The experimental determination of the kinetics of solid-liquid interfaces in transparent materials using temperature-gradient zone migration

Abstract

Detailed experimental measurements have been made of the rates at which liquid droplets of salol, containing between 2 and 10 mole % (mol %) of azobenzene, migrate through solid salol under the influence of a temperature gradient. Similar studies have been made for the migration through solid camphene of liquid droplets of camphene containing impurities. The concentrations of impurity in the latter droplets represented liquidus temperatures of between 17°K (K) and 5°K (K) below the melting point of pure camphene. The experimental results have been analysed according to the theory of Temperature-Gradient Zone Migration (TGZM). The analysis of the results showed that the migration of droplets through salol was limited by a screw-dislocation mechanism of dissolution, having a kinetic coefficient of (1·7±0·6) × 10⁻³ mm s⁻¹ °K⁻² (1·7±0·6 μm s⁻¹ K⁻²), taking place at the high-temperature interface. The dissolution kinetics were independent of the concentrations of azobenzene employed. The kinetics of the rate-controlling interfacial reaction were measured for the camphene system, but it was not possible to decide unambiguously the mechanism of interfacial motion in this material. Precise measurements were made of interfacial undercoolings of between 2·5 × 10⁻³ °K (K) and 1·6 × 10⁻³ °K (K), corresponding to interfacial velocities in the range 5·5 × 10⁻⁵ to 6·3 × 10⁻⁴ mm s⁻¹. If ‘normal growth’ is assumed to take place the measured undercoolings represent kinetic coefficients in the range 0·022–0·40 mm s⁻¹ °K⁻¹ (mm s⁻¹ K⁻¹). The kinetics for the camphene system dependend strongly on impurity concentration.