ISO 24687:2023

INTERNATIONAL ISO
STANDARD 24687
First edition
2023-04
Fine ceramics (advanced ceramics,
advanced technical ceramics) —
Measurement of Seebeck coefficient
and electrical conductivity of bulk-
type thermoelectric materials at room
and high temperatures
Céramiques techniques — Mesurage du coefficient de Seebeck et de
la conductivité électrique de matériaux thermoélectriques en vrac à
température ambiante et à haute température
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Significance and use .4
6 Apparatus . 4
7 Sampling . 5
7.1 Shape and dimension of specimen . 5
7.2 Pre-treatment . 6
7.3 Storage . 6
7.4 Number of specimens . 6
8 Procedure .6
8.1 Dimension measurement of specimen . 6
8.2 Placement of specimen . 6
8.3 Evacuating and purging the chamber . 7
8.4 Measurement of electrical conductivity . 7
8.5 Measurement of Seebeck coefficient . 7
9 Calculation . 7
9.1 Seebeck coefficient. 7
9.2 Electrical conductivity . 9
10 Expression of results .10
10.1 Seebeck coefficient and electrical conductivity . 10
10.2 Variation of Seebeck coefficient as a function of temperature . 11
10.3 Variation of electrical conductivity as a function of temperature . 11
11 Test report .12
Annex A (informative) Interlaboratory evaluation of Seebeck coefficient and electrical
conductivity of bulk-type thermoelectric materials .14
Annex B (informative) Periodic check of the apparatus (or equipment) by using a certified
reference material (CRM) or a reference material (RM) .20
Bibliography .21
iii
Foreword
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iv
INTERNATIONAL STANDARD ISO 24687:2023(E)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Measurement of Seebeck coefficient and
electrical conductivity of bulk-type thermoelectric
materials at room and high temperatures
1 Scope
This document specifies the measurement methods for the electronic transport properties of bulk-
type thermoelectric materials at room and elevated temperatures. The measurement methods
cover the simultaneous determination of Seebeck coefficient and electrical conductivity of bulk-type
thermoelectric materials in a temperature range from 300 K to 1 200 K. The measurement methods are
applicable to bulk-type thermoelectric materials used for power generation, energy harvesting, cooling
and heating, among other things.
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/IEC 17025, General requirements for the competence of testing and calibration laboratories
ISO 23331, Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for total
electrical conductivity of conductive fine ceramics
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
thermoelectric figure of merit
zT
dimensionless factor representing the thermoelectric conversion efficiency of a given material
3.2
thermoelectric power factor
S σ
characteristic value of a thermoelectric material given by the product of the square of Seebeck
coefficient (S) and electrical conductivity (σ)
Note 1 to entry: The units of the thermoelectric power factor are watts per metre per square kelvin (W/mK ).
3.3
Seebeck coefficient
S
intrinsic property which describes the induced voltage (thermal electromotive force, E) from a given
temperature difference (∆T) in a material
Note 1 to entry: The units of the Seebeck coefficient are microvolts per kelvin (μV/K).
3.4
electrical conductivity
σ
ability of a material to allow the transport of electric charges
Note 1 to entry: The units of electrical conductivity are Siemens per centimetre (S/cm).
4 Principle
This document is for simultaneously measuring the Seebeck coefficient and the electrical conductivity of
bulk-type thermoelectric materials using one measurement system. The off-axis four-terminal method
can be used to simultaneously measure the Seebeck coefficient and the electrical conductivity of bulk-
type thermoelectric material using one measurement system. As shown in Figure 1, the specimen is
set between two metal blocks in the heating zone and two thermocouple probes separately contact
the surface of the specimen. The measurement of the Seebeck coefficient of a bulk-type thermoelectric
material is necessary to measure the temperature difference between two positions (point H and
point C) on a specimen and the voltage across the two same positions (Figure 1). Seebeck coefficient
can be calculated by following Formula (1):
SE= /ΔT (1)
where
E is the induced thermoelectric voltage (thermal electromotive force) between the point H and
point C of the specimen;
∆T is the temperature difference between the point H and point C (= T - T ).
H C
For Seebeck coefficient measurement, measured temperature is the average temperature of the hot-
and cold-side thermocouple probes.
