Enhancement of Thermoelectric Figure of Merit for Bi0.5Sb1.5Te3 by Metal Nanoparticle Decoration

Introducing nanoinclusions in thermoelectric (TE) materials is expected to lower the lattice thermal conductivity by intensifying the phonon scattering effect, thus enhancing their TE figure of merit ZT. We report a novel method of fabricating Bi_0.5Sb_1.5Te₃ nanocomposite with nanoscale metal particles by using metal acetate precursor, which is low cost and facile to scale up for mass production. Ag and Cu particles of ??40?nm were successfully near-monodispersed at grain boundaries of Bi_0.5Sb_1.5Te₃ matrix. The well-dispersed metal nanoparticles reduce the lattice thermal conductivity extensively, while enhancing the power factor. Consequently, ZT was enhanced by more than 25% near room temperature and by more than 300% at 520?K compared with a Bi_0.5Sb_1.5Te₃ reference sample. The peak ZT of 1.35 was achieved at 400?K for 0.1?wt.% Cu-decorated Bi_0.5Sb_1.5Te₃.     

Thermoelectric Properties of Nano/microstructured p-Type Bi0.4Sb1.6Te3 Powders Fabricated by Mechanical Alloying and Vacuum Hot Pressing

Two kinds of Bi_0.4Sb_1.6Te₃ powder with different particle and grain sizes were fabricated by high-energy ball milling. Powder mixtures with varied weight ratios were consolidated by vacuum hot pressing (HP) to produce nano/ microstructured composites of identical chemical composition. From measurements of the Seebeck coefficient, electrical resistivity, and thermal conductivity of these composites, a figure of merit (ZT) value of up to 1.19 was achieved at 373 K for the sample containing 40% nanograin powder. This ZT value is higher than that of monolithic nanostructured Bi_0.4Sb_1.6Te₃. It is further noted that the ZT value of this sample in the temperature range of 450 K to 575 K is in the range of 0.7 to 1.1. Such ZT characteristics are suitable for power generation applications as no other material with a similar high ZT value in this temperature range has been observed until now. The achieved high ZT value can probably be attributed to the unique nano/microstructure, in which the dispersed nanograin powder increases the number of phonon scattering sites, which in turn results in a decrease of the thermal conductivity while simultaneously increasing the electrical conductivity, owing to the existence of the microsized powder that can provide a fast carrier transportation network. These results indicate that the nano/microstructured Bi_0.4Sb_1.6Te₃ alloy can serve as a high-performance material for application in thermoelectric devices.     

Effect of Suppression of Grain Growth of Hot Extruded (Bi0.2Sb0.8)2Te3 Thermoelectric Alloys by MoS2 Nanoparticles

Nanostructured bulk materials are regarded as a means of enhancing the performance of thermoelectric (TE) materials and devices. Powder metallurgy has the distinct advantage over conventional synthesis that it can start directly from nanosized particles. However, further processing, for example extrusion, usually requires elevated temperatures, which lead to grain growth. We have found that introduction of semiconductor nanoparticles of molybdenum disulfide (MoS₂), a well-known solid lubricant, suppresses grain growth in bismuth telluride-based alloys, thus improving the extrusion process. Scanning electron microscope images show that adding MoS₂ particles at concentrations of 0.2, 0.4, and 0.8 wt% to p-type (Bi_0.2Sb_0.8)₂Te₃, under otherwise identical extrusion conditions, reduces average grain size by a factor of four. Scherer’s formula applied to x-ray diffraction data indicates that average crystallite sizes (～17 nm) of powders are not significantly different from those of alloys extruded with MoS₂ (～18 nm), which is in stark contrast with those for conventional alloy (Bi_0.2Sb_0.8)₂Te₃ extruded under the same conditions (～80 nm). Harman measurements of TE properties reveal a decrease of the thermal conductivity accompanied by reduction of the room-temperature figure of merit (ZT) from 0.9 to 0.7, because of a lower power factor. Above 370 K, however, the performance of alloys containing MoS₂ surpasses that of (Bi_0.2Sb_0.8)₂Te₃, with reduction of the thermal conductivity which is more significant at temperatures above the cross point of the ZT values.     

