Enhancing the Thermoelectric Properties of p-Type Bulk Bi-Sb-Te Nanocomposites via Solution-Based Metal Nanoparticle Decoration

Embedding nanosized particles in bulk thermoelectric materials is expected to lower the lattice thermal conductivity by enhancing the degree of interface phonon scattering, thus improving their thermoelectric figure of merit ZT. We have developed a wet chemical process to fabricate Bi_0.5Sb_1.5Te₃-based thermoelectric nanocomposites which include nanometer-sized metal particles. By simple solution mixing of metal acetate precursors and Bi_0.5Sb_1.5Te₃ powders in ethyl acetate as a medium for homogeneous incorporation, it is possible to apply various types of metal nanoparticles onto the surfaces of the thermoelectric powders. Next, bulk Bi_0.5Sb_1.5Te₃ nanocomposites with homogeneously dispersed metal nanoparticles were fabricated using a spark plasma sintering technique. The lattice thermal conductivities were reduced by increasing the long-wavelength phonon scattering in the presence of metal nanoparticles, while the Seebeck coefficients increased for a few selected metal-decorated nanocomposites, possibly due to the carrier-energy-filtering effect. Finally, the figure of merit ZT was enhanced to 1.4 near room temperature. This approach highlights the feasibility of incorporating various types of nanoparticles into an alloy matrix starting by wet chemical routes, which is an effective means of improving the thermoelectric performance of Bi-Te-based alloys.     

Improved Thermoelectric Performance of Eco‐Friendly β‐FeSi2–SiGe Nanocomposite via Synergistic Hierarchical Structuring,Phase Percolation,and Selective Doping

A β‐FeSi₂–SiGe nanocomposite is synthesized via a react/transform spark plasma sintering technique, in which eutectoid phase transformation, Ge alloying, selective doping, and sintering are completed in a single process, resulting in a greatly reduced process time and thermal budget. Hierarchical structuring of the SiGe secondary phase to achieve coexistence of a percolated network with isolated nanoscale inclusions effectively decouples the thermal and electrical transport. Combined with selective doping that reduces conduction band offsets, the percolation strategy produces overall electron mobilities 30 times higher than those of similar materials produced using typical powder‐processing routes. As a result, a maximum thermoelectric figure of merit ZT of ≈0.7 at 700 °C is achieved in the β‐FeSi₂–SiGe nanocomposite.     

Generation of Nanosized Particles during Mechanical Alloying and Their Evolution through the Hot Extrusion Process in Bismuth-Telluride-Based Alloys

Our extensive studies of extruded alloys have shown that mechanically strong polycrystalline alloys also deliver surprisingly high thermoelectric performance with ZT > 1 for p-type material in the temperature range from 25°C to 90°C, which was traditionally attributed to the strong texture generated by the extrusion process. Optical and low-resolution scanning electron microscopy observations of the powder produced by mechanical alloying show a particle size distribution in the micrometric range. However, high-resolution transmission electron microscopy (HRTEM) observations of the powders obtained after milling clearly show nanosized crystal grains. The larger microparticles appear to be agglomerations of grains whose size goes down to 5 nm to 20 nm. Examination of bulk n-type and p-type materials using x-ray diffraction (XRD) combined with HRTEM shows that nanosized subgrains can also be found in materials after hot extrusion. In this article we present experimental evidence of the generation and evolution of nanocrystalline particles through the mechanical alloying and hot extrusion processes used to produce the thermoelectric alloys. We also discuss the possible influence of nanocrystalline inclusions on the thermoelectric performance of the produced material. The optimization of the hot extrusion process, in order to maximize the influence of the nanostructures, offers new opportunities to increase the thermoelectric performance of bulk materials.     

Synthesis and Characterization of Polythiophene/Bi2Te3 Nanocomposite Thermoelectric Material

To achieve low thermal conductivity, polythiophene (PTh)/bismuth telluride (Bi₂Te₃) nanocomposite has been prepared by spark plasma sintering using a mixture of nanosized Bi₂Te₃ and PTh powders. Bi₂Te₃ powder with spherical-shaped particles of 30 nm diameter and PTh nanosheet powder were first prepared by hydrothermal synthesis and chemical oxidation, respectively. X-ray diffraction analysis and scanning electron microscopy observations revealed that the hybrid composite consists of PTh nanosheets and spherical Bi₂Te₃. The organic PTh acts as an adhesive in the composite. Transport measurements showed that the PTh in the Bi₂Te₃ matrix can reduce its thermal conductivity significantly, but also dramatically reduces its electrical conductivity. As a result, the figure of merit of the composite is lower than that of pure Bi₂Te₃ prepared under the same conditions. The maximum value of ZT for the sample with 5% PTh (by weight) was 0.18 at 473 K, which is rather high compared with other polymer/inorganic thermoelectric material composites.     

