Electrical Strength of Talc

K. M. Kevroleva

The study of the electrical strength of talc is of both theoretical and purely practical interest. Talc crystals have a lamellar (packet) structure. The packets, consisting mainly of oxides of Mg and Si, are bound by molecules of water of crystallization, some of which also enter into the packets [1]. The structural formula of talc is

\[ \begin{array}{cccccccccccccccc} \mathrm{HO} &—& \mathrm{Si} &—& \mathrm{O} &—& \mathrm{Mg} &—& \mathrm{O} &—& \mathrm{Si} &—& \mathrm{O} &—& \mathrm{Mg} &—& \mathrm{O} &—& \mathrm{Si} &—& \mathrm{O} &—& \mathrm{Mg} &—& \mathrm{O} &—& \mathrm{Si} &—& \mathrm{OH} \\ && \| &&&&&&&& \| &&&&&&&& \| &&&&&&&& \| \\ && \mathrm{O} &&&&&&&& \mathrm{O} &&&&&&&& \mathrm{O} &&&&&&&& \mathrm{O} \end{array} \]

and the chemical formula is \(3\mathrm{MgO}\,4\mathrm{SiO}_2\mathrm{H}_2\mathrm{O}\). The temperature-frequency maxima of the loss angle, experimentally obtained and theoretically substantiated in the works of K. A. Vodop’yanov and his colleagues [2], are explained by relaxation polarization of the molecules of water of crystallization.

It is of interest to determine whether the molecules of water of crystallization manifest themselves at different rates of voltage application in strong fields, how the electrical strength of talc changes when the water of crystallization is removed, and with the physicochemical changes occurring in talc at high temperatures. There are no published data on the electrical strength of talc. We carried out a systematic study of the properties of talc under direct, alternating, and impulse voltage with durations of \(10^{-1}\)–\(10^{-7}\) sec. The material used for the investigation was lamellar talc from the Onot deposit and ground talc from the Chinese, Onot, and Miass deposits.

Figures 1 and 2 show the dependence of the electrical strength of lamellar talc on the thickness of the specimen under direct voltage and under impulse voltage with a duration of 7 μsec.

Under homogeneous-field conditions, the electrical strength under impulse voltage ranges from \(3.3\ \mathrm{MV/cm}\) at a specimen thickness of 45 microns to \(600\ \mathrm{kV/cm}\) for specimens 400 microns thick; under direct current, from \(480\ \mathrm{kV/cm}\) at a thickness of 115 microns to \(100\ \mathrm{kV/cm}\) for specimens 500 microns thick.

Such a clearly expressed dependence of the electrical strength of lamellar talc on thickness is explained by the layered structure of talc and by field nonuniformity.

The dependence of electrical strength on the duration of voltage application is shown in Fig. 3. Analogously to the data of G. A. Vorob’ev [3] for alkali-halide crystals, a significant increase in the electrical strength of talc is observed when the duration of impulse action is reduced to \(10^{-7}\) sec, which should be attributed to the delay in discharge formation.

To establish the nature of breakdown of talc under impulse, direct, and alternating voltages, the temperature dependence was measured.

electrical strength in the range from 20 to 400°C under direct and alternating voltage (Fig. 4) and from 20 to 650°C under impulse voltage (Fig. 5).

Under direct and alternating voltage the electrical strength is considerably lower than under impulse voltage.

Fig. 1. Dependence of the electrical strength of lamellar talc on the specimen thickness under direct voltage. Uniform field.

Fig. 2. Dependence of the electrical strength of lamellar talc on the specimen thickness under an impulse voltage of duration 7 μsec. Uniform field.

The impulse electrical strength of talc is practically independent of temperature in the interval from 20 to 400°C; then, with further increase to 650°C, the electrical strength decreases, which is probably connected with the transition of the electrical form of breakdown into a thermal one. Under direct and alternating voltage the breakdown is thermal. During breakdown under room-temperature conditions, heating of the specimen was observed. When the temperature is raised to 150°C, owing to the removal of hygroscopic water located between the packets of talc and part of the crystallization water, the electrical conductivity of talc probably decreases, which leads to an increase in electrical strength.

Fig. 3. Dependence of the electrical strength of lamellar talc on the duration of voltage application. Uniform field.

Under alternating voltage the rise of \(E_{\mathrm{br}}\) with increasing temperature is smaller than under direct voltage; this may be explained by the fact that thermal breakdown under alternating voltage is caused not only by the conduction current, but also by polarization phenomena (dipole polarization, etc.). The decrease in electric strength from \(200^\circ\mathrm{C}\) and above can be explained by the fact that, as the temperature increases, thermal breakdown is facilitated, as has been established for all dielectrics.

Fig. 4. Dependence of the electric strength of plate talc on temperature.

1—direct voltage; 2—alternating voltage. Uniform field. Breakdown in air.

The temperature dependence was measured under conditions of a uniform field in air. On pressed specimens, the dependence of the electric strength on the chemical composition of talc was investigated (Fig. 6). Breakdown was produced in a silicone-organic liquid at temperatures from \(-40\) to \(+250^\circ\mathrm{C}\) under direct voltage; the field was uniform, the diameter of the recess was 13 mm, the specimen thickness was 150 microns, and the pressing pressure was \(2000\ \mathrm{kg/cm^2}\). The specimens were fired in silite furnaces up to \(1300^\circ\mathrm{C}\), with a rate of temperature rise of \(100^\circ\) per hour.

