Oxygen Contamination Research Articles

In situ atomic-scale observation of oxidation and decomposition processes in nanocrystalline alloys

Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu–Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu–Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu–Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions.

A Markovian model for power transformer maintenance

The condition of the insulation paper is one of the key determinants of the lifetime of a power transformer. The winding insulation paper may deteriorate aggressively and result in the unexpected failure of power transformers, especially under the presence of high moisture, oxygen, and metal contaminants. Such types of scenarios can be prevented if the deterioration is detected on time. Various types of condition monitoring techniques have been developed to detect transformer condition such as dissolved gas analysis (DGA) and frequency response analysis (FRA). They are non-intrusive and provide early warning of accelerated deterioration both chemically and mechanically. However, the accuracy of those techniques is imperfect, which means periodic inspection is still indispensable. In this paper, we discuss the value of continuous condition monitoring for power transformers and present a way to estimate this value. Towards this, a continuous-time Markov decision model is presented to optimize periodic inspections, so that the cost is minimized and the availability is maximized. We then analyze the performance based on the information from both discrete inspection and continuous condition monitoring using DGA and FRA. The result shows the dissolved gas analysis can improve the availability and operation cost, while frequency response analysis can only improve the availability of power transformers.

One-step synthesis of graphene hollow nanoballs with various nitrogen-doped states for electrocatalysis in dye-sensitized solar cells

Nitrogen-doped graphene hollow nanoballs (N-GHBs) were synthesized in chemical vapor deposition (CVD) reaction using melamine as a chemical precursor via an in situ nitrogen-doping approach. In the CVD reaction, N-GHBs were deposited directly on carbon cloth (CC) to be used as an efficient metal-free electrocatalyst for dye-sensitized solar cell (DSSC) applications. The highly curved N-GHBs could avoid the self-assembly restacking of planar graphene sheets, which usually occurred during the film preparation. Keeping oxygen contaminations from N-GHBs, the characteristic electrical conductivity of graphene was preserved in the as-synthesized N-GHBs. By controlling the evaporation temperature of melamine, the nitrogen-doping content of 8.7–14.0% and different nitrogen-doped configurations in N-GHBs could be adjusted. The catalytic activities of different nitrogen-doped states in N-GHBs toward the triiodide (I3−) reduction in DSSCs were investigated, revealing that the pyridinic and quaternary nitrogens, rather than the total nitrogen doping level, in N-GHBs are mainly responsible for their catalytic activities in DSSCs. For solar cell applications, the high surface area and heteroatomic nitrogens of GHBs can remarkably improve the catalytic activity toward the triiodide reduction, lower the charge-transfer resistance, and enhance the corresponding photovoltaic performance (7.53%), which is comparable to that (7.70%) of a standard sputtered Pt counter electrode-based cell. These exceptional properties allow N-GHBs/CC to act as a promising electrocatalytic electrode for DSSC and other electrochemical energy applications.

Electronic excitations of manganese phthalocyanine molecules.

We have investigated the electronic excitation spectrum of individual manganese phthalocyanine (MnPc) molecules in a coronene matrix. Our results corroborate that the observed excitation spectrum is rather sensitive to oxygen contamination of the respective samples. Annealing in vacuum allowed the determination of the electronic excitations of individual MnPc molecules. These results confirm previous data from the literature and, in addition, we have identified a low energy excitation at about 0.8 eV, which had not been reported before.

Evolving Microbial Communities in Cellulose-Fed Microbial Fuel Cell

The abundance of cellulosic wastes make them attractive source of energy for producing electricity in microbial fuel cells (MFCs). However, electricity production from cellulose requires obligate anaerobes that can degrade cellulose and transfer electrons to the electrode (exoelectrogens), and thus most previous MFC studies have been conducted using two-chamber systems to avoid oxygen contamination of the anode. Single-chamber, air-cathode MFCs typically produce higher power densities than aqueous catholyte MFCs and avoid energy input for the cathodic reaction. To better understand the bacterial communities that evolve in single-chamber air-cathode MFCs fed cellulose, we examined the changes in the bacterial consortium in an MFC fed cellulose over time. The most predominant bacteria shown to be capable electron generation was Firmicutes, with the fermenters decomposing cellulose Bacteroidetes. The main genera developed after extended operation of the cellulose-fed MFC were cellulolytic strains, fermenters and electrogens that included: Parabacteroides, Proteiniphilum, Catonella and Clostridium. These results demonstrate that different communities evolve in air-cathode MFCs fed cellulose than the previous two-chamber reactors.

