YBaCuO and GdBaCuO + 15 wt% Ag large, single-grain, bulk superconductors have been fabricated via the top-seeded, melt-growth (TSMG) process using a generic NdBCO seed. The mechanical behavior of both materials has been investigated by means of three-point bending (TPB) and transversal tensile tests at 77 and 300 K. The strength, fracture toughness and hardness of the samples were studied for two directions of applied load to obtain comprehensive information about the effect of microstructural anisotropy on the macroscopic and microscopic mechanical properties of these technologically important materials. Splitting (Brazilian) tests were carried out on as-melt-processed cylindrical samples following a standard oxygenation process and with the load applied parallel to the growth-facet lines characteristic of the TSMG process. In addition, the elastic modulus of each material was measured by three different techniques and related to the microstructure of each sample using optical microscopy. The results show that both the mechanical properties and the elastic modulus of both YBCO and GdBCP/Ag are improved at 77 K. However, the GdBCO/Ag samples are less anisotropic and exhibit better mechanical behavior due to the presence of silver particles in the bulk, superconducting matrix. The splitting tensile strength was determined at 77 K and both materials were found to exhibit similar behavior, independently of their differences in microstructure.
In previous years, the R & D program between CERN and Columbus Superconductors SpA led to the development of several configurations of MgB2 wires. The aim was to achieve excellent superconducting properties in high-current MgB2 cables for the HL-LHC upgrade. In addition to good electrical performance, the superconductor shall have good mechanical strength in view of the stresses during operation (Lorenz forces and thermal contraction) and handling (tension and bending) during cabling and installation at room temperature. Thus, the study of the mechanical properties of MgB2 wires is crucial for the cable design and its functional use. In the present work we report on the electro-mechanical characterization of ex situ processed composite MgB2 wires. Tensile tests (critical current versus strain) were carried out at 4.2 K and in a 3 T external field by means of a purpose-built bespoke device to determine the irreversible strain limit of the wire. The minimum bending radius of the wire was calculated taking into account the dependence of the critical current with the strain and it was then used to obtain the minimum twist pitch of MgB2 wires in the cable. Strands extracted from cables having different configurations were tested to quantify the critical current degradation. The Young’s modulus of the composite wire was measured at room temperature. Finally, all measured mechanical parameters will be used to optimize an 18-strand MgB2 cable configuration.
The degradation of the superconducting properties of REBa 2 Cu 3 O 7−x coated conductors when heated above 150 °C is a topic of concern for any REBa 2 Cu 3 O 7−x -based applications. This study makes it clear the role of thermally activated oxygen out-diffusion processes in the modification of the superconducting properties of commercial coated conductors heat treated in air. We point out that oxygen can diffuse out of the REBa 2 Cu 3 O 7−x layer along two main channels: one associated with the grain boundaries and a second one associated with the superconducting grains. The activation energy is lower for the oxygen outdiffusion events that take place at the grain boundaries, and this is the main cause of the degradation of the critical current at temperatures lower than ∼250 °C. More specifically, through a systematic investigation of the electrical and magnetic properties, we demonstrate that the decrease of the critical current upon heating the coated conductor is initially due to a loss in the transparency of the REBa 2 Cu 3 O 7−x grain boundaries, which determines a reduction of the effective superconductor cross section available for the superconducting current. The degradation of the intragrain properties becomes important only above ∼250 °C and manifests itself through a simultaneous variation of the critical current and of the critical temperature.
High temperature superconducting (HTS) wires require a detailed characterization of the possible degradation of their properties by handling at room temperature as well as during their service life, establishing the limits for associated functional devices and systems. In this paper, we study the mechanical behavior of spliced joints between commercial HTS coated conductors based on YBCO at room (300 K) and service temperatures (77 K). Single lap shear tests were performed and the evolution of the critical current and electric resistivity of the joints were measured. The complete strain field for the tape and joints was also obtained by digital image correlation. In addition, tensile tests under an external magnetic field were performed, and the effect of the applied field on the critical current and electric resistivity of the joints were studied. Finally, finite element simulations were employed to reproduce the distribution of the stress field developed in the spliced joint samples during axial loading.
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