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Subsurface deuterium bubble formation in W due to low-energy high flux deuterium plasma exposure
No full textThe deuterium (D) bubbles formed in W exposed to high flux D plasma were researched by scanning electron microscopy and transmission electron microscopy. After D plasma exposure at 500 K and 1000 K, a layer of nano-sized bubbles were homogenously distributed in W subsurface region. The D bubbles were homogenously nucleated due to the high D concentration, and the nucleation process is not related to the vacancy defects. At low temperature (500 K), D bubbles can grow by surface blistering, which caused different nano scale morphologies on different surfaces. At high temperature (1000 K), D bubbles mainly grow by vacancy clustering, which caused pinholes on the surface.</p
Step-type and step-density influences on CO adsorption probed by reflection absorption infrared spectroscopy using a curved Pt(1 1 1) surface
Get PDFIn comparison to flat single crystals, the continuous variation of structure provided by curved crystals offers many benefits for the study of physical and chemical processes at surfaces. However, the curvature of the surface also creates experimental challenges. For infrared spectroscopy, in particular, adsorbates on metal samples are typically probed by grazing-incidence reflection-absorption infrared spectroscopy (RAIRS). In this geometry, a convex crystal acts as a strongly diverging mirror. The authors describe how the experimental difficulties introduced by a cylindrical surface can be resolved for RAIRS. A complementary mirror, placed directly downfield of the curved crystal within the vacuum chamber, minimizes the divergence created by the sample. By simply translating the infrared focus across the sample, the authors probe adsorbate vibrational spectra as a function of local step-type and step-density with high sensitivity and spatial resolution. Time-consuming sample exchange, and the concomitant sample-to-sample experimental errors, are eliminated. The authors apply this new technique to carbon monoxide adsorption on a curved Pt(1 1 1) crystal and use it to resolve the influence of step-type and step-density on the CO stretch vibration as a function of coverage.</p
Special issue: Plasma Conversion
No full textWith growing concern of energy and environmental issues, the combination of plasma and heterogeneous catalysts receives special attention in greenhouse gas conversion, nitrogen fixation and hydrocarbon chemistry. Plasma gas conversion driven by renewable electricity is particularly important for the electrification of the chemical industry, which is currently dominated by water electrolysis, producing CO2-free renewable hydrogen. Meanwhile, electrochemical conversion of CO2, CH4 and other gaseous materials has yet to be successful because the material and energy conversion efficiency as well as productivity of the electrochemical processes greatly depend on the electrolyte material in terms of the solubility of gaseous materials and mass transport capability, which needs to be dramatically improved. As for fuel processing, the majority of reactions are categorized as uphill (endothermic) reactions where energy input is indispensable due to the conservation of energy. Ideally, such energy should be supplied by low temperature heat while plasma-generated reactive species promote initiation reactions. One promising approach is the use of vibrationally excited molecules which enhance the probability of chemisorption onto the catalyst surfaces. The role of vibrational species has been well investigated through molecular beam studies. However, the way to create such species has not been studied yet. Non-thermal plasma creates an oversaturated amount of vibrational species even at low temperature, showing great promises towards low temperature plasma catalysis of stable molecules. Furthermore, vibrational species uniquely create reaction pathways in homogeneous reactions via stepwise energy transfer gradually populating higher vibrational levels which eventually leads to the dissociation of stable molecules such as CO2. The special issue entitled “Plasma Conversion” consists of three review articles and 11 original research papers, focusing on the state-of-the-art plasma technology that is used for enhancing homogeneous/heterogeneous plasma catalytic conversion of greenhouse gases, nitrogen fixation and hydrocarbon chemistry. Review articles include the new catalyst synthesis approach using nonthermal plasma processes and the potential application of such unique catalysts is introduced (Rahimpour et al.) [1]. CH4 and CO2 reforming is one of the hot topics in plasma catalysis and research has been conducted mostly by experimental approaches. Bogaerts et al. discuss CH4 reforming and CO2 splitting and various mixtures from a modeling perspective, paying special attention to plasma