Thermoelectric Performance and High-Temperature Creep Behavior of GeTe-Based Thermoelectric Materials

New, efficient thermoelectric materials (GeTe) _x (Mn_0.6Sn_0.4Te)_1−x (0.8 ≤ x ≤  1.0) were prepared by hot pressing, and the effect of MnTe and SnTe contents on thermoelectric and mechanical properties of GeTe was investigated. The maximum dimensionless figure of merit ZT of the prepared materials is 1.57 in the temperature range from 720 K to 770 K for x = 0.15. Niobium was added to the quasiternary GeTe-based materials to suppress creep without degradation of thermoelectric properties. The distortion of the material with added Nb was less than 0.4% under experimental conditions of 100 N load at 873 K for 100 h. The favorable thermoelectric properties of these materials are accompanied by their stability in long-term use and the possibility of widening the service temperature range as a result of decreasing their phase-transition temperature T _c.     

Thermal Expansion Studies of Selected High-Temperature Thermoelectric Materials

Radioisotope thermoelectric generators (RTGs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically plutonium-238) into electricity using a thermoelectric converter. RTGs have been successfully used to power a number of space missions and have demonstrated their reliability over an extended period of time (tens of years) and are compact, rugged, radiation resistant, scalable, and produce no noise, vibration or torque during operation. System conversion efficiency for state-of-practice RTGs is about 6% and specific power ≤5.1 W/kg. A higher specific power would result in more onboard power for the same RTG mass, or less RTG mass for the same onboard power. The Jet Propulsion Laboratory has been leading, under the advanced thermoelectric converter (ATEC) project, the development of new high-temperature thermoelectric materials and components for integration into advanced, more efficient RTGs. Thermoelectric materials investigated to date include skutterudites, the Yb₁₄MnSb₁₁ compound, and SiGe alloys. The development of long-lived thermoelectric couples based on some of these materials has been initiated and is assisted by a thermomechanical stress analysis to ensure that all stresses under both fabrication and operation conditions will be within yield limits for those materials. Several physical parameters are needed as input to this analysis. Among those parameters, the coefficient of thermal expansion (CTE) is critically important. Thermal expansion coefficient measurements of several thermoelectric materials under consideration for ATEC are described in this paper. The stress response at the interfaces in material stacks subjected to changes in temperature is discussed, drawing on work from the literature and project-specific tools developed here. The degree of CTE mismatch and the associated effect on the formation of stress is highlighted.     

The High-Temperature Thermoelectric Properties of Polycrystalline Ba8Ga x Al y Si46−x−y Type-I Clathrates

Thermoelectric materials suitable for practical thermoelectric power generators should, ideally, be based on light elements, for example Si and Al, which are abundantly available. For this reason, silicon clathrate compounds in which both Ga and Al were substituted for Si were synthesized and their thermoelectric properties were investigated. The temperature-dependent electrical resistivity of the samples indicated their metallic nature, and their negative Seebeck coefficient suggested that charge transport in the samples was mainly through electron transport. The maximum absolute value of the Seebeck coefficient achieved was ?180 μV/K at 1040 K for Ba_7.90Ga_13.8Al_2.29Si_30.0. Thus, these materials have potential for use in practical thermoelectric power generators.     

Methods for Enhancing the Thermal Durability of High-Temperature Thermoelectric Materials

Thermoelectric materials, for example skutterudites and magnesium silicides, are being investigated as promising materials for medium-to-high-temperature waste heat recovery in transport and in industry. A crucial aspect of the success of a thermoelectric material is its stability over time when exposed to rapid heating and cooling. In this work different aspects of the degradation of these thermoelectric materials at high temperature were examined. Initial thermal durability was studied, and several candidate coatings were evaluated to enhance durability by protecting the materials from oxidation and sublimation during thermal cycles in air for up to 500 h and up to 873 K. The samples were characterized by SEM and EDS. The results showed it is possible to reduce degradation of the thermoelectric material without compromising overall thermoelectric efficiency.     

Effects of High-Pressure High-Temperature Sintering on the Band Gap and Thermoelectric Properties of PbSe

In this study, PbSe bulk samples were prepared by a high-pressure high-temperature (HPHT) sintering technique, and the phase compositions, band gaps and thermoelectric properties of the samples were systematically investigated. The sintering pressure exerts a significant influence on the preferential orientation, band gap and thermoelectric properties of PbSe. With increasing pressure, the preferential orientation decreases, mainly due to the decreased crystallinity, while the band gap first decreases and then increases. The electrical conductivity and power factor decrease gradually with increasing pressure, mainly attributed to the decreased carrier concentration and mobility. Consequently, the sample prepared by 2 GPa shows the highest thermoelectric figure-of-merit, ZT, of 0.55 at ～ 475 K. The ZT of the HPHT-sintered PbSe could be further improved by properly doping or optimizing the HPHT parameters. This study further demonstrates that the sintering pressure could be another degree of freedom to manipulate the band structure and thermoelectric properties of materials.     

