Thermal Oxidation In Air Research Articles

Volta Potential of Oxidized Aluminum Studied by Scanning Kelvin Probe Force Microscopy

The Volta potential difference (VPD) of oxide layers formed on an aluminum surface as a result of anodization in a neutral solution of boric acid, thermal oxidation in air, and immersion in boiling water was studied by scanning Kelvin probe force microscopy. The measured VPD value was correlated with the layer thickness. It has been suggested that the measured VPD value is contributed by induced potential from the charges trapped by defects. Provided that the defects created during oxidation and thereby the embedded charges are mainly concentrated at the film borders, the induced potential depends linearly on film thickness. The observed difference in the linear dependence of VPD on the thickness of anodic films has been explained in terms of their charge distribution profiles. The effect of annealing in air on the VPD of the anodic films has been associated with charge redistribution and partial disappearance of the defects. It was shown that, on annealing, the VPD of the anodic films tends to the equili...

CoFe 2O 4 spinel protection coating thermally converted from the electroplated Co–Fe alloy for solid oxide fuel cell interconnect application

CoFe 2O 4 has been demonstrated as a potential spinel coating for protecting the Cr-containing ferritic interconnects. This spinel had an electrical conductivity of 0.85 S cm −1 at 800 °C in air and an average coefficient of thermal expansion (CTE) of 11.80 × 10 −6 K −1 from room temperature to 800 °C. A series of Co–Fe alloys were co-deposited onto the Crofer 22 APU ferritic steel via electroplating with an acidic chloride solution. After thermal oxidation in air at 800 °C, a CoFe 2O 4 spinel layer was attained from the plated Co 0.40Fe 0.60 film. Furthermore, a channeled Crofer 22 APU interconnect electrodeposited with a 40-μm Co 0.40Fe 0.60 alloy film as a protective coating was evaluated in a single-cell configuration. The presence of the dense, Cr-free CoFe 2O 4 spinel layer was effective in blocking the Cr migration/transport and thus contributed to the improvement in cell performance stability.

Effect of preparation conditions on the microstructural characteristics and optical properties of oxidized zinc films

Thin films of zinc (Zn) were deposited onto glass substrates (maintained at room temperature) by thermal evaporation under vacuum. The metallic zinc films were submitted to thermal oxidation in air at 670 K and 770 K, respectively, for 5–90 min, in order to obtain zinc oxide (ZnO) thin films. X-ray diffraction patterns revealed that the ZnO thin films were polycrystalline and had a wurtzite (hexagonal) structure. The morphology of the prepared ZnO thin films was investigated using atomic force microscopy and scanning electron microscopy techniques. Transmission spectra were recorded in the spectral domain from 300 nm to 1400 nm. The optical energy bandgap calculated from the absorption spectra (supposing allowed direct transitions) was in the range 3.05–3.30 eV. The dependence of the structural and optical characteristics on the preparation conditions of oxidized zinc films (oxidation temperature, oxidation time, etc.) was investigated.

Resistive Switching in Au/TiO2/Pt Thin Film Structures

The paper reports on the conditions of, and the mechanisms involved in pretreatment by strong electric field (“forming”) and subsequent resistive switching in Au/TiO 2 /Pt thin film structures on silicon. The thin TiO 2 films in these structures were prepared by different methods, namely, vacuum evaporation of metallic titanium followed by its thermal oxidation in air, and RF cathode sputtering of titanium dioxide from a powder target. Current-voltage and voltage-capacitance characteristics of the structures, as well as the relations connecting their conductivity with hold time under dc voltage of different polarities and temperature have been measured. The data obtained permit a conclusion that the physical mechanism underlying the forming consists in a sharp increase of the density of surface states in TiO 2 films initiated by electric breakdown of the Schottky barrier at the contact with the platinum electrode, while the resistive switching of the structures is governed by the variation of the occupation of surface states in the TiO 2 band gap and/or of defect concentration in the barrier region under the action of electric field.

