Journal articles on the topic 'Multilayer Stack Assemblies'

A linear-elastic analytical model has been developed to describe the deformed geometry of a multi-layered stack assembly subject to thermal loading. The model is based on Timoshenko’s bimetal thermostat analysis [1] and consists of a series of first-order polynomial equations. The radius of curvature, bending moment, force, horizontal and vertical displacements can be determined numerically. These quantities match well with finite element analysis. Calculations for silicon power transistor stacks are presented in order to demonstrate the model capability. The results from this analyitcal model have been found to correlate well with experimental measurements when an appropriate secant modulus is used to represent the nonlinear stress-strain behavior of solder.

Herein, we present a comparative study of thermo- and environmentally responsive TiO2/SiO2 one-dimensional photonic crystals (Bragg stacks) fabricated by different deposition methods and fabrication schemes, featuring various multilayer nanomorphologies. These include dense multilayer systems processed by physical vapor deposition and wet-chemistry protocols, as well as porous systems, namely, nanoparticle-based optical filters exhibiting textural porosity, and evaporation-induced self-assembled mesoporous Bragg stacks exhibiting predominantly structural porosity, as well as hybrid structures comprising both dense and porous layers. We investigate the spectral shift of the photonic stop band for the different Bragg stack nanomorphologies induced by the humidity-enhanced thermo-optic effect in a temperature range from 15 to 60 °C. We also demonstrate the response and recovery kinetics of the multilayer systems during external changes in ambient humidity. Notably, the choice of fabrication method plays a significant role in the thermal and humidity response of the system. Taking advantage of different material nanomorphologies we can tune the thermal shift of the photonic stop band in the range 0.2–32.9 nm for the Bragg stacks at ambient relative humidity. In addition, we can design dense multilayer systems nonresponsive to humidity and achieve time responses of the porous systems to external changes in humidity ranging from about 1 to 3 s.

In recent years, the use of hybrid composite stacks, particularly CFRP/Al assemblies, and fiber metal laminates (FMLs) has progressively become a convincing alternative to fiber-reinforced polymers (FRPs) and conventional metal alloys to meet the requirements of structural weight reduction in the modern aerospace industry. These new structural materials, which combine greater mechanical properties with low specific mass, are commonly assembled by riveted and bolted joints. The drilling operation, which represents the essential hole-making process used in the aerospace industry, proves particularly challenging when it comes to achieving damage-free holes with tight tolerances for CFRP/Al stacks in one-shot operations under dry conditions due to the dissimilar mechanical and thermal behavior of each constituent. Rapid and severe tool wear, heat damage, oversized drilled holes and the formation of metal burrs are among the major issues induced by the drilling of multi-material stacks. This paper provides an in-depth review of recent advancements concerning the selection of optimized strategies for high-performance drilling of multi-material stacks by focusing on the significant conclusions of experimental investigations of the effects of drilling parameters and cutting tool characteristics on the drilling performance of aerospace assemblies with CFRP/Al stacks and FML materials. The feasibility of alternative drilling processes for improving the hole quality of hybrid composite stacks is also discussed.

Electronic devices are high-demand commodities in today’s world, and such devices will continue increasing in popularity. Currently, batteries are implemented to provide power to these devices; however, the need for battery replacement, their cost, and the waste associated with battery disposal present a need for advances in self-powered technology. Energy harvesting technology has great potential to alleviate the drawbacks of batteries. In this work, a novel piezoelectret foam material is investigated for low-level vibration energy harvesting. Specifically, piezoelectret foam assembled in a multilayer stack configuration is explored. Modeling and experimentation of the stack when excited in compression at low frequencies are performed to investigate piezoelectret foam for multilayer energy harvesting. An equivalent circuit model derived from the literature is used to model the piezoelectret stack. Two 20-layer prototype devices and one 40-layer prototype device are fabricated and experimentally tested via harmonic base excitation. Electromechanical frequency response functions between input acceleration and output voltage are measured experimentally. Modeling results are compared to experimental measurements to assess the fidelity of the model near resonance. Finally, energy harvesting experimentation in which the device is subject to harmonic base excitation at the fundamental natural frequency is conducted to determine the ability of the stack to successfully charge a capacitor. For a 20-layer stack excited at 0.5 g, a 100-µF capacitor is charged to 1.45 V in 15 min, and produces a peak power of 0.45 µW. A 40-layer stack is found to charge a 100-µF capacitor to 1.7 V in 15 min when excited at 0.5 g, and produce a peak power of 0.89 µW.

