Thermoelectric properties of transition-metal oxide NaCo/sub 2/O/sub 4/ system

Yakabe, H.; Kikuchi, Keiko; Terasaki, Ichiro; Sasago, Y.; Uchinokura, K.

doi:10.1109/ict.1997.667584

Cited by 39 publications

(38 citation statements)

References 7 publications

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“…Layered cobaltites with a two-dimensional-triangular lattice (2DTL) of Co ions have been intensely investigated because of their structural variety, promising thermoelectric performance, [1,2,3,4,5,6,7,8] and unpredictable superconducting behavior induced by an intercalation of H 2 O. [9,10,11] Among them, the sodium cobaltite Na x CoO 2 is apparently the most basic compound, because of its simple structure, the single CoO 2 planes and the single disordered Na planes form alternating stacks along the c axis.…”

Section: Introductionmentioning

confidence: 99%

Electron correlation in the two-dimensional triangle lattice ofNaxCoO2

et al. 2004

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Magnetism of layered cobaltites NaxCoO2 with x = 0.6 and 0.9 has been investigated by a positive muon spin rotation and relaxation (µ + SR) spectroscopy together with magnetic susceptibility and specific heat measurements, using single crystal samples in the temperature range between 250 and 1.8 K. Zero-field (ZF-) µ + SR measurements on Na0.9CoO2 indicates a transition from a paramagnetic to an incommensurate spin density wave state at 19 K(=T SDW ). The anisotropic ZF-µ + SR spectra suggest that the oscillating moments of the IC-SDW directs along the c-axis. Since Na0.6CoO2 is paramagnetic down to 1.8 K, the magnitude of T SDW is found to strongly depend on x. This behavior is well explained using the Hubbard model within a mean field approximation on two-dimensional triangle lattice in the CoO2 plane. Also, both the appearance of the IC-SDW state by the change in x and the magnitude of the electronic specific heat parameter of Na0.6CoO2 indicate that NaxCoO2 is unlikely to be a typical strongly correlated electron system.

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Section: Introductionmentioning

confidence: 99%

Electron correlation in the two-dimensional triangle lattice ofNaxCoO2

et al. 2004

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“…On the other hand, phonons will be affected by the disorder in the Na layer, since the disorderd Na + ions make ionic bonding with adjacent O 2− ions. In fact, a preliminary measurement has revealed that κ for Na 1.1 Co 2 O 4 is as low as 10 mW/cmK, 15 suggesting that MFP of the phonons is of the order of the lattice spacing. Thus MFP of the carriers is much longer than MFP of the phonons in NaCo 2 O 4 .…”

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Na-site substitution effects on the thermoelectric properties ofNaCo2O4

et al. 1999

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The resistivity and thermopower of Na1+xCo2O4 and Na1.1−xCaxCo2O4 are measured and analyzed. In Na1+xCo2O4, whereas the resistivity increases with x, the thermopower is nearly independent of x. This suggests that the excess Na is unlikely to supply carriers, and decreases effective conduction paths in the sample. In Na1.1−xCaxCo2O4, the resistivity and the thermopower increase with x, and the Ca 2+ substitution for Na + reduces the majority carriers in NaCo2O4. This means that they are holes, which is consistent with the positive sign of the thermopower. Strong correlation in this compound is evidenced by the peculiar temperature dependence of the resistivity.PACS numbers: 72.15. Jf, 72.80.Ga, 72.15.LhThere appears a growing interest to a hunt for new thermoelectric (TE) materials, 1 reflecting urgent needs for a new energy-conversion system in harmony with our environments. A TE material generates electric power in the presence of temperature gradient through the Seebeck effect, and pumps heat in the presence of electric current through the Peltier effect. A serious drawback is the low conversion efficiency: It is characterized by the so-called "figure of merit" Z = S 2 /ρκ, where S, ρ and κ are the thermopower, resistivity and thermal conductivity of a TE material, respectively. In other words, a good TE material is a material that shows large S, low ρ and low κ. However, a high value of Z is difficult to realize, because the three parameters cannot be changed independently. To overcome this difficulty, a number of new concepts and new materials have been examined.Recently we have observed that a layered cobalt oxide NaCo 2 O 4 exhibits unusually large S (100 µV/K at 300 K) accompanied by low ρ (200 µΩcm at 300 K) along the direction parallel to the CoO 2 plane. 2 NaCo 2 O 4 belongs to a layered Na bronze Na x CoO 2 , which was studied as a cathode for sodium batteries. 3 During the characterization, Molenda et al. 4 first found a large S in Na 0.7 CoO 2 . Although they noticed that S was anomalously large, they did not mention a possibility for a TE material. Their samples were polycrystals, the resistivity of which was 2-4 mΩcm at 300 K, much higher than that of our crystals. Our finding is that the carrier density (n) is of the order of 10 21 − 10 22 cm −3 , and is two orders of magnitude larger than n of conventional TE materials. This is difficult to understand in the framework of a conventional one-electron picture, and may indicate a way to get a good TE material other than the conventional approach. We have proposed that strong electron-electron correlation plays an important role in the enhancement of the thermopower of NaCo 2 O 4 .Even in a correlated system, we can expect that a conductor of low n will have a large S, because the diffusive part of S is the transport entropy, of the order of k B T /E F , where E F is the Fermi energy. 5 Thus it would be tempting to improve the TE properties in NaCo 2 O 4 by decreasing n. We easily think of three ways to change n in NaCo 2 O 4 , i.e., (i) doping of e...

