氮化硼、碳化硅、锗纳米管的熔化与拉伸力学特性

该论文采用Tersoff势的分子动力学方法分析了单壁(5,5)氮化硼、碳化硅、锗纳米管的熔化与轴向拉伸力学特性,讨论了三种纳米管熔化与轴向拉伸力学性能的差异。 研究表明：氮化硼管熔化后呈现为网状,碳化硅管为疏松的不规则的团状, 锗管呈现为紧密排布的近似球状;相同温度下,碳化硅及氮化硼纳米管的熔点、比热容以及熔化热却均远高于锗管,但系统能量却远低于锗管;三种纳米管中,氮化硼管的抗变形抗能力最大,锗管的抗变形与抗载荷能力最小,而氮化硼、碳化硅管的抗载荷能力相当。     

温度、应变率和空位缺陷对氮化硼纳米管压缩性能的影响

采用分子动力学方法,分别模拟了完好的和含有缺陷的氮化硼纳米管的轴向压缩过程。原子间的相互作用采用Tersoff多体势函数来描述。结果表明,同尺寸的锯齿型氮化硼纳米管的临界轴向压缩强度高于扶手型氮化硼纳米管,这与碳纳米管的研究结果一致。发现纳米管的压缩强度,如临界轴向内力在低温下受温度影响明显,并且和应变率的大小有关。然而,应变率对纳米管的弹性变形没有影响。另外,还发现空位缺陷降低了纳米管的力学性能。与完好的纳米管相比,含有缺陷的纳米管轴向压缩强度对于温度的影响并不敏感。     

单壁氮化硼纳米管压缩特性的分子动力学模拟

采用分子动力学方法,分别模拟完好的和含有缺陷的(7,7)扶手椅型氮化硼纳米管的轴向压缩过程.原子间的相互作用采用Tersoff多体势函数来描述.为验证结果的可靠性,同时针对(7,7)扶手椅型碳纳米管进行对比计算.结果表明,氮化硼纳米管的压缩失效模式同碳纳米管相同,均表现为管壁的局部屈曲.发现纳米管的压缩强度,如临界轴向内力在低温下受温度影响明显,并且和应变率的大小有关.然而,应变率对纳米管的弹性变形没有影响.另外,还发现空位缺陷降低了纳米管的力学性能.与完好的纳米管相比,含有缺陷的纳米管轴向压缩强度对于温度的影响并不敏感.     

氧化物辅助生长法合成氮化硼纳米管

硼、氧化硅、硅的混合粉末与氨气在1423K反应生成了氮化硼纳米管.产物用X射线衍射(XRD),透 射电镜(TEM)和傅立叶红外光谱(FTIR)进行了表征.氮化硼纳米管的直径为20～50nm,长达几十微米.氮化硼 纳米管的头部是开口或封口的,没有观察到纳米颗粒被包覆在纳米管的端部.因此,氮化硼纳米管的生长被认为 是一种氧化物辅助生长的机理.同多晶六方氮化硼的红外光谱相比,在1520cm-1处的小吸收峰表现了氮化硼纳 米管的一维结构特征.     

单晶ZnO、TiO_2纳米线拉伸力学特性的分子动力学研究

采用分子动力学方法,模拟了不同加载速度、不同温度下单晶ZnO、TiO_2纳米线的拉伸破坏过程.通过模拟结果,对比、分析了两种单晶金属氧化物纳米线拉伸力学特性的差异.研究表明,1)ZnO纳米线的断裂机制为:表面微裂纹-微孔-微裂纹与微孔贯穿-断裂,而TiO_2纳米线的断裂机制为:局部屈服-颈缩-断裂;2)TiO_2纳米线的承载能力优于ZnO纳米线,而承受变形的能力劣于ZnO纳米线;3)温度较低的情况下,纳米线的抗拉性能较好;加载速度越高,纳米线的抗载性能越好,而抗变形能力越差.     

