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Lanthanide‐doped phosphors have garnered significant attention in the field of optics due to their robust upconversion luminescence capabilities. Enhancing the upconversion luminescence of these phosphors is of paramount importance. The recent advancements in fabricating ultrathin freestanding perovskite oxides present new opportunities for exploring coupled properties such as flexoelectricity. A lanthanide‐doped freestanding perovskite oxide scroll, specifically an SrTiO3:Er scroll is introduced herein, exhibiting an average strain gradient of 9.5 × 104 m−1. The developed scroll structure achieves a remarkable enhancement in upconversion luminescence, with a maximum increase of 7.4 times at the wavelength of 548 nm, driven by the synergistic effects of ferroelectricity and flexoelectricity. Theoretical model is adopted to elucidate the relationship between the scroll structure and strain, further investigating the mechanisms underlying the photoluminescence enhancement. This work unveils a novel three–dimensional (3D) tubular platform for enhancing the luminescence of phosphors without structural constraints, facilitating integration with silicon photonics. [ABSTRACT FROM AUTHOR]

Flexoelectricity, a universal electromechanical coupling phenomenon, has triggered new feasibilities of advancements in functional materials, especially for nanoscale materials. The strong flexoelectric response is initially discovered in ceramic materials with high permittivity, and then the past decades have witnessed the expansion of flexoelectricity to a broader range of material systems including semiconductors, polymers, and soft elastomers, which in turn raise emerging applications of flexoelectricity. Moreover, flexoelectricity is demonstrated to be significantly enhanced in thin films and nanostructures where ultra‐high strain gradients are easier to achieve, rendering flexoelectricity attractive for modifying the functional properties of advanced materials and devices at the nanoscale. To provide a comprehensive drawing of the above aspects, this review highlights the recent progress of flexoelectricity in diverse materials, covering the characterization of flexoelectricity, the fundamental mechanisms of the enhancement flexoelectric response as well as the multi‐functional applications. Finally, some open questions and perspectives are presented, underlining the fascinating future of this field. [ABSTRACT FROM AUTHOR]

The hydrogen embrittlement (HE) phenomenon occurring in drawn pearlitic steel wires sometimes results in dangerous delayed fracture and has been an important issue for a long time. HE is very sensitive to the amount of plastic deformation applied in the drawing process. Hydrogen (H) atom diffusion is affected by ambient thermal and mechanical conditions such as stress, pressure, and temperature. In addition, the influence of stress gradient (SG) on atomic diffusion is supposed to be crucial but is still unclear. Metallic materials undergoing plastic deformation naturally have SG, such as residual stresses, especially in inhomogeneous regions (e.g., surface or grain boundary). In this study, we performed molecular dynamics (MD) simulation using EAM potentials for Fe and H atoms and investigated the behavior of H atoms diffusing in pure iron (α-Fe) with the SG condition. Two types of SG conditions were investigated: an overall gradient established by a bending deformation of the specimen and an atomic-scale local gradient caused by the grain boundary (GB) structure. A bi-crystal model with H atoms and a GB structure was subjected to bending deformation. For a moderate flexure, bending stress is distributed linearly along the thickness of the specimen. The diffusion coefficient of H atoms in the bulk region increased with an increase in the SG value. In addition, it was clearly observed that the direction of diffusion was affected by the existence of the SG. It was found that diffusivity of the H atom is promoted by the reduction in its cohesive energy. From these MD results, we recognize an exponential relationship between the amount of H atom diffusion and the intensity of the SG in nano-sized bending deformation. [ABSTRACT FROM AUTHOR]

Ni-based superalloys, essential for high-temperature applications, derive strength from coherent second-order precipitates that impede dislocation motion through coherency misfit and elastic mismatch. This study employs multi-component phase-field crystal (PFC) simulations to explore the elastic deformation of such precipitates. Using a binary ordered square structure for the precipitate and a single species square structure for the matrix, elastic properties and lattice parameters are fitted to data from ab initio density functional theory calculations for Ni and Ni3Ti systems. Simulations reveal a smooth strain gradient across the matrix–precipitate interface with coherency misfit influenced by precipitate size and strain state. These findings highlight the utility of PFC simulations for understanding strain distribution and deformation in precipitate–matrix systems with the potential to offer insights for both experimental and computational studies. [ABSTRACT FROM AUTHOR]

Strain gradient is a normal phenomenon around a heterostructural interface in ultrathin film, and it is important to determine its effect on magnetic interactions to understand interfacial coupling. In this work, ultrathin Pr0.67Sr0.33MnO3 (PSMO) films on different substrates are studied. For PSMO film under different in-plane strain conditions, the saturated magnetization and Curie temperature can be qualitatively explained by double-exchange interaction and the Jahn–Teller distortion. However, the difference in the saturated magnetization with zero field cooling and 5 T field cooling is proportional to the strain gradient. Strain-gradient-induced structural disorder is proposed to enhance phonon–electron antiferromagnetic interactions and the corresponding antiferromagnetic-to-ferromagnetic phase transition via a strong magnetic field during the field cooling process. A non-monotonous structural transition of the MnO6 octahedral rotation can enlarge the strain gradient in PSMO film on a SrTiO3 substrate. This work demonstrates the existence of the flexomagnetic effect in ultrathin manganite film, which should be applicable to other complex oxide systems.

