Effects of particle grading on porous gelcasted and solid-state-sintered SiC ceramics with improved connectivity

Porous gelcasted and solid-state-sintered SiC (S-SiC) ceramics with significantly improved connectivity were successfully prepared through the synergistic pore-forming mechanism of direct foaming and particle packing, using fine (D50 = 0.5 μm) and coarse (D50 = 5.0 μm) graded SiC powders as the main raw materials. As the proportion of coarse SiC powder increased, the viscosities of aqueous SiC-B4C-C slurries sharply decreased, which thus flourished foaming and enlarged bubble-derived-pore diameter in porous S-SiC ceramics, accompanied with the increased amount and size of window. Moreover, abundant particle-packing pores in struts of bubble-derived pores were also formed and even interconnected with each other at 60 wt% of coarse SiC powder. Besides, there was a remarkable increase in porosity from 61.0% to 81.7% and accordingly a significant decline in density from 1.24 g cm−3 to 0.58 g cm−3 as the content of coarse SiC powder increased. The optimized multiple pore microstructures resulted in an amazing nitrogen gas permeability of 1.78×10−11 m2 under the condition of an excellent compressive strength of 10.8 ± 2.7 MPa.