ON THE QUESTION OF DIELECTRIC LOSSES IN MICA AT HIGH FREQUENCY

K. A. VODOPYANOV

Introduction

The study of the dielectric properties of mica is of interest for two reasons: they help to clarify the mechanism of dielectric losses in complex crystals, such as mica, and they provide a characterization of the most widely used electrical-insulating material in the electrical and radio-engineering industry.

In chemical composition and in the structure of its crystal lattice, mica has very much in common with talc, kaolin, and minerals.

In crystalline bodies of the mica type, silicon–oxygen tetrahedra form flat sheets parallel to one another. B. M. Tareev [1] points out that the structure of talc is usually taken as the basis in elucidating the structures of crystals of the mica type. The structure of mica differs from that of talc in that in it one of the four silicon atoms is replaced by an aluminum atom.

Crystals of talc and mica contain hydroxyl groups OH as a structural element. Depending on the strength of the bond of these hydroxyl groups in the crystal lattice, the dielectric polarization caused by the orientation of OH dipoles in an alternating electric field must have one value or another.

We [2] have shown that in crystalline talc there are temperature–frequency maxima of the dielectric-loss angle. The mechanism of dielectric losses caused by the orientation of hydroxyl groups of crystallization water has a relaxation character. Similar regularities were also observed by us [3] for phlogopite mica from certain deposits.

It is of interest to clarify the nature of dielectric losses in muscovite and phlogopite mica, taking into account the quantitative and qualitative characteristics of crystallization water and mineralogical inclusions.

Dielectric properties of muscovite and phlogopite mica

As a result of studying dielectric losses in muscovite mica at high frequency, we [5] have shown that for pure muscovite the absolute value of the dielectric-loss angle, measured perpendicular to the cleavage plane, is of the order of \(10^{-5}\) radians. Over a wide frequency range the loss angle does not change. The presence of impurities in the form of mineralogical inclusions increases the loss angle, the absolute value of which increases as the frequency decreases.

For pure muscovite the absolute value of the dielectric-loss angle, measured parallel to the cleavage plane, is of the order of \(10^{-4}\) radians. As the frequency decreases, the loss angle increases somewhat. The greatest

an increase in the loss angle with decreasing frequency is observed in micas that have mineral inclusions.

The influence of temperature on the loss angle of pure muscovite is small; the dielectric loss angle of spotted mica increases upon heating, and especially strongly in the case when the electric field is applied parallel to the cleavage plane.

The dielectric constant of pure mica depends little on temperature; in spotted mica it decreases with increasing temperature.

On the basis of the experimental data set forth above, the following conclusions may be drawn.

The nature of the dielectric losses of muscovite mica in the high-frequency region, in the case when the electric field is applied perpendicular to the cleavage plane, differs from ohmic losses. Depending on frequency and temperature, spotted mica may exhibit dielectric losses not only of an ohmic character, but also of a relaxation character, due to weakly bound impurity ions.

Dielectric losses of a relaxation character due to the orientation of hydroxyl groups OH did not appear, evidently because the OH groups are strongly fixed in the crystal lattice of muscovite mica.

Studies of the temperature dependence of the dielectric loss angle and dielectric constant in uncalcined and calcined at a temperature of 600°C specimens of muscovite mica, carried out in our laboratory by A. P. Izergin [4] at power and audio frequencies, showed that in calcined specimens maxima of the loss angle and the corresponding breaks in the frequency course of the dielectric constant are observed. Consequently, during the thermal treatment of muscovite, the bonding forces of the polar molecules of water of crystallization in it weakened, which facilitated their orientation in an alternating electric field.

E. K. Lashev [6], analyzing the quality of phlogopite mica from various deposits, drew the general conclusion that Pamir phlogopite is considerably better than Aldan phlogopite, and the latter better than Slyudyanka phlogopite. A comparative characterization of the electrical, mechanical, and thermal properties of these micas is given in Table 1.

It is evident from this table that the smallest dielectric loss angle is possessed by the most heat-resistant Pamir phlogopites, which have higher mechanical properties in comparison with the less heat-resistant phlogopites of the Aldan and Slyudyanka deposits.

