Kinetics of the Anatase‐Rutile Transformation

Transformation of three commercial anatase powders resulted in sigmoid curves of transformation vs. time, which were interpreted in terms of a nucleation‐growth process. Analysis of these curves by several rate laws led to activation energies for nucleation of 109, 136, and 148 kcal/mole and for growth of 100, 115, and 190 kcal/mole, respectively. The rate of transformation and activation energy are governed by the nature and amount of impurities which determine the defect structure of the TiO 2 , i.e. the concentration of oxygen vacancies or interstitials. It is suggested that, in general, oxygen vacancies accelerate and the interstitials inhibit the transformation. CuO additions and hydrogen atmospheres accelerated the transformation by the introduction of oxygen vacancies and/or by reduction to a second Ti n O 2n−1 Magnéli phase which could act as a nucleating agent. The transformation is retarded in vacuum; this retardation is attributed to the introduction of titanium interstitials.