This dissertation summarises the study of different aspects of the aerosol-assisted chemical vapour deposition (AACVD) technique for the production of multi-wall carbon nanotubes (MWCNTs). Upscaling the synthesis while retaining the quality of MWCNTs has been a prime objective throughout the work. A key aspect of this work was the study of different growth parameters and their influence on the homogeneity of the products across the reactor. The effect of the precursor composition on the yield and quality of MWCNTs were also investigated. It was shown that the synthesis rate can be significantly (60 – 80 %) increased by tuning the composition of the precursor. Moreover, by optimising the synthesis recipe and using a larger reactor, the synthesis rate and efficiency of the precursor were increased fivefold (up to 14 g/hr) and twice (up to 88 %) respectively. Large area (up to 90 cm²), mm-thick carpets of MWCNTs which were both free-standing and on substrate were produced. The carpets could withstand normal handlings without tearing apart, making them suitable for macroscopic characterisations and applications. By in-situ qualitative and quantitative gas analysis of the atmosphere of the reactor, the thermocatalytic cracking behaviour of 25 precursors was investigated and a mechanism for successive formation of different hydrocarbon fragments inside the reactor was proposed. A number of dedicated gas analysis methods and apparatuses such as a probe for zone-by-zone gas analysis of reactor and a heated chamber for preparation of standard gas analysis samples were developed to explore some of the least investigated aspects of the thermocatalytic cracking of precursors. Mapping the reactor revealed that some single-wall and double-wall carbon nanotubes (SWCNTs and DWCNTs) were also produced near the exhaust of the reactor. The SWCNTs were partly covered by fullerene-like species and resembled different forms of carbon nanobuds. In addition, the effect of the electron beam on the interaction of the SWCNTs and the fullerene-like species was studied in situ using high-resolution transmission electron microscopy (HRTEM).

This thesis presents studies on a novel ‘meta-material’ as a potential candidate to replace the traditional Electro-Optic crystals (GaP, ZnTe) used in ultrashort bunch monitors for electron/positron accelerators. This study is aimed at showing the linear and non-linear optical properties of such materials, and creating a toolbox for both optical characterisation and manipulation of their properties using an ultra-short pulsed laser. The material studied throughout this thesis is a composite of silver nanoparticles (a “nanocomposite”) embedded within soda-lime glass. The Surface Plasmon Resonance (SPR) is a feature of these particles that is responsible for its unique optical properties. It is shown in this work how SPR is utilised for shape modification of silver nanoparticles with the use of a ps- pulsed laser with various laser beam polarisations. The impact of linear polarisation irradiation is investigated. It is found that multipulse irradiation has the effect of elongating nanoparticles to form prolate spheroids, which results in a dichroic effect on the composite as a whole, caused by the anisotropic SPR band shift. It is also shown that changing the laser polarisation from linear to radial and/or azimuthal changes the character of the reshaped nanoparticles. It was observed that a localised change of ellipsoid orientation is achieved, resulting in a non-directionally-dependent SPR band shift. Second Harmonic Generation (SHG) has been observed from reshaped nanoparticles embedded in soda-lime glass. A comparison of the effect was made between ps-pulsed reshaped, fs-pulsed reshaped and mechanically stretched samples containing silver nanoparticles. Multiphoton Absorption Induced Luminescence (MAIL) was observed along with the SHG and characterised for the various laser polarisation components. The dependence of the aforementioned effects on the elongated nanoparticle aspect ratio was shown to have a great impact. A novel method for reshaped nanoparticles characterisation is presented. It is based on the laser-induced SHG and MAIL signal and is proved to give a precise measurement of the nanoparticle shape and orientation. Frequency Resolved Optical Gating (FROG) measurement of a fs-pulse is measured with great accuracy, in the case where the BBO nonlinear crystal is replaced by the reshaped nanoparticle composite. This was demonstrated to be caused by the anisotropic SHG of the ellipsoidal nanoparticles. Preliminary THz based measurements were performed as a part of a feasibility study of the application of these composites in the EO-based detection of ultrashort electron bunches. Future work is suggested in order to achieve more efficient EO detection.

