For the commercial success of polymer - multiwalled carbon nanotube (MWNT) composites the production of these materials on industrial scale by melt processing is of significant importance. The complete dispersion of primary MWNT agglomerates in a polymer melt is difficult to achieve, making it an important and challenging technological problem. Hence, it is necessary to understand the process of MWNT agglomerate dispersion in a polymer melt. Based on an intensive literature research on mechanisms and influencing factors on dispersion of other agglomerated nanostructured fillers (e.g. carbon black), the main dispersion steps were evaluated and investigated concerning the agglomerated MWNT.Consequently, systematic investigations were performed to study the effect of the melt infiltration on MWNT agglomerate dispersion and to analyse the corresponding main dispersion mechanisms, namely rupture and erosion. The states of MWNT agglomerate dispersion were assessed by quantifying the agglomerate area ratio and particle size distribution using image analysis of optical transmission micrographs. Additionally, the composite’s electrical resistivity was determined. In the prevailing study, polycarbonates (PC) varying in molecular weight were used to produce composites containing 1 wt% MWNT (Baytubes C150HP) as model systems and a discontinuous microcompounder was applied as melt mixing device. The agglomerate structure of the used MWNT material made them especially suitable for the reported investigations. The step of melt infiltration into the primary nanotube agglomerates plays a crucial role for their dispersion in the PC melt. During melt mixing when low shear rates were applied, better state of MWNT dispersion was obtained in high viscosity matrices because applied shear stresses were high. On the contrary, if high shear rates were applied, similar states of MWNT dispersion were obtained in low and high viscosity matrices although significantly lower shear stresses were applied in the low viscosity matrix as compared to the high viscosity matrix. The results indicate that if the applied shear stress values are compared, with increasing matrix viscosity the agglomerate dispersion gets worsen. This is attributed to the fact that low viscosity matrices can infiltrate relatively faster than high viscosity matrices into the agglomerate making them weaker and reducing the agglomerate strength. Thus, at sufficient shear rates MWNT agglomerates disperse relatively faster in low viscosity matrix. This illustrates a balance between the counteracting effects of viscosity on agglomerate infiltration and agglomerate dispersion. Additionally, the effect of matrix molecular weight on the size of un-dispersed MWNT agglomerates was investigated. Under similar conditions of applied shear stress, the composites based on low molecular weight matrix showed smaller sized un-dispersed primary agglomerates as compared to composites with higher molecular weight matrices. This again highlights the role of matrix infiltration as the first step of dispersion. Following the step of melt infiltration, agglomerate size gets reduced due to the dispersion mechanisms. To analyse the corresponding contributions of different dispersion mechanisms (rupture and erosion), the kinetics of MWNT agglomerate dispersion was investigated. If high mixing speeds are employed dispersion is quite fast and needs less time as compared to low mixing speed. A model is proposed to estimate the fractions of rupture and erosion mechanisms during agglomerate dispersion based on the kinetic study in the discontinuous mixer. Under the employed experimental conditions, at high mixing speeds, the dispersion was found to be governed by rupture dominant mechanism, whereas at low mixing speeds the dispersion was controlled by both mechanisms. As far as electrical resistivity is concerned, for a given content of MWNT as the state of dispersion improves, the resistivity values decrease significantly but only up to a plateau value. The composites produced using low viscosity matrices have lower resistivity values as compared to high viscosity matrices. Additionally, composites were prepared using additives, whereas the additives were found to be useful for improving filler dispersion and electrical conductivity.