The main aim of this paper is to provide the feasibility of non-destructive testing (NDT) method, such as scanning acoustic microscopy (SAM), for damage detection in ultrasound (US) probes for medical imaging during the manufacturing process. In a highly competitive and demanding electronics and biomedical market, reliable non-destructive methods for quality control and failure analysis of electronic components within multi-layered structures are strongly required. Any robust non-destructive method should be capable of dealing with the complexity of miniaturized assemblies, such as the acoustic stack of ultrasonic transducers. In this work, the application of SAM in an industrial scenario was studied for 24 samples of a phased array probe, in order to investigate potential internal integrity, to detect damages, and to assess the compliance of high-demanding quality requirements. Delamination, non-homogeneous layers with micron-thickness, and entrapped air bubbles (blisters) in the bulk of US probe acoustic stacks were detected and studied. Analysis of 2D images and defects visualization by means of ultrasound-based NDT method were compared with electroacoustic characterization (also following as pulse-echo test) of the US probe through an ad-hoc measurement system. SAM becomes very useful for defect detection in multilayered structures with a thickness of some microns by assuring low time-consuming (a limit for other NDT techniques) and quantitative analyses based on measurements. The study provides a tangible contribution and identifies an advantage for manufacturers of ultrasound probes that are oriented toward continuous improvement devoted to the process capability, product quality, and in-process inspection.

We have studied the use of self-assembled block copolymers to pattern multilayers of Co and Pd on silicon wafers. Stacks ranging from four to twelve bilayers of Co (0.3 nm)/Pd (0.8 nm) were sputtered onto Ta/Pd seed layers and capped with 3 nm of Ta and were found to have perpendicular magnetic anisotropy as-deposited. The block copolymer polystyrene- block-poly(ferrocenyl dimethylsilane) (PS-b-PFS) was dissolved in toluene and spun onto the wafers. The polymers were phase-separated by heat treatment, leaving self-assembled PFS spheres embedded in PS, which was removed by oxygen-plasma ashing. The PFS spheres were then used as masks to ion-mill the Co/Pd multilayers into nanopillars. To study the effect of etch time and etch angle on the coercivity distribution, we synthesized samples in a Design of Experiments-(DoE)- in these two factors. Scanning electron micrographs showed nanopillars ranging from 15 to 30 nm in diameter, depending primarily on etch time. M-H loops measured on both patterned and unpatterned wafers showed an increase of up to 130% in overall coercivity upon patterning. First Order Reversal Curves (FORC) were measured, and the resulting FORC distributions displayed using a smoothing program (FORCinel) and one that can display the raw data without smoothing (FORC+). We find that FORC+ reveals information about fine-scale structure and switching mechanism that cannot be seen in the smoothed display.

