Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1?10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1?10] direction fracture in a cleavage manner under tensile loading.     

不同拉伸应变率下金纳米线拉伸力学行为的分子动力学模拟

采用分子动力学方法模拟了不同拉伸应变率下金纳米线的拉伸力学行为。模拟结果表明,相同截面尺寸、拉伸温度、拉伸方向的金纳米线在较高的拉伸应变率下,屈服强度较大,屈服发生的较早;更高应变率下的金纳米线在屈服时对应的应变更大,并且更容易发生二次屈服。金纳米线的弹性模量随着拉伸应变率的增大有所增加,但是幅度不是很明显。     

Stress-induced martensitic phase transformation in Cu-Zr nanowires

A novel stress-induced martensitic phase transformation in an initial <100>/{100} B2-CuZr nanowire is reported for the first time in this letter. Such behavior is observed in a nanowire with cross-sectional dimensions of 19.44 × 19.44 Å² over a temperature range of 100-400 K and at a strain rate of 1 × 10⁹ s^− 1 using atomistic simulations. Phase transformation from an initial B2 phase to a BCT (Body-Centered-Tetragonal) phase is observed via nucleation and propagation of {100} twinning plane under high strain rate tensile deformation.     

Effects of Geometry and Shape on the Mechanical Behaviors of Silicon Nanowires

Molecular dynamics simulations have been performed to investigate the effects of cross section geometry and shape on the mechanical behaviors of silicon nanowires (Si NWs) under tensile loading. The results show that elasticity of <100> rectangular Si NWs depends on their cross section aspect ratios while the elastic limits of <110> and <111> wires show geometry independence. Despite the significant influence of axial orientation, both yield stress and Young's Modulus show the remarkable shape dependence for wires with various regular cross sections. Additionally, underlying mechanism for the geometry and shape effects on mechanical behavior are discussed based on the fundamental energy theory. From energy view, edge energy is the crucial factor that determines shape dependence of the elastic limits.     

Statistical analysis of the breaking processes of Ni nanowires

We have performed a massive statistical analysis on the breaking behaviour of Ni nanowires using molecular dynamic simulations. Three stretching directions, five initial nanowire sizes and two temperatures have been studied. We have constructed minimum cross-section histograms and analysed for the first time the role played by monomers and dimers. The shape of such histograms and the absolute number of monomers and dimers strongly depend on the stretching direction and the initial size of the nanowire. In particular, the statistical behaviour of the breakage final stages of narrow nanowires strongly differs from the behaviour obtained for large nanowires. We have analysed the structure around monomers and dimers. Their most probable local configurations differ from those usually appearing in static electron transport calculations. Their non-local environments show disordered regions along the nanowire if the stretching direction is [100] or [110]. Additionally, we have found that, at room temperature, [100] and [110] stretching directions favour the appearance of non-crystalline staggered pentagonal structures. These pentagonal Ni nanowires are reported in this work for the first time. This set of results suggests that experimental Ni conducting histograms could show a strong dependence on the orientation and temperature.     

Surface-stress-induced phase transformation in metal nanowires

Several researchers have demonstrated, through experiments and analysis, that the structure and properties of nanometre-scale materials can be quite different to those of bulk materials due to the effect of surfaces. Here we use atomistic simulations to study a surface-stress-induced phase transformation in gold nanowires. The emergence of the transformation is controlled by wire size, initial orientation, boundary conditions, temperature and initial cross-sectional shape. For a <100> initial crystal orientation and wire cross-sectional area below 4 nm(2), surface stresses alone cause gold nanowires to transform from a face-centred-cubic structure to a body-centred-tetragonal structure. The transformation occurs roughly when the compressive stress caused by tensile surface-stress components in the length direction exceeds the compressive stress required to transform bulk gold to its higher energy metastable crystal structure.     

