Fabrication and Properties of Porous RBSN

Porous silicon nitride was prepared by extrusion of silicon followed by nitridation at 1723 K. PMMA spheres with 20 µm in diameter were employed as the pore-forming precursors. b-silicon nitride whiskers were added to the dough for extrusion and their effect on the properties of porous RBSN were examined. The nitridation rate that was obtained from the weight change of the sample due to the nitridation process was between 75% and 80%. However, XRD patterns of the samples after nitridation had no Si peak. That means the actual nitridation rate of the sample was higher than that obtained from the weight change. Porosity of the sample was between 45% and 55%. The XRD patterns from the surfaces of the samples with the silicon nitride whiskers parallel and perpendicular to the extrusion direction showed a slight anisotropy. The pore size distributions of the samples showed a highly populated pores smaller than 3 micrometer, especially for the samples with the whiskers. The room temperature flexural strengths of the samples were between 25 MPa and 35 MPa, the sample with 5 wt% whiskers showing the highest value. The microstructures of the samples contained pores with about 100 micrometer in diameter as well as fine pores with a few micrometer in diameter. Closer observation of the fracture surface of the samples revealed that fine whiskers were inside the pores. A small honeycomb was fabricated by reaction bonding of silicon.