Carbon nanostructures synthesized by arc discharge between carbon and iron electrodes in liquid nitrogen

An idea of using pure iron and graphite electrodes was employed for synthesizing carbon nanoparticles by arc discharge in liquid nitrogen. The synthesized products consist of multiwalled carbon nanotubes (MW–CNT), carbon nanohorns (CNH), and carbon nanocapsules (CNC) with core–shell structure. Effect of metallic cathode and discharge current on product structure and yield had been experimentally investigated. Typical evidence of transmission electron microscopic images revealed that under some certain conditions of discharge in liquid nitrogen the synthesized products mainly consisted of CNCs with mean diameter of 50–400 nm. When conventional graphitic electrodes were employed, CNHs with some MW–CNTs were mainly synthesized. Meanwhile, MW–CNTs with diameter of 8–25 nm and length 150–250 nm became less selectively synthesized as cathode deposit under the condition of discharge in liquid nitrogen with higher arc current. The production yield of carbon nanoparticles synthesized by either carbon–carbon or carbon–iron electrodes became also lower with an increase in the arc current.     

燃烧法合成碳纳米管的实验方案设计

碳纳米管是一种新型的碳材料,其合成方法多种多样。燃烧法是一种新兴的合成方法,燃烧过程提供用于碳纳米管生长的高温环境,同时也提供足够的烃原料。目前,用于合成碳纳米管的原料包括气体燃料和液体燃料,火焰类型主要有层流扩散火焰、逆流扩散火焰和预混火焰等。影响炭纳米管火焰合成的因素主要有气体成分,温度,催化剂,燃氧比和采样条件。我们采用甲烷扩散火焰用于实验研究炭纳米管的合成条件。实验系统包括扩散火焰喷嘴,混和段,质量流量计,取样探针和基板,气源。内径5 mm的喷嘴与内径100 mm的钢筒同轴。实验测得在气量为0．20 SLM时火焰高度为 3．5 cm。涂覆有催化剂的基板水平朝下置于火焰中采样,并将采集的样品进行电镜分析。本文还对燃烧法合成碳纳米管的机理进行了分析。     

A comparative study of field-emission from different one dimensional carbon nanostructures synthesized via thermal CVD system

Different one dimensional (1D) carbon nanostructures, such as carbon nanonoodles (CNNs), carbon nanospikes (CNSs) and carbon nanotubes (CNTs) have been synthesized via thermal chemical vapour deposition (TCVD) technique. The different 1D morphologies were synthesized by varying the substrate material and the deposition conditions. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). FESEM and TEM images showed that the diameters of the CNNs and CNTs were ∼40 nm while the diameters of the CNSs were around 100 nm. Field emission studies of the as-prepared samples showed that CNSs to be a better field emitter than CNNs, whereas CNTs are the best among the three producing large emission current. The variation of field emission properties with inter-electrode distance has been studied in detail. Also the time dependent field emission studies of all the nanostructures have been carried out.     

Synthesis of carbon nanotubes on Ni-alloy and Si-substrates using counterflow methane–air diffusion flames

We have conducted an experimental study to investigate the synthesis of multi-walled carbon nanotubes (CNTs) in counterflow methane–air diffusion flames, with emphasis on effects of catalyst, temperature, and the air-side strain rate of the flow on CNTs growth. The counterflow flame was formed by fuel (CH₄ or CH₄ + N₂) and air streams impinging on each other. Two types of substrates were used to deposit CNTs. Ni-alloy (60% Ni + 26% Cr + 14% Fe) wire substrates synthesized curved and entangled CNTs, which have both straight and bamboo-like structures; Si-substrates with porous anodic aluminum oxide (AAO) nanotemplates synthesized well-aligned, self-assembled CNTs. These CNTs grown inside nanopores had a uniform geometry with controllable length and diameter. The axial temperature profiles of the flow were measured by a 125 μm diameter Pt/10% Rh–Pt thermocouple with a 0.3 mm bead junction. It was found that temperature could affect not only the success of CNTs synthesis, but also the morphology of synthesized CNTs. It was also found, against previous general belief, that there was a common temperature region (1023–1073 K) in chemical vapor deposition (CVD) and counterflow diffusion flames where CNTs could be produced. CNTs synthesized in counterflow flames were significantly affected by air-side strain rate not through the residence time, but through carbon sources available for CNTs growth. Off-symmetric counterflow flames could synthesize high-quality CNTs because with this configuration carbon sources at the fuel side could easily diffuse across the stagnation surface to support CNTs growth. These results show the feasibility of using counterflow flames to synthesize CNTs for particular applications such as fabricating nanoscale electronic devices.     

