Downloads & Free Reading Options - Results

The aims of this program were to synthesize increasingly high-quality thin film and single crystals of the high Tc and other novel superconductors. We also proposed to study their physical properties, understand the nature of their superconductivity, and search for new superconducting devices and device concepts, with a primary objective of developing a better understanding of the limits of the occurrence of superconductors with high transition temperatures. jg

Herein is presented the results of a comprehensive program of research aimed at understanding the materials science and the mechanistic physics of high-temperature superconducting oxides. This comprehensive research program has identified the materials properties that are consistently associated with high-Tc superconductors and has shown that the mechanism that gives rise to the phenomenon of high-Tc superconductivity is associated with bound holes that are due to charge-transfer excitations at high frequency. The latter are a result of the high internal electric field present in high-Tc materials, owing to the asymmetry of the crystal structure. The interaction of bound holes with free electrons and the interaction of local spin fluctuations with the spin of free electrons generate a charge density wave and a spin density wave that cause Cooper pairing.

We explored ways both to enhance J sub c and to optimize/simplify the manufacturing procedures for coated YBa2Cu3Ox (YBCO) conductors. For J sub c-enhancement, we found that the nano-size interface-irregularity through inserting a (Nd1/3Eu1/3Gd1/3) Ba2Cu3Ox sublayer is an effective way to enhance the critical current density (J sub c). Enhancements up to 50% have been observed, especially under high magnetic fields. We also found that the Ca-substituted overlayer on YBCO films with poor crystalline structure can significantly improve the grain misalignment and J sub c. Partial substitution of Y by rare earth elements as previously suggested, however, does not lead to better performance in our synthesis conditions. For manufacturing, copper-metal with electroplated Ni-layer and CeO2-buffer layer has been tested as a cheaper and more convenient alternative for the traditional textured Ni. High crystalline texture has been achieved. The in-plane misalignment angle, delta phi 5-10 deg, is comparable to those on traditional rolling-assisted MOCVD has also been tested in synthesizing both the buffer layer and YBCO films. Reasonable J sub c (77 K) up to 10 exp 6 A/sq cm has been achieved with a very fast process rate.

Progress under AFOSR Contract F49620-92-C-0004 for the period 15 October 1991 to 31 October 1992 is reported. Important results include studies of the grain boundaries in a-axis oriented high-Tc superconducting 123 YBCO thin films, the study of the vortex properties of high-Tc single crystals of the superconductor 2212 BSCCO and the artificially structured Mo-Ge/Ge quasi-two- dimensional superconductors. The latter provide an outstanding model system for the study of vortices in highly anisotropic superconductors, such at the high-Tc cuprates, without all the attendant difficult materials problems. Progress in the synthesis and study of the so-called infinite layer cuprate SrCuO4 and the normally conducting oxide SrRuO3 of interest as a barrier materials in high-Tc SNS devices are also reported. Finally we report the development of advanced approaches to rate control of the cation fluxes in the reactive coevaporation of the high-Tc superconductors.

Progress under AFOSR Contract F49620-92-C-0004 for the period 15 October 1991 to 31 October 1992 is reported. Important results include studies of the grain boundaries in a-axis oriented high-Tc superconducting 123 YBCO thin films, the study of the vortex properties of high-Tc single crystals of the superconductor 2212 BSCCO and the artificially structured Mo-Ge/Ge quasi-two- dimensional superconductors. The latter provide an outstanding model system for the study of vortices in highly anisotropic superconductors, such at the high-Tc cuprates, without all the attendant difficult materials problems. Progress in the synthesis and study of the so-called infinite layer cuprate SrCuO4 and the normally conducting oxide SrRuO3 of interest as a barrier materials in high-Tc SNS devices are also reported. Finally we report the development of advanced approaches to rate control of the cation fluxes in the reactive coevaporation of the high-Tc superconductors.

