| High-Tc superconducting (HTS) cuprates are highly anisotropic thus epitaxy along certain crystalline directions is essential to realize high-current-carrying capability at temperatures above 77 K. Hg-based HTS (Hg-HTS) cuprates have the record-high Tc up to 135 K, therefore are of great interest for fundamental research and practical applications. However, growth Of epitaxial Hg-HTS films is extremely difficult in conventional thermal-reaction process since Hg is highly volatile. Motivated by this, we first developed a cation-exchange process for growing epitaxial Hg-HTS films, which involves two steps: selection of precursor matrices with predesigned structure and composition followed by cation-exchange processing. New materials are formed via “atomic surgery” on an existing structure rather than thermal reaction among amorphous oxides in conventional process, thus the structural features of the precursor are inherited by the new material. Using epitaxial Tl-based HTS films as precursor and annealing them in Hg-vapor, epitaxial Hg-HTS films with superior quality have been obtained. This success encouraged us to develop epitaxy on metal tapes for coated conductors and On large-area wafers for electronic devices.; For coated conductors, we addressed three critical issues: epitaxy on metal substrates, enhancement of in-field Jcs and scale-up in thickness and length. First, using a fabrication scheme that combines two processes: cation-exchange and fast-temperature-ramping-annealing, epitaxial HgBa2CaCu2O6+δ films were grown on rolling-assisted-biaxially-textured Ni substrates buffered with CeO 2/YSZ/CeO2 for the first time. We fabricated HgBa2CaCu 2O6+δ coated conductors with Tc = 122–124 K and self-field Jc > 1 × 106A/cm2 at 92 K which are record-high for HTS coated conductors. Second, we demonstrated improved in-field J cs via overdoping HgBa2CaCu 2O6+δ films (by means Of charge “overdoped”), heavy-ion-irradiation and substrate engineering. Finally, thick HgBa 2CaCu2O6+δ films show high I c, and spool process also shows potential in middle-length tape fabrication. These results make Hg-HTS films good candidates as power transmission wires/tapes.; For large-area epitaxy, ½ inch x ½ inch HgBa2CaCu 2O6+δ films were synthesized on LaAlO3(100) with uniform and high Tcs and Jc s. A new crucible Hg-annealing technique that requires neither vacuum nor torch-sealing has been invented, promising for large-area wafers and long tapes/wires. So far HgBa2CaCu2O6+δ films with good quality have been reproducibly fabricated using this new technique. |
