Thermoelectric properties of novel skutterudites with didymium: DDy(Fe1−xCox)4Sb12 and DDy(Fe1−xNix)4Sb12

In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe_1−xCo_x)₄Sb₁₂ and (Fe_1−xNi_x)₄Sb₁₂. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD_0.4Fe₂Co₂Sb₁₂ was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material.     

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.     

Synthesis and thermoelectric properties of double-filled skutterudites CeyYb0.5−yFe1.5Co2.5Sb12

Ce–Yb double-filled skutterudites Ce_0.5?yYb_yFe_1.5Co_2.5Sb₁₂ (y = 0.1, 0.2, 0.4 and 0.5) were synthesized by a melting method with subsequent annealing. The thermal conductivity, electrical conductivity and Seebeck coefficient were measured from room temperature up to 773 K. The thermal conductivities of all the double-filled skutterudites were found to be lower than the Yb single-filled skutterudites. An enhancement in the dimensionless thermoelectric figure of merit ZT was also observed in all the double-filled skutterudites as compared to the Yb single-filled skutterudite. Ce_0.3Yb_0.2Fe_1.5Co_2.5Sb₁₂ has the highest dimensionless figure of merit ZT of 0.32 at 723 K, which is 55% higher than the Yb single-filled skutterudite at the same temperature.     

Dependence of thermoelectric behaviour on severe plastic deformation parameters: A case study on p-type skutterudite DD0.60Fe3CoSb12

High-pressure torsion (HPT), a severe plastic deformation technique, can effectively improve the thermoelectric performance of skutterudites, resulting in ZT values higher than for ball-milled and hot-pressed (BMHP) samples. In this paper the influence of the HPT parameters, i.e. the number of revolutions (equivalent to the applied strain), the processing temperature and the hydrostatic pressure on the microstructural and thermoelectric properties of the skutterudite DD_0.60Fe₃CoSb₁₂ are evaluated and compared with the BMHP samples before HPT processing. Whilst the three parameters have specific effects on (i) the crystallite size, (ii) the density of lattice defects and (ii) the density of cracks, a suitable combination thereof allows for an increase of the figure of merit by at least 20%.     

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.     

Thermoelectric properties of semi-conducting compound CoSb3 doped with Pd and Te

Hot-pressed samples of the semi-conducting compound CoSb₃-doped Pd and Te 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 900 K. All samples had n-type conductivity. The dimensionless thermoelectric figure of merit ZT increases with increasing temperature and reaches a maximum value of 1 at 873 K.     

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.     

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 and electronic structure of p-type Mg2Si and Mg2Si0.6Ge0.4 compounds doped with Ga

Mg₂Si:Ga_x and Mg₂Si_0.6Ge_0.4:Ga_x (x = 0.4% and 0.8%) solid solutions have been synthesized by direct melting in tantalum crucibles and hot pressing. The effect of Ga doping on the thermoelectric properties has also been investigated by measurements of thermopower, electrical resistivity, Hall coefficient and thermal conductivity in temperature range from 300 to 850 K. All samples exhibit a p-type conductivity evidenced by positive sign of both thermopower and Hall coefficient in the investigated temperatures. The maximum value of the dimensionless figure of merit ZT was reached for the Mg₂Si_0.6Ge_0.4:Ga(0.8%) compound at 625 K (ZT ∼ 0.36). The p-type character of thermoelectric behaviours of Ga-doped Mg₂Si and Mg₂Si_0.6Ge_0.4 compounds well corroborates with the results of electronic structure calculations performed by the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA), since Ga diluted in Mg₂Si and Mg₂Si_0.6Ge_0.4 (on Si/Ge site) behaves as hole donor due to the Fermi level shifted to the valence band edge. The onset of large peak of DOS from Ga impurity at the valence band edge, well corroborates with high Seebeck coefficient measured in Ga-doped samples.     

