STABILITY ANALYSIS OF HOPFIELD NEURAL NETWORKS WITH TIME DELAY

ＩｎｔｒｏｄｕｃｔｉｏｎＩｎｒｅｃｅｎｔｙｅａｒｓｔｈｅｄｙｎａｍｉｃｂｅｈａｖｉｏｒｏｆｔｈｅｆｏｌｌｏｗｉｎｇＨｏｐｆｉｅｌｄｎｅｕｒａｌｎｅｔｗｏｒｋｓｍｏｄｅｌｗｉｔｈｔｉｍｅｄｅｌａｙｈａｖｅｂｅｅｎｉｎｖｅｓｔｉｇａｔｅｄｔｈｏｒｏｕｇｈｌｙＣｉ ｘｉ(ｔ) =-ｘｉＲｉ + ∑ｎｊ=1Ｔｉｊｆｊ(ｘｊ(ｔ-τｉｊ) ) +Ｉｉ　　 (ｉ=1 ,2 ,… ,ｎ) ,( 1 )ｗｈｅｒｅＲｉ,ＣｉａｎｄＩｉｒｅｐｒｅｓｅｎｔｒｅｓｉｓｔａｎｃｅ ,ｃａｐａｃｉｔａｎｃｅａｎｄｅｌｅｃｔｒｉｃｃｕｒｒｅｎｔ.Ｒｉ,Ｃｉａｒｅｐａ…     

The use of a virtual configuration in formulating constitutive equations for residually stressed elastic materials

Residual stress is the stress present in the unloaded equilibrium configuration of a body. Because residual stresses can significantly affect the mechanical behavior of a component, the measurement of these stresses and the prediction of their effect on mechanical behavior are important objectives in many engineering problems. Common methods for the measurement of residual stresses include various destructive experiments in which the body is cut to relieve the residual stress. The resulting strain is measured and used to approximate the original residual stress in the intact body. In order to predict the mechanical behavior of a residually stressed body, a constitutive model is required that includes the influence of the residual stress.In this paper we present a method by which the data obtained from standard destructive experiments can be used to derive constitutive equations that describe the mechanical behavior of elastic residually stressed bodies. The derivation is based on the idea that for each infinitesimal neighborhood in a residually stressed body, there exists a corresponding stress free configuration. We refer to this stress free configuration as the virtual configuration of the infinitesimal neighborhood. The derivation requires that the constitutive equation for the stress free material be known and invertible; it is used to relate the residual stress to the deformation of the virtual configuration into the residually stressed configuration. Although the concept of the virtual configuration is central to the derivation, the geometry of this configuration need not be determined explicitly, and it need not be achievable experimentally, in order to construct the constitutive equation for the residually stressed body.The general mathematical forms of constitutive equations valid for residually stressed elastic materials have been derived previously for a number of cases. These general forms contain numerous unknown material-response functions or material constants that must be determined experimentally. In contrast, the method presented here results in a constitutive equation that is an explicit function of residual stress and includes only the material parameters required to describe the stress free material.After presenting the method for the derivation of constitutive equations, we explore the relationship between destructive experiments and the theory used in the derivation. Specifically, we discuss the use of the theory to improve the design of destructive experiments, and the use of destructive experiments to obtain the data required to construct the constitutive equation for a particular material.     

金属材料的激光表面改性及其在摩擦学中的应用

作者对激光表面处理的各种方法和现有技术水平进行了综述介绍,并就其在改善金属材料表面摩擦磨损性能方面的研究和应用,以及今后的发展都作了简要的分析和讨论。激光相变硬化能使处理的工件形成具有表面压应力的硬质马氏体表面,因而可以降低表面的磨损速率。在金属工件上通过激光包覆高抗磨、抗热、抗腐蚀和抗疲劳的高硬质合金甚至致密陶瓷,在严格控制工件材料对包覆合金稀释的情况下,可使底材获得高的摩擦学性能。激光合金化通过选择合金元素和基底材料能有选择地改进低成本工件的表面,使其具有优异的物理、化学和机械性能。激光上釉是通过细化铸造组织、减少偏析及形成高度过饱和固溶体等亚稳定相乃至非晶态而提高了材料的耐磨性、抗氧化性和抗腐蚀性能。激光冲击硬化是通过在材料表层内形成冲击波而引起“损伤”来改变表层的组织和性能。     

拓扑绝缘体(Bi0.5Sb0.5)2Te3薄膜中的线性磁阻

本文报道了拓扑绝缘体(Bi_0.5Sb_0.5)₂Te₃薄膜中线性磁阻问题的系统性研究工作. 此体系中, 线性磁阻在很宽的温度和磁场范围内出现: 磁场高达18 T时磁阻仍没有饱和趋势, 并且当温度不高于50 K时, 线性磁阻的大小对温度的变化不敏感. 栅压调控化学势可明显改变线性磁阻的大小. 当化学势接近狄拉克点时, 线性磁阻最为显著. 这些结果说明电荷分布的不均匀性是引起该材料线性磁阻的根源.     

