Hasil untuk "Electricity and magnetism"

E. N. Sidorenko, S. P. Shpanko, G. F. Zargano et al.

Dielectric spectra of protective organic films at different mechanical pressures in the frequency range of [Formula: see text] [Formula: see text]Hz have been experimentally investigated. The films were obtained on the surface of rolled steel from a unimolar solution of sulfuric acid using a new organic compound of the imidazole class as an inhibitor by varying the adsorption time of the additive and its concentration. The films are characterized by the presence of negative electrical capacitance and high through conductivity. Three dielectric dispersions are observed in the spectra, two of which have a relaxation character and one of which is resonant. The negativity of the electrical capacity of the investigated films is apparently due to three factors: High-frequency relaxation and resonance processes in electric fields with inverse field strength and low-frequency hopping-type conduction processes. To interpret the high-frequency relaxation polarization with inverse electric field strength, an impedance model of an organic film sample is used in the form of an equivalent current divider circuit consisting of series and parallel RC circuits. The comparison of experimentally measured electrical parameters of various films with approximated results within the framework of the described model showed their good agreement.

Ji Hwan Yoon, Jin Ju Won, Ji Eun Roh

In this paper, the clutter rejection threshold corresponding to the maximum Doppler frequency of mainlobe clutter in high pulse repetition frequency airborne radar with frequency modulation (FM) ranging is proposed. The clutter spreading due to range-Doppler coupling in FM ranging radar is analyzed, and based on that, four clutter rejection thresholds corresponding to the maximum sidelobe/mainlobe clutter frequencies without/with FM ranging are presented. In particular, the clutter rejection threshold for achieving mainlobe clutter-free condition with FM ranging is derived by analyzing the mainlobe clutter boundaries for various beam steering angles. The detection capability corresponding to each of the four thresholds is analyzed, and it is shown that the detection capability can be improve by applying the clutter rejection threshold proposed in this work.

Tong Zhang, Kazuya Kobayashi

Abstract The analysis of diffraction by a semi‐infinite parallel‐plate waveguide with partial material loading is rigorously carried out using the Wiener–Hopf technique for the H‐polarised plane wave incidence. In solving the Wiener–Hopf equations, the authors apply the Modified Residue Calculus Technique (MRCT) to achieve highly accurate solutions. The scattered field in real space is explicitly derived by performing the inverse Fourier transform of the solution in the transform domain. Within the waveguide, the scattered field is represented in terms of the waveguide TM modes, while the external field is asymptotically evaluated by applying the saddle‐point method to yield a far field expression. The authors present representative numerical examples of the radar cross section for various physical parameters and discuss the far‐field scattering characteristics in detail. Comparisons with the E‐polarisation are also given.

Zhuoheng Xie, Shi Chao Jin, Jia Xing Sun et al.

Abstract A K‐band eight‐element phased‐array receiver is proposed, featuring a reconfigurable beam count and high Tx‐band rejection, tailored for satellite communication applications. By precisely designing the inter‐stage and output matching networks of the four‐stage low‐noise amplifier, the Tx‐band rejection of the receiver can be effectively enhanced to align with the requirements of more than 35 dB in shared‐aperture phased‐array applications. Meanwhile, the reconfigurable power divider is meticulously designed to achieve three different operation modes, which allows the eight‐element receiver to be configured for one, two, or four simultaneous beams. In addition, an active vector‐modulated phase shifter is designed to meet the demands of precise phase shifts of phased‐array systems. Based on the proposed method, a K‐band eight‐element reconfigurable phased‐array receiver operating from 17.7 to 21.2 GHz is designed and measured. The measurement exhibits more than 35‐dB Tx‐band rejection, 21–24.5 dB for single‐channel gain, and lower than 2.6 dB for noise figure within the design bandwidth. Meanwhile, the measured root mean square (RMS) gain error and RMS phase error are confined within 0.8 dB and 3.7°, respectively.

Mikaela J. Surace, Jimmy Murillo-Gelvez, Mobish A. Shaji et al.

Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic organofluorine surfactants that are resistant to typical methods of degradation. Thermal techniques along with other novel, less energy-intensive techniques are currently being investigated for the treatment of PFAS-contaminated matrices. Non-equilibrium plasma is one technique that has shown promise for the treatment of PFAS-contaminated water. To better tailor non-equilibrium plasma systems for this application, knowledge of the energy required for mineralization, and in turn the roles that plasma reactive species and heat can play in this process, would be useful. In this study, fundamental thermodynamic equations were used to estimate the enthalpies of reaction (480 kJ/mol) and formation (−4640 kJ/mol) of perfluorooctanoic acid (PFOA, a long-chain legacy PFAS) in water. This enthalpy of reaction estimate indicates that plasma reactive species alone cannot catalyze the reaction; because the reaction is endothermic, energy input (e.g., heat) is required. The estimated enthalpies were used with HSC Chemistry software to produce a model of PFOA defluorination in a 100 mg/L aqueous solution as a function of enthalpy. The model indicated that as enthalpy of the reaction system increased, higher PFOA defluorination, and thus a higher extent of mineralization, was achieved. The model results were validated using experimental results from the gliding arc plasmatron (GAP) treatment of PFOA or PFOS-contaminated water using argon and air, separately, as the plasma gas. It was demonstrated that PFOA and PFOS mineralization in both types of plasma required more energy than predicted by thermodynamics, which was anticipated as the model did not take kinetics into account. However, the observed trends were similar to that of the model, especially when argon was used as the plasma gas. Overall, it was demonstrated that while energy input (e.g., heat) was required for the non-equilibrium plasma degradation of PFOA in water, a lower energy barrier was present with plasma treatment compared to conventional thermal treatments, and therefore mineralization was improved. Plasma reactive species, such as hydroxyl radicals (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>⋅</mo><mi>OH</mi></mrow></semantics></math></inline-formula>) and/or hydrated electrons (e⁻_(aq)), though unable to accelerate an endothermic reaction alone, likely served as catalysts for PFOA mineralization, helping to lower the energy barrier. In this study, the activation energies (E_a) for these species to react with the alpha C–F bond in PFOA were estimated to be roughly 1 eV for hydroxyl radicals and 2 eV for hydrated electrons.

Kai Wu, Wanye Xu, Peng Li et al.

Abstract A third‐order miniaturised element frequency selective surface (MEFSS) for high‐power microwave (HPM) systems is proposed. The MEFSS design is characterised by the excellent capability to handle high‐power waves and the stable frequency response at large incidence angles. Through combining the particle swarm optimisation algorithm and the equivalent circuit method to improve the efficiency, the design has low resonant temperature while maintaining a low maximum field enhancement factor. A theoretical guide for the selection of substrate parameters in high‐power systems is provided. Instead of being as small as possible, the loss tangent should be properly selected to reach a balance between metal loss and dielectric loss. To the author's best knowledge, it is the first time in‐band low temperature and out‐of‐band high temperature suppression at various incidence angles are realised. The results show that the frequency response remains stable for incident angles up to 60°, and the maximum temperature is reduced from 148°C to 44°C for incident power density of 10 kW/m2. A MEFSS prototype was fabricated, and the transmission coefficients were measured. The full‐wave simulation and measurement results agree well. It has significance to the engineering progress of FSS as radomes for HPM antenna systems.

Mengfang GUO, Xiang DU, Fei WANG

A measurement state equation was established for the integrated energy system (IES) of electricity gas heat under non Gaussian measurement noise. Considering the boundary conditions of coupled components, which serve as both equality constraints and virtual measurement equations, a robust state estimation method for electrical pneumatic thermal IES considering boundary equality constraints is proposed based on the maximum likelihood robust state estimation method. Conduct simulation verification under Gaussian and non Gaussian noise. The simulation results demonstrate that the proposed method can provide accurate global state awareness for online analysis and optimization scheduling of IES.

WANG Feng, LIU Tingwei, XU Yajie et al.

Temperature drift is an important factor affecting the measurement accuracy of desktop NMR spectrometers, and adding a field-locking coil to the probe to achieve field-frequency interlocking is a common means of suppressing temperature drift. In this paper, a dual-channel miniaturised RF probe with an external field-locking channel is designed based on a laboratory compact Halbach magnet. The coil diameter, height, number of turns, turn spacing and enamelled wire radius were optimized based on the solenoid structure. The optimum solenoid size was obtained with an enameled wire radius of 0.4 mm, a coil diameter and height of 8.2 mm, a turn spacing of 1.6 mm and a number of turns of 5. Based on the simulation results, the detection and field-locking coils were fabricated, and tested in conjunction with the peripheral circuitry. The results show that the crosstalk between the two coils is low, the signal-to-noise ratio of the detection channel is above 50 and the signal-to-noise ratio of the locking channel is above 20. Final field locking experiments were performed and the frequency drift of the overall system after equipping the locking field was approximately 0.2 ppm/h (1 ppm=10-6), verifying that this probe design can be applied in compact Halbach magnet-based NMR analysis facilities.

