Surface-induced structural transformation in nanowires

One of the unique features of nanomaterials is that they have large surface-to-volume atom ratios compared to bulk materials. The intrinsic compressive stress along the nanowire axis can be as large as tens of GPa, and spontaneous reorientation or phase transformation may occur in order for the nanowires to return to the low-energy state. Upon tensile loading, the nanowires can revert back to the original high-energy orientation or phase without introducing any defects. Two mechanisms are mainly involved in the deformation: (1) twinning/detwinning and (2) stress-induced martensitic phase transformation (MT)/inverse MT. Generally, this surface-induced behavior can only occur at a temperature higher than the critical temperature, T_c, due to the energy barrier for structural transformation. As a result, ordinary nanoscale metals can exhibit pseudo-elasticity and shape memory effects previously only observed from special alloys such as nickel titanium (NiTi). These nanowires have the predicted recoverable strain on the order of 40%–70% which is much larger than that of bulk NiTi (5%–10%), but have extremely low energy dissipation (2% for W nanowires, for example). Surface-induced structural transformation has been observed from fcc, bcc, and hcp single-element metal nanowires, intermetallic alloy nanowires, multilayered and core-shell composite nanowires, and even oxide and nitride compound semiconductor nanowires. This unique phenomenon enables the design of novel and flexible nanoelectromechanical systems (NEMS) having potential applications in nanomanipulators, energy storage, sensors, switches, and so on. We will review the breakthrough and development in this field in the past ten years, mainly focusing on the physical mechanisms and dominant factors governing this spontaneous structural transition. Future developments will also be discussed.     

Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.     

Highly Stretchable,Shape Memory Organohydrogels Using Phase‐Transition Microinclusions

Shape memory effect in polymer materials has attracted considerable attention due to its promising applications in a variety of fields. However, shape memory polymers prepared by conventional strategy suffer from a common problem, in which high strain capacity and excellent shape memory behavior cannot be simultaneously achieved. This study reports a general and synergistic strategy to fabricate high‐strain and tough shape memory organohydrogels that feature binary cooperative phase. The phase‐ transition micro‐organogels and elastic hydrogel framework act synergistically to provide excellent thermomechanical performance and shape memory effect. During shape memory process, the organohydrogels exhibit high strain capacity, featuring fully recoverable stretching deformation by up to 2600% and compression by up to 85% beneath a load ≈20 times the organohydrogel's weight. Furthermore, owing to the micro‐organogel and hydrogel heterostructures, the interfacial tension derived from heterophases dominates the shape recovery of the organohydrogel material. Simple processing and smart surface patterning of the shape memory behavior and multiple shape memory effects can also be realized. Meanwhile, these organohydrogels are also nonswellable in water and oil, which is important for multimedia applications.     

Intrinsic two-way shape memory effect in a Ni-Mn-Sn metamagnetic shape memory microwire

An intrinsic two-way shape memory effect with a fully recoverable strain of 1.0% was achieved in an as-prepared Ni_(50)Mn_(37.5)Sn_(12.5) metamagnetic shape memory microwire fabricated by Taylor-Ulitovsky method. This two-way shape memory effect is mainly owing to the internal stress caused by the retained martensite in austenite matrix, as revealed by transmission electron microscopy observations and highenergy X-ray diffraction experiments. After superelastic training for 30 loading/unloading cycles at room temperature, the amount of retained martensite increased and the recoverable strain of two-way shape memory effect increased significantly to 2.2%. Furthermore, a giant recoverable strain of 11.2% was attained under a bias stress of 300 MPa in the trained microwire. These properties confer this microwire great potential for micro-actuation applications.     

