On the Method for Determining the Magnitude of the Electric Strength of Solid Dielectrics

A. A. Vorob'ev, E. K. Zavadovskaya

The magnitude of the electric-field strength at which breakdown of a dielectric occurs in a uniform field is a physical characteristic of the dielectric and is called its electric strength. For the theory and practice of the use of dielectrics, it is important to determine the magnitude of the electric strength as accurately as possible. A substantial difficulty in measuring electric strength is obtaining a homogeneous dielectric and forming within it a uniform electric field. In practice this leads to the fact that the value of the electric strength of one and the same dielectrics, as determined by different authors, turns out to be different.

Modern technology, and the transition to higher voltages, require the development of new dielectrics with high electric strength. It is essential to develop a method for measuring the magnitude of the electric strength of dielectrics. In order to reduce the relative role of internal inhomogeneities in a dielectric during tests for electric strength, it should be taken in a thin layer and with a large surface area. When such a specimen is tested, the weakest point will be punctured first. After repairing the destroyed point, for example by filling it with bitumen, the next breakdown will occur already at a higher voltage. In this way, by successively eliminating all weakened points, with repeated use of the same specimens it will be possible to determine the magnitude of the breakdown voltage of the dielectric. The idea of such a method is present in Krasin’s experiments. A second possible way of reducing the relative role of inhomogeneity in the structure of the dielectric is to reduce the size of the dielectric surface. The probability of the appearance of a weakened region of the dielectric should decrease when the size of its surface is reduced. This proposition manifests itself especially strongly with point contact of the electrodes with the surface of the dielectric. Such a measurement method was developed in Hippel’s investigations.

Obtaining a homogeneous, uniform field throughout the volume of the entire dielectric under test is a difficult problem, for which a satisfactory solution has not yet been found. This latter statement is confirmed at least by the fact that many methods have been proposed for solving this problem. The difficulty consists in the fact that at the edges of the plates of a plane capacitor there is a concentration of the lines of force of the electric field—overvoltage. This phenomenon is known as the edge effect. When a plate of a solid dielectric is placed in a plane test capacitor, an increased field strength is formed in the dielectric near the edges of the plates. Breakdown occurs at the edge of the electrode at an increased field strength, substantially exceeding the average calculated electric-field strength in the dielectric. Proposed methods include…

various methods of equalizing the electric field when testing solid dielectrics for breakdown. The first group of methods consists in increasing the thickness of the dielectric specimen near the edges of the electrodes by selecting the shape of the specimen. The second group of methods consists in regulating the electric field in the dielectric by selecting the shape of the electrodes. The shape of the electrode surface is chosen by calculation. The third group of methods consists in regulating the distribution of the field in the dielectric by immersing the entire system in a medium with appreciable conductivity. If an overvoltage is present in some part of the field, conduction currents arise, equalizing the potential distribution. The same results can be achieved if the surface of the dielectric is coated with a semiconducting layer. The fourth group of methods consists in forced regulation of the potential over the dielectric by applying to its surface a system of electrodes whose potential is maintained at a definite value from an external source. Such a method of regulation is cumbersome. Other methods have also been proposed.

With surface charging of a dielectric, for example by electrons, as occurred in the experiments of Zernov, Krasin, Favorin, and others, a uniform field can be formed in the dielectric. In this case the electrodes will be the ionic or electronic surface charge on the dielectric. The specimen must be placed in vacuum. This latter condition already gives cleaner experimental conditions, eliminating a number of secondary phenomena, for example reducing the amount of moisture in the dielectric and, consequently, diminishing its role. If a local overvoltage arises in the dielectric, then the surface charge, under the influence of the longitudinal component of the field, will move until the potential difference that caused this displacement becomes equal to zero. The problem of the distribution of potential over the surface of a dielectric has been investigated by a number of authors. The theory of the distribution of potential over the surface of a dielectric is given in the literature. To equalize the potential distribution it is sufficient for the surface charge to be present on only one side of the dielectric. The other plate of the capacitor may be a metallic layer well applied to the surface of the dielectric.

As already indicated above, electrical breakdown under surface charging has been observed by many authors and is a well-known phenomenon in engineering, consisting in the breakdown of the glass bulbs of electron-vacuum devices. The most systematic experiments on the electrodeless breakdown of sheet mica were carried out by Krasin, whose results he explained in a different way from that used in the present article. He compared the breakdown voltage of mica \(10^{-3}—10^{-4}\ \mathrm{cm}\) thick in the case where voltage was applied to the specimen by two metallic electrodes and by one metallic electrode and an electron cloud. He established that the electric strength, in the case where voltage was applied to the specimen by an electron cloud, is higher than in the case of two applied metallic electrodes. This agrees with the ideas developed in this article concerning the equalization of the electric field in the dielectric in the presence of surface charge. Goldman obtained a considerable increase in the electric strength of mica \(0.1\ \mathrm{mm}\) thick and glass \(1\ \mathrm{mm}\) thick when semiconducting lacquer coatings were applied to the surface of the specimens. She established that, with increasing thickness of the dielectric, the difference in the breakdown voltages of specimens provided with semiconducting coatings and those not provided with them increases. In Krasin’s experiments the opposite picture was observed. With increasing thickness of the dielectric, the difference in breakdown strength determined with two metallic electrodes and with one metallic electrode and surface charging by electrons decreased.

Observance of the experimental procedures set forth above, i.e., breakdown under surface charging of the dielectric, should ensure a uniform electric field in the dielectric.

REFERENCES

1. Vorob’ev A. A. High-Voltage Engineering. GEI, 1945.

2. Goldman I. M. Weakening of the edge effect by means of semiconducting layers, ZhTF 1, no. 5, 402—408, 1931.

3. Ioffe A. F. and Vul B. M. The edge effect in electrical breakdown, Elektrichestvo 12, 650—651, 1931.

4. R. C. Buehl and A. Hippel. The Electrical Breacdown strength of jonic Crystals as a Function of Temperature, Phys. Rev. 56, 941—947, 1939.

5. Zernov D. V. On the influence of strong electric fields on the secondary electron emission of thin dielectric films and the prospect for practical application of this effect, ZhTF, XV, no. 7, 436—447, 1945.

6. Favorin V. N. Certain features of secondary electron emission from thin films of potassium chloride, ZhTF, XX, no. 8, 916—923, 1950.