The target of the development of thermoelectric sensor materials and systems at DLR is thermal sensors for measuring heat flow and heat transfer coefficients on the surface of structural components especially in thermal flow machinery, e.g. on structural parts of turbines, in car engines, and in industrial process technology for control of process conditions based on thermal process data. Systems development is targeted at sensors of small installation size for quick dynamic measurements. Arrays of that kind of sensors will allow monitoring of two-dimensional profiles of thermal quantities over critical surface areas of structural components.
Heat flow sensors of this type can be designed in a layered layout with a ramp-shaped thermally resistive coating. The continuous alteration of the coating’s thermal resistance on top of the semiconducting thermoelectric sensor material leads to the formation of a distributed lateral temperature gradient over the segmented sensor area.
In a thermoelectric sensor device, where the Seebeck coefficient (which is responsible for the formation of a thermo-voltage in a temperature gradient) is utilised for heat flow determination, a manifold higher detectivity compared to conventional metallic systems can be achieved by means of semiconductor application. Besides iron disilicide, further silicide materials and other doped semiconductors are taken into consideration as thermoelectric sensor materials, in case they combine high thermopower with the availability of highly developed technologies for fabrication and machining.
Shaping of sensor elements earns certain significance since reproducibility, responsively, and time response of the sensor are determined by the spatial distribution of heat flow. The magnitude of responsivity is given by the thermal resistance along the path of the heat flow. Numerical simulation calculations for responsivity and response time of thermal sensors with FEM are applied as an instrument for geometrical optimization. Recently established microprocessing techniques are utilised for precise shaping of miniaturized sensor structures.
The prototype of a DLR-developed sensor for a commercial measuring technique in materials characterisation reaches responsivity values 16 times larger than of comparable conventional systems using metallic sensors, at equal time response. Besides signal responsivity the achieved detectivity, the signal-to-noise ratio is practically relevant. The noise equivalent power of the DLR-sensor was about one tenth compared to conventional systems. The combination of all partial aspects leads to a prototypical solution of high maturity in practical use.
The current aim of ongoing technological work is to develop highly sensitive miniaturized high temperature thermopiles with linear sensitivity up to 800°C based on paste technologies and sintering methods, laser micro-processing and -contacting.