Showing papers on "Deposition (chemistry) published in 1997"

Effect of N deposition on decomposition of plant litter and soil organic matter in forest systems

Abstract: The effects of nitrogen (N) deposition on plant litter and soil organic matter decomposition differ depending on the stage of decomposition (early, late, and final stages). The effects can be divid...

Tribology of diamondlike carbon and related materials: an updated review

Abstract: Diamondlike carbon (DLC) has been studied for many years as a wear-resistant material with low friction coefficient. Its tribological behavior is strongly affected by the tribotesting environment and is controlled by tribochemical effects, which may in turn be dependent on the technique used for the deposition of the films. New variations of DLC films, with various dopings, and new deposition methods have been investigated in recent years. The paper presents an updated review of the tribological properties of DLC and related materials, and discusses the mechanisms suggested for the explanation of the wear and friction behavior of these materials.

Deposition of three-dimensional Ge islands on Si(001) by chemical vapor deposition at atmospheric and reduced pressures

Abstract: This report summarizes observations of Ge island formation during growth on Si(001) by chemical vapor deposition from germane in the pressure range from 10 Torr to atmospheric pressure in a conventional epitaxial reactor. A four-step growth process is observed: (1) uniform pseudomorphic overlayer (“wetting’’ layer) formation; (2) three-dimensional island growth with a constant aspect ratio; (3) continued island growth with a constant diameter and increasing height; (4) rapid growth of larger, faceted islands. Ostwald ripening of the islands during continued heat treatment after terminating the deposition is slow compared to island formation and growth during deposition for the experimental conditions used.

Transport of Humic-Coated Iron Oxide Colloids in a Sandy Soil: Influence of Ca2+ and Trace Metals

Abstract: Understanding colloid release, transport, and deposition in natural heterogeneous porous media is a prerequisite for evaluating the potential role of colloids in subsurface contaminant transport. In this study, we investigate the influence of adsorbed humic acid, solution Ca2+ concentration, and adsorbed trace metals (Cu2+, Pb2+) on the transport and deposition kinetics of colloidal hematite particles (α-Fe2O3; 122 nm diameter) in a sandy soil matrix. A short-pulse chromatographic technique was used to measure colloid deposition rate coefficients and collision efficiencies (α). At pH 5.7, pure hematite was positively charged and deposited rapidly (α ≈ 1) even at low electrolyte concentrations (10-4 M CaCl2). Adsorption of humic acid to the hematite caused reversal of surface charge from positive to negative. As a result, colloid deposition rates were decreased by approximately 2 orders of magnitude (α ≈ 0.01). Deposition rates of humic-coated hematite colloids strongly increased with increasing Ca2+ conce...

Effects of land use, climate variation, and N deposition on N cycling and C storage in northern hardwood forests

Abstract: We hypothesized that much of the variability in dissolved inorganic nitrogen (DIN) loss from forested catchments can be explained by land use history and interannual climatic variation, and that these factors determine the degree to which N deposition results in increased storage of C in forests. We used an existing model of C, N, and water balances in forest ecosystems in conjunction with long-term climate and N leaching loss data from several northern hardwood forest ecosystems to predict the effects of land use, climate variability and N deposition on C storage and N cycling and loss. Six sites from the White Mountains of New Hampshire with very different land use histories and annual stream DIN losses were used. The only model parameter that varied between sites was land use or disturbance history. Each site was simulated using both mean climate data for each year and actual time series climate data. Vegetation removal resulted in a period of increased DIN leaching, followed by losses below those in control stands for both measured and simulated data. One site with an extreme fire event over 170 years ago still showed reduced N losses in both modeled and measured data. Significant interannual variation in DIN loss is evident in the field data. Model predictions using actual climate time series data captured much of this variation. This high interannual variability along with the slow rate of change in DIN loss predicted by PnET-CN using mean climate throughout the simulations suggests that statistically significant increases in DIN leaching losses due to long-term increases in N deposition will not be detectable for several decades, given current rates of N deposition. N deposition increased C storage in all simulations, but the quantity stored was about 50% that predicted by another published model. This difference results from differences in the efficiency with which added N is retained in the ecosystem. The previous model used an 80% retention value, while retention was closer to 50% over most of the time period examined here.

Mechanism of WO 3 Electrodeposition from Peroxy‐Tungstate Solution

Abstract: Electrodeposition of amorphous electrochromic tungsten trioxide was studied. Good quality films were deposited cathodically onto tin‐doped indium oxide (ITO) from a water/isopropanol solution containing dissolved tungsten(VI) species. Three reducible species are present in this solution: unbound , polytungstate ions, and peroxy‐tungstate ions. The effects of the content, which is related to the age of the deposition solution, and of mass transport on the deposition current and deposition efficiency were determined and used to elucidate the mechanism of electrodeposition. Reduction of unbound and polytungstate did not result in film growth. This part of the current was under mixed diffusion‐kinetic control. The remainder of the current was due to reduction of peroxy‐tungstate and was kinetically limited. This process led to deposition. The current efficiency, defined as the number of electrons consumed by reduction of peroxy‐tungstate per W atom deposited, was about 1.6, in good agreement with the proposed deposition reaction.

