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The thermoelectric properties of the samples were investigated along with their microstructures. The results show that the plasma treatments reduced thermal conductivity and increased electrical conductivity and mobility, which led to an increase in thermoelectric figure of merit.

This clathrate had a maximum dimensionless figure-of-merit, ZT, of 0.93 at 637 K, which was slightly higher than that of 0.83 for the sintered type-VIII clathrate Ba8Ga16Sn30. We investigated the high-temperature thermoelectric properties, transport properties, electronic structures, and thermal stabilities of the clathrates. The type-II clathrate was found to be superior to the type-VIII clathrate as a thermoelectric material; it had a high thermal stability and melting point, 859 K, high mobility, 141 cm2V-1s-1 at 300 K, because of its low inertial mass, and low high-temperature lattice thermal conductivity, approximately 4 mW cm-1K-1, resulting from a larger unit cell and weaker bipolar thermal conduction. We discuss these properties in terms of the electronic structure and the differences between the two types of clathrate. [ABSTRACT FROM AUTHOR]

Analysis of the carrier mobilities and band structure calculations for the titled clathrates indicate that the primary carrier scattering mechanism around room-temperature is not acoustic phonon scattering but alloy disorder scattering due to substituting M atoms on the frameworks. Also, it is studied whether the rattling motion of the guest atoms in the cages affects carrier mobility due to optical phonon scattering. Electron densities of conduction-band bottoms exist not around the frameworks but between the guest and framework atoms and are anisotropic because of disordered placements of substituting atoms. Band-gap energies depend strongly on the presence of the substituting atoms. However, these features do not vary with the kind of substituting group-III elements very much. The maximum dimensionless figure-of-merits ZT were 0.65 at 940 K, 0.74 at 890 K, and 0.51 at 800 K for M = Al, Ga, and In, respectively, which depended on the experimentally achieved carrier concentrations. [ABSTRACT FROM AUTHOR]

Nominal Sr8AlxGa16-xGe30 samples with x=6, 8, and 10 crystallized in the type-VIII clathrate structure (I43m, No. 217), while the sample with x=4 crystallized in the type-I clathrate structure (Pm3n, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of Ba8Ga16Sn30, Eu8Ga16Ge30, and Sr8AlxGa16-xSi30 had been synthesized before. The type-VIII Sr8AlxGayGe46-x-y samples (6≤x≤7 and 10≤y≤11) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of n type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit ZT of 0.56 at 800 K was obtained for the type-VIII Sr8Al6.3Ga10.3Ge29.4 sample with a carrier concentration of 3.0×1020 cm-3. This ZT value is comparable to that of 0.62 at 800 K for the type-I Sr8Ga16.5Ge29.5 clathrate. The type-VIII clathrate had a smaller effective mass, a higher mobility, and a higher lattice thermal conductivity than those of the type-I clathrate. The difference in transport properties between the type-I and type-VIII clathrates is also discussed. [ABSTRACT FROM AUTHOR]

Nominal Sr8AlxGa16-xGe30 samples with x=6, 8, and 10 crystallized in the type-VIII clathrate structure (I43m, No. 217), while the sample with x=4 crystallized in the type-I clathrate structure (Pm3n, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of Ba8Ga16Sn30, Eu8Ga16Ge30, and Sr8AlxGa16-xSi30 had been synthesized before. The type-VIII Sr8AlxGayGe46-x-y samples (6≤x≤7 and 10≤y≤11) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of n type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit ZT of 0.56 at 800 K was obtained for the type-VIII Sr8Al6.3Ga10.3Ge29.4 sample with a carrier concentration of 3.0×1020 cm-3. This ZT value is comparable to that of 0.62 at 800 K for the type-I Sr8Ga16.5Ge29.5 clathrate. The type-VIII clathrate had a smaller effective mass, a higher mobility, and a higher lattice thermal conductivity than those of the type-I clathrate. The difference in transport properties between the type-I and type-VIII clathrates is also discussed. [ABSTRACT FROM AUTHOR]

