Synthetic Rutile

We performed experiments with thin film diffusion couples to simultaneously measure diffusion coefficients of Zr, Hf, Nb and Ta parallel to the a-and c-axes of synthetic rutile in a gas mixing furnace at controlled oxygen fugacity at temperatures between 800 and 1100 • C. Depth profiles of the diffusion couples were measured using secondary-ion mass spectrometry. Some of the diffusion profiles show a concentration dependence, which indicates different diffusion mechanisms above and below a particular trace-element concentration level (∼ 1000 μg∕g). The diffusion coefficients for the mechanism dominant at high-concentration levels are approximately two orders of magnitude smaller than for the low-concentration mechanism. Below the critical concentration the diffusion coefficient is constant, as consistently shown in all of the experiments. For this diffusion coefficient we have found that D Zr ∼ D Nb > D Hf >> D Ta , and diffusion is isotropic for the four elements at all investigated T and f O 2 conditions. At 1000 • C for log f O 2 < FMQ+1, the diffusion coefficients decrease with increasing oxygen fugacity where D is proportional to f O n 2 with exponents n ≈ −0.25 for Zr and Hf and n ≈ −0.30 for Nb and Ta. Diffusivites of Nb and Ta strongly differ from each other at all investigated conditions, thus providing the potential to fractionate these geochemical twins, as suggested earlier. The present data and literature data for Zr and Ti self diffusion are interpreted and predicted based on published quantitative point defect models. Two end-member diffusion mechanisms were identified for impurity diffusion of Zr: (i) an interstitialcy mechanism involving Ti 3+ on interstitial sites, which is dominant at approximately log f O 2 < FMQ+2; (ii) a vacancy mechanism involving Ti vacancies, which is dominant at approximately log f O 2 > FMQ+2. The point defect calculations also explain the observed effects of heterovalent substitutions, such as Nb 5+ for Ti 4+ at high concentration levels for changes in the diffusion mechanism and hence diffusion rates. In the case of rutile, this concentration effect becomes much more sensitive to the substitution level at lower temperature. In natural rutile penta-and hexavalent cations may largely be charge balanced by mono-, di-and trivalent cations, such that the doping effect on diffusion may be reduced or may even be reversed. The Arrhenius relationships established here may therefore not be directly applicable to natural rutile. We obtained the following Arrhenius relationships (with diffusion coefficients D in m 2 ∕s , f O 2 in Pascal and T in Kelvin), which are only applicable for log f O 2 < FMQ+2: Keywords TiO 2 · Diffusion · HFSE · Impurities · Experiments log D Zr =(−0.40 ± 0.47) + (−0.253 ± 0.019) log f O 2 10 −7 − 414 ± 11 kJ/mol RT ln 10 log D Hf =(−0.08 ± 0.63) + (−0.266 ± 0.023) log f O 2 10 −7 − 428 ± 15 kJ/mol RT ln 10 log D Nb =(−0.19 ± 0.36) + (−0.294 ± 0.014) log f O 2 10 −7 − 421 ± 9 kJ/mol RT ln 10 log D Ta =(0.45 ± 0.73) + (−0.304 ± 0.015) log f O 2 10 −7 − 463 ± 18 kJ/mol RT ln 10

Titania hydrolysates prepared from the hydrolysis of both unmodified and modified tetraisopropoxytitanium(iv) are shown by both FT-Raman spectroscopy and X-ray diffraction to be amorphous with no long range ordering. The Raman hydrolysate spectra show three broad bands, consisting of multiple components, at Raman shifts of ∼210, 440 and 570 cm−1. These bands do not correspond with any known phase of titania. Thermal analysis shows broad endothermic peaks centred at 106–136°C and extending to ≥200°C for each of the hydrolysates and is attributed to dehydration and loss of water. Strong, broad exothermic peaks are observed in the modified hydrolysate curves, in the 310–340°C temperature range and are attributed to the combustion of the residual surface adsorbed organics. Weight loss occurred during these exotherms. Crystallisation of the disordered hydrolysates into anatase occurs in the 500–600°C temperature range. The anatase → rutile phase transition was found between 665°C and 691°C.

The objective of this work is to utilize a low-grade synthetic rutile to produce high-grade titania nanoparticles. Due to the nature of the precursor, the hydrothermal method needs modification in order to accommodate the precursors and chemical reagents. The product will be characterized with the XRD (crystallite size and crystallinity), EDXRF (chemical composition), SEM (Morphology), N2 adsorption-desorption (Surface Area) and UV-Vis-NIR. Results revealed a crystallite size of less than 20 nm, a surface area of 186.8 m 2/g, a morphology that is a combination of agglomeration and particles, and an optical band gap of 3.23 eV. It is concluded that synthetic rutile is a viable precursor to produce high quality titania nanoparticles.

http://www.scientific.net/JNanoR.21.71