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Books > Science & Mathematics > Physics > Electricity, magnetism & electromagnetism
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Second-Generation HTS Conductors (Paperback, Softcover reprint of hardcover 1st ed. 2005)
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Second-Generation HTS Conductors (Paperback, Softcover reprint of hardcover 1st ed. 2005)
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Thediscoveryofhightemperaturesuperconductors(HTS)in 1986bytwoIBM
scientists led to an unprecedented explosion of research and
development efforts world-wide because of the signi?cant potential
for practical applications offered by these materials. However, the
early euphoria created by the exciting prospects was dampenedby the
dauntingtask of fabricating these materials into useful forms with
acceptablesuperconductingproperties. Progresstowardsthisgoalhasbeen
hindered by many intrinsic materials problems, such as weak-links,
?ux-creep, and poor - chanicalproperties.
TheearlieststudiesofcriticalcurrentdensityJ
inHTSmaterialsrevealedthatfor c
apolycrystallinematerialcontainingadistributionofgrainboundariesismuchlower
than that for a single crystal. High angle grain boundariesact as
Josephson coupled weak-links leading to a signi?cant ?eld-dependent
suppression of the supercurrent acrosstheboundary.
Forcleanstoichiometricboundaries, thegrainboundarycritical
currentdensity dependsprimarilyonthe grainboundarymisorientation.
The dep- denceof J (gb)onmisorientationanglehasbeendeterminedYBa O
Cu (YBCO) c 2 3 7 in boundary types which can be formed in
epitaxial ?lms on bicrystal substrates. Theseinclude 001]tilt,
100]tilt, and 100]twistboundaries. Ineachcasehighangle
boundarieswerefoundtobeweak-linked.
Theseexperimentshavealsobeenextended to arti?cially fabricated 001]
tilt bicrystals in Tl Ba CaCu O, Tl Ba Ca Cu O, 2 2 2 8 2 2 2 3 x
TlBa Ca Cu O (Tl-1223) and Nd Ce CuO . In each case it was found
that, 2 2 3 x 1. 85 0. 15 4 as in YBCO, J depends strongly on grain
boundary misorientation angle. Data on c
currenttransmissionacrossarti?ciallyfabricatedgrainboundariesinBi-2212also-
dicate that most large angle 001] tilt and 001] twist boundaries
are weak links. It is likely that the variation in J with grain
boundary misorientation is similar in all c high-T superconductors.
Hence, thelow J observedinrandomlyorientedpolycr- c c
tallineHTScanbeunderstoodonthebasisthatthepopulationoflowangleboundaries
issmallandthatfrequenthighangleboundariesimpedelong-rangecurrent?ow.
- ingconventionalprocessingtechniques,
threeHTSmaterialsweresuccessfullyfab-
catedinpolycrystallineformwithmodestJ 's.
ThesearetheBi-2223powder-in-tube c conductors,
theTl-1223spray-pyrolyzed?lmsandtheBi-2212melt-processedthick ?lms.
Thesethreetypesofconductorscomprisedthe First-GenerationHTScond-
torsorwires.
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