Mechanisms and optimization for the rapid fabrication method of polymeric microlens arrays.

This paper presents a simple and cost-effective rapid method to make defect-free polymeric microlens arrays at room temperature without applying external pressure. This method uses an optically clear and high-transparency Norland Optical Adhesive (NOA) monomer solution. This is realized by using a combination of a mold and an ultraviolet (UV) polymerization technique. NOA can cross-link in a tenth of a second upon UV exposure. The uniformity and surface quality of the manufactured microlens arrays are investigated through atomic force microscopy and optical microscopy techniques. Experimental results show that the microlens arrays manufactured by the polymerization process are of very high quality without any defects. Further, the surface quality of the lenses can be significantly enhanced by increasing the viscosity of the photosensitive monomer solution.

Numerical optimization platform for precision glass molding by the simplex algorithm.

Precision glass molding (PGM) can fabricate aspherical lens and irregular optical products in a single step, but its applicability is currently limited by the thermally induced residual stresses and lens shape derivation after molding. To remove this barrier, this paper develops a numerical optimization platform for PGM based on a simplex algorithm and finite element simulation. It was found that the platform can effectively reduce the residual stress in a molded lens through cooling process optimization and minimize the lens shape derivation by die shape compensation. The platform established can improve the lens quality by PMG and make molded lenses have better quality than those manufactured by ultraprecision machining processes.

Fabrication of Graphene-Polymer Nanocomposites Through Ionic Polymerization.

The extraordinary properties of graphene nanosheets (GNS) and the high performance of polymer-based composites have stimulated extensive research in the realm of polymer nanocomposites. This work examines the mechanisms and approach for the production of GNS-polymer composites by first principle ab initio calculations. The results show that GNS functionalized with anionic/cationic moieties can initiate anionic/cationic polymerization reactions, leading to chemically bonded GNS-polymer composites via the established anionic/cationic polymerization schemes. These outcomes deliver a solid theoretical basis for fabricating strong polymer nanocomposites.

Thermoforming mechanism of precision glass moulding.

Precision glass moulding (PGM) enables the production of an aspherical lens and irregular optical products in a single step, but its product quality depends highly on the control of both material properties and process parameters. This paper investigates the thermoforming mechanism of a glass lens in PGM. To precisely describe the material behavior in PGM, a modulus-based constitutive model was framed and integrated with the finite element analysis. This model can be parameterized conveniently by an impulse excitation technique. Key processing parameters that influence the final profile and residual stresses of a lens were identified with the aid of dimensional analysis. The study found that the cooling stage above the glass transition temperature can bring about large geometry deviations of a lens. The residual stresses in a lens depend mainly on the temperature history in the supercooled liquid region caused by the variability and heterogeneity of thermal expansion. However, the stresses can be reduced effectively by decreasing the cooling rate from moulding temperature to glass transition temperature.

A new lightweight deep neural network for surface scratch detection.

This paper aims to develop a lightweight convolutional neural network, WearNet, to realise automatic scratch detection for components in contact sliding such as those in metal forming. To this end, a large surface scratch dataset obtained from cylinder-on-flat sliding tests was used to train the WearNet with appropriate training parameters such as learning rate, gradient algorithm and mini-batch size. A comprehensive investigation on the network response and decision mechanism was also conducted to show the capability of the developed WearNet. It was found that compared with the existing networks, WearNet can realise an excellent classification accuracy of 94.16% with a much smaller model size and faster detection speed. Besides, WearNet outperformed other state-of-the-art networks when a public image database was used for network evaluation. The application of WearNet in an embedded system further demonstrated such advantages in the detection of surface scratches in sheet metal forming processes.

An Investigation into the Densification-Affected Deformation and Fracture in Fused Silica under Contact Sliding.

Subsurface damage of fused silica optics is one of the major factors restricting the performance of optical systems. The densification-affected deformation and fracture in fused silica under a sliding contact are investigated in this study, via three-dimensional finite element analysis (FEA). The finite element models of scratching with 70.3° conical and Berkovich indenters are established. A refined elliptical constitutive model is used to consider the influence of densification. The finite element models are experimentally verified by elastic recovery, and theoretically verified by hardness ratio. Results of densification and plastic deformation distributions indicate that the accuracy of existent sliding stress field models may be improved if the spherical/cylindrical yield region is replaced by an ellipsoid/cylindroid, and the embedding of the yield region is considered. The initiation sequence, and the locations and stages of radial, median, and lateral cracks are discussed by analyzing the predicted sliding stress fields. Median and radial cracks along the sliding direction tend to be the first cracks that emerge in the sliding and unloading stages, respectively. They coalesce to form a big median-radial crack that penetrates through the entire yield region. The fracture behavior of fused silica revealed in this study is essential in the low-damage machining of fused silica optics.

