Initial Stages Of Oxidation Research Articles

AFM Observation of Initial Oxidation Stage of 4H-SiC (0001) in Electrochemical Mechanical Polishing

Single-crystal SiC (4H-SiC) is a promising next-generation semiconductor power-device material because of its excellent electronic and thermal properties. Electrochemical mechanical polishing (ECMP) is an effective way of finishing SiC surfaces. Using the ECMP process, the realization of a scratch-free and subsurface damage (SSD)-free surface is expected because the surface of SiC is softened by anodic oxidation and then removed by soft abrasives. Compared with chemical mechanical polishing (CMP), ECMP has a higher material removal rate (MRR) and a lower cost. However, protrusions tend to be generated on the oxide/SiC interface in the anodic oxidation process, which significantly increase the surface roughness. In this study, we observed the generation process of the oxide layer in the anodic oxidation of 4H-SiC (0001) by atomic force microscopy (AFM) observation, and the mechanism of the initial anodic oxidation is discussed.

Effect of Si on the oxidation reaction of α-Ti(0 0 0 1) surface: ab initio molecular dynamics study

We present our ab initio molecular dynamics (MD) study of the effect of Si on the oxidation of α-Ti(0 0 0 1) surfaces. We varied the Si concentration in the first layer of the surface from 0 to 25 at.% and the oxygen coverage (θ) on the surface was varied up to 1 monolayer (ML). The MD was performed at 300, 600 and 973 K. For θ = 0.5 ML, oxygen penetration into the slab was not observed after 16 ps of MD at 973 K while for θ > 0.5 ML, oxygen penetration into the Ti slab was observed even at 300 K. From Bader charge analysis, we confirmed the formation of the oxide layer on the surface of the Ti slab. At higher temperatures, the Si atoms diffused from the first layer to the interior of the slab, while the Ti atoms moved from second layer to the first layer. The pair correlation function shows the formation of a disordered Ti-O network during the initial stage of oxidation. Si was found to have a strong influence on the penetration of oxygen in the Ti slab at high temperatures.

Initial stage oxidation on nano-trenched Si(1 0 0) surface

As the size of an electronic element shrinks to nanoscale, trench design of Si strongly influences the performance of related semiconductor devices. By reactive force field molecular dynamics (ReaxFF MD) simulation, the initial stage oxidation on nano-trenched Si(1 0 0) angled 60°, 90°, 120°, 150° under temperatures from 300 K to 1200 K has been studied. Inhomogeneous oxidation at the convex–concave corners of the Si surface was observed. In general, the initial oxidation process on the Si surface was that, firstly, the O atoms ballistically transported into surface, then a high O concentration induced compressive stress at the surface layers, which prevented further oxidation. Compared to the concave corner, the convex one contacted a larger volume of oxygen at the very beginning stage, leading an anisotropic absorption of O atoms. Afterwards, a critical compression was produced at both the convex and concave corners to limit the oxidation. As a result, an inhomogeneous oxide film grew on nano-trenched Si. Meanwhile, due to enhanced O transport and compression relaxation by increasing temperature, the inhomogeneous oxidation was more obvious under 1200 K. These present results explained the observed experimental phenomena on the oxidation of non-planar Si and provided an aspect on the design of nano-trenched electronic components in the semiconductor field.

Investigation of Ceramic-Like Coatings Formed on Aluminum Composites by Microarc Oxidation

Ceramic-like coatings with a thickness of up to 40 μm are formed on aluminum composites without additives and with copper additives (1 and 4.5%) in a silicate-alkaline electrolyte by microarc oxidation. The composites are prepared by powder metallurgy (cold pressing and sintering in forevacuum). An increase in the copper concentration in the composites to 4.5% leads to the retardation of anode voltage growth on the initial stage of oxidation corresponding to the formation of a barrier layer. The coatings are studied by scanning electron microscopy, X-ray microanalysis, X-ray photoelectron spectroscopy, and X-ray diffraction. The morphology of their surface corresponds to the morphology of the surface of coatings on compact aluminum alloys. According to X-ray photoelectron spectroscopy, a thin 1-μm layer forms on the surface. It consists predominantly of electrolyte components. X-ray diffraction analysis shows that the coatings mainly consist of γ-Al2O3 oxide as well as the η-Al2O3 phase, the peaks of which are broadened. This broadening is characteristic of the amorphous component and may be due to the presence of nanocrystalline regions in the coating structure. In the coatings on the composite Al + 4.5% Cu, mullite Al2SiO5 and copper oxide CuO are also found. The excess aluminum content may be associated with residual unoxidized aluminum inclusions in the structure of the coatings.

