Preparation And Performance Study Of Al₂O₃ Aerogels And Composite Materials

Aerogels are lightweight solid materials which consist of nanoparticles and continuous random dimensional network structure with low density, high porosity and high specific surface area. Because of unique structure, aerogels show very low thermal conductivity and good thermal stability, which are ideal heat preservation and heat insulation material. Therefore, aerogel is of important theoretical research significance and broad application prospects.Al₂O₃ aerogels, Al₂O₃-SiO₂ aerogel composites were synthe sized by sol- gel method via using AlC l3·6H2O as alumina precursor, alcohol（EtOH）and water（H2O） as solvent, isopropyl alcohol（IPA） as the solvent exchange agent and trimethylchlorosilane（TMCS） as the surface modification agent. The test and analysis have been carried on the structure and properties.First of all, various factors which may influence the Al₂O₃ aerogels were analyzed in this paper, with obtaining the optimum raw material proportioning and the optimal process to prepare Al₂O₃ aerogels. The results have shown the best mole ratio were Aluminium chloride for six water（AlC l3·6H2O）: water（H2O）: ethanol（EtOH）: epoxy propane（PO） = 1: 6.17:7.28:5. The best gel temperature was at 25℃. The effect of molar ratio of water, ethanol, six aluminium chloride, epoxy propane on the gel time and gel structure were mainly investigated.Secondly, the phase, microstructure, thermal stability and pore structure of Al₂O₃ aerogels were analyzed by XRD, SEM, TG-DSC, and nitrogen adsorption stripping test technology, compared with SiO₂ aerogels with atmospheric pressure drying method. The conclusions were as follows. The XRD results indicated that primary groups of alumina aerogels prepared by the atmospheric drying method were amorphous pseudo boehmite, after heat treatment at 500℃ and 800℃ still possesed amorphous, and after heat treatment at 1000℃, began to appear weak γ-Al₂O₃ crystal phase, θ-Al₂O₃ crystal phase started to appear after heat treatment at 1100℃, α-Al₂O₃ crystal phase started to appear after heat treatment at 1200℃, while primary groups of SiO₂ aerogels prepared by atmospheric drying were amorphous SiO₂. Under the macroscopic observation, alumina aerogel possessed milk-white and light blue, while silica aerogels possess sky- like blue. The results of SEM exhibited that of Al₂O₃ aerogels prepared show needle- like or flake particle aggregation in the experiment, many pores exits between uniform particles.After heat treatment at 1000℃, there were still flake or needle shaped particles, overall appearance have no change. After heat treatment at 1200℃, mullite crystal phase appeared. After heat treatment at 800℃,silica aerogel composed of globular particles began to gather. This implies Al₂O₃ aerogels have better thermal stability than SiO₂ aerogels.Finally Al₂O₃-SiO₂ aerogel composite materials were prepared by atmospheric pressure drying method. The structure and morphology of the aerogels were investigated by XRD, SEM, TG-DSC, and nitrogen adsorption stripping curve test.Compared with silica aerogel and alumina aerogel, the results showed that Al₂O₃-SiO₂ aerogel composite material prepared by atmospheric pressure drying method was composed of amorphous phase with long striped particles accumulating and crossing each other,with obvious three-dimensional network structure. And there were many pores between particles, specific surface area up to 703 m2/g, pores distributed in 3 to 30 nm, thermal conductivity below 0.073 W/m·K, finally it is concluded that the best molar ratio of Al : Si was in 3:1.At the same time, after heat treatment the mole ratio of Al : Si in 3:1 and 5:1, Al₂O₃-SiO₂ aerogel composites at 1200℃, XRD, FTIR and SEM analysis results show that θ-Al₂O₃ and γ-Al₂O₃ phase started to appear. FTIR showed that the sample of the Al : Si mole ratio in 3:1 after heat treatment at 1200℃exhibited stronger Al-Si absorption peak. The Al : Si mole ratio 3:1 of aerogel composite material has higher thermal stability, at the same time compared with the pure silica aerogels and alumina aerogel, Al₂O₃-SiO₂ aerogel composite material prepared by atmospheric pressure drying method had better thermal stability. After heat treatment at 1200℃,there was still good network structure for Al:Si mole ratio 3:1 of aerogel composite material.