Large Strain Field Research Articles

New strain configuration of convergence in the Arabia-Eurasia collisional zone: A comprehensive analysis of large shear zones inferred from geodetic and seismic observations

A new strain configuration is presented concerning the ongoing convergence deformation field in the Turkish-Iranian High Plateau (TIHP), E Anatolia-NW Iran. The strain configuration is derived from the contribution of geodetic strain-rate fields using GPS velocity data to reveal new constraints not documented previously in the region. This provides good evidence for novel “highland shear strain zones” by analysis of the combination of interpolated GPS velocity fields and seismological observations with maximum principal strain axes. Both the strain-rate fields and rotational elements of the estimated geodetic deformation exhibit juxtaposed, patchy strain rate partitioning in E Anatolia-NW Iran. These comprise both compressional strain lobes combined with local extensional areas and large shear strain fields in the core of the collision where the rotation rate fields present an unexpected propagation zone of continuous, counterclockwise rotation towards the W. The main results define approximately N-S- and W-E-trending, large, continuous, diagonal shear strain patterns with high total strain rate and maximum shear strain rate, consistent with right-lateral strike-slip and normal focal mechanisms. These are the Longitudinal Shear Zone (LSZ) extending from S to N and dividing the TIHP into the E Anatolia and NW Iran blocks, and the right-lateral Transcurrent Shear Zone (TSZ) extending from E (NW Iran) to W (E Anatolia). These differential movements of the inconsistent LSZ and TSZ shape both the highly buoyant plateau topography and the area, which undergoes migration to the W with respect to stable Eurasia by means of plateau push. Geodetic deformation compatible with fault focal data indicates that the LSZ and TSZ, in the framework of thermal and mechanical changes in the structure of the lithospheric thickness beneath the TIHP, are invaded and shaped by flowing mantle material that triggers and drives lateral escape of convergent crustal blocks, and configures the convergence with a shear-dominated deformation regime. The shear regime is primarily driven by NE- and NW-directed mantle flow fields and associated buoyancy forces due to slab fragmentation and tears in the region, exerting dominant control over the differential evolution of the LSZ and TSZ patterns. The new strain configuration for this convergence illuminates “the highland tectonics of large shear zones” resulting from slab thinning or its absence, and challenges conventional tectonic strain models in the TIHP.

Comparing the Impacts of Strain Types on Oxygen-Vacancy Formation in a Perovskite Oxide via Nanometer-Scale Strain Fields.

The utilization of an in-plane lattice misfit in an oxide epitaxially grown on another oxide with a different lattice parameter is a well-known approach to induce strains in oxide materials. However, achieving a sufficiently large misfit strain in this heteroepitaxial configuration is usually challenging, unless the thickness of the grown oxide is kept well below a critical value to prevent the formation of misfit dislocations at the interface for relaxation. Instead of adhering to this conventional approach, here, we employ nanometer-scale large strain fields built around misfit dislocations to examine the effects of two distinct types of strains─tension and compression─on the generation of oxygen vacancies in heteroepitaxial LaCoO3 films. Our atomic-level observations, coupled with local electron-beam irradiation, clarify that the in-plane compression notably suppresses the creation of oxygen vacancies, whereas the formation of vacancies is facilitated under tensile strain. Demonstrating that the defect generation can considerably vary with the type of strain, our study highlights that the experimental approach adopted in this work is applicable to other oxide systems when investigating the strain effects on vacancy formation.

The Impact of Local Strain Fields in Noncollinear Antiferromagnetic Films.

