Porous silicon nitride ceramics with difference volume fractions of porosity from 34.1% to 59.2% were produced by adding different amount of the pore-forming agent into initial silicon nitride powder. The microwave dielectric property of these ceramics at a frequency of 9.36 GHz was studied. The crystalline phases of the samples were determined by X-ray diffraction analysis. The influence of porosity on the dielectric properties was evaluated. The results show thatα-Si3N4 crystalline phase exists in all the samples while the main crystalline phase of the samples isβ-Si3N4, indicating that theα/βtransformation happens during the preparation of samples and the transformation is incomplete. There is a dense matrix containing large pores and cavities with needle-shaped and flakyβ-Si3N4 grains distributing. The dielectric constant of the ceramics reduces with the increase of porosity.
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