Low-Temperature Synthesis and Thermoelectric Properties of n-Type PbTe

n-Type PbTe compounds were synthesized at temperatures as low as 430°C. After synthesis, the materials were ground, cold pressed, and sintered at 600°C. The effect of this low-temperature synthesis on the structural features and thermoelectric properties of as-prepared and PbI₂-doped materials was investigated for the first time. The Seebeck coefficient, and electrical and thermal conductivity were measured in the temperature range 2 K ≤ T ≤  610 K. The results show that all materials exhibit n-type conduction and the thermoelectric properties are improved by doping. ZT values reach 0.5 at 610 K, and the discrepancies with the literature are discussed.     

Thermoelectric Properties of Nanocrystalline PbTe Synthesized by Mechanical Alloying

In this work, nanocrystalline lead telluride powder was synthesized from high-purity elements by mechanical alloying by means of a planetary ball-milling procedure. The milling medium was tungsten carbide, and the diameter of the balls was varied in order to investigate the effect on the structural features of the material. Phase transformations and crystallite evolution during ball-milling were followed by powder x-ray diffraction (PXRD). The broadened PXRD peaks were analyzed with Voigt functions, revealing small crystalline size and stress introduced during the mechanical alloying process. Transmission electron microscopy (TEM) studies confirmed the material’s nanostructure, as well as the effect of ball diameter on the size of the crystals. Thermoelectric properties are discussed in terms of the Seebeck coefficient and the nominal carrier concentration, as determined by Hall-effect measurements. The enhancement of the Seebeck coefficient is reported to be higher compared with other PbTe-based nanocomposites.     

Properties of p- and n-Type PbTe Microwires for Thermoelectric Devices

In thermopower measurements, microwires fabricated from as-purchased bulk PbTe exhibits p-type behavior between room temperature and ～600 K. At higher temperatures, it undergoes majority carrier inversion and exhibits n-type behavior. We report on the preparation and properties of potassium oxide and Zn-doped PbTe microwires, which exhibit stable p- and n-type behavior, respectively, between room temperature and 725 K. Thermoelectric figures of merit (ZT) are reported for device components prepared from bundles of such p- and n-type microwires in a glass matrix.     

Effect of Ce-Doping on Thermoelectric Properties in PbTe Alloys Prepared by Spark Plasma Sintering

Ce-doped Pb_1−x Ce _x Te alloys with x = 0, 0.005, 0.01, 0.015, 0.03, and 0.05 were prepared by induction melting, ball milling, and spark plasma sintering techniques. The structure and thermoelectric properties of the samples were investigated. X-ray diffraction (XRD) analysis indicated that the samples were of single phase with NaCl-type structure for x less than 0.03. The lattice parameter a increases with increasing Ce content. The lower Ce-doped samples (x = 0.005 and 0.01) showed p-type conduction, whereas the pure PbTe and the higher doped samples (x = 0, 0.015, 0.03, and 0.05) showed n-type conduction. The lower Ce-doped samples exhibited a much higher absolute Seebeck coefficient, but the higher electrical resistivity and higher thermal conductivity compared with pure PbTe resulted in a lower figure of merit ZT. In contrast, the higher Ce-doped samples exhibited a lower electrical resistivity, together with a lower absolute Seebeck coefficient and comparable thermal conductivity, leading to ZT comparable to that of PbTe. The lowest thermal conductivity (range from 0.99 W m⁻¹ K⁻¹ at 300 K to 0.696 W m⁻¹ K⁻¹ at 473 K) was found in the alloy Pb_0.95Ce_0.05Te due to the presence of the secondary phases, leading to a ZT higher than that of pure PbTe above 500 K. The maximum figure of merit ZT, in the alloy Pb_0.95Ce_0.05Te, was 0.88 at 673 K.     

Preparation of Ring-Shaped Thermoelectric Legs from PbTe Powders for Tubular Thermoelectric Modules

Waste heat recovery—for example, in automotive applications—is a major field for thermoelectric research and future application. Commercially available thermoelectric modules are based on planar structures, whereas tubular modules may have advantages for integration and performance in the field of automotive waste heat recovery. One major drawback of tubular generator designs is the necessity for ring-shaped legs made from thermoelectric material. Cutting these geometries from sintered tablets leads to considerable loss of thermoelectric material and therefore high cost. Direct sintering of ring-shaped legs or tubes of thermoelectric material is a solution to this problem. However, sintering such rings with high homogeneity and density faces some difficulties related to the mechanical properties of typical thermoelectric materials such as lead telluride (PbTe)—particularly brittleness and high coefficient of thermal expansion. This work shows a process for production of thermoelectric rings made of p- and n-doped PbTe. Long tubes of PbTe have been sintered in a current-assisted sintering process with specially designed sintering molds, coated with a diffusion barrier, and finally cut into ring-shaped slices. To demonstrate the technology, a tubular thermoelectric module has been assembled using these PbTe rings.     

