Enhancement of JC in oxide superconductors

Details Enhancement of JC in oxide superconductors Enhancement of JC in oxide superconductors

2000-03-14

2003-05-27

Kopec, Mark (Department: 1751)

Superconductor technology: apparatus, material, process

Processes of producing or treating high temperature...

Using magnetic field

C505S450000, C505S727000

Reexamination Certificate

active

06569811

ABSTRACT:

FIELD OF THE INVENTION
The present invention comprises novel nanophase orientation methods for the preparation of composite high-Tc superconductors which contain a dispersion of nanophases and thereby exhibit enhanced flux pinning and enhanced critical current in the superconducting state. The present invention also comprises novel composite high-Tc superconductors prepared by such methods.
BACKGROUND OF THE INVENTION
Superconductivity, ever since its discovery in 1911, has demonstrated enormous potential in industrial applications including nuclear magnetic resonance (NMR), magnetic levitation and propulsion, alternating and direct current power transmission, light weight generators, magnetic fission, high-field coils, and energy storage (Z. J. J. Stekly and E. Gregory, Chapter on Applications of A-15 S. C., “Intermetallic Compounds, Principles and Practices,” eds. J. H. Westbrook and R. L. Fleisher, J. Wiley & Sons, New York (1994)). All these applications have so far used the low-Tc superconductors (LTS) such as NbTi and NbSn, which are two commercially available compounds (D. Shi, “Properties and Defects of Type II Superconductors,” MRS Bulletin Vol. XVI, 37 (1991); Z. J. J. Stekly and E. Gregory, “High Temperature Superconducting Materials Science and Engineering,” (ed. D. Shi Pergamon, Oxford) p. 444, 1995). Although LTS materials offer unique properties, certain factors limit their usefulness in practice such as low Jc at high fields, brittleness, and an extremely low operating temperature at 4.2° K. These factors have contributed to the fact that, despite the intensive efforts, superconductors have not become common in industry.
Theoretical and experimental research in the field of superconducting materials by thousands of researchers has led to the discovery of a variety of oxide compounds which become superconducting at relatively high critical temperatures (Tc), i.e., above about 20°K. The widely known high temperature superconductors are oxides, and presently contain (1) copper and/or bismuth, (2) barium or other alkaline earths such as strontium or calcium, and (3) trivalent elements such as yttrium. Rare earth elements having atomic numbers ranging from 57 to 71 (lanthanum to lutetium), are substituted for yttrium in some materials, as are thallium or bismuth.
In 1986, a series of high temperature superconductors (“HTS”) including YBa
2
Cu
3
Ox (123) and Bi—Sr—Ca—Cu—O (BSCCO) were discovered with the Tc values exceeding 77 K (J. G. Bednorz and K. A. Mueller, Phys. Rev. B, 64 189 (1986); K. Wu et al., Phys. Rev. Lett. 58 908 (1987); H. Maeda, Y. Tanaka, M. Fukutorni and T. Asano, Jpn. J. Appl. Phys. 27 209 (1988)). These breakthroughs have indicated a promising future for superconductivity. With these HTS materials, it is possible to have applications at 77 K. Using liquid nitrogen as a coolant, the cryogenic systems can be greatly simplified making superconductivity application more realistic and economic. It has been found that not only the Tc's of HTS are much higher, the upper critical field, Bc2, has been measured to reach a value on the order of 100 T (U. Welp, W. Kwok, G. W. Crabtree, K. Vandervoort, and J. Z. Liu, Phys. Rev. Lett., 62 1908 (1989)), making them ideal candidates for high-field applications. However, it has also been found that HTS materials carry extremely low critical current densities in the unoriented polycrystalline form as a result of their crystal anisotropy and grain boundary weak links (R. J. Cava, et al., Phys. Rev. Lett. 58, 1676 (1987)). The research effort has been focused on enhancing Jc by texturing grains and identifying coupling mechanisms at interfaces (S. E. Babcock, X. Y. Cai, D. L. Kaiser, and D. C. Larbalestier, Nature 347, 167 (1990)). These efforts have resulted in significant improvement in critical current density, particularly in YBa
2
