The role of aluminium as an additive element in the synthesis of porous 4H-silicon carbide

Addition of 5 at.% aluminium to silicon and carbon in the process of forming SiC monoliths for use as diesel particle filters and catalyst supports, creates mechanically stable, well connected, and highly porous 4H-SiC structures. These monoliths have 65 % accessible porosity with pore diameters of 17 − 20 μm. Mixtures of silicon, graphite, aluminium and water, are extruded into honeycomb structures and heated under nitrogen and argon. The 2H-AlN crystalline assembly formed during initial heating under nitrogen at 850 °C acts as a template for the subsequent reaction between silicon and graphite under argon to yield 3C-SiC. During a final high temperature step under argon at 1950 °C, Al-vapour/liquid is crucial for the transformation of 3C-SiC to 4H-SiC. This final step also alters the SiC crystal morphology significantly and produces large by-product crystals of Al4C3·mSiC·nAlN. The polytypic conversion and recrystallization mechanism were found, in this case, to be independent phenomena.