Study of Atomic Hydrogen Concentration in Grain Boundaries of Polycrystalline Diamond Thin Films

This paper describes research focused on investigating the effect of hydrogen (H) atom insertion into the grain boundaries of polycrystalline diamond (PCD) films. This is required in order to understand the key morphological, chemical, physical, and electronic properties of the films. The PCD films were grown using the hot filament chemical vapor deposition (HFCVD) process, with flowing Ar gas mixed with CH4 and H2 gases to control film growth into microcrystalline diamond (MCD, 0.5–3 µm grain sizes), nanocrystalline diamond (NCD, 10–500 nm grain sizes), and ultrananocrystalline diamond (UNCD, 2–5 nm grain sizes) films depending on the Ar/CH4/H2 flow ratios. This study focused on measuring the H atom concentration of the PCD films to determine the effect on the properties indicated above. A simple model is presented, including a hypothesis that the two dangling bonds per unit cell of C atoms serve as the site of hydrogen incorporation. This correlates well with the observed concentration of H atoms in the films. Dangling bonds which are not passivated by hydrogen are postulated to form surface structures which include C double bonds. The Raman peak from these surface structures are the same as observed for transpolyacetyline (TPA). The data reveal that the concentration of H atoms at the grain boundaries is around 1.5 × 1015 atoms/cm2 regardless of grain size. Electrical current measurements, using a conductive atomic force microscopy (CAFM) technique, were performed using an MCD film, showing that the current is concentrated at the grain boundaries. Ultraviolet photo electron spectroscopy (UPS) confirmed that all the PCD films exhibited a metallic behavior. This is to be expected if the nature of grain boundaries is the same regardless of grain size.

Mechanical Properties of the Carbon Nanotube Modified Epoxy–Carbon Fiber Unidirectional Prepreg Laminates

The effect of plasma treatment of the multi-walled carbon nanotube (MWCNT) surface on the fracture toughness of an aerospace grade epoxy resin and its unidirectional (UD) carbon fiber prepreg laminates has attracted scientific interest. A prepreg route eliminates the possible risk of carbon nanotube filtration by unidirectional carbon fibers. X-ray photoelectron spectroscopy results suggested that oxygen atom concentration at the nanotube surface was increased from 0.9% to 3.7% after plasma modification of the carbon nanotubes. A low number (up to 0.5 wt.%) of MWCNTs was added to epoxy resin and their carbon fiber prepreg laminates. Transmission electron micrographs revealed that the plasma treatment resulted in a better dispersion and distribution of MWCNTs in the epoxy resin. Plasma-treated MWCNTs resulted in a more pronounced resistance to the crack propagation of epoxy resin. During the production of the reference and nanotube-modified prepregs, a comparable prepreg quality was achieved. Neat nanotubes agglomerated strongly in the resin-rich regions of laminates lowering the interlaminar fracture toughness under mode I and mode II loading. However, plasma-treated nanotubes were found mostly as single particles in the resin-rich regions of laminates promoting higher energy dissipation during crack propagation via a CNT pull-out mechanism.

Evaluating the sensitivity of radical chemistry and ozone formation to ambient VOCs and NO_<i>x</i> in Beijing

