Preparation of mullite whiskers from kaolin via the addition of tribasic calcium phosphate

Mullite whiskers are potential candidates for improving the mechanical and thermal properties of ceramic, glass, and composite material. In this work, well-developed whisker-shaped mullite has been produced by adding tribasic calcium phosphate (Ca3(PO4)2) into kaolin before calcining in air. In the raw kaolin, rod-like mullite crystal of ~0.5-1.0 μm length and ~0.05-0.2 μm diameter formed at 1350°C, and mullite whiskers were not be observed; however, by doping 6 wt% Ca3(PO4)2 into kaolin, mullite whiskers of ~5.5 μm length and ~0.05-0.10 μm diameter grew at 1250°C. The formation of high aspect ratio of mullite whiskers can probably be explained by the synergistic effect of P2O5 and CaO, resulting in the formation of liquid phase with a relatively higher content of Si and a lower viscosity at low temperature of 1250°C, which facilitates the growth of mullite displaying acicular morphology. The results are of interest in producing high aspect ratio of mullite whiskers from kaolin at reduced calcination temperatures.