This paper presents the culmination of an investigation on carbon nanotubes as catalysts for the hydrogenation of carbon monoxide. Carbon nanotubes (CNTs) have been found to have extraordinary physical properties and the potential for use in a variety of applications. They have been utilized as catalyst supports in many reactions, including the conversion of syngas to ethanol. The specific role played by CNTs in these reactions, aside from that of a support structure, has not been evaluated, however. Presented here are parametric studies on Fischer-Tropsch Synthesis with carbon nanotubes as active catalysts. The use of as-produced CNTs (containing trace amounts of iron from the synthesis process) resulted in a 100-fold increase in carbon monoxide conversion per unit mass of catalyst over a traditional Fe-Zn-K/γ-alumina catalyst. This value (CO conversion per unit mass of catalyst) was raised to nearly 1500 times as high as for Fe-Zn-K/γ-alumina when purified CNTs were used in the same FT synthesis. Because iron is a primary catalyst in the FT synthesis, it can be argued that the iron in the CNTs was responsible for the catalytic behavior. However, the iron content in the MWNTs (0.014 g, ≈ 5 mass%) and SWNTs (0.04 g, ≈ 27 mass%) compared to that of the traditional iron-loaded alumina support (2.5 g, ≈ 12.5 mass%), strongly suggests that iron alone cannot be responsible for the catalysis. Although single-walled nanotube (SWNT) catalysts provided high CO conversion, methane represented the bulk of the products. Conversely, multi-walled nanotubes (MWNTs) produced mostly liquid hydrocarbons and oxygenates, indicating that the CNT structure is an important factor in the hydrogenation of CO. The parametric experiments show that temperature, pressure and the syngas composition all play key roles in the distribution of liquid products. In general, an increase in temperature correlated to an increase in hydrocarbon chain length and the formation of more alcohols; above a certain temperature, the distribution shifted to 100% alcohols. Likewise, lower pressures resulted in hydrocarbons of shorter carbon chain length and at higher pressures, more alcohols were formed. Studies were also conducted on the effect of syngas composition and the effect of different types of CNTs. Syngas with 1:1 ratio (H2:CO) produced longer hydrocarbon chains than those produced by 3:1 syngas. The type of CNTs used in FT also affected the products but no clear relationships could be discerned.

Solid lubricants possessing low friction coefficients, low wear rates, and long wear lives are vital for significantly increasing the life span of instruments undergoing extreme frictional wear due to harsh environments. Solid lubricants currently used in high temperatures or excess humidity, such as MoS2, WS2, graphite, or noble metals, are unable to sustain superior frictional qualities over extended amounts of time or in changing environments. To ameliorate these limiting properties, a composite solid lubricant is produced to enable the favorable frictional properties of one lubricant to overcome the lacking frictional properties of the other. This composite uses the combined materials to produce a solid lubricant that can sustain a low friction coefficient and wear rate for a longer amount of time than each individual material. MoS2 electrodeposited on to carbon nanotubes (CNTs) has a lower friction coefficient in humid (~0.16), non-humid (~0.05) and non-humid/humid cycled (~0.075-0.2) environments than either bare MoS2 or bare CNTs. Similarly, silver deposited on CNTs, by electrodeposition, electroless deposition, and sputter coating, performs better in room temperature, high temperature (500°C), and room temperature/high temperature cycling environments than either of its individual materials. Using the techniques used to produce these solid lubricant composite coatings with appealing frictional properties will provide a variety of tribological applications involving high temperature and/or high humidity environments with necessary solutions and further facilitate the improvement of solid lubricants used in other extreme environments.

Orientador: Carlos Alberto Luengo / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-04T03:18:18Z (GMT). No. of bitstreams: 1 Maestro_LuisFernando_M.pdf: 1958027 bytes, checksum: cb56dc89c2faea48a95f3fa1350d5666 (MD5) Previous issue date: 2005 / Resumo: Desde a sua descoberta em 1991, os Nanotubos de Carbono (NTC) têm atraído muito a atenção da comunidade científica, devido as suas propriedades. Neste trabalho é apresentada uma breve revisão das pesquisas em NTC e algumas definições básicas relevantes para a sua estrutura e propriedades. Em vista da utilização deste material em uma futura aplicação e devido ao interesse do grupo na área de energia, é apresentado o estado da arte do armazenamento de Hidrogênio e, em particular, no armazenamento em sólidos de grande área superficial, classe a qual os NTC pertencem. Apresentam-se as modificações realizadas em um Reator de Arco Elétrico (Forar II) para se realizar a Síntese de NTC, são relatadas as experiências e a caracterização das amostras obtidas utilizando-se Microscopia Eletrônica de Varredura e Espectroscopia Raman. / Abstract: Since their discovery in 1991 Carbon Nanotubes (CNT) have received increasing attention by the scientific community due to their properties. Here is presented a brief review of ongoing CNT research, and basic definitions useful to understand their structure and significant properties. Because of future applications in the energy area, are presented developments in Hydrogen storage, more specifically its adsorption in solids with large internal surface areas, a characteristic of CNT materials. Modifications of the existing FORAR II to obtain CNT by the electric arc method are presented, and a description of the routines employed to obtain CNT. The characterization of catalysts and CNT by Scanning Electron Microscopy and Raman Spectroscopy are presented and discussed. / Mestrado / Física da Matéria Condensada / Mestre em Física

