Recent progress in oxide thermoelectric materials: p-type Ca3Co4O9 and n-type SrTiO3(-)

Thermoelectric energy conversion technology to convert waste heat into electricity has received much attention. In addition, metal oxides have recently been considered as thermoelectric power generation materials that can operate at high temperatures on the basis of their potential advantages over heavy metallic alloys in chemical and thermal robustness. We have fabricated high-quality epitaxial films composed of oxide thermoelectric materials that are suitable for clarifying the intrinsic "real" properties. This review focuses on the thermoelectric properties of two representative oxide epitaxial films, p-type Ca 3Co 4O 9 and n-type SrTiO 3, which exhibit the best thermoelectric figures of merit, ZT (= S (2)sigma Tkappa (-1), S = Seebeck coefficient, sigma = electrical conductivity, kappa = thermal conductivity, and T = absolute temperature) among oxide thermoelectric materials reported to date. In addition, we introduce the recently discovered giant S of two-dimensional electrons confined within a unit cell layer thickness ( approximately 0.4 nm) of SrTiO 3.     

Effect of precursor calcination temperature on the microstructure and thermoelectric properties of Ca3Co4O9 ceramics

Ca₃Co₄O₉ (CCO) powder precursors were prepared by the chemical sol–gel route and calcined at various temperatures between 923?K (CCO-923?K) and 1,073?K (CCO-1,073?K). The calcination temperature was found to be a critical factor affecting the microstructure and thermoelectric properties of CCO ceramic bulk samples. The grain size increases with calcination temperature. The nano-crystals with size about 100?nm in the powders calcined at 923?K promote large crystal growth and texture development during sintering. Bulk pellets made from CCO-923?K powder have large crystal grains, uniform grain size distribution, and a high degree of crystal alignment. By contrast, pellets made from CCO powders at higher calcination temperatures have a bimodal distribution of large and small grains and a large amount of randomly oriented grains. Transmission electron microscopy analysis shows that each crystal grain (identified in SEM images) consists of bundles of CCO nano-lamellas. The nano-lamellas within one bundle share the same c-axis orientation and have fiber texture. The electrical resistivity of CCO-923?K is weakly dependent on operating temperature. Compared to the CCO-1,073?K sample, the CCO-923?K sample has the highest power factor, a lower thermal conductivity, and higher electrical conductivity.     

Kinetics of thermal decomposition of Co3O4 powder and single crystals

The kinetics of the thermal decomposition of Co₃O₄ is described using a mixed-control model in which the reaction rate is controlled simultaneously by both a diffusional process and a chemical reaction. The observed dependence of the activation energy on the degree of decomposition, and the activation energies of the diffusion and of the chemical reaction are given.

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Zusammenfassung Es wurde die Kinetic der thermischen Zersetzung von Co₃O₄ mittels eines Modelles beschrieben, bei dem die Reaktionsgeschwindigkeit sowohl durch Diffusionsvorgänge als auch durch chemische Reaktionen beeinflut wird. Die gefundene Abhängigkeit der Aktivierungsenergie vom Grad der Zersetzung, von Diffusion und chemischer Reaktion wird gegeben.

     

Bulk textured Ca2.5(RE)0.5Co4O9 (RE: Pr,Nd, Eu,Dy and Yb) thermoelectric oxides by sinter-forging

Bulk textured Ca_2.5(RE)_0.5Co₄O₉ (RE, rare-earth element = Pr, Nd, Eu, Dy and Yb) ceramics have been prepared using sinter-forging technique. Both sintered and sinter-forged samples have been prepared and characterized. We present the thermoelectric properties, structural and microstructural investigations of the samples which show a strong degree of crystallite orientation. In addition, the effect of the rare-earth substitution on the properties is discussed.     

Synthesis of Co3O4 nanorod bunches from a single precursor Co(CO3)0.35Cl0.20(OH)1.10

Co₃O₄ nanorods were successfully synthesized from a single precursor via a thermal decomposition and oxidization route. The precursor used was Co(CO₃)_0.35Cl_0.20(OH)_1.10, which was prepared by a hydrothermal reaction using CoCl₂⋅6H₂O with CO(NH₂)₂ at 95–120 °C. Both the precursor and the as-prepared Co₃O₄ were characterized with XRD, TEM, SEM, TGA and XPS. The precursor, as well as Co₃O₄, was found to be composed of nanorods that were radially bunched. The Co₃O₄ nanorods obtained through a thermal treatment at 300 °C for 5 h were found to have a porous structure.     

Thermochemical studies on Bi4Ge3O12 and Bi4Ti3O12 single crystals

The specific heat values of Bi₄Ge₃O₁₂ and Bi₄Ti₃O₁₂ single crystals have been studied using a DSC instrument in the temperature range from 323 to 1273 K. The temperature range at which the anomaly associated with transition from polarized to non-polarized phase in Bi₄Ti₃O₁₂ occurred, has been estimated using the shape of the Bi₄Ge₃O₁₂ heat capacity curve as a “normal” one. The heat effect and the entropy change of the transition were evaluated.     

