Improvement of the thermoelectric characteristics of Fe-doped misfit-layered Ca3Co4−xFexO9+δ (x=, 0.05, 0.1, and 0.2)

Abstract

The authors report measurements of the electrical resistivity, Seebeck coefficient, and thermal conductivity for a series of misfit-layered oxides Ca3Co4−xFexO9+δ (x=0, 0.05, 0.1, 0.15, 0.2) prepared by solid state reaction. Structural parameters are refined with superspace group of X2∕m(0b0)s0 using powder x-ray diffraction data. With partial substitution of Fe+2 for Co+3, the resistivity decreases, while the thermopower increases simultaneously. The x=0.05 sample exhibits a higher figure of merit (Z=3.01×10−4K−1) than that of Ca3Co4O9+δ (0.33×10−4K−1) at 300K, indicating much improvement of thermoelectric characteristics via partial substitution of Fe for Co.

Thermoelectric properties of transition metals-doped Ca3Co3.8M0.2O9+δ (M = Co, Cr, Fe, Ni, Cu and Zn)

“…It was found from the Hall measurement that the hole carrier concentration in the polycrystalline doped CCO349 system is in the range of 2-4 9 10 20 cm -3 [9,24,29], and the mobility of hole is *1.0 cm 2 /Vs [24]. Putting in these values into Eq.…”

“…As mentioned prior that the major charge carrier of CCO349 was hole, and assuming the mobility of the carrier does not change significantly by doping, the conductivity is directly proportional to the carrier concentration. Previous studies have shown that the ionic state of the Co ions in the CCO349 structure can be ?2, ?3 and ?4, with the average valence between ?3 and ?4 [5,27,29]. The usual valence states of Fe and Cu ions are Fe 2?…”

“…However, elemental doping at the Co-site may have more influential effect on thermoelectric properties since the charge carrier transport of Ca 3 Co 4 O 9?d is restricted mostly to the CdI 2 -type CoO 2 layer [25], and also the Fermi level lies at the top of the valence band of the CoO 2 [26]. There have been a few experimental studies on partial substitution at the Co-site [6,9,24,[27][28][29][30][31][32], but very few studies reported a complete set of thermoelectric properties (resistivity, thermopower, and thermal conductivity) and the figure-of-merit at high temperature [9,24,30]. Thus, in this paper, following our previous work on synthesis, mechanical and magnetic properties [33], we focused on the thermoelectric properties of Ca 3 Co 4-x M x O 9?d (where M is Cr, Fe, Ni, Zn, and Cu).…”

Effects of Eu and Fe co-doping on thermoelectric properties of misfit-layered Ca3Co4O9+δ

“…can enhance the thermopoower, but also increase the resistivity [11][12][13][14][15][16]. In contrast, doping in Co-site is more complicated due to the intrinsic feature of multiple oxidation states of the transition metals as dopants as well as the presence of two nonequivalent Co sites in the [Ca 2 CoO 3 ] and [CoO 2 ] subsystems [7,9,[17][18][19]. The second approach focuses on advanced processing techniques, including spark plasma sintering (SPS), hot-pressing, multisheet corfiring (MSC), and sol-gel based electrospinning, which induce desired textures and nanocrystalline structures in the thermoelectric oxides [20][21][22][23][24][25].…”

“…concentrations, and suppresses the thermal conductivity [12,28]. On the other hand, the Fe substitution at Co-site could enhance the thermopower and decrease the electrical resistivity, which is believed to be associated with the increased carrier concentration and the enhanced electronic correlations [7,19]. These results motivated us to investigate the effect of co-doping on the thermoelectric properties by these two elements, i.e., Eu at Ca-site and Fe at Cosite.…”