A study of millimeter-wave sintering of fine-grained alumina compacts

A number of high-frequency microwave sintering studies of alumina have reported that sintering proceeds much faster in microwave furnaces when compared to conventional furnaces, and that densification can occur at lower temperatures. These differences have motivated the search for a nonthermal microwave enhancement effect such as the time-averaged microwave field-induced mass transport effect proposed by Rybakov and Semenov (1994). To assess the difference between microwave and conventional sintering, and the presence of a nonthermal effect in microwave sintering, a study of millimeter-wave (mm-wave) (35 GHz) sintering has been conducted at the Naval Research Laboratory (NRL) using a well-studied fine-grained (submicron) commercial alumina with reproducibly manufactured properties, Sumitomo AKP-50. This paper reports our results, which generally indicate no large differences in the required temperatures for densification of conventionally and microwave sintered compacts, or between the resulting microstructures. The nonthermal effect proposed by Ryakov and Semenov was evaluated for fine-grained alumina and found to be small compared to the surface energy driving force for sintering.