Papers by Keyword: NaCo₂O₄

Abstract: The electronic structures of thermoelectric material NaCo2O4 were studied by the first-principles calculations with plane-wave pseudopotential method and generalized approximation (GGA) based on density functional theory (DFT). On the basis of calculation for electronic structures, thermoelectric properties of NaCo₂O₄ were also investigated in this paper. The reasons for large thermoelectric potential are large effective mass of holes at the top of valence band and 6μB net spin magnetic moment. While high electrical conductivity should be attributed to small effective mass of electrons at the bottom of conduction band, large carrier concentration near the Fermi level and narrowed energy gap. In addition, covalent bond between Co atom and O atom promotes carrier mobility and thus benefits to electrical conductivity. Na1 and Na2 have different effects on the NaCo₂O₄. Na1 provides local state electrons and yet Na2 provides itinerant-electrons which play important role to strongly correlated system.

Abstract: NaCo₂O₄ and 0.5at%-Sb doped Mg₂Si have excellent thermoelectric properties. We tried to fabricate a thermoelectric module composed of these materials and using Ni plates as electrodes. The fine powder of NaCo₂O₄ was prepared by metal-citric acid complex decomposition. 0.5at%-Sb doped Mg₂Si bulk was ground to powder and sieved to a powder particle size of 75 micrometers or less. These powders were sintered using spark plasma sintering (SPS) to obtain a body of NaCo₂O₄ and 0.5at%-Sb doped Mg₂Si. These thermoelectric materials were connected to the Ni plates by using the SPS method. The whole process took a very short time (less than 2 min) and could be done at low temperature (below 873 K). The open-circuit voltage values were 82.7 mV, and the maxima, maximum output current and maximum output power, for the single module were 212.4 mA and 6.65 mW at ΔT = 470 K.

Abstract: The K, Ca, Sr and Ce doped and non-doped γ-NaxCo2O4 samples were prepared by the solid-state reaction method. Results show that the Na-site doping might lead to an increase in the Seebeck coefficient and a decrease in the electrical conductivity of the samples. The maximum power factors of the K, Ca and Sr doped samples are respectively 2.04, 1.93 and 1.9 W·m-1·K-2, corresponding to an increase by 58%, 50% and 47% compared with that of the non-doped samples. Thus, the Na-site doping can improve the thermoelectric properties of γ-NaxCo2O4 oxides.