Oxygen Diffusion Mechanism Research Articles

Diffusion mechanism of bound Schottky defect in magnesium oxide

In simple ionic crystals, intrinsic point defects must satisfy electrical neutrality and exist as Schottky defects. In magnesium oxide (MgO), a Schottky defect is then a combination of anionic and cationic vacancies. Since vacancies are charged, the stable configuration of the Schottky defect corresponds to a bound pair of vacancies of opposite signs. In this study, we investigate the kinetics of formation and migration of such a bound pair on long timescales reaching in some cases thousands of seconds using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with an event catalog built on-the-fly during static molecular simulations. We show that the diffusion of this bound Schottky defect involves the migration of vacancies bounded to the first and third neighbor sites of the crystal structure with an apparent migration energy which cannot be inferred from the migration energies expected from isolated defects. Overall, this study gives insights and constraints on the oxygen diffusion mechanism reported experimentally in high-purity MgO samples.

A FRACTAL MODEL FOR PREDICTING OXYGEN EFFECTIVE DIFFUSIVITY OF POROUS MEDIA WITH ROUGH SURFACES UNDER DRY AND WET CONDITIONS

Oxygen diffusion in porous media (ODPM) with rough surfaces (RS) under dry and wet conditions is of great interest. In this work, a novel fractal model for the oxygen effective diffusivity of porous media with RS under dry and wet conditions is proposed. The proposed fractal model is expressed in terms of relative roughness, the water saturation, fractal dimension for tortuosity of tortuous capillaries, fractal dimension for pores, and porosity. It is observed that the normalized oxygen diffusivity decreases with increasing relative roughness and fractal dimension for capillary tortuosity. It is found that the normalized oxygen diffusivity increases with porosity and fractal dimension for pore area. Besides, it is seen that that the normalized oxygen diffusivity under wet condition decreases with increasing water saturation. The determined normalized oxygen diffusivity is in good agreement with experimental data and existing models reported in the literature. With the proposed analytical fractal model, the physical mechanisms of oxygen diffusion through porous media with RS under dry and wet conditions are better elucidated. Every parameter in the proposed fractal model has clear physical meaning, with no empirical constant.

Effect of mineral fillers on ageing of bituminous mastics

Past research has shown that the ageing process of bitumen is not solely a function of the bitumen type itself, but rather, added effects have been identified attributed to the presence of fillers. Nevertheless, contradicting results exist between the various studies with respect to the implications of fillers on the ageing process of bitumen. The present study aims to develop a better understanding regarding the role of the fillers in the ageing process of bitumen. Bitumen-filler blends (mastics) were prepared, according to a single design protocol, and the resulting materials along with pure bitumen were subjected to accelerated laboratory ageing by means of the pressure ageing vessel. The ageing effect was studied by using dynamic shear tests and infrared spectroscopy measurements. The results revealed the ability of fillers to mitigate age-hardening of bitumen, despite the fact that formation of carbonyls in the bitumen matrix was observed. Oxygen diffusion and adsorption mechanisms were found to be responsible for the observed favourable effect of fillers on the ageing of bitumen, while the gradation, the mineralogical/elemental composition and, to some extent, the specific surface area of the fillers where established as the prevailing fillers’ properties regulating the oxidation mechanisms. The investigation of recovered binders from aged mastics did not lead to any further insight regarding the effect of fillers on the ageing of bitumen. Instead, features were revealed which manifest that the extraction and recovery of bitumen may not be a suitable approach to address the research question of this study.

Nuclear shell model for the ignition process of boron particle with binary surface oxide layer

