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

Abstract

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 LaAl11O18was obtained in 1 h only at temperatures >1500 °C after the formation and subsequent consumption of the intermediate perovskite, LaAlO3, which first appeared at ∼1150 °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 LaMnAl11O19and LaCuAl11O19were obtained at 1000 °C in 1 h directly from the amorphous powder without the formation of the intermediate perovskite. All other compositions could be obtained at 1100 °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 °C; the grain size of powders with dopants was significantly greater than that of pure LaAl11O18powder. 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.