Antiferromagnets hosting structural or magnetic order that breaks time reversal symmetry are of increasing interest for "beyond von Neumann" computing applications because the topology of their band structure allows for intrinsic physical properties, exploitable in integrated memory and logic function. One such group are the noncollinear antiferromagnets. Essential for domain manipulation is the existence of small net moments found routinely when the material is synthesized in thin film form and attributed to symmetry breaking caused by spin canting, either from the Dzyaloshinskii-Moriya interaction or from strain. Although the spin arrangement of these materials makes them highly sensitive to strain, there is little understanding about the influence of local strain fields caused by lattice defects on global properties, such as magnetization and anomalous Hall effect. This premise is investigated by examining noncollinear antiferromagnetic films that are either highly lattice mismatched or closely matched to their substrate. In either case, edge dislocation networks are generated and for the former case, these extend throughout the entire film thickness, creating large local strain fields. These strain fields allow for finite intrinsic magnetization in seemingly structurally relaxed films and influence the antiferromagnetic domain state and the intrinsic anomalous Hall effect.

Pneumatic Non-Equibiaxial Cell Stretching Device With Live-Cell Imaging.

Adherent cell behavior is influenced by a complex interplay of factors, including chemical and mechanical signals. In vitro experiments that mimic the mechanical environment experienced by cells in vivo are crucial for understanding cellular behavior and the progression of disease. In this study, we developed and validated a low-cost pneumatically-controlled cell stretcher with independent control of strain in two directions of a membrane, enabling unequal biaxial stretching and real-time microscopy during actuation. The stretching was achieved by two independent pneumatic channels controlled by electrical signals. We used finite element simulations to compute the membrane's strain field and particle tracking algorithms based on image processing techniques to validate the strain fields and measure the cell orientation and morphology. The device can supply uniaxial, equibiaxial, and unequal biaxial stretching up to [Formula: see text] strain in each direction at a frequency of [Formula: see text], with a strain measurement error of less than 1%. Through live cell imaging, we determined that distinct stretching patterns elicited differing responses and alterations in cell orientation and morphology, particularly in terms of cell length and area. The device successfully provides a large, uniform, and variable strain field for cell experiments, while also enabling real-time, live cell imaging. This scalable, low-cost platform provides mechanical stimulation to cell cultures by independently controlling strains in two directions. This could contribute to a deeper understanding of cellular response to bio-realistic strains and could be useful for future in vitro drug testing platforms.

The effect of ferroelastic domains and ultrahigh‐density dislocations on fracture toughness of high entropy niobates

AbstractThis study investigates the fracture toughness and toughening mechanism of (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4, a high entropy material characterized by a high thermal expansion coefficient and low thermal conductivity. Our finding demonstrates a noteworthy 50% increase in fracture toughness for (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 (∼3.09 MPa m0.5) compared to YNbO4 (∼2.11 MPa m0.5). The superior fracture toughness for (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 can be attributed to the two key factors: relatively easy ferroelastic domain switching and the presence of ultrahigh‐density dislocations (∼7 × 108 mm−2). The enhanced ferroelastic toughening in (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4 is associated with its distinct characteristics, large atomic radius (RA), significant tetragonality (c/a ratio), and low monoclinic β angle. These factors contribute to larger spontaneous strain, larger coercive strain, and lower transformation strain, respectively. These properties facilitate the formation and switching of the ferroelastic domains. In addition, the interactions between the large strain field surrounding the dislocations and the cracks facilitate crack deflection and bridging. The combined effect of ferroelastic toughening and ultrahigh‐density dislocations ultimately leads to the significant improvement in the fracture toughness of (La0.2Nd0.2Sm0.2Eu0.2Y0.2)NbO4.

Effect of heat treatment on microstructure and pseudoelasticity of a memory‐steel

