低维纳米材料热电性能测试方法研究

通过近几十年的研究,人们对于块体及薄膜材料的热电性能已经有了较全面的认识,热电优值ZT的提高取得了飞速的进展,比如碲化铋相关材料、硒化亚铜相关材料、硒化锡相关材料的最大ZT值都突破了2.但是,这些体材料的热电优值距离大规模实用仍然有较大的差距.通过理论计算得知,当块体热电材料被制作成低维纳米结构材料时,比如二维纳米薄膜、一维纳米线,热电性能会得到显著的改善,具有微纳米结构材料的热电性能研究引起了科研人员的极大兴趣.当块体硅被制作成硅纳米线时,热电优值改善了将近100倍.然而,微纳米材料的热电参数测量极具挑战,因为块体材料的热电参数测量方法和测试平台已经不再适用于低维材料,需要开发出新的测量方法和测试平台用来研究低维材料的热导率、电导率和塞贝克系数.本文综述了几种用于精确测量微纳米材料热电参数的微机电结构,包括双悬空岛、单悬空岛、悬空四探针结构,详细介绍了每一种微机电结构的制备方法、测量原理以及对微纳米材料热电性能测试表征的实例.     

Low-dimensional thermoelectric materials

The promise of low dimensional thermoelectric materials for enhanced performance is reviewed, with particular attention given to quantum wells and quantum wires. The high potential of bismuth as a low-dimensional thermoelectric material is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 755–758 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Thin-film thermoelectric cooling

Basic physical features of thermoelectric cooling in p-n structures of a submicrometer thickness, which is manifested even in the linear approximation in current, are investigated. It is shown that a multitem-perature model in which each subsystem of quasi-particles is characterized by its own temperature is effective for studying such structures. The criteria for applicability of the one-temperature model are established. The common structure of the boundary conditions for heat fluxes through the surface at which thermoelectric cooling or heating takes place is revealed. Physical regularities are studied for the most interesting cases; the criteria for their implementation are also considered.     

Template-based synthesis of nanomaterials

The large interest in nanostructures results from their numerous potential applications in various areas such as materials and biomedical sciences, electronics, optics, magnetism, energy storage, and electrochemistry. Ultrasmall building blocks have been found to exhibit a broad range of enhanced mechanical, optical, magnetic, and electronic properties compared to coarser-grained matter of the same chemical composition. In this paper various template techniques suitable for nanotechnology applications with emphasis on characterization of created arrays of tailored nanomaterials have been reviewed. These methods involve the fabrication of the desired material within the pores or channels of a nanoporous template. Track-etch membranes, porous alumina, and other nanoporous structures have been characterized as templates. They have been used to prepare nanometer-sized fibrils, rods, and tubules of conductive polymers, metals, semiconductors, carbons, and other solid matter. Electrochemical and electroless depositions, chemical polymerization, sol-gel deposition, and chemical vapour deposition have been presented as major template synthetic strategies. In particular, the template-based synthesis of carbon nanotubes has been demonstrated as this is the most promising class of new carbon-based materials for electronic and optic nanodevices as well as reinforcement nanocomposites. Received: 27 May 1999 / Accepted: 27 October 1999 / Published online: 8 March 2000     

Influence of spherical inclusions on effective thermoelectric properties of thermoelectric composite materials

A homogenization theory is developed to predict the influence of spherical inclusions on the effective thermoelectric properties of thermoelectric composite materials based on the general principles of thermodynamics and Mori–Tanaka method. The closed-form solutions of effective Seebeck coefficient, electric conductivity, heat conductivity, and figure of merit for such thermoelectric materials are obtained by solving the nonlinear coupled transport equations of electricity and heat. It is found that the effective figure of merit of thermoelectric material containing spherical inclusions can be higher than that of each constituent in the absence of size effect and interface effect. Some interesting examples of actual thermoelectric composites with spherical inclusions, such as insulated cavities, inclusions subjected to conductive electric and heat exchange and thermoelectric inclusions, are considered, and the numerical results lead to the conclusion that considerable enhancement of the effective figure of merit is achievable by introducing inclusions. In this paper, we provide a theoretical foundation for analytically and computationally treating the thermoelectric composites with more complicated inclusion structures, and thus pointing out a new route to their design and optimization.     

