Preparation And Properties Of Mullite Fibers/whiskers Frameworks With Hierarchical Porous Structures

With the rapid development of aerospace technology and the strengthening consciousness of energy conservation, the performances of thermal insulation materials have also made more requirements. Ceramic fiber insulation rigid tiles have high heatresistance, lightweight, high porosity, low thermal conductivity and some other requisite characteristics. In this paper, mullite fibers/whiskers frameworks were prepared through gelcasting and freeze-drying process. Mullite whiskers grew on the surface of mullite fibers via vapor deposition reaction. The structure and performance of mullite fibers/whiskers were controlled by varying the process parameters of gelcasting, freeze-drying and vapor deposition.First of all, agarose gel were used as frameworks and templates in the green body. The pore size and density of the materials could be effectively controlled by changing the content of agarose and solid. With the agarose content of 0.7² wt. % and the solid content of 5²0% against deionized water, the density of the materials varied between 0.118⁰.308 g/cm3 and the thermal conductivity were controlled between 0.0425⁰.1192 W/m·k. In addition, shrinkage of green body could be reduced by freeze-drying, which could lower the density and thermal conductivity. The IceTemplating technology could form different structures and sizes of pores and then change the microstructures of the porous materials. Mullite whiskers could uniformly deposit onto the mullite fibers by gelling at room temperature, freezing at-20 °C and freeze-drying.The chemical equilibrium of vapor deposition reaction could be controlled by changing heating temperatures, heating rates, mass ratio between raw material powders and mullite fibers, aluminum source, which could influence the structure and performance of mullite fibers/whiskers frameworks. The conclusions were as follows: Reducing the loss of aluminum source and silicon source by using a rapid heating rate of 5 °C/min was benefit to generate topaz and mullite whiskers. Mullite fibers/whiskers frameworks could be successfully synthesized when the heating temperature was between 1100¹400 °C. However, some of the mullite whiskers would turn into aluminum oxide crystal plate with heating temperature over 1300 °C. Increasing the mass ratio between raw material powders and mullite fibers could boost the growth of mullite whiskers and lower the density and thermal conductivity. When the mass ratio between raw material powders and mullite fibers was 10:1, the volume density and thermal conductivity of the material were 0.223 g/cm3 and 0.0619 W/m·k, respectively. Replacing part of AlF3 with Al₂O₃ as aluminum sources does not good to the vapor deposition reaction and mullite whiskers would deposit onto the surface of mullite fibers and unreacted Al₂O₃.