石墨烯在金刚石基体表面的纳米摩擦学行为研究

采用热化学气相沉积法(Thermal Chemical Vapor Deposition,TCVD)和机械剥离法分别制备了单层和少层石墨烯并转移至MPCVD制备的多晶金刚石基体表面,利用原子力显微镜研究了大气环境下石墨烯在金刚石基体上的纳米摩擦和磨损性能. 研究结果表明:单层和少层石墨烯在金刚石基体上具有良好的减摩作用,摩擦系数分别为0.03和0.014. 然而,由于石墨烯和金刚石表面之间的物理吸附作用较弱,其摩擦力会略高于SiO₂/Si基体表面石墨烯的摩擦力. 随扫描速度升高,金刚石表面的单层与少层石墨烯的摩擦力的变化可以分为自然对数正比上升,基本保持不变以及黏性阻尼增加三个阶段. 在磨损试验中,TCVD法制备和转移石墨烯的过程中产生的缺陷和污染物降低了单层石墨烯的耐磨性能,而机械剥离的少层石墨烯因为无缺陷的石墨烯晶体结构在金刚石基体上展现了优异的耐磨特性. 本研究可为以金刚石为基体的石墨烯固体润滑剂使用提供理论基础.      

球形金属表面球磨制备石墨烯薄膜摩擦特性研究

采用球磨的方法实现了在钢球表面制备大面积连续的石墨烯薄膜,考察其随球磨时间变化,石墨烯薄膜在钢球表面的包裹程度、形貌变化、结构演变过程、结合性能及摩擦学性能. 研究表明:随着球磨时间的增加,石墨烯在钢球表面团聚减少,包裹更加均一,结构趋于有序;当球磨时间达到50 h时,在钢球表面形成分布均匀且大面积连续的石墨烯薄膜,使与含氢类金刚石碳薄膜组成配伍的平均摩擦系数从裸钢球的0.043降至0.022,磨痕深度和宽度都显著降低. 经胶带粘取100次或乙醇中超声清洗30 min后球磨制备石墨烯薄膜仍然粘附于钢球表面,在氩气环境下石墨烯薄膜表现出优于钢球的摩擦磨损性能.      

Atomistic–continuum coupled model for nonlinear analysis of single layer graphene sheets

In this paper, atomistic–continuum coupled model for nonlinear flexural response of single layer graphene sheet is presented considering von-Karman geometric nonlinearity and material nonlinearity due to atomic interactions. The strain energy density function at continuum level is established by coupling the deformation at continuum level to that of at atomic level through Cauchy–Born rule. Strain and curvature dependent tangent in-plane extensional, bending–extension coupling, bending stiffness matrices are derived from strain energy density function constructed through Tersoff–Brenner potential. The finite element method is used to discretize the graphene sheet at continuum level and nonlinear bending response with and without material nonlinearity is studied. The present results are also compared with Kirchhoff plate model and significant differences at higher load are observed. The effects of other parameters like number of atoms in the graphene sheet, boundary conditions on the central/maximum deflection of graphene sheet are investigated. It is also brought out that the occurrence of bond length exceeding cutoff distance initiates at corners for CFCC, CFCF, SFSS, SFSF graphene sheets and near center for SSSS and CCCC graphene sheets.     

Harmonic model of graphene based on a tight binding interatomic potential

Like in many other materials, the presence of topological defects in graphene has been demonstrated to modify its behavior, thus enhancing features aimed at several technological applications, more specifically, its electronic and transport properties. In particular, pristine defect-free graphene has been shown to be of limited use for semiconductor-based electronics, whereas the presence of individual or cluster defect rings along grain boundaries hinders electron transport and introduce a transport gap, unveiling the possibility of novel electronic device applications based on the structural engineering of graphene-based materials. In this work, we present an atomic bondwise force-constant model from the tight binding potential by Xu et al. (1992), that accounts for the electron-mechanical coupling effects in graphene. First we verify that this computational scheme is capable of accurately predicting the defect energies and core structures of dislocation dipoles based on the theory of discrete dislocations of Ariza and Ortiz (2005). In order to demonstrate our ability to characterize the effect of patterned distributions of structural defects on the electronic structure of graphene, we present the electronic band structures and density of states curves of several defective graphene sheets.     

