Debye Length Scale Research Articles

N-Carbon-Doped Binary Nanophase of Metal Oxide/Metal-Organic Framework for Extremely Sensitive and Selective Gas Response.

Metal-organic frameworks (MOFs), which are highly ordered structures exhibiting sub-nanometer porosity, possess significant potential for diverse gas applications. However, their inherent insulative properties limit their utility in electrochemical gas sensing. This investigation successfully modifies the electrical conductivity of zeolitic imidazolte framework-8 (ZIF-8) employing a straightforward surface oxidation methodology. A ZIF-8 polycrystalline layer is applied on a wafer-scale oxide substrate and subjects to thermal annealing at 300°C under ambient air conditions, resulting in nanoscale oxide layers while preserving the fundamental properties of the ZIF-8. Subsequent exposure to NO2 instigates the evolution of an electrically interconnected structure with the formation of electron-rich dopants derived from the decomposition of nitrogen-rich organic linkers. The N-carbon-hybridized ZnO/ZIF-8 device demonstrates remarkable sensitivity (≈130ppm-1 ) and extreme selectivity in NO2 gas detection with a lower detection limit of 0.63ppb under 150°C operating temperature, surpassing the performance of existing sensing materials. The exceptional performances result from the Debye length scale dimensionality of ZnO and the high affinity of ZIF-8 to NO2 . The methodology for manipulating MOF conductivity through surface oxidation holds the potential to accelerate the development of MOF-hybridized conductive channels for a variety of electrical applications.

Effect of Debye length scale surface features on electro-osmosis and its use to devise a novel electro-microfluidic separation

Numerically validated analytical predictions for electro-osmosis over a charged surface decorated with a nanoscale groove pattern are developed for the situation when the electrical double layer thickness is comparable to the spatial period of the grooves. For the analytical predictions, the groove shape can be specified by any continuous periodic function, such as the triangular, trapezoidal, and sinusoidal waveforms, which are investigated as special cases. We discover that the classical Helmholtz–Smoluchowski expression for electrokinetic mobility, notwithstanding its widespread use in measurements, is rendered invalid by the presence of Debye-length-scale unevenness in the surface topography. Furthermore, we use the depth-resolved anisotropic response of oblique grooves to design and optimize a novel electro-microfluidic strategy for separating constituents of a nano-particulate mixture.

Enhanced triethylamine sensing performance of superfine NiO nanoparticles decoration by two-dimensional hexagonal boron nitride

The novel two-dimensional hexagonal boron nitride (h-BN) decorated nickel oxide (NiO) heterojunction was successfully synthesized by a facile solvothermal precipitation method combining with heat treatment. SEM and TEM analysis were used to corroborate the average size (~8 nm) and overall distribution of superfine NiO nanoparticles on h-BN. XRD, FT-IR and XPS characterization confirmed the configuration of highly crystallinity and p-n heterojunction as well as the presence of surface oxygen vacancy defects. Gas sensing test results revealed that the decoration of h-BN could significantly enhanced triethylamine (TEA) sensing property of NiO. The main contribution of such remarkable results lies in NiO nanoparticles that are close to Debye length scale were embedded on vacancy defects of functionalized h-BN nanosheets, which can optimize sensitivity and selectivity by taming two-dimensional (2D) interfacial electronic effects that strongly affect nonmetal-support interaction between grain boundaries. Meanwhile, the formation of p-n Schottky nanoscale heterojunction between NiO and h-BN can significantly enlarge resistance variation and efficiently promoted the adsorbed triethylamine molecules to oxidize into NO2, H2O, and CO2. Our work highlights the important role of coupling functionalized h-BN in gas sensors, which can also provide a valuable avenue in boosting the sensing performance.

