Anisotropic Oxidation Research Articles

Development of machine learning force field for thermal conductivity analysis in MoAlB: Insights into anisotropic heat transfer mechanisms

The MAB phases have garnered significant attention as a new generation high-temperature structural materials due to their outstanding high-temperature mechanical properties, resistance to oxidation at elevated temperatures, low-temperature synthesis, and machining capabilities. The chemically anisotropic bonding within MAB phases leads to unique anisotropic elastic properties, oxidation behavior, and electrical characteristics, posing challenges in understanding the structure-property relationships.To address this challenge, we leveraged recent advances in machine learning force fields based on quantum mechanics methods. Using ab initio molecular dynamics simulations, we constructed a comprehensive dataset and developed an accurate machine learning force field for the Mo–Cr–Al–B system based on deep neural network. We then employed non-equilibrium molecular dynamics simulations to calculate the phonon thermal conductivity of MoAlB, investigating its temperature dependence and anisotropy. Our analysis includes phonon density of states, phonon participation ratio, and spectral heat flux analysis, shedding light on the temperature-dependent anisotropy of phonon thermal conductivity in MoAlB. This work not only provides a reliable force field for large-scale molecular dynamics simulations but also advances our understanding of the composition-structure-performance relationship in MAB phases.

Enhancing oxidation resistance of MoAlB based on its anisotropic oxidation mechanism: A theoretical calculation guided experimental investigation

MoAlB is considered the most promising material for oxidation resistance among the MAB phase family. However, the anisotropic oxidation mechanism at the atomic scale has not been fully investigated, which may limit its practical applications. In this work the anisotropic oxidation mechanism of MoAlB was first explored through systematic theoretical calculations, based on which an effective strategy to enhance its oxidation resistance was identified and successfully implemented in the experimental work. The theoretical calculation revealed that the anisotropic oxidation of MoAlB was primarily related to the difference in free energy of surface oxidation reactions due to the degrees of electron interaction localization of O-Al bonding. A series of candidates of various transition metal-doped MoAlB materials were then designed to improve the weaker surface of MoAlB for oxidation resistance enhancement. Among these candidates, Zr-doped MoAlB was selected to further evaluate the effects of doping on the anisotropic oxidation through the electronic structure and Ab initio molecular dynamics (AIMD). Finally, Zr-doped MoAlB was experimentally synthesized to validate the theoretical design. Non-isothermal oxidation kinetics analysis indicated that Zr doping can enhance the oxidation resistance of MoAlB by promoting the formation of a protective oxide scale and increasing the formation activation energy of volatile products, which finally reduced the thickness of the oxidation scale on the surface after isothermal oxidation of the bulk material. This work not only contributes to an in-depth understanding of the oxidation mechanisms in MoAlB but also presents a new approach to enhance its oxidation resistance and other Al containing ceramic materials beyond MAB phases.

The anisotropic oxidation behavior of Hastelloy X alloy fabricated by laser powder-bed fusion (LPBF) during the cyclic oxidation process

The Hastelloy X alloy manufactured via LPBF has anisotropic oxidation behavior at 900 °C. The vertical cross section (V) specimen has worse oxidation resistance and more severe spalling behavior than the horizontal cross section (H) specimen as its low grain boundary density. During the oxidation process, larger voids was generated at Cr2O3-spinel interface of the V specimen to weaken the adhesive strength of oxidation film, and α-NiMoO4 with high cracking susceptibility only occurred in spinel layer of the V specimen, which broke down under high compressive stress in oxidation film to promote the spalling of oxides.

Оже-электронная спектроскопия поверхности MAX-пленок (Cr-=SUB=-0.5-=/SUB=-Mn-=SUB=-0.5-=/SUB=-)-=SUB=-2-=/SUB=-GaC, окисленной в результате хранения на воздухе

We have analyzed chemical bonding of atmospheric oxygen with chromium and manganese on the epitaxial MAX-phase (Cr0.5Mn0.5)2GaC surface using Auger electron spectroscopy combined with ion etching. It was found that the system has a specific anisotropic oxidation where oxygen atoms bind to chromium and manganese ones more actively at the edges of layered MAX phase crystallites in contrast to the (0001) basal plane. At the same time, the dominance of the Mn-O chemical bonding over Cr-O is observed on the latter.

