惯组小系统动力学建模与非线性特性分析

基于"三心重合"的设计思想和飞行器的姿态需求,建立了含立方项非线性刚度的惯组小系统动力学模型。提出了惯组小系统存在系统动刚度和减振器动刚度的"双层级"概念。利用龙格-库塔法求解渐软非线性系统对正弦扫频激励的响应,得出减振器动刚度存在对激励幅值和激励频率的敏感区域,采用动刚度曲线表征了减振器的非线性软化特性。利用虚弧长延拓法计算惯组小系统的非线性频响函数和传递特性,预示了高量级振动下减振器的动力失稳现象。通过惯组小系统传递特性试验,验证了减振器具有渐软刚度的非线性特性。扫频法计算结果与正弦扫频试验结果的吻合度达到96.5%,检验了构建模型的正确性。所建立的模型工程应用简便,对惯组小系统非线性特性的预示具有较高的精度,可供飞行器姿控系统设计时参考。     

Al2O3忆阻器中氧空位对阻值转换性能的影响研究

忆阻器能够在外加电压下实现高阻态与低阻态的转换,在存储器件及仿神经网络计算等方面有着重要的应用。本文通过在Si衬底上制备得到Pt-Al2O3-Pt的金属-绝缘层-金属结构的忆阻器器件,研究了氧空位对阻值转换性能的影响。利用原子层沉积技术工艺控制生长不同氧空位浓度的Al2O3薄膜,测量并比较其Ⅰ—Ⅴ循环曲线,发现仅有在氧空位浓度较高情况下忆阻器才能够实现在高阻态和低阻态之间的转换。本文实验结果表明氧空位对于实现阻值转换性能有着重要的影响,对生长制备忆阻器器件有着重要意义。     

一个R2上含双曲函数核的Hilbert型不等式

通过引入参数，构造了一个全平面上的、含双曲函数的非齐次核函数。利用正切函数的有理分式展开，建立了最佳常数因子与正切函数高阶导数相关联的Hilbert型积分不等式。 作为应用，通过赋予参数不同的值，建立了一些有意义的特殊结果。     

多元统计分析中一类矩阵迹函数最小化问题的有效算法

研究来源于多元统计分析中的一类矩阵迹函数最小化问题$\min c+ tr(AX)+\sum\limits_{j=1}^{m}tr(B_j X C_jX^{T}),\ \ {\rm s. t.} \ X^TX=I_p,$其中$c$为常数, $A\in R^{p\times n}\ (n\geq p)$, $B_j\in R^{n\times n}, C_j\in R^{p\times p}$为给定系数矩阵. 数值实验表明已有的Majorization算法虽可行, 但收敛速度缓慢且精度不高. 本文从黎曼流形的角度重新研究该问题, 基于Stiefel流形的几何性质, 构造一类黎曼非单调共轭梯度迭代求解算法, 并给出算法收敛性分析.数值实验和数值比较验证所提出的算法对于问题模型是高效可行的.     

广义Fermat型微分与微差分方程整函数解的存在性与形式

利用Nevanlinna值分布理论,主要讨论了几类广义Fermat型微分与微差分方程有限级超越整函数解,得到了二阶微分、微差分方程整函数解的存在性条件以及解的具体形式,所获定理推广了之前的结果,并举例说明了结果的精确性。     

Insights into the efficient charge separation over Nb2O5/2D-C3N4 heterostructure for exceptional visible-light driven H2 evolution

Two-dimensional carbon nitride(2 D-C₃ N₄)nanosheets are promising materials in photocatalytic water splitting,but still suffer from easy agglomeration and fast photogene rated electron-hole pairs recombination.To tackle this issue,herein,a hierarchical Nb₂ O₅/2 D-C₃ N₄ heterostructure is precisely constructed and the built-in electric field between Nb₂O₅ and 2 D-C₃ N₄ can provide the driving force to separate/transfer the charge carriers efficiently.Moreover,the strongly Lewis acidic Nb₂O₅ can adsorb TEOA molecules on its surface at locally high concentrations to facilitate the oxidation reaction kinetics under irradiation,resulting in efficient photogene rated electrons-holes separation and exceptional photocatalytic hydrogen evolution.As expected,the champion Nb₂O₅/2 D-C₃N₄ heterostructure achieves an exceptional H2 evolution rate of 31.6 mmol g^-1 h^-1,which is 213.6 times and 4.3 times higher than that of pristine Nb₂O₅ and2 D-C₃N₄,respectively.Moreover,the champion heterostructure possesses a high apparent quantum efficiency(AQE)of 45.08%atλ=405 nm and superior cycling stability.Furthermore,a possible photocatalytic mechanism of the energy band alignment at the hetero-interface is proposed based on the systematical characterizations accompanied by density functional theory(DFT)calculations.This work paves the way for the precise construction of a high-quality heterostructured photocatalyst with efficient charge separation to boost hydrogen production.     

