Technique for computer modeling of diamond crystallization process in
the reaction cell of high pressure apparatus is considered. The carbon
growth medium is supposed to be cleaned of 12C or 13C isotopes.
The diamond crystallization process is accompanied by the processes of
electrical and heat conduction, thermoelastoplastic deformation and
phase transitions. Kinetics of crystallization are greatly influenced
by the coupling between fields of electric potential, temperature,
pressure and various phase concentrations in a reaction zone. To
describe the diamond crystallization process we use the numerical
technique for FEM calculation of coupled nonlinear nonsteady problem
of electrical and heat conduction and thermoplasticity, taking into
account the large elastoplastic strains, phase transitions of
materials and high pressure effect. Phase transitions are determined
from the changes in properties, concentrations and abrupt change in
specific volume in the region where diamond synthesis is possible. The
coupling of the above problem is caused by the dependences of
physico-mechanical characteristics on pressure, temperature, phase
concentration and concentration fields on temperature and pressure
distributions. The following relationships are also taken into
account:
* diamond lattice thermal conductivity -- 12C or 13C isotopic
impurity content;
* diamond thermal conductivity -- crystal geometry;
* diamond lattice thermal conductivity -- temperature.
The temperature, pressure and concentration fields in the reaction
volume of HPA has been calculated. The results demonstrate a
significant coupling of these fields and the effect of self-regulation
of pressure in the HPA reaction zone consisting in pressure
oscillation with respect to graphite-diamond phase transition line.

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