Въведение
Известен също като коефициент на линейно разширение. Когато температурата на твърдо вещество се повиши с 1°C, удължението за единица дължина се нарича "коефициент на линейно разширение". Единицата е 1/℃ или 1/K. Символът е αl.
това е,
lt=l0(l+al△t).
Duetodifferentmaterials,thecoefficientoflinearexpansionisalsodifferent,anditsvalueisalsorelatedtotheactualtemperatureandthereferencetemperatureselectedwhendeterminingthelength1,butbecausethelinearexpansioncoefficientofsolidsdoesnotchangemuch,usuallyItcanbeignored,andaisregardedasaconstantindependentoftemperature.
Thecoefficientoflinearexpansionofmaterialsisavailableinthe"MachineDesignManual".
Вариационен закон
Thelawoflinearexpansioncoefficientchangingwithtemperatureissimilartothatofheatcapacity.Theavalueisverysmallatverylowtemperature,anditincreasesquicklywiththeincreaseoftemperature,andtendstobeconstantabovetheDebyecharacteristictemperature.Theabsolutevalueofthelinearexpansioncoefficientiscloselyrelatedtothecrystalstructureandbondstrength.Materialswithhighbondstrengthhavealowcoefficientoflinearexpansion.Comparedwithmetalmaterials,refractorymaterialshavestrongbondsandsmalllinearexpansioncoefficients.Generally,theαvalueofoxideisintherangeof(8～15)×10K,theαvalueofbinarysilicatematerialisgenerally(5.2～10)×10K,theavalueofcarbideis(5～7)×10Kdiamond1×1010Kquartzglassisduetotherelaxationofitsstructure,thelinearexpansionofthetetrahedroninthestructureisaccommodatedbythevoidsinthestructure,andhasaverysmallavalue(0.5×1010Knon-equaxialcrystalsalongdifferentcrystalaxesTheavalueisdifferent,especiallyformaterialswithalayeredstructuresuchasgraphite.Graphitehasastronginterlayerbondingforce,withasmalllayer-wiseavalue(1×1010K),andaweakinterlayerbondingforce,withavalueof27×intheinterlayerdirection.10KForcrystalswithstrongnon-equaxiality,thevalueofninacertaindirectionmaybenegative.Refractorymaterialscomposedofanisotropicpolycrystalsandrefractorymaterialscomposedofmultiphasepolycrystalswithdifferentphaseavalues,Internalstresswillbegeneratedinthematerialduringthefiringandcoolingprocess.Whenthegrainboundaryisinahighstressstate,thestrengthofthematerialwilldecrease,andevenmicrocrackswilloccur.Theporosityalsohasaneffectonthethermalexpansioncharacteristicsoftherefractory.Whentheporesmaketheparticlesinthematerialinter-particlesWhenthebondbecomesweaker,theavaluebecomessmaller.Theclosedsmallporesinthecontinuoussolidphasehardlyaffecttheavalue.Thelinearexpansioncoefficientofmultiphasepolycrystallineandcompositematerialscanbecalculatedbasedonthephasecomposition.AllcalculationformulasAllarebasedonthepremisethatnomicro-cracksaregeneratedundertheactionofinternalstressbetweenthephases,soitisactuallyanapproximateestimation.Forrefractorymaterialswithmultiplemicro-cracks,thedeviationofthemeasuredvalueandthecalculatedvalueofacanbeusedasameasureofmicro-crack.Ameasureofthenumberofdefectsinthestructure.
Метод на измерване
Thecommonlyusedmethodsformeasuringthelinearexpansioncoefficientofrefractorymaterialsaretheindirectmethodoftheejectorrodandthedirectreadingmethodofthetelescope.ThenewlasermethodThedeterminationofthecoefficientoflinearexpansionhasalsoreceivedmoreandmoreattention.
Индиректен метод на ежектора
Theejectormethodisaclassicmethod,whichusestheprincipleofmechanicalmeasurement,това е,oneendofthesampleisfixedOntheendofthesupporter,theotherendisincontactwiththeejectorrod,thesample,thesupporterandtheejectorrodareheatedatthesametime,andthethermalexpansiondifferencebetweenthesampleandthesepartsistransmittedbytheejectorrodandmeasured.Itcanbedividedintovarioustypesofinstrumentsaccordingtotheposition(verticalorhorizontal)andthemeasurementmethodofexpansion(directmeasurement,electronicoropticalmethod).Themostcommonapplicationistheinductivedilatometer.Itssensorisadifferentialtransformer,Alsocalleddifferentialtransformerthermaldilatometer.Duetothelongsizeoftheejectorrodandthesupporter,theheatingconditionsofthehigh-temperaturefurnacearedifficulttomakethetemperaturedistributionuniform,andtheexpansionbetweentheejectorrodandthesupporterisdifficulttooffseteachother,sothemeasuredvalueofexpansionNeedtobecorrected.
