The interplay of structural and optical properties in individual ZnO nanostructures

Semiconductor nanostructures exhibit unique properties distinct from their bulk counterparts by virtue of nanoscale dimensions; in particular, exceptionally large surface area-to-volume ratios relative to that of the bulk produce variations in surface state populations that have numerous consequences on materials properties. Of the low-dimensional semiconductor nanostructures, nanowires offer a unique prospect in nanoscale optoelectronics due to their one-dimensional architecture. Already, many devices based upon individual nanowires have been demonstrated, but questions about how nano-size and structural variations affect the underlying materials properties still remain unanswered. Here, we focus on understanding the growth mechanism and kinetics of ZnO nanowires and related nanowalls, and their effects on nanoscale structural and optical properties.     

纳米技术在相变储热材料中的应用

结合纳米颗粒的特殊尺寸效应,把纳米技术运用到相变储热材料的制备和改进中,可获得纳米流体、纳米胶囊和复合纳米相变材料等一系列性能优异的纳米相变储热材料。新型的纳米相变材料逐渐被利用到生产和生活的各个储热领域,引领着相变储热技术的发展方向。本文综述了纳米相变材料的研究进展,介绍了纳米相变材料的应用,展望了纳米相变材料的未来发展方向。     

Herstellung von Nanokomposit-Dünnschichten mit speziellen magnetischen Eigenschaften aus nanopartikulären Bausteinen

The combination of materials with different magnetic properties at the nanoscale can lead to phenomena such as exchange bias via coupling effects, which shows huge potential for applications, e.g., in storage media or magnetic sensors. Whilst such structures so far are obtained via complex processes of alternating deposition from the gas phase hampering their application, their preparation from nanoparticle dispersions would not only be simplified but also enable a broad variety of possible material combinations and structures. Thereby, the processing of hard magnetic nanoparticles is challenging but can be circumvented by the use of precursor materials with subsequent phase transformation.     

The effect of flexoelectricity on domain switching in the vicinity of a crack in ferroelectrics

Ferroelectrics are widely used in the manufacture of transducers, actuators, and memory devices, due to their attractive electromechanical properties. However, the reliability and failure of devices is greatly dependent on their brittleness. In view of the fact that both the polarization distribution and elastic field are at nanoscale and vary greatly in the vicinity of the crack tip, flexoelectricity is expected to strongly affect the domain configuration. In this work, Ginzburg-Landau (TDGL) theory and the phase field method (PFM) are employed to analyze the influence of flexoelectric effect on the domain switching process in the vicinity of the crack tip of ferroelectric materials. The results obtained show that, the domain configuration would become asymmetric with increasing flexoelectric coefficients, and the flexoelectric effect has a larger influence on the polarization field than on the elastic field in the vicinity of the crack tip of ferroelectric materials.     

复合相变储热材料的研究与发展

系统概括和评述了复合相变材料的制备方法及其研究进展，并介绍了研制有机／无机纳米复合相变储热材料的创新思路及初步研究成果，提出，有机／无机纳米复合技术是制备高性能复合相变储热材料的新途径。     

Nanoparticles in Liquid Crystals: Synthesis,Self-Assembly,Defect Formation and Potential Applications

Revolutionary developments in the fabrication of nanosized particles have created enormous expectations in the last few years for the use of such materials in areas such as medical diagnostics and drug-delivery, and in high-tech devices. By its very nature, nanotechnology is of immense academic and industrial interest as it involves the creation and exploitation of materials with structural features in between those of atoms and bulk materials, with at least one dimension limited to between 1 and 100 nm. Most importantly, the properties of materials with nanometric dimensions are, in most instances, significantly different from those of atoms or bulk materials. Research efforts geared towards new synthetic procedures for shape and size-uniform nanoscale building blocks as well as efficient self-assembly protocols for manipulation of these building blocks into functional materials has created enormous excitement in the field of liquid crystal research. Liquid crystals (LCs) by their very nature are suitable candidates for matrix-guided synthesis and self-assembly of nanoscale materials, since the liquid crystalline state combines order and mobility at the molecular (nanoscale) level. Based on selected relevant examples, this review attempts to give a short overview of current research efforts in LC-nanoscience. The areas addressed in this review include the synthesis of nanomaterials using LCs as templates, the design of LC nanomaterials, self-assembly of nanomaterials using LC phases, defect formation in LC-nanoparticle suspensions, and potential applications. Despite the seeming diversity of these research topics, this review will make an effort to establish logical links between these different research areas.     

