从常温到超临界条件下水的分子动力学模拟

采用MD模拟方法研究了从常温到超临界条件下不同状态水的微观结构、氢键结构及氢键松弛的动态性质。模拟结果表明,随着温度的升高,成键分子中O…O间的平均距离基本保持不变,O…H间距离增加,水分子的氢键作用逐渐减弱,形成氢键的两水分子O…O一H取向角的分布呈线型分布的概率降低;同一温度下密度的变化对O-O、O-H径向分布函数和氢键的键角分布的影响不大。由于温度升高使氢原子热运动增强,水的四面体结构缺陷增加,氢键的平均寿命显著缩短,温度是影响氢键平均寿命的主要因素。     

不同温度下水的分子动力学模拟

采用分子动力学模拟的方法对不同温度下水的氢键结构及扩散性质进行了研究。模拟发现随温度的升高,水分子平均氢键配位数减小,氢键键角分布变宽,键角分布峰值减小,即水分子间氢键作用不断减弱。扩散系数随温度的增加而上升,液体水的扩散系数与温度之间的关系符合Ａｒｒｈｅｎｉｕｓ行为。     

双电层对纳米流体润滑与摩擦影响的MD模拟 (2.东南大学MEMS教育部重点实验室 南京 210096）

采用分子动力学方法对带电和不带电两硅板之间的水分子润滑薄膜进行模拟研究,通过加双电层的水分子薄膜润滑与未加双电层薄膜润滑摩擦属性的对比,发现摩擦系数在存在双电层的情况下比未加双电层时要小,两板相对滑动速度对摩擦系数的影响与未加双电层时相似,相对滑动速度越大,摩擦系数在一定的速度范围内平稳增大.当速度大于某个数值时,摩擦系数增大变快.两板之间水分子以及离子密度或数目分布在靠近壁面的地方较大,中间密度相对较小.     

2,3-二甲基-5-(7’-十六烷基)苯磺酸钠油水界面的分子模拟

采用量子化学软件GAMESS(US)对2,3-二甲基-5-(7’-十六烷基)苯磺酸钠的结构进行了优化,应用分子动力学模拟的方法对其在油/水界面的聚集行为进行了研究,磺酸基中氧原子与水分子中氢原子间存在较强氢键作用形成第一水层和第二水层,反离子Na+分布于第一水层和第二水层,随着Na Cl浓度的增加,过量的Na+更倾向于扩散至水溶液中,Ca2+对扩散双电层有较强的压缩作用,2,3-二甲基-5-(7’-十六烷基)苯磺酸钠与辛烷分子的亲和能力较强。     

水分子团簇模拟及其在观测冷凝过程的应用

在气液相变过程中,水分子相互作用形成团簇,并产生分子间氢键,从而影响了羟基的振动频率。尺寸不同的团簇体系内氢键网络特性不同,红外吸收频率可反应OH所处的相邻氢键以及周围氢键的环境。本文采用密度泛函理论计算了不同尺寸水团簇的稳定结构和红外频率,分析了红外振动频率与团簇尺寸之间的演变关系。结果表明随着尺寸增大,内部水分子结构可能发生转变,(H2O)n团簇结构趋向于紧密的球形,并且伸缩振动频率向低频演化。研究结果可为通过红外光谱技术检测和确定团簇尺寸提供参考依据,并有可能用于水分子团簇在近壁区核化和生长过程的量化检测。     

量子化学方法研究阴离子表面活性剂在气液界面上的吸附

用量子化学方法中的密度泛函理论,在B3LYP/6-3IG水平上,对十二烷基硫酸钠（SDS）阴离子表面活性剂与水分子形成的水合物CH₃（CH₂）₁₁OSO₃ˉ（H₂O）_n（n=0～7）进行结构优化和频率计算。从分子水平上研究了CH₃（CH₂）₁₁OSO₃ˉ在气液界面上与水分子的相互作用。计算结果表明:（1）7个水分子与极性头均采用1:1型和2:1型,即极性头中一个氧原子或2个氧原子与水分子以氢键形式构成水合层;（2）CH₃（CH₂）₁₁OSO₃中的氧原子与水分子中的氧原子最短氢键的键长（O-O键长）在0.27～0.31 nm之间,H…O键长在0.19～0.21 nm之间,O-H…O键角在140°～167°之间,均属于中强氢键:（3）水合物R（S-O）平均键长比表面活性剂单体分子分别增长了,说明形成水合物后S-O间的键减弱;（4）结合能D₀从64.04 kJ/mol增加到.428.29 kJ/mol,说明随着水分子数的增加,所获得的7种水合物的稳定性依次增强,表明最终形成的水合层是稳定的:（5）随着水分子数增加疏水基链长收缩,亲水基总电荷增加,C12-O13-S14的键角增大;（6）由于烷烃链带有了弱电荷,使胶束内核带有了部分极性,此种极性介于烷烃油相和水相的极性之间,利于表面活性剂在溶液中的聚集。     

