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.     

Effects of Cooling Rate on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>(Se<Subscript>0.4</Subscript>Te<Subscript>0.6</Subscript>)<Subscript>3</Subscript> Compounds

A series of Bi₂(Se_0.4Te_0.6)₃ compounds were synthesized by a rapid route of melt spinning (MS) combined with a subsequent spark plasma sintering (SPS) process. Measurements of the Seebeck coefficient, electrical conductivity, and thermal conductivity were performed over the temperature range from 300 K to 520 K. The measurement results showed that the cooling rate of melt spinning had a significant impact on the transport properties of electrons and phonons, effectively enhancing the thermoelectric properties of the compounds. The maximum ZT value reached 0.93 at 460 K for the sample prepared with the highest cooling rate, and infrared spectrum measurement results showed that the compound with lower tellurium content, Bi₂(Se_0.4Te_0.6)₃, possesses a larger optical forbidden gap (E _g) compared with the traditional n-type zone-melted material with formula Bi₂(Se_0.07Te_0.93)₃. Our work provides a new approach to develop low-tellurium-bearing Bi₂Te₃-based compounds with good thermoelectric performance.     

Fabrication of an Fe<Subscript>80.5</Subscript>Si<Subscript>7.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>5</Subscript>Cu Amorphous-Nanocrystalline Powder Core with Outstanding Soft Magnetic Properties

In this study, the melt spinning method was used to develop Fe_80.5Si_7.5B₆Nb₅Cu amorphous ribbons in the first step. Then, the Fe_80.5Si_7.5B₆Nb₅Cu amorphous-nanocrystalline core with a compact microstructure was obtained by multiple processes. The main properties of the magnetic powder core, such as micromorphology, thermal behavior, permeability, power loss and quality factor, have been analyzed. The obtained results show that an Fe_80.5Si_7.5B₆Nb₅Cu amorphous-nanocrystalline duplex core has high permeability (54.8–57), is relatively stable at different frequencies and magnetic fields, and the maximum power loss is only 313 W/kg; furthermore, it has a good quality factor.     

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.     

Thermoelectric power of Bi<Subscript>92</Subscript>Sb<Subscript>8</Subscript> and Bi<Subscript>85</Subscript>Sb<Subscript>15</Subscript> thin films

The results of studying the galvanomagnetic and thermoelectric properties of thin block Bi₉₂Sb₈ and Bi₈₅Sb₁₅ films on mica and polyimide substrates are presented. The method used for measuring the thermoelectric power allowed us to study the temperature dependence the thermoelectric power, without introducing additional deformations into the substrate–film system. A significant difference in the temperature dependences of the galvanomagnetic and thermoelectric properties of films on mica and polyimide is found. The free charge-carrier concentrations and mobilities in the films on mica and polyimide and levels of the chemical potential for electrons and holes are calculated within the two-band approximation. The difference in the charge-carrier parameters for films on mica and polyimide is associated with strains in the film–substrate system.     

Influence of Group IV-Te Alloying on Nanocomposite Structure and Thermoelectric Properties of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Compounds

Thermoelectric compounds based on doped bismuth telluride and its alloys have recently attracted increasing interest. Due to their structural features they show increased values of the thermoelectric figure of merit (ZT). A promising approach to improve the thermoelectric properties is to manufacture nanocomposite materials exhibiting lower thermal conductivities and higher ZT. The ZT value of compounds can be shifted reasonably to higher values (>1) by alloying with IV-Te materials and adequate preparation methods to form stable nanocomposites. The influence of PbTe and Sn on the thermoelectric properties is studied as a function of concentration and preparation methods. Melt spinning and spark plasma sintering were applied to form nanocomposite materials that were mechanically and thermodynamically stable for applications in thermoelectric devices. The structural properties are discussed based on analysis by transmission electron microscopy and x-ray diffraction.     

