Diffusion Of Oxygen Atoms Research Articles

Research on the thermal oxygen aging mechanism of the composite curing system epoxy asphalt with different curing agent proportions on the molecular scale

AbstractTo study the influence of the cross‐linking network on the thermal oxygen aging performance of the epoxy asphalt, the laser copolymerization microscopy, Fourier transform infrared spectroscopy and element analysis tests were used to characterize the changes in chemical components of epoxy asphalt with composite curing system during thermal oxygen aging. According to the changes in chemical components, the epoxy asphalt molecular models were constructed. The diffusion behavior of oxygen atoms was analyzed, the aging degree was determined. Moreover, the Tg and radial distribution function were calculated to determine the effect of aging on the low‐temperature performance and component distribution of epoxy asphalt. The results show that a proper proportion of curing agent helped to uniform distribution of the asphalt phase. The rise in anhydride curing agent content increased the density of crosslinking network, limited the diffusion of oxygen atoms, and contributed to the resistance of epoxy asphalt chemical components to thermal oxygen aging. However, the high proportion of anhydride aggravated the oxidation reaction of the asphalt phase due to the uneven coating of the asphalt phase. After thermal oxygen aging, the distribution of asphaltene and epoxy resin phase showed a concomitant phenomenon due to polarity adsorption.

Simulations on the oxidation of Al-Mg alloy nanoparticles using the ReaxFF reactive force field

Nanoalloy holds multiple complex morphologies in combustion. However, the underlying mechanism, which is crucial to advance technical application of the nanoalloy, remains a challenging mystery. Herein, we performed a ReaxFF reactive molecular dynamics simulation on Al-Mg alloy nanoparticles (AMNPs) as a case study to further analyze oxidation behavior. Results showed that the evolution of AMNPs in combustion can be divided into three stages: (1) AMNPs first undergo phase separation and aggregate into Mg and Al phases; (2) Mg atoms rapidly diffuse to the surface and are oxidized; (3) Oxygen atoms diffuse inward and oxidize Al atoms. The inward diffusion of oxygen atoms is particularly hindered at the interface between Mg oxide shell and Al oxide layer. AMNPs prefer to form internal hollow structures at lower temperature and high oxygen concentration. The outward diffusion of Mg atoms causes the core to be gradually hollowed out. The spilled Mg atoms are oxidized and attached to the surface of alumina. The structure of Mg oxide (surface layer)-Al oxide (middle layer)-hollow (core) is finally formed. In addition, micro-explosion and external Mg oxide shell inhibit the agglomeration of nanoparticles. The study provides insight into the complex interactions between Al-Mg nanoalloy and oxygen.

Effects of Alloying Elements on the Solution and Diffusion of Oxygen at Iron Grain Boundary Investigated by First-Principles Study

The effects of alloying elements (Si, Cr, Mo) on the solution and diffusion of oxygen (O) atoms at the grain boundary of iron (Fe) Σ5(310)/[001] are investigated by the simulations of ab initio density functional theory (DFT). It is found that Si, Mo and Cr prefer to segregate to the grain boundary, and further affect the solution and diffusion of O atoms at Fe grain boundaries. The segregated Cr promotes the solution of O, while Si and Mo inhibit the solution of O at the grain boundary. Meanwhile, Cr and Si accelerate the diffusion of O, and Mo retards the diffusion of O in the grain boundary. Further analysis indicates that the effects are closely related to the interactions between the alloying elements and O atoms, which are determined by the competition between the distortion of local structure and the charge transfer between local atoms. Finally, the effects of alloying elements on the O concentration distribution near the grain boundary are explored by employing the Langmuir–McLean models. This work not only provides insights into the effects of alloying elements on the solution and diffusion of O at grain boundaries, but also provides parameters of the atomic interactions for the initial oxidation simulation on a large scale, which relates to the growth of oxide in polycrystalline systems with various grain sizes at experimental temperatures.

Reliability and Stability Improvement of MOS Capacitors via Nitrogen-Hydrogen Mixed Plasma Pretreatment for SiC Surfaces.

