A colloidal system usually refers to when very small particles are suspended within a solution. The study of these systems encompasses a variety of cases including bacteria in ground water, blood cells and platelets in blood plasma, and river silt transport. Taking a look at these kinds of systems using computer simulation can provide a great deal of insight into how they work. Most approaches to date do not look at the details of the system, however, and are specific to given system. In this study a program called OpenFOAM is used as a basis to build a computer simulation tool that is flexible and that provides a detailed look at what is happening with all of the particles within the colloidal solution. This code is run through a series of tests to verify its usefulness.

abstract: Solar energy has become one of the most popular renewable energy in human’s life because of its abundance and environment friendliness. To achieve high solar energy conversion efficiency, it usually requires surfaces to absorb selectivity within one spectral range of interest and reflect strongly over the rest of the spectrum. An economic method is always desired to fabricate spectrally selective surfaces with improved energy conversion efficiency. Colloidal lithography is a recently emerged way of nanofabrication, which has advantages of low-cost and easy operation. In this thesis, aluminum metasurface structures are proposed based on colloidal lithography method. High Frequency Structure Simulator is used to numerically study optical properties and design the aluminum metasurfaces with selective absorption. Simulation results show that proposed aluminum metasurface structure on aluminum oxide thin film and aluminum substrate has a major reflectance dip, whose wavelength is tunable within the near-infrared and visible spectrum with metasurface size. As the metasurface is opaque due to aluminum film, it indicates strong wavelength-selective optical absorption, which is due to the magnetic resonance between the top metasurface and bottom Al film within the aluminum oxide layer. The proposed sample is fabricated based on colloidal lithography method. Monolayer polystyrene particles of 500 nm are successfully prepared and transferred onto silicon substrate. Scanning electron microscope is used to check the surface topography. Aluminum thin film with 20-nm or 50-nm thickness is then deposited on the sample. After monolayer particles are removed, optical properties of samples are measured by micro-scale optical reflectance and transmittance microscope. Measured and simulated reflectance of these samples do not have frequency selective properties and is not sensitive to defects. The next step is to fabricate the Al metasurface on Al_2 O_3 and Al films to experimentally demonstrate the selective absorption predicted from the numerical simulation. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2019

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18522号 / 工博第3914号 / 新制||工||1601(附属図書館) / 31408 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 辻井 敬亘, 教授 金谷 利治, 教授 山子 茂 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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The utilization of membrane treatment for the production of potable water has become more prevalent in today's industry. As drinking water regulations become more stringent this trend is expected to continue. Widespread use is also a result of membrane treatment being the best available treatment in many cases. While membrane treatment is a proven technology that can produce a consistently superior product to conventional treatment methods, membrane fouling and concentrate disposal are issues that drive up the cost of membrane treatment and can effectively eliminate it from consideration as a treatment alternative. This research focused on membrane fouling. A series of filtration experiments were conducted on various membranes to investigate the physical and chemical factors that influence fouling. The effects of both organic and colloidal fouling were explored by conducting research on various commercial membranes and experimental membranes by Saehan Industries, Inc. (Saehan). Saehan's membranes were in various stages of development in their process of creating a more fouling resistant membrane (FRM). Various hydrodynamic and chemical conditions were used to characterize the evolution of the Saehan commercial products to the experimental FRMs. The developmental stage of the membrane tested included analysis of the various trade secret coating techniques termed single, double, and special. A proprietary post-treatment process was also utilized in combination with each of the coating techniques. The developmental membranes were also compared to commercially available FRMs. The existing FRMs showed better fouling resistance than Saehan's commercially available products in high organic surficial groundwater testing. Synthetic colloidal water testing demonstrated the superior performance of the FRMs, but was not acute enough to differentiate the fouling performance within the group of FRMs or Saehan products. Average roughness decreased slightly as coating technique progressed from single to double to special. Post-treatment increased roughness in single coated membranes and reduced the roughness in double and special coated membranes. The relative charge differences in the developmental membranes were exhibited among non post-treated membranes. Post-treatment membranes did not demonstrate relative surface charge differences consistent with the manufacturer. Initial mass transfer coefficient, determined by clean water testing, increased as coating moved from single to double to special. Clean water testing showed increased initial mass transfer coefficient for membranes with post-treatment. Single coated membranes showed the best salt rejection capability among non post-treated membranes. Post-treatment increased selectivity for all membrane coating techniques. The coating effect on fouling potential had an inverse relationship between single coated versus double and special coated membranes. The post-treatment increased fouling resistance for the single coated membranes, but decreased fouling resistance of double and special coated membranes. The SN7 membranes showed the best performance of the developmental membranes.

