Metal Oxide Coatings Research Articles

Anodic process for forming nanostructured metal-oxide coatings for large-value precise microfilm resistor fabrication

Nanoscale metal-oxide coatings have been produced from double layer Ta–Al film combinations by anodizing underlying tantalum through pores in an initially formed anodic alumina. The coatings developed have been studied by SEM and AES and examined as alternative materials for microfilm resistor fabrication. Periodical net-like structure and nanoscale topography of the coatings were found to result in high sheet resistance (to 75 kΩ/□) as well as low temperature resistance coefficient (in the order of 10 −5 K −1). The phenomena are explained by existence in the films of specific quantum mechanisms caused by metal-insulator composition and ultra-fine periodical structure of the coatings developed here. Additional electrochemical treatment allows precise resistor adjustment of 30% to within 1% of desired value without changes in resistor design geometry.

Chemical characteristics of sediments and interstitial waters in some small Hawkesbury Sandstone freshwater streams of the Sydney area

Sediments of three streams draining sandstone catchments in Sydney, Australia, were sampled under low flow conditions over a 2‐year period. Two streams receive urban contaminants, one (the reference creek) has an undeveloped catchment. Levels of sediment copper, zinc, cadmium and lead (total and speciated) and phosphorus (total and organic) were determined, as well as total organic carbon, particle size fractionation and cation exchange capacity. A range of quality parameters were also measured for interstitial waters. Sediments were characterized by very low organic matter (< 2% by weight) and low cation exchange capacity (< 3 mmol Na 100 g−1), due to high contents of coarse and fine sands (> 95%). Despite a low accumulation of heavy metals and phosphorus in the long term, the sediments were nevertheless a sink for these constituents. Greatest proportions of sediment zinc and lead were associated with hydrous metal oxide coatings, or coarse waste particles. Apart from a substantial residual component, copper was preferentially associated with organic matter. Cadmium was rarely detected in any stream. Multidimensional scaling showed that under low flow conditions the three sites had distinct sediment chemistries. However, high levels of temporal and spatial variability were apparent within the urbanized sites, consistent with those streams regularly receiving discharges of diverse composition. Poor sediment and interstitial water qualities were often observed in the two urban sites, although such conditions did not occur continuously. Poor interstitial water quality was also seen on occasion in the reference stream; however, poor sediment quality was never detected at this site. Considering surficial sediments of Hawkesbury Sandstone streams may be suspended during high flow conditions, it was concluded that surface waters were a more reliable indicator of stream condition for short‐term monitoring studies.

Analysis of Bacterial Deposition on Metal (Hydr)oxide-Coated Sand Filter Media

The aim of this study was to investigate the importance of surface potential in microbial deposition onto modified granular surfaces. Recent experimental and theoretical work has indicated that surfaces coated with metal oxides and hydroxide rich oxide/hydroxide mixtures ((hydr)oxides) have the potential to increase the capture efficiencies of commercial filtration systems. This study quantitatively compared different metal (hydr)oxide coatings in their abilities to enhance bacterial deposition. Specifically, the deposition rates of bacterial strainsStreptococcus faecalis, Staphylococcus aureus, Salmonella typhimurium,andEscherichia coliwere compared for Ottawa sand and surface coatings consisting of aluminum (hydr)oxide, iron (hydr)oxide, and mixed iron and aluminum (hydr)oxide. The metal-(hydr)oxide-modified granular media enhanced bacterial deposition relative to the noncoated Ottawa sand. The electropositive surfaces, the aluminum and the mixed (hydr)oxides, had similar average kinetic rate constants, five times larger than the rate constants observed for the untreated Ottawa sand. The measured kinetic rate constants for the positively charged systems of aluminum (hydr)oxide and mixed (hydr)oxide collectors suggested that the overall rate of deposition was limited by the transport of bacteria to the granular surface rather than the rate of attachment. For systems where the collector surfaces were negatively charged, as in the cases of Ottawa sand and the iron (hydr)oxide coating, large energy barriers to attachment were predicted from DLVO theory but these barriers did not totally inhibit bacterial deposition. The deposition results could not be fully explained by DLVO theory and suggested the importance of other factors such as collector charge heterogeneity, motility, and bacterial surface appendages in enhanced deposition.

