Surface Strain Energy Research Articles

Influence of Tb incorporation on the structural and the optical properties of ZnO nanoparticles

Structural and optical properties of the Tb doped ZnO nanoparticles are systematically studied as a function of the Tb mole-fraction. Our study suggests that the Tb incorporates mostly on the surface and affects the optical properties of the ZnO nanoparticles by influencing the attachment of certain adsorbed groups, which are found to be responsible for the appearance of a broad green luminescence (GL) band in the photoluminescence spectra recorded for these nanoparticles. It has been found that the accumulation of Tb on the surface of the nanoparticles not only enhances the band edge to green luminescence intensity ratio under the vacuum condition but also increases the band gap energy by introducing a hydrostatic compressive strain in individual nanoparticles, which provides a unique opportunity to study the pressure dependence of the optical properties of nanoparticles without applying any external pressure. The hydrostatic compressive strain is explained in terms of the increase of the surface strain energy as a result of the Tb accumulation on the surface of the nanoparticles. The average value of the surface energy density for the particles has been estimated as a function of Tb mole-fraction. The pressure coefficient of the band gap which is obtained from the variation of the band gap energy with the hydrostatic strain has been found to decrease significantly with the particle size for the ZnO nanoparticles.

Data-Driven Design Optimization for Composite Material Characterization

Abstract The main goal of the present paper is to demonstrate the value of design optimization beyond its use for structural shape determination in the realm of the constitutive characterization of anisotropic material systems such as polymer matrix composites with or without damage. The approaches discussed are based on the availability of massive experimental data representing the excitation and response behavior of specimens tested by automated mechatronic material testing systems capable of applying multiaxial loading. Material constitutive characterization is achieved by minimizing the difference between experimentally measured and analytically computed system responses as described by surface strain and strain energy density fields. Small and large strain formulations based on additive strain energy density decompositions are introduced and utilized for constructing the necessary objective functions and their subsequent minimization. Numerical examples based on both synthetic (for one-dimensional systems) and actual data (for realistic 3D material systems) demonstrate the successful application of design optimization for constitutive characterization.

Structure evolution and solid solubility extension of copper–niobium powders during mechanical alloying

The supersaturated Cu–Nb solid solutions were produced by mechanical alloying (MA) at room temperature (RT). The microstructural evolution and mechanical property of Cu 100− x Nb x ( x = 5, 10, 15, 20, wt.%) powder mixtures during milling were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) observations and microhardness measurement. The results show that the solid solubility limit of Nb in Cu can be extended to more than 10 wt.% Nb by MA at RT. The Cu crystallite size decreases with increasing milling time, and approaches 7 nm in Cu 80Nb 20 and 13 nm in Cu 95Nb 5 after 100 h milling, respectively. The maximum value of microhardness is about 4.8 GPa for Cu 80Nb 20 milled for 100 h. The mechanism of solid solubility extension by MA was discussed using a thermodynamical analysis. The surface and elastic strain energy produced by milling supply the main driving force for alloying.

Ta clustering and microstructural evolution in the A1 to L1Fe52PtX(Ta1−X) phase transformation

A series of Fe52Pt48,Fe52.3Pt46.3Ta1.4 and Fe52Pt40.7Ta7.3 thin films were sputter deposited and subsequently annealed at 550 and 750 °C for 30 min. The as-deposited films, which adopted the A1 phase, had a change from a predominate (111) fiber texture to (200) with the Ta additions. This has been explained in terms of the competition between the surface energy and strain energy. Annealing at 550 °C facilitated the L10 order in Fe52Pt48 and Fe52.3Pt46.3Ta1.4. Upon annealing at 750 °C, all three composition films phase transformed into L10. Atom probe tomography revealed nanoscale clustering in the annealed Ta containing films. The formation of these clusters appeared to be a necessary initial step to allow the L10 ordering reaction to occur but clustering in of itself is not sufficient for order. For the Fe52Pt40.7Ta7.3 film, the Ta must be depleted within the matrix to a sufficient level to allow the binary Fe–Pt to order. For the Fe52.3Pt46.3Ta1.4 film, these clusters were qualitatively observed within the grains at 550 °C and in the grain boundaries at 750 °C. The Fe52Pt40.7Ta7.3 film had clusters within and near grain boundaries. The clustering in the grain boundaries deterred grain growth through a Zener-based pinning mechanism.

