Bulk YBa 2 Cu 3 O x superconductors through pressurized partial melt growth processing
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A. N. Thorpe and D. D. Davis Howard University, Washington, DC 20059
S. Alterescu NASA Goddard Space Flight Center, Greenbelt, Maryland 20770 (Received 4 November 1991; accepted 26 November 1991)
We have developed a novel pressurized partial melt growth process for producing large pieces of bulk Y-Ba-Cu-0 superconductors. During long-time partial melt growth stage, an additional driving force for solidification is obtained by using pressurized oxygen gas. The microstructure and superconducting properties of the resulting samples were investigated. It was found that this new technique can eliminate porosity and inhomogeneity, promote large-scale grain-texturing, and improve interdomain coupling as well.
I. INTRODUCTION Large pieces of bulk high Tc superconductors have potential applications in magnetic bearings, microwave cavities, persistent permanent magnets, etc. Generally, all these applications require the capability of carrying high critical current density Jc throughout the sample. Based on the modified Ginzburg-Landau theory,1 the maximum possible supercurrent that can be carried by the high Tc superconductors can be expressed as,2 cm
(1)
where Hc(0) is the thermodynamic critical field and A(0) is the penetration depth at T = 0 K. In YBa 2 Cu 3 0 7 , for instance, Hc(0) « 1 tesla (T),3 Tc K, 92 K, and A(0) « 2 x 1(T5 cm.3-4 This gives J cmax = 2.3 x 108 A/cm 2 at 0 K and J cmax = 2.7 x 107 A/cm 2 at 77 K, respectively. However, the maximum achievable critical current densities are typically lower by a factor of 10 than the limiting calculated values, depending on the existence of effective flux pinning forces inside type-II superconductors. High critical current densities above 106 A/cm 2 , at T = 77 K and H = 1 T, have already been obtained in YBa2Cu3Ox thin films.5'6 Inside pure YBa2Cu30JC single crystals, maximum values of Jc have been found to be only about 5 x 103 A/cm 2 at 77 K and H = 1 T,7'8 apparently because of the lack of sufficient amount of effective defects to serve as pinning centers compared with the case of the thin films. It has also been found that flux pinning in YBa 2 Cu 3 0 x single crystals can be significantly improved by controlled introduction of defects. For instance, a hundredfold enhance808
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J. Mater. Res., Vol. 7, No. 4, Apr 1992
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ment of the critical current density to a value of 0.5 x 106 A/cm 2 has been achieved by irradiation with fast neutrons.9 However, when it comes to bulk polycrystalline high Tc superconducting samples, the situation is quite different. Since these bulk samples have a polycrystalline ceramic structure, weak links associated with the grain boundaries10"12 and, especially, the high angle boundaries, cause a more serious problem than the one of weak pinning inside the grains, dramatically reducing the actual achievable critical current densities. As a result, in the normally sintered bulk YBa 2 Cu 3 0 x samples, JC is usually less than 102 A/cm 2 , and it deteriorates seriously even in weak magne
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