退火温度对冷轧Ti-13V-3Al-0.5Cu形状记忆合金组织结构与马氏体相变的影响

使用XRD、TEM、DSC和室温拉伸等分析测试手段,对冷轧后经不同退火温度处理的Ti-13V-3Al-0.5Cu(%,原子分数)合金微观组织结构,马氏体相变行为,力学性能和形状记忆性能进行了研究。经冷轧、退火处理后,合金在室温下的组织主要为α"马氏体相,存在少量残余β母相、α相和Ti₂Cu第二相。随着退火温度的增加,合金形状记忆性能先升高后降低;当退火温度为750℃时,在预应变量为6%的前提下可实现5.3%的可回复应变。其组织结构观察结果表明,经冷轧、退火处理后,合金中α"马氏体形貌由“V”字型自协作组态向择优取向的单一取向马氏体板条转化,界面可动性提升,马氏体临界再取向应力降低,形状记忆性能提高。     

A numerical study on bolted end-plate connection using shape memory alloys

Superelastic shape memory alloys (SMAs) have the ability to recover their original shape after experiencing large strains. A new beam-to-column connection incorporating long shank SMA bolts is presented in this paper. By using the unique characteristics of SMAs, the connection possesses self-centering abilities. The 3D connection model is created using the software ANSYS, and Auricchio’s model is used to simulate the superelastic behavior of the SMA bolts. With cyclic loads applied on the beam ends, the behavior of the connection is studied. The results show the semi-rigid and moderate energy dissipation characteristics of the connection. Since the moment-carrying capacity of bolt cluster controlled below the elastic flexural capacity of connecting beam, a superelastic hinge forms just at the beam-to-column interface. The inelastic interstory drift angle of the connection reaches 0.035 rad, and 94% of the total rotations are recoverable upon unloading.     

Enhanced ferromagnetic stability in Cu doped passivated GaN nanowires

Density functional calculations are performed to investigate the room temperature ferromagnetism in GaN:Cu nanowires (NWs). Our results indicate that two Cu dopants are most stable when they are near each other. Compared with bulk GaN:Cu, we find that magnetization and ferromagnetism in Cu doped NWs are strongly enhanced because the bandwidth of the Cu t d band is reduced because of the one-dimensional nature of the NW. The surface passivation is shown to be crucial to sustain the ferromagnetism in GaN:Cu NWs. These findings are in good agreement with experimental observations and indicate that ferromagnetism in this type of systems can be tuned by controlling the size or shape of the host materials.     

Achieving 5.9% elastic strain in kilograms of metallic glasses: Nanoscopic strain engineering goes macro

The ideal elastic limit is the upper bound of the achievable strength and elastic strain of solids. However, the elastic strains that bulk materials can sustain are usually below 2%, due to the localization of inelastic deformations at the lattice scale. In this study, we achieved >5% elastic strain in bulk quantity of metallic glass, by exploiting the more uniform and smaller-magnitude atomic-scale lattice strains of martensitic transformation as a loading medium in a bulk metallic nanocomposite. The self-limiting nature of martensitic transformation helps to prevent lattice strain transfer that leads to the localization of deformation and damage. This lattice strain egalitarian strategy enables bulk metallic materials in kilogram-quantity to achieve near-ideal elastic limit. This concept is verified in a model in situ bulk amorphous (TiNiFe)-nanocrystalline (TiNi(Fe)) composite, in which the TiNiFe amorphous matrix exhibits a maximum tensile elastic strain of ∼5.9%, which approaches its theoretical elastic limit. As a result, the model bulk composite possesses a large recoverable strain of ∼7%, a maximum tensile strength of above 2 GPa, and a large elastic resilience of ∼79.4 MJ/m³. The recoverable strain and elastic resilience are unmatched by known high strength bulk metallic materials. This design concept opens new opportunities for the development of high-performance bulk materials and elastic strain engineering of the physiochemical properties of glasses.     

