Made available in DSpace on 2016-12-08T17:19:34Z (GMT). No. of bitstreams: 1 Pre-textuais.pdf: 74879 bytes, checksum: 6785c5543c0fdea74eb42ad4be2cded6 (MD5) Previous issue date: 2009-12-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Multi-wall carbon nanotubes (MWNT) functionalized with poly(ethylene glycol) (PEG) were used to prepare epoxy matrix nanocomposites based on Bisphenol A Diglycidyl Ether (DGEBA), in order to evaluate the effect of functionalization in nanotubes/matrix dispersion and adhesion. Firstly, two works were carried out to estimate the reaction between DGEBA and PEG using sulfuric acid (H2SO4) or dimethylbenzylamine (DMBA), as catalysts, aiming the evaluation of viable routes to reaction of PEG chains connected to nanotubes with epoxy resin. The reaction product was analyzed with infrared spectroscopy (FTIR), proton magnetic resonance spectroscopy (RMN) and viscosity. The results confirmed the reaction occurs between DGEBA epoxy groups and PEG hydroxyl groups in the presence of dimethylbenzylamine as catalyst at 100°C. DSC analyses and tensile tests of cured hardener systems based on polyamine show that the DGEBA reaction with PEG leads to a decrease of Tg, originating a more flexible material. Epoxy matrix nanocomposites were prepared using 0,1% m/m of NCPM functionalized with PEG and 0,5% m/m of dimethylbenzylamine. The samples were characterized with infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA). It was observed through SEM images a better adhesion of NCPM-PEG with matrix, in comparison with pristine MWNT, suggesting that NCPM-PEG reacted with the epoxy resin DGEBA. DSC analysis showed a Tg value decrease to prepared nanocomposites when compared to pure epoxy resin. / Nanotubos de carbono de paredes múltiplas (NCPM) funcionalizados com poli(etilenoglicol) (PEG), foram usados na preparação de nanocompósitos de matriz epóxi a base de diglicidil éter de bisfenol A (DGEBA), para avaliar o efeito da funcionalização na dispersão e adesão do nanotubo na matriz. Primeiramente dois estudos foram realizados para avaliar a reação entre DGEBA e PEG, utilizando ácido sulfúrico (H2SO4) ou dimetilbenzilamina (DMBA) como catalisadores, com o objetivo de se avaliar uma rota viável para a reação das cadeias de PEG ligadas aos nanotubos com a resina epóxi. O produto da reação foi avaliado por espectroscopia no infravermelho (FTIR), espectroscopia de ressonância magnética nuclear (RMN) de 1H e viscosidade. Os resultados das análises confirmaram a ocorrência da reação entre grupos epóxi do DGEBA e grupos hidroxila do PEG na presença de dimetilbenzilamina como catalisador, a 100°C. Análises de DSC e ensaios de tração dos sistemas curados com endurecedor a base de poliamina mostram que a reação de DGEBA com PEG leva a uma diminuição da Tg, gerando um material mais flexível. Nanocompósitos de matriz epóxi foram preparados usando 0,1% m/m de NCPM funcionalizados com PEG e catalisador dimetilbenzilamina na quantidade de 0,5% m/m. As amostras obtidas foram caracterizadas por Espectroscopia na região do Infravermelho (FTIR), Calorimetria Exploratória Diferencial (DSC), Análise termogravimétrica (TG), Microscopia Eletrônica de Varredura (MEV) e Análise dinâmico-mecânica (DMA). Através das imagens obtidas por MEV, sugere-se uma maior adesão dos NCPM-PEG com a matriz, indicando que os NCPM-PEG reagiram com a resina epóxi à base de DGEBA. Análises de DSC mostraram uma diminuição do valor da Tg para os nanocompósitos preparados quando comparados a resina epóxi pura.

