Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

Abstract The porous SiC–Si3N4 composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si3N4 powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s−1) and also suppressed the generation of NH3 and N2 from Si3N4 hydrolysis and reaction between Si3N4 and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si3N4 composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N2 and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si2N2O fibers formed in situ by the reaction of Si3N4 and amorphous SiO2 effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m1/2. Compared with monolithic Si3N4 ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO2 and Si3N4. The porous SiC–Si3N4 composite ceramics prepared by the unoxidized Si3N4 powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.