EN ISO 18755:2023
(Main)Fine ceramics (advanced ceramics, advanced technical ceramics) - Determination of thermal diffusivity of monolithic ceramics by flash method (ISO 18755:2022)
Fine ceramics (advanced ceramics, advanced technical ceramics) - Determination of thermal diffusivity of monolithic ceramics by flash method (ISO 18755:2022)
This document specifies the test method for the determination of thermal diffusivity from room temperature to at least 1 700 K by the flash method for homogeneous monolithic ceramics with porosity less than 10 %.
Flash methods, like laser flash, are applicable to homogeneous isotropic materials with thermal diffusivity values ranging from 0,1 to 1 000 mm2 s-1 within the temperature range from approximately 100 K to 2 300 K.
The method described in Annex G describes how to estimate, on the basis of the thermal diffusivity test, the specific heat capacity and the thermal conductivity of homogeneous monolithic ceramics with porosity less than 10 %.
Hochleistungskeramik - Bestimmung der Temperaturleitfähigkeit monolithischer Keramik mit dem Flash-Verfahren (ISO 18755:2022)
Dieses Dokument legt das Prüfverfahren für die Bestimmung der Temperaturleitfähigkeit von Raumtemperatur bis mindestens 1 700 K mittels des Flashverfahrens für homogene monolithische Keramik mit einer Porosität von weniger als 10 % fest.
Flashverfahren wie das Laserflash-Verfahren sind für homogene isotrope Werkstoffe mit Temperaturleitfähigkeitswerten im Bereich von 0,1 mm2 s−1 bis 1 000 mm2 s−1 innerhalb des Temperaturbereiches von etwa 100 K bis 2 300 K anwendbar.
Das Verfahren in Anhang G beschreibt, wie die spezifische Wärmekapazität und die Wärmeleitfähigkeit von homogener monolithischer Keramik mit einer Porosität von weniger als 10 % auf der Grundlage der Temperaturleitfähigkeitsprüfung abzuschätzen sind.
Céramiques techniques - Détermination de la diffusivité thermique des céramiques monolithiques par la méthode flash (ISO 18755:2022)
Le présent document spécifie la méthode d’essai pour la détermination de la diffusivité thermique de la température ambiante à au moins 1 700 K par la méthode flash pour les céramiques monolithiques homogènes dont la porosité est inférieure à 10 %.
Les méthodes flash, telles que flash laser, sont applicables aux matériaux isotropes homogènes dont les valeurs de diffusivité thermique sont comprises entre 0,1 mm2 s–1 et 1 000 mm2 s–1 dans une plage de températures allant d’environ 100 K à 2 300 K.
La méthode décrite à l’Annexe G décrit comment estimer, à partir de l’essai de diffusivité thermique, la capacité thermique massique et la conductivité thermique des céramiques monolithiques homogènes dont la porosité est inférieure à 10 %.
Fina keramika (sodobna keramika, sodobna tehnična keramika) - Ugotavljanje toplotne difuzivnosti monolitske keramike z bliskovno metodo (ISO 18755:2022)
Ta dokument določa preskusno metodo za ugotavljanje toplotne difuzivnosti iz sobne temperature vsaj v vrednosti 1700 K z bliskovno metodo za homogeno monolitsko keramiko s stopnjo poroznosti manj kot 10 %.
Bliskovne metode, kot je laserski blisk, se uporabljajo za homogene izotropne materiale z vrednostjo toplotne difuzivnosti med 0,1 in 1000 m2 s-1 znotraj temperaturnega območja med približno 100 K in 2300 K.