Design of layered-stacking graphene assemblies as advanced electrodes for supercapacitors

Graphene is a competitive electrode material for supercapacitors due to its unique two-dimensional structure,large surface area,high conductivity,and good physi...     

Plasticity of formable all-metal sandwich sheets: Virtual experiments and constitutive modeling

The mechanical behavior of a metallic sandwich sheet material composed of two flat face sheets and two bi-directionally corrugated core layers is analyzed in detail. The manufacturing of the sandwich material is simulated to obtain a detailed unit cell model which accounts for the non-uniform thickness distribution and residual stresses associated with the stamping of the core layers. Virtual experiments are performed by subjecting the unit cell model to various combinations of bi-axial in-plane loading including the special cases of uniaxial tension, uniaxial compression, equi-biaxial tension and shear. The results demonstrate that the core structure’s contribution to the in-plane load carrying capacity of the sandwich sheet material is similar to that of the face sheets. The numerical results are also used to identify the effective yield surface and hardening response of both the core layer and the face sheets. An anisotropic yield function with linear pressure dependency is proposed to approximate the equal-plastic work surfaces for the core structure and face sheets. Furthermore, a new two-surface model with non-linear interpolation based on plastic work density is presented to describe the observed combined isotropic-distortional hardening of the core structure.     

Balancing particle properties for practical lithium-ion batteries

As a state-of-the-art secondary battery, lithium-ion batteries (LIBs) have dominated the consumer electronics market since Sony unveiled the commercial secondary battery with LiCoO₂ as the negative electrode material in the early 1990s. The key to the efficient operation of LIBs lies in the effective contact between the Li-ion-rich electrolyte and the active material particles in the electrode. The particle properties of the electrode materials affect the lithium ion diffusion path, diffusion resistance, contact area with the active material, the electrochemical performance and the energy density of batteries. To achieve satisfied comprehensive performance and of LIBs, it is not only necessary to focus on the modification of materials, but also to balance the properties of electrode material particles. Therefore, in this review, we analyze the influence of particle properties on the battery performance from three perspectives: particle size, particle size distribution, and particle shape. A deep understanding of the effect and mechanism of particles on electrodes and batteries will help develop and manufacture practical LIBs.     

Synthesis and methane storage of binder-free porous graphene monoliths

Nanomesh graphene (NMG) obtained by template chemical vapor deposition was used to synthesize the binder-free graphene monoliths by simple tablet pressing. The stacking manner of the NMG sheets was crucial to the cohesion interaction between the graphene sheets, only the NMG materials with a loosely stacking manner could be pressed into binder-free monoliths. At the tableting pressure of 2–8 MPa, both the bulk densities and the specific surface areas of the monoliths keep nearly constant as the tableting pressure increases, indicating that the NMG monoliths have obvious elasticity and a porous structure due to the large corrugations and the mesh structures of the graphene sheets. As a result, an extraordinary methane storage capacity of 236 (v/v) at 9 MPa was obtained in the graphene monolith prepared by tableting at 4 MPa.     

