Papers by Author: Doh Hyung Riu

Abstract: Joining Si3N4 and Al2O3 using 15 layers has been achieved by a unique approach that introduces SiAlON polytypoids as a functionally graded material (FGM) bonding layer. Previously, the hot press sintering of multilayered FGM with 20 layers, each 500 µm thick, has been achieved successfully. In the present study, the number of layers for FGM was reduced from 20 to 15 to increase optimization. Samples were fabricated by hot pressing at 48 MPa during the temperature ramp to 1650°C and cooling at 2°C/min to minimize residual stresses from sintering. Moreover, a finite element method (FEM) program based on the maximum principal stress theory and the maximum tensile stress theory was applied to design optimized and reduced FGM layers that produced a crack-free joint. The sample had a 3-dimensional cylindrical shape that was transformed to a 2-dimensional axisymmetric mode. By determining the expected thermal stress from the calculated elastic modulus and coefficient of thermal expansion, we were able to predict and prevent damage due to thermal stresses. These analyses are especially useful for FGM samples where it is very difficult to measure the residual stresses experimentally. Finally, oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were deflected at the SiAlON layers, implying weak interfaces. In other areas, cracks were not deflected, implying strong interfaces.

Abstract: Magnetite (Fe3O4) particles were prepared for biomedical application through three different routes. Each route had the difference in the selection of pH controlling agents. Ammonia, Urea and combined use of ammonia and urea were tested in the synthesis through co-precipitation. With the combined use of urea and ammonia, the size of particles could be reduced as small as 10 nm while effectively suppressing the agglomeration among particles. The surface charge measured at physiological condition explained the good dispersion behavior of nano-magnetite particles. The superparamagnetism observed in nano magnetite particles was expected to be useful for biomedical application in the respect of magnetic localization and dispersion.

Abstract: Polycarbosilane was synthesized from polydimethylsilane in the presence of ZSM-5 (Si/Al=30) as a catalyst at 350oC-400oC. Characterization of synthesized polycarbosilane was performed with 29Si Solid NMR, FT-IR, and GPC analysis. Number average of molecular weight (Mn) of the polycarbosilane was ranged from 350 to 2340.

Abstract: Polycarbosilane is the most typical polymeric precursor for SiC ceramic. In this study, liquid type polycarbosilane was synthesized from polydimethylsilane in the presence of solid acid catalyst at 350oC. The molecular weights of the polycarbosilane were ranged between 350 and 530. Synthesized polycarbosilane was characterized with 29Si Solid NMR, FT-IR and GPC analysis. The synthesized polycarbosilane can be used as a good organometallic precursor for SiC coating via chemical vapor deposition or spin coating.