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 TiO2, 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 TinO2n−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.

Concentration profiles of AI2O3 in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system Si02-A1203. The intermediate compound mullite, 3A203. 2SiO,, melts incongruently at 1828' 5 10°C; its stable solid-solution region ranges from 70.5 to 74.0 wt% A1,03 below 1753'C and from 71.6 to 74.0 wt% at 1813°C. The microstructures of diffusion zones and heattreated specimens also indicate the incongruency of mullite. Additional information is given for 3 metastable systems: Si0,-A120, in the absence of mullite, SO,-"ordered"-mullite in the absence of alumina, and Si0,-"disordered"-mullite in the absence of alumina. Under metastable conditions, ordered mullite melts congruently at = 1880°C and its solid-solution range extends up to = 77 wt% A1203. The solid-solution range of disordered mullite extends to = 83 wt% A120, with an estimated congruent melting temperature of == 1900°C. The existence of metastable systems is associated with superheating of mullite above the incongruent melting temperature and with nucleation of alumina and mullite in supercooled aluminum-silicate liquids. ( I ) Diffusion-Couple ExperimentsDiffusion couples were prepared with sapphire and fused silica or sapphire and aluminum-silicate glasses of 10.9,22.8, and 42.2 wt% A1203. These binary glasses are in equilibrium with mullite at Turkey. f Commercial-grade fused cast aluminum-silicate, Carborundum C o p , Niagara Falls, NY. Wet chemical analysis performed by Coors Spectro Chemical Laboratory, Golden, CO. Spectrographic analysis (American Spectrographic Laboratory, San Francisco, CA) showed (in Wr% as oxides) 0.12 Na. 0.02 Fe, 0.01 Mg, 0.01 2, 0.006 Ca. 0.005 Ti, 0.003 Cr, <0.005 Cu. and <0.001 Mn.