Low temperature thermoelectric properties of Na-substituted Bi2Ca2Co2Oy ceramics fabricated via LFZ technique

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In this work, Bi2Ca2-xNaxCo2Oy with x = 0.0, 0.05, 0.10, and 0.125 thermoelectric ceramic materials have been produced by the classical solid-state route, followed by texturing through the laser floating zone (LFZ) method. All specimens showed similar XRD patterns, with the main peaks corresponding to the (00l) plane reflections of the thermoelectric Bi2Ca2Co2Oy phase, independently of Na substitution. In addition, small amount of Co-free Ca4Bi6O13 secondary phase has been identified. SEM micrographs showed that annealing procedure after the texturing process allows obtaining higher proportion of thermoelectric phase, decreasing the number and amount of secondary phases. Electrical resistivity is very similar for all samples at room temperature, about 0.62 mΩ m. However, below 50 K, Na-substitution decreases resistivity when compared to the undoped one, reaching values around 9.41, 6.48, 5.00, and 2.07 mΩ m for x = 0, 0.05, 0.1, and 0.125 samples at 50 K, respectively. Seebeck coefficient values at room temperature decrease from 180 to 145 μV/K, for the pure and 0.125Na doped samples, respectively. Thermal conductivity tends to linearly increase with temperature up to around 200 K, decreasing for higher ones. Thermal conductivity values are slightly higher in undoped samples (1.2 W/K m), when compared to the Na-substituted ones (1.10–1.30 W/K m) at room temperature. Finally, the highest ZT value is around 0.018 at 400 K for the 0.10Na doped samples.