Effect of alkali metal Na on the formation and superconductivity of Tl-1212-type phase (Tl0.85Cr0.15)Sr2−x Na x CaCu2O7

The effect of Na substitution on the formation and superconductivity of Tl-1212-type phase (Tl0.85Cr0.15)Sr2−xNaxCaCu2O7 (x = 0–0.30) is reported. The samples were prepared via the solid-state reaction method. X-ray diffraction patterns showed the presence of Tl-1212-type phase or Ca0.3Sr0.7CuO2 (CSCO) as a major or minor phase in the samples. The resistance versus temperature curves showed onset transition temperature, Tc-onset between 97 and 106 K and zero-resistance temperature, Tc-zero between 88 and 99 K. AC susceptibility measurement showed superconducting transition Tcχ′ between 89 and 98 K. The peak temperature, Tp, of the imaginary part of the susceptibility, χ″, was the highest for x = 0.02 sample (Tp = 96 K). The inter-grain critical current density, Jc, at Tp for x = 0 sample was Jc (Tp = 68 K) = 14 A cm−2 and for x = 0.02, 0.20 and 0.30 Jc (Tp) at 96, 96, and 88 K, respectively, is 19 A cm−2. Na substitution increased the grain size of the Tl-1212 samples. All Na-substituted samples showed higher transition temperature, enhanced critical current density, and improved inter-grain coupling compared with the non-substituted sample. This work showed that other than rare-earth elements, monovalent alkali metal such as Na could also be used to enhance the superconducting properties of the Tl-1212 phase.