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物理学类考研英语阅读分析详解.docx

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物理学类考研英语阅读分析详解物理学类考研英语阅读分析详解Oneofthesimplestandbestknownkindsofcrystalistheionicsalt,ofwhichatypicalexampleissodiumchloride,orordinarytablesalt.Thefundamentalcomponentsofanionicsaltareions:atomsormoleculesthathavebecomeelectricallychargedbygainingorlosingonemoreelectrons.Informingsodiumchloride,forexample,sodiumatomsgiveupanelectronandchlorineatomsgainanelectron.Theionsareattractedtooneanotherbytheiroppositecharges,andtheystacktogethercompactly,liketightlypackedspheres.Recently,scientistsatMichiganStateUniversitycreatedanewkindofcrystalcalledanelectride.Inelectrides,theanionsarecompletelyreplacedbyelectrons,whicharetrappedinnaturallyformedcavitieswithinaframeworkofregularlystackedcations.Electridesarethefirstexamplesofionicsaltsinwhichalltheseanionicsitesareoccupiedsolelybyelectrons.Unlikeothertypesofanions,anionicelectronsdonotbehaveasiftheyweresimplechargedspheres.Inparticular,becauseoftheirlowmassandtheirtendencytointeractwithoneanotherovergreatdistances,theycannotbepinneddowntoanyonelocation.Instead,theywanderclosetoandamongtheatomsliningthecavityandinteractwithelectronsinnearbycavities,perhapschangingplaceswiththem.Thepropertiesofanelectridedependlargelyonthedistancebetweenthecavitiesthatholdtrappedelectrons.Whenthetrappedelectronsarefarapart,theydonotinteractstrongly,andsobehavesomewhatlikeanarrayofisolatednegativecharges.Whentheyareclosertogether,theybegintodisplaypropertiesassociatedwithlargeensemblesofidenticalparticles.Whentheyarestillcloser,theensemblepropertiesdominateandtheelectronsdelocalize:theyarenolongertightlyboundwithininpidualcavitiesbutaremoreorlessfreetopassthroughthespaceswithintheframeworkofpositiveions.Bysynthesizingelectridesfromavarietyofmaterials,onecanvarythegeometryoftheanioniccavitiesandtheirrelationtothesurroundingcations.Theresultingpropertiesmaymakeitpossibleforelectridestobecomeabasisforeconomicallyusefulnewmaterialsanddevices.Forinstance,becausetheelectronsinsomeelectridesareveryweaklybound,thesecrystalscouldbeeffectiveasphotosens