Isotropic Structure Research Articles

Ultralight ZrB2/ZrC nanofiber aerogels with excellent elasticity and ultra-low thermal conductivity

In this work, ZrB2/ZrC nanofiber aerogels with excellent elasticity and ultra-low thermal conductivity were prepared by electrospinning and freeze-drying. The aerogels with isotropic cellular-like pore structure were obtained with pore walls composed of graphene oxide attached with ZrB2/ZrC nanofibers. The aerogels showed recoverable property under 60 % compressive strain. The thermal conductivity of the aerogels was 0.029 W·m−1·k−1, much lower than that of other carbide and boride aerogels. These intriguing results may offer considerable advantages for thermal insulation in extreme environments.

Cake layer 3D structure regulation to optimize water channels during Al-based coagulation-ultrafiltration process

The variation in cake layer three-dimensional (3D) structures and related water channel characteristics induced by coagulation pretreatment remains unclear; however, gaining such knowledge will aid in improving ultrafiltration (UF) efficiency for water purification. Herein, the regulation of cake layer 3D structures (3D distribution of organic foulants within cake layers) by Al-based coagulation pretreatment was analyzed at the micro/nanoscale. The sandwich-like cake layer of humic acids and sodium alginate induced without coagulation was ruptured, and foulants were gradually uniformly distributed within the floc layer (toward an isotropic structure) with increasing coagulant dosage (a critical dosage was observed). Furthermore, the structure of the foulant-floc layer was more isotropic when coagulants with high Al13 concentrations were used (either AlCl3 at pH 6 or polyaluminum chloride, in comparison with AlCl3 at pH 8 where small-molecular-weight humic acids were enriched near the membrane). These high Al13 concentrations lead to a 48.4% higher specific membrane flux than that seen for UF without coagulation. Molecular dynamics simulations revealed that with increasing Al13 concentration (Al13: 6.2% to 22.6%), the water channels within the cake layer were enlarged and more connected, and the water transport coefficient was improved by up to 54.1%, indicating faster water transport. These findings demonstrate that facilitating an isotropic foulant-floc layer with highly connected water channels by coagulation pretreatment with high-Al13-concentration coagulants (having a strong ability to complex organic foulants) is the key issue in optimizing the UF efficiency for water purification. The results should provide further understanding of the underlying mechanisms of coagulation-enhancing UF behavior and inspire precise design of coagulation pretreatment to achieve efficient UF.

Two new parameters for the ordinal analysis of images.

Local patterns play an important role in statistical physics as well as in image processing. Two-dimensional ordinal patterns were studied by Ribeiro et al. who determined permutation entropy and complexity in order to classify paintings and images of liquid crystals. Here, we find that the 2 × 2 patterns of neighboring pixels come in three types. The statistics of these types, expressed by two parameters, contains the relevant information to describe and distinguish textures. The parameters are most stable and informative for isotropic structures.

FORWARD MODELING THE GROUP AND PHASE VELOCITIES OF RAYLEIGH AND LOVE SURFACE WAVES BENEATH THE CENTRAL ANATOLIA: FIFTH PARAMETER FOR TRANSVERSE ISOTROPY

The complex dynamic processes such as the magma movements in the crust and uppermost mantle result in the anisotropic wave propagation. In case of the Rayleigh and Love surface waves, this anisotropy is known as the Rayleigh-Love wave discrepancy. The surface wave propagation beneath the Central Anatolia shows this discrepancy for which we utilize the vertical transverse isotropy in the form of forward solutions. We use single-station and two-station methods to attain the observed surface wave dispersion curves in the period range 7-40 s and then apply the two-dimensional (2-D) tomography to convert these curves into the velocity maps defined through a 0.05o x 0.05o – sized grid. The damped least-squares technique is used to invert the individual group and phase velocity curves for the one-dimensional (1-D) structure. The latter 1-D inversion provides the depth profiles for the Voigt isotropic average shear-wave velocity (𝑉𝑆) and the radial anisotropy (𝜉). The 𝑉𝑆 and 𝜉 depth profiles are employed to construct the vertical transverse isotropic (VTI) velocity structure beneath a grid point. Through the forward modeling, the VTI velocity structure is revised to jointly fit the observed Rayleigh and Love group and phase velocities. In the forward modeling, the fifth VTI parameter (𝜂𝐾) measuring the departure from the elliptic condition (𝜂𝐾=1.0) and the anisotropic velocity perturbations (𝑎𝑃2⁄ and 𝑎𝑆2⁄) are primarily adjusted to achieve the fit between the observed and theoretical dispersion curves. In the depth range ~20-30 km, the 𝜂𝐾 is generally found to be 𝜂𝐾<1.0 changing in the range 0.94≤𝜂𝐾≤0.95, which is consistent with the PREM and the expression 𝑎𝑃2⁄=0.5𝑎𝑆2⁄ appears to be valid for the mid-to-lower crust beneath the studied region. In particular, the Rayleigh group velocities require the setting 𝜂𝐾<1.0 in the mid-to-lower crustal depth range.

