High Current-carrying Capability Research Articles

3D Porous Silver Nonwoven Mat Prepared with Cellulosic Templates and Spray Equipment for Supercapacitor Current Collectors

Supercapacitors are energy conversion devices with higher power density compared to batteries and higher energy density compared to common capacitors.[1] These particular properties have attracted considerable attention with regard to numerous applications in such diverse fields as power electronics, military equipment, and hybrid electric vehicles (i.e. HEVs, in order to help the stop and go function and to provide peak power for improved acceleration).[2] In addition, supercapacitors can play an important role in complementing the energy storage functions of batteries and fuel cells by providing back-up power supplies to protect against power disruptions.[3,4]Supercapacitors have four major components, namely, current collectors, electrodes, electrolytes and separators.[3] However, although electrodes have been actively studied, there are far fewer studies of current collectors than the other there components. Typical current collectors are nickel (Ni), platinum (Pt), gold (Au), aluminum (Al), silver (Ag) and copper (Cu). Although Ag compared with the other materials offers many advantages including high current-carrying capability (i.e., lowest resistivity at 1.63 x 10-8 Ωm)[5] and good chemical and thermal stability,[6] Ag current collectors are rarely used in supercapacitors. In our previous study, which demonstrated the usefulness of a solution processed Ag current collector in supercapacitors, we used a 2-dimensional (2D) Ag plated polymer film as the current collectors.[7] In this study, we propose a method of enhancing the usefulness of a solution processed Ag current collector by making a 3-dimensional (3D) porous Ag nonwoven mat current collector from the cellulosic template in order to maximize the efficiency of the Ag current collector. This may have the result of maximizing the contact area between the electrode and the electrolyte, such as Ni foam.[8] This 3D porous Ag nonwoven mat would be also useful in such applications as the cathodes of alkaline fuel cells owing to its good chemical and thermal stability,[9] and might also be used in a variety of filtration applications where the antimicrobial and antibacterial properties of silver make silver membranes a very efficient filtration system.[10]In this study, we propose a simple method of making a 3D porous silver nonwoven mat as the current collector of supercapacitors, and investigate their super-capacitive properties using cyclic voltammetry in 1 M Na2SO4. For the purpose of comparison, the electrochemical properties of the 2D Ag plated current collector were also investigated. Cellulosic templates were used to make a 3D porous Ag nonwoven mat as the current collector of a supercapacitor with an Ag nanoparticle dispersed solution and simple spray equipment.

Mechanical-thermal coupling model of solidnitrogen cryostat for electrodynamic suspension system

High-temperature superconducting magnets wound by REBCO (rare-earth-barium-copper-oxide) coated conductors are promising candidates for electrodynamic suspension system due to their high current-carrying capability, excellent mechanical tolerance and low cooling cost. Solid nitrogen as coolant to protect HTS magnets can significantly promote their dynamic performance and reduce the use of auxiliary equipment, which reduce the system dimension and weight. In this study, we propose a mechanical-thermal coupling model of solid nitrogen cryostat for electrodynamic suspension system. This model is capable of calculating the temperature distribution during cooling process and the transient behavior for the rewarming process of the solid nitrogen cryostat. The cooling capacity map for the two-stage RDK-415D cryocooler was extended for accuracy. The steady state temperature distribution of cooling process and the thermal evolution of three different rewarming processes, including cryocooler reserved, 5 W losses applied and cryocooler detached, were calculated and verified experimentally, followed by the investigation of the structural stresses and displacement of the solid nitrogen cryostat due to thermal displacement among different components. The developed model can serve as a reference and guide for the design and optimization of solid nitrogen cryostat for electrodynamic suspension system.

