京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24003号 / エネ博第439号 / 新制||エネ||83(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 萩原 理加, 教授 佐川 尚, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Silicon oxycarbide (SiOC) is a unique polymer-derived ceramic (PDC) containing silicon, oxygen, and carbon atoms in the form of an amorphous network structure. The phase separation of SiOC is determined by polymeric precursors, pyrolysis temperatures, and atmosphere, which results in different compositions and microstructures. Because of its unique properties (high thermal stability, corrosion resistance, among others), SiOC has numerous applications in fields such as additive manufacturing, lithium-ion batteries, and advanced optics. In the SiOC system, SiO2 nanoclusters can be removed through the etching process, to create nanopores for increasing the surface area. By introducing the SiO2-forming filler (perhydropolysilazane) into SiOC, more SiO2 nanodomains with an average size of 1.72 nm were generated for an ultrahigh surface area of ~2100 m2/g material. Meanwhile, the distributions of domain wall thickness and pore distribution can be calculated by our modified model, to further understand the pore formation. The formation of porous SiOC ceramics with ultrahigh surface areas is greatly desired in numerous applications. Transition metal-containing SiOC composites have more functional properties over pure SiOC and receive more attention in different areas. High-temperature resistant TiC/SiOC was successfully synthesized by pyrolysis of polysiloxane (PSO) and titanium isopropoxide at 1200-1400 °C in argon. It had the first reported conductivity of >1000 S/m for TiC/SiOC ceramics. Nickel-containing SiOC magnetoceramics with soft ferromagnetism was fabricated from a base PSO with the addition of nickel 2,4‐pentanedionate. The effect of water vapor on the phase evolution of Ni/SiOC composites was studied at different pyrolysis temperatures, and the formation of nickel silicides was suppressed by the effect of water vapor during the pyrolysis. Our investigation showed the catalysts from transition metals induced the generation of metal silicides, silicon carbide, and turbostratic carbon with the catalytic activity corresponding to Fe > Co > Ni, which agrees with the activation energy calculation. Also, the phase separation of SiOC was proved to be predominant than local carbothermal reduction. In addition to these findings, a novel approach was developed through the Gibbs free energy minimization method to predict the phase content in PDCs with transition metal additives. And this work provides useful guidance to fabricate the transition metal-containing SiOCs with the desired phase content. Last, the state-of-the-art 4D-STEM technique, collaborated with Lawrence Berkeley National Laboratory, was applied to SiOC ceramics containing amorphous phase. The results showed that 4D-STEM is a valid approach to characterize the nanostructure of the amorphous phase as well as the crystallites. It solves the problem of analyzing SiOC materials at nanoscale due to the disordered atomic arrangement and properties. / Doctor of Philosophy / With the development of science and technology, some novel ceramics have begun to attract attention and become alternatives, such as polymer-derived ceramics (PDCs), due to more advantages over traditional ceramics. Silicon oxycarbide (SiOC) is the main part of the PDC family and possessing good combined thermophysical and mechanical properties. Highly porous SiOC ceramic has broad applications in the fields of catalyst, filters, and thermal insulation. A novel preparation to synthesize SiOC with a specific surface area above 2000 m2/g was investigated. Adding transition metals into the SiOC system can enlarge its application potentials to some extent. The bright spot of nickel-containing SiOC (Ni/SiOC) composites is in the magnetic area. Ni/SiOC composites show soft ferromagnetism and can be used as magnetic sensors, transformers, and so on. In this dissertation, the effect of water vapor on the phase evolution of Ni/SiOC was illustrated. The fabrication of high-temperature-resistant Ti/SiOC composite with large than 1000 S/m conductivity was studied. To further uncover the influence of transition metals on SiOC ceramics, the effects of transition metals on the phase and microstructure evolution of polysiloxane-derived SiOC ceramics were deeply demonstrated. A novel method was even developed to predict the phase content in SiOC ceramic with different transition metals. By working with Lawrence Berkeley National Laboratory, the nanoscale structures of SiOC ceramic was studied using state-of-the-art 4D-STEM. The findings of this dissertation shed light on more potential applications for SiOC ceramics in the future.

