A simple and efficient hydratable alumina gel‐casting method for the fabrication of high‐porosity mullite ceramics

Abstract In this study, we have developed a method for the preparation of porous alumina‐containing ceramics by inorganic gel‐casting of hydratable alumina system. The gelation behavior of hydratable alumina in water and the high‐temperature phase evolution of alumina gels were clarified. In addition, extremely high‐porosity mullite ceramics were prepared using the hydratable alumina gel‐casting method combined with carbon fibers as the pore‐forming agent method. The as‐prepared mullite ceramics have a pore size of less than 10 μm, controllable high porosity of 90.6%–78.1%, a lightweight of 0.30–0.70 g cm −3 , high compression strength of 1.41 ± 0.21–16.30 ± 0.20 MPa, low dielectric constants at 30 GHz of 1.34–1.79, the dielectric loss tangent of less than 3 × 10 −3 , and low thermal conductivities of 0.083–0.305 W/(m K). Lightweight, high strength, excellent dielectric properties, and outstanding thermal insulation indicate the potential of porous mullite ceramics for use in radar radomes or windows.