1、Designation:F8371(Reapproved 2013)Standard Practice forDefinition and Determination of Thermionic Constants ofElectron Emitters1This standard is issued under the fixed designation F83;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,t
2、he year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.INTRODUCTIONCathode materials are often evaluated by an emission test which in some ways measures thetemperature-limited emis
3、sion.A more basic approach to this problem is to relate the emission tofundamental properties of the emitter,in particular,the work function.Comparisons are convenientlymade between emitters using the thermionic constants,that is,the work function,the emissionconstant,and the temperature dependence
4、of the work function.These quantities are independent ofgeometry and field effects when properly measured.Although referred to as“constants”thesequantities show variations under different conditions.Considerable confusion exists over thedefinition,interpretation,and usage of these terms and,hence,th
5、ere is a need for at least a generalagreement on nomenclature.1.Scope1.1 This practice covers the definition and interpretation ofthe commonly used thermionic constants of electron emitters(1,2,3),2with appended standard methods of measurement.1.2 The values stated in SI units are to be regarded ass
6、tandard.No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the appli
7、ca-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:3F8 Recommended Practice for Testing Electron Tube Mate-rials Using Reference Triodes43.Terminology3.1 Definitions:3.1.1 effective work function,the work function obtainedby the direct substitution of experimen
8、tally determined valuesof emission current density and temperature into theRichardson-Dushman equation of electron emission of theform:J 5 AT2e2e/kT(1)For direct calculation of the work function,this is conve-niently put in the form:5kT/e!lnAT2/J!(2)where:J=emission current density in A/cm2measured
9、underspecified field conditions except zero field.(J0=emis-sion current density in A/cm2measured under zero fieldconditions.)A=the theoretical emission constant,which is calculatedfrom fundamental physical constants,with its valuegenerally taken as 120 A/cm2K2.A more exact calcu-lation(3)gives 120.1
10、7 which is used in determining theeffective work function.T=cathode temperature,K.e=electronic charge,C.e=natural logarithmic base.k=Boltzmanns constant.=work function,V.The form of Eq 1 is a simplified form of the emission1This practice is under the jurisdiction of ASTM Committee F01 on Electronics
11、and is the direct responsibility of Subcommittee F01.03 on Metallic Materials.Current edition approved May 1,2013.Published May 2013.Originallyapproved in 1967.Last previous edition approved in 2009 as F83 71(2009).DOI:10.1520/F0083-71R13.2The boldface numbers in parentheses refer to references at t
12、he end of thispractice.3For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.4Withdrawn.Copyright ASTM International,100
13、 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 equation which assumes zero reflection coefficient for electronswith energy normally sufficient for emission at the emittersurface.The effective work function is an empirical quantityand represents an average of the true w
14、ork function,giving themaximum information obtainable from a single measurementof the thermionic emission.3.1.2 Richardson work function,0the work functionusually obtained graphically from a Richardson plot,which isa plot of ln(J/T2)versus l/T using data of emission measure-ments at various temperat
15、ures.It is the work function obtainedfrom Eq 1,with the value of A determined graphically,insteadof using the theoretical value.For better visualization of theRichardson plot,Eq 1 may be put in the form:lnJ/T2!5 lnA 2e/kT!0(3)It can be seen(Fig.X1.4)that the Richardson work func-tion 0is obtained fr
16、om the slope of the graph,and theemission constant A from the intercept(l/T=0)on the ln(J/T2)axis.The Richardson work function is also an empiri-cal quantity.Its value is found with reasonable accuracyfrom the graph.However,large errors in the value of Amaybe expected(4).Considering only one factor,a slight inaccu-racy in the measurement of temperature introduces a largeerror in the value of A.Values of A obtained on practicalemitters can range from about 0.1 to 200 A/cm2K2.3.1.3 true work funct