Thermoelectric Materials Development

Within the HEMF-platform (Helmholtz Energy Materials Foundry) a laboratory for material synthesis and rapid evaluation of synthesis routes has been established for the study of thermoelectric compounds. It focuses on the development of novel materials employing new synthesis routes and new optimization strategies for higher thermoelectric efficiency. The laboratory offers an atmospherically controlled spark plasma sintering (SPS) machine for material synthesis of a wide range of material classes as well as equipment for characterization of the material properties such as Seebeck coefficient, electrical and thermal conductivity which are relevant for thermoelectric applications. The lab is open for internal as well as external users. Full user support will be offered upon request.

Atmospherically controlled Spark Plasma Sintering (SPS)

A spark plasma sintering (SPS) system automatically compacts thermoelectric powder materials by uniaxial pressure and high temperatures maintained by direct current pulses. By this method a high densification of the powders can be realized which is required for accurate measurements of the thermoelectric properties. At the same time, the micro- or nanostructure of the powdered sample can be preserved which affects the macroscopic transport properties. Furthermore, novel and facile synthesis routes are possible using a SPS system.

The SPS system HP D 10 GB (FCT Systeme GmbH) will be available in our laboratory. As thermoelectric and intermetallic compounds can oxidize before or during the sintering process, any form of oxygen, gaseous or in form of water, must be avoided to guarantee well-defined sinter products. Thus, an in situ preparation and sintering of the powder materials under an inert gas atmosphere is required. We accomplish this by a combined SPS-glovebox system, where the SPS is fully gas-proof and integrated into a glovebox.

To guarantee comparability and reproducibility of the SPS process, the sample temperature is precisely measured by a fixed axial and a fixed radial pyrometer as well as by three thermocouples. All process parameters are recorded an archived. All gas exhausts are connected to an extraction system which allows to handle and synthesize toxic samples.

The system will be available in spring 2018.

Specifications

 

Sample diameter range

Ø10 mm...Ø50 mm

Force range

3 kN…100 kN

Piston stroke range

0 mm…100 mm

Working temperature range

25 °C…2200 °C

Voltage range

0 V…7.2 V

Current range

0 kA…5.5 kA

Power range

0 kW…37 kW

Temperature measurement

 

2 pyrometers (one for radial and one for axial direction)

2 thermocouples type-K at the pistons; 1 type-K thermocouple at user-defined position; Type S thermocouples are available

Guaranteed oxygen content

< 1 ppm

Guaranteed water content

< 1 ppm

Process atmospheres

Vacuum, N2, Ar

Graphite tools

Ø10 mm, Ø12.7 mm, Ø20 mm, Ø30 mm, Ø40 mm, Ø50 mm

WC/Co tools

Ø10 mm, Ø20 mm, Ø30 mm

Steel tools

Ø10 mm, Ø20 mm, Ø30 mm

SPS P009 (Setzpfandt)

Fig. 1: Spark plasma sintering system type KCE®-FCT HP D 10-GB (copyright HZB/M. Setzpfandt)


Simultaneous measurements of the Seebeck coefficient and electrical conductivity

For thermoelectric material development, the knowledge of the power factor S2σ is essential as it determines the efficiency of the material. By measuring the Seebeck coefficient S and the electrical conductivity σ simultaneously, identical measurement conditions are ensured.

The laboratory platform provides a SBA458 Nemesis device (NETZSCH-Gerätebau GmbH) which allows simultaneous measurements of the Seebeck coefficient S and the electrical conductivity σ of bulk samples and thin films in the temperature range from room temperature up to 1100°C. The electrical conductivity is measured by the collinear 4-point probe method with fixed probe spacings as shown in Fig. 3. The probes are pressed to the rear surface of the sample with a variable contact pressure allowing a characterization even of fragile samples. Heaters are placed on the edge of the sample in order to create a temperature gradient along the sample. The electrical contacts are established by rhodium (current injection) and Inconel coated thermocouples, which also measure the sample temperature required for the determination of the Seebeck coefficient.

