Nanostructured Thermoelectric Materials and High-Efficiency Power-Generation Modules

For thermoelectric applications, the best materials have high electrical conductivity and thermopower and, simultaneously, low thermal conductivity. Such a combination of properties is usually found in heavily doped semiconductors. Renewed interest in this topic has followed recent theoretical predictions that significant increases in performance are possible for nanostructured materials, and this has been experimentally verified. During exploratory synthetic studies of chalcogenide-based bulk thermoelectric materials it was discovered that several compounds spontaneously formed endotaxially embedded nanostructures. These compounds have some of the best known properties for bulk thermoelectric materials in the 500–800 K temperature range. Here we report our continued efforts to better understand the role of the nanostructures while concurrently furthering the development of these new materials (for example n-type lead–antimony–silver–tellurium, and p-type lead–antimony–silver–tin–tellurium) into thermoelectric power-generation devices.     

Modeling Energy Recovery Using Thermoelectric Conversion Integrated with an Organic Rankine Bottoming Cycle

Engine and industrial waste heat are sources of high-grade thermal energy that can potentially be utilized. This paper describes a model system that employs thermoelectric conversion as a topping cycle integrated with an organic Rankine bottoming cycle. The model has many parameters that define combined system quantities such as overall output power and conversion efficiency. The model can identify the optimal performance points for both the thermoelectric and organic Rankine bottoming cycle. Key analysis results are presented showing the impact of critical design parameters on power output and system performance.     

Solid-State Self-Assembly of Nanostructured Oxide as a Candidate High-Performance Thermoelectric Material

We have focused on the recently reported nanostructured bulk ZnMn_2−x Ga _x O₄ to evaluate whether this type of nanostructured oxide can effectively reduce thermal conductivity. Firstly, powdered samples of ZnMn_2−x Ga _x O₄ (x = 0 to 2) were prepared and the effect of heat treatment on the obtained phases was examined. Secondly, we have picked out the composition of ZnMnGaO₄, in which two distinct types of rectangular nanorods with different compositions spontaneously interlace to form a cross-sectional checkerboard pattern. To confirm the effect of nanostructure on thermal transport properties, the room-temperature thermal conductivity of this nanostructured oxide was evaluated.     

Approach to the Practical Use of Thermoelectric Power Generation

The results of research and development in the Japanese national project “Development for Advanced Thermoelectric Conversion Systems” are summarized, and the approaches to practical use of advanced thermoelectric modules and power generation systems are presented. The 5-year national project was successfully completed in March 2007. Three kinds of high- efficiency cascaded thermoelectric modules and two kinds of innovative Bi-Te thermoelectric modules were successfully developed. Heat cycle tests for three types of modules were also completed. Moreover, four types of advanced thermoelectric power generation systems were experimentally demonstrated for recovery of waste heat from the industrial and private sectors. In order to proceed further, thermoelectric power generation systems using practical heat sources were followed after installation of the developed modules. In parallel, various approaches for practical use by private companies, as well as plans for the next-phase project by the National Institute of Advanced Industrial Science and Technology (AIST) and the Engineering Advancement Association (ENAA), were also followed. The scenarios to proceed to the commercial phase of thermoelectric power generation are discussed on the basis of the results of the national project.     

Microstructure‐Lattice Thermal Conductivity Correlation in Nanostructured PbTe0.7S0.3 Thermoelectric Materials

The reduction of thermal conductivity, and a comprehensive understanding of the microstructural constituents that cause this reduction, represent some of the important challenges for the further development of thermoelectric materials with improved figure of merit. Model PbTe‐based thermoelectric materials that exhibit very low lattice thermal conductivity have been chosen for this microstructure–thermal conductivity correlation study. The nominal PbTe_0.7S_0.3 composition spinodally decomposes into two phases: PbTe and PbS. Orderly misfit dislocations, incomplete relaxed strain, and structure‐modulated contrast rather than composition‐modulated contrast are observed at the boundaries between the two phases. Furthermore, the samples also contain regularly shaped nanometer‐scale precipitates. The theoretical calculations of the lattice thermal conductivity of the PbTe_0.7S_0.3 material, based on transmission electron microscopy observations, closely aligns with experimental measurements of the thermal conductivity of a very low value, ～0.8 W m^?1 K^?1 at room temperature, approximately 35% and 30% of the value of the lattice thermal conductivity of either PbTe and PbS, respectively. It is shown that phase boundaries, interfacial dislocations, and nanometer‐scale precipitates play an important role in enhancing phonon scattering and, therefore, in reducing the lattice thermal conductivity.     