Effect of micro-hole shape and arrangement on the impact properties of three-dimensional isotropic brittle structures

Brittle structures often fracture under shock wave compression, which will have a constraining effect on their structural design. By introducing and designing suitable three-dimensional micro-hole shapes, the ability of brittle structures to resist impacts can be effectively improved. This paper examines the impact characteristics of the three-dimensional isotropic brittle structure's micro-hole shape and arrangement using the lattice spring model with rotational freedom degrees. The model effectively simulates brittle materials’ mechanical properties and fracture characteristics and overcomes the lattice point-spring model's fixed Poisson's ratio limitation. The research results demonstrate that micro-hole shape strongly affects shock wave profile evolution. When arranged in a regular triangular prism, the concave decahedral holes, which have a negative Poisson’s ratio, can cause a strong impact deformation and a good energy absorbing effect. This paper’s results can provide some ideas for designing brittle structures with the best impact resistance.

Novel multiscale simulations on the membrane formation via hybrid induced phase separation process based on dissipative particle dynamics

A novel simulation methodology, that was based on dissipative particle dynamics (DPD) combined with information from molecular dynamics (MD) and calculations of an extended Maxwell-Stefan (MS) equations, was established to obtain multiscale analysis on the dynamics of hybrid induced phase separation (HIPS), taking polyvinylidene fluoride-diphenyl carbonate/water–ethanol (PVDF-DPC/water–ethanol) system as an example. Firstly, MS equations were derived to describe the heat transfer across the interface between the casting solution and coagulation to get the temperature profiles of the casting solution. Secondly, MD simulations for the PVDF-DPC solution were conducted to acquire bond and angle distributions of the PVDF chains. And compared to the DPD particle distributions, a novel DPD force field to describe the real flexibility of PVDF chains and a coarse-graining mapping between the MD and DPD were developed. Thirdly, based on the quenching time calculated by the MS equations and the DPD time scale, DPD simulations were carried out to demonstrate the dynamics of phase separation and membrane formation for different parts of the polymer solution via HIPS and compared to that via thermally induced phase separation (TIPS). The simulation results indicated that the mass transfer had a decisive effect on the phase separation of polymer solution near the interface and resulted in a highly asymmetric structure with a denser polymer top layer and porous sublayer, and this effect was together decided by the quenching rate and polymer solidification. For the inner parts of the polymer solution, a mainly isotropic structure was formed due to heat transfer. Based on experimental verification and simulation results, it can be known that the multiscale simulation was much closer to reality and can provide deep insights into the membrane formation prepared via the HIPS process.

Cost-effective natural graphite reengineering technology for lithium ion batteries

Graphite tailings produced by natural graphite is usually regarded as garbage to be buried underground, which would result in a certain waste of resources. Here, in order to explore the utilization of natural graphite tailings (NGT), a liquid-polyacrylonitrile (LPAN) is used to modify the NGT fragments and aggregate them together to form secondary graphite particles with low surface area and high tap density. Moreover, the modified NGT show much better electrochemical performances than those of original one. When tested in full cells coupled with NMC532 cathode, the material achieves a high rate capability and cycle stability at the cutoff voltage of 4.25 V as well as 4.45 V, which maintains 84.32% capacity retention after 500 cycles at 1 C rate (4.25 V), higher than that of the pristine one (73.65%). The enhanced performances can be attributed to the use of LPAN to create a unique carbon layer upon graphite tailings to reconstruct surface and repair defects, and also to granulate an isotropic structure of secondary graphite particles, which can help to weaken the anisotropy of Li+ diffusion pathway and form a uniform, complete and stable solid-electrolyte-interface (SEI) on the surface of primary NGT fragments to promote a fast Li+ diffusion and suppress lithium metal dendrites upon charge and discharge.