The flexoelectric effect, characterized by the induction of electric polarization by strain gradients, exhibits a remarkable size dependence. This makes flexoelectricity highly relevant for nanoscale electromechanical systems. Inevitably, flexoelectric solids, like all materials, are susceptible to various types of defects. These defects significantly influence the local electromechanical coupling phenomena, thereby affecting the performance of flexoelectric materials. This study investigates ellipsoidal inclusions in flexoelectric solids, a fundamental and classical defect type. We present Green's functions for flexoelectricity, which is the basis for formulating the eigen deformation problem within flexoelectricity theory. We then derive the expressions for strain dilatation, electric potential, and polarization magnitude under a constant eigenstrain dilatation scenario, which allows us to effectively address the ellipsoidal inclusion problem in flexoelectric solids. The investigation then extends to different ellipsoidal inclusions, shedding light on their distinctive shape and size effects. The insights gained from this study provide perspectives on the potential failure mechanisms in defective flexoelectric solids and lay a theoretical foundation for the design of nanoscale flexoelectric systems. [ABSTRACT FROM AUTHOR]

Spin texture in k -space is a consequence of spin splitting due to strong spin–orbit coupling and inversion symmetry breaking. It underlies fertile spin transport phenomena and is of crucial importance for spintronics. Here, we observe the spin texture in k -space of nominally centrosymmetric SrIrO3 grown on NdGaO3 (110) substrates, using non-linear magnetotransport measurements. We demonstrate that the spin texture is not only induced by the interface, which inherently breaks the inversion symmetry in strong spin–orbit coupled SrIrO3 films, but also originates from the film bulk. Structural analysis reveals that thicker SrIrO3 films exhibit a strain gradient, which could be considered as a continuous change in the lattice constant across different layers and breaks the inversion symmetry throughout the entire SrIrO3 films, giving rise to the spin texture in k -space. First-principles calculations reveal that the strain gradient creates large spin-splitting bands, inducing the spin texture with anisotropy, which is consistent with our experimental observations. Our results offer an efficient method for inducing the spin textures in k -space. [ABSTRACT FROM AUTHOR]

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The flexoelectric effect describes phenomena where strain gradients induce electrical polarization and electric field gradients cause strain in materials. This specific type of electromechanical coupling effect is remarkable for being independent of material symmetry, Curie temperature, and it exhibits notable effects in small‐sized materials. The study of flexoelectric effects has provided fresh insights into materials science, particularly highlighting how thin‐film structures, with their unique geometrical and dimensional attributes, are highly responsive to flexoelectric influences, thereby offering significant opportunities for modulating electrical properties. Herein, this paper presents the fundamental concepts and theories underlying the flexoelectric effect are presented. Various methods for inducing this effect in thin films are explored, including the optimization of growth and deposition conditions, and the application of external mechanical stresses to create strain gradients. Additionally, recent advances in utilizing the flexoelectric effect to modulate ferroelectric domains, modify properties of thin films, and enhance functionalities in photovoltaic systems, nanogenerators, sensors, and actuators are reviewed. Finally, the challenges and future prospects for flexoelectric effects in advanced electronics are briefly presented. [ABSTRACT FROM AUTHOR]

High sidewall cavern splitting failure is a critical engineering phenomenon that occurs during excavation and unloading under true three-dimensional high in-situ stress conditions. This issue significantly impacts the safety of underground cavern construction and excavation, yet its formation mechanism remains unclear. This research utilizes the high sidewall cavern of the main powerhouse at the Pubugou Hydropower Station as a case study to establish an elastic-plastic damage-softening model of splitting failure based on strain gradient theory and to develop a corresponding calculation program. The numerical simulation results effectively elucidate the formation conditions and failure mechanisms of high sidewall caverns. Through comprehensive numerical simulations under various working conditions, we systematically investigate the conditions for generation and the influencing factors of splitting failure. Our study successfully uncovers the generation conditions and formation mechanisms of high sidewall cavern splitting failure. A crucial condition for this failure is that the initial maximum principal stress of the underground cavern aligns parallel to the tunnel axis direction, with a magnitude between 0.21 and 0.7 times the compressive strength. The primary mechanical cause of splitting failure during excavation and unloading is the stress oscillation at the cavern's sidewall. [ABSTRACT FROM AUTHOR]

In recent years, a new class of metallic materials featuring heterogeneous structures has emerged. These materials consist of distinct soft and hard domains with significant differences in mechanical properties, allowing them to maintain high strength while offering superior ductility. Magnesium (Mg) alloys, renowned for their low density, high specific strength, exceptional vibration damping, and electromagnetic shielding properties, exhibit tremendous potential as lightweight and functional materials. Despite their advantageous properties, high-strength Mg alloys often suffer from limited ductility. However, the emergence of heterogeneous materials provides a fresh perspective for the development of Mg alloys with both high strength and ductility. This article provided a fundamental overview of heterostructured materials and systematically reviewed the recent research progress in the design of Mg alloys with strength-ductility balance based on heterostructure principles. The review encompassed various aspects, including preparation methods, formation mechanisms of diverse heterostructures, and mechanical properties, both within domestic and international contexts. On this basis, the article discussed the challenges encountered in the design and fabrication of heterostructured Mg alloys, as well as the urgent issues that require attention and resolution in the future.

