Doctor of Philosophy / Department of Chemical Engineering / Placidus B. Amama / The continued depletion of fossil fuels and concomitant increase in greenhouse gases have encouraged worldwide research on alternative processes to produce clean fuel. Fischer-Tropsch synthesis (FTS) is a heterogeneous catalytic reaction that converts syngas (CO and H₂) to liquid hydrocarbons. FTS is a well-established route for producing clean liquid fuels. However, the broad product distribution and limited catalytic activity are restricting the development of FTS. The strong interactions between the active metal catalyst (Fe or Co) and support (Al₂O₃, SiO₂ and TiO₂) during post-synthesis treatments of the catalyst (such as calcination at ~500°C and reduction ~550°C) lead to formation of inactive and unreducible inert material like Fe₂SiO₄, CoAl₂O₄, Co₂SiO₄. The activity of FTS catalyst is negatively impacted by the presence of these inactive compounds. In our study, we demonstrate the use of a modified photo-Fenton process for the preparation of carbon nanotube (CNT)-supported Co and Fe catalysts that are characterized by small and well-dispersed catalyst particles on CNTs that require no further treatments. The process is facile, highly scalable, and involves the use of green catalyst precursors and an oxidant. The reaction kinetic results show high CO conversion (85%), selectivity for liquid hydrocarbons and stability. Further, a gaseous product mixture from FTS (C1-C4) was utilized as an efficient feedstock for the growth of high-quality, well-aligned single-wall carbon nanotube (SWCNT) carpets of millimeter-scale heights on Fe and (sub) millimeter-scale heights on Co catalysts via chemical vapor deposition (CVD). Although SWCNT carpets were grown over a wide temperature range (between 650 and 850°C), growth conducted at optimal temperatures for Co (850°C) and Fe (750°C) yielded predominantly SWCNTs that are straight, clean, and with sidewalls that are largely free of amorphous carbon. Also, low-temperature CVD growth of CNT carpets from Fe and Fe–Cu catalysts using a gaseous product mixture from FTS as a superior carbon feedstock is demonstrated. The efficiency of the growth process is evidenced by the highly dense, vertically aligned CNT structures from both Fe and Fe–Cu catalysts even at temperatures as low as 400°C–a record low growth temperature for CNT carpets obtained via conventional thermal CVD. The use of FTS-GP facilitates low-temperature growth of CNT carpets on traditional (alumina film) and nontraditional substrates (aluminum foil) and has the potential of enhancing CNT quality, catalyst lifetime, and scalability. We demonstrate growth of SWCNT carpets with diameter distributions that are smaller than SWCNTs in conventional carpets using a CVD process that utilizes the product gaseous mixture from Fischer-Tropsch synthesis (FTS-GP). The high-resolution transmission electron microscopic (HR-TEM) and Raman spectroscopic results reveal that the use of a high melting point metal as a catalyst promoter in combination with either Co (1.5 nm ± 0.7) at 850ºC or Fe (1.9 nm ± 0.8) at 750ºC yields smaller-diameter SWCNT arrays with narrow diameter distributions. Scalable synthesis of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and onion like carbon (OLC) in a batch reactor using supercritical fluids as a reaction media is demonstrated. The process utilizes toluene, ethanol, or butanol as a carbon precursor in combination with ferrocene that serves as a catalyst precursor and a secondary carbon source. The use of supercritical fluids for growth does not only provide a route for selective growth of a variety of carbon nanomaterials, but also provides a unique one-step approach that is free of aggressive acid treatment for synthesis of CNT-supported metallic nanoparticle composites for catalysis and energy storage applications.

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).

