Mechanical properties of nano-MMT reinforced polymer composite and polymer concrete

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纳米材料与微型机器外文文献翻译、中英文翻译

外文资料Nanotechnology and Micro-machine原文(一):NanomaterialNanomaterials and nanotechnology have become a magic word in modern society.Nanomaterials represent today’s cutting edge in the development of novel advanced materials which promise tailor-made functionality and unheard applications in all key technologies. So nanomaterials are considered as a great potential in the 21th century because of their special properties in many fields such as optics, electronics, magnetics, mechanics, and chemistry. These unique properties are attractive for various high performance applications. Examples include wear resistant surfaces, low temperature sinterable high-strength ceramics, and magnetic nanocomposites. Nanostructures materials present great promises and opportunities for a new generation of materials with improved and marvelous properties.It is appropriate to begin with a brief introduction to the history of the subject. Nanomaterials are found in both biological systems and man-made structures. Nature has been using nanomaterials for millions of years,as Disckson has noted: “Life itself could be regarded as a nanophase system”.Examples in which nanostructured elements play a vital role are magnetotactic bacteria, ferritin, and molluscan teeth. Several species of aquatic bacteria use the earth’s magnetic field to orient thenselves. They are able to do this because they contain chains of nanosized, single-domain magnetite particles. Because they have established their orientation, they are able to swim down to nutriments and away from what is lethal to them ,oxygen. Another example of nanomaterials in nature is that herbivorous mollusks use teeth attached to a tonguelike organ, the radula, to scrape their food. These teeth have a complexstructure containing nanocrystalline needles. We can utilize biological templates formaking nanomaterials. Apoferritin has been used as a confined reaction environmentfor the synthesis of nanosized magnetite particles. Some scholars consider biologicalnanomaterials as model systems for developing technologically useful nanomaterials.Scientific work on this subject can be traced back over 100 years.In 1861 theBritish chemist Thomas Graham coined the term colloid to describe a solutioncontaining 1 to 100 nm diameter particles in suspension. Around the turn of thecentury, such famous scientists as Rayleigh, Maxwell, and Einstein studied colloids.In 1930 the Langmuir-Blodgett method for developing monolayer films wasdeveloped. By 1960 Uyeda had used electron microscopy and diffraction to studyindividual particles. At about the same time, arc, plasma, and chemical flame furnaceswere employed to prouduce submicron particles. Magnetic alloy particles for use inmagnetic tapes were produced in 1970.By 1980, studies were made on clusterscontaining fewer than 100 atoms .In 1985, a team led by Smalley and Kroto foundC clusters were unusually stable. In 1991, Lijima spectroscopic evidence that 60reported studies of graphitic carbon tube filaments.Research on nanomaterials has been stimulated by their technologicalapplications. The first technological uses of these materials were as catalysts andpigments. The large surface area to volume ratio increases the chemicalactivity.Because of this increased activity, there are significant cost advantages infabricating catalysts from nanomaterials. The peoperties of some single-phasematerials can be improved by preparing them as nanostructures. For example, thesintering temperature can be decreased and the plasticity increased on single-phase,structural ceramics by reducing the grain size to several nanometers. Multiphasenanostructured materials have displayed novel behavior resulting from the small sizeof he individual phases.Technologically useful properties of nanomaterials are not limited to theirstructural, chemical, or mechanical behavior. Multilayers represent examples ofmaterials in which one can modify of tune a property for a specific application bysensitively controlling the individual layer thickness. It was discovered that the resistance of Fe-Cr multilayered thin films exhibited large changes in an applied magnetic field of several tens of kOe.This effect was given the name giant magnetoresistance (GMR). More recently, suitably annealed magnetic multilayers have been developed that exhibit significant magnetoresistance effects even in fields as low as 5 to10 Oe (Oersted). This effect may prove to be of great technological importance for use in magnetic recording read heads.In microelectronics, the need for faster switching times and ever larger integration has motivated considerable effort to reduce the size of electronic components. Increasing the component density increases the difficulty of satisfying cooling requirements and reduces the allowable amount of energy released on switching between states. It would be ideal if the switching occurred with the motion of a single electron. One kind of single-electron device is based on the change in the Coulombic energy when an electron is added or removed from a particle. For a nanoparticle this enery change can be large enough that adding a single electron will effectively blocks the flow of other electrons. The use of Coulombic repulsion in this way is called Coulomb blockade.In addition to technology, nanomaterials are also interesting systems for basic scientific investigations .For example, small particles display deviations from bulk solid behavior such as reductios in the melting temperature and changes (usually reductions) in the lattice parameter. The changes n the lattice parameter observed for metal and semiconductor particles result from the effect of the surface free energy. Both the surface stress and surface free energy are caused by the reduced coordination of the surface atoms. By studying the size dependence of the properties of particles, it is possible to find the critical length scales at which particles behave essentially as bulk matter. Generally, the physical properties of a nanoparticle approach bulk values for particles containing more than a few hundred atoms.New techniques have been developed recently that have permitted researchers to produce larger quantities of other nanomaterials and to better characterize these materials.Each fabrication technique has its own set of advantages anddisadvantages.Generally it is best to produce nanoparticles with a narrow size distribution. In this regard, free jet expansion techniques permit the study of very small clusters, all containing the same number of atoms. It has the disadvantage of only producing a limited quantity of material.Another approach involves the production of pellets of nanostructured materials by first nucleating and growing nanoparticles in a supersaturated vapor and then using a cold finger to collect the nanoparticle. The nanoparticles are then consolidated under vacuum. Chemical techniques are very versatile in that they can be applied to nearly all materials (ceramics, semiconductors, and metals) and can usually produce a large amount of material. A difficulty with chemical processing is the need to find the proper chemical reactions and processing conditions for each material. Mechanical attrition, which can also produce a large amount of material, often makes less pure material. One problem common to all of these techniques is that nanoparticles often form micron-sized agglomerates. If this occurs, the properties of the material may be determined by the size of the agglomerate and not the size of the individual nanoparticles. For example, the size of the agglomerates may determine the void size in the consolidated nanostructured material.The ability to characterize nanomaterials has been increased greatly by the invention of the scanning tunneling microscope (STM) and other proximal probes such as the atomic force microscope (AFM), the magnetic force microscope, and the optical near-field microscope.SMT has been used to carefully place atoms on surfaces to write bits using a small number of atmos. It has also been employed to construct a circular arrangement of metal atoms on an insulating surface. Since electrons are confined to the circular path of metal atoms, it serves ad a quantum ‘corral’of atoms. This quantum corral was employed to measure the local electronic density of states of these circular metallic arrangements. By doing this, researchers were able to verify the quantum mechanical description of electrons confined in this way.Other new instruments and improvements of existing instruments are increasingly becoming important tools for characterizing surfaces of films, biological materials, and nanomaterials.The development of nanoindentors and the improvedability to interpret results from nanoindentation measurements have increased our ability to study the mechanical properties of nanostructured materials. Improved high-resolution electron microscopes and modeling of the electron microscope images have improved our knowledges of the structure of the the particles and the interphase region between particles in consolidated nanomaterials.Nanotechnology1. IntroductionWhat id nanotechnology? it is a term that entered into the general vocabulary only in the late 1970’s,mainly to describe the metrology associated with the development of X-ray,optical and other very precise components.We defined nanotechnology as the technology where dimensions and tolerances in the range 0.1～100nm(from the size of the atom to the wavelength of light) play a critical role.This definition is too all-embracing to be of practical value because it could include,for example,topics as diverse as X-ray crystallography ,atomic physics and indeed the whole of chemistry.So the field covered by nanotechnology is later narrowed down to manipulation and machining within the defined dimensional range(from 0.1nm to 100nm) by technological means,as opposed to those used by the craftsman,and thus excludes,for example,traditional forms of glass polishing.The technology relating to fine powders also comes under the general heading of nanotechnology,but we exclude observational techniques such as microscopy and various forms of surface analysis.Nanotechnology is an ‘enabling’ technology, in that it provides the basis for other technological developments,and it is also a ‘horizontal’or ‘cross-sectional’technology in that one technological may,with slight variations,be applicable in widely differing fields. A good example of this is thin-film technology,which is fundamental to electronics and optics.A wide range of materials are employed in devices such as computer and home entertainment peripherals, including magnetic disc reading heads,video cassette recorder spindles, optical disc stampers and ink jet nozzles.Optical and semiconductor components include laser gyroscope mirrors,diffraction gratings,X-ray optics,quantum-well devices.2. Materials technologyThe wide scope of nanotechnology is demonstrated in the materials field,where materials provide a means to an end and are not an end in themseleves. For example, in electronics,inhomogeneities in materials,on a very fine scale, set a limit to the nanometre-sized features that play an important part in semiconductor technology, and in a very different field, the finer the grain size of an adhesive, the thinner will be the adhesive layer, and the higher will be the bond strength.(1) Advantages of ultra-fine powders. In general, the mechanical, thermal, electrical and magnetic properties of ceramics, sintered metals and composites are often enhanced by reducing the grain or fiber size in the starting materials. Other properties such as strength, the ductile-brittle transition, transparency, dielectric coefficient and permeability can be enhanced either by the direct influence of an ultra-fine microstructure or by the advantages gained by mixing and bonding ultra-fine powders.Oter important advantages of fine powders are that when they are used in the manufacture of ceramics and sintered metals, their green (i.e, unfired) density can be greatly increased. As a consequence, both the defects in the final produce and the shrinkage on firing are reduced, thus minimizing the need for subsequent processing.(2)Applications of ultra-fine powders.Important applications include:Thin films and coatings----the smaller the particle size, the thinner the coating can beElectronic ceramics ----reduction in grain size results in reduced dielectric thicknessStrength-bearing ceramics----strength increases with decreasing grain sizeCutting tools----smaller grain size results in a finer cutting edge, which can enhance the surface finishImpact resistance----finer microstructure increases the toughness of high-temperature steelsCements----finer grain size yields better homogeneity and densityGas sensors----finer grain size gives increased sensitivityAdhesives----finer grain size gives thinner adhesive layer and higher bond strength3. Precision machining and materials processingA considerable overlap is emerging in the manufacturing methods employed in very different areas such as mechanical engineering, optics and electronics. Precision machining encompasses not only the traditional techniques such as turning, grinding, lapping and polishing refined to the nanometre level of precision, but also the application of ‘particle’ beams, ions, electrons and X-rays. Ion beams are capable of machining virtually any material and the most frequent applications of electrons and X-rays are found in the machining or modification of resist materials for lithographic purposes. The interaction of the beams with the resist material induces structural changes such as polymerization that alter the solubility of the irradiated areas.(1) Techniques1) Diamond turning. The large optics diamond-turning machine at the Lawrence Livermore National Laboratory represents a pinnacle of achievement in the field of ultra-precision machine tool engineering. This is a vertical-spindle machine with a face plate 1.6 m in diameter and a maximum tool height of 0.5m. Despite these large dimensions, machining accuracy for form is 27.5nm RMS and a surface roughness of 3nm is achievable, but is dependent both on the specimen material and cutting tool.