Temperature dependence of transport ac losses in Bi-2223/Ag multifilamentary tapes

Temperature dependence of transport ac losses in two Bi-2223/Ag multifilamentary tapes with different dc current–voltage characteristics was measured using a lock-in technique at power frequencies. At each temperature, different criteria for critical current determination were used. Comparisons of normalised ac losses with predictions of theoretical models for a tape with elliptical cross-section and a thin strip were made. It was found that the form of the current–voltage characteristic and the critical current criterion play an important role in comparison with theoretical data. A new normalisation procedure of ac loss data is proposed.     

Magnetic measurement of transport AC losses in Bi-2223/Ag tapes

Magnetic nature of the losses in superconducting wire carrying AC current implies that it should be possible to determine these losses in a contactless way. Ribbon-like samples are quite favorable for such an experiment, because a notable portion of magnetic flux related to losses ‘escapes' the sample volume and can be detected by an appropriate pick-up coil. In this case, a model describing the AC current penetration into the tape, based, e.g., on the critical state model, allows one to derive the losses from the pick-up coil signal. Because this signal is proportional to the number of coil turns, extension of the accessible range of measured voltages (and losses) can be achieved. We demonstrate the data obtained on a 1 cm long portion of a low-loss multifilamentary tape carrying AC current with frequency 35 Hz. The pick-up coil technique allowed us to reach loss level more than one order below the experimental limit for direct measurements.     

Experiment of enhancing critical current and reducing ac loss in pancake coil comprised of Ni-coated Bi-2223/Ag tape

An approach of realizing high performance HTS coil comprised of ferromagnetic material-coated BSCCO tape is proposed. The concept of influencing critical current and ac loss is based on the magnetic shielding effect resulting in redirection of self-field flux-lines. In the previous article, ac performance of Ni-coated tape was demonstrated where the Ni-coating was introduced at the edge-regime of the finished tape in order to redirect the perpendicular component of self-field lines. In order to investigate the shielding effect on ac performance in HTS coil, a two-turn pancake coil comprised of Ni-coated Bi-2223/Ag tape is demonstrated in the present article. About 6.4% of critical current was enhanced and 30% of transport current ac loss was reduced by means of 40 μm thick and 0.3 mm long (from the edge toward center of the tape) Ni-coating. This result suggests that additional ferromagnetic loss could be compensated well by the shielding effect of the partial Ni-coating. The degree of enhancement in critical current as well as ferromagnetic impact on ac losses depend on the volume and geometry of ferromagnetic coating introduced. Therefore, it is very important to control the parameter of ferromagnetic coating of the tape in order to balance the critical current and ac loss for optimum coil performance.     

Decrease of magnetic AC loss in twisted-filament Bi-2223 tapes

In AC power-engineering applications, a large part of the AC loss in the superconductor is due to magnetization by the external field. This magnetic AC loss has been well described for the low-T_c conductors. In Bi-2223 tapes the picture is different due to strong anisotropy, granularity and flux creep. Magnetic AC loss in various twisted and non-twisted Bi-2223 tapes has been measured at power frequencies by a pickup method. The results are compared to theoretical models of magnetization loss. When the field is parallel to the tape plane, the filaments in twisted tapes can be decoupled and the AC loss is decreased even when the matrix is pure silver. The extra effect of higher-resistance matrix materials is studied. In perpendicular field it is more difficult to decouple the filaments, due to the particular tape geometry. Contrary to a wire, there are essential differences between the AC loss mechanisms in a long twisted tape and those in a short piece of non-twisted tape. Finally, the dynamic resistance caused by the AC magnetic field is examined.     

高温超导带材Bi-2223/Ag的焊接接头电阻研究

针对高温超导带材Bi-2223/Ag的焊接问题进行了研究.利用ANSYS建立焊接接头电阻特性的有限元计算模型,仿真和计算分析了焊料材料、搭接长度和焊锡层厚度对接头电阻值的影响,并进行了实验验证.实验表明:可通过减少焊料厚度,增加焊接搭接长度来减少接头电阻,当焊接长度在2～5cm,焊料厚度为0.5mm时,Bi-2223/...     

Study of air heat treatment on Bi-2223/Ag superconducting tapes

Heat treatment has been carried out on commercially available Bi-2223/Ag superconducting tape under various temperature conditions in air. The critical currents (I_C) of tape after heat treatment at different annealing temperatures were tested at the liquid-nitrogen temperature. The microstructures and phase constituents of the tapes were investigated by SEM and XRD, respectively. The results indicate that annealing temperature has a very important influence on the superconducting properties of the tapes. When the temperature is above 840 °C, I_C value drops dramatically. Furthermore, when the temperature reaches up to 860 °C, the tape loses most of its superconducting property because of the decomposition of the superconducting phase Bi-2223.     

