Local Electric Currents Research Articles

Chemically-Driven Autonomous Janus Electromagnets as Magnetotactic Swimmers.

An electromagnet is a particular device that takes advantage of electrical currents to produce concentrated magnetic fields. The most well-known example is a conventional solenoid, having the form of an elongated coil and creating a strong magnetic field through its center when it is connected to a current source. Spontaneous redox reactions located at opposite ends of an anisotropic Janus swimmer can effectively mimic a standard power source, due to their ability to wirelessly generate a local electric current. Herein, we propose the coupling of thermodynamically spontaneous redox reactions occurring at the extremities of a hybrid Mg/Pt Janus swimmer with a solenoidal geometry to generate significant magnetic fields. These chemically driven electromagnets spontaneously transform the redox-induced electric current into a magnetic field with a strength in the range of μT upon contact with an acidic medium. Such on-board magnetization allows them to perform compass-like rotational motion and magnetotactic displacement in the presence of external magnetic field gradients, without the need of using ferromagnetic materials for the swimmer design. The torque force experienced by the swimmer is proportional to the internal redox current, and by varying the composition of the solution, it is possible to fine-tune its angular velocity.

Chemically‐Driven Autonomous Janus Electromagnets as Magnetotactic Swimmers

AbstractAn electromagnet is a particular device that takes advantage of electrical currents to produce concentrated magnetic fields. The most well‐known example is a conventional solenoid, having the form of an elongated coil and creating a strong magnetic field through its center when it is connected to a current source. Spontaneous redox reactions located at opposite ends of an anisotropic Janus swimmer can effectively mimic a standard power source, due to their ability to wirelessly generate a local electric current. Herein, we propose the coupling of thermodynamically spontaneous redox reactions occurring at the extremities of a hybrid Mg/Pt Janus swimmer with a solenoidal geometry to generate significant magnetic fields. These chemically driven electromagnets spontaneously transform the redox‐induced electric current into a magnetic field with a strength in the range of μT upon contact with an acidic medium. Such on‐board magnetization allows them to perform compass‐like rotational motion and magnetotactic displacement in the presence of external magnetic field gradients, without the need of using ferromagnetic materials for the swimmer design. The torque force experienced by the swimmer is proportional to the internal redox current, and by varying the composition of the solution, it is possible to fine‐tune its angular velocity.

Spin torque-driven electron paramagnetic resonance of a single spin in a pentacene molecule.

Control over quantum systems is typically achieved by time-dependent electric or magnetic fields. Alternatively, electronic spins can be controlled by spin-polarized currents. Here, we demonstrate coherent driving of a single spin by a radiofrequency spin-polarized current injected from the tip of a scanning tunneling microscope into an organic molecule. With the excitation of electron paramagnetic resonance, we established dynamic control of single spins by spin torque using a local electric current. In addition, our work highlights the dissipative action of the spin-transfer torque, in contrast to the nondissipative action of the magnetic field, which allows for the manipulation of individual spins based on controlled decoherence.

Ultra-stable dendrite-free Na and Li metal anodes enabled by tin selenide host material

Lithium/sodium metal anodes are considered promising candidates to realize high-energy–density batteries because of their high theoretical specific capacity and low potential. However, their cycling stability are hindered by uncontrolled dendrites growth. Herein, SnSe nanoparticles are tightly anchored on the fiber of carbon cloth (CC) to construct SnSe@CC host material in order to control Li/Na nucleation behavior and restrain dendrites growth. It is demonstrated that the alloying product of Li15Sn4/Na15Sn4 with strong metal affinity can provide abundant active nucleation sites, and three-dimensional structure of CC host can significantly decrease the local electric current, thereby guiding homogeneous metal deposition without Li and Na dendrites. Meanwhile, the conversion product of Li2Se/Na2Se will uniformly cover on the surface of metal to serve as ultra-stable solid state interface film. As a result, high-capacity Li metal anode (20 mAh·cm−2) and Na metal anode (10 mAh·cm−2) can work steadily with ultra-long lifespans over 5000 and 6000 h with low overpotentials of 7 mV and 141 mV, respectively. Moreover, the assembled Li and Na metal full batteries exhibit superior electrochemical performances, confirming the practicability of metal anode confined in composite host. Such a strategy of conversion-alloying-type materials as hosts opens up a new path for dendrite-free metal anode electrode.

