The mechanical performance evaluation of asphalt binder has always been a challenging issue for pavement engineers. Recently, the Phase Field Method (PFM) has emerged as a powerful computational tool to simulate the microstructure evolution of asphalt binder. PFM analyzes the structure from the free energy aspect and can provide a view of the whole microstructure evolution process. In this dissertation, asphalt binder performance is analyzed by PFM in three aspects: first, the relationship between asphalt chemistry and performance is investigated. The components of asphalt are simplified to three: asphaltene, resin and oil. Simulation results show that phase separation will occur under certain thermal conditions and result in an uneven distribution of residual thermal stress. Second, asphalt cracking is analyzed by PFM. The traditional approach to analyze crack propagation is Classic Fracture Mechanics first proposed by Griffith, which needs to clearly depict the crack front conditions and may cause complex cracking topologies. PFM describes the microstructure using a phase-field variable which assumes positive one in the intact solid and negative one in the crack void. The fracture toughness is modeled as the surface energy stored in the diffuse interface between the intact solid and crack void. To account for the growth of cracks, a non-conserved Allen-Cahn equation is adopted to evolve the phase-field variable. The energy based formulation of the phase-field method handles the competition between the growth of surface energy and release of elastic energy in a natural way: the crack propagation is a result of the energy minimization in the direction of the steepest descent. Both the linear elasticity and phase-field equation are solved in a unified finite element frame work, which is implemented in the commercial software COMSOL. Different crack mode simulations are performed for validation. It was discovered that the onset of crack propagation agrees very well with the Griffith criterion and experimental results. Third, asphalt self-healing phenomenon is studied based on the Atomic Force Microscopy (AFM) technology. The self-healing mechanism is simulated in two ways: thermodynamic approach and mechanical approach. Cahn-Hilliard dynamics and Allen-Cahn dynamics are adopted, respectively. / Ph. D.

This study presents a study on the binder film thickness effect on aggregate contact behavior. As a three-phase material composed of aggregates, asphalt binder and air voids, asphalt mixture could be considered as a visco-elastic material in the low stress level. Since the behavior of the mixture depends largely on the relationship of different components, a well developed contact model for micro-structural modeling is very important for understanding the deformation mechanism of the mixture. In this study, the contact modeling of asphalt mixture was reviewed and the numerical tools used to investigate the micromechanical behavior of asphalt mixture will also be introduced. By using the cabinet x-ray tomography system, the displacement and resistant force of a system of particles bonded by a thin layer binder are measured and recorded. Then, the results are compared with the theoretical solutions of a normal compliance model for a system comprised of two elastic particles bonded by a thin layer of visco-elastic binder. A closed-form time-dependent relationship between the contact forces and the relative particle/binder movements was developed. A reasonable agreement between experiments results and model predicted results is obtained combined with parametric analysis. / Master of Science

This work created a manufacturing process and material system based on binder jetting Additive Manufacturing to process pure copper. In order to reduce the sintered part porosity and shape distortion during sintering, the powder bed voids were filled with smaller particles to improve the powder packing density. Through the investigation of a bimodal particle size powder bed and nanoparticle binders, this work aims to develop an understanding of (i) the relationship between printed part properties and powder bed particle size distribution, and (ii) the binder-powder interaction and printed primitive formation in binder jetting of metals. Bimodal powder mixtures created by mixing a coarse powder with a finer powder were investigated. Compared to the parts printed with the monosized fine powder constituent, the use of a bimodal powder mixture improved the powder flowability and packing density, and therefore increased the green part density (8.2%), reduced the sintering shrinkage (6.4%), and increased the sintered density (4.0%). The deposition of nanoparticles to the powder bed voids was achieved by three different metal binders: (i) a nanoparticles suspension in an existing organic binder, (ii) an inorganic nanosuspension, and (iii) a Metal-Organic-Decomposition ink. The use of nanoparticle binders improved the green part density and reduced the sintering shrinkage, which has led to an improved sintered density when high binder saturation ratios were used. A new binding mechanism based on sintering the jetted metal nanoparticles was demonstrated to be capable of (i) providing a permanent bonding for powders to improve the printed part structural integrity, and (ii) eliminating the need for organic adhesives to improve the printed part purity. Finally, the binder-powder interaction was studied by an experimental approach based on sessile drop goniometry on a powder bed. The dynamic contact angle of binder wetting capillary pores was calculated based on the binder penetration time, and used to describe the powder permeability and understand the binder penetration depth. This gained understanding was then used to study how the nanoparticle solid loading in a binder affect the binder-powder interactions and the printed primitive size, which provided an understanding for determining material compatibility and printing parameters in binder jetting. / PHD

