Novel synthesis of low-agglomerated YAG:Yb ceramic nanopowders by two-stage precipitation with the use of hexamine

Abstract Novel technique for obtaining of low-agglomerated YAG:Yb ceramic powders by two-step precipitation using hexamine was developed. This method is based on the reverse precipitation of aluminum hydroxide from the sprayed salts solution and the subsequent precipitation of yttrium and ytterbium hydroxides during the alkalization of the mother liquor. The use of hexamine in the first stage of precipitation makes it possible to maintain the synthesis conditions almost stationary at the pH value about 5. In the first stage, an aluminum hydroxide gel with captured yttrium and ytterbium ions is formed. In the second stage, during the alkalization of the resulting gel, yttrium and ytterbium hydroxides precipitate. Thereby high homogeneity of the ceramic chemical composition at the micro-level and high monodispersity of the powder particles can be achieved. The mechanism of hydroxides precipitation in a hexamine solution and the effect of the hexamine concentration and the solution temperature on the morphology of the garnet ceramic powder were studied. The least agglomerated powders were obtained at the hexamine solution temperature of 60 °C and the excess of hexamine in a range of 1.5–6. An increase in the excess of hexamine to 6 or more leads to an increase in the viscosity of the gel in the first precipitation stage and, as a consequence, the degree of agglomeration of the ceramic powder rises. An increase in the temperature of the precipitant up to 90 °C leads to thermal destruction of the gel and decrease in its viscosity, so that the chemical homogeneity of the ceramic powder decreases. The developed synthesis method is also applicable for the preparation of low-agglomerated oxide ceramic powders of other compositions. A necessary condition is the presence of metals that form hydroxide gels at the pH below 5.