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Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) results for cementite in carbon steel suggest that the alloying elements containing Cr and Mn. In order to explore the influence of Cr and Mn on Fe3C (FC), FC and Fe6Cr4Mn2C4 (FCMC) atomic structures and their energies and properties were calculated and compared by first-principles method. The results show that Cr and Mn can easily dissolve in FC and form stable FCMC. The structures of FC and FCMC are electrically neutral, the C atoms in them can get electrons, while the metal atoms (Mn, Fe, Cr) lose electrons, it is worth noting that Cr atoms close to Mn will get a small number of electrons due to Mn atoms have the strongest ability to lose electrons. The magnetic moment results suggest that FC is ferromagnetic and FCMC is sub-ferromagnetic, their magnetic moments of them are 22.18 and 5.09 μB/cell, respectively. The C−M (M = Fe, Cr, and Mn) bonds in FC and FCMC are covalent, the covalent electrons mainly provide by metal atoms, with the increase of C-M bond length, the covalence of bonds decrease gradually. The metal bonds of Fe−Fe, Fe−Cr, Fe−Mn, and Cr−Mn have both ionic and covalent character, while Cr−Cr and Mn−Mn bonds are ionic, and bond spin of Mn−Mn is zero, indicating that the bond is paramagnetic. Compared with FC, the Poisson's ratio and bulk modulus of FCMC structure are significantly reduced, while the Young's modulus and shear modulus are increased, this means that FCMC has lower compressive deformation resistance and higher y-axis shear resistance capacity than FC, i.e., FCMC has a higher brittleness than FC. [ABSTRACT FROM AUTHOR]

The activation of CO on iron-based materials is a key elementary reaction for many chemical processes. We investigate CO adsorption and dissociation on a series of Fe, Fe3C, Fe5C2, and Fe2C catalysts through density functional theory calculations. We detect dramatically different performances for CO adsorption and activation on diverse surfaces and sites. The activation of CO is dependent on the local coordination of the molecule to the surface and on the bulk phase of the underlying catalyst. The bulk properties and the different local bonding environments lead to varying interactions between the adsorbed CO and the surface and thus yielding different activation levels of the C–O bond. We also examine the prediction of CO adsorption on different types of Fe-based catalysts by machine learning through linear regression models. We combine the features originating from surfaces and bulk phases to enhance the prediction of the activation energies and perform eight different linear regressions utilizing the feature engineering of polynomial representations. Among them, a ridge linear regression model with 2nd-degree polynomial feature generation predicted the best CO activation energy with a mean absolute error of 0.269 eV. [ABSTRACT FROM AUTHOR]

FeCx catalysts (Fe-CN-Py) were synthesized by co-pyrolyzing the mixture of iron nitrate and a CN source (melamine, bulk g-C3N4 (b-C3N4), or defective g-C3N4 (d-C3N4)). The physicochemical properties of Fe-CN-Py catalysts and their activities of CO2 hydrogenation to light hydrocarbon (C2-C6) were analyzed. The results indicated that Fe-d-C3N4-(0.3)-Py is the most promising with the highest CO2 conversion (47.2%), olefin yield (10.8%), and olefin space-time yield (STY = 4.5 µmol olefin/s/gFe). The promising activity of Fe-d-C3N4-(0.3)-Py was attributed to its high concentration of surface FeCx. The correlation between surface FeCx and the STY of hydrocarbons and olefins was established. [ABSTRACT FROM AUTHOR]

FeCx catalysts (Fe-CN-Py) were synthesized by co-pyrolyzing the mixture of iron nitrate and a CN source (melamine, bulk g-C3N4 (b-C3N4), or defective g-C3N4 (d-C3N4)). The physicochemical properties of Fe-CN-Py catalysts and their activities of CO2 hydrogenation to light hydrocarbon (C2-C6) were analyzed. The results indicated that Fe-d-C3N4-(0.3)-Py is the most promising with the highest CO2 conversion (47.2%), olefin yield (10.8%), and olefin space-time yield (STY = 4.5 µmol olefin/s/gFe). The promising activity of Fe-d-C3N4-(0.3)-Py was attributed to its high concentration of surface FeCx. The correlation between surface FeCx and the STY of hydrocarbons and olefins was established. [ABSTRACT FROM AUTHOR]

