Development and application of a green-chemistry solution deposition technique for buffer layer coating on cube-textured metal substrates in view of further deposition of rare-earth based superconductors

Superconductor based energy production has been thoroughly researched by many scientists all over the world, due to the advantage of zero electric resistance that will contribute to the energy saving capabilities. Recently successful developments have been reported in coated conductor architectures which consist of YBCO superconducting coatings on cube-textured Ni based alloy tapes.  Before the epitaxial deposition this superconducting layer, a buffer layer is applied on the metal substrate as a diffusion barrier which is also required to transfer the strong texture of the underlying substrate, allowing the epitaxial growth of the superconducting layer.

State-of-the-art coated conductor hetero structures are mainly based on CeO2 based buffer stacks that consist of a sequence of several different buffer layers. Buffer layers deposited by continuous chemical deposition techniques, which are the most suitable for large scale production, use to suffer from porosity and cracks that lead to material diffusion from the metal substrate into the superconductor as well as oxidation of the metal during high-temperature processing of the buffer layer.

SrTiO3 buffer layers have attracted interest due to its low lattice mismatch with YBCO and the possibility of using this material as a single-layer buffer. Nevertheless, generally SrTiO3 buffer layers for YBCO are still applied as SrTiO3 multilayers. In addition, the main chemical based fabrication routes of SrTiO3 buffer layers involve toxic and hazardous chemicals such as 2-methoxyethanol, and trifluroacetic acid (TFA).

Therefore, in our research the main focus was on the development of SrTiO3 single buffer layers based on environmentally safe chemicals, to reach the engineering requirements for continuous coating of long substrate tapes. A new precursor solution for SrTiO3 buffer layers has been successfully developed with the reagents of strontium acetate, acetic acid, titanium isopropoxide, 1-methoxy-2-propanol and 2,4-pentanedione.

Using this precursor with dip-coating, high cube-textured SrTiO3 mono-coatings on Cu-33at.%Ni tapes were reproducibly obtained using single step annealings at 950°C in N2-5%H2 ambient flow. These layers were found to have a pore-free, crack-free and smooth morphology with an excellent biaxially textured surface.

The simple annealing process combined with a relatively high film thickness per single coating is expected to be very advantageous in reel-to-reel applications. The thickness of these SrTiO3 monolayers was large enough to stop the nickel and copper diffusion from the Cu/Ni substrate. Hence, the developed high quality buffer layers are expected to be acting as efficient diffusion barriers and also as a texture template for the subsequent growth of biaxially textured YBCO nanostructure.