1、Designation: E 1461 01Standard Test Method forThermal Diffusivity by the Flash Method1This standard is issued under the fixed designation E 1461; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number i
2、n parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of the thermaldiffusivity of primarily homogeneous isotropic solid materials.Thermal diffusivity val
3、ues ranging from 10-7to 10-3m2/s arereadily measurable by this test method from about 75 to2800 K.1.2 This test method is a more detailed form of Test MethodC 714, but has applicability to much wider ranges of materials,applications, and temperatures, with improved accuracy ofmeasurements.1.3 This t
4、est method is applicable to the measurementsperformed on materials opaque to the spectrum of the energypulse, but with special precautions can be used on fully orpartially transparent materials (see Appendix X1).1.4 This test method is intended to allow a wide variety ofapparatus designs. It is not
5、practical in a test method of thistype to establish details of construction and procedures to coverall contingencies that might offer difficulties to a personwithout pertinent technical knowledge, or to stop or restrictresearch and development for improvements in the basictechnique.1.5 This test met
6、hod is applicable to the measurementsperformed on essentially fully dense materials; however, insome cases it has shown to produce acceptable results whenused with porous samples. Since the magnitude of porosity,pore shapes, sizes and parameters of pore distribution influencethe behavior of the ther
7、mal diffusivity, extreme 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 This test method can be considered an absolute (orprimary) method of measurement, sinc
8、e no reference standardsare required. It is advisable to use reference materials to verifythe performance of the instrument used.1.7 This method is applicable only for homogeneous solidmaterials, in the strictest sense; however, in some cases it hasshown to produce data which may be useful in certai
9、napplications.1.7.1 Testing of Composite MaterialsWhen substantialinhomogeneity 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 mat
10、e-rials of similar structure. Extreme caution must be exercisedwhen related properties, such as thermal conductivity, arederived, as composites may have heat flow patterns substan-tially different than uniaxial.1.7.2 Testing LiquidsThis method has found an especiallyuseful application in determining
11、 thermal diffusivity of moltenmaterials. For this technique, specially constructed sampleenclosures must be used.1.7.3 Testing Layered MaterialsThis method has alsobeen extended to test certain layered structures made ofdissimilar materials, where one of the layers is consideredunknown. In some case
12、s, contact conductance of the interfacemay also be determined.1.8 The values stated 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 establis
13、h appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:C 714 Test Method for Thermal Diffusivity of Carbon andGraphite by a Thermal Pulse Method2E 230 Temperature-Electromotive Force (EMF) Tables fo
14、rStandardized Thermocouples33. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 thermal conductivity, 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 u
15、nit temperature difference. Theproperty must be identified with a specific mean temperature,since it varies with 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.1This tes
16、t method 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 Feb. 10, 2001. Published May 2001. Originallypublished as E 1461 92. Last previous edition E 1461 92.2Annual
17、Book of ASTM Standards, Vol 05.05.3Annual Book of ASTM Standards, Vol 14.03.1Copyright ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.3.2 Description of Symbols and Units Specific to ThisStandard:3.2.1 Ddiameter, meters.3.2.2 kconstant in solution to diffusion equation.
18、3.2.3 Lspecimen thickness, meters.3.2.4 tresponse time, seconds.3.2.5 t*dimensionless time (t*=4ast/DT2).3.2.6 Ttemperature, Kelvin.3.2.7 athermal diffusivity, m2/s.3.2.8 lthermal conductivity, W/mK.3.2.9 bfraction of pulse duration required to reach maxi-mum intensity.3.2.10 Dt5T (5t12)/T (t12).3.2
19、.11 Dt10T (10t12)/T (t12).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 Test Method4.1 A small, thin disc specimen is subjected to a high-intensity shor
20、t duration radiant energy pulse. The energy of thepulse is absorbed on the front surface of the specimen and theresulting rear face temperature rise (thermogram) is recorded.The thermal diffusivity value is calculated from the specimenthickness and the time required for the rear face temperaturerise
21、 to reach certain percentages of its maximum value (Fig. 1).When the thermal diffusivity of the sample is to be determinedover a temperature range, the measurement must be repeated ateach temperature of interest. This test method is described indetail in a number of publications (1, 2)4and review ar
22、ticles (3,4, 5). A summary of the theory can be found in Appendix X5.5. Significance and Use5.1 Thermal diffusivity is an important property, requiredfor such purposes as design applications under transient heatflow conditions, determination of safe operating temperature,process control, and quality
23、 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, with a single apparatus, of ma
24、terialshaving a wide range of thermal diffusivity values over a largetemperature range.5.3 Under 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 (seeAppendix X4).5.4 Thermal diffusivi
25、ty results, together with specific heatcapacity (Cp) and density (r) values, can be used in many casesto derive thermal conductivity (l), according to the relation-ship:l5aCpr. (1)6. Interferences6.1 In principle, the thermal diffusivity is obtained from thethickness of the sample and from a charact
