Modeling of fishbone-type carbon nanofibers with cone-helix structures

Proposing a plausible model for fishbone-type carbon nanofiber (f-CNF) proves to be a challenge. A practical approach to the construction of a cone-helix model is suggested on the basis of early experimental observations and from a geometric perspective. With the introduction of disclination angle and overlap angle into graphene sheets, the resultant helical cones have variable morphologies which rationalize the broad distribution of the f-CNF apex angles. A fraction of overlap angles can produce energetically favorable cone-helix models with high densities of coincident lattice points. Once the nearest coincident points to the apex is identified, both the overlap angle and the degree of graphitic alignment can be obtained. Periodic boundary conditions are imposed on the cone-helix models to depict the f-CNF morphologies along the principal axes. The lattice strain induced by the multi-layered model is found to have a negative effect on the structural stability. After the central parts of f-CNFs are removed, the lattice strain around the cone tips is eliminated, and the cone-helix model of more graphite layers is energetically more favorable. X-ray diffraction simulations are finally conducted to evaluate the reliability of the proposed models and to reveal the identity of the reflections at the diffraction angle of 44.5°.     

Naturally occurring graphite cones

Carbon, boron nitride, and other materials that form nanotubes are also able to form conical shapes. Even though the potential applications of cone arrays as electron emitters and other devices are very promising, understanding of their structure and formation mechanisms is still very limited compared to nanotubes and other carbon structures. Moreover, the cones have only been synthesized in a mixture with other shapes, but never as continuous arrays. It appears, however, that we can learn from nature how to produce large carbon cone arrays. We here report the first-known natural occurrence of large arrays of conical graphite crystals. These occur on the surfaces of millimeter-sized polycrystalline spheroidal aggregates of graphite. Cone heights range from less then a micron to 40 μm, which is larger than any other carbon cones reported in the literature. They are also observed to dominate sample surfaces. The surface topography of the cones and petrologic relations of the samples suggest that the cones formed from a metamorphic fluid. Unlike most laboratory produced cones, the natural cones have a wide distribution of apex angles, which supports a disclination model for cone-helix structures.     

Cone-stacked carbon nanofibers with cone angle increasing along the longitudinal axis

Using porous anodic aluminum oxide as template and petroleum pitch as precursor, a massive amount of uniform carbon nanofibers was obtained after thermal treatment. The diameter and length were 300 nm and 60 μm, respectively. The difference between these and the classic herringbone structure is that the angle between the graphenes and the fiber axis increases regularly along the axis instead of being fixed. TEM observations show that the nanofiber consists of stacked conical graphenes with cone angles that steadily increase from 60° to 180° along the fiber axis. This structure is the first to be produced without using catalytic CVD, and has not been reported using template procedures. The large deformation of the graphene planes at the tip of the nanofiber may produce interesting electronic applications.     

Palladium catalyzed formation of carbon nanofibers by plasma enhanced chemical vapor deposition

A dc plasma enhanced chemical vapor deposition process is used to obtain vertically aligned carbon nanofibers (CNFs) from palladium catalysts using an ammonia-acetylene process gas mixture. Transmission electron microscopy is used to elucidate the microstructure of the as-grown fibers revealing different growth anomalies such as a new secondary growth phenomenon which we term hybrid tip growth. Also included in our analysis are conventional tip growth derived structures. In a few instances, the conventional tip growth derived structures possess elongated catalyst particles that impart small cone angles to the carbon nanofiber microstructure. Detailed microchemical analysis reveals that hybrid tip grown CNFs using thick Pd films are partially filled with Pd. Analysis of these growth phenomenon and implications for potential use as on-chip interconnects are discussed.     

