Large Strain Field Research Articles

Formation mechanism of twin domain boundary in 2D materials: The case for WTe2

Our scanning tunneling microscopy (STM) study observes, for the first time, twin domain boundary (TDB) formations on the surface of WTe2 single crystal, which is glued by solidifying indium to Si substrate. In these TDB regions, a large inhomogeneous strain field, especially a critical shear strain of about 7%, is observed by geometric phase analysis. This observation does not obey the old believe that a small mechanical stress is sufficient to drive thermally-induced TDB formations in two-dimensional materials. To resolve the contradiction, we perform density functional theory calculations combined with elasticity theory analysis, which show that TDBs on WTe2 are entirely displacement-induced, for which a critical strain is necessary to overcome the onset barrier.

Overexploitation of groundwater resources in the faulted basin of Querétaro, Mexico: A 3D deformation and stress analysis

The City of Querétaro is located on a continental basin filled since the Oligocene with lacustrine and alluvial sediments, pyroclastic deposits, and interbedded fractured basalts. The graben structure of the basin was formed by two major North-South trending normal faults, among which the thicknesses of the filling materials vary many tens of meters in close distances. Hence, important differences of hydraulic and mechanical properties characterize the various geologic units. Groundwater has been strongly withdrawn over the last three decades in the study area, with a decline of the piezometric level exceeding 100 m. Because of the high variability of the geologic deposits, the piezometric decrease and consequently the effective stress increase are characterized by a large spatial variability. Piezometric variations are also due to faults that strongly impact on groundwater flow dynamics. The deformation and effective stress variability has caused large differential subsidence causing ground fracturing that has damaged the urban infrastructure of the City of Querétaro. This complex geological setting has been properly accounted into a three-dimensional (3D) flow and geomechanical modeling approach to quantify the displacement, deformation, and stress fields caused by water withdrawal. The static geologic model was accurately defined using geological logs from extraction wells, field mapping of faults, fractures, and the integration of major structures reported in previous geophysical works. The model has been calibrated using observed groundwater levels and land settlement records. The simulations have spanned the period from 1970 to 2011. The modeling results highlight that the areas where large differential subsidence and horizontal displacements developed correspond to the portions of the city where ground fractures are observed. Normal and shear components of the stress field changes accumulate along the discontinuity surfaces at depth, providing evidence that large piezometric declines can be a key factor triggering the fault reactivation. The spatial relationship between major withdrawals, discontinuities of the geologic structure, and accumulation of large stress and strain fields clearly emerged from the outcomes of the 3D geomechanical model.

Effect of Small‐Angle Grain Boundary on the Mechanical Properties in Direct Silicon Bonded Wafer

The mechanical properties of small‐angle grain boundary (GB) in direct silicon bonded wafer are investigated by dislocation motion using the Vickers micro‐indentation combined with nano‐indentation technique. The dislocation motion is thoroughly hampered at the GB, due to the large strain field (comprehensive stress as high as ≈339 MPa), which is demonstrated by the micro‐Raman spectroscopy measurement. Nano‐indentation tests directly on GB show that Young's modulus and hardness are almost identical to those in the grain. The results indicate that electrically active small‐angle GB with high stress should not be the weak point of mechanical strength in multi‐crystalline silicon.

Nanowires with dislocations for ultralow lattice thermal conductivity.

Nanostructures grown by screw dislocations have been successfully synthesized in a range of materials, including thermoelectric materials, but the impact of these extended crystallographic defects on thermal properties of these nanostructures is not known. We investigate thermal transport in PbSe and SiGe nanowires storing screw dislocations via equilibrium molecular dynamics simulations. The inherent one dimensionality and the combined presence of a reconstructed surface and dislocation yield ultralow thermal conductivity values. Our simulations suggest that the large dislocation strain field in nanowires may play a key role in suppressing the thermal conductivity of thermoelectric nanomaterials to increase their thermoelectric figure of merit.

