Changes In Local Atomic Structure Research Articles

Electrochemical and In Situ Spectroscopic Study of the Effect of Prussian Blue as a Mediator in a Solid-State Supercapacitor

In situ X-ray absorption spectroscopy (XAS) along with electrochemical measurements were used to explore the redox activity of Prussian blue (PB) and its contribution to pseudocapacitance in an asymmetrical solid-state supercapacitor (SC). The SC, with a configuration of (activated carbon)/polymer electrolyte membrane/(activated carbon + PB), was fabricated with an embedded KCl saturated Ag/AgCl reference electrode. The potentials of the two electrodes were generally displaced proportionally with increase in the cell voltage. Upon negative polarization from the open circuit voltage (OCV) condition to −3 V, the charge capacity was greater than that upon positive polarization from the OCV condition to +3 V, in agreement with the higher fraction of Fe(III) than that of Fe(II) and the low concentration of K ions in PB. Detailed analysis of cyclic voltammetry data in the range between 0 and −3 V indicates that the battery-like contribution to the charge current was 96% that of the capacitor-like contribution at a scanning rate of 2.5 mV s−1 but only 15% at 100 mV s−1. The in situ XAS characterization results indicate that the majority of Fe was reversibly charged and discharged between Fe(III) and Fe(II) during positive and negative polarizations with minimal changes in local atomic structure.

Cooling rate dependence of the properties for Ti110Al14V4 alloy investigated by ab initio molecular dynamics

The structural evolution and kinetic properties of Ti110Al14V4 alloy under two constant cooling rates are comprehensively studied by ab initio molecular dynamics simulations. A variety of statistical methods are used to progressively analyze the changes in local atomic structure with temperature under different cooling rates. It is revealed that for slow cooling, the icosahedron and face-centered cubic (FCC) cluster dominate in the melt before the system transforms into a body-centered cubic (BCC) crystalline phase at a temperature of about 1600 K. For fast cooling, the system melts from 2600 to 1600 K, then turns into solid glass at 1400 K, and finally forms a crystalline and amorphous mixture. This is a process in which the icosahedron competes with the FCC structure, indicating that the Ti110Al14V4 alloy has a very poor glass-forming ability because of the fragility of Ti110Al14V4 liquids at high temperature. Furthermore, under a fast cooling rate, the self-diffusion coefficient obeys the Arrhenius function at high temperature but conforms to the mode coupling theory when approaching the critical temperature. Finally, through the five-fold local symmetry parameters and viscosity, a direct relationship between structure and dynamics is established, confirming that structural evolution is the root cause of slow dynamics. This discovery provides a new perspective on the structural mechanism of dynamic arrest in the Ti110Al14V4 melt.

Influence of Carbon interstitials to Ti1−xMexN (Me = Zr, Al, Cr) coatings by pulsed laser ablation on wear resistance

The wear resistance of Ti1−xMexN (Me = Zr, Al, Cr) coatings by the laser carburization process was investigated in terms of local atomic structural changes. The repeated pulsed laser ablation was performed to the Ti1−xMexN coating surfaces after Graphite paste was covered. The friction coefficients of the coating specimens were decreased from ∼0.7 to 0.2, and the formation of cracks and debris was suppressed by implementing the laser ablation process. ToF-SIMS depth profiles showed that the laser carburization helps Carbon penetrate into the coating layer as deep as ∼20 nm below its surface. XPS and XAFS analyses revealed that the improvement of the wear resistance of the coatings was achieved not by formation of TiC or ZrC lattices on the coatings surfaces but by Carbon interstitials to the Ti1−xMexN lattices.