Magnetism of purified amorphous carbon

In this work, magnetic properties of activated carbon coal, which is also named amorphous carbon, were investigated. Composition and structure analysis was performed by XRD, SEM and EDS techniques. Position and width of peaks on XRD pattern confirmed amorphous structure of coals with small degree of graphitization. Magnetic measurements were performed by SQUID magnetometer. According to thermomagnetic measurements, in part of the samples, antiferromagnetic transition was observed near 120 K. Nature of such phenomena, as well as effect of oxygen and magnetic contaminations on thermomagnetic measurements are discussed referring to similar results from previous researches and data from literature.

Highly efficient polyacetylene–based polyelectrolytes as cathode interfacial layers for organic solar cell applications

Abstract For application to inverted organic solar cells (IOSCs), three polyacetylene-based polyelectrolytes, poly(N-alkyl/aryl-2-ethynylpyridinium bromide) (P1-P3), were incorporated as cathode interfacial layers (CILs) between the ZnO and one of the following two photoactive layers: poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) or poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):PC71BM. This interfacial modification of the IOSCs by the CIL insertion enhanced their performance with an increase in power conversion efficiency (PCE) from 7.46% for a ZnO-based control device to 8.29% for PTB7 and to 9.37% for PTB7-Th. Characterizations of the IOSC devices by using electrical impedance spectroscopy (EIS), space-charge-limited current (SCLC) electron mobility, dark current density-voltage (J-V) curves, and film surface morphology study revealed that the CIL incorporation suppressed the charge recombination and thus facilitated charge extraction, as evidenced by the observed improvement of open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF). The devices with CILs showed higher electron mobility, in line with the higher FF and reduced series resistance. These CILs with ZnO exhibited significantly enhanced PCE and also served as an internal shield against oxygen contamination and humidity, which further increased the device stability.

The growth of the metallic ZrN x thin films on P-GaN substrate by pulsed laser deposition

Abstract Although metal nitride thin films have attractive prospects in plasmonic applications due to its stable properties in harsh environments containing high temperatures, shock, and contaminants, the effect of deposition parameters on the properties of the metallic ZrN grown on III-N semiconductors by pulse laser deposition still lacks of detailed exploration. Here we have successfully prepared metallic ZrN x films on p-GaN substrate by pulsed laser deposition in N 2 ambient of various pressures at a fixed substrate temperature (475 °C). It is found that the films exhibit quite smooth surfaces and (111) preferred orientation. The X-ray photoelectron spectroscopy measurements indicate that carbon contamination can be completely removed and oxygen contamination is significantly reduced on the film surfaces after cleaning using Ar + sputtering. The N/Zr ratio increases from 0.64 to 0.75 when the N 2 pressure increases from 0.5 Pa to 3 Pa. The optical reflectivity spectra measured by the UV–vis–NIR spectrophotometer show that the ZrN x is a typical and good metallic-like material and its metallic properties can be tuned with changing the film compositions.

Influence of increasing Al concentration on phase, microstructure and mechanical behaviors of Ni1.5CoFeCu1−xAlxV0.5 high entropy alloys

The present work systematically investigated the influence of increasing Al concentration on the phase composition, microstructure evolution and mechanical behaviors of Ni1.5CoFeCu1−xAlxV0.5 (x = 0.1, 0.2, 0.3mol) high entropy alloys (HEAs) synthesized by mechanical alloying (MA) followed by spark plasma sintering (SPS). In the MA powders, FCC phase reduced while BCC phase increased with increase in Al content. In the alloys consolidated by SPS, BCC phase disappeared with the formation of vanadium carbides as well as aluminum oxide due to carbon and oxygen contaminations. Microstructure characterization suggests that three bulk HEAs all displayed the bimodal grain microstructure, consisting of coarse-grained (CG) regions as well as fine-grained (FG) regions with uniform dispersion of aluminum oxide nanoparticles. Furthermore, with the increase of Al concentration in the alloy, the volume fraction of FG region increased gradually and the average grain sizes of CG and FG regions reduced dramatically. Since the microstructure refinement resulted in the enhancement of grain boundary strengthening, the yield strength of alloy was improved significantly with increasing Al. In addition, the increase of Al concentration remarkably changed the strain-hardening behavior of bulk HEA by influencing the strain-hardening stages during compression.

