발간논문

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The hot corrosion properties of heat-resistant steels were investigated in an oxidation atmosphere including artificial ash and sulfur dioxide. The heat-resistant steels of T22, T92, T122, T347HFG, Super304H and HR3C were evaluated at 620, 670 and 720℃ for 400 hours. The relationship between the corrosion rate and the temperature followed a bell-shaped curve with a peak rate at around 670℃. The corrosion rates showed a decreasing tendency as the chrome contents of these steels increased from 2.l5 wt.% to 24.5 wt.%, and austenitic steels had a lower corrosion rate than ferritic steels. Sulfidation by SO2 as well as molten salt corrosion also had an effect on the total corrosion rate, especially showing an increase in the corrosion rate in ferritic steels. Regardless of the chrome content in the steels and irrespective of the test temperature, the corrosion scale was composed of an outer oxide and an artificial ash mixed layer, a middle oxide layer andinner sulfide, and amixed oxide layer. As the chrome content increased, the proportion of chrome oxide in the corrosion scale increased. Before spalling of the corrosion scale, voids and cracks were initiated in the sulfide and the mixed oxide layer or at the interface with the substrate. (Received December 16, 2009)

keyword : oxides, aging, corrosion, scanning electron microscopy, SEM, X-ray diffraction

The effects of heat input and different microstructureswere investigated on the tensile-shear properties of an arc-brazed joint of theferritic stainless steel 429EM using a Cu-Si insert alloy. The brazing speed was fixed at 800 mm/min whilethe brazing current varied from 80 to l20A. For abrazing current lower than l00A, fracturing occurred at the joint root in the direction perpendicular to the tensile load. As the brazing current increased to l20A, fracturing occurred at the base metal or the joint root. The joint and the base metal had very similaryield and tensile load values. However, the amount of elongation was decreased considerably compared towhen the base metal was used. The fracturingbegan at the triple point of the root part and was classified into three types. The difference in the tensile-shear properties was closely related tothe three fracturetypes.

keyword : ferritic stainless steel, brazing, mechanical property, tensile test

The microstructure and hardness of CO2 laser welds were investigated in the Ti-stabilized ferritic stainless steel 409L. The observed specimen was welded in a fully penetrated condition in which the power was 5 kW and the welding speed 5 m/min. The grain structure near the bond line of the laser welds was produced by epitaxial growth. The grain size was the largest in the fusion zone, and HAZ showed nearly the same grain size as that of the base metal. The HAZ microstructure consisted of subgrains and precipitates that were less than l00 nm in size and that were located along the subgrain boundaries. On the other hand, the hardness was the highest in the fusion zone due to the large amount of small precipitates present. These were composed of TiN, Ti(C,N) and TiO2+Ti(C,N). The hardness decreased continuously from the fusion zone of the base metal. The HAZ hardness was slightly greater than that of the base metal due to the existence of subgrains and precipitates in the subgrain boundary. (Received January 13, 2010)

keyword : alloys, welding, microstructure, scanning/transmission electron microscopy, STEM, hardness

The development of textures and microstructures during plastic deformation in a hot-extruded AZ 31 Mg alloy was investigated using a compression test with such parameters as deformation temperature, strain rate. It was observed from true stress-strain curves that twinning involves changes of the flow stresses. In the early stages of deformation at temperatures lower than 200℃, the occurrence of twins resulted in a decrease of the work-hardening rate, which increased drastically at a true strain of -0.05. The evolution of the deformation textures were assessed with the aid of EBSD analyses in terms of the competition between twinning and slip activity. (Received November 26, 2009)

keyword : Mg alloy, EBSD, texture, twinning, extrusion

In this study, the hydrogen reduction behavior of ball-milled NiO nanopowder was investigated depending on the partial pressure of water vapor. The hydrogen reduction behavior was analyzed by thermogravimetry and hygrometry under heating to 873 K in hydrogen. In order to change the partial pressure of the water vapor, the dew point of hydrogen was controlled in the range of 248 K~293 K by passing high-purity hydrogen through a saturator that contained water. Interestingly, with the increase in the dew point of the hydrogen atmosphere, the first step of thehydrogen reduction process decreased and the second step gradually increased. After the first step, apore volume analysis revealed that the pore size distribution in the condition with ahigher water vapor pressure shifted to a larger size, whereasthe opposite appearedat a lower pressure. Thus, it was found that the decrease in the pore volume during the chemical reaction controlled process at a dew point of 248 K caused a reduction in retardation in the diffusion controlled process. (Received November 18, 2009)

keyword : NiO, nanostructured materials, H2 reduction, powder processing, thermodynamic properties

