Size-Dependent Transformation of α-Fe into γ′-Fe₄N in Nanocrystalline the Fe–NH₃–H₂ System

Abstract

On the basis of the results of experiments concerning nanocrystalline iron nitriding with ammonia−hydrogen mixtures, it was found that α-iron crystallites transform to iron nitride γ′-Fe 4 N in their whole volume in the order of their size, from the largest to the smallest ones. During the phase transition, the saturated solid solution of nitrogen in α-iron, being in the chemical equilibrium state, changes into an unsaturated solid solution of nitrogen in γ-iron, being in the transition state, and then to γ′-nitride, being again in the chemical equilibrium state. The phase transition is an adiabatic and isothermal process, and thus the change in Gibbs energy of the system is zero. On that basis the energy balance of the system was performed in this paper. The Gibbs energy required for phase transition was supplied to the iron nanocrystallite by the nitriding potential of the ammonia− hydrogen mixture. This energy depends on the ratio of the nanocrystallite active surface to its volume ratio and is the lowest for the largest nanocrystallites. Therefore, large crystallites have to accumulate the least energy, in comparison with the smaller ones. This explains the phenomenon that nanocrystallites undergo the phase transition in a specific order.

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

“…In the state of chemical equilibrium, when the chemical potential of nitrogen is equal to the absolute value of the potential change in the deformed crystal lattice of nanocrystalline iron, the saturated solution of α-Fe(N)-which is in a state of chemical equilibrium-is transformed to an unsaturated solution of nitrogen in the γ-iron crystallographic phase-which, in turn, is in a state of transition-and further to γ'-Fe 4 N nitride, which is again in the chemical equilibrium state [18,19]. The step change in the nanocrystalline iron nitriding rate (Figures 2 and 3) can be explained by the gradual change in the degree of coverage of the iron surface with nitrogen (Figure 4), caused by the gradual change in the free enthalpy of nitrogen segregation (Figure 3).…”

“…A hysteresis was observed for the dependence of the nitriding degree on nitriding potential [13][14][15][16][17]. It was also found that the chemical potentials of nitrogen occurring in all three parts of the system-in the gaseous phase, on the iron nanocrystallite surface and in nanocrystallite volume -equaled one another in the stationary states, i.e., chemical equilibrium was established [16,18,19]. There was only a reaction of catalytic ammonia decomposition [20].…”

“…There was only a reaction of catalytic ammonia decomposition [20]. The minimum nitriding potentials required to initiate nitriding of nanocrystalline iron and reduction of nanocrystalline nitrides are the linear functions of nanocrystallite sizes [8,18,19,21]. Phase transformation of nanocrystallites in close-to-equilibrium states takes place in the entire volume of nanocrystallites according to their sizes, starting from the largest down to the smallest [16,22].…”

“…Phase transformation of nanocrystallites in close-to-equilibrium states takes place in the entire volume of nanocrystallites according to their sizes, starting from the largest down to the smallest [16,22]. Based on theoretical calculations concerning thermodynamic parameters, it was found that such an order of nanocrystallite transformation occurred in the nanocrystalline iron nitriding process as well as in the reduction of nanocrystalline iron nitrides [18,19]. In the nanocrystalline iron nitriding process with ammonia-hydrogen mixtures, two solid phases occur simultaneously at the determined nitriding potential, and three solid phases in the wide ranges of nitriding potential occur simultaneously in the process of reducing the nanocrystalline ε iron nitrides [15,23].…”

Study of Phase Transitions Occurring in a Catalytic System of ncFe-NH3/H2 with Chemical Potential Programmed Reaction (CPPR) Method Coupled with In Situ XRD

“…Based on the results of gravimetric investigations of the nanocrystalline iron nitriding processes in atmospheres containing ammonia and hydrogen, it can be concluded that nanomaterials do not react chaotically but in a precisely defined order of increasing energy of transformation in the thermodynamic region of reaction, or of decreasing surface to volume ratio (geometrical aspect) in the kinetic region [31]. Based on these statements, a number of methods were developed to determine nanocrystallite size distribution, and it was stated that in the nanocrystalline iron doped with hardly reducible metals oxides the distribution was bimodal [32,33].…”