Enhancements in the properties of porous alumina materials by utilizing multi-sol-impregnated walnut shell

To further improve the properties of the porous alumina materials, which were prepared by using walnut shell as pore former with a multi-layer hierarchical structure, the multiple sol (zirconia sol, alumina sol, and silica sol)–impregnated walnut shell powder was utilized. The hygroscopicity and microstructure of walnut shell, and physical performances, thermal conductivity, and microstructure of porous alumina materials were characterized. The residual structure of walnut shells impregnated with multiple sols was left in the pores, which optimized the pore structure of materials. High-performance porous alumina materials were produced by successively impregnating with silica, alumina, and zirconia sols, which possesses low thermal conductivity of 0.166 W/(m·K) at 200 °C and ultra-high compressive strength of 84.4 MPa. The above excellent properties result in a great advantage for the existing materials. In addition, the feasibility of optimizing the structure and properties of the porous alumina materials, by using the residual structure of pore-forming agent, was further verified.