ELECTRICAL STRENGTH OF THE SURFACE OF ALKALI-HALIDE CRYSTALS IN VACUUM

O. S. KASSIROVA

(Presented by the scientific-technical seminar of the Department of “Physical Electrical Engineering”)

Questions concerning the participation of the bridged surface in the development of a discharge have been investigated and discussed in many works [1, 2, 3, 4, 5, 6, 7].

It has been established that the material of the bridged surface has a substantial influence on the electrical strength of the surface. The values of the surface-breakdown voltage obtained by various authors for the same dielectric materials differ from one another by a factor of 2–2.5, which makes it difficult to determine the true value of the discharge-field strength. The difference in the results of different authors may to a considerable extent be determined by peculiarities of the composition, structure of the material, and treatment of the bridged surface. Therefore, it is of interest to carry out experiments on electrical breakdown along the surface of a dielectric in vacuum under conditions in which the physical characteristics of the surface would remain constant from experiment to experiment.

In the present work the task was to determine the electrical strength of a uniform surface of a fresh cleavage of a crystalline dielectric under constant voltage. The discharge developed between the points of contact of spherical steel electrodes with the surface of the dielectric.

The investigations were carried out on ionic crystalline dielectrics of different chemical composition, namely NaCl, KCl, KBr, KJ. In the preceding work [8], results were presented on the determination of electrical strength for an NaCl crystal, and the method for determining the electrical strength was described; this method was also used in the present work. To determine the dependence of electrical strength on the number of discharges, 10 specimens were studied for each type of dielectric at an interelectrode distance of 8.5 mm. The results obtained are shown in Fig. 1.

Analyzing the experimental dependences of the electrical strength of the surface of single-crystal salts NaCl, KBr, and KJ on the number of discharges, we see that the surface of NaCl has the greatest electrical strength, and KJ the least. After 50–60 breakdowns the electrical strength for NaCl and KBr changes little. For KJ this dependence in Fig. 1 has a different form than the first two. The electrical strength increases rapidly, then, after reaching a maximum, decreases somewhat.

The scatter in the values of the surface breakdown voltages of KJ is considerably greater than for NaCl and KBr under the same conditions.

Fig. 1. Dependence of the flashover voltage on the number of discharges for crystals of different chemical composition: 1 — NaCl; 2 — KBr; 3 — KJ.
Distance between electrodes \(d = 8.5\) mm.

The investigation of the dependence of the electrical strength of the surface of alkali-halide salts on the dielectric material was carried out for NaCl, KCl, KBr, and KJ. The experimental dependences obtained are presented in Fig. 2.

From the results presented it is evident that the electrical strength of the surface of single-crystal dielectrics depends on the chemical composition of the dielectric. The surface electrical strength for the series of dielectrics studied decreases from NaCl to KJ, which confirms the dependences shown in Fig. 1.

In order to determine the comparative participation of the cathode and anode regions of the dielectric surface in the development of the discharge, experiments were performed to determine the surface electrical strength of a specimen consisting of two single-crystal dielectrics differing in electrical strength. For the study, single crystals with sharply different surface electrical strengths, NaCl and KJ, were selected. The specimens were prepared immediately before the experiment and consisted of two parts pressed tightly against one another. The discharge over the surface developed perpendicular to the plane of contact of the dissimilar dielectrics. To determine the electrical strength of composite dielectrics, 5–6 specimens were studied, for example, with negative polarity of the weak element. Then the same number of specimens was studied for the variant in which the polarity of the parts of the specimen was changed, i.e., the part that had been under the cathode was placed under the anode, and vice versa. The results obtained are shown in Fig. 3.

Fig. 2. Dependence of the flashover voltage on the interelectrode distance for crystals of different chemical composition: 1 — NaCl; 2 — KCl; 3 — KBr; 4 — KJ.

It was found that the electrical strength of a specimen consisting of two elements of different strength is greater when NaCl—the element possessing the higher electrical strength—adjoins the cathode and KJ adjoins the anode (curve 2), and is considerably lower when it adjoins the anode and the weak element (KJ) adjoins the cathode (curve 1). Curve 3 is the dependence of the breakdown

voltage on the number of breakdowns for pure NaCl; this dependence is given for comparison with composite dielectrics.

The results of the experiment described can be explained by the fact that the determining influence on the electrical strength of the surface of single-crystal dielectrics is exerted by the near-cathode region of the dielectric.

To understand the mechanism of development of a spark discharge over a surface in vacuum, it is of interest to establish the relationship between the physical properties of the dielectric and the electrical strength of its surface. The properties of alkali-halide crystals have been studied quite comprehensively [9, 10]. It has been established that the properties of alkali-halide crystals depend primarily on the internal lattice energy and the surface energy associated with it. In order of increasing lattice energy, the crystals studied are arranged in the following order: KI, KBr, KCl, NaCl. As the lattice energy increases, such characteristics as molecular weight, melting temperature, dielectric constant, and secondary-emission coefficient also increase. As

Fig. 3. Dependence of the flashover voltage on the number of discharges over the surface of composite and simple dielectrics:
1 — composite dielectric KI + NaCl, where KI is adjacent to the cathode, NaCl to the anode;
2 — composite dielectric KI + NaCl, where NaCl is adjacent to the cathode, KI to the anode;
3 — NaCl

Table 1

\(M\) — molecular weight;
\(\varepsilon\) — dielectric constant;
\(T_{\mathrm{m}}\) — melting temperature, °C;
\(\sigma\) — secondary-emission coefficient after cleaving the surface in vacuum.

established in the present work, the electrical strength of the samples increases in the same sequence (Table 1). This indicates the large participation of the sample surface in the development of the discharge.

Conclusions

1. The electrical strength of the surface of single-crystal dielectrics depends on the chemical composition of the single crystals. The single crystals with the higher surface energy possess the greater electrical strength.

2. The determining influence on the electrical strength of the surface of a dielectric in vacuum is exerted by the physical characteristics of the near-cathode region.

Literature

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