Dong-Geun Shin’s research while affiliated with Korea Institute of Ceramic Engineering and Technology and other places

The polymer-derived ceramics (PDCs) technique enables relatively low-temperature fabrication of Si-based ceramics, with silicon carbide fiber as a representative product. Polycarbosilane (PCS) has Si-C backbone structures and can be converted to silicon carbide. In the PDCs method, residual or excess carbon is generated from the precursor (C/Si ratio = 2 for polycarbosilane). Because of the non-stoichiometry of SiC, the physicochemical properties of polymer-derived SiC are inferior to those of conventional monolithic SiC. Herein, a silicon carbide fiber-hafnium carbide nanocomposite fiber was optimized by crosslinking oxygen into the PCS fiber by regulating the oxidation curing time. During pyrolysis, carbothermal reduction, and sintering, carbon was removed by reaction with hydrogen and cross-linked oxygen. Non-destructive techniques (X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and high-temperature thermomechanical analysis) were used to investigate the effects of excess carbon. The microstructure of the near-stoichiometric SiC-HfC nanocomposite fiber was more densified, with superior high-temperature properties.

Silicon carbide fiber is a representative structural material that has been studied as a major reinforcement material for ceramic matrix composites (CMCs). SiC fibers are classified by their operating temperature, manufacture process, composition, and crystallinity. Among the various SiC fibers, amorphous ones are composed of a Si–O–C amorphous matrix and nanoscale SiC crystals, and amorphous fibers exhibit amorphous fracture behavior. An amorphous fiber is almost always fractured by surface flaws generated during the manufacturing process. In this study, we aimed to strengthen the surface by etching the surface cracks and forming a SiO2 oxidation layer, and then evaluated the effect of the oxide layer thickness on the fracture behavior and control of surface defects. The passive oxidation layer was formed by exposing the fiber bundles to air atmosphere at a temperature higher than 1000 °C. The mechanical properties of the strengthened fibers were evaluated through single-filament tensile strength tests, and the fracture distribution was analyzed using Weibull distribution. Further, fractographic analyses were conducted on the cross-sectional microstructures observed by field-emission scanning electron microscopy. The surface-oxidized fibers exhibited enhanced flexibility owing to the crack-blunting effect of the SiO2 layer.

Silicon carbide fibers are used in silicon carbide fiber reinforced silicon carbide composite (SiCf/SiC) as reinforcement materials because of their outstanding mechanical properties, high-temperature durability and oxidation resistance. Owing to its excellent high-temperature performance, SiCf/SiC finds extensive application in materials exposed to extreme environments for long periods, such as nuclear fusion reactors, gas turbines, and rocket nozzles. Transition metal-based carbides, in particular hafnium carbide, have attracted considerable attention owing to their high melting point, high self-diffusion coefficient and chemical bonding energy. This study aims to improve the high-temperature creep characteristics of polycrystalline silicon carbide fibers by dispersing hafnium carbide nanocrystals in crystalline silicon carbide fibers and the changes in the microstructure, crystal phase and crystallite size were observed under various sintering conditions by different experiments. The results confirmed that hafnium carbide was uniformly dispersed, and that the crystal size grew to approximately 40 nm at a sintering temperature of 2000 °C. For high-temperature property evaluation, bend stress relaxation tests were conducted at temperatures in the range of 1000–1500 °C, and the effects of hafnium carbide were compared with those of other single silicon carbide fibers.

The polymer-derived SiC fibers are mainly used as reinforcing materials for ceramic matrix composites (CMCs) because of their excellent mechanical properties at high temperature. However, decomposition reactions such as release of SiO and CO gases and the formation of pores proceed above 1400 °C because of impurities introduced during the curing process. In this study, polycrystalline SiC fibers were fabricated by applying iodine-curing method and using controlled pyrolysis conditions to investigate crystallization and densification behavior. Oxygen and iodine impurities in amorphous SiC fibers were reduced without pores by diffusion and release to the fiber surface depending on the pyrolysis time. In addition, the reduction of the impurity content had a positive effect on the densification and crystallization of polymer-derived SiC fibers without a sintering aid above the sintering temperature. Consequently, dense Si-Al-C-O polycrystalline fibers containing β-SiC crystal grains of 50~100 nm were easily fabricated through the blending method and controlled pyrolysis conditions.

