Bi-2223 Multifilamentary Tapes Research Articles

Measurement of In-Field E-J Characteristics in Multi-Filamentary Bi-2223 Tapes at Ultra-Low Electric-Field Down to Around 10−13 V/m

We have developed a method to investigate in-field electric-field v.s. current-density (E-J) characteristics at ultra low electric field criterion around 10-13 V/m in a multi filamentary Bi-2223 tape at 5 K based on scanning Hall probe microscopy (SHPM). Bi-2223 is widely used in high field applications, e.g. a 30.5 T NMR magnet system is currently being developed in Japan. Ultra-low electric field is induced in such magnets operating in persistent current mode with extremely steady field. Namely, E-J characteristics need to be clarified around that criterion. E-J characteristics can be estimated from decay of magnetization in the whole sample, assuming the uniformity of J. But for multi filamentary Bi-2223 tape, magnetization current path should be clarified. In this study, we measured local magnetic field profile as a function of time for Bi-2223 tapes using SHPM. Additionally, we carried out flux annealing measurements to realize ultra-low electric field induced in the tape. As a result, two main messages we obtained. Experimental field profile or sheet current density obtained from the inverted Biot-Savart law indicated that filaments remain coupled up to high field, and even at ultra-low E criterion. Moreover, local E-J characteristics around 10-13 V/m are obtained at 5 K and 4.5 T of perpendicular magnetic field, which is comparable perpendicular field component in a Bi-2223 insert coil in the high field NMR magnet system. Ic properties at 10-12 V/m decrease to nearly 50%, in comparison to the transport Ic properties at 10-4 V/m.

The calculation and measurement of electromagnetic energy flows for investigation of critical current estimation for high-temperature superconducting coil by means of pick-up coils

For practical operations of high-temperature superconducting (HTS) devices, we have proposed a monitoring and diagnosing system to detect the degradations of superconducting properties before normal transitions occur locally in the HTS winding. The proposed system estimates the distributions of critical current indirectly from measurement results of the active power components of the Poynting vector around the monitoring HTS devices conducting ac currents by means of pick-up coils pairs mounted surrounding the HTS devices. For this purpose, it is necessary to solve the inverse problem. In order to estimate the critical current distribution, therefore, it is first necessary to be able to calculate the measurement signal distribution from the critical current distribution of the monitoring coil. This paper first proposes the calculation method of signal distributions measured by our system, next reports the results of the experiments and analysis, which were carried out to verify the validity of the calculation method. A pancake coil wound with Bi-2223 multifilamentary tape exposed to a uniform AC magnetic field was assumed as a monitoring coil in the experiments and analyses. The good agreement between the measured signals obtained from the experiments and the numerical analysis demonstrates the effectiveness of the proposed calculation method of measured signals.

Improvement of Critical Current in Superconducting Joint Between Multi-Filamentary Bi2223 Tapes

The superconducting joint between Bi2223 tapes has been developed with a method of joint by incongruent melting (JIM) in our Laboratory. The multi-junction method used to increase the contact area yielded a demonstrated increase in critical current. In previous studies, a simple boundary shape with lap-joint was chosen to connect several filaments in the tape, and this method is not suitable for connecting all about 100 filaments. Of course, more filaments are necessary to be joined with superconducting boundary, in order to increase the critical current of joint. However, due to the thin tape structure, there is difficult to use of butt joint between two cross-sections of tapes. In this study, we proposed a multi-junction method by using multiple boundaries with stepped structure. In experiments, joint samples with increasing of junction number to three from one were prepared, and the critical current for each unction number were measured at 77 K (self-magnetic field). In result, the critical current is increased with increasing of the junction number, and a highest critical current of 51.8 A was obtained in three-junction sample. in particular, the critical current at first junction (end of tapes) is highest, indicating that critical current at the end is important in total critical current of joint. Therefore, single junction at end of tapes was prepared and evaluated by using X-ray diffraction and critical current measurement. From X-ray diffraction patterns, the joint area is estimated to be about 1 cm. The critical current was 36.5 A at 77 K.