Ecodesigns and applications for noble metal nanoparticles by ultrasound process

Ecodesigns for nano-sized noble metal particles were investigated by a new liquid-solid (metal oxide-alcohol) system. We have reduced noble metal oxides as low-emission starting materials by ultrasound and tried to fabricate various noble metal nanoparticles (Ag, Au, Pt, Pd) at room temperature, Noble metal oxides were investigated during decomposition. These reductions are ecologically clean, because many noble metal oxides are not toxic, and during decomposition, O/sub 2/ is evolved. By choosing suitable conditions, it is reasonable to expect that this simple sonochemical process can be extended to obtain nano-sized metal particles.     

Microstructures and Thermoelectric Properties of an Annealed Ti<Subscript>0.5</Subscript>(Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)<Subscript>0.5</Subscript>NiSn<Subscript>0.998</Subscript>Sb<Subscript>0.002</Subscript> Ribbon

The thermoelectric half-Heusler compound Ti_0.5(Hf_0.5Zr_0.5)_0.5NiSn_0.998Sb_0.002 was fabricated by spin-casting and subsequent annealing. ZT at room temperature increased with annealing time through an increase in absolute Seebeck coefficients despite a decrease in electrical conductivity. ZT reached 0.10 after annealing at 1050 K for 48 h. In powder x-ray diffraction analysis, each half-Heusler peak was accompanied by a bump at the high-angle side, corresponding to a minor Ti-rich half-Heusler phase. The quantity and Ti composition of the minor phase increased with annealing time, although those of the major half-Heusler phase were almost constant. In transmission electron microscopic analysis, granular domains, several nanometers in size, with atomic ordering or disordering were observed. Thermoelectric properties were␣improved by annealing through the growth of heterogeneous microstructures of the Ti-rich and Ti-poor half-Heusler grains and of the granular domains.     

Titania Embedded with Nanostructured Sodium Titanate: Reduced Thermal Conductivity for Thermoelectric Application

Titania embedded with layer-cracking nanostructures (sodium titanate) was synthesized by a hydrothermal method and a subsequent sintering process. The structure and morphology were determined by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N₂ adsorption–desorption experiments. In thermoelectric investigations, this nanocomposite has reduced thermal conductivity, where the minimum reaches about 2.4 W/m K at 700°C. This value is relatively low among the transition-metal oxides. Strong boundary scattering at the interfaces of the layered nanostructures and point defect scattering resulting from volatilization of Na⁺ ions seem to be main reasons for the suppression of phonon heat transfer. On the other hand, the power factor shows no apparent deterioration. Our results suggest that introduction of proper layer-cracking nanostructures into thermoelectric hosts might be effective to enhance their performance.     

Reduced Grain Size and Improved Thermoelectric Properties of Melt Spun (Hf,Zr)NiSn Half-Heusler Alloys

Half-Heusler thermoelectric materials Hf(/Zr)NiSn were prepared by levitation melting followed by melt-spinning to refine the boundary structures, and then they were consolidated by spark plasma sintering. X-ray diffraction analysis and scanning electron microscopy showed that single phased half-Heusler compounds without compositional segregation had been obtained. It was found that the thermoelectric properties, especially the thermal conductivity, depended strongly on the boundary structures. The melt-spinning samples with refined boundary structures had a lower thermal conductivity but a power factor comparable to that of the sample prepared by levitation melting, thus providing good thermoelectric properties.     

Enhancement in Performance of the Tubular Thermoelectric Generator (TTEG)

For use in a tubular thermoelectric generator (TTEG), we fabricated tubular Bi_0.5Sb_1.5Te₃/Ni composite using a melt-spinning technique combined with the spark plasma sintering (SPS) process. With this method, powder sintering, joining of two different materials, and tubular shaping can be achieved simultaneously. The tilted laminate structure which is crucial for the transverse thermoelectric effect was successfully achieved in the sample after SPS densification. The sintered samples showed better mechanical stability and thermoelectric properties compared with the previously studied melt-cast sample. We confirmed larger open-circuit voltage of 240 mV and generating power of 2.5 W with a 100-mm-long TTEG under the small temperature difference of 83 K, and the corresponding power density for a unit heat transfer surface area was approximately 800 W m^?2.     

