Thermoelectric Properties of Zintl Compound YbZn<Subscript>2</Subscript>Sb<Subscript>2</Subscript> with Mn Substitution in Anionic Framework

The thermoelectric properties of the Zintl compound YbZn₂Sb₂ with isoelectronic substitution of Zn by Mn in the anionic (Zn₂Sb₂)²⁻ framework have been studied. The p-type YbZn_2−x Mn _x Sb₂ (0.0 ≤ x ≤ 0.4) samples were prepared via melting followed by annealing and hot-pressing. Thermoelectric property measurement showed that the Mn substitution effectively lowered the thermal conductivity for all the samples, while it significantly increased the Seebeck coefficient for x < 0.2. As a result, a dimensionless figure of merit ZT of approximately 0.61 to 0.65 was attained at 726 K for x = 0.05 to 0.15, compared with the ZT of ~0.48 in the unsubstituted YbZn₂Sb₂.     

The Zintl Compound Ca5Al2Sb6 for Low‐Cost Thermoelectric Power Generation

Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca₅Al₂Sb₆ is a charge‐balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ～2m_e, consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca₅Al₂Sb₆ possesses an extremely low lattice thermal conductivity (0.6 W mK^?1 at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca_4.75Na_0.25Al₂Sb₆ possesses a near‐optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca₅Al₂Sb₆ possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.     

Addressing the Challenges of Commercializing New Thermoelectric Materials

The time it takes for new thermoelectric materials to make the transition from first announcement in peer-reviewed publications to commercialization is undesirably long. As a result, universities, laboratories, government agencies, commercial users, and venture funding providers throughout the world have not supported research in the field to the level that would be expected for such an otherwise promising technology. This delay also has led to some misdirection of research efforts and a lack of availability of dependable long-term sponsorship commitments to research in the field. From the perspective of commercial users, this presentation discusses the challenges that the thermoelectric material research community faces in creating materials of commercial value. These challenges are broken down into objectives for both the traditional research activities related to improving ZT and those efforts needed to satisfy other, less recognized requirements which, if unaddressed, can significantly impede or even prevent commercialization. The ZT thresholds that enable much larger markets are presented for power generation, cooling, heating, and temperature control materials. Other important considerations, including semiconductor to metal interface (metallization) properties, material stability and constituent requirements, and costs and environmental-impact-related requirements are discussed. At the system level, factors that impede material development are identified, including challenges arising from a lack of property measurement repeatability among different organizations. Approaches and results are compared with that of the more heavily funded and rapidly developing photovoltaic field. The presentation concludes with recommendations for measures to accelerate thermoelectric material commercialization. International Conference on Thermoelectrics (August 3–7, 2008, Corvallis, Oregon, USA).     

2D Materials Based on Main Group Element Compounds: Phases,Synthesis, Characterization,and Applications

2D materials based on main group element compounds have recently attracted significant attention because of their rich stoichiometric ratios and structure motifs. This review focuses on the phases in various 2D binary materials including III–VI, IV–VI, V–VI, III–V, IV–V, and V–V materials. Reducing 3D materials to 2D introduces confinement and surface effects as well as stabilizes unstable 3D phases in their 2D form. Their crystal structures, stability, preparation, and applications are summarized based on theoretical predictions and experimental explorations. Moreover, various properties of 2D materials, such as ferroelectric effect, anisotropic optical and electrical properties, ultralow thermal conductivity, and topological state are discussed. Finally, a few perspectives and an outlook are given to inspire readers toward exploring 2D materials with new phases and properties.     

Evaluation of Half‐Heusler Compounds as Thermoelectric Materials Based on the Calculated Electrical Transport Properties

A theoretical evaluation of the thermoelectric‐related electrical transport properties of 36 half‐Heusler (HH) compounds, selected from more than 100 HHs, is carried out in this paper. The electronic structures and electrical transport properties are studied using ab initio calculations and the Boltzmann transport equation under the constant relaxation time approximation for charge carriers. The electronic structure results predict the band gaps of these HH compounds, and show that many HHs are narrow‐band‐gap semiconductors and, therefore, are potentially good thermoelectric materials. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level is investigated. Maximum power factors and the corresponding optimal p‐ or n‐type doping levels, related to the thermoelectric performance of materials, are calculated for all HH compounds investigated, which certainly provide guidance to experimental work. The estimated optimal doping levels and Seebeck coefficients show reasonable agreement with the measured results for some HH systems. A few HHs are recommended to be potentially good thermoelectric materials based on our calculations.     

