ON DIFFUSION AND ELECTRICAL CONDUCTIVITY IN ALKALI-HALIDE CRYSTALS

P. A. SAVINTSEV, V. Ya. ZLENKO, M. I. IGNAT'EVA

(Presented by the scientific seminar of the Radio Engineering Faculty)

When two or more dissimilar substances are in contact, the melting temperature may be lowered in comparison with the melting temperature of the initial substances. This phenomenon is known as contact melting \([1 \div 3]\). To elucidate the nature of contact melting it is necessary to study the properties of the contact of dissimilar crystals at different temperatures, paying particular attention to clarifying the question of particle mobility in the contact and the diffusion processes associated with it.

Diffusion phenomena were studied in the contact of alkali-halide crystals NaCl—NaBr, NaCl—KCl, KCl—KBr, which can form solid solutions. Single crystals were heated separately and then, after being brought into contact, were held for a certain time at a temperature lower than the temperature of contact melting. After cooling, the surface of the specimens was studied with a microscope, the microhardness was measured, and the chemical composition was determined at various distances from the contacting surface.

Table 1

Diffusion coefficient in cm² · sec⁻¹ of the KCl—NaCl system

For the KCl—NaCl system the coefficient of volume and surface diffusion was estimated. The results are given in Table 1, from which it is seen that the coefficient of surface diffusion exceeds the coefficient of volume diffusion by hundreds and thousands of times. However, in the temperature range 535—623 °C the coefficient of volume diffusion increases more rapidly than the coefficient of surface diffusion. Apparently, the role

surface diffusion in the NaCl—KCl contact is especially large at temperatures below 500 °C.

The microhardness values at various distances from the KCl—NaCl and KCl—KBr contacts are given in Table 2, from which it is seen that, as the distance from the contact surface increases, the microhardness decreases.

Microscopic investigations made it possible to detect changes in the relief of the contact surfaces KCl—NaCl, KCl—KBr, and NaCl—NaBr at temperatures close to the temperature of contact melting.

After holding for several minutes, stepped pyramids grow on the surface of the crystal with the smaller lattice constant, while pits appear on the surface of the crystal with the larger lattice constant. In this case it turns out that the faces of the steps and pits are parallel to the (100) planes.

With increasing holding time, growth of the pyramids and pores is observed. The latter occur not only on the surface of the crystal, but also in the bulk.

When NaCl—KCl and NaCl—NaBr crystals are brought into contact, pores are observed in KCl and NaBr. In experiments with the KCl—KBr pair, pores arise in the KBr crystal. Thus, pores are formed in crystals with the lower binding energy of the particles. During mutual diffusion, more particles leave these crystals than leave crystals with the higher binding energy.

The electrical properties of the contact of dissimilar crystals capable of forming eutectics and solid solutions at different temperatures were studied.

The temperature dependence of the logarithm of the electrical conductivity of the KCl—LiCl, KI—NaCl, and KI—NaBr systems is presented in Table 3, from which the following is seen. In the eutectic KCl—LiCl system, a sharp increase in electrical conductivity is observed beginning at a temperature of 320°. At a temperature close to 360°, the electrical conductivity of the mixture of eutectic powders exceeds the electrical conductivity of the pure substances making up the eutectic by tens and hundreds of times. At a temperature of 370° an even sharper increase in the current and melting of the KCl—LiCl mixture were observed.

In the eutectic KI—NaCl mixture, a bend in the electrical-conductivity curve was observed at a temperature of 410°. A sharp increase in current, associated with melting of the specimen, occurred at a temperature of 490°.

Thus, the special properties of the contact of dissimilar crystals begin to appear in the eutectic KCl—LiCl system 50° below the temperature of contact melting, and in the KI—NaCl system 80° below it.

In the KI—NaBr system, which gives a solid solution, the electrical-conductivity curve as a function of temperature has the same form as the electrical-conductivity curves of the KI—NaCl and KCl—LiCl systems. A bend is observed at a temperature of 470°, and the rapid increase in current corresponding to melting is observed at a temperature of 570°. On the graph of the electrical conductivity of the pure salts, no special points are observed in the temperature interval investigated.

The observed dependence of electrical conductivity on temperature is in good agreement with the diffusion experiments described above. The bend in the electrical-conductivity curve is explained by the formation, in the contact and near it, of a solid solution possessing increased electrical conductivity in comparison with the pure crystal.

Thus, in the contact of dissimilar crystals of systems with unlimited and limited mutual solubility in the solid state, diffusion processes are observed, as a result of which

...on the surface of the contacting crystals a layer is formed that has a lowered melting temperature.

Table 2

Microhardness of crystals at different distances from the contact
KCl—KBr and KCl—NaCl

Table 3

Dependence of the logarithm of the electrical conductivity of alkali-halide
crystals on temperature

REFERENCES

1. Saratovkin D. D. and Savintsev P. A., Formation of a liquid phase at the contact point of two crystals constituting a eutectic pair, Doklady AN SSSR, 33, No. 4, 1941.

2. Saratovkin D. D. and Savintsev P. A., The effect of contact melting as the cause of the low melting point of eutectics, Doklady AN SSSR, 52, No. 9, 1947.

3. Savintsev P. A. and Avericheva V. E., Contact melting of crystals, Izvestiya vuzov MVO SSSR, Fizika, No. 1, 1957.