Nucleation is a key step in the crystallisation process, where a new crystalline solid phase is created from a supersaturated solution. The applications of crystallisation as a purification and separation technique span many industries, yet still no definitive molecular mechanism for nucleation exists. This PhD is part of a critical mass research project involving researchers from both the Universities of Manchester and Leeds. The aim is to reveal the relationship between structural components of the nucleation transition state, solution phase molecular self-assembly and nano cluster formation, through to critically sized crystalline nuclei which then grow to crystals. All work has been carried out on a small organic molecule, p-aminobenzoic acid (PABA). This PhD has delivered successful characterisation of PABA in the solid and solution state, along with a detailed understanding of its nucleation kinetics and growth rates from a range of solvents. PABA has two enantiotropically related polymorphs, α and β, with the former constructed of carboxylic acid dimers and the latter of a hydrogen bonded tetramer network linking alternate acid and amine functionalities. New determinations of the crystal structures of both forms were submitted to the CCDC with Ref codes of AMBNAC07 and 08 for α and β PABA respectively. A detailed morphological study on both forms of PABA employing modelling and experimental methods has revealed the effect of solvent on the growth habit. In all polar solvents, α PABA displays a more important or slower growing (002) face than the calculated morphology implies. In water, β PABA has a much smaller (101 ̅) face in comparison to β PABA grown from alcohols. Crystallisation experiments demonstrate a clear solvent effect on the appearance of the two polymorphs. From organic solvents only α PABA is obtained, from water both α and β PABA are crystallised. A database search (CCDC) suggests that water may play an important role in the stabilisation of the nucleation transition state for both α and β PABA. This is not possible in organic solvents. Detailed nucleation and crystal growth kinetics have been measured for α PABA at 20°C in water, acetonitrile, ethyl acetate and 2-propanol. A clear solvent trend was observed in both the derived rates of molecular attachment and crystal growth. These were fastest in water, followed by acetonitrile, then ethyl acetate and finally slowest, in 2-propanol. This can be explained by the solvation of the carboxylic acid functional group, where 2-propanol is deemed the most effective solvator of building units in solution and on a crystal surface. This conclusion is supported by the solution FTIR spectroscopy, which clearly confirms strong solvation.

Le contrôle des caractéristiques des cristaux de glace, gouvernées par l'étape de nucléation, demeure un enjeu majeur dans les procédés de congélation et de lyophilisation industriels. Cependant la nucléation est par nature un phénomène aléatoire. L'application d'ultrasons de puissance, capable de générer de la cavitation acoustique, compte très certainement parmi les plus prometteuses méthodes permettant de contrôler le déclenchement de la nucléation, mais le matériel et les conditions opératoires à mettre en oeuvre afin d'industrialiser le procédé restent encore à définir.Le premier objectif de ce travail a été de corréler empiriquement les caractéristiques géométriques des cristaux de glace et les conditions opératoires dans le cas de la congélation d'une solution aqueuse de mannitol en vials. Il a été montré, grâce à une analyse de surface de réponses basée sur un plan d'expérience de deuxième ordre, que l'augmentation du sous refroidissement et de la puissance acoustique provoquent une diminution de la taille moyenne des cristaux et une augmentation de leur circularité.Le deuxième objectif était la modélisation de la cinétique de nucléation de glace dans de l'eau pure déclenchée par cavitation acoustique. D'après les simulations réalisées, la nucléation peut être initiée à de faibles sous-refroidissements (quelques degrés) au moyen de pressions acoustiques modérées (de l'ordre de un bar). Le nombre de nuclei créés est d'autant plus important que le sous-refroidissement et la pression acoustique sont élevés.Ces résultats sont cohérents si l'on considère que la taille des cristaux est inversement proportionnelle au nombre de nuclei générés. / Ice crystal characteristics control, which are governed by the nucleation step, remains a key issue in industrial freezing and freeze-drying processes. However nucleation is by nature arandom phenomenon. Power ultrasounds, able to generate acoustic cavitation, are one of the most promising methods of triggering nucleation, but operating conditions pertaining to an industrial freezing process have not yet been defined.This work aims first at empirically correlating the ice crystals' geometry to the operating conditions, in the case of mannitol aqueous solution in vials freezing. A response surface analysis based on a second order experimental design proved that increasing both supercooling and acoustic power resulted in decreasing ice crystals' mean size and increasing their circularity.The second goal was the modelling of ice nucleation triggered by acoustic cavitation in pure water. According to simulations, the nucleation could be initiated with moderate acoustic pressure amplitude (around one bar) even at low supercooling levels (around few degrees). The number of generated nuclei appeared to increase with the super cooling and with the acoustic power which should lead to a decrease of ice crystals' size as observed experimentally.These results are consistent if the ice crystals' size is considered to be inversely proportional to the number of generated nuclei.