Influence of ZnO Inclusions on the Low-Temperature Thermoelectric Properties of CoSb3

CoSb₃ composites with different amounts of ZnO nanoparticles (2?wt.% to 12?wt.%) were prepared from nanosized ZnO (commercial) and micron-sized CoSb₃ (obtained via solid-state reaction) particles mixed in solution and freeze dried. The resulting powders were densified by spark plasma sintering. The samples were characterized by x-ray diffraction and scanning electron microscopy. It was found that ZnO forms micron-sized clusters at the grain boundaries of the matrix material. The thermoelectric properties (electrical resistivity, thermopower, and thermal conductivity) were measured in the 2?K to 300?K temperature range. Both the electrical and thermal conductivities were observed to decrease with increasing ZnO content. The dimensionless figure of merit ZT was improved by up to 30% at 300?K for the sample containing 2?wt.% ZnO.     

掺杂PbTe晶体的热电性质研究

介绍了利用区熔法生长掺杂热电变换材料ＰｂＴｅ晶体,并对生长样品的杂质分布、热电特性进行了测量和分析。     

Effects of Ge Dopant on Thermoelectric Properties of Barium and Indium Double-Filled p-Type Skutterudites

A series of Ge-doped and (Ba,In) double-filled p-type skutterudite materials with nominal composition Ba_0.3In_0.2FeCo₃Sb_12?x Ge _x (x = 0 to 0.4, Δx = 0.1) have been prepared by melting, quenching, annealing, and spark plasma sintering methods. The effects of Ge dopant on the phase composition, microstructure, and thermoelectric properties of these materials were investigated in this work. A single-phase skutterudite material was obtained in the samples with 0 < x ≤ 0.2, and trace Fe₃Ge₂ was detected in the samples with x ≥ 0.3. The electrical conductivity increased and Seebeck coefficient decreased with increasing x in the range of 0 to 0.2, while the inverse behaviors of electrical conductivity and Seebeck coefficient were observed in the samples with x ≥ 0.3. The variations of electrical conductivity and Seebeck coefficient are attributed to the significant increase in the carrier concentration in the x range of 0 to 0.2 and the intensive impact of Fe₃Ge₂ when x ≥ 0.3. The lattice thermal conductivity of all the Ge-doped samples was considerably reduced as compared with the undoped Ba_0.3In_0.2FeCo₃Sb₁₂ sample, and the lowest value of lattice thermal conductivity of the Ba_0.3In_0.2FeCo₃Sb_11.8Ge_0.2 sample reached 1.0 W m^?1 K^?1 at 700 K. The highest ZT value of 0.54 was obtained at 800 K for the Ba_0.3In_0.2FeCo₃Sb_11.7Ge_0.3 sample, increased by 10% as compared with that of Ba_0.3In_0.2FeCo₃Sb₁₂.     

Effect of Indium Substitution on the Thermoelectric Properties of Orthorhombic Cu4SnS4

We report the thermoelectric properties of Cu₄In _x Sn_1?x S₄ (x = 0–0.02), which undergoes a first-order structural phase transition at ~230 K. Substitution of In³⁺ for Sn⁴⁺ suppresses the phase transition temperature (T _t). Indium substitution reduces the electrical resistivity, and degenerate conduction by the orthorhombic phase is observed. The Seebeck coefficient increases over the whole temperature range and a maximum value occurs in the monoclinic phase as a result of indium substitution. Thermal conductivity decreases as x increases, which enhances the dimensionless figure of merit, ZT. We therefore expect optimization of the chemical composition of indium-doped Cu₄SnS₄ to result in an even larger ZT value.     

Enhanced Thermoelectric Properties of (PbTe)0.88(PbS)0.12 Composites by Sb Doping

The effects of Sb doping on (PbTe)_0.88(PbS)_0.12 composites prepared by melting, ball milling, and spark plasma sintering were investigated. The x-ray diffraction results indicate that all samples Sb _x Pb_1?x Te_0.88S_0.12 with x = 0, 0.002, 0.004, 0.006 and 0.008 are composites containing PbTe with NaCl-type structure as the major phase and PbS with NaCl-type structure as the minor phase. The electrical resistivity is reduced with increasing Sb doping, from 1.95 × 10^?5 Ωm for Sb content x = 0 to 5.55 × 10^?6 Ωm for x = 0.008 at 298 K, showing that Sb is an efficient electron donor. However, the absolute Seebeck coefficient decreases, from 196 μV/K for x = 0 to 57.0 μV/K for x = 0.008 at 298 K, and the thermal conductivity increases, from 0.989 W/m K for x = 0 to 1.64 W/m K for x = 0.008, with Sb doping. The power factor and figure of merit ZT can be enhanced by proper Sb doping. The maximum dimensionless figure of merit ZT of 1.20 was obtained in the sample Sb_0.004Pb_0.996Te_0.88S_0.12 at 773 K.     

