Activation Energy For Oxygen Diffusion Research Articles

Oxidation kinetics of Fe-(2-10)Mn-xSb alloys during annealing

The selective oxidation kinetics of a series of Fe-(2-10)Mn-(0.00/0.01/0.03)Sb (at. pct) were determined and a model was proposed for analyzing oxide growth during intercritical annealing prior to galvanizing. The annealing conditions (temperature and isothermal holding time) affected the thickness and depth of the oxidation zone, which in turn influenced the MnO growth rate. Increasing the alloy bulk Mn content increased the Mn elemental flux to the external surface, thereby increasing the oxidation parabolic rate constant and decreasing the oxidation activation energy. Micro-additions of Sb to the Fe-Mn alloys reduced the parabolic rate constant, which was attributed to the reduction of oxygen permeability at the surface and interfaces by Sb segregation at both the external and internal oxide interfaces. This reduction was more significant in the Fe-10Mn alloys due to higher Sb segregation at the interfaces. The average activation energy of internal oxidation for the Fe-2Mn, Fe-6Mn and Fe-10Mn alloys were determined to be 216 ± 15 kJ/mol, 178 ± 18 kJ/mol and 152 ± 10 kJ/mol, respectively, which are consistent with the activation energy of oxygen diffusion through MnO interfaces and the bulk diffusion of oxygen in austenite. Furthermore, the average activation energy for external oxide growth was ∼113 ± 18 kJ/mol, which was attributed to the diffusion of Mn cations along the grain boundaries of the external Mn oxides.

Analysis of atomic structure, magnetic ordering, and oxygen diffusion in oxygen deficient Sr3Fe2O7−δ perovskite: Toward rational catalysts design

Mechanistic analysis of oxygen transfer in an atomic-scale structure is important for various applications, such as the three-way catalysts for treating ${\mathrm{NO}}_{x}$ in automotive exhaust and solid oxide fuel cell electrolyte materials. It has been shown that the $n=2$ Ruddlesden-Popper phases ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{7.0\ensuremath{-}\ensuremath{\delta}}$ have superior oxygen storage capacity compared to the conventional ${\mathrm{CeO}}_{2}\text{\ensuremath{-}}{\mathrm{ZrO}}_{2}$. Atomic structure, magnetic ordering, charged defects arising due to oxygen vacancies, and the effect of lattice parameters on the ionic transport in ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{7.0\ensuremath{-}\ensuremath{\delta}}$ within the oxygen content of 5.75 to 7.0 were investigated by using $\mathrm{DFT}+U$ calculations. In the case of ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{6.0\ensuremath{-}7.0}$, the oxidation reaction energies were found to be almost independent of the initial oxygen content. We investigated the activation energies of oxygen diffusion by using the equilibrium and compressively strained lattice parameters for ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{6.5}$. The activation energies were found to be nearly proportional to the volume of the unit cell; namely, as the volume of the unit cell increases, the activation energy decreases. It was found that in the oxygen diffusion process the activation energy increases as the bond length between iron and oxygen decreases and the bond length is minimum in the transition state, indicating that the iron-oxygen repulsive interaction is the origin of the activation energy. Moreover, in addition to the dominant migration pathway in ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{6.0\ensuremath{-}7.0}$, a new migration pathway in ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5.75\ensuremath{-}6.0}$ was found.

Effect of Uniaxial Strain on Oxygen Diffusion in Grain Boundaries of Polycrystalline YBa2Cu3O7-Coated Conductors