A grand canonical Monte Carlo study of hydrogen adsorption in carbon nanohorns and nanocones at 77 K

H₂ adsorption in carbon nanohorns and nanocones has been simulated at 77 K using the grand canonical Monte Carlo method. The models used for the solid adsorbents were nanosized curved graphene sheets of conical shape with five different apex angles, corresponding to the introduction of 1–5 pentagons to the hexagonal carbon network; nanohorns are a subclass of the carbon cone family (5 pentagons). Hydrogen molecules have been treated as Lennard-Jones spherical particles with quantum behavior. Details of the adsorption process have been revealed by studying carefully the cone–hydrogen interactions as well as the adsorption capacities and energies, in cones of different dimensions. Additionally, for comparison purposes similar studies have been carried out for carbon nanotube model structures and the effect of pore shape/size as well as elements of the role of confinement on sorption have been highlighted.     

Features of vapor-grown cone-shaped graphitic whiskers deposited in the cavities of wood cells

Vapor-grown graphitic whiskers in wood charcoal formed during heat treatment above 2000 °C are discussed through electron microscope observations. The whiskers were composed of conical stacked hexagonal carbon layers, with a cone apex angle of 136°. This angle probably helps the successive helical growth of the whiskers. To grow these whiskers, pyrolyzed gas from the walls of wood cells could be used as the carbon source, and the wood cell cavities functioned as a reactor that can store the pyrolyzed gas and concentrated it to supersaturation levels. Whiskers were produced under various conditions, and their features were compared using electron microscopy to examine the growth mechanism. Samples were prepared by precarbonizing wood along with SiC via a secondary heating process at 2000-2700 °C. The precarbonizing temperature controlled the potential supply of pyrolysate gas. Whisker growth began when the gas concentration reached the supersaturation level, and this was regulated by the secondary heat treatment temperature. The gas concentration can determine the size, features, and yield of the resulting whiskers. Since the carbon source was internal, originating from the surrounding cell walls, the regulation of the gas concentration was due entirely to the heat treatment and the features of the cell tissue.     

Growth of carbon nanofibers on activated carbon fiber fabrics

Activated carbon fiber fabrics, an excellent adsorbent, were used as catalyst supports to grow carbon nanofibers. Because of the microporous structure of the activated carbon fibers, the catalysts could be distributed uniformly on the carbon surface. Based on this concept, the carbon nanofibers can be grown directly on the activated carbon fiber fabrics. We demonstrate that carbon nanofibers with a diameter between 20 and 50 nm for most of the fibers can be synthesized uniformly and densely on activated carbon fiber fabrics, impregnated by nickel nitrate catalyst precursor, using catalytic chemical vapor deposition. Although the carbon nanofibers are not straight with a crooked morphology, they form a three-dimensional network structure. Structure characterizations by TEM and XRD indicate that the carbon nanofibers have a turbostratic graphite structure and the graphite layers are stacked with a herringbone structure.     

Diffusion-limited chronoamperometry at conical-tip microelectrodes

In this paper, we present simulated diffusion-limited time-variant currents at conical-tip microelectrodes fabricated by depositing a carbon film in and on pulled quartz capillaries. These mechanically strong microelectrodes are suitable probes for detecting neurotransmitters in vivo. The simulated results show that the currents obtained at conical-tip microelectrodes are larger than those at finite conical microelectrodes (e.g. etched carbon fibres protruding from an insulating plane) of comparable dimensions. The currents at conical-tip microelectrodes and finite conical microelectrodes both converge to that of a microdisk electrode at small cone heights and large cone angles, and to that of a cylindrical electrode portion of equal length and half the radius at large cone heights and small cone angles. At short times (scaled by the electrode dimensions), Cottrellian current is achieved at conical-tip microelectrodes and the current densities collapse to the expected chronoamperometric response at a microdisk electrode, subject to some simulation errors. Comparison between a simulated chronoamperogram and an experimental chronoamperogram then allows an estimate of parameters (such as electrode surface area and dimensions) that define the electrode geometry. Steady-state currents based on empirical functions have also been computed for conical-tip microelectrodes and finite conical microelectrodes.     

