Diffusion Of Oxygen Atoms Research Articles

Temperature-Dependent Oxidation Behavior of Silicon Carbide Surface: Reactive Molecular Dynamics Simulations.

The high-temperature oxidation mechanism of silicon carbide (SiC) is crucial for designing thermal protection systems in aircraft. This study explored the oxidative chemical reaction processes and mechanism on SiC surface and interfaces within the temperature range of 300-2300 K by using reactive molecular dynamics simulation. The oxygen impact on the silica (SiOx) growth of the SiC surface was analyzed, which shows a progressive increase of silica thickness. The simulation results indicated that the oxidation process of SiC was a typical passive oxidation mechanism. With the environmental temperature rising and oxygen impact, the increase of oxidation thickness on the SiC surface undergoes three oxidizing reaction processes: little chemical adsorption of oxygen molecules on the initial surface, rapid oxidation of silicon and carbon, and dramatic oxidation of the interface between the oxidation layer and SiC. Additionally, this work studied the mechanism of oxidation thickness growth and chemical diffusion of oxygen. The oxidation rate is weakened according to the oxygen atom diffusion barrier effect of silica repulsion. Moreover, the kinetic parameters were statistically calculated by fitting the growth of Si-O bonds and their reaction rate constants. Subsequently, the activation energy and pre-exponential factors were derived by using the Arrhenius equation to model the chemical reaction kinetics of the thermal oxidation process. The chemical reaction behaviors of the two stages could be concluded as follows: (i) in stage I, the initial oxidation is reaction rate limiting; (ii) in stage II, SiC oxidation is limited by both the oxidation reaction rate and the oxygen diffusion coefficient of the oxidation layer. The activation energy of stage II increased compared with stage I due to the oxygen atoms diffusion barrier between the oxidation layers. This study on the oxidation and ablation mechanism of the SiC surface at the atomic scale would provide insight into understanding thermal oxidation behavior and the design of ceramic materials.

Fully resolved simulations of micro-unit composite fuel in hydroxyl-terminated polybutadiene (HTPB): Al@AP

Micro-unit composite fuel (Al@AP) is a promising strategy for achieving stable and efficient combustion, due to its reduced pressure dependency and ability to prevent agglomeration. This research focuses on the heat and mass transfer as well as the reaction mechanism within the Al@AP structure through molecular modeling. The direct contact between the metal fuel and oxidizer leads to an immediate redox reaction at the corresponding interface, facilitating the diffusion of oxygen atoms from surrounding AP molecules into the Al particles. Heat is transferred from the Al particles to the surrounding oxidizer, instead of inward heat transfer from the AP-HTPB interface as observed in the conventional Al/AP composite. This novel assembly method significantly accelerates the reaction rate, more than doubling that of the conventional Al/AP composite. Moreover, the research explores combustion behavior at diverse pressure conditions (condensed phase vs. vacuum), revealing pressure’s minimal impact on the rapid burning stage primarily driven by diffusion mechanism. Under vacuum, reaction products propel burning Al within gas flow, transitioning to surface reactions on Al particles with gaseous products. Regarding agglomerations, the coalescence of two burning Al droplets is observed to form a larger product owing to the reduced distances within AP particle. The Al droplets are almost consumed before coalescence due to the rapid combustion, resulting in no negative impact on the Al combustion. The above findings highlight the unique features of micro-unit composite fuel in the application of solid propellants, and serve as an atomistic guide for propellant manufacturing and design.

Enhanced precision lapping techniques for fused quartz optical elements: Synergistic integration of mechanical and mechanochemical processes

