ISO 18755:2005
(Main)Fine ceramics (advanced ceramics, advanced technical ceramics) — Determination of thermal diffusivity of monolithic ceramics by laser flash method
Fine ceramics (advanced ceramics, advanced technical ceramics) — Determination of thermal diffusivity of monolithic ceramics by laser flash method
ISO 18755:2005 specifies the test method for the determination of thermal diffusivity from room temperature to 1700 K by the laser flash method for homogeneous monolithic ceramics with porosity less than 10 %.
Céramiques techniques — Détermination de la diffusivité thermique des céramiques monolithiques par la méthode flash laser
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 18755
First edition
2005-03-15
Fine ceramics (advanced ceramics,
advanced technical ceramics) —
Determination of thermal diffusivity of
monolithic ceramics by laser flash
method
Céramiques techniques — Détermination de la diffusivité thermique des
céramiques monolithiques par la méthode flash laser
Reference number
ISO 18755:2005(E)
©
ISO 2005
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ISO 18755:2005(E)
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ISO 18755:2005(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Apparatus. 3
4.1 Specimen holder . 4
4.2 Pulse laser . 4
4.3 Thermometer for measuring steady-state temperature of the specimen . 5
4.4 Detector for measuring transient temperature rise of rear face of the specimen. 5
4.5 Environment for measurements. 5
4.6 Temperature control unit. 5
4.7 Data acquisition unit. 5
5 Specimen . 5
5.1 Shape and dimension of specimens. 5
5.2 Coating on the specimen . 6
5.3 Reference specimen . 6
6 Measurement procedure . 6
6.1 Measurement of specimen thickness . 6
6.2 Surface treatment. 6
6.3 Determination of flash time of the laser pulse and the chronological profile of the laser
pulse. 6
6.4 Temperature and atmosphere control . 6
6.5 Stability of specimen temperature . 6
6.6 Energy of pulse heating . 7
6.7 Measurement temperature . 7
6.8 Record. 7
7 Data analysis . 7
7.1 Calculation based on the half-rise-time method. 7
7.2 Criteria for applicability of the half-rise-time method . 7
8 Measurement report. 10
Annex A (informative) Principle of laser flash thermal diffusivity measurements . 12
Annex B (informative) Correction for non-ideal initial and boundary conditions. 13
Annex C (informative) Data analysis algorithms to calculate thermal diffusivity from
observed temperature history curve under non-ideal initial and boundary conditions. 19
Annex D (informative) Other error factors . 22
Annex E (informative) Reference data and reference materials of thermal diffusivity. 28
Bibliography . 29
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ISO 18755:2005(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 18755 was prepared by Technical Committee ISO/TC 206, Fine ceramics.
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INTERNATIONAL STANDARD ISO 18755:2005(E)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Determination of thermal diffusivity of monolithic
ceramics by laser flash method
1 Scope
This International Standard specifies the test method for the determination of thermal diffusivity from room
temperature to 1700 K by the laser flash method for homogeneous monolithic ceramics with porosity less
than 10 %.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 3611, Micrometer callipers for external measurement
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
thermal diffusivity
thermal conductivity divided by the product of specific heat capacity and density
3.2
thermal conductivity
density of heat flow rate divided by temperature gradient under steady state condition
3.3
specific heat capacity
heat capacity per unit mass
3.4
pulse width
ττττ
p
full width of half maximum (FWHM) which is the time duration when the laser pulse intensity is larger than the
half of its maximum value on time basis
3.5
centroid of laser pulse
chronological centroid of laser light energy
3.6
spatial energy distribution of pulse heating
energy density of the laser beam incident at each point on the front face of the specimen
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ISO 18755:2005(E)
3.7
transient temperature curve
transient temperature change of the rear face of the specimen after the light pulse heating
3.8
transient radiance curve
transient change of the spectral radiance from the rear face of the specimen after the light pulse heating
NOTE It should be noted that the observed transient curve is proportional to the change of the spectral radiance
rather than the change of temperature when a radiation thermometer or a radiation detector is used to observe the
transient temperature rise of the specimen after the light pulse heating.