Electrokinetic modeling of mixtures: Impact of size and surface electrical property of particles

This study investigates mixture electrokinetics and explores its dependencies on the properties of constituent particles and network characteristics. Different networks with particles varying in size and type are developed, and a simulation study is conducted on the networks in both pore and network scales. Various dependencies between network parameters, particle properties and zeta potential are observed in the networks. The results also show that introducing even slight heterogeneity to a homogeneous network significantly transforms the local electric currents within the networks. The findings are utilized to develop a correlation predicting the zeta potential of mixture based on particle and network properties. Both the model and introduced correlation are evaluated using diverse experimental data. The comparisons demonstrate robust predictive capabilities of the model and correlation.

OTOMOTİV SEKTÖRÜNDE KULLANILAN DİRENÇ NOKTA KAYNAK İŞLEM PARAMETRELERİNİN KAYNAK MUKAVEMETİ ÜZERİNE ETKİLERİNİN İNCELENMESİ

The most widely used welding joining method in the automotive industry is resistance spot welding. Resistance spot welding (RSW) is the welding area where heat is produced by the effect of resistance against the flow as a result of exposing the welded parts to a local electric current. It has become an important issue to apply an effective approach method to guarantee weld quality and optimize welding parameters. This is of high value in theory and practice. In the literature, the effects of spot welding process parameters on the weld core diameter and electrode penetration depth are mostly investigated. Weld strength in RSW is affected by many process parameters such as applied electric current, compression force, welding time. Due to the contact resistance and joule heating, a molten weld nugget zone is formed on the work pieces. It is desirable to have the maximum temperature at the interface of the parts to be joined. In practice, the production of welds of acceptable quality depends on the definition of optimum weld parameters and the application of appropriate controls to ensure constant weld quality over a long production period. In the experimental studies, the optimization of the welding parameters was studied in order to obtain the maximum tensile-shear strength. The most prominent process parameters in experimental studies are electrode force, welding currents and welding times. Different algorithms have been developed for the optimization of welding parameters. The verification tests clearly show that it is possible to increase the tensile-shear strength of the connection with the combination of appropriate welding parameters. As a result, the experimental results confirm the validity of the algorithms used to improve welding performance and optimize welding parameters in resistance spot welding processes.

On grain growth and phase precipitation behaviors during W-Cr-Zr alloy densification using field-assisted sintering technology

Field-assisted sintering technology (FAST), as a fast densification method with low process temperature, was used to manufacture self-passivating tungsten alloys (SPTAs) of W-Cr-Zr in this work. To clarify the behaviors of grain growth and Cr-rich phase precipitation under the action of electric current during the densification process, interrupted sintering at different temperatures (600–1000 °C) were performed. According to the viscous flow theory, the activation energy of W-Cr-Zr sample for densification is ~23 kJ/mol. The differential form of power law was adopted to evaluate the grain growth behavior. It is found that the W-Cr-Zr alloy consolidated by FAST has a low activation energy for grain growth of 82 kJ/mol. The Cr-rich phase could be confirmed by XRD spectra even when the sintering was interrupted at 600 °C. From the characterization of the cross-sectional microstructure, the Cr-rich phases tend to precipitate at sintering necks and defects (cracks/voids) in particle interiors. The low formation temperature of the Cr-rich phase is attributed to local overheating caused by local high electric current. This work provides significant insight into the mechanisms underlying the densification and the evolution of the microstructure of the SPTAs during the FAST process.

