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1、Chapter3StructuresofMetalsandCeramics Highvelocityelectronbeamswavelike wavelengthsshorterthaninteratomicspacings diffractionbyatomicplanesincrystallinematerials Asx rays WhyStudyStructuresofMetalsandCeramics propertiesofsomematerials crystalstructures directlyrelatedby Forexample Magnesium Mg Beryl
2、lium Be Havingsamecrystalstructure morebrittlethan Gold Au Silver Ag Havingsamecrystalstructure CrystallineandNoncrystalline Differentproperty havingthesamecompositionbutdifferentstructure 3 1INTRODUCTION Chapter2wasconcernedatomicbonding theelectronstructureoftheindividualatoms Inthischapter nextle
3、velofthestructureofmaterialsthearrangementsofatomsinthesolidstate Introducingtheconcepts crystalandnoncrystal Forcrystalstructure unitcell crystallographicdirectionsandplanes metalsandceramics Singlecrystalsandpolycrystalline 非晶体 晶体 晶胞 单晶体 多晶体 3 2FundamentalConcepts Crystalline Thestateofasolidmater
4、ialcharacterizedbyaperiodicandrepeatingthree dimensionalarrayofatoms ions ormolecules long rangeorder theatomswillpositionthemselvesinarepetitivethree dimensionalpattern inwhicheachatomisbondedtoitsnearest neighboratoms Allmetals manyceramicmaterials andcertainpolymersformcrystallinestructures Mater
5、ials crystalline Noncrystallineoramorphous Nolong rangeatomicorder 长程有序 Thedescriptionofcrystalline atomichardspheremodel UnitCells晶胞 3 3UnitCells hardspherelatticeunitcells 晶格 atomichardspheremodel atoms orions arethoughtofasbeingsolidsphereshavingwell defineddiameters lattice Theregulargeometrical
6、arrangementofpointsincrystalspace meansathree dimensionalarrayofpointscoincidingwithatompositions unitcells Thebasicstructuralunitofacrystalstructure Itisgenerallydefinedintermsofatom orion positionswithinaparallelepipedorprismsvolume thelatticeparameters Thecombinationofunitcelledgelengthsandintera
7、xialanglesthatdefinestheunitcellgeometry a b c and 硬球模型 晶格 晶胞 晶格常数 平行六面体 棱柱 Crystalstructure Forcrystallinematerials themannerinwhichatomsorionsarearrayedinspace Itisdefinedintermsoftheunitcellgeometryandtheatompositionswithintheunitcell Aunitcellischosentorepresentthesymmetryofthecrystalstructure t
8、heunitcellisthebasicstructuralunitorbuildingblockofthecrystalstructure 对称性 3 4MetallicCrystalStructures norestrictionsastothenumberandpositionofnearest neighboratoms denseatomicpackings Threerelativelysimplecrystalstructures face centeredcubic body centeredcubic andhexagonalclose packed 1 TheFace ce
9、nteredCubicCrystalStructure Acrystalstructurefoundinsomeofthecommonelementalmetals Withinthecubicunitcell atomsarelocatedatallcornerandface centeredpositions 面心立方晶格 无限制 原子密堆 FIGURE3 1Fortheface centeredcubiccrystalstructure a ahardsphereunitcellrepresentation b areduced sphereunitcell and c anaggreg
10、ateofmanyatoms a b cand 90oatomslocatedateachofthecornersandthecentersofallthecubefaces theface centeredcubic FCC 1 thecubeedgelengthaandtheatomicradiusRarerelatedthrough 2 Thenumberofatominunitcell n 原子半径 晶胞原子数 3 CoordinationNumber CN Thenumberofatomicorionicnearestneighbors For FCC crystalstructur
11、e CN 12 配位数 4 AtomicPackingFactor APF Thefractionofthevolumeofaunitcellthatisoccupiedby hardsphere atomsorions 紧密系数 致密度 2 TheBody centeredCubicCrystalStructure body centeredcubic BCC a b cand 90oatomsarelocatedatcornerandcellcenterpositions FIGURE3 2Forthebody centeredcubiccrystalstructure a ahardsp
12、hereunitcellrepresentation b areduced sphereunitcell and c anaggregateofmanyatoms 体心立方晶格 1 thecubeedgelengthaandtheatomicradiusRarerelatedthrough 2 Thenumberofatominunitcell n 3 CN 8 4 3 TheHexagonalClose packedCrystalStructure hexagonalclose packed HCP Thetopandbottomfacesoftheunitcellconsistofsixa
13、tomsthatformregularhexagonsandsurroundasingleatominthecenter Anotherplanethatprovidesthreeadditionalatomstotheunitcellissituatedbetweenthetopandbottomplanes a b cc a 1 633anda b 90og 120o FIGURE3 3Forthehexagonalclose packedcrystalstructure a areduced sphereunitcelland b anaggregateofmanyatoms 密排六方晶
14、格 2 Thenumberofatominunitcell n 3 CN 12 4 1 thecubeedgelengthaandtheatomicradiusRarerelatedthrough a c 铬 镉 钴 3 5DensityComputations metals n numberofatomsassociatedwitheachunitcellA atomicweightVc volumeoftheunitcellNA Avogadro snumber 6 023 1023atoms mol EXAMPLE Copper R 0 128nm 1 28 FCC ACu 63 5g
15、mol Theliteraturevalueforthedensityofcopperis8 94g cm3 3 6CeramicCrystalStructures Ceramicsleasttwoelementsormore crystalstructuresmorecomplex atomicbondingfrompurelyionictototallycovalent sometimes twobondingtypes thedegreeofioniccharacterdependentontheelectronegativities ByEquation2 10andFigure2 7
16、 Table3 2thepercentioniccharacter Table3 2ForSeveralCeramicMaterials PercentIonicCharacteroftheInteratomicBonds crystalstructuresofceramicmaterialsbondedinpredominantlyionic Cations Anions Apositivelychargedmetallicion 阳离子 阴离子 Anegativelychargednonmetallicion Twocharacteristicsofinfluencethecrystalstructure themagnitudeoftheelectricalchargeions therelativesizesofthecationsandanions thecrystalmustbeelectricallyneutral thatis allthecationpositivechargesmustbebalancedbyanequalnumberofanionnegativec