Abstract The combustion of a single boron particle contains three continuous macroscopic processes, including ignition delay, ignition and combustion. Studies show that among these processes, the intermediate ignition process has the most complicated kinetic mechanism. On the other hand, conventional models have serious shortcomings to simulate this process. The main challenges in this regard are indeterminate initial thickness of the surface oxide layer, incomplete diffusion mechanism in the surface oxide layer, and neglected effects of the pressure and particle size on the control mechanism. In order to overcome the drawbacks of existing ignition models, a comprehensive kinetic ignition model is developed in the present study. To this end, the dual-beam focused ion beam micro/nanofabrication system is applied to etch and cut the heat-treated boron particles into slices. Then, the scanning transmission electron microscope with collateral energy dispersive spectrometer is applied for analyzing the microstructure and elementary composition of the slice. It is found that the average thickness of the surface oxide layer is higher than earlier assumed values, and the bidirectional diffusion mechanism of boron and oxygen in the liquid boron oxide is confirmed. During the model development, seven heterogeneous global reactions and two vapor-phase reactions are used in total for analyzing the kinetic mechanisms. In the present study, different indicators, including the vaporization process, O2 reactions, and H2O reactions on the internal and external surfaces of the binary oxide layer are investigated. Moreover, the conservation laws of mass and energy are used for the establishment of governing equations. Then, the accuracy of the developed model is evaluated by comparing the obtained results for the ignition time with that of the experiment, which shows a reasonable consistency between them. Then, the proposed model is applied to investigate the ignition characteristics of boron particles. Effects of different parameters, including the environment temperature, O2 concentration, and H2O concentration are studied on the ignition time. It is concluded that the established model, which is named as the nuclear shell (N-S) model is a powerful scheme to better understand the ignition mechanisms and characteristics of boron particles and can be further applied in the field of computational fluid mechanics.

Probing One-Dimensional Oxygen Vacancy Channels Driven by Cation-Anion Double Ordering in Perovskites.

Visualizing the oxygen vacancy distributions is highly desirable for understanding the atomistic oxygen diffusion mechanisms in perovskites. In particular, the direct observation of the one-dimensional oxygen vacancy channels has not yet been achieved in perovskites with dual ion (i.e., cation and anion) ordering. Here, we perform atomic-resolution imaging of the one-dimensional oxygen vacancy channels and their structural dynamics in a NdBaCo2O5.5 double perovskite oxide. An in situ heating transmission electron microscopy investigation reveals the disordering of oxygen vacancy channels by local rearrangement of oxygen vacancies at the specific temperature. A density functional theory calculation suggests that the possible pathway of oxygen vacancy migration is a multistep route via Co-O and Nd-Ov (oxygen vacancy) sites. These findings could provide robust guidance for understanding the static and dynamic behaviors of oxygen vacancies in perovskite oxides.

Understanding and Mitigating Stress Memorization Technique of Induced Layout Dependencies for NMOS HKMG Device

For the first time, this research addresses the notable layout proximity effects induced by stress memorization technique in planer high-k/Metal gate NMOS device systematically, including width effect, different shallow trench spacing effect, and length of diffusion effect. Based on the oxygen diffusion mechanism analysis of layout proximity effects in high-k/Metal gate NMOS device, an optimized process is proposed to suppress the layout dependency. The experiment result indicates that modified low temperature stress memorization technique process can suppress layout dependency efficiently without performance degradation of the devices.

Native point defects and oxygen migration of rare earth zirconate and stannate pyrochlores

Various excellent properties of rare earth zirconate and stannate pyrochlores are close related with their native point defects. First-principles calculations are performed to systematically investigate the point defect mechanism and the oxygen diffusion behavior of A2B2O7 (A=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd; B=Zr, Sn). The possible defect complexes and their associated reactions under stoichiometric and nonstoichiometric conditions are explored. The O Frenkel pairs are the most stable defect structure in stoichiometric zirconates, whereas the cation antisite defects are the predominant one in stoichiometric stannates. In the case of BO2 excess zirconates and stannates, the BA cation antisite defect with the A vacancy and/or the oxygen interstitial is energetically favorable, whereas the AB antisite defect together with the oxygen vacancy and/or the A interstitial is preferable under the A2O3 excess condition. Meanwhile, the maximum point defect concentrations of zirconates are much higher than those of stannates. Furthermore, the oxygen migration barriers are similar in these compounds, ranging in 0.68 eV∼ 0.80 eV. The predicted point defects and oxygen diffusion mechanisms play the critical role in their engineering applications and are expected to guide the future property improvement of pyrochlores through the control of point defects and/or composition.

Local Structures of Oxygen-Deficient Perovskite Sr2ScGaO5 Polymorphs Explored by Total Neutron Scattering and EXAFS Spectroscopy.

