Ag Tapes Research Articles

Development of a Conduction-Cooled HTS SMES

A 35-kJ/7-kW conduction-cooled high-temperature superconductor (HTS) superconducting magnetic energy storage (SMES) is developed. This paper presents the configuration of the HTS SMES and a part of important test results. The magnet of the SMES is a solenoid-type coil made of Bi-2223/Ag tapes, and cooled to about 20 K by conduction-cooled method. The SMES is assembled in the Electric Power System Dynamic Simulation Laboratory, Huazhong University of Science and Technology. A series of tests are successfully carried out to evaluate the performance of the SMES, including the current-carrying ability of the magnet, the active and reactive power exchange capability between the SMES and a simulated power system, as well as the power-compensating effect of the SMES to the simulated power system after short-circuit fault, etc.

Phenomenological description of flux pinning in non-uniform high-temperaturesuperconductors in magnetic fields lower than the self-field

Critical currents measured on a commercial Bi-2223/Ag tape in applied magnetic fieldsranging from zero to 100 mT are analysed in order to demonstrate the validity of thephenomenological description proposed by the authors for flux pinning in polycrystallinehigh-temperature superconductors in low magnetic fields. The four-parameter model is ageneralization of the Kim formula describing the magnetic field dependence of the criticalcurrent density. When combined with the finite-element calculation, it is able to predict thecritical currents measured in different field orientations with an average accuracy betterthan 0.2%. The volume density of the flux pinning force is easily evaluated from theparameters of the model. Particular attention is paid to the problem of a possible samplenon-uniformity. In fact, the experimental data can be better explained assuming thehypothesis of better filament texturing in outer filaments. This also means that theaccurate determination of critical currents in low magnetic fields can be used toreveal a non-uniformity in the current-carrying capability of the superconductingmaterial.

The effect of lead-rich phases on the microstructure and properties of (Bi, Pb)-2223/Agtapes

The effect of lead-rich phases on the microstructure and properties of (Bi, Pb)-2223/Agtapes has been studied. Powders containing different types of lead-rich phases wereprepared by controlling the processing parameters, including temperature, time and oxygenpartial pressure. Then, monofilament (Bi, Pb)-2223/Ag tapes were fabricated using thesepowders. The phase compositions of the powders and tapes were analysed by x-raydiffraction and the microstructure of these tapes was observed using scanning electronmicroscopy. Experimental results show that the lead-rich phases in powders played animportant role in the formation of the (Bi, Pb)-2223 phase, and the microstructure and thesuperconducting properties of (Bi, Pb)-2223/Ag tapes. The formation process of(Bi,Pb)2Sr2CaCu2Ox in tape fabricated using the powder containingCa2PbO4 is different from a tape fabricated using the powder containingPb3Sr2.5Bi0.5Ca2CuOy, which differs in the formation process and grain sizes of the (Bi, Pb)-2223 phase. Therefore,the critical currents of (Bi, Pb)-2223/Ag tapes containing various lead-rich phases are different.The highest critical current is obtained for tape fabricated using powder containing bothCa2PbO4 and 3321 phases.

Nanoceramic Processing for High-Power Multi-channel Electron Multiplier in Low Temperature Cofire Ceramic (LTCC)

Nanoceramic processing technologies are brought together with standard LTCC materials to demonstrate a dynode structured electron multiplier with integrated cooling. Process developments include the integration of numerous components, including an embedded passive, high-density interconnect, and high-performance thermal management system. Enhanced processing capabilities utilize nanoparticles to control sintering kinetics. Cofirable thick Ag tapes have been demonstrated that allow multilayer structures to include a thick (up to 0.5mm) solid Ag layer. The development of 3-D cavities using fugitive inserts have produced meso- and micro-scale channels, with X-Y-Z channels of 50 μm diameter demonstrated. Nano-sized MgO, processed using electrophoretic deposition, has been developed for a secondary electron emitter (SEE) with a gain factor of 2.0. To minimize thermal run-away and, subsequently, thermal failure, a positive temperature coefficient (PTC) resistor system compatible with LTCC processing is being developed to reduce supplied power to the individual multiplier structures to control localized heating. In this study the synthesis of nanostructure MgO is shaped by electrophoretic deposition that consists of the deposition of particles at dynodes submerged in a solution of magnesium methoxide. Charged particles of MgO are suspended in the solution and forced to move toward the dynode (which bears the opposite charge) by applying an electric field, thus forming a thin coating of collected MgO particles on the dynode. Different annealing conditions are used to optimize the microstructure and secondary electron emission (SEE) of the deposited materials.

