Initial Stages Of Thermal Oxidation Research Articles

Biohazard and dynamic features of different polar compounds in vegetable oil during thermal oxidation

Total polar compounds (TPC) have been considered as one of the most objective indicators for the evaluation of deterioration of deep-frying oils. However, the cytotoxic and dynamics features of different parts of polar compound were underestimated. The present study investigated the biohazard effect of different polar compounds from heated oils. The results indicated oxidize triglycerides monomer (ox-TGM) exhibited the most serious deleterious effect on THLE-2 cell viability, followed by triglycerides polymer (TGP) and degradation products. The investigation of polar compounds variation revealed that vegetable oil with higher saturation endured longer thermal oxidation time to achieve the discarded point of 27% TPC. As the increase of thermal oxidation time, the content of polymers and oxidized products increased linearly, while the level of degradation products was altered moderately. The canonical correspondence analysis uncovered significant positive associations between oleic acid and ox-TGM at the initial stage of thermal oxidation. Taken together, our overall findings offer an evidence that ox-TGM could be acted as an accurate indicator for the evaluation of deep-frying oil quality.

Effect of thermal oxidation on surface chemistry and elemental segregation of Al-Cu-Li alloy

The thermal stability of Al-Cu-Li alloys is critical for the extreme working conditions in the aircraft industry. The initial stages of thermal oxidation of an industrial AA 2050-T8 Al-Cu-Li alloy were investigated by in situ XPS and ToF-SIMS.Annealing the sample at 200 °C leads to a surface enrichment in metallic Li and Ag. When exposed to oxygen at low pressure (PO2 < 10−6 mbar) a mixed Li-Al oxide layer is formed. Photoemission intensities (recorded at 45° and 90° relative to the surface) are consistent with the formation of a continuous oxide layer for oxygen exposure from 0 to 5000 langmuirs (L). Assuming a homogeneous composition of the oxide layer, its thickness was 9 ± 1 Å at 5000 L. The relative content of Li and Al in the oxide layer evolves from an initial value of 50% to stabilize at 70% of Li above 100 L of oxygen exposure. Simultaneously, the content of Li at the alloy substrate surface is reduced from 10% to 0%. Moreover, specific segregation sites are evidenced by ToF-SIMS three-dimensional chemical mapping. For Li, a homogenous surface distribution while for Mg the accumulation at the grain boundaries were observed. The segregation of Ag at the oxide/alloy interface was confirmed by ToF-SIMS ion depth profiles.

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).

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).

Effects of γ-Fe2O3 on γ-Fe2O3/Fe3O4 composite magnetic fluid by low-temperature low-vacuum oxidation method

In this paper, the controllable γ-Fe2O3/Fe3O4 composite nanoparticles were prepared by low-temperature low-vacuum oxidation of Fe3O4 directly in different time. The Fe3O4 nanoparticles were synthesized by chemical co-precipitation, and they were almost spherical particles and had no significant changes during the thermal oxidation. The prepared samples were identified by X-ray powder diffraction (XRD), Raman spectra (RS), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The γ-Fe2O3/Fe3O4 was obtained by Fe3O4 oxidized from 0 to 24h, pure γ-Fe2O3 was obtained by Fe3O4 oxidized 24h and its saturation magnetization was 55.86emu/g. The magnetic nanoparticles were evenly dispersed in the silicone oil under the help of oleic acid and emulsifier, the Fe3O4, γ-Fe2O3/Fe3O4 and γ-Fe2O3 magnetic fluid with same viscosity were prepared. In the initial stage of thermal oxidation, the saturation magnetization of magnetic fluid increased with the oxidation time increasing, with the thermal oxidation going on, the saturation magnetization of γ-Fe2O3/Fe3O4 magnetic fluid kept growth until it reaches the maximum critical value and the corresponding oxidation time was 16h, and then it began to decrease. And the maximum saturation magnetization of prepared γ-Fe2O3/Fe3O4 magnetic fluid was 25.08emu/g, which was much higher than that of Fe3O4 magnetic fluid (12.45emu/g) and γ-Fe2O3 magnetic fluid (14.25emu/g).

Angle-resolved photoelectron spectroscopy study of initial stage of thermal oxidation on 4H-SiC(0001)

A key to improving the performance of SiC MOSFETs is to clarify the SiO2/SiC interface structure formed by thermal oxidation. We have investigated the initial stage of thermal oxidation on 4H-SiC(0001) by angle-resolved photoelectron spectroscopy. From the changes in the Si 2p3/2 and C 1s photoelectron spectra, the changes in the chemical bonding state of the SiO2/SiC structure with the progress of thermal oxidation were observed. We also found that the intensity of C–O bonds in the case of 4H-SiC(0001) was smaller than that in the case of 4H-SiC() with the same oxide thickness and that the oxidation rate of 4H-SiC(0001) is already slower than that of 4H-SiC() in the early stage of oxidation.