Electromechanical Imaging and Spectroscopy of Ferroelectric and Piezoelectric Materials: State of the Art and Prospects for the Future

Piezoresponse force microscopy (PFM) has emerged as a powerful and versatile tool for probing nanoscale phenomena in ferroelectric materials on the nanometer and micrometer scales. In this review, we summarize the fundamentals and recent advances in PFM, and describe the nanoscale electromechanical properties of several important ferroelectric ceramic materials widely used in memory and microelectromechanical systems applications. Probing static and dynamic polarization behavior of individual grains in PZT films and ceramics is discussed. Switching spectroscopy PFM is introduced as a useful tool for studying defects and interfaces in ceramic materials. The results on local switching and domain pinning behavior, as well as nanoscale fatigue and imprint mapping are presented. Probing domain structures and polarization dynamics in polycrystalline relaxors (PMN-PT, PLZT, doped BaTiO₃) are briefly outlined. Finally, applications of PFM to dimensionally confined ferroelectrics are demonstrated. The potential of PFM for studying local electromechanical phenomena in polycrystalline ferroelectrics where defects and other inhomogeneities are essential for the interpretation of their macroscopic properties is illustrated.     

添加碳素复(混)合相变储热材料的研究及应用进展

相变储热材料因具有储热密度大、相变温度变化小且过程易控制等优点而在许多领域具有重要应用。但传统的相变储热材料存在导热系数低及固-液相变过程中液态泄漏问题,阻碍了其实际应用。碳材料如石墨、碳纤维、碳泡沫和膨胀石墨,他们都具有高导热系数、低密度和良好的化学稳定性。将碳材料添加到相变储热材料中或与相变储热材料进行复合,从而构成碳素复(混)合相变储热材料,储热材料的导热系数及其性能可明显提高。本文综述了碳素复(混)合相变储热材料的研究进展。利用膨胀石墨的多孔特性吸附有机物制备膨胀石墨基复合相变储热材料,其储热密度大、导热系数高、性能稳定、成本低且在固-液相变过程中没有液态的流动性问题,是未来研究和应用最重要的碳素复合相变储热材料。     

Multi‐Component Self‐Assembly of Proteins and Inorganic Particles: From Discrete Structures to Biomimetic Materials

Due to their defined structure, proteins are suitable building blocks for the bottom‐up construction of multi‐component materials. Especially protein containers, with their inherent cavity, can be used to encapsulate synthetic components such as inorganic nanoparticles. This way, multi‐component discrete structures can be assembled. With recent advances in computational protein design, novel protein containers were successfully created, with a high potential for application in biomedicine and materials research. Moreover, engineered protein containers offer a unique building block for the self‐assembly of three‐dimensional materials. They combine the molecular precision of proteins with nanoscale dimensions. Designed interactions lead to novel protein scaffolds. In addition, nanoparticles encapsulated inside the container cavity introduce orthogonal functionality, important for the realization of nanostructured biomimetic materials with emergent properties.     

Porosity reduction of polyethylene glycol phase change materials by using nanoscale thermal‐energy‐conducting medium during crystallization process

Poly(ethylene glycols) (PEGs) are phase change materials (PCMs) that exhibit undesirable heat transfer properties, which restrict their industrial utility. Apart from the intrinsic material properties, a large quantity of micropores in the crystalline PEG polymers cause poor heat transfer performance. In this work, the formation and growth of micropores are reported through in situ characterization. The addition of a nanoscale thermal‐energy‐conducting medium into PCMs has been proposed for reducing their porosity. The mechanism for reducing porosity is reported for prepared composite PCMs. The intrinsic causes are thought to be the following. Metal oxide nanoparticles can migrate with the liquid PEG flow, which can reduce the thermal stresses in the crystal growth process. In addition, the nanoscale medium promotes heterogeneous nucleation. The results of this study show that reducing the porosity of the polymer crystals is an important approach for improving the heat transfer properties of the PEG PCMs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45446.     

