Insulations for metallic glasses in pulse power systems

By modeling the ring-like molecule as a pearl necklace of freely jointed hard sphere, we develop a new equation of state (EOS) for the ring-like fluids on the basis of generalized Flory-Huggins (GFH) theory. Before proposing the new EOS of the ring-like fluids, we first modify the generalized Flory-Huggins theory for the chain fluids by incorporating a function related to the packing fraction into the insertion probability. The results indicate that the modified GFH EOS can predict the compressibility factors more accurately than the GFH EOS, especially for the intermediate and high packing fractions (η ≥ 0.157). Subsequently, the modified GFH theory-based EOS for the ring-like fluids is proposed. Compared to the Monte Carlo data of 3-mer, 4-mer, 5-mer, 6-mer, 16-mer, and 32-mer ring-like fluids, our EOS exhibits the best prediction among four EOSs for the compressibility factors at intermediate and high packing fractions (η ≥ 0.157), although our EOS also shows a slight underestimation for the compressibility factors at low packing fractions. In summary, this is the first report on the generalized Flory-Huggins theory-based EOS for the ring-like fluids. It is expected that the same strategy can be applied to these fluids with more complex architectures.

A novel coarse-grained computational model for associating polymers is proposed that is based on a Gaussian "blob" representation of the polymer chains. The model allows a large number of model polymers to be simulated at moderate computational cost over a wide packing fraction range using the Brownian dynamics, BD, technique. The attraction of the hydrophobic part of the polymer to those on other molecules can lead to strong aggregation of the polymer molecules in real systems, and this is included in the model by an attractive potential felt by the Gaussian blobs to a common "nodal" point that represents the center of the micelle. Attention here is confined to model AB diblock copolymers in which the hydrophilic block, A, has a much higher mass than the hydrophobic moiety, B, which leads to relatively small aggregation numbers, Nagg, of approximately 8. The aggregation number at low packing fractions is found to increase with packing fraction, as observed in experiments, with a functional form that closely follows a simple theory derived here that is based on entropy-derived mean-field terms for the free-energy change associated with the incorporation of the polymer molecule into the micelle. The computational model exhibits an extremely low critical micelle concentration (cmc), and micelles with Nagg approximately 5 are observed at the lowest packing fractions, phi, simulated ( approximately 10-4), which is consistent with experiment. The long-time self-diffusion coefficient of the polymers (and hence micelles) decreases logarithmically with packing fraction, and the viscosity increased with concentration according to the Huggins equation. The spherical blob coarse graining results in the simulable time scales being longer than the Rouse time of the chain, and hence for the nonassociating polymers the intrinsic viscosity is an input parameter in the model. The introduction of association leads to the partial inclusion of the intrinsic viscosity in the simulation and has an effect on the computed Huggins coefficient, kH, which is found to be approximately 6 in those cases.

Co_(78-x)PtxB_(10)Si₁₂ alloys were produced using the melt-spin process in order to study the crystallization behavior and ensuing magnetic properties of the Co_(78-x)PtxB_(10)Si₁₂ (Co-Pt) amorphous alloys as a function of the Pt content. We showed that when x ＞ 15 well below its stoichiometric composition, CoPt crystallized in the amorphous alloy, thus greatly altering the crystallized microstructure and magnetic properties during annealing. Below this composition, the main crystallization product was Co with Pt dissolved in its lattice. In spite of the nucleation of CoPt with high magnetic anisotropy, the highest coercivity was obtained when x was 15. It was also concluded that the Pt addition deteriorated the glass stability, triggering the devitrification at a progressively lower temperature.

Summary form only given. In many high current pulsed power systems much, if not all, of the energy is inductive or magnetic rather than electric and the electric fields are dominantly inductive rather than electrostatic. That is, in the usual expression for generalized electric field, E=-grad(V)-dA /dt, V is the scalar potential that gives rise to the electrostatic field, and A is the magnetic vector potential, from which the inductive field is derived. In problems where there are regions without voltage sources or charge separation, the electrostatic component does not exist, and the usual technique of solving the scalar Laplace's equation for the potential is inappropriate for determining the electric fields. Calculation of the magnetic vector potential is plagued by choice of gauge condition and specification of correct boundary conditions. Especially for coaxial (axisymmetric) systems typical of many pulsed power components and systems, where the current flow is in the (r,z) plane, there are two components of the vector potential that must be solved-each with its own boundary conditions. Specification of all the correct boundary conditions is non-trivial. In this paper, we present a convenient technique for the calculation of inductive electric fields in coaxial systems. The technique is based on the introduction of a vector electric potential that is derived from Poisson's equation, in combination with Faraday's Law and the E-D constitutive relation. In coaxial geometry, the electric vector potential is only azimuthal and, therefore, quasi-scalar. It is conveniently calculated with any two-dimensional Poisson equation solver, and the resultant inductive field distribution easily derived. We have used the technique in several pulsed power system designs with success. Specific examples of the application of the technique are given.

