Preparation and Characterization of Sepiolite Microfibers with High Aspect Ratio

It is difficult to access exfoliated sepiolite(Sep)fibers with high aspect ratio from Sep ore.The traditional method used to purify Sep ore also reduces its aspect ratio.In this study,impurities in the Sep ore were removed by acid activation followed by a cetyltrimethylammonium chloride(C16)treatment to organically modify the purified Sep by cation exchange.Then,the organically-modified Sep(O-Sep)was stripped and processed by an ultrasonic cell crusher to obtain Sep microfibers at a specific frequency for a given period.These Sep samples had relatively high aspect ratio,compared with the Sep fibers gotten by traditional method.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)demonstrate the micro-morphology of exfoliated Sep samples in an intuitive way.Moreover,pure inorganic membrane prepared only with the exfoliated Sep fibers exhibited excellent flexibility,further demonstrating the excellent properties of Sep fibers with high aspect ratio.

Elliptical vibration chiseling:a novel process for texturing ultra-high-aspect-ratio microstructures on the metallic surface

High-aspect-ratio metallic surface microstructures are increasingly demanded in breakthrough applications,such as high-performance heat transfer enhancement and surface plasmon devices.However,the fast and cost-effective fabrication of high-aspect-ratio microstructures on metallic surfaces remains challenging for existing techniques.This study proposes a novel cutting-based process,namely elliptical vibration chiseling(EV-chiseling),for the high-efficiency texturing of surface microstructures with an ultrahigh aspect ratio.Unlike conventional cutting,EV-chiseling superimposes a microscale EV on a backward-moving tool.The tool chisels into the material in each vibration cycle to generate an upright chip with a high aspect ratio through material deformation.Thanks to the tool’s backward movement,the chip is left on the material surface to form a microstructure rather than falling off.Since one microstructure is generated in one vibration cycle,the process can be highly efficient using ultrafast(>1 kHz)tool vibration.A finite element analysis model is established to explore the process mechanics of EV-chiseling.Next,a mechanistic model of the microstructured surface generation is developed to describe the microstructures’aspect ratio dependency on the process parameters.Then,surface texturing tests are performed on copper to verify the efficacy of EV-chiseling.Uniformed micro ribs with a spacing of 1–10μm and an aspect ratio of 2–5 have been successfully textured on copper.Compared with the conventional EV-cutting that uses a forward-moving tool,EV-chiseling can improve the aspect ratio of textured microstructure by up to 40 times.The experimental results also verify the accuracy of the developed surface generation model of microstructures.Finally,the effects of elliptical trajectory,depth of cut,tool shape,and tool edge radius on the surface generation of micro ribs have been discussed.

A review of bird-like flapping wing with high aspect ratio

Bird-like flapping-wing vehicles with a high aspect ratio have the potential to fulfill missions given to micro air vehicles,such as high-altitude reconnaissance,surveillance,rescue,and bird group guidance,due to their good loading and long endurance capacities.Biologists and aeronautical researchers have explored the mystery of avian flight and made efforts to reproduce flapping flight in bioinspired aircraft for decades.However,the cognitive depth from theory to practice is still very limited.The mechanism of generating sufficient lift and thrust during avian flight is still not fully understood.Moving wings with unique biological structures such as feathers make modeling,simulation,experimentation,and analysis much more difficult.This paper reviews the research progress on bird-like flapping wings from flight mechanisms to modeling.Commonly used numerical computing methods are briefly compared.The aeroelastic problems are also highlighted.The results of the investigation show that a leading-edge vortex can be found during avian flight.Its induction and maintenance may have a close relationship with wing configuration,kinematics and deformation.The present models of flapping wings are mainly two-dimensional airfoils or three-dimensional single root-jointed geometric plates,which still exhibit large differences from real bird wings.Aeroelasticity is encouraged to consider the nonignorable effect on aerodynamic performance due to large-scale nonlinear deformation.Introducing appropriate flexibility can improve the peak values and efficiencies of lift and thrust,but the detailed conclusions always have strong background dependence.

