Effect of process conditions on the synthesis of carbon nanotubes by catalytic decomposition of methane

A new dual-composition catalyst based on Ni-Mo/MgO with high efficiency of producing carbon nanotubes （CNTs） from methane was reported recently. In the present article, with this type of catalyst, the impact of such experimental parameters as reaction temperature, reaction time, concentration of H2, flow rate ratio of CH4 to H2 on yield and graphitization were investigated, leading to the following optimal growth conditions： reaction time 60min, reaction temperature 900℃, CH4：H2 about 100：20mL/min, under which high-yield multi-walled CNTs bundles were synthesized. Raman measurement indicated that the as-synthesized product was well-graphitized, and the purity was estimated over 95% by TG-DSC analysis. In terms of the above results, an explanation of high-efficiency formation of CNTs bundles and the co-catalysis mechanism of Ni-Mo/MgO were suggested. 2007 Chinese Societv of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.     

Mechanical behaviour of carbon nanotubes under combined twisting–bending

The main objective of this paper is to investigate the mechanical behaviour (strength and stiffness) of carbon nanotubes (CNTs) under combinations of bending and twisting. In order to achieve this goal, molecular dynamics (MD) simulations of bended and twisted CNTs are performed. The LAMMPS code is used, the AIREBO potential is considered for CC bonds, the temperature is kept at 300 K and incremental bending and twisting rotations are imposed to the CNT. Two types of CNTs are analyzed, including zig-zag (8,0) and armchair (5,5) CNTs with similar radius and length. The CNTs are also analyzed for pure bending and pure twisting. The main results are shown in the form of diagrams of energy and moment against imposed rotations. Some relevant conclusions are drawn concerning the influence of loading (bending and twisting) on the stiffness, strength and failure of CNTs: namely, it is concluded that armchair CNTs possess higher strength and fracture toughness under twisting–bending loading than zigzag CNTs; additionally, it is found that both CNTs (armchair and zigzag) still support moderate-to-high bending levels without failure after being extremely twisted and torsionally buckled, even for twisting angles four times those corresponding to torsional buckling; finally, the results prove that CNTs, mostly armchair ones, exhibit very high twisting–bending stiffness and strength and can be used with confidence as torsional spring elements in nanoelectromechanical systems (NEMS).     

Characteristics of elemental carbon and organic carbon in PM10 during spring and autumn in Chongqing, China

PM10 （particulate matter with aerodynamic diameter less than 10 μm） samples were collected simultaneously at nine urban sites and one urban background site during two intensive observation campaigns in 2006. Concentrations of elemental carbon （EC） and organic carbon （OC） in PM10 were analyzed using an element analyzer. The characteristics regarding spatial and seasonal distribution patterns of OC and EC concentrations and their contributions to PM10 mass, as well as correlation between OC and EC, were investigated in detail. The average OC and EC concentrations for urban sites were 57.5 ± 20.8 and 8.3 ± 3.9 μg/m^3, respectively, both being around three times higher than those for urban background site. As a whole, EC concentrations did not show distinct seasonal variations, though OC concentrations were generally higher in autumn than in spring. For urban sites, total carbonaceous aerosol （TCA） accounted for 33.2% in spring and 35.0% in autumn of PM10 mass. The OC and EC concentrations were found significantly correlated to each other both in spring and in autumn, implying the existence of similar emission sources such as coal combustion. The OC/EC ratios generally exceeded 2.0, indicating the presence of secondary organic carbon （SOC）, whose estimated concentration for urban Chongqing was 26.7 and 39.4μg/m^3, accounting for 48.9 and 61.9% of the total OC observed in the samples, in spring and in autumn, respectively.     