Richtmyer-Meshkov instability of an imploding flow with a two-fluid plasma model

The two-fluid (ions and electrons) plasma Richtmyer-Meshkov instability of a cylindrical light/heavy density interface is numerically investigated without an initial magnetic field. Varying the Debye length scale, we examine the effects of the coupling between the electron and ion fluids. When the coupling becomes strong, the electrons are restricted to co-move with the ions and the resulting evolution is similar to the hydrodynamic neutral fluid case. The charge separation that occurs between the electrons and ions results in self-generated electromagnetic fields. We show that the Biermann battery effect dominates the generation of magnetic field when the coupling between the electrons and ions is weak. In addition to the Rayleigh-Tayler stabilization effect during flow deceleration, the interfaces are accelerated by the induced spatio-temporally varying Lorentz force. As a consequence, the perturbations develop into the Rayleigh-Taylor instability, leading to an enhancement of the perturbation amplitude compared with the hydrodynamic case.

Modelling the inclusion of swelling pressure in a tissue level poroviscoelastic model of cartilage deformation.

Swelling pressure in the interstitial fluid within the pores of cartilage tissue is known to have a significant effect on the rheology of cartilage tissue. The swelling pressure varies rapidly within thin regions inside pores known as Debye layers, caused by the presence of fixed charge, as observed in cartilage. Tissue level calculation of cartilage deformation therefore requires resolution of three distinct spatial scales: the Debye lengthscale within individual pores; the lengthscale of an individual pore; and the tissue lengthscale. We use asymptotics to construct a leading order approximation to the swelling pressure within pores, allowing the swelling pressure to be systematically included within a fluid-solid interaction model at the level of pores in cartilage. We then use homogenization to derive tissue level equations for cartilage deformation that are very similar to those governing the finite deformation of a poroviscoelastic body. The equations derived permit the spatial variations in porosity and electric charge that occur in cartilage tissue. Example solutions are then used to confirm the plausibility of the model derived and to consider the impact of fixed charge heterogeneity, illustrating that local fixed charge loss is predicted to increase deformation gradients under confined compression away from, rather than at, the site of loss.

Visualization of plasma fluxes by a measurement of own rotation of a dust particle

A research of own rotation of single dust particle which is in a dust trap in the stratified glow discharge under imposed magnetic field is carried out. It is experimentally shown that in the scale of Debye length an action of azimuthal torque from the fluxes of ions and electrons is mutually compensated; an angular velocity of rotation of dusty particle remains invariable up to magnetic fields of 200 G. The found effect was given theoretical interpretation on a basis of consideration of ambipolar diffusion of positive ions and electrons to the particle surface.

Collision frequency and mean free path for plasmas described by kappa distributions

The paper shows the surprising result of the universality of the mean-free-path formulation for plasmas described by kappa distributions. In particular, the ratio of the mean-free-path over the Debye length depends only on the Debye number – the number of particles within the Debye length scale. The derived formula appears to be identical for the cases of Maxwell-Boltzmann and kappa distributed plasmas; thus, it is not explicitly depended on kappa – the parameter that labels and governs the kappa distributions. The key aspect of these results is the specific functional dependence of the mean-free-path on the polytropic index; this is inextricably linked to the dependence of the Debye length on the polytropic index. The importance of the dependence of the mean-free-path on the polytropic index is demonstrated with applications in the solar wind plasma.

Electron Phase‐Space Holes in Three Dimensions: Multispacecraft Observations by Magnetospheric Multiscale

AbstractElectron phase‐space holes are kinetic plasma structures commonly observed in space plasmas on Debye length scales. Near the Earth's duskside flank at 10 Earth radii, a series of 32 electron holes (EHs) are detected within a 1‐s window on all four Magnetospheric Multiscale spacecraft. The spacecraft separation of <7 km is similar to the expected EH size in this region. Length, width, amplitude, and relative positions are determined for individual EHs using a cylindrically symmetric model fit to Magnetospheric Multiscale E field measurements. The model shows good agreement with observed E fields far from the EH center. Deviations in E⊥ from the model are present near the center, indicating observed EHs have complex, sometimes irregular, internal structure. Perturbation magnetic fields δB modeled assuming an E × B0 electron current reproduce the measured parallel perturbation in most cases, although there is a systematic variation due to geometric and finite gyroradius effects. Many EHs in this event have large amplitude for their size, reaching the theoretical lower limit in length parallel to the background magnetic field, which requires the electron phase‐space density to approach 0 in the center. It is possible that EHs of this type have recently formed, eventually weakening or becoming longer over time. This study provides the most detailed measurements of EHs to date. Their derived properties are largely in agreement with expectations from previous research. It remains unclear whether the few notable differences are due to rapid time evolution or are specific to the local environment.

