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1、AAA15phases—AnoxiczonesSuperconductivity.Superconductivityisapheno-A15phasesmenoninwhichametal,whencooledbelowitsAseriesofintermetalliccompoundsthathaveasuperconductingtransitiontemperatureTc,losesallparticularcrystalstructureandthechemicalfor-resistancetotheflowofanelectriccurr
2、ent.TheA15◦mulaA3B,whereArepresentsatransitionelementcompoundNb3GehasaTcof23.2K(−417.9F),andBcanbeeitheratransitionelementoranon-thehighestvalueoftheA15compounds,and,transitionelement.ManyA15compoundsexhibitpriorto1986,thehighestvalueeverobservedforthephenomenonofsuperconductiv
3、ityatrelativelyanyknownmaterial.OtherA15compoundswith◦hightemperaturesintheneighborhoodof20KhighTcincludeNb3Al(18.8Kor−425.8F),Nb3Ga◦◦(−424F)andinhighmagneticfieldsontheorder(20.3Kor−423.1F),Nb3Si(approximately19Kor◦◦ofseveraltensofteslas(severalhundredkilogauss).−426F),Nb3Sn(18
4、.0Kor−427.3F),V3Ga(15.9K◦◦High-temperature–high-fieldsuperconductivityhasor−431F),andV3Si(17.1Kor−428.9F).anumberofimportanttechnologicalapplicationsIn1986,evidenceofsuperconductivitywithaTc◦andisachallengingfundamentalresearchareainofapproximately30K(−405F)wasreportedforcondens
5、ed-matterphysics.amaterialcontainingtheelementslanthanum,bar-Crystalstructure.TheA15compoundscrystallizeium,copper,andoxygen.Sincethen,severalfami-inastructureinwhichtheunitcell,therepeatingliesofcompoundscontainingcopper,oxygen,andunitofthecrystalstructure,hastheoverallshapeof
6、acube.TheBatomsarelocatedatthecornersandinthecenterofthecube,whiletheAatomsarearrangedinpairsonthecubefaces(seeillus.).AspecialcharacteristicoftheA15crystalstructureisthattheAatomsformmutuallyorthogonallin-earchainsthatrunthroughoutthecrystallattice,asshownintheillustration.The
7、intrachaindistancebetweenAatomsistheshortestdistancebetweenatomsintheA15crystalstructure,andisabout22%lessthanthesmallestinterchaindistancebetweenAatoms.Theextraordinarysuperconductingproper-tiesoftheA15compoundsarebelievedtobeprimar-ilyassociatedwiththeselinearchainsoftransiti
8、on-elementAatoms.Atlowtemperature,someoftheA15phasesun
