MEMS-Based Nanomechanics: Influence of MEMS Design on Test Temperature

Microelectromechanical system (MEMS) devices based on electro-thermal actuation have been used over the past few years to perform tensile tests on nanomaterials. However, previous MEMS designs only allowed small (e.g., <100 nm) total displacement range without a significant increase in temperature near the nanospecimens (<20°C), thereby limiting the design of the load sensor or the range of nanomaterials to test. Here we characterize the thermo-mechanical behavior of three MEMS devices, using optical displacement measurements, micro-Raman temperature measurements, and finite element modeling. We observe the increase in temperature near the nanospecimen gap per displacement of thermal actuator to linearly decrease with the distance between nanospecimen gap and thermal actuator. We also present a MEMS device that can provide up to 1.6 μm of total displacement with less than 10°C increase in temperature near the nanospecimens, more than one order of magnitude improvement with respect to previously published MEMS material testing setups. This MEMS device can be used for accurate, temperature-controlled tensile testing of nanocrystalline metallic nanobeams.     

Radial filtration during constant-force squeeze flow of soft solids

Various soft solid suspensions were squeezed at constant force between polished and roughened circular glass plates and the time-dependence of the interplate separation was measured. The filterability of suspensions was quantified by their desorptivity S obtained from measurements of capillary suction time. The squeeze flow (SF) of suspensions for which S < 2 μm s^−1/2 was largely consistent with rheological theory, which neglected radial filtration: the relative motion between the liquid and solid phases of the suspension in the radial pressure gradient. Suspensions having S > 2 μm s^−1/2 showed SF behavior that was consistent with the presence of radial filtration.     

Experimental Investigation of Strain Rate Dependence of Nanocrystalline Pt Films

A new microscale uniaxial tension experimental method was developed to investigate the strain rate dependent mechanical behavior of freestanding metallic thin films for MEMS. The method allows for highly repeatable mechanical testing of thin films for over eight orders of magnitude of strain rate. Its repeatability stems from the direct and full-field displacement measurements obtained from optical images with at least 25 nm displacement resolution. The method is demonstrated with micron-scale, 400-nm thick, freestanding nanocrystalline Pt specimens, with 25 nm grain size. The experiments were conducted in situ under an optical microscope, equipped with a digital high-speed camera, in the nominal strain rate range 10⁻⁶–10¹ s⁻¹. Full field displacements were computed by digital image correlation using a random speckle pattern generated onto the freestanding specimens. The elastic modulus of Pt, E = 182 ± 8 GPa, derived from uniaxial stress vs. strain curves, was independent of strain rate, while its Poisson’s ratio was v = 0.41 ± 0.01. Although the nanocrystalline Pt films had the elastic properties of bulk Pt, their inelastic property values were much higher than bulk and were rate-sensitive over the range of loading rates. For example, the elastic limit increased by more than 110% with increasing strain rate, and was 2–5 times higher than bulk Pt reaching 1.37 GPa at 10¹ s⁻¹.     

Experimental investigation on turbulence modification in a horizontal channel flow at relatively low mass loading

Particle-laden flows in a horizontal channel were investigated by means of a two-phase particle image velocimetry (PIV) technique. Experiments were performed at a Reynolds number of 6 826 and the flow is seeded with polythene beads of two sizes, 60 μm and 110 μm. One was slightly smaller than and the other was larger than the Kolmogorov length scale. The particle loadings were relatively low, with mass loading ratio ranging from 5×10⁻⁴ to 4×10⁻² and volume fractions from 6×10⁻⁷ to 4.8×10⁻⁵, respectively. The results show that the presence of particles can dramatically modify the turbulence even under the lowest mass loading ratio of 5×10⁻⁴. The mean flow is attenuated and decreased with increasing particle size and mass loading. The turbulence intensities are enhanced in all the cases concerned. With the increase of the mass loading, the intensities vary in a complicated manner in the case of small particles, indicating complicated particle-turbulence interactions; whereas they increase monotonously in the case of large particles. The particle velocities and concentrations are also given. The particles lag behind the fluid in the center region but lead in the wall region, and this trend is more prominent for the large particles. The streamwise particle fluctuations are larger than the gas fluctuations for both sizes of particles, however their varying trend with the mass loadings is not so clear. The wall-normal fluctuations increase with increasing mass loadings. They are smaller in the 60 μm particle case but larger in the 110 μm particle case than those of the gas phase. It seems that the small particles follow the fluid motion to certain extent while the larger particles are more likely dominated by their own inertia. Finally, remarkable non-uniform distributions of particle concentration are observed, especially for the large particles. The inertia of particles is proved to be very important for the turbulence modification and particles behaviors and thus should be considered in horizontal channels. The project supported by the National Natural Science Foundation of China (50276021), and Program for New Century Excellent Talents in University, Ministry of Education (NCET-04-0708) The English text was polished by Yunming Chen.     

