High Compressive Strain Research Articles

Strain-tunable electronic and magnetic properties of two-dimensional gallium nitride with vacancy defects

Based on density functional theory, we have investigated the effects of in-plane biaxial strain on the electronic and magnetic properties of the two-dimensional GaN (2D GaN) with Ga- (VGa) or N-vacancy (VN). We considered two different levels of vacancy concentration, i.e., θ=1/62 and θ=1/34. While the pristine GaN 2D structures are intrinsically semiconducting, the 2D GaN with VGa defects under tensile/compressive biaxial strains is metallic, except at a high compressive strain of 6%. In addition, the 2D GaN exhibits a strain-tunable magnetic property by introducing the VGa defects, where the magnetic moment can be modulated by applying a biaxial strain on the material. A compressive strain larger than 2% tends to suppress the magnetic effect. A drastic reduction of the total magnetization from 2.21 μB to 0.16 μB is clearly visible for a lower VGa concentration of θ=1/62. On the other hand, the 2D GaN with VN defects is nonmagnetic, and this behavior is not affected by the biaxial strain.

Mechanisms of a Rectifying TiN Gate Contact for AlGaN$/$GaN HEMTs on Silicon Substrate

Rectifying Titanium Nitride (TiN) gate contact technology is developed for AlGaN $/$ GaN based micro and nanometer HEMTs. A high compressive strain occurring in thinner TiN films (ranging from 5 nm to 60 nm), deposited by sputtering, leads to a reduction in tensile strain at the surface of AlGaN barrier. The diminution in tensile strain forms a pseudo- $p$ -type layer (diode-like). This strain reduction has no effect on the bandgap of the AlGaN barrier layer, allowing the gate to withstand a reverse gate bias larger than 100 V. Characterization using the high-resolution transmission electron microscopy combined with the X-ray photoelectron spectroscopy reveals a good TiN $/$ AlGaN interface quality and no diffusion of TiN into AlGaN. The effective energy barrier of the rectifying nanoscale TiN gate contact has a relatively large height of 1.1 eV associated with an ideality factor of 1.4. A dramatic drop of the reverse-bias leakage current down to 11 pA $/$ mm is measured at −30 V. In addition, electrical measurements show very low gate and drain lag effects of 4.2 ${\%}$ and 6.7 ${\%}$ , respectively.

Secondary osteon structural heterogeneity between the cranial and caudal cortices of the proximal humerus in white-tailed deer.

Cortical bone remodeling is an ongoing process triggered by microdamage, where osteoclasts resorb existing bone and osteoblasts deposit new bone in the form of secondary osteons (Haversian systems). Previous studies revealed regional variance in Haversian systems structure and possibly material, between opposite cortices of the same bone. As bone mechanical properties depend on tissue structure and material, it is predicted that bone mechanical properties will vary in accordance with structural and material regional heterogeneity. To test this hypothesis, we analysed the structure, mineral content and compressive stiffness of secondary bone from the cranial and caudal cortices of the white-tailed deer proximal humerus. We found significantly larger Haversian systems and canals in the cranial cortex but no significant difference in mineral content between the two cortices. Accordingly, we found no difference in compressive stiffness between the two cortices and thus our working hypothesis was rejected. As the deer humerus is curved and thus likely subjected to bending during habitual locomotion, we expect that similar to other curved long bones, the cranial cortex of the deer humerus is likely subjected primarily to tensile strains and the caudal cortex is subject primarily to compressive strains. Consequently, our results suggest that strain magnitude (larger in compression) and sign (compression versus tension) affect the osteoclasts and osteoblasts differently in the basic multicellular unit. Our results further suggest that osteoclasts are inhibited in regions of high compressive strains (creating smaller Haversian systems) while the osteoid deposition and mineralization by osteoblasts is not affected by strain magnitude and sign.