The thermoelastic wave propagation analysis is developed in a Love–Bishop nanorod resonator employing the strain gradient elasticity and Green–Naghdi theories with energy dissipation for the first time. The assumed nanorod resonator is subjected to thermal shock loading. The variational principle is applied to derive the coupled system of partial differential equations for temperature and displacement fields in a Love–Bishop nanorod. An analytical solution is proposed to solve the governing equations in Laplace domain. To obtain the temporal variation of fields' variables, a proper Laplace inversion technique is employed in the problem. The size effects in the nano-sized Love–Bishop rods are taken into account with five small-scale parameters including three higher-order materials length parameters, the micro-length inertia and thermal parameters. Two types of thermal shock loadings such as laser pulse-induced suddenly increasing temperature are utilized in this work, and the laser shock-induced thermoelastic wave propagations in both temperature and displacement fields are illustrated. The effects of the higher-order materials length parameters and the micro-length inertia and thermal parameters on the propagation of thermal and elastic waves are studied in detail. [ABSTRACT FROM AUTHOR]

In this paper, a thermoelastic problem involving a viscous strain gradient beam is considered from the analytical and numerical points of view. The so-called type II thermal law is used to model the heat conduction and two possible dissipation mechanisms are introduced in the mechanical part, which is considered for the first time within strain gradient theory. An existence and uniqueness result is proved by using semigroup arguments, and the exponential energy decay is obtained. The lack of differentiability for the semigroup of contractions is also shown. Then, fully discrete approximations are introduced by using the finite element method and the backward time scheme, for which a discrete stability property and a priori error estimates are proved. The linear convergence is derived under suitable additional regularity conditions. Finally, some numerical simulations are presented to demonstrate the accuracy of the approximations and the behavior of the discrete energy decay. [ABSTRACT FROM AUTHOR]

This paper presented a physico-mathematical model for dynamic fracture propagation in brittle materials with a purely continuum mechanics hemi-variational-based strain gradient theory. As for the quasi-static case, the simulation results, obtained by means of finite elements, revealed that strain gradient effects significantly affected the fracture propagation, leading to finite fracture thickness that was independent of the mesh size. It was also observed that nonsymmetric loading rate lead to a deviation from standard mode-Ⅰ crack propagation that cannot be revealed in the quasi-static case. The model results were compared against experimental data from fracture tests on notched specimens taken from the literature. The comparison showed good agreement between the model predictions and the experimental measurements. The presented model and simulation results can be useful in the design and optimization of structural components subjected to dynamic loading conditions. [ABSTRACT FROM AUTHOR]

Biomechanical characteristics can be used to assist the early detection of many diseases, including breast cancer, thyroid nodules, prostate cancer, liver fibrosis, ovarian diseases, and tendon disorders. In this paper, a scale-dependent viscoelastic model is developed to assess the biomechanical behaviour of biological tissues subject to flexural waves. The nonlocal strain gradient theory, in conjunction with machine learning techniques such as extreme gradient boosting, k-nearest neighbours, support vector machines, and random forest, is utilised to develop a computational platform for biomechanical analysis. The coupled governing differential equations are derived using Hamilton's law. Transverse wave analysis is conducted to investigate different normal and pathological human conditions including ovarian cancer, breast cancer, and ovarian fibrosis. Viscoelastic, strain gradient, and nonlocal effects are used to describe the impact of fluid content, stiffness hardening caused by the gradients of strain components, and stiffness softening associated with the nonlocality of stress components within the biological tissues and cells. The integration of the scale-dependent biomechanical continuum model with machine learning facilitates the adoption of the developed model in practical applications by allowing for learning from clinical data, alongside the intrinsic mechanical laws that govern biomechanical responses. [ABSTRACT FROM AUTHOR]

This research investigates the free vibration of a rotating annular microplate under the flexoelectric effect. Initially, the Kirchhoff plate theory assumptions are used to express the displacement fields. After considering the displacement field, strains and their gradients are derived and substituted into the electric enthalpy and kinetic energy expressions. Subsequently, by applying Hamilton’s principle to the aforementioned equations, the electric and mechanical equations are computed. To derive the equations of motion, initially, the polarization vectors and their gradients are derived from the electric equations and associated boundary conditions. Subsequently, these are incorporated into the mechanical equations, which also encompass electric components. It is notable that by removing the time-dependent terms from the in-plane equations of motion, the static displacement due to rotation at each speed is obtained. After deriving the equations of motion and boundary conditions, these equations are non-dimensionalized using non-dimensionalizing relations. In the next step, Hamilton’s principle is used to discretize the equations and boundary conditions. Consequently, by applying the generalized differential quadrature method and extracting the stiffness and mass matrices resulting from the transverse equation of motion and boundary conditions, the natural transverse frequency of the rotating annular microplate under the flexoelectric effect is calculated. The results of this research are useful for promoting the use of rotating annular microplants under flexoelectric effect for microelectromechanical systems designers with high efficiency. [ABSTRACT FROM AUTHOR]