Effect of Anisotropy of Potassium Dihydrogen Phosphate Crystals on Its Deformation Mechanisms Subjected to Nanoindentation.

Potassium dihydrogen phosphate (KDP) is an important nonlinear material due to its excellent physical and optical properties. However, it is also a difficult-to-machine material due to its complex anisotropic microstructure. To better understand the deformation mechanisms under external stresses, this paper aims to carry out systematic nanoindentation simulations using molecular dynamics (MD). To facilitate the structural characterization of KDP, a machine learning-based method was developed. The results showed that the subsurface damage is obviously anisotropic. On the (001) surface, both tetragonal and monoclinic phases appear simultaneously and part of the monoclinic phase transfers to the tetragonal phase. The generated phases close to the surface undergo amorphization and are squeezed out to form pile-ups. On the (100) surface, however, an orthorhombic phase emerges directly from the original structure rather than transforming through the monoclinic phase. No amorphization happens and no pile-ups appear in this case. The first "pop-in" in the load-displacement curve of nanoindentation signified the emergence of phase transformation under the combined hydrostatic and shear stresses. After unloading, the recovery of the deformed KDP is also anisotropic. The maximum recovery takes place when the indentation is on the (100) surface.

Equivalent Circuit Model for a Large Array of Coupled Piezoelectric Micromachined Ultrasonic Transducers With High Emission Performance.

In this article, an analytical equivalent circuit model is established for the piezoelectric micromachined ultrasonic transducer (PMUT) cell and array with a combination of the annular and circular diaphragms used for structural optimization and complex array design. Based on this model, a comprehensive analysis is conducted on the acoustic-structural coupling of an annular and circular diaphragm-coupled PMUT (AC-PMUT) with a new excitation method. The model-derived results are in good agreement with the simulation and experimental results. Then, an optimized design has been presented to achieve high-output pressure and a good array working performance. In summary, a comparison of the array working performance is conducted between the arrays that consist of AC-PMUTs and traditional circular diaphragm PMUTs (C-PMUTs). The results indicate that the AC-PMUT array has a much lower crosstalk effect than that of the traditional C-PMUT array. By this means, the AC-PMUT array can fully use the high vibration amplitude achieved by each AC-PMUT cell to improve its output ability. As a result, the highest ultrasonic output pressure generated by the AC-PMUT array in its resonant condition can achieve an increase of 155%, compared with that generated by the C-PMUT array.

Advanced thermal metamaterial design for temperature control at the cloaked region.

The present study focuses on maintaining the temperature magnitude around heat-sensitive components (cloaked region) in advanced electronic devices by introducing convective elements using extended surface fins. A finite element analysis confirmed that with the aid of the convection component to thermal cloaking, heat flux can be redirected around the cloaked region as well as control the temperature. The simulation results were verified by experiment under natural convection corresponding to the simulation assumptions. It was found that when the heat source maintains its temperature at 100 °C and the heat sink remains at 0 °C, the temperature within the cloaked region can reduce by up to 15 °C, from ~ 50 °C with conventional cloaking to 35 °C with a well-designed array of surface fins. It is worth noting that experimental results are consistent with the simulation results.

Elastic modulus evolution of rocks under heating-cooling cycles.

Rocks decay significantly during or after heating-cooling cycles, which can in turn lead to hazards such as landslide and stone building collapse. Nevertheless, the deterioration mechanisms are unclear. This paper presents a simple and reliable method to explore the mechanical property evolutions of representative sandstones during heating-cooling cycles. It was found that rock decay takes place in both heating and cooling processes, and dramatic modulus changes occurred near the α - ß phase transition temperature of quartz. Our analysis also revealed that the rock decay is mainly attributed to the internal cracking. The underlying mechanism is the heterogeneous thermal deformation of mineral grains and the α - ß phase transition of quartz.

A New Polymer-Based Mechanical Metamaterial with Tailorable Large Negative Poisson's Ratios.

Mechanical metamaterials have attracted significant attention due to their programmable internal structure and extraordinary mechanical properties. However, most of them are still in their prototype stage without direct applications. This research developed an easy-to-use mechanical metamaterial with tailorable large negative Poisson's ratios. This metamaterial was microstructural, with cylindrical-shell-based units and was manufactured by the 3D-printing technique. It was found numerically that the present metamaterial could achieve large negative Poisson's ratios up to -1.618 under uniaxial tension and -1.657 under uniaxial compression, and the results of the following verification tests agreed with simulation findings. Moreover, stress concentration in this new metamaterial is much smaller than that in most of existing re-entrance metamaterials.

Array Design of Piezoelectric Micromachined Ultrasonic Transducers With Low-Crosstalk and High-Emission Performance.

