Presence Of Carbon Impurities Research Articles

Ion composition of nitrogen plasma jet with carbon admixture

With the aim of studying nitrogen plasma flow impacting graphite surfaces, we investigated the evolution of a plasma jet moving from the nozzle of the plasma torch to the target. The motion is accompanied by a decrease in temperature from 10 to 7 kK with a loss of ≈ 60% of the deposited energy and an increase in the density of carbon-containing impurities with partial preservation of the local thermodynamic equilibrium. The main reactions are thermal processes of the formation and decomposition of molecules, dissociative recombination and recharging of nitrogen ions. In the presence of carbon impurities, this set is supplemented with the substitution reaction C + N2 ↔ CN + N, electron-collision processes of stepwise ionization and recombination, and a whole ion-molecular cascades of reloads and substitutions. It has been established that a small admixture of carbon (∼ 0.1%) manifests itself only in ion-molecular kinetics, increasing the electron concentration, determining the ionic composition of the plasma, and thereby violating the equilibrium between its neutral and charged components. This leads, in particular, to the recombination nonequilibrium distribution of carbon atoms over electronic states. Other carbon-containing impurities (CN and C2) are not significant.

Low Temperature Reactive Flux Growth of SiC and SiC-AlNusing Al-Si Flux

Al - 12% Si alloy system was used as nutrient flux to grow silicon carbide at low temperature by reactive flux growth. Thick films were grown below 900°C on a SiC substrate having Al-Si melt rotated with a speed of 30rpm in a graphite cruciblein nitrogen atmosphere. A constant 2 atmospheric pressure of nitrogen was used on the melt. Using longer soak time of the melt in the presence of carbon impurity, the destruction of dendritic morphology was observed that subsequently resulted into the formation of cellular, colony and facetted crystal morphology. Photoluminescence data from film showed SiC with higher band gap in the presence of aluminum nitride. Keywords: A1.Crystallites; A1. Nucleation; A1. Optical morphology; A1.Solidification; A2. Growth from melt; B2.Semiconducting silicon compound;

Atomic processes leading to asymmetric divertor detachment in KSTAR L-mode plasmas

The experimentally observed in/out detachment asymmetry in KSTAR L-mode plasmas with deuterium (D) fueling and carbon walls has been investigated with the SOLPS-ITER code to understand its mechanism and identify important atomic processes in the divertor region. The simulations show that the geometrical combination of a vertical, inner target with a short poloidal connection from the X-point to the target and a much longer outer divertor leg on an inclined target lead to neutral accumulation towards the outer target, driving the outer target detachment at lower upstream density than is required for the inner target. This is consistent with available Langmuir probe measurements at both target plates, although the inner target profile is poorly resolved in these plasmas and further experiments with corroborating diagnostics are required to confirm this finding. The pressure and power loss factors defined in the two-point model (Stangeby 2018 Plasma Phys. Control. Fusion 60 4; Kotov and Reiter 2009 Plasma Phys. Control. Fusion 51 115002; Stangeby and Sang 2017 Nucl. Fusion 57 056007; Moulton et al 2017 Plasma Phys. Control. Fusion 59 6) of the divertor scrape-off layer (SOL) and the sources contributing to the loss factors are calculated through post-processing of the SOLPS-ITER results. The momentum losses are mainly driven by plasma-neutral interaction and the power losses by plasma-neutral interaction and carbon radiation. The presence of carbon impurities in the simulation enhances the pressure and power dissipation compared to the pure D case. Carbon radiation is a strong power loss channel which cools the plasma, but its effect on the pressure balance is indirect. Reduction of the electron temperature indirectly increases the momentum loss and increasing the volumetric reaction rates which are responsible for the loss of momentum. As a result, the addition of carbon saturates the momentum and power losses in the flux tube at lower upstream densities, reducing the roll-over threshold of the upstream density. The relative strengths of the various mechanisms contributing to momentum and power loss depend on the radial distance of the SOL flux tubes from the separatrix (near/far SOL) and the target (inner/outer target). This is related to the strong D2 molecule accumulation near the outer strike point, which makes the deuterium gas density at the outer target 2–10 times higher than that at the inner target. A large portion of the recycled neutral particles from both targets reach and accumulate in the outer SOL, which is predominantly attributed to the target inclination and gap structure between the central and outboard divertors and hence to the impact of geometry. The accumulated neutrals enhance the reactions involving D2, which causes momentum and power loss.

The effect of hydrocarbon chemistry on sputtering in mixed Be–C–H materials

Molecular dynamics simulations of low energy sputtering from mixed Be–C surfaces have been performed to gain an insight into the evolution and response of plasma facing components in current and projected fusion devices. An analytical bond-order potential (ABOP) for the ternary Be–C–H system was used to model the interactions. It was found that the presence of carbon required a more advanced description including chemical effects between carbon and hydrogen. To facilitate this, a method was constructed which combines the Brenner hydrocarbon potential [1] and the Be–C–H ABOP. To our knowledge this constitutes the first implementation of a method capable of simultaneously describing chemical hydrocarbon interactions and metallic bonding in molecular dynamics simulations. This model, which gives a more realistic description of low energy ion-surface interactions in the presence of carbon impurities, can also be extended to other fusion relevant mixed systems such as W–C–H.

