Remarkable weakness against cleavage stress for YBCO-coated conductors and its effect on the YBCO coil performance

Cleavage strength for an YBCO-coated conductor at 77 K was investigated with a model experiment. The nominal cleavage strength for an YBCO-coated conductor is extremely low, typically 0.5 MPa. This low nominal cleavage strength is due to stress concentration on a small part of the YBCO-coated conductor in cleavage fracture. Debonding by the cleavage stress occurs at the interface between the buffer layer and the Hastelloy substrate. The nominal cleavage strength for a slit edge of the conductor is 2.5-times lower than that for the original edge of the conductor; cracks and micro-peel existing over the slit edge reduce the cleavage strength for the slit edge. Cleavage stress and peel stress should be avoided in coil winding, as they easily delaminate the YBCO-coated conductor, resulting in substantial degradation of coil performance. These problems are especially important for epoxy impregnated YBCO-coated conductor coils. It appears that effect of cleavage stress and peel stress are mostly negligible for paraffin impregnated YBCO-coated conductor coils or dry wound YBCO-coated conductor coils.     

On some consequences of an external magnetic field applied to HTS coils

A computational model which enables to evaluate the distribution of the critical currents, electric fields and the voltage in the winding of a solenoidal high temperature superconducting (HTS) magnets subjected to an external magnetic field parallel with the magnet axis, was developed. The model comes out from the well-known power law between the electric field and the transport current of the HTS tape short sample. It allows to predict the voltage–current V(I) characteristics of both the pancake coils and the complete magnet. The model was applied to the magnet system consisting of 22 pancake coils made of multifilamentary Bi(2223)/Ag tape at 20 K, which is subjected to an external uniform magnetic field parallel with the coil axis. A rather unexpected behavior of the magnet at different operating conditions (operating current and external magnetic field strength) is predicted, analyzed and reported together with a theoretical explanation. On one hand, the external uniform magnetic field parallel with the coil axis increases the resulting magnetic field strength, however, on the other hand it simultaneously decreases the angle between the resulting magnetic field and the tape surface. Thus, the effect of higher magnetic loading caused by the presence of an external magnetic field strength which is acting on individual turns located close to the coil’s flanges is compensated by more favorable orientation of the tape with respect to the resulting magnetic field. As a result, increase in the critical currents of these turns is expected. Further, the results indicate, that in case of the high field HTS insert coils the anisotropy in the I_c(B) characteristic does not play a substantial role. As a consequence, the technology of the production of the tapes for high field insert HTS coils should concentrate rather on the tapes having the current carrying capacity as high as possible, than on the attempt how to decrease the anisotropy in the I_c(B) by changing the architecture of the filaments in the tape.     

I–V characteristics and magnetic field profile studies in high Tc BSCCO based Helmholtz coil

A double pancake wound high T_c Helmholtz coil has been fabricated using commercial grade BSCCO tape. The current voltage (I–V) characteristics of this HTS coil has been carried out using standard four probe technique. From the I–V characteristics, the critical current based on self field dependent of HTS coil and quality index “n” value have been calculated. Magnetic field profiles have been studied along the axis of this coil for various amplitudes of applied current ranging from 10 A to 50 A at 77 K. The measured field profiles have a very good agreement with that of theoretical values predicted in the literature.     

Advanced development of TFA-MOD coated conductors

American Superconductor is manufacturing 2G wire for initial commercial applications. The 2G wire properties satisfy the requirements for these initial projects; however, improvements in the critical current, field performance and cost are required to address the broad range of potential commercial and military applications. In order to meet the anticipated the performance and cost requirements, AMSC’s R&D effort is focused on two major areas: (1) higher critical current and (2) enhanced flux pinning.AMSC’s current 2G production wire, designed around a 0.8 μm thick YBCO layer deposited by a Metal Organic Deposition (MOD) process, carries a critical current in the range of 200–300 A/cm-w (77 K, sf). Achieving higher critical current requires increasing the thickness of the YBCO layer. This paper describes recent progress at AMSC on increasing the critical current of MOD-YBCO films using processes compatible with low-cost, high-rate manufacturing.     

YBCO films buffered by pyrochlore Nd2Mo2O7 layer on YSZ substrates by chemical solution deposition

A chemical solution deposition process was used to grow epitaxial Nd₂Mo₂O₇ (NMO) buffer layers on YSZ substrates to produce YBa₂Cu₃O_7?δ (YBCO) coated conductors. The NMO precursor solution prepared using metal acetylacetonates was spin-coated onto single crystal YSZ substrate of 10 mm × 10 mm in size at 3000 rpm for 30 s and heat-treated at 1000 °C for 2 h in Ar after calcinated at 550 °C for 1 h. The YBCO film was deposited by TFA-MOD route on top of the NMO/YSZ architecture. The phase purity and the crystalline orientation of NMO and YBCO films were evaluated by X-ray diffraction (XRD). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to observe their microstructure and their surface roughness. The critical current density (J_c) of YBCO film on NMO/YSZ is 1.8 MA/cm² at 77 K in self-field, which indicates that the Nd₂Mo₂O₇ is a potential buffer for YBCO coated conductor.     

