Effect of Transition-Metal Additives on Thermoelectric Properties of YB<Subscript>22</Subscript>C<Subscript>2</Subscript>N

The higher boride compound YB₂₂C₂N has been reported as a promising n-type high-temperature thermoelectric material and possible counterpart to boron carbide. To investigate the influence of transition-metal additives on the thermoelectric properties of YB₂₂C₂N, a series of Rh, Co, Cu, and Ni samples were prepared. The resistivity and Seebeck coefficient of the samples were measured in the temperature range of 323 K to 1073 K. Samples with Rh and Co additives showed a considerable reduction of resistivity in comparison with pure YB₂₂C₂N and maintained their semiconducting properties at high temperatures. A sample with Co, obtained using long-term ball milling, showed the highest absolute value of Seebeck coefficient among all previously studied YB₂₂C₂N-based materials. Analyses of the influence of transition-metal additives and processing methods such as ball milling on the thermoelectric properties of YB₂₂C₂N are presented.     

Solvent-Based Synthesis of <Emphasis Type="Italic">Nano</Emphasis>-Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript> for Low-Temperature Thermoelectric Applications

In this study we show a preparation method for nanostructured Bi_0.85Sb_0.15 powders via a chemical reduction route in a polyol medium, yielding material with particle sizes of 20–150 nm in scalable amounts. The powders were consolidated by spark plasma sintering (SPS) in order to maintain the nanostructure. To investigate influence of the sinter process, the powders were characterized by x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and scanning electron microscopy (SEM) measurements before and after SPS. Transport properties, Seebeck effect, and thermal conductivity were determined in the low temperature range below 300 K. The samples showed excellent thermal conductivity of 2.3–2.6 W/m × K at 300 K and Seebeck coefficients from ?97 μV/K to ?107 μV/K at 300 K with a maximum of ?141 μV/K at 110 K, thus leading to ZT values of up to 0.31 at room temperature. The results show that Bi-Sb-alloys are promising materials for low-temperature applications. Our wet chemical approach gives access to scalable amounts of nano-material with increased homogeneity and good thermoelectric properties after SPS.     

Thermoelectric Characterization of Zone-Melted and Quenched Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>

Because of its complex structure, Zn₄Sb₃ exhibits relatively low thermal conductivity. This, in combination with large values of the Seebeck coefficient and moderate to high electrical conductivity, makes the material especially interesting for thermoelectric application in temperatures up to 400°C. The phase purity and thermal stability of Zn₄Sb₃ are major issues for its thermoelectric performance and are strongly dependent on the synthesis method, atmosphere, density, and grain size. Therefore, Zn₄Sb₃ was prepared by both zone melting and quenching in this study, and pressed samples from crushed powders of three different grain sizes were compared. The effect of thermal cycling was studied, along with repeated structural analysis and Seebeck mapping. It was found that zone melting leads to improved thermal stability regarding decomposition via Zn loss, which finally may result in the formation of ZnSb. Larger grain size seems to reduce the degradation, because of lower concentration of grain boundaries, thus hindering diffusion inside the material.     

Thermoelectric properties of Bi<Subscript>2</Subscript>Te<Subscript>2.4</Subscript>Se<Subscript>0.6</Subscript> solid solutions of different particle-size composition

The thermoelectric properties of n-type Bi₂Te_2.4Se_0.6 solid solution prepared by the vacuum hot pressing of powder mixtures with different particle sizes are investigated. The powders were prepared by the mechanical grinding of ingots and melt spinning. The microstructure and fracture pattern of a sample cleavage surface are analyzed using scanning electron microscopy and optical microscopy. The thermoelectric characteristics (the Seebeck coefficient, electrical conductivity, and thermal conductivity) are measured at room temperature and in the temperature range of 100–700 K.     

CeB<Subscript>6</Subscript> thin films produced on different substrates by electron-beam deposition

Films of cerium hexaboride, a material promising for use in thermoelectric devices at liquidhelium temperatures, are produced by electron-beam deposition. Deposition is carried out from ceramic targets onto insulator, semiconductor, and metal substrates at different temperatures. The microstructure, the elemental and phase compositions, the temperature dependences of the resistivity and the Seebeck coefficient are thoroughly studied. CaB₆-structured films, for which the structure is characteristic of cerium hexaboride and the elemental composition is close to the stoichiometric composition, are obtained. At low temperatures, the resistivity of the films is somewhat higher than that of single-crystal samples, and the Seebeck coefficient is close to the corresponding coefficient for single-crystal samples. The main cause of the difference between the resistance values is a high concentration of oxygen impurity detected in the films.     