Enhancing the Deformation of Shape Memory Sandwich Panels

Abstract: Composite sandwich panels fabricated using a thermosetting shape memory polymer matrix material and a corresponding thermoset shape memory polymer foam core offer the potential to demonstrate large, recoverable, deformations in otherwise stiff structures, under flexural loading. However, as with flexure of thin, fibre‐reinforced shape memory matrix laminates, deflection is limited by fibre compression buckling because of the reduced shape memory matrix stiffness at elevated temperature. A hybrid matrix concept has been developed for sandwich panels loaded in flexure in a single direction. This concept uses a non‐shape memory resin as the matrix for a fraction of the plies on the surface of the facesheet loaded in compression. It is predicted that, at the elevated temperatures required for the generation of deformation in the shape memory structural sandwich panel, the shape memory matrix and foam moduli will be substantially reduced, while the modulus of the non‐shape memory resin will not. Thus, at elevated temperature this effectively leads to a shift of the neutral axis towards the non‐shape memory surface, keeping the low stiffness shape memory matrix material in tension and extending the range of deformation prior to onset of fibre buckling. The experiments performed demonstrate that this hybrid matrix approach enables a three‐fold increase in mid‐span deformation prior to buckling of fibres in the compression surface plies. Furthermore, the force measured to attain the deformed geometry, at elevated temperature, only increases approximately 10–15%, while the magnitude of the force required remains very low. Thus, the hybrid matrix approach functions as predicted and enables the development of sandwich panel structural elements which can undergo large, recoverable deformations.     

Ni50.1Mn24.1Ga20.3Fe5.5形状记忆合金多晶纤维的双程形状记忆效应

通过熔体抽拉技术制备Ni_50.1Mn_24.1Ga_20.3Fe_5.5多晶纤维,采用步进式热处理释放因快速凝固引起的内应力和缺陷,利用场发射扫描电子显微镜、透射电子显微镜、XRD衍射仪对其微结构和相结构进行表征,采用动态机械拉伸仪测试其相变行为和双程形状记忆性能。结果表明:热处理后原子有序度显著提高,孪晶界平直,在恒应力作用下一个热循环中母相和马氏体相的形状得到完全恢复。双程形状记忆曲线显示了热弹性马氏体相变的两个基本特征:可逆性和热滞性。在热循环实验中,纤维被加载到198 MPa时,其马氏体态总应变达到1.32%。根据热机械拉伸测量,发现相变温度遵循Clausius-Clapeyron关系式。与诸如Ti-Ni和Cu-Al-Ni的其他合金相比,Fe掺杂的纤维显示出较小的应变-应力依赖性,在恒应变输出的驱动中是有益的。     

Stable field emission from arrays of vertically aligned free-standing metallic nanowires

We present a fully elaborated process to grow arrays of metallic nanowires with controlled geometry and density, based on electrochemical filling of nanopores in track-etched templates. Nanowire growth is performed at room temperature, atmospheric pressure and is compatible with low cost fabrication and large surfaces. This technique offers an excellent control of the orientation, shape and nanowires density. It is applied to fabricate field emission arrays with a good control of the emission site density. We have prepared Co, Ni, Cu and Rh nanowires with a height of 3?μm, a diameter of 80?nm and a density of ～10(7)?cm(-2). The electron field emission measurements and total energy distributions show that the as-grown nanowires exhibit a complex behaviour, first with emission activation under high field, followed by unstable emission. A model taking into account the effect of an oxide layer covering the nanowire surface is developed to explain this particular field emission behaviour. Finally, we present an in situ cleaning procedure by ion bombardment that collectively removes this oxide layer, leading to a stable and reproducible emission behaviour. After treatment, the emission current density is ～1?mA?cm(-2) for a 30?V?μm(-1) applied electric field.     