The experimental data of Lashev presented in Table 1 have no theoretical substantiation.

It is unclear why Pamir phlogopites differ from phlogopites of other deposits.

According to chemical analysis data, Pamir phlogopite differs from phlogopites of other deposits only by a lower content of water of crystallization.

We believe that the quantitative and, chiefly, qualitative characterization of the polar molecules of water of crystallization in phlogopite mica determines, to some extent, its physical properties. The qualitative characterization of phlogopite is determined by the values of the activation energy of the polar molecules of water of crystallization or of their radicals. Its heat resistance depends on the magnitude of the activation energy of the polar molecules in phlogopite mica. The principal indices of the heat resistance of phlogopite are swelling and dehydration.

The considerations we have expressed concerning the role of water of crystallization in phlogopite are well confirmed by the experimental data presented in Table 1, from which it is evident that the maximum of swelling and the amount of water released in the temperature interval 600–1000°C in Pa-

for the Pamir phlogopite is five times less than for the Slyudyanka phlogopite. The principal mass of crystallization water in the Pamir phlogopite is released in the temperature range 1000–1200°C, which testifies to a strong bond of the polar molecules with the principal structural elements of the crystal lattice of phlogopite mica.

Table 1

Characteristics of phlogopites

Usually the degree of dehydration of phlogopite is characterized by the water released from phlogopites as a percentage of its total content. In doing so, both hygroscopic and crystallization water are taken into account. We believe that, for characterizing the physical properties of phlogopite, it is necessary to take into account only the crystallization water, which is released from the crystal at temperatures above 600°C. Hygroscopic water evaporates from phlogopite in the temperature range 100–300°C.

Hygroscopic or molecular water may increase dielectric losses by increasing conductivity; however, these losses will be noticeable only at low frequency, while at high frequency they will be considerably smaller than the relaxation losses caused by orientation of the polar molecules of crystallization water. At ultrahigh frequencies, a considerable increase in losses is possible due to weakly bound polar molecules of hygroscopic water. However, as far as we know, no one has yet carried out experiments on the investigation of dielectric losses in phlogopite at ultrahigh frequency.

Thus, it will be correct to determine the qualitative characteristics of phlogopite not by the features of the deposits, but by their physicochemical data. Such data are the qualitative characteristic of the crystallization water and heat resistance. These indices chiefly determine the magnitude and the character of the dependence of the loss angle on tempera-

ture and frequency. Consequently, the dielectric-loss angle is an important indicator of the physical properties of mica, i.e., it may serve as the principal criterion for evaluating phlogopite as a dielectric.

On the basis of these characteristics it may be considered that not only Pamir phlogopite possesses high electrical, thermal, and mechanical properties, but also all other micas, especially those with an insignificant content (less than one percent) of crystallization water. An example is the phlogopite of the Aldan (Medvedevka) deposit investigated by us; its electrical and mechanical properties, as is evident from Table 2, are close to those of Pamir phlogopite and of grade-2 and grade-3 muscovite mica.

Table

Analysis of the experimental data characterizing the dielectric properties of phlogopite mica [3] leads to the following conclusions.

The absolute value of the dielectric-loss angle of phlogopite depends on the following factors: a) chemical composition—the increase in the loss angle is proportional to the quantitative content of water, chiefly crystallization water; b) frequency of the electric field—the loss angle has a maximum in the region of audio frequencies; c) temperature—in the temperature interval 15–200°C the loss angle has a maximum, whose magnitude is greater the thicker the specimen and the lower the frequency.

The change in the dielectric-loss angle upon heating and cooling is reversible in the temperature interval 15–200°C. With increasing frequency, a displacement of the maximum of the loss angle toward higher temperatures is observed.

The dielectric permittivity as a function of temperature is represented by a curve having a break in the temperature interval where the loss angle has a maximum.

The regularities of the dielectric properties of phlogopite mica can be explained on the basis of concepts concerning the possibility of orientation of polar molecules of crystallization water or of their hydroxyl groups.

