Semimetal/semiconductor nanocomposites for thermoelectrics

In this work, we present research on semimetal-semiconductor nanocomposites grown by molecular beam epitaxy (MBE) for thermoelectric applications. We study several different III-V semiconductors embedded with semimetallic rare earth-group V (RE-V) compounds, but focus is given here to ErSb:In(x)Ga(1?x)Sb as a promising p-type thermoelectric material. Nanostructures of RE-V compounds are formed and embedded within the III-V semiconductor matrix. By co-doping the nanocomposites with the appropriate dopants, both n-type and p-type materials have been made for thermoelectric applications. The thermoelectric properties have been engineered for enhanced thermoelectric device performance. Segmented thermoelectric power generator modules using 50 μ m thick Er-containing nanocomposites have been fabricated and measured. Research on different rare earth elements for thermoelectrics is discussed.     

立体微型器件的微制造技术及其在微机电系统(MEMS)的应用

综述了近年来与微型机电系统（MEMS）相关的材料微制造和微加工技术的最新研究进展.重点介绍了如何利用硅晶片作为微型模具来制备压电陶瓷和热电材料的微型柱状阵列结构和反应烧结碳化硅微型转子等微制造技术,并展望了材料微制造技术在研制微型医疗器件和微型移动能源方面的应用前景.     

New promising Co-free thermoelectric ceramic based on Ba–Fe–oxide

Thermoelectric ceramics are based in a limited number of transition metal oxides (Co, Mn, Ni,…) which produce materials with high thermoelectric performances. Based on previously existing thermoelectric phases, the phase diagram equilibrium can help to design new thermoelectric ceramics based on other transition metals (for example, Fe). In this work, BaFeO_x ceramics have been prepared by the classical solid state method using different sintering temperatures. The produced materials have shown promising thermoelectric properties when treatment temperatures are in the perovskite zone domain of the phase equilibrium diagram. In spite of the good values for the Seebeck coefficients, power factor is low due to the high resistivities measured in all cases.     

AgSn18SbTe20无铅中温热电材料的粉末冶金法制备工艺及其对性能的影响

研究了制备p型AgSn₁₈SbTe₂₀无铅热电材料的机械合金化(MA)结合放电等离子烧结(SPS)工艺, 调查了MA过程中球磨时间和SPS温度对材料电热传输性能和热电优值的影响, 分析了样品的物相和显微结构。研究表明, 适当延长球磨时间和降低烧结温度, 可以有效提高材料的热电性能。优化制备条件可以实现59%的性能提升, 最佳条件(球磨12 h、SPS温度743 K)下制备的样品ZT值在723 K达到0.62。     

Self-heating study of bulk acoustic wave resonators under high RF power

The present work first provides an experimental technique to study self-heating of bulk acoustic wave (BAW) resonators under high RF power in the gigahertz range. This study is specially focused on film bulk acoustic wave resonators and solidly mounted resonators processed onto silicon wafers and designed for wireless systems. Precisely, the reflection coefficient of a one-port device is measured while up to several watts are applied and power leads to electrical drifts of impedances. In the following, we describe how absorbed power can be determined from the incident one in real time. Therefore, an infrared camera held over the radio frequency micro electromechanical system (RF-MEMS) surface with an exceptional spatial resolution reaching up to 2 microm/pixels gives accurate temperature mapping of resonators after emissivity correction. From theoretical point of view, accurate three-dimensional (3-D) structures for finite-element modeling analyses are carried out to know the best materials and architectures to use for enhancing power handling. In both experimental and theoretical investigations, comparison is made between film bulk acoustic wave resonators and solidly mounted resonators. Thus, the trend in term of material, architecture, and size of device for power application such as in transmission path of a transceiver is clearly identified.     

Solution-processed graphene/MnO2 nanostructured textiles for high-performance electrochemical capacitors

Large scale energy storage system with low cost, high power, and long cycle life is crucial for addressing the energy problem when connected with renewable energy production. To realize grid-scale applications of the energy storage devices, there remain several key issues including the development of low-cost, high-performance materials that are environmentally friendly and compatible with low-temperature and large-scale processing. In this report, we demonstrate that solution-exfoliated graphene nanosheets (～5 nm thickness) can be conformably coated from solution on three-dimensional, porous textiles support structures for high loading of active electrode materials and to facilitate the access of electrolytes to those materials. With further controlled electrodeposition of pseudocapacitive MnO(2) nanomaterials, the hybrid graphene/MnO(2)-based textile yields high-capacitance performance with specific capacitance up to 315 F/g achieved. Moreover, we have successfully fabricated asymmetric electrochemical capacitors with graphene/MnO(2)-textile as the positive electrode and single-walled carbon nanotubes (SWNTs)-textile as the negative electrode in an aqueous Na(2)SO(4) electrolyte solution. These devices exhibit promising characteristics with a maximum power density of 110 kW/kg, an energy density of 12.5 Wh/kg, and excellent cycling performance of ～95% capacitance retention over 5000 cycles. Such low-cost, high-performance energy textiles based on solution-processed graphene/MnO(2) hierarchical nanostructures offer great promise in large-scale energy storage device applications.     

