| We have identified the crystal growth mechanism of B(,2)O(,3). By seeding the surface of boron oxide glass with crystals, we have been able to eliminate the crystal nucleation barrier and to measure the growth rate of B(,2)O(,3)-I crystals as it varies over five orders of magnitude with changes in temperature and pressure. In conjunction with the thermodynamic information of MacKenzie and Claussen on the B(,2)O(,3) system and a crude free energy model for the crystal and glass phases, a theory of crystal growth limited by the rate of two-dimensional nucleation of monolayers accounts qualitatively for our results. By changing some of the thermodynamic parameters to values consistent with the work of other investigators, we account quantitatively for the measured growth rates. The model explains why crystal growth has never been observed at any temperature at atmospheric pressure, since nucleation is negligible at all temperatures below approximately five kilobars.;From laser-annealing data, the interface velocity-undercooling function for silicon crystal growth is deduced and the melting point of amorphous silicon is estimated. Good agreement is found with the values of others.;A model for nonequilibrium solute segregation during rapid solidification is presented in terms of a single unknown parameter, the interfacial diffusivity D(,i). The steady-state velocity of a planar liquid-solid interface is predicted by calculating the free energy dissipated by irreversible processes at the interface and equating it to the driving free energy. A solute drag term and an intrinsic interfacial mobility term are included in the dissipation calculations for a binary alloy. A transition from diffusion-controlled to diffusionless solidification occurs over approximately an order of magnitude in growth velocity, as the interface speed surpasses the maximum speed with which solute atoms can diffuse across the interface to remain ahead of the growing crystal. This diffusive speed is given by D(,i)/(lamda), where (lamda) is the interatomic spacing, and is typically of the order 10 m/s. No kinetic limit is found to solute trapping. However, with certain complicating assumptions the segregation coefficient can be made to appear to saturate, or stop increasing with velocity, at values less than unity. |
