Conversion from nano- to micron-sized structures: experimental observations

When consolidating nano-sized powders of various oxides and non-oxides susceptible or not susceptible to liquid-phase sintering by SPS technique, we observed a common feature of rapid conversion from nano- to micron-sized structures. The conversion is strongly temperature-dependent and above a critical temperature (Tg) grain growth occurs very rapidly. Below Tg there is a narrow temperature window where nano-sized powders can be fully densified without or with very limited grain growth. Above Tg rapid grain growth takes place in fully densified bodies owing to the high driving force provided by the large surface energy of nano-sized grains. This in turn favours the preparation of ceramics with tailored microstructures, e.g. in situ reinforced ceramics. In this presentation, examples will be given to demonstrate the possibility to utilize the benefit gained from nano-structure to produce complex shaped ceramic components by superplastic forming and then converting the ductile nano-sized structure into a rigid in situ reinforced microstructure by post annealing above Tg.