Metastable magnesium titanate phases synthesized in nanometric systems

Abstract

When heating mixtures of conventional magnesia and titania powders, geikielite (MgO. TiO₂) appears first at about 600°C; only above 1200–1300°C is either qandilite (2MgO. TiO₂) or karrooite (MgO. 2TiO₂) formed, depending on whether MgO or TiO₂ is in excess. A different behaviour pattern is observed when starting from nanometre precursors, consisting of hydroxides of magnesium and itanium, which lose water on heating to about 400°C, yielding nanocrystalline oxides. Coprecipitated nanometre precursors yield at 600°C qandilite, geikielite or karrooite, depending on the relative stoichiometries of magnesia and titania in the precursor powder. At higher temperatures, up to 1200°C geikielite is the only titanate formed, and only above 1300°C does qandilite or karrooite reappear, depending on the stoichiometry of the original mixture. Only karrooite is formed when independently prepared nanometre precursors of magnesia and titania are mixed together and heated to temperatures as low as 400°C and up to 600°C, the stoichiometric ratio of the mixture notwithstanding. Estimation of surface area changes taking place when nanometre particles coalesce with each other to form magnesium titanates show that substantial reductions in surface energy take place, even if the products remain of nanometre size. Such reductions depend on the absolute and relative particle sizes of the reacting oxides and the stoichiometry of the final product. The contribution of diffusion processes, surface energy and misfit volume to the formation of the various magnesium titanates from the oxides is discussed. All three magnesium titanates synthesized from nanometre precursors show a stable dielectric constant e over a wide range of frequencies, in contrast with an equivalent commerical powder used in the manufacture of microcondensers. Furthermore, the quality factor Q of magnesium orthotitanate (geikielite) synthesised from nanometre precursors is higher by about one order of magnitude than that of the commercial product.