We investigated the effect of nitrogen-hydrogen (NH) mixed plasma pretreatment of 4H-SiC surfaces on SiC surface properties, SiO2/SiC interface quality, and the reliability and voltage stability of metal-oxide-semiconductor (MOS) capacitors. The NH plasma pretreatment decreased the incomplete oxide and contaminants on the SiC surface and reduced the density of SiO2/SiC interface traps. Compared with the untreated sample, the dielectric insulating characteristics and reliability of samples pretreated by NH plasma were improved. We also demonstrated that the shift/hysteresis of the flat band voltage (Vfb) and the midgap voltage (Vmg) induced by bias temperature stress for SiC MOS capacitors after NH plasma pretreatment was significantly decreased. Furthermore, the mechanisms of NH plasma pretreatment to improve interface properties and device performances were determined by combining secondary ion mass spectrometry (SIMS) measurements, X-ray photoelectron spectroscopy (XPS), and first-principles calculations. The result indicates that the excessive oxidation at the SiO2/SiC interface was limited due to the reduction in the diffusion of oxygen atoms into SiC caused by the surface Si-H and Si-N; NH plasma pretreatment suppressed the generation of interfacial traps by reducing surface pollutants and passivating surface defects, and some N atoms introduced into the SiO2/SiC interface effectively passivated the interfacial electroactive traps.

Carbon nanotube network film-based field-effect transistor interface state optimization by ambient air annealing

Carbon nanotube field-effect transistors (CNTFETs) have been considered a strong candidate for post-Si era electronics due to the virtues of higher speed, lower power consumption, and multiple functionalities. The interface analysis based on the top gate structure has made little progress and lacks a reliable charge trap characterization model suitable for carbon tube devices. Quantitative extraction and analysis of the interface state are crucial for the integration of top-gate devices. Herein, a 5 nm thick Y2O3 thin film was selected as the gate dielectric layer in the top-gate CNTFETs device, and a post-annealing process in air ambience was utilized to optimize the Y2O3-CNT interface. A series of device performance evaluation results indicated that the post-annealing process in air ambience can effectively improve the on-state current and reduce the threshold voltage and subthreshold swing of the device, which are derived from diffusion of oxygen atom in the Y2O3 layer and optimization of the interface of Y2O3-CNT. Specifically, the maximum mobility, subthreshold swing, and threshold voltage are calculated to be 29 cm2/V s, 103 mV/dec, and −0.1 V, respectively, and the interface state density is reduced from 2.68 × 1012 to 1.51 × 1012 cm−2 in the gate insulator. These results not only are important to understand the dielectric impact on CNTFET devices but also are useful for future materials’ development and device optimization for high-performance CNT-based electronics.

Infrared Spectroscopic Studies of Oxygen Atom Quantum Diffusion in Solid Parahydrogen.

The thermally induced diffusion of atomic species in noble gas matrices was studied extensively in the 1990s to investigate low-temperature solid-state reactions and to synthesize reactive intermediates. In contrast, much less is known about the diffusion of atomic species in quantum solids such as solid parahydrogen (p-H2). While hydrogen atoms were shown to diffuse in normal-hydrogen solids at 4.2 K as early as 1989, the diffusion of other atomic species in solid p-H2 has not been reported in the literature. The in situ photogeneration of atomic oxygen, by ArF laser irradiation of an O2-doped p-H2 solid at 193 nm, is studied here to investigate the diffusion of O(3P) atoms in a quantum solid. The O(3P) atom mobility is detected by measuring the kinetics of the O(3P) + O2 → O3 reaction after photolysis via infrared spectroscopy of the O3 reaction product. This reaction is barrierless and is thus assumed to be diffusion-controlled under these conditions such that the reaction rate constant can be used to estimate the oxygen atom diffusion coefficient. The O3 growth curves are well fit by single exponential expressions allowing the pseudo-first-order rate constant for the O(3P) + O2 → O3 reaction to be extracted. The reaction rates are affected strongly by the p-H2 crystal morphology and display a non-Arrhenius-type temperature dependence consistent with quantum diffusion of the O(3P) atom. The experimental results are compared to H(2S) atom reaction studies in p-H2, analogous studies in noble gas matrices, and laboratory studies of atomic diffusion in astronomical ices and surfaces.

