Hexagonal Microstructures Research Articles

Homogenized and localized responses of coated magnetostrictive porous materials and structures

The homogenized properties and localized field distributions of coated magneto-strictive porous heterogeneous materials are extensively investigated using the extended Finite Volume Direct Averaging Micromechanics (FVDAM) with coupled mechanical-magnetic capabilities. Both square and hexagonal microstructures are employed to mimic different hollow coating arrangements. In order to validate the present technique, the corresponding finite element model is constructed in Abaqus to provide rigorous comparisons, and no visible difference is observed between two different models’ predictions. The effects of the thickness of coatings are tested on the effective and local responses, where the hollow coatings play significant roles in the variations of homogenized properties and transmissions of stress concentrations. Finally, a two-step multiscale framework is established on periodic wavy structures, wherein the layers are composed of CoFe2O4 material reinforced by BaTiO3 hollow coatings with two different volume fractions. The amplitude-to-wavelength ratio is varied to effectively study its impact on the field distributions of homogenized layers and local microstructures.

Surfactant-tuned phase crystallinity and morphologies of NaYF4:Yb3+,Er3+ hexagonal microstructures and their photoluminescence properties

Typical up-conversion materials of NaYF4:Yb3+,Er3+ microcrystals were synthesized by a hydrothermal route using different surfactants (ethylene diamine tetraacetic acid (EDTA) and citric acid (CA)) as chelating agents. The results and analysis reveal that surfactant as a shape modifier introduced into the reaction system plays an important role in altering the crystalline structures, morphologies, and up-conversion luminescence properties of target crystals. A possible growth mechanism for NaYF4:Yb3+,Er3+ microcrystals to confirm the influence of surfactant on morphology and size was proposed on the basis of the growth rate of different crystal facets and the difference in the type of organic ligand. The luminescence mechanisms for NaYF4:Yb3+,Er3+ microcrystals as well as the correlation between the grain size, defects, morphologies and up-conversion luminescence were thoroughly analyzed.

Broadband focusing of underwater sound using a transparent pentamode lens.

An inhomogeneous acoustic metamaterial lens based on spatial variation of refractive index for broadband focusing of underwater sound is reported. The index gradient follows a modified hyperbolic secant profile designed to reduce aberration and suppress side lobes. The gradient index (GRIN) lens is comprised of transversely isotropic hexagonal microstructures with tunable quasi-static bulk modulus and mass density. In addition, the unit cells are impedance-matched to water and have in-plane shear modulus negligible compared to the effective bulk modulus. The flat GRIN lens is fabricated by cutting hexagonal centimeter scale hollow microstructures in aluminum plates, which are then stacked and sealed from the exterior water. Broadband focusing effects are observed within the homogenization regime of the lattice in both finite element simulations and underwater measurements (20-40 kHz). This design approach has potential applications in medical ultrasound imaging and underwater acoustic communications.

Enhanced UV photosensing properties of ZnO nanowires prepared by electrodeposition and atomic layer deposition

A new process enabling the synthesis of zinc oxide (ZnO) and Al-doped ZnO nanowires (NWs) for photosensing applications is reported. By combining atomic layer deposition (ALD) for the seed layer preparation and electrodeposition for the NW growth, high-quality ZnO nanomaterials were prepared and tested as ultraviolet (UV) sensors. The obtained NWs are grown as arrays perpendicular to the substrate surface and present diameters between 70 and 130 nm depending on the Al doping, as seen from scanning electron microscopy (SEM) studies. Their hexagonal microstructure has been determined using X-ray diffraction and Raman spectroscopy. An excellent performance in UV sensing has been observed for the ZnO NWs with low Al doping, and a maximal photoresponse current of 11.1 mA has been measured. In addition, initial studies on the stability have shown that the NW photoresponse currents are stable, even after ten UV on/off cycles.

The rose petal effect and the role of advancing water contact angles for drop confinement

We studied the role of advancing water contact angles on superhydrophobic surfaces that exhibited strong pinning effects as known in nature from rose petals. Textured surfaces were engineered in silicon by lithographical techniques. The textures were comprised of hexagonal microstructures superimposed with randomly distributed nanospikes and were coated with a hydrophobic fluorocarbon agent. A step in the advancing water contact angle bounding specific areas was obtained by engineering a corresponding topographic step in the hexagonal micro-texture. This enabled a surface texture design confining drops to areas with a lower advancing contact angle.