AbstractThe pseudoelasticity behavior of memory‐steels has recently gained attention for applications in seismic‐resistant structures. The pseudoelasticity can be used to reduce residual deformations in building structures subjected to seismic loading. Fe‐based shape memory alloys (Fe‐SMA), which is also called as memory‐steel, have gained much attention because of their strong shape memory behavior and low material manufacturing cost, which justify their large‐scale application in construction. This research investigated the effects of heat treatment on microstructure and pseudoelasticity of a memory‐steel (Fe‐17Mn‐5Si‐10Cr‐4Ni‐1(V,C) %wt.). Various solution‐annealing treatments from 1000 to 1200 °C for 2 h and aging from 650, 750 and 850 °C were applied to investigate the microstructural evolution and pseudoelasticity behavior of the alloy. The results showed that increasing the solution annealing temperature caused grain growth and pseudoelasticity loss. On the other hand, the precipitation of (Cr,V)Cs after aging resulted in an improved pseudo‐elasticity. The large elastic strain field, which forms near the carbide precipitates, improves the pseudoelasticity. Furthermore, precipitates can provide preferential nucleation sites for the martensite phase and a high density of stacking faults in the austenite matrix. The study concludes that the best pseudoelasticity behavior was achieved after an aging at 750°C for 6 h.

Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Ge1–x–yBixCayTe with Ultrafine Ferroelectric Domain Structure

GeTe and its derivatives emerging as a promising lead-free thermoelectric candidate have received extensive attention. Here, a new route was proposed that the minimization of κL in GeTe through considerable enhancement of acoustic phonon scattering by introducing ultrafine ferroelectric domain structure. We found that Bi and Ca dopants induce strong atomic strain disturbance in the GeTe matrix because of large differences in atom radius with host elements, leading to the formation of ultrafine ferroelectric domain structure. Furthermore, large strain field and mass fluctuation induced by Bi and Ca codoping result in further reduced κL by effectively shortening the phonon relaxation time. The co-existence of ultrafine ferroelectric domain structure, large strain field, and mass fluctuation contribute to an ultralow lattice thermal conductivity of 0.48 W m-1 K-1 at 823 K. Bi and Ca codoping significantly enhances the Seebeck coefficient and power factor through reducing the energy offset between light and heavy valence bands of GeTe. The modified band structure boosts the power factor up to 47 μW cm-1 K-2 in Ge0.85Bi0.09Ca0.06Te. Ultimately, a high ZT of ∼2.2 can be attained. This work demonstrates a new design paradigm for developing high-performance thermoelectric materials.

Vacancy accumulation mechanism at iron grain boundaries: The influence of grain boundary character and its coupling with grain size

The interaction between the radiation defects of vacancies (Vs) and grain boundaries (GBs) is critical to the understanding of the radiation response of polycrystalline materials. In this work, by combing atomistic calculations with kinetic Monte Carlo methods, multiscale simulations of Vs behavior near different iron GBs were carried out to reveal the Vs accumulation mechanisms at GBs. On the one hand, the dependence of Vs accumulation on the GB character was clarified. At high temperature, new V escaping processes were revealed, including the emission from twin boundary and special coincidence site lattice (CSL) GB with a low V–GB binding strength and the leakage from the extremely low-angle GB with a bulk-like region between dislocation cores. These processes weaken the trapping efficiency of such GBs for Vs. Enhanced Vs trapping could be obtained for the general low-angle GB because of the large strain field. Due to the large V–GB binding energy and trapping site density, the general high-angle GB was found to capture Vs efficiently. On the other hand, the coupling of grain size with GB character via the combination of grain size, V formation energy at the GB and migration energy barrier within the GB was proposed, which leads to the competition between the V emission from the GB and its cruise along the GB. This accounts for the experimentally observed scattered relation of the radiation-performance to defect–GB binding strength. The present work provides some theoretical guidance for optimizing the radiation resistance of polycrystalline materials based on GB engineering.

