Anisotropy Ratio Research Articles

A novel method for preparing anisotropic negative Poisson's ratio composite foam with excellent structural stability and shape recovery property

Negative Poisson's ratio (NPR) foams are usually prepared by the transformation of positive Poisson's ratio (PPR) foams with honeycomb cell structure, which face a challenge for restraining the increase in volume and Poisson's ratio once they are stimulated by high temperature. In addition, they cannot restore to its reentrant structure after large deformation. In this work, a novel method for preparing anisotropic NPR foam with excellent structural stability and shape recovery property is established by synergistically controlling hierarchical pore structures. The obtained NPR silicone rubber/polylactic acid (SR/PLA) composite foam has large-size holes (auxetic structure) with concave hexagonal shape and small-size honeycomb cells, which are generated by 3D printing technology and salt leaching method, respectively. The auxetic structure with the orientation characteristic is directly formed rather than transformed from the honeycomb cell structure. Therefore, the dimension and Poisson's ratio value of the NPR SR/PLA composite foam keep constant after high temperature heating treatment, meaning an excellent structural stability. Furthermore the excellent shape recovery property of the NPR SR/PLA composite foam endows it restore to the initial auxetic structure after large deformation to a large extent. It should be mentioned that the anisotropy of the NPR SR/PLA composite foam further will bring a broad application prospect in the field of human body cushioning materials, such as prosthetic lining sleeve.

An ultrasonic study of the effect of uniaxial green compaction pressure on the elastic anisotropy of porous copper fabricated using the pore-former route

Porous copper is potentially very attractive for various functional applications such as filters and heat exchanger materials. Within the scope of this study, porous copper samples with dual pore morphologies were fabricated following the space holder technique using a 1:2 mixture ratio of K2CO3 to NaCl as pore formers. The initial compaction pressure required for green body fabrication was systematically varied between 120 and 180 MPa, and the influence of this pressure over porosity, pore morphology, volume shrinkage, and longitudinal elastic constants was systematically studied. Three longitudinal elastic constants of the porous Cu samples were determined following the non-destructive ultrasonic phase spectroscopy method through the measurement of the phase shift vs. frequency spectra along the three orthogonal directions in each sample. When the applied pressure is increased from 120 to 160 MPa and the Cu:PF mixture ratios are 70:30 and 60:40, respectively, the elastic constant values in the green compaction direction (C11) decrease from 34.2 GPa to 7.5 GPa and 23.7 GPa–17.9 GPa. However, for Cu:PF mixture ratios of 70:30 and 60:40, under 180 MPa green compaction pressure C11 increases to 30.9 GPa and 21.2 GPa, respectively. As the applied green compaction pressure increased from 120 to 160 MPa, the samples' average elastic anisotropy ratio initially increased from 0.29 to 0.59 and then decreased to 0.55 at 180 MPa pressure. While the initial increase in anisotropy is due to increased pore flattening with increasing green compaction pressure, the decrease in anisotropy ratio in the samples fabricated under 180 MPa green compaction pressure is due to enhanced sample densification. Contour maps for different properties have been proposed to identify the optimum combination of initial powder composition and green compaction pressure for the intended end application.

The influence of isotropic loading and unloading on anisotropic evolution of saturated hydraulic conductivity of bentonite-sand mixtures in a cube triaxial permeameter

Anisotropic permeability of bentonite mixtures governs water flow regime in buffer materials such as landfill cover systems, cutoff walls, and high-level waste repositories. Because of its widespread application, a series of 48 permeability tests were conducted in this study on specimens prepared from bentonite-sand mixtures with three bentonite contents. The tests were run along both isotropic loading and unloading paths to explore the evolution of anisotropic ratio from three distinct fabric anisotropy states. Accordingly, a new cubic sample-type, large-scale triaxial permeameter was introduced and employed. The results revealed an enhancement of as-compacted anisotropy ratio from 1 to almost 7 with bentonite contents. The anisotropy ratio was suppressed with isotropic loading for 40% and 70% bentonite while it was enhanced for 10% bentonite content. Unloading was revealed to reduce the anisotropy of higher bentonite contents due to the enhancement of fissures along horizontal and vertical directions. The practical implication in terms of the long-term performance of a landfill liner with medium to high bentonite contents is that despite a 10- to 30-fold reduction in permeability with loading, the leachate flow would be less anisotropic compared with the initial condition as supported by a reduction in the anisotropy ratio up to 3.5.