Design and numerical analysis: Effect of core and cladding area on hybrid hexagonal microstructure optical fiber in environment pollution sensing applications

This paper presents a micro-cored Hybrid Hexagonal shape photonic crystal fiber (PCF) for the higher sensitivity response in a wide range of wavelengths ranging from 1400 to 1640 nm. Numerical investigation is carried out by applying the full vectorial finite elements method (FV-FEM) with perfectly matched layer (PML) as an absorbing boundary condition regime. The proposed structure is suitable for highly sensible of standard multi-mode fiber (MMF) over E + S + C + L + U communication bands. Rigorous computational results are strongly evident that the proposed PCF acquires the highest relative sensitivity of about 62.24% at the controlling wavelength λ = 1.40 μm which will be applicable for monitoring environment in aspects of harmful and toxic chemical sensing. The proposed H-HPCF structure can be fabricated using a sol–gel technique method and is expected to be useful for wideband imaging and communicating applications too.

In situ growth of urchin-like NiCo2S4 hexagonal pyramid microstructures on 3D graphene nickel foam for enhanced performance of supercapacitors

Urchin-like NiCo2S4 hexagonal pyramid microstructures have been successfully fabricated on 3-D graphene nickel foam by a two-step hydrothermal method. These achieve a rather high mass loading, with enhanced specific capacitance, rate performance and cycling stability.

Phase-field study of pore-grain boundary interaction

During final stage sintering, a complex interplay of densification and grain growth dominates microstructural evolution. Grain growth starts, when pore drag effects become less important due to pore shrinkage. This grain growth then decreases the driving force available for sintering. Accordingly, the interplay of pores and grain boundaries needs to be considered in detail. A phasefield model was extended to treat pore dynamics under consideration of pressure stability. To study pore attachment and detachment at moving interfaces, an idealized hexagonal microstructure with a constant driving force relationship for pore migration is constructed. Additionally, realistic polycrystalline microstructures were used. The model is in good agreement with experiments and analytic equations. Three different cases were observed in the realistic microstructure: pore attachment at the moving interface, partial and total pore detachment. However, in the partial case, the initial location of pores was found to be important: pores tend to migrate from quadruple junctions over triple junctions to grain boundary planes, where they eventually detach. This results in a variation of pore detachment, which is not captured in analytic equations. Therefore large simulation setups are required to reflect the impact of initial pore location on pore drag effects.

Effect of Substrate on the Photodetection Characteristics of ZnO-PAni Composites

Heterojunction photodiodes involving a wide bandgap semiconductor on polymeric substrates have a number of potential advantages such as excellent blue response, simple processing steps, and low processing cost. We report on the chemical synthesis of ZnO–polyaniline (PAni) composite thin films and their use for UV detection. The morphological and structural properties of the composites were characterized using X-ray diffraction (XRD), Raman spectroscopy, and field-emission scanning electron microscopy (FESEM). The XRD results demonstrated the formation of hexagonal wurtzite ZnO structure, with no noticeable change upon the addition of PAni. The FESEM images revealed the hexagonal microstructure of the ZnO-PAni composite films, with high porosity. Raman spectroscopy revealed the interfacial interaction between ZnO and PAni. Thin films deposited on glass, PET, and Si substrates were used to fabricate photodetector devices based on the ZnO-PAni composites. The photodetector made using Si substrate showed the best performance with a high gain of 108, sensitivity of 9842, and responsivity of 0.707A/W.

