Strain status of epitaxial Ge film on a Si (001) substrate

An epitaxial Ge film was grown on a Si (001) substrate via a two-step process through the molecular beam epitaxy technique. The strain status of non-annealed and annealed epitaxial Ge films was determined by X-ray diffraction, Raman spectroscopy, and a combination of high-resolution transmission electron microscopy and geometric phase analysis. Results showed that the strain in non-annealed and annealed epitaxial Ge films is nonhomogeneous from the Ge/Si interface to the Ge film surface. The strain parallel to the interface in the non-annealed epitaxial Ge film is compressive; this strain reaches a minimum near the surface and a maximum at the interface. By contrast, the strain parallel to the interface in the annealed epitaxial Ge film is tensile; this strain reaches a minimum at the interface and a maximum near the surface.     

Measuring electrostatic properties of fibrous materials: A review and a modified surface potential decay technique

Electrostatic charging of fibrous materials are of concern for the performance of electret filters and comfort issues of textiles. However there is a huge controversy in the characterization techniques of ion beam irradiated or corona charged fibrous materials. In this study we reported a reliable, simple surface potential measurement method. Large variations in potential measurements were found to be mostly due to structural nonuniformity such as packing density, thickness and fiber–fiber proximity. Test samples were prepared after optimizing those parameters and we were able to reduce coefficient of variation below 15%. Methods that were developed so far were also reviewed.     

Application of optical 3D measurement on thin film buckling to estimate interfacial toughness

The shape-from-focus (SFF) method has been widely studied as a passive depth recovery and 3D reconstruction method for digital images. An important step in SFF is the calculation of the focus level for different points in an image by using a focus measure. In this work, an image entropy-based focus measure is introduced into the SFF method to measure the 3D buckling morphology of an aluminum film on a polymethylmethacrylate (PMMA) substrate at a micro scale. Spontaneous film wrinkles and telephone-cord wrinkles are investigated after the deposition of a 300 nm thick aluminum film onto the PMMA substrate. Spontaneous buckling is driven by the highly compressive stress generated in the Al film during the deposition process. The interfacial toughness between metal films and substrates is an important parameter for the reliability of the film/substrate system. The height profiles of different sections across the telephone-cord wrinkle can be considered a straight-sided model with uniform width and height or a pinned circular model that has a delamination region characterized by a sequence of connected sectors. Furthermore, the telephone-cord geometry of the thin film can be used to calculate interfacial toughness. The instability of the finite element model is introduced to fit the buckling morphology obtained by SFF. The interfacial toughness is determined to be 0.203 J/m² at a 70.4° phase angle from the straight-sided model and 0.105 J/m² at 76.9° from the pinned circular model.     

湍流边界层对数律区相干结构的Tomo-TRPIV实验研究

应用层析高时间分辨率粒子图像测速技术（tomographic time-resolved particle image velocimetry，Tomo-TRPIV），测量了Re_θ=2 460的平板湍流边界层三维3分量瞬时速度场的时间序列.提出湍流空间多尺度局部平均速度结构函数的概念，应用流向脉动速度沿流向的空间多尺度局部平均速度结构函数的正负过零点法，从瞬时脉动速度场中检测相干结构的喷射和扫掠事件，对检测到的喷射和扫掠事件的瞬态局部速度场、速度梯度场、涡量场、速度变形率场进行空间相位对齐叠加平均，获得喷射和扫掠事件的局部速度场、速度梯度场、涡量场、速度变形率场的典型特征.研究发现，相干结构喷射和扫掠时，速度梯度、速度变形率、涡量均表现为空间反对称分布的4极子结构.特别是流向涡量是沿流向、法向、展向均为反对称分布的法向多层4极子结构，表明法向各层相干结构是紧密联系，互相关联的.这种法向多层的4极子反对称结构导致强烈的动量、质量和能量交换，维持了相干结构的演化和发展过程.      

Experimental investigation on surface deformation of soft half plane indented by rigid wedge

In this work, the mechanical behavior of a block of soft material subject to large deformation from a series of wedge-shaped indenters is evaluated. Data fields acquired from digital image correlation (DIC) are compared with the existing theoretical models. The slope angles of the wedges vary from 5° to 73.5°, and the minimum measurement uncertainties of the DIC system are established in advance to define the accuracy. It is concluded that the assumptions underpinning the analytical theory make it difficult to characterize large deformation of soft materials during contact. The strain fields are also obtained from the measured displacement field and verify the previously postulated existence of two deformation sectors, namely, a so-called shrinkage sector symmetric to the loading axis and an expansion sector, which become smaller with the increasing load and decreasing wedge angle.     