Screened vacuum thermoelectric trap

A vacuum trap LVTÉ-1 is described, which contains guard elements in the form of a lightproof set of rings cooled by a two-stage thermoelectric battery and an antimigrator in the form of bellows connecting these elements with the outer case of the trap. The thermobattery is mounted in a separate pressurized compartment ensuring the possibility of its replacement without violating vacuum in the working volume. The effectiveness of the trap is elevating due to the screens protecting the cooled elements from external radiation and due to cooling of the screen and middle parts of the bellows of the antimigrator and the thermobattery compartment by its warmer stage.     

Risk-based classification system of nanomaterials

Various stakeholders are increasingly interested in the potential toxicity and other risks associated with nanomaterials throughout the different stages of a product’s life cycle (e.g., development, production, use, disposal). Risk assessment methods and tools developed and applied to chemical and biological materials may not be readily adaptable for nanomaterials because of the current uncertainty in identifying the relevant physico-chemical and biological properties that adequately describe the materials. Such uncertainty is further driven by the substantial variations in the properties of the original material due to variable manufacturing processes employed in nanomaterial production. To guide scientists and engineers in nanomaterial research and application as well as to promote the safe handling and use of these materials, we propose a decision support system for classifying nanomaterials into different risk categories. The classification system is based on a set of performance metrics that measure both the toxicity and physico-chemical characteristics of the original materials, as well as the expected environmental impacts through the product life cycle. Stochastic multicriteria acceptability analysis (SMAA-TRI), a formal decision analysis method, was used as the foundation for this task. This method allowed us to cluster various nanomaterials in different ecological risk categories based on our current knowledge of nanomaterial physico-chemical characteristics, variation in produced material, and best professional judgments. SMAA-TRI uses Monte Carlo simulations to explore all feasible values for weights, criteria measurements, and other model parameters to assess the robustness of nanomaterial grouping for risk management purposes.     

Solution combustion synthesis of nanomaterials

Solution combustion (SC) is an effective method for synthesis of nano-size materials and it has been used for the production of a variety (currently more than 1000) of fine complex oxide powders for different advanced applications, including catalysts, fuel cells, and biotechnology. However, it is surprising that while essentially all of the studies on SC emphasize the characterization of the synthesized materials, little information is available on controlling combustion parameters and the reaction mechanisms. This paper is devoted to the analysis of the combustion parameters for different SC reaction modes. First, the conventional volume combustion synthesis mode, which involves uniform reaction solution preheating prior to self-ignition, is briefly discussed. Second, for the first time, results of detailed experimental studies on steady-state self-propagating mode of SC synthesis of nano-powders are presented. Finally, the so-called solution + impregnation combustion mode is considered. The relationship between combustion parameters and product microstructures are emphasized. These results are crucial not only from the application stand-point, but more importantly lead to methodological benefits, allowing application of the developed approaches to investigate steady state heterogeneous combustion waves in new classes of reaction systems.     

Nonlinear optics of nontoxic nanomaterials

We studied the linear and nonlinear optical properties of halloysite nanotubes using Z-scan technique. Halloysite is alumina silicate clay rolled into 50 nm diameter hollow cylinders, where the silica layer is at the outer surface of the tube and alumina layer is in the inner surface. Optical absorption spectra show an absorption peak around 600 nm. Open aperture Z-scan measurements using 3 ns laser pulses at 532 nm reveal two-photon induced absorption. The closed aperture Z-scan indicates a positive nonlinear refractive index. As these nanotubes are nontoxic and biocompatible, they have advantages over conventional carbon nanotubes for biomedical applications.     

Photonic applications with photoanisotropic nanomaterials

Linearly polarized light beam can rotate the trans-azobenzene molecules into cis-form resulting in inducing photoanisotropy. This property is used to control the transmission of nanosecond and picosecond laser pulses and also for holographic data storage applications. Studies were performed using four commercially available azobenzene dyes in different solvents and also in custom prepared nanoporous azobenzene Dendron films. Nanopores are created in a silica matrix and azobenzene molecules are attached to the inner walls of these pores to mimic solution like environment for the azobenzene molecules such that they can undergo photoinduced trans-cis-trans isomerization process at a faster rate.     

Synthesis of camptothecin-loaded gold nanomaterials

Camptothecin-loaded gold nanomaterials have been synthesized by the sodium borohydride reduction method under a strong basic condition. The obtained gold nanomaterials have been characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and UV-vis absorption spectroscopy. The camptothecin-loaded gold colloidal solution was very stable and can be stored for more than two months at room temperature without obvious changes. The color of the colloidal solution can change from wine red to purple and blue during the acidifying process. It was revealed that the release of camptothecin and the aggregation of gold nanoparticles can be controlled by tuning the solution pH. The present study implied that the gold nanomaterials can be used as the potential carrier for CPT delivery.     

