Substrate Strain Research Articles

Comparison of DOFS Attachment Methods for Time-Dependent Strain Sensing.

Structural health monitoring (SHM) is a challenge for many industries. Over the last decade, novel strain monitoring methods using optical fibers have been implemented for SHM in aerospace, energy storage, marine, and civil engineering structures. However, the practical attachment of optical fibers (OFs) to the component is still problematic. While monitoring, the amount of substrate strain lost by the OF attachment is often unclear, and difficult to predict under long-term loads. This investigation clarifies how different attachment methods perform under time-dependent loading. Optical fibers are attached on metal, thermoset composite, and thermoplastic substrates for distributed strain sensing. Strains along distributed optical fiber sensors (DOFS) are measured by optical backscatter reflectometry (OBR) and compared to contact extensometer strains under tensile creep loading. The quality of the bondline and its influence on the strain transfer is analyzed. Residual strains and strain fluctuations along the sensor fiber are correlated to the fiber attachment method. Results show that a machine-controlled attachment process (such as in situ 3-D printing) holds great promise for the future as it achieves a highly uniform bondline and provides accurate strain measurements.

Mechanical strain and bias-stress compensated, 6T-1C pixel circuit for flexible AMOLED displays

This paper proposes a novel topology for a pixel circuit with a voltage-programmed threshold voltage compensation technique for amorphous indium gallium zinc oxide thin-film transistor (a-IGZO TFT) used in flexible AMOLED (Active-Matrix Organic Light-Emitting Diodes) displays. Simulation of the proposed circuit was done in Cadence Spectre using SPICE models for a-IGZO TFT, and the circuit utilizes six a-IGZO TFTs and one storage capacitor. Moreover, a detailed analytical analysis of the circuit is also done to estimate the error introduced during the emission phase. The simulation results showed that this pixel circuit offers excellent immunity to threshold voltage variation of driving TFT caused by bending and prolonged operation under gate bias. It can compensate for a wide range of VTH variations, i.e., 0 V–2.1 V, from the original 0.7 V of the TFT model, and the maximum percentage error in OLED current obtained was below 1.14%. The circuit proposed also alleviates the effect of electrical instability caused by threshold voltage shifts due to the compressive and tensile strain in the flexible substrate of the AMOLED display. The OLED current error was found to be below 0.0018% for a ±0.3% strain in the driving TFT.

A novel intrinsically strain sensor for large strain detection

Soft and stretchable electronic devices play an important role in the growing fields of “soft robotics”, electronic skin, prosthetics, etc. As one of the fundamental units for stretchable electronics, strain sensors have been widely investigated in previous studies, although large strain detection is still a well-known challenge. Intrinsically stretchable sensor could undergo large strain depending on the stretchable matrix, and have attracted lots of attentions in recent years. In this study, we introduce liquid metal into the volumetric matrix of the flexible substrates to construct a novel intrinsically strain sensor, which could undergo large strain of the flexible substrates, avoiding the conventional fracture failures at the interfaces of the sensor and the substrate. Due to the advantages of the extremely stretchable ability, intrinsically strain sensors, i.e., elongation sensor and bending sensor, are further developed. It shows that, up to 178% strain could be measured by these stretchable sensors, and 74 m−1 of curvature for the bending sensor. The proposed sensors also show excellent responses, less than 120 ms for steady, and without delay and flushness. The proposed intrinsically strain sensor provides an alternative way to construct new types of intelligent flexible electronics and artificial skins.

Solid-source metal–organic molecular beam epitaxy of epitaxial RuO2

A seemingly simple oxide with a rutile structure, RuO2, has been shown to possess several intriguing properties ranging from strain-stabilized superconductivity to a strong catalytic activity. Much interest has arisen surrounding the controlled synthesis of RuO2 films, but unfortunately, utilizing atomically controlled deposition techniques, such as molecular beam epitaxy (MBE), has been difficult due to the ultra-low vapor pressure and low oxidation potential of Ru. Here, we demonstrate the growth of epitaxial, single crystalline RuO2 films on different substrate orientations using the novel solid-source metal–organic (MO) MBE. This approach circumvents these issues by supplying Ru using a “pre-oxidized” solid MO precursor containing Ru. High-quality epitaxial RuO2 films with a bulk-like room-temperature resistivity of 55 μΩ cm were obtained at a substrate temperature as low as 300 °C. By combining x-ray diffraction, transmission electron microscopy, and electrical measurements, we discuss the effect of substrate temperature, orientation, film thickness, and strain on the structure and electrical properties of these films. Our results illustrating the use of a novel solid-source metal–organic MBE approach pave the way to the atomic-layer controlled synthesis of complex oxides of “stubborn” metals, which are not only difficult to evaporate but also hard to oxidize.