For polycrystalline YBa2Cu3O7 (YBCO)-coated conductors, the diffusion behavior of oxygen ions during the preparation process, especially in grain boundaries (GBs), results in the motion of oxygen vacancies, and finally has a strong impact on the critical current density (Jc) of YBCO conductors. Such the oxygen diffusion in GBs has been found to be very sensitive to applied strain. In order to deeply understand the strain effect, in this article, we have studied the oxygen diffusion in polycrystalline YBCO conductors under uniaxial strain by molecular dynamic simulation. On the two different bicrystal structural models, the calculated results display that the oxygen diffusion in the grain boundary region shows a downward trend with the loading of the uniaxial tensile or compressive strain. And also, we found that applied strain leads to the variation of the microstructure of GBs, and thus results in reducing the oxygen vacancy concentration. As our previous study concluded, the high diffusion of oxygen can be regarded as a medium for analyzing the supercurrent through grain boundary. This paper further illustrates that the relation between the activation energy of oxygen diffusion and the strain loading has an almost similar form as that between the critical current density and the strain. This indicates that the activation energy of oxygen diffusion in the grain boundary region could be closely related to the critical current density, which arouses our interest to further explore whether or not it can be used as an index of predicting the current-carrying capability of YBCO conductors.

Partial oxidation of methanol to formaldehyde in an annular reactor

A structured reactor with annular configuration was applied for studying methanol oxidation to formaldehyde over silver. By eliminating gas phase reactions, high formaldehyde selectivity (93–97%) was obtained at low methanol and oxygen conversion under practically isothermal reaction conditions. CH2O and CO2 were the only carbon containing products, and both may be claimed as primary products along with H2. It also proves that CO is formed by homogenous decomposition of CH2O and should not be considered a main precursor to CO2, as assumed in several reaction mechanisms. The analysis of H2/CO2 ratio as a function of temperature provides an estimate of contributions from dehydrogenation and partial oxidation of methanol, and clearly suggests presence of a dehydrogenation pathway to CH2O. Extracting kinetic parameters is challenging due to a correlation between activity, oxygen dissolution, and silver restructuring and morphology and its dependence on temperature. Nevertheless, the data indicate 1st order with respect to oxygen. Conditioning by reaction at high temperature followed by a temperature ramp was performed to minimize the impact of a gradually changing Ag catalyst. The resulting Arrhenius analysis implies two distinct regions of activity. The apparent activation energy was estimated to 41 kJ/mol for the high temperature region, a value close to the activation energy for oxygen diffusion in silver at high temperature. The investigation demonstrates benefits of using an annular reactor configuration in bridging lab scale investigations with industrial conditions. Collecting reaction data at low oxygen conversion is enabled, which has not been achievable in conventional lab scale reactors this far.

Sintering of Periclase with Brucite-Aluminum Phosphate Binder

The sintering kinetics of periclase with brucite-aluminum phosphate binder was studied. A model that takes into account the role of the physical compaction and chemical binding in the presence of a binder during the heating process was used to determine the kinetic parameters. The strength of a conglomerate was due to the sintering itself as well as the action of the binder. The effective energy of activation of the sintering of periclase was determined: Ea = 287 ± 9 kJ/mole, which is close to the activation energy of oxygen diffusion in MgO. Spinel MgAl2O4, which slows the sintering process, forms in the material at high temperatures.

Oxygen self-diffusion in near stoichiometric (U,Pu)O2 at high temperatures of 1673-1873 K

The oxygen self-diffusion coefficients in near stoichiometric (U,Pu)O2 at high temperatures were successfully measured by thermogravimetry combined with the oxygen isotope exchange method. The activation energy for oxygen diffusion in the stoichiometric composition of (U,Pu)O2 was evaluated from experimental data, and the value was determined to be 248 kJ/mol. In addition, the defect migration energies of (U,Pu)O2±x were derived, and the oxygen self-diffusion coefficients were evaluated using these. As a result, good agreement was found between the experimental data and the oxygen self-diffusion coefficients calculated using the defect migration energies.

Dynamics of Yb2+ to Yb3+ ion valence transformations in Yb:YAG ceramics used for high-power lasers

Due to the good thermal-mechanical and luminescence properties, Yb:YAG ceramics are suitable as thin-disk lasers; however, their efficiency is limited by the presence of Yb2+ ions, which entail parasitic energy transfer from Yb3+ to Yb2+. This article focuses on the Yb2+ formation in Yb:YAG ceramics prepared by solid-state reaction sintering. The samples were prepared by air annealing, the oxidation of the material leads to recharging Yb2+ ion to its trivalent state. The activation energy was determined by Jander to be Ea(D) = 2.7 ± 0.2 eV, which is in good agreement with the activation energy for oxygen diffusion in the YAG lattice. It was concluded that the recharging of Yb2+ ion to its trivalent state in YAG ceramics is limited by the oxygen self-diffusion through the grain volume, and the oxygen vacancy alone and/or together with the presence of antisites can be proposed as Yb2+ charge compensation mechanism in the YAG ceramics, unlike the YbAG single crystals, where tetravalent impurities are responsible for charge compensation.

