No abstract

In terms of steel alloying elements, generally, nickel is used as an austenite stabilizing element to increase the toughness of steel. The low temperature materials, such as nickel alloy steels with a nickel content of 3.5% to 9%, stainless steel and Invar, show excellent toughness at low (173 K) and cryogenic (108 K) temperatures. In particular, in the shipbuilding industry, it is mainly used for liquefied ethane and Liquefied Natural Gas (LNG) carriers, and research on low-temperature steels are attracting attention again as regulations on environmental issues are strengthened in recent years. Therefore, in this study, fatigue and fracture performances of nickel alloy steel containing 9% or less among nickel alloy steels are evaluated. Moreover, we assess the Fatigue Ductile to Brittle Transition (FDBT) of nickel alloy steels based on crack tip opening displacement (CTOD). In order to discuss the fatigue and fracture performances of nickel alloy steels, microstructure analysis carried out. As a result, CTOD and Fatigue Crack Growth Rate (FCGR) of nickel alloy steels increases as nickel contents increase. In addition, FDBT of 9% nickel alloy steel is the lowest compared to other nickel alloy steels.

Chemical converted graphene (CCG) were coated on 316L stainless steel as a bipolar plate which is a component of proton exchange membrane fuel cell (PEMFC) by electro spray coating (ESC). Scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to examine the thickness and surface properties of coating layer. Electrochemical potentiodynamic test was conducted in acidic atmosphere (0.1N H2SO4+2ppm F-) at 80°C using Versastat 4 and analysis program for corrosion resistance measurement. After packing bipolar plates for PEMFC stack, the electrical performances of graphite, bare SS316L and graphene coated SS316L bipolar plates were evaluated by PEMFC evaluating device. The chemical converted graphene was founded on the surface of coated SS316L, and the thickness was 12μm. Graphene coated bipolar plate showed high corrosion resistance of 1.32×10-7A/cm2beside bare SS316L bipolar plate. In electrical performance evaluation, the graphene coated bipolar plate was shown 0.978V on Voc and 0.5A/m2on the reduction potential (0.6V). Although the electrical performance of the graphene coated bipolar plate is lower than graphite bipolar plate, the thickness and weight is lower than graphite bipolar plate. These advantages can enable the PEMFC system more efficiently and economically.

Graphene was coated on low carbon steel (SS400) by electro spray coating method to improve its properties of corrosion resistance and contact resistance. Exfoliated graphite was made of the graphite by chemical treatment (Chemically Converted Graphene, CCG). CCG is distributed using dispersing agent, and low carbon steel was coated with diffuse graphene solution by electro spray coating method. The structure of the CCG was analyzed using XRD and the coating layer of surface was analyzed using SEM. Analysis showed that multi-layered graphite structure was destroyed and it was transformed in to fine layers graphene structure. And the result of SEM analysis on the surface and the cross section, graphene layer was uniformly formed with 3-5 microm thickness on the surface of substrate. Corrosion resistance test was applied in the corrosive solution which is similar to the polymer electrolyte membrane fuel cell (PEMFC) stack inside. And interfacial contact resistance (ICR) test was measured to simulate the internal operating conditions of PEMFC stack. As a result of measuring corrosion resistance and contact resistance, it could be confirmed that low carbon steel coated with CCG was revealed to be more effective in terms of its applicability as PEMFC bipolar plate.

Some composites of gold nanoparticles and graphite nanosheets were prepared by electrostatic interaction, and structurally and electrochemically characterized using X-ray diffraction, X-ray photoelectron spectroscopy, UVNis spectroscopy, transmission electron microscopy, and cyclic-voltammetry. Pristine graphite was chemically treated using aqueous acid solution, and dispersed inpoly(diallyldimethylammonium) chloride aqueous solution to prepare positively charged graphite nanosheets. The gold nanoparticles (GNPs) in this work were stabilized by sodium dodecyl sulfate, poly(sodium 4-styrene sulfonate), or poly(vinylpyrrolidone). Gold nanoparticles and graphite nanosheet composites with gold nanoparticles showed the characteristic surface plasmon band at -530 nm. The electrochemical properties of the graphite nanosheet composites with gold nanoparticles were studied by cyclic voltammetry, in which reduction potential and reduction current of gold nanoparticles were strongly dependent on the gold-wrapped stabilizer in the composites.