The search and research for high performance materials for aerospace applications is a continuous evolving process. Among several fibre reinforced polymers, carbon fibre reinforced polymer (CFRP) is well known for its high specific stiffness and strength. Though high modulus and high strength carbon fibre with structural resin systems have currently been established reasonably well and are catering to a wide variety of aerospace structural applications, these properties are generally directional with very high properties along the fibre direction dominated by fibres and low in other directions depending mainly on the resin properties. Thus, there is a need to enhance the mechanical properties of the resin systems for better load transfer and to improve the resin dominated properties like shear strength and properties in directions other than along the fibre. Use of carbon nanotubes (CNTs) with their extraordinary specific stiffness and strength apparently has great potential as an additional reinforcement in resin for development of CNT-CFRP nanocomposites. However, there are several issues that need to be addressed such as compatibility of a particular resin with CNTs, amount of CNTs that can be added, uniform dispersion of these nanotubes, surface treatment and curing process etc., for optimal enhancement of the required properties. Epoxy and cyanate ester resin systems are finding applications in aerospace structures owing to their desirable set of properties. Of these, bisphenol E cyanate ester (BECy) resin of low viscosity with its low moisture absorption, better dimensional stability, and superior mechanical properties can establish itself as potential structural resin system for these applications. BECy in particular has the advantage of being more suitable for out of autoclave manufacturing process such as Vacuum Assisted Resin Transfer Molding (VARTM). Literature shows that, significant work has been carried out by various researchers reporting improvements using CNTs in epoxy resins along with various associated problems. However, studies on effects of addition of CNTs /fCNTs to BECy-CFRP composite system are not well reported. Thus, objective of this work is to study the effects of adding pristine and functionalized CNTs to low viscosity cyanate ester as well as epoxy resin systems. Further, to study the effects on mechanical properties of nanocomposites with carbon fibre reinforcement in these CNT dispersed resin system through a combination of experimental and computational approaches. Multiwall carbon nanotubes (CNTs) without and with different chemical functionalization are chosen to be added to epoxy and BECy resins. The quantity of these CNTs /fCNTs is varied in steps up to 1% by weight. Different methods of mixing such as shear mixing, ultrasonication and combined mixing cycles are implemented to achieve uniform dispersion of these nanotubes in the resin system. Standard test samples are prepared from these mixtures of nanotubes in resin systems to study the variation in mechanical properties. Further, these nanotubes added resin systems are used in fabricating CFRP laminates by VARTM process. Both uni-directional and bi-directional laminates are made with the above modified resin systems with CNTs/fCNTs. Series of experimental investigations are carried out to study various aspects involved in making of nanocomposites and the effects of the same on different mechanical properties of the nanocomposites. Standard specimens are cut out from these laminates to evaluate them for tension, compression, flexure, shear and interlaminar shear strength. The main parameters investigated are the effects of varied quantity of CNTs and functionalized CNTs in the resin mix and in CFRP nanocomposites, effect of different mixing / curing cycles etc. on the mechanical properties of the nanocomposites. The investigations have yielded very interesting and encouraging results to arrive at optimum quantity of CNTs to be added and also the effects of functionalization to achieve enhanced mechanical properties. In addition, correlation of mechanical property enhancements with failure mechanisms, dispersion behaviour and participation of CNTs / fCNTs in load transfer are explained with the aid of scanning electron microscope images. Computational studies are carried out through atomistic models using computational tools to estimate the mechanical properties, understand and validate the effects of various parameters studied through series of experimental investigations. An atomistic model is built taking into consideration the nanoscale effects of the single wall carbon nanotubes (SWCNTs) and its reinforcement in the BECy resin. Using these atomistic models, mechanical properties of individual SWCNT, BECy polymer resin, polymer with different quantities of added SWCNT, and the CFRP laminates with improved resin are computed. As the interaction of CNT with the polymer is only at the outermost layer and the mechanical properties of either MWCNTs or SWCNTs are too high compared to resin systems, it is not expected to have any difference in the final outcome whether it is MWCNT or SWCNT. Hence, only SWCNTs are considered in computational studies as it helps in reducing the complexity of atomistic models and computational time when coupled with polymer resin. This is valid even for functionalized CNT as functionalization is also a surface phenomenon. To start with, the mechanical behaviour of SWCNT is studied using molecular mechanics approach. Molecular