Οι νανοσωλήνες άνθρακα (CNTs) εξακολουθούν να προσελκύουν το ενδιαφέρον πολλών ερευνητικών ομάδων λόγω των ασυνήθιστων μηχανικών, δομικών και ηλεκτρονικών τους ιδιοτήτων, που τους καθιστούν σοβαρούς υποψήφιους για μια πληθώρα εφαρμογών. Συνεπώς, έχουν αναφερθεί διάφορες τεχνικές, που αποσκοπούν στη μαζική παραγωγή νανοσωλήνων άνθρακα. Ο βασικός στόχος της παρούσας μελέτης ήταν να εξερευνηθεί η επίδραση καταλυτικών και λειτουργικών παραμέτρων στο ρυθμό της ανάπτυξης και την ποιότητα των παραγόμενων νανοσωλήνων άνθρακα μέσω της διεργασίας της χημικής εναπόθεσης ατμών (CVD) αιθυλενίου. Τα πειράματα εναπόθεσης διεξήχθησαν σε θερμοβαρυμετρικό αντιδραστήρα, που επιτρέπει τη συνεχή καταγραφή της μεταβολής του βάρους του δείγματος με τον χρόνο. Τα προϊόντα της αντίδρασης αποτελούνταν κυρίως από πολυφλοιικούς νανοσωλήνες άνθρακα (MWCNTs) και χαρακτηρίστηκαν με ηλεκτρονική μικροσκοπία σάρωσης (SEM) και Raman φασματοσκοπία. Υιοθετήθηκαν επίσης η ηλεκτρονική μικροσκοπία διέλευσης (ΤΕΜ) και η θερμοβαρυμετρική ανάλυση (ΤGA). Μια σειρά από μονομεταλλικούς καταλύτες Fe2O3/Al2O3, παρασκευασμένοι με διαφορετικές μεθόδους, εξετάστηκαν υπό συνθήκες ανάπτυξης CNTs. Η ελεγχόμενη εκρηκτική καύση (CEB) των πρόδρομων ενώσεων βρέθηκε ότι είναι η πιο αποδοτική μέθοδος για την παρασκευή του μονομεταλλικού καταλύτη σε σχέση με το ρυθμό ανάπτυξης και την απόδοση σε CNTs. Το αποτέλεσμα αυτό αποδόθηκε στην παρουσία σωματιδίων αιματίτη μικρής διαμέτρου στον καταλύτη. Η παρουσία του υδρογόνου στην αέρια τροφοδοσία της αντίδρασης επίσης αποδείχθηκε ότι προάγει το ρυθμό παραγωγής των MWCNTs. Η απόδοση και η ποιότητα των ΜWCNTs εξαρτώνται από τη συγκέντρωση της πηγής του άνθρακα (αιθυλένιο) στο ρεύμα της τροφοδοσίας καθώς και από τη θερμοκρασία της αντίδρασης. Υπό τις παρούσες πειραματικές συνθήκες, η βέλτιστη θερμοκρασία αντίδρασης βρέθηκε ότι είναι οι 650 0C. Αποδείχθηκε επίσης ότι η CVD του αιθυλενίου παράγει περισσότερους νανοσωλήνες άνθρακα από τη CVD του ακετυλενίου, σε όμοιες πειραματικές συνθήκες. Πολυφλοιικοί νανοσωλήνες άνθρακα αναπτύχθηκαν επίσης σε μια σειρά από καταλύτες Χ% κβ Fe2O3/Al2O3 με CVD αιθυλενίου στους 650 0C. Οι παραπάνω καταλύτες με συγκέντρωση οξειδίου σιδήρου που κυμαινόταν από 0 ως 100 %, παρασκευάστηκαν με ελεγχόμενη εκρηκτική καύση των πρόδρομων νιτρικών ενώσεων τους. Τα αποτελέσματα έδειξαν ότι η απόδοση και η ποιότητα των παραγόμενων νανοσωλήνων άνθρακα εξαρτώνται σε μεγάλο βαθμό από τη συγκέντρωση του Fe2O3. H βέλτιστη φόρτιση του οξειδίου του σιδήρου βρέθηκε ότι είναι η 75% κβ, η οποία οδήγησε σε δομή καταλύτη που απέφερε την μεγαλύτερη απόδοση (~2000 %), δηλαδή το βάρος των εναποτιθέμενων MWCNTs ήταν 20 φορές μεγαλύτερο από το βάρος του αρχικού καταλύτη. Το αποτέλεσμα αυτό αποδόθηκε στη δημιουργία μεγάλου αριθμού ενεργών κέντρων και στη μεγάλη διασπορά της φάσης του Fe2O3. Μια σειρά από διμεταλλικούς καταλύτες Μ-Fe2O3/Al2O3 (Μ: Ru, Ni, Co, Mo) παρασκευάστηκαν με διαφορετικούς μεθόδους και εξετάστηκαν υπό συνθήκες CVD αιθυλενίου. Η ελεγχόμενη εκρηκτική καύση των πρόδρομων ενώσεων τους βρέθηκε ότι είναι η πιο αποτελεσματική μέθοδος για την παρασκευή του Ni-Fe2O3/Al2O3 και του Co-Fe2O3/Al2O3, ενώ η μέθοδος της συγκαθίζησης για την