Growth Rate Of Carbon Nanofibers Research Articles

Parametric Study of Plasma Characteristics and Carbon Nanofibers Growth in PECVD System: Numerical Modeling

The aim of the present work is to develop a numerical model to understand and optimize the process parameters for the growth of carbon nanofibers (CNFs) inside the plasma enhanced chemical vapor deposition system containing acetylene, hydrogen, and argon gases. Two-dimensional axis-symmetrical inductive couple plasma module is implemented using COMSOL Multiphysics 5.2 simulation software to analyze the density profiles and temperatures of electrons, ions, and neutral species in the plasma at different gas pressures and input plasma powers. The outcomes of the COMSOL computational model show that the electron density in the plasma decreases with increase in gas pressure and increases with increase in plasma power. Other than the computational model, an analytical model is developed in the present paper that accounts the plasma sheath equations to study the fluxes and energies of the plasma species. The results obtained from the plasma sheath model at the catalyst-substrate surface boundary are fed as the input parameters to surface deposition model to investigate the growth characteristics of carbon nanofibers, i.e., poisoning of the catalyst nanoparticle, height, and diameter of carbon nanofiber at different gas pressures and input plasma powers. It is found that electron density decays at the faster rate when gas pressure is increased and decays at slower rate when input plasma power is raised. Moreover, it is also found that growth rate of CNFs increases with increase in gas pressure and plasma power. However, the significant drop in CNF growth rate is observed when the gas pressure is high enough (above and around 50 Torr). From the results obtained, it can be concluded that the CNFs having good growth characteristics can be obtained at some optimum pressure range, i.e., one order of the magnitude in the units of Torr. A good comparison between numerical simulation results and analytical results with each other and with existing experimental results confirms the adequacy of the computational and analytical approach.

Study the effect of various wash-coated metal oxides over synthesized carbon nanofibers coated monolith substrates.

In this research work, carbon nanofibers (CNFs) were synthesized on honeycomb monolith substrates using injection chemical vapor deposition (ICVD) technique. The effect of various wash-coated materials and catalyst promoter on the growth rate of CNFs on monolith substrates were examined. The characteristics of the synthesized CNFs-coated monolith composites were examined using Raman spectroscopy, Brunauer–Emmett–Teller (BET), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), and Transmission electron microscopy (TEM) techniques. According to the textural characterization study, the specific surface area and pore volume of CNFs-coated monolith composites were significantly improved as compared to bare monolith which might be attributed to the growth of highly pure and aligned CNFs over monolith substrate. Besides that, the synthesized CNFs-coated monolith possessed extremely well thermal stability up to the temperature of 550 °C which was corresponded to the strong attachment of highly graphitized CNFs over monolith substrates.

Synthesis of amorphous carbon nanofibers on a surface of commercial polymers under the action of a high power ion beam of nanosecond duration

The irradiation of various commercial polymers (chlorinated polyvinyl chloride, polyvinyl chloride, polyvinyl alcohol, polymethyl methacrylate) by a high-power ion beam of nanosecond duration has been investigated. The amorphous carbon nanofibers formation was detected on the surface of low-cost commercial polymers (chlorinated polyvinyl chloride, polyvinyl chloride) containing a catalyst under the high power ion beam irradiation at room temperature. Various iron compounds (organic and inorganic) were used as catalytic additions. The electron microscopy (scanning and transmission) with an energy dispersive analyzer and Raman spectroscopy were used to investigate the morphologies and structure of irradiated polymers. Carbon nanofibers (CNFs) had a most probable diameter in the range 50-90 nm and a maximum length of up to 10 μm. The maximum CNFs growth rate was estimated as 160 μm/μs. A possible growth mechanism for CNFs is discussed.

Ultrafast catalytic synthesis of carbon nanofibers on a surface of commercial chlorinated polymers under the action of a high power ion beam of nanosecond duration

Carbon nanofibers (CNFs) have been rapidly synthesized on the surface of low-cost commercial chlorinated polymers (chlorinated polyvinyl chloride, polyvinyl chloride), which contains iron compounds, under the action of a high power ion beam at room temperature. Organic (Fe(C5H5)2) and inorganic (FeCl3⋅6H2O и Fe(NO3)3·9H2O) iron compounds were used as catalytic additions. SEM, TEM and Raman spectroscopy were used to investigate the morphologies and structure of irradiated polymers. CNFs had a most probable diameter in the range 50–90 nm and a maximum length of up to 10 μm. The maximum CNFs growth rate was estimated as 160 μm/μs. A possible growth mechanism for CNFs is discussed.

