Investigation of solid/liquid interface temperatures via isenthalpic solidification

Isenthalpic solidification of a pure supercooled liquid is shown to result in either a two-phase solid/liquid mixture in invariant equilibrium or a single-phase, totally solid material in univariant equilibrium, depending on the level of supercooling prior to solidification. The critical supercooling above which univariant equilibrium obtains is large for metals (hundreds of degrees centigrade) but much smaller for certain molecular substances. Experiments on white phosphorus (alpha P4) show that the critical supercooling (25.6 °C) can be reached, and exceeded, easily. Solidification rate measurements taken above and below the critical supercooling for P4 show that the solid/liquid interface temperature varies smoothly with melt supercooling, although light-scattering experiments indicate that rapid changes occur in the extent of the dendritic zone as the critical supercooling is approached and exceeded. A method for extracting interface attachment kinetics from solidification rate data is explained in detail and applied to our rate measurements on P4. We find that above about 9 °C supercooling P4 solidifies with linear attachment kinetics having a rate constant of 17.7 ± 0.4 cm/sec °C. Below about 1 °C supercooling, P4 solidifies with a faceted morphology indicative of layer-passage limited kinetics. Between 1–9 °C supercooling, transitional growth kinetics occur. These results are in qualitative agreement with the crystal growth theory of Cahn et al., which predicts that attachment kinetics should change as the driving force for crystal growth is varied by substantial amounts.