N-doped carbon nanotube encapsulated nZVI as a high-performance bifunctional catalyst for oxidative desulfurization

Great efforts have been made to remove sulfur from fossil fuels to protect the environment. We proposed synthesis of high efficiency oxidation desulfurization (ODS) catalysts by encapsulating nano zero valent iron (nZVI) in self-catalyzed carbon nanotubes. The synthetic strategy features facile hydrothermal and pyrolysis process. The specific surface area, pore structure, and microstructure of the catalysts were characterized by series techniques, and the catalytic ability was evaluated by the reduction of sulfur after oxidation and reflux-extraction. The optimized nZVI@CNT catalyst exhibits outstanding catalytic performance (within 120 min, the oxidative removal rate of DBT reached 96%) and enhanced stability (a 80% retention of initial performance after six cycles.), revealing the effective optimization and modulation between carbon nanotubes and iron particles. This excellent ODS activity originated from the defects of N-doped nanotubes as well as excellent particle dispersion and material transport capacity, which excites highly active free radicals with the assistance of H₂O₂. In addition, the unique two-dimensional tube channel and mesoporous structure promoted the diffusion and transfer of reactants and electrons, leading to high density of active sites. The different experimental conditions confirmed that the material is a bifunctional catalyst integrating adsorption and catalysis. This work provides an creative ideas for the rational design and synthesis of advanced ODS catalysts for fuel oil.     

Open-pore LiFePO_4/C microspheres with high volumetric energy density for lithium ion batteries

LiFePO_4/C microspheres with different surface morphologies and porosities were prepared from different carbon sources.The effects of the surface morphology and pore structure of the microspheres on their electrochemical properties and electrode density were investigated.The electrochemical performance and electrode density depended on the morphology and pore structure of the LiFePO_4/C microspheres.Open-pore LiFePO_4/C microspheres with rough surfaces exhibited good adhesion with current collectors and a high electrode density(2.6g/cm~3).They also exhibited high performance in a half cell and full battery with a high volumetric energy density.     

Mechanical analysis of C/C composite grids in ion optical system

The ion thruster is an engine with high specific impulse for satellites and spacecrafts, which uses electric energy to boost the spacecraft. The ion optical system,also known as gate assemblies which consist of acceleration and screen grids, is the key component of the ion thruster. In this paper, the static mechanical properties of the C/C composite grids are evaluated based on the structural design. Representative volume element(RVE) is adopted to simplify the braded composite structure as a continuum material. The dynamical behavior of the 100 mm ion thruster optics in the launch environment(1 000 gshock-load) is numerically modeled and simulated with the half-sine pulse method. The impact response of the C/C and molybdenum gate assemblies on the stress distribution and deformation is investigated. The simulated results indicate that the magnitudes of the normal displacement of the composite grids subject to the uniformly distributed load are on the same level as molybdenum grids although the normal stiffness of the composite grids is much smaller. When sub ject to impact loading,the stress distribution in the C/C composite grids is similar to molybdenum grids while the stress magnitude is much smaller. This finding shows that the C/C gate assemblies outperform molybdenum grids and meet the requirement of long lifetime service in space travel.     

感性材料与结构分析的基本模型

感性材料是一类基于植物仿生思想,利用化学能产生机械能的高能量密度智能材料.与植物感性运动类似,感性材料能够运用细胞半透膜,有选择、可控地将物质传输到体内产生定向变形.感性材料由基体材料中夹杂液体腔组成,液体腔周围有一层包含离子传输通道(离子泵、离子通道、离子协运机制等)的人工合成细胞膜.本文对感性材料的基本建模过程进行描述,建立了多感性驱动单元与结构相互作用的分析模型,并给出了计算结果.在感性单胞层次,通过对细胞膜离子传输过程以及结构力学模型进行耦合计算,再现感性运动中离子传输和基体结构的力学响应情况;通过改变各初始输入参数,研究不同参数对感性材料变形和响应过程的影响.     

Diffusion induced stresses in buckling battery electrodes

Highly networked nanostructured battery electrode materials offer the possibility of achieving both rapid battery charge–discharge rates and high storage capacity. Recently, lithium ion battery (LIB) electrodes based on a 2-D honeycomb architecture were shown to undergo remarkable and reversible morphological changes during the lithiation process. Charge–discharge rates in 3-D composite electrode have also been shown to benefit from sandwiching the electrolytically active material between highly conductive ion and electron transport pathways to reduce electrical resistance and solid-state diffusion lengths. In the present work we simulate and analyze the observed morphological changes in honeycomb electrodes, with and without the presence of conductive pathways, during the lithiation–delithiation process. Diffusion induced stresses are analyzed for such structures undergoing elastic–plastic deformation during cycling. The results show that such a periodic, nanostructured electrode geometry allows for the presence of buckling-like deformation modes, which effectively reduce the resulting mechanical stresses that lead to electrode failure.     

