检索结果-南通市图书馆

Mechanical buckling induced periodic kinking/stripe microstructures in mechanically peeled graphite fla es from HOPG

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2015年第4期31卷 494-499页

作者： Manrui Ren Ze Liu quan-shui zheng Jefferson Zhe Liu Department of Engineering Mechanics and Center for Nano and Micro Mechanics Tsinghua University Department of Mechanical and Aerospace Engineering Monash University

Mechanical exfoliation is a widely used method to isolate high quality graphene layers from bulk graphite. In our recent experiments, some ordered microstructures, consisting of a periodic alternation of kinks and stripes, were observed in thin graphite flakes that were mechanically peeled from highly oriented pyrolytic graphite. In this paper, a theoretical model is presented to attribute the formation of such ordered structures to the alternation of two mechanical processes during the exfoliation：（1） peeling of a graphite flake and （2） mechanical buckling of the flake being sub- jected to bending. In this model, the width of the stripes L is determined by thickness h of the flakes, surface energy Y, and critical buckling strain ecr. Using some appropriate values of y and ecr that are within the ranges determined by other inde- pendent experiments and simulations, the predicted relations between the stripe width and the flake thickness agree reason- ably well with our experimental measurements. Conversely, measuring the L-h relations of the periodic microstructures in thin graphite flakes could help determine the critical mechan- ical buckling strain εcr and the interface energy γ.

关键词： HOPG Mechanical exfoliation Periodic microstructures Kinking Mechanical buckling

Control of surface wettability via strain engineering

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2013年第4期29卷 543-549页

作者： Wei Xiong Jefferson Zhe Liu Zhi-Liang Zhang quan-shui zheng Department of Engineering Mechanics and Center for Nano and Micro Mechanics Tsinghua University Department of Mechanical and Aerospace Engineering Monash University Department of Structural Engineering Norwegian University of Science and Technology (NTNU)

Reversible control of surface wettability has wide applications in lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a graphene sheet as a sam- ple material and molecular dynamic simulations, we demon- strate that strain engineering can serve as an effective way to control the surface wettability. The contact angles 0 of water droplets on a graphene vary from 72.5° to 106° under biaxial strains ranging from -10% to 10% that are applied on the graphene layer. For an intrinsic hydrophilic surface （at zero strain）, the variation of 0 upon the applied strains is more sensitive, i.e., from 0° to 74.8°. Overall the cosines of the contact angles exhibit a linear relation with respect to the strains. In light of the inherent dependence of the contact an- gle on liquid-solid interfacial energy, we develop an analytic model to show the cos 0 as a linear function of the adsorption energy Eads of a single water molecule over the substrate sur- face. This model agrees with our molecular dynamic results very well. Together with the linear dependence of Eads on bi- axial strains, we can thus understand the effect of strains on the surface wettability. Thanks to the ease of reversibly ap- plying mechanical strains in micro/nano-electromechanical systems, we believe that strain engineering can be a promis- ing means to achieve the reversibly control of surface wetta- bility.

关键词： Wettability ~ Strain engineering ~ Molecular dy-namics simulation

Effects of vacancies on interwall spacings of multi-walled carbon nanotubes

在线全文

学校读者我要写书评

暂无评论

Journal of Zhejiang University-Science A(Applied Physics & Engineering) 2010年第10期11卷 714-721页

作者： Ming-du MA Jefferson Zhe LIU Li-feng WANG Lu-ming SHEN quan-shui zheng Department of Engineering Mechanics Tsinghua University Beijing 100084 China Department of Mechanical and Aerospace Engineering Monash University Clayton Victoria 3168 Australia Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge Massachusetts 02139 USA School of Civil Engineering University of Sydney NSW 2006 Australia

We use molecular dynamics (MD) simulations to study the effects of vacancies on tube diameters and interwall spacings of multi-walled carbon nanotubes (MWCNTs). Two types of vacancies, double vacancy and three dangling-bond (3DB) single vacancy, are identified to have opposite effects on the tube size change, which explains the inconsistency of the experimentally measured interwall spacings of MWCNTs after electron beam irradiation. A theoretical model to quantitatively predict the shrunk structures of the irradiated MWCNTs is further developed. We also discuss the fabrications of prestressed MWCNTs, in which reduced interwall spacings are desired to enhance the overall elastic modulus and strength.

