Hot-Pressing of MgO with NaF

The results of using NaF additives in hot-pressing MgO are described and compared with the results when LiF is used. To obtain nearly theoretical density, the temperatures required for hot-pressing with NaF are ∼100°C higher than those required with LiF. Specimens made with NaF were generally translucent after hot-pressing or subsequent annealing. Sodium fluoride inhibits grain growth and can produce a duplex grain structure, whereas LiF enhances grain growth during annealing. Both NaF and LiF wet MgO and lubricate it well. The present study and other investigations of fluoride densification aids indicate that the formation of a liquid phase, with resultant wetting and lubrication, is important, especially in the early stages of densification. Possible solution of anions and cations is discussed.