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991.
在研究IEEE 802.16标准关于VoIP传输机制的基础上,指出语音流的静默特性严重制约了VoIP的容量.通过分析VoIP业务流的统计特性得出了在统计复用情况下VoIP的容量极限.提出了一种基于统计复用的带宽调度方法及其实现算法,以克服由于语音静默带来的语音速率波动.仿真结果表明,该调度方法与UGS(unsolicited grant service)调度相比能够显著提升VoIP的容量,同时使BS和SS之间的VoIP丢包率和传输时延保持在0.02%和5.1ms以下. 相似文献
992.
993.
点样机器人是制备微阵列生物芯片的核心设备。提高点样机器人的时效是非常重要的。通过分析点样机器人的系统,对影响点样机器人时间效率的各个因素,包括上下位机的数据传送,X、Y、Z三个轴上的速度和加速度,点样制备过程中的工作路径进行了分析。通过理论分析,阐述了数据通讯的方式,给出了工作路径的计算方法与公式,对主要影响因素进行了深入研究。其结论对点样机器人的协议规划和软件设计都具有指导意义。 相似文献
994.
Thin films of SrBi2Nb2O9 with and without the inclusion of Ag nanocrystals were prepared on MgO (001) substrates by alternative pulsed laser ablation of SrBi2Nb2O9 ceramic and Ag metal in a nitrogen atmosphere. The size of the Ag nanocrystals can be brought down to ca. 5.0 nm, as revealed by transmission electron microscopy and X-ray diffraction. We measured the third-order susceptibility χ(3) of the deposits by single beam z-scan technique at a wavelength of 532 nm. It is found that SrBi2Nb2O9 is an effectively nonlinear optical medium, exhibiting a very large χ(3). With the inclusion of Ag nanocrystals the χ(3) was nearly doubled (Reχ(3) = 8.052 × 10− 7 esu, and Imχ(3) = − 1.717 × 10− 7 esu), which comes very close to the maximum value available in the current time. This indicates that the Ag:SrBi2Nb2O9 composite structure can be an excellent candidate for nonlinear optical applications. A general approach for further enhanced third-order susceptibility by tailoring the dielectric constant of the metal inclusion is briefly discussed. 相似文献
995.
W. H. Zhong Y. C. Chan B. Y. Wu M. O. Alam J. F. Guan 《Journal of Materials Science》2007,42(13):5239-5247
This paper evaluates the shearing behavior of ball grid array (BGA) solder joints on Au/Ni/Cu pads of FR4 substrates after
multiple reflow soldering. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with Sn–3Ag–0.5Cu (SAC) and Sn–37Pb
(SP) solders, in terms of fracture surfaces, shearing forces and microstructures. Three failure modes, ball cut, a combination
of solder shear and solder/pad bond separation, and pad lift, are assessed for the different solders and reflow cycles. It
is found that the shearing forces of the SP and SAC solder joints tend to increase slightly with an increase in the number
of reflow cycles due to diffusion-induced solid solution strengthening of the bulk solder and augmentation of the shearing
area. However, the shearing forces of the SACI solder joints decrease slightly after four cycles of reflow, which is ascribed
to the thermal degradation of both the solder/intermetallic compound (IMC) and IMC/Ni interfaces. The SACI solder joints yield
the highest strengths, whereas the SP solder joints give the smallest values, irrespective of the number of reflow cycles.
Thickening of the interfacial IMC layer and coarsening of the dispersing IMC particles within the bulk solders were also observed.
Nevertheless, the variation of shearing forces and IMC thickness with different numbers of reflow cycles was not so significant
since the Ni under layer acted as an effective diffusion barrier. In addition, the initially-formed IMC layer retarded the
further extensive dissolution of the pad material and its interaction with the solder. 相似文献
996.
A stable and accelerated procedure for the electric field integral equation (EFIE) is developed. The procedure employs an implicit method in conjunction with a time averaging scheme to solve EFIE with the second-order time derivatives. Two examples are provided to demonstrate the superior stability and efficiency of the proposed procedure. 相似文献
997.
Xu Shubo Zhao Guoqun Wu Xin Ma Xinwu Guan Yanjin 《Frontiers of Materials Science in China》2007,1(2):187-196
Equal channel angular pressing (ECAP) has the capability of producing ultra fine-grained (UFG) materials bellow the dimension
of 1 μm. At present, it is one of the most important methods to get bulk UFG materials. Multi-pass ECAP processes for round
workpieces are investigated by using numerical simulations and experimental studies in this paper. The deformation mechanism
of ECAP for grain refinement is obtained. Three processing routes A, B and C are simulated in order to study the influence
of the processing routes to the deformation uniformity of the workpiece. The finite element (FE) analysis results of the multi-pass
ECAP process show that the different processing routes result in the different deformation distributions. The grain in the
workpiece is refined obviously after multi-pass pressing. The microstructures of the processed material are more different
than that of the microstructure of the annealing initial equiaxed grains. The microstructure evolution of the workpiece can
be changed via different processing routes. It is found that route B can get a high angle grain boundaries distribution in
the workpiece than other routes. The results of the analysis show that the process of grain refinement can be described as
a continuous dynamic recovery and recrystallization. The microstructure evolutions of the grain refinement mechanisms and
micro-structural characteristics for different multi-pass ECAP processing routes are verified by using OM (optical model)
and TEM (transmission electron microscope) analysis. In addition, the experimental microstructure results are also consistent
with FE analysis results. 相似文献
998.
The purpose of this study is to explore the character and pattern of the linkage between science and technology in China,
based on the database of United States Patent and Trademark Office (USPTO). The analysis is focused on the period 1995–2004,
a rapid increasing period for Chinese US patents. Using the scientific non-patent references (NPRs) within patents, we investigate
the science-technology connection in the context of Chinese regions as well as industrial sectors classified by International
Patent Classification (IPC). 11 technological domains have been selected to describe the science intensity of the technology.
The results suggest that the patents and the corresponding scientific citations are related in different ways. Finally, we
match the scientific NPRs to the Science Citation Index (SCI) covered publications to identify the core journals and categories.
It reveals that the scientific references covered by SCI show a skewed distribution not only in journals but also in categories. 相似文献
999.
Multiplexing Scheme for Self-Interfering Long-Period Fiber Gratings Using a Low-Coherence Reflectometry 总被引:1,自引:0,他引:1
Meng Jiang Zu-Guang Guan Sailing He 《IEEE sensors journal》2007,7(12):1663-1667
A novel multiplexing scheme based on a low-coherence reflectometry (LCR) is proposed for a sensing array (in parallel) of self-interfering long-period fiber gratings (SI-LPGs). Each SI-LPG sensor consists of an LPG and a section of fiber with a highly reflective end (mirror). The spectral information of each LPG is sensitive to some parameters of the surrounding measurand and can be reconstructed from the corresponding subreflectograms (obtained by the LCR) through a fast Fourier transformation. The sensing signals of multiple SI-LPG sensors can be multiplexed if the length of the fiber section in each sensor is set to a different value. Experiments of measuring the surrounding temperature at different sensors are demonstrated to show the good performance of our multiplexing system. 相似文献
1000.
针对阀控式密封铅酸蓄电池在变电站中的现行运行状况,以温度监控为例,提出了一种保障蓄电池安全运行的新策略.该策略大大提高了变电站用蓄电池的使用寿命及直流系统运行的稳定性,减少了日常维护工作,较大程度地减少了蓄电池发生故障的可能性. 相似文献