ELECTRIC STRENGTH OF SINGLE CRYSTALS OF SOLID SOLUTIONS OF ALKALI-HALIDE SALTS WITH A SUBSTITUTED HALIDE

A. M. TRUBITSYN

Increasing the electric strength of solid dielectrics is of great importance for modern practical electrical engineering. It is known that the mechanical strength of metals can be increased by introducing certain additives. Science has at its disposal a number of theoretical and experimental studies devoted to investigating the possibility of applying this method to the electrical strengthening of solid dielectrics.

On the basis of his theory of the electrical breakdown of solid dielectrics, Fröhlich \[1, 2\] carried out a quantitative calculation of the change in the electric strength of ionic crystals as a result of introducing impurities into their structure. In doing so, Fröhlich proceeded from the assumption that in solid dielectrics the behavior of electrons participating in electrical breakdown may be considered analogous to the behavior of electrons in metals, and he made use of Mott’s method for calculating the additional scattering of electrons by foreign atoms in metals. The shortening of the electron mean free path due to additional scattering makes it more difficult for an electron to accumulate energy sufficient for ionization. Thus, according to Fröhlich, the introduction of impurities should lead to an increase in the electric strength of ionic crystals.

From the standpoint of quantum theories of the electrical breakdown of solid dielectrics, impurities and other disturbances of the structure of a solid dielectric alter the energy spectrum of the crystal lattice and cause the appearance of discrete levels in the forbidden band. The presence of allowed energy levels situated close to the conduction band may facilitate the transition of electrons into that band and thereby lead to breakdown at a lower applied voltage.

According to the theory of A. A. Vorob’ev and E. K. Zavadovskaya, in addition to these two phenomena, which occur simultaneously in the dielectric, it is necessary to take into account the change in the interaction between the nodes of the crystal lattice. If the introduction of an impurity is accompanied by a weakening of the bonds between particles in the solid dielectric, that is, if the lattice energy decreases, one may expect a decrease in the electric strength of the solid dielectric. If, however, the impurities strengthen the interaction of particles in the solid dielectric, that is, increase the lattice energy, one may expect electrical strengthening of the dielectric.

Thus, theories of electrical breakdown, taking account of the influence of impurities in different ways, arrive at contradictory results.

The results of experimental investigations are also contradictory. According to Hipped and his collaborators \[3, 4\], the addition of approximately five molecular percent of AgCl leads to an increase in the electric strength of NaCl from \(1.7 \cdot 10^6\) to \(2.8 \cdot 10^6\) V/cm; in the system of solid solutions KCl—RbCl, the change in electric strength with chemical composition

is expressed by a curve with a maximum in the region of 20–30 mol% RbCl; additions of KCl to KBr and of KBr to KCl appreciably increase the electric strength relative to pure crystals.

The studies of N. B. Bogdanova [5] show that, for the system of solid solutions KJ—KBr, the graph of the dependence of electric strength on chemical composition has the form of a curve with a minimum at intermediate percentage contents.

Fig. 1. Oscillogram of a high-voltage pulse.

At intermediate contents. A systematic decrease in the electric strength of KJ was observed by N. B. Bogdanova upon introducing into KJ up to 40 mol% NaJ.

We formed the impression that the contradiction in the experimental data was due to the imperfection of the previously used method for measuring electric strength.

Fig. 2. Oscillogram of the voltage at breakdown of the specimen.

In analyzing the possibility of the influence of secondary effects on the electric strength of single crystals, we chose experimental conditions under which these influences are absent or reduced to a minimum.

The specimens were broken down on the front of a single high-voltage pulse. The time for the voltage to rise to the breakdown value did not exceed 4.3 μsec. Specimens with a recess 10 mm in diameter opposite the plane were prepared from single crystals grown from the salt melt by means of a cooled seed. The electrodes were applied by evaporation of metal in vacuum. The tests were carried out in transformer oil at room temperature. The use of oscillographic equipment with photographic recording of the breakdown oscillograms made it possible to increase the accuracy of the voltage measurement and to introduce an objective assessment of the quality of the experiments. Fig. 1 shows an oscillogram of the high-voltage pulse; Fig. 2 shows a typical oscillogram of specimen breakdown.