Low resistance ohmic contacts annealed at 600 °C on a InAlN/GaN heterostructure with SiCl4‐reactive ion etching surface treatment

AbstractIn this paper, the effect of the surface treatments on the performance of Ti/Al/Ni/Au ohmic contacts to InAlN/GaN has been investigated by electrical characterization and by normalized Auger electron spectroscopy (AES). Contact resistance and specific contacts resistivity as low as 0.7 Ωmm and 1.1×10–7 Ωcm2, respectively, have been achieved by pre‐treatment using SiCl4 plasma followed by rapid thermal annealing at at 600 °C. This is compared with the case where no plasma pre‐treatment was performed. Contacts without pretreatment and annealed at at 600 °C were still rectifying and at the optimized annealing temperature of 800 °C reach 1.7 Ωmm contact resistance. Moreover, AES investigations showed that the presence of Carbon impurities on the InAlN surface has been significantly reduced after SiCl4 RIE pre‐treatment. That may account for improved ohmic contacts resistance and lowered optimised temperature of annealing for pre‐treated contacts. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of a poloidal electric field on neoclassical transport in a multispecies tokamak plasma

The effects of a poloidal potential variation of order ε, which is likely to be produced during high-power cyclotron wave heating or neutral beam injection, upon neoclassical particle transport and plasma current are studied theoretically, for a realistic tokamak plasma with significant impurity content. Using an approximate collision operator, an analytic procedure is employed to calculate the transport coefficients in the low collisionality regime for a large aspect ratio tokamak. In the presence of carbon impurity, the ion diffusion coefficients are generally found to increase by a factor of ∼2. Inclusion of the effects of a poloidal electric field is found to result in an increase in the bootstrap current if the potential on the outside of the tokamak is greater than that on the inside [as during ion cyclotron resonance heating (ICRH) or neutral beam injection (NBI)] and the density profiles are more peaked than roughly the square root of the temperature profiles.

Influence of sulfur on the processing map for hot working of nickel

It is well known that sulfur segregates to grain boundaries in nickel and causes hot shortness [1-3]. At lower sulfnr levels (2-20 ppm), the concentration of sulfur at grain boundaries is sufficient to cause high temperature embrittlement while at higher concentrations, embrittlement occurs even at lower temperatures [2,3]. The aim of the present study is to examine the effect of sulfur impurity on the processing map for hot working of nickel. For this purpose, processing maps have been developed on pure nickel with no trace of sulfur and on commercial purity nickel with 20 ppm sulfiar. Since a separate study has revealed that the processing map is also influenced by the presence of carbon impurity, the carbon content in the two different nickel materials is kept nearly the same (0.02 wt%). The processing maps are developed using the approach of the Dynamic Materials Model [4,5] which considers the workpiece under hot working conditions as a dissipator of power. The variation of the efficiency of power dissipation given by [2m/(m+l)], where m is the strain rate senstivity of flow stress with temperature and strain rate, constitutes the power dissipation map. In addition, a continuum criterion for the onset of instabilities is used in combination with the power dissipation map for avoiding an unstable flow regime during hot deformation [5,6].

Atomic and electronic structure of Fe films grown on Pd{001}

The atomic and electronic structure of Fe films grown on Pd{l brace}001{r brace} is investigated by means of low-energy electron diffraction and angle-resolved photoemission spectroscopy (ARPES). The films grow pseudomorphically, probably by way of nucleation and growth of flat islands, which ultimately coalesce to form continuous Fe{l brace}001{r brace} films. The structure of these continuous films, if grown at slow rates (of the order of 0.1 A/min), is body-centered tetragonal and is shown to be a distortion from the stable bcc structure of Fe: the in-plane lattice constant is 2.75 A, as dictated by the Pd{l brace}001{r brace} substrate, and the bulk interlayer spacing is 1.50--1.53 A. In 10--12-layer films the first interlayer spacing is expanded by 3.6% above bulk, but with increasing thickness that spacing contracts progressively to about 6.3% below the bulk value in 40--50-layer films. Films as thick as 60--70 layers can be grown pseudomorphically at slow rates despite the large misfit (4.2%) between bcc Fe{l brace}001{r brace} and fcc Pd{l brace}001{r brace}. ARPES data indicate that these films are electronically indistinguishable from bulk bcc Fe. Thick (about 200-layer) films grown at fast rates are essentially bcc, with in-plane lattice constants of 2.87 A, but with slightlymore » expanded (3%) interlayer spacing, attributed to the presence of carbon impurities.« less

ChemInform Abstract: Catalysis and Surface Phase Transformation

Abstract The possible influence of a magnetic phase transition on the catalytic activity has been discussed for many years in the literature. Molecular beam studies on the H 2 /D 2 exchange reaction on Ni single crystal surfaces demonstrated that an apparent effect can be caused by the presence of carbon impurities, but is absent with rigorously clean surfaces. Structural phase transformations of the surface occur upon interaction of CO with Pt(100). The rate of catalytic CO oxidation on this surface exhibits sustained temporal oscillations under proper conditions which are associated with these hex ⇌ 1× 1 phase transformations. These propagate in the form of waves across the surface and thus cause spatial self-organization.

Catalysis and surface phase transformation

The possible influence of a magnetic phase transition on the catalytic activity has been discussed for many years in the literature. Molecular beam studies on the H 2/D 2 exchange reaction on Ni single crystal surfaces demonstrated that an apparent effect can be caused by the presence of carbon impurities, but is absent with rigorously clean surfaces. Structural phase transformations of the surface occur upon interaction of CO with Pt(100). The rate of catalytic CO oxidation on this surface exhibits sustained temporal oscillations under proper conditions which are associated with these hex ⇌ 1× 1 phase transformations. These propagate in the form of waves across the surface and thus cause spatial self-organization.