Abstract
The kinetics of nanocrystalline α-iron nitriding to γ’-iron nitride in an ammonia atmosphere was studied at 598–648 K and at atmospheric pressure. Oscillatory changes in nitriding reaction rates depending on nitrogen concentration in a solid sample were observed. This phenomenon was explained by a gradual change in the iron active surface coverage degree, with nitrogen resulting from a gradual change in the free enthalpy of nitrogen segregation. The α-Fe(N) nanocrystallites’ transformation into γ’-Fe4N went through six metastable FeNx states. The continuous function proposed by Fowler and Guggenheim was modified to a stepwise variable function.
Highlights
Due to the practical applications of the iron and steel nitriding process, it was studied in the presence of coarse-crystalline iron and thin iron foils [1,2,3,4,5]
An industrial iron catalyst for ammonia synthesis—KM1R (pre-reduced form containing nanocrystalline iron doped with hardly reducible oxides of aluminum (3.3 wt.%), calcium (2.8 wt.%), and potassium (0.65 wt.%))—was used in the study
Based on the results of thermogravimetric measurements and determination of hydrogen concentration in the gas phase, taking into account the mass balance of the reactor with perfect mixing, ammonia and nitrogen concentrations and nitriding potential were calculated at each process step [7]
Summary
Due to the practical applications of the iron and steel nitriding process, it was studied in the presence of coarse-crystalline iron and thin iron foils [1,2,3,4,5] During these studies, Grabke, among others, observed that the surface chemical reaction is a rate-limiting step only in the initial phase of the process [3,4]. In the case of nitriding of nanocrystalline iron, the effect of the diffusion of substances through the product layer on the process rate may be neglected due to the small size of nanoparticles and, short (compared to coarse-crystalline materials) mass transport paths. It was observed that nanocrystallites underwent the phase transformation according to their sizes, ranging from the smallest up to the largest
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