The aim of this paper is to propose an experimental procedure for determining the characteristic length of a strain gradient model. The identification problem is studied through a virtual pull-out test of a rigid bar along the symmetry axis of a cylindrical strain gradient elastic domain. To allow an accurate parameter identification based on measured data, we investigate the effect of the characteristic length on the mechanical fields for this problem. We see a significant sensitivity of the inflection point of the displacement profile evaluated on the cross section of the cylinder, with respect to the characteristic length. By adjusting the characteristic length of the strain gradient such that the theoretical models match best with experimental measurements of the surface displacement fields, the characteristic length of the strain gradient can be estimated. In order to allow for more efficient analysis and an almost real-time parameter identification, the initial three-dimensional (3D) problem is reduced to a one-dimensional (1D) problem by exploiting the cylindrical symmetry of the problem. As will be shown, an accurate 1D finite element method (FEM) strain gradient solution can be obtained for this simplified problem. Since the cylindrical symmetry is only true in an infinitely long cylinder, specific boundary conditions are constructed on a cylinder of finite length, which is then used for the comparison of the 1D and 3D problems. Results show, however, that the structural response at the inflection point is insensitive to whether the specific boundary conditions are considered or not, which is why the 1D model can be used for parameter identification. Since the proposed approach is methodological, it can be applied to any material. As a prototype problem in this paper, we consider the case of a bar embedded in Portland cement concrete. [ABSTRACT FROM AUTHOR]

• The review encompasses the processing techniques, heterogeneous microstructures, and mechanical properties, with a specific emphasis on the strengthening and toughening mechanisms of heterostructured Mg alloys. • Based on the concept of heterogeneous structure design, the construction of different heterostructures in Mg alloys can obtain high strength while maintaining good ductility. • Based on the challenges encountered in the design and fabrication of heterostructured Mg alloys, some urgent issues and future research directions for heterogeneous Mg alloys are suggested. In recent years, a new class of metallic materials featuring heterogeneous structures has emerged. These materials consist of distinct soft and hard domains with significant differences in mechanical properties, allowing them to maintain high strength while offering superior ductility. Magnesium (Mg) alloys, renowned for their low density, high specific strength, exceptional vibration damping, and electromagnetic shielding properties, exhibit tremendous potential as lightweight and functional materials. Despite their advantageous properties, high-strength Mg alloys often suffer from limited ductility. However, the emergence of heterogeneous materials provides a fresh perspective for the development of Mg alloys with both high strength and ductility. This article provided a fundamental overview of heterostructured materials and systematically reviewed the recent research progress in the design of Mg alloys with strength-ductility balance based on heterostructure principles. The review encompassed various aspects, including preparation methods, formation mechanisms of diverse heterostructures, and mechanical properties, both within domestic and international contexts. On this basis, the article discussed the challenges encountered in the design and fabrication of heterostructured Mg alloys, as well as the urgent issues that require attention and resolution in the future. [ABSTRACT FROM AUTHOR]

In this paper, a thermoelastic problem involving a viscous strain gradient beam is considered from the analytical and numerical points of view. The so-called type Ⅱ thermal law is used to model the heat conduction and two possible dissipation mechanisms are introduced in the mechanical part, which is considered for the first time within strain gradient theory. An existence and uniqueness result is proved by using semigroup arguments, and the exponential energy decay is obtained. The lack of differentiability for the semigroup of contractions is also shown. Then, fully discrete approximations are introduced by using the finite element method and the backward time scheme, for which a discrete stability property and a priori error estimates are proved. The linear convergence is derived under suitable additional regularity conditions. Finally, some numerical simulations are presented to demonstrate the accuracy of the approximations and the behavior of the discrete energy decay.

The hetero-zone boundary affected region (HBAR), with a high strain gradient, plays a crucial role in the synergistic deformation of layered materials. Our previous experimental study demonstrated that a decreasing interfacial spacing leads to a higher fraction of HBAR and an enhanced combination of strength and ductility. In this work, a conventional mechanism-based strain gradient (CMSG) plasticity model is adopted to simulate the tensile behavior of Cu–Fe layered materials with three different interfacial spacings. The simulation results indicated that strain/stress partitioning and strain banding are the main factors for the synergistic deformation behavior. Strain bands are more likely to be activated in the Cu–Fe layered materials with smaller interfacial spacing. In addition, the formation of HBAR near the layer boundary can be observed, consistent with the previous experiments. During deformation, the HBAR induces back stress and forward stress to strengthen the Cu layer and weaken the Fe layer, respectively. The simulation results indicate the stress transfer between the Cu and Fe layers, which benefits the strain hardening and enhances synergistic deformation. This study provides a valuable insight into the strength-ductility synergy of layered materials. It demonstrates that increasing the HBAR fraction is a viable approach to enhance the mechanical properties of hetero-structured materials.

Ni-based superalloys, essential for high-temperature applications, derive strength from coherent second-order precipitates that impede dislocation motion through coherency misfit and elastic mismatch. This study employs multi-component phase-field crystal (PFC) simulations to explore the elastic deformation of such precipitates. Using a binary ordered square structure for the precipitate and a single species square structure for the matrix, elastic properties and lattice parameters are fitted to data from ab initio density functional theory calculations for Ni and Ni3Ti systems. Simulations reveal a smooth strain gradient across the matrix–precipitate interface with coherency misfit influenced by precipitate size and strain state. These findings highlight the utility of PFC simulations for understanding strain distribution and deformation in precipitate–matrix systems with the potential to offer insights for both experimental and computational studies.

