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The oxidation of zone melted specimens of and was studied at temperatures of 1200°–2000°K and at oxygen partial pressures of 1–700 Torr in helium. Parabolic rate behavior was observed. For above 1400°K an activation energy of was determined. Below 1400°K the activation energy is much lower, and the mechanism of oxidation is different, probably because of the presence of boron oxide in the condensed phase. Below 1300°K, the parabolic rate constant appears to be directly proportional to oxygen partial pressure; at 1830°K, the rate constant is independent of oxygen pressure. For in the range 1480°–1870°K an activation energy of was observed. Around 1970°K, the transition temperature for the monoclinic to tetragonal phase change in , there is a sharp increase in the rate of oxidation. The parabolic rate constant for total oxygen consumption increases as the power of the oxygen partial pressure at 1630° and 1760°K.
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Joan B. Berkowitz-Mattuck (1966) studied this question.