Is it universal that the layered-spinel structure can improve electrochemical performance?

The introduction of spinel phase to form the layered-spinel structure(LSS)is an effective way to improve the electrochemical performance of Li-and Mn-rich layered oxides(LMR).But is this structure universal for all LMR systems?In this work,different Mn/Ni ratio systems with the LSS are discussed in detail.It is found that,high discharge capacity(200.8 mA h g^-1 at 1C rate;1C=250 mA h g^-1)as well as high capacity-retention(94%at 1C rate after 100 cycles)can be achieved by forming the LSS for low-Ni system(Mn/Ni=5.0).However,the capacity retention decreases severely in the high-Ni system(Mn/Ni=3.5,2.6).For example,when the ratio of Mn/Ni is 3.5,the capacity-retention of the layered-spinel sample was only 65.8%,compared to the 83%of the original LMR sample.The Ex-situ XRD,XPS,and HRTEM results demonstrate that the introduction of spinel phase in high-Ni system accelerates the transition and collapse of the crystal structure.This work provides guidance for optimizing the proportions of elements and the design of structures for the LMR.     

High temperature X-ray diffraction study of the formation of Na2Ti3O7 from a mixture of sodium carbonate and titanium oxide

Na2Ti3O7 has attracted much attention in the field of anode materials for Na-ion batteries thanks to its non-toxicity and very low working potential of 0.3 V vs Na0/Na+.Building a clearer picture of its formation from cheap Na₂CO₃ and TiO₂ starting materials is therefore of obvious interest.Here,we report new insights from an in-situ high temperature X-ray diffraction study conducted from room temperature to 800°C,complemented by ex-situ characterizations.We were thereby able to position the previously reported Na₄Ti₅O₁₂ and Na₂Ti₆O₁₃ intermediate phases in a reaction scheme involving three successive steps and temperature ranges.Shifts and/or broadening of a subset of the Na₂Ti₆O₁₃ reflections suggested a combination of intra-layer disorder with the well-established ordering of successive layers.This in-situ study was carried out on reproducible mixtures of Na₂CO₃ and TiO₂ in 1:3 molar ratio prepared by spraydrying of mixed aqueous suspensions.Single-phase Na₂Ti₃O₇ was obtained after only 8 h at 800°C in air,instead of a minimum of 20 h for a conventional solid-state route using the same precursors.Microstructure analysis revealed~15 mm diameter granules made up from rectangular rods of a fewmm length presenting electrochemical properties in line with expectations.In the absence of grinding or formation of intimate composites with conductive carbon,the specific capacity of 137 m Ah/g at C/5 decreased at higher rates.     

基于抛光基材的热蒸镀铜箔生长高平整单层石墨烯薄膜

在石墨烯的化学气相沉积工艺中,铜箔是决定石墨烯薄膜质量的重要因素。传统铜箔由于制备工艺的限制,存在大量的缺陷,导致石墨烯薄膜的成核密度较高。本工作选用抛光铝板、抛光不锈钢板、微晶玻璃和SiO₂/Si作为基材,用热蒸镀法制备了不同粗糙度的铜箔,并详细讨论了以该系列铜箔生长高平整度石墨烯薄膜的条件及铜箔对石墨烯薄膜品质的影响。实验结果表明,铜箔以(111)取向为主,与基材分离后,表面具有纳米级平整度。在生长石墨烯后,从SiO₂/Si剥离的铜箔成核密度是4种基材中最小的。同时,从SiO₂/Si剥离的铜箔晶体结构变化最不明显,具有良好的结晶性,表面几乎不存在铜晶界缺陷。当压强为3 000 Pa,氢气和甲烷流速分别为300 mL/min和0.5 mL/min时,可以获得约1 mm横向尺寸的石墨烯单晶晶畴。     

射频等离子制备石墨烯负载Co3O4催化剂及其析氧性能研究

氢能的引入能有效提升配电网的供电可靠性,而电解水制氢是实现低碳转型的关键技术,开发高效的电解水催化剂势在必行。过渡金属氧化物储量大、催化活性高,是具有广阔应用前景的析氧反应催化剂。本文通过射频等离子体处理制备石墨烯上负载Co₃O₄析氧催化剂,XRD、Raman和XPS测试结果显示,二维结构石墨烯的引入加速表面电子迁移,增大了反应面积。等离子体处理促进了纳米粒子在石墨烯上的负载,利用等离子体刻蚀作用在催化剂表面制造出大量碳结构缺陷和氧空位结构,改善了活性位点分布,有效调控Co₃O₄电子结构,提高析氧催化活性。电化学测试表明,本文中合成的Co₃O₄@rGO在电流密度为50 mA·cm^-2时的过电位为410 mV,动力学反应速率较快,表现出优于商业IrO₂的析氧催化活性。