Ag-Cu-MoS2复合材料的真空载流磨损性能

采用粉末冶金方法制备了含有不同Cu质量分数(2%~4%)的Ag-Cu-MoS_2复合材料,并对其力学性能、电学性能和真空载流磨损性能进行了研究.结果表明:较高Cu质量分数的Ag-Cu-MoS_2复合材料表现出较高的硬度和抗弯强度;中等Cu质量分数(2.5%和3%)的Ag-Cu-MoS_2复合材料电刷表现出较低的电压降和电噪声;滑动速度的增加会降低材料间的接触稳定性,进而导致较高的电压降和较大的电噪声.在真空载流条件下与Ag合金盘滑动摩擦过程,不同Cu质量分数的Ag-Cu-MoS_2复合材料电刷均表现出较低的磨损,其磨损率在1×10~(–14)m~3/(N·m)~2.5×10~(–14)m~3/(N·m)范围,其磨损机理主要为疲劳磨损以及黏着磨损.     

A new electronic circuit for a hot-cathode ionization gauge

Summary Details are given of a circuit, capable of measuring pressures in the range 10⁻³ mm Hg to 10⁻¹⁰ mm Hg. As the quotient of ion current over emission current is measured, which is proportional to the pressure in the above-mentioned range, the need for an emission stabilizer has been eliminated. At pressures lower than 10⁻⁴ mm Hg, meter indication varies by not more than 3% at emission currents between 10μA and 1 mA. The author wishes to thank Prof. Dr. J. Kistemaker for his stimulating interest, and J. Los, H. Zaaiman, C. Visser, R. Wunnink and T. Heise for carrying out many measurements. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie and was made possible by financial support from the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek.     

The effect of mixing particles of different size on the electrorheological response under steady shear flow

 The effect of mixing particles of different sizes on the electrorheological response of suspensions under steady shear flow was investigated. Two sizes, 15 μm and 50 μm, of monodisperse spherical sulfonated poly(styrene-co-divinylbenzene) particles were used. Several electrorheological fluids were made containing different proportions of small and large particles dispersed in silicone oil, but with constant overall particulate concentration. It was found that the mixed size system produced the highest electrorheological response under the shear rates used (10 s⁻¹ to 500 s⁻¹), which is the opposite trend to previous studies of bimodal systems with larger size ratios. Received: 21 December 2000 Accepted: 29 March 2001     

Dynamics of a Canonical Electrostatic MEMS/NEMS System

The mass-spring model of electrostatically actuated microelectromechanical systems (MEMS) or nanoelectromechanical systems (NEMS) is pervasive in the MEMS and NEMS literature. Nonetheless a rigorous analysis of this model does not exist. Here periodic solutions of the canonical mass-spring model in the viscosity dominated time harmonic regime are studied. Ranges of the dimensionless average applied voltage and dimensionless frequency of voltage variation are delineated such that periodic solutions exist. Parameter ranges where such solutions fail to exist are identified; this provides a dynamic analog to the static “pull-in” instability well known to MEMS/NEMS researchers.      

Microscopic high-speed liquid-metal jets in vacuum

The operation of microscopic high-speed liquid-metal jets in vacuum has been investigated. We show that such jets may be produced with good stability and collimation at higher speeds than previously demonstrated, provided that the nozzle design is appropriate and that cavitation-induced instabilities are avoided. The experiments with a medium-speed tin jet (u ∼ 60 m/s, Re=1.8×10⁴, Z=2.9×10⁻³) showed that it operated without any signs of instabilities, whereas the stability of high-speed tin jets (d=30 μm, u=500 m/s, Re=5.6×10⁴, Z=4.7×10⁻³) has been investigated via dynamic similarity using a water jet. Such a 500-m/s tin jet is required as the anode for high-brightness operation of a novel electron-impact X-ray source.     

STICTION AND ANTI—STICTION IN MEMS AND NEMS

Stiction in microelectromechanical systems (MEMS) has been a major failure mode ever since the advent of surface micromachining in the 80s of the last century due to large surfacearea-to-volume ratio. Even now when solutions to this problem are emerging, such as self-assembled monolayer (SAM) and other measures, stiction remains one of the most catastrophic failure modes in MEMS. A review is presented in this paper on stiction and anti-stiction in MEMS and nanoelectromechanical systems (NEMS). First, some new experimental observations of stiction in radio frequency (RF) MEMS switch and micromachined accelerometers are presented. Second, some criteria for stiction of microstructures in MEMS and NEMS due to surface forces (such as capillary, electrostatic, van der Waals, Casimir forces, etc.) are reviewed. The influence of surface roughness and environmental conditions (relative humidity and temperature) on stiction are also discussed. As hydrophobic films, the self-assembled monolayers (SAMs) turn out able to prevent release-related stiction effectively. The anti-stiction of SAMs in MEMS is reviewed in the last part. The project supported by the Distinguished Young Scholar Fund of NSFC (10225209), key project from the Chinese Academy of Sciences (KJCX2-SW-L2) and National “973” Project (G1999033103)     