Vibration analysis of thermally loaded isotropic and composite beam and plate structures

This work proposed the use of the Carrera Unified Formulation (CUF) for the vibration and buckling analysis of structures subjected to thermal loads. In detail, the variation of natural frequencies for progressively large thermal loads is investigated. Here, particular attention is focused on the study of buckling thermal loads as degenerate cases of the vibration analysis and on the mode aberration caused by thermal stresses. From this standpoint, the use of CUF for the development of high-order beam and plate models is fundamental. Indeed, Lagrange-like (LE) polynomials are considered for developing the kinematic expansion and Layerwise (LW) theories are employed to characterize the complex phenomena that may appear in composite structures. A linearized formulation to study the natural frequencies variation as a function of the progressive increasing thermal loadings is adopted. Different isotropic and laminated composite structures have been analyzed and compared with the Abaqus solution to validate the presented methodology and provide some benchmark solutions. In addition, a parametric study was conducted to evaluate the stacking sequence and thickness effect in the vibration modes and thermal buckling loads. The results document the excellent accuracy and reliability of the presented methodology and show the potentialities of this numerical tool able to analyze cases that are difficult to study experimentally.

Stability of Anisotropy Pressure in Self-Gravitational Systems in f(G) Gravity

This investigation aims to explore certain variables which are considered responsible for generating pressure anisotropy in dynamical spherically symmetric stellar systems against the background of the stringy-inspired Gauss–Bonnet modification of general relativity. We explore the hydrostatic equilibrium of self-gravitating systems by taking into account the modified form of Tolman–Oppenheimer–Volkoff for the quadratic-f(G) gravitational model. In this respect, we formulate a differential equation in terms of the Weyl curvature scalar, also described as an evolution equation, which is essential for understanding the evolution of the stellar structure. Finally, we conclude that the existence of some fluid variables such as shear, heat flux and the irregular behavior of energy density in the presence of an extra degree f(G)-terms in the fluid flow that are the elements that cause anisotropy in the initially isotropic stellar structure. The comparison of the presented results with those of the classical model shows that they are physically relevant and compatible.

MXene-containing anisotropic hydrogels strain sensors with enhanced sensing performance for human motion monitoring and wireless transmission

Conductive hydrogels have attracted great attention for flexible electronics and intelligent sensing. However, conventional hydrogels with isotropic components and structures merely exhibit isotropic sensing properties, while structural orientation and anisotropy are much more common in nature. Inspired by the anisotropic structure of human tissues, we propose a facile and efficient method to fabricate anisotropic conductive hydrogels. This endows the hydrogels with excellent mechanical properties and electrical conductivity compared to conventional isotropic conductive hydrogels. MXene-containing anisotropic hydrogels were prepared by mechanical stretching and exhibit anisotropic mechanical and electrochemical properties based on their anisotropic structure. The tensile stress and toughness along the parallel stretching direction of MXene-containing anisotropic hydrogel can reach 2.68 MPa and 7.99 MJ/m3, respectively, which are significantly higher than those of the hydrogel along the vertical stretching direction and MXene-containing isotropic hydrogel. Moreover, the conductivity of MXene-containing anisotropic hydrogel along the parallel stretching direction (0.096 S/m) is better than that of the hydrogel along the vertical stretching direction and the isotropic hydrogel. MXene-containing anisotropic hydrogels exhibit enhanced strain sensing performance along the parallel stretching direction. Meanwhile, the hydrogel sensors can realize wireless transmission. It is expected that MXene-containing anisotropic hydrogel sensors have wide application prospects in flexible wearable devices.

