Effect of carbon nanotubes on sintering behavior of alumina prepared by sol–gel method

Alumina (Al2O3)/carbon nanotube (CNT) (99/1 by weight) composite was prepared by mixing CNT dispersion with AlCl3-based gel, followed by high temperature sintering at a temperature up to 1150 °C in argon. Composite alumina precursor showed phase transition order from amorphous to γ-Al2O3 after sintered at 900 °C for 2 h, partially to θ-Al2O3 after sintered at 1000 °C for 2 h, and then partially to α-Al2O3 after sintered at 1150 °C for 2 h. By comparison, control alumina precursor directly transformed from amorphous to α-Al2O3 after sintered at a relatively low temperature of 600 °C for 2 h. Composite alumina showed porous structure with pore diameter ranging from 100 nm to 2 µm, whereas control alumina was relatively pore-free. The elevated alumina-crystal phase transition temperatures and the formation of porous structure were ascribed to the presence of CNTs in alumina precursor. The composite alumina sintered at 900 °C for 2 h containing only γ-Al2O3 had a BET surface area of 138 m2/g, which was significantly higher than that of control alumina sintered at 1150 °C for 2 h containing only α-Al2O3, ~15 m2/g.