Direct Current Field Research Articles

МНОГОФУНКЦИОНАЛЬНОЕ МАГНИТОГИДРОДИНАМИЧЕСКОЕ УСТРОЙСТВО

Three types of magnetohydrodynamic devices using in liquid metal circulating loops are known. These are electromagnetic pumps, flow meters, and throttles. Electromagnetic pumps and throttles are induction type, as a rule, using alternating current and alternating magnetic field. Electromagnetic flowmeters for liquid metals, as a rule, are conduction type using direct current and permanent magnetic field. Every of each device serves one function only. Operating principle and some designs of universal conduction magnetohydrodynamic devices, which are possible provide simultaneously two of three functions, (pump and flow meter, or throttle and flow meter) are described at the paper. They are called pump-flow meter and throttle-flow meter. These devices are proposed for small experimental facilities with flow rate of liquid metal 1-500 l/h and for auxiliary circulating loops of big facilities. Theoretical basis of fulfilment of two function simultaneously and needed mathematical formulas presented at the paper. Three variants of pump-flow meters tested and corresponding results are described.

Engineered Magnetization Dynamics of Magnonic Nanograting Filters

Magnonic crystals and gratings could enable tunable spin-wave filters, logic, and frequency multiplier devices. Using micromagnetic models, we investigate the effect of nanowire damping, excitation frequency and geometry on the spin wave modes, spatial and temporal transmission profiles for a finite patterned nanograting under external direct current (DC) and radio frequency (RF) magnetic fields. Studying the effect of Gilbert damping constant on the temporal and spectral responses shows that low-damping leads to longer mode propagation lengths due to low-loss and high-frequency excitations are also transmitted with high intensity. When the nanowire is excited with stronger external RF fields, higher frequency spin wave modes are transmitted with higher intensities. Changing the nanowire grating width, pitch and its number of periods helps shift the transmitted frequencies over super high-frequency (SHF) range, spans S, C, X, Ku, and K bands (3–30 GHz). Our design could enable spin-wave frequency multipliers, selective filtering, excitation, and suppression in magnetic nanowires.

A working-point perturbation method for the magnetoelectric sensor to measure DC to ultralow-frequency-AC weak magnetic fields simultaneously

Magnetoelectric (ME) sensors for the weak magnetic field measurement have attracted a lot of attention because of their high sensitivity and easy integration. However, the ME sensor performs well only at its working-point under a direct current (DC) bias field (Hbias) and mechanical resonance frequency (fres). The measurement of DC to ultralow frequency (0–100 Hz) weak magnetic fields has increasing demands, such as in geomagnetic anomaly fields, geological and mineral exploration, magnetocardiography, and magnetoencephalography. Unfortunately, fres of ME sensors is on the order of several tens of kilohertz, which is far higher than the ultralow frequency desired. Moreover, if the operation frequency deviates from fres, the sensitivity will deteriorate rapidly. In this study, a working-point perturbation method was used to measure the weak magnetic fields at 0–100 Hz with a high magnetic field resolution. (1) The perturbation of fres using an ultralow frequency (fac) magnetic field results in two modulation peaks with frequencies of fres ± fac. The frequency and resolution of the measured alternating current magnetic field can be obtained by varying fac and the modulation depth. A resolution around 1 nT for fac > 10 Hz and a lowest operation frequency of 0.1 Hz were achieved using our measurement system. (2) A high field resolution of 3 nT (better than the frequency perturbation method with a resolution of 16 nT at 0.1 Hz) can be achieved by the perturbation of Hbias at fres because the ME sensor is still working at the quasi-working-point and helped by lock-in amplifier technology.

Effect of the Two-Dimensional Magnetostrictive Fillers of CoFe2O4-Intercalated Graphene Oxide Sheets in 3-2 Type Poly(vinylidene fluoride)-Based Magnetoelectric Films.