Controlling Cellulose Membrane Performance via Solvent Choice during Precursor Membrane Formation

Fabrication of cellulose membranes by alkaline hydrolysis of cellulose acetate (CA) membranes is a simple alternative method to preparing cellulose membranes via phase inversion from their ionic liquid solutions. In this study, three different solvent systems were used to fabricate cellulose acetate membranes by phase inversion, which are dimethyl sulfoxide and its mixtures with acetone and acetic acid. Acetone as cosolvent led to an asymmetric morphology with the densest selective layer, whereas acetic acid led to a loose, almost isotropic structure with high porosity and pore connectivity, also reflected in the pure water permeance and molecular weight cutoff (MWCO) of the CA membranes. Comparing the effects of these cosolvents on the phase inversion process, acetic acid decreased the solvent quality, increased nonsolvent–solvent interactions, and in total decreased the polymer solutions’ stability against phase separation. The phase inversion rate when acetic acid was used as cosolvent was also markedly lower, which was attributed primarily to the solvent’s much higher viscosity. After alkaline hydrolysis of the CA membranes, which converted them into cellulose membranes, the permeance and MWCO of large-pore, small-pore, and dense membranes changed in different directions. This was attributed to a changing membrane structure due to partial degradation of cellulose chains in an alkaline medium, which in turn can affect transport through both the pores and the membrane matrix.

Geometry effect on mechanical properties and elastic isotropy optimization of bamboo-inspired lattice structures

Inspired by the geometry of bamboo, this study proposes a novel bamboo-inspired body-centered cubic (B-BCC) lattice structure consisting of tapered and hollow struts. Using representative volume elements applied with periodic boundary conditions, the mechanical properties and deformation behaviors of the B-BCC lattice structures are thoroughly evaluated by considering a large number of combinations of geometric parameters and volume fractions. Results reveal that the geometric parameters highly influence the deformation behavior of the B-BCC lattice structures under uniaxial compression (e.g, from bending- to stretching-dominated) but little under shear load. For this reason, tunable elastic modulus across a broad range can be realized via adjusting the geometric parameters and elastic isotropy can be obtained across all volume fractions. On this basis, a combination of artificial neural network and elastic isotropy optimization is proposed to obtain the isotropic B-BCC lattice structures with superior elastic modulus. The optimization results show that the elastic modulus of the isotropic B-BCC lattice structures increased by 271.24–1335 % and 17.72–43.63 %, as compared to the original BCC and isotropic hollow BCC lattice structures, respectively. Finally, the multi-layer simulation and compression experiments are applied to validate the optimization results. Good agreements are observed comparing the numerical and experimental results, demonstrating the effectiveness of the proposed bamboo-inspired design and optimization method for lightweight applications with desired properties.

Damage Identification for Isotropic Structures Using Lamb Wave Based on a Reference-Free Location Imaging Method

A Lamb wave-based reference-free damage localization imaging algorithm is proposed for online determination of the imperceptible defects due to harsh service environments, complex alternating, and impact loads in the plain isotropic structures (mainly aluminum alloys), which are crucial components of aircraft skin, bogies, and underframes of high-speed train, so as to overcome the deficiencies of the majority of localization imaging methods that rely on reference data in a healthy state. The analyses were implemented by configuring specific transducer arrays to pick alternate reference signals from the existing transmitter–receiver paths. A chirp excitation approach was introduced to choose the actuation frequency of ultrasonic Lamb wave more concisely and effectively. Simulations and experiments were conducted to recognize and pinpoint the defects in four damage scenarios in aluminum alloy plates with 12 surface-mounted piezoelectric transducers, and the damage area identified by the proposed methodology was compatible with the genuine position. The results between simulations and experiments are likewise essentially consistent, demonstrating that the proposed algorithm is able to identify and localize damage in plain isotropic structures without the requirement of healthy reference signals.

A spherical harmonics method for processing anisotropic X-ray atomic pair distribution functions

In this work, a general spherical harmonics method for the extraction of anisotropic pair distribution functions (PDFs) is described. In the structural study of functional crystallized materials, there is an urgent need for investigation of local structures under the application of external stimuli, such as an electric field and stress. A well established technique for local structural studies is PDF analysis, but the extraction of X-ray PDF data is usually based on angular integrations of isotropic X-ray structure functions, which is no longer valid for the anisotropic responses of materials under orientation-dependent stimuli. Therefore, an advanced spherical harmonics method has been developed, to transform 2D X-ray total-scattering data into anisotropic PDF data on the basis of the 3D diffraction geometry and a Fourier transform. The electric-field-induced local structural change in PbZr0.54Ti0.46O3 ceramics is presented to demonstrate the effectiveness of this method.