Copper is a material of interest to Additive Manufacturing (AM) owing to its outstanding material properties, which finds use in enhanced heat transfer and electronics applications. Its high thermal conductivity and reflectivity cause challenges in the use of Powder Bed Fusion AM systems that involve supplying high-energy lasers or electron beams. This makes Binder Jetting a better alternative as it separates part creation (binding together of powders) from energy supply (post-process sintering). However, it is challenging to fabricate parts of high density using this method due to low packing density of powder while printing. This work aims to investigate the effects of Hot Isostatic Pressing (HIP) as a secondary post-processing step on the densification of Binder Jet copper parts. By understanding the effects of HIP, the author attempts to create parts of near-full density, and subsequently to quantify the effects of the developed process chain on the material properties of resultant copper parts. The goal is to be able to print parts of desired properties suited to particular applications through control of the processing conditions, and hence the porosity. First, 99.47% dense copper was fabricated using optimized powder configurations and process parameters. Further, the HIP of parts sintered to three densities using different powder configurations was shown to result in an improvement in strength and ductility with porosity in spite of grain coarsening. The strength, ductility, thermal and electrical conductivity were then compared to various physical and empirical models in the literature to develop an understanding of the process-property-performance relationship. / Master of Science

Silicon has emerged as a promising alternative to traditional graphite as an anode material in battery technology, primarily due to its high theoretical capacity and abundance. However, its application is hindered by significant challenges, including severe volume expansion in the active material (~275%) during cycling, which can lead to a series of electrode failure issues. Polymer binder plays an essential role in addressing these challenges as it accommodates silicon's volume expansion and the rearrangement of particles. This work conducted an analysis of how different binders influence mechanical and electrochemical properties of silicon electrodes. Our findings are supported by a series of experiments, aimed at addressing the challenge of silicon volume expansion and improving the durability and efficiency of silicon-based anodes. Water-soluble polyacrylic acid (PAA) has emerged as a promising binder material for silicon anodes, with lithium hydroxide (LiOH) frequently added to improve the rheological properties of the slurry. However, literature presents varying results regarding the electrochemical performance of batteries incorporating LiOH in PAA binders. In addressing these discrepancies, our research investigates the role of LiOH in PAA, defining its impact through two primary factors: lithium-ion concentration and pH level. Our analysis involved conducting cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests, which confirmed our hypothesis that the addition of Li+ ions improves ion transport. Regarding pH, an optimal middle-ground pH level is identified, balancing the advantages shown at both lower and higher pH ranges. Despite the observed benefits of water-soluble PAA binder, such binders frequently result in uneven carbon distribution in coating, attributed to the poor wettability of nano-carbon in water. Consequently, the next portion of this work revisits the use of a traditional NMP (N-Methyl-2-pyrrolidone) soluble binder, PVDF (polyvinylidene fluoride), known for its widespread application in battery technology. However, PVDF-based silicon anodes often exhibit poor cycling performance. To address this issue and enhance the binder's flexibility, we attempted to chemically modify PVDF by incorporating carboxylic acid (-COOH) groups and reducing the polymer chain length. Despite these efforts, the experimental results did not show an improvement in cycling performance. The findings suggest that the deteriorated performance may be due to a weakened adhesion to the current collector for short-chain polymers. We then explore additional binder systems in an attempt to improve Si electrode performance. Our previous research suggests a trade-off between flexibility and adhesion in shortened polymers. To further verify this, we investigate the effect of two commercially available short-chain polymer binders, namely Jeffamine D-2000 and PAA(2000). Next, in order to mitigate the adverse effects of short polymer chain lengths on mechanical performance, we adopt an adhesion layer between the bulk electrode layer and the current collector. Finally, we evaluate several binders known for their promising results in other battery systems, including polyacrylonitrile (PAN), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and polyimide (PI). A series of mechanical and electrochemical characteristics of the as-mentioned binders are investigated. The findings confirm that shorter polymer chain length leads to a weaker adhesion between the electrode coating and the current collector. Additionally, we discovered that introducing an adhesion layer can enhance the cycling stability of silicon anodes.