Scale dependency of anisotropic thermal conductivity of heterogeneous geomaterials

The precise determination of subsurface thermal properties is critical for ground-source heating systems. The geomaterials are inherently heterogeneous, and their thermal conductivity measured in laboratory and field tests often exhibits anisotropic behaviours. However, the accurate measurement of thermal responses in geomaterials presents a challenging task due to the anisotropy’s variation with the observed scale. Hence, a numerical method is developed in this work and illustrated by taking a typical anisotropic structure of geomaterials with the porosity of 0.5 as an example. The differences in data from laboratory measurements and field tests are discussed to explore the scale effect on anisotropic thermal properties. A series of simulation tests are conducted on specimens with varying dimensions using the finite element method. Results indicate that the thermal properties show a substantial sensitivity to the observation scale, the variation of which decreases with the sample dimensions. By comparing in situ data and laboratory results, the values of average thermal conductivity and corresponding anisotropy ratio are lower than those at small scales, indicating that careful consideration should be given to the thermal properties to account for heterogeneity and anisotropy. In addition, four upscaling schemes based on the averaging method are discussed. This study sheds light on the gap between the laboratory results and the field’s inherent properties and provides guidelines for upscaling small-scale results to field-scale applications.

Highly Anisotropic to Isotropic Nature and Ultralow Out-of-Plane Lattice Thermal Conductivity of Layered PbClF-Type Materials.

Materials with an extreme lattice thermal conductivity (κl) are indispensable for thermal energy management applications. Layered materials provide an avenue for designing such functional materials due to their intrinsic bonding heterogeneity. Therefore, a microscopic understanding of the crystal structure, bonding, anharmonic lattice dynamics, and phonon transport properties is critically important for layered materials. Alkaline-earth halofluorides exhibit anisotropy from their layered crystal structure, which is strongly determined by axial bond(s), and it is attributed to the large axial ratio (c/a > 2) for CaBrF, CaIF, and SrIF, in which Br/I acts as a rattler, as evidenced from potential energy curves and phonon density of states. The low axial (c/a) ratio leads to relatively isotropic κl values in the BaXF (X = Cl, Br, I) series. MXF (M = Ca, Sr, Ba) compounds exhibit highly anisotropic (a large phonon transport anisotropy ratio of 10.95 for CaIF) to isotropic (a small phonon transport anisotropy ratio of 1.49 for BaBrF) κl values despite their iso-structure. Moreover, ultralow κl (<1 W/m K) values have been predicted for CaBrF, CaIF, and SrIF in the out-of-plane direction due to weak van der Waals (vdWs) bonding. Overall, this comprehensive study on MXF compounds provides insights into designing low κl layered materials with a large axial ratio by fine-tuning out-of-plane bonding from ionic to vdWs bonding.

Observation of giant room-temperature anisotropic magnetoresistance in the topological insulator β-Ag2Te

Achieving room-temperature high anisotropic magnetoresistance ratios is highly desirable for magnetic sensors with scaled supply voltages and high sensitivities. However, the ratios in heterojunction-free thin films are currently limited to only a few percent at room temperature. Here, we observe a high anisotropic magnetoresistance ratio of −39% and a giant planar Hall effect (520 μΩ⋅cm) at room temperature under 9 T in β-Ag2Te crystals grown by chemical vapor deposition. We propose a theoretical model of anisotropic scattering — induced by a Dirac cone tilt and modulated by intrinsic properties of effective mass and sound velocity — as a possible origin. Moreover, small-size angle sensors with a Wheatstone bridge configuration were fabricated using the synthesized β-Ag2Te crystals. The sensors exhibited high output response (240 mV/V), high angle sensitivity (4.2 mV/V/°) and small angle error (<1°). Our work translates the developments in topological insulators to a broader impact on practical applications such as high-field magnetic and angle sensors.