Devising techniques for reinforcing glued sausage casings by using different physical methods

This paper has substantiated the development and rationalization of techniques to manufacture sausage casings from natural raw materials with predefined functional and technological properties. It is noted that the issue related to the rational utilization of intestinal raw materials and the improvement of the production economic profitability could be resolved by implementing effective technologies of glued intestinal sausage casings.
 The strength has been investigated of the reinforcing seam between the layers of intestinal membranes obtained by such techniques as the local tanning, local thermal coagulation resulting from passing an electric current through wet raw materials, local thermal coagulation due to the arc discharge through dried raw materials.
 The rational concentration of tannin in tanning solution has been determined, at which it is recommended to make a reinforcing seam on glued intestinal casings by means of local tanning. A value of the breaking load for the reinforcing seam made by using local electric currents has been derived, which is 14 N/m. A 4.7-time increase in the breaking load has been established to occur, compared to the control sample. A value of the breaking load for the reinforcing seam obtained by applying an arc discharge has been found, which is 18 N/m. It was noted that the breaking load had increased compared to the control sample.
 Working bodies for an installation were designed aimed at reinforcing glued sausage casings by such techniques as local tanning; local thermal coagulation resulting from passing an electric current through wet raw materials; local thermal coagulation as a result of arc discharge through dried raw materials. It is noted that the advantages of techniques for the reinforcement of glued sausage casings are the high breaking load and the effective utilization of raw materials

Electromagnetic anapoles of a Cartesian expansion of localized electric currents

Electromagnetic anapoles of a Cartesian expansion of localized electric currents

A comparison of alendronate to varying magnitude PEMF in mitigating bone loss and altering bone remodeling in skeletally mature osteoporotic rats.

Pulsed electromagnetic field (PEMF) treatments stimulate bone formation activities though further work is needed to optimize its therapeutic benefit. PEMF can generate local potential gradients and electric currents that have been suggested to mimic bone electrochemical responses to load. In line with this reasoning, a recent publication reported that PEMF application on isolated bone tissue induced detectable micro-vibrations (doi:https://doi.org/10.1109/TMAG.2016.2515069). To determine the ability of PEMF to intervene in a rat model of osteoporosis, we tested its effect on trabecular and cortical bone following ovariectomy. Four PEMF treatments, with increasing sinusoidal amplitude rise with time (3850Hz pulse frequency and 15Hz repetition rate at 10 tesla/sec (T/s), 30T/s, 100T/s, or 300T/s), were compared to the efficacy of an osteoporosis drug, alendronate, in reducing levels of trabecular bone loss in the proximal tibia. Herein, the novel findings from our study are: (1) 30T/s PEMF treatment approached the efficacy of alendronate in reducing trabecular bone loss, but differed from it by not reducing bone formation rates; and (2) 30T/s and 100T/s PEMF treatments imparted measurable alterations in lacunocanalicular features in cortical bone, consistent with osteocyte sensitivity to PEMF in vivo. The efficacy of specific PEMF doses may relate to their ability to modulate osteocyte function such that the 30T/s, and to a lesser extent 100T/s, doses preferentially antagonize trabecular bone resorption while stimulating bone formation. Thus, PEMF treatments of specific magnetic field magnitudes exert a range of measurable biological effects in trabecular and cortical bone tissue in osteoporotic rats.

On-Demand Optical Generation of Single Flux Quanta.

Superconductors can host quantized magnetic flux tubes surrounded by supercurrents, called Abrikosov vortices. Vortex penetration into a superconducting film is usually limited to its edges and triggered by external magnetic fields or local electrical currents. With a view to novel research directions in quantum computation, the possibility to generate and control single flux quanta in situ is thus challenging. We introduce a far-field optical method to sculpt the magnetic flux or generate permanent single vortices at any desired position in a superconductor. It is based on a fast quench following the absorption of a tightly focused laser pulse that locally heats the superconductor above its critical temperature. We achieve ex-nihilo creation of a single vortex pinned at the center of the hotspot, while its counterpart opposite flux is trapped tens of micrometers away at its boundaries. Our method paves the way to optical operation of Josephson transport with single flux quanta.

Thermoelectric Driven Ring Currents in Single Molecules and Graphene Nanoribbons

In a Seebeck coefficient measurement with vanishing transport current, it is usually assumed that local currents cancel as well. Here we present the existence of local electric currents in single m...