J. D. Haller, D. L. Goodwin, B. Luy et al.

<p>A novel type of efficient broadband pulse, called second-order phase dispersion by optimised rotation (SORDOR), has recently been introduced. In contrast to adiabatic excitation, SORDOR-90 pulses provide effective transverse 90<span class="inline-formula">^∘</span> rotations throughout their bandwidth, with a quadratic offset dependence of the phase in the <span class="inline-formula"><i>x</i>,<i>y</i></span> plane. Together with phase-matched SORDOR-180 pulses, this enables the Böhlen–Bodenhausen broadband refocusing approach for linearly frequency-swept pulses to be extended to any type of 90<span class="inline-formula">^∘</span>/180<span class="inline-formula">^∘</span> pulse–delay sequence. Example pulse shapes are characterised in theory and experiment, and an example application is given with a <span class="inline-formula">¹⁹F</span>-PROJECT experiment for measuring relaxation times with reduced distortions due to <span class="inline-formula"><i>J</i></span>-coupling evolution.</p>

Decai ZHOU, Jinjin CHEN, Bo LIU

In view of the problem that the traditional co-frequency power prosperity index lacks timeliness and dynamism, a MF-MS-SW model is constructed, which can analyze both the quarterly and monthly frequencies at the same time. Leading, consistent and lagging mixed-frequency sample data consisting of 21 economic indicators are selected to construct China's mixed-frequency power prosperity index and early warning signal system. The results show that the proposed MF-MS-SW measurement model has a good description of the fluctuation characteristics of China’s power prosperity index, i.e., multiple-frequency and nonlinear characteristics, and has a high degree of consistency with China’s overall economic development status. The model can be used for early warning and forecasting. It is recommended that the energy and power sectors regularly compile and publish China's power prosperity index, reflecting the real-time changes in the power industry's economic status and promoting the optimization and adjustment of energy structure.

Bao-Huei Huang, Yu-Hsiang Fu, Chao-Cheng Kaun et al.

In our JunPy package, we have combined the first-principles calculated self-consistent Hamiltonian with divide-and-conquer technique to successfully determine the magnetic anisotropy (MA) in an Fe/MgO/Fe magnetic tunnel junction (MTJ). We propose a comprehensive analytical derivation to clarify the crucial roles of spin-orbit coupling that mediates the exchange and spin-orbit components of spin torque, and the kinetic and spin-orbit components of spin current accumulation. The angular dependence of cumulative spin-orbit torque (SOT) indicates a uniaxial MA corresponding to the out-of-plane rotations of magnetic moments of the free Fe layers. Different from the conventional MA energy calculation and the phenomenological theory for a whole MTJ, our results provide insight into the orbital-resolved SOT for atomistic spin dynamics simulation in emergency complex magnetic heterojunctions.

Dariusz Sztenkiel

Here we report on a new effect similar to the spin reorientation transition (SRT) that takes place at two magnetic fields of $B_{SORT1}$ and $B_{SORT2}$. The effect is observed in the magnetization curves of small Mn$^{3+}$ magnetic clusters in the wurtzite GaN (being in a paramagnetic state) calculated using crystal field model approach. The obtained results suggest that the computed magnetic anisotropy (MA) reverses its sign on increasing $B$ across $B_{SORT}$. Detailed analysis show however that MA is unchanged for high magnetic fields. We show that the observed effect arises from the interplay of the crystalline environment and the spin-orbit coupling $λ\hat{\textbf{S}} \hat{\textbf{L}}$, therefore we name it spin orbital reorientation transition (SORT). The value of $B_{SORT1}$ depends on the crystal field model parameters and the number of ions $N$ in a given cluster, whereas $B_{SORT2}$ is controlled mostly by the magnitude of the spin-orbit coupling $λ$. The explanation of SORT is given in terms of the spin $M_S$ and orbital momentum $M_L$ contributions to the total magnetization $M = M_S + M_L$. The similar effect should also be present in other materials with not completely quenched (non zero) orbital angular momentum $L$, a uniaxial magnetic anisotropy and the positive value of $λ$.