Properties of CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals and In/CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

Using the method of planar crystallization from the melt with deviations from the stoichiometric composition, p-CuIn₃Se₅ single crystals are grown. The electrical properties of the homogeneous crystals are studied. It is found that the resistivity of the p-CuIn₃Se₅ crystals depends on the excess Se content in the melt. It is established that the voltaic photosensitivity of the In/CuIn₃Se₅ structures is enhanced with an increasing excess of Se content in the melt. The energy spectrum and the character of interband transitions in the CuIn₃Se₅ crystals are discussed. It is concluded that the CuIn₃Se₅ ternary compound can be used in high efficiency photoelectric converters of solar radiation.     

Thermoelectric Properties of Spark Plasma-Sintered In<Subscript>4</Subscript>Se<Subscript>3</Subscript>-In<Subscript>4</Subscript>Te<Subscript>3</Subscript>

We report the thermoelectric properties of spark plasma-sintered In₄Se₃-In₄Te₃ materials. For comparison, pure In₄Se₃ and In₄Se₃ (80 wt.%)/In₄Te₃ (20 wt.%) mixture samples were prepared. In₄Se₃ and In₄Te₃ powders were synthesized by a conventional melting process in evacuated quartz ampoules, and a spark plasma method was used for the sintering of the pure In₄Se₃ and mixture samples. Thermoelectric and structural characterizations were carried out, and the mixing effect of In₄Se₃ and In₄Te₃ on the thermoelectric properties was investigated.     

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.     

Low-Temperature Thermoelectric Properties of Co<Subscript>0.9</Subscript>Fe<Subscript>0.1</Subscript>Sb<Subscript>3</Subscript>-Based Skutterudite Nanocomposites with FeSb<Subscript>2</Subscript> Nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher ZT due to effects arising from their nanostructure. Herein, we report low-temperature thermoelectric properties of Co_0.9Fe_0.1Sb₃-based skutterudite nanocomposites containing FeSb₂ nanoinclusions. These nanocomposites can be easily synthesized by melting and rapid water quenching. The nanoscale FeSb₂ precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to regular iron-substituted skutterudite samples. As a result, our best nanocomposite sample boasts a ZT = 0.041 at 300 K, which is nearly three times as large as that for Co_0.9Fe_0.1Sb₃ previously reported.     

Thermoelectric properties of the Mg<Subscript>2</Subscript>Ge<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript> solid solution with <Emphasis Type="Italic">p</Emphasis>-type conductivity

The thermoelectric properties of the Mg₂Ge_0.3Sn_0.7 solid solution doped with Ga and Li are studied. The samples with a hole concentration as high as 5 × 10²⁰ cm^–3 are obtained. The temperature dependences of the thermopower, electrical conductivity, and thermal conductivity are measured in the range from room temperature to 800 K. A higher mobility of free charge carriers is observed in the samples doped with lithium than in the samples doped with gallium. The largest thermoelectric figure of merit in the samples under study amounts to 0.42 at 700 K.     

First-Principles Study of Semiconducting Clathrate Ba<Subscript>8</Subscript>Al<Subscript>16</Subscript>Ge<Subscript>30</Subscript>

We calculated electronic structures of Ba₈Al₁₆Ge₃₀ in some Al-Ge framework configurations without nearest-neighbor Al-Al bonds by using a first-principles method. The calculated band structures are similar in outline but different in detail. We also calculated thermoelectric properties by using the electronic structures to analyze the experimental results on a sintered Ba₈Al₁₆Ge₃₀ sample. The calculated properties nearly agree with the experimental results; however, the calculated temperature dependences of electrical conductivity are slightly different from one another, because of differences in electronic structure. In this paper, we discuss the temperature dependence from the viewpoints of nonparabolic band effects.     

Effect of Ca Doping on the Thermoelectric Performance of Yb<Subscript>14</Subscript>MnSb<Subscript>11</Subscript>

Complex Zintl phases possess low thermal conductivity and can be easily doped to modify the transport properties. Therefore, these phases have the potential to be good thermoelectric materials by simply controlling carrier concentration. Yb₁₄MnSb₁₁ is a Zintl phase that has shown promise as a p-type thermoelectric material for high-temperature power generation. A Sn-flux synthetic route was used to make the new phase, Yb₁₃CaMnSb₁₁. The high-temperature thermoelectric properties were measured on polycrystalline hot-pressed pellets and compared with Yb₁₄MnSb₁₁. Substitution of the lighter isovalent Ca for Yb should reduce the lattice thermal conductivity by mass disorder scattering, and a noticeable reduction is seen in thermal diffusivity measurements at high temperature. There may also be a carrier concentration effect by employing the more electropositive Ca.     

