Hot‐Pressing Platelet Alumina to Transparency

Alumina powder with a platelet morphology was hot-pressed to transparency with preload pressures of 0-8 MPa, maximum temperatures of 1750-1825 degrees C, maximum pressures of 2.5-80 MPa, and isothermal hold times of 1-7 hours. Optical transmission (in-line and total), as well as optical losses (backward/forward scattering and absorption), of the hot-pressed samples were measured and related to the microstructure. Higher hot-pressing temperatures increase the in-line transmission. A gray discoloration of the samples (indicative of high absorption) was minimized by heat treating the powder in air prior to hot pressing and reducing the preload pressure. Maximum pressures above/below 10 MPa increased porosity, which decreased in-line transmission and increased backward/forward scattering. Lower densities at higher pressures are attributed to a pore-swelling phenomenon. Increasing isothermal hold time decreased porosity, which increased in-line transmission and reduced backward/forward scattering. Best optical properties with an in-line transmission of 65.3% at 645 nm (0.8 mm thick) were achieved by hot-pressing heat-treated platelet alumina powder with a preload pressure of 0 MPa, maximum temperature of 1800 degrees C, maximum pressure of 10 MPa, and an isothermal hold time of 7 hours. This high in-line transmission, despite its large grain size (65 mu m), is attributed to crystallographic orientation of the platelets during hot pressing.