Thermoelectric properties of directionally grown Bi₂Ba₂Co₂O_δ/Ag composites: effect of annealing

Bi₂Ba₂Co₂O_δ thermoelectric ceramics with Ag additions (0, 1, 3, and 5 wt%) have been successfully textured using the laser floating zone method. Microstructure has shown that samples are composed by thermoelectric grains, together with a high amount of secondary phases, and that 3 wt% Ag addition leads to the best grain orientation. These microstructural benefits have been reflected in the highest thermoelectric performances determined in these samples. It has also been found that annealing procedure leads to a drastic decrease of secondary phases amount and raises the oxygen content in the thermoelectric phase. These modifications are reflected in an important decrease of electrical resistivity, ρ (compared to the as-grown samples) without drastic modification of Seebeck coefficient, S. As a consequence, the highest thermoelectric performances, determined through the power factor, have been reached in the 3 wt% Ag textured and annealed samples in the whole measured temperature range. The maximum PF (=S²/ρ) values at 650 °C determined in these samples are very close to the highest reported in single crystals or very low rate LFZ grown Bi₂Ba₂Co₂O_δ compounds, making them very attractive for practical applications.