Formation and Properties of TiSi 2 as Contact Material for High-Temperature Thermoelectric Generators
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Formation and Properties of TiSi2 as Contact Material for High-Temperature Thermoelectric Generators Fabian Assion, Marcel Schönhoff and Ulrich Hilleringmann Department of Sensor Technology, University of Paderborn, 33098 Paderborn, Germany
ABSTRACT Thermoelectric generators (TEG) are capable of transforming waste heat directly into electric power. With higher temperatures the yield of the devices rises which makes high-temperature contact materials important. The formation of titanium disilicide (TiSi2) and its properties were analyzed and optimized for the use in TEG. Depending on a direct or an indirect transformation into the C54 crystal structure the process forms a layer with a resistivity of 20-22 μ cm. Process gases influence the resistivity and result in difference of 20 %. The growing rate of TiSi2 on silicon dioxide was determined; it shows a strong dependence on the used atmosphere and temperature. A maximum overgrowing length of 30 μm was found. INTRODUCTION It is estimated that between 20 to 50 % of industrial energy input is lost as waste heat in the form of hot exhaust gases, cooling water, and heat lost from hot equipment surfaces and heated products [1]. A thermoelectric generator (TEG) can transfer this heat directly into electrical energy without any moving parts. Therefore, those generators do not need any kind of maintenance or extra fuel which makes every single watt output power to a profit without subtractions. For a fast gain back of the investment costs and a better environmental balance the TEG efficiency should be as high as possible. The efficiency of TEG rises with the applied temperature difference across its thermocouples. Hence, it is reasonable to increase the thermal stability of such devices and to expand the field of applications to higher temperatures. Aiming on thermally stable contacts we investigate in titanium disilicide (TiSi2), which has a resistivity of 15-35 μ cm [2,3] and withstands temperatures up to 1150 K [4]. Thereby TiSi2 fulfills the requirements for an implementation of TEGs at the exhaust manifold of usual street cars where a temperature difference versus cooling water of up to 700 K could be applied (Figure 1).
Figure 1: Schematic of a thermoelectric generator for an automotive application
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EXPERIMENT The experiments covered in this paper deal with the influence on the TiSi2 sheet resistance depending on different process parameters and the TiSi2 overgrowth on silicon dioxide films (SiO2). A 200 nm titanium layer was deposited by magnetron sputtering on silicon wafers. After structuring the titanium layer it was heated up by rapid-thermal-annealing (RTA). This step forms titanium disilicide at between 600 °C till 700 °C in the instable C49 crystal structure (with resistivity of 60-300 μ cm [2]). Higher temperatures let the base-centered-orthorhombic C49 crystal structure directly switch to the thermodynamically favored C54, which has a facecentered-orthorhombic structure [3]. Table I is giving more details about the crystal structure. To be able to inse
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