Hiroyuki Kitagawa

Thermoelectric Properties of Single-Crystalline SiC and Dense Sintered SiC for Self-Cooling Devices

We investigated the thermoelectric properties of 4H-SiC substrates and dense sintered SiC, which are expected to be candidate materials for use in self-cooling devices because of their high Seebeck coefficient, low electrical resistivity, and high thermal conductivity. The carrier concentration of 4H-SiC samples doped with nitrogen is in the range of 1016 to 1019 cm-3. The sintered SiC samples of the α-type and β-type contain less than 1000 ppm of cation impurities and have a relative density higher than 98% with respect to single-crystalline SiC. 4H-SiC with a carrier concentration of 1019 cm-3 has the highest power factor of 2.7 ×10-3 WK-2m-1 and a high thermal conductivity of 260 WK-1m-1 at room temperature. One-dimensional calculations for heat distribution indicate that a Si chip in a self-cooling device, which consists of 4H-SiC with a carrier concentration of 1019 cm-3, could be refrigerated more strongly than one on a copper plate under specific operating conditions.

Magnetic and transport properties in Heusler-type Fe2TiSn compound

Abstract Magnetic and transport properties of Heusler-type Fe 3−x Ti x Sn (x=0.9,0.95,1.0,1.05 and 1.1) compounds have been investigated with DC magnetization, thermoelectric power and resistance measurements. The electrical resistivity of Fe3−xTixSn compounds with x=1,1.05 and 1.1 exhibit metallic behavior. On the other hand, Fe2.05Ti0.95Sn compound (x=0.95) exhibits semiconductor-like behavior above the Curie temperature of 120 K . These results suggest that the Fe2.05Ti0.95Sn compound should be a semimetal with a pseudogap in the density of states at the Fermi level.

Thermoelectric and transport properties of semi-conducting Bi88Sb12 alloy

Abstract Homogenized sample of Bi88Sb12 was prepared by quenching and annealing at 523 K for 200 days. It is an n-type semiconductor, and its properties show different temperature dependences between the low-temperature (T K ) and high-temperature (T>70 K ) regions. Chemical potential was calculated from the measured carrier concentration, and found to change abruptly at 70 K , increasing rapidly with temperature. This explains the above differences in the two temperature regions in the framework of the Boltzmann theory, indicating Bi88Sb12 is a strongly degenerate semiconductor.

Thermoelectric Properties of Cu-Doped n-Type Bi2Te2.85Se0.15 Prepared by Liquid Phase Growth Using a Sliding Boat

N-type Bi2Te2.85Se0.15 thermoelectric materials were prepared by liquid phase growth (LPG) using a sliding boat, a simple and short fabrication process for Bi2Te3-related materials. Cu was selected as a donor dopant, and its effect on thermoelectric properties was investigated. Thick sheets and bars of CuxBi2 Te2.85Se0.15 (x=0–0.25) of 1–2mm in thickness were obtained using the process. X-ray diffraction patterns and scanning electron micrographs showed that the in-plane direction tended to correspond to the hexagonal c-plane, which is the preferred direction for thermoelectric conversion. Cu-doping was effective in controlling conduction type and carrier (electron) concentration. The conduction type was p-type for undoped Bi2Te2.85Se0.15 and became n-type after Cu-doping. The Hall carrier concentration was increased by Cu-doping. Small resistivity was achieved in Cu0.02Bi2Te2.85Se0.15 owing to an optimized amount of Cu-doping and high crystal orientation. As a result, the maximum power factor near 310K for Cu0.02Bi2Te2.85Se0.15 was approximately 4×10−3W/K2m and had good reproducibility. Furthermore, the thermal stability of Cu0.02Bi2Te2.85Se0.15 was also confirmed by thermal cycling measurements of electrical resistivity. Thus, n-type Bi2Te2.85Se0.15 with a large power factor was prepared using the present LPG process.

Transport parameters of single crystalline SiC for self-cooling device

Cooling is important to keep the temperatures of the highly integrated silicon electronic devices and power devices e.g. power MOSFET, IGBT. Yamaguchi et al. have proposed a new scheme to cool down the devices by its own current named ldquoself-cooling devicerdquo, in which the cooling process uses Peltier effect. In the proposed scheme, we should use the materials that have high thermal conductivity, high Seebeck coefficient and low electrical resistivity. These requirements are different from the conventional Peltier materialspsila. SiC is one of the candidate materials, and we measured the electrical resistivity, the Seebeck coefficient and the thermal conductivity of single-crystalline 4H-SiC in the temperature of 300 K - 400 K. We also evaluated the performance of the proposed cooling for the present power MOSFET by using the experimental data, and discuss the direction of the future study.