In the last decade, magnetoelectric (ME) polymer films have been developed by including zero-dimensional or one-dimensional magnetostrictive fillers in a piezoelectric polymer matrix. Existing reports on ME polymer films reveal that the shape of the magnetostrictive fillers is a critical determinant of the polymeric phase conformation, strain transfer between the piezoelectric and magnetostrictive phases, and dipole alignment in the films. In this study, to investigate the effect of two-dimensional (2D) magnetostrictive fillers on piezoelectric, magnetic, and magnetoelectric responses, 3-2 type ME films were prepared using CoFe2O4-intercalated graphene oxide (CFO-i-GO) fillers and poly(vinylidene fluoride) (PVDF) polymers. The 2D fillers of CFO-i-GO were hydrothermally synthesized by CFO intercalation into the interlayers of GO sheets with different lateral sizes, which were controlled by ultrasonication treatment. It was found that the large-lateral-size GO (LGO), medium-lateral-size GO (MGO), and small-lateral-size GO (SGO) fillers in the PVDF-based ME films exhibited a lateral size effect on CFO intercalation, polymeric phase conformation, dipole alignment, and magnetoelectric responses. A maximum ME coefficient (αME) of 3.0 mV/cm∙Oe was achieved with a strong linearity (r2) of 0.9992 at an off-resonance frequency (f) of 1 kHz and applied direct current (dc) magnetic field (Hdc) of ± 1000 Oe. The 3-2 type polymer-based ME films with reliable ME responses have potential for use in high-feasibility ME devices for biomedical sensing applications.

Spark Plasma Sintering of LiFePO4: AC Field Suppressing Lithium Migration.

Our work proposes a comparison between Spark Plasma Sintering of LiFePO4 carried out using an Alternating Current (AC) and Direct Current (DC). It quantifies the Li-ion migration using DC, and it validates such hypothesis using impedance spectroscopy, X-ray photoelectron spectroscopy and inductively coupled plasma optical emission spectroscopy. The use of an AC field seems effective to inhibit undesired Li-ion migration and achieve high ionic conductivity as high as 4.5 × 10−3 S/cm, which exceeds by one order of magnitude samples processed under a DC field. These results anticipate the possibility of fabricating a high-performance all-solid-state Li-ion battery by preventing undesired Li loss during SPS processing.

Electrophoretic nuclei assembly of MOFs in polyamide membranes for enhanced nanofiltration

Thin film nanocomposite (TFN) polyamide (PA) membranes has drawn increasing attention due to the improved physicochemical properties of the top layer for versatile separation performance. However, the commonly used fabrication approach, lacking a controllable loading of nanofillers, leads to a random and insufficient distribution of nanofillers into the PA layer and generates interfacial defects that deteriorate the membrane performance. Herein, a rational strategy for incorporating metal-organic frameworks (i.e., ZIF-8) into the PA layer via electrophoretic deposition (EPD) was proposed. Driven by an external direct current field, the ZIF-8 nanoparticles were uniformly assembled onto a porous ultrafiltration (UF) substrate for fabrication of high-performance TFN nanofiltration membranes by vacuum-assisted interfacial polymerization (IP). The resultant TFN nanofiltration membrane yielded an enhanced water permeability of 22.4 ± 1.2 L·m−2·h−1·bar−1, due to incorporation of ZIF-8 nanoparticles which created extra nano-channels for fast water permeation. Additionally, such an EPD process effectively tailored the membrane surface properties (e.g., zeta potential) and conferred a considerably high rejection for Na2SO4 (96.9 ± 0.7%) but a low rejection for NaCl (18.9 ± 2.5%), demonstrating an excellent selectivity between SO42− and Cl− ions. This study highlights the impressive efficacy of the EPD process in precisely anchoring nanomaterials to design high-performance TFN nanofiltration membranes for target separations.

