铁磁材料作用于Bi-2223带材临界电流的研究

主要研究了铁磁材料对Bi-2223带材的临界电流的影响。通过计算铁磁材料在不同布局下对带材磁场分布的影响,进行了相应的临界电流测试实验。分析表明,将铁磁材料垂直放在带材边缘能有效地提高带材的临界电流。     

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.     

Investigation of Bi-2223 high temperature superconducting tape as the material for gradient coil in MRI

The use of Bi-2223 high temperature superconducting (HTS) tape as a material for gradient coils in MRI is evaluated in this paper. Bi-2223 tapes have a very high critical current and a very low power loss. A HTS tape gradient coil is expected to provide much higher gradient strength and generate much lower heating than a copper coil. Measurements of the AC power loss of Bi-2223 tapes at typical operating frequencies for gradient coils are presented. The degradation of the critical current and its effect on the increase of AC power loss is analyzed. Practical technical issues such as resistance, gradient strength and mechanical performance are also discussed. A prototype Bi-2223 HTS tape gradient coil is evaluated to verify the concept.     

Bi2223/Ag高温超导带材在脉冲电流下电磁特性分析

对高温超导带材在脉冲电流下的电磁特性进行了实验与仿真研究。实验部分对不同频率过电流下的伏安特性进行了测量,仿真部分运用有限元方法分析了电流和磁场分布特点。结果表明:在超导丝中,电流会因为频率的升高而呈现集肤效应,频率越高电流越向带材边缘集中,但与常规导体所不同的是,由于超导体各向异性的特点,边缘的非平行自场使该区域临界电流下降,导致电流密度最大的地方不是在带材的边缘,而是趋向于分布在磁场较弱的带材中部。     

Bi-2223带材有阻焊接接头电阻特性的研究

本文针对Bi-2223/Ag带材接头有阻焊接展开研究,考虑大电流时超导I~V特性非线性影响,建立焊接接头电阻特性的有限元计算模型,总结出不同搭接距离和焊锡层厚度对接头电阻的影响规律,并着重分析了接头电阻对临界电流的影响,最终通过实验验证了理论分析得到的一些基本规律.为降低Bi-2223/Ag带材接头电阻对高温超导磁体运行时的不利影响,实际焊接时搭接距离应保证在7~10cm以上.     

Bi-2223/Ag高温超导带临界电流各向异性的实验测量及分析

本文介绍了Bi-2223/Ag高温超导带临界电流各向异性的实验测量方法及原理,指出了在不同磁场方向(与超导带面夹角)下,Bi-2223/Ag带临界电流随直流背景磁场的变化规律以及实验应注意的问题.Bi-2223/Ag带材的各向异性决定了其临界电流对垂直于带材表面的磁场非常敏感,这是该带材在高温超导磁体等强电应用方面的不利因素.     

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.     

Bi-2223带材制备的高温超导电流引线

为了降低沿电流引线的漏热,用于制备高温超导电流引线的B i-2223带材要求具有较低的热导率。文中介绍了低热导率银金合金B i-2223高温超导带材;给出了其热导率的测试结果;并采用该带材制造了高温超导电流引线样机。液氮条件下测试该段高温超导引线的临界电流为332A。     

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。     

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/...     

Estimation of Jc(B) dependence from self-field alternating current (AC) losses measured on Bi-2223/Ag tapes

Transport AC losses measured in self-field conditions on multifilamentary Bi-2223 tapes are often found to be lower than those calculated within the framework of the critical state model for a bulk wire with elliptical cross section, though generally higher than predicted for a strip. This effect is sometimes ascribed to the non-ideal geometry of the tapes, which does not exactly reproduce either shape. Here we propose an alternative explanation assuming that the critical current density of superconducting material depends on magnetic field. In practice, we analyzed the AC loss curve and deduced different I_c values for the individual data points, using the standard Norris equation for elliptical conductor. This gives the relation between ‘calculated' I_c and the self-field associated to AC transport current, which can be regarded as an alternative way to qualify the dependence of J_c on magnetic field. Important is that this procedure covers the range of fields below the self-field at I_c where the measurement in background DC field can not be used to determine J_c(B).     

Bi-2223/AgAu带材及应用进展

Bi-2223/AgAu 高温超导带材因为较低的热导率和较高的载流性能, 被认为是制备高温超导电流引线的最佳材料之一. 这篇文章简单介绍了高温超导电流引线的工作特点和不同包套 Bi-2223 带材的热导率, 在此基础上详细介绍了目前 Bi-2223/AgAu 带材和高温超导电流引线国内外研究进展, 最后展望了 Bi-2223 带材的发展趋势.     

Magnetic ac loss in multi-filamentary Bi-2223 / Ag tapes

The ac loss in high-T_c superconducting tapes with twisted and non-twisted filaments has been studied by a magnetic method. A brief overview of the theoretical background and the experimental set-up is presented. Measurements were made at 77 K in a magnetic field of 50 Hz frequency and 0.001–0.7 T amplitude. Application of dc transport current made it possible to distinguish between the loss components, arising from intra-grain and from filament currents. The magnitude of the filament loss component indicates that the filaments are fully coupled, which agrees with theory. In other measurements, the orientation of the external field with respect to the tape was varied. Knowledge of the ac loss in parallel and in perpendicular field is sufficient to predict the ac loss for any intermediate orientations of the field.     