Multilayered Ge nanodots have drawn much attention due to their potential applications in optoelectronics, such as photodetectors and lasers. Many groups studied multilayered Ge nanodots with Si spacers on Si(001) grown by Stranski-Krastanov (SK) growth mode and vertically aligned by local tensile strain induced by buried Ge nanodots (1-2). However, to avoid plastic relaxation caused by a 4.2% lattice mismatch between Si and Ge, thick nanodots and/or large layer numbers are challenging. Additionally, laterally-aligned Ge nanodots without pre-structuring have not been reported. In this study, we demonstrate 3-dimensional (3D) self-ordered Ge nanodots on SiGe virtual substrate (VS) by SiGe/Ge cyclic epitaxial growth and show the effects of fabrication parameters. The 3D self-ordered Ge nanodots were fabricated by reduced-pressure chemical vapor deposition. A Si0.4Ge0.6 VS with step-graded buffer deposited on Si(001) wafer was used. This VS was post-annealed at 1000°C, followed by chemical-mechanical polishing. After HF dip, the substrate was baked at 850°C in H2, then cooled down to 550°C for epitaxial growth. A 52-82 nm thick Si0.48Ge0.52 layer was deposited using a H2-SiH4-GeH4 gas mixture, then a self-assembled Ge nanodots layer via SK growth mechanism was deposited with 7.5-15.0 nm Ge coverage using a H2-GeH4 gas mixture. This Si0.48Ge0.52/Ge deposition cycle was repeated 5 to 20 times to fabricate the 3D self-ordered Ge nanodot stack. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the morphology and alignment of the Ge nanodots. The facets of the Ge nanodots were studied by analyzing the cross-section cuts of AFM images and confirmed by scanning transmission electron microscopy (STEM). Nano-beam diffraction (NBD) was used to study strain in the superlattice. Fig. 1(a-c) show the AFM images of the 5-cycle superlattices with Ge coverage 7.5 to 15.0 nm. In fig. 1(a), we can see mainly two types of nanodots, diamond (32%) and dome (68%). The height of the diamond-like nanodots is 9 nm with a standard deviation (s) of 2.6 nm while that of the dome-like nanodots is 23 nm with s=1.8 nm. With increasing Ge coverage, dome-like nanodots dominate (fig. 1(b)) and some nanodots merge with the adjacent nanodots (fig. 1(c)). Since the dome-like nanodots show a lower s in height than the diamond-like nanodots do, engineering of self-ordering is more feasible with the dome-like nanodot. Fig. 2(a-b) show the angle view of SEM images of Ge nanodots on 5- and 20-cycle superlattice of 12.5 nm-Ge/52 nm-SiGe. The dome-like nanodots are dominant and laterally aligned. The alignment and the uniformity improve with the increasing cycle number of the superlattice. However, when the thickness of Si0.48Ge0.52 spacer increases, the lateral- and vertical alignment of nanodots become random, and the amount of diamond-like nanodots increases (not shown). To study facets of two types of nanodots, the tilt of each facet was calculated from the cross-section cuts of AFM images. Fig. 3(a) shows an AFM image of the dome-like nanodot. Fig. 3(b) shows the cross-section cuts of fig. 3(a) along <110>. These cross-section cuts are well-overlapped with the high-angle annular dark-field (HAADF) STEM image as shown in the background of fig. 3(b). Therefore, it is possible and reliable to estimate the facets from the cross-section cuts of our AFM images. By this method, we confirmed that the diamond-like nanodot is composed of {105} and the dome-like nanodot is composed of {113} and {159}. This is consistent with a study of Ge dot on Si substrate except for {159} facet (3). Instead of {159} facet, a relatively similar facet {3 15 23} was reported. This difference may result from the less compressive strain in our Ge nanodots because SiGe VS was used. To explain the vertical correlation of Ge nanodots, a HAADF STEM image and in- and out-of-plane strain distributions measured by NBD are shown in fig. 4(a-c). The nanodots are vertically aligned (fig. 4(a)). The SiGe on the nanodot shows a relatively higher lattice parameter along <110> (fig. 4(b)) and lower lattice parameter along <001> (fig. 4(c)) compared to that on Ge wetting layer, indicating tensile strain. This tensile strain area is the preferred position for Ge nanodot formation because of less lateral lattice mismatch. Consequently, the nanodots tend to grow above the buried nanodots. 3D self-ordered multilayered Ge nanodots on SiGe VS were successfully fabricated, and the facets and the vertical correlation of Ge nanodots were studied. References P.S. Chen et al. Materials Science and Engineering B 108 (2004) 213-218 K.L. Wang et al. Proceeding of the IEEE 95 (2007) 1866 J.T. Robinson et al. Nanotechnology 20 (2009) 085708 Figure 1

Lithium-ion batteries, containing flammable electrolytes, have become safer in many ways since their invention. As the technology matures, energy and power densities increase: the European Council for Automotive R&D set 2030 (cell level) targets for battery electric vehicle energy density of 1000 Wh L-1, with 450 Wh kg-1 specific energy and 1800 W kg-1 peak specific power. This rise in energy and power densities increases the risk of accidental release of energy from the batteries and thermal runaway (TR). Safety is thus a key concern when designing a lithium-ion battery system for electric vehicles (EV). TR relates to fast heating of a battery cell caused by exothermic chemical decomposition reactions of the materials inside the cells. It develops in a cell when heat is generated and cannot be dissipated quickly enough to the surroundings, giving rise to an abrupt (exponential) temperature increase and subsequent reaction rate increase. During TR, various exothermic side reactions can occur, leading to temperature increase, accompanied by pressure increase as electrolyte evaporates and venting. This can result in the emission of highly flammable gas and the formation of toxic atmosphere, as well as, in fire and, in very rare circumstances, in explosion. The corresponding temperature increase in adjacent cells (or modules) might then be sufficient to cause them to also go into TR – leading to a process known as thermal runaway propagation (TRP). Fit-for purpose testing procedures to assess the risks associated with TRP are therefore of utmost importance and the focus should always be the safety of the EV occupants, bystanders, first responders and property. Some tests in current standards and regulations try to simulate internally driven failures (e.g. internal short circuit), but whether these tests are suitable to represent field failures remains an open question. This work will give an insight into two selected TR initiation methods assessed within JRC’s TR initiation and propagation test campaigns, namely (a) localised rapid external heating, as developed and patented internationally by NRC (National Research Council Canada) and (b) ceramic nail penetration, as described in the IEC TR 62660-4 standard. It examines the response of short-stacks of pouch cells extracted from automotive batteries after TR is triggered in a single cell. Experimental data and analysis to better understand how TR propagates and the potential for TRP inhibition will be reported. The potential of inhibiting TRP is explored on 2- and 5-cell assemblies with a multi-layer, porous, composite insulation material between cells. Separating the cells can delay significantly TRP in adjacent cells in modules constructed with pouch cells, whereas TRP may be slowed and even inhibited as the thickness of the multilayer material varies.