单壁氮化硼纳米管的结构、对称性和晶格动力学

本文分析了氮化硼纳米管的结构对称性,并对其晶格振动模的对称性进行了分类。计算了单壁氮化硼纳米管的声子色散关系,给出了扶手椅管和锯齿管拉曼和红外活性模的频率随管径的变化规律。     

单、双原子空位缺陷对扶手椅型单层碳纳米管屈曲性能的不同影响

采用分子动力学方法,对完善和含缺陷扶手椅型单层碳纳米管进行轴向压缩的数值模拟,对比研究三种不同的温度环境下单、双原子空位缺陷对碳纳米管轴压变形性能的特殊影响.研究结果表明管壁缺陷显著降低了纳米管低温时的承载能力,由于单原子空位缺陷造成的特殊应力集中效应会引发纳米管过早发生局部屈曲,单原子缺陷管的屈曲强度反而小于双原子管的屈曲强度. 关键词： 分子动力学 碳纳米管 屈曲 缺陷     

C60、C60F60及C60H60“分子滚珠”的压缩特性与电子结构

采用量子分子动力学技术,模拟了C60、C60F60与C60H60分子的压缩过程,计算了这些"分子滚珠"受压变形后的电子结构.根据计算结果,对比、分析了三种分子的压缩力学特性以及压缩变形对其电子结构的影响.研究表明,1)三种分子的抗压缩载荷与能量吸收能力有C60F60＞C60H60＞C60的排序,但抗变形能力相当;2)随着压缩变形的增大,三种分子的化学活性增加,但相同应变下,C60F60和C60H60分子有着比C60更好的化学稳定性.     

掺氮碳化硅纳米管电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算,对本征碳化硅纳米管和掺氮碳化硅纳米管的电子结构进行了计算.计算表明本征（8,0）碳化硅为直接带隙半导体,能带间隙为0.94 eV;掺氮浓度为1.56%和3.12％的碳化硅纳米管的能带间隙减小为0.83 eV和0.74 eV.从差分电荷密度可以看出,能带间隙的减小是氮硅键与碳硅键相比共价成键能力降低的结果. 关键词： 碳化硅纳米管 掺氮 第一性原理 电子结构     

加氢单壁硅纳米管的热稳定性与拉伸力学特性

采用Tersoff势的分子动力学方法,研究外部加氢及内/外加氢单壁硅纳米管的热稳定性与拉伸力学特性,进而将两种单壁硅管与(14,14)碳纳米管的拉伸特性进行对比.研究结果表明:①外部加氢和内/外加氢单壁硅管的"骨架"结构近似于单晶硅的{110}晶面,两种硅管分别只能在低于150 K和75 K的温度下稳定存在;②外部加氢以及内/外加氢硅管硅的抗拉强度分别为4.0和1.2 GPa,断裂应变(抗拉强度对应的应变)分别为35%和32%,均远小于(14,14)碳管的抗拉强度和断裂应变.     

Electronic structure of SiC/BN composite segmented nanotubes

The electronic structure of segmented nanotubes composed of the alternating layers of (5,5) and (9,0) BN and SiC nanotubes in armchair and zigzag configurations, which differed in the orientation of the chemical bonds in the segments and the nature of the bonds (Si-N and B-C or Si-B and N-C) at the boundaries of BN and SiC regions, has been calculated using the linearized augmented cylindrical wave method. The calculations have been performed using the local density functional and the muffin-tin approximation for the electronic potential. It has been found that depending on the bonds at the segment boundaries, the (5,5) BN/SiC nanotubes are semiconductors with the energy gap E _g of 1 to 3 eV, whereas the (9,0) BN/SiC nanotubes exhibited a metal, semimetal, or semiconductor (E _g ～ 1 eV) type of band structures.     

Formation and structure of boron nitride conical nanotubes

Nanotubes exhibiting a novel structure - boron nitride (BN) conical nanotubes whose walls consist of conical layers with their cone axis parallel to the tube axis, as opposed to ordinary nanotubes, composed of concentric cylindrical layers with their normal perpendicular to the tube axis - were synthesized simultaneously with BN nanotubes by using carbon nanotubes (CNTs) as templates. The diameters of the BN conical nanotubes are typically about 15 nm, which is similar to those of the starting CNTs. Apex angles and inner diameters of most BN conical nanotubes are about 40° and 1 nm, respectively. The lengths of the BN conical nanotubes range from 50 nm to up to several micrometers.     

