This Thesis considers peculiarities of nanoparticle formation from the gas in different systems. The main role of the surface condensation in the nanoparticle growth in metal flames was established through a series of experiments and was described by the developed model. The stagnation of the post-nucleation nanoparticle growth was experimentally revealed and theoretically explained. The influence of generation conditions on the post-processing nanoparticle properties was examined. The non-isothermal approach to correct the homogeneous nucleation theory was developed. The results of this work can be summarized in 3 categories: (1) Nanoparticle formation in metal flames. In this work, it was demonstrated that the surface condensation is a main process responsible for nanooxides growth during metal combustion. It was shown that the rate of this condensation growth is consistent with the exponential law, which could lead to the formation of the lognormal particle size distribution in the system, where the Brownian coagulation is suppressed. The post-nucleation stagnation of the nanoparticle growth was found. The particle overheating was suggested as a cause of the growth stagnation. The found stagnation leads to the accumulation of the supercritical clusters in the system generating nanoparticles. The role of these supercritical clusters in the nanoparticle agglomeration was considered. (2) Study of properties of nanoparticles generated in different metal flames. The light absorption, photoluminescence and magnetic properties of nanoparticles produced in different metal flames were examined. The significant broadening of the absorption edge was found in nanooxides produced by direct metal combustion. This broadening allowed one to excite the unforeseen photoluminescence from these nanoparticles. The significant light absorption in the visible light found in the titania particles produced by metal combustion allows one to consider these particles as a prospective photocatalyst. The unusual optical properties revealed were related to the extreme conditions of the nanoparticle formation, namely, to high energy release (about 5 eV per condensing molecule). The stabilization of spinel structure was found in iron oxide particles synthesized by iron combustion. It allowed one to produce nanoparticles with magnetization close to the high-limit value of the bulk. (3) Approach to correct the homogeneous nucleation theory. The existing homogeneous nucleation theory implies that nucleation occurs at isothermal conditions, i.e. subcritical clusters have the same temperature as the ambient gas does. However, the theory overestimates the actual nucleation rate and underestimates the critical cluster size. It is understandable that due to release of the latent heat of condensation, the cluster temperature in the nucleating system should be higher than the environment temperature. In this work, it was suggested the method to account for the cluster overheating during nucleation. It was demonstrated that the consistent description of the detailed balance in the nucleating system may allow one to evaluate magnitudes of overestimation of the actual nucleation rate and underestimation of the number of molecules in the critical cluster, usually obtained by the isothermal nucleation theory. The numerical estimates are in good agreement with the wellknown experimental results. The implications of the results to generation of atmospheric aerosols were discussed.

Nucleation is the first step in the formation of a new phase in a thermodynamic system. The Classical Nucleation Theory (CNT) is the traditional theory used to describe this phenomenon. The object of this thesis is to investigate nucleation beyond one of the most significant limitations of the CNT: the assumption that the surface tension of a nucleating cluster of the new phase is independent of the cluster’s size and has the same value that it would have in the bulk of the new phase. In order to accomplish this, we consider a microscopic, two-dimensional Lattice Gas Automata (LGA) model of precipitate nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water particle bonding energy), η (next-to-nearest neighbour bonding coeffiicent in solid phase), and Cin (initial solute concentration). The LGA method was chosen for its advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system’s concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the system’s equilibrium concentration. A mean first-passage time (MFPT) technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation are evaluated using a modification to the CNT that incorporates a two-dimensional, radius-dependent surface tension term. The Tolman parameter, δ, which controls the radius-dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of η and Esw (Ess). On the other hand, δ increases as η increases while Ess and Esw are held constant. The constant surface tension term of the CNT, Σ0, increases (decreases) with increasing magnitudes of Ess (Esw) fixed values of Esw (Ess), and increases as η is increased. Together, these results indicate an increase in the radius-dependent surface tension, Σ, with respect to increasing magnitude of Ess relative to the magnitude of Esw. Σ0 increases linearly as a function of the change in energy during an attachment or detachment reaction, |ΔE|, however with a slope less than that predicted for a crystal that is uniformly packed at maximum density.

Nucleation is the first step in the formation of a new phase in a thermodynamic system. The Classical Nucleation Theory (CNT) is the traditional theory used to describe this phenomenon. The object of this thesis is to investigate nucleation beyond one of the most significant limitations of the CNT: the assumption that the surface tension of a nucleating cluster of the new phase is independent of the cluster’s size and has the same value that it would have in the bulk of the new phase. In order to accomplish this, we consider a microscopic, two-dimensional Lattice Gas Automata (LGA) model of precipitate nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water particle bonding energy), η (next-to-nearest neighbour bonding coeffiicent in solid phase), and Cin (initial solute concentration). The LGA method was chosen for its advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system’s concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the system’s equilibrium concentration. A mean first-passage time (MFPT) technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation are evaluated using a modification to the CNT that incorporates a two-dimensional, radius-dependent surface tension term. The Tolman parameter, δ, which controls the radius-dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of η and Esw (Ess). On the other hand, δ increases as η increases while Ess and Esw are held constant. The constant surface tension term of the CNT, Σ0, increases (decreases) with increasing magnitudes of Ess (Esw) fixed values of Esw (Ess), and increases as η is increased. Together, these results indicate an increase in the radius-dependent surface tension, Σ, with respect to increasing magnitude of Ess relative to the magnitude of Esw. Σ0 increases linearly as a function of the change in energy during an attachment or detachment reaction, |ΔE|, however with a slope less than that predicted for a crystal that is uniformly packed at maximum density.

