Yuzuru Miyazaki

Low-Temperature Thermoelectric Properties of the Composite Crystal [Ca 2CoO 3.34] 0.614[CoO 2]

Electric resistivity, thermoelectric power and thermal conductivity of a polycrystalline sample of the composite crystal [Ca2CoO3.34]0.614[CoO2], also known as Ca3Co4O9, have been measured below 300 K. Metallic conductivity accompanied by large thermoelectric power has been observed down to 50 K. At 300 K, the sample exhibits a thermoelectric power of S = 133 µVK-1, resistivity of ρ= 15 mΩcm and thermal conductivity of κ= 9.8 mWK-1cm-1. The resulting dimensionless figure of merit becomes ZT300 = 3.5×10-2, which is comparable to the value reported for a polycrystalline sample of NaCo2O4, indicating that the title compound is a potential candidate for a thermoelectric material.

Modulated Structure of the Thermoelectric Compound [Ca2CoO3]0.62CoO2

We have determined the crystal structure of the composite crystal [Ca 2 CoO 3 ] 0.62 CoO 2 , known as Ca 3 Co 4 O 9 , by a superspace group approach. Structural parameters were refined with a super...

Preparation of Bi2Te3 films by electrodeposition

Bismuth telluride films have been electrochemically deposited from solutions of Bi 2 O 3 and TeO 2 in diluted HNO 3 (pH = 0.50) onto Ti sheet working electrodes at 293 K. A conventional three-electrode cell was used with a platinum wire counter electrode and Ag/AgCl (saturated KCl) reference electrode. Single-phase films of Bi 2+x Te 3-x solid solution have been prepared from E = -200 to + 80 mV (versus Ag/AgCl). The films prepared at +20≤E≤ + 60 mV exhibit strong {110} orientation, while those prepared at +60 < E ≤ + 80 mV show remarkable {1 0 5} orientation. Both the a-axis and c-axis lengths are almost independent of the cathodic potential for E < - 100 mV. Above -100 mV, the a-axis length gradually decreases and the c-axis length increases with cathodic potential, implying the Bi concentration in the Bi 2-x Te 3-x solid solution moves from the stoichiometric value. The films prepared at E< + 20 mV show p-type conduction, while those prepared at +20 ≤ E ≤ + 80 mV show n-type conduction. The largest Seebeck coefficient, S = -63 μVK -1 (300 K), has been observed for the film deposited at E = +20 mV.

Preparation and crystal structure of Sr2CuO2(CO3)

Abstract Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material. The compound has a tetragonal structure with lattice constants a = 7.8045(1), and c = 14.993(1) A , and its space group is 14. The formula per unit cell is 8 Sr 2 CuO 2 (CO 3 ), and measured and calculated densities are D m = 4.71 g/cm 3 , and D x = 4.81 g/cm 3 , respectively. The crystal structure was refined by Rietveld analysis on X-ray powder diffraction and neutron powder diffraction data. The final residuals ( R F ) were 4.31 and 4.27% for the X-ray and neutron data, respectively. The structure consists of deformed [CuO 6 ] octahedrons and layers of ordered triangular CO 3 groups. Sr atoms having eight near oxygen neighbors are between the [CuO 6 ] octahedrons and the CO 3 layers.

The crystal structure of (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7

Abstract The crystal structure of a new compound, (C0.4Cu0.6)Sr2(Y0.86Sr0.14)Cu2O7, is derived from the structure of YBa2Cu3O7. Forty percent of CuO chains in the YBa2Cu3O7 structure are replaced by CO3 groups. This new compound has a superstructure along the a-axis and the c-axis. Diffuse superlattice reflections having periods of a∗/2-a∗/3 and c∗/2 were observed in electron diffraction patterns. Locally ordered distributions of C and Cu atoms were seen high-resolution images taken by transmission electron microscopy with an incident beam parallel to [010]. The basic structure of this superstructure was determined by neutron powder diffraction, assuming orthorhombic symmetry with the space group, Pmmm (lattice constants: a=3.8278(2), b=3.8506(2) and c=11.1854(5) A ).

