Sandwich Structure Research Articles

Optimization of Post-Annealing Temperature of RF Magnetron-Sputtered ZnO Thin Films for Enhancing Performances of UV Photodetectors

In this study, we examined the surface morphology and crystal structure of RF-sputtered ZnO thin films that were annealed at various temperatures. Also, we fabricated UV photodetectors with an Au-ZnO-ITO sandwich structure, utilizing the thin films annealed at 600 °C. The surface roughness of the film initially increases and then decreases as the annealing temperature rises, and the crystalline quality improves with an increase in the annealing temperature. Due to the Schottky heterojunction formed by the Au-semi contact, the fabricated UV photodetector exhibits a responsivity of 7.91 mA W−1 under 405 nm UV light. And the device demonstrates high response stability and rapid response sensitivity, with a swift rise time of 80 ms at a light intensity of 2.56 mW cm−2.

Transparent antenna with RCS tunability based on graphene and metasurface in S band

In this paper, we propose a graphene-based radar cross section (RCS) tunable antenna that utilizes metal mesh and graphene, both of which are optically transparent. The graphene sandwich structure is introduced to replace traditional components like diodes, micro-electro-mechanical systems (MEMS), and varactors, acting as an electromagnetic wave controller and significantly simplifying the device's complexity. By applying different voltages, the electrical properties of graphene are altered, enabling the regulation of reflection, transmission, and absorption of electromagnetic waves. This not only modifies the antenna's pattern but also achieves a substantial reduction in out-of-band RCS. The radiation and scattering mechanism of the antenna is carefully elaborated. The numerical and experimental results match well, which validates the effectiveness of the proposed method. The optical transmittance of the device is 27.9% at 550 nm, and the out-of-band RCS reduction is 10.01 dB at 3.83 GHz. The transparent, RCS tunable antenna proposed has significant potential applications in optical and microwave stealth technology.

Numerical Investigation on Dynamic Response of Carbon Fiber Honeycomb Sandwich Panels Subject to Underwater Impact Load

This study investigates the dynamic response and failure mechanisms of carbon fiber honeycomb sandwich structures under underwater impact loads using finite element numerical simulation. The geometric modeling was performed using HyperMesh, and the dynamic response simulations were carried out in ABAQUS, focusing on honeycomb core configurations with varying edge lengths, heights, and gradient forms. The Hashin damage model was employed to describe the damage evolution of the composite materials. The simulation results revealed that the dynamic response was significantly influenced by the initial shock wave pressure and the geometrical parameters of the honeycomb cells. Larger cell-edge lengths and heights generally resulted in improved energy absorption and reduced rear panel displacement. Among the different configurations, interlayer gradient honeycomb structures demonstrated superior impact resistance compared to homogeneous and in-plane gradient structures, particularly under higher initial shock wave pressures. These findings contribute to optimizing the design of carbon fiber honeycomb sandwich structures for enhanced impact resistance in relevant applications.

Sandwich-Structured Carbon Nanotube Composite Films for Multifunctional Sensing and Electrothermal Application.

Electro-conductive films with excellent flexibility and thermal behavior have great potential in the fields of wearable electronics, artificial muscle, and soft robotics. Herein, we report a super-elastic and electro-conductive composite film with a sandwich structure. The composite film was constructed by spraying Polyvinyl alcohol (PVA) polymers onto a buckled conductive carbon nanotube-polydimethylsiloxane (CNTs-PDMS) composite film. In this system, the PVA and PDMS provide water sensing and stretchability, while the coiled CNT film offers sufficient conductivity. Notably, the composite film possesses high stretchability (205%), exceptional compression sensing ability, humility sensing ability, and remarkable electrical stability under various deformations. The produced CNT composite film exhibited deformation (bending/twisting) and high electro-heating performance (108 °C) at a low driving voltage of 2 V. The developed CNT composite film, together with its exceptional sensing and electrothermal performance, provides the material with promising prospects for practical applications in wearable electronics.

Bending stiffness analysis of regular polyhedron sandwich structures

Abstract As a new type of sandwich structure, regular polyhedron sandwich structure has excellent structure performances such as high bending resistance and energy-absorbing features. However, the current design process of regular polyhedron sandwich structures often ignores the influence of unit-cell parameters on structural bending stiffness. Based on origami geometry, this study analyzes the geometric characteristics of polyhedron unit-cell structures. A simulation model was established to analyze the bending performance of regular polyhedron sandwich structures. The influence of the number of polyhedron surfaces and similarity transformation parameters on bending stiffness was analyzed by finite element software. Through finite simulation analysis, the influence graph of bending performance in the impact of various parameters was obtained. This analysis provides valuable insights for the optimization design of regular polyhedron sandwich structures.