Plasma-assisted deposition of metal and metal oxide coatings

A combination of physical vapour deposition and plasma-assisted chemical vapour deposition techniques were used to deposit Cu-, Ni- and Sn-rich SnO/SnO2 coatings on metal and ceramic substrates. Cu and Ni were deposited on Al alloy 6061 substrates and Ni deposition was also performed on glass microscope slides and commercially pure alumina substrates. Sn-rich SnO/SnO2, on the other hand, was coated on stainless steel and pure Cu substrates. A direct-current plasma system was used to deposit the pure metals in vacuum with a resistively heated tungsten boat that was coated with alumina. All samples were sputtered for 20 min in an argon:hydrogen (1:1) atmosphere at a pressure of 300 mTorr. To reduce contamination and oxidation of both substrates and deposited layers, Cu and Ni coatings were made with argon:hydrogen (2:1) carrier gas. Sn-rich tin oxide coatings were deposited in a pure argon atmosphere (no hydrogen) to allow for the oxidation of Sn deposits on the stainless steel and copper substrates. Investigations of coated surfaces by scanning electron microscopy and X-ray diffraction showed coatings to be smooth, continuous and pure. Deposition rates showed this application to provide a very high rate when compared with chemical vapour deposition and metal–organic chemical vapour deposition techniques. Scratch tests results prove good attachment of the coatings to their respective substrates. © 1998 Chapman & Hall

Surface and coatings effects in catalytic combustion in internal combustion engines

Various aspects of catalytic combustion relating to engines have been investigated. MoO 3 and coatings of transition metal oxides (prepared by flame pyrolysis) were confirmed to have combustion catalytic activity commencing at ~325–430 °C. Evidence from experiments and the literature indicates that in-cylinder catalysts can aid in fuel ignition (beneficial), but may also cause early quenching of the flame-front (detrimental) at combustion chamber surfaces. Catalytic combustion, when it occurs on thermocouple or heat probe surfaces, will cause surface temperature measurements in furnaces or engines to be erroneously high. This may explain the Woschni effect, a controversial finding wherein thermal insulation (which raises combustion chamber wall temperatures) appears to increase heat rejection from the combustion chamber.

Optical and structural characterization of tungsten-based CVD metal oxide coatings

Thin films of tungsten oxide coatings have been deposited by chemical vapor deposition (CVD) using W(CO) 6 as a precursor. By pyrolytical decomposition at 400 °C of W(CO) 6 in argon and oxygen atmospheres, thin tungsten oxides films were obtained on glass substrates. Layers were investigated by X-ray diffractometry, Auger analysis and by measurements of their absorption and reflectance in the range 0.2–15 μm. The films obtained were highly absorptive and possessed spectral selectivity. The structure studies showed that the as-prepared films possessed metal oxide crystalline phase structure.

Peculiarities of Field Electron Emission from CVD Diamond Films

Results are reported on characterization of the field electron emission from diamond films grown by dc arc discharge plasma CVD onto Si substrates from CH 4 -H 2 gas mixtures. The field electron emission was observed at 15-20 V/μm. Emission current-voltage dependences were studied for films prepared at different CVD conditions and post-growth surface treatment/ modification (ultrathin metal and metal oxide coatings, MW-plasma processing, laser-induced surface graphitization). Features of emission current versus applied field behaviour (including a hysteresis phenomenon, vacuum arc initiation) and ultralow (0.1-0.5 eV) values of effective work function derived from Fowler-Nordheim plot fitting are discussed. A high vacuum scanning tunneling-field emission microscope was applied for simultaneous mapping of field electron emission intern-sity. topography and work function to study electronic and structural properties of field emission centers.