Attenuation of interfacial pinning enhancement in YBCO using a PrBCO buffer layer

Numerous experimental results have suggested that the J_c of YBa₂Cu₃O₇ (YBCO) films is significantly higher near the film–substrate interface than in the remainder of the film. We previously proposed that this effect is due to interfacial pinning enhancement caused by stress and the resulting misfit dislocations at the heteroepitaxial interface. To test this hypothesis we have used a non-superconducting PrBa₂Cu₃O_7?δ (PrBCO) buffer layer to minimize the lattice mismatch with YBCO. We find that the PrBCO layers lower J_c of the 0.4 μm YBCO films in a predictable way, and that, if sufficiently thick (～0.5 μm), they eliminate interfacial enhancement altogether. Our interpretation of this result is that the defects responsible for interfacial enhancement of flux pinning originate at the bottom of the non-superconducting PrBCO layer, which screens the pinning centers from vortices in YBCO. This result demonstrates that the pinning enhancement arises from stress at the film–substrate interface.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

Epitaxial growth of Ce2Y2O7 buffer layers for YBa2Cu3O7−δ coated conductors using reel-to-reel DC reactive sputtering

Biaxially textured Ce₂Y₂O₇ (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa₂Cu₃O_7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO₂. The superconducting transition temperature T_c of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density J_c of about 0.7 MA/cm² indicate the potential of the single CYO buffer layer.     

Comparative study on the critical current performance of Bi-2223/Ag and YBCO wires in low magnetic fields at liquid nitrogen temperature

A comparative study on the critical current performance of Bi-2223/Ag and YBCO coated conductor wires in low magnetic fields at liquid nitrogen temperature was carried out in this work. Five commercial high temperature superconductor wires from different manufacturers were collected. Their critical currents were measured in magnetic fields, ranging from 0 to 0.4 T. On contrary to the common conception, the Bi-2223/Ag samples had better performance than YBCO coated conductor samples in the magnetic fields parallel to the wide surface of superconducting wires within the experimental scope. We also found similar results by collecting the concerned datum from the published literatures to confirm our measurement results. At the present stage, this fact made that the Bi-2223/Ag wires might be the preferred choice for the applications with mainly low parallel fields involved, unless other considerations were prioritized.     

Effects on Jc of pinning center morphology for multiple-in-line-damage in coated conductor and bulk,melt-textured HTS

The properties of discontinuous aligned pinning centers (PCs) created by high-energy heavy-ions are compared for bulk melt-textured and coated conductor HTS. Properties of PCs, which increase J_c (pinning potential and entanglement), and negative properties which decrease J_c (e.g., decreased T_c and percolation paths) are evaluated. Mechanisms are proposed to explain the very large increases in J_c resulting from multiple-in-line-damage (MILD) compared to continuous columnar pinning centers (CCPC). In particular, a mechanism which results in fluxoid entanglement, even for parallel (unsplayed) PCs, is discussed. The same mechanism is found to also account for restoration of much of the pinning potential expected to be lost due to the gaps in MILD PCs. It also accounts for the fact that at high fluence, J_c increases as fluence is increased, instead of decreasing as expected. The very low self-field in coated conductor permits separation of the negative and positive effects of PCs. It is found that parameters developed to quantify the negative effects in bulk melt-textured YBCO, by 63 GeV U²³⁸ ions, successfully describe damage to 2.1 μm thick coated conductor by 1 GeV Ru⁴⁴ ions. Coated conductor at 77 K and self-field is generally known to have J_c about 100 times that of melt-textured YBCO. However, at 77 K and applied field of 1 T, when both forms of HTS are processed with comparable numbers of near-optimum MILD PCs, the difference in J_c is reduced to a factor of 1.3–2. Whereas J_c for melt-textured YBCO increased sharply, by a factor of up to 16.8 for high-fluence MILD PCs, J_c in coated conductor increased by a smaller factor of 2.5–3.0. Nevertheless, 2.1 μm thick coated conductor, with near-optimum MILD PCs, exhibits J_c = 543 kA/cm² at 77 K and applied field of 1.0 T, and I_c = 114 A/cm-width of conductor. This is the highest value we find in the literature. The phenomenology developed indicates that for optimum MILD PCs in coated conductor, J_c ～ 700 ± 70 kA/cm² should be achievable at 77 K, 1.0 T.     

