Yoshikazu Shinohara

Thermoelectric Properties of Poly(3-Alkylthiophenes)

Typical conductive polymers of poly(3-alkylthiophenes) were synthesized by oxidative polymerization. Alkyl side chains were CnH2n+1 with n=4, 6, 8, 12. The regioregularity with the HT linkage was larger than 99% based on NMR analysis. Completely regioregular head-to-tail (HT) poly(3-alkylthiophenes) were obtained. We have evaluated the effect of side chain size on the thermoelectric properties of Seebeck coefficient and electrical conductivity. The results were as follows: 1) Seebeck coefficient decreased with an increasing electrical conductivity. 2) High Seebeck coefficient >1mV/K was observed at low electrical conductivity <10-2S/cm. 3) The small side chain caused the higher electrical conductivity in the range of electrical conductivity <10- 1S/cm.

Thermoelectric Properties of Sb-doped Mg2Si0.5Sn0.5

Mg2Si1-xSnx systems, an ecologically friendly semiconductor, are the perspective material for thermoelectric generators at temperatures range from 500 to 800K. The single phase of this system at the compositions range of 0.4 < x < 0.6 has not been reported until now. The single phase of Mg2Si0.5Sn0.5 has been successfully obtained by a Liquid-Solid reaction method and Hot-pressing method. The minimum value of thermal conductivity was identified at around x=0.5. The high thermoelectric performance can be attained by the controlling of carrier concentration for Mg2Si0.5Sn0.5 . In this present work, the thermoelectric properties for the single-phase of Sb-doped Mg2Si0.5Sn0.5 were investigated. Seebeck coefficient alpha, electrical resistivity p and thermal conductivity k were measured from room temperature to 850K. The sign of alpha showed negative for all samples and has been n-type conduction. The temperature dependency of alpha for non-and 5000ppmSb-doped samples increased up to the maximum value of -440muVK -1 at 440K and -328muVK-1 at 578K, respectively. The alpha of 7500ppmSb sample were increased linearly with temperature. For non- and 5000ppmSb-sample, the temperature dependency of the p showed semiconducting properties. On the other hand, the sample of 7500ppmSb or more showed metallic behavior. The difference of this behavior is result of the conduction mechanism changed by the increase of carrier concentration. The carrier component of thermal conductivity was increased, while the phonon component of thermal conductivity was decreased slightly with carrier concentration. The dimensionless figure of merit was showed markedly enhanced the maximum value of ZT=1.2 for Sb doped 7500ppm at 620K

Ecomaterials in the Global Eco-Society: Present Situation and Future Prospects

The concept of environmental conscious materials, ecomaterials, is reviewed. It has dispersed and recognized in the world as contributing to a sustainable society. Advanced models for secondary materials utilization, environmental benign processes and new material recycling systems are introduced.

Functionally Graded Materials

Functionally gradient materials (FGMs) are new type of body materials with continuous or stepwise change in functions inside them. They were developed as heat-resistant materials applied to space planes, which was proposed by the Science and Technology Agency of Japan in the mid-1980s. The metal and ceramics were the original constituent materials of FGMs, in which the composition changed from metal to ceramics in the thickness direction. They already have some applications in the fields of structural materials, biomaterials, semiconductors, coating materials, and electrode materials. In this chapter, the fundamentals of FGMs, natural materials with graded structures, fabrication processes, typical applications, and future of FGMs are introduced.

The effect of Bi doping on thermoelectric properties of Mg 2 Si 0.5 Sn 0.5

Mg2Si(1-x)Snx solid solution systems are an ecologically friendly semiconductor, and have been proposed to the probable materials for high-performance thermoelectric generators at temperatures range from 500 to 800 K. The single phase of this system at the compositions range of 0.4 < x < 0.6 has not been reported until now. The single phase of Mg2Si0.5Sn0.5 has been successfully obtained by a liquid-solid reaction method and hot-pressing method. The minimum value of thermal conductivity was identified at around x=0.5. The high thermoelectric performance can be attained by the controlling of carrier concentration for Mg2Si0.5Sn0.5. In this present work, the thermoelectric properties for the single-phase of Bi-doped Mg2Si0.5Sn0.5 were investigated. Seebeck coefficient alpha, electrical resistivity rho and thermal conductivity kappa were measured from room temperature to 850 K. The carrier concentration n increased lineally with the amount of Bi-doping, and the Bi atom acts as a singly ionizable substitutional donor. The reduction Fermi energy xi increased with increasing Bi amount. The undoped sample (xi=-2.808) was non-degenerated state, and 15000 ppm-Bi doping sample (xi=2.304) was heavily degenerated state. The absolute values of alpha for all samples showed a pronounced maximum which shifts to a higher temperature with increasing n. The rho of non-doping sample shows semiconducting behavior, and Bi-doping samples indicated the same behavior as a metal, which increased monotonously to the intrinsic region with increasing temperature. The carrier component of thermal conductivity was increased, while the phonon component of thermal conductivity was decreased slightly with carrier concentration. The dimensionless figure of merit was showed markedly enhanced the maximum value of ZT=0.87 for 7500 ppm-Bi doping at 630 K.

