Green Synthesis of Nanocrystalline Alumina and Influence of Barium and Titanium Oxides on Its Phase Transformation

Nanometer Al₂O₃ powders were prepared in direct precipitation method with sodium carbonate and aluminum chloride as raw materials and subsquent calcination at 500~1,200℃.Then TiO₂ and BaO were doped to control the phase transformation of nanometer alumina.X-ray diffractometer,differential scanning calorimeter and transmission electron microscopy were employed to analyze the phase transformation process of pure and doped nanometer alumina.It was indicated that boehmite obtained from the direct precipitation method was transformed to nanometer active alumina(γ-Al₂O₃) with the diameter of 10 nm.The pure and doped alumina were all still nanoparticles after calcined at 1,200 ℃ for 2 hours.Their grain sizes were 40~50 nm.The doping of BaO hindered the phase transformation of alumina since 800 ℃ and thereafter.As a result,alumina samples doped with BaO didn't transformed completely to α-Al₂O₃ even calcined at 1,200 ℃ for 2h.In contrast,the doping of TiO₂ facilitated significantly the phase transformation of alumina since 900 ℃ and thereafter.Consequently,alumina samples doped with TiO₂ were mainly composed of α-Al₂O₃ at 1,000 ℃ and transformed completely to α-Al₂O₃ at 1,100 ℃.