Improved alumina transparency achieved by high-pressure spark plasma sintering of commercial powder

Depending on starting powder and sintering conditions, alumina with extremely small grains and low residual porosity can be highly transparent. Combined with its excellent mechanical, thermal and chemical properties, transparent alumina is a promising material for a wide range of optical applications. In the present study, highly transparent alumina was fabricated by single-step high-pressure spark plasma sintering (HP-SPS) using raw commercial pure powder. The microstructure of the sintered alumina consists of fine grains with average size of ~170 nm and extremely fine residual porosity. The measured real in-line transmittance (RIT) was 72.6% at a wavelength of 640 nm, close to the calculated theoretical value (74.9%). The difference between theoretical and measured RIT was attributed to scattering by fine residual pores. This conclusion was supported by porosity analysis, and RIT calculations suggest that the porosity volume fraction is about 0.005%. The combination of fine grain size and high level of transparency could be suitable for potential advanced alumina-based laser materials.