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Functionally graded iron disilicide for linear high temperature thermal sensors

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Because of their extremely high Seebeck coefficient (thermopower) and their exceptional temperature characteristics, highly doped semiconductors (especially silicides and oxides) are suitable for application in highly sensitive thermal sensors used at elevated temperatures. Compared to metallic sensors, responsivity can be increased by more than one order of magnitude while maintaining the level of dynamic response. As a disadvantage, the generally substantial temperature dependence of the thermopower limits high responsivity to a narrow temperature range of application. This results in an essential nonlinearity of the characteristics, which necessitates complex on-top electronics plus processing of the measurement data to determine the desired thermal quantity from the signal voltage.
This can be essentially simplified if the sensor element is built up of segments of different functional properties along the direction of the temperature gradient (the principle of functionally grading). The functional gradient may be reached by variation of composition or doping, or arranging materials which exhibit maximal responsivity in adjacent temperature ranges. This principle allows linearization of the sensor characteristics, thus achieving a sensor responsivity which is independent of temperature over a wide temperature range. The temperature dependence of heat conductivity has to be tuned simultaneously to that of the Seebeck coefficient. FEM calculations allow the determination of the material’s optimal gradient function.
Functioning of this concept was proven by the DLR team “Thermal Sensors” in the framework of the DFG focal programme “Graded materials” (1996-2001) by means of stacks made of p-type iron disilicide. Furthermore, linearised p/n elements  are in the planning .  Multiplication of signal responsivity can be achieved by patterning thermopiles based on these elements.
Studies on long-term constitutional and functional stability of thermoelectric sensor materials under high temperatures as a basic condition of practical sensors’ application are an integral component of DLR’s work.