Depending on the synthesis route, the oxygen ion electrolyte Sr2ScGaO5 shows two polymorphs, a brownmillerite and a cubic perovskite framework. In order to better explore oxygen diffusion pathways and mechanisms, we report here on a multitechnical approach to characterize local structural changes for Sr2ScGaO5 polymorphs as a function of temperature, using a neutron pair distribution function (PDF) analysis together with an extended X-ray absorption fine structure (EXAFS) analysis. While for the brownmillerite type structure PDF and Rietveld refinements yield identical structural descriptions, considerable differences are found for the cubic oxygen-deficient polymorph. On a local scale a brownmillerite type vacancy structure could be evidenced for the cubic phase, suggesting a complex short-range ordering and respective microstructure. Both PDF and EXAFS data confirm an octahedral and tetrahedral coordination for Sc and Ga, respectively, at a local scale for both polymorphs. Related changes in the bond distances and oxygen vacancy ordering are discussed.

The first-principles calculations to explore the mechanism of oxygen diffusion on vacancy defective graphene in marine environment

Graphene can play an important role in the application of marine anticorrosion. However, the vacancy defects of graphene accelerate the diffusion of oxygen to deteriorate its corrosion resistance, especially in chlorine-containing marine environments. In present paper, the diffusion energy barriers of O on P, B or Si-doped vacancy defective graphene were calculated by the first principles simulation, it was found that B-doping in vacancy defective graphene could most effectively increase the diffusion barrier of oxygen in the whole diffusion stage. Combining Bader charge and differential charge density, the bonding properties of O, Cl and dopant atoms were analyzed systematically. Adsorption energy showed that the dopant P, Si or B atom could have a stronger effect on oxygen adsorption so that it inhibited the diffusion of oxygen for vacancy defective graphene. In particular, B-doped vacancy defective graphene with highest diffusion energy barrier could most effectively inhibit the diffusion of O, indicating that it could provide the most protection against oxidative corrosion in marine environment, such as the ocean and coastal area.

Investigation on the oxygen reduction reaction mechanism of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ@La2NiO4+δ core-shell structure cathode for solid oxide fuel cells

Excellent electrochemical performance is achieved by PrBa0.5Sr0.5Co1.5Fe0.5O5+δ@La2NiO4+δ (PBSCF@LN) core-shell structure cathode, LN coating's existence enhances the cathode catalytic activity by promoting the oxygen reduction reaction (ORR) surface process. Besides the LN's excellent oxygen adsorption-desorption property, roughness surface of LN coating provides more ORR active sites compared to pristine PBSCF cathode. Thorough investigation is carried out based on the PBSCF and LN oxide's oxygen diffusion mechanism and corresponding ORR process elementary steps are constructed. Combine the theoretical derivations with experimental results, rate-determining step of PBSCF and PBSCF@LN cathode can be ascertained. With LN coating's introduction, PBSCF cathode's rate-determining step transfers from surface oxygen exchange to oxygen species' crossing LN coating, reducing thickness of LN coating can further improve PBSCF@LN cathode’ performance.

Structural disorder and magnetic correlations driven by oxygen doping in Nd2NiO4+δ ( δ∼0.11 )

We investigated the influence of oxygen over-stoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystalizes in a tetragonal commensurate structure with the P42/ncm space group with excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase. Maximum Entropy analysis of the neutron single crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant 4b interstitial site, related to the phonon assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weak ferromagnetic component along the c-axis, while the long-range magnetic ordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at TN with the propagation vector k = (100) and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the P42/nc`m` Shubnikov space group. Refined magnetic moments of the Ni and Nd-sites at 2 K are 1.144(76) muB and 1.632(52) muB respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed.

Molecular dynamics simulations of point/line defect structures and oxygen diffusion mechanism in yttrium-doped barium zirconate

Yttrium-doped barium zirconate is one of the most attractive electrolyte candidates for solid oxide fuel cells due to its excellent chemical stability and ionic conductivity. For the first time, this paper studies the barium zirconate systems coexisting point defects and line defects by classical molecular dynamics simulations in combination with the Coulomb-Buckingham potential, and the influences of yttrium dopants and oxygen vacancies concentrations and the edge dislocations on the structures and oxygen transport properties. The calculated results have shown that the diffusion coefficient is related to concentration of dopant, and it is not conducive to oxygen conduction when the dopant concentration is too high or too low. And it is concluded that 30% Y-doped barium zirconates have the highest diffusion coefficients regardless of the presence of edge dislocation defects. In addition, at the temperature of 1073.15 K, the diffusion of oxygen ion has been accelerated by edge dislocations when the dopant concentration is less than 30%, which could be explained by the gathering-jogging transport mechanism among edge dislocation, oxygen vacancy and oxygen ion. Therefore, the dopant concentration of barium zirconate should not be too high in practical applications of solid electrolytes, and it is feasible to introduce line defects to enhance ion conductivity.