Surface band bending and interface alignment of plasma-enhanced atomic layer deposited SiO2 on AlxGa1-xN

AlxGa1-xN is characterized by a significant spontaneous and piezoelectric polarization, which increases with the aluminum content. As a result, a surface bound charge is present, which favors compensation by surface states and influences the reliability of AlGaN/GaN devices. This work, therefore, focused on the effects of the polarization charge for GaN and AlGaN with three different aluminum concentrations 15%, 25%, and 35%. The band bending of AlxGa1-xN surfaces was measured after a N2/H2 plasma pretreatment, which reduced the carbon and oxygen contamination below the detection limit of x-ray photoelectron spectroscopy. Surface band bending was then related to surface states, where the band bending of oxygen-free surfaces—as obtained with a high-temperature, immersed hydrogen/nitrogen plasma clean—scales with the aluminum content. In addition, the band offsets at the plasma-enhanced atomic layer deposited SiO2/AlxGa1-xN interface were measured, giving 3.4 eV, 3.3 eV, 3.3 eV, and 3.0 eV for respective 0%, 15%, 25%, and 35% aluminum concentrations. These values are in accordance with the charge neutrality level model, which implies that SiO2 will confine carriers over nearly the full range of the aluminum content.

EXAFS of Nickel Oxide in the Ionic Liquid Emic/AlCl3

Introduction Earlier research has shown that nickel metal and nickel alloys can be deposited from nickel chloride dissolved in the IL AlCl3/EMIC but this depended on Lewis acidity of the IL which in turn affects the structure of the Ni ions that form in the solution (1, 2). The structures of several transition metal chlorides have been studied in situ in ILs using EXAFS (3-6). However, the oxides of these metals have not received as much attention. When working with the water and air sensitive ILs, there is always a concern about oxygen and moisture contamination and its effect on the properties of the IL. Many of the transition metal chlorides are studied in the anhydrous state and thus they are also sensitive to oxygen and moisture contamination in the IL. In this study, we examine the structure of the nickel ions that form in the AlCl3/EMIC IL when nickel(II) oxide is added to simulate the effect of oxygen contamination. The structure of NiO was studied with CVs and EXAFS in both Lewis acidic and Lewis basic AlCl3/EMIC ILs. Experimental Solutions were prepared in a Vacuum Atmosphere nitrogen filled drybox. Voltammetric experiments were carried out at ambient temperature. The ionic liquid solutions were sealed in 2 mil thick polyethylene bags and were examined with EXAFS. The electrochemical experiments were conducted with an EG&G PARC model 283 potentiostat controlled with the EG&G PARC PowerSuite software package. The EXAFS experiments were conducted on beamline X-11A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). All of the EXAFS data were measured at room temperature with the storage ring operating at 2.8 MeV and beam currents in the range of 150 to 260 mA. The EXAFS data were analyzed using the standard XDAP data analysis package. Results and Discussion Data were collected for NiO and anhydrous NiCl2 powder samples for reference standards to be used in the analysis of the IL samples. The Fourier transform for the NiO powder standard shows several shells. The first shell is the nickel - oxygen interaction and the second shell is the nickel - nickel interaction. The shells that are at greater distances are additional oxygen and nickel interactions, but they also include multiple scattering contributions. The Fourier transform of the NiCl2 powder standard shows two primary shells. The first shell is the nickel - chlorine interaction and the second shell is the nickel – nickel interaction. The Fourier transform for NiO dissolved in the N=0.43 basic IL shows a single peak, indicating a single shell around the central nickel atom. The Fourier transform was phase-corrected with both the chloride shell from the NiCl2 standard, and oxide shell from the NiO standard, in order to determine which element was contributing to the backscattering. When the FT is phase-corrected with the correct backscattering atom, the peak in the imaginary part corresponds to the maximum of the positive peak in the magnitude of the FT. The phase corrected FT shows that the backscattering atoms must be primarily from a Ni–Cl interaction where the two peaks are mostly in phase. Acknowledgements The authors would like to acknowledge the financial support of the Naval Research Laboratory, the American Society for Engineering Education and the U.S Department of Energy. This research was carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.

