Superconductors application fields as energy distribution, fault current limiter

Objectives: The main goal of the studies  is to realize low cost superconducting wires for current transport and current limitation by using original and inexpensive processes like rolling for the elaboration of the substrate and chemical deposition methods MOD (metal organic decomposition) and MOCVD (metal organic chemical vapor deposition) for the different layers (buffer layers and superconducting film). The final material which remains as a candidate for HTS application is YBCO. But to realize high quality and high performance conductors, the main difficulty is that one has to perform the deposition of epitaxialy textured oxide films on a biaxially textured metallic substrate. The main challenge in doing this is to use cost effective chemical methods for obtaining the functional multilayer architecture and to control perfectly the metal oxide interface: this constitutes a radical advances in the processing of multifunctional films. In that context, the key objectives that we have followed during the last years were:

• to design materials for Superconducting Application: multifunctional multilayers for electrical, thermal and  mechanical stability with a simplified architecture
• to deposit a homogeneous and reproducible heteroepitaxial inorganic film on a metallic tape  
• to ensure a continuous (in line) quality control of nanostructured and textured long lengths of films

Outstanding results:   Our experience in Pulsed injection MOCVD technique has shown that this technique is well adapted for coated conductor processing: it allows obtaining reproducible epitaxial YBCO films and can be extrapolated for long length conductors. However the deposition of buffer layers by MOCVD on flexible metallic substrates causes its oxidation and destroys its crystalline structure. The epitaxial deposition of an oxide on a metal is not a trivial problem : it is actually the main bottleneck for the long length development of coated conductors. The transition layer, or buffer layer, from the metal to the superconducting oxide, can be obtained currently by physical deposition processes (IBAD, stacking of different metal and oxide adaptative layers by PLD), but all these processes are very expensive and difficult to translate for a large scale production.  We have concentrated therefore our efforts to find out a simple and efficient conductor architecture, which preserves the superconducting performances in a simplified technology process. The combination of both MOD and MOCVD techniques, developed during the PhD thesis of Tristan Caroff, solves actually this issue and represents a important step forward to realistic applications. We have shown that with just one single oxide buffer layer of La2Zr2O7 (LZO) obtained by MOD, good superconducting properties can be achieved in the subsequent YBCO layer. The deposition of a first buffer layer by MOD under reducing atmosphere (Ar + 5%H2) avoids the oxidation of the substrate during the following depositions by MOCVD. Figure 2 High magnification image of YBCO/LZO/LAO heterostructure and diffraction patterns obtained in the LAO [100] pseudocubic (pc) zone axis Two architectures have been developed and characterized: NiWRABiTS/LZOMOD/YBCOMOCVD et NiWRABiTS/LZOMOD/CeO2MOCVD/YBCOMOCVD. The study of these conductors proved such architectures can reach high critical current density: respectively Jc = 0.8 MA/cm2 et 1.2 MA/cm2 et Ic/cm = 34 A/cm et 54 A/cm on 800 nm thick YBCO tapes (Caroff, Superconductor Sci  & Tech, 21(7) (2008) N°075007). A TEM study allowed us to understand the growth of the LZO film and to observe the formation of pores during the pyrolysis of the precursors (Rapenne, J of Materials Res Vol 24, No4 (2009) 1480). Combined EBSD and MO studies demonstrated that the microstructure of the substrate (grain boundaries, scratches,...) is transferred to the super conducting film and has a negative influence YBCO inter-grain connectivity and thus on YBCO film quality. Figure 3 Effet of the thickness of the LZO buffer layer on the YBCO superconducting performances as detected by Magneto-optical observations The deposition of thick buffers layers (> 150 nm) smoothes these defects and improve superconducting properties of the tapes. Finally, successful tests in current limitation and in current transport under strain validated the fabrication process for such specific application. Figure 4 Test on a default current limiter consisting of YBCO/CeO2/LZO/Ni Perspectives : We keep a close collaboration with the end-user Nexans through the FUI project SUPERFACTS (TENERDISS labelled) starting on January 2009. This project is based on our previous results, but applied to a new cable shape patented by Nexans.  Round shape presents really advantages not only for optimisation of current transport (ac losses), but also for the cable manufacturer. Permanent Staff: C. Jimenez, L. Rapenne, P. Chaudouet, E. Sarigiannidou, F. Weiss Non Permanent Staff:  Tristan Caroff, Andrea Cavallaro,  Cyril Millon , Samir Guerroudj, Virginie Roche , Elena Santos, Didier Foué , Corentin Boulo, Rym Benaboud