Effect of α/β phase transformation on dielectric properties of porous silicon nitride ceramics

Low-density porous silicon nitride ceramics, which contained 0.8 mm spherical pores in diameter and had porosity of about 55%, were prepared through adding stearic acid by reactive sintering. The influence of α/β phase transformation on the dielectric properties of porous silicon nitride ceramics was investigated. Pleomorphic silicon nitride ceramics with different relative content (in mass, the same below) of β phase were obtained by setting the nitriding temperature and time. The results show that the α/β transformation occurs when the nitriding temperature is higher than 1400°C. The relative content of the β phase increases and the morphologies changes from an equiaxed shape to prismatic one with the increase of nitriding temperature and time. The dielectric constant ε′ and the dielectric loss tanδ of the samples increase because of the α/β transformation, and the change of the dielectric loss is more obvious. The increase of concentration of point defects due to the α/β transformation leads to the significant increase of the dielectric loss.