Nanoscale ferroelectric and piezoelectric properties of Sb2S3 nanowire arrays

We report the first observation of piezoelectricity and ferroelectricity in individual Sb(2)S(3) nanowires embedded in anodic alumina templates. Switching spectroscopy-piezoresponse force microscopy (SS-PFM) measurements demonstrate that individual, c-axis-oriented Sb(2)S(3) nanowires exhibit ferroelectric as well as piezoelectric switching behavior. Sb(2)S(3) nanowires with nominal diameters of 200 and 100 nm showed d(33(eff)) values around 2 pm V(-1), while the piezo coefficient obtained for 50 nm diameter nanowires was relatively low at around 0.8 pm V(-1). A spontaneous polarization (P(s)) of approximately 1.8 μC cm(-2) was observed in the 200 and 100 nm Sb(2)S(3) nanowires, which is a 100% enhancement when compared to bulk Sb(2)S(3) and is probably due to the defect-free, single-crystalline nature of the nanowires synthesized. The 180° ferroelectric monodomains observed in Sb(2)S(3) nanowires were due to uniform polarization alignment along the polar c-axis.     

On the volume change in Co-Ni-Al during pseudoelasticity

CoNiAl alloys are a new class of shape memory alloys, which exhibit pseudoelastic strains as high 6% over a broad range of temperatures. Based on the crystallographic lattice constants, a substantial volume change upon transformation is expected at the mesoscopic level, yet it has not been measured previously. Transformation strains are established in three mutually orthogonal directions in the [0 0 1]-oriented CoNiAl single crystals under compression. Experiments reveal that the transformation volume change is approximately 2% based on determination of strains on transformed and untransformed locations. Despite the high volumetric strain, the pseudoelastic stress-strain response represents full recoverability with small stress hysteresis. Additional factors that influence pseudoelasticity behavior are discussed particularly the M_d − A_f interval and the flow resistance, which are both higher for CoNiAl compared to other shape memory alloys.     

Smooth and conductive DNA-templated Cu₂O nanowires: growth morphology, spectroscopic and electrical characterization

DNA strands have been used as templates for the self-assembly of smooth and conductive cuprous oxide (Cu?O) nanowires of diameter 12-23 nm and whose length is determined by the template (16 μm for λ-DNA). A combination of spectroscopic, diffraction and probe microscopy techniques showed that these nanowires comprise single crystallites of Cu?O bound to the DNA molecules which fused together over time in a process analogous to Ostwald ripening, but driven by the free energy of interaction with the template as well as the surface tension. Electrical characterization of the nanowires by a non-contact method, scanned conductance microscopy and by contact mode conductive AFM showed the wires are electrically conductive. The conductivity estimated from the AFM cross section and the zero-bias conductance in conductive AFM experiments was 2.2-3.3 S cm?1. These Cu?O nanowires are amongst the thinnest reported and show evidence of strong quantum confinement in electronic spectra.     

铜纳米线阵列的模板组装

采用电解法溶解多孔阳极氧化铝(PAA)模板的阻挡层,用直流电沉积的方法在模板中组装了铜纳米线阵列.分别用扫描电镜和X射线衍射表征铜纳米线阵列的形貌和晶体结构,用电化学法表征了铜纳米线阵列的电催化性能.结果表明,PAA去阻挡层后,伏安图上出现一个阳极氧化峰.恒电位沉积的铜纳米线直径为22nm,沿(111)晶面择优取向.铜纳米线阵列电极能催化亚硝酸根的还原,其催化电流比本体铜电极上大2倍,峰电位正移80mV.纳米铜阵列电极可用于亚硝酸盐的电化学检测.     

Electrical and rheological percolation of polymer nanocomposites prepared with functionalized copper nanowires

The morphological, electrical and rheological characterization of polystyrene nanocomposites containing copper nanowires (CuNWs) functionalized with 1-octanethiol is presented. Characterization by SEM and TEM shows that surface functionalization of the nanowires resulted in significant dispersion of CuNWs in the PS matrix. The electrical characterization of the nanocomposites indicates that functionalized CuNWs start to form electrically conductive networks at lower concentrations (0.25?vol% Cu) than using unfunctionalized CuNWs (0.5?vol% Cu). The organic coating on the nanowires prevents significant changes in the electrical resistivity in the vicinity of the percolation threshold. Percolated nanocomposites showed electrical resistivity in the range of 10(6)-10(7)?Ω?cm. The transition from liquid-like to solid-like behavior (rheological percolation) of the nanocomposites was studied using dynamic rheology at 200?°C. Unfunctionalized CuNWs result in electrical and rheological percolation at similar concentrations. Functionalized CuNWs show rheological percolation at higher concentration (1.0-2.0?vol%) than that required for electrical percolation. This is attributed to the decrease in the interfacial tension between nanowires and polymer chains and its effect on the viscoelastic behavior of the combined polymer-nanowire networks.     