Spatial distribution of mechanical forces and ionic flux in electro-kinetic instability near a permselective membrane

This paper is devoted to the numerical investigation of electro-kinetic instability in a polarization layer next to a cation-exchange membrane. An analysis of some properties of the electro-kinetic instability is followed by a detailed description of the fluid flow structure and of the spatial distribution of the ionic flux. In this aim, the Stokes-Poisson-Nernst-Planck equation set is solved until the Debye length scale. The results show that the potential threshold of the marginal instability and the current density depend on the logarithm of the concentration at the membrane surface. The size of the stable vortices seems to be an increasing function of the potential drop. The fluid motion is induced by the electric force along the maximum concentration in the extended space charge (ESC) region and by the pressure force in the region around the inner edge of the ESC layer. Two spots of kinetic energy are located in the ESC region and between the vortices. The cationic motion, controlled by the electric field and the convection, presents counter-rotating vortices in the stagnation zone located in the fluid ejection region. The anion transport is also characterized by two independent layers which contain counter-rotating vortices. The first one is in contact with the stationary reservoir. In the second layer against the membrane, the convection, and the electric field control, the transversal motion, the Fickian diffusion, and the convection are dominant in the longitudinal direction. Finally, the longitudinal disequilibrium of potential and pressure along the membrane is analyzed.

Phase-Field Modeling of the Thermal Growth of an Oxide Film and the Electrochemical Performance of a Solid Oxide Fuel Cell: Similarities and Differences

The thermal growth of oxide film on top of a metal surface can be viewed as an electrochemical process and unless at very thin-film stage, the growth kinetics is largely controlled by mass transport (ionic) across the oxide layer. The latter process is also essential to solid oxide fuel cells (SOFCs). Differences are that for the former, the ionic transport is the slower the better for the sake of oxidation resistance, while for the latter, the faster the better in terms of cell performance. From the modeling perspective, both the processes involve a concern regarding the characteristic length scale that is the Debye length. We present an electrochemical phase-field model for thermal oxidation together with a multi-scale scheme which continuously models oxide film growth across orders of magnitude in length around the Debye length. On the other hand, phase-field modeling regarding SOFCs is approached at two length scales at and above the Debye-length scale. Issues in oxidation modeling especially the coupled-currents condition and local equilibrium, and their counterparts in SOFC modeling will be discussed.

A theoretical investigation of size-dependent composition and stresses in nanowires and nanotubes of ceria

In the past few decades, nanostructures of ceria have received significant attention for potential applications in a variety of technologies. Experiments have frequently shown that physical properties of ceria nanomaterials are strongly size-dependent. Here, by accounting for the highly non-linear coupling between electrical, chemical and mechanical driving forces, we develop a continuum model which allows us to investigate equilibrium distribution of defects in nanowires and nanotubes of ceria. It is shown that the model predicts strongly size-dependent non-stoichiometric composition, lattice constant and surface stresses in ceria nanostructures whose diameter is comparable with the Debye's length scale of the problem.

UV-enhanced hydrogen sensor based on nanocone-assembled 3D SnO2 at low temperature

In this paper, nanocone-assembled hedgehog-like SnO2 was synthesized via a facile hydrothermal process. Once under UV irradiation, the response of the as-prepared 3D SnO2 towards 100ppm H2 at low temperature (50°C) was improved almost 3.5 times (S=9.5) over that in dark. The enhancement may be attributed to the co-effect from three aspects: (1) the heavy electron depletion induced by oxygen chemisorption due to the Debye length scale dimensions of nanocones; (2) vast reaction sites and sufficient diffusion spaces provided by the 1D–3D configuration; (3) the possibility that the refreshed photoinduced oxygen ions on the surface of SnO2 are more energetic than chemisorbed oxygen ions and promote the photo-catalytic reaction of H2 even at low temperature.