Pt oxide and oxygen reduction at Pt(111) studied by surface X-ray diffraction

The influence of the oxygen reduction reaction on the oxidation of Pt(111) is studied by surface X-ray diffraction. The oxygen reduction reaction does not significantly influence the place-exchange process during the initial stages of oxidation and there is no change in the onset potential and kinetics.

Adhesion energy in the metal/oxide system for the case of high-temperature oxidation of nickel-based superalloys

The microstructure of the oxide layer is studied and the adhesion energy of the metal/oxide interface is determined after high-temperature isothermal oxidation (T = 1150°C) of a single-crystal commercial Rene N5 nickel alloy. Experiments upon heating and rapid cooling are performed to initiate failure of the oxide layer. The elastic energy of the metal/oxide interface is determined using the model of circular buckling followed by edge delamination. Calculations are conducted for a homogeneous alumina layer and with allowance for the multilayer structure that forms at the initial stages of alloy oxidation. The adhesion energy is 72 and 27 J/m2 for these cases, respectively.

Initial-Stage Oxidation of Ni3Al(100) and -(110) from Ab Initio Thermodynamics

The microscopic mechanisms of the initial-stage oxidation of the Ni3Al(100) and Ni3Al(110) surfaces are comparatively studied using ab initio calculations based on density-functional theory and thermodynamics considerations. The surface energies of the two surfaces as functions of aluminum and oxygen chemical potentials are constructed and show that the formation of any antisite defects is not favorable at the Ni3Al(100) surface, whereas Al antisite defects are favorable at the Ni3Al(110) surface. The surface phase diagrams of the Ni3Al(100) and -(110) surfaces with different antisite defects and at the various oxygen coverages are determined. These results show that oxygen adsorption enhances Al surface segregation at the initial stage of oxidation for both surfaces and that the Ni3Al(100) surface is thermodynamically more favored to oxidize completely at a lower oxygen coverage than the Ni3Al(110) surface.

Atomistic study of comblike structure on the MoO2/Mo(110) surface by scanning tunneling microscopy and density functional theory calculations

The origin of comblike step formation, which was previously observed in the initial oxidation stage of the Mo(110) surface, was investigated by comparing between scanning tunneling microscopy (STM) observations and ab initio density functional theory (DFT) calculations. The comblike steps were obtained by the formation and evaporation of cluster arrays during oxidation and thermal treatment. On the terraces surrounding the comblike steps formed by the evaporation of clusters, a characteristic “overstripe” pattern was found. On the basis of this pattern, it is assumed that a reliable unit cell with MoO2(010) placed on Mo(110) with in-plane rotation can be constructed and ab initio structural relaxations can be performed. The calculations included “small” model calculations for investigating stable interfacial oxygen sites and “large” model calculations for simulating STM images, and calculation results were compared with experimental results. The simulated STM images show good agreement with the experimental observations, indicating the relevance of the large model. From these analyses, it is pointed out that the local stresses on molybdenum in the oxide layer were important for the comblike step formation.

Detailed peak fitting analysis of the Zn 2p photoemission spectrum for metallic films and its initial oxidation stages

The metallic Zn 2p photoemission spectra hold a complex background that requires individual assignment of Shirley background for each peak comprising the spectra. For this reason, a close fit requires the use of the Shirley‐Vegh‐Salvi‐Castle background‐type under the active background approach. We found that the intensity of the plasmon peaks and their associated background cannot be described through existing energy loss (intrinsic and extrinsic) formalisms. We also analyzed the Zn 2p and O 1s spectra for the initial stages of oxide formation at various oxygen exposures. We found that the composition of the oxide layer is ZnO1.00±0.10 for all exposures, suggesting that our assessment of the primary function of metallic Zn is accurate and can be employed for quantitative studies. We also present a set of parameters to accurately fit and resolve the metallic and oxide Zn 2p peaks.