Ο κύριος σκοπός της διπλωματικής εργασίας αυτής είναι η εξοικείωση με τα γραφιτικά υλικά και τις μηχανικές τους ιδιότητες. Στο πρώτο μέρος της εργασίας, γίνεται μια εισαγωγή στους νανοσωλήνες άνθρακα και διεξάγεται μια βιβλιογραφική ανασκόπηση στο θέμα, με έμφαση στη μικροσκοπία Laser Raman ως μη-καταστρεπτική τεχνική μέτρησης μηχανικών ιδιοτήτων. Στο δεύτερο τμήμα, μελετώνται οι μηχανικές τους ιδιότητες με πειράματα εφελκυσμού σε πρωτότυπα ινίδια νανοσωλήνων, σε συσκευή δυναμικής μηχανικής ανάλυσης (DMA) και επίσης διεξάγονται μετρήσεις σε άλλα υλικά, όπως buckypapers και ίνες γραφίτη αλλά και αραμιδικές ίνες Kevlar®. Δίνεται ιδιαίτερη έμφαση στη διαδικασία παραγωγής ινιδίων νανοσωλήνων άνθρακα μονού τοιχίου με διηλεκτροφόρηση, την οποία ελπίζουμε να αναπαράγουμε στο μέλλον. / The main purpose of this thesis is to get acquainted with graphitic materials and their mechanical properties. The first part of the thesis is an introduction in carbon nanotubes and emphasis is put on Laser Raman spectroscopy as a non-destructive method to measure mechanical properties. In the second part, the mechanical properties of prototype carbon nanotube fibrils are measured in a dynamic mechanical analysis (DMA) machine and also measurements are carried out on other graphitic materials like buckypapers, graphitic fibres and aramidic polymer (Kevlar®) fibres. The production process of these carbon nanotubes fibrils via dielectrophoresis, which we hope to reproduce in the future, is specially stressed.

abstract: Nanofluidic devices in which one single-walled carbon nanotube (SWCNT) spans a barrier between two fluid reservoirs were constructed, enabling direct electrical measurement of the transport of ions and molecules. Ion current through these devices is about 2 orders of magnitude larger than that predicted from the bulk resistivity of the electrolyte. Electroosmosis drives excess current, carried by cations, and is found to be the origin of giant ionic current through SWCNT as shown by building an ionic field-effect transistor with a gate electrode embedded in the fluid barrier. Wetting of inside of the semi-conducting SWCNT by water showed the change of its electronic property, turning the electronic SWCNT field-effect transistor to "on" state. These findings provide a new method to investigate and control the ion and molecule behavior at nanoscale. / Dissertation/Thesis / Ph.D. Physics 2011

Abstract While numerous publications deal with the properties and applications of porous silicon (PS), some of the related topics are not complete or could be investigated from different aspects. Therefore, the main objective of this thesis is to provide novel information associated with the optical and chemical properties of PS. For the investigations, various PS samples are manufactured by electrochemical dark etching of boron-doped p+-type Si wafers. Amongst others, (i) the wavelength-dependent refractive indices of freestanding PS monolayers having different porosities were obtained from optical transmission and reflection spectra in the 700–1700 nm wavelength range, and compared to those calculated from Bruggeman's effective medium approximation (EMA). The refractive indices of the PS samples are shown to be described well with the EMA. In addition, optical scattering at the air-PS interface was demonstrated. (ii) Multilayer stacks are created by alternating the porosities of PS layers within the same sample to form Bragg filters. The Bragg conditions of the filters are calculated and compared to optical transmission measurements. (iii) The oxidation of PS membranes in dry air is investigated with emphases on the reaction kinetics and on the structural changes of the porous matter. As revealed, oxidation proceeds faster in PS than in Si bulk. The formed SiO2 is amorphous and causes stress in the lattice of the residual Si skeleton. (iv) The effect of oxidation extent of PS layers on the growth mechanism of multi-walled carbon nanotubes (CNTs) is investigated. The density of the CNT network is found proportional to the oxidation extent of the substrates. (v) Finally, the chemically-reductive nature of PS is studied and exploited via the immersion plating method to deposit palladium and silver nanoparticles in the nanopores and on the surface of PS samples. The presented novel results have potential in silicon-based technologies, including integrated active and passive optical components (waveguides, filters, antireflection coatings, optical gas/liquid sensors), electronic devices (electrochemical gas/liquid sensors, diodes, field effect devices) and selective chemical catalysis (substrates, growth templates).

No description available.

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.