Raman scattering and x-ray diffraction (XRD) measurements of iodine-doped cationic type-I clathrate I8Sb8Ge38 have been carried out at 1 bar and high pressures of up to 50 GPa at room temperature, by using a diamond anvil cell. The guest-iodine rattling vibrations were observed in a low-frequency region of 53–58 cm-1 at ambient conditions as well as the host vibrations in a higher-frequency region of 75–273 cm-1. High-pressure Raman spectroscopy showed the frequency softening of iodine rattling vibrations at pressures above 16 GPa, which originates from the weakening of guest-host interactions due to the transfer of electrons from the large-size guest I to the framework. High-pressure Raman and XRD studies revealed that the amorphization proceeds gradually around 36–44 GPa and is completed at about 50 GPa. The isostructural phase transition with a volume reduction was observed at 42 GPa, which is compared with other type-I Ge clathrates. [ABSTRACT FROM AUTHOR]

Thermoelectric properties of Al-substituted Ge clathrate compounds Ba8AlxGe46-x were investigated experimentally and theoretically. Polycrystalline Ba8AlxGe46-x (x=12–18) was synthesized by using an arc melting and a spark plasma sintering technique. The obtained samples show the n-type behavior and the carrier concentration decreases with increasing amount of Al composition from x=12 to 16. The reasonably large power factors was obtained: 900–1000 μW/cm K2 around 900 K. Theoretically electronic structure of Ba8Al16Ge30 was calculated by means of a first-principles method and then thermoelectric properties were analyzed on the basis of the result of the calculated electronic structure. The calculated thermoelectric properties agree with the experimental results very well in the wide range of temperature between room temperature and 900 K. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of FeTe2 substituted with Co were prepared by powder metallurgy, and their thermoelectric properties were investigated. When the substitution amount x increased to 0.4, their electrical conductivity increased to approximately 2000 S/cm at room temperature and their lattice thermal conductivity decreased to 10 mW cm-1 K-1 at high temperatures. A maximum dimensionless figure of merit of 0.13 was obtained for Fe0.8Co0.2Te2 at 700 K. The electronic structure of FeTe2 was also calculated, and the possibility of improving its thermoelectric performance of FeTe2 is discussed. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of Sr8GaxGe46-x clathrates with varied nominal Ga contents were prepared by powder metallurgy to produce n-type samples with carrier concentration n of 1.8×1020–1.1×1021 cm-3. Their thermoelectric properties were measured in the temperature range from 300 to 900 K. Their Seebeck coefficient and electrical conductivity were found to be typical for a heavily doped semiconductor, and varied systematically with their carrier concentration. The maximum value of the figure-of-merit ZT was 0.62 at 800 K for n=5.6×1020 cm-3. Other following properties were also measured or estimated, and their relationships with the thermoelectric properties are discussed. The band gap of the samples was estimated to be 0.4–0.5 eV; their effective mass was an almost constant value of 3.1me at room temperature, and this value decreases with increasing temperature; and their electron Hall mobilities were 8–13 cm2 V-1 s-1 at room temperature, increasing slightly with decreasing carrier concentration. [ABSTRACT FROM AUTHOR]

Thermoelectric properties of polycrystalline degenerate n-type PbTe films have been investigated in order to understand potential barrier scattering. The Seebeck coefficients of the PbTe films obtained in this study were larger than those of bulk samples having the same carrier concentrations in the temperature range from 100 to 450 K. Some of their power factors were larger than those of bulk samples having the same carrier concentrations in the temperature range from 200 to 450 K, while their electrical conductivities were smaller than those of bulk samples. From a comparison of these results with those previously reported, we concluded that potential barrier scattering occurred at grain boundaries in our films, resulting in the above favorable changes in thermoelectric properties. By analyzing their properties using the energy filtering model, we estimated the height of grain-boundary potential barriers, which probably influenced the increases in the Seebeck coefficient. We also examined the origin of the potential barriers accordingly. Consequently, we consider that the origin of the potential barriers was mainly due to point defects, probably Te vacancies, and that the barrier height may be controlled, for example, by the preparation conditions. [ABSTRACT FROM AUTHOR]