παρασκευή του Ru-Fe2O3/Al2O3 και του Μο-Fe2O3/Al2O3. Κατά τη μελέτη του διμεταλλικού καταλύτη Ru-Fe2O3/Al2O3, παρασκευασμένου με τη μέθοδο της συγκαθίζησης, βρέθηκε ότι η υψηλή του απόδοση σε MWCNTs οφείλεται στη μεγάλη διασπορά της φάσης του Fe2O3, που επιφέρει η παρουσία του ρουθινίου και στην χαμηλή συγκέντρωση της φάσης του RuO2. H ποσότητα και η ποιότητα των παραγόμενων MWCNTs εξαρτώνται από τη συγκέντρωση του αιθυλενίου στη τροφοδοσία και από τη θερμοκρασία της CVD διεργασίας. Η βέλτιστη συγκέντρωση βρέθηκε ότι είναι η 20% και η βέλτιστη θερμοκρασία οι 650 0C. Ακόμα, μεταξύ διάφορων υποστρωμάτων που εξετάστηκαν, βρέθηκε ότι το πιο αποδοτικό είναι η αλούμινα, η οποία προέρχεται από το ένυδρο νιτρικό αλουμίνιο. Επίσης, εξερευνήθηκε η επίδραση του λόγου Fe/Ni της δομής του διμεταλλικού καταλύτη Ni-Fe2O3/Al2O3 , παρασκευασμένου με τη CEB μέθοδο, στο ρυθμό της ανάπτυξης των MWCNTs μέσω της CVD του αιθυλενίου. Βρέθηκε ότι για φόρτιση σε μέταλλο ίση με 52.5 %, ο βέλτιστος λόγος ήταν Fe/Ni = 6, ο οποίος οδήγησε στην εντυπωσιακή απόδοση του 3600 % σε ΜWCNTs. Αυτό αποδόθηκε στο σχηματισμό χημικής ένωσης μεταξύ των στοιχείων Ni, Fe, Al και O και στο σχηματισμό πολλών νανοκρυσταλλιτών Fe2O3, όπως τεκμηριώθηκε από την ανάλυση TEM. Επίσης, αποδείχθηκε ότι η CVD του αιθυλενίου σε αυτόν τον καταλύτη είναι πιο αποδοτική από τη CVD του ακετυλενίου. Ένας φασματογράφος μάζας χρησιμοποιήθηκε για να καθοριστούν οι χημικές αντιδράσεις που λαμβάνουν χώρα κατά τη CVD του αιθυλενίου στον συγκεκριμένο διμεταλλικό καταλύτη. Βρέθηκε ότι οι ρυθμοί της διάσπασης του αιθυλενίου και της παραγωγής υδρογόνου είναι πολύ υψηλοί στο αρχικό στάδιο της εναπόθεσης, ενώ υδρατμοί παράγονται από την αντίδραση του υδρογόνου με τα μεταλλικά οξείδια. / Since their discovery in 1991, Carbon Nano-Tubes (CNTs) continue to draw significant attention due to their various potential applications, deriving from their extraordinary structural, electronic and mechanical properties. Thus, several methods of production of CNTs have been reported. The main goal of this research was the investigation of the influence of catalytic and operational parameters on the rate of growth and quality of CNTs via the process of Chemical Vapor Deposition (CVD) of ethylene. Deposition experiments were carried out in a thermogravimetric hot-wall reactor , which enables continuous monitoring of the evolution of carbon mass with time. The products of the deposition comprised mainly Multi-Wall Carbon Nano-Tubes (MWCNTs) and they were characterized using Scanning Electron Microscopy and Raman spectroscopy. Transmission Electron Microscopy and Thermo-Gravimetric Analysis were also employed. A series of Fe2O3/Al2O3 catalysts prepared by different methods were investigated under conditions of synthesis of CNTs. Controlled explosive burning (CEB) of precursor compounds was found to be the most effective method of preparation of the catalyst with respect to rate of deposition and yield of CNTs. This result has been attributed to the presence of hematite particles of small diameter on the catalyst. The presence of hydrogen in the gas feed mixture, even at small concentration, proved to be beneficial for