The kinetics of cubic boron nitride formation in the system BN-Mg2N2

Cubic boron nitride is synthesized using Mg3N2 as a catalyst at a pressure of 6.5 GPa in the temperature range 1600<T<2200 K. The temperature variations of growth and net transformation rates are estimated. Above 1650 K the transformation kinetics is well described by a nucleation and growth process. The theoretical description based on the homogeneous nucleation and layer growth mechanism yields reasonable results in comparison with the experimental data. The relevant parameters are estimated and discussed. The effect of the catalyst is suggested to consist in lowering the activation enthalpy for nucleation. Below 1650 K it is shown that the transformation kinetics changes qualitatively and the nucleation and growth mechanism is not the appropriate description. The physics of transformation in this temperature range have hitherto remained unexplained.