Iron Single Crystals Research Articles

Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods

A single crystal monolith of La 0.9Sr 0.1FeO 3 and thin pulsed laser deposited film of La 0.8Sr 0.2Fe 0.8Ni 0.2O 3 were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimens were not prepared in situ. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variation in the more surface representative valence band spectra near the Fermi energy. X-ray photoelectron diffraction images reflect the symmetry as expected from bulk iron perovskites. The correlation of spectral details in the valence band photoemission spectra (VB PES) and details of the conductivity during temperature variation suggest that valuable information on electronic structure and transport properties of complex materials may be obtained without in situ preparation.

In situ Raman spectroscopy and electrochemical techniques for studying corrosion and corrosion inhibition of iron in sodium chloride solutions

The corrosion of single crystal pure iron in 3.5% NaCl solutions and its inhibition by 3-amino-5-mercapto-1,2,4-triazole (AMTA) have been studied using in situ and ex situ Raman spectroscopy, cyclic voltammetry (CV), open-circuit potential (OCP), potentiodynamic polarization (PDP), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) measurements. CV experiments indicated that the iron electrode in the chloride solution alone showed an anodic peak at ∼−650 mV after the 5th cycle shifted to ∼−610 mV after the 20th cycle; another cathodic peak appeared at ∼−990 mV. In the presence of 1.0 mM AMTA, these two peaks shifted to ∼550 and −1050 mV, respectively. OCP, PDP, CA and EIS revealed that the presence of AMTA and the increase of its concentration move the corrosion potential to more positive values and decrease both the corrosion current and corrosion rate. This effect also increases with increasing the immersion time of iron electrode to 24 h in the test electrolyte. In situ and ex situ Raman investigations confirmed that the addition of AMTA molecules to the chloride solution strongly inhibits the iron corrosion through their adsorption onto the surface blocking its active sites and preventing its corrosion.

Stress and magnetic field-dependent Young's modulus in single crystal iron–gallium alloys

The variability in Young's modulus of single crystal iron–gallium (Galfenol) alloys having 16, 17.5, 19, 24.7 and 29 at% gallium is investigated using experiments and simulations. Some of these alloys showed more than 60% change in Young's modulus along the 〈1 0 0〉 directions on varying their magnetization and stress states compared to their modulus at magnetic saturation. A function, Δ E( σ, H), is defined such that the variability of modulus is bound between 0% and 100%. The observations are related to the inherent magnetomechanical coupling in the material. An energy-based non-linear constitutive model is used to predict the variable modulus in Galfenol as a continuous function of stress and magnetic field. Model predictions showed good correlation with experimental results.

Atomistic simulation of the bcc—hcp transition in iron driven by uniaxial strain

The bcc—hcp structural transition in single crystal iron under 〈001〉 uniaxial strain has been investigated by molecular dynamics simulation. The reversibility and the morphological characteristics are discussed. The stress history indicates a super-elastic deformation in the sample, while the change of temperature shows the heat release during both hcp and bcc nucleation. A laminated structure of bcc and hcp along {011} planes is obtained, where the phase boundaries for the bcc to hcp and hcp to bcc transition are found along the same plane, implying the memory effect of morphology. Stacking faults (fcc) can be formed at the interface between hcp nuclei. For the bcc to hcp transition, we observed the mergence of the stacking faults in an hcp grain and the position adjustment between hcp grains. No migration of stacking fault is found during the hcp to bcc transition. In addition, the bcc—hcp transition structure is analyzed by the radial distribution function.

Formation of envelope solitons of spin-wave packets propagating in thin-film magnon crystals

The formation of light envelope solitons has been experimentally observed under the pulsed excitation and propagation of radio-frequency spin-wave packets in a magnon crystal, a periodic magnetic film structure. The magnon crystal has been fabricated from a thin single-crystal yttrium iron garnet (YIG) film. The envelope solitons have been excited at frequencies corresponding to the edges of the Bragg-resonance-induced band gaps of the spin-wave spectrum of the magnon crystal. A theoretical explanation of the observed phenomenon has been proposed with the use of the numerical simulation of the formation of solitons based on the nonlinear Schrodinger equation.