Generalized Background Vector Method for Tailored Three-Dimensional Nonperiodic Woven Composite Designs

In critical aerospace applications such as Ceramic Matrix Composite structures, novel three-dimensional nonperiodic textile composite preforms hold great promise for creating advanced composite structures with strategically aligned fiber tows. However, the inherently challenging nature of determining tow locations, a large-scale combinatorial problem, presents a significant obstacle in textile architecture design. This study generalizes the previously developed Background Vector Method (BVM) to address diverse design requirements and constraints, effectively circumventing combinatorial design complexities via a game theoretic approach. This approach allows for the creation of tunable designs for woven architectures with complex geometries, such as channels and tapering features, through simple control parameter adjustments. The method demonstrates exceptional computational efficiency, making it suitable for large-scale nonperiodic textile structures. Case studies including woven sandwich and airfoil structures highlight the generalized BVM’s versatility and effectiveness in addressing complex design challenges within the aerospace sector.

Investigation of the Mechanical Properties of Composite Honeycomb Sandwich Panels after Fatigue in Hygrothermal Environments.

Since carbon fibre composite sandwich structures have high specific strength and specific modulus, which can meet the requirements for the development of aircraft technology, more and more extensive attention has been paid to their residual mechanical properties after subjecting them to fatigue loading in hygrothermal environments. In this paper, the compression and shear characteristics of carbon fibre-reinforced epoxy composite honeycomb sandwich wall panels after fatigue in hygrothermal environments are investigated through experiments. The experimental results show that under compressive loading, the load required for the buckling of composite honeycomb sandwich wall panels after fatigue loading in hygrothermal environments decreases by 25.9% and the damage load decreases by 10.5% compared to those at room temperature. Under shear loading, the load required for buckling to occur is reduced by 26.2% and the breaking load by 12.2% compared to those at room temperature.

Double-side debonding detection of honeycomb sandwich structure based on air-coupled ultrasonic guided waves

Abstract In structural health monitoring, fast, non-destructive, and non-contact testing has gradually become an important demand. This paper presents a double-side debonding detection method of honeycomb sandwich structure (HSS) based on the air-coupled guided wave. We analyze the dispersion characteristics and propagation law of guided waves in a honeycomb sandwich structure (HSS) using numerical simulation. Moreover, dispersion compensation was employed to reduce the stacking of signals. Finally, the debonding region was imaged. It realizes debonding detection on both upper and lower skins according to the amplitude changes of the direct wave signal and coda wave signal.

Study on ultrasonic lamb wave testing technology of honeycomb sandwich structures

Abstract Honeycomb sandwich composite panels have complex structures, large sizes, many types of defects, and significant sound attenuation, which lead to low efficiency and poor sensitivity of ultrasonic non-destructive testing. In the study, a non-linear ultrasonic lamb wave testing technique for honeycomb sandwich composite panels is proposed. Firstly, the anisotropic propagation characteristics and modal structure of lamb waves in honeycomb sandwich composite panels are analyzed, and the lamb wave modes with non-linear ultrasonic accumulation are excited. Secondly, the data extraction method and imaging algorithm of non-linear ultrasonic lamb wave imaging testing are proposed. Finally, an enhanced imaging algorithm is proposed to improve image quality based on the sliding window algorithm. Based on the comparative analysis of air-coupled ultrasonic testing images and metallographic tests, it can be seen that non-linear ultrasonic lamb wave testing signals can be used to characterize the damage distribution characteristics of honeycomb sandwich composite panels, display debonding and weak debonding defects, and have the advantages of being able to perform on the same side testing and high efficiency, which can be used for non-destructive testing of honeycomb sandwich composite panels.

Damage Assessment of 3-D Printed Ceramic Sandwich Structures with an Auxetic Honeycomb Core Subjected to Quasistatic Indentation Loading

Abstract Ceramic sandwich structures (CSS) have become an important material in the aerospace industry because of their high strength and excellent thermal insulation properties. However, the brittle nature of ceramic makes them vulnerable to damage from foreign objects, which can reduce their load-bearing capacity. In this paper, a series of tests were designed to investigate the response of CSS to impacts from foreign objects. To realize the damage characteristics and failure modes under the indentation force, a quasistatic indentation (QSI) test was conducted on CSS. Additionally, an acoustic emission device was used to capture damage signals during the QSI testing. Thereafter the extent of damage was evaluated by analyzing the damaged area and the compression after indentation properties. The results of these tests revealed the failure mechanism maps and indicated that the compressive strength of the damaged CSS had a stronger correlation with damage to the honeycomb core than to the face sheet.