Nonlinear finite element analysis for vibrations of double-walled carbon nanotubes

The large-amplitude free vibration analysis of double-walled carbon nanotubes embedded in an elastic medium is investigated by means of a finite element formulation. A double-beam model is utilized in which the governing equations of layers are coupled with each other via the van der Waals interlayer forces. Von-Karman type nonlinear strain-displacement relationships are employed where the ends of the nanotube are constrained to move axially. The amplitude-frequency response curves for large-amplitude free vibrations of single-walled and double-walled carbon nanotubes with arbitrary boundary conditions are graphically illustrated. The effects of material constant of the surrounding elastic medium and the geometric parameters on the vibration characteristics are investigated. For a double-walled carbon nanotube with different boundary conditions between inner and outer tubes, the nonlinear frequencies are obtained apparently for the first time. Comparison of the results with those from the open literature is made for the amplitude-frequency curves where possible. This comparison illustrates that the present scheme yields very accurate results in predicting the nonlinear frequencies.     

Preparation and Li^＋ storage properties of hierarchical hollow porous carbon spheres

Hollow ordered porous carbon spheres （HOPCS） with a hierarchical structure were prepared by templating with hollow ordered mesoporous silica spheres （HOMSS）. Scanning electron microscopy （SEM） and transmission electron microscopy （TEM） showed that HOPCS exhibited a spherical hollow morphology. High-resolution TEM, small angle X-ray diffraction （SAXRD） and N2 sorption measurements confirmed that HOPCS inversely replicated the unconnected hexagonal-stacked pore structure of HOMSS, and possessed ordered porosity. HOPCS exhibited a higher storage capacity for Li^＋ ion battery （LIB） of 527.6 mA h/g, and good cycling performance. A large capacity loss during the first discharge-charge cycle was found attributed to the high content of micropores. The cycling performance was derived from the hierarchical structure.     

Modeling of the catalytic properties of high-temperature surface insulation in a dissociated carbon dioxide — Nitrogen mixture

A model of the heterogeneous catalysis of a dissociated carbon dioxide — nitrogen mixture on high-temperature heat-shield coatings is developed; the model takes into account nonequilibrium adsorption-desorption reactions of nitrogen and oxygen atoms and carbon dioxide molecules and their recombination in the Eley-Rideal reactions. On the basis of a comparison of the calculated heat fluxes in dissociated carbon dioxide with those measured on the VGU-4 plasma generator of the Institute for Problems in Mechanics of the RAS, the parameters of the catalysis model in question are chosen for three modern oxidant-resistant coating materials. The performances of these coatings are compared for the conditions of Mars Miniprobe entry into the Martian atmosphere. Their usability for the entry path considered is shown. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 106–116, January–February, 2000. The study was carried out with the support of the Russian Foundation for Basic Research (project No. 99-01-00259) and the “Integratisya“ Federal Program (project No. 2.1–414).     

A structural mechanics approach for predicting the mechanical properties of carbon nanotubes

Based on molecular mechanics, a structural mechanics model of carbon nanotubes (CNTs) was developed with special consideration given to the bending stiffness of the graphite layer. The potentials associated with the atomic interactions within a CNT were evaluated by the strain energies of beam elements which serve as structural substitutions of covalent bonds in a CNT. In contrast to the original model developed by Li and Chou (Int. J. Solids Struct. 40(10):2487–2499, 2003), in the current model the out-of-plane deformation (inversion) of the bond was distinguished from the in-plane deformation by considering a rectangular cross-section for the beam element. Consequently, the model is able to study problems where the effect of local bending of the graphite layer in a carbon nanotube is significant. A closed-form solution of the sectional properties of the beam element was derived analytically. The model was verified through the analysis of rolling a graphite sheet into a carbon nanotube. Using the present model, the buckling behavior of nanotubes under bending is simulated. The predicted critical bending angle agrees well with molecular dynamics simulations.     

