Dong-Geun Shin

Low-temperature chemical vapour curing using iodine for fabrication of continuous silicon carbide fibres from low-molecular-weight polycarbosilane

In order to manufacture fine and continuous silicon carbide (SiC) fibres from low-molecular-weight polycarbosilane (PCS), a new chemical vapour curing process based on the use of iodine was developed. Its main advantages are short processing-time (∼1 h) and low processing-temperature (∼100 °C). The underlying curing mechanism was investigated by performing TG-DTA, GC-MS, solid-state NMR, FTIR, FE-EPMA, ESR, elemental analysis, and XRD. The results indicate that the curing reaction occurs upon diffusion of iodine into the PCS, where iodine plays the critical role of accelerating the cleavage of Si–H (mainly), Si–Si, and C–H bonds. This cleavage coincides with recombination to form cross-linked networks of –Si–C– and –CC–. The by-products were identified as oligomeric silanes, iodomethane, and aromatics. When the curing is conducted in air, the surface region in contact with oxygen is heavily oxidized to form a Si–O–Si network that is localized on the surface.

Dense Polycrystalline SiC Fiber Derived from Aluminum-doped Polycarbosilane by One-Pot Synthesis

Polyaluminocarbosilane was synthesized by direct reaction of polydimethylsilane with aluminum(Ⅲ)-acetylacetonate in the presence of zeolite catalyst. A fraction of higher molecular weight polycarbosilane was formed due to the binding of aluminium acetylacetonate radicals with the polycarbosilane backbone. Small amount of Si-O-Si bond was observed in the as-prepared polyaluminocarbosilane as the result. Polyaluminocarbosilane fiber was obtained through a melt spinning and was pyrolyzed and sintered into SiC fiber from 1200~2000℃ under a controlled atmosphere. The nucleation and growth of β-SiC grains between 1400 - 1600℃ are accompanied with nano pores formation and residual carbon generation. Above 1800℃, SiC fiber could be sintered to give a fully crystallized β-SiC with some α-SiC.

Wetting Behavior of Molten Salt on the Ceramic Filter Separators for Thermal Batteries

Ceramic Fiber separator is the promising material for thermal battery system because it reduces the production cost and offers the potential to a new application compared to a pellet type electrolyte. The molten salt electrolytes for thermal battery were prepared by the impregnation of the commercial glass filters such as GF-A, C and F (Whatman, USA) with two types of molten-lithium salts, LiCl-KCl and LiK-LiBr-LiF. The wetting properties were evaluated by wetting balance test and wetting angle measurement. The wetting behaviors were strongly affected by the composition of the molten salts and the pore structure of the glass separators. The optimum wetting conditions for maximum loading and effective retention of the molten electrolyte were also studied.

Reaction Behavior of Ceramic Mat with Lithium Salt for the Electrolyte Separators of Thermal Batteries

Lithium salt have been used mainly as electrolyte of thermal battery for electricity storage. Recently, The 3phase lithium salt(LiCl-LiF-LiBr) is tried to use as electrolyte of thermal battery for high electric power. It is reported that LiCl-LiF-LiBr salt have high ion mobility due to its high lithium ion concentration. Solid lithium salt is melt to liquid state at above 500℃. The lithium ion is easily reacted with support materials. Because the melted lithium ion has small ion size and high ion mobility. For the increasing mechanical strength of electrolyte pellet, the research was started to apply ceramic filter to support of electrolyte. In this study, authors used SiOC web and glass fiber filter as ceramic mat for support of electrolyte and impregnated LiCl-LiF-LiBr salt into ceramic mat at above 500℃. The fabricated electrolyte using ceramic mat was washed with distilled water for removing lithium salt on ceramic mat. The washed ceramic mat was observed for lithium ion reaction behavior with XRD, SEM-EDS and so on.

Densification Behavior of C/C Composite Derived from Coal Tar Pitch with Small Amount of Iodine Addition

We investigated the viscosity behavior and the carbon yield of coal tar pitch (CTP) treated with iodine. The viscosity of iodine treated pitch showed that the fluidity of iodine treated CTP did not increase within the iodine addition of 1.4%. DTG analysis showed that cross linking was accelerated at the temperature range from 400 to 500℃ with iodine treatment, which is due to the accelerated dehydrogenative reaction by iodine. The iodine treatment was mainly effective for β-resin content increase of CTP. The carbon yield of CTP increased from 40 to 60% by the iodine non-treated CTP.

