Deformation Of Nanostructures Research Articles

Continuum modeling of large-strain deformation modes in gold nanowires

Abstract

Plastic Deformation Drives Wrinkling, Saddling, and Wedging of Annular Bilayer Nanostructures

We describe the spontaneous wrinkling, saddling, and wedging of metallic, annular bilayer nanostructures driven by grain coalescence in one of the layers. Experiments revealed these different outcomes based on the dimensions of the annuli, and we find that the essential features are captured using finite element simulations of the plastic deformation in the metal bilayers. Our results show that the dimensions and nanomechanics associated with the plastic deformation of planar nanostructures can be important in forming complex three-dimensional nanostructures.

A multiscale finite element method for the dynamic analysis of surface‐dominated nanomaterials

AbstractThe purpose of this article is to present a multiscale finite element method that captures nanoscale surface stress effects on the dynamic mechanical behavior of nanomaterials. The method is based upon arguments from crystal elasticity, i.e. the Cauchy–Born rule, but significantly extends the capability of the standard Cauchy–Born rule by accounting for critical nanoscale surface stress effects, which are well known to have a significant effect on the mechanics of crystalline nanostructures. We present the governing equations of motion including surface stress effects, and demonstrate that the methodology is general and thus enables simulations of both metallic and semiconducting nanostructures. The numerical examples on elastic wave propagation and dynamic tensile and compressive loading show the ability of the proposed approach to capture surface stress effects on the dynamic behavior of both metallic and semiconducting nanowires, and demonstrate the advantages of the proposed approach in studying the deformation of nanostructures at strain rates and time scales that are inaccessible to classical molecular dynamics simulations. Copyright © 2010 John Wiley & Sons, Ltd.

Deformation of nanostructures on polymer molds during soft UV nanoimprintlithography

Soft nanoimprint lithography (soft NIL) relies on a mechanical deformation of a resist by apatterned polymer used as a mold. Here, we report on the investigation of the nanopatternfidelity of the high pressure imprint process based on a perfluorinated polyether (PFPE)soft mold material. The perfluorinated polyether material was found to be well suited totransfer the mold pattern into the resist by a direct imprinting process at low cost.Moderate deformations of the polymer mold structures occurring during the high pressureimprint are systematically studied. Features of decreased size are found to be moresensitive to pattern distortions. An optimized pattern design with increased structuredensity and constant pattern ratio is developed to minimize deformation effects.Imprints performed on the basis of these design rules result in reduced deformationsand repeal their size dependence. The improved pattern transfer, especially forsmall structural elements, turns the direct and cost-effective soft UV-NIL into aninteresting technique also for patterning tasks in the lower nanometer range.

Yield stress and plasticity of nanostructured titanium of different purity at 300, 77, and 4.2 K

Specimens of nanostructured titanium with different dopant concentrations were prepared by intense plastic deformation via equal-channel-angular pressing. The low-temperature mechanical characteristics of the specimens subjected to active deformation under uniaxial tension and compression were studied. The yield stress and the limit uniform deformation of nanostructured and coarse-grained polycrystalline titanium were compared.

Computer simulation of a twisted nanotube buckling

We develop procedures for solving the problems of dynamic nanostructure deformation and buckling numerically. The procedures are based on discretization with respect to time of the nonlinear equations of molecular mechanics whose matrices and vectors are determined using the Morse potential for the central forces of interaction between atoms and fictitious truss elements accounting for the variations of the angle between atomic bonds. To determine the critical values of deformation parameters and the shapes of buckling nanostructures we use a stability loss criterion for solutions to nonlinear ordinary differential equations on a finite time interval. We implemented our procedures in the PIONER code, using which we solve the problem of a twisted nanotube buckling in the conditions of a quasistatic deformation. To determine the postcritical equilibrium modes we solve the same problem in a dynamic formulation. We show that the modes of equilibrium configurations of the nanotube in the initial postcritical deformation correspond to a buckling mode obtained both at the bifurcation point of quasistatic solutions and at the quasibifurcation point of dynamic solutions.

