具有浮动结调制的SiC中子探测器研究

碳化硅(SiC)具有禁带宽度大、临界位移阈能高、临界击穿电场强以及热导率高等特性,被认为是高能中子探测的优选材料。目前SiC中子探测器主要采用内含本征(intrinsic)层结构的PN结二极管(PiN)或肖特基二极管形式,其灵敏区结构简单,需要施加高的反偏压才能获得高的探测效率,而高反偏压常会带来可靠性问题。本文基于美国新思公司的Sentaurus计算机辅助设计技术(TCAD)工具进行数值仿真,从灵敏区结构优化入手,研究了一种在灵敏区中内嵌浮动结(FJ)的PiN结构SiC中子探测器。仿真结果表明,浮动结的引入一方面能够优化灵敏区的纵向结构电场,使整体电场分布更加均匀,同时降低了近表面处的最大峰值电场强度,从而提高了探测器的工作可靠性。另一方面,浮动结能够加速灵敏区的耗尽,并在自身达到全耗尽的情况下,使探测器在较低偏压下实现电荷收集效率(CCE)的显著提升。

4H-SiC浮动结JBS器件的设计方法

研究了4H-SiC浮动结(FJ)结势垒肖特基(JBS)二极管的设计方法。提出在上外延层厚度一定的情况下得到外延层最佳掺杂浓度,然后以器件的功率优值(BFOM值)为依据确定出最佳下外延层厚度,进而设计出浮动结和表面结的最佳结构参数。否定了文献中认为浮动结位于器件中部为最佳设计的结论。仿真结果表明浮动结和表面结线宽比不仅影响器件导通特性,还会影响反向特性。浮动结线宽比在一定范围内会略微影响器件击穿电压,而表面结线宽比主要影响器件的反向泄漏电流。

套刻偏差对4H-SiC浮动结结势垒肖特基二极管的影响研究

4H-SiC浮动结结势垒肖特基二极管与常规结势垒肖特基二极管相比在相同的导通电阻条件下具有更高的击穿电压.由p^+埋层形成的浮动结与主结p^+区之间的套刻对准是实现该结构的一项关键技术.二维模拟软件ISE的模拟结果表明,套刻偏差的存在会明显影响器件的击穿特性,随着偏差的增大击穿电压减小.尽管主结和埋层的交错结构与对准结构具有相似的击穿特性,但是当正向电压大于2 V后,交错结构的串联电阻更大.

Numerical Investigations on Transient Behaviours of Two 3-D Freely Floating Structures by Using a Fully Nonlinear Method

Two floating structures in close proximity are very commonly seen in offshore engineering. They are often subjected to steep waves and, therefore, the transient effects on their hydrodynamic features are of great concem. This paper uses the quasi arbitrary Lagrangian Eulerian finite element method （QALE-FEM）, based on the fully nonlinear potential theory （FNPT）, to numerically investigate the interaction between two 3-D floating structures, which undergo motions with 6 degrees of freedom （DOFs）, and are subjected to waves with different incident angles. The transient behaviours of floating structures, the effect of the accompanied structures, and the nonlinearity on the motion of and the wave loads on the structures are the main focuses of the study. The investigation reveals an important transient effects causing considerably larger structure motion than that in steady state. The results also indicate that the accompanied structure in close proximity enhances the interaction between different motion modes and results in stronger nonlinearity causing 2hal-order component to be of similar significance to the fundamental one.

Mathematical simulation of steep waves at a focus point

Mathematical models simulating steep waves at a focus point are presented in this paper. Simulations of extreme waves in a model basin were used to determine the loads on floating structures induced by the waves. Based on a new wave theory, numerical test results show that the simulation procedure is effective and the induced motion of water particles in the front of waves is an important factor influencing impact loads on floating bodies.

