On the Relationship Between Electrical Strength and Certain Mechanical Characteristics
Abstract Generated abstract
This paper examines whether the electrical strength of dielectric crystals is related to mechanical properties, in contrast to earlier results suggesting that plastic deformation and mechanical stress do not directly determine breakdown strength. Using published data for alkali halide crystals and solid solutions, it compares breakdown field values with extrusion pressure, Brinell hardness, and elastic moduli. The comparisons indicate a generally direct correspondence between electrical strength and these mechanical characteristics, including composition dependent trends in several salt systems, although deviations are attributed to structural instability or decomposition effects. The study argues that mechanical parameters reflecting crystal energetics may provide useful indicators of electrical strength in ionic dielectrics.
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ON THE RELATIONSHIP BETWEEN ELECTRIC STRENGTH AND CERTAIN MECHANICAL CHARACTERISTICS
A. F. GORODETSKII
Many works have been devoted to the study of the influence of mechanical actions on a dielectric.
P. P. Kobeko and I. V. Kurchatov [1] investigated the influence of plastic deformations on the electric strength of rock salt.
Plastic deformation of the crystal was produced by pressure of 5–7 kg/cm² at a temperature of 700–750°C for one hour. The length of the deformed crystal proved to be 70–90% of the initial length. Cleavage of crystals treated in this way gave a scaly structure, which indicated a significant disturbance of the lattice. Breakdown was carried out both with direct current and with impulse voltage.
Comparison of the results obtained with the values of electric strength for well-annealed and unannealed (and consequently arbitrarily deformed) specimens showed no difference in the values of electric strength.
A. A. Vorob’ev [2] investigated, from the same point of view, crystals of rock salt, ebonite, and variously annealed glasses.
The degree of disturbance of the crystal lattice during plastic deformation of NaCl crystals does not affect the magnitude of the electric strength.
However, experiments showed that in some cases, with increasing pressure, there is first some increase in electric strength, and then a decrease. It is essential to note here that the increase in electric strength did not exceed the limiting values obtained for NaCl specimens that had been well pre-annealed.
Akahira and Gemant [4] investigated the influence of mechanical stresses on the magnitude of the breakdown voltage of glass, rubber, mica, cellophane, and celluloid, and likewise obtained some increase, followed by a decrease, in electric strength with increasing pressure.
In the opinion of A. A. Vorob’ev, this increase in electric strength is connected with a secondary effect, due in its origin to the inhomogeneity of the dielectric.
A microscopic investigation of the propagation of discharges in crystals in the presence of mechanical stresses in them was carried out by A. A. Vorob’ev [3] in a polarization apparatus. It turned out that mechanically stressed regions in NaCl crystals are not preferential directions for the development of breakdown. The discharges proceeded mainly along the boundaries between stressed and unstressed regions.
The absence of a direct connection between mechanical stresses and electric strength does not exclude the existence of a reduction in electric strength as a result of disturbance of the microscopic structure of dielectrics. This latter circumstance was emphasized by E. K. Zavadovskaya [5],
and also by us [6] in connection with consideration of the question of the electric strength of solid solutions.
On the other hand, one may expect that mechanical characteristics, in one way or another reflecting the energetics of crystals, may prove to be closely connected with electric strength.
In this connection we have carried out a comparison of the electric strength of some crystals of the alkali-halide series with the extrusion pressure and Brinell hardness.
Extrusion pressure, as a method of physicochemical analysis, was studied in detail by Kurnakov and Zhemchuzhnyi [7].
The substances tested were subjected to compression in the apparatus shown schematically in Fig. 1.
Fig. 1
“Substances were placed under the piston or spindle A (diameter \(D = 8.66\) mm and 15.25 mm) made of hardened steel, moving inside the steel cylinder B. The latter is screwed tightly into another cylinder F, in the lower part of which a conical plug C is inserted, with a cylindrical channel e (2.81 mm in diameter \((d)\) and up to 10 mm long), serving for extrusion. To guide the jet and eliminate dead space, the lower end of the spindle is sharpened to a cone and a recess of corresponding shape is cut in plug C. The apparatus is placed on the stand E with a slot A, intended for observing the extrusion process.”
The pressure on the piston was set and measured with a Gagarin press, with recording of the diagram.
Study of the diagrams obtained made it possible to choose, as the measure of the extrusion pressure, that maximum stress on the spindle at which extrusion of the substance through channel e begins.
The results of studies carried out by Kurnakov’s school indicate the presence of a parallelism between the values of extrusion pressure and Brinell hardness.
A comparison of the values of electric strength with the magnitude of the extrusion pressure and Brinell hardness is shown in Figs. 2 and 3 and in the table.