AC Susceptibility Studies under DC Fields in Superspinglass Nanomaghemite-Multiwall Carbon Nanotube Hybrid

Magnetic properties of maghemite (γ-Fe2O3) nanoparticles grown on activated multiwall carbon nanotubes have been studied by alternating current (AC) magnetic susceptibility experiments performed under different temperatures, frequencies, and applied magnetic fields. Transmission electron images have suggested that the γ-Fe2O3 nanoparticles are not isolated and have an average size of 9 nm, but with a relatively broad size distribution. The activation energies of these 9 nm γ-Fe2O3 nanoparticles, determined from the generalized Vogel–Fulcher relation, are reduced upon increasing the direct current (DC) field magnitude. The large activation energy values have indicated the formation of a superspinglass state in the γ-Fe2O3 nanoparticle ensemble, which were not observed for pure γ-Fe2O3 nanoparticles, concluding that the multiwall carbon nanotubes favored the appearance of highly concentrated magnetic regions and hence the formation of superspinglass state. Magnetic relaxation studies, using Argand diagrams recorded for DC probe fields (<20 kOe) below the magnetic blocking temperature at 100 and 10 K, have revealed the presence of more than one relaxation process. The behavior of the ensemble of γ-Fe2O3 nanoparticles can be related to the superspinglass state and is also supported by Almeida–Thouless plots.

AC/DC dual-mode magnetoelectric sensor with high magnetic field resolution and broad operating bandwidth

In this study, a dual-mode Metglas/Pb(Zr,Ti)O3 magnetoelectric (ME) sensor was prepared for measuring weak magnetic fields. It is interesting to note that this ME sensor can work at alternating current (AC) and direct current (DC) dual-modes with high field resolution. In AC mode, a very accurate AC magnetic field resolution of 0.8 nT was achieved at a mechanical resonance frequency of 72.2 kHz; moreover, the operating frequency band for resolution better than 1 nT is as wide as 3.4 kHz. We proposed a DC bias field perturbation (DBFP) method to detect the DC magnetic field using lock-in amplifier technology. As a result, an ultra-accurate DC field resolution of 0.9 nT with noise power spectral density as low as 100 pT/Hz was obtained in the studied ME sensor via the DBFP method. The dual-mode ME sensor enables simultaneous measurement for DC and AC magnetic fields with wideband and accurate field resolution, which greatly enhances the measurement flexibility and application scope.

Experimental study on variation law of electrical parameters and temperature rise effect of coal under DC electric field

Joule heats which are generated by coals in an applied electric field are directly correlated with variation resistivity of electrical parameters of coals. Moreover, the joule heating effect is closely related with microstructural changes and relevant products of coal surface. In the present study, a self-developed applied direct current (DC) field was applied onto an experimental system of coals to investigate variation resistivity of electrical parameters of highly, moderately and lowly metamorphic coal samples. Moreover, breakdown voltages and breakdown field intensities of above three coal samples with different metamorphic grades were tested and calculated. Variation resistivity of electrical parameters of these three coal samples in 2 kV and 4 kV DC fields were analyzed. Results show that internal current of all coal samples increases continuously and tends to be stable gradually after reaching the “inflection point” at peak. The relationship between temperature rise effect on anthracite coal surface in an applied DC field and electrical parameters was discussed. The temperature rise process on anthracite coal surface is composed of three stages, namely, slowly warming, rapid warming and slow cooling to stabilize. The temperature rise effect on anthracite coal surface lags behind changes of currents which run through coal samples. There’s uneven temperature distribution on anthracite coal surface, which is attributed to the heterogeneity of coal samples. In the experiment, the highest temperature on anthracite coal surface 65.8 ℃ is far belower than the lowest temperature for pyrolysis-induced gas production of coals 200 ℃. This study lays foundations to study microstructural changes and relevant products on coal surface in an applied DC field.

Two Decanuclear DyIIIxCoII10-x (x = 2, 4) Nanoclusters: Structure, Assembly Mechanism, and Magnetic Properties.