the rate of production of MWCNTs. Yield and quality of MWCNTs depend on the concentration of the carbon source (ethylene) in the feed mixture and on temperature of deposition. Under the present experimental conditions, the optimal reaction temperature was proved to be 650 0C. It was also found that ethylene CVD process is more productive than acetylene CVD process, under identical experimental conditions. MWCNTs were also grown on a series of X % wt Fe2O3/Al2O3 catalysts by thermal cracking of ethylene at 650 0C. The above catalysts with composition ranging from 0 to 100 % wt Fe2O3 were prepared by controlled explosive burning (CEB) of their nitrate precursors. Results show that the final yield and quality of the MWCNTs are highly dependent on the iron oxide concentration of the catalyst. The optimal iron oxide loading was found to be 75%wt, which led to a yield of ~2000 % relative to the initial weight of the catalyst. This result was attributed to the formation of large number of active sites for CNTs growth as well as to the high dispersion of the Fe2O3 phase. A series of bimetallic catalysts M-Fe2O3/Al2O3 (M: Ru, Ni, Co, Mo), were also prepared by different methods and investigated under conditions of CVD reaction of ethylene. Controlled explosive burning (CEB) of precursor compounds was found to be the most effective method of preparation of the catalysts Ni-Fe2O3/Al2O3 and Co-Fe2O3/Al2O3 with respect to rate of deposition and yield of CNTs. Co-precipitation of nitrate precursors (CP-W(H)) was the most effective method of preparation of the catalysts Ru-Fe2O3/Al2O3 and Mo-Fe2O3/Al2O3. During the study of the Ru-Fe2O3/Al2O3 catalyst, prepared by CP-W(H) method, its high catalytic activity ( ~2600% MWCNTs yield) was attributed to the high dispersion of Fe2O3 particles, due to the presence of ruthenium, and the low concentration of RuO2 phase. Yield and quality of MWCNTs depend on the concentration of ethylene in the feed mixture and on temperature of CVD reaction. Under the present experimental conditions, the optimal concentration of ethylene and reaction temperature was found to be 20% and 650 0C, respectively. Furthermore, the most effective substrate, among others tested, was proved to be the aloumina deriving from aluminum nitrate. The influence of the ratio Fe/Ni in the structure of Ni-Fe2O3/Al2O3 catalyst, prepared by CEB method, on rate of growth of MWCNTs was investigated. It was found that the optimal value was Fe/Ni = 6 for 52.5 % metal loading, which led to the impressive yield of ~3600% . This was attributed to the generation of a chemical compound comprising Fe, Ni, Al and O, which in turn induced the formation of Fe2O3 nanocrystallites, as established by TEM analysis. Furthermore, it was evidenced that CVD of ethylene is more efficient process than CVD of acetylene. A mass spectrometer was also employed as a way to determine the chemical reactions that take place during carbon deposition in this bimetallic catalyst. It was found that ethylene decomposition and hydrogen production rates are high at the first stages of deposition, while H2O(g) is produced due to the reaction of hydrogen with the metal oxides.