Manipulation of skyrmion motion by magnetic field gradients

Magnetic skyrmions are particle-like, topologically protected magnetisation entities that are promising candidates as information carriers in racetrack memory. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Here, we demonstrate that chiral skyrmions in Cu2OSeO3 can be effectively manipulated under the influence of a magnetic field gradient. In a radial field gradient, skyrmions were found to rotate collectively, following a given velocity–radius relationship. As a result of this relationship, and in competition with the elastic properties of the skyrmion lattice, the rotating ensemble disintegrates into a shell-like structure of discrete circular racetracks. Upon reversing the field direction, the rotation sense reverses. Field gradients therefore offer an effective handle for the fine control of skyrmion motion, which is inherently driven by magnon currents. In this scheme, no local electric currents are needed, thus presenting a different approach to shift-register-type operations based on spin transfer torque.

Quantitative nanoscale mapping of three-phase thermal conductivities in filled skutterudites via scanning thermal microscopy

Abstract In the last two decades, a nanostructuring paradigm has been successfully applied in a wide range of thermoelectric materials, resulting in significant reduction in thermal conductivity and superior thermoelectric performance. These advances, however, have been accomplished without directly investigating the local thermoelectric properties, even though local electric current can be mapped with high spatial resolution. In fact, there still lacks an effective method that links the macroscopic thermoelectric performance to the local microstructures and properties. Here, we show that local thermal conductivity can be mapped quantitatively with good accuracy, nanometer resolution and one-to-one correspondence to the microstructure using a three-phase skutterudite as a model system. Scanning thermal microscopy combined with finite element simulations demonstrate close correlation between sample conductivity and probe resistance, enabling us to distinguish thermal conductivities spanning orders of magnitude, yet resolving thermal variation across a phase interface with small contrast. The technique thus provides a powerful tool to correlate local thermal conductivities, microstructures and macroscopic properties for nanostructured materials in general and nanostructured thermoelectrics in particular.

Magneto-static Modeling from Sunrise/IMaX: Application to an Active Region Observed with Sunrise II

Abstract Magneto-static models may overcome some of the issues facing force-free magnetic field extrapolations. So far they have seen limited use and have faced problems when applied to quiet-Sun data. Here we present a first application to an active region. We use solar vector magnetic field measurements gathered by the IMaX polarimeter during the flight of the Sunrise balloon-borne solar observatory in 2013 June as boundary conditions for a magneto-static model of the higher solar atmosphere above an active region. The IMaX data are embedded in active region vector magnetograms observed with SDO/HMI. This work continues our magneto-static extrapolation approach, which was applied earlier to a quiet-Sun region observed with Sunrise I. In an active region the signal-to-noise-ratio in the measured Stokes parameters is considerably higher than in the quiet-Sun and consequently the IMaX measurements of the horizontal photospheric magnetic field allow us to specify the free parameters of the model in a special class of linear magneto-static equilibria. The high spatial resolution of IMaX (110–130 km, pixel size 40 km) enables us to model the non-force-free layer between the photosphere and the mid-chromosphere vertically by about 50 grid points. In our approach we can incorporate some aspects of the mixed beta layer of photosphere and chromosphere, e.g., taking a finite Lorentz force into account, which was not possible with lower-resolution photospheric measurements in the past. The linear model does not, however, permit us to model intrinsic nonlinear structures like strongly localized electric currents.

Electromagnetic homeostasis and the role of low-amplitude electromagnetic fields on life organization

ABSTRACTThe appearance of endogenous electromagnetic fields in biological systems is a widely debated issue in modern science. The electrophysiological fields have very tiny intensities and it can be inferred that they are rapidly decreasing with the distance from the generating structure, vanishing at very short distances. This makes very hard their detection using standard experimental methods. However, the existence of fast-moving charged particles in the macromolecules inside both intracellular and extracellular fluids may envisage the generation of localized electric currents as well as the presence of closed loops, which implies the existence of magnetic fields. Moreover, the whole set of oscillatory frequencies of various substances, enzymes, cell membranes, nucleic acids, bioelectrical phenomena generated by the electrical rhythm of coherent groups of cells, cell-to-cell communication among population of host bacteria, forms the increasingly complex hierarchies of electromagnetic signals of different frequencies which cover the living being and represent a fundamental information network controlling the cell metabolism. From this approach emerges the concept of electromagnetic homeostasis: that is, the capability of the human body to maintain the balance of highly complex electromagnetic interactions within, in spite of the external electromagnetic noisy environment. This concept may have an important impact on the actual definitions of heal and disease.