The right part of the Fe3C–100% C diagram has a certain incompleteness and fragmentation of the phase structure, which is associated with the impossibility of obtaining and studying alloys containing more than 5.0% C using known metallurgical smelting technologies under normal conditions. In this paper, three groups of problems are considered in the study of the right side of the Fe3C–100% C diagram. The first group of problems includes the absence of a sufficiently voluminous line of iron carbides on the diagram: FeC, Fe2C (except for Fe3C), Fe3C2, Fe4C, Fe5C2, Fe6C, Fe7C3, Fe8C, Fe20C9, Fe23C6. From this group of carbides, in addition to cementite θ-Fe3C, only three significant types of carbides are objectively distinguished: ε-carbide Fe2C, Hegg carbide χ-Fe5C2 and Ekström–Adcock carbide æ–Fe7C3, located to the right of the cementite line in chemical composition and not displayed on the Fe3C–100% C state diagram as full phases or diagram components, which are considered by default to be metastable, transitional phases. The absence of these carbides on the Fe3C–100% C diagram is explained by the fact that they cannot be obtained by metallurgical methods through the iron-graphite alloying and, therefore, their complete physicochemical and metallographic study can not be carried out with the subsequent introduction of the corresponding phase equilibrium fields into the Fe3C–100% C state diagram. The second group of problems is the fact that the "liquid–vapor" phase equilibrium area of the Fe–100% C diagram is currently not scientifically convincing, has not been studied, and no attempts have been made to study it. The third group of problems is that carbon as the main component of Fe–100% C alloys is currently considered as a pure graphite chemical in general with a "crystalline hexagonal layered lattice". It has been established that the structure of graphite inclusions in gray and high-strength cast iron does not correspond to an ideal hexagonal layered graphite lattice, but to a turbostratic, chaotic one. It is also shown that when assessing the graphite crystallization, the cast iron melt should be considered as a single-phase system in the form of a carbon-iron polymer, the structural basic elements of which are fullerenes and carbon nanoparticles based on them. The use of carbon allotropes to explain the problems of graphite structure formation is considered. It is proposed to designate the existing graphite line on the Fe3C–100% C diagram as a line of atomic carbon—Catom. [ABSTRACT FROM AUTHOR]

The effect of the action of temperature on the structure and phase composition of an Fe3C/Fe7C3/P-phase/Cam composite obtained by mechanosynthesis of Fe–75 at % C has been studied by the methods of X‑ray diffraction, Mössbauer spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. It is shown that the structural and phase changes at heating have a multistage character. In the temperature range 315–400°C crystallization of the paramagnetic P-phase with generation of Fe3C and/or Fe7C3 occur. In the course of heating to higher temperatures, complete decomposition of carbide Fe7C3 (in the range 450–550°C) and partial decomposition of Fe3C (at 600°C and above) are observed. After cooling from 800–1000°C the mechanocomposite consists of α-Fe, cementite Fe3C, and graphite. The phase transformations are accompanied by the processes of composite oxidation with the formation of Fe3O4 oxide and its subsequent reduction. The P-phase is a disorded amorphous carbide Fe1 – xCx, which is characterized by magnetic ordering at the temperature of liquid nitrogen. [ABSTRACT FROM AUTHOR]

Comparative studies of the iron–75fullerite and iron–75graphite composites obtained by the mechanosynthesis method have been performed by X-ray diffraction and Mössbauer spectroscopy. It has been shown that a carbon polymorph and its deformation stability predetermine the kinetics of formation for the phase composition of composites. In the case of disordering in the crystal structure of fullerite with the further destruction of fullerene molecules, iron–75fullerite powders are characterized by the same phase composition as for the iron–75graphite composite: amorphous carbon, Fe3C, Fe7C3, and the P-phase. [ABSTRACT FROM AUTHOR]

