ON THE ELECTRIC STRENGTH OF ROCHELLE SALT CRYSTALS

K. M. KEVROLEVA

The literature contains incomplete and contradictory information on the electric strength of Rochelle salt.

According to N. A. Prikhodko [1], the electric strength of Rochelle salt in a nonuniform field under alternating current depends on the crystallographic direction; moreover, along the \(a\) axis the strength is 2 times lower than along the \(b\) and \(c\) axes. This author relates this to the high value of the dielectric permittivity along this axis.

B. V. Gorelik [2] investigated the electric strength of Rochelle salt under constant, alternating, and impulse voltages, under conditions of a uniform and nonuniform field at atmospheric pressure, placing the specimens in transformer oil and in compressed air at a pressure of 5–10 atmospheres.

B. V. Gorelik obtained the following values of the electric strength of Rochelle salt: for pulses of duration \(10^{-5}\) sec—310 kV/cm in a nonuniform field; under direct current in transformer oil—610 kV/cm; under alternating voltage—50 Hz—70 kV/cm.

In this case no dependence of \(E_{\text{br}}\) on temperature or crystallographic direction was observed. We carried out a systematic investigation of the electric strength of Rochelle salt. The absence of definite data on the relaxation times of the dipoles of Rochelle salt prompted interest in the dependence of electric strength on the duration of voltage application.

The method for determining the electric strength of Rochelle salt deserves special attention.

1. Rochelle salt is sharply anisotropic in its properties and has an anomalously high value of dielectric permittivity along the “\(a\)” axis.

According to I. V. Kurchatov [3], the high value of the dielectric permittivity along the \(a\) axis is preserved even in strong fields. According to the investigations of B. V. Gorelik [2], the value of the dielectric permittivity along the \(a\) axis in fields close to breakdown decreases and becomes close to the values along the \(b\) and \(c\) axes; specifically, the dielectric permittivity along the \(a\) axis at a field strength of 600 kV/cm, according to B. V. Gorelik [2], is equal to 12, while along the \(b\) and \(c\) axes, according to Mason [4], it is respectively equal to 8 and 9 units.

1. Rochelle salt is a crystalline hydrate containing 4 molecules of water of crystallization, which, according to Fowler’s assumptions [4], are responsible for the anomalous properties of Rochelle salt. At a temperature of \(55.6^\circ\), Rochelle salt melts in its water of crystallization.

Our investigations, apart from taking the temperature dependence of the electric strength of Rochelle salt, were carried out at temperatures

14–18°C and a relative air humidity of 78–81%, which corresponds to the moisture-stability region of the crystals.

Breakdown was carried out under conditions of a uniform field, which was ensured by the shape of the specimen.

The radius of the recess exceeded the thickness of the specimen at its thinnest point by 25–40 times; thus the condition for creating a uniform field was fully satisfied.

The crystal was sawn with a wet thread into plates perpendicular to the axes \(a\), \(b\), and \(c\). The recess in the specimen was made with a special drill; it was polished with a wooden ball with cotton wool moistened with an unsaturated solution of Rochelle salt at room temperature.

Adjustment of the thickness was performed by grinding on wet material stretched over a smooth surface. The thickness was measured twice: with a thickness gauge accurate to 0.01 mm during the initial treatment, and on an IZV-1 vertical optimeter with a reading accuracy of up to 1 micron during the final measurement. After measurement the specimens were examined in transmitted light and, selectively, under a microscope.

Fig. 1. Dependence of the electric strength \((E_{\mathrm{br}})\) of Rochelle salt on specimen thickness.
Axis \(a\). \(\tau = 10^{-6}\) sec. \(t = 18^\circ\).

Electrodes were applied to the specimen by evaporating metal (tin) in vacuum at a pressure of \(10^{-4}\)–\(10^{-5}\) mm of mercury. This ensured tight contact of the high-voltage electrode with the crystal, which was checked directly with a tester, one lead of which was connected to the spherical high-voltage electrode and the other to the layer of metal extending beyond the limits of the recess.

Breakdown at temperatures above \(0^\circ\)C was carried out in transformer oil, and at negative temperatures (below \(0^\circ\)C) in alcohol in a thermostat equipped with temperature control. The pulsed breakdown voltage was measured with an electronic oscillograph, and for direct and alternating current with an S-96 electrostatic voltmeter.

Figures 1 and 2 present the dependence of the electric strength of Rochelle-salt crystals, cut perpendicular to the \(a\) axis, on the thickness of the specimen.