(2) GrindingFixed Abrasive Grinding The term“fixed abrasive” denotes that a grinding wheel is employed in which the abrasive particles, such as diamond, cubic boron nitride or silicon carbide, are attached to the wheel by embedding them in a resin or a metal. The forces generated in grinding are higher than in diamond turning and usually machine tools are tailored for one or the other process. Some Japanese work is in the vanguard of precision grinding, and surface finishes of 2nm (peak-to-valley) have been obtained on single-crystal quartz samples using extremely stiff grinding machinesLoose Abrasive Grinding The most familiar loose abrasive grinding processes are lapping and polishing where the workpiece, which is often a hard material such asglass, is rubbed against a softer material, the lap or polisher, with abrasive slurry between the two surfaces. In many cases, the polishing process occurs as a result of the combined effects of mechanical and chemical interaction between the workpiece, slurry and polished.Loose abrasive grinding techniques can under appropriate conditions produce unrivalled accuracy both in form and surface finish when the workpiece is flat or spherical. Surface figures to a few nm and surface finishes bettering than 0.5nm may be achieved. The abrasive is in slurry and is directed locally towards the workpiece by the action of a non-contacting polyurethane ball spinning at high speed, and which replac es the cutting tool in the machine. This technique has been named “elastic emission machining” and has been used to good effect in the manufacture of an X-ray mirror having a figure accuracy of 10nm and a surface roughness of 0.5nm RMS.3）Thin-film production. The production of thin solid films, particularly for coating optical components, provides a good example of traditional nanotechnology. There is a long history of coating by chemical methods, electro-deposition, diode sputtering and vacuum evaporation, while triode and magnetron sputtering and ion-beam deposition are more recent in their wide application.Because of their importance in the production of semiconductor devices, epitaxial growth techniques are worth a special mention. Epitaxy is the growth of a thin crystalline layer on a single-crystal substrate, where the atoms in the growing layer mimic the disposition of the atoms in the substrate.The two main classes of epitaxy that have ben reviewed by Stringfellow (1982) are liquid-phase and vapour-phase epitaxy. The latter class includes molecular-beam epitaxy (MBE), which in essence, is highly controlled evaporation in ultra high vacuum. MBE may be used to grow high quality layered structures of semiconductors with mono-layer precision, and it is possible to exercise independent control over both the semiconductor band gap, by controlling the composition, and also the doping level. Pattern growth is possible through masks and on areas defined by electron-beam writing.4. ApplicationsThere is an all-pervading trend to higher precision and miniaturization, and to illustrate this a few applications will be briefly referred to in the fields of mechanical engineering,optics and electronics. It should be noted however, that the distinction between mechanical engineering and optics is becoming blurred, now that machine tools such as precision grinding machines and diamond-turning lathes are being used to produce optical components, often by personnel with a backgroud in mechanical engineering rather than optics. By a similar token mechanical engineering is also beginning to encroach on electronics particularly in the preparation of semiconductor substrates.(1) Mechanical engineeringOne of the earliest applications of diamond turning was the machining of aluminum substrates for computer memory discs, and accuracies are continuously being enhanced in order to improve storage capacity: surface finishes of 3nm are now being achieved. In the related technologies of optical data storage and retrieval, the toler ances of the critical dimensions of the disc and reading head are about 0.25 μm. The tolerances of the component parts of the machine tools used in their manufacture, i.e.the slideways and bearings, fall well within the nanotechnology range.Some precision components falling in the manufacturing tolerance band of 5～50nm include gauge blocks, diamond indenter tips, microtome blades, Winchester disc reading heads and ultra precision XY tables (Taniguchi 1986). Examples of precision cylindrical components in two very different fields, and which are made to tolerances of about 100 nm, are bearing for mechanical gyroscopes and spindles for video cassette recorders.The theoretical concept that brittle materials may be machined in a ductile mode has been known for some time. If this concept can be applied in practice it would be of significant practical importance because it would enable materials such as ceramics, glasses and silicon to be machined with minimal sub-surface damage, and could eliminate or substantially reduce the need for lapping and polishing.Typically, the conditions for ductile-mode machining require that the depth of cutis about 100 nm and that the normal force should fall in the range of 0.1～0.01N. These machining conditons can be realized only with extremely precise and stiff machine tools, such as the one described by Yoshioka et al (1985), and with which quartz has been ground to a surface roughness of 2 nm peak-to-valley. The significance of this experimental result is that it points the way to the direct grinding of optical components to an optical finish. The principle can be extended to other materials of significant commercial importance, such as ceramic turbine blades, which at present must be subjected to tedious surface finishing procedures to remove the structure-weakening cracks produced by the conventional grinding process.(2) OpticsIn some areas in optics manufacture there is a clear distinction between the technological approach and the traditional craftsman’s approach, particul arly where precision machine tools are employed. On the other hand, in lapping and polishing, there is a large grey area where the two approaches overlap. The large demand for infrared optics from the 1970s onwards could not be met by the traditional suppliers, and provided a stimulus for the development and application of diamond-turning machines to optic manufacture. The technology has now progressed and the surface figure and finishes that can be obtained span a substantial proportion of the nanotechnology range. Important applications of diamond-turned optics are in the manufacture of unconventionally shaped optics, for example axicons and more generelly, aspherics and particularly off-axis components. Such as paraboloids.The mass production(several million per annum) of the miniature aspheric lenses used in compact disc players and the associated lens moulds provides a good example of the merging of optics and precision engineering. The form accuracy must be better than 0.2μm and the surface roughness m ust be below 20 nm to meet the criterion for diffraction limited performance.(3) ElectronicsIn semiconductors, nanotechnology has long been a feature in the development of layers parallel to the substrate and in the substrate surface itself, and the need for precision is steadily increasing with the advent of layered semiconductor structures.About one quarter of the entire semiconductor physics community is now engaged in studying aspects of these structures. Normal to the layer surface, the structure is produced by lithography, and for research purposes ar least, nanometre-sized features are now being developed using X-ray and electron and ion-beam techniques.5. A look into the futureWith a little imagination, it is not difficult to conjure up visions of future developments in high technology, in whatever direction one cares to look. The following two examples illustrate how advances may take place both by novel applications and refinements of old technologies and by development of new ones.(1) Molecular electronicsLithography and thin-film technology are the key technologies that have made possible the continuing and relentless reduction in the size of integrated circuits, to increase both packing density and operational speed. Miniaturization has been achieved by engineering downwards from the macro to the micro scale. By simple extrapolation it will take approximately two decades for electronic switches to be reduced to molecular dimensions. The impact of molecular biology and genetic engineering has thus provided a stimulus to attempt to engineer upwards, starting with the concept that single molecules, each acting as an electronic device in their own right, might be assembled using biotechnology, to form molecular electronic devices or even biochip computers.Advances in molecular electronics by downward engineering from the macro to the micro scale are taking place over a wide front. One fruitful approach is by way of the Langmure-Biodgett (LB) film using a method first described by Blodgett (1935).A multi-layer LB structure consists of a sequence of organic monolayers made by repeatedly dipping a substrate into a trough containing the monolayer floating on a liquid (usually water), one layer being added at a time. The classical film forming materials were the fatty acids such as stearic acid and their salts. The late 1950s saw the first widespread and commercially important application of LB films in the field of X-ray spectroscopy (e.g, Henke 1964, 1965). The important properties of the films that were exploited in this application were the uniform thickness of each film, i.e.one molecule thick, and the range of thickness, say from 5to 15nm, which were available by changing the composition of the film material. Stacks of fifty or more films were formed on plane of curved substrates to form two-dimensional diffraction gratings for measuring the characteristic X-ray wavelengths of the elements of low atomic number for analytical purposes in instruments such as the electron probe of X-ray micro-analyzer.(2) Scanning tunneling engineeringIt was stated that observational techniques such as microscopy do mot, at least for the purposes of this article, fall within the domain of nanotechnology. However,it is now becoming apparent that scanning tunneling microscopy(STM) may provide the basis of a new technology, which we shall call scanning tunneling engineering.In the STM, a sharp stylus is positioned within a nanometre of the surface of the sample under investigation. A small voltage applied between the sample and the stylus will cause a current to foow through the thin intervening insulating medium (e.g.air, vacum, oxide layer). This is the tunneling electron current which is exponentially dependent on the sample-tip gap. If the sample is scanned in a planr parallel to ies surface and if the tunneling current is kept cnstant by adjusting the height of the stylus to maintain a constant gap, then the displacement of the stylus provides an accurate representation of the surface topographyu of the sample. It is relevant to the applications that will be discussed that individual atoms are easily resolved by the STM, that the stylus tip may be as small as a single atom and that the tip can be positioned with sub-atomic dimensional accuracy with the aid of a piezoelectric transducer.The STM tip has demonstrated its ability to draw fine lines, which exhibit nanometre-sized struture, and hence may provide a new tool for nanometre lithography.The mode of action was not properly understood,but it was suspected that under the influence of the tip a conducting carbon line had been drawn as the result of polymerizing a hydrocarbon film, the process being assisted by the catalytic activity of the tungsten tip. By extrapolating their results the authors believed that it would be possible to deposit fine conducting lines on an insulating film. The tip would operatein a gaseous environment that contained the metal atoms in such a form that they could either be pre-adsorbed on the film or then be liberated from their ligands or they would form free radicals at the location of the tip and be transferred to the film by appropriate adjustment of the tip voltage.Feynman proposed that machine tools be used to make smaller machine tools which in turn would make still smaller ones, and so on all the way down to the atomic level. These machine tools would then operate via computer control in the nanometre domain, using high resolution electron microscopy for observation and control. STM technology has short-cricuired this rather cumbrous concept,but the potential applications and benefits remain.原文(二)Micro-machine1. IntroductionFrom the beginning, mankind seems instinctively to have desired large machines and small machines. That is, “large” and “small” in comp arison with human-scale. Machines larger than human are powerful allies in the battle against the fury of nature; smaller machines are loyal partners that do whatever they are told.If we compare the facility and technology of manufacturing larger machines, common sense tells us that the smaller machines are easier to make. Nevertheless, throughout the history of technology, larger machines have always stood ort. The size of the restored models of the water-mill invented by Vitruvius in the Roman Era, the windmill of the middle Ages, and the steam engine invented by Watt is overwhelming. On the other hand, smaller machined in history of technology are mostly tools. If smaller machines are easier to make, a variety of such machined should exist, but until modern times, no significant small machines existed except for guns and clocks.This fact may imply that smaller machines were actually more difficult to make. Of course, this does not mean simply that it was difficult to make a small machine; it means that it was difficult to invent a small machine that would be significant to human beings.。