Effects of Excess Sb on Thermoelectric Properties of Barium and Indium Double-Filled Iron-Based p-Type Skutterudite Materials

A series of Ba and In double-filled iron-based p-type skutterudite thermoelectric (TE) materials with nominal composition BaInFe_3.7Co_0.3Sb_12+m (0.72????m????2.4) have been prepared by melting, quenching, annealing, and spark plasma sintering (SPS) methods. The effects of excess Sb on the phase composition, microstructure, and TE transport properties of these materials were investigated in this work. All the SPS bulk materials are composed of the main skutterudite phase and trace InSb and FeSb₂. The content of FeSb₂ in the SPS bulk materials gradually decreased and that of InSb remained nearly invariable with increasing m. The impurities InSb and metallic Sb are found at grain boundaries. The amount of metallic Sb at grain boundaries gradually increased with increasing m. The excess Sb had no effect on the growth of grains. The dependence of the TE properties on m indicates that preventing the formation of FeSb₂ by adjusting the excess Sb value may significantly improve the TE properties of Ba and In double-filled iron-based p-type skutterudite materials. The significant increases in the carrier concentration and electrical conductivity as well as the remarkable reduction in the lattice thermal conductivity of the sample with m?=?0.96 are due to the significant reduction in the FeSb₂ content induced by the excess Sb. The gradual increase in ZT with increasing m from 0.72 to 1.44 is attributed to the gradual decrease of the FeSb₂ content, and the gradual decrease in ZT in the m range of 1.44 to 2.4 is due to the gradual increase of the Sb content in the Sb-In alloy impurity occurring at grain boundaries. The lowest lattice thermal conductivity of 0.31?W?m^?1?K^?1 and the highest ZT value of 0.63 were obtained at 800?K for the sample with m?=?1.44.     

Thermoelectric Figure of Merit Enhancement in Bi2Te3-Coated Bi Composites

This study examines the thermoelectric behavior of composites containing hydrothermally processed tellurium-coated bismuth particles of various sizes. Since only a very thin layer of Bi₂Te₃ forms on the particle surface, the high-pressure compacted composite is still dominated by bismuth as the main ingredient (??96% Bi). Thermoelectric figure of merit ZT values are derived from measurements of thermal conductivity, electrical resistivity, and Seebeck coefficient. As expected, a ZT value almost three times higher than that of bismuth is found. This enhancement appears to be caused mainly by lowered thermal conductivity due to the significant number of grain boundaries, short phonon mean free path in the coating layers, and lattice mismatch.     

Enargite Cu3PS4: A Cu–S‐Based Thermoelectric Material with a Wurtzite‐Derivative Structure

Compound semiconductors derived from ZnS (zincblende and wurtzite) with tetrahedral framework structures have functions for various applications. Examples of such materials include Cu–S‐based materials with zincblende‐derivative structures, which have attracted attention as thermoelectric (TE) materials over the past decade. This study illuminates superior TE performance in polycrystalline samples of enargite Cu₃P_1?xGe_xS₄ with a wurtzite‐derivative structure. The substitution of Ge for P dopes holes into the top of the valence band composed of Cu‐3d and S‐3p, whereby its multiband characteristic leads to a high TE power factor. Furthermore, a reduction in the grain size to 50–300 nm can effectively decrease phonon mean free paths, leading to low thermal conductivity. These features result in a dimensionless TE figure of merit ZT of 0.5 at 673 K for the x = 0.2 sample. Environmentally benign and low‐cost characteristics of the constituent elements of Cu₃PS₄, as well as its high‐performance thermoelectricity, make it a promising candidate for large‐scale TE applications. Furthermore, this finding extends the development field of Cu–S‐based TE materials to those with wurtzite‐derivative structures.     