Reel-to-reel fabrication of meter-long YBCO coated conductor

YBa₂Cu₃O_7?δ (YBCO) superconductors were coated on the CeO₂/YSZ/Y₂O₃ buffered Ni-5at%W tapes by a reel-to-reel pulsed laser deposition (PLD). The process of a multi-layer deposition of YBCO film was explored. X-ray diffraction texture measurements showed good both in-plane and out of plane crystalline orientations in YBCO films. The average values calculated at a full width at half maximum (FWHM) of the peaks from phi-scans (φ) and omega (ω) scans for one meter-long YBCO tape were 7.49° and 4.71°, respectively. The critical current (I_c) was over 200 A/cm-width at 77 K and under self-field for meter-long YBCO tape. The critical transition temperature of the YBCO tape was typically as 90.1 K with 0.5 K transition widths.     

Texture analysis of RF-sputtered CeO2 buffer layers and superconducting MOD-YBCO films

CeO₂ buffer layers were deposited on YSZ single-crystal substrates using an RF-sputtering method. The development of crystalline textures of sputtered CeO₂ films at different sputtering pressure and their effects on YBCO films, deposited by Metal Organic Deposition (MOD), were investigated. Both CeO₂ and subsequent YBCO films grew well epitaxially. The relative XRD peak intensities of CeO₂ (2 0 0) to substrate YSZ (2 0 0) increased with deposition pressure in the range of 3–5 mTorr and were inversely proportional to the θ–2θ scan FWHM values of CeO₂ (2 0 0). Also, the reaction layers of BaCeO₃ were thicker in the samples with lower CeO₂ (2 0 0) intensities and poor out-of-plane alignment when CeO₂ were deposited at the lower pressure of 3.3 mTorr. It is noted, however, that the superconducting layer grew well epitaxially on these BaCeO₃ layers, possibly due to the epitaxial relation between CeO₂ and YBCO. The superconducting critical currents of MOD-YBCO films showed an increasing tendency as both the Δ2θ (CeO₂) and BaCeO₃ peak intensities decreased.     

Preparation of solution-based YBCO films with BaSnO3 particles

The YBCO films with BaSnO₃ (BSO) particles were prepared on LAO (0 0 1) substrates by metal organic deposition using trifluoroacetates (TFA-MOD) via introducing SnCl₄ powders into the YBCO precursor solution. It was found that with the increase of the SnCl₄ contents, the slower decomposition and higher temperature for nucleation during the reaction were requested compared to that of pure YBCO film. The YBCO films with different contents of Sn with dense surface and well c-alignment were obtained under optimized heat treatment, and the BaSnO₃ phases were detected by XRD analysis. Litter effect of BSO particles on the T_c and J_c values of YBCO films was found. All YBCO films with BSO particles had T_c values over 90 K and J_c values over 1 MA/cm². A significant enhancement of J_c was observed for YBCO films with BSO particles compared to that of pure YBCO film by the field dependence of J_c values. The best property was obtained for YBCO film with 6 mol.% Sn at 77 K under magnetic field. The results showed that the J_c value of YBCO film with 6 mol.% Sn was enhanced by a factor of 2 in 2 T, and over a factor of 10 beyond 4 T compared to that of pure YBCO film.     

Stable barium compounds in YBa2Cu3Oy superconductors

The stability of various amounts of Ba₃Cu₃In₄O₁₂ (334) or BaTbO₃ (BTO) in a sintered YBa₂Cu₃O_y (YBCO) matrix was examined. Samples with added 334 or BTO exhibited critical temperatures (T_c) above 90 K for up to 20 vol.% addition and improved critical current densities (J_c) under a magnetic field. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analysis indicated that 334 and BTO did not react with the YBCO matrix under the sintering conditions used. The normalized J_c under a magnetic field of 1 T reached a maximum at 14 vol.% of 334 addition and 20 vol.% BTO addition. YBCO thin films with added BTO showed a gradual decrease in the T_c with increasing BTO content. YBCO films with added 334 showed a constant T_c of 87 K up to a 334 content of 4 vol.%.     

Enhancement of the critical current density in YBCO/Ag composites

YBa₂Cu₃O_7 + xAg?(x = 0.0, ?5.0, ?15.0 and 20 wt%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean's formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density (?J_C) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force (F_P) by a factor of ten is also reported. The enhancement in ?J_C is investigated over a wide range of magnetic fields. These significant changes in ?J_C and F_P are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO.     

Effect of air humidity on the microstructure and properties of large-area YBa2Cu3O7−x films by TFA-MOD process

Large-area YBa₂Cu₃O_7?x (YBCO) films had been successfully fabricated on LaAlO₃ (LAO) substrate by metalorganic deposition using trifluoroacetates (TFA-MOD) and the effect of air humidity on the structure and properties of YBCO films was systematically investigated. The precursor coating process was performed in the air humidity range of 70–40%. According to the X-ray diffraction (XRD) and scanning electron microscopy (SEM) images, the films prepared at high air humidity showed poor electrical performance due to large pores and impurity phases. In contrast, dense, homogeneous and epitaxially grown YBCO films prepared at low air humidity with the critical current densities of 3.8–5.0 MA cm^?2 at 77 K had been obtained.     

