Synthesis and Characterization of Nanoparticles Added CuTl-based Superconductors

Abstract

Low anisotropic (Cu 0.5 Tl 0.5 )Ba 2 Ca 2 Cu 3 O 10-δ (CuTl-1223) phase of Cu 1-x Tl x Ba 2 Ca 2 Cu 3 O 10- δ [CuTl-12(n-1)n]; n=1, 2, 3, ... high temperature superconducting (HTSC) family was synthesized by solid-state reaction method. Nanoparticles (NPs) were prepared separately by different techniques (i.e. co-precipitation, sol-gel, and colloidal solution methods). Different kinds and sizes of NPs such as non-magnetic metallic (Ag, Au), non-magnetic metallic oxide (Al 2 O 3 ) and magnetic ferrite (CoFe 2 O 4 ) were added with different wt.% during the final sintering process of CuTl-1223 superconducting matrix to get the required (NPs) x /CuTl-1223 nanoparticles-superconducting composites. These composites were characterized by different experimental techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray (EDX) spectroscopy, resistivity versus temperature measurements, critical current density (J c ) measurements, magnetic ac-susceptibility measurements, Fourier transform infrared (FTIR) spectroscopy etc. The experimental data were analyzed by different well established theoretical models such as Aslamazov-Larkin (AL), Lawerence-Doniach (LD) and Maki-Thompson (MT) models. It was observed that tetragonal structure and stoichiometry of the host CuTl-1223 superconducting phase remained unaltered after the addition of these NPs, which indicates about the occupancy of these NPs at the inter-granular spaces. Non-magnetic metallic NPs (Ag, Au) have improved the superconducting transport properties (T c , J c , etc) up to certain optimum concentration of these NPs in CuTl-1223 matrix. The improvement in the superconducting properties can be attributed to an increase in the inter-grains connectivity by healing up the inter-grains voids and pores after the addition of these NPs. The improved inter-grains connections can facilitate the carriers transport processes across the inter-crystallite sites due to their metallic nature. But the superconducting volume fraction starts to be decreased after certain optimum inclusion level of these non-magnetic non-superconducting metallic NPs, which causes the suppression of superconductivity parameters. The effects of highly coercive cobalt ferrite (CoFe 2 O 4 ) nanoparticles addition on superconducting properties of CuTl-1223 matrix were also explored. The magnetic behavior of CoFe 2 O 4 nanoparticles was determined by MH-loops with the help of superconducting quantum interference device (SQUID). The resistivity versus temperature measurements showed an increase in T c (0), which could be most probably due to improvement of weak-links by the addition of these nanoparticles. The increase of mass density with xxviincreasing content of these NPs can also be an evidence of filling up the voids in the matrix. But the addition of these NPs beyond an optimum level caused the agglomeration and produced additional stresses in material and suppressed the superconductivity. We observed non- monotonic variation of superconducting properties after the inclusion of nano-Al 2 O 3 particles, which can be associated with inhomogeneous distribution of these NPs at the grain-boundaries in CuTl-1223 matrix. But overall suppression of superconducting properties was attributed to a pair-breaking mechanism caused by reflection/scattering of carriers across these insulating nano- Al 2 O 3 particles present at the grain-boundaries of the host CuTl-1223 matrix. The presence of Al 2 O 3 nanoparticles at the grain-boundaries possibly reduced the number of flux pinning centers, which were present in the form of weak-links in pure CuTl-1223 superconducting matrix. The suppression of activation energy {U (eV)} may be due to weak flux pinning in the nano-Al 2 O 3 particles added samples. The superconducting microscopic parameters (i.e. zero temperature coherence length along c-axis {c (0)}, inter-layer coupling (J), inter-grain coupling () etc.) deduced from fluctuation induced conductivity (FIC) analysis with the help of above mentioned theoretical models explained the experimental findings very well. For example, the increase in the values of inter-grain coupling () deduced from FIC analysis is a theoretical evidence of improved inter-grain coupling in the host CuTl-1223 matrix with the increased contents of these nanoparticles. We also calculated the activation energy {U (eV)} of (NPs) x /CuTl-1223 nanoparticles-superconducting composites. The increase in T c (0), J c , U (eV) etc and decrease in normal state resistivity {ρ 300 K (Ω-cm)} were observed after the addition Ag, Au and CoFe 2 O 4 NPs in CuTl-1223 superconducting phase. The suppression of superconducting properties (i.e. T c (0), J c , U (eV) etc) after the addition of Al 2 O 3 nanoparticles in host CuTl-1233 superconducting matrix was observed.