热处理气氛对Bi2223带材中Bi2201相的影响

采用常规的形变热处理工艺制备Bi2223带材,通过X射线衍射仪、超导量子干涉仪和标准四引线法研究热处理气氛对带材中Bi2201相含量及载流能力的影响.结果表明,Bi2201相是影响带材临界电流的重要因素,而热处理气氛影响Bi2201相含量,通过一种复合热处理(热处理过程中使用2种气氛)可以消除带材中的Bi2201相.优化热处理工艺后,带材临界电流从78A增加到103A.     

Alternating Current Losses in Ag-Sheathed (Bi, Pb)-2223 Multifilamentary Tape at Liquid-Nitrogen Temperature

The alternating current losses have been investigated at 77 K on the silver-sheathed (Bi, Pb)-2223 multifilamentary tape having 37 filaments using an inductive technique under ac magnetic field with frequencies between 100 and 510 Hz. The measured data on the multifilamentary tape have been discussed vis-à-vis that reported for monofilamentary tapes. The results obtained could be accounted fairly well by considering contributions from the eddy current losses in the silver matrix along with the hysteresis losses in the superconducting filaments.     

热压处理对室温轧制Ag/Bi(2223)超导带材显微组织的影响

采用SEM和TEM观测了经热压处理后的Ag/Bi(2223) 带材(Jc＝56000A/cm2,在77K、自场)和只经过室温轧制处理的带材.与室温轧制的带材比较发现:即使在临近第二相的区域,热压处理后的带材结构也非常密实;热压处理后,晶体缺陷诸如裂缝、亚晶界、残余无定形态和中间相等,易阻碍或中断电流传输的缺陷显著减少;热压处理后带材的位错密度基本相同,但在某些区域分布不均匀或呈网状结构;在一定方向上晶界能捕获途经它的位错;热压过程促使第二相部分转变为超导相Bi(2223),进而提高带材的载流能力.     

Detection of temperature rise in Bi2223/Ag tape by time-frequency visualization of AE signals

This paper presents the electric insulated type detection method of the temperature rise in the Bi2223/Ag tape using the time-frequency visualization of AE signals based on the wavelet transform. At the high operation temperature, the specific heat and the normal zone propagation velocity of the Bi2223/Ag tape are high and low, respectively, and therefore the local and excessive temperature rise is easy to generate in the superconducting-to-normal transition. As a result, the Bi2223/Ag tape is degraded or burned. In the proposed method, AE signals accompanying with the temperature rise are measured by an AE sensor, and furthermore AE signals are precisely and visually measured and estimated by the time-frequency visualization using the wavelet transform. Experimental results show the feasibility of the proposed method as the detection method of the temperature rise in the Bi2223/Ag tape.     

Development of Bi(2223) Multifilamentary Tapes with Low ac Losses

A significant reduction of ac losses in twisted Bi(2223) multifilamentary tapes with Ag sheaths has been achieved by using oxide (BaZrO₃ and SrZrO₃) barriers between filaments. These barriers have two important effects: they increase the transverse resistivity, which suppresses induced coupling currents, and they reduce filament bridging, which in pure Ag sheath tapes largely cancels the beneficial effect of filament twisting. The decoupling can be gauged by the frequency at which loss shows a maximum in a low-amplitude ac field applied perpendicular to the tape. So far, the frequency of the loss maximum, f _m, in Ag-sheathed tapes has been enhanced from 5 Hz (untwisted) to 82 Hz (11 mm in twist pitch length). Different ways to introduce oxide barriers in tapes with 19–95 filaments are presented. The critical current density in the filaments varied between 10,000 and 20,000 A/cm². Ac loss measurements as well as the electrical and mechanical characterization are discussed in detail. The variation of the critical current density with bending strain is shown to be similar to that of tapes without barriers.     

Bi2223/Ag带双饼线圈失超电压耦合补偿测试

为排除双饼线圈两端的自感电压对其失超信号的影响，把它降低到测量所允许的范围内，本文介绍双饼线圈外加补偿线圈的耦合测试原理和在此基础上制成的Ic的测试装置。分别对研制的9个高温超导双饼线圈的Ic进行测试，获得非常好的实验结果，为将9个双饼线圈组合成磁体提供了依据。     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Bi（2223）超导体的磁屏蔽效应

研究了Ｂｉ（２２２３）超导体磁屏蔽筒的磁屏蔽效应，从而得出磁屏蔽效果与超本的屏蔽空间大小有关，以及将０．３ｍｍ厚的软铁筒放于两超导体屏蔽筒之间，可使磁屏蔽效果提高为单超导体蔽筒的２１倍的结论。     

Effect of Bi and Sr concentrations on the formation of the 2223 phase in Bi-Pb-Sb-Sr-Ca-Cu-O materials, obtained under different conditions of thermal treatment

The superconducting properties and the phase and chemical composition of Bi _x Pb_0.3Sb_0.1Sr _y Ca₂Cu₃O_z (x=1.5, 1.6, and 1.7;y=1.9, 2.0, and 2.1) materials synthesized by one-step and multistep thermal treatments have been investigated. The multistep annealing between 825–875°C promotes the 2223 phase. The change of Bi concentration has little effect on the 2223 phase formation, regardless of the kind of thermal treatment used, but increasing the Sr concentration (y> 2) strongly inhibits this phase. It has also been established that the loss of Pb after synthesis depends on the initial Bi concentration. The loss of Pb decreases when the initial Bi content is lower.     

