环境扫描电子显微镜中真空系统特点及成像信号分析

环境扫描电子显微镜(ESEM)是针对常规扫描电子显微镜(SEM)在使用过程中暴露出来的一些重大缺陷而设计的.它淘汰了SEM中繁琐、复杂的样品准备工作,扩大了仪器的使用范围,并使图像更清晰.着重介绍了ESEM的工作原理、与此有关的真空系统构成及成像信号分析.     

调频信号发射超声成像的参数优化与成像实验

研究调频信号发射超声成像中影响成像质量的各种参数,提高现有超声图像的信号噪声比.用仿真计算和声学实验来研究不同调频超声信号发射参数下的成像质量.增加超声发射能量,提高图像的信号噪声比;同时,采用相关算法对接收的超声信号解码,保证图像的轴向精度不会下降.优化了调频信号发射超声成像的发射参数,并得到了仿体的B型超声图像,此图像的质量优于传统成像方法的图像质量.该成像方法能够提高超声图像的信号噪声比,尤其是提高超声衰减严重的深部组织图像的信号噪声比.     

一种用于水声运动目标成像的空间散射模型

水声成像过程中为了分析所接收的运动目标回波、理解接收信号的形成机理,提出了一种用于该情况的空间散射模型,并利用此模型仿真了"T"型阵成像声呐接收的回波信号。对信号的分析说明了空间散射模型的正确性。以空间散射模型为基础,利用傅里叶变换波束形成算法对不同条件下的球体目标进行了水声成像,并分析了成像性能。性能分析说明了水声目标大小、距离等因素对成像的影响情况,还特别强调了在运动情况下目标的成像情况。以上工作完成了对"T"型阵水声成像过程的模拟,为成像过程回波信号研究和成像声呐研制提供了理论基础和仿真手段。     

应用CZT成像算法处理非线性调频SAR信号

在雷达领域,非线性调频(NLFM)信号正越来越广泛地被应用．对于综合孔径雷达(SAR),利用NLFM脉冲压缩既可改善脉压系统性能,又可提高系统的安全性和抗干扰性,但是如何应用现有的成像算法处理NLFM SAR信号仍少有研究．本文提出了一种改进的Chirp Z-Transform(CZT)算法．该算法适用于处理NLFM SAR信号．作者从NLFM函数族中选取了一组NLFM信号进行处理,不失普遍性地证明了应用改进的CZT算法处理NLFM信号和线性调频(LFM)信号,可以得到相同的距离单元迁移(RCM)校正效果．     

基于ADSP—TS201的实时成像处理器

本文介绍基于ADSP—TS201信号处理板实现的实时成像处理系统。对使用不同硬件资源情况下的成像性能进行了分析。     

基于Shannon 复数小波的复合材料结构时间反转聚焦多损伤成像方法

研究了一种基于压电传感器阵列和主动Lamb 波的结构损伤成像方法,有助于克服Lamb 波在板结构中、特别是在复合材料板结构中存在的频散、多种模式及模式转换的现象给结构健康监测带来的困难。分析了结构多损伤散射信号的时间反转聚焦原理,在此基础上提出了一种基于Shannon 复数小波和时间反转聚焦的信号合成成像方法。该方法中,确定Lamb 波响应信号的到达时刻是信号能够准确聚焦的关键因素之一。提出了利用Shannon 复数小波变换计算Lamb 波响应信号到达时刻的方法。在碳纤维复合材料板结构上对整套信号合成成像方法进行了验证。研究结果表明,该方法能够有效地对同一个监测区域中的多个损伤进行成像定位。相对于30 cm ×30 cm 的监测区域,定位误差不超过2 cm。该方法有助于结构健康监测技术的工程应用。      

基于阵列传感器的复合材料结构损伤MUSIC成像方法

提出了一种基于传感器线阵的多重信号分类(Multiple Signal Classification,MUSIC)损伤成像方法用于航空复合材料的损伤监测。该方法采用MUSIC阵列信号处理方法,通过对传感器阵列信号进行协方差特征值分解,在结构上进行方向扫描并构建监测区域的空间谱,从而实现对结构损伤的成像,具有一维传感器阵列易于布置的优点。所提出的方法在变厚度航空复合材料油箱结构上进行了验证,结果表明,该方法能够准确实现航空复合材料结构上的损伤成像,定位误差小于2cm。     

