ASTM E2585-09(2022)

Standard Practice for Thermal Diffusivity by the Flash Method

SIGNIFICANCE AND USE
5.1 Thermal diffusivity is an important property, required for such purposes under transient heat flow conditions, such as design applications, determination of safe operating temperature, process control, and quality assurance.  
5.2 The flash method is used to measure values of thermal diffusivity, α, of a wide range of solid materials. It is particularly advantageous because of simple specimen geometry, small specimen size requirements, rapidity of measurement and ease of handling.  
5.3 Under certain strict conditions, specific heat capacity of a homogeneous isotropic opaque solid sample can be determined when the method is used in a quantitative fashion (see Test Method E1461, Appendix 1).  
5.4 Thermal diffusivity results, together with related values of specific heat capacity (Cp) and density (ρ) values, can be used in many cases to derive thermal conductivity (λ), according to the relationship:
SCOPE
1.1 This practice covers practical details associated with the determination of the thermal diffusivity of primarily homogeneous isotropic solid materials. Thermal diffusivity values ranging from 10–7 to 10-3 m2/s are readily measurable by this from about 75 K to 2800 K.  
1.2 This practice is adjunct to Test Method E1461.  
1.3 This practice is applicable to the measurements performed on materials opaque to the spectrum of the energy pulse, but with special precautions can be used on fully or partially transparent materials.  
1.4 This practice is intended to allow a wide variety of apparatus designs. It is not practical in a document of this type to establish details of construction and procedures to cover all contingencies that might offer difficulties to a person without pertinent technical knowledge, or to stop or restrict research and development for improvements in the basic technique. This practice provides guidelines for the construction principles, preferred embodiments and operating parameters for this type of instruments.  
1.5 This practice is applicable to the measurements performed on essentially fully dense materials; however, in some cases it has shown to produce acceptable results when used with porous specimens. Since the magnitude of porosity, pore shapes, and parameters of pore distribution influence the behavior of the thermal diffusivity, extreme caution must be exercised when analyzing data. Special caution is advised when other properties, such as thermal conductivity, are derived 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 are required. It is advisable to use only reference materials to verify the performance of the instrument used.  
1.7 This method is applicable only for homogeneous solid materials, in the strictest sense; however, in some cases it has been shown to produce data found to be useful in certain applications:  
1.7.1 Testing of Composite Materials—When substantial non-homogeneity and anisotropy is present in a material, the thermal diffusivity data obtained with this method may be substantially in error. Nevertheless, such data, while usually lacking absolute accuracy, may be useful in comparing materials of similar structure. Extreme caution must be exercised when related properties, such as thermal conductivity, are derived, as composite materials, for example, may have heat flow patterns substantially different than uniaxial. In cases where the particle size of the composite phases is small compared to the specimen thickness (on the order of 1 to 25 % of thickness) and where the transient thermal response of the specimen appears homogenous when compared to the model, this method can produce accurate results for composite materials. Anisotropic materials can be measured by various techniques, as long as the directional thermal diffusivities (two dimensional or three dimensional) are mutually orthogonal and the measureme...

General Information

• Status: Published
• Publication Date: 30-Jun-2022
• ICS: 17.200.10 - Heat. Calorimetry
• Technical Committee: E37 - Thermal Measurements
• Drafting Committee: E37.05 - Thermophysical Properties

Relations

• Refers: ASTM E228-11(2016) - Standard Test Method for Linear Thermal Expansion of Solid Materials With a Push-Rod Dilatometer
• Effective Date: 01-Sep-2016
• Refers: ASTM E1461-11 - Standard Test Method for Thermal Diffusivity by the Flash Method
• Effective Date: 01-Dec-2011
• Refers: ASTM E1461-07 - Standard Test Method for Thermal Diffusivity by the Flash Method
• Effective Date: 01-Nov-2007
• Refers: ASTM E228-06 - Standard Test Method for Linear Thermal Expansion of Solid Materials With a Push-Rod Dilatometer
• Effective Date: 01-Sep-2006
• Refers: ASTM E1461-01 - Standard Test Method for Thermal Diffusivity of Solids by the Flash Method
• Effective Date: 10-Feb-2001
• Refers: ASTM E1461-92 - Standard Test Method for Thermal Diffusivity of Solids by the Flash Method
• Effective Date: 10-Feb-2001
• Refers: ASTM E228-95 - Standard Test Method for Linear Thermal Expansion of Solid Materials With a Vitreous Silica Dilatometer (Withdrawn 2005)
• Effective Date: 01-Jan-1995