Multi-scale periodic homogenization of ionic transfer in cementitious materials

A multi-scale periodic homogenization procedure of the ionic transfers in saturated porous media is proposed. An application on a multi-scale porous material was achieved for establishing models describing a ionic transfer from Nernst–Planck–Poisson–Boltzmann system. The first one is obtained by homogenization from the scale of Debye length to the capillary porosity scale, by taking into account the electrical double layer phenomenon. The second one results from another homogenization procedure from the capillary porosity scale to the scale of the material, where the electrical double layer effects are naturally negligible. A numerical parametric study is conducted on three dimensional elementary cells in order to highlight the effects of the electrical double layer on the ionic transfer parameters. Comparisons with existing experimental data are also presented and discussed. The double homogenization procedure gives homogenized diffusion coefficients very close to those obtained experimentally for chlorides ions from electrodiffusion tests carried out in laboratory.

Laboratory investigation of lunar surface electric potentials in magnetic anomaly regions

AbstractTo gain insight into lunar surface charging in the magnetic anomaly regions, we present the results of laboratory experiments with a flowing plasma engulfing a magnetic dipole field above an insulating surface. When the dipole moment is perpendicular to the surface, large positive potentials (close to ion flow energies in eV) are measured on the surface in the dipole lobe regions, charged by the unmagnetized ions while the electrons are magnetically excluded. The potential decreases exponentially with distance from the surface on the ion (flow) Debye length scale. The surface potentials become much smaller when the dipole moment is parallel to the surface, likely due to collisionality. We discuss the implications of our laboratory results for the lunar surface charging in the magnetic anomaly regions, suggesting that the surface potential may be much higher than the generally expected several volts positive due to photoemission.

Uniform derivation of Coulomb collisional transport thanks to Debye shielding

The effective potential acting on particles in plasmas being essentially the Debye-shielded Coulomb potential, the particles collisional transport in thermal equilibrium is calculated for all impact parameters b, with a convergent expression reducing to Rutherford scattering for small b. No cutoff at the Debye length scale is needed, and the Coulomb logarithm is only slightly modified.

Connection coefficients for cold plasma wave propagation near metallic surfaces

Sheaths tend to form when immersing metallic objects in plasmas. As it avoids the need to capture the sheath details, which occur on the Debye length scale while antennas are typically various orders of magnitude larger, the sheath boundary condition due to D'Ippolito and Myra (2006 Phys. Plasmas 13 102508, 2008 Phys. Plasmas 15 102501) offers antenna designers a major reduction in the numerical problem size they face. The sheath boundary condition was derived by making a number of simplifying assumptions to enable finding an analytical approximation of the conditions rapidly oscillating waves have to satisfy beyond the sheath that forms close to such objects. This paper discusses the solution of the cold plasma wave equation for sheath relevant density profiles, e.g. highlighting the role of the orientation of the static magnetic field and of oblique incidence, and underlining the impact the density profile has on the wave physics. It illustrates that the cross-talk between the waves impinging on and those excited at the wall and in the sheath sensitively depends on a number of parameters. The 2 × 2 connection coefficient matrix that is numerically obtained captures the sheath region fast time scale wave physics for a given density profile. When supplemented with a satisfactory model for the slow time scale variation it is a numerical tool that permits upgrading the realism of the fast time scale wave physics contained in the sheath boundary condition and that can help delimiting the range of applicability of simplified models, and assessing if a sufficiently general set of boundary conditions to describe the effect of the sheath can at all be constructed.