Thermoelectric properties of B-doped SrTiO3 singe crystal

Effect of boron on the electric and thermoelectric properties of SrTiO3 has been studied. Boron-doped SrTiO3 was prepared by vacuum annealing of SrTiO3 single crystals in presence of boron vapors. The crystals show low resistivity ~0.1 Ωcm and high Seebeck coefficient of several hundreds μV/K at a room temperature. It was estimated that almost every boron atom incorporated in SrTiO3 crystal provided a charge carrier. Temperature dependence of Seebeck coefficient and figure of merit ZT were measured as a function of carrier concentration.

Thermoelectric Measurements on Bi0.5Sb1.5Te3 Under Hydrostatic Pressure

The thermoelectric power alpha and the electric resistivity rho of Bi0.5Sb1.5Te3 were measured under hydrostatic pressure. Alumel-chromel thermocouples (used for the temperature detection and current leads) and Pt voltage leads were spot-welded on the both ends of the long and narrow sample and a film heater was fixed on the one end for generating the temperature gradient. The wired sample was set into a piston-cylinder type pressure cell together with pressure-transmitting medium and then compressed up to 2.5 GPa. The thermoelectric power slightly decreases with increasing pressure. Otherwise the electrical resistivity is reduced by about 50% at 2.5 GPa compared with 0.5 GPa. Consequently, the power factor alpha 2/rho increases by a factor of about 1.5 at 2.5 GPa relative to 0.5 GPa. The results indicate that the thermoelectric figure of merit is possibly improved by application of hydrostatic pressure

Preparation of p- and n-type SiC-based thermoelectric materials by spark plasma sintering

We report on the preparation of Si/sub 3/N/sub 4/ or Al/sub 4/C/sub 3/ added SiC by spark plasma sintering (SPS) and their thermoelectric properties. The relative densities of all the sintered materials were more than 80% of the theoretical value. The conduction types were controlled by the addition of Si/sub 3/N/sub 4/ for n-type and Al/sub 4/C/sub 3/ for p-type. The electrical resistivity decreased and the Seebeck coefficient increased with temperature. We also found that the power factor depends on the composition of the Si/sub 3/N/sub 4/ or Al/sub 4/C/sub 3/ and takes the maximum value at 7wt% addition at 973K.

Novel electrical and mechanical characteristics of composites composed of electrically conducting Ni-Cr alloy particles in non-conducting soda-lime glass

The bulk resistivity of Ni-Cr alloys is inherently constant. Therefore, it is necessary to regulate the cross-section and length of Ni-Cr alloys to achieve the desired electrical resistance. Here, we describe a composite, comprising a soda-lime glass matrix and disk-like Ni-Cr alloy particles, that has variable bulk resistivity. The bulk resistivity of the composite can be controlled accurately by adjusting the volume fraction (30–80 vol% Ni-Cr alloy) and aspect ratio of the particles. Furthermore, the composite’s strength and fracture toughness are both improved by microdispersion of the disk-like Ni-Cr alloy particles. A 1/100-scale model resistor based on this composite was found to have attractive properties for electrical power applications, such as an approximately 50% reduction in volume, a 65% reduction in weight, and a 95% reduction in inductance in comparison with a conventional neutral grounding resistor made from special cast iron. Clearly, use of this composite material for fabrication of ubiquitous electrical components would greatly reduce the demand and consumption of Ni and Cr for this application. Several benefits are envisioned from this development, including the fabrication of downsized devices and the availability of lower-cost home appliances and industrial products.

Preparation of Functionally Graded Materials (FGMs) Using Coal Fly Ash and NiCr‐Based Alloy Powder by Spark Plasma Sintering (SPS)

Functionally Graded Materials (FGMs) were prepared by spark plasma sintering (SPS) using coal fly ash and NiCr alloy powder. The coal fly ash was produced by the Misumi Coal Thermal Power Station (Chugoku Electric Power Co., Inc.), with 80 wt% nickel and 20 wt% chromium (Fukuda Metal Foil & Powder Co., Ltd.) used as source materials. The sintering temperature in the graphite die was 1000 °C. X‐ray diffraction patterns of the sintered coal fly ash materials indicated that mullite (3Al2O3⋅2SiO2) and silica (SiO2) phases were predominant. Direct joining of coal fly ash and NiCr causes fracture at the interface. This is due to the mismatch in the thermal expansion coefficients (CTE). A crack in the FGM was observed between the two layers with a CTE difference of over 4.86×10−6 K−1, while a crack in the FGM was difficult to detect when the CTE difference was less than 2.77×10−6 K−1.