Papers by Author: Hitoshi Kohri

Abstract: Thermoelectric generator is expected as an independent source, an energy converter for co-generation with Refuse Derived Fuel (RDF) and so on. Thermoelectric materials were required high Seebeck coefficient, low electrical resistivity and low thermal conductivity. Thermoelectric oxides are suitable at the high temperature range because of chemical stability. Authors focus attention on Aurivillius compounds. The Aurivillius compounds consist of Perovskite layers and Bi-O layers. It is expected that nano-layered structure shows high Seebeck coefficient due to the quantum confinement of electron in Perovskite layers. It was reported that the Seebeck coefficient of Aurivillius phase Bi₂VO_5.5 was high value of -28.3 mVK^-1 at 1010 K, and the electrical resistivity of the one was also high value of 0.033 Ωm at 1010 K. We investigated about element substitution effects at V site on thermoelectric properties. Bi₂V_1-xM_xO_5.5 (M=Cu, Cr, x=0, 0.05, 0.1, 0.2) were prepared by solid-state reaction and hot pressing. From the results of the electrical resistivities and the Seebeck coefficients, Cu and Cr behaved as acceptor to Bi₂VO_5.5. Cr was effective for reducing the thermal conductivity of Bi₂VO_5.5. The maximum value of dimensionless figure of merit for Bi₂VO_5.5 was 0.06 at 910 K.

Abstract: Research and development of thermoelectric generators have been actively carried out to use waste heat. It is well known some p-type oxides show high thermoelectric performance. However, an n-type oxide with high performance has not been found. An n-type CaMnO3 is a promising material because of its high Seebeck coefficient. The electrical resistivity of this oxide is, however, too high to use it practically. Not only high Seebeck coefficient but also low electrical resistivity is required for practical use. At first, we investigated the effects of element substitution in order to decrease the resistivity. N-type CaMn0.9M0.1O3 (M=Cu, In) compounds were prepared by solid-state reaction and hot pressing. The maximum value of power factor for CaMn0.9In0.1O3 was 0.204 mWm-1K-2, which was the largest of all specimens at 673 K. This value was, however, not enough to use it practically. Secondly, we focus attention on Aurivillius compounds. The Aurivillius compounds consist of Perovskite layers and Bi-O layers. We expect that this crystal structure shows large Seebeck coefficient due to the quantum confinement of electron in Perovskite layer. Bi2VO5.5 with Aurivillius structure was prepared by solid-state reaction and hot pressing. The Seebeck coefficient of Bi2VO5.5 decreased with increasing temperature and was positive value below 600 K and was negative value above 600 K. The power factor of annealed Bi2VO5.5 showed the highest value of all specimens at the temperature range above 800 K.

Abstract: In modern age, much thermal energy is emitted from ceramic and/or steel industries. Their temperature range is between 500 K and 1300 K. Thermoelectric materials are promising to utilize the waste heat, because of no CO2 emission and long life due to no moving parts. The thermoelectric properties of every thermoelectric material have temperature dependence and high performance appears at a specific temperature range. If the proper materials are placed and joined along the temperature gradient to form an FGM, the performance should be higher than a monolithic material. The performance of a thermoelectric material is expressed by the dimensionless figure of merit ZT=α2ρ-1κ -1T, where α is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is absolute temperature. Thermoelectric oxides are suitable for high temperature materials because of chemical stability. NaxCoO2 shows relatively high ZT value in thermoelectric oxide at the temperature range below 800 K. Ca3Co4O9 shows ZT ~1 at 1000 K. Recently, it is reported that Ca3Co2O6 that is formed by decomposition of Ca3Co4O9 at 1173 K has high performance at 1300 K. The properties and fabrication condition of high density Ca3Co2O6 are, however, not reported in detail. In order to improve the thermoelectric properties and to shift the temperature range for Ca3Co2O6, we investigated the effects of element substitution. In this experiment, the sintered Ca3Co2-xMxO6 (x=0 or 0.2, M= Mn, Mo or V) were prepared by solid-state reaction or hot pressing. Relative density of Ca3Co2O6 by hot-pressing (HP) was over 94% which is larger than one of Ca3Co2O6 by solid-state reaction (SSR). The resistivity of Mo- or V-substituted Ca3Co2O6 (HP-Mo or HP-V) were lower than one of non-substituted Ca3Co2O6 (HP). The resistivity of Mo-substituted Ca3Co2O6 (HP-Mo) showed the lowest value of 4.3×10-2 Ωcm in all specimens at 1181 K. The power factor α2ρ-1 of Ca3Co2O6 (HP-Mo) was 64.2 Wm-1K-2, which is the largest of all specimens at 1178 K, and this value is approximately 1.3 times higher than 48.8 Wm-1K-2 for Ca3Co2O6 (HP).

