Fundamental studies in the millimeter-wave and microwave sintering of alumina compacts

Summary form only given. Prior work at the Naval Research Laboratory (NRL) has focused on microwave sintering of fine- and ultra-fine-grained alumina (Al/sub 2/O/sub 3/) compacts at both 35 and 2.45 GHz. Although the volumetric microwave heating of the compacts facilitated rapid processing, results from these studies have not indicated the presence of a significant non-thermal "microwave effect". These results are consistent with our estimates of the nonthermal microwave interaction with grain lattice vacancies using the model formulated by Rybakov and Semenov (RS) (1994), since the resulting microwave stress was calculated to be much smaller than the surface energy stress, the usual driving force for sintering. Moreover, comparison of conventional and microwave sintering of a fine-grained commercial alumina (Sumitomo AKP-50) showed similar microstructures. The RS model predicts that the microwave stress effect should be affected, and, perhaps, enhanced, if the defect structure at the grain boundary is modified. This defect structure modification can be due to either increased vacancy concentrations at the surfaces of ultra-finegrained materials, e.g., nano-crystalline alumina, or to increased oxygen vacancies caused by the deliberate removal of oxygen via a thermochemical procedure. This presentation will discuss work in progress on the sintering of reduced alumina at 35 GHz and 2.45 GHz. The latter frequency is of interest because the RS model predicts that there should be a more than 100:1 increase in its effect at the lower frequency compared to the higher. This effect may be mitigated somewhat by the reduced coupling strength at the lower frequency.