ASTM E2585-09(2015)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E2585 − 09 (Reapproved 2015)
Standard Practice for
Thermal Diffusivity by the Flash Method
This standard is issued under the fixed designation E2585; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope required. It is advisable to use only reference materials to
verify the performance of the instrument used.
1.1 This practice covers practical details associated with the
determination of the thermal diffusivity of primarily homoge-
1.7 This method is applicable only for homogeneous solid
neous isotropic solid materials. Thermal diffusivity values
materials, in the strictest sense; however, in some cases it has
-7 -3 2
ranging from 10 to 10 m /s are readily measurable by this
been shown to produce data found to be useful in certain
from about 75 to 2800 K.
applications:
1.2 This practice is adjunct to Test Method E1461. 1.7.1 Testing of Composite Materials—When substantial
non-homogeneity and anisotropy is present in a material, the
1.3 This practice is applicable to the measurements per-
thermal diffusivity data obtained with this method may be
formed on materials opaque to the spectrum of the energy
substantially in error. Nevertheless, such data, while usually
pulse, but with special precautions can be used on fully or
lacking absolute accuracy, may be useful in comparing mate-
partially transparent materials.
rials of similar structure. Extreme caution must be exercised
1.4 This practice is intended to allow a wide variety of
when related properties, such as thermal conductivity, are
apparatus designs. It is not practical in a document of this type
derived, as composite materials, for example, may have heat
to establish details of construction and procedures to cover all
flow patterns substantially different than uniaxial. In cases
contingencies that might offer difficulties to a person without
where the particle size of the composite phases is small
pertinent technical knowledge, or to stop or restrict research
compared to the specimen thickness (on the order of 1 to 25 %
and development for improvements in the basic technique.
of thickness) and where the transient thermal response of the
This practice provides guidelines for the construction
specimen appears homogenous when compared to the model,
principles, preferred embodiments and operating parameters
this method can produce accurate results for composite mate-
for this type of instruments.
rials. Anisotropic materials can be measured by various
1.5 This practice is applicable to the measurements per-
techniques, as long as the directional thermal diffusivities (two
formed on essentially fully dense materials; however, in some
dimensional or three dimensional) are mutually orthogonal and
cases it has shown to produce acceptable results when used
the measurement and specimen preparation produce heat flow
with porous specimens. Since the magnitude of porosity, pore
only along one principle direction. Also, 2D and 3D models
shapes, and parameters of pore distribution influence the
and either independent measurements in one or two directions,
behavior of the thermal diffusivity, extreme caution must be
or simultaneous measurements of temperature response at
exercised when analyzing data. Special caution is advised
different locations on the surface of the specimen, can be
when other properties, such as thermal conductivity, are
utilized.
derived from thermal diffusivity obtained by this method.
1.7.2 Testing Liquids—This method has found an especially
1.6 The flash can be considered an absolute (or primary) useful application in determining thermal diffusivity of molten
method of measurement, since no reference materials are
materials. For this technique, specially constructed specimen
enclosures must be used.
1.7.3 Testing Layered Materials—This method has also
This practice is under the jurisdiction of ASTM Committee E37 on Thermal
Measurements and is the direct responsibility of Subcommittee E37.05 on Thermo-
been extended to test certain layered structures made of
physical Properties.
dissimilar materials, where the thermal properties of one of the
Current edition approved Sept. 1, 2015. Published September 2015. Originally
layers are considered unknown. In some cases, contact con-
approved in 2009. Last previous edition approved in 2009 as E2585 – 09. DOI:
10.1520/E2585-09R15. ductance of the interface may also be determined.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2585 − 09 (2015)
1.8 The values stated in SI units are to be regarded as 3.3 Description of Subscripts Specific to This Standard:
standard. No other units of measurement are included in this 3.3.1 C—Cowan.
standard.
3.3.2 m—maximum.
1.9 This standard does not purport to address all of the
3.3.3 o—ambient.
safety concerns, if any, associated with its use. It is the
3.3.4 R—ratio.
responsibility of the user of this standard to establish appro-
3.3.5 s—specimen.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. 3.3.6 t—time.
