Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

p-Type thermoelectric properties of the oxygen-deficient perovskite Ca2Fe2O5 in the brownmillerite structure

Brownmillerite calcium ferrite was synthesized in air at 1573 K and thermoelectric properties (direct current electrical conductivity σ, Seebeck coefficient α, thermal conductivity κ, thermal expansion α_L) were measured from 373 to 1050 K in air. Seebeck coefficient was positive over all temperatures indicating conduction by holes, and electrical properties were continuous through the Pnma-Imma phase transition. Based on the thermopower and conductivity activation energies as well as estimated mobility, polaron hopping conduction was found to dominate charge transport. The low electrical conductivity, <1 S/cm, limits the power factor (α²σ), and thus the figure of merit for thermoelectric applications. The thermal conductivity values of ∼2 W/mK and their similarity to Ruddlesden-Popper phase implies the potential of the alternating tetrahedral and octahedral layers to limit phonon propagation through brownmillerite structures. Bulk linear coefficient of thermal expansion (∼14×10⁻⁶ K⁻¹) was calculated from volume data based on high-temperature in situ X-ray powder diffraction, and shows the greatest expansion perpendicular to the alternating layers.     

Concerted Rattling in CsAg5Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S²σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p‐type thermoelectric material, CsAg₅Te₃, is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm^?1 K^?1) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state‐of‐the‐art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.     

Improvement of the thermoelectric properties of [Bi1.68Ca2O4−δ][CoO2]1.69 cobaltite by chimie douce methods

[Bi_1.68Ca₂O_4−δ]^RS[CoO₂]_1.69 has been obtained by different chimie douce methods and uniaxially or isostatically pressed. The influence of these parameters on the thermoelectric properties has been investigated. Contrary to the Seebeck coefficient, which remains unchanged, the electrical conductivity is greatly modified. In particular, spray-drying synthesis followed by uniaxial pressing results in an electrical conductivity two times larger than in the case of conventional solid state synthesis. Our results suggest that a narrow particle size distribution is beneficial to the thermoelectric properties of the layered compounds. The spray-drying technique seems to be promising to improve the electrical conductivity of layered materials. Moreover, this method presents other advantages (homogeneous samples and less energetic processing) which could be interesting to the future manufacturing of thermoelectric devices.     

Preparation and thermoelectric properties of AgPbmSbSem+2 materials

Hydrothermally synthesized AgPb_mSbSe_m+2 (m=10, 12, 16, 18) nanoparticles with diameters of 20-50 nm were compacted by pressureless sintering. The Seebeck coefficient and electrical conductivity of the samples were measured from room temperature up to ∼750 K. The samples show large and positive values of the Seebeck coefficient and moderate electrical conductivity. The thermoelectric properties of Ag_xPb₁₈SbSe₂₀ (x=0.8, 0.85) and AgPb₁₈SbSe_20−yTe_y (y=1, 3) samples have also been studied. It has been found that Ag_0.85Pb₁₈SbSe₂₀ sample has a higher thermoelectric power factor. A significant difference in thermoelectric properties has also been observed for the AgPb₁₈SbSe₂₀ samples prepared with pressureless sintering and spark plasma sintering.     

Enhanced thermoelectric performance of hydrothermally synthesized polycrystalline Te-doped SnSe

In this study,large-scale Te-doped polycrystalline SnSe nanopowders were synthesized by a facile hydrothermal approach and the effect of Te doping on the thermoelectric properties of SnSe was fully investigated.It is found that the carrier concentration increases due to the reduction of band gap by alloying with Te,which contributes to significant enhancement of electrical conductivity especially at room temperature.Combined with the moderated Seebeck coefficient,a high power factor of 4.59μW cm ~1 K ~2 is obtained at 773 K.Furthermore,the lattice the rmal conductivity is greatly reduced upon Te substitution owing to the atomic point defect scattering.Benefiting from the synergistically optimized both electrical-and thermal-transport properties by Te-doping,thermoelectric performance of polycrystalline SnSe is enhanced in the whole temperature range with a maximum ZT of-0.79 at a relatively low temperature(773 K) for SnSe_(0.85)Te_(0.15).This study provides a low-cost and simple lowtemperature method to mass production of SnSe with high thermoelectric performance for practical applications     

Transport coefficients of titanium-doped Sb2Te3 single crystals

Titanium-doped single crystals (c_Ti=0-2×10²⁰ atoms cm⁻³) were prepared from the elements Sb, Ti, and Te of 5 N purity by a modified Bridgman method. The obtained crystals were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 3-300 K. It was observed that with an increasing Ti content in the samples the electrical resistivity, the Hall coefficient and the Seebeck coefficient increase. This means that the incorporation of Ti atoms into the Sb₂Te₃ crystal structure results in a decrease in the concentration of holes in the doped crystals. For the explanation of the observed effect a model of defects in the crystals is proposed. The data of the lattice thermal conductivity were fitted well assuming that phonons scatter on boundaries, point defects, charge carriers, and other phonons.     