Analysis of premixed flame kernel/turbulence interactions under engine conditions based on direct numerical simulation data

Although the evolution of premixed flames in turbulence has been frequently studied, it is not well understood how small flames interact with large-scale turbulent flow motion. Since this question is of practical importance for the occurrence of cycle-to-cycle variations in spark ignition engines, the objective of the present work is to fundamentally differentiate early flame kernel development from well-established turbulent flame configurations. For this purpose, a DNS database consisting of three flames propagating in homogeneous isotropic turbulence (Falkenstein et al., Combust. Flame, 2019) is considered. The flames feature different ratios of the initially laminar flame diameter to the integral length scale. To quantify flame kernel development, the time evolution of flame topology and flame front geometry are analysed in detail. It is shown that some realizations of the early flame kernel are substantially influenced by high compressive strain caused by large-scale turbulent flow motion with characteristic length scales greater than the flame kernel size. As a result, the initial spherical kernel topology may become highly distorted, which is reflected in the stochastic occurrence of excessive curvature variance. Two mechanisms of curvature production resulting from early flame kernel/turbulence interactions are identified by analysis of the mean curvature balance equation. Further, it is shown that the curvature distribution of small flame kernels becomes strongly skewed towards positive curvatures, which is contrary to developed turbulent flames. Hence, the transition of ignition kernels to self-sustaining turbulent flames is very different in nature compared with the development of a statistically planar flame brush.

Tuning the mechanical properties of Fex(CoMoNi)100-x high-entropy alloys via controlled formation of hard μ phase

Single-phase face-centered cubic (fcc) high-entropy alloys (HEAs) exhibits outstanding ductility but low strength, which hinders their practical applications. To overcome this dilemma, we propose a strategy to achieve an excellent combination of high compressive strength and failure strain via controlled formation of hard intermetallic compound in HEAs. Specifically, we vary the Fe content in FeCoMoNi HEAs to control the relative content of μ phase embedded within the ductile fcc matrix and achieve a yield strength of 1877 MPa and an appreciable fracture strain of ~40% in Fe40Co20Mo20Ni20 HEA. The present work provides deep insight into the design of high-performance HEAs.

InGaN based tunable green light-emitting diodes using InAlN interlayer and strain compensated AlGaN interlayer for better device performance

The effect of employing an InyAl1−yN and AlyGa1−yN interlayer (IL) in the InxGa1−xN/GaN active region for the green light-emitting diodes (LEDs) has been studied. The high compressive strain in InxGa1−xN/GaN QW is compensated by the tensile strained AlyGa1−yN IL. The InyAl1−yN IL reduces the strain between InxGa1−xN and GaN layers. The AlyGa1−yN or the InyAl1−yN layer in between the QW and the GaN barriers increase the barrier height, which potentially suppresses the electron and hole leakage from the QW. These increase the square of the overlap of electron and hole wave functions (Meh2) of the QW LEDs. By changing the well width and aluminum or indium content in the IL, we obtained better optical properties. It has found that 10 Ǻ thick In0.2Al0.8N and 10 Ǻ thick Al0.8Ga0.2N layers give the maximum Meh2 for the green LEDs, using InyAl1−yN and AlyGa1−yN ILs. Furthermore, we have compared the Meh2, using AlyGa1−yN and InyAl1−yN IL with the current. The best result is obtained for the 10 Ǻ Al0.8Ga0.2N IL, which increases the Meh2up to 2 times, as compared to the QW LEDs, without any IL.

Impact Experiment on Gneiss: The Effects of Foliation on Cratering Process

AbstractTwo impact experiments were carried out on blocks of Maggia gneiss to assess the influence of mechanical target anisotropies on the cratering process. The experiments were done within the framework of Multidisciplinary Experimental and Modeling Impact Research Network. In one experiment the metamorphic foliation was vertical, that is, oriented perpendicular to the target surface, and in the other horizontal, that is, parallel to the target surface. This study reveals that due to preferred spallation along the foliation, the cratering efficiency is higher if the foliation is horizontal. The study further presents a comprehensive quantitative strain analysis in the subsurface of these craters. We use biotite kink bands as strain markers. The distribution of strain differs strongly in both experiments. If the foliation is vertical, the area beneath the central crater floor shows a high compressive strain. This is attributed to the ability of biotite to kink more easily when shortened parallel to its basal plane. If the foliation is horizontal, surface kinking is reduced and tensile failure along the foliation deepens the crater during unloading.

