Oxygen Vacancy Diffusion Coefficient Research Articles

Study on growth kinetics of passivation film of Ti-6Al-3Nb-2Zr-1Mo alloy Based on point defect model

This study aims to explore the passivation behavior and growth mechanism of the Ti-6Al-3Nb-2Zr-1Mo alloy in an oxygen-free environment, taking into account various film-forming potentials and solution pH levels in the process. The point defect model is employed to study the growth mechanism of the passivation film. A film-forming potential is selected within the stable passivation region for polarization, followed by conventional electrochemical tests, Localized Electrochemical Impedance Spectroscopy, X-ray photoelectron spectroscopy characterization, and Glow Discharge Optical Emission Spectrometry. Experimental findings indicate that the passivation film on the Ti-6Al-3Nb-2Zr-1Mo alloy surface demonstrates characteristics of an n-type semiconductor. Increasing the film-forming potential leads to a decrease in the donor point defect density. Meanwhile, elevating the pH of the solution decreases the oxygen vacancy diffusion coefficient from 7.8 × 10−19 to 4.3 × 10−19, thus improving the corrosion resistance of the passivation film.

(Invited) Oxide Ion Conductivity and Oxide Vacancy Diffusion Coefficient of Stabilized ZrO2 and Doped CeO2 Discussed Based on Classical Defect Chemistry with Particular Focus on Valence State (2+, 3+, and 4+) of Doped Cations

So far tremendous works have been reported on the oxide-ion conductivity of fluorite-type oxides typically stabilized zirconia and doped ceria. It is well known that the oxide-ion conductivity vs. dopant concentration plot shows maximum around 10 mol percent dopant concentration and the maximum conductivity varies with the dopant species. Interests tend to be focused on the higher and the stabler conductivity. For the aim, explorations are made on the dopant survey and the phase stability of the solid solution.Generally, conductivity is considered proportional to the product of carrier concentration and carrier mobility. For the fluorite-type oxide ion conductors, it is considered that carrier is oxygen vacancy, V O ··, and mobility is proportional to oxygen vacancy diffusion coefficient, DV . However, such defect chemical approach to the fluorite-type oxides is not popular. When we assume that the mobility is constant following the classical hypothesis, we cannot explain the conductivity maximum.The author has been trying to discuss the oxide-ion conduction in fluorite-type oxide solid solutions in terms of oxygen vacancy concentration, cV ,and DV .Using the literature data on conductivity and composition relationship, the author has already calculated DV for some compositions of stabilized zirconia and doped ceria as a function of composition. The data have been published in a short review [1] and books [2,3]. However, the published graphical displays were so crowded that the physical meanings seem still unclear.In this presentation, discussion is made on the conductivity of stabilized zirconia and doped ceria in the following order to make clear the characteristic feature of DV with the variation of valence state and chemical species of dopant oxides.1: zirconia base solid solutions 1a: with divalent cation oxides 1b: with trivalent cation oxides2: ceria base solid solutions 2a: with divalent cation oxides 2b: with trivalent cation oxides3: stabilized zirconia – ceria solid solutions 3a: stabilized zirconia base solid compositions 3b: ceria base solid compositions 3c: trivalent cerium formation by partial reductionIt is clarified that DV of fluorite type oxide is affected by the dopant cation not only by different valent but also by the same valent cations.

Modeling oxygen diffusion in barium titanate using molecular dynamics: Comparison between Mg and Sc dopants

In this study, the diffusion coefficient of oxygen vacancies in barium titanate doped with 2.0% Sc was calculated by using molecular dynamics. The temperature was varied from 1273 K to 2500 K, and the simulation box consisted of 10 × 10 × 10 unit cells subject to periodic boundary conditions. The Sc dopants were incorporated into the B-sublattice and compensated for by using the randomly distributed oxygen vacancies on the oxygen sublattice. The diffusivity of the vacancies was determined from the slope of the mean-squared displacement of the oxygen ions over time. The Arrhenius plot of the diffusion coefficient showed a clear linear behavior, with an activation energy of 0.84 eV. The results were interpreted by computing radial pair distribution functions for various correlations (e.g., Ti–O and Sc–O) and by static lattice (nudged elastic band) calculations of energy barriers for the migration of oxygen. While Mg-doped BaTiO3 exhibited a strong trend of the formation of defect associates between the acceptor dopant and the oxygen vacancies that lead to a clear reduction in the observed activation energy for oxygen transport with increasing temperature (non-linear Arrhenius behavior), defect-induced interactions (associates) in case of Sc doping were nearly negligibly small, and gave rise to a linear Arrhenius plot with a single activation energy.

