Low-temperature formation of α-alumina from various polyhydroxoaluminum–hydroxy acid composite gels

Low-temperature α-alumina formation was attempted using various polyhydroxoaluminum (PHA)–hydroxy acid composite gels, which were prepared from PHA solutions containing different amounts of hydroxy acids, such as lactic acid, glycolic acid, malic acid, citric acid or mandelic acid. The composite gels began to transform into α-alumina when heated at lower temperatures of around 500 °C and the α-alumina fraction of the heat-treated products increased with increasing temperature. The α-alumina fraction was also dependent on both the type and amount of hydroxy acid additive. Among the composite gels studied, significant low-temperature α-alumina formation was observed for the PHA–mandelic acid, PHA–citric acid and PHA–lactic acid series. Low-temperature α-alumina formation was further promoted by employing a two-step heat-treatment method. The interaction between the functional groups of PHA and hydroxy acid and the seeding effect appear to play important roles in the course of the gelation and calcination processes for low-temperature α-alumina formation.