Yukihiro Isoda

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.

Problems of conductive polymers as thermoelectric materials

Thermoelectric energy conversion is paid attention to as an environmental technology. Research and development has been performed only for inorganic materials for a long time. From a point of view of environmental burden through material life cycle, organic/polymer materials are worthy to be pioneered. Thermoelectric eco-device could be realized by organic/polymer materials. We evaluated thermoelectric properties of typical conductive polymers to expose what are the problems of polymer applications to thermoelectric materials.

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

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.

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.

Synthesis and transport properties of ternary type-I Si clathrate K8Al7Si39

A ternary type-I Si clathrate, K8AlxSi46-x, which is a candidate functional material composed of abundant non-toxic elements, was synthesized and its transport properties were investigated at temperatures ranging from 10 to 320 K. The synthesized compound is confirmed to be the ternary type-I Si clathrate K8Al7Si39 with a lattice parameter of a = 10.442 A using neutron powder diffractometry and inductively coupled plasma optical emission spectrometry. Electrical resistivity and Hall coefficient measurements revealed that K8Al7Si39 is a metal with electrons as the dominant carriers at a density of approximately 1x10^27 /m3. The value of Seebeck coefficient for K8Al7Si39 is negative and its absolute value increases with the temperature. The temperature dependence of the thermal conductivity is similar to that for a crystalline solid. The dimensionless figure of merit is approximately 0.01 at 300 K, which is comparable to that for other ternary Si clathrates.

Dissociation and sublimation of tellurium from the thermoelectric tellurides

Abstract The authors have recently reported that Te in Pb(1 − x)SnxTe (0 ≤ x ≤ 1) system compounds will be dissociated and sublimated at high temperatures over 400°C on the basis of experimental results. In the experiment, the thermal expansion was tested and the thermogravimetry-differential thermal analysis was carried out from the room temperature to 500°C. In the present work, a high-temperature X-ray diffraction profiles are observed to investigate the change in the crystal structure of PbTe and Bi2Te3. It is concluded from the experimental results that Te will be dissociated and move onto the sample surface to form a liquid or amorphous state at temperatures ranging from 200 to 400°C and that Te on the sample surface will be volatilised at higher temperatures than 400°C. It is also concluded that in Bi2Te3, Te might be sublimated even at lower temperatures than 400°C.

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.