The effects of the method of fabricating thermoelecteric materials, such as sintering or direct fabrication, have been the focus of recent attention. The low-dimensional materials sush as two-dimensional film or skutterudite structure has been made use of recently in the higher performance. A computer simulation by DV-Xα calculation using the molecular orbital method was carried out to investigate the electronic structures related to the thermoelectric properties such as electrical conductivity and the Seebeck coefficient of FeSi2. As regards electrical conductivity, the energy gap was estimated to be 0.53 eV when conventional HOMO and LUMO levels were considered. On the other hand, a new definition was provided for the Seebeck coefficient; it is the energy difference between HOMO and LUMO levels which have the same symmetry at the twofolded- and threefolded-degenerated energy structures. Furthermore, computer simulation and experiment gave the same trends concerning the thermoelectric properties of metal-added materials.
Sintered materials of (1−x)FeSi2+xBSi2 in the composition range of 0≤x≤0.08 were prepared by the hot-pressing technique. The solid solutions of Fe1−xBxSi2, which were a β-FeSi2 single-phase, were obtained in the range of x≤0.03. The electrical resistivity (ρ) and Seebeck coefficient (α) were measured as a function of temperature over the range from 77 to 1241 K. It was found that the solid solutions possess semiconducting properties below the semiconductor-to-metal transition temperature (Tc). Logarithmic ρ versus temperature 1⁄T curves showed an S-shaped variation in the temperature range of 200 to 600 K for the β-FeSi2 containing boron. The curves could be explained in terms of a small polaron model and a impurity conduction in which boron atoms formed a localized state as a donor in the β-FeSi2. The energy gap at 0 K (Eg0) decreased with increasing the boron concentration. The energy gap (Eg) estimated around Tc was smaller than the Eg0 and decreased with increasing the boron concentration. Tc was of 1241 K and independent of the boron concentration. From these facts it was found that the transition from semiconductor to metal phases was mainly caused by expanding the width of a narrow valence band with increasing hole concentrations. The α of β-FeSi2 with borons showed a remarked peak larger than that of un-doped, and the peak temperature shifted to lower temperatures with increasing boron concentrations.
We studied whether or not Si-based materials, which were prepared by arc melting Si with Ge and a small amount of Group 3B or 5B elements, could be produced with high performance thermoelectric properties. The present materials have not only strong alloy segregation but also strong dopant segregation along the grain boundaries at high carrier concentration n of above 1019 cm−3. The combination of alloying with 5 at%Ge and doping with 0.3 at%B or 0.4 at%P decreased the thermal conductivity κ to 5.7% or 6.7% of pure Si at 298 K, respectively, in spite of strong segregation. The electrical resistivities ρ of doped Si and Si0.97Ge0.03 decrease linearly with increasing n. However the Seebeck coefficient S of doped Si have a local maximum at (3∼4)×1019 cm−3, while no S maximum was observed in doped Si0.97Ge0.03. At n\fallingdotseq2×1020 cm−3 the S values of Si0.97Ge0.03 and Si0.95Ge0.05 with strong segregation are about 50% higher than those measured previously on Si0.95Ge0.05 and Si0.7Ge0.3 with little segregation. Such interesting phenomena of S may probably result from the grain boundary segregation of dopants and/or Ge. The thermoelectric figures of merit (ZT=S2T⁄κρ) of n- and p-type Si0.95Ge0.05 increase linearly with temperature T, and were 0.90 and 0.57 at 1073 K, respectively. These values correspond to 92% and 81% of those obtained by Dismukes et al. for Si0.7Ge0.3 alloys with little segregation.
We produced Si0.8Ge0.2 multiple quantum wells using MBE and evaluated the in-plane electrical properties. Si0.8Ge0.2 quantum wells with various well widths separated by 20 nm Si barrier layers and other samples with the same well/barrier ratio were grown on high resistive Si(100) substrates with a 20 nm buffer layer. Both well and barrier layers were uniformly boron-doped. The electrical conductivity σ and Seebeck coefficient S were measured at room temperature and showed a size effect in power factor σS2, depending on the quantum well period.
Electrical transport and magnetic properties of polycrystalline ceramic specimens of the system La0.9(Sr1−xCax)1.1CoO4 have been investigated as a function of temperature by means of complex-plane impedance analysis which distinguishes the bulk resistivity from the resistivity across grain boundaries, four-probe dc resistivities, Seebeck coefficients and magnetic susceptibilities below room temperature. The temperature dependencies of the magnetic susceptibilities indicate that the ratio of intermediate-spin Coiii ions (S=1) is dominant in a specimen with x=0.1. This suggests that La0.9(Sr1−xCax)1.1CoO4 at x=0.1 has a high potential as a thermoelectric material. Therefore, it is necessary to search for thermoelectric materials in transition-metal perovskites using a spin-state control.