Plasma oxidation as an effective method in etching copper interconnect lines at room-temperature

The oxygen plasma-copper reaction has been proved effective in preparing copper fine patterns at room temperature. The oxidized copper film is composed of crystalline CuO and Cu2O with a granular structure due to the volume expansion. The time-dependent oxide growth progresses in three stages with different mechanisms: penetration of native oxide, oxidation kinetics, and transportation of precursors through the porous oxide layer. The vertical copper profile is obtained from the anisotropic oxidation due to the acceleration of oxygen ions from the cathode self-bias voltage. The plasma oxidation etching method is applicable to a wide range of nano and microelectronics.

A physiochemical model for the combustion of aluminum nano-agglomerates in high-speed flows

A theoretical model of aluminum nanoparticle (ANP) and agglomerates has been developed to highlight the effect of flow-particle interactions and heat conduction within particles. To understand the atomic physicochemical mechanism, a series of reactive molecular dynamics (MD) simulations have been performed on single ANP and agglomerates combustion in high-speed flows. The results show that both single ANP and agglomerates experience the transition from diffusive oxidation mode to anisotropic and explosive oxidation modes as flow velocities increase. All the fundemental mechanisms in the model development are derived from fully atomic insights, including thermal and convective heat transfer between flow and particle, core-shell reaction, heat conduction, particle dispersion and drag force. The model predictions are compared with MD simulations, and the results show that our model can capture the ignition behaviors of ANP and agglomerates in a wide range of flow rates. The combustion of a serial of agglomerates (3–180 primary particles) is simulated to reveal the effects of agglomerates at different flow conditions and agglomerate sizes. At low flow rate conditions, the core-shell reactions contribute about two-thirds of the heat triggering the ignition of single ANP. At high flow rate conditions, the gas-particle convective heat transfer and heat conduction between particles become the major heat source rather than the core-shell reactions. In addition, the ignition delay of agglomerates is much shorter than that of single particle with the same equivalent radius, and the ignition becomes almost independent with the agglomerate size due to the heat accumulation inside at the flow-particle interface. Our study reveals the atomic mechanism of the ignition of ANP agglomerates and proposes a theoretic model of aluminum nanoparticles and agglomerates in high-speed flows, which is applicable to the numerical simulations of aluminum propellants and explosives.

Impact of the Surface and Microstructure on the Lubricative Properties of MoS2 Aging under Different Environments.

Molybdenum disulfide (MoS2) with a hydrophobic property and layered structure possesses an excellent lubricative property and has been widely used as a lubricant in various areas, including satellites, aircraft, and new energy vehicles. Aging is a ubiquitous phenomenon in MoS2 and plays a key role in its tribological application for shortening its service life. The effect of the surface and microstructure on the lubricative properties of MoS2 aging under different environments, including deionized water (DI water), ultraviolet/ozone (UV/Ozone), and high-temperature, was investigated. First, the lubrication of MoS2 transiently degrades because of physical adsorption and recovers after mechanical removal. The lubrication of MoS2 also degrades slightly when its surface becomes hydrophilic, thereby enhancing the adhesion energy due to atomic oxygen interaction under UV/Ozone exposure. Second, the lubrication of MoS2 degrades irreversibly because of the formation of stripes with the destroyed structures under accelerated aging. The lubrication of MoS2 further degrades with the formation of small triangular pits under high-temperature annealing. Finally, the lubrication of MoS2 deteriorates due to the destroyed structure and complete oxidation. The severe aging of MoS2 is accompanied with large triangular pits due to anisotropic oxidation etching of MoS2. The lubrication failure of MoS2 was determined on the basis of structural defect formation and surface property degradation induced by the extent of oxygen diffusion. The enhanced out-of-plane deformation due to the reduced out-of-plane stiffness and the increased energy barriers of defects are fundamentally responsible for the lubrication degradation of MoS2 at the atomic scale. These findings can provide new insights into the atomic-scale mechanism underlying the lubrication failure of MoS2 and pave the way for the realization of MoS2-based lubrication application under various environments.