Carbon nanotube induced polymer crystallization: The formation of nanohybrid shish-kebabs

Carbon nanotubes (CNTs) have attracted tremendous attention in recent years because of their superb optical, electronic and mechanical properties. In this article, we aim to discuss CNT-induced polymer crystallization with the focus on the newly discovered nanohybrid shish-kebab (NHSK) structure, wherein the CNT serves as the shish and polymer crystals are the kebabs. Polyethylene (PE) and Nylon 6,6 were successfully decorated on single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Polymer CNT nanocomposites (PCNs) containing PE, Nylon 6,6 were prepared using a solution blending technique. Both pristine CNTs and NHSKs were used as the precursors for the PCN preparation. The impact of CNTs on the polymer crystallization behavior will be discussed. Furthermore, four different polymers were decorated on CNTs using the physical vapor deposition method, forming a two-dimensional NHSK structure. These NHSKs represent a new type of nanoscale architecture. A variety of possible applications will be discussed.     

SPATIAL DISTRIBUTION OF GAS AND SOLID PHASES IN CONICAL SLURRY BUBBLE COLUMNS

In this work we perform an experimental study of the spatial distribution of phases in slurry bubble columns with conical distributors that have a volume comparable to that of the cylindrical section. Three different distributors were used whose apex angles were 13°, 22° and 34°. In gas-liquid operation, the gas holdups are axially uniform in the cylindrical section and decrease towards the wall, whereas in the conical section they increase towards the inlet. These trends are observed in the three cones for all the operating conditions explored. The solids distributions in the conical sections are qualitatively different depending on whether the operation is semibatch or continuous with respect to the flow of solid-liquid suspension: in semibatch operation, the concentration monotonically increases towards the bottom of the cone and exhibits a slight increase as the wall is approached; in continuous operation, an absolute maximum in solids concentration is obtained at a point located on the wall of the cone and intermediate height. The location of this maximum moves upwards as the total solids content in the column increases and as the apex angle decreases. The maximum in solids concentration signals the most probable site for the onset of solids sedimentation and the presence of low mixing levels and reduced mass transfer rates in a slurry reactor. In the range of conditions explored in the present work, the lowest apex angle (13°) yields a more uniform solids distribution throughout the system     

Flammability of thermoplastic carbon nanofiber nanocomposites

Five commodity thermoplastics (polyethylene, polypropylene, thermoplastic polyurethane, poly(butylene terephthalate), and poly(amide 6)) were melt compounded with vapor grown carbon nanofibers via twin screw extrusion. These materials were then analyzed for flammability behavior by cone calorimeter to determine how the nanofibers would reduce flammability of the polymers. It was found by cone calorimeter that the nanofibers greatly reduced peak heat release rate and improved many other flammability parameters of the samples. However, smoke release was increased in all samples, which may be one drawback of using these materials. Interestingly, the amount of flammability reduction was not uniform across all samples, with nanofiber reducing flammability the most in the thermoplastic polyurethane sample. The mechanism of flammability reduction in the polymers tested in this paper is shown again to be a mass loss rate reduction induced by the formation of thick tangled networks of carbon nanofibers during polymer decomposition. This mechanism was confirmed by studying the mass loss rate curves and electron microscopy analysis of the final chars collected from the cone calorimeter experiments. Copyright © 2010 John Wiley & Sons, Ltd.     

Growth of Y-shaped Carbon Nanofibers from Ethanol Flames

Y-shaped carbon nanofibers as a multi-branched carbon nanostructure have potential applications in electronic devices. In this article, we report that several types of Y-shaped carbon nanofibers are obtained from ethanol flames. These Y-shaped carbon nanofibers have different morphologies. According to our experimental results, the growth mechanism of Y-shaped carbon nanofibers has been discussed and a possible growth model of Y-shaped carbon nanofibers has been proposed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Motion and stability of cones in a yield stress fluid

