Pressure drop across filter materials is critical for the effective performance of various kinds of filtering system. To evaluate the feasibility of lowering the pressure drop of porous sintered reaction-bonded silicon nitrides, permeability tests were conducted for specimens with different porosity and pore structure. The results showed that pore former size is a minor factor, while permeability (k) had a strong dependency on the amount of pore former. Prolonged sintering time (2 h vs 10 h at 17001C) increased the permeability by increasing the effective pore diameter, which compensates for the observed decrease in total porosity.

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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.

Prediction of porosity level for the Si3N4 filter substrate manufactured by reaction bonding of Si compact and subsequent post-sintering was attempted and its validity was verified by comparison between calculation and measurement. Both are well matched in the specimens of asnitrided and low temperature post-sintering, while few percents higher measurement in high temperature post-sintered specimens resulted from weight loss during the process.