Concentration Of Fe Atoms Research Articles

Intergranular fracture induced by helium bubbles segregation in NiFe single-phase concentrated solid solution alloys

Single-phase concentrated solid solution alloys (SP-CSAs) are potential nuclear structural materials with excellent irradiation resistance. Under irradiation environment, helium (He) atoms are easily segregated and stored at grain boundaries (GBs), thus causing He embrittlement of materials. The intergranular fracture behavior of NiFe SP-CSAs caused by He bubbles segregation is studied with molecular dynamics (MD) method. The effects of Fe atom concentration, GB type and He atom number in He bubbles on the intergranular fracture are analyzed. Since the disordered atoms impede crack initiation and propagation, NiFe SP-CSAs exhibit stronger fracture resistance than pure Ni. With the increase of strain, the number of disordered atoms at the GB increases further, so that the fracture strain of NiFe SP-CSAs with symmetric tilt GB decreases. With the increase of Fe atom concentration, Fe atoms inhibit the intergranular crack initiation and propagation in NiFe SP-CSAs more powerfully. In addition, the increase of the He atom number in the He bubble promotes the fracture. The formation of SP-CSAs is also beneficial to enhancing intergranular fracture resistance of alloys with twist GBs and asymmetrically tilt GBs, and the impact on the latter is more pronounced.

Impact of Fe-doped H2/O2 flame equivalence ratio on the fate and temperature history of early particles

The temperature and species concentration history experienced by the gas-borne nanoparticles during their evolution in the flame has a major impact on their size, morphology, composition, and crystallinity. In our recent work (Combust. Flame, 244 (2022) 112251), we have reported optical emission measurements of a Fe(CO)5-doped H2/O2/Ar fuel-lean (ɸ = 0.5) flame, revealing that the temperature of the early-formed nanoparticles exceeds the gas temperature by several hundred degrees, while the particle volume fraction increases sharply, followed by rapid disintegration in the reaction zone. This behavior, modeled by single particle Monte-Carlo simulations indicates involvement of heterogeneous reactive processes at the particle surface, such as particle reduction and oxidation, growth and etching. Within the refined approach of the current study, reactive and non-reactive collisions were treated consistently, assuming rapid thermalization between the impinging molecule and the particle, with subsequent random energy sampling to determine reactivity. In the present work, we test the limits and validity of the heterogeneous flame-particle interaction model by manipulating the oxidation–reduction and growth-etching balance by varying the equivalence ratio (0.25<ɸ<1.5). For the entire range of equivalence ratios studied in experiments and simulations, we find a deviation between the particle and gas phase temperatures with significantly higher particle temperature, which is continued until a full degree of iron oxidation within the particle (O/Fe ratio=3/2) is reached. Validating the simulations against the measurements of particle temperature and volume fraction over a wide range of equivalence ratios, emphasized the necessity to account for gas-phase Fe-atom concentration depletion. We incorporated nucleation theory to estimate initial cluster population, linking Fe-concentration variation in the gas phase and the stochastic particle evolution model. The surface reaction parameters in our current work were updated using density functional theory literature data, and validation of the model predictions against experimental data, across the entire range of equivalence ratios.

Interactions between displacement cascades and grain boundaries in NiFe single-phase concentrated solid solution alloys

Single-phase concentrated solid solution alloys (SP-CSAs) are promising structural materials with excellent irradiation resistance. It is very important for their application in nuclear reactor structures to understand the interactions between their grain boundaries (GBs) and irradiation-induced defects. With molecular dynamics (MD) simulations, the displacement cascades in NiFe SP-CSAs with Σ5(210) symmetrical tilt GB were studied. Compared with pure Ni, NiFe has higher sink strength of GB for vacancies but has little effect on the interstitial absorption of GB. The formation energies of interstitials and vacancies near the GB are more dispersed, so that the point defects are more difficult to aggregate, which is beneficial for the recombination of interstitials and vacancies near the GB. Both the formation energies of point defects and the sink strengths of GB for point defects could be changed with the SP-CSA compositions. When the Fe atom concentration is higher than 30 at.%, the point defects are easier to annihilate. Both the effects of the PKA energy and the PKA-GB distance on the displacement cascades damage can be explained with the size of overlapping region between the cascade region and GB. In addition, the formation of SP-CSAs helps to annihilate the vacancies in NiFe with Σ37(750) symmetrical tilt GB and Σ5 twist GB.

