Seok Min Kang

Fabrication and Ion Irradiation Characteristics of SiC-Based Ceramics for Advanced Nuclear Energy Systems

SiC-based ceramics are considered as candidate materials for the advanced nuclear energy systems such as the generation IV reactors and the fusion reactors due to their excellent high-temperature strength and irradiation resistance. The advanced nuclear energy systems and their main components adopting ceramic composites were briefly reviewed. A novel fabrication method of SiC f /SiC composites by introducing SiC whiskers was also described. In addition, the charged-particle irradiation (Si 2+ and H + ion) into CVD SiC was carried out to simulate the severe environments of the advanced nuclear reactors. SiC whiskers grown in the fiber preform increased the matrix infiltration rate by more than 60% compared to the conventional CVI process. The highly crystalline and pure SiC showed little degradation in hardness and elastic modulus up to a damage level of 10 dpa at 1000℃.

Microstructure and Properties of SiC Nanowire-Reinforced SiCf/SiC Composites

Microstructure and mechanical/thermal properties of SiCf/SiC composites additionally reinforced with SiC nanowires were investigated. SiC nanowires could be grown successfully within the SiC fiber preform by a chemical vapor infiltration (CVI) process through a control of the deposition parameters. The incorporation of the SiC nanowires into the fiber perform was clearly shown to be effective in increasing the efficiency of the matrix infiltration, and thus resulted in a higher density in a shorter CVI time. The modification of the pore structure and the reduction of the macro-pores in the composite resulted in higher mechanical and thermal properties than the conventional CVI counterpart. The matrix deposition on both surfaces of the SiC fibers and nanowires induced smaller matrix grains in the nanowire-reinforced composite, and thus a higher hardness and elastic modulus than the conventional one after an ion irradiation.

Whisker Growing Assisted 화학침착 공정으로 제조된 SiC f /SiC 복합체의 파괴거동과 기계강도 평가

SiCf/SiC composites with whiskers and pyrolytic carbon (PyC) coated whiskers in the matrix were fabricated for enhancement of the fracture behaviors by the whisker growing assisted chemical vapor infiltration (WA-CVI) process, respectively. SiC f /SiC composites were also prepared by the conventional CVI process as reference material. The mechanical properties and fracture behaviors were analyzed by comparison of the two types of composites prepared by WA-CVI and conventional CVI. The densities of SiC f /SiC composites were in the range of 2.6~2.65 g/㎤. The flexural strengths of composite with whiskers and with those coated by PyC were 650 ㎫ and 600 ㎫, respectively. The tensile strength of composites with whiskers was ~285 ㎫.

Characterization of MeV Ion-Irradiated SiCf/SiC Composites Prepared with Different Methods

To estimate the ion-irradiation effect on various types of SiCf/SiC composites, a silicon self-ion irradiation was performed at temperatures of 600 °C and 1200 °C and at doses of 5 dpa and 20 dpa, respectively. These SiCf/SiC composites were prepared by different processes such as CVI (chemical vapor infiltration), WA-CVI (SiC whisker assisted CVI) and hot-pressing (HP) method. Hardness was measured by a nano-indentation tester and microstructural changes were observed by TEM with SAD(selected area diffraction) technique for the specimens prepared by FIB (Focused Ion Beam) milling. The damage dose was calculated by the SRIM2003 code and then compared with microstructureal observation.

Densification Behavior and Microstructure Evolution of SiCf/SiC Composites Incorporated with In Situ Grown SiC Nanowires

SiC nanowires could be grown homogeneously within SiC fiber preforms by controlling the concentration of a reactant gas. The morphology and the growth behavior of the SiC nanowires were largely dependent on the degree of the reactant supersaturation. The SiC nanowires incorporated in the fiber preform increased the surface area at which the matrix deposition could take place, enhancing an efficiency of the matrix infiltration and altering the remaining pore structure of the SiCf/SiC composite.

Corrosive Degradation of MgO/Al 2 O 3 -Added Si 3 N 4 Ceramics under a Hydrothermal Condition

Silicon nitride () ceramics have been considered for various components of nuclear power plants such as the mechanical seal of a reactor coolant pump (RCP), the guide roller for a control rod drive mechanism (CRDM), and a seal support, etc. Corrosion behavior of ceramics in a high-temperature and high-pressure water must be elucidated before they can be considered as components for nuclear power plants. In this study, the corrosion behaviors of ceramics containing MgO and as sintering aids were investigated at a hydrothermal condition (, 9.0 MPa) in pure water and 35 ppm LiOH solution. The corrosion reactions were controlled by a diffusion of the reactive species and/or products through the corroded layer. The grain-boundary phase was preferentially corroded in pure water whereas the grain seemed to be corroded at a similar rate to the grain-boundary phase in LiOH solution. Flexural strengths of the ceramics were significantly degraded due to the corrosion reaction. Results of this study imply that a variation of the sintering aids and/or a control (e.g., crystallization) of the grain-boundary phase are necessary to increase the corrosion resistance of ceramics in a high-temperature water.

Corrosion Behavior of Si3N4 Ceramics under High-Temperature and High-Pressure Water Condition

Silicon nitride (Si3N4) ceramics have been considered for various components of nuclear power plants such as mechanical seal of reactor coolant pump (RCP), guide roller for control rod drive mechanism (CRDM), and seal support, etc. Corrosion behavior of Si3N4 ceramics in high-temperature and high-pressure water must be elucidated before they can be considered for components of nuclear power plants. In this study, the corrosion behaviors of Si3N4 ceramics at hydrothermal condition (300°C, 9.0 MPa) were investigated in pure water. The grain-boundary phase was preferentially corroded and the corrosion reaction was controlled by the diffusion of the reactive species and/or products through the corroded layer. Results of this study imply that the variation of sintering aids and/or the control (e.g., crystallization) of the grain-boundary phase are necessary to increase the corrosion resistance of Si3N4 ceramics in high-temperature water.