Determination of solid solubility level of Ho nanoparticles in Y-123 superconducting matrix and strong Cu1 site preference of nanoparticles

Abstract

This comprehensive study reports the effect of the Ho inclusions on the microstructural, electrical, mechanical and superconducting characteristics of YBa2Cu3O7−δ ceramic superconductors with the aid of standard characterization methods including the bulk density, dc resistivity (ρ–T), transport critical current density (Jc), X-ray diffraction (XRD), electron dispersive X-ray (EDX), scanning electron microscopy (SEM) and Vickers microhardness (HV) investigations. The experimental results such as the degree of granularity, hole (filling) localization effect, room temperature resistivity, onset–offset critical transition temperature, degree of the broadening, thermodynamic fluctuations (spin-gap opening temperature), crystallinity, crystal plane alignments (texturing), crystal structure, grain size, phase purity and lattice parameters, appearance of flux pinning centers, grain boundary weak-links (interaction between the superconducting grains), surface morphologies (grain size distribution), real (load independent) microhardness values, elemental compositions and distributions belonging to the pure and Y-site Ho substituted Y-123 superconducting samples are discussed in detail for the first time. Moreover, mechanical characterization enables us to theoretically determine the elastic (Young’s) modulus and yield strength being in charge of the potential mechanical applications. Additionally, the load dependent microhardness values of the Y-site Ho substituted Y-123 materials have not been modeled by the available theoretical methods (Hays–Kendall and indentation-induced cracking approach) up to the present. All the experimental findings show that the microstructural, electrical, mechanical and superconducting properties improve regularly with the increment in the Ho concentration level at the Y-site in the superconducting matrix until a certain value of x=0.100 (optimum) beyond which the characteristics tend to retrograde rapidly. This is attributed to the fact that excess (0.100<x) penetration of the homovalent Ho-sites on the Y-sites damages the crucial properties given above. In other words, from the Ho content level of x=0.100 onwards in the Y-123 matrix, the oxygen content in the Cu–O chains begins to increase considerably and new induced oxygen atoms reorder the unit cell structure (rapid increment of the a axis length), so the structural phase transits from the orthorhombic to pseudotetragonal, being one of the most striking points deduced from this paper. Another vital discussion relies on the fact that the degradation of the mobile hole concentration (hole localization effect) in the Cu-sites leads to transition from optimally doped to the underdoped position in the crystal structure. Besides, the rapid degradation in the mechanical properties due to the increase of the specimen cracking/porosity, grain boundary weak-links and irregular grain orientation distribution confirms the relationships between the microstructure and mechanical properties of the Y-123 superconducting materials. Furthermore, all the samples exhibit the typical Reverse Indentation Size Effect (RISE) behavior under the applied indentation test load. As for the theoretical modeling of the hardness evidences, the calculations performed by IIC model are quite closer to the values of the plateau region as compared to those of HK approach. Based on the results, the IIC model is found to be superior to HK approach for the description of the real microhardness values belonging to all the superconducting samples.