Electronic transport characteristics and nanodevice designs for β-HfNCl monolayer

The mechanical properties, electronic structure, electric transport and optoelectronic properties of a recently predicted wide bandgap semiconductor β-HfNCl monolayer are systematically studied by means of first-principles calculations. β-HfNCl monolayer is isotropic in mechanical properties, whose calculated Young’s modulus, shear modulus, and layer modulus of β-HfNCl monolayer are 128.9–129.2, 44.28, and 119.46 N m−1, respectively. An appropriate tensile strain (i.e., beyond 3 %) can induce a transition from indirect bandgap to direct bandgap. In addition, we construct several conceptual nanodevice structures based on β-HfNCl monolayer, such as pn-junction diodes, pin-junction field-effect transistors (FETs) and phototransistors. The electronic transport results reveal that the pn-junction diodes have obvious rectification effect and strong electric anisotropy. Their rectification ratios and electric anisotropy ratio (η) can reach up to 106 and 3.69, respectively. The FETs have an obvious field-effect behavior with a slightly lower rectification ratio (105). Moreover, we investigate the photoelectric response of the phototransistors of β-HfNCl monolayer under the illumination of light. They have a strong response to the light whose energy is larger than the violet light, indicating that the β-HfNCl monolayer can be a platform to detect the ultraviolet light. These findings provide crucial insights into the potential applications of β-HfNCl monolayer in electronic and optoelectronic devices.

Nanoimprinted Quasi‐2D Perovskites toward High‐Performance Polarization‐Sensitive Photodetectors

AbstractThe demand for high‐performance polarization‐sensitive perovskite photodetectors (PSPPDs) has driven research to develop new materials that enable efficient light harvesting and polarization detection. Herein, a PSPPD based on nanoimprinted quasi‐2D halide perovskite thin films with vertical gradient phase distribution is reported. The nanoimprinting method is utilized to create nanoscale linear patterns on the surface of the film. It enables polarized light detection and enhances absorption. According to the results of transient absorption analysis, a vertical gradient phase distribution along the longitudinal direction of quasi‐2D perovskite (PEA)2(MA)n‐1PbnI3n+1 (PEA+ = C6H5CH2CH2NH3+, MA+ = CH3NH3+, n = 3) is confirmed, which facilitates efficient carrier transfer and separation, leading to improved device performance. These PSPPDs demonstrate outstanding photo‐response with a large responsivity of ≈90 A W−1 and a high detectivity reaching the order of 1012 Jones. The photoluminescence of the quasi‐2D perovskite film exhibits an anisotropy ratio of 2.05, and the PSPPD reaches a polarization sensitivity of 1.68. These findings offer insight into the feasibility of developing perovskite materials for PSPPDs, which have potential applications in imaging, sensing, and communication technologies.

Ta2NiSe5/MoTe2/Graphene van der Waals Heterostructures Toward Ultrabroadband and Polarization‐Sensitive Imaging

AbstractUltrabroadband and polarization‐sensitive imaging are essential for pioneering advancements in intelligent technology, offering a pivotal pathway to multidimensional information extraction. However, the dearth of appropriate photosensitive semiconductors in the infrared region and the lack of suitably efficient device architecture for the separation and collection of photocarriers significantly impede the development of ultra‐broadband and polarization‐sensitive photodetectors. To address these challenges, a sandwiched Ta2NiSe5/MoTe2/Graphene heterostructure device is engineered. In this device, efficient and polarization‐sensitive photocarriers are generated in the top Ta2NiSe5 layer, rapidly separated through the middle MoTe2 layer, and effectively collected by the bottom graphene layer. As a result, the developed photodetector exhibits an ultra‐broadband and polarization‐sensitive photoresponse that extends from visible light (520 nm) to short‐wave infrared (2200 nm). At 2200 nm, the device displays a notable responsivity of 5.79 A W−1, a specific detectivity of 1010 Jones, and an anisotropic ratio of 1.44. Furthermore, this device successfully demonstrates high‐resolution ultra‐broadband and polarized light imaging capabilities. This study thus presents an intriguing blueprint for the development of advanced 2D imaging platforms for future‐generation intelligent systems.

