基底对硅纳米线热导率影响的分子动力学模拟

采用非平衡态分子动力学模拟的方法研究了硅纳米线平放在基底上的轴向热导率,结果表明基底的存在降低了纳米线的热导率,且随着温度的升高,热导率逐渐降低.通过改变纳米线与基底之间的范德华作用力强度,研究了基底约束对硅纳米线热导率的影响,结果表明,随着作用力强度的增加,纳米线热导率逐渐减小.模拟结果还显示,基底维数的降低能够进一...     

碳纳米管包裹的硅纳米线复合结构的热稳定性研究

采用经典分子动力学方法模拟一定直径[111]晶向的硅纳米线填充不同扶手椅型单壁碳纳米管复合结构的加热过程, 通过可视化和能量分析的方法判断复合结构中硅纳米线和碳纳米管的热稳定性. 通过讨论碳纳米管的空间限制作用和分子间相互作用力的关系, 对碳纳米管和硅纳米线的热稳定性变化进行初步解释. 研究发现碳纳米管中硅纳米线的热稳定性和碳纳米管的直径关系密切: 当管径较小时, 硅纳米线的热稳定性有所提高, 当管径增大到一定大小时, 硅纳米线的热稳定性会突然显著地下降, 直到硅纳米线与管壁不存在分子间相互作用力, 硅纳米线的热稳定性才会恢复. 而硅纳米线填充到碳纳米管中对碳纳米管的热稳定性有着明显的降低作用.     

X射线光电子能谱法研究硅纳米线的能带结构

制备了氧辅助热分解法,以一氧化硅为原料,以氩气为载气,维持管内压强为1000Pa,在高温炉中于1250℃下反应5 h后得到硅纳米线。硅纳米线经5%氢氟酸水溶液处理5 min后,与1×10-3 mol·L-1的氯化金溶液中反应5 min,在硅纳米线的表面上修饰了金纳米粒子,用X射线粉末衍射表征了产物的结构,同时观察到单质硅和金的XRD图谱;用电子扫描和透射显微镜观察了产物的形貌,表明氧辅助方法可制得大量均匀的硅纳米线,修饰在硅纳米线上的金纳米点形状整齐,尺寸均匀,平均直径约8 nm;并用X射线光电子能谱分析了修饰过程中能带结构的变化。结果表明,金纳米粒子表面带负电,它在施主能级和受主能级上都有电子存在;由于氧杂质的存在,硅纳米线的费米能级移向价带顶。     

Al纳米线不同晶向力学行为和变形机制的模拟

采用分子动力学模拟计算方法,考察具有较高层错能的Al纳米线沿不同晶向的力学行为和变形机制。在相同计算条件下与具有较低层错能的Ni、Cu、Au和Ag等FCC金属纳米线进行比较。结果表明：在力学行为方面,Al纳米线的弹性模量呈现明显的结构各向异性,满足E_[111] > E_[110] > E_[100]的关系,这一关系在FCC金属纳米线中普遍成立;Al纳米线的屈服应力随晶向呈现σ_y[100] > σ_y[111] > σ_y[110]的关系,这一关系在具有较低层错能的FCC金属纳米线中不具有普遍性,这与体系中位错形成机制密切相关。根据拉伸变形过程微观结构的演变规律,阐明Al纳米线不同晶向的变形机制,并与具有较低层错能的Ni、Cu、Au和Ag等FCC金属纳米线的变形机制进行比较。结果表明,对于尺度较小的高层错能Al纳米线,Schmid因子和广义层错能均难以准确预测其变形机制。     

表面修饰对硅锗合金纳米线的内部键长分布及能带影响的机理研究

采用密度泛函第一性原理方法,研究了三种不同官能团修饰对硅锗111方向纳米线内部键长分布的不均匀性及能带影响,计算结果表明CH_3修饰加剧了键长分布不均性,与其它两个官能团相比,键长峰值弱化,键长范围扩大,也就是键长发生了再分布,F修饰对键长表现出了拉力效应,但并没有引起键长的再分布.同时,通过计算电子性质,以H修饰纳米线为参考,因不同官能团修饰对键长不均匀性的影响不同,使能带Z点位置下移幅度不同,CH_3修饰纳米线下移幅度最大,F次之,H修饰的下移幅度最小,这为利用不均匀性调控电子结构提供了理论依据.     