Brownmillerite phase formation and evolution in epitaxial strontium ferrite heterostructures

By controlling the synthesis and process conditions, we show that the oxygen vacancy channels (OVCs) of brownmillerite-structured SrFeO2.5+δ (0 ≤ δ < 0.5) thin films can be oriented differently on the same LaAlO3(001) substrate due to either a bottom up or top down synthesis route. The dynamic brownmillerite structure formation and evolution processes yield distinctively different optical and electronic properties, owing to the difference in both the oxygen stoichiometry and the OVC configuration in the resultant films. With the established recipe, we generated insulating SrFeO2.5 with vertically aligned OVCs and metallic SrFeO3 standards so that the other intermediate states can be directly compared and understood. Our results offer further insight into the phase stability and oxygen-diffusion mechanisms, which is important for the predictive synthesis of novel functional materials.

First-principles calculations of the oxygen-diffusion mechanism in mixed Fe/Ti perovskites for solid-oxide fuel cells

Abstract The ionic conductivity of transition-metal (TM) oxide perovskites plays an essential role in their applications in solid-oxide fuel cells. The conductivity of oxygen ions can increase the reaction rate on the electrode surface and improve the performance of fuel-cell devices. Herein, the oxygen-diffusion mechanism in a series of Fe/Ti oxide perovskites, especially oxygen-vacancy formation and migration, was investigated using density functional theory calculations. By simulating various vacant sites, we found that the oxygen vacancy adjacent to the Fe cation has lower formation energy than that between two Ti cation sites. Analysis of differential charge densities demonstrated that electrons originating from oxygen vacancies are reorganized at the d -state of Fe through the weak covalency of TM–O bonds, resulting in the low formation energy of these vacancies. The energy profiles of oxygen migration around Fe and Ti cations were calculated in the rocksalt supercell of SrFe0.5Ti0.5O3 through the nudged-elastic-band method. The oxygen migration barrier around Ti cations was lower than that of Fe cations, which suggests that Ti cations also play an important role in oxygen diffusion in the perovskites. Overall, our study elucidated the oxygen-diffusion mechanism in the B-sites of mixed Fe/Ti oxide perovskites and provides a theoretical basis for the design of TM oxide perovskites with mixed ionic and electronic conductivity for solid-state fuel cells.

Enhanced surface strengthening of titanium treated by combined surface deep-rolling and oxygen boost diffusion technique

Thermal oxidation and oxygen boost diffusion were respectively applied on the rolled and unrolled titanium to improve the mechanical properties of titanium. After deep-rolling process, the gradient plastic deformation layer was generated. The deep-rolling technique speeded up the thermal oxidation of titanium. Rolled titanium exhibits a better hardening effect than unrolled titanium by thermal oxidation or oxygen boost diffusion treatment. By means of SEM, XRD and first-principles calculations, the oxygen diffusion mechanisms of rolled and unrolled titanium at different stages were quantitatively analyzed. Finally, the optimized processing parameters of oxygen boost diffusion treatment for titanium were proposed.

Effect of α phase on evolution of oxygen-rich layer on titanium alloys

Abstract In order to understand the evolution of oxygen-rich layer (ORL) on titanium alloys, the near α titanium alloy TA15 and α+β type titanium alloy TC4 were thermally exposed in air at 850 °C to evaluate the effect of α phase content on formation and evolution of ORL, and the stability and diffusion of oxygen in α- and β-Ti were investigated by first principles calculations to reveal the oxygen diffusion rate. TA15 with more α phases has a higher diffusion coefficient of ORL evolution than TC4, resulting in forming thicker ORL on TA15 under the same thermal exposure condition. The first principles calculations indicate that octahedral interstice of α-Ti is the most stable site for oxygen atom. The nearest neighbor diffusion between octahedral interstices along the [0001] direction in α-Ti presenting the lowest activation energy is the most favorable oxygen diffusion mechanism in α- and β-Ti.

