Sintering and Grain Growth of Alpha‐Alumina

The sintering (densification) and grain growth of alumina were studied to determine the effect of the variables raw material, particle size, grinding in acid media, molding pressure, various single additives in different amounts, and firing temperature. Fine grinding promoted sintering and the growth of large grains and caused the grains to be more elongated in habit. Sintering was facilitated by additions of iron oxide, manganese oxide, copper oxide, and titanium oxide, provided the amounts of these oxides and the temperature of firing were within certain bounds. The growth of large grains was facilitated by additions of iron oxide and manganese oxide. Nineteen other additives had no effect or retarded sintering and large‐grain growth. Both magnesium oxide and silica had a marked effect in inhibiting the growth of large grains. The alkali metal oxides, added singly, were especially deleterious to the production of strong alumina bodies. The maximum density and maximum strength of the fired body were attained approximately simultaneously with the onset of large‐grain growth. The habit of the large grains was markedly altered by increasing amounts of each additive; the grains lost their characteristic crystalline shape and became nearly spheroidal particles. It is suggested that two grain‐growth phenomena exist which are independent of each other. One is termed “small‐grain growth” and is associated with densification; the other is referred to as “large‐grain growth” and occurs in certain specimens, depending on the additions to the alumina, after the sintering (densification) is substantially complete.