One-pot low temperature solution synthesis, magnetic and microwave electromagnetic properties of single-crystal iron submicron cubes

Large quantities of single-crystal iron submicron cubes with controllable dimensions and surface structures were synthesized by a facile low-temperature solution reduction approach under normal atmosphere. The influence of kinetic parameters such as NaOH concentration, reaction temperature, reaction time, species of solvents and reducing agents, etc. on the morphology, size and crystal structure of the as-synthesized products were investigated in detail. The resultant products experience multistep phase transformations and morphology evolutions from Fe(OH)2 nanosheets, to weak crystalline Fe3O4 ultrafine particles and to high crystalline Fe cubes. It is believed that both the control over the quasi-equilibrium crystal nucleation and growth rate by adjusting kinetic parameters, and the selective interaction between ethylenediamine and various crystallographic planes of bcc iron play crucial roles in the formation of the slightly truncated cubes with {100} planes at the sides and small {111} planes on the corners. The as-synthesized iron cubes exhibit a strong and wide resonance behavior for the imaginary permittivity and imaginary permeability over the frequency range of 2–14 GHz as a result of the submicron size and anisotropy morphology. The paraffin-based composites containing 26 vol% iron submicron cubes show good electromagnetic wave absorbing characteristics and the reflection loss values less than −20 dB are obtained in the frequency range of 6.5–18.0 GHz when the thickness of the composites is 1.0–2.5 mm, suggesting that the iron submicron cubes synthesized here are promising as a strong-absorption, thin-thickness, light-weight and low-cost microwave absorber.

A dislocation density-based single crystal constitutive equation

Single crystal constitutive equations based on dislocation density (SCCE-D) were developed from Orowan’s strengthening equation and simple geometric relationships of the operating slip systems. The flow resistance on a slip plane was computed using the Burger’s vector, line direction, and density of the dislocations on all other slip planes, with no adjustable parameters. That is, the latent/self-hardening matrix was determined by the crystallography of the slip systems alone. The multiplication of dislocations on each slip system incorporated standard 3-parameter dislocation density evolution equations applied to each slip system independently; this is the only phenomenological aspect of the SCCE-D model. In contrast, the most widely used single crystal constitutive equations for texture analysis (SCCE-T) feature 4 or more adjustable parameters that are usually back-fit from a polycrystal flow curve. In order to compare the accuracy of the two approaches to reproduce single crystal behavior, tensile tests of single crystals oriented for single slip were simulated using crystal plasticity finite element modeling. Best-fit parameters (3 for SCCE-D, 4 for SCCE-T) were determined using either multiple or single slip stress–strain curves for copper and iron from the literature. Both approaches reproduced the data used for fitting accurately. Tensile tests of copper and iron single crystals oriented to favor the remaining combinations of slip systems were then simulated using each model (i.e. multiple slip cases for equations fit to single slip, and vice versa). In spite of fewer fit parameters, the SCCE-D predicted the flow stresses with a standard deviation of 14 MPa, less than one half that for the SCCE-T conventional equations: 31 MPa. Polycrystalline texture simulations were conducted to compare predictions of the two models. The predicted polycrystal flow curves differed considerably, but the differences in texture evolution were insensitive to the type of constitutive equations. The SCCE-D method provides an improved representation of single-crystal plastic response with fewer adjustable parameters, better accuracy, and better predictivity than the constitutive equations most widely used for texture analysis (SCCE-T).

First Principles Calculation of Hydrogen Embrittlement in Iron

Hydrogen embrittlement in iron and steels has been studied for long years. Although a few mechanisms of it have been proposed, it has not been clearly understood yet. Especially, the decohesion theory and the hydrogen enhanced localized plasticity theory have been discussed in the center of attention. Hydrogen is considered to induce a cleavage fracture in the former, but enhance slips in the latter. It is essential problem which hydrogen induces cleavage or slip in hydrogen embrittlement. The purpose of the present paper is to clarify the effects of hydrogen in iron single crystal on the fracture or deformation using first principles calculation. In the cleavage model, the changes of total energy were estimated with a distance of a pair of {001} planes in a unit cell composed of 12 iron atoms and a hydrogen atom. In the slip model, the changes of total energy were estimated with a displacement of a few {110} layers toward &lt;111&gt; direction. Hydrogen reduces the total energy in the cleavage model, but does not change that in the slip model.

