Papers by Author: Jong Won Yoon

Abstract: Dense 8mol% yttria-stabilized zirconia (8YSZ) consisting of submicrometer-sized grains was prepared using spark plasma sintering (SPS) along with Al2O3 additives. The starting powder with average particle size of 50nm was densified to 98% of the relative density with short sintering time (5min) at 1200
while preserving a submicrometer grain size. The fracture toughness and bending strength showed maximum values of 2.54MPam1/2 and 380MPa at 2vol% alumina-added 8YSZ, due mainly to the higher relative density and small grain size. The electrical conductivity of 2vol% alumina-added 8YSZ was 0.0278 S/cm at 700
 in airThus, alumina additions in 8YSZ using the SPS method are an effective process to improve the mechanical strength and electrical conductivity.

Abstract: Uniformly nano-sized macroporous silica consisting of well-defined pore sizes were successfully fabricated. A modified sol-gel process, using mono-dispersed polystyrene(PS) spheres as templates, was employed for configuring three-dimensionally ordered macroporous structure. The nano-sized macroporous silica structure was formed when the templates were removed by the calcination at high temperature. The pH value affected uniformly stacked order in the PS/silica matrix due to the PS being uniformly coated by silica film at appropriate pH value. Also this pH value affected the iso-electrics points between the PS and silica particles surface charges. The heating temperatures and the reaction times were considered to conform the microstructural morphology, the thermal deposition of organic components, the matrix wall thickness and densification behavior. Three-dimensionally assembled porous silica consists of uniform-sized pores of 200nm and high specific surface area of 163 m2/g.

Abstract: The nitrogenated diamond-like carbon films (a-C:H:N) were deposited on Si-wafer by the rf-PECVD method with the addition of nitrogen to the mixture gas of methane and hydrogen. We were investigating the effect of the additive nitrogen gases and annealing in relationship between bonding structure and electrical properties of the deposited films. The electrical conductivity of films increased with the flow rate of nitrogen increasing up to 10 sccm. Also as annealing temperature was increased, the electrical conductivity of films increased. The structure analysis results show that an increase of the flow rate of nitrogen and annealing temperature favor the formation of sp2 bonding in the films. Therefore, we confirmed that the increase of the electrical conductivity is due to structure change by graphitization of the films.

Abstract: Amorphous/pseudoamorphous GaN was prepared by pulsed-laser ablation at room temperature without any heat treatment. The structure and chemical composition of the specimens were systematically investigated. Laser ablation at low Ar pressure (<50Pa) led to deposition of smooth Ga-rich films, which is independent with laser energy. Under same pressures, as laser energy increased, the film stoichiometry changed from Ga-rich to near stoichiometric composition. Varying background Ar pressure strongly affected the product structure showing little effect on the chemical composition. Under higher pressure than 100 Pa, fine nanoparticles with a size of 5 nm rather than films were deposited on substrate due to the increased collision by plume confining. The optical band-gap of the deposited a-GaN is 2.8 eV for thin films and 3.9 eV for nanoparticles.

Abstract: The TiN/DLC nanocomposite coatings were grown on Si wafers using Ar/CH4/TDMAT (Ti[(CH3)2N]4N2) gas mixtures by r.f. plasma enhanced chemical vapor deposition. The sliding friction tests were carried out using a ball-on-flat type tribometer. The different test parameters such as applied loads, counterpart materials and environment were applied to understand the tribological behavior in terms of friction and wear. The coatings provided a low friction coefficient and high wear resistance depending on the friction test conditions.

Abstract: Nano-sized MWO4 and MMoO4 (M = Ca, Ni) powders, which have scheelite and wolframite type structure, were successfully synthesized at low temperatures by a modified citrate complex method assisted by microwave irradiation. The citrate complex precursors were heattreated at temperatures from 300 to 600 °C for 3 h. Crystallization of the MWO4 and MMoO4 precursors was detected at 400 °C and completed at a temperature of 500 °C. Most of the MWO4 and MMoO4 powders heat-treated between 300 and 600 °C showed primarily spherical and homogeneous morphology. The average crystallite sizes of MWO4 (M = Ca, Ni) were between 22 and 39 nm, and those for MMoO4 (M = Ca, Ni) were between 19 and 35 nm respectively, showing an ordinary tendency to grow with temperature.

Abstract: The dense Pb(Zr0.52Ti0.48)O3 (PZT) piezoelectric ceramics have been prepared at a low temperature by a spark plasma sintering (SPS) method without excess PbO addition and their structural features including domains were systematically investigated. The fine microstructure consisting of submicrometer-sized grains as well as relative density reaching 99% was achieved by sintering at 950°C which is 400°C lower than that of pressureless sintering (PLS). Transmission electron microscopy (TEM) results confirmed that the sintered specimen contained very dense domain structures inside each grain, showing the nanoscaled single-domains even at the small grains (below 100 nm). The SPS-processed PZT exhibited better piezoelectric properties than those of the PLS-processed one, which is attributed to its fine-microstructural feature.