Progress under AFOSR Contract F49620-92-C-0004 for the period 15 October 1991 to 31 October 1992 is reported. Important results include studies of the grain boundaries in a-axis oriented high-Tc superconducting 123 YBCO thin films, the study of the vortex properties of high-Tc single crystals of the superconductor 2212 BSCCO and the artificially structured Mo-Ge/Ge quasi-two- dimensional superconductors. The latter provide an outstanding model system for the study of vortices in highly anisotropic superconductors, such at the high-Tc cuprates, without all the attendant difficult materials problems. Progress in the synthesis and study of the so-called infinite layer cuprate SrCuO4 and the normally conducting oxide SrRuO3 of interest as a barrier materials in high-Tc SNS devices are also reported. Finally we report the development of advanced approaches to rate control of the cation fluxes in the reactive coevaporation of the high-Tc superconductors.

Superplastic deformation is being explored as a means to turn the new ceramic superconducting YBa2Cu3O(6+x) into useful Naval hardware. The deformation experiments hat were conducted on fine YBa2Cu3O(6+x) powders suggest that the maximum deformation that can be achieved without the degradation of superconductivity is about 62 % at 850 deg C. The addition of silver improves the maximum deformation and the addition of alumina has a devastating effect on the deformation mode.

Superplastic deformation is being explored as a means to turn the new ceramic superconducting YBa2Cu3O(6+x) into useful Naval hardware. The deformation experiments hat were conducted on fine YBa2Cu3O(6+x) powders suggest that the maximum deformation that can be achieved without the degradation of superconductivity is about 62 % at 850 deg C. The addition of silver improves the maximum deformation and the addition of alumina has a devastating effect on the deformation mode.

A three year research effort aimed at exploring the potential of the new high-Tc superconductors for SQUID applications is described. Over the course of the contract advances in multilayer materials and junction devices were made, particularly in the reduction of hysteresis and in the demonstration in high-Tc technology of a three squid gradiometer. The latter approach is particularly well suited for further development and application for Naval gradiometer purposes.

This program was broadly focused on the materials science of high temperature superconducting coated conductors, which are of potential interest for application in electric power systems of interest to the Air Force. The three main elements of the program were as follows: (1) understanding the basic materials science underlying the deposition of 123 YBCO at the high rates needed for economic manufacture of coated conductors; (2) exploration of variants of and alternatives to 123 YBCO for possible application as coated conductors; and (3) (in the early parts of the program) development and application of in situ deposition process monitoring tools relevant to coated conductor deposition. Each of these topics is discussed in turn. In addition, other topics are addressed as special opportunities that presented themselves as the program evolved.

High-temperature superconducting conductors for power application have been identified as a critical enabling technology for future Air Force systems. Work on the materials science and factors limiting current carry capacity of 123 YBCO coated conductor materials are reported. A new model of the high-rate electron-beam coevaporation of 123 YBCO has been developed based on our experiments. It now appears that the process involves an amorphous precursor that is subsequently oxidized. Also the crystallization of the film appears to involve a liquid flux of BaCuOx. The first successful route of synthesis of 248 YBCO thin films has also been developed. We have also carried out scanning tunneling spectroscopy on the surfaces of 2212 BSCCO that reveal the presence of nanoscale inhomogeneities of the superconducting energy gap that could be a source of vortex pinning and hence the critical current in this material. A scanning tunneling potentiometer system has been constructed. This system has yielded the first transport measurements of a complete oxide material on a nanometer scale.

The success of high temperature superconductor technology in the efficient use, generation, transmission, and storage of electrical energy is determined by the performance of the coated conductor and by the ability to fabricate it in long lengths at a reasonable cost. Our program on materials and processing of coated high temperature superconducting conductors has had two components in the past year: 1) improving the Jc by introducing effective pinning centers to increase the interlayer coupling and reduce the compound anisotropy by cation substitution and introduction of nano-particles; and 2) electroplated Ni layer on textured Cu substrate for Cu- based HTS coated conductors.

The events that led to the discovery of high temperature superconducting materials is reviewed in a chronological fashion beginning with the discovery of superconductivity in mercury by H.K. Onnes. The signature properties of superconductivity are described, the basic theoretical framework necessary to understand superconductivity is briefly sketched, and the properties that lead to technological applications are discussed. Finally, the properties of LaBCO and YBCO are described, and the possibility of technological applications is discussed. Keywords: Superconductivity, Meissner effect, Diamagnetism, London theory, Cooper pairs, Ginsberg-Landau theory, Josephson effect, High temperature superconductors.