Low- and High-temperature Processes and Mechanisms in the Preparation of Porous Ceramics via Starch Consolidation Casting

Starch consolidation casting (SCC) is a versatile and well established process for shaping ceramic green bodies by casting starch-containing suspensions into impermeable molds. The process is based on the ability of starch to swell and absorb water from aqueous ceramic suspensions after heating to approx. 80°C. The swelling factor is highest for potato starch (3.7) and significantly lower for the other starch types (down to approx. 2). A simple mathematical model is presented that allows the minimum consolidation time to be estimated on the basis of the swelling kinetics of starch granules as measured via laser diffraction. The critical temperature range for starch burnout before sintering the ceramic is 300–500°C, as determined by thermal analysis (thermogravimetry and differential thermal analysis). In this temperature range gaseous products may ignite, leading to strongly exothermic thermal effects. If the bodies are large and the starch content high this may lead to defects in the (still unsintered) ceramic green bodies. Therefore this temperature range has to be carefully controlled in the production of ceramics prepared by SCC. The residual pores (remaining after starch burnout) are too large to be closed during the firing step (insufficient driving force for sintering due to too low surface curvature). Therefore they do not contribute to shrinkage, and shrinkage is determined by the ceramic matrix alone. Porosities higher than 50% (corresponding to the maximum nominal starch content) may be obtained by combining SCC with partial sintering (resulting in porosities up to 70%).