Thermal oxidation of Zr–Cu–Al–Ni amorphous metal thin films

The initial stages of thermal oxidation for Zr–Cu–Al–Ni amorphous metal thin films were investigated using X-ray photoelectron spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The as-deposited films had oxygen incorporated during sputter deposition, which helped to stabilize the amorphous phase. After annealing in air at 300°C for short times (5min) this oxygen was found to segregate to the surface or buried interface. Annealing at 300°C for longer times leads to significant composition variation in both vertical and lateral directions, and formation of a surface oxide layer that consists primarily of Zr and Al oxides. Surface oxide formation was initially limited by back-diffusion of Cu and Ni (<30min), and then by outward diffusion of Zr (>30min). The oxidation properties are largely consistent with previous observations of Zr–Cu–Al–Ni metallic glasses, however some discrepancies were observed which could be explained by the unique sample geometry of the amorphous metal thin films.

Identification by photoelectrochemistry of oxide phases grown during the initial stages of thermal oxidation of AISI 441 ferritic stainless steel in air or in water vapour

Room temperature photoelectrochemistry was used to characterise oxide phases grown during the initial stages of oxidation of the ferritic stainless steel AISI441 at 650°C and 850°C in synthetic air or in water vapour. Grazing incidence X-ray diffraction and Raman spectroscopy were additionally used to discuss PEC results. Haematite Fe 2O 3 (∼2.0 eV), chromia Cr 2O 3 (3.0 and 3.5 eV) and their mutual solid solution (∼ 2.5 eV) were detected by their respective bandgap values determined from photocurrent vs. energy curves. The Cr/Fe ratio of the films increased with time/temperature and was higher in air-grown than in H 2O-grown oxides. Observation of photocurrent vs. potential curves indicated that chromia was N-type in all specimens, resulting from thermodynamic equilibrium with the metallic substrate and not with the gas phase.

Synchrotron investigations of the initial stage of tin nanolayers oxidation

Initial stages of thermal oxidation of metal tin nanolayers (100 nm) in the air were investigated using X-ray absorption near edge structure (XANES) technique. Analysis of the XANES data revealed that in the metal film exposed to air at room temperature an oxygen K-edge is observed. Its structureless shape indicates the formation of amorphous Sn–O phase in the surface layer. At the same time from Sn M 4,5-spectra of the same tin film one can see M 4,5 absorption edges characteristic of the elementary tin and for its oxide as well. The fine structure near the absorption edges for both elements (tin and oxygen) in the sample annealed at 170 °C correlates quite well and it corresponds to the results of calculations of the density of p-states for tetragonal SnO monoxide. The changes in the fine structures of O K-edge and Sn M 4,5-spectra with the increase of the oxidation temperature up to 240 °C indicates the appearance of SnO 2 in the nanolayers, which becomes a predominant one under the further increase of annealing temperature.

The initial oxidation of zirconium—oxide‐film microstructure and growth mechanism

AbstractThe initial stage of thermal oxidation of bare Zr within the temperature range of 373–773 K at a pO2 of 2 × 10−6 Pa was investigated by combined application of angle‐resolved XPS (AR‐XPS) and in situ spectroscopic ellipsometry (SE). A model description of the oxide‐film growth kinetics was achieved adopting coupled currents of cations and electrons within a surface‐charge field. The model could be fitted very well to the experimental data, thereby yielding profound knowledge of the relation between the oxidation mechanism and the developing oxide‐film microstructure. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Initial Stages of Thermal Oxidation of 4H-SiC (11-20) Studied by Photoelectron Spectroscopy

Initial Stages of Thermal Oxidation of 4H-SiC (11-20) Studied by Photoelectron Spectroscopy

