Application of laser technology on CoO-based materials for the improvement of their thermoelectric performances

The laser radiation has been found to be a very useful tool in the materials processing during the last years. Between all these processes, some of them, probably less known, have been used to produce texture in the processed materials. This grain orientation can be easily produced in the whole material volume by the laser floating zone (LFZ) technique. With the help of this process, based on the directional growth from the melt, it is possible to enhance the performances of highly anisotropic materials. In the last years this process has been successfully applied on different materials characterized by their high crystalline and electrical anisotropy, as high-Tcsuperconductors and oxide thermoelectric materials. This is the case for the pure and doped Bi-AE-Co-O (AE: Alkaline earth) misfit layered thermoelectric (TE) compounds. In spite of the formation of multiphase materials after the directional solidification, the high-temperature measured properties are higher than the usually obtained on classically textured materials (sinter-forging). Moreover, when these materials are subjected to an annealing process, their TE performances are further improved. This technique has also been successfully applied on the Ca3Co4O9 (349). In this system, in accordance with the phase diagram, a dense CaO-ss and CoO-ss eutectic microstructure is produced. After an annealing process, performed under air, the solid solutions react to produce the 349 TE phase. It has been found that the optimal annealing process lead to the highest reported TE performances measured on bulk 349 materials.