改变飞艇重心位置控制纵向运动

本文针对目前平流层飞艇的运动控制效率较低的问题,重点研究了改变平流层飞艇重心的轴向位置来控制飞艇的纵向运动。首先讨论了考虑飞艇重心位置的力和力矩平衡,并利用德国斯图加特大学风洞试验的数据,进行了算例分析。然后和常用的升降舵控制进行效率比较,得出了在空气密度较低的平流层采用改变重心位置比舵面控制具有优越性的结论。最后提出了改变飞艇重心位置的方法。     

Sample Entropy as a Tool to Assess Lumbo-Pelvic Movements in a Clinical Test for Low-Back-Pain Patients

Low back pain (LBP) obviously reduces the quality of life but is also the world’s leading cause of years lived with disability. Alterations in motor response and changes in movement patterns are expected in LBP patients when compared to healthy people. Such changes in dynamics may be assessed by the nonlinear analysis of kinematical time series recorded from one patient’s motion. Since sample entropy (SampEn) has emerged as a relevant index measuring the complexity of a given time series, we propose the development of a clinical test based on SampEn of a time series recorded by a wearable inertial measurement unit for repeated bending and returns (b and r) of the trunk. Twenty-three healthy participants were asked to perform, in random order, 50 repetitions of this movement by touching a stool and another 50 repetitions by touching a box on the floor. The angular amplitude of the b and r movement and the sample entropy of the three components of the angular velocity and acceleration were computed. We showed that the repetitive b and r “touch the stool” test could indeed be the basis of a clinical test for the evaluation of low-back-pain patients, with an optimal duration of 70 s, acceptable in daily clinical practice.     

Dislocation Analysis for Large-sized Sapphire Single Crystal Grown by SAPMAC Method

In this paper,large-sized sapphire (230×210 mm,27.5 kg) was grown by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at the cooled center). Dislocation peculiarity in large sapphire boule (0001) basal plane was investigated by chemical etching,scanning electron microscopy and X-ray topography method. The triangular dislocation etch pit measured is 7.6×101～8.0×102 cm-2,in which relative high-density dislocations were generated at both initial and final stages of crystal growth. The analysis of single-crystal X-ray topography shows that there are no apparent sub-grain boundaries; the dislocation lines are isolated and straight. Finally,the origins of low-density dislocation in sapphire crystal are discussed by numerical analysis method.     

动态载荷下剪切变形局部化、微结构演化与剪切断裂研究进展

总结和评述了近年来金属与合金变形局部化的形成、微结构演化与剪切断裂方面作者和相关的研究工作成果. 材料包括低碳钢, SS304不锈钢, Fe-15%Ni-15%Cr单晶, Al-Li合金,α-Ti和Ti-6Al-4V, Al/SiCp复合材料等.综述内容主要包括:采用改进的Hopkinson扭杆装置,对剪切变形局部化形成、发展和演化过程进行了实验观察与数值模拟;采用"侧剖"与"对接"等定点方法制备电子显微镜薄膜试样,对剪切带内相变与再结晶、非晶转变、旋涡结构等动态变形现象,以及与宏观动态力学行为对应的位错胞的形成、发展和坍塌等微结构特征进行了观测;提出了应变和应变率同时作为剪切带形成的两个必要条件的直接实验证据;在剪切带内发现了α'$-马氏体相变现象,以及相变产物与母体之间的晶体学关系;通过位错单滑移或交滑移等微观剪切最后发展成为宏观剪切的机制;对剪切带内再结晶结构的观测和对再结晶动力学本构关系的定量描述;对剪切带特别是``白色'腐蚀带(或相变带)的形成机制的分析和新的解释,指出 ``白色'是带内亚结构取向趋于一致,其在光学或扫描显微镜下很难辨认这些亚结构的取向差所致,并非表明剪切带内一定发生了相变;通过截断实验和实时跟踪观测发现,剪切带内微裂纹的萌生与聚合是材料承载能力骤然下降并导致最后断裂的主控因素.此外,本文对近年来在准静态和循环加载下材料的局部化形变与剪切断裂的实验结果予以简要评述,指出其微观机制与动态载荷下的截然不同, 是由位错的平面滑移所控制的,与热效应无关的等温变形.      

