Doh-Hyung Riu

Effect of Cr2O3 addition on microstructural evolution and mechanical properties of Al2O3

Abstract The effects of chromia (Cr 2 O 3 ) additions on the microstructural evolution and the mechanical properties of alumina (Al 2 O 3 ) were investigated. When small amounts ( 2 O 3 were added in samples hot pressed at 1500°C, the grain size distribution became bimodal; large platelike grains were dispersed in a relatively small grained matrix. The large grains were composed of a core region that is free of Cr and a surrounding shell region rich in Cr. The interface between the core and the shell was composed of misfit dislocations. The high diffusion rate of Cr ions through the surface of alumina was attributed to this microstructural evolution. The mechanical properties of the specimens were strongly influenced by this microstructural change. The fracture toughness and the flaw tolerance (R-curve behavior) of Al 2 O 3 were improved markedly by the formation of the large platelike grains. The hardness and the elastic modulus also increased, however, the fracture strength decreased by the addition of Cr 2 O 3 .

Oxidation behaviour and strength of B4C-30 wt% SiC composite materials

The oxidation behaviour and the effect of oxidation on the room-temperature flexural strength of B4C-30 wt% SiC composite material were investigated. The weight changes of the samples exposed to air at temperatures between 500 and 1000 °C were continuously monitored with a microbalance. At temperatures below 800 °C, the weight change of the specimen was negligible. As the temperature was increased to 800 °C, parabolic weight gain was observed. The rate of the weight gain increased with exposure temperature. The oxidation product formed on the surface was found to be a crystalline boric oxide (B2O3) by X-ray analyses. The oxide layer was severely cracked due to the thermal expansion mismatch between the oxide layer and the substrate. However, the room-temperature flexural strength was increased when the samples were exposed at temperature between 700 and 900 °C, apparently due to the blunting of strength-limiting defects at the surface. When the temperature was higher than 1000 °C, a severe reduction in strength was observed. The reliability of the composite material was also improved significantly by such exposures.

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.

Preparation of shape-controlled copper oxide powders from copper-containing solution

Abstract Shape-controlled copper oxides have been recovered from copper-containing waste etchant by neutralization with alkali hydroxide. Large amounts of copper-containing waste etchant composed of copper chloride, hydrochloric acid and water are generated from the printed circuit board (PCB) industry. In an environmental and economic point of view, the retrieval of the valuable natural resource from waste is important. In the recycling process of copper oxide from the waste etchant, reaction temperature controls the shapes and sizes of the products. Copper oxide recovered below the reaction temperature of 40 °C was of the needle shape, while copper oxide comes in a platy shape above 40 °C. As a result of the experiments, more than 99% of the copper in the waste etchant was recovered as copper oxide, and its by-products are only sodium chloride and water. Physical properties of the samples have been characterized using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA) and atomic absorption spectroscopy. The particle size scatters in the range of 0.5–10 μm. Shape-controlled copper oxides are expected to be promising precursors for synthesizing copper powder by reduction.

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.

Fast responsive gas sensor of vertically aligned fluorine-doped tin oxide nanorod thin film

We prepared fluorine-doped tin oxide (FTO) nanorod films and a conventional FTO thin film for the application of a semiconducting gas sensor by spray pyrolysis method. The lengths of FTO nanorods (FTON, 100 and 500 nm) were controlled by changing deposition times, and FTO thin film (FTOT) was also prepared as a reference. The gas sensitivity test shows FTON with long nanorods had higher sensitivity for both hydrogen and ethanol gases but slow response and recovery times, despite an advantage of the higher gas sensitivity. FTO nanorod film with short length about 100 nm showed relatively lower sensitivity, but fast gas response and recovery characteristics. The fast response and recovery for the analyte gases are attributed to the conductance of FTO nanorods, which is closely related to the diameter and length of nanorods.

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.