SO_4~(2-)微观水合结构DFT研究

运用密度泛函理论(DFT)计算方法系统研究硫酸根阴离子水合团簇,[SO_4(H_2O)_n]~(2-)(n=1-18),微观水合结构。理论计算采用ωB97XD泛函和6-311++G(d,p)基组。每个尺寸团簇结构,尽可能考虑所有合理初始结构,以此进行结构优化和频率分析等。结果表明,气相和液相条件下SO_4~(2-)与H_2O之间的相互作用能和溶剂稳定化能均与团簇尺寸n成线性关系,因此优化得到稳定结构均是合理构型。在[SO_4(H_2O)_n]~(2-)水合团簇稳定结构中,氢键的形式有两种,即溶质-溶剂(SO_4~(2-)…HOH)之间形成的对称双氢键,以及溶剂-溶剂(H_2O... HOH)之间形成的氢键。当SO_4(H_2O)_n]~(2-)水合团簇的尺寸为5个水分子时,由于水分子内氢键的形成,出现氢键环状结构,且团簇结构中出现水分子内氢键时,结构较为稳定。随着n增加,氢键结构逐渐变为网状直至稳定的笼状。通过能量参数和键参数分析可知,当团簇尺寸n≥18时,SO_4~(2-)第一水合层水分子数≥9,而第二水合层还有待进一步研究。同时随着浓度降低SO_4~(2-)与H_2O分子之间总相互作用能增加,溶液中溶剂-溶质相互作用增强。     

化合物(C10H8N2)·2（H2O）的晶体结构及量子化学研究

以水热法制备了标题化合物(C10H8N2).2(H2O)单晶并用X射线单晶衍射仪测定了晶体结构,该晶体属于单斜晶系,C2空间群,晶胞参数为a=1.58589(16)nm,b=0.37770(4)nm,c=0.920210(11)nm,α=90.00°,β=114.0500(10)°,γ=90.00°,V=0.50335(7)nm3;最终偏差因子R1=0.0525,wR2=0.1311[对I>2θ(I)的衍射点]和R1=0.0661,wR2=0.1404[对所有衍射点]。化合物分子与水分子间由弱的O-H…N和O-H…O氢键作用形成了一维线形结构,该对称结构中两个吡啶环平面之间的夹角为40.7o。依据晶体结构数据使用G03程序对化合物进行了量子化学计算,探讨了化合物的分子优化结构、前线轨道、电荷分布、成键特征和稳定性。计算得到的分子键长、键角和X射线衍射的晶体结构数据基本符合,其差值证实晶体分子间氢键的存在。     

量子化学方法研究丝氨酸与酪氨酸水合团簇几何构型

为研究水合氨基酸几何构型,利用从头计算(ab initio),多极距法(multipole moments)等量子化学方法,在分子水平上对几何优化后的水分子,水合丝氨酸及酪氨酸团簇进行系统的比较。此外,针对目前流行的多种点电荷模型AMBER,CHARMM,OPLS,MMFF,TAFF的精确性进行了检测。结果表明:(1)ab initio得到的水分子偶极距和四极距与实验结果相一致,可以作为精确度评判标准与多极距、点电荷等方法进行比较;(2)丝氨酸与酪氨酸均可与周围水分子以氢键形式构成水合团簇,且氢键键长范围(O-O键)在2.7A～2.9A之间,H…O键长在1.9A～2.0A之间,均属于中强氢键。(3)当n=1～5时,酪氨酸与水分子间均形成1:1型氢键;n≤3时,丝氨酸与周围水分子以1:1型氢键构成环形水合团簇;n>3时,以2:1型氢键构成环形水合团簇;(4)多极距法阶数L增加,平均绝对误差(mean absolute error,MAE)先降低后升高,L=5时MAE最小,结果最精确。(5)量子化学方法预测丝氨酸与酪氨酸水合团簇精确度:Multipole moments>TAFF>OPLS-AA>AMBER99>CHARMM27>MMFF94X     

一种电容式微加速度计的结构设计和工艺仿真

微硅电容式加速度计是目前微硅加速度传感器发展的主流,本文在硅-玻璃阳极键合工艺同深槽刻蚀工艺相结合的加工技术基础之上设计了一种电容式微加速度计,该结构将两种改变平行板电容量的方式有效的结合在一起,提高了结构的灵敏度并具有较好的线性度.最后,对所设计的结构进行了工艺仿真,通过虚拟工艺仿真结果与设计进行比较,论证了结构的基本可行性.     