3.3.7 T—thermocouple.
2. Referenced Documents
3.3.8 x—percent rise.
2.1 ASTM Standards:
E228 Test Method for Linear Thermal Expansion of Solid
4. Summary of Practice
Materials With a Push-Rod Dilatometer
4.1 A small, thin disc specimen is subjected to a high-
E1461 Test Method for Thermal Diffusivity by the Flash
intensity short duration radiant energy pulse (Fig. 1). The
Method
energy of the pulse is absorbed on the front surface of the
3. Terminology
specimen and the resulting rear face temperature rise (thermo-
gram) is recorded. The thermal diffusivity value is calculated
3.1 Definitions of Terms Specific to This Standard:
from the specimen thickness and the time required for the rear
3.1.1 thermal conductivity, λ, of a solid material—the time
face temperature rise to reach certain percentages of its
rate of steady heat flow through unit thickness of an infinite
maximum value. When the thermal diffusivity of the sample is
slab of a homogeneous material in a direction perpendicular to
to be determined over a temperature range, the measurement
the surface, induced by unit temperature difference. The
must be repeated at each temperature of interest. This is
property must be identified with a specific mean temperature,
described in detail in a number of publications (1, 2) and
since it varies with temperature.
review articles (3, 4, 5).Asummary of the theory can be found
3.1.2 thermal diffusivity,α, of a solid material—theproperty
in Test Method E1461, Appendix 1.
givenbythethermalconductivitydividedbytheproductofthe
density and heat capacity per unit mass.
5. Significance and Use
3.2 Description of Symbols and Units Specific to This
5.1 Thermal diffusivity is an important property, required
Standard:
for such purposes under transient heat flow conditions, such as
3.2.1 C —specific heat capacity, J/(kg·K).
p
design applications, determination of safe operating
3.2.2 D—diameter, metres.
temperature, process control, and quality assurance.
3.2.3 k—constant depending on percent rise.
3.2.4 K—correction factors.
3.2.5 K ,K —constants depending on β.
1 2
The boldface numbers given in parentheses refer to a list of references at the
3.2.6 L—specimen thickness, m.
end of the text.
3.2.7 t—response time, s.
3.2.8 t ⁄2 —half-rise time or time required for the rear face
temperature rise to reach one half of its maximum value, s.
3.2.9 t*—dimensionless time (t*=4α t/D ).
s T
3.2.10 T—temperature, K.
3.2.11 α—thermal diffusivity, m /s.
3.2.12 λ—thermal conductivity, (W/m·K).
3.2.13 β—fraction of pulse duration required to reach maxi-
mum intensity.
3.2.14 ρ—density, kg/m .
1 1
3.2.15 ∆t —T(5t ⁄2)/T(t ⁄2 ).
1 1
3.2.16 ∆t —T(10t ⁄2)/T(t ⁄2 ).
3.2.17 ∆T —temperature difference between baseline and
max
maximum rise, K.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. FIG. 1 Block Diagram of a Flash System
E2585 − 09 (2015)
5.2 The flash method is used to measure values of thermal 7.2.2 Most commonly used lasers are: ruby (visible red),
diffusivity, α, of a wide range of solid materials. It is particu- Nd: glass, and Nd: YAG (near infrared); however, other types
larly advantageous because of simple specimen geometry, of lasers may be used. In some instances, properly engineered
small specimen size requirements, rapidity of measurement Xenon flash sources can provide comparable performance for
and ease of handling.
all but the shortest rise times. Xenon flash sources, when
properly focused, provide a lower cost and lower maintenance
5.3 Under certain strict conditions, specific heat capacity of
alternative to lasers for many applications.
a homogeneous isotropic opaque solid sample can be deter-
mined when the method is used in a quantitative fashion (see
7.3 An environmental control chamber is required for mea-
Test Method E1461, Appendix 1).
