Local Vacancy Concentration Research Articles

Towards in Operando XAS Investigation of CO2 Electrolysis in Solid Oxide Cells: Synchrotron Studies of La0.3Ca0.7Fe0.7Cr0.3O3-δ Perovskites

Solid oxide cells (SOCs) are poised to provide a significant contribution to the emerging field of green technology. SOCs could form the basis for a closed loop, sustainable energy system in which fuel cells generate power efficiently and any waste products are converted back into fuel or useful products.Early solid oxide fuel cell (SOFC) systems utilized ceramic-metal composites, such as Ni-YSZ, as the fuel electrodes. Ni metal provides electronic conduction while YSZ plays the role of oxygen anion conductor. This requires the catalyst target (e.g., H2) to come into contact with both Ni and YSZ, forming a so-called triple phase boundary.Cermet systems perform well but are typically unstable under oxidizing conditions and are susceptible to passivating phenomena, such as sulfur poisoning. Since then, mixed ionic and electronic conductors (MIECs) have been developed, where a single material such as a perovskite replaces both cermet components. The gas-MIEC interface acts as a double phase boundary, theoretically providing electrochemical reaction sites on the entire surface of the electrode. This makes them less susceptible to poisoning by impurities, and the oxide composition can be tailored to improve stability in reducing or oxidizing environments. SOFCs based on mixed-conducting catalysts have shown marked performance increases over room-temperature fuel cells in ORR catalysis, often with an order of magnitude advantage in current response. Further work led to MIEC compositions being used at SOFC fuel electrodes, and more recently, as active catalysts for the carbon dioxide reduction (CO2RR) reaction in electrolysis cells (SOECs).Our research group recently demonstrated that the perovskite material, La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr), shows excellent stability and activity during the OER and ORR. This single composition is also highly active for the CO2RR, opening the door for reversible and symmetrical SOC devices (RSOFCs) that promise to greatly reduce device complexity. This should ease the difficulty of scaling up these technologies for industrial applications. While we have observed performance gains in both SOC modes of operation using LCFCr, we sought to better understand the underlying mechanism of catalysis in CO2 electrolysis. This has led us to begin a thorough investigation of LCFCr-based reversible SOFCs using synchrotron x-ray absorption spectroscopy (XAS), culminating with in operando studies, where CO2 electrolysis can be tracked as it happens in fully operational electrolysis cells. Harsh operating conditions have made in situ or in operando SOC studies more challenging than those for RT cells, which also share the barriers of access to electrode surfaces and the attenuation effect of feedstocks on incident energy used for spectroscopy.This presentation will introduce our initial efforts in synchrotron XAS studies of the as-synthesized LCFCr material, which was prepared by combustion synthesis of precursor metal nitrates. Synchrotron energy provided by the Canadian Light Source (CLS) was used to study powder samples of this composition; experiments were carried out at both hard (SXRMB) and soft (SGM) x-ray sources at the CLS. Preliminary experiments consisted of ex situ XAS testing of samples heat treated using conditions similar to those encountered in operation (800 °C in air or CO2). When compared to spectra of as-synthesized compositions, this provides a conventional “before and after” comparison and has demonstrated that overall changes in oxidation state and coordination environment could be observed. In situ testing was also carried out at SXRMB on La0.3Ca0.7Fe0.7Cr0.3O3-δ powder samples. Samples were placed in a “six-shot” apparatus that provided a versatile test bed for semi-realistic conditions. In these studies, materials were heated in air to 800 °C, while XANES spectra (Fe K-edge) were collected simultaneously. This process was repeated as the gas was changed to CO2, and then upon cooling. These transitionary XAS measurements allowed us to observe both structure and valence state of the material responding in real time. The results showed changes in the pre-edge feature and edge intensity that we have attributed to a change in local vacancy concentration around Fe metal centers. At each stable set of conditions, full EXAFS spectra were collected for all four metals; initial analysis suggests that chromium undergoes redox changes while the iron valence state remains largely unchanged. Together, this set of experiments is allowing us to assemble a dynamic picture of the chemical and structural state of the catalyst during operation.This study underscores the value of in situ and in operando investigation of SOC catalysts compared to traditional ex situ analysis. Insights into efficient SOC electrolysis of CO2 could give us the ability to reduce carbon emissions. It is both urgent and vital that the underlying mechanisms are understood more completely.