Behavioral response of pyrite structured Co0.2Fe0.8S2 nano-wires under high-pressure up to 8 GPa – Mössbauer spectroscopic and electrical resistivity studies

Pyrite-structured Co0.2Fe0.8S2 nano wires with aspect ratio 45:1, synthesized using solution colloid method were studied under high pressure up to 8GPa using 57Fe Mössbauer spectroscopy (using diamond anvil cell) and electrical resistivity (using tungsten carbide cell) techniques. Room temperature S K-edge XANES studies at INFN-LNF synchrotron beam line signified the changes in the electronic structure owing to Co substitution. Magnetic measurements at 5K demonstrated disordered ferromagnetic behavior similar to Griffith phase. The value of isomer shift identified Fe in divalent, low spin state corresponding to pyrite structure. Higher value of quadrupole splitting observed at ambient condition was due to large lattice strain and electric field gradient generated by large surface to volume ratio of the nano size of the system. With applied pressure, the value followed the expected trend of increase up to 4.3GPa, then to decrease till 6.4GPa. Such change in the trend suggested a phase transition. On decompression to ambient pressure, the system seemed to retain high pressure phase and nano structure. The pressure coefficient of electrical resistivity varying from −0.0454 to −0.166Ω-cm/GPa across the transition pressure of ~4.5GPa was sluggish suggesting second order phase transition. The pressure-dependent variations by Mössbauer parameters and electrical resistivity showed identical result. This is the first report of pressure effect on nano sized Co0.2Fe0.8S2. Effect of particle size on transition pressure could not be evaluated due to lack of available reports on bulk system.

Differential uplift and settlement between inner column and diaphragm wall in top-down excavation

Top structure and basement will confront the risk of being damaged on account of large stress and strain fields incurred by differential uplift and settlement between inner column and diaphragm wall in top-down method. Top-down excavation of the Metro Line 10 in Shanghai was modeled with finite element analysis software ABAQUS and parameters of subsoil were obtained by inverse analysis. Based on the finite element model and parameters, changes in the following factors were made to find more effective methods to restrain differential uplift and settlement: length of diaphragm wall, thickness of jet-grouting reinforcement layer, ways of subsoil reinforcement, sequence of pit excavation, connection between slabs and diaphragm wall or column and width of pit. Several significant results are acquired. The longer the diaphragm wall is, the greater the differential uplift between column and diaphragm wall is. Rigidity of roof slab is in general not strong enough to keep diaphragm wall and column undergoing the same uplift during excavation; Uplift at head of column and differential uplift between column and diaphragm wall decrease when subsoil from −16.6 to −43 m in pit is reinforced through jet-grouting. But, as excavation proceeds to a lower level, benefit from soil reinforcement diminishes. During the process applying vertical load, the larger the depth of diaphragm wall is, the smaller the settlement is at head of column and diaphragm wall, and the greater the differential settlement is between column and diaphragm wall. When friction connection is implemented between column, diaphragm wall and floor slabs, uplifts at head of column and diaphragm wall are larger than those of the case when tie connection is implemented, and so does differential uplift between column and diaphragm wall. The maximum deflection of diaphragm wall decreases by 58% on account of soil reinforcement in pit. The maximum deflection of diaphragm wall decreases by 61.2% when friction connection is implemented instead of tie connection.

Thermoelectric transport properties of diamond-like Cu1−xFe1+xS2 tetrahedral compounds

Polycrystalline samples with the composition of Cu1−xFe1+xS2 (x = 0, 0.01, 0.03, 0.05, 0.1) were synthesized by a melting-annealing-sintering process. X-ray powder diffraction reveals all the samples are phase pure. The backscattered electron image and X-ray map indicate that all elements are distributed homogeneously in the matrix. The measurements of Hall coefficient, electrical conductivity, and Seebeck coefficient show that Fe is an effective n-type dopant in CuFeS2. The electron carrier concentration of Cu1−xFe1+xS2 is tuned within a wide range leading to optimized power factors. The lattice phonons are also strongly scattered by the substitution of Fe for Cu, leading to reduced thermal conductivity. We use Debye approximation to model the low temperature lattice thermal conductivity. It is found that the large strain field fluctuation introduced by the disordered Fe ions generates extra strong phonon scatterings for lowered lattice thermal conductivity.

