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The interaction between mechanics and chemistry plays an essential and critical role in the surface segregation and relaxation in nanoscale alloys. Following the thermodynamics analysis based on surface eigenstress, the present study takes the free-standing nanometer thick films of Ni1–xCux solid solutions with face-centered cubic (fcc) crystalline structures as an example to investigate surface segregation of Cu and relaxation of the films. Hybrid Monte Carlo and Molecular Dynamics (MCMD) simulations are conducted on free-standing Ni1–xCux alloys of (100) and (111) nanofilms. The MCMD simulations verify the theoretical analytic results and determine the values of parameters involved in the theoretical analysis. Especially, the parameter of the differentiation in reference chemical potential behaves like the molar free energy of segregation in the McLean adsorption isotherm, and the differentiation in chemical composition induced eigenstrain plays also an important role in surface segregation and relaxation. The integrated theoretical and numerical study exhibits that both surface excess Cu concentration and apparent biaxial Young's modulus of Ni1–xCux nanofilms depend on the nominal Cu concentration and the film thickness. [ABSTRACT FROM AUTHOR]

The present study discovers the significant enhancement of critical current density by borohydride pinning in H-doped MgB2 bulks. Second-phase borohydrides are formed by synthesizing MgB2 bulks ex situ in an H2 atmosphere and in situ with H-trapped boron based on the concept of gas doping. Second phase Mg(BH4)2 appears in the samples ex situ sintered in an H2 atmosphere, leading to an enhancement of critical current density at magnetic fields over 3 T. Moreover, the in situ synthesized samples from H-trapped boron also exhibit a significantly enhanced critical current density of 1.8 × 104 A cm−2 at 20 K and 3 T, due to Mg(BH4)2 pinning centers that are embedded in the MgB2 grains. In contrast, the critical current density is only 9.6 × 103 A cm−2 at 20 K and 3 T in an un-doped MgB2 sample. [ABSTRACT FROM AUTHOR]

The effect of grain boundaries on the electromechanical response of a ferroelectric polycrystal subjected to an electric field or stress is investigated numerically by using a phase field model. The grain boundaries in the phase field model are regarded as dielectrics in which the ferroelectric properties are degraded completely. The phase field simulations show that the presence of dielectric grain boundaries results in a large build-in depolarizing field in grains. The depolarizing field has a significant influence on the coercive field, the switching behaviour of ferroelectric domain under an electric field or stress, and the piezoelectric and dielectric properties of the ferroelectric polycrystal. It is found that both coercive field and remnant polarization decrease with the increase of the thickness of dielectric grain boundary. However, the piezoelectric coefficient and permittivity of the ferroelectric polycrystal become larger when the thickness of dielectric grain boundary increases. The enhancement of dielectric and piezoelectric properties by the dielectric grain boundary suggests a new degree of freedom to tune the electromechanical response of ferroelectric polycrystalline materials. [ABSTRACT FROM AUTHOR]

The present work carries out mechanics analysis and atomistic simulations of nanobridge tests under large deformation. The nanobridge test is a three-point bending test on a nanowire sample with two clamped ends. The bending behavior of a tested nanowire under large deformation is mechanically analyzed, by treating a nanowire as a composite of a three-dimensional core, two-dimensional surfaces, and one-dimensional edges, and approximated by two loading steps corresponding to pure bending and tension, respectively. The two step loadings can be easily implemented in atomistic simulations and yield the strain energy of the tested nanowire in a closed form so that the analysis of simulation results becomes straightforward. Atomistic simulations of nanobridge tests on Si and SiC nanowires were conducted to demonstrate the developed approach. The surface stiffness and edge stiffness determined from the simulations of nanobridge tests are consistent with those determined from the tensile-compressive and pure bending simulations, thereby indicating that the surface stiffness and edge stiffness are material properties, independent of the loading condition, although the size-dependent Young’s modulus is more significant in the nanobridge test than that in the tensile-compressive test. [ABSTRACT FROM AUTHOR]

The nonlinear optical absorption in a three-subband step asymmetric semiconductor quantum well driven by a strong terahertz (THz) field is investigated theoretically by employing the intersubband semiconductor-Bloch equations. We show that the optical absorption spectrum strongly depends on the intensity, frequency, and phase of the pump THz wave. The strong THz field induces THz sidebands and Autler-Townes splitting in the probe absorption spectrum. Varying the pump frequency can bring not only the new absorption peaks but also the changing of the energy separation of the two higher-energy levels. The dependence of the absorption spectrum on the phase of the pump THz wave is also very remarkable. [ABSTRACT FROM AUTHOR]

