多壁碳纳米管对p型Ba0.3FeCo3Sb12化合物热电性能的影响

用两步固相反应法合成了P型Ba填充方钴矿化合物Ba0.3FeCo3Sb12，并采用放电等离子烧结法(SPS)制备了Ba0.3FeCo3Sb12／多壁碳纳米管复合材料．研究了Ba0.3FeCo3Sb12／多壁碳纳米管复合材料的结构及多壁碳纳米管对其热电性能的影响规律：SEM分析表明多壁碳纳米管比较均匀地分布在Ba0.3FeCo3Sb12基体中；随着碳纳米管添加量的增加，Ba0.3FeCo3Sb12／多壁碳纳米管复合材料的电导率下降、塞贝克系数略微下降、晶格热导率大幅度降低，当碳纳米管含量为5％时其晶格热导率最小；当碳纳米管的含量为3％时，本研究得到的Ba0.3FeCo3Sb12／碳纳米管复合材料的最大ZT值达0．78．     

Modulation of thermoelectric power of individual carbon nanotubes

Thermoelectric power (TEP) of individual single walled carbon nanotubes (SWNTs) has been measured at mesoscopic scales using a microfabricated heater and thermometers. Gate electric field dependent TEP modulation has been observed. The measured TEP of SWNTs is well correlated to the electrical conductance across the SWNT according to the Mott formula. Strong modulations of TEP were observed in the single-electron conduction limit. In addition, semiconducting SWNTs exhibit large values of TEP due to the Schottky barriers at SWNT-metal junctions.     

Breakdown of Fourier's law in nanotube thermal conductors

We present experimental evidence that the room temperature thermal conductivity (kappa) of individual multiwalled carbon and boron-nitride nanotubes does not obey Fourier's empirical law of thermal conduction. Because of isotopic disorder, kappa's of carbon nanotubes and boron-nitride nanotubes show different length dependence behavior. Moreover, for these systems we find that Fourier's law is violated even when the phonon mean free path is much shorter than the sample length.     

Thermal conductivity for single-walled carbon nanotubes from Einstein relation in molecular dynamics

Equilibrium molecular dynamics based Einstein relation with an appropriate definition for integrated heat current (i.e., with modified energy moment) are combined to quantify the thermal conductivity of individual single-walled carbon nanotubes, armchair, zigzag and chiral tubes. The thermal conductivity has been investigated as a function of three parameters, tube radius, length and chirality at and near room temperature with Brenner potential model. Thermal conductivity is found to have unusually high value and varies with radius, length and chirality of tubes. Also the thermal conductivity at temperature range from 50 to 100 K is found to have a maximum value. For 12.1 nm tube length, the thermal conductivity has converging trend which its value dependents on the tube radius and chirality. Tubes with large radius have lower values of thermal conductivity. Furthermore, the results show that armchair tubes have large values of the thermal conductivity comparing with zigzag and chiral tubes. It seems possible to uncover carbon nanotubes thermal properties based on measurements having heat dependence by adding another methods for calculations.     

Effect of carbon nanotubes addition on thermoelectric properties of Ca3Co4O9 ceramics

Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity. In this study, a series of Ca$_{3}$Co$_{4}$O$_{9}/x$ wt.% CNTs ($x=0$, 3, 5, 7, 10) polycrystalline ceramic thermoelectric materials by adding carbon nanotubes (CNTs) were prepared with sol-gel method and cold-pressing sintering technology. The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure. The effect of adding carbon nanotubes to the thermoelectric properties of Ca$_{3}$Co$_{4}$O$_{9}$ was systematically measured. The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material. Nevertheless, the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes. At 625 K, the thermal conductivity of Ca$_{3}$Co$_{4}$O$_{9}$/10 wt.% CNTs sample is reduced to 0.408 W$\cdot$m$^{-1}\cdot$K$^{-1}$, which is about 73% lower than that of the original sample. When the three parameters are coupled, the figure of merit of Ca$_{3}$Co$_{4}$O$_{9}$/3 wt.% CNTs sample reaches 0.052, which is 29% higher than that of the original sample. This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca$_{3}$Co$_{4}$O$_{9}$ ceramic samples and improve their thermoelectric properties.     

