Non-conducting Boundaries Research Articles

Effect of high pressure-high temperature treatment on the microstructure and dielectric properties of cobalt doped СaСu3Тi4О12

The results of studying the effect of high pressures and high temperatures (8 GPa, 1273 K) used in the process of thermobaric treatment of CaCu3-xCoxTi4O12 ceramics (x = 0; 0.4; 0.6), initially prepared by solid-phase synthesis, on the structure and electrical characteristics are presented. The dielectric parameters were evaluated in a temperature interval (293–1000) K in a frequency range 1 Hz–32 MHz. It was established that the value of dielectric permittivity ε of samples upon thermobaric treatment is an order of magnitude higher than ε of samples prepared by solid-phase synthesis. The main reasons for the high values of dielectric permittivity are both the intragrain effects related, among other things, to hopping mechanism of polaron motion, and the polarization effects provided by the presence microstructure inhomogeneities. The latter are nonconducting boundaries of conducting grains and, in some examined materials, microscopic-scale surfaces of inhomogeneities, which appeared due to deformation during thermobaric treatment. At high temperatures, oxygen vacancies make the main contribution to the observed values of dielectric permittivity.

Peristaltic mechanism of hydromagnetic Jeffrey fluid having variable thermal conductivity and slip conditions

In this study, we focus on the slip effects on the peristaltic unsteady flow of magnatohydromagnetic Jeffrey fluid in a flow passage with non-conducting and flexible boundary walls. The effect of the magnetic field with varying thermal conductivity is taken under the influence of heat transfer analysis. The dimensionless system of PDEs is solved analytically, and the obtained results are computed for the temperature, pressure drop, the axial pressure gradient, axial velocity, and then these results are discussed for different values of the physical parameters of our interest. For the stream functions, the contour plots are also obtained which indicates the exact flow behavior within the flow channel, and the effects of the physical parameters on Jeffery fluid within the flow channel are discussed briefly. Our results indicate that the heat transfer coefficient decreases with an increase in thermal slip and velocity slip parameters. Furthermore, it shows that the size of the trapped bolus is greater for the inclined magnetic field as compared to the transverse magnetic field.

Individualized ablation strategy to treat persistent atrial fibrillation: Core-to-boundary approach guided by charge-density mapping

Noncontact charge-density mapping allows rapid real-time global mapping of atrial fibrillation (AF), offering the opportunity for a personalized ablation strategy. The purpose of this study was to compare the 2-year outcome of an individualized strategy consisting of pulmonary vein isolation (PVI) plus core-to-boundary ablation (targeting the conduction pattern core with an extension to the nearest nonconducting boundary) guided by charge-density mapping, with an empirical PVI plus posterior wall electrical isolation (PWI) strategy. Forty patients (age 62 ± 12 years; 29 male) with persistent AF (10 ± 5 months) prospectively underwent charge-density mapping-guided PVI, followed by core-to-boundary stepwise ablation until termination of AF or depletion of identified cores. Freedom from AF/atrial tachycardia (AT) at 24 months was compared with a propensity score-matched control group of 80 patients with empirical PVI + PWI guided by conventional contact mapping. Acute AF termination occurred in 8 of 40 patients after charge-density mapping-guided PVI alone and in 21 of the remaining 32 patients after core-to-boundary ablation in the study cohort, compared with 8 of 80 (10%) in the control cohort (P<.001). On average, 2.2 ± 0.6 cores were ablated post-PVI before acute AF termination. At 24 months, freedom from AF/AT after a single procedure was 68% in the study group vs 46% in the control group (P = .043). An individualized ablation strategy consisting of PVI plus core-to-boundary ablation guided by noncontact charge-density mapping is a feasible and effective strategy for treating persistent AF, with a favorable 24-month outcome.

