Several advances were made in a program focused on the science and technology of high-temperature superconductors [especially yttrium barium copper oxide ("YBCO")] deposited on normal electrical conductors. This program was part of a continuing effort to develop superconductor-coated normal conductors for use in electric-power systems of interest to the Air Force.
The advances are summarized as follows:
•Characterization of high-temperature superconductors in general and YBCO in particular.
A method, based on in situ Fouriertransform infrared (FTIR) reflectivity measurements, of monitoring the temperature and optical properties of high-critical- temperature (high-Tc) superconducting films in real time during the deposition and post-deposition processing of those films was developed. The method was then used to track the evolution of material phases during a high-rate deposition process in which YBCO was formed by electron-beam co-evaporation from Y, Ba, and Cu targets in the presence of oxygen. In somewhat simplified terms, it was found that the deposit starts as a glassy precursor of partially oxidized Y, Ba, and Cu ions and, as the oxygen pressure is increased, evolves to the desired superconducting YBa2Cu3O7 phase (sometimes called "123 YBCO"). The method is expected to be utilized in future efforts to optimize processing to obtain 123 YBCO.
A profiling technique was developed as a means of studying selected transport properties (specifically, the normal-state resistivity and the superconducting critical current density) as functions of position through the depth of a high-Tc film. The basic idea of the technique is to measure the transport properties as successive layers are etched away and to obtain the desired values through calculation of differences between successive measurements after each etching step. By use of this technique, it was determined that each of several YBCO films formed as described above was not homogeneous and, instead, consisted of a top layer having excellent high-Tc superconducting properties and a lower layer that was nearly electrically insulating. The origin of the layered nature of the films later came to be understood in the light of the knowledge gained through the FTIR study described above plus supplementary knowledge gained by use of x-ray diffraction.
•248 YBCO as a superconductive coating material.
Another superconducting phase of YBCO, known in the art as "248 YBCO," was investigated as an alternative to 123 YBCO. Prior to this research, it was considered to be difficult to synthesize 248 YBCO in film form because of high oxygen pressure needed for equilibrium growth. This research led to a nonequilibrium process for growing a 248 YBCO film. In this process, a film consisting of a glassy precursor of the 248 phase is formed by pulsed laser deposition at a high temperature. The precursor film is then subjected to a heat treatment in which it is exposed to oxygen at temperature greater than the deposition temperature.
This research program also included other activities not directly connected with the advances described above. One such activity involved characterization of thin films of MgB2. Another such activity was collaboration in invention of a new memory cell based on switching of a magnetic dot with a single Josephsonjunction readout. The advantage of such a memory cell is that it holds promise to be scalable to sizes much smaller than those of traditional memory cells.
This work was done by M. R. Beasley of Stanford University for the Air Force Research Laboratory
This Brief includes a Technical Support Package (TSP).
Some Advances in High-Temperature Superconductor Coatings
(reference AFRL-0065) is currently available for download from the TSP library.
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