Nanosized Cr[sub 2]O[sub 3] as pinning center in (Bi,Pb)-Sr-Ca-Cu-O superconductor tapes

Abstract

Nanosized Cr2O3 (6 nm) has been introduced into (Bi,Pb)-Sr-Ca-Cu-O/Ag tapes fabricated by the powderin-tube method. The starting powder with composition (Bi,Pb)2Sr2Ca2Cu3O10(Cr2O3)0.05 was prepared by the solid state reaction method. The superconductor tapes with thickness of about 0.3 mm and length about 2 cm were heated at 845°C for 96, 144, 192 and 240 h. The samples were characterized by the four point probe electrical measurements, X-ray powder diffraction method and scanning electron microscopy. The transport critical current density, Jc was measured from 30 to 65 K using the 1 μ/cm criterion. The highest Jc was obtained for the tape heated for 240 h with Jc = 1700 A/cm2 at 30 K. XRD patterns showed that the samples consisted of the 2212 and 2223 phase. SEM micrographs showed the plat-like structure. The role of nano-Cr2O3 in the superconductor system is discussed. The size of the nanoparticle (R) was larger than the coherence length, ξ but smaller than the penetration depth, λ(ξ< R <λ). The temperature dependent Jc from 30 to 65 K showed a linear relation for all except for the tapes heated at 240 h. Using the self-field approximation together with the Jc dependence on temperature, it was observed that between 30 and 65 K the characteristic length (Lc) associated with the pinning force Fp is approximately the same as the average grain size (Rg) in all samples except for the tapes heated for 240 h where Rg < Lc. This shows that a change in the flux pinning mechanism occurred in the tapes when the heating time was prolonged.