The aggregation and formation of heterometallic nanoclusters usually involves a variety of complex self-assembly processes; thus, the exploration of their assembly mechanisms through process tracking is more challenging than that for homometallic nanoclusters. We explored here the effect of solvent on the formation of heterometallic clusters, which gave two heterometallic nanoclusters, [Dy2Co8(μ3-OCH3)2(L)4(HL)2(OAc)2(NO3)2(CH3CN)2]·CH3CN·H2O (1) and [Dy4Co6(L)4(HL)2(OAc)6(OCH2CH2OH)2(HOCH2CH2OH)(H2O)]·9CH3CN (2), with the H3L ligand formed from the in situ condensation reaction of 3-amino-1,2-propanediol with 2-hydroxy-1-naphthaldehyde in the presence of Co(OAc)2·4H2O and Dy(NO)3·6H2O. It is worth noting that the skeleton of cluster 1 has a high stability under high-resolution electrospray ionization mass spectrometry (HRESI-MS) conditions with a gradually increasing energy of the ion source. Cluster 2 underwent a multistep fragmentation even under a zero ion-source voltage for the measurement of HRESI-MS. Further analysis showed that cluster 2 underwent a possible fragmentation mechanism of Dy4Co6L6 → Dy2Co6L5/DyL → DyCo2L3/DyCo2L → DyL/Co2L2. Most notably, the species emerging in the formation process of cluster 1 were tracked using time-dependent HRESI-MS, from which we proposed its possible formation mechanism of H2L → Co2L2 → Co2DyL2/Co3L2 → Co3DyL2 → Co4DyL2 → Co5Dy2L4 → Co8Dy2L6. As far as we know, it is the first time to track the formation process of Dy-Co heterometallic clusters through HRESI-MS with the proposed assembly mechanism. The magnetic properties of the two titled DyIIIxCoII10-x (x = 2, 4) clusters were studied. Both of them exhibit slow magnetic relaxation, and 1 is a single-molecule magnet at zero direct-current field.

Magnetization relaxation of uniaxial anisotropic ferromagnetic particles with linear reaction dynamics driven by DC/AC magnetic field* *Project supported by the National Natural Science Foundation of China (Grant Nos. 62001398 and 61731013) and the China West Normal University of Sichuan, China (Grant Nos. 416681 and 416682).

The response of uniaxial anisotropic ferromagnetic particles with linear reaction dynamics subjected to alternating current (AC) or direct current (DC) bias magnetic field is evaluated by the reaction–diffusion equation for the probability distribution function of the molecular concentration in the spherical coordinate system. The magnetization function and the probability distribution function of the magnetic particles in the reaction system are derived by using the Legendre polynomials and Laplace transform. We discuss the characteristics of magnetization and probability distribution of the magnetic particles with different anisotropic parameters driven by a DC and AC magnetic fields, respectively. It is shown that both the magnetization and the probability distribution decrease with time increasing due to the reaction process. The uniformity of the probability distribution and the amplitude of the magnetization are both affected by the anisotropic parameters. Meanwhile, the difference between the case with linear reaction dynamics and the non-reaction case is discussed.

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

CH3NH3PbI3 as a radio frequency decoupling capacitor: interplay between Maxwell–Wagner polarization and a pseudo inductive response

Frequency-dispersive impedance analysis of CH3NH3PbI3 perovskite is carried out under the external Direct current (DC) field to investigate the interplay of dielectric polarization and delocalized carrier transport. Switching of capacitance from positive to negative values is observed in the radio frequency range (42.1–42.5 MHz) for the external bias ranging from 0–4 V. The switching frequency outlined a decreasing trend with an increase in bias. Upon fitting the experimentally obtained dispersions, a bi-relaxation mechanism is unveiled. One of its constituents arises due to the typical Maxwell–Wagner interfacial polarization between the grain cores and boundaries and acts at the lower frequencies. The other one is manifested via hopping of delocalized carriers, resulting in a high frequency degenerative pseudo inductive response. The interference of these two mechanisms is manifested into an asymmetric Breit–Wigner–Fano profile of the dielectric susceptance spectra. The results are further elaborated from a theoretical point of view involving the energy band structure, electron localization function, and Mulliken charge distribution.

