Nucleation and Growth of Mullite Whiskers from Lanthanum‐Doped Aluminosilicate Melts

Abstract

Nucleation and growth of mullite whiskers in the La2O3−Al2O3−SiO2 system were investigated in the 1500°–1700°C temperature range. A differentiaC Bhermal analysis (DTA) showed that the mullitization temperature decreases from 1350°C to 1240°C as a result of lanthania doping. In the temperature range of 1250°–1500°C, most of the mullite grains have an Al2O3/SiO2= 1.5 composition throughout the ceramic body; however, from 1400°C upward, the number of anisotropic grains with the Al2O3/SiO2= 1.3 composition begins to increase. The concentration of alumina in the composition of the grain‐boundary phase decreases as firing temperatures increase. At temperatures > 1500°C, alumina grains and whiskers grow on the internal and external surfaces of the ceramic body with the characteristic Al2O3/SiO2= 1.3 composition. Removal of the mullite whisker layer by acid attack revealed an alumina‐rich, rosace‐like patterned microstructure correlated with the process of whisker nucleation and growth. In the early stages, whisker growth rates were found to be near 60 μm/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface.