Preparation of High Seebeck Coefficient Calcium Cobaltate Thermoelectric Powders
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Preparation of High Seebeck Coefficient Calcium Cobaltate Thermoelectric Powders Sidney Lin1 and Jiri Selig1 1 Dan F. Smith Department of Chemical Engineering, Lamar University P.O. Box 10053, Beaumont, TX 77710, U.S.A.
ABSTRACT Thermoelectric calcium cobalt oxide (Ca1.24Co1.62O3.86) with high Seebeck coefficient was prepared by self-propagating high temperature synthesis (SHS) followed by a short post treatment process. Synthesized samples were analyzed by XRD for their phase purity for samples prepared from different reactants mixtures. A final element model of SHS of calcium cobalt oxide was developed to study the temperature history and reaction rate change during the synthesis. This model can be used to predict reaction temperatures for various initial conditions. INTRODUCTION The temperatures in automobile exhaust can reach 600 ºC. This high temperature exhaust causes a large amount of waste heat (20.90x1015 Btu) released every year [1]. Thermoelectric devices are used to convert automobile exhaust heat to electric power which not only improve automobile fuel efficiency but also reduce heat pollution, an important factor in climate change. Thermoelectric devices utilize a temperature difference to generate electrical current. The Seebeck effect describes the generation of an electrical current in a closed loop when there is a temperature difference between two leads [2]. In recent years, the Seebeck effect is used to generate electrical power for electronic equipment in automobiles from automobile exhaust heat [3]. Thermoelectric materials are characterized by their thermoelectric figures of merit, ZT, defined by equation 1: ZT =
ασ T κ
(1)
where α is Seebeck coefficient, σ is electrical conductivity, T is temperature, and κ is thermal conductivity. From equation 1 can be seen that a good thermoelectric material should have a high Seebeck coefficient to produce a substantial voltage, a high electrical conductivity to minimize the Joule heating, and a low thermal conductivity to keep a high temperature difference within the material. However, these properties are closely related. Optimizing one property could degrade another one and yield no or negative net effect. Calcium cobaltates are good candidates for thermoelectric material for automobile applications because of their stability at a high temperature and oxidizing environment [4]. Their intricate layered structure, which combines the electronic layers of CoO2 and insulating Ca2CoO3 substructure [5]. This substructure approach helps to separate the electronic and thermal properties of the material. The electronic layer keeps a good electrical conductivity in the material while the layered structure increases phonon scattering and thus lowers thermal conductivity [6]. The highest reported ZT values of calcium cobaltates at 800 ºC are close to 0.3
[7,8]. This value is not as large value as convectional thermoelectric materials, but it is still makes calcium cobaltate good thermoelectric material. There are several widely used techniques t
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