Herbicides and their metabolites in rainfall: Origin, transport, and deposition patterns across the midwestern and northeastern United States, 1990-1991

Abstract: Herbicides were detected in rainfall throughout the midwestern and northeastern United States during late spring and summer of 1990 and 1991. Herbicide concentrations exhibited distinct geographic ...

Dust deposition and particle size in Mali, West Africa

Abstract: Dust deposition samples were collected during April and May (1990) at four sites on the inland delta region of Mali in West Africa. High dust deposition rates were measured and characterised by considerable variations between sites. A major dust plume passed over the area, following a dust storm near Gao, 500 km to the east, however the contribution of this plume to overall deposition was surprisingly small. Dust particle-size characteristics indicate mixing of dusts from three sources. Long-distance dust deposition producing fine deposits (mainly < 5 μm); dust from regional sources producing deposits mainly in the size range 20–40 μm; and local dust arising from human activities, particularly vehicle and livestock movements, producing relatively coarse material in the size range 50–70 μm, plus some fines. Local dusts appear to have made the greatest contribution to deposition, with a moderate (though less than expected) contribution from the plume and a minor contribution from long-distance dusts. These results have implications for dust deposition studies aimed at quantifying dust contributions to soils. Although these Mali dust deposition rates are high (2.5–28.6 t−1 km−2 day−1), the significant proportion of locally derived dust means that new dust inputs to the soils of the Inland Delta region of Mali are small.

Atomic layer epitaxy growth of aluminum oxide thin films from a novel Al(CH3)2Cl precursor and H2O

Abstract: The deposition of Al2O3 thin films by atomic layer epitaxy was investigated using Al(CH3)2Cl as a new aluminum precursor. All the films grown in the temperature range of 125–500 °C were amorphous as examined by x-ray diffraction analysis. The residual contents of carbon, chlorine, and hydrogen in the film deposited at 200 °C were 0.2, 2.1, and 12 at. %, respectively, and diminished rapidly with increasing growth temperature. The refractive index of the films increased with deposition temperature, stabilizing at the highest value of 1.68 above 300 °C. The permittivity of the films increased from 7.3 to 8.7 with increasing growth temperature from 200 to 500 °C. The leakage current density was lowest in the film deposited at 200 °C and increased markedly at higher deposition temperatures.

Biomass allocation and canopy development in spruce model ecosystems under elevated CO2 and increased N deposition.

Abstract: Ecosystem-level experiments on the effects of atmospheric CO2 enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m2 of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO2 concentrations (280, 420, 560 μl l−1) and three different rates of wet N deposition (0, 30, 90 kg ha−1 year−1) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO2 concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO2 leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO2. In contrast to CO2 enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO2 × N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO2 l−1, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO2 largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO2 (−18% at 420 and −31% at 560 compared to 280 μl CO2 l−1). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha−1 year−1). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO2 enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO2 suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO2 may indicate CO2-induced reductions of soil N availability.

Variation partitioning by partial redundancy analysis (RDA)

Abstract: Partial redundancy analysis was applied to assess the effects of sulphur (S) and nitrogen (N) deposition on epiphytic green algae (mainly Protococcus viridis) on Picea abies needles. Data were collected from 15 sites over the country during 12 years by the Swedish National Environmental Monitoring Program (PMK). The green algae colony thickness and colonization rate depend on three intercorrelated environmental variable groups, namely, geographic location, climatic condition and S and N deposition. The collinearity between the environmental variables complicates the assessment of deposition-induced impact. The total explained variation within algae distribution patterns (eigenvalue=0.811 as 100 per cent) was partitioned among the three environmental variable groups. Analysis showed that a large part of the explained variation (67 per cent) was due to the joint affects of geographic factors, climatic factors and S, N deposition. Climatic factors (temperature sum and relative moisture) alone, and joint affects of climatic and S, N deposition accounted for 13 per cent and 12 per cent of the explained variation, respectively. The pure geographic factors and the pure deposition accounted for less than 5 per cent, respectively. This analysis suggests that the response of green algae to S and N deposition is mainly climate dependent, stronger in a warm and moist climate than in a cool and dry one. © 1997 John Wiley & Sons, Ltd.