chemistry and the role of plasma-generated vibrationally excited molecules [2]. Reactions involving vibrationally excited species need to be explored further and a numerical approach is truly beneficial providing insightful information towards the reaction mechanisms. Liquefaction of CH4 is to produce higher hydrocarbons via CH4 polymerization at ambient conditions (not for cryogenic methane condensation) [3]. Carbon containing liquid fuels such as gasoline and diesels produced via renewable electricity improve transportability and storage of renewable energy. Two research papers are related to heterogeneous plasma catalysis of CH4 and CO2 where nonthermal plasma is combined with Ni-supported reforming catalysts [4,5]. Research was conducted experimentally and plasma induced synergistic effects are highlighted. Van Rooij and Bongers discuss the homogeneous CO2 conversion with warm discharge, or low pressure microwave plasma [6,7]. Special emphasis on the vibrationally excited CO2 chemistry and CO2 dissociation by the latter mechanism is discussed. A novel heterogeneous plasma catalysis approach of CO2, where nonthermal plasma is combined with an oxygen permeating solid electrolyte membrane, is presented by Mori [8]. O2 is extracted directly from the plasma reaction field, and thus the CO2 conversion and energy efficiency were greatly improved. Moreover, CO2 microwave plasma diagnostics by optical emission spectroscopy [9], diagnostics plasma-catalyst interfacial phenomena [10], and atomistic simulations of plasma catalytic interactions [11] are included in the collection of this special issue. Three other papers focus on ammonia synthesis with atmospheric pressure plasma combined with heterogeneous catalysts [12–14]. Finally we would like to thank all contributors to this special issue and the editorial staff of Plasma Processes and Polymers for their great support. We hope that this issue contributes to exploring the new frontiers of plasma chemistry, and also to promoting the interdisciplinary interaction among the plasma science, chemical catalysis and surface science community, as well as other science specialists.</p
Capacitively coupled hydrogen plasmas sustained by tailored voltage waveforms: vibrational kinetics and negative ions control
Get PDFA comprehensive hybrid model of a hydrogen capacitively coupled plasmas (CCP), including a detailed description of the molecular vibrational kinetics, has been applied to the study of the effect of tailored voltage waveforms (TVWs) on the production kinetics and transport of negative ions in these discharges. Two kinds of TVWs are considered, valleys-to-peaks and saw-tooth, with amplitude and slope asymmetry respectively. By tailoring the voltage waveform only, it is possible to exert substantial control over the peak density and position of negative ions inside the discharge volume. This control is particularly effective for saw-tooth waveforms. Insight into the mechanisms allowing this control is provided by an analysis of the model results. This reveals the roles of the vibrational distribution function and of the electron energy distribution and their correlations, as well as changes in the negative ion transport in the electric field when using different TVWs. Considering the chemical reactivity of H<sup>-</sup> ions, the possibility of a purely electrical control of the negative ion cloud in a reactor operating with a feedstock gas diluted by hydrogen may find interesting applications. This is the first study of vibrational kinetics in the context of TVWs in molecular gases.</p
Atomistic simulations of graphite etching at realistic time scales
No full textHydrogen-graphite interactions are relevant to a wide variety of applications, ranging from astrophysics to fusion devices and nano-electronics. In order to shed light on these interactions, atomistic simulation by Molecular Dynamics (MD) has been shown to be an invaluable tool. It suffers, however, from severe time-scale limitations. In this work we employ the recently developed Collective Variable-Driven Hyperdynamics (CVHD) method to hydrogen etching of graphite for varying inter-impact times up to a realistic value of 1 ms, which corresponds to a flux of ~1020 m-2s-1. The results show that the erosion yield, hydrogen surface coverage and species distribution are significantly affected by the time between impacts. This can be explained by the higher probability for C-C bond breaking due to the prolonged exposure to thermal stress and the subsequent transition from ion- to thermal-induced etching. This latter regime of thermal-induced etching - chemical erosion - is here accessed for the first time using atomistic simulations. In conclusion, this study demonstrates that accounting for long time-scales significantly affects ion bombardment simulations and should not be neglected in a wide range of conditions, in contrast to what is typically assumed