Superconducting joint of Bi-2223/Ag superconducting tapes by diffusion bonding

61-Filaments Bi-2223/Ag superconducting tapes have been joined by diffusion bonding. The critical currents (I_Cs) of the joints are obtained by using standard four probe method under no magnetic field in the liquid nitrogen. The microstructures of the joints are evaluated by the electron microscope in electron backscatter diffraction mode and the phase compositions of the superconducting cores of the joint and the original tape are determined by X-ray diffraction (XRD). The results show diffusion bonding is effective bonding technique for HTS tapes, and the bonding time is reduced greatly from hundreds of hours to a few hours, and the bonding pressure also changes from 140–4000 MPa to 3 MPa. Furthermore, the diffusion bonding joints sustain superconducting properties, and the critical current ratios (CCR_O) of the joints are in the range of 35%–80%. Microstructures of the typical joint display a good bonding and some defects existed in traditional method are avoided. XRD results show that the phase compositions of the superconducting cores have no obvious changes before and after diffusion bonding, which offers physical and material bases for high superconducting property of the joints.     

Electrical and magnetic properties of hot extruded Bi-2223/Ag concentric tapes

Summary In this work we report on the anisotropic physical properties of silver-sheathed Bi-2223 tapes fabricated by means of hot extrusion and repeated pressing and sintering processes. The obtained Bi-2223/Ag short tapes, having critical current densitiesJ _c of 20–30 kA/cm² at 77 K, 0 T, were measured in external magnetic fields up to 0.5T applied in two different orientations (i.e. μ₀H‖(a,b)-planes and μ₀H ⊥(a,b)-planes). The magnetic characterizations were performed in a wide range of temperatures and magnetic fields to study the first magnetization curve of tapes evaluating the lower critical fields μ₀H_c1⊥ab and ⊥₀H_c1#x2016;ab and their dependences on temperature. TheJ _c values at different fields in the temperature range 4.6–90 K, calculated from the magnetization data by the critical state model, are also presented. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Comparison of transport and magnetic AC losses in Bi-2223/Ag tapes — the role of superconducting core geometry

AC losses were measured by 4-probe transport method and by external magnetization method in three samples of Bi-2223/Ag tape: a multifilamentary tape with separated filaments, another multifilamentary tape with ‘bridges' between filaments, and a two-shell tape. The transport losses agreed with those calculated using I_c from DC experiment. Magnetization experiments gave indications about the various paths of induced currents. For the tape with well separated filaments the main part of screening current closes inside individual filaments. Additional screening of the whole filamentary zone involves the normal metal matrix, leading to frequency dependent losses. In the case of tape with ‘bridged' filaments, supercurrents interconnect the filaments into bundles whose screening (and loss) is frequency-independent. Matching the experimental data indicates that a typical bundle was composed of 8 filaments. Magnetic losses of the two-shell tape were explained by a model for magnetization of superconducting wire with elliptical cross-section.     

Reactivity and oxygen diffusion property of resistive barriers for Bi-2223/Ag tapes

Reactivity of several oxide materials (OM) with BSCCO powder and oxygen diffusion through OM layer has been tested at temperature ≈840 °C in air. The OM (e.g.: BaZrO₃, SrCO₃, MgO and ZrO₂) showing the low or no reactivity with BSCCO have been mixed (10 wt.%) with precursor powder and used for single-core tapes. Bi-2223/Ag/OM/Ag single-core tapes with oxide barriers made of BaZrO₃, SrCO₃, ZrO₂ and Al₂O₃ have been also prepared by a standard powder-in-tube technique. The used OM in the direct contact with BSCCO influences the electrical properties of Bi-2223 phase differently. These is because the oxides react with BSCCO during the heat treatment and simultaneously affect the 2212→2223 phase transformation, the Bi-2223 grain growth and so also grain connectivity. SrCO₃ powder has been evaluated as the best material from the point of no destructive effect on 2223 phase transport current property. The oxide barrier controls the oxygen diffusion during the tape heat treatment and simultaneously the HTS phase formation kinetics, its purity and content within the superconducting core. For single-core Bi-2223/Ag/OM/Ag tapes, the highest current density was measured for Al₂O₃ due to only slightly reduced oxygen diffusion through the barrier.     

Influence of tape-specific properties on local stress and strain in Bi-2223/Ag tapes

Superconductors are typically fragile, and their superconducting properties are reduced under strain. Moreover, superconductor wires operate under large mechanical loads in most applications. Thus, a careful mechanical analysis is essential when designing superconducting applications. So far, plenty of experimental and computational studies concerning stress and strain in, e.g. Bi-2223/Ag tapes have been carried out. However, Bi-2223 tapes contain several properties that vary between different tapes and are often unknown when designing a specific application. The largest uncertainties are related to the creep at high temperatures, stiffness and anisotropy of filaments and the yield strength of silver. In this paper, the influence of uncertainties on the local stress and strain in Bi-2223/Ag tapes is studied computationally to determine efficient strategies to reduce inaccuracy of the results. The results suggest that the filaments can be described as an isotropic material and the filament stiffness is the most important input parameter to estimate the total stiffness of the tape.     

