Microstructure, mechanical and dielectric properties of highly porous silicon nitride ceramics produced by a new water-based freeze casting

Novel graded porous Si ₃N ₄ ceramics with different solid content have been fabricated by a new water-based freeze-casting method. A 25 μm-thick surface dense layer on the porous layer was successfully formed without any noticeable defects. A great number of fibrous grains protruding from the internal walls of the macroscopic aligned channels were observed in the porous Si ₃N ₄ samples with 20-25 vol.% solid content, but little fibrous grains were formed in the sample with 15 vol.% solid content at the similar processing route. As the solid content increasing from 15 to 25 vol.%, the porosity decreased from 76.8% to 66.4%, while the flexural strength and dielectric constants at 10 GHz increased from 14.3 MPa to 33.5 MPa and 1.78 to 2.64, respectively. The dense/porous Si ₃N ₄ ceramic is feasible in broadband radomes application at high temperature due to ultra-low dielectric constants and moderate strength.