Interfacial properties of hybrid nanomaterials

A brief summary of our ongoing efforts to understand the surface properties of nanoparticles using fluorophores, namely pyrene alkanethiols, is presented. Excited state interactions were investigated by varying the length of the spacer group and the concentration of fluorophore. The flexible long alkyl chain tethering pyrene inAu-P2/Au-P3 allows free interaction between fluorophores resulting in excimer formation whereas the intermolecular interactions are limited in theAu-P 1 system due to the restriction imposed by the curvature of spherical gold nanoparticle. A gradual increase in the peak intensity ratio of III/I band of the normal fluorescence of pyrene was observed indicating that the surface of nanoparticle is more polar than the bulk solvent (toluene)     

Optimization and fabrication of a planar thermoelectric generator for a high-performance solar thermoelectric generator

A planar thermoelectric generator (TEG) could be used as an energy harvester for wireless sensor networks (WSNs). Its planar configuration offers the advantages of miniaturization and low-cost fabrication. Herein, we report the fabrication of a device based on a co-evaporated thick film of Bi–Te. Before fabrication we studied quantitative optimization dimensions for the planar TEG via finite element method (FEM) simulations. The planar TEG was also used in a high-performance solar thermoelectric generator (STEG) that concentrated heat from solar radiation via a solar absorber to generate a temperature difference (ΔT) between two thermoelectric and electrode junctions. When exposed to a solar simulator, the STEG produced a ΔT of about 6 °C and generated 2.3 μW of power. The demonstrated high flexibility and mechanical stability of this device suggests applications in wearable electronics.     

Measuring thermoelectric property of nano-heterostructure

A method of measuring the thermoelectric power of nano-heterostructures based on four-probe scanning tunneling microscopy is presented. The process is composed of the it in-situ fabrication of a tungsten-indium tip, the precise control of the tip-sample contact and the identification of thermoelectric potential. When the temperature of the substrate is elevated, while that of the tip is kept at room temperature, a thermoelectric potential occurs and can be detected by a current-voltage measurement. As an example of its application, the method is demonstrated to be effective to measure the thermoelectric power in several systems. A Seebeck coefficient of tens of μV/K is obtained in graphene epitaxially grown on Ru (0001) substrate and the thermoelectric potential polarity of this system is found to be the reverse of that of bare Ru (0001) substrate.     

Macro-performance of multilayered thermoelectric medium

The effective properties of thermoelectric composites are well known to depend on boundary conditions, which causes the macro performance of thermoelectric composite to be difficult to assess. The overall macro-performance of multilayered thermoelectric medium is discussed in this paper. The analytical solutions are obtained, including the heat flux, temperature,electric potential, and the overall energy conversion efficiency. The results show that there are unique relationships between the temperature/electric potential and the electric current/energy flux in the material, and whether the material is independent of or embedded in thermoelectric composites. Besides, the Peltier effect at the interface can significantly improve the overall energy conversion efficiency of thermoelectric composites. These results provide a powerful tool to analyze the effective behaviors of thermoelectric composites.     

Anisotropic thermoelectric power of TTF-TCNQ

We have measured the thermoelectric power along the a-axis as well as the highly conducting b-axis in TTF-TCNQ crystals. Measurements were taken both parallel and perpendicular to the long axis on two sets of crystals. One set grew along the a-axis and the other along the b-axis. The thermopowers are of opposite sign except near 60°K where both cross zero at slightly different temperatures. The a-axis thermopower is consistent with non-metallic diffusive transport in the a direction.     

Recent advances on thermoelectric materials

By converting waste heat into electricity through the thermoelectric power of solids without producing greenhouse gas emissions, thermoelectric generators could be an important part of the solution to today’s energy challenge. There has been a resurgence in the search for new materials for advanced thermoelectric energy conversion applications. In this paper, we will review recent efforts on improving thermoelectric efficiency. Particularly, several novel proof-of-principle approaches such as phonon disorder in phonon-glass-electron crystals, low dimensionality in nanostructured materials and charge-spin-orbital degeneracy in strongly correlated systems on thermoelectric performance will be discussed.