TGA-MS study of steam oxidation resistance of HCM12A steel at 750 and 800 °C

Initial oxidation stages of the ferritic/martensitic HCM12A steel at 750 and 800 °C in Ar + 40%H2O for 190 h were studied in a closed steam loop by means of mass spectrometry (MS) and thermogravimetric analysis (TGA). TGA measures verified that mass gained by the HCM12A oxidized steel at 750 °C was up to two times lower than for HCM12A oxidized steel at 800 °C. Thermo-Calc program under same oxidation conditions of experiments was used for study of some theoretical oxidation simulations. The MS measurements at 750 °C of the HCM12A steel for 190 h indicated the presence of low intensity (in order of ppm) volatile species Mn(OH)3(g), Mn(OH)7(g), and CrOOH(g). While the HCM12A oxidized steel in steam at 800 °C and under same oxidation conditions emitted only a volatile CrOOH(g) species of double intensity. Therefore, it confirmed that the loss and rupture of the chromium scale of the steel can accelerate when the oxidation temperature increases. The morphology, composition, and structure of samples by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), X-ray energy dispersive spectrometry (EDAX), and X-ray diffraction (XRD) are described.

High-temperature oxidation of AZ91-0.3%Ca-0.1%Y alloy in air

AZ91 magnesium alloys containing 0.3% Ca and 0.1% Y were cast, and their oxidation behavior was investigated between 425 and 600 °C in atmospheric air to examine roles of Ca and Y during oxidation. During casting, Ca formed Al2Ca particles intergranularly, and reduced the amount of Al12Mg17 particles, while most of yttrium existed as Al2Y particles inter- and intra-granularly in the alloy. The AZ91 alloy oxidized fast above 425 °C, leading to complete ignition. By contrast, AZ91-0.3Ca-0.1Y alloy oxidized very slowly up to 550 °C. Calcium, which is more active than Mg, preferentially oxidized to CaO at the surface of the MgO-rich oxide to suppress the oxidation, evaporation and diffusion of Mg during the initial oxidation stage. Such suppression was due to the quite low vapor pressure and high stoichiometry of CaO. Calcium also suppressed the formation of less oxidation-resistant Al12Mg17 through forming oxidation-resistant Al2Ca in the alloy from the initial oxidation stage.

Metabolic peculiarities of the citric acid overproduction from glucose in yeasts Yarrowia lipolytica

Comparative study of 43 natural yeast strains belonging to 20 species for their capability for overproduction of citric acid (CA) from glucose under nitrogen limitation of cell growth was carried out. As a result, natural strain Yarrowia lipolytica VKM Y-2373 was selected. The effect of growth limitation by biogenic macroelements (nitrogen, phosphorus, or sulfur) on the CA production by the selected strain was studied. It was shown that yeasts Y. lipolytica grown under deficiency of nitrogen, phosphorus, or sulfur were able to excrete CA in industrially sufficient amounts (80–85g/L with the product yield (YCA) of 0.70–0.75g/g and the process selectivity of 92.5–95.3%). Based on the obtained data on activities of enzymes involved in the initial stages of glucose oxidation, the cycle of tricarboxylic acids, and the glyoxylate cycle, the conception of the mechanism responsible for the CA overproduction from glucose in Y. lipolytica was formulated.

Effects of Interstitial Nitrogen Atoms on Atomic Oxygen Adsorption on Fe (001) Surface from <i>Ab Initio</i> Calculations

The effects of interstitial nitrogen atoms on the adsorption of atomic oxygen on fcc Fe (001) surface have been studied using ab initio density functional theory calculations to understand the initial stage of oxidation on nitrogenous austenitic stainless steel. It has been found out that the N atoms can improve the adsorption ability of the O atom at the hollow site on the surface, and thus promote the rapid passivation of nitrogenous austenitic stainless steel. This improvement is possibly because the Coulombic interactions between the O atom and the neighboring Fe and Cr atoms are enhanced due to the electron transfer from the Fe and Cr atoms to the N atoms.

