Spatial distribution of the local resistive transition and the critical current density in YBCO coated conductors using Low-temperature Scanning Laser Microscopy

Low-temperature Scanning Laser Microscopy (LTSLM) was carried out to reveal the spatial distribution of the local resistive transition temperature and the local current density in commercial YBCO coated conductors near the superconducting transition. The result of the sample with an array of holes of various sizes shows that the signal δV is proportional to the current density. The distribution of the critical temperature of the sample with two parallel bridges is quite homogeneous and the transport current flows mainly along the outer edge of the sample. Using LTSLM we directly image the current path in YBCO coated conductors of different patterns.     

Experimental and numerical analysis of high resistive coated conductor for conceptual design of fault current limiter

Resistive superconducting fault current limiters (FCLs) using coated conductor (CC) have been developed to reduce fault current which exceeds ratings of circuit breaker in power grid. Our group has participated in the development of distribution level non-inductive winding type FCL using stainless steel-stabilized CC, as one of the 21st century Frontier R&D program. Recently, stabilizer-free CC with Hastelloy substrate was developed for FCL application. Since the CC has higher average resistivity than existing CCs with metallic stabilizer, required amount of wire can be reduced. Short-circuit tests were performed by increasing voltage applied to the small-scale FCL coil using stabilizer-free CC in sub-cooled liquid nitrogen of 65 K. Experimental results of the tests were compared with numerical analysis of current limiting characteristics of the CC by using finite element method (FEM). Conceptual design of the FCL was performed using test results and was compared with FCL using existing CC in regards to current limiting characteristics.     

AC loss characteristics of copper-stabilized YBCO coated conductor

Influence of copper layer and mechanical stress-strain on AC losses in copper-stabilized YBCO coated conductor (CC) was investigated. The uni-axial tensile stress-strain was applied to the sample CC in liquid nitrogen at atmospheric pressure, and the AC losses (transport, magnetization and total AC losses) were measured by an electric method. Experimental results showed that the stress-strain dependence of the AC losses can be estimated by knowing stress dependent critical current and magnetization losses in the non-stressed CC. Influence of the copper layer was investigated by measuring frequency dependence of the AC losses. The experimental results showed that there are some influences of the copper layer. However, the influence of the copper layer was not significant from the stand point of practice applications.     

Degradation of critical current in an HTS coated conductor considering curvature of ellipse for rotating flux pump

A rotating HTS flux pump is used to study the charging characteristics of superconducting synchronous motor. In order to design the rotating flux pump, the elliptical winding is essential. Numerous studies have been conducted on the degradation of the critical current with respect to the bending condition of the second generation (2G) high-temperature superconducting (HTS) tape. These studies focused on the same radius of curvature depending on the bending position. Therefore, in this paper, the critical current in a HTS coated conductor (CC) considering the curvature of elliptical shape is measured with regard to nine mandrels. These are a tall prolate ellipsoid, oblate ellipsoid and semicircle. In the tall prolate ellipsoid, the degradation of the critical current was the least. The diameters of each shape are 20, 30 and 40 mm, respectively. Finally, the experimental observations have been compared with the theoretical predictions.     

TFA-MOD方法制备YBCO超导薄膜生长研究

TFA-MOD方法是制备YBCO涂层导体的最有应用前景的方法之一.本文采用TFA-MOD方法在LaAlO3单晶基片上生长YBCO薄膜并系统地研究了高温退火不同的停留时间对其影响.XRD分析表明退火时间较短时仍有明显的YBCO(00l)取向,但也有其它杂相峰存在,而延长退火时间可以使杂相峰消失而得到纯的且强度更高的YBCO(00l)取向.SEM分析表明未完全结晶的YBCO薄膜表面晶粒大小不一致,而且气孔较多.而结晶良好的YBCO薄膜表面平整致密,空洞较少.随着退火时间的增加YBCO颗粒尺寸在增大.但是,退火时间太长,会有大的空洞,甚至出现裂纹.超导电性能测试表明,随着退火时间的增加超导临界电流(Ic)在增加,但是退火时间达到60min后,Ic即达到最大值,表明60min就能使前驱膜在该实验条件下完全的结晶形成取向良好的YBCO薄膜,YBCO薄膜的生长速度达到0.16nm/s.     