Ceramic colloidal processing routes such as slip casting, gelcasting and direct ink writing provide valuable insight into the role of interaction forces between particles, solvents, and polymeric additives in the rheology, particle packing, and strength of a ceramic green body. For difficult-to-densify ceramics such as the UHTCs, which find their place in extreme environment applications, precise control of each step of the manufacturing process is key. In this work, a fundamental study on the interaction between particles in non-aqueous slip casting is performed comparing the rheological behavior and consolidation with current models for interaction potential within a suspension. The advantages and drawbacks of such a model are discussed in relation to formulating a colloidal process for advanced ceramics such as ZrB2, and a case for a cyclohexane slip casting system resulting in low viscosity, shear-thinning behavior and green density of 64%, is made. The focus on non-aqueous colloidal processing is extended to gelcasting, involving three different sets of chemically curable polymer systems: HEMA+MBAM, TMPTA, and PEGDA. Merits of the gelcasting process including homogeneity, green strength, and processing time reduction are discussed, with the HEMA+MBAM system resulting in nearly an order of magnitude increase in green density from slip casting. Gelcast samples were also sintered to a density of 88% and capable of being processed in a variety of complex shapes with fine feature size on the mm scale. The properties examined in slip casting and gelcasting, as well as others pertaining to the setup of an extrusion-based additive manufacturing system, are carefully considered to design an ink that has been used to print ZrB2. The role of each additive as well as the solvent in creating an ink that is not only within the correct viscosity range for extrusion and shape retention, but also produces a strong and densely packed green body, is discussed. Finally, adjustment of printing parameters, and the method of using a low-cost rheology match to tune the settings of a pneumatic screw-extrusion printing setup, are explained. Each of these processes points to new and practical methods of complex shaping ZrB2 that can provide insight into processing of these challenging materials and create new avenues for their use in extreme environment applications, such as thermal protection systems in atmospheric re-entry vehicles. / Doctor of Philosophy / This work examines the use of ultra-high temperature ceramics (UHTCs), which are materials with some of the highest melting points in existence. These are an intriguing option for extreme environment applications. One such application is the protection of rockets, scramjets, and other hypersonic (speed > Mach 5) vehicles from the high temperatures experienced during flight and re-entry. In this work, the UHTC Zirconium diboride (ZrB2) is used as a reference material. For many of the same reasons UHTCs such as ZrB2 have extreme melting points, they can be difficult to manufacture, particularly in complex shapes. Like many ceramics, UHTCs are not melted and cast as metals are, but rather are processed in powder form to a compact known as a green body. The green body is placed in a high-temperature furnace at 2/3 - 3/4 of the melting point, where the powder undergoes sintering, or consolidation into a dense part. The manufacture of a green body that is versatile in its capacity to be molded into any shape, and allows for close packing of the particles in the powder compact to avoid failure-inducing flaws in the final component under intense loads, remains a challenge for UHTCs. Most UHTCs are hot pressed, where the powder alone is consolidated under intense heat and pressure, but this process offers very little complex shaping capacity or control of the uniformity of the part. In this work, three methods for green body manufacture using colloid-based routes, which all have unique capabilities and challenges, are described. The first process is slip casting, which is a centuries-old process that has been used for the manufacture of pottery, whitewares, and art ceramics. When used effectively, slip casting ensures that the forces between ceramic particles in a suspension, or "slip", are well-controlled such that the ceramic particles will not form clumps, or agglomerates, which create non-uniformities that weaken the final component. With information about the powder, solvent, and additives in a slip, the extent to which this will be effective can be predicted with mathematical models. This work compares the results of these models with slip casting suspensions in different solvent environments to gain knowledge about slip casting as an option for complex shaping of ZrB2. The second colloidal process discussed is gelcasting, in which the suspension of ceramic powder can undergo chemical gelation, or a reaction that transitions the suspension from a liquid to a solid, not unlike that of a natural gel such as gelatin, agarose, or albumin (egg white). The gel, which is loaded with ceramic powder, allows for more versatile shaping than slip casting, and shorter processing time; a gelcast ceramic is generally solidified in less than an hour, while a slip cast typically dries overnight. The presence the gel also provides strength to the green body, which is advantageous in handling as well as any machining to adjust the shape that may be necessary prior to sintering. The final process detailed in this work is direct ink writing, a type of additive manufacturing (or 3D printing). Knowledge gained from slip casting and gelcasting was used to carefully design a ceramic colloid that could be deposited in a layer-by-layer fashion to create a complex shape with high uniformity and control, as well as minimal surface cracking. The printed green bodies were compared in strength and sintering behavior to the gelcasts from previous chapters, and the expansion of shaping capacity for each route as it relates to aerospace applications, is described.