3.9
maximum temperature rise
∆∆∆∆T
max
difference between the steady temperature before the pulse heating and the maximum temperature of the rear
face of the specimen after the pulse heating
NOTE See Figure 1.
3.10
half rise-time
t
l/2
time until ∆T /2 is attained from the pulse heating
max
3.11
characteristic time of heat loss
ττττ
c
time of heat loss determined when the cooling region is fitted with an exponential function, ∆T exp( - t/τ )
0 C
NOTE See Figure 1.
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ISO 18755:2005(E)
Key
X: time
Y: temperature rise
1 exponential function ∆−Ttexp /τ
()
0 c
2 initial noise
Figure 1 —Transient temperature curve of the rear face of the specimen after a light pulse heating
onto the front face of the specimen
3.12
extrapolated temperature rise
∆∆T
∆∆
0
temperature rise determined when the cooling region is fitted with an exponential function, ∆T exp( - t/τ )
0 C
3.13
initial noise superimposed on transient temperature curve
initial spike and/or hump superimposed on the initial part of the transient temperature curve, due to
transmitted and/or scattered light from the heating laser pulse and/or electrically induced noise associated
with the laser pulse discharge
3.14
homogeneity of specimen
degree of homogeneity of local thermal diffusivity over the specimen
4 Apparatus
The apparatus shall be designed for obtaining the thermal diffusivity from the transient temperature curve of
the rear face of a specimen after the laser pulse is irradiated onto the front face of the specimen, and shall
consist of the following principal components as shown in Figure 2.
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ISO 18755:2005(E)
4.1 Specimen holder
The specimen holder shall hold the specimen stable, with minimum thermal contact, and shall be designed to
suppress stray lights from the laser beam being transmitted to the transient detector.
A diaphragm with aperture diameter slightly larger than the specimen diameter should be placed close to the
front face of the specimen, and another diaphragm with aperture diameter smaller than the specimen diameter
and larger than the target size of radiative detection should be placed close to the rear face of the specimen.
Key
1 pulsed laser 4 specimen holder
2 data analysis 5 power supply
3 detector 6 heater
a
Trigger signal.
b
Transient temperature response.
Figure 2 — Block diagram of laser flash apparatus for measuring thermal diffusivity
4.2 Pulse laser
The pulse laser shall be capable of emitting the light pulse with pulse duration preferably shorter than 1,0 ms
in full width at half maximum (FWHM). The specimen should be irradiated uniformly by the light pulse.
When a pulse laser is used for the light pulse, the direct beam profile is often irregular because of multi-mode
oscillation. In this case, the beam should be converted to a uniform beam using beam-homogenizing optics.
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ISO 18755:2005(E)
4.3 Thermometer for measuring steady-state temperature of the specimen
The steady-state temperature of the specimen before pulse heating should be measured by a thermocouple,
or an equally or more reliable thermometer.
The thermocouple shall be positioned such that it does not interrupt the light pulse heating onto the front face
of the specimen, or the radiation from the rear face of the specimen. If the specimen does not react with the
thermocouple, a thin thermocouple should be contacted with the specimen to measure the specimen
temperature with minimal uncertainty. If the thermocouple junction cannot be allowed to contact the specimen
because of chemical reaction with the specimen, or because it interrupts the setting of the specimen, or
because of the system design, the tip should be placed as close as practical to the specimen in the same
plane.
4.4 Detector for measuring transient temperature rise of rear face of the specimen
The transient temperature rise curve on the rear face of the specimen shall be observed with a non-contact
radiation thermometer or a radiation detector. The frequency response of the detector and its associated
electronics should be faster than 10 kHz. The target diameter of the radiation detector should be smaller than
50 % of the diameter for disk specimens, or 50 % of the shortest side-length for square and rectangular
specimens.
