Issues and progress related to the continuous Ex-Situ BaF₂ processing of long-length YBCO coated conductors

For the past several years, an increasing amount of effort in the high temperature superconductor (HTS) community has been focused on the development of YBCO coated conductors. Very high values of critical current density (J _c) in excess of 106 A/cm2 at 77 K and self-field have been reported on YBCO films grown on biaxially textured metallic substrates. These excellent results were typically obtained on a variety of short-length (less than 2cm) textured templates such as ion beam assisted deposition (IBAD) substrates (Iijima et al., 1992; Reade et al., 1992; Wu et al., 1995; Bauer et al., 1999b), rolling assisted biaxially textured substrates (RABiTS) (Goyal et al., 1996; Norton et al., 1996; Petrisor et al., 1999; Wang et al., 2000) and, to a lesser extend, inclined substrate deposition (ISD) substrates (Fukutomi et al., 1994; Bauer et al., 1999a). YBCO deposition methods employed to obtain these high J_cs include pulsed laser deposition (PLD) (Mathis et al., 1998), thermal evaporation (Bauer et al., 1999a), metalorganic chemical vapor deposition (MOCVD) (Ignatiev et al., 1996; Selvamanickam et al., 2000), BaF2 precursor (Paranthaman et al., 2000; Feldmann et al., 2001) and TFA (Rupich et al., 2001). While there are ample demonstrations of high J_cs on short-length YBCO coated conductors, there has been only limited investigation into the feasibility and issues related to the fabrication of meter-length YBCO coated conductors. This is mainly due to the limited availability of long-length substrates as well as the resources necessary to investigate the epitaxial YBCO formation process in a moving manner. Nevertheless, meter-length YBCO with high end-to-end J_cs have been produced on longlength substrates using the PLD technique (Foltyn et al., 1999; Iijima et al., 2000; Sato et al., 2001). Although this in-situ technique is a well proven method, it requires simultaneous control of film composition and HTS epitaxial growth within a relatively narrow process window. Still, much knowledge has been gained in the continuous deposition of YBCO by PLD. The ex-situ approach of YBCO formation, on the other hand, separates the precursor deposition step (i.e. compositional control) from the YBCO epitaxial growth process. By separating these steps, further simplification in processing procedures such as non-vacuum deposition of precursor film (Rupich et al., 2001) or batch conversion of YBCO is possible. Unfortunately, most of the ex-situ works have been performed on short-length samples to date. Consequently, issues related to the continuous ex-situ processing of YBCO are largely unknown and thus, the feasibility of this method in the manufacturing of long-length YBCO coated conductor cannot be evaluated. In this paper, we will present the progress and known issues that are important in the BaF₂ ex-situ processing of long-length YBCO coated conductor in a moving manner.