Heat and mass transfer in LPE processes

Abstract

Current LPE processes for the preparation of magnetic oxide films for “bubble” technology have been developed empirically. However, control of layer perfection and properties can only be achieved by a better understanding of the growth process which involves heat and mass transfer and necessitates consideration of hydrodynamic factors in solution. Solution stability and flow processes which can occur are characterised by the use of dimensionless numbers; the most relevant are the Prandtl and Grashof numbers, which together define the better known Rayleigh number. Until recently, lack of data has restricted the application of hydrodynamic principles to fluxed melt systems. However, many of the important parameters such as solubility, density, viscosity, thermal and mass diffusivity, and their variations with temperature, can be determined using simple experimental techniques. Consideration of the possible modes of growth in the “dipping” process indicates that though dissipation of the heat of crystallisation at the interface is not a problem, forced cooling of the substrate via the holder should offer improved interface stability. Measurement of the growth rate as a function of rotation rate and time show the Burton, Prim and Slichter model to be applicable to this process. Growth occurs on an atomically rough surface and is largely controlled by diffusion through the boundary layer, the thickness of which is determined by the rotation rate. These considerations are used to predict qualitatively the most suitable experimental growth conditions.