2.5

t h e D i v a l e n t C a t i o n in A r b i t r a r y Uni t s

DIELECTRIC A FUNCTION

DIVALENT

Ba + +•

LOSS

OF T H E G L A S S

18 Na2 O H R 0 - 6 8 S i 0 2

OF THE P O L A R I Z A B I L I T Y

CATI ON

OF T H E

63 glasses containing the latter was always lower than the loss of those which contained a comparable noble gas type auxiliary cation.

Figure 1*1-.

Substitutions in the RgO content of the experimental glass, 16 RgO - 1*J- CaO - 63 SiOg, resulted in large differ­ ences in the dielectric loss.

K A a t 60 cps decreased as the

RpO was changed in the order NagO, Li20, KgO, (Li, K ^ O . The sodium oxide glass had the highest dielectric loss at both frequencies of measurement, 60 cps and 600 KC. the order of decreasing loss was Na20, KgO,

At 600 KC,

(Li, K)gO, LigO.

From the general experiments,two broad conclusions were obvious.

The dielectric loss in a simple silicate glass is

dependent on the kind or number of different kinds of alkali cations present.

The

energy loss of glasses islowered by

the substitution of a

non-noble gas type divalent cation for

its noble counterpart

or by the substitution of

noble-gas type cation

of larger size.

C.

Cations.

Role of Univalent

a divalent

In order to Investigate the role of alkalis, several ser­ ies of silicate and phosphate glasses were measured.

Figure

15 (Table I) shows the increase in dielectric loss observed in silicate glasses of Increasing alkali content.

The effect

of mixed alkalis on the dielectric loss in glasses Is shown in Figures 16, 17, 13 and 19.

(Tables B, C, P, Q)

In all cases,

a minimum loss was observed around a 1:1 ratio of mixed alka­ lis at both 60 cps and 600 KC.

The dielectric loss in the

0.2

•

Noble

■

Non

Gas

Type

Cations

Nobl e G as T ype Cations

D ielectric Loss per Uni t Vo lume, K A

6 0 0 Kc

Mg + +

Sr Cd + +

0.5

. 375 Ionic

Noble Non

Cations

Nobl e G a s T y p e

Cations

6 0 cps

-

+ +-

Dielectric Loss per Unit Vol ume, K A

2.0

Gas T y p e

Rad i i A

++ Cd +■+ Pb + +

Ionic FIG.

14

DIELECTRIC AS

LOSS

A FUNCTION

DIVALENT

Radii

A°

OF T H E G L A S S

l 8 N a g0 - 14 RO - 6 8 S i 0 2

OF T H E I O N I C R AD IUS

CATION

OF

THE

600

0.2

Kc

CU

Ql to » to O c

3 a> C i Un ) O l

.04

.03

02

o 0I a> O 0

Co m p o s i t i o n 12 C a O

12 Ca O Si O 2

6 8 SiOo

6 8

c

3

2 6 0 cps

ai a.

o a

0 Com posi ti on

20 LuO I 2 CaO

12 Ca 0 6 8

S i O2

FI G.

16

6 8

DIELECTRIC

LOSS

SILICATE

6

IN

POTASS. L I T H I U M

LASSES

CALCIUM

Si02

2.0

6 0 0 Kc

Dielectric Loss

per Uni t Volume, K A

3.0 r

C o m p o s i t i on

L i2 0 3 Si 0 2

Dielectric

Loss per Unit Volume, K A

3Si O 2

6.0

4.0

3.0

6 0 cps

2.0

C o mp o s i t i o n

Li2 0

3 Si 0 2

FI G.

3 Si 0 2

17

DI E L E CT RI C

LOSS

SILICATE

I N S ODI UM GLASSES

LITHIUM

.20

W

S.

u > 05 O

_J

o W ■* —

.08

0 _Q> 1

-06

.04

600 *c

.02

Co m p o s i t i o n

Na

K

Al M g (P 0 3)6

FI G.

