Thermoelectric generators (TEG) exhibit high potential as renewable energy sources transforming waste heat into electricity. Most devices rely on semiconductor thermoelectric materials, however, oxides also 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 advantageous properties, e.g. stability at increased operating temperatures. The application of the ceramic multilayer technology is an interesting option enabling the fabrication of miniaturized oxide-based multilayer generators. Such multilayer TEGs might be used for energy conversion from waste heat into electricity in the low-power range to drive autonomous sensors and microsystems.
We report on an experimental study to fabricate multilayer thermoelectric generators. Examples of n- and p-type conducting oxides that are discussed in detail include n-type Nd2CuO4 and CaMnO3 as well as p-type La2CuO4 and Ca3Co4O9. The materials were prepared by the mixed-oxide route with dopants to tailor the electric properties. The shrinkage behaviors of the materials were adapted using appropriate additives to enable co-firing of different materials. The preparation of the individual oxide materials, their sintering behaviors and thermoelectric properties are presented and correlations between structure and properties will be discussed.
A concept of an oxide-based ceramic multilayer thermoelectric generator is demonstrated. Thermoelectric oxides were adapted according to the requirements of the multilayer process. Green tapes of the thermoelectric oxides were prepared using a doctor blade tape casting process. Multilayer generators were assembled by stacking n- and p-type thermoelectric oxide layers which are separated by insulating glass layers and connected by screen-printed metal contacts. However, the co-firing of such ceramic multilayer modules turned out to be very challenging. Alternatively, the concept of a transversal oxide multilayer generator was developed; here only one thermoelectric oxide is combined with metal electrodes. This allows facile fabrication of oxide multilayer TEGs.