Fused silica is a material of great importance in the field of precision optical equipment due to its excellent optical properties, e.g. photolithography, laser fusion devices, etc. However, the material's inherent hardness and brittleness render it a challenging material to process. Therefore, an enhanced precision lapping technique was proposed, i.e., a synergistic integration of mechanical and mechanochemical lapping processes. Firstly, a synergistic lapping tool with an outer layer of porous diamond abrasive and an inner layer of CeO2 abrasive was fabricated by hot press forming method. The reduction of layer thickness due to abrasion and detachment of the porous diamond abrasive layer was analyzed, and the lapping mechanism of porous diamond on fused silica materials was revealed. The surface creation mechanism of the CeO2 abrasive layer on fused silica material was analyzed by molecular dynamics simulation and the results were validated by enhanced precision lapping experiment. The results demonstrated that the surface atoms of fused silica are removed in a ‘remote disordered’ manner by conventional diamond abrasive grains, whereas the surface atoms of fused silica are removed in a ‘proximally ordered’ manner by the fine porous diamond abrasive grains through the micro-multi-flute cutting. Consequently, the grinding of a porous diamond abrasive layer can significantly reduce the processing damage, with the surface roughness of the lapping Ra reaching 199 nm. The lapping of CeO2 induces the formation of an oxygen bridge on the surface of fused silica, which then produces bending vibration. This is followed by the adsorption and diffusion of oxygen atoms, which form the -Ce-O-Si intermediate with the oxygen atoms shared with the fused silica. This process results in the formation of CeSiO3 and CaSiO3 compounds. The final surface quality with a surface roughness of Ra 21 nm was obtained by mechanochemical lapping of the cerium oxide abrasive layer. The single mechanical-mechanochemical composite ultra-precision lapping process provides technical support for the efficient and low-damage processing of hard and brittle components.

Effect of Au ion irradiation on microstructure, mechanical, and lead‐bismuth eutectic corrosion properties of Cr/ZrO2 coatings

AbstractThe effect of ion irradiation on the microstructure, mechanical, and corrosion properties of Cr/ZrO2 coating was investigated. The monoclinic to tetragonal phase transition is observed under heavy ion irradiation, and the tetragonal phase is stabilized by the irradiation‐induced defect clusters. With increasing irradiation fluence, grain growth, and irradiation‐induced hardening effects are observed. After lead‐bismuth eutectic (LBE) corrosion at 650°C for 1000 h, the irradiated coatings can still maintain structural integrity and protect the substrate from lead/bismuth corrosion. However, in contrast to the unirradiated coating, the irradiation‐induced defects provide a channel for the rapid inward diffusion of oxygen atoms from the LBE, leading to the formation of a thicker Cr‐rich oxide layer beneath the ZrO2 coating. This study also provides insight into how irradiation‐induced microstructural changes affect zirconia's properties as a corrosion protection layer.

Investigation of the effect of temperature and applied electric field on the aluminum-water reaction by ReaxFF molecular dynamics

Hydrogen energy is gaining attention as a non-polluting source. Aluminum’s reaction with water for propulsion and hydrogen production is widely used, but its nanoscale characteristics are unexplored. This study investigated aluminum hydrolysis in water vapor by ReaxFF-MD, examining effects of temperature and electric field. The oxidation of the aluminum surface is accompanied by aluminum dissolution, hydrogen generation and H3O+ formation, Formed hydrides and oxides were AlH3 and Al2O3. Hydrogen disappearance at 700 K and 900 K was due to conversion to AlH3. The diffusion of hydrogen and oxygen atoms in the Z direction was enhanced at increasing temperatures, Outer aluminum atoms preferentially reacted with oxygen to form Al2O3, while inner atoms reacted with hydrogen to form AlH3, the loosely packed outer oxide layer contains inclusions of AlH3. Studies at room temperature under different electric fields showed −Z field promoted the reaction while + Z field inhibited it, attributed to different electric field strengths’ effect on OH– ion electron-sharing Coulomb equilibrium, resulting in varying rates of hydroxide polar covalent bond cleavage. Our results clarify the atomic-scale effects of temperature and electric field on the aluminum-water reaction, providing guidance for optimizing experimental conditions and enhancing the efficiency of hydrogen production.

Unveiling the feature of deposition of oxygen and water molecules on the zirconium surface: A molecular dynamic study

Unveiling the feature of deposition of oxygen and water molecules on the zirconium surface at microscale is crucial during the initial stages of zirconium oxide layer formation in Pressurized Water Reactors (PWRs) or Light Water Reactors (LWRs). In this work, we use molecular dynamic simulations to study the feature of deposition of oxygen and water molecules on the zirconium surface. A special focus of this work is a formula our built for the number of atoms adsorbed by the metal surface at the considered moment of time. The results show that the surface density of atoms for the crystallographic plane (10−10) in is 3 times greater than that for the crystallographic plane (11−20). The effective surface density σ(10−10) for the crystallographic plane (10−10) is greater than the similar value σ(11−20) for the crystallographic plane (11−20). Furthermore, in a sufficiently wide range of temperatures, the mechanical properties of zirconium crystals during tensile deformation are practically independent of the diffusion of oxygen atoms along the grain boundaries. This distinct feature may be providing basis understanding for the experimental puzzle of oxidation rate of zirconium alloys.

