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Number of results: 40
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Abstract

Nano-sized yttria (Y2O3) powders were synthesized by a polymer solution route using polyvinyl alcohol (PVA) as an organic carrier. The PVA polymer affected the dispersion of yttrium ions in precursor sol. In this study, three kinds of PVA polymer (different molecular weight) were applied for synthesis of yttria powder. The PVA type as well as calcination temperature had a strongly influence on the particle morphology. Single crystal nano wire particles were observed at the temperature of polymer burn out range and the size was dependent on the PVA type. The stable, fully crystallized yttria powder was obtained through the calcination at 800°C for 1 h. The yttria powder prepared with the high weight PVA (MW: 153,000) revealed a particle size of 30 nm with a surface area of 18.8 m2/g.
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Abstract

Macroporous silica fibers having spherical cavities were fabricated by electrospinning using the spinning solution prepared from the mixed dispersion of tetraethylorthosilicate (TEOS) and polystyrene nanospheres as precursor and sacrificial templates, respectively, by injection through metallic nozzle. By applying electric field, the electro-spun fibers obtained by evaporation-driven self-assembly were collected on flat substrate or rotating drum, followed by the removal of the templates by calcination. The sound absorption coefficient of the porous fibers was measured by impedance tube, and the measured value was larger than 0.9 at high frequency region of incident waves. The surface of the resulting fibers was modified using fluorine-containing silane coupling agent to produce superhydrophobic fibrous materials to prevent the infiltration of humidity.
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Abstract

Y2O3-MgO nanocomposites are one of the most promising materials for hypersonic infrared windows and domes due to their excellent optical transmittance and mechanical properties. In this study, influence of the calcination temperature of Y2O3-MgO nanopowders on the microstructure, IR transmittance, and hardness of Y2O3-MgO nanocomposites was investigated. It was found that the calcination temperature is related to the presence of residual intergranular pores and grain size after spark plasma sintering. The nanopowders calcined at 1000°C exhibits the highest infrared transmittance (82.3% at 5.3 μm) and hardness (9.99 GPa). These findings indicated that initial particle size and distribution of the nanopowders are important factors determining the optical and mechanical performances of Y2O3-MgO nanocomposites.
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Abstract

The effect of additives on the densification behavior and mechanical properties of pure and additive (Zr, B and Mg)-added silica ceramics were investigated for their application to the matrix phase of a silica fiber reinforced silica (SiO2/SiO2f) composite. The additives affected the rate of densification and crystallization (or transformation) of the amorphous silica. Among the compositions, pure silica ceramics sintered at 900°C for 1 h showed the maximum flexural strength. Based on the results, SiO2/SiO2f was fabricated by a repeated vacuum-assisted infiltration method followed by the heat treatment at 900°C for 1 h. The relative density of the composite was 78.2% with a flexural strength of 22.4 MPa. Fractography revealed that the composite was damaged by strong bonding at the fiber/matrix interface and the fracture of fiber.
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Abstract

This study attempted to manufacture an Y2O3 ceramic coating layer on a ceramic (AlN) substrate using aerosol deposition (AD) and investigated its macroscopic properties. Pure Y2O3 powder with a polygonal shape and average size of 5.0 μm was used as initial feedstock. Using aerosol deposition with suitable process conditions, an Y2O3 coating layer was successfully fabricated on aluminum nitride (AIN). The thickness of the manufactured coating layer was approximately 10 mm. The coating layer consisted of Y2O3 phase identical to that in the initial powder, and no additional oxides were identified. In regard to the roughness of the Y2O3 coating layer, the average roughness (Ra) measured 1.32 μm, indicating that the surface roughness was relatively even compared to the initial powder size (5 μm). Mechanical properties of the Y2O3 coating layer were measured using nano indentation equipment, and the indentation modulus of the Y2O3 coating layer fabricated by aerosol deposition measured 136.5 GPa. The interface of the coating layer was observed using TEM, and the deposition mechanism of the Y2O3 coating layer manufactured by aerosol deposition was also discussed.
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Abstract

Fe-Cr-B alloy is a material with precipitation of boride inside Fe matrix, and it features outstanding hardness and wear resistance properties. However, Fe-Cr-B alloy is a difficult material to process, making it difficult to use as a bulk type structure material which requires delicate shapes. This study attempted to manufacture Fe-Cr-B alloy using a 3D printing process, laser metal deposition. This study also investigated the microstructure, hardness and compression properties of the manufactured alloy. Phase analysis results is confirmed that α-Fe phase as matrix and (Cr, Fe)2B phase as reinforcement phase. In the case of (Cr, Fe)2B phase, differences were observed according to the sample location. While long, coarse, unidirectional needle-type boride phases (~11 μm thickness) were observed in the center area of the sample, relatively finer boride phases (~6 μm thickness) in random directions were observed in other areas. At room temperature compression test results confirmed that the sample had a compression strength is approximately 2.1 GPa, proving that the sample is a material with extremely high strength. Observation of the compression fracture surface identified intergranular fractures in areas with needle-type boride, and transgranular fractures in areas with random borides. Based on this results, this study also reviewed the deformation behavior of LMD Fe-Cr-B alloy in relation to its microstructures.
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