Reliable Growth Of Vertically Aligned Carbon Nanotube Arrays By Chemical Vapor Deposition And In Situ Measurement Of Fundamental Growth Kinetics In Oxygen Free Conditions
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Reliable Growth Of Vertically Aligned Carbon Nanotube Arrays By Chemical Vapor Deposition And In Situ Measurement Of Fundamental Growth Kinetics In Oxygen Free Conditions
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Author : JUNG BIN. IN
language : en
Publisher:
Release Date : 2011
Reliable Growth Of Vertically Aligned Carbon Nanotube Arrays By Chemical Vapor Deposition And In Situ Measurement Of Fundamental Growth Kinetics In Oxygen Free Conditions written by JUNG BIN. IN and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2011 with categories.
Vertically-aligned carbon nanotube (VACNT) arrays are both an important technological system, and a fascinating system for studying basic principles of nanomaterial synthesis. However, despite continuing efforts for the past decade, important questions about this process remain largely unexplained. Recently, nanotube research investigations have been conducted, aiming at revealing the underlying growth mechanisms, rather than merely studying the feasibility on new growth methods. Nonetheless, growth deactivation and the accompanying termination mechanisms still remain a topic of nanotube synthesis science. Due to the extremely small size, however, direct characterization of various transport and conversion events occurring at the catalyst surface is not an easy task. Thus investigations on growth kinetics are the first step to resolve questions about growth mechanism. Before exploring kinetic aspects of the growth process, one must achieve reliable growth conditions since growth non-reproducibility retards obtaining reliable growth data and undermines the scientific value of the data. In order to improve growth reliability, several factors that may contribute to growth non-reproducibility were identified and thereafter mitigated. Firstly, a simulation study was conducted to achieve insight into temperature and velocity profile of gases inside the reactor since gas flow dynamics can render growth environment near the substrate non-uniform. Interestingly, when argon gas was used as the main carrier gas, natural convective flow emerged, generating flow circulation before the gas reached the substrate placed at the center of the tube reactor. This flow circulation was not favorable for controlled gas introduction. This problem could be resolved by using a more heat- and momentum- conductive gas such as helium. Secondly, atomic force microscopy of annealed catalyst revealed that the aluminum sub-layer was not thermally stable at the growth temperature although this material has been widely used as a barrier layer to avoid silicide formation of catalyst on silicon substrates. In this respect, aluminum oxide should be a better choice, but under-stoichiometry of the aluminum oxide layer, which originated from sputter target degradation, affected thermal stability of the layer. Reactive sputtering by oxygen addition greatly enhanced thermal stability, and finally defect-free catalyst nanoparticles were formed by thermal annealing. Thirdly, the effect of the small part-per-million levels of oxygen-containing species on VACNT growth revealed that oxygen-containing gas impurities in nominally pure gas sources have a great influence on growth kinetics in a positive way; their presence increases catalyst lifetime and growth yield. However, the kinetic behavior that is highly sensitive to gas purity is prone to showing an interfering kinetic trend where the real mechanism is masked by the significant gas impurity effect. The stark difference in catalytic lifetime after the introduction of high-performance gas purifiers shows that extremely tight control of the reaction gas composition purity is necessary to obtain controlled growth of CNTs under atmospheric chemical vapor deposition (CVD) conditions. Finally, more reliable growth of VACNTs was achieved, and thereafter the next step for fundamental growth kinetics measurement was followed. Finally, the CVD system was equipped with an optical micrometer that enables in-situ measurement of the height of growing VACNTs, which have advantageous structure facilitating measurement of growth kinetics since the array height has a robust correlation with growth yield and thereby growth rate. Various ethylene and hydrogen combinations were examined to capture growth kinetics related to different gas environment. The measured initial growth rates were linearly proportional to ethylene concentration, whereas a reciprocal relation was observed with respect to hydrogen concentration. The apparent activation energy was higher than reported in references. Flow rate variation experiments revealed that gas phase reaction is involved as the crucial growth step, which supports the observed high activation energy. Consequently, a growth model was proposed so that it could reasonably fit the initial growth rate data. Kinetic aspects related to growth deactivation were explored by measuring the final growth height and catalyst lifetime. Unlike growth with unpurified gases, growth became much less sensitive to gas composition after purification. Importantly, it was observed that growth deactivates by deficit of carbon source when relatively low ethylene was introduced. This result is surprising since ethylene pressure should be high enough at the catalyst, considering the calculated sticking coefficient of ethylene is very low, approximately 10^-5. Thus it substantiates the idea that catalyst-mediated gas pretreatment process is critical to sustain nanotube growth. Importantly, this idea challenges the widely accepted growth termination concept whereby nanotube stops growing due to catalyst encapsulation by excessive carbon. Indeed, reduced flow rate of gas mixture increased growth yield remarkably by promoting the gas pretreatment over the catalyst. Catalyst ripening, or steric hindrance by interaction of nanotubes can be an alternative reason for growth termination, but analysis of morphologies of the annealed catalyst and as-grown nanotubes revealed that their effects were not significant for the corresponding growth conditions.
