Negative Seebeck Coefficient Research Articles

High-temperature thermoelectric properties of thallium-filled skutterudites

The high temperature thermoelectric (TE) properties of polycrystalline Tl-filled skutterudites TlxCo4Sb12 (x=0, 0.05, 0.10, 0.15, 0.20, and 0.25) were examined from room temperature to 750 K. All samples exhibited negative Seebeck coefficients. The electrical resistivity, absolute values of the Seebeck coefficient, and the lattice thermal conductivity were found to decrease with increasing Tl content. Tl0.25Co4Sb12 exhibited the best TE performance; the maximum value for the dimensionless figure of merit ZT was 0.90 at 600 K.

Preparation and Thermoelectric Properties of LaCoO<sub>3</sub> Ceramics

LaCoO3 ceramics were prepared by conventional solid state reaction and normal sintering at the temperatures ranging from 1373 to 1523 K. The sintered densities increased with increasing sintering temperature and exceeded 90 % of the theoretical values when sintered above 1473 K. The thermoelectric properties of the samples sintered at different temperatures were investigated from 323 to 673 K. The LaCoO3 samples showed a negative Seebeck coefficient, whose absolute values decreased dramatically with increasing temperature in the range of 323 to 460 K, then changed to a positive value and lightly decreased above 460 K. The electrical conductivity increased with increasing temperature, indicating a semiconducting behavior. The Seebeck coefficients showed little difference between the samples sintered at different temperatures, but the power factor of the sample sintered at a higher temperature was larger because of the higher electrical conductivity.

Electrical and magnetic properties of quaternary compounds LnMnPO (Ln=Nd, Sm, Gd) with ZrCuSiAs-type structure

Electronic and optical properties of layered compounds LnMnPO (Ln = Nd, Sm, Gd) are investigated in comparison with an antiferromagnetic semiconductor LaMnPO. Diffuse reflectance spectra and temperature dependent electrical conductivity indicate that the LnMnPO compounds are semiconductors with band gap energies of 0.7–0.8 eV. Electrical conductivity at RT is varied from 4 × 10−3 to 2.5 S cm−1 by Cu doping for NdMnPO. Nominally undoped LnMnPO compounds exhibit negative Seebeck coefficients due presumably to P deficiency, while the sign of the Seebeck coefficients is changed to positive by doping of Ca or Cu to the Ln sites, indicating that the polarity of the predominant carriers in LnMnPO is controlled by doping. Mn spins in NdMnPO are already in an antiferromagnetic ordering state below 350 K, while antiferromagnetic ordering of Nd spins occurs at ∼10 K.

Interpenetrating icosahedra chains based zinc-rich ternary phases Ru4.0Sn2.9Zn11.6 and Ru3.0Sb0.97Zn11.0: synthesis, structures and physical properties

Substituting of Ru with Sn and Sb in the phase RuZn(3) led to two new zinc-rich ternary compounds: Ru(4)Sn(2.9)Zn(11.6) (SnRZ: Monoclinic, C2/m, Pearson symbol oC46-delta, a = 6.8011(14), b = 7.9760(16), c = 11.083(2) A, beta = 97.56(3) degrees, Z = 2) and Ru(3.0)Sb(0.97)Zn(11.0) (SbRZ: Orthorhombic, Cmcm, Pearson symbol oC60, a = 7.8936(16), b = 6.8347(14), c = 16.904(3) A, Z = 4). The structures of SnRZ and SbRZ are composed of four identical atomic layers but with different stacking sequences. They exhibit structural units of either three or two interpenetrating icosahedra fusing to form chains along the c axis. Their a and b lattice parameters are alike-interchanged in the chosen conventional setting-while the c parameters are related by c(SbRZ)/2 x (1 + tau) approximately = 2c(SnRZ) with tau = (1 + square root(5))/2 being the golden mean (known as tau-inflated phases). Both two compounds exhibit diamagnetic properties, and SnRZ and SbRZ congruently melt at 1296 K and 1233 K, respectively. Temperature-dependent resistivities reveal metallic behavior. The negative Seebeck coefficients indicate transport processes are dominated by electrons as carriers.

