Non-fluorine based bulk solution techniques to grow superconducting YBa₂Cu₃O_7-δ films

Major advances have been made in the last 14 years in the area of hightemperature superconductor (HTS) research, resulting in increasing use of HTS materials in commercial and pre-commercial applications. HTS conductors are expected to be useful for numerous electric power applications, including underground transmission cables, oil-free transformers, high efficiency motors, compact generators, and superconducting magnetic energy storage systems for smoothing voltage fluctuations in the power grid. Research on the YBa ₂Cu₃O_7?σ (YBCO) based second generation conductors is now intensively carried out in the world. Recently, the US Department of Energy conducted a Coated Conductor Technology Development RoadmapWorkshop in St. Petersburg, Florida. This roadmap identified specific near-term activities that are needed to advance techniques for continuous processing of high quality, low-cost coated conductors that will lead to industrial-scale commercial manufacturing (Energetics, Inc., Columbia, MD, 2001). The activities specified in this roadmap are focused on achieving the following vision. Low-cost, high-performance YBCO Coated Conductors will be available in 2005 in kilometer lengths. For applications in liquid nitrogen, the wire cost will be less than $50/kA-m, while for applications requiring cooling to temperatures of 2060 K the cost will be less than $30/kA-m. By 2010 the cost-performance ratio will have improved by at least a factor of four. One of the important critical needs that came out of this workshop was to develop alternative nonvacuum processes for fast, reliable and economic deposition of YBCO. The traditional in-situ process, in which pulsed laser deposition (PLD) of oxide or co-evaporation of Y, Ba, and Cu metals under appropriate oxygen atmospheres, could be used to fabricate YBCO films. However, it may be difficult to scale up these processes to produce low-cost conductors. This is mainly due to the initial investment of a high cost laser, the high cost of large vacuum chambers with pumping system, etc. The control of substrate temperature during deposition is also difficult. To circumvent this problem, ex-situ precursor processes can be used. Chemical solution epitaxy has emerged as a viable, low-cost, non-vacuum process for fabricating long lengths of YBCO coated conductors. In these processes, YBCO precursors can be deposited at room temperature and later post-annealed in a controlled atmosphere furnace. The advantages of ex-situ processes are the separation of the deposition and post-annealing steps, and a wider processing window by combining temperature, and oxygen partial pressures. The dependence of oxygen partial pressure and the YBCO process temperatures is shown in Figure 12.1. Also, the precursor stoichiometry, and dopant concentration can be easily controlled and the post-annealing step can be a batch process. The growth rate of YBCO generally varies from 13 /sec. This could be a rate-limiting step in these processes. However, it is possible to overcome these limitations by suitably modifying the furnace designs to process large quantities of wires (hence, large area) in a single step. The most commonly used bulk solution techniques to fabricate YBCO coated conductors are: i) Sol-gel processing (a) Sol-gel alkoxide approach (b) Metalorganic decomposition (MOD) (ii) Electrodeposition (iii) Electrophoresis (iv) Spray (Aerosol) pyrolysis techniques (v) Chemical vapor deposition (a) Combustion chemical vapor deposition (CCVD) (b) Metalorganic chemical vapor deposition (MOCVD) (vi) Powder suspension techniques (vii) Liquid phase epitaxy (LPE) Each film deposition process will have some maximum rate, beyond which defects or other problems such as supplying source material or removing by-products may become limiting. The properties of YBCO films will depend critically upon the microstructures that develop during the nucleation and growth of the films. These microstructures depend on the substrate properties, the particular deposition technique, the processing conditions and the film thickness. The Trifluoroacetate (TFA), MOCVD, CCVD, and LPE techniques are reviewed elsewhere in this book. In this review, we will report only the recent achievements in growing YBCO films using non-fluorine containing solution precursors. In addition, the recent developments in the buffer layer work at Oak Ridge National Laboratory are also highlighted.