Investigation on Solidification in Cu-20wt%Fe Alloy through In Situ Observation

The performance of Cu-Fe alloy is related to the solidification structure, which is directly determined by the microstructure evolution during solidification. The solidification sequence, solid–liquid interface variation, and microstructural evolution of Cu-20wt%Fe alloy at three cooling rates (0.3, 1.5, and 5.0 °C/s) were investigated. The results indicate that the remelting of primary γ-Fe dendrites was directly observed through the solidification experiment, and the partial γ-Fe dendrite was fragmented owing to remelting. The Fe phase morphology changed from the cellular structure to the typical finer and longer dendrite structure with the cooling rate increasing. As the cooling rate increased, the constitutional undercooling caused by the decrease in the Fe atom concentration and the increase in the Cu atom concentration increased in the solidifying interface. There was a parabolic relationship between the growth rate of the dendrite tip and time. Meanwhile, the growth of the primary γ-Fe phase was inhibited by the insufficient diffusion of Fe and Cu at the solidification front, which resulted in a decrease in the Fe phase volume fraction, and the Fe content in the Fe dendritic phase decreased slightly.

Effect of substitution of Mn and Ga atoms by Fe atom in the Mn2GaC MAX phase

The magnetic properties of ordered and disordered MAX-phase Mn2-xFe2xGaC and Mn2Ga1-xFexC (x = 12.5, 25, and 50 at.%) have been studied within DFT-GGA. The investigation of phase stability of M2AX phases is performed by comparing the total energy of MAX phases to that of the set of competitive phases for calculation of the phase formation enthalpy. At the small concentration of Fe atoms (x = 12.5 %) compound remains stable. We have found that introducing Fe atom at A-site leads to the forming of ferromagnetic phase with large magnetic moments on magnetic atoms and magnetization. Through detailed group-theoretical analysis we have obtained that only ferromagnetic ordering is possible when Fe atom ordering over Ga sites. The study of exchange constants shows that the out-of-plane Fe-Mn exchange gives the main contribution in appearance of ferromagnetic phase. The temperature dependences of magnetization reveal the increase of Curie temperature in Mn2GaC with Fe atom incorporated into Ga-site.

Dynamic interactions between low-angle grain boundary and stacking fault tetrahedron in Ni-Fe solid solution alloys

A stacking fault tetrahedron (SFT) is a very common kind of defects in face-centered cubic (FCC) metals under irradiation environment. The migrating GB can heal the SFT through dislocation reactions. To explore the healing mechanisms and conditions required, the low-angle GB interactions with an SFT in pure Ni and Ni-Fe bicrystals were investigated using molecular dynamics (MD) simulations. Ni-Fe bicrystals have higher healing efficiency on SFT than pure Ni due to their more diverse dislocation reactions. The addition of Fe atom can reduce the stacking fault energy (SFE), so that most of stair-rod dislocations (SRDs) of SFT in Ni-Fe bicrystals can be transformed into Shockley partial dislocations (SPDs) through the energetically unfavorable reactions. As a result, the vacancies of SFT are redistributed and some of them are carried away by migrating GBs. In pure Ni, however, only a small part of SRDs can undergo the energetically unfavorable reactions, and the SPDs generated would spontaneously be transformed back to SRDs after the GB-SFT interaction. The healing efficiency on SFT can be gradually enhanced with the increasing Fe atom concentration and elevated temperature. • Dislocation reactions of low-angle GB and SFT in Ni-Fe are more diverse than in Ni. • Stair-rod dislocations of SFT can be converted into Shockley partial dislocations. • The vacancies of SFT are redistributed and carried away by migrating GB in Ni-Fe. • The SFT absorbed by low-angle GB is regenerated after GB-SFT interaction in Ni. • The healing efficiency on SFT can be enhanced with increasing Fe atom concentration.