流体组装制造的金属-碳纳米管-金属结构

基于碳纳米管的纳米器件制造中,金属-碳纳米管-金属结构是重要部分通过流体组装多壁碳纳米管,并在其上沉积、光刻图形化金属制造了金属-碳纳米管-金属结构．对比不同流体组装条件下的排列效果发现,较大的样品台倾角和较高的气流速度可实现较好的碳纳米管组装．通过对碳纳米管与金属间的接触电阻进行估算和比较,金属包覆碳纳米管比碳纳米管沉积在全电极上的接触方式具有更小的接触电阻和接触电阻分散性,且碳纳米管与金电极接触状态比钛电极好最终通过该方法制造出了接触良好的金属-碳纳米管-金属结构。     

Temperature dependence of transport anisotropy of planar-type graphite nanostructures fabricated by focused ion beam

We report in this paper on the observation of temperature-dependent anisotropic transport behavior for planar-type nanostructures (in-plane and out-of-plane) fabricated on a thin graphite layer using a three-dimensional focused-ion-beam (FIB) etching technique. The transport characteristics were studied for several in-plane areas with sizes of 6 x 6 microm2, 6 x 4 microm2 and 6 x 2 microm2 planar-type structures/patterns and out-of-plane structures with the dimensions of 2 x 1 x 0.3 microm3. Both in-plane (rho(a)) and out-of-plane (rho(c)) resistivities are measured for these structures and the ratio of resistivity anisotropy is determined. The observed values of anisotropy ratio rho(c)/rho(a) were approximately 12.5 at 300 K and approximately 54 at 25 K. The room temperature value of rho(c)/rho(a) varies by a few orders from the values of previously reported anisotropy results of bulk pyrolytic graphite. However, the value of resistivity anisotropy increases with decreasing temperature, which is an identical behavior to bulk pyrolytic graphite. From current (I)-voltage (V) characteristics, we observed an ohmic behavior at 300 K for both low- and high-current biasing. This behavior turns into nonlinear characteristics when the temperature goes down. As these fabricated structures consist of multiple elementary junctions along the c-axis, nonlinear I-V characteristics result. The impurity assisted interlayer hopping conduction and thermal excitation of carriers play a key role in this effect.     

Micromachining of a bimorph Pb(Zr,Ti)O3 (PZT) cantilever using a micro-electromechanical systems (MEMS) process for energy harvesting application

We designed and fabricated a bimorph Pb(Zr,Ti)O3 (PZT) cantilever with an integrated Si proof mass to obtain a low resonant frequency for an energy harvesting application. The cantilevers were fabricated on the micro-electromechanical systems (MEMS) scale. A mode of piezoelectric conversions were d31 and d33 mode in cantilever vibration Therefore, we designed and fabricated a single cantilever with d31 unimorph, d31 bimorph, d33 unimorph, and d33 bimorph modes. Finally, we fabricated a device with beam dimensions of about 5,400 microm x 480 microm x 14 microm (< +/- 5%), and an integrated Si proof mass with dimensions of about 1,481 microm x 988 microm x 450 microm (< +/- 5%). In order to measure the d31 and d33 modes, we fabricated top and bottom electrodes. The distance between the top electrodes was 50 microm and the resonant frequency was 89.4 Hz. The average powers of the d31 unimorph, d31 bimorph, d33 unimorph, and d33 bimorph modes were 3.90, 9.60, 21.42, and 22.47 nW at 0.8 g (g = 9.8 m/s2) and optimal resistance, respectively.     

Inorganic-Based Printed Thermoelectric Materials and Devices

One of the simplest ways to generate electric power from waste heat is thermoelectric (TE) energy conversion. So far, most of the research on thermoelectrics has focused on inorganic bulk TE materials and their device applications. However, high production costs per power output and limited shape conformity hinder applications of state-of-the-art thermoelectric devices (TEDs). In recent years, printed thermoelectrics has emerged as an exciting pathway for their potential in the production of low-cost shape-conformable TEDs. Although several inorganic bulk TE materials with high performance are successfully developed, achieving high performance in inorganic-based printed TE materials is still a challenge. Nevertheless, significant progress has been made in printed thermoelectrics in recent years. In this review article, it is started with an introduction signifying the importance of printed thermoelectrics followed by a discussion of theoretical concepts of thermoelectricity, from fundamental transport phenomena to device efficiency. Afterward, the general process of inorganic TE ink formulation is summarized, and the current development of the inorganic and hybrid inks with the mention of their TE properties and their influencing factors is elaborated. In the end, TEDs with different architecture and geometries are highlighted by documenting their performance and fabrication techniques.     