• The GDH-PD is proposed to address size-dependent effects and fracture problems. • The lower- and higher-order micro-modulus states of GDH-PD are firstly derived. • GDH-PD eliminates zero-energy modes of traditional higher-order peridynamics. • HOBCs are applied by constructing quadratic functional on the boundary points. • Nonlocal effects, size effects, strain gradient effects and damage are considered. In this paper, we propose the implicit stabilized dual-horizon peridynamics-based strain gradient damage model (GDH-PD) to describe the cross-scale fracture behavior of materials. To this end, firstly, the strain energy density function of GDH-PD is reformulated by considering the energy compensation to eliminate zero-energy modes of the traditional higher-order peridynamics. And then, the constitutive force state of GDH-PD is derived and explicitly expressed with the help of the proposed special dimension reduction of the nonlocal higher-order tensors. To solve the steady-state crack propagation problems, the implicit GDH-PD is developed by deriving the lower- and higher-order micro-modulus double state, such that the linearization of the equilibrium equation of GDH-PD is established. At last, the bond length-dependent energy-based failure criterion is used to characterize the cross-scale fracture in the form of bond breakage. The effectiveness of GDH-PD to characterize microstructure size effects and macrostructure strain gradient effects are investigated by numerical simulations. The numerical results are in good agreement with the analytical solutions or the available experimental results. We believe that the proposed GDH-PD may pave the way to an increased application of peridynamics to be used in the cross-scale fracture predictions for the advanced material. [ABSTRACT FROM AUTHOR]

Of concern is a problem of strain gradient porous elastic theory with nonlinear damping terms, whose constitutive equations contain higher-order derivatives of the displacement in the basic postulates. The paper is based on the theory of 'consistency' due to Aouadi et al. [J. Therm. Stress. 43(2)(2020), 191–209] and Ieşan [American Institute of Physics, Conference Proceedings, 1329 (2011), 130–149], and contains four results. We firstly show that the system is global well posed by using maximal monotone operator. The second main result is the existence of global attractors which is proved by the method developed by Chueshov and Lasiecka [Long-time behavior of second order evolution equations with nonlinear damping. Mem. Amer. Math. Soc. vol. 195, no. 912, Providence, 2008; Von Karman evolution equations: well-posedness and long-time dynamics. Springer Monographs in Mathematics, Springer, New York, 2010]. By showing the system is gradient and asymptotically smooth, we establish the existence of global attractors with finite fractal dimension via a stabilizability inequality. Then we study the continuity of global attractors regarding the parameter in a residual dense set. The above results allow the damping terms with polynomial growth. Finally we discuss the exponential decay and global boundedness to the linear case of damping terms of the system. The assumption of equal-speed wave propagations is not needed for all of results obtained. [ABSTRACT FROM AUTHOR]

In this work, we study a strain gradient problem involving mixtures. The variational formulation is written as a first-order in time coupled system of parabolic variational equations. An existence and uniqueness result is recalled. Then, we introduce a fully discrete approximation by using the finite element method and the implicit Euler scheme. A discrete stability property and a priori error estimates are proved. Finally, some one- and two-dimensional numerical simulations are performed. [ABSTRACT FROM AUTHOR]

In this study, a new anisotropic elastic shell model with a nonlocal strain gradient is developed to investigate the vibrations of simply supported single-walled carbon nanotubes (SWCNTs). The Sanders–Koiter shell theory is used to obtain strain–displacement relationships. Eringen's nonlocal elasticity and Mindlin's strain gradient theories are adopted to derive the constitutive equations, where the anisotropic elasticity constants are expressed via Chang's molecular mechanics model. An analytical method is used to solve the equations of motion and to obtain the natural frequencies of SWCNTs. First, the anisotropic elastic shell model without size effects is validated through comparison with the results of molecular dynamics simulations reported in the literature. Then, the effects of the nonlocal and material parameters on the natural frequencies of SWCNTs with different geometries and wavenumbers are analyzed. From the numerical simulations, it is confirmed that the natural frequencies decrease as the nonlocal parameter increases, while they increase as the material parameter increases. As new results, the reduction in natural frequencies with increasing SWCNT radius and the increase in natural frequencies with increasing wavenumber are both amplified as the material parameter increases, while they are both attenuated as the nonlocal parameter increases. [ABSTRACT FROM AUTHOR]

Abstract Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices.

Ultrafine-grained (UFG) materials typically exhibit high strength but disappointingly low ductility due to the limited dislocation accumulation. To evade the strength-ductility tradeoff of UFG pure aluminum (Al), four kinds of UFG/coarse-grained (CG)/UFG sandwich-structured pure Al laminates were designed via extrusion and cold rolling. Microstructure characterization displayed that the grain size of the CG in the four designed laminates always remained ∼5 times that of UFG, indicating significant grain size heterogeneity between the constituent layers. The finite element analysis indicated that as the CG/UFG layer thickness ratio increased, the distribution of residual stress between the constituent layers was inhomogeneous and became more pronounced near the interface, which reduced the tensile ductility of the laminates. Tensile tests indicated that when the CG:UFG layer thickness ratio was 1:3, the laminate obtained a desirable strength and ductility compared to other laminates and UFG sample, with a yield strength of 150 MPa and a tensile ductility of 13.5 %. The optical microscope (OM)-digital image correlation (DIC) results revealed that the significant strain gradient near the layer interface caused by the heterogeneous deformation contributed to hetero-deformation induced (HDI) strengthening and HDI strain hardening, thereby strengthening and toughening the laminate.