High-Temperature Thermoelectric Properties of p-Type Yb-Filled Skutterudite Nanocomposites with FeSb2 Nanoinclusions

Bulk thermoelectric (TE) nanocomposite materials have attracted considerable attention due to their great potential to exhibit higher dimensionless figure of merit ZT. Filled skutterudites of both n-type and p-type have already demonstrated their excellent high-temperature TE performance, good mechanical properties, and thermal stability. Herein, we extend this work to Yb-filled p-type skutterudite nanocomposites with in?situ precipitated FeSb₂ nanoinclusions. Such a nanocomposite material can be easily synthesized by fine control of starting stoichiometry and the subsequent heat treatment process. By taking advantage of these naturally occurring FeSb₂ nanoparticles, we achieve ZT _max?=?0.74 in Yb_0.6Fe₂Co₂Sb₁₂/0.05FeSb₂ at 780?K. We apply the method of four coefficients to calculate the density-of-states effective mass and the carrier scattering parameter. We find that a larger effective mass induced by the presence of nanoparticles is the origin of the enhanced Seebeck coefficient.     

Fabrication and Evaluation of a Skutterudite-Based Thermoelectric Module for High-Temperature Applications

We report a straightforward methodology for the fabrication of high-temperature thermoelectric (TE) modules using commercially available solder alloys and metal barriers. This methodology employs standard and accessible facilities that are simple to implement in any laboratory. A TE module formed by nine n-type Yb _x Co₄Sb₁₂ and p-type Ce _x Fe₃CoSb₁₂ state-of-the-art skutterudite material couples was fabricated. The physical properties of the synthesized skutterudites were determined, and the module power output, internal resistance, and thermocycling stability were evaluated in air. At a temperature difference of 365 K, the module provides more than 1.5 W cm^?3 volume power density. However, thermocycling showed an increase of the internal module resistance and degradation in performance with the number of cycles when the device is operated at a hot-side temperature higher than 573 K. This may be attributed to oxidation of the skutterudite thermoelements.     

Thermoelectric Oxides: Effect of Doping in Delafossites and Zinc Oxide

The electrical resistivity (ρ) and Seebeck coefficient (S) of the three delafossites CuFe_0.9Cr_0.1O₂, CuCr_0.98Mg_0.02O₂, and CuRh_0.9Mg_0.1O₂ have been measured and their power factor (PF) calculated. These p-type oxides show PF values at 800 K from 1.4 × 10⁻⁴ W K⁻² m⁻¹ to 6.9 × 10⁻⁴ W K⁻² m⁻¹. In contrast to delafossites containing Fe or Cr, for which ρ exhibits a regime, the Rh-based delafossite shows a metallic regime from 5 K to 1000 K. This points toward the role of the transition-metal electronic configuration in the transport properties. Otherwise, similar PF values are obtained in the case of the n-type Al-doped ZnO. For these oxides, the ρ and PF values are minimum and maximum, respectively, for x = 0.01. However, the presence of spinel impurities even for x = 0.01 in Zn_1−x Al _x O or for x = 0.02 in CuCr_1−x Mg _x O₂ calls into question the role of the doping element in the physical properties. This should motivate a deeper insight into the physics of thermoelectric oxides.     

Microstructure and Thermoelectric Properties of Screen-Printed Thick Films of Misfit-Layered Cobalt Oxides with Ag Addition

Thermoelectric properties of thick (~60???m) films prepared by a screen-printing technique using p-type misfit-layered cobalt oxide Ca₃Co₄O_9+?? with Ag addition have been studied. The screen-printed films were sintered in air at various temperatures ranging from 973?K to 1223?K. After each sintering process, crystal and microstructure analyses were carried out to determine the optimal sintering condition. The results show that the thermoelectric properties of pure Ca₃Co₄O_9+?? thick film are comparable to those of cold isostatic pressing (CIP) samples. We found that the maximum power factor was improved by about 67% (to 0.3?mW/m?K²) for film with proper silver (Ag) metallic inclusions as compared with 0.18?mW/m?K² for pure Ca₃Co₄O_9+?? film under the same sintering condition of 1223?K for 2?h in air.     