Metoda, opisana v prilogi G, opisuje, kako na podlagi preskusa toplotne difuzivnosti oceniti specifično toplotno kapaciteto in toplotno prevodnost homogene monolitske keramike s stopnjo poroznosti manj kot 10 %.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-december-2023
Fina keramika (sodobna keramika, sodobna tehnična keramika) - Ugotavljanje
toplotne difuzivnosti monolitske keramike z bliskovno metodo (ISO 18755:2022)
Fine ceramics (advanced ceramics, advanced technical ceramics) - Determination of
thermal diffusivity of monolithic ceramics by flash method (ISO 18755:2022)
Hochleistungskeramik - Bestimmung der Temperaturleitfähigkeit monolithischer Keramik
mit dem Laserflash-Verfahren (ISO 18755:2022)
Céramiques techniques - Détermination de la diffusivité thermique des céramiques
monolithiques par la méthode flash (ISO 18755:2022)
Ta slovenski standard je istoveten z: EN ISO 18755:2023
ICS:
81.060.30 Sodobna keramika Advanced ceramics
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EN ISO 18755
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2023
EUROPÄISCHE NORM
ICS 81.060.30 Supersedes EN 821-2:1997
English Version
Fine ceramics (advanced ceramics, advanced technical
ceramics) - Determination of thermal diffusivity of
monolithic ceramics by flash method (ISO 18755:2022)
Céramiques techniques - Détermination de la Hochleistungskeramik - Bestimmung der
diffusivité thermique des céramiques monolithiques Temperaturleitfähigkeit monolithischer Keramik mit
par la méthode flash (ISO 18755:2022) dem Laserflash-Verfahren (ISO 18755:2022)
This European Standard was approved by CEN on 22 October 2023.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18755:2023 E
worldwide for CEN national Members.
Contents Page
European foreword . 3
European foreword
The text of ISO 18755:2022 has been prepared by Technical Committee ISO/TC 206 "Fine ceramics” of
the International Organization for Standardization (ISO) and has been taken over as EN ISO 18755:2023
by Technical Committee CEN/TC 184 “Advanced technical ceramics” the secretariat of which is held by
DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2024, and conflicting national standards shall be
withdrawn at the latest by April 2024.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 821-2:1997.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 18755:2022 has been approved by CEN as EN ISO 18755:2023 without any modification.
INTERNATIONAL ISO
STANDARD 18755
Second edition
2022-12
Fine ceramics (advanced ceramics,
advanced technical ceramics) —
Determination of thermal diffusivity
of monolithic ceramics by flash
method
Céramiques techniques — Détermination de la diffusivité thermique
des céramiques monolithiques par la méthode flash
Reference number
ISO 18755:2022(E)
ISO 18755:2022(E)
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 18755:2022(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Apparatus . 3
4.1 General . 3
4.2 Specimen holder . 4
4.3 Flash source . 4
4.4 Thermometer for measuring steady-state temperature of the specimen . 5
4.5 Detector for measuring transient temperature rise of rear face of the specimen . 5
4.6 Environment for measurements . . 5
4.7 Temperature control unit . 5
4.8 Data acquisition unit . 5
5 Specimen . 5
5.1 Shape and dimension of specimens . 5
5.2 Density of the specimen . 6
5.3 Coating on the specimen . 6
5.4 Reference specimen . 6
6 Measurement procedure . 6
6.1 Measurement of specimen thickness . 6
6.2 Surface treatment . 6
6.3 Determination of the flash time of the laser or light pulse and the chronological
profile of the laser or light pulse . . 7
6.4 Temperature and atmosphere control . 7
6.5 Stability of specimen temperature . 7
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 . 8
8 Measurement report .10
Annex A (informative) Principle of flash thermal diffusivity measurements .13
Annex B (normative) Correction for non-ideal initial and boundary conditions .14
Annex C (informative) Data analysis algorithms to calculate thermal diffusivity
from observed transient temperature curve under non-ideal initial and
boundary conditions .21
Annex D (informative) Other error factors .23
Annex E (informative) Procedure to determine intrinsic thermal diffusivity.29
Annex F (informative) Reference data and reference materials of thermal diffusivity .32
Annex G (informative) Evaluation of specific heat capacity and thermal conductivity .34
Annex H (informative) Example data including precision and uncertainty up to high
temperature .36
Bibliography .39
iii
ISO 18755:2022(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 206, Fine ceramics.
This second edition cancels and replaces the first edition (ISO 18755:2005), which has been technically
revised.