Perpendicular magnetization of long iron carbide nanowires inside carbon nanotubes due to magnetocrystalline anisotropy

Single crystal iron carbide nanowires contained in multiwalled carbon nanotubes have been prepared by aerosol-based thermal chemical vapor deposition. Investigations by transmission electron microscopy reveal the crystallographic [010] axis of the orthorhombic Fe3C nanowires to be predominantly aligned along the nanotube axis. Despite the high aspect ratio of the Fe3C nanowires, magnetic force microscopy measurements imply single domain behavior with the easy magnetic axis of the nanowires perpendicular to the wire axis. In agreement with the structural results, these findings show that the magnetic behavior is dominated by the magnetocrystalline anisotropy contribution, causing the easy axis to be along the [001] direction.

Figures of merit of magnetostrictive single crystal iron–gallium alloys for actuator and sensor applications

Energy density, magnetomechanical coupling factor and a dimensionless sensing gage factor of 〈1 0 0〉 oriented single crystal iron–gallium alloys with 16, 17.5, 19, 24.7 and 29 at% gallium were studied as functions of stress and magnetic field. To estimate these quantities, the samples were characterized under different quasi-static stress and magnetic field conditions. The experimental behavior was modeled using an energy-based non-linear approach. Both the experimental data and the model simulations were used to calculate material parameters such as magnetic permeability, piezo-magnetic strain coefficient ( d 33), inverse piezo-magnetic coefficient (stress sensitivity, d* 33) and Young's modulus in the material. These quantities were used to obtain energy density, magnetomechanical coupling factor and sensing gage factor as functions of magnetomechanical conditions. Maximum energy density of around 3 kJ/m 3, magnetomechanical coupling factor higher than 0.75 and sensing gage factor on the order of 10 3 were calculated.

Monolayer iron oxide film on platinum promotes low temperature CO oxidation

CO oxidation on a clean Pt(1 1 1) single crystal and thin iron oxide films grown on Pt(1 1 1) was studied at different CO:O 2 ratios (between 1:5 and 5:1) and partial pressures up to 60 mbar at 400–450 K. Structural characterization of the model catalysts was performed by scanning tunnelling microscopy, low energy electron diffraction, Auger electron spectroscopy and temperature-programmed desorption. It was found that monolayer FeO(1 1 1) films grown on Pt(1 1 1) were much more active than clean Pt(1 1 1) and nm-thick Fe 3O 4(1 1 1) films at all reaction conditions studied. Post-characterization of the catalysts revealed that at CO:O 2 > 1 the FeO(1 1 1) film dewets the Pt surface with time, ultimately resulting in highly dispersed iron oxide particles on Pt(1 1 1). The film dewetting was monitored in situ by polarization-modulated infrared reflection absorption spectroscopy. The reaction rate at 450 K exhibited first order for O 2 and non-monotonously depended on CO pressure. In O 2-rich ambient the films were enriched with oxygen while maintaining the long-range ordering. Based on the structure-reactivity relationships observed for the FeO/Pt films, we propose that the reaction proceeds through the formation of a well-ordered, oxygen-rich FeO x (1 < x < 2) film that reacts with CO through the redox mechanism. The reaction-induced dewetting in fact deactivates the catalyst. The results may aid in our deeper understanding of reactivity of metal particles encapsulated by thin oxide films as a result of strong metal–support interaction.

Evolution of Microstructure and Internal Stresses in Multi-Phase Oxide Scales Grown on (110) Surfaces of Iron Single Crystals at 650 °C

The evolution of microstructure and growth stresses in oxide scales grown on a (110) iron single crystal surface at 650 °C was studied by electron backscatter diffraction and in situ energy-dispersive diffraction with synchrotron radiation. Within this high temperature regime, the oxidation kinetics and scale microstructure were not significantly different from those encountered in the oxidation of ferrous polycrystals. Thus, epitaxial strains did not determine the stress state within the oxide scale. Relevant sources of growth stresses were inferred to be volumetric differences between the iron oxides in the early stages, and later, inner oxide formation, scale consumption as well as pore formation. These sources caused time-dependent stress cycles in magnetite and wüstite during oxidation. In the hematite layer stress cycles did not occur and creep appeared to be the predominant stress relieving mechanism. On cooling, the differences in thermal expansion caused residual stress gradients through the oxide scale.