Rheology of carbon black suspensions. IV. Effect of suspending media on the sol�Cgel transition behavior

Linear viscoelastic properties of carbon black (CB) suspensions with various CB concentrations (c_CB) in two suspending media, mixtures of a rosin-modified phenol resin-type varnish (Varnish-1)/an alkyd resin-type varnish (Varnish-2) and Varnish-1/petroleum solvent (AF) were investigated. The former medium has higher affinity and the latter has poorer affinity toward CB particles than Varnishi-1, although these two media have almost the same viscoelastic properties. Both CB/(Varnish-1/Varnish-2 = 60/40) and CB/(Varnish-1/AF = 80/20) suspensions exhibited a sol?Cgel transition on an increase in c_CB, as similar to the behavior of CB/Varnish-1 suspensions. But, for the CB/(Varnish-1/Varnish-2 = 60/40) suspensions the critical gel concentration (c_gel) was lower and the critical relaxation exponent (n) was higher than that of CB/Varnish-1 suspensions. In contrast, for the CB/(Varnish-1/AF = 80/20) suspensions c_gel and n, respectively, were almost the same value as those of CB/Varnish-1 suspensions. These results strongly suggest that c_gel and n are attributable to the medium affinity, but not to the medium viscosity, and the CB fractal structure changes from plane-like to straight-line structure with increasing medium affinity.     

Viscoelastic melt rheology and time–temperature superposition of polycarbonate–multi-walled carbon nanotube nanocomposites

This work investigates the linear and non-linear viscoelastic melt rheology of four grades of polycarbonate melt compounded with 3 wt% Nanocyl NC7000 multi-walled carbon nanotubes and of the matching matrix polymers. Amplitude sweeps reveal an earlier onset of non-linearity and a strain overshoot in the nanocomposites. Mastercurves are constructed from isothermal frequency sweeps using vertical and horizontal shifting. Although all nanocomposites exhibit a second plateau at ～10⁵ Pa, the relaxation times estimated from the peak in loss tangent are not statistically different from those of pure melts estimated from cross-over frequencies: all relaxation timescales scale with molar mass in the same way, evidence that the relaxation of the polymer network is the dominant mechanism in both filled and unfilled materials. Non-linear rheology is also measured in large amplitude oscillatory shear. A comparison of the responses from frequency and amplitude sweep experiments reveals the importance of strain and temperature history on the response of such nanocomposites.     

Dispersion of multi-walled carbon nanotubes in poly（p-phenylene） thin films and their electrical characteristics

Dispersion of multi-walled carbon nanotubes in poly（p-phenylene） composite exposed to toluene was experimentally investigated. 3 mg of multi-walled carbon nanotubes with nominal size of 20 nm was compounded with 30 mg of poly（p-phenylene） with the presence of terpineol as binding initiator. To investigate an optimal condition for homogenizing all constituents, ultrasonication with an output power of 750W was employed with compounding time of 3, 10, 20 and 30 min. With FTIR analyses, it could be confirmed that homogeneous composite of multi-walled carbon nanotubes and poly（p-phenylene） could be prepared. SEM analyses were also conducted to examine the dispersion of multi-walled carbon nanotubes in the polymer matrix. Then intrinsic electrical resistance of the composites after being exposed to toluene was also investigated. It was found that the composite film prepared with ultrasonication for 20 min could provide sufficiently sensitive response with respect to varied concentration of toluene.     

Investigation of guided waves propagation in orthotropic viscoelastic carbon–epoxy plate by Legendre polynomial method

The effect of viscoelasticity on the guided waves propagation in viscoelastic plate has been investigated according to multi-aspect. To this purpose, an extension of the Legendre polynomial method is proposed to formulate the guided waves equation in orthotropic viscoelastic plate composed of carbon–epoxy. The validity of the proposed Legendre polynomial method is illustrated by comparison with available data. The convergence of the method is discussed through a numerical example. The hysteretic and Kelvin–Voigt viscoelastic models are used to integrate the imaginary part of the complex stiffness matrix associated with the viscoelastic plate in this study. Accordingly, both viscoelastic models do not affect on the dispersion curves results. However, appreciable effects are seen in the attenuation curves. Also, the sensitivity of the guided waves propagation caused by variations of elastic and viscoelastic modulus has been studied in detail. Finally, the advantages of the Legendre polynomial method are described.     