Understanding cell behavior at the material-host tissue interface is a fundamental prerequisite for designing the next generation of biomaterials capable of directing cellular events towards a desired biological outcome (e.g. faster tissue integration). In addition, unraveling the relationship between cell activity and nanoscale surface features will further the present knowledge of the fundamental cellular mechanisms that control how cells sense and respond to natural (e.g. extracellular matrix) and synthetic (e.g. biomaterials) surfaces. It is now well-known that the nanoscale physicochemical features of surfaces dictate cell fate by affecting phenomena such as proliferation, differentiation, genetic transcription and protein translation. In particular, nanotopographical features play a pivotal role during cell-surface interactions by exerting a direct mechanotransductive effect on cells, which, in turn, dictate biochemical signaling. In this context, several studies have addressed different aspects of the relationships between nanofeatures and specific cellular functions, including morphological changes, the establishment of focal adhesions (FAs, clusters of adhesion molecules that regulate cell structure and activity, determining how cells sense and respond to natural and synthetic substrates) and differentiation. However, the precise interplay between the morphological characteristics of nontopographical features not only on the surface but also along a third dimension (height) and cellular response still needs to be fully elucidated. Once revealed, such knowledge will shed new light on how cells sense and respond to 2- and 3-dimensional nanoscale patterns. In this context, anodization, a simple yet effective electrochemical treatment, allows to engender on titanium, the gold standard in medicine, arrays of nanotubes with tailor-made diameters. Notably, although nanotubular surfaces on anodized titanium have been extensively studied in relation to their effect on cell response, none of the previous studies has precisely assessed the effects of the morphological features and geometrical arrangement of the nanotubes. This is an important aspect, since the morphological characteristics and the spatial placement of nanofeatures has been shown to control cell response. In addition, by employing the same technique (i.e. anodization), a 3-dimensional hierarchical surface that mimics the frustule (i.e. silicified cell wall) of diatoms (a type of microalgae) can be created. Aside from enabling, for the first time, cellular studies on such bioinspired surface, this hierarchical nanoscale substrate will also allow to probe the effects of a 2-tier nanotopographical gradient along the depth of the nanotubular layer.

No description available.