Crystal grain size dependence of thermoelectric properties for sintered PbTe by spark plasma sintering technique

Thermal and electrical conduction parameters of the sintered PbTe with different crystal grain sizes Ds were measured over the temperature range from 77 to 350 K, to examine the possibility of improvement in thermoelectric performance for sintered PbTe by reducing D. The starting powders with particle sizes of 6, 39, 180 and 380 /spl mu/m were obtained by pulverizing an undoped PbTe boule prepared by the Bridgman method. Sintering was carried out by the spark plasma sintering technique (SPS) under the conditions of sintering temperature of 732-803 K, sintering duration of 60-90 min and sintering pressure of 45 MPa. All the sintered compacts, whose conduction type was p-type, had an apparent relative density of more than 99%. Thermal conductivity /spl kappa/ decreased with reducing D below 200 K, but showed no dependence on D above 200 K. Long-wave phonons are scattered efficiently at grain boundaries only below 200 K. Hall mobility /spl mu//sub H/ of the sintered compacts with different Ds were almost same above 200 K. It is concluded that thermoelectric performance for sintered PbTe is not directly affected by controlling D.

The effect of carrier conduction between main chains on thermoelectric properties of polythiophene

Polythiophene films were synthesized by electrolytic polymerization using nitrobenzene as a solvent and tetra-n-butylammonium perchrorate as an electrolyte. We have evaluated thermoelectric properties of Seebeck coefficient, electrical conductivity, Hall mobility and carrier concentration of the films in the in-plane direction. The films showed a tendency that Seebeck coefficient decreased with an increasing electrical conductivity. The figure-of-merit was calculated assuming the thermal conductivity was 0.1 W/Km. The figure-of-merit increased with an increasing electrical conductivity up to 1.5x10-4/K. The electrical conductivity was mainly changed with Hall mobility, which was related to molecular spacing of the mains chains. Variable range hopping was a determining process of carrier conduction. The easiness of carrier conduction between main chain molecules directly affected the figure-of-merit of the polythiophene polymers.

A new challenge of polymer thermoelectric materials as ecomaterials

Typical conductive polymers of poly(3-alkylthiophenes) were synthesized by oxidative polymerization. Polythiophene with no side chain was also electrolyticaly polymerized. Alkyl side chains were CnH2n+1 with n=4, 6, 8, 12. The regioregularity with the HT linkage was larger than 99% based on NMR analysis. We have evaluated the effect of side chain size on the thermoelectric properties of Seebeck coefficient and electrical conductivity. The results were as follows: 1) Seebeck coefficient decreased with an increasing electrical conductivity. 2) High Seebeck coefficient >1mV/K was observed at low electrical conductivity <10-2S/cm. 3) Small side chains, especially no side chain caused higher thermoelectric properties of polythiophene series.

Power Evaluation of PbTe with Continuous Carrier Concentration Gradient

In this paper, we reported that lead telluride (PbTe) with continuous carrier concentration gradient, in which PbI2, Al and Zr were doped, were successfully fabricated by the unidirectional solidification method. The carrier concentration was optimized by adjusting the relation between the dopants and the carrier concentration gradient. The carrier concentration for the ingots was estimated from the resistance results which were measured by the one-probe method. The result shows that the carrier concentration was large at the initiation side and small at termination side of the solidified ingots. The degree of the carrier concentration gradient can be controlled by the holding time at a liquid state and the cooling rate from the liquid state. The carrier concentration gradient can be largely affected by the Al-dopant. The samples 0.07mol%PbI2-0.05mol%Zr-0.07mol%Al-PbTe, which were made from a liquid phase at 1200K held for 1h and cooled at 98K/h, showed a carrier concentration gradient ranging from 2×1024 to 1.5×1025 /m3. The effective maximum power for this continuous FGM is 20% larger than that of jointed FGM.

Thermoelectric properties of segmented Pb-Te systems with graded carrier concentrations

Thermoelectric properties have been investigated on n-type PbTe material composed of 2 segments with different electron concentrations ne. The 2-segment material was prepared by liquid state diffusion bonding of two kinds of solidified ingots. ne of the ingots were 3/spl times/10/sup 24/ and 6/spl times/10/sup 25//m/sup 3/. The ingot of 3/spl times/10/sup 24//m/sup 3/ had higher power factor below 500 K, while the ingot of 6/spl times/10/sup 25//m/sup 3/ had higher factor above 500 K. The ingot with higher ne was for higher temperature use. When the temperature difference was given with the jointed interface of more than 450 K, the 2-segment material was found to give higher maximum power than the original ingots. From these results, the segmentation with graded ne is effective in improving the power generating performance of n-type PbTe.