Self-field redistributions and strain effects observed by magneto-optical imaging in Bi2212 multifilaments

Magneto-optical (MO) imaging was used to study and characterize superconducting Bi2212 multifilaments melted in a 5 T magnetic field. MO investigations were performed in presence of a transport current, with and without the superposition of an external magnetic field (30 mT). Transport current redistributions via the Ag cladding have been observed in MO self-field experiments for the first time in Bi2212 multifilaments. MO in situ investigations under tensile stress reveal a slow progression of the cracks, that do not cross the entire section of the multifilamentary tape. This crack formation is correlated with the transport critical current versus strain curves: the critical current drop under tensile stress is relatively smooth for Bi2212 multifilaments.     

MICE超导耦合磁体线圈等效弹性模量辨识

应用于"μ介子离子化冷却实验装置(MICE)"的超导耦合磁体系统是MICE中的三大关键设备之一。耦合磁体线圈内径为1 500mm,长度为285mm,厚度为110.4mm,采用方形截面1.65×1.00mm2的NbTi复合超导线。每层导线之间为环氧和绝缘玻璃纤维布,每匝导线之间为环氧,线圈具有复杂的正交各向异性性质。根据线圈横截面细观结构的周期性,选出单根导线与周围的绝缘材料为代表性体积元(Representative Volume Element,RVE),基于能量等效原理得出线圈等效弹性模量的含义,然后根据细观力学有限元法,采用有限元软件计算了不同边界条件下细观结构代表性体积元的力学响应,得出了线圈平面径向和环向等效弹性模量,结果已经作为基础数据运用在MICE超导耦合磁体的力学计算中。     

Doping effect of Ag nanoparticles on critical current of YBa2Cu3O7−δ bulk superconductor

The objective of this work is to investigate the effect of Ag nanoparticles on critical current of YBa₂Cu₃O_7?δ (YBCO) superconductor. Ag nanoparticles with different particle sizes from 30 to 1000 nm were prepared through the chemical reduction of AgNO₃ in an alcohol solution. Then, samples of YBCO superconductors were doped by 1 and 2 wt.% of Ag nanoparticles with different sizes. Samples were characterized with XRD, SEM and EDX measurements. Critical current measurements were performed using a standard four-probe technique at liquid nitrogen temperature. The results showed by increasing of Ag nanoparticles up to 700 nm the J_c increases, but decreases by further increase in Ag particles size. The critical current enhancement is attributed to the improved connectivity between the grain boundaries and better crystallization of the grains.     

Modified Lanthanum Zirconium Oxide buffer layers for low-cost,high performance YBCO coated conductors

The pyrochlore Lanthanum Zirconium Oxide, La₂Zr₂O₇ (LZO), has been developed as a potential replacement barrier layer in the standard RABiTS three-layer architecture of physical vapor deposited CeO₂ cap/YSZ barrier/Y₂O₃ seed on Ni–5%W metal tape. The main focus of this research is to ascertain whether: (i) we can further improve the barrier properties of LZO; (ii) we can modify the LZO cation ratio and still achieve a high level of performance; and (iii) it is possible to reduce the number of buffer layers. We report a systematic investigation of the LZO film growth with varying compositions of La:Zr ratio in the La₂O₃–ZrO₂ system. Using a metal–organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of La_xZr_1?xO_y (x = 0.2–0.6) on standard Y₂O₃ buffered Ni–5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial LZO phase with only (0 0 1) texture can be achieved in a broad compositional range of x = 0.2–0.6 in La_xZr_1?xO_y. Both CeO₂ cap layers and MOD–YBCO films were grown epitaxially on these modified LZO barriers. High critical currents per unit width, I_c of 274–292 A/cm at 77 K and self-field were achieved for MOD–YBCO films grown on La_xZr_1?xO_y (x = 0.4–0.6) films. These results indicate that LZO films can be grown with a broad compositional range and still support high performance YBCO coated conductors. In addition, epitaxial MOD La_xZr_1?xO_y (x = 0.25) films were grown directly on biaxially textured Ni–3W substrates. About 3 μm thick YBCO films grown on a single MOD–LZO buffered Ni–3W substrates using pulsed laser deposition show a critical current density, J_c, of 0.55 MA/cm² (I_c of 169 A/cm) at 77 K and 0.01 T. This work holds promise for a route for producing simplified buffer architecture for RABiTS based YBCO coated conductors.