Stages Of Carbon Nanotube Growth Research Articles

Bubble characteristic of carbon nanotubes growth process in a tapered fluidized bed reactor without a distributor

In this work, an imaging processing technique was used to study bubble distribution and evolution in the carbon nanotube (CNT) growth process for the first time. The Fractal analysis method is used to analyze the degree of gas-solid mixing and particles movement. A superposition imaging method was proposed and used to investigate the effect of bubbles on gas-CNT particle mixing. Using a tapered fluidized bed reactor (TFBR) without a distributor, we found that bubbles play different roles in particle mixing at different stages of CNT growth in tapered fluidized bed reactor (TFBR) without a distributor. The degree of mixing of gas-CNT particles and the degree of particle movement are related to the growth time of the CNT. In the growth process of agglomerated multi-walled CNT (@A), strong gas-solid contact dominates in the 1-5 min and 15-20 min windows. Stable fluidization of @A plays an important role in 35-40 min and 45-50 min windows. The degree of dispersion of bubbles of vertical array multi-walled CNT (@A) decreases over time. Importantly, efficient heat and mass transfer are ensured in the three time periods before 15.28 min. After 35.28 min, the @V growth process exhibits stable fluidization. Based on the probability density and joint probability density of bubble aspect ratio and shape factor, we found that different growth time of CNT differentially influences the bubble characteristics in the bed. Additionally, the distribution tendency of @A particle and particle productivity were strikingly similar, confirming that the image processing technique and fractal analysis method adopted in this study are feasible and reliable.

Investigation of Early Stage of Carbon Nanotube Growth on Plasma-Pretreated Inconel Plates and Comparison with Other Superalloys as Substrates.

We investigate the early stage of carbon nanotube (CNTs) growth on Inconel 600 to address the effect of pretreatments such as annealing and plasma pretreatment on growth behavior. In addition, we compare the growth results to other Ni-based superalloys including Invar 42 and Hastelloy C276. The growth substrates were prepared using mechanical polish, thermal annealing and plasma pretreatment. The air annealing was performed at 725 °C for 10 min and plasma pretreatment was subsequently undergone with 10.5 W at 500 °C for 30 min. The annealed and plasma-pretreated substrates exhibited different surface morphologies on the surface and enhanced growth behavior of CNT was observed from the region of particulate surface. The optimized growth temperature, which produces the highest CNT height, was determined at 525 °C for Ni and Inconel 600 and 625 °C for Invar 42 and Hastelloy C276 substrates. The difference of optimal growth temperature is expected to the existence of high temperature elements such as Mn or Mo in the alloys. X-ray diffraction spectroscopy revealed that the formation of roughened oxide layers caused by the pretreatments would promote the nucleation and growth of the CNTs.

Ab initio molecular dynamics simulation of ethanol dissociation reactions on alloy catalysts in carbon nanotube growth

The dissociation reactions of ethanol molecules on Fe, Co and FeCo catalysts are investigated by ab initio molecular dynamics simulation to clarify the initial stages of carbon nanotube growth. It is found that the local arrangement of atoms determines which bond is likely to dissociate. For the dissociation of a CC bond, the site with adjacent iron and cobalt atoms is advantageous. With regard to the dissociation of CO bonds, if iron atoms are concentrated on the dissociated oxygen bonding site, the reaction is more likely to occur.

Dissociation dynamics of ethylene molecules on a Ni cluster using ab initio molecular dynamics simulations

The atomistic mechanism of dissociative adsorption of ethylene molecules on a Ni cluster is investigated by ab initio molecular-dynamics simulations. The activation free energy to dehydrogenate an ethylene molecule on the Ni cluster and the corresponding reaction rate is estimated. A remarkable finding is that the adsorption energy of ethylene molecules on the Ni cluster is considerably larger than the activation free energy, which explains why the actual reaction rate is faster than the value estimated based on only the activation free energy. It is also found from the dynamic simulations that hydrogen molecules and an ethane molecule are formed from the dissociated hydrogen atoms, whereas some exist as single atoms on the surface or in the interior of the Ni cluster. On the other hand, the dissociation of the C-C bonds of ethylene molecules is not observed. On the basis of these simulation results, the nature of the initial stage of carbon nanotube growth is discussed.

Ab initio molecular dynamics simulation of ethanol decomposition on platinum cluster at initial stage of carbon nanotube growth

Ethanol decomposition on a platinum cluster is investigated by ab initio MD simulation. As the dehydrogenation proceeds, the Mulliken charge of the methylene carbon becomes a positive value, whereas that of the methyl carbon keeps a negative value. Especially, the Mulliken charge of the methylene carbon in CHxCO (x=0, 1, 2 and 3) fragment molecules takes a large positive value. These fragment molecules correspond to those with CC bond that dissociated in the MD simulation. It suggests the large deviation in the Mulliken charge between methylene and methyl carbons is the key factor inducing the CC bond dissociation.

Early stages of the carbon nanotube growth by low pressure CVD and PE-CVD

The early stages of single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) growth were investigated by chemical vapour deposition (CVD) and plasma enhanced CVD (PE-CVD). SWCNT and MWCNT nuclei were observed using low pressure and short duration synthesis process. Microscopic evidences of the quite long duration, and hence complexity, of the MWCNT nucleation compared to SWCNT growth was provided. Besides, Raman spectroscopy pointed out significant differences in terms of disorder between the CVD and PE-CVD samples at the early stage of the growth process. The CVD process was found to be more selective for carbon nanotube growth, without significant disordered carbon material deposited on the substrate.

STM and XPS studies of early stages of carbon nanotube growth by surface decomposition of 6H–SiC(000-1) under various oxygen pressures

The effects of oxygen on the early stage of carbon nanocap and nanotube growth by surface decomposition of 6H–SiC(000-1) were investigated, using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Our results showed that the presence of oxygen enhances the decomposition of the SiC surface, promoting carbon nanocap and nanotube growth. We also observed active-to-passive oxidation transition below 10 − 1 Pa, which is consistent with the pressure–temperature phase diagram previously estimated from experimental results above 10 − 1 Pa.

Direct Dynamics Studies of CO-Assisted Carbon Nanotube Growth

Semiempirical direct dynamics simulations are used to study the potential role of CO in the gas-phase catalytic growth of carbon nanotubes. Calculations employing a fully self-consistent AM1 semiempirical electronic Hamiltonian predict that CO molecules can chemically adsorb to the open edge of partially grown zigzag and armchair nanotubes. The adsorbed CO molecules can form either pentagonal or hexagonal carbon rings by an electrocyclic reaction with neighboring adsorbed CO groups. Formation of hexagonal carbon rings in zigzag nanotubes is kinetically and thermodynamically favored by a CO insertion reaction with CO-saturated ends. These results provide some computational evidence that gas-phase CO can adsorb and contribute to the intermediate and latter stages of carbon nanotube growth. This study assumes that growth proceeds by the addition of carbon to the nanotube edge held open by metal catalyst particles, as opposed to a root growth or capped end growth mechanism.