Large Compressive Strain Research Articles

Poro-Viscoelastic Effects During Biomechanical Testing of Human Brain Tissue

Brain tissue is one of the softest tissues in the human body and the quantification of its mechanical properties has challenged scientists over the past decades. Associated experimental results in the literature have been contradictory as characterizing the mechanical response of brain tissue not only requires well-designed experimental setups that can record the ultrasoft response, but also appropriate approaches to analyze the corresponding data. Due to the extreme complexity of brain tissue behavior, nonlinear continuum mechanics has proven an expedient tool to analyze testing data and predict the mechanical response using a combination of hyper-, visco-, or poro-elastic models. Such models can not only allow for personalized predictions through finite element simulations, but also help to comprehensively understand the physical mechanisms underlying the tissue response. Here, we use a nonlinear poro-viscoelastic computational model to evaluate the effect of different intrinsic material properties (permeability, shear moduli, nonlinearity, viscosity) on the tissue response during different quasi-static biomechanical measurements, i.e., large-strain compression and tension as well as indentation experiments. We show that not only the permeability but also the properties of the viscoelastic solid largely control the fluid flow within and out of the sample. This reveals the close coupling between viscous and porous effects in brain tissue behavior. Strikingly, our simulations can explain why indentation experiments yield that white matter tissue in the human brain is stiffer than gray matter, while large-strain compression experiments show the opposite trend. These observations can be attributed to different experimental loading and boundary conditions as well as assumptions made during data analysis. The present study provides an important step to better understand experimental data previously published in the literature and can help to improve experimental setups and data analysis for biomechanical testing of brain tissue in the future.

Multifunctional Superelastic Cellulose Nanofibrils Aerogel by Dual Ice‐Templating Assembly

AbstractA superelastic aerogel with fast shape recovery performance from large compressive strain is highly desired for numerous applications such as thermal insulation in clothing, high‐sensitive sensors, and oil contaminant removal. Fabrication of superelastic cellulose nanofibrils (CNF) aerogels is challenging as the CNF can assemble into non‐elastic sheet‐like cell walls. Here, a dual ice‐templating assembly (DITA) strategy is proposed that can control the assembly of CNF into sub‐micrometer fibers by extremely low temperature freezing (–196 °C), which can further assemble into an elastic aerogel with interconnected sub‐micron fibers by freezer freezing (−20 °C) and freeze drying. The CNF aerogel from the DITA process demonstrates isotropic superelastic behavior that can recover from over 80% compressive strain along both longitudinal and cross‐sectional directions, even in an extremely cold liquid nitrogen environment. The elastic CNF aerogel can be easily modified by chemical vapor deposition of organosilane, demonstrating superhydrophobicity (164° water contact angle), high liquid absorption (489 g g−1 of chloroform absorption capacity), self‐cleaning, thermal insulating (0.023 W (mK)−1), and infrared shielding properties. This new DITA strategy provides a facile design of superelastic aerogels from bio‐based nanomaterials, and the derived high performance multifunctional elastic aerogel is expected to be useful for a wide‐range of applications.

Strain-induced structural phase transition in GeN monolayer

The recent synthesis of SiP, SiAs, GeP, and GeAs monolayers has brought two-dimensional (2D) group IV–V systems into the limelight. To date, all the fabricated structures of this class belong to the C2/m space group which has a low structural symmetry, while the class could exist in more symmetric phases (i.e., P3¯m1 and P6¯m2). The realization of more symmetric phases can enhance the intrinsic properties of these materials and increase their potential field of usage. In this study, the possibility of a structural phase transition in GeN monolayer by application of mechanical strain is investigated. Based on ab initio simulations, we first confirm the stability of the GeN monolayer in all phases, then demonstrate how a large enough compressive strain (∼12%) can transform C2/m into P3¯m1 phase. The results are interpreted by analyzing the geometry, bond order, electron localization functions, and net atomic charges of the structures. Upon transition into the P3¯m1 phase, tensile strength and in-plane stiffness double, while the compressive strength quadruples. On the other hand, the effect of the phase transition on the electronic properties is not substantial and similar band structure profiles with narrowed band gap are obtained. Our study provides insight on how to experimentally achieve the P3¯m1 phase of the GeN monolayer, which is in principle applicable to other group IV–V monolayers under suitable conditions involving the optimization of pressure, temperature, and impurity concentration. These unique features of the GeN monolayer render them ideal candidates for a variety of high technological nanoscale applications.