Concurrent topology optimization of sandwich structures with multi-configuration and variable-diameter lattice infill

The superior stiffness-to-weight and strength-to-weight mechanical advantages of sandwich structures can be fully exploited through concurrent design of entire topology, infill configuration and density, where the high-performance yet complicated structure can be fabricated through additive manufacturing. However, the emerging design challenges are concurrent design updating related to sandwich topology, infill configuration and density, which is a design problem with continuous and discrete variables mathematically. In this paper, a concurrent topology optimization is proposed for sandwich structures with multi-configuration and variable-diameter lattice infill. Three design variable fields are employed to describe the fundamental topology considering sandwich structural topology, infill configuration and density simultaneously. Corresponding material interpolation model is developed by combining DSP-based shell-infill description and multi-response latent-variable surrogate model based effective material property calculation. Two-stage design model is formulated as a rough design model considering all design variables followed by a refined design model with only infill density variables, which is developed to strictly satisfy the material allowance constraint due to the mapping between discrete infill configuration variables and continuous latent configuration variables. Corresponding sensitivities of compliance and constraint with respect to the structural topology, infill configuration and density variables are derived, and the method of moving asymptotes (MMA) is employed to solve the design model efficiently. Several numerical examples are presented to systematically demonstrate the effectiveness of the proposed approach.

General synthesis of porous metal oxides nanosheets for gas sensing by gradient crystallization induced spatial confinement approach

Inorganic-template synthesis of crystalline porous metal oxides nanosheets with fantastic properties is a powerful approach, but it is limited in the systems with strong interaction between inorganic crystals and guest species. Here, we report a gradient crystallization induced spatial confinement method to achieve a general synthesis of porous metal oxides nanosheets regardless of host–guest interaction. Taking the synthesis of porous SnO2 nanosheets as an example, the freezing of KCl/SnCl2 aqueous solution induces the gradient crystallization of lamellar ice and KCl structure, and subsequently expelled KCl/SnCl2 crystals can be limited in the confined region to form the ice/SnCl2/KCl sandwich structure. Followed by freeze-drying, calcination and water washing, the porous SnO2 nanosheets structure can be obtained. Notably, this general method enables the synthesis of functional porous nanosheet with specific designed component and surface properties, and the obtained porous Pt-SnO2 nanosheets with abundant accessible active sites and lower d-band center exhibit superior gas sensing performance to 3-hydroxy-2-butanone.

Modified honeycomb cores for enhancing the durability of sandwich structures under low-velocity impact

Honeycomb cores are essential components of composite sandwich structures, and enhancing their ability to withstand out-of-plane loads can improve the resilience of sandwich structures under low-velocity transverse impacts. Therefore, this study computationally investigates the potential for improving the out-of-plane strength of honeycomb cores by superposing periodic sinusoidal perturbations to their cell walls. Such perturbations have been used to enhance the acoustic properties of honeycomb cores. Results demonstrated that introducing these perturbations can improve the strength of honeycomb cores under localized out-of-plane loadings resembling colliding with small objects at low impact speeds. Superposed perturbations increased the out-of-plane strength by a maximum of 28.5 % and eliminated the post-buckling softening behavior. Moreover, they increased the toughness, represented by the area under the force-displacement curve, under localized out-of-plane loads by a maximum of 56.7%. The perturbed honeycomb cores showed more sensitivity to the frequency of the superposed perturbations than their magnitude.

Memristive gas sensor (gasistor) based on Ag/ordered TiO2 nanorods/FTO sandwich structure for evaluation of ethanol concentration in mixed ambient

Herein, we designed a memoristor-type gas sensor (gasistor) with Ag/ordered TiO2 nanorods/FTO sandwich structure for recognizing ethanol concentration in ethanol-methanol mixtures. The gas adsorption characteristics of different TiO2 substrates were first investigated by first-principle calculations. The results showed that the TiO2 surface in the low resistance state exhibited better immunity to other interfering gas molecules. However, similar adsorption energies and charge transfers calculated for adsorption of methanol and ethanol mean that the interference of methanol to the response of ethanol cannot be completely ignored. In next experiments, the Ag/TiO2/FTO gasistor with well-aligned TiO2 nanorods as the resistive layer was constructed successfully and demonstrated the stable resistive switching function and excellent retention characteristic. Further device tests confirm the ability of the gasistor in the low resistance state to discriminate between ethanol and methanol, as well as better anti-interference for other organic vapors. Also, an ultra-short recovery time (∼1.8 s) was observed by applying a direct current scanning voltage to the gasistor. Finally, to improve the recognition rate of ethanol in ethanol-methanol mixed atmosphere, a neural network algorithm based on back-propagation was incorporated and good prediction accuracy was obtained at ethanol concentrations ≥ 5 ppm.