Effect of Substrate Grain Size on the Growth, Texture and Internal Stresses of Iron Oxide Scales Forming at 450 °C

The oxidation behavior of iron polycrystals and single crystals with (110) surface orientation was studied at 450 °C. Energy-dispersive diffraction with synchrotron radiation provided in situ information regarding the evolution of stress gradients and fiber texture in the oxide scales. Within this low-temperature regime, grain boundaries caused the oxidation kinetics of polycrystalline iron to be more rapid than iron single crystals only during the first minutes of oxidation. Epitaxial growth of iron oxides occurred only on single crystal substrates during the initial oxidation. In situ stress analyses suggested that stress relief occurred invariably in the magnetite layer due to the formation of a fine-grained seam near the iron substrates. Above the magnetite and in the hematite layer, the growth stresses depend initially on volumetric strains and later on inner oxide formation and creep of the hematite.

Atomic-scale anisotropy of nanoscratch behavior of single crystal iron

In this paper a parallel molecular dynamics (MD) model has been developed to investigate the nanoscratch process of single crystal iron. The simulations were performed for two cases with different crystallographic orientations and scratch directions. In Case I the scratch plane is (1 0 0) and the scratch direction is [0 0 1]. In Case II the scratch plane and the scratch direction are (1,−1,2) and [1 1 1], respectively. To validate the MD simulation the nanoscratch testing was conducted using the TriboIndenter. The simulation results reveal that the vertical force and the lateral force tend to increase with the scratch displacement for both cases. Case I has smaller forces than Case II. However, the coefficient of friction for both cases is similar, which is in good agreement with the experimental value. The crystallographic orientation also affects the scratch hardness. The scratch hardness of Case I is smaller than that of Case II.

Dynamic properties of structural transition in iron under uniaxial compression

By using molecular dynamics simulations, we have successfully simulated the bcc hcp structural transition in single-crystal iron under isothermal compressionalong the [001] direction. The results reveal a distinct softening ofC33 and ahardening of C31 (or C32) prior to the transition and an over-relaxation of the stress after transition. Above thecritical stress the morphology evolution of structural transition is analyzed, which can bedivided into four stages: hcp homogeneously nucleated, columnar grains formed,nuclei competed and merged, and a laminar structure formed along {110} planes.Besides, our simulations demonstrate that in mixed phases the hcp phase hasnegative shear stress and the potential of the hcp phase is higher than the bccphase, and the shear stress of the system keeps a linear decrease with hcp massfraction. The effect of temperature on the structural transition is also discussed.

Low temperature deformation in iron studied with dislocation dynamics simulations

Tensile tests of iron single crystals reported in the literature were analyzed to deduce the effective stress associated to the double-kinks mechanism in order to establish a mobility law for screw dislocations. This mobility law is used to carry out the first dislocation dynamics simulations of low temperature deformation in iron. The sensitivity of the flow stress on temperature and strain rate is investigated and compared to experiment. First simulation results give an insight into the effect of temperature on the microstructure evolution and on the dislocation–dislocation interactions.

Direct measurements of the α-ϵ transition stress and kinetics for shocked iron

Iron undergoes a polymorphic phase transformation from the α-phase (bcc) to the ϵ-phase (hcp) when compressed to stresses exceeding 13 GPa. Because the epsilon phase is denser than the alpha phase, a single shock wave is unstable and breaks up into an elastic wave, a plastic wave, and a phase transition wave. Examination of this structured wave coupled with various phase transformation models has been used to indirectly examine the transition kinetics. Recently, multimillion-atom molecular dynamics (MD) simulations have been used to examine the shock-induced transition in single crystal iron illustrating an orientation dependence of the transition stress, mechanisms, and kinetics. The objective of the current work was to perform plate impact experiments to examine the shock response of polycrystalline and single crystal iron with nanosecond resolution for impact stresses spanning the α-ϵ transition. The current data reveal an orientation dependence of the transition stress coupled with a transition time that is nonlinearly dependent on the impact stress with a duration ranging from picoseconds to hundreds of nanoseconds. The higher transition stress for iron shocked along the [100] direction is in agreement with the predictions from MD calculations that describe an orientation dependence of the transition stress. However, MD calculations do not capture the complexity of the continuum states achieved or the transition kinetics. Further results and implications are discussed in this paper.

