Ag Coil Research Articles

E–I characteristics and critical currents of small Bi-2223/Ag coil thermally stabilized by solid and liquid nitrogen compared to water ice

A small-sized coil of helically wound Bi-2223/Ag tape was measured in liquid/solid nitrogen (LN2/SN2), and also in water (H2O) ice at external fields of 0–8 T and in a temperature range of 10–77 K. This work is especially focused on the coil stability for current amplitudes above the critical current criterion of 1 µV cm−1. While the E–I characteristics measured under the critical current criterion did not show any substantial variances at these different cooling conditions, significant differences were observed above the critical current magnitude, mainly upon cooling by solid nitrogen and water ice. The results confirm improved thermal stability for the coil measured in sub-cooled water ice compared to solid nitrogen. Consequently, cooling by water ice could be interesting for future applications of high-temperature superconducting coils.

Effect of Electrode Geometry on Selectivity and Activity in CO2 Electroreduction

We investigated the effect of the electrode geometry on the product selectivity and catalytic activity in CO2 electroreduction (CO2ELR) for equal surface area Cu and Ag electrodes. Three different geometries (wire coil, foil coil, and flag) with relatively smooth surfaces were chosen for this study. The electrode geometry impacts the current density distribution on the electrode and the different current densities create an electric field distribution on the electrode surface. The geometry that causes the higher current density distribution (coil wire) showed higher total current density and lower charge transfer resistance in comparison with the flag-shaped electrodes for both Cu and Ag. The results showed that the current density distribution changes due to the geometry influences the product selectivity as well. Cu coil electrodes showed higher faradaic efficiency (FE) for C2 products in comparison to the Cu flag. Ag coil showed higher FE% for CO compared to the Ag flag.

The Effect of Temperature Dependence of AC Losses in a Bi-2223/Ag Insert of an 8-T Superconducting Magnet

A conduction-cooled split-gap superconducting magnet system with a center field of 8 T has been designed and fabricated in the Institute of Electrical Engineering, Chinese Academy of Sciences. The system consists of two Bi-2223/Ag coils and six NbTi coils. Due to a large aspect ratio of the high-temperature superconducting tape, there will be large ac losses when the magnet is ramped up and down. An accurate estimation of the total ac losses in the high-temperature superconducting coils is essential for the cryogenic system design. In the Bi-2223/Ag coils, the total ac losses mainly originate from two parts: One is transport loss caused by the variation of transport current with respect to time, and the other is magnetization loss due to alternating external magnetic field. In this paper, the effect of temperature variation on ac losses generated in the Bi-2223/Ag coils is studied. The magnetic field and temperature dependence relations of the critical current density $J_{c}$ are considered. The calculations are carried out in three steps. First, to estimate the magnitude of ac losses rapidly in the Bi-2223/Ag coils, the ac losses are calculated when the Bi-2223/Ag and NbTi coils are simultaneously charged, in which the temperature distribution is not considered. Second, the temperature variation is considered to calculate the ac losses under the same operating conditions with those in the first case. Finally, the NbTi coils are charged first, followed by the Bi-2223/Ag coils; the ac losses calculated are less than those in the second case. Hence, it is a good way to reduce the ac losses by changing the charging sequences of the Bi-2223/Ag and NbTi cols. Afterward, the calculated results are compared with the experimental data, and they show a good agreement.

Co/Ti-substituted M-type hexagonal ferrites for high-frequency multilayer inductors

The sintering behavior, microstructure and permeability of Co/Ti-substituted M-type hexagonal ferrites BaCoxTixFe12−2xO19 (1.0≤x≤1.4) was studied for applications as multilayer inductors in the high-frequency range up to 2GHz. Single-phase M-type ferrites were obtained after calcination at 1000°C. The saturation magnetization and coercivity decrease with x and hysteresis measurements illustrate a gradual transition of the anisotropy from uniaxial to planar. Addition of 5wt% of a BBSZ glass allows densification at 900°C through liquid-phase sintering. The permeability of samples sintered at 900°C increases with the Co/Ti substitution and reaches its maximum of µ′=16 at 1MHz at x=1.3 with a resonance frequency fr≥1GHz. Monolithic ferrite multilayer inductors were fabricated with printed Ag coil patterns by co-firing at 900°C. It is shown that Co/Ti-substituted hexagonal M-type ferrite is an excellent material for the high-frequency multilayer inductors.