Atomically Precise Ternary Cluster: Polyoxometalate Cluster Sandwiched by Gold Clusters Protected by N-Heterocyclic Carbenes.

Coinage metal (Au, Ag, Cu) cluster and polyoxometalate (POM) cluster represent two types of subnanometer "artificial atoms" with significant potential in catalysis, sensing, and nanomedicine. While composite clusters combining Ag/Cu clusters with POM have achieved considerable success, the assembly of gold clusters with POM is still lagging. Herein, we first designedly synthesized two cluster structural units: an Au3O cluster stabilized by diverse N-heterocyclic carbene (NHC) ligands and an amine-terminated POM linker. The subsequent reaction involved amine substitution in the POM linker for the central O atom in the Au3O cluster, resulting in the first ternary composite cluster - a POM cluster sandwiched by two Au clusters protected by NHCs. Single-crystal X-ray diffraction and other characteristic methods characterized their atomically precise structures. Furthermore, altering the NHC ligands decreased the number of gold atoms in the sandwich structures, accompanying the different protonated degrees of amine ligand in the terminal end of the POM linker. These composite clusters showed excellent performances in catalytic H2O2 conversion through the synergistic effect between gold clusters and POM clusters. This work opens a new avenue to functional composite metal clusters and would promote their enhanced catalysis applications through intercluster synergistic interactions within composite systems.

Pull-out strength of thin perfobond connectors and C-ties in steel-concrete-steel sandwich composite structures

Due to the remarkable mechanical property, short construction period, and labor-saving features, steel-concrete-steel (SCS) sandwich composite structures are widely used in assembled buildings, modularized immersed tunnels, and bridge cable pylons. In SCS sandwich structures, sufficient pull-out resistance in connectors is necessary to effectively prevent outward buckling of the outer steel faceplates. However, the existing connectors remain deficient in terms of load bearing capacity, ductility, and construction convenience. A novel type of connector named thin perfobond connector and C-tie (PBL-CT) was proposed to address these problems in this paper. While retaining the superior shear performance of traditional perfobond connectors (PBLs), the addition of C-ties in PBL-CTs enhances structural shear resistance. Through 18 monotonic pull-out tests, the pull-out performance of shallow-embedded thin PBL-CTs was investigated. Test results revealed that the primary failure mode of shallow-embedded thin PBLs was cracking of the surrounding concrete, with C-ties significantly improving the pull-out performance and shifting the failure mode to a combination of concrete breakout and perforated rib rupture. A nonlinear finite element (FE) model was developed and calibrated to conduct parameter analyses on concrete strength, embedment depth, and perforated rib parameters. This led to the establishment of a mixed failure model for shallow-embedded thin PBL-CTs. The ultimate pull-out bearing capacity was found to comprise two components: the pull-out resistance of the concrete cone and the shear resistance of the perforated rib. A regression-based prediction formula was derived, providing accurate and conservative predictions of the ultimate pull-out bearing capacity of shallow-embedded thin PBL-CTs.

Hydrophilic and Insulative Interface Strategy Against Side Reactions for Dendrite‐Free Zinc Metal Anodes

AbstractThe zinc dendrite growth and the parasitic hydrogen evolution reactions (HER) hinder the commercialization of zinc batteries. To address this, a hydroxyl‐rich Boehmite coating (HR‐BC) strategy is developed that combines excellent electrical insulation and high zinc ion conductivity. The hydrophilic hydroxyl groups facilitate hydrogen bond formation with the hydrated zinc ions, accelerating the de‐solvation process and suppressing the HER. Additionally, the electrically insulative nature of Boehmite prevents the reduction of zinc ions within HR‐BC and results in preferential Zn deposition underneath it, leading to a dendrite‐free “sandwich structure” of HR‐BC//Zn deposition//Zn foil. Symmetric cells using HR‐BC‐Zn electrodes obtain an ultralong and stable cycling lifetime of 1700 h at 5 mA cm−2, along with a high cumulative plating capacity of 4250 mAh cm−2. When paired with a V2O5 cathode, the HR‐BC‐Zn anode demonstrates a high capacitance retention of 90% and an average Coulombic efficiency (CE) of 99.8% after 4000 cycles. Furthermore, when combined with a heteroatoms‐doped carbon (HDC) cathode, the HR–BC–Zn//HDC pouch‐type cell exhibits superior lifetime performance with nearly 100% average CE after 15000 cycles at 3.0 A g−1. This work highlights the effectiveness of hydrophilic and insulative coating strategies in fostering the progression of long‐lasting zinc‐based energy storage systems.

