Modelling of electron energy absorption and backscattering during EBW

A three-dimensional heat flux model for deep-penetrating electron beam welding (EBW) is established to mathematically describe the physical heat generation process during interaction between electrons and the dynamic molten pool free surface. Monte Carlo method is used to determine the electron-target interaction, and random distribution of initial electrons, progressive trajectory tracing and electron backscattering models are used to describe the spatial distribution of electrons absorption. The model is verified in preset keyholes and applied in the simulation on electron beam welding process, and the calculated bead shape shows a good consistency with experimental results.     

基于后向散射和软件定义的无线传感器网络通信

针对需要低比特率和超低成本传感器的无线传感器网络(WSN),提出了一种基于后向散射无线电和软件定义的通信体系结构,它由实现射频载波生成、零差检测和复杂信息处理的中心Hub和具有载波调制功能的传感器构成。前者构建了一种软件定义的收发器来发送载波和接收来自于各个传感器的反射,并提取和处理它们发送的信息;后者每个传感器的发射机被简化为一个连接到天线上的晶体管,且每个传感器的设计都围绕着一个驱动低功率射频(RF)开关的低功率微控制器进行,实现传感器的信息被调制到其唯一的子载波上;从而建立起一个完整的后向散射无线电链路通信系统,实现每个传感器和中心Hub之间的通信;基于理论的仿真结果和原型机的实验结果表明,提出的WSN通信系统不仅能够实现较大范围的可靠通信和数据传输及处理,而且具有低复杂度和超低成本。     

EEMD在激光测云仪后向散射信号处理中的应用

激光测云仪后向散射信号是典型的非线性、非稳态信号,容易受噪声污染。针对该问题采用集成经验模态分解(EEMD)去噪算法进行处理,首先对含噪信号进行经验模态分解(EMD),将分解后的IMF分量进行自相关性分析,找出含噪占有量较大的IMF分量,对其进行SG(savitzky-golay)滤波,最后将滤波后的IMF分量和剩余分量进行信号的重构。经仿真实验结果表明,与传统的EMD方法相比,EEMD方法处理含噪信号后的输出信噪比提高了1.695 dB,均方误差平均降低了30%以上,说明该方法可以适用于非线性、非稳态的后向散射回波信号去噪处理,能为激光测云仪下一级的云底高度反演提供高信噪比的初始数据。     

Optical fiber distributed acoustic sensing based on the self-interference of Rayleigh backscattering

In order to detect the weak underwater acoustic signal, a Distributed Acoustic Sensing (DAS) scheme based on the self-interference of Rayleigh backscattering is presented. Rayleigh backscattered light which contains a phase change induced by acoustic signal along the sensing fiber which is a standard telecom single-mode fiber is split and fed into an imbalance Michelson interferometer. With the self-interference of two Rayleigh backscattered beams, the phase change is amplified theoretically compared with phase-sensitive OTDR. We designed an experiment to prove the scheme, and successfully restored the acoustic information, meanwhile, the DAS system has preliminary realized around the acoustic phase sensitivity of −151 dB (re rad/μPa) at 600 Hz, and the minimum detectable acoustic pressure of 6 Pa in the experiment.     

The effect of microstructure and texture evolution on mechanical properties of low carbon steel in a non-circular drawing sequence

In this study, a new process sequence of non-circular and circular drawing is designed using finite element simulations and is proposed to produce strengthened wires in a simple continuous way for industrial applications. The developed non-circular drawing (NCD) sequence was experimentally applied to low carbon steel at room temperature. Mechanical properties, microstructure and texture evolution of the specimen processed by the newly proposed process and conventional wire drawing (WD) were investigated by tension test, electron backscattering diffraction (EBSD), and X-ray diffraction (XRD) for comparison. According to the present investigation, the specimen processed by the NCD sequence achieved 10.7% higher ultimate tensile strength (UTS) with slightly higher reduction of area at fracture than the one processed by the WD for the two-pass with the same area reductions. Furthermore, the UTS value (612 MPa) of the drawn wire by the two-pass NCD sequence was equivalent to the level of 602 MPa processed by the three-pass WD. From the EBSD results, the areal fraction of the low angle grain boundaries (1.12 μm⁻¹) of the specimen processed by the NCD was higher than that (0.78 μm⁻¹) of the specimen processed by the WD for the two-pass. The pole figures and ODFs of the specimen processed by the NCD sequence showed typical drawing and rolling textures. It is demonstrated that the non-circular drawing sequence could be beneficial in producing high-strength wires with comparable ductility through grain refinement according to the observations made in the present work.     