Thermoelectric Properties of Chevrel-Phase Sulfides M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript>6</Subscript>S<Subscript>8</Subscript> (M: Cr,Mn, Fe,Ni)

Chevrel-phase sulfides M _x Mo₆S₈ (M, Cr, Mn, Fe, Ni; x: 1.3, 2.0) were prepared by reacting appropriate amounts of M, Mo, and MoS₂ powders. The samples were then consolidated by pressure-assisted sintering to fabricate dense compacts. While Cr_1.3Mo₆S₈ crystallized in a triclinic structure, Mn_1.3Mo₆S₈, Fe_1.3Mo₆S₈, and Ni_2.0Mo₆S₈ crystallized in a hexagonal structure. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the sintered samples were measured over the temperature range of 300 K to 973 K. All the samples exhibited a positive Seebeck coefficient. The Seebeck coefficient, electrical resistivity, and thermal conductivity of M_1.3Mo₆S₈ (M: Cr, Mn, Fe) were almost identical and increased with temperature. However, the corresponding values and temperature dependent behavior of Ni_2.0Mo₆S₈ were different from those of M_1.3Mo₆S₈ (M: Cr, Mn, Fe). For Ni_2.0Mo₆S₈, as temperature increased, the Seebeck coefficient and thermal conductivity increased while the electrical resistivity decreased. The highest value of the thermoelectric figure of merit (0.17) was observed in Cr_1.3Mo₆S₈ at 973 K.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Effects of Microstructural Evolution on the Thermoelectric Properties of Spark-Plasma-Sintered Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Compound

The MNiSn (M = Ti, Zr, Hf) half-Heusler semiconducting compounds are widely investigated due to their good potential for thermoelectric (TE) power generation applications. In the current work, the evolution of the transport and structural properties of the Ti_0.3Zr_0.35Hf_0.35NiSn compound upon various thermal treatments was studied. The nominal composition was arc melted, ball milled, and spark plasma sintered (SPS). Following SPS, large Hf-rich domains were found by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Subsequently, the samples were subjected to homogenization treatments at 1163 K for 480 h and 610 h under argon atmosphere. Following these thermal treatments, the relative amount of the Hf-rich domains was reduced and they became smaller in size, with increasing thermal treatment duration. Nevertheless, no uniphased structure was reached. The dissolution of the Hf-rich domains in the half-Heusler matrix resulted in increase of both the Seebeck coefficient and electrical resistivity values and a decrease of the carrier concentration, attributed to elimination of these metallic domains. Resulting from the high atomic disorder caused by substitution at the M site, low thermal conductivity values of ~3.8 W m^?1 K^?1 were obtained leading to high ZT values of up to 0.82 following SPS.     

Preparation and Thermoelectric Properties of Nanoporous Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Alloys

n-Type nanoporous Bi₂Te₃-based thermoelectric materials with different porosity ratios have been prepared by spark plasma sintering (SPS). The microstructure and phase morphology have been analyzed by x-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), and the thermoelectric properties of the SPS samples have been measured. Experimental results show that the nanoporous structures lying in the sheet layers and among the plate grains of the Bi₂Te₃ bulk material can lead to an increase in the Seebeck coefficient and a decrease in the thermal conductivity, thus leading to an enhanced figure of merit.     

High-Pressure Torsion to Improve Thermoelectric Efficiency of Clathrates?

High-pressure torsion (HPT), as a technique to produce severe plastic deformation, has been proven effective to improve the thermoelectric performance of skutterudites. In this report, we present microstructural and thermoelectric properties of the clathrate Ba₈Cu_3.5Ge₄₁In_1.5 processed by HPT. The sample was synthesized from high-purity elements, subsequently annealed, ball milled, and hot pressed, and finally subject to HPT. Compared with the ball-milled and hot-pressed sample, the HPT-processed sample has higher electrical resistivity and Seebeck coefficient, and lower thermal conductivity, electron concentration, and mobility, which is attributed to the reduced grain size and increased density of dislocations, point defects, and cracks. No essential improvement of the dimensionless thermoelectric figure of merit is observed in the investigated temperature range, questioning the universal versatility of this technique for improvement of thermoelectric materials.     