A novel method to prepare gold-nanoparticle-modified nanowires and their spectrum study

In this paper, a practical and mild sol–gel self-assembly method was successfully developed for the synthesis of plain and gold-nanoparticle-modified silica nanowires (AuSiNWs). In this new method, the 3-mercaptopropyltrimethoxysilane (MPTMOS) was used as the key linker through disulfide bonds and mediated the reorganization of the particles into elongated clusters. The high surface strain energy of the silica nanoparticles, as a driving force, deformated the interconnected nanoparticles into nanowire. All the procedures were performed in solution and under mild conditions (below 100 °C at atmosphere pressure), and no special apparatus was needed. This promising method made it possible to synthesize the SiNWs in large scale. Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and scanning electron microscopy (SEM) were used to characterize the obtained nanostructures. The microscopy was used to investigate the products at each stage in the synthetic process and the formation mechanism of the new method was discussed. The investigated AuSiNWs displayed an intensive absorption at 528 nm, characteristic of gold nanoparticles, and a strong luminescence peak at 440 nm.

Modeling diffusion-induced stress in nanowire electrode structures

There is an intense, worldwide effort to develop durable lithium ion batteries with high energy and power densities for a wide range of applications, including electric and hybrid electric vehicles. One of the critical challenges in advancing lithium ion battery technologies is fracture and decrepitation of the electrodes as a result of lithium diffusion during charging and discharging operations. When lithium is inserted in either the positive or negative electrode, a large volume change on the order of a few to several hundred percent, can occur. Diffusion-induced stresses (DISs) can therefore cause the nucleation and growth of cracks, leading to mechanical degradation of the active electrode materials. Our work is aimed at developing a mathematical model relating surface energy with diffusion-induced stresses in nanowire electrodes. With decreasing size of the electrode, the ratio of surface area to volume increases. Thus, surface energy and surface stress can play an important role in mitigating DISs in nanostructured electrodes. In this work, we establish relationships between the surface energy, surface stress, and the magnitude of DISs in nanowires. We find that DISs, especially the tensile stresses, can decrease significantly due to the surface effects. Our model also establishes a relationship between stress and the nanowire radius. We show that, with decreasing size, the electrode material will be less prone to mechanical degradation, leading to an increase in the life of lithium ion batteries, provided other phenomena are unaffected by increased surface area (e.g., chemical degradation reactions). Also we show that, in the case of nanostructures, surface strain energy is significant in magnitude comparing with bulk strain energy. A mathematical tool to calculate total strain energy is developed that can be used to compare strain energy with the fracture energy of that material in electrode system.

Mechanisms of periodic deformation of the film-substrate system under compressive stress

The paper considers elastic and plastic deformation of metal, oxide and semiconductor films during their growth, thermal annealing and mechanical loading. It is shown that in a compressed thin film-compliant substrate system, wrinkling of the film surface and coherent deformation of the substrate take place. With a rigid substrate, compressive stress leads to elastic buckling of the film and its local or periodic delamination from the substrate. Plastic deformation of thin films is governed by competitive changes in their surface energy and strain energy. It is found that the periodic stress-strain distribution at the interface of two media plays a fundamental role and underlies the mechanisms for degradation of thin films under various external actions.

Interlayer correlation of embedded quantum-dot arrays through their surface strain energy distributions

We propose an interlayer correlation of multilayer quantum-dot (QD) distributions from a rigorous strain energy calculation. This leads to a map of correlations, or interlayer alignment, based solely on lateral and vertical spacing ($xdist/b$ versus $hdist/h$). We identify four distinct correlation regimes---aligned correlation, antialigned correlation, noncorrelation, and the transition zone between the aligned and antialigned correlation. Our prediction matches well with available experimental data for a broad range of semiconductors with low elastic anisotropy $[A=2{\text{C}}_{44}/({\text{C}}_{11}\ensuremath{-}{\text{C}}_{12})<2]$ and can further predict the QD array distribution for those with high elastic anisotropy $(A\ensuremath{\ge}2)$ by a simple shift in $hdist/h$. The agreement spans both IV-VI and III-V systems. Moreover, the aligned correlation regime produces a large decay in strain energy magnitudes in subsequently grown layers, which may contribute to their observed larger nucleation domains.

