A number of experimental and theoretical studies have been performed to understand the mechanism of high-Tcsuperconductivity and to enhance Tc. High-pressure techniques have played a very important role for these studies. In this paper,the high-pressure techniques and physical properties of high-Tc superconductor under high pressure are presented.
Superconducting filaments in composite wires/tapes are subjected to residual stresses arising from the mismatch of the coefficient of thermal expansion among the constituents, tensile and bending stresses during winding, and electromagnetic (Lorentz) force during operation. Such stresses affect the superconducting properties due to the change in strain itself and stress-induced damage. The present article reviews the recent experimental and analytical results on the accumulation of residual strain in the sample length direction (current transport direction). In addition, this article reviews the prediction of strain dependence for critical current with applied tensile/bending strain from tensile stress-strain curve in the multifilamentary BSCCO-superconducting composite tapes.
The strain effect on critical current (Ic) in REBCO coated conductors has been intensively investigated owing to its importance for practical applications. The change in Ic with applied strain can be divided into two phenomena; namely reversible variation within elastic deformation for superconducting film, and irreversible degradation related to brittle fracture. REBCO coated conductor is the first material among various high temperature superconductors in which the non-linear reversible change in Ic with uniaxial strain is markedly observed. High stress (strain) tolerance in coated conductors is one of the advantages compared with other superconducting composites, and there are high expectations for application to high-field magnets based on such superior mechanical properties. In this review article, recent research results on the mechanical properties of REBCO coated conductors are summarized, including the stress-strain characteristic for substrate materials, the strain effect on Ic and the experimental techniques for evaluating these properties. Delamination has been recognized as one of the most critical issues for coil applications. Several evaluation methods for delamination strength and interlaminar fracture toughness were introduced.
YBCO-coated conductor tape is a composite material that consists of two or more layers. The superconducting layer is generally a thin film. In this study, the YBCO consisted of columnar aggregates of orthorhombic crystals with a texture whose axis was 001 to the film surface. Due to the difficulty of obtaining such a thin film for mechanical testing, the mechanical elastic constant is unknown. The elastic constant of the thin film can be obtained only via analysis. In this study, finite element models of bi-textured thin films were constructed. Through the analysis, the uniaxial mechanical elastic constant and in-plane Poisson's ratio were obtained. The strain dependence of the elastic properties of the YBCO-coated conductor with surrounding Cu stabilized were investigated precisely using white X-ray diffraction. The diffraction elastic constants estimated from (0k0) were larger than those estimated from (h00), and this tendency corresponded to the FEM analysis. Using X-ray diffraction, the obtained diffraction strain was smaller than the macroscopic strain. The diffraction strain - macroscopic strain slope theoretically become one. Strain relaxation will result if the slope does not become one. Our observations strongly suggest that strain relaxation is caused in the interface of twin crystals. The effective mechanical elastic constant was predicted by correcting the calculated mechanical elastic constant.
Mechanical properties of melt growth GdBa2Cu3Ox (Gd123) superconducting samples with Ag2O of 10 wt% and Pt of 0.5 wt% were evaluated at 77 K through flexural tests for specimens cut from the samples in order to estimate the mechanical properties of the Gd123 material without metal substrates, buffer layers or stabilization layers. We discuss the mechanical properties; the Young's modulus and flexural strength with stress-strain behavior at 77 K. The results show that the flexural strength and fracture strain of Gd123 at 77 K are approximately 100 MPa and 1000μ , respectively, and that the origin of the fracture is defects such as pores, impurities and non-superconducting compound. We also show that the Young's modulus of Gd123 superconductor is estimated to be 160 to 165 GPa.
There are high expectations for RE123 wires to be used as future commercial superconductors in the future because of their excellent performance under high magnetic fields and of low cost capabilities. ISTEC, national institutes, universities and companies are taking part in a Japanese national project on electric power devices known as, "Materials & Power Applications of Coated Conductors (M-PACC)". The target applications of the project are SMES, cables and transformers. ISTEC, Fujikura, Showa, Sumitomo and Furukawa are developing RE123 wires for mass-production using various fabrication processes. One of the most important characteristics for practical superconducting wires is, the bending-strain dependence of the critical current. The IC dependencies of RE123 wires for various bending strains were measured using the "Goldacker type" continuous bending apparatus. For tensile bending with the superconductor layer on the outside of the bend, the diameter of the 95% IC retentions was approximately 20 mm for all wires. On the other hand, for compressive bending with the superconductor layer on the inside of the bend, the IC dependencies for the bending diameter were each different. The reversibility strain under bending and straightening was also evaluated using the continuous bending apparatus. These results demonstrate that the IC dependencies for the bending diameter of all wires are suitable for the applications of the M-PACC project.
This study investigates superconducting and mechanical properties of monolayer GdBa2Cu3Oy-coated conductor coils under large electromagnetic stress. We found that reversible superconducting properties can be achieved in the large hoop stress/strain states up to 1288 MPa/0.63% for the Hastelloy-outside-winding coil (coil A). This limit is larger than the results for the tensile test of the short sample. However, damage occurred near the current terminal at 640 MPa for the Hastelloy-inside-winding coil (coil B). It is considered that the degradation was due to the stress concentration at the edge of the current terminal. In the case of the Hasetelloy-inside-winding coil with a lap joint (coil C), the coil performance is limited by the shear stress of the lap joint.