Search

[1.1][n]Paracyclophanes ([1.1][n]PCPs) (3) with n=2, 3, and 4, which consist of two [n]paraphenylene units connected by methylene bridges, were synthesized using short synthetic pathways with good overall yields. Single‐crystal X‐ray diffraction analyses reveal that the paraphenylene unit in 3 is bent, resulting in an elliptic core structure of 3. The facing bridgehead carbons of the paraphenylene units are separated by a distance much shorter than the sum of the van der Waals radii of sp2‐carbon atoms. UV‐vis absorption spectra, fluorescence spectra, electrochemical measurements, and theoretical calculations were used to demonstrate the presence of through‐space (TS) conjugation in 3. Furthermore, host‐guest complex formation between 3D (n=3) and tetracyanoquinodimethane in the solid state was revealed. [ABSTRACT FROM AUTHOR]

The development of novel strained spiro heterocycles (SSHs) as bioisosteres for aromatic or non‐strained aliphatic rings is highly sought after for improving drug design. Their high molecular rigidity and predictable vectorization can enhance drug‐likeness, target selectivity and clinical success. Towards this goal, 1‐oxa‐2,6‐diazaspiro[3.3]heptane (ODASE) is reported as a novel potential SSH‐bioisostere. We demonstrate through theoretical studies the potential of this strained spiro heterocycle to act as a bioisostere for piperazine. We have developed its synthesis from the highly strained azabicyclo[1.1.0]butyl intermediate through a robust and mild flow technology‐assisted two‐step protocol. Its tolerance and stability towards medicinally relevant N‐functionalisation protocols are studied, as well as its mild reduction to the C3‐aminoalkylazetidinol motif found in the anti‐cancer drug cobimetinib. [ABSTRACT FROM AUTHOR]

In this study, the stabilization of ergodicity and electromechanical strain properties of lead‐free ternary 0.75Na1/2Bi1/2TiO3–0.24SrTiO3–0.01ABO3 piezoceramics (BNT–24ST, ABO3 = BaZrO3 (BZ), CaTiO3 (CT), and BiFeO3 (BF)) were investigated. The relaxor state could be stabilized in BZ‐and BF‐modified BNT–24ST ceramics even at room temperature, whereas pure BNT–24ST and CT‐modified ceramics exhibited ferroelectric (FE) behavior. These phenomena were rationalized by analyzing the temperature‐dependent permittivity; detecting the FE‐relaxor phase transition temperature (TF‐R); and investigating features in strain, polarization, and switching current curves. Furthermore, high‐temperature conductivity data showed that relaxor behavior and realization of large strain properties strongly correlated with the concentration of oxygen vacancies in ABO3‐modified BNT–ST ternary systems. [ABSTRACT FROM AUTHOR]

Intercalation anodes usually exhibit better cyclability than conversion and alloying anodes for lithium or sodium storage due to the robust layered structure. However, for larger‐sized potassium accommodation, these intercalation anodes usually undergo huge layer expansion and structural distortion, triggering severe capacity fading. Herein, the novel 1T"' MoS2 is revealed the intercalation anode for stable K+ storage, in which metallic Mo─Mo bonds and puckered S layers accelerate the charge transfer and homogeneous K+ insertion. Moreover, the ultralow strain (3.5%) induced by the non‐detachable potassium ions pillar sustains the layered structure. Consequently, 1T"' MoS2 achieves a reversible capacity of 125 mA h g−1 at 0.2 C and keeps nearly 100% capacity retention at 1 C over 500 cycles. In situ characterizations and density functional theory simulations reveal the in‐depth intercalation reaction accompanied by the MoS2 phase transformation between 1T"' and 1T' during cycling. Furthermore, a 1T"' MoS2//MCMB K‐dual ion battery displays a superior cycling lifespan with 98% capacity retention over 250 cycles. This study provides a new intercalation anode and contribute to the electrode design for stable potassium ion storage. [ABSTRACT FROM AUTHOR]

Delving into the influence of strain on organic reactions in small molecules at the molecular level can unveil valuable insight into developing innovative synthetic strategies and structuring molecules with superior properties. Herein, we present a molecular‐strain engineering approach to facilitate the consecutive [1,2]‐aryl shift (formal [1,3]‐aryl shift) in molecular bows (MBs) that integrate 1,4‐dimethoxy‐2,5‐cyclohexadiene moieties. By introducing ring strain into MBs through tethering the bow limb, we can harness the intrinsic mechanical forces to drive multistep aryl shifts from the para‐ to the meta‐ to the ortho‐position. Through the use of precise intramolecular strain, the seemingly impractical [1,3]‐aryl shift was realized, resulting in the formation of ortho‐disubstituted products. The solvent and temperature play a crucial role in the occurrence of the [1,3]‐aryl shift. The free energy calculations with inclusion of solvation support a feasible mechanism, which entails multistep carbocation rearrangements, for the formal [1,3]‐aryl shift. By exploring the application of molecular strain in synthetic chemistry, this research offers a promising direction for developing new tools and strategies towards precision organic synthesis. [ABSTRACT FROM AUTHOR]