Modal Pursuit to Detect Large Displacements and Strain Fields by Digital Image Correlation

AbstractDigital image correlation offers a useful approach to resolve displacements and strains by picture comparisons. However, many of the several methods proposed in the past suffer the occurrence of large displacements between the two compared pictures. The modal pursuit approach here introduced intends giving a possible strategy to prevent this limitation. The main advantage is that the method uses a global approach for the solution strategy. The procedure uses a displacement basis that derives from a modal analysis on a membrane structure whose shape is the same as the image region under investigation. The modes are introduced a little at a time so that the procedure is facilitated to converge, even when only two pictures (undeformed and final state) are used for the matching.

Conditions associated with large rain-field areas for tropical cyclones landfalling over Florida

To adequately warn the public about the potential for heavy rainfall, it is important to understand the conditions associated with large rain fields as tropical cyclones (TCs) move over land. This study examines rain-field areas during 29 landfalls over Florida utilizing data from the Tropical Rainfall Measuring Mission’s 3B42 product. A GIS was used to calculate the areas of regions delineated by 5 mm h−1 rain rates occurring within 48 h pre- or post-landfall. Thirteen conditions were evaluated, including intensity, motion, vertical wind shear, cause of demise, month, and time of day. After dividing observations into groups for each condition, Mann–Whitney U and Kruskal–Wallis tests determined that statistically significant differences existed among the groups for 11 conditions. The largest rain fields belonged to TCs that occurred in October or November, followed by those that were hurricanes and/or were intensifying at landfall. TCs nearing the end of an extratropical transition as well as those observed at 1100 Eastern Daylight Time also had relatively large areas of moderate-to-high rain rates. TCs meeting these conditions should be monitored closely as they move towards and over Florida due to their potential for widespread moderate-to-heavy rainfall, which may begin prior to the arrival of the fastest winds.

Characterization of InGaN quantum disks in GaN nanowires

AbstractCatalyst‐free GaN nanowires with InGaN quantum disks (QDs) were characterized by scanning/transmission electron microscopy (S/TEM) and photoluminescence. A variety of structures, from QDs with large strain fields to apparently strain free QDs were observed. TEM lattice imaging revealed dislocations in the QD regions of the samples. Stacking faults were also observed and appear to be associated with the GaN growth temperature rather than the presence of indium. Energy dispersive X‐ray spectroscopy in the TEM revealed indium concentrated in stacking faults but not always localized in the QD regions. Photoluminescence spectra show a red‐shift, possibly associated with indium concentration. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-pressure electrical resistivity and Mössbauer spectroscopic studies on narrow band Co0.8Fe0.2S2 nanoparticles up to 8 GPa

Pyrite-structured Co0.8Fe0.2S2 colloid nanoparticles, synthesized using solution method with particle distribution around 3–4 nm determined by transmission electron microscope, were studied under high pressure up to 8 GPa using 57Fe Mössbauer spectroscopy and electrical resistivity techniques. Drastic decrease in TC by magnetic measurements indicated nanosize of the particle. Higher quadrupole splitting (QS) at ambient condition was due to large lattice strain and electric field gradient generated by higher surface to volume ratio of these nanosized particles. Under pressure, Mössbauer parameters – isomer shift and QS – showed an expected trend up to ∼5.6 GPa. Above 6.4 GPa, the QS remained constant up to 8 GPa with decreased value. Even after decompression, the high-pressure phase is retained. The variation of pressure coefficient of electrical resistivity from −0.021/GPa to −0.151/GPa across 6.8 GPa suggested a second-order phase transition. The coincidence of observed value with that of the bulk suggested that the particle size does not impart much influence on transition pressure. This is the first report on nanoparticles of Co0.8Fe0.2S2 under pressure.

Independent control of exciton and biexciton energies in single quantum dots via electroelastic fields

We investigate the effect of large in-plane strain and vertical electric fields on the binding energies of excitonic complexes confined in single InGaAs/GaAs quantum dots (QDs) and we find that the two independently tunable perturbations modify the interaction energies among electrons and holes in a different manner. By taking advantage of this difference, we frequency-lock the QD fundamental excitation (the neutral exciton) at a predefined value, while the biexciton transition is actively tuned from a binding to an antibinding configuration. Our electrically controlled dual-knob device demonstrates unprecedented control over the electronic properties of the few-particle states in a QD and may be applied to create novel energy-tunable sources of entangled photons using the time-reordering or the time-bin scheme.

