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.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101789 |
| Date | 07 January 2021 |
| Creators | Yang, Ni |
| Contributors | Materials Science and Engineering, Lu, Peizhen, Pickrell, Gary R., Reynolds, William T. Jr., Bortner, Michael J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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