Topological dislocation-type defects and mechanisms of plasticity and failure of nanostructured and amorphous materials

Possible mechanisms of plastic deformation and failure of nanostructured and cluster amorphous materials have been considered. It is shown that the most probable carriers of plastic deformation in these materials are macrodislocations—linear topological defects of the regular nanocrystallite packing in the nanostructure or cluster packing in amorphous materials. Continuum models are proposed to describe the processes of plastic deformation and failure of nanostructured and cluster amorphous materials.

Surface Polarity and Shape‐Controlled Synthesis of ZnO Nanostructures on GaN Thin Films Based on Catalyst‐Free Metalorganic Vapor Phase Epitaxy

With the rapid developments in nanotechnology, there has been tremendous interest in ZnO-related nanomaterials because ZnO is a wide band-gap semiconductor and a piezoelectric oxide. As a result, it is useful for optoelectronics, transparent electronics, sensors, and transducers. ZnO crystals have a noncentral symmetric wurtzite structure and are composed of close packed O and Zn2þ layers piled alternatively along the c-axis, producing positively charged Zn-terminated (0001) polar surfaces and negatively charged O-terminated (0001) polar surfaces. Together with the polar surfaces, three fast growth directions of [0001], [0110], and [21 10] facilitate anisotropic growth of ZnO nanocrystals which have various one-dimensional (1D) structures including c-axis oriented nanowires and a-axis oriented nanobelts. For ZnO at the nanometer scale, meanwhile, the surface to volume ratio increases significantly with size reduction and thus, the effect of the surface, especially the surface polarity, plays an important role in determining the structures as well as the chemical and physical properties of the ZnO nanostructures. For example, electrostatic interactions driven by the polar surfaces leads to the deformation of ZnO nanostructures to form unique shapes such as seamless nanorings, nanobows, nanosprings, and nanohelices. In addition, ZnO nanocrystals with varying surface configurations and polarities display different optical and electronic properties. These phenomena make control of the surface is a crucial factor in designing nanomaterials for various applications.

Integrated MD simulation of scratching and shearing of 3D nanostructure

An integrated MD simulation of scratching and shearing with one specimen is conducted to analyze the nanomachining mechanism and mechanics properties of nanostructures. Simulation results indicate that during scratching the onset and propagation of dislocations depend on the scratch depths; during shearing, the yielding stress of a small-size nanostructure is more sensitive to nanomachining imperfection and residual defects. Dislocations nucleate first near the burr in a scratched specimen. In an ideal nanostructure or a nanostructure with shallow scratched groove, the distribution of stress is generally lower and flatter. As the depth of groove increases, high stresses transit from the corner to either end of groove, especially near the burr or around the location of tool withdraw. During the deformation of nanostructures, shear stress plays a leading role in the elastic stage, and both normal stress gradients and shear stress determine the onset and evolvement of defects in the plastic stage. When the ratio of the depth of groove to the height of specimen is up to one third, the scratched groove determines the breakpoint of a nanostructure. The fluctuation of shear stress during the plastic deformation of specimen is caused by the competition between atoms which form new atomic planes and slip on different slip planes.

Plastic deformation of a nanostructured and ultra-fine grained Fe-1%Cr with a bimodal grain size distribution

The plastic deformation of nanostructured and ultra-fine grained Fe-1%Cr of bulk specimens obtained from mechanically milled iron powder has been investigated by means of compression and tensile tests. The resulting microstructure was related to the mechanical behaviour. It was found that nanocrystalline grained specimens presented strain hardening in compression tests whereas they had brittle behaviour in tensile tests. However, when a bimodal grain size distribution is obtained by heat treatments, an improvement in the strain hardening capacity in both tensile and compression tests was observed.