纳米氮化硼增强金属基复合材料的研究进展

第14卷第9期精密成形工程收稿日期：2022–05–11基金项目：国家自然科学基金（52105259）；中国科学院海洋新材料与应用技术重点实验室浙江省海洋材料与防护技术重点实验室开放课题（2020K06）；江苏大学优秀青年人才基金（19JDG021，18JDG030）；江苏省研究生科研与实践创新计划（KYCX21_3328）；江苏省高校自然科学基金（19KJB460012）；江苏省博士后基金（2021K389C ）作者简介：刘振强（1996—），男，博士生，主要研究方向为金属基复合材料。

刘振强，王匀，李瑞涛，何培瑜，刘宏，刘为力（江苏大学机械工程学院，江苏镇江 212013）摘要：在金属中添加陶瓷增强相是调控和改善金属材料结构和性能的重要途径。

传统硬质陶瓷增强相难以满足金属材料日益严苛的应用需求。

以氮化硼纳米片（boron nitride nanosheet ，BNNS ）和氮化硼纳米管（boron nitride nanotube ，BNNT ）为代表的纳米氮化硼具有极大的比表面积和优异的力学性能、热稳定性、化学稳定性等，是制备性能优异的金属基复合材料的理想增强相。

系统总结了纳米氮化硼的种类和特征，综述了纳米氮化硼增强金属基复合材料的制备方法，归纳了纳米氮化硼增强Cu 、Al 、Ti 复合材料的研究成果，总结了纳米氮化硼/金属复合材料的力学和摩擦学性能，并揭示了复合材料性能改善的机理。

最后，展望了纳米氮化硼/金属复合材料的发展趋势。

关键词：纳米氮化硼；金属基复合材料；力学性能；摩擦学性能DOI ：10.3969/j.issn.1674-6457.2022.09.017中图分类号：TB331 文献标识码：A 文章编号：1674-6457(2022)09-0119-12Research Progress of Nano-boron Nitride Reinforced Metal Matrix CompositesLIU Zhen-qiang , WANG Yun , LI Rui-tao , HE Pei-yu , LIU Hong , LIU Wei-li(School of Mechanical Engineering, Jiangsu University, Jiangsu Zhenjiang 212013, China)ABSTRACT: The introduction of ceramic fillers into metal is an effective way to optimize the microstructure and enhance the properties of metal. Traditional hard ceramic reinforcements are difficult to meet the rising application requirements of metal materials. Nano-boron nitrides such as boron nitride nanosheet (BNNS) and boron nitride nanotube (BNNT) are ideal fillers for high-performance MMCs due to the large specific surface areas and excellent mechanical, chemical and thermal properties. The types and performance of nano-boron nitrides were systematically reviewed. The preparation method of nano-boron nitride re-inforced metal matrix composites was introduced. The research works that led to the advances in nano-boron nitride reinforced Cu, Al, and Ti matrix composites were summarized. The mechanical and wear properties of nano-boron nitride/metal composites were concluded, and the mechanisms improving performance of composites were also revealed. Finally, the promising outlook of nano-boron nitride/metal composites is prospected.KEY WORDS: nano-boron nitride; metal matrix composite; mechanical properties; wear properties航空航天、深海舰船、汽车交通、核电、化工、能源等领域的迅猛发展使金属基复合材料的服役条件日趋复杂和苛刻。

我国揭示锯齿形边缘石墨烯纳米带中的电声子耦合效应

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离子刻蚀技术，首次实现了可控的石墨烯面内各向异性刻蚀技术；并结合人工缺陷工程，首次实现了对石墨烯纳米结构的精确加工和剪裁，制备出了尺寸可控（小线宽达最５米以下）、边缘可控（有原子级平整的纳具锯齿形边缘结构）的石墨烯纳米结构。最近，张广宇研究组的杨蓉博士等在先前的工作基础上，利用拉曼散射光谱技术，研究了具有锯齿形边缘结构的石墨烯纳米带的电声子耦合特性。他们首次在这种结构中观察到了Ｇ的劈裂（一１８软化的Ｅｇ式；峰Ｇ５３２模Ｇ＋１９本征的Ｅｇ式）。这种非应力效应一５４２模导致的Ｇ劈裂可归因于锯齿形边缘结构独特峰
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定量电子显微学方法与氧化钛纳米结构研究获国家自然科学二等奖

据介绍，包括美国、Ｈ本、欧盟、俄罗斯在内的５０多拓宽，释许多传统理论无法描述的物质体系，且将理论解并
个国家和地区都有各自明确的纳米科技发展战略，投入巨成果成功应用到纳米结构的研究领域。在科学家眼中，食盐不是食盐，而是一种典型的离子资、抢占战略制高点。美国国家纳米讨‘ （划ＮＮＩ）２１０１晶体。么一种最最普通的东西，电子显微镜去观察它的这用年财政预算ｌ美元，历年投入稳定增长。日本尤其重视８亿结构时，也没有好的理论去描述电子在这种离子晶体表面纳米技术在信息、金属、环境、能源、生命科学以及基础技的衍射现象。彭练矛团队给出了描述诸如离子晶体表面衍
仿生智能纳米界面材料、碳纳米管宏观薄膜及纤维结构的贝尔物理学奖就颁发给了成功地分离出单层石墨烯材料的
制备与性能研究、纳米结构金属材料达到高性能极限等诸两名英国物理学家。
多方面取得突破。
像科技研发和产业化的整体协调。同时，入并持续开展纳米重，多数情况下我们在电子显微镜中所看到的 ‘ ’不能直深接反映物体的真正结构。练矛说，用量子力学的原理， ”彭利科技领域战略研究，提高决策能力；整合资源，形成在纳米