Spatially Regulated Multiphenotypic Differentiation of Stem Cells in 3D via Engineered Mechanical Gradient.

Within the osteochondral interface, cellular and extracellular matrix gradients provide a biomechanical and biochemical niche for homeostatic tissue functions. Postnatal joint loading is critical for the development of such tissue gradients, leading to the formation of functional osteochondral tissues composed of superficial, middle, and deep zones of cartilage, and underlying subchondral bone, in a depth-dependent manner. In this regard, a novel, variable core-shell electrospinning strategy was employed to generate spatially controlled strain gradients within three-dimensional scaffolds under dynamic compressive loading, enabling the local strain-magnitude dependent, multiphenotypic stem cell differentiation. Human mesenchymal stem cells (hMSCs) were cultured in electrospun scaffolds with a linear or biphasic mechanical gradient, which was computationally engineered and experimentally validated. The cell/scaffold constructs were subjected to various magnitudes of dynamic compressive strains in a scaffold depth-dependent manner at a frequency of 1 Hz for 2 h daily for up to 42 days in osteogenic media. Spatially upregulated gene expression of chondrogenic markers (ACAN, COL2A1, PRG4) and glycosaminoglycan deposition was observed in the areas of greater compressive strains. In contrast, osteogenic markers (COL1A1, SPARC, RUNX2) and calcium deposition were downregulated in response to high local compressive strains. Dynamic mechanical analysis showed the maintenance of the engineered mechanical gradients only under dynamic culture conditions, confirming the potent role of biomechanical gradients in developing and maintaining a tissue gradient. These results demonstrate that multiphenotypic differentiation of hMSCs can be controlled by regulating local mechanical microenvironments, providing a novel strategy to recapitulate the gradient structure in osteochondral tissues for successful regeneration of damaged joints in vivo and facile development of interfacial tissue models in vitro.

Comparison of the isothermal and cyclic oxidation behavior of Cr and Mo containing γ-TiAlNb alloys

The isothermal and cyclic oxidation behavior of γ-Ti-46.5Al-5Nb-0.3B alloys containing 2 at.% Cr, 2 at.% Mo, 1 at.% each of Cr and Mo, is compared at 900 °C until 500 h. The alloys exhibit parabolic oxidation, similar isothermal oxidation rates, but contrasting cyclic oxidation characteristics. The Mo-containing alloys form more Al2O3, exhibit extensive oxide spallation and weight loss; and vice-versa is observed for the Cr-rich alloy. The residual stresses, strains and spallation mechanisms in the multi-layered oxide scale are analyzed. Higher Al2O3 formation, high compressive stress (∼1 GPa) and strain (> 0.2 %) induce early delamination of oxide scale and weight degradation.

Microstructural material characterization of hypervelocity-impact-induced pitting damage

In a hypervelocity impact (HVI) between the micrometeoroids/orbital debris (MMOD) and multi-layered shielding mechanisms of spacecraft, the debris cloud, formed by shattered materials of the outer bumper layer and projectile, commits multitudinous pitting craters and cracks that are disorderedly scattered in the rear wall layer. Material degradation due to the pitting damage is a precursor of structural fragmentation and system failure of the space assets. In this study, microscopic material degradation of the rear wall of a typical dual-layered Whipple shield, initiated and intensified by the debris cloud-engendered pitting damage, is characterized using metallographic analysis including optical microscope (OM), laser scanning microscope (LSM), scanning electron microscope (SEM) and X-ray diffraction (XRD). Results have revealed that (1) the degree of material degradation shows difference in the central cratered area, the ring cratered area, and the spray area, respectively; (2) the dynamic recrystallization gives rise to the formation of fine grains adjacent to pitting craters; (3) the extents of recrystallization and dislocation depend on the strain rate levels during HVI; and (4) the temperature elevation, caused by the heat transformed from the adiabatic plastic deformation energy and shock heating, warrants the recrystallization. Two types of damage, namely micro-voids and micro-cracks, are identified beneath the pitting damage area; under the extremely high compressive strain rate induced by HVI, micro-voids are initiated by the nucleation of grains or deteriorate from existing material defects, and these micro-voids further expand at the grain boundaries and within the grains to form micro-cracks under a tensile-type wave converted from the HVI-induced shock wave.