B1−B2 phase transition of ferropericlase at planetary interior conditions

Using ab initio simulations based on density functional theory, we have analyzed the crystal structure and thermodynamic stability of ${\mathrm{Mg}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{O}$ ferropericlase, showing how the $P\text{\ensuremath{-}}T$ phase diagram associated with the $B1\text{\ensuremath{-}}B2$ phase transition of pure MgO is influenced by the presence of iron substitutional alloys. We find that a small concentration of Fe atoms contribute to an increase of the transition pressure at fixed temperature, extending the stability of $B1$ crystalline structure. Moreover, we find a significant nonhomogeneous distribution of the iron atoms between the two phases at low temperatures, with strong partitioning in the $B1$ phase, an interesting phenomena that could lead to important dynamic consequences. Finally, we analyze the effect of the iron impurities on the volume thermal expansion.

Диаграммы Эвери, Блэкмана, давление Коши и анализ упругих свойств кристаллической решетки сплавов Ti49Ni51 (ат. %) и TiNi – TiFe с мартенситными превращениями и без них

In this paper, to analyze the elastic properties of Ti49Ni51 (at. %) and TiNi – TiFe crystals with and without martensitic transformations Every’s and Blackman’s diagrams are used as two ways to characterize the elastic properties of cubic materials. Paired correlations between the parameters (s2, s3), (s3, s1) и (F44, F12) are calculated. The evolution of the Cauchy pressure pC value depending on the content of iron atoms in the crystal lattice of alloys is considered. It is established that, regardless of the composition of the alloys, the Cauchy pressure is greater than zero and decreases almost monotonically from 100.5 to 35.0 GPa with an increase in the content of Fe atoms in the B2 crystal lattice of the TiNi – TiFe phase and an increase in its stability with respect to martensitic transformations (up to complete stabilization). This evolution of pC corresponds to the data according to which a significant component of the bonding forces in TiNi is a metallic bond, and with an increase in the concentration of Fe atoms in alloys instead of Ni atoms and approaching the composition of TiFe, the proportion of the covalent component of the bonding forces increases.

Multiple magnetic states and irreversibilities in the FexTiS2 system

The structural and concentration conditionality of magnetic states and transport properties of layered chalcogenide compounds ${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.75$) have been studied by means of x-ray and neutron diffraction, magnetization, electrical resistivity, and magnetoresistance measurements performed on polycrystalline samples synthesized by solid-phase reaction method with prolonged homogenization heat treatment. It has been revealed that various magnetic states [spin-cluster glass state at $x&lt;0.25$, antiferromagnetic (AFM) order at $x\ensuremath{\approx}0.25--0.28$, cluster glass state at $x\ensuremath{\approx}0.33$, AFM at $x\ensuremath{\approx}0.45--0.5$, and ferrimagnetic ordering above $x=0.5$] are realized in the $\mathrm{Fe}{}_{x}\mathrm{TiS}{}_{2}$ system with increasing Fe content. The absence of a long-range magnetic order in ${\mathrm{Fe}}_{0.33}{\mathrm{TiS}}_{2}$ is confirmed by neutron diffraction measurements. The magnetic states observed in this system are closely related not only to the concentration of Fe atoms, but also, to a greater extent, to their distribution over the cation layers. It is assumed that the single crystals obtained by chemical vapor deposition are practically unique objects with only their inherent properties, which leads to inconsistency between the literature data on the magnetic properties of these compounds. Changes in the magnetic state of ${\mathrm{Fe}}_{x}{\mathrm{TiS}}_{2}$ with Fe concentration are accompanied by nonmonotonic dependencies of the magnetoresistance and coercivity with maximal absolute values in compounds with the AFM virgin state. The ferrimagnetic order in compounds with $x&gt;0.5$ is suggested to originate in Fe-Ti mixing in cationic layers. The AFM ordered compounds undergo the field-induced phase transitions to the metastable high-coercive ferromagnetic (FM) state, which is accompanied by a large remnant magnetoresistance. The enhanced coercivity observed in compounds with the AFM virgin state is ascribed to the intrinsic exchange bias together with the Ising-type spin state of Fe ions.

