Single- and Mixed-Phase TiO2 Powders Prepared by Excess-Hydrolysis of a Titanium Alkoxide

To investigate excess-hydrolysis of titanium alkoxides, TiO2 powders were fabricated from titanium-tetra-isopropoxide using 6:1 and 100:1 H2O:Ti (r) ratios. Powders were dried and fired at a range of temperatures ( up to 800 C). Hydroxylation and organic content in powders were characterised using ATR-FTIR, laser Raman microspectroscopy, and elemental microanalysis; surface area and pore size distribution were evaluated using N2 gas adsorption; phase composition was analysed using XRD and laser Raman microspectroscopy; and crystallite size was evaluated by XRD, TEM and SEM. Results showed near-complete hydrolysis in a predominantly aqueous medium (r = 100), resulting in precipitated crystalline powders exhibiting brookite and anatase, which begin to transform to rutile below 500 C. Powders precipitated in a predominantly organic medium (r = 6) underwent partial hydrolysis, were highly porous and exhibited an amorphous structure, with crystallisation of anatase occurring at 300 C and transformation to rutile beginning at 500 to 600C.

Solution based synthesis of mixed phase materials in the Li2TiO3 - Li4SiO4 system

As candidate tritium breeder materials for use in the ITER helium cooled pebble bed, ceramic multiphasic compounds lying in the region of the quasi-binary lithium metatitanate- lithium orthosilicate system may exhibit mechanical and physical advantages relative to single phase materials. Here we present an organometallic solution-based synthesis procedure for the low-temperature fabrication of compounds in the Li2TiO3 - Li4SiO4 region and investigate phase stability and transformations through temperature varied X-ray diffraction and scanning calorimetry. Results demonstrate that the metatitanate and metasilicate phases Li2TiO3 and Li2SiO3 readily crystallise in nanocrystalline form at temperatures below 180{deg}C. Lithium deficiency in the region of 5% results from Li sublimation from Li4SiO4 and/or from excess Li incorporation in the metatitanate phase and brings about a stoichiometry shift and product compounds with mixed lithium orthosilicate/ metasilicate content towards the Si rich region and predominantly Li2TiO3 content towards the Ti rich region. Above 1150{deg}C the transformation of monoclinic to cubic {gamma}-Li2TiO3 disordered solid-solution occurs while the melting of silicate phases indicates a likely monotectic type system with a solidus line in the region 1050{deg}-1100{deg}C. Synthesis procedures involving a lithium chloride precursor are not likely to be a viable option for breeder pebble synthesis as this route was found to yield materials with a more significant Li-deficiency exhibiting the crystallisation of the Li2TiSiO5 phase at intermediate compositions.