Abstract
The effects of nanosized silicon oxide nanoparticles and nanowires additions on the microstructure and the normal state transport properties of polycrystalline YBa2Cu3Oy (YBCO, or Y-123) were systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nanosized entities up to 3 wt.%. Phases, microstructure, superconductivity, have been systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrical measurements. When nanosized SiO2 entities are added to the YBCO the orthorhombic structure is maintained. SEM results reveal that the grain size is reduced with increasing the content of SiO2. TEM investigation shows the presence of inhomogeneities embedded in the superconducting matrix along with the presence of columnar defects in the case of SiO2 nanoparticles added samples. Nanowires tend to agglomerate by entangling with each other in the intergrain regions. To analyze the normal state properties of the samples, the percolation theory based on localized states is applied. A change from Coulomb gap to variable-range hopping mechanisms is observed as a result of increasing the nano-entities concentration. The SiO2 nanowires addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to SiO2 nanoparticles addition. The localization length d and the range hopping R of samples are estimated. (C) 2013 Elsevier B.V. All rights reserved.