Effect of a powder compaction process on the thermoelectric properties of Bi2Sr2Co1.8Ox ceramics

Bi–Sr–Co–O ceramics, as members of a thermoelectric cobaltite family, do not reach the quality of thermoelectric compounds used at low temperatures. Their thermoelectric performance can be, however, boosted by the microstructure tailoring or by the variation of cobalt valence. In this work, we compare Bi2Sr2Co1.82Ox polycrystalline samples that were compacted using different procedures. The precursor powder was prepared by a chelating sol–gel method. Three compaction methods were subsequently tested: (i) cold uniaxial pressing, (ii) hot uniaxial pressing on BELT equipment and (iii) spark plasma sintering, all three followed by additional annealing in air. The thermoelectric ceramics were characterized by XRD, SEM and electric transport measurement up to 950 K. The temperature dependence of the power factor and the figure of merit were also calculated. The spark plasma sintering process yielded a material with an improved value of Seebeck coefficient – 190 μV/K at 300 °C – the highest value reported for the polycrystalline Bi-222 material so far.