Application of microwave heating to ceramic processing: Design and initial operation of a 2.45-GHz single-mode furnace

High-power microwave and millimeter-wave sources are currently being applied to ceramic processing studies at the Naval Research Laboratory (NRL). A single-mode cavity microwave furnace, operating in the TE₁₀₃ mode at 2.45 GHz, is operational and is being used to investigate sintering of nanocrystalline ceramics. This paper reports the design of the 2.45-GHz furnace and its use in initial microwave sintering experiments on nanocrystalline alumina and titania compacts. The high purity Al₂O₃ and TiO₂ nanocrystalline powders used in the sintering experiments were prepared by the sol-gel method. These powders were first uniaxially pressed to 14 MPa, cold isostatically pressed (CIP´ed) to various pressures ⩾420 MPa, and finally sectioned into wafers. The density of the green compacts was 30 to 38% theoretical density (TD). The compacts were placed in insulating fiberboard caskets which were sufficiently lossy to provide hybrid heating at room temperature. The compacts were heated in the microwave furnace for up to three hours at temperatures ⩽1720°C. The temperature of the workpiece was monitored using an optical pyrometer. Final densities up to 80% TD have been obtained to date for Al₂O₃ and up to 52% TD for TiO₂. The sintered compacts were characterized by X-ray diffraction and by scanning electron microscopy (SEM) to determine the phase and grain size