A thermovisco-hyperelastic constitutive model of HTPB propellant with damage at intermediate strain rates

The uniaxial compressive tests at different temperatures (223–298 K) and strain rates (\(0.40\mbox{--}63~\mbox{s}^{-1}\)) are reported to study the properties of hydroxyl-terminated polybutadiene (HTPB) propellant at intermediate strain rates, using a new INSTRON testing machine. The experimental results indicate that the compressive properties (mechanical properties and damage) of HTPB propellant are remarkably affected by temperature and strain rate and display significant nonlinear material behaviors at large strains under all the test conditions. Continuously decreasing temperature and increasing strain rate, the characteristics of stress-strain curves and damage for HTPB propellant are more complex and are significantly different from that at room temperature or at lower strain rates. A new constitutive model was developed to describe the compressive behaviors of HTPB propellant at room temperature and intermediate strain rates by simply coupling the effect of strain rate into the conventional hyperelastic model. Based on the compressive behaviors of HTPB propellant and the nonlinear viscoelastic constitutive theories, a new thermovisco-hyperelastic constitutive model with damage was proposed to predict the stress responses of the propellant at low temperatures and intermediate strain rates. In this new model, the damage is related to the viscoelastic properties of the propellant. Meanwhile, the effect of temperature on the hyperelastic properties, viscoelastic properties and damage are all considered by the macroscopical method. The constitutive parameters in the proposed constitutive models were identified by the genetic algorithm (GA)-based optimization method. By comparing the predicted and experimental results, it can be found that the developed constitutive models can correctly describe the uniaxial compressive behaviors of HTPB propellant at intermediate strain rates and different temperatures.     

Anisotropic and temperature effects on mechanical properties of copper nanowires under tensile loading

Atomistic simulations are used to investigate the mechanical properties of copper nanowires (NWs) along 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 crystallographic orientations under tensile loading at different temperatures. The inter-atomic interactions are represented by employing embedded-atom potential. To identify the defects evolution and deformation mechanism, a centrosymmetry parameter is defined and implemented in the self-developed program. The simulations show that Cu NWs in different crystallographic orientations behave differently in elongation deformations. The stress–strain responses are followed by a particular discussion on yield mechanism of NWs from the standpoint of dislocation moving. Generally, the study on the incipient plastic deformation will be helpful to further understanding of the mechanical properties of nanomaterials. In addition, the Young’s modulus decreased linearly with the increase of temperature. The crystal structure is less stable at elevated temperatures.     

Anisotropy effect on strain-induced instability during growth of heteroepitaxial films

The use of misfit strain to improve the electronic performance of semiconductor films is a common strategy in modern electronic and photonic device fabrication. However, pursuing a favorable higher strain could lead to mechanical instability, on which systematic and quantitative understandings are yet to be achieved. In this paper, we investigate the anisotropy effects on strain-induced thin-film surface roughening by phase field modeling coupled with elasticity. We find that compared with films grown along {111} and {100} surfaces, the instability of {110} film occurs at a much lower strain. Our simulations capture the evolution of interface morphology and stress distribution during the roughening process. Similar characterizations are performed for heteroepitaxial growth from a surface pit. Finally, from 3D simulations, we show that the surface roughening pattern on {110} film exhibits a clear in-plane orientation preference, consistent with experimental observations.     

Mechanical and electronic properties of diamond nanowires under tensile strain from first principles

The atomic and electronic structures, heat of formation, Young's modulus, and ideal strength of hydrogenated diamond nanowires (DNWs) with different cross-sections (from 0.06 to 2.80 nm(2)) and crystallographic orientations ((100), (110), (111), and (112)) have been investigated by means of first-principles simulations. For thinner DNWs (cross-sectional area less than 0.6 nm(2)), preferential growth orientation along (111) is observed. The Young's modulus and ideal strength of these DNWs decrease with decreasing cross-section and show anisotropic effects. Moreover, the band gap of DNWs is sensitive to the size, crystallographic orientation and tensile strain, implying the possibility of a tunable gap. The effective mass at the edges of the conduction band and valence band are also obtained. These theoretical results are helpful for designing novel optoelectronic devices and electromechanical sensors using diamond nanowires.     

