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.     

Design of a Miniaturized Thermoelectric Generator Using Micromachined Silicon Substrates

This paper presents the design of a compact (~1 cm³) thermoelectric (TE) generator intended to generate power locally for sensor/electronic device applications using hot gases (~100°C to 400°C). The design employs 13-mm-diameter, ~0.36-mm-thick (48 mm³) silicon-micromachined TE modules that are stacked to form a cylindrical, finned heat exchanger. The stacked structure is intended to establish a large, uniform temperature gradient across radially oriented thermopiles in each module. Analytical heat transfer and electrical circuit models are used to design and optimize the thermopile for maximum output power under microfabrication and system-level constraints. Optimized structures using PbTe and Bi₂Te₃ thin films are predicted to achieve output power levels of 1.3 mW per module (26.7 mW/cm³) and 0.83 mW per module (17.4 mW/cm³), respectively, for hot gas at 400°C.     

Microstructure and Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-Type Doped Mg<Subscript>2</Subscript>Sn Compounds Prepared by the Modified Bridgman Method

Mg₂Sn compounds were prepared by the modified vertical Bridgman method, and were doped with Bi and Ag to obtain n- and p-type materials, respectively. Excess Mg was also added to some of the ingots to compensate for the loss of Mg during the preparation process. The Mg₂Sn samples were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), and their power factors were calculated from the Seebeck coefficient and electrical conductivity, measured from 80 K to 700 K. The sample prepared with 4% excess Mg, which contains a small amount of Mg₂Sn + Mg eutectic phase, had the highest power factor of 12 × 10⁻³ W m⁻¹ K⁻² at 115 K, while the sample doped with 2% Ag, in which a small amount of eutectics also exists, has a power factor of 4 × 10⁻³ W m⁻¹ K⁻² at 420 K.     

Effects of surface treatments on the electrical and the microstructural changes of Pd contact on p-type GaN

We investigated the effects of surface treatments by aqua regia and (NH₄)₂S_x on the electrical and the microstructural changes of Pd contact on p-type GaN during annealing. The formation of a surface oxide was suppressed by the (NH₄)₂S_x treatment, and S-Ga and S-N bonds with binding energy of 162.1 eV and 163.6eV were formed, degrading the structural ordering of Pd. After 300°C annealing, the contact resistivity in the aqua regia-treated sample increased significantly. This could be attributed to the outdiffusion of N atoms leaving N vacancies below the contact, as confirmed by the increase of the Pd (111) plane spacing probably due to the dissolution of N atoms in Pd interstitial sites. Meanwhile, the contact resistivity in the (NH₄)₂S_x-treated sample was not degraded and no change was observed in the Pd (111) plane spacing. These results suggest that S-Ga and S-N bonds formed on (NH₄)₂S_x-treated GaN could act as a diffusion barrier for the outdiffusion of N atoms. The contact resistivity for the aqua regia-treated sample decreased again, probably due to the outdiffusion of Ga as well as N atoms at 500°C.     

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.     

Zintl Phases as Thermoelectric Materials: Tuned Transport Properties of the Compounds CaxYb1–xZn2Sb2

Zintl phases are ideal candidates for efficient thermoelectric materials, because they are typically small‐bandgap semiconductors with complex structures. Furthermore, such phases allow fine adjustment of dopant concentration without disrupting electronic mobility, which is essential for optimizing thermoelectric material efficiency. The tunability of Zintl phases is demonstrated with the series Ca_xYb_1–xZn₂Sb₂ (0 ≤ x ≤ 1). Measurements of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity (in the 300–800 K temperature range) show the compounds to behave as heavily doped semiconductors, with transport properties that can be systematically regulated by varying x. Within this series, x = 0 is the most metallic (lowest electrical resistivity, lowest Seebeck coefficient, and highest carrier concentration), and x = 1 is the most semiconducting (highest electrical resistivity, highest Seebeck coefficient, and lowest carrier concentration), while the mobility is largely independent of x. In addition, the structural disorder generated by the incorporation of multiple cations lowers the overall thermal conductivity significantly at intermediate compositions, increasing the thermoelectric figure of merit, zT. Thus, both zT and the thermoelectric compatibility factor (like zT, a composite function of the transport properties) can be finely tuned to allow optimization of efficiency in a thermoelectric device.     

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.     

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.     

