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Characterisation of Functional Materials
Characterisation of Functional Materials
CTEM – Combined ThermoElectric Measurement
The CTEM – an in-house development of DLR in cooperation with the University of Halle-Wittenberg (Germany) – provides automated simultaneous measurements of the Seebeck coefficient, the thermal and electric conductivity, and the thermoelectric figure of merit (immediate measurement by the HARMAN method). Within one temperature sweep the CTEM joins temperature dependent measurements of four essential material quantities of a thermoelectric over a temperature range from 80-450 K on prismatic and cylindrical bulk samples.
LFA – Laser flash apparatus
Measurement of thermal diffusivity The laser flash method is applied for in-house material development but also offered to external customers (industrial and institutes) as a reliable and convenient measurement technique. The thermal diffusivity is determined on homogenous materials as well as for coatings on known substrates using small disc-shaped specimens. Covering an application range from room temperature up to 1450 °C, the LFA fulfils the requirements for almost any material and system problem, including ceramic thermal barrier coatings and thermoelectric materials.
Measurement of the Seebeck coefficient at intermediate temperature (80–800 K)
The Seebeck coefficient is an essential feature of thermoelectric materials. It is the decisive parameter for the generation of a thermoelectric voltage by a TEG and for the signal responsivity of a thermoelectric sensor. A facility developed and constructed at DLR determines the temperature-dependent Seebeck coefficient on plate-shaped specimens between liquid nitrogen temperature and about 500 °C. It was designed for quick and reliable Seebeck measurement on specimens of variable geometry allowing for quick and easy exchange of specimen.
High temperature measurement of Seebeck coefficient and electrical conductivity (300 K–1000 K)
An essential component of relevant characterisation of thermoelectric functional materials is the relation between Seebeck coefficient and electrical conductivity as well as their variation with temperature. A high-temperature facility for simultaneous measurement of both quantities under vacuum on cylindrical specimens was developed in the institute. Crucial points for its reliability are stable contacting of the specimen, precise temperature measurement, as well as quick and precise temperature control.
Automated Hall facility - Determination of electric transport properties
Semiconductor properties such as concentration and mobility of the charge carriers provide important information for application-oriented improvement of thermoelectric materials. DLR´s Hall facility is equipped with a low temperature and high temperature probe head which allow for temperature-dependent determination of transport properties (Hall coefficient, electric conductivity, Seebeck coefficient) in low magnetic fields (up to 1 T) in the range from very low to very high temperatures (10 K–1200 K).
Thermogenerator test facility - Efficiency determination of thermoelectric converters
The priority performance parameter of thermoelectric generators (TEG) is the efficiency, which denotes the ratio of the passing heat flow to the gained electric power. Testing of generator elements of various kinds and of complete modules can be performed by a TEG test facility developed at DLR. The efficiency together with other TEG system parameters (thermoelectric figure of merit, effective thermopower, internal electrical resistance and thermal resistance) can be measured under various thermal and electrical conditions.
Analysis of the functional homogeneity of electrically conducting materials Scanning of the Seebeck coefficient over surface areas of a sample material provides a unique tool for characterisation of the homogeneity with respect to electrical functional properties. DLR has implemented this unique technique in several set-ups and is using it as a diagnostic tool for a multitude of transports-physical and technological tasks. Since the Seebeck coefficient is systematically correlated to the electrical conductivity, this technique is also suitable to evaluate materials of modern semiconductor-technology concerning homogeneity of doping distribution.
Temperature-controlled Seebeck micro-thermoprobe
Functionality and the application scope of the Seebeck micro-thermoprobe are essentially broadened if the temperature of the specimen is stabilised not only near room temperature but can be varied over a certain temperature range. An improved set-up was installed at DLR covering material homogeneity testing at different (stabilised) temperatures (-20°C – +60°C). Simultaneously, the temperature dependence for an entire spectrum of carrier density values (thus for various material qualities) can be recorded within one measurement.
Uniaxial hot pressing
In preparation of thermoelectric materials, focus at DLR is put on hot pressing technology for consolidation of powders of (semiconducting) functional materials. Powders of various material families (silicides, Skutterudites, zinc antimonide, tellurides, oxides) are processed coming from different sources such as gas-atomising, rapid solidification, reaction sintering and from different pre-treatments such as annealing, milling, mixing etc. Special gentle press methods were developed for nano-structured powders to keep their nano-structure during the consolidation process. Composite powders of a functional component and a ceramic filler were pressed and advantageous combinations of properties were selected. The technical equipment these studies are based on, are uniaxial hot pressing machines that can be operated under controllable thermal and atmospheric conditions.
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