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“…7) Additionally, Na atoms, which number about 50-70% of all possible Na sites, randomly occupy the Na sites in the Na layer. 8) This random occupation results in significantly low thermal conductivity. Thus, the Na x Co 2 O 4 polycrystal shows good performance as a thermoelectric oxide.…”

Section: Introductionmentioning

confidence: 99%

“…6,[8][9][10][11] Generally, it is considered that partial substitution for a Na site by metals such as alkali metals, alkaline earth metals or rare earth metals changes the valence of the Co ion, which affects the carrier concentration. On the other hand, partial substitution for a Co site by 3d or 4d transition metals is considered to change the electric spin state of a Co ion, which affects the carrier concentration, the mobility or/and the electron effective mass.…”

Section: Introductionmentioning

confidence: 99%

“…6) Especially, in the case of the hot-pressed samples with partial substitution for a Co site, it was reported that Cu substitution significantly enhanced the thermoelectric performance; the high figure-of-merit, 0.88 × 10 −3 K −1 at 300 K, was obtained. 8) However, effects of substitution for Co by other transition metals on the thermoelectric properties, especially in the range from room temperature to higher temperatures, where it is expected to be used practically, have not been investigated systematically. Additionally, in the case of the sintered Na x Co 2 O 4 polycrystal, the thermoelectric prop-erties are also affected by the quantity, size, number etc.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Partial Substitution of 3d Transition Metals for Co on the Thermoelectric Properties of NaxCo2O4

et al. 2002

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Layered P2 type Na x Co 2 O 4 thermoelectric oxides partially substituted by 3d transition metal (up to 15 at%: Cr, Mn, Fe, Ni, or Zn) for Co were synthesized through calcination and sintering in air. The Cr-substituted samples were mostly decomposed into the NaCrO 2 + Co 2 CrO 4 phases. On the other hand, the X-ray diffraction patterns of the Mn-substituted samples showed a single P2 type γ -Na x Co 2 O 4 phase. The Fe-and Ni-substituted samples were mostly composed of the Na x Co 2 O 4 phase with small amounts of Co 3 O 4 and CoNiO 2 phases. In the case of the Zn-substituted samples, only the Na x (Co 0.95 Zn 0.05 ) 2 O 4 was mostly composed of the Na x Co 2 O 4 phase with a small amount of the NaO phase. The effects of partial substitution of these 3d transition metals for Co except for Cr in the layered Na x Co 2 O 4 on thermoelectric properties were investigated. The electrical resistivity of these partially substituted samples significantly increased over the entire temperature range, as compared to the non-substituted sample, indicating reduction in carrier concentration. Except for Na x (Co 1−y Fe y ) 2 O 4 , the Seebeck coefficient was enhanced by the partial substitution, and in the case of Na x (Co 1−y Fe y ) 2 O 4 , it is suggested that the decrease in the Seebeck coefficient is associated with reduction in the electron-electron correlation due to the substitution. For the Mn-substituted samples, a small grain size and many pores compared to the non-substituted sample were effective for reducing the thermal conductivity. The figure-of-merit Z was enhanced in the case of the Na x (Co 0.95 Zn 0.05 ) 2 O 4 above 550 K and the maximum dimensionless figure-of-merit ZT, 0.68, was obtained at 965 K.

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Thermoelectric properties of transition-metal oxide NaCo/sub 2/O/sub 4/ system

Cited by 39 publications

References 7 publications

Electron correlation in the two-dimensional triangle lattice ofNaxCoO2

Electron correlation in the two-dimensional triangle lattice ofNaxCoO2

Na-site substitution effects on the thermoelectric properties ofNaCo2O4

Effect of Partial Substitution of 3d Transition Metals for Co on the Thermoelectric Properties of Na<SUB>x</SUB>Co<SUB>2</SUB>O<SUB>4</SUB>

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