6英寸低位错锗单晶生长热场设计

锗片作为衬底材料已在空间太阳电池领域得到广泛的应用,新型锗基空间太阳能电池对锗片的需求由4英寸(1英寸=2.54 cm)提高到6英寸后,低位错锗单晶的生长难度增大.本文设计开发了一种适用于直拉法生长大尺寸、低位错锗单晶的双加热器热场系统,模拟研究了不同形状主加热器的热场分布,从而得到最优的热场环境.研究发现:渐变长度为...     

Si中30度部分位错弯结运动特性的分子模拟

首先使用分子动力学方法(MD)得出了左弯结(LK)和右弯结(RK)在不同温度和剪应力作用下的速度特性和运动过程.然后利用基于紧束缚势(TB)的nudged elastic band(NEB)方法计算LK和RK的迁移势垒.由计算结果得出,单个LK或RK的势垒很高,运动速度相对较慢;LK中的多弯结对结构和由RK分解产生的右弯结-重构缺陷(RC)能够加速位错运动;其中,RC能促进30°部分位错更快地迁移.     

基于生存概率的保险基金最优投资策略研究

利用破产理论和随机控制理论研究保险基金最优投资策略,建立生存概率最大化的目标函数,得到最优投资策略满足的随机微分方程;在初始金逼近0时得到保险基金的最优投资策略的显示解;采用递推算法,得到初始准备金为任意值时的最优投资策略.     

Testing of Multifractional Brownian Motion

Fractional Brownian motion (FBM) is a generalization of the classical Brownian motion. Most of its statistical properties are characterized by the self-similarity (Hurst) index 0<H<1. In nature one often observes changes in the dynamics of a system over time. For example, this is true in single-particle tracking experiments where a transient behavior is revealed. The stationarity of increments of FBM restricts substantially its applicability to model such phenomena. Several generalizations of FBM have been proposed in the literature. One of these is called multifractional Brownian motion (MFBM) where the Hurst index becomes a function of time. In this paper, we introduce a rigorous statistical test on MFBM based on its covariance function. We consider three examples of the functions of the Hurst parameter: linear, logistic, and periodic. We study the power of the test for alternatives being MFBMs with different linear, logistic, and periodic Hurst exponent functions by utilizing Monte Carlo simulations. We also analyze mean-squared displacement (MSD) for the three cases of MFBM by comparing the ensemble average MSD and ensemble average time average MSD, which is related to the notion of ergodicity breaking. We believe that the presented results will be helpful in the analysis of various anomalous diffusion phenomena.     

Highly sensitive,stretchable and durable strain sensors based on conductive double-network polymer hydrogels

Hydrogel-based strain sensors have been attracting immense attention for wearable electronic devices owing to their intrinsic soft characteristics and flexibility. However, developing hydrogel sensors with hightensile strength, stretchability, and strain sensitivity remains a great challenge. Herein, we report a technique to synthesize highly sensitive hydrogel-based strain sensors by integrating carbon nanofibers (CNFs) with a double-network (DN) polymer hydrogel matrix comprising of a physically cross-linked agar network and a covalently cross-linked polyacrylamide (PAAm) network. The resultant nanocomposite sensors display superior piezoresistive sensitivity with a hightrue gauge factor (GF_T = 1.78) at an ultrahigh strain of 1,000%, a fast response time and linear correlation of ln(R/R₀) and ln(L/L₀) up to 1,000% strain. Most significantly, these sensors possess highmechanical strength (~0.6 MPa) and superb durability (>1,000 cycles at strain of 100%), stemming from the effective energy dissipation mechanism of the first agar network acting as sacrificial bonds and the CNFs serving as dynamic nanofillers. The combination of highstrain sensitivity and ultrahigh stretchability of hydrogel sensors makes it possible to sense both small mechanical deformations induced by human motions and large strain up to 1,000%.     

H2+分子双H核对高次谐波辐射的贡献

理论研究了H2+分子双H核对高次谐波辐射的贡献。结果表明：在少周期激光场下，由于激光场的反对称性，负向H核辐射谐波强度高于正向H核。随着激光脉宽增大，激光波形趋于对称，因此导致双H核辐射谐波的反对称结构减小。谐波辐射的时频分析图显示，当激光场为正向时(E(t) > 0.0)，负向H核辐射谐波强度高于正向H核；当激光场反向时(E(t) < 0.0)，正向H核辐射谐波强度高于负向H核。最后，通过分析含时电子波包及H2+的缀饰态给出了电子在双H核之间转移的原因。