Spinodal decomposition in (CaxBa1−x)yFe4Sb12

The thermoelectric and structural properties of a double filled skutterudite solid solution (Ca_xBa_1?x)_yFe₄Sb₁₂ were investigated. Using X-ray powder and X-ray micro analyses (EPMA) an immiscibility gap was established with a critical point at x ≈ 0.45 and a critical temperature that depends on the filling level (T_C = 590 ± 5 °C at y = 0.8 and T_C = 610 ± 5 °C at y = 0.9). The thermoelectric properties were measured for samples prepared in four different states: (i) a single phase solid solution (Ca_xBa_1?x)_yFe₄Sb₁₂; (ii) a two phase microcrystalline mixture of Ca_yFe₄Sb₁₂ and Ba_yFe₄Sb₁₂; (iii) a single phase structure obtained after annealing of the latter sample at 600 °C for 200 h; (iv) a spinodally demixed sample after annealing at 400 °C for 672 h. The thermoelectric properties of the phase mixture (ii) are compatible with data reported for the microcrystalline end members (Ca_yFe₄Sb₁₂ and Ba_yFe₄Sb₁₂), whilst the single phase (i and iii) and spinodally decomposed (iv) samples show increased thermopower and decreased thermal conductivity, similarly to those observed for nano-structured Ca_yFe₄Sb₁₂ and Ba_yFe₄Sb₁₂.     

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.     

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 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 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.     

The effect of Cu addition on the thermoelectric properties of Cu2CdGeSe4

Recently, research in copper based quaternary chalcogenide materials has focused on the study of thermoelectric properties due to the complexity in the crystal structure. In the present work, stoichiometric quaternary chalcogenide compounds Cu_2+xCd_1−xGeSe₄ (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by solid state synthesis. The powder X-ray diffraction patterns of all the samples showed a tetragonal crystal structure with the space group I-42m of the main phase, whereas the samples with x = 0 and x = 0.025 revealed the presence of an orthorhombic phase in addition to the main phase as confirmed by Rietveld analysis. The elemental composition of all the samples characterized by Electron Probe Micro Analyzer showed a slight deviation from the nominal composition. The transport properties were measured in the temperature range of 300 K–723 K. The electrical conductivity of all the samples increased with increasing Cu content due to the enhancement of the hole concentration caused by the substitution of Cd (divalent) by Cu (monovalent). The positive Seebeck coefficient of all the samples in the entire temperature ranges indicates that holes are the majority carriers. The Seebeck coefficient of all the samples decreased with increasing Cu content and showed a reverse trend to the electrical conductivity. The total thermal conductivity of all the samples decreased with increasing temperature which was dominated by the lattice contribution. The maximum figure of merit ZT = 0.42 at 723 K was obtained for the compound Cu_2.1Cd_0.9GeSe₄.     

A new semiconductor Al2Fe3Si3 with complex crystal structure

Al₂Fe₃Si₃, a new semiconductor with complex triclinic structure was synthesized by arc melting and spark plasma sintering, followed by heat treatment. The nominal compositions of samples have been changed to compensate Al evaporation during synthesis process, and single Al₂Fe₃Si₃ phase has been obtained with the nominal composition of Al: Fe: Si = 26: 37: 37 (6 at.% Al excess against stoichiometry). In this study, we measured the sound velocity, thermal expansion coefficient, Vickers hardness, fracture toughness, electrical conductivity, Seebeck coefficient, and thermal conductivity of the new semiconductor Al₂Fe₃Si₃. The Al₂Fe₃Si₃ sample displayed positive Seebeck coefficient from 300 to 850 K, with a maximum Seebeck coefficient of 110 μV/K at 430 K. The Debye temperature of Al₂Fe₃Si₃ was 640 K, which was similar to or higher than those of other Al, Fe, Si based thermoelectric materials, but the lattice thermal conductivity was lower, 4–5 W/mK, due to the complex crystal structure of Al₂Fe₃Si₃. The maximum ZT value was 0.06 at 580 K.     