Decoration of carbon nanotube with size-controlled L10-FePt nanoparticles for storage media

In this work, first multi-wall carbon nanotubes (MWCNTs) with outer diameter about 20–30 nm are synthesized by a CVD method; they have been purified and functionalized with a two-step process. The approach consists of thermal oxidation and subsequent chemical oxidation. Then, monosize FePt nanoparticles along carbon nanotubes surface are synthesized by a Polyol process. The synthesized FePt nanoparticles are about 2.5 nm in size and they have superparamagnetic behavior with fcc structure. The CNTs surfaces as a substrate prevent the coalescence of particles during thermal annealing. Annealing at the temperature higher than 600 ^°C for 2 h under a reducing atmosphere (90 % Ar + 10 % H₂) leads to phase transition from fcc to fct-L1₀ structure. So, the magnetic behavior changes from the superparamagnetic to the ferromagnetic. Furthermore, after the phase transition, the FePt nanoparticles have finite size with an average of about 3.5 nm and the coercivity of particles reaches 5.1 kOe.     

Enhanced interface interaction between modified carbon nanotubes and magnesium matrix

Carbon nanotube (CNT)/metal interface interaction is critical to the mechanical properties of CNT-reinforced metal matrix composites (MMCs). In this paper, in order to realize the chemical modification of the interface interaction between CNTs and Mg matrix, different types of defects (monovacancy, carbon and oxygen adatoms, as well as p-type boron and n-type nitrogen substitution) are introduced in CNTs to investigate the effect of the defects on the interface interaction (E_ib) between CNT and Mg (0 0 0 1) surface. Moreover, two models (adsorption model and interface model) are compared and validated to investigate the interface interaction. It is revealed that the CNT with the carbon adatom has the highest E_ib with the Mg (0 0 0 1), and the effect of boron doping on E_ib is superior to the intermediate oxygen which has already been proved experimentally in the enhancement of the interface interaction in MMCs. In terms of the electronic structure analysis, we reveal the micro-mechanism of the increase of E_ib under the action of different types of defects, and propose that the presence of holes (boron dopant) and the unsaturated electrons in CNTs can generate the chemical interaction between CNT and Mg matrix effectively. Our results are of great scientific importance to the realization of robust interfacial bonding between CNTs and Mg matrix via the reinforcement modification, so as to enhance the mechanical properties of CNTs reinforced Mg matrix composites.     

Temperature and carbon source effects on methane–air flame synthesis of CNTs

We have conducted experimental and numerical studies on flame synthesis of carbon nanotubes (CNTs) to investigate the effects of three key parameters – selective catalyst, temperature and available carbon sources – on CNT growth. Two different substrates were used to synthesize CNTs: Ni-alloy wire substrates to obtain curved and entangled CNTs and Si-substrates with porous anodic aluminum oxide (AAO) nanotemplates to grow well-aligned, self-assembled and size-controllable CNTs, each using two different types of laminar flames, co-flow and counter-flow methane–air diffusion flames. An appropriate temperature range in the synthesis region is essential for CNTs to grow on the substrates. Possible carbon sources for CNT growth were found to be the major species CO and those intermediate species C₂H₂, C₂H₄, C₂H₆, and methyl radical CH₃. The major species H₂, CO₂ and H₂O in the synthesis region are expected to activate the catalyst and help to promote catalyst reaction.     

Control of morphology for energy dissipation in carbon nanotube forests

This study focuses on the effect of carbon precursor on the carbon nanotube (CNT) morphology and energy dissipation. Benzene, toluene, and m-xylene were used as carbon precursors for the synthesis of CNT forests following a chemical vapor deposition process. The results indicate that substituents on the benzene ring increase entanglement in the CNT forests. The absorbed energy was slightly greater for CNT forests synthesized using m-xylene than for toluene, but was much smaller for benzene. When compressed to a strain of 0.67, the toluene CNTs absorbed more energy than the m-xylene CNTs. The restitution was much higher for the forests synthesized with m-xylene than toluene while it further decreased for the forests made with benzene. A strong correlation is also observed between the average diameter of the CNTs and the number of methyl substituents on the benzene ring. The control of the entanglement of the CNT forests can potentially be used to design high energy absorbing composites for blast energy dissipation.     

