Showing papers by "Thomas Henry Tiefel published in 1990"

Synthesis and properties of the YBa2Cu4O8 superconductor

Abstract: A new simple synthesis route was used to produce the ‘124’ superconductor YBa 2 Cu 4 O 8 . The method utilizes YBa 2 Cu 3 O 7 powder (‘123’) as a precursor, which is then converted to the 124 superconductor by reaction with a stoichiometric amount of CuO through normal grinding and sintering, without the need for high oxygen pressure processing. Sintered bars of the 124 superconductor exhibited a T c of ∼ 75 K in resistivity and AC magnetic susceptibility measurements. Transport critical current density was measured to be ∼ 150 A/cm 2 at 60 K, and showed a strong field dependence. This behavior, in combination with a relatively high J c (magn.) of 4 × 10 4 A/cm 2 at 60 K and H = 0.9 T, is indicative of Josephson weak links at grain boundaries, which is similarly observed in the 123 phase. It is also noted that the intragrain J c in the twin-free 124 superconductor is about the same as that in the twinned 123 superconductor at ∼ 15 degrees below their respective T c .

Recovery of high-Tc superconductivity in Y-Ba-Cu-O doped with reacting metals

Abstract: The chemical reactions of YBa 2 Cu 3 O 7−δ with intimately mixed platinum-group metals (Pt, Pd, Rh, Ru) or Re during sintering severely suppress and broaden the superconducting transition to below ∼ 40 K. However, it has been discovered that an essentially complete recovery of 90 K T c can be obtained in some samples (doped with Pt, Rh, Re) by proper melt processing of the superconductor. SEM and X-ray analysis indicates that a phase separation occurs during melting of these doped materials, with one of the phases actually draining the doped elements away from the YBa 2 Cu 3 O 7−δ phase, making the latter essentially chemically pure. The results are of technological interest in view of the lack of high melting-point metals which are non-poisonous in contact with Y-Ba-Cu-O. Some of the metals could be used as a substrate or clad material during high temperature melt processing.

Flux pinning effect in Y-123 superconductor containing

Abstract: Silicon-doped Y-123 superconductors were prepared by melting and rapid solidfication process. The as-solidified samples essentially consisted of nonsuperconductive, metastable phases. Subsequent annealing at 945°C crystallized the sample into the 1-2-3 superconducting phase with T c =80–90 K. Manetization measurements indicate that Si-doping (e.g. with 0.7 at.% Si) increases intragrain J c at 77 K, H =0.9 T by a factor of six. This significant improvement in J c (enhanced flux pinning) is tentati attributed to unknown defect structures induced by finely dispersed Si-rich particles.