Effect of Water Vapor on Initial Sintering of Magnesia

The effects of water vapor on the initial sintering of magnesia powder compacts were studied from 800° to 1107°C. The magnesia was obtained by thermal decomposition of magnesium oxalate. Water vapor partial pressures from 8 × 10 –4 to 658 mm were used. Initial sintering occurred by a grain‐boundary vacancy‐diffusion mechanism with magnesium assumed to be the slow moving species. Increasing water vapor partial pressure increased sintering rates. A model based on the solubility of the hydroxide ion in MgO relates this increase to the increase in the cation vacancy concentration. The grain‐boundary diffusion coefficient, D G , varied with P H2O . For partial pressures up to about 5 mm, n was approximately ½ and for partial pressures above 5 mm it was between 1.0 and 1.5. The activation energy for densification was 80 kcal/mole for partial pressures up to about 5 mm and 48 kcal/mole above this partial pressure.