Development of a solute and defect concentration dependant Ising model for the study of transmutation induced segregation in neutron irradiated W–(Re, Os) systems

In this study, radiation-induced precipitation of transmutation products is addressed via the development of a new solute and vacancy concentration dependant Ising model for the W–Re–Os system. This new model includes interactions between both Os and Re atoms with vacancies, thus facilitating more representative simulations of transmutation in fusion reactor components. Local solute concentration dependencies are introduced for the W–Re, W–Os and Re–Os pair interactions. The model correctly accounts for the repulsion between small clusters of vacancies and the attraction between larger clusters/voids, via the introduction of local vacancy concentration dependant interaction coefficients between pairs of atoms and vacancies. To parameterise the pair interactions between atoms and/or vacancies, the enthalpy of mixing, ΔH mix, for various configurations and solute/defect concentrations, was calculated using density functional theory, within 6 binary systems: W–Re, W–Os, Re–Os, W–vacancy, Re–vacancy and Os–vacancy. The new energy model was implemented into the SPPARKS Monte Carlo code, and successfully used to predict the formation of voids decorated with Re and Os solute atoms. Analysis suggests that there is a strong thermodynamic tendency for Os to bind to these voids with a comparatively weaker binding from Re atoms. The binding energies of various solute/vacancy clusters were calculated and showed that Re and Os solute atoms tend to stabilise small clusters of vacancies, increasing the attractive binding energy between the constituents.

Equilibrium distribution of point defects in Fe-Y-O as a typical representative of nanocluster-strengthened alloys

The statistical mechanic approach [1, 2] has been used to estimate the equilibrium defect distribution in a multiphase system by minimizing its free energy using microstructural characteristics and a finite set of defects obtained by first-principles calculations. Recently the approach was extended to take in consideration defects at the interface of the precipitate and matrix [3]. Herein we apply the developed approach to the investigation of the stability of bcc Fe containing yttria, Y2O3, nanoclusters as a prototype of nanostructured ferritic alloys. It has been obtained that (100)Fe-O interface is unstable with respect to vacancies production. In a contrast to (100)Fe-O interface, the so called Klim interface is stable, i.e. local vacancy concentration at this interface at 600 K is below 10−12. It has been demonstrated that due to large defect formation energies the ODS particles are extremely stable and the main defect corresponds to Fe atoms substitute Y in Y2O3 precipitate. Moreover, under thermodynamic equilibrium condition, the preexist vacancies in bulk Fe do not accumulate oxygen atom. The later observation not necessarily forbid the existence of a large amount of preexisting Fe vacancy - oxygen atom clusters at the initial stages of alloy formation far from equilibrium.

First-principles calculations of Ag addition on the diffusion mechanisms of Cu–Fe alloys

The effect of Ag addition on the diffusion mechanisms of aging precipitation processed Cu–Fe alloys were investigated by first-principles calculations. The calculation of solute–solute binding energies accurately revealed an attractive binding for the Fe–Fe at the first NN site, and Ag–Ag at the first NN site as well as the second NN site, however the Fe–Ag interactions are repulsive both at the first NN and the second NN distance. The investigation of binding energy between X and V (X represents Fe or Ag and V a vacancy) showed that vacancy is more likely to be in the proximity of the solute Ag atom, and the addition of Ag to dilute Cu–Fe system will increase the local vacancy concentration close to a Ag–Fe dimer. The calculated migration energy for an X–V exchange indicated that the Ag atom diffused more easily in Cu than Fe atom; and the addition of Ag decreased the energy of the transition state, thereby significantly reduced the migration energy of Fe. The results agree well with the available experimental observation. The calculation gives way to find other suitable alloying elements for preparation of high strength and high conductivity Cu–Fe alloys or in situ composites.

Effects of solute and vacancy segregation on antiphase boundary migration in stoichiometric and Al-rich Fe 3Al: A phase-field simulation study

Effects of segregation of solute atoms and vacancies on migration of antiphase boundaries (APBs) in stoichiometric (Fe-25 at%Al) and Al-rich (Fe-28 at%Al) Fe 3Al at 673 K have been studied using a phase-field method in which local vacancy concentration is taken into account [Koizumi Y, Allen SM, Minamino Y. Acta Mater 2008;56:5861, ibid. 2009;57:3039]. Boundary mobility ( M) of APBs having different phase-shift vectors of a/4<111> and a/2<100> (hereafter denoted as B2-APB and D0 3-APB, respectively) was evaluated by measuring the boundary velocity of shrinking circular APBs. Similar effects of the segregation on the migration of B2-APBs were observed in both compositions. Vacancies segregated and Al-atoms were depleted at B2-APBs in both compositions. Vacancy concentration at B2-APBs was up to 80% higher than that in the bulk. As a result, the migration of B2-APBs was greatly enhanced by the vacancy segregation. In contrast, the segregation to D0 3-APBs showed a marked composition dependence. Vacancies were depleted and Al-atoms segregated at D0 3-APBs in the Al-rich Fe 3Al, whereas vacancies segregated and Al-atoms were depleted at D0 3-APB in the stoichiometric Fe 3Al. The Al segregation in the Al-rich Fe 3Al decreased M of D0 3-APBs much more significantly than the Al-depletion in the stoichiometric Fe 3Al. As the APDs shrank, D0 3-APBs broke away from the segregation atmospheres and the M increased rapidly in both compositions. A greater increase in the M due to the breakaway was observed in the Al-rich Fe 3Al than in Fe 3Al with the stoichiometric composition.

