传输交流的Bi2223/Ag高温超导带在不同取向交流外场下的交流损耗

在高温超导的电力应用中，如超导电机、变压器等，多数情况下，高温超导带材在通以交流传输电流的同时还处于交变磁场中。此时，超导体的交流损耗不仅依赖于磁场的大小，还与磁场相对于超导带面的取向有关。本文在77K及工频50Hz情况下，实验研究了单根多芯Bi2223/Ag高温超导带及两带并联时的交流损耗随着外磁场与带面夹角的变化情况；以及交流磁场对临界电流的影响情况；并对单根带及两带并联的实验结果进行了比较与分析。     

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.     

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.     

Reduction of AC losses applying novel resistive interfilamentary carbonate barriers in multifilamentary Bi(2223) tapes

Hysteresis losses and coupling losses, a main component of the AC losses in Bi(2223) tapes, can effectively be reduced by enhancing the resistivity of the matrix material between the filaments and applying a filament twist. Since through alloying the sheath, as using AgAu(8 wt.%), the resistivity can only be raised by a factor <10 (77 K), a new conductor configuration with a quite novel composite matrix having resistive SrCO₃ barriers inside the Ag matrix between the filaments was developed. These new barriers, a cheap and commercial material, withstand the tape annealing, do not react with the superconductor, sinter dense and have a good bonding to Ag. Applying two different preparation techniques for 19 filament prototype tapes, critical current densities up to 20.7 kA cm⁻² were achieved. We report on tape preparation, the effect on the phase texture and the superconducting properties of such barrier tapes.     

Evaluation of geometrical effect on magnetization loss in GdBCO coated conductor

The magnetization loss density in stacked GdBa₂Cu₃O_7−δ coated conductor was measured using a SQUID magnetometer in a magnetic field normal to the tape surface to investigate the effect of stack on the magnetization loss. The AC loss characteristics are successfully collapsed to a single curve by the normalization at all the measurement temperatures. The losses are similar at high magnetic fields regardless the number of tapes. In addition, the AC loss decreases with increasing the number of tapes in stack at low magnetic fields.     

Self-field AC losses of textured Bi2Sr2CaCu2O8+δ thin rods

Self-field AC losses of polycrystalline Bi-2212 thin rods textured by a Laser Floating Zone (LFZ) melting technique have been measured at 77 K. With the optimal processing parameters, these rods, of 1.6–2 mm diameter and 10 cm length, have a transport critical current density of 3 kA/cm² in the self-field which decreases to about 1.5 kA/cm² in fields of 0.02 T applied perpendicular to the rod axis. The self-field AC losses have been measured in DC magnetic fields up to 0.03 T. The measurements in zero field show that for a large current range the losses are dominated by hysteresis losses as described by the Critical State Model for a cylinder. For the measurements in DC fields the losses show an increasingly resistive-like dependence with current, while the hysteretic component expected from the CSM becomes less important. Measurements at different frequencies also indicated that the loss per cycle in fields is strongly frequency dependent.     

AC losses in multifilamentary HTS-composite tapes based on BiPbSrCaCuO

Some results of AC loss measurements are presented for 19, 61, 127-filamentary Bi-2223/Ag tapes prepared by the ‘powder-in-tube' method. All measurements have been made at T=77 K under sinusoidal transport current with frequency in the range of 30–600 Hz and the current amplitude up to 30 A. The measurements have been carried out both in self field conditions and at the external magnetic field applied to the tape at the different angles. The dependencies of the AC losses on current amplitude and frequency have been obtained. It is found that for all tapes the current amplitude dependencies of the AC losses show good agreement with the Norris prediction for an elliptical or strip geometry. The AC loss dependencies on frequency were linear. The measurements of AC losses in external magnetic field show that the change of AC losses is only through the change of the critical current. So the transport AC losses in the tapes are the ‘saturation losses' that is they are different from classic hysteresis losses.     