ToF-SIMS Study on the Initial Stages of the Halogen Effect in the Oxidation of TiAl Alloys

Intermetallic titanium aluminide samples implanted with fluorine using plasma immersion ion implantation were investigated after high-temperature exposure for short times at 500 and 900 °C in air by ToF-SIMS as a surface analytical method. The ToF-SIMS method was applied in order to obtain information regarding the location of fluorine with respect to the oxide layer formed and the substrate after ion implantation and after high-temperature exposure, respectively. The aim of the present work was to obtain further insight into the mechanisms active during the initial stages of oxidation. The scale structure in the initial oxidation phase consists of titanium and aluminum oxides/fluorides/oxyfluorides. The arrangement of their layered structure can be interpreted according to their thermodynamic stabilities in the partial pressure gradient of oxygen over the scale and according to the metal activities and the presence of fluorine at the metal/scale interface.

Effects of boron addition on the high temperature oxidation resistance of dense sSiBCN monoliths at 1500 °C

The high-temperature oxidation behavior of various boron addition SiBCN monoliths has been explored. Boron addition promotes the formation of new BxC phase and growth of SiC gains. At the initial stage of oxidation, surface BxC and BN(C) are prior to SiC generating B2O3. As time progresses, SiC oxidizes to SiO2 further reacted with B2O3 forming stable SiO2-B2O3 binary melt or borosilicate. Boron-enhanced SiC oxidation occurs immediately with the rapid consumption of BN(C), BxC and loss of B2O3 from borosilicate decomposition, thereby promoting oxidation rate. Substantial SiC and BN(C) in amorphous SiO2 for transition layer is the main structure feature.

Kinetics and oxidation mechanism of MoSi2–MoB ceramics in the 600–1200 °C temperature range

Abstract The aim of this work was to study the kinetics and oxidation mechanism of MoSi 2 -MoB SHS-ceramics in the temperature range of 600–1200 °C. Due to the low rate of change in the mass of samples during the initial stage of oxidation, the kinetic curves at 600 °C suggest that the oxidation process occurs according to a parabolic rate law. At this temperature, MoO 2 and MoO 3 are formed in samples with the greatest MoB content due to its oxidation, and their intensive evaporation only begins after 700 °C. A thin film consisting of a mixture of Mo, Si and B oxides forms on the sample surface as a result of oxidation with a minimum MoB content. The presence of B 2 O 3 in the oxide film leads to the formation of a borosilicate barrier layer, which prevents oxygen diffusion to the sample surface and MoO 3 evaporation. At temperatures of 900 and 1200 °C, the oxidation process proceeds according to a logarithmic law. At these temperatures, an oxide film based on SiO 2 is formed on the surface layer, which in the case of the maximum MoB content sample, is a mixture of B 2 O 3 -SiO 2 oxides in an amorphous state. At lower MoB concentrations, the formed SiO 2 film has a crystalline structure and contains boron. In addition, during oxidation of the materials at the maximal temperature, a Mo 5 Si 3 silicide layer is formed at the interface between the oxide layer of B 2 O 3 -SiO 2 or B-SiO 2 and the sample material due to the oxidation of MoSi 2 , whose thickness and continuity increase with increases of the temperature and duration of oxidation.

Angle-Resolved Photoelectron Spectroscopy Studies of Initial Stage of Thermal Oxidation on 4H-SiC (0001) on-Axis and 4° Off-Axis Substrates

It is a key for improving the performance of SiC MOSFET to clarify SiO2/SiC interface structure formed by thermal oxidation. We have investigated the initial stage of thermal oxidation on 4H-SiC (0001) on-axis and 4° off-axis substrates using angle-resolved photoelectron spectroscopy. The changes of the Si 2p3/2 and C 1s photoelectron spectra show that the off-axis has an influence on the chemical bonding state of SiO2/SiC. On the other hand, there isn't difference in the oxidation rate between on-axis 4H-SiC(0001) and 4° off-axis 4H-SiC(0001).

