1、Designation: E 2585 09Standard Practice forThermal Diffusivity by the Flash Method1This standard is issued under the fixed designation E 2585; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in p
2、arentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers practical details associated with thedetermination of the thermal diffusivity of primarily homoge-neous isotropic solid mater
3、ials. Thermal diffusivity valuesranging from 10-7to 10-3m2/s are readily measurable by thisfrom about 75 to 2800 K.1.2 This practice is adjunct to E 1461.1.3 This practice is applicable to the measurements per-formed on materials opaque to the spectrum of the energypulse, but with special precaution
4、s can be used on fully orpartially transparent materials.1.4 This practice is intended to allow a wide variety ofapparatus designs. It is not practical in a document of this typeto establish details of construction and procedures to cover allcontingencies that might offer difficulties to a person wi
5、thoutpertinent technical knowledge, or to stop or restrict researchand development for improvements in the basic technique.This practice provides guidelines for the construction prin-ciples, preferred embodiments and operating parameters forthis type of instruments.1.5 This practice is applicable to
6、 the measurements per-formed on essentially fully dense materials; however, in somecases it has shown to produce acceptable results when usedwith porous specimens. Since the magnitude of porosity, poreshapes, and parameters of pore distribution influence thebehavior of the thermal diffusivity, extre
7、me caution must beexercised when analyzing data. Special caution is advisedwhen other properties, such as thermal conductivity, arederived from thermal diffusivity obtained by this method.1.6 The flash can be considered an absolute (or primary)method of measurement, since no reference materials arer
8、equired. It is advisable to use only reference materials toverify the performance of the instrument used.1.7 This method is applicable only for homogeneous solidmaterials, in the strictest sense; however, in some cases it hasbeen shown to produce data found to be useful in certainapplications:1.7.1
9、Testing of Composite MaterialsWhen substantialnon-homogeneity and anisotropy is present in a material, thethermal diffusivity data obtained with this method may besubstantially in error. Nevertheless, such data, while usuallylacking absolute accuracy, may be useful in comparing mate-rials of similar
10、 structure. Extreme caution must be exercisedwhen related properties, such as thermal conductivity, arederived, as composite materials, for example, may have heatflow patterns substantially different than uniaxial. In caseswhere the particle size of the composite phases is smallcompared to the speci
11、men thickness (on the order of 1 to 25 %of thickness) and where the transient thermal response of thespecimen appears homogenous when compared to the model,this method can produce accurate results for composite mate-rials. Anisotropic materials can be measured by various tech-niques, as long as the
12、directional thermal diffusivities (twodimensional or three dimensional) are mutually orthogonal andthe measurement and specimen preparation produce heat flowonly along one principle direction. Also, 2D and 3D modelsand either independent measurements in one or two directions,or simultaneous measurem
13、ents of temperature response atdifferent locations on the surface of the specimen, can beutilized.1.7.2 Testing LiquidsThis method has found an especiallyuseful application in determining thermal diffusivity of moltenmaterials. For this technique, specially constructed specimeenclosures must be used
14、.1.7.3 Testing Layered MaterialsThis method has alsobeen extended to test certain layered structures made ofdissimilar materials, where the thermal properties of one of thelayers are considered unknown. In some cases, contact con-ductance of the interface may also be determined.1.8 The values stated
15、 in SI units are to be regarded as thestandard.1.9 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 applica-bility of regu
16、latory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:21This practice is under the jurisdiction of ASTM Committee E37 on ThermalMeasurements and is the direct responsibility of Subcommittee E37.05 on Thermo-physical Properties.Current edition approved March 15, 2009. Published Ju
17、ly 2009.2For 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.1Copyright ASTM International, 100 Barr Harbor Drive,
18、 PO Box C700, West Conshohocken, PA 19428-2959, United States.E 228 Test Method for Linear Thermal Expansion of SolidMaterials With a Push-Rod DilatometerE 1461 Test Method for Thermal Diffusivity by the FlashMethod3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 thermal conduc
