Low-temperature synthesis of mullite by molten molybdenum trioxide assisted mesoporous aluminum source dissolution

Porous ceramics with more than 99% mullite (3Al2O3·2SiO2) content were fabricated at the low sintering temperature of 800 °C using silicon debris and mesoporous aluminum oxide (Al2O3) obtained from calcining aluminium hydroxide (Al(OH)3) as silicon source and aluminium source, respectively. The study found that adding ammonium molybdate tetrahydrate (AMT) as sintering additive allowed the complete mullitization reaction temperature of the raw materials to be reduced to approach the melting point of molybdenum trioxide (MoO3). Specifically, the silicon debris is continuously oxidized to silicon dioxide (SiO2) in the sintering process, prompting the formation of a silicon-rich liquid phase in the molten MoO3. Meanwhile, a large number of mesopores in the Al2O3 contributed to the dissolution of aluminum source in the silicon-rich liquid phase. Resultantly, synthesizing mullite can be performed at a temperature as low as 800 °C. Additionally, the research showed that the porous mullite ceramics sintered at this temperature had a flexural strength of 22.07 MPa, an open porosity of 52.58%, a linear shrinkage rate of 0.9%, and a bulk density of 1.49 g/cm3. This study not only presents a novel low-cost method for preparing porous mullite ceramics at low temperatures but also provides a simple method for the complete recycling of waste silicon debris.