Low-temperature (70 °C) ambient air plasma-fabrication of inkjet-printed mesoporous TiO2 flexible photoanodes

Titania/silica electron-generating and -transporting nanocomposite 300 nm layers of high porosity were deposited onto ITO/PET flexible foils using inkjet printing. Prior to printing, the ITO surface had been modified by novel low-temperature ambient air roll-to-roll plasma in order to enhance its surface properties by removing carbon and oxygen contaminants, a process that led to rapid improvement of surface energy. Consequently the ITO work function, an important parameter involving charge injection efficiency in energy harvesting systems, increased by 1 eV. Afterwards, the TiO2/methyl-silica ink exhibited excellent wetting on a 2 s plasma-treated ITO surface. The coating was further processed/mineralized by an additional low-temperature ambient air plasma treatment step. The plasma processing of raw photoanodes led to the mineralization of the methyl-silica binder which resulted in the formation of a fully inorganic TiO2/SiO2 mesoporous structure and significantly increased electrophotocatalytic activity, leading to increased photocurrents. The entire two-step plasma process was performed at low-temperature (70 °C) and high speeds, enabling practical applications of such a procedure for large-area fabrication of flexible photoanodes.

Thermal stability and grain boundary strengthening in ultrafine-grained CoCrFeNi high entropy alloy composite

Thermal stability of CoCrFeNi high entropy alloy in as-milled and sintered conditions was investigated using X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. Composite microstructure consists of FCC and carbide with a fine dispersion of oxide was observed in the sintered condition. Unsolicited contamination of carbon and oxygen in the as-milled powder due to the milling medium had led to the formation of composite microstructure. An exceptional thermal stability was observed upon exposure of sintered compact to higher temperatures (0.56Tm to 0.68Tm) for the prolonged duration of 600h. Sintered compact exposed to 700°C (0.56Tm) for 600h showed negligible change in hardness and grain size. Analysis based on the modified Hall-Petch model for two phase alloy indicates the phase boundaries act as a strong obstacle while the major contribution to strengthening comes from grain boundaries.

Laser-induced surface recrystallization of polycrystalline PbI2 thick films for X-ray detector application

In this work, laser-induced surface recrystallization process was developed to improve the surface properties and device performance of the polycrystalline PbI2 thick films prepared by using close space vapor deposition method. A continuous polycrystalline PbI2 recrystallized layer with a better mechanical strength and reflectivity improved from 2% to 4%–6% was obtained by this recrystallization process for the films with mechanical pretreatment. Other polytypes is absent in the recrystallized layer with the 2H-polytype remaining before and after treatment and obtaining improved electrical and X-ray photoelectric response performance. The pretreatment such as mechanical cutting/polishing and hydrogenation is necessary to lower the non-wetting crystallization behavior during the recrystallization process due to the rough surface state and oxygen contamination.

Oxygen and nitrogen concentrations in the Ti-6Al-4V alloy manufactured by direct metal laser sintering (DMLS) process

Two machines from two scientific centers (Russia and South Africa) were used for the manufacturing of the Ti6Al4V alloys by the direct metal laser sintering. The chemical composition of powders complies with the ASTM F-136 (grade 5), ASTM B348 (grade 23) standard for medical applications. Analysis of the oxygen and nitrogen contamination in DMLS alloys was done with Van de Graaff accelerator with two Mega Volts. It is found that structures of the samples manufactured with two different machines used the same regimes are close to each other. TEM studies found the metastable martensitic structure and silicon nitride Si3N4. It was found that the oxygen and nitrogen contents in both samples are within the normal range for medical grade titanium alloys.

A sequential pre-cracking procedure to measure the mode-I fracture toughness of ultra pure bulk metallic glasses

A dedicated fatigue pre-cracking method, performed successively under mode II and mode I, has been set up to initiate and propagate a crack close to the notch plane, which is particularly difficult to achieve using pure mode I loading on some ultra pure bulk metallic glasses, free from oxygen contamination. This method, was applied to a pure Zr55Cu30Al10Ni5 (at.%) amorphous alloy, and allowed successful pre-cracking and reliable measurement of the apparent mode I fracture toughness.