First-principles simulation of neutral and charged oxygen vacancies in m-ZrO2: an origin of filamentary type resistive switching

Metal oxide ZrO2 has been widely explored for resistive switching application due to excellent properties like high ON/OFF ratio, superior data retention, and low operating voltage. However, the conduction mechanism at the atomistic level is still under debate. Therefore, we have performed comprehensive insights into the role of neutral and charged oxygen vacancies in conduction filament (CF) formation and rupture, which are demonstrated using the atomistic simulation based on density functional theory (DFT). Formation energy demonstrated that the fourfold coordinated oxygen vacancy is more stable. In addition, the electronic properties of the defect included supercell confirm the improvement in electrical conductivity due to the presence of additional energy states near Fermi energy. The CF formation and rupture using threefold and fourfold oxygen vacancies are demonstrated through cohesive energy, electron localization function, and band structure. Cohesive energy analysis confirms the cohesive nature of neutral oxygen vacancies while the isolated behavior for +2 charged oxygen vacancies in the CF. In addition, nudged elastic band calculation is also performed to analyze the oxygen vacancy diffusion energy under different paths. Moreover, we have computed the diffusion coefficient and drift velocity of oxygen vacancies to understand the CF. This DFT study described detailed insight into filamentary type resistive switching observed in the experimentally fabricated device. Therefore, this fundamental study provides the platform to explore the switching mechanism of other oxide materials used for memristor device application.

Onset of electronic conductivity in nanometer thick films of yttria stabilized zirconia (YSZ) at high electric fields

Electrochemical cells were prepared on a silicon wafer consisting of a 100 nm thick solid electrolyte of yttria stabilized zirconia (YSZ) prepared by atomic layer deposition (ALD), a bottom Ruthenium and an upper Gold electrode. Constant voltages were applied to the cells and currents were measured at temperatures between 120 °C and 170 °C. After an incubation time stationary currents were measured corresponding to conductivities well above the ionic one of the YSZ. After the polarization an open circuit voltage remained, which was reduced to zero by passing a constant depolarization current through the cell. All results were explained qualitatively and quantitatively assuming reactions with the electrodes and internal reactions within the YSZ. The latter lead to virtual electrodes with high electronic conductivity between which unreacted YSZ exists, i.e. a n/i/p junction forms. A stationary flux of single charged vacancies through the unreacted YSZ is assumed leading to the final stationary current. The calculated characteristic time for reaching this steady state is shown to be the incubation time, which in agreement with the experiment is inverse proportional to both the second power of electric field strength and the diffusion coefficient of oxygen vacancies.

An influence of mechanical stresses on the magnitude of the internal field in lead zirconate titanate thin films

In self-polarized lead zirconate titanate thin films formed on platinized silicon substrates, a significant increase in the internal electric field was observed as a result of long-term aging. To explain this phenomenon, a mechanism is proposed for the formation of the internal field associated with the diffusion of charged oxygen vacancies, which, in turn, is due to the action of a mechanical stress gradient. The diffusion coefficient of charged oxygen vacancies was estimated to be ~ 3·10-16 cm2/s. Keywords: thin films, lead zirconate titanate, internal field, Gorsky effect.

Влияние механических напряжений на величину внутреннего поля в тонких пленках цирконата-титаната свинца

In self-polarized lead zirconate titanate thin films formed on platinized silicon substrates, a significant increase in the internal electric field was observed as a result of long-term aging. To explain this phenomenon, a mechanism is proposed for the formation of the internal field associated with the diffusion of charged oxygen vacancies, which, in turn, is due to the action of a mechanical stress gradient. The diffusion coefficient of charged oxygen vacancies was estimated to be ∼ 3 × 10–16 cm2/s.

Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial La1-xSrxCoO3-δ Films.

Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO3-δ and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO2.5. This is emerging as a paradigmatic example of the power of electrochemical gating (using, e.g., ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO3 films are challenging with respect to stability, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P → BM transformations across almost the entire La1-xSrxCoO3 phase diagram (0 ≤ x ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all x, including x ≤ 0.50, where both P and BM phases are highly stable. Under small compressive strain, the transformation threshold voltage decreases from approximately +2.7 V at x = 0 to negligible at x = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low x. The P → BM threshold voltage is thus tunable, via both composition and strain. At x = 0.50, voltage-controlled ferromagnetism, transport, and optical transmittance are then demonstrated, achieving Curie temperature and resistivity modulations of ∼220 K and at least 5 orders of magnitude, respectively, and enabling estimation of the voltage-dependent Co valence. The results are analyzed in the context of doping- and strain-dependent oxygen vacancy formation energies and diffusion coefficients, establishing that it is thermodynamic factors, not kinetics, that underpin the decrease in the threshold voltage with x, that is, with increasing formal Co valence. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications.