Sub-micrometer structure formation during spin coating revealed by time-resolved in situ laser and X-ray scattering
No full textSolution-processed thin polymer films have many applications, such as organic electronics and block-copolymer nanofabrication. These films are often made by spin coating a solution that contains one or more solids and can show different phase-separated structures. The formation mechanism of the droplet-like morphology is studied here by processing polystyrene (PS) and a fullerene derivative ([6,6]-phenyl-C71-butyric acid methyl ester, [70]PCBM) from o-xylene. The final structure consists of [70]PCBM droplets partially embedded in a PS-rich matrix showing interdomain distance of 100–1000 nm as determined from transmission electron microscopy and grazing incidence small angle X-ray scattering (GISAXS). To elucidate the formation of these morphologies in real time, ultrafast in situ GISAXS coupled with laser interferometry and laser scattering is performed during spin coating. In situ thickness measurements and laser scattering show that liquid–liquid phase separation occurs at ≈70 vol% solvent. Subsequently, in only 100–400 ms, almost dry [70]PCBM domains start to protrude from the swollen PS-rich matrix. These results are used to verify the ternary phase diagram calculated using Flory–Huggins theory. The discussed multitechnique approach can be applied to study fundamental aspects in soft matter such as phase separation in thin films occurring at very short time scales
Tearing mode formation induced by internal crash events at different β N
No full textTearing mode formation after internal crash events like sawteeth or fishbones is one of the most important MHD processes that results in a big island structure and associated confinement degradation. The process implies magnetic reconnection at the rational surface, which has been investigated in great detail in the ASDEX Upgrade tokamak. Using direct local measurements, it is found that the crash leads to the formation of an ideal kink mode with large saturated amplitude at the resonant surface immediately after the sawtooth crash. This kink mode transforms into a tearing mode on a much longer timescale than the crash itself. The ideal kink mode, formed at the resonant surface after the crash, provides the driving force for the magnetic reconnection. The conversion of the ideal kink mode into a tearing mode after the internal crash is similar for various values of plasma rotation and normalized pressure.</p
Rate constants of quenching and vibrational relaxation in the OH (A2Sigma+, v = 0, 1), manifold with various colliders
No full textDetachment evolution on the TCV tokamak
No full textDivertor detachment in the TCV tokamak has been investigated through experiments and modelling. Density ramp experiments were carried out in ohmic heated L-mode pulses with the ion ∇B drift directed away from the primary X-point, similar to previous studies [1]. Before the roll-over in the ion current to the outer strike point, C III and Dα emission from the outer leg recede slowly from the strike point toward the X-point, at a rate of ∼2.0 × 10−19 m/m−3 along the magnetic field as the electron temperature along the leg reduces with increasing density. Around the onset of detachment, the upstream density profile and outer target Dα profiles broaden, possibly leading to an increase in radiation in the SOL by increased interaction between the SOL and the carbon tiles lining the outer wall. The plasma conditions upstream and at various locations along the detached outer divertor leg have been characterised, and the consistency of this data has been checked with the interpretive OSM-EIRENE-DIVIMP suite of codes [2] and are broadly found to be consistent with measured Dγ/Dα emissivity profiles along the detached outer divertor leg.</p
Plasma-catalyst interaction studied in a single pellet DBD reactor: dielectric constant effect on plasma dynamics
No full textA novel single dielectric pellet DBD that is designed to facilitate studying the interaction between plasmas and catalysts is presented. The influence of material dielectric constant on plasma dynamics across a range of applied voltages is determined through the use of electrical characterisation combined with videos of the discharge. Different discharge modes in nitrogen are observed and their behaviour is characterised. A particular focus is given to the phenomenon known as \u27partial discharging\u27. This is where incomplete plasma formation occurs between the electrodes of the reactor, which may have implications for the fair testing of catalysts in packed bed reactors. Additionally, the occurrence of an \u27almond shaped\u27 QV plot in the event of point to-point discharging in PBRs is explained. This work provides easily implemented analytical techniques that can be applied to understand the behaviour of plasmas within packed bed DBD reactors