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Title Enhancing Thermoelectric Figure-of-Merit of Polycrystalline NayCoO2 by a Combination of Non-stoichiometry and Co-substitution
 
Names MALLICK, MM
VITTA, S
Date Issued 2018 (iso8601)
Abstract Co-oxides with a layered structure are of interest for high-temperature thermoelectric applications as they can be tuned to enhance their electrical conductivity while retaining their low thermal conductivity. The figure-of-merit of NayCoO2 has been enhanced using the combined effects of Na-non-stoichiometry and non-isoelectronic Co-substitution. A series of compounds Na0.7Co1-xNixO2 with x <= 0.1 have been synthesized using conventional techniques. Structural analysis using x-ray diffraction and Rietveld refinement shows the formation of a gamma-NaCoO2-type phase in all the compounds. The presence of a small amount of NiO for x > 0.05 indicates that the solubility limit of Ni in Na0.7CoO2 is 5 at.%. All the compounds have been found to be p-type with the thermopower reaching a maximum of 220 mu V K-1 at 1023 K for x = 0.1. The thermopower has been found to vary linearly with temperature for all the compounds; a degenerate metallic behavior. The electrical resistivity varies between 3 and 10 m Omega cm at all temperatures and has a metallic temperature dependence in agreement with the thermopower results. The power factor for the x = 0.1 compound reaches a maximum value of 0.55 mW m(-1) K-2 at similar to 900 K compared to 0.45 mW m(-1) K-2 for the compound with no substitution. The thermal conductivity at 1023 K decreases from 1.2 to 0.9 W m(-1) K-1 for x = 0.1. These factors lead to an increase of the figure-of-merit, zT, to 0.58 at 1023 K for x = 0.1, an increase of 57% compared to the unsubstituted compound. The magnetic studies show that Na0.7CoO2 is paramagnetic with an antiferromagnetic transition at similar to 36 K. Substitution of Ni2+ for Co3+ has been found to induce a ferromagnetic-like transition at similar to 240 K which is suppressed at high fields.
Genre Article; Proceedings Paper
Topic Thermoelectric material
Identifier JOURNAL OF ELECTRONIC MATERIALS,47(6)3230-3237