The approach advocated in this work implements energy/exergy analysis and indirectly an irreversibility evaluation to a continuous manufacturing process involving discrete net shape production of compact heat exchangers through a complex controlled atmosphere brazing (CAB) process. The system under consideration involves fifteen cells of a continuous ramp-up heating, melting, reactive flow, isothermal dwell, and rapid quench solidification processing sequence during a controlled atmosphere brazing of aluminum compact heat exchangers. Detailed mass, energy, and exergy balances were performed. The irreversibility sources were identified and the quality of energy utilization at different processing steps determined. It is demonstrated that advanced thermodynamics metrics based on entropy generation may indicate the level of sustainable energy utilization of transient open systems, such as in manufacturing. This indicator may be related to particular property uniformity during materials processing. In such a case, the property uniformity would indicate systems distance from equilibrium, i.e., from the process sustainable energy utilization level. This idea is applied to net shape manufacturing process considered. A metric based on exergy destruction is devised to relate the heat exchanger temperature uniformity and the quality. The idea advocated in this thesis will represent the coherent framework for developing energy efficient, economically affordable and environmentally friendly manufacturing technology.

Exergy utilization calculations have been in the past repeatedly used to quantify the quality and quantity of energy used in thermal energy processes. This thesis is an attempt to derive a common language exergy utilization and compare for the first time two entirely different manufacturing processes, namely material processing by a mechanical method of straining of the material and simple heating of the same mass of the material using exergy utilization as a metric. The exergy utilization of material processing is determined by performed work and utilized heat transfer using 1) Ramberg-Osgood equation and 2) Lumped heat capacitance method. A comparison of these two methods is presented.

Al93Fe3Cr2Nb2 (at.%) nanoquasicrystalline alloys have been shown to have the potential to push the applications of aluminium alloys to more elevated temperatures, by maintaining a high strength. They also have more thermally stable microstructures than previous nanoquasicrystalline alloys from similar systems (the most studied of which is Al93Fe3Cr2Ti2 (at.%)). Al93Fe3Cr2Nb2 (at.%) alloys have never previously been produced in samples on a scale larger than melt-spun ribbon. This study examines the production parameters of bulk nanoquasicrystalline Al-Fe-Cr-Nb alloys. Firstly an attempt was made to reduce the melting temperatures of thermally stable nanoquasicrystalline alloys through additional alloying. The melting processes of binary, ternary, quaternary and quinary nanoquasicrystalline alloys was analysed though DTA, with endothermic reactions up to 1034oC observed. Rapidly solidified Al-Fe-Cr-Nb alloys were then produced in kilogram quantities through gas atomisation at an industrial scale. The smallest atomised powder particles contained fine scale microstructures consisting of nano-scale quasicrystals embedded in an aluminium matrix. As the cooling rate of the powder particles decreased new phases, including the theta phase (Al13(Fe,Cr)2-4) and Al3Nb were produced. 0-25μm, 25-50μm and 50-75μm (diameter) size fractions of atomised powder were each consolidated through extrusion to produce nanoquasicrystalline Al-Fe-Cr-Nb bars. Composite bars of the nanoquasicrystalline alloy mixed with 10(vol.)% and 20(vol.)% pure aluminium were also produced. The consolidation of the nanoquasicrystalline atomised powders through extrusion led to precipitation of intermetallics including (Al13(Fe,Cr)2-4) and Al3Nb, particularly in the smallest powder size fractions with the most metastable microstructures. Finally the effects of the atomisation and extrusion conditions on the microstructure and its mechanical properties were studied. Improved strength, coupled with reduced ductility was observed with decreases in the elemental aluminium composition of the Al-Fe-Cr-Nb bars and the powder size fraction they were produced from. There was however improvements in toughness of the extruded composite bars, over the nanoquasicrystalline alloy bars.

