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Porous silicon nitride ceramics with various porosities were fabricated by liquid phase sintering of mixtures containing fibrous and equiaxed α- Si3 N4 powder with a various content ratios. The effects of the contents of the fibrous α- Si3 N4 powder (0%-100%) on the microstructure and mechanical properties of porous Si3 N4 ceramics were studied. As the increase of the fibrous α- Si3 N4 powder content, both the density of green bodies and the linear shrinkage decreased, resulting in increased porosity due to the inhibited densification by the fibrous Si3 N4 particle. XRD analysis proved the complete formation of single β- Si3 N4 phase. SEM analysis revealed that the microstructure of the low content of fibrous α- Si3 N4 porous ceramics was almost composed of fine elongated β- Si3 N4 grains with high aspect ratio while numerous coarse elongated β- Si3 N4 grains with low aspect ratio surrounding fine grains were formed as the content of the fibrous α- Si3 N4 powder increased. With the increase in content of the fibrous α- Si3 N4 powder from 0% to 100%, the porosity changed from 47.8% to 56.6%, and the flexural strength decreased from 146 to 62 MPa correspondingly, indicating a flexural adjustment of the porosity and mechanical properties. [ABSTRACT FROM AUTHOR]

Porous β- Si3 N4 ceramics are sintered at 1600°C in N2 and postheat treated at 1500°C under vacuum using Li2 O and Y2 O3 as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li2 O. The addition of Y2 O3 is advantageous for the formation of high aspect ratio β- Si3 N4 grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β- Si3 N4 porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li2 O and 0.33 wt% Y2 O3, and it is suitable for high-temperature applications. [ABSTRACT FROM AUTHOR]

Porous β- Si3 N4 ceramics are sintered at 1600°C in N2 and postheat treated at 1500°C under vacuum using Li2 O and Y2 O3 as the sintering additives. The partial sintering and phase transformation are promoted at low temperature by the addition of Li2 O. The addition of Y2 O3 is advantageous for the formation of high aspect ratio β- Si3 N4 grains. After postheat treatment, a large amount of intergranular glassy phase is removed, and the Li content in the samples is decreased. By this method, the β- Si3 N4 porous ceramic with a porosity of 54.1% and high flexural strength of 110 ± 8.1 MPa can be prepared with a small amount of sintering additives, 0.66 wt% Li2 O and 0.33 wt% Y2 O3, and it is suitable for high-temperature applications. [ABSTRACT FROM AUTHOR]