《電子與通信工程專業(yè)英語》課件第14章

Unit14TemperatureSensorsNEWWORDSANDPHRASES

NOTES

EXERCISES

參考譯文

EXTENSIVETEXT

Oneoftheprimarymethodsforelectricalmeasurementoftemperatureinvolveschangesintheelectricalresistanceofcertainmaterials.Inthis,aswellasothercases,theprinciplemeasurementtechniqueistoplacethetemperature-sensingdeviceincontactwiththeenvironmentwhosetemperatureistobemeasured.Thesensingdevicethentakesonthetemperatureoftheenvironment.Thusameasureofitsresistanceindicatesthetemperatureofthedeviceandtheenvironmentmustwaituntilthedevicecomesintothermalequilibriumwiththeenvironment.ThetwobasicdevicesusedaretheResistance-TemperatureDetector(RTD),basedonthevariationofmetalresistancewithtemperature,andthethermistor,basedonthevariationofsemiconductorresistancewithtemperature[1].

Thereexistsanotherdependenceofelectricalbehaviorofmaterialsontemperaturethatformsthebasisofalargepercentageofalltemperaturemeasurement.Thiseffectischaracterizedbyavoltage-generatingsensorinwhichanelectromotiveforce(emf)isproducedthatisproportionaltotemperature.Suchanemfisfoundtobealmostlinearwithtemperatureandveryrepeatableforconstantmaterials.Devicesthatmeasuretemperatureonthebasisofthisthermoelectricprinciplearecalledthermocouples(TCs).

1．Metalresistanceversustemperature

Ametalisassemblageofatomsinthesolidstateinwhichtheindividualatomsareinanequilibriumpositionwithsuperimposedvibrationinducedbythethermalenergy[2].Thechiefcharacteristicofametalisthefactthateachatomgivesuponeelectron,calleditsvalenceelectron,thatcanmovefreelythroughoutthematerial;thatis,itbecomesaconductionelectron.Wesay,then,forthewholematerial,thatthevalencebandofelectronsandconductionbandofelectronsinthematerialoverlapinenergy,asshowninFig

14.1(a).Contrastthiswithasemiconductor,whereasmallgapexistsbetweenthetopelectronenergyofthevalencebandandthebottomelectronenergyoftheconductionband,asshowninFig

14.1(b).Similarly,Fig

14.1(c)showsthataninsulatorhasalargegapbetweenvalenceandconductionelectrons.Whenacurrentistobepassedthroughamaterial,itistheconductionbandelectronsthatcarrythecurrent.

Aselectronsmovethroughoutthematerial,theycollidewiththestationaryatomsormoleculesofthematerial.Whenathermalenergyispresentinthematerialandtheatomsvibrate,theconductionelectronstendtocollideevenmorewiththevibratingatoms.Thisimpedesthemovementofelectronsandabsorbssomeoftheirenergy;thatis,thematerialexhibitsaresistancetoelectricalcurrentflow.Thus,metallicresistanceisafunctionofthevibrationoftheatomsandthusofthetemperature.Asthetemperatureisraised,theatomsvibratewithgreateramplitudeandfrequency,whichcausesevenmorecollisionwithelectrons[3],furtherimpedingtheirflowandabsorbingmoreenergy.Fromthisargument,wecanseethatmetallicresistanceshouldincreasewithtemperatures,anditdoes.

Fig14.1Energybandsforsolids

ThegraphinFig14.2showstheeffectofincreasingresistancewithtemperatureforseveralmetals.Tocomparethedifferentmaterials,thegraphshowstherelativeresistanceversustemperature.Foraspecificmetalofhighpurity,thecurveofrelativeresistanceversustemperatureishighlyrepeatable,andthuseithertablesorgraphscanheusedtodeterminethetemperaturefromaresistancemeasurementusingthatmaterial.Itispossibletoexpresstheresistanceofaparticularmetalsampleataconstanttemperature(T)analyticallyusingtheEquation(14.1).

