Numerical investigation of the effects of thermal creep in physical vapor transport

It has been recently recognized that the nonisothermal conditions present in physical vapor transport ampoules can give rise to a slip flow of gas over the side walls of the ampoule. This phenomenon, known as thermal creep, is usually insignificant relative to buoyancy-induced flows under similar nonisothermal conditions, and has therefore been neglected in previous PVT numerical models. However, thermal creep can, in principle, become a dominant convection mechanism in buoyancy-free environments such as those encountered in microgravity experiments. We present here a numerical investigation of the effects of thermal creep on the growth process in axisymmetric, binary component PVT systems. A continuum-based model, which includes buoyancy and Soret diffusion, is developed. We show that thermal creep can result in recirculating bulk flows within the ampoule. For relatively high values of the Schmidt number and large wall temperature gradients, these flows can result in significantly nonuniform distributions of mass flux at the crystal interface, and can also be comparable to or exceed the flow velocities generated by buoyancy under normal gravity. The effects of thermal creep on buoyant convection, and on the Sorer transport of the vapor, are examined.