压缩科学Putting the squeeze on materials.docx

压缩科学Putting the squeeze on materials.docx

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压缩科学Puttingthesqueezeonmaterials

压缩科学：

Puttingthesqueezeonmaterials

Fewgemstonesareasmesmerizingasdiamonds.Livermorephysicistsalsofinddiamondsattractivebutforreasonsotherthantheirbeauty.Theresearchersuseflawless,polisheddiamondsinopposingpairs,oranvils,toslowlycompresssamplesofmaterialsatextremepressures.Thisdevice,calledadiamondanvilcell（DAC）,forcesmaterialstorevealnewinformationabouthowtheirstructureandelectricalandmagneticpropertieschange--sometimesdrastically--inresponsetoincreasingpressure.

ADACisasmallmechanicalpressthatforcestogetherthesmall,flattips（calledculets）oftwobrilliant-cutdiamondanvils.Thediamondtipspressonamicrogramsampleofamaterial,heldwithinametalgasket,tocreateextremelyhighpressures.Diamondsareusedbecausetheyarethehardestknownsolidandsocanwithstandultrahighpressures.Theyalsopermitdiagnosticradiation,suchasxraysandvisiblelight,topassunhamperedthroughtheircrystallinestructure. 

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. 

Clickforahighresolutionphotograph.    

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. 

Clickforahighresolutionphotograph.    

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