Thermoelectric Materials Discovery Using Combinatorial Chemistry
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Thermoelectric Materials Discovery Using Combinatorial Chemistry Matin Amani, Ian Tougas and Otto J. Gregory University of Rhode Island, Department of Chemical Engineering, 15 Greenhouse Rd, Kingston, RI 02881, U.S.A.
ABSTRACT Transparent conducting oxides have been previously investigated for both bulk and thin film thermoelectric applications, and have shown promising results due to their thermal stability and electrical conductivity. Alloys of two or more transparent conducting oxides have been deposited using pulsed laser deposition (PLD) and combinatorial sputtering, and the resulting films were optimized for optical applications. In this study, thermoelectric materials were prepared by co-sputtering techniques, whereby a chemical gradient was formed across an alumina substrate that was patterned using photolithography to form hundreds of microthermocouples. The systems indium tin oxide (ITO), indium zinc oxide (IZO), and zinc tin oxide (ZTO) were investigated for this purpose and the resulting combinatorial libraries were rapidly screened to establish room temperature resistivity, Seebeck coefficient, and power factor as functions of both composition and heat treatment, in nitrogen and air ambients. Due to their chemical stability, oxidation resistance, and large Seebeck coefficients relative to metal thermocouples, these materials are ideal for temperature measurement or energy harvesting in harsh environments such as gas turbine engines. INTRODUCTION Thin film thermoelectric devices based on semiconducting oxides are being investigated for energy harvesting and temperature measurement in gas turbine engines used for propulsion and power generation. Thin film thermocouples for example have a rapid response time (less than one μs) and can be directly deposited onto the surface of components without the need for adhesives or surface preparation [1]. Furthermore, thin films add negligible mass to the components and thus, do not disturb the vibrational modes in smaller blades. Due to the existing thermal gradient across TBC coated turbine blades, it is possible to fabricate thermoelectric generators capable of powering active wireless transmitters. Thermoelectric devices are characterized by a dimensionless figure of merit, ZT which is defined according to eq. (1) as ZT= S2 T/
(1)
where , , S, and T are the electrical conductivity, thermal conductivity, Seebeck coefficient, and absolute temperature, respectively. Since thermal conductivity measurements are difficult to make on thin films, the thermoelectric power factor given by eq. (2) below is normally used to screen promising candidates for thermoelectrics, = S2 = Z
1
(2)
Since conventional thermoelectrics such as PbTe, Bi2Te3, and SiGe either have relatively limited operating temperature ranges or cannot be used in oxidizing environments, significant research is being done to find alternative thermoelectric materials based on refractory oxides [2]. Several attempts have been made to replace platinum/rhodium based thermocouples, which cannot be used at
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