Fig. 5. Dependence of electric strength on temperature under impulse voltage \((10^{-6}\ \mathrm{sec})\).

In this same figure, the dashed line shows the dependence of electric strength on temperature for ceramic materials S-61, SK-1, TK-21 based on talc, obtained by Kh. S. Valeev [4]. As is evident from Fig. 6, the character of the temperature dependence of electric strength for talc and for the ceramic materials is repeated, while the absolute values of \(E_{\mathrm{br}}\) for the ceramic materials are lower. Consequently, the breakdown mechanism in the ceramic materials and in pressed talc specimens is the same, and the high electric strength of the indicated ceramic materials is due to the high electrical properties of the talc itself. The greatest

electrical strength in our investigations is possessed by samples of Chinese talc, which, according to chemical-analysis data, has a smaller percentage loss on ignition and a larger percentage of basic oxides than talc from the Onot and Miass deposits.

Fig. 6. Dependence of the electrical strength of pressed talc on temperature and chemical composition. Breakdown under direct current. Field uniform.

1. Onot talc. 2. Miass talc.
2. Chinese talc.

To establish the influence of water of crystallization on the electrical strength of talc, the dependence on the dehydration temperature was measured. In talc, two silicic-oxygen tetrahedra are linked by a layer of Mg(OH)₂, forming a three-layer packet 6.4 Å thick.

Fig. 7 — Dependence of the electrical strength of plate talc under pulsed and constant voltage on the dehydration temperature. Field uniform.

○ — maximum value, ● — mean value under pulsed voltage \(10^{-6}\) sec; Δ — direct current.

The connection between packets is effected by weak van der Waals forces. The total amount of water of crystallization in plate talc is 5.5%; of this, 0.4% is molecular water located within and between

packages, and 5.1% constitutional water, included in the form of hydroxyl groups.

The removal of water of crystallization occurs in the following stages: [5]

80–150°C — removal of hygroscopic water.

150–200°C — removal of molecular water between the packages.

300–500°C — removal of molecular water within the layers.

600–1050°C — removal of hydroxyl water.

Up to 600°C no changes occur in the crystal lattice of talc, while from 600°C a rearrangement of ions within the lattice begins [6].

Chemical analysis of talc from various deposits indicates that the amount of water of crystallization ranges from 2.3 to 6%.

For the talc samples used by us in the investigation, thermographic curves were taken by the staff of GIEKI; these curves marked the differing intensity of the dehydration periods, as well as curves of the change in weight and volume.

Thus, we were able to determine the presence of water at different dehydration temperatures.

Figure 7 gives the values of the electrical strength of Onot talc under impulse and direct voltage as a function of dehydration temperature.

Under impulse voltage of duration 7 μsec, no dependence of \(E_{\mathrm{br}}\) of talc on the degree of calcination was found for samples from 40 to 150 microns thick; under direct voltage the electrical strength increases with increasing dehydration temperature.

Thus, water of crystallization, both molecular and constitutional, does not manifest itself under the action of an impulse voltage \(\tau = 10^{-6}\) sec and lowers the electrical strength under direct voltage.

Conclusions

1. Platy Onot talc has high electrical strength. \(E_{\mathrm{br}}\) of talc samples 40–50 microns thick in a uniform field under impulse voltage \(\tau = 10^{-6}\) sec reaches 3 MV/cm.

2. Breakdown of platy talc under impulse voltage is electrical; under direct and alternating voltage it is thermal.

3. Water of crystallization in talc does not manifest itself when the duration of voltage application is varied from \(10^{-7}\) to \(10^{-4}\) sec and when talc is dehydrated in the temperature interval from 100 to 1300°C.

4. Water of crystallization lowers the electrical strength of talc under direct and alternating (50 cps) voltage.

5. The electrical strength of pressed talc samples increases with increasing percentage content of basic oxides and with decreasing losses on calcination.

6. The electrical strength of pressed talc samples, with subsequent annealing at a temperature of 1300°C, under direct voltage is considerably higher than the electrical strength of talc samples without heat treatment. The electrical strength of Onot talc under impulse voltage, both platy and pressed, calcined at 1300°C, is practically the same.

The present work was carried out at the suggestion and under the scientific supervision of A. A. Vorob’ev, to whom the author expresses sincere gratitude.

References

1. Gassel O. Crystal Chemistry. ONTI, 1931.
2. Vodopyanov K. A. ZhTF, vol. XXIV, issue 1, 1954.
3. Vorob’ev G. A. ZhETF, vol. XXX, issue 2, 1956.
4. Valeev Kh. S. Elektrichestvo, No. 4, pp. 56–61, 1955.
5. Avgustinnik A. I. and Vigdertauz V. S. Proceedings of the Leningrad Technological Institute, issue 20, pp. 69–83, 1951.
6. Sverchkova L. A. Dissertation. Leningrad Technological Institute, 1949.

Tomsk Polytechnic Institute