Making Honeycomb Microcomposites by Soft Lithography

Elastomeric stamps and molds provide a great opportunity to eliminate some of the disadvantages of photolithograpy, which is currently the leading technology for fabricating small structures. In the case of "soft lithography" there is no need for complex laboratory facilities and high-energy radiation. Therefore, this process is simple, inexpensive, and accessible even to molecular chemists. The current state of development in this promising area of research is presented here.

The use of atomistic methods, such as the continuous cellular automaton (CCA), is currently regarded as an accurate and efficient approach for the simulation of anisotropic etching in the development of micro-electro-mechanical systems (MEMS). However, whenever the targeted etching condition is modified (e.g. by changing the substrate material, etchant type, concentration and/or temperature) this approach requires performing a time-consuming recalibration of the full set of internal atomistic rates defined within the method. Based on the level set (LS) approach as an alternative and using the experimental data directly as input, we present a fully operational simulator that exhibits similar accuracy to the latest CCA models. The proposed simulator is tested by describing a wide range of silicon and quartz MEMS structures obtained in different etchants through complex processes, including double-sided etching as well as different mask patterns during different etching steps and/or simultaneous masking materials on different regions of the substrate. The results demonstrate that the LS method is able to simulate anisotropic etching for complex MEMS processes with similar computational times and accuracy as the atomistic models.

Recently we have developed novel polyimide adhesive sheet materials (PI sheet) which have advantages such as planarization, via-filling, low shrinkage (low stress) during curing and easy fabrication of multi-layer and cavity structures compared to spin-coating polyimide materials. The PI sheets are expected to be used in wafer level package (WLP) applications such as 5G telecommunication systems (5G) and Micro Electro Mechanical Systems (MEMS). To meet 5G and MEMS requirements, we recently have developed novel low-dielectric loss (low Df) and high mechanical properties PI sheets with good adhesion to Cu and polyimide layers on a substrate. We have achieved Df values of 0.003 and 0.007 at 20GHz for non-photosensitive (NP) PI sheet and photo-sensitive (PS) PI sheet, respectively. The use of low Df PI sheet as a redistribution layer can help realize low transmission loss due to low Df value itself and also to low conductor loss due to good adhesion to the smooth copper surface of the wiring.

Films and thin films are exploited by an ever increasing number of technologies. The properties of films can be different from those of the same material in bulk form, and can depend on the preparation process, and on thickness. Specific techniques are needed for their measurement. Whenever films or thin layers have structural functions, as in micro electro-mechanical systems (MEMS), a precise characterization of their stiffness is crucial for the design of devices. The same can be said for the design of devices which exploit thin layers to support surface acoustic waves (surface acoustic wave filters). More generally, knowledge of the elastic properties is interesting because such properties depend on the structural properties. The most widespread technique for the mechanical characterization of films, instrumented indentation, induces both elastic and inelastic strains. It also characterizes irreversible deformation, but the extraction of the information concerning the elastic behaviour is non trivial. To overcome this difficulty, several measurement methods have been developed, which exploit vibrations as a probe of the material behaviour. These methods intrinsically involve only elastic strains, and are non destructive. This is true at any length scale, and is peculiarly useful at micrometric and sub-micrometric scales, where the exploitation of other types of probes can become critical. Both propagating waves and standing waves can be exploited, with excitation which can be either monochromatic (e.g. resonance techniques) or impulsive, therefore broadband, requiring an analysis of the response either in the time domain (the so called picoseconds ultrasonics) or in the frequency domain (the so called laser ultrasonics). The propagation velocities of vibrational modes are obtained, from which the stiffness is derived if an independent value of the mass density (the inertia) is available. Older resonance techniques have been developed to be operated with thin slabs, also exploiting optical measurements of displacement. In the measurement of films and small structures, the advantages of light, a contact-less and inertia-less probe, are substantial, and are increasingly exploited. The laser ultrasonics technique, commercially available since some years ago, measures waves travelling along the film surface. The so called picosecond ultrasonics technique measures waves travelling across the film thickness; it is a relatively sophisticated optical technique, which exploits femtosecond laser pulses in a pump-and-probe measurement scheme. For best performance it needs the deposition of

Arrays of silicon honeycombs are evaluated as a negative electrode material for lithium-ion microbatteries. The morphological changes of the structure are investigated by means of scanning electron microscopy (SEM) and it is revealed that the honeycomb structure can reversibly withstand huge mechanical deformations. Free-standing structures are envisioned to serve advanced future applications, such as switchable sieves and microelectromechanical systems.