Effects of chemical complexity on the initial oxidation resistance of HfC1−xNx ceramics

The initial oxidation process of refractory alloy ceramics is closely related to their intrinsic properties such as surface adsorption or diffusion of oxygen atoms. We devise a machine learning model that predicts the full spectrum of adsorption energies for an oxygen atom on HfC1−xNx ceramic surfaces with quantum accuracy. With this approach, we show that the chemical complexity of carbonitride makes HfC1−xNx ceramics exhibit multiple types of adsorption sites with competing oxygen adsorption energies, leading to fewer preferable adsorption sites. In particular, we find that heavily doped N can change the stable adsorption site from the 3-fold hollow between metals and C atoms (MMC) to the top of Hf atoms (top-Hf), and the total number of preferable adsorption sites is regulated by their competing energies. In this scenario, we predict HfC0.76N0.24 has superior anti-oxidation performance, consistent with existing experimental measurements. Our findings can stimulate new strategies to enhance the oxidation resistance of refractory alloy ceramics.

Possible handle for broadening the catalysis regime towards low temperatures: proof of concept and mechanistic studies with CO oxidation on surface modified Pd-TiO2.

The present work demonstrates the effect of temperature-dependent surface modification (SM) treatment and its influence in broadening the catalysis regime with Pd-TiO2 catalysts prepared by various methods. Due to SM induced changes, a shift in the onset of CO oxidation activity as well as broadening of the oxidation catalysis regime by 30 to 65 K to lower temperatures is observed compared to the temperature required for virgin counterparts. SM carried out at 523 K for PdPhoto-TiO2 exhibits the lowest onset (10% CO2 production - T10) and T100 for CO oxidation at 360 and 392 K, respectively, while its virgin counterpart shows T10 and T100 at 393 and 433 K, respectively. The SMd Pd-TiO2 catalysts were investigated using X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and atomic force microscopy (AFM). It is observed that diffusion of atomic oxygen into Pd-subsurfaces leads to SM and changes the nature of the surface significantly. These changes are demonstrated by work function (ϕ), surface potential, catalytic activity, and correlation among them. UPS results demonstrate the maximum increase in ϕ by 0.5 eV for PdPhoto-TiO2 after SM, compared to all other catalysts. XPS study shows a moderate to severe change in the oxidation states of Pd due to atomic oxygen diffusion into the subsurface layers of Pd. Kelvin probe force microscopy (KPFM) study also reveals corroborating evidence that the surface potential increases linearly with increasing temperature deployed for SM up to 523 K, followed by a marginal decrease at 573 K. The ϕ measured by KPFM and UPS shows a similar trend and correlates well with the changes in catalysis observed. Our results indicate that there is a strong correlation between surface physical and chemical properties, and ϕ changes could be considered as a global marker for chemical reactivity.

The initial oxidation of the 4H-SiC (0001) surface with C-related point defects: Insight by first-principles calculations

The initial oxidation of 4H-SiC (0001) surfaces with C-related point defects (SurfCD) is studied by using first-principles coupled with thermodynamics calculations to gain deeper insights into the effects of C-related point defects on the formation of SiO2 and SiO2/SiC interface. Three types of SurfCD, including a surface with C interstitial defect (SurfCi), a surface with antisite defect (SurfCsi) and a surface with C vacancy (SurfVc) are considered. The initial oxidation of SurfCD commences on the chemisorption of oxygen atoms at the bridge sites between the Si atoms adjacent to the C-related point defects. As the oxygen coverage increased, the effects of C-related point defects on the adsorption sites and energies of oxygen atoms, the diffusion of oxygen atoms and the interfacial lattice strain are elucidated. The thermodynamic diagrams of O/4H-SiC structures are performed to investigate the stability of surface during the actual oxidation conditions for SurfCi, SurfCsi and SurfVc. Thermodynamically stable intermediate structures are observed on these surfaces during the initial formation of SiO2, the 1 ML structures are the most stable; the Si dangling bonds, Si-O-C structure and C-clusters are observed in the 1 ML structures. Moreover, the evolution mechanisms and electronic properties of the defects are further investigated.