The main evolution stages of the phase diagram of Fe–C in the carbide region (on the right from the cementite line) is a recently discussed theoretical topic that has been considered. It has been determined that previously isolated ε-Fe2C, χ-Fe5C2, and g-Fe7C3 carbides (except for cementite Θ-Fe3C) are nonstoichiometric compounds. Thus, they are variable composition phases containing stoichiometric composition or second-type interstitial solid solutions based on daltonides and berthollides. It has been stated that the iron–cementite phase diagram can be identified as the iron–ε-Fe2C carbide phase diagram in the concentration range of 0–9.7% C. The following phase transformations are introduced to the diagram: reaction of nonvariant syntectic equilibrium of cementite Θ-Fe3C crystallization; reaction of peritectic equilibrium of carbide χ-Fe5C2 crystallization; low-temperature peritectoid transformation of carbide, at which solid solutions of ferrite and cementite form solid solution possessing broad homogeneity range based on ε-carbide Fe2C berthollide. It has been shown that the carbide phases represent a uniform isomorphous quasi-carbide solid solution, while the carbide phase crystallizes in the form of the carbide phase mixture as quasi-eutectics. [ABSTRACT FROM AUTHOR]

Experimental results and geological observations suggesting the existence of a deep hydrocarbon cycle are presented. During immersion hydrocarbons accumulated in the Earth's crust in the slab generally maintain stability to a depth of 50 km. With deeper immersion, the hydrocarbon fluids contact the surrounding ferrous minerals forming a mixture of iron hydride and iron carbide on the depths of 100–290 km. Iron carbide transported into the asthenosphere by convective flows can act as carbon donors and react with hydrogen donors presented in the asthenosphere and form an aqueous-hydrocarbon fluid. This fluid can migrate through deep faults into the Earth's crust and form oil and gas deposits. [ABSTRACT FROM AUTHOR]

N-doped iron carbide (N-doped Fe3C) nanomaterials have a wide range of applications. In this study, a hexadecyltrimethylammonium bromide (CTAB)-assisted two-step process was used to prepare N-doped Fe3C nanomaterials. The effect of CTAB addition on the phase, structure, and morphologies of the products has been investigated in detail based on the characterization results of X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectra (Raman), N2 adsorption–desorption, and elemental analysis. By tuning the amount of CTAB, N-doped Fe3C nanowires were prepared. The magnetic hysteresis loops at room temperature of the N-doped Fe3C nanomaterials show ferromagnetic characteristics. [ABSTRACT FROM AUTHOR]

Abstract—X-ray diffraction, Mössbauer and Raman spectroscopy have been used to study structural phase transformations during mechanosynthesis of Fe–С60/70 in toluene with 20 and 25 at % С60/70 fullerite. The quantitative carbon content determines the phase composition of mechanocomposites. (Fe3C)D forms at 20 at % С60/70, whereas Fe7C3 and a paramagnetic phase form at 25 at % С60/70 at the initial stage of mechanical alloying. Free carbon is in an amorphous state in the sample. The formation mechanism of carbides and a paramagnetic component is discussed. [ABSTRACT FROM AUTHOR]

The stability, electronic structure, magnetism and hardness of iron carbides in eight different space groups were investigated by the first-principles calculations. The electron-ion interactions were modelled using the Vanderbilt ultra-soft pseudopotentials (USPP) and norm-conserving pseudopotentials (NCPP). Generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formulation was used to describe the exchange-correlation energy. The calculated lattice parameters for USPP were in good agreement with experimental results. The formation enthalpy, cohesive energy and elastic constants revealed that with the exception of hex-FeC, all Fe-C compounds were both thermodynamically and mechanically stable. Meanwhile, the iron carbides' bulk and shear moduli were also calculated and compared. Furthermore, magnetic properties were studied and it was found that the average magnetic moment of Fe C (2.123 μ ) was the largest among these compounds, with a value close to that of pure α-Fe (2.24 μ μ. As expected, hex-FeC is a paramagnetic metal based on the obtained densities of states and partial densities of states. Furthermore, the magnetic behaviour of iron carbides with polymorphic transformation was almost identical. Finally, several semi-empirical methods were used for estimating the hardness of the iron carbides. [ABSTRACT FROM AUTHOR]

The kinetics of the formation of iron carbides during the activation of iron-coated catalyst for Fischer-Tropsch synthesis promoted by copper and potassium, and by carbon monoxide and syngas, is studied. It is established that the presence of copper lowers the initial temperature of hematite reduction to magnetite and leads to the formation of carbide in both CO and СО/Н. Potassium slows the rate of magnetite formation, but it accelerates the formation of iron oxide. It is shown that the rate of carbide formation during magnetite reduction for catalysts is half that in the reaction of hematite reduction to magnetite in both CO and СО/Н. [ABSTRACT FROM AUTHOR]