Chemical Vapor Deposition of MoS2 and TiS2 Films from the Metal-Organic Precursors Mo(S-t-Bu)4 and Ti(S-t-Bu)4

Abstract: The deposition of MoS2 and TiS2 thin films from the metal-organic precursors Mo(S-t-Bu)4 and Ti(S-t-Bu)4 has been investigated. Stoichiometric films with low levels of oxygen and carbon contaminants can be grown at temperatures between 110 and 350 °C and low pressure. The films are amorphous when grown at these low temperatures and have granular morphologies in which the grains are 30−90 nm in diameter, the larger grain sizes being observed at higher deposition temperatures. For the MoS2 deposits, the electrical conductivity was ∼1 Ω-1cm-1. For both precursors, the organic byproducts generated during deposition consist principally of isobutylene and tert-butylthiol; smaller amounts of hydrogen sulfide, isobutane, di-tert-butyl sulfide, and di-tert-butyl disulfide are also generated. A β-hydrogen abstraction/proton-transfer mechanism accounts for the distributions of the organic byproducts seen during the deposition of MoS2 and TiS2 films. Our results differ in some respects from those of a previous study ...

Measurements of pesticide spray drift deposition into field boundaries and hedgerows: 2. Autumn applications

Abstract: The drift of fluorescent tracer in water and formulated pesticide sprays was measured at different heights and distances within 6-m-wide buffer strips (“Conservation Headlands”) and field boundaries surrounding mature winter cereal fields. Spray deposits were taken from artificial collectors. There was a diminution in deposition at the hedgerow, where a 6-m-wide buffer strip of unsprayed crop was employed between the sprayer and the hedgerow compared to where the entire crop edge was fully sprayed. There was some evidence that a mature crop would absorb some of this spray drift, reducing deposition on the hedge flora below crop level and, therefore, the nature of the buffer strip as well as it's width were thought to be important in determining hedgerow deposition rates. Frequency distributions of deposition along strips of hedgerow revealed that the unsprayed buffer zone also served to reduce the peaks in drift that might occur as a result of the pitch and yaw of travelling tractor booms. The vegetational complexity of the hedge bottom was also shown to determine spray deposition within field boundaries. Larvae of the cabbage white butterfly (Pieris brassicae[L.]) showed higher mortalities when exposed to hedgerow grasses adjacent to conventionally sprayed headlands compared to those adjacent to a Conservation Headland. A simple methodology for spray deposition studies is outlined, and implications of pesticide drift into field boundaries discussed.

Prevalence of Obesity in the Saudi Population

Abstract: Obesity, i.e., excessive deposition of fat in the body, is the m ost common nutritional disorder in the developed countries. The aim of this study was to determine the prevalence of overweight and ...

Functional Gradient Metallic Prototypes through Shape Deposition Manufacturing

Abstract: Stanford's SDM laser deposition system has been recently improved to enable the deposition of functionally graded metals through the use of powder mixing. While Shape Deposition Manufacturing has always had the capability to produce multimaterial artifacts, powder mixing enables the deposition of single layers in which material properties can be smoothly varied without discrete interfaces between dissimilar materials. It has been shown that certain materials will completely mix during deposition and form alloys which exhibit material properties intermediate to those of the constituent feed powders. To date, oxidation and hardness have been effectively controlled through appropriate mixing of powders. Functional gradient material deposition has been exploited to construct an advanced injection molding tool which transitions from Invar in the center to stainless steel on the outside. The resulting tool exhibited minimal distortion from thermal stress and excellent exterior corrosion resistance.

ENVIRONMENTAL AUDITING Exceedance of Critical Loads for Lakes in Finland, Norway, and Sweden: Reduction Requirements for Acidifying Nitrogen and Sulfur Deposition

Abstract: The main objectives of this study were to identify the regions in Fennoscandia where the critical loads of sulfur (S) and acidifying nitrogen (N) for lakes are exceeded and to investigate the consequences for deposition reductions, with special emphasis on the possible trade-offs between S and N deposition in order to achieve nonexceedance. In the steady-state model for calculating critical loads and their exceedances, all relevant processes acting as sinks for N and S are considered. The critical loads of N and S are interrelated (defining the so-called critical load function), and therefore a single critical load for one pollutant cannot be defined without making assumptions about the other. Comparing the present N and S deposition with the critical function for each lake allows determination of the percentage of lakes in the different regions of Fennoscandia where: (1) S reductions alone can achieve nonexceedance. (2) N reductions alone are sufficient, and (3) both N and S reductions are required but to a certain degree interchangeable. Secondly, deposition reduction requirements were assessed by fixing the N deposition to the present level, in this way analyzing the reductions required for S, and by computing the percentage of lakes exceeded in Finland, Norwaymore » and Sweden for every possible percent deposition reduction in S and N, in this way showing the (relative) effectiveness of reducing S and/or N deposition. The results showed clear regional patterns in the S and N reduction requirements. In practically the whole of Finland and the northern parts of Scandinavia man-made acidification of surface waters could be avoided by reducing S deposition alone. In the southern parts of Sweden some reductions in N deposition are clearly needed in addition to those for S. In southern Norway strong reductions are required for both N and S deposition. 55 refs., 5 figs.« less