Calculating transport AC losses in stacks of high temperature superconductor coated conductors with magnetic substrates using FEM

In this paper, the authors investigate the electromagnetic properties of stacks of high temperature superconductor (HTS) coated conductors with a particular focus on calculating the total transport AC loss. The cross-section of superconducting cables and coils is often modeled as a two-dimensional stack of coated conductors, and these stacks can be used to estimate the AC loss of a practical device. This paper uses a symmetric two dimensional (2D) finite element model based on the H formulation, and a detailed investigation into the effects of a magnetic substrate on the transport AC loss of a stack is presented. The number of coated conductors in each stack is varied from 1 to 150, and three types of substrate are compared: non-magnetic weakly magnetic and strongly magnetic. The non-magnetic substrate model is comparable with results from existing models for the limiting cases of a single tape (Norris) and an infinite stack (Clem). The presence of a magnetic substrate increases the total AC loss of the stack, due to an increased localized magnetic flux density, and the stronger the magnetic material, the further the flux penetrates into the stack overall. The AC loss is calculated for certain tapes within the stack, and the differences and similarities between the losses throughout the stack are explained using the magnetic flux penetration and current density distributions in those tapes. The ferromagnetic loss of the substrate itself is found to be negligible in most cases, except for small magnitudes of current. Applying these findings to practical applications, where AC transport current is involved, superconducting coils should be wound where possible using coated conductors with a non-magnetic substrate to reduce the total AC loss in the coil.     

Granularity, microstructure, and flux creep in TlBa2Ca2Cu3O9+δ superconducting powder and silver-sheated tapes

Silver-sheathed TlBa₂Ca₂Cu₃O_9+δ (Tl-1223) tapes, with a transport critical current density, J_ct, of 6200 A/cm² at 75 K under zero magnetic field, were fabricated by the oxide-powder-in-tube (OPIT) method and characterized using an X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and superconducting quantum interference device (SQUID) magnetometer. The results of the magnetization and SEM study indicate that, in these tapes, individual grains are distributed randomly in orientation and weak links exist. However, strongly linked percolative current paths within the tape persisting in increasing fields, accompanied by strong intrinsic interplanar coupling, sustain a significant J_ct at high fields and lead to the plateau in the J_ct-H curve. Dislocation networks, which may act as effective pinning centers, are the common features observed in the tapes. To investigate the effects on flux pinning due to thermomechanical processing, magnetic relaxation at 1 and 2 T over 5–50 K was measured. The tape shows slightly lower normalized relaxation rates (S=-(1/M_o)dM/ dln t) than the cauliflower-like precursor powder. Analyses of the relaxation data obtained from the tape, after incorporating temperature dependence and field scaling, yield an expression for the effective pinning energy U_eff(J,T,H) = (U₁/H^0.54)[1-(T/ 72.5)²]⁴(J/J_i)^μ. This result was compared with the prediction of the collective flux-creep model, which suggests that Tl-1223 has a three-dimension-like (3D-like) vortex lattice. Presumably, a more plate-like powder morphology may result in improved texturing by the OPIT process. Tl_0.5Bi_0.5Sr₂CaCu₂O_7+δ ((Tl,Bi)-1212) powder with this morphology was therefore synthesized for comparison.     

Bi-axial texture in Ca0.1Y0.9Ba2Cu4O8 composite wires made by metallic precursors

High filament count, silver-sheathed composite wires of Ca_0.1Y_0.9Ba₂Cu₄O₈ (Y–124) were prepared by a metallic precursor route. The ductility of the metallic precursor enabled one to manufacture tapes containing up to 962 407 filaments, with filament dimensions as fine as 0.25 μm thick and 1 μm wide. By using a thermal-mechanical treatment to texture the Y–124 grains, transport critical current densities in the oxide filaments of 69 500 A/cm² at 4.2 K in self-field were obtained. Moreover, in an applied field of 0.1 T, the samples retained 39% of their self-field critical current density. A TEM investigation revealed significant bi-axial crystallographic texture: in areas viewed, c-axis alignment of adjacent grains was within 10° and oriented perpendicular to the tape face; a-axis alignment of adjacent grains was within 15° and oriented parallel to the longitudinal direction of the filaments. Furthermore, c-axis texture alone did not adequately predict the performance of these Y−124 composite conductors. Instead, performance scaled with the degree of bi-axial texture. These wires exhibited among the best reported J_c for a polycrystalline, sintered wire of YBCO in an applied magnetic field.     

