Papers by Author: Jong Heun Lee

Abstract: Two different BaTiO₃ (160nm) nanopowders coated with Y and with Dy were fabricated by an aqueous chemical coating method, and their dielectric properties and microstructures were investigated with X-ray diffraction, Impedance analyzer, SEM and TEM. Y and Dy were coated on the BaTiO₃ powder using nitrates. Coated BaTiO₃ powders were pressed in a disk shape and sintered at 1150~1200°C for 2 hours in reduced atmosphere of 10%H₂ - 90%N₂. Coating layer of the BaTiO₃ particle was thin with a thickness of 3 ~ 5nm. Coated BaTiO₃ sintered sample exhibited a larger lattice parameter (a, c) and smaller tetragonality (c/a) than pure BaTiO₃ one. Y coated BaTiO₃ sample sintered at 1200°C showed good dielectric properties with a high dielectric constant of around 2000 and a stable temperature coefficient of capacitance (TCC).

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: The optimal stacking sequence and wall thickness of the composite strut tubes were determined to minimize thermal strains during orbital operation using generic algorithms and finite element analyses. From the results of previous thermal analyses of composite struts with various stacking sequences, the axial deformation is a matter of prime importance. For this reason, the optimization focuses to minimize the axial strains. The balanced and symmetric stacking sequences are used to minimize the radial and the twisting deformations. The genetic algorithm is known to be very effective for the discrete optimization such as stacking sequences of composite materials. As a result, the thermal deformations of the strut with an optimal stacking sequence are almost zero. The optimal strut tube consists of 6 plies and the weight of a composite strut is 22.4% that of aluminum strut. Finite element analyses showed that the optimal design of composite strut tubes withstood combined launch loads without buckling and failure. To validate the analyses, four composite struts were fabricated and their thermal strains were measured under the temperature increase of 100°C. The thermal and vibration experiments showed excellent correlations with analytical results.

Abstract: In order to optimize the recrystallization texture of ferritic stainless steel sheets, the crystallographic texture was modified by means of cross rolling. The as-received hot band displayed pronounced through-thickness texture gradients with a strong rotated cube orientation in the sheet center layer. After the conventional normal rolling, the strong initial texture was retained. Pronounced{001}<110> in the rolling textures led to the formation of {334}<483> in the final recrystallization texture. Cross rolling in the present work was performed by a 45° rotation of RD around ND. Cross-rolling led to a weakening of {001}<110> orienations. After recrystallization annealing the cross-rolled samples displayed stronger {111}//ND orientations. The orientation stability during the rolling deformation was tackled by means of Taylor deformation model.