Effect of precursors and dopants on the synthesis and grain growth of calcium hexaluminate

A variety of colloidal and polymeric sols, some doped with transition-metal oxides, were investigated as potential precursors to calcium hexaluminate (CaAl12O19) in an attempt to enhance reactivity and lower the formation and texturing temperatures of phase-pyre powders and films. One undoped polymeric sol was found to yield CaAl12O19, free of any intermediate phases, at temperatures as low as 1300 degrees C in 1 h, Doping the same mixed-metal, citrate-based precursor with a variety of transition-metal cations was found either to enhance the gamma- --> alpha-Al2O3 phase transformation or to enhance the formation of CaAl12O19. Incorporation of MnO, Fe2O3, CoO, NiO, CuO, ZnO, or SiO2 into the citrate precursor resulted in significant hexaluminate formation within 1 h at 1100 degrees C, compared with negligible formation using the undoped precursor. Fe2O3-doped precursors gave the highest hexaluminate yields at 1100 degrees C, Phase-pure hibonite was obtained at temperatures as low as 1000 degrees C with 25 at,% Fe substituting for Al in the hexaluminate structure. Textured CaAl11.5Fe0.5O19 films with (0001) planes parallel to the substrate surface were obtained on YAG substrates in 1 h at 1200 degrees C. In comparison, undoped citrate-derived films first textured at 1300 degrees C, Enhanced CaAl12O19 formation was attributed to rapid reaction of the constituents, due to the uniform dispersion of cations on an atomistic scale throughout the citrate-based precursor, prior to completion of the gamma- --> alpha-Al2O3 phase transformation, Solid-solution dopants were believed to further enhance reactivity and grain growth by increasing diffusive mass transport.