Aligned Carbon Nanotubes
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Author : Zhifeng Ren
language : en
Publisher: Springer Science & Business Media
Release Date : 2012-09-05
Aligned Carbon Nanotubes written by Zhifeng Ren and has been published by Springer Science & Business Media this book supported file pdf, txt, epub, kindle and other format this book has been release on 2012-09-05 with Science categories.
This book gives a survey of the physics and fabrication of carbon nanotubes and their applications in optics, electronics, chemistry and biotechnology. It focuses on the structural characterization of various carbon nanotubes, fabrication of vertically or parallel aligned carbon nanotubes on substrates or in composites, physical properties for their alignment, and applications of aligned carbon nanotubes in field emission, optical antennas, light transmission, solar cells, chemical devices, bio-devices, and many others. Major fabrication methods are illustrated in detail, particularly the most widely used PECVD growth technique on which various device integration schemes are based, followed by applications such as electrical interconnects, nanodiodes, optical antennas, and nanocoax solar cells, whereas current limitations and challenges are also be discussed to lay the foundation for future developments.
Kinetics Of Carbon Nanotube Growth With Applications In Hydrogen Storage
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Author : Ranadeep Bhowmick
language : en
Publisher:
Release Date : 2010
Kinetics Of Carbon Nanotube Growth With Applications In Hydrogen Storage written by Ranadeep Bhowmick and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2010 with categories.
Carbon nanotubes (CNTs) have unique transport and elastic properties due to their high aspect ratios. Hence there is considerable interest in using these tubes as field emitter cathodes, composite materials with enhanced electrical and mechanical properties, electronic components and recently for hydrogen storage applications taking advantage of the high specific surface areas. In this thesis three different aspects of carbon nanotubes were studied: (1) Controlled growth of single walled nanotubes (SWNTs), (2) Electric field directed Chemical Vapor Deposition (CVD) of multi walled (MWNTs) and (3) "Spillover" Mechanism of hydrogen storage in Pt-SWNT composites. The kinetics of carbon nanotube growth was studied in the context of the CVD process. A generic model for growth of 1-d nano structures via the Vapor-Liquid-Solid (VLS) mechanism is applied to the nanotube growth. This model considers the energetics of individual mass transfer steps through each phase and at the phase interfaces. The flux is then written in terms of the change in chemical potential. Laser interferometry was applied in a cold-wall thermal CVD reactor to measure the growth of the MWNT films in-situ. Temperature dependent studies in the steady-state regime were used to obtain activation energies which are consistent with the interfacial transport step. Consideration of the catalyst activation/de-activation process in the non-steady regimes requires the rate limiting step to be in the vapor-liquid transition. Application of an electric field during the MWNT growth was found to enhance both the growth rate and alignment of the MWNTs. Temperature dependent studies in the presence and absence of the electric field show that there are actually two activated processes involved, with rate-limiting step being independent of applied field at high temperature. At higher temperatures, the rate-limiting step is the carbon dissolution into the catalyst particle, while at lower temperatures it is the carbon dissociation at the catalyst-vapor interface that limits the growth. Application of an electric field enhances the decomposition of the C precursor in the vapor phase, thus circumventing this low temperature activation barrier. The enhanced alignment of the MWNTs with the electric field is explained by tensile stretching overcoming the defect-induced kinking of the MWNTs. Calculations show that this benefit is obtained at a minimum field level, with no benefit arising from further increase in field strength. The catalyst particle size is one of the key parameters that determine the morphology of the 1D carbon nanostructures in both processes studied. The thermodynamics of the nano-particle formation and carbon dissolution are studied and applied to these processes. While the diameter serves to template the CNT diameter, the Gibb's Thompson effect predicts a size dependent suppression of melting point which determines the nature of CNT formed. In the CVD process, higher pressures were found to form larger particle sizes which led to nanofiber growth. At these diameters, the melting point suppression puts the Fe-C particle in a dual solid-liquid phase. Carbon flux accumulates in the dual phase during growth until the dual phase becomes energetically unfavorable. At this point, the particle reverts to a single solid phase regime by discarding excess carbon, resulting in a discontinuous graphitic structure characteristic of Carbon nanofibers. For smaller particles, the phase is entirely liquid and leads to steady state carbon flux and CNT growth. Controlling the iron bearing precursor concentration of the solution fed into the floating catalyst reactor was found to control particle size, and hence SWNT diameter, within this regime. For similar catalyst particle size distributions, increasing the temperature increased the range of SWNT diameters and chiralities obtained. The thermodynamic energy barrier for SWNT formation at the different diameters was calculated and shown to be consistent with the observed variation. Finally, the mechanism of hydrogen uptake in transition metal-doped SWNT was studied. Molecular hydrogen, dissociated by metal catalyst nanoparticles, diffuses to the nanotube surface forming stronger bonds. In-situ 4-probe conductivity tests were performed on mats of Pt doped SWNT during hydrogen uptake. On hydrogen charging the resistivity of the Pt doped SWNT mat increased. This is due to the formation of C-H bonds, which breaks the symmetry of the CNT electronic structure resulting in formation of localized defects, thereby increasing the resistivity. Initial studies of the temporal dependence of hydrogen uptake suggest a diffusion-limited process. XPS was employed to measure the extent of sp3 C-H bonding.