Impact of In3+ ion substitution on microstructural, magnetic and dielectric responses of nickel–cobalt spinel ferrite nanocrystals

In this article, indium ion-doped mixed nickel–cobalt ferrite nanoparticles with chemical composition Ni0.7Co0.3InxFe2−xO4 (x = 0.00, 0.05, 0.10, 0.15 and 0.30) were synthesized using soft co-precipitation method. All prepared nanoferrites showed pure cubic spinel structure without containing any impurity phases as ensured by X-ray diffraction profiles. Mean diameters of nanocrystals were found within the range of 5 nm to 13 nm as estimated from Williamson–Hall (W–H) plots. Due to large size of In3+ ions, the compressive microstrain inside the nanocrystals became tensile in nature. Both the coercive field and saturation magnetization were reduced with increasing In3+ ion concentration in nickel–cobalt ferrite nanoparticles. Overall effective anisotropy was noticed to decrease with In3+ ion doping. This is due to the diamagnetic response of indium ions and weakening of magnetic interactions. Hopping of electrons between sublattice sites was the charge conduction process for all nanoferrites as confirmed by the AC conductivity measurement. Single semi-circular arc was observed in Cole–Cole plot for all samples which also revealed that the non-conductive grain boundaries was most effective in comparison to grains in dielectric responses. Decrement in the radius of semi-circles with respect to pristine sample also indicated the enhancement in DC conductivity with indium doping.

Application of rotating circular obstacles in improving ferrofluid heat transfer in an enclosure saturated with porous medium subjected to a magnetic field

In this paper, the evaluation of the heat transfer rate and fluid flow in an enclosure with rotating circular obstacles has been studied. The enclosure is filled with a porous medium and subjected to the magnetic field. Fe3O4/water nanofluid has been used to simulate the effect of magnetism. The finite volume method has been applied to solve the equations. To velocity–pressure coupling, the SIMPLE algorithm has been applied. The influence of magnetism on the enclosure in the conductive and non-conductive boundaries along the magnetic field has been investigated. The streamlines and isotherm-lines contours in the conductive and non-conductive boundaries along the magnetic field, the dimensionless angular velocities of the circular obstacles, and their direction have been obtained. The results show that the different temperature cases of the circular obstacles and their direction play an essential role in the flow and heat transfer. The highest and lowest heat transfer rates occur in cold circular obstacles and hot circular obstacles, respectively. Also, the composition of the porous medium and the magnetic field show different behaviors at the heat transfer rate.

Elastic distortion determining conduction in BiFeO3 phase boundaries.

It is now well-established that boundaries separating tetragonal-like (T) and rhombohedral-like (R) phases in BiFeO3 thin films can show enhanced electrical conductivity. However, the origin of this conductivity remains elusive. Here, we study mixed-phase BiFeO3 thin films, where local populations of T and R can be readily altered using stress and electric fields. We observe that phase boundary electrical conductivity in regions which have undergone stress-writing is significantly greater than in the virgin microstructure. We use high-end electron microscopy techniques to identify key differences between the R–T boundaries present in stress-written and as-grown microstructures, to gain a better understanding of the mechanism responsible for electrical conduction. We find that point defects (and associated mixed valence states) are present in both electrically conducting and non-conducting regions; crucially, in both cases, the spatial distribution of defects is relatively homogeneous: there is no evidence of phase boundary defect aggregation. Atomic resolution imaging reveals that the only significant difference between non-conducting and conducting boundaries is the elastic distortion evident – detailed analysis of localised crystallography shows that the strain accommodation across the R–T boundaries is much more extensive in stress-written than in as-grown microstructures; this has a substantial effect on the straightening of local bonds within regions seen to electrically conduct. This work therefore offers distinct evidence that the elastic distortion is more important than point defect accumulation in determining the phase boundary conduction properties in mixed-phase BiFeO3.

