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Oxidation of MgB 2 under air conditions of room temperature to 1000 • C was studied by thermogravimetry, x-ray diffraction and scanning electron microscopy equipped with energy dispersion spectrum. The experiment shows that the obvious oxidation process begins at about 400 • C and becomes very strong when the temperature reaches beyond 700 • C. X-ray diffraction shows that after oxidation for a long time at high temperature, most of the magnesium in MgB 2 becomes MgO. The analyses of the microstructure and components show that the oxidation of MgB 2 is a very complex process accompanying the evaporation of boron and magnesium and the formation of MgO. Long MgO whiskers with diameters in the nanometre scale also appear, mostly on the grain surface, in the oxidation process due to the reaction of evaporated magnesium with oxygen. The oxidation activation energy of MgB 2 is estimated by using the Freeman-Carroll method based on the Arrhenius equation. It is shown that in the range of strong oxidation temperature the activation energy is about 70-90 kJ mol −1 .

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