Thermoelectric properties of In<Subscript>z</Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> skutterudites

In this study, indium-filled CoSb₃ skutterudite is synthesized via encapsulated induction melting and subsequent annealing at 823 K for six days, and the crystal structure, lattice constant, filler position, phase homogeneity and stability were investigated. All of the In-filled CoSb₃ samples were n-type conducting samples. The temperature dependence of the electrical resistivity showed In_zCo₄Sb₁₂ is a highly degenerate semiconducting material. The thermal conductivity was reduced considerably by In filling. The highest thermoelectric figure of merit value was achieved when the In filling fraction is 0.25. It was found that the ZT of the In-filled CoSb₃ (In_zCo₄Sb₁₂) was higher than that of the In-substituted CoSb₃ (Co_3.75In_0.25Sb₁₂ and Co₄Sb_11.75In_0.25). This is mainly due to the lower thermal conductivity and higher Seebeck coefficient.     

Thermoelectric properties of hot-pressed Zn4Sb3−xTex

A series of samples have been fabricated through vacuum melting method followed by hot-pressing for Zn₄Sb_3−xTe_x (x = 0.02–0.08), XRD patterns indicated that all the samples were single-phased β-Zn₄Sb₃. Electrical conductivity and Seebeck coefficient were evaluated in the temperature range of 300–700 K, showing p-type conduction. The thermoelectric figure of merit (ZT) was increased with the increase of Te content. ZT values of 0.8 and 1.0 were obtained at 673 K for Zn_4.08Sb₃ and Zn₄Sb_2.92Te_0.08, respectively.     

Effects of nano-TiO<Subscript>2</Subscript> dispersion on thermoelectric properties of Co<Subscript>4</Subscript>Sb<Subscript>11.7</Subscript>Te<Subscript>0.3</Subscript> composites

Nano-TiO2/Co4Sb11.7Te0.3 composites were prepared by mechanical alloying (MA) and cold isostatic pressing (CIP) process.The phase composition,microstructure,and thermoelectric properties were characterized.The diffraction spectra of all samples well corresponds to CoSb3 skutterudite diffraction plane.TiO2 agglomerates into irregular clusters.They locate at the grain boundaries or some are distributed on the surface of Co4Sb11.7Te0.3 particles.For composites with high TiO2 content (0.6% and 1.0% TiO2),the phonon scattering by TiO2 particle,pores,and small size grains can result in a remarkable reduction in thermal conductivity.The maximum value of ZT is 0.79 for sample with 0.6 wt.% TiO2 at 700 K,which is 11% higher than that of non-dispersed sample.     

Electronic transport properties of Ni-doped CoSb<Subscript>3</Subscript> prepared by encapsulated induction melting

Ni-doped CoSb₃ skutterudites were prepared by encapsulated induction melting and their thermoelectric and electronic transport properties were investigated. The negative signs of Seebeck and Hal coefficients for all Ni-doped specimens revealed that Ni atoms successfully acted as n-type dopants by substituting Co atoms. The carrier concentration increased as the Ni doping content increased, and the Ni dopants could generate excess electrons. However, the carrier mobility decreased as the doping content increased, which indicates that the electron mean free path was reduced by the impurity scattering. The Seebeck coefficient and the electrical resistivity decreased as the carrier concentration increased, as the increase in carrier concentration by doping overcame the decrease in the carrier mobility by impurity scattering. The Seebeck coefficient showed a negative value at all temperatures examined and increased as the temperature increased. The temperature dependence of electrical resistivity suggested that Co_1−xNi_xSb₃ is a highly degenerate semiconducting material. Thermal conductivity was considerably reduced by Ni doping, and the lattice contribution was dominant in the Ni-doped CoSb₃.     