Vertical strain tuning and perpendicular magnetic anisotropy in La0.5Ca0.5MnO3 vertically aligned nanostructured thin films by the application of high magnetic fields

Epitaxial La0.5Ca0.5MnO3 (LCMO) thin films with vertically aligned nanostructures (VANs) have been achieved with the application of high magnetic fields in pulsed laser deposition processing. More interestingly, the microstructures, vertical strain, and perpendicular magnetic anisotropy (PMA) in the LCMO VAN films can be systematically manipulated by changing the applied high magnetic field strength. With increasing the high magnetic field, an increase in growth rate and a decrease in nanocolumn dimension are observed in the LCMO VAN films, which can be attributed to the enhanced suppression effect on adatom mobility and surface diffusion. Different from the LCMO planar-structured films, the intrinsic stress behaviors associated with vertical interfaces and grain boundaries instead of substrate strain in the LCMO VAN films are dominant in vertical strain evolution, indicating an out-of-plane compressive state, which is highly correlated with the growth rate and nanocolumn dimension. A core-shell model based on phase separation is proposed to understand the nanoscale size effect on magnetic properties in the LCMO VAN films. A tunable and enhanced PMA effect in the LCMO VAN films is also demonstrated. The results suggest that the application of a high magnetic field in pulsed laser deposition processing is a practicable route to achieve tunable VANs, and manipulate physical properties in manganite films.

Control of substrate strain transfer to thin film photovoltaics via interface design

Brittle thin film photovoltaics (PV) that are integrated with load-bearing structures can be subjected to large strains that lead to fragmentation and performance degradation. Such adverse effects can be mitigated, or altogether eliminated, by mechanically isolating a thin PV module from the underlying load-bearing structure. We developed an effective and efficient strain attenuation strategy to eliminate strain transfer from a stiff unidirectional Carbon Fiber Reinforced Polymer (CFRP) laminate to an integrated thin amorphous Si PV module. An analytical model of the three-layer material system (CFRP laminate, interface layer and PV module) was employed to create strain attenuation maps that depend on the length of the PV module and capture the coupled effects of the shear modulus and the thickness of the interface layer (G2, t2), and the Young’s modulus and the thickness of the PV module (E3, t3) through the terms G2/t2 and E3t3, respectively. Based on these strain attenuation maps, polydimethylsiloxane (PDMS) was identified as an effective and versatile interface material, which provided up to 100% strain attenuation between the CFRP laminate and the PV module. After accounting for confinement effects on the effective modulus of the PDMS interface layer, a very good agreement was achieved between the measured and the predicted strain attenuation. Importantly, the PDMS interfacial layer preserved the initial fill factor of the PV module until CFRP laminate failure at 1.7% strain, while in the absence of the PDMS interface the fill factor decreased when the CFRP laminate strain exceeded 0.80%.

Electric field control of the reversible magnetocaloric effect in strain-mediated Ni37.5Co12.5Mn35Ti15/PMN-PT composite

In this study, the martensite transformation of Ni37.5Co12.5Mn35Ti15 ribbon was studied and regulated by the electric field-induced strain in the PMN-PT substrate. The reversible magnetocaloric effect in the Ni37.5Co12.5Mn35Ti15/PMN-PT composite was enhanced by manipulating transition paths under the coupling of magnetic field and electric field, which is significantly important for applications in solid-state refrigeration. With the assistance of E = +8 kV/cm, the reversible entropy change increased from 19.5 to 25.7 J/kgK at 294.5 K. Meanwhile, the reversible cooling temperature span was also enlarged. Hence, the reversible refrigerant capacity was enhanced from 83 to 131 J/kg, and the increase ratio reached 58%. On this basis, an active magnetic regenerating cycle facilitated by an electric field was designed. This work provides an effective method for enhancing reversible caloric effects in first-order phase transition materials via electric fields.