Oxidation of Thin Titanium Films: Determination of the Chemical Composition of the Oxide and the Oxygen Diffusion Factor

The morphologies and local electronic structures of titanium coatings deposited on the surfaces of highly oriented pyrolytic graphite were determined. Chemical compositions of the oxides formed on the coating surfaces were established. A theoretical model was developed describing the changes in the TiOx oxides (1.75 < x < 2) band gap depending on the duration and temperature of the titanium film annealing procedure in oxygen. The effective activation energy of oxygen diffusion in TiOx (1.75 < x < 2) was determined, and the pre-exponential factor of the diffusion coefficient was estimated.

Valence State of Iron and Molybdenum Cations under Conditions of Anionic Deficiency in Sr2FeMoO6–δ

The activation energy of oxygen diffusion in strontium ferromolybdate Sr2FeMoO6–δ is determined by the Merzhanov technique based on the temperature dependences of the oxygen desorption dynamics. The activation energy has a minimum value of 76.7 kJ mol−1 at δ = 0.005 and a maximum value of 156.3 kJ mol−1 at δ = 0.06. It is suggested that with an increase in the concentration of oxygen vacancies, an interaction occurs between them and the nearest cations, with the subsequent formation of associates of various types that are less mobile than the single anion vacancies. According to Mössbauer spectroscopy data, it has been established that the appearance of oxygen vacancies and their ordering contribute to the isomer shift, and some of the iron ions occupy the tetrahedral (or close to it) positions in the lattice. This indicates the formation of associates of oxygen vacancies. The results of X‐ray photoelectron spectroscopy (XPS) studies show that the increase in the concentration of oxygen vacancies results in a decrease of the Mo6+ and Fe2+ concentrations. At the same time, the number of Mo5+ and Fe3+ cations increases due to the redistribution of the electron density, and molybdenum cations in a different valence state (Mo4+) appear.

A- and B-site Codoped SrFeO3 Oxygen Sorbents for Enhanced Chemical Looping Air Separation.

Chemical-looping air separation has numerous potential benefits in terms of energy saving and emission reductions. The current study details a combination of density functional theory calculation and experimental efforts to design A- and B-site codoped SrFeO3 perovskites as "low-temperature" oxygen sorbents for chemical-looping air separation. Substitution of the SrFeO3 host structure with Ca and Co lowers oxygen vacancy formation energy by 0.24-0.46 eV and decreases the oxygen release temperature. As a result, Sr1-x Cax Fe1-y Coy O3 (SCFC; x=0.2, 0.0<y<1.0) spontaneously releases oxygen at 400-500 °C even under a relatively high oxygen partial pressure (e.g. P =0.05 atm). Sr0.8 Ca0.2 Fe0.4 Co0.6 O3 exhibits a significantly higher oxygen capacity of 1.2 wt % at 400 °C and under a P swing between 0.05 and 0.2 atm, when compared to the <0.2 wt % capacity for undoped a SrFeO3 (SF) and Ca-doped Sr0.8 Ca0.2 FeO3 (SCF). Electrical conductivity relaxation (ECR) study demonstrates that codoping of Ca and Co lowers the activation energy of oxygen diffusion and surface oxygen exchange by 26.6 or 137.9 kJ mol-1 , respectively, resulting in faster redox kinetics for SCFC than for SCF perovskite. The SCFC oxygen sorbent also exhibits excellent stability for 2000 redox cycles for air separation.

