Effect of NaF doping on the thermoelectric properties of Ca3Co4O9.

We report on thermoelectric properties of layered misfit cobaltites Ca 3− x Na x Co 4 O 9− x F x ( x = 0, 0.3, 0.45, 0.6) prepared by a two steps solid state reaction route followed by spark plasma sintering. Structural characterization revealed that a minor amount of Co 3 O 4 is presented in all the samples as an impurity phase. Besides, a trace of NaF phase was detected in the sample with x = 0.6 which can be an indication of the solubility limit of NaF into Ca 3 Co 4 O 9 . Measurements of electrical resistivity and Hall effect showed that double substitution of Ca 2+ by Na + and O 2− by F − results in a decrease of the electrical resistivity which is due to an increase in the carrier concentration. Thermal conductivity of Ca 3− x Na x Co 4 O 9− x F x decreases with increasing temperature which is due to the enhancement of the phonon scattering at high temperatures and slightly increases with x which is caused by the increase in the carrier concentration and the growth of the mean grain size in the doped samples. Seebeck coefficient S of Ca 3− x Na x Co 4 O 9− x F x is positive in the whole temperature range measured. At high temperatures (above 800 K) value of S in the x = 0.3 and x = 0.45 samples exceeds that in the pristine sample. Thermoelectric figure of merit ZT = 0.13 was achieved at 873 K, which is of about 20% higher than that in the pristine Ca 3 Co 4 O 9 . [ABSTRACT FROM AUTHOR]