mechanics based finite element analysis is adopted to evaluate the mechanical properties of armchair, zigzag and chiral SWCNT of different diameters. Three different types of atomic bonds, i.e., carbon-carbon covalent bond and two types of carbon-carbon van der Waals bonds are considered in the carbon nanotube system. The stiffness values of these bonds are calculated using the molecular potentials, namely Morse potential function and Lennard-Jones interaction potential function respectively and these stiffness values are assigned to spring elements in the finite element model of the SWCNT. The importance of inclusion of Lennard-Jones interactions is highlighted in this study. Effect of these non-bonded interactions is studied by making the numerical stiffness of these bonds to negligible levels and found that they significantly reduce the mechanical properties. The effect of non-bonded Lennard-Jones atomic interactions (van der Waal interactions) considered here is a novelty in this work which has not been considered in previous research works. The finite element model of the SWCNT is constructed, appropriate boundary conditions are applied and the behaviour of mechanical properties of SWCNT is studied. It is found that the longitudinal tensile strength and maximum tensile strain of armchair SWCNTs is greater than that of zigzag and chiral SWCNTs and its value increases with increasing SWCNT diameter. The estimated values of the mechanical properties obtained agree well with the published literature data determined using other techniques. As the systems become more complicated with the inclusion of polymers, molecular dynamics (MD) method using well established codes is more adoptable to study the effect of SWCNTs on BECy. Hence, it is used to model and solve the nanosystems to generate their stress-strain behavior. Further, MD approach followed here can effectively include interfacial interaction between polymer and the CNTs as well. Mechanical properties of SWCNT functionalized SWCNT (fSWCNT), pure BECy resin and that of the CNT nanocomposite consisting of specific quantity of SWCNT / fSWCNT in BECy are estimated using MD method. Atomistic models of SWCNT, fSWCNT, BECy, BECy with specific quantities of CNT / fSWCNT are constructed. A monomer of BECy is modelled and stabilized before its usage as a building block for modelling of BECy resin and to compute its properties. A cell of specific size containing monomers of BECy and another cell of same size with SWCNT at centre surrounded by BECy monomer molecules are built. The appropriate quantity of SWCNT in resin is modelled. This model captures the required density of the composite resin. The models so constructed are subjected to geometric optimization satisfying the convergence criteria and equilibrated through molecular dynamics to obtain a stable structure. The minimized structure is subjected to small strain in different directions to calculate the Young’s modulus and other moduli of the CNT-BECy resin composite. The process is repeated for different quantities of SWCNT in BECy resin to obtain their moduli. Further, tensile and shear strengths of CNT-BECy are obtained by subjecting the equilibrated structure to a series of applied strains from 0 to 10% in steps of 1%. The stress values corresponding to each strain are obtained and a stress – strain curve is plotted. From the stress- strain curve, the strengths of the CNT -BECy which is the stress corresponding to the modulus after which the material starts to soften are determined. Effects of functionalization on mechanical properties of SWCNT are observed. Further, effects of functionalization of SWCNT are studied with a specific quantity of fSWCNT on different moduli and strengths of BECy are investigated. The properties of enhanced CNT–BECy nanocomposite resin with different quantities of added CNT obtained through MD are used to estimate the mechanical properties of the CNT-BECy-CFRP nanocomposite using micromechanics model. Further, validation with experimental results is attempted comparing the trends in enhancement of properties of the CNT-BECy resin and CNT-BECy-CFRP nanocomposite system. The outcome of this research work has been significantly positive in terms of i) Development of an appropriate process establishing different parameters for dispersing CNTs in the resin system, mixing, curing cycle for making of nanocomposites demonstrating significant and consistent enhancement of mechanical properties of BECy based resin system and CFRP nanocomposites using optimum quantity of CNTs /fCNTs through a series of well planned and executed experimental investigations. Evaluation of mechanical properties for each of the cases has been carried out experimentally. ii) Establishing a computational methodology involving intricate atomistic modelling and molecular dynamics of nanosystems for estimation of mechanical properties of BECy polymer resin and to study the effects by addition of SWCNT / functionalized SWCNT on the properties. Results obtained through series of experimental investigations have been validated through this computational study. This could be an important step towards realising the potential of this resin system for high performance aerospace applications. Thus, in brief, detailed experimental work combined with computational studies performed as presented in this thesis resulted in achieving structurally efficient cyanate ester based nanocomposites which is unique and not reported in open literature.