放射光リアルタイム光電子分光を用いた酸素ガスによるＳｉ（００１）表面の熱酸化初期過程の“その場”観察

放射光リアルタイム光電子分光を用いた酸素ガスによるＳｉ（００１）表面の熱酸化初期過程の“その場”観察

SURFACE AND INTERFACE INVESTIGATION OF NANOMETRIC DIELECTRIC FILMS ON Si AND ON SiC

Dielectric films on Si or SiC were investigated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and ion beam analysis techniques, namely high and low energy ion scattering (RBS and LEIS) and narrow nuclear resonance profiling (NRP) combined with isotopic substitution. For the Si substrate, attention was focused on the thermal stability of materials candidate to replacing SiO 2 in Si-based microelectronic devices (so-called "high-k dielectrics"): Al 2 O 3, ZrSi x O y, ZrAl x O y, HfSi x O y, and GdSi x O y. Mobility of different atomic species — especially Si and O — was observed at both the surface of the films and the interface with Si. Such atomic transport may have serious consequences concerning application of these materials in the microelectronics industry. For the SiC substrate, attention was focused on the initial stages of thermal oxidation in O 2, seeking an understanding of its poorer electrical quality as compared to SiO 2– Si . It was found that the initial oxidation products are silicon oxycarbides ( SiC x O y), while for longer oxidation times a mixture of SiC x O y and SiO2 is formed in the near surface region of the growing film. The composition of the near surface region of such thin films is very similar to that reported in previous investigations for the near interface region when thicker oxides films are grown on SiC.

Recent Progress in Theoretical Study of Formation of Semiconductor Surfaces and Interfaces Based on Microscopic Processes. Reaction Kinetics during Initial Stage of Thermal Oxidation on Si(100) Surface.

Autocatalytic-reaction model, a chemical-kinetics model that provides an integrated description of various dynamical processes in the very initial oxidation of Si(100) surface, is illustrated. The nonlinear rate equation of this model has an analytical solution. With only two fitting parameters as it is, the solution quantitatively reproduces the experimental time-evolution of the surface oxide coverage in various oxidation modes, which range from the Langmuir-type in the low-temperature/high-pressure regime to the two-dimensional-island-growth-type in the high-temperature/low-pressure regime, or even to the one in the etching regime containing simultaneous decomposition of grown oxides in addition to their growth. Microscopic background behind the model is discussed in comparison with existing oxidation models and thin-film-growth models.

Time evolution of interface roughness during thermal oxidation on Si(0 0 1)

The surface morphological change at an initial stage of thermal oxidation on Si(0 0 1) surface with O 2 was investigated as a function of oxide coverage by a real-time monitoring method of Auger electron spectroscopy (AES) combined with reflection high energy electron diffraction (RHEED). At 653 °C where oxide islands grow laterally, protrusions were observed to develop under the oxide islands as a consequence of concurrent etching of the surface. The rate of etching was measured from a periodic oscillation of RHEED half-order spot intensity I (1/2,0) and I (0,1/2). At 549 °C where Langmuir-type adsorption proceeds, it was observed that both I (1/2,0) and I (0,1/2) decrease more rapidly in comparison with an increase of oxide coverage and the intensity ratio between them decreases gradually with O 2 exposure time. These suggest that Langmuir-type adsorption occurs at sites where O 2 adsorbs randomly, leading to subdivision of the 2×1 and 1×2 domains by oxidized regions, and that Si atoms are ejected due to volume expansion in oxidation to change the ratio between 2×1 and 1×2 domains.

Load-Independent Friction: MoO3 Nanocrystal Lubricants

The behavior of the model lubricant system molybdenum disulfide (MoS2) has been characterized as a function of thermal oxidation on the atomic to nanometer scales with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). STM studies of single-crystal MoS2 surfaces showed that the initial stages of thermal oxidation produce atomic-scale pits and then molybdenum oxide (MoO3) nanocrystals 1−5 nm in diameter. The densities of pits and MoO3 nanocrystals and the size of MoO3 nanocrystals increased with increasing oxidation time. UHV AFM friction measurements made on these same samples exhibited a direct relationship between defect density and friction for short oxidation times. After longer oxidation times when well-defined MoO3 nanocrystals were present, load-independent friction was observed. A model, which involves adhesion of MoO3 nanocrystals to the AFM probe tip end, has been developed to explain this first observation of load-independent friction. The implications of these results are discussed.

Initial stages of cobalt oxidation by FTIR spectroscopy

The behaviour of specular reflectance of cobalt oxides on metal has been studied theoretically (CoO/Co system) and experimentally (Co304/Co system; the thin films are obtained by chemical spray pyrolysis) in the infrared region. COO may be identified by a LO mode at 585 cm-I and by a T O mode at 345 cm-l. Co304 is characterized by five bands respectively at 690 (LO), 660 (TO), 605 (LO), 560 (TO) and 390 cm-l. These experimental results allow the characterization of COO and Cos04 grown on metal during the initial stages of thermal oxidation (400-750 OC). For film thicknesses in the range 10-1000 nm, the amounts of COO and Co304 in the duplex structure layers may be estimated by FTIR reflectance spectroscopy. FTIR and XRD studies reveal a cyclic behaviour in the growth of the Co304/CoO/Co system.