We show that a flat two dimensional network of connected vertices, when stretched, may deform plastically by producing "pleats", system spanning linear structures with width comparable to the lattice spacing, where the network overlaps on itself. To understand the pleating process, we introduce an external field that couples to local non-affine displacements, i.e., those displacements of neighbouring vertices that cannot be represented as a local affine strain. We obtain both zero and finite temperature phase diagrams in the strain–field plane. Pleats occur here as a result of an equilibrium first-order transition from the homogeneous network to a heterogeneous phase where stress is localised within pleats and eliminated elsewhere. We show that in the thermodynamic limit, the un-pleated state is always metastable at vanishing field for infinitesimal strain. Plastic deformation of the initially homogeneous network is akin to the decay of a metastable phase via a dynamical transition. We make predictions concerning local stress distributions and thermal effects associated with pleats which may be observable in suitable experimental systems. [ABSTRACT FROM AUTHOR]

Sodium‐ion oxide cathodes with triphase heterostructures have attracted intensive attention, since the sodium‐storage performance can be enhanced by utilizing the synergistic effect of different phases. However, the composite structures generally suffer from multiple irreversible phase transitions and high lattice strain because of interlayer‐gliding during the charge/discharge process. Here, the concept of strain engineering via manipulating the local chemistry of heterostructured oxide cathode is proposed to regulate the relevant physical and chemical properties, resulting in highly reversible structural evolution (P2/P3/spinel → P2/P3″/spinel) and low intrinsic stress in the potential window of 1.5–4.0 V. Also, the simple structural evolution at a relatively high cut‐off potential of 4.3 V can be detected by in situ X‐ray diffraction and other electrochemical characterization techniques during Na+ extraction/insertion. Meanwhile, the electrode exhibits a high reversible capacity (169.4 mAh g−1 at 0.2 C) and excellent rate performance from 1.5 to 4.3 V. Overall, this study reveals the mechanisms of regulating local chemistry to realize strain engineering of the cathode materials and paves the way for the further improvement of high‐performance sodium‐ion batteries. [ABSTRACT FROM AUTHOR]

Nanostructures with specific crystallographic planes display distinctive physico-chemical properties because of their unique atomic arrangements, resulting in widespread applications in catalysis, energy conversion or sensing. Understanding strain dynamics and their relationship with crystallographic facets have been largely unexplored. Here, we reveal in situ, in three-dimensions and at the nanoscale, the volume, surface and interface strain evolution of single supported platinum nanocrystals during reaction using coherent x-ray diffractive imaging. Interestingly, identical {hkl} facets show equivalent catalytic response during non-stoichiometric cycles. Periodic strain variations are rationalised in terms of O2 adsorption or desorption during O2 exposure or CO oxidation under reducing conditions, respectively. During stoichiometric CO oxidation, the strain evolution is, however, no longer facet dependent. Large strain variations are observed in localised areas, in particular in the vicinity of the substrate/particle interface, suggesting a significant influence of the substrate on the reactivity. These findings will improve the understanding of dynamic properties in catalysis and related fields. Understanding strain dynamics and their relationship with crystallographic facets have been largely unexplored. Here the authors demonstrate how the 3D lattice displacement and strain evolution depend on the crystallographic facets of Pt nanoparticles during CO oxidation reaction, providing new insights in the relationship between facet-related surface strain and chemistry. [ABSTRACT FROM AUTHOR]

The optical emission of germanium-based luminescent and/or laser devices can be enhanced by tensile strain and n-type doping. In this work, we study by simulation the interplay between electrical transport and optical gain in highly n-doped and intrinsic germanium p-n heterostructure diodes under tensile strain. The effects of strain and doping on carrier mobilities and energy distribution are taken into account. Whereas the n-doping of Ge enhances the filling of the indirect L and Brillouin zone-center conduction band states, the n-doping also reduces the carrier injection efficiency, which is detrimental for the achievement of optical gain at reduced current densities. For applied biaxial strains larger than 1.25%, i.e., far before reaching the cross-over from indirect to direct band gap regime, undoped germanium exhibits a lower optical gain threshold as compared to doped germanium. We also show that the threshold current needed to reach transparency in germanium heterostructures has been significantly underestimated in the previous works. [ABSTRACT FROM AUTHOR]

We theoretically investigate the impact of uniaxial strain in extremely thin Si and Ge p-type doublegate transistors. Quantum transport modeling is treated using a 6-band k.p Hamiltonian and the nonequilibrium Green's function formalism including phonon scattering. Based on this framework, we analyze the influence of strain on current characteristics considering different transport directions and gate lengths. Our results first confirm the superiority of Ge over Si in long devices (15nm gate length) for which best electrical performances are obtained considering channels along 〈110〉 with a uni-axial compressive strain. For this configuration, Si devices suffer from inter-subband coupling which generates a strong hole-phonon scattering. Material dominance is reversed for shorter devices (7nm gate length) where the small effective masses of Ge deteriorate the off-regime of the nanotransistor regardless of strain and crystallographic options. Due to weaker hole-phonon-scattering, 〈100〉-Si devices with a tensile strain are interestingly found to be more competitive than their 〈110〉-compressive counterparts. These results show that Si is still the most relevant material to reach the ultimate nanometer scale. More importantly, the same tensile strain can be considered to boost performances of both p- and n-type planar transistors which would lead to a significant simplification of the technological strain manufacturing. [ABSTRACT FROM AUTHOR]