The goal of this work is to provide a detailed description of the freezing mechanism in gold clusters. This is accomplished by using constrained Monte Carlo simulations combined with parallel tempering algorithms to evaluate the free energy barriers for various temperatures with respect to crystalline order parameters on a 456 atom cluster. <p>Our simulation results help us to challenge the usual assumption of classic nucleation theory where nucleation starts at the center of a cluster, showing instead that nucleation is favored by freezing started at the surface. We study simplistic phenomenological models for surface freezing and find that the three phase contact line free energy term must be included in order to properly describe the features of the free energy barriers. <p>Furthermore, we propose an alternative free energy parameter with which we are able to identify a kinetic spinodal temperature where the nucleation barrier disappears and find that the critical cluster size remains finite at the limit of stability of the fluid phase. This result is supported by Molecular Dynamics simulations.

The goal of this work is to provide a detailed description of the freezing mechanism in gold clusters. This is accomplished by using constrained Monte Carlo simulations combined with parallel tempering algorithms to evaluate the free energy barriers for various temperatures with respect to crystalline order parameters on a 456 atom cluster. <p>Our simulation results help us to challenge the usual assumption of classic nucleation theory where nucleation starts at the center of a cluster, showing instead that nucleation is favored by freezing started at the surface. We study simplistic phenomenological models for surface freezing and find that the three phase contact line free energy term must be included in order to properly describe the features of the free energy barriers. <p>Furthermore, we propose an alternative free energy parameter with which we are able to identify a kinetic spinodal temperature where the nucleation barrier disappears and find that the critical cluster size remains finite at the limit of stability of the fluid phase. This result is supported by Molecular Dynamics simulations.

Nucleation is the first step in the formation of a new phase in a thermodynamic system. The Classical Nucleation Theory (CNT) is the traditional theory used to describe this phenomenon. The object of this thesis is to investigate nucleation beyond one of the most significant limitations of the CNT: the assumption that the surface tension of a nucleating cluster of the new phase is independent of the cluster’s size and has the same value that it would have in the bulk of the new phase. In order to accomplish this, we consider a microscopic, two-dimensional Lattice Gas Automata (LGA) model of precipitate nucleation in a supersaturated system, with model input parameters Ess (solid particle-to-solid particle bonding energy), Esw (solid particle-to-water particle bonding energy), η (next-to-nearest neighbour bonding coeffiicent in solid phase), and Cin (initial solute concentration). The LGA method was chosen for its advantages of easy implementation, low memory requirements, and fast computation speed. Analytical results for the system’s concentration and the crystal radius as functions of time are derived and the former is fit to the simulation data in order to determine the system’s equilibrium concentration. A mean first-passage time (MFPT) technique is used to obtain the nucleation rate and critical nucleus size from the simulation data. The nucleation rate and supersaturation are evaluated using a modification to the CNT that incorporates a two-dimensional, radius-dependent surface tension term. The Tolman parameter, δ, which controls the radius-dependence of the surface tension, decreases (increases) as a function of the magnitude of Ess (Esw), at fixed values of η and Esw (Ess). On the other hand, δ increases as η increases while Ess and Esw are held constant. The constant surface tension term of the CNT, Σ0, increases (decreases) with increasing magnitudes of Ess (Esw) fixed values of Esw (Ess), and increases as η is increased. Together, these results indicate an increase in the radius-dependent surface tension, Σ, with respect to increasing magnitude of Ess relative to the magnitude of Esw. Σ0 increases linearly as a function of the change in energy during an attachment or detachment reaction, |ΔE|, however with a slope less than that predicted for a crystal that is uniformly packed at maximum density.

