Enhanced Thermoelectric Performance Induced by Cr Doping at Ca‐Sites in Ca₃Co₄O₉ System

Abstract

Thermoelectric performance of Ca3Co4O9 system has been enhanced through the misplaced substitution with Cr doping at Ca‐sites. The study of Cr doping at Co‐sites has also been performed for comparing. Base on the analysis of the structural data, we conclude that the doped Cr ions should be in the form of Cr+6, and enter into Ca‐sites of Ca2CoO3 layers in Ca3−xCrxCo4O9 lattice and Co‐sites of CoO2 layers in Ca3Co3.90Cr0.10O9 lattice, respectively. As Cr ions are doped into Ca‐sites in Ca3−xCrxCo4O9 lattice, both resistivity and thermopower increase obviously, while thermal conductivity decreases. Among all samples, Ca2.95Cr0.05Co4O9 shows the maximum ZT value, which is enhanced by 12% than that of Ca3Co4O9. The results show that the misplaced substitution with a proper Cr doping at Ca‐sites is beneficial to the enhancement of the thermoelectric performance in Ca3Co4O9 system compared to the traditional idea: Cr doping at Co‐sites, which is suggested to originate from the combined action of the carrier concentration, the electronic correlation, and the lattice disharmony.

Enhanced Thermoelectric Performance Induced by Misplaced Substitution in Layered Ca3Co4O9

“…Actually, such a misplaced substitution idea has been explored. For example, the substitutions of Cr for Li in LiFePO 4 structure, Cr for Ca and Ag for Co in Ca 3 Co 4 O 9 one, respectively. , In the former, the electric conductivity was enhanced by 8 orders of magnitude via Cr doping. Accordingly, such an enhancement would be greatly conductive to the improvement of TE performance.…”

Enhancing the Thermoelectric Performance of Calcium Cobaltite Ceramics by Tuning Composition and Processing

“…Figure 6b presents data for the variation of cobalt species in Ca 2.7 Bi 0.3 Co 3.92 O 9+δ ceramics before and after annealing at 1203 K. Cobalt is present in three valences in the Ca 3 Co 4 O 9 lattice: Co 2+ exists in the rock salt layers while Co 3+ and Co 4+ coexist in the CoO 2 layers. 42 The Co 2p 3/2 spectra are deconvoluted into three peaks, where the first peak at 779.26− 779.30 eV corresponds to Co 3+ , the second peak at 780.26− 780.30 eV is assigned to Co 2+ , and the third at 782.06−782.10 eV is ascribed to Co 4+ . These results are consistent with published Co 2p XPS data 43−45 and confirm a mixture of valence for cobalt in the calcium cobaltite lattice.…”

Calcium cobaltite, a promising oxide for energy harvesting: effective strategies toward enhanced thermoelectric performance

“…In fact, the misfit ratio between the [CoO 2 ] layer and the [Ca 2 CoO 3 ] layer is not exactly a rational number but approaches the golden ratio τ ≈ (1: 0.618) [25]. It was reported that mixed cobalt valence states occur in the calcium cobaltite lattice, where Co 2+ exists in the rock salt layers, while Co 3+ and Co 4+ coexist in the [CoO 2 ] layers [27]. By preserving charge neutrality in the different layers, electrons are transferred from the negatively charged [CoO 2 ] layers to the positively charged [Ca 2 CoO 3 ] layers, thereby generating hole carriers in the [CoO 2 ] layers [28].…”

“…As the substitution of Ca 2+ by higher valence ions (e.g. Cr 6+ , La 3+ , Eu 3+ , Bi 3+ ) [27,39,50,51] generates electrons, the hole concentration is expected to be reduced. However, these substituting ions have larger atomic mass or size than Ca 2+ ions, which gives rise to obvious strain and mass variations.…”

“…where τ D is the relaxation time for point defect scattering, A is a constant independent of temperature and frequency, ω is the phonon frequency, V is the volume per atom, v is the velocity of sound, Γ is the disorder scattering parameter, Γ MF and Γ SF are scattering parameters related to mass variation and strain field, C i is the relative degeneracy of the site, Mi and ri are the average mass and radii of dopants, f i is the fractional occupation and ε i is the amplitude of thermal vibration of the atoms. It was reported that by substituting 1.7 at.% Cr 6+ or 10.0 at.% Eu 3+ at the Ca 2+ site, the thermal conductivity at 300 K reduced by 35% [27,39]. In addition to lowering thermal conductivity, Bi 3+ doping at the Ca 2+ site also increases carrier mobility, through enlarging the inter-site hopping distance, and enhances the preferred grain orientation, thereby leading to a high ZT; for example a value of 0.25 at 973 K was reported by Liu et al [54].…”