External DC Research Articles

Transient Analysis of High-Power Microwave Air Breakdown under External DC Magnetic Field

This article concentrates on the high-power microwave (HPM) breakdown process and behaviors in air-filled rectangular waveguide under the external dc magnetic field. To solve the persuasive issue, a three-dimensional (3-D) multiphysics model coupling full-wave Maxwell’s equations with plasma fluid equations is established and solved by the spectral-element time-domain (SETD) method. In our simulation, the Galerkin’s method is employed for the space discretization and the central difference scheme is used for the temporal discretization during the SETD process. The numerical results reveal that the external dc magnetic field applied in breakdown volume can significantly enhance the breakdown threshold to suppress the HPM breakdown, which is beneficial for the propagation of HPM pulse in air-filled waveguide. Our research provides theoretical guidance for suppressing the impact of HPM breakdown for HPM devices protection.

P1377Substrate-based ablation in patients with frequent appropriate ICD therapy and dilated cardiomyopathy: long-term experience with high-density mapping

Abstract Recurrent ventricular tachydisrhythmias (VT) episodes have a negative impact in the outcome of patients (P) already with an implantable cardioverter-defibrillator (ICD). Elimination of arrhythmic reentry circuits represents a difficult challenge, mainly due to the induction of intolerable VTs, with multiple ECG morphologies, requiring rapid interruption. Substrate guided ablation has been used as a promising approach strategy to treat recurrent VTs. Aim: to assess long-term results of a VT substrate-based ablation using high-density mapping in P with an ICD, severe left ventricular (LV) dysfunction and recurrent appropriate ICD therapy. Methods: 16P (12 men, non-ischemic cardiomyopathy 67%, 55 ± 13 years, LV ejection fraction 32 ± 6%) and recurrent appropriate shocks despite antiarrhythmic drug therapy and optimal heart failure medication. All P underwent a protocol of ventricular programmed stimulation (600 ms/S3) to obtain baseline VT documentation. A sinus rhythm (SR) voltage map was created using a 3D electroanatomic mapping system (CARTO) with a high-density mapping catheter (PentaRay) to delineate areas of scarred myocardium (ventricular bipolar voltage ≤0,5 mV – dense scar; 0,5-1,5 mV – border zone; ≥1,5 mV – healthy tissue) and provide high-resolution electrophysiological mapping. The substrate modification included catheter elimination of local abnormal ventricular activities (LAVA) - fractionated, splited, low-amplitude/long-lasting, late potentials, pre-systolic potentials - and linear ablation to obtain scars homogenization and scar dechanneling. Pace-mapping techniques were used when capture was possible. LV approach was retrograde in 6 cases, transeptal in 4 and endo-epicardial in 2 cases. In 2P the ablation was performed in the right ventricle. Results: VTs were induced and interrupted with bursts or external DC shocks. LAVA were identified and ablated in all P. Eleven P underwent modification of scar areas. The mean duration of the procedure was 153 mn (103-218 mn), with radiofrequency ranging from 18 to 60 mn (mean 33 min), and a mean fluoroscopy time of 16 mn. Non-inducibility was achieved in 75% of the cases. There was 1 pericardial tamponade drained successfully. During a follow-up of 48 ± 18 months, 75% had no VT recurrences, 2P underwent redo ablation, 1P died from stroke. Conclusion: Catheter ablation of VT based on substrate modification guided by high-density mapping is feasible and safe in P with LV dysfunction. This approach may be of clinical relevance, with potential benefits in reducing VT burden.

Piezoelectric Nanocellulose Thin Film with Large-Scale Vertical Crystal Alignment.