Due to recent legislation aimed at reducing carbon emissions into the environment through weight reduction, the automotive and aerospace industries are using light alloys such as aluminium silicon (Al–Si) and aluminium zirconium (Al–Zr) instead of steel due to their excellent mechanical properties and low weight to strength ratio. In order to further improve mechanical and metallurgical properties in these alloys, grain refinement is usually used in industry. However, the current and most widely used grain refiner Al–5Ti–B is unable to refine Al–Si alloys with silicon content greater than 3 wt.%., and Al–Zr alloys due to poisoning of the refiner by silicon and zirconium. The Al–5Ti–B refiner also contains larger Al3Ti particles and agglomerates of TiB2 which affect its efficiency and suitability in industrial applications where thin sheets are required. In this study, a new technique which improves the microstructure and efficiency of the Al–5Ti–B refiner has been developed. This involves the reaction of potassium tetrafluoroborate (KBF4) and potassium hexafluorotitanate (K2TiF6) salts at shorter reaction time before ultrasonic processing during solidification. This leads to the formation of a new Al3Ti morphology and de-agglomeration of TiB2 particles which enhances its grain refinement efficiency by 20%. Secondly, through phase diagram analysis of Al grain refining systems and crystallography studies, it was observed that Al3Ti and Al3Nb display similar lattice parameters with atomic misfit of 4.2% and would undergo a peritectic reaction with α-Al at low contact angles. Based on this, and using the duplex nucleation theory and poisoning by Si and Zr, a new quaternary grain refiner containing aluminium, titanium, niobium and boron (Al–4Ti–Nb–B) has been developed. This novel grain refiner has been found to be efficient in Al–Si alloys and Al–Zr, both at laboratory and industrial scales, and to improve the mechanical properties of the alloys despite the presence of Ti in the alloy. It was observed that the addition of Nb to an Al–Ti–B system leads to the formation of solid solution phases of Al3Ti1-xNbx, Al3Nb1-xTx, and (Ti1-xNbx)B2 which prevents poisoning by Si and Zr. Experimental simulations showed that Al3Nb1-xSix rather than Ti(Al1-xSix)3 are formed in Al–Si alloys, and Al3(Ti1-xNbx) and (Al3Ti1-xNbx)B2 phases are formed in Al–Zr alloys rather than Al3(Zrx,Ti1-x), B2(Zrx,Ti1-x) or ZrB2 phases. A new grain refining mechanism, ‘The Quad Nucleation Theory’ based on four nucleation events in Al–4Ti–Nb–B has been proposed. Other newly developed quaternary and ternary novel grain refiners capable of refining aluminium silicon alloys are also presented in this thesis. This includes a novel method of refining Al–Si alloys using phosphorus and niobium.

No description available.

The melting behavior of polymers can provide information on their crystallization mechanism. However, the origin of the time-dependent low endotherm, or annealing peak, and the extent of melting-recrystallization-remelting during heating are still debated. The crystallization and subsequent melting behavior of isotactic polystyrene are explored in the context of Stroblâ "s 3-Phase model using differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and wide angle X-ray diffraction. DSC experiments confirm the existence of a crystallization time-dependent low endotherm, and melting-recrystallization-remelting processes during heating. SAXS analysis using the correlation function confirms that the lamellar thickness increases with crystallization temperature and is independent of time. The spread between equilibrium melting and crystallization temperatures determined in this work (Tfâ"" = 533K, Tcâ"" = 544K) is much smaller than reported by Strobl et al. (Tfâ"" = 562K, Tcâ"" = 598K). These differences are partially attributed to overestimation in lamellar thicknesses calculated through the interface distribution function. Analysis of diffraction broadening shows that the apparent crystal size decreases with crystallization time, suggesting the formation of smaller/less perfect crystals during secondary crystallization. These results are consistent with observations that the glass transition temperature increases with crystallization time and supports the idea that secondary crystallization leads to increased amorphous conformational constraints. These results also suggest that the upward shift of the annealing peak during secondary crystallization is associated with increased amorphous constraints rather than increased crystal dimensions. The lack of distinction between Tfâ"" and Tcâ"" and the evolution of crystal size during crystallization stand in direct contrast with Stroblâ "s model. / Master of Science

Living systems possess seemingly bottomless complexity. Attempts to parse the details of one cellular process from all other concurrent processes are challenging, if not daunting undertakings. The apparent depth of this problem, as it pertains to biomineralization, is related to the small number of existing studies focused on the development of a mechanism-based understanding of intracellular mineralization processes. Molecular biologists and geneticists have only begun to turn their attention towards identification and characterization of molecules involved in regulating and controlling biomineral formation. With this new knowledge, a number of new and exciting research opportunities are currently awaiting development upon a barren landscape. Silica biomineralization is one of these emerging frontiers. As new information about the chemical and structural nature of the macromolecules involved in biosilicification is revealed, the means these species employ to control the temporal and spatial onset of silica deposition in vivo become available for exploration. The first chapter of this dissertation outlines those aspects of silicate metabolism that are directly relevant to the controlled biomineralization of silica in eukaryotic organisms and identifies pervasive and unanswered questions surrounding biosilica formation. Particular attention is paid to the diatoms, which are the most abundant, and extensively investigated silica-mineralizing organisms in modern seas. The extent, and mechanism through which specific organic moieties work individually or in concert to direct mineral formation at biological interfaces is a central concern of modern biomineralization research. Chapter two addresses this forefront issue for silica mineralizing systems, and reports the results of an experimental investigation designed to measure the effects of individual surface-bound organic functional groups on the rate of surface-directed silica nucleation. Chapter three discusses an additional aspect of this research aimed at investigating the reactivity of nanoparticulate biogenic silica produced by marine phytoplankton and terrestrial plants in natural environments. Density Functional Theory and ab initio molecular orbital calculations are employed to explore potential mechanisms underlying the catalytic activity of divalent metal cations during the hydrolysis of Si – O bonded networks. / Ph. D.