Thermoelectric Properties of Sn-Substituted AgPb m SbTe m+2 via the Route of Mechanical Alloying and Plasma-Activated Sintering

Starting from elemental powders of Ag, Sn, Pb, Sb, and Te, Sn-substituted single-phase LAST (AgPb _m SbTe _m+2) materials were synthesized by a combined process of mechanical alloying and plasma-activated sintering. The effect of Sn content on thermoelectric (TE) properties of the Sn-substituted LAST materials was investigated in detail. With increase of Sn/Pb ratio, the fraction of SnTe in the (Pb,Sn)Te-based solid solution increased, and the lattice spacing decreased accordingly. The electrical conductivity, thermal conductivity, and power factor also increased with increasing substitutional Sn content. It was found that the composition AgPb₉Sn₉SbTe₂₀ had superior TE properties compared with other compositions, and the maximum ZT of 0.70 was achieved at 675?K for this composition.     

Effect of Cooling Conditions on the Microstructure and Thermoelectric Properties of Zn/Si-Codoped InSb

InSb is a good candidate thermoelectric (TE) material owing to its high carrier mobility and narrow band gap around 0.18 eV. However, a high figure of merit (ZT) value has not been achieved with InSb because of its high lattice thermal conductivity (κ _lat). To reduce the κ _lat of InSb, we prepared a ZnIn₁₈SiSb₂₀ alloy by Zn/Si codoping into the In lattice sites of InSb. Polycrystalline samples of ZnIn₁₈SiSb₂₀ were prepared by a solid-state reaction method combined with hot pressing. To investigate the microstructures and TE properties resulting from different cooling conditions, samples were prepared by water quenching or slow cooling after an annealing process. The different cooling conditions led to different ZnIn₁₈SiSb₂₀ microstructures and TE properties. The electrical transport properties showed that both samples exhibited metal-like behavior and p-type conduction. The thermal conductivity values of the quenched and slow-cooled samples at room temperature were 8.7 W m^?1 K^?1 and 11.7 W m^?1 K^?1, respectively. A maximum ZT value of 0.23 was obtained at 723 K for the quenched ZnIn₁₈SiSb₂₀ sample.     

Improvement in Thermoelectric Properties of Se-Free Cu3SbS4 Compound

Recently, Cu-based chalcogenides such as Cu₃SbSe₄, Cu₂Se, and Cu₂SnSe₃ have attracted much attention because of their high thermoelectric performance and their common feature of very low thermal conductivity. However, for practical use, materials without toxic elements such as selenium are preferable. In this paper, we report Se-free Cu₃SbS₄ thermoelectric material and improvement of its figure of merit (ZT) by chemical substitutions. Substitutions of 3 at.% Ag for Cu and 2 at.% Ge for Sb lead to significant reductions in the thermal conductivity by 37% and 22%, respectively. These substitutions do not sacrifice the power factor, thus resulting in enhancement of the ZT value. The sensitivity of the thermal conductivity to chemical substitutions in these compounds is discussed in terms of the calculated phonon dispersion and previously proposed models for Cu-based chalcogenides. To improve the power factor, we optimize the hole carrier concentration by substitution of Ge for Sb, achieving a power factor of 16 μW/cm K² at 573 K, which is better than the best reported for Se-based Cu₃SbSe₄ compounds.     

Bulk Nanostructured Thermoelectric Materials: Preparation,Structure and Properties

Bulk nanostructured materials have recently emerged as a new paradigm for improving the performance of existing thermoelectric materials. Here, we fabricated two kinds of bulk nanostructured thermoelectric materials by a bottom-up strategy and an in situ precipitation method, respectively. Binary PbTe was fabricated by a combination of chemical synthesis and hot pressing. The grain sizes of the hot pressed bulk samples varied from 200 nm to 400 nm, which significantly contributed to the reduction of thermal conductivity due to the enhanced boundary phonon scattering. The highest figure of merit ZT of the binary PbTe sample reached 0.8 at 580 K. Mg₂(Si,Sn) solid solutions have shown great promise for thermoelectric application, due to good thermoelectric properties, non-toxicity, and abundantly available constituent elements. The nanoscale microstructure observation of the compounds showed the existence of nanophases formed in situ, which is believed to be related to the relatively low lattice thermal conductivity in this material system. The highest ZT of Sb-doped Mg₂(Si,Sn) samples reached 1.1 at 770 K.     