用于白光LED的KNaCa2(PO4)2:Eu2+蓝色荧光粉的发光特性

采用高温固相法合成了蓝色荧光粉KNaCa₂(PO₄)₂:Eu²⁺,利用X射线衍射(XRD)和光谱技术等表征了材料的性能。结果显示,少量Eu 2+的掺入并没有影响KNaCa₂(PO₄)₂的晶体结构。 在399nm近紫外光激发下,KNaCa₂(PO₄)₂:Eu²⁺材料发 射蓝光,发射光谱为400～600nm, 主发射峰位于471nm,对应Eu²⁺的4f^65d1→ 4f⁷跃迁发射;471nm发射峰,对应的激发光 谱为250～450nm,主激发峰位于399nm,与近紫外芯片匹配很好。 以365nm近紫外光作为 激发源时,KNaCa₂(PO₄)₂:Eu²⁺材料的发射强度约为商用蓝色荧光粉BAM:Eu 2+的85%;而以 399nm近紫外光作为激发源时,相较于BAM:Eu²⁺,KNaCa₂(P O₄)₂:Eu²⁺材料具有更强的发射强 度。此外,KNaCa₂(PO₄)₂:Eu²⁺和BAM:Eu²⁺的CIE色坐标接近,均位于蓝 色区域,色坐标分别 为(0.154,0.154)和(0.141,0.112)。研究结果 表明,KN aCa₂(PO₄)₂:Eu²⁺是一种在三基色白光LED中有应用前景的蓝色荧光粉。     

Electronic Structure and Thermoelectric Properties of Si-Based Clathrate Compounds

Thermoelectric properties of Au-substituted Si-based clathrates, Ba₈AuGa₁₃Si₃₂ and Ba₆A₂AuGa _x Si_45−x (A = Sr, Eu, x = 13, 14), were experimentally and theoretically investigated. The polycrystalline samples of the Au-substituted Si-based clathrates were prepared by using the spark plasma sintering technique. The electronic structure of Ba₆A₂AuGa₁₃Si₃₂ was theoretically calculated by ab initio calculations, and the thermoelectric properties of Ba₆A₂AuGa _x Si_45−x were estimated through the calculated electronic structure. The effective mass of Ba₆A₂AuGa _x Si_45−x was experimentally estimated to be greater than that of Ba₈AuGa₁₃Si₃₂. Experimentally observed electronic properties agree with the calculations for Ba₆A₂AuGa _x Si_45−x . The maximum ZT value of Ba₆Sr₂AuGa₁₄Ge₃₁ is about 0.5 at 850 K. The calculated thermoelectric properties agree very well with the experimental results in the range from room temperature to 900 K.     

空气氧化法对纳米碳管场发射性能的影响

以镍金属为催化剂,在600℃条件下,采用化学气相沉积法(CVD)制备碳纳米管。将制得的碳纳米管用高能球磨法处理0.5～1h后,以空气氧化法进行提纯,并研究了氧化温度对碳纳米管形貌和场发射性能的影响。用扫描电镜、Raman光谱分别对300～500℃的氧化提纯后的碳纳米管的形貌和结构进行了表征。结果表明:碳纳米管的场发射性能随温度的升高而升高,经400～450℃加热10min后,非晶碳成分减少,碳管纯度得到提高,场发射性能达到最高;当氧化温度继续升高时,碳纳米管的缺陷密度增大,非晶化程度增加,场发射特性变差。因此,通过控制氧化温度可以有效提高碳纳米管的纯度和场发射性能。     

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO₂ layer at the Ta₂O₅/Si interface is observed by annealing in dry O₂ or N₂ and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta₂O₅ films decreases as the annealing temperature increases when annealed in dry O₂ or N₂. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta₂O₅. The effect is larger in thicker Ta₂O₅ films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO₂ layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO₂. The electrical properties of Ta₂O₅ as a function of annealing conditions do not depend on the fabrication method of Ta₂O₅ but strongly depend on the thickness of Ta₂O₅ layer.     

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.     

In0.53Ga0.47As表面氮化和硫钝化对其Al/Al2O3/InGaAs MOS电容特性的影响

本文通过对比频散特性和滞回特性,计算界面态密度D_it和有效边界缺陷密度ΔN_bt,分析界面缺陷和漏电流等方法,系统的研究了In_0.53Ga_0.47As表面氮化和硫钝化对其Al/Al₂O₃/InGaAs结构MOS电容特性的影响。实验结果表明,这两种方法都能够在InGaAs表明形成一层界面钝化层。相比较于未处理的样品,经过氮气等离子体处理的样品表现出较好的界面特性,得到了最小的积累区频散、滞回电压,以及良好的I-V性能。经过(NH₄)₂S_x处理的样品则获得了最小的平带电压区频散以及最低的界面态密度Dit=2.6E11cm_-2eV_-1.     

Thermoelectric Properties of Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> and Prospects for Other Oxide Thermoelectrics

We discuss the thermoelectric properties of Na _x CoO₂ using the electronic structure, as determined in first principles calculations, and Boltzmann kinetic transport theory. The Fermi energy lies near the top of a manifold of Co t _2g bands. These t _2g bands are separated by a large gap from the higher-lying e _g states. Although the large crystal-field splitting implies substantial Co–O hybridization, the bands are narrow. Application of standard Boltzmann transport theory to such a narrow band structure yields high thermopowers in accord with experimental observations, even for high metallic carrier densities. The high thermopowers observed for Na _x CoO₂ can therefore be explained by standard band theory and do not rely on low dimensionality or correlation effects specific to Co. We also present results for the cubic spinel structure ZnRh₂O₄. Like Na _x CoO₂, this compound has very narrow valence bands. We find that if it could be doped with mobile carriers, it would also have a high thermopower, comparable with that of Na _x CoO₂.     

