Thermoelectric Properties Of Ca Research Articles

First-principles calculation to investigate half metallic ferromagnetism and thermoelectric properties of Ca0.75Ti0.25X (X = S, Se) alloys

The electronic, magnetic and thermoelectric properties of Ca0.75Ti0.25S and Ca0.75Ti0.25Se alloys are explored comprehensively. The stabilityin ferromagnetic state (FM) has been ensured by energy released from optimized structures and enthalpy of formation (ΔH). The Heisenberg classical model has been used to calculate Curie temperature (Tc). The band structures (BS) as well as densities of states (DOS) are analyzed to explore half metallic ferromagnetism and 100% spin polarization. The Δx(d) (exchange energy) overcome ΔEcrystal (Crystal field energy) due to ferromagnetism induced by strong hybridization. The negative values of Δx(pd) and exchange constant N0β lesser exchange energy which satisfy the double exchange model. The thermoelectric properties of the studied compounds are elaborated in terms of electrical conductivity, thermal conductivity and Seebeck coefficient. The increase in power factor with the replacement of S by Se has been noted.

Defect Chemistry, Phase Transitions, and Thermoelectric Properties of Ca1–xCexAg1–ySb (0 ≤ x ≤ 1; 0 ≤ y ≤ 1)

Zintl phases Ca1–xCexAg1–ySb (0 ≤ x ≤ 1; 0 ≤ y ≤ 1) have exhibited high-temperature thermoelectric potentials in previous studies. In this work, defect chemistry and thermoelectric properties related to the Ca1–xCexAg1–ySb series were carefully investigated. The results revealed that the Ce contents in Ca1–xCexAg1–ySb were very critical in governing the formation of various Ag-defect structures. With the Ce concentrations increased, several phase transition processes were observed, corresponding to closely related but obviously different [Ag1–ySb] anionic superlattices. Such a defect model makes these compounds very flexible in the optimization of thermoelectric properties. For a material with the nominal composition of Ca0.83Ce0.17Ag0.85Sb, a high figure of merit (ZT ∼ 0.6) can be maintained over a wide temperature range from 800 to 1025 K, which is a favorable characteristic for practical applications.

Effects of Ag-Doping on Thermoelectric Properties of Ca(2−x)AgxSi Alloys

Ca(2−x)AgxSi (0 ≤ x ≤ 0.1) with 47.5% excess of Ca alloys were fabricated by melting in a tantalum tube and hot pressing technique. Phase structures of the samples were studied by means of x-ray diffraction. The electrical conductivity and Seebeck coefficient of Ca(2−x)AgxSi alloys were studied in the temperature range of 300–873 K. The electrical conductivity of the Ag-doped samples increases within the whole test temperature range. All samples show p-type semiconductor behavior. The electrical conductivity decreases with increasing temperature from 300 K to 873 K, which is typically observed for a degenerate semiconductor. Compared with the undoped samples, Ag-doping (x = 0.04–0.1) results in decreases of Seebeck coefficient, especially Ca(2−x)AgxSi with x = 0.1. The thermal conductivity of the doped samples gradually increases with increasing the Ag-doping content. The Ca(2−x)AgxSi with x = 0.02 sample exhibits the lowest thermal conductivity within the whole test temperature range. The ZT values of Ca(2−x)AgxSi with x = 0.02 sample have an enhancement in the temperature range of 300–873 K by contrast with those of the Ca2Si sample. The maximum ZT value is 0.16 at 837 k, which is observed for the Ca(2−x)AgxSi with x = 0.04 sample.

Electrical transport properties of Ca0.9La0.1−xBixMnO3−δ (0 ≤ x ≤ 0.1) thermoelectric materials