Tensile strength for high-temperature superconducting joints fabricated with incongruent melting

We recently suggested a joint method by using incongruent melting for high-temperature superconducting (HTS) wires, such as joint between RE123-coated conductors and between Bi2223 multi-filamentary tapes. It is important to obtain a suitable mechanical performance in joint to develop a persistent current HTS magnet, and this joint method with the melting is considered to have a higher tensile strength than the solid diffusion. In this study, the tensile strength of such a joint at critical current under tension was investigated toward application in a HTS magnet, such as Bi2223-NMR. We prepared two types of lap joint samples between RE123-coated conductors and between Bi2223 multi-filamentary tapes, with wrapping Ni sheets on junction (joined part in joint sample). The critical current of the samples was a similar value about 15 A at 77 K. The Ic of the samples showed a dependence on a small tension of junction below 40 MPa. With the reinforcement effect of the Ni sheet, the tensile stress tolerance of both joints that Ic of joint is reversible was higher than 90 MPa.

Non-twisted stacks of coated conductors for magnets: Analysis of inductance and AC losses

Almost all present High Temperature Superconducting (HTS) cable designs for magnets are based on twisted or transposed concepts that were developed for Low Temperature Superconducting (LTS) cables. However, requirements for LTS materials (like filament twisting) are in general not valid for HTS materials, which are extremely stable; for example, non-twisted multifilamentary Bi-2223 tapes have been successfully used in several magnets. Is twisting necessary for HTS cables? We investigated inductance mismatches and AC losses by numerical and analytical methods in twisted and non-twisted stacks of coated conductors; various experiments reported in the literature support the analysis. Large (hysteretic) losses are common in all magnets built with tapes and are far larger than in magnets built with LTS multifilamentary conductors, because of the aspect ratio and large width of the tape. In small magnets, losses and residual magnetisation could be reduced by replacing a wide tape with a non-twisted stack of narrow tapes. In large cables, we have found that twisting a stack of tapes reduces losses only marginally. Therefore, non-twisted stack cables could be designed to have losses comparable to those of twisted ones. Some examples of non-twisted large cables for fusion applications are discussed: non-twisted stack designs can be simpler, more robust and cost effective than twisted ones, but would require additional R&D.

The Method to Diagnose Soundness of the High Temperature Superconducting Coil by Pick-Up Coil Pairs

We have proposed a new system using pick-up coil pairs to detect normal transitions in the high temperature superconducting (HTS) coils which are applied to several applications, high field magnets, SMES, transformer, and so on. In the previous study, we have demonstrated that our system can detect local normal transitions in the winding of the test transformer wound by Bi-2223 multifilamentary tapes even if the operation of the transformer is ongoing. However, it is not sufficient to detect normal transitions for operations of practical devices because sudden breaks are not allowable for practical operation. It is important that soundness of the coil, such as critical currents, are monitored and diagnosed for actual operation of practical superconducting devices. In this paper, the external magnetic fields and temperature dependences of signals measured by our system are investigated, in order to make possible to diagnose soundness of the windings from the signals. And then it is shown that critical current of the windings, as an indicator of soundness, can be estimated under ac operation. The diameter of the sample coil is 70 mm. The results indicate that the critical currents of the winding can be estimated from our measured signals.

Superconducting joint interface for multi-filamentary Bi2223 tapes fabricated by JIM method

Recently, we proposed a method of joint by incongruent melting (JIM) for making a superconducting connection between two multi-filamentary Bi2223 tapes. At high temperatures, the Bi2223 phase separates into solid Bi2212 and a liquid phase by incongruent melting. In the JIM method, the Bi2223 filaments in a tape are first melted to Bi2212 and liquid. Then, Bi2212 is connected by the liquid to form a superconducting current pathway. In the present experiment, a joint between multi-filamentary Bi2223 tapes was prepared, resulting in a superconducting joint with a critical current of 8 A at 77 K. We then investigated the microstructure of the superconducting joint interface using X-ray diffraction and SEM/EDS, to clarify the superconducting joint formation. In examining samples prepared using the JIM method, Bi2212 and Sr-Ca-Cu-O system compounds were found at the joint interface, which had a high crystalline orientation.