Magister Scientiae - MSc / The spectroscopic, SEM and Raman measurements on carbon nanotubes under the exact conditions of which OES analysis were made showed that at a pressure of 400 Torr and a flow rate of 200 sccm, the quality and quantity of single-walled carbon nanotubes was the highest. / South Africa

Este estudo teve como objetivo principal desenvolver uma técnica para síntese de um nanocompósito de Y-TZP/MWCNT-COOH (Zircônia estabilizada por 3 mol% de ítria reforçada por nanotubos de carbono funcionalizado em -COOH) com propriedades mecânicas e ópticas que permitam a sua futura utilização como infraestrutura de próteses fixas dentárias e pilares protéticos para implantes. Assim, foram avaliados a microestrutura, resistência à flexão, tenacidade à fratura, limite de fadiga e propriedades ópticas do nanocompósito e comparada àquelas medidas para Y-TZP convencional (controle). O material Y-TZP/MWCNT-COOH foi desenvolvido pelo processo de co-precipitação de hidróxidos mistos associado ao tratamento hidrotérmico/solvotérmico e prensagem uniaxial em formato de blocos para sistemas CAD/CAM. O pó de MWCNT-COOH foi caracterizado por meio de MEV-FEG, TEM, TGA, DRX e FRX previamente a sua utilização para desenvolvimento do nanocompósito. Espécimes foram obtidos a partir do material Y-TZP/MWCNT-COOH para caracterização por meio de DRX, MEV-FEG e TEM, e comparação de suas propriedades estruturais (densidade e contração), ópticas, resistência à flexão, tenacidade à fratura e limite de fadiga com a Y-TZP convencional. O MWCNT-COOH apresentou-se em feixes de nanotubos de carbono recobertos por sílica tendo comprimento médio de 5,10 ± 1,34 ?m, com 90% dos comprimentos medidos (D90) estando abaixo de 6,9 ?m. Foi verificado a não possibilidade da utilização de líquidos orgânicos em nenhum passo da fabricação dos compósito Y-TZP/MWCNT-COOH por levar ao escurecimento do compósito, inviabilizando sua futura aplicação clínica. O tratamento hidrotérmico sem uso de líquidos orgânicos mostrou-se eficaz em proporcionar o revestimento do nanotubo de carbono por partículas de óxido de zircônio e ítrio. Entretanto, ocasionou a formação de aglomerados e partículas de Y-TZP com tamanho maiores que 5 ?m. Uma densidade relativa de 97,4% foi alcançada para o compósito experimental de Y-TZP contendo MWCNT-COOH, tendo uma razão de contraste de 0.9929 ± 0.0012 e um valor de diferença de cor da Y-TZP convencional de 6,1 ± 3,1 ( ?E). As propriedades mecânicas da Y-TZP/MWCNT-COOH, dureza Vickers (10,14 ± 1,27 GPa; p=0,25) e tenacidade à fratura (4,98 ± 0,30 MPa.m1/2; p=0,39), não apresentaram diferença significativa da Y-TZP convencional (dureza: 8,87 ± 0,89; tenacidade à fratura: 4,98 ± 0,30 MPa.m1/2). Entretanto, para a resistência à flexão (p=0,003) e limite de fadiga cíclica (LFC) foram obtidos valores inferiores para o material experimental Y-TZP/MWCNT-COOH (resistência à flexão: 299,4 ± 30,5 MPa; LFC: 179,4 ± 22,5 MPa) quando comparado à Y-TZP controle (resistência à flexão: 623,7 ± 108,8 MPa; LFC: 439,0 ± 56,4 MPa). Com base nos resultados apresentados, é possível concluir que a síntese de um nanocompósito de Y-TZP/MWCNT-COOH com propriedades ópticas adequadas para aplicação na odontologia restauradora foi possível por meio dos métodos descritos, entretanto algumas adequações nos métodos de síntese e processamento para criação do nanocompósito devem ser realizadas para se evitar a acentuada diminuição de importantes propriedades mecânicas do material. / This study aim was to develop a technique for synthetize nanocomposite of Y-TZP/MWCNT-COOH (3 mol% Yttria-Stabilized Tetragonal Zirconia reinforced with COOH functionalized carbon nanotubes) with mechanical and optical properties that allow their future use as fixed dental prosthesis infrastructure and implant abutments. Thus, the following properties of the nanocomposite were investigated and compared to those measured for conventional Y-TZP (control): microstructure, flexural strength, fracture toughness, fatigue limit and optical