The influence of two different additives, glycerol and polyvinyl alcohol (PVA), on the rheological behaviour and freeze casting performance of 35 vol.-% aqueous alumina suspensions is studied. Suspensions with PVA were prepared by either mixing all the components together or adding the PVA in a second step on the dispersed slurry. Although the slight increase in viscosity suggests that competitive adsorption occurs, the microstructure seems not to change depending on the order of addition of additives. Considering its lower molecular weight, glycerol provides lower viscosity, and as a consequence of its cryoprotective character, sintering leads to smaller pore size, being the porosity 35% for an added content of 10 wt-% with respect to solids. In the case of PVA, contents of 2 wt-% are enough to obtain solid firm bodies with a porosity of 48%. The porosity and the size of the pores decrease with increasing concentration of PVA.

Freeze casting is a well-known shaping technique to produce materials with directional porosity. One of the major problems is the difficulty to control the cooling rate thus leading to gradients in pore size and homogeneity. This work deals with the manufacture of alumina ceramics with directional porosity by freeze casting of aqueous suspensions. An experimental set-up was prepared in order to apply different cooling rates. Freeze casting tests were done with an aqueous alumina suspension after optimization of its rheological behavior. The porosity and microstructural features of sintered bodies produced under different experimental conditions were studied and analyzed. It is concluded that the cooling rate influences the microstructure while final temperature has a much lower influence. Also, the microstructural analysis shows that there is a gradient in the directionality of pores, being lower at the bottom and the top and higher in the central region of the specimens