Laser-induced oxidation assisted micro milling of high aspect ratio microgroove on WC-Co cemented carbide

Severe tool wear and poor surface quality are the main problems during micro machining of cemented carbide.In this work,an innovative hybrid process of laser-induced oxidation assisted micro milling(LOMM)was proposed to solve the problems.A nanosecond laser was utilized to induce oxidation of the WC-20%Co material,producing loose oxide which was easy to remove.The micro machinability of the material was improved by laser-induced oxidation.The oxidation mechanisms of cemented carbide were studied.A microgroove with a depth of 2.5 mm and aspect ratio of 5 was fabricated successfully.The milling force,surface quality and tool wear mechanisms were investigated.For comparison,a microgroove was also fabricated with conventional micro milling(COMM)using identical milling parameters.Results revealed that in LOMM the milling force and tool wear rate were extremely low during removing the oxide.The machined surface quality and dimensional accuracy achieved by LOMM were superior to those obtained by COMM.The surface roughness Saof the microgroove bottom reached 88 nm in LOMM,while the cross-sectional geometry of the microgroove was a trapezoid.Perpendicularity of the microgroove sidewall machined by LOMM was better than that by COMM.The tool wear forms in LOMM were coating spalling and slight tool nose breakage.Compared with COMM,the tool life in LOMM was prolonged significantly.It indicates that the proposed hybrid process is an effective and efficient way to fabricate high aspect ratio micro-features with high dimensional accuracy.

Drilling high aspect ratio holes by femtosecond laser filament with aberrations

A near-infrared femtosecond laser is focused by a 100 mm-focal-length plano-convex lens to form a laser filament,which is employed to drill holes on copper targets.By shifting or rotating the focusing lens,additional aberration is imposed on the focused laser beam,and significant influence is produced on the aspect ratio and cross-sectional shape of the micro-holes.Experimental results show that when proper aberration is introduced,the copper plate with a thickness of 3 mm can be drilled through with an aspect ratio of 30,while no through-holes can be drilled on 3-mm-thickness copper plates by femtosecond laser with minimized aberration.In addition,when femtosecond laser filament with large astigmatism is used,micro-holes that had a length to width ratio up to 3.3 on the cross-section are obtained.Therefore,the method proposed here can be used to fabricate long oval holes with high aspect ratios.

Electroplating for high aspect ratio vias in PCB manufacturing:enhancement capabilities of acoustic streaming

This paper considers the copper electrodeposit ion processes in microvias and investigates whether the quality of the electroplating process can be improved by acoustic streaming using megasonic transducers placed into a plating cell. The theoretical results show that acoustic streaming does not take place within the micro-via （either through or blind-via＇ s）, however it does help improve cupric ion transport in the area close to the mouth of a via. This replenishment of cupric ions at the mouth of micro-via leads to better quality filling of the micro-via through diffusion compared to basic conditions. Experiments showing the improved quality of the filling of vias are also presented.

A Rapid Synthesis of High Aspect Ratio Silver Nanowires for High-Performance Transparent Electrodes

A rapid,simple and cost-effective polyol method has been developed for the synthesis of silver nanowires with high aspect ratio and high purity.The aspect ratios of the silver nanowires as high as ca.1000(average length 40μm and some even as long as 80μm,diameter 50-100 nm)were obtained via optimizing the reaction conditions.Transparent electrodes with excellent optoelectronic performances(optical transmittance of 90%,sheet resistance of 23.2Ω/□and optical transmittance of 87%,sheet resistance of 19.7Ω/□)comparable to commercial ITO were fab-ricated via simple spin coating the resulting silver nanowires onto the glass substrates.The high optoelectronic per-formances and the facile all-solution process of the as-prepared transparent electrodes render them rather promising candidates for use in cost-effective large-area optoelectronic devices.

Aeroelastic Analysis and Optimization of High-aspect-ratio Composite Forward-swept Wings

In order to analyze the effects of forward-swept angle and skin ply-orientation on the static and dynamic aeroelastic characteristics, the aeroelastic modeling and calculation for high-aspect-ratio composite wings with different forward-swept angles and skin ply-orientation are performed. This paper presents the results of a design study aiming to optimize wings with typical forward-swept angles and skin ply-orientation in an aeroelastic way by using the genetic/sensitivity-based hybrid algorithm. Under the conditions of satiated multiple constraints including strength, displacements, divergence speeds and flutter speeds, the studies are carried out in a bid to minimize the structural weight of a wing with the lay-up thicknesses of wing components as design variabies. In addition, the effects of the power of spanwise variation function of lay-up thicknesses of skins and iugs on the optimized weights are also analyzed.

Structural Modeling and Aeroelastic Analysis of High-Aspect-Ratio Composite Wings

A unified structural model for high-aspect-ratio composite wing with arbitrary cross-section is developed. Two types of lay-ups of the composite wing, namely, circumferentially uniform stiffness （CUS） configuration and circumferentially asymmetric stiffness （CAS） configuration, are investigated. The present structural modeling method is validated through ANSYS FEM software for the case of a composite box beam. Then, the case of a single-cell composite wing with NACA0012 airfoil shape is considered. To investigate the aeroelastic problem of high-aspect-ratio composite wings, the linear ONERA aerodynamic model is used to model the unsteady aerodynamic loads under the case of small angle of attack. Finally, flutter speeds of the high-aspect-ratio wing with various composite ply angles are determined by using U-g method.