Scanning Electron Microscopy for Quantitative Small and Large Deformation Measurements Part II: Experimental Validation for Magnifications from 200 to 10,000

A combination of drift distortion removal and spatial distortion removal are performed to correct Scanning Electron Microscope (SEM) images at both ×200 and ×10,000 magnification. Using multiple, time-spaced images and in-plane rigid body motions to extract the relative displacement field throughout the imaging process, results from numerical simulations clearly demonstrate that the correction procedures successfully remove both drift and spatial distortions with errors on the order of ±0.02 pixels. A series of 2D translation and tensile loading experiments are performed in an SEM for magnifications at ×200 and ×10,000, where both the drift and spatial distortion removal methods described above are applied to correct the digital images and improve the accuracy of measurements obtained using 2D-DIC. Results from translation and loading experiments indicate that (a) the fully corrected displacement components have nearly random variability with standard deviation of 0.02 pixels (≈25 nm at ×200 and ≈0.5 nm at ×10,000) in each displacement component and (b) the measured strain fields are unbiased and in excellent agreement with expected results, with a spatial resolution of 43 pixels (≈54 μm at ×200 and ≈1.1 μm at ×10,000) and a standard deviation on the order of 6 × 10⁻⁵ for each component.

Seismic efficiency of a contact explosion and a high-velocity impact

The seismic energy transferred to an elastic half-space as a result of a contact explosion and a meteorite impact on a planet’s surface is estimated. The seismic efficiency of the explosion and impact are evaluated as the ratio of the energy of the generated seismic waves to the energy of explosion or the kinetic energy of the meteorite. In the case of contact explosions, this ratio is in the range of 10⁻⁴–10⁻³. In the case of wide-scale impact effects, where the crater in the planet’s crust is produced in the gravitational regime, a formula is derived that relates the seismic efficiency of an impact to its determining parameters. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 2, pp. 3–12, March–April, 2007.     

Miniaturized Compression Test at Very High Strain Rates by Direct Impact

A modified miniaturized version of the Direct Impact Compression Test (DICT) technique is described in this paper. The method permits determination of the rate-sensitive plastic properties of materials up to strain rate ∼10⁵ s⁻¹. Miniaturization of the experimental setup with specimen dimensions: diameter d _S = 2.0 mm and thickness l _S = 1.0 mm, Hopkinson bar diameter 5.2 mm, with application of a novel optical arrangement in measurement of specimen strain, makes possible compression tests at strain rates from ∼10³ s⁻¹ to ∼10⁵ s⁻¹. In order to estimate the rate sensitivity of a low-alloy construction steel, quasi-static, Split Hopkinson Pressure Bar (SHPB) and DICT tests have been performed at room temperature within the rate spectrum ranging from 5*10⁻⁴ s⁻¹ to 5*10⁴ s⁻¹. Adiabatic heating and friction effects are analyzed and the final true stress versus true strain curves at different strain rates are corrected to a constant temperature and zero friction. The results have been analyzed in the form of true stress versus the logarithm of strain rate and they show two regions of a constant rate sensitivity : relatively low up to the strain rate threshold ∼50 s⁻¹, and relatively high above the threshold, up to strain rate ∼4.5*10⁴ s⁻¹.     

微重力环境下硅基纳米器件碰撞滑动接触问题研究

随着纳米技术的发展,微机电系统被广泛应用于微纳卫星、皮卫星以及各种高精密仪器. 单晶硅广泛应用于微机电系统,考虑微重力环境空间机构无规则碰撞的运动特性,建立刚性金刚石压头与弹性硅基体之间碰撞滑动接触的分子动力学模型,对比研究压头不同振动频率和振幅对平均摩擦力的影响. 结果表明:压头振动频率低于基体固有频率时,平均摩擦力无明显变化,而高于固有频率时,平均摩擦力随振动频率增大呈现先减小后不变的趋势;振幅的增大导致压头和基体的碰撞更加剧烈,剧烈的碰撞导致基体表面更多原子晶格结构破坏,失效原子数增多,降低了平均摩擦力;在基体表面引入纹理,发现纹理表面能够有效降低平均摩擦力.      

