Chuanyi Bai

Low-Resistance and Strong-Adhesion Soldering of Second-Generation High-Temperature Superconductor Tapes Within a Short Time

An effective soldering process of second-generation coated conductors (CCs) with low joint resistance was presented by using home-made equipment in the present study. The benefit of preprocessing, i.e., polishing, was proved with respect to the reduced joint resistances based on the voltage–current curves. It is revealed that the joint resistance of 10-8 Ω with an overlapping length as short as 2 cm can be achieved, while the joint resistances would further decrease with increasing the overlapping area, in agreement with Ohms law. In the case of the overlapping length of 8 cm, the order of nΩ can be achieved for the resistances of solder joints. The heating time in joint fabrication process can be shortened to 1 min without negative effects on joint resistance, speeding up the soldering process largely. The effectiveness was evidenced further by the hard degradation in Ic through the joint part of the studied tapes and the enhanced adhesion between solder layer and Cu layer according to the cross-section observation. Temperature dependence of the joint specimen resistances suggested that the lattice constant of superconducting layer after jointing may be preferable to attain a higher critical temperature than that of original tape. The fragmentation of the superconducting layer and decohesion of the interface between hastelloy/buffer and ceramic layers, not the Cu-solder interface for the joint by external force, implied that the valid soldering was achieved using the present process.

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Y₂O₃ film was deposited as the seed layer by dc reactive sputtering technique on a Ni-5at%W (NiW) substrate. The subsequent oxide buffer layers were introduced as well such as Gd₂Zr₂O₇ (GZO) and La₂Zr₂O₇ (LZO). The effects of Y₂O₃ seed layer on the epitaxial growth of buffer layers and the performance of YBa₂Cu₃O_7-δ (YBCO) films were investigated. The atomic force microscopy images showed that the surfaces of Y₂O₃ seed layers were homogeneous and flat with the root mean square roughness of about 1.2 nm, which is significant for the subsequent layers. In contrast, root mean square roughness values of GZO films on bare NiW or Y₂O₃ seeded NiW were normally about 2.7-3.4 nm. A series of X-ray studies indicated that the GZO and LZO oxide barrier layers on Y₂O₃ can be epitaxially grown easily with a wide processing condition of water vapor partial pressure. Furthermore, the effect of Y₂O₃ seed layer on subsequent YBCO films was investigated by comparing the transition temperature Tc and the critical current density .Ic between YBCO/GZO/NiW and YBCO/GZO/Y₂O₃/NiW. These results demonstrate the notable role of the Y₂O₃ seed layer, not only on high-efficiency production of subsequent functional, but also on the improvement of superconducting performances of YBCO films.

\hbox{Y}_{2}\hbox{O}_{3}

Thick superconducting multilayer architectures consisting of alternate superconducting and superconducting layers DyBCO/YBCO and alternate superconducting and insulating layers YBCO/CuO(Y₂O₃) are developed on buffered Hastelloy tapes using a metal organic deposition process. It is revealed that multilayer intermittent structures for superconducting layers are effective to avoid the presence of microcracks that occur with increasing thickness, which are frequently observed in monolayer films of REBa₂Cu₃O_7−δ (REBCO, RE = Y, Gd, or other rare earth elements). This paper shows that the thicknesses of DyBCO/YBCO multilayer films can reach 2.8 μm with a homogeneous, dense and crack-free surface morphology. The out-plane FWHM of the YBCO (005) peaks and the critical current value reach 1.1° and 365 A/cm (77 K, self-field) for such a thick film consisting of DyBCO/YBCO multilayers. It is also revealed that the introduction of a CuO layer is effective to avoid the presence of a-axis grains and voids of the YBCO films, which are frequently observed in multilayer films. Note that the introduction of CuO shows little negative influence on the texture of multilayer structure, especially for the bottom CuO. However, the in-plane texture deteriorates seriously for Y₂O₃/YBCO/YBCO multilayer film. The critical current density of YBCO films with middle CuO and bottom CuO are 2.0 MA/cm² and 2.5 MA/cm² (77 K, self-field), respectively. The microstructure optimization including the texture, uniformity, crack, defect, and porosity in the multilayered films is probably the reason for the enhancement of superconducting performance.

Seed Layer on Epitaxial Growth of Oxide Barrier Layer for YBCO Coated Conductor

BaHfO3 (BHO) has been proposed as a new cap layer material for YBa2Cu3O7−δ (YBCO) coated conductors. Highly c-axis oriented BHO cap layer has been deposited on ion-beam assisted deposition-MgO buffered Hastelloy tapes by direct-current-magnetron sputtering method. The epi-growth of BHO films combined with its properties is investigated in details. The degenerated cubic crystal structure of BHO film is confirmed by Raman spectrum analysis. XRD θ–2θ scan, φ-scan and ω-scan reveal an excellent c-axis alignment with good in-plane and out-of-plane textures for BHO cap layers. SEM and AFM investigations show BHO cap layer a dense and crack-free morphology. Subsequently pure c-axis orientation YBCO film was epitaxial grown on such BHO cap layer successfully, shown BaHfO3 a potential cap layer material for coated conductors.