(T=constant)(14.1)

Where：R—sampleresistance(

)；

l

—length(m)；

A

—cross-sectionalarea(m2)；

ρ

—resistivity(

·m).

InEquation(14.1)theprincipalincreaseinresistancewithtemperatureisduetochangesintheresistivity(ρ)ofthemetalwithtemperature.Iftheresistivityofsomemetalisknownasafunctionoftemperature,thenEquation(14.1)canbeusedtodeterminetheresistanceofanyparticularsampleofthatmaterialatthesametemperature.Infact,curvessuchasthoseinFig14.2arecurvesofresistivityversustemperaturebecause,forexample,

(14.2)

TheuseofeitherEquation(14.1),resistanceversustemperaturegraphs,orresistanceversustemperaturetablesispracticalonlywhenhighaccuracyisdesired[4].Formanyapplicationswecanuseananalyticalapproximationofthecurves,forwhichwesimplyinsertthetemperatureandquicklycalculatetheresistance[5].

Fig14.2Metalresistanceincreasesalmostlinearlywithtemperature

2．Resistanceversustemperatureapproximations

AnexaminationoftheresistanceversustemperaturecurvesofFig14.2showsthatthecurvesareverynearlylinear,thatis,astraightline.Infact,whenonlyshorttemperaturespansareconsidered,thelinearityisevenmoreevident.Thisfactisemployedtodevelopapproximateanalyticalequationsforresistanceversustemperatureofaparticularmetal.

Alinearapproximationmeansthatwemaydevelopanequationforastraightlinethatapproximatestheresistanceversustemperature(R-T)curveoversomespecifiedspan.InFig

14.3,weseeatypicalR-Tcurveofsomematerial.AstraightlinehasbeendrawnbetweenthepointofthecurvethatrepresenttemperatureT1andT2asshown,andT0representsthemidpointtemperature.

TheequationofthisstraightlineisthelinearapproximationtothecurveoverthespanT1toT2.Theequationforthislineistypicallywrittenas

(14.3)

Where： R(T)—

approximationofresistanceattemperatureT；

R(T0)—

resistanceattemperature；

T—T-T0；

—

fractionalchangeinresistanceperdegreeoftemperatureatT0.

Notethat

dependsonthemidpointtemperatureT0,whichsimplysaysthatastraight-lineapproximationoversomeotherspanofthecurvewouldhaveadifferentslope.

Thevalueofcanbefoundfromvaluesofresistanceandtemperaturetakenfromagraph,asgiveninFig14.2.Ingeneral,then,

(slopeatT0)

(14.4)

Thus,theunitsofaretypically1/℃or1/F.

Fig14.3AlinearapproximationofresistanceversustemperaturebetweenT1andT2

NEWWORDSANDPHRASES

thermal a. 熱的，熱量的

equilibrium n. 平衡，均衡

thermistor n. 熱敏電阻

superimpose vt. 重疊(安裝，添加)

thermocouple n. 熱電偶

electromotive adj. 電的，電動的

vibration n. 震動，顫動，振動

vibrate vt.,vi. (使)振動，(使)搖擺；搖動，震動

valence n. 化合價

impede vt.,vi. 妨礙，阻礙，阻止

metallic adj. 金屬的

repeatable adj. 重復的

analytical adj. 分析的，解析的

approximation n. 近似值

span n. 跨距

sensitivity n. 靈敏度

platinum n. 白金

nickel n. 鎳

inevitably adv. 不可避免地

erroneous adj. 錯誤的，不正確的

delicate adj. 細致優(yōu)雅的，微妙的，美味的

valenceelectron 價電子

valenceband 價(電子)帶

conductionband 傳導帶

electromotiveforce 電勢

NOTES

[1]ThetwobasicdevicesusedaretheResistance-TemperatureDetector(RTD),basedonthevariationofmetalresistancewithtemperature,andthethermistor,basedonthevariationofsemiconductorresistancewithtemperature.