Hydrothermal synthesis of hexagonal ZnO microstructures in HPMC polymer matrix and their catalytic activities

ZnO microcrystals with hexagonal morphologies are synthesized via a fast and facile hydrothermal route using hexamethylene tetramine (HMTA) as reducing agent and hydroxylpropyl methyl cellulose (HPMC) as morphology directing agent. In the absence of HPMC, hexagonal rod shaped ZnO microcrystals are formed whereas hexagonal bar and both end open hexagonal bar shaped structures are obtained in the presence of different amounts of HPMC. Synthesized ZnO microstructures are characterized using XRD, SEM and fluorescence spectroscopic study. The strong asymmetric blue emission band from ZnO microrod has been explained due to the presence of extended Zni states within the microcrystals. Photocatalytic activities of the microcrystals are investigated by monitoring the photochemical degradation of Methylene Blue. It has been observed that the catalytic efficiency of hexagonal both end open bar shaped ZnO microcrystals is higher than the other ZnO structures.

Functional anatomy of the pretarsus in whip spiders (Arachnida, Amblypygi)

Whip spiders (Amblypygi) are a small, cryptic order of arachnids mainly distributed in the tropics. Some basal lineages (families Charinidae and Charontidae) have adhesive pads on the tips of their six walking legs. The present study describes the macro- and ultrastructure of these pads and investigates their contact mechanics and adhesive strength on smooth and rough substrates. Furthermore, the structure of the pretarsus and its kinematics are compared in Charon cf. grayi (with an adhesive pad) and Phrynus longipes (without an adhesive pad). The adhesive pads exhibit an elaborate structure with a unique combination of structural features of smooth and hairy foot pads including a long transversal contact zone performing lateral detachment, a thick internally-branched cuticle with longitudinal ribs and hexagonal surface microstructures with spatulate keels. The contact area of the pads on smooth glass is discontinuous due to the spatulate microstructures with a discontinuous detachment, which could be observed in vivo by high speed videography at a rate of up to 10,000 fps. Adhesive strength was measured with vertical whole animal pull-off tests, obtaining mean values between 55 and 200 kPa. The occurrence of viscous lipid secretions between microstructures was occasionally observed, which, however, seems not to be a necessity for good foothold. The results are discussed in relation to the whip spider's ecology and evolution. Structure–function relationships of the adhesive pads are compared to those of insects and vertebrates.

Computational homogenization of nano‐materials accounting for size effects via surface elasticity

AbstractThe objective of this contribution is to establish a first‐order computational homogenization framework for micro‐to‐macro transitions of porous media that accounts for the size effects through the consideration of surface elasticity at the microscale. Although the classical (firstorder) homogenization schemes are well established, they are not capable of capturing the well‐known size effects in nano‐porous materials. In this contribution we introduce surface elasticity as a remedy to account for size effects within a first‐order homogenization scheme. This proposition is based on the fact that surfaces are no longer negligible at small scales.Following a standard first‐order homogenization ansatz on the microscopic motion in terms of the macroscopic motion, a Hill‐type averaging condition is used to link the two scales. The averaging theorems are revisited and generalized to account for surfaces. In the absence of surface energy this generalized framework reduces to classical homogenization. The influence of the length scale is elucidated via a series of numerical examples performed using the finite element method. The numerical results are compared against the analytical ones at small strains for tetragonal and hexagonal microstructures. Furthermore, numerical results at small strains are compared with those at finite strains for both microstructures. Finally, it is shown that there exists an upper bound for the material response of nano‐porous media. This finding surprisingly restricts the notion of “smaller is stronger”. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of thermally induced microstress and microstructural randomness on transverse strength of unidirectional composites

An investigation is performed into the interaction between thermally and mechanically induced stresses and their relation to failure in a unidirectional composite under transverse tension and longitudinal shear. Multiple realizations of periodic unit cells with randomly positioned fibers are used. It is found that for microstructures with close fiber spacing, thermally induced stresses from cooling after cure tend to relieve the critical mechanically induced stress in the matrix for transverse tensile load, while they tend to worsen the critical stress for microstructures with space between fibers. Matrix strength properties are obtained through the solution of an inverse problem and are then used to perform progressive failure analyses at various temperatures. These analyses indicate that for random microstructures with closely spaced fibers, cooling after cure tends to increase the transverse tensile strength of the unidirectional composite, while cooling tends to weaken models using a hexagonal microstructure.