脂肪酸相变储能材料热性能研究进展

在以往所研究的相变材料中,脂肪酸由于展现出优越的性能,得到了研究者更多的关注,但同样存在相变温度不适宜和导热性能差等热性能问题。本工作通过现有文献对脂肪酸相变储能材料的热性能进行系统分析,提出了脂肪酸与脂肪酸、脂肪醇及石蜡复合3种有效解决相变温度不适宜的方法;针对导热性能差提出了多孔材料吸附、添加碳材料或金属粒子和微胶囊化3种高效易行的强化传热方式,进而说明这一领域目前研究重点。同时,对脂肪酸储能材料的相变性能、导热增强方法及导热增强剂进行了比较,分析了各自的优缺点。最后,对脂肪酸相变储能材料热性能研究的不足之处进行了探究,并指出了制备出更多能应用于建筑节能和纺织等领域的脂肪酸相变储能材料和着重研究脂肪酸与石蜡的复合等进一步研究方向。     

Manipulating crystal growth and polymorphism by confinement in nanoscale crystallization chambers

The phase behaviors of crystalline solids embedded within nanoporous matrices have been studied for decades. Classic nucleation theory conjectures that phase stability is determined by the balance between an unfavorable surface free energy and a stabilizing volume free energy. The size constraint imposed by nanometer-scale pores during crystallization results in large ratios of surface area to volume, which are reflected in crystal properties. For example, melting points and enthalpies of fusion of nanoscale crystals can differ drastically from their bulk scale counterparts. Moreover, confinement within nanoscale pores can dramatically influence crystallization pathways and crystal polymorphism, particularly when the pore dimensions are comparable to the critical size of an emerging nucleus. At this tipping point, the surface and volume free energies are in delicate balance and polymorph stability rankings may differ from bulk. Recent investigations have demonstrated that confined crystallization can be used to screen for and control polymorphism. In the food, pharmaceutical, explosive, and dye technological sectors, this understanding and control over polymorphism is critical both for function and for regulatory compliance. This Account reviews recent studies of the polymorphic and thermotropic properties of crystalline materials embedded in the nanometer-scale pores of porous glass powders and porous block-polymer-derived plastic monoliths. The embedded nanocrystals exhibit an array of phase behaviors, including the selective formation of metastable amorphous and crystalline phases, thermodynamic stabilization of normally metastable phases, size-dependent polymorphism, formation of new polymorphs, and shifts of thermotropic relationships between polymorphs. Size confinement also permits the measurement of thermotropic properties that cannot be measured in bulk materials using conventional methods. Well-aligned cylindrical pores of the polymer monoliths also allow determination and manipulation of nanocrystal orientation. In these systems, the constraints imposed by the pore walls result in a competition between crystal nuclei that favors those with the fastest growth direction aligned with the pore axis. Collectively, the examples described in this Account provide substantial insight into crystallization at a size scale that is difficult to realize by other means. Moreover, the behaviors resulting from nanoscopic confinement are remarkably consistent for a wide range of compounds, suggesting a reliable approach to studying the phase behaviors of compounds at the nanoscale. Newly emerging classes of porous materials promise expanded explorations of crystal growth under confinement and new routes to controlling crystallization outcomes.     

Characterization of nanocellulose‐ reinforced shape memory polyurethanes

BACKGROUND: Shape memory polymers are capable of fixing a transient shape and of recovering their original dimensions by the application of an external stimulus. Their major drawback is their low stiffness compared to smart materials based on metals and ceramics. To overcome this disadvantage, nanocellulose was utilized as reinforcement. RESULTS: Composites were prepared by casting stable nanocellulose/segmented polyurethane suspensions. The heat of melting of the polyurethane soft segment phase increased on cellulose addition. Composites showed higher tensile modulus and strength than unfilled films (53% modulus increase at 1 wt% nanocellulose), with higher elongation at break. Creep deformation decreased as cellulose concentration increased (36% decrease in 60‐minute creep by addition of 1 wt% nanocellulose). The nanocomposites displayed shape memory properties equivalent to those of the neat polyurethane, with recoveries of the order of 95% (referred to second and further cycles). CONCLUSIONS: It is possible to markedly improve the rigidity of shape memory polymers by adding small amounts of well‐dispersed nanocellulose. However, this improvement did not have substantial effects on the material shape fixity or recovery. Shape memory behavior seems to continue to be controlled by the polymer properties. Copyright © 2007 Society of Chemical Industry     