多晶银纳米线拉伸变形的分子动力学模拟研究

纳米尺度金属Ag以其独特的导电和导热性,广泛应用于微电子、光电子学、催化等领域,特别是在纳米微电极和纳米器件方面的应用.本文采用分子动力学方法模拟了不同晶粒尺寸下多晶银纳米线的拉伸变形行为,详细分析了晶粒尺寸对多晶银纳米线弹性模量、屈服强度、塑性变形机理的影响.发现当晶粒尺寸小于13.49 nm时,多晶Ag纳米线呈现软化现象,出现反Hall-Petch关系,此时的塑性变形机理主要以晶界滑移、晶粒转动为主,变形后期形成五重孪晶;当晶粒尺寸大于13.49 nm时,塑性变形以位错滑移为主,变形后期产生大量的孪晶组织.     

Pt-Ag合金纳米线热力学性质的分子动力学模拟研究

基于原子嵌入势(EAM),采用分子动力学方法,对临界尺寸下的Pt_(0.95)Ag_(0.05)合金纳米线多边形结构的熔化行为进行了计算模拟.结果表明:径向尺寸对Pt_(0.95)Ag_(0.05)合金纳米线的熔点影响较为显著,而长度对其影响较小;引入林德曼因子得到的熔点和用势能-温度变化曲线找到的熔点基本一致;合金纳米线的染色原子由外向内运动;综合分析发现Pt_(0.95)Ag_(0.05)合金纳米线以先外后内的模式进行熔化.     

Pt-Ag合金纳米线热力学性质的分子动力学模拟研究

基于EAM原子嵌入势,采用分子动力学方法,对临界尺寸下的Pt0.95Ag0.05合金纳米线多边形结构的熔化行为进行了计算模拟.结果表明:径向尺寸对Pt0.95Ag0.05合金纳米线的熔点影响较为显著,而长度对其影响较小;引入林德曼因子得到的熔点和用势能-温度变化曲线找到的熔点基本一致;合金纳米线的染色原子由外向内运动;综合分析发现Pt0.95Ag0.05合金纳米线以先外后内的模式进行熔化.     

单层石墨烯杨氏模量的理论模型

石墨烯力学性能的研究对其在半导体技术中的应用是十分重要的,本文基于半连续体模型并结合石墨烯纳米结构特性,通过对原子的描述构建了石墨烯形变分量和位移分量的新关系,从而给出了单层石墨烯结构形变能,并计算了不同尺寸单层石墨烯的杨氏模量值.通过对不同方向杨氏模量的分析,讨论了单层石墨烯的手性行为.结果表明:随着尺寸的增加,单层石墨烯两个方向的杨氏模量分别趋于0.746 TPa和0.743 TPa,当尺寸相同时,两方向杨氏模量的最大差值不超过0.003 TPa,此结果与文献报道结果相符.在小应变情况下,单层石墨烯薄膜呈各向同性,且薄膜尺寸变化对该特性影响不大.该计算结果对研究石墨烯的其它力学特性提供一定的参考价值.     

Thermal stability and Young's modulus of mechanically exfoliated flexible mica

In recent years, mica has been successfully used as a substrate for the growth of flexible epitaxial ferroelectric oxide thin films. Here, we systematically investigated the flexibility of mica in terms of its thickness, repeated bending/unbending, extremely hot/cold conditions, and successive thermal cycling. A 20-μm-thick sheet of mica is flexible even up to the bending radius of 5 mm, and it is durable for 20,000 cycles of up- and down-bending. In addition, the mica shows flexibility at 10 and 773 K, and thermal cycling stability for the temperature variation of ca. 400 K. Compared with the widely used flexible polyimide, mica has a significantly higher Young's modulus (ca. 5.4 GPa) and negligible hysteresis in the force-displacement curve. These results show that mica should be a suitable substrate for piezoelectric energy-harvesting applications of ferroelectric oxide thin films at extremely low and high temperatures.     

