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 mu m. 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 degrees 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 degrees 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 center dot mSiC center dot nAlN. The polytypic conversion and recrystallization mechanism were found, in this case, to be independent phenomena. (C) 2016 Elsevier Ltd. All rights reserved.