properties. Y-TZP/MWCNT-COOH material was developed by the co-precipitation of mixed hydroxides associated with the hydrothermal/solvothermal treatment and uniaxial pressing to form blocks for CAD/CAM systems. The MWCNT-COOH powder was characterized by SEM-FEG, TEM, TGA, XRD and XRF prior to its use for the development of nanocomposite. Specimens were obtained from the Y-TZP/MWCNT-COOH material and characterized by XRD, SEM-FEG and TEM. After characterization, the material had their structural properties (density and contraction), optical, flexural strength, fracture toughness and fatigue limit compared to a conventional Y-TZP. The MWCNT-COOH material was observed to be a bundle formation of carbon nanotube covered with silica with an average length of 5.10 ± 1.34 ?m, with 90% of the measured lengths (D90) being below 6.9 ?m. It has been found to be not possible to use organic liquids on any step of the Y-TZP/MWCNT-COOH manufacturing process due to darkening of the composite, making it unfeasible to future clinical application. The hydrothermal treatment without the use of organic liquids was effective in providing the carbon nanotube coating by zirconium and yttrium oxide particles. However, this treatment led to the formation of agglomerates and particles of Y-TZP with larger than 5 ?m. A relative density of 97.4% was achieved for the Y-TZP/MWCNT-COOH composite, having a contrast ratio of 0.9929 ± 0.0012, and a color difference value from the conventional Y-TZP of 6.1 ± 3.1 (?E). The mechanical properties of Y-TZP/MWCNT-COOH, Vickers hardness (10.14 ± 1.27 GPa; p = 0.25) and fracture toughness (4.98 ± 0.30 MPa.m1/2; p = 0.39), showed no significant difference from the conventional Y-TZP (hardness: 8.87 ± 0.89; fracture toughness: 4.98 ± 0.30 MPa.m1/2). However, flexural strength (p = 0.003) and cyclic fatigue limit (CFL) showed lower values for the experimental material Y-TZP/MWCNT-COOH (flexural strength: 299.4 ± 30.5 MPa; CFL: 179.4 ± 22.5 MPa) compared to Y-TZP control (flexural strength: 623.7 ± 108.8 MPa; CFL: 439.0 ± 56.4 MPa). Based on the results presented, it could be conclude that the synthesis of a nanocomposite of Y-TZP/MWCNT-COOH with optical properties suitable for application in restorative dentistry was made possible by the described methods, however some adjustments in synthesis and processing methods for the nanocomposite creation should be taken; to avoid the sharp decrease of important mechanical properties of the material.

Στόχος της διδακτορικής αυτής διατριβής ήταν η ανάπτυξη μιας μεθόδου παραγωγής νανοσωλήνων άνθρακα η οποία στηρίζεται στη χημική απόθεση ατμών, χρησιμοποιώντας ως πηγή του άνθρακα ενώσεις σε υγρή μορφή, όπως οι αλκοόλες. Επιπρόσθετα μελετήθηκαν διάφορες παράμετροι της πειραματικής διαδικασίας (πηγή άνθρακα, θερμοκρασία απόθεσης, είδος και συγκέντρωση μετάλλου και υποστρώματος, παρουσία υδρογόνου κ.ά), τόσο ως προς την επίδρασή τους στο ρυθμό εξέλιξης της διεργασίας, όσο και ως προς το είδος των παραγόμενων προϊόντων, με σκοπό τη στοχευμένη παραγωγή νανοσωλήνων άνθρακα με συγκεκριμένες ιδιότητες. Η κύρια πειραματική διάταξη αποτελούνταν από έναν θερμοβαρομετρικό αντιδραστήρα, ο οποίος επέτρεπε τη συνεχή μέτρηση των μεταβολών του βάρους του δείγματος σε συνάρτηση με το χρόνο, ενώ και φασματογράφος μάζας ήταν συνδεδεμένος στην έξοδο του αντιδραστήρα για να μελετηθεί η αέρια φάση των αντιδράσεων. Τα προϊόντα προκειμένου να πιστοποιηθούν ως προς το είδος των νανοσωλήνων που παρήχθησαν, χαρακτηρίστηκαν με τη βοήθεια ηλεκτρονικής μικροσκοπίας σάρωσης (SEM) και διερχόμενης δέσμης (TEM), καθώς και με φασματοσκοπία Raman και θερμοσταθμική ανάλυση (TGA). Τα αποτελέσματα των πειραμάτων οδήγησαν στο συμπεράσμα πως οι ατμοί της αιθανόλης είναι καλύτερη πηγή άνθρακα σε σύγκριση με της μεθανόλης, οδηγώντας μάλιστα στη παραγωγή μίγματος πολυφλοιϊκών και μονοφλοιϊκών νανοσωλήνων άνθρακα, με καθαρότητες που ξεπερνούσαν το 90%. Επιτακτική αποδείχθηκε η παρουσία του μετάλλου, το οποίο και αποτελεί το κέντρο πυρημοποιήσης για την ανάπτυξη των νανοσωλήνων, ενώ καθοριστική είναι και η χρήση υποστρώματος προκειμένου ο άνθρακας να αποτεθεί με τη μορφή αυτή. Επιπρόσθετα, η παρουσία του υδρογόνου αύξησε σημαντικά το ποσοστό του άνθρακα που αποτέθηκε οδηγώντας μάλιστα στο σχηματισμό μεταλλικών μονοφλοικών νανοσωλήνων άνθρακα πολύ μικρής διαμέτρου, η οποία υπολογίστηκε ίση με 0.45nm. / The main goal of this research was the development of a new method for the production of carbon nanotubes, based on chemical vapor deposition (CVD), which employs a liquid carbon source. In addition, a detailed investigation of the effect of several parameters (carbon source, deposition temperature, kind and metal concentration and support, hydrogen addition e.t.c.) on both the process and the final carbon product was carried out. For this purpose, a CVD experimental apparatus was developed, which uses vapors of liquid precursors and allows the continuously recording of sample weight changes in correlation with time. In some cases, a mass spectrometer was used as a way to determine the kind of processes that take place in the gas phase during carbon deposition. The solid product was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and thermogravimetric analysis (TGA). The experimental results led to the conclusion that ethanol vapors are preferable because they lead to higher yield of both multi-wall and single-wall carbon nanotubes. Also, it was proved that the presence of a metal catalyst and support is necessary, because the first one is the active site of carbon nanotubes formation, and the second leads to the deposition of carbon in this form. Additionally, it was proved that the use of hydrogen in the gas mixture of the process is very important, as a way to reduce catalyst, leading to the formation of metalic single-wall carbon nanotubes of very small diameter (0.45nm).

Este estudo teve como objetivo principal desenvolver uma técnica para síntese de um nanocompósito de Y-TZP/MWCNT-COOH (Zircônia estabilizada por 3 mol% de ítria reforçada por nanotubos de carbono funcionalizado em -COOH) com propriedades mecânicas e ópticas que permitam a sua futura utilização como infraestrutura de próteses fixas dentárias e pilares protéticos para implantes. Assim, foram avaliados a microestrutura, resistência à flexão, tenacidade à fratura, limite de fadiga e propriedades ópticas do nanocompósito e comparada àquelas medidas para Y-TZP convencional (controle). O material Y-TZP/MWCNT-COOH foi desenvolvido pelo processo de co-precipitação de hidróxidos mistos associado ao tratamento hidrotérmico/solvotérmico e prensagem uniaxial em formato de blocos para sistemas CAD/CAM. O pó de MWCNT-COOH foi caracterizado por meio de MEV-FEG, TEM, TGA, DRX e FRX previamente a sua utilização para desenvolvimento do nanocompósito. Espécimes foram obtidos a partir do material Y-TZP/MWCNT-COOH para caracterização por meio de DRX, MEV-FEG e TEM, e comparação de suas propriedades estruturais (densidade e contração), ópticas, resistência à flexão, tenacidade à fratura e limite de fadiga com a Y-TZP convencional. O MWCNT-COOH apresentou-se em feixes de nanotubos de carbono recobertos por sílica tendo comprimento médio de 5,10 ± 1,34 ?m, com 90% dos comprimentos medidos (D90) estando abaixo de 6,9 ?m. Foi verificado a não possibilidade da utilização de líquidos orgânicos em nenhum passo da fabricação dos compósito Y-TZP/MWCNT-COOH por levar ao escurecimento do compósito, inviabilizando sua futura aplicação clínica. O tratamento hidrotérmico sem uso de líquidos orgânicos mostrou-se eficaz em proporcionar o revestimento do nanotubo de carbono por partículas de óxido de zircônio e ítrio. Entretanto, ocasionou a formação de aglomerados e partículas de Y-TZP com tamanho maiores que 5 ?m. Uma densidade relativa de 97,4% foi alcançada para o compósito experimental de Y-TZP contendo MWCNT-COOH, tendo uma razão de contraste de 0.9929 ± 0.0012 e um valor de diferença de cor da Y-TZP convencional de 6,1 ± 3,1 ( ?E). As propriedades mecânicas da Y-TZP/MWCNT-COOH, dureza Vickers (10,14 ± 1,27 GPa; p=0,25) e