Transient elongational viscosities of aqueous polyacrylamide solutions measured with an optical rheometer

An anionic polyacrylamide solution was characterized in elongational flow by combining laser-Doppler velocimetry to determine the strain rate in the flow direction and the two-color flow-induced birefringence method to measure the first normal stress difference along the axial centerline of a hyperbolic die. The elongational rate was constant along the axial centerline of the planar hyperbolic die as long as vortices at the die entrance did not occur. The transient elongational viscosity μ ⁺ was determined as a function of the elongational rate. The parameters varied are the Hencky strain rate and the polymer concentration. μ ⁺ showed a pronounced increase over the linear viscoelastic behavior above critical Hencky strains of 1.2 to 1.5; that is, a significant strain hardening could be observed for polyacrylamide solutions. This strain hardening is stronger the higher the elongational rate. A slight enhancement of strain hardening was found by increasing the concentration from 0.5 to 1 g/l. The stress optical coefficient was determined as 1.8 × 10⁻⁷ Pa⁻¹ (0.5 g/l) and 1.2 × 10⁻⁷ Pa⁻¹ (1 g/l).

A Hybrid Experimental/Numerical Investigation of the Response of Multilayered MEMS Devices to Dynamic Loading

In order to probe the mechanical response of microelectromechanical systems (MEMS) subjected to dynamic loading, a modified split Hopkinson pressure bar was used to load MEMS devices at accelerations ranging from 10³–10⁵  g. Multilayer beams consisting of a PZT film sandwiched between two metal electrodes atop an elastic layer of silicon dioxide were studied because of their relevance to active MEMS devices. Experiments were conducted using the modified split Hopkinson pressure bar to quantify the effects of dynamic loading amplitude, duration, and temporal profile on the failure of the multilayered cantilever beams. Companion finite element simulations of these beams, informed by experimental measurements, were conducted to shed light into the deformation of the multilayered beams. Results of the numerical simulations were then coupled with independent experimental measurements of failure stress in order to predict the material layer at which failure initiation occurred, and the associated time to failure. High-speed imaging was also used to capture the first real-time images of MEMS structures responding to dynamic loading and successfully compare the recorded failure event with those predicted numerically.     

Temperature and deformation measurements in transient metal cutting

Metal cutting is a thermomechanically coupled process in which plasticity induced heating and friction play a critical role. The objective of this work is to develop a methodology to understand and quantify this coupling. Temperatures of the workpiece and the chip during transient cutting processes are measured using a linear array of 16 InSb infrared detectors with 200 ns rise time and 27 μm spatial resolution. Three different materials, 1018 CR steel, Al6061-T6 and Ti-6Al-4V, are tested at a cutting speed of 4.3 m s⁻¹. A grid method is used to measure deformations during the above set of experiments. Measured values of temperature and deformation are compared to results of finite element simulations of the experiments.     

A Device Enhancement for the Dry Sliding Friction Coefficient Measurement Between Steel 1080 and VascoMax with Respect to Surface Roughness Changes

An enhancement of an existing tribometer device developed by Philippon et al. (Wear 257:777–784, 2004) is presented in this work. This experimental device is made up of a dynamometer ring and a specific load sensor allowing to apply an apparent normal force on specimens and to measure frictional forces respectively. A set of strain gauges are added to the upgraded dynamometer ring in this new configuration. The apparent normal force can be recorded accurately during the sliding process. The setup is adapted on a hydraulic testing machine to carry out steel-on-steel dry sliding tests. The first set of standard Steel on standard Steel specimens (XC 38 French standard steel) with two apparent normal pressures are imposed (8 and 80 MPa) as the range of sliding velocities varies from 0.12 to 3.72 m/s for the same contact conditions. The main set of experiments with low sliding velocities (varying from 0 to 3 m/s) for the Steel 1080 on Steel VascoMax are performed in the same tested setup. The recordings of normal and tangential forces leading to the friction coefficient determination are discussed. The values of dry friction coefficient μ according to the experimental parameters are in good agreement with those observed in the literature. Using this new configuration, the effects of the sliding velocity on the surface roughness changes and on the dry fiction coefficient are also investigated. Additionally the surface roughness changes are also investigated. Performing the scans with use of the scanning electron microscope in particular locations of the specimens show the roughness decrease and reveal the occurrence of the wear phenomenon. Moreover, very interesting relations between wear and sliding velocity are observed.     

A fibre sensor based frequency analysis of surface waves at hollow cone nozzles

The characterization of the disintegration process caused by sheet break up, e.g. at hollow cone nozzles (HCN) is of fundamental importance for a better understanding of the drop formation process. This paper introduces our new measuring device capable of resolving surface wave formation and propagation on the sheet along main streamlines of the flow. Furthermore, it reveals the development of adapted algorithms for the data and image evaluation. Two different frequency ranges are found for the oscillations on the sheet at higher viscosities, showing a superposition of different spectra in each case. The wave bands in the lower frequency range 10<f _low<80 s⁻¹ are considered to be irrelevant for the disintegration process. Surface waves with higher energy content at given conditions are found within a range 300<f _high<1,500 s⁻¹ probably leading to sheet disintegration. Results obtained from a photo-based evaluation show good agreement compared to the frequency analysis.