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>

Abstract

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.

Effect of Partial Substitutions of Rare-earth Metals for Na-site on the Thermoelectric Properties of Na<SUB><i>x</i></SUB>Co<SUB>2</SUB>O<SUB>4</SUB> Prepared by the Polymerized Complex Method

“…Recently, metal oxides such as ZnO, Ca-Co-O, and Na x Co 2 O 4 systems [1][2][3][4][5] have been recognized as encouraging thermoelectric materials because they have good stability at high temperature and a low toxicity. Na x Co 2 O 4 has a layered structure, where the Na layer, randomly occupied by 50% Na ions in the regular sites, and the CoO 2 layer of the edgesharing CoO 6 octahedra, are alternately stacked along the caxis.…”

Synthesis of NaxCo2O4thermoelectric oxide with crystallographic anisotropy by chemical solution process

“…have been recognized as potential candidates for a new thermoelectric material [1][2][3][4][5][6]. These thermoelectric oxides can be used at high temperatures without deterioration of their performance due to oxidation, and their production costs are comparatively low.…”

Effect of the Polymerized Complex Process on Doping Limit of Thermoelectric Na<I><SUB>x</SUB></I>Co<SUB>1&minus;<I>y</I></SUB>M<I><SUB>y</SUB></I>O<SUB>2</SUB> (M=Mn, Ni)

“…For the Mn substitution, the ceramic sample with y ¼ 0:10 was also found to be the single phase of -Na x CoO 2 , as well as in the case of the Mn-substituted SSR ceramic sample. 12) On the other hand, in the XRD pattern of the Ni-substituted ceramic sample with y ¼ 0:10, several peaks from the CoNiO 2 phase appeared, indicating the nominal composition of Na x Co 0:90 Ni 0:10 O 2 was beyond the Ni soluble limit in the phase. However, for the Ni-substituted SSR ceramic sample with y ¼ 0:05, the CoNiO 2 phase was clearly detected in its XRD pattern.…”

“…The Nisubstituted ceramic sample showed a thermal conductivity greater than that of the non-doped ceramic sample above 700 K. For the Ni-substituted SSR ceramic samples, the precipitation of a large amount of the CoNiO 2 phase significantly increased the thermal conductivity, indicating that the thermal conductivity of the CoNiO 2 phase is higher than that of the Na x CoO 2 phase. 12) In addition, the relative density of the Ni-substituted ceramic sample was 2% higher than that of the non-substituted ceramic sample. The small amount of CoNiO 2 and the slight increase in the relative density are considered to contribute to the increase in thermal conductivity.…”

“…The substitution of several 3d transition metals at the Co site was tried using the SSR process. 12) However, except for Mn, the concentration of the substitutive elements in the Na x CoO 2 phase was quite lower than the nominal composition and large amount of second phases associated with the substitutive metals precipitated, resulting in a deterioration of the thermoelectric performance, especially in the case of the Ni and Fe substitutions. The PC process is also expected to be effective for promoting the substitution of 3d transition metals, such as Ni, etc., for the Co site, because the constituent elements can be mixed at an atomic level during the PC process, as described above.…”