Метод на директно четене с телескоп
Thetelescopedirectreadingmethodusesbinocularstodirectlyobservethechangevalueofthesampleexpansionunderhightemperatureinthefurnace,andobtainthelinearexpansioncoefficientthroughcalculation.Themeasurementtemperaturecanbeashighas2000℃,andthemicrometerontheeyepiecedirectlymeasurestheelongationofthesample.Thesampleusedislong,andtheheatingfurnacemusthaveenoughconstanttemperaturezone.Thedisadvantageofthismethodisthatitisgenerallynoteasytorecordautomatically.NowithasbeendevelopedAutomaticrecordingsystemfortimedphotography.
Лазерно измерване
Thermalexpansionhasdevelopedinrecentyears.Itscansthesamplewithalaserbeamandcontinuouslymeasuresthechangeinlengthofthesampleduringtheheatingprocess.Itispopularbecauseofitshighmeasurementaccuracyandthefullyautomaticcontrol,recordingandmulti-functionsystemcomposedofcomputers.Whenchoosingathermalexpansionmeasurementmethod,themainconsiderationisthetestrange,thetypeandcharacteristicsofthematerialtobetested,themeasurementaccuracyandsensitivity,etc.
Приложение за живот
Thecoefficientoflinearexpansionisoneoftheimportantpropertiesthatshouldbeconsideredwhenusingrefractories.Thefurnaceisusuallybuiltatroomtemperature,andthefurnacebodyexpandswhenusedathightemperature.Inordertooffsetthestresscausedbythermalexpansion,expansionjointsneedtobereserved.Thecoefficientoflinearexpansionisakeyparameterforthestructuraldesigncalculationofthereservedexpansionjointsandtheoverallsizeofthemasonry.Itiscloselyrelatedtothethermalshockresistanceofthematerialandthedistributionandsizeoftheinternalthermalstressofthematerialduringthermalshock.Inthemanufactureofcompositematerialsandmultiphasematerials,theinfluenceofthematchinganddifferenceoftheirlinearexpansioncoefficientsonthestructureandperformancemustbeconsidered.Inaddition,bymeasuringthecurveofthelinearexpansioncoefficientofthematerialwiththetemperature,itispossibletostudythematerialmineralanalysis,phasetransition,andthehealingandpropagationofmicrocracks.
Влияещи фактори
1:Chemicalmineralcomposition.Thecoefficientofthermalexpansionisrelatedtothechemicalcomposition,crystallinestate,crystalstructure,andbondstrengthofthematerial.Substanceswiththesamecompositionanddifferentstructurehavedifferentexpansioncoefficients.Undernormalcircumstances,crystalswithatightstructurehavealargeexpansioncoefficient;whilesimilartoamorphousglass,theyoftenhaveasmallexpansioncoefficient.Materialswithhighbondstrengthgenerallyhavealowcoefficientofexpansion.
2:Phasechange.Whenamaterialundergoesaphasechange,itsthermalexpansioncoefficientalsochanges.Whenthepuremetalallotropetransforms,thelatticestructurerearrangementisaccompaniedbythemutationofthemetalspecificvolume,whichleadstothediscontinuouschangeofthelinearexpansioncoefficient.
3:Alloyingelementshaveaneffectonthethermalexpansionofthealloy.Theexpansioncoefficientofasingle-phaseuniformsolidsolutionalloycomposedofsimplemetalsandnon-ferromagneticmetalsisbetweentheexpansioncoefficientsoftheinternalcomponents.Theexpansioncoefficientofamultiphasealloydependsonthenatureandquantityoftheconstituentphases,andcanberoughlycalculatedaccordingtothevolumepercentageoccupiedbyeachphaseusingthemixingrule.
4:Theinfluenceoftexture.Singlecrystalsorpolycrystalshavetexture,whichleadstodifferencesintheatomicarrangementdensityofthecrystalsineachcrystaldirection,resultinginthermalexpansionanisotropy.Thethermalexpansioncoefficientparalleltothemainaxisofthecrystalislarge,andthethermalexpansioncoefficientissmallintheverticaldirection.
5:Internalcracksanddefectswillalsoaffectthethermalexpansioncoefficient.