Comparison between Bi-2223 tape and RE-123 coated conductor from the view point of current transport properties influencing thermal stability

We have investigated flux flow dissipation in typical two kinds of HTS tapes, i.e., a Bi-2223 multi-filamentary tape and a RE-123 coated conductor (CC) from the view point of heat load under over current conditions. Based on systematic measurements on current-voltage characteristics, the nonlinear flux flow dissipation has been described analytically by taking into account current sharing in metallic sheath or stabilization layer. Flux flow dissipation in the RE-123 CC shows much steeper temperature dependence than that of the Bi-2223 tape. As a result, attainable cooling power becomes smaller in the RE-123 CC in comparison with that of Bi-2223 tape even if the same cooling condition. In other word, acceptable temperature rise in the RE-123 CC is small at over current condition, whereas moderate temperature dependence in the Bi-2223 tape allows stable operation even if the bias current exceeds the critical current. Influence of spatial inhomogeneity in the both HTS tapes has also been investigated. Longitudinal variation of local critical current, Ic, and its statistical behavior have been characterized by use of reel-to-reel scanning Hall probe microscopy. It has been found that the flux flow dissipation is possibly localized more than one order higher than that of the average value due to discrete local Ic drops.

Influences of Pb content on the critical current of Bi-2223 multi-filamentary tapes

Multifilament Bi-2223/Ag tapes with different Pb contents in precursor powders were fabricated by powder in tube (PIT) method. The influence of Pb contents on the phase formation, microstructure and critical current density of Bi-2223/Ag tapes were systematically investigated. X-ray photoelectron spectroscopy (XPS) demonstrated that the content and oxidation states change of Pb ions can influence the carrier concentration of superconducting phase. Scanning electron microscopy (SEM) revealed that the variation of Pb content can lead to the difference in microstructure due to the change of thermodynamic properties. The results showed that with the Pb content of x=0.35 in the nominal composition of Bi2−xPbxSr2Ca2Cu3Oy, proper content of liquid phase can be formed during the sintering process, thus maximum critical current (Ic) was obtained. The correlation between the transport property, carrier concentration and Pb content was discussed.

Detection of Local Abnormalities in Windings of High-Temperature Superconducting Transformers by Using Pick-Up Coils

We have proposed a new method for monitoring the condition in windings of high-temperature superconducting transformers during operation. In this method, pick-up coil pairs arranged outside the cryostat for measuring electric fields and magnetic fields are used. This indirect method has benefits such as high reliability, safe detection, and easy maintenance. The purpose of this paper is to confirm experimentally whether the system using pick-up coil pairs can detect unusual local conditions in the windings of high-temperature superconducting transformers. For the purpose of this paper, a test transformer of 800 VA class, which was wound with Bi-2223 multifilamentary tapes, was fabricated. The transformer was monitored by our system. When a local temperature rise in the primary winding of the transformer in liquid nitrogen was generated, the monitoring test of the transformer by our system was carried out, which confirmed that our system could detect the local abnormalities in windings of the transformer during operation.

A Measurement Method of AC Losses in Superconducting Coils Using Poynting's Vector Method

A new measurement method to determine ac losses in superconducting coils has been proposed, and its validity confirmed experimentally. In this method, the ac losses in the coil can be obtained from measured Poynting's vectors around the coil windings. The Poynting's vectors can be obtained by local magnetic fields and electric fields around the coil. Local magnetic fields and electric fields are measured by pick-up coils and potential leads, which are mounted around the sample coil. These pick-up coil and potential lead pairs are arranged on the inner side, outer side, top, and bottom of the coil windings to cover the entire coil windings. This measurement method has the benefit that ac losses in a coil in a superconducting coil system consisting of multiple coils can be measured. Not only ac losses in the coils but also information on local electromagnetic phenomena in the coils can be obtained from profiles of the Poynting's vectors. In this paper, the details of the measuring method are described, and experiments using a coil wound with Bi-2223 multifilamentary tapes were carried out in liquid nitrogen to confirm the validity of the measurement method.