tenacidade à fratura (4,98 ± 0,30 MPa.m1/2; p=0,39), não apresentaram diferença significativa da Y-TZP convencional (dureza: 8,87 ± 0,89; tenacidade à fratura: 4,98 ± 0,30 MPa.m1/2). Entretanto, para a resistência à flexão (p=0,003) e limite de fadiga cíclica (LFC) foram obtidos valores inferiores para o material experimental Y-TZP/MWCNT-COOH (resistência à flexão: 299,4 ± 30,5 MPa; LFC: 179,4 ± 22,5 MPa) quando comparado à Y-TZP controle (resistência à flexão: 623,7 ± 108,8 MPa; LFC: 439,0 ± 56,4 MPa). Com base nos resultados apresentados, é possível concluir que a síntese de um nanocompósito de Y-TZP/MWCNT-COOH com propriedades ópticas adequadas para aplicação na odontologia restauradora foi possível por meio dos métodos descritos, entretanto algumas adequações nos métodos de síntese e processamento para criação do nanocompósito devem ser realizadas para se evitar a acentuada diminuição de importantes propriedades mecânicas do material. / This study aim was to develop a technique for synthetize nanocomposite of Y-TZP/MWCNT-COOH (3 mol% Yttria-Stabilized Tetragonal Zirconia reinforced with COOH functionalized carbon nanotubes) with mechanical and optical properties that allow their future use as fixed dental prosthesis infrastructure and implant abutments. Thus, the following properties of the nanocomposite were investigated and compared to those measured for conventional Y-TZP (control): microstructure, flexural strength, fracture toughness, fatigue limit and optical properties. Y-TZP/MWCNT-COOH material was developed by the co-precipitation of mixed hydroxides associated with the hydrothermal/solvothermal treatment and uniaxial pressing to form blocks for CAD/CAM systems. The MWCNT-COOH powder was characterized by SEM-FEG, TEM, TGA, XRD and XRF prior to its use for the development of nanocomposite. Specimens were obtained from the Y-TZP/MWCNT-COOH material and characterized by XRD, SEM-FEG and TEM. After characterization, the material had their structural properties (density and contraction), optical, flexural strength, fracture toughness and fatigue limit compared to a conventional Y-TZP. The MWCNT-COOH material was observed to be a bundle formation of carbon nanotube covered with silica with an average length of 5.10 ± 1.34 ?m, with 90% of the measured lengths (D90) being below 6.9 ?m. It has been found to be not possible to use organic liquids on any step of the Y-TZP/MWCNT-COOH manufacturing process due to darkening of the composite, making it unfeasible to future clinical application. The hydrothermal treatment without the use of organic liquids was effective in providing the carbon nanotube coating by zirconium and yttrium oxide particles. However, this treatment led to the formation of agglomerates and particles of Y-TZP with larger than 5 ?m. A relative density of 97.4% was achieved for the Y-TZP/MWCNT-COOH composite, having a contrast ratio of 0.9929 ± 0.0012, and a color difference value from the conventional Y-TZP of 6.1 ± 3.1 (?E). The mechanical properties of Y-TZP/MWCNT-COOH, Vickers hardness (10.14 ± 1.27 GPa; p = 0.25) and fracture toughness (4.98 ± 0.30 MPa.m1/2; p = 0.39), showed no significant difference from the conventional Y-TZP (hardness: 8.87 ± 0.89; fracture toughness: 4.98 ± 0.30 MPa.m1/2). However, flexural strength (p = 0.003) and cyclic fatigue limit (CFL) showed lower values for the experimental material Y-TZP/MWCNT-COOH (flexural strength: 299.4 ± 30.5 MPa; CFL: 179.4 ± 22.5 MPa) compared to Y-TZP control (flexural strength: 623.7 ± 108.8 MPa; CFL: 439.0 ± 56.4 MPa). Based on the results presented, it could be conclude that the synthesis of a nanocomposite of Y-TZP/MWCNT-COOH with optical properties suitable for application in restorative dentistry was made possible by the described methods, however some adjustments in synthesis and processing methods for the nanocomposite creation should be taken; to avoid the sharp decrease of important mechanical properties of the material.

Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.

