Hyun Chul Jung

Spinel LiMn2O4/reduced graphene oxide hybrid for high rate lithium ion batteries

A well-crystallized and nano-sized spinel LiMn2O4/reduced graphene oxide hybrid cathode material for high rate lithium-ion batteries has been successfully synthesized via a microwave-assisted hydrothermal method at 200 °C for 30 min without any post heat-treatment. The nano-sized LiMn2O4 particles were evenly dispersed on the reduced graphene oxide template without agglomeration, which allows the inherent high active surface area of individual LiMn2O4 nanoparticles in the hybrid. These unique structural and morphological properties of LiMn2O4 on the highly conductive reduced graphene oxide sheets in the hybrid enable achieving the high specific capacity, an excellent high rate capability and stable cycling performance. An analysis of the cyclic voltammogram data revealed that a large surface charge storage contribution of the LiMn2O4/reduced graphene oxide hybrid plays an important role in achieving faster charge/discharge.

Characteristics of microstructure and electrical resistivity of inkjet-printed nanoparticle silver films annealed under ambient air

The microstructure and electrical resistivity of inkjet-printed silver (Ag) films annealed under ambient air were characterized. Analyses of the impurity amounts in the films using secondary-ion mass spectrometry showed that the decomposition temperature of the capping molecules was just below 170°C. Both the characteristics of the microstructure and electrical resistivity when annealed at low temperatures (lower than the decomposition temperature) were significantly different from those when annealed at high temperatures. The results show that neither microstructural features, such as grain size, nor the amounts of impurities can explain both the magnitude and characteristic decrease in electrical resistivity. The changes in electrical resistivity can be described using exponential decay kinetics. The corresponding activation energy of 0.44 eV when annealed at the high temperatures is explained by the migration of point defects such as vacancy–oxygen pairs. On the other hand, negligible dependence on temperature was identified when annealed at low temperatures, which was attributed to decomposition of the capping molecules. The results indicate the importance of controlling the defects of nanoparticles and the properties of capping molecules from the viewpoint of electrical optimization of metallization fabricated using inkjet printing.

Sintering and Consolidation of Silver Nanoparticles Printed on Polyimide Substrate Films

We investigated the sintering and consolidation phenomena of silver nanoparticles under various thermal treatment conditions when they were patterned by a contact printing technique on polyimide substrate films. The sintering of metastable silver nanoparticles commenced at 180 °C, where the point necks were formed at the contact points of the nanoparticles to reduce the overall surface area and the overall surface energy. As the temperature was increased up to 250 °C, silver atoms diffused from the grain boundaries at the intersections and continued to deposit on the interior surface of the pores, thereby filling up the remaining space. When the consolidation temperature exceeded 270 °C, the capillary force between the spherical silver particles and polyimide flat surface induced the permanent deformation of the polyimide films, leaving crater-shaped indentation marks. The bonding force between the patterned silver metal and polyimide substrate was greatly increased by the heat treatment temperature and the mechanical interlocking by the metal particle indentation.

Ultra-drawing of gel films of ultra high molecular weight polyethylene/low molecular weight polymer blends containing BaTiO3 nanoparticles

The ultra-drawing process of an ultra high molecular weight polyethylene (UHMWPE) gel film was examined by incorporating linear low-density polyethylene (LLDPE) and BaTiO3 nanoparticles. The effects of LLDPE and the draw ratios on the morphological development and mechanical properties of the nanocomposite membrane systems were investigated. By incorporating BaTiO3 nanoparticles in the UHMWPE/LLDPE blend systems, the ultra-drawing process provided a highly extended, fibril structure of UHMWPE chains to form highly porous, composite membranes with well-dispersed nanoparticles. The ultra-drawing process of UHMWPE/LLDPE dry-gel films desirably dispersed the highly loaded BaTiO3 nanoparticles in the porous membrane, which could be used to form multi-layered structures for electronic applications in various embedded, printed circuit board (PCB) systems.