Synthesis, crystal structure and characterization of iron pyroborate (Fe 2B 2O 5) single crystals

Single crystals of iron(II) pyroborate, Fe 2B 2O 5, were prepared at 1000–1050 °C under an argon atmosphere. The crystals were transparent, yellowish in color and needle-like or columnar. The crystal structure of Fe 2B 2O 5 was analyzed by single-crystal X-ray diffraction. Refined triclinic unit cell parameters were a=3.2388(2), b=6.1684(5), c=9.3866(8) Å, α=104.613(3)°, β=90.799(2)° and γ=91.731(2)°. The final reliability factors of refinement were R1=0.020 and wR2=0.059 [ I > 2 σ( I)]. Transmittance over 50% in the visible light region from 500 to 750 nm was observed for a single crystal of Fe 2B 2O 5 with a thickness of about 0.3 mm. The light absorption edge estimated from a diffuse reflectance spectrum was at around 350 nm (3.6 eV). Magnetic susceptibility was measured for single crystals at 4–300 K. Fe 2B 2O 5 showed antiferromagnetic behavior below the Néel temperature, T N≈70 K, and the Weiss temperature was T W=36 K. The effective magnetic moment of Fe was 5.3 μ B.

Specific heat and phase diagrams of single crystal iron pnictide superconductors

We present specific heat measurements on single crystals of the pnictide superconductors NdFeAsO 1− x F x and Ba 1− x K x Fe 2As 2. Low-temperature measurements on Ba 1− x K x Fe 2As 2 reveal that the Sommerfeld coefficient of the quasiparticle specific heat increases linearly with applied magnetic field. This is the signature of a fully gapped superconducting state as is expected for instance in the extended s-wave scenario. The large value of the normal state Sommerfeld coefficient indicates a large electronic density of states and significant mass enhancements. We determine the phase diagram using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature. Both materials display clear mean-field steps in the specific heat at T c of 47 K and 34.6 K, respectively, proving bulk superconductivity. They are characterized by a low superconducting anisotropy near T c of Γ ∼ 4 for NdFeAsO 1− x F x and 2.6 for Ba 1− x K x Fe 2As 2, which is promising for potential applications. We observe extraordinarily high upper critical field slopes of μ 0 ∂ H c 2 c / ∂ T = - 6.5 T / K and μ 0 ∂ H c 2 ab / ∂ T = - 17.4 T / K for Ba 1− x K x Fe 2As 2 which points to the emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with κ c ∼ 100 and κ ab ∼ 260.

Intermetallic charge transfer inFeTiO3probed by resonant inelastic soft x-ray scattering

Resonant inelastic soft x-ray inelastic-scattering (RIXS) spectra of single-crystal iron ilmenite ${\text{FeTiO}}_{3}$ were measured at the $\text{Ti}\text{ }2p$ resonance, and additional peaks at $\ensuremath{-}2.5$ and $\ensuremath{-}4.5\text{ }\text{eV}$ in energy loss were observed. In order to investigate the origin of these peaks, we performed a double-cluster model calculation based on full-multiplet calculation in which, in addition to the interaction with the $\text{O}\text{ }2p$ ligands of the Ti ion, which is already included in the more conventional single-cluster model, the charge-transfer contribution to $\text{Ti}\text{ }3d$ from $\text{Fe}\text{ }3d$ is taken into account. The calculation shows that the peaks may be attributed to the $\text{Fe}\text{ }3d\ensuremath{\rightarrow}\text{Ti}\text{ }3d$ intermetallic charge-transfer scheme that previously had not been accounted for in RIXS.