In-Situ X-Ray Diffraction Study of Pt Oxidation during Oxygen Reduction Reaction

ORR is one of the most studied electrochemical reactions due to its tremendous fundamental and practical importance. Oxygen is a common, readily accessible oxidizing agent and, therefore, the Pt ORR cathode is part of many energy conversion devices, e.g. fuel cells. Unfortunately, slow kinetics of the ORR negatively affects the performance and it is currently one of the main bottleneck in large scale fuel cells commercialization. It is partly caused by the presence of surface Pt oxides, which slow the reaction rate and trap reaction intermediates on the surface. The oxide formation and reduction is also known to cause dissolution of the Pt catalyst, which further degrades the performance. Even though the electrochemical formation of surface oxides on platinum surface has been extensively studied in the past, there are still many questions unanswered, mainly about the detailed structure of the oxide and its growth mechanism [1 and references therein]. Most of the studies were performed in the absence of O2, the fuel cell oxidant, and therefore they are less relevant to the fuel cell operation, as gaseous O2 can modify the oxidation potentials and mechanism. Given the above, further fundamental understanding of the Pt oxidation mechanism and its atomistic picture is clearly needed in order to determine the role of surface oxides in ORR and its effect on the fuel cell performance.Here we show the results of in-situ study of electrochemical oxide formation on Pt(111) and how it is influenced by presence of O2 during ORR. Furthermore, we show that surface reorganization processes taking place during the oxidation/reduction cycle are governed by the ad-atom surface diffusion dynamics and closely resemble the dynamics found under the vacuum conditions. The place exchange process associated with the initial stages of oxidation is followed dynamically during cyclic voltammetry (CV) and potential step experiments in the presence and absence of oxygen. Detailed analysis at two potentials shows that the reconstruction is consistent with a place exchange process between Pt and O atoms, in which the exchanged Pt atoms are directly above their original positions in the Pt(111) lattice. The reconstruction initiates with the CV peak at 1.06 V vs RHE, even though repeated cycling to 1.15 V leads to no changes in the CV. Adding O2 to the electrolyte does not have any significant effect on the oxidation behavior, in contrast to some literature reports, and the O2 accelerated Pt dissolution is not caused by the negative shift in the oxidation potential. Furthermore, the ORR current decreases before oxidation, implying that the presence of the surface oxide is not the limiting factor in the ORR and the high ORR overpotential is solely due to the slow ORR mechanism on an unreconstructed surface.The surface restructuring upon electrochemical oxidation/reduction shows a characteristic ripening behavior where Pt islands grow and become more prominent and homogeneous in size with increasing number of cycles. Their characteristic lateral dimensions primarily depend on the upper potential limit of the cycle and only slightly increase with cycle number. The structural evolution of the Pt surface morphology strongly resembles that found in studies of Pt(111) homoepitaxial growth and ion erosion in ultrahigh vacuum. This finding shows that the electrolyte does not need to be included in the ab-initio investigations of the Pt surface structure change during ORR. Figure 1

Angle-Resolved Photoelectron Spectroscopy Studies of Initial Stage of Thermal Oxidation on 4H-SiC (0001) on-Axis and 4° Off-Axis Substrates

