磁共振无线能量传输拓扑结构特性研究

研究磁共振无线能量传输拓扑结构的一般特性,即变输入(激励、损耗)条件下谐振能量在时域上的存储、振荡的谐振规律。建立了基于输入项的链式拓扑结构、分支拓扑结构的能量谐振模型,通过求解两种拓扑结构变激励、变损耗输入条件下的能量振荡时域解,分别得到链式拓扑条件下的距离增强条件和分支拓扑结构下的能量分配条件。以单一中继拓扑结构和两负载拓扑为实例,在相隔线圈无耦合、负载之间无耦合的假设条件下,建立了基于能量的最大传输容量模型和负载间能量"抢夺"模型,实验验证了链式拓扑结构最大传输距离增强条件和分支拓扑结构能量分配模型的正确性。     

(3~2.4.3.4)晶格声子晶体的多点缺陷试验研究

由于复式晶格能够降低结构对称性,使晶体比较容易打开多个完全带隙。设计有限尺寸新型多点缺陷形式的复式晶格声子晶体,可以形成宽频段声波的局域态和波导。通过试验研究二维(32.4.3.4)晶格钢/水声子晶体的Zigzag型多点缺陷性质,结合超胞技术,并利用有限元法数值计算该声子晶体缺陷体系的能带结构、本征模态和透射谱。结果表明:数值计算和试验结果符合较好。对Zigzag型多点缺陷分析可知,其数量和形式决定高频(或低频)带隙内声波局域效应、本征模态和波导特性。因此研究复式晶格声子晶体缺陷性质,丰富了调控声波传播的手段,为设计新型声波器件提供了重要的思路。     

一维声子晶体振动带隙的带边模式研究

从一维声子晶体的波动方程出发,选用保持晶格对称性的原胞对声子晶体的能带结构进行解析分析.结果表明一维声子晶体带隙边缘的振动模式与其单个对称性原胞的固有振动模式完全一致,从而对一维声子晶体带隙特性的分析可以简化为对其单个对称性原胞固有模式的分析.研究表明,一维声子晶体中的振动模式是其各对称性原胞固有振动模式相互作用的耦合模式,在带隙边缘由于布拉格散射出现解耦效应,振动模式为解耦的驻波模式,从而与单个对称性原胞的固有振动模式一致.以有机玻璃及铝构成的声子晶体细直梁为例,采用振动试验对其振动传输特性进行测试,试验结果同理论分析结果基本吻合.这为声子晶体复杂梁杆结构的带隙特性研究及减振降噪设计提供了新的、简化的思路.     

多源不确定性下平面变胞机构运动可靠性分析

变胞机构可根据环境和工况的变化及需求,进行重组和重构,在机器人领域得到广泛应用.变胞机构可变拓扑结构特性,使得分析多源不确定性下变胞机构全构态运动可靠性更为困难.以多源不确定性下平面五杆变胞机构为研究对象,提出一种适应交变温度工况的变胞机构全构态运动可靠性分析方法.建立温度载荷下连杆制造公差-热变形、关节配合公差-热变形的耦合模型,采用服从截断正态分布的随机参数和区间参数描述连杆尺寸、转动副和移动副间隙的不确定性,基于变胞特性建立变胞机构全构态运动误差传递模型,结合区间优化算法和蒙特卡洛算法建立多源不确定性下变胞机构全构态运动可靠性模型.基于提出的方法,分析了交变温度工况下变胞机构全构态运动可靠性分布规律,并分析了不同孔轴配合、连杆公差等级及其热膨胀系数对变胞机构全构态运动可靠性的影响规律.     