昆明理工大学于晓华

昆明理⼯⼤学于晓华Characteristics and Corrosion Behavior of Pure Titanium Subjected to Surface Mechanical AttritionTIANLIN FU,1XIAO WANG,1JIANXIONG LIU,1LI LI,1XIAOHUA YU,2,3and ZHAOLIN ZHAN 1,41.—Faculty of Material Science and Engineering,Kunming University of Science and Technology,Kunming 650093,China.2.—Solid Waste Utilization National Engineering Center,Kunming University of Science and Technology,Kunming 650093,China.3.—e-mail:xiaohua_y@/doc/1b4f5662effdc8d376eeaeaad1f34693daef1090.html .4.—e-mail:zl_zhan@/doc/1b4f5662effdc8d376eeaeaad1f34693daef1090.htmlA stable passive ?lm exhibiting good corrosion resistance in a 3.5wt.%NaCl solution was formed on the surface of pure titanium (Ti)subjected to a surface mechanical attrition treatment (SMAT).The corrosion potential (à0.21V)of the ?lm was signi?cantly higher than that (à0.92V)of the untreated sample.Moreover,the corrosion current density was an order of magnitude lower than that of the untreated sample.SMAT resulted in a decrease in the vacancy condensation in the TiO 2?lm,thereby inhibiting the invasion and diffusion of Cl àin the ?lm.INTRODUCTIONIn recent years,surface mechanical attrition treat-ment (SMAT)techniques have been extensively investigated as grain-size re?nement methods for fabricating nanostructured surface layers of alloys.1–3Previous studies have shown that grain-re?ned mate-rials exhibit excellent properties.For example,the temperature of pure iron nitriding was signi?cantly reduced (to 300°C)after SMAT,4indicating that the nanostructured surface promotes diffusion and in?u-ences the growth mechanism of coatings or oxide ?lms.The corrosion behavior of nanocrystallized mate-rials has also been investigated.Li et al.fabricated a nanostructured layer via fast multiple rotation rolling and found that the corrosion resistance of a Ti-6Al-4V alloy improved with formation of this layer.5Pan et al.revealed that the nucleation mechanism of the passive ?lm changed (i.e.,from progressive to instantaneous)with nanocrystalliza-tion.6Most previous reports have revealed improve-ments in the corrosion resistance,7–11but the opposite effect has also been reported in some cases.For example,the corrosion resistance of AISI 409stainless steel improved after ultrasonic shot peen-ing treatment with 2-mm balls,but decreased when 5-and 8-mm balls were used.12Therefore,under-standing the characteristics of the oxide ?lms formed on the surface of SMAT alloys is essential for optimization of the treatment parameters.Passive oxide ?lms are formed on the surface of reactive metals,such as titanium (Ti),aluminum (Al),and chromium (Cr).However,these ?lms consist of defects,undergo dissolution and breakdown and,hence,exhibit low corrosion resistance in corrosive environments.Despite being extremely thermody-namically reactive in air and aqueous environments,Ti is passivated by a very stable oxide (Ti +O 2=TiO 2,D G 0=à888.8kJ mol à1),13which grows spon-taneously on its surfaces.This oxide has resulted in the widespread use of Ti and its alloys in various ?elds.14–16The chemical stability of the TiO 2?lm is crucial for the use of Ti in applications.17Previous studies have shown that SMAT promotes oxygen diffusion in the surface layer of alloys and the formation of a relatively stable oxide ? lm.This is attributed to the high density of grains and defects,induced by severe plastic deformation,which act as rapid-diffusion paths and promote oxide-?lm forma-tion on the surface of reactive metals.Therefore,the aim of this work was to determine the effect of SMAT on the properties of the surface oxide ?lms of TA1and identify the mechanism governing the corresponding improvement in the corrosion resistance.ExperimentalThe pure Ti plate (100910093mm 3)investi-gated in this work had a nominal composition (wt.%)of C 0.02,Fe 0.10,O 0.15,N 0.02,H 0.0011,JOM,Vol.69,No.10,2017DOI:10.1007/s11837-017-2511-7ó2017The Minerals,Metals &Materials Society1844(Published online August 9,2017)and a balance of Ti.Samples of this plate were subjected to a15-min SMAT conducted at room temperature and a vibration amplitude of50kHz, using8-mm-diameter balls(composed primarily ofGCr15).The electrochemical behavior of samples in a3.5wt.%NaCl solution was evaluated via electro-chemical tests conducted on an electrochemical workstation(CHI760E;Shanghai Chenhua Instru-ment,Shanghai,China).After1h of immersion in the solution,samples were subjected to polarization (scan rate:3mV/s)at potentials ranging from à3000mV to4000mV(SCE).RESULTS AND DISCUSSIONFigure1shows the XPS spectrum of the samples. The Ti2p spectra recorded from the sample without SMAT were resolved into eight peaks(see Fig.1a), which are attributed to Ti,Ti2+,Ti3+,and Ti4+.18–20 The Ti2+and Ti3+species are associated with Ti sub-oxides and the Ti4+species correspond to TiO2. Furthermore,O occurs as O2à,OHà,and adsorbed water and,hence,the corresponding chemical states of Ti are Ti,TiO,Ti2O3,TiO2,Ti(OH)4,and TiO2?H2O.The peaks associated with the SMAT sample are both very well-?tted by the Gaussian–Lorenzian curve(Fig.1b).The Ti2p3/2region con-sists of one main overlapping peak,attributed to Ti4+,indicating that SMAT promotes the formation of titanium dioxide(TiO2)on the surface of the sample.The corrosion behavior of samples immersed ina3.5wt.%NaCl solution was investi-gated via open-circuit potential(OCP)measure-ments and potentiodynamic polarization curves,as shown in Fig.2.The OCP values of the pure Ti sample decreased initially,then increased sharply,reached a steady-state value,and decreased gradu-ally thereafter.In contrast,the OCP values of the SMAT sample decreased slowly in the initial stage, reached a steady-state value,and increased gradu-ally thereafter(Fig.2a).These results indicate that a passive?lm is formed on the surface of each sample.This?lm acts as a barrier to metal disso-lution,leading to a decrease in the corrosion rate, thereby resulting in improved stability of the immersed material.21Moreover,the increasingly negative OCP value of the pure Ti sample indicates that the passive?lm formed on this sample is less stable,and therefore considerably less effective, than that formed on the SMAT sample.The high stability of the?lm formed on the SMAT sample is attributed to the formation(as suggested by the XPS results)of a stable phase(Ti2O)on the surface of the sample.The potentiodynamic polarization curves of pure Ti and the SMAT sample are shown in Fig.2b.The similarity in the shape of the curves suggests that the same fundamental reactions occur on both samples during cathodic and anodic polarization (see Table I for a list of the corresponding corrosion parameters).Passivation occurs when a protective oxide?lm forms on the surface of a metal.18In the case of the SMAT sample,the passive region occursat potentials ranging fromà0.21V to4V,and is characterized by an almost constant current den-sity.In the case of TA1,passivation occurs atà0.92V and the current density in the passivation region increases with increasing potential.Thecorrosion current density(i corr)and the passivecurrent density(i pass)of the SMAT sample are almost one order of magnitude lower than those of the pure Ti sample.In addition,the increase in the passive current density with increasing applied potential may be attributed to the gradual dissolu-tion of the oxide layer;the jump is attributed to a sudden breakdown of this layer.22These results indicate that the corrosion resistance of the SMAT sample is signi?cantly higher than that of TA. Figure2c shows the Mott–Schottky plots of the samples.23In region1,Cà2is almost independent of E(as evidenced by the near-zero slope of the plot).This trend is consistent with the?at band region.24In region2(0.5–3.5V SCE),Cà2exhibits a linear dependence on E.The positive slopes associ-ated with region1are indicative of an n-type semiconductor,which is characterizedby:Fig.1.XPS spectra of the(a)pure Ti and(b)SMAT samples.Characteristics and Corrosion Behavior of Pure Titanium Subjected to Surface MechanicalAttrition18451C SC2ee 0eN D E àE fb àkT ewhere,e is the relative dielectric constant,e 0thepermittivity of free space,e the charge of the electron (1.6910à19C),N D the donor density (n -type semiconductor),E the applied potential,E fb the ?at band potential,k the Boltzmann constant(1.38910à23J/K),and T the absolute temperature.Based on the character of n -type semiconductors,the oxide layer consists primarily of donor-type defects (in this case,oxygen vacancies and/or Ti in-terstitials).25E fb values of 1–0.52V SCE have been estimated for the samples,and N D values of 2.6491021cm à3and 1.6891019cm à3were obtained for the TA1and SMAT samples,respec-tively.Therefore,the donor density of the pas-sive layer formed on the SMAT sample is lower than that of the layer formed on the pure Ti sample.This relatively low density may have contributed to the high corrosion resistance of the SMAT sample.The corrosion rate of Ti depends on the diffusion of Ti,O,and Cl ions and vacancies in the oxide ?lms (see Fig.3for the point defect model(PDM)26,27description of these diffusion processes).According to this model,vacancies are generated and annihi-lated at the metal/barrier layer and barrier layer/outer layer interfaces.Cation vacancies are gener-ated and annihilated via Ti x Ti !Ti 4ttTi 4tTi and Ti tv 40Ti !Ti x Ti tv Ti t4e 0,and oxygen vacancies aregenerated and annihilated via Ti !Ti x Ti t2v ::O t4eand v ::O tH 2O !O O t2H t.Oxygen diffusion and cation-vacancy diffusion through the oxide ?lms are driven by the electric ?eld and the concentrationgradient (Fig.3).Incomplete cation-vacancy anni-hilation leads to oxygen vacancy/cation vacancy interactions,which are driven by theelectrostaticFig.2.(a)Open-circuit potential versus time,(b)potentiodynamic polarization curves,and (c)Mott–Schottky plots of pure Ti and SMAT samples immersed in 3.5wt.%NaCl solution. Table I.Corrosion potential,corrosion current density,and passive current density of samples Sample E corr (V)i corr (A cm 22)i pass (A cm 22)TA1à0.92 2.51910à4 3.46910à3SMAT sampleà0.215.01910à51.58910à4Fig.3.Schematic illustration based on PDM description of Ti,O,and Cl ionic and vacancy diffusion in the passive ?lms.Fu,Wang,Liu,Li,Yu,and Zhan1846attraction between high concentrations of oppo-sitely charged defects.28These interactions lead to defect elimination,and eventually to void formation(v40Ti t2v::O!0).Moreover,Cl defects(Cl?tv::Ote0!Cl:OTmay form if Clàions diffusing and migrat-ing through the outer layer(Clàaq t?!Cl?te0)areabsorbed by oxygen vacancies.In addition,a cation vacancy/oxygen vacancy is generated(via a Schot-tky-pair type of reaction),because of this absorption.24,29The high density of vacancies in the thin oxide?lm formed on the pure Ti sample would easily reach the surface layer,leading to Clàinvasion.Vacancy condensation in this layer results in?lm breakage and rapid Clàinvasion.This process is manifested as an increase and a jump in the passive current density associated with the passive region of the potentiody-namic polarization curve(Fig.2b).The donor density of the SMAT sample(1.6891019cmà3)is lower than that of the pure Ti sample(2.6491021cmà3).There-fore,compared with the passive?lm formed on the Ti sample,the?lm formed on the SMAT sample is more effective in inhibiting Clàinvasion and diffusion.In addition,the probability of vacancy condensation decreases signi?cantly and the?lm exhibits good corrosion resistance.CONCLUSIONA stable passive?lm(Ti2O),exhibiting good corrosion resistance,is formed on the surface of pure Ti subjected to SMAT.The corrosion potential (à0.21V SCE)of the SMAT sample is signi?cantly higher than that(–0.92V SCE)of the untreated Ti. Moreover,the corrosion current density(5.019 10à5A cmà2)is one order of magnitude lower than that of the untreated sample(2.51910à4A cmà2). The passive? lm formed on the SMAT Ti is an n-type semiconductor and,hence,the oxide layer consists primarily of donor-type defects.The diffu-sion of ions and vacancies in the dense passive?lm formed on the SMAT sample hinders vacancy condensation and,in turn,the invasion and diffu-sion of Clàin the?lm.ACKNOWLEDGEMENTThis work was?nancially supported by the Nat-ural Science Foundation of China(Grant Nos. 51665022and51601081).REFERENCES1.H.W.Zhang,Z.K.Hei,G.Liu,J.Lu,and K.Lu,Acta Mater.51,1871(2003).2.K.Y.Zhu,A.Vassel,F.Brisset,K.Lu,and J.Lu,ActaMater.52,4101(2004).3.Y.Liu,B.Jin,and J.Lu,Mat.Sci.Eng.A.636,446(2015).4.W.P.Tong,N.R.Tao,Z.B.Wang,J.Lu,and K.Lu,Science299,686(2003).5.Y.Li,K.N.Sun,P.Liu,Y.Liu,and P.F.Chui,Vacuum101,102(2014).6. C.Pan,L.Liu,Y.Li,S.G.Wang,and F.H.Wang,Elec-trochim.Acta56,7740(2011).7.S.Kumar,K.Chattopadhyay,and V.Singh,Mater.Char-act.121,23(2016).8.S.Jindal,R.Bansal,B.P.Singh,R.Pandey,S.Narayanan,M.R.Wani,and V.Singh,J.Oral Implantol.40,347(2014).9.Y.Shadangi,K.Chattopadhyay,S.B.Rai,and V.Singh,Surf.Coat.Technol.280,216(2015).10.T.Chen,H.John,J.Xu,Q.H.Lu,J.Hawk,and X.B.Liu,Corros.Sci.77,230(2013).11.R.Huang and Y.Han,Mater.Sci.Eng.,C33,2353(2013).12.T.Balusamy,S.Kumar,and T.S.N.Sankara Narayanan,Corros.Sci.52,3826(2010).13.Z.L.Jiang,X.Dai,T.Norby,and H.Middleton,Corros.Sci.53,815(2011).14.V.M.C.A.Oliveira,C.Aguiar,A.M.Vazquez,A.Robin,andM.J.R.Barboza,Corros.Sci.88,317(2014).15.S.Jelliti,C.Richard,D.Retraint,T.Roland,M.Chemkhi,and C.Demangel,Surf.Coat.Technol.224,82(2013). 16.T.L.Fu,Z.L.Zhan,L.Zhang,Y.R.Yang,Z.Liu,J.X.Liu,L.Li,and X.H.Yu,Surf.Coat.Technol.280,129(2015). /doc/1b4f5662effdc8d376eeaeaad1f34693daef1090.html osˇev,M.Metikosˇ-Hukovic′,and H.-H.Strehblow, Biomaterials21,2103(2000).18.Z.L.Jiang,X.Dai,and H.Middleton,Mater.Sci.Eng.,B176,79(2011).19. B.V.Senkovskiy, /doc/1b4f5662effdc8d376eeaeaad1f34693daef1090.html achov, A.V.Fedorov,O.Yu.Vilkov, A.V.Shelyakov,and V.K.Adamchuk,J.Alloy.Compd.537,190(2012).20.J.F.Moulder,W.F.Stickle,P.E.Sobol,and K.D.Bomben,Handbook of X-Ray Photoelectron Spectroscopy(Eden Prairie:Perkin-Elmer Corporation,1992).21.I.Cvijovic′-Alagic′,Z.Cvijovic′,S.Mitrovic′,V.Panic′,and M.Rakin,Corros.Sci.53,796(2011).22.M.K.Han,M.J.Hwang,M.S.Yang,H.S.Yang,H.J.Song,and Y.J.Park,Mater.Sci.Eng.,A616,268(2014).23.Z.C.Feng,X.Q.Cheng,C.F.Dong,L.Xu,and X.G.Li,Corros.Sci.52,3646(2010).24.R.Leiva-Garc?′a,J.C.S.Fernandes,M.J.Mun?oz-Portero,and J.Garc?′a-Anto′n,Corros.Sci.94,327(2015).25.Z.L.Jiang,X.Dai,and H.Middleton,Mater.Chem.Phys.126,859(2011).26.R.M.Ferna′ndez-Domene,E.Blasco-Tamarit,D.M.Garc?′a-Garc?′a,and J.Garc?′a-Anto′n,Electrochim.Acta95,1 (2013).27.R.Cabrera-Sierra,J.Vazquez-Arenas,S.Cardoso,R.M.Luna-Sa′nchez,M.A.Trejo,J.Mar?′n-Cruz,and J.M.Hallen,Electrochim.Acta56,8040(2011).28.P.Kofstad,Oxid.Met.44,3(1995).29. D.D.Macdonald,J.Electrochem.Soc.12,3434(1992).Characteristics and Corrosion Behavior of Pure Titanium Subjected to Surface MechanicalAttrition 1847。

