非金属与金属的协同作用对镁基储氢合金电化学性能的影响

用机械球磨法分别以Ti、B、复合物TiB对非晶态Mg45Ti3V2Ni50储氢合金进行了表面修饰.实验结果表明,恰当比例的TiB球磨修饰对镁基储氢合金循环稳定性远好于Ti、B同比例单独修饰合金电极的效果.Mg45Ti3V2Ni50与TiB质量比为2∶1的Mg45Ti3V2Ni50-TiB(2∶1)复合合金电极的初始放电容量为529.4mAh·g-1,第50次循环放电容量仍为277.1mAh·g-1.复合物TiB中Ti、B元素之间和复合合金中合金元素与TiB之间产生了金属与非金属的协同作用,导致复合合金新的立体褶皱结构的生成,增强了修饰层与合金间的作用,Mg45Ti3V2Ni50-TiB(2∶1)合金电极表面活性增强,循环稳定性显著提高.     

Locally finite graphs with ends: A topological approach, II. Applications

This paper is the second of three parts of a comprehensive survey of a newly emerging field: a topological approach to the study of locally finite graphs that crucially incorporates their ends. Topological arcs and circles, which may pass through ends, assume the role played in finite graphs by paths and cycles. The first two parts of the survey together provide a suitable entry point to this field for new readers; they are available in combined form from the ArXiv [20]. They are complemented by a third part [31], which looks at the theory from an algebraic-topological point of view.The topological approach indicated above has made it possible to extend to locally finite graphs many classical theorems of finite graph theory that do not extend verbatim. This second part of the survey concentrates on these applications, many of which solve problems or extend earlier work of Thomassen on infinite graphs. Numerous new problems are suggested.     

Vertex-disjoint directed cycles of prescribed length in tournaments with given minimum out-degree and in-degree

In a recent paper, Bessy, Sereni and the author (see [3]) have proved that for r≥1, a tournament with minimum out-degree and in-degree both greater than or equal to 2r−1 contains at least r vertex-disjoint directed triangles. In this paper, we generalize this result; more precisely, we prove that for given integers q≥3 and r≥1, a tournament with minimum out-degree and in-degree both greater than or equal to (q−1)r−1 contains at least r vertex-disjoint directed cycles of length q. We will use an auxiliary result established in [3], concerning a union of sets contained in another union of sets. We finish by giving a lower bound on the maximum number of vertex-disjoint directed cycles of length q when only the minimum out-degree is supposed to be greater than or equal to (q−1)r−1.     

Liénard型系统解的有界性与极限环的存在性

本文讨论比Lienard系统更广的一类非线性系统 x=h(y)-F(x) y=-g(x)解的有界性与极限环的存在性,得到了解正向有界的充要条件和存在极限环的充分条件,推广和改进了文[1-5]的工作.     

Rational design of freestanding necklace-like ZnSe with double N-doped carbon layer protection for efficient sodium storage

ZnSe, as a promising electrode material for sodium storage, has a high theoretical capacity, low cost, and excellent physicochemical properties. The poor reaction kinetics and huge volume variation of ZnSe hinder its practical applications. Therefore, in this study, ZnSe electrode materials embedded in carbon nanofibers (ZnSe@NC@NCNFs) were synthesized through electrospinning and selenization using a Zn-based metal-organic framework (Zn-MOF) precursor. During the calcination process, the MOF-derived porous N-doped carbon layer wraps the ZnSe nanoparticles, and the one-dimensional (1D) carbon nanofiber forms a second N-doped carbon protective layer. The interwoven nanofibers can be severed as a freestanding electrode for sodium storage without conductive and binder agents. In situ X-ray diffraction (XRD) demonstrates the formation of irreversible NaZn₁₃ during the initial discharge process, which can act as sodiophilic sites and buffering matrices for subsequent Na⁺ insertion/extraction. The ZnSe@NC@NCNFs exhibit significant electrochemical performance for sodium storage with high reversible capacity and desired rate performance.     

Investigation of non-equilibrium adsorption character in solid adsorption refrigeration cycle

The adsorber is the key element in an adsorption refrigeration system. Its characteristic coupled with heat and mass transfer and adsorption dominats the cycle performance. In general, the velocity of adsorption in adsorber is not faster than that of heat transfer, that is, the adsorption rate does not reach the equilibrium value at each status point in real operation. The feature of non-equilibrium adsorption results of apparent different characteristic on adsorption refrigeration performance comparing with the case of equilibrium adsorption model. Based on numerical simulation, both models with equilibrium and non-equilibrium adsorption are compared with each other. The simulations condition includes various surface diffusing velocity coefficients. This paper shows the characteristic of temperature and adsorbed mass rate distribution in adsorber and some thermodynamic analysis on adsorption refrigeration cycle. It could be observed that non-equilibrium adsorption should be taken into account in adsorption refrigeration particularly for short cycle time and vacuum system.     

Enhanced Electrochemical Performance of Na0.67Fe0.5Mn0.5O2Cathode with SnO2 Modification

P2-type layered oxide Na_0.67Fe_0.5Mn_0.5O₂ is recognized as a very promising cathode material for sodium-ion batteries due to the merits of high capacity, high voltage, low cost, and easy preparation. However, its unsatisfactory cycle and rate performances remain huge obstacles for practical applications. Here, we report a strategy of SnO₂ modification on P2-type Na_0.67Fe_0.5Mn_0.5O₂ to improve the cycle and rate performance. Scanning electron microscope(SEM) and transmission electron microscope(TEM) images indicate that an insular thin layer SnO₂ is coated on the surface of Na_0.67Fe_0.5Mn_0.5O₂ after medication. The coating layer of SnO₂ can protect Na_0.67Fe_0.5Mn_0.5O₂ from corrosion by electrolyte and the cycle performance is well enhanced. After 100 cycles at 1 C rate(1 C=200 mA/g), the capacity of SnO₂ modified Na_0.67Fe_0.5Mn_0.5O₂ retains 83 mA·h/g(64% to the initial capacity), while the capacity for the pristine Na_0.67Fe_0.5Mn_0.5O₂ is only 38 mA·h/g(33.5% to the initial capacity). X-Ray photoelectron spectroscopy reveals that the ratio of Mn⁴⁺ increases after SnO₂ modification, leading to less oxygen vacancy and expanded lattice. As a result, the capacity of Na_0.67Fe_0.5Mn_0.5O₂ increases from 178 mA·h/g to 197 mA·h/g after SnO₂ modification. Furthermore, the rate performance of Na_0.67Fe_0.5Mn_0.5O₂ is enhanced with SnO₂ coating, due to high electronic conductivity of SnO₂ and expanded lattice after SnO₂ coating. The capacity of SnO₂ modified Na_0.67Fe_0.5Mn_0.5O₂ at 5 C increases from 21 mA·h/g(pristine Na_0.67Fe_0.5Mn_0.5O₂) to 35 mA·h/g.     

二次微分系统极限环的分布

在假设文中命题Ａ成立的条件下证明了一般二次微分系统的极限环所有可能的分布为（３,１）,（１,３）,（３,０）,（０,３）,（２,１）,（１,２）,（２,０）,（１,１）,（０,２）,（１,０）,（０,１）和（０,０）。