Kinetics of the oxidation of bismuth tellurite, Bi2TeO5 -: Empirical model and least squares evaluation strategies to obtain reliable kinetic information

The oxidation kinetics of Bi2TeO5, a special non-linear optical material, was investigated. Thermogravimetric (TG) experiments combining non-isothermal (1-10degreesC/min) and isothermal (600-700degreesC) sections at oxygen partial pressures of 21-101 kPa were evaluated. The model contained an empirical f(alpha) function to describe the dependence of the reaction rate on the fraction reacted (alpha). The unknown parameters were determined by the least-squares evaluation of 12 experiments. Different evaluation strategies were employed to ensure the reliability of the obtained kinetic information. A least-squares baseline correction method was also proposed. The results indicated that the reaction rate is highly influenced by the solid phase diffusion of the oxygen. Several combinations of the experimental conditions resulted in fl(alpha) functions increasing to a maximum at alpha approximate to 0.3-0.5 while monotonously decreasing f(alpha) functions were observed in other cases. This behavior indicates that the size, variation and topography of the reaction surface depend on the experimental conditions. (C) 2002 Elsevier Science B.V. All rights reserved.