Non-layer-wise fracture and deformation mechanism in beta titanium cubic lattice structure manufactured by selective laser melting

The layer-wise fracture is a common fracture in porous metal structures, where the whole layer with stress concentration fractures firstly, and then the adjacent layers fracture sequentially. The brittle layer-wise fracture has always been a major obstacle that restricts the development of metallic porous structures, which may induce catastrophic failure under occasional overloading. This work reports that the non-layer-wise fracture was achieved in a beta-type Ti–25Nb–3Zr–3Mo–2Sn (TLM) titanium alloy with 50% porosity cubic lattice structure manufactured by selective laser melting (SLM), mainly due to the stress-induced progressive martensitic transformation (β→α″) supplemented by deformation twinning, resulting in uniform deformation without cracks in the struts even under a large compression strain of ~50%. The in situ synchrotron high-energy X-ray diffraction (HE-XRD) was utilized to study the dynamic deformation behavior of a strut, revealing that the martensitic transformation initiates at the early stage of the compression with a strain of 1.47%, followed by progressive martensitic transformation upon further straining, which probably contributes to the “double-yielding” platform. TEM microstructural observations of the {112}<111>β twins in the strut implies that deformation twinning was activated as a supplementary deformation mechanism for the accommodation of large strain. The findings in this work provide an alternative strategy of introducing non-layer-wise fracture in porous metallic materials through the coupled effect of phase transformation and twinning.

Large Tunnel Electroresistance with Ultrathin Hf0.5Zr0.5O2 Ferroelectric Tunnel Barriers

AbstractHafnia‐based ferroelectric tunnel junctions (FTJs) hold great promise for nonvolatile memory and emerging data storage applications. In this article, a large tunnel electroresistance effect with ultrathin Hf0.5Zr0.5O2 (HZO) barrier based FTJs is reported. Robust ferroelectricity is achieved with ≈1 nm films by stabilizing the rhombohedral polar phase of HZO (R‐HZO) through a large compressive strain, induced by growing the film epitaxially on a SrTiO3 (001) substrate. The OFF/ON ratio of the junction resistance at zero bias is about 135 with ≈1 nm thick barrier, which increases to ≈105 with increasing the barrier thickness to ≈2.5 nm. The resistance‐area product (RA) of tunnel junctions is reduced by nearly three orders of magnitude by using an ≈1 nm R‐HZO barrier as compared with typically reported RA values for doped‐HfO2 barrier based FTJs, which significantly improves signal‐to‐noise ratio during the read operation. These results set the stage for further exploration of Hafnia‐based FTJs for non‐volatile memory applications.

Effect of biaxial strain and hydrostatic pressure on the magnetic properties of bilayer CrI3

Two-dimensional van der Waals magnetic materials are intriguing for applications in the future spintronics devices, so it is crucial to explore strategy to control the magnetic properties. Here, we carried out first-principles calculations and Monte Carlo simulations to investigate the effect of biaxial strain and hydrostatic pressure on the magnetic properties of the bilayer CrI3. We found that the magnetic anisotropy, intralayer and interlayer exchange interactions, and Curie temperature can be tuned by biaxial strain and hydrostatic pressure. Large compressive biaxial strain may induce a ferromagnetic-to-antiferromagnetic transition of both CrI3 layers. The hydrostatic pressure could enhance the intralayer exchange interaction significantly and hence largely boost the Curie temperature. The effect of the biaxial strain and hydrostatic pressure revealed in the bilayer CrI3 may be generalized to other two-dimensional magnetic materials.

Coexisting morphotropic phase boundary and giant strain gradient in BiFeO3 films

Large compressive strains may introduce giant tetragonality and morphotropic phase boundaries in perovskite BiFeO3 films, where the coexisting tetragonal and rhombohedral phases (T like and R like phases) are identified to exhibit large piezoelectric response. Here, we have further achieved mechanical bending deformations in R like BiFeO3 through its neighboring T like BiFeO3 phases, where a strain gradient of ∼106/m was identified. Aberration-corrected scanning transmission electron microscopy revealed not only the strain distributions but also the atomic scale Fe polar displacement in the gradient R like BiFeO3. In spite of the giant strain gradient, the polarization direction in each R like BiFeO3 unit cell was found mainly along its diagonal direction, suggesting that potential flexoelectric coupling induced polarization in BiFeO3 is smaller than its spontaneous polarizations, while a large built-in electric field can be obtained via the large strain gradient. Our results indicate that the common phase coexistences in oxide materials could be further manipulated to introduce elastic strain gradients and tune the properties for oxide films.

Harnessing Wrinkling Patterns Using Shape Memory Polymer Microparticles.