AC loss in a high-temperature superconducting coil

In a typical superconducting coil made of BSCCO/Ag tape, both amplitude and direction of the magnetic field determine the critical current, resistive voltage and AC loss. The distribution of the magnetic field along and across the superconducting tape in a coil is rather complex. This gives rise to the question: how accurate can one predict the critical current, V–I characteristic and AC loss of the AC coil from results of short sample measurements? To answer this question, we have measured and compared the characteristics of a short sample and a small coil employing 14 m of the same tape at 77 K. The comparison is performed as follows. First, a short sample is characterised with regard to the field dependence of the critical current, V–I characteristic and the AC loss. Second, the distribution of the magnetic field along the tape in a coil is accurately calculated. From the data, the voltage along the tape and the loss of the tape in the coil are found. Finally, the resistive voltage and the AC loss of the complete coil are calculated and compared to measured AC losses in the frequency range of 0 to 160 Hz, typical for power applications.     

控轧控冷技术应用在Bi-2223／Ag超导带材制备过程中的研究

控轧控冷技术在钢铁中厚板材的应用已趋成熟，采用控轧控冷技术制备的中厚板材具有组织均匀、晶粒细小、机械性能高等优点．Bi-2223／Ag超导带材制备过程需经过多次反复的热处理和中间轧制过程，本文结合控轧控冷工艺特点，通过控制带材中间轧制制度和控制热处理冷却速度，研究了压下量和冷却条件对Bi-2223相变和其微观结构的影响，并借助SEM、XRD等方法对带材样品进行了观察，实验结果表明：合理的控轧控冷条件能够改善Bi-2223／Ag的微观结构并能够有效提高其电性能．这为控轧控冷技术在Bi-2223／Ag超导带材制备中的应用提供了初步理论依据，同时为形变热处理工艺提供了新的优化方向．     

Manifestations of damage to a silver–gold alloy sheathed Bi-2223 tape through the AC losses and V–I characteristics

AC losses are measured using the null calorimetric method on a silver–gold sheathed Bi-2223 tape. The dependencies on current and on frequency give the usual results, showing the predominance of hysteretic losses. After damaging the tape, the same measurements are repeated. Loss variations with current still follow a power law but with an increased coefficient. Furthermore, the loss now depends on the square root of the frequency. This result is attributed to the presence of cracks in the superconducting core. Some of them reduce locally the cross-section at a point where the transport current is shared nearly equally between the few remaining superconducting grains, and the silver–gold alloy. This explanation is supported by a micrograph of the peeled tape, which shows a thin layer of superconducting material adhering to the silver–gold alloy, even where large cracks are present. The comparison of critical current distributions, extracted from V–I characteristics of the tape before and after the damage, shows a broadening of the good condition peaks. Also a sharp peak at very low current appears and is attributed to a huge fracture in the core.     

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.     

Bi2223/Ag带HTS双饼线圈中的接头焊接

降低双饼线圈间的搭接电阻 ,可有效提高 HTS磁体的稳定性并降低制冷费用。文中在分析影响搭接电阻因素的基础上 ,对双饼线圈间的焊接工艺和接头性能进行了实验研究 ,结果表明所用焊接技术满足磁体稳定运行的要求     

AC-induced DC voltage in HTS coil

The interplay between AC and DC currents in a High-T_c Superconducting (HTS) coil, made of multifilamentary silver-sheathed Bi-2223 tape, was investigated. We observed that the application of a small sinusoidal current in the frequency range of 50–500 Hz into the coil, while it is already carrying a DC current in the range of 16–22.5 A, caused an increase in the coil DC voltage. The DC voltage increment due to the AC signal is found to increase linearly with frequency and quadratically with amplitude. The DC voltage increment increases as the coil current grows towards its critical value of 22.2 A. This result may be important in some power applications such as fault current limiters (FCL) and superconducting magnet energy storage (SMES) based on HTS coils.     

Indirect measurement of AC current distributions in HTS tapes by a pickup coil method

It is necessary to develop HTS conductors with a large current capacity and low AC loss characteristics for practical use of HTS power devices. For large capacity power applications, HTS tapes such as Bi-2223 tapes and YBCO coated conductors are assembled. So, it is important to evaluate the characteristics of current distributions in such assembled HTS tapes. However, an adequate measurement method has not been established. In this study, we have tried the development of an indirect measurement method to evaluate the current distributions in stacked HTS tapes. In this method, the current distributions are indirectly found from the results of the field distributions measured by a pickup coil located around the surface of the sample tapes. To confirm the accuracy and the sensitivity of this method, current distributions in a test conductor made of copper tapes are measured. Numerical and measured current distributions are compared. Using this method, the current distributions in stacked Bi-2223 tapes are measured at room temperature and 77 K. On the basis of the obtained results, the characteristics of the AC current distributions in the HTS tapes are discussed.