The Thesis reports charge transport studies on conducting polymers, polymer carbon nanotube composites and organic semiconductor devices. Conducting and semiconducting polymers consisting of π-conjugated chains have attracted considerable attention as they combine the optoelectronic properties of semiconductors with mechanical properties and processing advantages of plastics. The chemical/electrochemical/photodoping of these semiconducting polymers can tune the Fermi levels and conductivity in a controlled way, and hence the properties of devices can be easily tailored to suit in several applications. Carbon nanotube (CNT) is another another novel promising material for electronic/optoelectronic applications. Lately there has been a great interest in developing composites of polymer and CNTs to utilize the advantages of both CNTs and polymers. The inclusion of CNTs in polymers improves the mechanical, electrical and thermal properties since the aspect ratio (ratio of length to diameter) is very large, as well its density is rather low. The Thesis consists of 6 chapters. First chapter is a brief introduction of general and transport properties of conducting polymers and polymer-carbon nanotube composites. In Chapter 2, the sample preparation and experimental techniques used in this work are discussed. The charge transport in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) is presented in Chapter 3. Chapter 4 focuses on the transport measurements in the polymer-CNT composite samples. Chapter 5 elaborates the ac and dc characterization of organic field-effect transistors (OFETs). And chapter 6 presents the conclusion and future directions of the work that has been presented in the Thesis. Chapter 1: In the scientific and technological revolution of the last few years, the study of high performance materials has been steadily increasing including the study of carbon-based materials. Conducting polymers have special properties that are interesting for this new technology. The charge transport in conjugated polymers is important to optimize the performance of devices. The discovery of CNTs with exceptional thermal, mechanical, optical, electrical and structural properties has facilitated the synthesis of new type of nanocomposites with very interesting properties. Nanocomposites represent a guest-host matrix consisting of easily processible functionalized conjugated polymer as host, incorporating CNTs as fillers with versatile electronic and magnetic properties, which provide a wide range of technological applications. To optimize their electrical properties it is essential to understand the charge transport mechanism in detail. Chapter 2: The multi-wall carbon nanotubes (MWNTs) grown by thermal chemical vapor deposition (CVD) are mixed with a 1:1 mixture of 98% H2SO4 and 70% HNO3 to produce sulfonic acid functionalized multi-wall carbon nanotubes (s-MWNTs). The s-MWNTs are dispersed in a solution of Nafion by ultrasonication and then cast on a glass substrate and slowly dried by moderate heating to obtain the composite films. Polyaniline (PANI)-MWNT composites were obtained by carrying out the chemical synthesis of nanofibrilar PANI in the presence of CNTs. This water dispersible PANIMWNT composite contains well segregated MWNTs partially coated by nanofibrilar PANI. The ac and dc charge transport measurements suggest hopping transport in these materials. OFETs are fabricated with pentacene, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)(PBTTT) and poly(3-hexylthiophene) (P3HT) as active materials. A novel technique is used to characterize the acphotoresponse of these OFETs. Chapter 3: Charge transport studies on PEDOT-PSS have been carried out and found that it correlates with the morphology. The dc conductivity of PEDOT–PSS shows enhanced delocalization of the carriers upon the addition of dimethyl sulfoxide (DMSO) and this is attributed to the extended chain conformation. PEDOT-PSS is known to form a phase-segregated material comprising highly conducting PEDOT grains that are surrounded by a sea of weakly ionic-conducting PSS and a wide variation in the charge transport properties of PEDOT-PSS films is attributed to the degree of phasesegregation of the excess insulating polyanion. The magnetotransport and temperature dependent ac transport parameters across different conducting grades of PEDOT-PSS processed with DMSO were compared. Depending on the subtle alterations in morphology, the transport at low temperatures is shown to vary from the hopping regime (Baytron P) to critical regime of the metal-insulator transition (Baytron PH510) There is a significant positive magnetoresistance (MR) for P–films, but this is considerably less in case of PH510-film. From the low temperature ac conductance it is found that the onset frequency for PH510 is nearly temperature independent, whereas in P type it is strongly temperature dependent, again showing the superior transport in PH510. The presence of ‘shorter network connections’ together with a very weak temperature dependence down to ~ 5 K, suggest that the limitation on transport in PH510 arises from the connectivity within the PEDOT-rich grain rather than transport via the PSS barriers. Chapter 4: DC and AC charge transport properties