Oxidative post-treatments for enhanced corrosion resistance of aluminium electrodeposited from ionic liquids

In this paper we report the results of a study on the corrosion properties of carbon steel (UNI Fe360B) coated with a thin aluminium layer electrodeposited from ionic liquids and subsequently post-treated in an oxidizing environment. Two different types of treatments were investigated: thermal oxidation in air and electrochemical anodization. We found that heat treatments provided only a modest improvement of the anticorrosion properties while the electrochemical treatment produced a significant improvement.

Single-Walled Carbon Nanotubes Prepared by Large-Scale Induction Thermal Plasma Process: Synthesis, Characterization, and Purification

Characterization and purification studies have been performed on the single-walled carbon nanotubes (SWNTs), prepared by the large-scale (i.e., at production rates of ∼100 g h−1) induction thermal plasma process. The main impurities present in the as-produced soot materials are first identified by various material analyses and turn out to be amorphous carbon, graphitic carbon particles, and metal/metal−oxide catalysts, but without significant content of fullerenes. In particular, the amounts of graphitic particles present were high in comparison with those of other plasma arc or laser-grown SWNTs, and it is believed that these graphitic particles mainly originate from the remains of the feedstock materials. On the basis of this observation, different purification protocols have been explored by combining the two-step processes of thermal oxidation in air and liquid oxidation in a variety of acids. At each step of the purification, the changes in the SWNT samples are monitored by various analytical techniques, and the results reveal that the air oxidation at 350 °C for 1 h, followed by refluxing in 7 M HNO3 for 6 h, provides for the best efficiency in conducting the purification of the thermal plasma-grown SWNT. The material properties of the purified samples, such as the electrical resistivity and the specific surface area (SSA), have also been analyzed by using the Brunauer−Emmett−Teller (BET) and the four-probe methods, the measured values being ∼500 m2 g−1 and ∼2 × 10−3 Ω cm, respectively.

Resistive switching in Au/TiO2/Pt thin film structures on silicon

The conditions and mechanisms of preliminary treatment in strong electric fields (forming) and subsequent resistive switching in Au/TiO2/Pt thin-film structures on silicon were investigated. The thin TiO2 films in these structures were prepared by different methods, namely, vacuum evaporation of metallic titanium followed by thermal oxidation in air and radio-frequency cathode sputtering of titanium dioxide from a powder target. The current-voltage and voltage-capacitance characteristics of the structures, as well as the dependences of their conductivity on the time of exposure to a dc voltage of different polarities and on the temperature were measured. The data obtained permitted the conclusion that the physical mechanism underlying the forming process consists in a sharp increase in the density of surface states in TiO2 films due to the electric breakdown of the Schottky barrier at the contact with the platinum electrode, whereas the resistive switching of the structures is governed by the variation in the population of surface states in the TiO2 band gap and/or in the defect concentration in the barrier region of the structures acted upon by voltage pulses of different polarities.

Purification of single-walled carbon nanotubes using a fixed bed reactor packed with zirconia beads

Single-walled carbon nanotube (SWCNT) soot produced by arc discharge was purified through gas and liquid phase oxidations. In the gas-phase oxidation, zirconia beads with different diameters of 1, 5, and 10 mm were packed together with raw SWCNT soot inside a vertical quartz tube to enhance air flow uniformity and an exposed surface area of the raw soot during thermal oxidation in air. A decrease of the bead sizes led to such a stronger oxidation of carbonaceous impurities that ∼10 wt.% higher weight loss was then achieved with the 1 mm beads than without them. A subsequent HNO 3 treatment and the second thermal oxidation were engaged to improve further the purity of SWCNTs. Thermogravimetric (TG) analysis, scanning electron microscopy, high resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the samples. The derivative TG (DTG) curves were deconvoluted to quantitatively determine the SWCNT purity of the samples. Our final purified samples showed a yield of ∼26%, a metal impurity of ∼7% and a SWCNT purity of ∼83% as calculated from the deconvoluted DTG curves.