The texturing of ultra-high temperature ceramics (UHTCs) using weak magnetic fields is studied and developed for the first time. Textured UHTCs were prepared by magnetically assisted slip casting (MASC) in weak magnetic field (B ~ 0.5 T). Analytical calculations describing the balance of torques acting on the suspended particles suggested that texture would form at such low magnetic fields. The calculations include a novel contribution of Stokes drag arising from the inhomogeneous velocity profile of the fluid during slip casting. Experimental proof-of-concept of the theoretical calculations was successfully demonstrated. Calculations of Lotgering orientation factor (LOF) based on the intensities of the (00l) family of peaks measures by XRD revealed strong c-axis crystalline texture in TiB2 (LOF = 0.88) and ZrB2 (LOF = 0.79) along the direction of the magnetic field. Less texture was achieved in HfB2 (LOF = 0.39). In all cases, the density of the textured materials was less than that of control untextured materials, indicating that texturing hindered the densification. The findings from this work confirm the potential for more cost-effective, simple, and flexible processes to develop crystalline texture in UHTCs and other advanced ceramics and give new insight into the mechanisms of magnetic alignment of UHTCs under low magnetic fields. The microstructural evolution during slip casting and pressureless sintering is investigated. The interplay between magnetic alignment and particle packing was investigated using XRD and SEM. During MASC, the suspended particles rotate into their aligned configuration. Particles that deposit at the bottom of the mold near the plaster of Paris substrate have their alignment slightly disrupted over a ~220 μm-thick region. The aligned suspended particles lock into an aligned configuration as they consolidate, leading to a uniform degree of texturing across the entire sample height of several millimeters upon full consolidation of the particle network. If the magnetic field is removed before the particles fully consolidate, the suspended particles re-randomize their orientation. Grain size measurements done using the ASTM E112 line counting method on SEM images revealed anisotropic microstructures in green and sintered textured ZrB2 materials. Smaller effective grain sizes were observed in the direction of c-axis texture than the directions perpendicular to the texture. Grain aspect ratios of 1.20 and 1.13 were observed in materials where the c-axis texture directions were parallel (PAR) and perpendicular (PERP) to the slip casting direction, respectively. Constraint of the preferred a-axis grain growth direction in the textured materials inhibited their densification compared to the untextured material. The PERP material with the preferred grain growth direction constrained along the casting direction had smaller average grain sizes than the PAR material which contained the preferred grain growth directions in the circular plane normal to the casting direction. Compression testing suggests a trend towards higher strength and stiffness in materials with higher density. Classical catastrophic brittle failure was observed in the untextured materials, but in the textured materials some samples exhibited a multiple failure mode. The PERP material tended to exhibit superior strength and stiffness to the PAR material in the classical brittle failure mode due to the orientation of the stiffer a-axis along the loading direction and smaller average grain size in the plane normal to the loading direction in the PERP condition. In the multiple failure mode, the PAR material tended to reach higher strength values after the initial failure and reach slightly higher strains before ultimate failure due to the orientation of the compliant c-axis along the loading direction and ability of the grains elongated in the plane normal to the loading direction to rearrange themselves after initial failure(s). Regardless of density or texture condition, all ZrB2 samples survived thermal shock resistance (TSR) testing. Samples were heated to 1500°C in air, held for 30 minutes, then quenched in room temperature air. After TSR testing, oxide layers formed on the surface of the materials. The specific mass gain and oxide layer thickness tended to increase with increasing porosity and were dramatically increased when open porosity was dominant as in the CTRL 1900 condition. After TSR testing, the compressive strength and strain at failure were both higher compared to the as-sintered materials. The increases in the average compressive strength were 20%, 76%, and 57% in the CTRL, PAR, and PERP conditions, respectively. The combination of the presence of the oxide layer shifting the onset of macroscale damage to higher strain values, the dissipation of load in the more porous region near the oxide layer, and the constraining effect of the oxide layer acting against the expansion of the material contributed to reinforcement of the samples after TSR testing. The CTRL material outperformed the textured materials on average in terms of strength and stiffness due to the higher density. The results suggest that reinforcement was more effective in the PAR condition than the PERP, which may be caused by the formation of a homogenous oxide layer on the PAR while the PERP formed an anisotropic layer. The work presented in this dissertation lays the foundation for affordable, energy efficient preparation of UHTCs and other ceramic materials. Equipment costs are reduced by 3 orders of magnitude, and the operating costs and energy consumption are greatly reduced. Facilitation of the preparation of textured materials opens the door to renewed investigations into their processing and performance. This work describes in detail for the first time the relationships between processing, microstructure, and properties of a textured UHTC part, providing a model for future research. Finally, the findings in this work can be used to guide process optimization, exploration of complex shapes and microstructures, and design of manufacturing schemes to create specialty textured parts for demanding structural and functional applications. / Doctor of Philosophy / Textured ultra-high temperature ceramics (UHTCs), special materials with melting temperatures above 3000°C and potential for use in thermal protection of Mach 5+ aircraft and spacecraft, were prepared