Cellulosic materials are attractive candidates for nature piezoelectrics. Vertically aligned cellulose nanocrystal (CNC) films are expected to show strong piezoelectricity as the largest dipole moment in CNCs exists along the cellulose chain. In this work, we adapted the confinement cell technology that was used to fabricate colloidal opal structures to align CNC rods vertically on a large scale. The high interfacial energy between the CNC-poly(tetrafluoroethylene) (PTFE) surface and torque induced by the shear force led to a large degree to the vertical alignment of CNC rods. An external DC electric field was added to further align the dipole moment of each CNC to the same direction. The as-obtained CNC film displayed excellent piezoelectric performance, and the piezoelectric coefficient was found to be 19.3 ± 2.9 pm/V, comparable to the piezoelectric coefficient d33 of poly(vinylidene difluoride) (PVDF) (20-30 pm/V). This work presents a new class of high-performance piezoelectric polymeric materials from renewable and biocompatible natural resources.

Improvement of grain alignment in Bi2Sr2Co1.8Oy thermoelectric through the electrically assisted laser floating zone

Thermoelectric performances of cobaltite ceramics were improved by increasing grain sizes and alignment through the Electrically Assisted Laser Floating Zone method. Bi2Sr2Co1.8Oy ceramics were directionally grown at 30 mm/h, applying external DC (0-500 mA) during the growth process. Microstructure and thermoelectric properties are significantly influenced by the external current direction and intensity. Negative currents/pole on the seed rod prevent the formation of thermoelectric phase, while positive pole on the seed rod, decreases the amount of secondary phases and increase grain orientation when compared to samples grown without current. Annealing procedure decreases secondary phases content, without modifying grain alignment. The best thermoelectric performances were obtained for samples grown with +300 mA, reaching the highest ZT values of 0.09 at 650 °C. Moreover, it should be highlighted that these processed samples can be directly applied in thermoelectric modules, avoiding the long and expensive steps necessary when other preparation processes are used.

Effect of Coordination Geometry on Magnetic Properties in a Series of Cobalt(II) Complexes and Structural Transformation in Mother Liquor.

The three Co(II) complexes [Co(bbp)2][Co(NCS)4]·4DMF (1), [Co(bbp)(NCS)2(DMF)]·2DMF (2), and [Co(bbp)(NCS)2] (3) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, and various spectroscopic techniques. Complexes 1 and 3 are obtained by the reaction of Co(NCS)2 with 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine (bbp), and complex 1 undergoes a structural transformation to form complex 2. A single-crystal X-ray study revealed that complex 1 is comprised of two Co(II) centers, a cationic octahedral Co(II) unit and an anionic tetrahedral Co(II) unit, while the Co(II) ion is in a distorted-octahedral environment in 2. Moreover, in complex 3, the Co(II) ion is in a distorted-square-pyramidal geometry. The effect of coordination geometry on the magnetic properties was studied by both static and dynamic magnetic measurements. Direct current (dc) magnetic susceptibility measurements showed that all of the Co(II) ions are in high-spin state in these complexes. Alternating current (ac) magnetic susceptibility measurements indicated that complexes 2 and 3 display slow relaxation of magnetization in an external dc magnetic field, while complex 1 displayed no such property. EPR experiments and theoretical calculations were consistent with the above findings.

Dynamic susceptibility in torus nanoring with canted external DC magnetic field

The dynamic susceptibility spectra of torus nanoring with a perpendicular or canted external DC magnetic field are studied by means of micromagnetic simulation. Due to the shape anisotropy of the torus nanoring, the system has a vortex spin structure when there is no external DC magnetic field, and the imaginary part of the dynamic susceptibility shows a single resonance peak. With the increase of DC magnetic field which is perpendicular to the plane of the ring, the resonant frequency decreases firstly and then increases monotonically. When the angle between the direction of the DC magnetic field and the center axis of the nanoring increases, the imaginary part of the magnetic susceptibility gradually shows one low frequency and three high frequency resonance peaks. By calculating the dynamic magnetic susceptibility of different parts of the nanoring, the corresponding region of each resonance mode is determined. Finally, we also discuss the effect of size on the magnetic susceptibility of the torus nanoring.

