反相微乳液体系中电子束辐射制备Fe3O4/聚丙烯酰胺磁性核壳微球

在共沉淀法制备Fe3O4纳米磁粉的基础上,以丙烯酰胺(Acrylamide,AM)为单体在反相微乳液中通过电子束辐照的方法制备了具有核壳结构的四氧化三铁/聚丙烯酰胺磁性核壳微球(Fe3O4/Polyacrylamide,PAM),用X射线衍射仪、原子力显微镜、傅立叶变换红外光谱、动态激光光散射仪表征样品.结果表明,制备的磁粉为Fe3O4单相,粒径为9nm左右,磁性高分子微球Fe3O4/PAM直径范围为80-150 nm,呈球形.分析了乳化剂用量,单体浓度,磁粉用量,吸收剂量等对Fe3O4/PAM微球粒径的影响规律.     

A novel autofocus algorithm based on maximum total variation criteria for SAR images

A novel autofocus algorithm for synthetic aperture radar (SAR) based on total variation is presented in this paper. The method, which starts with a complex phasedegraded SAR image, after the phase errors model is introduced into the range-compressed phase-history domain, carries out phase errors correction by changing the focus till the total variation of the azimuth profile is maximized. Compared with the minimum entropy autofocus algorithm, the autofocus algorithm has less computational complexity and is easier to implement. The simulation and the processing results of the measured data show the validity of the proposed method. Translated from Journal of Xidian University, 2006, 33(2): 167–172 [译自: 西安电子科技大学学报]     

利用有功功率扰动的微网孤岛下垂控制策略

对于孤岛模式运行的微网中的并联逆变器,其功率分配主要受逆变器输出阻抗和输出端与公共连接点之间线路阻抗差异的影响。以感性输出阻抗为例,从逆变器并联的功率传输特性出发,分析了线路阻抗差异对并联逆变器无功功率分配的影响。为了提高无功功率的均分精度,提出了一种改进型下垂控制策略,在有功频率下垂中加入无功功率,利用有功扰动反映无功分配偏差,在无功幅值下垂中加入可调下垂系数,通过消除有功扰动调节下垂系数补偿线路阻抗差异实现稳态时无功均分。仿真和实验验证了改进下垂控制策略的正确性和有效性。     

Quadratic stabilizability of switched linear systems with polytopic uncertainties

In this paper, we consider quadratic stabilizability via state feedback for both continuous-time and discrete-time switched linear systems that are composed of polytopic uncertain subsystems. By state feedback, we mean that the switchings among subsystems are dependent on system states. For continuous-time switched linear systems, we show that if there exists a common positive definite matrix for stability of all convex combinations of the extreme points which belong to different subsystem matrices, then the switched system is quadratically stabilizable via state feedback. For discrete-time switched linear systems, we derive a quadratic stabilizability condition expressed as matrix inequalities with respect to a family of non-negative scalars and a common positive definite matrix. For both continuous-time and discrete-time switched systems, we propose the switching rules by using the obtained common positive definite matrix.     

Clutter suppression method for heterogeneous environment

Clutter suppression is a key process in ground moving target indication (GMTI). Adaptive filtering is an effective method for clutter suppression but the performance degrades severely in heterogeneous clutter environment. To solve this problem, a new clutter suppression method is proposed. The approach uses subspace tracking technique to update clutter subspace of different clutter patches which may mitigating the heterogeneous effects. Simulation results illustrate that the method performs well when lacking of the independent and identically distributed (i.i.d.) samples.     