Conductive AFM study of the local current in thin ferroelectric sol-gel PZT films

The local current distribution across the grains and their boundaries in ferroelectric PZT films are studied using conductive AFM. The films were formed with various grain structures and different lead excess content by chemical solution deposition. C-AFM measurements have shown the influence of the lead excess and seed sublayer crystallization conditions on the grain-boundary conductivity. PZT films with fixed 0-15 wt% Pb excess demonstrate non-conductive grain boundaries, whereas in films with fixed 30 wt% Pb excess the grain-boundary conductivity is found to be much higher than that of the grains themselves. Conductive grain-boundaries was also found in PZT films without lead excess in crystallized sublayer. A study of the transient current at varied preliminary polarization revealed the current peaks in the current-voltage curves. The appearance conditions and magnitude of these peaks depend on the grain-boundary conductivity. The correlation between the grain-boundary conduction and the current polarization dependences is confirmed.

P978Core to block: a new ablation strategy for treating persistent atrial fibrillation

Abstract Background Ablation strategy targeting the core of atrial fibrillation (AF) rotors alone can lead to both stabilisation and destabilisation of rotational activity. Non-contact dipole density mapping system is designed to rapidly identify dynamic regional atrial activation patterns of interest (API) during AF. Purpose To assess the feasibility of an ablation strategy consisted of pulmonary vein isolation + targeting the core of APIs followed by linear ablation to the nearest non-conducting boundary to treat persistent AF. Methods The ablation strategy includes: 1. Antral pulmonary vein electrical isolation (APVI); 2. APIs (focal, localised rotational and localised irregular activation; Figure1) detection by dipole density mapping; 3. API core ablation followed by linear ablation to the nearest non-conduction barrier (APVI/mitral valve); 4. repeat step 2 and 3 in LA (RA, if necessary) until sinus rhythm is achieved. Results Consecutive 40 persistent AF patients (mean 62±12 years, 29 males, AF duration 10±4 months, LA diameter 42±9 mm) were included from 2 centres. An average of 2.0±0.7 APIs per patient were targeted post-APVI. Acute AF termination by ablation was achieved in 27/40 (68%) patients. The mean ablation time of APVI and “Core to block” was 33±12 mins and 31±22 mins, respectively. No major complication occurred. During a mean follow-up of 12±5 months, 32/40 patients (80%) maintained sinus rhythm. Conclusion APVI + “Core to block” guided by the dipole density mapping is feasible for treating persistent AF. A larger randomised study is needed to test the effectiveness of this ablation strategy. Acknowledgement/Funding None

Mechanisms for the Termination of Atrial Fibrillation by Localized Ablation: Computational and Clinical Studies.

Human atrial fibrillation (AF) can terminate after ablating localized regions, which supports the existence of localized rotors (spiral waves) or focal drivers. However, it is unclear why ablation near a spiral wave tip would terminate AF and not anchor reentry. We addressed this question by analyzing competing mechanisms for AF termination in numeric simulations, referenced to clinical observations. Spiral wave reentry was simulated in monodomain 2-dimensional myocyte sheets using clinically realistic rate-dependent values for repolarization and conduction. Heterogeneous models were created by introduction of parameterized variations in tissue excitability. Ablation lesions were applied as nonconducting circular regions. Models confirmed that localized ablation may anchor spiral wave reentry, producing organized tachycardias. Several mechanisms referenced to clinical observations explained termination of AF to sinus rhythm. First, lesions may create an excitable gap vulnerable to invasion by fibrillatory waves. Second, ablation of rotors in regions of low-excitability (from remodeling) produced re-entry in more excitable tissue allowing collision of wavefront and back. Conversely, ablation of rotors in high-excitability regions migrated spiral waves to less excitable tissue, where they detached to collide with nonconducting boundaries. Third, ablation may connect rotors to nonconducting anatomic orifices. Fourth, reentry through slow-conducting channels may terminate if ablation closes these channels. Limited ablation can terminate AF by several mechanisms. These data shed light on how clinical AF may be sustained in patients' atria, emphasizing heterogeneities in tissue excitability, slow-conducting channels, and obstacles that are increasingly detectable in patients and should be the focus of future translational studies.