Bond‐curvature effect on burning of single‐wall carbon nanotubes

AbstractChirality dependent burning of single‐wall carbon nanotubes (SWCNTs) was monitored using optical absorption and photoluminescence spectroscopy for the HiPco and the CoMoCAT SWCNTs. Results have indicated that nanotubes with a higher chiral angle and smaller diameter decomposed more rapidly through thermal oxidation in air. A “local curvature radius” along the C–C bond is found to be a major rate‐determining factor in the combustion process of SWCNTs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Change of properties after oxidation of IG-11 graphite by air and CO 2 gas

Artificial graphite is typically manufactured by carbonization of a shaped body of a kneaded mixture using granular cokes as a filler and pitch as a binder. It undergoes a pitch impregnation process if necessary and finally applying graphitization heat treatment. The effect of thermal oxidation in air or a CO 2 atmosphere on IG-11 graphite samples is investigated in this study. The results show a localized oxidation process that progressively reveals the large coke particles with increasing level of overall weight loss in air. The surface of the graphite was peeled off and no change was found in the specific gravity after air oxidation. However, the specific gravity of graphite was continuously decreased by CO 2 oxidation. The decrease in the specific gravity by CO 2 oxidation was due to CO 2 gas that progressed from the surface to the interior. The pore shape after CO 2 oxidation differed from that under air oxidation.

A multi-step process of oxygen diffusion to improve the wear performance of a gamma-based titanium aluminide

A gamma-based titanium aluminide with the composition of Ti–45Al–2Nb–2Mn–1B (at%) was surface treated through a process in which thermal oxidation in air and diffusion were carried out in several steps to investigate the effect of oxide forming elements and diffusion of oxygen on the wear performance of surface layers. The microstructure and phases on the surface were characterized using optical and electron microscopy, X-ray diffraction (XRD) and glow discharge spectroscopy (GDS). The constituents of the top surface layers were several layers of oxide compounds of TiO 2 and Al 2O 3, beneath which a small amount of nitride compounds of TiN and Ti 2AlN were appeared. After diffusion of oxygen a layer of Ti 3Al with diffused oxygen appeared as a hard layer under the oxide compound layer. The ball on disk wear tests showed the dominant adhesive wear on the untreated surfaces. The treated surfaces were in most cases hard enough to be worn by the steel ball counterface and in fact the material of steel was transferred to the surface. The wear of steel counterface was also due to the ploughing action of the projected angular crystals of TiO 2 and Al 2O 3 on the surface. The Ti 3Al layer should also have played a major role in resistance to wear after the removal of the compound layer as well as supporting the top layer during the wear. The wear resistance and load bearing capacity of the treated samples improved up to two orders of magnitudes higher than those of untreated samples.

Electrolytic loading of hydrogen in Zr65(Pd80Rh20)35 and Zr65Pd35 alloys prepared by mechanical grinding and in their oxidised derivatives

Abstract Zr65(Pd80Rh20)35 and Zr65Pd35 alloys were prepared by mechanical grinding of stoichiometric amounts of either ZrH2 and Pd80Rh20 or ZrH2 and Pd. Following a suggestion in the literature, these alloys were converted by thermal oxidation in air into the corresponding ZrO2(Pd80Rh20) and ZrO2Pd compounds, which are probably composed of Pd80Rh20 and Pd nanostructures embedded in ZrO2. The aim of our work thereafter was to investigate hydrogen storage in these nanostructures. Both the alloys and their oxidation derivatives were thus loaded with hydrogen by cathodic reduction at 25 °C in 6 M KOH, and loaded hydrogen was then determined by anodic extraction. On comparing the hydriding extent of the alloys with that of the corresponding oxidation derivatives, the metal clusters which formed after thermal oxidation are probably much larger in PdRh than in Pd. The former exhibits the high decomposition pressure typical of massive Pd80Rh20 hydride, and the maximum [H]/[Pd0.8Rh0.2] atom ratio is ≈0.82. Conversely, the amount of hydrogen extracted from reduced ZrO2Pd samples prepared in optimal conditions fits [H]/[Pd] atom ratios between 1 and 2. The shape of the electrochemical hydrogen desorption isotherms indicates that the hydrogen in excess of the β-Pd hydride phase is probably stored in a new, very stable form.