The adsorption, diffusion, and molecular dissociation of hydrogen on the biaxially strained Mg (0001) surface have been systematically investigated by the first principle calculations based on density functional theory. When the strain changes from the compressive to tensile state, the adsorption energy of H atom linearly increases while its diffusion barrier linearly decreases oppositely. The dissociation barrier of H2 molecule linearly reduces in the tensile strain region. Through the chemical bonding analysis including the charge density difference, the projected density of states and the Mulliken population, the mechanism of the strain effect on the adsorption of H atom and the dissociation of H2 molecule has been elucidated by an s-p charge transfer model. With the reduction of the orbital overlap between the surface Mg atoms upon the lattice expansion, the charge transfers from p to s states of Mg atoms, which enhances the hybridization of H s and Mg s orbitals. Therefore, the bonding interaction of H with Mg surface is strengthened and then the atomic diffusion and molecular dissociation barriers of hydrogen decrease accordingly. Our works will be helpful to understand and to estimate the influence of the lattice deformation on the performance of Mg-containing hydrogen storage materials. [ABSTRACT FROM AUTHOR]

An analysis of the combustion process along gradient trajectories is presented for a set of Direct Numerical Simulations of turbulent premixed jet flames at different Reynolds numbers and approximately constant Karlovitz numbers. The variation of the jet Reynolds number, ranging from 5600 to 22400, is achieved by increasing the width of the jet and keeping the bulk velocity constant, which also implies approximately constant values of the turbulence intensities across the flames. The flames considered are nominally in the thin-reaction zone regime of the Borghi-Peters diagram of turbulent combustion. The thickening of the inner reaction layer and its enhancement with increasing Reynolds number, observed in previous works on the same database, is linked here to the presence of extremal points in the temperature field (local maxima in the low temperature side and minima in the high temperature side) in the vicinity or inside the inner reaction layer. Therefore, an enhancement of the turbulent flame speed is linked to the interruption of the flamelet structure by turbulence and not by the thickening of the entire flamelet itself. In addition to the expected large thickening of the formaldehyde layer, the layers of species which are characteristic of the inner reaction zone, such as OH and O, show a significant thickening and these species are observed several flame thicknesses ahead of the turbulent flame surface. Finally, it is shown that, in the regions where the inner reactive layer is thickened, the local fuel consumption rate is reduced in comparison to a laminar planar flame, but the total burning rate integrated over the entire flame structure is larger due to the increased volume of the reactive layer. These observations highlight the importance of assessing the balance between two competing phenomena related to high Karlovitz numbers. The first is the effect of turbulence strain and stirring on the chemistry, which usually decreases the local burning rate. The second is the effect on the species fields, which might experience modifications in their topology, such as the introduction of local extremal points, and significant thickening of the volumes between their isosurfaces. [ABSTRACT FROM AUTHOR]

Materials' mechanical properties highly depend on their internal structures. Designing novel structure is an effective route to improve materials' performance. One-dimensional disordered (ODD) structure is a kind of particular structure in silicon carbide (SiC), which highly affects its mechanical properties. Herein, we show that SiC nanowires (NWs) containing ODD structure (with an occupation ratio of 32.6%) exhibit ultrahigh tensile strength and elastic strain, which are up to 13.7 GPa and 12% respectively, approaching the ideal theoretical limit. The ODD structural occupation ratio effect on mechanical properties of SiC NWs has been systematically studied and a saddle shaped tendency for the strength versus occupation ratio is firstly revealed. The strength increases with the increase of the ODD occupation ratio but decreases when the occupation ratio exceeds a critical value of ~ 32.6%, micro twins appear in the ODD region when the ODD segment increases and soften the ODD segment, finally results in a decrease of the strength. [ABSTRACT FROM AUTHOR]

The β grain growth behavior of two newly developed titanium alloys with composition Ti-10V-1Fe-3Al and Ti-10V-2Cr-3Al (wt.%) isothermally annealed at 900°C for a different time was studied, and its effect on the triggering stress of stress-induced martensitic (SIM) transformation and related mechanical properties was evaluated. The results show that the grain size increases with the increase of annealing time. The measured grain size of Ti-10V-1Fe- -3Al alloy is larger than that of Ti-10V-2Cr-3Al alloy under the same annealing conditions. The increase of the grain size leads to an increase of the triggering stress resulting in the martensitic transformation. The work hardening ability and compressive failure strain of the studied alloys decrease rapidly. [ABSTRACT FROM AUTHOR]