surements above and below room temperature. This chamber
must be gas or vacuum tight if operation in a protective
5.4 Thermal diffusivity results, together with related values
atmosphere is desired. The enclosure shall be fitted with a
of specific heat capacity (C ) and density (ρ) values, can be
p
window, which has to be transparent to the flash source. A
used in many cases to derive thermal conductivity (λ), accord-
second window is required if optical detection of the rear face
ing to the relationship:
temperature rise is used. In such cases it is recommended that
λ 5α C ρ (1)
p
the optical detector be shielded from direct exposure to the
6. Interferences energy beam with the use of appropriate filter(s).
6.1 In principle, the thermal diffusivity is obtained from the
7.4 The furnace or cryostat should be loosely coupled
thickness of the sample and from a characteristic time function
(thermally) to the specimen support and shall be capable of
describing the propagation of heat from the front surface of the
maintaining the specimen temperature constant within4%of
sample to its back surface. The sources of uncertainties in the
the maximum temperature rise over a time period equal to five
measurement are associated with the sample itself, the tem-
halves of the maximum rise time. The furnace may be
perature measurements, the performance of the detector and of
horizontal or vertical. The specimen support shall also be
the data acquisition system, the data analysis and more
loosely coupled thermally to the specimen. Specimen supports
specifically the finite pulse time effect, the nonuniform heating
may be constructed to house one specimen or several at a time,
of the specimen and the heat losses (radiative and conductive).
with the latter providing substantial improvements in data and
Thesesourcesofuncertaintycanbeconsideredsystematic,and
testing speed.
should be carefully considered for each experiment. Errors
7.5 The detector can be a thermocouple (see Appendix X1),
random in nature (noise, for example) can be best estimated by
infrared detector, optical pyrometer, or any other means that
performing a large number of repeat experiments. The relative
can provide a linear electrical output proportional to a small
standard deviation of the obtained results is a good represen-
temperature rise. It shall be capable of detecting 0.05 K change
tation of the random component of the uncertainty associated
above the specimen’s initial temperature. The detector and its
with the measurement. Guidelines for performing a rigorous
associated amplifier must have a response time substantially
evaluation of these factors are given in (6).
smaller than2%ofthe half-rise time value. When intrinsic
7. Apparatus
thermocouples are used, the same response requirements shall
apply. Electronic filters, if used, shall be verified not to distort
7.1 The essential components of the apparatus are shown in
the shape of the thermogram. Several precautions are required
Fig. 1. These are the flash source, specimen holder, environ-
when using optical temperature sensing. The sensor must be
mental enclosure (optional), temperature response detector and
focused on the center of the back surface of the specimen. It
recording device.
also must be protected from the energy beam, to prevent
7.2 The flash source may be a pulse laser, a flash lamp, or
damage or saturation. When the specimen is housed in a
other device capable to generate a short duration pulse of
furnace, the energy beam may bounce or shine past the edges
substantial energy. The duration of the pulse should be less
and enter the detector. To avoid this, proper shielding is
than 2 % of the time required for the rear face temperature rise
necessary. For protection against lasers, dielectric spike filters
to reach one half of its maximum value, to keep the error due
that are opaque at the selected wavelength are very useful.The
to finite pulse width less than 0.5 %, if pulse width correction
viewing window and any focusing lenses must not absorb
(7, 8, 9) is not applied.
appreciably the radiation in the wavelength region of the
7.2.1 The pulse hitting the specimen’s surface must be
detector. This is particularly important for infrared detectors,
spatially uniform in intensity. Most pulse lasers exhibit hot
and means should be provided to ensure that during high
spotsandasubstantiallyhigherintensityinthecenterregionof
temperature measurements all window surfaces are monitored
the beam than in the periphery. For this reason, systems using
and kept free of deposits, which might lead to absorption of
unmodifiedbeamsdirectlyfromapulselasershouldusebeams
energy. Such build-ups can lead to loss of signal intensity and
somewhat larger in diameter than the largest diameter of the
may cause non-uniform specimen heating from the energy
specimens to be tested. The use of an optical fiber between the
source.
laserandthespecimenimprovessubstantiallytheuniformityof
the beam (up to 95 %). Since this method produces almost no 7.6 The signal conditioner includes the electronic circuit to
edge effects, a larger portion of the energy can be directed to bias out the ambient temperature reading, spike filters, ampli-
the specimen than from natural beam lasers. fiers and analog-to-digital converters.
E2585 − 09 (2015)
7.7 Data Recording: tem
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