Overview

ASTM E2585-09(2022) - Standard Practice for Thermal Diffusivity by the Flash Method provides a comprehensive and practical framework for determining the thermal diffusivity of homogeneous, isotropic solid materials. Developed by ASTM International, this standard is crucial for industries and laboratories where precise thermal property measurements are required under transient heat flow conditions. The flash method described enables rapid, accurate measurements with minimal sample requirements, playing a significant role in design, quality assurance, process control, and the assessment of safe operating temperatures.

Key Topics

• Thermal Diffusivity Measurement:
  The standard outlines procedures for determining thermal diffusivity (α) of solid materials, typically within the range of 10⁻⁷ to 10⁻³ m²/s and temperatures from 75 K up to 2800 K.
• Flash Method Advantages:
  • Simple specimen geometry and small sample size
  • Fast measurements with easy handling
  • Minimal edge effects and broad applicability to various apparatus designs
• Material Applicability:
  • Primarily for homogeneous, isotropic solids
  • Can be extended, with precautions, to composite materials, anisotropic samples, porous specimens, and even liquids or layered materials
• Derivation of Thermal Conductivity:
  Thermal diffusivity data, combined with specific heat capacity (Cp) and density (ρ), enables calculation of thermal conductivity (λ) through the relationship λ = αCpρ.
• Calibration and Verification:
  Flash method is considered absolute-no reference materials required, though periodic verification is advised for instrument performance.
• Reporting and Quality:
  Guidance is provided for specimen preparation, apparatus setup, measurement protocol, and uncertainty analysis to ensure reliable, repeatable results.

Applications

• Transient Heat Flow Design:
  Engineers use thermal diffusivity data for modeling and designing equipment or structures exposed to rapid temperature changes, ensuring safety and performance.
• Quality Assurance and Process Control:
  Manufacturers monitor the thermal properties of raw materials and finished products to maintain consistent quality and meet industry specifications.
• Material Research and Development:
  Researchers rely on the flash method to evaluate new materials, composites, and coatings, optimizing performance for applications such as aerospace, automotive, and electronics.
• Comparison of Similar Materials:
  Even when absolute accuracy is reduced (e.g., for composites or anisotropic materials), the method remains valuable for comparative studies and material selection.
• High and Low Temperature Testing:
  The procedure supports measurement across a vast temperature range, making it suitable for everything from cryogenics to high-temperature ceramic and metal testing.

Related Standards

• ASTM E1461:
  Test Method for Thermal Diffusivity by the Flash Method - the primary procedural document for the flash technique referenced throughout ASTM E2585.
• ASTM E228:
  Test Method for Linear Thermal Expansion of Solid Materials - important for calculating density variations with temperature.
• ASTM E2585:
  Supplements ASTM E1461 with broader practical guidance, especially for non-ideal, inhomogeneous, or complex specimens.

Practical Value

Adopting ASTM E2585 improves thermal property measurement consistency and reliability, supporting product safety, process optimization, and accelerated material innovation. Its flexibility addresses real-world challenges, from specimen preparation to apparatus design, making it widely recognized and applicable across many scientific and industrial sectors.