Maxym Dudka, Mariana Krasnytska, Juan J. Ruiz-Lorenzo et al.

Changes in magnetic critical behaviour of quenched structurally-disordered magnets are usually exemplified in experiments and in MC simulations by diluted systems consisting of magnetic and non-magnetic components. By our study we aim to show, that similar effects can be observed not only for diluted magnets with non-magnetic impurities, but may be implemented, e.g., by presence of two (and more) chemically different magnetic components as well. To this end, we consider a model of the structurally-disordered quenched magnet where all lattice sites are occupied by Ising-like spins of different length $L$. In such random spin length Ising model the length $L$ of each spin is a random variable governed by the distribution function $p(L)$. We show that this model belongs to the universality class of the site-diluted Ising model. This proves that both models are described by the same values of asymptotic critical exponents. However, their effective critical behaviour differs. As a case study we consider a quenched mixture of two different magnets, with values of elementary magnetic moments $L_1=1$ and $L_2=s$, and of concentration $c$ and $1-c$, correspondingly. We apply field-theoretical renormalization group approach to analyze the renormalization group flow for different initial conditions, triggered by $s$ and $c$, and to calculate effective critical exponents further away from the fixed points of the renormalization group transformation. We show how the effective exponents are governed by difference in properties of the magnetic components.

Xing-xing NIU, Zhi-jie BAI, Yi YANG et al.

In this work, operando low-field nuclear magnetic resonance (LF-NMR) technology was used to quantitatively study the photocatalytic reduction of Cr(Ⅵ) in a real solid-liquid reaction environment. The performance of Ag/g-C3N4 photocatalysts with different loadings of Ag for the photocatalytic reduction of aqueous Cr(Ⅵ) to Cr(Ⅲ) under visible-light irradiation was systematically investigated. The results showed that the addition of Ag cocatalysts (Ag loading amount: 1 wt.%, 2 wt.%, 5 wt.%, and 10 wt.%) significantly enhanced the photocatalytic performance of g-C3N4 photocatalysts, while the optimal content of Ag loading was 5 wt.%, which was greater than that of pristine g-C3N4 by a factor of 4.0. Furthermore, we carried out a quantitative analysis on the concentration of paramagnetic Cr(Ⅲ) ions in solution by using the transverse relaxation time (T2) values, and proved the viability of employing operando LF-NMR relaxometry for monitoring the photocatalytic Cr(Ⅵ) reduction performance.

Haihong FAN

With the rapid development of the power industry, the power load prediction is becoming more and more important in recent years, and short-term load prediction plays an extremely important role in dispatching and market operation of the power system. Power load prediction can effectively improve the utilization of power generation equipment. The selective ensemble learning method based on Kappa statistic and the glowworm swarm optimization algorithm (GSO) to forecast short-term load is proposed. This proposed method firstly generates multiple learners by bootstrap sampling, and then use glowworm swarm optimization algorithm to select some learners with large differences and high accuracy to participate in the integration. Compared with a single learner, the accuracy of the proposed method is significantly improved. The daily average load curves of two enterprises in Wuhan from 2015 to 2016 are used as a case study to carry out load forecasting. Comparing with other forecasting methods, the prediction accuracy of the proposed method is proved to be higher.

Jorge A. Ardila‐Rey, Matias Cerda‐Luna, Rodrigo Rozas‐Valderrama et al.

Abstract One of the main tools for monitoring the condition of high voltage equipment is the measurement of partial discharges (PD). The electromagnetic (EM) radiation originated from this degradation phenomenon can be captured by various types of ultra‐high frequency (UHF) antennas carefully designed and optimised for specific frequency bands. However, the presence of environmental noise may limit the use of this technique. Different types of monopole antennas normally used in UHF PD measurement have been evaluated in order to validate the performance of a novel separation technique of EM sources. Accordingly, a new separation technique based on the energy ratio of the captured signals was developed, considering noise interferences. The results revealed that the new technique allows an adequate separation, even when three sources act simultaneously.

Pei CAO, Peng XU, Jianming HE et al.