Study on the micro-heater geometry in In,2O3 micro electro mechanical systems gas sensor platforms and effects on NO2 gas detecting performances

Micro electro mechanical systems (MEMS) platforms for gas sensing devices with the co-planar type micro-heaters were designed, fabricated and its effects on the In2O3 gas sensors were investigated. Micro-heaters in MEMS gas sensor platforms were designed in the four-type heater patterns with different geometries. Electro-thermal characterizations showed that the designed platforms had highly thermal efficiency because the micro hot-plate structures were formed in the diaphragm and the thermal efficiencies were analyzed for all of 16 models and compared with each other, respectively. The designed micro-platforms were fabricated by MEMS process, and Indium oxide (In2O3) nanoparticles were synthesized by sol-gel process and dropped on the MEMS platforms for detecting the noxious oxide gas (NO2) Fabricated micro-platforms had a very low power consumption in the fabricated 16-type models, especially, the minimum power consumption was 41 mW at the operating temperature of 250 degrees C. After experiments on gas sensing characteristics to NO2 gases, fabricated In2O3 gas sensors had almost the same gas sensitivity (Rs) at the operation temperature of 250 degrees C. It is concluded that the micro-heater geometries, pattern shapes and sizes, can be influential on the power consumption of the devices and its gas sensing characteristics.     

适用于微机电系统的自适应红外探测器的可调谐滤光器设计与开发

微机电系统与高品质红外探测技术联合运用为国防、商业、通信、生物医学检测及环境监测等许多应用领域面临的一系列具有挑战性的问题提供了唯一可行的解决方案．可调谐法布里．玻罗滤光片是适用于微机电系统的红外探测器的核心部件．滤光片的结构设计和关键结构件的材料对于滤光片的性能和整个装置的完善性有重要影响．阐述了利用有限元建模进行法布里-玻罗滤光片机械设计和分析的方法．报告了滤光片的结构材料——用低温等离子增强化学沉积法制造的氮化硅的结构表征和机械性能测定方法和结果．最后展示了一些所制作的滤光片阵列．     

Notching效应在MEMS单步干法制造工艺中的应用

研究了一种基于深反应离子刻蚀（DRIE）中notching效应的MEMS单步干法制造工艺.首先,基于DRIE刻蚀SOI硅片时notching现象产生的机理,设计了多种不同线宽的槽结构,验证notching效应的发生条件.实验结果表明,对于所采用的具有30μm器件层的SOI硅片,发生notching现象的临界槽宽为12μm,而notching释放的极限结构宽度同样为12μm.其次,为实现大面积结构的notching释放,研究了正方形、矩形、三角形及六边形等4种典型释放孔结构的干法释放效果.实验结果表明,六边形释放孔不但能够快速有效地释放结构,同时还能降低notch-ing效应的磨损,有利于惯性MEMS器件的加工.最后,设计了一种Z轴微机械陀螺结构以验证提出的设计及工艺.加工及测试结果表明,所提出的单步干法制造工艺完全满足微机械陀螺设计加工要求,工艺简单、成品率高,所测试的陀螺在常压下即可达到122的品质因数.     

n型填充方钴矿热电材料的快速制备研究

如何有效控制方钴矿基热电材料的制备成本成为其商业化应用的瓶颈。本课题组采用一种简单并且可放量的方法来制备n型填充方钴矿热电材料。该法由感应熔融淬、火和放电等离子烧结(SPS)组成, 制备周期(少于30 min)远小于传统制备方法: 电阻炉熔融(超过24 h),退火(1 w)和SPS。该法同传统制备工艺相当, 制备的方钴矿块体材料具有相对均匀的物相成份和组织结构, 以及良好的热电性能, 这得益于将经历感应熔融、淬火冷凝工艺形成的Sb/CoSb/CoSb₂包晶偏析结构破坏, 能同时实现快速反应和致密化。良好的热电性能和较少的生产周期及能耗, 使该法有望发展成为具有潜在应用前景的填充方钴矿热电材料工业化制备工艺。     

Probabilistic Weibull behavior and mechanical properties of MEMS brittle materials

