Ji Yeon Park

Synthesis of Li2TiO3 ceramic breeder powders by the combustion process

Abstract The synthesis of the ultra-fine Li 2 TiO 3 powder by the combustion reaction of lithium nitrate, titanium nitrate and specific fuels was investigated. Ultrafine Li 2 TiO 3 powders could be synthesized using glycine or a mixture of urea and citric acid. A pure Li 2 TiO 3 phase was obtained by the simple process without further calcination reaction. The specific surface area of the as-synthesized powder was 10 to 14 m 2 /g and the primary particle size was about 30 nm. The Li 2 TiO 3 body sintered at 800°C for 3 h had dense agglomerates which were formed by the inter-agglomerate sintering process. Each of the agglomerates consisted of very fine grains with a size of 0.3 to 0.5 μm.

Low temperature degradation of yttria-stabilized tetragonal zirconia polycrystals under aqueous solutions

Abstract Degradation of 3 mol% Y 2 O 3 doped tetragonal zirconia polycrystals (3Y-TZPs) was investigated under either water or lithiated solutions at low temperatures of 90 to 200°C. Li content in the lithiated solution was varied from 3.5 to 350 ppm. The degradation of 3Y-TZPs induced by the tetragonal to monoclinic (t → m) transformation is enhanced under LiOH solutions compared to that under water. The larger the Li contents in the lithiated solution, the thicker the transformed/porous layer. Based on the demonstration that the transformed layer is strongly related to the diffusion distance of OH − as well as the confirmation of the existence of OH − in the transformed layer, degradation mechanism of 3Y-TZPs under aqueous solutions is proposed.

Fabrication and characterization of SiCf/SiC composite by CVI using the whiskering process

Abstract The chemical vapor infiltration (CVI) process is an effective method for fabricating SiC fiber-reinforced SiC matrix composites but it is slow with an inherent drawback of substantial residual porosity. To obtain the dense SiC f /SiC composite by the CVI process, in situ whisker growing and matrix filling (called whiskering process) was applied. The process was designed to reduce the canning effect during the CVI process and may serve to divide the large natural pores between fibers or bundles by the grown whisker and then fill the matrix inside the modified pore structure. This process was performed using MTS (CH 3 SiCl 3 ) as a source gas and H 2 or N 2 as a diluent and the amounts of the whiskers into the matrix phases were changed by controlling the whiskering cycles. The infiltration effects, that is, increasing of the density of the SiC f /SiC composite were investigated. And the improved SiC f /SiC composites were fabricated using this process.

Deposition of compositionally graded SiC/C layers on C–C composites by low pressure chemical vapor deposition

Abstract To improve the oxidation and erosion resistance of the carbon-fiber reinforced carbon–carbon (C–C) composites, SiC/C compositionally graded layers were deposited between the C–C composite and SiC layer by the low pressure chemical vapor deposition method. The compositions of the graded layers were easily controlled in various steps by manipulating the input ratio of CH3SiCl3 to C2H2. The compositions of the SiC/C deposited layers were consistent with calculated compositions. All the SiC/C layers had dense microstructures and uniform distributions of C and SiC phases.

Effect of soft substrate on the indentation damage in silicon carbide deposited on graphite

Indentation-induced damage is investigated in silicon carbide (SiC) deposited on graphite substrate. The SiC films have been grown by LPCVD (Low Pressure Chemical Vapor Deposition) method using MTS (CH3SiCl3) as a source gas and H2 as a diluent gas to provide highly dense deposited layer and strong interfacial bonding. The elastic-plastic mismatch is very high to induce distinctive damages in the coating and the substrate layer. The specimens with various coating thicknesses are prepared by changing CVD condition or mechanical polishing. Indentation damages with different sizes are introduced by controlling indentation load in Nanoindentation, Vickers indentation and Hertzian indentation test. Basic mechanical properties such as hardness, toughness, elastic modulus are evaluated against coating thickness. Mechanical properties are sensitive to the indentation load and coating thickness. The results indicate that coating thickness has a vital importance on the design of hard coating/soft substrate system because the soft substrate affects on the mechanical properties.

Microstructure of Al2O3-ZrO2 nanocomposites prepared by combustion process

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