太赫兹金属光子晶体结构研究

鉴于金属光子晶体结构在滤波、极化、传感、成像等领域的重要作用,研究了基于三角形晶格的金属光子晶体的超常透射、光子能带和表面波特性。采用三维时域有限差分法对金属光子晶体结构建模,同时解释超常透射机理;推导和讨论光子能带沿着ΓK和ΓM晶格方向的能带图,分析不同的表面波特征,并解释非对称入射下观察到的三角形的两个晶格方向模式分裂效应;建立了描述这种模式分裂效应的精确的表面模式谐振频率公式,并对金属光子晶体的色散进行了定量分析。研究结果可为太赫兹功能器件的研究提供参考。     

几何约束驱动的可重构位移子群建模理论

提出基于位移子群的构态重构理论模型,由此探讨约束驱动的空间变胞运动链连续衍生设计策略。通过结合机构构态含拓扑与功能两方面变化特性分析,定义变胞机构中可重构特性的内涵,并将其封装进变胞结构单元,即变胞单元,依据此构建空间运动链构态特性的约束模型,分别构建构态约束矩阵与构态约束特征图,分析约束驱动生成运动链群的构型思想。具体是通过分析子群元素组合对应的构态变化,讨论约束作用性质与子群组合规律,确定几何约束引导单元的交、并运算规则,得出子群约束模型理论,驱动构态方案变化时对应的约束规则与方案衍生过程。深入研究构态约束图对空间变胞链中的拓扑与功能特征尤其含自由度特征的作用机制及约束操作规则、子群运动子集的组合规律,使得变胞运动链的设计转化为子群构态约束特征中元素的改变问题,由此构建变胞机构的多构态结构模型向约束驱动几何模型映射的建模方法,为变胞运动链群的构型方案自动生成与设计提供理论支持。     

一种新型并联变胞机构的设计与分析

3-UPU并联机构的动静平台虎克铰关节轴线在满足不同的几何约束条件下可分别具有平动和转动的输出特性。根据变胞机构理论中改变运动副方位特征实现构态变换的原理,设计一种3-UPU并联机构构态转换实现机构。此机构由结构相同的上下两部分运动链组成,通过改变其连接关系可实现3-UPU并联机构动静平台虎克铰轴线在平行和汇交之间变化,达到改变其运动输出的目的,从而形成一种新型并联变胞机构。由于变胞机构具有多组耦合关系,合适的构态转换位置是影响后续构态机构工作性能的重要因素,应用螺旋理论分别进行其各构态机构的自由度分析,并以机构工作空间为例分析构态转换位置点对其工作性能的影响。     

应变对耦合量子点空穴基态混合特性的影响

考虑应变对量子点结构产生的重要影响,采用六带K·P理论模型计算了耦合量子点系统在不同耦合距离下空穴基态及激发态的能态特性,探讨了应变效应对耦合量子点空穴基态反成键态特性的影响.计算结果表明,单轴应变对量子点的空穴能带有主要影响:首先它使重空穴(HH),轻空穴(LH)能级分裂增加,减少了HH,LH的混合;同时,改变了LH的束缚势垒,使得空穴基态波函数较多局限在底部量子点中.在不考虑应变的情况下,随着量子点之间耦合强度的减小,价带基态能级和激发态能级发生反交叉现象,基态从成键态翻转为反成键态.应变效应使得量子点的重空穴及轻空穴的能带发生改变,轻重空穴耦合减弱,基态和激发态之间发生成键、反成键态翻转的临界距离明显减小.     

变拓扑机构结构组成理论

研究得出了变拓扑机构(含变胞机构)新的结构组成理论。提出并定义变拓扑机构的不变元、不变子链、变元、可变子链、变胞器等新概念;给出并严格证明变拓扑机构结构组成的4个新定理,它们分别是变拓扑机构结构与其各阶段构态的关系定理;变拓扑机构的不变子链定理;变拓扑机构的可变子链定理;变拓扑机构的组成定理。研究得出并严格证明关联关系变胞器的等效转化定理。研究变胞器的分类,并给出构件变胞器、连接变胞器和关联关系变胞器的图形描述。变拓扑机构结构组成理论为变拓扑机构的结构分析和综合奠定了新的理论基础。     

