Dynamic elastic constants of anisotropic materials by resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy (RUS) has been applied to the anisotropic elastic stiffness determination of SiC/Al composites and highly textured Zr-2.5 Nb alloys. To determine the elastic stiffness of anisotropic materials, the resonant frequencies of a rectangular parallelepiped specimen were measured and compared with the calculated frequencies based on the input data of the estimated stiffness, dimensions, and density. The initial estimates of the elastic stiffness of SiC/Al composites were calculated using the Mori-Tananka (MT) theory and the concept of effective aspect ratio of reinforcements. For highly textured Zr-2.5 Nb alloy, the initial estimates were obtained from its orientation distribution function, determined by X-ray diffraction, and the reported elastic stiffness of a single crystal zirconium. Through a comparison of calculated frequencies with those measured by RUS, elastic stiffness values have been determined very accurately by iteration and convergence processes.     

Stochastic identification of elastic constants for anisotropic materials

This paper presents an energy‐based characterization technique that stochastically identifies the elastic constants of anisotropic materials by modeling the measurement noise and removing its effect unlike conventional deterministic techniques, which deterministically identify the elastic constants directly from noisy measurements. The technique recursively estimates the elastic constants at every acquisition of measurements using Kalman Filter. Owing to the non‐linear expression of the measurement model, a Kalman gain has been newly derived and achieves optimal estimation. Since the variances in addition to the means are computed, the proposed technique can not only identify the elastic constants but also describe their certainty as an additional advantage. The validity of the proposed technique and its superiority to the conventional technique were first demonstrated via parametric studies of low‐dimensional problems. The proposed technique was then successfully applied to the identification of elastic constants of an anisotropic material. Copyright © 2009 John Wiley & Sons, Ltd.     

Study by resonant ultrasound spectroscopy of the elastic constants of the β phase in Cu---Al---Ni shape memory alloys

The evolution with temperature of the elastic constants of the metastable β phase in a Cu-27.96 at.%Al-3.62 at.% Ni shape memory alloy (SMA) has been studied by resonant ultrasound spectroscopy (RUS). The corresponding elastic constants of this cubic phase have been measured near the martensitic transformation temperature in a single crystal. Above the martensitic transformation temperature, an anomalous behavior has been found in the C′elastic constant which shows a softening as the temperature decreases. The internal friction value Q⁻¹ has been obtained in this temperature range from the RUS spectra. The mechanisms associated with the Q⁻¹ increase must be related to {1 1 0} shear.     

Optical constants of CdS thin films prepared by spray pyrolysis

CdS thin films were prepared by spray pyrolysis techniques. Variable angle spectroscopic ellipsometry was used for optical constant calculations. Multiple angle measurements were taken in the most sensitive angle of incidence region. The sensitive regions of angle of incidence were obtained theoretically using 3-dimensional graph ofδψ andδΔ. Real partn and imaginary partk of the complex refractive index of the samples were calculated in the wavelength range 470–650 nm, taking into account surface roughness. Bruggeman’s effective medium approximation is used for analysis of the surface rough layer of the thin films.     

氧化锌薄膜的拉曼光谱研究

利用拉曼光谱结合X射线衍射分析对未掺杂和掺杂的ZnO薄膜，陶瓷薄膜进行了研究，ZnO薄膜及ZnO陶瓷薄膜均由sol-gel法制备，掺杂组份有Bi2O3,Sb2O3,MnO和Cr2O3等。结果表明，未掺杂的薄膜的ZnO主晶相均表现出显著的定向生长特征，其拉曼光谱特征谱峰为437cm^-1，谱峰强度随薄膜退火温度的提高略有增强，掺杂后ZnO的拉曼谱峰发生了红移，掺Bi2O3后ZnO的拉曼谱峰由347cm^-1移质移至434cm^-1，掺Sb2O3后ZnO的拉曼谱峰移至435cm^-1，而掺杂Bi2O3,Sb2O3,MnO和Cr2O3等组份的ZnO陶瓷薄膜的ZnO拉曼谱峰则移至434cm^-1，说明掺杂元素进入了ZnO晶格，引起了晶格的变化，ZnO薄膜性能不仅受次晶相组成的影响，而且受因掺杂元素进入而引起的ZnO晶格畸变的影响。     

Numerical calculations of elastic wave propagation in anisotropic thin films deposited on substrates

In this work, we present a generalized numerical model to study the dispersion curves of elastic waves in anisotropic thin layer/substrate systems. Dispersion curves as a function of wave frequency for different propagation angles are calculated. The nature of the mode (Rayleigh or Love) is identified by analyzing the polarization of the three-dimensional displacement field. This numerical model has been applied to the system Au(0 0 1)/Ni(0 0 1).     

