J. de Boor

Data analysis for Seebeck coefficient measurements.

The Seebeck coefficient is one of the key quantities of thermoelectric materials and routinely measured in various laboratories. There are, however, several ways to calculate the Seebeck coefficient from the raw measurement data. We compare these different ways to extract the Seebeck coefficient, evaluate the accuracy of the results, and show methods to increase this accuracy. We furthermore point out experimental and data analysis parameters that can be used to evaluate the trustworthiness of the obtained result. The shown analysis can be used to find and minimize errors in the Seebeck coefficient measurement and therefore increase the reliability of the measured material properties.

Simultaneous measurement of all thermoelectric properties of bulk materials in the temperature range 300–600 K

Thermoelectric materials can directly convert heat into electrical energy. The characterization of different materials is an important part in thermoelectric materials research to improve their properties. Usually, different methods and setups are combined for the temperature dependent determination of all thermoelectric key quantities - Seebeck coefficient, electrical conductivity, and thermal conductivity. Here, we present a measurement system for the simultaneous determination of all of these quantities plus the direct determination of the figure of merit by means of the Harman method (zT)H in a temperature range from room temperature up to 600 K. A simultaneous measurement saves time and reduces the measurement error, and the change of all material properties can be monitored even for unstable materials. Thermal conductivity measurements are inherently affected by undesired thermal losses, in particular, through radiation at higher temperatures. We show a simple experimental approach to measure radiation losses and correct for those. Comparative measurements on traditional systems show good agreement for all measured quantities.

Thermoelectric transport and microstructure of optimized Mg2Si0.8Sn0.2

Solid solutions of magnesium silicide and magnesium stannide exhibit excellent thermoelectric properties due to favorable electronic band structures and reduced thermal conductivity compared to the binary compounds. We have optimized the composition Mg2Si0.8Sn0.2 by Sb doping and obtained a thermoelectric figure of merit close to unity. The material comprises of several phases and exhibits intrinsic nanostructuring. Nevertheless, the main features of electronic transport can be understood within the framework of a single parabolic band model. Compared to Mg2Si we observe a comparable power factor, a drastically reduced thermal conductivity and an increased effective mass.

Electrical conductivity measurements on disk-shaped samples

We have developed a sample holder design that allows for electrical conductivity measurements on a disk-shaped sample. The sample holder design is based on and compatible with popular measurement systems that are currently restricted to bar-shaped samples. The geometrical correction factors which account for the adjusted measurement configuration were calculated using finite element modeling for a broad range of sample and measurement geometries. We also show that the modeling results can be approximated by a simple analytical fit function with excellent accuracy. The proposed sample holder design is compatible with a concurrent measurement of the Seebeck coefficient. The chosen sample geometry is furthermore compatible with a thermal conductivity measurement using a laser flash apparatus. A complete thermoelectric characterization without cutting the sample is thus possible.

Mass production of magnesium silicide as a TEG material

Despite the promising property of TEGs to convert waste heat into electrical energy, there is currently no commercial break-through for TEGs although high research efforts are present all around the world. One rather neglected barrier is a possible mass production of the requested materials, which makes TEGs commercially interesting for industry. Instead of in research almost exclusively used processes like spark plasma sintering (SPS) and field activated sintering technology (FAST) this paper presents a method to produce Mg2Si with separated compacting and sintering steps. Therefore, an already in industry used sintering process for insulating or metallic materials was transferred to TE-materials by changing the process atmosphere and adding a new developed special stacked set-up. The density-issue of this process was minimized by higher sintering temperatures. RAMAN-spectra, EDX and XRD analysis confirm the desired composition.