Silicon Nanocrystals as an Enabling Material for Silicon Photonics

Silicon nanocrystals (Si-nc) is an enabling material for silicon photonics, which is no longer an emerging field of research but an available technology with the first commercial products available on the market. In this paper, properties and applications of Si-nc in silicon photonics are reviewed. After a brief history of silicon photonics, the limitations of silicon as a light emitter are discussed and the strategies to overcome them are briefly treated, with particular attention to the recent achievements. Emphasis is given to the visible optical gain properties of Si-nc and to its sensitization effect on Er ions to achieve infrared light amplification. The state of the art of Si-nc applied in a few photonic components is reviewed and discussed. The possibility to exploit Si-nc for solar cells is also presented. In addition, nonlinear optical effects, which enable fast all-optical switches, are described.     

扩展电阻技术在硅,硅基材料测试中的应用

本文利用扩展电阻技术对半导体硅、硅基材料进行测试分析 ,从而用以开发新材料和评估材料的质量。     

Confinement of Thermoresponsive Hydrogels in Nanostructured Porous Silicon Dioxide Templates

A thermoresponsive hydrogel, poly(N‐isopropylacrylamide) (poly(NIPAM)), is synthesized in situ within an oxidized porous Si template, and the nanocomposite material is characterized. Infiltration of the hydrogel into the interconnecting nanoscale pores of the porous SiO₂ host is confirmed by scanning electron microscopy. The optical reflectivity spectrum of the nanocomposite hybrid displays Fabry–Pérot fringes characteristic of thin film interference, enabling direct, real‐time observation of the volume phase transition of the confined poly(NIPAM) hydrogel. Reversible optical reflectivity changes are observed to correlate with the temperature‐dependent volume phase transition of the hydrogel, providing a new means of studying nanoscale confinement of responsive hydrogels. The confined hydrogel displays a swelling and shrinking response to changes in temperature that is significantly faster than that of the bulk hydrogel. The porosity and pore size of the SiO₂ template, which are precisely controlled by the electrochemical synthesis parameters, strongly influence the extent and rate of changes in the reflectivity spectrum of the nanocomposite. The observed optical response is ascribed to changes in both the mechanical and the dielectric properties of the nanocomposite.     

Nanostructured Silicon Anodes for High‐Performance Lithium‐Ion Batteries

Despite the high theoretical capacity of Si anodes, the electrochemical performance of Si anodes is hampered by severe volume changes during lithiation and delithiation, leading to poor cyclability and eventual electrode failure. Nanostructured silicon and its nanocomposite electrodes could overcome this problem holding back the deployment of Si anodes in lithium‐ion batteries (LIBs) by providing facile strain relaxation, short lithium diffusion distances, enhanced mass transport, and effective electrical contact. Here, the recent progress in nanostructured Si‐based anode materials such as nanoparticles, nanotubes, nanowires, porous Si, and their respective composite materials and fabrication processes in the application of LIBs have been reviewed. The ability of nanostructured Si materials in addressing the above mentioned challenges have been highlighted. Future research directions in the field of nanostructured Si anode materials for LIBs are summarized.     

Enhancement of Thermoelectric Figure‐of‐Merit by a Bulk Nanostructuring Approach

Recently a significant figure‐of‐merit (ZT) improvement in the most‐studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high‐energy ball milling and a direct‐current‐induced hot‐press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.     

Automotive Applications of Thermoelectric Materials

This report reviews several existing and potential automotive applications of thermoelectric technology. Material and device issues related to automotive applications are discussed. Challenges for automotive thermoelectric applications are highlighted.     

SCLC法测量非晶硅有效隙态密度

提出了用空间电荷限制电流（SCLC）法测量非晶硅材料的有效隙态密度的新方法，并且报告了用4061A型半导体综合测试仪测量有效隙态密度的结果。测量结果发现与用低频电容法所得结果相符。     

Phonon Scattering and Thermal Conductivity in p‐Type Nanostructured PbTe‐BaTe Bulk Thermoelectric Materials

Transmission electron microscopy studies show that a PbTe‐BaTe bulk thermoelectric system represents the coexistence of solid solution and nanoscale BaTe precipitates. The observed significant reduction in the thermal conductivity is attributed to the enhanced phonon scattering by the combination of substitutional point defects in the solid solution and the presence of high spatial density of nanoscale precipitates. In order to differentiate the role of nanoscale precipitates and point defects in reducing lattice thermal conductivity, a modified Callaway model is proposed, which highlights the contribution of point defect scattering due to solid solution in addition to that of other relevant microstructural constituents. Calculations indicate that in addition to a 60% reduction in lattice thermal conductivity by nanostructures, point defects are responsible for about 20% more reduction and the remaining reduction is contributed by the collective of dislocation and strain scattering. These results underscore the need for tailoring integrated length‐scales for enhanced heat‐carrying phonon scattering in high performance thermoelectrics.     

