Sintering of ceramic compacts in a 35 GHz gyrotron-powered furnace

Summary form only given, as follows. The development of powerful gyrotrons has opened up the millimeter-wave regime (/spl ges/28 GHz) for processing ceramic materials. Millimeter-waves couple more strongly than pure oxides, eliminating the need for auxiliary heating at low temperatures, and highly uniform fields intensities can be achieved in compact overmoded cavity applicators. A number of low and high frequency microwave sintering studies have generally indicated that sintering proceeds much faster in microwave furnaces than in conventional furnaces, and that densification occurs at lower temperatures. Lower sintering temperatures are desirable for minimizing grain growth which usually has a negative effect on mechanical properties of ceramics. To assess the potential of high frequency microwave sintering, and to investigate the possibility of a specific microwave mechanism, the Naval Research Laboratory (NRL) has recently undertaken a systematic study focused on alumina because of its industrial importance, large data base, and challenging microwave properties. This paper presents 35 GHz sintering data obtained at NRL, using a gyrotron-powered furnace, for fine grain (submicron) alumina compacts and compares the data with results from other high frequency microwave and conventional sintering studies. The results indicate that the microwave sintering process appears to densify more quickly at a given temperature than in a conventional furnace.