碳、碳氮和硼碳氮纳米管场发射性能的比较研究

采用高温热解法在860℃分别制备出了碳、碳氮和硼碳氮纳米管，提纯后利用丝网印刷工艺分别将它们制备成薄膜，并测试了它们的场发射性能.结果表明：碳纳米管、碳氮纳米管和硼碳氮纳米管薄膜的开启电场分别为2.22，1.1和4.4V/μm，当电场增加到5.7V/μm时，它们的电流密度分别达到1400，3000μA/cm²和小于50μA/cm².碳和碳氮纳米管薄膜的场增强因子分别为10062和11521.可见，碳氮纳米管的场发射性能优于碳纳米管，而硼碳氮纳米管的场发射性能比前两者要差.解释了这三种纳米管场发射性能差别的原因. 关键词： 碳纳米管 碳氮纳米管 硼碳氮纳米管 场发射     

Growth of carbon nanotubes from ring carbon clusters

The possibility of growing single-wall carbon nanotubes from ring carbon clusters that appear at a certain stage of cooling carbon vapor is discussed. Such a technique could allow one to grow single-wall nanotubes without introducing a macroscopic amount of a catalyst and to retain nanotubes open during their growth. An analysis performed using semiempirical quantum-chemical methods shows that, when catalyst atoms interact with the edge of an already formed nanotube surface, the bonds of these atoms with carbon tend to occupy positions normal to the generatrix of the nanotube. This situation is natural for transition-metal atoms, since they favor the destruction of pentagonal cycles at the edge of the surface. The destruction mechanism consists in the fact that pentagons incorporate carbon atoms from the outside and become hexagons. The dependence of this tendency on the type of catalyst atom is considered.     

Growth of carbon nanotubes using nanocrystalline carbon catalyst

The basic growth of carbon nanotubes (CNTs) involves dissociation of hydrocarbon molecules over a metal layer as a catalyst. Generally, the metals used for the catalyst include nickel, cobalt, gold, iron, platinum, and palladium. However, the metal catalyst used with CNTs could have a harmful influence on the electrical properties of electronic devices. Therefore, we propose the use of nanocrystalline carbon (nc-C) as the catalyst for the growth of CNTs. We used a nc-C catalyst layer deposited by the closed-field unbalanced magnetron (CFUBM) sputtering method, and CNTs were grown by the hot filament plasma-enhanced chemical vapor deposition (HF-PECVD) method with ammonia (NH₃) as a pretreatment and acetylene gas (C₂H₂) as a carbon source. The CNTs were grown on the nc-C layers pretreated with a variation of the pretreatment time. The characteristics of the pretreated nc-C layers and the grown CNTs were investigated by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) measurements. Also, the structural variation of the pretreated nc-C layers was investigated by Raman measurement. We used the nc-C catalyst without metal, and we confirmed that our CNTs were composed with only carbon elements through an EDS measurement. Also, the pretreatment time was attributed to the growth of CNTs.     

Sonochemical preparation of carbon nanosheet from carbon black

Preparation of carbon nanosheet via ultrasound irradiation of carbon black under ambient conditions was reported for the first time. The structure of resulting carbon nanosheet was characterized by TEM, HRTEM, EDX and AFM. The experimental results showed that the carbon nanosheet is composed of ordered graphite carbon layers with a thickness of several nanometers.     

Gasification of porous carbon particle by carbon dioxide

The diffusive-kinetic model of porous carbon particles gasification is developed. The model considers the processes of heat and mass transfer both inside the porous particle and above it. Analysis of the model shows that heat and mass transfer have an influence to the gasification process to a marked degree. Gasification of carbon particle by carbon dioxide is impossible if particle temperature is lower about 850 K because concentration of carbon dioxide at the particle surface becomes lower than its equilibrium concentration. The rate of the carbon particle gasification is determined as a function of the porous particle internal surface area for different pressures and furnace temperatures.     

Rydberg states of carbon dioxide and carbon disulfide

The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and (npσ) and (npπ) Rydberg series have been assigned. For both molecules, the lowest excited ¹Π_g and ¹Π_u states are identified as Rydberg states. The lowest ${}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state is mainly Rydberg for CO₂, but contains some valence character for CS₂, There is no evidence for transitions to additional valence states of these symmetries.It is shown that $\text{LCAO}\text{MO}$ predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO₂ and CS₂ predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.     