The objective of this work is to present a brief overview of a probabilistic design methodology for brittle structures, review the literature for evidence of probabilistic behavior in the mechanical properties of MEMS (especially strength), and to investigate whether evidence exists that a probabilistic Weibull effect exists at the structural microscale. Since many MEMS devices are fabricated from brittle materials, that raises the question whether these miniature structures behave similar to bulk ceramics. For bulk ceramics, the term Weibull effect is used to indicate that significant scatter in fracture strength exists, hence requiring probabilistic rather than deterministic treatment. In addition, the material's strength behavior can be described in terms of the Weakest Link Theory (WLT) leading to strength dependence on the component's size (average strength decreases as size increases), and geometry/loading configuration (stress distribution). Test methods used to assess the mechanical properties of MEMS, especially strength, are reviewed. Four materials commonly used to fabricate MEMS devices are reviewed in this report. These materials are polysilicon, single crystal silicon (SCS), silicon nitride, and silicon carbide.     

Three-dimensional fluidic self-assembly by axis translation of two-dimensionally fabricated microcomponents in railed microfluidics

A method for high-throughput 3D self-assembly of 2D photopatterned microstructures using railed microfluidics is presented. Vertical device patterning of heterogeneous materials requires high-level integration using conventional microelectromechanical system (MEMS) technology; however, 3D railed assembly enables easy and fast self-assembly via a fluidic axis-translation process and simple material exchange in microfluidic channels. Individually photopatterned 2D microstructures are axis-translated from in-plane to out-of-plane and fluidically self-assembled, guided by side-rails in microfluidic channels to form a 3D morphology. Since the structures are fabricated in fluidic environments, there are no fixed initial points on the channel substrate allowing fluidic horizontal stacking of erected 2D structures. The guiding mechanism of railed microfluidics enables efficient fluidic handling and deterministic 3D self-assembly of heterogeneous components such as electronic components or polymeric microstructures using only fluidic force.     

Bi2Te3基可穿戴温差发电器件的制备及性能

利用人体体温发电的热电器件因其结构简单、可靠性高,有望为可穿戴电子产品等低功耗设备提供免维护、长期稳定的能源。以高性能无机块体热电材料和低热导环氧树脂/玻璃微珠复合粘结剂作为原料,采用切割粘结法和磁控溅射/电化学镀铜技术,制备了热电臂高度不同的48对温差发电器件。由于该技术不需使用陶瓷覆铜板,在给定的器件厚度条件下,可提高热电臂高度。性能表征结果显示,在实际穿戴条件下,随热电臂高度的增加,器件的输出功率密度持续增加。在相当于一级风的空气对流条件下或正常行走状态下,热电臂高度为3.14mm的器件输出功率密度超过40μW/cm²。     

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power.Moreover,the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades.Among these compounds,layered two-dimensional(2D)materials,such as graphene,black phosphorus,transition metal dichalcogenides,IVA–VIA compounds,and MXenes,have generated a large research attention as a group of potentially high-performance thermoelectric materials.Due to their unique electronic,mechanical,thermal,and optoelectronic properties,thermoelectric devices based on such materials can be applied in a variety of applications.Herein,a comprehensive review on the development of 2D materials for thermoelectric applications,as well as theoretical simulations and experimental preparation,is presented.In addition,nanodevice and new applications of 2D thermoelectric materials are also introduced.At last,current challenges are discussed and several prospects in this field are proposed.     

Metastable structures and isotope exchange reactions in polyoxometalate ions provide a molecular view of oxide dissolution

Reactions involving minerals and glasses in water are slow and difficult to probe spectroscopically but are fundamental to the performance of oxide materials in green technologies such as automotive thermoelectric power generation, CO2 capture and storage and water-oxidation catalysis; these must be made from geochemically common elements and operate in hydrous environments. Polyoxometalate ions (POMs) have structures similar to condensed oxide phases and can be used as molecular models of the oxide/water interface. Oxygen atoms in POM exchange isotopes at different rates, but, at present, there is no basis for predicting how the coordination environment and metal substitution influences rates and mechanisms. Here we identify low-energy metastable configurations that form from the breaking of weak bonds between metals and underlying highly coordinated oxygen atoms, followed by facile hydroxide, hydronium or water addition. The mediation of oxygen exchange by these stuffed structures suggests a new view of the relationship between structure and reactivity at the oxide/solution interface.     

微型热电器件研究进展

历经了30年的低谷期后,伴随着新型热电材料的出现及增大材料优值系数ZT新思路的提出,热电材料和器件这一研究领域开始了复兴.由于自身的诸多优点,微型热电器件已成为该领域的研究热点.作为新型的热电器件,微型热电器件在发电、制冷和传感器等领域获得了重要应用.综述了微型热电器件在应用、结构和制备技术方面的研究进展.