Macroporous ceramics: Novel route using partial sintering of alumina-powder agglomerates obtained by spray-drying

This paper presents a new method for manufacturing macroporous ceramics by partial sintering of controlled ceramic powder agglomerates packing. Using this route, the final ceramic material is characterized by a hierarchical porous network that can contain up to three levels of interconnected pores: (i) the voids existing between the agglomerates (typically ≥10 µm in size), (ii) the porosity remaining inside the agglomerates after partial sintering (typically ≈100–1000 nm in size), (iii) the pores that may exist within the initial ceramic particles (typically <100 nm in size). Such porous ceramic structures are developed to be applied in the fields of fluid transfer/mixing, catalysis or filtration. This paper reports the first results obtained by applying this process to α-alumina granules obtained by spray drying. Conventional sintering, with or without load, and spark plasma sintering are used. It is shown that minimum load must be applied on the granules packing during sintering in order to achieve a satisfactory “bridging” at the contact points: the lower the maximum sintering temperature, the higher the pressure to apply. This makes it not possible to obtain specimens with high bimodal porosity level and good mechanical resistance by conventional sintering. On the opposite, by using SPS it is possible to lower the temperature and the load applied during sintering while keeping sufficient mechanical performances. A porous ceramic exhibiting a bimodal porosity level of 52 vol% and a compressive strength of 30 MPa has been obtained.