Catalyst-Free and Rapid Chemical Approach for in Situ Growth of "Chemically Reactive" and Porous Polymeric Coating.

Design of "chemically reactive" coating with a tailored topography is a simple basis for optimizing various physical and chemical parameters, which is essential for achieving different biomimicked liquid wettability. In general, the essential topography and appropriate chemistry in the superhydrophobic coating is optimized following various top-down and bottom-up approaches, where various hydrophilic building blocks are associated using electrostatic interaction, hydrogen bonding, and other weak bonding (e.g., metal-thiol etc.), for both developing the desired hierarchical features and optimizing the appropriate chemistry on top of this featured interface. Such designs are inappropriate to sustain practically relentlessly harsh settings. So, further development for the synthesis of a durable and substrate-independent superhydrophobic coating is essential for various prospective applications in "real-world" scenarios. However, the design of highly abrasion-tolerant and "absolutely" substrate-independent artificial superhydrophobicity following a simple and scalable synthesis procedure is rare in literature. In this current work, a catalyst-free and facile chemical approach is adopted for an in situ and rapid deposition of a "chemically reactive" nanocomplex for decorating a wide range of substrates, including water-soluble, water-sensitive, highly flexible, rigid, and fibrous substrates with a highly tolerant biomimicked superhydrophobicity property. Branched poly(ethylenimine) (BPEI) and dipentaerythritol pentaacrylate (5Acl) mutually react through 1,4-conjugate addition reaction, and a hierarchically featured "chemically reactive" dip-coating is synthesized by the appropriate selection of the alcoholic solvent that is 1-heptanol. Furthermore, the choice of small alkylamines for post-covalent modifications of the "chemically reactive" dip-coating provided superhydrophobicity with a tailored water adhesion. A gradual increase in both roll-off angles, and the contact angle hysteresis (from 5° to 30°) was noted with a decrease in the hydrocarbon tail of selected alkylamines. The synthesized biomimicked interfaces are capable of performing under various practically relevant, severe physical and chemical challenges including bending, creasing, twisting, different physical abrasions (i.e., adhesive tape peeling test, abrasive sand paper test, etc.), high compressive strain, highly acidic and alkaline aqueous phases, artificial sea water, river water, etc. Moreover, this current approach was extended in developing various relevant functional materials, including superhydrophilic/superhydrophobic physical patterns on flexible papers and highly compressible super-oil-absorbent, etc.

Laboratory investigation on the influence of high compressive strain rates on the hybrid fibre reinforced self-compacting concrete

The influence of hybrid fibres on the response of self-compacting concrete to quasi-static and high strain rates was laboratory investigated. The Split Hopkinson Pressure Bar enabled to achieve the strain rates in specimens in the range of 41–254 s−1. Two kinds of steel fibres: micro and coming from end-of-life tires were combined with glass fibres to create a hybrid mix. The sensitivity of mixes to high strain rates increased with the increase of strain rate. The experiments revealed that the dynamic strength of hybrid fibre reinforced SCC was lower than the plain SCC.

Shaking table tests of a half-scale 2-storey URM building retrofitted with a high ductility fibre reinforced concrete overlay system

The dynamic response and seismic behavior of unreinforced masonry (URM) buildings retrofitted with high ductility fibre reinforced concrete (HDC) overlay is investigated in this study. The HDC is a class of fibre-reinforced cementitious composite with tensile strain-hardening behavior and high compressive strain abilities. 17 shaking table tests, divided into three phases, were conducted on a half-scale two-storey URM model building. In phase I, testing was performed on the model without any retrofitting until it reached a near-local-collapse state. The damaged model was retrofitted with welded wire reinforcement (WWR) and tested in phase II. The model was retrofitted again with an HDC overlay for the tests in phase III. The comparisons between the WWR and the HDC overlay were analysed regarding the natural frequency, drift response and base shear. Backbone curve, torsion effect and seismic behavior evaluations were also conducted for the model retrofitted with the HDC overlay. The experimental results indicated that the HDC overlay retrofit system is more effective in improving seismic behavior than WWR overlay. The model that was retrofitted with the HDC overlay exhibited clear enhancements in shear capacity and damage tolerance.