Fluidized Bed Chemical Vapor Deposition of Zirconium Nitride Films

A fluidized bed–chemical vapor deposition (FB-CVD) process was designed and established in a two-part experiment to produce zirconium nitride barrier coatings on uranium-molybdenum particles for a reduced enrichment dispersion fuel concept. A hot-wall, inverted fluidized bed reaction vessel was developed for this process, and coatings were produced from thermal decomposition of the metallo-organic precursor tetrakis(dimethylamino)zirconium (TDMAZ) in high-purity argon gas. Experiments were executed at atmospheric pressure and low substrate temperatures (i.e., 500 to 550 K). Deposited coatings were characterized using scanning electron microscopy, energy dispersive spectroscopy, and wavelength dispersive spectroscopy. Successful depositions were produced on 1 mm diameter tungsten wires and fluidized ZrO2-SiO2 microspheres (185 to 250 µm diameter) with coating thicknesses ranging from 0.5 to 30 μm. The coating deposition rate was nominally estimated to be 0.04 ± 0.02 µm/h. The ZrN coating adhered to the microspheres, but there was a significant oxygen and possible carbon contamination.

Characterization of the interfacial-microstructure evolution and void shrinkage of Ti-22Al-25Nb orthorhombic alloy with different surface roughness during diffusion bonding

Ti-22Al-25Nb alloy specimens with different surface roughness were joined, through diffusion bonding at 975 °C at 12.5 MPa. The interfacial-microstructure evolution during this process was characterized via scanning electron microscopy combined with electron probe microanalysis and electron backscatter diffraction analysis. Further, the interfacial void-shrinkage mechanism and the quality of the bonded joints were determined through atomic force microscopy, which revealed the three-dimensional morphologies of the surfaces, and shear strength testing of the joints. The results revealed that fine equiaxed α2 grains are precipitated in the bonding interface of specimens with ground surfaces. These interfacial α2 grains were formed via phase transformation and recrystallization processes, which were triggered by asperity deformation at the contact plane and unavoidable oxygen contamination. Two types of fracture occurred during the shear strength tests, where the bonds generated from (i) polished surfaces failed predominantly along the bond line, and (ii) ground surfaces failed predominantly in the base material away from the bond line. This indicated that the mechanism controlling the void-shrinkage process associated with the contact between two rough surfaces during diffusion bonding varied with the surface roughness: the void-shrinkage process of specimens with (i) polished surfaces is controlled by diffusion, and (ii) ground surfaces was controlled by both diffusion and plastic deformation.

Process-structure-property relationships of the comminution processing of Al scrap

The comminution of Al scrap was investigated under different processing conditions to assess its ability to tailor powders for the powder metallurgy (PM) industry. Specifically, the influence of the circumferential velocity and ventilation power over the properties of the pulverized particles was highlighted. These parameters controlled the residence time and deformation experienced by the feedstock chips, which, in turn, governed the size, morphology and composition of the pulverized particles. The pulverized chips consisted of heavily passivated oxide-dispersion strengthened (ODS) particles with varying contents of Fe, Si and Mn, depending on the extent of oxygen accumulation and contaminants embedding taking place during comminution. Upon screening, the −140 mesh pulverized powders were also characterized. The passivation layer thickness and Al2O3 dispersoids content of these powders ranged, respectively, from 6.81±0.31nm to 7.58±0.38nm and from 2.13±0.11wt% to 5.20±0.11wt% as circumferential velocity decreased, while they ranged from 4.96±0.41nm to 8.67±0.36nm and from 2.02±0.09wt% to 7.12±0.09wt% as ventilation power decreased. These tendencies highlighted that the oxygen enrichment process occurring during comminution changed from a passivation layer thickening regime to an Al2O3 dispersoids accumulation regime, for a residence time of ~210s. Furthermore, all pulverized powders contained nanograins whose fractions depended on the comminution parameters used.

Synthesis of highly conductive cobalt thin films by LCVD at atmospheric pressure

We have deposited very low resistant Co films on SiO2-coated Si substrates using UV pulsed laser pyrolytic decomposition of Co2(CO)8 with 355nm laser radiation at atmospheric pressure. Facile decomposition of the precursors and the use of Ar curtain enable the deposition of relatively pure Co (with O less than 7% and negligible C) at the power of 1.11–3.33W, and of pure Co at 6.67W. The resistivity decreases from 58 to 19µΩ-cm as the power increases from 2.22 to 3.33W, showing inverse-linear dependence on grain size. In addition, further increase of the power to 6.67W decreases the resistivity to 9µΩ-cm, due to both the growth of large grains with negligible contaminants, and the adverse effect of surface roughness. The effects of oxygen contaminants on the resistivity can be minimal, because of its presence in the form of oxide. These low resistant fine metal lines deposited by a direct-writing laser chemical vapor deposition technique at atmospheric pressure have been reported for the first time.