Deposition of SiO 2 and Ta 2O 5 films by electron-beam-excited plasma ion plating

Refractory metal-oxide coatings are widely used for optical interference applications. Conventional evaporation films have inferior optical and structural properties compared with the bulk form. These films often have a columnar microstructure with voids which leads to the instability of the optical properties (e.g., the spectral characteristics). Hence, electron-beam-excited plasma (EBEP) ion plating has been developed. The process, operated by injecting a low-energy (~ 150 eV) high-current (~30 A) electron beam into a vacuum chamber, produces high-density plasma at low pressure. SiO 2 and Ta 2O 5 films for optical applications were deposited by the process. Films prepared at an oxygen pressure of 1–2×10 −4 Torr showed good stoichiometry and dense microstructure, which lead to the absence of significant spectral shifts resulting from changes in the temperature and the humidity of the ambient atmosphere. The above results prove that this process is a promising tool for the large-area deposition of refractory metal-oxides at low temperatures.

Oxidation behavior of aqueous contaminants in the presence of hydrogen peroxide and filter media

Hydrogen peroxide has been used as an oxidant to degrade contaminants in solutions and soils. A poor understanding of the numerous variables that are involved makes it difficult to determine dominant contaminant removal mechanisms. Our primary objective was to examine the relationship between contaminant (quinoline and nitrobenzene) degradation rate and the rate of hydrogen peroxide decomposition on filter media. Both batch and continuous flow column experiments were conducted. In general, the rate of contaminant degradation was proportional to the rate of hydrogen peroxide decomposition, but the mass of contaminant removed depended on the amount of hydrogen peroxide decomposed, filter medium concentration, and filter medium characteristics. For increasing filter medium concentration and equivalent loss of hydrogen peroxide, the mass of contaminant degraded was found to decrease. In addition, acid-hydroxylamine treatment of the selected filter medium, to examine the role of reducible metal oxide coatings, resulted in greater contaminant removals than the parent material despite a slower hydrogen peroxide decomposition rate. The observed hydrogen peroxide decomposition and contaminant oxidation results are consistent with a reaction scheme whose central elements include: (1) a rate limiting filter medium surface catalyzed reaction initiating hydrogen peroxide decomposition with the formation of a reactive intermediate, (2) a competing reaction of the intermediate with the filter medium surface, and (3) reaction of the same intermediate with the aqueous organic contaminant. Loss of quinoline and nitrobenzene is most likely a solution phase reaction because sorption of these compounds was small over the pH range 7–8 and oxidation efficiency did not increase with increasing filter medium concentration, which would be expected if the reactions were occurring on the surface. Finally, enhanced oxidation of quinoline and nitrobenzene on the treated material is explained by more efficient use of the reactive intermediates for contaminant oxidation due to a reduction in the number of scavenging sites associated with reducible metal oxide coatings.

Adsorption of natural organic matter onto goethite

The factors influencing the association of two natural organic matter (NOM) samples (from Redwater Creek in Australia and Suwannee River in USA) with goethite were studied. Sorption occurred very rapidly (minutes) and at very low NOM concentrations (about 0.5 mg C 1 −1), implying that mineral surfaces in the natural aquatic environment would not remain uncoated by NOM for long. The sorption behaviour of the two NOM samples followed the Langmuir adsorption equation. The maximum sorption density for the unfractionated Redwater Creek NOM was 1.1 μmol C mg −1 of goethite. The hydrophobic fractions (both acidic and neutral) separated from the Redwater Creek sample had much greater sorption affinities for the goethite surface than did the hydrophilic fraction. Calculation of the surface crowding indicates that a monolayer of sorbed NOM exists over the goethite surface. Sorption was influenced by solution pH and calcium ion concentration. Maximum sorption density occurred at around pH 3–4, and decreased at higher pH. The sorption density of NOM also appears to be influenced by the presence of non-ionic hydrophobic solutes. This component could shield the electrostatic repulsive forces between neighbouring adsorbed anionic NOM molecules on the goethite surface, thus leading to an increase in the maximum amount of NOM adsorbed. Calcium ion concentration increased sorption of NOM up to calcium concentrations of around 1 mM but higher calcium concentrations resulted in reduced sorption. Sorption of the hydrophobic acid fraction was most affected by calcium concentration. The main effect of NOM sorption on the surface characteristics of goethite is alteration of the surface charge. The isoelectric point of the NOM-coated goethite is shifted markedly to lower pH, compared to that of uncoated goethite. Thus, at the normal pH of natural waters, NOM-coated goethite has a net negative charge which would significantly influence the types of interaction occurring at the surface of the NOM-coated particles in natural aquatic systems. The model advanced for suspended particulate matter (SPM) in aquatic systems, namely a mineral core with coatings of hydrous metal oxides and NOM, appears to explain adequately many aspects of the behaviour of natural SPM.