Sparkgap are most widely used closing switches in various high-voltage pulsed power systems and its reliable operation at desired voltage level is very essential. Conventionally by adjusting the filling gas pressure inside sparkgap switch, breakdown voltage level is altered but switching characteristics such as stability in hold-off voltage at various pressures, breakdown delay, plasma channel formation, and erosion rate are mainly dictated by adopted electrode profile and its dimensions, inter-electrode gap length and polarity. In this paper, experimental results obtained on breakdown characteristics of four different electrode geometries—Plane Parallel, Hemi-spherical, Bruce, and Rogowski and also a generalized criterion for fixing major dimensions of electrode and inter-gap length to ensure uniform electric field in the inter-electrode region are reported. All electrodes are of brass material and have common radius and thickness of 25 mm and 18 mm, respectively (surface finish <1 µm). Experiments performed on various electrode profiles in gap lengths of 2 mm to 5 mm range with pure nitrogen (N2) gas pressurization up to 50 psi reveal that among all profiles, Rogowski performs most reliably having stable hold-off voltage in wide operating range. Hold-off voltage magnitude and breakdown delay was commonly obtained higher for negative polarity in all trials. A comprehensive overview of experimental investigation reported herein compares suitability of various electrode profiles and polarity for reliable switching.

The collective behavior of self-propelled particles (SPPs) under the combined effects of a circularly patterned substrate and circular confinement is investigated through coarse-grained molecular dynamics simulations of polarized and disjoint ring polymers. The study is performed over a wide range of values of the SPPs packing fraction ϕ[over ¯], motility force F_{D}, and area fraction of the patterned region. At low packing fractions, the SPPs are excluded from the system's center and exhibit a vortical motion that is dominated by the substrate at intermediate values of F_{D}. This exclusion zone is due to the coupling between the driving force and torque induced by the substrate, which induces an outward spiral motion of the SPPs. For high values of F_{D}, the SPPs exclusion from the center is dominated by the confining boundary. At high values of ϕ[over ¯], the substrate pattern leads to reversals in the vorticity, which become quasiperiodic with increasing ϕ[over ¯]. We also found that the substrate pattern is able to separate SPPs based on their motilities.

We study gelation in suspensions of model colloidal particles with short-ranged attractive and long-ranged repulsive interactions by means of three-dimensional fluorescence confocal microscopy. At low packing fractions, particles form stable equilibrium clusters. Upon increasing the packing fraction the clusters grow in size and become increasingly anisotropic until finally associating into a fully connected network at gelation. We find a surprising order in the gel structure. Analysis of spatial and orientational correlations reveals that the gel is composed of dense chains of particles constructed from face-sharing tetrahedral clusters. Our findings imply that dynamical arrest occurs via cluster growth and association.

Directed self-assembly of nanoparticles using topographical templates has demonstrated great capabilities of ordering particles at their maximum packing fraction resulting from template confinement effects and free energy minimization. However, to self-assemble nanostructures at a lower packing fraction with a precise control over particle's positioning is challenging due to the high entropy of such a system. Here, by fabricating templates of irregular cavities together with appropriate choice of solvent, we demonstrate the positioning of 8 nm Au nanoparticles within individual cavities at a low filling factor. In addition to the first-order of ordering dictated by the template topography, there is a second-order of ordering induced by the interplay between the evaporation of the residual solvent trapped within the cavities and their intrinsic geometries. The experimental results show that the cavities shaped as equilateral triangles exhibit a random positioning of the particles at the corners; in comparison, right-angled scalene (or irregular) triangles show a more controllable positioning of the particles within the corners of the smallest angle. Finally, this technique has been successfully used to fabricate arrays of dimers with a controllable center-to-center distances at sub-5 nm length scales.