Thermoelectric properties of doped and undoped mixed phase hydrogenated amorphous/nanocrystalline silicon thin films

AbstractThe Seebeck coefficient and dark conductivity for undoped, and n-type doped thin film hydrogenated amorphous silicon (a-Si:H), and mixed-phase films with silicon nanocrystalline inclusions (a/nc-Si:H) are reported. For both undoped a-Si:H and undoped a/nc-Si:H films, the dark conductivity is enhanced by the addition of silicon nanocrystals. The thermopower of the undoped a/nc-Si:H has a lower Seebeck coefficient, and similar temperature dependence, to that observed for undoped a-Si:H. In contrast, the addition of nanoparticles in doped a/nc-Si:H thin films leads to a negative Seebeck coefficient (consistent with n-type doping) with a positive temperature dependence, that is, the Seebeck coefficient becomes larger at higher temperatures. The temperature dependence of the thermopower of the doped a/nc-Si:H is similar to that observed in unhydrogenated a-Si grown by sputtering or following high-temperature annealing of a-Si:H, suggesting that charge transport may occur via hopping in these materials.

Substitution effect on the thermoelectric properties of reduced Nb-doped Sr 0.95La 0.05TiO 3 ceramics

Ceramic samples of Sr 0.95La 0.05Ti 1− x Nb x O 3 (0 ≤ x ≤ 0.10) have been prepared by the solid-state reaction method. From their crystal structures determined by means of the powder X-ray diffraction, we found that the lattice parameters increase with increasing niobium content. The electrical resistivity, Seebeck coefficient and thermal conductivity have been measured in the temperature range between 300 K and 1100 K. All samples show large negative Seebeck coefficient, which is indicative of a n-type conduction. The high-temperature electrical resistivity and the absolute Seebeck coefficients decrease with the increase of niobium content. The electrical resistivity decreases from 10.3 mΩ cm for x = 0 at 1043 K to 5.4 mΩ cm for x = 0.10 at 1042 K. The thermal conductivity decreases with increasing temperature for all samples, and shows no distinct influence by the niobium substitution. The power factor is decreased in niobium doping samples, as there is no effectively decreasing of the electrical resistivity but reduction of Seebeck coefficient. Therefore, the highest thermoelectric figure of merit still observed in Sr 0.95La 0.05TiO 3, reaches 0.21 at 973 K, which is 40% larger than that of the stoichiometric single crystal.

Characterization and properties of microwave plasma-treated SrTiO 3

Nitrogen was successfully incorporated into the surface of a SrTiO 3(1 1 1) single crystal by exposing it to a microwave-induced glow discharge plasma of ammonia (NH 3). The layers generated by the flux of active species in the plasma varied in composition and structure depending on the treatment time, t. Substitution of oxygen by nitrogen along with the reduction of the crystal surface was observed by X-ray photoelectron spectroscopy. The 0–150 nm thick top layer showed Sr and O deficiencies and was identified as a mixture of nanocrystalline cubic rocksalt-type TiN, perovskite-type SrTiO 3− δ –SrTiO 3− x N y and amorphous phases. Deeper nitrogen diffusion into the crystal yielded perovskite-type nitrogen-doped strontium titanate. The diffusion of N into the perovskite structure was characterized by the diffusion coefficient D = (2.1 ± 0.4) × 10 −15 cm 2 s −1. The growth of the N-containing perovskite layers showed the time dependency L [ μ m ] = 6.3 t [ min ] . Examination of the electronic properties revealed metallic-like electrical conductivity, superconductivity with T c of up to 5 K, and negative Seebeck coefficients of up to S = −465 μV K −1 at room temperature.

