Characterizing the Structure and Properties of Individual Wire‐Like Nanoentities

A new approach to the characterization of the mechanical and electrical properties of individual nanowires and nanotubes is demonstrated by in‐situ transmission electron microscopy (TEM). The technique allows a one‐to‐one correlation between the structure and properties of the nanowires. Recent developments include the determination of the Young's modulii of carbon nanotubes and semiconductor nanowires, femtogram nanobalance of a single fine particle, field emission of carbon nanotubes, and quantum ballistic conductance in carbon nanotubes.     

Mechanical characterization of nickel nanowires by using a customized atomic force microscope

A new experimental method to characterize the mechanical properties of metallic nanowires is introduced. An accurate and fast mechanical characterization of nanowires requires simultaneous imaging and testing of the nanowires. However, existing mechanical characterization techniques fail to accomplish this goal due either to the lack of imaging capability of the mechanical test setup or the difficulty of individual alignment and manipulation of single nanowires for each test. In this study, nanowire specimens prepared by an electroplating technique are located on a silicon substrate with trenches. A customized atomic force microscope is located inside a scanning electron microscope (SEM) in order to establish the visibility of the nanowires, and the tip of the atomic force microscope cantilever is utilized to bend and break the nanowires. The ability to visualize the nanowires in an SEM improves the speed and accuracy of the tests. Experimentally obtained force versus bending displacement curves are fitted into existing analytical formulations to extract the mechanical properties. Experimental results reveal that nickel nanowires have significantly higher strengths than their bulk counterparts, although their elastic modulus values are comparable to bulk nickel modulus values.     

一维单体纳米材料弹塑性变形机理的原位研究进展EI北大核心CSCD

近年来,随着研究技术手段的发展,纳米线表现出了大量具有潜在应用价值的新现象。清晰描绘纳米线结构与力学性能的构效关系对纳米器件的设计、服役以及性能优化具有重要的指导意义。本文首先归纳了纳米线力学性能几种常用的原位测试方法,其次介绍了各类纳米线在拉伸实验中的弹性和强度等力学性能,阐述了纳米线与尺寸相关的塑性变形,此外简述了纳米线在原位测试中所表现出的奇特力学行为。今后,系统地研究原位电镜表征过程中电子束辐照对纳米线变形行为的影响,探究纳米线在复杂外场环境下所展现的力学性能,从而建立一套完备的理论指导体系,是纳米材料性能原位表征领域的重要发展方向。     

Metallurgical and mechanical characterization of electron beam welded DP600 steel joints

Several new commercial advanced high-strength steels exhibit high strength and enhanced formability. These materials have the potential to affect cost and weight saving while improving performance. However, welding, by modifying the microstructure of the steel, has in general a detrimental effect on the mechanical properties of structural components. If high power density technologies are used, the result is that the mechanical properties of such kind of joints can be improved. This article presents a metallurgical and mechanical characterization of electron beam welded joints in advanced high-strength steel DP600. The experimental analysis was supported by a thermal numerical model obtained through the Sysweld^? code. Results show that mechanical properties of the electron beam welded joints are comparable with those of parent metal both in terms of static strength and ductility.     

Nano-mechanical properties of individual mineralized collagen fibrils from bone tissue

Mineralized collagen fibrils (MCFs) are distinct building blocks for bone material and perform an important mechanical function. A novel experimental technique using combined atomic force microscopy and scanning electron microscopy is used to manipulate and measure the mechanical properties of individual MCFs from antler, which is a representative bone tissue. The recorded stress–strain response of individual MCFs under tension shows an initial linear deformation region for all fibrils, followed by inhomogeneous deformation above a critical strain. This inhomogeneous deformation is indicative of fibrils exhibiting either yield or strain hardening and suggests possible mineral compositional changes within each fibril. A phenomenological model is used to describe the fibril nano-mechanical behaviour.     

In situ tensile testing of nanofibers by combining atomic force microscopy and scanning electron microscopy

A nanomechanical testing set-up is developed by integrating an atomic force microscope (AFM) for force measurements with a scanning electron microscope (SEM) to provide imaging capabilities. Electrospun nanofibers of polyvinyl alcohol (PVA), nylon-6 and biological mineralized collagen fibrils (MCFs) from antler bone were manipulated and tensile-tested using the AFM-SEM set-up. The complete stress-strain behavior to failure of individual nanofibers was recorded and a diversity of mechanical properties observed, highlighting how this technique is able to elucidate mechanical behavior due to structural composition at nanometer length scales.     