Consistent boundary conditions at nonconducting surfaces of planetary bodies: Applications in a new Ganymede MHD model

AbstractThe interaction of planetary bodies with their surrounding magnetized plasma can often be described with the magnetohydrodynamic (MHD) equations, which are commonly solved by numerical models. For these models it is necessary to define physically correct boundary conditions for the plasma mass and energy density, the plasma velocity, and the magnetic field. Many planetary bodies have surfaces whose electrical conductivity is negligibly small and thus no electric current penetrates their surfaces. Magnetic boundary conditions, which consider that the associated radial electric current at the planetary surface is zero, are difficult to implement because they include the curl of the magnetic field. Here we derive new boundary conditions by a decomposition of the magnetic field in poloidal and toroidal parts. We find that the toroidal part of the magnetic field needs to vanish at the surface of the insulator. For the spherical harmonics coefficients of the poloidal part, we derive a Cauchy boundary condition, which also matches a possible intrinsic field by including its Gauss coefficients. Thus, we can additionally include planetary dynamo fields as well as time‐variable induction fields within electrically conductive subsurface layers. We implement the nonconducting boundary condition in the MHD simulation code ZEUS‐MP using spherical geometry and provide a numerical implementation in Fortran 90 as supporting information on the JGR website. We apply it to a model for Ganymede's plasma environment. Our model also includes a consistent set of boundary conditions for the other MHD variables density, velocity, and energy. With this model we can describe Galileo spacecraft observations in and around Ganymede's minimagnetosphere very well.

MHD flow past a circular cylinder using the immersed boundary method

The immersed boundary method (IB hereafter) is an efficient numerical methodology for treating purely hydrodynamic flows in geometrically complicated flow-domains. Recently Grigoriadis et als. [1] proposed an extension of the IB method that accounts for electromagnetic effects near non-conducting boundaries in magnetohydrodynamic (MHD) flows. The proposed extension (hereafter called MIB method) integrates naturally within the original IB concept and is suitable for magnetohydrodynamic (MHD) simulations of liquid metal flows. It is based on the proper definition of an externally applied current density field in order to satisfy the Maxwell equations in the presence of arbitrarily-shaped, non-conducting immersed boundaries. The efficiency of the proposed method is achieved by fast direct solutions of the two poisson equations for the hydrodynamic pressure and the electrostatic potential. The purpose of the present study is to establish the performance of the new MIB method in challenging configurations for which sufficient details are available in the literature. For this purpose, we have considered the classical MHD problem of a conducting fluid that is exposed to an external magnetic field while flowing across a circular cylinder with electrically insulated boundaries. Two- and three-dimensional, steady and unsteady, flow regimes were examined for Reynolds numbers Re d ranging up to 200 based on the cylinder’s diameter. The intensity of the external magnetic field, as characterized by the magnetic interaction parameter N, varied from N = 0 for the purely hydrodynamic cases up to N = 5 for the MHD cases. For each simulation, a sufficiently fine Cartesian computational mesh was selected to ensure adequate resolution of the thin boundary layers developing due to the magnetic field, the so called Hartmann and sidewall layers. Results for a wide range of flow and magnetic field strength parameters show that the MIB method is capable of accurately reproducing integral parameters, such as the lift and drag coefficients, as well as the geometrical details of the recirculation zones. The results of the present study suggest that the proposed MIB methodology provides a powerful numerical tool for accurate MHD simulations, and that it can extend the applicability of existing Cartesian flow solvers as well as the range of computable MHD flows. Moreover, the new MIB method has been used to carrry out a series of accurate simulations allowing the determination of asymptotic laws for the lift and drag coefficients and the extent of the recirculation length as a function of the amplitude of the magnetic field. These results are reported herein.

Plasma Cathode for a Short-Pulse Dielectric Wall Accelerator

The Beam Research Program at Lawrence Livermore National Laboratory is continuing development of the dielectric wall accelerator (DWA), a type of accelerator which uses stacked pulse-forming lines (PFLs) to apply an accelerating field directly to the beam through a nonconducting vacuum boundary. Here, we report operation of a DWA as an electron diode using a surface flashover plasma cathode. Peak perveances in excess of 6 times 10-6A/V3/2 were measured, with current extraction and pulse train format depending on flashover source timing and PFL switching speed.

