Mechanical analysis of double-layered circular graphene sheets as building material embedded in an elastic medium

Journal of Central South University - Possessing the unique and highly valuable properties, graphene sheets (GSs) have attracted increasing attention including that from the building engineer due...     

球形LiMn1.5Ni0.5O4材料的嵌脱动力学

为明晰Li Mn1.5Ni0.5O4正极材料的动力学性能,采用水热辅助共沉淀法合成了尖晶石Li Mn1.5Ni0.5O4正极材料,并采用扫描电镜(SEM)、X射线粉末衍射(XRD)和电化学阻抗(EIS)研究了材料的结构和锂离子嵌脱动力学.实验结果表明:共沉淀法制备的Li Ni0.5Mn1.5O4材料颗粒呈均匀球形,且平均粒径较小,粒度分布较窄.在循环过程中,Li Ni0.5Mn1.5O4的电荷转移电阻增大,锂离子扩散系数减小,进而电子电导率和离子电导率下降.温度升高后,Li Ni0.5Mn1.5O4材料的溶液电阻变化不大,但是电荷转移电阻逐渐增大,锂离子扩散系数逐渐减小;此外,随着温度的升高,Li Ni0.5Mn1.5O4材料的溶解速度加快,从而导致SEI膜的厚度增大.Li Ni0.5Mn1.5O4材料的嵌脱锂动力学与温度和循环次数有密切关系.     

多信道认知无线电频谱感知时间和门限联合分配

为提高多信道认知无线电的吞吐量,提出采用交替方向优化联合分配次用户的频谱感知时间和子信道感知门限.基于"先听后传"的次用户帧结构,建立了感知时间和门限的优化分配模型.该模型在保证满足主用户通信需求和子信道频谱感知性能的前提下,最大化次用户各子信道的吞吐量总和.联合优化算法通过交替优化感知时间和门限能够获得模型的最优解.仿真结果表明:存在最优的感知时间和门限最大化次用户的吞吐量,并且相比之前的方案,联合分配能够提高次用户的吞吐量.     

Rheological behaviors of fresh cement pastes with polycarboxylate superplasticizer

The rheologicalbehaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated.Influentialfactors including superplasticizer to cement ratio(Sp/C),water to cement ratio(w/c),temperature,and time were discussed.Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0,20 and 40 °C,respectively.Flowability and rheologicaltests on cement pastes were conducted to characterize the development of the rheologicalbehavior of fresh cement pastes over time.The exprimentalresults indicate that the initialflowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer.Higher temperature usually leads to a sharper drop in initialflowability and flowability retention.However,for the cement paste with high Sp/C or w/c,the flowability is slightly affected by temperature.Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influentialfactors at low Sp/C.In the case of high Sp/C,yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature.Moreover,two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C,w/c,temperature and time are developed on the basis of the existing models,in which experimentalconstants can be extracted from a database created by the rheologicaltest results.     

区域电磁环境的动态特性表征

区域电磁环境中各种电磁干扰辐射信号种类繁多、调制体制各异,信号参数的动态变化,传播路径及其相应影响的不确定性,表现为电磁信号在时间、能量上相互干扰发生异常变化造成电磁环境动态性异常激烈,导致区域内接受设备系统性能下降。针对这一问题,提出基于分形理论与数学形态学相结合的方法,研究区域电磁环境动态特征,利用分形维数表征电磁环境动态性。首先,基于信号基础理论对区域接收点处电磁信号的集合进行建模;其次,利用数学形态学滤波原理对接收的电磁信号集合进行交替滤波,滤除电磁信号中的脉冲噪声和白噪声;最后,运用分形理论求解电磁环境接收点集合的关联维数,并对计算结果进行仿真分析,表明该方法能从时间、空间和能量上为电磁环境的动态性提供有利支撑。     

The Dynamical FSI Simulation for Bellows Inflating to Disperse Bomblets

According to the characteristics of fluid-structure interaction(FSI) in the process of metal bellows inflating to disperse bomblets,a 3D dynamical FSI model(W model) which describes the interaction between a viscous compressible flow and a structure undergoing large deformationis established.Then the dynamic characteristics of the metal bellows deformation, the changing law of the internal flow field and the motion law of the bombs are acquired.Where the internal pressure and bombs'' moving law are approximate to interior ballistic results, which indicates the W model established reasonably.Besides, considering gaps existing atthe bellows'' tow ends, the dynamical FSI modelcontaining gaps(Y model) is also built.The results of the W and Y models are compared and the results show that the existence of the gaps has little influence to the flow field and the stress distributionin the bellows,but obviously reduces the separation speed of the bomblets about 10%.     

全局优化的蝴蝶优化算法

针对基本蝴蝶优化算法中存在的易陷入局部最优值、收敛速度慢等问题,提出一种全局优化的蝴蝶算法,引入limit阈值来限定蝴蝶优化算法陷入局部最优解的次数,从而改变算法易陷入早熟的问题,结合单纯形策略优化迭代后期位置较差的蝴蝶使种群能够较快地找到全局最优解;将正弦余弦算法作为局部算子融入BOA中,改善迭代后期种群多样性下降的缺陷,加快算法跳出局部最优。在仿真模拟实验中与多个算法进行对比,结果表明改进算法的寻优性能更好。     

基于可信期望的跨域访问安全性研究

访问控制作为保护信息安全的主要手段,能够有效保证用户合法地访问网络资源。随着移动互联网的发展,跨域和跨系统等多域环境下的安全问题面临严峻挑战。为了满足云计算多域环境的访问需求,基于角色访问控制技术,提出一种适用于云计算多域环境的访问控制模型。该模型利用贝叶斯理论得出访问者的可信期望值,然后与预先设定的访问阈值进行比较,决定用户的访问请求是否被允许,且访问权限随着用户可信度动态变化而改变,避免了之前获得高信任值的用户因信任度变化而进行恶意攻击的风险。实验结果表明,提出模型不仅能减少高风险用户的访问请求量,且能满足为用户动态授权的需求。因此该模型可以有效解决云计算多域环境中的安全问题。     

基于GEP的高速公路通行费预测方法研究

高速公路通行费未来收入状况的预测对于高速公路运营管理、建设规划有着重要的指导意义。然而,通行费收入水平的变化受到多方面因素的影响,具有较强的非线性和复杂性,传统预测模型无法准确表达通行费收入的发展规律。本文针对复杂的高速公路通行费预测问题,建立了基于基因表达式编程算法(GEP)的高速公路通行费预测模型。该模型利用GEP算法建立通行费当前收入与历史数据之间复杂的函数关系,准确地刻画通行费收入随时间的发展规律。此外,针对节假日期间通行费减免政策的影响,提出了有效的修正模型。最后,本文采集了浙江沪杭甬高速公路股份有限公司等12家公司通行费收入的历史数据进行仿真实验,对比传统的ARIMA以及神经网络预测模型,结果充分验证了本文算法的有效性和准确性。     

基于用户分类的隐含因子模型研究*

针对现有隐含因子模型存在的新用户和项目的冷启动问题,提出基于用户分类的隐含因子模型,将用户分类信息融入到隐含因子的矩阵分解当中,先在原评分矩阵和用户分类信息的基础上使用指示函数和数据归一化等方法构建一个分类评分矩阵,再将分类评分矩阵融入到隐含因子模型的评分预测中。通过与传统隐含因子模型等方法在多个不同隐含因子个数上的实验比较分析,实验结果表明,改进模型能够不仅能解决新用户和项目的冷启动问题,还能有效降低预测评分的均方根误差,并提高预测推荐的准确度。