Trapping and releasing bidirectional rainbow at terahertz frequencies

Rainbow trapping arisen from slow light effect has been attracted lots of attention for the past two decades. However, until now only unidirectional rainbow trapping has been proposed, which relies on a complicated configuration or technique. Here we propose and investigate a metal–dielectric-semiconductor-dielectric-metal structure, where the semiconductor layer is applied with an external DC magnetic field. We show that in such a waveguide configuration there exist branches of dispersion curves showing slow-light bands at finite propagation constants. The positions of the slow-light bands can be tuned by varying the thickness of the semiconductor or the dielectric layers in the waveguide. By tapering the waveguide, we first demonstrate bidirectional rainbow trapping in such a simple waveguide configuration, and more importantly the trapped rainbow can be easily released by changing the external magnetic field.

Field-Induced Single-Ion Magnet Phenomenon in Hexabromo- and Hexaiodorhenate(IV) Complexes

Two mononuclear ReIV complexes of general formula (PPh4)2[ReX6] [PPh4+ = tetraphenylphosphonium cation, X = Br (1) and I (2)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group Pī. Their structures are made up of hexahalorhenate(IV), [ReX6]2−, anions, and bulky PPh4+ cations. Each ReIV ion in 1 and 2 is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX6]2− anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2 show a very similar magnetic behaviour, which is typical of noninteracting mononuclear ReIV complexes with S = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for 1 and 2, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.

PI-PBC Cascade Control Tuning For Active Power Filter Application

Because of the non-linear load increase in the electrical grid, the use of active power filter has gone increasing, this last is to improve the power quality. In the study of control applied to the active power filter, cascade control is the most used; this control scheme consists of two control loops: one external dc voltage control loop and one internal current control loop. For this work, in the external loop, a proportional-integral (PI) control is used, whereas in the inner loop a passivity-based controller (PBC) is used. The main objective of this work is to present a proposal for tuning of PI and PBC controllers. This tuning methodology allows computing the gain k of the traditional PBC based on switching frequency, taking into a count the error dynamics. From step response analysis, the PI parameters are obtained. This tuning proposal is simple and has shown to be consistent for different switching frequency values. Through the measurement of the active power filter performance, the tuning proposal has been corroborated. Simulation and experimental results are presented.

Dielectric and magnetic properties of Sr3Co2Fe24O41 thin hexaferrite samples in the microwave range

We present studies on the changes of the dielectric properties of 0.7 – 1.2-mm thick-layers of Sr3Co2Fe24O41 Z-type hexaferrite in the frequency range 5 – 40 GHz; to the best of our knowledge, for the first time we used a combination of two methods – a resonance method, which yields infomation on the dielectric and magnetic anisotropies of the samples, and a broadband transmission-line method – covered coplanar and microstrip lines. We measured an increase of the dielectric constant and dielectric loss tangent above 20 GHz and a measurable anisotropy of the permittivity and permeability. The changes in the two parameters were also investigated (a magneto-electric effect was observed) in an external dc magnetic field in directions parallel and perpendicular to the sample at room temperature. The investigations presented are complex and intricate and should be considered as a first step in the characterization of thick Sr3Co2Fe24O41 Z-type hexaferrite slabs above 3 GHz up to the mm-wave range.

Microwave magnetoabsorption in R0.6Sr0.4MnO3 (R = Pr and Nd)

We report microwave (MW) magnetoabsorption (ΔP) at room temperature in R0.6Sr0.4MnO3 (R = Pr and Nd) samples. ΔP as a function of dc magnetic field (− 2.5 kOe ≤ Hdc ≤ 2.5 kOe) was measured for a broad frequency range ( f = 0.1–4 GHz) of an MW magnetic field using a vector network analyzer and a copper strip coil which encloses one of the above samples. As the external dc magnetic field decreased from the maximum value, ΔP initially increases and shows a maximum for a critical field and then decreases as the field approaches zero. The critical field value increases with increasing frequency of the MW signal. Line shape analysis of the obtained spectra suggests that the observed features in ΔP are caused by ferromagnetic resonance in R = Pr and electron paramagnetic resonance in R = Nd samples, which were later confirmed using a coplanar waveguide-based broadband magnetic resonance spectrometer.