The effect of activation on the electrochemical behaviour of graphite felt towards the Fe 3+/Fe 2+ redox electrode reaction

This paper is dedicated to the study of the effect of graphite felt activation by thermal oxidation in air on its electrocatalytic activity towards Fe 3+/Fe 2+ redox electrode reaction. For the first time, the exchange current densities and electron transfer coefficients determined from the Tafel equation were obtained within the wide range of burn-off levels (0–50%). The maximal catalytic activity was obtained at the burn-off of 17%. The cathode having this burn-off level expressed almost three-fold enhancement in the galvanic cell performance (criterion for the performance evaluation in our case was a cell voltage at the current density of 300 mA cm −2) as compared to that with the non-activated graphite felt, and allowed to obtain current densities up to 670 mA cm −2 at the cathode polarization as low as 150 mV. The correlation between electrocatalytic activity and a surface oxide chemistry of graphite felt was established. The cell performance was found to be the best when the pH at a point of zero charge and the amount of surface quinoid groups per unit area were minimal. The results obtained are of significant importance for practical applications, including the development of electrodes in redox flow batteries.

Oxidation temperature dependent optical properties of bismuth oxide thin films: Effect of vapour chopping and air exposure

The optical properties of vapour chopped and nonchopped bismuth oxide thin films of two thicknesses 1500 Å and 2000 Å have been studied. The films were prepared by thermal oxidation in air; of vacuum evaporated vapour chopped and nonchopped bismuth thin films. As revealed by XRD studies, multiphase and polycrystalline bismuth oxide thin films were obtained. The refractive index was found to increase with the thickness and exposure to air for 40 days. The vapour chopped films showed higher refractive index, band gap and lower grain size than those of nonchopped films. The films showed high transmittance in the visible spectrum. The ageing effect on the vapour chopped films was found low.

Enrichment of Small-Diameter Double-Wall Carbon Nanotubes Synthesized by Catalyst-Supported Chemical Vapor Deposition Using Zeolite Supports

A highly efficient enrichment of thin double-wall carbon nanotubes (DWNTs) having an average outer diameter of ca. 1.4 nm by thermal oxidative purification has been reported. Thin DWNTs of ca. 1% with respect to single-wall carbon nanotubes (SWNTs), which are synthesized by catalyst-supported chemical vapor deposition (CCVD) on zeolite supports, are enriched up to ca. 90% (vs SWNTs) by thermal oxidation in air. The present thin DWNTs grown on zeolite are several hundreds of times as resistant as SWNTs toward the oxidation at 700 °C. When oxidative purification is applied for DWNTs with larger diameters, the amount of DWNTs lost during the purification process is large compared to thinner DWNTs lost. The present oxidative purification is much more effective for thin DWNTs with diameters smaller than 2 nm.

CuO nanowires synthesized by thermal oxidation route

CuO nanowires were prepared on the copper foil by thermal oxidation in air. The effect of annealing temperature and growth time on the morphology of the nanowires is investigated. It is found that the annealing temperature and the growth time play an important role in the morphology of CuO nanowires such as the density, the length and the diameter. The length and the density of nanowires increase with prolonging growth time; but if the time is too long, CuO crystallite grains form instead of nanowires. Annealing copper foils at lower or higher temperature, the density of nanowires is lower. For comparison, the Cu films on Si substrate deposited by direct current (dc) magnetron sputtering are oxidized, but we do not find large-scale of nanowires. The possible mechanism is also discussed for the growth of CuO nanowires.

Surface elastic properties of Ti alloys modified for medical implants: A force spectroscopy study

We report here the first nanoscale surface elasticity measurements on surface-modified titanium alloys using the force spectroscopy mode in scanning force microscopy. Samples of three vanadium-free titanium alloys, Ti–7Nb–6Al, Ti–13Nb–13Zr and Ti–15Zr–4Nb, were investigated. Surface modification of the three alloys was produced by thermal oxidation in air at 750 °C for different times, which resulted in the formation of protective oxide layers with different surface composition and morphology. The elastic properties of the surface layers were studied comparatively in the as-received Ti alloys and after the oxidation process using cantilevers with different stiffness to evaluate the influence of the indentation depth. In all cases, Young’s modulus of the sample surfaces was found to be lower than 65 GPa, and as low as 20 GPa for some of the oxidized samples. Variations observed for the three oxidized Ti alloys can be related to the different chemical composition of the outer layers generated for the different oxidation times.

