Kinetics of cBN crystallization in the Li3N-BN system at 6.6 GPa

Kinetics of cBN crystallization from the Li3N-BN melt being in equilibrium with hBN has been studied in situ at 6.6 GPa in the 1720-1820 K range using diffraction of synchrotron radiation. The process under consideration has been found to have the activation energy of 115 +/- 5 kJ mol(-1) and to be controlled by BN diffusion in the melt. Kinetics data might be best fitted by a model that assumes an instantaneous nucleation in the initial stage of crystallization and nucleation at constant rate when hBN-to-cBN conversion degree is higher than 0.2. Using the model of regular solutions, the maximum solubilities of hBN and cBN in the melt of the Li3N-BN system were calculated, which allowed the BN diffusion coefficient in the melt al 6.6 GPa and 1770 K to be evaluated at (3+/-1) x 10(-7) cm(2)s(-1). (C) 1998 Elsevier Science S.A.