Formation mechanism and microstructure development in acicular mullite ceramics fabricated by controlled decomposition of fluorotopaz

The Dow Chemical Company has developed a ceramic wall flow filter for use as a diesel particulate filter (DPF). It is composed of mullite grown from the low temperature (1000–1200 °C) decomposition of fluorotopaz in the presence of silicon tetrafluoride. Mullite grains are acicular (aspect ratio ∼20), and their size can be controlled by raw material and processing variables (diameters chosen from 3 to 50 μm). The acicular mullite grains interlock forming an efficient filter with low back pressure. Heating precursors in air or introducing SiF4 at temperature does not lead to a similar mullite microstructure. Characterization of samples quenched during the transformation from fluorotopaz to mullite indicates that growth of mullite was facilitated by a melt. The mullite formed is alumina-poor and chemically zoned from a molecular ratio Al2O3/SiO2 = 1.25 in the core to Al2O3/SiO2 = 1.7 in the rim. This mullite composition (5:4–7:4) appears to be stable across a broad range of temperatures.