Texture evolution in Copper film at high temperature studied in situ by electron back-scatter diffraction

Changes in texture and microstructure during the thermal treatment of Cu films have been studied in situ using electron back-scatter diffraction (EBSD). A partially recrystallized Cu film which still had its microstructure evolving at room temperature was investigated using orientation imaging microscopy. Two separate investigations were conducted—the first one at different locations of the film and at different temperatures and a second one at the same location of the film and at different temperatures. The orientation of the (111), (110) and (100) grains within the plane of Cu film was investigated from the orientation distribution functions. There was an increased tendency of the (111) and (110) grains to form either {111}<112> or {111}<110> and {110}<100> texture respectively at higher temperature. The impact of elastic strain energies and dislocation glide in formation of these textures at higher temperatures has been analyzed in the light of some recent observations reported in literature. The variation in the area fraction of different fiber texture components, as a function of temperature, has been discussed in correlation with the measured mean grain size, grain boundary misorientation distribution and stress states. Stress state during the entire thermal cycle was monitored by wafer curvature technique and the traces of additive impurities at the surface were measured using X-ray photoelectron spectrometry. The possible role of impurities in affecting the behavior of texture components at high temperature is discussed. Comparison was made between the EBSD and X-ray diffraction texture data.     

Deformation and Dislocation Structure in L1_2 Titanium Trialuminide Alloys

The deformation behaviour and the nature of dislocations of the Al3Ti-base L12 alloya modified with Fe and Mn etc, were investigated. The results show that the deformation and fracture character istics are closely related to the alloy compositions. The effect of hot-working process on the room tem perature ductility is remarkable, not only resulting in an appreciable improvement of compressive properties but also showing a 0.28% plastic strain in tensile test. The SISF dissociation of a < 110>dislocations on {111} planes was found at room temperature. The determined dissociation scheme is consistent with the mechanical behaviour of these alloys in the lower temperature region.     

The effect of surface relaxation and atomic vibration on the equilibrium shape of gold and copper crystallites

Summary The free energy of surfaces along the <011> pole in gold and copper is determined to assess the effect of surface relaxation and atomic vibration on the equilibrium crystal shape of gold and copper. The Wulff construction is performed on the -plots to determine the equilibrium shape of gold and copper crystallites at different temperatures. It is shown that surface relaxation and atomic vibration do not have any discernible effect on the equilibrium shape of EAM gold or copper crystallites. The equilibrium shape of EAM gold crystallites is formed entirely from {111} and {100} facets, while that of EAM copper shows small {110} facets in addition to the {111} and {100} facets.     

Molecular dynamics simulation of mechanical properties of Ni–Al nanowires

We employed molecular dynamics simulations to study mechanical properties of Ni–Al nanowires by calculating the stress–strain response of the wires under various loading conditions. For this purpose, nanowires were subjected to uniaxial strain at different strain rates and temperatures using embedded atom model potential. The behaviour of the wires at lower and higher strain rates was investigated, and the yield and rupture strain values and also Young’s Modulus were obtained which are essential factors for the ductility of the wires. This work indicates that how the stress–strain response of the nanowires are affected by varying strain rates and temperatures.     

低碳钢形变强化相变时铁素体织构类型的分析

利用背散射电子衍射取向成像技术分析了在热模拟单向压缩条件下Q235碳素钢形变强化相变时铁素体织构的类型。结果表明,在利用形变强化相变实现铁素体的超细化过程中会出现铁素体的相交织构和形变织构,在大应变条件下还会出现动态再结晶织构。在形变强化相变后细晶铁素体在整体上表现为以〈111〉方向为主的线织构。主要的相交织构在粗晶奥氏体内部形变带形核时产生并与〈111〉织构对应。形变织构是在形变时形成的铁素体受到继续变形所致,在形变强化相变过程中及完成后都会产生,对应〈111〉及〈100〉方向的线织构,随着形变的加大,〈100〉方向的织构增加得更快,形变温度的降低有利于形变织构的加强。在形变量很大且形变温度比较合适时(但不能过低)会发生铁素体的动态再结晶,它以连续的方式进行,导致形变织构的进一步加强,并使晶粒均匀细化。     

A study of recrystallisation textures in hot band of a continuous cast Al–Mn–Mg alloy

Abstract

The hot band of a commercial continuous cast (CC) Al–Mg–Mn alloy was annealed at different temperatures ranging from room temperature to 510°C for 3 h. The evolution of microstructure and crystallographic texture was investigated during the annealing treatment. It was found that the recrystallised alloy exhibited a severely elongated grain structure and a texture that consisted of a new type of component ({113}〈110〉) and two fibre components (〈100〉//ND and 〈110〉//ND), the axes of which were along the normal direction of the rolling plane (ND) in 〈100〉 and 〈110〉 respectively. The 〈100〉//ND fibre was dominated by a ND rotated cube orientation {001}〈310〉, while the 〈110〉//ND fibre was mainly composed of Goss and P orientation ({011}〈566〉). The formation of the {113}〈110〉 texture and two fibre textures was likely to be attributed to the concurrent precipitation effect taking place upon heating in annealing treatment of the alloy.     