β-rhombohedral boron (β-boron) is expected to be one of the candidates for high-temperature thermoelectric materials because of its hopping conduction mechanism. To reveal the controllability of the thermoelectric properties of boron-rich semiconductors which consisted mainly of B12 icosahedral clusters, β-boron was doped with V, Cr, Fe, Co or Zr, and the composition and temperature dependence of the electrical conductivity and the Seebeck coefficient were investigated. V or Cr dopant which preferentially occupies the A1 sites in β-boron structure causes an increase of the electrical conductivity and a negative Seebeck coefficient. On the other hand, Zr dopant which does not occupy the A1 sites causes a large positive Seebeck coefficient. The electrical conductivity and Seebeck coefficient of Co- and Zr-doubly-doped p-type β-boron are nearly determined by the Co content. These results have been discussed with the concept of metallic-covalent bonding conversion for the B12 cluster. The temperature dependence of the electrical conductivity of metal-doped β-boron is well explained as the variable range hopping conduction. The Seebeck coefficient as well as the electrical conductivity increases with increasing temperatures up to room temperature, contrary to the cases of ordinary metals and semiconductors. Consequently, the power factors of both Co-doped p-type β-boron and V- or Cr-doped n-type β-boron increase with increasing temperature, and materials whose power factor is 3-4 orders of magnitude larger than that of pure β-boron at room temperature were obtained by doping with about 1 at% metal atoms.
CoSb3 has been doped with Ni, Pd, and Pt as donor impurities, and the effects on the electronic structure and the electrical and thermoelectrical properties have been investigated so as to improve the thermoelectric figure of merit. It was found that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on the impurities. Our theoretical analysis of these results suggests that the electron effective mass and the conduction band deformation potential are significantly affected by doping. These doping effects in CoSb3 can be attributed to (1) the influence of doping on the electronic structure and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band, which can be explained in terms of a Kane model. On the other hand, the lattice thermal conductivity decreased to about 4 Wm−1K−1 with increasing carrier concentration, almost independently of the impurities. Our analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron-phonon scattering plays an important role in reducing the lattice thermal conductivity of heavily doped n-type CoSb3. A large value of the dimensionless thermoelectric figure of merit ZT=about 0.85 (at 800 K) could be obtained for Pd and Pt double doping. Therefore, the double doping with Pd and Pt is considered to be effective in optimizing the electronic properties and the carrier concentration of CoSb3.
The filled skutterudite compounds, CeyFexCo4−xSb12(x=0-1.0, y=0-0.15), were prepared from powders of Co, Fe, Sb and CeCl3·6H2O by solid state reaction, and their thermoelectric properties were investigated. CeyFexCo4−xSb12(x=0-1.0, y=0-0.15) compounds showed p-type conductivity. Carrier concentration (p) increased with increasing substitution Fe for Co site, and decreased with increasing Ce content. Electrical conductivity (σ) increased with increasing Fe content. The seebeck coefficient (α) increased with increasing Fe content for CeFexCo4−xSb12, but decreased with increasing Fe content for FexCo4−xSb12. Thermal conductivity (κ) decreased significantly with the substitution of Fe for Co and filling of Ce into Sb dodecahedron voids. The thermoelectric figure of merit was improved by the combination of Fe substitution and Ce filling. The maximum ZT value of 0.8 was obtained for Ce0.12Fe0.71Co3.29Sb12 at 750 K.
Influence of oxygen content and grain size on the thermoelectric properties of hot pressed bismuth-tellurium based compounds has been investigated. Bismuth, tellurium, antimony and selenium powders were mixed to the stoichiometric compositions of (Bi, Sb)2Te3 for p-type and Bi2(Te, Se)3 for n-type, respectively. The powder mixtures were ball-milled in acetone and consolidated by hot pressing at 733 K for the p-type and 773 K for the n-type compound in an argon or hydrogen atmosphere. Sintering in a hydrogen atmosphere provided a lower oxygen content, particularly for the p-type compound. Coarser grain size (2-10 μm) was observed for the p-type sintered samples while the n-type samples exhibited a finer grained microstructure (about 1 μm). Seebeck coefficient, electrical resistivity and thermal conductivity of the sintered samples were measured at room temperature. The p-type compound exhibited dependence of the figure of merit (Z value) on grain size while the figure of merit of the n-type compound could be expressed as a function of oxygen content. The obtained maximum Z values were 2.5×10−3 K−1 for both of the p-type and n-type compounds.