This experimental study focuses on the creeping flow of a shear thinning yield stress fluid around conical obstacles. The flow has been analyzed in steady state and with adherence conditions. Firstly, the influences of the cone apex angle and of the Oldroyd number, that is the ratio between plastic and viscous effects, on the drag coefficient have been analyzed. Correlations have been proposed to model the evolution of this coefficient as a function of these two parameters. The analysis provides a new alternative for measuring the yield stress. Then, the kinematic fields around the cones have been analyzed. These fields enable to describe the rigid zones and the sheared zone developing around the lateral edge of the cones as a function of the cone apex angle. Moreover, the wall shear stresses estimated from the particle image velocimetry measurements have enabled to quantify the contribution of the lateral drag force in the drag force. © 2014 American Institute of Chemical Engineers AIChE J, 61: 709–717, 2015     

Excavating the unique synergism of nanofibers and carbon black in Natural rubber based tire tread composition

The article portrays the synergistic reinforcement of new generation nanofillers such as silicon carbide nanofibers (SiCs), carbon nanotubes (CNTs), and graphite nanofibers (GNFs) when used along with carbon black (CB) in a typical tire tread composition. The unique synergism in these composites, which were fabricated by a liquid phase mixing method, was reflected in their enhanced failure resistance and dynamic mechanical properties. At 4 phr loading of the nanofiber, the tensile strength, tear strength, modulus at 300% elongation, storage modulus, rolling resistance, and abrasion resistance were improved by 29, 45, 36, 110, 15, and 14%, respectively. The role of nanofibers in the development of a hybrid microstructure was investigated by scanning and transmission electron microscopy. Tribological characteristics were studied using a Laboratory Abrasion Tester (LAT 100), and the abrasion loss of the samples was correlated with energy dissipation occurring during the process. The fatigue properties indicated the ability of the CB-nanofiber dual filler system to arrest crack growth. The study also serves to establish a correlation between the wear loss and fatigue properties of the hybrid nanocomposites containing different fibrous nanofillers. A mechanism of reinforcement by hybrid fillers is proposed.     

Excellent Field Emission Properties of Short Conical Carbon Nanotubes Prepared by Microwave Plasma Enhanced CVD Process

Randomly oriented short and low density conical carbon nanotubes (CNTs) were prepared on Si substrates by tubular microwave plasma enhanced chemical vapor deposition process at relatively low temperature (350–550 °C) by judiciously controlling the microwave power and growth time in C₂H₂ + NH₃ gas composition and Fe catalyst. Both length as well as density of the CNTs increased with increasing microwave power. CNTs consisted of regular conical compartments stacked in such a way that their outer diameter remained constant. Majority of the nanotubes had a sharp conical tip (5–20 nm) while its other side was either open or had a cone/pear-shaped catalyst particle. The CNTs were highly crystalline and had many open edges on the outer surface, particularly near the joints of the two compartments. These films showed excellent field emission characteristics. The best emission was observed for a medium density film with the lowest turn-on and threshold fields of 1.0 and 2.10 V/μm, respectively. It is suggested that not only CNT tip but open edges on the body also act as active emission sites in the randomly oriented geometry of such periodic structures.     

Microstructural changes induced in “stacked cup” carbon nanofibers by heat treatment

Systematic studies of structural changes in stacked cup carbon nanofibers by heat treatment from 1800 to 3000 °C are carried out. The most prominent feature upon heat treatment of these nanofibers is the formation of energetically stable loops between adjacent active end planes both on the inner and outer surfaces. The appearance of the jagged outer and inner surfaces at 3000 °C is due to a combinational effect, perhaps caused by improved stacking within domains connected by loops having limited crystallite size, accompanied by structural reorganization between domains. Consequently, a low degree of graphitizability is ascribed to this unusual stacked cup morphology of pristine carbon nanofibers.     

