Evaporation of KC1 crystals in the presence of a.c. and d.c. fields

Abstract

The evaporation of cleaved (100) faces of KC1 has been investigated in the presence of externally applied a.c. and d.c. fields. The effect of d.c. fields depended on the magnitude of the imposed potential gradient and on temperature, relative to the isoelectric temperature T _i of the samples. Below T _i the evaporation rate increased in the presence of d.c. fields at or above a threshold value. In contrast, the presence of d.c. fields at T > T _i resulted in a decrease in the evaporation rate. Threshold values and the degree of evaporation enhancement were temperature-dependent. The thresholds decreased and the degree of enhancement increased with increasing temperature of evaporation. The imposition of a.c. fields, on the other hand, had no measurable effect on the evaporation rate of KCI at the highest experimentally attainable potential gradient. An analysis is presented in which the effect of the field on the evaporation process is assumed to be related to surface diffusion enhancement. On this basis, the presence of a d.c. field can reduce the activation energy of surface diffusion by 3·9 kJ mol⁻¹.