Machine learning approaches for stability prediction of rectangular tunnels in natural clays based on MLP and RBF neural networks

In underground space technology, the issue of tunnel stability is a fundamental concern that significantly causes catastrophe. Owing to sedimentation and deposition processes, the strengths of clays are anisotropic, where the magnitudes of undrained shear strengths in the vertical and horizontal directions are different. The anisotropic undrained shear (AUS) model is effective at considering the anisotropy of clayey soils when analyzing geotechnical stability issues. This study aims to assess the stability of rectangular tunnels by adjusting the dimensionless overburden factor, cover-depth ratio, and width-depth ratio in clay with various anisotropic strength ratios. The stability analysis of these tunnels involves employing finite element limit analysis and the AUS model to identify the planes of soil collapse in response to the aforementioned variations. In addition, this study presents the development of soft-computing models utilizing artificial neural networks (ANNs) to forecast the stability of rectangular tunnels across various combinations of input parameters. The findings of this study are presented in the form of design charts, tables, and soft-computing models to facilitate practical applications.

A series of auxetic metamaterials with negative thermal expansion based on L-shaped microstructures

Mechanical metamaterials are facing various multi-functional requirements. In this study, a series of auxetic metamaterials with enhanced negative thermal expansion and improved stiffness are proposed. These metamaterials consist of four novel L-shaped microstructures connected in various clockwise or counterclockwise directions. Two length ratios and four angle parameters are utilized to define the geometry of these metamaterials. Both analytical analysis and validated numerical homogenization methods are employed to determine their effective thermoelastic properties, including effective Young's modulus, Poisson's ratio, and coefficient of thermal expansion. It is demonstrated that tailoring these microstructural geometries can modulate the effective thermoelastic properties of these metamaterials accordingly. By appropriately tailoring the geometric parameters, these metamaterials exhibit increased effective stiffness as well as more pronounced auxeticity and negative thermal expansion compared to traditional triangle and trapezoid microstructures. Notably, the in-plane effective anisotropic or isotropic Poisson's ratio and coefficient of thermal expansion can be adjustable over a broader range from negative to positive values. High stiffness, High strength, and near-zero thermal expansion can be achieved concurrently in these metamaterials, which is very conducive for structural designing in thermal environments. These metamaterials with a coupled design of stiffness, thermal expansion, and Poisson's ratio hold significant potential for advancing applications in satellite platforms, space structures, composite sandwiches, and precision equipment.

Highly anisotropic spin transport in ultrathin black phosphorus.

In anisotropic crystals, the direction-dependent effective mass of carriers can have a profound impact on spin transport dynamics. The puckered crystal structure of black phosphorus leads to direction-dependent charge transport and optical response, suggesting that it is an ideal system for studying anisotropic spin transport. To this end, we fabricate and characterize high-mobility encapsulated ultrathin black-phosphorus-based spin valves in a four-terminal geometry. Our measurements show that in-plane spin lifetimes are strongly gate tunable and exceed one nanosecond. Through high out-of-plane magnetic fields, we observe a fivefold enhancement in the out-of-plane spin signal case compared to in-plane and estimate a colossal spin-lifetime anisotropy of ∼6. This finding is further confirmed by oblique Hanle measurements. Additionally, we estimate an in-plane spin-lifetime anisotropy ratio of up to 1.8. Our observation of strongly anisotropic spin transport along three orthogonal axes in this pristine material could be exploited to realize directionally tunable spin transport.

In-plane and out-of-plane vibration analysis of laminated composite frames with warping effects

This paper addresses the free vibration analysis of a general planar frame structure, consisting of an assembly of laminated composite beam members. A mathematical model is established for the beams by introducing a novel displacement field, which encompasses the influence of shear deformation, rotary inertia, material coupling, warping phenomenon, as well as in-plane and out-of-plane deformations. By incorporating these various factors, a comprehensive representation of the beam's behavior is achieved. The continuity equations for displacements and rotations of two adjacent members are formulated to derive the equations for the entire frame structure. The assembled equations are solved using the Finite Element Method (FEM). The numerical results obtained are compared with existing results in the literature and new results from 3D models in ANSYS. The present results for torsional modes are closer to the ANSYS results than the results of previous studies. An extensive analysis is performed to investigate the impact of various parameters, including stacking sequence, frame angle, relative frame length, and anisotropy ratios on the system response.

