TRANSMISSION OPTICAL-PROPERTIES OF POLYCRYSTALLINE ALUMINA WITH SUBMICRON GRAINS

Translucent alumina ceramics with extremely high in-line optical transmittance to visible light and high mechanical strength were produced by applying HIP process to a sample which was densified at a relatively low temperature after injection molding. Measurements of bulk density and average grain size, X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) analysis, and SEM and TEM observations were conducted. The bulk density was higher than 3.985 kg/m3, average grain size was between 0.82 and 1.43-mu-m, and the lattice constants were a = 0.476 nm and c = 1.300 nm. The segregation of impurities at grain boundaries and the grain boundary interface was not observed. The in-line transmittance increased with the reduction of the average grain size of the translucent alumina ceramics. It was confirmed that in the translucent alumina ceramics, optical scattering does not occur due to the increase in the number of grain boundaries and pores, segregation of impurities and distortion of lattices as conventionally reported, and hence does not conform to Fresnel's law. Based on these findings, it was concluded that the improvement of optical transmittance can be realized by reducing the crystal grains to smaller than 1-mu-m as well as by the conventional method of growing them to larger than 20-30-mu-m.