准分子激光引起的非晶硅薄膜晶化行为的研究

利用KrF准分子激光对非晶硅薄膜的表层进行了晶化.研究了激光能量密度和照射脉冲数对薄膜结晶度的影响,并对晶化后薄膜的形貌和结构进行了表征.结果表明:该非晶硅薄膜晶化阈值约为110 mJ/cm2,且不受照射脉冲数的影响;激光能量密度是影响薄膜结晶度的首要因素,但在较低的能量密度时,增加照射脉冲数也会显著的提高薄膜结晶度;结构及形貌表征发现,薄膜晶化层厚度约为400～500 nm,平均晶粒尺寸为30～50 nm.     

Modelling of anisotropic elastic properties in alloy 718 built by electron beam melting

ABSTRACT

Owing to the inherent nature of the process, typically material produced via electron beam melting (EBM) has a columnar microstructure. As a result of that, the material will have anisotropic mechanical properties. In this work, anisotropic elastic properties of EBM built Alloy 718 samples at room temperature were investigated by using experiments and modelling work. Electron backscatter diffraction data from the sample microstructure was used to predict the Young’s modulus. The results showed that the model developed in the finite element software OOF2 was able to capture the anisotropy in the Young’s modulus. The samples showed transversely isotropic elastic properties having lowest Young’s modulus along build direction. In addition to that, complete transversely isotropic stiffness tensor of the sample was also calculated.

This paper is part of a thematic issue on Nuclear Materials.     

First-principles calculations of elastic constants for epsilon-carbide and the consequences

ABSTRACT

The elastic constants of ?-carbide at 0?K and zero pressure have been calculated using first-principle methods. The iron to carbon ratio for the carbide is not established and may, in fact, vary between 3 and 2. As a consequence, the calculations have been conducted as a function of the chemical composition using the special quasi-random structures method. In all cases, the elastic constants obtained are consistent with ?-carbide being mechanically stable. The analysis indicates that in comparison with cementite, ?-carbide should be more brittle; that when present as a precipitate in steel, it would be less effective in participating in the overall plastic deformation, and that its coherency strain field in the surrounding matrix should be less extensive than cementite.     

AlN薄膜的KrF准分子脉冲激光沉积

研究了采用脉冲激光沉积(PLD)技术在Si(100)衬底上AlN薄膜的制备及其性质。结果表明,衬底温度从室温到800℃的范围内,所得到的AlN薄膜为(002)择优取向的纤锌矿结构。随着衬底温度的升高,AlN薄膜从纳米晶结构转为多晶结构,同时表面微粗糙度上升。AlN晶粒呈柱状生长机制。     

Optical emission spectroscopy as a process control tool during plasma enhanced chemical vapor deposition of microcrystalline silicon thin films

The decisive criterion associated with the species emission intensity ratio (Hα/SiH*) which characterizes the crystallinity of microcrystalline silicon (μc-Si) film was found to display an unstable behavior resulting from species concentration variation during μc-Si film growth with optical emission spectroscopy (OES) tool. In this study, a real-time process control system i.e. closed-loop system was developed. It aims to control the species intensity ratio with OES device in a very high frequency (VHF) plasma enhanced chemical vapor deposition reactor, via modulating the VHF power and silane dilution to improve μc-Si film growth for high efficiency a-Si/μc-Si tandem solar cell. The experiment results show that the closed-loop system stabilized the Hα/SiH* intensity ratio within a variation of 5% during the μc-Si film deposition process. Higher growth rate of μc-Si film with the same crystallinity was obtained in the closed loop system which consumed less power and SiH4 gas than in the open loop system, i.e. without process control.     

铁电薄膜退火方法的研究

以锆钛酸铅薄膜（PZT）为例，分析了国内外铁电薄膜退火的各种方法。针对解决铁电薄膜基底高温生长工艺与硅集成电路承受温度较低的不兼容及器件性质劣化的难题，分别对普通炉子退火、快速热退火及激光退火进行了详细的分析比较。激光低温退火技术有望成功地在未来应用PZT铁电薄膜制作组件时，增加其制备工艺设计的弹性和可行性。     

脉冲激光沉积法制备Pt薄膜的研究

采用脉冲激光沉积技术在(0001)取向的蓝宝石基片上外延生长了Pt单晶薄膜,研究了沉积温度和激光能量对Pt薄膜的晶体结构,表面形貌及电学性能的影响规律.X射线衍射(XRD)分析结果表明,在沉积温度650℃、激光脉冲频率1Hz和激光能量280mJ的条件下,制备得到的Pt(111)单晶薄膜,其(111)面ω摇摆曲线半高宽(FWHM)仅为0.068°.原子力显微镜(AFM)分析表明外延的Pt薄膜表面具有原子级平整度,其表面均方根粗糙度(RMS)约为1.776nm.四探针电阻测试结果显示薄膜方阻为1/962Ω/□,满足铁电薄膜的制备工艺对Pt底电极的要求.     