Immersed boundary method for the MHD flows of liquid metals

Wall-bounded magnetohydrodynamic (MHD hereafter) flows are of great theoretical and practical interest. Even for laminar cases, MHD simulations are associated with very high computational cost due to the resolution requirements for the Hartmann and side layers developing in the presence of solid obstacles. In the presence of turbulence, these difficulties are further compounded. Thus, MHD simulations in complex geometries are currently a challenge. The immersed boundary (IB hereafter) method is a reliable numerical tool for efficient hydrodynamic field simulations in arbitrarily geometries, but it has not yet been extended for MHD simulations. The present study forms the first attempt to apply the IB methodology for the computation of both the hydrodynamic and MHD fields. A consistent numerical methodology is presented that is appropriate for efficient 3D MHD simulations in geometrically complicated domains using cartesian flow solvers. For that purpose, a projection scheme for the electric current density is presented, based on an electric potential correction algorithm. A suitable forcing scheme for electric density currents in the vicinity of non-conducting immersed surfaces is also proposed. The proposed methodology has been first extensively tested for Hartmann layers in fully-developed and developing channel and duct flows at Hartmann numbers Ha = 500 - 2000 . In order to demonstrate the potential of the method, the three-dimensional MHD flow around a circular cylinder at Reynolds number Re = 200 is also presented. The effects of grid resolution and variable arrangement on the simulation accuracy and consistency were examined. When compared with existing numerical or analytic solutions, excellent agreement was found for all the cases considered. The proposed projection and forcing schemes for current densities were found capable of satisfying the charge conservation law in the presence of immersed non-conducting boundaries. Finally, we show how the proposed methodology can be used to extend the applicability of existing flow solvers that use the IB concept with a staggered variable arrangement.

Preparation and field emission property of nanodiamond-cluster-embedded diamondlike carbon film

The authors prepared nanodiamond-cluster-embedded diamondlike carbon (DLC) composite films by electrophoresis deposition followed by magnetic filtered cathodic vacuum arc plasma deposition. Nanodiamond clusters are uniformly embedded into the DLC films. The authors investigated the location of the emission sites with nanometer scale accuracy and the emission mechanism using scanning probe microcopy (SPM) based technique. These composite films show better field emission properties than the as-deposited DLC film and as-deposited nanodiamond clusters. It is revealed that electrons are emitted primarily from the nonconducting boundaries surrounding the embedded nanodiamond cluster. Thus, by depositing nanodiamond clusters, an emission site may be predetermined; this is very useful in fabricating nanoelectron sources.

Shortening of cardiac action potential duration near an insulating boundary

It is known, from both experiments and simulations, that cardiac action potentials are shortened near a non-conducting boundary. In the present paper, this effect is studied in a simple, two-current ionic model, with propagation restricted to a 1D fibre. An asymptotic approximation for the dependence of action potential duration on distance to the boundary is derived. This estimate agrees well with simulations.

Modeling signal and background components of electrosensory scenes

Weakly electric fish are able to detect and localize prey based on microvolt-level perturbations in the fish's self-generated electric field. In natural environments, weak prey-related signals are embedded in much stronger electrosensory background noise. To better characterize the signal and background components associated with natural electrolocation tasks, we recorded transdermal voltage modulations in restrained Apteronotus albifrons in response to moving spheres, tail bends, and large nonconducting boundaries. Spherical objects give rise to ipsilateral images with center-surround structure and contralateral images that are weak and diffuse. Tail bends and laterally placed nonconducting boundaries induce relatively strong ipsilateral and contralateral modulations of opposite polarity. We present a computational model of electric field generation and electrosensory image formation that is able to reproduce the key features of these empirically measured signal and background components in a unified framework. The model comprises an array of point sources and sinks distributed along the midline of the fish, which can conform to arbitrary body bends. The model is computationally fast and can be used to estimate the spatiotemporal pattern of activation across the entire electroreceptor array of the fish during natural behaviors.

