压缩科学Putting the squeeze on materialsWord下载.docx

压缩科学Putting the squeeze on materialsWord下载.docx

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However,DACstudiesofsuchpropertiesaselectricalconductivityandmagneticsusceptibilityareextremelydifficulttoperform.The1-microgramsampleshaveadiameterofabout75micrometers,anddiagnosticinstrumentscannotbeplacedcloseenoughtothemtomaketherequiredmeasurements.Problemsespeciallyarisewhenresearcherstrytoobtaininformationaboutmaterialsatstaticpressuresabove1millionatmospheres,or100gigapascals（GPa）.（Forcomparison,theatmosphericpressureatsealevelisabout1/10,000thof1GPa,andthepressureatthecenterofEarthisabout3.6millionatmospheres.） 

Toovercometheproblemsposedbystandarddiamondanvils,Livermoreresearchershavetakenadvantageofrecentimprovementsindiamondsynthesistechnologytofabricatemicrocircuitswithinthediamondanvilsthemselves.Thetungstenmicrocircuitsserveastinydiagnosticinstrumentsthatmeasuredataaboutmaterials'

fundamentalphysicalandmechanicalpropertiesunderhighpressures.Theresearcherscallthismodifiedtooladesignerdiamondanvilbecausethemicrocircuitscanbealteredtosuittheneedsoftheexperimenter.

Scanningelectronmicrographsshowacompleteddesignerdiamondanvilformeasuringelectricalconductivity.（a）Tungstenmicrocircuitsleadfromthesidesofthediamond,wheretheyformelectricalcontactpadswithinstruments,tothetipofthediamondface,orculet,wheretheymonitorvariouspropertiesofthesample.（b,c）Progressivemagnificationofthediamondtipwithalightmicroscopeshowstheterminationofthetungstenwires. 

Clickforahighresolutionphotograph. 

PressuringMaterialstoChange

Materialsbehavequitedifferentlyunderextremepressuresthantheydoatnormalatmosphericpressure.Oxygen,forexample,becomesashinymetalunderultrahighpressure.InsupportoftheNationalNuclearSecurityAdministration'

sStockpileStewardshipProgram,Livermoreresearchersareparticularlyinterestedinbetterunderstandinghownuclearweaponmaterials,suchasplutoniumanduranium,behaveunderhighpressures.

ExperimentswithDACsprovidestockpilestewardshipdatathatcomplementdatafromshockexperimentsandtestsdrivenbyhighexplosives.Allofthesedataimprovetheprecisionofcomputercodesthatscientistsusetomodelweaponperformanceandthus,helptoensurethesafetyandreliabilityofthenation'

sagingnuclearweaponsstockpile.Inparticular,experimentaldataareusedtorefineamaterial'

spressureolumeemperaturerelationship（itsequationofstate,orEOS）andtheresultingstructuralchanges（itsphasediagram）.

WithDACs,researcherscanmeasurematerialpropertiesdirectlyunderstaticpressure,andtheycanvarypressuresandtemperaturesslowlyoverthecourseofmanyhours.LivermorescientistsareusingdesignerDACstolearnhowhighpressurescausematerialstochangetheirmagneticproperties,switchfrominsulatorstometals,andaltertheirmolecularstructures.

"

Itisdifficulttolearnaboutelectricalconductivityandmagneticpropertieswithstandarddiamondanvilsathighpressures,"

saysLivermorephysicistanddesigneranvilinventorSamWeir."

Untilrecently,wewerelimitedtotryingtomaneuverwiresintoplacewithtweezers,butthesewiresdeform,break,andshort-circuit.Ourapproachnowistobuildtinytungstenwiresinsidethediamondssotheysurvivethehighpressures.Welithographicallyfabricatethin-filmwiresontopoftheanvilandthen'

grow'

alayerofdiamondontopofthewirestoprotectthem."

Adesignerdiamondanvilusesaone-third-caratdiamond.Tungstenmetalmicrocircuitsarefabricatedonthediamond抯300-micrometer-widepolishedtip.Thesemicrocircuitsarecoveredwithathinfilmofdiamondandthenpolishedtorevealthetipsofthemicrocircuitsonthetopofthediamondface. 

DesignerDiamondsHand-Fashioned

Everydesignerdiamondanviliscustom-fabricatedbyresearchersfromLivermoreandtheUniversityofAlabamaatBirmingham.Theproductionteammakesthreetypesofdesignerdiamondanvils:

oneforhigh-pressureelectricalconductivityexperiments,anotherformagneticsusceptibilityexperiments,andathirdforelectricallyheatinghigh-pressuresamplestohightemperatures.Eachtypefeaturesauniquepatternofmicrocircuits,usuallymadeoftungsten,whicharefabricatedonthediamondtipandthenencapsulatedwithinadiamondfilm.Thesemicrocircuitsterminateonthediamond'

ssides,wheretheycanbeconnectedtoinstrumentsthatcollectdatawithhighaccuracyandsensitivity.