18

AIMgtPOg^

DIELECTRIC ALUMINO

LOSS IN SODI UM

POTASSI UM MAGNESI UM

METAPHOSPHATE GLASSES

■ Volume, k A

.08

.07

.06

Loss

per Unit

.09

Dielectric

.05

.04

. 03

.02

600^°

Composition

Na Mg Al ( P 0 3 ) g

FIG.

19

Mg Al l P 0 3 ) g

DIELECTRfC

LOSS

ALUMINO

IN

SODIUM

METAPHOSPHATE

LITHIUM GLASSES

MA G N E S I U M

69 mixed alkali alumino phosphates at 60 cps reached a minimum value so low that it could not he measured. It was desirable to examine a series of glasses in which a univalent non-noble gas type cation was substituted for its alkali cation counterpart.

In a series of phosphate

glasses which have a relatively high tolerance for metals, silver ion was substituted for sodium ion.

The dielectric

loss, Figure 20. (Table R) first decreased on the addition of silver, then increased again as more sodium ion was re­ placed. D.

Substitution of Various Cations for Mg** Several groups of glasses of the general composition

Na.20 - RO - 5 Si02 were made in which various divalent cat­ ions were substituted for Mg++.

These glasses were espe­

cially interesting because low temperature viscosity data was being obtained on them range plan to accumulate as much data as possible on some selected simple glasses.

The parallel between the dielectric

and viscosity effect of the various divalent cations intro­ duced was striking.

The dielectric loss, K A , was decreased

strongly by the substitution of Gu++ for Mg++, less strongly by the substitution of Fe++, Co++ or Ni++ for Mg++ and was +4* 4-4* practically unaffected by the substitution of Zn for Mg The low temjperature viscosities of these glasses were found to decrease in the same order.

The results of the dielectric

Volume, K A

.08

per Unit

06

07

Dielectric

Loss

.05

.04

600 Kc .03

.02 .01

Composition

No Mg Al ( P 0 3 FI G. 2 0

A( Mg Al 1 P ° 3 > 6

)6

DIELECTRIC

LOSS

IN S O D I U M

ALU M IN O META PHOSPHATE

SILVER

MAGNE S I UM

GLASSES

70 measurements are given in Figures 21, 22, 23, 2K and 25. (Tables D, E, F, G-, and H) It was desirable to examine the effect of the substi­ tution of Cu++ for Mg++ on dielectric loss in another type of base glass.

This was done in a series of alumino phos­

phate glasses.

The results are shown in Figure 26. (Table

S)

At 60 ops, the energy loss, K

, went through a minimum

value and at 600KC decreased in an almost lineai* fashion with increasing copper content. E.

Substitution of Cations for S i ^ Several series of silicate glasses were made in which

various cations or combinations of cations were substituted for silicon ion in a NagO - 3 S3-°2 base glass. Il+

two series, Ti

In the first

li^

and Zr

were substituted for Si

results are given in Figure 27. (Tables J and K).

.

The

Titanium

lowered the dielectric loss sharply at both frequencies. Zir­ conium decreased the dielectric loss at 60 cps, but increased the loss at 600 KC. Low temperature viscosity measurements 60/ 1l+ by R. L. Thakur— showed an increase in viscosity as Ti re­ placed Si^+in a sodium trisilicate.

The substitution of

Zr^+ for Si^+ in the same glass gave a sharper increase in low temperature viscosity. In the remaining series of the glasses, Al-^+ , (Na,Al)^"* (r j l