of Nafion s-MWNT and PANI-MWNT composites are studied. Such a detailed investigation is required to optimize the correlation among morphology and transport properties in these composites towards applications in field-effect transistors, antistatic coating, electromagnetic shielding, etc. The conductivity in Nafion s-MWNT shows a percolative transport with percolation threshold pc = 0.42 whereas such a sharp percolation is absent in PANI-MWNT composite since the conduction via PANI matrix smears out the onset of rapid increase in conductivity. Three-dimensional variable range hopping (VRH) transport is observed in Nafion s-MWNT composites. The positive and negative MR data on 10 wt. % sample are analyzed by taking into account forward interference mechanism (negative MR) and wave-function shrinkage (positive MR), and the carrier scattering is observed to be in the weak limit. The electric-field dependence, measured to high fields, follows the predictions of hopping transport in high electric-field regime. The ac conductivity in 1 wt. % sample follows a power law: ( )  A s , and s decreases with increasing temperature as expected in the correlated barrier hopping (CBH) model. In general, Mott’s VRH transport is observed in PANI-MWNT samples. It is found that the MWNTs are sparingly adhered with PANI coatings, and this facilitates inter-tube hopping at low temperatures. The negative MR of MWNT-PANI composites suggest that the electronic transport at low temperatures is dominated by MWNT network. AC impedance measurements at low temperatures with different MWNT loading show that ac conductivity become temperature independent as the MWNT content increases. The onset frequency for the increase in conductivity is observed to be strongly dependent on the MWNT weight percentage, and the ac conductivity can be scaled onto a master curve given by  ( )  0[1 k( 0 )s ]. Chapter 5: Organic field-effect transistors (OFETs) based on small molecules and polymers have attracted considerable attention due to their unique advantages, such as low cost of fabrication, ease of processing and mechanical flexibility. Impedance characterization of these devices can identify the circuit elements present in addition to the source-drain (SD) channel, and the bottlenecks in charge transport can be identified. The charge carrier trapping at various interfaces and in the semiconductor can be estimated from the dc and ac impedance measurements under illumination. The equivalent circuit parameters for a pentacene OFET are determined from low frequency impedance measurements in the dark as well as under light illumination. The charge accumulation at organic semiconductor–metal interface and dielectric semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under SD electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. Similar charge trapping is observed in an OFET with PBTTT as the active material. This novel method can be used to differentiate the photophysical phenomena occurring in the bulk from that at the metal-semiconductor interface for the polymer. Chapter 6: The conclusions from the various works presented in the thesis are coherently summarized in this chapter. Thoughts for future directions are also summed up.

Indiana University-Purdue University Indianapolis (IUPUI) / Carbon nanotubes (CNTs) are one of the most common nanoparticles (NP) found in workplace air. Therefore, there is a strong chance that these NP will enter the human body. They have similar physical properties to asbestos, a known toxic material, yet there is limited evidence showing that CNTs may be hazardous to human barrier epithelia. In previous studies done in our laboratory, the effects of CNTs on the barrier function in the human airway epithelial cell line (Calu-3) were measured. Measurements were done using electrophysiology, a technique which measures both transepithelial electrical resistance (TEER), a measure of monolayer integrity, and short circuit current (SCC) which is a measure of vectorial ion transport across the cell monolayer. The research findings showed that select physiologically relevant concentrations of long single-wall (SW) and multi-wall (MW) CNTs significantly decreased the stimulated SCC of the Calu-3 cells compared to untreated cultures. Calu-3 cells showed decreases in TEER when incubated for 48 hours (h) with concentrations of MWCNT ranging from 4µg/cm2 to 0.4ng/cm2 and SWCNT ranging from 4µg/cm2 to 0.04ng/cm2. The impaired cellular function, despite sustained cell viability, led us to investigate the mechanism by which the CNTs were affecting the cell membrane. We investigated the interaction of short MWCNTs with model lipid membranes using an ion channel amplifier, Planar Bilayer Workstation. Membranes were synthesized using neutral diphytanoylphosphatidylcholine (DPhPC) and negatively charged diphytanoylphosphatidylserine (DPhPS) lipids. Gramicidin A (GA), an ion channel reporter protein, was used to measure changes in ion channel conductance due to CNT exposures. Synthetic membranes exposed to CNTs allowed bursts of currents to cross the membrane when they were added to the membrane buffer system. When added to the membrane in the presence of GA, they distorted channel formation and reduced membrane stability.