| dc.description.abstract | The kinetics and energetics of the transformation of pure anatase (tetragonal) to rutile (tetragonal) have been studied. The rate data fit a first-order rate equation, and the activation energy is approximately 90 kcal·mol?¹. The transformation is immeasurably slow below 610?°C. The first-order rate data have been interpreted by drawing analogies to polymerization reactions and thermal decomposition of solids. A more satisfactory interpretation is obtained using the general rate equation derived from an extension of the order-disorder theory. This interpretation indicates negligible phase aggregation and shows that the nucleation rate constant is much larger than the propagation rate constant. Apparently, the activation energy is consumed mainly in producing active nucleation sites.
The effect of thermal treatment on pure anatase, prepared by peroxide oxidation of spectroscopically pure titanium metal, has been studied in detail from its preparation stage (<100?°C) to about 1000?°C. Peroxy titania decomposes around 150?°C, giving rise to anatase. The anatase thus formed is initially amorphous and becomes crystalline around 250?°C. The crystallization reaction is associated with a large evolution of heat. The transformation of anatase to rutile at 630?°C is accompanied by a small heat evolution (~100 cal·mol?¹). Particle size and crystallite size of anatase increase markedly while surface area decreases above 600?°C. The anatase lattice appears to expand prior to transformation to rutile. This expansion is attributed to a displacive transformation of the type described by Buerger. The rutile formed undergoes contraction with increasing temperature. Expansion of the anatase lattice prior to transformation may be taken as an indirect indication that this crystal structure transformation is preceded by the formation of a disordered lattice. The anatase–rutile transformation may be visualized as follows:
620–650?°C: Anatase I (collapsed form, ordered lattice) ? Displacive transformation ? Anatase II (open form, disordered lattice)
Above ~650?°C: Reconstructive transformation ? Rutile II (open form, disordered lattice) ? Displacive transformation (?) ? Rutile I (collapsed form, ordered lattice)
The effects of particle size and surface area on the kinetics of anatase transformation have been investigated. Smaller particle size seems to favor the transformation, possibly due to dislocations produced during grinding, which act as nucleation sites. Surface area appears to have negligible effect. The activation energy remains essentially the same within experimental error for samples of varying surface area and particle size.
The characteristics of anatase prepared by hydrolysis of titanium tetrachloride are similar to those of pure anatase, except that the transformation temperature is slightly higher. The rate curves are exponential, as in pure anatase, and the activation energy is slightly higher (~110 kcal·mol?¹). Doping pure anatase with 0.1–5% chloride ion impurity has little effect on the transformation, and the chloride impurity remains intact even after heating to the transformation temperature.
The kinetics of anatase prepared by hydrolysis of titanium sulfate differ considerably from those of pure anatase. The transformation becomes immeasurably slow below ~695?°C compared to pure anatase. An induction period is observed, which decreases with increasing temperature. At sufficiently high temperatures (>740?°C for this sample), the rate becomes exponential. The activation energy is ~120 kcal·mol?¹, higher than that for pure anatase–rutile transformation. These observations are represented as a special case of the general rate equation derived by applying order-disorder theory to diffusionless transitions in solids. For large values of Z relative to c, the curve becomes more sigmoidal. The induction time observed in the rate curves for anatase from sulfate hydrolysis is interpreted as due to a relatively large ratio of nucleation to propagation rate constants. The decrease in induction period with increasing temperature is explained by the greater sensitivity of nucleation to temperature changes. Nucleation is therefore associated with a high activation energy. The activation energy for transformation at high temperature is ~120 kcal·mol?¹. From rate data at lower temperatures, low values of activation energy can be calculated: ~215 kcal·mol?¹ for the induction period and <40 kcal·mol?¹ after the induction period. It is concluded that the simple exponential rate curves observed in crystal structure transformations represent a combination of two processes: nucleation (high activation energy) and propagation (low activation energy). Studies on anatase samples doped with different amounts of sulfate impurity (1–5%) show that sulfate progressively inhibits transformation with increasing temperature, and activation energy also increases with impurity concentration.
Brookite, the orthorhombic modification of titanium dioxide, also transforms to rutile on heating. The kinetics and energetics of this transformation have been studied. Below 715?°C, the rate of transformation is extremely slow, with little or no induction time. The kinetic data fit a first-order rate equation. The activation energy is ~60 kcal·mol?¹, and the heat of transformation is about 100 cal·mol?¹. The kinetic results of the brookite–rutile transformation can also be explained by applying order-disorder theory to diffusionless transformations in solids. The ratio of propagation to nucleation rate constants is small, and there is negligible phase aggregation, as in the anatase–rutile transformation.
The decomposition of several carbonates has been studied using differential thermal analysis and powder X-ray analysis to understand the origin of the small exothermic reaction observed immediately after carbonate decomposition. The heat evolution is probably due to the removal of internal crystal defects in the metastable oxide lattice formed immediately after decomposition. This annealing process, resulting in ordering of the oxide lattice, may explain a major part of the thermodynamic anomalies observed in certain oxides. | |
