MgOの結晶成長におけるフッ化物添加の影響

概要

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Crystal growth of MgO doped with AlF_3 and MgF_2 has been studied by BET method and X- ray powder diffractometry. The fluorides were hydrolyzed by the water contained in MgO powder above 500℃ and generated HF gas, which was adsorbed on the surface of MgO. Fluorides inhibited crystal growth of MgO below 900℃, but accelerated the growth above 1000℃, 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. MgF_2 crystallized again at 1000℃ for MgO doped with MgF_2 in dry air. The HF coverage of MgO grains in a specimen containing 2 mol% MgF_2 (evaluated as monolayer) was constant up to 1000℃ and decreased above 1100℃. 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℃ and that the crystal growth couldn't follow because of the rapid evaporation of HF above 1100℃.
1989-08-01