The Al <sub> 2 </ Sub> O <sub> 3 </ Sub> Aerogels Dried At Ambient Pressure Preparation Methods

In this paper, the white monolithic alumina precursor-aluminum hydroxide aerogels with low density were synthesized by sol-gel technology and ambient drying method while AlCl₃.6H₂O was introduced as precursor and the mixed solution of water and ethanol were chosen as reaction medium and propylene oxide was added as gelation promoter and formamide as drying control chemical additive (DCCA). The TEOS/ethanol solution was used to immerse the wet gel, and then the ethanol solution was used to exchange liquid fluids. The alumina aerogels were achieved after calcinating at 500℃.The effects of parameters such as concentratiob of propylene oxide, ethanol, water and formamide on surface area were studied. The optimized reaction ratio was determined by orthogonal method as n (Al): n (propylene oxide): n (ethanol): n (water): n (formamide) =1: 8: 8: 20: 0.8. The density of the aerogel product synthesized by the optimized condition was 138kg/m³, the surface area was 560m²/g, and the pore volume was 1.982cm³/g. The product had a particular three-dimensional network microstructure which was formed by lots of strip cross linked grains with the length of 10-100nm and the width of about 2-5nm.The alumina aerogels product with the surface area of 423m²/g and the pore volume of 1.638cm³/g was achieved by calcining precursor aerogels at 500℃. The main pore width range of the product was about 20nm and three-dimensional framework was formed by many one-dimensional crossed strip nano-lines with the width of about 25nm.The use of TEOS/ethanol solution to immerse aerogels resulted in the existing of SiO₂ in the aerogel, which restrained the agglomeration and phase-transformation of Al₂O₃ crystal particles. One side, TEOS reacted with the hydroxyl on the surface of Al₂O₃. On the other hand, the surface hydroxyl was replaced by Si-OH, which was hard to remove; and then formed Si-O-Al or Si-O-Si which was inserted firmly on the surface ofγ-Al₂O₃ and restrained the agglomeration of grains. Based on that process, the aerogels had possessed agglomeration-resistant and phase transformation-resistant capabilities under high temperature. The aerogels also had a high surface area (122m²/g) and a high pore volume (0.564cm³/g) even after calcinations at 1000℃.The preparation method was simplified by adding different contents of silicon directly during the process of preparing the alumina sol. The results revealed that the product with high performance was achieved by adding 10% content of silicon. The alumina aerogel product with the surface area of 289m /g and the pore volume of 1.127cm /g was achieved by calcinations at 500℃. Compared to the products prepared by TEOS/ethanol solution immersion method, the ones prepared by the simplified had poor performances such as surface area and pore volume. The optimized procedure remains to be explored further.