Inclusion of an arbitrary polygon with graded eigenstrain in an anisotropic piezoelectric half plane

This paper presents an exact closed-form solution for the Eshelby problem of a polygonal inclusion with graded eigenstrains in an anisotropic piezoelectric half plane with traction-free on its surface. Using the line-source Green’s function, the line integral is carried out analytically for the linear eigenstrain case, with the final expression involving only elementary functions. The solutions are applied to the semiconductor quantum wire (QWR) of square, triangular, and rectangular shapes, with results clearly illustrating various influencing factors on the induced fields. The exact closed-form solution should be useful to the analysis of nanoscale QWR structures where large strain and electric fields could be induced by the non-uniform misfit strain.

The effect of high-In content capping layers on low-density bimodal-sized InAs quantum dots

The structural and morphological features of bimodal-sized InAs/(In)GaAs quantum dots with density in the low 109 cm−2 range were analyzed with transmission electron microscopy and atomic force microscopy and were related to their optical properties, investigated with photoluminescence and time-resolved photoluminescence. We show that only the family of small quantum dots (QDs) is able to emit narrow photoluminescence peaks characteristic of single-QD spectra; while the behavior of large QDs is attributed to large strain fields that may induce defects affecting their optical properties, decreasing the optical intensity and broadening the homogeneous linewidth. Then, by using a rate-equation model for the exciton recombination dynamics, we show that thermal population of dark states is inhibited in both QD families capped by high In content InGaAs layers. We discuss this behavior in terms of alloy disorder and increased density of point defects in the InGaAs pseudomorphic layer.

Localization of Folds and Cracks in Thin Metal Films Coated on Flexible Elastomer Foams

Thin metal films coated on soft elastomeric foam substrates exhibit enhanced electromechanical performance. The open-cell foam structure conveys highly anisotropic mechanical properties within the top, thin capping elastomer at the surface of the foam. Upon stretching, large strain fields inducing cracks and folds localize above the foam cells, while the surrounding cell ligaments remain almost strain-free, enabling stable electrical conduction in the metallic coating.

Characterization of Reinforced Carbon Composites with Full Field Measurements: Long Gauge Length Compressive Apparatus

A new compressive testing apparatus is developed and used in this research. It has long gauge length to allow digital image correlation monitoring and anti buckling guides to prevent buckling. It allows the optical recording of strains and displacements. The novel setup is used to study the compressive response of tufted and untufted Carbon non crimp fabric composites with full field measurements. Experimental results show that the specimens are not bending in the apparatus under compression. Results also show reduced strain concentrations and a large strain field that provides a good environment for material compressive stiffness characterization. The test proves particularly successful for bias direction layup of [+45/-45] for which large damage mechanism occurs. However for [0/90] specimens a scatter in compressive ultimate strength was noticed which is due to the difficulty to prepare specimens with best minute accurate geometry. The compressive apparatus has shown to be a good alternative to existing setups and to provide significantly more information as well as having the possibility to be used in dynamics with a drop tower.

Universal Recovery of the Energy-Level Degeneracy of Bright Excitons in InGaAs Quantum Dots without a Structure Symmetry

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

Raman spectroscopy and strain mapping in individualGe-SixGe1−xcore-shell nanowires

Core-shell ${\text{Ge-Si}}_{x}$${\text{Ge}}_{1\ensuremath{-}x}$ nanowires (NWs) are expected to contain large strain fields due to the lattice mismatch at the core-shell interface. Here we report the measurement of core strain in a NW heterostructure using Raman spectroscopy. We compare the Raman spectra, and the frequency of the Ge-Ge mode measured in individual ${\text{Ge-Si}}_{0.5}$${\text{Ge}}_{0.5}$ core-shells, and bare Ge NWs. We find that the Ge-Ge mode frequency is diameter independent in Ge NWs with a value similar to that of bulk Ge, 300.5 ${\text{cm}}^{\ensuremath{-}1}$. On the other hand, ${\text{Ge-Si}}_{0.5}$${\text{Ge}}_{0.5}$ core-shell nanowires reveal a strain-induced blue shift of the Ge-Ge mode, dependent on the relative core and shell thicknesses. Using lattice dynamical theory we determine the strain in the Ge core and show that the results are in good agreement with values calculated using a continuum elasticity model.