Improved Poynting's Vector Method: AC Loss Measurement of HTS Tapes Formed into a Short Straight or a Solenoidal Coil

Abstract Improved Poynting's vector method without movements was proposed to measure ac losses of HTS tapes, formed into a short straight or a solenoidal coil, which were carrying ac transport currents under transverse ac magnetic fields. In the previous Poynting's vector methods, for a short straight case two sets of both potential lead pairs and pickup coils were rotated around tapes in order to measure local energy flow densities, i.e., the distribution of Poynting's vector; for a solenoidal coil case two sets of both potential lead coils and spiral pickup coils were shifted to the coil axis direction by one winding pitch of the solenoidal coil. During moving these sets at low temperature, the data acquisition system was working and the value of ac losses was finally obtained. These previous methods, therefore, have inconveniences such as much time to get the value, much space needed for moving sets, or frequent troubles of movements due to low temperature deformation. The present method based on the following idea can resolve them: as for pillar parts of potential lead lines which are standing on the tape surface, only one plays a substitute role for all as long as all lines are connected together. As a result, immovable arrays of sets can be rigidly arranged, so that the signal measured at each position can be simultaneously. Moreover, information enough to clarify the electromagnetic behavior inside the tape can be obtained, because the sets can put more closely to the tape surface. The validity of the present method was confirmed by preliminary experiments using Bi-2223 multifilamentary tapes in liquid nitrogen.

Relation of Shunting Current at Cracked Part to Critical Current and <i>n</i>-Value in Multifilamentary Bi2223 Composite Tape

Relation of transport current and n-value to collective crack-induced current shunting in BSCCO (Bi2223) multi-filamentary composite tape pulled in tension was studied experimentally and analytically. For analysis, the partial crack-current shunting model of Fang et al. was used by placing the collective crack as the partial one. It was shown that (a) collective crack-induced shunting current increases with increasing current (and voltage) and with increasing collective crack size, (b) the transport current normalized with respect to the transport current in noncracked state is described with the modified ratio of non-cracked area to overall cross-sectional area of superconducting filaments at low voltage where shunting current is low, while it deviates upward from the modified ratio at high voltage where the shunting current is enhanced, and (c) the enhanced shunting current acts to reduce n-value at high voltage. These features were similar to those of cracked coated conductor.