Anomalous charge transport in CeB6

The comprehensive study of conductivity σ, Hall coefficient R_H and Seebeck coefficient S has been carried out on high-quality single crystals of CeB₆ in a wide range of temperatures 1.8-300 K. An anomalous behavior of all transport characteristics (σ, R_H, S) was found for the first time in the vicinity of T^*≈80 K. The strong decrease of conductivity σ as well as the unusual asymptotic behavior of Seebeck coefficient S(T)∼−ln T observed below T^* allowed us to conclude in favor of crossover between different regimes of charge transport in CeB₆. The pronounced change of Hall mobility μ_H, which diminishes from the maximum value of 20 cm²/(V s) at T^* to the values of ∼6 cm²/(V s) at T∼10 K, seems to be attributed to the strong enhancement of charge carriers scattering due to fast spin fluctuations on Ce-sites. The low-temperature anomalies of the charge transport characteristics are compared with the predictions of the Kondo-lattice model.     

Anisotropic thermoelectric properties of Bi1.9Lu0.1Te2.7Se0.3 textured via spark plasma sintering

Spark plasma sintering method was applied to prepare bulk n-type Bi_1.9Lu_0.1Te_2.7Se_0.3 samples highly textured along the 001 direction parallel to the pressing direction. The texture development is confirmed by X-ray diffraction analysis and scanning electron microscopy. The grains in the textured samples form ordered lamellar structure and lamellar sheets lie in plane perpendicular to the pressing direction. The average grain size measured along the pressing direction is much less as compared to the average grain size measured in the perpendicular direction (∼50 nm against ∼400 nm). A strong anisotropy in the transport properties measured along directions parallel and perpendicular to the pressing direction within the 290 ÷ 650 K interval was found. The specific electrical resistivity increases and the thermal conductivity decreases for the parallel orientation as compared to these properties for the perpendicular orientation. The Seebeck coefficient for both orientations is almost equal. Increase of the electrical resistivity is stronger than decrease of the thermal conductivity resulting in almost three-fold enhancement of the thermoelectric figure-of-merit coefficient for the perpendicular orientation (∼0.68 against ∼0.24 at ∼420 K). The texturing effect can be attributed to (i) recovery of crystal structure anisotropy typical for the single crystal Bi₂Te₃-based alloys and (ii) grain boundary scattering of electrons and phonons. An onset of intrinsic conductivity observed above T_d ≈ 410 K results in appearance of maxima in the temperature dependences of the specific electrical resistivity, the Seebeck coefficient and the thermoelectric figure-of-merit coefficient and minimum in the temperature dependence of the total thermal conductivity. The intrinsic conductivity is harmful for the thermoelectric efficiency enhancement since thermal excitation of the electron-hole pairs reduces the Seebeck coefficient and increases the thermal conductivity.     

Epitaxial Growth and Thermoelectric Measurement of Bi2Te3/Sb Superlattice Nanowires

Superlattice nanowires are expected to show further enhanced thermoelectric performance compared with conventional nanowires or superlattice thin films. We report the epitaxial growth of high density Bi₂Te₃/Sb superlattice nanowire arrays with a very small bilayer thickness by pulse electrodeposition. Transmission electron microscopy, selected area electron diffraction and high resolution transmission electron microscopy were used to characterize the superlattice nanowires, and Harman technique was employed to measure the figure of merit (ZT) of the superlattice nanowire array in high vacuum condition. The superlattice nanowire arrays exhibit a ZT of 0.15 at 330 K, and a temperature difference of about 6.6 K can be realized across the nanowire arrays.     

Thermoelectric properties of Ag-doped n-type (Bi2Te3)0.9-(Bi2−xAgxSe3)0.1 (x=0-0.4) alloys prepared by spark plasma sintering

Ag-doped n-type (Bi₂Te₃)_0.9-(Bi_2−xAg_xSe₃)_0.1 (x=0-0.4) alloys were prepared by spark plasma sintering and their physical properties evaluated. When at low Ag content (x=0.05), the temperature dependence of the lattice thermal conductivity follows the trend of (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1; while at higher Ag content, a relatively rapid reduction above 400 K can be observed due possibly to the enhancement of scattering of phonons by the increased defects. The Seebeck coefficient increases with Ag content, with some loss of electrical conductivity, but the maximum dimensionless figure of merit ZT can be obtained to be 0.86 for the alloy with x=0.4 at 505 K, about 0.2 higher than that of the alloy (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1 without Ag-doping.     

