Study of the effect of Ni additive in YBa2Cu3O7-δ superconducting composite employing ultrasonic measurement

•A new technique was developed for reproducible ultrasonic velocities measurements.•The microstructure of composite YBCO/Ni affects notably its mechanical properties.•Voids growing among YBCO grains influence the values of the ultrasonic velocities.•VS is much more sensitive than VL to connectivity among YBCO grains.•Calculations of ϴD showed the effective role of Ni on the YBCO lattice vibrations. A series of superconducting composites (100-x) YBa2Cu3O7-δ + x Ni metal (x = 0, 1, 2.5, 5, 7.5, 10 and 15 wt.%) was prepared by the solid state reaction method. Density and molar volume were determined using the immersion method. X-Rays-Diffraction (XRD) and Scanning Electron Microscope (SEM) characterizations were terminated for structural and microstructural analysis. The results showed the presence of orthorhombic peaks of YBa2Cu3O7-δ (YBCO) in the whole composite samples with different lattice constants and how Ni addition affects the distortions in the orthorhombic unit cell of YBCO. A new technique has been used to measure the ultrasonic wave velocities, longitudinal (VL) and transverse or shear (VS), at 4 MHz ultrasonic frequency at room temperature. The mechanical parameters of the sintered superconductor samples including longitudinal modulus (L), shear modulus (G), Young’s modulus (Y), Poisson’s ratio (σ) and microhardness (H) were investigated from the view point of the porosity and the connectivity between the formed phases. Besides, the Debye temperature (ϴD) was also determined as a function of Ni additions. The elastic moduli, H and ϴD have been improved up to 2.5 wt.% Ni, then a decrease in all data observed with further addition of Ni content. Quantitative analysis has been implemented to obtain more details about the structure of the composite samples, based on orthorhombic phases, the oriented grain size, the porosity and the agglomeration between grains.