Microdroplet-SERS platform for single cell-secreted VEGF and extracellular pH analysis in oxidative stress event

Although oxidative stress processes based on reactive oxygen species have been the research focus, analyzing this phenomenon at the single-cell level is challenging. A microfluidic platform combined with highly sensitive surface-enhanced Raman scattering (SERS) technology was constructed. The secretion of the vascular endothelial growth factor (VEGF) and the extracellular microenvironmental pH fluctuations of a single cell during the oxidative stress process were assessed and analyzed. The immune sandwich structure formed between the capture probe and the reporter probe with the linkage of secreted VEGF was built above the probed cell surface in each drop. The self-driven collection behavior of the magnetic bead-based capture probe toward the cell surface significantly amplified the SERS signals of the reporter probe, thus improving the sensitivity and accuracy of detection. 4-Mercaptopyridine (4-Mpy) responding sensitively to pH was applied to label the reporter probe, which endows the extracellular microenvironmental pH sensing during oxidative stress events according to relative SERS intensity. Combined with machine learning to analyze the spectral characteristics, the distinction between normal cells and different types of tumor cells was finally realized. This study makes full use of the advantages of machine learning to process spectral data to explore deep information and provides an auxiliary tool for diagnosing cancer and other medical conditions in the future.

A water strider-inspired intestinal stent actuator for controllable adhesion and unidirectional biofluid picking

Soft-bodied aquatic organisms exhibit extraordinary navigation and mobility in liquid environments which inspiring the development of biomimetic actuators with complex movements. Stimulus-responsive soft materials including hydrogels and shape-memory polymers are replacing traditional rigid parts that leading to dynamic and responsive soft actuators. In this study, we took inspiration from water strider to develop a biomimetic actuator for targeted stimulation and pH sensing in the gastrointestinal tract. We designed a soft and water-based Janus adhesive hydrogel patch that attaches to specific parts of the intestine and responds to pH changes through external stimulation. The hydrogel patch that forms the belly of the water strider driver incorporates an inverse opal microstructure that enables pH responsive behavior. The hydrogel patch on the water strider's leg uses a sandwich structure of Cu particles to convert light into heat and bend under infrared light to mimic the water strider's leg simulating the efficient and steady movement of the water strider's leg which transporting the biological fluid in one direction. This miniature bionic actuator demonstrates controlled adhesion and unidirectional biofluid delivery capabilities, proving its potential for targeted stimulus response and pH sensing in the gastrointestinal tract, thus opening up new possibilities for medical applications in the growing field of soft actuators.

A machine learning approach for predicting flexural strength of 3D printed hexagon lattice-cored sandwich structures

The development of sandwich structures using Fused Filament Fabrication (FFF) technology is an effective method for the rapid construction of complex profiles and is widely recognized for various structural applications. In this study, hexagon lattice-cored sandwich structures are developed by placing the lattice core in the center portion of the PLA polymeric specimen. The performance is analyzed with respect to varying levels of 3D-Printing Factors (3D-PFs) such as Nozzle Temperature (NT), Line Width (LW), Printing Speed (PS), and Layer Height (LH). The levels of the respective 3D-PFs are varied as follows: NT (180, 190, 200, and 210 °C), LH (0.15, 0.2, 0.25, and 0.3 mm), PS (15, 20, 25, and 30 mm/s), and LW (0.1, 0.2, 0.3, and 0.4 mm). The sandwich flexural samples are 3D printed using an FFF 3D printer, and their flexural properties are examined using a Universal Testing Machine (UTM). In the present work, various Machine Learning (ML) algorithms, such as Least Angle Regression (LARS), Support Vector Machine (SVM), Decision Tree (DT), and Ridge Regression (Ridge), are employed to predict the Flexural Strength (FS) of the 3D-printed sandwich structures and help in determining the optimized levels of 3D-PFs for achieving higher FS. The findings indicate that the LARS algorithm, particularly when applied within the Bagging Ensemble Learning Technique (ELT), exhibits the highest precision, showcasing a Mean Absolute Error (MAE) of 0.163, Root Mean Square Error (RMSE) of 0.153, and Median Absolute Error (MedAE) of 0.129. With the help of the LARS algorithm under Bagging ELT, the impact of 3D-PFs on the resulting FS is analyzed, which helps in determining the optimized levels of 3D-PFs concerning FS. The sandwich structures fabricated at the optimized levels show improved flexural properties, making them suitable for various structural applications.