engleski

Thermal behaviour of titanium dioxide

Temperature dependence of microstructure of TiO2 and the phase transition anatase (A) to rutile (R) were studied by X-ray powder diffraction (in situ measurements using a Philips diffractometer, having a high-temperature attachment, proportional counter, monochromatized CuK radiation, air pressure 10-2 Pa). A commercial TiO2 p.a. was used and three experiments were performed with (1) powder as-synthesized ; (2) high-energy ball-milled powder ; (3) high-energy ball-milled powder pressed in a form of thin pellet (pressure 0.18 GPa). In all experiments, the heating and cooling was performed at a rate of about 3-4 °C/minute, and the process was stopped at a series of temperatures in order to scan prominent parts of diffraction patterns. The results are as follows: (1) The dominant phase at room temperature (RT) was A, while R was present in traces. During heating, the intensities of diffraction lines slowly decreased due to increased thermal vibrations of atoms. Thermal expansion of A was found anisotropic. A negligible broadening of diffraction lines was observed. Above 950 °C a gradual transition A to R took place. At the highest reached temperature, 1300 °C, the fraction of R was bigger than that of A. During cooling, the transition A to R continued and gradually ceased below  900 °C. At RT, the sample contained R as the dominant phase, with several percent of A. R also showed an anisotropy in thermal expansion. The width of diffraction lines of A and R phases depended very little on temperature during the heating and cooling cycle. (2) The process of milling induced the transition A to R, and the milled sample contained R as the dominant phase, about 15 % of A and about 10 % of the high-pressure phase, TiO2 II. Diffraction lines were broad as the milling process introduced lattice strains and caused a decrease of the crystallite sizes. During heating, a sharpening of diffraction lines was noticed above 500 °C, and the transition to R took place above about 700 °C. At 1100 °C the sample contained only few percent of A and of the phase TiO2 II. In the cooling cycle to RT the sharpening of diffraction lines went on, due to a continuous annealing of strains. At RT diffraction lines of R were as sharp as the ones of the non-milled sample. The fractions of A and TiO2 II were 2 to 3 percent. (3) The milled and pressed sample showed similar molar fractions of R and A as the milled sample with diffraction lines similarly broad. The thermal behaviour of that sample was more or less similar to that of the milled sample. The present work confirms the literature citation that, in the transition A  R, the nuclei of R form both at the surface and throughout the bulk of the A crystallites, and these nuclei grow with increasing heating time.

TiO2 ; XRD ; phase transition

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