Uranium doping of silver sheathed bismuth-strontium-calcium-copper-oxide superconducting tapes for increased critical current density through enhanced flux pinning

A combination of uranium doping with thermal neutron irradiation has been well demonstrated to be one of the most effective means to introduce pinning centres in (Bi,Pb)[subscript2]Sr[subscript2]Ca[subscript2]Cu[subscript3]O[subscriptx]/Ag tapes (Bi-2223/Ag). A substantial improvement in flux pinning and reduction of anisotropy in uranium doped Bi-2223/Ag tapes has been achieved due to thermal neutron irradiation. However, the radioactivity of the silver sheath is too high for practical application of this technique. This work aimed at minimising the detrimental effects of doping to allow for later irradiation and fission to create columnar defects for strong flux pinning. A number of uranium containing compounds, U[subscript3]O[subscript8], U(Ca,Sr)[subscript2]O[subscript5], and U(Ca,Sr)[subscript3]O[subscript6] have been identified in the U-Bi-Sr-Ca-Cu-O system. These equilibrium compounds have been synthesised and added to Bi-2223/Ag composite tapes in progressively greater amounts. The effects on the transport critical current density of Bi-2223/Ag of doping with a variety of uranium compounds up to 2 at% was determined. The compatibility and interaction between uranium compounds and the Bi-2223 matrix was systematically studied under various thermal processing conditions. Bi-2223/Ag doped with compounds using compositions closer to the equilibrium composition was shown to have superior superconducting properties to that doped with uranium oxide compounds. The effects of chemical degradation by uranium compound dopants via strontium and calcium removal from the Bi-2223 structure have been indirectly determined by critical current density (J[subscriptc]) measurements. Losses in J[subscriptc] can be minimised by stabilising uranium into appropriate compounds. Uca[subscript1.5]Sr[subscript1.5]O[subscript6] shows a remarkable compatibility with Bi-2223 phase, with critical current density reduced by about 15% at a doping level of 1.1 at%, compared with the 85% reduction in critical current density at the same doping level of pure uranium oxide. X-ray diffraction (XRD) results show that Uca[subscript1.5]Sr[subscript1.5]O[subscript6] has no effect on Bi-2223 formation, while U[subscript3]O[subcript8] caused degradation of Bi-2223 phase up to 50% at a doping level of 1.1 at%. The uranium compound doping also widens the processing window to 15 [celsius degrees]. Microstructural changes due to doping were investigated. Removal of elements from the Bi-2223 matrix by uranium compound dopants was studied by energy dispersive X-ray spectroscopy (EDS). It was found that Uca[subscript1.5]Sr[subscript1.5]O[subscript6] least degraded electrical performance and microstructure, as it removed the least elements from the matrix. It would appear that Uca[subscript1.5]Sr[subscript1.5]O[subscript6] most closely approximates an ideal Bi-2223 compatible uranium compound, but it still removes some amount of copper.