Thermoelectric Properties of Ca3Co2−xMnxO6 (x = 0.05, 0.2, 0.5, 0.75, and 1) (original) (raw)
Related papers
Chemical stability of Ca3Co4−xO9+δ/CaMnO3−δ p–n junction for oxide-based thermoelectric generators
RSC Advances, 2020
An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. Ca 3 Co 4Àx O 9+d and CaMnO 3Àd exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca 3 Co 1+y Mn 1Ày O 6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca 3 Co 1+y Mn 1Ày O 6 varies and is close to 1 (y ¼ 0) at the Ca 3 Co 1+y Mn 1Ày O 6-CaMnO 3Àd boundary. The existence of a wide homogeneity range of 0 # y # 1 for Ca 3 Co 1+y Mn 1Ày O 6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.
Synthesis and Characterization of Bulk Nanostructured Thermoelectric Ca3Co4O9
Journal of Nanoscience and Nanotechnology, 2017
Nanostructuring has been proposed as an effective strategy for the reduction of the phonon contribution to the thermal conductivity, resulting in an increase in the figure of merit of thermoelectric materials. However, obtaining bulk samples presenting high relative density and nanometric grain size can be quite challenging, particularly in the case of ceramic phases. Only few examples have been reported and none in the case of Ca 3 Co 4 O 9. In this work, we used a sol-gel synthesis coupled with ball milling to prepare powders of Ca 3 Co 4 O 9 presenting a grain size as small as 4 nm. These nanopowders were then sintered at different temperature and pressures using the High-Pressure Field-Assisted Sintering Technique (HP-FAST). Relative densities up to 95 vol% where obtained while maintaining a nanometric grain size. The microstructural and electrical properties of the sintered samples have been characterized. The results show that in this oxide a reduction to the nanometric grain size produces a drastic reduction in the electrical conductivity, which cannot be compensated by the reduction in the thermal conductivity. The Seebeck effect, on the other hand, appears to be affected only marginally by the grain size and porosity.
High-temperature thermoelectric properties of Ca1−xPrxMnO3−δ (0⩽x<1)
Physica. B, Condensed matter, 2004
Ca 1Àx Pr x MnO 3Àd (x=0, 0.05, 0.15, 0.1, 0.2, 0.4, 0.67; d=0.02) samples were prepared by a solid-state reaction method. X-ray diffraction analysis showed that all samples prepared were of single phase with orthorhombic structure. Electrical resistivity measurements from room temperature to 1300 K showed that a metallic conducting tendency dominated at high temperatures. The hopping nature of the charge carriers was well interpreted in the framework of polaron theory. The Seebeck coefficient was measured in the same temperature interval, and its concentration dependence was analyzed using the high-temperature (HT) thermopower theory proposed by Marsh-Parris. The thermal conductivity and the figure of merit of the prepared samples were also compared with those of other similar perovskite compounds. The observed figure of merit of the sample with x=0.15 was Z=1.5 Â 10 À4 K À1 at T=1100 K, indicating a good potential for application as a HT thermoelectric material.
Temperature dependent electrical resistivity, crystal structure and heat capacity measurements reveal a resistivity drop and metal to semiconductor transition corresponding to first order structural phase transition near 400 K in Ca 3 Co 4 O 9 . The lattice parameter c varies smoothly with increasing temperature, while anomalies in the a, b 1 and b 2 lattice parameters occur at ~ 400 K. Both Ca 2 CoO 3 and CoO 2 layers become distorted above ~ 400 K associated with the metal to semiconductor transport behavior change. Resistivity and heat capacity measurements as a function of temperature under magnetic field indicates low spin contribution to this transition. Reduced resistivity associated with this first order phase transition from metallic to semiconducting behavior enhances the thermoelectric properties at high temperatures and points to the metal to semiconductor transition as a mechanism for improved ZT in high temperature thermoelectric oxides.