Synthesis and functionalization of large-area graphene and its structural, electrical and electrochemical properties has been investigated. First, the graphene films, grown by thermal chemical vapor deposition (CVD), contain three to five atomic layers of graphene, as confirmed by Raman spectroscopy and high-resolution transmission electron microscopy. Furthermore, the graphene film is treated with CF4 reactive-ion plasma to dope fluorine ions into graphene lattice as confirmed by X-ray photoelectron spectroscopy (XPS) and UV-photoemission spectroscopy (UPS). Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction enhanced with increasing plasma treatment time, which is attributed to increase in catalytic sites of graphene for charge transfer. The fluorinated graphene is characterized as a counter-electrode (CE) in a dye-sensitized solar cell (DSSC) which shows ~ 2.56% photon to electron conversion efficiency with ~11 mAcm−2 current density. Second, the large scale graphene film is covalently functionalized with HNO3 for high efficiency electro-catalytic electrode for DSSC. The XPS and UPS confirm the covalent attachment of C-OH, C(O)OH and NO3- moieties with carbon atoms through sp2-sp3 hybridization and Fermi level shift of graphene occurs under different doping concentrations, respectively. Finally, CoS-implanted graphene (G-CoS) film was prepared using CVD followed by SILAR method. The G-CoS electro-catalytic electrodes are characterized in a DSSC CE and is found to be highly electro-catalytic towards iodine reduction with low charge transfer resistance (Rct ~5.05 Wcm2) and high exchange current density (J0~2.50 mAcm-2). The improved performance compared to the pristine graphene is attributed to the increased number of active catalytic sites of G-CoS and highly conducting path of graphene. We also studied the synthesis and characterization of graphene-carbon nanotube (CNT) hybrid film consisting of graphene supported by vertical CNTs on a Si substrate. The hybrid film is inverted and transferred to flexible substrates for its application in flexible electronics, demonstrating a distinguishable variation of electrical conductivity for both tension and compression. Furthermore, both turn-on field and total emission current was found to depend strongly on the bending radius of the film and were found to vary in ranges of 0.8 – 3.1 V/μm and 4.2 – 0.4 mA, respectively.

Fabrication of carbon infiltrated carbon nanotubes (CI-CNT) can result in large mechanical property variation, and methods to characterize properties usually involve destructive testing. Finding a non-destructive way to test for stiffness of this material reduces the number of parts that have to be made. It also simplifies testing of complex parts. The stiffness of CI-CNT beams is related to the type of carbon material infiltrated between the carbon nanotubes (CNTs), how it interacts with the CNTs, and how much of it there is. The amount of material can be estimated using the density of the beam, and both the type of material and its interaction with the carbon nanotubes can be approximated through analysis of the Raman spectra taken at the surface. A combination of these two observations can be related to the effective material stiffness. The relationship can be fitted with a power function, with a variance of 1.41 GPa, which is about 11% of the maximum stiffness of the samples tested. This variance is similar to the larger variations in CI-CNT beam stiffnesses found in a single batch of beams.

This diploma thesis deals with monitoring the properties of concrete with carbon nanoparticles. The theoretical part describes properties of nanoparticles, their dosing and dispersion. In the practical part of the diploma thesis the physico-mechanical properties of the concrete with the addition of carbon nanotubes were determined after 7 and 28 days of aging. The frost resistance test was then determined after 100 cycles. All results were compared with the reference samples.

The interlaminar region is a contributing factor to the limited electrical conductivity of carbon fiber/epoxy composites. Consisting of electrically insulating epoxy matrix between conductive layers of carbon fiber, the interlaminar region prevents electrical interaction between the carbon fiber layers and electrical conduction in the through thickness direction.The interlaminar region in thin [0,0] carbon fiber/epoxy composites has been reinforced by carbon nanotubes (CNT) by two methods. First by aligned CNT forests from N12 Technologies and secondly by self-produced Buckypapers, porous CNT films, of different areal densitites. Two batches of laminates modified by aligned CNTs, having different curing conditions, and laminates modified with Buckypapers were manufactured. The laminates were evaluated by their electrical conductivity and electromagnetic interference shielding efficiency (EMI SE). The addition of external pressure to the laminates during curing brought an increase in longitudinal conductivity, a consequence of higher fiber packing. Also, both reinforcement methods increased the longitudinal conductivity through improved electrical interaction between the carbon fiber layers. However, only the Buckypaper reinforcement augmented the transversal conductivity significantly, acting as a highly conductive route in the interlaminar region. Both batches of aligned CNT modified laminates exhibited equal EMI SE, questioning the influence of the conductivity of the laminate on its EMI SE. Also, the increase in EMI SE brought by the aligned CNT forests were negligible compared to the reference. However, the reinforcement by Buckypapers proved successful, reaching -45/-50 dB at 1000 MHz, improving from 30 dB of the unmodified reference at the same frequency.