Effects of Sn-doping on the electrical and thermal transport properties of p-type Cerium filled skutterudites

p-type Sn-doped CoSb₃-based skutterudite compounds have been prepared using melting-quenching-annealing method and spark plasma sintering technique. Sn atoms in our samples are completely soluted on Sb-site with a fixed charge state and non-magnetic feature, providing a better choice to ascertain the effect of element doping at the [Co₄Sb₁₂] framework on the electrical and thermal transport properties in p-type skutterudites. Doping Sn at the framework introduces additional ionized impurity scattering to affect the electron transport greatly. Similar electrical transport properties between Ce_0.2Co₄Sb_11.2Sn_0.8 and Co₄Sb₁₁Sn_0.6Te_0.4 suggest that Ce fillers contribute little to the valence band edge. Filling Ce into the voids and doping Sn at the framework introduce additional phonon resonant and point defect scattering mechanisms, thereby reducing lattice thermal conductivity remarkably. Moreover, our data suggest that combining these two effects is more effective to suppress lattice thermal conductivity through scattering broad range of phonons with different frequencies.     

Thermoelectric Properties of Cu-doped Bi<Subscript>2−x</Subscript>Sb<Subscript>x</Subscript>Te<Subscript>3</Subscript> Prepared by Encapsulated Melting and Hot Pressing

P-type Bi_2?xSb_xTe₃:Cu_m (x = 1.5–1.7 and m = 0.002–0.003) solid solutions were synthesized using encapsulated melting and were consolidated using hot pressing. The effects of Sb substitution and Cu doping on the charge transport and thermoelectric properties were examined. The lattice constants decreased with increasing Sb and Cu contents. As the amount of Sb substitution and Cu doping was increased, the electrical conductivity increased, and the Seebeck coefficient decreased owing to the increase in the carrier concentration. All specimens exhibited degenerate semiconductor characteristics and positive Hall and Seebeck coefficients, indicating p-type conduction. The increased Sb substitution caused a shift in the onset temperature of the intrinsic transition and bipolar conduction to higher temperatures. The electronic thermal conductivity increased with increasing Sb and Cu contents owing to the increase in the carrier concentration, while the lattice thermal conductivity slightly decreased due to alloy scattering. A maximum figure of merit, ZT_max = 1.25, was achieved at 373 K for Bi_0.4Sb_1.6Te₃:Cu_0.003.     

p-Type skutterudites RxMyFe3CoSb12 (R,M = Ba,Ce, Nd,and Yb): Effectiveness of double-filling for the lattice thermal conductivity reduction

Ab-initio calculations of the resonant modes and frequencies for a number of possible fillers in p-type RFe₃CoSb₁₂ and RFe₄Sb₁₂ were carried out. The results indicate that, although the exact values of fillers’ resonant frequencies in p-type skutterudites are somewhat different from those in n-type Co-based skutterudites, the Einstein-like resonant modes of the fillers are similar to those in n-type materials. Experimentally, several pairs of the fillers were selected and double-filled p-type skutterudite compounds R_xM_yFe₃CoSb₁₂ (R, M = Ba, Ce, Nd, and Yb) were successfully synthesized. The reduction in the lattice thermal conductivity was realized by extending the range of resonant frequencies. As a result, enhanced ZT values above unity were achieved in these double-filled p-type skutterudites.     

Thermoelectric properties of tin-filled skutterudites prepared by mechanical alloying process

Sn-filled (Fe, Co)Sb₃ skutterudites of the form of Sn_yFe₃Co₅Sb₂₄ (0≤y≤1.5) were synthesized by the mechanical alloying of elemental powders followed by vacuum hot pressing. The phase transformations that occur during both mechanical alloying and vacuum hot pressing were examined by X-ray diffraction. Single-phase Sn-filled skutterudite was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. The thermoelectric properties of the hot-pressed specimens were evaluated as a function of temperature and tin content. The void filling of tin (up to y=1.0) in Fe₃Co₅Sb₂₄ appeared to increase the thermoelectric figure of merit.     