Large variation in Young's modulus of carbon nanotube yarns with different diameters

Twist-spun carbon nanotube (CNT) yarns are composed of numerous CNTs and their bundles with entangled and twisted structures. In this paper, we studied the mechanical properties of CNT yarns. The individual CNT, a component of yarn, is well known to have an extremely high mechanical strength. However, CNT yarns are very flexible and relatively free to transform their shapes, showing the potential for application in the design of wearable devices. Since CNT yarns have two opposing characteristics at the same time, a wide range of Young's modulus can be achieved by simply changing the geometrical structure while using the same fabrication process. We also suggest that CNT yarns can be utilized as the base material for several applications that require different stresses in a structure, such as bioimplants or foldable devices.     

Young's modulus of ZnO microwires determined by various mechanical measurement methods

The mechanical properties of ZnO microwires have been studied using three different methods: quasi-static flexural measurements using atomic force microscopy, static measurements using a nano indenter, and dynamic flexural measurements using optical interferometry. ZnO microwires were synthesized by chemical vapor deposition method, and the crystal structure and quality were examined using x-ray diffraction and photoluminescence spectroscopy. The Young's moduli were estimated using the measurement results from the three methods, and they showed consistent values in the range 67.5–79.4 GPa for microwires with diameters of 1.8 μm ± 100 nm.     

Crystal orbital study on the combined carbon nanowires constructed from linear carbon chains encapsulated in zigzag double-walled carbon nanotubes

This work has presented first-principle self-consistent field crystal orbital studies of combined carbon nanowires (CNWs) consisted of linear carbon chains encapsulated in zigzag double-walled carbon nanotubes (DWCNTs). The geometrical structures, relative stabilities and electronic properties of CNWs made of DWCNTs and single-walled carbon nanotubes are investigated and compared in details. As adding second outer tube, enhanced stabilities of the CNWs made of DWCNTs are detected from viewpoint of energies. The calculated band structures show that all CNWs studied are metals with zero energy gap. It is found that the atomic density of the carbon chain and the size of the tube are important to modulate the electronic properties of the CNWs. Since chemical bonding is not formed among the constitute parts of CNWs, the interaction among the subsections are analyzed based on orbital hybridization and charge transfer, which both play the leading roles on the energies and band structures of the CNWs. The computed charge carrier mobility of encapsulated carbon chain is much larger than that of the free carbon chain, reaches 10⁵–10⁶ cm² V⁻¹ at room temperature. The filling carbon chain can be considered as one of the narrowest one-dimensional electronic nanowires covered by outer DWCNT. Moreover, the elastic properties of CNWs are studied based on the results of Young's modulus.     

Thermal stability of silicon nanowires: atomistic simulation study

Using the Stillinger--Weber (SW) potential model, we investigate the thermal stability of pristine silicon nanowires based on classical molecular dynamics (MD) simulations. We explore the structural evolutions and the Lindemann indices of silicon nanowires at different temperatures in order to unveil atomic-level melting behaviour of silicon nanowires. The simulation results show that silicon nanowires with surface reconstructions have higher thermal stability than those without surface reconstructions, and that silicon nanowires with perpendicular dimmer rows on the two (100) surfaces have somewhat higher thermal stability than nanowires with parallel dimmer rows on the two (100) surfaces. Furthermore, the melting temperature of silicon nanowires increases as their diameter increases and reaches a saturation value close to the melting temperature of bulk silicon. The value of the Lindemann index for melting silicon nanowires is 0.037.     