Formation of Nanostructured Carbon Layers on the Surface of Chlorinated Polymers under High-Power Ion Beam Irradiation

A comparative study of the effect of a high-power ion beam of nanosecond duration on layers of reactive industrial chlorinated polymers (chlorinated polyvinyl chloride, polyvinyl chloride) containing a catalytic additive (ferrocene) is carried out. The formation of amorphous carbon nanofiber layers on the surface of chlorinated polymers is revealed. The characteristic diameter and length of nanofibers depends on the type of polymer and irradiation mode and is equal to 30–250 nm and 10 μm, respectively. The growth rate of carbon nanofibers is estimated. Its value might reach ~160 μm/μs. A possible mechanism for carbon nanofiber formation on the surface of chlorinated polymers with the addition of ferrocene under high-power ion beam irradiation is discussed.

Theoretical modeling to study the impact of different oxidizers (etchants) on the plasma-assisted catalytic carbon nanofiber growth

An analytical model based on the various surface deposition processes and plasma sheath kinetics of the plasma species (electrons, positively charged ions, radicals, and neutrals) has been developed to investigate the effects of different plasmas (different etchants) on the catalyzed plasma aided growth of carbon nanofibers (CNFs). In particular, the model accounts the poisoning of the catalyst nanoparticle, i.e., the formation of the amorphous carbon layer on the catalyst active surface due to the continuous dissociation of incoming hydrocarbon species from the plasma. It is observed that oxidizers (H2O and O2) in the typical hydrocarbon/hydrogen (C2H2 + H2) plasma act as the dominant etchants and remove the amorphous carbon layer from the catalyst surface and, thus, preserve and enhance the catalyst activity. However, the growth rate of CNFs is much higher when O2 is added as an etchant in the reactive plasma as compared to H2O. This is due to the dual role played by the oxygen, i.e., (i) removal of amorphous carbon from the catalyst active surface, (ii) removal of hydrogen radicals that interact with the carbon species generated on the catalyst surface and suppress their diffusion through the catalyst nanoparticles. The CNF grows much longer in the presence of O2, therefore, etching of CNF tip and deformation of catalyst nanoparticle is the maximum, and hence, the CNF tip diameter is least. Moreover, in the present investigation, we also found that the relative concentrations of H2O or O2 species in the reactive plasma have significant effects on the CNF growth. Our theoretical results are in good agreement with the experimental observations.

Mechanisms of plasma-assisted catalyzed growth of carbon nanofibres: a theoretical modeling

A theoretical model is developed to study the nucleation and catalytic growth of carbon nanofibers (CNFs) in a plasma environment. The model includes the charging of CNFs, the kinetics of the plasma species (neutrals, ions and electrons), plasma pretreatment of the catalyst film, and various processes unique to a plasma-exposed catalyst surface such as adsorption of neutrals, thermal dissociation of neutrals, ion induced dissociation, interaction between neutral species, stress exerted by the growing graphene layers and the growth of CNFs. Numerical calculations are carried out for typical glow discharge plasma parameters. It is found that the growth rate of CNFs decreases with the catalyst nanoparticle size. In addition, the effect of hydrogen on the catalyst nanoparticle size, CNF tip diameter, CNF growth rate, and the tilt angle of the graphene layers to the fiber axis are investigated. Moreover, it is also found that the length of CNFs increases with hydrocarbon number density. Our theoretical findings are in good agreement with experimental observations and can be extended to enhance the field emission characteristics of CNFs.

Kinetically controlled synthesis of carbon nanofibers with different morphologies by catalytic CO disproportionation over iron catalyst

Carbon nanofibers (CNFs) were synthesized by CO disproportionation on iron catalyst at CO concentration between 58.3% and 75.0%, H 2 concentration between 8.3% and 25.0% and reaction temperature between 833 and 913 K. The time-depending rate of CNFs growth as a function of time was determined by an on-line mass spectrometer and the morphologies of all CNFs products were observed by electronic microscopy. Not only the CNFs growth rate but also the morphology of the grown CNFs were shown to vary with the three operating variables. SEM and TEM images showed that the three-dimensional morphologies of the CNFs were twist, helical or straight and an interesting relationship between the maximal growth rate and the morphology was observed. When the growth rate was between 0.8 and 0.9 mmol/(s gcat), the CNFs were twist. As the growth rate increased to 1.0 mmol/(s gcat), more helical nanofibers appeared. Straight nanofibers were produced when the growth rate reached the level of 1.2 mmol/(s gcat). Finally when the rate of CNFs growth was high at 1.8 mmol/(s gcat), the absolute majority of the solid products was amorphous carbon coexisting with some short and thick nanofibers. Under different operating conditions, the crystal faces of the catalysts had different anisotropy properties for carbon deposition, thus producing CNFs with different morphologies.