Pulsed lasers are studied as new tools to realize competitive nanomanufacturing. The capabilities of pulsed lasers as promising tools for research, design, manufacturing, and control rely on the flexibility due to the great variety of operation parameters, and the inherent precision in aspects of time, spatial resolution, and energy input. As new tools, the fundamental understanding and technological capabilities of pulsed laser-induced chemical synthesis were explored in this dissertation research. In order to study the capabilities of pulsed laser in controlled synthesis, a thermal model was developed to predict the local temperature change due to the very short period of irradiation by a pulsed laser. And combining with the classical Gibbs free energy theories, a set of guidelines were developed for precision control for pulsed laser-induced chemical synthesis. Zinc oxide crystals were studied as an example case, showing the relationship between the wide range variables of pulsed laser including repetition rate, energy area density, power density, irradiation duration, etc. and the material structures of deposited crystals in aspects of crystal density, size, shape, crystalline properties, surface morphologies, growth rate, etc. Mechanisms from thermodynamic and kinetic aspects were explored. Pulsed laser-induced different heating conditions were found to separate two crystallization processes with different energy barriers, one dominated by a burst of nucleation and the other dominated by crystal growth through particle aggregation. For the study of the fundamental mechanisms in crystallization, pulsed laser initiated and controlled the crystallization in its early stage, and the crystal evolution were observed and analyzed by transmission electron microscopy (TEM). Crystal growth from intermediate monomers was first studied by an electron beam under the condition without precursor solution environment, providing crucial process information of crystal evolution, indicating multistage processes by continuous mass and phase transfer among intermediate monomers. This dissertation shows the capabilities of pulsed laser in realizing precision control for the targeted synthesis in nanomanufacturing, providing unique insight to crystallization mechanisms, and extending prospects to scientific research of other energy beam induced processes.

