Satoshi Sodeoka

An Oxide Single Crystal with High Thermoelectric Performance in Air.

An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µVK-1 at 100 K and increases with temperature up to 210 µVK-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (ZT) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

Thermoelectric properties of Bi2Sr2Co2Ox polycrystalline materials

Bi2Sr2Co2O9 (BC-2202) polycrystalline materials with a layered structure have been prepared by partial melting. The chemical compositions of the samples are Bi2Sr2Co2Ox (2202), Bi1.8Sr2Co2Ox (Bi-1.8), and Bi2Sr1.8Co2Ox (Sr-1.8). All three samples are p-type conductors. The electric properties, namely, the Seebeck coefficient (S) and electric resistivity (ρ), of the samples are dependent on chemical composition. The S values increase with temperature at T>673 K and, at 973 K, reach 100, 110, and 150 μV K−1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. Thermal conductivity (κ) for all samples is lower than for ordinary conducting oxides. The figure of merit (Z) increases with temperature for all samples. Z values at 973 K are 0.77×10−4, 0.61×10−4, and 2.0×10−4 K−1 for the 2202, Bi-1.8, and Sr-1.8 samples, respectively. The thermoelectric properties depend on the chemical composition of the BC-2202 phase. The BC-2202 material thus appears to be a promising thermoelectric material due to its...

Fabrication of an all-oxide thermoelectric power generator

An oxide thermoelectric device was fabricated using Gd-doped Ca3Co4O9 p-type legs and La-doped CaMnO3 n-type legs on a fin. The power factors of p legs and n legs were 4.8×10−4 Wm−1 K−2 and 2.2×10−4 Wm−1 K−2 at 700 °C in air, respectively. With eight p–n couples the device generated an output power of 63.5 mW under the thermal condition of hot side temperature Th=773 °C and a temperature difference ΔT=390 °C. This device proved to be operable for more than two weeks in air showing high durability.

Thermoelectric properties of spark plasma sintered Ca2.75Gd0.25Co4O9 ceramics

Spark plasma sintering (SPS) was applied for preparing dense Ca2.75Gd0.25Co4O9 ceramics, and their thermoelectric properties were investigated from room temperature to 700 °C in air. The SPS process was effective in obtaining dense Ca2.75Gd0.25Co4O9 ceramics, typically 98% of the theoretical x-ray density, with lower resistivity, without any degradation of the thermoelectric power. The power factor and figure of merit of the spark plasma sintered samples attain 4.8×10−4 W m−1 K−2 and ZT=0.23 at 700 °C, respectively.

Synthesis and thermoelectric properties of the new oxide ceramics Ca3-xSrxCo4O9+δ (x = 0.0-1.0)

Abstract A new series of oxides Ca3−xSrxCo4O9+δ, (x=0.0–1.0) were synthesized, and their structures, electrical properties and Seebeck coefficients were measured from room temperature to 700°C. All the oxides have Ca2Co2O5 type structure. For all the oxides, both the electrical conductivity and the Seebeck coefficients increase with increasing temperature. The Seebeck coefficients are all positive. The values of Seebeck coefficient decrease with increasing Sr content for the Sr substituted samples. The power factor decreases with increasing Sr content for the samples with x 0.5. The Sr substituted sample (x=1.0) has lower thermal conductivity than Ca3Co4O9+δ due to its lower electrical conductivity. At 600°C, the figure of merit for Ca2SrCo4O9+δ is 0.9×10−4 K−1, and the value increases toward higher temperature.

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.

Magnetic and thermoelectric properties of NaCo2−xMxO4 (M = Mn, Ru)

Abstract A new series of oxides NaCo2−xMxO4 (M = Mn, 0.0 ≤ x ≤ 1.0; M = Ru, 0.0 ≤ x ≤ 0.5) were synthesized, and their crystal structure, electrical conductivity, thermoelectric power, and magnetic properties were studied. All the samples crystallized in hexagonal systems. The electrical resistivity and the Seebeck coefficient of NaCo2−xMxO4 (M = Mn, Ru) increase with increasing Mn content. The magnetic properties of NaCo2−xMnxO4 change with increasing Mn content.

Mechanical properties and fracture behavior of fibrous Al2O3/SiC ceramics

Abstract Fibrous Al 2 O 3 ceramics with a mixture of SiC and Al 2 O 3 as the cell boundaries were fabricated by extrusion-molding and hot-pressing techniques. The effects of the cell boundary composition on the mechanical properties and fracture behavior are investigated. It is shown that a 65:35 mixture of SiC:Al 2 O 3 can act as a suitable cell boundary for Al 2 O 3 cells. In bending tests, such a ceramic displays a non-catastrophic fracture behavior with reasonable load-carrying capability, and its fracture energy and apparent toughness are up to 1349 J/m 2 and 6.0 MPa m 1/2 , respectively.

Effects of High-Pressure Plasma Spraying for Yttria-Stabilized Zirconia Coating

Plasma spraying of yttria-stabilized zirconia was carried out under chamber pressures ranging from low (30 kPa) to high pressure (300 kPa) to investigate pressure effects on the plasma jet and to clarify the potential of high-pressure plasma spraying (HPPS) as a high performance coating tool. Plasma flame length and velocity of the particles were measured in situ, and the coating characteristics including its microstructure, density, and hardness were studied. A condensed plasma flame under high pressure facilitated sufficient melting of zirconia particles, resulting in high deposition efficiency and a dense coating with improved hardness, in spite of reduced particle velocity. High-pressure plasma spraying was found to be suitable for thermal spraying of high-melting-point materials such as zirconia.

Micro Gas Turbine With Ceramic Nozzle and Rotor

A series of operation tests of a ceramic micro gas turbine has been successfully carried out. The baseline machine is a small single-shaft turbojet engine (J-850, Sophia Precision Corp.) with a centrifugal compressor, an annular type combustor, and a radial turbine. As a first step, an Inconel 713C alloy turbine rotor of 55 mm in diameter was replaced with a ceramic rotor (SN-235, Kyocera Corporation). A running test was conducted at rotational speeds of up to 140,000 rpm in atmospheric air. At this rotor speed, the compression pressure ratio and the thrust were 3 and 100 N, respectively. The total energy level (enthalpy and kinetic energy) of the exhaust gas jet was 240 kW. If, for example, it is assumed that 10% of the total power of the exhaust jet gas was converted into electricity, the present system would correspond to a generator with 24 kW output power. The measured turbine outlet temperature was 950°C (1,740°F) and the turbine inlet temperature was estimated to be 1,280°C (2,340°F). Although the ceramic rotor showed no evidence of degradation, the Inconel nozzle immediately in front of the turbine rotor partially melted in this rotor condition. As a second step, the Inconel turbine nozzle and casing were replaced with ceramic parts (SN-01, Ohtsuka Ceramics Inc.). The ceramic nozzle and case were supported by metal parts. Through tests with the ceramic nozzle, it became evident that one of the key technologies for the development of ceramic gas turbines is the design of the interface between the ceramic components and the metallic components, because the difference between the coefficients of linear thermal expansion of the ceramic and metal produces large thermal stress at their interface in the high-temperature condition. A buffer material made of alumina fiber was therefore introduced at the interface between the ceramic and metal.Copyright