Comparative Characterization of the Dielectric Properties of Muscovite and Phlogopite Mica

Table 3 gives the principal qualitative characteristics, obtained by us, of the dielectric properties of muscovite and phlogopite as functions of specimen thickness, electric-field frequency, and temperature.

The absolute value of the loss angle in pure muscovite is many times smaller than in other micas containing inclusions. The loss angle does not

from the sample thickness, frequency, and temperature over a wide interval of these quantities. The dielectric constant changes only slightly under the influence of the above-mentioned factors.

Muscovite mica with inclusions in the form of black and yellow spots has approximately the same characteristics; the difference lies only in the nature of the dependence of the loss angle and the dielectric constant on temperature. In the case of a black spot, an increase in the loss angle is observed,

and the dielectric permittivity with increasing temperature; for the yellow spot an increase is also observed, but a maximum of the loss angle and an inflection on the dielectric-permittivity curve are also possible. In our opinion, muscovite mica with a yellow spot should behave similarly to phlogopite from various deposits, in which temperature maxima of the loss angle and corresponding changes in dielectric permittivity are observed.

The character of the temperature-frequency dependence of the loss angle of phlogopite mica of different grades is almost the same; only the absolute values differ.

Muscovite mica has considerably better electrical and mechanical properties than phlogopite. On the other hand, muscovite has lower heat resistance than phlogopite and, moreover, does not exhibit the relation between mechanical properties and thermal stability that occurs in phlogopite.

We associate the physical properties of phlogopite and the regularities in dielectric losses as functions of temperature and frequency with the presence of crystallization water and with the activation energy of its polar molecules. We related the degree to which the polar molecules of the crystallization water are bound to the basic structural elements of the phlogopite crystal lattice to its heat resistance.

How, then, are the different regularities in the characteristics of dielectric losses in muscovite and phlogopite to be explained? The large amount of crystallization water and the low value of heat resistance in muscovite appear, at first glance, to contradict our ideas about the role of crystallization water.

In fact, in chemical composition muscovite mica differs from phlogopite. It contains more aluminum, whereas phlogopite contains magnesium. The replacement of magnesium by aluminum, as is known, gives rise to the great chemical stability of muscovite [6], which improves the electrical and mechanical properties of muscovite mica. In addition, pure muscovite mica has a narrow temperature interval of maximum release of crystallization water, which testifies to its homogeneity. On this basis it may be considered that up to 600–700°C muscovite contains a small number of weakly bound polar molecules, which accounts for the high quality of muscovite.

In the temperature interval 600–700°C, however, there occurs a sharp weakening of the binding forces between the polar molecules of the crystallization water and the structural elements of the lattice. This leads to a vigorous release of water, as a result of which muscovite sharply worsens its electrical and mechanical properties.

Thus, proceeding from data on the crystal structure of muscovite and phlogopite, their chemical composition, the character of inclusions, and their thermal and mechanical properties, it may be assumed that the nature of dielectric losses in muscovite and phlogopite is the same. Only under some conditions (frequency and temperature) do losses caused by conductivity manifest themselves, while under others relaxation processes appear, caused by the displacement of weakly bound impurity ions, as well as by the orientation of hydroxyl groups. The activation energy of the latter depends on the energy bonds in the crystal lattice.

The conclusions we have drawn regarding the nature of dielectric losses in mica are of interest not only for characterizing the electrical properties of mica, but also for obtaining new high-quality dielectrics of the type of micanite and micalex. In their manufacture it is necessary to carry out heat treatment of the mica powder in such a way as either completely to remove

polar molecules of crystallization water, or increase their binding energy with other components of the fillers.

REFERENCES

1. B. M. Tareev. VEP, No. 3, 1943.
2. K. A. Vodop’yanov. ZhTF, 24, issue 1, 1954.
3. K. A. Vodop’yanov. Proceedings of the SFTI, 7, issue 2, 1947.
4. A. P. Izergin. Abstract of dissertation, Tomsk, University, 1955.
5. K. A. Vodop’yanov. Elektrichestvo, No. 11, 1950.
6. E. K. Lashev. Mica, Moscow, 1948.

Siberian Physico-Technical
Institute.

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