Study On Preparation And Performance Of An Alumina-Fiber-Based Expandable Insulation Material

The passive thermal protection system has been used wildly in the dynamic equipment of hypersonic vehicles at present. The typical passive thermal protection system is composed of metals on the top, insulation materials in the middle and structure materials on the bottom. When the dynamic equipment working under high temperature, because of the tremendous differences of thermal expansion coefficient of each components, there are a large separation formed between the materials of top and middle. In this paper, a new kind of aluminafiber-based expandable insulation material was designed using expandable graphite, alumina fiber and aluminum phosphate adhesives, and fabricated through wet mixing, vacuum suction filter, pressure drying. And the material was systematically studied.The main research contents in this paper are as follows:A new kind of alumina-fiber-based expandable insulation material was designed and fabricated, which had 100%-300% of volume expansion ratio under temperatures from 400 to 1000℃, well heat insulation performance and mechanical property after expansion. Under high temperature, expandable graphite will expand in an instant, result in the volume increasing of material, then aluminum phosphate adhesives will begin condensation and crystallization, forming rigid connections in the intersection of alumina fiber. The condensation and crystallization process is much slower than the expansion of the expandable graphite process, which makes the strength increasing of material after expansion.The influence of material composition on the material properties had been systematically studied. As particle size decreasing of expandable graphite, the expansion ratio of material decreased, the compression strength increased, and the coefficient of thermal conductivity first increased and then decreased. The properties of material with particle size 0.053-0.075 mm of expandable graphite were the better one. At 800℃, the expansion ratio was 227.4%, while the compression strength was 0.853 MPa. The heat conductivity was 0.099 W/m?k at 900℃. As expandable graphite content increasing, the expansion ratio of material increased slightly, which increment was only 10%-15%, the compression strength decreased, the heat conductivity had little change, The properties of material with the mass ratio of expandable graphite and alumina fiber was 1:16.2 were the better one. The compression strength was 0.808 MPa at 800℃ and the heat conductivity were maintained at around 0.098 W/m?k at 900℃. As aluminum phosphate content increasing, the expansion ratio of material decreased, the compression strength increased, and the heat conductivity decreased slightly. The properties of material the mass ratio of aluminum phosphate and alumina fibers was 1:2.2 were the better one. The expansion ratio was 228.5% at 800℃, the compression strength was 0.736 MPa at 600℃, the heat conductivity was 0.100 W/m?k at 900℃. The influence of application environment on the material properties had been studied. When storaging in high humidity conditions, the moisture absorption of material happened. The biggest moisture absorption rate was 32%. After moisture absorption, the expansion ratio declined slightly, the compression strength decreased by 50% and the heat conductivity had little change. The influence of two different heating-up modes on the material properties had been studied. At slowly heating-up mode, the expansion ratio almost maintained at 115%, and the compression strength increased visibly comparing with fast heating-up. At one-side heating-up mode, the expansion ratio was lower than the all-side heating-up mode, which respectively were 164.9 % and 226.9% at 800℃, and the compression strength increased slightly because of the extension of heat treatment time.