BRUCITE-BASED MAGNESIUM PHOSPHATE BONDING AGENT, ITS ANALYSIS AND APPLICATION FOR PERICLASE SINTERING

Some results on magnesium phosphate binder obtained from natural brucite (mainly Mg(OH)2) and H3PO4 were discussed. Hereafter, it was named brucite phosphate (BPB). This binder then was used to the sintering of MgO (periclase). MgO is one of the most high-refractory materials and chemically stable to various aggressive mediums, so the aim was presented as actual. To determine the phase composition data on sample weight loss in different temperature intervals were compared. All weight losses were related to the multistage water moving away. Up to 140 °С it was the removal of crystalline hydrate water. Then monosubstituted magnesium hydrophosphate transformed into MgH2P2O7 and Mg2P4O12, that in principle was in accordance with literature data. Dehydration was accompanied with appropriate endothermal effects. There was a slight exothermal effect which could relate to a structure rearrangement (cyclization) of primary magnesium metaphosphates and/or some crystallization of anhydrous cyclophosphates from primarily obtained amorphous phase non-connected with a weight loss in the interval of 450-575 °С. At 500–1000 °С the phase composition remained constant that was in a good accordance with data obtained by other methods. By means of IR-spectroscopy, it was confirmed the formation of structure cycles composed from oxygen-phosphor tetrahedrons. To determine kinetics parameters of periclase sintering with BPB it was used a model being proposed that took the role of a physical consolidation and a chemical binding in the presence of binder into account under heating. The obtained parameters analysis showed that the forming conglomerate strength was provided with the sintering properly as well as with binder action; an estimated chemical factor contribution was maximal at low temperatures (1100-1200 °C). The activation energy value was practically identical to the activation energy of oxygen diffusion in MgO.

Thermally stimulated oxygen desorption in Sr2FeMoO6-δ

Polycrystalline Sr2FeMoO6-δ specimens have been obtained by solid state synthesis from partially reduced SrFeO2.52 and SrMoO4 precursors. It has been shown that during oxygen desorption from the Sr2FeMoO6-δ compound in polythermal mode in a 5%H2/Ar gas flow at different heating rates, the oxygen index 6–δ depends on the heating rate and does not achieve saturation at T = 1420 K. Oxygen diffusion activation energy calculation using the Merzhanov method has shown that at an early stage of oxygen desorption from the Sr2FeMoO6-δ compound the oxygen diffusion activation energy is the lowest Еа = 76.7 kJ/mole at δ = 0.005. With an increase in the concentration of oxygen vacancies, the oxygen diffusion activation energy grows to Еа = 156.3 kJ/mole at δ = 0.06. It has been found that the dδ/dt = f(Т) AND dδ/dt = f(δ) functions have a typical break which allows one to divide oxygen desorption in two process stages. It is hypothesized that an increase in the concentration of oxygen vacancies V ·· leads to their mutual interaction followed by ordering in the Fe/Mo–O1 crystallographic planes with the formation of various types of associations.

High temperature oxidation behaviour of V-Ti-Ta alloys

The high temperature oxidation behavior of V and V-4Ti-(1–7) Ta alloys was investigated under air atmosphere in the temperature range from 250 to 750 °C using isothermal oxidation testing. The oxidized samples were characterized with Grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS). Oxidation kinetics of the investigated samples at different temperatures (250, 300 and 500 °C) have been established. The activation energies for oxidation were estimated for V, V-4Ti-(1–7)Ta alloys and compared with reference V-4Cr-4Ti alloy. V-4Ti-7Ta alloy exhibited 20% lesser weight gain compared to V-4Cr-4Ti alloy. The improved oxidation performance of V-Ti-Ta alloys was attributed to decreased activity of vanadium in V-Ti-Ta alloy. Ta increases the activation energy for oxygen diffusion in V-Ti-Ta alloys, which contributes to enhance the oxidation performance of the alloys.