The Impact of Nanostructuring on the Thermal Conductivity of Thermoelectric CoSb3

The high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric figure‐of‐merit, ZT. A novel chemical alloying method has been used for the synthesis of nanoengineered‐skutterudite CoSb₃. The CoSb₃ powders were annealed for different durations to obtain a set of samples with different particle sizes. The samples were then compacted into pellets by uniaxial pressing under various conditions and used for the thermoelectric characterization. The transport properties were investigated by measuring the Seebeck coefficient and the electrical and thermal conductivities in the temperature range 300 K to 650 K. A substantial reduction in the thermal conductivity of CoSb₃ was observed with decreasing grain size in the nanometer region. For an average grain size of 140 nm, the thermal conductivity was reduced by almost an order of magnitude compared to that of a single crystalline or highly annealed polycrystalline material. The highest ZT value obtained was 0.17 at 611 K for a sample with an average grain size of 220 nm. The observed decrease in the thermal conductivity with decreasing grain size is quantified using a model that combines the macroscopic effective medium approaches with the concept of the Kapitza resistance. The compacted samples exhibit Kapitza resistances typical of semiconductors and comparable to those of Si–Ge alloys.     

Cost-Efficient Preparation and Enhanced Thermoelectric Performance of Bi0.48Sb1.52Te3 Bulk Materials with Micro- and Nanostructures

A cost-efficient method has been developed based on the combination of hydrothermal exfoliation and spark plasma sintering (SPS) to fabricate Bi_0.48Sb_1.52Te₃ bulk material with multiscale microstructures composed of micro- and nanosized microstructures. The thermoelectric (TE) transport properties of the bulk material with multiscale microstructures were measured along the directions parallel (||) and perpendicular (⊥) to the SPS pressing direction. It is confirmed that the anisotropy of the electrical conductivity (σ) and thermal conductivity (κ) was decreased by the transformation of the microstructure from a single microscale structure to multiscale microstructures. As compared with Bi_0.48Sb_1.52Te₃ bulk material with single microscale microstructures, the κ value of the Bi_0.48Sb_1.52Te₃ bulk material with multiscale microstructures was significantly reduced, the σ value was slightly decreased, while the α value was slightly increased. Thus, a maximum ZT value of 1.1 was achieved at 350 K along the direction perpendicular to the pressing direction, increased by 20%. The enhanced ZT value was mainly attributed to the significant decrease in κ induced by the multiscale microstructures. This work offers a new approach to improve TE performance by multiscale microstructural engineering.     

Improving the Power Factor and the Role of Impurity Bands

The thermoelectric figure of merit, Z, is proportional to the ratio of the power factor to the thermal conductivity. In the past, efforts to improve Z have largely been directed towards reduction of the thermal conductivity by lowering the lattice component, λ _L. This approach has been so successful that λ _L is now sometimes no larger than it is in a typical amorphous material. Any further improvement would require the development of thermoelectric materials with larger power factors. Here, we consider some of the ways in which the power factor might be enlarged. The carrier mobilities and density- of-states effective masses for different semiconductors are reviewed briefly, and the relevance of these properties to the power factor is discussed. It is shown that a semiconductor with the mobility and effective mass of electrons in silicon and with the minimum lattice conductivity might have a ZT value of about 6. Preferential scattering to improve the Seebeck coefficient is then considered. Finally, the effect on the power factor of a modification of the density of states through the introduction of impurity bands is calculated. It is found that such bands are not beneficial when they lie close to the edge of a main band. However, they significantly improve the power factor when they lie several kT from the band edge, and their effect can be enhanced by counterdoping.     

