Effect of divalent cations on the synthesis of citrate-gel-derived lanthanum hexaluminate powders and films

Low-temperature synthesis of hexaluminate phases for fiber-matrix interphases in ceramic-matrix composites is necessary to minimize processing temperatures to prevent fiber-strength degradation. Citrate-gel-derived lanthanum hexaluminate was synthesized using divalent transition-metal cations to stabilize the magnetoplumbite structure. Pure, undoped LaAl11O18 was obtained in 1 h only at temperatures >1500 degrees C after the formation and subsequent consumption of the intermediate perovskite, LaAlO3, which first appeared at similar to 1150 degrees C. Powders of LaMAl11O19, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn, were prepared at much lower temperatures. Highly crystalline, phase-pure powders of LaMnAl11O19 and LaCuAl11O19 were obtained at 1000 degrees C in 1 h directly from the amorphous powder without the formation of the intermediate perovskite. All other compositions could be obtained at 1100 degrees C in 1 h except for the nominal LaNiAl11O19 which formed primarily LaAlO3, NiAl2O4, and Al2O3. Powders containing dopants all had similar grain sizes and morphologies at 1200 and 1500 degrees C; the grain size of powders with dopants was significantly greater than that of pure LaAl11O18 powder. The introduction of a second charge-compensating quadrivalent dopant for excess divalent cation did not greatly influence synthesis or grain growth below the eutectic temperature but did seem to enhance the [0001] texture of films on single-crystal yttrium-aluminum garnet substrates compared with singly doped films.