Ab initio modeling of oxygen ion migration in non-stoichiometric bismuth titanate pyrochlore Bi1.5Ti2O6.25

Ab initio investigation of the possibility of oxygen ion migration in non-stoichiometric bismuth titanate pyrochlore Bi1.5Ti2O6.25 has been performed. Density functional theory calculations together with the nudged elastic band method were used to estimate the O2– migration barriers. The migration energy barriers for the most favorable paths of oxygen migration related to O48f–O48f jumps are in the range 0.67–0.92 eV with the corresponding vacancy formation energy in the range 1.76–2.60 eV. These paths form extensive migration network in non-stoichiometric Bi1.5Ti2O6.25 which is sufficient for 3-dimensional O2– diffusion. The results show a good agreement with the experimentally obtained activation energy of oxygen diffusion in doped bismuth titanate pyrochlores with low dopant concentration.

Activation energy of oxygen diffusion: A possible indicator of supercurrents through YBa2Cu3O7 grain boundaries

Oxygen diffusion is the main transportation form during the preparation process of polycrystalline superconductor YBa2Cu3O7 (YBCO) thin films. Especially the diffusion of oxygen ions in grain boundaries (GBs) can lead to the formation of oxygen vacancies, and finally have an impact on the critical current density (Jc) of YBCO conductors. To deeply understand the supercurrent-limiting mechanism of GBs, for an YBCO bicrystal (containing a single GB), in this article we intensively studied the diffusion of oxygen ions through GB region by molecular dynamics simulations. The calculated results show that the oxygen diffusion in the YBCO bicrystals can be enhanced in GBs, and specifically which is closely related to the grain boundary angle (θ). The most interesting finding is that the activation energy of oxygen diffusion shows an exponential fall with the grain boundary angle, which is exactly consistent with the relation of the critical current density versus the grain boundary angle (Jc ~ θ) in GBs. We firmly believe that the activation energy of oxygen diffusion should have an essential relevance with the Jc especially in YBCO GBs, and can be as an indicator for predicting supercurrents. For these results we further gave an in-depth discussion.

Oxygen ion conducting pyrochlore oxides prepared by an ultrasound-assisted wet chemistry route: Ca-doped Gd2Ti2O7 nanocrystals

Abstract Different compositions of pyrochlore-type oxides of general formula Gd2-xCaxTi2O7 (x = 0, 0.05 and 0.1), interesting solid oxide ion conducting materials, were prepared by a novel wet-chemistry method assisted by ultrasound. Titanium isopropoxide (TTIP), and Gd3+ and Ca2+ hydrated nitrates were used as metal sources; whereas, ethanol and ethylene glycol were used as solvents. The results corroborate the feasibility of this method to obtain nanometric particles of lanthanide titanates. Electrical properties of uniaxially pressed and sintered pellets (1200 and 1500 °C) were measured as a function of temperature (280–700 °C) and frequency (100 Hz-1MHz) by using impedance spectroscopy. The Analysis of electrical properties revealed an increasing of dc conductivity (σ) for higher contents of Ca2+ in the system, reaching a maximum of 3.55 × 10−3 S cm−1 at 700 °C for x = 0.1. Calculated activation energies for oxygen diffusion in the Ca-doped samples, are in the 0.68–0.89 eV range, well below that of pristine Gd2Ti2O7 (2.23 eV), which explains their enhanced conductivity. Increasing of dc conductivity is promoted by substitution of Gd3+ by Ca2+ in these titanates, generating oxygen vacancies in the structure, which facilitates the ion migration process.

A limiting current oxygen sensor prepared by a co-pressing and co-sintering technique

La0.8Sr0.2Ga0.8Mg0.2O3–δ (LSGM) and La0.8Sr0.2(Ga0.8Mg0.2)1–xCrxO3–δ (LSGMC, 0.1 ≤ x ≤ 0.9) were synthesized by a solid-state reaction and characterized by X-ray powder diffraction (XRD), thermal expansion, and DC four-probe method. A limiting current oxygen sensor using LSGM solid electrolyte and LSGMC (x = 0.7) dense diffusion barrier was prepared by a co-pressing and co-sintering technique and the sensing performances were evaluated. XRD shows the presence of a crystal structure in the series belonging to Pm-3m (No. 221) for x = 0–0.9. Electrical conductivity and thermal expansion coefficients first decrease and then increase with increasing x. LSGMC (x = 0.7) is the most suitable ceramic material and is selected as dense diffusion barrier. There is a directly proportional relationship between limiting current and oxygen concentration, and the equation iL(mA)=3.65CO2(%) is established from the measured data by simple regression analysis. The Arrhenius model for limiting current is logiL(mA)=3.11−2.16/T(K), and the activation energy for oxygen diffusion is 0.43 eV.

