Abstract: Packed bed dielectric barrier discharge (PB-DBD) plasma reactors are very promising for various plasma catalysis applications, but the exact mechanisms of plasma-catalyst interaction are far from understood, because the plasma discharge and catalyst/packing properties are mutually dependent. To better understand the effect of packing dielectric material on the electrical plasma properties, we study here a single bead DBD plasma reactor operating in dry air, with beads of different dielectric constant and for different applied voltages, by means of fluid modelling validated by optical imaging experiments. Our study reveals that the plasma in the single bead DBD reactor can manifest itself in two different modalities, i.e. (a) polar discharges at the bead poles in contact with the electrodes, and (b) a streamer discharge caused by surface ionization waves, which bridges the gas gap. Beads with high dielectric constant result in localised electric field enhancement and hence yield a reduction of the applied voltage required for plasma production. At low applied voltage, the discharge appears as polar discharges between the bead and the electrodes, and upon higher voltage it undergoes a transition into a bridging streamer discharge. The transition voltage to the streamer mode rises for beads with higher dielectric constant. These observations are important for plasma catalysis applications. A higher dielectric constant yields a higher electric field and thus higher average electron energy and density, giving rise to more reactive species, but it also yields a confined discharge near the contact points of packing beads, limiting the interaction area between the catalyst and the active plasma species. In addition, our model reveals that the dielectric bead behaves as a capacitor and traps charges, which can explain the significant occurrence of partial discharging in PB-DBDs and non-parallelogram shaped Lissajous plots. Hence, equivalent circuit modelling of PB-DBDs should take into account the role of packing beads in charge trapping as a capacitor.

Abstract: The discharge properties of the plasma bulk flow near the surface of heated compound-materials strongly affects the kinetic layer parameters modeled and manifested in the Knudsen layer. This paper extends the widely used two-layer kinetic ablation model to the ablation controlled non-equilibrium discharge due to the fact that the local thermodynamic equilibrium (LTE) approximation is often violated as a result of the interaction between the plasma and solid walls. Modifications to the governing set of equations, to account for this effect, are derived and presented by assuming that the temperature of the electrons deviates from that of the heavy particles. The ablation characteristics of one typical material, polytetrafluoroethylene (PTFE) are calculated with this improved model. The internal degrees of freedom as well as the average particle mass and specific heat ratio of the polyatomic vapor, which strongly depends on the temperature, pressure and plasma non-equilibrium degree and plays a crucial role in the accurate determination of the ablation behavior by this model, are also taken into account. Our assessment showed the significance of including such modifications related to the non-equilibrium effect in the study of vaporization of heated compound materials in ablation controlled arcs. Additionally, a two-temperature magneto-hydrodynamic (MHD) model accounting for the thermal non-equilibrium occurring near the wall surface is developed and applied into an ablation-dominated discharge for an electro-thermal chemical launch device. Special attention is paid to the interaction between the non-equilibrium plasma and the solid propellant surface. Both the mass exchange process caused by the wall ablation and plasma species deposition as well as the associated momentum and energy exchange processes are taken into account. A detailed comparison of the results of the non-equilibrium model with those of an equilibrium model is presented. The non-equilibrium results show a non-equilibrium region near the plasma-wall interaction region and this indicates the need for the consideration of the influence of the possible departure from LTE in the plasma bulk on the determination of ablation rate.

Abstract: A gliding arc plasma is a potential way to convert CO2 into CO and O2, due to its non-equilibrium character, but little is known about the underlying mechanisms. In this paper, a self-consistent two-dimensional (2D) gliding arc model is developed, with a detailed non-equilibrium CO2 plasma chemistry, and validated with experiments. Our calculated values of the electron number density in the plasma, the CO2 conversion and energy efficiency show reasonable agreement with the experiments, indicating that the model can provide a realistic picture of the plasma chemistry. Comparison of the results with classical thermal conversion, as well as other plasma-based technologies for CO2 conversion reported in literature, demonstrates the non-equilibrium character of the gliding arc, and indicates that the gliding arc is a promising plasma reactor for CO2 conversion. However, some process modifications should be exploited to further improve its performance. As the model provides a realistic picture of the plasma behaviour, we use it first to investigate the plasma characteristics in a whole gliding arc cycle, which is necessary to understand the underlying mechanisms. Subsequently, we perform a chemical kinetics analysis, to investigate the different pathways for CO2 loss and formation. Based on the revealed discharge properties and the underlying CO2 plasma chemistry, the model allows us to propose solutions on how to further improve the
CO2 conversion and energy efficiency by a gliding arc plasma.

