Systematic study on densification of alumina fine powder during milliwave sintering at 28 GHz

Pure ultrafine alumina powder (0.2 μm average grain size (GS)) was sintered with a 28 GHz millimeter-wave radiation source by systematically varying sintering temperature (ST), holding time (HT), and heating rate (HR). Densification, microstructure and grain growth effects were analyzed after millimeter-wave sintering (MMWS). Alumina compacts were densified under rapid heating at 1100 °C without any HT up to 96% TD by retaining sub-micrometric GS (∼0.4 μm) and homogeneous microstructure. The ST was increased to 1200 °C (without HT, 50 °C/min HR) to achieve densification up to 98.6% TD, however, at the expense of an increased GS of ∼2 μm. Microstructure was maintained uniform throughout the sample even under extremely fast firing conditions of 200 °C/min HR except for the case in which HT was changed. Remarkable grain growth was observed when either ST or HT increased above the reference values.