Impurity-induced disordering of AlxGa1−xAs-GaAs quantum well heterostructures with (Si2)x(GaAs)1−x barriers

Recent work indicates that the alloy (Si₂)_x(GaAs)_1−x can be formed within the GaAs quantum well of an Al_xGa_1−xAs-GaAs quantum well heterostructure (QWH) and results in a shift of laser operation to higher energy. In this paper we show, by SIMS and EDS measurements, that the Si concentration in the (Si₂)_x(GaAs)_1−x layer far exceeds typical “doping” levels. The stability of these QWHs has been investigated with respect to thermal annealing and Zn impurity-induced layer disordering (Zn-IILD). Data are presented showing that the (Si₂)_x(GaAs)_1−x alloy is stable against thermal annealing unless a rich source of Ga vacancies is provided, and that relatively low temperature Zn diffusion greatly enhances the disordering process of the alloy layer.     

Thermoelectric power of SnO2 anisotropic thin films

A calculation of thermoelectric power in fluorine-doped SnO₂ thin films is carried out using the Boltzmann transport theory and including anisotropic effects over material as well as the degeneracy grade of the samples. Curves of thermoelectric power in dependence of the temperature and concentration of F-impurities are reported. Finally, the computed values are compared with those obtained experimentally.     

Thermal Cycling Effects on the Thermoelectric Properties of n-Type In,Ce-Based Skutterudite Compounds

n-Type In-filled CoSb₃ is a known skutterudite compound that has shown promising thermoelectric (TE) properties resulting in high dimensionless figure of merit values at elevated temperatures. Use in various waste heat recovery applications will require survival and operation after exposure to harsh thermal cycling environments. This research focused on uncovering the thermal cycling effects on TE properties of n-type In_0.2Co₄Sb₁₂ and In_0.2Ce_0.15Co₄Sb₁₂ skutterudite compositions as well as quantifying their temperature-dependent structural properties (elastic modulus, shear modulus, and Poisson??s ratio). It was observed that the Seebeck coefficient and resistivity increased only slightly in the double-filled In,Ce skutterudite materials upon thermal cycling. In the In-filled skutterudites the Seebeck coefficient remained approximately the same on thermal cycling, while the electrical resistivity increased significantly after thermal cycling. Results also show that the thermal conductivity marginally decreases in the case of In-filled skutterudites, whereas the reduction is more pronounced in In,Ce-based skutterudite compounds. The possible reason for this kind of reduction can be attributed to grain pinning effects due to formation of nanoinclusions. High-temperature structural property measurements (i.e., Young??s modulus and shear modulus) are also reported. The results show that these structural properties decrease slowly as temperature increases and that the compounds are structurally stable after numerous (up to 200) thermal cycles.     

Ca3Y2(Si3O9)2:Tb3+绿色荧光粉的光谱特性

采用高温固相法制备了Ca₃Y₂(Si₃O₉)₂: Tb³⁺绿色荧光粉,研究了材料的光学性能。X 射线衍射(XRD)结果显示,掺杂少量的Tb³⁺,并未影响Ca₃Y₂(Si₃O₉)₂材料 的晶相结构。Ca₃Y₂(Si₃O₉)₂:Tb³⁺ 荧光粉的激发光谱由较强的4f^75d1宽带吸收(200～300 nm )和较弱的4f-4f电子跃迁吸收 (300～500 nm)构成,主激发峰位于236nm。取波长分别为236、376和482nm的光 作为激发源时,发现样品的主发射峰均位于544 nm,对应Tb³⁺的⁵D 4→⁷F₅跃迁发射。以236nm 紫外光作为激发源,监测544nm主发射峰,随Tb³⁺浓度 的增大,Ca₃Y₂(Si ₃O₉)₂:Tb³⁺的荧光寿命逐渐减小,但在实验范围内并未出现浓度猝灭现象。     

AgIn共掺杂(AgIn)xPb1-2xTe化合物的合成及热电传输性能

用高温熔融法合成了Ag和In共掺的单相n型（AgIn）x Pb1-2x Te化合物,研究了（AgIn）掺杂量x对（AgIn）x Pb1-2x Te（x=0.01～0.05）物相组成及热电性能的影响.结果表明：掺杂量x≤0.04时得到单相四元化合物,x=0.05时样品中出现了组成为AgInTe2第二相;（AgIn）x Pb 1-2x Te化合物的Seebeck系数随着x增加而增大,电导率随着掺杂量x增加而降低;化合物的热导率随着掺杂量x增大而减小;当x=0.01时,（AgIn） x Pb1-2x Te化合物的热电性能指数值最大,在800K时达到1.1.