Effect of an external electric field on the diffusion of oxygen ions in zirconium dioxide doped with yttrium oxide. Molecular dynamics study

The influence of an external electric field on the character of ion motion, diffusion coefficients, and diffusion activation energies in cubic ZrO2 doped with Y2O3 with varying dopant content, temperature, and electric field strength has been studied by the molecular dynamics method. The diffusion coefficient of oxygen vacancies for models with different Y2O3 contents in the temperature range 873–1273 K without an imposed field decreases with an increase in the molar fraction of yttrium oxide in the crystal, while the dependence of the diffusion coefficient of O2− ions has a maximum (2.4∙10−11 m2/s) with an yttrium oxide content of 3 mol%. The activation energy of ionic conductivity, estimated in the absence of a field on the basis of diffusion of O2− ions, increases from 0.45 eV at a Y2O3 content of 4.6 mol% to 0.61 eV at a dopant content of 15.7 mol%. An external electric field of 0.02–0.08 V/Å noticeably changes the frequency of jumps of oxygen ions. At the electric field strength of 0.08 V/Å, a dopant content of 4.6 mol%, and a temperature of 1073 K, the О2− diffusion coefficient increases to 1.41∙10−9 m2/s. The dependence of diffusion coefficients on the field strength is close to quadratic.

Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at −20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (∼30 nm in diameter) inside a ceria grain is reduced to the C-type CeO1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transportare determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO2, Ce0.94Sm0.06O1.97, Ce0.89Sm0.11O1.945, and Ce0.8Sm0.2O1.9, respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.

Effects of temperature and dopant concentration on oxygen vacancy diffusion coefficient of yttria-stabilized zirconia

Oxygen vacancy diffusion coefficient in yttria-stabilized zirconia (YSZ) is investigated by statistical moment method. Vacancy-dopant association energy, vacancy migration energy are calculated as a function of temperature and dopant concentration. Oxygen vacancy diffusion coefficient increases with the increasing temperature but decreases with the dopant concentration. This degradation of the diffusion coefficient arises from the limitation for oxygen vacancy migration across cation barriers with the increasing dopant concentration. Calculated results are in good agreement with other theoretical and experimental results.

Enhancement of electrical and magnetic properties of nanocomposite BaTiO3 - NiZnFe2O4 irradiated by Gamma ray

The effects of the gamma ray irradiation on the, microstructural and magnetic properties of the composite (1-x) (Ni0.8Zn0.2Fe2O4) + x (BaTiO3) where x = 0%, 20%, 40%, 60%, 80% and 100 %BT content have been investigated. All samples were exposed to gamma radiation dose in range 310 kGy with dose rate 1.4 kGy/h at room temperature using 60Co radioactive source at irradiation cell (medical sterilizer type CM-20). From the X-ray diffraction analysis, it was found obviously that the lattice parameters and average grain size have been affected, for all irradiated samples. The initial permeability at Curie temperature for all samples were sharply decreases after irradiation. The calculated values of diffusion coefficient of Oxygen vacancies, the concentration of Oxygen vacancies and the activation energy of diffusion process were calculated before and after irradiation. From the results we conclude that the material after irradiation can be used for the Gamma ray detection.

Импеданс анодных процессов на пассивном NiSi-электроде в сернокислом фторидсодержащем электролите

The mechanism and kinetics of anodic oxidation of the Ni-Si electrode in solutions of 0.5 M H2SO4 + (0,005 – 0,05)M NaF in the passive state were investigated by methods of polarization and impedance measurements.Theimpedance spectra are interpreted on the assumption about the formation of the bilayer oxide film on the surface of the silicide nickel, the outer layer which has a porous structure.The growth of the porous layer with the increase of the electrode potential in the investigated solutions is linearly (constant anodizing is 2.2 nm/V).The increase in NaF concentration leads to a decrease in the thickness of the porous layer. The growth of the barrier layer of the oxide film was described in the framework of the model of point defects.The diffusion coefficient of oxygen vacancies inside the barrier layer of the film is 8.5∙10-16 cm2/s and varies weakly with the potential and the content of sodium fluoride in solution. Microscopic and profilometric studies show the development of the surface of the NiSi electrode during anodic etching.