Comment on “Undamped electrostatic plasma waves” [Phys. Plasmas 19, 092103 (2012)

The relevance of linear “corner modes” for the description of coherent electrostatic structures, as proposed by Valentini et al. [Phys. Plasmas 19, 092103 (2012)], is questioned. Coherency in their on-dispersion simulation is instead found to be caused by particle trapping in agreement with Schamel's nonlinear wave model [Phys. Plasmas 19, 020501 (2012)]. The revealed small amplitude structures are hence of cnoidal electron hole type exhibiting vortices in phase space. They are ruled by trapping nonlinearity rather than by linearity or quasi-linear effects, as commonly assumed. Arguments are presented, which give preference to these cnoidal hole modes over Bernstein-Greene-Kruskal modes. To fully account for a realistic theoretical scenario, however, at least four ingredients are mandatory. Several corrections of the conventional body of thought about the proper kinetic wave description are proposed. They may prove useful for the general acceptance of this “new” nonlinear wave concept concerning structure formation, updating several prevailing concepts such as the general validity of a linear wave Ansatz for small amplitudes, as assumed in their paper. It is conjectured that this nonlinear trapping model can be generalized to the vortex structures of similar type found in the more general setting of driven turbulence of magnetized plasmas. They appear as eddies in both, the phase and the position spaces, embedded intermittently on the Debye length scale.

High-temperature correlation functions: Universality, extraction of exchange interactions, divergent correlation lengths, and generalized Debye length scales

We derive a universal form for the correlation function of general n component systems in the limit of high temperatures or weak coupling. This enables the extraction of effective microscopic interactions from measured high temperature correlation functions. We find that in systems with long range interactions, there exist diverging correlation lengths with amplitudes that tend to zero in the high temperature limit. For general systems with disparate long range interactions, we introduce the notion of generalized Debye length (and time) scales and further relate it to the divergence of the largest correlation length in the high temperature (or weak coupling) limit.

Equation Free Projective Integration: A multiscale method applied to a plasma ion acoustic wave

Multiscale problems such as magnetic reconnection and turbulence are notoriously hard to simulate because the physics of micro and macroscales are strongly linked. This study is the first application of a novel numerical scheme called Equation Free Projective Integration (EFPI) to a plasma system: we simulate the propagation and steepening of a 1D ion acoustic wave. In EFPI, the simulations act on two scales, an “inner” microscale and an “outer” coarse scale. The long timescale dynamic behavior of the system is determined by extrapolating forward estimates of the coarse scales obtained from short duration simulations of the microscale dynamics. There are no explicit closed-form equations governing the coarse scales, hence the name “Equation Free.” In this study, a kinetic particle-in-cell code called P3D is used to simulate the microscale dynamics. We find excellent agreement between P3D and EFREE, the EFPI code. The differences which occur between the two are not due to numerical accuracy issues, but instead are caused by an incomplete representation of the system at coarse scales. More specifically, as the ion acoustic wave becomes a shock with Debye length scales, the assumptions of quasi-neutrality and maxwellian ions become invalid. Generalizing the “outer” coarse representation of the system to include non-maxwellian ions and Debye length structures should minimize the differences between P3D and EFREE. The largest EFREE run in this study showed a speedup of about 13 times over its P3D counterpart. Remarkably, the speedup of EFREE over P3D scales linearly with system size: the larger the simulation, the greater the speedup. The results indicate that EFPI is a potential candidate for simulating multiscale plasma problems.

Structure of the separatrix region close to a magnetic reconnection X‐line: Cluster observations

We use Cluster spacecraft observations to study in detail the structure of a magnetic reconnection separatrix region on the magnetospheric side of the magnetopause about 50 ion inertial lengths away from the X‐line. The separatrix region is the region between the magnetic separatrix and the reconnection jet. It is several ion inertial lengths wide and it contains a few subregions showing different features in particle and wave data. One subregion, a density cavity adjacent to the separatrix, has strong electric fields, electron beams and intense wave turbulence. The separatrix region shows structures even at smaller scales, for example, solitary waves at Debye length scale. We describe in detail electron distribution functions and electric field spectra in the separatrix region and we compare them to a numerical simulation. Our observations show that while reconnection is ongoing the separatrix region is highly structured and dynamic in the electric field even if the X‐line is up to 50 ion inertial lengths away.