Analysis of Defect Irrelevancy in a Non-Insulated REBCO Pancake Coil Using an Electric Network Model

High-temperature superconducting REBCO coated conductor is one of the main candidates for next-generation high field magnets in fusion reactors and particle accelerators owing to their high current-carrying capability. Although these materials can operate at higher temperatures and generate higher magnetic fields than their counterparts with lower critical temperatures, protecting the REBCO magnet against quench is challenging. A variety of candidate technologies that may be able to enable self-protection, including no-insulation technology and insulative coatings with temperature-dependent resistance, are in development. In order to understand current sharing and thermal processes during a quench, we model a REBCO pancake coil as an electrical circuit, considering power generation and heat transfer along conductor turns, and study the current distribution around a local defect with lower critical current. The magnetic field and coil terminal voltage predicted by the simulation was compared to published experimental results. Our results provide useful insights into how current sharing occurs around defects.

Properties of Nb3Sn Superconductors of Various Designs

To manufacture a high field magnet it is required to have Nb₃Sn superconductors with high current-carrying capability in magnetic fields up to 18 T. For example, CERN has chosen Nb₃Sn together with HTS as potential material for high field magnet of Future Circular Collider (FCC) project. Also, these superconductors must have an RRR of more than 150 and a low level of energy loss. One of the ways to achieve high current density is to use Internal Tin method to manufacture Nb₃Sn superconductors. In this paper, Internal Tin Nb₃Sn strands with 37, 54, 84 and 120 sub-elements are studied. Using TEM and SEM analysis, the structure and elemental composition of the superconducting layer were investigated. The analysis of the measurement results of electrophysical parameters showed that with an increase in the number of subelements from 37 to 120, at the same heat treatment mode of 370°C, 100 h + 665°C, 40 h, the critical current density measured in the range of magnetic fields from 12 to 23 T for strands of different designs practically does not changes significantly, and the RRR parameter decreases from 280 to 110 units and the effective filament diameter decreases from 139 to 70 micrometers.

The performance of first CORC cable solenoid insert for development of CFETR high-field magnet

A ReBCO coated conductor maintains high current-carrying capability under a high magnetic field and superior mechanical performance. As a promising candidate material for high-field high-temperature superconductor magnets, it has been designed for application in the China Fusion Engineering Test Reactor CS coil, with a maximum magnetic field requirement higher than 17 T. In practical application of the ReBCO tapes, the characteristic Ic response with respect to electromagnetic (EM) and thermal stress is important for safe operation of the magnet. In this paper, a nine-turn solenoid magnet was wound from 4 m of CORC cable and tested at 4.2 K in a background magnetic field of up to 19 T. The aim is to check the stability of the current-carrying properties of the ReBCO cable under combined thermal and EM loads. The solenoid coil results in a combined peak magnetic field on the conductor of 19.4 T at a critical current of 888 A. Furthermore, no performance degradation was observed after 20 cycles of operation at 800 A (90% Ic) under a background field of 19 T and 10 cycles of warm-up–cool-down between 77 K and room temperature. This demonstrates the stable performance of ReBCO conductors for high-field magnet application with EM and thermal cycles.

Fluoroelastomer encapsulation for enhanced reliability of solution-processed carbon nanotube thin-film transistors

Single-walled carbon nanotube (SWCNT) has attracted considerable attention as a promising semiconducting material due to its high field-effect mobility, high current-carrying capability, and superior mechanical durability. However, SWCNT-based thin-film transistors (SWCNT-TFTs) generally show insufficient electrical reliability in terms of the drain-current hysteresis and threshold voltage shift under the bias stress. To facilitate the great advantages of SWCNT in commercial electronic devices, strategies are required for enhancing the electrical reliability of SWCNT-TFTs without compromising their mechanical flexibility. In this work, we demonstrate solution-processed, hysteresis-free, and bias-stable SWCNT-TFTs encapsulated with elastomeric poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) using a lamination method. The dipolar nature of e-PVDF-HFP suppresses the charge trapping, resulting in elimination of hysteresis as well as enhancement of field-effect mobility and subthreshold slope. The bias instability under the negative gate bias stress is also alleviated because the hydrophobic encapsulation layer can exclude physisorbed water/oxygen molecules on the hydrophilic surface. Besides, our solvent-free lamination method effectively prevents the potential damages of the active and electrode layers from the use of the solvents. The proposed encapsulation method with laminating the e-PVDF-HFP film can provide a possibility for realization of wearable applications based on highly reliable solution-processed SWCNT-TFT.