The effect of element substitution on high-temperature thermoelectric properties of Ca3Co2O6 compounds

Polycrystalline Ca₃Co_1.8M_0.2O₆ (M=Mn, Fe, Co, Ni, Cu) and Ca_2.7Na_0.3Co₂O₆ were synthesized by solid-state reaction to evaluate the effect of substitution on the thermoelectric properties of Ca₃Co₂O₆. Substitution by Mn, Cu and Na appears to increase carrier density, given that electrical resistivity (ρ) and the Seebeck coefficient (S) were simultaneously reduced. Conversely, Fe substitution seems to reduce carrier density, resulting in a simultaneous increase in S and ρ. Cu and Na substitution resulted in a significant decrease in ρ due to enhancement of grain size and grain boundary connectivity, which could have a strong impact on ρ. Not only the intrinsic substitution effect on the electronic state but also this modification of the microstructure plays an important role in improvement of the thermoelectric power factor, particularly in the case of the Na-substituted sample.     

Structure, Stoichiometry, and Properties of the Chimney-Ladder Phases Ru2Ge3+x (0<x<1)

The preparation and physical characterization of non-stoichiometric Ru₂Ge_3+x (0≤x≤1) are reported for the first time. The defect TiSi₂-type chimney-ladder structure is maintained for the full stoichiometry range. The resistivity of Ru₂Ge_3+x increases systematically with x from 300 mΩ cm, x=0 -3 Ω cm, x=1 at 300 K. The temperature dependence is consistent with a variable range-hopping mechanism for x≥0.6. The Seebeck coefficients of samples do not evolve simply with x. A low thermal conductivity (κ_{300 K}=0.03 W/K cm) suggests that Ru₂Ge₃ has some of the properties of a phonon-glass-electron-crystal. The low value of the thermoelectric figure of merit ZT=3.2×10⁻³ (T=300 K) calculated for Ru₂Ge₃ is due primarily to a low conductivity.     

Structure and high-temperature thermoelectric properties of SrAl2Si2

Single crystals of SrAl₂Si₂ were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl₂Si₂ is isostructural to CaAl₂Si₂ and crystallizes in the hexagonal space group P-3m1 (90 K, a=4.1834 (2), c=7.4104 (2) Å, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at ∼1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of ∼7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from ∼−78 μV K⁻¹ at room temperature to −34 μV K⁻¹ at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm⁻¹ K, and contributes to the overall low zT for this compound.     

Enhanced thermoelectric performance and novel nanopores in AgSbTe2 prepared by melt spinning

We report a melt-spinning spark-plasma-sintering synthesis process of the polycrystalline p-type material composed of AgSbTe₂ coarse grains and evenly formed 5-10 nm pores that occur primarily on the surface of matrix grains. The formation mechanism of nanopores and their influences on the thermoelectric properties have been studied and correlated. Microstructure analysis shows that the as-prepared sample can be regarded as a nanocomposite of matrix and in situ generated nanopores evenly coated on matrix grains. For the single-phase component and the possible energy-filter effect caused by the nanopores, the electrical transport properties are improved. Moreover, the thermal conductivity is significantly reduced by strong phonon scattering effect resulted from the nanopores. The thermoelectric performance of the as prepared sample enhances greatly and a ZT of 1.65 at 570 K is achieved, increasing∼200% compared with the sample prepared by traditional melt and slow-cooling method.     

Synthesis and evaluation of lead telluride/bismuth antimony telluride nanocomposites for thermoelectric applications

Heterogeneous nanocomposites of p-type bismuth antimony telluride (Bi_2−xSb_xTe₃) with lead telluride (PbTe) nanoinclusions have been prepared by an incipient wetness impregnation approach. The Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient were measured from 80 to 380 K in order to investigate the influence of PbTe nanoparticles on the thermoelectric performance of nanocomposites. The Seebeck coefficients and electrical resistivities of nanocomposites decrease with increasing PbTe nanoparticle concentration due to an increased hole concentration. The lattice thermal conductivity decreases with the addition of PbTe nanoparticles but the total thermal conductivity increases due to the increased electronic thermal conductivity. We conclude that the presence of nanosized PbTe in the bulk Bi_2−xSb_xTe₃ matrix results in a collateral doping effect, which dominates transport properties. This study underscores the need for immiscible systems to achieve the decreased thermal transport properties possible from nanostructuring without compromising the electronic properties.     