基于改进空间滤波器的复合材料结构损伤成像方法

为降低Lamb波在复合材料结构中传播时存在的频散现象和各方向传播特性的不一致性给损伤监测带来的困难, 提出一种利用Hilbert变换改进的空间滤波器对复合材料结构损伤进行成像的方法。分析了基于空间滤波器的信号合成成像原理, 在此基础上利用Hilbert变换构造传感器时域响应信号的解析信号参与到信号合成成像过程中。通过对仿真声源的成像实验, 验证了该方法的可行性, 通过对碳纤维复合材料板结构上激励源和损伤的成像实验, 验证了该方法的功能。研究表明该方法能够识别损伤相对于压电传感器阵列的角度位置和损伤散射信号的到达时刻。      

超声相控阵全聚焦无损检测技术概述

超声相控阵全聚焦成像技术基于接收信号后处理的思想,对检测回波数据进行离线分析成像,是超声检测领域里的一项新技术。因其成像分辨率高、覆盖面广、对小缺陷灵敏度高等优势,在航空航天、高铁、石油管道、核电站等工业领域已有初步应用。文章介绍了全聚焦成像技术的检测原理及优缺点,介绍了其国内外发展历程,总结了研究热点及发展趋势。     

基于量子关联的显微成像研究进展

简要回顾了量子关联成像的基本原理和发展历程,从量子光源和经典光源的角度详细介绍了量子关联成像在显微成像中的研究进展。做出了基于经典源的量子关联成像因易于实施、成本较低,在显微成像中更具应用前景的判断。     

Research on Damage Localization of Plate-like Structure Using Mode Acoustic Emission and Matching Pursuit

Based on mode acoustic emission theory,the paper analyses the acoustic emission analog signal of thin steel plate using matching pursuit,then obtains the characteristics interpretation of the different frequency signal energy concentration degree; Combined with four-point arc positioning method,the papers researches the damage localization of the plate-like structure. Simulation experiment shows that this method can accurately detect and locate the damage. This can provide data support for further imaging research based on time reverse theory.     

Monte Carlo simulation of the electron beam scattering under gas mixtures environment in an HPSEM at low energy

The effects of various gas environment used in high pressure SEM inside the specimen chamber were investigated using Monte Carlo simulation. In order to improve the signal to noise ratio for the electron detection, we suggest to use helium gas-based mixture. The Helium gas is well known to reduce the skirt effect due to its low elastic scattering cross-section. The addition of an ionizing gas such as hydrogen or nitrogen is proposed to increase the inelastic scattering cross section which is mainly responsible for the ionisation process taking place during the beam-gas interactions. For all the mixtures (except He-Argon), the main results show that the skirt is slightly modified with the increase of the pressure. For the BSE detection, the signal to noise can remain high and gives a good contrast in imaging. Moreover, the presence of an ionizing gas will favour the ionizing process which is very important in beam-based electron detection. In this case, an increase of the signal to noise ratio can be expected.     

基于深度学习的超声成像技术研究现状

超声成像技术以其无损、无辐射、实时性好、成像深度深和检查费用低等优点,是目前临床医学最常应用的影像技术之一。随着医疗技术的日益发展,兼顾高帧率和高质量的超声成像已成为临床上的迫切需求。深度学习作为能快速提取信号特征的技术,近年来已经在超声成像领域展开多种应用研究并产生了较好的效果,具有很大的应用前景。文章总结了基于深度学习的超声成像技术现状,重点介绍了超声成像技术中常用深度学习架构,列举了深度学习技术在超声成像中的应用,最后总结了目前深度学习在训练以及实现临床应用中可能会遇到的挑战。     

Chemical Imaging Beyond the Diffraction Limit: Experimental Validation of the PTIR Technique

Photothermal induced resonance (PTIR) has recently attracted great interest for enabling chemical identification and imaging with nanoscale resolution. In this work, electron beam nanopatterned polymer samples are fabricated directly on 3D zinc selenide prisms and used to experimentally evaluate the PTIR lateral resolution, sensitivity and linearity. It is shown that PTIR lateral resolution for chemical imaging is comparable to the lateral resolution obtained in the atomic force microscopy height images, up to the smallest feature measured (100 nm). Spectra and chemical maps are produced from the thinnest sample analyzed (40 nm). More importantly, experiments show for the first time that the PTIR signal increases linearly with thickness for samples up to ≈ 1 μm (linearity limit); a necessary requirement towards the use of the PTIR technique for quantitative chemical analysis at the nanoscale. Finally, the analysis of thicker samples provides the first evidence that the previously developed PTIR signal generation theory is correct. It is believed that the findings of this work will foster nanotechnology development in disparate applications by proving the basis for quantitative chemical analysis with nanoscale resolution.     