Enhanced high-temperature thermoelectric performance of Yb(14-x)Ca(x)MnSb11

The high temperature p-type thermoelectric material Yb(14)MnSb(11) has been of increasing research interest since its high temperature thermoelectric properties were first measured in 2006. Subsequent substitutions of Zn, Al, and La into the structure have shown that this material can be further optimized by altering the carrier concentration or by reduction of spin-disorder scattering. Here the properties of the Yb(14-x)Ca(x)MnSb(11) solid solution series where isovalent Ca(2+) is substituted for Yb(2+) will be presented. Crystals of the Yb(14-x)Ca(x)MnSb(11) solid solution series were made by Sn-flux (x = 2, 4, 6, 8) with the following ratio of elements: (14-x)Yb: xCa: 6 Mn: 11Sb: 86Sn, and their structures determined by single crystal X-ray diffraction. The density of the material significantly decreases by over 2 g/cm(3) as more Ca is added (from x = 1 to 8), because of the lighter mass of Ca. The resulting lower density is beneficial from a device manufacturing perspective where there is often a trade-off with the specific power per kilogram. The compounds crystallize in the Ca(14)AlSb(11) structure type. The Ca substitution contributes to systematic lengthening the Mn-Sb bond while shortening the Sb-Sb bond in the 3 atom linear unit with increasing amounts of Ca. Temperature dependent thermoelectric properties, Seebeck, electrical resistivity, and thermal conductivity were measured from room temperature to 1273 K. Substitution of Yb with Ca improves the Seebeck coefficient while decreasing the thermal conductivity, along with decreasing the carrier concentration in this p-type material resulting in an enhanced thermoelectric figure of merit, zT, compared to Yb(14)MnSb(11).     

Modeling of electronic structure of Co3O4 by Xα-scattered wave method

A previously developed procedure for modeling the electronic structure of oxide systems, using a quantum-chemical calculation in the X-scattered wave approximation, has been applied to the investigation of cobaltous-cobaltic oxide Co₃O₄. As a model of octahedral and tetrahedral sublattices of the oxide we took CoO₆ ^9– and CoO₄ ^6–, respectively. An energy scheme has been constructed for Co₃O₄, and a quantitative interpretation has been given for the photoelectron and x-ray electronic spectra of the valence band, optical excitation spectra, and satellite shake-up lines.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 291–300, May–June, 1990.     

Electron spin resonance of gamma-irradiated (CH3)4NPF6 and (CH3)4NBF4 single crystals

The electron spin resonance spectra of γ-irradiated tetramethylammonium hexafluorophosphate and tetramethylammonium tetrafluoroborate single crystals and powders have been investigated between 120–350 K. The dominant species in both compounds have been attributed to (CH₃)₃N⁺ radical. All hyperfine coupling constants of the methyl protons and the nitrogen nucleus have been found to be temperature independent and determined. It is concluded that the motional behaviour of (CH₃)₃N⁺ does not differ in tetramethylammonium compounds. The outer lines of the main spectrum of (CH₃)₄NPF₆ have been attributed to PF^-₅ and FPO^-₂ radicals and the anisotropic behaviour of the hyperfine coupling constants has been presented.     

Low-lying electronic states of M(3)O(9)(-) and M(3)O(9)(2-) (M = Mo, W)

Multiple low-lying electronic states of M(3)O(9)(-) and M(3)O(9)(2-) (M = Mo, W) arise from the occupation of the near-degenerate low-lying virtual orbitals in the neutral clusters. We used density functional theory (DFT) and coupled cluster theory (CCSD(T)) with correlation consistent basis sets to study the structures and energetics of the electronic states of these anions. The adiabatic and vertical electron detachment energies (ADEs and VDEs) of the anionic clusters were calculated with 27 exchange-correlation functionals including one local spin density approximation functional, 13 generalized gradient approximation (GGA) functionals, and 13 hybrid GGA functionals, as well as the CCSD(T) method. For M(3)O(9)(-), CCSD(T) and nearly all of the DFT exchange-correlation functionals studied predict the (2)A(1) state arising from the Jahn-Teller distortion due to singly occupying the degenerate e' orbital to be lower in energy than the (2)A(1)' state arising from singly occupying the nondegenerate a(1)' orbital. For W(3)O(9)(-), the (2)A(1) state was predicted to have essentially the same energy as the (2)A(1)' state at the CCSD(T) level with core-valence correlation corrections included and to be higher in energy or essentially isoenergetic with most DFT methods. The calculated VDEs from the CCSD(T) method are in reasonable agreement with the experimental values for both electronic states if estimates for the corrections due to basis set incompleteness are included. For M(3)O(9)(2-), the singlet state arising from doubly occupying the nondegenerate a(1)' orbital was predicted to be the most stable state for both M = Mo and W. However, whereas M(3)O(9)(2-) was predicted to be less stable than M(3)O(9)(-), W(3)O(9)(2-) was predicted to be more stable than W(3)O(9)(-).     