Preparation and properties of Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>-based thermoelectric material

The effect of synthesis conditions on the structure and thermoelectric properties of zinc-antimonide- based materials is investigated. The effects of Zn excess, the modes of spark plasma sintering, and In doping on the phase composition and the thermal stability of the properties of the obtained material are considered. The material is prepared by the method of the direct alloying of components and spark plasma sintering. It is shown that, at certain modes of spark plasma sintering, the introduction of an excess amount of Zn and In doping make it possible to obtain β-Zn₄Sb₃ with the thermoelectric efficiency ZT ≈ 1.47 at a temperature of 720 K, which shows the stability of characteristics under the performed tests.     

Anomalous thermoelectric power in Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals

Based on data on the Hall coefficient, it is shown that the existence of potential barriers in the region of impurity conductivity of highly compensated Hg₃In₂Te₆ crystals is possible. The role of barriers in the anomalous behavior of transport phenomena is discussed qualitatively. Extremely large values of the thermoelectric power are related to the combination of thermoelectric powers of contact potentials for regions with different concentrations of electrons.     

Crystal Structure and High-Temperature Thermoelectric Properties of the Mo<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>Ru<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript>7</Subscript> Compounds

Zintl phases are currently receiving great attention for their thermoelectric potential typified by the discovery of a high ZT value in Yb₁₄MnSb₁₁-based compounds. Herein, we report on the crystallographic characterization via neutron and x-ray diffraction experiments, and on the thermoelectric properties measured in the 300 K to 1000 K temperature range, of Mo₃Sb₇ and its isostructural compounds Mo_3−x Ru _x Sb₇. Even though Mo₃Sb₇ displays rather high ZT values given its metallic character, the partial substitution of Mo by Ru substantially improves its thermoelectric properties, resulting in a ZT value of ∼0.45 at 1000 K for x = 0.8.     

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 Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript> and Ni<Subscript>0.06</Subscript>Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript>

Mo₃Sb₇, crystallizing in the Ir₃Ge₇ type structure, has poor thermoelectric (TE) properties due to its metallic behavior. However, by a partial Sb-Te exchange, it becomes semiconducting without noticeable structure changes and so achieves a significant enhancement in the thermopower with the composition of Mo₃Sb₅Te₂. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the thermal conductivity by the so-called rattling effect. As part of the effort to verify this idea, we report herein the growth as well as measurements of the thermal and electrical transport properties of Mo₃Sb_5.4Te_1.6 and Ni_0.06Mo₃Sb_5.4Te_1.6.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Thermoelectric Properties of <Emphasis Type="Italic">p</Emphasis>-Type Mg<Subscript>2.00</Subscript>Si<Subscript>0.25</Subscript>Sn<Subscript>0.75</Subscript> with Li and Ag Double Doping

Mg₂Si_1−x Sn _x -system solid solutions are ecofriendly semiconductors that are promising materials for thermoelectric generators in the middle temperature range. To produce a thermoelectric device, high-performance p- and n-type materials must be balanced. In this paper, p-type Mg_2.00Si_0.25Sn_0.75 with Li and Ag double doping was prepared by the liquid–solid reaction method and hot-pressing. Effects of Li and Ag double doping on thermoelectric properties were investigated in the temperature range from room temperature to 850 K. All sintered compacts were identified as single-phase solid solutions with anti-fluorite structure. The carrier concentration increased with the double doping. The temperature dependence of resistivity of the double-doped samples was similar to that of a metal. The seebeck coefficient increased with temperature to a maximum value and then decreased in the intrinsic region. Thermal conductivity decreased linearly with increasing temperature, reaching a minimum near the intrinsic region, and then increased rapidly because of the contribution of the bipolar component. The dimensionless figure of merit reached 0.32 at 610 K for Mg_2.00Si_0.25Sn_0.75 double-doped with Li-5000 ppm and Ag-20000 ppm.