This study is focus on solute concentration ahead of solidification front and base on holographic method by using holographic interferometry. Heterogeneous nucleation of bubbles on an advancing solidification front during freezing of water containing a dissolved gas has been experimentally and analytically studied. The formation of bubbles resulting from supersaturation of liquids is commonly encountered in different fields such as heat transfer, manufacturing, and bioscience.

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The industrial use of recycled aluminium is greatly limited by the degraded mechanical properties due to the increased impurities. Fe, one of the common impurity content in Al alloys, is difficult to eliminate once introduced into aluminium during primary production or recycling processes. Due to the low solid solubility of Fe in Al, the formation of Fe-rich intermetallic compounds (Fe-IMCs) is inevitable, which is one of the main causes for the deterioration of mechanical properties in various cast Al alloys. In order to obtain desirable mechanical properties of recycled Al alloys, modification and refinement of the Fe-IMCs are urgently required as the compact and refined morphologies of such intermetallics are generally non detrimental to Al alloy's performance. However, manipulating the solidification behaviour of the Fe-IMCs phases, including nucleation and growth, is very challenging because of the inherently more difficult heterogeneous nucleation of the Fe-IMCs compared with that of a pure metal or a solid solution; and the strong growth anisotropy. Limited understanding on mechanisms of nucleation and growth of the multicomponent Fe-IMCs is available in the literature. The aim of this study is to gain a deeper understanding on the heterogeneous nucleation and growth behaviour of Fe-IMCs in various Al alloys. The nucleation and growth of both primary and eutectic Fe-IMCs have been investigated during various solidification conditions including a number of different cooling rates and casting temperatures. Based on the experimental results of the solidification of several ternary and quaternary alloys, effect of Mg on the solidification behaviour of Fe-IMCs was investigated. Further the surface modified TiB2 particles were used to enhance the heterogeneous nucleation of Fe-IMCs in order to refine the Fe-IMCs particles. The dominant Fe-IMC in Al-5Mg-2Si-1.2Fe-0.7Mn alloy is identified, using transmission electron microscopy (TEM), as α-AlFeMnSi with a body centred cubic (BCC) lattice structure and lattice parameter of 1.256nm. In the current alloy system, the nucleation of primary α-AlFeMnSi occur at lower cooling rate (≤0.8K/s) when required nucleation undercooling is reached, as the slower cooling rate allows longer diffusion time for the solute to form a stable nucleation embryo. When casting with 20K superheat, the size of primary α-AlFeMnSi increases gradually from 24.5±3.1μm (870K/s) to 251.3±75.3μm (0.02K/s) and the size of α-AlFeMnSi eutectic increased gradually from 102.0μm (870K/s) to 623.3μm (0.02K/s). The Fe and Mn concentration in α-AlFeMnSi appears to reduce with the increased cooling rate due to the relatively insufficient solute supply when solute concentration is low (1.2wt.% Fe and 0.7wt.% Mn). Microstructure observation reveals that the {011} plane, especially on <111> orientation, is the preferred growth orientation of BCC primary α-AlFeMnSi, resulting in rhombic dodecahedral in 3D. The eutectic α-AlFeMnSi, prefers to initiate on the primary α-AlFeMnSi. In addition to the substantial nucleation undercooling, the research revealed that the nucleation of primary α-AlFeMnSi also rely on the local solute concentration and the solute diffusion. Compared with α-Al, the growth of α-AlFeMnSi is less sensitive to the cooling rate changes due to the complexities in multi-components interaction and different diffusion efficiency of different elements. The addition of Mg to Al-1.2Fe-0.7Mn and Al-2Si-1.2Fe-0.7Mn alloys was found to lead to a morphology change of Fe-IMCs. Al6(Fe,Mn), the predominant Fe-IMC in the Al-1.2Fe-0.7Mn-xMg alloy, changed from needle morphology to interconnected lamellar morphology when Mg composition increased from 0.004wt.