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Rigid SiC-Si 3 N 4 foams with hierarchical porosity were prepared through protein-based gel-casting followed by radiant sintering in a modified spark plasma sintering (SPS) set-up. The porous bodies sintered at 1500-1700 C for only 10 minutes achieved a compressive strength of 15-21 MPa while keeping a porosity of 60-70 vol%. Gradient porous structures, with pore sizes ranging between 1 to 100 mm, were intersected by the growth of hybrid SiC and Si 3 N 4 nanowires inside the pores resulting in a nasal cavity-like appearance. Gas permeability at room temperature (25 C) and 600 C was evaluated.Darcian permeabilities and non-Darcian permeabilities of all the prepared foams at room temperature fell within (0.354-1.55) Â 10 À12 m 2 and (1.60-6.33) Â 10 À8 m, respectively. Measurement of the Darcian and non-Darcian permeabilities at 600 C were much higher, at 1.71 Â 10 À11 m 2 and 2.68 Â 10 À7 m, respectively. The microstructure, stability, gas flow properties and the green synthesis route reveal the potential of these ceramic foams to be used as industrial PM filters for airborne pollutions.

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Silicon nitride foams were prepared by direct foaming and subsequent rapid sintering at 1600 °C. The intense thermal radiation generated under the pressureless spark plasma sintering condition facilitated necking of Si3N4 grains. The prepared foams possessed a porosity of ∼80 vol% and a compressive strength of ∼10 MPa, which required only ∼30 min for the entire sintering processes. Rapid growth of one-dimensional SiC nanowires from the cell walls was also observed. Thermodynamic calculations indicated that the vapor–liquid–solid model is applicable to the formation of SiC nanowires under vacuum.

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