Stall Flutter Analysis of High-Aspect-Ratio Composite Wing

The stall flutter characters of high-aspect-ratio composite wing are investigated, and the effects of structure geometric nonlinearity and stiffness couple created by composite anisotropy on them also are discussed. Firstly, the high-aspect-ratio wing is modeled as a composite thin-walled closed section Euler beam whose displacement and rotation both could be of finite value, and the nonlinear dynamic equations is build up on it with all the effects of geometric nonlinearity, aerodynamic nonlinearity and anisotropy of material being considered. Then vibration equations are deduced through perturbing the dynamic equations at wing＇s equilibrium position, and coupled with unsteady stall aerodynamic model and ONERA model, to obtain the nonlinear stall flutter analysis equations of wing. Finally, the flutter stabilities with various wind speeds are determined by the harmonic balance method. With several exampies, the validity of the stall flutter model is proved, and the significant effects of geometric nonlinearity on the stall flutter various characters as wall as the effects of ply angle on the stall flutter speed and frequency also are discussed.

Development of high-aspect-ratio microchannel heat exchanger based on multi-tool milling process

A high-aspect-ratio microchannel heat exchanger based on multi-tool milling process was developed. Several slotting cutters were stacked together for simultaneously machining several high-aspect-ratio microchannels with manifold structures. On the basis of multi-tool milling process, the structural design of the manifold side height, microchannel length, width, number, and interval were analyzed. The heat transfer performances of high-aspect-ratio microchannel heat exchangers with two different manifolds were investigated by experiments, and the influencing factors were analyzed. The results indicate that the magnitude of heat transfer area per unit volume dominates the heat transfer performances of plate-type micro heat exchanger, while the velocity distribution between microchannels has little effects on the heat transfer performances.

Stiffness Distribution and Aeroelastic Performance Optimization of High-Aspect-Ratio Wings

The relationship between stiffness distribution and aeroelastic performance for a beam-frame model and a3-D model is investigated based on aeroelastic optimization of global stiffness design for high-aspect-ratio wings.The sensitivity information of wing spanwise stiffness distribution with respect to the twist angle at wing tip,the vertical displacement at wing tip,and the flutter speed are obtained using a sensitivity method for both models.Then the relationship between stiffness distribution and aeroelastic performance is summarized to guide the design procedure.By using the genetic/sensitivity-based hybrid algorithm,an optimal solution satisfying the strength,aeroelastic and manufacturing constraints is obtained.It is found that the summarized guidance is well consistent with the optimal solution,thus providing a valuable design advice with efficiency.The study also shows that the aeroelastic-optimization-based global stiffness design procedure can obtain the optimal solution under multiple constraints with high efficiency and precision,thereby having a strong application value in engineering.

Electrodeposition of Gold to Conformally Fill High-Aspect-Ratio Nanometric Silicon Grating Trenches: A Comparison of Pulsed and Direct Current Protocols

Filling high-aspect-ratio trenches with gold is a frequent requirement in the fabrication of X-ray optics as well as micro-electronic components and other fabrication processes. Conformal electrodeposition of gold in sub-micron-width silicon trenches with an aspect ratio greater than 35 over a grating area of several square centimeters is challenging and has not been described in the literature previously. A comparison of pulsed plating and constant current plating led to a gold electroplating protocol that reliably filled trenches for such structures.

Design and Simulation of BTT Missile with High-Aspect-Ratio Wing Robust H_∞ Autopilot

For the strong coupling among the channels of bank-to-turn （BTT） missile with high-aspect-ratio wing, an autopilot is designed with a two loop control structure robust autopilot design methods. By the inner loop design, the question of pole-zero cancellation is solved, and the stabilization of structured uncertainty is achieved. Through the outer loop of H∞ controller design, the flying performance and robustness can be guaranteed. The nonlinear simulation results show that the autopilot designed has perfect time domain response, and can suppress bad influence of the inertial and kinematics couplings. It can make the missile fly stably in the large flying areas. The control is very effective.

