Oxidation of Fine Tantalum Particles: Metastable Intermediates and Multistep Kinetics

Abstract

The metastable tantalum suboxides have been found to be a cause of complicated oxidation behaviour of fine tantalum particles when heated in air at moderate temperatures. Fine Ta 0.02–10 μm particles of bimodal size distribution with average volume maxima of 0.12 and 4 μm were prepared via electric explosion of thin Ta wire in Ar atmosphere. The oxidation reaction proceeded via several intermediate suboxides (Ta6O, Ta4O, Ta2O and TaO2) in which chemical and structure transformations caused multistep kinetics of the oxidation process in a narrow temperature interval of 250–570 °C. Within the interval of 250–400 °C, the oxidation process occurred under diffusion control with an apparent activation energy Ea,1 = 31.6 ± 1.2 kJ/mol and the main oxidation products were Ta(O) solid solutions and suboxides Ta6O and Ta4O. In the interval of 400–520 °C, oxidation was accompanied by a low-barrier martensite-like transformation of the lower suboxides resulting in reducing of diffusion limitations of the process (Ea,2 = 4.8 ± 1.2 kJ/mol). In the interval of 520–570 °C, oxidation of the larger Ta particles was completed to form Ta2O (Ea,3 = 64.7 ± 1.9 kJ/mol). An abrupt decay of Ta2O at temperatures greater than ~ 570 °C induced the high-rate oxidation of micron-sized Ta particles, resulting in self-heating of the sample.