Laboratory for Transport Measurements on Thermoelectric Materials

For thermoelectric research pursued in the Working Group Methods for the Characterization of Transport Phenomena (EM-AMCT), the characterization of thermoelectric materials with respect to thermal and electrical properties is essential for a complete understanding of the material and for the optimization of their properties.

Our laboratories, located at Lise-Meitner-Campus, offer a laser-flash-apparatus (LFA), differential scanning calorimeter (DSC), dilatometer and Hall effect measurement system. The labs are open for in-house and external collaborating partners upon request.

Laser-Flash-Apparatus (LFA)

A Laser-Flash-Apparatus (LFA) is able to measure the thermal diffusivity D of a sample in a wide temperature range without time-consuming sample preparation steps. The technique uses a short laser pulse, which is absorbed at the front surface of the sample. The temperature of the rear surface of the sample is recorded as a function of time. From the evolution of the temperature signal, the thermal diffusivity D can be determined. Furthermore, by measuring a standard sample simultaneously, the specific heat Cp can be obtained, which is crucial for the determination of the thermal conductivity κ=ρDCp, where ρ is the sample density.

Our laboratory offers a LFA 457 MicroFlash (NETZSCH-Gerätebau GmbH). High and a low temperature furnaces are available to cover the temperature range between -100°C and 1100 °C. An automatic sample change allows measurements of three samples at the same time. The software includes various models to determine the diffusivity D. Further, the specific heat can be determined if a standard sample is measured simultaneously. Standard samples are available for the sample holders with a diameter of 6, 8, 10, and 12.7 mm. The software also calculates the thermal conductivity κ if the sample density ρ and the specific heat Cp are known.

Specifications

 

Temperature range

-100 °C…500 °C (low temperature furnace)

room temperature…1100 °C (high temperature furnace)

Geometry of quadratic samples

6 x 6 mm2, 8 x 8 mm2

Geometry of circular samples

Ø6, Ø8, Ø10, Ø12.7 mm

Thickness of the samples

0.1 mm…6 mm depending on thermal conductivity

Gas atmospheres

He, N2, Ar

Measurement range D

0.01 mm2/s … 1000 mm2/s

Measurement range κ

0.1 W/(m*K)…2000 W/(m*K)

Accuracy of D

±3%

Accuracy of Cp

±5%

Repeatability of D

±2%

Repeatability of Cp

±3%

TE-Lab Laserblitz-Apparatur

Fig. 1: Laser-Flash-Apparatus


Differential Scanning Calorimetry (DSC)

Heat flux differential scanning calorimetry is a technique which measures the temperature difference between an empty crucible and a crucible containing a sample during heating with a constant rate. Due to the specific heat of the sample, the crucible containing the sample heats up slower than the empty crucible. Furthermore, if the sample undergoes a phase transition a considerable temperature difference is observed due to the absorbed (endothermic process) or released (exothermic process) heat. Hence, a differential scanning calorimeter is perfectly suited for the determination of the specific heat Cp of a sample and for studying phase transitions.

The available DSC 404 F1 Pegasus (NETZSCH-Gerätebau GmbH) is equipped with two exchangeable furnaces which cover the temperature range between -120 °C and 1650 °C. Standard sapphire samples for the determination of Cp using the ratio method are available.

Specifications

 

Temperature range

-120 °C…675 °C (low temperature furnace)

 25 °C…1650 °C (high temperature furnace)

Sample crucibles

Al (85 ml), Al2O3 (85 ml), PtRh (85 ml), inner diameters: 5.2...6 mm

Gas atmospheres

N2, Ar, He

Accuracy of Cp

±2.5 % (RT…1400 °C)

±3.5 % (RT…1500 °C)

Dilatometer

A dilatometer measures the change of a sample length Δl temperature dependently. Thus, it provides information on the thermal expansion of a material, phase transitions and sintering properties.

The laboratory includes a DIL 402 Expedis Supreme (NETZSCH-Gerätebau GmbH), which is a push rod dilatometer with controlled and adjustable contact force. Two furnaces are available in order to reach temperatures below room temperature and well above room temperature.

Specifications

 

Temperature range

-180 °C … 500 °C (low temperature furnace)

RT…1600 °C (high temperature furnace)

Diameter of the samples

max. 12 mm

Length of the samples

0 mm…52 mm

Gas atmosphere

N2, Ar, He

Measuring range

±25000 μm

Δl resolution

0.1 nm

Accuracy of Δl/l0

0.002 %

Repeatability of Δl/l0

0.001 %

Force range

10 mN…3 N in steps of 0.2 mN

TE-Lab Dilatometer + DSC

Fig. 2: Dilatometer (left) and DSC (right)


Hall Effect Measurement System

For a complete understanding of the electrical properties of a material, the knowledge of the electrical conductivity σ alone is often not sufficient. In fact, as the electrical conductivity is the product of the charge carrier density n, the mobility μ and the elementary charge, it is important to know n and μ as well. For measurements of the charge carrier density, the Hall effect is used.

The Hall effect measurement system 8404 (Lake Shore Cryotronics, Inc.) is able to perform DC as well as AC field Hall measurements in the van der Pauw and the Hall bar geometry. The measurement of the electrical resistivity is also implemented in order to obtain all quantities σ, n and μ at the same time. In this set-up the AC field measurement option extends the mobility measurements down to 0.001 cm2/Vs. Furthermore, the system is equipped with a DC loop closed field control and an oven option. Samples are electrically contacted by four Inconel probes.

Specifications

 

Temperature range

RT…1000 °C (oven option)

Size of the samples

max. 12 mm x 12 mm x 3 mm

Magnetic field

0.88 T (DC), 0.62 T (AC), 1.2 T (RT insert only)

Gas atmosphere

Ar (oven option)

Measurement range μ

1…106 cm2/Vs (DC), 10-3…106 cm2/Vs (AC)

Measurement range n

8x102…8x1023 cm-3

Measurement range ρ

10-5…105 Ωcm

TE-Lab Hall-Effekt-Mess

Fig. 3: Hall Effect Measurement System