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|Transverse Multilayer Thermoelectric Generators|
|Keywords: thermoelectrics, multilayer thermoelectric generator, oxides|
|Thermoelectric generators (TEG) exhibit high potential as renewable energy sources transforming waste heat into electricity. Oxides represent a versatile class of thermoelectric materials. Unfortunately, their thermoelectric performance is inferior compared to classic semiconductor thermoelectric materials (lower ZT values). On the other hand, oxide thermoelectrics exhibit other advantages, e.g. stability at increased operating temperatures. Thermoelectric oxide ceramics are attractive because of their lower price, availability and environment friendliness. Ceramic TEGs require p- and n-conducting oxides with high Seebeck coefficient and high electrical and low thermal conductivity. The p- and n-oxides are arranged as individual blocks and are electrically connected by a metallization pattern. TEG modules with a high level of integration and miniaturization can be prepared as multilayer TEGs from tapes of the thermoelectric materials. The ceramic multilayer technology, which is a standard technology for functional ceramics components, enables fabrication of miniaturized oxide-based generators. We report on the fabrication of multilayer thermoelectric generators. Examples of n- and p-type conducting oxides that are discussed in detail include n-type CaMnO3 (CMO) as well as p-type La2CuO4 and Ca3Co4O9 (CCO). The shrinkage behavior of the materials is adapted using appropriate additives to enable co-firing of different materials as required in a multilayer process. The synthesis of individual oxide materials, their sintering behavior and thermoelectric properties will be discussed. Green sheets of n- and p-doped oxide ceramics and glass insulators were made by tape casting with a doctor blade. Multilayer TEG�s were made by stacking thermoelectric oxide green sheets and screen-printing of metal conductors. Sintering was performed as co-firing of all components with glass insulation layers between n- and p-type thermoelectric layers. Conductor pattern were printed using silver, silver/palladium or gold inks. However, co-firing of such multilayer thermoelectric generators consisting of at least four different materials turned out to be very challenging. We report on oxide-based ceramic Transversal Multilayer Thermoelectric Generators (TMLTEGs). Green tapes of the thermoelectric oxides were prepared using a doctor blade casting process. Multilayer generators were assembled by stacking n- or p-type thermoelectric oxide layers and by screen-printing metal stripe layers. This concept of a transversal multilayer generator is based on the combination of one thermoelectric oxide only with metal electrodes. The electrode design consists of stripes, which are oriented in a certain angle to the current flow. This enables the use of the transversal thermoelectric effect, with the directions of the heat and charge flow perpendicular to each other. This concept allows facile fabrication of oxide multilayer TEGs. Demonstrators of transversal TEGs provide some mW power at T = 100 k or less and might be used for energy conversion from waste heat into electricity in the low-power range to drive autonomous sensors and microsystems. TMLTEGs were fabricated based on n-type Gd/W-doped CMO or p-type CCO in combination with Ag/Pd metallization. We report on the synthesis, microstructure and texture formation in multilayers.|