High temperature thermoelectric properties of skutterudite-Bi2Te3 nanocomposites

Nanocomposite engineering has been proved effective in diverse regimes of material research to attain a performance beyond each constituent phase. In this work, Yb-filled CoSb₃ (bulk matrix/host)-Bi_0.4Sb_1.6Te₃ (secondary inclusion) thermoelectric nanocomposites have been synthesized via an ex situ process. Bi_0.4Sb_1.6Te₃ inclusions are mainly distributed at the grain boundaries of Yb_0.2Co₄Sb₁₂ matrix in the composites. In particular, Te diffuses in situ from Bi_0.4Sb_1.6Te₃ through Yb_0.2Co₄Sb₁₂ matrix during the hot pressing process. This, combined with the grain boundary effect, results in favorable changes in the carrier concentration, carrier mobility, electrical resistivity, Seebeck coefficient, and thermal conductivity. Such synergistic changes are notably absent in the stand-alone Te-doped Yb-filled CoSb₃, suggesting the key role of diffusion and grain boundaries. As a result, a maximum ZT value of 0.96 has been attained for Yb_0.2Co₄Sb₁₂-2 wt% Bi_0.4Sb_1.6Te₃ at 650 K. The present work opens a new avenue towards high performance thermoelectric composites via controlled inter-constituent diffusion and grain boundary effect.     

Electronic transport properties of Te-doped CoSb<Subscript>3</Subscript> skutterudites prepared by hot pressing

Te-doped CoSb₃ (CoSb_3−yTe_y) skutterudites were prepared by hot pressing and their electronic transport properties examined. A single δ-phase was successfully obtained. The Seebeck and Hall coefficients confirmed that all the Te-doped CoSb₃ showed n-type conduction. The Te atoms successfully acted as electron donors by substitution of the Sb atoms. The carrier concentration increased an order of 10²⁰ cm⁻³ by Te doping, whereas the carrier mobility decreased as the doping content increased. The Seebeck coefficient and electrical resistivity decreased with an increase in the Te content. The doping considerably reduced the thermal conductivity due to electron-phonon scattering. The lattice contribution was dominant over the electronic contribution.     

Effects of Te doping on the transport and thermoelectric properties of Zn4Sb3

The transport and thermoelectric properties of Te-doped Zn₄Sb₃ compounds, Zn₄(Sb_1?xTe_x)₃ (x = 0, 0.005 and 0.02), were investigated. The results showed that thermal conductivity of the Te-doped compounds were reduced remarkably as compared to that of Zn₄Sb₃ presumably due to enhanced impurity (dopant) scattering of phonons. Thermopower S was found to decrease with increase in Te content, which could be ascribed to the excess Zn in the doped compounds acting as p-type dopant that leads to an increase in the carrier concentration. Moreover, it was found that the β to α phase transition of Zn₄Sb₃ could be completely prohibited by Te doping. The figure of merit, ZT, for doped compounds was greater than the un-doped Zn₄Sb₃ for the temperature range investigated. In particular, the ZT of Zn₄(Sb_0.995Te_0.005)₃ reached a value of 1.08 at 680 K, which is 69% greater than that of the un-doped Zn₄Sb₃ obtained in this study.     

The Microstructure and Thermoelectric Properties of Yb-Filled Skutterudite Yb<Subscript>0.1</Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Under Cyclic Thermal Loading

This paper is devoted to investigating the microstructure and thermoelectric properties of Yb-filled skutterudite Yb_0.1Co₄Sb₁₂ under a cyclic thermal loading from room temperature to 773 K. The results indicate after 1000 cycles, the surface morphology changes dramatically, and clear grain boundaries appear on the surface of the sample. The grain sizes of the sample change little after 1000 cycles, and the main phase is still skutterudite; however, a trace amount of YbSb also exists. In addition, the electrical conductivity and thermal conductivity decrease distinctly after 1000 cycles, but the absolute value of the Seebeck coefficient increases a little. Consequently, the ZT value decreases slightly from 0.75 at 800 K before cycling to 0.69 after 1000 cycles. It indicates that the effect of the cyclic thermal loading on the ZT of the Yb_0.1Co₄Sb₁₂ material is not distinct.     

