Parameters of iron and aluminum nano- and micropowder activity upon oxidation in air under microwave irradiation

Iron nanopowders and iron and aluminum micropowders exposed to microwave radiation with a frequency of 9.4 GHz and a power density of 80 W/cm$^2$ at a pulse repetition rate of 400 Hz have been investigated. According to the results of differential thermal analysis, the microwave radiation caused nonmonotonic changes in the thermal properties of the A1 and Fe powders. After irradiation of the iron nanopowder, the temperature of the onset of its oxidation increased from 150.01 to 158.75$^{\circ}$C; in the case of the micropowder, the temperature nonmonotonically changed from 150.00 to 275.38$^{\circ}$C. The specific heat of oxidation of the Fe nanopowder increased by 17.3% at maximum, while in the Fe micropowder the maximum attained increase was 13%. For the Al micropowder, the maximum increase in the specific heat of oxidation was found to be 59.7%. Microwave irradiation leads to the formation of electron avalanches, which reduce metal ions in their oxides. At the same time, at certain irradiation doses the generated electron flows oxidize the reduced metals, which is reflected in the nonmonotonic variation in the properties of a material. The increase in the specific heat of oxidation is related to the participation of energy-saturated states of the metals in the oxidation processes.