High critical current density YBa2Cu3O7−δ tapes prepared by the surface-oxidation epitaxy method

YBa₂Cu₃O_7−δ (YBCO) films with high critical current density (J_c) were successfully fabricated on nickel tapes buffered with epitaxial NiO. NiO was prepared on the textured nickel tape by the surface-oxidation epitaxy (SOE) method. We have reported so far a critical temperature (T_c) of 87 K and J_c=4–6×10⁴ A/cm² (77 K, 0 T) for the YBCO films on NiO/Ni tapes. To enhance the superconducting properties of the YBCO films on the SOE-grown NiO, depositions of thin oxide cap layers such as YSZ, CeO₂, and MgO on NiO were investigated. These oxide cap layers were epitaxially grown on NiO and provided the template for the epitaxial growth of YBCO films. Substantially improved data of T_c=88 K and J_c=3×10⁵ A/cm² (77 K, 0 T) and 1×10⁴ A/cm² (77 K, Hc, 4 T) were obtained for YBCO film on NiO, by using a MgO cap layer with a thickness of 50 nm. The method described in this paper is a simple way to produce long YBCO tape conductors with high-J_c values.     

外场下机械振动对高温超导带材交流损耗的影响

高温超导带材在磁场中传输交变电流时,将受到电磁力的作用而产生机械振动,振动对带材的交流损耗将产生影响.本文讨论了振动情况下交流损耗的测量方法,在平行于带面的直流磁场下,测量了Bi-2223/Ag高温超导带材在不同振动情况下的交流损耗.结果显示:当传输电流频率偏离样品的共振频率时,振动对带材的交流损耗影响不大;只有当电流频率在共振频率附近时,样品产生剧烈振动,交流损耗才有明显的增加;另外,带材振动时的交流损耗随频率变化曲线的斜率比不振动时略有增加.     

AC losses in multifilamentary Bi(2223) tapes with an interfilamentary resistive carbonate barrier

For the most common AC application frequencies, the main component of the AC losses in multifilamentary Bi(2223) tapes are caused by hysteresis- and coupling losses. These losses can be reduced enhancing the matrix resistivity and applying a twist to the filaments. We report on the AC loss properties of 37-filament tapes with AgAu (8 wt.%) matrix, and novel 19-filament tapes with SrCO₃ barriers between the filaments. We performed transport AC loss and magnetic AC loss measurements in parallel and perpendicular magnetic fields. Both kinds of tapes were also prepared with filament twists below a twist pitch of 20 mm. The influence of the different tape modifications on the AC loss behaviour is presented and compared with theoretical models to understand the effect of the resistive matrix. In the case of magnetic AC loss measurements, reduced AC losses due to decoupled filaments were observed for the twisted tapes with a resistive matrix in low parallel fields.     

AC transport and magnetic characterisation of multifilamentary Ag-BSCCO(2223) tapes with different filament arrangements

AC losses in multifilamentary tapes depend on various parameters. Among them, geometrical factors such as overall tape width and thickness as well as the precise arrangement of the filaments are expected to have an important influence. Several theoretical models describe this dependency. In order to study these geometrical effects experimentally, we prepared a series of Bi(2223)/Ag tapes with gradually changing filament arrangements and tape aspect ratio, and characterised them by AC transport and magnetic measurements. The results are compared to model predictions.     

具有不同临界电流的Bi2223/Ag多芯超导带材的自场损耗和交流电流电压特性

对具有不同临界电流的Ｂｉ２２２３／Ａｇ多芯超导材的交流电流电压特性进行了测量和评价,当传输电流的有效值Ｉｎｎｓ小于临界电流Ｉｃ时,交流电压的大小与传输电流的频率成正比;但Ｉｎｎｓ接近临界电流时,不同频率所对应的交流电压的大小之间的差别减小了,所有的曲线都汇聚成一条曲线,传输电流的频率分别为４０Ｈｚ,６０Ｈｚ,８０Ｈｚ,２００Ｈｚ及３００Ｈｚ时,我们测量了交变传输电流在Ｂｉ２２２３／Ａｇ带材中产生的自场损耗。结果表明当传输电流的频率较低时,实验结果与基于Ｂｅａｎ模型的Ｎｏｒｒｉｓ方程预期的结果一致;另外,实验结果表明存在一个电流Ｉ′,它的值小于任何一个样品的临界电流,本实验中Ｉ′的值是１０安培,在传输相同大小的电流Ｉｎｎ且Ｉｎｎｓ〈Ｉ′时,Ｂｏｉ２２２３／Ａｇ带材的交流损耗与它的临界电流成正比;但当Ｉｎｎｓ〉Ｉ′     