Inkjet-Printed High Mobility Transparent–Oxide Semiconductors

In this paper, we report a general and low-cost process to fabricate high mobility metal–oxide semiconductors that is suitable for thin-film electronics. This process use simple metal halide precursors dissolved in an organic solvent and is capable of forming uniform and continuous thin films via inkjet-printing or spin-coating process. This process has been demonstrated to deposit a variety of semiconducting metal oxides include binary oxides (${hbox{ZnO}}, {hbox{In}}_{2}{hbox{O}}_{3}$ , ${hbox{SnO}}_{2}$ , ${hbox{Ga}}_{2}{hbox{O}}_{3}$ ), ternary oxides (ZIO, ITO, ZTO, IGO) and quaternary compounds (IZTO, IGZO). Functional thin film transistors with high field-effect mobility were fabricated successfully using channel layers deposited from this process. This synthetic pathway opens an avenue to form patterned metal oxide semiconductors through a simple and low-cost process and to fabricate high performance transparent thin film electronics via digital fabrication processes on large substrates.      

激光直接烧结成形金属零件的试验研究

激光直接烧结成形技术是在激光熔覆技术和快速原型及制造技术的基础上发展起来的一项先进的制造技术,其发展趋势是直接制造金属零件.本文采用Ni基和Co基合金粉末进行了激光烧结实验,利用激光烧结直接成形工艺进行了单道烧结试验,研究了不同工艺参数对成形性和表面质量的影响规律,同时使用SEM分析了单道涂覆层的组织特征,并得出获得致密组织较为理想的激光烧结工艺参数.     

纳米Al2O3激光烧结快速成型试验初探

将快速成型技术引入纳米材料成型领域,在Al2O3纳米粉末的选择性激光烧结试验基础上,系统分析了纳米陶瓷材料激光烧结工艺的影响因素,初选了烧结参数,得到了较为合理的纳米Al2O3粉末激光烧结工艺。通过多层烧结试验对其进行了验证,对烧结制件进行了成分、微观组织等检测分析。试验表明,采用得到的选择性激光烧结工艺,可以实现纳米Al2O3的自由成型,烧结制件内部组织保持纳米结构,材料晶粒尺寸基本不长大。     

Flexible Thermoelectric Devices of Ultrahigh Power Factor by Scalable Printing and Interface Engineering

Printing is a versatile method to transform semiconducting nanoparticle inks into functional and flexible devices. In particular, thermoelectric nanoparticles are attractive building blocks to fabricate flexible devices for energy harvesting and cooling applications. However, the performance of printed devices are plagued by poor interfacial connections between nanoparticles and resulting low carrier mobility. While many rigid bulk materials have shown a thermoelectric figure of merit ZT greater than unity, it is an exacting challenge to develop flexible materials with ZT near unity. Here, a scalable screen‐printing method to fabricate high‐performance and flexible thermoelectric devices is reported. A tellurium‐based nanosolder approach is employed to bridge the interfaces between the BiSbTe particles during the postprinting sintering process. The printed BiSbTe flexible films demonstrate an ultrahigh room‐temperature power factor of 3 mW m^?1 K^?2 and ZT about 1, significantly higher than the best reported values for flexible films. A fully printed thermoelectric generator produces a high power density of 18.8 mW cm^?2 achievable with a small temperature gradient of 80 °C. This screen‐printing method, which directly transforms thermoelectric nanoparticles into high‐performance and flexible devices, presents a significant leap to make thermoelectrics a commercially viable technology for a broad range of energy harvesting and cooling applications.     

Synthesis of Thermoelectric Manganese Silicide by Mechanical Alloying and Pulse Discharge Sintering

Manganese silicide is a candidate for low-cost thermoelectric materials with low-environmental load. MnSi_1.73 compound was studied as a thermoelectric material available for thermoelectric power generation using waste heat. Manganese and silicon powders were mechanically alloyed under three different conditions (at 200 r.p.m. for 36 ks, at 400 r.p.m. for 3.6 ks and at 400 r.p.m. for 36 ks) with planetary ball milling equipment. Then, the mechanically alloyed powder was consolidated by a pulse discharge sintering process. Phases of MnSi_1.73 (primary phase) and MnSi were synthesized by mechanical alloying and pulse discharge sintering. Thermoelectric properties were dependent on the mechanical alloying condition. The sample mechanically alloyed at 400 r.p.m. for 3.6 ks gave the best thermoelectric performance. The maximum dimensionless figure of merit ZT of 0.47 was achieved at 873 K.     