面向变胞机构动力学建模的变拓扑构型数学描述方法

针对变胞机构的变拓扑特性,以拓扑学和多体系统动力学理论为基础,提出一种可由变胞源机构动力学模型自动生成任意子机构动力学模型的建模方法。首先,从变胞机构的拓扑组成和所受约束角度出发,重新定义了变胞源机构的内涵,将变胞源机构向子机构的切换方式简化为构件数减少和构件数不变两种情况。其次,运用关联矩阵描述变胞机构的构态切换过程,提出实现构态切换所需的单位消元矩阵Ei,j、关系消去矩阵Ij和关联关系变化矩阵G,将变胞机构的拓扑结构变化转化为一系列的矩阵运算,并给出了统一的数学模型。最后,建立关联矩阵、Huston低序体阵列和通路矩阵三者之间的转换关系,结合R-W方法分析变胞机构子构态动力学模型的自动生成机理,以具有3个构态的闭环五杆变胞机构为例,建立变胞源机构动力学模型,通过编程自动生成所有子构态动力学模型,并对机构的动力学特性进行分析,验证所提方法的有效性和实用性。     

Model-based quantification of EELS spectra: Including the fine structure

An extension to model-based electron energy loss spectroscopy (EELS) quantification is reported to improve the possibility of modelling fine structure changes in electron energy loss spectra. An equalisation function is used in the energy loss near edge structure (ELNES) region to model the differences between a single atom differential cross section and the cross section for an atom in a crystal. The equalisation function can be shown to approximate the relative density of unoccupied states for the given excitation edge. On a set of 200 experimental h-BN spectra, this technique leads to statistically acceptable models resulting into unbiased estimates of relative concentrations and making the estimated precisions come very close to the Cramér-Rao lower bound (CRLB). The method greatly expands the useability of model-based EELS quantification to spectra with pronounced fine structure. Another benefit of this model is that one also gets an estimate of the unoccupied density of states for a given excitation edge, without having to do background removal and deconvolution, making the outcome intrinsically more reliable and less noisy.     

Advances in EELS spectroscopy by using new detector and new specimen preparation technologies

First results obtained with a Gatan UHV Enfina system, which was attached to a VG HB 501 UX dedicated STEM, are reported. The Enfina system is based on a CCD detector and offers, compared to the previously used photodiode array, a narrower point‐spread function, higher sensitivity, and faster read‐out capabilities. These improvements are demonstrated with electron energy‐loss measurements on various oxides, such as Al₂O₃, TiO₂ and SrTiO₃. It is shown that a better energy resolution is achieved and that acquisition of high‐energy absorption edges with a reasonable signal‐to‐noise ratio becomes possible. Furthermore, we report on the influence of the TEM specimen quality on the energy‐loss spectra. Thin amorphous layers at the specimen surfaces, which are induced by ion‐milling processes, can modify specific electron energy‐loss near‐edge structure features. We found that for the investigated ceramics the use of low‐energy ion‐milling systems is highly recommended, since the loss of energy‐loss near‐edge structure details by the presence of the amorphous layers is considerably reduced. This is especially true for very thin specimens.     

Electron energy-loss near edge structure (ELNES) of InGaN quantum wells

Lasers and light‐emitting diodes (LEDs) that emit in the blue to green region are often based on In_xGa_1–xN quantum well structures. Ionization edges in the electron energy‐loss spectrum contain fine structures (called the energy‐loss near edge structure (ELNES)) and provide information about the electronic structure. In this paper we compare the experimental and calculated ELNES for the N‐K ionization edge of In_xGa_1–xN quantum wells. When the effects of the core‐hole are included in the calculations, agreement between experimental and calculated spectra is very good. Strain has been shown to accentuate the effects of In on the ELNES and moves the ionization edge onset down in energy, relative to the other features. These results suggest that ELNES may provide an alternative method to lattice imaging to determine the presence of strain in this system.     

Combining FIB milling and conventional Argon ion milling techniques to prepare high‐quality site‐specific TEM samples for quantitative EELS analysis of oxygen in molten iron

This paper reports a procedure to combine the focused ion beam micro‐sampling method with conventional Ar‐milling to prepare high‐quality site‐specific transmission electron microscopy cross‐section samples. The advantage is to enable chemical and structural evaluations of oxygen dissolved in a molten iron sample to be made after quenching and recovery from high‐pressure experiments in a laser‐heated diamond anvil cell. The evaluations were performed by using electron energy‐loss spectroscopy and high‐resolution transmission electron microscopy. The high signal to noise ratios of electron energy‐loss spectroscopy core‐loss spectra from the transmission electron microscopy thin foil, re‐thinned down to 40 nm in thickness by conventional Argon ion milling, provided us with oxygen quantitative analyses of the quenched molten iron phase. In addition, we could obtain lattice‐fringe images using high‐resolution transmission electron microscopy. The electron energy‐loss spectroscopy analysis of oxygen in Fe_0.94O has been carried out with a relative accuracy of 2%, using an analytical procedure proposed for foils thinner than 80 nm. Oxygen K‐edge energy‐loss near‐edge structure also allows us to identify the specific phase that results from quenching and its electronic structure by the technique of fingerprinting of the spectrum with reference spectra in the Fe‐O system.     

