High thermoelectric performances in Co-oxides processed by a laser floating zone technique

Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit high thermopower and relatively low electrical resistivity, as well as high chemical stability at high temperatures. They show anisotropic thermoelectric properties linked to their layered structures. As a consequence, different texturing methods developing oriented grains, such as sinterforging, and template grain growth processes have been successfully used to improve the performances of these materials. There are some other less used methods, like the directional growth from the melt, which have already shown their applicability to this kind of compounds. One of these methods is the laser floating zone (LFZ) melting technique. This process has demonstrated its suitability in the Co-oxide based thermoelectric materials processing in the last years in our laboratories. In this work, two examples of the versatility and usefulness of the LFZ technique will be shown. The first one will be based on the processing of the well known Ca3Co4O9 ceramic material. In this case, a very dense and homogeneous material is obtained in two steps (growth and annealing), reaching PF values as high as 0.42 mW/K2m at 800 ºC. In the second case, a new and well textured thermoelectric composite (Bi2Ca2Co1.7Ox + Ca3Co4O9) is obtained, reaching PF values of 0.31 mW/K2m at 650 ºC.