Enhanced thermoelectric cooling properties of Bi2Te3−xSex alloys fabricated by combining casting,milling and spark plasma sintering

Bi₂Te_3−xSe_x alloys are extensively used for thermoelectric cooling around room temperature, but, previous studies have reported peak thermoelectric efficiency of the material at higher temperature around 450 K. This study presents the casting followed by high energy ball milling and spark plasma sintering as a thriving methodology to produce efficient and well-built Bi₂Te_3−xSe_x material for the thermoelectric cooling around room temperature. In addition, changes in electrical and thermal transport properties brought up by amount of Se in the Bi₂Te_3−xSe_x material for this methodology are measured and discussed. Although Seebeck coefficient and electrical conductivity showed irregular trend, power factor, thermal conductivity and figure of merit ZT gradually decreased with the increase in amount of Se. A maximum ZT value of 0.875 at 323 K was obtained for x = 0.15 sample owing to its higher power factor. This value is 17% and 38% greater than for x = 0.3 and x = 0.6 samples respectively. At 323 K, herein reported ZT value of 0.875 is higher than the state of art n-type Bi₂Te₃ based thermoelectric materials produced by the time consuming and expensive methodologies.     

Thermoelectric properties of Fe0.2Co3.8Sb12−xTex skutterudites

Skutterudites Fe_0.2Co_3.8Sb_12?xTe_x (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by induction melting at 1273 K, followed by annealing at 923 K for 144 h. X-ray powder diffraction and electron microprobe analysis confirmed the presence of the skutterudite phase as the main phase. The temperature-dependent transport properties were measured for all the samples from 300 to 818 K. A positive Seebeck coefficient (holes are majority carriers) was obtained in Fe_0.2Co_3.8Sb₁₂ in the whole temperature range. Thermally excited carriers changed from n-type to p-type in Fe_0.2Co_3.8Sb_11.9Te_0.1 at 570 K, while in all the other samples, Fe_0.2Co_3.8Sb_12?xTe_x (x = 0.2, 0.3, 0.4, 0.5, 0.6) exhibited negative Seebeck coefficients in the entire temperature range measured. Whereas for the alloys up to x = 0.2 (Fe_0.2Co_3.8Sb_11.8Te_0.2) the electrical resistivity decreased by charge compensation, it increased for x > 0.2 with an increase in Te content as a result of an increase in the electron concentration. The thermal conductivity decreased with Te substitution owing to carrier–phonon scattering and point defect scattering. The maximum dimensionless thermoelectric figure of merit, ZT = 1.04 at 818 K, was obtained with an optimized Te content for Fe_0.2Co_3.8Sb_11.5Te_0.5 and a carrier concentration of ～n = 3.0 × 10²⁰ cm^?3 at room temperature. Thermal expansion (α = 8.8 × 10^?6 K^?1), as measured for Fe_0.2Co_3.8Sb_11.5Te_0.5, compared well with that of undoped Co₄Sb₁₂. A further increase in the thermoelectric figure of merit up to ZT = 1.3 at 820 K was achieved for Fe_0.2Co_3.8Sb_11.5Te_0.5, applying severe plastic deformation in terms of a high-pressure torsion process.     

Thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1−x prepared by spark plasma sintering

High performance (Bi₂Te₃)_x(Sb₂Te₃)_1?x bulk materials have been prepared by combining fusion technique with spark plasma sintering, and their thermoelectric properties have been investigated. With the increase of Bi₂Te₃ content, the electrical resistivity and Seebeck coefficient increase greatly and the thermal conductivity decreases signi?cantly, which lead to a great improvement in the thermoelectric ?gure of merit ZT. The maximum ZT value reaches 1.33 at 398 K for the composition of 20% Bi₂Te₃–80%Sb₂Te₃ with 3 wt% excess Te.     