Phase field study on the effect of substrate elasticity on tip-force-induced domain switching in ferroelectric thin films

Tip-force-induced domain switching in ferroelectrics has recently attracted extensive interest as it provides an alternative switching strategy that might ease the problems brought by electrical switching. From the viewpoint of mechanics, substrate elasticity can largely modify the tip-induced deformation of ferroelectric thin films. However, so far, discussions on the influence of substrate elastic properties on such domain switching still remain exclusive. Here, a phase-field model is employed to study the influence of substrate stiffness on the domain switching in BaTiO3 (BTO) thin films, with the strain and stress distributions in BTO thin films and substrates solved by the finite element method. The results demonstrate that the substrate stiffness and loading modes (i.e., pressing and sliding) have a great influence on the symmetry of strain and stress distributions. The switched domain size is highly dependent on the substrate stiffness and loading modes. The switching is more efficient for thin films on a softer substrate. Moreover, the domain could be switched more effectively by the sliding mode under relatively large forces. Our study thus provides a strategy to increase the mechanical switching efficiency of ferroelectric thin films via tuning the substrate elasticity.

Engineered microenvironment for the study of myofibroblast mechanobiology.

Myofibroblasts are mechanosensitive cells and a variety of their behaviours including differentiation, migration, force production and biosynthesis are regulated by the surrounding microenvironment. Engineered cell culture models have been developed to examine the effect of microenvironmental factors such as the substrate stiffness, the topography and strain of the extracellular matrix (ECM) and the shear stress on myofibroblast biology. These engineered models provide well-mimicked, pathophysiologically relevant experimental conditions that are superior to those enabled by the conventional two-dimensional (2D) culture models. In this perspective, we will review the recent advances in the development of engineered cell culture models for myofibroblasts and outline the findings on the myofibroblast mechanobiology under various microenvironmental conditions. These studies have demonstrated the power and utility of the engineered models for the study of microenvironment-regulated cellular behaviours. The findings derived using these models contribute to a greater understanding of how myofibroblast behaviour is regulated in tissue repair and pathological scar formation.

(Invited) Progress in Using Carbon Nanotube Spectra for Mechanical Strain Sensing

A characteristic property of semiconducting single-wall carbon nanotubes (SWCNTs) is distinct near-infrared photoluminescence following excitation by visible light. Theory and experiment show that these optical emission peaks shift predictably in wavelength as nanotubes are compressed or stretched along their axis. We are exploiting this effect in a powerful new method intended for measuring mechanical strain in critical infrastructure components such as airframes, pressurized vessels, pipelines, support beams, etc. The method involves applying a dilute dispersion of nanotubes in a polymeric host onto the surface of the specimen to form a sub-micron thick film in which SWCNTs act as strain sensors. This layer is overcoated with a transparent protective top coat such as a polyurethane varnish. Subsequent strains in the substrate are transmitted to the nanotubes by load transfer. The substrate strain magnitude and direction are then measured by illuminating the surface at any point of interest with a small visible laser beam and spectrally analyzing the resulting near-IR nanotube emission. Single-point measurements currently provide strain magnitude resolution of ca. 50 microstrain, strain angle resolution of ca. 5 degrees, and spatial resolution of ca. 50 mm. Each reading takes less than one second, allowing compilation of strain maps from scanned data. Unlike digital image correlation, which is currently the only commercial non-contact strain technology, the new method can measure strains accumulated when the specimen is not under observation. It thus has the potential for routine use in industrial structural health monitoring as well as in testing and development laboratories. Results will be presented showing strain measurements in metallic, plastic, and concrete specimens. We will also describe recent progress in adapting the technology to camera-based measurements using spectrally resolved fluorescence imaging.