Stability analysis of high temperature superconducting coil in liquid hydrogen

Recently, it is expected that hydrogen plays an important role in energy source including electric power in near future. Liquid hydrogen has high potential for cooling down superconducting coil wound with high temperature superconductors (HTS), such as BSCCO, YBCO. In this paper, we study stabilities of the coils wound with BSCCO tapes, which are immersed in the liquid hydrogen, and compare stability results with those cooled by liquid helium. We treat a minimum propagation zone (MPZ) theory to evaluate the coil stability considering boiling heat flux of the liquid hydrogen, and specific heat, heat conduction and resistivity of HTS materials as a function of temperature. It is found that the coil cooled by the liquid hydrogen has higher stability margin than that cooled by the liquid helium. We compare the stability margins of both coils wound with Bi-2223/Ag tape and Bi-2212/Ag tape in liquid hydrogen. As a result, it is found that the stability of Bi-2212 coil is equivalent to that of Bi-2223 coil in low and high magnetic field, while the maximum current of Bi-2212 coil exceeds a little bit that of Bi-2223 coil in both magnetic fields.

A Study on the Performance Improvement of an HTS Coil by the Outer Corner Modification

In designing an HTS coil, it is necessary to secure the large magnetic field and/or the stored energy with shorter tape length. From this point of view, the transport current performance of the HTS coil needs to be improved. The critical current and value depends on the magnitude and direction of the traversing magnetic field against the Bi-2223/Ag tapes. The critical current and value of the tape was measured with various magnitude and direction of the traversing magnetic field. From this, the fitting equations of the critical current and value for any magnitude and direction of the traversing magnetic field were obtained. Considering this anisotropic property of the tape, we considered the analytical model to obtain the coil critical current. As an initial coil shape, the minimum volume coil derived from the Fabry Factor constant curve is taken, which is often employed in low temperature superconducting coils. The electric field distribution within the coil cross-section was calculated to examine its effect on a current carrying capability. It was clear that high electric field portions appear at the coil edges due to the inclined magnetic flux against the tapes. In this study, the external corner portions of the coil are displaced under the condition that the total coil volume is kept constant. As a result of this, the coil critical current increased and the stored energy was improved by 20%.

Fault Current Limiting Properties of ${\rm MgB}_{2}$ Superconducting Wires

We have investigated magnesium diboride wires as an element in a resistive superconducting fault current limiter using a pulsed system, which could deliver a fixed number of AC cycles to avoid burnout due to thermal instabilities. Experiments have been carried out in both liquid helium and at 27 K on magnesium diboride mono-core wires in stainless steel tubes, in a CuNi sheath with an Fe barrier and also on multifilamentary wires with Nb barriers. Experiments showed good current limiting in the first cycle and no damage after six cycles. Also wires with a number of voltage contacts spaced at about 1 cm intervals to assess the effect of inhomogeneity were tested. Relatively small differences in local J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> led to large differences in the local temperature rise although in no case did this lead to failure of the conductor. The results were compared with those on Bi-2223/Ag tapes and YBCO coated conductors measured in liquid nitrogen.

Quench Protection of Bi-2223/Ag Tape Using SFCL Made of YBCO Thin Film

As a fundamental study for the safe operation of high temperature superconducting (HTS) machines, AC over-current characteristics of HTS tapes and the method for the protection using superconducting fault current limiters (SFCLs) were investigated. Firstly, the AC over-current characteristics of a Bi-2223/Ag tape were investigated. Various amounts of AC over-currents were applied and the quench behavior was analysed, measuring resistance increase with applying time. Secondly, based on the over-current characteristics of the HTS tapes, SFCLs made of YBCO thin films was applied to the protection of the HTS tapes for the safe operation. At this time, the quench characteristics of the HTS tape and the SFCL were considered simultaneously and each electrical operation was compared. At the transition range from super- to normal state in the HTS tape, quench behavior of both materials was investigated in detail. From the experiments, the safe operation state of HTS tapes was achieved and the optimal design factors for the SFCL applications were acquired.

“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.

Performance Improvement of Superconducting Tapes Due to Ferromagnetic Cover on Edges

Improvement of critical current and reduction of AC loss is presented for a common multifilamentary Bi-2223/Ag tape with thin nickel layer electroplated at the edges. Numerical calculations using a commercial finite-element code have been carried out to find the distributions of electrical current and magnetic field when the whole section is filled with the critical current density. The dependence of critical current density on local magnetic field and its orientation was taken into consideration, allowing to understand the mechanism of Ic increase. Optimization of the cover thickness and width would be possible due to these calculations. AC loss was investigated experimentally in the condition of AC transport and simultaneous action of AC magnetic field in phase with transport current. Reduction of both the transport loss and the magnetization loss has been observed, as well as the loss at simultaneous action of transport AC and applied AC field.