The quadrupole moments of carbon dioxide and carbon disulphide

The electric quadrupole moments of carbon dioxide and carbon disulphide have been measured through the birefringence induced in gaseous samples over a range of temperature. The value for CO₂, Θ = (-14·98 ± 0·50) × 10^-40 C m², is consistent with earlier measurements, showing that the temperature-independent hyperpolarizability contribution to the birefringence is insignificant. For CS₂, Θ = (+12·0 ± 0·6) × 10^-40 C m². The positive sign reflects the increased importance of the π electron contribution to Θ in CS₂.     

Dimensional changes of carbon fiber fabrics reinforced phenolic resin-based carbon/carbon composites during pyrolysis

The dimensional changes of two-dimensional phenolic resin-based carbon/carbon composites during pyrolysis were investigated. The carbon/carbon composites were obtained by pyrolyzing the carbon fiber/phenolic resin composites that were fabricated using a vacuum bag hot pressing technique. Length, width and thickness of the rectangular composite samples were measured carefully before and after pyrolysis. The effects of heat treatment temperature and fiber pre-heat treatment on the dimensional changes of carbon/carbon composites were investigated. The measurement results indicated that different behavior in dimensional changes could be obtained for carbon/carbon composites with different fiber–matrix bonding in the interface.     

Monte Carlo simulation of carbon monoxide,carbon dioxide and methane adsorption on activated carbon

In this study, the adsorption capacity of pure and activated carbon with regard to carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄) gases at 298?K and pressure from 0.01 up to 2.0?MPa has been investigated computationally. Computational work refers to Monte Carlo (MC) simulation of each adsorbed gas on a graphite model with varying density of activation sites. The Grand Canonical Monte Carlo (GCMC) simulation technique was employed to obtain the uptake of each adsorbed gas by considering a graphite model of parallel sheets activated by carboxyl and hydroxyl groups, as observed experimentally. The simulation adsorption data for these gases within the examined carbon pore material are presented and discussed in terms of the adsorbate fluid molecular characteristics and corresponding interactions between adsorbate species and adsorbent material. We found that the simulated adsorption uptake of the examined graphite model under these conditions with regard to the aforementioned fluids increases in the order CO?<?CH₄?<?CO₂.     

Comparison of carbon fiber and cesium iodide-coated carbon fibercathodes

Presents results of an experimental comparison of a bare carbon fiber cathode and the same cathode when coated with cesium iodide salt (CsI). An annular cathode was constructed by arranging carbon fibers in an annular tuft pattern. The cathode was then operated as a bare carbon fiber cathode and in a configuration with a CsI coating. The cathode was tested at electric field strengths ranging from 50 kV/cm to 265 kV/cm at anode-cathode (A-K) gaps of 3.175 cm. The applied voltage had a 1-μs duration and the modulator was operated at up to 1 Hz repetition rate. The system had a low base pressure (<1.0×10^-7 torr). The article reports on results concerning the conditioning of the cathodes, the shot-to-shot reproducibility of the cathodes and the pressure evolution of the diode under 1 Hz operation. We also report on the impedance evolution of each of the diodes     

Laser vaporization of carbon in the presence of carbon dioxide

2 . The dependence of the ion production on the laser ablation parameters is investigated and the expansion dynamics of the ablated species is studied through time and space resolved measurements of the ion yield. We discuss our observations on the basis of reactions involving neutral and ionized carbon-based species. Received: 11 March 1997/Accepted: 30 June 1997     

Low-density carbon material modified with pyrolytic carbon

Here we report a physicochemical investigation of low-density carbon materials modified with pyrolytic carbon (PC). Exfoliated graphite (EG) obtained by nitrate expandable graphite thermal destruction was pressed into low-density graphite materials (LDGMs) with densities of 0.045-0.50 g/cm³ and saturated with PC by impact CVI technique. LDGM infiltration with PC leads to sample weight and density growth. The amount of deposited PC strictly depends on synthesis conditions. The maximum surface and volume deposition of PC occurred for samples with density of 0.05 g/cm³. XRD, Raman spectroscopy and scanning electron microscopy revealed that the deposited PC is of smooth laminar (SL) type. Composite thermal conductivity is about 2-3.5 Wt/m K.     

Fabrication of a three-dimensional complex carbon nanoneedle from carbon nanowalls

A three-dimensional complex carbon nanoneedle has been fabricated from carbon nanowalls by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibits novel three-dimensional complex nanostructure, the center of which is a carbon nanoneedle, and many carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10⁷/cm². The I-V characteristic addressed an emission current density of 314 mA/cm² at the electric field of 2.5 V/μm.     