When initial conditions are uniformly random, numerical simulations of Motility-Induced Phase Separation (MIPS) in active suspensions with average packing fractions above a specific (spinodal) threshold require relatively short run times. The question remains of how phase separation can be achieved within a feasible run time at lower packing fractions (binodal region). We demonstrate that defects functioning as attractive chemotactic centers can rapidly initiate the formation of local particle aggregates, which, under certain conditions, can detach and grow until phase separation is fully realized. Here, we provide a detailed investigation of defect-triggered MIPS in the binodal region. We suggest that this mechanism may facilitate the aggregation of bacteria into biofilms, with nutrient particles acting as chemotactic attractors.

Nonequilibrium active matter made up of self-driven particles with short-range repulsive interactions is a useful minimal system to study active matter as the system exhibits collective motion and nonequilibrium order-disorder transitions. We studied high-aspect-ratio self-propelled rods over a wide range of packing fractions and driving to determine the nonequilibrium state diagram and dynamic properties. Flocking and nematic-laning states occupy much of the parameter space. In the flocking state, the average internal pressure is high and structural and mechanical relaxation times are long, suggesting that rods in flocks are in a translating glassy state despite overall flock motion. In contrast, the nematic-laning state shows fluidlike behavior. The flocking state occupies regions of the state diagram at both low and high packing fraction separated by nematic-laning at low driving and a history-dependent region at higher driving; the nematic-laning state transitions to the flocking state for both compression and expansion. We propose that the laning-flocking transitions are a type of glass transition that, in contrast to other glass-forming systems, can show fluidization as density increases. The fluid internal dynamics and ballistic transport of the nematic-laning state may promote collective dynamics of rod-shaped micro-organisms.

Structure and density of transition metal-metalloid amorphous alloys

Summary form only given. We report studies of mini (6.3 mm diameter electrodes) back-lighted thyratron (BLT) switches, operating with currents up to 10 kA, and hold off voltages in excess of 40 kV for the mini-BLT. The pseudospark is a thyratron with a cathode variant: the cathode emission is provided by an initially hollow cathode mode that transitions to a self-heated super-emissive mode. As a result of this mechanism, the pseudospark does not require externally supplied power for heating the cathode. The BLT is an optically-triggered version of this switch. The switch studies reported here are of (optically triggered) BLTs with helium or hydrogen fill. Hold-off voltage and optical triggering studies of these small switches suggest that they can be optimized for miniaturized high voltage pulse generation. Overvolted operation was also studied for potential applications such as implementation into Marx generators. These studies suggest that the small pseudospark BLT represents progress towards ultra compact high peak power switches for compact repetitive pulsed power applications. Research to develop sealed versions will be reported, and a new plasma source based from the BLT structure will also be discussed.

Magnetic field annealing of FeCo‐based amorphous alloys to enhance thermal stability and Curie temperature

High-voltage pulsed power is nowadays a very important research field and an expanding industrial technology with major worldwide economical impact, this technology being used in different applications (food processing, medicine…). For the development of PEF technology, it is necessary to have a wide variation interval of the different pulse parameters (amplitude, pulse width, frequency …). Conventional pulse generators do not allow it, prompting the researchers to focus on the development of pulse generators to make them more versatile to be used in different applications. With the development of solid-state electronics, solid-state devices are becoming more and more suitable for pulsed power application. They could provide the pulsed power systems with compactness, reliability, high repetition rate, and long life time. The rising of pulsed power generators using solid-state devices eliminates limitations of conventional components, and promises pulsed power technology to be widely used in commercial applications. However, solid-state switching devices such as Insulated Gate Bipolar Transistor (IGBT) available now are only rated up to a few kilo Volts. Most of pulsed power systems demand much higher voltage ratings. The aim of the present work is the development of a Marx modulator which is a unique circuit intended for voltage multiplication. Its functioning is based on charging capacitors in parallel and discharging them in series into the treatment chamber. The Marx bank developed comprises four stages, was constructed using 1200 V IGBTs and 10 µF capacitors. This generator is versatile; several factors could be varied such as the voltage level up to 3.6 kV, the repetition rate and the pulse width.

Atomic structure, thermal stability and isothermal crystallization kinetics of novel Co-based metallic glasses with excellent soft magnetic properties