Continuation of the Discussion on Issues of Electrical Breakdown of Dielectrics
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This discussion examines unresolved questions in the electrical breakdown of dielectrics, especially the dependence of breakdown strength on composition, aging, decomposition, and mechanical stresses in mixed crystals. The contributors compare conflicting interpretations of experimental curves, including whether decomposition toward a mechanical mixture should increase or decrease electrical strength, and suggest that microcracks formed during decomposition may help account for observed breakdown values. The text argues for separating structural factors through new experiments on solution-grown single crystals and for broadening the experimental basis of breakdown theory through thermodynamic, radiographic, optical, ultramicroscopic, viscosity, and conductivity studies in strong pre-breakdown electric fields.
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CONTINUATION OF THE DISCUSSION
ON QUESTIONS OF THE ELECTRICAL BREAKDOWN
OF DIELECTRICS
A. A. Vorob'ev says that all solid solutions decompose at a greater or lesser rate; decomposition occurs even during the process of crystal growth. On the other hand, in grown crystals there are always mechanical stresses, which are removed to some extent during annealing. Decomposition and mechanical stresses may affect the electrical strength of crystals. The factors of decomposition and mechanical stresses must be separated and taken into account. Therefore it is necessary to grow single crystals from solution and repeat the experiments on such specimens.
The various results obtained in our laboratory in studying the course of the curve of the dependence of electrical strength on composition require explanation and further investigation.
Two opposite views have taken shape regarding the consequences of the decomposition of crystals. According to our ideas (Vorob'ev, Kislina, Trubitsyn), the electrical strength of a mechanical mixture is higher than that of a single crystal, and aging of a single crystal, when decomposition is considerable, when it approaches a mechanical mixture, should lead to an increase in electrical strength.
According to Gorodetskii's ideas, the alloy should have greater electrical strength than the mechanical mixture, and as the specimens age their electrical strength should decrease.
The question of aging of specimens is very important and must be specially studied. Could the discrepancy between Hippel's experimental results and those of our laboratory perhaps be explained by aging of Hippel's specimens?
In our laboratory, measurement of the strength of fresh crystals gave a curve with a minimum (Kislina, Bogdanova, Trubitsyn, Ivankina).
The curve of the dependence of \(E_{\text{br}}\) on composition was also obtained for old specimens. It was found that the absolute values of \(E_{\text{br}}\) decrease.
Curves with a maximum are obtained.
I. G. Skanavi says that, in experiments on the study of electrical strength as a function of the composition of mixed crystals, there is disagreement and various assumptions for explaining the course of the curves.
It is possible that in the concentration ranges where decomposition proceeds at a greater rate, microcracks are formed that affect electrical breakdown. It would be interesting, using the Gorovits formula, to obtain from experiment the value of the surface energy and, specifying the size of a microcrack, to calculate the electrical strength. If one takes the crack length to be of the order of \(10^{-4}\) cm, then the calculated \(E_{\text{br}}\) is of the same order as the experimentally found value.
R. L. Müller notes the following: progress in the study of the phenomenon of electrical breakdown over the many years that have passed has been very small; no new problems have appeared during these years. This is explained by the difficulty of the problem, by the fact that breakdown is a rapidly proceeding process, in which some reactions develop rapidly, while others rapidly die away.
Technical dielectrics do not possess ideal, purely ionic lattices; yet what must be studied are precisely these technical, complex dielectrics.
The study of electrical breakdown must be approached by conducting investigations in the following directions.
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Study the dependence of the heat capacity of a dielectric on temperature in a strong electric field, i.e., in pre-breakdown fields.
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Study the dielectric by the radiographic method in the presence of a strong electric field in the dielectric.
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Study the optical spectrum of vibrations, the combination scattering of light by a dielectric situated in a strong electric field.
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An ultramicroscopic investigation of the site of breakdown is necessary.
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It is necessary to investigate the viscosity of dielectrics in a strong electric field.
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Study the temperature dependence of the electrical conductivity of dielectrics in the pre-breakdown state.
Such consideration of the phenomenon signifies a thermodynamic approach to the study of the breakdown phenomenon; it would broaden the experimental basis for constructing a theory of breakdown.
The questions of theory have turned out to be much broader than the experimental basis. Therefore one often operates with facts, even nonexistent ones, draws groundless conclusions, and constructs abstract schemes. The theories operate with ideal ionic crystals, whereas real crystals differ considerably from them.
The valuable mathematical apparatus should be adapted to a larger number of experimental facts.