Exploring the possibility of enhancing the high figure-of-merit ( >>> 2) of Na${0.74}$CoO${2}$ by using combined experimental and theoretical studies (original) (raw)
Related papers
The European Physical Journal B, 2020
Search of new thermoelectric (TE) materials with high figure-of-merit (ZT) is always inspired the researcher in TE field. Here, we present a combined experimental and theoretical study of TE properties of Na0.74CoO2 compound in high temperature region. The experimental Seebeck coefficient (S) is found to vary from 64 to 118 µV/K in the temperature range 300 − 620 K. The positive values of S are indicating the dominating p-type behaviour of the compound. The observed value of thermal conductivity (κ) is ∼ 2.2 W/m-K at 300 K. In the temperature region 300 − 430 K, the value of κ increases up to ∼ 2.6 W/m-K and then decreases slowly till 620 K with the corresponding value of ∼ 2.4 W/m-K. To understand the experimental transport properties, we have carried out the theoretical calculations using spin-polarized and spin-unpolarized DFT and DFT+U methods. The best matching between experimental and calculated values are observed when the spin-polarized calculation is done by chosen U = 4 eV in DFT+U. By taking calculated S and electrical conductivity (σ) along with experimental κ, we have optimized the ZT values up to 1200 K and the maximum value is found to be ∼ 0.67 at 1200 K. Our computational study suggests that the possibility of n-type behaviour of the compound which can lead to a large value of ZT at higher temperature region. Electron doping of ∼ 5.1×10 20 cm −3 is expected to give rise the high ZT value of ∼ 2.7 at 1200 K. Using these temperature dependent ZT values, we have calculated the maximum possible values of efficiency (η) of thermoelectric generator (TEG) made by p and n-type Na0.74CoO2. The present study suggests that one can get the efficiency of a TE cell as high as ∼ 11% when the cold and hot end temperature are fixed at 300 K and 1200 K, respectively. Such high values of ZT and efficiency suggest that Na0.74CoO2 can be used as a potential candidate for high temperature TE applications.
Synthesis and Rietveld analysis of new thermoelectric oxides F-doped Na1.6Co2O4
Transactions of the Materials Research Society of Japan, 2008
A series of five bulk samples of NaxCo 2 0 4 .yFy (x = 1.6, y = 0 to 0.20) compounds was successfully synthesized by employing the solution precursor method using CH 3 COONa, Co(CH1C00)2 and NaF dissolved in aqua regia. The XRD study with the Rietveld analysis revealed that the lattice parameter a slightly decreased with increasing F content y, while the c increased with increasing y. The electrical conductivity (a) of the compounds continuously decreased with increasing y. The See beck coefficient (S) once increased from y = 0 to 0.1 0, and decreased above y = 0.15. No improvement in the power factor (S 2 o-) was observed in the compositional range of 0 :S y :S 0.20. The carrier concentration was unchanged over the compositional range of 0 :S y :S 0.20. A drastic reduction in the thermal conductivity (K) of the compounds was observed by substituting 0 atoms with F atoms at the anion site in the Na1. 6 Co20 4 compounds.
Ab-initio study of thermoelectric properties of Co2XGa (X = Mn, Mo, Pt)
Nucleation and Atmospheric Aerosols, 2020
Transparent conductive oxides, such as tin dioxide (SnO 2), have recently shown to be promising materials for thermoelectric applications. In this work we studied the thermoelectric properties of Fe-, Sb-and Zn-uniformly doping and co-doping SnO 2 , as well as of Sb and Zn planar (or delta)-doped layers in SnO 2 forming oxide superlattices (SLs). Based on the semiclassical Boltzmann transport equations (BTE) in conjunction with ab initio electronic structure calculations, the Seebeck coefficient (S) and figure of merit (ZT) are obtained for these systems, and are compared with available experimental data. The delta doping approach introduces a remarkable modification in the electronic structure of tin dioxide, when compared with the uniform doping, and colossal values for ZT are predicted for the delta-doped oxide SLs. This result is a consequence of the two-dimensional electronic confinement and the strong anisotropy introduced by the doped planes. In comparison with the uniformly doped systems, our predictions reveal a promising use of delta-doped SnO 2 SLs for enhanced S and ZT, which emerge as potential candidates for thermoelectric applications.