Thermal Stability of Barium and Indium Double-Filled Skutterudite Ba<Subscript>0.3</Subscript>In<Subscript>0.2</Subscript>Co<Subscript>3.95</Subscript>Ni<Subscript>0.05</Subscript>Sb<Subscript>12</Subscript> Coated by SiO<Subscript>2</Subscript> Nanoparticles

Filled skutterudite thermoelectric (TE) materials have been extensively studied to search for better TE materials in the past decade. However, there is no detailed investigation about the thermal stability of filled skutterudite TE materials. The evolution of microstructure and TE properties of nanostructured skutterudite materials fabricated with Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ core–shell composite particles with 3 nm thickness shell was investigated during periodic thermal cycling from room temperature to 723 K in this work. Scanning electronic microscopy and electron probe microscopy analysis were used to investigate the microstructure and chemical composition of the nanostructured skutterudite materials. TE properties of the nanostructured skutterudite materials were measured after every 200 cycles of quenching in the temperature range from 300 K to 800 K. The results show that the microstructure and composition of Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ nanostructured skutterudite materials were more stable than those of single-phase Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂ bulk materials. The evolution of TE properties indicates that the electrical and thermal conductivity decrease along with an increase in the Seebeck coefficient with increasing quenching up to 2000 cycles. As a result, the dimensionless TE figure of merit (ZT) of the nanostructured skutterudite materials remains almost constant. It can be concluded that these nanostructured skutterudite materials have good thermal stability and are suitable for use in solar power generation systems.     

Nanostructure and High Thermoelectric Performance in Nonstoichiometric AgPbSbTe Compounds: the Role of Ag

High-performance nanostructured Ag_1−x Pb_22.5SbTe₂₀ thermoelectric materials have been fabricated using mechanical alloying and spark plasma sintering. A decrease in Ag content causes a great reduction in thermal conductivity and a prominent increase in ZT value. A minimum thermal conductivity of 0.86 W/m K and a high ZT value of 1.5 (700 K) have been obtained for the Ag_0.4Pb_22.5SbTe₂₀ sample. The smaller and denser nanoscopic regions with reduced Ag content are thought to enhance phonon scattering, resulting in decreased thermal conductivity and enhanced thermoelectric performance.     

Thermoelectric Performance of AgPb x SbTe20 (x?=?17 to 23) Bulk Materials Derived from Large-Particle Raw Materials

AgPb _x SbTe₂₀ (x?=?17 to 23) bulk materials with nanometer-sized grains were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) intentionally using large-sized lead and tellurium particles as raw materials to reduce oxygen impurity from surface oxides. It was found that the nominal Pb content in the starting powder mixture greatly affected the thermoelectric (TE) properties of the sintered bulk samples, with excess amount relative to the stoichiometric ratio of AgPb _m SbTe _m+2 less than previously reported. x-Ray diffraction experiments revealed that the lattice parameter increases linearly with increasing Pb content until x?=?20, which corresponds to the optimal nominal Pb content in the starting composition, showing the highest ZT value. A high ZT value of 1.2 at 675?K was achieved in the x?=?20 sample due to its lowest electrical resistivity and reduced thermal conductivity. The present study shows that using large-sized Pb particles containing less oxygen is suitable for fabrication of AgPb _m SbTe _m+2-system TE materials by MA and SPS processes.     

Preparation and Enhanced Thermoelectric Properties of p-Type BaFe12O19/CeFe3CoSb12 Magnetic Nanocomposite Materials