Abstract: It is difficult to use an ordinary plain bearing under a high load or at a high friction speed because lubricant oil is pushed out from the friction surface or deterioration of the lubricant oil is caused by heat of friction. A solid lubricant, MoS2, is promising in such condition. When the lubricant is dispersed in a matrix, the solid lubricant is always supplied from the matrix. Such a composite bearing needs a back metal to maintain its shape. The heat of friction may cause a crack between the bearing and the back metal due to thermal stress. A bearing with low coefficient of friction is necessary to decrease the heat of friction, and an FGM structure is also promising to decrease the stress. The aim of this experiment is to fabricate and to examine friction properties of the composites. Cu was plated on the lubricant particles by electroless deposition. The lubricant volume fraction (hereafter Vf) was up to Vf 30 %. The Cu plated lubricant particles were hot-pressed to form a composite at 873 K under 30 MPa in a vacuum. Friction properties of the composites were determined by using a ball-on-disk type testing machine. The test was performed in the air without oil at room temperature. The solid lubricant in the composites was effective to decrease the coefficients of friction under a high load when the Vf was higher than 20 %.

Abstract: In bio-systems, nano-composites with complex micro-structures are formed by self-assembly only using low energy at room temperature. If these mechanisms of biological tissue are identified, we can possibly propose a new process to fabricate composites by mimicking tissue formation in vivo. As a bio-material, we paid attention to bio-tissue reinforced with collagen fibrils. Collagen fibrils are of baculiform; Thus the self-assembly process through liquid crystalline transition has been proposed by a French group [1]. In the present study, factors controlling liquid crystalline transition, e.g. concentration and pH, are discussed using collagen solution. When liquid crystalline phase is produced, aligned molecules exhibits optical anisotropy. This anisotropy was observed with a polarized optical microscopy (POM). By observations with POM, development of cholesteric phase in collagen solution was clarified.

Abstract: Thermoelectric materials can directly convert thermal energy into electrical energy. Research and development of thermoelectric generators have been actively carried out to use waste heat. Electrodes are necessary to take out the electrical power from the thermoelectric couples. However, large portion of the generated electrical power is often lost at the interface between electrode and thermoelectric materials. Though oxide materials are promising for a thermoelectric generator at a high temperature, they are not practically used as the joining technique is not established. Not only low contact resistance but also sufficient mechanical strength is required for the joining. In this report, tin alloy solder was attempted for cold side junction to obtain low contact resistance and high mechanical strength at the interface. Wettability of the solder to Ca3Co2O6 and the thermoelectric generating properties were improved by adding titanium to tin alloy.