Optimization and scale-up of CNF production based on intrinsic kinetic data obtained from TEOM

Optimizing the operating parameters for growing carbon nanofibers (CNFs) is an important step toward the large-scale production of CNFs with a high degree of structure control. The present work demonstrates that a tapered-element oscillating microbalance (TEOM) is an excellent tool for the kinetic study of the highly dynamic process of CNF growth. Initial CNF growth rates, deactivation rates, and yields can be studied simultaneously as a function of temperature, pressure, hydrogen partial pressure, or residence time. The kinetic data from TEOM studies are directly implemented to scale up the production of CNF. For a hydrotalcite (HT)-derived Ni catalyst (77 wt% Ni), the highest CNF growth rate was obtained at a temperature of around 580 °C and a hydrogen partial pressure of around 0.1 bar. High temperatures and low partial pressures of hydrogen resulted in a high initial growth rate and deactivation rate, whereas low temperatures and high partial pressures of hydrogen resulted in low initial growth rate and deactivation rate. A carbon capacity (maximum carbon yield) of 50 g/g cat could be achieved at optimized conditions. Growth rates and CNF yields were found to increase with increasing total pressure (0.3–3.8 bar). The mechanisms of CNF growth and deactivation were explored based on kinetic data and DFT calculation studies from the literature. The results obtained from the TEOM studies were verified in a PFR fixed-bed reactor and a horizontally placed fixed-bed reactor similar to a CSTR reactor. The residence time of methane was identified as the most important parameter for scaling up the process. The data on the effect of hydrogen from the TEOM were reproduced, and a successful scale-up of 10,000 times was achieved.

Experimental characterization of an inductively coupled acetylene/hydrogen plasma for carbon nanofiber synthesis

A plasma-enhanced chemical-vapor deposition process was employed to synthesize carbon nanofibers (CNFs) on glass substrates patterned with Ni catalytic films. At the gas pressure of 20mTorr and the substrate temperature (surface) of ∼500°C, the isolated and vertically aligned carbon nanofibers have been successfully synthesized. This paper reports experimental investigation of plasma properties characterized by the optical emission spectroscopy of the spectral line intensities of the various species such as hydrogen, C2, and CH, as well as the rf characteristics at the biased substrate stage measured by an impedance meter. The measurement results reveal that the C2 density increases with the acetylene/hydrogen flow ratio and the inductively coupled plasma (ICP) source power, as expected. The atomic hydrogen density, however, decreases with the flow ratio but increases with the ICP power. The resulting growth rate of CNFs increases with the C2 density if atomic hydrogen density also increases accordingly, e.g., as the ICP power increases. The trend is reversed if the atomic hydrogen density decreases, due to too much amorphous carbon (a-C) layer formed as a result of oversupply of carbon but not enough atomic hydrogen to remove a-C. The experimental results also show that the etch effect upon the effective removing of the a-C on the surface of catalytic nanoparticles is further enhanced by ion bombardment, e.g., when either the flux (or current) or energy of the ions incident on the substrate surface increases, to give rise to an increase in growth rate. In our ICP reactor, the ion current increases with the ICP power, but it changes little when the bias power is varied. The ion energy increases with the bias power, but it decreases as the ICP power increases while the bias power is fixed. The latter one is because the plasma density increases with the inductively coupled plasma power.

Surface Diffusion: The Low Activation Energy Path for Nanotube Growth

We present the temperature dependence of the growth rate of carbon nanofibers by plasma-enhanced chemical vapor deposition with Ni, Co, and Fe catalysts. We extrapolate a common low activation energy of 0.23-0.4 eV, much lower than for thermal deposition. The carbon diffusion on the catalyst surface and the stability of the precursor molecules, C2H2 or CH4, are investigated by ab initio plane wave density functional calculations. We find a low activation energy of 0.4 eV for carbon surface diffusion on Ni and Co (111) planes, much lower than for bulk diffusion. The energy barrier for C2H2 and CH4 dissociation is at least 1.3 eV and 0.9 eV, respectively, on Ni(111) planes or step edges. Hence, the rate-limiting step for plasma-enhanced growth is carbon diffusion on the catalyst surface, while an extra barrier is present for thermal growth due to gas decomposition.

The effects of pre-treatment and catalyst composition on growth of carbon nanofibers at low temperature

Carbon nanofiber films have been grown on Fe–Ni alloy catalysts at 400°C by a thermal chemical vapor deposition (CVD) method using acetylene as a carbon source. Fe–Ni–C alloy catalysts were found to be more suitable for carbon nanofiber growth at 400°C than pure Ni or Fe–Ni catalysts. When Fe–Ni catalysts were pre-treated in hydrogen at a low temperature (400°C), the catalyst lost its activity in growing carbon nanofibers at 400°C by forming an amorphous carbon layer covering catalytic sites on the surface. The growth rate of carbon nanofibers was improved significantly by pre-treating the catalyst with a low concentration of carbon source (e.g. acetylene, carbon monoxide) forming Fe–Ni–C alloy during the temperature-rising step before growth. Carbon nanofiber walls consisted of imperfect graphite structure as characterized by transmission electron microscopy.