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排序方式: 共有193条查询结果,搜索用时 578 毫秒
91.
In this paper, we study the tail behavior of the stationary queue length of an M/G/1 retrial queue. We show that the subexponential
tail of the stationary queue length of an M/G/1 retrial queue is determined by that of the corresponding M/G/1 queue, and
hence the stationary queue length in an M/G/1 retrial queue is subexponential if the stationary queue length in the corresponding
M/G/1 queue is subexponential. Our results for subexponential tails also apply to regularly varying tails, and we provide
the regularly varying tail asymptotics for the stationary queue length of the M/G/1 retrial queue.
AMS subject classifications: 60J25, 60K25 相似文献
92.
Hua Q Shang D Zhang W Chen K Chang S Ma Y Jiang Z Yang J Huang W 《Langmuir : the ACS journal of surfaces and colloids》2011,27(2):665-671
The morphological evolution of uniform Cu(2)O nanocrystals with different morphologies in a weak acetic acid solution (pH = 3.5) has been studied for cubic, octahedral, rhombic dodecahedral, {100} truncated octahedral, and {110} truncated octahedral nanocrystals. Cu(2)O nanocrystals undergo oxidative dissolution in weak acid solution, but their morphological changes depend on the exposed crystal planes. We found that the stability of Cu(2)O crystal planes in weak acid solution follows the order of {100} ? {111} > {110} and determines how the morphology of Cu(2)O nanocrystals evolves. The stable {100} crystal planes remain, and new {100} facets form at the expense of the less stable {111} and {110} crystal planes on the surface of Cu(2)O nanocrystals. Density functional theory calculations reveal that the Cu-O bond on Cu(2)O(100) surface has the shortest bond length. These results clearly exemplify that the morphology of inorganic crystals will evolve with the change of local chemical environment, shedding light on fundamentally understanding the morphological evolution of natural minerals and providing novel insights into the geomimetic synthesis of inorganic materials in the laboratory. 相似文献
93.
LiFePO4/C composite cathode material has been synthesized by a carbothermal reduction method using β-FeOOH nanorods as raw materials and glucose as both reducing agent and carbon source. The results indicate that the content of carbon and the morphology of raw material have effect on the electrochemical performance of the final LiFePO4/C material. Sample LFP14 with a carbon content of 2.79 wt.% can deliver discharge capacities of 158.8, 144.3, 111.0, and 92.9 mAh g?1 at 0.1, 1, 10, and 15 C, respectively. When decreasing the current from 15 C back to 0.1 C, a discharge capacity of 157.5 mAh g?1 is recovered, which is 99.2 % of its initial capacity. Therefore, as a kind of cathode material for lithium ion batteries, this LiFePO4/C material synthesized via a carbothermal reduction method is promising in large-scale production, and has potential application in upcoming hybrid electric vehicles or electric vehicles. 相似文献
94.
Xuanyu Zhang Rui You Zeyue Wei Xiao Jiang Jiuzhong Yang Yang Pan Peiwen Wu Qingdong Jia Zhenghong Bao Lei Bai Mingzhou Jin Bobby Sumpter Victor Fung Weixin Huang Zili Wu 《Angewandte Chemie (International ed. in English)》2020,59(21):8042-8046
Although hexagonal boron nitride (h‐BN) has recently been identified as a highly efficient catalyst for the oxidative dehydrogenation of propane (ODHP) reaction, the reaction mechanisms, especially regarding radical chemistry of this system, remain elusive. Now, the first direct experimental evidence of gas‐phase methyl radicals (CH3.) in the ODHP reaction over boron‐based catalysts is achieved by using online synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV‐PIMS), which uncovers the existence of gas‐phase radical pathways. Combined with density functional theory (DFT) calculations, the results demonstrate that propene is mainly generated on the catalyst surface from the C?H activation of propane, while C2 and C1 products can be formed via both surface‐mediated and gas‐phase pathways. These observations provide new insights towards understanding the ODHP reaction mechanisms over boron‐based catalysts. 相似文献
95.
Zhenhua Zhang Hong Wu Zongyou Yu Rui Song Kun Qian Xuanye Chen Jie Tian Wenhua Zhang Weixin Huang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(13):4320-4324
The identification of the contribution of different surface sites to the catalytic activity of a catalyst nanoparticle is one of the most challenging issues in the fundamental studies of heterogeneous catalysis. We herein demonstrate an effective strategy of using a series of uniform cubic Cu2O nanocrystals with different sizes to identify the intrinsic activity and contributions of face and edge sites in the catalysis of CO oxidation by a combination of reaction kinetics analysis and DFT calculations. Cu2O nanocrystals undergo in situ surface oxidation forming CuO thin films during CO oxidation. As the average size of the cubic Cu2O nanocrystals decreases from 1029 nm to 34 nm, the dominant active sites contributing to the catalytic activity switch from face sites to edge sites. These results reveal the interplay between the intrinsic catalytic activity and the density of individual types of surface sites on a catalyst nanoparticle in determining their contributions to the catalytic activity. 相似文献
96.