Applied-strain-promoted oxidation of Nickel: Insights from ReaxFF molecular dynamic simulation

Ni-based superalloy components are often subjected to applied stress in service. Besides creep and fatigue, the impact of applied stress on metal oxidation corrosion should not be underestimated. In this work, we studied the oxidation behavior of Ni under different applied strains by molecular dynamics simulation with ReaxFF potential. Results show that the oxidation begins with the outward diffusion of random Ni atoms from the surface, and both applied compressive strain and applied tensile strain can activate the Ni surface, advancing the oxidation start time and promoting oxidation. The applied tensile strain promotes oxidation by promoting the inward diffusion of oxygen atoms and inhibiting the outward diffusion of nickel atoms, while the applied compression strain does the opposite. This work provides a better understanding of the effect of mechanical loading on oxidation kinetics, which is helpful for the improvement of anti-corrosion measures of Ni-based superalloys.

Investigation on Microstructures and High-Temperature Oxidation Resistance of Cr Coatings on Zircaloy-4 by Multi-Arc Ion Plating Technology.

Introducing the oxidation-resistant coating on Zr alloy is considered to be one of the potential solutions for accident-tolerant fuel (ATF) materials. In this study, pure Cr coatings were prepared on a Zircaloy-4 (Zry-4) alloy surface by multi-arc ion plating under different process parameters. The ability of Cr coating on Zry-4 alloy cladding to improve the oxidation resistance to prevent a loss-of-coolant accident (LOCA) was studied. The microstructure of Cr coating was analyzed using the EBSD technique, and the high-temperature steam oxidation was tested at 800, 1000 and 1200 °C. Compared with the original Zry-4 alloy, the samples with Cr coatings exhibited much better oxidation resistance under different high-temperature steam oxidation conditions. However, the Cr coating exhibited columnar grain, strong preferred orientation and (001) fiber texture. The columnar grain boundaries provided paths for the diffusion of oxygen atoms to the Zry-4 alloy matrix at high temperatures. The results showed that the oxidation film of Cr coating with relatively random grain orientation was compact and uniform and exhibited the best oxidation resistance at high temperatures.

Memory Properties of Zr-Doped ZrO2 MOS-like Capacitor

The high-k-based MOS-like capacitors are a promising approach for the domain of non-volatile memory devices, which currently is limited by SiO2 technology and cannot face the rapid downsizing of the electronic device trend. In this paper, we prepare MOS-like trilayer memory structures based on high-k ZrO2 by magnetron sputtering, with a 5% and a 10% concentrations of Zr in the Zr–ZrO2 floating gate layer. For crystallization of the memory structure, rapid thermal annealing at different temperatures between 500 °C and 700 °C was performed. Additionally, Al electrodes were deposited in a top-down configuration. High-resolution transmission electron microscopy reveals that ZrO2 has a polycrystalline–columnar crystallization and a tetragonal crystalline structure, which was confirmed by X-ray diffraction measurements. It is shown that the tetragonal phase is stabilized during the crystallization by the fast diffusion of oxygen atoms. The capacitance–voltage characteristics show that the widest memory window (ΔV = 2.23 V) was obtained for samples with 10% Zr annealed at 700 °C for 4 min. The charge retention characteristics show a capacitance decrease of 36% after 10 years.

Exploring the Interfacial Reaction of Nano Al/CuO Energetic Films through Thermal Analysis and Ab Initio Molecular Dynamics Simulation.

