Hitoshi Kohri

Thermoelectric Generating Properties of Aurivillius Compounds

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 Bi2VO5.5 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. Bi2V1-xMxO5.5 (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 Bi2VO5.5. Cr was effective for reducing the thermal conductivity of Bi2VO5.5. The maximum value of dimensionless figure of merit for Bi2VO5.5 was 0.06 at 910 K.

Thermoelectric Properties of High Density Sintered Ca3Co2O6

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.

Fine Bi2Te3 wires fabricated by glass sealed melt spinning

Applying a small size cooling system is essential to avoid thermal noise in the field of micro-electronics. Fine thermoelectric materials are required to construct the system. A glass sealed melt spinning process was attempted to fabricate the fine materials. Bi2Te 3 wires of 75 mum in diameter with oval cross section were successfully obtained. The original electrical resistivity and Seebeck coefficient of the wires were maintained after the process

Control of grain size and crystal orientation for Bi-Sb-Te compounds

Reducing the grain size of Bi/sub 0.5/Sb/sub 1.5/Te/sub 3/ was attempted by mechanical alloying under ultra low oxygen atmosphere. The powder prepared by mechanical alloying was sintered by hot pressing at 650 K under an atmosphere of 4% H/sub 2/ with Ar balance (0.1 MPa). The crystal orientation in the sintered block was random. Then the block was deformed by ultra high pressure hot pressing at 650 K or 700 K or 750 K to obtain a desirable orientation for thermoelectric properties. Improvement of the crystal orientation in the deformed specimens was not confirmed by TEM. The results of electrical resistivity and Hall coefficient measurements, however, revealed that better orientation was indicated in the electrical properties.

Thermoelectric Generating Properties of Perovskite Like Materials

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.

Ca3Co2O6 for High Temperature Side of Thermoelectric FGM

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).

Fabrication of Electrode for Thermoelectric Oxide Materials

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.

Development of Thermoelectric Cooling Devices with Graded Structure

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.

Thermal stability of thermoelectric properties for nondoped PbTe

The undoped PbTe compounds were prepared by the vertical Bridgman method. Hall coefficient R/sub H/ and /spl rho/ were measured by a d.c. four-terminal method. The temperature dependence of /spl rho/ and R/sub H/ were measured in the temperature range from 700 to 300K in an Ar atmosphere. PbTe showed p-type conduction at 300K and was transformed to n-type conduction at about 500K. No thermal hysteresis was observed in the temperature dependence of /spl rho/ for the heating temperature below 500K at slower cooling rates than 8.88/spl times/10/sup -2/K/s and at cooling rates above 6.45/spl times/10/sup -1/ K/s from 700K. The thermal hysteresis in the temperature dependence of /spl rho/ was confirmed at a slower cooling rate than 8.88/spl times/10/sup -2/K/s at temperatures higher than 600K. The Hall concentration of a specimen which showed thermal hysteresis in the /spl rho/-curve at the slow cooling rate of 8.88/spl times/10/sup -2/K/s from 700K was found to be 1.25/spl times/10/sup 24//m/sup 3/ smaller, 1/3 of that with no thermal hysteresis.

Thermoelectric Properties and Thermal Stability of BiCuSeO

Although BiCuSeO has become of interest as a high performance thermoelectric material, it has been reported to be unstable in air above 548 K. However, the details of the thermal degradation of BiCuSeO have not yet been determined. In this study, the time dependence of the thermoelectric properties of BiCuSeO at 550 K was assessed along with the accompanying thermal degradation behavior. Variations in ρ and α over time at 550 K were determined using the direct current four-terminal method and small temperature difference method, respectively. In addition, the crystalline phases of the specimen after thermoelectric measurements were identified by x-ray diffraction (XRD) analysis, and the surface chemical states were analyzed by x-ray photoelectron spectroscopy (XPS). The results of thermoelectric property measurements over time showed that ρ abruptly increased between 135 min and 182 min then decreased from 182 min to 210 min, while the value of α exhibited the opposite trend. XPS and XRD data suggested that BiCuSeO was pyrolyzed upon heating at 550 K for 6 h in air. In contrast, thermogravimetric analysis demonstrated that BiCuSeO was stable below 800 K under inert gas flow, thus the pyrolysis observed at 550 K was the result of reaction with atmospheric oxygen.