Microstructural characteristics and mechanical properties of magnetron sputtered nanocrystalline TiN films on glass substrate

Nanocrystalline TiN thin films were deposited on glass substrate by d.c. magnetron sputtering. The microstructural characteristics of the thin films were characterized by XRD, FE-SEM and AFM. XRD analysis of the thin films, with increasing thickness, showed the (200) preferred orientation up to 1·26 μm thickness and then it transformed into (220) and (200) peaks with further increase in thickness up to 2·83 μm. The variation in preferred orientation was due to the competition between surface energy and strain energy during film growth. The deposited films were found to be very dense nanocrystalline film with less porosity as evident from their FE-SEM and AFM images. The surface roughness of the TiN films has increased slightly with the film thickness as observed from its AFM images. The mechanical properties of TiN films such as hardness and modulus of elasticity (E) were investigated by nanoindentation technique. The hardness of TiN thin film was found to be thickness dependent. The highest hardness value (24 GPa) was observed for the TiN thin films with less positive micro strain.

Effect of symmetry of substrate surfaces on the orientation and growth mode of ZnO films

The growth behaviors of ZnO films prepared by laser molecular beam epitaxy (L-MBE) were investigated on the substrates with different symmetry. The growth mode, the out-of-plane and in-plane orientations of ZnO thin films have been estimated by in-situ reflected high-energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD). ZnO films on amorphous glass substrates were polycrystalline with mixed (1 0 0) and preferred (0 0 1) crystal orientation. Single-domain ZnO films were arranged on (0 1 1)-, (1 1 1)-cubic perovskite and (0 0 1)-hexagonal substrates. On (0 0 1)-cubic perovskite surfaces, ZnO films contained poly-domain twined structure. The orientation difference of ZnO films could be attributed to the competition between the factors of the surface and interface influence including surface energy, strain energy and electrostatic energy. The crystal quality and growth mode were found to be dependent on not only the interface mismatch but also domain symmetry.

Investigation of substrate bias effects on the reactively sputtered ZrN diffusion barrier films

ZrN diffusion barrier films were prepared by DC reactive magnetron sputtering under different negative substrate bias. The composition, microstructure, resistivity and diffusion barrier properties of ZrN films, with respect to substrate bias, were studied by means of X-ray diffraction, electron probe microanalyzer, Auger electron spectroscopy, and four point probe method. Results showed that the deposition rate and impurity oxygen content of ZrN films were substantially influenced by the resputtering effects due to the ion bombardment on the film surface. The competition between surface energy and strain energy made the preferred orientation of ZrN films change from (1 1 1) to (2 0 0) and then back to highly (1 1 1) preferred orientation as a function of substrate bias. The application of negative substrate bias could effectively decrease the electrical resistivity due to the decrease of impurity oxygen content and the densification of films, resulting from the moderate-energy ion irradiation. The biased ZrN films could successfully be used as a diffusion barrier layer, between Cu and SiO 2, even up to the high temperature of 800 °C for 30 min.

Surface Segregation of PdxNi100-x Alloy Nanoparticles

Alloy nanoparticles of PdxNi100-x (x = 20, 30, 70, and 80) were prepared by the coprecipitation method. The surface segregation of nanoparticles, driven by the surface energy, strain energy, and reduction potential, was studied systematically. The surface composition, surface species, and alloy structure of as-prepared PdxNi100-x and heated Pd30Ni70 alloy nanoparticles was characterized by ESCA, TPR, and XRD, respectively. The structure of as-prepared PdxNi100-x alloy nanoparticles is not homogeneous and changes with the alloy content x. The surface is Pd enriched for as-prepared Pd80Ni20 alloy due to the strain energy effect. In contrast, the surface is Ni enriched for as-prepared PdxNi100-x alloy with x = 20, 30, and 70 due to the reduction potential effect. In addition, a combinatory process of surface segregation and homogenization during the heat treatment was observed on the surface of Pd30Ni70 alloy nanoparticles after heating at T > 620 K.