Local atomic structures and degradation behaviors of Ti1 − xZrxN coatings under laser thermal shock

The degradation behaviors of Ti1−xZrxN coatings due to repeated thermal stress were investigated in terms of local atomic structural changes according to Zr substitution. The coating specimens deposited by TiZr compound targets with different Zr wt% ratios had the same growth orientations of TiN and ZrN, namely (111), (200), and (220). The lattice parameter and microhardness increased as the Zr ratio increased, which was attributed to the lattice distortion and increase in texture degree of the crystallites in the coating layer by Zr substitution to Ti sites. The coating with 10wt% of Zr showed spalling of the coating layer after repeated thermal shocks, and delamination was suppressed as ZrN formation increased. It was expected that the interatomic distance between (Ti, Zr)-N would decrease as Zr substitution to Ti sites increased, indicating an atomic diffusion path. The depth profiles revealed that the diffusion distance of the coating (Ti, Zr, N) and the substrate (Fe, Cr) elements decreased according to the Zr substitution.

Structure and transport properties of the liquid Al80Cu20 alloy – A molecular dynamics study

Structural and transport properties of the liquid Al80Cu20 alloy have been investigated performing molecular dynamics simulations using a new parametrization of modified embedded atom model (MEAM) potential method retained to the available ab initio molecular dynamics (AIMD) and experimental data. Local atomic structural changes, including chemistry and topology, have been studied and discussed with the literature data. Strong chemical bonding of unlike atoms in comparison to like-ones is found and confirmed by the available literature data. High degree of topological complexity and icosahedron-like type atomic clusters of increasing abundance with temperature decrease are observed, and their influence on short-range chemical ordering as well as transport properties have been defined. Almost perfect agreement between experimental data and the presented MEAM-MD simulations has been registered for the self-diffusion coefficient of Cu atoms in comparison to significant underestimation of experimentally determined inter-diffusion coefficient of the liquid Al80Cu20 alloy. Viscosities simulated, using two MD-based methods, exhibit a slight overestimation of experimental data.

Thermal stability and local atomic structure of (Na0.5K0.5)NbO3 doped BaTiO3 ceramics

Dielectric properties of x(Na0.5K0.5)NbO3–(1−x)BaTiO3 (x=0.00 and 0.06) specimens were investigated in terms of changes in local atomic structure, according to the phase transition by elevating the overall temperature. A 0.06(Na0.5K0.5)NbO3–0.94BaTiO3 (NKN–BT) specimen exhibited enhanced temperature stability along with an increased dielectric constant. The degree of reduction in tetragonality (c/a) at the Curie temperature was smaller in NKN–BT compared to that in pure BaTiO3, as calculated by Rietveld refinement. From a comparison of the pre-edge region in the Ti K-edge, it was determined that the off-center displacement of the Ti atom was also raised to 13.4% through NKN substitution, with a change in local orientation from the [001] to the [111] directions. The substitution by NKN, which has a different ionic radius and electrical charge compared with BaTiO3, causes structural distortion of the TiO6 octahedra in the NKN–BT lattice, resulting in local polarization. These structural changes lead to the temperature stability of the dielectric constant and an overall improvement in the electrical properties of BaTiO3.

Role of Magnesium-Stabilized Amorphous Calcium Carbonate in Mitigating the Extent of Carbonation in Alkali-Activated Slag

Oil well cements have received a significant amount of attention in recent years due to their use in high-risk conditions combined with their exposure to extremely aggressive environments. Adequate resistance to temperature, pressure, and carbonation is necessary to ensure the integrity of the well, with conventional cements prone to chemical degradation when exposed to CO2 molecules. Here, the local atomic structural changes occurring during the accelerated carbonation (100% CO2) of a sustainable cement, alkali-activated slag (AAS) have been investigated using in situ X-ray diffraction and pair distribution function analysis. The results reveal that the extent of carbonation-induced chemical degradation, which is governed by the removal of calcium from the calcium-alumino-silicate-hydrate (C-A-S-H) gel, can be reduced by tailoring the precursor chemistry; specifically the magnesium content. High-magnesium AAS pastes are seen to form stable magnesium-containing amorphous calcium carbonate phases, which pr...