New technology opens the door for deepwater development

The paper provided an updated status of technology for deepwater field development, demonstrated the importance of its application through actual project example, and discussed some future technical development trends. The focus was on the floating structures. By reviewing some of the engineering aspects of the project, the technology advancement, innovations and challenges in offshore engineering were discussed and demonstrated. The author’s view of technical challenges facing deepwater forwarding was discussed, which covered water depth limitations, new material application, installation methods, riser development and operational issues. An overview of technologies that will enable deepwater projects to be extended into new frontiers was presented.

Suspension principle of deep-ocean nodule collector

To make sure that the nodule collector can walk on the soft sediments of seafloor effectively, suspension principle of deep-ocean nodule collector is proposed. The supporting and driving mechanisms are separated to avoid collector sinking. The supporting mechanism is a streamlined body with large and smooth supporting area. The grounding pressure is reduced to 0.5- 1 N/cm2 to make sure that the sinkage is limited. The impellers serve as the driving mechanism to supply enough driving power. The position between the supporting mechanism and the driving mechanism can be adjusted according to the operating condition to decrease the walking resistance and to increase driving efficiency. The test results indicate that the collector can walk on the surface of the soft sediments with the limited sinkage. The traction forces were up to 800 kg and the sinkage of the impeller was under the limitation.

Simplified method for predicating consolidation settlement of soft ground improved by floating soil-cement column

A simplified method is presented for predicting consolidation settlement of soft ground improved by floating soil-cement column on the basis of double soil-layer consolidation theory. Combining the axisymmetric consolidation model and equal strain assumption, the governing equation was derived for the consolidation of clayey subsoil reinforced by soil-cement column. By modifying the boundary condition of the interface between the improved layer and underlying layer on seepage and pore-water pressure, the analytical solution of consolidation of soft ground improved by floating soil-cement column was developed under depth-dependent ramp load. The results of the parameter analysis of consolidation behavior show that the consolidation rate is closely related with the depth replacement ratio by the column and the permeability of upper layer. The influence of column-soil constrained modulus ratio and radius ratio of the influence zone to the column on consolidation is also affected by depth replacement ratio. The column-soil total stress ratio increases with time and approaches the final value accompanied with the dissipation of excess pore water pressure.

Time Domain Calculation of Connector Loads of a Very Large Floating Structure

Loads generated after an air crash, ship collision, and other accidents may destroy very large floating structures （VLFSs） and create additional connector loads. In this study, the combined effects of ship collision and wave loads are considered to establish motion differential equations for a multi-body VLFS. A time domain calculation method is proposed to calculate the connector load of the VLFS in waves. The Longuet-Higgins model is employed to simulate the stochastic wave load. Fluid force and hydrodynamic coefficient are obtained with DNV Sesam software. The motion differential equation is calculated by applying the time domain method when the frequency domain hydrodynamic coefficient is converted into the memory function of the motion differential equation of the time domain. As a result of the combined action of wave and impact loads, high-frequency oscillation is observed in the time history curve of the connector load. At wave directions of 0° and 75°, the regularities of the time history curves of the connector loads in different directions are similar and the connector loads of C1 and C2 in the X direction are the largest. The oscillation load is observed in the connector in the Y direction at a wave direction of 75° and not at 0° This paper presents a time domain calculation method of connector load to provide a certain reference function for the future development of Chinese VLFS

Seasonal Community Structure of Mesozooplankton in the Daya Bay,South China Sea

Mesozooplankton are key components of coastal ecosystems,linking the microbial food web to the classic food chain.In this study,species composition and abundance of mesozooplankton is studied for the Daya Bay in April(spring) and October(fall),2006.A total of 27 species of mesozooplankton were identified in spring and 58 species in fall.Dominant species were Oithona tenuis,Flaccisagitta enflata,Penilia avirostris and Centropages tenuiremis in spring,shifting to Microsetella norvegica,Oithona tenuis and Parvocalanus crassirostris in fall.Higher mesozooplankton abundance was found at Aotou Cove and Dapeng'ao Cove compared to other stations,indicating the influence of eutrophication on mesozooplankton community in the Daya Bay.The outbreak of Noctiluca scintillans bloom in spring reduced the species diversity and abundance of mesozooplankton.