<p>The overall aim of this project is to study the heat transfer inside the reaction chamber of the Hot-Wire Chemical Vapor Deposition (HWCVD) system with a commercial software package FLUENT6.3 / it is one of the most popular Computational Fluid Dynamics solvers for complex flows ranging from incompressible to mildly compressible to even highly compressible flows. The wealth of physical models in FLUENT allows us to accurately predict laminar and turbulent flows, various modes of heat transfer, chemical reactions, multiphase flows and other phenomena with complete mesh flexibility and solution-based mesh adaptation. In this study the 3-D HWCVD geometry was measured and created in GAMBIT which then generates a mesh model of the reaction chamber for the calculation in FLUENT. The gas flow in this study was characterized as the steady and incompressible fluid flow due to the small Mach number and assumptions made to simplify the complexity of the physical geometry. This thesis illustrates the setups and solutions of the 3-D geometry and the chemically reacting laminar and turbulent gas flow, wall surface reaction and heat transfer in the HWCVD deposition chamber.</p>

Vertically aligned Multiwalled Carbon Nanotubes (MWCNTs) were grown in the pores of Anodized Aluminum Oxide (AAO) templates and investigated for resistive sensor applications. High Sensitivity of 23% to low concentration (100 ppm) of ammonia was observed. An equivalent circuit model was developed to understand the current flow path in the resistive sensor. This helped us in achieving high sensitivities through amorphous carbon (a-C) layer thickness tailoring by employing post-growth processing techniques like plasma etching. A simulation model in MATLAB was developed to calculate the device resistance and the change in the sensitivity as a function of device parameters. The steady state response and transient response of the model to the number of ammonia molecules and its adsorption rate were studied. Effects of oxygen plasma, argon plasma and water plasma etch on thinning of the a-C layer were studied. In order to enhance the sensitivity, the top and bottom a-C layers were replaced by a more conductive metal layer. This also helped in understanding the current flow in the device and in the estimation of the resistivity of the a-C layer.

In this thesis a tool to be used in experimental design for batch processes is presented. Specifically, this method is to aid in the development of a process model. Currently, experimental design methods are either empirical in nature which need very little understanding of the underlying phenomena and without the objective of more fundamental understanding of the process. Other methods are model based which assume the model is correct and attempt to better define the model parameters or discriminate between models. This new paradigm for experimental design allows for process optimization and process model development to occur simultaneously. The methodology specifically evaluates multiple models as a check to evaluate whether the models are capturing the trend in the experimental data. A new tool for experimental design developed here is called the grid algorithm which is designed to constrain the experimental region to potential optimal points of the user defined objective function for the process. It accomplishes this by using the confidence interval on the objective function value. The objective function value is calculated using the model prediction of the best performing model among a set of models at the predicted optimal point. This new experimental design methodology is tested first on simulated data. The first simulation fits a model to data generated by the modified Himmelblau function (MHF). The second simulation fits multiple models to data generated to simulate a film growth process. In both simulations the grid algorithm leads to improved prediction at the optimal point and better sampling of the region around the optimal point. This experimental design method was then applied to an actual chemical vapor deposition system. The films were analyzed using atomic force microscopy (AFM) to find the resulting film roughness. The methodology was then applied to design experiments using models to predict roughness. The resulting experiments were designed in a region constrained by the grid algorithm and were close to the predicted optimum of the process. We found that the roughness of a thin film depended on the substrate temperature but also showed a relationship to the nucleation density of the thin film.

The deposition of hydrocarbons on silicon substrates is a topic of wide interest. This is generally related to the technological importance of silicon carbide (SiC) and a growing interest in graphene and graphitic materials. Methods for producing these materials predominantly involve high processing temperatures. In the case of SiC, these high processing temperatures often result in the formation of surface defects, which compromise the electronic properties of the material. In an effort to grow SiC films at low temperatures, a technique known as electron-beam chemical vapor deposition (EBCVD) has been developed. Most electron beam deposition techniques employ a focused beam of high-energy (20-30 keV) electrons to form nanometer-sized solid deposits on a surface. However, in an effort to deposit macroscale films, a broad beam of low-energy electrons was used. In addition to investigating the applications of low-energy electrons in semiconductor film growth, the fundamental chemical and physical processes induced by the bombardment of adsorbate-covered surfaces with low-energy electrons were examined. Specifically, the electron-stimulated desorption of various adsorbate-substrate systems such as acetylene adsorbed on silicon, graphene oxide on silicon, and ultrathin graphite films on silicon carbide have been investigated. The yields of cation and neutral desorbates as a function incident electron energy were measured, appearance thresholds were determined and mechanisms of desorption were proposed.