The satellite-shaped Co-15 Polyoxotungstate, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(beta-SiW8O31)3}]5-

The 15-cobalt-substituted polyoxotungstate [Co(6)(H(2)O)(30){Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}](5-) (1) has been characterized by single-crystal XRD, elemental analysis, IR, electrochemistry, magnetic measurements, and EPR. Single-crystal X-ray analysis was carried out on Na(5)[Co(6)(H(2)O)(30){Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}].37H(2)O, which crystallizes in the hexagonal system, space group P6(3)/m, with a = 19.8754(17) A, b = 19.8754(17) A, c = 22.344(4) A, alpha= 90 degrees, beta = 90 degrees, gamma = 120 degrees, and Z = 2. The trimeric polyanion 1 has a core of nine Co(II) ions encapsulated by three unprecedented (beta-SiW(8)O(31)) fragments and two Cl(-) ligands. This central assembly {Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}(17-) is surrounded by six antenna-like Co(II)(H(2)O)(5) groups resulting in the satellite-like structure 1. Synthesis of 1 is accomplished in a simple one-pot procedure by interaction of Co(II) ions with [gamma-SiW(10)O(36)](8-) in aqueous, acidic NaCl medium (pH 5.4). Polyanion 1 was studied by cyclic voltammetry as a function of pH. The current intensity of its Co(II) centers was compared with that of free Co(II) in solution. Our results suggest that 1 keeps its integrity in solution. Magnetic susceptibility results show the presence of both antiferro- and ferromagnetic coupling within the (Co(II))(9) core. A fully anisotropic Ising model has been employed to describe the exchange-coupling and yields g = 2.42 +/- 0.01, J(1) = 17.0 +/- 1.5 cm(-1), and J(2) = -13 +/- 1 cm(-(1). Variable frequency EPR studies reveal an anisotropic Kramer's doublet.     

Enhanced photocatalytic degradation of sulfamethoxazole by stable hierarchical Fe2O3/Co3O4 heterojunction on nickel foam

A facile approach was successfully employed to prepare Fe₂O₃/Co₃O₄ nanosheet arrays on nickel foams (Fe₂O₃/Co₃O₄@NF), which owned such advantages as narrow band gap energies and high separation rate of photoexcited electron-hole pairs. The combination of Fe₂O₃ and Co₃O₄ dramatically enhanced the photocatalytic activity towards sulfamethoxazole (SMZ) degradation, with the highest catalytic efficiency of k = 0.0538 min⁻¹, which was much higher than that of Fe₂O₃@NF (0.0098 min⁻¹) and Co₃O₄@NF (0.0094 min⁻¹). The introduction of Ni foam could not only act as the support to anchor photocatalyst, but also work as the electron mediator to promote the transition of electron-hole pairs. Reactive species trapping experiments combined with electron paramagnetic resonance analysis confirmed ^•O₂⁻ was primarily responsible for SMZ degradation. Furthermore, Fe₂O₃/Co₃O₄@NF was effective and almost unaffected by inorganic cations and anions in aqueous solution. This study could provide a facile and promising path for the construction of self-supported metal oxide-based heterojunction with high efficiency and strong stability.     

THERMAL DEHYDRATION OF BARIUM AND CALCIUM TRIMETAPHOSPHATE HEXAHYDRATES: Ba3(P3O9)26H2O AND Ca3(P3O9)26H2O

Abstract

Ba₃(P₃O₉)₂6H₂O and Ca₃(P₃O₉)₂6H₂O have mobile H₂O's which, under a dynamic vacuum of about 0.133 Pa, leave the lattice without disrupting it. Under water-vapor pressure, dehydration is accompanied by hydrolysis of P₃O₉ ^3- rings. The final product of dehydration is the β polyphosphate.     

Crystal structure of NaBa2[Co(Edta)]2(ClO4)3 · 9H2O

Crystals of Ba Na ethylenediaminetetraacetatocobaltate(III) perchlorate, NaBa₂[Co(Edta)]₂(ClO₄)₃ · 9H₂O, contain two crystallographically nonequivalent Ba²⁺ cations and two complex cations [Co(Edta)]^? (Edta^4? is ethylenediaminetetraacetic acid anion), the latter anions showing opposite chirality. The nearest surrounding of the Ba²⁺ ions involves three water molecules (including two bridging water molecules), six O atoms of four complexes [Co(Edta)]^? and the perchlorate O atom. Tetrameric fragments Ba₂(H₂O)₄[Co(Edta)]₂(ClO₄)₂ are united through the Ba-O bonds into layers with the Ba atoms in the middle of the layers and the perchlorate ions and complex anions at the periphery; in the latter anions, noncoordinated O atom of one of the R-metallocycles is directed outside. The Na atom of the Na(H₂O)(ClO₄) group located between the layers is bonded to these O atoms of the neighboring layers.