Effects of presintering on properties of reaction-bonded porous silicon nitride ceramics

This paper presents the dielectric and mechanical properties of porous silicon nitride ceramics. Silicon powders were mixed with pore-forming agent of 30wt% and agglomerant (PVA) of 1 wt% and shaped into discs of Φ 70 mm × 5 mm by mould pressing under pressure of 40 Mpa. After volatilization of pore-forming agent at 200°C, the green bodies were subjected to presinter at different temperature for different holding time and then sintered at a maximum nitridation temperature of 1380°C for 28 h in a home-made furnace in a flowing gas mixture of 95% N₂/5% H₂ under a positive pressure of 0.1 MPa. The heating-up rate is 3°C/min. The experimental results demonstrate that the dielectric constant ε′, the dielectric loss tanδ increase diminutively but the bending strength of the samples increase evidently with increase of temperature and holding time of presintering. The dielectric constant ε′ of porous silicon nitride ceramics with 5wt% Y₂O₃ and 5wt% Al₂O₃ presintered in vacuum atmosphere at 1200°C for different holding time before sintered in a home-made furnace with same sintering technology reduce evidently with increase of holding time of presintering.