ON THE ELECTRICAL STRENGTH OF SINGLE CRYSTALS OF SOLID SOLUTIONS OF ALKALI-HALIDE SALTS

A. A. VOROB'EV, A. N. KISLINA

Experimental studies showing an increase in electrical strength when impurities are introduced into a crystal in the form of a solid solution were carried out at constant voltage, when secondary effects could have influenced the results [1, 2].

The results of experimental studies of electrical strength by the more advanced pulse method [3] are few in number and are in contradiction with those indicated above. Below we describe the determination of the impulse electrical strength of certain solid solutions of alkali-halide salts of varying degrees of stability.

One of the quantities characterizing the stability of a chemical compound is the heat of formation. The greater the amount of energy that must be expended on the formation of a solid solution, the lower its chemical stability will be. It has been established [3] for certain systems of solid solutions that, with an increase in the heat of formation of the solid solution, its electrical strength decreases.

Solid solutions formed with absorption of heat spontaneously decompose over time with the evolution of heat [4].

According to observations by Academician N. S. Kurnakov [4, 5], solid solutions of the KCl—RbCl system are chemically more stable than, for example, solid solutions of the KCl—KBr and NaCl—NaBr systems. This is also evidenced by the lower heats of formation of solid solutions of KCl—RbCl than of KCl—KBr and NaCl—NaBr (Table 1).

Single crystals of solid solutions were grown from the melt. Control of the percentage content of components in the single crystals of solid solutions was carried out by the refractive index and molecular refraction. The structure of the solid solution was monitored by X-ray analysis. No decomposition of the solid solutions was detected either in fresh specimens or in specimens subjected to thermal and natural aging.

Testing of single crystals of solid solutions for electrical strength was carried out on specimens with a spherical depression against a plane. The thickness of the specimens at the thinnest point was brought to 0.150–0.250 mm by polishing the flat surface of the specimen on a slightly moistened cloth.

After measuring the thickness of the specimens on a vertical optical length gauge “438—1,” the specimens were examined under a microscope. Speci-

crystals were broken down either 2–5 days after the single crystals had been grown, or after more prolonged storage in a desiccator, or after annealing at a specified temperature, depending on the aim of the investigation.

Breakdown was carried out on the rectilinear portion of the front of a single high-voltage pulse with a front duration of the order of 7 μsec. For plotting the graphs, the maximum and arithmetic-mean values of the electrical strength were taken.

Table 1

Data on the heats of formation of solid solutions of alkali-halide salts, found by various authors

Fig. 1 gives the diagram “Electrical strength—composition” of fresh single crystals of solid solutions KCl—RbCl.

It is seen from Fig. 1 that solid solutions of the KCl—RbCl system are formed in such a way that their electrical strength increases in comparison with the electrical strength of the pure components.

In the course of the dependence of electrical strength on chemical composition for fresh specimens of solid solutions KCl—KBr, a minimum is observed, falling approximately in the region of equal content of the components in the solid solution (Fig. 2).

The electrical strength of single crystals of solid solutions containing 30, 40, 50, 60, 70, and 80 mol% KBr in KCl, as a function of composition after 3 months of storage, is distributed along a curve with a maximum (curve 3 in Fig. 2).

In the same Fig. 2 is shown the electrical strength of KCl—KBr specimens stored for 6 and 12–14 months, as well as for 3 years.

Fig. 1. Change in electrical strength (\(E_{\mathrm{br}}\)) under pulses as a function of the chemical composition of single crystals of solid solutions \( \mathrm{KCl—RbCl} \) (fresh).
— \(E_{\mathrm{br}}\), max.
- - - \(E_{\mathrm{br}}\), avg.

Fig. 2. Dependence of the electrical strength of solid solutions of the \( \mathrm{KCl—KBr} \) system on chemical composition.
1. \( \times \)—\(E_{\mathrm{br}}\), avg., of freshly grown crystals.
2. \( \circ \)—\(E_{\mathrm{br}}\), after 3 months of storage of the crystals.
3. \( \square \)—\(E_{\mathrm{br}}\), avg., after 6 months of storage of the crystals.
4. \( \Delta \)—\(E_{\mathrm{br}}\), avg., after 12–14 months of storage of the crystals.
4. \( \Delta \)—\(E_{\mathrm{br}}\), avg., after three years of storage of the crystals.

Fig. 3. 1.—\(E_{\mathrm{br}}\), avg., of freshly grown crystals. 2.—\(E_{\mathrm{br}}\), avg., after annealing the crystals at a temperature of \(500^\circ\mathrm{C}\) (6 hours).
3.—\(E_{\mathrm{br}}\), avg., after annealing at a temperature of \(300^\circ\mathrm{C}\) (40 hours).

A comparison of the electrical strength of crystals stored before breakdown for 6 months and 3 months with the electrical strength of fresh specimens shows that, for small impurity concentrations within the range of 10–20 mol%, the electrical strength changes only slightly during the aging of solid solutions. After 1 year and 3 years, the electrical strength of solid solutions of this composition exceeds the electrical strength of fresh specimens.

Annealing of KCl—KBr crystals is likewise accompanied by an increase in their electrical strength (Fig. 3).

Natural aging manifests itself in an analogous way in crystals of NaCl—NaBr solid solutions.

Table 2 gives the arithmetic mean values of the electrical strength of single crystals of equimolar NaCl—NaBr solid solutions, both fresh and subjected to aging, as well as of NaCl and NaBr crystals.

Table 2

Mean electrical strength in \(10^6\) V/cm.

As can be seen from Table 2, the electrical strength of fresh crystals of 50 mol% NaCl—50 mol% NaBr is considerably lower than the electrical strength of the individual components, while after 1.5–2 months of storage in a desiccator their electrical strength already exceeds the electrical strength of pure crystals.

Consequently, the electrical strength of a solid solution may depend not only on the percentage ratio of its components, but also on the state of the solid solution. Failure to take this factor into account leads to contradictory results in experimental studies of the electrical strength of solid solutions in comparison with pure components.

Solid solutions that form with the smallest thermal effect have higher values of electrical strength.

However, if solid solutions are formed in such a way that their electrical strength is reduced in comparison with the strength of the components, then by appropriate treatment of these solid solutions one can obtain for them a “electrical strength—composition” diagram in the form of a curve with a maximum.

REFERENCES

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4. Kurnakov N. S., Collected Selected Works, 1. ONTI, 1938.
5. Kurnakov N. S., Zhemchuzhnyi S. ZhRKhO, 38, 49, 1900.
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Errata

Izv. TPI, vol. 95.