Aggregate nanostructures of organic molecular materials

Conjugated organic molecules are interesting materials because of their structures and their electronic, electrical, magnetic, optical, biological, and chemical properties. However, researchers continue to face great challenges in the construction of well-defined organic compounds that aggregate into larger molecular materials such as nanowires, tubes, rods, particles, walls, films, and other structural arrays. Such nanoscale materials could serve as direct device components. In this Account, we describe our recent progress in the construction of nanostructures formed through the aggregation of organic conjugated molecules and in the investigation of the optical, electrical, and electronic properties that depend on the size or morphology of these nanostructures. We have designed and synthesized functional conjugated organic molecules with structural features that favor assembly into aggregate nanostructures via weak intermolecular interactions. These large-area ordered molecular aggregate nanostructures are based on a variety of simpler structures such as fullerenes, perylenes, anthracenes, porphyrins, polydiacetylenes, and their derivatives. We have developed new methods to construct these larger structures including organic vapor-solid phase reaction, natural growth, association via self-polymerization and self-organization, and a combination of self-assembly and electrochemical growth. These methods are both facile and reliable, allowing us to produce ordered and aligned aggregate nanostructures, such as large-area arrays of nanowires, nanorods, and nanotubes. In addition, we can synthesize nanoscale materials with controlled properties. Large-area ordered aggregate nanostructures exhibit interesting electrical, optical, and optoelectronic properties. We also describe the preparation of large-area aggregate nanostructures of charge transfer (CT) complexes using an organic solid-phase reaction technique. By this process, we can finely control the morphologies and sizes of the organic nanostructures on wires, tubes, and rods. Through field emission studies, we demonstrate that the films made from arrays of CT complexes are a new kind of cathode materials, and we systematically investigate the effects of size and morphology on electrical properties. Low-dimension organic/inorganic hybrid nanostructures can be used to produce new classes of organic/inorganic solid materials with properties that are not observed in either the individual nanosize components or the larger bulk materials. We developed the combined self-assembly and templating technique to construct various nanostructured arrays of organic and inorganic semiconductors. The combination of hybrid aggregate nanostructures displays distinct optical and electrical properties compared with their individual components. Such hybrid structures show promise for applications in electronics, optics, photovoltaic cells, and biology. In this Account, we aim to provide an intuition for understanding the structure-function relationships in organic molecular materials. Such principles could lead to new design concepts for the development of new nonhazardous, high-performance molecular materials on aggregate nanostructures.     

基于超声衰减和声速动态监测石蜡的相变过程

为研究石蜡在相变过程中内部结构和状态的改变特性,用中心频率为5 MHz的脉冲式超声波动态测量石蜡的相变过程,采集并分析不同升降温速率下的声速和声衰减信号的变化规律,结果与差示扫描量热仪(DSC)测量的热力学性质比较,并拍摄石蜡溶解过程的图像为辅助,探讨了二者由于测量原理不同导致的差异和特点。结果表明,两种方法均得到约50℃的初凝点,且二者信号反映的相变规律一致,表明利用声衰减和声速能够较好的表征石蜡在相变过程中的声学特性。超声可能成为一种新的蜡化物性质原位测量手段。     

Symmetry of Bioinspired Short Peptide Nanostructures and Their Basic Physical Properties

Supramolecular bioinspired peptide nanostructures are considered as a new frontier in materials science and engineering. The nano-crystalline packing of various peptide nanostructures, and especially those lacking a center of symmetry at the nanoscale, give rise to exceptional physical properties. Specifically, native aromatic diphenylalanine (FF) and aliphatic dileucine (LL) based nanotubes, which are related to hexagonal and orthorhombic non-centrosymmetric crystalline groups respectively, exhibit fundamental physical phenomena, such as piezoelectricity and second harmonic generation (SHG). This review covers our latest findings on the physical properties of FF and LL nanostructures. We show that heat treatment at the temperature range of 140–180 °C induces irreversible phase transition in FF and LL nanotubes, wherein all their physical properties and structure at all levels (molecular, electronic, optical, space symmetry, morphology, wettability) change. Using high resolution microscopy tools, based on Kelvin probe force microscopy (KPFM), piezoresponse force microscopy (PFM), and SHG, as well as Raman spectroscopy, we demonstrate that the phase-transition phenomena in FF and LL nanotubes leads to full reconstruction and reassembling of native open-end nanotubes into new fiber-like structures, followed by deep variation of non-centrosymmetric to centrosymmetric space symmetry. As a result, the newly generated centrosymmetric phase in FF and LL nanostructures demonstrates neither piezoelectric effect nor nonlinear optical activity.     