The effect of the interface layer on energy release in nanoenergetic composite films is important and challenging for the utilization of energy. Nano Al/CuO composite films with different modulation periods were prepared by magnetron sputtering and tested by differential scanning calorimetry. With the increase in the modulation period of the nano Al/CuO energetic composite films, the interface layer contained in the energetic composite film decreased meaningfully, increasing the total heat release meaningfully. Ab initio molecular dynamics (AIMD) simulation were carried out to study the preparation process changes and related properties of the nano Al/CuO energetic composite films under different configurations at 400 K. The results showed that the diffusion of oxygen atoms first occurred at the upper and lower interfaces of CuO and Al, forming AlOx and CuxAlyOz. The two-modulation-period structure changed more obviously than the one-modulation-period structure, and the reaction was faster. The propagation rate and reaction duration of the front end of the diffusion reaction fronts at the upper and lower interfaces were different. The Helmholtz free energy loss of the nano Al/CuO composite films with a two-modulation-period configuration was large, and the number of interfacial layers had a great influence on the Helmholtz free energy, which was consistent with the results of the thermal analysis. Current molecular dynamics studies may provide new insights into the nature and characteristics of fast thermite reactions in atomic detail.

The Role of Grain Boundaries in the Corrosion Process of Fe Surface: Insights from ReaxFF Molecular Dynamic Simulations

Intergranular corrosion is the most common corrosion phenomenon in Fe-based alloys. To better understand the mechanism of intergranular corrosion, the influence of grain boundaries on Fe-H2O interfacial corrosion was studied using molecular dynamics simulation based on a new Fe-H2O reaction force field potential. It is found that the corrosion rate at the polycrystalline grain boundary is significantly faster than that of twin crystals and single crystals. By the analysis of stress, it can be found that the stress at the polycrystalline grain boundary and the sigma5 twin grain boundary decreases sharply during the corrosion process. We believe that the extreme stress released at the grain boundary will promote the dissolution of Fe atoms. The formation of vacancies on the Fe matrix surface will accelerate the diffusion of oxygen atoms. This leads to the occurrence of intergranular corrosion.

Control of Transfer Characteristics of Atomic Layer Deposited Al-Doped SnO2 Thin Film through a Post-Annealing Process

In this paper, changes in transfer characteristics for an Al-doped SnO2 thin film using the annealing process was studied. Generally, as-deposited SnO2 thin film has electrical properties similar to those of the conductor due to high carrier concentration. However, various electrical properties such as on/off current ratio could be obtained as Al was doped and the annealing process was performed in an air atmosphere. As a result of XPS analysis, it was confirmed that the concentration of oxygen vacancy concentration decreased when the Al-doped SnO2 thin film was annealed. Compared to the as-deposited Al-doped SnO2 thin film, the annealed Al-doped SnO2 thin film had superior channel characteristics because the oxygen vacancy concentration in the SnO2 thin film was lowered by the oxygen atom diffusion during the annealing process. This is due to the lower carrier concentration. As a result of analyzing the electrical properties of the annealed Al-doped SnO2 thin film, that film annealed at 500 °C, an on/off current ratio of 106 was obtained. And the field effect mobility was also 1.56 cm2 Vs−1.

Locating the rate-limiting step of hydrogen conversion on Sr2Fe1.5Mo0.5O6 (0 0 1) surface: Implications for efficient SOFC anode design

Density functional theory (DFT) based first-principles calculations have been employed to explore the hydrogen conversion mechanism on Sr2Fe1.5Mo0.5O6 (SFM) surface. Results show that hydrogen molecule exhibits weak physical adsorption behavior while hydrogen atom strongly chemisorbed on SFM (001) surface. Hydrogen molecule preferably dissociates at surface (Fe)-Fe-(Fe) site with an activation barrier of 0.69 eV. Hydrogen oxidation on SFM (001) surface is favorable both in kinetics and thermodynamics, and the resultant H2O molecule desorption only needs to overcome energy barrier of 0.22–0.24 eV. Oxygen atom diffusion from bulk to surface prefers to follow the path from subsurface (Fe)-O-(Fe) site to outer-surface (Fe)-VO-(Mo) vacancy site, which needs to overcome an energy barrier of 0.49 eV. The rate-limiting step of hydrogen conversion on the SFM surface is determined to be hydrogen dissociation at temperature below 775 °C while it switches to hydrogen adsorption at temperature above 775 °C. The theoretical findings in current study are expected to offer fundamental basis for designing efficient perovskite-based SOFC anodes.