Insulation fault accounts for a large proportion in electrical equipment faults, so it is an important strategy to detect and eliminate the faults in their latent phase. Insulation defects are usually accompanied by temperature rise or partial discharge, which can be used as an important basis for judging the insulation status of the equipment. The infrared photoelectric sensor can detect the temperature of the equipment, and the ultraviolet photoelectric sensor can detect the ultraviolet pulse signal generated by partial discharge of the equipment. In this paper, taking the cable terminal defects in the switch cabinet as an example, an infrared and ultraviolet photoelectric sensor synchronous acquisition device is constructed. Based on the adaptive fuzzy neural network, an intelligent detection method is proposed with combining two information sources of temperature rise and partial discharge. The experimental results show that compared to the information detection with single sensor, the diagnosis algorithm based on multi-source sensing significantly improves the accuracy of equipment defect diagnosis. The proposed method can provide a new research idea for identification and diagnosis of insulation defects of switchgear.

Jiahao LU, Sizhe CHEN

In view of the problem of uncontrollable rotor voltage of variable frequency transformer, a non grid side converter-based variable frequency transformer (NGSC-VFT) configuration is proposed. Based on this configuration, an integrated mathematical model of NGSC-VFT under dual-side symmetrical and asymmetrical grid conditions is established, and the control strategy of NGSC-VFT is further studied. The stator side series compensation converte (SCC ) aims to maintain the DC-link voltage, independently control reactive power, and eliminate the stator’s negative sequence voltage, while the objective of the rotor side SCC is to eliminate the rotor’s negative sequence voltage. The simulation results show that, with the proposed NGSC-VFT configuration, the DC-link voltage can be regulated without GSC with faster dynamic tracking effect of the active power and lower DC capacitor voltage, and the reactive power can be independently controlled. In addition, the fluctuations of torque and power under dual-side asymmetrical conditions can be further suppressed. The novel configuration improves the asymmetrical fault ride-through capability of VFT system.

Saifullah YASIR, Guomin YANG, Feng XU

In this paper, a novel four-leaf clover-shaped coding metasurface is proposed and applied to achieve an ultra-wideband diffusion-like scattering. The proposed metasurface element has rotational symmetry; hence, it produces similar reflection characteristics for both x- and y-polarized waves. To realize a 1-bit coding metasurface, two elements are chosen that have a phase difference of 180°±37° from 15.5 to 40.5 GHz. An optimization algorithm is applied to get the best arrangement of unit cells in the array to attain the wideband RCS reduction. The four-leaf clover-shaped metasurface can attain more than 10 dB RCS reduction from 15.5 to 26.5 GHz and 30.5 to 40.5 GHz. A prototype of the proposed design is fabricated, and an experiment is carried out to validate the performance of the metasurface. The proposed concept of four-leaf clover-shaped coding metasurface is an effective solution for wideband RCS reduction applications.

Katarzyna Gas, Janusz Sadowski, Maciej Sawicki

Here we report on detailed studies of the magnetic properties of the wurtzite (Ga,Mn)As cylindrical shells. Ga$_{0.94}$Mn$_{0.06}$As shells have been grown by molecular beam epitaxy at low temperature as a part of multishell cylinders overgrown on wurtzite (Ga,In)As nanowires cores, synthesized on GaAs (111)B substrates. Our studies clearly indicate the presence of a low temperature ferromagnetic coupling, which despite a reasonably high Mn contents of 6\% is limited only to below 30~K. A set of dedicated measurements shows that despite a high structural quality of the material the magnetic order has a granular form, which gives rise to the dynamical slow-down characteristic to blocked superparamagnets. The lack of the long range order has been assigned to a very low hole density, caused primarily by numerous compensation donors, arsenic antisites, formed in the material due to a specific geometry of the growth of the shells on the nanowire template. The associated electrostatic disorder has formed a patchwork of spontaneously magnetized (macrospin) and nonmagnetic (paramagnetic) volumes in the material. Using high field results it has been evaluated that the total volume taken by the macrospins constitute about 2/3 of the volume of the (Ga,Mn)As whereas in the remaining 1/3 only paramagnetic Mn ions reside. By establishing the number of the uncoupled ions the two contributions were separated. The Arrott plot method applied to the superparamagnetic part yielded the first experimental assessment of the magnitude of the spin-spin coupling temperature within the macrospins in (Ga,Mn)As, $T_{\mathrm{C}}=28$~K. In a broader view our results constitute an important contribution to the still ongoing dispute on the true and the dominant form(s) of the magnetism in this model dilute ferromagnetic semiconductor.