The synthesis of structurally complex and highly strained natural products provides unique challenges and unexpected opportunities for the development of new reactions and strategies. Herein, the synthesis of (+)‐[5]‐ladderanoic acid is reported. En route to the target, unusual and unexpected strain release driven transformations were uncovered. This occurrence required a drastic revision of the synthetic design that ultimately led to the development of a novel stepwise cyclobutane assembly by an allylboration/Zweifel olefination sequence. [ABSTRACT FROM AUTHOR]

In contrast to usual assumptions, it is shown that even when ferroelastic domain walls are inactive or absent, the motion of ferroelectrically active interfaces in ferroelectric materials contributes, at subcoercive electric fields, not only to the polarization but also to the strain. Specifically, in polycrystalline samples, strain coupling between adjacent grains, or mediated through the substrate in thin films, influences both the dielectric and piezoelectric response. The model developed explains the unexpected observation of piezoelectric nonlinearity in films even in cases in which the domain variants' projections are equivalent along the direction of the external driving field. [ABSTRACT FROM AUTHOR]

First-principles electronic structure methods are used to predict the mobility of n-type carrier scattering in strained SiGe. We consider the effects of strain on the electron-phonon deformation potentials and the alloy scattering parameters. We calculate the electron-phonon matrix elements and fit them up to second order in strain. We find, as expected, that the main effect of strain on mobility comes from the breaking of the degeneracy of the six Δ and L valleys, and the choice of transport direction. The non-linear effects on the electron-phonon coupling of the Δ valley due to shear strain are found to reduce the mobility of Si-like SiGe by 50% per % strain. We find increases in mobility between 2 and 11 times that of unstrained SiGe for certain fixed Ge compositions, which should enhance the thermoelectric figure of merit in the same order, and could be important for piezoresistive applications. [ABSTRACT FROM AUTHOR]

The effect of misfit strain and deadlayer on the polarization switching of epitaxial ferroelectric thin film is investigated using a phase field model. Simulation results show that a compressive misfit strain increases the coercive field and remanent polarization of ferroelectric thin film with deadlayers, whereas a tensile misfit strain decreases these factors. The presence of a deadlayer between the ferroelectric thin film and the electrode prevents charge compensation on the ferroelectric surface and reduces the coercive field and remanent polarization of the film. A periodic a/c/a/c multiple domain structure is found in ferroelectric thin film with tensile misfit strain when there is no deadlayer. However, when a deadlayer is present in the film, the c domains vanish and the out-of-plane component of polarization degrades. The degradation of the out-of-plane polarization makes ferroelectric thin films with tensile misfit strain and deadlayers lose their ferroelectric property in the thickness direction, which can be attributed to the combined effects of the deadlayer and tensile strain. The coercive field and remanent polarization of ferroelectric thin film decrease with the thickness of the deadlayer. [ABSTRACT FROM AUTHOR]

In a previous study by Asay et al. [J. Appl. Phys. 106, 073515 (2009)], the inelastic response of annealed and cold-rolled pure polycrystalline tantalum at intermediate strain rates was characterized with ramp wave loading to peak longitudinal stresses of 17 GPa. It was found that the annealed Ta at strain rates of about 106/s exhibited pronounced elastic overshoot, followed by rapid stress relaxation and the amplitude of the elastic precursor depicted essentially no dependence on sample thickness for samples with controlled initial properties, in contrast to the precursor attenuation typically observed in shock wave experiments. The precursor for the cold-rolled sample was more dispersive and did not exhibit the characteristics depicted by the annealed samples. A principal objective of the present study was to gain some insights into this behavior and its implication on the deformation mechanisms for tantalum. Another objective was to gain a fundamental understanding of the dynamic inelasticity of polycrystalline tantalum, its evolution with the processing history, and the resultant thermomechanical behavior. The approach used to achieve these objectives was to first develop a material model that captured the observed material characteristics and then to use numerical simulations of dynamic experiments to gain additional insights into the observed material behavior. The constitutive model developed is based on the concept of dislocation generation and motion. Despite its simplicity, the model works quite well for both sets of data and serves a valuable tool to achieve the research objectives. The tantalum studied here essentially exhibits a strong rate sensitivity and this behavior is modeled through the low dislocation density and the strong stress dependence of the dislocation velocity. For the annealed material, the mobile dislocation density is assumed to be essentially zero in the model. This low dislocation density combined with strong stress dependence of dislocation velocity results in a metastable elastic response and a precursor that shows little attenuation. The increase of mobile dislocations through the cold-rolling process leads to a less rate-sensitive behavior for the cold-rolled tantalum and also the disappearance of the precursor behavior observed for the annealed samples. Both the low dislocation density and the strong rate dependence of the dislocation velocity may be related to the low mobility of the screw dislocations in bcc metals. This low mobility results from its extended, three-dimensional core structure. [ABSTRACT FROM AUTHOR]