Two-dimensional phase field simulations of ferroelectric nanoparticles with different long-range (LR)-electrostatic and -elastic interactions and different domain wall energy densities are conducted based on the time-dependent Ginzburg–Landau equation. The phase field simulations exhibit vortex patterns of polarizations, which have purely toroidal moments of polarizations and macroscopically negligible averaged polarizations, in nanoparticles without or with weak LR-elastic interactions when LR-electrostatic interactions are fully taken into account. However, a single-domain structure without any toroidal moment of polarizations is formed in small nanoparticles if LR-electrostatic interactions are completely ignored or LR-elastic interactions are fully taken into account. The polarization gradient energy or domain wall energy density plays also an important role in the formation of polarization structure. The vortex structure transits from a multivortex structure to a single-vortex structure as the domain wall energy density increases. [ABSTRACT FROM AUTHOR]

The present work studies the size-dependent surface stress, surface stiffness, and Young’s modulus of a prism crystalline nanowire, which is theoretically treated to be composed of a hypothetical nanowire phase, a true two-dimensional geometric surface phase, and a true one-dimensional geometric edge phase. The hypothetical nanowire phase could be elastically deformed due to relaxation of a free-standing nanowire, without any applied load, with respect to its bulk counterpart. The initially deformed nanowire phase is taken as reference in the present work in the determination of excess surface and edge energies. The theoretical results indicate that the edge phase causes the nominal specific surface energy, surface stress, and surface stiffness to be size dependent, and the surface phase and the edge phase make the nominal Young’s modulus size dependent. The edge and surface effects are more significant as the cross-sectional area of a nanowire becomes smaller. Molecular dynamics simulations on hexagonal prism [111] β-SiC nanowires were conducted and the results verified the theoretical approach and illustrated the intrinsic mechanism of the size-dependent surface properties and Young’s modulus of nanowires. The theoretical analysis and methodology are universal when the continuum concepts of surface energy, surface stress, and Young’s modulus are used to characterize mechanical properties of nanowires. [ABSTRACT FROM AUTHOR]

Chemical stresses induced during a diffusion process have a great influence on the diffusion process by enhancing the diffusion coefficient. The effective diffusion coefficient is a function of the concentration when chemical stresses are taken into account. The diffusion equation with the effective diffusion coefficient is numerically solved for the three cases, namely, permeation through a membrane, one-side charging and two-side charging of membranes, with the flux continuity boundary conditions, which include the surface effect, i.e., the adsorption, absorption and desorption reactions. The results indicate that chemical stresses enhance the diffusion process. If chemical stresses are neglected, both two-side charging and one-side charging have the same concentration profiles. However, when the chemical stresses are fed back to the diffusion, the concentration level for the two-side charging is higher than that for the one-side charging. The surface effect is also investigated by changing the ratio of the drift velocity in surface to the drift velocity in bulk, showing that the diffusion process is promoted by the increase of the drift velocity ratio. [ABSTRACT FROM AUTHOR]

Based on the dislocation theory of twinning, an analytical solution is given to determine the critical thickness of the misfit twin formation in an epilayer with different elastic constants from its substrate. For a given epilayer thickness and lattice mismatch strain, the twin formation energy should reach its minimum to determine the twin width and a zero minimum formation energy determines the critical thickness for misfit twinning. The effect of elastic mismatch between the epilayer and the substrate on the critical thickness is studied comprehensively, revealing that an elastically soft epilayer has a large critical thickness. Moreover, a misfit-twin-and-perfect-dislocation predominance chart is constructed to predict the predominant regions of misfit twinning and perfect dislocation in the mismatch strain and the specific twin-boundary energy domain. [ABSTRACT FROM AUTHOR]

The equilibrium morphology of misfit twins in an epilayer with different elastic constants from its substrate is systematically investigated by energy minimization based on the dislocation mechanism of twinning. Explicit formulas for period arrays of misfit twins with the same orientation and with alternating orientations are derived by superposition of the solutions reported in Part I of this series. The equilibrium morphology for a periodic array of misfit twins is characterized by the twin width and the twin mean spacing. The effects of elastic mismatches between the epilayer and the substrate on the equilibrium morphology of twins are also studied comprehensively. The theoretical predictions agree with experimental observations. [ABSTRACT FROM AUTHOR]

