Remarks on the Effect of Plastic Deformation of Alkali-Halide Single Crystals on Their Dielectric Strength

A. A. Vorob'ev

The hypothesis of an influence of plastic deformation on the dielectric strength of solid dielectrics has been advanced many times. Experimental results have for the most part led to the conclusion that there is no primary influence of plastic deformation on the dielectric strength of solid dielectrics. At the same time, investigators have emphasized that secondary phenomena accompanying plastic deformation, such as densification of the material or, conversely, disruption of its continuity and the formation of cracks, alter the value of the dielectric strength of the dielectric [1]. Recently this question has again been taken up. Some authors, adhering to the ionization theory of electrical breakdown of solid dielectrics proposed by Fröhlich, attempt to show that plastic deformation leads to an increase in the dielectric strength of solid dielectrics.

Calderwood, Cooper, and Wallace [2], in the paper “Experimental Study of Factors Affecting the Dielectric Strength of KCl,” indicate that it depends only weakly on the electrode material. The dielectric strength of NaCl and KCl crystals increases appreciably after treatment that introduces structural defects associated with residual mechanical stresses and plastic deformation.

These authors are inclined to explain the scatter of points observed in measuring dielectric strength by the influence of the mechanically stressed state.

They carried out experiments on the breakdown of annealed and unannealed NaCl and KCl specimens made from a single crystal grown from the melt. The experiments were conducted at constant voltage; specimens with pits were used. Electrodes of colloidal graphite or silver were applied after annealing the specimen. The thickness of the crystal layer subjected to breakdown was 0.01–0.2 mm. The maximum value of the dielectric strength of annealed specimens was almost 50% lower than for unannealed specimens.

The authors state that the specimens that showed high dielectric strength had mechanical stresses, which were recorded by means of a special analyzer. The annealed specimens had no mechanical stresses. Their structure is more homogeneous; therefore the scatter of points in measurements of dielectric strength is smaller. In the opinion of the authors of this work, high values of dielectric strength are obtained as a consequence of disturbance of the crystal structure. This explanation is in accord with the theory of impact ionization by electrons.

In another paper, Calderwood and Cooper no longer distinguish between the electric strength of annealed and unannealed NaCl and KCl crystals. They constructed the dependence of the breakdown voltage on thickness, where there are points for annealed and unannealed crystals. These results are in contradiction with others obtained by various authors.

As E. K. Zavadovskaya [3] has shown, dielectrics in an electric field before breakdown experience large mechanical forces. The magnitude of the mechanical stresses in a dielectric before breakdown may not only significantly exceed the yield point of the material, but in individual cases may also exceed the ultimate strength.

At a high magnitude of mechanical forces in a dielectric situated in a strong electric field, plastic deformation must occur. The occurrence of plastic deformation in a single crystal can be determined by an optical method.

Cooper and Wallace [4] have recently made interesting observations of the occurrence of plastic deformation in KCl single crystals in a homogeneous electric field under the action of a pulsed voltage of \(1/50\ \mu\mathrm{sec}\).

To obtain a homogeneous field, specimens with dimples were used. The electrodes were transparent, made from a solution of KCl in glycerine. The region of greatest electric-field intensity at the center of the dimple was examined with a microscope under illumination by polarized light in the direction of the electric field.

The observations were carried out on three groups of specimens, respectively, with the field acting in the directions \([100]\), \([110]\), and \([111]\).

Before voltage was applied to a specimen, no brightening was observed in them. Breakdown of specimens in the \([100]\) direction occurred at field strengths of \(6\text{--}7 \cdot 10^5\ \mathrm{V/cm}\), at which plastic deformation and brightening of the field are still not observed. After the action on the specimens of a pulsed voltage of the order of 80% of the breakdown voltage, brightening of the field was observed for all specimens broken down in the \([110]\) and \([111]\) directions.

Breakdown occurred at a field strength of the order of \(1.1 \cdot 10^6\ \mathrm{V/cm}\). With increasing field strength, the region of brightening in polarized light also increased. Specimens that exhibited higher strength broke down with stronger brightening. Thus, after the action of short electric pulses, residual mechanical stresses arose in the specimens and brightening of the region persisted with crossed polarizer and analyzer. It is obvious that if the specimens had been preliminarily annealed, then after the electric-field strength in them reached a certain value, plastic deformation of the material occurred, and the specimens broke down in a deformed state.

Consequently, the assertion that there is a difference in the measured electric strength of preliminarily deformed and undeformed specimens has no meaning, since the specimens always break down deformed.

The cited materials on observations of the plastic deformation of single crystals in a strong electric field are of considerable scientific interest. The calculations of E. K. Zavadovskaya have been experimentally confirmed, and it has been shown that breakdown of a specimen having high electric strength is always preceded by its plastic deformation and, possibly, also by its mechanical rupture. The experimental results of Cooper and Wallace contradict the opinion, again expressed by them in this same article, concerning the strengthening influence of plastic deformation. In this article Cooper and Wallace, trying to support their idea, clearly reversing cause and effect, indicate that the greatest electric strength was obtained for specimens that showed, before ...

the samples probed the greatest lightening of the field, i.e., the most highly deformed specimens had the greatest dielectric strength. It is obvious that the higher-quality specimens, withstanding a greater electrical load and larger mechanical stresses, showed greater lightening of the field in the polarization apparatus.

In a number of published works [5] it was shown that the dielectric strength of solid dielectrics increases with increasing lattice energy. The magnitude of the lattice energy under plastic deformation of single crystals changes by several percent. Such a change in lattice energy will be accompanied by a correspondingly small change in the magnitude of the dielectric strength and is practically unlikely to be determinable. The published experimental materials confirm this proposition; this proposition is also consistent with the latest experimental data of Cooper and Wallace, despite the opposite tendency of their authors.

REFERENCES

1. Vorob'ev A. A. Izvestiya TPI, vol. 63, 1944.
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   Vorob'ev A. A. DAN, vol. XXVI, issue 8, 1940.
   Vorob'ev A. A. Trudy SFTI, vol. VI, issue 2, 1939.
   Vorob'ev A. A. Elektrichestvo, 1940.

2. L. H. Calderwood, R. Cooper and Wallace Proc. Phys. Soc. B. 65, 301–302, 1952; 66, 74–80, 1953.

3. Zavadovskaya E. K. DAN, 81, 541–543, 1951.

4. R. Cooper and A. A. Wallance Proc. Phys. Soc. 66, 1113–1115, 1953.

5. Vorob'ev A. A. and Zavadovskaya E. K. DAN 81, 375–377, 1951.
   Zavadovskaya E. K. DAN 82, 5, 709–712, 1952.