基于金硅原电池保护电容加速度计的制作

提出并实现了一种利用SoI结合金硅原电池保护和反熔丝制作电容式加速度计的新工艺方法。该工艺用SoI顶层硅制作梁和上电极,用衬底制作质量块。采用DRIE从正面刻蚀形成释放孔,TMAH腐蚀实现质量块的释放,在TMAH腐蚀过程中利用金硅原电池保护实现梁和表面极板的保护。在TMAH腐蚀完成前,反镕丝保持断开状态,腐蚀完成后,击穿反镕丝形成导通状态。通过测量金和硅的极化曲线得到60℃25%TMAH中实现原电池保护的金硅面积比不小于5∶1。成功制作成电容式加速度计结构,释放前后梁宽度均在9.4～10μm范围内,表明原电池保护有效。击穿后反熔丝并联导通电阻为5～25 kΩ之间。     

Piezoelectric control of composite plate vibration: Effect of electric potential distribution

The paper deals with the vibration analysis of active rectangular plates. The plates considered are composites containing piezoelectric sensor/actuator layers, which operate in a velocity feedback control to achieve transverse vibration suppression. The piezoelectric layers are poled through the thickness and equipped with traditional surface electrodes. In order to satisfy the Maxwell electrostatics equation the widely used simplification of the electric potential distribution in the actuator layer (linear across the thickness) is replaced by a combination of a half-cosine and linear distribution in the transverse direction. The in-plane spatial variation of the potential instead of applying uniform distribution is determined by the solution of the coupled electromechanical governing equations with the natural boundary conditions corresponding to both the flexural and electric potential fields. The analysis is performed for simply supported plates. Two models of the plate are considered. In the first case the displacement field is based on the Kirchhoff hypothesis. For the second the Mindlin plate model is applied. The governing coupled equations describing the active plate behaviour are derived. The influence of the electric potential distribution and also the thickness of piezoelectric layers on the plate dynamics including the natural frequencies modification is numerically investigated and discussed.     

Momentum transfer to nanoobjects between isothermal parallel plates

A small-scale, trapezoidal rigid body in the gas-filled gap between two parallel plates at different temperatures is considered. An analytical expression for the thermally induced force onto the body in the direction parallel to the plates valid for an infinite Knudsen number is derived. For this purpose, diffuse reflection of the gas molecules at the solid walls is assumed. Simultaneously, Monte Carlo simulations are performed allowing an extension of the analysis to Knudsen numbers of the order of one. The numerical and the analytical results show excellent agreement, indicating that a temperature gradient orthogonal to the plates can induce a significant force in parallel direction, a phenomenon without analogy in the macroworld. This force is only slightly reduced when a Knudsen number of one is considered. In addition to the diffuse-reflection boundary condition, a mixture of diffuse and specular reflection is studied. The practical relevance of the results is exemplified by considering two scenarios with bodies of a specific geometry, among others a nanoscopic platelet.     

Gas motion in a microgap between a stationary plate and a plate oscillating in its normal direction

Unsteady motion of a gas between two parallel plates is investigated in the case where one of the plates starts (harmonic) oscillation in its normal direction. A kinetic–theoretic approach is employed under the condition that the distance between the two plates is comparable to the mean free path of the gas molecules and/or the frequency of oscillation of the plate is comparable to their mean collision frequency. More specifically, the Bhatnagar–Gross–Krook model of the Boltzmann equation is solved numerically for wide ranges of parameters, such as the Knudsen number and the Mach number, with special interest in the fully nonlinear wave motion. As the result, the time evolution of the local flow field and the periodic state attained at later times are obtained accurately. It is shown that, in the periodic state, one-period average of the momentum (or energy) transferred from the oscillating to the stationary plate takes a nonzero value in contrast to the linear theory, and it becomes minimum at an intermediate Knudsen number (for a given oscillation of the plate and for a given distance between the center of the oscillating plate and the stationary plate).     

Studying on water nanojet ejection and the wetting phenomena on the nozzle surface

In this research, molecular dynamics simulations of water nanojet ejection out of nozzle holes with various sizes under various pressing forces are performed. The water molecules are ejected out the nozzle by a back plate on which a constant force is applied. The results of MD simulations of water ejection show that after one ejection, about 1.3?C2.5% of total molecules accumulate on the nozzle plate surface. These molecules affect the ejection of water jet thereafter. The cause of the accumulation of wetting water is investigated by analyzing the trajectories of these molecules. It is found that in the firing chamber near the nozzle plate wall, the arrangement of water molecules is aligned by the surface topology of the metal wall. Water molecules are packed into filamentous structure and these lines stack up at equal distances to each other. Water molecules drift along these lines, the trajectories of these molecules are sinuous, the velocity directions of them are random; molecules drift along the parallel lines until they reach a region of low pressure beneath the nozzle opening. These molecules eject out through the edge of the nozzle, they fall back on to the nozzle surface and eventually deposit on the nozzle surface due to low ejection velocity.     

