M. Jergel

Preparation and properties of precursor Ba–Ca–Cu–(O, F) thin films deposited from fluorides for superconducting Tl- and Hg-based films

Abstract Precursor Ba–Ca–Cu–(O, F) thin films prepared by thermal evaporation of BaF2, CaF2 and Cu show good chemical resistance against the atmosphere even after a partial defluorination by means of vacuum annealing with subsequent oxidation. The prepared precursor films were characterized by X-ray diffraction (XRD) analyses and Raman scattering measurements. A possible explanation for the observed properties is presented. According to the XRD measurements performed in the grazing incidence set-up, a polycrystalline cubic BaCuO2 is the main phase after defluorination followed by oxidation. No peaks attributable to carbides (as a consequence of atmospheric exposure of the samples) were identified. Raman analysis confirmed the presence of another phase, namely CuO.

Rutherford backscattering analysis of Bi-based superconducting films

The elemental composition, film thickness and concentration depth profiles of precursor and superconducting (Bi ,Pb)-SrCa-Cu-O films were studied by the Rutherford Backscattering Spectrometry (RBS) technique. The precursor films were deposited on MgO single-crystalline substrates from an aerosol atomized ultrasonically from an aqueous nitrate solution. Precursor films, about 5 to 5.5 μm thick, were then annealed in air at temperatures ranging from 835oC to 855oC during 10 h. XRD studies revealed mainly the presence of the 2212 phase (for bulk Tc is about 85 K). Films annealed at temperatures Ta ≥ 850oC were superconducting with Tc in the range of 60 to 71 K, showing a double Tc onset at 85 K and 110 K. The RBS study of the Bi-profile of precursors showed a maximum content of Bi at a depth around 1 to 2 μm from the film surface. After the film annealing, the Bi content was found to be constant from the surface to about 1 μm depth, decreasing then its value towards the film-substrate interface.

Tl-based films grown on silver tape substrates

The Tl-based superconductors belong to potential candidates for high-current and high-field applications. It is therefore of importance to prepare them in the shape of tapes and wires. It seems that silver tape is a very convenient material having suitable mechanical properties as well as chemical compatibility with superconducting films deposited and grown on top of them. In our case, Tl-based films were grown using a two-step procedure. First, the Ba-Ca-Cu precursors were deposited on a commercial silver tape from an aerosol, and then the precursors were thallinated in a closed crucible with a source of Tl-oxide in the form of a crude pellet. Results are reported on superconducting samples consisting so far of 2212, 2223 and 1223 phase mixtures. SEM observations reveal the growth of plate-like grains. Metallurgical processing of silver tape is underway to obtain a cubic texturized substrate which should help to improve transport properties of the grown films.