Molecular investigation on the adhesion and deformation behaviors of asphalt binder under nanoindentation

The adhesion of asphalt binders at micro/nano-scale remains one of the least understood and explored areas of the constitutive characteristic of asphaltic composites. In this study, the displacement-controlled nanoindentations were performed using a rigid, conical quartz indenter on the asphalt binder substrate by molecular dynamics (MD) simulation. To accurately describe the mechanical response of asphalt substrate, a reasonable force-displacement (f-d) curve was depicted via determining the initial contact point. Meanwhile, the deformation response of asphalt substrate can be intuitively analyzed from the atomic strain and displacement of the asphalt substrate. Significant curvilinear motion and isotropic plastic deformation in asphalt substrate were observed. The adhesion forces between indenter and asphalt substrate under different indentation depths were also investigated. An adhesion strength model was established. From this model, the size effect was clearly observed, which shows that the adhesion strength reaches a peak value and then gradually declines with an increase of indentation depth. The laboratory nanoindentation test on asphalt binders indicated that the calculated adhesion force is in good agreement with that from the measurement. Moreover, the influence of surface contamination was further investigated. It is shown that the capillary force and interaction from the contamination layer contribute to the rise of adhesion force. The present study raises the practicability of using MD to explore the nanoindentation processes and assists in elucidating the basic adhesion mechanisms involved.

Isopentenol Utilization Pathway for the Production of Linalool in Escherichia coli Using an Improved Bacterial Linalool/Nerolidol Synthase.

Linalool is a monoterpenoid used as a fragrance ingredient, and is a promising source for alternative fuels. Synthetic biology offers attractive alternative production methods compared to extraction from natural sources and chemical synthesis. Linalool/nerolidol synthase (bLinS) from Streptomyces clavuligerus is a bifunctional enzyme, producing linalool as well as the sesquiterpenoid nerolidol when expressed in engineered Escherichia coli harbouring a precursor terpenoid pathway such as the mevalonate (MVA) pathway. Here we identified two residues important for substrate selection by bLinS, L72 and V214, where the introduction of bulkier residues results in variants with reduced nerolidol formation. Terpenoid production using canonical precursor pathways is usually limited by numerous and highly regulated enzymatic steps. Here we compared the canonical MVA pathway to the non‐canonical isopentenol utilization (IU) pathway to produce linalool using the optimised bLinS variant. The IU pathway uses isoprenol and prenol to produce linalool in only five steps. Adjusting substrate, plasmid system, inducer concentration, and cell strain directs the flux towards monoterpenoids. Our integrated approach, combining enzyme engineering with flux control using the artificial IU pathway, resulted in high purity production of the commercially attractive monoterpenoid linalool, and will guide future efforts towards efficient optimisation of terpenoid production in engineered microbes.

Co-deposition growth and superconductivity of La2−xSrxCuO4 films by reactive molecular beam epitaxy

Reactive molecular beam epitaxy has been employed to prepare ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ cuprate films via a Co-deposition method. Precise control of flux ratios among the solid sources leads us to establish a very narrow growth window for the cation stoichiometry. The hole doping by divalent ${\mathrm{Sr}}^{2+}$ cation is found to occur in a substitution-limited manner, accompanied by partial oxidation of Sr flux into peroxide $\mathrm{Sr}{\mathrm{O}}_{2}$ on the top surface. Through interface engineering, we explore the impact of substrate strain on the crystal parameters and superconductivity of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ thin films. The present study provides a time-saving, effective approach to preparing high-quality cuprate films, and further understanding the effect of surface/interface microstructures on cuprate superconductivity.

Strain transfer mechanism in surface-bonded distributed fiber-optic sensors subjected to linear strain gradients: Theoretical modeling and experimental validation

Strain transfer analysis is an important means of assessing the measurement accuracy of embedded or surface-bonded fiber-optic sensors; however, the effect of complex strain fields in substrates has not been well elucidated. Here, a theoretical model was proposed for the analysis of strain transfer mechanisms in surface-bonded distributed fiber-optic sensors due to linear strain gradients. Closed-form solutions were obtained for both single linear and bilinear strain distributions, which were validated through controlled laboratory testing. High-resolution strain profiles acquired with optical frequency-domain reflectometry allowed also the establishment of a simple approach for determining the strain transfer coefficient at the turning point of a bilinear-type strain. Moreover, parametric analyses were conducted to investigate the influences of geometric and mechanical properties of protective and adhesive layers on the strain transfer efficiency, shedding light on the design, installation, and measurement accuracy improvement of fiber-optic sensors after accounting for the effect of substrate strain patterns.