Unraveling the characteristics of adsorption and diffusion of oxygen atoms on the surface of Zr and ZrO2 crystals with point defects from molecular dynamic simulations

Unveiling the characteristics of adsorption and diffusion of oxygen atoms on the surface of zirconium alloys at microscale is significant during the initial stages of zirconium oxide layer formation in PWRs or LWRs. In this work, Adsorption and diffusion of oxygen atoms on the surface of Zr and ZrO2 crystals have been investigated using molecular dynamic simulations. A special focus of this work is on the diffusion coefficient calculation based on our developed unbiased approach. The results show that a weak effect of temperature on the adsorption rate of ZrO2. The greater influence of grain boundary orientation than the concentration of Frenkel defects on the adsorption rate of oxygen atoms. The number of oxygen atoms that diffuse deep into the studied samples after adsorption on the free surface increases with increasing temperature. Moreover, for crystals that have a certain concentration of Frenkel defects in the volume, the diffusion coefficients are greater than for defect-free crystals. The diffusion coefficient of adsorbed oxygen atoms is smaller for crystal system with grain boundary of Zr/Zr. This distinct feature may be solving the experimental puzzle of oxidation rate of zirconium alloys under neutron irradiation.

Study on the impact of hydrogen storage temperature on iron-based thermochemical hydrogen storage technology

Thermochemical hydrogen storage technology based on iron-based materials has received significant attention owing to its low cost, safety features, and high volumetric hydrogen storage density. However, no consensus has been established on the optimal temperature range for hydrogen storage. This study investigated the impact of hydrogen storage temperature on hydrogen consumption capacity, hydrogen release capacity, and hydrogen storage energy consumption. The hydrogen storage temperature was optimized with the objectives of low energy consumption and high hydrogen storage density. The experiments were conducted using thermogravimetric analyzer and gram-scale packed-bed reactor setups, accompanied by thermodynamic and kinetic calculations. The research indicated that at 550 °C, the optimal hydrogen storage temperature, the lowest relative energy demand was 32.30 %, with the hydrogen consumption capacity reaching the theoretical maximum value of 4.8 %, and the hydrogen release capacity reaching 4.04 %. Furthermore, achieving the theoretical maximum value of hydrogen consumption capacity became exceedingly difficult when the hydrogen storage temperature exceeded 570 °C. Microstructural characterization and kinetic calculations revealed that this phenomenon was attributed to the formation of a dense iron layer on the particle surface, which impeded the solid-state diffusion of oxygen atoms. The formation of the intermediate phase wüstite was identified as the cause of the formation of the dense iron layer. Overall, this study provided theoretical support for iron-based thermochemical hydrogen storage processes and offered a perspective into the reaction behavior of the reduction of magnetite with hydrogen.

Effect of Adding Al on the Phase Structure and Gettering Performance of TiZrV Non-Evaporable Getter Materials.

Titanium zirconium vanadium (TiZrV) is a widely used non-evaporable getter (NEG) material with the characteristics of a low activation temperature and a large gas absorption capacity. At present, the research on TiZrV getters mainly focuses on the thin-film state, with little research on the bulk state. In this paper, a TiZrV getter was optimized by adding Al, and the phase structure, activation properties, and gettering performance were studied. With the addition of Al, the α-Zr phase and Ti2Zr phase changed into the Ti-Zr phase and Al-Zr, Al-Ti phase. The newly generated phase promoted the diffusion of hydrogen and oxygen atoms. The activation temperature decreased significantly, as shown in the in situ XPS results. The H2 and CO gettering performance of TiZrVAl samples was promoted to 2073 cm3·s-1 and 1912.8 cm3·s-1, increased by 40.7% and 40.3%. This paper provides valuable ideas for optimizing the properties of bulk TiZrV getters.