Keywords: ASTM E2585, thermal diffusivity, flash method, transient heat flow, thermal conductivity, quality assurance, material testing, specific heat capacity, homogeneous solids, ASTM standards.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2585 − 09 (Reapproved 2022)
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 K 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 July 1, 2022. Published July 2022. Originally approved
layers are considered unknown. In some cases, contact con-
in 2009. Last previous edition approved in 2015 as E2585 – 09 (2015). DOI:
10.1520/E2585-09R22. 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 (2022)
1.8 The values stated in SI units are to be regarded as 3.2.14 ρ—density, kg/m .
standard. No other units of measurement are included in this
1 1
3.2.15 ∆t —T(5t ⁄2)/T(t ⁄2 ).
standard.
1 1
3.2.16 ∆t —T(10t ⁄2)/T(t ⁄2 ).
1.9 This standard does not purport to address all of the
3.2.17 ∆T —temperature difference between baseline and
max
safety concerns, if any, associated with its use. It is the
maximum rise, K.
responsibility of the user of this standard to establish appro-
3.3 Description of Subscripts Specific to This Standard:
priate safety, health, and environmental practices and deter-
3.3.1 C—Cowan.
mine the applicability of regulatory limitations prior to use.
1.10 This international standard was developed in accor-
3.3.2 m—maximum.
dance with internationally recognized principles on standard-
3.3.3 o—ambient.
ization established in the Decision on Principles for the
3.3.4 R—ratio.
Development of International Standards, Guides and Recom-
3.3.5 s—specimen.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
3.3.6 t—time.
3.3.7 T—thermocouple.
2. Referenced Documents
2 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
Method intensity short duration radiant energy pulse (Fig. 1). The
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.
3.2 Description of Symbols and Units Specific to This
Standard:
3.2.1 C —specific heat capacity, J/(kg·K).
The boldface numbers given in parentheses refer to a list of references at the
p
end of the text.
3.2.2 D—diameter, metres.
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
3.2.6 L—specimen thickness, m.
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.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. 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 (2022)
5. Significance and Use specimens to be tested. The use of an optical fiber between the
laserandthespecimenimprovessubstantiallytheuniformityof
5.1 Thermal diffusivity is an important property, required
the beam (up to 95 %). Since this method produces almost no
for such purposes under transient heat flow conditions, such as
edge effects, a larger portion of the energy can be directed to
design applications, determination of safe operating
the specimen than from natural beam lasers.
temperature, process control, and quality assurance.
7.2.2 Most commonly used lasers are: ruby (visible red),
5.2 The flash method is used to measure values of thermal
Nd: glass, and Nd: YAG (near infrared); however, other types
diffusivity, α, of a wide range of solid materials. It is particu-
of lasers may be used. In some instances, properly engineered
larly advantageous because of simple specimen geometry,
Xenon flash sources can provide comparable performance for
small specimen size requirements, rapidity of measurement
all but the shortest rise times. Xenon flash sources, when
and ease of handling.
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-
7.3 An environmental control chamber is required for mea-
mined when the method is used in a quantitative fashion (see
surements above and below room temperature. This chamber
Test Method E1461, Appendix 1).
must be gas or vacuum tight if operation in a protective
atmosphere is desired. The enclosure shall be fitted with a
5.4 Thermal diffusivity results, together with related values
window, which has to be transparent to the flash source. A
of specific heat capacity (C ) and density (ρ) values, can be
p
second window is required if optical detection of the rear face
used in many cases to derive thermal conductivity (λ), accord-
temperature rise is used. In such cases it is recommended that
ing to the relationship:
the optical detector be shielded from direct exposure to the
λ 5α C ρ (1)
p
energy beam with the use of appropriate filter(s).
6. Interferences
7.4 The furnace or cryostat should be loosely coupled
(thermally) to the specimen support and shall be capable of
6.1 In principle, the thermal diffusivity is obtained from the
thickness of the sample and from a characteristic time function maintaining the specimen temperature constant within4%of
the maximum temperature rise over a time period equal to five
describing the propagation of heat from the front surface of the
sample to its back surface. The sources of uncertainties in the halves of the maximum rise time. The furnace may be
measurement are associated with the sample itself, the tem- horizontal or vertical. The specimen support shall also be