and Sb^

Ji +

were substituted for Si

•

In the series where

(Na, Al)^"*" was substituted for silicon, the number of oxygen

Dielectric L os s per Unit Vo lum e, K A

Dielectric L o s s per Unit V o l u m e , K A

CCOO H m _ CD

o r c o t» o co

CO m

co £> CD 2

T 3

m co

c

5 ?NI n rsi 2

o o

co N

2

600

per

.06

.04

Dielectric

Kc

08

Loss

Unit

Vol ume, K&

.12

.0 2

Comp o si ti on

Na20

Cu0

Mg 0 5 Si 0 2

Volume,KA

5 S i Og

cps

0.8

Dielectric

Loss

per

Unit

60

G o m po s i t i o n Mg

5 Si 0 FIG.

Cu 0

0

5 Si 0 2

2

22

DIELECTRIC

LOSS

IN

SILICATE

SODIUM GLASSES

MAGNESIUM

COPPER

KA

Kc

.10

.05

D ielect ric

Loss

per

Uni t Vo I u m e ,

600

Compositi o n Mg 0

Fe

5 S i0 ,

per Unit Vol ume} KA

5 Si0

Loss

60 c ps

Dielectric

0.5

Composition

Nag 0

FeO 5 Si O,

Mg 0 5 S i0 o FIG.

0

2 3

DIELECTRIC IRON

LOSS

SILICATE

IN

SODIUM

GLASSES

MAGNESIUM

2

Kh

.15 Kc

, 05

Dielectric

LossperUnif

Volume,

600

Composition CoO 5 Si Og

Dielectric

Los?

per

Unit

Vol ume, K-A

Mg 0 5 S i02

Composi ti on Mg 0

CoO

5 Si O 2

5 Si Og

FI G

2 4

DIELECTRIC COBALT

LOSS

IN SODI UM

S ILIC A TE

MAGNESIUM

GLASSES

KA p e r Un i t Vol ume,

6 0 0 Kc

.10

.05

c

t r i c

Loss

.15

Ni 0

MgO

5 Si0o

5 Si02

5

l u

m

e

, D

i e

l e

C o m posi t i o n

c ps

0

Loss

per

Uni t

V

o

6

Dielectric

5

0 Composition

No„0

MgO

Ni 0

5 Si0 o

5 S iO g

FIG.

25

DIELECTRIC NICKEL

LOSS

SILICATE

IN

SODIUM

GLASSES

MAGNESIUM

Vol ume, K Zi Unit per Loss Dielectric

600

Kc

Composition

Mg

Cu

No Al ( P 0 3 L 0 6

FI G. 2 6

No Al ( PO, ) 0

DIELECTRIC ALUMINO

LOSS

IN

SODIUM

METAPH.OSPHATE

MAGNESIUM GLASSES

COPPER

6

\ f o l u me , K Z l

.2

Loss

per Uni t

6 0 0 Kc

Di el ect r i c

I

O

Composi tion

Na q 0 3 Si Og

No? 0 . 7 5 Ti 0 2 ( Z r 0 2 ) 2.25 Si02

0

6 0 cps

6.0 5.0

Diel ectri c

Loss

perUmf

V o l u me , K

A

7.0

2.0

C o mp o s i t i o n 3 Si0

. 75 Ti O 2 ( Z r 0 2 ) 2 . 2 5 Si 0 2

2

FIG.2 7

DIELECTRIC

LOSS

(ZIRCONIUM)

IN

SODIUM

SILICATE

TITANIUM

GLASSES

71 + anions per Al + Si was kept constant, but the number of Na / v increased. Dielectric loss increased rapidly when (Na, Al) )l+ was substituted for Si in a sodium trisilicate, Figure 2S (Table L).

At the lower frequency dielectric loss increased

beyond the range of measurement of the Sobering Bridge at only .05 (NaAlJOg substitution for Si^+ in the .25 Na.gO .75 Si02 base glass.

At 600 KC, the loss Increased steadily.

A direct substitution of Al^+ for Si^+, Figure 29, (Table M), gave glasses of increasing loss.

In this series

of glasses, the number of oxygens per Al + Si decreased since (A10]_#9 ) was substituted for S102 . The replacement of Si^+ by Sb5+ with the attendant in­ crease in oxygen anions per cation in sodium trisilicate glass gave glasses of decreasing dielectric loss, Figure 30, (Table N).

R. L. Thakur found decreasing low-temperature vis­

cosities for the same series of glasses. F.