Investigation of the Effect of Stress on Oxygen Diffusion in Pure Titanium Using a Phase-Field Model.

Diffusion plays a vital role during the fabrication of many materials. It is a well-known fact that stress can influence diffusion behavior. In order to optimize material processing techniques, a quantitative evaluation of the effect of stress on diffusion is essentially required. By analyzing the free energy change in a Ti-O system during diffusion, a phase-field model was developed to address this issue. Using this model, the diffusion of oxygen atoms in pure titanium under different stress states was investigated. It was observed that the true equilibrium concentration of oxygen was proportional to its hydrostatic pressure. Tensile stress can increase the oxygen concentration. This raise in concentration decreased with temperature. However, the promotion of diffusion can be attained in deeper regions at a higher temperature. On the contrary, compressive stress inhibited the diffusion of oxygen in pure titanium. Under a certain compressive stress, the decrease in the oxygen concentration at the surface layer was more significant at a lower temperature, while a decrease could be observed at a deeper distance from the surface at a higher temperature. A thermodynamic explanation of the effect of stress on diffusion was given based on the proposed phase-field model.

Unveiling the oxidation mechanism of CrTaTiMo refractory medium-entropy alloys: A synergy of density functional theory and ab initio molecular dynamics

The demand for advanced materials capable of withstanding extreme high-temperature conditions has led to the development of novel High-Entropy Alloys (HEAs). Recent works indicate that forming a CrTaO4 protection layer gives HEAs excellent long-term oxidation resistance. In this study, the high-temperature oxidation resistance properties of CrTaTiMo Refractory Medium-Entropy Alloys (RMEAs) were assessed by Density Functional Theory (DFT) and ab initio Molecular Dynamics (AIMD). Through the oxygen adsorption and diffusion calculations, we demonstrated the preferential oxygen adsorption sequence on the surface of the RMEA as Ti, Ta, Cr, and Mo. Furthermore, our analysis identified the sites featuring Ta as subsurface atoms were the weakest locations for oxygen atom diffusion. The dynamic oxidation mechanism of oxygen molecules on CrTaTiMo RMEA was investigated by AIMD simulations. The results confirmed that the adsorption and dissociation of O2 molecules on the alloy surface. Additionally, the diffusion of the O atom took place at temperatures greater than 873 K and confirmed the O-attracting feature of Ta atoms. Moreover, electronic structure calculations confirmed the bonding of oxygen atoms with those four metal elements. This study could serve as a valuable reference for the strategic development of the CrTaTiMo-based RMEAs or RHEAs for high-temperature, long-term oxidation resistance applications.

Coverage-Dependent Site-Specific Placement and Correlated Diffusion of Atomic Oxygen on Moiré-Patterned Graphene on Ru(0001).

Nano-periodic arrays of atomic oxygen are visualized on epitaxial graphene on Ru(0001) via STM following supersonic beam exposure to non-equilibrium fluxes of atomic oxygen. Self-organization of atomic oxygen on graphene is directed by the intrinsic moiré pattern of the ruthenium-graphene interface. Atom-resolved STM imaging reveals the richness of multiparticle interactions, leading to correlated atomic diffusion and placement. Pair-distribution functions demonstrate that repulsive oxygen-oxygen interactions play an increasingly important role in the site specificity and diffusivity of atomic oxygen on the moiré lattice with increasing coverage. Atomic visualization shows the number of oxygen atoms in a local region changes overall diffusion rates and promotes the correlated motion of oxygen atoms. Understanding the site specificity of oxygen adsorption and diffusive behavior of atomic oxygen on epitaxial graphene on Ru(0001) provides insight for both the synthesis and stability of moiré-templated two-dimensional materials which show promise as platforms for next-generation quantum materials and catalysts.