用于ESD防护的PDSOI NMOS器件高温特性

研究了基于0.18μm部分耗尽型绝缘体上硅(PDSOI)工艺的静电放电(ESD)防护NMOS器件的高温特性。借助传输线脉冲(TLP)测试系统对该ESD防护器件在30~195℃内的ESD防护特性进行了测试。讨论了温度对ESD特征参数的影响,发现随着温度升高,该ESD防护器件的一次击穿电压和维持电压均降低约11%,失效电流也降低近9.1%,并通过对器件体电阻、源-体结开启电压、沟道电流、寄生双极结型晶体管(BJT)的增益以及电流热效应的分析,解释了ESD特征参数发生上述变化的原因。研究结果为应用于高温电路的ESD防护器件的设计与开发提供了有效参考。     

Effects of In Substitution for Ga on the Thermoelectric Properties of Type-VIII Clathrate Ba8Ga16Sn30 Single Crystals

We have prepared single crystals of type-VIII clathrate Ba₈Ga_15.9?x In _x Sn_30.1 for x ≤ 0.60 by the Sn-flux method. As x is increased from 0 to 0.60, the lattice parameter increases by 0.2%, which is consistent with the larger covalent diameter for In than for Ga. The Seebeck coefficient α, electrical resistivity ρ, and thermal conductivity κ were measured in the temperature range from 300 K to 600 K. For all samples, α is negative, indicating the dominant charge carriers are electrons. With increasing x from 0 to 0.20, ρ and \(\left| \alpha \right|\) decrease by 50% and 30%, respectively. As a result, the lattice thermal conductivity at 300 K decreases from 0.58 W/Km to 0.41 W/Km, which is ascribed to enhancement of rattling of the guest atoms. It is found that the maximum of the dimensionless figure of merit ZT reaches 1.05 at 540 K for x = 0.20.     

Fabrication of Metallic Glass Powder for Brazing Paste for High-Temperature Thermoelectric Modules

Metallic glass (MG) offers the advantage of outstanding oxidation resistance, since it has disordered atomic-scale structure without grain boundaries. We fabricated Al-based MG ribbons (Al_84.5Y₁₀Ni_5.5) by a melt spinning process. We evaluated the adhesion strength of interfaces between the Al-based MG and a Ni-coated Cu electrode formed under various conditions at high temperature. In addition, we attempted to optimize the process conditions for pulverizing MG ribbons to < 100 micrometers by combining high-energy ball milling and planetary milling. We confirmed that the electrical resistivity of the Al-based MG ribbon was substantially reduced after annealing at high temperature (over 300°C) due to crystallization.     

The Influence of Spark Plasma Sintering Temperature on the Microstructure and Thermoelectric Properties of Al,Ga Dual-Doped ZnO

ZnO dual-doped with Al and Ga was prepared by spark plasma sintering using different sintering temperatures. The microstructural evolution and thermoelectric properties of the samples were investigated in detail. The samples obtained with sintering temperature above 1223 K had higher relative densities and higher electronic conductivity than the sample sintered at 1073 K. These results were supported by the solid-state reaction completion rate, which suggested that sintering temperature above 1223 K would be preferable for complete solid-state reaction of the samples. The sintering mechanism of ZnO particles and microstructure evolution at different sintering temperatures were investigated by simulation of the self-Joule-heating effect of the individual particles.     

热电式MEMS微波功率传感器的热学特性研究

热电转换效率直接影响热电式MEMS微波功率传感器的性能。着重对衬底掏空结构的热电式微波功率传感器进行了研究。将热电式微波功率传感器分成三个区域,建立了傅里叶模型,研究背面刻蚀的长度与厚度对热电堆热端温度的影响,发现热电堆两端温差与背面刻蚀的长度、厚度成正比。利用有限元仿真软件ANSYS,对不同刻蚀长度、厚度的传感器进行热学仿真。结果表明,背面刻蚀尺寸越大,热电堆两端的温差越大,传感器的灵敏度得到提高。仿真结果与模型结果具有较高的一致性,验证了模型的准确性。     

Characteristics of P-channel SOI LDMOS Transistor with Tapered Field Oxides

A new tapered TEOS oxide technique has been developed to use field oxide of the power integrated circuits. It provides better uniformity of less than 3 % and reproducibility. On-resistance of P-channel RESURF (REduced SURface Field) LDMOS transistors has been optimized and improved by using a novel simulation and tapered TEOS field oxide on the drift region of the devices. With the similar breakdown voltage, at V_gs = ?5.0 V, the specific on-resistance of the LDMOS with the tapered field oxide is about 31.5 mΩ · cm², while that of the LDMOS with the conventional field oxide is about 57 mΩ · cm².     