This article presents a resonant cavity-based array design for piezoelectric micromachined ultrasonic transducers (PMUTs). The cavity depth is designed to ensure that its open end achieves a considerably smaller acoustic impedance than the surrounding PMUT cells. The interference acoustic wave generated between every two adjacent PMUT cells at the near surface of the array will take an easy path down to the cavity bottom. As such, the crosstalk effect among different adjacent cells in the array can be largely reduced. An equivalent circuit model of the proposed array is established for its design and optimization. In addition, the solutions for circuit parameters in the electromechanical domain are analytically derived and verified via FEM simulations. Given the low crosstalk effect achieved by the proposed array design, the output sensitivity of the proposed PMUTs can be improved by 259% compared with the traditional PMUTs with a high distribution density of the same size. The cavity-based array design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.

Equivalent Circuit Models of Cell and Array for Resonant Cavity-Based Piezoelectric Micromachined Ultrasonic Transducer.

This article presents a design of resonant cavity-based piezoelectric micromachined ultrasonic transducers (PMUTs), including impedance matching tube-integrated (T) and Helmholtz resonant (HR) cavity-integrated PMUTs. In addition, equivalent circuit models for single PMUT cell and PMUT array are developed for structural optimization and complex array design. The model-derived results agree well with the FEM results. On the basis of the proposed models, an optimized design is established to achieve high output pressure and a good array working performance. The working performance of arrays that consist of HR-PMUTs and traditional circular diaphragm PMUTs (C-PMUTs) is compared. Results indicate that the HR-PMUT array has a lower crosstalk effect than the traditional C-PMUT array. Furthermore, the highest ultrasonic output pressure of HR-PMUT array at the resonant frequency can be achieved with an increase of up to 163% compared with that of the C-PMUT array because of the liquid amplification effect. Also, the cavity-based design and its model can be used for further advanced PMUT cell structures in other arrays to improve their performance.

Tensile and Interfacial Loading Characteristics of Boron Nitride-Carbon Nanosheet Reinforced Polymer Nanocomposites.

The discovery of hybrid boron nitride-carbon (BN-C) nanostructures has triggered enormous research interest in the design and fabrication of new generation nanocomposites. The robust design of these nanocomposites for target applications requires their mechanical strength to be characterized with a wide range of factors. This article presents a comprehensive study, with the aid of molecular dynamics analysis, of the tensile loading mechanics of BN-C nanosheet reinforced polyethylene (PE) nanocomposites. It is observed that the geometry and lattice arrangement of the BN-C nanosheet influences the tensile loading characteristics of the nanocomposites. Furthermore, defects in the nanosheet can severely impact the tensile loading resistance, the extent of which is determined by the defect's location. This study also found that the tensile loading resistance of nanocomposites tends to weaken at elevated temperatures. The interfacial mechanics of the BN-C nanocomposites are also investigated. This analysis revealed a strong dependency with the carbon concentration in the BN-C nanosheet.

An Analytical Equivalent Circuit Model for Optimization Design of a Broadband Piezoelectric Micromachined Ultrasonic Transducer With an Annular Diaphragm.

This paper presents an equivalent circuit model, a systematic design, and optimization method for developing a broadband annular diaphragm piezoelectric micromachined ultrasonic transducer (A-PMUT). By utilizing array analysis methods, an annular diaphragm is regarded as an array consisting of equally spaced sector diaphragms influencing each other by crosstalk effect. The model successfully explains the phenomenon of multi-resonance peaks in the frequency response curve, sharing the same vibration mode. The study finds that the analytical predictions of the model are in good agreement with the simulation and experimental results. Meanwhile, based on the phenomenon of multi-resonance peaks, a systematic design method is proposed to extend the bandwidth of the A-PMUT. In this method, the radiation impedance of the A-PMUT is separated into crosstalk-free and crosstalk contributed parts. This method enables the determination of the optimal structure counting for the influences on the frequency response of A-PMUTs for broadband applications. The model here can also be further generalized to be a guideline for the design and optimization of broadband PMUTs.

Novel sphene coatings on Ti-6Al-4V for orthopedic implants using sol-gel method.