SMES용 전도냉각형 고온초전도 자석의 설계, 제작 및 평가

본 논문은 초전도 에너지 저장장치(SMES)용 전도냉각형 고온초전도 자석의 설계, 제작 및 평가에 대해 기술한다. 고온초전도 자석은 황동 안정화재를 갖는 2개의 Bi-2223 다심 선재가 적층된 4-ply 도체로 제작된 22개의 double pancake coil(DPC)로 구성된다. 그리고, 각 DPC는 내경과 외경이 각각 500 mm, 691 mm이고 높이가 10 mm인 2개의 single solenoid coil로 구성된다. 코일 내부의 전기적 손실에 의해 발생된 열의 냉각을 위하여 DPC 사이에 두께 3 mm의 알루미늄 판이 내재된다. 고온초전도 자석은 2단 Gifford McMahon 냉동기에 의해 5.6 K까지 냉각된다. 충전전류가 증가할수록 방전시 고온초전도에서의 최대 온도가 증가 하였다. 충전전류가 360 A일 때 ��치 없이 고온초전도 자석에 1 MJ의 자기에너지가 성공적으로 저장되었다. 본 연구에서는 SMES용 전도 냉각형 고온초전도자석에 대한 열적, 전자기적 특성을 보이고, 본 연구를 통해 얻어진 결과는 전도냉각형 고온초전도자석의 최적설계 및 안정도 평가에 활용될 것이다. This paper describes design, fabrication, and evaluation of the conduction cooled high temperature superconducting (HTS) magnet for superconducting magnetic energy storage (SMES). The HTS magnet is composed of twenty-two of double pancake coils made of 4-ply conductors that stacked two Bi-2223 multi-filamentary tapes with the reinforced brass tape. Each double pancake coil consists of two solenoid coils with an inner diameter of 500 mm, an outer diameter of 691 mm, and a height of 10 mm. The aluminum plates of 3 mm thickness were arranged between double pancake coils for the cooling of the heat due to the power dissipation in the coil. The magnet was cooled down to 5.6 K with two stage Gifford McMahon (GM) cryocoolers. The maximum temperature at the HTS magnet in discharging mode rose as the charging current increased. 1 MJ of magnetic energy was successfully stored in the HTS magnet when the charging current reached 360A without quench. In this paper, thermal and electromagnetic behaviors on the conduction cooled HTS magnet for SMES are presented and these results will be utilized in the optimal design and the stability evaluation for conduction cooled HTS magnets.

Development of a Compact Measurement System of Transport Characteristics in HTS Conductors by Using an HTS Current

In order to realize HTS power machines and devices with high performance, it is important to evaluate the characteristics of HTS conductors quantitatively. We developed a compact system to measure the transport characteristics of HTS conductors under various experimental conditions. This system enables us to measure the transport characteristics of straight HTS conductors up to a current capacity of 1 kA class at 77 K, 66 K, and 4.2 K. The measurement of transport current losses is done by the Poynting vector method. The distribution of Poynting vectors around the sample conductor can be obtained by using many sets of both potential leads and pickup coils. An HTS current transformer (HTS-CT) is used to feed current to the sample conductor for the measurement. A secondary current of about 1 kA at 77 K can be induced when the primary current is 40 A. The sample conductor is connected to the secondary coil of this transformer. As a sample conductor, we used a conductor consisting of 16 parallel Ag-sheathed Bi-2223 multifilamentary tapes. The joint method between the HTS-CT and sample conductors is also examined. Based on these results, the usefulness of this measurement system is discussed.

Transport AC Loss Measurements and Simulations in Bi-2223/Ag Tape with Ni Cover at the Edges

Measurements on Bi-2223 multifilamentary tapes covered on edges by nickel showed a substantial AC loss reduction (Gomory et al. in Supercond. Sci. Technol. 22: 034017, 2009) and increasing critical current of the tape. Here we present a systematic study of this effect involving numerical simulations. The samples were prepared by deposition of Ni layer on commercial Bi-2223 tape. After measuring the critical current and the transport AC loss, cross-section photos are taken and the properties of nickel cover determined to be used in the calculations. We observed that with increasing Ni thickness the transport AC loss of the wire is decreasing in the high current region. In low current region because of the magnetic hysteresis in the Ni cover the transport AC loss of the Ni covered tapes are larger. Our calculations according to the method which is detailed in Gomory et al. (Supercond. Sci. Technol. 22:034017, 2009) show good agreement with measurements.

Effect of number of filaments on the critical current density in multifilamentary Bi-2223 tape

The critical current property of Bi2Sr2Ca2Cu3Ox(Bi-2223) silver-sheathed tape has been remarkably improved by application of ConTrolled Over Pressure (CT-OP) process with adjusting temperature and oxygen partial pressure. For further improvement of the property of Bi-2223 tapes, the effect of number of filaments on the critical current density Jc was investigated. It was found that Jc was higher and its decrease with increasing magnetic field was weaker in the tape with larger number of filaments. In addition, the irreversibility field Bi increased with increasing filament number. The field-angle anisotropy Jc(0°)/Jc(90°) of the tape with the largest filament number was smallest, where the field-angle was measured from the a–b plane. However, the out-of-plane misorientation angle is fairly small in this tape. Hence, the low field-angle anisotropy in this tape seems to be attributed to improved superconductivity in the block layers in the superconducting region near the interface with silver matrix.