Pulsed laser deposition of multiwalled carbon nanotubes thin films

The physical/chemical properties of multiwalled carbon nanotubes have attracted much interest for applications in different fields, from micro-electronic to coating technology due, in particular, to their peculiar conductivity properties, to their hardness and high resistance to thermal stress. The technology to produce carbon nanotubes thin films with the desired properties, however, is still under development. In this work, we report on multiwalled carbon nanotubes thin films deposited by pulsed laser deposition techniques ablating commercially polystyrene-nanotubes pellets on alumina substrates. MicroRaman spectroscopy and high resolution Transmission Electron Microscopy provide the experimental confirmation that carbon nanotubes-like structures are present on the alumina surface with both minimal morphological damage of the tubes and structural changes induced by laser beam.     

Is the intrinsic thermoelectric power of carbon nanotubes positive?

The thermoelectric power (TEP) of single-walled carbon nanotubes (SWNTs) is extremely sensitive to gas exposure history. Samples exposed to air or oxygen have an always positive TEP, suggestive of holelike carriers. However, at fixed temperature the TEP crosses zero and becomes progressively more negative as the SWNTs are stripped of oxygen. The time constant for oxygen adsorption/desorption is strongly temperature dependent and ranges from seconds to many days, leading to apparently "variable" TEP for a given sample at a given temperature. The saturated TEP can be accounted for within a model of strong oxygen doping of the semiconducting nanotubes.     

Thermal properties of continuously spun carbon nanotube fibres

As indicated by theory and experimental measurements individual carbon nanotubes (CNTs) have very high values of thermal conductivity. One of the challenges is to achieve high thermal conductivity in macroscopic assemblies of CNTs such as fibres, films and composites, paving the way to a wide range of applications. CNT fibres have tremendous potential in succeeding as the future materials for a variety of applications when properties at the nanoscale are translated to their macroscopic assemblies. In this paper we report the measurements of thermal conductivity of continuously spun CNT fibres and its dependence on temperature. Thermal conductivity measurements were performed using in-house built temperature sensing microscope probe. Specific thermal conductivity of CNT fibres showed an order of magnitude advantage over the traditional materials used for heat dissipation.     

Thermal transport in the intermetallic compound CeNi<Subscript>4</Subscript>Cr

Electrical resistivity, thermopower (TEP), thermal conductivity and the thermoelectric figure of merit are studied for the CeNi₄Cr compound, which has been previously suggested to be a fluctuating valence system with a tendency to the increase of the effective mass at low temperatures. The analysis of the thermoelectric properties confirms such a possibility and provides characteristic parameters like the Debye temperature, Fermi energy and the position of the f band. Both the thermopower and the magnetic part of the electrical resistivity could be analyzed within a similar model assuming a narrow f-band of the Lorentzian form near the Fermi energy. The thermal conductivity shows that the phonon contribution exceeds the electronic one below 220 K.     

Heat conduction in extended X-junctions of single-walled carbon nanotubes

Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which are responsible for the larger temperature gradient and reduction in thermal conductivity when compared to standard X-junction. Quantum corrected results show that the longitudinal thermal resistance of the X-junction and EX-junction decreases monotonically with increasing temperature which makes the longitudinal thermal conductivity of the tube with junction less sensitive to temperature above 400 K comparing with the individual pristine tube. The origin of the significant decrease of thermal conductivity of EX-junction is discussed through phonon spectra analysis.     

Effect of gases on the temperature dependence of the electric conductivity of CVD multiwalled carbon nanotubes

The influence of various gaseous media on the temperature dependence of the electric conductivity σ of multiwalled carbon nanotubes (MWNTs) synthesized using the method of catalytical chemical vapor deposition (CVD) has been studied. The σ(T) curves were measured in a temperature range from 4.2 to 300 K in helium and its mixtures with air, methane, oxygen, and hydrogen. The introduction of various gaseous components into a helium atmosphere leads to a significant decrease in the conductivity of MWNTs in the interval between the temperatures of condensation and melting of the corresponding gas. Upon a heating-cooling cycle, the conductivity restores on the initial level. It is concluded that a decrease in σ is caused by the adsorption of gases on the surface of nanotubes.     

Effect of annealing on the microwave characteristics of carbon nanotubes and the nanocomposite materials based on them

Transmission and reflection spectra of electromagnetic microwave radiation are used to determine the complex permittivity of the composite materials consisting of a dielectric matrix and multiwalled carbon nanotubes subjected to high-temperature annealing in an inert atmosphere. The dependence of the electrical conductivity of multiwalled carbon nanotubes on the annealing temperature in an inert atmosphere is shown to be nonmonotonic.     