The influence of hydrogen and oxygen vacancy concentrations on diffusion coefficients of oxide and hydride ions in reduced barium titanate oxyhydride using molecular dynamics simulation

A molecular dynamics method with a reax force field has been employed to investigate the effect of hydrogen and oxygen vacancy concentrations on the diffusion of oxygen and hydrogen atoms in BaTiO3−xHy□(x-y). The result shows that by using this force field, the BaTiO3−xHy□(x-y) samples are stable under the condition of x < 0.18. Therefore, the samples have been simulated under conditions of (x-y) <0.15, y < 0.18 and x < 0.18. The results reveal that the dominant diffusion dynamics of hydrogen atoms is the motion between next-nearest neighboring oxygen vacancies in the form of hydride. The diffusion coefficients and activation energies are evaluated using mean square displacements and Arrhenius plots. The results show that the activation energies of oxide and hydride ions are close together and are in the range of 0.03–0.9 eV and they both decrease by increasing y. The results also show that the pre-exponential factors of hydrogen and oxygen diffusion are in the range of 10−6 to 0.0045 cm2s-1 and 1.7710−6 to 0.0017 respectively.

Effect of FeCrAl sublayer composition on the microstructure and corrosion resistance of FeCrAl/yttria-stabilized zirconia composite coatings

The effect of FeCrAl sublayer composition on the microstructure and lead-bismuth eutectic corrosion behavior of the FeCrAl/YSZ (Yttria-stabilized zirconia) composite coatings was systematically investigated by SEM and XRD. The results show that the FeCrAl sublayer as a diffusion barrier and a bonding layer is beneficial to improve the corrosion resistance of the coatings. After corrosion, a slow-growing, continuous, and protective Al-rich oxide is formed at the interface between the YSZ and FeCrAl sublayer, which effectively hinders the inward diffusion of oxygen atoms and the outward diffusion of iron atoms. Especially, the Fe-12Cr-7Al/YSZ coating exhibits the best corrosion resistance even after corrosion at 700 °C. The microscopic corrosion mechanism of FeCrAl/YSZ coatings is elucidated.

Adsorption and diffusion behavior of oxygen atoms on Ti3SiC2 (0001) surface

Ti3SiC2 is the most typical 312 MAX phase, which has attracted more attentions in many fields. In this paper, the (0001) plane of Ti3SiC2 was built. The adsorption energy and diffusion path of oxygen atom at different sites were calculated by first principles. There are three different adsorption sites for oxygen atom on Ti3SiC2 (0001) surface. By calculating the adsorption energy of the three adsorption sites, it was found that the adsorption energy of the oxygen atom at the center site is the lowest. The diffusion path of the oxygen atom was simulated and showed that oxygen atom located at the center of Ti-Si tetrahedron after the diffusion into the bulk phase. The effects of solid solution elements on the diffusion activation energy of oxygen atoms were calculated. suggesting that the solid solution of V, Mo and W will greatly increase the energy barrier, thus preventing the inward diffusion of oxygen atoms.

Cyclic Oxidation Kinetics and Thermal Stress Evolution of TiAl Alloys at High Temperature

The oxidation resistance of TiAl alloys is crucial for their commercial application. In this paper, a cyclic oxidation test with stable air circulation was designed to investigate the cyclic oxidation behavior of the TNM alloy and 4822 alloy at 800 °C and to analyze the phase, morphology, and thermal stress evolution of the oxide layer. The oxidation weight gain curves of both alloys are found to be in parabolic form, and the oxidation reaction orders of the TNM alloy and 4822 alloy are 2.374 and 1.838, respectively. The Nb and Mo elements enhance the antioxidant performance of the TNM alloy by inhibiting the dissolution and diffusion of oxygen, Ti, and Al atoms in the TiAl alloy. The thermal stress evolution of the two alloys during the heating and cooling phases of the cyclic oxidation process are calculated separately, and it is found that the thermal stresses in the TNM alloy are smaller than those in the 4822 alloy, while the maximum thermal stresses appear at the oxide/substrate interface rather than inside the oxide scale, which quantitatively explains the oxidation peeling resistance of the two alloys.

Modeling of CO Oxidation by Diffusion of Oxygen Atoms in Ceria-Zirconia Particulates in a Three-Way Catalyst Particle Membrane Filter

In order to design a microporous membrane filter comprising Three-way Catalyst (TWC) particles with a size distribution of 1 to 2 microns, isothermal CO oxidation experiments and numerical simulations were conducted to investigate the transport of oxygen atoms within primary Ceria-Zirconia (CZ) particulates. These spherical TWC particles were fabricated through the agglomeration of primary CZ and alumina particulates, incorporating Pd and Rh catalysts. By comparing experimental CO2 emissions with simulation results over time, a temperature-dependent diffusion coefficient was determined. The simulation results reveal that the effective distance of oxygen atom transport within CZ particulates, heterogeneously distributed in the spherical TWC particle, is limited to approximately 100 nm from the surface of agglomerated spherical TWC particles within a temperature range of 175 to 225°C.