Preparation and characterization of unimorph actuators based on piezoelectric Pb(Zr0.52Ti0.48)O3 materials

This paper describes the preparation and characterization of unimorph actuators for deformable mirrors, based on Pb(Zr_0.52Ti_0.48)O₃ (PZT52) thin film. As comparison, two different designs, where the PZT layer in the unimorph actuators was driven by either interdigitated electrodes (IDT-mode) or parallel plate electrodes (d₃₁-mode), were investigated. The actuators utilize a unimorph membrane (diaphragm) structure consisting of an active PZT piezoelectric layer and a passive SiO₂/Si composite layer. To fabricate the diaphragm structures, n-type (1 0 0) silicon-on-insulator (SOI) wafers with 1 μm thermal SiO₂ were used as substrates (for d₃₁-mode actuators, the upper Si part of SOI need to be heavily doped and used as bottom electrodes simultaneously). Sol-gel derived PZT piezoelectric layers with PbTiO₃ (PT) bufferlayer in total of 0.86 μm were then fabricated on them, and 0.15 μm Al reflective layers were deposited and patterned into top electrode geometries, subsequently. The diaphragms were released using orientation-dependent wet etching (ODE) with 5-10 μm residual silicon layers. The complete unimorph actuators comprise 4 × 4 discrete units (4 mm² in size) with patterned PZT films for parallel plate configuration or 3 × 3 individual pixels (2 mm in IDT diameter) with continuous PZT films in graphic region for IDT configuration. The measurement results indicated that both of the two configurations can generate considerable deflections at low voltage. The measured maximum central deflections at 15 V were approximately 2.5 μm and 2.8 μm, respectively. The intrinsic strain conditions shaping the deflection profiles for the diaphragm actuators were also analyzed. In this paper, the behaviors of clamped parallel plate configuration without a diaphragm were also evaluated.     

Linear Bilayer ALD Coated MEMS Varactor With High Tuning Capacitance Ratio

A curl-up-plate microelectromechanical system (MEMS) varactor with an almost linear response and high tuning capacitance ratio is presented. The curl-up in the top plate is realized by the residual stress in the two layers that construct the top plate of the varactor. The linear response is achieved by having the curl-up plate designed to relax on the bottom plate and by having unanchored cantilever beams that prevent the pull-in, while applying a dc bias voltage. The developed varactor exhibits a low parasitic capacitance through etching the lossy substrate underneath the varactor's plates. A thin alumina dielectric layer of 100-nm thickness is deposited using an atomic-layer-deposition technique to provide electrical isolation between the two plates. This MEMS varactor exhibits an almost linear capacitance with a tuning ratio of 5 : 1. $hfill$[2008-0161]      

曲面体间隙电涡流检测的有限元分析

为研究曲面体间隙电涡流检测受目标导体曲率的影响,基于电磁场有限元分析方法对平面线圈用于不同曲率目标导体间隙测量时的交流阻抗进行了数值计算,得到了线圈的归一化阻抗平面图,并讨论了由于曲率不同引起的数据误差处理方法,为曲面体间隙的准确测量提供了理论依据.计算结果表明:保持线圈位置曲面体间的平均间隙不变,当目标导体曲率不同时,平面线圈的归一化阻抗变化近似为一条直线,随曲率增大,线圈的归一化阻抗将沿着远离零曲率点的方向变化,且凹面和凸面时远离的方向相反.     

Molecular dynamics characterization of n-octyl-beta-D-glucopyranoside micelle structure in aqueous solution

n-Octyl-beta-D-glucopyranoside (OG) is a non-ionic glycolipid, which is used widely in biotechnical and biochemical applications. All-atom molecular dynamics simulations from two different initial coordinates and velocities in explicit solvent have been performed to characterize the structural behaviour of an OG aggregate at equilibrium conditions. Geometric packing properties determined from the simulations and small angle neutron scattering experiment state that OG micelles are more likely to exist in a non-spherical shape, even at the concentration range near to the critical micelle concentration (0.025 M). Despite few large deviations in the principal moment of inertia ratios, the average micelle shape calculated from both simulations is a prolate ellipsoid. The deviations at these time scales are presumably the temporary shape change of a micelle. However, the size of the micelle and the accessible surface areas were constant during the simulations with the micelle surface being rough and partially elongated. Radial distribution functions computed for the hydroxyl oxygen atoms of an OG show sharper peaks at a minimum van der Waals contact distance than the acetal oxygen, ring oxygen, and anomeric carbon atoms. This result indicates that these atoms are pointed outwards at the hydrophilic/hydrophobic interface, form hydrogen bonds with the water molecules, and thus hydrate the micelle surface effectively.