Taking advantage of the strong charge interactions between negatively charged graphene oxide (GO) sheets and positively charged poly(diallyldimethylammonium chloride) (PDDA), self-assembled multilayer films of (GO/PDDA)n were created on hydroxylated silicon substrates by alternating electrostatic adsorption of GO and PDDA. The formation and structure of the films were analyzed by means of water contact angle measurement, thickness measurement, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Meanwhile, tribological behaviors in micro- and macro- scale were investigated by AFM and a ball-on-plate tribometer, respectively. The results showed that (GO/PDDA)n multilayer films exhibited excellent friction-reducing and anti-wear abilities in both micro- and macro-scale, which was ascribed to the special structure in (GO/PDDA)n multilayer films, namely, a well-stacked GO–GO layered structure and an elastic 3D crystal stack in whole. Such a film structure is suitable for design molecular lubricants for MEMS and other microdevices.

We have investigated the effect of post growth rapid thermal annealing on self-assembled InAs/GaAs mul tilayer QDs (MQD) overgrown with a combination barrier of InAlGaAs and GaAs for their possible use in photovoltaic device application. The samples were characterized by transmission electron microscopy and photoluminescence measurements. We noticed a thermally induced material interdiffusion between the QDs and the wetting layer in the MQD sample up to a certain annealing temperature. The QD heterostructure exhibited a thermal stability in the emission peak wavelength on annealing up to 700 ◦C . A phenomenological model has been proposed for this stability of the emission peak. The model considers the effect of the strain field, propagating from the underlying QD layer to the upper layers of the multilayer QD and the effect of indium atom gradient in the combination barrier layer due to the presence of a quaternary InAlGaAs layer.

AbstractEbola virus (EBOV) glycoprotein (GP) can be recognized by neutralizing antibodies (NAbs) and is the main target for vaccine design. Here, we first investigate the contribution of the stalk and heptad repeat 1-C (HR1C) regions to GP metastability. Specific stalk and HR1C modifications in a mucin-deleted form (GPΔmuc) increase trimer yield, whereas alterations of HR1C exert a more complex effect on thermostability. Crystal structures are determined to validate two rationally designed GPΔmuc trimers in their unliganded state. We then display a modified GPΔmuc trimer on reengineered protein nanoparticles that encapsulate a layer of locking domains (LD) and a cluster of helper T-cell epitopes. In mice and rabbits, GP trimers and nanoparticles elicit cross-ebolavirus NAbs, as well as non-NAbs that enhance pseudovirus infection. Repertoire sequencing reveals quantitative profiles of vaccine-induced B-cell responses. This study demonstrates a promising vaccine strategy for filoviruses, such as EBOV, based on GP stabilization and nanoparticle display.

Abstract With the recent discovery of three dimensional Dirac semimetals, their integrations with the optoelectronic devices allow the novel optical effects and functionalities. Here, we theoretically report the photonic spin Hall effect in a periodic structure, where three dimensional Dirac semimetals and the dielectric materials are assembled into the stack. The incident angle and frequency dependent spin shift spectrum reveals that the spin shifts of the transmitted wave in this structure emerge the obvious peaks and valleys for the horizontal polarized wave and their magnitudes and positions display a distinct dependence on the incident angle around the specific frequency. These observations originate from its zero value of the effective perpendicular permittivity and its greatly reduced transmission in the multilayered structure, whose mechanism is different from those in the previous works. Moreover, both the peaks and valleys of the transmitted spin shift are significantly sensitive to the Fermi energy of three dimensional Dirac semimetals, whose magnitudes and positions can be tuned by varying it. Our results highlight the vital role of three dimensional Dirac semimetals in their applications of the spin photonic devices and pave the way to explore the tunable photonic spin Hall effect by engineering their Fermi energies.