Evaluating Local Primary Dendrite Arm Spacing Characterization Techniques Using Synthetic Directionally Solidified Dendritic Microstructures

Microstructure characterization continues to play an important bridge to understanding why particular processing routes or parameters affect the properties of materials. This statement certainly holds true in the case of directionally solidified dendritic microstructures, where characterizing the primary dendrite arm spacing is vital to developing the process–structure–property relationships that can lead to the design and optimization of processing routes for defined properties. In this work, four series of simulations were used to examine the capability of a few Voronoi-based techniques to capture local microstructure statistics (primary dendrite arm spacing and coordination number) in controlled (synthetically generated) microstructures. These simulations used both cubic and hexagonal microstructures with varying degrees of disorder (noise) to study the effects of length scale, base microstructure, microstructure variability, and technique parameters on the local PDAS distribution, local coordination number distribution, bulk PDAS, and bulk coordination number. The Voronoi tesselation technique with a polygon-side-length criterion correctly characterized the known synthetic microstructures. By systematically studying the different techniques for quantifying local primary dendrite arm spacings, we have evaluated their capability to capture this important microstructure feature in different dendritic microstructures, which can be an important step for experimentally correlating with both processing and properties in single crystal nickel-based superalloys.

Dew condensation on desert beetle skin.

Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collect water droplets from wind-blown fogs. We aim to determine whether dew water collection is also possible for desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionidæ) beetle, where the macro-structure appears as a series of "bumps", with "valleys" between them. Dew formation experiments were carried out in a condensation chamber. The surface properties (infra-red emissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, its micro-structure. We performed scanning electron microscope on histological sections and determined the infra-red emissivity using a scanning pyrometer. The emissivity measured (0.95±0.07 between 8-14 μm) was close to the black body value. Dew formation occurred on the insect's elytra, which can be explained by these surface properties. From the surface coverage of the condensed drops it was found that dew forms primarily in the valleys between the bumps. The difference in droplet nucleation rate between bumps and valleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surface of the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insect's mouth.

Evaluation of the AZ31 cyclic elastic-plastic behaviour under multiaxial loading conditions

Components and structures are designed based in their material’s mechanical properties such as Young's modulus or yield stress among others. Often those properties are obtained under monotonic mechanical tests but rarely under cyclic ones. It is assumed that those properties are maintained during the material fatigue life. However, under cyclic loadings, materials tend to change their mechanical properties, which can improve their strength (material hardening) or degrade their mechanical capabilities (material softening) or even a mix of both. This type of material behaviour is the so-called cyclic plasticity that is dependent of several factors such as the load type, load level, and microstructure. This subject is of most importance in design of structures and components against fatigue failures in particular in the case of magnesium alloys. Magnesium alloys due to their hexagonal compact microstructure have only 3 slip planes plus 1 twining plane which results in a peculiar mechanical behaviour under cyclic loading conditions especially under multiaxial loadings. Therefore, it is necessary to have a cyclic elastic-plastic model that allows estimating the material mechanical properties for a certain stress level and loading type. In this paper it is discussed several aspects of the magnesium alloys cyclic properties under uniaxial and multiaxial loading conditions at several stress levels taking into account experimental data. A series of fatigue tests under strain control were performed in hour glass specimens test made of a magnesium alloy, AZ31BF. The strain/stress relation for uniaxial loadings, axial and shear was experimentally obtained and compared with the estimations obtained from the theoretical elastic-plastic models found in the state-of-the-art. Results show that the AZ31BF magnesium alloy has a peculiar mechanical behaviour, which is quite different from the steel one. Moreover, the state of the art cyclic models do not capture in full this peculiar behaviour, especially the cyclic magnesium alloys anisotropy. Further, an analysis is performed to identify the shortcomings inherent to the actual cyclic models in the capture of the magnesium alloys cyclic behaviour. Several conclusions are drawn.