A honeycomb-tube packing medium and its application to column flotation

We address problems in the development of large-scale flotation columns that use short cylinders. As a starting point, we investigated the packing medium to identify a highly efficient internal packing for the flotation column. The chosen packing was a honeycomb structure with an aperture diameter of 80 mm, a web thickness of 0.80 mm, a film height of 1000 mm, packed into a 400 mm diameter space, which completely filled the vessel at optimal cost. The column consisted of a modular ring of single-hole hexagonal honeycomb tube packing made from atactic polyproplene (PP-R). The packing was tested in a cyclonic, static micro-bubble flotation column. Computational fluid dynamic modeling was used to analyze the flotation fluid in a honeycomb tube packed flotation column. Our results show that the fluid axial movement was maximized and that the transverse fluid velocities were zero in the vicinity of axial flow. Using the honeycomb tube packing for copper sulfide flotation we observed that the average concentration in the product was increased to 25.41%, from an average feed concentration of 0.729%, with an average recovery of 92.92%. The demands of on-site industrial production were met.

Structural Features of Zooplankton Cenosis of Southern Kazakhstan Surface Waters

The present condition of Southern Kazakhstan natural reservoirs is represented in the article. The research was held in summer time of 2010. Quantitative and qualitative features and dominating types of zooplankton of the individual reservoirs are shown. It is detected that the waters of the South Kazakhstan oblast （region） are inhabited by 70 types of zooplankton organisms of different taxonomic groups, whose association to certain reservoir depends on the hydrochemical indicators of the aquatic environment.

Dynamic Behaviors of a Moored Floating Structure with Flexible Skirts

A numerical model was developed by using the dual boundary element method to investigate the dynamic behavior of a moored floating structure with a pair of vertical and flexible skirts attached at its bottom in the linear wave field. Theoretical conception is based on potential theory with linear external forces. The motions of the structure were assumed to be small and linear. The flexible skirts mounted beneath the structure were assumed uniform flexural rigidity and the thickness of the skirts was negligible. Comparison between the present model and Gesraha＇s solution was made to verify the results for a moored floating structure with or without rigid skirts. The influence of the skirt rigidity on the moored floating structure, moored lines and waves is investigated in this study. The results show that, the natural frequencies of structure＇s oscillation, moored force, wave reflection and transmission tend to the region of short-period waves when the flexible rigidity gradually decreases. Positive correlation exists between the aft mooring force and the pitch motion of the floating structure.

Design of Ocean Floating Structures:Prediction of Hydrodynamic Coefficients

[Introduction] Accurate calculation of the hydrodynamic coefficients for floating structures and the investigation of the flow field distribution around floating bodies on the marine free surface are essential for improving the engineering design and application of marine structures.[Method] This study utilized the computational fluid dynamics(CFD) approach and the Reynolds Averaged NavierStokes(RANS) method and considered the effects of viscosity and free surface interactions on the hydrodynamic behavior of floating structures.By employing the dynamic mesh technique,this study simulated the periodic movements of simplified three-dimensional(3D)shapes:spheres,cylinders,and cubes,which were representative of complex marine structures.The volume of fluid(VOF) method was leveraged to accurately track the nonlinear behavior of the free surface.In this analysis,the added mass and damping coefficients for the fundamental modes of motion(surge,heave,and roll) were calculated across a spectrum of frequencies,facilitating the fast determination of hydrodynamic forces and moments exerted on floating structures.[Result] The results of this study are not only consistent with the results of the 3D potential flow theory but also further reflect the role of viscosity.This method can be used for precise calculation of the hydrodynamic coefficients of floating structures and for describing the flow field of such structures in motion on a free surface.[Conclusion] The methodology presented goes beyond the traditional potential flow approach.