Unveiling oxidation mechanism of bulk ZrS2

Transition metal dichalcogenides have shown great potential for next-generation electronic and optoelectronic devices. However, native oxidation remains a major issue in achieving their long-term stability, especially for Zr-containing materials such as ZrS2. Here, we develop a first principles-informed reactive forcefield for Zr/O/S to study oxidation dynamics of ZrS2. Simulation results reveal anisotropic oxidation rates between (210) and (001) surfaces. The oxidation rate is highly dependent on the initial adsorption of oxygen molecules on the surface. Simulation results also provide reaction mechanism for native oxide formation with atomistic details.Graphic

Study of local anodic oxidation regimes in MoSe2

Scanning probe microscopy is widely known not only as a well-established research method but also as a set of techniques enabling precise surface modification. One such technique is local anodic oxidation (LAO). In this study, we investigate the LAO of MoSe2 transferred on an Au/Si substrate, focusing specifically on the dependence of the height and diameter of oxidized dots on the applied voltage and time of exposure at various humidities. Depending on the humidity, two different oxidation regimes were identified. The first, at a relative humidity (RH) of 60%–65%, leads to in-plane isotropic oxidation. For this regime, we analyze the dependence of the size of oxidized dots on the oxidation parameters and modify the classical equation of oxidation kinetics to account for the properties of MoSe2 and its oxide. In this regime, patterns with a maximum spatial resolution of 10 nm were formed on the MoSe2 surface. The second is the in-plane anisotropic oxidation regime that arises at a RH of 40%–50%. In this regime, oxidation leads to the formation of triangles oxidized inside the zigzag edges. Based on the mutual orientation of zigzag and armchair directions in successive oxidized layers, the stacking type and phase of MoSe2 flakes were determined. These results allow LAO to be considered not only as an ultra-high-resolution nanolithography method, but also as a method for investigating the crystal structure of materials with strong intrinsic anisotropy, such as transition metal dichalcogenides.

Anisotropy of local anodic oxidation process in thin MoSe2 films

In this work, various regimes of local anodic oxidation (LAO) of MoSe2 were studied. Here we show that there is a certain set of oxidation parameters that results in the anisotropic oxidation of MoSe2. In this mode, LAO leads to the formation of oxidized triangles. The triangles have the same orientation on the surface of the flakes, which indicates that MoSe2 is oxidized mainly along the crystallographic directions of the zigzag edges. These results can be useful for determining crystallographic directions of zigzag/armchair edges and the degree of single-crystallinity of MoSe2 flakes.

MEMS blazed gratings fabricated using anisotropic etching and oxidation sharpening

Micro-electro-mechanical system (MEMS) blazed gratings are the critical optical elements in high resolution micro-spectrometer systems. The MEMS blazed gratings fabricated by conventional methods are not ideal saw-tooth profiles. After the photolithographic and anisotropic etching process, a tiny platform of silicon will be left on the top of the grating groove because of the silica mask, which will lead to amounts of stray light and reduce the diffraction efficiency of gratings. This paper presents a novel design and fabrication method for fabricating MEMS blazed grating, which utilizes off-cut silicon (111) wafers. The blazed angle of gratings is revised from 7.54° to 8.0° according to the tiny platform width on the top of grating grooves. In the procedure of grating fabrication, the oxidation sharpening method is used to cut down the width of the tiny platform. The experimental result shows that the platform width of the grating groove has sharpened from 1000 nm to 540 nm, and the fabricated blazed grating has high diffraction efficiency. The result indicates that the process to fabricate blazed gratings through the anisotropic etching and oxidation sharpening technology is quite effective.

The anisotropic oxidation behaviors of Ti2AlC ceramics: a DFT study

The solution and diffusion behaviors of oxygen in Ti2AlC have been revisited using first principle calculation, to construct a theoretical foundation for understanding, thus controlling the oxidation behavior of Ti2AlC ceramics. In presence of intrinsic vacancies, the stablest configuration is found when oxygen fills the carbon vacancy site. However, for a stoichiometric crystal containing no vacancies, there exists two stable configurations for accommodating the oxygen interstitial. Some possible diffusion paths along z axis in Ti2AlC have been examined. It is found that the diffusion of oxygen through carbon planes is difficult, only possible with the presence of carbon vacancies. In consequence, the Ti2AlC ceramics are deduced to behavior anisotropically during the oxidation process.