The main changes are as follows:
— a change of title and scope to enable the use of flash lamps to generate the energy pulse;
— the addition of three new informative annexes: one dealing with the determination of the intrinsic
thermal diffusivity; the second with the determination of specific heat and thermal conductivity of
the samples tested; and the third providing precision data for the method on the basis of an inter-
laboratory study carried out by seven European laboratories in 2020-2021 in the framework of the
project Hi-TRACE;
— an additional normative reference to provide clear instructions on the determination of the density
of the materials to be analysed;
— relevant specifications added concerning the size and the density of the specimen;
— improvement of Annex F, with an updated list of potential reference material and incorporation of a
validation method.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
INTERNATIONAL STANDARD ISO 18755:2022(E)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Determination of thermal diffusivity of
monolithic ceramics by flash method
1 Scope
This document specifies the test method for the determination of thermal diffusivity from room
temperature to at least 1 700 K by the flash method for homogeneous monolithic ceramics with porosity
less than 10 %.
Flash methods, like laser flash, are applicable to homogeneous isotropic materials with thermal
2 -1
diffusivity values ranging from 0,1 to 1 000 mm s within the temperature range from approximately
100 K to 2 300 K.
The method described in Annex G describes how to estimate, on the basis of the thermal diffusivity
test, the specific heat capacity and the thermal conductivity of homogeneous monolithic ceramics with
porosity less than 10 %.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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, Geometrical product specifications (GPS) — Dimensional measuring equipment: Micrometers for
external measurements — Design and metrological characteristics
ISO 18754, Fine ceramics (advanced ceramics, advanced technical ceramics) — Determination of density
and apparent porosity
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
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
ISO 18755:2022(E)
3.4
pulse width
τ
p
full width at half maximum (FWHM), which is the time duration when the laser or light pulse intensity
is larger than half of its maximum value on time basis
3.5
centroid of laser pulse
chronological centroid of laser light energy
3.6
centroid of light pulse
chronological centroid of light energy
3.7
spatial energy distribution of pulse laser beam
energy density of the laser beam or light flash incident at each point on the front face of the specimen
3.8
transient temperature curve
transient temperature change of the rear face of the specimen after the light pulse heating
3.9
transient radiance curve
transient change of the spectral radiance from the rear face of the specimen after the light pulse heating
Note 1 to entry: 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.
3.10
maximum temperature rise
ΔT
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 1 to entry: See Figure 1.
ISO 18755:2022(E)
Key
X time
Y temperature rise
exponential function ΔTe xp −t /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
3.11
half rise-time
t
l/2
time until ΔT /2 is attained from the pulse heating
max
3.12
characteristic time of heat loss
τ
c
time of heat loss determined when the cooling region is fitted with an exponential function,
[]ΔTe xp()−t /τ
0 c
Note 1 to entry: See Figure 1.
3.13
extrapolated temperature rise
ΔT
temperature rise determined when the cooling region is fitted with an exponential function,
[]ΔTe xp()−t /τ
0 c
4 Apparatus
4.1 General
The apparatus shall be designed for obtaining the thermal diffusivity from the transient temperature
curve of the rear face of a specimen after the light pulse is irradiated onto the front face of the specimen.
It shall consist of the principal components as shown in Figure 2.
ISO 18755:2022(E)
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 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/light flash 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.
4.3 Flash source
The flash source shall be a pulse laser, a flash lamp or another device capable of generating a short
duration pulse of substantial energy 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.
ISO 18755:2022(E)
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 by using beam-
homogenizing optics.
4.4 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.
4.5 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.
4.6 Environment for measurements
Measurements can 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 or phase changes and compatibility problems.
4.7 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.
4.8 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 1 000 data points should
be sampled with the sampling time faster than 1 % of the half rise-time “t1/2”.
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 larger than 5 mm and up to typically 20 mm.
The specimen thickness shall be chosen to be as follows:
a) sufficiently thick that the t value is larger than five times the pulse width.
1/2
b) the diameter-or-side-to-thickness ratio shall be equal to or higher than 5:2.
ISO 18755:2022(E)
NOTE In most cases, experience shows that the diameter-or side-to-thickness ratio is in the order of
magnitude of 4:1. However, some reference materials supplied from NMIJ (see Annex F) include specimens
of 10 mm diameter and 4,0 mm thickness. They would be out of the ratio of 4:1. Therefore, in order to also
include the reference materials with the above-mentioned sizes, the selected ratio is identified as 5:2.
c) The uniformity of the specimen thickness shall be smaller than 1,0 %.
5.2 Density of the specimen
The porosity of the specimen as determined by ISO 18754 shall be lower than 10 %.