Electric field induced strain is one of the most important performance parameters for multilayer ferroelectric actuators. In these devices, donor doped morphotropic lead zirconate titanate (PZT) ceramics are the materials currently used. Strain and, more general, electromechanical properties of these piezoceramics are grain size dependent. Any attempts to decrease the sintering temperature will suffer from the drawback, that changing Ts will also change the grain size and therefore the electromechanical properties. A series of NbSr-doped PZT materials with 11 different Zr/Ti ratios over the whole range of the morphotropic phase boundary has been prepared, and sintered at temperatures between 975 and 1100 °C. Grain size and x-ray diffraction patterns were evaluated and the strain behavior was measured and analyzed. Qualitative similar effects of grain size on strain behavior were found for all morphotropic PZT 1Nb2Sr compositions: When lowering the sintering temperature, field induced strain decreases whereas the remanent strain increases. Quantitative differences between the materials indicate that, in addition to the influence from the grain size, there are effects specific for the compositions, which are most probably related to changes in phase composition. From the inverse behavior of remanent strain and field induced strain versus grain size, on a macroscopic level, a description of the mechanisms which govern the effects of grain size on strain is derived. [ABSTRACT FROM AUTHOR]

Prior studies have shown that a field-induced ferroelectricity in ceramics with general chemical formula (1-x-y)(Bi1/2Na1/2)TiO3–xBaTiO3–y(K0.5Na0.5)NbO3 and a very low remanent strain can produce very large piezoelectric strains. Here we show that both the longitudinal and transverse strains gradually change with applied electric fields even during the transition from the nonferroelectric to the ferroelectric state, in contrast to known Pb-containing antiferroelectrics. Hence, the volume change and, in turn, the phase transition can be affected using uniaxial compressive stresses, and the effect on ferroelectricity can thus be assessed. It is found that the 0.94(Bi1/2Na1/2)TiO3–0.05BaTiO3–0.01(K0.5Na0.5)NbO3 ceramic (largely ferroelectric), with a rhombohedral R3c symmetry, displays large ferroelectric domains, significant ferroelastic deformation, and large remanent electrical polarizations even at a 250 MPa compressive stress. In comparison, the 0.91(Bi1/2Na1/2)TiO3–0.07BaTiO3–0.02(K0.5Na0.5)NbO3 ceramic (largely nonferroelectric) possesses characteristics of a relaxor ferroelectric ceramic, including a pseudocubic structure, limited ferroelastic deformation, and low remanent polarization. The results are discussed with respect of the proposed antiferroelectric nature of the nonferroelectric state. [ABSTRACT FROM AUTHOR]

Electronic structures of two-dimensional (2D) hexagonal boron nitride (h-BN) with different planar strain distributions have been studied using the first principles methods. We found that the 2D h-BN without strain has a large direct bandgap and its bandgap structure strongly depends on the strength and direction of the strain. The bandgap width can be reduced significantly under both symmetrical and asymmetrical strain distributions. Moreover, the bandgap feature exhibits strong anisotropic behaviors. The bandgap remains direct under large symmetrical tensile strain or asymmetrical tensile strain perpendicular to B–N bonds. However, a small amount of symmetrical compressive strain larger than 1.5% or asymmetrical tensile strain parallel to B–N bonds larger than 1.2% turns the direct bandgap into indirect. Our results indicate that optical and electronic transport properties of 2D h-BN can be effectively tuned by applying different planar forces, offering a unique route for designing nanoscale tunable ultrathin optoelectronic devices only one atomic layer thick. [ABSTRACT FROM AUTHOR]

The mechanical behavior of Zr41.25Ti13.75Cu12.5Ni10Be22.5 (LM-1) has been extensively characterized under quasistatic loading conditions; however, its mechanical behavior under dynamic loading conditions is currently not well understood. A Split–Hopkinson pressure bar (SHPB) and a single-stage gas gun are employed to characterize the mechanical behavior of LM-1 in the strain-rate regime of 102–105/s. The SHPB experiments are conducted with a tapered insert design to mitigate the effects of stress concentrations and preferential failure at the specimen-insert interface. The higher strain-rate plate-impact compression-and-shear experiments are conducted by impacting a thick tungsten carbide (WC) flyer plate with a sandwich sample comprising a thin bulk metallic glass specimen between two thicker WC target plates. Specimens employed in the SHPB experiments failed in the gage-section at a peak stress of approximately 1.8 GPa. Specimens in the high strain-rate plate-impact experiments exhibited a flow stress in shear of approximately 0.9 GPa, regardless of the shear strain-rate. The flow stress under the plate-impact conditions was converted to an equivalent flow stress under uniaxial compression by assuming a von Mises–like material behavior and accounting for the plane strain conditions. The results of these experiments, when compared to the previous work conducted at quasistatic loading rates, indicate that the peak stress of LM-1 is essentially strain rate independent over the strain-rate range up to 105/s. [ABSTRACT FROM AUTHOR]