International Conference on Thermoelectrics, 2002
Four different structure types depending on composition and reaction temperature are known of the highly anisotropic non-stoichiometric oxide NaxCoO2. Only γ-NaxCoO2 (0.55≤x≤0.74) was reported to possess large thermopower and studied in detail. Despite the corresponding stoichiometry of γ-, α'-NaxCoO2 and γ-, β-NaxCoO2, respectively, no phase transition has been reported for γ-NaxCoO2 while investigating its thermoelectric properties. This aspect motivated our
Journal of Applied Physics, 2010
Synthesis and characterization of bulk Na x CoO 2 samples substituted by K and Rb is reported. Phase formation studies revealed a narrow stable region for Na-alkali metal-Co system. Whisker and platelike single crystalline structures have been found to form on the surface of the pellets in case of K doping. All samples were metallic and no characteristic anomaly in R-T curves was observed for Rb doped sample. Magnetoresistance measured has a pronounced positive response only for K-doped and pure Na x CoO 2 phases, reaching 11% and 7% at 5 K temperature, respectively.
Resistivity and thermoelectric power of Na[sub x]CoO[sub 2] (x=1.0, 0.7, and 0.6) system
Journal of Applied Physics, 2005
Results of thermo-electric power (S) and electrical resistivity (ρ) measurements are reported on Na x CoO 2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P6 3 /mmc) at room temperature. Thermo-electric power values at 300K (S 300K ) are, ≅ 80µV/K, 39µV/K and 37µV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of ρ 300K for x = 1.0 sample is ~0.895 mΩ-cm and the power factor (S 2 /ρ) is ~ 7.04 x 10 -3 W/mK 2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300−5K 2 temperature range and decreases monotonically to zero as temperature T→ 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T → 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.
Physica Status Solidi A-applications and Materials Science, 2016
Sodium cobaltate (Na x CoO 2 , NCO) is a model type thermoelectric material for high temperature applications, which is representative for the class of non-stoichiometric and mixed ion-electron conducting thermoelectrics (e.g., Cu 2x Se). The present study deals with the kinetic instability of the originally homogeneous chemical composition when a temperature gradient is applieda common situation in thermoelectric materials with a mobile component (element), but rarely considered in thermoelectric materials research so far. In order to investigate a well reproducible system, highly epitaxial thin films of Na x CoO 2 with an atmosphere protective capping layer of alumina are prepared via pulsed laser deposition on sapphire (001) and lanthanum aluminate (111). A self-designed non-isothermal setup allows the precise determination of the heat of transport for mobile sodium as 8.3 kJ mol À1 and, therefore, the quantification of thermodiffusion (i.e., the Ludwig-Soret effect in the stationary state). The experiments also allow to estimate the chemical diffusion coefficient of sodium at 422 K as about e D Na ¼ 5 Â 10 À4 cm 2 s À1 .
Electronic Texture of the Thermoelectric Oxide Na0.75CoO2
Physical Review Letters, 2008
From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering on the cobalt electronic states in Na0.75CoO2: at long time scales, there is neither a disproportionation into 75 % Co3+ and 25 % Co4+ states, nor a mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an intermediate configuration in which 30 % of the lattice sites form an ordered pattern of localized Co3+ states. Above 180 K, an anomalous mobility of specific Na+ sites is found to coexist with this electronic texture, suggesting that the formation of the latter may contribute to stabilizing the Na+ ordering. Control of the ion doping in these materials thus appears to be crucial for fine-tuning of their thermoelectric properties.
Sodium and oxygen nonstoichiometry and thermoelectric properties of NaxCoO2+d
The sodium-cobalt oxide Na x CoO 2+δ is a promising candidate for thermoelectric applications since it may possess simultaneously large thermoelectric power and low resistivity. It is difficult to understand the simultaneous appearance of the two properties within the framework of conventional one-electron models. It has been suggested that the spin state of cobalt ions plays a crucially important role in enhancing the thermoelectric power. In order to study the relationship between the cobalt spin state and thermoelectric properties, Na x CoO 2+δ samples with precisely controlled sodium and oxygen contents are indispensable. Recently, we established an unconventional sample-synthesis method that enables us to precisely control the Na content in Na x CoO 2+δ samples. In this article, we discuss the effects of sodium and oxygen nonstoichiometry on the thermoelectric properties of Na x CoO 2+δ .