ABSTRACT
Layered structured Ca3Co4O9 has displayed great potential for thermoelectric (TE) renewable energy applications, as it is nontoxic and contains abundantly available constituent elements. In this work, we study the crystal structure and high-temperature TE properties of Ca3-2y Na2y Co4-y Mo y O9 (0 ≤ y ≤ 0.10) polycrystalline materials. Powder X-ray diffraction (XRD) analysis shows that all samples are single-phase samples and without any noticeable amount of the secondary phase. X-ray photoelectron spectroscopic (XPS) measurements depict the presence of a mixture of Co3+ and Co4+ valence states in these materials. The Seebeck coefficient (S) of dual-doped materials is significantly enhanced, and electrical resistivities (ρ) and thermal conductivities (κ) are decreased compared to the pristine compound. The maximum thermoelectric power factor (PF = S 2/ρ) and dimensionless figure of merit (zT) obtained for the y = 0.025 sample at 1000 K temperature are â¼3.2 × 10-4 W m-1 K-2 and 0.27, respectively. The zT value for Ca2.95Na0.05Co3.975Mo0.025O9 is about 2.5 times higher than that of the parent Ca3Co4O9 compound. These results demonstrate that dual doping of Na and Mo cations is a promising strategy for improving the high-temperature thermoelectric properties of Ca3Co4O9.
ABSTRACT
A new series of Ba2-xBixCoRuO6 (0.0 ≤ x ≤ 0.6) hexagonal double perovskite oxides have been synthesized by a solid-state reaction method by substituting Ba with Bi. The polycrystalline materials are structurally characterized by the laboratory X-ray diffraction, synchrotron X-ray, and neutron powder diffraction. The lattice parameters are found to increase with increasing Bi doping despite the smaller ionic radius of Bi3+ compared to Ba2+. The expansion is attributed to the reduction of Co/Ru-site cations. Scanning electron microscopy further shows that the grain size increases with the Bi content. All Ba2-xBixCoRuO6 (0.0 ≤ x ≤ 0.6) samples exhibit p-type behavior, and the electrical resistivity (ρ) is consistent with a small polaron hopping model. The Seebeck coefficient (S) and thermal conductivity (κ) are improved significantly with Bi doping. High values of the power factor (PF â¼ 6.64 × 10-4 W/m·K2) and figure of merit (zT â¼ 0.23) are obtained at 618 K for the x = 0.6 sample. These results show that Bi doping is an effective approach for enhancing the thermoelectric properties of hexagonal Ba2-xBixCoRuO6 perovskite oxides.
ABSTRACT
We report the electrical, magnetic and magnetotransport properties of Na and Mo dual doped Ca3-2x Na2x Co4-x Mo x O9 (0 ≤ x ≤ 0.15) polycrystalline samples. The results indicate that the strength of ferrimagnetic interaction decreases with increase in doping, as is evident from the observed decrease in Curie temperatures (T C). The substitution of non-magnetic Mo6+ ions (4d0) in CoO2 layers and the presence of oxygen vacancies are responsible for decrease in ligand field strength, which results in an enhanced magnetization in the low doped x = 0.025 sample due to a change from the low spin to partial high spin electron configuration. The electrical resistivity of samples exhibits a semiconducting-like behavior in the low temperature range, a strongly correlated Fermi liquid-like behavior in the intermediate temperature range, and an incoherent metal-like behavior in the temperature range 210-300 K. All the samples show a large negative magnetoresistance (MR) at low temperature with a maximum MR value of -59% for the x = 0.025 sample at 2 K and 16 T applied field. The MR values follow the observed trend in magnetization at 5 K and sharply increase below the Curie temperatures of the samples, suggesting that the ferrimagnetic interactions are mainly responsible for the decrease in electrical resistivity under an applied magnetic field.
ABSTRACT
The detailed crystal structures and high temperature thermoelectric properties of polycrystalline Ca3-2x Na2x Co4-x W x O9 (0 ≤ x ≤ 0.075) samples have been investigated. Powder X-ray diffraction data show that all samples are phase pure, with no detectable traces of impurity. The diffraction peaks shift to lower angle values with increase in doping (x), which is consistent with larger ionic radii of Na+ and W6+ ions. X-ray photoelectron spectroscopy data reveal that a mixture of Co2+, Co3+ and Co4+ valence states are present in all samples. It has been observed that electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) are all improved with dual doping of Na and W in Ca3Co4O9 system. A maximum power factor (PF) of 2.71 × 10-4 W m-1 K-2 has been obtained for x = 0.025 sample at 1000 K. The corresponding thermoelectric figure of merit (zT) for x = 0.025 sample is calculated to be 0.21 at 1000 K, which is â¼2.3 times higher than zT value of the undoped sample. These results suggest that Na and W dual doping is a promising approach for improving thermoelectric properties of Ca3Co4O9 system.