Sorption of Chlorobenzenes to Cape Cod Aquifer Sediments

Sorption of tetra- and pentachlorobenzene by sediment from a glacial outwash aquifer on Cape Cod, MA, was evaluated. Particle size and mineralogical fractions (separated based on paramagnetic susceptibility) were characterized with respect to sediment organic carbon (SOC), mineralogy, surface area, metal oxide coatings, and spatial variability. SOC increases by a factor of 10 as particle size decreases from 500-1000 to 25% in the <63-μm fraction, and SOC is preferentially associated with the magnetic minerals

Optical properties and environmental stability of oxide coatings deposited by reactive sputtering

Refractory metal-oxide coatings are deposited by reactive dc magnetron sputtering in an oxygen environment. The optical constants and the environmental stability of silicon oxide, aluminium oxide, hafnium oxide, zirconium oxide, tantalum oxide, titanium oxide, and a blend of hafnium oxide with silicon oxide are investigated. Properties of both single-layer and multilayer interference filters are examined.

Soil washing: Practical considerations and pitfalls

AbstractThe use of soil washing to remove petroleum hydrocarbon contamination from the soil matrix is becoming more widely used. When viewed as a volume reduction tool, this technology shows some promise. However, ongoing research and treatability studies indicate that without further treatment, even larger‐sized soil fractions (sands and cobbles) may retain hydrocarbon contamination at levels that require further cleaning prior to permanent disposal or reuse. The perception has been that by removing the sand from the soil matrix, thus achieving a 30 percent to 60 percent volume reduction, expensive post‐washing treatment or approved disposal of the finer materials (silts and clays) would be cost‐effective. There exists evidence to the contrary, however. Hydrocarbon retention after soil washing may be influenced by a number of factors unrelated to particle size. Soil characteristics that may play a role include soil humic acids, metal oxide coatings, geologic origin of the soil particles, and clay type. In this article the authors describe a laboratory study designed to evaluate the “cleanability” of two soils.

Phase maps for sputter deposited refractory metal oxide ceramic coatings: review of niobium oxide, yttrium oxide, and zirconium oxide growth

AbstractReactive sputter deposition is a widely used glow discharge process for growing high melting point coatings near room temperature, and metastable and multiphase structures not attainable in bulk material grown under conditions of thermodynamic equilibrium. It is therefore ideally suited for growing refractory metal oxide coatings. In this study, ‘phase maps’ are constructed for the sputter deposition of the refractory metal oxides of Nb, Y, and Zr. These diagrams interrelate process parameters, the growth environment, and metallurgical phase in the growth regime of near room substrate temperature, low surface diffusion, and sticking coefficient of unity. Phase boundaries are discussed in terms of: (i) the fractional flux of metal atoms and metal oxide molecules to the substrate; (ii) a complete oxide layer at the metal target surface; (iii) oxygen species in the plasma available for reaction at the substrate.MST/1693

Chromogenic materials for transmittance control of large-area windows

Abstract Chromogenic materials have optical properties that can be changed in a persistent and reversible manner under the action of an external stimulus. They can regulate the throughput of radiant energy between widely separated extrema, and hence they can be used in future “smart windows” in buildings, cars, etc., and for many other applications. This article summarizes the state-of-the-art for: Photochromic (light sensitive) materials, with a focus on glass and spirooxazine compounds Thennochromic (temperature sensitive) materials, particularly vanadium-oxide-based coatings and “cloud gels” Electrochromic (electric charge dependent) materials integrated in multilayer configurations, with emphasis on transition metal oxide coatings and ion intercalation into these Certain liquid-crystal-based (electrically switched) materials of the guest-host as well as the polymer dispersed and encapsulated types