Material combinations and parametric study of thermal and mechanical performance of pyramidal core sandwich panels used for hypersonic aircrafts

A novel kind of lightweight integrated thermal protection system, named pyramidal core sandwich panel, is proposed to be a good safeguard for hypersonic aircrafts in the current study. Such system is considered as not only an insulation structure but also a load-bearing structure. In the context of design for hypersonic aircrafts, an efficient optimization should be paid enough attention. This paper concerns with the homogenization of the proposed pyramidal sandwich core panel using two-dimensional model in subsequent research for material selection. According to the required insulation performance and thermal–mechanical properties, several suitable material combinations are chosen as candidates for the pyramidal core sandwich panel by adopting finite element analysis and approximate response surface. To obtain lightweight structure with an excellent capability of heat insulation and load-bearing, an investigation on some specific design variables, which are significant for thermal–mechanical properties of the structure, is performed. Finally, a good balance between the insulation performance, the capability of load-bearing and the lightweight has attained.     

Open-pore LiFePO4/C microspheres with high volumetric energy density for lithium ion batteries

LiFePO₄/C microspheres with different surface morphologies and porosities were prepared from different carbon sources. The effects of the surface morphology and pore structure of the microspheres on their electrochemical properties and electrode density were investigated. The electrochemical performance and electrode density depended on the morphology and pore structure of the LiFePO₄/C microspheres. Open-pore LiFePO₄/C microspheres with rough surfaces exhibited good adhesion with current collectors and a high electrode density (2.6 g/cm³). They also exhibited high performance in a half cell and full battery with a high volumetric energy density.     

On analysis of physical processes on the electrode surface in a rail launcher

This paper analyzes some physical effects that occur on the electrode surface in plasmaarmature rail launchers when the linear current density is higher than critical value. It is shown that under typical experimental conditions, Rayleigh–Taylor and Kelvin–Helmholtz instabilities and magnetohydrodynamic instabilities, which arise from the interaction of the current with the selfmagnetic field, can develop over times much smaller than the launcher operation time and can be responsible for the entry of the electrode material into the discharge. Flash radiography of the electrode surface confirmed the presence of inhomogeneities and ejection of the material from the surface. Under certain conditions, the emergence of conducting metal jets from the electrode surface was detected.     

采用非协调元的连续体拓扑优化设计

介绍了满足分片检验条件的一种非协调元，推导了结构拓扑优化设计中数值计算和敏度分析的基本方程，给出了数值算例，并对协调等参元和非协调元的拓扑优化结果进行了对照，最后的优化结果表明：采用非协调元所得的优化解已经能够使用，如果再实施过滤技术，设计区域中的中间密度单元明显减少，优化结果会更加精致；使用两类单元的求解效率和优化迭代次数相近；非协调元比等参元具有更高精度的拓扑优化结果。能进一步克服棋盘格式。     

Electrokinetic Salt Removal from Porous Building Materials Using Ion Exchange Membranes