Spin Qubits Confined to a Silicon Nano-Ridge

Electrostatically-defined quantum dots (QDs) in silicon are an attractive platform for quantum computation. Localized single electron spins define qubits and provide excellent manipulation and read-out fidelities. We propose a scalable silicon-based qubit device that can be fabricated by industry-compatible processes. The device consists of a dense array of QDs localized along an etched silicon nano-ridge. Due to its lateral confinement, a simple dense array of metallic top-gates forms an array of QDs with controllable tunnel-couplings. To avoid potential fluctuations because of roughness and charged defects at the nano-ridge sidewall, the cross-section of the nano-ridge is trapezoidal and bounded by atomically-flat {111} facets. In addition to side-gates on top of the low-defect oxidized {111} facets, we implement a global back-gate facilitated by the use of silicon-on-insulator. The most relevant process modules are demonstrated experimentally including anisotropic wet-etching and local oxidation of the silicon nano-ridge, side-gate formation with chemical-mechanical polishing, and top-gate fabrication employing the spacer process. According to electrostatic simulations, our device concept allows forming capacitively-coupled QD double-arrays or adjacent charge detectors for spin-readout. Defining a logical qubit or realizing a single electron conveyor for mid-range qubit-coupling will be future applications.

Oxidation of Suspended Graphene: Etch Dynamics and Stability Beyond 1000 °C.

We study the oxidation of clean suspended mono- and few-layer graphene in real time by in situ environmental transmission electron microscopy. At an oxygen pressure below 0.1 mbar, we observe anisotropic oxidation in which armchair-oriented hexagonal holes are formed with a sharp edge roughness below 1 nm. At a higher pressure, we observe an increasingly isotropic oxidation, eventually leading to irregular holes at a pressure of 6 mbar. In addition, we find that few-layer flakes are stable against oxidation at temperatures up to at least 1000 °C in the absence of impurities and electron-beam-induced defects. These findings show, first, that the oxidation behavior of mono- and few-layer graphene depends critically on the intrinsic roughness, cleanliness and any imposed roughness or additional reactivity from a supporting substrate and, second, that the activation energy for oxidation of pristine suspended few-layer graphene is up to 43% higher than previously reported for graphite. In addition, we have developed a cleaning scheme that results in the near-complete removal of hydrocarbon residues over the entire visible sample area. These results have implications for applications of graphene where edge roughness can critically affect the performance of devices and more generally highlight the surprising (meta)stability of the basal plane of suspended bilayer and thicker graphene toward oxidative environments at high temperature.

Surface Modification and Microstructuring of 4H-SiC(0001) by Anodic Oxidation with Sodium Chloride Aqueous Solution.

Anodic oxidation is a promising surface modification technique for the manufacture of SiC wafers owing to its high oxidation rate. It is also possible to fabricate porous SiC by anodic oxidation and etching owing to the material properties of SiC. In this study, the anodic oxidation of a 4H-SiC(0001) surface was investigated by performing repeated anodic oxidation and hydrofluoric acid etching on a 4H-SiC(0001) surface, during which the formation of porous SiC was observed and studied. Anodic oxidation is very effective for removing the surface damage formed by mechanical polishing, and the surface after removing the surface damage can be oxidized uniformly and has a higher oxidation rate than a surface newly finished by chemical mechanical polishing (CMP). We proposed a model based on the electrochemical impedance method to explain the difference in the oxidation between an as-CMP-finished surface and an oxidized/etched surface. Porous SiC was obtained in this study, which was due to the anisotropy of the SiC crystal. The structure of the porous SiC was significantly dependent on the etch pits generated at the beginning of anodic oxidation and can be controlled via anodic oxidation parameters. Anodic oxidation and hydrofluoric acid etching cannot remove porous SiC owing to the anisotropic oxidation of the SiC surface and the difficulty of anodizing SiC fibers. This study shows that anodic oxidation is a promising technique for the modification of SiC surfaces and the fabrication of porous SiC.

