Refinement of microstructure and highly improved electrical properties of Bi_(1.6)Pb_(0.5)Sr_(1.925)Ho_(0.075)Ca_(1.1)Cu_(2.1)O_(8+δ) superconductor

摘要

The importance of rare-earth (RE) doping and precise tuning of sintering temperature on the microstructural features, electrical properties, and magnetic performance of a (Bi,Pb)-2212 superconductor sintered between 846 and 860 ℃ has been investigated. The results show that the RE (holmium) atoms enter into the crystal structure and induce significant changes in the hole concentration, lattice parameter values, microstructure, normal state conductivity, self- and in-field critical current densities, and flux pinning properties of the system. The microstructural analysis shows that the samples sintered at a comparatively lower sintering temperature (846 ℃) have more grain boundaries with smaller grains, while those sintered at a higher sintering temperature (856 ℃) contain larger grains with good texturing. Also, the irreversibility lines are shifted to higher temperatures and magnetic fields to different extents depending on the sintering temperature. The flux pinning force density (F_P) curves are theoretically fitted using the Dew-Hughes model, which shows that the main pinning mechanism is the point defect arising due to the substitution of holmium atoms at the strontium site of the (Bi,Pb)-2212 system. Also, for samples sintered at 846 ℃, F_(Pmax) appeared at a reduced field value of h=0.39, which shows that in addition to point pinning, grain boundary pinning is also effective in these samples. The changes in microstructure followed by a very high enhancement of superconductivity and flux pinning properties due to RE doping within a small sintering temperature range are of great scientific and technological significance.