聚焦离子束技术雕刻出的纳米级图案

直径和电子结构决定其漂浮密度。单壁和多壁碳纳米管拥有有趣的特性，但是双壁碳纳米管是透明且导电的，因而具有单
壁、多壁碳纳米管混合特性的双壁碳纳米管更具有吸引力。西北大学的研究人员发现，ＤＷＮＴ的长度比ＳＴ长４％。由于双壁碳纳米管长度更ＷＮ４长，因而透明导电的ＤＴ在用于太阳能电池板时，其电导率提高了２ＷＮ．４倍。（英惠杨
内的发育。该项发现的关键之处在于，这种表面可直接激励细胞，因而无需借助药物，从而排除药物不良影响。科学家们利用化学试剂改变通常的医用金属（如钛）的表面。将这些金属置于选定的酸和氧化物混合液中，使金属表面形成海绵状、由纳米尺寸小坑组成的图案。研究表明，经过处理的表面使骨细胞生长加速、抑制了无用细胞的生长，并激发了支撑细胞。此外，由于与
纳米多孔表面的接触，有利于细胞附着，且生长的基因表达能力也提高了。．（英惠摘译）杨
聚焦离子束技术雕刻出的纳米级图案
美国国家标准与工艺研究所（ＩＴＮＳ）报道，将离子流聚焦成一个ｌｍ大小的光点技术ｎ在纳米工艺中有广泛用途，利用这项技术可在半导体上刻出比目前所有方法都更精小的图形，它还能进行以比目前电子显微镜所达到的分辨率更为细小的非破坏成像。研究人员制成了一个小的原子 “ ” 云，而后联合使用磁场及激光捉住这些原子并将其冷冻至极低温度。利用另一个激光器将原子离化，并使荷电粒子加速通过一个小孔，从而制成了带能离子束。这一装置被命名为ＭＯＩ，用作磁光离子源陷阱。ＭＯＩＴＳＴＳ首先以铬原子进行了演示，并确定除镓外其他元素均可达到相应亮度与强度，宛如聚集离子束。该项技术还可用于许多其他原子。这些离子束可用于切削纳米级图案，而不产生污染，还可用于提高离子束显微镜的对比度。（英惠摘译）杨

螺旋碳纳米管的制备

西南交通大学曲南父逋大罕学位论文版权使用授权书本学位论文作者完全了解学校有关保留、使用学位论文的规定，同意学校保留并向国家有关部门或机构送交论文的复印件和电子版，允许论文被查阅和借阅。

本人授权西南交通大学可以将本论文的全部或部分内容编入有关数据库进行检索，可以采用影印、缩印或扫描等复印手段保存和汇编本学位论文。

本学位论文属于１．保密口，在年解密后适用本授权书；２．不保密团，使用本授权书。

（请在以上方框内打“√”）学位论文作者签名：指导老师签名：ＣⅥ幻日期：加Ｉ≥．ｏｒ．ｆ６西南交通大学硕士研究生学位论文第３页形式：单螺旋，双螺旋；发辫型，弹簧型：共轴螺旋，非共轴螺旋等。

图１－２［５１列出了多种不同螺旋形貌的一维螺旋结构碳材料。

莲图１．２各种螺旋结构碳纳米材料的微观形貌照片【５１团图１．３螺旋碳纤维和螺旋碳纳米管‘叼在微观上，螺旋碳纳米纤维是一种致密的、无内在中空的结构；而螺旋碳纳米管是一种中空管状结构，如图１．３【６】所示。

Ｗ．Ｄａｖｉｓ［７】在１９５３年首次报道了螺旋碳纤维，西南交通大学硕士研究生学位论文第３６页４４．５。

附近出现的峰，对应于镍（１１１）晶面。

图３—４不同催化剂制备的产物的ＸＲＤ衍射图谱进一步分析所得产物中分子结构的变化过程，我们对三种催化剂所得产物分别进行瓜分析。

对比发现，三种尺寸催化剂所得产物较之催化剂前驱体并没有出现新的峰，即在该反应条件下，不同尺寸催化剂纳米Ｎｉ颗粒制备得到的产物分子结构没有发生明显差异。

Ｗａｖｅｎｕｍｂｅｒｓ（ｃｍ’１）图３—５氧化镍前驱体及不同催化剂制备的产物的ｍ图谱３．２．３催化剂对螺旋碳纳米管收率的影响表３—１是三种催化剂所得产物相关结果的比较。