The mass of the specimen shall be measured before and after measurement in order to detect possible
mass changes, in particular for high-temperature measurements, due to reactions which can occur
during the measurements, even if they ought to be avoided.
NOTE If the porosity is higher than 10 % other approaches can be applied, see References [43] to [47].
5.3 Coating on the specimen
If the specimen does not have a high absorption coefficient for the heating laser beam/light flash 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/light flash or thermal radiation at the observed wavelength, and should be resistive against
laser/light pulse heating at high temperatures. Coating thickness should be a minimum commensurate
with excluding directly transmitted laser/light pulse.
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, can alternatively be used. The surface of the test specimen can, with
advantage, be roughened to improve adhesion of the coating. The coating thickness dependence should
be evaluated for the observed thermal diffusivity, if the contribution of coatings is not negligible.
5.4 Reference specimen
Reference specimens can be used to evaluate uncertainty of thermal diffusivity measurements by a
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 Several materials are used as reference (see Annex F).
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
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 5.3.
ISO 18755:2022(E)
6.3 Determination of the flash time of the laser or light pulse and the chronological
profile of the laser or light pulse
The chronological trace of the laser or light pulse versus the same trigger signal to initiate flash thermal
diffusivity measurements shall be observed. If the FWHM of the laser/light 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.
6.6 Energy of pulse heating
Irradiate the specimen with the laser or light pulse at an intensity of as low energy as possible,
commensurate with an acceptable noise level.
NOTE See Annex D regarding non-linearity of spectral radiance on temperature.
6.7 Measurement temperature
Record the measurement temperature as TT+Δ , where T is the initial steady-state temperature
0max 0
and ΔT is the maximum temperature rise of the specimen recorded by the thermocouple in contact
max
with the 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 ΔT .
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 non-uniform heating effect.
7 Data analysis
7.1 Calculation based on the half-rise-time method
The standard algorithm to calculate thermal diffusivity from the flash method is the half-rise-time
method, in which the analytical formula 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
baseline ΔT /2 over the half-rise-time.
max
If the measurement is valid when made under the above-mentioned ideal initial and boundary
conditions, the thermal diffusivity, α, is represented by Formula (1), based on the half-rise-time method:
0, 138 8d
α = (1)
t
ISO 18755:2022(E)
where
d is the specimen thickness, in metres;
t is the time delay when the temperature of the rear face reaches one-half of the maximum
temperature rise, ΔT , 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 shall be
satisfied:
— The duration of the laser/light 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 laser/light pulse.
— The specimen is adiabatic during the period of measurement after the laser/light pulse heating.
— The specimen is uniform (in geometry) and is homogeneous.
— The specimen is opaque (non-transparent and non-translucent) to the laser/light pulse and to
thermal radiation.
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 formula (see Annex A).
The thermal diffusivity value shall be determined by fitting this formula 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 analysed to yield the
thermal diffusivity, α. This is given by Formula (2).
Kd
x
α = (2)
t
x
where
d is the specimen thickness, in metres;
is the time for the specimen rear face to reach a fraction of the maximum temperature rise, in
t
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 ,
1/2 0,3
t and t calculated using the relevant values of K in Table 1 are all within ±2 %, then it can be
0,5 0,7 x
assumed that the 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 flash apparatus, or problems associated with the
specimen, shall be considered.
ISO 18755:2022(E)
Table 1 — Values of constant K for a range of transient times
X
x K t
x x
%
10 0,066 2 t
0,1
20 0,084 3 t
0,2
30 0,101 2 t
0,3
40 0,119 0 t
0,4
50 0,138 8 t
1/2
60 0,162 2 t
0,6
70 0,191 9 t
0,7
80 0,233 2 t
0,8
90 0,303 6 t
0,9
Key
X time
Y temperature rise
solid curve transient temperature curve
broken curve observed half rise-time
Figure 3 — Averaged deviation of the transient temperature curve from
the Parker’s formula having the observed half rise-time
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 formula 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 ΔT . If the averaged deviation is within ±1 % then it can be assumed that no corrections apply.
max
ISO 18755:2022(E)
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 flash apparatus, or problems associated with the
specimen, shall be considered as follows:
a) Imperfect design and/or operation of the 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.
c) Non-ideal initial and boundary conditions:
1) non-uniform heating effects;
2) finite pulse time effect;
3) radiation heat loss.
