Critical currents after thermal neutron irradiation of uranium doped superconductors
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I. INTRODUCTION 1
In a previous paper we reported the large increase in critical current Jc, within the grains of randomly oriented sintered YBa2Cu3Ox, obtained with thermal neutron irradiation of samples doped with natural uranium. Jc was increased by twenty times at 63 K. This increase was shown to be due to the increased flux pinning produced by uranium fission fragment damage, with a much smaller effect due to the primary thermal neutrons. There have been many reports on the effect of various radiation exposures in attempts to introduce flux pinning centers in the oxide superconductors. The references through Dec. 1988 have been summarized previously.1 More recent work follows. We define JC(M) as Jc calculated from magnetization measurements and JC{T) as Jc measured by transport technique. A flux of heavy ions of Si4+ or Cu5+ into Y-Ba-Cu-O deteriorated both Tc and JC(T).2 Irradiation with oxygen or nitrogen produced very small improvements in JC(T), for example, from 1.4 x 106 A/cm2 before irradiation to 2 x 106 A/cm2 after irradiation.3"5 Helium ion irradiation of sintered Y-Ba-Cu-O also produced no improvement in superconducting properties.6"9 Electron irradiation resulted in a small increase in JC{M) from 7.4 x 102 A/cm2 before irradiation to 11.3 x 102 A/cm2 after irradiation.10 Similar results were obtained on melt processed samples.11 Irradiation with fast neutrons produced significant increases in critical currents within grains.12"16 Van Dover et al.12 reported increases in JC(M) of up to 95 times on irradiation of single crystals with fast neutrons although previous work17 on irradiation of single crystals with fast neutrons had produced only a twofold improvement. This large change on irradiation obtained by van Dover reflects the lack of pin28
J. Mater. Res., Vol. 6, No. 1, Jan 1991
ning centers in the original single crystal since the Jc after irradiation was in the range typically obtained for epitaxial thin films, similar to the values produced by fission induced damage. Okada and Kawakubo15 reported measurements of JC(T) after fast neutron irradiation of polycrystalline Y-Ba-Cu-O samples. JC(T) was increased up to 4 times the values before irradiation. Waliszewski16 reported increases in/ c (M) on polycrystalline samples of 20 times after fast neutron irradiation. In a previous paper1 we reported increases in JC(M) of up to 20 times on irradiation of uranium doped polycrystalline samples, with thermal neutrons. This irradiation causes fission fragment damage to the crystals. In this paper we report on the changes in Jc produced by slow neutron irradiation of uranium-doped YBa2Cu3Ox for aligned sintered samples, for the 80 K phase of Bi2Sr2Ca!Cu2Oy, for the 110 K phase of Bij.7Pbo.3Sr2Ca2Cu3Oz, and for epitaxial thin films of YBa2Cu3O^ on (100)SrTiO3. For polycrystalline sintered samples the transport critical-current density is strongly dependent on the usually very poor intergrain coupling. It was hoped that because the ratio of the Jc due to grain boundaries to the Jc within the grain in th
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