A computational investigation of some optoelectronic applications of carbon nanotubes (CNT) is presented, including CNT-based solar cells and biosensors. The results could be used to evaluate the performance of CNT devices and clarify the necessity of further experimental research in this area. A coaxially-gated CNT field-effect transistor (CNFET) forms the basic structure of the devices modeled in this thesis. Diffusive transport is present in long-channel devices, as in our case, while the quantum mechanical effects are mainly present in the form of tunneling from Schottky-barrier contacts at the metal-CNT interfaces. Band-to-band recombination of electron-hole pairs (EHP) is assumed to be the source of electroluminescence. In a first-order approximation, protein-CNT interactions are modeled as the modification of the potential profile along the longitudinal axis of CNTs due to electrostatic coupling between partial charges, in the oxide layer of the CNFET, and the nanotube. The possibility of electronic detection is evaluated. The electroluminescence of the CNT is proposed as an optical detection scheme due to its sensitivity to the magnitude and the polarity of the charge in the oxide. The validity of the model is argued for the given models. A value for the minimum required size of a computational window in a detailed simulation is derived. The structure of an electrostatically gated p-i-n diode is simulated and investigated for photovoltaic purposes. The absorbed power from the incident light and the interaction between the nanotubes is modeled with COMSOL. The results are interpreted as a generation term and introduced to the Drift-Diffusion Equation (DDE). We have observed behavior similar to that in an experimentally-realized device. The performance of CNT-based solar cells under standard AM 1.5 sunlight conditions is evaluated in the form of an individual solar cell and also in an array of such devices. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Hydroxyapatite (HA) is a biologically active ceramic that is used in surgery to replace bone. While HA promotes bone growth along its surface, its mechanical properties are not sufficient for major load-bearing medical devices. Carbon nanotubes (CNTs), as one of the strongest and stiffest materials available, have the potential to strengthen and toughen HA, thus expanding the range of clinical uses for the material. Furthermore, studies have suggested that the nanotubes themselves possess some bioactive properties. This work sought to develop and characterise HA-CNT composites in four main areas: 1) production and characterisation of green materials, 2) investigation of appropriate sintering atmospheres, 3) evaluation of mechanical properties, and 4) assessment of biological response to in vitro cell culture. HA was synthesised by a precipitation reaction between Ca(OH)2 and H3PO4, and multi-walled CNTs were produced by chemical vapour deposition. Composites were produced by adding the CNTs to the Ca(OH)2 solution as the HA was precipitating. Both as-made (nfCNTs) and acid-treated CNTs (fCNTs) were used to make composites with loadings of 0.5-5 wt.% CNTs. The resulting slurry was shear mixed and then processed to make a powder. The powder was then uniaxially pressed into tablets of ~45% theoretical density. Characterisation of the green material with XRD and FTIR found that the primary phase was HA which was well hydroxylated. The powder particles were found to have a bimodal size distribution, and all materials had similar surface areas, as determined by BET. Composites made with fCNTs were found to have a better dispersion of CNTs in the HA matrix and better interaction between the HA and CNTs compared with nfCNT composites. CNTs oxidise at the high temperatures needed to sinter HA, yet water is necessary to prevent dehydroxylation and decomposition of the HA. Using 5 wt.% fCNT composite, fourteen sintering atmospheres were investigated to determine their effect on phase purity, hydroxylation, sintered density, and remaining CNT content after sintering. An atmosphere of CO + H2 bubbled through ice water resulted in optimal properties. Additionally, it was found that increasing the gas flow rate and the number of samples sintered in one batch increased CNT retention. However, this came at the expense of the density of the sintered samples, as composites with a higher CNT content were more porous. To optimise the composite microstructure for mechanical studies, six different sintering time/temperature profiles were examined to determine their effect on density (balanced with CNT retention) and grain size. HA and both nfCNT and fCNT composites with CNT loadings of 0.5, 1, 2 and 5 wt.% were produced using the optimised atmosphere and profile, and then tested to determine tensile strength (using diametral compression) and hardness, and to look for evidence of toughening. It was found that CNTs had little reinforcing effect; instead, mechanical behaviour results were mainly attributed to differences in porosity, due at least in part to the CNTs' presence. The in vitro cellular response to the materials was examined by culturing human osteoblast-like cells on HA and nfCNT (0.88 wt.%) and fCNT (3.3 wt.%) composites for 12 days. Cells were found to attach and grow well on HA and the nfCNT composite, with slightly enhanced response on the composite. The fCNT composite, on the other hand, showed a decrease in cell viability between days 1 and 12. These results were mainly attributed to the effects of a lower local pH due to remnant acid on the fCNTs and differences in material characteristics, such as CNT loading and surface roughness. This systematic study of the production and properties of HA-CNT composites has resulted in improved understanding of the production and processing of these materials and the effects of a wide range of sintering atmospheres on their characteristics. Additionally, it has yielded interesting preliminary results of their mechanical reinforcement potential and biological behaviour.