Preparation and characterization of single-phase SiC nanotubes and C-SiC coaxial nanotubes

Preparation conditions of single-phase SiC nanotubes and C-SiC coaxial nanotubes were investigated. The characterization of single-phase SiC nanotubes and C-SiC coaxial nanotubes were carried out. The SiC nanowires, which were made of the catenated SiC grains of 50–200 nm in diameter, were obtained in carbon nanotubes reacted at 1450 °C. The only C-SiC coaxial nanotubes were formed at 1300 °C. A few single-phase SiC nantoubes were synthesized at 1200 °C for 100 h. More than half number of nanotubes reacted at 1200 °C for 100 h were altered to single-phase SiC nantoubes by heat treatment of 600 °C for 1 h in air since the remained carbon was removed. The energy dispersive X-ray spectroscopy analysis revealed that the atomic ratio of Si to C in single-phase SiC nanotubes was almost 1; these single-phase SiC nanotubes consisted of near-stoichiometric SiC grains.     

铬银氧共掺n型，p型特性氮化硼纳米管第一性原理研究

基于第一性原理的平面波超软赝势法对(6, 0)单壁氮化硼纳米管、Cr掺杂、Ag掺杂、以及Cr-O共掺纳米管进行电子结构和光学性质的计算。结果表明：Cr掺杂和Cr-O共掺体系相比于本征体系的带隙值均减小,掺杂体系的导带底穿过费米能级从而实现了氮化硼纳米管的n型掺杂。Ag掺杂实现了纳米管的p型掺杂。本征氮化硼纳米管、Ag掺杂、Cr掺杂、以及Cr-O共掺纳米管的静态介电常数分别为1.17、1.61、1.32和1.48,相对于本征体系静介电性能有所提高。     

Theoretical insights into the effects of the diameter and helicity on the adsorption of formic acid on silicon carbide nanotube

The anchoring of small organic molecules onto the semiconductor surface has a great application for developing various molecular devices, such as novel solar cells, fuel cells, hybrid systems, sensors, and so on. In the present work, by carrying out detailed density-functional theory calculations, we have investigated the adsorption of the formic acid (HCOOH) molecule on planar and various curved silicon carbide (SiC) nanotubes. By considering both the molecular and dissociative adsorptions of HCOOH on these SiC nanomaterials, we found that the HCOOH molecule prefers to dissociate into HCOO and H group. Interestingly, different adsorption modes were found for HCOOH on SiC nanotubes, i.e. dissociative monodentate or bidentate adsorption, which depends on the tube diameter and helicity. For (n, 0) SiC nanotube, the monodentate adsorption mode is energetically favorable when n is less than 10. However, HCOOH prefers to be adsorbed on other (n, 0) SiC nanotubes in a bridged bidentate mode, which is similar to those of on (n, n) SiC nanotubes or planar SiC sheet. Moreover, upon HCOOH adsorption, these SiC nanomaterials remain to be of the semiconducting nature and their band gaps are decreased to different degrees. In addition, we also explored the effects of HCOOH coverage on its adsorption on SiC nanotube.     

Boron nitride nanotubes as templates for half-metal nanowires

We investigate by means of a GGA + U implementation of density functional theory the electronic and structural properties of magnetic nanotubes composed of an iron oxide monolayer and (n,0) boron nitride (BN) nanotubes, with n ranging from 6 to 14. The formation energy per FeO molecule of FeO covered tubes is smaller than the formation energy of small FeO nanoparticles, which suggests that the FeO molecules may cover the BN nanotubes rather than aggregating locally. Both GGA (PBE) and Van der Waals functionals predict an optimal FeO-BN interlayer distance of 2.94 ?. Depending on the diameter of the tube, novel electronic properties for the FeO covered BN nanotubes were found. They can be semiconductors, intrinsic half-metals or semi-half-metals that can become half-metals if charged with either electrons or holes. Such results are important in the spintronics context.     