句子中的“basedonthevariationofmetalresistancewithtemperature”修飾theResistance-TemperatureDetector,basedonthevariationofsemiconductorresistancewithtemperature修飾thethermistor，theResistance-TemperatureDetector和thethermistor相并列。本句可譯為：使用的兩個測溫器件是電阻-溫度檢測器(RTD)和熱敏電阻，其中電阻-溫度檢測器是基于金屬的電阻隨溫度變化而變化的原理，而熱敏電阻則基于半導體的電阻隨溫度變化而變化的原理。

[2]Ametalisassemblageofatomsinthesolidstateinwhichtheindividualatomsareinanequilibriumpositionwithsuperimposedvibrationinducedbythethermalenergy.

本句可譯為：金屬在固體狀態(tài)下由許多原子組成，每個原子都處于平衡位置，當這些原子受到熱能作用時就會圍繞這個平衡位置產生振動。

[3]Asthetemperatureisraised,theatomsvibratewithgreateramplitudeandfrequency,whichcausesevenmorecollisionwithelectrons,...

本句可譯為：隨著溫度的上升，原子的振動幅度和頻率逐漸變大，這就加劇了原子與電子之間的碰撞，……

[4]TheuseofeitherEquation(14.1),resistanceversustemperaturegraphs,orresistanceversustemperaturetablesispracticalonlywhenhighaccuracyisdesired.本句可譯為：實際應用時，一般使用公式(14.1)，即電阻-溫度曲線。只有當期望獲得高精度的測量結果時才使用電阻-溫度表格。

[5]Formanyapplicationswecanuseananalyticalapproximationofthecurves,forwhichwesimplyinsertthetemperatureandquicklycalculatetheresistance.

句中“which”指“ananalyticalapproximationofthecurves”。

本句可譯為：對于很多應用，我們可以使用曲線的解析近似，只需簡單地代入溫度值，即可快速計算出電阻值。

EXERCISES

Ⅰ.

TrueorFalse.

(1)TheslownessofresponseforaRIDisdueprincipallytotheslownessofthermalconductivityinbringingthedeviceintothermalequilibriumwithitsenvironment.()

(2)

Onlylinearequationcanbeusedtoexpresstheresistanceversustemperature.()

(3)

Aresistance-temperaturedetectorisatemperaturesensorthatdependsonthedecreaseinmetallicresistancewithtemperature.()

(4)

Metalresistanceincreasesalmostlinearlywithtemperature,buttheslopeissmall.()

(5)

BecausetheRIDisaresistance,thereisanI2Rpowerdissipatedbythedeviceitself.()

Ⅱ.

AnalyseandCalculation.

ARTDhas(20℃)

=

0.004/°C.IfR

=

106

at20℃,findtheresistanceat30℃.

參考譯文

第十四單元溫

度

傳

感

器

測量材料電阻值的變化是實現(xiàn)電子測溫的一種基本方法。這種方法的測量技術原理與其他方法類似，就是將溫度傳感器放置在需要測溫的環(huán)境中，接著傳感器就會傳回環(huán)境的溫度。因此必須等到傳感器與環(huán)境達到熱平衡時才能用測量的電阻值指示溫度。常用的兩個測溫器件是電阻-溫度檢測器(RTD)和熱敏電阻，其中電阻-溫度檢測器是基于金屬電阻隨溫度變化而變化的原理，而熱敏電阻則基于半導體電阻隨溫度變化而變化的原理。物質的電特性還與溫度有關，所有的溫度測量大多基于此來進行。該效應在電壓產生傳感器中表現(xiàn)得尤為明顯，在電壓產生傳感器中，電動勢(emf)與溫度成比例增長。該電動勢與溫度幾乎呈線性關系，并且對于特定材料是可復驗的?；谶@種熱電原理測溫的器件稱為熱電偶(TC)。