% to 6.04wt%. A Mg-rich layer at about 5-20nm in thickness was commonly observed on the Fe-IMC/α-Al interface in the alloys with Mg content. The eutectic lamellar spacing for Al6(Fe,Mn) increases from 1.8±0.3μm to 4.5±0.8μm when Mg content increased from 0.004wt.% to 6.04wt.%. In the case of α-Al12(Fe,Mn)3Si, the predominant Fe-IMC in Al-2Si-1.2Fe-0.7Mn-yMg alloys, its lamellar spacing of the eutectic increased from 1.4±0.3μm to 3.25±0.8μm when Mg increased from 0.04wt.% to 5.41wt.%. Owing to the strong anisotropy of the Fe-IMC crystals, the segregation of solute Mg on preferred growth orientation is higher, causing greater growth restriction on this orientation. Consequently, the growth velocity on other orientations becomes relatively more significant. To optimise the morphology of Fe-IMCs in Al alloys, a novel Αl-Ti-B(Fe) grain refiner for Fe-IMCs has been developed to enhance the heterogeneous nucleation of Fe-IMCs. The addition of the novel grain refiner to an Al-5Mg-2Si-1.2Fe-0.7Mn alloy under controlled solidification condition results in a considerable refinement of the primary Fe-IMCs from 251.3±75.3μm to 110.9±45.5μm and from 127.3±36.2μm to 76.5±18.2μm at cooling rates of 0.02K/s and 0.15K/s, respectively. TEM investigations on the refiner reveal a Fe-rich adsorption monolayer in a zigzag fashion on the prismatic planes on the boride particles. This surface modification is beneficial for the heterogeneous nucleation of the Fe-IMCs. Further investigation of the Al alloy with this grain refiner addition revealed that there existed specific orientation relationships (ORs) between TiB2 and Fe-IMCs: (001)[020]Al13Fe4 // (11-20)[10-10]TiB2, and (001)[120]Al13Fe4 ∠6.05˚ (11-20)[10-11]TiB2; (0-11)[100]α-AlFeMnSi // (0001)[-2110]TiB2, and (0-11)[111]α-AlFeMnSi ∠4.5˚ (0001)[10-10]TiB2. The Fe adsorption on substrate particle, the observed ORs between TiB2 and Fe-IMCs, and the refinement of primary α-AlFeMnSi with the addition of modified TiB2 provide evidence of structure templating and composition templating required by heterogeneous nucleation of Fe-IMCs. This research has delivered contribution to the understanding and new approach for optimizing the morphology of Fe-IMCs in the Fe-containing Al alloys. Using the slow cooling rates (≤0.15K/s), the formation compact primary α-AlFeMnSi can be considerably encouraged. With a lower casting temperature, the size and volume fraction of large Chinese-script α-AlFeMnSi can be significantly reduced. With addition of reasonable Mg content the morphology of Fe-IMC can be modified. Particularly, with the addition of the Al-Ti-B(Fe) grain refiner in well-controlled condition, the primary α-AlFeMnSi can be significantly refined. Thus, by implementing these approaches, the optimized Fe-IMC morphology in the microstructure of Fe-containing Al alloy is able to offer promising mechanical performance.

Nucleation of ice crystals in the atmosphere often occurs through heterogeneous freezing processes facilitated by an atmospheric aerosol that acts as the ice nuclei (IN). Depending on ambient conditions and aerosol composition, heterogeneous nucleation will occur through one of several mechanisms including the contact and immersion freezing mechanisms. Through a series of contact freezing experiments, we have characterized the ability of aerosols composed of volcanic ash, soot, and peat soil, to act as ice nuclei (IN) as a function of temperature. The immersion freezing ability of the ash particles has also been measured. In these studies, an optical microscope apparatus equipped with a cooling stage and a digital camera was used to observe the freezing events. For each experiment, a particular IN was placed in contact with the surface, or immersed in the bulk, of an ultra pure water droplet. The droplet was then subjected to freezing-melting cycles resulting in 25 independent measurements of the freezing temperature of the droplet. In the volcanic ash experiments, we observed contact freezing at warmer temperatures than immersion freezing. As contact freezing IN, the peat was the most effective with an average contact freezing temperature of -10.5 �C, followed by volcanic ash (-11.2 �C), and then soot (-25.6 �C). In addition, we have used classical nucleation theory to identify the contact parameters and nucleation rates for the compositions explored.