The frequency dependence of AC transport losses in stacked Bi—2223／Ag superconducting tapes

AC transport losses in a single superconducting tape, double-and triple-stacked Bi-2223/Ag superconducting tapes were measured by use of electrical method. The measurements were carried out at 77K with the frequency of AC transport currents ranging from 50 to 100Hz. The dependence of AC losses on frequency and the number of tapes in the stack were presented and analysed.     

多芯Bi-2223/Ag带材交流损耗的特性研究

Bi-2223/Ag是一种很有前景的高温超导带材,交流损耗是高温超导在大规模电力应用方面的瓶颈,如何降低Bi-2223/Ag中的交流损耗是人们一直关注的问题。带材超导部分的横截面形状对交流损耗有很大的影响,前人曾通过双轴轧制等工艺制造矩形截面的带材,以降低其交流损耗;另有人在单芯带材中得到了环状结构,从中发现可降低交流损耗。本论文尝试利用抽换芯工艺制造多芯环状截面的Bi-2223/Ag高温超导带材,以期降低带材的交流损耗。论文中,通过解析方法计算了临界态模型下,临界电流密度在径向上分布不均匀的超导体的交流损耗特性,并得到结论:带材的外围临界电流密度越高,交流损耗性能越好。     

Magnetic coupling and self-field AC losses of two neighbouring Ag sheathed PbBi2223 tapes

The magnetic interaction between two neighbouring superconducting tapes has significant influences on their self-field AC losses. While the two tapes are independent of each other when separated far apart, they are expected to be fully coupled and behave as a single tape when placed very near. One of the consequences of such coupling is the increase in the self-field loss per tape, which is double of that for an uncoupled tape carrying the same current. In order to assess the interactions among tapes in systems such as a power cable, a better understanding is necessary on the critical distance where the coupling becomes important. Experimental measurements on the self-field losses in two neighbouring Ag sheathed PbBi2223 tapes were carried out with the two tapes separated by various distances, while placed either side by side (side-configuration) or one on top of the other (top-configuration). The results indicates that the critical coupling distance for the top-configuration is about 5 mm, where the increase in loss per tape is about 10%. The critical length for the side-configuration is found to be of the same order of magnitude.     

Bi—2223带材在拉伸应力下的临界电流

研究了不锈钢加强的Bi-2223/Ag高温超导带材在不同的拉伸应力下临界电流的变化情况。采用改造后的拉伸设备,在温度77K下对超导带材施加拉伸应力的同时,利用四引线法测量带材的临界电流,得到了超导带材的应力—应变曲线以及临界电流—应变曲线。利用理论公式计算了应力对超导带材临界电流的影响,与实验结果进行对比,并对两者的差别进行了分析。     

Electrical AC loss measurements of Bi-2223 tapes, performed under the Brite EuRam Research programme SACPA

A series of electrical AC loss measurements on Bi-2223 tapes have been performed under the Brite Euram project SACPA. This included, for the first time, a round-robin of independent self-field AC loss measurements between four laboratories. The very close agreement of data demonstrates the validity of the electrical technique and lays the basis for a measurement standard. Other preliminary measurements in SACPA showing the variation of losses with frequency, temperature and applied DC field are also reported.     

Fabrication of joint Bi-2223/Ag superconducting tapes with BSCCO superconducting powders by diffusion bonding

61-Filaments Bi-2223/Ag superconducting tapes have been successfully joined with BSCCO superconducting powder interlayer by diffusion bonding. The electrical properties of the diffusion bonding joints were tested by standard four probe method and the microstructures of the joints were also examined by SEM. Additionally, the phase constituents of the superconducting powders between the tapes before and after bonding process were evaluated by XRD analysis. The result shows that the diffusion bonding joints are superconductive. The microstructures of the joint display a good bonding with no cracks and discontinuities. The joining zones are mainly composed of Bi-2223 phase, Bi-2212 phase and a small amount of CuO, Ca₂PbO₄. At last, the phase transformations of the superconducting powders in the bonding process are discussed.     

AC loss measurements on model Bi-2223 conductors

Measurements of the AC loss in applied magnetic fields at 77 K have been made on model composite Bi-2223 conductors. A vibrating sample magnetometer (VSM) and a dual Hall sensor magnetometer (HSM) were used to cover the frequency range from below 0.01 Hz to over 250 Hz at AC fields up to 0.05 T rms. The VSM was limited to the frequency range below 0.2 Hz. A comparison of the two measurement techniques was possible at intermediate frequencies. The samples consisted of vertical stacks of well separated flat filaments of superconductor in Ag and Ag-alloy matrix, allowing a range of filament coupling conditions to be explored.