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

Abstract

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.

Mullite phase formation in oxide mixtures in the presence of Y2O3, La2O3 and CeO2

“…Regiani et al observed a mullitization temperature of ∼1240 • C in a 3 mol%-La 2 O 3 doped Al 2 O 3 /SiO 2 mixture and claimed that the temperature was very close to the eutectic temperature in the La 2 O 3 -Al 2 O 3 -SiO 2 ternary system [20]. The presence of glass phase, which was responsible for the lowering of the mullite formation temperature, was directly evidenced in the SEM images of the samples' surfaces.…”

Mullite crystallization kinetics of lanthanum doped sol–gel derived precursors

“…Mullite formation in the presence of lanthanum was recently investigated, [15][16][17] but to the best of our knowledge, crystallization kinetics of lanthanum doped pre-mullite gels has not been reported.…”

Tailoring the microstructure of mechanoactivated Al2O3 and SiO2 mixtures with TiO2 addition

“…An alternative way to produce interlocking microstructured mullite ceramics is via in situ processing technique. For a conventional solid-state reaction process, in situ anisotropic grain growth usually takes place at very high temperatures (>1600 • C) [5][6][7][8]. This is because mullite ceramics prepared in this way experienced densification before mullite phase formation.…”

“…This suppressed anisotropic growth of mullite grains. In this case, ultra high temperature is essential for the formation of a eutectic liquid to maintain the anisotropic grain growth of mullite in a fully densified sample [5][6][7][8].…”