Papers by Author: Chan Park

Abstract: Influence of heat input on the tensile strength and impact toughness of multipass weld metal made with AWS E81T1-Ni1 metal-cored wire was investigated. Welding parameters such as current, voltage and travel speed were varied independently to get different heat inputs. When it was increased by varying current, tensile strength of the weld metal increased even if more primary ferrite and wider columnar grains were observed. The increase is attributed to the higher recovery ratio of deoxidizing elements such as carbon, manganese and silicon due to the shorter reaction time in both wire tip and arc column. It also showed that impact toughness was influenced by the formation of reheated weld metal by subsequent passes and it decreased continuously with an increase of the amount of coarse grained region in the reheated weld metal.

Abstract: Lead zinc niobate (PZN) added lead zirconate titanate (PZT) thick films with thickness of 5~10 μm were fabricated on silicon and sapphire substrates using aerosol deposition method. The contents of PZN were varied from 0, 20 and 40 %. The as deposited film had fairly dense microstructure without any crack, and showed only a perovskite single phase formed with nano-sized grains. The as-deposited films on silicon were annealed at temperatures of 700oC, and the films deposited on sapphire were annealed at 900oC in the electrical furnace. The effects of PZN addition on the microstructural evolution were observed using FE-SEM and HR-TEM, and dielectric and ferroelectric properties of the films were characterized using impedance analyzer and Sawyer-Tower circuit, respectively. The PZN added PZT film showed poor electrical properties than pure PZT film when the films were coated on silicon substrate and annealed at 700oC, on the other hand, the PZN added PZT film showed higher remanent polarization and dielectric constant values then pure PZT film when the films were coated on sapphire and annealed at 900oC. The ferroelectric and dielectric characteristics of 20% PZN added PZT films annealed at 900oC were comparable with the values obtained from bulk ceramic specimen with same composition sintered at 1200oC.

Abstract: Well-defined block copolymers consisting of a hydrophilic poly (ethylene oxide) (PEO) and lipophobic poly (1H,1H-perfluorooctyl methacrylate) (PFOMA) were synthesized with controlled molecular weight via atom transfer radical polymerization (ATRP). The block copolymers formed stable micelles where PEO is the shell and PFOMA is the core in chloroform. The micellar morphology of PEO5K-b-PFOMA5K was found to be spherical with average diameter of ca. 12 nm, while meandering cylindrical with average diameter of ca. 13 nm was observed for PEO5K-b-PFOMA13K by TEM analysis. The self-assembled structures were reorganized to thermodynamically stable morphologies upon annealing above glass transition temperature (Tg). For example, spherical shape of PEO5K-b-PFOMA5K and cylindrical domain of PFOMA block in PEO5K-b-PFOMA5K became ordered cylindrical and continuous phase, respectively.

Abstract: The interaction between gas atmosphere and silicon during reaction bonded silicon nitride(RBSN)process leads to a non-uniform band formation of alpha silicon nitride. The reaction layer, α -Si3N4, was formed near the surface of the sample in the early stage of RBSN. Reactive nitrogen gas was supplied as static state using computer controlled gas delivery system. The formation of α -Si3N4 band near the surface of the sample can be explained thermodynamically, based on the nitrogen partial pressure in the gas mixtures.

Abstract: The effects of nitrogen content on weld metal impact toughness in submerged arc welding were investigated and interpreted in terms of microstructural changes and solid solution hardening. The weld metal impact toughness in as-welded condition decreased with increasing nitrogen content from 110 to 200 ppm. The weld metal microstructure changed with increasing nitrogen content; ferrite with second phase increased at the expense of tough acicular ferrite. In addition to microstructural changes, the microhardness of acicular ferrite increased gradually with the nitrogen content. Therefore, the loss of impact toughness can be attributed to a combination of the effects of microstructural changes and solid solution hardening.

Abstract: Silicon nitride samples without and with 3 wt% of the aligned b-silicon nitride whisker seeds were prepared with 8.2 wt% Er2O3 and 1.9 wt% AlN. After sintering at 2148 K for 4h, the samples exhibited densities higher than 99.5% TD. The microstructures and properties of the samples were compared with those of the samples sintered with 4.8 wt% Y2O3 and 2.2 wt% Al2O3 at 2273 K for 4h. For samples without the whiskers, the sample with 4.8 wt% Y2O3 + 2.2 wt% Al2O3 had coarser microstructures than those with with 8.2 wt% Er2O3 + 1.9 wt% AlN. However, the samples with the whisker seeds, the former sample appeared to have only slightly larger grains than the latter sample in spite of the significant difference in the sintering temperatures. For the samples without the whisker seeds, the room temperature flexural strength was higher for the sample with Er2O3 + AlN. However, for the samples with the aligned whisker seeds, the sample with Y2O3 + Al2O3 exhibited higher room temperature flexural strength than that with Er2O3 + AlN although the average grain width of the former sample was larger than that of the latter sample. In case of the high temperature flexural strength at 1673 K, the flexural strengths of the samples with the whisker seeds were higher than double the strengths of the samples without the whisker seeds. For samples without the whisker seeds, the sample with Er2O3 + AlN exhibited better mechanical properties than that with Y2O3 + Al2O3. However, for the samples with the aligned whisker seeds, the sample with Y2O3 + Al2O3 exhibited better mechanical properties than those with Er2O3 + AlN. The results were explained in terms of the microstructures of the samples.