Periodically twinned SiC nanowires

Twinning has been recognized to be an important microstructural defect innanoscale materials. Periodically twinned SiC nanowires were largely synthesizedby the carbothermal reduction of a carbonaceous silica xerogel prepared fromtetraethoxysilane and biphenyl with iron nitrate as an additive. The twinnedβ-SiC nanowires, with a hexagonal cross section, a diameter of 50–300 nm anda length of tens to hundreds of micrometers, feature a zigzag arrangement ofperiodically twinned segments with a rather uniform thickness along the entire growthlength. Computer simulation has been used to generate three-dimensional atomicstructures of the zigzag columnar twin structure by the stacking of hexagonal discs of{111} planes of SiC. A minimum surface energy and strain energy argument isproposed to explain the formation of periodic twins in the SiC nanowires. Thethickness of the periodic twinned segments is found to be linearly proportional to thenanowire diameter, and a constant volume model is proposed to explain the relation.

Theoretical Investigation of the Structure and Coverage of the Si(111)−OCH3 Surface

The surface structure, strain energy, and charge profile of the methoxylated Si(111) surface, Si(111)-OCH3, has been studied using quantum mechanics, and the results are compared to those obtained previously for Si(111)-CH3 and Si(111)-C2H5. The calculations indicate that 100% coverage is feasible for Si(111)-OCH3 (similar to the methylated surface), as compared to only approximately 80% coverage for the ethylated surface. These differences can be understood in terms of nearest-neighbor steric and electrostatic interactions. Enthalpy and free energy calculations indicate that the formation of the Si(111)-OCH3 surface from Si(111)-H and methanol is favorable at 300 K. The calculations have also indicated the conditions under which stacking faults can emerge on Si(111)-OCH3, and such conditions are contrasted with the behavior of Si(111)-CH3 and Si(111)-CH2CH3 surfaces, for which stacking faults are calculated to be energetically feasible when etch pits with sufficiently long edges are present on the surface.

Compositional and structural changes in TiB 2 films induced by bias, in situ and post-deposition annealing, respectively

Structural changes in TiB 2 films were induced at relatively low temperatures by the application of bias and in situ annealing or by post-deposition heat treatment of samples subjected to bias with simultaneous in situ annealing. In situ annealing by itself evoked only partial crystallization. Application of bias by itself only modified the composition of the as deposited film. A simple model is presented to explain the variation of the composition when RF bias is applied to a cold substrate. The crystallized films had a (0 0 0 1) texture. A model has been suggested to explain the observed preferred orientation, based on the contribution of surface and strain energies. Both, the surface energy and strain energy are direction dependent. These were evaluated for two film orientations reported in the literature, namely, the (0 0 0 1) and ( 1 0 1 ¯ 1 ) orientations. The preferred orientation of the film is determined by the lowest overall free energy resulting from the competition between the surface energy and the strain energy on different lattice planes. The surface energy is not film thickness dependent while the strain energy is thickness dependent and increases with it. For small film thickness, as in this work, the surface energy term is significant and (0 0 0 1) orientation with a minimum surface energy is preferred. At large film thicknesses the strain energy becomes dominant and the ( 1 0 1 ¯ 1 ) preferred orientation is observed. Under certain experimental conditions strain energy effects may tip the preferred orientation to ( 1 0 1 ¯ 1 ) . The elastic moduli in the (0 0 0 1) and ( 1 0 1 ¯ 1 ) directions were determined as 435 and 538 GPa, respectively.

Control of catalyst particle crystallographic orientation in vertically aligned carbon nanofiber synthesis

Vertically aligned carbon nanofibers (VACNF) have been synthesized where the crystallographic orientation of the initial catalyst film was preserved in the nanoparticle that remained at the nanofiber tip after growth. A substantial percentage of catalyst particles (75%), amounting to approximately 200 million nanofibers over a 100 mm Si wafer substrate, exhibited a sixfold symmetry attributed to a cubic Ni(1 1 1)∥Si(0 0 1) orientation relationship which was verified by X-ray diffraction studies. The Ni catalyst films were prepared by rf-magnetron sputtering under substrate bias conditions to yield a single (1 1 1) film texture. The total energy of the Ni thin film was estimated by calculating the sum of the surface free energy and strain energy. The total film energy was minimized by the evolution of the plane of lowest surface free energy, the (1 1 1) texture. This result was in agreement with X-ray diffraction measurements. The preferred orientation present in the Ni catalyst film prior to nanofiber growth was preserved in the Ni catalyst particles throughout the VACNF growth process. The Ni catalyst particles at the nanofiber tips were not pure single crystals but rather consisted of a mosaic structure of Ni nanocrystallites embedded within Ni catalyst nanoparticles (200–400 nm). The tip-located nanoparticles exhibited a faceted, crystal morphology with the faceting transferred to the underlying carbon nanofiber during the growth process. The possibility of precisely and accurately controlling VACNF growth velocity over macroscopic wafer dimensions with uniformly aligned catalyst particles is discussed.

