Features of binary melt solidification at variable crystal pulling rate

The impurity diffusion in the melt, out of which the crystal is pulled into a cold zone, has been explained in the framework of one-dimensional non stationary solidification model in the second approximation by the amplitude of pulling speed oscillating term. It has been found that in the first approximation the crystallization and pulling rates have the same oscillation frequency, differing in phase shift phi(v). A dependence of phi(v) on oscillation frequency has been obtained with following parameters: melt impurity diffusion coefficient, impurity distribution coefficient k and concentration overcooling coefficient, which value shows whether overcooling takes place in the melt. In the second approximation, along side with the second harmonic, the impurity diffusion is explained by the presence of time-independent admixture which makes the impurity concentration decrease on the interface border at k<1 at average time and increase at k>1. Thus, a crystal being pulled at variable rate has the effect of a partially mixed melt.