Test research and mechanism analysis of vacuum fluxless laser soldering for SMD

Fluxless soldering can solve a series of problems caused by sude-effects of flux essentially.Feasibility research on vacuum laser soldering and mecharvism analysis on fluxless action of vacuum were carried out.Fluxlesssoldering succeeded in spreading and wetting on Cu pad laser heating source in vacuum surroundings.What’smore,this fluxless technolgy was applied in surface mounting of chip resistance successfully.     

Synthesis and characterization of urea bridged hybrid periodic mesoporous organosilica materials

Urea bridged organic–inorganic hybrid mesoporous SiO₂ materials (U-BSQMs) were synthesized through a sol–gel procedure by co-condensation of bis(triethoxysilyl propyl) urea (BSPU) under basic conditions using cetyltrimethylammonium bromide (CTAB) as organic template. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the mesoporous structure of the sample. Fourier-transform infrared spectroscopy (FT-IR), solid state CP-MAS NMR spectroscopy of ²⁹Si (²⁹Si CP-MAS NMR) and ¹³C (¹³C CP NMR) indicated that most of the Si–C bonds are unbroken during the synthesis process. The N₂ adsorption–desorption results revealed that these hybrid mesoporous SiO₂ materials have bimodal distribution of pores with pore diameters of 2.4 and 3.8 nm, respectively. Thermogravimetric analysis (TG) demonstrated that about 16% Si–C bonds have been broke during the synthesis progress. This kind of material is expected to find possible application in ion supporting, drug delivery and catalysis.     

压力开关的过载与限位保护研究

介绍了压力开关存在的过载现象和过载对压力开关寿命及可靠性的影响,提出了通过限位措施解决过载问题的办法。通过将推动微动开关机械触头的顶杆设计成两体结构,解决了限位不可靠的难题,并给出了2种限位方法的实验结果比较。试验结果表明:该结构设计合理,限位准确。该设计对同类产品具有借鉴和指导意义。     

基于分割的蠕虫传播抑制方法研究

通过对蠕虫的深入分析，发现了蠕虫的传播速度与网络的规模具有一定的关系。在此基础上，为了有效地遏制蠕虫在大规模网络上的爆发，提出了对网络进行划分以减小网络规模的方法，在划分边界上对蠕虫进行检测和隔离。建立了基于图的网络划分的模型，并提出了一种适于抑制蠕虫传播的网络划分的算法。     

Formation of the ζ phase at an interface between an Fe substrate and a molten 0.2 mass% Al–Zn during galvannealing

An interstitial free (IF) steel was dipped in a bath of molten Zn containing 0.2 mass% Al. The as-dipped (galvanized) steel was then annealed above the melting point of Zn (galvannealed), say at 773 K, for a period ranging from 1 to 30 s. Evolution of the microstructure of the Zn-containing coating was examined by transmission electron microscopy (TEM) and the chemical composition around the Zn(Al)/Fe interface was analyzed by energy dispersion spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). In the as-dipped (galvanized) steel a continuous layer of Fe₂Al₅ existed at the Zn(Al)/Fe interface, but no Fe–Zn intermetallic compounds were observed. Galvannealing at 773 K for 1 s resulted in the formation of the ζ phase at the Zn(Al)/Fe₂Al₅ interface, which grew at the expense of the Zn(Al) coating during the subsequent galvannealing. Possible mechanisms of nucleation of Fe–Zn intermetallic compounds are discussed.     

低碳钢双丝焊平板横向残余应力超声波法测量

介绍了超声波法应力测量的理论基础声弹性方程,特殊条件下简化临界折射纵波声弹性方程.建立了超声波法测量焊接残余应力系统,实现了高精度信号提取,使用特制变角度超声波探头,选用对应力变化敏感的临界折射纵波作为测量波形,对低碳钢双丝焊对接平板横向残余应力场实现了无损测量.结果表明,测量结果可靠,测量过程实时无损快速.分析了测量数据波动的原因.建立的超声波法应力无损测量系统为焊接结构服役状态在线可靠性评估奠定了基础.     

循环小波变换及其应用

介绍了循环小波的概念及其循环小波变换的快速算法,详细描述了由原正交小波获得其相应的循环小波的过程,从其中的缠绕叠加过程中,给出了信号的循环小波分解的一般公式,对任意长度数据的信号使用任意偶数的Daubechies小波的变换矩阵的构成给出了统一的描述。接着对使用循环小波变换识别结构系统脉冲响应函数的思想进行了仿真研究。在仿真中以两自由度和悬臂梁结构系统为例考虑了不同的小波对识别精度的影响,还讨论了循环小波变换方法的总体平均性能。     

信息共享在建筑供应链决策中的价值测度

科学测度信息共享在建筑供应链决策中的价值,对提高承包商的运作绩效具有重要意义。借鉴传统供应链信息共享价值分析的研究思路,结合建筑供应链管理实践,建立建筑供应链信息共享价值测度模型。以总承包商和材料设备供应商组成的供应链为例,建立供应商是否同总承包商共享供货信息对总承包商的服务水平和库存成本的影响的信息共享价值测度模型;以钢筋的采购管理为例,对该模型进行模拟分析。结果表明:共享信息可以提高总承包商的服务水平,降低总库存成本;供应商服务水平越低、供货提前期越大,信息共享对总承包服务水平和总库存成本的影响越大。     

The mechanism characterizations of methane steam reforming under coupling condition of temperature and ratio of steam to carbon

To elucidate the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming process, the characterizations of methane steam reforming at different temperature and ratio of steam to carbon in term of distribution of H₂ and CO, and the elementary reaction rate were investigated. Meanwhile, the formation mechanisms of H₂ and CO via sensitivity analysis and reaction path analysis were obtained. The results showed that the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming were higher than that of individual factor. The effects of temperature on the methane steam reforming were higher than that of the ratio of steam to carbon. The adsorption and desorption reaction of CH₄ on the surface of Ni-based catalyst had the most obvious effect on the sensitivity of H₂, CH₄ and CO. Besides, the effects of adsorption and desorption reaction of H₂O on the sensitivity of H₂ were higher than that of CH₄ and CO. Hydrogen was generated by the desorption reaction of H(s) in the adsorbed state and from three generating paths: a) CH₄(s) dissociated directly or reacted with O(s) to form H(s); b) The dissociation reaction of H₂O(s) produced H(s); c) OH(s) dissociated directly or reacted with C(s) to form H(s). Carbon monoxide was generated from single path: CH₄(s)→CH₃(s)→CH₂(s)→CH(s)→C(s)→CO(s)→CO(g).     

Evaluation of Plastic Deformation During Metal Forming by Using Lode Parameter

In the present research, the relationship between the strain increment and Lode parameter was obtained by using Lode parameter and Levy-Mises equation. The quantitative relation between different strain increments was presented and the relationship between plastic deformation types and Lode parameter was demonstrated. The relations between plastic deformation types and stress states were revealed and they were applied into typical metal forming process. The strain types in the workpiece were analyzed based on the numerical visualization of the values of Lode parameter. This research reveals the important role of the Lode parameter in the analysis of metal flow behavior during metal forming process.