Fabrication and characterization of Bi2223 tapes with interfilamentarySrZrO3+Bi2212 barriers for AC loss reduction

To suppress interfilamentary coupling in(Bi,Pb)2Sr2Ca2Cu3Ox (Bi2223) multifilamentary tapes under an AC external magnetic field it is necessary toincrease the transverse resistivity by using a high-resistivity Ag alloy sheath as a matrix.Alternatively, oxide layers can be introduced as resistive barriers among the filaments,together with filament twisting. In this paper, we fabricated twisted Bi2223 tapeswith interfilamentary oxide barriers and evaluated the effect of the introductionof such a barrier on the AC magnetization loss properties in a perpendiculartransverse magnetic field. The conventional powder-in-tube method was used for tapefabrication. In order to avoid affecting the Bi2223 phase during the sintering process,SrZrO3 was selected as the barrier material. Moreover, 20 wt%Bi2Sr2CaCu2Oy (Bi2212) wasmixed with SrZrO3 to improve its ductility for cold working. Monocore Ag-sheathed rodswere coated with an oxide barrier using a slurry, before being stacked intohoneycomb structures. By twisting the filament with a twist pitch lengthbelow 9 mm and introducing interfilamentary barriers, the coupling frequency (fc) in a perpendicular field, which is inversely proportional to the decay time constant (τc) of coupling current, exceeded 100 Hz and perpendicular field losses around thepower-grid frequency were reduced from the analytical prediction for fully coupledfilaments. Avoiding physical connections among the irregular deformed filaments intwisted barrier tapes should lead to further loss reduction in a perpendicularfield.

Magnetic Flux Penetration and Transport AC Loss in Superconductor Coated Conductor on Ferromagnetic Substrate

The current distribution and the AC loss in a composite superconducting tape containing a layer from magnetic material is calculated and compared with experiments, showing a very good agreement. The situations of an alternating uniform applied field or a transport current are studied. The newly developed numerical model is an approximation to the critical state model, adapted for the applicability to commercial finite elements codes that solve the vector potential. Substantial feature of this procedure is that it can be carried out in the case when the critical current density in superconductor depends on the magnetic field and the magnetic layer material is non- linear. Additionally, the hysteresis loss in the magnetic material is estimated, based on its measured magnetization loops. Measurements on Bi-2223 multifilamentary tapes covered on edges by nickel confirmed our predictions, showing a substantial ac loss reduction in both the investigated regimes.

Measurement of Transport Current Losses in HTS Conductors With Large Current Capacity by Using HTS Current Transformer

In order to evaluate the transport current loss characteristics in HTS conductors with large current capacity, we developed a new compact measurement system. This system enables us to measure the transport current losses electrically in the straight HTS conductors with the current capacity of 1 kA class by using the small capacity AC power supply of about 50 A. In this system, an HTS current transformer is used to feed current to the sample conductor for the measurement. The secondary current of this current transformer can be induced at about 1 kA at 77 K when the primary current is 40 A. The current transformer is made very compactly, and the externals size is 70 mm in diameter, and 110 mm in length. The sample conductor is connected with the secondary coil of this current transformer. The loss measurement is done by the Poynting vector method. The distribution of Poynting vectors around the sample conductor can be obtained by using many sets of both a potential lead and a pickup coil. To confirm the validity of this measurement system, we measured the transport current losses in an HTS conductor. As a sample conductor, we used a parallel conductor composed of 16 Ag-sheathed Bi-2223 multifilamentary tapes. The obtained results showed that this compact system was useful for measuring AC loss characteristics of large scale HTS conductors.