Preliminary Study of Single Phase Preparation and Doping Effect of <Emphasis Type="Italic">β</Emphasis>-Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>

Single phase β-Zn₄Sb₃ was prepared by the application of a two-stage heat treatment, and impurity elements were doped. The undoped and doped samples were prepared by direct melting followed by two-stage heat treatment at 450°C and 400°C after solidification of the samples in sealed quartz ampoules. Impurity doping of the samples was performed by the addition of 1 at.% of Se, In, Pb, Te, or Bi. The resulting samples were characterized by x-ray diffraction (XRD), differential thermal analysis (DTA), optical microscopy, and electron probe microanalysis, and their Seebeck coefficients were determined at room temperature. The undoped samples were determined by XRD and DTA to comprise single phase β-Zn₄Sb₃, while the doped samples were composed of multiple phases. From the measurements of the Seebeck coefficient, all samples were found to be p-type and all were found to have almost the same values. These results indicate that β-Zn₄Sb₃ has limited solubility for these impurity elements.     

Thermoelectric Properties of NaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

NaCo₂O₄ has one of the highest figures of merit among all ceramic thermoelectric materials. Because of its large thermopower and low resistivity, the ceramic oxide NaCo₂O₄ is a promising candidate for potential thermoelectric applications. NaCo₂O₄ is, moreover, a ceramic compound with high decomposition temperature and chemical stability in air and it does not contain any toxic elements. Like all 3-d transition ions, Co ions have multiple spin and oxidation states. In this investigation, thermopower and electrical conductivity of NaCo₂O₄ as a function of substitution of Co by Fe ions were measured. Fe substitution for Co causes resistivity to increase, whereas the Seebeck coefficient remained nearly invariant, especially above 330 K. An erratum to this article can be found at     

Solution-Chemical Syntheses of Nano-Structured Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and PbTe Thermoelectric Materials

In this work, nano-structured Bi₂Te₃ and PbTe thermoelectric materials were synthesized separately via solvothermal, hydrothermal and low-temperature aqueous chemical routes. X-ray diffraction (XRD), field-emission scanning-electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) were used to analyze the powder products. Results showed that the as-prepared Bi₂Te₃ samples were all single-phased and consisted of irregular spherical granules with diameters of ∼30 nm whereas the PbTe samples were mainly composed of well-crystallized cubic crystals with average size of approximately 100 nm. Some nanotubes and nanorods were found in Bi₂Te₃ and PbTe samples, respectively; these were identified as Bi₂Te₃ nanotubes and PbTe nanorods by EDS analysis. Possible reaction mechanisms for these syntheses are discussed in detail herein.     

Low-Temperature Transport Properties of Sn<Subscript>24</Subscript>P<Subscript>19.3</Subscript>Br<Subscript>8</Subscript> and Sn<Subscript>17</Subscript>Zn<Subscript>7</Subscript>P<Subscript>22</Subscript>Br<Subscript>8</Subscript>

We report on temperature-dependent thermal conductivity, resistivity, and Seebeck coefficient of two polycrystalline Br-containing Sn-clathrate compounds with the type I crystal structure. Interstitial Br atoms reside inside the polyhedral cavities formed by the framework, resulting in hole conduction. The framework bonding directly influences the transport properties of these two compositions. The transport properties of these two clathrates are compared with those of other Sn-clathrates. We also discuss our results in terms of the potential for thermoelectric applications.     

Thermal Stress Analysis and Structure Parameter Selection for a Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Thermoelectric Module

The output power and conversion efficiency of thermoelectric modules (TEMs) are mainly determined by their material properties, i.e., Seebeck coefficient, electrical resistivity, and thermal conductivity. In practical applications, due to the influence of the harsh environment, the mechanical properties of TEMs should also be considered. Using the finite-element analysis (FEA) model in ANSYS software, we present the thermal stress distribution of a TEM based on the anisotropic mechanical properties and thermoelectric properties of hot-pressed materials. By analyzing the possibilities of damage along the cleavage plane of Bi₂Te₃-based thermoelectric materials and by optimizing the structure parameters, a TEM with better mechanical performance is obtained. Thus, a direction for improving the thermal stress resistance of TEMs is presented.     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Theoretical Study of Electronic Structure and Thermoelectric Properties of Doped CuAlO<Subscript>2</Subscript>