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     

Multishell conduction in multiwalled carbon nanotubes

The full electronic complexity of multiwalled carbon nanotubes may be explored by sequentially removing individual carbon shells. This technique is employed to directly measure the number of shells contributing to conduction at room temperature, as well as the contribution of each shell to the overall conductance. By exploring the gate dependence of the conductance, the random alternation between semiconducting and metallic shells can also be observed. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Electrical properties of multiwalled carbon nanotube film

The multiwalled carbon nanotube films examined in this study are produced by two methods: current annealing of carbon paper and dc magnetron sputtering. The conductivity and the temperature dependence of resistance of the samples are measured. The thermal conductivity of the film-substrate system is evaluated.     

Two-carrier conduction in Rb-TCNQ-II

Single crystal electrical resistivity and thermoelectric power (TEP) measurements as a function of temperature are repotted in Rb-TCNQ-II. Results suggest intrinsic conductivity by holes above 360 K with an activation energy of 0.44 eV. Below room temperature conductivity is extrinsic and is due to donors located 0.18 eV below the conduction band.     

Room-temperature power factor of CuAlO2 composite tablets enhanced by MWCNTs

CuAlO₂ with high theoretical thermoelectric performance has potential applications in thermal energy conversion. Herein, multi-wall carbon nanotubes (MWCNTs)/CuAlO₂ composite tablets are prepared by using different amounts of MWCNTs and solid paraffin binder, where MWCNTs served as a conductive agent and rendered three orders of magnitude increase in electrical conductivity. Seebeck coefficient of the composites was reduced with increasing MWCNTs content. Consequently, an optimal room-temperature thermoelectric power factor (PF) of 1.31 μW m⁻¹K⁻² has been rendered by MWCNTs/CuAlO₂ composite tablet with 1 wt % MWCNTs. Moreover, PF value increased with increasing temperature after a slight decrease at 333 K, which can be ascribed to the modulation of electrical conductivity. Current work provides an effective strategy to improve thermoelectric performance of CuAlO₂ materials.     

Growth of multiwalled carbon nanotube arrays by chemical vapour deposition over iron catalyst and the effect of growth parameters

Multiwalled carbon nanotube (CNT) arrays were grown by catalytic thermal decomposition of acetylene, over Fe-catalyst deposited on Si-wafer in the temperature range 700-750 °C. The growth parameters were optimized to obtain dense arrays of multiwalled CNTs of uniform diameter. The vertical cross-section of the grown nanotube arrays reveals a quasi-vertical alignment of the nanotubes. The effect of varying the thickness of the catalyst layer and the effect of increasing the growth duration on the morphology and distribution of the grown nanotubes were studied. A scotch-tape test to check the strength of adhesion of the grown CNTs to the Si-substrate surface reveals a strong adhesion between the grown nanotubes and the substrate surface. Transmission electron microscopy analysis of the grown CNTs shows that the grown CNTs are multiwalled nanotubes with a bamboo structure, and follow the base-growth mechanism.     

Linear conductance of multiwalled carbon nanotubes at high temperatures

We have studied the electrical conductance of gas-desorbed multiwalled carbon nanotubes (MWNTs) at high temperatures, and found a peculiar linear temperature dependence of conductance over a wide temperature range from 100 to 800 K. We interpret this phenomenon by using a thermal activation picture of conduction channels below the gap in the vicinity of the Dirac points. The result also indicates a very short and temperature-independent electron mean free path in our MWNTs, and provides a way to determine the number of residual conduction channels in the MWNTs.     

Characterization of fluorinated multiwalled carbon nanotubes by x-ray absorption spectroscopy

The C 1s and F 1s x-ray absorption spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents and reference compounds (highly oriented pyrolytic graphite crystals and “white” graphite fluoride) were measured using the equipment of the Russian-German beamline at the BESSY II storage ring with a high energy resolution. The spectra obtained were analyzed with the aim of characterizing multiwalled carbon nanotubes and their products formed upon treatment of the nanotubes with fluorine at a temperature of 420°C. It was established that, within the probing depth (～15 nm) of carbon nanotubes, the process of fluorination occurs uniformly and does not depend on the fluorine concentration. The interaction of fluorine atoms with multiwalled carbon nanotubes in this case proceeds through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton and is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp ²) to tetrahedral (sp ³) hybridization.