3D Porous Silver Nonwoven Mat Prepared with Cellulosic Templates and Spray Equipment for Supercapacitor Current Collectors

Supercapacitors are energy conversion devices with higher power density compared to batteries and higher energy density compared to common capacitors.[1] These particular properties have attracted considerable attention with regard to numerous applications in such diverse fields as power electronics, military equipment, and hybrid electric vehicles (i.e. HEVs, in order to help the stop and go function and to provide peak power for improved acceleration).[2] In addition, supercapacitors can play an important role in complementing the energy storage functions of batteries and fuel cells by providing back-up power supplies to protect against power disruptions.[3,4] Supercapacitors have four major components, namely, current collectors, electrodes, electrolytes and separators.[3] However, although electrodes have been actively studied, there are far fewer studies of current collectors than the other there components. Typical current collectors are nickel (Ni), platinum (Pt), gold (Au), aluminum (Al), silver (Ag) and copper (Cu). Although Ag compared with the other materials offers many advantages including high current-carrying capability (i.e., lowest resistivity at 1.63 x 10-8 Ωm)[5] and good chemical and thermal stability,[6] Ag current collectors are rarely used in supercapacitors. In our previous study, which demonstrated the usefulness of a solution processed Ag current collector in supercapacitors, we used a 2-dimensional (2D) Ag plated polymer film as the current collectors.[7] In this study, we propose a method of enhancing the usefulness of a solution processed Ag current collector by making a 3-dimensional (3D) porous Ag nonwoven mat current collector from the cellulosic template in order to maximize the efficiency of the Ag current collector. This may have the result of maximizing the contact area between the electrode and the electrolyte, such as Ni foam.[8] This 3D porous Ag nonwoven mat would be also useful in such applications as the cathodes of alkaline fuel cells owing to its good chemical and thermal stability,[9] and might also be used in a variety of filtration applications where the antimicrobial and antibacterial properties of silver make silver membranes a very efficient filtration system.[10] In this study, we propose a simple method of making a 3D porous silver nonwoven mat as the current collector of supercapacitors, and investigate their super-capacitive properties using cyclic voltammetry in 1 M Na2SO4. For the purpose of comparison, the electrochemical properties of the 2D Ag plated current collector were also investigated. Cellulosic templates were used to make a 3D porous Ag nonwoven mat as the current collector of a supercapacitor with an Ag nanoparticle dispersed solution and simple spray equipment.

Design of a Superconducting DC Demonstrator for Wind Generators

The trend to reduce the cost of offshore wind energy is to develop large wind turbines. Turbine sizes of 6–8 MW have already been seen in the wind market. Even larger turbine sizes are managing to pave their way from studies to market. Due to the high current-carrying capability and no dc losses of the superconductors, superior power to weight/volume ratio of a superconducting generator can be achieved. Hence, in this paper, a superconducting dc generator is proposed. In this generator, the superconducting coils are used mainly to generate the main air gap flux density and silicon steel iron cores are employed by the stator and rotor. In order to study the feasibility of the proposed generator and to design the generator, the critical current of the superconducting material and electromagnetic properties of the silicon steel iron core are first measured. Then, a demonstrator of the superconducting dc generator is designed through the developed process, which integrates the analytical method into the particle swarm optimization. As a preliminary step toward the demonstrator, a test for a key component of the demonstrator, the superconducting coil, has been made.

