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

Abstract

Polycrystalline (Na 1Ày M y ) x Co 2 O 4 (M=Nd, Sm, Dy and Yb; y ¼ 0:01 $ 0:05) samples were synthesized by a polymerized complex method. The effects of partial substitution of rare-earth metals for a Na-site on the thermoelectric properties of Na x Co 2 O 4 were investigated, and their differences with the sample prepared by a conventional solid-state reaction (SSR) method were evaluated. In case of the Nd, Sm and Dysubstituted samples, each rare-earth metal content in the matrix phase was slightly higher than that of the sample prepared by the SSR method, resulting in the slight increase in the thermoelectric power. Dy-substituted samples with y ¼ 0:03 and 0.05 showed the decrease in the electrical resistivity compared to the non-substituted sample because of the enhanced c-axis orientation. The power factors of the Dy-substituted samples were higher than that of the non-substituted sample over the entire temperature range, and the sample with y ¼ 0:05 showed the maximum power factor value of 1:5 Â 10 À3 Wm À1 K À2 at 1073 K. On the other hand, Yb-substituted samples with y 0:03 were almost single Na x Co 2 O 4 phase. The maximum Yb content in the matrix phase corresponded to y ; 0:03 from the EDX analysis. The value was remarkably higher than that of the sample prepared by the SSR method. For the Yb-substituted samples with y 0:03, the c-axis orientation was enhanced with increasing the nominal Yb composition in conjunction with the rise in the lattice parameter, c. The Yb substitution caused the increase in the electrical resistivity and thermoelectric power compared to the non-substituted sample over the entire temperature range, which is considered to be due to the reduction in the carrier density resulted from the Yb substitution for Na-site. The power factor of the sample with y ¼ 0:03 was enhanced over the entire temperature range, and the maximum power factor, 1:5 Â 10 À3 Wm À1 K À2 was obtained at 1073 K.

Restoration of long range order of Na ions in NaxCoO2 at high temperatures by sodium site doping

“…The data are taken from the following works: K [5], Mn [10], Ni [17]. Cu [14], Zn [16,18], Sr and Y [5], Ru [10], Ag [12,15], Nd [5], Sm [5,11], Dy [11], Yb [5,11], Au [15]. Cu doping, the most stable configuration was Y Co with a formation energy of 3.94 eV for x = 0.75 and 3.74 eV for x = 1.00.…”

“…21 Real-space global cut-off radii were set for all elements at 6. The data are taken from the following works: K, 5 Mn, 10 Ni, 17 Cu, 14 Zn, 16,18 Sr and Y, 5 Ru, 10 Ag, 12,15 Nd, 5 Sm, 5,11 Dy, 11 Yb, 5,11 and Au. 15 found that the interstitial dopants are more stable when located on a vacant Na2 (or Wyckoff d) position.…”

“…Fig. 2 shows the Seebeck coefficient (a) and electrical resistivity (b) of Na x CoO 2 doped with a wide variety of dopants as reported in the literature for T = ∼ 800 K. 5,[10][11][12][13][14][15][16][17][18] After inspection, we notice that (i) the highest Seebeck coefficient is obtained for late transition elements dopants like Cu and Zn for which the Seebeck coefficient exceeds ∼ 320 µVK −1 , followed by noble metals dopants such as Ag and Au for which the Seebeck coefficient is ∼ 200 µVK −1 and the rare earth dopants for which the Seebeck coefficient is ∼ 180 µVK −1 ; (ii) The highest Seebeck coefficient is achieved when x = ∼ 0.5 as higher Na concentrations consistently result in lower Seebeck coefficient; (iii) the lowest resistivity is achieved for dopant concentrations of ∼ 2.5% or lower. Higher dopant concentrations result in the higher resistivity (ρ), sometimes by few orders of magnitude.…”

“…The shade of the circles represents the concentration of the dopants as expressed in the legend of (b). The data are taken from the following works: K, 5 Mn,10 Ni,17 Cu,14 Zn,16,18 Sr and Y, 5 Ru, 10 Ag,12,15 Nd, 5 Sm,5,11 Dy,11 Yb,5,11 and Au 15.…”

Suppression of magnetism and Seebeck effect in Na0.875CoO2 induced by SbCo dopants

“…In spite of the efforts, doping Na x CoO 2 , as a strategy to enhance the thermoelectric performance, delivered mixed results. For instance, heavier ions such as rare earth Yb and noble metal Ag were successfully used to decrease the lattice thermal conductivity (k l ) and thus improving ZT [17,18]. Doping Ru and Mn in Na 0.5 CoO 2 , however, increased the resistivity to ~ 300 µΩ m, three times higher than that of the undoped Na 0.5 CoO 2 [19].…”