Mechanism of electric conductivity change in BaCo0.7Fe0.2Nb0.1O3 − δ at intermediate temperature

In this paper, the change mechanism of electric conductance behavior of BaCo0.7Fe0.2Nb0.1O3−δ (BCFN) materials at the temperature range 550–650°C is studied. The ionic conductivity separated by the electrode-blocking method exhibits a gradual increase at this temperature range. However, the electronic conductivity shows a steady curve. According to the characterization of thermogravimetry (TG) and O2-Temperature Programmed Desorption (O2-TPD), the desorption of oxygen and the formation of oxygen vacancies also occur at this temperature range. Using the oxygen vacancy diffusion coefficients Dchem (obtained by Electric Conductivity Relaxation (ECR)) and the oxygen vacancy content (obtained by TG), the theoretical ionic conductivity is calculated by Einstein–Nernst equation. The theoretical value is in good agreement with the experimental results, which means that the increase of BCFN ionic conductivity can be explained by the mostly ionized oxygen vacancies. The defect reaction of BCFN suggests that there's consumption of 4+ charge B sites ions accompanied with the formation of oxygen vacancies, which will stabilize the electrical conductivity according to the Zener double exchange effects. Besides, the X-ray photoelectron spectroscopic (XPS) results suggest that the B sites ions are also greatly influenced by the formation of oxygen vacancies. This may increase the exchange energy in the Zener double exchange effect and stabilize the electrical conductivity.

Defect Chemical Models for Temperature Dependence of Oxygen Stoichiometry and Electrical Conductivities of Ba0.5Sr0.5Co0.8Fe0.2O3-δ

Ba0.5Sr0.5Co0.8Fe0.2O3-δ is a well-known cathode material of intermediate-temperature solid oxide fuel cell. To elucidate its electrical and electrochemical properties, three chemical defect models were adopted in this work for fitting the data of oxygen stoichiometry and electrical conductivities as a function of temperature. Reasonable values of concentrations and mobility of charge carriers, and were obtained with the cluster defect model. The p-type electronic conductivities and oxygen ionic conductivities were separated from the total electrical conductivities. The temperature-dependence of oxygen vacancy diffusion coefficient of BSCF were also obtained, and the results were comparable with the data measured experimentally.

Oxygen diffusion in single crystal barium titanate.

Oxygen diffusion in cubic, nominally undoped, (100) oriented BaTiO3 single crystals has been studied by means of (18)O2/(16)O2 isotope exchange annealing and subsequent determination of the isotope profiles in the solid by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Experiments were carried out as a function of temperature 973 < T/K < 1173, at an oxygen activity of aO2 = 0.200, and as a function of oxygen activity 0.009 < aO2 < 0.900 at T = 1073 K. The oxygen isotope profiles comprise two parts: slow diffusion through a space-charge zone at the surface depleted of oxygen vacancies followed by faster diffusion in a homogeneous bulk phase. The entire isotope profile can be described by a single solution to the diffusion equation involving only three fitting parameters: the surface exchange coefficient ks*, the space-charge potential Φ0 and the bulk diffusion coefficient D*(∞). Analysis of the temperature and oxygen activity dependencies of D*(∞) and Φ0 yields a consistent picture of both the bulk and the interfacial defect chemistry of BaTiO3. Values of the oxygen vacancy diffusion coefficient DV extracted from measured D*(∞) data are compared with literature data; consequently a global expression for the vacancy diffusivity in BaTiO3 for the temperature range 466 < T/K < 1273 is obtained, with an activation enthalpy of vacancy migration, ΔHmig,V = (0.70 ± 0.04) eV.

Macro- and microscopic properties of strontium doped indium oxide

Solid state synthesis and physical mechanisms of electrical conductivity variation in polycrystalline, strontium doped indium oxide In2O3:(SrO)x were investigated for materials with different doping levels at different temperatures (T = 20–300 °C) and ambient atmosphere content including humidity and low pressure. Gas sensing ability of these compounds as well as the sample resistance appeared to increase by 4 and 8 orders of the magnitude, respectively, with the doping level increase from zero up to x = 10%. The conductance variation due to doping is explained by two mechanisms: acceptor-like electrical activity of Sr as a point defect and appearance of an additional phase of SrIn2O4. An unusual property of high level (x = 10%) doped samples is a possibility of extraordinarily large and fast oxygen exchange with ambient atmosphere at not very high temperatures (100–200 °C). This peculiarity is explained by friable structure of crystallite surface. Friable structure provides relatively fast transition of samples from high to low resistive state at the expense of high conductance of the near surface layer of the grains. Microscopic study of the electro-diffusion process at the surface of oxygen deficient samples allowed estimation of the diffusion coefficient of oxygen vacancies in the friable surface layer at room temperature as 3 × 10−13 cm2/s, which is by one order of the magnitude smaller than that known for amorphous indium oxide films.