Specifications

 

Temperature range

25 °C…1100 °C

Geometry of rectangular samples

l x w: 12.7 mm...25.4 mm x 2.0 mm...25.4 mm

Geometry of quadratic samples

l x w: 10 mm x 10 mm

Geometry of circular samples

Ø12.7 mm...25.4 mm

Thickness of the samples

100 nm...3 mm depending on thermoelectric properties

Gas atmospheres

N2, Ar

Measurement range S

10 µV/K…2000 µV/K

Measurement range σ

0.05 S/cm...150 000 S/cm

Uncertainty of S

±7%

Uncertainty of σ

±5%

Reproducibility of S

±3%

Reproducibility of σ

±3%

SBA (Setzpfandt)

Fig. 2: The SBA458 set-up is shown (copyright HZB/M. Setzpfandt)

HEMF SBA458 (von oben)

Fig. 3: Top-view of the SBA458 Nemesis sample holder showing the fix co-linear arrangement of the probes. Here, the sample holder is covered by millimeter paper to show distances between the probes.


Potential-Seebeck-Microprobe (PSM)

For studying samples which show an inhomogeneous sample composition, spatially resolved measurements of the electrical conductivity σ and the Seebeck coefficient S are of special interest. In our laboratory a Potential-Seebeck-Microprobe PSMII (PANCO GmbH) is available which measures S and σ of a sample surface spatially resolved with a resolution of up to 5 μm.

Specifications

 

Temperature

room temperature

Scanning area

max. 100 mm x 100 mm

Local resolution

up to 5 μm

Position accuracy

0.05 μm unidirectional

1 μm bidirectional

Signal resolution

100 nV

Measurement range

100 S/cm...10000 S/cm

Uncertainty of S

±3% for semiconductors

±5% for metals

Uncertainty of s

±4%

Reproducibility of S

±3%

Reproducibility of s

±3%

3ω-measurement methods for thermal conductivity characterization of thin films

The 3ω-method allows the measurement of the thermal conductivity of bulk material and thin films. This measurement technique requires a micro heater on top of the sample which can be defined by means of lithography and subsequent metallization. Fig. 4 shows a micro heater set-up on top of a thin film. The micro heater is periodically heated by an AC current which leads to a temperature oscillation. The temperature change of the micro heater leads to a resistance change of the micro heater that creates higher harmonic voltage signals. Those higher harmonics are detected with state-of-the art lock in amplifiers. In particular the voltage signal of the threefold frequency of the input signal is used to determine thermal diffusivity and thermal conductivity. Recent literature shows that anisotropic thermal conductivity can be measured by a modified heater arrangement and detection of the second harmonic of the voltage signal [1].

We will provide the sample environment for 2ω and 3ω methods in the temperature range from 10 K to 500 K (800 K will be available in future). To exclude parasitic heat we host an evacuable closed cycle refrigerator with versatile options for thermal shielding. For best signal quality 24 coaxial wires are placed inside the cryostat and attached to the sample which is mounted inside a standard dual inline package. We can provide those packages, so that users can perform the sample preparation in the home institute.

The system will be available in summer 2018.

Specifications

 

Temperature range

10 K…500 K (in the future 800 K)

Geometry of rectangular samples

l x w: max. 16 mm x 10.5 mm

Thickness of the samples

100 nm...1 mm depending on thermoelectric properties

Process atmosphere

Vacuum

Pitch size of the package

2.54 mm

HEMF micro heater

Fig. 4: A micro heater set-up for 3ω-measurements of a thin film. The AC-current is injected over the outer contact pads whereas the voltage drop is measured at between the inner pads.


References

[1] A.T. Ramu, J.E. Bowers, A “2-omega” technique for measuring anisotropy of thermal conductivity, Rev. Sci. Instrum. 83 (2012) 124903.