Ablation behaviors of carbon/carbon composites with C-SiC-TaC multi-interlayers

Carbon/carbon composites with C-SiC-TaC multi-interlayers were prepared by isothermal chemical vapor infiltration. The ablation behaviors of the composites were tested with an oxyacetylene flame. The mass loss rate increases markedly in the initial 10 s, then reaches a steady state or decreases slightly in 10–40 s; while after 40 s, the mass loss rate increases remarkably. A similar trend is observed in the linear loss rate, except that it begins to increase only after 60 s. After ablation for 5 s, the composite surface consists in black carbon fibers and white ceramic oxides. After 20 s, three different regions with different ablation behaviors are observed: central, transition and border. After 100 s, the composites are severely ablated and the shape is completely destroyed. A cross-section of the composites after ablation for 20 s shows three distinct regions: a rugged oxide layer, a smooth oxide layer and the matrix. The tantalum compounds have not been able to protect efficiently the material from constant oxide evolution, possibly because of a too large pore volume fraction.     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

Ablation properties of carbon/carbon composites with tungsten carbide

The ablation properties and morphologies of carbon/carbon (C/C) composites with tungsten carbide (WC) filaments were investigated by ablation test on an arc heater and scanning electron microscopy. And the results were compared with those without tungsten carbide (WC) filaments tested under the same conditions. It shows that there is a big difference between C/C composites with and without WC filaments on both macroscopic and microscopic ablation morphologies and the ablation rates of the former are higher than the latter. It is found that the ablation process of C/C composites with WC filaments includes oxidation of carbon fibers, carbon matrices and WC, melting of WC and WO₃, and denudation of WC, WO₃ and C/C composites. Oxidation and melting of WC leads to the formation of holes in z directional carbon fiber bundles, which increases the coarseness of the ablation surfaces of the composites, speeds up ablation and leads to the higher ablation rate. Moreover, it is further found that the molten WC and WO₃ cannot form a continuous film on the ablation surface to prevent further ablation of C/C composites.     

Field emission of carbon nanotubes and electronic structure of carbon nanopeapods

We calculate the electron emission from nanostructures under an applied electric field using the ab initio pseudopotential method. The transition rates of the electrons are calculated by integrating the time-dependent Schrödinger equation for the states initially inside the emitter. A localized basis set is used for obtaining the eigenstates before emission and the potential that drives the field emission. The calculated electron-tunneling graph is quite linear in the short-time region, giving the transition rate within the simulation time. We have applied this new method to the field emission of carbon nanotubes as a test. Then we calculate the electronic structure of the fullerenes encapsulated inside the carbon nanotubes, the so-called carbon “nanopeapods”. The fullerenes may or may not contain metallic atoms such as gadolinium or potassium. There is an interesting effect of strain when the diameter of the carbon nanotube is smaller than that of the inserted fullerene plus twice the van der Waals distance.     

Amorphous carbon

The properties of various types of amorphous carbon and hydrogenated amorphous carbon are reviewed with particular emphasis on the effect of atomic structure on the electronic structure. It is shown how the proportion of sp³ and sp² sites not only defines the short-range order but also a substantial medium-range order. Medium-range order is particularly important in amorphous carbon because it is the source of its optical gap, whereas short-range order is usually sufficient to guarantee a gap in other amorphous semiconductors. The review discusses the following properties: short-range order and the radial distribution function, the infrared and Raman spectra, mechanical strength, the electronic structure, photoemission spectra, optical properties, electron energy-loss spectra, core-level excitation spectra, electrical conductivity, electronic defects and the electronic doping of hydrogenated amorphous carbon.     

Oscillation of nested fullerenes (carbon onions) in carbon nanotubes

Nested spherical fullerenes, which are sometimes referred to as carbon onions, of I _h symmetries which have N(n) carbon atoms in the nth shell given by N(n) = 60n ² are studied in this paper. The continuum approximation together with the Lennard-Jones potential is utilized to determine the resultant potential energy. High frequency nanoscale oscillators or gigahertz oscillators created from fullerenes and both single- and multi-walled carbon nanotubes have attracted much attention for a number of proposed applications, such as ultra-fast optical filters and ultra-sensitive nano-antennae that might impact on the development of computing and signalling nano-devices. Further, it is only at the nanoscale where such gigahertz frequencies can be achieved. This paper focuses on the interaction of nested fullerenes and the mechanics of such molecules oscillating in carbon nanotubes. Here we investigate such issues as the acceptance condition for nested fullerenes into carbon nanotubes, the total force and energy of the nested fullerenes, and the velocity and gigahertz frequency of the oscillating molecule. In particular, optimum nanotube radii are determined for which nested fullerenes oscillate at maximum velocity and frequency, which will be of considerable benefit for the design of future nano-oscillating devices.