Preparation and Characterization of High Temperature Thermoelectrics Based on Metal/Oxide Nanocomposites
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1044-U07-04
Preparation and Characterization of High Temperature Thermoelectrics Based on Metal/Oxide Nanocomposites Otto J. Gregory1, Ximing Chen1, Gustave C. Fralick2, and John Wrbanek2 1 Chemical Engineering, University of Rhode Island, Kingston, RI, 02881 2 NASA Glenn Research Center, 21000 Brookpark Rd, Cleveland, OH, 44135 ABSTRACT Thermoelectric devices based on “n-type” oxide semiconductors and metal/oxide nanocomposites are being considered for high temperature thermocouples, heat flux sensors and energy harvesting devices. In terms of energy harvesting, preliminary 2D thermoelectric calculations indicated that enough electrical energy can be generated from the large thermal gradients that exist within a gas turbine engine to power active wireless devices. Several promising bi-ceramic junctions based on this concept were investigated in terms of their high temperature thermoelectric properties. The most promising bi-ceramic junction was based on indium tin oxide (ITO) and a NiCrCoAlY/alumina nanocomposite. The thermoelectric responses of these individual elements were evaluated relative to a platinum reference electrode. A maximum electromotive force (emf) of 77 mV was achieved for a NiCrCoAlY/alumina nanocomposite/platinum thermocouple for an imposed temperature gradient of 1111 °C. The thermoelectric power for this couple was 78 µV/°C. When this NiCrCoAlY/alumina nanocomposite was combined with ITO to form a bi-ceramic junction, thermoelectric powers on the order of 700 µV/°C were obtained. A maximum emf of 291 mV was achieved for a hot junction temperature of 1100 °C. The thermoelectric response after repeated thermal cycling to 1200 °C was both repeatable and reproducible. The ITO was prepared in varying nitrogen, oxygen and argon partial pressures, which was used to control the charge carrier concentration, stability and thermoelectric response of the bi-ceramic junctions. The thermoelectric response decreased with increasing nitrogen partial pressure and increased with oxygen partial pressure in the plasma with the argon partial pressure constant. The relationship between the sputtering parameters and thermoelectric properties was investigated and the application of these biceramic junctions as thermocouples and energy harvesting devices is discussed. INTRODUCTION The evolutionary development of new materials and designs has allowed gas turbine engines to be operated at much higher temperatures to achieve higher efficiencies. To evaluate engine performance and monitor operating characteristics, it is necessary to measure the surface temperature of components in the hot section of turbine engines. Platinum based thermocouples are expensive, have a limited temperature range and are prone to errors due to catalytic effects which can yield results that can deviate by as much as 50 °C from the actual temperature [1]. Thin film thermocouples deposited directly on the blades and vanes are ideally suited for measuring the surface temperature of engine components during operation due to their small thermal ma
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