Microstructural control by means of plasma treatment was performed on sintered Cr-doped β-FeSi[SUB2] in order to improve its thermoelectric properties. The micrograins were exposed to rf plasmas of germane (GeH[SUB4]) or a mixture of germane and silane (SiH[SUB4]) and were then sintered. The thermoelectric properties of the samples were investigated along with their microstructures. The results show that the plasma treatments reduced thermal conductivity and increased electrical conductivity and mobility, which led to an increase in thermoelectric figure-of-merit. In addition, the results show that the plasma treatments also formed small SiGe crystals distributed throughout the samples, removed most of the grain boundaries, and sometimes formed FeSi crystals. The relation between the changes in the thermoelectric properties and the modification of the microstructure was discussed. It was found that the microstmctural modification by the plasma treatment caused favorable changes in thermoelectric properties. [ABSTRACT FROM AUTHOR]

Sintered degenerate n-type PbTe samples with small grain sizes ranging from 0.7 to 4 µm were prepared and the effects of grain size on their thermoelectric properties were then investigated. The Seebeck coefficient of the sintered samples increased almost double when the grain size decreased from 4 to 0.7 µm. On the other hand, their electrical and thermal conductivity decreased with decreasing grain size. Accordingly, decreasing their grain size increased their thermoelectric figure-of-merit. A maximum value of the figure-of-merit of the obtained small grain-size samples was significantly higher than that of large grain-size samples with the same carrier concentration reported. This favorable result was caused mainly by the increase in the Seebeck coefficient. The influences of grain boundaries on the increase in the Seebeck coefficient were discussed. It is concluded that the Seebeck coefficient was increased by potential barrier scattering, which occurred at the grain boundaries in the sintered samples. [ABSTRACT FROM AUTHOR]

The type-VIII [Sr.sub.8][Al.sub.x][Ga.sub.y][Ge.sub.46-x-y] samples with various concentrations are prepared to examine their thermoelectric properties. The type-VIII clathrate had a smaller effective mass, a higher mobility and a higher lattice thermal conductivity than those of the type-I clathrate and the difference in transport properties between the type-I and type-VIII clathrates is discussed.

Raman scattering and x-ray diffraction (XRD) measurements of iodine-doped cationic type-I clathrate [I.sub.8][Sb.sub.8][Ge.sub.38] are carried out at high pressures at room temperature by using a diamond anvil cell. The studies have shown that the amorphization has proceeded gradually around 36-44 GPa and the isostructural phase transition with a volume reduction is observed at 42 GPa.

Thermoelectric properties of Al-substituted Ge clathrate compounds [Ba.sub.8][Al.sub.x][Ge.sub.46-x] are examined. The studies have shown that the thermoelectric properties can be improved by controlling the carrier concentration.

Polycrystalline samples of Fe[Te.sub.2] substituted with Co are prepared by powder metallurgy and their thermoelectric properties are investigated. The substitution of Co for Fe has increased the electron carrier concentration, leading to the increase in their electrical conductivity, and has also caused alloy scattering of phonons, decreasing their lattice thermal conductivity.

Polycrystalline samples of [Sr.sub.8][Ga.sub.x][Ge.sub.46-x] clathrates with various carrier concentrations are prepared and their thermoelectric and transport properties are measured. The band gap of the samples is estimated to be 0.4-0.5 eV, their effective mass is a constant value of 3.1 [m.sub.e] at room temperature which decreases with increasing temperature and their electron Hall mobilities are 8-13 [cm.sup.2] [V.sup.-1] [s.sup.-1] at room temperature, increasingly slightly with decreasing carrier concentration.