Tomonari Takeuchi

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.

All-solid-state lithium secondary battery with ceramic/polymer composite electrolyte

Abstract Dense lithium lanthanum titanate, (Li,La)TiO 3 , pellets were prepared using a spark-plasma sintering (SPS) method. The obtained (Li,La)TiO 3 pellets showed relatively high lithium ion conductivity, typically 10 −3 S cm −1 at 22 °C, with an activation energy of 30.1 kJ mol −1 . Lithium manganese oxide, LiMn 2 O 4 , was deposited on (Li,La)TiO 3 pellets by an electrostatic spray deposition (ESD) method at 400 °C without significant formation of by-products at the interface (Li,La)TiO 3 /LiMn 2 O 4 . An all-solid-state battery system, LiMn 2 O 4 /(Li,La)TiO 3 /SPE/Li, where a solid polymer electrolyte (SPE) was sandwiched between (Li,La)TiO 3 and Li, showed good charge/discharge characteristics over 100 cycles at 60 °C.

Thermoelectric properties of highly grain-aligned and densified Co-based oxide ceramics

Highly grain-aligned Ca3Co4O9 and (Ca2.7Sr0.2La0.1)(Co3.9Cu0.1)O9 ceramics were prepared by the magnetic alignment technique, and then treated by a spark plasma sintering process to increase their bulk densities. Thermoelectric properties were investigated from room temperature to 700 °C in air. Grain alignment is effective in lowering the electrical resistivity and has no obvious influence on the Seebeck coefficient, thus resulting in enhancement of the thermoelectric power factor. Substitution of Sr, La and Cu does not appreciably change the electrical resistivity and Seebeck coefficient, but significantly reduces the thermal conductivity.

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.

ZnRh2O4: A p-type semiconducting oxide with a valence band composed of a low spin state of Rh3+ in a 4d6 configuration

It is demonstrated that a ZnRh2O4 normal spinel with a band gap of ∼2.1 eV is a unique material as a p type wide-gap semiconductor. The electrical conductivity of the sputtered film was 0.7 S cm−1 at 300 K with no intentional doping. The electronic structure was investigated by photoemission and inverse photoemission measurements and indicated that the band gap is composed mainly of ligand field splitting of an octahedrally coordinated Rh3+ octahedron between fully occupied t2g6 and empty eg0 sets.

Grain size dependence of dielectric properties of ultrafine BaTiO3 prepared by a sol-crystal method

Ultrafine BaTiO3 prepared by a decomposition of an organometallic crystal with unity of Ba/Ti ratio (sol-crystal method) has been characterized. While the as-prepared product resulting from the decomposition of the organometallic crystal at room temperature was BaTiO3 with pseudo-cubic structure, the well-crystallized tetragonal polymorph was obtained by firing the as-prepared product above 1000°C. Residual organic compounds, CO2-3 and OH- ions in the samples prevent the grain growth and tetragonal distortion of BaTiO3. We obtained quite higher room temperature permittivity (3700) at 1 kHz for the sample fired at 1200°C than that (630) prepared by conventional solid-state reaction starting from BaCO3 and TiO2. Such a high value was probably due to the accomplishment of homogeneous cation stoichiometry, which was achieved by this preparation method via the organometallic crystal with stoichiometric Ba/Ti ratio.

All-Solid-State Lithium Secondary Battery with Li2S – C Composite Positive Electrode Prepared by Spark-Plasma-Sintering Process

Electrochemically active lithium sulfide-carbon (Li 2 S-C) composite positive electrodes, prepared by the spark plasma sintering process, were applied to all-solid-state lithium secondary batteries with a Li 3 PO 4 -Li 2 S-SiS 2 glass electrolyte. The electrochemical tests demonstrated that In/Li 2 S-C cells showed the initial charge and discharge capacities of ca. 1010 and 920 mAh g -1 -Li 2 S, respectively, which showed higher discharge capacity and coulombic efficiency (ca. 91%) than the Li/Li 2 S-C cells with nonaqueous liquid electrolytes (ca. 200-380 mAh g -1 -Li 2 S and ca. 27%, respectively). The ex situ S K-edge X-ray absorption fine structure measurements suggested the appearance and disappearance of elemental sulfur in the positive electrodes after charging and discharging, respectively, indicating that the ideal electrochemical reaction Li 2 S ⇔ 2Li + S proceeded in the present all-solid-state cells. Such ideal electrochemical reaction, due probably to the suppression of the dissolution of Li 2 S in the form of polysulfides into the electrolytes, would result in higher coulombic efficiency and discharge capacity as compared with those of the liquid-electrolyte cells.

Dielectric properties of spark-plasma-sintered BaTiO3

Spark-plasma-sintering (SPS) has been applied to BaTiO3 to prepare dense ceramics consisting of submicrometre-sized powder. Relatively dense (typically 97% of the theoretical X-ray density) pellets with an average grain size remaining similar to that of the starting powder, approximately 0.6 μm, were obtained by the SPS process. Fixed frequency (1 kHz) measurements show the room temperature permittivity of SPS ceramics to be relatively high, approximately 3500, and at least double the value of conventionally sintered ceramics, approximately 1500. Alternating current (a.c.) impedance spectroscopy measurements show that SPS is an effective process to reduce the influence of intergranular (grain boundary) effects on the permittivity and direct current (d.c.) resistance characteristics of BaTiO3 ceramics substantially.

Plasma polymerization of thiophene and 3-methylthiophene

Abstract Thin polymeric films have been prepared by the plasma polymerization of thiophene and 3-methylthiophene. The electrical conductivities, optical properties and structures of the polymeric products are examined and compared with those of the electrochemically polymerized thiophenes. Upon iodine doping, the electrical conductivities increased by four to six orders of magnitude. Based on the present observation data the molecular structure of the sample films obtained is discussed.

Preparation of Fine-grained BaTiO 3 Ceramics by Spark Plasma Sintering

Dense BaTiO 3 ceramics consisting of fine grains were prepared using fine powder (average grain size of 0.06 μm; BT006) as a starting material and the spark plasma sintering (SPS) method. The powder was densified to >95% of theoretical x-ray density by the SPS process, and the average grain size of the resulting ceramics was <0.5 μm; the particle size of the initial powder significantly affects the grain size of the resulting SPS pellets. Fixed-frequency (100 kHz), room-temperature permittivity measurements of the BT006-SPS ceramics showed relatively low values (3000-3500) compared with those (typically 5000) for SPS ceramics consisting of larger grains (approximately 1 μm). Lower permittivity was attributed to poor development of ferroelectric domains in the ceramics, which originated from incomplete development of the tetragonal structure as well as the presence of a local orthorhombic structure.