In situ measurements of growth strains and creep relaxation in α-Al2O3 films, isothermally grown on β-NiAl alloys at 1100 °C, are reported and analyzed. Samples containing the reactive element Zr, and Zr-free samples, are examined. For Zr-free samples, steady state growth strains are compressive, whereas the growth strains are tensile when the reactive element (RE) is added to the alloy. This behavior is attributed to the counterflow of oxygen and aluminum interstitials, and to simultaneous counterflow of oxygen and aluminum vacancies, all moving through the grain boundaries. Cross diffusing oxygen and aluminum interstitials may merge and combine within the film, forming new oxide along grain boundary walls, a mechanism that leads to an in-plane compressive stress. Cross diffusing oxygen and aluminum vacancies will also merge and combine within the film; in this case material is removed from grain boundary walls, a mechanism that leads to an in-plane tensile stress. When no RE is present, the interstitial mechanism dominates and the resultant stress is compressive. Consistent with the “dynamic segregation model,” the RE slows the outdiffusion of Al interstitials permitting the tensile mechanism to dominate. This interpretation invokes the unconventional view that oxygen and aluminum interstitials and vacancies, created in and driven by the strong chemical gradient, all participate meaningfully in the scale growth process. Grain boundary diffusion measurements were obtained from low stress creep data, interpreted using the Coble model of grain boundary diffusion. Reported diffusion measurements of oxygen through grain boundaries of α-Al2O3, which are known to be inconsistent with oxide scale growth, are critically examined. A simple picture, a “balanced defect model,” emerges that is consistent with the dynamic segregation model, observed growth stresses and their dependence on the presence of a reactive element, sequential oxidation experiments, and our best knowledge about grain boundary diffusion coefficients. [ABSTRACT FROM AUTHOR]

The potential of Si1-xGex source and drain strained silicon p-channel transistors is investigated for ultrathin body silicon-on-insulator (SOI) substrates. We used process simulations to quantify and understand the role of device design parameters such as raised source and drain, source and drain length, and recess depth in the context of SOI transistors. Simultaneous strain calculations were performed with the process flow to capture not only lattice mismatch, but all process induced strain. Germanium condensation technique was found suitable for enhancing strain in ultrathin body transistors by increasing germanium concentration and driving germanium deeper into the source and drain. The scalability of germanium condensation process is discussed. [ABSTRACT FROM AUTHOR]

Multiple cracking of brittle films deposited on the relatively soft substrate was commonly used in different applications. In the present paper, multiple film cracking in the film/substrate systems was analyzed. The system was subjected to the combination of the residual stresses due to the mismatch strain between the film and substrate and the applied stress due to the unidirectional strain on the substrate. First, a simply analytical model was developed to derive the closed-form solutions for the residual stress distribution and redistribution due to the edge effects in the film segment. Second, the energy and strength criteria were adopted to perform the analysis on the multiple film cracking on the basis of the knowledge on the residual stress distributions. Third, the case of SiOx film/polyethylene terephthalate substrate system was used to illustrate the implementation of this model. The relationship between the fracture energy as well as the film strength and the film thickness was obtained. The predicted crack density against the applied strain was compared with the existing experimental measurements. The effect of the mismatch strain on the crack density of the film was also discussed. [ABSTRACT FROM AUTHOR]

We report a first-principles study on the electronic structures of deformed graphene nanoribbons (GNRs). Our theoretical results show that the electronic properties of zigzag GNRs are not sensitive to uniaxial strain, while the energy gap modification of armchair GNRs (AGNRs) as a function of uniaxial strain displays a nonmonotonic relationship with a zigzag pattern. The subband spacings and spatial distributions of the AGNRs can be tuned by applying an external strain. Scanning tunneling microscopy dI/dV maps can be used to characterize the nature of the strain states, compressive or tensile, of AGNRs. In addition, we find that the nearest neighbor hopping integrals between π-orbitals of carbon atoms are responsible for energy gap modification under uniaxial strain based on our tight binding approximation simulations. [ABSTRACT FROM AUTHOR]

This paper experimentally and theoretically studies the magneto-electric (M-E) responses of layered magnetostrictive and piezoelectric composites. A total of three different piezoelectric volume fractions of 0.17, 0.29, and 0.44 were prepared to investigate the strain variation in the sample and the effective M-E voltage coefficient [formula] as a function of bias magnetic field Hbias. Analytical modeling involving uniform strain, shear lag only, and shear lag plus demagnetization was performed and compared with experimental measurements. The results demonstrate that the uniform strain theory significantly overestimates the measured strain and α values. This discrepancy is physically attributed to shear lag and demagnetizing effects. The analytical models involving shear lag and demagnetizing effects agree with the experimental results well within 5%. [ABSTRACT FROM AUTHOR]

We describe a method of applying large persistent biaxial in-plane tensile stress to thin films by loading a Ta substrate with hydrogen. Using this technique, we studied the influence of biaxial stress on the effective perpendicular magnetic anisotropy (Keff) of Ni and Co50Fe50 thin films. The Keff of Ni increases by 40% whereas for Co50Fe50 it decreases by 15% with respect to that of zero stress state upon applying biaxial stresses of 1.24 and 1.79 GPa, respectively. The results are analyzed using a simple model including shape and magnetoelastic anisotropy contributions. The influence of the saturation magnetization and the mechanical properties of the thin films on the maximum achievable relative change of Keff is discussed. [ABSTRACT FROM AUTHOR]