A study of fine-grained ceramics based on complex oxides ZrO2-Ln2O3 (Ln = Sm, Yb) obtained by Spark Plasma Sintering for inert matrix fuel

Fine-grained ceramics based on solid solutions of ZrO2-x-Ln2O3 (where Ln = Sm, Yb), which are used to simulate inert matrix fuel (IMF), were obtained by colloidal chemical synthesis of powders and their Spark Plasma Sintering (SPS). It has been established that the SPS activation energy (Qs) for fine-grain ceramics based on ZrO2 corresponds to the activation energy of oxygen diffusion on grain boundaries of zirconium oxide (zirconia). We have demonstrated that introducing oxides of samarium (Sm2O3) and ytterbium (Yb2O3) into zirconia decreases SPS activation energy and stabilizes the phase composition of zirconia. Ceramics with densities up to 100% were obtained by the following modes of SPS: sintering temperature 1000–1250 °C, sintering time 6–10 min. Hydrolysis testing in the static mode (Т = 20 °C, P = 1 atm) in distilled water revealed no leaching of lanthanide ions at detection thresholds of 5 ppm for Sm ions and 0.1 ppm for Yb ions.

Thermally stimulated oxygen desorption in Sr2FeMoO6-δ

Polycrystalline Sr2FeMoO6-δ specimens have been obtained by solid state synthesis from partially reduced SrFeO2,52 and SrMoO4 precursors. It has been shown that during oxygen desorption from the Sr2FeMoO6-δ compound in polythermal mode in a 5%H2/Ar gas flow at different heating rates, the oxygen index 6-δ depends on the heating rate and does not achieve saturation at T = 1420 K. Oxygen diffusion activation energy calculation using the Merzhanov method has shown that at an early stage of oxygen desorption from the Sr2FeMoO6-δ compound the oxygen diffusion activation energy is the lowest Еа = 76.7 kJ/mole at δ = 0.005. With an increase in the concentration of oxygen vacancies, the oxygen diffusion activation energy grows to Еа = 156.3 kJ/mole at δ = 0.06. It has been found that the dδ/dt = f (Т) and dδ/dt = f (δ) functions have a typical break which allows one to divide oxygen desorption in two process stages. It is hypothesized that an increase in the concentration of oxygen vacancies Vo•• leads to their mutual interaction followed by ordering in the Fe/Mo-01 crystallographic planes with the formation of various types of associations.

Effect of annealing on structural changes and oxygen diffusion in amorphous HfO2 using classical molecular dynamics

Non-volatile memory is a promising alternative to present memory technologies. Oxygen vacancy diffusion has been widely accepted as one of the reasons for the resistive switching mechanism of transition-metal-oxide based resistive random access memory. In this study, molecular dynamics simulation is applied to investigate the diffusion coefficient and activation energy of oxygen in amorphous hafnia. Two sets of empirical potential, Charge-Optimized Many-Body (COMB) and Morse-BKS (MBKS), were considered to investigate the structural and diffusion properties at different temperatures. COMB predicts the activation energy of 0.53 eV for the temperature range of 1000–2000 K, while MBKS predicts 2.2 eV at high temperature (1600–2000 K) and 0.36 eV at low temperature (1000–1600 K). Structural changes and appearance of nano-crystalline phases with increasing temperature might affect the activation energy of oxygen diffusion predicted by MBKS, which is evident from the change in coordination number distribution and radial distribution function. None of the potentials make predictions that are fully consistent with density functional theory simulations of both the structure and diffusion properties of HfO2. This suggests the necessity of developing a better multi-body potential that considers charge exchange.