Synthesis, Structure, and High-Temperature Thermoelectric Properties of Boron-Doped Ba8Al14Si31 Clathrate I Phases

Single crystals of boron-doped Ba8Al14Si31 clathrate I phase were prepared using Al flux growth. The structure and elemental composition of the samples were characterized by single-crystal and powder X-ray diffraction; elemental analysis; and multinuclear (27)Al, (11)B, and (29)Si solid-state NMR. The samples' compositions of Ba8B0.17Al14Si31, Ba8B0.19Al15Si31, and Ba8B0.32Al14Si31 were consistent with the framework-deficient clathrate I structure Ba8Al(x)Si(42-3/4x)cube(4-1/4x) (x = 14, cube = lattice defect). Solid-state NMR provides further evidence for boron doped into the framework structure. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates that transport processes are dominated by electrons. Thermal conductivity is low, but not significantly lower than that observed in the undoped Ba8Al14Si31 prepared in the same manner.

Effects of Sb compensation on microstructure, thermoelectric properties and point defect of CoSb3 compound

Volatilization of Sb during the fabrication of CoSb3 by mechanical alloying and then spark plasma sintering has been successfully compensated by adding excess Sb. The average grain size increases apparently with excess Sb content, abnormally grown by about 100 times as the excess Sb is up to 4 at.%. A liquid-phase-related mechanism is used to explain the abnormal growth. The uncompensated sample shows a negative Seebeck coefficient near room temperature, while the sample compensated with 6 at.% excess Sb shows an intrinsic positive Seebeck coefficient and an enhanced ZT value, which has a maximum of about 0.1 at 350 °C, which is two times higher than the uncompensated one. The transition of electrical conductivity from n- to p-type relative to the Sb compensation is discussed in relation to the point defect. A defect equation is given to show the nature of electron generation due to Sb deficiency. The Sb-vacancy not only provides extrinsic carrier but also generates a significant impact on the band gap and hence on the Seebeck coefficient.

Moderate-temperature thermoelectric properties of TiCoSb-based half-Heusler compounds Ti1−xTaxCoSb

Ta-doped Ti1−xTaxCoSb (0⩽x⩽0.08) half-Heusler compounds were synthesized by melting and annealing process. Their thermoelectric properties were studied in the temperature range of 300–900K. The Ti1−xTaxCoSb compounds exhibit negative Seebeck coefficients with considerably large absolute values. With increasing Ta substitution, the electrical conductivity was greatly increased, but the thermal conductivity was reduced. Because of the combined effects of increased electrical conductivity and reduced thermal conductivity, the thermoelectric performance of Ti1−xTaxCoSb alloys was apparently improved by doping Ta. The dimensionless figure of merit of 0.3 was obtained for Ti0.92Ta0.08CoSb compound at 900K. This value is about ten times larger than that of the undoped TiCoSb compound.

Structure and Thermoelectric Characterization of AxBa8-xAl14Si31 (A = Sr, Eu) Single Crystals

Single crystals of AxBa8-xAl14Si31 (A = Sr, Eu) were grown using a molten Al flux technique. Single-crystal X-ray diffraction confirms that AxBa8-xAl14Si31 (A = Sr, Eu) crystallize with the type I clathrate structure, and phase purity was determined with powder X-ray diffraction. Stoichiometry was determined to be Sr0.7Ba7.3Al14Si31 and Eu0.3Ba7.7Al14Si31 by electron microprobe analysis. These AxBa8-xAl14Si31 phases can be described as framework-deficient clathrate type I structures with the general formula, AxBa8-xAlySi42-3y/4[]4-1/4y. DSC measurements indicate that these phases melt congruently at 1413 and 1415 K for Sr0.7Ba7.3Al14Si31 and Eu0.3Ba7.7Al14Si31, respectively. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity of these phases remains low with Sr0.7Ba7.3Al14Si31 having the lowest values.

