Analysis of the low gravity tolerance of Bridgman-Stockbarger crystal growth: I. Steady and impulse accelerations

Abstract

The effects of steady and impulse-type residual accelerations on dopant distributions during directional solidification in 2D and 3D “generic” models of the Bridgman-Stockbarger technique have been investigated using numerical models. The calculations are based on the thermo-physical properties of molten germanium doped with a low concentration of gallium. A novel Chebyshev collocation pseudo-spectral method was used for the solution of the governing momentum-, mass-, species- and heat-transfer equations. Only convection caused by temperature gradients was considered. It was found that lateral non-uniformity in composition is very sensitive to the orientation of the steady component of the residual gravity vector and to the particular operating conditions under consideration. For growth rates of several microns per second a steady background level of 10 -6 times that of normal gravity can be tolerated provided that the acceleration vector is aligned with the axis of the growth ampoule. For reduced growth rates, higher (steady) background acceleration levels are acceptable. It was also found that laterally or radially averaged composition profiles are alone insufficient to describe the extent of residual convection in a spacecraft environment. The effects of impulse-type disturbances can be severe and can extend for times on the order of one thousand seconds after the termination of the impulse. A so-called “compensating double pulse” will not result in completely offsetting effects.