Study of high temperature superconducting films with high critical current density on metallic substrates

Author

Qing He

Date of Award

5-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

A.J. Pedraza

Committee Members

D.K. Christen, J.R. Thompson, D.H. Lowndes, C.R. Brooks

Abstract

High temperature superconductors (HTS) hold great promise for widespread commercial applications, most of which require the fabrication of flexible long HTS tapes with high critical current density J_c values. In this thesis, two new processes have been developed. One is a spin-coating process, which can provide a simple, inexpensive and rapid means to fabricate c-axis textured high J_c Tl₁Ba₂Ca₂Cu₃O₉ (Tl-1223) thick films on flexible polycrystalline Ag substrates. A limited matrix study has been conducted to determine how the Tl-1223 films are affected by the Tl-source temperature, sample temperature, and reaction time. The pressing process is the key point in the technique to get c-axis orientation and high J_c value for the Tl-1223 films. The superconducting properties, microstructure, and structural orientation of the Tl-1223 films have been analyzed. The other process is RABiTS (Rolling- Assisted Biaxially Textured Substrates), by which biaxially textured HTS films with high J_c values could be grown on buffered flexible Ni substrates. The deposition of biaxially textured buffer layers on the aligned Ni substrates plays an important role in the RABiTS process, which should ensure epitaxy and chemical compatibility of the buffer layers with both the underlying nickel and the final HTS layer. To grow the biaxially oriented buffer layers on the Ni substrates, two approaches have been taken. The first is to grow the biaxially oriented metal or oxide buffer layers, such as Ag and CeO₂, on the Ni tapes by using oxidation-resistant Pd (or Pt) film as an intermediate layer, which transfers the biaxially oriented structure from Ni to the top buffer layers. The second approach is to grow biaxially oriented oxide films, such as CeO₂ and LaNiO₃, directly on the Ni substrate by introducing forming gas (Ar/4%H₂2) which might prevent the formation of NiO while assuring stability of the sputtered oxides. It is found that some biaxially textured films, such as Pd and CeO₂, can be grown at room temperature and that post-annealing can improve the biaxial orientations. The effects of deposition temperature on the degree of biaxial orientations of the CeO₂ films were investigated. SEM observations show that cracking may occur on the CeO₂ buffer layer during oxygen-annealing at high temperature. To prevent the cracking, an additional YSZ layer is deposited on the CeO₂ buffered Ni substrates. It is found that good Ni surface smoothness can improve the out-of-plane orientation of the deposited films. Electrically conductive LaNiO₃ buffer layers with biaxial orientations have also been grown on the textured Ni at low temperature. Subsequent oxygen anneals at 850°C neither change the biaxial orientations of the film significantly nor cause any cracks on the surface of the films. The resistivity of the LaNiO₃ films can be reduced by oxygen-annealing at high temperature. Biaxially textured YBCO films with J_c values over 10⁶ A/cm² at 77 K have been successfully grown on the buffered flexible Ni tapes.

Recommended Citation

He, Qing, "Study of high temperature superconducting films with high critical current density on metallic substrates. " PhD diss., University of Tennessee, 1998.
https://trace.tennessee.edu/utk_graddiss/9268