Measurements of OH, HO2, complex RO2 (alkene- and aromatic-related RO2) and total RO2 radicals taken during the integrated Study of AIR Pollution PROcesses in Beijing (AIRPRO) campaign in central Beijing in the summer of 2017, alongside observations of OH reactivity, are presented. The concentrations of radicals were elevated, with OH reaching up to 2.8×107moleculecm-3, HO2 peaking at 1×109moleculecm-3 and the total RO2 concentration reaching 5.5×109moleculecm-3. OH reactivity (k(OH)) peaked at 89 s−1 during the night, with a minimum during the afternoon of ≈22s-1 on average. An experimental budget analysis, in which the rates of production and destruction of the radicals are compared, highlighted that although the sources and sinks of OH were balanced under high NO concentrations, the OH sinks exceeded the known sources (by 15 ppbv h−1) under the very low NO conditions (<0.5 ppbv) experienced in the afternoons, demonstrating a missing OH source consistent with previous studies under high volatile organic compound (VOC) emissions and low NO loadings. Under the highest NO mixing ratios (104 ppbv), the HO2 production rate exceeded the rate of destruction by ≈50ppbvh-1, whilst the rate of destruction of total RO2 exceeded the production by the same rate, indicating that the net propagation rate of RO2 to HO2 may be substantially slower than assumed. If just 10 % of the RO2 radicals propagate to HO2 upon reaction with NO, the HO2 and RO2 budgets could be closed at high NO, but at low NO this lower RO2 to HO2 propagation rate revealed a missing RO2 sink that was similar in magnitude to the missing OH source. A detailed box model that incorporated the latest Master Chemical Mechanism (MCM3.3.1) reproduced the observed OH concentrations well but over-predicted the observed HO2 under low concentrations of NO (<1 ppbv) and under-predicted RO2 (both the complex RO2 fraction and other RO2 types which we classify as simple RO2) most significantly at the highest NO concentrations. The model also under-predicted the observed k(OH) consistently by ≈10s-1 across all NOx levels, highlighting that the good agreement for OH was fortuitous due to a cancellation of missing OH source and sink terms in its budget. Including heterogeneous loss of HO2 to aerosol surfaces did reduce the modelled HO2 concentrations in line with the observations but only at NO mixing ratios <0.3 ppbv. The inclusion of Cl atoms, formed from the photolysis of nitryl chloride, enhanced the modelled RO2 concentration on several mornings when the Cl atom concentration was calculated to exceed 1×104atomscm-3 and could reconcile the modelled and measured RO2 concentrations at these times. However, on other mornings, when the Cl atom concentration was lower, large under-predictions in total RO2 remained. Furthermore, the inclusion of Cl atom chemistry did not enhance the modelled RO2 beyond the first few hours after sunrise and so was unable to resolve the modelled under-prediction in RO2 observed at other times of the day. Model scenarios, in which missing VOC reactivity was included as an additional reaction that converted OH to RO2, highlighted that the modelled OH, HO2 and RO2 concentrations were sensitive to the choice of RO2 product. The level of modelled to measured agreement for HO2 and RO2 (both complex and simple) could be improved if the missing OH reactivity formed a larger RO2 species that was able to undergo reaction with NO, followed by isomerisation reactions reforming other RO2 species, before eventually generating HO2. In this work an α-pinene-derived RO2 species was used as an example. In this simulation, consistent with the experimental budget analysis, the model underestimated the observed OH, indicating a missing OH source. The model uncertainty, with regards to the types of RO2 species present and the radicals they form upon reaction with NO (HO2 directly or another RO2 species), leads to over an order of magnitude less O3 production calculated from the predicted peroxy radicals than calculated from the observed peroxy radicals at the highest NO concentrations. This demonstrates the rate at which the larger RO2 species propagate to HO2, to another RO2 or indeed to OH needs to be understood to accurately simulate the rate of ozone production in environments such as Beijing, where large multifunctional VOCs are likely present.

The role of gold atom concentration in the formation of Cu–Au nanoparticles from the gas phase

The synthesis of bimetallic nanoparticles need to be controlled in order to obtain particles of a desired size, spatial structure, and chemical composition. In the synthesis of the Cu–Au nanoparticles studied here, nanoparticles can be obtained through either chemical or physical methods, each of which has its own drawbacks. Although it is very difficult to achieve the required target chemical composition of nanoparticles during chemical synthesis, their size can be stabilized quite well. In turn, physical synthesis methods mainly allow to maintain the required chemical composition; however, the size of the resulting particles varies significantly. To solve this issue, we studied the formation of Cu–Au nanoparticles with different chemical compositions from a gaseous medium using computer molecular dynamics (MD) simulation. The aim was to determine the effect of the concentration of gold atoms on the size and on the actual chemical composition of the formed bimetallic nanoparticles. The modeled region had a cubic shape with a face length of 1350 Bohr radii and contained a total of 91125 copper and gold atoms uniformly distributed in space. Thus, based on the results of the MD simulation, it was concluded that an increase in the percentage of gold atoms in the initial vapor phase led to a decrease in the size of the synthesized nanoparticles. In addition, it was found that clusters with a size of more than 400–500 atoms, regardless of the chemical composition of the initial vapor phase, basically corresponded to a given target composition.

The role of gold atom concentration in the processes of formation of Cu-Au nanoparticles from the gas phase

Currently, bimetallic nanoparticles have been widely used in various fields of nanotechnology, but the main area of their application continues to be the catalysis of chemical reactions. However, it soon became clear that the catalytic activity of nanoalloys very much depends on their size, chemical composition, and shape. Therefore, the question of controlling the synthesis of bimetallic nanoparticles to obtain particles of the desired size, spatial structure, and chemical composition is very acute. In the synthesis of the Cu-Au studied by us, nanoparticles can occur either through chemical or physical methods, each of which has its own drawbacks. Though it is very difficult to achieve the required target chemical composition of nanoparticles during chemical synthesis, their size can be stabilized quite well. In turn, physical synthesis methods mainly allow you to withstand the required chemical composition, but the size of the resulting particles varies significantly. To solve this contradiction, we studied the formation of Cu-Au nanoparticles of different chemical compositions from a gaseous medium using computer Molecular Dynamics simulation to determine the effect of the concentration of gold atoms on the size and actual chemical composition of the formed bimetallic nanoparticles.