26、eristic time functiondescribing the propagation of heat from the front 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
27、 data analysis and morespecifically the finite pulse time effect, the nonuniform heatingof the sample and the radiative heat losses. These sources ofuncertainty can be considered systematic, and should becarefully considered for each experiment. Errors random innature (noise, for example) can be bes
28、t estimated by perform-ing a large number of repeat experiments and comparing theirresults. The relative standard deviation of the obtained resultsis a good representation of the random component of theuncertainty associated with the measurement. Guidelines inperforming a rigorous evaluation of thes
29、e factors are given in(31).7. ApparatusThe essential components of the apparatus are shown in Fig.2. These are the flash source, sample holder, environmentalenclosure (optional), temperature response detector and record-ing device.7.1 The flash source may be a pulse laser, a flash lamp, orother devi
30、ce capable to generate a short duration pulse ofsubstantial energy. The duration of the energy flash should beless than 2 % of the time required for the rear face temperaturerise to reach one half of its maximum value (see Fig. 1).7.1.1 The pulse hitting the samples surface must be spa-tially unifor
31、m in intensity. Most pulse lasers exhibit hot spotsand a substantially higher intensity in the center region of thebeam than in the periphery. For this reason, systems usingunmodified beams directly from a pulse laser should use beamssomewhat larger in diameter than the largest diameter of thesample
32、s to be tested. The use of an optical fiber between thelaser and the sample improves substantially uniformity of thebeam (up to 95 %). Since this method produces almost no edgeeffects, a larger portion of the energy can be directed to thesample than for natural beam lasers.7.1.2 Most commonly used l
33、asers are: ruby (visible red),4The boldface numbers given in parentheses refer to a list of references at theend of the text.FIG. 1 Characteristic Thermogram for the Flash MethodE 14612Nd: glass, and Nd: YAG (near infrared); however, other typesof lasers may be used. In some instances properly engin
34、eeredXenon 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-surements above an
35、d below room temperature. This chambermust be gas or vacuum tight if operation in 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 facetemperature rise is used. In
36、 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 temperature constan
37、t within 4 % 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 sample or several sampl
38、es ata time, with the latter providing substantial improvements indata and testing speed.7.4 The detector can be a thermocouple (see Appendix X2),infrared detector, optical pyrometer, or any other means thatcan provide a linear electrical output proportional to a smalltemperature rise. It shall be c
39、apable of detecting 0.05 K changeabove the samples initial temperature. The detector and itsassociated amplifier must have a response time substantiallysmaller than 2 % of the half time value. When intrinsicthermocouples are used, the same response requirements shallapply. Electronic filters, if use
40、d, 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 sample. It alsomust be protected from the energy beam, to prevent damage orsaturation. When the s
41、pecimen is housed in a furnace, theenergy beam may bounce or shine past the edges and enter thedetector. To avoid this, proper shielding is necessary. Forprotection against lasers, dielectric spike filters that are opaqueat the selected wavelength are very useful. The viewingwindow and any focusing
42、lenses must not absorb appreciablythe radiation in the wavelength region of the detector. This isparticularly important for infrared detectors, and means shouldbe provided to ensure that during high temperature measure-ments all window surfaces are monitored and kept free ofdeposits, which might lea
43、d to absorption of energy. Suchbuild-ups can lead to loss of signal intensity and may causenon-uniform specimen heating from the energy source.7.5 The signal conditioner includes the electronic circuit tobias out the ambient temperature reading, spike filters, ampli-fiers and analog-to-digital conve
44、rters.7.6 Data Recording7.6.1 The data collection system must be of adequate speedto 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 must contain in
45、formation 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 discha
46、rge 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 each pulse. This also takesinto account the varying energy of
47、 each pulse whether con-trolled or uncontrolled.7.6.3 It is desirable to employ a data recording system thatis capable of preprogrammed multiple speed recording withina single time period. This enables high-resolution (fast) record-ing prior to and during the rising portion of the thermogram,and low
48、er resolution (slow) recording of the prolonged cool-down of the sample. (The cool-down portion of the thermo-gram is used for heat loss corrections see later sections.)7.6.4 In case the recording device does not have accuratebuilt-in training (such as for digital systems), the timingaccuracy must b
49、e verified periodically to ensure that the half ofthe maximum rise time is measured within 2 % for the fastestexpected signal.7.7 It is practical to incorporate an alignment device such asa He-Ne laser or a laser diode into the system, to aid withverifying proper positioning of the sample. The alignmentbeam must be at all times co-linear with the energy pulse pathwithin 1 %.7.8 An aperture must be provided in close proximity of thesample, to ensure that no portion of the energy beam will shineFIG. 2 Block Diagram of a Flash SystemE 14613by the sample. I