The application of metal additive manufacturing technology and products in the aviation field requires optimized structural design at macroscale and precise manufacture control at microscale. As one of the typical features of additive manufacture, microstructure inevitably affects the material performance. The research has shown that the uniformity, plasticity, and fatigue fracture characteristics of additive manufacturing materials are often inferior to traditional materials, while their strength, hardness, wear resistance, and some microscale properties are often better than traditional materials. The size effect in the micro/nano-scale and the heterogeneous characteristics of materials have a significant impact on metal materials with microstructures. Under different microstructures, materials can achieve a better balance between strength and ductility, which is also applicable to additively manufactured metals. Therefore, the process characteristics of additive manufacturing and the heterogeneities introduced by human design are both expected to significantly improve the comprehensive performance of metals, which have important guiding value for the application of metal additive manufacturing in the aviation field. However, since many of the mechanism of these phenomena are still unclear, the strength-ductility synergistic and antagonistic relationships with other properties of the materials are also worth further research.

Scientists have explored alternative reinforcements to improve the mechanical properties of composite structures in recent years due to the high financial and environmental costs associated with the synthesis of carbon nanotubes and graphene plates. A sandwich composite beam reinforced with carbon nanorods (CNRRCs) synthesized from potato waste with a porous core is examined in this study. The nonlocal strain gradient theory and general strain theory are applied to this porous core structure under axially variable force. Based on the sinusoidal theory, shear deformation theory is used to calculate displacement fields of reinforced composite sandwich beams. Equilibrium sandwich beam equations are derived using higher order shear deformation theory. The mixture rule is used to determine the properties of the face sheet, including Young's modulus, shear modulus, and Poisson's ratio. A variable axial compression force is used to calculate the external work. Based on the Ritz method and different boundary conditions, the final equations are derived, and then the buckling and stiffness matrices are derived, and finally, the equations are solved and the critical buckling load is determined. An analysis of the critical buckling load for this structure is conducted using carbon nanorods made from potato waste. In addition, various parameters such as the strain gradient parameter, the non-local parameter, the volume fraction of carbon nanorods, and the thickness ratio are discussed. In this study, carbon nanorods made from recycled materials are found to increase the stiffness of sandwich beams and then increase critical buckling loads, which is lower than carbon nanotubes and more economical than carbon nanotubes. Graphene platelets and carbon nanotubes have a higher stiffness-to-cost ratio than carbon nanorods. Carbon nanorods, on the other hand, have a lower cost than graphene platelets and carbon nanotubes.

The local scaling symmetry of the Lagrange density is exploited to investigate the myriad electro-mechanical coupling effects observed in the elastic dielectrics. In contrast to most known flexoelectricity theories, this approach has also explicated on the geometric underpinnings of the induced polarization and electric potential. Due to the inhomogeneous scaling of the metric, the invariance of the Lagrange density will be lost. To restore the gauge invariance of the Lagrange density, we introduce the notion of minimal replacement, viz., define a gauge covariant operator in place of the ordinary partial derivative. Minimal replacement introduces gauge compensating one form field. We relate the exact part of the gauge compensating one form field with electric potential. However, the gauge compensating one form field's anti-exact part is correlated with the electric polarization vector. We constructed different components of the gauge invariant energy density using the geometric objects like scale invariant gauge curvature. We appropriately contracted the skew symmetric gauge curvature tensor with the metric to construct the scale invariant energy associated with the gauge field. Finally, we derive the governing coupled equations employing Hamilton's principle. In order to assess how the theory performs, we carry out a few numerical simulations and validation. Model prediction of the pressurized flexoelectric disk with a central hole shows good agreement with the analytical solution. We also investigated flexoelectric cantilever beam subjected to a point load and different earthing conditions. Explorations of this kind of coupling may have notable implications in many industrial and laboratory applications. • Studied electro-mechanical coupling in dielectrics using the scaling symmetry. • Explored geometric underpinnings of induced polarization and electric potential. • Also recovered elastic Lagrangian assuming Euclidean manifold with gauge fixing. • Correspondence with Mindlin's polarization gradient theory for dielectrics. • This kind of coupling may have notable implications in many applications. [ABSTRACT FROM AUTHOR]

The phenomenon of flexoelectricity, wherein the generation of electrical polarization results from strain gradients, has garnered significant attention in electromechanics. This phenomenon holds immense potential for diverse applications in nanoelectromechanical systems. Doubly curved shells present a particularly compelling structure for implementing flexoelectric devices, representing the most general scenario. To comprehensively understand the mechanical behavior of nanoscale flexoelectric doubly curved shells, we present a novel analytical model in this study. Our model incorporates a reformulated strain gradient elasticity theory, a modified expression of electric enthalpy density considering Maxwell's self-field gradient, and a moderately thick shell configuration. To further elucidate this model, we establish complete formulations and analytical solutions of static bending and free vibration problems. This study reveals the crucial roles that various parameters such as principal curvature radii, length scale parameters, transverse structural sizes, and the Winkler–Pasternak foundation play in controlling the mechanical behavior of nanoscale flexoelectric doubly curved shells. These theoretical findings offer valuable insights into the design of nanodevices based on the direct flexoelectric effect. [ABSTRACT FROM AUTHOR]

