Thermal Stability And Oxidation Kinetics Of Nanocrystalline Iron

Nanocrystalline materials are equipped with a series of superior physical, chemical and mechanical properties. They possess wide application potentials, such as electronics, energy resources, biological engineering, etc. Our study focused on pure iron. The samples were prepared by mechanical milling and characterized by means of XRD, SEM and TG. The thermal stability of nanocrystalline iron (nc Fe) powder and the oxidation kinetics of nc Fe powder and surface nanocrystallized iron (snc Fe) were investigated. The main results are as follows:1. Thermal stability and oxidation behavior of nc Fe powderThe nc Fe powder with a grain size of 16nm was obtained by mechanical milling for 24h. After isothermally annealed at 200℃,250℃,300℃,400℃,500℃for 60min, the grain sizes are 21nm,22nm,23nm,24nm,30nm, respectively. The initial growth temperature of ball milled nc Fe power is low. The grain sizes increase at 200℃. Therefore the nc Fe powder is of reasionable good thermal stability below 500℃.(2) Isothermally oxidized at 450℃, the oxidation rate of the nc Fe powder is larger than that of the coarse grain iron (cg Fe) powder. While oxidized at 500℃, the oxidation rate of nc Fe powder is larger than that of the cg Fe powder in the binginning 4700s. With the increase of time, the oxidation rate of the nc Fe powder decreases. The gain in weight per mg is less than that of the cg Fe powder. Furthermore, oxidized at 550℃, the oxidation rate of the nc Fe is much smaller than that of the cg Fe powder.(3) The relation between gain in weight per mg and isothermal oxidation time of both the nc Fe powder and the cg Fe powder follows the Arrhenius relation:(ΔW)n=Kt. The orders of reaction of the nc Fe and the cg Fe are 3.24 and 2.92, respectively. While the energy of the nc Fe and the cg Fe are 94 kJ/mol and 124 kJ/mol, respectively.2. The oxidation behavior of snc Fe(1) While isothermally oxidized at 800℃,900℃and 1000℃, an abrupt increase showed in each TG curve and a corresponding exothermic peak was observed in each DSC curve. Moreover the higher the isothermal temperature is, the earlier the abrupt in TG curve and the exothermic peak in DSC curve. Furthermore the explosive oxidation of the nc Fe is earlier in time than that of the cg Fe.(2) Before and after the abrupt in TG curve, the relation between the gain in weight per cm2 and the isothermal oxidation time of both the snc Fe and the cg Fe follows the Arrhenius relation:(ΔW)n=Kt. Before the abrupt the orders of reaction of the nc Fe and the cg Fe are 2.17 and 2.13, respectively. The active energy of the snc Fe and the cg Fe are 270kJ/mol and 306kJ/mol, respectively. However after the abrupt the orders of reaction change to 4.73 and 4.46, respectively. And the active energy change to 453kJ/mol and 470kJ/mol, respectively.