Evolution of photoluminescence (PL) and strain is investigated in GaIn(N)As/Ga(N)As multiple quantum wells (MQWs) over a wide range of annealing temperatures from 700 to 900 °C. We observe two optimal annealing temperatures (Topt) that result in reduced PL linewidth and increased PL intensity irrespective of the thermal-induced strain relaxation. The decrease of PL intensity accompanied by peak splitting after the first Topt is mainly associated with the deterioration of the GaAs cap layer and the optical quenching after the second Topt is due to the overannealing-induced degradation of the bottom QWs. The strain relaxation in GaInAs/GaAs MQWs at elevated temperatures, which gives rise to extrinsic defects at the bottom of the MQWs stack, only plays a minor role in PL evolution, while no strain relaxation is observed in GaInNAs/GaNAs MQWs. [ABSTRACT FROM AUTHOR]

The shape of the self-assembled GeSi/Si(001) islands changed from a dome to a pyramid bounded with {103} or {105} facets after initial Si capping at 640 °C. The strains in the islands with initial Si capping are investigated by Raman spectroscopy. Compared with those of the uncapped islands, both peaks of Ge-Ge and Ge-Si vibration modes in the capped islands show blueshifts, corresponding to the Ge content decrease and the compressive strain increase in the capped islands. The total strain energy in an island is found to increase remarkably after Si capping. After simple analysis, it is found that the surface energy change could not overwhelm this large strain energy increase, making the shape transition favorable. It implies that the strain energy in the substrate in association with an island formation as well as evolution should be considered in accounting for the resulting island shape changes after Si capping. [ABSTRACT FROM AUTHOR]

Ion channeling technique using MeV C++ ions was used to study strain in self-assembled InAs quantum dots (QDs) buried in GaAs matrix. Because of the use of heavy ions, we were able to observe an angular shift in the angular scan of the In signal relative to that of the Ga/As signal. This provided a direct evidence that the InAs lattice is larger than that of GaAs in the growth direction. Combining the channeling results in [100] and [110] directions and the photoluminescence emission spectrum, we conclude that the InAs QDs are under tensile strain in the growth direction and have the same lattice constant as that of GaAs in the lateral direction. Thermal annealing causes the strain to relax, first in the growth direction and then in the lateral direction as the annealing temperature increases. The photoluminescence spectra of the QDs before and after annealing indicate, however, that composition intermixing also takes place during annealing and is the dominant factor in determining the band gap energy of the QDs. [ABSTRACT FROM AUTHOR]

Oxide films of BaTiO3 and MgO were deposited on SrTiO3 single crystal substrates by laser molecular beam epitaxy. Reflection high-energy electron diffraction was performed in situ to investigate the change of growth mode and the lattice relaxation. Due to the different mismatches between epitaxial oxide films and substrates, two kinds of strain relaxation behavior during the growth process can be observed. For the BaTiO3/SrTiO3 system with the small mismatch of 2.18%, the coherent epitaxial growth can be maintained within the critical thickness. The experimental data were well in accord with the elastic strain theory in the small mismatch system. The coherent epitaxial growth is crucial to fabricate the functional oxide superlattice. However, strained island can form at the initial period to release strain energy in the case of MgO/SrTiO3 system with the large mismatch of 7.8%. It provides a useful practical method to realize self-organized nanostructures by introducing the moderate strain at interface. Our systematic study on the interface strain of functional oxide heteroepitaxial growth could give an instructive method to realize different heterostructures, e.g., superlattice and nanostructures. [ABSTRACT FROM AUTHOR]

We present a systematic study of the effect of strain (equivalent to uniform pressure) on the thermal conductivity of an insulating solid. Following a theoretical analysis that uncovers the dependence of the thermal conductivity on temperature and strain, we present classical molecular dynamics calculations of the thermal conductivity. We find that the molecular dynamics results closely match the theoretical result. [ABSTRACT FROM AUTHOR]

Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure (∼10 GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostatic pressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder suppresses PT which compensates the promotion effect of plastic shear. The existence of transformation-induced plasticity (TRIP) was also proved during strain-induced PT. The degree of disorder is proposed to be used as a physical measure of plastic straining. This allows us to quantitatively separate the conventional plasticity and transformation-induced plasticity. Surprisingly, it is found that TRIP exceeds the conventional plasticity by a factor of 20. The cascade structural changes were revealed, defined as the reoccurrence of interacting processes including PTs, disordering, conventional plasticity, and TRIP. In comparison with hydrostatic loading, for the same degree of disorder, plastic shear indeed reduces the PT pressure (by a factor of 3–4) while causing a complete irreversible PT. The analytical results based on coupled strain-controlled kinetic equations for disorder and PT confirm our conclusions. [ABSTRACT FROM AUTHOR]