Nano‐Scale Mechanics of Nanotubes,Nanowires, and Nanobelts

One‐dimensional (1D) nanostructures have numerous potential applications in science and engineering. Nanocomposites made of nanowires, such as carbon nanotubes, are likely to decrease material’s density and increase its strength,^[1] which are of critical importance to space technology. To investigate the uniqueness offered by these materials, new techniques must be developed to quantitatively measure the properties of individual wire‐like structures whose structures are well characterized by electron microscopy techniques, because their properties may sensitively depend on their geometrical shape/configurations and crystal as well as surface structures. Within the framework of in‐situ TEM we have recently developed a novel approach that relies on electric field induced mechanical resonance for measuring the properties of individual wire‐like structures, such as Young’s modulus, electron field emission, tip work function, and electrical quantum conductance. This is a new technique that provides the properties of a single nanowire with well characterized.     

玻璃纤维织物/聚苯硫醚粉叠层模塑工艺与性能

研究聚苯硫醚(PPS)树脂粉与玻璃纤维织物(GF)叠层模塑(粉末工艺)制复合材料的工艺与性能。测试各种工艺条件制GF/PPS层板的弯曲力学性能、动态力学性能,用扫描电镜(SEM)探查树脂对纤维的浸渍及纤维/基体的界面粘合情况。SEM分析结果表明,粉末工艺制得的PPS基复合材料呈现高的力学性能,是由于树脂对纤维的均匀浸渍和良好的纤维/基体的界面粘合。熔前热压、高温成型、退火处理是粉末工艺制高质量GF/PPS层板的工艺要点。      

Carbon nanotube yarn and 3-D braid composites. Part I: Tensile testing and mechanical properties analysis

Macroscopic textile preforms were produced with a multi-level hierarchical carbon nanotube (CNT) structure: nanotubes, bundles, spun single yarns, plied yarns and 3-D braids. The 3-D braided preform was the first of its kind produced by textile processing technique and used as a composite reinforcement consisting solely of carbon nanotubes. Four different epoxy systems that possessed a wide range of mechanical properties (owed to an added modifier) were infused into the CNT yarns and 3-D braids. Mechanical characterization of the resulting composites was conducted through the use of tensile testing. It was found that the tensile strength, stiffness and, especially, strain-to-failure values for each preform type were close regardless of the properties of the matrix whose strain-to-failure values ranged from 3.6% to 89%. This is hypothetically attributed to the nano-scale interaction between individual nanotubes and polymeric macromolecules in the composites. This hypothesis is validated by the Dynamic Mechanical Analysis results in Part II.     

Development of tantalum scaffold for orthopedic applications produced by space-holder method

In the present study, production of tantalum porous scaffolds using the space holder technique was performed. The effect of size and content of sodium chloride particles, used as space holder, as well as compacting pressure on foam structure and mechanical properties have been investigated. The morphological characterization was carried out by means of scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and micro-CT technique. The relationship between the elastic modulus and yield strength of the tantalum porous scaffold and the pore structure was evaluated. Space holder technique allows obtaining tantalum open-cell structure (70% of porosity) and modulus of elasticity similar to cancellous bone, with reproducible processability into three-dimensional structures and reasonable manufacturing costs.     

跨尺度模拟硬化水泥净浆力学性能:微观物相的影响

采用纳米压痕实验测量硬化水泥净浆中未水化物、水化产物和孔隙等微观物相的力学性能,并基于背散射电镜（BSE）图像的灰度分析计算各微观物相的含量。在得到各微观物相含量和力学性能的基础上,针对水泥净浆的弹性模量进行均一化建模,并讨论各微观物相及其模量的选取对跨尺度模拟硬化水泥净浆力学性能的影响。通过微米压痕的实测净浆模量验证模型及参数选取的可靠性。提出在硬化水泥净浆力学性能的多尺度模型中,需要选取12 GPa作为孔隙有效模量,并将水化产物划分为低密度的水化硅酸钙凝胶（LD-CSH）和高密度的水化硅酸钙凝胶（HD-CSH）两种物相,而不同种类的未水化物可被视作一种物相。在此基础上,使用Mori-Tanaka模型或自洽模型计算得到的净浆模量与实测净浆模量吻合。     

Felling of individual freestanding nanoobjects using focused-ion-beam milling for investigations of structural and transport properties

We report that, to enable studies of their compositional, structural and electrical properties, freestanding individual nanoobjects can be selectively felled in a controllable way by the technique of low-current focused-ion-beam (FIB) milling with the ion beam at a chosen angle of incidence to the nanoobject. To demonstrate the suitability of the technique, we report results for zigzag/straight tungsten nanowires grown vertically on support substrates and then felled for characterization. We also describe a systematic investigation of the effect of the experimental geometry and parameters on the felling process and on the induced wire-bending phenomenon. The method of felling freestanding nanoobjects using FIB is an advantageous new technique enabling investigations of the properties of selected individual nanoobjects.     