Automating model construction and visualization of results of numerical simulation of deformation of nanostructures

The models and results of visualization of mathematical modeling of quasi-static/dynamic deformation of a nanotube subject to twisting are presented. Automating the model construction and visualization of numerical simulation have been carried out using the software packages MSC Patran 2007 and VMD, and the programming languages PCL and TCL. The advantages and deficiencies of both software packages are illustrated by visualizing the quasi-static/dynamic deformation of the nanotube. Computations have been made using the package PIONER.

Preparation and electrochemical performance of nanosized Co3O4 via hydrothermal method

The hydrotalcite-type cobalt compounds were prepared through oxidation of Co(OH)2 gel using NH3O4 as precipitating agent and H2O2 as oxidant. These hydrotalcite-type cobalt compounds were transformed into Co3O4 through hydrothermal decomposition with nanostructural deformation. The precursor and product were characterized by Fourier-transform infrared(FT-IR) spectrum, X-ray diffractometry(XRD) and transmission electron microscopy(TEM). The electrochemical performances of as-prepared nanosized Co3O4 as anode materials in lithium-ion batteries were tested by charge-discharge test in the voltage range of 0–3.0 V. The influence of morphology of Co3O4 particle on the capacity and cycling performance was studied. The results show that the shape and size of the final product can be controlled by altering cobalt sources. The irregular cubic Co3O4 with the average particle size of about 10 nm shows the best electrochemical performance. After 10 charge-discharge cycles, the specific charge capacity retains 555 mA-h/g.

Accelerating the molecular time steps for nanomechanical simulations: Hybrid Monte Carlo method

A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at a time scale of the order of microseconds in a reasonable computing time. Additionally, MD simulations have to be carried out at very high loading rates (∼108 s−1) rarely observed during experiments. In this investigation, a modified hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain-rate-dependent) atomistic mechanical deformation of nanostructures at higher time scales than currently possible using MD is established for a Cu nanowire and for a nanocrystalline Ni sample. In this method, there is no restriction on the size of MD time step except that it must ensure a reasonable acceptance rate between consecutive Monte Carlo (MC) steps. In order to establish the method, HMC analyses of a Cu nanowire deformation at two different strain rates, viz., 108 and 109 s−1, and of a nanocrystalline Ni sample deformation at a strain rate of 109 s−1 with three different time steps, viz., 2, 4, and 8 fs, are compared with the analyses based on MD simulations at the same strain rates and with a MD time step of 2 fs. MD simulations of the Cu nanowire as well as nanocrystalline Ni deformations reproduce the defect nucleation and propagation results as well as strength values reported in the literature. Defect formation and stress-strain responses of the Cu nanowire, as well as of the nanocrystalline Ni sample during HMC simulations with a time step of 8 fs, are similar to that observed in the case of MD simulations with the maximum permissible time step of 2 fs (for the interatomic potential used, 2 fs is the highest MD time step). Simulation time analyses show that by using HMC approximately 4 times saving in computational time can be achieved bringing the atomistic analyses closer to the continuum time scales.

Microstructural characteristics of post-shear localization in cold-rolled 316L stainless steel

The microstructural evolution in a cold-rolled 316L stainless steel during shear localization was comprehensively studied using transmission electron microscopy (TEM). The TEM results indicate that the main substructure inside a shear band consists of elongated lath, fine rectangular and equiaxed subgrains. The substructure at an early stage of shear banding was found to strongly depend upon existing defects, especially deformation twin patterns. These twin structures determine the initial dimensions of the elongated subgrains inside the shear bands. The coexistence of both rectangular and equiaxed subgrains suggests that no melting occurred though the predicted temperature was much higher than the bulk melting temperature. Dynamic/static recovery and continuous dynamic recrystallization are thought to be the main mechanisms by which these substructures form inside the shear bands. A new mechanism for nanostructure deformation of subgrains within shear bands is proposed to explain the temperature divergence between the experimental and calculated results.