4.5 Environment for measurements
Measurements may be performed under open air, under an inert gas atmosphere, or under vacuum at room
temperature. For higher temperature measurements, an appropriate inert atmosphere or vacuum shall be
used, when necessary, to protect furnace parts and specimen holders from oxidation and to protect the
specimen and its coating from structure/phase changes and compatibility problems.
4.6 Temperature control unit
For higher temperature measurements, the specimen should be kept at a stable temperature by electric
heaters before pulse heating. Drift and fluctuation of the temperature should be less than 0,01 K/s.
4.7 Data acquisition unit
The transient detector signal should be amplified and converted to the digital signal using a digital
oscilloscope or an AD converter which is input to a personal computer for computation of the thermal
diffusivity. The frequency response of the amplifier and the AD conversion should be faster than 10 kHz. The
resolution of the AD conversion should be larger than 10 bits, more than 1000 data points should be sampled
with the sampling time faster than 1 % of the half rise-time “t ”.
1/2
5 Specimen
5.1 Shape and dimension of specimens
The specimen shall be a flat plate of circular, square or rectangular shape. The specimen diameter or side
shall be between 5 mm and 15 mm.
The specimen thickness shall be chosen to be as follows:
a) thicker than 0,5 mm and thinner than 5 mm;
b) sufficiently thick that the t value is larger than 5 times the pulse width.
1/2
The uniformity of the specimen thickness shall be smaller than 1,0 %.
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ISO 18755:2005(E)
5.2 Coating on the specimen
If the specimen does not have a high absorption coefficient for the heating laser beam or a high emissivity for
radiative temperature detection, the surfaces of the specimen shall be coated with a thin, opaque, preferably
black layer. The coating shall be dense enough to prevent penetration of the laser beam or thermal radiation
at the observed wavelength, and should be resistive against laser pulse heating at high temperatures. Coating
thickness should be a minimum commensurate with excluding directly transmitted laser pulse.
NOTE Suitable coatings for many ceramic materials include evaporated, sputtered carbon or sprayed colloidal
graphite. If the test specimen reacts with carbon at high temperatures, a metal coating, such as platinum, gold or nickel
may alternatively be used. The surface of the test specimen may, with advantage, be roughened to improve adhesion of
the coating. The coating thickness dependence must be evaluated for the observed thermal diffusivity, if the contribution of
coatings is not negligible.
5.3 Reference specimen
Reference specimens can be used to evaluate uncertainty of thermal diffusivity measurements by a laser flash
apparatus. The uncertainty is obtained as the difference between the measured value and the reference value
of thermal diffusivity of the reference specimen.
NOTE There are no certified reference materials for thermal diffusivity measurements authorized by national or
international organizations yet, although several materials are used as such (see Annex E).
Care should be taken in the use of these references to ensure that the half rise-time and the thermal diffusivity
value are similar to those of the test materials.
6 Measurement procedure
The specimen shall be measured under the following procedures.
6.1 Measurement of specimen thickness
Measure the thickness of the specimen to an accuracy of 0,5 % or better, using a micrometer in accordance
with ISO 3611.
6.2 Surface treatment
Carry out the surface treatment in accordance with 5.2.
6.3 Determination of flash time of the laser pulse and the chronological profile of the laser
pulse
The chronological trace of the laser pulse versus the same trigger signal to initiate laser flash thermal
diffusivity measurements shall be observed. If the FWHM of the laser pulse duration is larger than 1 % of the
half rise-time, correction for the finite pulse time shall be made following one of the procedures stated in
Annex B.
6.4 Temperature and atmosphere control
Insert the test specimen in the apparatus and position the thermocouples. The atmosphere should be such
that the specimen is not subjected to any chemical change under the measured temperature range.
6.5 Stability of specimen temperature
The specimen temperature shall be controlled with drift smaller than 0,01 K/s.
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ISO 18755:2005(E)
6.6 Energy of pulse heating
Irradiate the specimen with the laser pulse at an intensity of as low energy as possible, commensurate with an
acceptable noise level.
NOTE Refer to Annex D about nonlinearity of spectral radiance on temperature.