Abstract: The conversion of atmospheric nitrogen into valuable compounds, that is, so-called nitrogen fixation, is gaining increased interest, owing to the essential role in the nitrogen cycle of the biosphere. Plasma technology, and more specifically gliding arc plasma, has great potential in this area, but little is known about the underlying mechanisms. Therefore, we developed a detailed chemical kinetics model for a pulsed-power gliding-arc reactor operating at atmospheric pressure for nitrogen oxide synthesis. Experiments are performed to validate the model and reasonable agreement is reached between the calculated and measured NO and NO2 yields and the corresponding energy efficiency for NOx formation for different N2/O2 ratios, indicating that the model can provide a realistic picture of the plasma chemistry. Therefore, we can use the model to investigate the reaction pathways for the formation and loss of NOx. The results indicate that vibrational excitation of N2 in the gliding arc contributes significantly to activating the N2 molecules, and leads to an energy efficient way of NOx production, compared to the thermal process. Based on the underlying chemistry, the model allows us to propose solutions on how to further improve the NOx formation by gliding arc technology. Although the energy efficiency of the gliding-arc-based nitrogen fixation process at the present stage is not comparable to the world-scale HaberBosch process, we believe our study helps us to come up with more realistic scenarios of entering a cutting-edge innovation in new business cases for the decentralised production of fertilisers for agriculture, in which low-temperature plasma technology might play an important role.

Abstract: The laser-modulated magnetopolaron effect on shallow donors in semiconductors is investigated in the presence of a magnetic field in the Voigt configuration. A nonperturbative approach is used to describe the electron-photon interaction by including the radiation field in an exact way via a laser-dressed interaction potential. Through a variational approach we evaluate the donor binding energy. We find that the interaction strength of the laser-dressed Coulomb potential in the z direction cannot only be enhanced but also weakened by the radiation field, while that in the x-y plane is only weakened. In this way, the binding energy of the states with odd z parity, like 2p(z) can be decreased or increased with respect to its static binding energy by the radiation field, while that of the other states can be only decreased. Furthermore, all binding energies become insensitive to the magnetic field if the radiation field is strong. The magnetopolaron effect on these energies is studied within second-order time-dependent perturbation theory. In the nonresonant region, a laser-modulated magnetopolaron correction, including the effect of single-photon processes, is observed. In the resonant region, a laser-modulated magnetopolaron effect, accompanied by the emission and absorption of a single photon, is found. Moreover, the 1s -> 2p(+) transition, accompanied by the emission of a single photon, is tuned by the radiation field into resonance with the longitudinal-optical phonon branch. This is electrically analogous to the magnetopolaron effect, and therefore we name it the dynamical magnetopolaron effect. Finally, by changing the frequency of the radiation field, these interesting effects can be tuned to be far away from the reststrahlen band and, therefore, can be detected experimentally. This in turn provides a direct measure of the electron-phonon interaction.

Abstract: A Ti-50.8at.%Ni wire produced using a co-drawing method and a commercial Ti-50.8at.%Ni wire were annealed at different temperatures between 450°C and 700°C. Grains with diameter less than 100nm were revealed by transmission electron microscopy for both wires before annealing treatment. However, the microstructural heterogeneity of the co-drawn wire is more obvious than that of the commercial wire. Multi-stage martensitic transformation was observed in the co-drawn wire, compared with the one-stage A↔M transformation in the commercial wire after annealing at 600°C for 30min. The differences of total elongation, plateau strain and pseudoelastic recoverable strain between the commercial wire and the co-drawn wire were also observed. The differences of the transformation behavior and mechanical properties between the commercial wire and the co-drawn wire are attributed to the microstructural difference between these two wires.

Abstract: A cold-drawn Ti50.8 at.% Ni wire was annealed at 600 °C for 30 min, followed by aging at 250 °C for different times. A microstructure with small grains and nanoscaled precipitates was obtained. The thermally induced martensite transformation is suppressed in the samples aged for 4 h or longer, leaving a one-stage R-phase transition between −150 and +150 °C. The transformation behavior, work output and recovery stress associated with the R-phase transition are presented.

