Megumi Akoshima

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.

Exploring Advantages of Diverse Carbon Nanotube Forests with Tailored Structures Synthesized by Supergrowth from Engineered Catalysts

We explored advantages of diverse carbon nanotube forests with tailored structures synthesized by water-assisted chemical vapor deposition (CVD) growth (supergrowth) from engineered catalysts. By controlling the catalyst film thickness, we synthesized carbon nanotube (CNT) forests composed from nanotubes with different size and wall number. With extensive characterizations, many interesting dependencies among CNT forest structures and their properties, which were unknown previously, were found. For example, multiwalled carbon nanotubes (MWNTs) showed superior electronic conductivity while single-walled carbon nanotubes (SWNTs) showed superior thermal diffusivity, and sparse MWNTs achieved lower threshold voltage for field emission than dense SWNTs. These interesting trends highlight the complexity in designing and choosing the optimum CNT forest for use in applications.

Thermal Diffusivity of Single-Walled Carbon Nanotube Forest Measured by Laser Flash Method

The science and technology of carbon nanotubes are very interesting topics in the field of nanotechnology. It is considered that such nanotubes have excellent properties of electrical conduction, tensile strength, and thermal conduction. However, these properties are not well understood yet. Techniques for handling a nanotube and measuring these properties have not yet been established. Recently, a technique for synthesizing supergrowth carbon nanotubes, which form a highly pure single-walled carbon nanotube (SWNT) forest, has been developed. These supergrowth carbon nanotubes grow to as long as a millimeter scale. As-grown supergrowth carbon nanotubes include about 5×1011 SWNTs per square centimeter. The solid of supergrowth carbon nanotubes is prepared by reducing the distance between SWNTs. We have investigated the thermal conduction of such carbon nanotubes and were successful in measuring the thermal diffusivity of self-standing samples of as-grown supergrowth carbon nanotubes and their solid using the laser flash method. It was found that the carbon nanotube samples show a comparable thermal diffusivity to isotropic graphite. We have also measured the temperature dependence of the thermal diffusivity of the supergrowth carbon nanotube samples.

Transport and {mu}SR studies at p{approximately}1/8 in the Bi-2212 and Y-123 phases

The so-called 1/8 effect, namely, the anomalous suppression of superconductivity at the hole concentration per Cu in the CuO2plane, p, ∼ 1/8 has been investigated in the partially Zn-substituted Bi-2212 and Y-123 phases by measuring the Hall effect and the muon spin relaxation ( μSR ). The Hall angle has been found to exhibit large enhancement at p ∼ 1/8 in Bi2Sr2Ca1−xYx(Cu1−yZny)2O8+δwith y=0.025. In YBa2Cu3−2yZn2yO7−δwith y=0.025, anomalous enhancement of the muon spin depolarization has been observed at p ∼ 1/8, indicating that the magnetic correlation between Cu spins is enhanced at p ∼ 1/8. These results support the stripe model and the proposition that the 1/8 effect is common to all high-Tccuprates with the CuO2plane.

Development of Spatial Inhomogeneity of Internal Magnetic Field Above Tc in Bi2Sr2Ca1−xYxCu2O8+δ Observed by Longitudinal-Field Muon Spin Relaxation

Longitudinal-field muon-spin-relaxation measurements have revealed inhomogeneous distribution of the internal magnetic field at temperatures above the bulk superconducting (SC) transition temperature, Tc, in slightly overdoped Bi2Sr2Ca1−xYxCu2O8+δ. The distribution width of the internal magnetic field, Δ, evolves continuously with decreasing temperature toward Tc. The origin of the increase in Δ is discussed in terms of the generation of SC domains in a sample.

Thermal Diffusivity Measurement for Thermal Spray Coating Attached to Substrate Using Laser Flash Method

Ceramic-based thermal barrier coatings are used as heat and wear shields of gas turbine blades. There is a strong need to evaluate the thermal conductivity of coating for thermal design and use. The thermal conductivity of a bulk material is obtained as the product of thermal diffusivity, specific heat capacity, and density above room temperature in many cases. Thermal diffusivity and thermal conductivity are unique for a given material because they are sensitive to the structure of the material. Therefore, it is important to measure them in each sample. However it is difficult to measure the thermal diffusivity and thermal conductivity of coatings because coatings are attached to substrates. In order to evaluate the thermal diffusivity of a coating attached to the substrate, we have examined the laser flash method with the multilayer model on the basis of the response function method. We carried out laser flash measurements in layered samples composed of a CoNiCrAlY bond coating and a 8YSZ top coating by thermal spraying on a Ni-based superalloy substrate. It was found that the procedure using laser flash method with the multilayer model is useful for the thermal diffusivity evaluation of a coating attached to a substrate. # 2011 The Japan Society of Applied Physics