As a result of increasing cost, efficiency and environmental pressures ferrochrome producers strive towards lower overall energy consumption. Increases in local electricity prices have placed particular pressure on South African ferrochrome producers. Pelletised chromite pre–reduction is likely the currently applied ferrochrome production process option with the lowest specific electricity consumption. In this process fine chromite, together with a carbonaceous reductant and a clay binder is milled, pelletised and pre–reduced. In this dissertation it is demonstrated that the functioning of the clay binder in this process is not as straightforward as in conventional metallurgical pelletisation processes, since the cured pre–reduced pellets are characterised by an oxidised outer layer and a pre–reduced core. Conventional performance characteristics of clay binders (e.g. compressive strength and abrasion resistance) therefore have to be evaluated in both oxidative sintering and reducing environments. Two clay samples, i.e. attapulgite and bentonite, were obtained from a local ferrochrome producer and investigated within the context of this study. Results indicated that the compressive and abrasion resistance strengths of oxidative sintered pellets for both clays were substantially better than that of pre–reduced pellets. Thus, although the objective of the chromite pre–reduced process is to achieve maximum pre–reduction, the strength of pre–reduced chromite pellets is significantly enhanced by the thin oxidised outer layer. The strength of the bentonite–containing pellets was found to be superior in both pre–reducing and oxidative sintering environments. This is significant, since the attapulgite clay is currently the preferred option at both South African ferrochrome smelting plants applying the pelletised chromite pre–reduction process. Although not quantitatively investigated, thermo–mechanical analysis indicated that the hot strength of the attapulgite pellets could be weaker than the bentonite–containing pellets. The possible effects of clay binder selection on the level of pre–reduction were also investigated, since it could have substantial efficiency and economic implications. For both case study clays investigated, higher clay contents resulted in lower pre–reduction levels. This has relevance within the industrial process, since higher clay contents are on occasion utilised to achieve improved green strength. The average pre–reduction of the bentonite–containing pellets were also consistently higher than that of the attapulgite–containing pellets. Again, this is significant, since the attapulgite clay is currently the preferred option. In general the case study results presented in this dissertation indicated that it is unlikely that the performance of a specific clay binder in this relatively complex process can be predicted; based only on the chemical, surface chemical and mineralogical characterisation of the clay. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012.