Filling boron nitride nanotubes with metals

The authors’ endeavors over the last few years with respect to boron nitride (BN) nanotube metal filling are reviewed. Mo clusters of 1–2 nm in size and FeNi Invar alloy (Fe ∼60 at. %; Ni ∼40 at. %) or Co nanorods of 20–70 nm in diameter were embedded into BN nanotube channels via a newly developed two-stage process, in which multi-walled C nanotubes served as templates for the BN multi-walled nanotube synthesis. During cluster filling, low-surface-tension and melting-point Mo oxide first filled a C nanotube through the open tube ends, followed by fragmentation of this filling into discrete clusters via O₂ outflow and C→BN conversion within tubular shells at high temperature. During nanorod filling, C nanotubes containing FeNi or Co nanoparticles at the tube tips were first synthesized by plasma-assisted chemical vapor deposition on FeNi Invar alloy or Co substrates, respectively, and, then, the nanomaterial was heated to the melting points of the corresponding metals in a flow of B₂O₃ and N₂ gases. During this second stage, simultaneous filling of nanotubes with a FeNi or Co melt through capillarity and chemical modification of C tubular shells to form BN nanotubes occurred. The synthesized nanocomposites were analyzed by scanning and high-resolution transmission electron microscopy, electron diffraction, electron-energy-loss spectroscopy and energy-dispersive X-ray spectroscopy. The nanostructures are presumed to function as ‘nanocables’ having conducting metallic cores (FeNi, Co, Mo) and insulating nanotubular shields (BN) with the additional benefit of excellent environmental stability. Received: 10 October 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/51-6280, E-mail: golberg.dmitri@nims.go.jp     

Structural and electronic properties of composite BxCyNz nanotubes and heterojunctions

x C_yN_z nanotubes and related heterojunctions have been studied using both ab initio and semi-empirical approaches. Pure BN nanotubes present a very stable quasiparticle band gap around 5.5–6.0 eV independent of the tube radius and helicity. The bottom of the conduction bands is controlled by a nearly-free-electronn state localized inside the nanotube, suggesting interesting properties under doping. In the case of nanotubes with BC₂N stoichiometry, we show that in the thermodynamic limit the system is driven towards segregation of pure C and BN sections. This demixing significantly affects the electronic properties of such materials. The same process of segregation into BC₃ islands is evidenced in the case of B-doped carbon nanotubes. These spontaneous segregation processes lead to the formation of quantum dots or nanotube heterojunctions. In particular, C/BN superlattices or isolated junctions have been investigated as specific examples of the wide variety of electronic devices that can be realized using such nanotubes. Received: 27 November 1998 / Accepted: 14 December 1998     

Theoretical study of defects of BN nanotubes: A molecular-mechanics study

We investigate the structure and the formation energy of the pentagon–heptagon pairs (5775) defects of BN nanotubes using a molecular-mechanics method. For (n, 0) BN nanotubes, the homoelemental bonds of the 5775 defect may be located nearly either parallel or perpendicularly in the direction of tube axis. The former is energetically favored and strongly affects the reduction of the strain with decreasing radius. The formation energy of the 5775 defect, which increases with increasing radius, is lower than that of the tetragon–octagon pairs (4884). The 5775 defect of BN nanotubes is structurally and energetically stable, as compared to the 4884 defect.     

锯齿型与扶手椅型碳化硅纳米管光电性质第一性原理研究

本文基于密度泛函理论计算分析了手性参数为(17,0)、(20,0)、(26,0) (10,10)、(12,12)、(15,15)的碳化硅纳米管的能带图,态密度及主要光学性质。结果表明：锯齿型与扶手椅型碳化硅纳米管均具有明显的半导体性质;在相近直径下,扶手椅型碳化硅纳米管带隙宽度要大于锯齿型碳化硅纳米管的带隙宽度;碳化硅纳米管的光吸收峰在100nm~200nm之间,可用于制作紫外线探测器件。