Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide-silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600°C for 1.0 -8.0 h resulted in a high surface concentration of mullite whiskers. Their c-axes were aligned preferentially along the pellet surface; the maximum length was 50 m, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm 3 in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30 -1.45 (an average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio.

Nucleation and growth of mullite whiskers in the La 2 3 _AlA 2 0 3 _SiO 2 system were investigated in the 1500°-1700°C temperature range. A differential thermal 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 AI 2 OJSiO 2 = 1.5 composition throughout the ceramic body; however, from 1400°C upward, the number of anisotropic grains with the AI 2 ,O/SiO 2 = 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 AI203/SiO 2 = 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 jim/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface.

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