The interplay between damage- and chemical-induced isolation mechanism in Fe+-implanted AlGaN/GaN HEMT structures

This work presents the development of Fe ion implantation processes for the fabrication of thermally stable isolation of AlGaN/GaN high electron mobility transistors (HEMTs). Experiments, carried out by the triple Fe+ implantation into AlGaN/GaN HEMT epilayers on sapphire substrates, showed that it is possible to fabricate device isolation that is stable up to the temperatures typical for the formation of ohmic contacts to the AlGaN/GaN heterostructure. Thermal stability of the formed isolating regions was tested by electrical measurements on cTLM test structures, in a wide range of annealing temperatures (400–1100 °C) and concentration of Fe atoms of 1×1018 cm−3 and 1×1019 cm−3. Sheet resistance values over 1×1015Ω\\□ have been achieved. The obtained highly resistive isolation was thermally stable up to the temperature of 1100 °C. Moreover, after annealing at the temperature of 1000 °C and 1100 °C the sheet resistance of implanted regions was almost an order of magnitude larger than previously presented in the literature. Due to the mastering of the technology of making thermally stable isolation through Fe ion implantation, it will be possible to raise the thermal budget and increase the flexibility in the design of individual steps in the fabrication process of AlGaN/GaN HEMTs.

The Degree‐of‐Order Dependent Electronic Structures and Magnetic Properties of Fe3Si Alloys

The effect of ordering degree on the electronic structures and magnetism of Fe3Si alloys are investigated scientifically using first‐principles calculations based on plane‐wave pseudopotential theory. The studied results of the heat of formation and cohesive energy for D03 and B2 structures of Fe3Si reveal that the order of structural stability from high to low is B2‐9, B2‐8, B2‐7, B2‐6, D03/B2‐1, B2‐2, B2‐3, B2‐4, and B2‐5. Both D03 and B2 structures of Fe3Si exhibit the metallic feature. A wider breadth of the pseudogap with occupation of Si sites by more Fe[B] atoms except B2‐5 implies that the hybridization and covalent bond are intensified. Meanwhile, with substitution of Fe[B] atoms by more Si atoms, the stability of the system improves because the Fermi level is nearer to the bottom of the pseudogap. The origin of ferromagnetism for ordered Fe3Si except B2‐9 is mainly from the Fe 3d spin polarization. Compared with D03 structure of Fe3Si, an increment in the concentration of Fe atoms with occupation of Si sites by more Fe[B] atoms causes the change of interaction between Fe and Si atoms and the total magnetic moment increases. On the contrary, the total magnetic moment decreases with gradual substitution of Fe[B] atoms by Si atoms.

MgFe and Mg–Co–Fe mixed oxides derived from hydrotalcites: Highly efficient catalysts for COx free hydrogen production from NH3

Mg–Fe Layered Double Hydroxide (LDH) with M2+: M3+ 3:1 stoichiometric ratio was synthesized and employed as catalyst precursor for COx-free hydrogen production from ammonia. The resulting catalyst showed good catalytic activity. A series of Mg/Co–Fe layered double hydroxides were synthesized by replacing Mg2+ with Co2+ without disturbing M2+:M3+ ratio. The influence of nature and extent of Co(II) substitution on structure, morphology and surface properties were studied. A systematic study was carried out using these materials as catalyst precursors for ammonia decomposition. BET, XRD, TPR, XPS, CO2-TPD and TEM techniques were used to characterize the synthesized catalysts. These Fe-based catalysts are highly active, highly stable and not promoting any stable surface nitridation during the ammonia decomposition reaction. Among all catalysts, the Mg3Co3Fe2 catalyst showed the highest activity i.e. 100% conversion at 6,000 h−1 and 60% at 50,000 h−1 space velocities at 550 °C. The registered superior catalytic activity was result of the formed specific catalyst's properties like high surface area, high surface Co and Fe atomic concentration and suitable basicity. These Fe-based materials are, cost-effective, easily synthesize and highly stable, thus attractive for large-scale operation.

