Sintering transparent and other sub-μm alumina:: The right powder

New cutting tools, high-strength bioceramics and armor materials all request small sub-mum grain sizes at high sintered densities > 99,5 %, and the business is still more difficult when for highly transparent corundum ceramics the small grain size has to be associated with a residual porosity of < 0,05 %. Contrary to a general assumption of an increased sintering activity of nanopowders, the present results show that powders < 100 nm may give sintered microstructures with larger absolute grain sizes than observed using coarser powders, and the nanopowders < 100 nm exhibit rather unfavorable ratios close to 1:10 between the powder particle size and the size of grains in the dense sintered microstructure (this ratio is about 4...5 for conventional solid state sintering). On the other hand, a ratio as I close as 1:2 was surprisingly observed with a grain size of 0,4 pm in highly dense sintered products (> 919,95 %) starting from a powder in the range of 200 nm. It is, therefore, concluded that it is important to distinguish carefully between the diffusive properties of the one individual nanoparticle and the "sintering activity" of a multitude of particles in a sintering body.