Co-extruded multilayer shape memory materials: Nano-scale phenomena

Shape memory polymers (SMPs), which demonstrate the ability to possess multiple shapes, are traditionally produced from copolymers and recently from blends. These materials often have phase separated morphologies that possess domain sizes on either the nano- or micro-scale. The observed properties, specifically the shape memory behavior, can be significantly altered by a change in the domain size; however, doing this often requires modification to the materials or material production process. Forced assembly multilayer co-extrusion was used to produce shape memory materials with a continuous layered structure that can be easily tailored to cover layer thicknesses ranging from the nano- to the micro-scale. Upon decreasing the layer thickness of polyurethane/polycaprolactone (PU/PCL) layered films, improvement in the shape fixity and recovery ratios tracked with layer thickness. The improvement in properties was attributed to a change in the PCL crystal orientation from randomly oriented in microlayers to in-plane lamella orientation in nanolayers.     

Nanoceramics: Issues and Opportunities

The goal of this study is to review developments in nanoceramics as discussed during presentations on the subject during the International Ceramic Congress (ICC4) held in Chicago on July 2012. As nanotechnology is such a broad subject affecting many industrial sectors, some presentations in other sessions were included as well. The study has been organized according to the dimensions of the nanoscale material starting with nanoscale entities or particulates followed by nanothin films and finally bulk nanoceramics where the nanoscale character lies in the structure. Developments on hybrid nanocomposites containing at least one ceramic compound were often included in the presentations and are discussed in a separate section. The promise of nanotechnology is also to discover materials with totally new properties or combinations of properties. One such example presented in depth at the conference is presented. The study concludes with a section on issues and opportunities. In general, it can be said that the very large investments that were made more than a decade ago into nanotechnology start to pay off dividends. Products are appearing on the market and can have a substantial impact there. Other commercial applications are on the horizon. Because of the wide scope of the technology, there are still plenty of opportunities for research and development. A particular issue that was mentioned by a number of speakers is that of upscaling, which can be understood literally as interfacing the nanostructure with the micro‐, meso‐, and macroworld. However, often one means the implementation on the factory floor of processes that have been developed within the laboratory.     

基于多孔支撑体的形状稳定复合相变储能材料的研究进展

固-液相变材料（PCMs）是热能储存（TES）技术发展的关键因素,然而一些固有的问题如泄漏和热导率低等严重制约了相变材料的性能。因此,选择合适的方法构建形状稳定的复合相变材料（FSCPCMs）,并有效地提高其热导率是实现相变材料实用化的重要前提。多孔载体封装相变材料为构建具有高储能密度和优异热传输性能的定形复合相变材料提供了一条有效的途径。本文对不同FSCPCMs的制备、结构热学性能、应用等方面进行了综述,详细总结和讨论了孔径和几何形状、表面改性、作用力、组成等因素对FSCPCMs相变行为的影响。重点介绍了具有高热导率、高负载率和高潜热的新型多孔复合相变材料的设计和应用。最后,基于理论、数值和实验方法,展望了FSCPCMs在约束结构中的相变和多尺度传热方面未来的研究方向及其在能源转换方面的商业化应用。     

相变储热技术用于被动式建筑节能的研究进展

相变潜热储能系统具有储热密度高、工作温度稳定和工艺流程简单等优点。相变材料可以通过多种方式与建筑物相结合，利用自身吸热/放热的特性对热能进行储存和释放，从而提高可再生能源的利用率。研究表明，相变储热单元能够有效地降低室内的温度波动，提高室内环境的热舒适性，减少建筑能耗。本文基于相变储热技术在建筑围护结构中的墙体、屋顶、地板以及窗户的研究现状，对近年来被动式储热建筑节能的研究现状进行了综述。阐述了适用于建筑物的相变材料的特点、优化相变材料热性能的方法、相变储热技术调控室内热环境的原理以及相变材料应用于建筑物的节能效果。文章指出，未来的研究应注重高性能相变材料的开发、复合工艺的简化以及室内热环境的综合评价。