Vacancy distribution around isolated implantation sites

Microscopic experimental data on the distribution of vacancies created in isolated implantation events are reported for the model system B in Cu. An interstitial-to-substitutional conversion process of the implanted B12 ions is utilized to detect lattice damage. We observe vacancy clouds with well-defined edges and a mean radius of 13.5(9)nm. The radial profile of the local vacancy concentration never exceeds 10−4; it is almost flat in the central region before it falls off rapidly, a behavior which cannot be explained by simple binary-collision computer models.

Investigation of the effects of He bubbles on the nucleation, growth and thermal stability of Al–Cu nanoprecipitates in ion implanted Al foils

The effects of He bubbles on the thermal evolution of θ ′ Al 2Cu precipitates in Al matrix are investigated. The Al–Cu precipitates and the He bubbles were produced at the same depth region (≈200 nm below the sample surface) by Cu + and He + ion implantation followed by furnace annealing in vacuum at temperatures of 200, 250 and 280 °C for 2–5 h. The Cu and He concentration–depth profiles were determined by Rutherford backscattering and elastic recoil detection analysis, respectively. The precipitates and bubbles morphologies were examined by means of transmission electron microscopy. It is demonstrated that the presence of He bubbles reduces the nucleation and the growth rate of the precipitates. Both effects are discussed taking into account that the processes of (a) precipitate nucleation, (b) diffusion interaction between precipitates and (c) precipitate growth/dissolution are affected by the decrease of the local vacancy concentration in the matrix, which is caused by the formation and the thermal relaxation of the bubble system. Based on the above results He implantation is proposed as an alternative tool to control the mean sizes of precipitate systems.

Interfacial stresses at inclusions during grain-boundary sliding

Abstract Grain-boundary sliding may be controlled by the interfacial diffusion of material around boundary inclusions. The sliding or diffusion process leads to a redistribution of local vacancy concentration, and therefore stress, over the surface of the inclusion. In this paper, these stresses are calculated for the case of a rectangular inclusion located symmetrically in the plane of the boundary. The sliding rate is predicted as a function of the aspect ratio of the inclusion and a novel feature, inclusion rotation, is also revealed by the analysis. For small and large aspect ratios, this rotation is minimal but, for ratios close to unity, it can be significant. Rotation is predicted to be zero at a certain critical ratio, when the inclusion experiences only pure shear, with the direction of rotation being reversed at values above and below this critical ratio.

Diffusion in multi-component systems with no or dense sources and sinks for vacancies

General equations for the multi-component diffusion in crystalline systems are derived in the framework of Onsager's non-equilibrium thermodynamics. The aim is to provide explicit equations needed for computer-modeling of, e.g., diffusion-controlled phase transformations, but avoiding the usual simplifying assumptions, such as the independence of the fluxes of different atomic species. An additional difficulty is introduced into the problem by the fact that the concentration of vacancies, which mediate diffusion of substitutional atoms, may follow different rules, depending on whether there is a sufficient density of sources and sinks to keep the local vacancy concentration in equilibrium. We treat two limiting cases, one where the total vacancy number is conserved (that is, there are no available sources and sinks) and one where the vacancy concentration is kept in equilibrium (the case of dense sources and sinks). First, the diffusive fluxes of all components and of vacancies are expressed by the well known Onsager relation. The kinetic coefficients L ik from the Onsager relation are derived by means of an extremal thermodynamic principle with respect to the atomic mobilities of individual components and taking into account the constraint amongst fluxes resulting from the vacancy diffusion mechanism. In the case where a dense network of sources and sinks of vacancies is active, the number of lattice positions is not necessarily conserved in every region of the specimen. This means, that the second Fick law, derived for the local conservation of lattice positions, is not applicable in this case. Using mass conservation considerations, the second Fick law is modified to account for this effect. The lattice may shrink or expand for two reasons—either due to the generation or annihilation of vacancies or to the change of the molar volume connected with the change of chemical composition. The deformation of the system is expressed quantitatively by strain rates. Finally, the equations for the system evolution are expressed in both the lattice-fixed and in the laboratory-fixed frames of reference.