Inhibiting the bipolar effect via band gap engineering to improve the thermoelectric performance in n-type Bi2-Sb Te3 for solid-state refrigeration

• Se replacement can enlarge the E g and thus effectively mitigate the bipolar effect. • The manipulation of donor-like effect retains the optimized carrier concentration. • The large strain field and mass fluctuation reduce lattice thermal conductivity. • A state-of-the-art zT = 1.0 at 300 K is attained for n -type Bi 1.5 Sb 0.5 Te 2.8 Se 0.2 . • n -type Bi 2- x Sb x Te 3 becomes a promising candidate for solid-state cooling field. To date, the benchmark Bi 2 Te 3 -based alloys are still the only commercial material system used for thermoelectric solid-state refrigeration. Nonetheless, the conspicuous performance imbalance between the p -type Bi 2- x Sb x Te 3 and n -type Bi 2 Te 3- x Se x legs has become a major obstacle for the improvement of cooling devices to achieve higher efficiency. In our previous study, novel n -type Bi 2- x Sb x Te 3 alloy has been proposed via manipulating donor-like effect as an alternative to mainstream n -type Bi 2 Te 3- x Se x . However, the narrow bandgap of Bi 2- x Sb x Te 3 provoked severe bipolar effect that constrained the further improvement of zT near room temperature. Herein, we have implemented band gap engineering in n -type Bi 1.5 Sb 0.5 Te 3 by employing isovalent Se substitution to inhibit the undesired intrinsic excitation and achieve the distinguished room-temperature zT . First, the preferential occupancy of Se at Te 2 site appropriately enlarges the band gap, thereby concurrently improving the Seebeck coefficient and depressing the bipolar thermal conductivity. In addition, the Se alloying mildly suppresses the compensation mechanism and essentially preserves the already optimized carrier concentration, which maintains the peak zT near room temperature. Moreover, the large strain field and mass fluctuation generated by Se alloying leads to the remarkable reduction of lattice thermal conductivity. Accordingly, the zT value of Bi 1.5 Sb 0.5 Te 2.8 Se 0.2 reaches 1.0 at 300 K and peaks 1.1 at 360 K, which surpasses that of most well-known room-temperature n -type thermoelectric materials. These results pave the way for n -type Bi 2- x Sb x Te 3 alloys to become a new and promising top candidate for large-scale solid-state cooling applications.

High symmetry structure and large strain field fluctuation lead enhancement of thermoelectric performance of quaternary alloys by tuning configurational entropy

Quaternary Cu2MnSnSe4 alloys are promising thermoelectric materials due to their earth-abundant nature and environmentally friendly components. Recently, effectiveness of entropy engineering in phonon transport regulation has been demonstrated, while maintaining well electrical properties remains challenging. Here, first principles calculation predicts Fe Co and Zn have weak effect on electronic band edges of Cu2.1Mn0.9SnSe4, so compositions of Cu2.1Mn0.8Fe0.1SnSe4, Cu2.1Mn0.7Fe0.1Co0.1SnSe4, Cu2.1Mn0.6Fe0.1Co0.1Zn0.1SnSe4 are designed with tetragonal structure. Their thermodynamic stabilities are confirmed by analysis of entropy, enthalpy and Gibbs free energy. The increased configurational entropy leads to a well-maintained tetragonal structure with high symmetry, thus obtaining high Seebeck coefficients, which compensate for reduction in electrical conductivity. A highest power factor of 732.1 μW m−1 K−2 at 673 K is obtained for Cu2.1Mn0.9SnSe4 sample. Meanwhile, increased configurational entropy also results in large strain field fluctuation and strong anharmonicity (γ = 1.59–1.80) in crystal lattice, which suppress lattice thermal conductivity (κL) together. Consequently, the Cu2.1Mn0.7Fe0.1Co0.1SnSe4 sample obtains the lowest κL of 0.80 W m−1 K−1 at 673 K. Finally, a high Vickers Hardness of 206 Hv and zTmax of 0.40 are achieved in Cu2.1Mn0.8Fe0.1SnSe4. Entropy engineering opens up a channel to optimize the electrical-thermal properties of complex compounds simultaneously.

Theoretical Study of Intrinsic and Extrinsic Point Defects and Their Effects on Thermoelectric Properties of Cu2SnSe3.