Control of iron nanoparticle size by manipulating PEG–ethanol colloidal solutions and spin-coating parameters for the growth of single-walled carbon nanotubes

Iron catalyst nanoparticles were prepared on silicon wafers by spin-coating colloidal solutions containing iron nitrate, polyethylene glycol (PEG) and absolute ethanol. The effects of various spin-coating conditions were investigated. The findings showed that the size of the iron particles was governed by the composition of the colloidal solution used and that a high angular speed was responsible for the formation of a thin colloidal film. The effect of angular acceleration on the size and distribution of the iron particles were found to be insignificant. It was observed that a longer spin-coating duration provoked the agglomeration of iron particles, leading to the formation of large particles. We also showed that single-walled carbon nanotubes could be grown from the smallest iron catalyst nanoparticles after the chemical vapor deposition of methane.     

Effect of the Stone–Wales defect on the structure and mechanical properties of single-wall carbon nanotubes in axial stretch and twist

The effect of the Stone–Wales defect due to the rotation of a pair of neighboring atoms on the equilibrium structure and mechanical properties of single-wall carbon nanotubes in axial stretch and twist is considered. The position of carbon atoms in a test section consisting of a number of repeated units hosting a solitary Stone–Wales defect is computed by minimizing the Tersoff–Brenner potential. The energy invested in the defect is found to decrease as the radius of the nanotube becomes smaller. Numerical computations for nanotubes with zigzag and armchair chiralities show that inclined, axial, and circumferential defect orientations have a strong influence on the mechanical response in axial stretch and twist. Stretching may cause the defect energy to become negative, revealing the possibility of spontaneous defect formation leading to failure. In some cases, stretching may eliminate the defect and purify the nanotube. When the tube is twisted around its axis, a neck develops at the location of the defect, signaling possible disintegration.     

Predicting ductile fracture of low carbon steel sheets: Stress-based versus mixed stress/strain-based Mohr–Coulomb model

Two distinct implementations of the Mohr–Coulomb failure model are used in conjunction with a non-associated quadratic plasticity model to describe the onset of fracture in low carbon steel sheets. The stress-based version corresponds to the original Mohr–Coulomb model in stress space. For the mixed stress/strain-based version, the Mohr–Coulomb failure criterion is first transformed into the space of stress triaxiality, Lode angle parameter and equivalent plastic strain and then used as stress-state dependent weighting function in a damage indicator model. Basic fracture experiments including tensile specimens of different notch radii and a punch test are performed to calibrate the material parameters of the respective models. Subsequently, the models are used to predict the crack initiation in a Hasek test and during the stamping of an anticlastic structure. Unlike for the calibration experiments, the loading history during stamping is highly non-linear. Both models can be calibrated with similar accuracy, but the strain-based model predicts the instant of onset of fracture with greater accuracy in the stamping experiment which is an advantage of the empirical damage accumulation rule.     

Influence of temperature on a carbon–fibre epoxy composite subjected to static and fatigue loading under mode-I delamination

In this paper, interlaminar crack initiation and propagation under mode-I with static and fatigue loading of a composite material are experimentally assessed for different test temperatures. The material under study is made of a 3501-6 epoxy matrix reinforced with AS4 unidirectional carbon fibres, with a symmetric laminate configuration [0°]_16/S. In the experimental programme, DCB specimens were tested under static and fatigue loading. Based on the results obtained from static tests, fatigue tests were programmed to analyse the mode-I fatigue behaviour, so the necessary number of cycles was calculated for initiation and propagation of the crack at the different temperatures. G–N curves were determined under fatigue loading, N being the number of cycles at which delamination begins for a given energy release rate. G_ICmax–a, a–N and da/dN–a curves were also determined for different G_cr rates (90%, 85%, 75%, etc.) and different test temperatures: 90 °C, 50 °C, 20 °C, 0 °C, ?30 °C and ?60 °C.     