Joule heating-induced sp2-restoration in graphene fibers

Joule heating can instantaneously achieve high temperatures (>2000 °C) by applying electrical current on a resistive material. This ohmic heating by the passage of an electric current through a conducting domain may effectively heal the defective sites (i.e., vacancies, structural defects, and sp3 oxygen groups) of graphene oxide (GO) and revert them into sp2 domains. Indeed, the direction of electric field controls the texture of GO with preferential alignment, which significantly affects the transport properties along the fiber axis. Here we present electrical current-induced manipulation of resistive domain (i.e. Joule heating) as an effective healing method for the defect sites in GO fibers (GOFs). Systematic control of input current restores the sp2 lattice structures within fibers in a well-controlled manner. Structural evolution mechanism is proposed for multilayer stacked graphitic structures as well as graphene sheet plane under the reduction process. This defect-healing principle is rapid, environmentally benign and energy efficient, compared to other defect restoration methods, and yields tailored-aligned fibers with a high current-carrying capability and facile charge transport, which is potentially beneficial for power cables and other relevant applications.

Solution-processed nanometers thick amorphous carbon-coated boron as an efficient precursor for high-field performance of MgB2

We report the fabrication of high-performance MgB2 bulk superconductors doped with amorphous carbon (aC), derived from PMMA polymer, a safe and cost effective carbon source as compared to explosive gaseous and expensive C-containing nanoadditives. The commonly used C-containing nanoadditives and boron (B) precursor nanopowders are usually prone to agglomeration in solid state mixing which causes poor reactivity and non-uniform distribution that leads to deterioration of critical current properties of MgB2. We have overcome this problem by achieving uniform 3–4 nm thin coating of aC on B nanopowders by pyrolysis of PMMA through solution process. Compared to undoped MgB2, significantly high current-carrying capability (Jc (5 K, 8 T) ∼3.1 × 104 A/cm2 and Jc (20 K, 6 T) > 103 A/cm2) was obtained for aC-doped MgB2, which is comparable to the record high in-field Jc reported for SiC-doped MgB2 and much better than those reported in literature with other C sources. Furthermore, significant improvement in Hirr and Hc2 were also observed for aC-doped MgB2. The improved high-field properties in PMMA-derived aC-doped MgB2 bulk suggest that the solution process could be a powerful technique to obtain uniform mixture of C-coated B to develop high-performance Mg(B1−xCx)2 wires for practical applications.

Ripple Field AC Losses in 10-MW Wind Turbine Generators With a MgB2Superconducting Field Winding

Superconducting (SC) synchronous generators are proposed as a promising candidate for 10-20-MW direct-drive wind turbines because they can have low weights and small sizes. A common way of designing an SC machine is to use SC wires with high current-carrying capability in the dc field winding and the ac armature winding is made with copper conductors. In such generators, the dc field winding is exposed to ac magnetic field ripples due to space harmonics from the armature. In generator design phases, the ac loss caused by these ripple fields needs to be evaluated to avoid local overheating and an excessive cooling budget. To determine the applicability of different design solutions in terms of ac losses, this paper estimates the ac loss level of 10-MW wind generator designs employing a MgB 2 SC field winding. The effects on ac losses are compared between nonmagnetic and ferromagnetic teeth with different numbers of slots per pole per phase. The necessity of an electromagnetic shield is then discussed based on the obtained loss levels. The results show that the total ac loss is so small that ferromagnetic teeth can be applied in the generator design without using an electromagnetic shield.