Performance of Inverter Fed HTS Induction-Synchronous Motor Operated in Liquid Nitrogen

We studied the effects of changing the driving of a squirrel-cage type induction motor with high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconducting (HTS) rotor windings. Bi-2223/Ag tape conductors were utilized for the secondary windings and the conventional (normal conducting) stator, 3-phase and 4-pole, was introduced. The fabricated motor was installed in the metal cryostat, and no-load tests were carried out in liquid nitrogen (77 K). A PWM (pulse width modulation) inverter was utilized in order to drive the motor. The test results are discussed based on the nonlinear electrical equivalent circuit. It is shown that the fabricated motor operates not only at slippage mode but also at synchronous mode for the frequency range from 10 Hz to 60 Hz. Furthermore, the rotating characteristics had frequency dependence, and higher input voltage was necessary for synchronism as the driving frequency increased. These characteristics were explained based on the conventional theoretical discussion and the nonlinear current transport property of HTS windings.

The effect of post-annealing on the critical current recovery of bending-deformed (Bi,Pb)-2223/Ag tapes

(Bi,Pb)-2223/Ag tape fabricated by PIT methods is a promising candidate for electrical power application. In these applications, the tape often undergoes a winding process, which leads to the degradation of the critical current. In this paper, we report the effect of post-annealing on the critical current (Ic) recovery of bent-deformed (Bi,Pb)-2223/Ag multi-filamentary tapes. The tapes were bent at room temperature with different curvatures, then straightened and post-annealed with different processing parameters, including post-annealing temperature, dwelling time and oxygen partial pressure. The experiment results show that rather high Ic value could be obtained by post-annealing even for the tapes whose bending strain was 0.8%. SEM observation shows that the decreasing in Ic value is caused by the formation and propagation of transverse micro-cracks in the (Bi,Pb)-2223 filaments and the effect of post-annealing could be related to the healing of micro-cracks caused by bending strain as well as the improving of grain connectivity.

The effect of high-pressure processing on unsealed Bi-2223/Ag tape

High-pressure (HP) processing of Ag-sheathed Bi2Si2Ca2Cu3Ox (Bi-2223) tape was investigated. In Bi-2223/Ag tapes, the outer Ag sheath might act as a sealing barrier against the penetration of the high-pressure atmosphere. In this work, short tapes after final heat treatment (FHT) were not sealed hermetically with Ag foil when HP processing was applied. The results show that the average thickness of tapes after HP processing is 2.5% lower than that of FHT tapes. However, the critical current Ic for tapes is not improved by HP processing. There are still many cracks and porosity in the superconducting core and coarse superconducting filaments are sometimes observed in transverse cross section of HP processed tapes.

Influence of Sheath Materials on the Strain Tolerance of Bi-2223 Superconducting Tapes

The critical current versus tensile strain characteristics of mono- and multi-filament Bi-2223 tapes with Ag and AgMn and AgMg sheath has been studied. The strain tolerance ε0.9 of naked mono- and 37-filament Bi-2223 tapes were 0.13% and 0.20%, respectively. Ag sheath increased the strain tolerance ε0.9 of critical current by 0.08% and 0.16% for mono-filament and 37-filament Bi-2223/Ag tapes. The ε0.9 of Bi-2223 with Ag and AgMn and AgMg sheath increased with filament number. AgMn and AgMg sheath, increasing ε0.9 of Bi-2223 tapes by 0.07-0.09%, are effective to improve strain tolerance of critical current for multifilamentary Bi-2223 tapes.

The improvement of YBCO film properties by two-step deposition using pyrolysis method

The YBCO films were directly deposited on the {1 1 0} 〈0 1 1〉 textured Ag substrates by ultrasonic spray pyrolysis method. The observed white points in YBCO film by one-step deposition at 900 °C consisted in high content of Ag on the surface of this film, which was due to the evaporation and diffusion of Ag atoms at high deposition temperature. While the performance of YBCO films was significantly enhanced using two-step deposition route. A 15 cm length of coated YBCO film on {1 1 0} 〈0 1 1〉 textured Ag tape was prepared by this two-step deposition method, which exhibited a transport J c value of more than 10 4 A/cm 2 (at 77 K).

