Oxidation kinetics of aluminum diboride

The oxidation characteristics of aluminum diboride (AIB2) and a physical mixture of its constituent elements (AI+2B) were studied in dry air and pure oxygen using thermal gravimetric analysis to obtain non-mechanistic kinetic parameters. Heating in air at a constant linear heating rate of 10 C/min showed a marked difference between Al +2B and AlB2 in the onset of oxidation and final conversion fraction, with AlB2 beginning to oxidize at higher temperatures but reaching nearly complete conversion by 1500 degrees C. Kinetic parameters were obtained in both air and oxygen using a model-free isothermal method at temperatures between 500 and 1000 degrees C. Activation energies were found to decrease, in general, with increasing conversion for AlB2 and Aid-2B in both air and oxygen. AlB2 exhibited 02-pressureindependent oxidation behavior at low conversions, while the activation energies of Al + 2B were higher in O-2 than in air. Differences in the composition and morphology between "oxidized Al-F2B and AlB2 suggested that Al2O3 B2O3 interactions slowed AI + 2B oxidation by converting Al2O3 on aluminum particles into a A1413209 shell, while the same Al4B209 developed a needle-like morphology in AlB2 that reduced oxygen diffusion distances and increased conversion. The model-free kinetic analysis was critical for interpreting the complex, multistep oxidation behavior for which a single mechanism could not be assigned. At low temperatures, moisture increased the oxidation rate of Al + 2B and AlB2, but both appear to be resistant to oxidation in cool, dry environments. (C)2013 Elsevier Inc. All rights reserved.