Abstract: <script type='text/javascript'>document.write(unpmarked('The motion of Abrikosov vortices in type-II superconductors results in a finite resistance in the presence of an applied electric current. Elimination or reduction of the resistance via immobilization of vortices is the \u0022holy grail\u0022 of superconductivity research. Common wisdom dictates that an increase in the magnetic field escalates the loss of energy since the number of vortices increases. Here we show that this is no longer true if the magnetic field and the current are applied parallel to each other. Our experimental studies on the resistive behavior of a superconducting Mo0.79Ge0.21 nanostrip reveal the emergence of a dissipative state with increasing magnetic field, followed by a pronounced resistance drop, signifying a reentrance to the superconducting state. Large-scale simulations of the 3D time-dependent Ginzburg-Landau model indicate that the intermediate resistive state is due to an unwinding of twisted vortices. When the magnetic field increases, this instability is suppressed due to a better accommodation of the vortex lattice to the pinning configuration. Our findings show that magnetic field and geometrical confinement can suppress the dissipation induced by vortex motion and thus radically improve the performance of superconducting materials.'));

Abstract: Surface chemistry plays a pivotal role in regulating the morphology of nanoparticles, maintaining colloidal stability, and mediating the interaction with target analytes toward practical applications such as surface-enhanced Raman scattering (SERS)-based sensing and imaging. The use of a binary ligand mixture composed of 1,4-benzenedithiol (BDT) and hexadecyltrimethylammonium chloride (CTAC) to provide gold nanostars with long-term stability is reported. This is despite BDT being a bifunctional ligand, which usually leads to bridging and loss of colloidal stability. It is found however that neither BDT nor CTAC alone are able to provide sufficient colloidal and chemical stability. BDT-coated Au nanostars are additionally used as seeds to direct the encapsulation with a gold outer shell, leading to the formation of unusual nanostructures including semishell-coated gold nanostars, which are characterized by high-resolution electron microscopy and electron tomography. Finally, BDT is exploited as a probe to reveal the enhanced local electric fields in the different nanostructures, showing that the semishell configuration provides significantly high SERS signals as compared to other coreshell configurations obtained during seeded growth, including full shells.

Abstract: Surface and interface control is fundamentally important for crystal growth engineering, catalysis, surface enhanced spectroscopies, and self-assembly, among other processes and applications. Understanding the role of ligands in regulating surface properties of plasmonic metal nanocrystals during growth has received considerable attention. However, the underlying mechanisms and the diverse functionalities of ligands are yet to be fully addressed. In this contribution,
we report a systematic study of ligand-mediated interface control in seeded growth of gold nanocrystals, leading to diverse and exotic nanostructures with an improved surface enhanced Raman scattering (SERS) activity. Three dimensional transmission electron microscopy (3D TEM) revealed an intriguing gold shell growth process mediated by the bifunctional ligand 1,4-benzenedithiol (BDT), which leads to a unique crystal growth mechanism as compared to other ligands, and subsequently to the concept of interfacial energy control mechanism. Volmer-Weber growth mode was proposed to be responsible for BDT-mediated seeded growth, favoring the strongest interfacial energy and generating an asymmetric island growth pathway with internal crevices/gaps. This additionally favors incorporation of BDT at the plasmonic nanogaps, thereby generating strong SERS activity with a maximum efficiency for a core-semishell configuration obtained along seeded growth. Numerical modeling was used to explain this observation. Interestingly, the same strategy can be used to engineer the structural diversity of this system, by using gold nanoparticle seeds with various sizes and shapes, and varying the [Au3+]/[Au0] ratio. This rendered a series of diverse and exotic plasmonic nanohybrids such as semishell-coated gold nanorods, with embedded Raman-active tags and Janus surface with distinct surface functionalities.
These would greatly enrich the plasmonic nanostructure toolbox for various studies and applications such as anisotropic nanocrystal engineering, SERS, and high-resolution Raman bioimaging or nanoantenna devices.

Abstract: Nonlinear electron plasma waves driven by a finite-charged particle pulse or rigid object moving at relativistic speeds are investigated. Quasi-stationary smooth and spiky wake waves comoving with the object are found. Localized soliton-like solutions are also shown to exist. Relativistic effects tend to prevent their formation because of the electron mass increase. The application of the very-large-amplitude wake density waves as a source of ultrahigh-energy cosmic-ray events is discussed.