Magnetoelectric coupling in multiferroic Z-type hexaferrite revealed by electric-field-modulated magnetic resonance studies

Temperature dependence of ferromagnetic resonance (FMR) frequency fFMR was revealed in Z-type hexaferrite (BaxSr1−x)3Co2Fe24O41 using microwave spectroscopy in zero external magnetic field. A linear decrease in fFMR toward magnetic phase transition temperature Tc2 = 500 K was observed, implicating magnetic anisotropy decrease. Ferromagnetic resonance studies performed near 9 GHz with the magnetic field up to 10 kOe also confirmed FMR anomaly near 500 K, where the magnetic structure changes from conical to collinear. FMR spectra are tunable by external dc and ac (100 kHz) electric fields which allowed us to determine the value of magnetoelectric coefficient αE = 390 ps/m at 170 K. αE is at about 0.5 kOe one order of magnitude lower than the reported value at 10 K, because in our case the electric field was applied along c-axis, while the previously reported value was obtained with the field in hexagonal plane. Nevertheless, our αE is still one or even four orders of magnitude higher than in other “high-temperature” multiferroics.

Characterisation of tunable graphene antenna

Graphene is a unique material that can display tuning characteristics. This characteristic originates from its surface complex conductivity, which is controlled by a chemical potential. Most characteristics of tunable graphene antenna have been studied on Terahertz frequency range, thus making it difficult to be realised practically. Besides that, the standard antenna that uses switching components may have trouble during installation, and consume size as can be seen in the reconfigurable antenna. With the integration of this material, a tunable antenna based on graphene operating at microwave frequency range is proposed. The antenna was designed and fabricated at 15 GHz with a gate electrode placed behind it. They are connected to external DC bias during the measurement. The biasing is applied from 0 V to 30 V. The result shows that resonance frequency tuned about 20 MHz and reflection coefficient magnitude improved 1.24 dB. Following this, an analytical calculation on chemical potential was also derived for enhancing the graphene tunability. The count shows that at least 2.85 kV of the gate voltage is needed to vary the chemical potential and less than 0.26 µm of dielectric spacer thickness is suitable for tuning with a given condition, respectively.

DC bias electric field and stress dependence of piezoelectric parameters in lead zirconate titanate ceramics – Phenomenological approach

The piezoelectric material behavior, including loss mechanism, under external DC electric field and stress biases have been extensively studied in lead zirconate titanate ceramics. However, in order to achieve a better understanding of these effects, development of a comprehensive model is required. The purpose of this paper is to develop a phenomenological model to explain the nonlinear nature of piezoelectric parameters under DC bias electric field and stress. Therefore, the Landau-Devonshire energy function considering external biases was developed and accordingly the material properties were derived, based on first order approximation. In order to explain the compliance and piezoelectric constants change higher order elastic and electrostrictive terms were introduced into the Gibb's free energy function.Accordingly, thermodynamic theory parameters, including higher order parameters, of a soft PZT were measured and characterized based on inverse permittivity behavior near Curie temperature. Finally, the phenomenological model predictions of material property changes under DC bias electric field and compressive stress were compared with experimental data reported by our group. The proposed model succeeded to explain the material properties change under external biases. Furthermore, the effect of DC electric field and compressive stress on loss parameters were explained based on domain wall motion.

Engineering of optical, magneto-optical and magnetic properties of nickel-based one-dimensional magnetoplasmonic crystals

Magnetoplasmonic crystals consisted of the combination of noble and ferromagnetic thin films deposited on diffraction gratings represent a special class of nanostructures that can utilize the magneto-optical Kerr effect enhanced by surface plasmon-polaritons excitation for the probing of an external DC magnetic field. Optical and magneto-optical properties of a magnetoplasmonic crystal are formed by magnetic behavior. This article represents ways to manipulate optical, magneto-optical and magnetic properties of nickel-based magnetoplasmonic crystals by the variation of the substrate parameters, the composition of magnetoplasmonic crystals as well as the compressive mechanical stresses on the surface of a ferromagnetic layer.

Spatial Ordering of the Structure of Polymer-Capped Gold Nanorods under an External DC Electric Field.