紫外纳秒激光加工高深宽比且高一致性微槽的研究

高深宽比微槽广泛应用于生物学以及电子学等各领域。然而,在制备高深宽比微槽时,微槽的一致性仍然是一个技术上的难题。通过一种新颖的多次扫描技术,利用355纳米纳秒激光成功制备了同时具有高深宽比和高一致性的微槽。研究了脉冲能量、离焦量和扫描次数对微槽深度和宽度的影响。结果表明:略微增加负离焦量和脉冲能量可以显著提高微槽的深宽比,但过大的脉冲能量会导致一致性下降。提出一种多次扫描的方法,通过在每次扫描的同时调整激光离焦量,制备了一组高深宽比且高一致性的微槽。所获得的微槽的平均深宽比高达6.84,深度和宽度的一致性也得到了显著提高。深度和宽度的相对标准偏差分别低至4.8%、4.4%。由于其高深宽比和高一致性,这些微槽有望在生物医学科学、3D微型电池和超疏水表面等领域得到广泛应用。

大展弦比飞行器变形辅助高度控制策略

针对变形飞行器结构/飞行耦合动力学的飞行控制问题,以大展弦比飞行器为研究对象开展了变形辅助高度控制策略研究。采用单元翼多体假设,构建了大展弦比飞行器二面角变形结构,建立了结构/飞行纵向耦合动力学模型,基于线性二次型(LQR)控制器设计方法设计了主动变形(AM)和被动变形(PM) 2种控制策略。其中,被动变形控制策略仅采用升降舵作为控制输入,结构二面角动态行为由耦合动力学驱动;主动变形控制策略以二面角铰链处的扭矩作为额外输入对高度跟踪进行协调控制。仿真分析了2种控制策略下的高度跟踪效果及状态变化,研究结果表明:采用主动变形控制策略可有效改善高度跟踪暂态过程的性能,降低姿态回路的阻尼特性,为未来先进变形飞行器的结构/飞行一体化控制提供研究基础。

大展弦比混杂纤维复合材料机翼的强度及失效分析

以大展弦比机翼为研究对象,基于经典层合板理论和Tsai-Wu失效准则,考虑几何非线性效应,用流固耦合数值模拟方法,重点研究碳/玻纤维混杂比、铺层相对位置、铺层角度对机翼强度及失效特性的影响。研究表明:碳/玻纤维混杂比=8∶2为最佳纤维混杂比,机翼可兼顾高强度、低成本;玻璃纤维铺设在蒙皮对称中面位置时,机翼的强度及安全性更高,玻璃纤维铺设位置的变化不会对机翼的弹性变形产生显著影响;在不同混杂比下,以±45°铺层角度铺设的机翼整体力学性能更稳定,失效概率更低。

大展弦比板弯扭耦合受迫振动分析

对于工程广泛存在的大展弦比结构,首次研究了大展弦比板弯扭耦合受迫振动响应.考虑弦向相对弯曲刚度远大于展向弯曲刚度,将弦向截面等效为刚体,考虑其绕展向轴线的扭转,应用Hamilton原理分别计算大展弦比板的势能与动能,建立两端简支的大展弦比板弯扭耦合受迫振动的动力学控制方程.将动力学控制方程进行Galerkin截断后,通过谐波平衡法和微分求积法/微分求积单元法,计算得到了大展弦比模型弯扭耦合受迫振动稳态响应的解析解及数值解,同时验证了解的截断收敛性及谐波收敛性.最后研究了外激励幅值对结构受迫振动响应的影响.结果将为工程中广泛存在的大展弦比结构设计提供指导.

长径比大于1200的超长AgNWs制备及影响因素

采用改进多元醇法制备了长径比大于1200的超长银纳米线(AgNWs)。系统研究了反应温度(120~170℃)、反应时间(0.5~3h)、搅拌速率(0~300rpm)、聚乙烯吡咯烷酮(PVP)分子量(58000~1280000 g/mol)、氯化铁(FeCl_(3))乙二醇溶液添加量(0~5.6 g,5 mM)和PVP添加量(0.2~1 g)对AgNWs形貌的影响。在最佳实验条件下,制备的超长Ag NWs平均长度约96.6μm,平均直径约80.0 nm。此外,通过简单的调控PVP添加量可以获得不同长度和直径的Ag NWs。

氨硼烷制备的高宽厚比氮化硼纳米片及其形成机理

受限于商业氮化硼(BN)晶体尺寸及BN独特的层间相互作用,传统的剥离法难以实现高宽厚比(>5000)氮化硼纳米片(BNNSs)的制备。以氨硼烷为前驱体,经烧结和分离得到了高宽厚比的BNNSs。通过FTIR,TG,TEM,XRD等手段对BNNSs的组成结构及形成机理进行表征。结果表明:当温度超过80℃时,氨硼烷之间开始交联脱氢,氢气释放使得在交联过程中形成泡沫结构,并最终在1000℃完成无机化,得到含有BNNSs的BN泡沫,其中的膜结构即为BNNSs。所制备的BNNSs为六方晶型结构,厚度约为1.5 nm,宽度约30μm,宽厚比达到20000。将高宽厚比的BNNSs与聚乙烯醇(PVA)复合制备成复合膜,其导热及力学性能相较于小宽厚比BNNSs所制备的复合膜有显著提升,充分说明了高宽厚比BNNSs在BN基复合材料领域的应用优势。