Effect of presintering on the dielectric and mechanical properties of porous reaction-bonded silicon nitride

This paper presents the influence of presintering on the dielectric and mechanical properties of porous silicon nitride ceramics which were prepared by reaction sintering in flowing nitrogen atmosphere. Si3N4 ceramics containing 0.8 mm spherical pores in diameter and with porosity of about 53% were made by adding 30% in mass pore-forming agent grains prepared by spray prilling. Green bodies were subjected to presintering in vacuum atmosphere at temperature 1250 °C and then sintered. The experimental results demonstrate that the dielectric constant ɛ′ and the dielectric loss tan δ have small increase but the flexure strength of the samples increase evidently with increase in holding time of presintering. When presintered with 5% in mass additives of Y2O3 and Al2O3, ɛ′ reduce evidently, but tan δ increases all the time with increase in holding time of presintering, and different phases and microstructures are formed with change of holding time of presintering.