A series of Ca0.9La0.1−xBixMnO3−δ (0 ≤ x ≤ 0.1) was fabricated by cold compaction and tape casting methods. The microstructural and thermoelectric properties of Ca0.9La0.1−xBixMnO3−δ were studied, with respect to the partial substitution of Bi3+ for La3+. All of the sintered Ca0.9La0.1−xBixMnO3−δ crystallized in the orthorhombic perovskite structure, belonging to the Pnma space group. The substituted Bi3+ significantly increased grain size and density because it acted as a sintering additive. The electrical conductivities of tape casting-processed Ca0.9La0.1−xBixMnO3−δ were much higher than those of cold compaction-processed Ca0.9La0.1−xBixMnO3−δ. On the other hand, the absolute values of the Seebeck coefficient for tape casting-processed Ca0.9La0.1−xBixMnO3−δ were similar to those of cold compaction-processed Ca0.9La0.1−xBixMnO3−δ. Consequently, tape casting-processed Ca0.9La0.1−xBixMnO3−δ showed a much higher power factor in comparison with cold compaction-processed Ca0.9La0.1−xBixMnO3−δ. The partial substitution of La3+ by Bi3+ up to x = 0.05 led to an increase in the power factor. The highest power factor (3.01 × 10−4 Wm−1·K−2) was obtained for tape casting-processed Ca0.9La0.05Bi0.05MnO3−δ at 800 °C.

Influence of rare-earth elements doping on thermoelectric properties of Ca0.98Dy0.02MnO3 at high temperature

Ca0.98Dy0.02MnO3 and Ca0.96Dy0.02Re0.02MnO3 (Re=La, Pr, Sm, Eu, Ho, and Yb) have been synthesized by the solid state reaction method. Samples with relative densities all over 96% have been obtained. Thermoelectric properties are evaluated between 300 and 1000K. The electrical resistivity shows a typical metal-like conductivity behavior, and at high temperature, 973K, decreases from 36.1mΩcm for Ca0.98Dy0.02MnO3 to 8.6mΩcm for Ca0.96Dy0.02Yb0.02MnO3. Both the absolute values of Seebeck coefficient and thermal conductivity are reduced by the introduction of second rare-earth element. The highest power factor of 415μW/(K2m) is obtained for Ca0.96Dy0.02Yb0.02MnO3 sample, resulting in the highest dimensionless figure of merit (ZT) 0.25 at 973K. This value shows an improvement of 144% compared with that of Ca0.98Dy0.02MnO3 ceramics at the same temperature.

Thermoelectric Properties of Ca_3Co_4O_9-Based Ceramics Doped with Fe and/or Y

Thermoelectric Properties of Ca3Co4O9-Based Ceramics Doped with Fe and/or Y A.S. Fedotov, A.K. Fedotova,∗, A.V. Mazanik, I.A. Svito, A.M. Saad, I.O. Troyanchuk, M.V. Bushinski, V.V. Fedotova, P. Zukowski and T.N. Koltunowicz Belarusian State University, Independence av. 4, 220030 Minsk, Belarus Al-Balqa Applied University, P.O. Box 4545, Amman 11953, Jordan Scienti c-Practical Materials Research Center of NASB, 220072 Minsk, Belarus Department of Electrical Devices and High Voltages Technologies, Lublin University of Technology Nadbystrzycka 38a, 20-618 Lublin, Poland

Effect of Mixed Grain Sizes on the Thermoelectric Properties of Ca0.9Yb0.1MnO3

Ca0.9Yb0.1MnO3 has been identified as a material that might be suitable for thermoelectric applications. We fabricated micro/nanograined Ca0.9Yb0.1MnO3 composites, with the aim of controlling the passage of electrons and phonons simultaneously. Micro/nanograined Ca0.9Yb0.1MnO3 composites containing various fractions of nanosized powder were prepared by sintering mixtures of microparticulate and nanoparticulate Ca0.9Yb0.1MnO3, obtained by solid-state reaction and by gas-phase reaction, respectively. The electrical resistivity increased markedly when the weight fraction of nanosized powder exceeded 50%, probably as a result of a percolation phenomenon. However, the thermal conductivity was considerably reduced when the weight fraction of nanosized powder exceeded 25%, but then remained almost constant. The absolute values of the Seebeck coefficient of micro/nanograined Ca0.9Yb0.1MnO3 composites were larger than those of monolithic micro- or nanograin Ca0.9Yb0.1MnO3, probably as a result of the effects of potential-barrier scattering. The highest dimensionless figure of merit ZT value of 0.09 at 973 K was achieved with a sample containing 50% nanosized powder, and this value is 10% larger than that of monolithic micrograined Ca0.9Yb0.1MnO3.