太阳能和厂房余热综合利用节能方案研究

针对南方某烟厂联合厂房的实际情况和环境特点,提出了利用太阳能资源和余热回收作为辅助热源对生活用水进行加热的方案。通过对厂房热水需求量和耗热量以及可用余热量进行计算,设计了两种余热利用方案,分别为地源热泵对冷却循环水余热利用和对烟气余热直接利用。通过对两种方案节能效果的比较,分析得出了两种方案的可行性,对厂房的节能设计有一定的指导意义。     

Printable Single‐Crystal Silicon Micro/Nanoscale Ribbons,Platelets and Bars Generated from Bulk Wafers

This article demonstrates a method for fabricating high quality single‐crystal silicon ribbons, platelets and bars with dimensions between ～ 100 nm and ～ 5 cm from bulk (111) wafers by using phase shift and amplitude photolithographic methods in conjunction with anisotropic chemical etching procedures. This “top‐down” approach affords excellent control over the thicknesses, lengths, and widths of these structures and yields almost defect‐free, monodisperse elements with well defined doping levels, surface morphologies and crystalline orientations. Dry transfer printing these elements from the source wafers to target substrates by use of soft, elastomeric stamps enables high yield integration onto wafers, glass plates, plastic sheets, rubber slabs or other surfaces. As one application example, bottom gate thin‐film transistors that use aligned arrays of ribbons as the channel material exhibit good electrical properties, with mobilites as high as ～ 200 cm² V^–1 s^–1 and on/off ratios > 10⁴.     

Performance Results of a High-Power-Density Thermoelectric Generator: Beyond the Couple

This paper describes the development of a high-power-density thermoelectric generator (TEG) with a power output of greater than 100 W. Previous papers have described the development of the generator made of high-power-density TE couples. In this discussion, initial thermal cycling results for the TE couples are described. The building blocks are then scaled and integrated into a complete TEG. The design, build, and test of the TEG are discussed. The high-power-density design produces power at greater than 250 W/L and 80 W/kg. Test results are shown for varying flow rates, temperatures, and electrical loads.     

Fabrication and Evaluation of a Thermoelectric Microdevice on a Free-Standing Substrate

Using shadow masks prepared by standard microfabrication processes, we fabricated in-plane thermoelectric microdevices (4 mm × 4 mm) made of bismuth telluride thin films, and evaluated their performance. We used Bi_0.4Te_3.0Sb_1.6 as the p-type semiconductor and Bi_2.0Te_2.7Se_0.3 as the n-type semiconductor. We deposited p- and n-type thermoelectric thin films on a free-standing thin film of Si₃N₄ (4 mm × 4 mm × 4 μm) on a Si wafer, and measured the output voltages of the microdevices while heating at the bottom of the Si substrate. The maximum output voltage of the thermoelectric device was 48 mV at 373 K.     

Fabrication of a Class of Nanostructured Materials Using Carbon Nanowalls as the Templates

Well‐aligned carbon nanowalls with a thickness of a few nanometers and a lateral size in the micrometer range have been grown on various types of substrates. The nanowalls exhibit a remarkably different surface morphology as compared to fullerenes and carbon nanotubes, in particular their two‐dimensionality and high surface area. In this work, we focused on the second aspect and developed a templating method to fabricate a class of nanostructured materials based on the novel surface morphology of the carbon nanowalls. These structures may have potential applications in batteries, gas sensors, catalysts, and light‐emission/detection, field‐emission, and biomedical devices.     

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.     

Porous Silicon Nanostructures as Effective Faradaic Electrochemical Sensing Platforms

The electrochemical performance of porous silicon (pSi) stabilized via thermal decomposition of acetylene gas is investigated for the first time. In this study, pSi undergoes two thermal treatments at either 525 or 800 °C, which result in hydrogen‐terminated thermally hydrocarbonized pSi (THCpSi) and hydroxyl‐terminated thermally carbonized pSi (TCpSi), respectively, the latter upon dipping in hydrofluoric acid to activate the surface termination. Electrochemical characterization, using cyclic voltammetry, chronocoulometry, and electrochemical impedance spectroscopy in the presence of several redox pairs, [Fe(CN)₆]^3/4?, [Ru(NH₃)₆]^2/3+, and hydroquinone/quinone, is used to demonstrate the versatility and high stability to degradation of carbon‐stabilized pSi nanostructures and their excellent electrochemical performance. Added to the large surface area, adjustable pore morphology and tailorable surface chemistry of THCpSi and TCpSi, these nanostructures demonstrate fast electron‐transfer kinetics, providing key advantages over conventional carbon electrodes. The versatile surface chemistry of THCpSi and TCpSi offer various possibilities to introduce multiple functional groups depending on the nature of the bioreceptor to be immobilized. For proof of principle, the use of a THCpSi‐based immunosensor to detect MS2 bacteriophage is demonstrated by means of electrochemical impedance spectroscopy, showing a detection limit of 4.9 pfu mL^?1. Carbon‐stabilized pSi structures represent a new class of nanostructured electrodes for biosensing applications.