perature measurements, the performance of the detector and of loosely coupled thermally to the specimen. Specimen supports
the data acquisition system, the data analysis and more may be constructed to house one specimen or several at a time,
specifically the finite pulse time effect, the nonuniform heating with the latter providing substantial improvements in data and
of the specimen and the heat losses (radiative and conductive). testing speed.
Thesesourcesofuncertaintycanbeconsideredsystematic,and
7.5 The detector can be a thermocouple (see Appendix X1),
should be carefully considered for each experiment. Errors
infrared detector, optical pyrometer, or any other means that
random in nature (noise, for example) can be best estimated by
can provide a linear electrical output proportional to a small
performing a large number of repeat experiments. The relative
temperature rise. It shall be capable of detecting 0.05 K change
standard deviation of the obtained results is a good represen-
above the specimen’s initial temperature. The detector and its
tation of the random component of the uncertainty associated
associated amplifier must have a response time substantially
with the measurement. Guidelines for performing a rigorous
smaller than2%ofthe half-rise time value. When intrinsic
evaluation of these factors are given in (6).
thermocouples are used, the same response requirements shall
apply. Electronic filters, if used, shall be verified not to distort
7. Apparatus
the shape of the thermogram. Several precautions are required
7.1 The essential components of the apparatus are shown in
when using optical temperature sensing. The sensor must be
Fig. 1. These are the flash source, specimen holder, environ-
focused on the center of the back surface of the specimen. It
mental enclosure (optional), temperature response detector and
also must be protected from the energy beam, to prevent
recording device.
damage or saturation. When the specimen is housed in a
7.2 The flash source may be a pulse laser, a flash lamp, or furnace, the energy beam may bounce or shine past the edges
other device capable to generate a short duration pulse of and enter the detector. To avoid this, proper shielding is
substantial energy. The duration of the pulse should be less necessary. For protection against lasers, dielectric spike filters
than 2 % of the time required for the rear face temperature rise that are opaque at the selected wavelength are very useful.The
to reach one half of its maximum value, to keep the error due viewing window and any focusing lenses must not absorb
to finite pulse width less than 0.5 %, if pulse width correction appreciably the radiation in the wavelength region of the
(7, 8, 9) is not applied. detector. This is particularly important for infrared detectors,
7.2.1 The pulse hitting the specimen’s surface must be and means should be provided to ensure that during high
spatially uniform in intensity. Most pulse lasers exhibit hot temperature measurements all window surfaces are monitored
spotsandasubstantiallyhigherintensityinthecenterregionof and kept free of deposits, which might lead to absorption of
the beam than in the periphery. For this reason, systems using energy. Such build-ups can lead to loss of signal intensity and
unmodifiedbeamsdirectlyfromapulselasershouldusebeams may cause non-uniform specimen heating from the energy
somewhat larger in diameter than the largest diameter of the source.
E2585 − 09 (2022)
7.6 The signal conditioner includes the electronic circuit to 7.11 The temperature controller and/or programmer are to
bias out the ambient temperature reading, spike filters, ampli- bring the specimen to the temperatures of interest. While it is
fiers and analog-to-digital converters. desirable to perform the measurements at exact temperatures,
in most cases it is not necessary to exactly settle at those
7.7 Data Recording:
temperatures when the testing program covers a temperature
7.7.1 The data acquisition system must be of an adequate
range. It is uneconomical time-wise to try to reach an exact
speed to ensure that time resolution in determining half of the
temperature when the thermal diffusivity is expected to behave
maximum temperature rise on the thermogram is at least 1 %,
monotonically in the range. In cases when the specimen is
for the fastest thermogram for which the system is qualified.
expected to undergo internal transformations during the test,
7.7.2 The recorded signal must contain information that
the temperatures of interest must be closely observed.
enables the precise definition of the starting time of the energy
8. Test Specimen
pulse.
8.1 The usual specimen is a thin circular disc with a front
7.7.2.1 If no other means are available, the inevitable spike
surface area less than that of the energy beam. Typically,
caused by the trigger pulse (for a laser of flash lamp) may be
specimens are 10 to 12.5 mm in diameter, however, there is no
used. This, however, is considered marginal, as it uses the
fundamental limitation for using smaller or larger specimens.
beginning of the capacitor discharge as “time zero.”