TaC, which is an ultra-high-temperature structural material, was derived from two types of organometallic precursors: Me3CCH = Ta(CH2CMe3)3 and Cp*-TaMe4. Both compounds are kinds of single-source precursors composed only of tantalum, carbon, and hydrogen, which should be converted into carbide material without oxygen contamination. The carbide material was formed through several processes such as pyrolysis, nucleation, and crystal growth, which are dependent on heat treatment temperature. However, organometallic precursor usually leaves residual carbon, and the crystallization of carbide is affected by the carbon. In this study, two types of organometallic precursors containing different amounts of carbon in the organic part were used to prepare TaC, and the crystallization behavior of TaC in rich carbon was investigated. The amount of residual carbon was estimated by thermogravimetry (TG) analysis, and the TaC crystals were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nano-sized TaC particles were obtained by heat treatment without any by-products but with excess carbon. The TaC crystal was formed starting from 1300 °C, but the crystals grew better with less carbon, while the carbon barrier prevented agglomeration of the atoms.

By iodine curing of polycarbosilane fibers followed by sintering under a controlled atmosphere of carbon monoxide, a unique strategy is developed for the in situ growth of graphene networks inside silicon carbide fibers. In the resulting fibers, three-dimensionally interconnected few-layered graphene sheets are well-dispersed in the nanocrystalline SiC, allowing for fast electron transport through the graphene networks. The roles of iodine and carbon monoxide in fabricating the graphene-network embedded SiC fibers are elucidated. The distinct evolution of graphene structure was observed in the iodine-treated Si(O)C using transmission electron microscopy and Raman spectroscopy. The iodine incorporated in the fibers induces the sp²-hybridization of carbon, generating carbon–carbon double bonds and graphene seeds such as reduced graphene oxide, which are supposed to grow into graphene layers at elevated temperatures. Carbon monoxide is employed as a component of the atmospheric gas mixture during the decomposition of Si(O)C to suppress the evolution of SiO and CO gases, thereby restraining coarsening of SiC nanocrystallites and maintaining the integrity of the graphene network. These processes pave the way for designing graphene structures in polymer-derived ceramic materials for a broad range of applications.

Carbon fibers, which have excellent mechanical and thermal properties, are used in many fields; however, they are very vulnerable to oxidation and have limited service life. Various studies have attempted to address this. In this study, a SiC–C composite material was prepared using a silica sol to coat the carbon surface and improve the oxidation resistance of a carbon-fiber insulator as a material for solar-cell ingot-growing crucibles. The SiC coating was formed on the carbon surface under various conditions by controlling the composition of the silica sol, and its characteristics were examined. Via SiC-conversion coating through a carbothermal reaction, a film of thickness 30–80 nm film was uniformly formed over the entire sample. In addition, the oxidation characteristics were enhanced by a factor of three to five, when compared with conventional carbon materials.

SiC fibers can be obtained by the spinning, curing, and heat treatment of polycarbosilane (PCS); however, the properties of the PCS precursor must be considered to set the correct spinning conditions. Although many studies have focused on the synthesis conditions, the characterization (in particular, the structural characteristics) of PCS fibers, and the polymer itself has limitations. In this study, PCS was prepared in two steps, and the growth of the polymer with respect to the reaction conditions was analyzed. We found that PCS is formed and grown by the rearrangement and subsequent condensation reactions of polydimethylsilane (PDMS). Further, fiber formation was affected by the reaction temperature, time, and pressure. Three types of PCS were obtained under different synthetic conditions, and they were all characterized. Regardless of the structural similarity of the PCS fibers (based on the spectroscopic analysis), the polymers showed different thermal and rheological properties. Our findings will be important in improving the production of PCS fibers (and subsequent SiC fibers) with finely controlled properties.

The structural evolution of silicon carbide phase from polycarbosilane fibers cured with iodine in air was investigated using nuclear magnetic resonance (NMR) together with in situ gas analysis up to 1400 °C by thermogravimetry coupled with mass spectroscopy (TG-MS). The investigation with solid-state ¹H, ¹³C, and ²⁹Si NMR analyses showed the influence of the oxygen affinity of Si atoms on the chemical structural changes of the SiOCH system during pyrolysis (up to 800 °C). In particular, the mechanism of phase segregation (SiOC → β-SiC + SiO2 + C) in amorphous SiOC structure at 800–1250 °C was determined. Carbon in the Si–O–C networks is replaced by silicon, forming the Si-O-Si network, while the cleaved carbon atoms, which have unpaired electrons, combine, forming C=C bonds. This mechanism accounts for the structural rearrangement from O2SiC2 to O3SiC to SiO4 (from the silicon-centered standpoint, i.e., SiO2 phase), the growth of β-SiC crystallites, and the carbon clustering.