可以看出，采用了溶胶凝胶法制备的催化剂Ｃ所得产物的收率最高，达到了２０３（ｇ．ＨＣＮＴｓ／ｇ。

ｃａｔａｌｙｓｔｓ），而未采取任。

非局部应变梯度理论下纳米梁的力学特性研究

非局部应变梯度理论下纳米梁的力学特性研究张英蓉;沈火明;张波【摘要】基于非局部应变梯度理论,建立了一种具有尺度效应的高阶剪切变形纳米梁的力学模型.其中,考虑了应变场和一阶应变梯度场下的非局部效应.采用哈密顿原理推导了纳米梁的控制方程和边界条件,并给出了简支边界条件下静弯曲、自由振动和线性屈曲问题的纳维级数解.数值结果表明,非局部效应对梁的刚度产生软化作用,应变梯度效应对纳米梁的刚度产生硬化作用,梁的刚度整体呈现软化还是硬化效应依赖于非局部参数与材料特征尺度的比值.梁的厚度与材料特征尺度越接近,非局部应变梯度理论与经典弹性理论所预测结果之间的差异越显著.%A size-dependent mechanical model of nanobeam is built within the framework of the nonlocal strain gradient theory. The present model considers the nonlocal effects of the strain field and first gradient strain field, as well as the high-order shear deformation effect. Governing equations and boundary conditions are derived simultaneously by using Hamilton's principle. The Navier-type solutions are developed for nanobeams with simply-supported boundary conditions. Parametric studies are performed to exhibit the static bending, free vibration and linear buckling behaviors of nanobeams with different groups of geometrical and material parameters. It is found that the non-local effect produces a softening effect on the stiffness of the beam while the strain gradient effect produces a hardening effect, the stiffness of nanobeams is significantly dependent on the ratio between the nonlocal parameter and strain gradient parameter. In addition, the stiffness-hardening or stiffness-softingeffects become increasingly significant as the thickness is close to the material characteristic and can be negligible when the thickness is sufficient large.【期刊名称】《力学与实践》【年(卷),期】2018(040)002【总页数】6页(P167-172)【关键词】非局部应变梯度理论;尺度效应;高阶剪切变形;纳米梁【作者】张英蓉;沈火明;张波【作者单位】西南交通大学力学与工程学院,成都610031;应用力学与结构安全四川省重点实验室,成都610031;【正文语种】中文【中图分类】O343.5随着工程结构逐渐向微型化、智能化的方向发展，纳米梁在微机电系统、生物传感器和原子力显微镜等领域得到日益广泛的应用[1].在研究微尺度结构力学性能的诸多方法中，实验研究由于对试样、仪器和测试方法的严苛要求，以及对精度控制的困难性而备受局限.分子动力学模拟和离散位错动力学模拟也因程序计算量巨大，计算效率较低而难以进行[2].在微纳米尺度下，材料特征长度尺寸接近材料颗粒尺寸，结构的尺度效应不可忽略，传统连续介质理论已无法准确预测微纳米尺度结构的力学性能 [3]，因此，考虑尺度效应的非局部理论 [4]，偶应力理论[5]，应变梯度理论[6]等高阶理论成为微纳米力学领域的研究重点.非局部理论认为，结构内某一点的应力不仅与该点的应变有关，而且与该点附近区域内所有点的应变有关 [7].应变梯度理论则将连续体中的每一个物质点看作含有高阶应变的胞元，据此引入长度尺度参数来表征其对结构力学性能的影响 [8].Lim等[9]在2015年提出了非局部应变梯度理论，该理论考虑了非局部效应和应变梯度效应.基于该理论，Li等[10]分析了尺寸相关杆的轴向振动，得到了不同边界条件下杆的固有频率解析解.Li等 [11]建立了非局部应变梯度非线性欧拉--伯努利纳米梁模型，并进行了屈曲分析，获得了简支梁的后屈曲挠度和临界屈曲力.Ebrahimi 等[12]探究了热环境下功能梯度纳米梁内的波传播行为，以及温度、非局部效应、应变梯度效应对波频和相速度的影响.Ebrahimi等[13]还建立了非局部应变梯度理论下的非均匀功能梯度纳米板的波传播模型，并与经典弹性理论模型做了对比.S¸im¸sek[14]研究了功能梯度纳米梁的非线性振动，通过新型哈密顿法给出了非线性振动频率的近似解.本文基于非局部应变梯度理论和Reddy高阶剪切变形理论，建立了纳米梁的动静力学问题理论模型，以简支纳米梁为例，给出了梁的弯曲、振动和屈曲的纳维级数解，探讨非局部参数、材料特征长度参数及结构尺寸对纳米梁挠度、固有频率和临界屈曲力的影响.1 控制方程建立考虑一个两端简支矩形截面Reddy梁，梁的横截面积为A,其余尺寸参数及坐标设置如图1所示，u1,u2,u3分别为纳米梁沿x,y,z方向的位移式中u,φ,w分别为梁的平移位移，转角位移和挠度.相应的非零应变为其中，c1=4/3h2,c2=4/h2.在Reddy梁理论下，无需额外引入剪切修正系数，且在梁的上下表面(即z=±h/2时)，剪应变等于0，相较于欧拉--伯努利梁和铁木辛柯梁而言更符合实际情况[15].图1 纳米梁尺寸参数及坐标设置示意图根据非局部应变梯度理论，应力可以表示为其中，▽是拉普拉斯算子，σij和分别是经典非局部应力和高阶非局部应力式中，和分别为应变和应变梯度.ea是表征非局部效应的长度参数，α0(|r−r′|,ea)和α1(|r−r′|,ea)是非局部核函数，|r−r′|是弹性体内不同两点间的距离，l是表征高阶应变梯度效应的材料特征长度.鉴于非局部应变梯度积分本构方程求解困难，实际计算中，常使用其微分形式其中，E为材料的弹性模量，G为剪切模量.当ea=0时，式(8)和式(9)退化为应变梯度理论本构方程；当l=0时，退化为非局部弹性理论本构方程.非局部应变梯度理论下的应力和高阶应力沿横截面的内力分别为式中，P,R,P(1)和R(1)只在高阶理论中出现.纳米梁应变能的变分形式可写为动能的变分形式为考虑轴向压力 FN和横向均布力 q所做外力功，其变分形式为根据哈密顿原理将式(12)～式(14)分别代入式(15)中进行分部积分，由变分基本引理提取δu,δw和δφ项相关系数可得到梁的运动方程其中同时，根据积分边界项，可得在x=0和x=L处的边界条件为控制方程(16)与经典理论下相同，对式(8)、式(9)进行积分和变换，可以将式(17)和式(18)转化为位移形式其中为方便起见，引入以下参数来简化方程2 弯曲、振动及屈曲求解对于简支梁，其挠度，转角和分布载荷可以展开为傅里叶级数其中，Wn和Φn为傅里叶系数，Qn是载荷幅值，ωn是固有频率.显然，式(25)满足梁的边界条件.将式(25)和式(26)代入式(21)和式(22)可得其中对于纳米梁的静弯曲问题，挠度和转角与轴向力、频率及时间相关项无关，令式(27)和式(28)中FN,ωn和所有关于时间的求导项为0，可以得到对于自由振动问题，高阶惯性项对频率影响较小，不予考虑 (即令m2=m4=m6=0)，并令式(27)和式(28)中Qn和FN等于0，可以得到纳米梁的固有频率对于屈曲问题，令式 (27)和式(28)中Qn,ωn和所有关于时间的求导项等于0，并设n=1,可以得到纳米梁的临界屈曲力3 数值算例及分析为了探究非局部应变梯度理论下的剪切变形梁模型特点，采用如下几何和材料参数：L=20h,b=2h,E=30×106MPa,ν=0.3,ρ=1kg/m2,q=1N/m,并对相关变量做无量纲化处理为满足精度要求，取傅里叶级数前 100项，分别研究非局部参数和材料特征长度的比值对纳米梁弯曲、振动和屈曲的影响，如图2～图4所示.图2 非局部参数与材料特征长度的比值对挠度的影响图3 非局部参数与材料特征长度的比值对固有频率的影响图4 非局部参数与材料特征长度的比值对临界屈曲力的影响纳米梁刚度总体随非局部长度参数的增加而降低，随特征长度参数的增加而增强.但当ea/l＜1时，最大弯曲挠度随材料特征长度的增大而减小，固有频率和临界屈曲力随材料特征长度的增大而增大；当ea/l＞1时，最大弯曲挠度随材料特征长度的增大而增大，固有频率和临界屈曲力随材料特征长度的增大而减小；当ea/l=1时，纳米梁挠度、固有频率和临界屈曲力值保持不变.当l=0时，所得结果与非局部理论下结果一致[16]，当ea=0时，与应变梯度理论下结果相同[17].取不同尺寸纳米梁，分别计算经典弹性理论(图中②)和非局部应变梯度理论(图中①)下的挠度、固有频率和临界屈曲力，如图5～图7所示.在非局部应变梯度理论中，当ea/l＜1时，纳米梁挠度与经典弹性理论值相比偏小，固有频率和临界屈曲力偏大；当ea/l＞1时，纳米梁挠度与经典弹性理论值相比偏大，固有频率和临界屈曲力偏小；当ea/l=1时，两种理论下所得结果相同.随着纳米梁高度与材料特征尺度越接近，结构尺度效应越明显，非局部应变梯度理论下结果与经典弹性理论结果相比偏差越大，该现象与实验结果相符 [18].而在经典弹性理论中，纳米梁尺寸的等比例变化对无量纲的挠度、固有频率和临界屈曲力没有影响，纳米结构的尺度效应在经典弹性理论中无法得以体现.图5 两种理论下结构尺寸对挠度的影响(①为非局部应变梯度理论，②为经典弹性理论)图6 两种理论下结构尺寸对固有频率的影响(①为非局部应变梯度理论，②为经典弹性理论)图7 两种理论下结构尺寸对临界屈曲力的影响(①为非局部应变梯度理论，②为经典弹性理论)4 结论本文从非局部应变梯度理论本构关系出发，建立了能同时反映剪切变形效应和尺度效应的Reddy梁模型，并通过哈密顿原理得到了梁的控制方程和边界条件，求得纳米梁最大弯曲挠度、固有频率和临界屈曲力的解析解，结合数值算例分析发现：(1)非局部效应的引入对纳米梁起刚度软化作用，而应变梯度效应的引入对纳米梁起刚度硬化作用.(2)当非局部参数大于材料特征长度 (ea＞l)时，结构呈现刚度软化效应；当非局部参数小于材料特征长度(ea＜l)时，结构呈现刚度硬化效应；当其相等(ea=l)时，硬化效应和软化效应相互抵消，挠度、固有频率和临界屈曲力值保持不变并与经典弹性理论下的值相等.(3)纳米梁高度与材料特征尺度越接近，非局部应变梯度理论下结果与经典弹性理论结果相比偏差越大.参考文献1 高世桥,刘海鹏.微机电系统力学(第1版).北京:国防工业出版社,20082 尹春松,杨洋.基于非局部铁木辛柯梁模型的碳纳米管弯曲特性研究.固体力学学报,2015(S1):165-1693 尹莉.微尺度下结构的静动力学行为研究.[博士论文].武汉：华中科技大学,20104 Eringen AC.Nonlocal Continuum Field Theories.New York:Springer Science&Business Media,20025 Yang F,Chong ACM,Lam DCC,et al.Couple stress based strain gradient theory for elasticity.International Journal of Solids andStructures,2002,39(10):2731-27436 Mindlin RD.Micro-structure in linear elasticity.Archive for Rational Mechanics and Analysis,1964,16(1):51-787 Reddy JN,Pang SD.Nonlocal continuum theories of beams for the analysis of carbon nanotubes.Journal of AppliedPhysics,2008,103(2):0235118 Kong S,Zhou S,Nie Z,et al.Static and dynamic analysis of micro beams based on strain gradient elasticity theory.International Journal of Engineering Science,2009,47(4):487-4989 Lim CW,Zhang G,Reddy JN.A 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Mechanical properties of nano-MMT reinforced polymercomposite and polymer concreteByung-Wan Joa,1,Seung-Kook Parkb,*,Do-Keun Kimaa Department of Civil Engineering,2303Structural Engineering Laboratory,Hanyang University,17Haengdang-dong,Seongdong-gu,Seoul 133-791,South KoreabKorea Research Institute for Construction Policy,Specialty Construction B/D,14F,395-70Sindaebang-dong,Dongjak-gu,Seoul 156-714,Republic of KoreaReceived 15December 2006;received in revised form 12February 2007;accepted 23February 2007Available online 19April 2007AbstractUnsaturated polyester (UP)resin is widely used for the matrix of composites such as ﬁber reinforced plastic (FRP)and polymer con-crete.Consequently,inexpensive and high performance resins are important for the future of polymer composites.One recent method for enhancing the performance of polymer composites is the manufacture of MMT (montmorillonite)-UP nanocomposite synthesized by intercalating the UP resin into the silicate layers of MMT.This study investigates the mechanical and thermal properties of MMT-UP nanocomposites,and those of polymer concretes using these nanocomposites.Test results indicate that the mechanical properties and thermal stability of MMT-UP nanocomposites are better than those of pure UP.The glass transition and main chain decomposition temperatures of the MMT-UP nanocomposite exceed those of pure UP.The compressive strength,elastic modulus,and splitting tensile strength of the polymer concrete using MMT-UP nanocomposites exceeded those of polymer concrete using pure UP.Also,the polymer concrete made with MMT-UP nanocomposite has better thermal performance than that of pure UP.The improved performance of UP is very important for the future of polymer concrete.Ó2007Elsevier Ltd.All rights reserved.Keywords:Polymer concrete;Unsaturated polyester resin;MMT-UP nanocomposites;Mechanical properties1.IntroductionPolymer composites are increasingly considered as structural components for use in civil engineering due to their excellent strength-to-weight ratios.Due to its excel-lent adhesion properties,unsaturated polyester (UP)resin is widely used for the matrix of composites such as FRP and polymer composites.However,compared to other res-ins,unsaturated polyester (UP)resin has relatively poor mechanical properties,thermal stability,and ﬁre retardant properties,which limits its use in advanced composites.The modiﬁcation of polymers is of considerable signiﬁ-cance from a material science and engineering point ofview.The performance of unsaturated polyester (UP)resin may be enhanced by the addition of inorganic ﬁllers [5,7].Conventional particulate polymer composites,often called ﬁlled polymers,are of signiﬁcant commercial importance as materials in industrial applications.Polymer nanocompos-ites are a new class of composites derived from nano-scale inorganic particles.Their dimensions typically range from 1to 1000nm and they are homogeneously dispersed in the polymer matrix.Owing to the high aspect ratio of the ﬁllers,the mechanical,thermal,ﬂame retardant and barrier properties of polymers may be enhanced without a signiﬁ-cant loss of clarity,toughness or impact strength.The lay-ered silicate is generally made organophilic by exchanging the inorganic cation,which is located between the layers (d-spacing),with an organic ammonium cation.Clay–poly-mer composites can be classiﬁed into three types:conven-tional composite,intercalated nanocomposites and0950-0618/$-see front matter Ó2007Elsevier Ltd.All rights reserved.doi:10.1016/j.conbuildmat.2007.02.009*Corresponding author.Tel.:+82222200327;fax:+82222920321.E-mail address:plus_skpark@ (S.