Effect of divalent cations on the synthesis of citrate-gel-derived lanthanum hexaluminate powders and films
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Effect of divalent cations on the synthesis of citrate-gel-derived lanthanum hexaluminate powders and films Michael K. Cinibulka) Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio 45433-7817 (Received 31 December 1998; accepted 10 June 1999)
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 °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 LaMnAl11O19 and LaCuAl11O19 were 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 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.
I. INTRODUCTION
An oxide-based alternative to carbon and boron nitride, which has the magnetoplumbite structure, is being considered as a fiber coating in ceramic–matrix composites. Initial work in this area focused on potassium– magnesium hexaluminate, 1 having the -alumina structure, and calcium hexaluminate,2–7 having the magnetoplumbite structure. Other work on evaluating related hexaluminates as a fiber–matrix interphase has been reported as well.8–11 The -alumina and magnetoplumbite structures are similar; they both consist of layered spinel blocks separated by mirror planes containing the stabilizing cation (K+ or Ca2+, respectively).12,13 Recent work6 on calcium hexaluminate (the mineral hibonite), CaAl12O19, has shown that the reaction temperature to form phase-pure hibonite can be lowered considerably by
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UES, Inc., Dayton, OH 45432. J. Mater. Res., Vol. 14, No. 9, Sep 1999
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the incorporation of small amounts of transition-metal cations into the spinel block. Also, the improvement in the degree of texture, which is achieved by abnormal grain growth and driven by surface/interface energy anisotropy, of thin films of hibonite doped with certa
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