Experimental and numerical investigation of screening currents induced in a Bi-2223/Ag double pancake coil for space applications

We modelled the screening currents (Is) induced in high-temperature superconducting (HTS) coils to develop a method for the characterization and design of HTS magnets for space applications. The analysis made use of so-called percolation depinning and flux creep models to describe the current density versus the electric field in HTS tapes. We compared the model results with the experimental data obtained from a Bi-2223/Ag double pancake coil. The experimental residual magnetization due to the Is in the Bi-2223/Ag coil can be effectively modelled, assuming an equivalent loop length of approximately 9 mm for the Is in the coil. The values calculated from the method quantitatively agreed with the results for various experimental conditions. We also successfully modelled the hysteresis of the magnetization due to the Is. These results demonstrate the validity of our model for the Is, which considers the effects of flux creep and smaller Is loops in the multi-filamentary Bi-2223/Ag tape.

Numerical Investigation on Thermal Stability of Conduction-Cooled Bi-2223/Ag Coil Under AC Ripple Current for Space Applications

This study investigated the transport ac loss in a high-temperature superconducting (HTS) coil under ac ripple currents with dc offsets for space applications of the HTS coil. We developed an analysis method to evaluate the effective ac loss in the HTS coil on the basis of the percolation depinning model. Our analysis clarified that larger dc offsets greatly increase the effective ac loss even under a smaller ac current. In addition, we investigated the effect of the ac loss with the ripple current on the thermal behavior of a conduction-cooled Bi-2223/Ag coil. As a result, the ac loss decreased the thermal stability of the conduction-cooled coil in case that HTS tapes in the coil are in the flux-flow state such as the load factor of 80%. However, at a lower load factor such as less than 60%, the ripple current did not have much effect on the thermal stability of the conduction-cooled coil because most of the ac power were consumed as a reactive power. These results suggested that, in order to apply the ac ripple current to HTS coils, the operational load factor must be properly selected. This study leads to a design of the light-weight HTS coil system for space missions.

Design and low-temperature cofired ceramic fabrication of a novel vibration-induced electromagnetic microgenerator with enhanced power output

We present a novel vibration-induced electromagnetic microgenerator with enhanced power output, which uses a unique inducer made of multilayer silver (Ag) coils and a magnet with a neodymium-iron-boron (NdFeB) central core. The device converts mechanical power into electric power using electromagnetic induction that arises from relative motion between the magnet and the vibrating induction coil. The Ag coils are combined with the ceramic microspring to build the inducer. The configuration of magnets is chosen such that the magnetic field in the radial direction for electromotive force is perpendicular to the velocity vector within the vibrating induction coil. Ansoft Maxwell (Pittsburgh, Pennsylvania) modal analysis is used to predict the magnetic field. In this work, the manufacturing technology of multilayer Ag coils of low temperature cofired ceramic (LTCC) is first used and feasible for the fabrication of vibration-induced electromagnetic devices. The experimental results produce a maximum electromotive force of 25.19 mV and a power output of 1 mW at a frequency of 120 Hz and amplitude of 0.03 mm. The volumetric and mass power densities per frequency are found to be 1.97×10−2 mW/cm3·Hz and 314 mW/Kg·Hz. The new design mechanism has been build and is valid for power output.

Simulation of high frequency properties of planar magnetic inductors prepared by direct write techniques

High frequency characteristics of the inductance (L) and the quality factor (Q) of square spiral type planar magnetic inductors prepared by direct write techniques were simulated using a Maxwell three-dimensional field simulator. The materials, cross-section shape, thickness and conductivity of the coil were varied to estimate the trend of the inductors' performances. The area dimension of inductors utilised was 1·3 × 1·3 mm. Al2O3 was used as the substrate material. The used inductors with a permalloy (Ni81Fe19) core, silver (Ag) coils, trapezoid shaped cross-section of 30 μm thick coil and coil conductivity of 70% of Ag bulk value exhibit L=12·3 nH, maximum Q=33 near the frequency of 50 MHz and a self-resonant frequency of 0·78 GHz. The simulated high frequency data of inductors agreed well with those obtained from the equivalent circuit model of inductors.

Influence of current ramp rate on voltage current measurement of a conduction-cooled HTS magnet

High-temperature superconductors (HTS) have notably different voltage current characteristic compared to the low-temperature superconductors (LTS). Due to the anisotropy and slanted electric field – current density characteristics the loss of stability in a Bi-2223/Ag magnet is viewed as a global temperature increase inside the coil rather than a local normal zone. Therefore, the quench current depends strongly on the cooling conditions. In this paper a finite element method based analysis method is presented and example runs are carried out in order to explain in detail the influence of the current ramp rate and cooling on the voltage current characteristics of a conduction-cooled Bi-2223/Ag coil at 20 and 45 K. The results show that in certain operation conditions the coil critical current has a maximum value with respect to the ramp rate used in the measurements.