1．金屬的電阻與溫度的關系

金屬在固體狀態(tài)下由許多原子組成，每個原子都處于平衡位置，當這些原子受到熱能作用時就會圍繞這個平衡位置產生振動。金屬最重要的特性就是它的每一個原子都可以釋放一個電子，稱為價電子。價電子能在金屬中自由游走，也即它變成了一個傳導電子。也就是說，對于所有的金屬物質，其電子價帶和電子導帶在能量上有重疊，如圖14.1(a)所示。而對于半導體材料，其電子價帶的頂端電子能量與導帶的底端電子能量間存在一條小的間隙，如圖14.1(b)所示。同樣的，絕緣體的價帶和導帶電子間存在一條大的間隙，如圖14.1(c)所示。當電流經(jīng)過物質時，是導帶電子負責傳輸電流。

圖14.1固體能量帶示意圖在電子穿過物質時，它們與物質的靜態(tài)原子或分子發(fā)生碰撞。當物質中存在熱能且原子振動時，導帶電子將與振動著的原子產生更多的碰撞。這將阻止電子的移動并吸收掉它們的部分能量，即物質會阻礙電流流過。因此，金屬電阻是原子振動，也是溫度的函數(shù)。隨著溫度的上升，原子的振動幅度和頻率逐漸變大，這就加劇了原子與電子之間的碰撞，進而會阻止電子的移動并吸收更多的能量。根據(jù)上述論述，顯而易見，金屬的電阻會隨著溫度的上升而增大。圖14.2金屬電阻隨溫度升高呈線性增長圖14.2所示為幾種金屬的電阻受溫度上升而增大的情況。為了方便比較不同的金屬材料，圖中給出了電阻隨溫度變化的關系。對于高純度的特殊金屬，相對電阻與溫度的曲線是可以重復獲得的，因此無論是表或圖都可以用來根據(jù)該金屬的電阻測量得到溫度值。一個特殊金屬樣品在一個恒定溫度(T)下的電阻值可以用公式(14.1)表示。

(T為常量)

(14.1)

式中：R

——樣品電阻；

l

——長度(m)；

A

——橫截面(m2)；

——電阻率(

·m)。公式(14.1)中，金屬的電阻隨溫度升高的主要原因是其電阻率受到溫度的影響。若已知某種金屬的電阻率是溫度的函數(shù)，則公式(14.1)可以用來確定在同一溫度條件下該種金屬材料任何特殊樣品的電阻值。實質上，圖14.2所示的曲線就是電阻率隨溫度變化的曲線，例如：

(14.2)實際應用時，一般使用公式(14.1)，即電阻-溫度曲線。只有當期望獲得高精度的測量結果時才使用電阻-溫度表格。對很多應用，我們可以使用曲線的解析近似，只需簡單地代入溫度值，即可快速計算出電阻值。

2．電阻與溫度的近似法

圖14.2中通過實驗得到的電阻隨溫度變化的曲線表明，該曲線非常趨近于線性，即一條直線。實質上，如果只考慮很小的溫度變化幅度，它的線性度會更明顯。由該特性可推導出一個近似分析方程，可用于分析特種金屬的電阻隨溫度變化的規(guī)律。

所謂線性近似，是指建立一個直線方程，使其能在指定的范圍近似表示電阻隨溫度變化的曲線。圖14.3所示為某種材料的一條典型的電阻-溫度曲線。圖中，畫了一條直線來表示溫度T1

到T2間的曲線，T0表示中點溫度。

圖14.3電阻值在T1至T2間呈線性增長該直線方程就是T1

到T2間曲線的線性近似。該直線方程通常記為

(14.3)

式中：R(T)——

在溫度為T時電阻的近似值；

R(T0)——

T0溫度時的電阻值；

T——T-T0；

——在溫度為T0時電阻每度的百分比變化。

注意：a0取決于中點溫度T0，也就是說，曲線上不同區(qū)間段的近似直線斜率不同。

的值可以從圖14.2中的電阻和溫度值中查到。通常，

(T0處的斜率)