Magnetostriction effect of Co substitution in the Nd6Fe13Si intermetallic compound

We studied the magnetostriction of Nd₆Fe_13−xCo_xSi (x = 0, 1) intermetallic compounds with tetragonal Nd₆Fe₁₃Si-type structure, using the strain gauge method in the temperature range of 77–600 K under applied magnetic fields up to 1.5 T. The anisotropic magnetostriction (Δλ) versus temperature of the studied samples has shown almost similar field-dependence behavior. Below the spin reorientation temperature (T_SR), Δλ changes its sign from positive to negative value at an applied threshold field which increases with decreasing temperature. This behavior may originate from the reduction of the magnetocrystalline anisotropy with temperature. It is also observed that absolute value of Δλ increases by Co substitution. On the other hand, the volume magnetostriction (ΔV/V) versus field shows different behavior. The ΔV/V curves of Nd₆Fe₁₂CoSi tend to have a nearly quadratic dependence on applied field near magnetic ordering temperature as expected for the parastrictive behavior. The temperature dependence of magnetostriction values is discussed based on the magnetostriction relation of the tetragonal structure to determine the signs of some of magnetostriction constants for these polycrystalline compounds.     

Structure and thermoelectric performance of β-Cu2Se doped with Fe,Ni, Mn,In, Zn or Sm

Cu_1.99A_0.01Se (A = Fe, Ni, Mn, In, Zn or Sm) alloys with high thermoelectric performance were prepared through a conventional melting, ball milling and quenching route, followed by a spark plasma sintering technique. Elemental doping did not change the structure type of Cu₂Se. All the samples showed p-type conduction. All the doping elements except Indium reduced the electric resistivity and modified the carrier concentration, leading to a significant increase in the power factor. The lattice distortion and point defects due to the substitution of Cu became new phonon scattering centers, leading to a significant decrease in thermal conductivity. All the samples except the In-doped sample obtained better thermoelectric properties compared with the undoped Cu₂Se sample. The values of the figure of merit ZT of the samples doped with Zn, Mn, Ni, Fe and Sm were 1.25, 1.28, 1.51, 1.07 and 1.07 at 823 K, respectively. In Cu_2−xNi_xSe system, High ZT value of 1.51 is obtained for the sample of x = 0.0075 and 0.010 at 823 K.     

Synthesis and thermoelectric properties of hydrochloric acid-doped polyaniline

A series of hydrochloric acid-doped polyaniline (PANI) were prepared by chemical oxidative polymerization. And the effects of HCl-doping concentration on the thermoelectric properties in the temperature range of 303–423 K were discussed. The results show that an increase in HCl-doping concentration will lead to a trend of first increase and then decrease in both the electrical conductivity and thermoelectric figure-of-merit ZT, accompanied by the opposite trend of the Seebeck coefficient. The maximum ZT can reach 2.67 × 10^?4 at 423 K when HCl-doping concentration is 1.0 M. Moreover, the temperature dependence of the electrical conductivity shows a transition from non-metallic to metallic sign with doping level increasing, while the Seebeck coefficient of all the samples has a metallic character.     

Effects of Co substitution for Fe on the glass forming ability and properties of Fe80P13C7 bulk metallic glasses

Bulk magnetic Fe_80−xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) glassy alloy rods were prepared by the combination method of fluxing treatment and J-quenching technique, and the attainable maximum diameter for fully glass formation gets to 2.5 mm for x = 5. The effects of Co substitution for Fe on the glass formation ability (GFA), thermal stability, mechanical properties and magnetic properties have been investigated systematically. It was found that the partially substitution of Co for Fe can enhance the GFA of Fe₈₀P₁₃C₇ alloy, while excessive substitution will lead to the degradation of GFA. The compressive test shows that the substitution of Co for Fe results in the decease of fracture strength, and then significantly enhance the room temperature plastic strain of the present Fe-based BMGs, which can be identified that the plastic strain at room temperature gets to 2.5% and 3.0% for x = 5 and 10, respectively. The saturation magnetization of Fe_80−xCo_xP₁₃C₇ (x = 0, 5, 10, 15, 20 at.%) BMGs firstly increases from 1.477 T to 1.550 T with increasing Co content from x = 0 to 5, and then deceases from 1.549 T to 1.519 T with increasing Co content from x = 5 to 20. The Curie temperature of the present FeCoPC BMGs quickly increases with the substitution of Co for Fe.