19、tivity, l, of a solid materialthe timerate of steady heat flow through unit thickness of an infiniteslab of a homogeneous material in a direction perpendicular tothe surface, induced by unit temperature difference. Theproperty must be identified with a specific mean temperature,since it varies with
20、temperature.3.1.2 thermal diffusivity, a, of a solid materialthe prop-erty given by the thermal conductivity divided by the productof the density and heat capacity per unit mass.3.2 Description of Symbols and Units Specific to ThisStandard:3.2.1 Ddiameter, meters.3.2.2 Cpspecific heat capacity, J/(k
21、gK).3.2.3 kconstant depending on percent rise.3.2.4 Kcorrection factors.3.2.5 K1,K2constants depending on b.3.2.6 Lspecimen thickness, m.3.2.7 tresponse time, s.3.2.8 t12 half-rise time or time required for the rear facetemperature rise to reach one half of its maximum value, s.3.2.9 t*dimensionless
22、 time (t*=4ast/DT2).3.2.10 Ttemperature, K.3.2.11 athermal diffusivity, m2/s.3.2.12 lthermal conductivity, (W/mK).3.2.13 bfraction of pulse duration required to reachmaximum intensity.3.2.14 rdensity, kg/m3.3.2.15 Dt5T (5t12 )/T (t12 ).3.2.16 Dt10T (10t12 )/T (t12 ).3.2.17 DTmaxtemperature differenc
23、e between baseline andmaximum rise, K.3.3 Description of Subscripts Specific to This Standard:3.3.1 oambient.3.3.2 sspecimen.3.3.3 Tthermocouple.3.3.4 xpercent rise.3.3.5 CCowan.3.3.6 Rratio.3.3.7 mmaximum.3.3.8 ttime.4. Summary of Practice4.1 A small, thin disc specimen is subjected to a high-inten
24、sity short duration radiant energy pulse (Fig. 1). Theenergy of the pulse is absorbed on the front surface of thespecimen and the resulting rear face temperature rise (thermo-gram) is recorded. The thermal diffusivity value is calculatedfrom the specimen thickness and the time required for the rearf
25、ace temperature rise to reach certain percentages of itsmaximum value (Fig. 2). When the thermal diffusivity of thesample is to be determined over a temperature range, themeasurement must be repeated at each temperature of interest.This is described in detail in a number of publications (1, 2)3and r
26、eview articles (3, 4, 5). A summary of the theory can befound in E 1461, Appendix 1.5. Significance and Use5.1 Thermal diffusivity is an important property, requiredfor such purposes under transient heat flow conditions, such asdesign applications, determination of safe operating tempera-ture, proce
27、ss control, and quality assurance.5.2 The flash method is used to measure values of thermaldiffusivity, a, of a wide range of solid materials. It isparticularly advantageous because of simple specimen geom-etry, small specimen size requirements, rapidity of measure-ment and ease of handling.5.3 Unde
28、r certain strict conditions, specific heat capacity ofa homogeneous isotropic opaque solid sample can be deter-mined when the method is used in a quantitative fashion (seeE 1461, Appendix 1).5.4 Thermal diffusivity results, together with related valuesof specific heat capacity (Cp) and density (r) v
29、alues, can beused in many cases to derive thermal conductivity (l), accord-ing to the relationship:l5aCpr. (1)6. Interferences6.1 In principle, the thermal diffusivity is obtained from thethickness of the sample and from a characteristic time functiondescribing the propagation of heat from the front
30、 surface of thesample to its back surface. The sources of uncertainties in themeasurement are associated with the sample itself, the tem-perature measurements, the performance of the detector and ofthe data acquisition system, the data analysis and morespecifically the finite pulse time effect, the
31、nonuniform heatingof the specimen and the heat losses (radiative and conductive).3The boldface numbers given in parentheses refer to a list of references at theend of the text.FIG. 1 Block Diagram of a Flash SystemE2585092These sources of uncertainty can be considered systematic, andshould be carefu
32、lly considered for each experiment. Errorsrandom in nature (noise, for example) can be best estimated byperforming a large number of repeat experiments. The relativestandard deviation of the obtained results is a good represen-tation of the random component of the uncertainty associatedwith the meas
33、urement. Guidelines for performing a rigorousevaluation of these factors are given in (31).7. ApparatusThe essential components of the apparatus are shown in Fig.1. These are the flash source, specimen holder, environmentalenclosure (optional), temperature response detector and record-ing device.7.1
34、 The flash source may be a pulse laser, a flash lamp, orother device capable to generate a short duration pulse ofsubstantial energy. The duration of the pulse should be lessthan 2 % of the time required for the rear face temperature riseto reach one half of its maximum value, to keep the error duet