“Tolerance Current” of Bi-2223/Ag Coil

Different from low temperature superconductors, Bi-2223/Ag HTS tapes have lower ldquonrdquo value. This characteristic brings rather ldquofuzzyrdquo notification of the critical current. It depends on the cooling condition. This means that the critical current of Bi-2223/Ag tape can not be decided absolutely. Rather we should focus on the temperature rise characteristics of the coil against over-current exceeding the critical current. If the temperature of the coil keeps a finite value without thermal runaway, the coil can be used continuously. We call the uppermost current without thermal runaway as ldquotolerance currentrdquo. Considering the situation mentioned above, the authors have studied the temperature rise characteristics of HTS coil for both DC and AC currents. Two cooling conditions are adopted. One is a cooling by immersion in liquid nitrogen bath, and the other is a conduction cooling by GM refrigerator. From the experimental result, the ldquotolerance currentrdquo of the Bi-2223/Ag coil for various conditions are obtained.

Analysis of Discharging Characteristics in a Bi-2223/Ag Coil for SMES With Consideration of Cooling Capacity of a Cryocooler

We investigated discharging characteristics of a Bi-2223/Ag coil by means of thermo-electromagnetic coupled analysis. The objective was to evaluate the thermal behavior of conduction-cooled high temperature superconducting (HTS) coils when they are applied to superconducting magnetic energy storage (SMES) systems for voltage dip compensation. The electromagnetic analysis adopted finite element method (FEM) considering the J-E expressions, which can quantitatively estimate the experimental data obtained from the tape in wide ranges of temperature (20-77 K), external magnetic field (0.02-3 T) and its applied direction (arbitrary). The thermal analysis also employed FEM, and was performed under new boundary condition where heat flow rate into a cryocooler does not exceed its cooling capacity. It was shown that transient temperature rise occurred in the coil during discharging, and such temperature rise was underestimated for generally applied boundary condition, i.e., fixed value of temperature

The effect of epoxy impregnation on the behaviour of a Bi-2223/Ag coil carrying DC or ACcurrent

We studied the behaviour of a superconducting coil consisting of three doublepancake coils made of Bi-2223/Ag multifilamentary tape with high criticalcurrent, cooled by liquid nitrogen at atmospheric pressure. One double pancakecoil located at one edge of the winding had a lower value of critical currentthan the other ones, as occurs in real coils quite often. We measured DCI–V curvesof the complete coil and of all pancake coils and the temperature increase in several parts of the winding atvarious d I/d t. The coil impregnation resulted in larger heating of the coil. The critical currents of theindividual pancakes after impregnation were reduced by 5% to 15% in comparison with thevalues measured before impregnation. The thermal runaway current was approximately twotimes larger than the coil critical currents defined by the mean value of the electric field1 µV cm−1. The values of safe operation currents are discussed. We also measured the dependence ofAC losses and the coil temperature on the coil current with a frequency of 50 Hz. Thecontribution of the resistive losses in the double pancake coil with reduced critical currentto the total AC losses of the whole coil was very small, which is very important for ACapplications of Bi-2223/Ag tapes.

Optimal design of a Bi-2223/Ag coil for superconducting magnetic energy storage atdifferent operating temperatures

We carried out optimal design of a Bi-2223/Ag toroidal coil for a superconducting magneticenergy storage system. The objective was to minimize the total length of Bi-2223/Ag tapeat fixed conditions of operating temperature, stored energy and upper limit of the loss. Theenergy loss was calculated by means of the 3D finite element method (FEM) with the use ofJ–E expressions, which can quantitatively estimate the experimental data obtained from thetape in wide ranges of temperature (20–77 K), external magnetic field (0.02–3 T) and itsapplied direction (arbitrary). On the other hand, the optimal solution was obtained by agenetic algorithm (GA), which was a particularly effective optimization method when theobjective function had a number of local minimum points. We performed suchoptimal design under various constraint conditions, and found that the optimalconfiguration of the coil drastically changed as its operating temperature varied.

Design of Bi-2223/Ag Coil Based on Genetic Algorithm and Finite Element Method

We designed Bi-2223/Ag coils based on genetic algorithm and finite element method. The winding geometry as well as the operating current of the coil was optimized for the minimization of its loss, on condition that its operating temperature, stored energy and total length of the conductor were constant. The loss was calculated with the use of the analytical expressions that could quantitatively describe the current transport characteristics in high T/sub c/ superconductor including their magnetic anisotropy and temperature dependence. Some optimized results were shown and discussed in this paper.