Shape memory polymers (SMPs) are the simplest and most attractive alternatives for soft substrates of typical bilayer wrinkle systems because of shape fixity and recovery capabilities. Herein, we have successfully programmed large compressive strains in chemical cross-linking shape memory polystyrene (PS) microparticles via nanoimprint lithography, which acted as the substrate of a wrinkle system using a gold nanoparticle (Au NP) film as the top layer. When triggered by two different stimuli (direct heating and toluene vapors), the thin Au NP film could transform into various wrinkle structures atop the recovered PS particles. In addition, we also investigated the evolution mechanisms of wrinkling by heating and toluene vapors and tuned the wrinkled surfaces through altering the Au NP thickness and stimulation methods (direct heating and toluene vapors), which utilized the structural adjustability of Au NPs to program the amplitude, wavelength, and morphology of the wrinkles. The concept presented here provides a cost-effective approach to realize the surface wrinkling and can be extended to other available SMPs.

Strain-Induced Tunable Band Offsets in Blue Phosphorus and WSe2 van der Waals Heterostructure

The electronic structure and band offsets of blue phosphorus/WSe2 van der Waals (vdW) heterostructure are investigated via performing first-principles calculations. Blue phosphorus/WSe2 vdW heterostructure exhibits modulation of bandgaps by the applied vertical compressive strain, and a large compressive strain of more than 23% leads to a semiconductor-to-metal transition. Blue phosphorus/WSe2 vdW heterostructure is demonstrated to have a type-II band alignment, which promotes the spontaneous spatial separation of photo-excited electrons and holes. Furthermore, electrons concentrating in BlueP and holes in WSe2 can be enhanced by applied compressive strain, resulting in an increase of carrier concentration. Therefore, these properties make blue phosphorus/WSe2 vdW heterostructure a good candidate for future applications in photodetection.

Investigating Effect of Strain and Geometric Defects on Single Polarization Vortex in PbTiO3 Nanowires

The single polarization vortex structure in nanowire can be used to store binary data in Non-Volatile Ferroelectric Random Access Memories (NVFRAM or FRAM). However, at the nanoscale, mechanical strains or geometry defects (cracks) can affect the polarization vortex and they are one of the reasons to reduce the service life as well as the reliability of the device. In this study, the atomic simulation method using the interactive potential function based on the core-shell model is selected to investigate the effects of strain, cracks and domain wall deviations (DW) on the single polarization vortex in PbTiO3 (PTO) nanowires. The results obtained showed that the polarization vortex can appear or disappear depending on the position and size of the crack. Deviations in the DW position make the polarization vortex change the size and shape. Besides, the magnitude of the vortex investigated increases under tension strain and decreases under compression strain. Especially, in large compression strain (10%), the vortex can be disappeared.

Characterization of the strain rate effect under uniaxial loading for nanoporous gold

While several studies assessed the behavior of nanoporous gold (NP-Au) under different loading conditions for various material characteristics and loading scenarios, very limited attention was given to the effect of strain rate on material response. In this study, the effect of strain rate is investigated by performing novel atomistic simulations on NP-Au under uniaxial loading up to large compressive and tensile strains (60% strain) for strain rates in the range of 106/s and 109/s. This paper explores the material response under uniaxial loading and proposes a size, relative density, and strain rate dependent dislocation based constitutive model that describes the plastic flow in NP-Au. In addition, modified Gibson and Ashby (G-A) scaling relations that capture the effect of strain rate are proposed to predict the elastic modulus, yield stress and ultimate stress. The simulation results show that the elastic modulus is strain rate independent similar to that of bulk materials. Additionally, the yield stress and its compression-tension asymmetry are strain rate dependent. Under compression, strain hardening is found to be strain rate dependent, and it is controlled by the amount of dislocation density for strain rates below 108/s; whereas, it is controlled by the coupling effect of dislocation density and dislocation mobility for higher strain rates. Under tension, the material shows higher ductility and softening with increasing strain rate. Also, the material deformation and failing mechanisms change at strain rates exceeding 108/s due to the transition in dislocation activity within the ligaments.

Tunable Schottky contact in graphene/InP3 van der Waals heterostructures

To date, the study of contact behavior between graphene and two-dimensional semiconductors is an open topic. Here, we carry out the density-functional-theory to calculate graphene/InP3 van der Waals heterostructures. The results share that low binding energy and mechanical stability ensure that graphene/InP3 heterostructures can be prepared in the experiment. Moreover, n-type Schottky barrier is found in graphene/InP3 heterostructures. Shortening the layer spacing induces a shift of Schottky contact from n- to p-type, and finally makes graphene/InP3 heterostructures to be semiconductors under a large compressive strain. In addition, we design the Schottky barrier transistor on the basis of modulated contact type (Schottky to Ohmic contact) under external electric field, which also can tune the doping of graphene in graphene/InP3 heterostructures. These findings are of utmost significance to guiding the design of new generation graphene-based contact.