Critical current measurement of commercial REBCO conductors at 4.2 K

REBCO coated conductors are now available from several industrial manufacturers and are expected to be promising conductors for high-field-magnet applications. Using these conductors, the development of solenoids capable of generating high magnetic fields of 20–30 T is ongoing in major high-field laboratories in the world. In addition, CERN recently launched a conceptual design study for the Future Circular Collider, in which a 20-T dipole magnet is listed as a candidate for the bending magnet of the main ring. However, there has been limited research published on the electrical transport properties of commercially available REBCO conductors in a high-field, low-temperature environment. For magnet designers, the transport properties are of the highest importance in choosing a suitable conductor, and the data form the bases for high-field magnet development. Therefore, in this work, a new sample holder, which allows the measurements of full-width conductors to be carried out relatively easily, was developed, and the transport properties of commercial REBCO conductors from seven manufacturers (AMSC, Fujikura, Shanghai Superconductor, SuNAM, SuperOx, SuperPower, and SWCC Showa) were investigated at 4.2 K in perpendicular fields of up to 18 T. The results show that the I_c values at 4.2 K clearly vary to some extent among these commercial conductors and the higher-current 4-mm-wide conductors have I_c values in the range of 230–305 A at 18 T and in the range of 320–424 A at 12 T.     

Test results of 12/18 kA ReBCO coated conductor current leads

A pair of hybrid current leads (brass + stacked & soldered ReBCO tapes) rated for 12 kA in steady state and for up to 18 kA at pulsed over current conditions was designed, developed and tested at NRC “Kurchatov Institute” (NRC “KI”). During the experiment at LN2 temperature, the current leads (CLs) were successfully charged with 18 kA at 100 A/s ramp rate. To date, as far as we know, this is the highest current capacity achieved for 2G HTS current leads. The feasibility of “stack-and-soldering technique” for 10 kA+ class coated conductor CLs for accelerators and fusion was demonstrated. This paper gives an overview of the leads design and presents the preliminary test results. Detailed studies of magnetic properties and current sharing process for the stacked and staggered HTS joints are also reported.     

脉冲激光法制备涂层导体YBCO超导层的研究

报道了用脉冲激光沉积技术(PLD)在CeO2/YSZ/Y2O3/NiW衬底上连续制备YBCO超导层的研究结果。分析了衬底温度、薄膜厚度和退火时间分别对YBCO的织构、表面形貌及c轴晶格常数的影响。实验发现温度较低将导致a轴晶粒的生长,薄膜太厚将引起表面形貌变差,而YBCO薄膜c轴晶格常数随退火时间的增长而减小。最终得到了高质量的YBCO涂层导体,超导转变宽度(ΔTc)为1.6K,临界电流密度(Jc)达1.3MA/cm2(77K,SF)。     

The role of intergrain junctions in transport critical current irreversibility

The magnetic field dependence of voltage at constant measuring currents of YBa₂Cu₃O_7– bulk ceramics and Bi(Pb)SrCaCuO screen printed films were studied at different temperatures so as to obtain information on critical current irreversibility. Two ranges of these curves were found to show sensitivity to magnetic field. They suggest that the two types of superconducting medium, i.e., the Josephson and Abrikosov medium, result in two different voltage generation mechanisms. It is shown that the Josephson medium plays a role in both low and high magnetic field critical current irreversibility.     