Exploring B-site high-entropy configuration of spinel oxides for improved cathode performance in solid oxide fuel cells

High-entropy engineering is an emerging strategy to enhance material properties beyond their individual components. In this study, we synthesized a high-entropy spinel oxide cathode material, Ni(Fe0.2Mn0.2Co0.2Cr0.2Ni0.2)2O4, for solid oxide fuel cells (SOFCs) using a solution combustion method. The material exhibits a spinel structure with diverse surface metal valence states and a high concentration of oxygen vacancies. Density functional theory calculations reveal the B-site high-entropy design can significantly improve the oxygen adsorption capability. Electrochemical tests demonstrate that Ni(Fe0.2Mn0.2Co0.2Cr0.2Ni0.2)2O4 has superior conductivity and oxygen reduction activity compared to binary spinel oxides. An anode-supported SOFC with Ni(Fe0.2Mn0.2Co0.2Cr0.2Ni0.2)2O4 cathode achieved a peak power density of 583 mW cm-2 at 800 °C. Impedance and relaxation time distribution (DRT) analyses indicate that the oxygen molecular dissociation and oxygen atom diffusion dominates the cathode polarization. This study demonstrates that high-entropy engineering of the B-site in spinel oxides is a promising strategy to optimize cathode performance for SOFCs.

Physical mechanism of oxygen diffusion in the formation of Ga2O3 Ohmic contacts

The formation of low-resistance Ohmic contacts in Ga2O3 is crucial for high-performance electronic devices. Conventionally, a titanium/gold (Ti/Au) electrode is rapidly annealed to achieve Ohmic contacts, resulting in mutual diffusion of atoms at the interface. However, the specific role of diffusing elements in Ohmic contact formation remains unclear. In this work, we investigate the contribution of oxygen atom diffusion to the formation of Ohmic contacts in Ga2O3. We prepare a Ti/Au electrode on a single crystal substrate and conduct a series of electrical and structural characterizations. Using density functional theory, we construct a model of the interface and calculate the charge density, partial density of states, planar electrostatic potential energy, and I–V characteristics. Our results demonstrate that the oxygen atom diffusion effectively reduces the interface barrier, leading to low-resistance Ohmic contacts in Ga2O3. These findings provide valuable insights into the underlying mechanisms of Ohmic contact formation and highlight the importance of considering the oxygen atom diffusion in the design of Ga2O3-based electronic devices.

Improvement of high-temperature oxidation resistance of WC–Co–Ni alloy by Cr3C2–CeO2–WS2 mixed additives and its mechanism

Whether used as hot rollers or hot forging dies, the high-temperature oxidation resistance of cemented carbides is closely related to their lifetime. In the context of the above applications, a WC–Co–Ni alloy with Cr3C2–CeO2–WS2 mixed additives is designed based on the principles of oxidation resistance and self-lubrication at elevated temperatures. The results of continuous oxidation at 700 °C for 16 h show that the oxidation rate of WC–6Co–6Ni–1.2Cr3C2–1.0CeO2–0.4WS2 (alloy 2#) with ultrafine dispersion phase Ce2O2S2 decreases by 23.7% and 15.4%, respectively, compared with WC–6Co–6Ni (alloy 1#) and WC–6Co–6Ni–1.2Cr3C2 (alloy 3#). The additives of Cr3C2 and CeO2 can promote the in situ atomic recombination in the binder phase and rapid nucleation of (Ni,Co,Cr)WO4. As a result, a 2.3 times and 1.6 times higher MWO4 (M = Co, Ni, Cr) in quantity is formed on the oxidized surfaces compared with the counterpart alloy 1# and alloy 3#. The high proportion of dense (Ni,Co,Cr)WO4 phase and the Ce2O2S2 third phase resistant to high-temperature oxidation can form a double obstacle to the diffusion of oxygen atoms and delay the kinetic process of oxidation reaction.