The present work analyzes hydrogen permeation through a slab sample containing isolated hydrogen traps. The analysis is based on a flux continuity model that considers both the absorption and desorption processes. An analytical solution in series is derived for a dilute system with a low trap coverage fraction. According to this solution, we are able to characterize isolated hydrogen traps using only hydrogen permeation tests on two groups of samples with each group having different thicknesses. One group of samples, serving as a reference, is made of a pure matrix material without any hydrogen traps, while the other group of samples is made of the same matrix material with traps. The proposed multisample method is demonstrated by numerical calculations and plots, from which we determine the hydrogen diffusion coefficient, the desorption rate, the absorption parameter, the trap density, and the trap enthalpy from the permeation data of the steady flux and the time lag. The maximum error caused by the numerical calculations is less than 7%. [ABSTRACT FROM AUTHOR]

The present work studies the effects of absorption and desorption on the chemical stress field in a membrane during permeation or both-side charging of a diffusing species. The permeation and both-side charging processes are analyzed with the consideration of the absorption and desorption processes, wherein the flux continuity boundary conditions are adopted. The results show that the chemical stresses are compressive near the surfaces and tensile in the center of the membrane. The maximum magnitude of stress occurs at the entry surface for the permeation process and at both surfaces for the both-side charging process. For the permeation process, the compressive stress at the entry surface increases with time to a maximum and then decreases with time gradually to zero. A similar phenomenon is found at both surfaces in the both-side charging process. In both processes, the stress magnitude depends upon the ratio of the drift velocity through surface to the drift velocity through bulk. When this ratio approaches infinity, the flux continuity boundary conditions reduce to the concentration boundary condition and the highest chemical stress field will be produced. Therefore, reducing the drift velocity through surface would be an efficient means to reduce the strength of the chemical stress. In general, designed surface alloying can introduce diffusing species traps at the surfaces of a membrane, which may efficiently decrease the drift velocity through surface. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study that examined critical epilayer thickness for the formation of misfit dislocations at the interface between an epilayer and a substrate with finite thickness. Theoretical analysis on stress distribution; Details on the contours of stress components arising from an edge dislocation; Conclusion.

Presents information on a study which investigated the elastic interaction of an edge dislocation with a wedge crack using Muskhelishvili's complex potential treatment of elasticity. Methods; Results; Discussion.

Presents information on a study which investigated the critical thickness of an epilayer on a substrate with different elastic constants using a treatment of anisotropic elasticity. Methods; Results; Discussion.

Resolves the inconsistencies in the limiting solutions regarding self energy by deriving the energy components using complex potentials. Equation of stress and displacement fields under antiplane strain; Limiting cases for the self energy of dislocation array; Implication of a constant interaction energy per unit length of dislocation line.

Considers the sequential generation of dislocations in a strained epitaxial layer. Derivation of self energy and interaction energy between the fresh dislocation and the mismatch strain; Interaction energy as a function of vertical component of the distance between the dislocations; Nucleation of a fresh dislocation at a position midway between any two pre-existing dislocations in the array.

Presents a study which investigated the elastic interaction between an edge dislocation and a macrocrack with microcrack by means of the Muskhelishvili potential method. Mathematical expression of the stress and displacement field of a plane strain; Equation of the stress intensity factors.

Presents a study that formulated the stress displacement fields of an edge dislocation near a semi-infinite or a finite interfacial crack by using the complex potential theory of Muskhelishvili's elasticity treatment of plane strain problems. Role of the interaction between dislocations and cracks in a fracture; Information on the stress intensity factor produced by an edge dislocation; Equations used in the study.

Explores the boundary conditions appropriate to a crack filled with a dielectric medium and reformulated the stress and electric fields around such a mode-III crack. Summary of a three-dimensional formulation of linear piezoelectricity; Illustration on the effect of a nonconducting medium in the crack; Evaluation on the mechanical displacement and the static electrical potential.

Presents a study that examined the interaction between a screw dislocation and an interface cylindrical hole. Investigation of the boundary problem of a screw dislocation near a cylindrical hole; Formulation of the stress field of a screw dislocation; Description of the image force.