Properties of the Electrolyte Separators for Thermal Batteries Using SiOC Mat

Ceramic fiber separator is the promising material for thermal battery system because it reduces the production cost and offers the potential to a new application compared to a pellet type electrolyte. The electrolyte separator for thermal battery should be easily handled and loaded a large amount of the molten lithium salt. Ceramic fibers were used as an electrolyte separator and the lithium based molten salts were infiltrated into the ceramic filters. Leakage of molten salt (several lithium salts) leads to short-circuit during the thermal battery operation. In this study, a uniform and fine SiOC mat with fibers ranging from 1 to 3 ㎛ was obtained by electrospinning of polycarbosilane and pyrolysis. The optimum spinning conditions for obtaining fine diameters of SiOC fiber were controlled by the solution composition and concentration, applied voltage and spinning rate, release rate by porosity. The pore structures of the ceramic filter and the melting properties of the lithium salts affected to the electrolyte loading and leakage. The importance of the fiber size and porosity and their control was discussed and the mechanical properties were also discussed.

Room temperature reaction of polycarbosilane with iodine under different atmospheres for polymer-derived silicon carbide fibres

The reaction of iodine with polycarbosilane (PCS) in air and N2 atmospheres at room temperature was investigated with the goal of limiting oxygen uptake during iodine curing, in order to fabricate SiC fibres with low oxygen contents. The investigation shows that the PCS fibres are well cured by iodisation even at room temperature, regardless of the presence of oxygen in the atmosphere. Although it was previously reported that the amount of oxygen incorporated is affected proportionally by the processing temperature, when PCS fibres were iodised in air at room temperature, they contained significant amounts of oxygen owing to the very long processing time despite the low temperature. On the other hand, when iodised in a N2 atmosphere, the fibres were successfully cured without oxygen uptake, thus demonstrating that iodisation under an oxygen-free atmosphere is a successful curing method for fabricating SiC fibres with low oxygen contents. During the cross-linking process, the iodine causes fragmentation of certain groups from the PCS polymer chains, which mostly contributes to the cross-linking between the chains, but leaves reactive sites that readily allow oxidation in air.

Effects of asperities and organic-inorganic interactions on the strength of nacre-mimetic composites.

Nacre is a natural organic-inorganic hybrid composite, whose hierarchical structure has a complex influence on its high strength. Many structural features have been discovered, which influence the mechanical properties of nacre, and the authors have a particular interest in the role of the asperities and organic-inorganic interactions. In this study, a composite was prepared which mimics the asperity structure using clay minerals. Organic-inorganic bonding was induced with silane treatment. Both factors increased the yield strength of the composites; however, different deformation behavior was exhibited. It was found that asperities improved the strength of the composite, and that composition influences the stiffness of the composite. The organic-inorganic interaction between the resin and the other components of the composite reduced the deformation of the composite and consequently improved strength.

Formation of ZSM-5 on Silicon Carbide Fibers for Catalytic Support

ZSM-5 crystals were grown on the SiC fiber surface which was prepared by melt-spinning of polycarbosilane, silicon based preceramic polymer. To improve the bonding property between ZSM-5 crystals and SiC fiber, 200 nm of SiO2 layer was formed on the fiber surface by simple oxidation before hydrothermal reaction in the autoclave because it was compositionally similar to ZSM-5 and relatively easy to form on the SiC fiber. During the process, 1 μm of zeolite crystals were very uniformly grown on the fiber surface. Weak X-ray diffraction (XRD) peaks observed between 10° and 30° showed that crystal structure of them were accorded to ZSM-5 structure. They were broken away with SiC fiber support during fracture test which mean that they were strongly bonded to fiber surface. This type of SiC based catalysts are expected for high-temperature gas-phase reaction, especially strong acid base conditions.

Uniform Zeolite Layer Formation on Glass Fiber Surface

왔는데, Tang 등은 광섬유 표면에 제올라이트 층을 형성하 고 암모니아의 흡착 특성을 확인하였으며[9], Xu 등은 세 라믹 중공사(hollow fiber)에 약 5 μm 두께의 제올라이트 멤브레인을 올리고 기체 투과 특성을 확인하였다[10]. 또한 Shu 등은 알루미나와 YSZ 중공사에 나노크기의 제올라이 트 입자를 5 μm 내외 두께로 층을 올리고 에탄올에 대한 selectivity를 평가하였다[11]. 섬유 표면에 제올라이트 층을 형성하는 방법으로는 dipcoating 법[11], seed를 이용한 성장법[12], 또는 seed 없이 수열반응하여 in situ로 제올라이트 층을 형성하는 방법[13] 등이 있다. 이때, 제올라이트 중 촉매로 가장 널리 사용되 는 ZSM-5의 경우 일반적으로 수열합성 반응에 의하여 제 조되며 반응의 초기에 지지체를 함께 첨가하여 ZSM-5의 불균질성장(heterogeneous growth)을 유도할 수 있어 손쉽 게 지지체와 복합화 할 수 있다. 본 연구에서는 섬유상의 글라스 울(또는 매트)을 지지체 Corresponding Author: Dong-Geun Shin E-mail: dgshin73@kicet.re.kr