Cu-site-substitution effects on the 1/8 anomaly in the high-Tc cuprates and on the anomaly at x=0.21 in La2−xSrxCuO4

Effects of the Cu-site substitution on the 1/8 anomaly in the Bi-2212 and Y-123 phases and also on the anomaly at x = 0.21 in La 2-x Sr x CuO 4 (LSCO) have been investigated from the transport, muon-spin relaxation and neutron-scattering measurements. It is suggested that the dynamical stripe correlations of spins and holes exist in the Bi-2212 and Y-123 phases as well as in the La-214 phase and that they tend to be pinned by Zn at the hole concentration of ∼1/8 per Cu in the high-T c cuprates and also at x = 0.21 in LSCO. Based upon the stripe-pinning model, universal properties in the high-T c cuprates that partially substituted Zn is a strong scatterer and that the Zn substitution markedly suppresses the superconductivity are also discussed.

μSR study on the magnetic state of the Zn-substituted Bi-2212 system

Abstract Zero-field muon spin relaxation (μSR) measurements were applied to the Zn-substituted Bi-2212 system, Bi 2 Sr 2 Ca 1− x Y x (Cu 1− y Zn y ) 2 O 8+ δ , to study a possibility of the existence of the so-called “1/8 Anomaly” which was established in the La-systems. The muon spin depolarization rate of a Zn-substituted ( y =0.025) system at a hole density of about 1/8 per Cu-atom ( x =0.3125) increased with decreasing temperature below 5 K . The depolarization behavior at low temperatures was well described by an exponential-type relaxation function. The possibility of the existence of the “1/8 anomaly” in the Bi-system is discussed.

Thermal Diffusivity Measurements of the Layered Materials by the Laser Flash Method

Ceramics-based thermal barrier coatings are used as heat and wear shields of blades of gas turbine. There are strong needs to evaluate thermophysical properties of coatings, such as thermal conductivity, thermal diffusivity and heat capacity of them. Since coatings are attached on substrates, it is not easy to measure these properties separately. In order to evaluate the thermal diffusivity of coating attached on substrate, we have tried to apply the multi-layer model based on the response function method and established a procedure for the measurement by the laser flash method. We verified the procedure by the measurements from room temperature to about 1000 K for two-layer ceramics sample prepared by the doctor blade tape casting method. The thermally sprayed CoNiCrAlY coating on the SUS304 substrate was also used for verification. The thermal diffusivity of coating attached on substrate approximately agreed with that of the single-layer coating removed from substrate. In the case of the ceramics sample, the thermal diffusivity of the coating including the interfacial thermal resistance determined within about 20 % uncertainty. We compared the laser flash measurement signals of the samples prepared by the thermal spraying with variant thickness and found the difference among them. It was found that the procedure has enough resolution to detect the heat shield effect caused by the change with about 200 m in thickness. The result shows that the procedure and analysis were practically effective for the thermal diffusivity estimation of coating attached on the substrate without remove from substrate.

1/8 anomaly in the Bi-2212 and Y-123 phases

Abstract Recent experimental work on the 1/8 anomaly in the Bi-2212 and Y-123 phases by our group is reviewed. In the partially Zn-substituted Bi2Sr2Ca1−xYx(Cu1−yZny)2O8+δ, we have found anomalous suppression of superconductivity at x=0.30−0.35, where the hole concentration per Cu in the CuO2 plane, p, ∼1/8. Moreover, it has been revealed from the muon spin relaxation (μSR) measurements that the Cu-spin fluctuations exhibit slowing-down behavior at low temperatures singularly in these samples. In Y1−xCaxBa2Cu3O7−δ, the so-called 60 K plateau of Tc has been found not to be correlated with the oxygen content but to be interpreted as being due to the suppression of superconductivity at p ∼1/8. The slowing-down behavior of the Cu-spin fluctuations has also been observed singularly at p ∼1/8 in the partially Zn-substituted YBa2Cu3−2yZn2yO7−°. These results are suggestive of the dynamical stripe correlations of holes and spins tending to be pinned by a small amount of Zn at p ∼1/8 in the Bi-2212 and Y-123 phases as well as in the La-214 phase. Accordingly, we conclude that the 1/8 anomaly is common to all high-Tc cuprates with the CuO2 plane.