DEVELOPMENT OF SURFACE DISCHARGE ON ALKALI-HALIDE CRYSTALS

L. A. PRUDNIKOVA

(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)

At the present time, the regularities of electrical breakdown of alkali halides have been investigated quite fully; in this connection, a dependence has been established of the magnitude of the electric strength of crystals on the lattice energy [1].

At the same time, questions connected with the laws governing the development of electrical discharges on the surface of a crystal remain little studied, whereas the processes in the surface layers of a material determined by these laws most often accelerate the onset of dielectric breakdown [2].

The information available in the literature on surface breakdown of materials pertains mainly to organic and ceramic insulation, and the regularities obtained have been constructed for specific insulating structures [3, 4, 5].

Separate data on surface breakdown of crystals [6] are considered without relation to the physicochemical composition of the dielectric.

The present article sets forth the results of work aimed at establishing the dependence of the degree of damage produced by surface electric discharges in NaCl, KCl, and KBr crystals on the magnitude of the lattice energy of the dielectrics and on the strength of the applied electric field.

The method for solving these questions is based on the fact, now considered established, that crystalline structures—in particular alkali-halide crystals—respond to the action of mechanical and electric fields and ionizing radiations by the formation of dislocation defects, which are revealed by various methods of crystal etching [7].

The work used crystals prepared by the Kyropoulos method in the crystallization laboratory of the Tomsk Polytechnic Institute from chemically pure grade reagents.

Preparation of the crystals for the experiments included cleaving, along the (100) plane, specimens of dimensions \(20 \times 20 \times 2\ \mathrm{mm}^3\), and etching to reveal the initial dislocation pattern; the following etchant compositions were used:

NaCl—glacial acetic acid with an addition of calcined ZnO;
KCl, KBr—absolute ethyl alcohol with an addition of \(\mathrm{PbSO}_4\).

The dislocation pattern of the crystal obtained as a result of etching was observed and photographed with MIM-6 and MBU-1 microscopes equipped with an MF-1 microphoto attachment. In counting the dislocation density, an object micrometer with an accuracy of up to 0.01 mm was used.

The final stage of the investigations was the study of specimens re-etched after exposure to the field.

The specimens were placed in a sharply nonuniform electric field between “point—plane” electrodes.

The use of pulsed voltage made it possible to exclude the effect of the humidity of the surrounding air [8].

The onset of the discharge was recorded by an electronic oscilloscope connected in parallel with the specimen.

Fig. 1. Cracks during flashover on a KCl crystal at a voltage of 14 kV:
a) at the center, point positive, 80×; b) at the edge, point negative, 80×.

Microscopic studies of the specimens revealed, at voltages above 4 kV, changes in the initial dislocation pattern of the crystal. The maximum distance over which this change propagated was taken as the discharge length.

Measurements with an object micrometer showed that the discharge length increases from crystal to crystal for NaCl, KCl, and KBr, and also with increasing applied voltage. Data on the dependence of \(l_{\text{disch}}\) on voltage are given in Table 1.

Table 1

Dependence of the discharge length on the magnitude of the voltage

The occurrence of surface breakdown at 14–16 kV was accompanied by cracking of the crystal; in this case, for a positive point the characteristic pattern is the development of cracks from the center (Fig. 1, a), while for a negative one it occurs at some distance from it, with maximum density at the edge of the specimen (Fig. 1, b).

The destruction of the crystal lattice of a solid in an electric field is due not only to the action of electric forces, but also to the mechanical stresses arising in the dielectric; for the magnitude of these stresses a linear dependence on the dielectric permittivity is valid [9].

An empirical dependence on the dielectric permittivity of the substance also exists for the voltage of creeping discharges [5, 8].

A considerable part of the static dielectric permittivity is due to displacements of positive ions relative to negative ions occurring in the lattice in an external field—the ionic component of the dielectric permittivity, \(\varepsilon - \varepsilon_\infty\), i.e., a characteristic of the dielectric that is related to the coefficient of all-round mechanical compression of the substance [10].

Fig. 2. Dependence of the discharge length on the ionic polarizability for NaCl, KCl, and KBr crystals:

a—at a voltage of 12 kV, with the point positive; b—at a voltage of 10 kV, with the point positive.

Fig. 3. Dependence of the discharge length on the magnitude of the crystal-lattice energy for NaCl, KCl, and KBr:

a—at a voltage of 12 kV, with the point positive; b—at a voltage of 10 kV, with the point positive; v—at a voltage of 10 kV, with the point negative; g—at a voltage of 6 kV, with the point negative.

Comparison of the quantity \(\varepsilon - \varepsilon_\infty\) with the discharge length on NaCl, KCl, and KBr crystals (Fig. 2) made it possible to conclude that there is a dependence between the length of the surface discharge and the magnitude of the lattice energy, since the ionic component of the dielectric permittivity is related to the latter by a definite relation [1].

The constructed relationship is shown in Fig. 3, from which it follows that, at a given value of the applied voltage, discharge development is most probable for crystals with a lower lattice energy.

The action of the tangential electric field on alkali-halide crystals was accompanied by an increase in the dislocation density.

The photographs presented in Fig. 4 demonstrate a successive increase in dislocation density from \(10^4\ \text{cm}^{-2}\) for the initial KBr specimen (a) to \(2.5 \times 10^6\ \text{cm}^{-2}\) after application to it of a voltage of 14 kV.

The cracks formed in the crystal during surface breakdown consist of pointed etch pits (Fig. 5). The flat-bottomed pits observed in this case characterize the sites of departed dislocations,

Fig. 4. [[unclear: photographs of surface discharge]] on a KBr crystal:
a — sample without field, 600×; b — at a voltage of 14 kV, 600×.

Obviously, the electric field causes the motion of dislocations in the crystal, orienting them in the direction of the field.

Fig. 5. Cracks during surface breakdown on an NaCl crystal at a voltage of +14 kV, 400×.

Conclusions

1. The electric field is one of the sources of dislocation formation in alkali-halide crystals.

2. The length of the surface discharge is inversely proportional to the magnitude of the lattice energy of the crystal.

REFERENCES

1. A. A. Vorob’ev. Physical Properties of Ionic Crystalline Dielectrics, book 1, Tomsk University Press, Tomsk, 1960.

2. Corona Discharges and Breakdown of Insulation, Electrical Engineering Industry Abroad, TsINTI, Moscow, 1963.

3. M. I. Mantrov. Elektrichestvo, No. 9, 54, 1940.

4. A. N. Gubkin. Izvestiya TPI, 91, p. 145, 1956.

5. A. A. Vorob’ev. High-Voltage Engineering, Gosenergoizdat, 1957.

6. J. Gilman and Staff. J. Appl. Phys., vol. 29, No. 2, p. 120, 1958.

7. E. V. Kolontsova and L. K. Mikhailova. Crystallography, vol. 8, issue 6, 1963.

8. L. A. Bibergal’, E. A. Nagi, S. S. Solomonik. Cables and Wires for Electronic Equipment, Energiya Publishing House, Moscow—Leningrad, 1964.

9. E. K. Zavadovskaya. Izvestiya TPI, vol. 77, 16, 1953.

10. A. Dekker. Physics of Electrical Engineering Materials, translated from English, ed. by B. M. Tareev, Gosenergoizdat, Moscow—Leningrad, 1962.