Numerically simulated behavior of radiative Fe3O4 and multi-walled carbon nanotube hybrid nanoparticle flow in presence of Lorentz force

Free convection in hybrid nanomaterial-saturated permeable media is crucial in various engineering applications.The present study aims to investigate the free convection of an aqueous-based hybrid nanomaterial through a zone under the combined effect of the Lorentz force and radiation.The natural convection of the hybrid nanomaterial is modeled by implementing a control volume finite element method(CVFEM)-based code,whereas Darcy assumptions are used to model the porosity terms in the momentum buoyancy equation involving the average Nusselt number Nu_ave,flow streamlines,and isotherm profiles.A formula for estimating Nu_ave is proposed.The results show that the magnetic force retards the flow,and the fluid tends to attract the magnetic field source.Nu_ave is directly correlated with the Rayleigh number and radiation;however,it is indirectly dependent on the Hartmann number.Conduction is the dominant mode at larger Darcy and Hartmann numbers.     

Nanohybrid TiO2/carbon black sensor for NO2 gas

A nanohybrid sensor of nanosized TiO2-coated carbon black particles, prepared by sol-gel technology for the detection of NO2 gas, has been developed. The response of the electric resistance of the hybrid sensor to NO2 concentration is investigated, showing that the sensitivity of the hybrid sensor is raised as certain ratio of the TiO2 content in the sensor. Easy and cheap to fabricate, the hybrid TiO2/carbon black promises to be a practical sensor for detecting NO2 gas.     

Can a single-wall carbon nanotube be modeled as a thin shell?

Single-wall carbon nanotubes (SWCNT) have been frequently modeled as thin shells, but the shell thickness and Young's modulus reported in literatures display large scattering. The order of error to approximate SWCNTs as thin shells is studied in this paper via an atomistic-based finite-deformation shell theory, which avoids the shell thickness and Young's modulus, but links the tension and bending rigidities directly to the interatomic potential. The ratio of atomic spacing (Δ≈0.14 nm) to the radius of SWCNT, Δ/R, which ranges from zero (for graphene) to 40% [for a small (5,5) armchair SWCNT (R=0.35 nm)], is used to estimate the order of error. For the order of error O[(Δ/R)³], SWCNTs cannot be represented by a conventional thin shell because their constitutive relation involves the coupling between tension and curvature and between bending and strain. For the order of error O[(Δ/R)²], the tension and bending (shear and torsion) rigidities of SWCNTs can be represented by an elastic orthotropic thin shell, but the thickness and elastic modulus cannot. Only for the order of error O(Δ/R), a universal constant shell thickness can be defined and SWCNTs can be modeled as an elastic isotropic thin shell.     

A heat transfer model for incorporating carbon foam fabrics in firefighter’s garment

In the present work, a numerical study was performed to predict and investigate the performance of a thermal protection system for firefighter’s garment consisting of carbon foam fabric in both the outer shell and the thermal liner elements. Several types of carbon foam with different thermal conductivity, porosity, and density were introduced to conduct a parametric study. Additionally, the thickness of the introduced carbon foam fabrics was varied to acquire optimum design. Simulation was conducted for a square planar 2D geometry of the clothing comprising of different fabric layers and a double precision pressure-based implicit solver, under transient state condition was used. The new anticipated thermal protection system was tested under harsh thermal environmental conditions that firefighters are exposed to. The parametric study showed that employing carbon foam fabric with one set of designed parameters, weight reduction of 33 % in the outer shell, 56 % in the thermal liner and a temperature reduction of 2 % at the inner edge of the garment was achieved when compared to the traditional firefighter garment model used by Song et al. (Int J Occup Saf Ergon 14:89–106, 2008). Also, carbon foam fabric with another set of designed parameters resulted in a weight reduction of 25 % in the outer shell, 28 % in the thermal liner and a temperature reduction of 6 % at the inner edge of the garment. As a result, carbon foam fabrics make the firefighter’s garment more protective, durable, and lighter in weight.     

Investigation of the Stability Problems of Elastic Bodies Using the Method of Mathematical Theory of Elasticity

In this paper,using the equilibrium equations and boundary conditionsof elastic stability problem of Новожилов and the method of mathematicaltheory of elasticity,we solve some elastic stability problems,which werestudied byищлинскииandвоицеховская,and obtained more reason-able results than theirs.