Thermoelectric properties of some metal borides

Polycrystalline AlMgB₁₄ and some hexaborides (CaB₆, SrB₆, YbB₆, SmB₆, and CeB₆) were synthesized to examine their thermoelectric properties. Single phase of orthorhombic AlMgB₁₄, which contains B₁₂ icosahedral clusters as building blocks, was obtained at sintering temperatures between 1573 and 1823 K. Seebeck coefficient (α) and electrical conductivity (σ) of the phase were about 500 μV/K and 10⁻¹ 1/Ω m at room temperature, respectively. These values are comparable to those of metal-doped β-rhombohedral boron. On the other hand, metal hexaborides with divalent cation possessed large negative α ranging from −100 to −270 μV/K at 1073 K. Calculated power factors of CaB₆ and SrB₆ exceeded 10⁻³ W/K² m within the entire range of temperature measured. As a result, they can be thought as promising candidates for n-type thermoelectric material.     

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.     

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.     

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.     

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.     

Highly enhanced thermoelectric performance of WS2 nanosheets upon embedding PEDOT:PSS

Two‐dimensional (2D) WS₂ nanosheets (NSs) as a promising thermoelectric (TE) material have gained great concern recently. The low electrical conductivity significantly limits its further development. Herein, we reported an effective method to enhance the TE performance of WS₂ NSs by combining poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS). The restacked WS₂ NSs thin film with 1T phase structure obtained by a common chemical lithium intercalation show a high Seebeck coefficient of 98 μV K^?1 and a poor electrical conductivity of 12.5 S cm^?1. The introduction of PEDOT:PSS with different contents obviously improve the electrical conductivity of WS₂ NSs thin films. Although a declining Seebeck coefficient was observed, an optimized TE power factor of 45.2 μW m^?1 k^?1 was achieved for WS₂/PEDOT:PSS composite thin film. Moreover, the as‐prepared WS₂/PEDOT:PSS thin film can be easily peeled off and transferred to other substrate leading to a more promising application. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 997–1004     

Effects of Rh on the thermoelectric performance of the p-type Zr0.5Hf0.5Co1−xRhxSb0.99Sn0.01 half-Heusler alloys

We show that Rh substitution at the Co site in Zr_0.5Hf_0.5Co_1−xRh_xSb_0.99Sn_0.01 (0≤x≤1) half-Heusler alloys strongly reduces the thermal conductivity with a simultaneous, significant improvement of the power factor of the materials. Thermoelectric properties of hot-pressed pellets of several compositions with various Rh concentrations were investigated in the temperature range from 300 to 775 K. The Rh “free” composition shows n-type conduction, while Rh substitution at the Co site drives the system to p-type semiconducting behavior. The lattice thermal conductivity of Zr_0.5Hf_0.5Co_1−xRh_xSb_0.99Sn_0.01 alloys rapidly decreased with increasing Rh concentration and lattice thermal conductivity as low as 3.7 W/m*K was obtained at 300 K for Zr_0.5Hf_0.5RhSb_0.99Sn_0.01. The drastic reduction of the lattice thermal conductivity is attributed to mass fluctuation induced by the Rh substitution at the Co site, as well as enhanced phonon scattering at grain boundaries due to the small grain size of the synthesized materials.     

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.     

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.     

High-temperature thermoelectric studies of A11Sb10 (A=Yb, Ca)

Large samples (6-8 g) of Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀ have been synthesized using a high-temperature (1275-1375 K) flux method. These compounds are isostructural to Ho₁₁Ge₁₀, crystallizing in the body-centered, tetragonal unit cell, space group I4/mmm, with Z=4. The structure consists of antimony dumbbells and squares, reminiscent of Zn₄Sb₃ and filled Skutterudite (e.g., LaFe₄Sb₁₂) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity (ρ), Seebeck coefficient (α), thermal conductivity (κ), and thermoelectric figure of merit (zT) from room temperature to at minimum 975 K are presented for A₁₁Sb₁₀ (A=Yb, Ca). DSC/TG were measured to 1400 K and reveal the stability of these compounds to ∼1200 K. Both A₁₁Sb₁₀ (A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10 mW/cm K for both Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀) that can be attributed to the complex crystal structure. Yb₁₁Sb₁₀ is a poor metal with relatively low resistivity (1.4 mΩ cm at 300 K), while Ca₁₁Sb₁₀ is a semiconductor suggesting that a gradual metal-insulator transition may be possible from a Ca_11−xYb_xSb₁₀ solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.     

