Yeo-Joo Yoon

Preparation of macroporous carbon nanofibers with macroscopic openings in the surfaces and their applications.

Macroporous carbon nanofibers with mesoscale surface openings were produced by electrospinning. During the electrospinning of polyacrylonitrile (PAN) solution including crosslinked polymer colloids, the polymer colloids were concentrated in the center of PAN fibers. Carbonization left interconnected spherical pores inside the carbon fibers and mesoscale openings in the fiber surfaces. The existence of surface openings facilitated inward diffusion of various solvent molecules, nanoparticles, and large molecules such as proteins. The porous fibers could be dispersed in both hydrophilic and hydrophobic solvents and materials, which enabled production of polymer composites in which the fibers and polymers were interpenetrating through the pores. Silica coating on the macroporous carbon fibers enriched the surface chemistry to effectively immobilize proteins helped by easy diffusion through surface openings.

Formation of Core-Shell Structure in BaTiO 3 Grains

To understand the formation of core-shell structure in BaTiO₃ (BT) grains in multilayer ceramic capacitors, specimens were prepared with BT powders mixed with Y and Mg, and their microstructures were investigated with scanning electron microscopy, x-ray diffractometry, and transmission electron microscopy. Microstructural investigation showed that Y dissolved easily in BT lattice to a certain depth inside of the grain, whereas Mg tended to stay at grain boundaries rather than become incorporated into BT. It was considered that in case of Y and Mg addition in a proper ratio, Y could play a dominant role in the formation of shell leading to a slight dissolution of Mg in the shell. Next, the effects of ball-milling conditions on the core-shell formation were studied. As the ball-milling time increased, the milled powders did not show a significant change in size distribution but rather an increase of residual strain, which was attributed to the milling damage. The increase in milling damage facilitated the shell formation, leading to the increased shell portion in the core-shell grain.

Formation of Core-Shell Structure of BaTiO 3 Grains in MLCC

To understand the formation of core-shell structure in BaTiO3 (BT) grains in multilayer ceramic capacitors, specimens were prepared with BT powders mixed with Y and Mg, and their microstructures were investigated in terms of scanning electron microscopy, x-ray diffractometry, and transmission electron microscopy. Compared to BT without Y and Mg, the addition of additives inhibited growth of the BT grains. Microstructural investigation showed that Y dissolved easily in BT lattice to a certain depth inside of the grain whereas Mg tended to stay at grain boundaries rather than to be incorporated into BT. It was considered that in case of Y and Mg addition in a proper ratio, Y could play a dominant role in the formation of shell leading to a slight dissolution of Mg in the shell. Next, the effects of ball-milling condition on the core-shell formation were studied. As the ball-milling period increased, the milled powders did not show a significant change in size distribution but an increase of residual strain which was attributed to the milling damage. The increase in milling damage facilitated the shell formation leading to the increased shell portion in the core-shell grain and, in turn, the deteriorating dielectric property.

A Study on Phase Separation in InGaN/GaN Multi-quantum Wells

Phase separation in InGaN/GaN multi-quatum wells (MQWs) grown with various annealing time is studied for obtaining the high brightness green LED. For this purpose, transmission electron microscopy (TEM), and photoluminescence (PL) are used. InGaN/GaN MQWs for the Green LED with high Indium composition up to 30% are grown at 800-900°C by metal organic chemical vapor deposition (MOCVD) and annealed during 0, 5, 12.5 minutes at 1000°C. It is found that the phase separation in InGaN/GaN multi-quantum wells under the condition of annealing for 5minutes is severe according to PL spectrum and indium ratio map. It is directly obtained indium concentration map images with energy filtered transmission electron microscopy (EFTEM) which can be interpreted as indium compositional fluctuation in Fig 1. The PL intensities of MQWs with annealing for 5 minutes increased when compared to MQWs prepared without annealing at 1000°C and The PL spectra separated blue and green emission peaks as the result of indium phase separation in Fig. 2. As the annealing time increases, the emission peak shows a blueshift in Fig 2. The presence of indium clustering with high indium composition results in high luminescence efficiency. It is likely to see the recombination center of excitons localized in indium clustering regions.