CVD synthesis and radial deformations of large diameter single-walled CNTs

Single-walled carbon nanotubes (CNTs) were synthesized by a chemical vapor deposition (CVD) method on transmission electron microscopy (TEM) silica coated nickel grids using carbon monoxide as carbon source and iron nanoparticles as catalyst. The produced CNTs were as large as 11 nm in diameter. Investigations on the CNT deformations based on high-resolution TEM images showed that the deformation of CNTs due to their interaction with the substrate occurs at diameters larger than 2.7 nm. Small deformation of free standing tubes was found to occur at diameters above approximately 4.5 nm.     

The interaction between unique hyperbranched polyaniline and carbon nanotubes,and its influence on the dielectric behavior of hyperbranched polyaniline/carbon nanotube/epoxy resin composites

Novel hybridized multi-walled carbon nanotubes (CNTs), consisting of a unique hyperbranched polyaniline (HSiPA) and CNTs, were prepared. The interaction between HSiPA and CNTs was investigated by many techniques, and results show that there are strong π–π and electrostatic interactions between HSiPA and CNTs, so HSiPA can stack firmly onto the surface of CNTs to form a coating. Based on this, a new kind of ternary composites made up of hybridized CNTs and epoxy (EP) resin was prepared, the influence of the ratio of HSiPA to CNTs on the structure and properties of the HSiPA/CNT/EP composites was intensively studied. The percolation threshold of HSiPA/CNT/EP composites is very low (1.26 wt%); besides, with a suitable ratio of HSiPA to CNTs, the HSiPA/CNT/EP composite has much higher dielectric constant and lower dielectric loss than the CNT/EP composite with the same loading of CNTs. When the ratio of HSiPA to CNTs is 0.5:1, the dielectric constant and loss at 100 Hz of the resultant HSiPA/CNT0.5/EP composite are 711 and 1.53, about 7.1 and 4.3 × 10^?3 times the corresponding value of CNT0.5/EP composite, respectively. In addition, compared with traditional CNT/EP composites, the HSiPA/CNT0.5/EP composites have different equivalent circuit models. These attractive results are attributed to unique structure of hybridized CNTs, and thus leading to greatly different structures between the CNT0.5/EP and HSiPA/CNT0.5/EP composites. This investigation reported herein suggests a new approach to prepare new CNTs and related composites with controllable dielectric properties.     

Thermal resistance of CNTs-based thermal interface material for high power solid state device packages

Vertically aligned carbon nanotubes (VACNTs) were synthesized over copper substrate. The diameter and length of the CNTs were 100 nm and 2–3 μm, respectively. Synthesis of CNTs was confirmed by Raman spectrum and verified by TEM as multi walled CNTs. SEM images showed the vertically aligned CNTs over Cu substrate. Strengthening of CNTs was performed by filling with Cu and SU-8 epoxy sealant in gap between the CNTs. The observed density was high for epoxy sealed CNTs. The bending ability of CNTs was checked and observed as low for epoxy sealed CNTs. The thermal resistance of the samples was measured by JESD51-2 standard for various loads. The observed resistance was low (0.277 cm²?K/W) for epoxy sealed CNTs at 1100 kPa. The calculated resistance of CNTs alone was 0.097 cm²?K/W for epoxy sealed at 900 kPa.     

NiFe LDH-CoPc/CNTs as novel bifunctional electrocatalyst complex for zinc–air battery

A new type of composite electrocatalyst was designed and prepared with NiFe layered double hydroxides (LDHs) for oxygen evolution reaction (OER) and CoPc for oxygen reduction reaction (ORR) supported on carbon nanotubes (CNTs). The NiFe LDH–CoPc/CNT composite exhibits higher electrocatalytic activity and stability than the commercial precious metal catalyst Pd/C + Ru/C in 6 M KOH electrolyte. The resulting rechargeable Zn–air battery showed high discharge voltage at 195 mW cm^?2. The discharge voltage is around 1.08~0.95 V and the charge voltage is lower, 2.07 V, after the cycle of 300 h at 80 mA cm^?2, indicating that zinc–air battery possessed high reversibility and durability over long charge and discharge cycles.     