As a result of increasing cost, efficiency and environmental pressures ferrochrome producers strive towards lower overall energy consumption. Increases in local electricity prices have placed particular pressure on South African ferrochrome producers. Pelletised chromite pre–reduction is likely the currently applied ferrochrome production process option with the lowest specific electricity consumption. In this process fine chromite, together with a carbonaceous reductant and a clay binder is milled, pelletised and pre–reduced. In this dissertation it is demonstrated that the functioning of the clay binder in this process is not as straightforward as in conventional metallurgical pelletisation processes, since the cured pre–reduced pellets are characterised by an oxidised outer layer and a pre–reduced core. Conventional performance characteristics of clay binders (e.g. compressive strength and abrasion resistance) therefore have to be evaluated in both oxidative sintering and reducing environments. Two clay samples, i.e. attapulgite and bentonite, were obtained from a local ferrochrome producer and investigated within the context of this study. Results indicated that the compressive and abrasion resistance strengths of oxidative sintered pellets for both clays were substantially better than that of pre–reduced pellets. Thus, although the objective of the chromite pre–reduced process is to achieve maximum pre–reduction, the strength of pre–reduced chromite pellets is significantly enhanced by the thin oxidised outer layer. The strength of the bentonite–containing pellets was found to be superior in both pre–reducing and oxidative sintering environments. This is significant, since the attapulgite clay is currently the preferred option at both South African ferrochrome smelting plants applying the pelletised chromite pre–reduction process. Although not quantitatively investigated, thermo–mechanical analysis indicated that the hot strength of the attapulgite pellets could be weaker than the bentonite–containing pellets. The possible effects of clay binder selection on the level of pre–reduction were also investigated, since it could have substantial efficiency and economic implications. For both case study clays investigated, higher clay contents resulted in lower pre–reduction levels. This has relevance within the industrial process, since higher clay contents are on occasion utilised to achieve improved green strength. The average pre–reduction of the bentonite–containing pellets were also consistently higher than that of the attapulgite–containing pellets. Again, this is significant, since the attapulgite clay is currently the preferred option. In general the case study results presented in this dissertation indicated that it is unlikely that the performance of a specific clay binder in this relatively complex process can be predicted; based only on the chemical, surface chemical and mineralogical characterisation of the clay. / Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012.

Die Arbeit befasst sich mit der Untersuchung des Kernformstofffließens als nichtnewtonsche Fluidströmung. Dazu werden verschiedene Formgrundstoffe und Kernformstoffe rheologisch untersucht. Als Bindersysteme kommen PUR Coldbox und Wasserglas zum Einsatz. Für diese Untersuchungen wird ein eigens für diesen Zweck entwickeltes Messgerät verwendet. Die gewonnenen Daten werden in ein numerisches Modell implementiert, welches mit den Methoden der numerischen Strömungsmechanik den Kernschießvorgang abbildet. Dabei kommt der Open Source CFD Code OpenFOAM zum Einsatz. Der Kernschießvorgang wird mit verschiedenen Kernkastengeometrien numerisch und experimentell untersucht und die Ergebnisse verglichen. Die Ergebnisse der rheologischen Untersuchungen zeigen deutlich den Einfluss der Beschaffenheit des Formgrundstoffes und des Bindersystems auf die Fließfähigkeit des Kernformstoffes. Der Vergleich zwischen Experiment und Simulation zeigt gute Übereinstimmung. Das formulierte Modell gibt die Möglichkeit, Probleme in der Kernqualität vorherzusagen.

Binder jetting (BJT) is a powder bed based additive manufacturing (AM) process where the interaction of inkjetted droplets of a binder and particles in the powder bed create 3D geometries in a layerwise fashion. The fabricated green parts are usually thermally post-processed for densification and strengthening. BJT holds distinct advantages over other AM processes as it can fabricate parts with virtually any materials (metals, ceramics, and polymers) in a fast and cost-effective way, while achieving isotropic material properties in the parts. However, broad adoption of this process for production is still lagging, partially due to the lack of repeatable part quality, which largely stems from the limited understanding of the process physics, namely binder-powder (B/P) interaction. To bridge this knowledge gap, it is necessary to understand the implications of B/P interaction on process-structure-property relationships