碳化硅中点缺陷对热传导性能影响的分子动力学研究

碳化硅(SiC)由于性能优异,已广泛应用于核技术领域.在辐照环境下,载能入射粒子可使材料中的原子偏离晶体格点位置,进而产生过饱和的空位、间隙原子、错位原子等点缺陷,这些缺陷将改变材料的热物性能,劣化材料的服役性能.因此,本文利用平衡分子动力学方法(Green-Kubo方法)采用Tersoff型势函数研究了点缺陷对立方碳化硅(β-SiC或3C-SiC)热传导性能的影响规律.研究过程中考虑的点缺陷包括:Si间隙原子(Si_Ⅰ)、Si空位(Si_Ⅴ)、Si错位原子(Si_C)、C间隙原子(C_Ⅰ)、C空位(C_Ⅴ)和C错位原子(C_Si).研究结果表明,热导率(λ)随点缺陷浓度(c)的增加而减小.在研究的点缺陷浓度范围(点缺陷与格点的比例范围为0.2%—1.6%),额外热阻率(Δ_R-R_defect-R_perfect,R=1/λ,R_defect为含缺陷材料的热阻率,R_perfec...     

Molecular dynamics study of coupled layer thickness and strain rate effect on tensile behaviors of Ti/Ni multilayered nanowires

Novel properties and applications of multilayered nanowires(MNWs) urge researchers to understand their mechanical behaviors comprehensively.Using the molecular dynamic simulation,tensile behaviors of Ti/Ni MNWs are investigated under a series of layer thickness values(1.31,2.34,and 7.17 nm) and strain rates(1.0 × 10~8 s~(-1) ≤ε≤5.0 × 10~(10) s~(-1)).The results demonstrate that deformation mechanisms of isopachous Ti/Ni MNWs are determined by the layer thickness and strain rate.Four distinct strain rate regions in the tensile process can be discovered,which are small,intermediate,critical,and large strain rate regions.As the strain rate increases,the initial plastic behaviors transform from interface shear(the shortest sample) and grain reorientation(the longest sample) in small strain rate region to amorphization of crystalline structures(all samples) in large strain rate region.Microstructure evolutions reveal that the disparate tensile behaviors are ascribed to the atomic fractions of different structures in small strain rate region,and only related to collapse of crystalline atoms in high strain rate region.A layer thickness-strain rate-dependent mechanism diagram is given to illustrate the couple effect on the plastic deformation mechanisms of the isopachous nanowires.The results also indicate that the modulation ratio significantly affects the tensile properties of unequal Ti/Ni MNWs,but barely affect the plastic deformation mechanisms of the materials.The observations from this work will promote theoretical researches and practical applications of Ti/Ni MNWs.     

熔融Cu55团簇在Cu(010)表面上凝固过程的分子动力学模拟

采用基于嵌入原子方法的分子动力学,模拟了熔融Cu55团簇在Cu衬底(010)表面上以两个不同降温速率降温过程中结构的变化.模拟结果表明,降温速率对团簇结构的变化有很大影响.较快的降温速率使得降温过程中团簇原子具有较低的能量;较慢的降温速率有助于高温时位于衬底内的原子向衬底表面扩散,排列形成面心立方结构.     

Molecular dynamics simulations of point defects in plutonium grain boundaries

A modified analytic embedded atom method (MAEAM) potential is constructed for fcc updelta-Pu. Molecular dynamics (MD) simulations with the potential are performed to investigate the interactions between two symmetrical tilt grain boundaries (GBs) and point defects such as He atom, vacancy and self-interstitial atom (SIA) in Pu. The calculated results show that point defect formation energies are on average lower than those in the lattice but variations from site to site along the GBs are very remarkable. Both substitutional and interstitial He atoms are trapped at GBs. Interstitial He atom is more strongly bound at the GB core than the substitutional He atom. The binding energy of SIA at GB core is higher than those of He atom and vacancy. GB core can bind many He atoms and SIAs due mainly to the fact that it contains many vacancies. Compared with He atom and SIA, the vacancy far from GB core is difficult to diffuse into the core. The GBs can act as sinks and sources of He atoms and SIAs, which may be a reason for the swelling of Pu after a period of self-irradiation because of the higher concentration of vacancy in the bulk.