Structural characterization of ZnO thin films deposited by dc magnetron sputtering

In this work we present recent results on ZnO thin films grown by dc magnetron sputtering technique at room temperature (RT), focusing on structural and surface characterization using conventional cross-section transmission electron microscopy (XTEM) and high resolution cross section transmission electron microscopy (HRXTEM) in an attempt to understand the thickness influence on film, mechanical and optical properties as well as photoreduction/oxidation conductivity changes. Films were found to be polycrystalline with a columnar mode of growth. For films with thickness over 100 nm, XTEM and HRTEM analysis evidenced the presence of a small grains transition layer near interface with the substrate, feature which plays an important role in ZnO thin films for gas sensing application. The control of such structural parameters is proved to be critical for the improvement of their gas sensing performance.     

Kinetics and Nanostructure Dependence of High Temperature-Low Stress Creep of Al and Al-0.3%Fe

The novel nanostructure of Al and Al-Fe were prepared by ball milling alumina with elemental Fe.The kinetics and nanostructure dependence of high temperature low stress Newtonian creep of Al and Al-0.3%Fe have been investigated and compared with the predications of the Nabarro-Herring(N-H) theory of directional diffusion.A simple theory based on the climb controlled generation of dislocations from a fixed density of sources is developed to explain the observed behavior.The dislocation density increases and subgrains form during the creep.Also,the presence of precipitates of FeAl 3 reduces the creep rate of Al by absolute faster of 100 at the same stress and temperature,in spite of the fact that the grain size in the Al-0.3%Fe alloy is smaller by a factor of about 100 nm.The reduction of grain size to the nanometer scale improves their mechanical properties.Electron diffraction methods combined with transmission electron microscopy(TEM) and scanning electron microscopy(SEM) studies are a convenient and powerful technique for the characterization of the phases and grain structure of the resulting materials.     

Single‐Particle Cryo‐EM and 3D Reconstruction of Hybrid Nanoparticles with Electron‐Dense Components

Single‐particle cryo‐electron microscopy (cryo‐EM), accompanied with 3D reconstruction, is a broadly applicable tool for the structural characterization of macromolecules and nanoparticles. Recently, the cryo‐EM field has pushed the limits of this technique to higher resolutions and samples of smaller molecular mass, however, some samples still present hurdles to this technique. Hybrid particles with electron‐dense components, which have been studied using single‐particle cryo‐EM yet with limited success in 3D reconstruction due to the interference caused by electron‐dense elements, constitute one group of such challenging samples. To process such hybrid particles, a masking method is developed in this work to adaptively remove pixels arising from electron‐dense portions in individual projection images while maintaining maximal biomass signals for subsequent 2D alignment, 3D reconstruction, and iterative refinements. As demonstrated by the success in 3D reconstruction of an octahedron DNA/gold hybrid particle, which has been previously published without a 3D reconstruction, the devised strategy that combines adaptive masking and standard single‐particle 3D reconstruction approach has overcome the hurdle of electron‐dense elements interference, and is generally applicable to cryo‐EM structural characterization of most, if not all, hybrid nanomaterials with electron‐dense components.     

Process Optimization for Pulse Reverse Electrodeposition of Graphene-Reinforced Copper Nanocomposites

In the present study, processing of graphene-reinforced copper nanocomposite foils with homogenous dispersion of graphene throughout the matrix, exhibiting good mechanical properties by a simple, cost-effective, and scalable pulse reverse electrodeposition technique (PRED) with special focus on the influence of graphene content in the electrolyte to tailor the properties. A systematic approach has been adopted for enhancing the properties. Distribution of graphene nanosheets in the copper metal matrix and the microstructural properties have been studied by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). Interesting observations have been made from nanoindentation studies, where hardness (～2.7 GPa) enhanced mainly with increase in graphene content (0–0.75 g/L), while maximum elastic modulus (～139 GPa) is achieved for a graphene content of 0.5 g/L in the electrolyte. Four-point probe testing has been adopted to evaluate the electrical features. The major contribution in enhancement of properties is found to be the presence of graphene and its uniform individual dispersion and distribution as nanosheets in the copper matrix.     