Electrical conductivity, thermal expansion and electrochemical performances of Ba-doped SrCo0.9Nb0.1O3−δ cathodes for IT-SOFCs

Perovskite-type oxides BaxSr1−xCo0.9Nb0.1O3−δ (BSCNx, x = 0.0–0.8) were synthesized and investigated as cathodes for IT-SOFCs. Ba doping improves chemical compatibility between BSCNx oxides and Ce0.9Gd0.1O1.95 (GDC) electrolyte. Effects of Ba doping on electrical conductivity, thermal expansion and electrochemical performances were systematically elucidated and discussed. Both thermal expansion coefficient (TEC) and polarization resistance (Rp) decrease with increasing Ba doping level up to x = 0.6, attain a minimum at x = 0.6 and then increase with further increasing x > 0.6. The decrease of TEC with the incorporation of Ba can be attributed to the weakened chemical expansion and the decrease of Rp with Ba is due to the increase of oxygen vacancy concentration and oxygen vacancy diffusion coefficient. With a 300 μm-thick GDC as electrolyte and BSCN0.6 as the cathode, the maximum power density of a single-cell achieves 778 mW cm−2 at 800 °C. All these results indicate that the BSCN0.6 oxide is a promising cathode material for IT-SOFC.

Comparison of oxygen exchange kinetics of the IT-SOFC cathode materials La0.5Sr0.5CoO3 − δ and La0.6Sr0.4CoO3 − δ

La0.5Sr0.5CoO3−δ and La0.6Sr0.4CoO3−δ are characterized with respect to application as cathode materials in intermediate temperature solid oxide fuel cells (IT-SOFCs). Surface exchange and transport parameters of oxygen are determined by the conductivity relaxation technique between 525°C and 725°C at oxygen partial pressures of 0.1, 0.01 and 0.001bar. Electrical conductivities of both compounds range between 1000 and 2400Scm-1 and are slightly higher for La0.6Sr0.4CoO3−δ. However, La0.5Sr0.5CoO3−δ shows superior performance with regard to oxygen diffusion, ionic conductivity and oxygen surface exchange within the investigated range of temperatures and oxygen partial pressures. At 725°C the chemical surface exchange coefficient of oxygen is 2×10-3cms-1 for La0.5Sr0.5CoO3−δ and 4×10-4cms-1 for La0.6Sr0.4CoO3−δ. At lower oxygen partial pressures a strong decrease in the surface exchange coefficient is observed and diffusion data cannot be obtained from relaxation measurements. Oxygen vacancy diffusion coefficients are similar for both compounds and range between 10-6 and 10-8cm2s-1 with activation energies around 100kJmol-1. At 725°C and 0.1bar oxygen partial pressure, the ionic conductivity of La0.5Sr0.5CoO3−δ is 1×10-2Scm-1 with an activation energy of 118kJmol-1. The results show that both compositions meet the requirements for the application as IT-SOFC cathodes in the short-term range.

Electrical Conductivity and Oxygen Diffusivity of Perovskite-Type Solid Solution La0.6Sr0.4Co1-yFeyO3-δ (y=0.2, 0.4, 0.5, 0.6, 0.8)

The electrical conductivity of La0.6Sr0.4Co1-yFeyO3-δ (y=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) were measured as a function of P(O2) and temperature. The conductivity variation can be well explained only by change in electron-hole concentration due to oxygen nonstoichiometry regardless of temperature. Conductivity relaxation analyses were also conducted to determine chemical diffusion coefficients by non-linear least squares fitting using the solution of Fick's diffusion equation. Oxygen vacancy diffusion coefficients were calculated using the chemical diffusion coefficients and oxygen nonstoichiometry. It should be emphasized that vacancy diffusion coefficients are not independent of oxygen vacancy concentration at lower temperature but steeply decrease with increase in oxygen vacancy concentration. These phenomena may cause the performance degradation of La0.6Sr0.4Co1-yFeyO3-δ cathode.