Hydrogen storage capacity at high pressure of raw and purified single wall carbon nanotubes produced with a solar reactor

Samples of single wall carbon nanotubes (SWNTs) were prepared using a solar reactor. Graphite targets containing different catalysts (Ni/Co, Ni/Y, Ni/Ce) allowed the synthesis of SWNTs soot in which nanotubes had different diameter distributions. Several consecutive stages of HCl treatment and thermal oxidation in air (HCl protocol) purified the samples. Another protocol involving HNO 3 treatment and H 2O 2 oxidation (HNO 3 protocol) was also used. Isotherms of hydrogen adsorption were volumetrically measured at 253 K under pressures below 6 MPa on raw and treated samples. The highest adsorption capacity (0.7 wt%) was measured on raw soot. HCl protocol clearly increases the BET surface area ( S BET ) and the microporous volume ( W 0 ( N 2 ) ) measured by N 2 at 77 K of the treated samples with respect to the as-produced materials, whereas HNO 3 protocol decreases them. A correlation between textural properties and hydrogen storage capacities is discussed.

Synthesis and purification of single-walled carbon nanotubes by methane decomposition over iron-supported catalysts

This article describes the synthesis and the purification of carbon nanotubes. Large quantities of single-walled carbon nanotubes (SWNTs) with few defects and very small amount of amorphous carbon coating have been synthesized by catalytic decomposition of methane over well-dispersed iron particles supported on MgO at the temperature range from 800to1000°C. It was found that the quality of carbon nanotubes was slightly increased as increasing growth temperature while the yield was decreased to some degree. It was also found that the as-grown SWNTs can be separated easily from the support by a simple acidic treatment rather than thermal oxidation to obtain a product with high yields of SWNTs. Above 94wt% high purity SWNTs were obtained by combining two-step processes of thermal oxidation in air and acid treatment without multistep filtration. The as-grown and purified SWNTs were also characterized by field emission scanning electron microscopy, x-ray diffraction, and Raman spectroscopy.

Distinct oxidative stabilities of char versus soot black carbon: Implications for quantification and environmental recalcitrance

Sequestration in sediments of black carbon (BC) from vegetation fires and fuel combustion may constitute a significant sink of otherwise rapidly cycling carbon from the atmosphere‐biosphere cycle. It also has the potential to provide a historical record of atmospheric BC loadings. Previous treatments of BC as one homogeneous entity are being replaced with the growing awareness of a BC combustion continuum, a range spanning from slightly charred biomass to soot and graphite. Here the relative recalcitrance of different BC forms is evaluated, and implications for both BC quantification and environmental stability are considered. The stabilities of four BC reference materials against thermal oxidation in air were quite distinct with T50%BC values (i.e., the temperature where 50% BC remained in the residue) of 444°C (diesel soot–BC), 388°C (n‐hexane soot–BC), 338°C (wood char–BC), and 266°C (grass char–BC). The implications for BC quantification have been illustrated for a thermal oxidation (the CTO‐375) method commonly applied to study BC in sediments. This technique measured BC:TOC ratios of 78.3 ± 1.3% for the diesel soot–BC and 45.3 ± 6.1% for n‐hexane soot–BC, whereas no CTO375‐BC was detected for the two analyzed char‐BC materials. The greater lability of char‐BC compared to soot‐BC likely reflects higher accessibility to internal microporosity in char‐BC, facilitating internal O2 transfer. Decreasing the temperature cutoff below 375°C to also include char‐BC is not possible as thermograms of nonpyrogenic reference materials indicated that such material would then be artifactually quantified as BC. The presence of mineral oxides in the sediment matrix may lead to a catalytically mediated lowering of the activation energy for soot‐BC oxidation but not for char‐BC or nonpyrogenic organic material. Several recent studies combine to challenge the proposition of complete recalcitrance of BC. Particularly, the thermal lability of char‐BC from grassland fires deserves further attention in order to improve the understanding of BC in the global carbon cycle.