碳纳米管和碳化硅纳米管热导率的分子动力学研究

采用Tersoff势测试和研究了反向非平衡分子动力学中的Müller-Plathe法和Jund法在一维纳米管热传导中的应用.在相同的模拟步数中,Müller-Plathe法可以得到很好的结果,热导率在交换频率大于50时对参数的选择并不敏感.然而,Jund法并不能得到良好的线性温度梯度,其热导率在一定程度上依赖于选择的热流大小.在此基础上,运用Müller-Plathe法进一步研究了碳纳米管和碳化硅纳米管的长度、直径和温度对热导率的影响.结果表明,无论是碳纳米管还是碳化硅纳米管,其长度、直径和温度对热导率的影响是一致的.只要长度增加,纳米管的热导率相应增大,但增长速率不断降低.直径对热导率的影响很大程度上还取决于温度,在高温时,直径对热导率几乎没有影响.除此之外,纳米管的热导率随着温度的增加总体上也是不断降低的,但峰值现象的出现还受纳米管长度的影响.     

Polaron effects on the thermal conductivity of zigzag carbon nanotubes

Thermal conductivity of metallic zigzag carbon nanotube is investigated in the context of Holstein model. Green's function approach is implemented to calculate the electronic contribution of thermal conductivity as a function of radius of carbon nanotube, temperature and electron phonon coupling strength. Our results show that electronic thermal conductivity increases as a function of temperature at low temperature and gets a maximum value then decays at high temperature. Also the effect of radius of both metallic and semiconductor zigzag carbon nanotube on the thermal conductivity is studied. Our results show thermal conductivity increases when CNT diameter increases and decreases with electron phonon interaction strength.     

Influence of chirality on the thermal conductivity of single-walled carbon nanotubes

The influence of chirality on the thermal conductivity of single-walled carbon nanotubes(SWNTs) is discussed in this paper, using a non-equilibrium molecular dynamics(NEMD) method. The tube lengths of the SWNTs studied here are 20, 50, and 100 nm, respectively, and at each length the relationship between chiral angle and thermal conductivity of a SWNT is revealed. We find that if the tube length is relatively short, the influence of chirality on the thermal conductivity of a SWNT is more obvious and that a SWNT with a larger chiral angle has a greater thermal conductivity. Moreover, the thermal conductivity of a zigzag SWNT is smaller than that of an armchair one. As the tube length becomes longer, the thermal conductivity increases while the influence of chirality on the thermal conductivity decreases.     

Molecular dynamics studies on the thermal conductivity of single-walled carbon nanotubes

We have studied the thermal conductivity of single-walled carbon nanotubes (SWCNTs) using the NEMD method. The results indicate that the thermal conductivity values are not profoundly influenced by the specific simulation-technique used in the MD simulations. Some possible reasons, which could be responsible for the discrepancy on thermal conductivity values of SWCNTs in the literatures, are discussed.      

Electronic thermal conductivity of armchair graphene nanoribbons and zigzag carbon nanotubes

Through the Green's function formalism and tight-binding Hamiltonian model calculations, the temperature dependent electronic thermal conductivity (TC) for different diameters of zigzag carbon nanotubes and their corresponding unzipped armchair graphene nanoribbons is calculated. All functional temperature dependencies bear crossovers, for which, at higher temperatures, nanotubes have a slightly higher TC than their derived nanoribbons, while below that crossover, both systems exhibit a significant coincidence over a moderate range of lower temperatures. Noticeably, TC decreases with increasing the width or diameter of the corresponding systems. Also, at low temperatures TC is proportional to the density of states around the Fermi level, and thus increasing for metal or semiconductors of narrower gap cases.     

Optical conductivity of carbon nanotubes

The effect of electron–phonon interaction on the optical conductivity of semiconducting carbon nanotubes is studied. In this manner, the Kubo–Greenwood formula, Green's function technique and the Holstein Hamiltonian model are used. The optical conductivity of the system shows different behaviors between low and high frequency region. In the low frequency, the optical conductivity increases with electron–phonon coupling strength increasing while it has no noticeable change in the high frequency region. The results also show that the optical conductivity increases with increasing of nanotube's diameter.     

Thermal conductivity of carbon supracrystalline nanotubes

A principally new method of numerical calculation of the surface thermal conductivity of single-layer nanotubes taking into account their monoatomic thickness has been proposed. The method has been tested for graphene and cupracrystalline carbon nanotubes. The results for graphene nanotubes agree with the experimental data.     