Study of transport properties in Se-deficient and Fe-intercalated NbSe2 single crystals: experiment and theory

In this study, the magnetoresistance measurements of Se-deficient (i.e., NbSe1.85) as well as Fe-incorporated NbSe2 (Fe0.0015NbSe2) were performed to observe the effect of both intrinsic and extrinsic defect in the thermally activated flux flow region (TAFF) of NbSe2. In TAFF region, NbSe1.85 shows nonlinear response of thermal activation energy (TAE) with temperature following the modified TAFF method. For NbSe2 and Fe0.0015NbSe2, TAE depends linearly on temperature and hence was evaluated using Arrhenius relation. NbSe1.85 can be considered as the 2D-like system in the TAFF region. The magnetic field dependence of TAE shows parabolic nature in Fe0.0015NbSe2 in contrast to the power-law dependence of TAE in NbSe1.85. The power-law dependence of TAE in NbSe1.85 indicates the plastic deformation flux lines. The parabolic dependence indicates the elastic deformation of flux lines in pure as well as in Fe0.0015NbSe2. The band structures and density of states (DOS) of the above mentioned two cases were calculated using first-principle density functional theory. The number of bands and the DOS at the Fermi level decreases remarkably for both Se vacancy and Fe doping cases, indicating to the degradation of superconductivity. A peak shift in the partial density of state of Nb was observed at the Fermi level of Fe0.0015NbSe2. Spin-polarized optimization of first-principle calculations implies large Fe–Se overlaps and contradicts the Kondo mechanism due to the low concentration of Fe atoms. The spin polarization calculation indicates the negligible effect of magnetism of Fe atoms in Fe0.0015NbSe2.

Effect of Fe content on atomic and electronic structure of complex oxides Sr(Ti,Fe)O3 − δ

We present a study of the electronic and atomic structure of two series of SrTi1−xFexO3−δ (STFO) powders with different Fe content produced by two different methods, spray pyrolysis or modified Pechini synthesis, by means of soft X-ray absorption spectroscopy. Partial substitution of Ti by Fe atoms in SrTiO3 were found to cause asymmetric distortion of TiO6 octahedrons, which increases with increasing Fe content that may violate the cubic symmetry of STFO. The presence mainly of Fe3+states in octahedral environment at small concentration of Fe atoms along with essentially smaller content of Fe4+ states in octahedral environment where the latter contribution increases with increasing Fe content was traced. It is found that the modified Pechini method allows to synthesize more stable structures but a tendency of the SrOx formation in the structure prepared by this technique was marked. The spray pyrolysis method gives the structure free of SrO precipitates but the presence of Fe3+ states in tetrahedral environment with Fe content higher than 50% and even a certain amount of Fe2+ ions in an octahedral environment at concentrations higher than 75% in the STFO prepared by this method was established. The O1s (K)-absorption spectra point to increase in oxygen vacancy concentration with increasing Fe content. The lowest degree of structure distortions with enough high oxygen vacancy concentration was traced in STFO (x=0.25 to x=0.35) produced by modified Pechini synthesis, which makes it mostly appropriate for technical applications, e.g., as gas sensors, oxygen separation membranes or as fuel cell material.

Effect of Fe Doping on Magnetic Properties and Lattice Structure of Ferromagnetic EuO

The structural, electronic, and magnetic properties of Fe-doped EuO (Eu0.75Fe0.25O, Eu0.50Fe0.50O, and Eu0.25Fe0.75O) in the rock salt ferromagnetic phase have been studied. Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) was used for structural analysis of these compounds. The lattice constant linearly decreased with increase in doping concentration of Fe atoms. Spin-polarized calculations were carried out using the modified Becke-Johnson exchange potential with Hubbard U correction (mBJ + U method). Semiconductor EuO showed a metallic character when doped with Fe atoms. The calculated density of states (DOS) of Eu0.75Fe0.25O, Eu0.50Fe0.50O, and Eu0.25Fe0.75O exhibit extra peaks of Fe atom 3 d states in the valance band between −10 and 3.5 eV. With an increase in Fe doping, the band gap was decreased due to an increase in spd exchange interaction between the band electrons and localized 3d electrons of transition atoms. Moreover, the properties of the doped EuO were found to be dependent on the location of dopant atoms.