Evolution of ZnO microstructures from hexagonal disk to prismoid, prism and pyramid and their crystal facet-dependent gas sensing properties

Herein, the evolution of ZnO structures from hexagonal disk to prismoid, prism and pyramid was found via a facile two-step low temperature hydrothermal reaction, and the evolution was achieved by only adjusting the pH value of the reactive solution without the assistance of a template or a surfactant. The characterization results showed that the precursor (hexagonal Zn5(OH)8Cl2·2H2O disk) played a key role in the morphology evolution of ZnO during the early stage of the growth process and that the disks tended to stack together layer by layer in both directions (up and down) to form prismoid, prism and pyramid structures with the increase in pH value from 7 to 10. After calcination, the corresponding hexagonal ZnO microstructures were obtained. This structure evolution resulted in the weakening dominance of the (0001) plane in the total exposed crystal facets. Furthermore, despite the similar specific surface areas of the four hexagonal ZnO microstructures, the gas sensing properties of the sensors based on these microstructures deteriorated sequentially. At a working temperature of 330 °C, the ZnO disk with the most exposed (0001) plane showed the highest gas response toward ethanol, which was nearly 2, 3, and 6 times higher than those of the prismoid, prism and pyramid structures, respectively. This superior gas sensing performance strongly depends on the predominantly exposed polar facets (0001), which can provide more active sites for oxygen adsorption and subsequent reaction with the detected gas than other apolar facets. It demonstrates that the (0001) crystal facet plays a significant role in the gas sensing behavior of ZnO. This research will bring some inspiration to researchers for the fabrication of a high performance ZnO gas sensor as well as other metal oxides.

The influence of microstructure randomness on prediction of fiber properties in composites

Elastic properties were predicted for AS4, IM7, T300, and T650-35 graphite fibers. An inverse method was employed using lamina and epoxy properties taken from literature. Fiber properties predicted using finite element analysis based on a hexagonal microstructure, finite element analysis based on a random microstructure, and the Mori-Tanaka averaging scheme were compared. It was observed that the Mori-Tanaka averaging scheme and finite element analysis based on a hexagonal microstructure predicted nearly identical fiber properties. In contrast, randomness in the arrangement of fibers results in significantly different predictions for the longitudinal shear modulus of the fiber. The three microstructural models were also used to predict properties for isotropic E-glass 21xK43. It was observed that the three models predicted nearly the same properties for the glass fibers.

Multicore Fiber Optimization for Application to Chip-to-Chip Optical Interconnects

We present the design of a holey microstructured multicore optical fiber optimized to meet the stringent requirements of chip-to-chip optical interconnects, namely, be compatible with high-speed vertical-cavity surface-emission lasers, feature ultrahigh channel density, low crosstalk, and millimeter-bend resistance to sustain the tight bends required on an electronic circuit board. We show that the shortcomings of the standard hexagonal microstructure can be overcome by the use of seven-rod cores. We present the detailed simulation results of the crosstalk and bend loss as a function of all the important microstructure parameters that led to the optimized solution. We discuss the crosstalk dependence on the bending radius. To our knowledge, the multicore fiber presented here achieves the highest normalized core density proposed to date, low enough crosstalk for meter-long transmission at 10 Gb/s and bend loss <; 0.02 dB/loop at a 1-mm bend radius. Maximizing space-division multiplexing, it has the potential to allow Tb/s·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> optical interconnects without the need for wavelength-division multiplexing.

Design optimization of porous fibrous material for maximizing absorption of sounds under set frequency bands

In this paper, a methodology is proposed for designing porous fibrous material with optimal sound absorption under set frequency bands. The material is assumed to have a rigid frame and a hexagonal arrangement of fibers, and the analytical model derived by Johnson, Champoux and Allard (“JCA model”) is used to investigate the influences of the micro-structural parameters (fiber radius r and gap w) on sound absorption performance, and the macro-acoustic parameters used in JCA model is determined via finite element analysis for the hexagonal micro-structure. Moreover, a mathematical model is constructed to obtain the optimized micro-structure design, with fiber radius and gap as design parameters and average absorption performance of the porous fibrous material under set frequency band as target. Utilizing the constructed optimization model, the microstructure parameters are derived with optimal sound absorption under low frequency (20⩽f<500Hz), medium frequency (500⩽f<2000Hz) and high frequency (2000⩽f<15,000Hz), respectively. On top of that, for a given thickness of porous fibrous material layer, the analytical relationship between fiber radius and optimal porosity under set frequency bands is constructed.