EXAFS and XPS studies of germanium and gallium arsenide nanostructures in porous aluminum oxide matrices

The local atomic structure and chemical bonds of germanium and gallium arsenide, specimens of an isoelectronic series, are studied. Both materials are obtained via thermal evaporation of the materials’ powders onto porous matrices of anodic aluminum oxide and polycorr substrates. The mechanism responsible for the formation of local atomic structure is shown to be almost identical for Ge and GaAs. When compared to the films on polycorr surfaces, changes in local atomic structure are observed in the material obtained in porous matrices due to the difference between the mechanisms of condensation on rough and smooth surfaces.

Fischer−Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts

The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hägg carbide and Fe3O4. After 10 h of CO/He treatment at 290 °C, the main component observed was Hägg carbide. A model containing Hägg and ε-carbides, and Fe3O4, was also investigated. However, the r-factor was not significantly impacted by including ε-carbide in the fitting, and the resulting contribution of ε-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.

Structural evolution in the supercooled liquid of Zr36Ti24Be40 metallic glass

Local atomic structural changes associated with the exothermic heat flow in the supercooled liquid of Zr36Ti24Be40 metallic glass have been studied by using pair distribution function analysis. The exothermic heat flow that used to be interpreted as double glass transition is attributed to the evolution of short range ordering towards the final crystallization product in the supercooled liquid.

Electronic and local atomic structures of(La1−xPrx)0.85Zr0.15MnO3 studied by x-ray absorption spectroscopy

Mn L3,2-, K-,O K- and Pr M5,4-edge x-ray absorption near-edge structure (XANES) spectra of(La1−xPrx)0.85Zr0.15MnO3 (LPZMO)with x = 0, 0.05, 0.1, 0.15 and 0.2 were measured to study the electronicstructure and the effect of Pr substitution. An analysis of MnL3,2- and K-edge spectra indicates that LPZMO is an electron-doped system. Pr substitution isfound to increase the average effective charge of Mn ions and to induce an additionalunoccupied majority spin ( spin) eg subband in theMn L3,2-edge XANESspectrum. Mn L3,2-, O K- and Pr M5,4-edge XANES results demonstrate that the substitution of La ions by smaller Pr ions changesO 2p–Mn 3d and O 2p–Pr 4f hybridized states. Extended x-ray absorption fine structure(EXAFS) analysis at the Mn K edge reveals clear changes in local atomic structure around Mnions due to the Pr substitution. XANES and EXAFS analyses show a small distortion in theMnO6 octahedron or a disorder in the lattice, which may be due to localization of charge carriersand/or the increased effective positive charge of Mn ions.

Change in local atomic structure during formation of the icosahedral quasicrystalline phase in Zr70Pd30glassy alloy

The local atomic structures in glassy, supercooled liquid and quasicrystalline phases for a Zr70Pd30binary alloy have been examined by the x-ray diffraction method. It was found that the local atomic structure in the glassy phase can be identified as the distorted icosahedral-like structure around Zr and remains almost unchanged by the phase transformation into the supercooled liquid. In the formation process of the icosahedral phase, approximately one Zr atom substitutes with one Pd atom in this local structure. This kind of atomic rearrangement may improve the perfectibility of the as-quenched icosahedral-like cluster, leading to the phase evolution of the icosahedral phase.

A ReflEXAFS study of the local structural changes around implanted yttrium ions during the high temperature oxidation of Ni[sbnd]20Cr

A study has been made of the initial stages of oxidation of yttrium ion implanted Ni20Cr in an attempt to shed some light on the so-called “reactive element effect” observed in chromia forming alloys. ReflEXAFS was used in order to observe the initial local atomic structural changes around the implanted ions, while glancing angle X-ray diffraction was used to monitor the longer range structural transformations taking place near the surface. The results indicated that the implantation amorphised the alloy surface. Following a short oxidation at 700°C Y2O3 nanocrystals formed throughout the implanted region. Simultaneously a highly oriented Cr2O3 layer formed on the sample surface. Preliminary measurements reported here on the thermal expansion of hexagonal chromium oxide indicate a pronounced anisotropy with respect to the crystallographic direction which could explain the superior adhesion to the substrate alloy of oriented scales under conditions of thermal cycling.