By using the measuring system illustrated in Figure 2, electrical conductivity is also measured based
on the four-terminal method. This method is conducted by placing four probes. Constant current is
applied through the two outmost probes, causing a measurable voltage drop, V, between the two inner
probes. The electrical resistance, R, is calculated using Ohm’s law following Formula (2):
RV= /I (2)
where
V is the voltage;
I is the current.
The resistivity, ρ, is be calculated following Formula (3):
ρ=RA /l (3)
where
A is the cross-sectional area of the specimen;
l is the separation between the two inner probes.
The electrical conductivity is the reciprocal of the resistivity. For electrical conductivity measurement,
measured temperature is the actual temperature of the specimen, which generally can be measured by
furnace temperature.
Key
1 upper metal block 2 specimen
3 lower metal block 4 heater
5 point C 6 point H
7 upper thermocouple probe (cold side) 8 lower thermocouple probe (hot side)
9 current electrode 10 heating furnace
Figure 1 — Schematic diagram of off-axis four-terminal method for simultaneous measurement
of Seebeck coefficient and electrical conductivity
Key
1 inner probes 2 outer probes
3 specimen 4 electrodes
Figure 2 — Schematic diagram of four-terminal method to measure the electrical resistivity
The results of an interlaboratory test are given in Annex A.
5 Significance and use
This document gives guidance for simultaneously measuring the high-accuracy and low-error Seebeck
coefficient and electrical conductivity of thermoelectric materials. Therefore, this standard is intended
to be used for the development, characterization and quality control of thermoelectric materials, data
acquisition for high-efficiency thermoelectric system design, etc.
Thermoelectric materials show Seebeck effect, Peltier effect and Thomson effect. The Seebeck effect is
the direct conversion of heat into electricity. The conversion efficiency of a thermoelectric material is
determined by the dimensionless thermoelectric figure of merit, zT, calculated following Formula (4):
zT =STσκ/ (4)
where
S is the Seebeck coefficient;
σ is the electrical conductivity;
κ is the thermal conductivity;
T is the absolute temperature.
Thermoelectric materials show a trade-off relation between Seebeck coefficient and electrical
conductivity according to carrier concentration. Therefore, the accuracy of the power factor, S σ, where
S is the Seebeck coefficient and σ is the electrical conductivity, can be improved through simultaneous
measurement of Seebeck coefficient and electrical conductivity in one run.
6 Apparatus
6.1 Current source, accurate to ±0,5 % on ranges of −1 A to +1 A used in the measurement.
−7
6.2 Electronic voltmeter, at least capable of measuring potential differences from 10 V to 0,05 V
-7
with a resolution below 10 V.
6.3 Conducting metal blocks.
The contact surface of conducting metal blocks shall be sufficiently large compared to a measurement
specimen. A specimen shall be placed between two conducting metal blocks, such as platinum or
tungsten. One end of the specimen is heated while the other acts as a heat sink, dispersing heat, thus
cooling that side. In addition, the conducting metal blocks play a role as the electrodes for applying the
current when measuring the electrical conductivity.
NOTE Pt or Pt – Pd alloy is the best electrode material due to high measuring temperatures.
6.4 Thermocouple probes.
The diameter of thermocouple probes shall be 0,5 mm or less to obtain reproducible Seebeck
coefficient value. Thermocouples should have a resolution of at least 0,01 K or better. Thermocouple
probes integrating electrical probes for measuring the voltage and thermal probes for measuring the
temperature should be designed for working from 300 K to 1 200 K. Thermocouple probes should be
checked periodically as their output may drift with usage or contamination.
NOTE In some equipment, the voltage can be measured only with thermocouple wires without additional
electrical probes.
6.5 Test chamber.
The test chamber shall be capable of heating both the specimen and the conducting metal blocks up
to at least 1 200 K as well as maintaining the test temperature within ±1 K during the test, by which
vacuum environment shall be available for test requirement. The test chamber should be evacuated
below 3 Pa and can be backfilled with a variety of gases such as helium, argon, nitrogen and oxygen or a
mixture of these. Low-pressure helium can be used to improve the thermal contact between the probe
and the sample. However, low pressure may affect the m
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