An important factor in system longevity is service-phase evolvability, which is defined as the ability of a system to physically transform from one configuration to a more desirable configuration while in service. These transformations may or may not be known during the design process, and may or may not be reversible. A study of 210 engineered systems was performed and found that system excess and modularity allow a system to evolve while in service. Building on these observations, this thesis introduces mathematical relationships that map a system's excess and modularity to that system's ability to evolve. These relationships are derived from elastic potential energy theories. The use of the evolvability measure, and other related measures presented herein, are illustrated with simple numerical examples and applied to the design of US Navy nuclear aircraft carriers. Using these relationships, it is shown that the Navy's new Ford-class aircraft carrier is the most evolvable carrier designed to date. Though the evolvability relationships introduced here are generically derived based on excess and modularity, the aircraft carrier example presented considers only the system excess.

<p>Al-Si cast alloys have been in the fore-front of commercial casting application for more than a century. Iron containing intermetallic phases that evolve during the solidification of these alloys play a major role in the resultant mechanical properties and performance of the cast products. Changes in alloy composition and casting parameters significantly alter the evolution of the Al-Si-Fe intermetallic phases. There was a lack of clear understanding of the complex relationships between the solidification parameters and nature intermetallic phases in these alloys. Current thermodynamic model predictions for the nature of these intermetallic phases in the Al corner of the Al-Si-Fe ternary system are strikingly different from the experimental results in this project. Trace levels of Sr (about 0.02wt%) are typically added to the Al-Si commercial alloys to effect a morphological modification of the eutectic phases to improve the properties and performance of the cast products.</p> <p>The nature of the Fe containing intermetallic phases have been characterized as a function of alloy composition (Si, Fe and Sr) and cooling rates during solidification. There was an anomalous evolution of the t₅-Al₈Fe₂Si phase which transformed into the t₆-Al₉Fe₂Si₂ phase during solidification at lower cooling rates and higher Fe concentration in the alloy, alike. Further, Sr addition to these alloys prevented the evolution of the t₅ phase and promoted the evolution of an unidentified Al₅Fe₂Si₃ phase which was noted as k in this dissertation; the k phase also transformed into the t₆ phase at lower cooling rates and higher Fe concentration in the alloy, alike.</p> / Doctor of Philosophy (PhD)

Carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) are attractive reinforcements for lightweight and high strength metal matrix composites due to their excellent mechanical and physical properties. The present work is an attempt towards investigating the effect of CNT and GNP reinforcements on the mechanical properties of nickel matrix composites. The CNT/Ni (dry milled) nanocomposites exhibiting a tensile yield strength of 350 MPa (about two times that of SPS processed monolithic nickel ~ 160 MPa) and an elongation to failure ~ 30%. In contrast, CNT/Ni (molecular level mixed) exhibited substantially higher tensile yield strength (~ 690 MPa) but limited ductility with an elongation to failure ~ 8%. The Ni-1vol%GNP (dry milled) nanocomposite exhibited the best balance of properties in terms of strength and ductility. The enhancement in the tensile strength (i.e. 370 MPa) and substantial ductility (~40%) of Ni-1vol%GNP nanocomposites was achieved due to the combined effects of grain refinement, homogeneous dispersion of GNPs in the nickel matrix, and well-bonded Ni-GNP interface, which effectively transfers stress across metal-GNP interface during tensile deformation. A second emphasis of this work was on the detailed 3D microstructural characterization of a new class of Ni-Ti-C based metal matrix composites, developed using the laser engineered net shaping (LENSTM) process. These composites consist of an in situ formed and homogeneously distributed titanium carbide (TiC) as well as graphite phase reinforcing the nickel matrix. 3D microstructure helps in determining true morphology and spatial distribution of TiC and graphite phase as well as the phase evolution sequence. These Ni-TiC-C composites exhibit excellent tribological properties (low COF), while maintaining a relatively high hardness.