Growth of thick chemical solution derived pyrochlore La2Zr2O7 buffer layers for YBa2Cu3O7−x coated conductors

The preparation of several 100 nm thick La₂Zr₂O₇ (LZO) buffer layers on biaxially textured Ni-5 at.%W substrates using chemical solution deposition is studied. This oxide material is currently of great interest for the fabrication of YBa₂Cu₃O_7−x (YBCO) coated conductors. Buffer layers for these coated conductors are required to have thicknesses greater than 100 nm in order to guarantee a sufficient barrier function against metal diffusion from the substrate. In this work, single LZO buffer layers with thicknesses exceeding 200 nm have been prepared. Detailed investigations were carried out in order to study the texture development with increasing thickness as well as the microstructure of these layers. Independent of the thickness, high quality buffer layers showing a distinct biaxial texture up to the surface, smooth surfaces, and a sufficient barrier function against Ni diffusion from the substrate have been reproducibly obtained. The high performance of these chemical solution derived LZO buffer layers was confirmed by a YBCO critical current density J_c of 1.0 MA/cm² (77 K, 0 T) achieved for a coated conductor sample with a layer sequence YBCO/CeO₂/LZO(CSD)/Ni-5 at.%W where CeO₂ and YBCO were deposited by pulsed laser deposition.     

环氧树脂固化工艺条件对YBCO超导带材临界电流的影响

为了研究固化条件对超导带材临界电流的影响,采用电测法实验测量了多种固化工艺条件下YBCO高温超导带材的临界电流。分析得到热处理温度140℃时,超导带临界电流出现明显退化,温度越高退化越明显;超导带临界电流随着固化时间的延长而降低,处理温度越高,随时间下降的幅度越大,并从超导体钉扎效应的角度对实验结果进行了初步的分析。     

Angular dependence of transport current near critical at fields to 4 T in metalorganic thin films

Values of the transport current have been obtained as a function of angle of the magnetic field to the plane of the thin film YBa₂Cu₃O_7–d (YBCO) prepared by metalorganic deposition. Current flowed in thea, b planes at various angles to the applied magnetic fields to 4.2 T. Films with thicknesses near 350 nm were studied. For these thin films andHc a secondary maximum is observed whenJ _c> 1 MA/cm₂. NearHa, b a large increase inI _c is observed, and the sharpness, field dependence, and field directional dependence is discussed. The response to currents aboveI _c is determined and the fit to a power lawVI _n shows a strong correlation betweenI _c andn.     

Critical current data of NbTi conductors at sub-4.2 K temperatures and high fields

Critical current data in the temperature and field ranges of 1.8 to 4.2 K and 8.5 to 12.5 T are presented for two size-groups of commercial NbTi conductors. The first group of conductors had critical currents at 4.2 K and 9 T of 6 to 77 A, while the second group's ranged from 1400 to 1570 A.     

Magnetic-field angle dependent critical current densities and flux pinning in commercial YBCO tapes at liquid nitrogen temperatures

The magnetic-field angle dependence of critical current densities J_c(H,θ) in commercial YBCO tapes grown by MOCVD and MOD was examined at liquid nitrogen temperatures. We first measured J_c(H,θ) in MOCVD-YBCO tapes at 70 and 77.3 K in fields up to 2 T using both transport and inductive (the third harmonic voltage) methods and compared the results. It was observed that, in low magnetic fields, the transport measurements gave higher J_c than the inductive ones; however, in high fields they agreed well, which is well explained by the effects of weak links due to low-angle grain boundaries. We then investigated J_c(H,θ) in MOCVD- and MOD-YBCO tapes at 77.3 K in fields up to 2 T. All the tapes exhibited peaks at H//ab in the shape of Mount Fuji, which shows that small random pinning plays a major role. However, an anisotropic scaling analysis showed that the flux pinning mechanisms in those tapes were different, resulting in distinctive angular behaviors of J_c(H,θ). It is suggested that the difference in the flux pinning mechanisms of the two types of tapes came from the different sizes of point defects originating from the growing processes.     

Forced flow He vapor cooled critical current testing facility for measurements of superconductors in a wide temperature and magnetic field range

As superconducting materials find their way into applications, there is increasing need to verify their performance at operating conditions. Testing of critical current with respect to temperature and magnetic field is of particular importance. However, testing facilities covering a range of temperatures and magnetic fields can be costly, especially when considering the cooling power required in the cryogenic system in the temperature range below 65 K (inaccessible for LN₂). Critical currents in excess of 500 A are common for commercial samples, making the testing of such samples difficult in setups cooled via a cryocooler, moreover it often does not represent the actual cooling conditions that the sample will experience in service. This work reports the design and operation of a low-cost critical current testing facility, capable of testing samples in a temperature range of 10–65 K, with magnetic field up to 1.6 T and measuring critical currents up to 900 A with variable cooling power.     