Unlocking large compressive strains in thin active elastocaloric layers

• • Durable composites enabling compression of elastocaloric layers are demonstrated. • • Elastocaloric materials can be in any thin-layer geometry, e.g., strip, plate, foil. • • Composites achieve 8.1 K temperature change at 2.5% strain. • • Mechanical and thermal properties are stable after 104 loading cycles at 2% strain. Elastocaloric cooling attracts broad interest and rapidly growing attention due to its potential for high efficiency and low environmental impact. While it is common knowledge that triggering reversible entropy and temperature changes with stress applied in compression prevents rapid failures of materials, realizing this regime in elastocaloric systems is highly challenging because nearly all geometries suited for efficient heat transfer are prone to buckling even under modest loads. This work describes a concept of a novel composite, where an active NiTi layer is embedded into a polymer support structure such that the elastocaloric material is entirely in compression when the assembly is subjected to bending. The active layer achieves 8.1 K temperature change at 2.5% compressive strain without buckling. After 10,000 cycles at 2% compressive strain, the composite maintains mechanical integrity without degradation of the elastocaloric effect. The results show that NiTi and, potentially, other elastocalorically active materials in geometries previously thought impossible can be successfully implemented in regenerative cooling systems operating in compression.

Mechanical characterisation and modelling of a thermoreversible superamolecular polyurethane over a wide range of rates

Understanding the mechanical response of thermo-recoverable elastomers to applied deformation at different strain rates and temperatures is crucial for more effective exploitation of these reusable materials in industrial applications. The research presented in this paper aims to understand the thermal and mechanical responses of a healable, elastomeric supramolecular polyurethane, examining morphology, rheology and mechanical responses from low to high strain rates. In particular, measurement of the high strain rate response of low modulus, or low strength, materials is challenging, and the current paper addresses this by incorporating a modelling framework based on low-rate characterization. To support this research, significant characterization experiments have been performed, which also show, for the first time, the excellent reusability of the polymer. The structure of specimens with different thermal histories was characterized using nuclear magnetic resonance spectroscopy, gel permeation chromatography and small angle X-ray scattering. Differential scanning calorimetry and Rheometry were used to investigate thermomechanical performance during heating and cooling cycles, whilst large strain compression characterization was performed on a commercial screw-driven test frame, a hydraulic loading system and a split-Hopkinson pressure bar. To describe the mechanical response of the polymer, a viscoelastic softening model was developed and characterized. The model incorporates data from rheometry and dynamic mechanical analysis using the principle of time-temperature superposition in a Prony series, combined with a non-linear large strain response, and recovery. The model was able to describe low strain rate behaviour under monotonic and cyclic loading, as well as predicting the response to monotonic loading at medium-to-high strain rates from 40 to 1220 s−1. The paper therefore demonstrates the effectiveness of using the viscoelastic theory and its analytical model to calibrate and further predict the mechanical behaviour of polymers at different strain rates: this is particularly useful for low modulus materials for which high strain rate characterization is challenging.

Properties of (001) NaNbO3 films under epitaxial strain: A first-principles study

First-principles calculations are performed to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely, $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ (NNO), and are subject to epitaxial strain. In particular, an energy-versus-misfit strain phase diagram is constructed and reveals the existence of three different ground states for different strain regimes. For large compressive strain and up to moderate tensile strain, a monoclinic $Cc$ phase occurred, with its polarization and axis of antiphase tilting both rotating within a ($\overline{1}10$) plane with the magnitude of the strain. For large tensile strain, a ferroelectric orthorhombic state of $Pmc{2}_{1}$ symmetry emerges with polarization lying along the [110] in-plane direction together with an octahedral tilting adopting the ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{+}$ pattern and an antiferroelectric vector associated with the reciprocal zone-border $X$ point. Finally, in between and for a narrow region of strain, a complex ground state is found. It has orthorhombic $Pca{2}_{1}$ symmetry, for which a complex tilting pattern coexists with polarization pointing along the [001] out-of-plane direction and antiferroelectric displacement associated with the $\mathrm{\ensuremath{\Delta}} k$ point located halfway between the zone-center $\mathrm{\ensuremath{\Gamma}}$ and zone-border $X$ point. Ferroelectric, antiferroelectric, and antiferrodistortive properties are also reported and discussed as a function of misfit strain.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