The effects of He implantation on the thermal stability of Cu–Al precipitates in aluminum

In this contribution we report on the effects of He on the formation and thermal evolution of Cu–Al precipitates produced after Cu + and He + ion implantation in pure Al foils. The profiles of Cu and He were measured via Rutherford Backscattering (RBS) and Elastic Recoil Detection Analysis (ERDA) techniques, respectively, and the precipitate and bubble morphology were investigated by TEM. After thermal annealing, we observe a retardation of the Cu–Al precipitates coarsening and a Cu redistribution governed by the presence of He bubbles. Our results are discussed in terms of an overpressurized bubble system, which reduces the local vacancy concentration and affects the diffusional process.

Transient homologous structures in nonstoichiometric YBa2Cu

We propose that a homologous series of transient ordered structures may form during the spinodal decomposition of the nonstoichiometric superconducting oxide $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$. These transient phases provide the best kinetic accommodation of the local vacancy concentration. Their structures should be similar to those of the ${\mathrm{Ti}}_{m}{\mathrm{O}}_{2m\ensuremath{-}1}$ and ${\mathrm{Mo}}_{m}{\mathrm{O}}_{2m\ensuremath{-}1}$ series of Magneli phases. The transient phases in the oxide have the stoichiometry $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{\frac{7\ensuremath{-}n}{(2n+1)}}$, where $n$ is an integer. The structure of the $n$ phase has a diffraction maximum at the point ($\frac{n}{(2n+1) 00}$). Examples have been experimentally observed.

Cascade diffusion theory as a general method for modeling radiation damage phenomena

Much of radiation damage theory is based on a rate equation formalism which assumes temporally and spatially homogeneous defect concentrations. Recently, the cascade diffusion theory has been developed to achieve a more realistic picture of point-defect concentrations. Under neutron irradiation, the local vacancy concentration is characterized by fluctuating concentration composed of spikes of varying size with significant overlap. Interstitial concentrations, on the other hand, consist of brief spikes having high concentration and with virtually no overlap. For most of the time the local interstitial concentration is zero. In this paper, we outline a general approach for using cascade diffusion theory to model materials behavior during irradiation. We also summarize some published results applying this method to swelling and irradiation creep. We suggest an approach for applying the method to other problems such as analysis of pulsed irradiation devices and defect cluster nucleation.

Diffusion ofAg110in single-crystalline silver in a temperature gradient

The diffusion of $^{110}\mathrm{Ag}$ into the cold end of silver single crystals exposed to a temperature gradient was studied in an effort to determine the steady-state local vacancy concentration in this metal under nonisothermal conditions. The temperature of the hot end of the specimens was 930\ifmmode^\circ\else\textdegree\fi{}C and the cold-end temperature was varied between 930 and 705\ifmmode^\circ\else\textdegree\fi{}C for the centimeter-long specimens. The least-squares line of the nonisothermal silver diffusion is described by ${D}_{0}=0.21$ ${\mathrm{cm}}^{2}$/sec and ${Q}^{\ensuremath{'}}=177.7\ifmmode\pm\else\textpm\fi{}5.4$ kJ/mol; this may be compared to the isothermal results of Peterson, Rothman, and Robinson who obtain ${D}_{0}=0.67$ ${\mathrm{cm}}^{2}$/sec and $Q=188.9\ifmmode\pm\else\textpm\fi{}0.8$ kJ/mol. Our further work in terms of isothermal data agrees well with the latter results. The nonisothermal data are interpreted to imply the existence of a slight withdrawal of vacancies from equilibrium due to the temperature gradient. Such an effect was found in aluminum single crystals. Both sets of data are consistent with the vacancy concentration as follows: ${C}_{v}=\mathrm{exp}[\frac{{S}_{v}}{K\ensuremath{-}(1\ensuremath{-}\ensuremath{\gamma})}\frac{{H}_{v}}{K{T}_{H}\ensuremath{-}\ensuremath{\gamma}}\frac{{H}_{v}}{\mathrm{KT}}]$, where $T$ is the local temperature, ${T}_{H}$ is the temperature at the hot end of the crystal, and $\ensuremath{\gamma}$ is the nonequilibrium vacancy concentration factor. Herein, an average value for $\ensuremath{\gamma}$ is 0.89 for silver, which may be compared to 0.35 for aluminum.