Graphene, a honeycomb structure with single layer sp2 hybridization carbon atom, has extraordinary mechanical and electrical properties, exhibiting great potential in sensing area. The ultrahigh electron mobility in room temperature as well as the ultrahigh specific surface area, endows graphene a promising candidate for ultrasensitive gas sensor. As a typical two dimensional material, every atom in graphene can be regarded as surface atom. Therefore, every atom will be able to interact with gas molecule, which provides an ultrahigh sensitivity and the ultralow detection limit (as low as for single molecule detection). The current researches for improvement of gas sensing mainly focus on two aspects: (1) Design of different working principle devices; (2) surface modification on graphene and composite with other materials (i.e . metal, metal oxide and organic polymer). The specific adsorption sites can be achieved, resulting the improvement of selectivity. This review will present and summarize recent achievements of graphene-based gas sensor in both aspects and also predict the potential research direction in the future. According to different working mechanism, graphene-based gas sensor can be classified as resistive type, field-effect-transistor (FET) type, mass sensitive type, and micro-electromechanical system (MEMS) type. Each kind has its advantages. Thanks to the simple manufacture process, resistive gas sensor is investigated broadly. Compared to passive resistive sensor, the active FET sensor exhibits better performance in sensitivity and stability generally. As for mass sensitive sensor, the unique mechanism can provide a new device structure different from the other three types. However, concerning the compatibility with integrated circuits (IC) manufacture, MEMS sensor can be a good choice. Materials with high specific surface area are able to provide more adsorption sites. Consequently, sensor performance can be improved sharply. Therefore, surface modification on graphene and composite with other materials attracts broad attention recently. By changing the density and the type of functional groups on graphene surface, specific adsorption sites can be achieved. Hence, the ability for selective gas detection is enhanced. Besides, graphene-based composite with other materials (i.e. metal, metal oxide and organic polymer) is another effective strategy to optimize sensor performance. In conclusion, lots of achievements and big breakthroughs for graphene-based gas sensor have been made in recent years. Especially the obvious enhancement for sensitivity (as low as for single molecule detection) and gas selective detection, graphene-based gas sensor exhibits great advantage compared with traditional sensors. However, the time-consuming process of gas adsorption and desorption hampers the real-time measurements due to the long response and recover time. Therefore, improvement in response and recover performance will be a potential research direction in the future. In addition, the integration of different kinds of gas sensors even other types of sensors will be a trend. Also, it will be a desirable fundamental research to give explanations for the sensing mechanism as well as to investigate the dynamic interaction between gas molecules and graphene. Recently, it is believed that with the help of special in - situ holder, researchers can launch experiments in transmission electron microscope (TEM) to explore this fundamental study.

The present review explores an overall view of the vibration damping materials ranging from traditionally used viscoelastic materials for macroscale damping to hybrid thin film heterostructures for micro-electro-mechanical systems (MEMS). Vibration damping materials like rubbers, polymers, metals, metal-matrix composites and smart materials are reviewed in terms of damping capacity, stiffness, mechanical strength and figure of merit. Nanoscale shape memory alloys, piezoelectric materials, carbon nanotubes, their composites and thin films are promising materials for future nanoscale damping devices. The main focus of this article is on our development of new vibration damping approach for MEMS structures comprising of ferroelastic/ferroelastic thin film heterostructures. For the first time, nanoindentation has been explored as an alternative tool to evaluate the damping capability of actual components (e.g., thin films for MEMS) where production of dynamic mechanical analyzer (DMA) test samples is not feasible. A comprehensive insight on the existing vibration damping materials and our new approach would definitely trigger some important applications in nano- and micro-electro-mechanical systems.