Electrochemical stability and bio-mineralization capability of zinc substituted and elemental zinc reinforced calcium phosphate composite coatings synthesized through pulsed electro-deposition

Recent studies showed that zinc, as one of the essential element in the human body, is able to promote osteoblast proliferation and bone growth when present with calcium phosphate phases. In this study, zinc substituted and zinc reinforced calcium phosphate composites were prepared by electro chemical deposition from aqueous electrolyte bath. Nucleation rate and reaction time were varied to understand the relation between in-situ formation mechanism of various phases with the varying reaction kinematics and kinetics. The thin functional composite coatings were compared in terms of their surface morphology, lattice structure, surface elemental composition and related electrochemical corrosion resistance along with bio-mineralization capability under simulated body fluid (SBF). On a particular parameter of 10 mA/cm2 current density and 20% duty cycle, nano tubes enriched in elemental zinc; hopeite and calcium orthophosphate composite phases are formed. This unique microstructural morphology and composition make the coating surface 10 times more electrochemical corrosion resistant compared to all other coatings. A high amount of zinc substitution in calcium phosphate phases leads to the development of high compressive lattice strain, which in turn results in rapid dissolution of the coating under contact with SBF. During bio-mineralization, the zinc loading ratio is rapidly decreased for the initial 7 days through preferential dissolution and enrichment in calcium ions and subsequently stabilized at 60% from the initial 83% in nano tube structures. The effective adherence of osteoconduction product on the nano tube structure is achieved post 14 days. In summary, electrochemical deposition was found as an effective way to design various tailor-made/functionally graded zinc-calcium phosphate composite coatings based on specific requirements of biodegradability, biocompatibility and surface properties.

Microstructure and mechanical properties of a rapid solidified Mg-Gd-Zn alloy containing Mg3(Gd, Zn) nanoparticles and LPSO phase using spark plasma sintering

In this work, a Mg-Gd-Zn alloy containing both long period stacking ordered (LPSO) phase and intermetallic Mg-RE nanoparticle was successfully developed using rapid solidified Mg96.9Gd2.7Zn0.4 ribbons before spark plasma sintering (SPS) treatment at 450 °C. The microstructural evolution and compressive properties of the SPSed alloy were systematically investigated. The results showed that the mean grain size of the bulk alloy was about 7.0 μm with plentiful intragranular lamellar LPSO phase and Mg3(Gd, Zn) nanoparticle. In addition, the yield compressive strength (YCS) and ultimate compressive strength (UCS) of the sintering alloy were 316 MPa and 408 MPa, respectively, as well as a satisfied high compressive strain of 0.14. The strengthening mechanism included precipitation (fine LPSO phase and nano-scale Mg3(Gd, Zn) particles) strengthening, grain refinement and partial solid solution strengthening, and each contribution to YCS were 37%, 36%, and 27%, respectively.

Optimization of powder metallurgy parameters to obtain low corrosion rate and high compressive strength in Al-MoO3 composites using SN ratio and ANOVA analysis

The powder metallurgy technique is considered and proved to be one of the successful fabrication methods in fabricating metal matrix composites. In this work aluminium matrix was reinforced with 5, 10, 15 wt% MoO3 through powder metallurgy route. The objective of this paper is to optimize the powder metallurgy process parameters to obtain low corrosion rate, high compressive strength and high compressive strain percentage. The significance and the effect of process parameters on corrosion rate, compressive strength and compressive strain percentage was studied. Taguchi method and ANOVA were employed to perform optimization and the run order to conduct experiments was chosen using L9 orthogonal array. Through analysis of the experimental data, the reinforcement weight percentage was identified to be the most influencing factor for attaining maximum compressive strength and compressive strain percentage and sintering temperature plays vital role in corrosion rate. The optimum condition to obtain low corrosion rate is 5 wt% MoO3, 300 MPa compaction pressure, 400 °C sintering temperature, high compressive strength is 15 wt% MoO3, 300 MPa compaction pressure, 400 °C sintering temperature, and for high compressive strain percentage is 5 wt% MoO3 , 250 MPa compaction pressure, 400 °C sintering temperature. The uniform distribution of reinforcement over the matrix and the absence of clusters and agglomeration of reinforcement particles were confirmed through SEM test.