基于运动学的涡旋型线固有耦合机理研究

在涡旋压缩机型线设计研究中，针对涡旋型线暂啮合角固有耦合机理理论，提出了涡旋型线节点的概念，分析了涡旋型线节点的速度突变和加速度突变，通过微分几何理论分析并证明了涡旋型线固有耦合机理理论的正确性。结果表明：当特征几何边数增加时，曲率半径变化随之变小，速度突变相应随之变小，涡旋压缩机运转趋于平稳，振动和能量损失变小；当特征几何边数趋于无穷大时，曲率半径和速度突变趋向于零，涡旋压缩机运转最平稳，振动与能量损失最小；反之，特征几何边数减小时，曲率半径变化随之变大，速度突变大，涡旋压缩机运转不平稳，振动与能量损失变大。     

基于局部样本增益优化的α-shape曲面拓扑重建

在曲面重建中,提高棱边特征重建精度是逆向工程和计算机辅助设计制造等领域的难点问题。采用样点的近似拓扑近邻点集作为曲面局部样本,对α-shape算法进行优化,使α-shape尺度阈值能更为准确地反映样点分布密度,从而提高α-shape曲面拓扑重建结果的正确性。样点的近似拓扑近邻点集的获取本质上是欧氏近邻点集的增益优化,使后者向邻近的稀疏区域适度延伸,从而弥补因数据分布不均匀而导致的拓扑邻域信息缺失。基于增益优化后的样点近邻点集并结合曲面重建先验知识可确定α-shape尺度阈值,使α-shape曲面拓扑重建过程中尺度阈值可自适应调整。试验表明:该算法使所得网格曲面基本不含孔洞和棱边凹痕,能更好保持棱边特征的形位精度,可减少初次过滤结果中的非流形面片,同时具有与主流Delaunay网格过滤算法相近的重建效率。     

EELS quantification of M edges by using oxidic standards

A new method for quantifying electron energy loss spectra containing M edges is proposed in which the scattering cross-section of the M edge is determined by the use of a stoichiometric oxidic standard. A spectrum of MoO₃ was used to determine the Mo M cross-section with respect to the calculated hydrogenic model cross-section of the O K edge. For example spectra of MoSi₂ are quantified using Mo M cross-sections determined with MoO₃ and Si L cross-sections calculated with the hydrogenic model.     

碰摩故障转子-滚动轴承耦合系统非线性动力学研究

针对滚动轴承支承下的转子碰摩故障机理分析和故障诊断问题,考虑滚动轴承非线性赫兹接触和轴承径向间隙,建立了含碰摩故障的转子-滚动轴承系统动力学模型。应用数值积分方法得到系统的非线性响应,利用时间波形图、分叉图、频谱图以及Poincaré映射图,研究了系统响应随转速、轴承间隙、碰摩刚度、偏心量以及碰摩间隙的变化规律,为有效诊断滚动轴承支承下的转子碰摩故障提供了理论依据。     

Momentum-resolved EELS and EMCD spectra from the atomic multiplet theory: Application to magnetite

While the energy loss near edge structures of metallic crystals can be calculated with a good accuracy using density functional theory based codes, core-level spectra of transition metal oxides show pronounced multiplet effects which are better described by atomic multiplet codes. We describe the formalism which allows to calculate momentum-resolved electron energy loss spectra in the electric dipole approximation from the atomic multiplet theory, and we apply this formalism to the calculation of energy loss magnetic chiral dichroic spectra of magnetic transition metal oxides. Explicit results are given for magnetite Fe₃O₄.