and discover ways to achieve new functionalities for enhanced properties. Thus, this research is broadly focused in establishing understanding in (i) binder-powder interaction and (ii) the impact of binder on part densification. Prior studies have focused on the effects of powder interaction with micro/meso-scale binder droplets, despite commercial BJT systems featuring picoliter-scale droplets. These studies have explored the effects of B/P interaction on printed primitive formation, but it's implication on final part properties have not been studied. In this work, the effects of particle size distribution and droplet size variation on final part properties are explored. Additionally, the effects of B/P interaction on accuracy and the resolution of the printed parts are investigated. Densification of parts is a primary focus of many BJT studies as it dictates the final part properties and is influenced by factors from both the printing process and post-processing treatments. Binder plays an integral role in the shaping of parts and maintaining part integrity until densification through sintering. Prior studies on the effects of binder content on densification are inclusive. In this work, a new approach termed as "shell printing" is introduced to vary the binder content in the parts. The process-structure-properties influenced by this approach are investigated. It was found that binder hinders densification, and through the selective variation of binder content throughout the part volume, this new approach is introduced as a means for enhancing part properties. Finally, the insights from the impact of binder on densification are leveraged to create an anti-counterfeiting tagging strategy by controlling the pores and grain microstructures inside a part. In this novel approach, binder concentration is controlled in a manner that the stochastically formed pores are clustered to create a designed domain that represents a secret 'tag' within the part volume. The created tagging domains, and the feature resolvability of this approach are investigated through metallographic characterization and non-destructively evaluated through micro-computed tomography. / Doctor of Philosophy / Binder jetting (BJT) is an additive manufacturing (AM) process to create 3D geometries from powder particles. Liquid droplets of binder from an inkjet printhead are jetted on a bed of packed powders, binding the particles. The as-printed parts, known as green parts, are generally fragile and require thermal post-processing (through sintering) for densification and strengthening. BJT holds distinct advantages over other AM processes as it can fabricate parts with virtually any powdered materials (metals, ceramics, and polymers) in a fast and cost-effective way. However, broad adoption of this process for production is still lagging, partially due to the lack of repeatable part quality, which largely stems from the limited understanding of the process physics, namely binder-powder (B/P) interaction. In this study the implications of B/P interaction on part quality (e.g., density, strength) and dimensional accuracy are studied. Additionally, the impact of binder on sintering densification is studied. Specifically, the effects of varying amount of binder on sintered part density, strength and internal pore and grain microstructures are empirically investigated. Finally, a novel anti-counterfeiting method for BJT printed parts is introduced based on the insights gained from the study of the impact of binder on densification. Through control over binder placement throughout the part, porous regions can be generated selectively throughout the part volume, which can be detected through x-ray computed tomography. Overall, an improved understanding of BJT processing conditions is achieved through this research, which can guide future designers to fabricate BJT parts with enhanced part properties and functionality.

O presente trabalho está relacionado com o desenvolvimento e análise do desempenho eletroquímico de eletrodos compósitos do tipo nonwoven também chamados de free-standing binder/metal-free electrodes, em eletrólito líquido orgânico que contem íons de lítio. Os eletrodos de excelente resistência mecânica, livre de metais e binder, e que podem conter vários miligramas dematerial eletroativo por cm3, são constituídos por substratos de fibras de carbono derivado de poliacrilonitrila, e a carga eletroativa composta por nanotubos de carbono de parede múltipla (NTC) e nanotubos de MnO2 (NT). Foram utilizados dois tipos de substrato (denominados aqui de feltro e tecido de carbono) de diferentes condutividades eletrônicas e geometrias tridimensionais. O recobrimento das fibras de carbono dos nonwovens com NTC foi realizado por decomposição química de vapor (CVD) mantendo-se constante as variáveis operacionais, o que resultou em NTC do mesmo tipo para todas as amostras e um bom controle da massa depositada. O MnO2 foi incorporado por eletrodeposição em eletrólito