97.
Fuqing Zhang Yihui He Weixin Li Lei Zhao Hui Chen Xuan He Jia Guo 《Chemical Papers》2018,72(6):1417-1426
The fabrication of low-cost and environmentally friendly photocatalysts with superior photodegradation efficiency remains one of the most pressing challenges in present research. Herein, we elaborately designed and synthesized a promising N-doped carbon quantum dots/CeO2 hollow microsphere (NCQDs/h-CeO2) photocatalyst with superior photoresponse property and photoactivity in visible light. The removal efficiencies of rhodamine B (RhB) under visible light illumination for NCQDs/h-CeO2 exhibits a 30.3% enhancement compared with pure h-CeO2. The results of EIS and PL imply that the excellent performance may be attributed to the strong synergistic effect between NCQDs and h-CeO2, thus effectively promoting charge transfer and restraining the recombination of photogenerated holes and electrons. Accordingly, a synergistic photocatalysis mechanism was proposed to explain the photocatalytic reaction process. Besides, the NCQDs/h-CeO2 exhibits better cycle stability than common CQDs/h-CeO2 after a four-cycle photocatalytic test. Therefore, the NCQDs/h-CeO2 may represent a promising strategy for the current water pollution issues. 相似文献
98.
仿生超疏水性表面的生物应用 总被引:1,自引:0,他引:1
自然给科学家和工程师带来仿生的灵感和启发. 近年来, 受自然界中荷叶的启发, 在充分考虑表面形貌和化学组成协同效应的基础上, 人们已经制备出许多仿生超疏水性表面, 这些表面在抗结冰、微流体、生物相容性等领域具有很多潜在的应用价值. 仿生超疏水性表面在生物领域的应用逐渐崭露头角, 研究发现, 超疏水性表面所俘获的空气能够减缓药物释放的速率, 因此利用此类表面作为药物的载体有望实现长期供药. 超疏水特性能在一定程度改善和提高生物体与材料表面之间的相互作用, 例如, 血小板几乎不在超疏水表面上进行粘附和活化避免了造成血栓和血凝, 因此仿生超疏水性表面可用于制备人造血管和与血液相接触的仪器. 细胞和生物分子在不同特殊润湿性表面具有不同的行为和现象, 如粘附、繁殖、吸附等差异, 这有助于进一步探索研究细胞和生物分子的信息功能, 是当前仿生超疏水性表面应用的重要研究方向之一. 本综述简单介绍了经典的润湿模型, 重点总结了仿生超疏水表面在生物领域的应用, 其主要包括控制药物释放、提高血液相容性、蛋白质吸附研究、细胞行为研究、生物分子和细胞微图案化等. 最后, 对仿生超疏水性表面在生物领域研究应用进行了展望. 相似文献
99.
In this article, we propose and study a new class of semiparametric mixture of regression models, where the mixing proportions and variances are constants, but the component regression functions are smooth functions of a covariate. A one-step backfitting estimate and two EM-type algorithms have been proposed to achieve the optimal convergence rate for both the global parameters and the nonparametric regression functions. We derive the asymptotic property of the proposed estimates and show that both the proposed EM-type algorithms preserve the asymptotic ascent property. A generalized likelihood ratio test is proposed for semiparametric inferences. We prove that the test follows an asymptotic \(\chi ^2\)-distribution under the null hypothesis, which is independent of the nuisance parameters. A simulation study and two real data examples have been conducted to demonstrate the finite sample performance of the proposed model. 相似文献
100.
Shilong Chen Ali M. Abdel‐Mageed Dan Li Joachim Bansmann Sebastian Cisneros Johannes Biskupek Weixin Huang R. Jürgen Behm 《Angewandte Chemie (International ed. in English)》2019,58(31):10732-10736
Ru/TiO2 catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO2‐ and H2‐rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non‐deactivating Ru/TiO2 catalysts by engineering the morphology of the TiO2 support. Using anatase TiO2 nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO2‐{100} and Ru/TiO2‐{101} are very stable, while Ru/TiO2‐{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal–support interactions (MSIs). The stronger MSIs on the defect‐rich TiO2‐{100} and TiO2‐{101} supports stabilize flat Ru nanoparticles, while on TiO2‐{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO2‐{001}. 相似文献