Anisotropic distributions of current density and electric field in Bi-2223/Ag coil with consideration of multifilamentary structure

Bi-2223/Ag double pancake coil with a stack number of 6 is analysed with the use of quantitative J–E expressions that can describe the magnetic anisotropy as well as temperature dependence of J–E curves in the tape. The practical multifilamentary structure of the tape is directly considered for the precise analysis. It is shown that the magnetic anisotropy greatly and complicatedly influences the distribution of the current density inside the tape as well as that of the electric field in the coil. Furthermore, the current transport performance of the coil is also shown to be largely influenced by the magnetic field generated by the coil itself.

Improved critical currents in a Bi-2223/Ag coil using splayed columnar defects

We have irradiated a double pancake coil of Bi-2223/Ag sheathed tape with high energy protons, which yield splayed columnar defects in the superconductor core of the tape. We report the critical current I c and current–voltage power law n value as a function of magnetic field at temperatures of 75–64 K for the coil and for a short sample taken from the coil. A novel apparatus using permanent magnets is used to apply a magnetic field in the coil radial direction. Both the coil and short sample show a strong enhancement in I c for magnetic fields along the c axis (tape normal or radial direction for the coil) compared to the performance of unirradiated tape. These results represent the first time a real superconducting device has been irradiated with high energy protons for the purpose of introducing splayed columnar defects to enhance performance.

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I– V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I– V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm −2 during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I– V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.

Current-voltage characteristics of an HTS coil having a hot spot

Due to a slanted electric field-current density characteristics some power dissipation exists in HTS magnets even at operation currents below a critical current determined by using some electric field criterion. Thus, the temperature inside an HTS magnet does not remain constant during a single voltage (V)-current (I) measurement. Furthermore, a sufficiently large operation current will produce a hot spot in the turn of the maximum electric field inside the coil. In this paper the effect of the temperature increase during a V (I)-measurement on the V (I)-characteristics of a Bi-2223/Ag coil is studied at 20, 40 and 56 K. The V (I)-calculations take into account both the material inhomogeneity and anisotropy and are based on the short sample data measured with the Bi-2223/Ag tape. The investigation is concentrated at overcritical currents, where the effect of temperature increase on the V (I) -characteristics of the coil is significant.

Energy and critical current considerations of Bi(2223)/Ag coils formicro-superconducting magnetic energy storage: influence of operatingtemperature and winding geometry within the same overall tape length

A detailed theoretical qualitative, as well as quantitative,analysis of the influence of the dimensions of Bi(2223)/Ag based cylindricalmagnets on the energy stored in the magnet's winding was performed for variousoperating temperatures. The results achieved can be used to consider thepotential suitability of the high-temperature superconductivity magnets forthe purposes of micro-superconducting magnetic energy storage applications. Amathematical model which enables one to calculate the values of basicparameters, such as the critical current and stored energy of cylindricalmagnets consisting of the set of Bi(2223)/Ag pancake coils, was developed withrespect to the real distribution of the magnetic field in the winding and theangular dependence (anisotropy) of the Ic(B) characteristic of the tape. Anexample of a detailed analysis of the influence of the winding geometry, whichis changed within the same overall length of 1, 2 and 5 km of themultifilamentary Bi(2223)/Ag tape, was performed at the temperatures of 77, 65and 4.2 K. The most interesting and important result achieved is that thegeometry of the winding that corresponds to the maximum stored energy differsaccording to the temperature. The disc-shaped magnets, which consist of a verylow number of pancake coils, are the most suitable solution at 77 and 65 K.Simultaneously, the value of the stored energy is practically independent ofthe bore diameter of the magnet. On the other hand, when looking for theoptimum winding geometry at 4.2 K, the magnets with the smallest bore diameterare more suitable, while the value of the energy stored does not depend on thenumber of pancake coils.

Optimization of winding geometry of Bi(2223)Ag coils with respect to external magnetic field

A theoretical model, which enables to estimate the critical currents distribution in the winding of HTS cylindrical coils exposed to external magnetic field, was developed. The model takes into account the real angular dependence (anisotropy) of the I/sub c/(B) characteristic of Bi(2223)Ag tape. It is shown how the change in the winding geometry within the same overall tape length and the value of applied external magnetic field parallel with the magnet axis influences both, the critical current and the resulting magnetic field generated by the magnet itself. It is predicted that applying a low magnetic field parallel to the HTS magnet axis may result in a relatively high increase in the magnet critical current (up to 30-40%) if compared with the magnet critical current achieved without the presence of external magnetic field. An example of optimizing the magnet winding geometry by using the different lengths of the Bi(2223)Ag multifilamentary tape of Vacuumschmelze GmbH production is presented.