Polycrystal modelling of fatigue: Pre-hardening and surface roughness effects on damage initiation for 304L stainless steel

The 304L stainless steel is a major component of residual heat removal circuits of pressurized water reactors (PWRs). The main purpose of this study is to understand the risk of thermal fatigue damage resulting from the machining of the 304L steel pipes inner surface (pre-hardening gradient, residual stresses and scratches), at the scale of the microstructure. This work is based on previous results obtained for pipe specimens thanks to a macroscopic elasto-visco-plastic model. Applied to the pipe specimens, this modelling showed that a thermal loading with temperature gradient, induced a cyclic non-linear biaxial loading at the inner surface of the pipe. In this paper, a polycrystal plasticity model, implemented in a Finite Element (FE) code, is adapted to cyclic loading. An elementary volume (3D aggregate), representing the inner surface and sub-surface of the 304L steel tube, is built from successive polishings and orientation mappings thanks to an Electron Back Scattering Diffraction method. At the grain scale, the polycrystal model is used as a “numerical microscope” to compute the local mechanical fields. Different fatigue criteria are tested to determine their sensitivity to surface properties (roughness, residual stress and pre-hardening) and to the microstructure of the material (crystallographic orientation and grain size). Pre-hardening leads to a lower and more homogeneous distribution of local strain amplitudes in the aggregate, but slightly higher stresses when compared to initial material without hardening. By contrast, surface roughness leads to large localized strain and stress fields in grains located at the bottom of scratches. To determine the surface micro-structural “hot spots” features and to test the sensitivity of different surface conditions, three different fatigue criteria (Manson-Coffin, Fatemi–Socie and Dissipated Energy criteria) have been computed. We point out that the pre-hardening may have a complex effect on fatigue resistance, since it reduces local plastic strain amplitudes, but increases local stresses. Moreover, the pre-hardening has a positive effect on fatigue since it delays damage initiation. By contrast, the surface roughness leads to a negative effect. However, we have shown that the three different fatigue criteria do not deliver similar quantitative predictions. Relevant criteria for high cycle fatigue, such as stress based criteria, are not considered in this paper, since the thermal loading used for computation is large enough to reduce cyclic plastic strain straining within all grains of 304L pipe inner surface for midlife of experiments.

Noninvasive high resolution mechanical strain maps of the spine intervertebral disc using nonrigid registration of magnetic resonance images

High resolution strain measurements are of particular interest in load bearing tissues such as the intervertebral disc (IVD), permitting characterization of biomechanical conditions which could lead to injury and degenerative outcomes. Magnetic resonance (MR) imaging produces excellent image contrast in cartilaginous tissues, allowing for image-based strain determination. Nonrigid registration (NRR) of MR images has previously demonstrated sub-voxel registration accuracy although its accuracy and precision in determining strain has not been evaluated. Accuracy and precision of NRR-derived strain measurements were evaluated using computer generated deformations applied to both computer generated images and MR images. Two different measures of registration similarity—the cost function which drives the registration algorithm—were compared: Mutual Information (MI) and Least Squares (LS). Strain error was evaluated with respect to signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and strain heterogeneity. Additionally, the creep strain response from an in vitro loaded porcine IVD is shown and comparisons between similarity measures are presented. MI showed a decrease in strain precision with increasing CNR and decreasing SNR while LS was insensitive to both. Both similarity measures showed a decrease in strain precision with increasing strain heterogeneity. When computer generated heterogeneous strains were applied to MR images of the IVD, LS showed substantially lower strain error in comparison to MI. Results suggest that LS-driven NRR provides a more accurate image-based method for mapping large and heterogeneous strain fields and this method can be applied to studies of the IVD and, potentially, other soft tissues which present sufficient image texture.