The paper presents results of experiments aimed at diamond synthesis in the Fe-C-S system at 5.3-5.5 GPa and temperatures of 1300-1370°C and detailed data on the microtextures of the experimental samples and the composition of the accompanying phases (FeC and FeC carbides, graphite, and FeS). It is demonstrated that diamond can be synthesized after temperatures at which carbides are formed are overcome and can crystallize within the temperature range of 1300°C (temperature of the peritectic reaction melt + diamond = FeC) to 1370°C (of thermodynamically stable graphite) under the appearance experimental pressure. The possible involvement of natural metal- and sulfur-bearing compounds in the origin of natural diamond is discussed. [ABSTRACT FROM AUTHOR]

Methods of X-ray diffraction, differential thermal analysis, and measurements of the dynamic magnetic susceptibility have been used to investigate the sequence of phase transformations upon the mechanical alloying of a mixture of powders of the initial components of the composition (FeCr)C. It has been shown that, at later stages of mechanical alloying, the phase composition is determined by the conditions of the dynamic equilibrium between the crystalline and amorphous phases. A change in the conditions of mechanical alloying leads to a shift in this equilibrium and to a change in the phase composition of the alloy. A comparison of carbide formation in the Fe-C system upon the mechanosynthesis, tempering of martensite, the saturation of iron with carbon from the gaseous medium, the quenching of the melt, and the sputtering deposition of films has been performed. Some general regularities have been established, from which it follows that an important role in phase formation upon the mechanosynthesis, just as in other abovementioned processes, is played by the thermally activated phenomena. [ABSTRACT FROM AUTHOR]

Methods of differential thermal analysis and Mössbauer spectroscopy (Fe) have been used to study the process of the formation of cementite in α-Fe upon the low-temperature mechanosynthesis ( T < 375 K) in the medium of liquid hydrocarbons. It has been established that this process occurs in the absence of austenite and corresponds to a two-stage model suggested previously for describing the mechanism of the decomposition of the quench martensite α″-(Fe)C with the precipitation of the θ phase in the process of aging or tempering. Upon dilatometric studies of the single-phase samples of cementite in the range of 750 ≤ T ≤ 925 K, a significant increase was revealed in the linear elongation Δ l/ l and linear thermal expansion coefficient (LTEC). It has been assumed that this increase is not connected with the localization of carbon atoms in the positions C(4 a) and C(4 b), but rather is determined by the anomalously high concentration of equilibrium carbon vacancies V in the unit cell of cementite. The concentration of this type of vacancies can be sufficient for the growth of a graphite component of the carbon layer on the surface of the particles of the mechanosynthesized cementite (θ phase). [ABSTRACT FROM AUTHOR]

Methods of thermomagnetic analysis (TMA) and Mössbauer spectrometry (Fe) have been used to study the processes of the carburizing of α-Fe under the conditions of mechanical milling in a medium of liquid hydrocarbons. It has been established that, under the chosen conditions of the mechanical synthesis of carbides, the process of carbonization at T < 375 K occurs through the decomposition of the deformation-induced martensite, i.e., the supersaturated bct solid solution α″-Fe(C) with the formation of transitional hcp ε and ε′ phases that precede the formation of cementite. The milling of the metallic iron in the toluene medium substantially enhances the catalytic capability of disperse powders of α-Fe in the process of conversion of cyclic structures of hydrocarbons into other chemical forms. The increase in the dispersity of the iron powder to a nanocrystalline state leads to an increase in the chemical activity of carbon and an increase in the rate of diffusion sufficient for the formation in the Fe-C mixture of both primary cementite (θ′) with an anomalously low Curie temperature T(θ′)(first stage) and secondary cementite (θ″) at the second stage of mechanosynthesis. The parameters of hyperfine interactions have been calculated for a number of synthesized carbides. It has been shown that the change in the carbon concentration in iron carbides is determined by the following inequality: c(θ′) > c(ε) > c(ε′). The boundary of the temperature stability of cementite has been established. The effect of the decomposition of the θ phase (FeC) upon thermal cycling θ ⇔ γ in the temperature range of 300 < T < 1075 K has been revealed. Based on the results obtained, a scheme of the sequence of phase transformations that occur in the Fe-C system under the conditions of low-temperature mechanosynthesis has been derived. [ABSTRACT FROM AUTHOR]