The SERS and TERS effects obtained by gold droplets on top of Si nanowires

We show that hemispherical gold droplets on top of silicon nanowires when grown by the vapor-liquid-solid (VLS) mechanism, can produce a significant enhancement of Raman scattered signals. Signal enhancement for a few or even just single gold droplets is demonstrated by analyzing the enhanced Raman signature of malachite green molecules. For this experiment, trenches (approximately 800 nm wide) were etched in a silicon-on-insulator (SOI) wafer along <110> crystallographic directions that constitute sidewalls ({110} surfaces) suitable for the growth of silicon nanowires in <111> directions with the intention that the gold droplets on the silicon nanowires can meet somewhere in the trench when growth time is carefully selected. Another way to realize gold nanostructures in close vicinity is to attach a silicon nanowire with a gold droplet onto an atomic force microscopy (AFM) tip and to bring this tip toward another gold-coated AFM tip where malachite green molecules were deposited prior to the measurements. In both experiments, signal enhancement of characteristic Raman bands of malachite green molecules was observed. This indicates that silicon nanowires with gold droplets atop can act as efficient probes for tip-enhanced Raman spectroscopy (TERS). In our article, we show that a nanowire TERS probe can be fabricated by welding nanowires with gold droplets to AFM tips in a scanning electron microscope (SEM). TERS tips made from nanowires could improve the spatial resolution of Raman spectroscopy so that measurements on the nanometer scale are possible.     

Phase segregation in AlInP shells on GaAs nanowires

We have studied morphology and phase segregation of AlInP shells on GaAs nanowires. Photoluminescence measurements on single core-shell nanowires indicated variations in the shell composition, and phase segregation was confirmed by cross-sectional scanning transmission electron microscopy on 30 nm thin slices of the wires. It was discovered that Al-rich domains form in the <112> directions where two {110} shell facets meet during growth. We propose that the mechanism behind this phase segregation is a variation in the chemical potential along the circumference of the nanowire together with a difference in diffusion lengths for the different growth species. From the morphology of the core and the shell, we conclude that the side facet growth is temperature dependent forming {112}facets at low growth temperature and {110} facets at high growth temperature.     

Hot deformation and annealing of cube oriented aluminium single crystals

Abstract

Single crystals of the {001}〈100〉 orientation of an Al–0.05Si single phase alloy have been deformed in plane strain compression at temperatures of 300–500°C and strain rates of 0.5–50 s^-1, and annealed in a salt bath at temperatures of 300–450°C. Quantitative texture measurements by electron backscatter diffraction (EBSD)show that, in agreement with previous work, the cube orientation is stable at lower strain rates and higher temperatures (lower Zener–Hollomon parameter Z), whereas this orientation is unstable at higher values of Z. During annealing of the deformed crystals there is a competitive migration of subgrain boundaries of a wide range of orientations, and recrystallisation starts preferably at deformation bands of high orientation gradient. Measurement of subgrain growth has enabled the dependence of the mobility of low angle grain boundaries on misorientation to be determined. The results are in accord with those obtained for lower angle (<6°)boundaries in the same material.     

Fabrication of oriented arrays of porous gold microsheaths using aligned silver nanowires as sacrificial template

A simple one-step process for preparation of oriented arrays of porous gold microsheaths has been developed by dissolution of sacrificial templates of aligned Ag nanowires in a mixture solution of HAuCl₄ and NaCl at room temperature. The morphology and crystal structure of the product were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Its composition was estimated by energy dispersive X-ray spectroscopy (EDX). The results indicated that the gold microsheaths had generally preserved the original orientation of Ag nanowires and their orientation was robust enough to survive the centrifugal process. The gold microsheaths consist of nanoparticles (ca. 100 nm) that form nanovoids (tens to hundreds of nanometers) between them, giving them a porous nature. Such arrays of well oriented gold microsheaths are expected to show interesting anisotropic optical and electronic properties, and their hollow porous structures might find broad potential applications in surface plasma resonance (SPR), catalysis and chemical sensing.