On the Nature of the Formation of Space Charge in Ionic Crystals

G. A. Vorob'ev, V. D. Kuchin

(Presented by Professor, Doctor A. A. Vorob'ev)

In several works [1, 2] it is indicated that, when testing at constant voltage, higher values of the electric strength of crystals of alkali-halide salts are obtained than the strength under pulses. In our experiments [3, 4] an increase in the electric strength of NaCl, KCl, KBr, and KJ was also obtained at an exposure of \(10^{-5}\) sec and more, in comparison with the electric strength of these same single crystals at an exposure of \(10^{-6}\) sec. The increase in the electric strength of ionic crystals with increasing duration of the action of the voltage is explained by the influence of the space charge that forms. There are various explanations of the action of space charge on the electric strength of ionic crystals.

Ioffe et al. [5] consider that the space charge in a dielectric creates a field directed opposite to the applied field. In this case the average field strength in the dielectric decreases, which should lead to an increase in electric strength.

Hippel and Alger [6] suppose that in ionic crystals two types of space charges occur: electronic at times greater than \(10^{-7}\) sec and ionic at times exceeding \(10^{-4}\) sec. Each of these space charges separately causes a decrease in electric strength. If both types of space charges have formed in the dielectric (at an exposure of more than \(10^{-4}\) sec), then their influences compensate each other, and the electric strength obtained under such conditions corresponds to the true value.

Inuishi and Suita [2] propose the following explanation of the action of space charge. In the region of the dielectric adjacent to the cathode there are microcracks, at the boundaries of which there must be high electric-field strengths, causing intensive emission of electrons from the electrodes into the dielectric. These electrons are captured at the sites of the crystal lattice and form a negative space charge, which will hinder the emission of electrons from the cathode and thus should increase the electric strength.

Fowler [7] believes that a positive space charge, being located at the cathode, facilitates the tearing out of electrons from the electrode, which should lead to a decrease in electric strength. However, such a decrease in electric strength was observed only for calcite [8, 9], where the space charge is concentrated in the electrode-adjacent layers.

The influence of space charges on the electric strength of ionic crystals is indicated by the experimental fact that the electric strength under the simultaneous action of direct and pulse voltages of the same polarity is greater than when measured only under pulse voltage [1, 2]. We believe that in ionic crystals the space charge is formed owing to the movement of ions toward one of the electrodes.

In [3] a small calculation is given. It is known that the mobility of electrons in air is 1,000 times greater than the mobility of ions. If the same ratio is adopted for rock salt and it is taken into account that the time of formation of the discharge (an electronic process) in the investigated rock-salt specimens is of the order of \(10^{-8}\) sec [3], then the time of formation of an ionic space charge, provided the ions pass through the thickness of the specimen, should be \(10^{-5}\) sec. At this time, according to our data, the influence of the space charge begins.

The assumption of the ionic nature of the formation of the space charge also finds some confirmation in additional experimental data (figure). The figure presents the dependences

Temporal dependences of the electric strength of rock salt, punctured in a homogeneous field at different temperatures.

of the electric strength of rock salt on exposure at different temperatures. With increasing temperature the ions are more easily torn from their sites, which should increase the concentration of the space charge and, consequently, the electric strength. On the other hand, with increasing temperature the dissipation of the space charge should occur more readily, which should lead to a decrease in electric strength. The data in the figure show that, with increasing temperature, the influence of the space charge on the electric strength first grows and then decreases (150° C). Evidently, at temperatures up to 100° C the first factor predominated (an increase in the number of detached ions), while at a temperature of 150° C the second factor did (dissipation of the space charge).

The authors express their sincere gratitude to Professor, Doctor A. A. Vorob'ev for scientific guidance.

Literature

1. Zavadovskaya E. K., Izv. TPI, 63, 127, 1944.
2. Inuishi Y. and Suita T., J. Inst. El. Eng. Japan, 74, 150, 1954.
3. Vorob'ev G. A., ZhETF, 30, 257, 1956.
4. Kuchin V. D., Report at the All-Union Conference on the Physical Properties of Solid and Liquid Dielectrics, Dnepropetrovsk, 1956.
5. Ioffe A. F., Physics of Crystals, Gosizdat, 1929.
6. Hippel A. and Alger R. S., Phys. Rev., 76, 127, 1949.
7. Fauler K., Proc. Roy. Soc., A141, 56, 1933.
8. Venderovich A. M. and Vorob'ev A. A., The influence of concentrated space charge on electric strength, Proceedings of the Siberian Physico-Technical Institute, 4, 15, 1936.
9. Zavadovskaya E. K., Electric breakdown of polarized solid dielectrics, Izv. TPI, 63, 136, 1944.