A series of p-type xBaFe₁₂O₁₉/CeFe₃CoSb₁₂ (x = 0, 0.05%, 0.10%, 0.20%, 0.40%) magnetic nanocomposite thermoelectric (TE) materials have been prepared by the combination of ultrasonic dispersion and spark plasma sintering (SPS). The effects of BaFe₁₂O₁₉ magnetic nanoparticles on the phase composition, microstructure, and TE properties of the nanocomposite materials were investigated in this work. x-Ray diffraction analysis shows that all the SPSed bulk samples are composed of main phase skutterudite besides a small amount of FeSb₂ and Sb. The TE transport measurements demonstrated that remarkable enhancements in electrical conductivity and Seebeck coefficient can be simultaneously realized by optimizing the doping content of BaFe₁₂O₁₉ magnetic nanoparticles. The lattice thermal conductivity was significantly reduced because of enhanced phonon scattering induced by BaFe₁₂O₁₉ nanoparticles. The highest ZT value reached 0.75 at 800 K for the sample with x = 0.05%, increased by 41.5% as compared with that of p-type CeFe₃CoSb₁₂ bulk material without BaFe₁₂O₁₉ magnetic nanoparticles. This work confirms that doping a small amount of BaFe₁₂O₁₉ magnetic nanoparticles can significantly improve the ZT value of p-type xBaFe₁₂O₁₉/CeFe₃CoSb₁₂ magnetic nanocomposite TE materials.     

Self-Assembled Germanium Quantum-Dot Supercrystals in Silicon with Extremely Low Thermal Conductivities for Thermoelectrics

Superlattices with one-dimensional (1D) phonon confinement were studied to obtain a low thermal conductivity for thermoelectrics. Since they are composed of materials with a lattice mismatch, they often show dislocations. Like 1D nanowires, they also decrease heat transport in only one main propagation direction. It is therefore challenging to design superlattices with a thermoelectric figure of merit ZT higher than unity. Epitaxial self-assembly is a major technology to fabricate three-dimensional (3D) Ge quantum-dot (QD) arrays in Si. They have been used for quantum and solar-energy devices. Using the atomic-scale phononic crystal model, 3D Ge QD supercrystals in Si also present an extreme reduction of the thermal conductivity to a value that can be under 0.04 W/m/K. Owing to incoherent phonon scattering, the same conclusion holds for 3D supercrystals with moderate QD disordering. As a result, they might be considered for the design of highly efficient complementary metal–oxide–semiconductor (CMOS)-compatible thermoelectric devices with ZT possibly much higher than unity. Such a small thermal conductivity was only obtained for two-dimensional layered WSe₂ crystals in an experimental study. However, electronic conduction in the Si/Ge compounds is significantly enhanced. The 0.04 W/m/K value can be computed for different Ge QD filling ratios of the Si/Ge supercrystal with size parameters in the range of current fabrication technologies.     

Enhancement of Thermoelectric Figure‐of‐Merit by a Bulk Nanostructuring Approach

Recently a significant figure‐of‐merit (ZT) improvement in the most‐studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high‐energy ball milling and a direct‐current‐induced hot‐press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.     

Thermoelectric Properties of Melt-Spun Zn x Sb3 Ribbons

Melt-spun ribbons composed of Zn _x Sb₃ (3.4????x????4.3) were fabricated through a single-wheel melt-spinning process at wheel velocities of 0.6?m?s^?1 to 4.2?m?s^?1 and annealed for 2?h at 673?K. The structures were investigated using x-ray diffraction. The dimensionless figure of merit ZT, Seebeck coefficient, and electrical conductivity were measured to estimate the power factor and thermal conductivity. ??-Zn₄Sb₃ in the as-spun ribbons coexisted with ZnSb or Zn at 0.6?m?s^?1, while it coexisted with ??-Zn₃Sb₂ in x????3.8 at 4.2?m?s^?1, where ??-Zn₃Sb₂ disappeared in the annealed ribbons. The Seebeck coefficient in the as-spun and annealed ribbons tended to decrease slightly with increasing x at all the wheel velocities. At 0.6?m?s^?1, the ZT and power factor of as-spun and annealed ribbons increased with increasing x at x?<?4.0 because of increase in the electrical conductivity. At 4.2?m?s^?1, ZT was smaller than that at 0.6?m?s^?1 because the electrical conductivity was small in the as-spun ribbons and the thermal conductivity was large in the annealed ribbons.