Current understanding of the intrinsic point defects and potential extrinsic dopants in p-type Cu2SnSe3 is limited, which hinders further improvement of its thermoelectric performance. Here, we show that the dominant intrinsic defects in Cu2SnSe3 are CuSn and VCu under different chemical conditions, respectively. The presence of VCu will damage the hole conduction network and reduce hole mobility. Besides, we find that the substitution of Al, Ga, In, Cd, Zn, Fe, and Mn for Sn can inhibit the formation of VCu; introducing CuSn, FeSn, MnSn, and NiCu defects can significantly enhance electronic density of states near the Fermi level due to the contribution of 3d orbitals. Therefore, increasing the Cu content and/or introducing the above beneficial dopants appropriately are expected to cause enhancement of carrier mobility and/or thermopower of Cu2SnSe3. Furthermore, introducing AgCu, AlSn, ZnSn, GeSn, and MnSn defects can induce large mass and strain field fluctuations, lowering lattice thermal conductivity remarkably. Present results not only deepen one's insights into point defects in Cu2SnSe3 but also provide us with a guide to improve its thermoelectric properties.

Understanding the pitting mechanism of super ferritic stainless steel in bromide solutions: The role of Ti/Nb–Mo precipitates with a core–shell structure

The core–shell structure of Ti/Nb precipitates in UNS S44660 was revealed by focused ion beam–transmission electron microscopy (FIB–TEM). In the bromide solution, the Nb in the shell is preferentially and selectively eluted, leaving a micro-crevice. The combined effect of the strong anodic adsorption of Br− and the large strain field in the shell promotes the free dissolution of residual Mo. Micro-crevices surround the Ti-rich nucleus and expand into pits. Consequently, pitting preferentially initiates at the Ti/Nb precipitates, which was verified by scratching electrode tests and immersion tests in bromide solutions.

Synergistically Enhanced Thermoelectric Performance of Cu2SnSe3-Based Composites via Ag Doping Balance.

As an ecofriendly and low-cost thermoelectric material, Cu2SnSe3 has recently drawn much attention. In this work, the thermoelectric properties of Cu2SnSe3-based materials have been synergistically optimized. Ag doping at the Cu site enables strong phonon scattering via large strain field fluctuations and increases the effective mass of carriers through band engineering, which results in a reduced lattice thermal conductivity and enhanced Seebeck coefficient. Thus, a peak ZT value of 1.04 at 800 K is obtained for Cu1.85Ag0.15SnSe3. Then, In is further doped at the Sn site to further increase the carrier concentration and power factor; the ZT values of Cu1.85Ag0.15Sn1-yInySe3 samples are highly improved in the temperature range of 300-800 K, and a peak ZT value of 1.12 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3 sample. Considering that Ag2S decomposes to Ag and S completely under vacuum and high current field, the sulfur vapor would be pumped away by a vacuum pump, and the generated Ag not only enriches at the grain boundary of the Cu1.85Ag0.15Sn0.91In0.09Se3 bulk material but also enters the matrix and occupies the Cu site, leading to the extruding Cu and Se forming the second phase of CuSe nanoparticles. Thus, the power factor greatly improves to 13.8 μW cm-1 K-2 at 700 K, and the lattice thermal conductivity is as low as 0.12 W m-1 K-1 at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite. Finally, a high ZT value of 1.58 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite, which is nearly an increase of 204% compared to that of Cu2SnSe3. This work provides an effective solution to optimize the conflicting material properties for Cu2SnSe3-based thermoelectric materials.

An analysis of flexoelectric coupling associated electroelastic fields in functionally graded semiconductor nanobeams

