Dependence of optical properties on annealing process of transparent nano-structured γ-Al2O3 ceramics

Nano-structured gamma-Al2O3 ceramics have been pelletized under a pressure of 2.5 GPa on uniaxial compaction of nano-size particles. Although the obtained gamma-Al2O3 ceramics show low density, they have a good transparency. For estimating the influence of particle size on the transparency of nano-structured gamma-Al2O3 ceramics, nano-sized particles with four different sizes have been used. Nano-structured gamma-Al2O3 ceramics made of particles whose size is < 10 nm show a transmittance above 40%. Transmittance decreases with increasing particle diameter. In the case of particle size > 20 nm the transmittance is less than 20%. This phenomenon is explained by Rayleigh scattering, which is applicable to a particle diameter much smaller than the wavelength of the incident light. The change in transmittance of nano-structured alumina ceramics on heat treatment is not observed below 1123 K. However the transmittance remarkably decreases at above 1223 K because of rapid grain growths with phase transition, although this temperature is 250 K lower than the gamma- > alpha-phase transition temperature (1473 K) of bulk alumina. When the sample produced from 11.2 nm particles was heat-treated at 1173 K, the transmittance was maintained for 2 h and then decreased with increasing the annealing time. At this time, the transmittance slowly decreased with grain growth without phase transition between 2 h and 6 h, and then it steeply decreased with grain growth after 6 h with phase transition.