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1、CHAPTER3StructureofCarbonFibersThepropertiesofcarbonfibersstronglydependonthestructure.Thepropertiesincludetensilemodulus,tensilestrength,electricalresistivity,andthermalconductivity.Thestructuralaspectsthatareparticularlyimportantare(1)thedegreeofcrystallinity,(2)theinterlayerspacing(doo2)
2、,(3)thecrystallitesizesor,moreaccurately,thecoherentlengthsperpendicular(L,)andparallel(La)tothecarbonlayers,(4)thetexture(preferredorientationofthecarbonlayers)parallelandperpendiculartothefiberaxis,(5)thetransverseandlongitudinalradiiofcurvature(r,andr,)ofthecarbonlayers,(6)thedomainstruc
3、ture,and(7)thevolumefraction,shapeandorientationofmicrovoids.Ahighdegreeofcrystallinity,alowinterlayerspacing,largecrystallitesizes,astrongtextureparalleltothefiberaxis,andalowdensityofin-planedefects(disclinations)generallyresultinahightensilemodulus,alowelectricalresistivity,andahightherm
4、alconductivity.Aweaktextureperpen-diculartothefiberaxis,smallvaluesofr,andrl,alargeamountofdefectsanddistortionswithinalayer,alargevalueofL,,andalowvolumefractionofmicrovoidscontributetoahightensilestrength.However,alargeL,valuemaybeaccompaniedbyreducedlateralbondingbetweenthestacksofcarbon
5、layers,therebydegradingthestrength.Thestructureisaffectedbytheprocessingofthefibers,particularlytheheattreatmenttemperatureandtheeaseofgraphitizationofthecarbonfiberprecursor.PAN-basedcarbonfibers,evenafterheattreatmentbeyond2OOO'C,remainturbostratic(i.e.,nographiticABABstackingofthecarbonl
6、ayers)[I].TheinterlayerspacingdecreaseswhileL,andLaincreasewiththeheattreatmenttemperature.Figure3.1[2]showsthatL,increaseswiththeheattreatmenttemperature,suchthatitsvalueishigherforpitch-basedcarbonfibersthanPAN-basedcarbonfibersthathavebeenheat-treatedatthesametemperature.ForPAN-basedfibe
7、rs,L,increasessharplyabove220O0C,whereasL,increasessmoothlywithincreasingtemperatureforpitch-basedfibers.Figure3.2[2]showsLaparalleltothefiberaxis(i.e.,La,,)andLaperpendiculartothefiberaxis(Le.,L&bothplottedagainstL,.BothLulland5556CARBONFIBERCOMPOSITESF