Thermoelectric property of fine-grained CoSb3 skutterudite compound fabricated by mechanical alloying and spark plasma sintering

Skutterudite CoSb3 polycrystalline materials were prepared using a combined process of mechanical alloying (MA) and spark plasma sintering (SPS). The influence of SPS temperature on the thermoelectric properties was focused in this work with a special emphasis on the analysis of the size effects of grains. The average grain sizes decreased from 300 to 50 nm with decreasing SPS temperatures from 600 to 300 °C. The electrical resistivities of samples spark plasma sintered at 300–600 °C all decreased with increasing temperature, indicating a classic intrinsic conduction behaviour of semiconductors. The samples spark plasma sintered at 300–500 °C showed a positive Seebeck coefficient while the sample spark plasma sintered at 600 °C showed a negative Seebeck coefficient. The room-temperature thermal conductivities were reduced from 4.30 to 2.92 W m−1 K−1 as the grain sizes were decreased from 300 to 100 nm corresponding to SPS at 600 and 400 °C, respectively. The present work indicates that MA and SPS is a good combination for fabricating fine-grained CoSb3 thermoelectric materials.

Thermoelectric properties of perovskites: Sign change of the Seebeck coefficient and high temperature properties

Abstract The possibility to change the Seebeck coefficient sign has been evidenced in the LaCoO3 perovskites. A small hole doping (Co3+/Co4+) will result in a large positive Seebeck coefficient, while a small electron doping (Co2+/Co3+) will give a large negative Seebeck coefficient at room temperature. This mechanism is shown to be efficient as well in 1D Ca3Co2O6 deriving from hexagonal perovskites. By doping Ca3Co2O6 with Ti4+, a mixed valency Co2+/Co3+ is introduced and the thermopower turns negative. At high temperature, the Seebeck coefficients of LaCoO3 and related compounds decrease to small values due to the spin state transition. The values converge towards a positive value, close to +20 μV/K at 800 K. This suggests that at high T, the Seebeck coefficients in the case of localized charges do not depend on the doping, but only on the spin and orbital degeneracies. On the other hand, in the case of metallic-like samples as electron-doped manganites, the properties can be described up to high T in terms of a single-band metal. Due to the linear variation of S as a function of T and the almost constant value of ρ, the ratio S2/ρ which is crucial for high temperature applications increases.

Transport properties of heavy-fermion systems

We calculate the temperature dependence of the transport properties of heavy-fermion systems such as resistivity, optical conductivity, thermoelectric power, the electronic part of the thermal conductivity, and the ``figure of merit.'' The one-particle properties of the periodic Anderson model are obtained within dynamical mean-field theory for the paramagnetic phase using Wilson's numerical renormalization group and the modified perturbation theory as impurity solvers. We discuss the dependence of the transport properties on the band filling, valence, and Coulomb correlation $U$. The typical experimental findings can be reproduced and understood, in particular the temperature dependence of the resistance and the thermoelectric power and their absolute magnitude for both metallic heavy-fermion systems and Kondo insulators. For large values of $U$, we find a negative Seebeck coefficient $S(T)$ for an intermediate-temperature regime as observed in $S(T)$ of $\mathrm{Ce}{\mathrm{Cu}}_{2}{\mathrm{Si}}_{2}$. We analyze different estimates for possible characteristic low-temperature scales of the lattice. Our results indicate a one-parameter scaling of thermodynamic and some transport properties with a strongly occupancy-dependent scaling function. This is consistent with a strong-coupling local Fermi-liquid fixed point of the effective site governing all low-lying excitations for $T\ensuremath{\rightarrow}0$ in the paramagnetic phase.

Structure and Thermoelectric Characterization of Ba8Al14Si31

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba8Al14Si31. Single-crystal neutron diffraction data for Ba8Al14Si31 were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba8Al14Si31 confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state (27)Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba8Al14Si31, with the general formula, Ba(8)Al(x)Si(42-3/4x)[](4-1/4x) ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.

Thermoelectric Properties and Microstructure of Ba8Al14Si31 and EuBa7Al13Si33

Powder samples of the type I clathrate phases, with the proposed stoichiometry Ba8Al16Si30 and Eu2Ba6Al16Si30, were synthesized using direct reaction of stoichiometric amounts of the elements. Rietveld refinement of powder X-ray diffraction data is consistent with the clathrate type I structure. Composition, microstructure, and thermoelectric property measurements were made on hot-pressed pellets. Stoichiometry of the Ba8Al16Si30 sample was determined from microprobe data to be Ba8Al14Si31, close to the deficient framework solid solution of the general formula, Ba8AlxSi42-3/4x[]4-1/4x (x = 14; [], open square indicates lattice defect). In the case of the rare-earth-substituted compound, microprobe analysis of the microstructure indicates that it is not single phase, but contains multiple components, most of the general clathrate stoichiometry. The majority phase has the stoichiometry EuBa7Al13Si32. Both phases show conductivity typical of heavily doped semiconductors with negative Seebeck coefficients. Th...

