Abstract
Alumina prepared by millimeter-wave sintering, compared to that by electric-furnace sintering, was densified at a 250 K lower temperature and exhibited twice the bending strength (800 MPa). However, this alumina showed a 7% lower value of Young's modulus than alumina by electric-furnace sintering. The reasons for these properties were clarified by microstructural analysis using a transmission electron microscope. The lower Young's modulus was caused by the higher porosity. The relationship between bending strength and porosity or grain size showed a lower correlation, but the reduction in pore size corresponded to the increase in bending strength. At the grain boundaries of all of the alumina ceramics, no thin films or impurities were observed, and, therefore, no significant difference in these features among aluminas could be attributed to sintering conditions. However, a difference in the distribution of pores was clearly observed. Many small pores existed in the grains of the alumina by millimeter-wave sintering, while larger pores existed at the grain boundaries of the alumina by electric-furnace sintering. The small pores in the grains contributed to the improved bending strength. This result was caused by the millimeter-wave sintering process, in which the pore velocity was less than the boundary velocity due to the rapid diffusion of atoms.