基片表面粗糙度对坡莫合金AMR以及磁性能的影响

利用霍尔离子源轰击Si基片获得了不同表面粗糙度的基片,然后利用磁控溅射方法制备了一系列Ta(5nm)/Ni 80Fe 20(12nm)/Ta(2nm)薄膜样品,重点研究了基片表面粗糙度对薄膜结构和各向异性磁阻效应的影响。采用AFM测量基片的表面粗糙度,采用四探针法测量薄膜的各向异性磁阻效应。研究结果表明,随着基片表面粗糙度的增加,坡莫合金的AMR值显著降低,且ΔH显著增加。     

A1N薄膜的KrF准分子脉冲激光沉积

研究了采用脉冲激光沉积（PLD）技术在Si（100）衬底上A1N薄膜的制备及其性质。结果表明，衬底温度从室温到800℃的范围内，所得到的A1N薄膜为（002）择优取向的纤锌矿结构。随着衬底温度的升高，A1N薄膜从纳米晶结构转为多晶结构，同时表面微粗糙度上升。A1N晶粒呈柱状生长机制。     

提高碳纤维增强树脂基复合材料弹性常数超声表征精度的方法

为提高碳纤维增强树脂基复合材料（Carbon Fiber Reinforced Plastic,CFRP）单向板弹性常数的超声表征精度,基于超声液浸背反射法开展了实验测量和数值模拟。首先,搭建了角度旋转测试装置,并结合信号互相关处理技术实现了不同方向上的延时测量和声速计算;然后,通过模拟退火优化算法反演声速数据,得到了5个独立弹性常数;最后,利用理论计算和CIVA软件模拟仿真对实验测量结果进行了验证。结果表明:1-3平面内准纵波和准横波声速随入射角度而变化,与1-2平面内的表现出明显差异,由此显示材料为横观各向同性的;独立弹性常数分别为12.88、6.39、126.83、4.85、2.97 GPa;理论计算、CIVA软件模拟仿真及实验测量结果的声速偏差小于6.4 m/s,实验数据波动范围小于0.33%,对应的弹性常数误差均小于0.47%。相关方法同样适用于其他正交各向异性介质的弹性常数测试。      

Femtosecond laser spectroscopy of spins: Magnetization dynamics in thin magnetic films with spatio-temporal resolution

Based on the Magneto-Optical Kerr Effect (MOKE), we have developed an experimental set-up that allows us to fully characterize the magnetization dynamics in thin magnetic films by measuring all three real space components of the magnetization vector M. By means of the pump-probe technique it is possible to extract the time dependence of each individual projection with sub-picosecond resolution. This method has been exploited to investigate the temporal evolution of the magnetization (modulus and orientation) induced by an ultrashort laser pulse in thin epitaxial iron films.According to our results, we deduced that the initial, sub-picosecond demagnetization is established at the electronic level through electron-magnon excitations. The subsequent dynamics is characterized by a precessional motion on the 100 ps time scale, around an effective, time-dependent magnetic field. Following the full dynamics of M, the temporal evolution of the magneto-crystalline anisotropy constant can be unambiguously determined, providing the experimental evidence that the precession is triggered by the rapid, optically-induced misalignment between the magnetization vector and the effective magnetic field.These results suggest a possible pathway toward the ultrarapid switching of the magnetization.     

Investigation of hexadecanethiol self-assembled monolayers on cadmium tin oxide thin films

This study reports the use of variable angle reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy to investigate the formation of a 1-hexadecanethiol adlayer on cadmium tin oxide (CTO) thin film surfaces. These adlayers appear to be robust, ordered monolayers. The optical and electronic properties of CTO thin films chemically vapor deposited onto glass substrates were also investigated. The reflectance of the CTO films was dependent upon the incident angle of the impinging radiation and revealed a reflectance decrease indicative of a plasma frequency in the mid-IR using p-polarized radiation.     

二维机织复合材料弹性常数的有限元法预测

为了预测二维机织复合材料的弹性性能,建立了有限元力学分析模型。基于二维机织复合材料的几何特征,建立了参数化的单胞模型;考虑了织物纤维束呈现出的各向异性材料特征,将有限元中材料主方向转化到纤维屈曲方向,建立其力学分析有限元模型;分析了单胞边界面保持平面假设的不足,提出了对于二维机织复合材料通用的周期边界条件,获得了更为准确的二维机织复合材料的工程弹性常数。结果表明:织物衬垫单胞边界面,在单向拉伸载荷和纯剪切载荷下,呈凹凸翘曲变形,即为周期边界;应用给出的织物参数化几何建模方法与有限元求解方法,可以精确地获得工程弹性常数,数值计算结果与实验值吻合较好。