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.     

The effects of quaternary additions on thermoelectric properties of TiNiSn-based half-heusler alloys

Half-Heusler-type compounds have gained increasing attention as promising thermoelectric materials. In the present work, a focus is placed on TiNiSn with additions of Hf, Zr, Si, or Pt. Nominally stoichiometric TiNiSn alloys were prepared using arc melting and subsequent annealing at 1,073 K for 2 weeks. The thermoelectric properties, such as thermoelectric power, electrical resistivity, and thermal conductivity, were measured in a temperature range from 300 K to 1,000 K. As-cast materials show metallic transport properties, while annealed ones exhibit semiconductor behavior. Microstructures of TiNiSn alloys basically consist of nonequilibrium four-phase; half-Heusler TiNiSn, Heusler TiNi₂Sn, metallic Ti₆Sn₅, and Sn solid solution. The volume fraction of the half-Heusler TiNiSn phase significantly increases by annealing. It is revealed that coexisting metallic phases degrade the thermoelectric properties of half-Heusler TiNiSn. Alloy additions strongly affect not only thermoelectric properties but also phase stability. The thermal conductivity of TiNiSn alloys with alloy additions decreases because of the point-defect phonon scattering.     

纳米金属氧化物粉体的液相制备和表征进展

根据国内外研究者的报道及作者近几年来对纳米金属氧化物的制备研究结果,介绍了纳米金属氧化物液相法制备中常用的方法及表征手段,并探讨了纳米粉体制备过程中的团聚和反团聚措施。     

Development of a Thermoelectric Module Using the Heusler Alloy Fe<Subscript>2</Subscript>VAl

A thermoelectric module was constructed using a Heusler Fe₂VAl sintered alloy prepared by pulse-current sintering (PCS) using mechanically alloyed powder. A large-amplitude vibration mill was developed for powder preparation. The processing rate was increased 50-fold over that of laboratory planetary ball milling. In all, 25 pieces of millimeter-sized sintered alloy, which is useful for thermoelectric devices without cutting, were fabricated simultaneously using a single PCS process. The thermoelectric module consisting of the high-strength sintered alloy and electrode joint showed high durability. This module stably generated electric power on the exhaust pipe of a running motorcycle.     

Thermoelectric Properties of Half-Heusler Bismuthides ZrCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi (<Emphasis Type="Italic">x</Emphasis> = 0.0 to 0.1)

The usefulness of half-Heusler (HH) alloys as thermoelectrics has been mainly limited by their relatively large thermal conductivity, which is a key issue despite their high thermoelectric power factors. In this regard, Bi-containing half-Heusler alloys are particularly appealing, because they are, potentially, of low thermal conductivity. One such a material is ZrCoBi. We prepared pure and Ni-doped ZrCoBi by a solid-state reaction. To evaluate thermoelectric potential we measured electrical resistivity (ρ = 1/σ) and thermopower (σ) up to 1000 K and thermal conductivity (κ) up to 300 K. Our measurements indicate that for these alloys resistivity of approximately a few mΩ cm and thermopower larger than a hundred μV K⁻¹ are possible. Low κ values are also possible. On the basis of these data we conclude that this system has a potential to be optimized further, despite the low power factors (α ² σT) we have currently measured.     

A Comparison Between the Effects of Sb and Bi Doping on the Thermoelectric Properties of the Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Alloy

We deal here with Sb and Bi doping effects of the n-type half-Heusler (HH) Ti_0.3Zr_0.35Hf_0.35NiSn alloy on the measured thermoelectric properties. To date, the thermoelectric effects upon Bi doping on the Sn site of HH alloys have rarely been reported, while Sb has been widely used as a donor dopant. A comparison between the measured transport properties following arc melting and spark plasma sintering of both Bi- and Sb-doped samples indicates a much stronger doping effect upon Sb doping, an effect which was explained thermodynamically. Due to similar lattice thermal conductivity values obtained for the various doped samples, synthesized in a similar experimental route, no practical variations in the thermoelectric figure of merit values were observed between the various investigated samples, an effect which was attributed to compensation between the power factor and electrical thermal conductivity values regardless of the various investigated dopants and doping levels.