820 細胞分離における生体細胞の破壊とその評価方法(1)(OS8-5 医療援用工学における流動ダイナミクス,OS8 医療援用工学における流動ダイナミクス,オーガナイズドセッション)

In this paper, we propose a fracture model of islets in a shear flow to evaluate the mechanical damage of islets. The fracture model was developed using the relationship between surface area increase and strain energy induced into the islet through the fracture. The islet diameter reduction during the purification process was predicted using the fracture model and numerical calculation. Comparing experimental and numerical results in terms of the islet size reduction, it is confirmed that the fluid dynamical stress during the islet purification damages islets.

The asymmetric effect of stress in the morphological instability of a growing thin film

Many experiments have shown that films growing under tensile stress and compressive stress have different morphological instability behaviours. Departing from earlier theoretical works, which treated surface diffusion as the only kinetic process for bringing out the morphology change and assumed uniform growth rate along the surface, this paper studies the coupling between the deposition process and surface diffusion, to offer another possible mechanism for the asymmetric effect of stress. A nonlinear kinetic law, in which the activation energy of growth process is modified by local stress, is adopted. Linear stability analysis shows that if the growth rate is enhanced by tensile stress while being slowed by compressive stress, the surface instability, for a growing film under tension, could either be fully suppressed for perturbations of any wavelength, or occurs only when the wavelength is within an intermediate range; if the film is under compressive stress, the instability behaviour is similar to that obtained from considering surface diffusion alone, but with different amplitude on growth rate. Two characteristic lengths, one characterizing the competition between surface energy increase and strain energy reduction and the other representing the competition between surface diffusion and growth process, are identified. The thin film instability behaviour is largely determined by the ratio of these two lengths.

On the mechanisms of epitaxial island alignment on patterned substrates

A recent approach to fabricating self-assembled epitaxial nanostructures involves the use of topographically patterned substrates to control the formation of material deposits during growth. This article investigates the energetic and kinetic mechanisms that guide the positioning of strained epitaxial islands in these systems. A continuum-level model is used wherein the free energy of system consists of surface energy and strain energy. The substrate shape is represented by small amplitude sinusoidal features. An energetic phase diagram is calculated to determine which sites are most favorable—peaks, valleys, or side walls—depending on factors as substrate shape, lattice mismatch, surface energy, and the amount of material deposited. The kinetic evolution through the processes of deposition and surface diffusion is then simulated. It is found that the resulting morphology depends largely on the rate of deposition relative to surface mobility. Relatively slow deposition rates produce configurations of minimum free energy while higher deposition rates give rise to novel metastable states. Mechanisms by which various configurations arise are discussed in detail. Results are found to be in excellent agreement with published experiments.

Surface-energy-driven abnormal grain growth in Cu and Ag films

A comparative investigation has been made for Cu and Ag films in three states (as-deposited, attaching films after annealing, and free-standing films after annealing) by XRD and TEM. XRD patterns show that, after annealing at 300 °C for 2.5 h, (1 1 1) peak increased obviously in both free-standing Cu and Ag films, on the contrary, (2 0 0) and (2 2 0) peak increased obviously in attaching films. In addition, a (1 1 1)-oriented abnormal large grain was observed in both free-standing Cu and Ag films with TEM, while (1 0 0) and (1 1 0)-oriented abnormal large grains appeared in attaching Cu films and a (2 1 l)-oriented abnormal grain appeared in attaching Ag films. The experimental results have been explained satisfactorily by the minimizations of surface energy and strain energy.