Fabrication and catalytic growth mechanism of mullite ceramic whiskers using molybdenum oxide as catalyst

Abstract

Mullite ceramic whiskers are successfully synthesized by a facile solid-state reaction process, using kaolin and Al(OH)3 as raw materials and MoO3 as catalyst of mullite whisker growth. The effect of MoO3 catalyst on mullitization reaction and mullite morphology is investigated by XRD analysis and SEM observation. XRD results indicate that the addition of MoO3 alters the mullitization reaction path, which leads to a decrease by 200°C in the initial temperature of mullitization reaction, accelerates the reaction process and raises the reaction extent. SEM images demonstrate that MoO3 encourages growth of mullite whiskers. The average aspect ratio of mullite whiskers increases from 4.7 to 18.4 with the increase of firing temperature from 1100 to 1500°C. The combined results from TG-DSC and XRD suggest that the formation of Al2(MoO4)3 delays the sublimation temperature of MoO3 from 600 to 800°C. Through analyzing the experimental results, the MoO3-assisted catalytic growth mechanism of mullite whiskers is proposed based on the “dissolution-precipitation” process. The MoO3 decreases the viscosity of Si-rich liquid and thus promote alumina solubility, resulting the mullite nucleation and growth. The assumption that MoO3 containing MoO6 octahedral chains acts as the role of template agent inducing mullite whisker growth is proposed.