by magnetically assisted slip casting (MASC) in a weak magnetic field for the first time. The magnetic field was supplied by commercially available permanent magnets which was applied to a liquid-like slurry with UHTC particles floating in it to orient the UHTC particles with their c-crystal axis along the magnetic field direction. Calculations which described the balance of rotational forces acting to align or misalign the suspended particles suggested that the UHTC particles would align in the weak magnetic field. This prediction was realized. After the liquid in the slurry was removed during MASC to leave behind an aligned particle network, the samples were densified by heating in the absence of air to 2100°C for one hour. In titanium diboride (TiB2) and zirconium diboride (ZrB2), two of the most relevant UHTC materials, strong texture was achieved; 88% and 79% of the crystals in the material were aligned along the original magnetic field direction. This is the first time that this has been reported in the scientific literature. In hafnium diboride (HfB2), only 39% of the grains were aligned. The textured materials all had lower density than the untextured materials prepared alongside them using conventional slip casting. The relationship between magnetic alignment and particle packing was investigated by observing the microstructure. During MASC, the suspended particles rotate into their aligned configuration. Particles that deposit at the bottom of the mold near the plaster of Paris substrate have their alignment slightly disrupted over a ~220 μm-thick region. The aligned suspended particles lock into an aligned configuration as they consolidate, leading to a uniform degree of texturing over across the entire sample height of several millimeters upon full consolidation of the particle network. If the magnetic field is removed before the particles fully consolidate, the suspended particles re-randomize their orientation. The findings from this work confirm the potential for more cost-effective, simple, and flexible processes to develop crystalline texture in UHTCs and other advanced ceramics and give new insight into the mechanisms of magnetic alignment of UHTCs under low magnetic fields. Because of the magnetic alignment of the particles, it is expected that the microstructure would show some difference along and across the direction that the alignment formed along the applied magnetic field. In order to determine that, the size of the grains (particles joined to each other during densification) in the materials are measured along different directions in the sample chosen for their orientational relationship to the magnetic field and casting directions. Smaller effective grain sizes were observed along the direction of magnetically aligned crystalline texture than the directions perpendicular to the texture. Because of how the crystal axes of the particles are aligned, there are differences in how the particles join each other during densification, and that results in an anisotropic microstructure where different grain sizes as a function of the magnetic field direction and the texture direction. Compression testing conducted by squeezing the samples at a fixed rate suggests a trend that indicates the samples are stronger and stiffer when the density is higher, as expected. Untextured samples abruptly failed after reaching their maximum strength value in a manner typical of brittle ceramics. Some textured samples failed in this way, but some failed at low strength values then climbed back up in strength repeatedly until they eventually gave out completely, in a crumbly mode. In the classical brittle failure mode, the PERP material with c-axis texture aligned along the sample diameter, perpendicular to the loading direction, tended to exhibit superior strength and stiffness to the PAR material with c-axis texture oriented along the height and loading directions of the sample because the stiffer crystal axis was oriented along the loading direction and the average grain size seen by the load head was smaller. In the crumbly mode, the PAR material tended to reach higher strength values after initial failure and ultimately fail later in a crumblier mode because the more compliant crystal axis was oriented along the loading direction and the grains elongated in the plane perpendicular to the loading direction could rearrange themselves better after initial failure(s) to bear more load. Regardless of density or texture condition, all ZrB2 samples survived thermal shock resistance (TSR) testing, meaning that the samples remained fully intact after experiencing a big difference in temperature in very short time. Samples were heated in a furnace to 1500°C in air, held for 30 minutes, removed from the furnace, and cooled in air. After TSR testing, the samples developed an oxide layer on the outside, in a similar manner to rust forming on a piece of metal. How much it oxidized per unit area and how thick that oxide layer was increased with increasing porosity. These quantities increased dramatically when the pores connected the interior of the sample to the outside, as in the CTRL 1900 condition. After TSR testing, the samples were stronger by 20%, 76%, and 57% in the CTRL, PAR, and PERP conditions, respectively, indicating that the oxide layer was responsible for an enhancement in strength. The results suggest that increase of strength of the oxide layer was more effective in the PAR condition than the PERP, which is believed to be caused by the formation of a homogenous oxide layer on the PAR while the PERP formed an anisotropic layer. The work presented in this dissertation reduces the start-up equipment costs associated with magnetic alignment processes by 1000 times and lays the foundation for affordable, energy efficient preparation of UHTCs and other ceramic materials. The simplicity of this technique makes it easier for future researchers to study textured materials. This work describes in detail for the first time the relationships between processing, microstructure, and properties of a textured UHTC part, providing a model for future research. Finally, the findings in this work can be used to guide process optimization, exploration of complex shapes and microstructures, and design of manufacturing schemes to create specialty textured parts for demanding applications.

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