Tuning Magnetocaloric Effect of a Mn-Cr-Sb-Ga alloy by the Nonvolatile Residual Strain of a Ti-Ni Shape Memory Alloy

The multiferroic composites with a layer of magnetocaloric materials (MCMs) on a piezoelectric substrate have recently been developed to tune the magnetocaloric effect (MCE) through the stress from the piezoelectric substrate, which could help achieve high-efficiency and miniaturized refrigeration. The small strain of piezoelectric substrates is sufficient to tune the MCE of alloys whose transformation temperature (TM) is sensitive to the external stress. However, it might not work effectively for those stress-insensitive MCMs, which are also important MCMs with considerable MCE and small hysteresis. In this paper, we design a new composite with a Mn1.95Cr0.05Sb0.95Ga0.05 ribbon on a Ti50Ni50 shape-memory-alloy substrate. The Ti50Ni50 substrate can provide much larger nonvolatile stress than a piezoelectric substrate, which thus successfully tunes the MCE of the Mn1.95Cr0.05Sb0.95Ga0.05, despite its low stress sensitivity. With a 2% residual strain of the Ti50Ni50 substrate, TM of the Mn1.95Cr0.05Sb0.95Ga0.05 is increased by ~6 K, which ~7.5 times that (0.8 K) achieved via a Mn1.95Cr0.05Sb0.95Ga0.05/PMN-PT composite. With different residual strains of Ti50Ni50 substrate from 0% to 2%, a wide working temperature window of 31 K is obtained at a low magnetic field of 1 T. Moreover, this composite exhibits an almost constant refrigeration capacity and a small hysteresis of 4.9 K under different stress, which promotes homogeneous heat transfer and improves the working efficiency. This work provides an effective method to tune the MCE of both stress-sensitive and stress-insensitive MCMs, which could be used in miniaturized refrigerators.

Mechanical Analysis and Experimental Studies of the Transverse Strain in Wrinkled Metallic Thin Films

The wrinkling structures, which can greatly improve the stretchability of the metallic thin films, have been widely used in the preparation of stretchable devices. However, the artificial wrinkling structures are often accompanied by the generation of microcracks, which seriously affect the performance of the devices. In this work, by establishing the corresponding model, the transverse strain of the longitudinally prestrained continuous film and the strip film is mechanically analyzed, which is verified by experimental results; for the strain of blank substrate, the error of the model was about 3.7%. It is difficult to avoid the generation of microcracks with continuous films, but strip films can avoid the generation of microcracks to a certain extent. The experimental results illustrate the various factors affecting the generation of microcracks. The transverse strain of the film is proportional to the substrate’s Young’s modulus, Poisson’s ratio, thickness, and prestrain and is basically inversely proportional to the strip film’s Young’s modulus, thickness, and strip interval. Our results provide deeper knowledge for choosing proper metallic materials to fabricate stretchable wrinkled devices.

Compositionally graded Ga1−xInxP buffers grown by static and dynamic hydride vapor phase epitaxy at rates up to 1 μm/min

We demonstrate Ga1−xInxP compositionally graded buffers (CGBs) grown on GaAs with lattice constants between GaAs and InP by hydride vapor phase epitaxy (HVPE). Growth rates were up to ∼1 μm/min, and the threading dislocation density (TDD) was as low as 1.0 × 106 cm−2. We studied the effect of the substrate offcut direction, growth rate, and strain grading rate on the CGB defect structure. We compared the effect of a “dynamic” grading style, which creates compositional interfaces via mechanical transfer of a substrate between two growth chambers, vs “static” grading where the CGB grows in a single chamber. Dynamic grading yielded smoother grades with higher relaxation, but TDD was not significantly different between the two styles. The substrate offcut direction was the most important factor for obtaining CGBs with low defect density. (001) substrates offcut toward (111)B yielded smoother CGBs with lower TDD compared to CGBs grown on substrates offcut toward (111)A. Transmission electron microscopy of static and dynamic CGBs grown on A- and B-offcuts only found evidence of phase separation in a static A-offcut CGB, indicating that the B-offcut limits phase separation, which, in turn, keeps TDD low. Reductions in growth rate led to the appearance of CuPt-type atomic ordering, which affected the distribution of dislocations on the active glide planes but did not alter TDD significantly. Higher growth rates led to smoother CGBs and did not appreciably increase TDD as otherwise predicted by steady-state models of plastic relaxation. These results show HVPE's promise for lattice-mismatched applications and low-cost InP virtual substrates on GaAs.

