Tomoki Ariga

Thermoelectric Properties of Mineral Tetrahedrites Cu10Tr2Sb4S13 with Low Thermal Conductivity

We have investigated thermoelectric properties of synthesized mineral Cu10Tr2Sb4S13 (Tr = Mn, Fe, Co, Ni, Cu, and Zn) tetrahedrites, which have a cubic and complex crystal structure. The mother phase Tr = Cu shows metal–semiconductor transition and anomalous hysteresis. Through various Tr substitutions, the thermopower was increased and thermal conductivity was decreased. Results show that Tr = Ni had the largest dimensionless figure of merit Z T of 0.15 at 340 K. The main advantage for the large Z T is the quite low lattice thermal conductivity. Because of the large Z T and the environmentally friendly components, tetrahedrites are anticipated as a good thermoelectric material.

Variable-range-hopping conduction and low thermal conductivity in chalcogenide spinel CuyFe4Sn12X32 (X = S, Se)

Low-temperature thermoelectric properties are reported for polycrystalline samples of chalcogenide spinel CuyFe4Sn12X32 (X = S, Se; y = 8.0 to 4.0). For all samples with X = S, the electrical resistivity ρ behaves similarly to that of a doped semiconductor at high temperatures and shows a variable-range-hopping-type (VRH) temperature dependence at low temperatures. The values of ρ and the thermopower S decrease concomitantly with decreasing initial Cu composition y. For y=6.0 of the sample with X = Se, ρ shows VRH behavior in a wide temperature range. Because of the monotonic increase of S as T1/2 and the exponential decrease of ρ with increasing temperature due to the the VRH conduction, the sample with X = Se can be a good high-temperature thermoelectric material. Furthermore, both systems show a low thermal conductivity of 1.5 W/Km because of their complex structures.

Investigation of solution-processed bismuth-niobium-oxide films

The characteristics of bismuth-niobium-oxide (BNO) films prepared using a solution process were investigated. The BNO film annealed at 550 °C involving three phases: an amorphous phase, Bi3NbO7 fluorite microcrystals, and Nb-rich cubic pyrochlore microcrystals. The cubic pyrochlore structure, which was the main phase in this film, has not previously been reported in BNO films. The relative dielectric constant of the BNO film was approximately 140, which is much higher than that of a corresponding film prepared using a conventional vacuum sputtering process. Notably, the cubic pyrochlore microcrystals disappeared with increasing annealing temperature and were replaced with triclinic β-BiNbO4 crystals at 590 °C. The relative dielectric constant also decreased with increasing annealing temperature. Therefore, the high relative dielectric constant of the BNO film annealed at 550 °C is thought to result from the BNO cubic pyrochlore structure. In addition, the BNO films annealed at 500 °C contained approximate...

Single-Crystal Growth of Bi-Sb-Te Thermoelectric Materials by Halide Chemical Vapor Transport Technique

This report describes synthesis of binary Bi2Te3 and ternary Bi0.5Sb1.5Te3 single crystals using the halide chemical vapor transport technique. For synthesis of ternary Bi0.5Sb1.5Te3, BiBr3 is a more effective transport agent compared with iodine I2. The single crystal includes a few atomic percent of Br. The Bi0.5Sb1.5Te3 crystal shows p-type conduction and has a comparatively large residual resistivity ratio. The crystal exhibits relatively high electrical resistivity and high Seebeck coefficient. These high values are attributed to decrease of the hole concentration p due to doping of the transport agent Br.

High-relative-dielectric-constant bismuth–niobium–oxide films prepared using Nb-rich precursor solution

Various ceramic materials have been developed for electronic devices. Bismuth–niobium–oxide (BNO) films prepared by a chemical solution deposition (CSD) method have the cubic pyrochlore phase, high relative dielectric constant, and low tangent loss (tan δ). We found that a BNO cubic pyrochlore crystal was Nb-rich, even though its pyrochlore formula is A2B2O7. The crystallization temperature of BNO increased with increasing Nb ratio. The relative dielectric constants of BNO films were related to the Nb ratio in the precursor solution. The dielectric constant of the BNO films was 250 when the Bi and Nb ratios in BNO precursor solutions were 4 and 6, respectively, and the sintering temperature was 600 °C. In addition, the tan δ was less than 0.01 at 1 kHz, which is higher than the reported values of BNO systems despite using the CSD method. These results show that the properties of BNO films prepared by the CSD method were associated with the Nb ratio in the precursor solution. Furthermore, the dielectric characteristics indicated that the Nb-rich BNO films have potential applications in electronic devices.