35、o finite pulse width less than 0.5 %, if pulse width correction(17, 18, 19) is not applied.7.1.1 The pulse hitting the specimens surface must bespatially uniform in intensity. Most pulse lasers exhibit hotspots and a substantially higher intensity in the center region ofthe beam than in the peripher
36、y. For this reason, systems usingunmodified beams directly from a pulse laser should use beamssomewhat larger in diameter than the largest diameter of thespecimens to be tested. The use of an optical fiber between thelaser and the specimen improves substantially the uniformity ofthe beam (up to 95 %
37、). Since this method produces almost noedge effects, a larger portion of the energy can be directed tothe specimen than from natural beam lasers.7.1.2 Most commonly used lasers are: ruby (visible red),Nd: glass, and Nd: YAG (near infrared); however, other typesof lasers may be used. In some instance
38、s, properly engineeredXenon flash sources can provide comparable performance forall but the shortest rise times. Xenon flash sources, whenproperly focused, provide a lower cost and lower maintenancealternative to lasers for many applications.7.2 An environmental control chamber is required for mea-s
39、urements above and below room temperature. This chambermust be gas or vacuum tight if operation in a protectiveatmosphere is desired. The enclosure shall be fitted with awindow, which has to be transparent to the flash source. Asecond window is required if optical detection of the rear facetemperatu
40、re rise is used. In such cases it is recommended thatthe optical detector be shielded from direct exposure to theenergy beam with the use of appropriate filter(s).7.3 The furnace or cryostat should be loosely coupled(thermally) to the specimen support and shall be capable ofmaintaining the specimen
41、temperature constant within4%ofthe maximum temperature rise over a time period equal to fivehalves of the maximum rise time. The furnace may behorizontal or vertical. The specimen support shall also beloosely coupled thermally to the specimen. Specimen supportsmay be constructed to house one specime
42、n or several at a time,with the latter providing substantial improvements in data andtesting speed.7.4 The detector can be a thermocouple (see Appendix X1),infrared detector, optical pyrometer, or any other means thatcan provide a linear electrical output proportional to a smalltemperature rise. It
43、shall be capable of detecting 0.05 K changeabove the specimens initial temperature. The detector and itsassociated amplifier must have a response time substantiallysmaller than2%ofthehalf-rise time value. When intrinsicthermocouples are used, the same response requirements shallapply. Electronic fil
44、ters, if used, shall be verified not to distortthe shape of the thermogram. Several precautions are requiredwhen using optical temperature sensing. The sensor must befocused on the center of the back surface of the specimen. Italso must be protected from the energy beam, to preventdamage or saturati
45、on. When the specimen is housed in afurnace, the energy beam may bounce or shine past the edgesand enter the detector. To avoid this, proper shielding isnecessary. For protection against lasers, dielectric spike filtersthat are opaque at the selected wavelength are very useful. Theviewing window and
46、 any focusing lenses must not absorbappreciably the radiation in the wavelength region of thedetector. This is particularly important for infrared detectors,and means should be provided to ensure that during hightemperature measurements all window surfaces are monitoredand kept free of deposits, whi
47、ch might lead to absorption ofenergy. Such build-ups can lead to loss of signal intensity andmay cause non-uniform specimen heating from the energysource.7.5 The signal conditioner includes the electronic circuit tobias out the ambient temperature reading, spike filters, ampli-fiers and analog-to-di
48、gital converters.7.6 Data Recording7.6.1 The data acquisition system must be of an adequatespeed to ensure that time resolution in determining half of themaximum temperature rise on the thermogram is at least 1 %,for the fastest thermogram for which the system is qualified.7.6.2 The recorded signal
49、must contain information thatenables the precise definition of the starting time of the energypulse.7.6.2.1 If no other means are available, the inevitable spikecaused by the trigger pulse (for a laser of flash lamp) may beused. This, however, is considered marginal, as it uses thebeginning of the capacitor discharge as “time zero.”7.6.2.2 More accurate results are obtained if the center ofgravity for the energy pulse is used as “time zero.” This can bedetermined only with actual recording of the pulse shape andderivation of the point of start for