Observations of structured and long-range transport in a large volume dusty (complex) plasma experiment

DUPLEX—the DUsty PLasma EXperiment at the Naval Research Laboratory—is a large volume dc glow discharge plasma device. The DUPLEX chamber is a transparent polycarbonate cylinder that is 40 cm in radius and 80 cm in height. Argon dc glow discharge plasmas are generated between a grounded cathode and a biased anode. The anode and cathode are separated by 15 to 20 cm. Clouds of 1 μm diameter alumina microparticles are suspended in the plasma—with the highest density directly above the cathode. However, particles have been detected throughout the entire plasma volume. This article reports on previously unidentified, and possibly unknown, phenomena observed in a dusty plasma. Two specific features will be the focus of this article. First, large microparticle clouds—up to 15 cm in diameter in some locations—with highly complex internal structures are identified. Second, long range—20 to 30 cm distances—periodic transport of microparticles from one region of the plasma to another is identified. It is believed that the large size and nonconducting boundary of the DUPLEX chamber facilitates the observation of these phenomena.

About the equations of non-stationary magneto-hydrodynamics with non-conducting boundaries

About the equations of non-stationary magneto-hydrodynamics with non-conducting boundaries

Catheter ablation of clinical intraatrial reentrant tachycardias resulting from previous atrial surgery: Localizing and transecting the critical isthmus

We sought to evaluate the efficacy of anatomically based radiofrequency catheter ablation for the treatment of intraatrial reentrant tachycardia in patients with previous atrial surgery. Intraatrial reentrant tachycardias, a common late complication of atrial surgery, are often refractory to standard medical management. Data from experimental animals and from humans indicate that anatomic barriers resulting from residual atrial scars provide a substrate for intraatrial reentry. We speculated that these tachycardias require a narrow isthmus of tissue between surgical scars and native nonconductive boundaries and that transection of this isthmus with radiofrequency ablation would therefore constitute an effective treatment. Fourteen patients with a history of atrial surgery and clinical intraatrial reentrant tachycardia underwent electrophysiologic testing. From activation mapping, putative surgical scars and patches that served as boundaries of reentrant circuits were identified. Radiofrequency lesions were then placed to transect the narrowest isthmus of conducting tissue between a surgical scar and an anatomic barrier. Catheter ablation was attempted only for tachycardias consistent with the patient's clinical arrhythmias. Radiofrequency catheter ablation was attempted for 17 (55%) of 31 tachycardias identified; it successfully terminated tachycardias in 13 (93%) of 14 patients (95% confidence interval [CI] 79% to 99%). There were clinical recurrences in six patients (46%, 95% CI 19% to 73%), each of whom underwent a repeat ablation that was successful. Twelve (86%) of 14 patients (95% CI 67% to 99%) have remained free of intraatrial reentrant tachycardia for a mean of 7.5 +/- 5.3 months. Anatomically guided radiofrequency catheter ablation is an effective technique for definitive management of intraatrial reentrant tachycardia in patients with previous atrial surgery.

ENERGY STABILITY OF A STATIONARY CONDUCTING THERMOMICROPOLAR FLUID LAYER UNDER THE INFLUENCE OF A MAGNETIC FIELD

The effect of an orthogonal applied magnetic field on an electrically conducting thermomicropolar fluid layer which is heated from below, is investigated. The linear stability and the nonlinear stability of the stationary state are considered when the fluid layer lies between two nonconducting rigid boundaries. The nonlinear stability study is performed for different cases of the Alfven No. A which depends on the strength of the magnetic field. A generalized energy is used for the general case A different from 1. From a variational and numerical analysis the nonlinear stability threshold is obtained and compared with the linear stability results and the nonconducting case. Both micro-rotation and magnetic field tend to stabilize the fluid layer.