Effects of oxygen concentration and irradiation defects on the oxidation corrosion of body-centered-cubic iron surfaces: A first-principles study

Oxidation corrosion of steels usually occurs in contact with the oxygen-contained environment, which is accelerated by high oxygen concentration and irradiation. The oxidation mechanism of steels is investigated by the adsorption/solution of oxygen atoms on/under body-centered-cubic (bcc) iron surfaces, and diffusion of oxygen atoms on the surface and in the near-surface region. Energetic results indicate that oxygen atoms prefer to adsorb at hollow and long-bridge positions on the Fe(100) and (110) surfaces, respectively. As the coverage of oxygen atoms increases, oxygen atoms would repel each other and gradually dissolve in the near-surface and bulk region. As vacancies exist, oxygen atoms are attracted by vacancies, especially in the near-surface and bulk region. Dynamic results indicate that the diffusion of O atoms on surfaces is easier than that into near-surface, which is affected by oxygen coverage and vacancies. Moreover, the effects of oxygen concentration and irradiation on oxygen density in the near-surface and bulk region are estimated by the McLean’s model with a simple hypothesis.

Oxidation of Supported Nickel Nanoparticles at Low Exposure to O2: Charging Effects and Selective Surface Activity.

The oxidation of Ni nanoparticles supported on highly oriented pyrolytic graphite was investigated under conditions of low exposure to oxygen by methods of scanning tunneling microscopy and spectroscopy. It was found that charge transfer effects at the Ni-C interface influenced the surface activity of the nanoparticles. The O2 dissociation and the Ni oxidation were shown to occur only at the top of the nanoparticle, while the border of the Ni-C interface was the less preferable area for these processes. The O2 dissociation was inhibited, and atomic oxygen diffusion was suppressed in the given nanosystem, due to the decrease in holes concentration.

Charging properties of thin gate dielectrics, obtained by the method of rapid thermal processing

The charge properties of thin dielectrics, obtained by rapid thermal processing (RTP), and their interfaces with silicon for MOS transistors are investigated. The production of insulator layers was carried out by a two- or three-stage RTP with photon processing regimes similar for each stage (duration – 12 s, maximum temperature – 1250 °C). After the third stage of RTP in a nitrogen atmosphere of the gate oxides, obtained by a two-stage process in oxygen atmosphere, the defects responsible for local charge centers are partially eliminated. There is also an increase in the relative value of the surface potential by an average of 100 relative units. The elimination of defects is a consequence of the rearrangement of the structure of the dielectric, its interface with silicon, and the diffusion of oxygen and silicon atoms along the interface of the insulator layer. For samples obtained by a two-stage RTP in an oxygen atmosphere and subjected to the third stage of processing in a forming gas, there is an almost complete elimination of local charge centers and an increase in the relative value of the surface potential by an average of 300 relative units. In this case, in addition to the processes occurring during the treatment of SiO2 by the RTP method in an nitrogen atmosphere, the liquidation of charge centers is a consequence of the passivation of defects by hydrogen atoms.

Resolving atomic diffusion in Ru(0001)−O(2×2) with spiral high-speed scanning tunneling microscopy

An intermediate state in atomic diffusion processes in the $\mathrm{O}(2\ifmmode\times\else\texttimes\fi{}2)$ layer on Ru(0001) is resolved with spiral high-speed scanning tunneling microscopy (STM). The diffusion of atomic oxygen in the adlayer has been studied by density functional theory and STM. Transition state theory proposes a migration pathway for the diffusion in the oxygen adlayer. With spiral scan geometries---a new approach to high-speed STM---the oxygen vacancy mobility on the highly covered Ru(0001) surface is determined to be in the range of 0.1 to 1 Hz. Experimental evidence for the intermediate state along the oxygen diffusion pathway is provided in real space and real time.