Papers by Author: Seoung Soo Lee

Abstract: In this study, the relationship between microstructural evolution and mechanical properties of thermal barrier coatings (TBCs) has been investigated through different thermal fatigue systems, electric thermal fatigue (ETF) and flame thermal fatigue (FTF), including the thermal stability through the interface between the bond and top coats. The TBC system with the thicknesses of 300 µm in both the top and bond coats was prepared with METCO 204 NS and AMDRY 962, respectively, with the air plasma spray (APS) system using 9MB gun. To observe the oxidation resistance and thermal stability of TBC, the thermal exposure tests were performed with both thermal fatigue tests at a surface temperature of 850 °C with a temperature difference of 200 °C between the surface and bottom of sample, for 12,000 EOH in designed apparatuses. The hardness values are slightly increased due to the densification of top coat with increasing the thermal exposure time in both thermal fatigue tests. The influence of thermal fatigue condition on the microstructural evolution and interfacial stability of TBC is discussed.

Abstract: WC nanosized powders are synthesized by the plasma arc discharge process and annealing under an inert (Ar) atmosphere. The high temperature used during discharging at local region causes the work-piece and electrode to melt and evaporate. The melted tungsten on the arc discharge electrode and carbon arising from decomposition CH4 gas forms WC1-x nanosized powders. The WC1-x phase is encapsulated in a amorphous carbon shell. The WC1-x nanosized particles are annealed at 1200~1400°C under an Ar atmosphere. The WC1-x nanosized powder is transformed to the W2C phase, and then it becomes WC above at 1400°C.

Abstract: In this study, shrinkage behavior and coefficient of thermal expansion (CTE) of novel nickel (Ni) powder with an addition of dielectric material, BaTiO3, have been investigated to reduce the large shrinkage mismatch between Ni electrode and dielectric material and to control the thermal and/or residual stresses created by CTE mismatch in MLCCs (multilayer ceramic capacitors). For which two kinds of Ni powders were used. The component of Ni powders is analyzed by XRF, and the thermal behavior is measured by TG/DTA. The Ni and BaTiO3 powders were mixed with 9:1, 8:2, and 7:3 volume ratios. The BaTiO3-added Ni green bodies were fabricated through cold isostatic pressing, and then sintered to 1300°C in a reduction atmosphere. The shrinkage behavior with volume ratio was checked during sintering from 700 to 1300°C with 300°C interval. The CTE was measured in inert (argon) atmosphere with sintered samples. It is found that the shrinkage behavior and the CTE of Ni electrode are dependent on the volume of BaTiO3 added. The particle size of Ni powder also affects the microstructure and its sintering density, with less effect of its component.

Abstract: Silicon carbide (SiC) nano-powders are successfully synthesized by a reaction between carbon nano-powders (carbon black) and SiO gas at 1300°C for 9 hrs in dynamic argon atmosphere (flow rate; 400 cm3 min-1), using the solid-vapor reaction method. The particle size of synthesized SiC nano-powders is below 40 nm and the shape is uniform. Unexpectedly, SiC nano-fibers are also coexisted in the SiC nano-powders. The quantitative and qualitative properties of the SiC nanopowders and nano-fibers are analyzed by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). Carbon residuals removed by heating over 700°C in air are estimated by thermogravimetry analysis (TGA). It is found that the SiC nanopowders are easily produced by direct synthesis via the solid-vapor reaction method. The morphological characteristics of the resulting SiC nano-powders are dependent upon the morphology of carbon black used as precursor.

Abstract: A novel process in the synthesis of SiC microtubes is proposed, based on modified CVD process. This process was adopted to siliconize the carbon fibers that perform with relatively small surface area, either partially or completely, into the SiC microtubes with large surface area. An oxidic vapor generated from a metal reacts with a carbon solid preform, which leads to a carbide possessing with a unique morphology. The carbon fiber preforms have been unidirectionally arranged into alumina tube, and then a solid-vapor reaction between carbon fiber preforms and SiO vapor originated from the mixture powder pack of Si and SiO2 has been carried out in restoration atmosphere (argon gas/hydrogen gas = 80/20) at 1450°C. The synthesized SiC.C composites were re-sintered with the modified mixture powder pack of Si, SiO2, Al, and Al2O3 at the same condition, in which amounts of Al and Al2O3 employed as a impurity have been controlled under 1 wt% with different mole ratios (Al2O3 : Al = 1:1, 2:1, 3:1). Two kinds of SiO and AlO vapors are generated in the second step reaction process, which react with carbon residuals. The quantitative and qualitative properties of the SiC micorotubes were analyzed by using TGA, SEM, and XRD. The inner morphology of the SiC micorotubes is dependent on the impurity concentration and the gas nature, showing the grains grown with membrane type in the second reaction process because of the impurity.

Abstract: A modified sol-gel method is proposed for the preparation of silicon carbide nano-particles and nano-whisker. Tetraethoxysilane (TEOS), SiO2 bead, and PMMA as precursor of carbon source were used for preparing a ternary carbonaceous silicon xerogel, and morpholine is employed in the sol-gel process as a catalyst reagent. SiC nano-particles and nano-whisker were obtained from carbothermal reduction and vapor-solid (VS) reaction of the ternary xerogel at 1300°C for 9h in a dynamic argon atmosphere (flow rate; 600 cm3/min), and then purified by removing excess silica, residual carbon, and other impurities. The purified SiC sample was characterized by TG/DTA, XRD, SEM, and HRTEM. The nano-particles and the nano-whisker were synthesized by different growth mechanisms, with an initial nucleation via a VS reaction between SiO vapor and carbon precursor (PMMA) and a partially subsequent growth via a vapor-vapor (VV) reaction between SiO and CO, respectively.

Abstract: Ferroelectric properties of Pb-free (Bi,La)4Ti3O12 (BLT) films were optimized on a newly developed MTP cell structure. BLT films were coated on Pt/IrOx/Ir bottom electrode using sol-gel solutions. The composition of the optimized BLT film was about Bi3.25La0.75Ti3.0O12, which was analyzed by ICP-MS method. The switchable polarization obtained in a 100nm-thick BLT film was about 20 uC/cm2 at the 3 V applied voltage, and the optimized BLT film showed little fatigue loss about 10% up to 1×1011 cycles. The imprint properties of the BLT film were also characterized at 25 °C and 90 °C operating temperature after 125 °C data storage. Regardless of operating temperature, switchable polarization of BLT had a sufficiently large margin for device operation up to 10 years.

Abstract: Silicon carbide (SiC) microtubes with villus-like morphology have been successfully fabricated through a reaction between carbon fiber and SiO gas via the pack cementation process. The effect of gas flow nature (argon and/or hydrogen) on silicon carbide microtube synthesis has been studied. The morphology of the microtubes and the production yield are greatly influenced by the nature of gas flow. Mixed ratio of argon to hydrogen gas has a strong influence on the thickness of the microtubes.