TR PIV INVESTIGATION ON THE LAGRANGIAN HAIRPIN VORTICES INDUCED BY A CUBIC DISTURBANCE

运用时分粒图仪 (time-resolved particle imagevelocimetry)对平板边界层中立方体扰动诱导产生的发卡涡涡结构进行了实验测量, 得到了立方体扰动尾迹区中展向平面内的速度矢量分布.基于拉格朗日坐标系, 运用李亚普诺夫指数对流场中的相干结构进行了辨识, 比较欧拉坐标系下的结果发现拉格朗日体系下李亚普诺夫指数场不仅有效地包含欧拉坐标系下涡强所表征的发卡涡涡头信息外, 并且能够呈现出涡颈在空间中的分布, 能够完全刻画出发卡涡涡结构在展向平面内的信息, 还发现李亚普诺夫指数对流场拟序结构的表征有很好的鲁棒性.另外, 李亚普诺夫指数场的互相关分析表明诱导产生的发卡涡涡结构随着向下游的传播, 其涡头不断抬升, 发卡涡涡结构与壁面的倾角也逐渐变大, 有效地刻画了拟序结构的空间演化过程.     

Improvement for expansion of parabolized stability equation method in boundary layer stability analysis

An improved expansion of the parabolized stability equation(iEPSE) method is proposed for the accurate linear instability prediction in boundary layers. It is a local eigenvalue problem, and the streamwise wavenumber α and its streamwise gradient dα/dx are unknown variables. This eigenvalue problem is solved for the eigenvalue dα/dx with an initial α, and the correction of α is performed with the conservation relation used in the PSE. The i EPSE is validated in several compressible and incompressible boundary layers. The computational results show that the prediction accuracy of the i EPSE is significantly higher than that of the ESPE, and it is in excellent agreement with the PSE which is regarded as the baseline for comparison. In addition, the unphysical multiple eigenmode problem in the EPSE is solved by using the i EPSE. As a local non-parallel stability analysis tool, the i EPSE has great potential application in the eNtransition prediction in general three-dimensional boundary layers.     

Shock tube study of the interaction between ammonia and nitric oxide at high temperatures using laser absorption spectroscopy

The interaction between ammonia (NH₃) and nitric oxide (NO) at high temperatures is studied in this work using a shock tube combined with laser absorption diagnostics. The system simultaneously measured the NH₃ and NO time-histories during the reaction processes of the shock-heated NH₃/NO/CO/Ar mixtures (NH₃:NO ≈ 0.9:1.0 and 1.4:1.0). The absorption cross-sections of NH₃ near 1122.10 cm^–1 and NO at 1900.52 cm^–1 (characterized in this study) were used for measuring NH₃ and NO time-histories with the temperature measured by two CO absorption lines. The measured NH₃ and NO time-histories at 1614–1968 K and 2.4–2.8 atm were compared with predictions of seven recent kinetics models. The predictions that based on different mechanisms are very different and the measured profiles are within the range of the predictions. The Glarborg, NUI Galway Syngas-NOx, and Mathieu mechanisms give the closest predictions to the measurements. Kinetics analyses indicate that the NH₃ and NO consumption rates are extremely sensitive to the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH, which are more reliably represented in the Glarborg and NUI Galway Syngas-NOx mechanisms. The performances of Glarborg mechanisms at lower initial temperatures can be apparently improved by revising the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH. These two reactions are also the primary pathways for NO reduction and NH₃ is mainly consumed via NH₃ + OH = NH₂ + H₂O and NH₃ + H = NH₂ + H₂. Trace amounts of NO₂ and N₂O impurities decompose to form O radical followed by the generation of OH radical via H-abstraction reactions, which significantly affects the predictions of NH₃ and NO according to kinetics analyses.     

Analysis on diffusion-induced stress for multi-layer spherical core–shell electrodes in Li-ion batteries

Silicon-based carbon composites are believed as promising anodes in the near future due to their outstanding specific capacity and relatively lower volume effect compared to pure silicon anodes. Herein, a multilayer spherical core–shell(MSCS) electrode with a graphite framework prepared with Si@O-MCMB/C nanoparticles is developed, which aims to realize chemically/mechanically stability during the lithiation/delithiation process with high specific capacity. An electrochemical-/mechanical-coupling model for the M-SCS structure is established with various chemical/mechanical boundary conditions.The simulation of finite difference method(FDM) has been conducted based on the proposed coupling model, by which the diffusion-induced stress along both the radial and the circumferential directions is determined. Moreover, factors that influence the diffusion-induced stress of the M-SCS structure have been discussed and analyzed in detail.     

湍流边界层相干结构剩余脉动雷诺应力项的实验测量

用IFA300恒温热线风速仪和X形二分量热线探针以采样间隔小于湍流耗散时间尺度的分辨率精细测量了风洞中平板湍流边界层不同法向位置的瞬时流向、法向速度分量的时间序列信号。用条件采样和相位平均技术提取了相干结构猝发过程中相干结构剩余脉动雷诺应力和随机脉动对相干结构贡献的雷诺应力的条件相位平均波形。基于理论上对湍流相干结构动量方程中随机脉动对相干结构贡献的雷诺应力和相干结构剩余脉动雷诺应力项的分析,对两种雷诺应力项进行了对比研究。研究发现,相干结构剩余脉动雷诺应力项在数值上具有和随机脉动对相干结构贡献的雷诺应力相同的数量级,表明在相干结构动力学模型方程中,相干结构剩余脉动雷诺应力项并不像以前估计的那样可以忽略不计。