-K.Park).1Tel.:+82222200327;fax:+82222920321./locate/conbuildmatAvailable online at Construction and Building Materials 22(2008)14–20Construction and BuildingMATERIALSexfoliated nanocomposites(Fig.1).In a conventional com-posite the registry of the clay nanolayers is retained when mixed with the polymer,but there is no intercalation of the polymer into the clay structure(see Fig.2a).Conse-quently,the clay fraction in conventional clay composites plays little or no functional role and acts mainly as aﬁlling agent for economic considerations.An improvement in modulus is normally achieved in a conventional clay com-posite,but this reinforcement beneﬁt is usually accompa-nied by a sacriﬁce in other properties,such as strength or elasticity.Two types of clay–polymer nanocomposites are possible[1,3,6].Intercalated nanocomposites(Fig.2b)are formed when one or a few molecular layers of polymer are inserted into the clay galleries withﬁxed interlayer spacings.Exfoliated nanocomposites(Fig.2c)are formed when the silicate nanolayers are individually dispersed in the polymer matrix,where the average distance between segregated layers is dependent on the clay loading.The sep-aration between the exfoliated nanolayers may be uniform (regular)or variable(disordered).Exfoliated nanocompos-ites show greater phase homogeneity than intercalated nanocomposites.More importantly,each nanolayer in an exfoliated nanocomposite contributes fully to interfacial interactions with the matrix.This structural distinction is the primary reason why the exfoliated clay state is especially eﬀective in improving the reinforcement and other performance properties of clay composite materi-als.The key to the extraordinary performance of polymer–clay nanocomposites is dependent on the complete dispersal(exfoliation)of the clay nanolayers in the polymer matrix.The structure of the montmorillonite clay used as theﬁl-ler comprises an octahedral alumina sheet sandwiched between two tetrahedral silica sheets.Alkylammonium ions lower the surface energy of the clay so that monomers and polymers with diﬀerent polarities can enter the space between the layers and cause further separation of the sili-cate layers to form the nanocomposite[2,4].The objective of this study is to enhance the performance of polymer composites using unsaturated polyester(UP) resin based recycled PET(poly ethylene terephthalate)[8]. Therefore,this work investigates the mechanical properties and thermal stability of MMT-UP nanocomposites and polymer concrete using the MMT-UP nanocomposite.The results are supported by mechanical testing,X-ray diﬀraction(XRD),transmission electron microscopy (TEM),diﬀerential scanning calorimetry(DSC),and thermo gravimetric analysis(TGA).2.Research signiﬁcanceThis study contributes to the understanding of the prop-erties of MMT-UP nanocomposite and polymer concrete using MMT-UP nanocomposite as follows.B.-W.Jo et al./Construction and Building Materials22(2008)14–20151.It proves possible the manufacture of a higher gradepolymer concrete using nano-MMT.2.It posits methods for enhancing the performance ofpolymer concrete with the addition of nano-MMT. 3.It suggests that polymer concrete made with recycledPET and nano-MMT may be useful materials for pro-ducing polymer concrete products.3.Experiments3.1.MaterialsThree diﬀerent kinds of MMT were used.Southern Clay Products Inc.,USA,supplied non-treated Na+-MMT and organophilic treated MMT under the trade names of Cloi-site30B and25A.Cloisite30B is a montmorillonite modi-ﬁed with methyl,tallow,bis-2-hydroxyethyl,quaternary ammonium chloride;Cloisite25A is a montmorillonite modiﬁed with dimethyl,dehydrogenated tallow,2-ethyl-hexyl,quaternary ammonium chloride.Table1shows some of the manufacturer’s data on these MMTs.The unsaturated polyester resin based on recycled plas-tic(PET)was used as the matrix[8].A styrene content of 40%in unsaturated polyester was chosen for its low viscos-ity(1300mPa s at25°C)and to achieve improved resin dif-fusion into the galleries of the MMT.To start the curing process,1%(by weight of resin)of10.7%active oxygen methylethy ketone peroxide initiator and0.1%(by weight of resin)of8%solution cobalt octoato promoter(used as an accelerator)were added to the resin.The following coarse andﬁne inorganic aggregates were used in the experimental study of polymer concrete:8mm pea gravel;siliceous river sand with aﬁneness modulus of 2.48,and CaCO3(calcium carbonate).The aggregate was oven-dried for a minimum of24h at200°C to reduce its moisture content to less than0.3%by weight,thus ensuring a perfect bond between the polymer matrix and the inor-ganic aggregates.The use of calcium carbonate greatly improved the workability of the fresh mix.Theﬁne and spherical calcium carbonate particle provided the fresh mix with better lubricating properties,thus improving its plasticity and cohesiveness.The better gradation obtained with calcium carbonate also resulted in a hardened mate-rial with improved strength properties and surface appear-ance.The properties of the aggregate and resin are shown in Tables2and3,respectively.3.2.UP-MMT NanocompositesThere are two steps for manufacturing the UP-MMT nanocompostite.First,in the mixing process,the UP linear chains are mixed with styrene monomers and layered sili-cate.Second,in the curing process,the crosslinking reac-tion is started by decomposing the initiators.The unsaturated polyester chains,styrene monomers and nano-MMTs were mixed for3h at60°C.The weight per-centages of MMT in UP-MMT nanocomposite used were 2%,5%,8%and10%,respectively.The mixture was then cooled to room temperature.1%by weight of initiator (MKPO)was added and the mixture was stirred for 2min.The mixture was poured into molds,cured at room temperature for12h and post-cured at120°C for4h.X-ray diﬀraction(XRD)patterns were obtained using a Rigaku X-ray diﬀractometer equipped with CuK a radia-tion and a curved graphite crystal monochromator.Sam-ples were prepared by applying the pre-intercalated mixture and nanocomposite of UP-MMT in sheet form on a slide.All XRD data were collected with an X-ray gen-erator(k=1.5406A˚).Bragg’s law(k=2d/sin h)was used to compute the crystallographic d-spacing.In order to evaluate the change in the glass transition temperature,T g,associated with increases in the MMT content,a diﬀerential scanning calorimeter(DSC)analysis was carried out using a General V4.1C DuPont2000.The measurement was carried out from30°C to300°C using a heating rate of10°C/min in a nitrogen atmosphere.The thermal behavior was determined with a thermogravimetric analyzer(TGA).Microscopic investigation was performed with a transmission electron microscope(TEM)with an acceleration voltage of100kV.3.3.Polymer concrete using UP-MMT NanocompositesTensile tests were performed according to ASTM D638M-91a at a crosshead speed of5mm/min.The poly-mer concrete cylinders used for compression and splitting tensile tests were76mm in diameter and152mm in length. Specimens were tested in a hydraulic load machine at a constant loading rate of44,500N/min.The mix design of polymer concrete,proportioned by weight,was as follows: 11%resin(MMT-UP),45%oven-dried coarse aggregate, 33%oven-dried sand,and11%CaCO3.The compressiveTable1Properties of modiﬁed montmorilloniteProperties Na+Cloisite30B Cloisite25AOrganic modiﬁer None MT2EtOH2MHTL8Speciﬁc gravity(g/cc) 2.86 1.87 1.98%Weight loss on ignition(%)73430X-ray results(d001)(A˚)11.718.518.6MT2EtOH(methyl,tallow,bis-S-hydroxyethyl,quaternaryammonium),2MHTL8(dimethyl hydrogenated tallow2-ethylhexylammonium).Table2Properties of the aggregatesType Size Speciﬁc gravity Fineness modulus Absorption(%) Coarse aggregate Maximum size8mm 2.63 6.420.08Fine aggregate0.1–0.6mm 2.60 2.480.0516 B.-W.Jo et al./Construction and Building Materials22(2008)14–20modulus of elasticity was ﬁrst obtained using a compress-ometer with a 76-mm gauge length using two diametrically opposite sides.The compression elastic modulus was calcu-lated where the stress was 40%of the maximum strain on the stress–strain (load–deﬂection)graph.Flexural specimens were mixed and compacted in a steel mold with dimensions of 50·50·305-mm.The beams were loaded in third-point loading at a uniform rate of 2225N/min.The specimens were cast,cured,and tested at room temperature.Testing of the specimens was performed at 7days.Tests were per-formed to determine the eﬀect of temperature on the PC compressive strength,splitting tensile strength,modulus of elasticity,and ﬂexural strength.After curing,specimens were put in an environmental chamber at the desired tem-perature for a period of 2days prior to testing.Selected tem-peratures were À15°C,25°C,and 65°C.Actual testing was performed at room temperature immediately after removing the specimens from the environmental chamber.4.Results and discussion 4.1.UP-MMT NanocompositeSilicate layer dispersion in the MMT-UP nanocomposite was analyzed by XRD.As shown in Fig.3,the XRDs of MMTs and MMT-UP composites investigated diﬀerent peak with the types of MMTs.The peaks for Na +,Cloisite 25A,Na +-UP and Cloisite 25A-UP nanocomposite are shown at 7.5°,3.5°,5.2°and 2.6°,respectively.These 2h val-ues correspond to interlayer spacings of 11.7,18.6,17.0and34.6A˚,respectively.A new peak was observed in the Na +-UP composite.This indicates that UP by polymerization is intercalated between the MMT layers.However,for theCloisite 30B-UP composite,the peak at the lower angle dis-appeared,suggesting that either the silicate layer platelets were exfoliated in the polymer matrix or they disappeared because the spacing between the layers was too large.It is important to note that polymerization of Na +-UP and Cloi-site 25A-UP composite led only to an intercalated structure,while Cloisite 30B promoted the delamination process of layered silicates to achieve exfoliation.More direct evidence for the formation of a nanocom-posite is provided by the TEM.The dark lines in the TEM image in Fig.4b are individual silicate layers.In the case of the MMT (Cloisite 30B)layers,some irregular dis-persions exist in the silicate layer.Also,the relatively exfo-liated and well-dispersed portion of the nanocomposite wasTable 3Unsaturated polyester resin formulation ComponentsRecycled PET Propylene glycol,diethylene glycol,dipropylene glycol Terephthalic acid,maleic anhydride Styrene monomer (SM)Percentage by weight (%)29.116.014.940.0Fig.4.Transmission electron micrographs (TEM)of MMT-UP nanocomposite:(a)Na +-UP,(b)Cloisite 30B-UP.B.