A wafer-scale etching technique for high aspect ratio implantable MEMS structures

Hysteresis in Piezoelectric Actuators: Modeling and Compensation

In this article, we present a high-performance aluminum nitride (AlN)-based microelectromechanical system (MEMS) hydrophone sensor, in which the sensing cells are designed and arranged as an innovative honeycomb architecture for achieving large acoustic pressure sensitivity and high fill-factor. An 8×9 array of 360 μm in characteristic size, 0.92-MHz MEMS hydrophone sensor is developed based on an AlN-on-cavity silicon-on-insulator (CSOI) platform. The size of the MEMS hydrophone sensor is 3.2 mm ×3.2 mm. The MEMS hydrophone sensor and its preamplification circuit are integrated on a printed circuit board and packaged together with an acoustically transparent material, in order to meet the stringent requirements of underwater applications. The packaged MEMS hydrophone is fully characterized through using an industry-standard hydrophone calibration instrument. The MEMS hydrophone achieves an acoustic pressure sensitivity of -178 dB (re: 1 V/ μ Pa), with a maximum nonlinearity of 0.1%, and a noise resolution of 58.7 dB (re: 1 μPa/Hz). The measurement results show that the reported MEMS hydrophone sensor with bioinspired honeycomb structure is compared favorable with advanced commercially available bulky hydrophones.

Lattice structures are promising materials in the terms of energy absorption, acoustic and vibrational damping, high strength-to-weight ratios and thermal management capabilities. In particular, auxetic lattice structures, among others, show high energy absorption performances due to their characteristic negative Poisson’s ratio. In this study, it is aimed to compare auxetic deformation mechanisms under quasi-static crush loads. Ti6Al4V tensile test specimens were produced with Electron Beam Melting (EBM) Additive Manufacturing (AM) technology. Moreover, a constitutive equation was derived and calibrated according to tensile results. The calibrated data were used to generate non-linear computational crush models including elastoplastic material data, damage initiation criterion, damage evaluation law and element deletion. The computational models are utilized for optimum topology design and mechanical performance prediction of different auxetic cells, including anti-tetrachiral, hexachiral, re-entrant and honeycomb lattice structures that are prone to prematurely fail under crush loading conditions. Consequently, it was found that the chiral auxetic deformation mechanism experienced better energy absorption ability over re-entrant deformation mechanism for metallic lattice structures.

Existence of periodic solutions for a class of damped vibration problems

Impact damage processes in composite sheet and sandwich honeycomb materials

The impact performance of several porous polymeric and metallic foam core sandwich composite systems were evaluated for their suitability for protecting vehicle occupants in the event of a low velocity impact. The material systems evaluated were glass/phenolic face sheets reinforced with Nomax honeycomb core, cross-ply carbon-fiber face sheets reinforced with aluminum honeycomb cores of different cell sizes, and aluminum metallic foam cores of different cell sizes. Lastly, an exploratory study using an extrusion type 3D-printed polystyrene foam structure that customized pore size, pore distribution were undertaken. The peak load and energy dissipation of the composite materials were experimentally measured. An instrumented large semispherical impactor (48 mm diameter) applied loads at constant strain rate on the order of 0.1 m/sec to a 50 mm × 50 mm coupon sized composite specimen with varying thicknesses. The impact damage to materials were also visually examined. The current material system used for some interior components (glass/phenolic face sheets reinforced with Nomax honeycomb core) reaches a maximum load in a small time duration and displacement, causing catastrophic local crushing and delamination events. It is expected that the failure can be spread out with these alternative material systems with varying pore size distribution so that the energy dissipation can be accomplished with a lower peak force to improve occupant safety.

Recently, engineers and researchers reconsider honeycomb sandwich structures due to their vast application in industries and aviation arenas. In this study, a new honeycomb sandwich material was developed and tested. The purpose of the present work is to investigate numerically and experimentally with a comparative study on the effects of heat transfer on design parameters and geometry for different types of exotic honeycomb structures taking in account radiation within the cell and conduction in the cell walls. The numerical solution for temperature profiles for different types of exotic honeycomb structures and solid disk are performed in order to inspect the variation of heat transfer. The modeling results show a good agreement with the experimental results. The present work demonstrates that the temperature profile for reentrant is the highest one compared to splined and stiffened which reaches around 10% at temperature of the front surface Tin = 100 °C. It was found that the rib length enhances significantly heat transfer. Results showed also that stiffened honeycomb has a good insulation and metallic honeycomb core structure has a good thermal insulation characteristic for the highest instantaneous temperature, whereas reentrant honeycomb has a good heat transmission.