关键词： Multi-walled carbon nanotubes (MWCNTs) Noise carbon nanotubes Electron beam irradiation Defects

Changes of Leaf Morphological, Anatomical Structure and Carbon Isotope Ratio with the Height of the Wangtian Tree (Parashorea chinensis) in Xishuangbanna, China

在线全文

学校读者我要写书评

暂无评论

Journal of Integrative Plant Biology 2008年第2期50卷 168-173页

作者： Chun-Xia He Ji-Yue Li Ping Zhou Ming Guo quan-shui zheng Key Laboratory for Silviculture and Conservation Ministry of Education Beijing Forestry University Beijing 100083 China Viikki Tropical Resources Institute University of Helsinki FIN-00014 Helsinki Finland Department of Engineering Mechanics of Tsinghua University Beijing 100084 China

Leaf morphological and anatomical structure and carbon isotope ratio （δ^13C） change with increasing tree height. To determine how tree height affects leaf characteristics, we measured the leaf area, specific leaf mass （ratio of leaf mass to leaf area [LMA]）, thickness of the total leaf, cuticle, epidermis, palisade and sponge mesophyll, stomata traits and δ^13C at different heights of Parashorea chinensis with methods of light and scanning electron microscopy （SEM） and isotope-ratio mass spectrometry. The correlation and stepwise regression between tree height and leaf structure traits were carried out with SPSS software. The results showed that leaf structures and δ^13C differed significantly along the tree height gradient. The leaf area, thickness of sponge mesophyll and size of stomata decreased with increasing height, whereas the thickness of lamina, palisade mesophyll, epidermis, and cuticle, ratios of palisade to spongy thickness, density of stomata and vascular bundles, LMA and δ^13C increased with tree height. Tree height showed a significant relationship with all leaf indices and the most significant relationship was with epidermis thickness, leaf area, cuticle thickness, δ^13C. The δ^13C value showed a significantly positive relationship with LMA （R = 0.934）. Our results supported the hypothesis that the leaf structures exhibited more xeromorphic characteristics with the increasing gradient of tree height.

关键词： δ^13C anatomy morphology Parashorea chinensis ratio of leaf mass to leaf area.

Interlayer shear strength of single crystalline graphite

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2012年第4期28卷 978-982页

作者： Ze Liu Shou-Mo Zhang Jia-Rui Yang Jefferson Zhe Liu Yan-Lian Yang quan-shui zheng Department of Engineering Mechanics and Center for Nano and Micro Mechanics Tsinghua University100084 BeijingChina Department of Mechanical and Aerospace Engineering Monash UniversityClayton3800 VICAustralia National Center for Nanoscience and Technology Beijing 100190China

Reported values （0.2 MPa-7.0 GPa） of the interlayer shear strength （ISS） of graphite are very dispersed. The main challenge to obtain a reliable value of the ISS using conventional measuring methods was the unavailability of sufficiently large single crystalline graphite. Here we present a novel experimental method to measure the ISS, and obtain the value as -0.14 GPa. Our result can serve as an important basis for understanding mechanical behavior of graphite or graphene-based materials.

关键词： Shear strength Single crystalline graphite

Nonlinear Evolution of Aggregates with Inextensible Constraints

在线全文

学校读者我要写书评

暂无评论

Communications in Nonlinear Science and Numerical Simulation 1996年第3期1卷 8-12页

作者： Ming-Xiang CHEN Wei YANG and quan-shui zheng(Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China)E-mail: demyw@tsinghua. edu. cn Department of Engineering Mechanics Tsinghua University Beijing 100084 China

Crystalline and sendcrystalline polymers are formed as aggregates of grainswith evolving inextensible axes. This inextensible constraint leads to texture evolution underlarge plastic deformation, This paper reveals th... 详细信息

关键词： Nonlinear texture evolution crystalline polymers inextensible constraint

Progress of research in the application of ultrasound technology for the treatment of Alzheimer’s disease