Electricalconductivityexperimentsusefourtoeighttungstenwires,magneticsusceptibilityexperimentsrequireamicroloopofabouttenturnsofwire,andhigh-temperatureexperimentsuseeightwires.

Thedesignerdiamondanvilisplacedinaberyllium朿oppercellabout6centimeterstalland3centimetersindiameter.Thecell,inturn,isplacedinasmalldeviceconsistingofagear-drivenpistonandcylindermechanismthatcanpushdiamondtipstogetherwithacontrolledforcegreatenoughtogenerateultrahighpressuresbetweenthetips.Turningtheknobonthismechanismpushesthedesignerdiamondanvil（usuallylocatedonthebottom）againstastationary,standarddiamondanvil,increasingthepressureandmaintainingitindefinitely. 

Becausediamondsaretransparent,scientistscanuseDACstomakeopticalandx-raymeasurements.Livermoreresearchersusealightmicroscopetomonitoranexperiment.Inaddition,theyplaceatinychipofrubynexttothesampletomeasurepressure.Whengreenorbluevisiblelaserlightshinesontheruby,therubyemitsredlightatawavelengthofabout694nanometers.Asthepressureincreases,thewavelengthincreases.

Forsomeexperiments,theresearcherstransporttheDACtoasourceofverybright,highlycollimatedxrays,suchastheNationalSynchrotronLightSourceatBrookhavenNationalLaboratoryinNewYork.Thescientistspassabeamofxraysthroughthesampleandbothdiamondsandrecordtheresultingdiffractionpatternonanx-rayfilmordetector.Changesinthediffractionpatternrevealhowamaterial'

sstructurerespondstopressure.

Eachtypeofdesignerdiamondanvilfeaturesauniquepatternofmicrocircuitsthatarefabricatedonthediamondtip.Alightmicroscopeshowsthetipfor（a）anelectricalconductivityexperimentand（b）amagneticsusceptibilityexperiment. 

FocusonTwoElementGroups

ManydesignerDACexperimentsfocusontwogroupsofelements--thelanthanidesandtheactinides--whichincludethenuclearweaponmetalsuraniumandplutonium.TheexperimentsprovidedataaboutlanthanidesandactinidesthatstandardDACtechniquesanddynamicexperimentscannotsupply. 

Mostofthepressure-drivenchangestheresearchersseecanbeexplainedbythebehaviorofamaterial'

selectrons.Weirexplainsthatunderextremepressures,certainelectrons,whicharenormallytightlyheldwithinanatom'

sinnerelectronbandsorshells,canmoveabout,resultinginchangesinmaterialpropertiesandmolecularstructures.Inlanthanidesandactinides,theseelectronsbelongtoanatom'

s4fand5fbands."

Mostexperimentsdon'

tgiveinsightaboutthecauseofvolumechanges,"

saysWeir."

Ourexperimentsdobecausewecanexplainthechangesbythedelocalizationofelectronsfromspecificbandstheynormallyoccupy."

LivermorescientistChantelAracnemonitorsahigh-pressureexperimentusingadesignerdiamondanvilcell. 

Clickhereforahighresolutionphotograph. 

HowInsulatorsBecomeMetals

PostdoctoralresearcherReedPattersonperformedoneofthefirstexperimentswithadesignerDACtodeterminewhycompoundssuchasmanganeseoxide（MnO）areinsulators--thatis,whytheyresistthemovementofelectrons.Electricalconductivityexperiments,whichprobematerials'

insulatingnature,canonlybeaccomplishedatultrahighpressuresusingDACsequippedwithdesignerdiamondanvils.

Pattersonperformedseveralhigh-pressureelectricalconductivityexperimentsonaMnOsample.Theexperimentsusedadesignerdiamondanvilwitheighttungstenprobesmeasuring10micrometerswideand0.5micrometerthick.Theprobeswerecoveredwithdiamondfilmandexposedonlyatthesurfacenearthecenterofthediamondanvil'

sculet,wheretheymakecontactwiththeMnOsample. 

Electricalconductivitywasdeterminedbypassingadirectcurrentthroughthewirestothesampleandmeasuringtheelectricalresistanceasafunctionofpressure.Theresearchersnotedthatthesample'

selectricalresistancerapidlydecreasedbyafactorof100,000between85and106GPa,signalingthetransformationofMnO