(14.4)

因此，a0的單位通常為1/℃或1/F。

EXTENSIVETEXT

Thermistors

Thethermistorrepresentsanotherclassoftemperaturesensorthatmeasurestemperaturethroughchangesofmaterialresistance.ThecharacteristicsofthesedevicesareverydifferentfromthoseofRTDsanddependonthepeculiarbehaviorofsemiconductorresistanceversustemperature.

1．Semiconductorresistanceversustemperature

Incontrasttometals,electronsinsemiconductormaterialsareboundtoeachmoleculewithsufficientstrengththatnoconductionelectronsarecontributedfromthevalencebandtotheconductionband.Wesaythatagapofenergy

Wgexistsbetweenvalenceandconductionelectrons.Suchamaterialbehavesasaninsulatorbecausetherearenoconductionelectronstocarrycurrentthroughthematerial.Thisistrueonlywhenthereisnothermalenergypresentinthesample,thatis,atatemperatureof0K.Whenthetemperatureofthematerialisincreased,themoleculesbegintovibrate.Inthecaseofasemiconductor,suchvibrationprovidesadditionalenergytothevalenceelectrons.Whensuchenergyequalsorexceedsthegapenergy

Wg,theseelectronsbecomefreeofthemolecules.Thus,theelectronisnowintheconductionbandandisfreetocarrycurrentthroughthebulkofthematerial.Asthetemperatureisfurtherincreased,moreandmoreelectronsgainsufficientenergytoentertheconductionband.Itisthenclearthatthesemiconductorbecomesabetterconductorofcurrentasitstemperatureisincreased,thatis,asitsresistancedecreases.Fromthisdiscussionweformapictureoftheresistanceofasemiconductormaterialdecreasingfromverylargevaluesatlowtemperaturetosmallerresistanceathightemperature.Thisisjusttheoppositeofametal.Animportantdistinction,however,isthatthechangeinsemiconductorresistanceishighlynonlinear,asshowninFig14.4.Thereasonsemiconductors(butnotinsulatorsandothermaterials)behavethiswayisthattheenergygapbetweenconductionandvalencebandsissmallenoughtoallowthermalexcitationofelectronsacrossthegap.

Fig14.4Thermistorresistanceversustemperatureishighlynonlinear

Itisimportanttonotethattheeffectjustdescribedrequiresthatthethermalenergyprovidesufficientenergytoovercomethebandgapenergy

Wg.Ingeneral,amaterialisclassifiedasasemiconductorwhenthegapenergyistypically0.01

-4

eV(1

eV

=

l.6

×

10-19J).Thatthisistrueisexemplifiedbyaconsiderationofsilicon,asemiconductorthathasabandgapof

Wg

=

1.107

eV.

2．Thermistorcharacteristics

Athermistorisatemperaturesensorthathasbeendevelopedfromtheprinciplesjustdiscussedregardingsemiconductorresistancechangewithtemperature.Theparticularsemiconductormaterialusedvarieswidelytoaccommodatetemperatureranges,sensitivity,resistanceranges,andotherfactors.

(1)

Sensitivity.Thesensitivityofthethermistorsisasignificantfactorintheirapplication.Changesinresistanceof10%per℃arenotuncommon.Thus,athermistorwithanominalresistanceof10k

atsometemperaturemaychangeby1

k

fora1℃changeintemperature.

(2)

Construction.

Becausethethermistorisabulksemiconductor,itcanbefabricatedinmanyforms.Thus,commonformsincludediscs,beads,androds,varyinginsizefromabead1mmindiametertoadiscseveralcentimetersindiameterandseveralcentimetersthick.Byvariationofdopinganduseofdifferentsemiconductingmaterials,amanufacturercanprovideawidera