Since hydration forces become very strong at short range and are particularly important for determining the magnitude of the adhesion between two surfaces or interaction energy, the influences of the hydration force and elastic strain energy due to hydration-induced layering of liquid molecules close to a solid film surface on the stability of a solid film in a solid-on-liquid (SOL) nanostructure are studied in this paper. The liquid of this thin SOL structure is a kind of water solution. Since the surface forces play an important role in the structure, the total free energy change of SOL structures consists of the changes in the bulk elastic energy within the solid film, the surface energy at the solid–liquid interface and the solid–air interface, and highly nonlinear volumetric component associated with interfacial forces. The critical wavelength of one-dimensional undulation, the critical thickness of the solid film, and the critical thickness of the liquid layer are studied, and the stability regions of the solid film have been determined. Emphasis is placed on calculation of critical values, which are the basis of analyzing the stability of the very thin solid film. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A mechanics-based model is developed to examine the effects of clamping during wafer bonding processes. The model provides closed-form expressions that relate the initial geometry and elastic properties of the wafers to the final shape of the bonded pair and the strain energy release rate at the interface for two different clamping configurations. The results demonstrate that the curvature of bonded pairs may be controlled through the use of specific clamping arrangements during the bonding process. Furthermore, it is demonstrated that the strain energy release rate depends on the clamping configuration and that using applied loads usually leads to an undesirable increase in the strain energy release rate. The results are discussed in detail and implications for process development and bonding tool design are highlighted. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The strain relaxation due to cracks of different depths in AlGaN layers grown on GaN template layers has been investigated using spatially resolved cathodoluminescence spectroscopy, high-resolution x-ray diffraction and two-dimensional finite element simulations. The experimental data consistently show that the relief of tensile stress increases with decreasing crack spacing. The measured strain profiles between the cracks are well described by the theoretical calculations for small crack spacings; whereas, deviations for larger crack spacings have been found. The latter is discussed in terms of inelastic strain relaxation mechanisms, the reliability of the deformation potential for AlGaN employed in this article, and the spatial variations in the properties of the AlGaN, e.g., its composition. [ABSTRACT FROM AUTHOR]

High-quality strain-relaxed SiGe templates with a low threading dislocation density and smooth surface are critical for device performance. In this work, SiGe films on low temperature Si buffer layers were grown by solid-source molecular beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, photoluminescence spectroscopy, and Raman spectroscopy. Effects of the growth temperature and the th density as well as smooth surface was obtained by this method. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study of how the strain relaxation of the multilayer grating evolves toward the free surface and how the local strain distribution is modulated due to the compositional profile of the layered setup. Experimental details; Fit of the experimental results; Conclusions.

Presents information on a study which described the strain relaxation in large wafer diameter, that includes heat transfer effects and the upper yield point of the silicon material. Force balance model for strain relaxation in silicon wafers; Maximum allowable ramp rate; Simulation of thermal batch processing for large silicon wafer diameters; Conclusions.

Proposes segment connectivity, chain-length breathing, segmental stretch and constraint release in reptation models for analyzing scaling laws of polymer chain gels. Description of the viscoelastic behavior of entangled polymer chains; Theory of self-consistent reptation; Comparison on simulation and experimental observations in single step strain flows.

Interprets the manner of entropy change at the ice Ih-ice XI transformation in terms of an equilibrium involving the strain energy. Calculations of strain energy using measure entropy; Thermodynamics of phase transformation; Factors affecting the strain at the ice Ih-ice XI transformation.

Focuses on a study which developed a numerical model for the heat transfer and stress-strain analysis of laser heating of thin films coated on a glass substrate. Dependency of chromium (Cr) film fracture and peeling off on the temperature and the thermally induced stress strain; Surface morphology of Cr films; Temperature and thermal stress strain; Conclusion.

Presents a study on the effects of misfit strain on the microstructure and properties of ferroelectric lead zirconate titanate thin films. Experimental details; Results and discussion; Conclusions.

Presents information on a study which calculated the effects of strain and alloying on the Hall scattering factor for holes in silicon using a Monte Carlo method and a method based on relaxation time approximation. Model and calculation; Hall scattering factor in strained silicon; Conclusions.

A two‐step, one‐flask synthesis of central seven‐membered borondifluoride‐3,3‐dimethyl‐2‐[2‐(2‐pyrrolyl)ethenyl] indole (BOPYIN) ligands has been developed by using the unexplored 3,3‐dimethyl‐2‐[2‐(2‐pyrrolyl)ethenyl] indole. The simple synthetic approach has enabled modification of the electronic structure by changing the substituents on the indole unit. X‐ray analysis indicated that conformations of the seven‐membered BF2 complex including BOPYIN and diazaborepin differ from that of the five‐ and six‐membered organoboron complexes. Interestingly, the bond angle of the N⋅⋅⋅B−N bond increases with the number of atoms in the core ring, based on Baeyer strain theory. These unsymmetric BOPYIN derivatives have excellent photophysical properties, including high fluorescence quantum yields, except for BOPYIN‐4 in the solution state, large Stokes shifts, and good molar absorptivity. The dipole moment of BOPYIN‐3 in the first excited singlet state and ground state was demonstrated by a linear Lippert–Mataga plot. The absorption and emission spectra were not mirror images for BOPYIN‐1–3 and 5, in contradiction to Kasha's rule, as determined by TDDFT. The synthesized BOPYINs have been shown to be biocompatible fluorophores in cell bioimaging. [ABSTRACT FROM AUTHOR]