Strong and tunable anisotropy in monolayer graphene with broken symmetry

Low-symmetry two-dimensional materials have attracted much attention in recent years due to their exotic and incredible properties. Here, we construct Ta2NiS5-based van der Waals heterostructures to introduce in-plane polarization in monolayer graphene through symmetry engineering. Angle-dependent conductance and mobility measurement combined with theoretical calculations all verify that the pristine Ta2NiS5 possesses prominent electrical anisotropy, with anisotropic ratio (a-axis/c-axis) of about 2.1 in conductivity and 3.28 in mobility. Furthermore, we demonstrate that van der Waals interlayer coupling has induced explicit in-plane polarization in monolayer graphene according to angle-dependent Raman spectroscopy and electrical transport measurement on monolayer-graphene/Ta2NiS5 heterostructures. In polarized graphene, the conductivity and mobility ratios are up to 3.21 and 4.84, respectively. The induced polarization in monolayer graphene within heterostructures probably originates from strain, lattice reconstruction, induced symmetry reduction and charge transfer through van der Waals interfaces. These results might provide inspiration to study symmetry-related van der Waals heterostructures and pave their way to novel nano-electronic devices.

Strong mechanical anisotropy and an anisotropic Dirac state in 2D C5N3.

Two-dimensional (2D) carbon nitride materials have emerged as a versatile platform for the design of high-performance nanoelectronics, but strong anisotropy in 2D carbon nitrides has rarely been reported. In this work, a 2D carbon nitride with strong anisotropy composed of tetra-, penta-, and hexa-rings (named as TPH-C5N3) is proposed. This TPH-C5N3 exhibits both dynamical and mechanical stability. Furthermore, it also showcases remarkable thermal stability, reaching up to 2300 K, as evidenced by AIMD simulations conducted in an NVT environment utilizing the Nosé-Hoover thermostat. Significantly, TPH-C5N3 demonstrates high anisotropic ratios in its mechanical properties, positioning it as the frontrunner in the current carbon nitride systems. In addition, a Dirac cone with an anisotropic ratio of 55.8% and Fermi velocity of 7.26 × 105 m s-1 is revealed in TPH-C5N3. The nontrivial topological properties of TPH-C5N3 are demonstrated by a non-zero Z2 invariant and topologically protected edge states. Our study offers theoretical insights into an anisotropic 2D carbon nitride material, laying the groundwork for its design and synthesis.

Single-Point Incremental Forming (SPIF) of AA5754 Aluminium Alloy Blanks: Experimental and numerical studies

This study presents a Finite Element (FE) simulation model for Single-Point Incremental Forming (SPIF) of a panel part using AA5754 Aluminium Alloy. The precision of model was highly enhanced through experimental data integration by uniaxial tensile tests (UATT) to determine material flow stress and anisotropic properties. The Voce model accurately predicted material flow stress within a 5% error margin, and equations were concluded to convert Lankford coefficients (R-values) into anisotropic stress ratios for Abaqus/Explicit simulation. Experimental validation demonstrated a close agreement in major strain and thickness distribution, with a maximum error below 5.8% across high and low forming speeds.

Prediction of novel ground-state structures and analysis of phonon transport in two-dimensional GexSy compounds.

We conducted this study to explore the ground-state structures of two-dimensional (2D) variable-composition GexSy compounds, driven by the polymorphic characteristics of bulk germanium sulfides and the promising thermoelectric performance of 2D GeS (Pmn21). To accomplish this, we utilized the highly successful evolutionary-algorithm-based code USPEX in conjunction with VASP for total energy calculations, leading to the discovery of three previously unexplored structures of Ge2S (P2/c), GeS (P3̄m1), and GeS2 (P21/c). These 2D materials exhibit significantly lower formation energies compared to their reported counterparts. We thoroughly scrutinized the structural stability and subsequently analyzed their electronic structures. Our analysis reveals a nearly direct band gap of 0.12/0.84 eV with the PBE/HSE06 functional for 2D Ge2S and an indirect band gap for 2D GeS and GeS2. Their semiconducting nature highlights the crucial importance of lattice thermal conductivity (κl), which we determined by solving the Boltzmann transport equation for phonons. Importantly, we predict a room temperature κl value of 6.82 W m-1 K-1 for GeS, lower than its 2D orthorhombic counterpart. In the case of GeS2, we observed an anisotropic κl value of 16.95/10.68 W m-1 K-1 along the zigzag/armchair directions at 300 K, with an in-plane anisotropy ratio of 1.59, surpassing that of 2D IV-VI compounds. We delve into detailed discussions regarding the role of lattice anharmonicity, group velocities, phonon lifetimes, and three-phonon weighted phase space in the overall thermal conductivity analysis.