Cu
3
Ox (123) (J. W. Ekin, Adv. Cer. Mater. 2, 586 (1987); D. Shi et al., Appl. Phys. Lett., 57 2606, (1990); K. Salama and D. F. Lee, Supercon. Sci. Technol. 7, 177 (1994)) and Bi—Sr—Ca—Cu—O (BSCCO) systems (Q. Li, H. A. Hjuler and T. Freltoft, Physica C, 217 360 (1993); U. Balachandran, A. Iyer, P. Haldar, J. Hoehn, L. Motowidlo, G. Galinsid, Appl. Supercon. 2 251 (1994); H. Santhage, G. N. Riley Jr., and W. L. Carter, J. Metals, 43 21 (1991); R D. Ray II and E. E. Hellstrom, Physica C, 172 227 (1993)). For the 123 compound, Jc has reached the order of 104 A/cm2 at 5 T (P. G. McGinn et al. Appl. Phys. Lett. 57 1455 (990); R. L. Meng, C. Kinalidis and Y. Y. Sun, Nature 345 326 (1990); D. Shi, S. Sengupta, J. Luo, C. Varanasi, and P. J. McGinn, Physica C, 213 179 (1993)) indicating strong pinning strength in the system. But the current density in the BSCCO system is still limited, particularly at high fields above 30 K as a result of 2D vortex nature (K. E. Gray, Appl. Supercon., 2 295 (1994)).
Introduction of defects in intermetallic type II superconductors was proposed to increase their critical current density. See, for example, Campbell et al., “Pinning of Flux Vortices in Type II Superconductors,” Phil. Mag., 18, 313 (1968).
However, in the case of high-temperature superconductors, the introduction of defects to increase critical current density to a useful level has met with only limited success. For example, in Gammel et al., Phys. Rev. Lett., 59, 2592 (1987), an increased density of twin boundaries provides only moderate improvement in flux pinning. Some increase in low temperature Jc in YBa
2
Cu
3
O
7
in strong magnetic fields was achieved by the introduction of point defects by neutron irradiation in, for example, Willis et al., “Radiation Damage in YBa
2
Cu
3
O
7-x
By Fast Neutrons”, High Temperature Superintroductors, MRS Symposium Proceedings Vol. 99, 391-94 (1988). However, even in Willis et al., the increase in Jc was limited and at 70 K and B=4 T, increased to only about 10
4
A/cm
2
after about 1018n cm
−2
above which value superconductivity was adversely effected by the neutron dose. This may limit the wide application of neutron irradiation to provide improvement in flux pinning. Critical currents in polycrystalline high-temperature superconductors are still further reduced by weak links at the grain boundaries, which are made worse by high porosity, misalignment of the crystalline axis of adjacent grains, and by formation and accumulation of non-superconductor phases (compounds) at boundaries between superconducting grains.
The need for additional high temperature superconductors and methods of manufacturing superconductors is great, not only to achieve superconductors with higher Tc 's, but also to achieve superconductors with improved Jc's in magnetic fields, improved mechanical properties, stability, and ease of processing.
Previous findings show that the high-current applications of BSCCO are limited to low temperatures (<30 K) since its Jc is rapidly reduced at 77K, particularly at appreciable applied fields. According to the previous experimental measurements, the Jc of Bi
2
Sr
2
Ca
2
Cu
3
O
x
(Bi2223) tape is on the order of 104 A/cm
2
at 77 K and self filed, but it is rapidly dropped to less than 1000 A/cm
2
below 0.5 T for the field perpendicular to the surface of the tape. At 20 K for the same configuration, however, the decrease of Jc is within 10%, showing significant pinning strength. To increase the Jc in the higher temperature range, extensive effort has been devoted to the enhancement of flux pinning by manipulation of micro structure (D. Shi, M. Blank, M. Patel, D. Hinks, K. Vandervoort, and H. Claus Physica C 156, 822 (1988); K. C. Goretta, B. P. Brandel, M. T. Lanagan, J. G. Hu, D. J. Miller, S. Sengupta, J. C. Parker, M. N. Ali, and Nan Chen, IEEE Transactions on Apply. Superconductivity, 5, 1309 (1995)) and radiation damage (H. Safar et el., Appl. Phys. Lett. 67 130 (1995)). The former has been successful only at low temperatures and the latter is considered not practical for industrial applications. It is, therefore, important to develop new methods by which flux pinning can be sufficiently increased in th

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