Particle distribution and microstructure of IN718/WC composite coating fabricated by electromagnetic compound field-assisted laser cladding

In this study, IN718/WC composite coatings were fabricated by electromagnetic compound field-assisted laser cladding. The macrostructure, microstructure, and microhardness of the coating were systematically investigated. The results show that the electromagnetic compound field induces the downward Ampere force, enhancing the Marangoni convection in the molten pool. The enhanced Marangoni convection makes the WC particles uniformly distributed in the composite coating. However, excessive Marangoni convection causes the WC particles to escape from the molten pool, especially the small WC particles. The application of 20 mT steady magnetic field in combination with 100 A direct current field is appropriate. It is found that the WC particles are decomposed in the molten pool, resulting in the formation of sub-micron eutectic carbides, feather-like eutectic carbides, and equiaxial eutectic carbides. The WC particle decomposition is related to the spatial position of WC particles and the distance between WC particles. The higher the spatial position of the WC particles (relative to the molten pool) and the greater the inter-WC particle distance, the more severe the decomposition. In addition, the direct current increases the nucleation rate of eutectic carbides, and the enhanced Marangoni convection breaks the long columnar eutectic carbides, which collectively refine the microstructure of the coating. The diffraction intensities of the reinforcing phases in the above case (20 mT, 100 A) are the highest, and the microhardness (530 HV0.2) exceeds that of the coating (400 HV0.2) fabricated by unassisted laser cladding.

2-D Polarimetry of Visualized Radio Frequency Waves Using Cesium Vapor Atoms

A powerful tool for users of radio frequencies (RFs) is the ability to visualize those distributions. Here, the 2-D visualization of RF waves on a circuit board is demonstrated using the fluorescence associated with the double resonance of cesium atoms. The invisible RF waves are upconverted into near-infrared light that can be viewed with an optical camera. In addition, a 2-D visualization image is captured separately for each polarization of the RF magnetic field component using a direct-current magnetic field. The images of the 2-D polarimetry are consistent with finite-element calculations of the distribution of the magnetic field strength.

Electric current dependence of plastic flow behavior with large tensile elongation in tetragonal zirconia polycrystal under a DC field

This study systematically investigated the electric direct current (DC) dependence of the plastic flow behavior in a densified tetragonal ZrO2 polycrystal (TZP). Consequently, we observed an optimal current density for tensile ductility. The maximum elongation of 135% was achieved under the current density of 250 mA∙mm−2 at a furnace temperature of 1000°C and an initial strain rate of 1 × 10−3 s−1. The diffusional mass transport in the TZP was accelerated by the combination of Joule heating and applied current effect, resulting in a reduced flow stress and an improved ductility.

Structural changes induced by direct current magnetic field improve water holding capacity of pork myofibrillar protein gels

The study was to investigate the role of direct current magnetic field (DC-MF) for water-holding capacity (WHC) of myofibrillar protein gels and to understand potential mechanisms. Samples were subjected to DC-MF with different intensities (3.5, 3.8, 9.5 and 10.4 mT), and DC-MF treatment significantly improved WHC compared with control (46.09%), reaching the maximum value of 50.36% at 3.8 mT. The main reason for the increase in WHC might be that DC-MF modified the protein structure via unfolding, re-crosslinking and aggregation of proteins, which was supported by the increased intensity of tyrosine, aliphatic and tryptophan residues, and reduced reactive sulfhydryl (2.97 to 1.94). And the re-crosslinking between molecules was maintained mainly through hydrophobic interactions and disulfide bonds. Besides, DC-MF treatment helped to generate a relatively loose and uniform microstructure to trap more water as shown by electron microscope image, which was consistent with the highest WHC at 3.8 mT.

Manipulation of Molecular Qubits by Isotope Effect on Spin Dynamics

Controlling spin behavior via external stimuli is a key route to develop molecular spintronics devices. Photons, temperature, pressure, chemicals, and electric field are the possible stimuli. Herei...

Secondary Metal Coordination Using a Tetranuclear Complex as Ligand Leading to Hexanuclear Complexes with Enhanced Thermal Barriers for Electron Transfer

Postsynthesis of the paramagnetic square-shaped complex {[(Tp*Me)Fe(μ-CN)2(CN)][Co(dmbpy)2]}2(BPh4)2·6MeCN·H2O [1, Tp*Me = tris(3,4,5-trimethylpyrazole)-borate; dmbpy = 4,4′-dimethyl-2,2′-bipyridin...

A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...