The Impact of Flexural/Torsional Coupling on the Stability of Symmetrical Laminated Plates

In this study, we will evaluate the effect of bending/torsion coupling on the buckling instability and free vibration behavior of symmetrical laminated plates. We will load these plates in-plane with bi-axial or uni-axial, uniform or non-uniform mechanical loads. To quantify this behavior, we’ll compare the results obtained with those of specially orthotropic symmetrical plates (where bending/torsion coupling is absent). A parametric study will be carried out by varying the plate’s aspect ratio, anisotropy ratio and/or lamination angle. The aim of these studies is to construct a planar loading margin for the plate while remaining elastically stable, and to determine a physically admissible limit where we can approximate the behavior of symmetrical laminates to that of specially orthotropic plates (easy to study). We will base ourselves on a Rayleigh-Ritz energy formulation of the problem because of the difficulty of finding closed-form solutions. Following validation of this formulation, a numerical survey of the results will be carried out to quantify the effect of bending/torsion coupling on the instability of this type of plate. Various conditions on the plate boundaries will be used.

Polarization‐Sensitive Photodetectors Based on Highly In‐Plane Anisotropic Violet Phosphorus with Large Dichroic Ratio

AbstractAnisotropic 2D layered materials with distinctive anisotropic electronic band structures and optical response are promising for practical applications in polarization imaging and sensors. Nonetheless, many of the currently reported polarized photodetectors based on anisotropic 2D materials are constrained by the low polarization responsivity and low linear dichroism ratio. Here a high‐performance polarization‐sensitive photodetector based on the novel in‐plane anisotropy 2D violet phosphorus (VP) crystal is reported. The angle‐resolved polarized Raman spectroscopy (ARPRS) study and the anisotropic electrical transport measurements revealed the highly in‐plane anisotropy phonon vibration and electrical properties of VP crystal. The anisotropic ratio of electron mobility is calculated to be 2.7 at room temperature. Moreover, a polarization‐sensitive photodetector based on the VP nanosheet shows high photoresponsivity of up to 341 A W−1 and a linearly dichroic ratio of up to 3.9, which is much larger than most other anisotropic 2D materials. These results indicate that the anisotropic VP crystal would be suitable for polarization‐sensitive photodetectors, which leads to a promising perspective for future novel electronic and optoelectronic devices.

Asymmetrical LaO9 polyhedron inducing anisotropic positive thermal expansion and moderate lattice thermal conductivity in LaBO3 with isolated planar [BO3] group

Thermal physical properties of inorganic borates are important in application of optical device working under high temperature. Herein, we investigate the temperature-dependent thermoelasticity, thermal expansion(α), and lattice thermal conductivity(κ) of simple orthohomibic LaBO3 with a two dimension like crystal structure though first principles for the first time. The calculated rate of temperature-dependent second elastic constant C11 is significantly larger than that of C22 and C33, and the theoretical slopes of corresponding bulk, shear, and Young's moduli in the range of 300–900 K are −0.0289 GPa•K−1, −0.0156 GPa•K−1, and −0.0398 GPa•K−1, respectively. The predicted α is positive and increases with the rise of temperature. The anisotropic ratio αc/αa or αc/αb is approximately 2 at 300 K, which might be attributed to the difference of elastic compliance S12 and S13. Besides, the anisotropic lattice thermal conductivity κa, κb and κc of LaBO3 is predicted to be 1.43, 1.40 and 0.80 W/m-K at 300 K via Debye-Callaway model. Most importantly, the atomic projected Grüneisen parameters γ reveal that asymmetrical LaO9 polyhedra in the crystal lattice dominate the thermal expansion and lattice thermal conductivity. Our results indicate that LaBO3 has good thermal physical properties working under high temperature.