Anisotropic lateral oxidation of Al-III–V semiconductors: inverse problem and circular aperture fabrication

The lateral wet oxidation of aluminum-containing III–V-semiconductors is a technological process which converts a buried (thin) cristalline material into an amorphous insulator and, as such, tends to exhibit an anisotropic behavior. As a result, the shape of the interface between the semiconductor and the insulator, often referred as the oxide aperture, differs from the etched mesa contour from which the oxidation proceeds. This, in turn, complicates the design of the devices relying on this process especially when specific oxide patterns are needed. In this paper, we introduce a method based on a morphological dilatation to determine the shape of the mesas which will lead to a specific targetted contour upon an anisotropic oxidation over a modest extent. The approach is experimentally validated by demonstrating the fabrication of circular oxide apertures which are inherently difficult to make because of their high degree of symmetry but which also turn out to be of critical importance in obtaining efficient single-mode vertical-cavity surface-emittting lasers.

Polarization-controlled and single-transverse-mode vertical-cavity surface-emitting lasers with eye-shaped oxide aperture

We presented a single-transverse-mode and single-polarization vertical-cavity surface-emitting laser (VCSEL) with an eye-shaped oxide aperture, obtained by enhanced anisotropic oxidation of oxide layer. For apertures with dimensions of 2 × 4.6 and 3 × 6 µm2, the orthogonal polarization suppression ratio (OPSR) of the VCSEL was 22 and 19 dB, respectively. A single-mode suppression ratio (SMSR) of more than 25 dB at an output power of 0.5 mW was also achieved for the VCSEL with aperture dimension of 2 × 4.6 µm2. We believe that the proposed method to realize the mode and polarization control of VCSELs has great potential in future applications.

Synthesis and oxidation resistance of MoAlB single crystals

Needle and plate like single MoAlB crystals were synthesized by reacting MoB and Al powders. The growth of MoAlB single crystals was promoted by either increasing dwell temperature or by adding excess of Al. When raising the temperature from 1100 to 1300 °C, the shape of MoAlB single crystals transformed from needle to plate like. The single crystals exhibited anisotropic oxidation resistance where the (0l0) plane showed a better oxidation resistance compared to (l00) and (00l) planes. The oxidization weight gain tested at 1100 °C and weight loss at 1300 °C suggests the beneficial role of a dense Al2O3 scale formed on the plate-like single crystals.

Modelling anisotropic lateral oxidation from circular mesas

In this paper, an iterative method to model the anisotropic lateral oxidation of circular structures is proposed and validated by confrontation to experimental data. The described model enables the efficient calculation of the temporal bi-dimensional evolution of the oxidation front shape, starting from a circular mesa, and progressing inward as a result of an anisotropic process combining an isotropic diffusion with an anisotropic reaction. The result of the developed model shows that the oxide aperture smoothly deforms from a circle to become more diamond-like, mimicking the experimental situation encountered when fabricating vertical-cavity surface-emitting lasers (VCSELs) on (100) wafers or, more generally, when oxidizing circular mesas of aluminum-containing III-V semiconductor on similarly oriented substrates.

Elucidation of the atomic-scale mechanism of the anisotropic oxidation rate of 4H-SiC between the (0001) Si-face and (0001¯) C-face by using a new Si-O-C interatomic potential

Silicon carbide (SiC) is an attractive semiconductor material for applications in power electronic devices. However, fabrication of a high-quality SiC/SiO2 interface has been a challenge. It is well-known that there is a great difference in the oxidation rate between the Si-face and the C-face and that the quality of oxide on the Si-face is greater than that on the C-face. However, the atomistic mechanism of the thermal oxidation of SiC remains to be solved. In this paper, a new Si-O-C interatomic potential was developed to reproduce the kinetics of the thermal oxidation of SiC. Using this newly developed potential, large-scale SiC oxidation simulations at various temperatures were performed. The results showed that the activation energy of the Si-face is much larger than that of the C-face. In the case of the Si-face, a flat and aligned interface structure including Si1+ was created. Based on the estimated activation energies of the intermediate oxide states, it is proposed that the stability of the flat interface structure is the origin of the high activation energy of the oxidation of the Si-face. In contrast, in the case of the C-face, it is found that the Si atom at the interface is easily pulled up by the O atoms. This process generates the disordered interface and decreases the activation energy of the oxidation. It is also proposed that many excess C atoms are created in the case of the C-face.