Flexoelectric effect describes the electromechanical coupling between the strain gradient and its internal polarization in all dielectrics. Despite this universality, the resulting flexoelectric field remains small at the macroscopic level. However, in nanosystems, the size-dependent effect of flexoelectricity becomes increasingly significant, leading to a notable flexoelectric field that can strongly influence the material's physical properties. This review aims to explore the flexoelectric effect specifically at the nanoscale. We achieve this by examining strain gradients generated through two distinct methods: internal inhomogeneous strain and external stimulation. In addition, advanced synthesis techniques are utilized to enhance the properties and functionalities associated with flexoelectricity. Furthermore, we delve into other coupled phenomena observed in thin films, including the coupling and utilization of flexomagnetic and flexophotovoltaic effects. This review presents the latest advancements in these areas and highlights their role in driving further breakthroughs in the field of flexoelectricity. [ABSTRACT FROM AUTHOR]

Flexoelectricity, a remarkable size-dependent effect, means that strain gradients can give rise to electric polarization. This effect is particularly pronounced near defects within flexoelectric solids, where large strain gradients exist. A thorough understanding of the internal defects of flexoelectric devices and their surrounding multiphysics fields is crucial to comprehend their damage and failure mechanisms. Motivated by this, strain gradient elasticity theory is utilized to investigate the mechanical and electrical behaviors of flexoelectric solids with cylindrical cavities under biaxial tension. Closed-form solutions are obtained under the assumptions of plane strain and electrically impermeable defects. In particular, this study extends the Kirsch problem of classical elasticity theory to the theoretical framework of higher-order electroelasticity for the first time. Our research reveals that different length scale parameters of the strain gradient and bidirectional loading ratios significantly affect the hoop stress field, radial electric polarization field, and electric potential field near the inner cylindrical cavity of the flexoelectric solid. Furthermore, we validate our analytical solution by numerical verification using mixed finite elements. The congruence between the two methods confirms our analytical solution's accuracy. The findings presented in this paper provide deeper insights into the internal defects of flexoelectric materials and can serve as a foundation for studying more complex defects in flexoelectric solids. [ABSTRACT FROM AUTHOR]

This study couples peridynamic (PD) theory with strain gradient elasticity (SGE) theory to investigate the combined effect of PD and SGE length scale parameters on size effect. The SGE theory introduces a length scale parameter in the stress–strain relations to account for size effect. The solution to the classical form of SGE equation of motion requires two additional non-classical boundary conditions arising from the presence of fourth-order spatial derivatives. The wave dispersions level off as the wave number increases as observed in real materials. The PD theory allows for nonlocal interactions of a material point within its horizon which serves as the PD length scale parameter. The equation of motion emerges in the form of an integral equation and the internal forces are expressed through nonlocal interactions (bonds) between the material points within a continuous body. It is a reformulation of classical continuum mechanics which is free of spatial derivatives. The numerical results concern a previously considered carbon nanotube (CNT). Under quasi-static axial loading, the results indicate an increased strengthening effect along the length of the tube. Under dynamic loading, its longitudinal vibration response is captured through explicit time integration. The numerical predictions capture the reference solutions corresponding to the classical SGE equation. [ABSTRACT FROM AUTHOR]

In this paper, the Johnson-Cook constitutive model is modified by considering the influence of hard point such as TiC and NbC in the matrix of Inconel718 on the deformation stress. The theoretical model of cutting specific energy in the main deformation zone of Inconel718 is modified based on the new constitutive model by combining the strain gradient theory. The effect of different cutting parameters on cutting specific energy is studied, and the effect of cutting specific energy on cutting deformation and the resulting dimensional effect are also analyzed. The research results show that cutting specific energy increases with the increase of cutting speed. With the increase of cutting thickness, the cutting specific energy reduced, and the trend is non-linear. The change of undeformed chip thickness will cause size effect. The cutting specific energy increases with the reduction of the thickness of undeformed chip, and the impact of the thickness of undeformed chip on the cutting specific energy becomes smaller and smaller as the speed increases. The existence of hard points makes the main deformation zone generate a large amount of heat energy and deformation energy, which leads to dimensional effects and makes the material be more prone to adiabatic shear instability, then leads to the increase of cutting specific energy. With the increase of cutting specific energy, the width of adiabatic shear band is narrowed, the degree of serration is aggravated, and the chip morphology is closer to the experimental results. [ABSTRACT FROM AUTHOR]

Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Materials are of prime importance for designing and manufacturing structures and components in numerous industries, including aviation, aerospace, military, automotive, machine construction, electronics, and telecommunications, among others. Throughout the industrial transformations in human history, it is evident that the materials industry had the most significant impact on scientific and technological progress. In recent years, the Fourth Industrial Revolution has altered the infrastructure and character of production in a number of global industries. Materials science has been contributing a significant and essential role in the global competitiveness of all industries, particularly those utilizing electronic domains such as semiconductors, microprocessors, and sensors for industrial and social applications. Consequently, nanoscale materials with exceptional properties have garnered the interest of numerous researchers. One of these phenomena in dielectric materials is flexoelectricity. This phenomenon was discovered in the 1950s of the previous century, but it wasn't until the early 2000s, when materials science and other disciplines flourished, that many researchers began to focus on it. In recent years, the applicability of flexoelectric materials has increased across all disciplines. In addition, as a consequence of the importance of novel electrical materials to the flexoelectric effect, the research problem for this material broadly and the analysis of the mechanical responses of flexoelectric structures are being investigated and developed at a rapid rate. This paper provides an overview of the flexoelectric phenomenon, together with potential applications and recommendations for further study. The article's content will serve as a valuable resource for scientists interested in dielectric materials with unique electromechanical effects, which are extensively used in contemporary electronic disciplines. [ABSTRACT FROM AUTHOR]

Biomechanical characteristics can be used to assist the early detection of many diseases, including breast cancer, thyroid nodules, prostate cancer, liver fibrosis, ovarian diseases, and tendon disorders. In this paper, a scale-dependent viscoelastic model is developed to assess the biomechanical behaviour of biological tissues subject to flexural waves. The nonlocal strain gradient theory, in conjunction with machine learning techniques such as extreme gradient boosting, k-nearest neighbours, support vector machines, and random forest, is utilised to develop a computational platform for biomechanical analysis. The coupled governing differential equations are derived using Hamilton’s law. Transverse wave analysis is conducted to investigate different normal and pathological human conditions including ovarian cancer, breast cancer, and ovarian fibrosis. Viscoelastic, strain gradient, and nonlocal effects are used to describe the impact of fluid content, stiffness hardening caused by the gradients of strain components, and stiffness softening associated with the nonlocality of stress components within the biological tissues and cells. The integration of the scale-dependent biomechanical continuum model with machine learning facilitates the adoption of the developed model in practical applications by allowing for learning from clinical data, alongside the intrinsic mechanical laws that govern biomechanical responses.

Flexoelectric effect describes the electromechanical coupling between the strain gradient and its internal polarization in all dielectrics. Despite this universality, the resulting flexoelectric field remains small at the macroscopic level. However, in nanosystems, the size-dependent effect of flexoelectricity becomes increasingly significant, leading to a notable flexoelectric field that can strongly influence the material’s physical properties. This review aims to explore the flexoelectric effect specifically at the nanoscale. We achieve this by examining strain gradients generated through two distinct methods: internal inhomogeneous strain and external stimulation. In addition, advanced synthesis techniques are utilized to enhance the properties and functionalities associated with flexoelectricity. Furthermore, we delve into other coupled phenomena observed in thin films, including the coupling and utilization of flexomagnetic and flexophotovoltaic effects. This review presents the latest advancements in these areas and highlights their role in driving further breakthroughs in the field of flexoelectricity.

In the context of architected materials, it has been observed that both long-wavelength instabilities leading possibly to localization and short-wavelength instabilities leading to the apparition of a deformation pattern could occur. This work proposes for the first time a comparison of the ability of two families of higher order equivalent media, namely strain-gradient and micromorphic media, to capture both patterning and long-wavelength macroscopic instabilities in those materials. The studied architected material consists in a simple one-dimensional arrangement of non-linear springs, thus allowing for analytical or nearly analytical treatment of the problem, avoiding any uncertainties or imprecisions coming from a numerical method. A numerical solution of the problem is then used to compare the post-buckling prediction of both models. The study concludes that, micromorphic media are the appropriate choice of equivalent continuum models to emply when dealing with the possibility of patterning inside a structured medium, but if long-wavelength global instability is of interest, a strain-gradient type equivalent medium is well suited. [ABSTRACT FROM AUTHOR]

An electrical‐biased or mechanical‐loaded scanning probe written on the ferroelectric surface can generate programmable domain nanopatterns for ultra‐scaled and reconfigurable nanoscale electronics. Fabricating ferroelectric domain patterns by direct‐writing as quickly as possible is highly desirable for high response rate devices. Using monolayer α‐In2Se3 ferroelectric with ≈1.2 nm thickness and intrinsic out‐of‐plane polarization as an example, a writing‐speed dependent effect on ferroelectric domain switching is discovered. The results indicate that the threshold voltages and threshold forces for domain switching can be increased from −4.2 to −5 V and from 365 to 1216 nN, respectively, as the writing‐speed increases from 2.2 to 10.6 µm s−1. The writing‐speed dependent threshold voltages can be attributed to the nucleations of reoriented ferroelectric domains, in which sufficient time is needed for subsequent domain growth. The writing‐speed dependent threshold forces can be attributed to the flexoelectric effect. Furthermore, the electrical‐mechanical coupling can be employed to decrease the threshold force, achieving as low as ≈189±41 nN, a value smaller than those of perovskite ferroelectric films. Such findings reveal a critical issue of ferroelectric domain pattern engineering that should be carefully addressed for programmable direct‐writing electronics applications. [ABSTRACT FROM AUTHOR]