Role of substrate strain to tune energy bands–Seebeck relationship in semiconductor heterostructures

In doped semiconductors and metals, the Seebeck coefficient or thermopower decreases monotonically with increasing carrier concentration in agreement with the Pisarenko relation. Here, we establish a fundamental mechanism to modulate and increase the thermopower of silicon (Si)/germanium (Ge) heterostructures beyond this relation, induced by the substrate strain. We illustrate the complex relationship between the lattice strain and the modulated thermopower by investigating the electronic structure and cross-plane transport properties of substrate strained [001] Si/Ge superlattices (SLs) with two independent theoretical modeling approaches: first-principles density functional theory and the analytical Krönig–Penny model in combination with the semi-classical Boltzmann transport equation. Our analysis shows that the SL bands, formed due to the cubic structural symmetry, combined with the potential perturbation and the intervalley mixing effects, are highly tunable with epitaxial substrate strain. The strain tuned energy band shifts lead to modulated thermopowers, with a peak approximately fivefold Seebeck enhancement in strained [001] Si/Ge SLs in the high-doping regime. As a consequence, the power factor of a 2.8% substrate strained SL shows a ≈1.8-fold improvement over bulk Si at high carrier concentrations, ≈12×1020cm−3. It is expected that the fundamental understanding discussed here, regarding the complex effect of lattice strain to control energy bands of heterostructures, will help to exploit strain engineering strategies on a class of future technology-enabling materials, such as novel Si/Ge heterostructures as well as layered materials, including van der Waals heterostructures.

Effect of Substrate Strain, Aluminum Thickness and Corona Pretreatment on the Electrical Resistance of Physical Vapor Deposited Aluminum Coatings

Aluminum coatings applied to polymer films by physical vapor deposition should be defect-free for applications such as packaging and electronic devices. However, cracks can appear in the aluminum coating as the polymer film stretches, becoming manifest as an increase in resistance. We evaluated the effect of different aluminum thicknesses (10–85 nm), polymer films (polyethylene terephthalate or polypropylene), and corona doses (0–280 W∙min/m2) on the relative increase in resistance during stretching (strain = 0–100%). We found that the thickness of the aluminum coating was inversely related to the increase in resistance. Corona pretreatment led to an increase in surface energy (≤40 mN/m for polypropylene; ≤50 mN/m for polyethylene terephthalate) although high corona doses resulted in overtreatment, which limited the adhesion of aluminum to the substrate and led to a greater increase in resistance. Varying the coating thickness had a much greater effect than the corona pretreatment, suggesting that thicker aluminum coatings are more effective than corona pretreatment as a strategy to increase coating stability. The effect of aluminum thickness and strain on resistance was described using a fit function containing three fit factors.

Molecular determinant of substrate binding and specificity of cytochrome P450 2J2

Cytochrome P450 2J2 (CYP2J2) is responsible for the epoxidation of endogenous arachidonic acid, and is involved in the metabolism of exogenous drugs. To date, no crystal structure of CYP2J2 is available, and the proposed structural basis for the substrate recognition and specificity in CYP2J2 varies with the structural models developed using different computational protocols. In this study, we developed a new structural model of CYP2J2, and explored its sensitivity to substrate binding by molecular dynamics simulations of the interactions with chemically similar fluorescent probes. Our results showed that the induced-fit binding of these probes led to the preferred active poses ready for the catalysis by CYP2J2. Divergent conformational dynamics of CYP2J2 due to the binding of each probe were observed. However, a stable hydrophobic clamp composed of residues I127, F310, A311, V380, and I487 was identified to restrict any substrate access to the active site of CYP2J2. Molecular docking of a series of compounds including amiodarone, astemizole, danazol, ebastine, ketoconazole, terfenadine, terfenadone, and arachidonic acid to CYP2J2 confirmed the role of those residues in determining substrate binding and specificity of CYP2J2. In addition to the flexibility of CYP2J2, the present work also identified other factors such as electrostatic potential in the vicinity of the active site, and substrate strain energy and property that have implications for the interpretation of CYP2J2 metabolism.