Examining the anomalous electrical characteristics observed in InN nanowires

Research interest in InN has intensified in recent years because of its unique material properties and promising applications in electronic and photonic devices. Measurements on InN nanowires presented by Chang et al., [J. Electron. Mater. 35, 738 (2006)] showed an anomalous resistance behavior in InN nanowires with diameters less than 90 nm. We examine possible theories presented in literature to explain this intriguing observation. We propose that the presence of a high density electron accumulation layer at the surface of thin InN nanowires is the most probable cause for the uncharacteristic relationship between the total measured resistance and the ratio of length-to-area. High density surface electron accumulation layer, characteristic of InN films and nanowire, promotes a surface conduction path distinct from the bulk conduction. For large diameter nanowires, bulk conduction is likely to be the dominant mechanism while surface conduction is proposed to play a major role for small diameter InN nanowires.     

Surface stress effects on the critical buckling strains of silicon nanowires

The objective of this paper is to quantify how nanoscale surface stresses impact the critical buckling strains of silicon nanowires. These insights are gained by using nonlinear finite element calculations based upon a multiscale, finite deformation constitutive model that incorporates nanoscale surface stress and surface elastic effects to study the buckling behavior of silicon nanowires that have cross sectional dimensions between 10 and 25 nm under axial compressive loading. The key finding is that, in contrast to existing surface elasticity solutions, the critical buckling strains are found to show little deviation from the classical bulk Euler solution. The present results suggest that accounting for axial strain relaxation due to surface stresses may be necessary to improve the accuracy and predictive capability of analytic linear surface elastic theories.     

Self-assembly of conductive Cu nanowires on Si(111)'5 × 5 '-Cu surface

Upon room-temperature deposition onto a Cu/Si(111)'5 × 5' surface in ultra-high vacuum, Cu?atoms migrate over extended distances to become trapped at the step edges, where they form Cu?nanowires (NWs). The formed NWs are 20-80?nm wide, 1-3?nm high and characterized by a resistivity of ～8?μΩ?cm. The surface conductance of the NW array is anisotropic, with the conductivity along the NWs being about three times greater than that in the perpendicular direction. Using a similar growth technique, not only the straight NWs but also other types of NW-based structures (e.g. nanorings) can be fabricated.     

Thermal conductivity and secondary porosity of single anatase TiO₂ nanowire

Single anatase TiO? nanowire is synthesized using the electrospinning technique with the sol-gel method and is suspended over a pre-processed 100 μm-wide TEM grid for further characterization. The diameters of the nanowires fall in the range of 250-400 nm. The transient electrothermal (TET) method is adopted to acquire the voltage-time (U-t) profile of the Ir-coated nanowire under step Joule heating. The intrinsic thermal diffusivity of single anatase TiO? nanowires varies from 1.3 to 4.6 × 10?? m2 s?1, and the thermal conductivity changes distinctly from 1.3 to 5.6 W m?1 K?1, much lower than the value of the bulk counterpart: 8.5 W m?1 K?1. The density and thermal conductivity increase significantly with the diameter, largely because at larger diameters less secondary porosity is left by decomposition of organic composites and their escape from the wire during calcination. The density of TiO? nanowires is found to be much lower than that of the bulk counterpart. This is supported by the SEM image of the secondary porous surface. High secondary porosity is observed for TiO? nanowires, ranging from 18% to 63%. This very high secondary porosity confirms that the decomposition of PVP content may distort the fibrous matrix and leave vacancies. In addition, the transition from amorphous to anatase phase could also create a porous state due to crystal particle aggregation.