Effect of microgravity and hypergravity on deposition of 0.5- to 3-μm-diameter aerosol in the human lung

Abstract: We measured intrapulmonary deposition of 0. 5-, 1-, 2-, and 3-micron-diameter particles in four subjects on the ground (1 G) and during parabolic flights both in microgravity (microG) and at approximately 1.6 G. Subjects breathed aerosols at a constant flow rate (0.4 l/s) and tidal volume (0.75 liter). At 1 G and approximately 1.6 G, deposition increased with increasing particle size. In microG, differences in deposition as a function of particle size were almost abolished. Deposition was a nearly linear function of the G level for 2- and 3-micron-diameter particles, whereas for 0.5- and 1.0-micron-diameter particles, deposition increased less between microG and 1 G than between 1 G and approximately 1.6 G. Comparison with numerical predictions showed good agreement for 1-, 2-, and 3-micron-diameter particles at 1 and approximately 1.6 G, whereas the model consistently underestimated deposition in microG. The higher deposition observed in microG compared with model predictions might be explained by a larger deposition by diffusion because of a higher alveolar concentration of aerosol in microG and to the nonreversibility of the flow, causing additional mixing of the aerosols.

Thermal Analysis Of Fused Deposition

Abstract: Fused Deposition processes involve successive melting, extrusion and solidification of thermoplastic polymer melts. Fluid mechanics and heat transfer of neat or particle-filled polymeric melts, viscoelastic deformation and solidification of the roads that are being produced, and repetitive thermal loading of the growing part are important physical processes that control the final quality of the part. Previous computational process models investigated deposition and cooling processes for single and multiple filaments. In the current study, complimentary computational models are presented for the extrusion phase of the process. Impact of liquefier and nozzle design on thermal hardware behavior and operational stability has been quantified. Also a detailed study of temperature field near the vicinity of deposition point is presented with particular emphasis on dimensional analysis and deposition ofmultiple material systems. BACKGROUND Fused Deposition process involves deposition of thermoplastic melts by a computer controlled minirobot to produce arbitrary geometry three dimensional objects (Crump, 1992.) Fabricated objects possess a two level hierarchical meso-structure, which consists of curvilinear roads packed into discrete layers. The thermal energy stored in the molten material is redistributed into the part through conduction, and is consumed by lateral convection cooling. The redistribution of thermal energy ensures bonding at the interfaces of deposited roads, and generates the structural integrity of the part. Material delivery into the workvolume can be achieved by a liquefier which employs a self-extruding filament (Comb et.al, 1994), a fluid metering rotary pump (Batchelder et.al, 1994) or through a high-pressure plunger system (Hilmas, 1996). The road cross-sections are shaped through fountain flow of polymer melt between the previously deposited material and nozzle tip, resulting in flattened ellipsoids. These ellipsoid shapes are basically rectangles with two semi-circles attached at each lateral side of the road, which have the layer thickness as their diameter Recently computer controlled surface forming mechanisms, trowels, have been proposed to shape the road cross-sectional geometries (Khoshnevis, 1997). The available material set for filament deposition techniques has been increased through introduction of particles into the feed-stock, which enabled the production of intermediate powder processing products, particulate polymeric composites or green bodies, that" can be processed further to obtain porous and/or fully dense metallic and ceramic articles (Geiger, 1994), (Agarwala et.al, 1995). Previous process modeling effort for Fused Deposition was concentrated on the cooling behavior of single and multiple filaments (Yardimci et.al, 1995), (Yardimci and Guceri, 1996),

A Multiscale Simulator for Low Pressure Chemical Vapor Deposition

Abstract: An integrated simulator for chemical vapor deposition is introduced. In addition to reactor scale and feature scale simulators, it includes a mesoscopic scale simulator with the typical length scale of a die. It is shown that the three-scale integrated simulator used is a proper extension of two-scale deposition simulators that consist of reactor scale and feature scale simulation models. Moreover, it is demonstrated that information is provided on a new length scale, for which no information is available from the two-scale approach, as well as important corrections to the simulation results on the reactor scale. This enables, for instance, studies of microloading. Thermally induced deposition of silicon dioxide from tetraethyoxysilane is chosen as the application example. The deposition chemistry is modeled using six gaseous reacting species involved in four gas-phase and eight surface reactions.

Method of controlling multi-species epitaxial deposition

Abstract: An integrated dual beam multi-channel optical-based flux monitor and method of monitoring atomic absorption of a plurality of atomic species during epitaxial deposition. Light from multiple sources is simultaneously passed through a region of deposition of material such that atomic absorption takes place. The light that passed through the region is then compared to light in a reference arm that did not pass through a region of atomic absorption. From this comparison the deposition of an epitaxial layer can be carefully monitored and controlled.