Aluminium applications can be found in the vast majority of industries – particularly the automotive, aerospace and building sectors. Light weight, good corrosion resistance, high strength with good machining and weldability has led 6xxx series alloy to be the most widely used for extrusion products. Semi-continuous direct-chill (DC) casting is a well established process and the most widely used in the production of wrought aluminium extrusion billets. The techniques have continuously evolved since its invention in the 1930s. To ensure high productivity and a quality billet by DC casting, grain refiners are added during casting prior to solidification. It is efficient, cost effective and considered optimized in modern production techniques. However, some problems still persist, for example, macrosegregation, centerline cracking, porosity, hot tearing, etc. For surface finish critical products, particles in added grain refiners may cause surface defects during downstream processing. Additions of grain refiners are also not desirable for recycling of the end use products. As a novel DC casting technology, the melt conditioned DC casting (MC-DC) technology is developed to achieve uniform fine equiaxed grains without deliberate additions of grain refiners. The MC-DC process is implemented by submerging a rotor-stator high shear device into the mould assembly of a conventional hot-top vertical DC caster. In this work, the fundamentals of MC-DC process has been investigated by studying the flow patterns in the sump using computer modelling in combination with thermal field measurement and delineation of the sump profile. Followed is the microstructural evolution of the MC-DC castings. Then the formation of Fe-bearing intermetallics which are critical to the arrangement of homogenisation treatment are presented. The grain refining mechanism by MC-DC is due to enhanced heterogeneous nucleation on dispersed oxides and grain fragments by intensive melt shearing, in combination with dendrite fragmentation and transportation in a uniform temperature and solute field. By optimising MC-DC parameters, alleviation of macrosegregation can be achieved even compared with DC-GR castings. Another finding is the correlation between grain structure and the distribution of the Fe-intermetallic particles. It has been demonstrated that equiaxed dendritic grains with fine secondary dendritic arm spacings achieved in MC-DC are preferred rather than finer granular grains in grain refined material. MC-DC also promotes the formation of α- Fe-bearing intermetallics. All these offer the potential for the reduction of homogenisation practices currently required as part of the DC process.

(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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The heterogeneous condensation of a vapour onto a substrate is a key step in a wide range of chemical and physical process that occur in both nature and technology. For example, dust and pollutant aerosol particles, ranging in size from several microns down to just a few nanometers, serve as cloud condensation nuclei in the atmosphere, and nanoscale structured surfaces provide templates for the controlled nucleation and growth of variety of complex materials. While much is known about the general features of heterogeneous nucleation onto macroscopic surfaces, much less is understood about both the dynamics and thermodynamics of nucleation involving nanoscale heterogeneities. The goal of this thesis is to understand the general features of condensation of vapours onto different types of nanoscale heterogeneity that range in degree of solubility from being insoluble, to partially miscible through to completely miscible. The heterogeneous condensation of the Lennard-Jones vapour onto an insoluble nanoscale seed particle is studied using a combination of molecular dynamics simulations and thermodynamic theory. The nucleation rate and free energy barrier are calculated from molecular dynamics using the mean first passage time method. These results show that the presence of a weakly interacting seed has no effect on the formation of small cluster embryos but accelerates the rate by lowering the free energy barrier of the larger clusters. A simple phenomenological model of film formation on a small seed is developed by extending the capillarity based liquid drop model. It captures the general features of heterogeneous nucleation, but a comparison with the simulation results show that the model significantly overestimates the height of the nucleation barrier while providing good estimates of the critical film size. A non-volatile liquid drop model that accounts for solution non-ideality is developed to describe the thermodynamics of partially miscible and fully miscible droplets in a solvent vapour. The model shows ideal solution drops dissolve always spontaneously, but partially miscible drops exhibit a free energy surface with two minima, associated with a partially dissolved drop and a fully dissolved drop, separated by a free energy barrier. The solubility transition between the two drops is shown to follow a hysteresis loop as a function of system volume similar to that observed in deliquescence. A simple lattice gas model describing the absorption of mono-layers of vapour onto the particle is also developed. Finally, molecular dynamics simulation of miscible and partially miscible binary Lennard-Jones mixtures are also used to study this system. For all cases studied, condensation onto the drop occurs spontaneously. Sub-monolayers of the solvent phase form when the system volume is large. At smaller system volumes, complete film formation is observed and the dynamics of film growth are dominated by cluster-cluster coalescence. Some degree of mixing into the core of the particle is observed for the miscible mixtures for all volumes. However, mixing of the solvent into the particle core only occurs below an onset volume for the partially miscible case, suggesting the presence of a solubility transition similar to the one described by the thermodynamic model.