On the yielding and densification of nanoporous Au nanopillars in molecular dynamics simulations

We report on the plastic deformation characteristics of nanoporous gold nanopillars under compression in molecular dynamics simulations. The stress-strain curves demonstrate an initial non-linear regime up to compressive true strains of about 5%, followed by a stress plateau and a strong hardening stage. The plateau stress is increasing with the ligament size. In addition, a power-law is fitted to the stress variation during the plateau and hardening stages. In order to relate the mechanical response to the dislocation activity, an in-house post-processing technique was developed to skeletonize the structure and to identify the yielded ligaments. We found that the elastic-plastic transition starts at a strain substantially lower than that of the plateau. The load-bearing capacity is further retained by the unyielded ligaments within the three-dimensionally connected network, allowing further increase in the stress and at the onset of the plateau stress 40% of the ligaments have already yielded plastically. Calculation of the genus of the subnetwork of unyielded ligaments shows that it drops to zero at the onset of the plateau stage. Additionally, the rate of dislocation nucleation is maximum at the onset of the plateau, allowing to continue the plastic flow without increasing the stress. The stress starts increasing again as the ligaments coalesce and the nanoporous structure densifies. Based on the analysis of the genus of the whole structure during compression, we propose a correlation between the hardening and the topology of the structure, suggesting that hardening is dominated by the coalescence of ligaments. During coalescence, grain boundaries were formed between coalesced ligaments, some are either removed or retained. As a result, at very large compressive strains the single crystal nanoporous nanopillar turns into a polycrystalline specimen with some disconnected pores.

Mechanical performance of 3D printed interpenetrating phase composites with spinodal topologies

The mechanical response of interpenetrating phase composites (IPCs) with stochastic spinodal topologies is investigated experimentally and numerically. Model polymeric systems are fabricated by Polyjet multi-material printing, with the reinforcing phase taking the topology of a spinodal shell, and the remaining volume filled by a softer matrix. We show that spinodal shell IPCs have comparable compressive strength and stiffness to IPCs with two well-established periodic reinforcements, the Schwarz P triply periodic minimal surface (TPMS) and the octet truss-lattice, while exhibiting far less catastrophic failure and greater damage resistance, particularly at high volume fraction of reinforcing phase. The combination of high stiffness and strength and a long flat plateau after yielding makes spinodal shell IPCs a promising candidate for energy absorption and impact protection applications, where the lack of material softening upon large compressive strains can prevent sudden collapse. Importantly, in contrast with all IPCs with periodic reinforcements, spinodal shell IPCs are amenable to scalable manufacturing via self-assembly techniques.

Superlubricity of molybdenum disulfide subjected to large compressive strains

The friction between a molybdenum disulphide (MoS2) nanoflake and a MoS2 substrate was analyzed using a modified Tomlinson model based on atomistic force fields. The calculations performed in the study suggest that large deformations in the substrate can induce a dramatic decrease in the friction between the nanoflake and the substrate to produce the so-called superlubricity. The coefficient of friction decreases by 1–4 orders of magnitude when a high strain exceeding 0.1 is applied. This friction reduction is strongly anisotropic. For example, the reduction is most pronounced in the compressive regime when the nanoflake slides along the zigzag crystalline direction of the substrate. In other sliding directions, the coefficient of friction will reduce to its lowest value either when a high tensile strain is applied along the zigzag direction or when a high compressive strain is applied along the armchair direction. This anisotropy is correlated with the atomic configurations of MoS2.

Strain tuning of negative capacitance in epitaxial PbZr&lt;sub&gt;0.2&lt;/sub&gt;Ti&lt;sub&gt;0.8&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; thin films

In order to continue Moore’s Law, the reducing of power consumption is concerned by many researchers, and the discovery of ferronegative negative capacitance effect (NCE) provides a solution. Strain engineering has been widely studied as an effective means to regulate the physical properties of ferroelectric thin films. But the relevant mechanism of strain to ferroelectric negative capacitance regulation is not clear. Recently, the experimental results have shown that it is possible to stabilize the transient NCE in resistance-ferroelectric networks. In this work, we use the Landau-Khalatnikov theory to study the microscopic domain evolution and the influence of strain and temperature on NCE in a ferroelectric film. It is shown that compressive strain enhances NCE while NCE becomes weaker under a tensile strain. However, a larger compressive strain will give rise to a higher coercive voltage that hinders the NCE from forming. In addition, under a certain strain, the NCE becomes stronger at lower temperature. This work provides the theoretical basis for designing the negative capacitance devices and scaling towards nanoscale dimensions in future.