Preparation and thermoelectric properties of sintered iodine-containing clathrate compounds Ge38Sb8I8 and Sn38Sb8I8

The iodine-containing cationic type-I clathrates Ge38Sb8I8 and Sn38Sb8I8 were prepared and their thermodynamic properties as well as their thermoelectric properties were investigated. Their atomic displacement parameters were as large as those of anionic clathrates such as Ba8Ga16Ge30 and Cs8Cd4Sn42. The room temperature thermal conductivities of Ge38Sb8I8 and Sn38Sb8I8 were 7 and 12mWcm−1K−1, respectively; these values were as low as that of the above anionic clathrates. Both cationic clathrates had negative Seebeck coefficients; the band gap energies of Ge38Sb8I8 and Sn38Sb8I8 were 1.16 and 0.80eV, respectively.

Thermoelectric Properties of Indium‐Filled Skutterudites.

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Thermoelectric Properties of Indium-Filled Skutterudites

Structural, electrical, and thermal transport properties of CoSb3 partially filled with indium are reported. Polycrystalline samples of InxCo4Sb12 (0 ≤ x ≤ 0.3) were prepared by solid-state reaction under a gas mixture of 5% H2 and 95% Ar. The solubility limit of the indium filling voids in CoSb3 was found to be close to 0.22. Synchrotron X-ray diffraction refinement of the x = 0.2 sample showed that the indium is located in the classic rattler site and has a substantially larger thermal factor than those of Co and Sb. The electrical resistivity, Seebeck coefficients, and thermal conductivity of the InxCo4Sb12 samples were measured in the temperature range of 300−600 K. All samples showed metal-like behavior, and the large negative Seebeck coefficients indicated n-type conduction. The thermal conductivity decreased with increasing temperature for all samples. A thermoelectric figure-of-merit (ZT) ≥ 1 (n-type) has been achieved when x ≥ 0.2 in InxCo4Sb12 at 575 K.

Doping dependence of transport properties inFe1−xCoxSi

The positive magnetoresistance has been investigated for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ single crystals in a wide range of doping $(0.05\ensuremath{\leqslant}x\ensuremath{\leqslant}0.7)$. Most of the magnetoconductivity data are found to scale well with the magnetization. This is inconsistent with the quantum interference scenario proposed by Manyala et al. [Nature 404, 581 (2000)]. We have shown that the decrease of density of the minority spin band with high mobility in the course of Zeeman splitting is relevant to the positive magnetoresistance. The nearly half-metallic nature in this system seems to enhance the magnetoresistance. The pressure dependence of resistivity has been measured for ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}\mathrm{Si}$. $T$-linear behavior has been found in the resistivity above $7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, where the helical spin order is completely suppressed. This temperature dependence reproduces that of the hypothetical resistivity of the nonmagnetic state deduced by the analysis of the magnetoresistance. We have investigated the large Hall conductivity in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ ($\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}{\ensuremath{\Omega}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at a maximum). The doping dependence of the Hall conductivity is almost parallel with those of the critical field and the wave vector of the helical spin state. This suggests that the Hall conductivity is proportional to the effective spin-orbit interaction. We have also observed the doping dependence of the Seebeck coefficient for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$. In the underdoped region $(x\ensuremath{\leqslant}0.1)$, the negative Seebeck coefficient is enhanced at low temperature below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, corresponding to the steep doping variation of the resistivity in this temperature region. In the higher doping region $(x\ensuremath{\geqslant}0.2)$, the Seebeck coefficient shows a gradual upturn at low temperatures $(\ensuremath{\lesssim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. This is caused by the electronic structural change occurring with the transition from the paramagnetic to the ferromagnetic state.