Stratified Metal-Glass Dielectric. Table 0 shows the results of the attempt to produce a

stratified dielectric by introducing metallic platinum into a sodium borosilicate glass.

Stratified dielectrics are

known for their high dielectric constant which is related to the number of interfaces between the dielectrics which make up the whole.

The gray glass containing the platinum showed

no appreciable difference in dielectric properties from those

Q Gomposi tion

. 25 Na 2 0

. 2 5 N a 20

. 7 5 Si 0 2

7.0

l 5 ( Na AI ) Q> .60 S i O 2

Very

High

^ it 6.0 qT

e

2o 5.0 >

6 0

cpc

o v. 2.0

Gomposi ti on

.25Na,0

.I5(NaA0Oj

.7 5 Si Oc

. 6 0 Si02

FIG. 2 8

DIELECTRIC GLASSES

IN

LOSS WHICH

IN

SODIUM

ALUMINUM

(NoAI)02 W A S

SILICATE

SUBSTITUTED

0. 3

0.2,

Unit

Volume, K9 (I876) b. Phil. Mag., 2, 314- (1876) c. Proc. Roy. Soc.,A25 J4-96 (1^76) d. Phil. Trans., 167, 599 (1^77)

36.

Gevers, M., Philips Research Reports (194-5-4-6)

37. Curie, J.,

Ann. chem. phys., IS,

36. vonSchweidler,

203 (1669)

E.,Wein. Ber., 116,

1019 (1907)

Ann. Physik,

24;, 711 (1907)

Ann. Physik,

13, 766 (1907)

39.

Guyer, E. M., J. Am. Ceram. Soc., 16, 607 (1953)

4-0.

Weyl, W. A., Office of Naval Research, Task Order 6 , Tech. Rep. 7 (194-9)

4-1.

Gutman, F., Rev. Mod. Phys., 20, 4-57 (194-6)

4-2.

Pellat, H., Compt. rend., 123 1312 (1399)

i4-3. Wagner, K. W., Ann. Physik, 4o, S17 (1913) 44-.

Siemens, W., Pogg. Ann., 125, (1864)

11-5•

Hopkinson, J., "Original Papers", 27 (1697)

46.

Hess, A., Eclairage Elec. 4-, 205 (1&95)

k-J. Beaulard, F., Compt. rend., 119, 263 (1394) 43.

Grover, F. W., Bull. Bur. Stand. Washington, 7,^95 (1911)

4-9. Hoch, E., Bell System tech. J., 1, 110 (1922) 50.

Barth, Cited by Wells, A.F., "Structural Inorganic Chem­ istry", Clarendon Press, Oxford (194-7) p. 4-65

V

51.

Littleton, J. T., Morey, G. W., "The Electrical Prop­ erties of Glass", Wiley, New York (1933)

52.

Navias, L., Green, R. L., J.Am. Ceram. Soc., 29,, 267 (194-6)

53-

Thurnauer, H., Badger, A. E., J. Am. Ceram. Soc., 25, 9 (1940)

54-.

Humphrys, J. M., Morgan, W. R., J. Am. Ceram., Soc., 2H-, 123 (i9lil)

55.

Skanavi, G. I., Martyushov, K. I., J. Tech. Phys., (USSR), 9, 1024-31 (1939); C. A. 22, 2369 (1939)

56.

Val*ter, A., Gladkikh, Martyushov,K,J. Tech. Phys. (USSR), 10, 1593 (1940)

57.

Armistead, W. H., Patents US 2393W

(1-22-4-$; US 2393449

(1-22-4-6); US 23934-50 (1-22-4-6) 5S.

ASTM III B (194-6) P 64-7

59 . Kupinsky, T., Division, Am.

Paper presented to meeting of the Glass Ceram. Soc., Oct 6 (194-9). . In print.

60.

Thakur, R. L., private communication.

61.

Poole, J. P.,

Note;

J. Am. Ceram. Soc., 2£, 232 (194-9)

In order to give the original worker credit, some of the inaccessible references were taken from secondary sources.