A nonlinear electroporation (EP) model is proposed to study the electro-mechanobiological behavior of cell membrane during EP, by taking the nonlinear large deformation of the membrane into account. The proposed model predicts the critical transmembrane potential and the activation energy for EP, the equilibrium pore size, and the resealing process of the pore. Single-cell EP experiments using a micro EP chip were conducted on chicken red blood cells at different temperatures to determine the activation energy and the critical transmembrane potential for EP. The experimental results are in good agreement with the theoretical predictions. [ABSTRACT FROM AUTHOR]

Phase field simulations were conducted to investigate the effects of unequally biaxial misfit strains on domain stability diagrams and equilibrium domain structures in an epitaxial ferroelectric PbTiO3 thin layer, which is sandwiched in a nonferroelectric medium. The simulations reveal a multidomain structure in the layer and allow constructing “misfit strain-misfit strain” and “temperature-misfit strain” phase diagrams. It is found that unequally biaxial misfit strains may lead to the presence of a single tetragonal variant, either a-domains or b-domains, which do not exist if the misfit strains are equally biaxial. [ABSTRACT FROM AUTHOR]

From phase field simulations, we investigate the size-dependent polarization distribution in ferroelectric nanostructures embedded in a nonferroelectric medium. The simulation results exhibit that vortex structures of polarizations and single-domain structures are formed in ferroelectric nanodots and nanowires, respectively. Furthermore, a single-vortex structure is formed in the ferroelectric nanodots if the aspect ratio of thickness to lateral size is less than a critical value, whereas the ferroelectric nanodots are in a multivortex state if the aspect ratio exceeds the critical value. When the aspect ratio approaches infinity, nanodots will become nanowires, in which polarizations are homogeneous. [ABSTRACT FROM AUTHOR]

A technique for impeding Cu outdiffusion in Cu alloy based preplated lead frames has been developed by electroplating a 3–4 nm thick Sn layer on a Cu alloy base prior to electroplating a Ni layer. A 10–14 nm thick epitaxylike and dense (Cu,Ni)3Sn intermetallic-compound (IMC) layer is automatically formed en route of diffuse reaction, which leads to a drastic reduction in Cu outdiffusion and hence improves significantly the protection of the lead frames against oxidation and corrosion attack. The estimated Cu diffusion coefficient in the IMC interlayer is about 1.6×10-22 m2/s at 250 °C. [ABSTRACT FROM AUTHOR]

The effect of long-range (LR) elastic interactions on the toroidal moment of polarization in a two-dimensional ferroelectric particle is investigated using a phase field model. The phase field simulations exhibit vortex patterns with purely toroidal moments of polarization and negligible macroscopic polarization when the spontaneous strains are low and the simulated ferroelectric size is small. However, a monodomain structure with a zero toroidal moment of polarization is formed when the spontaneous strains are high in small simulated ferroelectrics, indicating that, because of the LR elastic interactions, high values of spontaneous strains hinder the formation of polarization vortices in ferroelectric particles. [ABSTRACT FROM AUTHOR]

This study demonstrates that electric fields can fracture depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to measure the electrical fracture toughness. The electrical fracture toughness (G[sup E][sub C] = 263 ± 35 N/m) is about nine times higher than the mechanical fracture toughness (G[sup M][sup IC] = 30.4±3.9N/m). The highly electrical fracture toughness arises from the greater energy dissipation around the conductive sharp notch tip under purely electric loading, which is impossible under mechanical loading in brittle depoled ceramics. © 2001 American Institute of Physics. [DOI: 10.1063/1.1427437]. [ABSTRACT FROM AUTHOR]

The present work uses Raman spectra to measure residual stresses in Pb(Zr[sub 0.53]Ti[sub 0.47])O[sub 3] thin films. Based on thermodynamic analysis, a linear relationship is found between the stress and the square of the Raman frequency in the A[sub 1] [transverse optical[sub 3] (TO[sub 3])] and E [longitudinal optical[sub 3] (LO[sub 3])] modes. We calibrate the linear relationship by measuring the Raman spectra of stressed bulk Pb(Zr[sub 0.53]Ti[sub 0.47])O[sub 3] samples. Then, we assess residual stresses in the lead zirconate titanate thin films at different thicknesses and different annealing temperatures. The residual stresses extracted from the A[sub 1](TO[sub 3]) mode are consistent with those from the E(LO[sub 3]) mode, which are more or less the same as those measured by the x-ray diffraction sin[sup 2] ψ method. © 2001 American Institute of Physics. [DOI: 10.1063/1.1426271]. [ABSTRACT FROM AUTHOR]

Fracture of piezoelectric materials can be caused by either mechanical loads or by electric fields. It is shown in lead zirconate titanate ceramics that conductive cracks driven by electric fields can be described by the same fracture mechanics concepts used for mechanical loads, but with a much higher fracture toughness. The electrical fracture toughness G[sub IC][sup E]=223.7±17.0 N/m (95% confidence) is 25 times higher than the mechanical fracture toughness G[sub IC][sup M]=8.7±0.4 N/m (95% confidence). This is due to the greater energy dissipation that is possible under electric loading but not possible under mechanical loading in the brittle ceramics of lead zirconate titanate. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]