Microstructure and thermoelectric properties of Ca0.9Yb0.1MnO3/Pd nanocomposite prepared by electroless plating

We have fabricated a Ca0.9Yb0.1MnO3/Pd nanocomposite in which a palladium layer made up of particles with a size of about 50nm exists at grain boundaries in the Ca0.9Yb0.1MnO3 matrix. The nanocomposite was produced by plating Ca0.9Yb0.1MnO3 powder with palladium and then carrying out spark plasma sintering. The palladium particles contained 10wt.% phosphorus, independent of the palladium content, and had an amorphous structure. The as-plated phosphorous-containing palladium particles transformed into crystalline palladium and amorphous palladium phosphide after sintering at 1023K. The thermoelectric properties of the Ca0.9Yb0.1MnO3/Pd nanocomposite bulk degraded with increasing palladium content, mainly as a result of the high electrical resistivity. This is probably because the palladium-phosphorus phase located at the grain boundaries inhibits electrical conductance, which leads to lower electrical mobility.

Thermoelectric properties of Ca0.9Yb0.1MnO3−x prepared by spark plasma sintering in air atmosphere

In this paper we report the densification of Ca0.9Yb0.1MnO3-x ceramics by spark plasma sintering (SPS) in different atmospheres and using different tools. The thermoelectric properties of these ceramics are reported and compared to those of ceramics obtained by conventional sintering. This study clearly demonstrated the possibility for processing oxide materials by SPS under air atmosphere and overcomes the problem of reduction due to the low oxygen partial pressure (graphitic environment, dynamic vacuum or neutral atmosphere) commonly encountered.

Chemical pressure effect on thermoelectric properties of Ca and Ba substituted SrSi2 alloys

We report the study of temperature-dependent electrical resistivity, Seebeck coefficient, and thermal conductivity of the Sr1−xCaxSi2 and Sr1−xBaxSi2 alloys to elucidate the chemical pressure effect on the thermoelectric performance, characterized by the figure-of-merit, ZT. It is found that the room-temperature electrical resistivity and thermal conductivity are slightly reduced through the substitution of Ca and Ba (x < 0.10) onto the Sr sites of SrSi2. Moreover, the room-temperature Seebeck coefficients of these samples are substantially enhanced upon substitution. These promising results lead to a significant enhancement in the ZT value of the substituted SrSi2 alloys as compared to the parent compound SrSi2. Namely, the highest room-temperature ZT values of ∼0.17 and ∼0.11 were obtained for the Sr0.92Ca0.08Si2 and Sr0.93Ba0.07Si2 alloys, much larger than that of pure SrSi2.

Thermoelectric properties of Ca0.8Dy0.2MnO3 synthesized by solution combustion process

High-quality Ca0.8Dy0.2MnO3 nano-powders were synthesized by the solution combustion process. The size of the synthesized Ca0.8Dy0.2MnO3 powders was approximately 23 nm. The green pellets were sintered at 1150-1300°C at a step size of 50°C. Sintered Ca0.8Dy0.2MnO3 bodies crystallized in the perovskite structure with an orthorhombic symmetry. The sintering temperature did not affect the Seebeck coefficient, but significantly affected the electrical conductivity. The electrical conductivity of Ca0.8Dy0.2MnO3 increased with increasing temperature, indicating a semiconducting behavior. The absolute value of the Seebeck coefficient gradually increased with an increase in temperature. The highest power factor (3.7 × 10-5 Wm-1 K-2 at 800°C) was obtained for Ca0.8Dy0.2MnO3 sintered at 1,250°C. In this study, we investigated the microstructure and thermoelectric properties of Ca0.8Dy0.2MnO3, depending on sintering temperature.

Thermoelectric properties of Ca 2.8Cu 0.2Co 4O 9 ceramics fabricated by solution combustion method

Nano-sized Ca 2.8Cu 0.2Co 4O 9 powders were synthesized by the solution combustion process, using five different fuels, i.e., aspartic acid, alanine, glutamic acid, glycine, and urea. The combustion process lowered both the calcination and sintering temperatures. The sintered Ca 2.8Cu 0.2Co 4O 9 consisted of plate-like grains that were favorable for high energy conversion efficiency. In the comparative study of various different fuels, aspartic acid-processed Ca 2.8Cu 0.2Co 4O 9 showed an extremely high Seebeck coefficient, thus resulting in an enhanced power factor. The highest power factor value (8.05 × 10 −4 Wm −1 K −2) was attained for the aspartic acid-processed sample at 800 °C.

