Effect of β-Si3N4 seeds on microstructure and properties of porous Si3N4 ceramics prepared by gelcasting using DMAA system

Porous Si3N4 ceramics fabricated by gelcasting with DMAA demonstrated excellent mechanical and dielectric properties after introducing beta-Si3N4 seeds into ceramic matrix. The microstructure and properties of slurries, green and sintered bodies were highly correlated with beta-Si3N4 seed size, content and morphology. The slurry viscosity reached the maximum of 0.87 Pa s when introducing 10 wt% elongated seeds, but overall, exhibiting good fluidity. The green body strength dropped remarkably after adding small equiaxial seeds, but improved within 6 wt% elongated seeds and afterwards declined to the minimum of 25.99 MPa, which was quite ideal for processing. The elongated seeds impeded liquid phase flow and particle rearrangement more pronouncedly than equiaxial ones during sintering, thereby causing the highest porosity of 41.77% and the largest pore size of 1.04 pm when employing 10 wt% elongated seeds. The elongated seeds were more helpful in developing the strong self-reinforced bimodal microstructure due to the original hexagonal prism morphology and achieved the highest bending strength of 378.50 MPa and fracture toughness of 8.54 MPa m(1/2) at 2 wt% and 6 wt% elongated seeds, respectively. The dielectric constant and loss tangent depended strongly on the porosity and separately varied in 4.70-4.82 and 0.041-0.047 ranges in 8.2-12.4 GHz when using 6 wt% elongated seeds, demonstrating superior microwave-penetrating properties.