Improvements in the critical currents of high temperature superconductors through an uranium doping and thermal neutron irradiation method

Full text: For practical utilisation in commercial and industrial applications, high temperature superconductors (HTS) materials must meet a number of important criteria, such as mechanical stability and sufficient yield of transport critical current, Jc. Silver-sheathed Bi2-Sr2-Ca2-Cu3-Ox tape is an example of a HTS material that has been developed with the required properties. However, at the present its potential for use, as well as that of other similar HTS materials, is limited by the reduction in Jc caused by an applied magnetic field. The decrease in Jc is a result of the motion of flux lines within the HTS material. To overcome this problem, flux lines must be effectively pinned within potential energy wells to stop their motion. Irradiation methods have been used to create displacement damage tracks, which act as flux pinning sites. Uranium-235 can be doped into the superconductor before processing of the tape. When the tape is then irradiated with thermal neutrons, the resulting 235U fission fragments create randomly oriented columnar pinning tracks, which improve the flux pinning of the tape and hence increase the critical current under applied magnetic fields. There are two problems with this process. Firstly, the silver sheath also captures thermal neutrons, and as a result becomes radioactive. The total neutron fluence used must thus be reduced as far as possible. To achieve the same degree of pinning with a lower fluence, the amount of 235U added must be increased. This brings the second problem, that the initial Jc in zero field is degraded by the addition of increasing amounts of U-oxide. A balance between the amount of U added and the fluence used must therefore be reached for a sufficient improvement in the flux pinning. Ag/Bi-2223 tapes were created with small amounts of UO4 added. They were then irradiated at the HIFAR reactor to various fluence levels shows the field behaviour of Jc normalised to its initial zero field value, Jc0 The normalisation highlights the resultant improvements in flux pinning. It can be seen that increasing the amount of U-235 added can significantly reduce the required fluence for a similar pinning strength, but that a moderate doping and fluence level can provide the best pinning performance