Understanding High Critical Currents in YBa2Cu3O7 Thin Films and Coated Conductors

The main challenges for the success of high temperature superconducting wires, the YBa₂Cu₃O₇ (YBCO) coated conductors (CC), are to avoid the the weak-link problem through the production of biaxially textured films, and to increase the critical current density (J _c) through the introduction of large densities of appropriate defects. To that end, it is essential to understand the pinning mechanisms and their correlation with the microstructure of the CC. We first present a brief overview of the main methods currently used to produce YBCO CC, and we describe the architecture of the YBCO on IBAD fabricated at Los Alamos, summarizing the recent improvements of their structural and superconducting properties. Then, we analyze some aspects of the J _c dependence on temperature and magnetic field (orientation and intensity) for the best CC available, and we compare and contrast the results with those of YBCO thin films on single crystal substrates, in order to determine if the defects controlling the pinning mechanisms are the same in both cases. Our results indicate that over large field and angular ranges J _c on CC is higher than J _c in thin films on SCS.     

Progress in Nanoengineered Microstructures for Tunable High‐Current,High‐Temperature Superconducting Wires

High critical current densities (J_c) in thick films of the Y₁Ba₂Cu₃O_7–δ (YBCO, T_c ≈ 92 K) superconductor directly depend upon the types of nanoscale defects and their densities within the films. A major challenge for developing a viable wire technology is to introduce nanoscale defect structures into the YBCO grains of the thick film suitable for flux pinning and the tailoring of the superconducting properties to specific, application‐dependent, temperature and magnetic field conditions. Concurrently, the YBCO film needs to be integrated into a macroscopically defect‐free conductor in which the grain‐to‐grain connectivity maintains levels of inter‐grain J_c that are comparable to the intra‐grain J_c. That is, high critical current (I_c) YBCO coated conductors must contain engineered inhomogeneities on the nanoscale, while being homogeneous on the macroscale. An analysis is presented of the advances in high‐performance YBCO coated‐conductors using chemical solution deposition (CSD) based on metal trifluoroacetates and the subsequent processing to nano‐engineer the microstructure for tuneable superconducting wires. Multi‐scale structural, chemical, and electrical investigations of the CSD film processes, thick film development, key microstructural features, and wire properties are presented. Prospects for further development of much higher I_c wires for large‐scale, commercial application are discussed within the context of these recent advances.     

On the Mechanisms Governing Aluminum-Mediated Solid-Phase Epitaxy of Silicon

Mechanisms governing the aluminum-mediated solid-phase epitaxy of Si on patterned crystalline Si substrates have been identified by studying the deposited material as a function of growth conditions when varying parameters such as temperature, growth time, and layer-stack properties. Early growth stages can be discerned as first formation of “free” Si at the Al/α-Si interface, then diffusion of Si along the Al grain boundaries, nucleation at the Si substrate surface, nuclei rearrangement, and finally crystal growth. The acquired understanding is applied to control the selectivity and completeness of single-crystal growth in various sizes of contact windows to the Si substrate.     

Aligned carbon nanotubes sandwiched in epitaxial NbC film for enhanced superconductivity

Highly aligned carbon nanotube (CNT) ribbons were sandwiched in epitaxial superconducting NbC films by a chemical solution deposition method. The incorporation of aligned long CNTs into NbC film enhances the normal-state conductivity and improves the superconducting properties of the assembly.     

Irradiation-enhanced pinning in YBa2Cu3O7−x crystals

Deliberately introducing defects by particle irradiation is an effective way to increase the flux pinning in high-critical-temperature superconductors, which is a requirement for technological applications of these materials. Proton irradiation generates a random distribution of point defects, which largely enhances the critical current in YBa₂Cu₃O_7?x single crystals; but it is not effective in shifting the irreversibility line to higher magnetic fields. The aligned columnar defects created by high-energy heavy-ion irradiation generate even stronger vortex pinning, resulting in higher critical currents at high temperatures and fields and a large displacement of the irreversibility line to higher fields.     

Sub-500/spl deg/C solid-phase epitaxy of ultra-abrupt p/sup +/-silicon elevated contacts and diodes

A well-controlled low-temperature process, demonstrated from 350/spl deg/C to 500/spl deg/C, has been developed for epitaxially growing elevated contacts and near-ideal diode junctions of Al-doped Si in contact windows to the Si substrate. A physical-vapor-deposited (PVD) amorphous silicon layer is converted to monocrystalline silicon selectively in the contact windows by using a PVD aluminum layer as a transport medium. This is a solid-phase-epitaxy (SPE) process by which the grown Si is Al-doped to at least 10/sup 18/ cm/sup -3/. Contact resistivity below 10/sup -7/ /spl Omega//spl middot/cm/sup 2/ is achieved to both p/sup -/ and p/sup +/ bulk-silicon regions. The elevated contacts have also been employed to fabricate p/sup +/-n diodes and p/sup +/-n-p bipolar transistors, the electrical characterization of which indicates a practically defect-free epitaxy at the interface.