Influence of Ball-Milling,Nanostructuring, and Ag Inclusions on Thermoelectric Properties of ZnSb

In the last few years much attention has been given to the promising thermoelectric material Zn₄Sb₃. The related ZnSb phase features a high Seebeck coefficient at room temperature. Its thermoelectric conversion efficiency, however, is low due to its relatively high thermal conductivity. ZnSb has potential as a thermoelectric material if this can be reduced. Nanostructuring of bulk materials and introducing extrinsic particles have been shown to lower lattice thermal conductivity. In this study we created the microstructure by ball-milling of bulk ZnSb and added Ag particles which attain sizes in the micrometer range in this milling process. Hot-pressing was used to obtain dense samples. Several techniques were used for structural characterization. Here we report on scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analysis. Thermoelectrical measurements were conducted around room temperature. Thermal conductivity was reduced by up to 40% by the reported nanostructuring. However, the electrical conductivity and the Seebeck coefficient were adversely affected, leading to no overall improvement in performance.     

Thermoelectric and Mechanical Propertiesof Novel Hot-Extruded PbTe n-Type Material

Lead telluride-based materials demonstrate the highest thermoelectric performance in the temperature range from 200°C to 400°C, and they are of interest for numerous waste heat recovery applications. Unfortunately, these conventionally grown materials are usually very brittle, which results in significant material loss during module manufacturing and a decrease in module reliability when subjected to continuous vibrations common for automotive applications. We present a hot extrusion process developed for the first time for PbTe which yields polycrystalline materials with strong mechanical properties combined with high thermoelectric performance. n-Type lead telluride was extruded from conventionally synthesized and powdered material at temperatures in the range of 450°C to 520°C depending on material stoichiometry. The extruded rods were of cylindrical shape with 2.54?cm diameter and lengths up to 40?cm. Young??s modulus measured using mechanical spectroscopy varied from 59?GPa to 51?GPa for temperatures in the range of 20°C to 300°C. Slicing and dicing of extruded rods to obtain cubical samples with 2?mm side demonstrated no difficulties, illustrating the material homogeneity and its potential for manufacturing module legs. The microstructure of the material was studied by scanning electron microscopy. Doping with antimony iodide during the milling process controls the conduction electron concentration in the range from 1?×?10¹⁹?cm^?3 to 6?×?10¹⁹?cm^?3. For optimized doping of 0.08?wt.% SbI₃, the maximum thermoelectric figure of merit (ZT) reaches a value of 0.99 at 380°C, as measured by the Harman method. The combination of high thermoelectric performance and improved fracture toughness makes this novel hot-extruded polycrystalline PbTe material highly competitive for many applications.     

Interfacial Reaction Between Nb Foil and n-Type PbTe Thermoelectric Materials During Thermoelectric Contact Fabrication

PbTe is a high-conversion-efficiency thermoelectric (TE) material that is commonly used in space exploration applications. Integration of PbTe in TE devices has a significant impact on the conversion efficiency and reliability of TE devices. Hence, our effort focuses on developing novel approaches for bonding metallic contacts to PbTe to improve device performance and reliability. In this study, pure Nb foil was directly bonded to PbTe-based TE materials to fabricate the hot-side contacts of TE elements using a rapid hot-press. The materials were sintered at 700°C under pressure of 40 MPa for various holding times. We found that a reaction layer of needle-like Nb₃Te₄ mixed with Pb forms at the interface of the Nb/PbTe joints and that Pb is distributed in the gaps of the Nb₃Te₄ grains. We analyze the resulting microstructure and finally calculate the time exponent of the growth kinetics of the Nb₃Te₄ layer. Fracture surface analysis showed that the Nb/PbTe joint fractures at the interface between Nb and Nb₃Te₄ and within the PbTe matrix, indicating that the bonding between Nb and Nb₃Te₄ is weak.     

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.     