Since the fortuitous discovery of the existence of quantum size effects on the band structure of colloidal semiconductor nanocrystals, the development of synthetic methods that can form nanoscale crystalline materials of controllable size, shape, and composition has blossomed as an empirical scientific achievement. The fact that the term “recipe” is commonly used within the context of describing these synthetic methods is indicative of the experimentally driven nature of the field. In this respect, the highly attractive photophysical properties of semiconductor nanocrystals—as cheap wavelength tunable and high quantum yield absorbers and emitters of light for various applications in lighting, biological imaging, solar cells, and photocatalysis—has driven much of the interest in these materials. Nevertheless, a more rigorously predictive first-principles-grounded understanding of how the basic processes of nanocrystal formation (nucleation and growth) lead to the formation of semiconductor nanocrystals of desired size and size dispersity remains an elusive practical and fundamental goal in materials chemistry. In this thesis, we describe efforts to directly study these dynamic nucleation and growth processes for lead chalcogenide nanoparticles, in many cases in-situ, using a mixture of X-ray scattering and UV-Vis/NIR spectroscopy. The lack of a rigorously predictive and verified mechanism for nanocrystal formation in solution for many material systems of practical interest is due both to the inherent kinetic complexity of these reactions, as well as the spectroscopic challenge of finding in-situ probes that can reliably monitor nanoscale crystal growth. In particular, required are direct time-resolved structural probes of metastable inorganic amorphous and crystalline intermediates formed under the high temperature inert conditions of nanocrystal synthesis. It is, at the very least, highly challenging to apply many of the standard spectroscopic tools of mechanistic inorganic and organic chemistry such as ¹H NMR spectroscopy, IR vibrational spectroscopy, and mass spectrometry to this task. A notable counterexample is, of course, UV-vis/NIR absorbance and emission spectroscopies, which are of great value to the studies described herein. Nevertheless, to address this relative dearth of conventional spectroscopic probes, here we explore the use of X-ray Total Scattering real space Pair Distribution Function (PDF) analysis and Small Angle X-ray Scattering (SAXS) techniques to directly probe the crystallization process in-situ. Time-resolved measurements of the small angle reciprocal space scattering data allow mapping of the time evolution of the colloidal size and concentration of the crystals during synthesis, while the Fourier transform of scattering data over a wide range of reciprocal space provides direct insight into the local structure. Through this approach, we compare direct observations of these nucleation and growth processes to the widely cited theoretical models of these processes (Classical Nucleation Theory and LaMer “Burst Nucleation”) and find a number of stark differences between these widely cited theories and our experiments. The first two chapters cover the results of these 𝘪𝘯-𝘴𝘪𝘵𝘶 diffraction studies. Chapter 1 focuses on small angle X-ray scattering data collection and modeling. Chapter 2 focuses upon lead sulfide and lead selenide real space PDF analysis of local structural evolution during synthesis. Finally, Chapter 3 discusses a project in which we examine the origins of emergent semiconducting electronic structure in an increasing size series of atomically precise oligomers of [Ru₆C(CO)₁₆]²⁻ bridged by Hg²⁺ and Cd²⁺ atoms. Using an atomically well-defined series of molecules that bridge the small molecule and nanoscale size regimes, we discuss the factors that give rise to controllable semiconductor electronic structure upon assembly into extended periodic structures in solution. In all these projects, we seek to highlight the value of applying concepts of molecular inorganic chemistry—ligand binding models, relative bond strengths, in addition to kinetics and thermodynamics—to explain our observations regarding nanocrystal nucleation and growth. Consideration of the chemistry of nanocrystal formation processes provides a valuable compliment to the physics-based classical models of nucleation and growth that do not explicitly consider the system specific molecular structure and bonding.

(1) Yoshikawa, S., Kida, H. & Sato, K. Promotional effects of new types of additives on fat crystallization. J. Oleo Sci. 63(4), 333–345, © 2014 Japan Oil Chemists’ Society, Tokyo., (2) Yoshikawa, S., Kida, H. & Sato, K. Fat crystallization with talc particles is influenced by particle size, concentration, and cooling rate. Eur. J. Lipid Sci. Technol. 117(6), 858–868, © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., (3) Yoshikawa, S., Kida, H., Matsumura, Y. & Sato, K. Adding talc particles improves physical properties of palm oil-based shortening. Eur. J. Lipid Sci. Technol., DOI: 10.1002/ejlt.201500283 (in press), © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. / 京都大学 / 0048 / 新制・論文博士 / 博士(農学) / 乙第13044号 / 論農博第2837号 / 新制||農||1044(附属図書館) / 学位論文||H28||N5008(農学部図書室) / 33036 / (主査)教授 松村 康生, 教授 裏出 令子, 教授 安達 修二 / 学位規則第4条第2項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

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Magnesium alloys are excellent material candidate to reduce mass of automotive structures, and as such to meet the Department of Energy's targets in fuel economy and clean energy. However, magnesium alloys show poor ductility at room temperature, which is one of the most important impediments to achieving cost-effective manufacturing of wrought alloys and insuring good energy absorption in crash structures. This Master thesis aims to identify the mechanisms behind the low ductility of magnesium. Therefore, non-destructive EBSD analyses upon tension of both a strong and weak textured magnesium alloy were conducted with a focus on the role of twinning in fracture initiation. This study revealed five mechanisms responsible for early fracture, all of which relate to twinning activity. These mechanisms were involved directly in the shear incompatibility arising from interactions between twin-twin, twin-slip, twin-grain boundary, and double twinning. Backstress played a major role in twin-grain boundary and twin-twin boundary interactions.