Effect of Carrier-Doping on the Thermoelectric Properties of Narrow-Bandgap (Fe,Ru)Ga3 Intermetallic Compounds

We have focused on the binary narrow-bandgap intermetallic compounds FeGa₃ and RuGa₃ as thermoelectric materials. Their crystal structure is FeGa₃-type (tetragonal, P4₂/mnm) with 16 atoms per unit cell. Despite their simple crystal structure, their room temperature thermal conductivity is in the range 4–5–W–m^?1–K^?1. Both compounds have narrow-bandgaps of approximately 0.3–eV near the Fermi level. Because their Seebeck coefficients are quite large negative values in the range 350–<–|S _373K|–<–550–μV–K^?1 for undoped samples, it should be possible to obtain highly efficient thermoelectric materials both by adjusting the carrier concentration and by reducing the thermal conductivity. Here, we report the effects of doping on the thermoelectric properties of FeGa₃ and RuGa₃ as n and p-type materials. The dimensionless figure of merit, ZT, was significantly improved by substitution of Sn for Ga in FeGa₃ (electron-doping) and by substitution of Zn for Ga in RuGa₃ (hole-doping), mainly as a result of optimization of the electronic part, S ² σ.     

Effect of Ag on the Microstructure of Sn-8.5Zn-xAg-0.01Al-0.1Ga Solders Under High-Temperature and High-Humidity Conditions

The effect of Ag on the microstructure and thermal behavior of Sn-Zn and Sn-8.5Zn-xAg-0.01Al-0.1Ga solders (x from 0.1 wt.% to 1 wt.%) under high-temperature/relative humidity conditions (85°C/85% RH) for various exposure times was investigated. Scanning electron microscopy (SEM) studies revealed that, in all the investigated solders, the primary α-Zn phases were surrounded by eutectic β-Sn/α-Zn phases, in which fine Zn platelets were dispersed in the β-Sn matrix. SEM micrographs revealed that increase of the Ag content to 1 wt.% resulted in coarsening of the dendritic plates and diminished the Sn-9Zn eutectic phase in the microstructure. Differential scanning calorimetry (DSC) studies revealed that the melting temperature of Sn-8.5Zn-xAg-0.01Al-0.1Ga solder decreased from 199.6°C to 199.2°C with increase of the Ag content in the solder alloy. Both ZnO and SnO₂ along with Ag-Zn intermetallic compound (IMC) were formed on the surface when Sn-8.5Zn-0.5Ag-0.01Al-0.1Ga solder was exposed to high-temperature/high-humidity conditions (85°C/85% RH) for 100 h. The thickness of the ZnO phase increased as the Ag content and exposure time were increased. Sn whiskers of various shapes and lengths varying from 2 μm to 5 μm were extruded from the surface when the investigated five-element solder with Ag content varying from 0.5 wt.% to 1 wt.% was exposed to similar temperature/humidity conditions for 250 h. The length and density of the whiskers increased with further increase of the exposure time to 500 h and the Ag content in the solder to 1 wt.%. The Sn whisker growth was driven by the compressive stress in the solder, which was generated due to the volume expansion caused by ZnO and Ag-Zn intermetallic compound formation at the grain boundaries of Sn.     

Electrical Transport Properties of Nb and Ga Double Substituted Fe2VAl Heusler Compounds

Semiconductors - In this work the results of studying the electrical transport properties of Fe2V1 –xNbxAl1 –yGay (0.1&nbsp;≤ x ≤ 0.2 and 0.1 ≤ y ≤ 0.2) are...     

Introduction of Metal Oxides into Mg2Si Thermoelectric Materials by Spark Plasma Sintering

Oxide incorporation into thermoelectric Mg₂Si-based materials was performed starting from commercial Mg₂Si and commercial metal oxides by applying ball milling and spark plasma sintering (SPS) processing. The SPS conditions, such as sintering temperature, pressure, and holding time, were optimized with the aim of obtaining both full densification and oxide incorporation. Thermoelectric characterizations, such as Seebeck coefficient and electrical and thermal conductivity, were carried out and related to the pellet compositions. The morphology, composition, and crystallographic structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectrometry, and x-ray diffraction analyses, respectively.