Structurally Perfect Metal-Oxide Superconductors of the Y-Ba-Cu-O System with Improved Superconducting Characteristics
Tech Area / Field
- MAT-SYN/Materials Synthesis and Processing/Materials
- PHY-SSP/Solid State Physics/Physics
3 Approved without Funding
Institute of Microelectronics Technology and High Purity Materials, Russia, Moscow reg., Chernogolovka
- VNIIKhT (Chemical Technology), Russia, Moscow
- Ecole Polytechnique de Montreal / Centre for Research in Computational Thermochemistry, Canada, QC, Montreal\nUniversity of Alberta, Canada, AB, Edmonton\nLos Alamos National Laboratory / Superconductivity Technology Center, USA, NM, Los-Alamos
Project summarySuperconducting materials are extremely important for production of strong-current (cables being capable to transmit electric power with low losses, solenoids generating super-strong magnetic fields, compact powerful electric motors, etc.) and weak-current (ultra-high frequency filters, sensitive electromagnetic field sensors, memory and logical computer cells etc.) devices of electrical engineering and electronics. The use of superconducting materials is non-alternative in a number of cases.
Unique physical properties of superconducting materials and possibilities of their effective application generate persistent interest in the development of fabrication technology of superconductors and studying their properties. A new stage in studying superconductors has begun since the discovery of oxide superconductors with critical temperatures higher than that of liquid nitrogen in 1986, whereas hitherto known superconductors needed to be cooled by liquid helium to achieve the superconducting state. The changeover from liquid helium temperature to liquid-nitrogen one simplifies design of superconducting devices, decreases their weight, size and expenses for cooling, etc. And this reduces essentially the maintain costs and make a wide use of superconductors economically justified. Several types of oxide superconductors have been discovered up to now. The leading superconducting material in the initial stage of the development of industrial strong-current devices is based on oxides of the Bi(Pb)-Sr-Ca-Cu-O system. At present studies are underway, which are related to the development of the second generation of industrial superconductors on the basis of the Re-Ba-Cu-O system oxides, where Re – a rare earth element (Y, Nd). The main crystallochemical distinction of these superconductors from the first-generation ones is a lower crystal lattice anisotropy that provides higher critical current density (~ 106 A/cm2 at 77 K) in the second-generation superconductors. It is expected also that superconductors based on ReBaCuO oxides will be much cheaper.
The properties of oxide superconductors have been the subject of many investigations during all years since their discovery. The results of the first structural studies were interpreted as if synthesized superconductors were homogeneous and single-phased. However, gradual accumulation of the experimental data on properties of superconductors and comprehensive reviews of the obtained results set one suggesting the existence of structural and electronic inhomogeneity of oxide superconductors. From the known spatial resolutions of employed structural methods it follows that the inhomogeneity size lies in the range of a few nm (10-9 m), which explains the impossibility to detect such fine inhomogeneity by standard long-range structural methods. One of the most prominent results of recent experimental studies is the direct detection of nm-sized inhomogeneity with use of high resolution tunneling microscopy.
The existence of inhomogeneity obliges reconsidering the widely spread views about phase composition and structure of superconducting oxides. Structural and electronic inhomogeneity determines physical characteristics of superconductors, which are important in practice, e.g., critical current density, and high frequency surface impedance. A number of anomalous properties of superconducting oxides are also explained by the existence of inhomogeneity. The inhomogeneity problem must be regarded as the principal one both from an industrial standpoint and in respect to revealing the superconductivity mechanism acting in oxide superconductors, which is still the subject of discussions.
The participants of this project have carried out detailed investigations of phase formation in perovskite metal-oxide systems, including superconducting ones. They concluded that inhomogeneous state of oxide superconductors results from their chemical inhomogeneity. It arises due to the fact that simultaneous formation of superconducting and non-superconducting phases takes place during synthesis of superconductors by universally used procedures. Superconducting and non-superconducting phases are the members of the same homologous series of oxides having common crystal lattice elements. By virtue of very close crystallographic similarity, nm-sized crystallites of these phases are coherently intergrown, that produces illusion of homogenous material on a macroscopic scale. The synthesized product contains most commonly a few isostructural phases of different conductivity. The solution of chemical inhomogeneity problem resides in finding conditions of single-phase formation of a superconducting phase of the definite composition.
The main objects of the project:
- Detailed investigation of phase equilibria in the Y-Ba-Cu-O system with use of the complex of physical, chemical and structural methods,
- Analysis of phase composition of synthesized superconductors by the methods of high spatial resolution, which can detect fine structure and physical characteristics of superconductors with resolution of a few nm and less,
- Development of physical and chemical bases of fabrication technology of structurally perfect superconductors with high technical parameters.
The following scientific tasks will be approached to meet the project objects:
1. Synthesis of oxides of the Y-Ba-Cu-O system in wide ranges of cationic composition, temperature and oxygen partial pressure. Visual-polythermic, differential-thermic and thermogravimetric analyses of phase transformations in the system.
2. Electron diffraction and X-ray phase analysis of synthesized oxides. Revealing discrete phases existing in the Y-Ba-Cu-O system, refinement of their cationic compositions. Determination of oxygen content and copper valence state in discrete phases by methods of chemical analyses.
3. Investigation of crystal structures of synthesized oxides by high resolution methods including high resolution electron microscopy and scanning tunneling microscopy.
4. Investigation of physical characteristics of synthesized superconductors. Measurement of magnetic susceptibility, electrical conductivity, high frequency surface impedance, critical current density.
5. Comparison of physical properties, integral and fine structure. Revealing correlation between physical characteristics and the degree of structural inhomogeneity.
The basic expected results:
1. Diagrams of phase equilibria in the Ba-Cu-O system, which is the parent system for the Y-Ba-Cu-O superconducting system.
2. Diagrams of phase equilibria in the Y-Ba-Cu-O system at different oxygen partial pressure.
3. Models of crystal structures of ordered phases of the Ba-Cu-O and Y-Ba-Cu-O systems.
4. Characteristics of physical properties of YBaCuO superconductors with different degree of structural perfection.
5. Physical and chemical parameters of synthesis procedure of structurally perfect YBaCuO superconductors.
A practical result of the project will be data on physical and chemical processes of phase formation in the Y-Ba-Cu-O system, which form the basis for development of fabrication technology of structurally perfect superconductors.
The team of researches involved in the project consists of specialists in solid state chemistry and physics, chemical technology, material science, investigation of ideal crystalline and defect structure. They completed complex fundamental studies of superconducting oxides of the K-Ba-Bi-O system and related compounds, worked out the fabrication problem of homogeneous ordered KBaBiO superconductors, carried out a number of investigations of the Y-Ba-Cu-O oxide system. A general approach for studying phase equilibria in metal-oxide systems has been developed on the basis of the results obtained. The concept of chemical nature of structural inhomogeneity of oxide superconductors, its roots and the ways of solving the inhomogeneity problem have been proposed. The experience and experimental facilities of the team may provide the successful realization of the project.
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