Dual formation of carpets of large carbon nanofibers and thin crystalline carbon nanotubes from the same catalyst-underlayer system

A dense, micron-tall layer of carbon nanofibers (CNFs) was grown above a layer of carbon nanotubes (CNTs) during the same synthesis using a thick cobalt catalyst (15 nm). The CNFs had large diameters (100 nm) and were amorphous while the CNTs had small diameter (10–20 nm) and were crystalline. Base growth mechanism was at play for both the nanofibers and the nanotubes. High-resolution transmission electron microscopy characterization suggested that the main mechanisms leading to the growth of the two structures were based on the dewetting of the catalyst layer and its subsequent alloying with the Ta underlayer. We can extend these principles to grow diverse carbon nanostructures during the same synthesis using appropriate multilayer thin films for different applications, especially for electrochemical cells and supercapacitors.     

Carbon nanonodules fewer than ten graphenes thick grown on aligned amorphous carbon nanofibers

A novel structure of carbon nanonodules containing fewer than 10 layers graphene has grown on amorphous carbon nanofibers by carbonization-induced self-assembly. It is found that a successive processes containing pre-oxidation in air at 220 °C and carbonization in a high vacuum (1 × 10⁻⁴ Pa) at 750 °C are necessary for the fabrication of the carbon nanonodules. Possible mechanism for the evolution of amorphous nanofibers to carbon nanonodules is presented. It is also found that the temperature of the collector during electrospinning of the fiber and the pressure of carbonization are critical factors for growth of the nanonodules. With these mechanisms, carbon nanonodules can be selectively grown on the prepared amorphous carbon nanofibers using pre-oxidation and carbonization of an electrospun glycerol–polyacrylonitrile fiber.     

Investigating the mechanism of collective bidirectional growth of carbon nanofiber carpets on metallic substrates

Bidirectional-growth of carbon nanofibers is a rare phenomenon found on free-standing catalyst particles, in contrast to the most commonly studied tip- and base-growth mechanisms for carbon nanostructures synthesized through thermal chemical vapor deposition. We reveal the underlying mechanisms of collective bidirectional growth in Ni_xPd_1−x-catalyzed carbon nanofiber carpets grown on a palladium substrate with varying nickel film thickness by monitoring the fiber growth evolution. The results show that the collective bidirectional growth is promoted and controlled by the chemical and physical restructuring of the sub-surface portion of the metal stack which undergoes micro-fragmentation as a result of the incorporation, diffusion, and precipitation of carbon. Carbon nanofiber growth can be controlled by engineering the catalyst-underlayer materials properties such as grain size, chemical composition and alloying. Since the determining factor whether carbon nanofibers or nanotubes are obtained is a strong function of catalyst size, the understanding of this growth mechanism can be transferred to the field of carbon nanotube synthesis. By keeping the grain size small enough to ensure carbon nanotube instead of carbon nanofiber growth, achieving dense, vertically aligned carbon nanotube carpets on metallic substrates might be possible, which is a prerequisite for carbon nanotube integration in integrated circuits.     

Higher‐order structural analysis of nylon‐66 nanofibers prepared by carbon dioxide laser supersonic drawing and exhibiting near‐equilibrium melting temperature

Extended chains and/or extended chain crystals (ECC) are important structures for improving the mechanical properties of polymer fibers. ECC have so far been produced using specially prepared materials or manufacturing methods. In our study on the production of nanofibers by carbon dioxide (CO₂) laser supersonic drawing, we succeeded in producing nylon‐66 nanofibers having a high melting point near the equilibrium melting point (T_m⁰). Two melting points (T_m) of 260 and 276°C were observed for the nanofibers, with the latter temperature being close to the T_m⁰ (280°C) of nylon‐66. A nanofiber that was heat treated at 279°C for 10 min displayed a large stacked lamellar structure with an average crystal thickness of 140 nm. That value was close to the average molecular chain length of 212 nm, which was calculated from the average molecular weight of the nanofibers. It was inferred from these results that ECC corresponding to the average molecular chain length were present in the nanofibers. The CO₂ laser supersonic drawing process is applicable to general purpose thermoplastic polymers and uses a simple drawing system. It is expected that this drawing method will help to improve the fundamental performance of general purpose polymers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40361.