Effect of individual and combined addition of micro/nano-sized metallic elements on the microstructure and mechanical properties of pure Mg

In this study, the effects of the addition of individual micro- and nano-sized metallic elements (micron-Ti and nano-Cu) and their combination on the microstructure and mechanical properties of pure Mg were investigated. The materials were produced by rapid microwave sintering assisted powder metallurgy technique followed by hot extrusion. From the microstructural characterization and mechanical property evaluation, it was identified that the solid solubility, size and the method of addition of elements significantly influenced the properties. While the insoluble micron-Ti enhanced the strength and reduced the ductility due to weak interface, the addition of nano-Cu with relative solid solubility enhanced the strength due to the formation of Mg₂Cu intermetallics and retained the ductility. The positive effect of their combined addition was found to be more pronounced when they were pre-processed, rather than their direct addition. The change in particle morphology, Ti₃Cu intermetallic formation and good interfacial bonding with the matrix achieved due to pre-processing contributed towards its superior strength and ductility.     

Nanoscale scraping and dissection of collagen fibrils

The main function of collagen is mechanical, hence there is a fundamental scientific interest in experimentally investigating the mechanical and structural properties of collagen fibrils on the nanometre scale. Here, we present a novel atomic force microscopy (AFM) based scraping technique that can dissect the outer layer of a biological specimen. Applied to individual collagen fibrils, the technique was successfully used to expose the fibril core and reveal the presence of a D-banding-like structure. AFM nanoindentation measurements of fibril shell and core indicated no significant differences in mechanical properties such as stiffness (reduced modulus), hardness, adhesion and adhesion work. This suggests that collagen fibrils are mechanically homogeneous structures. The scraping technique can be applied to other biological specimens, as demonstrated on the example of bacteria.     

Influence of metal nanoparticle decorated CNTs on polyurethane based electro active shape memory nanocomposite actuators

Polymer nanocomposites based on thermoplastic polyurethane (PU) elastomer and metal nanoparticle (Ag and Cu) decorated multiwall carbon nanotubes (M-CNTs) were prepared through melt mixing process and investigated for its mechanical, dynamic mechanical and electro active shape memory properties. Structural characterization and morphological characterization of the PU nanocomposites were done using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Morphological characterization revealed better dispersion of M-CNTs in the polyurethane, which is attributed to the improved interaction between the M-CNTs and polyurethane. Loading of the metal nanoparticle coated carbon nanotubes resulted in the significant improvement on the mechanical properties such as tensile strength of the PU composites in comparison to the pristine carbon nanotubes (P-CNTs). Dynamic mechanical analysis showed that the glass transition temperature (Tg) of the polyurethane increases slightly with increasing loading of both pristine and metal nanoparticle functionalized carbon nanotubes. The metal nanoparticles decorated carbon nanotubes also showed significant improvement in the thermal and electrical conductivity of the PU/M-CNTs nanocomposites. Shape memory studies of the PU/M-CNTs nanocomposites exhibit remarkable recoverability of its shape at lower applied dc voltages.     

Development of high strength magnesium based composites using elemental nickel particulates as reinforcement

Magnesium based materials due to their inherently low density and ensuing potential to exhibit high specific mechanical properties are actively sought for weight-critical structural application. In the present study, elemental and nickel reinforced magnesium materials were synthesized using an innovative disintegrated melt deposition technique followed by hot extrusion. Microstructural characterization of the composite samples showed uniform distribution of nickel particulates in the matrix material, good interfacial integrity of magnesium matrix with nickel particulates and Mg-Ni based intermetallics, and the presence of minimal porosity. Physical properties characterization revealed that addition of nickel as reinforcement improves the dimensional stability of pure magnesium. Mechanical properties characterization revealed that the presence of nickel reinforcement lead to significant improvement in hardness, elastic modulus, 0.2% yield strength and UTS while the ductility was adversely affected. The results further revealed that the combination of 0.2% yield strength, UTS, and ductility exhibited by nickel reinforced magnesium remained much superior even when compared to high strength magnesium alloy AZ91 reinforced with much higher volume percentage of SiC. An attempt is made in the present study to correlate the effect of nickel as reinforcement and its increasing amount with the microstructural, physical and mechanical properties of magnesium.