磁场中的拓扑绝缘体边缘态性质

用数值方法研究了拓扑绝缘体薄膜体系在外加垂直磁场 作用下其边缘态的性质. 磁场的加入通过耦合k+eA, 即Peierls势替换关系和 该作用导致的Zeeman交换场体现在哈密顿量中. 考虑窄条圆环状结构的二维InAs/GaSb/AlSb薄膜量子阱材料, 当其处于拓扑非平庸状态, 即量子自旋霍尔态时, 会出现受时间反演对称性保护的两支简并边缘态, 而在垂直磁场的作用下, 时间反演对称性被破坏, 这时能带将形成一条条的朗道能级, 原来简并的两支边缘态也会分开到朗道能级谱线的两侧, 从电子态密度的空间分布情况则可以看到边缘态分别局域在材料的两个边界. 随着磁场的增大, 位于同一边界上的不同 自旋极化的边缘态将出现分离: 一支仍然局域在边缘, 另一支则随外加磁场的增加而有逐渐演化到材料内部的趋势. 文中还计算了同一边界上的两支边缘态之间的散射, 结果表明由于两个边缘态在空间发生分离, 相互之间的散射被很大的压制, 得到了其散射随磁场增加没有明显变化的结论, 所以磁场并不会增强散射过程, 也没有破坏体拓扑材料的性质, 说明了量子自旋霍尔态在没有时间反演对称的情况下也可以有较强的稳定性.     

Cu对用于高速相变存储器的Sb2Te薄膜的结构及相变的影响研究

采用原位X射线衍射仪、拉曼光谱仪和X射线反射仪分别研究了Cu-Sb₂Te 薄膜的微结构、成键结构和结晶前后的密度变化. Sb₂Te薄膜的结晶温度随着Cu含量的增加而增大. 在10 at.%和14 at.% Cu的Sb₂Te薄膜中, Cu与 Te 成键, 结晶相由六方相的Cu₇Te₄、菱形相的Sb及六方相的Sb₂Te构成. 10 at.% 和14 at.% Cu 的Sb₂Te薄膜在结晶前后的厚度变化分别约为3.2%和 4.0%, 均小于传统的Ge₂Sb₂Te₅ (GST)薄膜. 制备了基于Cu-Sb₂Te薄膜的相变存储单元, 并测试了其器件性能. Cu-Sb₂Te器件均能在10 ns的电脉冲下实现可逆SET-RESET操作. SET和RESET操作电压随着Cu含量的增加而减小. 疲劳测试结果显示, Cu 含量为10 at.%和14 at.%的PCRAM单元的循环操作次数分别达到1.3×10⁴和1.5×10⁵, RESET和SET态的电阻比值约为100. Cu-Sb₂Te可以作为应用于高速相变存储器(PCRAM)的候选材料.     

混合堆第一壁中子辐照损伤模拟

利用蒙特卡罗程序和辐照损伤程序,通过构建模型,对常用的第一壁材料W,Fe,Be的中子辐照造成的离位损伤、裂变气体产生量进行了模拟计算,结果表明,混合堆与纯聚变堆相比,可以明显降低对第一壁材料的损伤要求。在W,Fe,Be三种材料之中,对于纯聚变堆来说,Be的离位损伤最小;对于混合堆来说,W的离位损伤、裂变气体产生量最低。从中子辐照损伤的角度来说,Be更适宜作纯聚变堆的第一壁材料,而W则更适宜作混合堆的第一壁材料。     

Novel 4-Aryl-2-amino-6-(naphthalene-1-yl)-3-cyanopyridine Derivates as Potential Organic Fluorescent Materials Prepared Under Microwave Irradiation via Three-Component Domino Reactions

A green and efficient method for direct preparation of 3-cyanopyridine from aromatic aldehydes is reported. A series of novel 4-aryl-2-amino-6-(naphthalene-1-yl)-3-cyanopyridine derivates were successfully prepared by one-pot synthesis of three-component domino reactions including aromatic aldehydes, 1-acetylnaphthalene, and dicyanomethane in the presence of ammonium acetate in a solution of ethanol under microwave irradiation. This synthetic method for these compounds possesses many advantages including greater yield and shorter reaction times compared with traditional methods. The structures of these compounds were confirmed by means of high-resolution mass spectrometry, ¹H NMR, ¹³C NMR, and infrared. Moreover, the results of their fluorescent spectra showed that these compounds have good fluorescence intensity excited by the visible light wavelength of 416 nm.     

氢气还原制备高电容多孔Mn3O4/rGO复合材料

Mn3O4资源丰富、结构稳定,具有较高的理论电容,是一种较有潜力的超级电容器材料,但其较差的导电性却阻碍了其在这方面应用。本论文以实心ε-MnO2微米粉体和具有亲水性的GO粉体为原料,在去离子水中通过磁力搅拌和冷冻干燥制备出具有良好接触的MnO2/GO复合粉体。然后经一步氢气还原处理,使实心ε-MnO2和GO同步转化为具有纳米多孔结构的多孔Mn3O4和RGO,从而获得分散均匀、具有高比表面积和优良导电性的高性能Mn3O4/RGO复合粉体。经电化学测试可知,Mn3O4/RGO复合粉体在2 mV/s的扫描速率下的比电容为25.2 F/g且具有较好的电容留存率。     

核壳纳米球钴基金属有机聚合物的制备及其储锂性能

以偏苯三甲酸和六水合硝酸钴为原料,通过水热法合成了2种反应时间不同的钴基金属有机聚合物（Co-MOP）。采用X射线衍射（XRD）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）和N₂吸附-脱附对Co-MOP材料进行了结构和形貌表征。将2种Co-MOP材料用作锂离子电池负极材料,并进行了电化学性能测试。结果表明,Co-MOP-12（水热反应12 h）展示出了优异的电化学性能,在100 mA·g^-1的电流密度下,Co-MOP-12电极的首圈可逆比容量达到979 mAh·g^-1,循环100圈后比容量高达1 345 mAh·g^-1。