Shiwoo Lees footprints

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C/SiC composites

- One of my research interests is C/SiC composites. C/SiC composites can maintain their outstanding strength at high temperature. Also, the fabric structure makes these composites have higher fracture toughness than ceramic monolith.

- C/SiC can be processed by liquid silicon infiltration (LSI) method. C/SiC can be mass-produced with LSI, because this method utilizes the fast reaction between carbon and liquid silicon. Furthermore, the LSI method is cheaper than chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP) method.

- The LSI method consists of three steps.

① Fabricating CFRP

② Pyrolyzing CFRP

③ Infiltrating liquid Si into the pyrolyzed CFRP

- There are several ways to enhance the mechanical properties of C/SiC composites.

① Reducing the graphene-like crystallinity of pyrolyzed carbon preform

② Increasing the surface area of the pyrolyzed preform to react liquid Si

③ Modifying the interfacial bonding strength between the fibers and the matrix

I have studied how cellulose nanofiber (CNF) can be used to enhance the mechanical properties of C/SiC composites.

3D Woven Textile Composites

- 3D weaving technology gives us the possibility of the composite materials with high damage tolerance and strong capability to fabricate complex-shaped parts. Since the layers inside the fabric structure are connected by the binding yarns, the composites with 3D woven carbon preforms have intensive resistance to delamination.

- With 3D weaving, complex-shaped composite parts can be produced with consistent properties regardless of the fabricator's skill level, because the preform is formed in three dimensions. In contrast, it is difficult to fabricate composite laminates with complex shape due to the laminates are 2D materials.

- I have researched optimizing the fabrication process of C/SiC composites with various 3D woven carbon preforms.

Cellulose Nanofiber-Based Composites

- Cellulose is abundant in nature (10^11-10^12 tons/year). It has strong mechanical properties and high crystallinity. Depending on the size, the crystallinity, and the method of production, cellulose is classified into

① Micro fibrillated cellulose (MFC), ② Cellulose nanofiber (CNF)

③ Microcrystalline cellulose (MCC), ④ Cellulose nanocrystal (CNC)

⑤ Bacterial cellulose (BC), ⑥ Electrospun cellulose nanofiber (ECNF)

- I have used CNF produced by the aqueous counter collision (ACC) method. The advantage of ACC method is that it consumes less energy than other methods and produce CNF with high crystallinity.

- To disperse CNF into the phenolic resin, we can try various techniques such as ball milling and three roll milling.

- I am interested in applying nanomaterials (cellulose nanofiber, carbon nanotube, graphene, carbon black, and etc) to polymer composites, fiber-reinforced plastics, and pyrolysis behavior of them.

Composites for Stealth and EMI Shielding Applications

- Since the carbon fiber is highly conductive materials, it reflects electromagnetic wave (EM wave). With the high resistance to erosion and lightweight, CFRP is promising candidate for EMI shielding materials.

- FRP can be used to stealth application. By decreasing reflection of EM wave through absorption and the impedance mismatch, or by reflecting the wave in a different direction, FRP-based radar absorbing structure (RAS) can reduce an object's radar cross section (RCS).

- I have studied how CNF affect shielding effectiveness (SE) of C/Phenolic composites by measuring electrical conductivity and calculating S-parameters based on measured value.