Journal of Alloys and Compounds, 2014
Misfit-layered Ca 3 Co 4Àx Cr x O 9+d (0 6 x 6 0.2) powders were synthesized by a simple thermal hydrodecomposition method and then fabricated to form highly dense ceramics using spark plasma sintering. X-ray diffraction, and a field emission scanning electron microscope equipped with an energy dispersive X-ray spectrometer were used to verify the single phase of Ca 3 Co 4 O 9+d for the samples with x = 0, 0.05, 0.10 and 0.15. The characteristic plate-like grain structure was observed. The chemical composition of the fabricated samples was found to be close to nominal composition. The thermoelectric measurement showed that Cr doping has an influence on the thermoelectric properties. The resistivity and the Seebeck coefficient increased while the thermal conductivity was suppressed when Cr was added into the system. Explanation of the change in thermoelectric properties was discussed regarding the difference in the charge states and the ionic radii between Co and Cr ions. The highest ZT of 0.19 at 1073 K was obtained for the Ca 3 Co 3.85 Cr 0.15 O 9+d sample. In addition, the paramagnetic property was observed in all samples at room temperature.
TOWARDS HIGHLY EFFICIENT THERMOELECTRICS: Ca 3 Co 4 O 9+δ · n CaZrO 3 COMPOSITE
Ceramics Silikaty
We successfully prepared Ca 3 Co 4 O 9+δ · n CaZrO 3 composites by a ceramic route. These composites were characterized by X-Ray diffraction, differential thermal analysis, thermogravimetric analysis and scanning electron microscopy. Moreover, transport properties (Seebeck coefficient, electrical resistivity and thermal conductivity) were measured and the thermoelectric figure of merit ZT was determined. Addition of CaZrO 3 led to a suppression of thermal conductivity of the samples. A high thermal stability connected to the interesting thermoelectric properties made this material a potential candidate for the p type cell in the high-temperature thermoelectric batteries.
Materials Science and Engineering: B, 2003
In the Ca-Co-O system, there exists at least two identified phases, Ca 3 Co 4 O 9 and Ca 3 Co 2 O 6 exhibiting 2D CoO 2 planes and 1D Co-Co chains, respectively. Both oxides are characterized by large thermopower values at room temperature, the largest value being obtained for Ca 3 Co 2 O 6 (Seebeck coefficient, S 300K = 450 V K −1 ). However, the electrical resistivity of the latter is too large (typically 50 cm) for applications as thermopower materials. Interestingly, the substitutions of Ir 4+ or Rh 4+ for one cobalt out of two in the chain do not greatly affect the electronic properties in contrast to the substitution by Sc 3+ (3d 0 ). This emphasizes that the Ir or Rh cations can play a similar role to that of the Co cations. This opens a new route for the search of new thermoelectric materials alternative to the cobaltites.
Electronic structure and large thermoelectric power in Ca3Co4O9
Journal of Electron Spectroscopy and Related Phenomena, 2004
Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (E ≈ 10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca 3 Co 4 O 9 , which has attracted interests as one of the potential thermoelectric materials because of its possession of high thermoelectric power S, relatively low electrical resistivity ρ, and small thermal conductivity κ. A narrow band of 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level (E F). This peak turns out to be less significant when Co 2p-3d resonance takes place, indicating that it consists mainly of O 2p and of small amount of Co 3d component. Since E F is located near the high-energy edge of this narrow band, the density of states at E F is finite but negligibly small at room temperature. An energy-gap across E F opens below 50 K with decreasing temperature. This development of the energy-gap causes the insulating behavior; divergence both in electrical resistivity and Hall coefficient. We calculated thermoelectric power S(T) using the photoemission spectra near E F. The calculated S(T) shows almost half of the measured value. Large thermoelectric power up to 200 V/K observed for the Ca 3 Co 4 O 9 is closely related to the metallic electron transport in a less dispersive band with E F near its band-edge.
Journal of Physics: Condensed Matter, 2011
The incommensurately layered cobalt oxide Ca 3 Co 4 O 9 exhibits an unusually high Seebeck coefficient as a polycrystalline bulk material, making it ideally suited for many high temperature thermoelectric applications. In this paper, we investigate properties of Ca 3 Co 4 O 9 thin films grown on cubic perovskite SrTiO 3 , LaAlO 3 , and (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 substrates and on hexagonal Al 2 O 3 (sapphire) substrates using the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy analysis indicate strain-free growth of films, irrespective of the substrate. However, depending on the lattice and symmetry mismatch, defect-free growth of the hexagonal CoO 2 layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca 2 CoO 3 buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO 2 stacking faults is observed, possibly due to weak interlayer interaction in this layered material. We propose that these stacking faults have a significant impact on the Seebeck coefficient and we report higher values in thinner Ca 3 Co 4 O 9 films due to additional phonon scattering sites, necessary for improved thermoelectric properties.