Nano-crystalline CVD diamond films deposited on cemented carbide using high current extended DC arc plasma process

Nano-crystalline diamond (NCD) films have been grown on cemented carbide substrates by high current extended DC arc plasma process using Ar/H₂/CH₄ gas mixture at low gas pressure. The plain view and cross section of films are characterized with scanning electron microscopy. A uniform and smooth surface morphology of NCD thin films is observed. Raman spectroscopy has been used to investigate purity of the NCD films. Experimental results on the synthesis and characterization of the NCD films on cemented carbide substrates are discussed in this article.     

Effect of the local defects induced by bending strain on the quench properties for YBCO tapes

The effect of local defects on the quench properties for YBCO tapes after applying bending strain was investigated at self-field in 77 K. The minimum quench energy (MQE) was related to the position of defects in the tape and the smallest MQE appears where the region of the defects existed in the position of the heater at the same transport current. The normal zone propagation velocity (NZPV) was related to the size and quantity of the regions of defects. The more defects were in the tape, the faster the normal-zone propagation velocity.     

Simple technique for increasing the conductivity of copper for current lead conductors

Current leads are used mainly as conductors for electrical current to be supplied to superconducting magnets. Low liquid helium losses in the current leads are of great importance and therefore the geometry of the current lead must correspond to a specific optimized current. Here it is shown that, by altering the residual resistivity of the conductor material, one can modify the optimized current of the current lead without changing its original geometry. A relatively easy technique for decreasing the resistivity of commercial electrolytic tough-pitch copper is described. Experimental results of copper annealing in a vacuum of 2.7 × 10⁻³ Pa (2 × 10⁻⁵ torr) in the temperature range of 295–703 K are discussed.     

Effect of composition on critical current density of Bi2212/Ag round wires

We have fabricated Bi2212/Ag round wires using three kinds of precursor to study the effect of a narrow variation of composition. Slightly different compositions - Bi_2.17Sr_1.94Ca_0.89Cu_2.0O_x(N13), Bi_2.15Sr_1.94Ca_0.89Cu_2.0O_x(N14), and Bi_2.17Sr_1.98Ca_0.89Cu_2.0O_x(N15) - were used and Sr/Ca ratio of them were 2.18, 2.18, and 2.22, respectively. The Ag ratios of the wires were 2.7-2.8 and average filament diameter was 19-21 μm. DTA analysis of the wire showed the peritectic temperature of three wires was very similar value of the range of 880-881 °C. The best engineering critical current density (J_e) of three wires at 4.2 K and 0 T was 414-448 A/mm² at the maximum process temperature range of 884-892 °C. The n-value of N14 showed 13.6, whereas other two wires showed lower n-value, estimating the existence of micro-cracks. Although Bi2212/Ag round wires fabricated by three kinds of composition showed similar J_e value, n-value was quite different. It is likely that the fabrication process such as the drawing as well as the composition of precursor will affect on J_e of Bi2212/Ag round wire.     

Fracture of filaments and its influence on critical current and residual strength of fatigued Nb-Ti/Cu superconducting composite

Fatigue behavior at room temperature and its influence on critical current at 4.2 K and residual strength at room temperature of multifilamentary Nb-Ti/Cu superconducting composite wire with a filament volume fraction of 0.49 (copper ratio of 1.04) were studied. The fatigue crack nucleated in the copper in the circumferential region and propagated stably into the inner region, causing fracture of the Nb-Ti filaments in the late stage of the fatigue life. Once the fracture of the filaments started, the number of the fractured filaments increased steeply with increasing number of stress cycles, and correspondingly, the current-transportable and stress-carrying capacity of the composite decreased steeply. In this process, both the critical current and residual strength of the fatigued composite decreased nearly linearly with decreasing fraction of surviving filaments. Thus, the critical current of the fatigued composite was proportional to residual strength as a first approximation.