Identification of oxygen diffusion mechanisms in Nd1-xAExBaInO4-x/2 (AE = Ca, Sr, Ba) compounds through molecular dynamics.

Molecular dynamics simulations have been widely adopted to study oxygen ion diffusion mechanisms in materials for application in solid oxide fuel cells. Indeed, understanding the fundamental aspects of oxygen diffusion is important to develop new materials for this application. In this work, Nd1-xAExBaInO4-x/2 (AE = Ca, Sr, Ba) compounds have been studied by MD simulations focusing on oxygen diffusion mechanisms. Two general clustering methods were used, namely a convex hull classification method and a DBSCAN machine learning algorithm, to identify oxygen ion diffusion pathways. Here, relevant details are provided for an efficient use of these two approaches during MD analysis of ion conductors. The calculations show that Ca is the most favorable dopant for substituting Nd in NdBaInO4, while Ba is the least desired. Indeed, the substitution of Nd by Ca hardly changes the pristine lattice parameters of NdBaInO4 and leads to the highest oxygen diffusion coefficient compared to other dopants. The oxygen vacancies induced by doping mainly locate on two specific oxygen sites over four oxygen sites available. Concerning the diffusion process, jumps involving these two sites play the main role and are associated with smaller migration enthalpies. For the main diffusion path, ions migrate along the b (2 routes) and c (4 routes) directions. Some other oxygen sites can be considered as barriers for the diffusion process inducing a strong anisotropy in the diffusion process. Additionally, the residence time analysis of oxygen ions confirms that ions at different sites have different motion abilities. As a whole, the approach presented here can be extrapolated to other ion conductors for gaining detailed information about the diffusion process.

Research on the efficient and stable sinking electrical discharge machining ablation process of Ti-6Al-4V

When machining Ti-6Al-4V, electrical discharge machining ablation (EDMA) using a pure oxygen medium can greatly improve the relatively low machining efficiency in EDM, especially when machining a deep-type blind hole. However, Ti-6Al-4V is prone to an explosive phenomenon in the process of ablation, which makes stable processing unsustainable. In this paper, the relations of this phenomenon to the oxide layer and to oxygen concentration were revealed by analyzing the formation process of the surface oxide layer, the explosion mechanism, and the oxygen diffusion mechanism. The results show that high-concentration oxygen enters the alloy substrate through the broad and deep crack channel on the oxide layer, inducing an explosion phenomenon given the high temperature of the discharge. To reduce the oxygen concentration and the crack width and depth, nitrogen is added to the oxygen medium, which prevents the oxygen from entering the matrix and allows stable processing. Experimental results showed that the crack width and depth increase with the increase of the oxygen concentration. Stable and efficient processing can be achieved when the concentration ratio of oxygen to nitrogen is 5:1, and the machining efficiency is more than 8 times higher than in the air medium. Finally, a deep-type blind hole with a deep diameter ratio of 10 was machined continuously and stably.

Oxygen diffusion kinetics of an advanced three step heat treatment for zirconium alloy ZrNb7

ZrNb7 was exposed to a very specific three step heat treatment. The alloy is first oxidized in ambient air. The second step is a solid state reduction in vacuum, during which the oxide scale is dissolved and the oxygen diffuses into the substrate. Within a third step, the alloy is oxidized again to generate the desired ceramic coating on the outside followed by an oxygen hardened diffusion zone, that gradually reduces the hardness of the material from the outer surface to the inside. The high oxygen solubility of up to 29 at.% is distinctive in zirconium. Only titanium and hafnium show similar properties. Therefore, this rare phenomenon is further investigated with the main focus on the oxygen diffusion mechanisms through the oxide scale and the substrate. The reaction kinetics are evaluated and activation energies, parabolic oxidation constants and diffusion coefficients are determined for the two diffusion controlled processes: the oxidation and deoxidation of zirconium. This article reveals why the diffusion hardened zone does not appear in the desired thickness within a simple oxidation step and ideal process parameters are given concerning oxide thickness, cracks in the oxide scale and oxygen diffusion depth.

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

AbstractOxygen diffusion mechanisms during oxidation of ZrB2‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an 18O tracer technique. Double oxidation experiments in 16O2 and 18O2 were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO2 oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO2 was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.