Fabrication of Poly(3-hexylthiophene) Thin Films by Vapor-Phase Polymerization for Optoelectronic Device Applications

The vapor-phase polymerization (VPP) of poly(3-hexylthiophene) (P3HT) was achieved successfully as an alternative method to conventional solution-based thin film fabrication. Using Fe(III)Cl(3).6H(2)O, a spontaneous reaction of 3-hexylthiophene monomers resulted in the rapid formation of conducting P3HT thin films directly on substrates, such as glass, indium-tin-oxide, and poly(ethylene terephthalate), at thicknesses ranging from 50 to 1000 nm. The VPP of P3HT was achieved using ferric chloride hexahydrate and a 1:1 ratio of a methanol/ethanol mixture as the solvent system. The developed VPP technique can provide good processing consistency with an electrical conductivity, a transmittance, and a surface roughness of approximately 10(-2) S/cm, >90%, and <10 nm, respectively.

Effect of microstructure on electrical and mechanical properties: Impurities of inkjet-printed Ag and Cu interconnects

Inkjet printing technology is a pattern-on-demand technology which has numerous advantages. However, this technology needs an additional thermal treatment, i.e., drying process. This treatment results in microstructure evolution which is expected to relate to properties of film. The microstructure, electrical and mechanical properties of the inkjet printed Ag and Cu films were characterized as drying process. Model study on electrical resistivity of Ag film shows that the decomposition of capping molecule plays a key role in microstructure evolution and electrical resistivity. The effect of ambient in thermal treatment of inkjet printed Cu film also investigated in this purport. The adhesion strength as a mechanical property was measured by 4 point bend test using sandwiched structure. Strengthening of adhesion was observed as densification of inkjet printed film.

Thermal transformation of solvent-processable organosilicon compounds into inorganic silicone-based thin films

In this study, a solution-processable precursor, decaphenycyclopentasilane (DPCPS) was synthesized and successfully converted into an inorganic thin film through the heat treatment process. When the DPCPS coating was heat-treated at various temperatures up to 900 °C, the organic DPCPS was converted into an inorganic compound composed of Si, C and O atoms in the form of SiCxOy. The relative compositions of Si, C, and O changed with the heat-treatment temperatures changing the carbon content, for example, from 88.9% for the pristine DPCPS to 3.0% for the specimen heat-treated over 700 °C, consequently becoming SiC0.05O1.7. The dielectric constant of the heat-treated DPCPS could be tailored with the heat treatment temperatures from high- to low-dielectric materials, i.e., 10.8 at 300 °C and 2.3 at 900 °C, providing tunable dielectric properties for various optoelectronic applications. The resulting inorganic thin films had excellent coating characteristics with low RMS roughness (<1.0 nm).

Double-Layer Structured CO2 Adsorbent Functionalized with Modified Polyethyleneimine for High Physical and Chemical Stability

CO2 capture using polyethyleneimine (PEI)-impregnated silica adsorbents has been receiving a lot of attention. However, the absence of physical stability (evaporation and leaching of amine) and chemical stability (urea formation) of the PEI-impregnated silica adsorbent has been generally established. Therefore, in this study, a double-layer impregnated structure, developed using modified PEI, is newly proposed to enhance the physical and chemical stabilities of the adsorbent. Epoxy-modified PEI and diepoxide-cross-linked PEI were impregnated via a dry impregnation method in the first and second layers, respectively. The physical stability of the double-layer structured adsorbent was noticeably enhanced when compared to the conventional adsorbents with a single layer. In addition to the enhanced physical stability, the result of simulated temperature swing adsorption cycles revealed that the double-layer structured adsorbent presented a high potential working capacity (3.5 mmol/g) and less urea formation under CO2-rich regeneration conditions. The enhanced physical and chemical stabilities as well as the high CO2 working capacity of the double-layer structured adsorbent were mainly attributed to the second layer consisting of diepoxide-cross-linked PEI.