P-type Pb1−xSnxTe single crystals were prepared by a Bridgman method with various Sn content (x). Electrical conductivity (σ), Hall coefficient (RH) and thermal conductivity (κ) were measured in the temperature range of 300 to 700 K. Hole concentration at 77 K changed from 2.0×1024 to 4.9×1026 m−3 as x varied from 0 to 1.0. The σ value increased monotonously with increasing x. The minimum of κ found at x=0.25 were resulted from that of the lattice thermal conductivity (κlattice), since the phonon-phonon scattering in the solid solution may have decreased the lattice thermal conductivity. The ratio of electrical conductivity to thermal conductivity (σ⁄κ), to which the figure of merit Z is proportional, increased with increasing x and then reached its maximum at x=0.5. It was revealed that the thermoelectric performance of Pb1−xSnxTe can be improved by controlling its composition.
Thermoelectric properties of ZnO and (Zn0.98Al0.02)O alloys sintered under different pressures, from ultra fine powder have been investigated. The samples are with a grain size under 10 μm. Electrical conductivity, Seebeck coefficient and thermal conductivity of the samples are measured at temperatures ranging from 337 to 973 K. Electrical conductivity of the samples decreases with increasing temperature. Seebeck coefficient of the samples showed negative and the absolute value increased with increasing temperature. Thermal conductivity of the samples decreased with increasing temperature. The electrical conductivity and Seebeck coefficient appeared relatively insensitive to grain size, while the thermal conductivity did.
Thermoelectric properties of Fe3Si with the DO3 structure where some Fe were replaced with Mn and V were examined. By adding 5 mol%Mn or V, the Seebeck coefficients, α, reach the maximum values of about+22.5 μV/K, while they decreased to a large negative value by the further replacement of Fe by these transition elements. The minimum values of α were −36 μV/K and −45 μV/K at Fe-30 mol%Mn-25 mol%Si and Fe-20 mol%V-25 mol%Si, respectively. These values are smaller than the minimum value reported for the iron-based alloys. The compositional dependency of α corresponds to the fact that the added transition metal atoms replaced iron atoms at a particular site in the DO3 lattice. By combining an approximate model approximation suitable for simple metals and the density of state reported, however, the obtained compositional dependency of thermoelectricity could not be explained. The absolute values of α in the Fe-Mn-Si system were small, both in the samples with the α-Fe solid solution and with the γ-Fe solid solution.
Thermoelectric properties and electrical transport parameters of the Si and Ni co-doped SiC have been studied as functions of both Si and Ni doping concentration and temperature. It is intended to increase the figure of merit easily by addition of isoelectric element Si and transition metal element Ni. The samples were fabricated by conventional wet mixing and powder sintering process. Measurements of electrical resistivity and thermoelectric power were made in the temperature range from room temperature to around 1000 K in He atmosphere. Thermal conductivity were measured in the temperature range from room temperature to around 600 K in air. Measurements of Hall coefficient, X-ray crystallography and EPMA were also made. The thermoelectric power increases drastically by addition of Si and Ni. The maximum value of thermoelectric power reached to about 600 μV. Thermal conductivity reached to the minimum value of 8.0 W/(mK) at 40.0 mass% of Si concentration and 1.0 mass% of Ni concentration. Enhancement of the figure of merit Z was achieved by co-doping of Si and Ni. The figure of merit Z reached to the maximum value of 2.5×10−4 K−1.