Chemical Vapour Deposition Growth Of Carbon Nanotube Forests Kinetics Morphology Composition And Their Mechanisms
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Author : Phillip A. Vinten
language : en
Publisher:
Release Date : 2013
Chemical Vapour Deposition Growth Of Carbon Nanotube Forests Kinetics Morphology Composition And Their Mechanisms written by Phillip A. Vinten and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013 with University of Ottawa theses categories.
This thesis analyzes the chemical vapour deposition (CVD) growth of vertically aligned carbon nanotube (CNT) forests in order to understand how CNT forests grow, why they stop growing, and how to control the properties of the synthesized CNTs. In situ kinetics data of the growth of CNT forests are gathered by in situ optical microscopy. The overall morphology of the forests and the characteristics of the individual CNTs in the forests are investigated using scanning electron microscopy and Raman spectroscopy. The in situ data show that forest growth and termination are activated processes (with activation energies on the order of 1 eV), suggesting a possible chemical origin. The activation energy changes at a critical temperature for ethanol CVD (approximately 870°C). These activation energies and critical temperature are also seen in the temperature dependence of several important characteristics of the CNTs, including the defect density as determined by Raman spectroscopy. This observation is seen across several CVD processes and suggests a mechanism of defect healing. The CNT diameter also depends on the growth temperature. In this thesis, a thermodynamic model is proposed. This model predicts a temperature and pressure dependence of the CNT diameter from the thermodynamics of the synthesis reaction and the effect of strain on the enthalpy of formation of CNTs. The forest morphology suggests significant interaction between the constituent CNTs. These interactions may play a role in termination. The morphology, in particular a microscale rippling feature that is capable of diffracting light, suggest a non-uniform growth rate across the forest. A gas phase diffusion model predicts a non-uniform distribution of the source gas. This gas phase diffusion is suggested as a possible explanation for the non-uniform growth rate. The gas phase diffusion is important because growth by acetylene CVD is found to be very efficient (approximately 30% of the acetylene is converted to CNTs). It is seen that multiple mechanisms are active during CNT growth. The results of this thesis provide insight into both the basic understanding of the microscopic processes involved in CVD growth and how to control the properties of the synthesized CNTs.
In Situ And Ex Situ Analysis For The Optimization Of Vertically Aligned Carbon Nanotubes Grown By Thermal Catalytic Chemical Vapor Deposition Of Acetylene And Alcohol
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Author : Jeremy Joseph Jackson
language : en
Publisher:
Release Date : 2006
In Situ And Ex Situ Analysis For The Optimization Of Vertically Aligned Carbon Nanotubes Grown By Thermal Catalytic Chemical Vapor Deposition Of Acetylene And Alcohol written by Jeremy Joseph Jackson and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2006 with Chemical vapor deposition categories.
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Author :
language : en
Publisher:
Release Date : 1957
written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1957 with categories.
Molecular Jet Growth Of Carbon Nanotubes And Dense Vertically Aligned Nanotube Arrays
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Author :
language : en
Publisher:
Release Date : 2010
Molecular Jet Growth Of Carbon Nanotubes And Dense Vertically Aligned Nanotube Arrays written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2010 with categories.
A method of growing a carbon nanotube includes the step of impinging a beam of carbon-containing molecules onto a substrate to grow at least one carbon nanotube on the catalyst surface.
Investigation Into The Growth Mechanisms Of Carbon Nanotubes Formed Using Thermal Chemical Vapour Deposition
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Author : William Murray Whyte
language : en
Publisher:
Release Date : 2011
Investigation Into The Growth Mechanisms Of Carbon Nanotubes Formed Using Thermal Chemical Vapour Deposition written by William Murray Whyte and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2011 with Carbon categories.
Growth And Characterization Of Carbon Nanotubes Grown Via Chemical Vapor Deposition
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Author : Christian Peter Deck
language : en
Publisher:
Release Date : 2004
Growth And Characterization Of Carbon Nanotubes Grown Via Chemical Vapor Deposition written by Christian Peter Deck and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2004 with categories.
Controlled Growth Of Carbon Nanotube Architectures And Devices
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Author : Nathan Robert Franklin
language : en
Publisher:
Release Date : 2002
Controlled Growth Of Carbon Nanotube Architectures And Devices written by Nathan Robert Franklin and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2002 with categories.