Advanced LiTi<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C anode by incorporation of carbon nanotubes for aqueous lithium-ion batteries

Inferior rate capability is a big challenge for LiTi₂(PO₄)₃ anode for aqueous lithium-ion batteries. Herein, to address such issue, we synthesized a high-performance LiTi₂(PO₄)₃/carbon/carbon nanotube (LTP/C/CNT) composite by virtue of high-quality carbon coating and incorporation of good conductive network. The as-prepared LTP/C/CNT composite exhibits excellent rate performance with discharge capacity of 80.1 and 59.1 mAh g^?1 at 10 C and 20 C (based on the mass of anode, 1 C = 150 mA g^?1), much larger than that of the LTP/C composite (53.4 mAh g^?1 at 10 C, and 31.7 mAh g^?1 at 20 C). LTP/C/CNT also demonstrates outstanding cycling stability with capacity retention of 83.3 % after 1000 cycles at 5 C, superior to LTP/C without incorporation of CNTs (60.1 %). As verified, the excellent electrochemical performance of the LTP/C/CNT composite is attributed to the enhanced electrical conductivity, rapid charge transfer, and Li-ion diffusion because of the incorporation of CNTs.     

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT

The diffusion of a CO₂/CH₄ mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO₂ and CH₄ are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO₂ and CH₄ molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO₂ in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO₂ and CH₄ in the pores. For CO₂, the interaction energies (CO₂–CO₂ and CO₂–CNT) are larger than that of CH₄ in all cases. But only a very small difference in the diffusion coefficient was observed between CO₂ and CH₄. Temperature has a negligible effect on the difference between diffusion coefficients of CO₂ and CH₄ in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO₂/CH₄ in CNT bundle membranes.     

Aligned Ni nanoparticle arrays encapsulated in carbon nanotubes

Using the carbon nanotube (CNT) arrays embedded in anodic aluminum oxide (AAO) template as an electrode, large amounts of Ni nanoparticles have been encapsulated into the CNTs by an alternating current (AC) electrodepostion technique. As deposited Ni nanoparticles with a typical size of 50–60 nm randomly nucleated on the CNT walls, thus inhomogeneously distributed in the CNTs. After annealing at 600 ^°C, the nanoparticles transformed into quasi-spherical structures with the diameter increasing to 60–80 nm. The quasi-spherical nanoparticles were aligned in orderly rows along the axis of the CNT channels. Magnetic hysteresis measured at 5 K showed that the coercivity was 450 Oe for the as-deposited sample and 385 Oe for annealed sample, with the applied magnetic field parallel with the CNT’s axis. The structures and magnetic properties were discussed for both as-deposited and annealed samples.     

Carbon nanotube synthesis in a flame using laser ablation for in situ catalyst generation

Laser ablation of either Ni or Fe is used to create nanoparticles within a reactive flame environment for catalysis of carbon nanotubes (CNTs). Ablation of Fe in a CO-enriched flame produces single-walled nanotubes, whereas, ablation of Ni in an acetylene-enriched flame produces carbon nanofibers. These results illustrate that the materials for catalyst particle formation and CNT, SWNT or nanofiber, inception and growth in the aerosol phase can be supplied from separate sources; a metal-carbon mixture produced by condensation is not necessary. Both particle formation and CNT inception can begin from molecular species in a laser-ablation approach within the complex chemical environment of a flame. Moreover, SWNTs and nanofibers can be synthesized within very short timescales, of the order of tens of milliseconds. Finally, high-intensity pulsed laser light can destroy CNTs through either vaporization or coalescence induced by melting. PACS 42.62 Fi; 81.05.Tp; 82.80.Ch; 81.15 Fg     

Size distribution and morphology of nascent soot in premixed ethylene flames with and without benzene doping

Particle size distribution functions of nascent soot formed in four burner-stabilized, premixed ethylene-oxygen-argon flames were studied in a spatially resolved manner by online sampling/scanning mobility particle sizer. Particle morphology was analyzed by atomic force microscopy (AFM) of substrate-deposited samples. Two of the four flames were doped with benzene. An aerosol electrometer is introduced to extend the lower detection limit to around 1.5 nm in diameter. The results show that the bimodal behavior of particle size is applicable to all premixed ethylene flames studied. The variation of the size distribution from flame to flame is conclusively attributed to flame temperature variation. Under the condition of an equal carbon concentration, benzene doping leads to negligible changes in the characteristics of the size distribution. For all flames studied, AFM observations show that nascent soot is liquid-like and spreads extensively upon impact on a substrate surface.     