High temperature transport and thermoelectric properties of Ca 3− xEr xCo 4O 9+ δ

Ca 3− x Er x Co 4O 9+ δ ( x=0, 0.05, 0.15, 0.3 and 0.5) samples were prepared using a sol–gel method followed by hot-pressing sintering technique. Powder X-ray diffraction analysis showed the single-phases of Ca 3− x Er x Co 4O 9+ δ were obtained up to x=0.3 and the crystallinity of the samples decreased with increasing Er-doping amount. The high-temperature (323–1073 K) thermoelectric properties of the samples were investigated. The substitution of Er 3+ for Ca 2+ resulted in the decrease of electrical conductivity, thermal conductivity and the increase of Seebeck coefficient for all the samples except the x=0.05 one. The dimensionless figure of merit ZT reached 0.28 at 1073 K for Ca 2.7Er 0.3Co 4O 9+ δ , indicating the thermoelectric properties of CaCo 4O 9+ δ can be enhanced by doping Er in the system.

High-temperature thermoelectric properties of late rare earth-doped Ca 3Co 4O 9+ δ

Misfit-layered oxides Ca 3− x Ln x Co 4O 9+ δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca 3− x Ln x Co 4O 9+ δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln 3+. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca 2.8Lu 0.2Co 4O 9+ δ sample, which is about 1.6 times larger than that for the non-doped counterpart.

Microstructure control on thermoelectric properties of Ca 0.96Sm 0.04MnO 3 synthesised by co-precipitation technique

Nanopowder of Ca 0.96Sm 0.04MnO 3 (CSM) perovskite was prepared by chemical co-precipitation technique. It was characterized and studied by X-ray diffraction and Transmission Electron Microscopy. Nano-crystalline CSM phase having grain size of 60–70 nm can be prepared by calcination at 1073 K for 6 h in air. The obtained nanopowder was pressed, sintered at 1373 K for 24 h to obtain a dense sample. In comparison, CSM samples prepared from normal powder need sintering temperature of 1623 K to reach a similar microstructure, showing the efficiency of nanopowder to decrease the sintering temperature. The thermoelectric properties of samples prepared with two different processes were studied. The Seebeck coefficient and the thermal conductivity slightly decrease by using the nanopowder while the value of the electrical resistivity does not change. This results in the same figures of merit for the two materials.

Thermoelectric properties of Bi, Nb co-substituted CaMnO 3 at high temperature

Ca 1− x Bi x Mn 1− y Nb yO 3 (0 ≤ x = y ≤ 0.1) was prepared using a conventional solid-state reaction method. The microstructures and homogeneity were confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and its high temperature thermoelectric properties were also characterized. The lattice parameter of orthorhombic perovskite increased with additional amounts of Bi and Nb. The electrical conductivity ( σ) increased, and Seebeck coefficient ( α) decreased, as the amount of Bi and Nb increased up to x = y = 0.08. The electrical conductivity decreased as the temperature increased, indicating metallic behavior, and the Seebeck coefficient increased with increasing temperature. As a result, thermoelectric properties were improved by substituting Bi and Nb into CaMnO 3. It showed maximum values at x = y = 0.04, in which the power factor (PF) was 200 μW/m K 2 and dimensionless figure of merit ( ZT) was approximately 0.1 at 873 K. The thermoelectric properties of Ca 0.96Bi 0.04Mn 0.96 Nb 0.04O 3 were improved nearly twofold over un-doped CaMnO 3.

Fabrication and thermoelectric properties of Ca 3− xDy xCo 4O 9+ δ system

Ca 3− x Dy x Co 4O 9+ δ ( x=0, 0.15, 0.3 and 0.45) polycrystalline samples have been prepared using a sol–gel process followed by SPS sintering. Their thermoelectric properties have been carefully studied from room temperature to 1000 K. The substitution of Dy for Ca results in increasing of both thermopower and electrical resistivity, which could be attributed to the decreasing of carrier concentrations. The Dy substituted samples ( x=0, 0.3) have lower thermal conductivity than Ca 3Co 4O 9+ δ due to their lower electronic and lattice thermal conductivities. The dimensionless figure of merit ZT reaches 0.27 at 1000 K for the sample of Ca 2.7Dy 0.3Co 4O 9+ δ .