Advanced techniques for characterization of ion beam modified materials

Understanding the mechanisms of damage formation in materials irradiated with energetic ions is essential for the field of ion-beam materials modification and engineering. Utilizing incident ions, electrons, photons, and positrons, various analysis techniques, including Rutherford backscattering spectrometry (RBS), electron RBS, Raman spectroscopy, high-resolution X-ray diffraction, small-angle X-ray scattering, and positron annihilation spectroscopy, are routinely used or gaining increasing attention in characterizing ion beam modified materials. The distinctive information, recent developments, and some perspectives in these techniques are reviewed. Applications of these techniques are discussed to demonstrate their unique ability for studying ion-solid interactions and the corresponding radiation effects in modified depths ranging from a few nm to a few tens of μm, and to provide information on electronic and atomic structure of the materials, defect configuration and concentration, as well as phase stability, amorphization and recrystallization processes. Such knowledge contributes to our fundamental understanding over a wide range of extreme conditions essential for enhancing material performance and also for design and synthesis of new materials to address a broad variety of future energy applications.     

The effect of SiO2 addition on the development of low-Σ grain boundaries in PTC thermistors

Electron backscatter pattern analysis has been used to characterise, using the coincidence site lattice model, the distribution of grain boundary structures in a series of BaTiO₃ based positive temperature coefficient of resistance (PTC) thermistors, prepared with 0, 1.0, 2.0 and 3.0 at.% SiO₂ additions. As the SiO₂ content was raised, the proportion of random, high-angle grain boundaries in the microstructure increased steadily from 85.7% to 89.6%, while the proportion of grain boundaries indexable in the range Σ3–Σ29 decreased from 14.3% to 10.4%, and the Σ3 grain boundary population fell from 5.9% to 3.6%. At the same time the proportion of Σ3 twin boundaries remained approximately constant at 3.0 ± 0.3%. Significantly more Σ3 grain boundaries than would be expected in a randomly oriented, untextured material were observed in all samples. The variation in grain boundary types with SiO₂ addition is discussed in terms of grain boundary energy and its effect on PTC performance.     

Ion beam analysis of a structural phase transition in porous TiO2/V2O5 ceramics with rough surfaces

TiO₂/V₂O₅ based ceramic materials applied in catalysis were investigated. Structural properties like the grain diameter of these pressed ceramic powders were analysed by means of Rutherford backscattering (RBS) making use of an analytic model to describe the energy spectra of porous rough samples. Grain diameters of these samples were deduced as function of process temperature and chemical composition and related to a phase transition from the TiO₂-Anatase/V₂O₅-Shcherbinaite to Rutile solid solution (Rutile-ss) structure. RBS data were compared to results of scanning electron microscopy (SEM). The activation energy for the sintering at the phase transition was estimated to 5.4 eV.     

Determination of sulphur and copper depth distribution in patina layers using nuclear reaction techniques

A method for Cu and S profiling in patina layers was developed by applying a combination of nuclear reaction analysis (NRA) and Rutherford backscattering spectroscopy (RBS). The copper profiling was performed by using the 1327 keV γ-ray deexciting the third excited state to the ground state of ⁶³Cu produced by the reaction ⁶³Cu(p,p^′γ)⁶³Cu. For the determination of sulphur the 2230 keV γ-ray was used deexciting the first excited state to the ground state of ³²S formed through the reaction ³²S(p,p^′γ)³²S, which exhibits three sharp resonances at projectile energies 3.094, 3.195 and 3.379 MeV. The relevant cross-sections were measured in the energy range between 3.0 and 3.7 MeV in steps of 20 keV at 125° to the incident proton beam direction. The technique was tested using artificially produced and natural copper patina layers. Supporting information on the depth distribution of the constituent elements of the patina samples was obtained by p-RBS (E_p: 1.5 MeV, θ: 160°).     

Preprocessing of ionospheric echo Doppler spectra

The real-time information of the distant ionosphere can be acquired by using the Wuhan ionospheric oblique backscattering sounding system (WIOBSS), which adopts a discontinuous wave mechanism. After the characteristics of the ionospheric echo Doppler spectra were analyzed, the signal preprocessing was developed in this paper, which aimed at improving the Doppler spectra. The results indicate that the preprocessing not only makes the system acquire a higher ability of target detection but also suppresses the radio frequency interference by 6–7 dB. __________ Translated from Journal of Wuhan University (Natural Science), 2006, 52(1): 114–118 [译自: 武汉大学学报(理学版)]