6.7 Measurement temperature
Record the measurement temperature as TT+∆ , where T is the initial steady-state temperature and
0max 0
∆T is the maximum temperature rise of the specimen recorded by the thermocouple in contact with the
max
specimen or the calibrated radiation thermometer.
NOTE A thermocouple below 0,15 mm in diameter, which is directly contacted to the rear or side surface of a
specimen mechanically or with a paste, is preferable to estimate ∆T .
max
6.8 Record
The transient temperature curve should be recorded for a duration at least until 10 times the half-rise-time, in
order to make reliable evaluation of measurements, including heat-loss correction and evaluation of
nonuniform heating effect.
7 Data analysis
7.1 Calculation based on the half-rise-time method
The standard algorithm to calculate thermal diffusivity from the laser flash method is the half-rise-time method,
in which the analytical equation is fitted to the transient temperature curve at t, the height of a half of maximum
temperature rise of the transient temperature or radiance response curve above the base line “ ∆T / 2 ” over
max
the half-rise-time.
If the measurement is valid when made under the above-mentioned ideal initial and boundary conditions, the
thermal diffusivity, α, is represented by the following equation based on the half-rise-time method:
2
0,1388 d
α = (1)�
t
1/ 2
where t is the time delay when the temperature of the rear face reaches one-half of the maximum
1/2
temperature rise, ∆T , after the front face was heated by the laser pulse.
max
7.2 Criteria for applicability of the half-rise-time method
In order that the rise-time can be validly applied, the following initial and boundary conditions must be
satisfied:
The duration of the laser pulse is short, compared with the characteristic time of heat diffusion
(FWHM < 1 % of t ).
1/2
The front face of the specimen is uniformly heated by the light pulse.
The specimen is adiabatic during the period of measurement after the light pulse heating.
The specimen is uniform (in geometry) and is homogeneous.
The specimen is opaque (nontransparent and nontranslucent) to the light pulse and to thermal radiation.
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ISO 18755:2005(E)
If these conditions are satisfied, the heat flow becomes one-dimensional and the temperature of the rear face
of the specimen changes according to an analytical equation (See Annex A).
The thermal diffusivity value shall be determined by fitting this equation to the observed transient temperature
curve. Theoretically, if the measurement is made under the above-mentioned ideal conditions, the calculated
thermal diffusivity value should be independent of the position along the transient curves. Therefore, any point
on the transient temperature curve can be analyzed to yield the thermal diffusivity, α. This will be given by
Equation (2) as follows;
2
K d
x
α = (2)
t
x
where
d is the specimen thickness, in metres;
t is the time for the specimen rear face to reach a fraction of the maximum temperature rise, in
x
seconds (see Table 1);
x is the percentage of the maximum rise in temperature;
K is a constant relating α to d and t in the case of ideal measurements.
,
x x
Calculate the thermal diffusivity at fractional temperature rises other than t . If the values at t , t and t
1/2 0,3 0,5 0,7
calculated using the relevant values of K in Table 1 are all within ± 2 % then it can be assumed that the
x
half-rise-time method is applicable without any correction. If the spread of thermal diffusivity values so
calculated is greater than ± 2 %, the possibility of non-ideal initial and/or boundary conditions, imperfect
design and/or operation of the laser flash apparatus, or problems associated with the specimen, must be
considered.
Table 1 — Values of constant K for a range of transient times
X
x % K t
x
x
10 0,0662 t
0,1
20 0,0843 t
0,2
30 0,1012 t
0,3
40 0,1190 t
0,4
50 0,1388 t
1/2
60 0,1622 t
0,6
70 0,1919 t
0,7
80 0,2332 t
0,8
90 0,3036 t
0,9
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ISO 18755:2005(E)
Key
X: time
Y: temperature rise
Figure 3 — Averaged deviation of the transient temperature curve (solid curve) from
the Parker’s equation having the observed half rise-time (broken curve)
The applicability of the half-rise-time method can alternatively be checked through the averaged deviation of
the transient temperature curve from the Parker’s equation corresponding to the experimentally determined
half rise-time as shown in Figure 3. The averaged deviation is calculated over the region from the half rise-
point to the maximum point normalized by the maximum temperature rise “∆T ”. If the averaged deviation is
max
within ± 1 % then it can be assumed that no corrections apply.