Physicochemical changes in stabilized, orientated polypropylene films during the initial stages of thermal oxidation

AbstractOrientated, stabilized, isotactic polypropylene films have been oven‐aged at 130°C for times shorter than required for their embrittlement. The structural changes occurring during this induction period have been studied using tensile tensiometric, density, thermal analytical (DTA and TGA), and infrared spectrophotometric techniques. After an initially rapid annealing, films show similar time‐dependent changes in tensile, density‐derived crystalline and thermogravimetric behaviors. Comparison with vacuum‐aged sample properties shows that these changes reflect the effects of oxidative attack of polypropylene chains. Infrared examination shows that, even after the longest aging times studied and close to the induction period for embrittlement, the presence of oxidized chain groups is only just discernible. Oven aging reduces post‐fusion oxidative onset and maximum mass loss temperatures determined by TGA. Kinetic analysis demonstrates that the apparent activation energies and Arrhenius factors are reduced during aging as the post‐fusion oxidation mechanism becomes more complex. The results suggest that, during the induction period, changes in both physical and chemical structural features of orientated polypropylene occur which have possible importance in determining the lifetime of the polymer in use.

Initial stage of thermal oxidation of the Si(111)-(7 x 7) surface.

We report a systematic study of the early oxidation process in a wide temperature range (20--700?deC) for the Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) annealed surface. Oxygen uptake data obtained by Auger-electron spectroscopy indicate that the surface oxygen uptake increases and then gradually saturates with ${\mathrm{O}}_{2}$ exposure at 20?deC. At intermediate temperatures, there is no saturation and the oxygen uptake rate is enhanced. At 700?deC, the uptake rate is enhanced in the high-exposure (or high-${\mathrm{O}}_{2}$-pressure) region, but is significantly reduced in the low-exposure region, presumably because of oxygen desorption as volatile SiO.Low-energy electron diffraction pattern transition from 7\ifmmode\times\else\texttimes\fi{}7 to no pattern through a 1\ifmmode\times\else\texttimes\fi{}1 intermediate pattern seems to be governed only by the amount of surface oxygen at any temperature. Using photoelectron spectroscopy we found that ${\mathrm{O}}_{2}$ exposures corresponding to this 1\ifmmode\times\else\texttimes\fi{}1 pattern result in the disappearance of surface states centered about 0.6 eV below the valence-band maximum. A chemical-shift study of the Si 2p core level shows that not only the oxygen uptake rate, but also the way oxygen bonds to Si is strongly dependent on temperature. Our results show that at room temperature (20?deC) all the Si atoms are oxidized at more or less the same rate, so that the adsorbed oxygen atoms are spread out uniformly over the entire Si surface. At high temperatures, certain Si atoms are being oxidized faster (i.e., have more oxygen atoms bonded to them) than others, so that the distribution of adsorbed oxygen atoms is highly nonuniform; we find microscopic regions of ${\mathrm{SiO}}_{2}$-like material interdispersed with regions of Si-like material. Therefore, in the early stage of oxidation, heating the substrate causes nonuniform oxidation with microscopic phase separation of ${\mathrm{SiO}}_{2}$ and Si in the oxide film. Tight-binding calculations reported in the literature predict the oxygen 2p orbital to produce these valence states: O \ensuremath{\pi} (nonbonding), O \ensuremath{\sigma} (\ensuremath{\sigma} bonding), and O i (intermediate nature between nonbonding and \ensuremath{\sigma} bonding). By studying the O i--O \ensuremath{\pi} peak height ratio as a function of oxidation temperature and referring to theoretical calculations, we again come to the conclusion of nonuniform oxidation at high temperatures.

Inhibition of thermal oxidative breakdown of poly-4-methylpent-1-ene

A kinetic study was made of the initial stage of thermal oxidation of poly-4-methylpent-l-ene (PMP) at different temperatures and initial oxygen pressures. An empirical relation was established between the induction period, temperature and oxygen pressure. The activation energy and other kinetic parameters of the initial stage of oxidation of PMP were determined. A study was made of the relation between the induction period of thermal oxidation of PMP and temperature and the concentration of antioxidants: phenyl-β-naphthylamine (AO-I), 2,2′-methylene-bis-(4-methyl-6-tert. butylphenol) (AO-II), KBr, Kl and S. It was shown that in PMP AO-I behaves as a “weak” antioxidant and the effect of AO-II is lower in this polymer than in PE and PP, i.e. the effect of the antixidant also depends on polymer structure. It was established that KBr, Kl and S antioxidants function at temperatures higher than 250°.