NOx Emissions and Nitrogen Fate at High Temperatures in Staged Combustion

Staged combustion is an effective technology to control NOx emissions for coal-fired boilers. In this paper, the characteristics of NOx emissions under a high temperature and strong reducing atmosphere conditions in staged air and O2/CO2 combustion were investigated by CHEMKIN. A methane flame doped with ammonia and hydrogen cyanide in a tandem-type tube furnace was simulated to detect the effects of combustion temperature and stoichiometric ratio on NOx emissions. Mechanism analysis was performed to identify the elementary steps for NOx formation and reduction at high temperatures. The results indicate that in both air and O2/CO2 staged combustion, the conversion ratios of fuel-N to NOx at the main combustion zone exit increase as the stoichiometric ratio rises, and they are slightly affected by the combustion temperature. The conversion ratios at the burnout zone exit decrease with the increasing stoichiometric ratio at low temperatures, and they are much higher than those at the main combustion zone exit. A lot of nitrogen compounds remain in the exhaust of the main combustion zone and are oxidized to NOx after the injection of a secondary gas. Staged combustion can lower NOx emissions remarkably, especially under a high temperature (≥1600 °C) and strong reducing atmosphere (SR ≤ 0.8) conditions. Increasing the combustion temperature under strong reducing atmosphere conditions can raise the H atom concentration and change the radical pool composition and size, which facilitate the reduction of NO to N2. Ultimately, the increased OH/H ratio in staged O2/CO2 combustion offsets part of the reducibility, resulting in the final NOx emissions being higher than those in air combustion under the same conditions.

The Comparison of Nitrogen Atom Solubility in the Formation of Surface Layer on Stainless Steel and Alloy Steel

The chrome alloy has better affinity with nitrogen atom in nitriding process than any other alloy elements in steel. The higher content of chrome element binds larger number of nitrogen atoms. However, the higher concentration of nitrogen atom on the surface of the steel does not often make thicker case depth. This study clarified the phenomenon. The nitriding process was performed in two stages, namely the boost stage in the fluidized bed with the composition of 80 % NH3 and 20 % of N2 in 4 hours at 550 °C. Subsequently, the diffusion process was carried out in fluidized bed with the gas composition of 100 % N2 HP (high purity) in 2 hours. The chemical composition was measured by spectrometry and EDAX. The case depth was identified by micro-Vickers, and metallography was observed by SEM. The concentration of nitrogen atom on the surface of AISI 316 L is twice higher than that on the AISI 4140. The result shows that 0.1 to 1 wt % of nitrogen atom creates diffusion layer, 1 to 5 wt % of nitrogen atom produces nitride layer of γꞌ and ε, and nitrogen atom above 5 wt % forms white layer. The layer strongly depends on the percentage of nitrogen atom concentration. The nitrogen atom concentration is determined by the concentration of chromium element within the steel (AISI 316 L). Meanwhile, the depth of nitrogen atom diffusion is highly determined by the alloy element of Fe (AISI 4140).

Thermal expansion coefficient of BCC defective substitutional alloy AB with interstitial atom C

The analytic expressions for the free energy, the concentration of equilibrium vacancies, the isothermal compressibility, the thermal expansion coefficient and the heat capacities at constant volume and at constant pressure of BCC substitutional alloy AB with interstitial atom C and with vacancy under pressure are derived by the statistical moment method. The theoretical results are applied to the thermal expansion coefficient of alloy FeCrSi in the interval of temperature from 700 K to 1100 K, in the interval of substitutional atom concentration from 0% to 10%, in the interval of interstitial atom concentration from 0% to 5% and in the interval of pressure from 0 GPa to 10 GPa. Our calculated results for main metal Fe are compared with the experimental data.

Образование дефектов в структурах GaAs с непокрытой и покрытой пленкой AlN поверхностями при имплантации ионов азота и последующем отжиге

The concentration profiles of defects produced in structures upon the implantation of nitrogen ions into GaAs epitaxial layers with an uncovered surface and that covered with an AlN film and subsequent annealing are studied. The ion energies and the implantation doses are chosen so that the nitrogen-atom concentration profiles coincided in structures of both types. Rutherford proton backscattering spectra are measured in the random and channeling modes, and the concentration profiles of point defects formed are calculated for the samples under study. It is found that the implantation of nitrogen ions introduces nearly the same number of point defects into structures of both types, and the formation of an AlN film by ion-plasma sputtering is accompanied by the formation of an additional number of defects. However, the annealing of structures of both types leads to nearly the same concentrations of residual defects.