First Theoretical Realization of a Stable Two-Dimensional Boron Fullerene Network

Successful experimental realizations of two-dimensional (2D) C60 fullerene networks have been among the most exciting latest advances in the rapidly growing field of 2D materials. In this short communication, on the basis of the experimentally synthesized full boron B40 fullerene lattice, and by structural minimizations of extensive atomic configurations via density functional theory calculations, we could, for the first time, predict a stable B40 fullerene 2D network, which shows an isotropic structure. Acquired results confirm that the herein predicted B40 fullerene network is energetically and dynamically stable and also exhibits an appealing thermal stability. The elastic modulus and tensile strength are estimated to be 125 and 7.8 N/m, respectively, revealing strong bonding interactions in the predicted nanoporous nanosheet. Electronic structure calculations reveal metallic character and the possibility of a narrow and direct band gap opening by applying the uniaxial loading. This study introduces the first boron fullerene 2D nanoporous network with an isotropic lattice, remarkable stability, and a bright prospect for the experimental realization.

Utilizing Angle‐Dependent Photovoltage Effect in a CH3NH3PbCl3/Si Heterojunction toward High‐Performance Polarized Light Detection in Ultraviolet Region

AbstractOrganic–inorganic hybrid MAPbCl3 perovskite (MA+ = CH3NH3+) in UV photodetection is drawing interest due to its superior semiconductor properties and UV‐matchable optical bandgap. However, MAPbCl3‐based photodetector targeting high‐performance polarized light detection has remained unexplored due to the isotropic structure of MAPbCl3. The photovoltaic effect in the heterojunction, which shows an angle dependence on light polarization, provides opportunities to break the restriction of intrinsic structure for realizing polarization‐sensitive photodetection. Herein, an effective strategy is reported to realize high‐performance polarization‐sensitive UV photodetection by constructing a heterojunction with two isotropic materials MAPbCl3 and Silicon (Si). Emphatically, the photovoltage in MAPbCl3/Si heterojunction changes with the angle of polarized light with the maximum (minimum) value of 0.15 V (0.05 V), originating from the built‐in electric field. More interestingly, driven by the photovoltage, the device thus exhibits a large polarization ratio (Imax/Imin) of 2.9 at 377 nm under the self‐driven mode. Besides, the present device also delivers high‐performance photodetection in UV region owing to the superior photoresponse of MAPbCl3, including a high detectivity (D*) of 2.93 × 1011 Jones, a superior responsibility of 8 mA W–1, a substantial switching ratio of 900, and an ultrafast response speed of 40/98 µs at 0 V bias. This work opens a new avenue for high‐performance polarization‐sensitive photodetection by utilizing angle‐dependent photovoltage effect.

In Situ Formation of Soft–Rigid Hybrid Fibers Decorated by Sparse Lamellae of PLLA: Achieving Ductile and Heat-Resistant Materials with High Strength

Constructing an oriented crystalline structure with high crystallinity in poly(l-lactide) (PLLA) is one efficient way to improve its mechanical strength, but flexibility is sacrificed because orderly arrangement restricts molecular mobility seriously. In this work, a unique hierarchical structure of soft–rigid hybrid fibers decorated by sparse lamellae of PLLA is in situ constructed successfully to improve mechanical strength and flexibility simultaneously. Through interfacial stereocomplexation between the PLLA matrix and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)–poly(d-lactide) (PHBV-PDLA) copolymers, followed by a strong flow field during the multistage stretching extrusion (MSE), soft–rigid hybrid fibers of PHBV/interfacial stereocomplex crystallites (i-SCs) are formed to not only act as a reinforcement filler but also favor energy-dissipating deformation mechanisms. Moreover, under the coupling effect of hybrid fibers and intense flow field, the sparse PLLA lamella with a stiff shish is obtained. Consequently, nanofibrillar PLLA/PHBV-PDLA composites with a highly oriented structure exhibit high strength (94.1 MPa), considerable ductility (65.7%), and excellent heat resistance (E′140 °C > 310 MPa), compared to sea–island structured PLLA/PHBV-PDLA composites with an isotropic structure (59.3 MPa, 11.7%, and E′140 °C > 60 MPa). Furthermore, nanofibrillar PLLA/PHBV-PDLA composites can maintain ductility (57.7%) and strength (98.3 MPa) for 886 days at ambient temperature. The strengthening and toughening mechanisms based on the unique structure are proposed by investigating the microstructural changes before and after a tensile test. This work provides significant guidance for strong, ductile, and heat-resistant PLLA-based materials for durable applications.

