Kanishchev V. Transients of the directed crestallization at crestals growing from the melt

Thesis for scientific degree of Doctor of Technical Science on specialty 05.02.01 - Materials Science. - Institute for Single Crystals of National Academy of Science of Ukraine, Kharkov, 2014. This thesis studies directional crystallization transitional processes and their connec-tion with the unlike distribution of impurity in crystals and optimizes conditions for grow-ing crystals from melt when the conditions deviate. It has been proposed that the temperature profile shift rate is the time function w(t) and the crystallization rate V(t) is a sought quantity in the one dimensional non-stationary model. Thanks to the introduction of these rates the impurity distribution has been calcu-lated at the temperature profile shifting at a constant rate (initial and final transition pro-cesses), sinusoidal rate, a constant acceleration as well as at a decreasing temperature gra-dient at the crystallization interface. It has been calculated that the crystallization rate changes non-monotonously and can have a decaying oscillation form at the initial transitional process under the constant tem-perature profile shift rate at which the concentration overcooling takes place. An average interface impurity concentration differs at varying and constant temperature profile shifting rates. So, at an impurity distribution coefficient k<1 the average interface impurity concentration is lower at varying than constant temperature profile shifting rate, and at k>1 the concentration is higher, respectively. Thus, the varying temperature profile shifting rate is as effective as a partial melt stirring. It has been experimentally found that the crystallization rate changes periodically and concurs with the impurity bands when sapphire was being constantly pulled by a horizon-tal directional crystallization method in an over-critical mode. A crystal growing method has been developed due to the data obtained, which allows optimizing the technology of sapphire production