aquoso, esse método garantiu um bom controle de massa eletrodepositada de NT. Os eletrodos obtidos foram caracterizados estruturalmente empregando-se microscopia de varredura (MEV), difração de raios-X e microscopia Raman. Para análise de desempenho eletroquímico e mecanismo de armazenagem/conversão de energia nos eletrodos empregadas as técnicas de voltametria e cronopotenciometria cíclicas. Os resultados mostram que os eletrodos compósitos são híbridos, podem atuar como capacitores e eletrodos de baterias de íons lítio. As metodologias aplicadas se mostram extremamente reprodutíveis reprodutivas e controláveis. Depedendo das composições e combinações foi possível obter capacidades específicas associadas com armazenagem/estocagem de lítio em altas densidades de corrente (A/g) na janela de potencial de 0,005 - 3,5V vs Li/Li+ (por exemplo, 800 mAh/g em 1 A/g, taxa C-rate = 1,25C, 400 mAh/g em 2,66A/g, taxa C-rate = 5C). A eficiência faradaica para o primeiro ciclo carga/descarga variou entre 83% e 54%, dependendo da quantidade de MnO2 e da corrente aplicada. Foi observado que é possível melhorar ainda mais os resultados com adição de outros constituintes, como por exemplo, a adição de partículas de prata (<1 % em peso). Neste caso os eletrodos forneceram eficiência faradaica de 83%, 1.100 mAh/g em 1,7A/g, em taxa C-rate = 1,66C e 550 mAh/g em 2,8A/g em taxa C-rate = 5C). Em termos de capacitância os compósitos também se mostram muito positivos. Valores de capacitância da ordem de 180F/g foram facilmente obtidos em tempos de descarga de 58s e num intervalo de potencial em relação ao Li/Li+ (~3,05 V vs H2/H+) de 1,4 a 3,8V vs Li/Li+, o que permite gerar densidade de energia e potência da ordem de 63 Wh/kg e 3,6 kW/kg respectivamente. Os eletrodos estudados podem atuar como eletrodo em baterias de íons lítio e em dispositivos de capacitores, o que significa que pode ser útil para o desenvolvimento de sistemas híbridos de armazenamento/conversão de energia, particularmente, de sistemas híbridos bipolar bateria-supercapacitor. / The present work is correlated with the development and electrochemical analisys of a nonwoven kind of electrode, also called as free-standing binder/metal-free electrodes, into lithium-ion liquid organic electrolyte, whereas the constituents are the substrate made of carbon fiber derived from carbonization of polyacrylonitrile, and the electroactive material which are defective multi-walled carbon nanotubes (MWCNT) and MnO2 nanotubes. Two types of nonwoven substrates (here denominated felt and cloth) with different electronic conductivity and three-dimensional geometry were employed. MWCNT coating of the nonwoven carbon fibers was achieved with chemical vapor decomposition (CVD) of methanol at same growth conditions, which resulted in electrodes with same type of MWCNT and a good control of the deposited mass. MnO2 was incorporaded by electrodeposition in aqueous electrolyte and this methodology was found appropriate to provide electrodes with same MnO2 NT loading, although the structural phase of MnO2 was affect by nonwoven substrate type. The robusts electrodes able to support several miligrams of electroactive material per cm3 obtained were structurally characterized using scanning electron microscopy (SEM, TEM), X-ray diffraction and Raman microscopy. It was employed cyclic voltammetry at different scan rate and chronopotentiometry (discharge/charge curves at galvanostatic conditions) aiming the understanding of the electrochemical performance and mechanism of energy storage/conversion of MnO2/MWCNT coated nonwoven electrodes. The results show that the composite electrode is hybrid, can act like capacitor or lithium ion battery electrode. It can provide very high specific capacity associated with storage/extraction of Li same in elevated gravimetric current density of A/g in the potential window of 0.005-3.5V vs Li/Li+ (e.g 800 mAh/g at 1 A/g, rate = 1,25C, 400 mAh/g at 2,66A/g, rate = 5C). The Faradic efficiency measure during the first charge/discharge cycle was between 83% to 54% depending on amount of MnO2 constituent and applied current. It was also observed a gain in the electrochemical performance of MnO2/MWCNT coated nonwoven electrode with Ag nanoparticles addition (about 1% wt). With presence of Ag constituent into the composites nonwovens it was found for instance 83% of Faradic efficiency at 1st discharge/charge cycle, 1,100 mAh/g at 1,7A/g rate = 1,66C and 550 mAh/g at 2,8A/g rate = 5C. In terms of capacitance the nonwoven were able to provide values like 180 F/g during 58s in high voltage window (1.4-3.8V vs LI/Li+) which correspond to energy and power density of 63 Wh/kg e 3.6 kW/kg, respectively. The electrodes developed in the present study could therefore act both as an electrode for Li intercalation and for capacitors devices, which means that it can be useful for the development of hybrid energy storage/conversion systems, particularly, bipolar battery-supercapacitor hybrid single.