Composites are increasingly used in aerospace applications where performance is the foremost priority of industry. Research on carbon nanotube (CNT)-reinforced polymers conducted in the past decade showed promising results for the improvement of mechanical, thermal and electrical properties of composites. This thesis was undertaken in the context of a larger project, the main goal of which is to develop a complete solution for the manufacturing of carbon fibre-epoxy composites using CNT-reinforced epoxies, referred to as multi-scale composites. This thesis focuses on the thermal aspect of this project under three topics: 1) thermal conductivity of dry carbon fibre fabrics for understanding heat diffusion in composites and similar fabric materials 2) thermal conductivity of CNTreinforced polymers and composites for determining the effect of parameters including CNT addition, and 3) modelling of heat transfer during composite manufacturing for ensuring that their temperature distribution remains controlled. In-plane k rip and through-thickness k rtt thermal conductivity data were measured for two dry carbon fibre fabrics as a function of fibre volume fraction Vf . Results showed that k rip varies linearly with Vf whilst k rtt varies in an exponential recovery trend with Vf . An existing analytical model was used successfully for predicting k rip and simulations developed for predicting k rtt values demonstrated that k rtt depends on the evolution of heat conduction paths in the through-thickness direction as a result of improvements in the fibre contact network. A procedure was developed for manufacturing composites using the RFI process. Thirty-two composites and multi-scale composite plates were manufactured and characterised for investigating the effects of eleven material and manufacturing parameters on fibre volume fraction, porosity, k rip and k rtt . Results showed that the effect of using multi-walled CNT-reinforced epoxy on thermal conductivity of composites is negligible at 0.3% CNT loading. However, this reduced the porosity of the composites significantly. Results also showed that using fabrics with higher surface densities led to a slight increase in k cip . A heat transfer model coupled with cure kinetics was developed for predicting temperature profiles of the laminate during RFI manufacturing. The model was validated experimentally and eleven simulation cases were run for investigating the effects of five material and manufacturing parameters on temperature profiles in the laminate. Results showed that the epoxy resins used in this project combined with the cure cycle recommended by the manufacturer are well-suited for manufacturing laminates with a typical thickness of approximately 5 mm as well as thick laminates of 15 mm to 20 mm.

Carbon nanotubes have great strength, high conductivities, and very large aspect ratios. Their physical, mechanical, and electrical properties are unique and ideally suited for use in structural materials, nano-electronic devices, and as a conductive filler. The homogeneous incorporation of carbon nanotubes in bulk materials such as polymers is difficult to achieve. This is further complicated by the inhomogeneity of carbon nanotube samples. The desire to incorporate carbon nanotubes in a wide variety of devices has been the impetus for carbon nanotube chemistry over the last decade. This requires techniques for dispersal and processing, as well as methodologies for producing monodisperse samples. In Chapter 1, this thesis discusses the fundamental properties of carbon nanotubes and gives a brief overview of the state-of-the-art in carbon nanotube separation, dispersion, and the incorporation of carbon nanotubes in bulk polymers. Chapters 2, 3, and 4 outline our efforts in the area of bulk polymer suspensions; in polystyrene (Ch. 2) and in crosslinked polydimethylsiloxane elastomers (Ch. 3 and 4). Chapter 2 describes our efforts to gain an understanding of the factors limiting the graft density of polymers on the surface of carbon nanotubes, and our insights on the ability of polymer grafts to compatibilize carbon nanotubes with a host polymer. Chapter 3 discusses the application of the Piers-Rubinsztajn reaction as a method of functionalizing the surface of nanotubes with silanes, and crosslinking them within silicone rubbers. Chapter 4 outlines the development of a supramolecular strategy for the dispersion of carbon nanotubes within silicone elastomers using conjugated and di-block co-polymers. Lastly, Chapters 5 through 6 explore the initial stages of development of a “universal” polymer for the dispersion of carbon nanotubes based on highly reactive cyclooctyne monomer units (Ch. 5) and the precursor chemistry required to make this unit on sufficient scale (Ch. 6). / Thesis / Doctor of Philosophy (PhD)

Various techniques to synthesize nanowires and nanotubes as a function of growth temperature and time were investigated. These include growth of nanowires by a chemical vapor deposition (CVD) system using vapor-liquid-solid (VLS) growth mechanism and electro-chemical synthesis of nanowires and nanotubes. Narrow band gap InSb Eg = 0.17 eV at room temp) nanowires were successively synthesized. Using a phase diagram, the transition of the nanowire from metallic- semiconducting- semi-metallic phase was investigated. A thermodynamic model is developed to show that the occurrence of native defects in InSb nanowires influenced by the nanowire growth kinetics and thermodynamics of defect formation. Wide band gap ZnO (Eg = 3.34 eV) and In2O3 (3.7 eV) were also synthesized. ZnO nanowires and nanotubes were successfully doped with a transition metal Fe, making it a Dilute Magnetic Semiconductor of great technological relevance. Structural and electronic characterizations of nanowires were studied for different semiconducting, metallic and semi-metallic nanowires. Electron transport measurements were used to estimate intrinsic material parameters like carrier concentration and mobility. An efficient gas sensing device using a single In2O3 nanowire was studied and which showed sensitivity to reducing gas like NH3 and oxidizing gas like O2 gas at room temperature. The efficiency of the gas sensing device was found to be sensitive to the nature of contacts as well as the presence of surface states on the nanowire.