Particle emissions from laboratory activities involving carbon nanotubes

This site study was conducted in a chemical laboratory to evaluate nanomaterial emissions from 20–30-nm-diameter bundles of single-walled carbon nanotubes (CNTs) during product development activities. Direct-reading instruments were used to monitor the tasks in real time, and airborne particles were collected using various methods to characterize released nanomaterials using electron microscopy and elemental carbon (EC) analyses. CNT clusters and a few high-aspect-ratio particles were identified as being released from some activities. The EC concentration (0.87 μg/m³) at the source of probe sonication was found to be higher than other activities including weighing, mixing, centrifugation, coating, and cutting. Various sampling methods all indicated different levels of CNTs from the activities; however, the sonication process was found to release the highest amounts of CNTs. It can be cautiously concluded that the task of probe sonication possibly released nanomaterials into the laboratory and posed a risk of surface contamination. Based on these results, the sonication of CNT suspension should be covered or conducted inside a ventilated enclosure with proper filtration or a glovebox to minimize the potential of exposure.     

Experimental and numerical study of chemiluminescent species in low-pressure flames

Chemiluminescence has been observed since the beginning of spectroscopy, nevertheless, important facts still remain unknown. Especially, reaction pathways leading to chemiluminescent species such as OH^?, CH^?, $\mathrm{C}_{2}^{*}$ , and $\mathrm{CO}_{2}^{*}$ are still under debate and cannot be modeled with standard codes for flame simulation. In several cases, even the source species of spectral features observed in flames are unknown. In recent years, there has been renewed interest in chemiluminescence, since it has been shown that this radiation can be used to determine flame parameters such as stoichiometry and heat release under some conditions. In this work, we present a reaction mechanism which predicts the OH^?, CH^? (in A- and B-state), and $\mathrm{C}_{2}^{*}$ emission strength in lean to fuel-rich stoichiometries. Measurements have been performed in a set of low-pressure flames which have already been well characterized by other methods. The flame front is resolved in these measurements, which allows a comparison of shape and position of the observed chemiluminescence with the respective simulated concentrations. To study the effects of varying fuels, methane flame diluted in hydrogen are measured as well. The 14 investigated premixed methane–oxygen–argon and methane–hydrogen–oxygen–argon flames span a wide parameter field of fuel stoichiometry (?=0.5 to 1.6) and hydrogen content (H₂ vol%=0 to 50). The relative comparison of measured and simulated excited species concentrations shows good agreement. The detailed and reliable modeling for several chemiluminescent species permits correlating heat release with all of these emissions under a large set of flame conditions. It appears from the present study that the normally used product of formaldehyde and OH concentration may be less well suited for such a prediction in the flames under investigation.     

Electrochemical synthesis and lithium storage properties of three-dimensional porous Sn–Co alloy/CNT composite

Three-dimensional (3-D) porous copper with stable pore structure is prepared by electroless plating. 3-D porous Sn–Co alloy/carbon nanotube (CNT) composite is synthesized by electrodeposition using 3-D porous copper as the substrate. The scanning electron microscope results indicate that 3-D porous Sn–Co alloy/CNT composite contains a large amount of interconnected pores with the diameter size of ~3 μm. Upon cycling, the pore structure gradually disappears, but no serious exfoliation appears due to porous structure and reinforcement by CNT. The charge/discharge results demonstrate that the 3-D porous Sn–Co alloy/CNT composite electrode delivers high first reversible specific capacity of 490 mAh g^?1, and remains 441 mAh g^?1 after 60 cycles tested at different current densities. Even at the current density of 3,200 mA g^?1, the reversible specific capacity remains 319 mAh g^?1, which is 65 % of the first specific capacity cycled at the current density of 100 mA g^?1.