Negative differential conductivity in carbon nanotubes

A theoretical model and computations of the I-V characteristics of long carbon nanotubes in a strong axial dc field at room temperature is presented. Negative differential conductivity is predicted. It is shown that |dI/dV| for metal carbon nanotubes in the region of the negative differential conductivity significantly exceeds corresponding values for semiconducting ones. The predicted effect would enable the design of wave-generating nanotube-based diodes for submillimeter and infrared ranges.     

单根单壁碳纳米管导热系数随长度变化尺度效应的实验和理论

考虑纳米碳管与基体之间的热损失,采用四焊盘-3ω法测量了室温下基体表面不同长度单根单壁碳纳米管(SWNT)的导热系数.SWNT的导热系数在测试长度范围(05—7μm)内随长度的增大而增大,增加的幅度逐渐减小.考虑二阶3-声子过程的影响,采用改进的WV模型预测了SWNT导热系数随长度的变化规律.理论预测的声子平均自由程～175nm.导热系数的测量结果与室温下不同长度SWNT的实验结果相吻合.理论预测结果与实验结果均说明SWNT导热系数随长度变化具有尺度效应. 关键词： ω法')" href="#">3ω法 单壁碳纳米管 导热系数 二阶3-声子过程     

Nonlinear conductivity of single-walled zigzag carbon nanotubes

The conductivity of the single-walled zigzag carbon nanotube system was studied in an alternating electric field with the intensity vector along the axis of nanotubes. The electronic carbon nanotube system was macroscopically considered in terms of the Boltzmann kinetic equation in the constant relaxation time approximation while omitting the interaction with the phonon subsystem. The nonlinear responses to the applied harmonic field were calculated and analyzed.     

功能化单壁碳纳米管的热导率

采用基于BrennerⅡ势的非平衡态分子动力学方法,模拟研究了300K温度下经氢化学修饰的(10,0)单壁碳纳米管的热导率.研究显示功能化后碳管的热导率有明显减小,当有一列碳原子被氢化后（功能化程度为5%）,碳管的热导率减小了大约1/3,为了进一步解释这种功能化对碳纳米管热导率的影响,计算了不同功能化程度下碳纳米管的声子谱.     

功能化单壁碳纳米管的热导率

采用基于BrennerⅡ势的非平衡态分子动力学方法,模拟研究了300K温度下经氢化学修饰的(10,0)单壁碳纳米管的热导率.研究显示功能化后碳管的热导率有明显减小,当有一列碳原子被氢化后（功能化程度为5%）,碳管的热导率减小了大约1/3,为了进一步解释这种功能化对碳纳米管热导率的影响,计算了不同功能化程度下碳纳米管的声子谱.     

Effects of doping, Stone Wales and vacancy defects on thermal conductivity of single-wall carbon nanotubes

The thermal conductivity of carbon nanotubes with certain defects (doping, Stone-Wales, and vacancy) is investigated by using the non-equilibrium molecular dynamics method. The defective carbon nanotubes (CNTs) are compared with perfect tubes. The influences of type and concentration of the defect, length, diameter, and chirality of the tube, and the ambient temperature are taken into consideration. It is demonstrated that defects result in a dramatic reduction of thermal conductivity. Doping and Stone-Wales (SW) defects have greater effect on armchair tubes, while vacancy affects the zigzag ones more. Thermal conductivity of the nanotubes increases, reaches a peak, and then decreases with increasing temperature. The temperature at which the thermal conductivity peak occurs is dependent on the defect type. Different from SW or vacancy tubes, doped tubes are similar to the perfect ones with a sharp peak at the same temperature. Thermal conductivity goes up when the tube length grows or diameter declines. It seems that the length of thermal conductivity convergence for SW tubes is much shorter than perfect or vacancy ones. The SW or vacancy tubes are less sensitive to the diameter change, compared with perfect ones.     

Far-infrared to visible optical conductivity of single-wall carbon nanotubes

The reflectance of unoriented single-wall carbon nanotube films has been measured over a wide wavelength range (far-IR–UV). The results are consistent with the film being a mixture of conducting (armchair), small bandgap (n≡m, mod 3) and semiconducting nanotubes. The optical conductivity shows peaks corresponding to transitions between density-of-states peaks of these tubes, at energy locations consistent with 1.4 nm diameter tubes. In addition optical absorption spectroscopy of aligned single-wall carbon nanotubes shows that the optical transitions are well-aligned along the tube axis. This behavior is consistent with polarized resonant Raman and electronic structure calculations.     