Microstructure refinement was observed in the fusion zone of AZ31 magnesium (Mg) alloy resistance spot welds when an inoculant was added, either Ti, Al8Mn5, or Mn. The dependence of inoculant potency on the lattice disregistry between inoculants and matrix, and on the liquid cooling rate was studied. Microstructural characterization was performed via optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thin foils containing the interface of the inoculant particles and Mg matrix were prepared using a focused ion beam (FIB) technique. Columnar dendritic structures in the vicinity of the fusion boundary and equiaxed dendritic structures in the central area were observed in the fusion zone of welds in the SA and SB AZ31Mg alloys from different suppliers. However, the columnar dendritic zone (CDZ) was well restricted, and the width of the CDZ and the diameters of equiaxed dendrites were much smaller in the SA alloy than those in the SB alloy due to the earlier columnar-equiaxed-transition (CET) in the SA alloy. The refined microstructure in the fusion zone of the SA alloy welds is attributed to the pre-existence of the larger Al8Mn5 particles of 4-10 microns in length in the SA alloy which act as an inoculant for alpha-Mg heterogeneous nucleation. Fatigue life and dislocation substructure were compared between the SA and SB welds. The SA welds with the refined microstructure displayed an enhanced fatigue resistance compared to the SB welds, when the interfacial failure took place across the fusion zone. The increased number and dispersion of slip systems in the fine-grained SA welds contributed to the improvement of fatigue life. The well-developed columnar dendritic grains were successfully restricted and the coarse equiaxed dendritic grains were efficiently refined by intentionally adding Ti or Mn inoculant particles into the as-received SB alloy welds. The Ti and Mn particles of about 8μm diameter were observed to promote the nucleation of alpha-Mg grains during welding. TEM examinations showed the existence of local orientation relationships between the respective inoculants Ti, Mn, and Al8Mn5 with the Mg matrix. The further lattice matching was observed between the Al8Mn5 particles and Mg. The diameter of the added inoculant should be larger than 1.8 microns to make it a potent inoculant based on the thermodynamic calculation. Microstructural examinations of samples with different inoculant additions and under different cooling rates showed that the inoculant potency was high for the Ti inoculant, medium for the Al8Mn5, but low for the Mn, when the cooling rate was low. This order in the decrease of grain refinement efficiency is inversely proportional with the order of crystallographic lattice disregistry between inoculants and matrix, which is calculated based on a crystallographic matching model. This implies that the lattice disregistry determines the potency of inoculants at the low cooling rates. In comparison, the lattice disregistry did not influence the heterogeneous nucleation, when the cooling rate was high. It could be inferred that an extremely high cooling rate produces a large supercooling, and provides a sufficient driving force for heterogeneous nucleation.

Cement clinker production is a highly resource and energy intensive process and contributes substantially to annual global anthropogenic greenhouse gas emissions. One potential pathway to reduce the environmental footprint of cement-based materials is through the reduction of clinker content in concrete by partial replacement of cement with fillers. In this investigation, the partial replacement of cement with chemically inert nano and microsized fillers of titanium dioxide (TiO₂) and limestone was examined. The effects of nano and micro fillers on early-age properties, long-term properties, photocatalytic properties (for TiO₂-cement mixtures) and life cycle costs were measured and compared. Investigation of early-age properties shows that nanoparticles increase rate and degree of early cement hydration and chemical shrinkage due to heterogeneous nucleation effect. In contrast, coarser microparticles (>3µm in this research) maintain or marginally decrease the rate and degree of early cement hydration and decrease chemical shrinkage due to a dilution effect. In addition, temperature sensitivity of hydration reactions increases in the presence of nanoparticles. Investigation of long-term properties shows that pore size refinement is possible with the partial replacement of cement with nanoparticle fillers. But the long-term tests of filler-cement mixes also demonstrate that, compared to ordinary portland cement mix, the strength decreases and permeability increases. Analysis of photocatalytic properties of TiO₂-cement mixtures showed a lack of an appropriate testing procedure for nitrogen oxide (NOₓ) gas conversion by cement-based materials. Thus, a new standardized procedure and photocatalytic efficiency factor for characterizing photocatalytic NOₓ binding by cementitious materials is proposed. Life cycle analysis demonstrates that although inclusion of TiO₂ increases initial environmental impact of cementitious materials, the innovative photocatalytic properties of TiO₂ could improve sustainability. Life cycle analysis also shows that partial replacement of cement with limestone decreases environmental impact of cementitious mixtures due to lower processing “costs” of limestone compared to cement. Thus, the results from the current research demonstrate that variation of dosage and particle size of inert fillers can be used to tailor properties and structure of cement-based materials and that environmental sustainability can be improved by partial replacement of cement with inert fillers that introduce additional functionalities or fillers with lower embodied-energy and emissions.