Secondary electron hyperspectral imaging: nanostructure and chemical analysis for the LV-SEM

ABSTRACT

Materials engineers have increasing control over nanoscale chemical composition and nanostructures. The latter can be easily analysed by the scanning electron microscope (SEM) yet nanoscale chemical analysis in the SEM poses challenges. Nevertheless, nanoscale chemical analysis capabilities have been developed in the LV-SEM by secondary electron hyperspectral imaging (SEHI). In this article, the insights that SEHI has provided into well characterised semiconductor, photovoltaic, polymer and hard carbon materials are reviewed. Instrument and experimental considerations for the obtainment of secondary electron hyper-spectra are discussed and recommendations for future analyses in the LV-SEM by SEHI are made.     

New Developments in Transmission Electron Microscopy for Nanotechnology

High‐resolution transmission electron microscopy (HRTEM) is one of the most powerful tools used for characterizing nanomaterials, and it is indispensable for nanotechnology. This paper reviews some of the most recent developments in electron microscopy techniques for characterizing nanomaterials. The review covers the following areas: in‐situ microscopy for studying dynamic shape transformation of nanocrystals; in‐situ nanoscale property measurements on the mechanical, electrical and field emission properties of nanotubes/nanowires; environmental microscopy for direct observation of surface reactions; aberration‐free angstrom‐resolution imaging of light elements (such as oxygen and lithium); high‐angle annular‐dark‐field scanning transmission electron microscopy (STEM); imaging of atom clusters with atomic resolution chemical information; electron holography of magnetic materials; and high‐spatial resolution electron energy‐loss spectroscopy (EELS) for nanoscale electronic and chemical analysis. It is demonstrated that the picometer‐scale science provided by HRTEM is the foundation of nanometer‐scale technology.     

High-resolution and high-voltage electron microscopy at the University of California,Berkeley

After 20 years of exciting growth in our theoretical and experimental expertise in transmission electron microscopy, the development of new generations of instruments over the past few years has opened up new opportunities for sophisticated studies of microstructure under well-controlled conditions and at near-atomic levels. High-voltage electron microscopy now provides unique capabilities for dynamic studies of bulk specific behaviour during exposure to ‘ real world ’ conditions, e.g., varying environments, deformation, irradiation, or combinations of these, while developments in lattice-imaging techniques afford new insights into structures and mechanisms operating at a near-atomic level. Both these approaches are being explored and exploited at Berkeley in a variety of applications to practical problems in materials science. Several examples are presented in this paper which illustrate the power of high resolution lattice-imaging in studies of interfacial structures, and in its combination with laser optical microdiffraction for studying lattice parameters and compositional changes over small (10 Å) regions. Other examples will underline the importance of imaging techniques and environmental control duringin-situ studies of ceramic materials in the high-voltage electron microscope. Finally, future directions and anticipated developments are briefly discussed.     

航空锻件的超声成像技术

由于锻件表面不平整,且厚度不均,目前超声扫查系统普遍采用手工超声波探伤法检测锻件内部缺陷,无法应用标准的B扫描成像和C扫描成像,存在着效率低、探伤质量较差等不足之处。提出了以“厚度-灰度”超声检测为核心的基本成像过程,包括路径规划、全波采样、信号处理、波形分析和缺陷扫描成像等方法,并示例实现。该检测手段有助于促进航空锻件超声检测的智能化,提高检测的效率和可靠性。     

浅海波导中垂直时反线阵聚焦研究

在浅海波导中,垂直阵可以跨越整个波导,并能对大多数简正波模态进行精确采样.在给定声源及海洋环境信息的条件下,通过MOATL简正波模型仿真计算出到达垂直时反阵位置处的波形,即垂直时反阵的接收信号,将接收信号时间上取反,通过以KRAKEN简正波模型计算得出的返回声信道,在声源处取得了较好的聚焦效果.并分析了垂直阵基元数、基...     

Energy-Filtered High-Resolution Electron Microscopy for Quantitative Solid State Structure Determination

Energy-filtered (or selected) electron imaging is one of the future directions of high-resolution electron microscopy (HREM). In this paper, the characteristics and applications of energy-selected electron imaging at high-resolution for structure determinations are illustrated. It is shown that image contrast can be dramatically improved with the use of an energy filter. High-resolution chemical-sensitive imaging using ionization-loss electrons is demonstrated in studies of Ni/Ti and Al/Ti multilayer thin films. It is also shown that the spatial resolution of energy-selected ionization edge electron images is dominated by the signal-to-noise ratio. Experimental parameters which may be selected to improve the signal-to-noise ratio are discussed.