Magnetic properties and hyperfine interactions in the carbide FeC prepared by mechanical synthesis (MS) of α-Fe in toluene have been studied. It has been found that the boundary of temperature stability of the MS carbide does not exceed 775 K. The Curie temperature of the synthesized phase varies between 509 and 525 K. The Mössbauer experiments (Fe) have demonstrated that the unit cell of FeC contains 32 iron atoms distributed over five types of crystallographic positions with a multiplicity of 4: 8: 8: 8: 4. The hyperfine interaction parameters have been calculated for each nonequivalent position. It may be conjectured from the specific features of the decomposition at T ≥ 775 K that the most probable sequence of the phase transitions in the MS carbides Fe-C is → θ, where -Adcock carbide FeC. [ABSTRACT FROM AUTHOR]

Thermal treatment of amorphous iron(III) oxide nanopowder (2–3 nm) in various atmospheres is presented as a simple route towards α-Fe and iron carbides nanoparticles. The chemical composition and size of nanoparticles can be controlled using the reaction temperature, annealing time and atmosphere (hydrogen, carbon monoxide, hydrocarbons). To monitor the structure, phase composition, size, morphology and magnetic properties of nanoparticles, XRD, Mössbauer spectroscopy, and magnetic measurements were carried out. The measurements of temperature dependence of magnetic moment yield information on the critical temperatures of magnetic and phase transformations. Application of hydrogen atmosphere results in the formation of α-Fe, while iron carbides (∼Fe3C) can be obtained in atmospheres of carbon monoxide or hydrocarbons. [ABSTRACT FROM AUTHOR]

Thermal treatment of amorphous iron(III) oxide nanopowder (2–3 nm) in various atmospheres is presented as a simple route towards α-Fe and iron carbides nanoparticles. The chemical composition and size of nanoparticles can be controlled using the reaction temperature, annealing time and atmosphere (hydrogen, carbon monoxide, hydrocarbons). To monitor the structure, phase composition, size, morphology and magnetic properties of nanoparticles, XRD, Mössbauer spectroscopy, and magnetic measurements were carried out. The measurements of temperature dependence of magnetic moment yield information on the critical temperatures of magnetic and phase transformations. Application of hydrogen atmosphere results in the formation of α-Fe, while iron carbides (∼Fe3C) can be obtained in atmospheres of carbon monoxide or hydrocarbons. [ABSTRACT FROM AUTHOR]

Thin films are playing a more and more important role for technological applications and there are many aspects of materials surface processing and thin film production, ranging from simple heat treatments to ion implantation or laser surface treatments. These methods are often very complicated, involving many basic processes and they have to be optimized for the desired application. Nuclear methods, especially Mössbauer spectroscopy, can be successfully applied for this task and some examples will be presented for laser-beam and ion-beam based processes. [ABSTRACT FROM AUTHOR]

Slurry phase Fischer-Tropsch synthesis was conducted with an ultrafine iron oxide catalyst promoted with either 0.5 at% K or 1.0 at% Zr or both. Pretreatment in CO yielded higher conversions and a more stable catalyst than activation in hydrogen or synthesis gas. Hydrogen pretreatment of K promoted catalysts and synthesis gas activation in general were less effective. Mössbauer spectroscopy and XRD showed-FeC and ε′-FeC were formed during pretreatment in CO and did not depend on promoters present. Catalysts pretreated in H2 were reduced to metallic Fe and FeO; promotion with K and Zr decreased the extent of reduction. Hydrogen pretreated catalysts, promoted with K, lost surface area and carbided rapidly under synthesis conditions. Activation in synthesis gas reduced all catalysts to FeO. Subsequent synthesis did not affect the phase present for the unpromoted and Zr promoted catalysts while those promoted with K formed χ-FeC and ε′-FeC. It is concluded that pretreatment type is more important to the catalyst activity during the early period of synthesis than the impact of promotion with K and/or Zr and that changes in the bulk composition of iron catalysts do not necessarily correlate with changes in activity. [ABSTRACT FROM AUTHOR]