First, check items a), b) and c) 1) and improve them to make the measurements closer to the ideal
conditions.
NOTE During practical measurement by the 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
can 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.
Annex F gives information on a method which allows evaluating the intrinsic thermal diffusivity
regardless of the measurement conditions.
Finally, Annex H provides details concerning the precision data, reproducibility and repeatability of the
thermal diffusivity measurements.
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 document;
ISO 18755:2022(E)
5) the name of the 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) density or porosity of the specimen;
5) diameter or side length of the specimen;
6) thickness of the specimen.
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;
2) 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 Annexes B or C;
2) calculated values of non-uniform heating corrections, if any, giving full details if the methods
are not given in Annexes B or C;
ISO 18755:2022(E)
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 non-linearity of spectral radiance correction, if any, giving full details if
the methods are not given in Annexes 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;
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 18755:2022(E)
Annex A
(informative)
Principle of flash thermal diffusivity measurements
A.1 Ideal condition
[1]
The analytical solution for flash thermal diffusivity measurements is given by Parker et al. under the
following conditions.
a) The duration of the pulse is negligibly short compared to the characteristic time of heat diffusion.
b) The front face of the specimen is uniformly heated by a light pulse.
c) The specimen is adiabatic during the measurement after the light pulse heating.
d) The specimen is uniform (in geometry) and homogeneous.
e) The specimen is opaque (non-transparent and non-translucent) to the light pulse and to thermal
radiation.
If these conditions are satisfied, the heat flow becomes one-dimensional and the temperature of the
rear face of the specimen changes according to Formula (A.1):
∞ t
n 2
Tt =+ΔTn12 ()−−1 exp ()π (A.1)
()
∑
n=1
τ
0
where
Q
ΔT=
C
Q is the total energy absorbed by the specimen;
C is the heat capacity of the specimen;
is the characteristic time of heat diffusion across the specimen;
d
τ =
α
α is the thermal diffusivity of the specimen.
ISO 18755:2022(E)
Annex B
(normative)
Correction for non-ideal initial and boundary conditions
B.1 General
The calculation of thermal diffusivity using Formula (1) is modified if the pulse transit time is less than
100 times the duration of the heat pulse, or if heat is lost from the specimen. Examples of modifications
applying to the calculation method are given in B.2 and B.3.
The mathematical derivation given in Annex A assumes that no heat is lost from the specimen during
[2-5]
the time taken for the heat pulse to pass through it. For good conductors at temperatures close
to ambient, this is a reasonable approximation, but for poor conductors and for most samples at high
temperatures, corrections for heat losses will almost certainly be applicable.
Provided that use of a suitable holder design has minimized heat lost from the specimen by conduction,
and that the duration of the transient is short enough for heat lost by convection to be neglected (there
are no convective losses if the measurement is performed in a vacuum), the main source of heat loss
is by radiation from the specimen surfaces. The best way to analyze heat loss is to compare the entire
experimental curve with one or more of the many theoretical models available. Examples of analytical
methods are given in Annex C.
B.2 Effect of finite pulse time
B.2.1 General
Several analytical calculations have been reported in order to correct the effect of the finite duration
[6-8]
time of the heating laser/light pulse energy. If the pulse width is much shorter than the half rise-
time of the transient temperature curve, the laser/light pulse heating can be approximated by Dirac’s
[9]
delta function located at the chronological centroid of laser/light pulse energy .
When t is less than 100 times the heat pulse duration, the finite pulse time effect should be corrected.
1/2
Two widely used correction methods are the centroid method and the triangular pulse method
described as follows.
[9]
B.2.2 Centroid method
If t is larger than three times the pulse width (τ ), the finite pulse time effect can be corrected by
1/2 p
shifting the time origin of the data analysis to the chronological centroid of the laser/light pulse (t ).
g
Thus, t should be replaced by tt− .
1/2 12 g
B.2.3 Determination of chronological centroid of laser/light pulse
B.2.3.1 General
The chronological centroid of the laser beam/light flash (t ) should be determined by one of the
g
following procedures.
B.2.3.2 Real time method
Measure the waveform of the laser beam /light pulse by a detector of frequency response faster than
10 μs and c
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.
Loading comments...