We studied the alignment changes of polymer-capped gold nanorods (GNRs@PS) under an applied electric field by visible-near-infrared absorption and small-angle X-ray scattering (SAXS) measurements. Monodispersed GNRs with an aspect ratio of 4.0 were produced by the seed-mediated growth method using cetyltrimethylammonium bromide and sodium oleate binary surfactants. We investigated the phase transition between the ordered structure of GNRs@PS induced by the external electric field. At appropriate field strengths (>3 V/μm), the SAXS profiles of GNRs@PS showed a smectic ordered structure. Increasing the electric field strength densified the ordered structure and greatly increased the Raman signals (the 298 and 445 cm-1 bands) of the carbon tetrachloride (solvent) between the GNRs@PS. The insights gained are potentially applicable to catalysts, future displays, optical filters, and data storage devices.

Electrokinetic‐vortex formation near a two‐part cylinder with same‐sign zeta potentials in a straight microchannel

Vortex formation near a two-part cylinder with zeta potentials of different values but the same sign under an external DC electric field is numerically investigated in this paper. The cylinder, inserted in a straight microchannel filled with an aqueous solution, is composed of an upstream part and a downstream part. When a DC electric field is applied in the channel, under certain conditions, the vortex will form near the cylinder due to the different velocities of electroosmotic flow generated on the cylinder surface. The numerical results reveal that the larger the velocity difference of electroosmotic flow generated on the two-part cylinder and the smaller the channel width, the more conducive to vortex formation in the channel. In addition, if the zeta potential ratios of cylinder downstream part to upstream part and channel wall to cylinder upstream part are unchanged, the DC electric field strength and the zeta potential value do not affect the pattern of vortices formed in the channel. This study provides a way for vortex formation in microchannels and has the potential application in microfluidic devices.

An Electronically Adjustable Waveform Generator

A novel electronically adjustable square/triangular waveform generator has been introduced in this paper. The proposed circuit employs one active element, the multi-output current-controlled current conveyor transconductance amplifier, and one passive component grounded capacitor only. The resistorless realization of the presented generator provides a good advantage in terms of integrated circuit fabrication. In the offered circuit, the frequency and amplitude of the output square wave are electronically tunable by means of relevant bias currents. Additionally, the upper and lower threshold levels are electronically controllable by the respective bias current. On the contrary, electronically adjusting of the duty cycle of output waveform is possible via the external DC current. The generator circuit is simulated with TSMC 0.18[Formula: see text][Formula: see text]m technology parameters and SPICE. Moreover, the introduced circuit is implemented by using commercially available active devices and thus it is also verified experimentally.

Electrically tunable optical refractive index of a WS2 film on a SiC substrate

For applications in portable electronic devices and machine vision, a small sized focus-tunable lens or lens system is highly desired. Materials with electrically tunable optical refractive indices are required in these fields. A functional material consisting of a WS2 thin film on a SiC substrate was synthesized and reported in our investigation. A 15-nm-thick WS2 film was deposited on an n-doped SiC substrate (7.37 × 1019 cm − 3) by pulsed laser deposition. The optical refractive index of the WS2 / SiC material was found to be electrically tunable. The value of the effective optical refractive index ranged from 4.03 to 5.84, which implies a 44.9% variation in the refractive index change, at a wavelength of 460 nm under an applied external DC voltage modulated in the range of 0 to 7 V. We also found experimentally that the focal length of the plane lens was tunable from 184 to 156 mm at a wavelength of 650 nm with the applied voltage ranging from 0 to 7 V. Our investigation presents a new way to modulate the refractive index and make the compact focus-tunable lens design convenient.

Destroying synchrony in an array of the FitzHugh–Nagumo oscillators by external DC voltage source

A control method for desynchronizing an array of mean-field coupled FitzHugh–Nagumo-type oscillators is described. The technique is based on applying an adjustable DC voltage source to the coupling node. Both, numerical solution of corresponding nonlinear differential equations and hardware experiments with a nonlinear electrical circuit have been performed.