Orientation Distribution of Grain Planes in Polycrystal Ag/Bi-2223 Tape

For polycrystalline Ag/Bi-2223 tapes, the preferential orientation of grains is a very important issue. The platelike grains in the tapes are generally believed in a high-order alignment. However, microstructural observation by SEM shows that the grain alignment is far from perfect. Theoretically, upper critical field, H _c2, for H parallel to ab plane and c axis was calculated form the relation of M(H). However, the ratio of H _{c2 (H//ab)} to H _c2(H//c) depends on the angle () between the ab plane of grains and the broad surface of the tapes. Based on the ratio, the orientation distribution of grains is obtained. The results show that grains in Ag/Bi-2223 tapes can grow with ab plans at any angle between 75° > > –75°, but no grain can grow with its ab plane perpendicular to the tape broad surface. The overall distribution is that: >90% grains orient in the angle range of || < ± 75°, about 5% grains at ±75° || ±85°, and no grains at || > ±85°.     

Fabrication of a working Bi-2223 superconducting magnet cooled by liquid nitrogen

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N₂), was designed and fabricated. Bi-2223 tape with a critical current of 147 A was prepared by a controlled overpressure (CT-OP) process at 77.3 K in self-field. Ten double-pancake coils were resistively connected by copper terminals. The bore diameter was 54 mm?, the magnet outer diameter was 122 mm?, the height of the magnet was 124 mm, and the weight of the magnet was about 3 kg. The maximum magnetic field at the center of the bore was 0.48 T with an operating current of 50 A. The experimental results agree well with design predictions calculated by finite element method. AC operation was also performed, and no distortion of the voltage waveform was observed. Therefore, this Bi-2223 superconducting magnet is a suitable replacement for copper magnets designed for applications in science and technology.     

The angular dependence of AC losses in BSCCO/Ag tapes under AC magnetic fields and AC transport currents

The measurement results of AC transport current losses in BSCCO/Ag tapes caused by AC transport currents and the simultaneously presented AC magnetic fields were reported. The measurements were carried out at 77 K and power frequency (50 Hz) for AC applied magnetic fields of 48, 80 and 120 mT, respectively. The angle between the direction of the magnetic field and the wide side of the tape varied from 0° to 90° with a step of 10°. Analyses on the angular dependency of the AC losses for BSCCO/Ag tapes were given. A theoretical model to describe the angular dependency of AC losses was proposed and compared with the experimental results.     

Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Oz tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^–0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

弯曲应变对Bi2223/Ag带材临界电流的影响

本文主要研究了Bi223/Ag带材的弯曲应力-应变特征及弯曲疲劳对其在77K自场下临界电流的影响.分析临界电流Ic降低的原因是应变和热循环引起的超导陶瓷芯内部的微裂纹.实验研究发现当带材的弯曲应变超过0.3％以后,Ic显著降低;当带材受到多次弯曲时,前四次弯曲会使Ic急剧降低,然后Ic降低非常缓慢.因此,在实际应用过程中,应使Bi223/Ag带材的弯曲应变不超过0.3％,且在Bi223/Ag带材的生产和使用过程中,均应尽量减少其弯曲的次数.     

Thermal Conductivity of 2223 BSCCO Superconductor–Polyethylene Glycol Composites Between 20 and 300 K

Thermal conductivity measurements have been taken between 20 and 300 K on composites prepared from polyethylene glycol (PEG) containing various amounts of superconducting (Bi,Pb)2223 powder. The nature of temperature variation of thermal conductivity (_C) of the composites and its magnitude depend strongly on the volume concentration of the high-temperature superconductor (HTSC) filler present in the material. The results have been discussed in the light of models known for polymer–metal powder composites. It is shown that for composites with 2223 powder content <44 vol. %, the measured data can be accounted well with Hamilton–Crosser model, taking the sphericity factor into consideration. Failure of Hamilton–Crosser expression for composites with higher filler concentration is thought to be associated with direct contact between the superconducting grains, which shortcircuits the acoustic mismatch resistance in the composites.     

Effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes

The effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes are investigated computationally and experimentally at 77 K and self-field. Generally the current-voltage characteristics of superconductors are described by the standard power law model with parameters such as critical current I_c and index n. By measuring the critical current {I_ci} and index {n_i} values of local tapes, we can get the critical current I_c and n value of full length tapes by means of statistical method. The results show that the distribution of local critical currents are non-uniform, and local critical currents have important effect on the performance of the entire tapes. The critical current of the entire tape is different from the mean value of local critical current based on Gaussian statistical distribution along the long tape.     

International round Robin test for mechanical properties of BSCCO-2223 superconductive tapes at room temperature

International RRT was carried out in order to establish the test method for mechanical properties of commercial BSCCO superconductive tapes under cooperation of seven research laboratories. From the stress versus strain curve, the following quantities were evaluated; modulus of elasticity, 0.2% proof strength, fracture strength and stresses at fixed strains. The scatter of measured values was analyzed by evaluating the relative standard uncertainty (RSU), which is the standard uncertainty divided by the average. The expected value of RSU for N = 3 was derived for each mechanical quantities. In order to make clear the major contribution to the scatterings, the F test was applied. The major source of RSU’s was attributed mostly to the influence of inter-laboratory scattering.     

Thermal Transport in BPSCCO System Between 20 and 170 K

Thermal conductivity of superconducting (Bi,Pb)2223 pellet in the temperature range 20–170 K is reported. Electronic contribution to thermal conductivity in the normal state is estimated to be 25%. Considering both phonon and phonon + electron approach, we attempted to examine the observed nature of the temperature dependence of thermal conductivity. Our analysis strongly supports the role of phonons as well as electrons in the origin of the thermal conductivity peak in the superconducting state. Some of the microscopic quantities evaluated from the best-fit parameters obtained from phonon + electron approach give reasonable values.