Analysis of Nanostructuring in High Figure‐of‐Merit Ag1–xPbmSbTe2+m Thermoelectric Materials

Thermoelectric materials based on quaternary compounds Ag_1?xPb_mSbTe_2+m exhibit high dimensionless figure‐of‐merit values, ranging from 1.5 to 1.7 at 700 K. The primary factor contributing to the high figure of merit is a low lattice thermal conductivity, achieved through nanostructuring during melt solidification. As a consequence of nucleation and growth of a second phase, coherent nanoscale inclusions form throughout the material, which are believed to result in scattering of acoustic phonons while causing only minimal scattering of charge carriers. Here, characterization of the nanosized inclusions in Ag_0.53Pb₁₈Sb_1.2Te₂₀ that shows a strong tendency for crystallographic orientation along the {001} planes, with a high degree of lattice strain at the interface, consistent with a coherent interfacial boundary is reported. The inclusions are enriched in Ag relative to the matrix, and seem to adopt a cubic, 96 atom per unit cell Ag₂Te phase based on the Ti₂Ni type structure. In‐situ high‐temperature synchrotron radiation diffraction studies indicated that the inclusions remain thermally stable to at least 800 K.     

Thermoelectric Properties of Heavily Doped <Emphasis Type="Italic">n</Emphasis>-Type SrTiO<Subscript>3</Subscript> Bulk Materials

Three Ta-doped strontium titanates were prepared as potential candidates for n-type thermoelectric oxides. The purity of the polycrystalline samples of SrTi_1−x Ta _x O₃ (x = 0.05 to 0.14) were characterized by means of powder x-ray diffraction and electron probe micro analysis (EPMA). We present the results of Seebeck coefficient, electrical conductivity, and thermal conductivity measurements performed at high temperatures.     

Thermoelectric Characteristics of <Emphasis Type="Italic">p</Emphasis>-Type (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript>/(Pb,Sn)Te Functional Gradient Materials with Variation of the Segment Ratio

The p-type (Bi,Sb)₂Te₃/(Pb,Sn)Te functional gradient materials (FGMs) were fabricated by hot-pressing mechanically alloyed (Bi_0.2Sb_0.8)₂Te₃ and 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te powders together at 500°C for 1 h in vacuum. Segment ratios of (Bi,Sb)₂Te₃ to (Pb,Sn)Te were varied as 3:1, 1.3:1, and 1:1.6. A reaction layer of about 350-μm thickness was formed at the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGM interface. Under temperature differences larger than 340°C applied across a specimen, superior figures of merit were predicted for the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs to those of (Bi_0.2Sb_0.8)₂Te₃ and (Pb_0.7Sn_0.3)Te. With a temperature difference of 320°C applied across a specimen, the (Bi,Sb)₂Te₃/(Pb,Sn)Te FGMs with segment ratios of 3:1 and 1.3:1 exhibited the maximum output powers of 72.1 mW and 72.6 mW, respectively, larger than the 63.9 mW of (Bi_0.2Sb_0.8)₂Te₃ and the 26 mW of 0.5 at.% Na₂Te-doped (Pb_0.7Sn_0.3)Te.     

p型方钴矿化合物BayFeCo3Sb12的热电性能研究

用两步固相反应法合成了单相的p型BayFeCo3Sb12化合物,并系统地研究了Ba不同填充分数对方钴矿化合物热电性能的影响:化合物载流子浓度强烈地依赖于填充原子的填充分数,随Ba填充分数y的增加,载流子浓度及电导率降低;塞贝克系数随温度T的上升而增加,比CoSb3的塞贝克系数有一定程度的提高,尤其是在中温部分有大幅度提高,得到的最大塞贝克系数由CoSb3的107μVK-1提高到Ba1.0FeCo3Sb12的235μVK-1晶格热导率随Ba的填充分数y的增加而进一步下降,Ba08FeCo3Sb12甚至降到2.2 Wm1K1;Ba08FeCo3Sb12化合物显示最大热电性能指数,在850K左右其最大无量纲热电性能指数ZT值达0.75.     