Electrostatically actuated MEMS relay arrays for high-power applications

Micro-electromechanical systems (MEMS) relays should have high current-carrying capability and high device reliability for high-power applications. The current-carrying capability of MEMS relays is mainly limited by the thickness of metal contacts fabricated by micromachining process. Another significant limiting factor is the high contact resistance resulted from the low driving force, which is provided by microactuators. This paper presents a matrix configuration of electrostatically actuated microcantilever relay arrays that are connected in parallel to shunt high currents to individual relay elements. This method allows the matrix to be configured for different power requirements. The proposed novel hollow suspended spring lowers the driving voltage and enhances the device stability considerably because of its low longitudinal stiffness while high lateral stiffness. The average of pull-in voltages is approximately 39.4 V and the overdamped switching-on time is approximately 180 μs when 42 V of driving voltage is applied. Contact resistance of each relay is less than 1 Ω, and the equivalent contact resistance of the 2 × 2 relay arrays is lower than 250 mΩ. Each relay can operate in over 6.5 × 103 hot-switching cycles without failing at a current of 20 mA with 0.3 μm thick Au contacts, thus the 2 × 2 relay arrays can carry four times currents more than one single relay. Therefore, the proposed MEMS relay arrays are suitable for applications in high-power switching systems that require high reliability and stability, such as space applications.

High-Field Pinning Potential in YBCO Films with Nanoengineered Pinning Centres

Frequency-dependent AC susceptibility measurements have been used to estimate the critical current densities and pinning potential of two 1.1-μm-thick YBCO films with nanoengineered pinning centres. The films were grown by pulsed laser deposition using a target with 4 % BaZrO3 nanoinclusions. Data were analysed in the framework of the Anderson-Kim model, collective pinning model, and a model due to Zeldov that implies a logarithmic dependence of pinning potential on the current density. It was found that the latter describes better our experimental data, and that the resulting pinning potential has reasonable high values in high magnetic field up to 9 T, indicating a high current-carrying capability in high magnetic fields.

The scaling of transport AC losses in Roebel cables with varying strand parameters

A Roebel cable is a good candidate for low-voltage windings in a high-temperature superconductor (HTS) transformer because of its high current-carrying capability and low AC loss. Transport AC loss measurements were carried out in 1.8 m long 15/5 (fifteen 5 mm wide strands) and 15/4 Roebel cables. The results were compared with those in many Roebel cables composed of 2 mm wide Roebel strands. Comparison of the AC losses hinted that the intrinsic difference in normalized transport AC losses is due to differences in the g/w (ratio of the horizontal gap between the Roebel strands over the Roebel strand width) values. The intrinsic difference was confirmed by measuring transport AC loss in a series of horizontally arranged parallel conductor pairs with various g values. A method to scale transport AC losses in Roebel cables with varying strand parameters was developed. The scaling method will be useful for a rough assessment of AC loss in one-layer solenoid winding coils, such as in a HTS transformer.

Investigation of a Rutherford cable using coated conductor Roebel cables as strands

Coated conductor applications such as fusion magnets, particle accelerator magnets and generator windings require high current-carrying capabilities. This requirement can be fulfilled by various cable concepts using commercial long length REBCO coated conductors with high current-carrying performance. In the past few years, our group has successfully developed the Roebel cable concept for coated conductors. The design advantages of such a cable are high current-carrying capability and low alternating current (AC) losses. Unfortunately, for large-scale applications, the possibilities of a simple scale-up of the Roebel geometry are limited and additional design ideas are needed. One way to reach the required high currents is the Rutherford cable concept. In this concept a conductor is wound with transposition on a flat metal former. In order to design the former, the bending properties of the Roebel assembled coated conductor cables (RACC) must be measured and characterized. This allows the identification of a destruction-free interval for the Roebel cable, in terms of bending angle and transposition length. In this work we designed and assembled a demonstrator of a coated conductor Rutherford cable (CCRC) with three RACC cables. We measured the critical current and the AC losses of the cable demonstrator. Our results show that, despite still needing efforts in terms of reproducibility of the assembly process and of AC loss reduction, this design is a promising and viable solution for high current-capacity cables made of coated conductors.