The doping level dependence of thermoelectric properties of delafossite CuAlO₂ has been investigated in the constant scattering time (τ) approximation, starting from the first principles of electronic structure. In particular, the lattice parameters and the energy band structure were calculated using the total energy plane-wave pseudopotential method. It was found that the lattice parameters of CuAlO₂ are a = 2.802 Å and c = 16.704 Å, and the internal parameter is u = 0.1097. CuAlO₂ has an indirect band gap of 2.17 eV and a direct gap of 3.31 eV. The calculated energy band structures were then used to calculate the electrical transport coefficients of CuAlO₂. By considering the effects of doping level and temperature, it was found that the Seebeck coefficient S(T) increases with increasing acceptor doping (A _d) level. The values of S(T) in our experiments correspond to an A _d level at 0.262 eV, which is identified as the Fermi level of CuAlO₂. Based on our experimental Seebeck coefficient and the electrical conductivity, the constant relaxation time is estimated to be 1 × 10^?16 s. The power factor is large for a low A _d level and increases with temperature. It is suggested that delafossite CuAlO₂ can be considered as a promising thermoelectric oxide material at high doping and high temperature.     

High-Performance (Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>SbTe<Subscript><Emphasis Type="Italic">x</Emphasis>/2+1.5</Subscript>)<Subscript>15</Subscript>(GeTe)<Subscript>85</Subscript> Thermoelectric Materials Prepared by Melt Spinning

A new preparation process combining melt spinning and hot pressing has been developed for the (Ag _x SbTe _x/2+1.5)₁₅(GeTe)₈₅ (TAGS-85) system. Compared with samples prepared by the traditional air-quenching and hot-pressing method, electrical conductivity and thermal conductivity are lowered. The thermoelectric performance of the TAGS-85 samples varied with changing Ag content and reached the highest ZT of 1.48 when x was 0.8 for the melt-spun sample, compared with the maximum ZT of 1.36 for the air-quenched sample. The Seebeck coefficient of the melt-spun TAGS-85 alloys was improved, while both the electrical conductivity and thermal conductivity were decreased. The net result of this process is to effectively enlarge the temperature span of ZT > 1, which will benefit industrial application.     

Strain-Controlled Transport Mechanism in Strongly Correlated LaNiO<Subscript>3</Subscript>

A density functional theory + Hubbard U (DFT + U) method is employed to investigate the effect of strain on the electronic and transport properties of the correlated metal LaNiO₃. LaNiO₃ without strain is characterized by a low temperature Fermi liquid behaviour of resistivity, a negative Seebeck coefficient and a positive Hall coefficient. Density of states, resistivity, thermopower and Hall coefficient obtained within the DFT + U approach reveal that LaNiO₃ under both compressive and tensile strain is more metallic compared to the unstrained system. However, LaNiO₃ under tensile strain is found to be more strongly correlated than that under compressive strain. Electron localization function calculation shows that there is a substantial increase in the covalent part of the chemical bonding, which corroborates an increase in the resistivity for LaNiO₃ under tensile strain. Our first-principle–based calculation clearly demonstrates that the transport properties of LaNiO₃ can be tuned by applying suitable strain.     

Cage-Shaped Mo<Subscript>9</Subscript> Chalcogenides: Promising Thermoelectric Materials with Significantly Low Thermal Conductivity

Thermoelectric properties of molybdenum selenides containing Mo₉ clusters have been investigated between 300 K and 800 K. Ag _x Mo₉Se₁₁ (x = 3.4 and 3.8) have been synthesized by solid-state reaction and spark plasma sintering. X-ray diffraction and scanning electron microscopy reveal high purity and good homogeneity of the samples. The thermoelectric power of the samples is positive over the whole investigated temperature range, indicating that the majority of charge carriers are holes. The Seebeck coefficient increases with temperature, and the temperature coefficient of the resistivity is positive. Significantly low thermal conductivity, comparable to values reported for state-of-the-art thermoelectric materials, is observed in this new system, and this is assumed to be associated with the rattling effect from the Ag filler atoms. It has been demonstrated that the electrical and thermal properties correlate to the Ag concentration. For x = 3.8, a promising dimensionless thermoelectric figure of merit of ∼0.7 is obtained at 800 K.