The strengthening in thinner specimens, as well as the increased scatter in the range 110–220 microns, is explained by the inhomogeneity of Rochelle salt. In subsequent experiments, equivalent thickness intervals were chosen, namely: for determining the temperature dependence, 160–180 microns; for determining the volt-second characteristics, 140–200 microns.

Figure 3 shows the dependence of electric strength on temperature.

Figures 4 and 5 present changes in electric strength at different breakdown probabilities, plotted for different crystallographic—

Fig. 2. Dependence of \(E_{\mathrm{br}}\) of Rochelle salt on the thickness of the specimen under direct current; specimen pit diameter \(10\ \mathrm{mm}\); \(t = 18^\circ\).

Fig. 3. Dependence of \(E_{\mathrm{br}}\) of Rochelle salt on temperature under impulse voltage; \(\tau = 10^{-6}\ \mathrm{sec}\).

Breakdown along the \(a\) axis. Uniform field.

Fig. 4. Electric strength of Rochelle salt for different crystallographic directions under direct current. The field is close to homogeneous. Specimen thickness \(0.25 \div 0.50\ \mathrm{mm}\). Temperature \(t = 20^\circ\mathrm{C}\), \(x\)—\(a\) axis; \(\Delta\)—\(b\) axis; \(o\)—\(c\) axis.

crystallographic directions. From these results it is not apparent that breakdown developed preferentially along any crystallographic direction. How rapidly does the polarization process occur in Rochelle salt; does the orientation of domains have time to occur during the action of a voltage pulse; and how will this be reflected in the electric strength?

Fig. 5. Electric strength of Rochelle salt for different crystallographic directions under pulses of \(10^{-6}\) sec. Field close to uniform. Specimen thickness \(0.25 \div 0.50\) mm. \(x\)—axis \(a\); \(\Delta\)—axis \(b\); \(o\)—axis \(c\).

We attempted to find answers to these questions by recording the volt-second characteristics of Rochelle salt under pulses of duration \(10^{-4}\), \(10^{-5}\), \(10^{-6}\), \(10^{-7}\) sec.

Fig. 6 presents the dependence of the electric strength on the duration of voltage application. The volt-second characteristics were recorded for all three directions.

Fig. 6. Volt-second characteristic of Rochelle salt under pulses in a uniform field. Temperature \(18^\circ\)C.

Analogously to the data of G. A. Vorob’ev [5] for alkali-halide crystals, an increase in the electric strength of Rochelle salt is observed when the pulse duration is reduced to \(10^{-7}\) sec, which should be attributed to the delay in discharge formation.

Both at a pulse duration of \(10^{-6}\) sec and at the other exposure times, no dependence of the electric strength on crystallographic direction was observed.

Figure 7 presents the results of statistical processing of the volt-second characteristic of Rochelle salt for the \(a\) axis and the \(b\) and \(c\) axes.

Fig. 7.

Results of the Experiments

1. The electric strength of Rochelle salt under pulses and under direct current decreases with increasing specimen thickness.

2. The electric strength of Rochelle salt does not depend on temperature in the interval from \(+50\) to \(-60^\circ\mathrm{C}\), and no changes in electric strength are observed at the upper and lower Curie points \((-18\) and \(+24^\circ\mathrm{C})\).

3. The electric strength of Rochelle salt does not depend on crystallographic direction either under direct current or for durations of exposure to single pulses of \(10^{-7}\), \(10^{-6}\), \(10^{-5}\), \(10^{-4}\) sec.

4. The electric strength of Rochelle salt increases when the time of exposure to the impulse voltage is decreased.

5. Despite the presence in Rochelle salt of 4 molecules of water of crystallization, its electric strength is high and, in a uniform field, is equal to 1300 kV/cm for an exposure time to single pulses of \(10^{-6}\) sec.

In conclusion, I express my deep gratitude to Professor Doctor A. A. Vorob’ev, under whose scientific supervision the present work was carried out, and to the staff members of the department G. A. Vorob’ev and A. M. Trubitsyn.

for discussion of the results of the work and for the use of the high-voltage apparatus assembled by them.

References

1. Prikhodko N. A. Proceedings of the SFTI, vol. VI, issue 2, 1942.
2. Gorelik B. M. ZhTF, vol. X, issue 5, 1940.
3. Kurchatov I. V. ZhTF, vol. I, issue 121, 1931.
4. W. Cady, Piezoelectricity and Its Practical Applications. I. L. Publishing House, 1949.
5. Vorob’ev G. A. ZhTF, vol. XXX, issue 2, 1956.

Tomsk Polytechnic
Institute