Thermal sensors used at high temperature require the availability of an active material of high sensitivity, linearity and functional long- term stability, of high-temperature stable contacting between active sensor material and metallic signal leads, as well as thermomechanically stable joints between carrier structure, semiconductor and coatings. Suitable calibration or test apparatuses are required for system evaluation.The team “Thermal Sensors” considers all of these partial aspects in the development of a layer-structured linear heat flow sensor based on semiconducting iron disilicide. System development for sensor applications of laboratory measuring techniques extends to fabrication of prototypes in industrial co-operation applying modern micro-structuring technologies, optimised semiconductor materials and numerical modelling tools.
Novel thermoelectric materials
The target quantity of thermoelectric materials development is the thermoelectric figure of merit. Thermoelectrics with a high figure of merit (possessing high Seebeck coefficient, high electric conductivity and low thermal conductivity) achieve highest detectivity in sensors, high efficiency in thermoelectric energy conversion and best coefficient of performance (C.O.P.) in Peltier applications.In a European cooperation (EU FP5 project “NanoThermEl”) the “Thermal Sensors” team develops Skutterudites based on CoSb3 with nano-scaled microstructure via compaction of nano-powders, powder-compacted zinc antimonide and single crystalline Clathrates in the BaGaGe system showing a high thermoelectric figure of merit. Furthermore, thermoelectric cobaltates (DFG project) and optimised crystalline Peltier-materials are under development.
Thermoelectric energy conversion
Thermoelectric energy conversion means direct conversion from thermal to electrical power by means of a semiconductor effect without involvement of moving media or mechanical moving parts. This technique is gaining more and more technical importance with respect to mobile auxiliary current sources and integrated miniaturized generators (self-powered microdevices).The research team “Thermal Sensors” is part of an international cooperation for the development of prototypes of functionally graded energy converter modules for waste heat with the ultimate goal of generating mobile thermoelectric power. Extensive work is dedicated to iron disilicide as an inexpensive long-term stable high temperature material qualified for the application from room temperature to up to 800 °C, suitable for industrial fabrication and processing methods. Improved functional materials provide reserves for augmenting the output power and efficiency of thermoelectric energy converters via the principle of functionally grading, via control of microstructure by means of rapid solidification and additional thermal treatment.
Thermoelectric measurement technology
Efficient experimental set-ups for temperature-dependent determination of thermoelectric material and system properties are complex facilities and commercially not readily available. The research team “Thermal Sensors” has designed, constructed, automated, and tested a family of modular PC-controlled measuring systems capable of recording the Seebeck coefficient, electrical conductivity, thermal conductivity, and the thermoelectric figure of merit in a wide temperature range as well as spatial homogeneity of thermoelectric properties. Facilities for system characterisation determine temperature-dependent sensor responsivity at high temperature for heat flow sensors and the efficiency of thermogenerators under realistic and variable thermal operation conditions.