High-Density Frenkel Defects as Origin of N-Type Thermoelectric Performance and Low Thermal Conductivity in Mg3Sb2-Based Materials

Mg₃Sb₂-based intermetallic compounds with exceptionally high thermoelectric performance exhibit unconventional n-type dopability and anomalously low thermal conductivity, attracting much attention to the underlying mechanisms. To date, investigations have been limited to first-principle calculations and thermodynamic analysis of defect formation, and detailed experimental analysis on crystal structure and phonon modes has not been achieved. Here, a synchrotron X-ray diffraction study clarifies that, against a previous view of a simple crystal structure with a small unit cell, Mg₃Sb₂ is inherently a heavily disordered material with Frenkel defects, charge-neutral defect complexes of cation vacancies and interstitials. Ionic charge neutrality preserved in Mg₃Sb₂ is responsible for exotic n-type dopability, which is unachievable for other Zintl phase materials. The thermal conductivity of Mg₃Sb₂ exhibits deviation from the standard T⁻¹ temperature dependency with strongly limited phonon transport due to a strain field. Inelastic X-ray scattering measurement reveals enhanced phonon scattering induced by disorder. The results will draw renewed attention to crystal defects and disorder as means to explore new high-performance thermoelectric materials.     

Doping Effects on the Thermoelectric Properties of AgSbTe<Subscript>2</Subscript>

A doping study of ternary alloys AgSbTe₂ doped with excess AgTe, NaTe, NaSe, TlTe, BiTe, and excess Pb showed that carrier concentrations can be effectively manipulated. Measured thermopower and resistivity indicate a shift of the power factor peak toward the lower-temperature regime. The measured figure of merit ZT increases from 0.5 to 1.2 after doping with NaSe, and 1.05 with TlTe, at 400 K. We also show that doping with Bi and Pb has a negative effect on the thermoelectric properties of these alloys.     

硫系相变材料Ge1Sb2Te4和Ge2Sb2Te5薄膜相变速度及电学输运性质研究

利用磁控溅射方法制备了Ge1Sb2Te4和Ge2Sb2Te5两种相变存贮材料的薄膜.原位X射线衍射(XRD)的结果表明,随着退火温度的升高,Ge1Sb2Te4和Ge2Sb2Te5薄膜都逐步晶化,材料结构发生了从非晶态到面心立方结构、再到六角密堆结构的转变.由衍射峰的半宽高可以看出,在达到第一次相变温度后,Ge2Sb2T...     

Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in α‐MgAgSb Thermoelectric Materials

Understanding the lattice dynamics and phonon transport from the perspective of chemical bonds is essential for improving and finding high‐efficiency thermoelectric materials and for many applications. Here, the coexistence of global and local weak chemical bonds is elucidated as the origin of the intrinsically low lattice thermal conductivity of non‐caged structure Nowotny–Juza compound, α‐MgAgSb, which is identified as a new type of promising thermoelectric material in the temperature range of 300–550 K. The global weak bonds of the compound lead to a low sound velocity. The unique three‐centered Mg? Ag? Sb bonds in α‐MgAgSb vibrate locally and induce low‐frequency optical phonons, resulting in “rattling‐like” thermal damping to further reduce the lattice thermal conductivity. The hierarchical chemical bonds originate from the low valence electron count of α‐MgAgSb, with the feature shared by Nowotny–Juza compounds. Low lattice thermal conductivities are therefore highly possible in this series of compounds, which is verified by phonon and bulk modulus calculations on some of the compositions.     

Effects of Ball-Milling Atmosphere on the Thermoelectric Properties of TAGS-85 Compounds

Inspired by the high ZT value lately attained in Ar-protected ball-milled nanocrystalline p-BiSbTe bulk alloy, we report herein an investigation of the effects of ball-milling atmosphere on the thermoelectric (TE) properties of the traditional TE material (GeTe)₈₅(AgSbTe₂)₁₅ (TAGS-85). TAGS-85 samples were prepared via a melting–quenching–annealing process, and then ball-milled in different atmospheres and subsequently densified using a spark plasma sintering technique. The Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient were measured as a function of temperature from 10 K to 310 K. It was found that different ball-milling atmospheres, i.e., air, liquid N₂ (LN₂), and Ar, profoundly affected the TE properties. A state-of-the-art figure of merit ZT ≈ 0.30 was attained at 310 K in the Ar-ball milled sample. The results are discussed in terms of the carrier concentration, mobility, crystallinity, and the grain boundary scattering.     

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.     