Boundary scavenging in the Pacific Ocean: a comparison of 10Be and 231Pa

Concentrations of U, Th, 231Pa and 10Be were measured in Holocene sediments from two cores collected off the west coast of South America, two cores from the East Pacific Rise, two from the equatorial Pacific and one from the south Pacific central gyre. Our results, together with data from 5 cores reported in the literature, show that boundary scavenging plays a major role in the removal of 10Be from the Pacific Ocean. Deposition rates of 10Be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central Pacific. Deposition of 231Pa is 4 to 5-fold greater at the margin sites. The residence time of 10Be with respect to chemical scavenging, defined as its inventory in the water column divided by its rate of removal to the sediments, varies regionally from > 1000 years at the red-clay sites in the deep central Pacific to ∼ 100 years at the margin sites. Different factors control boundary scavenging of Pa and Be. For example, scavenging of 231Pa is enhanced by metal-oxide coatings of particles, whereas this seems to have little influence on the scavenging of 10Be.

Microencapsulation of oil droplets by aerosol techniques—I. Metal oxide coatings

The solubility and reactivity of titanium and aluminum alkoxides in dodecane, diesel oil and distillation fractions of the latter are reported. When solutions of alkoxide in oil are nebulized and the thus formed droplets reacted with water vapor, metal oxide microcapsules form around the oil solvent and have morphologies and porosities dependent on reaction conditions.

Oxidized Nickel-Iron and the Effect of Absorption Modes on Infrared Emission

Metal oxide coatings are used on the surface of electronic components to improve heat dissipation through emission of infrared radiation. The physical properties of these films, e.g., composition and thickness, affect the spectral distribution of the emission. In addition, in the spectral region of the characteristic oxide vibrational modes, infrared emission is substantially reduced. In this paper, the infrared emissions for oxides on iron and Invar (36% Ni-Fe alloy) are compared. These metals are used in TV manufacture (shadow mask), and their oxide film properties are important to the operation of TV kinescopes.

Fabrication of electrically conductive metal oxide coatings by reactive ion plating

Electrically conductive metal oxide (ceramic) coatings produced by reactive ion plating are being investigated at USA-CERL for potential application in cathodic protection systems. Ceramic materials are advantageous because of their very low dissolution rates (typically less than 1 g/A/yr in 3.5% NaCl solution) and ease of fabrication. Among the ceramic anode materials currently under investigation are two systems: (1) a mixture of titanium oxide and ruthenium oxide and (2) a mixture of titanium oxides ‘‘doped’’ with niobium, each ion plated on niobium substrates. The mixed oxide coatings were fabricated by a reactive ion plating process involving oxygen and dual electron beam evaporation sources. An enhanced plasma was used to increase reactivity. X-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy have been used for characterization of the microstructure, crystallography, and elemental composition of the coating.

Sputtered coatings for protection of spacecraft polymers

Kapton® polyimide oxidizes at significant rates (4.3 × 10 −24 g per incident oxygen atom) when exposed in low earth orbit to the ram atomic oxygen flux. Ion-beam-sputter-deposited thin films of Al 2O 3 and SiO 2 as well as a codeposited mixture of predominantly SiO 2 with a small amount of polytetrafluoroethylene were evaluated and found to be effective in protecting Kapton from oxidation both in laboratory plasma ashing tests as well as in space on board shuttle flight STS-8. A protective film of 96% SiO 2 or more and 4% polytetrafluoroethylene or less was found to be very flexible compared with the pure metal oxide coatings and resulted in mass loss rates that were 0.2% of that of the unprotected Kapton. The optical properties of Kapton for wavelengths investigated between 0.33 and 2.2 μm were not significantly altered by the presence of the coatings or changed by exposure of the coated Kapton to the low earth orbital ram environment.