From a practical standard point, 12.5 mm was found to be
7.7.2.2 More accurate results are obtained if the center of
ideal.
gravity for the energy pulse is used as “time zero.”This can be
8.1.1 Specimens that are very small tend to provide small
determined only with actual recording of the pulse shape and
amounts of energy from the rear face, especially at low
derivation of the point of start for each pulse. This also takes
(<400°C) temperatures. For systems that have an appreciable
into account the varying energy of each pulse whether con-
distance from the specimen to the detector, such as most high
trolled or uncontrolled.
temperature systems, this is a serious problem that should be
7.7.3 It is desirable to employ a data recording system that
avoided simply by using 10-mm diameter or larger specimens.
is capable of preprogrammed multiple speed recording within
Under all circumstances, one must not expect the same
asingletimeperiod.Thisenableshigh-resolution(fast)record-
performance for sub-size specimens, under all conditions.
ing prior to and during the rising portion of the thermogram,
Larger specimens on the other hand, may suffer from insuffi-
and lower resolution (slow) recording of the prolonged cool-
cient energy density, and produce more widely scattered data.
down of the sample. (The cool-down portion of the thermo-
8.1.2 The optimum thickness depends upon the magnitude
gram is used for heat loss corrections — see Test Method
of the estimated thermal diffusivity, and should be chosen so
E1461.)
that the time to reach half of the maximum temperature falls
7.7.4 In case the recording device does not have accurate
within the 10 to 1000 ms range.Thinner specimens are desired
built-in training (such as for digital systems), the timing
at higher temperatures to minimize heat loss corrections;
accuracy must be verified periodically to ensure that the
however, specimens should always be thick enough to be
half-rise time is measured within 2 % for the fastest expected
representative of the test material. Typically, thicknesses are in
signal.
the 1 to 6-mm range.
7.8 It is practical to incorporate an alignment device such as 8.1.3 Since the thermal diffusivity is proportional to the
a He-Ne laser or a laser diode into the system, to aid with square of the thickness, it may be desirable to use different
verifying proper positioning of the specimen. The alignment thicknesses in different temperature ranges. In general, one
beam must be at all times co-linear with the energy pulse path thickness will be far from optimum for measurements at both
within 1 %. cryogenic and high temperatures.
8.2 Inappropriately selected specimen thickness will not
7.9 An aperture must be provided in close proximity of the
only cause unnecessary frustration for the experimenter, but
specimen, to ensure that no portion of the energy beam will
also can be a major source of error in the measurement. As a
shine by the specimen. It is desirable to keep this aperture’s
general guideline, one can start with 2 to 3-mm thick
diameter within approximately 95 % of the specimen diameter.
specimens, and later change them based on the information
Providing a too small aperture will cause uneven specimen
obtained from the thermogram. (An overly thick specimen can
heating and promote bi-axial heat-flow within the specimen.A
totally extinguish the signal.)
too large aperture will defeat the purpose. Systems with pin
type specimen suspensions are especially in need of accurate
8.3 Specimens must be prepared with faces flat and parallel
alignment and effective aperture size.
within 0.5 % of their thickness, in order to keep the error in
thermal diffusivity due to the measurement of average
7.10 Measurementofspecimentemperatureistobedoneby
thickness, to less than 1 %. Non-uniformity of either surface
accepted means, such as calibrated thermocouple, optical
(craters, scratches, markings) of significant depth compared to
pyrometer, platinum RTD, etc. whichever is appropriate for the
the specimen thickness should be avoided.
temperature range. In all cases, such a device must be in
intimate contact with or trained on the specimen holder, in 8.4 Proper surface preparation of specimens is imperative
close proximity of the specimen. Touching the specimen with for obtaining reliable results.
thermocouples is not recommended. Embedding thermo- 8.4.1 Shiny surfaces, in large part, reflect light and, as a
couples into the specimen is not acceptable. consequence, only a small amount of the total pulse energy is
E2585 − 09 (2022)
absorbed.To combat this, it is customary to deposit a very thin Appendix 3).While most materials used are not true certified
layer of highly energy absorbing (high emissivity) coating on standards, they are generally accepted industry-wide with the
the surface. Graphite has been found to work well, and is best available literature data.
available in aerosol spray, paint, or colloidal suspension form.
9.2.1 It must be emphasized that the use of reference
Other materials, such as boron nitride powder, have also been
materialstoestablishvalidityofthedataonunknownmaterials