Electric power grid application requirements for superconductors

Abstract

Growth and Properties of YBCO-Coated Conductors Fabricated by Inclined-Substrate Deposition

YBCO-coated conductors with high current-carrying capability are desirable for electric power transmission applications. Inclined-substrate deposition (ISD) is capable of producing high-quality biaxially textured template films, which are important for fabrication of YBCO-coated conductors. We have grown biaxially textured ISD-MgO template films on flexible metallic substrates at deposition rates of 2-10 nm/sec. Columnar grains with a roof-tile-shaped surface structure were observed on the ISD-MgO films. X-ray pole figure analysis revealed that the ISD-MgO film is biaxially textured and its c-axis is titled at an angle from the substrate normal. Strontium ruthenium oxide (SRO) buffer films were epitaxially grown on ISD-MgO by pulsed laser deposition prior to the deposition of YBCO. Low /spl phi/-scan full-width at half maximum (FWHM) values of 6/spl deg/ and 7/spl deg/ were observed for YBCO and SRO, respectively. T/sub c/ of 91 K with a sharp transition and transport J/sub c/ over 1.4 MA/cm/sup 2/ at 77 K in self-field were measured on YBCO coated conductors grown with ISD MgO architectures using a SRO buffer.

Investigation of a microtriode with a planar field emitter-extractor source fabricated by direct-write nanolithography using electron beam induced deposition

A simple microtriode with planar field emission cathode is investigated. Electron optical performance and field distribution are simulated using numerical electron optics. A field emitter tip is fabricated by direct write nanolithography with electron beam induced deposition. The emission properties are studied in UHV and high vacuum. Measurements of currents from deposited emitters are recorded with milli-second resolution to monitor short-term variations in long-term measurements. Emission currents of a few nanoamperes are observed over several days without and with the use of an ion mirror. Depending on vacuum conditions the currents show a characteristic variation of up to 50%. The work is aimed to develop a planar micro-triode with very high current-carrying capability and low transconductance.

Ballistic carbon nanotube field-effect transistors.

A common feature of the single-walled carbon-nanotube field-effect transistors fabricated to date has been the presence of a Schottky barrier at the nanotube--metal junctions. These energy barriers severely limit transistor conductance in the 'ON' state, and reduce the current delivery capability--a key determinant of device performance. Here we show that contacting semiconducting single-walled nanotubes by palladium, a noble metal with high work function and good wetting interactions with nanotubes, greatly reduces or eliminates the barriers for transport through the valence band of nanotubes. In situ modification of the electrode work function by hydrogen is carried out to shed light on the nature of the contacts. With Pd contacts, the 'ON' states of semiconducting nanotubes can behave like ohmically contacted ballistic metallic tubes, exhibiting room-temperature conductance near the ballistic transport limit of 4e(2)/h (refs 4-6), high current-carrying capability (approximately 25 micro A per tube), and Fabry-Perot interferences at low temperatures. Under high voltage operation, the current saturation appears to be set by backscattering of the charge carriers by optical phonons. High-performance ballistic nanotube field-effect transistors with zero or slightly negative Schottky barriers are thus realized.

Growth and transport properties of c-axis-oriented MgB 2 thin films

We report the growth of high quality c-axis-oriented MgB 2 thin films on Al 2O 3 substrates by using a pulsed laser deposition technique. The thin films show an onset transition temperature of 39.2 K with a sharp transition width of ∼0.15 K. X-ray diffraction patterns indicate a c-axis-oriented crystal structure perpendicular to the substrate surface. We observed high critical current densities ( J c) of ∼16 MA/cm 2 at 15 K and under self-field, which is comparable to or exceeds those of cuprate high-temperature superconductors. The extrapolation J c at 5 K was observed to be ∼40 MA/cm 2, which is the highest record for MgB 2 compounds. At a magnetic field of 5 T, the J c of ∼0.1 MA/cm 2 was detected at 15 K, suggesting that this compound is very promising candidate for the practical applications at high temperature with lower power consumption. As a possible explanation for the high current-carrying capability, the vortex-glass transition is considered.