Artificial Multilayers of REBaCuO/YBaCuO and YBCO/CuO(Y2O3) for High-Performance Long Tapes of Coated Conductors Derived by Metal Organic Solution Deposition

Surface morphology evolution of the trifluoroacetate metal-organic precursor for YBa2Cu3O7-δ during pyrolysis was observed by in situ optical microscopy, and a mechanism was proposed based on an analogy to the gel-swelling-induced surface instability. The cracking and wrinkling phenomena were ascribed to the competition between reaction and diffusion. The critical condition for wrinkling was obtained, and the dependence between the heating ramp and the surface morphology was identified. Experiments showed that three types of cracks existed, and one of them exhibited healing behavior during drying. The wrinkles may exhibit healing behavior as well under certain condition. These results are believed to arise from the suppression of the precursor surface instability during pyrolysis treatment, which is relevant to the manufacturing of the coated conductor.

Epitaxial growth of BaHfO3 buffer layer and its structure degeneration analysed by Raman spectrum

Various precursor solutions with different fluorine content have been developed, including the conventional low-fluorine (CLF) solution with fluorine content of 54% (F-54%), the super low-fluorine (SLF) solution with F-31% and the extreme low-fluorine (ELF) solution with F-7% in contrast to the all trifluoroacetates (All-TFA) solution with F-100%. The DTA analysis shows that the precursor solution with lower fluorine content endures a faster pyrolysis rate. The influence of pyrolysis rate on the microstructure and transport properties of YBa2Cu3O7-δ (YBCO) films prepared by various low-fluorine (LF) solutions was investigated, revealing that smooth and homogeneous YBCO films can be achieved with high-rate decomposition process in the case of LF solutions. Also, high-performance YBCO films can be obtained at 5 °C/min using CLF solution or 20 °C/min using ELF solution. Subsequently, the superconducting properties and production rate of YBCO film are improved effectively in the case of various LF solutions, suggesting a promising route to scale up and rapid preparation of YBCO coated conductors.

Direct Observation of Wrinkling and Healing Evolution for YBa2Cu3O 7−δ Precursor Films Prepared by the Metalorganic Solution Method

Various precursor solutions with different fluorine content have been developed, including the conventional low-fluorine (CLF) solution with fluorine content of 54% (F-54%), the super low-fluorine (SLF) solution with F-31% and the extreme low-fluorine (ELF) solution with F-7% in contrast to the all trifluoroacetates (All-TFA) solution with F-100%. The DTA analysis shows that the precursor solution with lower fluorine content endures a faster pyrolysis rate. The influence of pyrolysis rate on the microstructure and transport properties of YBa2Cu3O7-δ (YBCO) films prepared by various low-fluorine (LF) solutions was investigated, revealing that smooth and homogeneous YBCO films can be achieved with high-rate decomposition process in the case of LF solutions. Also, high-performance YBCO films can be obtained at 5 °C/min using CLF solution or 20 °C/min using ELF solution. Subsequently, the superconducting properties and production rate of YBCO film are improved effectively in the case of various LF solutions, suggesting a promising route to scale up and rapid preparation of YBCO coated conductors.

Controlled-growth of YBa2Cu3O7-δfilm using modified low-fluorine chemical solution deposition

Various precursor solutions with different fluorine content have been developed, including the conventional low-fluorine (CLF) solution with fluorine content of 54% (F-54%), the super low-fluorine (SLF) solution with F-31% and the extreme low-fluorine (ELF) solution with F-7% in contrast to the all trifluoroacetates (All-TFA) solution with F-100%. The DTA analysis shows that the precursor solution with lower fluorine content endures a faster pyrolysis rate. The influence of pyrolysis rate on the microstructure and transport properties of YBa2Cu3O7-δ (YBCO) films prepared by various low-fluorine (LF) solutions was investigated, revealing that smooth and homogeneous YBCO films can be achieved with high-rate decomposition process in the case of LF solutions. Also, high-performance YBCO films can be obtained at 5 °C/min using CLF solution or 20 °C/min using ELF solution. Subsequently, the superconducting properties and production rate of YBCO film are improved effectively in the case of various LF solutions, suggesting a promising route to scale up and rapid preparation of YBCO coated conductors.

Controlled-Growth of Film Using Modified Low-Fluorine Chemical Solution Deposition

YBa2Cu3O7-δ (YBCO) coated conductor has been a promising candidate for high-current transportation material in industry, while the critical current (Ic) becomes a limitation for large-scale application. An efficient way to improve Ic is to increase the thickness of YBCO film without much reduction in critical current density (Jc). Metallorganic solution deposition (MOD) is a traditional method to fabricate YBCO films while it takes time as long as 10h during the pyrolysis process. In comparison, Low-Fluorine MOD (LF-MOD) method is applied to reduce the pyrolysis time to less than 2h. In this paper, we successfully obtain YBCO thick films up to 1.7μm by double dip coating process using low-fluorine MOD method. Though wrinkles are frequently observed on the surface of the first layer after pyrolysis at 400 °C for 0.5h, the surface of the final double coated YBCO thick film is homogeneous and cracks are hardly found. Further tests show relatively good c-axis orientation and Jc value of about 105 A/cm2 at 77K. With an optimization for the pyrolysis and crystallization processes, better superconducting properties of YBCO thick film are expected.