Hydroxyapatite (HAp) is commonly used to coat titanium alloys (Ti-6Al-4V) for orthopedic implants. However, their poor adhesion strength and insufficient long-term stability limit their application. Novel sphene (CaTiSiO5) ceramics possess excellent chemical stability and cytocompatibility. The aim of this study is to use the novel sphene ceramics as coatings for Ti-6Al-4V. The sol-gel method was used to produce the coatings and the thermal properties, phase composition, microstructure, thickness, surface roughness and adhesion strength of sphene coatings were analyzed by differential thermal analysis-thermal gravity (DTA-TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), atom force microscopy (AFM) and scratch test, respectively. DTA analysis confirmed that the temperature of the sphene phase formation is 875 degrees C and XRD analysis indicated pure sphene coatings were obtained. A uniform structure of the sphene coating was found across the Ti-6Al-4V surface, with a thickness and surface roughness of the coating of about 0.5-1 microm and 0.38 microm, respectively. Sphene-coated Ti-6Al-4V possessed a significantly improved adhesion strength compared to that for HAp coating and their chemical stability was evaluated by testing the profile element distribution and the dissolution kinetics of calcium (Ca) ions after soaking the sphene-coated Ti-6Al-4V in Tris-HCl solution. Sphene coatings had a significantly improved chemical stability compared to the HAp coatings. A layer of apatite formed on the sphene-coated Ti-6Al-4V after they were soaked in simulated body fluids (SBF). Our results indicate that sol-gel coating of novel sphene onto Ti-6Al-4V possessed improved adhesion strength and chemical stability, compared to HAp-coated Ti-6Al-4V prepared under the same conditions, suggesting their potential application as coatings for orthopedic implants.

Mechanisms of the Complex Thermo-Mechanical Behavior of Polymer Glass Across a Wide Range of Temperature Variations.

This paper aims to explore the mechanisms of the complex thermo-mechanical behavior of polymer glass across a wide range of temperature variations. To this end, the free vibration frequency spectrum of simply supported poly(methyl methacrylate) (PMMA) beams was thoroughly investigated with the aid of the impulse excitation technique. It was found that the amplitude ratio of the multiple peaks in the frequency spectrum is a strongly dependent on temperature, and that the peaks correspond to the multiple vibrational modes of the molecular network of PMMA. At a low temperature, the vibration is dominated by the overall microstructure of PMMA. With increasing the temperature, however, the contribution of the sub-microstructures is retarded by ß relaxation. Above 80 °C, the vibration is fully dominated by the microstructure after relaxation. The relaxation time at the transition temperature is of the same order of the vibration period, confirming the contribution of ß relaxation. These findings provide a precise method for establishing reliable physical-based constitutive models of polymer glass.

Effective Mechanical Properties and Thickness Determination of Boron Nitride Nanosheets Using Molecular Dynamics Simulation.

Research in boron nitride nanosheets (BNNS) has evoked significant interest in the field of nano-electronics, nanoelectromechanical (NEMS) devices, and nanocomposites due to its excellent physical and chemical properties. Despite this, there has been no reliable data on the effective mechanical properties of BNNS, with the literature reporting a wide scatter of strength data for the same material. To address this challenge, this article presents a comprehensive analysis on the effect of vital factors which can result in variations of the effective mechanical properties of BNNS. Additionally, the article also presents the computation of the correct wall thickness of BNNS from elastic theory equations, which is an important descriptor for any research to determine the mechanical properties of BNNS. It was predicted that the correct thickness of BNNS should be 0.106 nm and the effective Young's modulus to be 2.75 TPa. It is anticipated that the findings from this study could provide valuable insights on the true mechanical properties of BNNS that could assist in the design and development of efficient BN-based NEMS devices, nanosensors, and nanocomposites.

High-fidelity replication of thermoplastic microneedles with open microfluidic channels.

Development of microneedles for unskilled and painless collection of blood or drug delivery addresses the quality of healthcare through early intervention at point-of-care. Microneedles with submicron to millimeter features have been fabricated from materials such as metals, silicon, and polymers by subtractive machining or etching. However, to date, large-scale manufacture of hollow microneedles has been limited by the cost and complexity of microfabrication techniques. This paper reports a novel manufacturing method that may overcome the complexity of hollow microneedle fabrication. Prototype microneedles with open microfluidic channels are fabricated by laser stereolithography. Thermoplastic replicas are manufactured from these templates by soft-embossing with high fidelity at submicron resolution. The manufacturing advantages are (a) direct printing from computer-aided design (CAD) drawing without the constraints imposed by subtractive machining or etching processes, (b) high-fidelity replication of prototype geometries with multiple reuses of elastomeric molds, (c) shorter manufacturing time compared to three-dimensional stereolithography, and (d) integration of microneedles with open-channel microfluidics. Future work will address development of open-channel microfluidics for drug delivery, fluid sampling and analysis.

Fabrication and application of polymer composites comprising carbon nanotubes.

Carbon nanotubes are being used in place of carbon fibers in making composites due to their high strength, high aspect-ratio and excellent thermal and electrical conductivity. Although carbon nanotubes were discovered more than a decade ago, works on preparation of satisfactory composites reinforced by carbon nanotubes have encountered difficulties. This review will discuss some registered patents and relevant papers on the fabrication of carbon nanotube-polymer composites on improving material properties such as electrical conductivity, mechanical strength, and radiation detection which have a broad range of applications in nano-electronic devices, and space and medical elements.