-W.Jo et al./Construction and Building Materials 22(2008)14–2017made with Closite30B,as shown in Fig.4b.However,the MMT(Na+)layers shown in Fig.4a are more regular and some of the silicate layers maintained their original order.With the addition of the MMT,the tensile modulus of the composites shown in Fig.5increases up to5%loading since MMT is more rigid than the matrix resin.The modu-lus of the Cloisite30B and Cloisite25A composites exceed that of the Cloisite Na+composite owing to their higher degree of exfoliation and better adhesion at the MMT-UP interface.Above5%MMT content,the tensile modulus starts to decrease with MMT content in both types of com-posites,due to a lower degree of exfoliation and a lower degree of polymer–MMT surface interactions at higher MMT content.For the Cloisite Na+composite,the increase in the tensile modulus is not signiﬁcant compared to the ten-sile modulus of pure unsaturated polyester.This is because there is little or no intercalation/exfoliation between the sil-icate layers of the Cloisite Na+composite,so these materi-als act as conventional composites,especially at high MMT contents.Also,the cross-link density might be lower with a higher MMT content,leading to a lower modulus.The increase in tensile strength associated with increases in MMT content is demonstrated in Fig.6.The variation in tensile strength of the composite with MMT contents is sim-ilar to that of the tensile modulus.The maximum tensile strength emerged at5%MMT content.Work done on the thermal properties of polymers has shown that the glass transition of polymer–MMT nano-composites increases with increasing MMT content.The eﬀect of nano-MMT(Closite30B)content on the T g is shown in Fig.7.The T g increases with increasing of MMT(Closite30B)content.This implies improved adhe-sion between the UP and MMT surfaces.Also,the nano-MMT prevents segmental motions of the polymer chains. It is known that the primary factor aﬀecting the T g of cured UP is the crosslinked density in the same UP resin.There-fore,it can be concluded that the UP-MMT nanocomposite has a high crosslinking density.However,beyond a certain MMT content(approximately in the range of5–7%),T g decreases with increasing MMT content.Thus,the cross-linking density might decrease at a high MMT content.TGA curves of the pure UP and the Closite30B-UP nanocomposite are shown in Fig.8.The onset of degrada-tion is slightly but progressively hastened in the pure UP compared to the Closite30B-UP nanocomposite.Thermal degradation of pure UP and MMT-UP has three distinct18 B.-W.Jo et al./Construction and Building Materials22(2008)14–20steps.Theﬁrst is the decompostition of relatively weak head-to-head linkages,impurities,and styrene monomers in the UP.The second is the decomposition of the UP chain-end,and the third is the decomposition of the UP main chains.The three degradation steps occurred at 161°C,272°C,and321°C in the pure UP and at224°C, 326°C,and408°C in the MMT-UP nanocomposite.The temperature of the main chain decomposition of the Cloi-site30B-UP nanocomposite exceeds that of the pure UP by about80°C.Pure UP is completely decomposed at 400°C.The nanocomposites show slower degradation above400°C since only inorganic MMT is left in the sys-tem at that stage.This demonstrates that the MMT-UP nanocomposite has better thermal stability than pure UP.4.2.Eﬀect of UP-MMT(Closite30B)nanocomposite on polymer concrete4.2.1.Testing temperatureÀ25°CThe strength of polymer concrete specimens cast with Cloisite30B-UP nanocomposite containing5%MMT con-tent was estimated.The compressive strength,elastic mod-ulus,and splitting tensile strength of polymer concrete using Cloisite30B-UP nanocomposite exceeded the corre-sponding properties of polymer concrete using pure UP, suggesting that the use of exfoliated MMT-UP nanocom-posite enhances polymer concrete strength.Theﬂexural strength of the polymer concrete does not signiﬁcantly increase with the use of the Cloisite30B-UP nanocompos-ite.The compressive strength,elastic modulus,and split-ting tensile strength of polymer concrete were found to be correlated with the tensile strength and tensile modulus of the MMT-UP nanocomposite.However,theﬂexural strength of the polymer concrete was not signiﬁcantly cor-related with the tensile strength and tensile modulus of the MMT-UP nanocomposite.4.2.2.The eﬀect of temperature on strength and modulus of elasticityThe eﬀects of temperature on the compressive strength, modulus of elasticity,and splitting tensile andﬂexural strength of the polymer concrete using UP-MMT nano-composite are shown in Figs.9and10.The modes of failure of polymer concrete diﬀered depending on the temperature at which the materials were pression cylinders had a sudden,brittle failure when tested atÀ15°C and25°C.Conversely,cylinders tested at65°C had a slow,ductile failure resulting in an excessive bulging of the specimens.This behavior arises from decreases in the modulus of the resin binder in the polymer concrete specimens under increasing temperature. That is,the modulus of the polymer concrete specimen decreases with increases in temperature,as shown in Fig.9.Increase in temperature eﬀected a loss in strength and modulus of elasticity in the polymer concrete specimens because the resin binder decreased in strength with an increase in temperature.In the case of the polymer concrete specimens using pure UP,an increase in temperature from 25°C to65°C decreased the compressive strength by about 33%,modulus of elasticity by about36%,splitting tensile strength by about31%,andﬂexural strength by about 38%.In the case of the polymer concrete specimens using UP-MMT nanocomposite,an increase in temperature from 25°C to65°C decreased the compressive strength by about 18%,modulus of elasticity by about22%,splitting tensile strength by about18%,andﬂexural strength by about22%.This result demonstrates that the polymer concrete made with MMT-UP nanocomposite has mechanical prop-erties that are better than those of pure UP.The improved performance of UP is very important for the future of poly-mer concrete.Therefore,the enhancement of the mechani-cal and thermal performance of polymer concrete aﬀorded by the use of nano-MMT is remarkable.5.ConclusionsThe main objective of this study was to enhance the per-formance of polymer concrete using unsaturated polyester resin.This work investigated whether MMT-UP nanocom-posite can be used to produce polymer concrete that exhibitsB.-W.Jo et al./Construction and Building Materials22(2008)14–2019excellent mechanical and thermal performance.The follow-ing conclusions can be drawn from the results of this study: 1.The mechanical and thermal properties of the compos-ites,which have their maximum tensile strength,tensile modulus,and T g with5%nano-MMT,were dramati-cally improved by the addition of nano-MMT dispersed in the polymer matrix.Also,the elastic modulus of MMT-UP nanocomposite was enhanced by the addition of nano-MMT.However,beyond a certain MMT con-tent(approximately in the range of5–7%),the mechan-ical and thermal performance of the composites decreased with increasing nano-MMT content.2.In composites with Na+,the mechanical and thermalproperties did not show a signiﬁcant change because the degree of exfoliation is less than that of Cloisite 30B-UP nanocompostites.3.The strength and elastic modulus of the polymer con-crete was enhanced by the use of exfoliated MMT-UP nanocomposite.It is important to note that the exfoli-ated MMT-UP nanocomposite greatly aﬀects the per-formance of the polymer concrete.Also,the strength and elastic modulus of polymer concrete was found to be positively correlated with the tensile strength and ten-sile modulus of the MMT-UP nanocomposite.4.Unsaturated polyester resins made with recycled PETand nano-MMT may be used to greatly enhance the per-formance of polymer composites at a relatively low cost. 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