Junctions are the key component for 3D carbon nanotube (CNT)-graphene seamless hybrid nanostructures. Growth mechanism of junctions of vertical CNTs growing from graphene in the presence of iron catalysts was simulated via quantum mechanical molecular dynamics (QM/MD) methods. CNTs growth from graphene with iron catalysts is based on a ‘‘base-growth’’ mechanism, and the junctions were the mixture of C-C and Fe-C covalent bonds. Pure C-C bonded junctions could be obtained by moving the catalyst during CNT growth or etching and annealing after growth. The growth process of 3D CNT-graphene junctions on copper templates with nanoholes was simulated with molecular dynamic (MD) simulation. There are two mechanisms of junction formation: (i) CNT growth over the holes that are smaller than 3 nm, and (ii) CNT growth inside the holes that are larger than 3 nm. The growth process of multi-layer filleted CNT-graphene junctions on the Al2O3 template was also simulated with MD simulation. A simple analytical model is developed to explain that the fillet takes the particular angle (135°). MD calculations show that 135° filleted junction has the largest fracture strength and thermal conductivity at room temperature compared to junctions with 90°,120°, 150°, and 180° fillets. The tensile strengths of the as-grown C–C junctions, as well as the junctions embedded with metal nanoparticles (catalysts), were determined by a QM/MD method. Metal catalysts remaining in the junctions significantly reduce the fracture strength and fracture energy. Moreover, the thermal conductivities of the junctions were also calculated by MD method. Metal catalysts remaining in the junctions considerably lower the thermal conductivity of the 3D junctions.

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 1976 / Bibliography: p. 361-366. / Sc. D. / Sc. D. Massachusetts Institute of Technology, Department of Chemical Engineering

[pt] A síntese de Fischer-Tropsch converte o gás de síntese (H2 + CO), em uma variedade complexa de hidrocarbonetos na presença de um catalisador (principalmente Co/Al2O3). Neste trabalho foram estudados catalisadores de Co e o Fe (1 e 5% em massa), incorporados aos suportes: zeólitas KL, HL 0,1M e HL 1,0 M, além da peneira molecular mesoporosa MCM- 41, pelo método de impregnação úmida incipiente, para a reação de Fischer- Tropsch. As amostras preparadas foram analisadas pelas técnicas de: Espectometria de Emissão Atômica de Plasma Acoplado Induzido, Adsorção Física de N2 pelo método BET, Difração de Raios-X, Redução com Temperatura Programada, Microscopia Eletrônica de Transmissão (MET), Quimissorção de Hidrogênio e Espectroscopia no Infravermelho de piridina adsorvida. Além disso, as amostras foram avaliadas em um reator de leito fixo na reação de Fischer-Tropsch. Para as amostras de ferro, com mesmo teor e suportes diferentes, pode-se observar que a amostra suportada na MCM-41 apresentou um grau de redução menor. Entre as amostras de ferro suportadas na KL, a 5% Fe/KL apresentou maior grau de redução e foi observado por microscopia eletrônica de transmissão (MET) que as partículas de ferro apresentaram diâmetro em torno de 6 nm. Para as amostras de cobalto foi observado que a temperatura de redução da amostra suportada na MCM-41 foi mais alta. A amostra 5% Co/KL apresentou um maior grau de redução. Foi possível observar por MET que as partículas de cobalto apresentaram diâmetro variando entre 8 e 20 nm. Verificou-se que o catalisador com maior teor de ferro proporcionou uma maior conversão de CO, tendo sido a distribuição de produtos deslocada para as frações mais leves. Comparando os catalisadores de ferro suportados na zeólita KL e na MCM-41 pode-se concluir que as conversões são da mesma ordem de grandeza. Foi observado que o ferro foi mais ativo que o cobalto em termos de conversão do CO, sendo que o cobalto promoveu a formação de uma maior quantidade de produtos na faixa de diesel, assim como uma menor quantidade de leves. / [en] The Fischer- Tropsch synthesis converts the synthesis gas (H2 + CO), in a complex variety of hydrocarbons, using a catalyst (Co/Al2O3 normally) were introduced to the used supports by the wetness incipient impregnation. The samples were analyzed by several techniques such as: plasma- emission spectrometry (ICP- EAS), N2 physical adsorption by BET method, X-ray diffraction (XRD), temperature programmed reduction (TPR), transmission electronic microscopy (TEM), hydrogen chemisorption and Infrared Spectroscopy of adsorbed pyridine. The catalysts were evaluated using a fixed bed reactor in the Fischer-Tropsch synthesis. For the iron samples, with the same metal content and different supports, it was observed that the MCM-41 sample presented the lowest reduction level. Among the iron samples supported in KL zeolite, the 5% Fe/KL sample presented the largest reduction level. It was observed by transmission electronic microscopy that the iron particles diameter measured around 6 nm. For the cobalt samples, it was observed that the reduction temperature of the MCM-41 supported was the highest one. The 5% Co/KL sample presented the largest reduction level. It was observed by TEM that the cobalt particles presented diameters in the range from 8 to 20 nm. It was verified that the catalyst with the largest iron percentage promoved the highest CO conversion. The products distribuition was shifted to light fractions. It was observed similar conversions to iron catalysts supported in the KL zeolite and in the MCM- 41 mesoporous molecular sieve. The iron catalysts were more active than the cobalt ones in the CO conversion, but tha cobalt catalysts promoted a higher content of diesel fraction and lesser light fractions.

Thesis (Ph. D.)--Ohio State University, 2009. / Title from first page of PDF file. Includes bibliographical references (p. 316-323).