Abstract: Ca3Co4O9 is a promising material for thermoelectric generation, as it is stable up to 1173 K in the air, and shows good thermoelectric properties. Recently, it was found that Ca3Co2O6 was stable up to 1300 K in the air. The Ca3Co2O6 is decomposed phase of Ca3Co4O9 and the temperature limit is higher than one of Ca3Co4O9. The electrical resistivity of Ca3Co2O6 was, however, higher than the one of Ca3Co4O9. Not only high power generation performances but also excellent strength is required for practical use of the thermoelectric oxide materials. Polycrystalline samples of Ca3Co2O6 were prepared by solid-state reaction (SSR) and hot pressing (HP). Relative density of Ca3Co2O6 (HP) was over 98%, which is larger than the one of Ca3Co2O6 (SSR). Ca3Co2O6 (HP) showed larger strength and lower resistivity than Ca3Co2O6 (SSR). The resistivity (ρ) of Ca3Co2O6 (HP) in perpendicular to the pressurized direction decreased from 64 Ωcm to 4.0×10-2 Ωcm at the temperature range between 373 and 1173 K. In addition, the resistivity of this sample was decreased by heat treatment in the air. The Seebeck coefficients (S) of Ca3Co2O6 (HP) was positive value and more than 160 μVK-1 at the temperature range between 373 and 1173 K. Ca3Co1.8M0.2O6 (M= Mn or V) were prepared by solid state reaction and hot pressing. The resistivity of Mn-substituted Ca3Co2O6 (HP-Mn) and V-substituted Ca3Co2O6 (HP-V) were lower than the one of non-substituted Ca3Co2O6 (HP) at the temperature below 523 K for the Mn-substituted sample or 723 K for V-substituted sample. The latter showed the lowest value 1.53 Ωcm of all specimens at 383 K. The power factor (S2ρ-1) of Ca3Co2O6 (HP) was 88.3 μWm-1K-2, which is the largest of all specimens at 1176 K, but S2ρ-1 of V-substituted Ca3Co2O6 (HP-V) is the largest of all specimens up to 773 K.

Abstract: It is difficult to use an ordinary bearing in a vacuum and/or at a high temperature, because of evaporation or deterioration of the lubricant oil. A solid lubricant, MoS2 or BN, is promising in such condition. Recently, a thin solid lubricant film is often applied on a hard material by PVD or CVD. The thin film is, however, easily removed when the load or friction speed is too high. On the other hand, when the lubricant is dispersed in a matrix, the solid lubricant always exists even the surface of the bearing is worn out. The aim of this experiment is to examine friction properties of the composites. Cu was plated on the lubricant particles by electroless deposition. The lubricant volume fraction (hereafter Vf) was up to 30 vol%. The Cu plated lubricant powder was hot-pressed to form composites at 873 K under 30 MPa in a vacuum. Friction properties of composites were determined by using a ball-on-disk type testing machine. The test was performed either in air or in a vacuum without oil at the room temperature. The coefficients of dynamical friction of the composites were decreased by the lubricant in the air. The effect of the lubricant was much remarkable in vacuum. The wear rates of the composites were, however, larger than Cu in the air. On the contrary, the wear rates of the composites were much less than one of Cu in a vacuum.

Abstract: Bi2Te3 is the best compound for thermoelectric materials around the room temperature. If the temperature range is shifted to higher side, it is useful to obtain electrical energy from waste heat source which is abundant at the temperature around 500 K. In this experiment, Bi2Te3-GeTe pseudo binary compounds were investigated to shift the temperature range. The lattice thermal conductivity was remarkably decreased at 50 or 75 mol%GeTe by synergy effect of solid solution and grain boundaries. The peak temperature of figure of merit Z for Bi2Te3-GeTe pseudo binary compounds was higher than Bi2Te3.

Abstract: Every thermoelectric material shows high performance at a specific narrow temperature range. The temperature range with high performance can be expanded by joining the materials with different peak temperature. This is the concept of a functionally graded material (FGM) for thermoelectric materials. Bismuth telluride is the best material for cooling devices at around room temperature. Then we investigated the thermoelectric cooling properties for bismuth telluride with two step graded structure. FGM samples were fabricated by three methods. The first FGM was synthesized by in situ method. The second one was fabricated by joining in a hot-press equipment. The last one was composed by joining with solder. Thermoelectric cooling properties were evaluated by observing the maximum temperature drop to electric current when the high temperature side was kept constant. The large temperature difference was obtained when the proper configuration of thermoelectric materials along the temperature gradient were performed. The coincidence of optimum electrical currents of composing materials is also essential to obtain the high cooling performance.