The thermoelectric materials, n-PbTe, p-Pb1−xSnxTe (x=0.00-0.50) and p-AgSbTe2 were prepared by melt-growth and plasma-activated sintering(PAS). Compositional analysis and thermoelectric measurements were carried out on as-grown ingots. The electron concentration in the n-PbTe ingots was controlled in the range 3.0×1023∼5.0×1025 m−3 by PbI2-doping, while the concentration and Hall mobility of the sintered materials were lower than those of the melt-grown samples. For both the Pb1−xSnxTe and AgSbTe2 ingots, compositional uniformity was confirmed over a region of 80% of the total length of the ingots, while some fluctuations in electronic properties were observed in AgSbTe2 ingot. The carrier concentration of the Pb1−xSnxTe changed by two orders of magnitude from 1024 to 1026 m−3 with an increase of x, whilst that of AgSbTe2 was 1025 m−3. The properties of the AgSbTe2 were characterized by larger thermoelectric power and lower Hall mobility than those of the Pb1−xSnxTe. By using available thermal conductivity data, the estimated figure of merit for PAS-AgSbTe2 at 300 K was 9.72×10−4 K−1, which is 2-7 times larger than that of PAS-Pb1−xSnxTe
The development of mass production process of thermoelectric materials is necessary to practical use. The most important thing is how to make large amounts of high quality thermoelectric materials with high productivity and low cost. Skutterudite structure materials are new kinds of thermoelectric materials which is recently paid much attention. From these view points, Skutterudite structure materials were produced by gas atomizing and sintering process. The materials examined in this study are CoSb3 of the typical Skutterudite structure material, (CoPdPt)Sb3 of the substituted structure n-typed material and CeFe3CoSb12 of the filled Skutterudite structure p-typed material. Sintering was tried with hot pressing process or spark plasma sintering process. Each gas atomized powder contains every necessary element with fine structure almost the same stoichiometric ratio of aimed thermoelectric material. Accordingly gas atomized powders could be sintered into thermoelectric materials in a short time without mixing and grinding process by both sintering methods. These lead to high productivity and low cost in manufacturing. Although there is no remarkable difference of thermoelectric properties between hot press sintered and spark plasma sintered materials, it was confirmed that sintering time required can be shorter for spark plasma sintering than for hot press sintering. CeFe3CoSb12 has some problems to attain high ZT value. On the other hand, (CoPdPt)Sb3 produced by gas atomizing and sintering process had almost same value of ZT which was obtained in laboratory scale. The combination process of gas atomizing and sintering methods is useful for mass production of thermoelectric materials.
Sintered PbTe materials with average grain sizes of 28-309 μm were prepared by spark plasma sintering technique. The apparent densities of the sintered PbTe were 8.17-8.23 Mg/m3, which were higher than 99% of the theoretical one. Thermoelectric properties of the sintered materials and the as-grown boule by Bridgman method were measured in the temperature range from 77 to 350 K. Resistivity, ρ, of the sintered materials increased with decreasing grain size. Temperature dependence of ρ of the sintered PbTe was remarkably different from that of the as-grown boule below 250 K because of potential barriers at grain boundaries. Hall coefficient, RH, of the sintered materials at room temperature increased from 1.4×10−6 to 3.4×10−6 m3/C, as the average grain size increased from 28 to 309 μm, which suggested that oxidation in the crystal grains was caused in the sintering process. Temperature dependence of the thermoelectric power, α, of the sintered PbTe below 250 K reflected the effect of carrier scattering at grain boundaries. Lattice thermal conductivity of the sintered PbTe decreased with decreasing grain size below 250 K, while it was independent of the grain size above that temperature. Above 250 K, thermoelectric properties of the sintered PbTe except those in the 28 μm grain case were consistent with those of the as-grown boule. The values of α and RH at 295 K were calculated by using a two-valence-band model involving a non-parabolic and a parabolic valence band. The calculation results were in good agreement with the experimental data.
A novel sintering process has been developed for porous thermoelectric devices such as gas-combustion-type and heat-exchange-type devices. Mixture of PbTe and KCl particles with a certain particle diameter and volume fraction of KCl was sintered at 973 K, 7.2 ks, 35 MPa. The KCl particles were removed by water and porous PbTe was obtained. Porosity ratio, specific surface area and permeability of gas were controlled by changing volume fraction of KCl and diameter of PbTe particles. Addition of KCl during sintering did not affect the Seebeck coefficient of PbTe. Therefore, this process can be applied to production of porous PbTe thermoelectric devices.
A thermoelectric generator has been made consisting of an inner shell as a exhaust pipe, an outer shell as a water jacket and 16 Bi2Te3 modules working at low temperature established between the inner shell and the outer shell. Heat-exchanging fins were formed inside the inner shell with heat transfer area ratios of 0.92, 1.21, 1.65, and 1.99 along the exhaust gas flow, in order to reduce the temperature distribution at the hot side of the modules without over overheating of the module. When the exhaust gas was introduced to the inner shell of the generator under conditions corresponding to a 2-3 litre gasoline engine vehicle running at 60 km/h up a 3-5% hill, the electric power generated by the generator was 193 W. The heat change conversion efficiency of the generator was estimated at to be 37% of the primary exhaust gas energy flux. The generated power was 2.9% of the heat flux through the generator.