Large area deposition of YBCO thick films for applications in resistive fault current limiting devices

The preparation of the switching element of a resistive superconducting fault current limiter requires the up-scaling of deposition techniques for thin YBa₂Cu₃O_7-a (YBCO) films to large areas and high film thicknesses. For a projected 100 kVA limiter model an area of about 400 cm² and a film thickness of up to 5 μm will be necessary, depending on the material properties like critical current density j_c and normal state resistivity ρ_n. Within a joint project various deposition methods including pulsed laser deposition (PLD), magnetron sputtering(MS), thermal evaporation (TE) and plasma flash evaporation (PFE) are evaluated as possible candidates for the operation of deposition systems capable of coating 20x20 cm² substrates. Both single crystalline wafers and polycrystalline ceramic plates are considered as substrates. To achieve high j_c on polycrystalline substrate materials an additional zirconia buffer layer consisting of biaxially orientated crystallites has to be prepared by ion beam assisted deposition (IBAD). In small samples with IBAD buffer critical currents above 10⁵ A cm^-2 with a maximum of 1x10⁶ A cm^-2 have been achieved. The presently available sample sizes depend on the installed systems for YBCO and buffer deposition, respectively and on the commercial availability of the substrate material. The largest samples which have been prepared and characterised have sizes of 1x25 cm² and 5x5 cm² for PLD, 10x10 cm² for TE and IBAD, 2 in, (1 in,=2.54 cm) diameter for MS, and 3x7 cm² for PFE. The corresponding highest film thicknesses are 4.5 μm for PLD, 1.4 μm for TE, 1.6 μm for IBAD and 0.4 μm for MS.