Microstructure and properties of highly porous mullite materials fabricated by direct foaming-gelcasting

Highly porous ceramics are of interest for various industrial applications, including filtration, supports for catalysts and biomedical applications, and insulating materials. In this study, porous mullite materials were fabricated utilizing the direct foaming-gelcasting approach with industrial-grade mullite powder and different sintering additives as the main starting materials. The phase compositions, pore microstructures, mechanical, thermal and dielectric properties were investigated in order to elucidate the relationships between the structure and performance of the porous materials. The results indicated that the porous mullite materials contained a hierarchical pore structure including interconnected and approximately spherical cells (40–230 μm) with no preferred orientation, along with small pores (5–40 μm) within the skeleton structure. The developed porous ceramics showed thin struts, large cells and internal windows, and a higher degree of total porosity. The porous mullite materials prepared with the addition of sintering aids (1 wt% SiO2 and 2 wt% TiO2) exhibited high porosity (~ 83%), good compressive strength (4.6–5.3 MPa) and low dielectric constant (1.5–2.0). Good thermal insulation and low dielectric loss are also available. These porous mullite materials are particularly applied for broadband microwave-transparent radome structures.