在线全文

同方期刊数据库

学校读者我要写书评

暂无评论

Neural Regeneration Research 2025年第10期20卷 2823-2837页

作者： Qiuquan Cai Lianghui Meng Meina quan Ling Wang Jing Ren Chenguang zheng Jiajia Yang Dong Ming Academy of Medical Engineering and Translational Medicine Tianjin UniversityTianjinChina Innovation Center for Neurological Disorders Xuanwu HospitalCapital Medical UniversityBeijingChina Brain-Computer Interaction and Human-Machine Fusion Haihe Laboratory Tianjin UniversityTianjinChina Tianjin Key Laboratory of Brain Science and Neural Engineering Tianjin UniversityTianjinChina

Alzheimer’s disease is a common neurodegenerative disorder defined by decreased reasoning abilities,memory loss,and cognitive *** presence of the blood-brain barrier presents a major obstacle to the development of effective drug therapies for Alzheimer’s *** use of ultrasound as a novel physical modulation approach has garnered widespread attention in recent *** a safe and feasible therapeutic and drug-delivery method,ultrasound has shown promise in improving cognitive *** article provides a summary of the application of ultrasound technology for treating Alzheimer’s disease over the past 5 years,including standalone ultrasound treatment,ultrasound combined with microbubbles or drug therapy,and magnetic resonance imaging-guided focused ultrasound *** is placed on the benefits of introducing these treatment methods and their potential *** found that several ultrasound methods can open the blood-brain barrier and effectively alleviate amyloid-βplaque *** believe that ultrasound is an effective therapy for Alzheimer’s disease,and this review provides a theoretical basis for future ultrasound treatment methods.

关键词： Alzheimer’s disease blood-brain barrier drugs magnetic resonance imaging-guided focused ultrasound microbubbles scanning ultrasound ultrasound ultrasound stimulation

ORIENTATION DISTRIBUTION FUNCTIONS FOR MICROSTRUCTURES OF HETEROGENEOUS MATERIALS (Ⅱ)──CRYSTAL DISTRIBUTION FUNCTIONS AND IRREDUCIBLE TENSORS RESTRICTED BY VARIOUS MATERIAL SYMMETRIES

在线全文

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2001年第8期22卷 885-903页

作者： zheng quan-shui(郑泉水) FU Yi-bin(傅依斌) Department of Engineering Mechanics Tsinghua University Beijing 100084 P R China Department of Mathematics University of Keele Staffordshire ST5 586 UK

The explicit representations for tensorial Fourier expansion of 3_D crystal orientation distribution functions (CODFs) are established. In comparison with that the coefficients in the mth term of the Fourier expansion of a 3_D ODF make up just a single irreducible mth_order tensor, the coefficients in the mth term of the Fourier expansion of a 3_D CODF constitute generally so many as 2m+1 irreducible mth_order tensors. Therefore, the restricted forms of tensorial Fourier expansions of 3_D CODFs imposed by various micro_ and macro_scopic symmetries are further established, and it is shown that in most cases of symmetry the restricted forms of tensorial Fourier expansions of 3_D CODFs contain remarkably reduced numbers of mth_order irreducible tensors than the number 2m+1 . These results are based on the restricted forms of irreducible tensors imposed by various point_group symmetries, which are also thoroughly investigated in the present part in both 2_ and 3_D spaces.

关键词： crystal orientation distribution function irreducible tensor Fourier expansion microstructure material symmetry

同方期刊数据库

Genus Expanded Cut-and-join Operators and Generalized Hurwtiz Numbers

在线全文

学校读者我要写书评

暂无评论

Acta Mathematica Sinica,English Series 2016年第9期32卷 1089-1098页

作者： quan zheng College of Mathematics Sichuan University Chengdu 610064 P. R. China

To distinguish the contributions to the generalized Hurwitz number of the source Riemann surface with different genus, by observing carefully the symplectic surgery and the gluing formulas of the relative GW-invariants, we define the genus expanded cut-and-join operators. Moreover all normalized the genus expanded cut-and-join operators with same degree form a differential algebra, which is isomorphic to the central subalgebra of the symmetric group algebra. As an application, we get some differential equations for the generating functions of the generalized Hurwitz numbers for the source Riemann surface with different genus, thus we can express the generating functions in terms of the genus expanded cut-and-join operators.

关键词： Genus expanded cut-and-join operator differential algebra Hurwitz number generating function