Preparation of alumina precursor sols with a high solid content for alumina fibers

Precursor sol with a high solid content is significant for alumina fiber synthesis. In the present work, alumina precursor sols used for preparing alumina fibers were synthesized by dissolving aluminum powder (AP) into a mixture of formic acid, acetic acid and deionized water. The effects of preparation temperatures on solubility of AP, structure of the synthesized sols and gels were investigated. The reaction mechanism of hydrolysis and polymerization was also studied. Based on the reaction mechanism, alumina precursor sol with a high solid content was acquired through the adjustment of temperature. The results revealed that all the sols synthesized at the temperature from 85 °C to 115 °C consisted of polymers, linear oligomers and monomeric species. Colloidal particles presented spherical shape with a uniform size. However, AP cannot dissolve entirely at 85 °C and white precipitates (HO)Al(CH3COO)2 was produced at 105 °C and 115 °C. AP had the highest dissolution degree at 95 °C. A colorless and transparent alumina sol with the highest solid content was acquired at this temperature. The structure of concentrated gels was also discussed. It was revealed that network polymers increased when using sols synthesized at temperature above 105 °C, which were negative for spinning. Continuous green fibers were obtained successfully using the alumina sol prepared at 95 °C.