Electrical conductivity of zigzag carbon nanotubes including Holstein polarons

We report molecular dynamics calculations on the evolution of Co-Au and Ag-Au alloy nanowires stretched along the [100], [110] and [111] crystallographic directions. The strong tendency of chain formation has been found for Ag-Au alloy. On the contrary the Co-Au alloy presents a different breaking pattern. In particular, we have found the formation of tetramer alloy nanowires. Finally, we present the mechanical properties of alloy nanocontacts.     

碳纳米管的可控长度拾取及导电性分析

拾取指定长度的半导体性碳纳米管对大规模制造碳纳米管场效应管具有重要意义.本文提出了一种利用原子力显微镜探针和钨针对碳纳米管进行可控长度拾取的方法并进行了碳纳米管导电性分析.在扫描电子显微镜下搭建微纳操作系统,针对切割操作过程中原子力显微镜探针、钨针和碳纳米管的接触情况进行了力学建模和拾取长度误差分析.建立了单根金属性碳纳米管、单根半导体性碳纳米管及碳纳米管束与钨针接触的电路模型,推导了接入不同性质碳纳米管后电路的电流电压特性方程.使用原子力显微镜探针对碳纳米管的空间位姿进行调整,控制钨针对碳纳米管上目标位置进行通电切割,同时获取切割电路中的电流电压数据.实验结果表明,本文提出的方法能够有效控制所拾取碳纳米管的长度,增加碳纳米管与原子力显微镜探针的水平接触长度能够减小碳纳米管形变导致的拾取长度误差,建立的电流电压特性方程能够用于分析碳纳米管的导电性.     

Nonmetallic crystals with high thermal conductivity

Nonmetallic crystals transport heat primarily by phonons at room temperature and below. There are only a few nonmetallic crystals which can be classed as high thermal conductivity solids, in the sense of having a thermal conductivity of > 1 W/cmK at 300K. Thermal conductivity measurements on natural and synthetic diamond, cubic BN, BP and AIN confirm that all of them are high thermal conductivity solids. Studies have been made of the effect on the thermal conductivity of nitrogen impurities in diamond, and oxygen impurities in AIN. The nitrogen impurities scatter phonons mostly from the strain field, the oxygen impurities scatter phonons mostly from the mass defects caused by aluminum vacancies. Pure A1N as well as pure SiC, BeO, BP and BeS conduct heat almost as well as does copper at room temperature, while pure natural and synthetic diamonds conduct heat five times better than copper.All of the nonmetallic solids that are known to possess high thermal conductivity have either the diamond-like, boron carbide, or graphite crystal structure. There are twelve different diamond-like crystals, a few boron carbide-type crystals, and two graphite structure crystals that have high thermal conductivity. Analyses of the rock-salt, fluorite, quartz, corundum and other structures show no candidates for this class. The four rules for finding crystals with high thermal conductivity are that the crystal should have (1) low atomic mass, (2) strong bonding, (3) simple crystal structure, and (4) low anharmonicity. The prime example of such a solid is diamond, which has the highest known thermal conductivity at 300K.     

Adjustable thermal conductivity in carbon nanotube nanofluids

Homogeneous and stable nanofluids have been produced by suspending well dispersible multi-walled carbon nanotubes (CNTs) into ethylene glycol base fluid. CNT nanofluids have enhanced thermal conductivity and the enhancement ratios increase with the nanotube loading and the temperature. Thermal conductivity enhancement was adjusted by ball milling and cutting the treated CNTs suspended in the nanofluids to relatively straight CNTs with an appropriate length distribution. Our findings indicate that the straightness ratio, aspect ratio, and aggregation have collective influence on the thermal conductivity of CNT nanofluids.     

Isotope effect on the thermal conductivity of boron nitride nanotubes

We have measured the temperature-dependent thermal conductivity kappa(T) of individual multiwall boron nitride nanotubes using a microfabricated test fixture that allows direct transmission electron microscopy characterization of the tube being measured. kappa(T) is exceptionally sensitive to isotopic substitution, with a 50% enhancement in kappa(T) resulting for boron nitride nanotubes with 99.5% 11B. For isotopically pure boron nitride nanotubes, kappa rivals that of carbon nanotubes of similar diameter.