The automotive industry's search for stronger lighter materials has been hampered in its desire to make greater use of Magnesium alloys by their poor formability below 150°C. One current challenge is to identify the complex structure and deformation mechanisms at work and determine which of these are primary contributors to the nucleation of defects. Orientation Imaging Microscopy has been the most accessible tool for microstructural analysis over the past 15 years. However, using OIM to analyze defect nucleation sites requires prior knowledge of where the defects will occur because once the defects nucleate the majority of microstructural information is destroyed. This thesis seeks to contribute to the early detection of nucleation sites via three mechanisms: 1. Detection of cracks that have already nucleated, 2. Detection of surface topography changes that may indicate imminent nucleation and 3. Beam control strategies for efficiently finding areas of interest in a scan. Successive in-situ OIM scans of a consistent sample region while strain is increased, while using the three techniques developed in this thesis, will be employed in future work to provide a powerful defect analysis tool. By analyzing retrieved EBSD patterns we are able to locate defect / crack sites via shadowing on the EBSD patterns. Furthermore, topographical features (and potentially regions of surface roughening) can be detected via changes in intensity metrics and image quality. Topographical gradients are currently only detectable in line with the beam incidence. It is therefore suggested that the tensile specimens to be examined are orientated such that the resulting shear bands occur preferentially to this direction. The ability to refine the scan around these areas of interest has been demonstrated via an off-line adaptive scan routine that is implemented via the custom scan tool. A first attempt at a defect detection framework has been outlined and coded into MATLAB. These tools offer a first step to accessing the information about defect nucleation that researchers are currently seeking.

Quantum dots are semiconductor nanocrystals that have tunable emission through changes in their size. Producing bright, efficient quantum dots with stable fluorescence is important for using them in applications in lighting, photovoltaics, and biological imaging. This study aimed to optimize the process for coating CdSe quantum dots (which are colloidally suspended in octadecene) with a ZnS shell through the pyrolysis of organometallic precursors to increase their fluorescence and stability. This process was optimized by determining the ZnS shell thickness between 0.53 and 5.47 monolayers and the Zn:S ratio in the precursor solution between 0.23:1 and 1.6:1 that maximized the relative photoluminescence quantum yield (PLQY) while maintaining a small size dispersion and minimizing the shift in the center wavelength (CWL) of the fluorescence curve. The process that was developed introduced a greater amount of control in the coating procedure than previously available at Cal Poly. Quantum yield was observed to increase with increasing shell thickness until 3 monolayers, after which quantum yield decreased and the likelihood of flocculation of the colloid increased. The quantum yield also increased with increasing Zn:S ratio, possibly indicating that zinc atoms may substitute for missing cadmium atoms at the CdSe surface. The full-width at half-maximum (FWHM) of the fluorescence spectrum did not change more than ±5 nm due to the coating process, indicating that a small size dispersion was maintained. The center wavelength (CWL) of the fluorescence spectrum red shifted less than 35 nm on average, with CWL shifts tending to decrease with increasing Zn:S ratio and larger CdSe particle size. The highest quantum yield was achieved by using a Zn:S ratio of 1.37:1 in the precursor solution and a ZnS shell thickness of approximately 3 monolayers, which had a red shift of less than 30 nm and a change in FWHM of ±3 nm. Photostability increased with ZnS coating as well. Intense UV irradiation over 12 hours caused dissolution of CdSe samples, while ZnS coated samples flocculated but remained fluorescent. Atomic absorption spectroscopy was investigated as a method for determining the thickness of the ZnS shell, and it was concluded that improved sample preparation techniques, such as further purification and complete removal of unreacted precursors, could make this testing method viable for obtaining quantitative results in conjunction with other methods. However, the ZnS coating process is subject to variations due to factors that were not controlled, such as slight variations in temperature, injection speed, and rate and degree of precursor decomposition, resulting in standard deviations in quantum yield of up to half of the mean and flocculation of some samples, indicating a need for as much process control as possible.