Labels in the figure: number of specimens; specimen thickness 0.100–0.150 mm; specimen thickness 0.151–0.200 mm; specimen thickness 0.201–0.258; specimen thickness 0.100–0.258; electric strength in \(10^6\) V/cm; \(E_{\mathrm{br}}\).

Fig. 3. Distribution of specimens from NaCl single crystals according to their electric strengths.

As a result of the experiments we obtained diagrams of the distribution of specimens according to their electric strengths (Fig. 3) like those obtained in measuring tensile mechanical strength according to data of other authors [6].

The measured values of the electric strength of single crystals of pure salts are given in Table 1.

Table 1

The results of measuring the electric strength on annealed and unannealed specimens lie within one and the same range of scatter.

Figures 4, 5, and 6 present the graphs we obtained for the variation of electric strength with the chemical composition of single crystals of solid solutions of the systems NaCl—NaBr, KCl—KBr, and KJ—KBr. It follows from the graphs that, for all three solid-solution systems investigated by us:

a) the dependence of electric strength on chemical composition, constructed from the maximum and mean values of the measured electric strength, is expressed by curves with a minimum in the region of intermediate molecular-percentage contents of the components;

b) the lower boundary of the measured values of electric strength is formed by straight lines parallel to the abscissa axis and lying in the region of 300–400 kV/cm.

Fig. 4. Dependence of electric strength on composition for single crystals of the NaCl—NaBr system.

Deviations from the indicated situation are observed only for the KBr—KJ system. In the region of small percentage contents of KBr in KJ, the mean value of the electric strength increases as the amount of KBr impurity increases.

The measurements were carried out over 3–5 days after the single crystals had been grown. During this time the specimens were kept in desiccators. Single crystals of NaBr and NaCl—NaBr, in the presence of moisture, become covered with a white film. To eliminate this phenomenon, the specimens were stored and treated in dry transformer oil. In tests under dc voltage of specimens whose surface was covered with a white film, surface flashover of the specimen was observed before the breakdown voltage was reached. With pulses having a front duration of the order of \(10^{-6}\) sec this phenomenon was absent, and the electric strength in some cases was higher than that of transparent specimens. In the KJ—KBr system, some of the grown crystals had a cloudy-white tint. The measured electric strength of such crystals fit well into the general regularity.

The results of our measurements of the electric strength of solid solutions are in good agreement with the experiments of N. B. Bogdanova (the KJ—KBr system) and do not confirm the data of Hippel and Buehl (the KCl—KBr system). The electric strength of solid solutions of the NaCl—NaBr systems was measured by us for the first time.

In the figure: ordinate—Electric strength, \(10^6\ \mathrm{V/cm}\); abscissa—mol % KBr / mol % KCl.

Fig. 5. Dependence of electric strength on composition for single crystals of the KCl—KBr system.

On the basis of the results of our measurements and those of N. B. Bogdanova, we have arrived at the following conclusion:

1. The introduction of impurities with the formation of solid solutions of alkali-halide salts of the halide-substitution type is accompanied by a decrease in

In the figure: ordinate—Electric strength, \(10^6\ \mathrm{V/cm}\); abscissa—mol % KBr / mol % KJ.

Fig. 6. Dependence of electric strength on composition for single crystals of the KJ—KBr system.

electric strength of the single crystals in comparison with the pure initial salts. The lowest electric strength is possessed by single crystals of solid solutions with intermediate molecular percentages of the components.

1. Fröhlich’s theory, predicting only strengthening of ionic crystals when impurities are introduced into their structure, does not fully reflect the actual regularities that occur in electrical breakdown. No analogy can be drawn between the processes leading to electrical breakdown of solid dielectrics and the electrical conductivity of metallic alloys.

References

1. H. Fröhlich, Proc. Roy. Soc. A., vol. 172, 94, 1939.
2. H. Fröhlich, Proc. Roy. Soc. A., vol. 178, 493, 1941.
3. A. von Hippel and C. M. Lee, Phys. Rev. vol. 59, 824, 1941.
4. R. C. Buehl, A. von Hippel, Phys. Rev, vol. 56, 941, 1939.
5. N. B. Bogdanova. JETP, vol. 4, 30, 1944.
6. V. D. Kuznetsov. Physics of the Solid State, vol. 2, Tomsk, 1941.

Tomsk Polytechnic
Institute

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