Nano-processing for larger fine-grained windows of transparent spinel

A clear transparency of components free of scattering losses needs < 0.01% of porosity which is frequently achieved by high sintering temperatures associated with extended grain growth whereas an extreme mechanical stability is bound to fine-grained microstructures < 2 μm. Thus, improvements are expected by technologies which achieve an extreme sintering densification at low temperature. Nanopowders with short diffusion distances do not generally match this target because their high surface curvature gives rise to increased diffusion but is also associated with strong forces of physical interaction (= increasing agglomeration at high specific surface). Therefore, a successful use of such powders for transparent windows needs a high degree of de-agglomeration and processing approaches which provide an extreme homogeneity of the particle coordination in the green shaped bodies. For this end, commercial spinel raw materials from America, Europe and Japan were modified by the manufacturing companies upon specifications of our laboratory in order to promote a "defect-free processing" by an improved dispersibility of the powders. Highly transparent windows up to ~ 240mm and with thickness > 10 mm were obtained with powders covering a wide surface range of 10-80 m2/g. However, only successfully de-agglomerated powders with higher surfaces > 30 m2/g were able to achieve a perfect in-line transmission (= density > 99.99%) at lowest sintering temperatures resulting in smallest grain sizes (e.g. 270 nm) and a hardness of the spinel on the level of sapphire. The measured spectra l transmission agrees with Mie calculations and an experimentally verified residual porosity in the ppm range.