Oscillatory Kinetics in the Process of Reduction of Nanocrystalline Iron Nitride γ′-Fe₄N

Abstract

The kinetics of the reduction of nanocrystalline γ′-iron nitride to α-iron in mixtures of nitrogen and hydrogen of different hydrogen concentration was studied. Kinetic measurements were carried out in a differential chemical reactor with a thermogravimetric system for measuring the weight of a sample and an analyzer of hydrogen at 400 °C and at atmospheric pressure. The occurrence of cyclic changes in the rate of chemical reaction in a solid phase has been observed. Such an oscillatory phenomenon was not observed so far in the reaction between gases and a solid with phase transition in a solid phase, which results in a solid products. The mechanism of the observed phenomenon is explained, taking into account processes occurring in the volume of the solid phase. The oscillatory rate changes of the described processes prove a nonmonotonic coverage degree of the surface with nitrogen, which is caused by a step change in the free enthalpy of segregation of nitrogen in the reduction of γ′-Fe4N nitride. On the basis of model calculations the changes of free enthalpy of segregation during reduction of nitrides have been specified, and modification of the Fowler–Guggenheim equation has been proposed.

Oscillatory Mechanism of α-Fe(N) ↔ γ’-Fe4N Phase Transformations during Nanocrystalline Iron Nitriding

“…Saraev et al [42] observed oscillatory phenomena in solid-phase-gas-phase reaction systems with phase transformation of a solid substrate in the surface layer (a range of several dozens of atomic layers). As a result of detailed kinetic studies of the nitride reduction process, we observed oscillatory changes in the rate of the reduction reaction of iron γ'-Fe 4 N iron nitride to α-iron [43]. The observed oscillatory phenomenon has been explained by a gradual variation in the free enthalpy of nitrogen segregation.…”

Synthesis and Physical Properties of Antiperovskite CuNFe3 Thin Films via Solution Processing for Room Temperature Soft-Magnets

“…Room temperature soft-magnets are materials that quickly respond to changes in external magnetic fields at room temperature and can obtain high magnetization with low loss. They have important applications in power transformers, high frequency inductors, electric motor rotors, flux gate magnetometers, and high-density recorders [13][14][15][16]. There are two different types of Fe atoms in the γ -Fe 4 N unit cell.…”