1. Introduction The performance of a SiC MOSFET has not reached the level predicted based on the physical properties of SiC. One of the reasons is that interface state density (Dit) at SiO2/SiC is more than an order of magnitude higher than that at SiO2/Si. [1] Therefore, it is a key for improving the performance of SiC MOSFET to clarify the origin of Dit and SiO2/SiC interface structure formed by thermal oxidation, and to control interface structure based on the knowledge. We have reported the results of the changes in chemical bonding state of SiO2/4H-SiC (0001(_)) (C-face) and 4H-SiC (0001(-)) (Si-face) structure with the progress of thermal oxidation using angle-resolved X-ray photoelectron spectroscopy (AR-XPS). [2, 3] In this paper, we report the results the initial stage of thermal oxidation on 4H-SiC (0001) on-axis and 4° off-axis substrates. 2. Experimental Method 4H-SiC (0001) epitaxial films with on-axis and 4° off-axis were used in this study. We used the flattened 4H-SiC(0001) surface with on-axis which is composed of alternating wide and narrow terraces with single-bilayer-height steps. [4] The samples were prepared as follows. The sample was cleaned in the mixture of H2SO4 and H2O2 (H2SO4:H2O2=4:1) at 80-85 ºC, and the native oxide was removed by dipping in 5% hydrofluoric acid (HF) followed by a rinse in deionized water. The sample was oxidized at 850 °C in dry oxygen with a pressure of 133 Pa. Then, the sample was oxidized in dry oxygen with a pressure of 133 Pa at 900 °C. Subsequently, the sample was oxidized in dry oxygen with a pressure of 133 Pa at 950 °C, 1000 °C and 1050 °C. The Si 2p and C 1s photoelectron spectra, excited by monochromatic AlKα radiation, were measured at a photoelectron take-off angle of 15 and 90 with an energy resolution of 0.37 eV and an acceptance angle of 3.3°, using an ESCA-300 manufactured by Scienta Instruments AB [5]. 3. Result and Discussion The analyses of Si 2p 3/2 photoelectron spectra arising from the sample of on-axis 4H-SiC(0001) (Fig. 1) show that the oxide increases with the increase of oxidation time. The analyses of C 1s photoelectron spectra show that the component with higher binding energy (chemical shift of about 2eV) is reduced by the oxidation. The measured spectra in Si 2p and C 1s regions were also decomposed into peaks convolution of Gaussian and Lorentzian as shown in Fig. 1 (b) and Fig. 2 (b). Fig. 3 shows the oxide thickness dependence of spectral intensity ratio in the case of samples of the on-axis and 4° off-axis 4H-SiC(0001). As seen in Fig. 3, the intensity ratio ISi3+/ISiC in the case of the on axis 4H-SiC(0001) is larger than that in the case of the 4° off-axis 4H-SiC(0001). In the addition, the intensity ratio ISi1+/ISiC and ISi2+/ISiC in the case of the on axis 4H-SiC(0001) is smaller than that in the case of the 4° off-axis 4H-SiC(0001). These imply that there is the defference in SiO2/SiC interfce structure between on-axis 4H-SiC(0001) and 4° off-axis 4H-SiC(0001). Here, ISi1+, ISi2+, ISi3+, and ISiC denote the intensity of component related to Si1+, Si2+, Si3+ and SiC, respectively. Figure 4 shows the oxidation time dependence of oxide thickness in the case of the on-axis and 4° off-axis 4H-SiC(0001) oxidized at the range from 850℃ to 1050℃. As seen in Fig. 4, the oxidation rate decreases at the oxide thickness of 0.6nm in both the on-axis and 4° off-axis 4H-SiC(0001). This implies that the oxidation rate decreases when the 1 ML SiO2was formed by the oxidation. There isn't difference in the oxidation rate between on-axis and 4° off-axis 4H-SiC(0001). 4. Conclusions From the changes of the Si 2p 3/2 and C 1s photoelectron spectra, a changes in chemical bonding state of SiO2/SiC structure with the progress of thermal oxidation were observed. We found that there is the defference in SiO2/SiC interfce structure between on-axis 4H-SiC(0001) and 4° off-axis 4H-SiC(0001).

Oxidation behavior of Ti2AlC in the temperature range of 1400 °C–1600 °C in steam

The oxidation behavior of bulk Ti2AlC ceramic in steam has been investigated in the temperature range of 1400 °C–1600 °C. The oxidation kinetics followed a sub-parabolic law at the early stage of oxidation, then tended to be a linear law beyond 18 h at 1400 °C, and obeyed a linear law during the whole exposure up to 24 h at 1500 °C. At the initial stage of oxidation at 1400 °C and 1500 °C, randomly Al2TiO5 isolated islands with large elongated grains were observed on the surface. A continuous inner α-Al2O3 layer with a thin discontinuous outer layer of Al2TiO5 formed with prolonged exposure time. Outward diffusion of Ti and C through grain boundaries of the α-Al2O3 scale during steady-state oxidation result in segregation of TiO2 at the grain boundaries of α-Al2O3 and formation of gaseous CO and CO2, respectively. The scale adhesion was reduced in steam compared to that in air due to the accumulation of stresses, and generation of voids at the scale/substrate interface. The mechanical disruption of the oxide scale to relief the stresses contribute to the breakaway oxidation of Ti2AlC at 1400 °C and to the non-protective effect at 1500 °C. The sample was rapidly and completely consumed during isothermal oxidation at 1600 °C accompanied by release of heat and hydrogen. The maximum tolerant temperature of Ti2AlC in steam was approximate 1555 °C, which can be extended via a tailored pre-oxidation process.