Millimeter-wave sintering of ceramics with applied pressure

Pressure-free microwave processing of polycrystalline materials has demonstrated rapid, efficient sintering attributed to volumetric heating and microwave-activated mass transport. However, the potential for improved sintering in the nanoparticle regime has often not been realized in part because of the tendency of nanoparticles to form agglomerates that reduce the density of the green compact leading to large pores and excessive grain growth. The application of pressure during sintering, i.e., hot pressing and hot isostatic pressing, has been shown to be an effective sintering method as the stress resulting from an external load can easily exceed the stress caused by surface tension. The application of pressure can result in faster sintering and lower sintering temperature. The current experimental investigations are being carried out in an 83 GHz, 15 kW gyrotron-based materials processing facility with a hydraulic press integrated into the millimeter-wave applicator in order to exert regulated pressure on the samples being heated by the beam. The pressing fixtures are fabricated from refractory ceramic materials with low microwave absorptivity such as alumina and boron nitride. Uniaxial pressures up to 10 MPa have been applied during sintering. Sample temperature is monitored by a two-color pyrometer. The processing environment can be vacuum or special atmosphere. Results of pressure-enhanced sintering of advanced ceramics will be presented showing rapid sintering rates and high final densification. We compare the measured sintering rate with the predictions of a micro-mechanical theory of sintering based on grain-boundary diffusion [1]. The model shows the sintering rate increases with applied load. The ponderomotive microwave effect [2] included in the model also contributes to increasing the sintering rate.