The Effect of Deformation Stress-Strain and Temperature on The I&lt;sub&gt;c&lt;/sub&gt; Degradation of Bi-2223/Ag Tapes

Bi-2223/Ag tapes are usually used for superconducting power cables and magnets. Bi- 2223 ceramic superconducting core can be damaged under complicated stress and strain conditions such as winding tension, bending and twisting. In this study, we have presented the effect of axial stress and bending strain on the superconducting properties of Bi-2223/Ag tapes. In order to establish the value for 95% retained Ic of the Bi-2223/Ag tapes under various stress-strain conditions, the tension apparatus with bending former was used to apply the tension and bending stress-strain. Tension and bending stress-strain simultaneously applied to the tapes could reduce the critical current even though each applied stress and strain values were not higher than that of 95% retained Ic of the tapes. Complex stress-strain conditions including the thermal stress-strain have accelerated the degradation of the Bi-223/Ag tapes. The deformation temperature was important to maintain the 95% retained Ic of the Bi-2223/Ag tapes after bending or tension deformation because mechanical strength of the tapes can be changed drastically between room temperature and 77 K.

Investigation of the influence on residual stresses of porosity in high temperature ZrO 2 coatings on Ag tape for magnet technologies

The present paper reports on the effect on residual stresses of porosity in high temperature ZrO 2 coatings on Ag tape for magnet technologies. ZrO 2 coatings were fabricated on Ag tape substrate using a reel-to-reel sol–gel system. The microstructural evolution of high temperature ZrO 2 coatings was investigated by a scanning electron microscope (SEM). SEM observations revealed that ZrO 2 coatings with crack had some porosity and mosaic structure. Stress analysis was carried out on ZrO 2 coatings with porosity on Ag tape substrates under cryogenic conditions by using classical lamination theory (CLT) for elastic solution and finite element method (FEM) for elasto-plastic solution in the temperature range of 0 °C to −223 °C in liquid helium media. Because of the static equilibrium, tensile force is applied to the Ag substrate, by ZrO 2 coating. The stress component ( σ x ) values change rapidly at coating-substrate interface owing to the different moduli of elasticity and thermal expansion coefficient. In spite of the thickness of Ag substrate, the stress components vary from tensile to compressive. In addition, along the thickness of ZrO 2 coating and Ag substrate system, the stress distribution changes linearly. FEM results demonstrate that the failure does not occur in ZrO 2 coating for all porosities due to its high yield strength.

Microstructure and superconducting properties of Bi-2223/Ag tapes fabricated in high magnetic fields

The microstructure and superconducting properties of Bi-2223/Ag tapes fabricated in high magnetic fields were investigated. The results show that the partialmelting temperature has influence on the formation of Bi-2223 phase and the Jc. The optimum partial-melting temperature is 855°C. The magnetic field during sintering at 830°C has no influence on the formation of Bi-2223 phase and the Jc. However, the magnetic field during partial-melting process has obvious influence on the formation of Bi-2223 phase and the Jc. The tape partially melted with 10 T magnetic field shows stronger c-axis alignment of Bi-2223 phase and higher Jcvalue than that partially melted without magnetic field.

Stability measurements of LTS/HTS hybrid superconductors

A new kind of low temperature superconductor (LTS) and high temperature superconductor (HTS) hybrid conductor is proposed. In LTS/HTS hybrid conductor, a multifilamentary NbTi/Cu LTS cable and Bi-2223/Ag HTS tapes are combined. The objective is to use Bi-2223/Ag tapes as stabilizers in composite-type superconductors in place of low resistive metals, such as pure aluminum. Firstly, the stability measurement experiments on short sample LTS/HTS hybrid conductors have been done as a proof-of-principle using a 9 T bias magnet. A significant increase in minimum propagation currents has been observed in case of the LTS/HTS hybrid conductor as compared with NbTi/Cu cable.

Electromagnetic behavior of HTS coils in persistent current operations

The current decay behaviors in high-temperature superconducting (HTS) coils using Bi-2223/Ag tapes are investigated. Generally, the resistance of a superconducting closed loop is determined by the joints and electric field versus current density relationship of the HTS tape. In this paper, we demonstrate that the resistance is enhanced due to the decay of the shielding current by the flux creep and flux flow in HTS tapes. Experiments with HTS coils with persistent current operations were carried out and the results are compared with numerical analysis using a finite element method. Furthermore, the improvement of the current decay behavior is also discussed by choosing an overshooting excitation technique.