Papers by Author: Kee Sun Sohn

Abstract: Microfracture mechanisms of Zr-based bulk metallic glass (BMG) alloy containing ductile crystalline particles were investigated by directly observing microfracture processes using an in situ loading stage. Strength of the BMG alloy containing crystalline particles was lower than that of the monolithic BMG alloy, while ductility was higher. According to the direct microfracture observation, crystalline particles initiated shear bands, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

Abstract: The luminance level of thin film Y2O3:Eu3+ phosphors deposited by a pulsed laser deposition (PLD) technique are not acceptable for field emission display (FED) because of so-called wave guiding effect. Corrugated glass plates were employed as a substrate in order to relieve the wave guiding effect and in turn improve light extraction efficiency of Y2O3:Eu3+ thin film phosphors. Integrated photoluminescent efficiency, film surface roughness and crystallinity were monitored as function of the pitch of corrugation. The integrated photoluminescence efficiency of Y2O3:Eu3+ thin film phosphor deposited on the corrugated substrate was promoted significantly. The corrugated substrate makes it possible not only to enhance the roughness of phosphor film surface but also to facilitate the film growth in a specific orientation if the pitch size condition was met.

Abstract: The present investigation aimed at visualizing the propagating crack in a mechano-luminescence (ML) material, which enabled us to measure instantaneous R-curves and observe directly the bridging (shielding) stress in the fast-propagating crack system. The well-known ML compound, SrAl2O4:Eu,Dy,Nd, was adopted as a testing material. The crack initiation and growth from the mechanically machined sharp notch tip in the disc shaped compact tension (CT) specimen at a relatively fast loading rate were found to be associated with the extent of light emission around the crack. The in-situ measurement of crack length and applied load for 0.3 sec yielded an instantaneous R-curve at the conventional crack propagation speed.

Abstract: An evolutionary optimization process involving a genetic algorithm and combinatorial chemistry (combi-chem) was tailored exclusively for the development of LED phosphors. The genetic algorithm assisted combi-chem (GACC) is a well-known, very efficient heuristic optimization method. Therefore the combination of a genetic algorithm and combi-chem would enhance the searching efficiency when applied to phosphor screening. The ultimate goal of our study was to develop oxide red and green phosphors, which are suitable for three-band white light emitting diodes (LED). In this regard, promising red and green phosphors for three-band white LED applications, such as Eu0.14Mg0.18Ca0.07Ba0.12B0.17Si0.32Od and Tb0.01Gd0.02Ce0.04B0.1Si0.83Od, were obtained.