Made available in DSpace on 2014-10-09T12:53:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:47Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:53:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:47Z (GMT). No. of bitstreams: 0 / Um requisito do processo de manufatura é a necessidade de se obter peças processadas, caracterizadas pela boa qualidade de acabamento superficial, baixa rugosidade e a conservação de suas propriedades metalúrgicas. Essas condições motivaram o desenvolvimento deste estudo, no qual selecionou-se o processamento de materiais a laser para o corte do titânio, unindo uma tecnologia a um metal de recente aplicação. Além disso, a versatilidade e as vantagens, como também a tendência global do setor industrial, tornaram-se fatores preponderantes na utilização do laser como ferramenta de usinagem. Neste presente trabalho foram investigados os efeitos da usinagem por laser pulsado de Nd:YAG sobre a qualidade, como também a formação de fases na superfície de corte e analisados pela aplicação do planejamento experimental. Para isso, chapas de titânio comercialmente puro (grau 2) e da liga Ti-6Al-4V (grau 5), com espessuras de 0,5 e 1,0 milímetros, foram empregadas na realização dos ensaios sob ação do laser. As amostras obtidas foram analisadas por microscopia ótica (MO), microscopia eletrônica por varredura (MEV), ensaios de microdureza e inspeção superficial da rugosidade. Capturaram-se digitalmente as imagens do material ressolidificado, aderido na superfície de corte para determinação da formação de rebarbas. Com base nestes dados construíram-se arranjos fatoriais, por meio da metodologia de planejamento experimental (DOE), a fim de avaliar o grau de influência dos parâmetros e suas possíveis interações e assim averiguar sua significância estatística. Verificou-se um endurecimento superficial na região do corte a laser com nitrogênio, em virtude da formação de nitretos (TiN) sob uma fina camada da zona de ressolidificação. Apesar da complexidade das interações entre os diversos parâmetros envolvidos no processamento a laser, os resultados corroboram que a otimização do processo de corte a laser do titânio pode ser factível. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Currently, there is not enough information available on the effect of inclusions on extrusion alloys. Theoretical calculations in the past demonstrated the probable role of oxides in Fe-intermetallic phase selection (Cao et al., 03). However, no concrete evidence can be found in the literature to support this argument. This study investigates the role of in-situ oxides in intermetallic phase selection. Various Mg oxides (spinel and MgO) were formed in-situ by adding different levels of Mg. A special intermetallic extraction process was used for 3D analysis. SEM, EDS and XRD analysis were used for qualitative and quantitative analysis. Dry and wet surfaces of the oxide bi-films were observed with the wet surfaces highly associated with MgO and spinel particles. MgO particles had spherical morphology and there average diameter was observed to be in the range 200nm-400nm, whereas spinel particles had octahedral morphology with average length of the side in the range 1-2μm. MgO was found in locations which appear to be the most probable nucleation points of α-AlFeSi intermetallics and Mg₂Si. These results provide a new and more distinctive perspective on the actual morphology of Fe-rich intermetallics and Mg oxides than the ones that exist in the literature. It also provides direct evidence of the role of inclusions (oxides) in intermetallic phase nucleation. This information can be utilised to improve the surface properties in 6xxx extrusion alloys.