Plasma detachment in a propulsive magnetic nozzle via ion demagnetization

Plasma detachment in propulsive magnetic nozzles is shown to be a robust phenomenon caused by the inability of the internal electric fields to bend most of the supersonic ions along the magnetic streamtubes. As a result, the plasma momentum is effectively ejected to produce thrust, and only a marginal fraction of the beam mass flows back. Detachment takes place even if quasineutrality holds everywhere and electrons are fully magnetized, and is intimately linked to the formation of local electric currents. The divergence angle of the 95%-mass flow tube is used as a quantitative detachment performance figure.

Seismogenic frictional melting in the magmatic column

Abstract. Lava dome eruptions subjected to high extrusion rates commonly evolve from endogenous to exogenous growth and limits to their structural stability hold catastrophic potential as explosive eruption triggers. In the conduit, strain localisation in magma, accompanied by seismogenic failure, marks the onset of brittle magma ascent dynamics. The rock record of exogenous dome structures preserves vestiges of cataclastic processes and thermal anomalies, key to unravelling subsurface processes. Here, a combined structural, thermal and magnetic investigation of a shear band crosscutting a large block erupted in 2010 at Soufrière Hills volcano (SHV) reveals evidence of faulting and frictional melting within the magmatic column. The mineralogy of this pseudotachylyte vein offers confirmation of complete recrystallisation, altering the structure, porosity and permeability of the material, and the magnetic signature typifies local electric currents in faults. Such melting events may be linked to the step-wise extrusion of magma accompanied by repetitive long-period (LP) drumbeat seismicity at SHV. Frictional melting of Soufrière Hills andesite in a high velocity rotary shear apparatus highlights the small slip distances (< 15 cm) thought to be required to bring 800 °C magma to melting point at upper conduit stress conditions (10 MPa). We conclude that frictional melting is a common consequence of seismogenic magma fracture during dome building eruptions and that it may govern the ascent of magma in the upper conduit.

Nanostripe length dependence of plasmon-induced material deformations

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations-or "nanobumps"-are created. We have studied the dependence of these nanobumps on the length of nanostripes and verified the link with plasmons. More specifically, local electric currents can melt the nanostructures in the hotspots, where hydrodynamic processes give rise to nanobumps. This process is further confirmed by independently simulating local magnetic fields, since these are produced by the same local electric currents.

Spin-Polarized Electron Transport Across Metal-Organic Molecules: A Density Functional Theory Approach.

In the field of molecular spintronics, experimental techniques have achieved a stage where it is feasible to explore the interplay between quantum electron transport and magnetism at the single molecule level. An example is a spin-polarized STM, which can probe local electrical currents through organic molecules deposited on magnetic surfaces. The atomistic complexity of nanoscale systems calls for a first-principles description of spin-dependent transport phenomena, e.g., based on the nonequilibrium Green's function (NEGF) formalism merged with density functional theory (DFT). However, for the case of molecular junctions with transition metal electrodes, a computation of the underlying Kohn-Sham Hamiltonian can be a challenging problem: a simultaneous and accurate description of spin ordered magnetic surfaces together with the electronic structure of a molecule is required. In the present work, we provide a solution for this problem. We present an implementation, within a standard quantum chemistry package, of the NEGF formalism with an efficient approximation for the self-energy, which accounts both for absorbing boundary conditions and for exchange splitting of the energy bands in ferromagnetic electrodes. We demonstrate an ability to simulate a variety of magnetic configurations including nanoscale domain walls, which are realized when a molecule with few spin centers is brought in contact with differently magnetized reservoirs. The magnetoresistance effect arising on the molecular scale is discussed based on examples including Ni atomic-sized contact, electron transport across a prototypical molecular magnet (vanadium-benzene multidecker cluster), and tunneling through Co-phthalocyanine. Furthermore, we verify the stability of magnetically nontrivial solutions against electron correlations within the DFT+U approach.