Preparation of high-performance Ca3Co4O9 thermoelectric ceramics produced by a new two-step method

High-performance Ca3Co4O9 thermoelectric ceramic has been prepared from a Ca1−xCoxO/CayCo1−yO divorced eutectic structure produced by a directional melt-grown using the laser floating zone technique. This material has been grown at very high solidification rate in order to produce a very fine microstructure to reduce the necessary annealing time to recover the Ca3Co4O9 thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases and estimate the extent of Ca3Co4O9 formation with time and related with their thermoelectric performances. The optimum annealing time, 72 h, has been determined by the maximum power factor value (about 0.42 mW K−2m−1), which is around the best values reported in textured materials (∼0.40 mW K−2m−1). This high power factor outcome from the high Ca3Co4O9 phase content, apparent density and Co3+/Co4+ relationship determinations performed in the present work.