Microstructural change in the course of decomposition of an amorphous Pd 82Si 18 alloy

The medium range order (MRO) and its relation to the structural decomposition on annealing in PdSi amorphous alloys have been studied by high resolution electron microscopy, nano-beam electron diffraction and nano-beam elemental analysis. Amorphous Pd 82Si 18 thin film specimens were prepared by Ar-beam sputtering. The f.c.c. MRO domains were observed in the as-sputtered films. On annealing, observation frequency and the size of the MRO domains increased. The f.c.c. MRO domains grew to nano-precipitates of α-Pd after annealing at 573 K. At this stage, the second halo diffraction ring tends to split into two in the selected area electron diffraction patterns. The Si atoms are thought to occupy octahedral interstices in the small precipitates and diffuse out to the matrix with increasing the precipitate sizes. The Si composition in the nano-precipitates decreases in the course of growth. It is concluded that the atomic free volume is annihilated by the atomic rearrangement in the formation and development of MRO domains, and that chemical ordering is developed in the surrounding matrix by the local compositional change. The structural relaxation must be corresponding to these local atomic structural changes.

Local atomic structural changes on structural relaxation in a Pd 82Si 18 amorphous alloy

The microstructural changes on low temperature annealing of amorphous Pd 82Si 18 ribbon specimens were observed using high-resolution electron microscopy and the results were correlated with the structural relaxation. At the initial stage of annealing up to 573 K, atomic medium-range order (MRO) develops and grows. At temperatures higher than 573 K but below the glass transition temperature, the MRO domains grow to f.c.c. nanocrystals which change the profile of the electron diffraction pattern, especially of the second halo ring. According to nano-beam elemental analysis on annealing using sputter-deposited Pd 82Si 18 thin films, the Si content in the MRO domains and nano-precipitates decreases as the domain or particle size increases. It is concluded that the atomic free volume introduced during amorphization is annihilated by atomic rearrangement in the formation and development of the MRO domains, and the chemical order must be developed in the dense random packing (DRP) matrix by the diffusion of Si from MRO into the matrix. The structural relaxation corresponds to these local atomic structural changes.

The effect of salt concentration on the local atomic structure and conductivity of PEO-based NiBr 2 electrolytes

High molecular weight poly (ethylene oxide) (PEO) ( MW = 5 × 10 6) based NiBr 2 electrolytes of compositions, NiBr 2(PEO) n , n = 2, 4, 8, 16, 32, 100, where n is the number of oxygen per Ni(II) ion, were prepared by solution casting. An unusually high oxygen/metal ratio of 2 could be achieved with NiBr 2 salt. X-ray powder diffraction study and pair distribution function (PDF) analysis indicate that no crystalline PEO-salt complex phase exists until n≤16. At low salt concentration ( n> 16), the electrolyte consists of two components: amorphous PEO containing dissolved salt and crystalline PEO in which salt ions may exist as imperfections. The interchain correlation of PEO is virtually unaffected by dilute concentrations of salt. With increasing salt concentration, a new PEO-salt crystalline complex starts to form and the interchain correlation of PEO is drastically increased. These results are consistent with those obtained from differential scanning calorimetry (DSC) analysis and show a possible correlation between changes in local atomic structure and ionic conductivity.

Measurement of local structural configurations associated with reversible photostructural changes in arsenic trisulfide films.

Extended x-ray-absorption fine-structure measurements have been made on three reversible and reproducible cycles of thermally annealed and light-soaked amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ films. Associated with the light-soaked material are (1) a very small increase in the population of wrong bonds in the first shell, (2) an enlarged As---S---As bond angle with an expansion of As---As distance in the second shell, (3) a larger spread in the distribution of As---S---As bond angles, and (4) an absence of any change in the third As---S shell. From these data, we present the first quantitative correlation between observed local atomic structural changes and measured macroscopic properties that are associated with photodarkening. Our data demonstrate that the photoinduced structural changes mainly involve bonding alterations at S atoms as well as a change in the dihedral angle relationship between adjacent ${\mathrm{AsS}}_{3}$ pyramids joined at S atoms.