Superconductivity in the new compound (Y1-xCax)0.95Sr2.05Cu2.4(CO3)0.6Oy

Abstract A new Cu-oxide superconductor (Y 1− x Ca x ) 0.95 Sr 2.05 Cu 2.4 (CO 3 ) 0.6 O y has been found by resistivity and magnetization measurements. The transition temperature is about 63 K. The crystal structure consists of the periodic replacements of the Cu(I) site with a Co 3 group in the basic YBa 2 Cu 3 O y structure.

Preparation and low-temperature thermoelectric properties of the composite crystal [Ca2(Co0.65Cu0.35)2O4]0.624CoO2

A new composite crystal cobaltite, [Ca2(Co0.65Cu0.35)2O4]0.624CoO2, has been synthesized and its thermoelectric properties below 300 K have been investigated. The compound consists of a CdI2-type CoO2 conduction sheet and a quadruplicated rock-salt-type [Ca2(Co0.65Cu0.35)2O4] layer alternatively stacked along the c-axis. Metallic conductivity accompanied by large thermoelectric power (S) has been observed down to 100 K. At 300 K, the sample exhibits S=150 µVK-1 and resistivity of ρ=15 mΩcm. The resulting power factor (S2/ρ) becomes 1.5×10-4 WK-1m-2 at 300 K, which is more than 20% larger than the value reported for a polycrystalline sample of another composite crystal, [Ca2CoO3]0.62CoO2.

Excellent p-n control in a high temperature thermoelectric boride

Polycrystalline samples of YxAlyB14 (x ∼ 0.57) with different fractional occupancies y (0.41 ≤ y ≤ 0.63) were synthesized and their thermoelectric properties investigated. Electrical conductivities generally followed three-dimensional variable range hopping with a rapid delocalization indicated as electrons were increased. Positive Seebeck coefficients were obtained for the Al-poor sample, y = 0.41, which was shifted in the negative direction with increase of y. Maximum Seebeck coefficient values were approximately 400 μV K−1 at 850 K and −200 μV K−1 at 1000 K, for p-type and n-type, respectively. Excellent control of p-n characteristics was achieved in a system with the same crystal structure and consisting of the same elements.

Bi-Substitution Effects on Crystal Structure and Thermoelectric Properties of Ca3Co4O9 Single Crystals

We grew single crystals of partially Bi-substituted Ca3Co4O9 phase in a solution consisting of K2CO3–KCl solvent. All the X-ray diffraction patterns of the single crystals with different Bi contents were attributable to the Ca3Co4O9 structure, although weak diffraction peaks from a secondary phase of Bi2Ca2Co2Ox were observed in the crystals grown from a starting composition molar ratio of Ca:Bi:Co=2.5:0.5:4.0 (BC-0.5 crystal). Thermoelectric properties of the crystals in the ab-plane were measured at various temperatures. Seebeck coefficient (S) was increased by the partial Bi-substitution at room temperature, whereas electrical resistivity (ρ) was decreased at room temperature except for BC-0.5 crystals. The simultaneous increase in S and decrease in ρ suggest an increase in carrier mobility. Although Bi atoms are heavier than the replaced Ca or Co atoms, the phonon part of thermal conductivity is increased by the Bi-substitution. We suggest that these effects of the Bi-substitution on thermoelectric properties are largely governed by changes in the peculiar crystal structure, such as the misfit relationship between the CoO2 and Ca2CoO3 layers, which constitute the layered structure of the Co3O4O9 phase.

Low-Temperature Synthesis and Electric Properties of New Layered Cobaltite, SrxCoO2

A new layered strontium cobaltite, SrxCoO2, was synthesized by a low-temperature ion-exchange technique from a γ-Na0.7CoO2 precursor. The strontium content was determined to be x = 0.345 by inductively coupled plasma (ICP) analysis. Rietveld analysis of X-ray powder diffraction data indicates that strontium ions are preferentially distributed at the 2d (2/3, 1/3, 1/4) site (space group P63/mmc). The chemical content of strontium ions was determined as x = 0.346(3) relative to cobalt ions. A polycrystalline sample of Sr0.35CoO2 shows metallic electric resistivity below 300 K. The Seebeck coefficient is about 70 µVK-1 at 300 K, which is comparable to that of γ-Na0.7CoO2.