Strain gradient with strong size dependency and structural association (geometry or microstructure) can efficiently tune the performances of semiconductors by the flexoelectric coupling effect. In this work, we studied a novel asymmetric beam-like semiconductor made by functionally graded (FG) flexoelectric materials. When being applied with pure bending loads at two ends, it can generate a relatively large inhomogeneous strain field to achieve obvious semiconducting behaviors. Unlike the analysis for piezoelectric semiconductor materials, we considered the effects of flexoelectricity and strain gradient elasticity in constitutive equations for flexoelectric semiconductor materials. Then, the complicated mutual coupling governing equations and associated boundary conditions are rederived strictly. By the Fourier series expansion and spatial integration methods, we obtained the solutions of the set of partial differential equations with non-constant coefficients. Results show that the semiconducting electromechanical coupling performances of the beam-like FG flexoelectric semiconductor depend heavily on the ratio and structural distributions of its constituent. Moreover, it is found that the inner carrier distributions and electromechanical characteristics can be significantly tuned by the strain gradient elasticity, the flexoelectricity, and the structural size. We believe this work provides a useful guideline for the practical design and manufacture of novel electromechanical semiconductor devices.

Dopant Segregation Inside and Outside Dislocation Cores in Perovskite BaSnO3 and Reconstruction of the Local Atomic and Electronic Structures.

Distinct dopant behaviors inside and outside dislocation cores are identified by atomic-resolution electron microscopy in perovskite BaSnO3 with considerable consequences on local atomic and electronic structures. Driven by elastic strain, when A-site designated La dopants segregate near a dislocation core, the dopant atoms accumulate at the Ba sites in compressively strained regions. This triggers formation of Ba vacancies adjacent to the core atomic sites resulting in reconstruction of the core. Notwithstanding the presence of extremely large tensile strain fields, when La atoms segregate inside the dislocation core, they become B-site dopants, replacing Sn atoms and compensating the positive charge of the core oxygen vacancies. Electron energy-loss spectroscopy shows that the local electronic structure of these dislocations changes dramatically due to segregation of the dopants inside and around the core ranging from formation of strong La-O hybridized electronic states near the conduction band minimum to insulator-to-metal transition.

Ultralow Lattice Thermal Conductivity and Superhigh Thermoelectric Figure-of-Merit in (Mg, Bi) Co-Doped GeTe.

High-efficiency thermoelectric (TE) technology is determined by the performance of TE materials. Doping is a routine approach in TEs to achieve optimized electrical properties and lowered thermal conductivity. However, how to choose appropriate dopants with desirable solution content to realize high TE figure-of-merit (zT) is very tough work. In this study, via the use of large mass and strain field fluctuations as indicators for low lattice thermal conductivity, the combination of (Mg, Bi) in GeTe is screened as very effective dopants for potentially high zTs. In experiments, a series of (Mg, Bi) co-doped GeTe compounds are prepared and the electrical and thermal transport properties are systematically investigated. Ultralow lattice thermal conductivity, about 0.3 W m-1 K-1 at 600 K, is obtained in Ge0.9 Mg0.04 Bi0.06 Te due to the introduced large mass and strain field fluctuations by (Mg, Bi) co-doping. In addition, (Mg, Bi) co-doping can introduce extra electrons for optimal carrier concentration and diminish the energy offset at the top of the valence band for high density-of-states effective mass. Via these synthetic effects, a superhigh zT of ≈2.5 at 700 K is achieved for Ge0.9 Mg0.04 Bi0.06 Te. This study sheds light on the rational design of effective dopants in other TE materials.

Determination of acoustic nonlinearity parameters using thermal modulation of ultrasonic waves

This study presents a test method and its theoretical framework to determine the acoustic nonlinearity parameters (α, β, δ) of material using thermal modulation of ultrasonic waves. Temperature change-induced thermal strain excites the nonlinear response of the material and modulates the ultrasonic wave propagating in it. Experimental results showed a strong correlation between the relative wave velocity change and the temperature change. With a quadratic polynomial model, the acoustic nonlinearity parameters were obtained from the polynomial coefficients by curve fitting the experimental data. Their effects on thermal-induced velocity change were discussed. The parameters α, β, and δ govern the hysteretic gap, average slope, and curvature of the correlation curve, respectively. The proposed theory was validated on aluminum, steel, intact and damaged concrete samples. The obtained nonlinear parameters show reasonable agreement with values reported in the literature. Compared to other nonlinear acoustic methods using vibration or acoustic excitation, the thermal modulation method generates more uniform, slow changing, and larger strain field in the test sample. Employing the thermal effect as the driving force for nonlinearity instead of an undesired influencing factor, this method can measure the absolute values of α, β, and δ with good accuracy using a simple ultrasonic test setup.