Thermoelectric properties of P-type Yb-filled skutterudite YbxFeyCo4-ySb12

While intensive work has been done on n-type Yb filled skutterudites in the past, very little is known about their p-type counterparts for potential applications as thermoelectric materials. In this paper, we report a systematic study of high temperature thermoelectric transport properties of p-type Yb-filled Fe-compensated skutterudites Yb_xFe_yCo_4-ySb₁₂ with the aim to complement the knowledge base for the Yb-filled skutterudite family. The highest ZT_max = 0.6 was found in Yb_0.6Fe₂Co₂Sb₁₂ at 782 K. YbFe₄Sb₁₂ exhibits the second highest ZT_max = 0.57 at 780 K, which is much higher than the previous estimate of 0.4 for the same composition.     

Thermoelectric properties of semi-conducting compound Zn4Sb3

Hot-pressed samples of the semi-conducting compound Zn₄Sb₃ with the stoichiometric composition were prepared and characterized by X-ray and microprobe analysis. Thermoelectric characterization was done through measurements of the electrical and thermal conductivities as well as the Seebeck coefficient between room temperature and 650 K. All samples had p-type conductivity. High thermoelectric figures of merit (ZT) were obtained between 450 and 650 K and a maximum of about 1.3 were obtained at a temperature of 650 K.     

High-pressure synthesis of phonon-glass electron-crystal featured thermoelectric LixCo4Sb12

Li-filled CoSb₃, which is inaccessible under ambient pressure, was successfully synthesized with a high-pressure synthesis technique, demonstrating a fast and effective way to broaden elemental species that can be filled into voids of skutterudites. The optimized Li_0.36Co₄Sb₁₂, with a greatly enhanced thermal power factor and much reduced thermal conductivity, has a ZT value of 1.3 at 700 K, the highest among all single elemental filled CoSb₃ materials at this temperature. In addition, an instructive linear relationship between the Einstein temperatures of the distinct rattling fillers and their ionic radii is revealed, which as a reference can easily be applied to the multiple elemental filling strategy for selecting suitable filling elemental species to reduce the lattice thermal conductivity more effectively.     

Solvothermal synthesis and thermoelectric properties of skutterudite compound Fe（0.25）Ni（0.25）Co（0.5）Sb3

Nanostructured skutterudite-related compound Fe_0.25Ni_0.25Co_0.5Sb₃ was synthesized by a solvothermal method using FeCl₃, NiCl₂, CoCl₂, and SbCl₃ as the precursors and NaBH₄ as the reductant. The solvothermally synthesized powders consisted of fine granules with an average particle size of tens of nanometers. The bulk material was prepared by hot pressing the powders. Transport property measurements indicated a heavily doped semiconductor behavior with n-type conduction. The thermal conductivity is about 1.83 W·m⁻¹·K⁻¹ at room temperature and decreases to 1.57 W·m⁻¹·K⁻¹ at 673 K. The low thermal conductivity is attributed to small grain size and high porosity. A maximum dimensionless figure of merit of 0.15 is obtained at 673 K.     

Substitution of Antimony by Tin and Tellurium in n-Type Skutterudites CoSb2.8Sn x Te0.2−x

We have studied the substitution of antimony by tin and tellurium in n-type skutterudites CoSb_2.8Sn _x Te_0.2?x . The samples were made by ball milling ingots and hot pressing the ball-milled nanopowder. Rather than filling the cage of the structure, we aimed to use disorder in pnictogen rings by elemental substitution of Sb by Sn and Te. In skutterudite CoSb₃, the dominant heat-carrying phonons are associated with the vibrational modes of the Sb-rings; disorder in the rings can be an effective way to suppress the thermal transport. By suitably tuning the contents of Sn and Te in the skutterudites, we have suppressed the thermal conductivity and achieved a power factor of ~42 μW cm^?1 K^?2 at 530°C. A peak thermoelectric figure of merit (ZT) reaches ~1.1 at 530°C for CoSb_2.8Sn_0.005Te_0.195. This ZT value is comparable with that of some of the single-filled skutterudites.