Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconducting wire – tape – cable – or fiber – per se
Patent
1999-03-18
2000-09-19
King, Roy V.
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconducting wire, tape, cable, or fiber, per se
505231, 505430, 505500, 505501, 505704, 505742, 29599, 1741251, H01B 1200, H01L 3924
Patent
active
061225347
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to composite oxide superconductor articles having reduced AC losses and to a process for preparing such composite superconductor articles having reduced AC losses. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
BACKGROUND OF THE INVENTION
Recent advances in the development of high temperature superconducting (HTS) oxide composites have demonstrated the feasibility of oxide superconductor applications in power transmission cables, fault current limiters, utility inductors, motors and generators. However, a major impediment to such commercial applications of HTS conductors is the energy dissipation within the conductor associated with time varying electric and magnetic fields, referred to as AC losses. This limitation has commercial significance since many of the superconductor applications that have the greatest potential for energy conservation involve operating the superconductor in the presence of an AC magnetic field or require that the superconductor carry an AC current.
HTS wires typically are composite wires composed of superconductor filaments sheathed in a matrix of NORMAL or non-superconducting metal. In the presence of time varying, magnetic fields or currents, that is, alternating currents, a variety of mechanisms give rise to energy dissipation in composite superconductors. In particular, eddy current losses occur in the metal matrix that is enhanced by coupling between superconducting filaments.
In multifilamentary superconductors, eddy currents can be quite large. The physics governing AC losses in low temperature superconductor composite materials have been described and analyzed (see Wilson, "Superconducting Magnets", Ch. 8 (1983, 1990).
To minimize eddy currents, the matrix resistivity is preferably increased and the twist pitch of the superconducting filaments in a multifilamentary superconductor are preferably tightened, i.e., reduced. Eddy current losses may be reduced by increasing the resistivity of the metal matrix in close proximity to the superconductor filaments. High resistivity within the matrix diminishes eddy current (i.sub.e) flow and consequently diminishes power loss (that is, reduced AC losses).
The prior art has not demonstrated the ability to increase matrix resistivity to the required level for reduction of AC losses while retaining desirable superconducting properties. Bulk resistivity within the matrix should be on the order of at least 1 micro-ohm centimeter (.mu..OMEGA.-cm), and preferably 10 .mu..OMEGA.-cm, at T.ltoreq.T.sub.c, where T.sub.c is the critical current temperature of the HTS oxide, in order to have a significant effect on lowering eddy current losses. Pure silver has a resistivity on the order of about 0.1 .mu..OMEGA.-cm. An approach to increasing matrix resistivity includes oxidation of a silver alloy (typically AgMg) to introduce oxide particles (MgO) into the silver matrix, known as oxygen dispersion strengthened (ODS) silver. Oxidation of Mg to MgO occurs in the early heat treatments of an HTS precursor composite during processing to the oxide superconductor. This produces resistivities on the order of 0.7 .mu..OMEGA.-cm. Another approach to increasing matrix resistivity includes introducing a metallic barrier layer into the interior of an HTS precursor composite which is converted into a high resistivity phase (typically by oxidation) during processing of the composite into an oxide superconductor. Both of these approaches attempt to increase the bulk resistivity of the matrix while processing the precursor materials into the desired oxide superconductor.
There are technical difficulties in attempting to manufacture an oxide superconductor in the presence of or concurrent with the formation of a high resistivity matrix. For example, processing steps associated with formation of the high resistivity matrix may not be compatible with processing of the oxide superconductor. In particular, at the high temperatures used to form the oxide sup
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Shamoto et al, Jpn. J. Appl. Phys. vol. 26, No. 4 pp. L493-497, Apr. 1987.
Cotton James D.
Holesinger Terry G.
Riley, Jr. Gilbert N.
Cottrell Bruce H.
King Roy V.
The Regents of the Univeristy of California
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