Y-123 Films on technical substrates

For high-current applications, homogeneous well-textured high-temperature superconducting Y₁Ba₂Cu₃O_7−δ (Y-123) films on technical ceramic or metallic substrates are required. The ion-beam-assisted deposition (IBAD) of yttria-stabilized zirconia (YSZ) buffer layers, which serve as a template for Y-123, was extended to large-area substrates as well as differently shaped long substrates. TEM, as well as XRD investigations, was performed to observe the relevant growth mechanisms. The influence of nucleation, growth selection, and homoepitaxial growth on the development or the maintenance of a preferential orientation of the YSZ buffer is discussed. Y-123 films were prepared by a modified pulsed-laser-deposition (PLD) technique based on a quasi-equilibrium substrate heating and variable azimuth scanning of the target. The method allows a PLD coating of long pieces of tapes and tubular substrates because no direct mechanical contact of a substrate with the heater is required. Critical current densities, J_c, above 10⁶ A cm⁻² were achieved on polycrystalline Ni tapes as well as on polycrystalline YSZ sheets at 77 K and self fields. The dependence of J_c on magnetic field resembles that of Y-123 deposited on single-crystalline substrates and demonstrates the absence of weak links due to grain boundaries. The thickness dependence of J_c is interpreted in terms of a nucleation layer appearing in the early stage of film growth. A growth model is proposed which seems to be in agreement with the experimental observations. The measured strain tolerance of the films does not impede their application.