The removal of salt from porous building materials under the influence of an applied voltage gradient normally results in high pH gradients due to the formation of protons and hydroxyl ions at the electrodes. The formed acidic and alkaline regions not only lead to disintegration of the porous material, but also affect the salt transport. In this work we use ion exchange membranes between the electrodes and the porous material to prevent the protons and hydroxyl ions from intruding into the material. The porous material used in this study is fired clay brick, which has been saturated with a 4?mol/l sodium chloride solution prior to the desalination treatment. In order to experimentally determine the salt removal, we monitored the sodium ion concentration profiles across the material with nuclear magnetic resonance (NMR). In addition, we present theoretical predictions for the salt removal according to a model based on the Poisson?CNernst?CPlanck theory for ion transport. From the work reported here, we can conclude that the use of ion exchange membranes to desalinate porous building materials is not useful since it reduces the salt removal rate to such an extent that desalination with poultices, which is driven by diffusion only, is more efficient. The reason behind this is twofold. First, the ion exchange membranes provide a penalty for the ions to leave the material. Second, in the absence of acidic and alkaline regions, the salt concentration at the edges of the porous material will reduce to almost zero, which leads to a locally increased electrical resistance, and thus a reduction of the electrical field in the bulk of the material. Due to this reduction the effect of the applied voltage gradient across the material vanishes, and the salt removal is limited by diffusion.     

面内随机堆叠石墨烯复合材料压阻传感机理与压阻性能

面内随机堆叠石墨烯复合材料(graphene composites, GC)是可穿戴柔性传感器的基础材料之一, 但是其压阻传感机理与压阻性能仍然有待深入研究. 本文基于GC的微观结构特征, 利用0 $\sim$ 1间均匀分布随机数获得石墨烯在复合材料中的位置和方向, 建立了二维GC压阻传感器模型. 根据GC均匀变形的特点和有限单元法发展了GC压阻性能的计算方法, 计算得到了相对电阻、灵敏度系数、石墨烯片的微观形态与电流密度云图. 研究结果表明, GC中的压阻效应是由于在变形过程中石墨烯形态的改变, 包括GC中石墨烯片的密度随应变变化、石墨烯片滑移、分离导致电子迁移路径和无效片数量的改变, 而GC中石墨烯片密度随应变的变化是影响压阻效应的主要因素. 石墨烯片间的相对滑移产生线性传感特征, 分离反之. 高面分比GC与大尺寸石墨烯的GC拥有较大的感知范围, 低面分比GC和小尺寸石墨烯的GC具有更高的灵敏度系数. 最后将接触面的面内电阻率设为应变的函数, 研究了石墨烯片的接触效应对GC压阻性能影响, 解释了GC压阻性能的接触效应和影响机理. 研究结论可为GC生产方法的改进与创新、以及GC压阻传感器件的制备提供理论依据和技术参考.      

PIEZORESISTIVE SENSING MECHANISM AND PIEZORESISTIVE PERFORMANCE OF IN-PLANE RANDOM STACKED GRAPHENE COMPOSITES 1)

面内随机堆叠石墨烯复合材料(graphene composites, GC)是可穿戴柔性传感器的基础材料之一, 但是其压阻传感机理与压阻性能仍然有待深入研究. 本文基于GC的微观结构特征, 利用0 $\sim$ 1间均匀分布随机数获得石墨烯在复合材料中的位置和方向, 建立了二维GC压阻传感器模型. 根据GC均匀变形的特点和有限单元法发展了GC压阻性能的计算方法, 计算得到了相对电阻、灵敏度系数、石墨烯片的微观形态与电流密度云图. 研究结果表明, GC中的压阻效应是由于在变形过程中石墨烯形态的改变, 包括GC中石墨烯片的密度随应变变化、石墨烯片滑移、分离导致电子迁移路径和无效片数量的改变, 而GC中石墨烯片密度随应变的变化是影响压阻效应的主要因素. 石墨烯片间的相对滑移产生线性传感特征, 分离反之. 高面分比GC与大尺寸石墨烯的GC拥有较大的感知范围, 低面分比GC和小尺寸石墨烯的GC具有更高的灵敏度系数. 最后将接触面的面内电阻率设为应变的函数, 研究了石墨烯片的接触效应对GC压阻性能影响, 解释了GC压阻性能的接触效应和影响机理. 研究结论可为GC生产方法的改进与创新、以及GC压阻传感器件的制备提供理论依据和技术参考.