The formation of crystalline needles of rutile by flux evaporation

Abstract

In searching for a high-temperature (>700 °C liquid medium in which titanium oxycompounds could be dissolved for effecting subsequent reactions, a promising candidate appeared to be the flux Na20. 3K20.5B203, used by Kajiwara to prepare potassium titante fibres [1]. Kajiwara reported that a mixture with a weight ratio of flux/K2Ti6013 = 1.3, heated to 1050 °C, produced a glass that remained in that state even after approximately 15 wt% had been lost by evaporation [1]. Tests were made to establish the limits of solubility of TiO2, a representative compound, in fluxes composed of aNa20"bK20.cB203 at temperatures near 1000 °C. The composition of the fluxes was varied by manipulating the ratio of the initial reagents and by evaporation of some of the components at high temperature. It was found that although the solids crystallizing out of the flux had high aspect (length/diameter) ratios, their composition was dependent on the initial composition of the flux. Crystalline needles of K2Ti6013 and TiO2, rutile, were obtained. The preparation of crystalline needles of rutile is emphasized below. The sources of boron oxide used, H3BO 3 and Na2B4OT.10H20, were heated at 1000-1100 °C to constant weight before the addition of other reagents. The NazCO3, K2CO3-1.5H20 and TiO 2 were of analytical grade purity. K2Ti2Os, K2Ti409 and K2Ti6013 were prepared by methods described previously [2]. The reagents were contained in platinum, heated in air (to 1100-1200°C) and quenched in water. The resultant phases were examined visually to determine whether they consist of a single colourless transparent glass phase or two or more phases that included white or yellow opaque solids. All of the phases were additionally examined by Raman spectroscopy. The colourless glasses, which in general exhibited low viscosity at temperatures near 900 °C, were subsequently heated further, either at higher temperature (1350 °C) or for longer times at 1100-1200°C, in order to volatilize some of their components and cause crystallization of the remaining liquid. The solids resulting from the thermal treatments were extracted with hot water to remove B20 3 and alkali-