Mechanical behaviour of a novel biomimetic lattice structure for bone scaffold

In this research, a new lattice structure based on the octagonal geometry and produced by Additive Manufacturing (AM) technique was proposed. Eight octagons with the same dimensions are combined to each other forming a ring. To obtain an isotropic lattice structure, cubic symmetry was imposed; thus, the unit cell is made of three rings mutually perpendicular, one ring for each principal direction.The aim of this study is the morphological and mechanical characterization of the novel unit cell to check its suitability to the biomechanical field, along with its comparison with other lattice structures currently used as bone scaffold.Electron Beam Melting (EBM) technique was used to produce Ti6Al4V ELI alloy specimens of the novel unit cell and of the truncated octahedron (Kelvin) cell. Three different unit cell sizes were selected to investigate the effect of cell dimensions on the mechanical properties.Morphological analysis was performed through a scanning electron microscope (SEM), to compare the actual structures to the designed ones.On the whole, the new lattice structure provided adequate mechanical properties to be considered as a bone substitute; further tests will be focused on its osteointegration capability.

Nondestructive Examination of Carbon Fiber-Reinforced Composites Using the Eddy Current Method.

This paper presents the results of experiments using the eddy current system designated for nondestructive inspection of carbon fiber-reinforced composites. For this purpose, the eddy current testing system with a differential transducer with two pairs of excitation coils oriented perpendicularly and a central pick-up coil was utilized. The transducer measures the magnetic flux difference flowing through the pick-up coil. The transducer of this design has already been successfully utilized to inspect isotropic metal structures. However, the anisotropy of the composites and their lower conductivity compared to metal components made the transducer parameters adjustment essential. Thus, various excitation frequencies were considered and investigated. The system was evaluated using a sample made of orthogonally woven carbon fiber-reinforced composites with two artificial flaws (the notches with a maximum relative depth of 30% and 70%, respectively, thickness of 0.4 mm, and a length of 5 mm). The main goal was to find a configuration suitable for detecting hidden flaws in such materials.

Three Dimensional Coupling between Elastic and Thermal Fields in the Static Analysis of Multilayered Composite Shells

This new work aims to develop a full coupled thermomechanical method including both the temperature profile and displacements as primary unknowns of the model. This generic full coupled 3D exact shell model permits the thermal stress investigation of laminated isotropic, composite and sandwich structures. Cylindrical and spherical panels, cylinders and plates are analyzed in orthogonal mixed curved reference coordinates. The 3D equilibrium relations and the 3D Fourier heat conduction equation for spherical shells are coupled and they trivially can be simplified in those for plates and cylindrical panels. The exponential matrix methodology is used to find the solutions of a full coupled model based on coupled differential relations with respect to the thickness coordinate. The analytical solution is based on theories of simply supported edges and harmonic relations for displacement components and sovra-temperature. The sovra-temperature magnitudes are directly applied at the outer faces through static state hypotheses. As a consequence, the sovra-temperature description is assumed to be an unknown variable of the model and it is calculated in the same way as the three displacements. The final system is based on a set of coupled homogeneous differential relations of second order in the thickness coordinate. This system is reduced in a first order differential relation system by redoubling the number of unknowns. Therefore, the exponential matrix methodology is applied to calculate the solution. The temperature field effects are evaluated in the static investigation of shells and plates in terms of displacement and stress components. After an appropriate preliminary validation, new benchmarks are discussed for several thickness ratios, geometrical data, lamination sequences, materials and sovra-temperature values imposed at the outer faces. Results make evident the accordance between the uncoupled thermo-mechanical model and this new full coupled thermo-mechanical model without the need to separately solve the Fourier heat conduction relation. Both effects connected with the thickness layer and the related embedded materials are included in the conducted thermal stress analysis.