Understanding the fundamentals of the growth of nanostructures is key to controlling their size and morphology. This thesis investigated the supersaturation effect as well as other effects on the growth of two one-dimensional nanomaterials - Magnesium hydroxide sulfate hydrate(MHSH) nanobelt and ZnO nanorod. It was found that the supersaturation for the growth of 1D nanomaterial had to be controlled at low level and it could be controlled through sparingly soluble carbonate salts. To examine the supersaturation effect on one-dimensional nanostructure growth, this study developed a quantitative kinetic model was developed to describe the one-dimensional nanostructure growth. The qualitative investigation and the quantitative model should provide more insight to the processes and some guide-lines for one-dimensional nanostructure growth in the future. This thesis also examines other factors such as agitation and temperature. Some other process parameters can have a significant effect on the growth process. The Zn(NO3)2 and HMT system was used as a model system to demonstrate that agitation had dramatic effects on the final morphology of the nanostructure. Spherical morphology could be obtained through mixing the reaction solution. The building blocks of the nanospheres were similar to those forming the nanorods. Thus, the spherical morphology was believed to result from the breakdown of the free diffusion of building blocks to the growing nanocrystal surface.

This report details work toward the fabrication of a single-electron transistor created from a single-walled carbon nanotube (SWNT). Specifically discussed is a method for growing carbon nanotubes (CNTs) via carbon vapor deposition (CVD). The growth is catalyzed by a solution of 0.02g Fe(NO3)3·9H2O, 0.005g MoO2(acac)2, and 0.015g of alumina particles in 15mL methanol. SWNT diameter ranges from 0.6 to 3.0 nm. Also discussed is a method to control nanotube growth location by patterning samples with small islands of catalyst. A novel “maskless” photolithographic process is used to focus light from a lightweight commercial digital projector through a microscope. Catalyst islands created by this method are approximately 400 μm2 in area.

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Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2015-06-09T18:48:47Z No. of bitstreams: 0 / Made available in DSpace on 2015-06-09T18:48:47Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Orientador: Lourdes Aparecida Martins dos Santos-Pinto / Banca: Evaldo José Corat / Banca: Celso Luiz de Angelis Porto / Resumo: A tecnologia CVD (Chemical Vapor Deposition) aplicada à confecção de pontas odontológicas diamantadas possibilitou aderência do diamante à haste metálica suficiente para suportar o efeito do ultra-som, surgindo então uma técnica alternativa para corte de tecido dentário. O objetivo deste estudo in vitro foi avaliar a efetividade dos corte realizados com as pontas CVD cilíndrica e esférica para ultra-som (pontas CVDentUSÒ), em esmalte e dentina de dentes permanentes, variando-se o sentido de movimento (frontal e lateral). Foram utilizados 40 terceiros molares, num total de 80 preparos cavitários confeccionados por meio de um dispositivo padronizador. Os cortes foram realizados com 30 movimentos consecutivos da ponta sobre a superfície dentária, correspondendo a 27 segundos de utilização das pontas CVDentUSÒ. A profundidade e largura dos preparos cavitários foram analisadas em microscopia eletrônica de varredura e medidas por meio de uma análise cefalométrica modificada no programa Radiocef 4.0 (Radiocef Memory Ltda). A análise estatística pelo teste não paramétrico de Kruskal-Wallis ao nível de significância de 5% mostrou que as maiores médias de largura e profundidade foram observadas nos preparos de dentina, sendo a ponta esférica responsável por cavidades mais profundas e a cilíndrica por cavidades mais largas, não havendo influência dos sentidos de movimento da ponta. / Abstract: The CVD (Chemical Vapor Deposition) technology applied in dental diamond burs manufacture allowed to the diamond to have enough adherence to the metallic stem supporting the ultrasound effect. This technique is bringing back an alternative method to cut dental tissue. The aim of this in vitro study was to evaluate the cutting effectiveness of cylindrical and round CVD-coated diamond burs for ultrasound (CVDentUSÒ system), in enamel and dentine of permanent teeth, changing the movement sense (frontal and lateral). Forty third molars were used, in a total of 80 cavity preparations made using a standard device. The cuts were made with 30 consecutive movements of the bur on the dental surface, corresponding to 27 seconds of CVDentUSÒ tips use. The depth and width of the cavity preparations were analyzed by scanning electronic microscopy and the measures were made using a modified cefalometric analysis in a Radiocef 4.0 program (Radiocef Memory Ltda). The statistical analysis applying Kruskal-Wallis non-parametric test, at 5% significance level, showed that the largest width and depth averages were observed in dentine cavities, being the round bur responsible for deeper cavities and the cylindrical for wider cavities. The movement senses had no influence on cavity size. / Mestre