Magnetism and Phase Formation in the Candidate Dilute Magnetic Semiconductor System In2 − xCrxO3: Bulk Materials are Dilute Paramagnets

Well‐characterized bulk materials in the candidate dilute magnetic semiconductor system In_{2 − x}Cr_xO₃ are prepared for 0 ≤ x < 0.15, with cation site preferences in the bixbyite structure identified by diffraction methods. Small ferromagnetic moments are observed; their size (<10⁻² µ_B/dopant ion) is not consistent with bulk ferromagnetism. The resulting bulk materials display dilute paramagnetic behaviour, with all of the moment expected per Cr³⁺ cation dopant being involved in this paramagnetic response.     

Synthesis,Crystal Structure,and Magnetic Properties of ϵ‐InxFe2–xO3 Nanorod‐Shaped Magnets

A series of ?‐In_xFe_2–xO₃ nanorods are prepared by combining the reverse‐micelle and the sol–gel methods. Metal replacement was achieved in the region of 0 ≤ x ≤ 0.24. The crystal structures are orthorhombic structures (space group: Pna2₁), which are pyroelectric with an electric polarization along the c axis. The transmission electron microscopy images show that the particle sizes are (80 ± 40) × (23 ± 5) nm (x = 0), (65 ± 30) × (30 ± 10) nm (x = 0.12), and (80 ± 40) × (35 ± 15) nm (x = 0.24). The magnetization versus temperature curves of the samples with x = 0, x = 0.12, and x = 0.24 show spontaneous magnetization with Curie temperatures of 495 K, 456 K, and 414 K, respectively. Their coercive fields at 300 K are 20 kOe (x = 0), 14 kOe (x = 0.12), and 9 kOe (x = 0.24). These samples show a spin reorientation with reorientation temperatures (T_p) of 102 K (x = 0), 149 K (x = 0.12), and 180 K (x = 0.24). In particular, the samples with x = 0.12 and x = 0.24 show antiferromagnetic behavior below T_p. This series of ?‐In_xFe_2–xO₃ is the first example of a pyroelectric material that exhibits a phase transition between ferrimagnetism and antiferromagnetism.     

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.     

Template‐Free Preparation of Hollow Sb2S3 Microspheres as Supports for Ag Nanoparticles and Photocatalytic Properties of the Constructed Metal–Semiconductor Nanostructures

A simple and convenient Ostwald ripening route to the morphology‐ and phase‐controlled preparation of hollow Sb₂S₃ microspheres is developed. The hollow spheres are clusters of smaller microspheres if orange amorphous Sb₂S₃ colloid is used as the precursor, whereas, if starting from the yellow precursor, the products are regular hollow spheres. By selecting appropriate experimental conditions for ripening, the phase of the hollow Sb₂S₃ microspheres can be controlled. Amorphous and orthorhombic hollow spheres are prepared by ripening the colloidal precursors at ambient temperature and in an autoclave, respectively. The closed shell of hollow Sb₂S₃ spheres can be easily eroded by hydrochloric acid to form an open structure. By the in situ reduction of adsorbed Ag⁺ on the surface and interior of the hollow spheres, Ag nanoparticles are introduced into them, to form functional metal–semiconductor composites, the weight content of which is controlled by regulating the concentration of the Ag⁺ source and the adsorption time. The composite structures composed of Ag nanoparticles and hollow Sb₂S₃ spheres exhibit a remarkably enhanced absorption covering the UV and visible regions of the electromagnetic spectrum. A study of the photocatalytic properties of the composite structures demonstrates that exposure to both UV and visible light enables them to induce the rapid decomposition of 2‐chlorophenol. The degradation rate increases with a larger weight content of Ag in the composite structure.     

Precisely Designed Gold Nanoparticles by Surface Polymerization – Artificial Molecules as Building Blocks for Novel Materials

The synthesis of gold nanoparticles (2–4 nm) carrying a single functional polystyrene chain and its characterization by gel permeation chromatography are reported. This has been achieved by a new type of macromolecular azo‐initiator based on telechelic polystyrene with containing α,ω‐methylcoumarin endgroups and an azo group in the middle of the polystyrene. The structure and the near‐quantitative functionality of the initiator have been verified by performing NMR, GPC, and UV–vis measurements. This macroinitiator has been used to initiate a surface polymerization of 4‐vinylthiophenol molecules immobilized on the surface of gold nanoparticles. As a product, gold nanoparticles carrying exactly one polystyrene chain have been synthesized with functionalization degrees of up to 90% (crude yield). Proof and quantification of the functionalization degree have been demonstrated by application of a GPC setup with a diode array detector for online UV–vis spectra.