Structure and properties of the CaFe2O4-type cobalt oxide CaCo2O4

The calcium cobalt oxide CaCo₂O₄ was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo₂O₄ crystallizes in the CaFe₂O₄ (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO₆ octahedral network. The structure of CaCo₂O₄ belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) Å, b=2.9006(7) Å and c=10.282(3) Å. Curie-Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co³⁺, 3d, S=0), semiconductor-like temperature dependence of resistivity (ρ=3×10⁻¹ Ω cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant (γ=4.48(7) mJ/Co mol K²), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides.     

Conductive polymers as a new type of thermoelectric material

Thermoelectric properties were investigated for the films of electrically conductive doped polyanilines. The thermoelectric performance, evaluated by thermoelectric figure-of-merit (ZT = T (S² σ) / κ), of various protonic acid-doped polyaniline bulk films was found to depend on the electrical conductivity σ of the film. Thus, the higher the electrical conductivity, the higher the figure-of-merit is, because the thermal conductivity κ of polyaniline films does not depend on the electrical conductivity. Among the conductive bulk films of polyaniline, the highest figure-of-merit (ZT = 1 × 10⁻⁴) was observed for (±)-10-camphorsulfonic acid (CSA)-doped polyaniline in an emeraldine form (σ - 188 S cm⁻¹) at room temperature. The multilayered film, composed of electrically insulating emeraldine base layers and electrically conducting CSA-doped emeraldine salt layers, exhibited 6 times higher ZT at 300 K than that of a bulk film of CAS-doped polyaniline, showing the highest ZT value of 1.1 × 10⁻² at 423 K. Stretching of the CAS-doped polyaniline film also increased the figure-of-merit of doped polyaniline films along the direction of the stretching.     

Cu22Bi12S21Cl16—A mixed conductor with fast one-dimensional copper(I) ion transport

Melting reactions of Cu, CuCl, S, and Bi₂S₃ yield black, shiny needles of Cu₂₂₍₁₎Bi₁₂S₂₁₍₁₎Cl₁₆₍₁₎. The compound decomposes peritectically at 649(5) K. Oxidation state +I of the copper atoms is supported by Cu-K-XANES. The compound crystallizes in the hexagonal space group P6/m with a=2116.7(7) pm and c=395.17(5) pm. Seven anions coordinate each of the two independent bismuth cations in the shape of mono-capped trigonal prisms. These polyhedra share edges and faces to form trigonal and hexagonal tubes running along [0 0 1]. The hexagonal tubes are centered by chloride ions, which are surrounded by disordered copper cations. The majority of copper cations are distributed over numerous sites between the tubes. The Joint Probability Density Function (JPDF) reveals a continuous pathway along [0 0 1]. The high mobility of the copper cations along [0 0 1] was demonstrated by impedance spectroscopy and DC polarization measurements on single crystals. The ionic conductivity at 450 K is about σ_ion=0.06 S cm⁻¹, and the activation energy for Cu⁺ ion conduction is E_a=0.44 eV. The chemical diffusion coefficient of copper is in the order of D_cu^δ=10¹⁹ cm⁻³ at 420 K. The electronic band gap (p-type conductor) was determined as E_g=0.06 eV. At room temperature the thermal conductivity of a pressed pellet is about κ=0.3 W K⁻¹ m⁻¹ and the Seebeck coefficient is S=43 μV K⁻¹.     

Etude du pouvoir thermoélectrique des composés Cr3Se4−xTex (0 ≤ x ≤ 4)

Seebeck coefficient (S) determination has been carried out on nine polycrystalline samples in the Cr₃Se_4?xTe_x series (x = 0; 0.5; 1; 1.5; 2; 2.5; 3; 3.5; 4). A thermoelectric cell was constructed, which works automatically between 14 and 320 K with a precision better than 5%. For high concentration in selenium, the thermoelectric power remains negative over the whole temperature range. Such is not the case with tellurium-rich compounds where the Seebeck coefficient, negative at 320 K, becomes positive at lower temperatures. Anomalies are observed on the S = f(T) curves at temperatures T_t in the vicinity of the magnetic transitions. Transport properties for selenium-rich compounds are discussed in terms of narrow-band conduction. Hole conduction, which appears for x ≥ 3 at low temperature, is interpreted as due to an overlapping band conduction between 3d levels and the valence band.