Direct Evidence of Cation Disorder in Thermoelectric Lead Chalcogenides PbTe and PbS

The extraordinary thermoelectric properties of lead chalcogenides have attracted huge interest in part due to their unexpected low thermal conductivity. Here, it is shown that anharmonicity and large cation disorder are present in both PbTe and PbS, based on elaborate charge density visualization using synchrotron powder X‐ray diffraction (SPXRD) data analyzed with the maximum entropy method (MEM). In both systems, the cation disorder increases with increasing temperature, whereas the Te/S anions appear to be centered on the expected lattice positions. Even at the lowest temperatures of 105 K, the lead ion is on average displaced by ≈0.2 Å from the rock‐salt lattice position, creating a strong phonon scattering mechanism. These findings provide a clue to understanding the excellent thermoelectric performance of crystals with atomic disorder. The SPXRD–MEM approach can be applied in general opening up for widespread characterization of subtle structural features in crystals with unusual properties.     

Synthesis of Nanocomposites with Improved Thermoelectric Properties

Bulk thermoelectric materials are of interest for commercial application in both power generation and Peltier refrigeration. Various synthesis approaches have been developed by our group for high performance bulk thermoelectric materials, such as solvo- or hydrothermal synthesis for nanopowders, hot-pressing, and spark plasma sintering for nanostructured bulk materials, and rapid solidification for metal silicides. In this article we report some of our recent results in the development of high ZT thermoelectric materials, including Bi₂Te₃-Sb₂Te₃ nanocomposites and CoSb₃ micro/nanocomposites prepared by a powder blending route, and GeTe-AgSbTe₂ and Mg₂Si-Mg₂Sn nanocomposites prepared by an in situ route. The results show various possibilities for improved microstructures and therefore enhanced properties of bulk thermoelectric materials through optimization of the preparation processing based on simple synthesis routes. A high ZT of approximately 1.5 has been obtained in both Bi₂Te₃-Sb₂Te₃ and GeTe-AgSbTe₂ nanocomposites. Further ZT enhancement of the materials should be possible through the control of the nanopowder morphology during synthesis and the hindering of␣grain growth during sintering, as well as through the optimization of composition and doping.     

液封直拉富铟InP中铟夹杂的纵向和横向分布

采用光学显微镜与红外透射显微镜相结合的方法,研究了磷化铟(InP)晶片中铟夹杂的形貌,通过比较含有铟夹杂的晶体与晶片,总结了铟夹杂在富铟磷化铟中的纵向与横向分布行为,并分析了熔体组分配比度、固液界面形貌以及温度梯度对铟夹杂分布行为的影响。在含有铟夹杂的单晶片中发现铟夹杂的一种特殊环形分布行为,对比和它相邻的晶片,发现这种环形分布呈现出沿生长方向直径逐渐变小的趋势,初步分析与固液界面呈微凸的形貌有关。在这种特殊分布中,发现部分铟夹杂呈四方对称分布。结合磷化铟的晶体结构,分析这种对称分布与固液界面处的{111}晶面有关。     

Thermoelectric Properties of Nanograined ZnO

Bulk ZnO with a grain size of 20 nm was successfully obtained by pulsed electric current sintering. The crystalline size was almost identical to that of the raw particles, because the sintering temperature was as low as 200°C. A pressure of 500 MPa effectively enhanced densification, leading to a relative density of >90% at 200°C. The small grain size led to a low thermal conductivity of 3 W/m K at room temperature, due to enhanced boundary scattering. The Seebeck coefficient was higher than that of micrograined ZnO with similar Ga doping (0.3 at.% Ga). However, the resistivity was increased by more than 1000 times. The temperature dependence of conductivity showed thermally activated conduction behavior, while that of micrograined ZnO exhibited metallic-like behavior. The thermoelectric properties suggest that a carrier trap in the nanograined ZnO hinders carrier transport. Surface modification of the ZnO nanoparticles by heat treatment in H₂ resulted in observable photoluminescence which was quenched in the starting nanoparticles, and led to a decrease in the resistivity of the sintered bulk, which indicates that control of surface defects on the nanoparticles is crucial for enhancement of the thermoelectric properties of nanograined ZnO.     

Thermoelectric Properties of Zn-Substituted Magnetite

Thermoelectric properties of Zn-substituted magnetite were investigated experimentally. Since Zn is incorporated in the A-site of magnetite for 0 ≤ x ≤ 0.2 in Zn _x Fe_3−x O_4−δ , electrical resistivity remained constant in this region and the thermoelectric power factor (PF) increased with Zn content. At x = 0.2, it attained 1.66 μW/K² cm at 700°C. Above x = 0.2, where Zn began to enter the B-site, the PF decreased with x.