If the averaged deviation is greater than ± 1 %, the possibility of non-ideal initial and/or boundary conditions,
imperfect design and/or operation of the laser flash apparatus, or problems associated with the specimen,
shall be considered, as follows.
a) Imperfect design and/or operation of the laser flash apparatus:
1) superposition of stray light or electrical noise on the transient temperature response curve;
2) excessive drift of steady-state temperature;
3) insufficient response time of radiation detector and/or amplifier;
4) non-negligible heat exchange with the specimen holder;
5) effect of non-linearity of spectral radiance.
b) Problems associated with the specimen:
1) thermal resistance of coating;
2) poor flatness of the specimen;
3) large void or inhomogeneous distribution of pores.
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ISO 18755:2005(E)
c) Non-ideal initial and boundary conditions:
1) nonuniform heating effects;
2) finite pulse time effect;
3) radiation heat loss.
First, check Items a), b) and item c) 1) and improve them to make the measurements closer to the ideal
conditions.
NOTE During practical measurement by the laser flash method it is difficult always to satisfy the ideal initial and
boundary conditions. Annex C and the Bibliography give examples of analyses for non-ideal conditions which may be
applied as appropriate. Annex D gives information on other sources of error. If the position is still not acceptable, then
corrections for items c) 1) and c) 2) are necessary via appropriate algorithms.
Examples of such analyses are given in Annex C. Details of all the procedures employed shall be given in the
measurement report.
8 Measurement report
The following information should be recorded in the measurement report:
a) General information
1) The name and address of the testing establishment.
2) The date of measurements.
3) A unique identification of the report.
4) A reference to this International Standard.
5) The name of the laser flash apparatus used.
b) Light pulse
1) The type of the pulse light source.
2) Duration of the light pulse in full width at half maximum (optional).
3) Energy of one light pulse (optional).
4) Statement of spatial profile of the laser beam.
c) Specimen
1) A description of the material; (material type, manufacturing code, batch number, date of receipt).
2) Method of cutting, grinding and/or polishing specimens from supplied material.
3) Shape of the specimen (disk, square plate or rectangular plate).
4) Diameter or side length of the specimen.
5) Thickness of the specimen.
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ISO 18755:2005(E)
d) Coating
1) Use of coating (Yes or No).
2) Coated material.
3) Coating procedure.
4) Thickness of coating (optional).
e) Thermometry
1) Thermometer used for steady-state temperature measurements.
2) Thermometer or radiation detector used for measuring transient temperature or radiance rise of the
specimen rear face after light pulse heating.
f) Data acquisition (optional)
1) Response time of the transient temperature measurements.
g) Data analysis
1) The type of the analytical solution on which the data analysis is founded.
2) The data analysis algorithm (half-rise-time method, least-square-fit method, non-linear least-square
method, equiareal method or logarithmic method).
h) Corrections
1) Calculated values of heat-loss corrections, if any, giving full details if the methods are not given in
Annex B or C.
2) Calculated values of nonuniform heating corrections, if any, giving full details if the methods are not
given in Annex B or C.
3) Calculated values of finite pulse time correction, if any, giving full details if the methods are not given
in Annex B.
4) Calculated values of nonlinearity of spectral radiance correction, if any, giving full details if the
methods are not given in Annex B or C.
5) Calculated values of coating thermal resistance corrections, if any, giving full details.
i) Measured results
1) The measurement temperature(s), in kelvins or degrees Celsius.
2) The half rise-time, in seconds.
2
3) The calculated thermal diffusivity value(s) in m /s.
j) Other important information
1) Discussion of errors and correction procedures.
2) Comments about the measurement and measurement results.
© ISO 2005
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