Raman scattering study of CaB6 and YbB6

Phonon spectra of CaB₆ and RB₆ (R=Yb, Ce, and Pr) have been investigated by Raman scattering. We found clear spectral difference between divalent cation hexaboride and trivalent one. E_g mode shows the doublet spectra for only the divalent crystals of CaB₆ and YbB₆. The doublet spectra are understood by the two-dimensional charge distribution on B₆ without lattice distortion. In addition, the scattered intensities of the phonons change at around the ferromagnetic Curie temperature for YbB₆ and at T?600 K for CaB₆. These are the characteristic temperatures due to the change of the electronic system.     

Transport Properties of Bi2−xInxSe3 Single Crystals

The paper reports on the temperature dependence of the electrical and thermal conductivity, Hall constant, and Seebeck coefficient of Bi_2−xIn_xSe₃ (x=0, 0.2, 0.4) single crystals measured over the temperature range from 2 to 300 K. One single-valley conduction band model is used to interpret relations among transport coefficients. The data analysis relies on the use of a mixed carrier scattering mechanism consisting of acoustic scattering and scattering on ionized impurities. The effect of In incorporation into the Bi₂Se₃ crystal lattice on the individual components of thermal conductivity is evaluated and discussed.     

Crystal structure, characterization and thermoelectric properties of the type-I clathrate Ba8−ySryAl14Si32 (0.6≤y≤1.3) prepared by aluminum flux

The title compound was prepared as single crystals using an aluminum flux technique. Single crystal and powder X-ray diffraction indicate that this composition crystallizes in the clathrate type-I structure, space group Pm3?n. Electron microprobe characterization indicates the composition to be Ba_8−ySr_yAl_14.2(2)Si_31.8(2) (0.77<y<1.3). Single-crystal X-ray diffraction data (90 and 12 K) were refined with the Al content fixed at the microprobe value (12 K data: R₁=0.0233, wR₂=0.0441) on a crystal of compositions Ba. The Sr atom preferentially occupies the 2a position; mixed Al/Si occupancy was found on all framework sites. These refinements are consistent with a fully occupied framework and nearly fully occupied cation guest sites as found by microprobe analysis. Temperature dependent electrical resistivity and thermal conductivity have been measured from room temperature to 1200 K on a hot-pressed pellet. Electrical resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity is between 22 and 25 mW/cm K. The sample shows n-type conductivity with a maximum figure of merit, zT of 0.3 at 1200 K. A single parabolic band model predicts a five-fold increase in zT at 800 K if carrier concentration is lowered.     

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.     

MeV Si ions bombardments effects on thermoelectric properties of SiO2/SiO2+Ge nanolayers

The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT=S²σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. ZT can be increased by increasing S, increasing σ, or decreasing K. We have prepared the thermoelectric generator device of SiO₂/SiO₂+Ge multilayer superlattice films using the ion beam assisted deposition (IBAD). The 5 MeV Si ion bombardments have been performed using the AAMU Pelletron ion beam accelerator at five different fluences to make quantum structures (nanodots and/or nanoclusters) in the multilayer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after MeV Si ions bombardments at the different fluences we have measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity, Raman spectra to get some information about the sample structure and bond structures among the used elements in the superlattice thin film systems.     

Thermoelectric properties and microstructure of Mg3Sb2

Mg₃Sb₂ has been prepared by direct reaction of the elements. Powder X-ray diffraction, thermal gravimetric, differential scanning calorimetery, and microprobe data were obtained on hot pressed samples. Single phase samples of Mg₃Sb₂ were prepared and found to contain oxygen at the grain boundaries and to lose Mg and oxidize at temperatures above 900 K. Thermoelectric properties were characterized by Seebeck, electrical resistivity, and thermal conductivity measurements from 300 to 1023 K, and the maximum zT was found to be 0.21 at ∼875 K.     

The preparation and thermoelectric properties of molten salt electrodeposited boron wafers

We have prepared electrodeposited boron wafer by molten salts with KBF₄-KF at 680°C using graphite crucible for anode and silicon wafer and nickel plate for cathodes. Experiments were performed by various molar ratios KBF₄/KF and current densities. Amorphous p-type boron wafers with purity 87% was deposited on nickel plate for 1 h. Thermal diffusivity by ring-flash method and heat capacity by DSC method produced thermal conductivity showing amorphous behavior in the entire temperature range. The systematical results on thermoelectric properties were obtained for the wafers prepared with KBF₄-KF (66-34 mol%) under various current densities in the range 1-2 A/cm². The temperature dependencies of electrical conductivity showed thermal activated type with activation energy of 0.5 eV. Thermoelectric power tended to increase with increasing temperature up to high temperatures with high values of (1-10) mV/K. Thermoelectric figure-of-merit was 10⁻⁴/K at high temperatures. Estimated efficiency of thermoelectric energy conversion would be calculated to be 4-5%.     

