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

The porous SiC–Si 3 N 4 composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si 3 N 4 powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s −1 ) and also suppressed the generation of NH 3 and N 2 from Si 3 N 4 hydrolysis and reaction between Si 3 N 4 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–Si 3 N 4 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, N 2 and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si 2 N 2 O fibers formed in situ by the reaction of Si 3 N 4 and amorphous SiO 2 effectively enhanced the mechanical properties , achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m 1/2 . Compared with monolithic Si 3 N 4 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 SiO 2 and Si 3 N 4 . The porous SiC–Si 3 N 4 composite ceramics prepared by the unoxidized Si 3 N 4 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.