Abstract
Crystal growth of MgO doped with AlF3 and MgF2 has been studied by BET method and X-ray powder diffractometry. The fluorides were hydrolyzed by the water contained in MgO powder above 500°C and generated HF gas, which was adsorbed on the surface of MgO. Fluorides inhibited crystal growth of MgO below 900°C, but accelerated the growth above 1000°C, because of evaporation of adsorbed HF. Departure of HF from the surface increases the surface energy of MgO and/or forms active points on the surface, and then migration of MgO between grains, mainly through evaporation and condensation, is promoted and would result in rapid crystal growth. MgF2 crystallized again at 1000°C for MgO doped with MgF2 in dry air. The HF coverage of MgO grains in a specimen containing 2mol% MgF2 (evaluated as monolayer) was constant up to 1000°C and decreased above 1100°C. It was inferred that crystal growth of MgO proceeded so as to maintain the balance of surface energy for the evaporation of HF from the surface at 1000°C and that the crystal growth couldn't follow because of the rapid evaporation of HF above 1100°C.