used.
has often led to unwarranted statements on accuracy. The use
of references is only valid when the properties of the reference
NOTE 1—Material compatibility between the specimen and the coating
(including half-rise times and thermal diffusivity values) are
must be investigated in all cases before a particular use, especially in high
temperature applications. For example, graphite coating will react with an closely similar to those of the unknown specimen, and the
iron specimen, making the coating disappear at elevated temperatures, as
temperature-rise curves are determined in an identical manner
well as potentially changing the composition of the specimen.
for the reference and unknown.
8.4.2 For transparent materials, it is customary to deposit a
9.2.2 One important check of the validity of data (in
metal film (gold, platinum, silver, etc.) on both faces of the
additiontothecomparisonoftherisecurvewiththetheoretical
specimen, to make it opaque. Highly reflective materials are
model), when corrections have been applied, is to vary the
favored so that only a minute amount of the absorbed energy
specimen thickness. Since the half-rise times vary as L ,
willbere-radiatedbytheotherfaceofthemetalfilmacrossthe
decreasing the specimen thickness by one-half should decrease
transparent medium, and the bulk of the energy will traverse it
the half-rise time to one-fourth of its original value. Thus, if
by heat conduction.
one obtains the same thermal diffusivity value with represen-
8.4.3 Since the highly reflective metal coating would not
tative specimens from the same material of significantly
allow full absorption of the energy pulse, it is necessary to coat
different thicknesses, the results can be assumed valid.
the specimen as in accordance with 8.4.1.
8.4.4 Conversely, since the shiny metal surface, due to its
10. Procedure
low emissivity, would produce a very weak optical target for
10.1 For commercially produced systems, follow manufac-
obtainingthethermogram,thebackfaceofthespecimenhasto
turer’s instructions.
be coated too as in accordance with 8.4.1.
8.4.5 In all cases, the combined effect of the coatings must
10.2 As a minimum, any system must ensure the following,
be a negligible fraction of the total signal for any specimen,
either by design or by adjustment procedure:
unless multi-layer analysis is applied.
10.2.1 Verification of specimen concentricity with energy
8.4.6 Light sandblasting of specimen surfaces greatly en-
beam when properly mounted in holder.
hances film adherence, and for some opaque reflective mate-
10.2.2 Verificationofapertureandenergybeamcoverageon
rials can provide sufficient pulse absorption and emissivity,
specimen.
especially at higher temperatures, where coatings may not be
10.2.3 Permanent alignment features for detector or means
stable or may react with the material.
to properly align detector on center of rear surface.
8.4.7 For specific heat capacity determinations, where two
10.2.4 Safety interlocks in case lasers are used, to prevent
different surfaces are present (unknown and reference), proper
the escape of laser beam directly or reflections thereof.
and completely identical surface preparation for both specimen
and reference is imperative. Since in this quantitative measure- 10.3 The testing procedure must contain the following
ment the energy absorbed is fully controlled by the emissivity functions:
of the surface, both surfaces must present identical properties
10.3.1 Determine and record the specimen thickness.
to the incoming energy pulse, to ensure a truly differential
10.3.2 Mount the specimen in its holder.
determination.
10.3.3 Establish vacuum or inert gas environment in the
8.4.8 Encapsulated specimens should not be used for spe-
chamber if necessary.
cific heat capacity tests, as the contribution of the capsule can
10.3.4 Determine specimen temperature, unless the system
not be mitigated via multi-layer calculations, and therefore the
will do it automatically.
direct data will be in substantial error.
10.3.5 Especially at low temperatures, use the lowest level
of power for the energy pulse able to generate a measurable
9. Calibration and Verification
temperature rise, in order to ensure that the detector functions
9.1 Calibrate the micrometer used to measure the specimen
within its linear range.
thickness, so that the thickness measurements are accurate to
10.3.6 After the pulse delivery, monitor the raw or pro-
within 0.2 %.
cessed thermogram to establish in-range performance. In case
ofmultiplespecimentesting,itisadvisable(fortimeeconomy)
9.2 The Flash Method is an absolute (primary) method by
to sequentially test specimens at the same temperature (includ-
itself, therefore it requires no calibration. However, actual
ing replicate tests) before proceeding to the next test tempera-
execution of the measurement itself is subject to random and
ture.
systematic errors. It is therefore important to verify the
performance of a device periodically, to establish the extent 10.3.7 Inallcases,thetemperatu
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