It is regrettable that, owing to the absence of data, one has to limit oneself to only a small number of substances.
As is readily seen from the graphs presented (Figs. 2 and 3), there is a more or less direct dependence between the quantities being compared.
It is interesting that, as applied to solid solutions, a correspondence is also obtained between the values of extrusion pressure and electric strength as a function of composition.
Table
| Salt | Breakdown voltage, MV/cm | Brinell hardness, kg/mm² | Flow pressure, kg/mm² |
|---|---|---|---|
| KF | 1.8 | — | 123.8 |
| NaCl | 1.5 | 14 | 72.0 |
| KCl | 1.0 | 6.69 | 50.5 |
| RbCl | 0.8 | — | 70.5 |
| KBr | 0.7 | 5.40 | 28.2 |
| RbBr | 0.6 | — | 35.3 |
| NaJ | 0.7 | — | 37.1 |
| KJ | 0.6 | 4.08 | 31.0 |
The flow pressure as a function of composition for the systems KCl—KBr and KBr—KI was investigated by I. B. Vrzheshevskii [8], whose results are presented in Figs. 4, 5.
The investigation of the electric strength for the same solid solutions was carried out by Hippel [9] for the KCl—KBr system (Fig. 6) and by N. B. Bogdanova [10] for the KBr—KI system (Fig. 7).
Fig. 2. Flow pressure
From the curves it is evident that, for these systems, the specific action of admixtures leads to an increase in the flow pressure, which also corresponds to the expected results for electric strength. For the KBr—KI system, the opposite course of the curves of flow pressure and electric strength merely indicates that the decomposition process had a sharper effect on the magnitude of the electric strength, in full agreement with the views expressed earlier [6].
This opinion is further confirmed by the fact that, in the KCl—KI system, characterized by especially low stability at normal temperatures, the flow-pressure—composition curve, according to the data of [8], is analogous to the curve in Fig. 7, which expresses the dependence of electric strength on composition for the KBr—KI system (see Fig. 8).
Extrusion pressure
\( \mathrm{kg/mm^2} \)
0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88
Composition
0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 mol. %
\(\mathrm{HCl}+\mathrm{NaBr}\)
\(\mathrm{KCl}\)
Fig. 4
Electric strength
\(E,\ \mathrm{MV/cm}\)
\(\mathrm{KI}\)
\(\mathrm{KBr}\)
\(\mathrm{KCl}\)
\(\mathrm{NaCl}\)
5, 10, 15
Fig. 3. Brinell hardness
Electrical strength
\(E,\ \mathrm{kV/cm}\)
KJ — KBr
mol %
Fig. 7. Composition
Yield pressure
\(\mathrm{kg/mm^2}\)
Yield pressure
\(\mathrm{kg/mm^2}\)
KBr + KJ
KJ
Fig. 5. Composition
Yield pressure
\(\mathrm{kg/mm^2}\)
KJ
KBr + KJ
mol %
Fig. 8. Composition
\(E,\ \mathrm{kV/cm}\)
KCl — KBr
mol %
Fig. 6
It is also of interest to make a further comparison of electric strength with the elastic moduli. Using data for elastic moduli measured by the pulsed acoustic method for the salts LiF, NaCl, KBr, KI [11], a graph was constructed for the dependence of electric strength on the elastic moduli (Fig. 9).
Fig. 9
As is easy to see, in this case as well a clearly expressed dependence is obtained.
Thus, the relation of electric strength to certain mechanical characteristics appears quite definite.
In conclusion I express my gratitude to Professor, Doctor A. A. Vorob'ev for discussion of the questions touched upon in the article.
References
- Kobeko P. P. and Kurchatov I. V. Journal of the Russian Physico-Chemical Society, 61, 378, 1929.
- Vorob'ev A. A. Electricity, 4, 22, 1940.
- Vorob'ev A. A. Reports of the Academy of Sciences of the USSR, 26, No. 8, 1940.
- Akasihira T. und Coeman t A. Arch f. Electrot., 27, 577, 1933.
- Zavodovskaya E. K. Reports of the Academy of Sciences of the USSR, 81, 541, 1951.
- Gorodetskii A. F. Proceedings of TPI, 71, 21, 1952.
- Kurnakov N. S. and Zhemchuzhnyi S. F. Journal of the Russian Physico-Chemical Society, 45, 1004, 1913.
- Vrzhesnevskii I. Journal of the Russian Physico-Chemical Society, 43, 1376, 1911.
- Hippel A. V. Zs. f. Phys., 88, 358, 1934.
- Bogdanova N. B. JETP, 14, 30, 1944.
- H. Hutington. Phys. Rev. 72, 321, 1947.