洞庭湖盆地线性影象特征分析

本文从基底时代、基底结构、沉积盖层、构造格局和水系分布等特征,论述了洞庭湖盆地北部与南部的差异性及与江汉盆地的相似性,从遥感图象上提取大量信息,经处理、解译认为,可把洞庭湖盆地北部和江汉盆地看作为一个整体,这对石油天然气勘探将是有益的。     

Effects of shell thickness of polystyrene‐encapsulated Mg(OH)2 on flammability and rheological properties of high‐impact polystyrene composites

Polystyrene (PS)‐encapsulated magnesium hydroxide (Mg(OH)₂) particles with various shell thicknesses were successfully prepared using a method of in situ polymerization of styrene in a high‐speed mixer. High‐impact polystyrene (HIPS)/Mg(OH)₂ composites were prepared by melt blending. They were characterized using cone calorimetry, horizontal burning rate, rheology and scanning electron microscopy in order to investigate the effects of the shell thickness of the PS‐encapsulated Mg(OH)₂ on the flame retardancy and rheological properties of the resulting composites. Rheological tests showed that the composites containing encapsulated Mg(OH)₂ had a stronger solid‐like response at low frequency than that of the sample containing untreated Mg(OH)₂. However, with PS/Mg(OH)₂ ratio increasing up to 6.0 wt% and above, the dynamic viscosity, loss modulus and storage modulus of HIPS/Mg(OH)₂ composites decreased. The optimum PS/Mg(OH)₂ ratio, 4.5 wt%, was determined using a new ‘crossover point’ rheological method. The combustion tests showed that compared to the composites containing untreated Mg(OH)₂, the fire retardancy of the composites containing PS‐encapsulated Mg(OH)₂ was improved significantly. Also, there appeared to be a critical PS/Mg(OH)₂ ratio, namely 6.0 wt%, for optimum flame‐retarding properties. However with the continuous increase of PS/Mg(OH)₂ ratio, the fire resistance of the composites declined somewhat, which can be explained by acceleration of combustion of the composites due to the introduction of free PS chains of low molecular weight on the surface of Mg(OH)₂. Copyright © 2007 Society of Chemical Industry     

基于温度场的冻土路基变形数值分析及对比

考虑相变和全球气温升高的影响,利用数值分析方法,对青藏铁路典型路基的变形问题进行了分析和数值模拟。通过建立路基有限元模型,选取适当的边界条件、初始条件及热学计算参数,在计算了典型路基断面温度场变化的前提下,对三年的路基变形场进行了数值分析并与实际测试资料进行了对比分析。分析表明,融化沉降在一年内已基本完成,并逐渐开始回冻,两三年后,除路堤填料和地基的进一步压密外,蠕变效应开始显现。因此,对高温、高含冰量冻土而言,蠕变产生的沉降变形是不可忽略的。     

云计算热点问题分析

在云计算的研究和应用中一些热点问题比较突出,如：如何理解计算资源及其虚拟化、云计算与网格计算的差异、云计算中心与高性能计算机的关系、云安全和云标准等。文章对此给出了一些见解：计算资源的虚拟化促使信息服务走向规模化、集约化和专业化;网格计算是＂多为一＂,而云计算是＂一为多＂;部署于高性能计算中心的高性能计算机未必适合云计算;云安全已经将传统安全问题发展为服务方和被服务方之间的信任和信任管理问题;在现有标准的基础上,云计算标准将需更加关注服务的互操作等。     

Crystallization,melting behavior,and wettability of poly(ε‐caprolactone) and poly(ε‐caprolactone)/ poly(N‐vinylpyrrolidone) blends

Both wettability and crystallizability control poly(ε‐caprolactone)'s (PCL) further applications as biomaterial. The wettability is an important property that is governed by both chemical composition and surface structure. In this study, we prepared the PCL/poly(N‐vinylpyrrolidone) (PVP) blends via successive in situ polymerization steps aiming for improving the wettability and decreasing crystallizability of PCL. The isothermal crystallization of PCL/PVP at different PVP concentrations was carried out. The equilibrium melting point (T), crystallization rate, and the melting behavior after isothermal crystallization were investigated using differential scanning calorimetry (DSC). The Avrami equation was used to fit the isothermal crystallization. The DSC results showed that PVP had restraining effect on the crystallizability of PCL, and the crystallization rate of PCL decreased clearly with the increase of PVP content in the blends. The X‐ray diffraction analysis (WAXD) results agreed with that. Water absorptivity and contact angle tests showed that the hydrophilic properties were improved with the increasing content of PVP in blends. The coefficient for the water diffusion into PCL/PVP blends showed to be non‐Fickian in character. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010