AC transport current loss of assembled conductor of HTS tapes

We study experimentally and theoretically the AC transport current loss characteristics of a tape in multiple tapes assembled in single layer and subject to external field produced by transport currents of adjacent tapes. We measured the AC transport current losses of a Bi2223 silver-sheathed tape in a single layer arrangement of three tapes using our newly developed potential leads arrangement to avoid spurious loss components caused by the magnetization in the adjacent tapes. In the paper, the influence of the external AC field produced by adjacent tapes on the loss characteristics is studied based on the experimental results and theoretical analysis.     

Numerical minimization of AC losses in coaxial coated conductor cables

Power cables are one of the most promising applications for the superconducting coated conductors. In the AC use, only small resistive loss is generated, but the removal of the dissipated heat from the cryostat is inefficient due to the large temperature difference. The aim of this work is to minimize the AC losses in a multilayer coaxial cable, in which the tapes form current carrying cylinders. The optimized parameters are the tape numbers and lay angles in these cylinders. This work shows how to cope with the mechanical constraints for the lay angles and discrete tape number in optimization. Three common types of coaxial cables are studied here to demonstrate the feasibility of optimization, in which the AC losses were computed with a circuit analysis model formulated here for arbitrary phase currents, number of phases, and layers. Because the current sharing is practically determined by the inductances of the layers, the optima were obtained much faster by neglecting the nonlinear resistances caused by the AC losses. In addition, the example calculations show that the optimal cable structure do not usually depend on the AC loss model for the individual tapes. On the other hand, depending on the cable type, the losses of the optimized cables may be sensitive to the lay angles, and therefore, we recommend to study the sensitivity for the new cable designs individually.     

Performance at 10–50 K of Bi-2212/Ag multilayer tape fabricated by using pre-annealing and intermediate rolling process

High-transport critical current density (J_c-oxide)>500 kA/cm² at 4.2 K, 10 T can be obtained for Bi-2212/Ag tapes fabricated by using pre-annealing and intermediate rolling (PAIR) and melt-solidification process. In this paper, we report high-temperature properties of PAIR-processed Bi-2212/Ag multilayer tape in order to show their potential for practical applications operated at cryocooling temperatures. Magnetic field dependence and angular dependence of critical current (I_c) are investigated at temperatures ranging 10–50 K by using helium gas cooling and liquid neon. Field-temperature curves for I_c=0.2 and 2.0 A are also determined in order to show the approximation of the irreversible field. High-temperature performance of the tape is attractive to consider future applications. For example, the best sample carries I_c=267 A (engineering-J_c=303 A/mm², J_c-oxide=151 kA/cm²) and 92 A (104 A/mm², 52 kA/cm²) at 27.1 K (in liquid neon), in magnetic fields (parallel to the tape surface) of 2 and 10 T, respectively. Engineering-J_c of 100 A/mm² is obtained even in the perpendicular field of 0.5 T at 27.1 K.     

AC losses in BiPbSrCaCuO-2223/Ag multifilamentary tapes in conditions similar to those in superconducting transmission lines

Transport AC losses in BiPbSrCaCuO-2223/Ag multifilamentary tape with 19 filaments were measured. The sample was 1.05 m long, wound in form of a helix with a gap between the tapes comparable with the tape width. Two different types of potential leads—tape following, axis following—with taps positioned in the centre of the tape were mounted on inner as well as outer surface of the helical sample. AC losses were measured at power frequencies by an electrical method using a lock-in nanovoltmeter. The influence of potential wires arrangement and the potential taps position on measured AC loss level and its frequency dependence was analysed.     

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.