Introduction of nanocarbons, such as graphene and carbon nanotubes, to metal matrices, may enhance the electrical and thermal transport, mechanical properties and some other properties of the composite materials. However, uniform distribution of the nanocarbon phase in the matrix material and manufacturing the composites in large scale can be challenging using traditional mixing methods. In this study, a facile method to fabricate metal-nanocarbon composites was developed. Firstly, copper (Cu)-polydopamine (PDA) composite was fabricated by coating Cu powders with the bioinspired PDA polymer, which was then converted to a graphite-like structure during the subsequent sintering. In terms of the properties, compared to the pure Cu sample, the Cu-PDA composite showed increased electrical and thermal conductivity, higher microindentation hardness, and enhanced wear resistance. These findings suggest the inclusion of nanocarbon phase converted from PDA can simultaneously improve the electrical, thermal, and mechanical properties of sintered Cu materials. Effect of sintering temperature and coating time (carbon content) on the microstructure and properties of the composites were discussed. Secondly, aluminum (Al)-copper nanoparticles (CuNP)-PDA composite was fabricated with a new method, to improve the sintering behavior of Al for serving as feedstock materials of additive manufacturing (AM). CuNPs were synthesized by directly reducing Cu ions in the aqueous solution. With the assistance of the PDA coating, the CuNPs can be better attached to the Al powder surfaces. The composite samples showed better sintering behavior by exhibiting higher electrical conductivities and mechanical properties, which may be due to local nanosized alloying phases generation after sintering. These findings illustrated that the composite powders could be a good candidate feedstock material for AM. The structural characterizations of the metal nanocarbon powders and the composites were performed with SEM, TEM, XRD and Raman spectroscopy. With the help of these techniques, the formation of the targeted structures in the composite was studied, including graphite-like structures of cPDA and nano alloying phases in Al-CuNP-PDA composites. Apart from the composite materials fabrication, a novel and facile manufacturing method based on metal powders was also developed. In this study, a new type of Cu- binder paste was formed, which not only can be utilized with direct ink/paste printing but also can be casted into a soft silicone rubber mold. Three-dimensional (3D) metal parts can then be subsequently obtained after sintering. Comparing to other additive manufacturing methods that involve high energy laser or electron beams, this new approach does not require expensive facilities, and it is less time-consuming. Moreover, the silicone rubber molds can be easily removed and reused. In summary, the composite powders fabricated in this study can be utilized as feedstock materials for additive manufacturing of metals and alloys. The new soft-mold casting could be used as an alternative method to manufacture 3D metal components. Therefore, the materials and the processing methods developed in the current study could have broad applications in various metal industries. / Mechanical Engineering

Fundamental research is required to support the commercialization of 3rd Generation Advanced High Strength steels (3G AHSS). Mid-manganese 3G AHSS steels can contain up to 11wt% manganese and are expensive if traditional ferroalloying practices are used; reductive alloying is a promising alternative. This study has researched the fundamental science behind possible processing methods. Silicothermic reduction of MnO from slag was studied. The reaction is fast but can be blocked by a stagnant layer of SiO bubbles cutting the rate of reaction by one order of magnitude. A theoretical model for mixed mass transport control was tested against original experimental data. Across nine datasets, the mass transfer coefficient for metal species, kMetal, was 2.3∙10-4m/s and the slag mass transfer coefficient, kSlag, was 6.7∙10-4m/s. In real industrial systems, gas blockage should not have an effect because stirring will dislodge these bubbles. Carbothermic reduction is dramatically different and has been qualitatively documented in this work. The reaction occurs in two stages: the first approximately three times faster than the second. The first stage is characterized by internal CO nucleation and growth and is rate-limited by the formation and growth of these CO bubbles. The second stage occurs along the metal interface and shows that the slag and metal are essentially separated by an intermediary gas phase. This reaction is controlled by decomposition of metal oxides at the gas-slag boundary, decomposition of CO2 at the gas-metal boundary, and transport of CO2 across the gas bubble; this mechanism is nearly identical to the carbothermic reduction of FeO. Reductive alloying can be utilized with the silicothermic reduction process to obtain high levels of manganese in steel but the carbothermic reduction may be too slow to be a viable process. / Thesis / Doctor of Philosophy (PhD) / 3rd Generation Advanced High Strength steels (3G AHSS) are a promising opportunity to produce steels with improved mechanical properties. These steels are alloyed with up to 11wt% manganese; traditional alloy additions are added as ferroalloys which may not be the most economical solution to achieve the required concentrations of manganese. Reductive alloying is a potential method for achieving high concentrations of manganese in the metal. By adding manganese oxide to slag, and reductants like carbon or silicon to the molten metal, manganese can be reduced from slag to metal. This work has determined the kinetics (rate of reaction) during the silicothermic and carbothermic reduction of manganese oxide from slag. The silicothermic reduction of manganese oxide is fast and can achieve high levels of manganese in the metal. The carbothermic reduction is much slower with questionable viability.