Tunable electronic structure of monolayer semiconductor g-C2N by adsorbing transition metals: A first-principles study

The electronic structure of transition metal (TM) adsorbed two-dimensional g-C2N is investigated by first-principles calculations. Most of TM-adsorbed g-C2N show metallic properties, but Ni and Zn adsorptions show a band gap of 0.6 and 1.6 eV. The maximum spin splitting of TM-adsorbed g-C2N from Sc to Zn is 110, 186, 117, 100, 0, 515, 147, 64, 95 and 0 meV, respectively. The magnetic moments of adsorbed structure from Sc to Co are in the range from 0.15 to 2.11 μB, while in Ni, Cu and Zn adsorbed cases no magnetic moments appear. In Fe-adsorbed g-C2N, the adsorption structure at site of Fe12 generates a 90 meV band gap, where the adsorption sites at CC and CN rings are marked as Fe1 and Fe2. The maximum spin splitting is 515, 412 and 750 meV as Fe atomic concentration is 5.56%, 11.11% and 16.67%. Our results can bring more significant basis on the design of spintronic and valleytronic devices.

Magnetic field induced precipitation behaviour of (Fe,Mo)6C alloy carbides in molybdenum alloyed steel

Three kinds of alloy carbides Fe6-xMoxC were precipitated during the transformation of austenite-to-bainite in molybdenum alloyed steel under the conditions of 12-T magnetic field and conventional treatment. The magnetic field induced Fe atom concentration in Fe6-xMoxC carbides was increased. The higher Fe atom content among three carbides (Fe2Mo4C, Fe3Mo3C and Fe4Mo2C) makes magnetic Gibbs free energy decreased remarkably. The measured aspect ratio of (Fe,Mo)6C carbides was ranged from 1∶1·16 to 1∶1·25, which was consistent with the calculated result according to the minimum energy principle. It was suggested that the high magnetic field had no visible influence on the morphology of (Fe,Mo)6C carbides.

Investigation of composition-induced strain effect in FexPt1−x films grown on different substrates

Different composition FexPt1−x films were fabricated on three typical single crystal substrates (MgO, KTaO3, and SrTiO3), and the composition-induced strain evolution of FexPt1−x films was systematically investigated. The study showed that different Fe compositions in Fe–Pt films resulted in different strain status and crystallographic textures, and thus influenced the magnetic properties. Under the tensile strain between the Fe–Pt and (MgO, KTaO3, and SrTiO3) substrates, Fe–Pt films preferred to form ordered Fe–Pt (001) texture. Decrease of the Fe atom concentration caused the Fe–Pt films to be further relaxed, and an obvious increase of lattice constant c. Moreover, with reducing the mismatch between Fe–Pt and substrate from MgO to KTaO3 and SrTiO3, the strain status of Fe–Pt films changed from completely strained to partially relaxed. The perpendicular anisotropy of Fe55Pt45 films grown on STO was larger than that grown on MgO and KTO, which was attributed to better epitaxial quality of the FePt (001) texture induced by less lattice mismatch.

The effect of high magnetic field on metal solute substitution in M23C6 alloy carbide

Alloy carbide precipitation during tempering at 200 °C in a 2.25Cr–Mo steel was investigated using transmission electron microscope. The high magnetic field promoted the precipitation of M23C6 alloy carbides, and also increased the concentration of Fe atoms in the carbides. Calculated results indicated that the magnetic moment of alloy carbides increases with increasing Fe atom content, which is consistent with the experimental results. The magnetic Gibbs free energy is reduced by increasing the Fe atom content in the M23C6 alloy carbide.

Crystallographic plane dependent Fe and Si dopant incorporation and activation in InP

AbstractVarious crystallographic planes are exposed when growing along mesa sidewalls during metalorganic vapor phase epitaxy. Dopant incorporation and activation on these higher index surfaces can be quite different from that observed on (001) surfaces. Incorporation and activation of Si and Fe in MOVPE grown InP on (001), (110), and (311)A and B surfaces are reported. The Si donor concentration is higher on the P‐terminated B surface than the A surface while the opposite behavior is observed for Fe incorporation. In the fabrication of optoelectronic devices, it must be remembered that it is the carrier concentration, that is, activated atomic species, that is relevant for device performance and not the incorporation of atomic species alone. Activation of Si is &gt;1 on all surfaces; however, Fe is shown to be an effective compensating species only on the (001) and (311)A crystallographic surfaces. Data for the presence of oscillations in the iron concentration on (001) surfaces are also reported. The active Fe concentration is shown to be related to the Fe atomic concentration relative to the donor density on the different crystallographic surfaces, suggesting that Fermi level position sets a limit to the fraction of Fe centers which are electrically active as a deep trap. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)