Gradually Crosslinking Carbon Nanotube Array in Mimicking the Beak of Giant Squid for Compression‐Sensing Supercapacitor

AbstractPortable and wearable electronics are undergoing rapid development, but these flexible devices may break and fail to work under high compressive strains. Here, presented is a new free‐standing compressible carbon nanotube array (CCNA) with a unique gradually crosslinking structure, which mimics the gradient structure of the beak of the giant squid. The CCNAs can tolerate various compressive strains and demonstrate high reversible compressibility up to 1 00 000 cycles with high electrical conductivities. On the basis of the CCNA, a novel all‐solid‐state compression‐sensing supercapacitor (CSS) that can store energy and tolerate and sense the external strain change is produced. It demonstrates a high capacitance of 93.2 mF cm−2 and can be maintained by 94% even after 3000 continuous compressing cycles at a strain of 60%. In addition, it also shows superior strain sensing capability and stability up to 1900 compressive cycles. These flexible CSSs promise a wide range of applications including electronic skins and advanced bioelectronic devices.

Wearable, Ultrawide-Range, and Bending-Insensitive Pressure Sensor Based on Carbon Nanotube Network-Coated Porous Elastomer Sponges for Human Interface and Healthcare Devices.

Flexible and wearable pressure sensors have attracted a tremendous amount of attention due to their wider applications in human interfaces and healthcare monitoring. However, achieving accurate pressure detection and stability against external stimuli (in particular, bending deformation) over a wide range of pressures from tactile to body weight levels is a great challenge. Here, we introduce an ultrawide-range, bending-insensitive, and flexible pressure sensor based on a carbon nanotube (CNT) network-coated thin porous elastomer sponge for use in human interface devices. The integration of the CNT networks into three-dimensional microporous elastomers provides high deformability and a large change in contact between the conductive CNT networks due to the presence of micropores, thereby improving the sensitivity compared with that obtained using CNT-embedded solid elastomers. As electrical pathways are continuously generated up to high compressive strain (∼80%), the pressure sensor shows an ultrawide pressure sensing range (10 Pa to 1.2 MPa) while maintaining favorable sensitivity (0.01-0.02 kPa-1) and linearity ( R2 ∼ 0.98). Also, the pressure sensor exhibits excellent electromechanical stability and insensitivity to bending-induced deformations. Finally, we demonstrate that the pressure sensor can be applied in a flexible piano pad as an entertainment human interface device and a flexible foot insole as a wearable healthcare and gait monitoring device.

Formation and mechanical properties of a Zr73Al8Cu6Ni13 bulk metallic glass composite containing in-situ beta-Zr dendrites

The variations of structure and mechanical properties with sample diameter (d) for ZrAlCuNi alloys containing a high Zr content of 73 at.% were investigated. A Zr73Al8Cu6Ni13 alloy forms a single glassy phase with a d of 1.0 mm and changes to a composite of β-Zr dendrites dispersing homogeneously in metallic glass matrix when the d increases to 1.5–3.0 mm. The Zr73Al8Cu6Ni13 bulk metallic glass composites (BMGCs) exhibit high compressive plastic strain of over 35% without final failure, superior to that of 4.5% for the ϕ1.0 mm bulk metallic glass (BMG) sample, and the yield strength of ∼1510 MPa is comparable to the BMG. The mechanisms of formation of the β-Zr dendrites and plasticity improvement of the BMGCs were also discussed.

Probing the uniformity of hydrogen intercalation in quasi-free-standing epitaxial graphene on SiC by micro-Raman mapping and conductive atomic force microscopy

In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer graphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 °C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (ΦB). The ΦB values evaluated from local I–V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky–Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.