Transport properties and lithium insertion study in the p-type semi-conductors AgCuO2 and AgCu0.5Mn0.5O2

The transport properties and lithium insertion mechanism into the first mixed valence silver-copper oxide AgCuO₂ and the B-site mixed magnetic delafossite AgCu_0.5Mn_0.5O₂ were investigated by means of four probes DC measurements combined with thermopower measurements and in situ XRD investigations. AgCuO₂ and AgCu_0.5Mn_0.5O₂ display p-type conductivity with Seebeck coefficient of Q=+2.46 and +78.83 μV/K and conductivity values of σ=3.2×10⁻¹ and 1.8×10⁻⁴ S/cm, respectively. The high conductivity together with the low Seebeck coefficient of AgCuO₂ is explained as a result of the mixed valence state between Ag and Cu sites. The electrochemically assisted lithium insertion into AgCuO₂ shows a solid solution domain between x=0 and 0.8Li⁺ followed by a plateau nearby 1.7 V (vs. Li⁺/Li) entailing the reduction of silver to silver metal accordingly to a displacement reaction. During the solid solution, a rapid structure amorphization was observed. The delafossite AgCu_0.5Mn_0.5O₂ also exhibits Li⁺/Ag⁺ displacement reaction in a comparable potential range than AgCuO₂; however, with a prior narrow solid solution domain and a less rapid amorphization process. AgCuO₂ and AgCu_0.5Mn_0.5O₂ provide a discharge gravimetric capacity of 265 and 230 mA h/g above 1.5 V (vs. Li⁺/Li), respectively, with no evidence of a new defined phases.     

Phase transition and mixed oxide-proton conductivity in germanium oxy-apatites

La_9.75□_0.25(Ge₆O₂₄)O_2.62 oxy-apatite shows a phase transition from triclinic to hexagonal symmetry at approximately 1020 K that has been characterised by high-temperature synchrotron X-ray and neutron powder diffraction, and ionic conductivity measurements. The crystal structure at 1073 K has been determined from joint Rietveld refinements of synchrotron X-ray and neutron powder diffraction data. The study shows that hexagonal-La_9.75□_0.25(Ge₆O₂₄)O_2.62 contains interstitial oxygen at the position previously reported for other oxy-germanates. Changes in the oxide conductivity associated with this structural transition are discussed. The thermal analyses showed a weight loss on heating close to 600 K very likely due to water release. The synchrotron thermodiffractometric study shows an anomaly in the cell parameters evolution at that temperature, which indicates that this residual water is located into the apatite channels. The electrical characterisation under different atmospheres (dry and wet synthetic air) indicates that there is a significant proton contribution to the overall conductivity below 600 K, mainly under wet atmosphere.     

Preparation and characterization of graphite nano-platelet (GNP)/epoxy nano-composite: Mechanical,electrical and thermal properties

Epoxy based polymer nano-composite was prepared by dispersing graphite nano-platelets (GNPs) using two different techniques: three-roll mill (3RM) and sonication combined with high speed shear mixing (Soni_hsm). The influence of addition of GNPs on the electrical and thermal conductivity, fracture toughness and storage modulus of the nano-composite was investigated. The GNP/epoxy prepared by 3RM technique showed a maximum electrical conductivity of 1.8 × 10⁻⁰³ S/m for 1.0 wt% which is 3 orders of magnitude higher than those prepared by Soni_hsm. The percentage of increase in thermal conductivity was only 11% for 1.0 wt% and 14% for 2.0 wt% filler loading. Dynamic mechanical analysis results showed 16% increase in storage modulus for 0.5 wt%, although the Tg did not show any significant increase. Single edge notch bending (SENB) fracture toughens (K_IC) measurements were carried out for different weight percentage of the filler content. The toughening effect of GNP was most significant at 1.0 wt% loading, where a 43% increase in K_IC was observed. Among the two different dispersion techniques, 3RM process gives the optimum dispersion where both electrical and mechanical properties are better.