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 (CaAl 12 O 19 ) in an attempt to enhance reactivity and lower the formation and texturing temperatures of phase‐pure powders and films. One undoped polymeric sol was found to yield CaAl 12 O 19 , free of any intermediate phases, at temperatures as low as 1300°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 γ‐ →α‐Al 2 O 3 phase transformation or to enhance the formation of CaAl 12 O 19 . Incorporation of MnO, Fe 2 O 3 , CoO, NiO, CuO, ZnO, or SiO 2 into the citrate precursor resulted in significant hexaluminate formation within 1 h at 1100°C, compared with negligible formation using the undoped precursor. Fe 2 O 3 ‐doped precursors gave the highest hexaluminate yields at 1100°C. Phase‐pure hibonite was obtained at temperatures as low as 1000°C with 25 at.% Fe substituting for Al in the hexaluminate structure. Textured CaAl 11.5 Fe 0.5 O 19 films with (0001) planes parallel to the substrate surface were obtained on YAG substrates in 1 h at 1200°C. In comparison, undoped citrate‐derived films first textured at 1300°C. Enhanced CaAl 12 O 19 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 γ‐ →α‐Al 2 O 3 phase transformation. Solid‐solution dopants were believed to further enhance reactivity and grain growth by increasing diffusive mass transport.