Scattering Mechanisms and Compositional Optimization of High‐Performance Elemental Te as a Thermoelectric Material

Abstractp‐Type elemental tellurium (Te) has been found to be a promising thermoelectric (TE) material due to its high band degeneracy near the valence band maximum, and has exhibited a high zT ≈ 1.0 above 600 K. However, when forming Te1−xSex solid solutions, the maximal zTs are reduced because of the severely decreased carrier concentration. It is demonstrated that Se alloying is beneficial for enhancing TE performance of elemental Te provided the carrier concentration is optimized. Through Se alloying, the lattice thermal conductivity is remarkably suppressed by the induced large mass and strain field fluctuation, while the power factor can be maintained at a relatively high value as a result of the moderate alloying scattering potential, the unchanged density‐of‐state effective mass, and the optimized carrier concentration in Te1−xSex alloys. Notably, a positive temperature dependence of carrier mobility is observed near room temperature in Te1−xSex, which is proven to be caused by grain boundary scattering. A maximal zT ≈ 1.05 at 625 K is realized in Te0.93Se0.04As0.03 alloys, about 9% higher than the Se‐free Te. The conversion efficiency between 300 and 625 K is also improved ≈18% via Se alloying.

Size-dependent electromechanical coupling in functionally graded flexoelectric nanocylinders

Flexoelectricity is an electromechanical coupling between polarization and strain gradient, which not only exhibits strong size dependency but is structure associated (geometry or microstructure). By the definition of flexoelectric coefficients, the flexoelectricity-related strain gradients can be generated by tailoring mechanical structures, such as the traditional designed truncated pyramid. In this work, a novel asymmetric nanocylinder is composed of functionally graded materials presented with uniform pressures on the top surface to create a relatively large inhomogeneous strain field for the achievement of obvious flexoelectric polarization. Based on the power-law-distributed material property assumption, we investigate the flexoelectricity of the proposed functionally graded nanocylinder. Based on the extended linear theory of piezoelectricity, the closed-form solutions are obtained, which can specifically characterize the size-dependent flexoelectricity. The most common setups are applied to quantify the flexoelectric response. From the numerical results, we can conclude that the electromechanical properties can be significantly influenced by the given FG configuration with graded material parameters, which can be a guideline for the design of novel flexoelectric devices.

Ferroelectric switching and scale invariant avalanches in BaTiO3

Ferroelectric-field switching in $\mathrm{BaTi}{\mathrm{O}}_{3}$ generates ``Barkhausen noise'' when domain walls are displaced. We show by acoustic emission spectroscopy that electric-field switching of ${90}^{\ensuremath{\circ}}$ boundaries generates large strain fields, which emit acoustic phonons during ferroelectric hysteresis measurements. We use highly sensitive receivers (microphones) to measure the time sequences of noise in close analogy to noise patterns in ferroelastic and magnetic materials. Domain-wall interactions and jamming generate the ``crackling noise'' that follows scale invariant avalanche dynamics: the avalanche energy and amplitude probability distribution functions follow power laws with exponents $\ensuremath{\varepsilon}=1.65$ (energy) and \ensuremath{\tau}\ensuremath{'} = 2.25 (amplitudes). Aftershocks are very common and follow Omori law with probability $\ensuremath{\sim}{t}^{\ensuremath{-}p}$ where $t$ is the time elapsed after the main shock and $p$ is the Omori exponent $p\ensuremath{\sim}1$. The interevent times follow a double power-law distribution with exponents 0.9 for small times and 2.2 for the larger times. The scaling behavior is consistent with predictions of mean field theory.