Atomistic Simulations of Strain Rate Dependent Deformation Behavior at Continuum Timescales

AbstractA majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at the timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (∼108 s−1) rarely observed in experiments. In this investigation, a modified Hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain rate dependent) atomistic mechanical deformation of nanostructures at continuum timescales is established. In this method there is no restriction on the size of MD timestep except that it must be such that to ensure a reasonable acceptance rate between consecutive Monte-Carlo (MC) time-steps. For the purpose of comparison HMC analyses of Cu nanowires deformation at two different strain rates (108 s−1 and 109 s−1) (each with three different timesteps 2 fs, 4fs, and 8 fs) are compared with the analyses based on MD simulations at the same strain rates and a MD timestep of 2 fs. As expected, the defect formation is found to be strain rate dependent. In addition, HMC with timestep of 8 fs correctly reproduces defect formation and stress-strain response observed in the case of MD with 2 fs (for the interatomic potential used 2 fs is the highest MD timestep). Simulation time analyses show that by using HMC a saving of the order of 4 can be achieved bringing the atomistic analyses closer to the continuum timescales.

Fabrication of morphology and crystal structure controlled nanorod and nanosheet cobalt hydroxide based on the difference of oxygen-solubility between water and methanol, and conversion into Co3O4

Films of brucite-type cobalt hydroxide with nanorod morphology and hydrotalcite-type cobalt hydroxide with nanosheet morphology films were fabricated by heterogeneous nucleation in a chemical bath using water and a mixed solution of water–methanol as solvents, respectively. Since oxygen is around 25 times more soluble in methanol than in water, a methanol solution was used to convert a part of divalent cobalt ions into trivalent cobalt ions through oxidation, due to the amount of dissolved oxygen. The resultant cobalt hydroxides were of the hydrotalcite type, with a sheet-like morphology, and di- and trivalent cobalt ions. On the other hand, brucite-type hydroxides with a rod morphology, constructed using only divalent cobalt ions, were fabricated due to the scarcity of dissolved oxygen in a water-only solvents. Both the brucite and hydrotalcite types of cobalt hydroxide films were transformed into Co3O4 through pyrolysis without nanostructural deformation. The Co3O4 films were porous structures with a large surface area because both rod and sheet were constructed through nanoparticles and nanopores once the self-template was removed.

Three-Dimensional Chord Distribution Function SAXS Analysis of the Strained Domain Structure of a Poly(ether ester) Thermoplastic Elastomer

The nanostructure deformation mechanism of a predrawn and annealed poly(ether ester) (PEE) thermoplastic elastomer comprising poly(butylene terephthalate) as hard segments and poly(ethylene glycol) as soft segments of a weight ratio 57/43 was studied by means of synchrotron radiation. For this purpose a recently proposed method by Stribeck for extraction of the topological domain structure information contained in the small-angle X-ray scattering (SAXS) patterns was applied to patterns observed as a function of elongation of PEE when measured in the strained and in the relaxed state after straining. The SAXS fiber diagram as a whole was transformed into a three-dimensional chord distribution function (CDF) with fiber symmetry. Assignment of the observed peaks to the various structural characteristics of the PEE is suggested.

Nanostructure Deformation Behavior in Poly(ethylene terephthalate)/Polyethylene Drawn Blend As Revealed by Small-Angle Scattering of Synchrotron X-Radiation

The nanostructure deformation mechanism of a predrawn and annealed blend of poly(ethylene terephthalate) (PET) and polyethylene (PE) (PET/PE: 50/50 by wt %) was studied. For the first time six-point small-angle X-ray scattering (SAXS) patterns were observed as a function of elongation in the investigation of a drawn polymer blend fiber sample. It was found that the meridional two-point pattern arises from PET and the inclined four-point one from PE. Long spacings and the angle between inclined reflections and fiber axis were determined as a function of draw ratio. At lower external strain (elongation up to 13%) the macrodeformation is related to conformational changes located only in amorphous material outside the lamellar stacks, whereas at higher elongation destruction of layer crystallites is observed, possibly due to pull-out of chains.