NiO thin films were deposited by radio frequency magnetron sputtering on glass substrates. The processing variables of the oxygen content, sputtering power, and pressure were varied to investigate the electrical properties and surface morphology of NiO films. It was found that the resistivity of NiO films at 1.22xl0(2) Ωcm (2.5% O2in Ar gas) was greatly reduced to 2.01x10(-1) Ωcm (100% oxygen) under a typical sputtering condition of 6 mTorr and 200 watts. In an effort to observe the resistivity variances, the sputtering power was varied from 80 to 200 watts at 6 mTorr with 100% O2. However, the resistivity of the NiO films changed in the range of 10(-1)-10(-2) Ωcm. The dependence on the sputtering power was therefore found to be weak in this experiment. When the sputtering pressure was changed from 3 to 60 mTorr at 200 watts with 100% O2, the resistivity of the NiO films showed the lowest value of 5.8xl0(-3) Ωcm at 3 mTorr, which is close to that of commercial ITO films (~10(-4) Ωcm). As the sputtering pressure increased, the resistivity also increased to 4.67 cm at 60 mTorr. The surface morphology of the NiO films was also checked by Atomic Force Microscopy. It was found that the RMS surface roughness values ranged from 0.6 to 1.5 nm and thtthe dependence on the sputtering parameters was weak. (Received October 29, 2009)

keyword : optoelectronic materials, sputtering, electrical properties, resistivity, AFM

Porous and net-shaped tantalum powder for a capacitor was formulated in a SHS (self-propagating high-temperature synthesis) process. However, this powder, which has weak strength among its particles and low flow ability, cannot be used for a capacitor. Therefore, this powder was sintered in a high-vacuum furnace to increase agglomeration to improve the flow ability, bonding strength among the particles, and shrinkage during pellet sintering. Finally, it was deoxidated with 2 wt% Mg powder to remove the increased surface oxygen that arose during the sintering process. The final product was analyzed in terms of its chemical and physical properties and was compared with a commercial powder used by a capacitor manufacturer. (Received October 29, 2009)

keyword : SHS, capacitor, tantalum powder, deoxidation, porous tantalum

High-quality single-walled carbon nanotubes (SWCNTs) were synthesized by catalytic decomposition of C2H2 using Fe-Mo/MgO catalyst at 800℃. The as-synthesized SWCNTs typically occurred in the form of a bundle with a diameter of 10~20 nm together with amorphous carbon and catalytic impurities, which were removed by a two-step purification process consisting of oxidation and an acid treatment. The oxidation step, using an O2-Ar mixture at 380℃ for 5 hr in a vertical-type furnace and aHNO3 treatment at 100℃ for one hour, was utilized to remove the amorphous carbon particles. Subsequently, metallic catalysts were removed in HCl at room temperature for 5 hr under magnetic stirring. The SWCNT suspension was prepared by dispersing the purified SWCNTs in an aqueous sodium dodecyl benzene sulfonate solution with horn-type sonication. This was then air-sprayed on glass to fabricate CNT field emitters. The samples had a turn-on field value of 4 V/μm and a current density of 0.67 mA/cm2 at 9 V/μm. Increasing the HCl treatment time improved the field emission properties. (Received October 29, 2009)

keyword : nanostructured materials, purification, electrical properties, activation analysis, carbon nanotube

Pd nanoparticles (NPs) were successfully functionalizedon the surfaces of single-walled carbon nanotubes (SWNTs) by dendrimer-mediated synthesis. The hydrogen sensing properties of the Pd NPs functionalized SWNTs were investigated. Pd NPs-dendrimer-SWNTs sensors show much better speedsand superior recovery rates but lower sensitivity compared toPd NPs-functionalized SWNTs directly fabricated due to the existence of dendrimers. Pyrolysis of the dendrimers by heat treatment resulted in afast response time and high sensitivity owing to the reduced length of the dendrimers. Theseresults demonstrate that the heat treatment of dendrimers in Pd NPs-dendrimer-SWNTs sensors can enable significant electrical conductance modulation upon exposure to extremely low concentrations (10 ppm) of hydrogen gas (H2) in air. (Received October 27, 2009)

keyword : Carbon and graphite, chemical synthesis, hydrogen, transmission electron microscopy, TEM, hydrogen absorbing materials

This study reports a novel method off abricating highly sensitive hydrogen gas sensors based on PdQ thin films. The PdQ thin films with a thickness of 40 nm were deposited on Si substrates under Ar and Q2 ambient conditions using a reactive dc magnetron sputtering system. Considerable changes in the resistance of the palladium oxide thin films were observed when they were initially exposed to hydrogen gas, as a result of the reduction process. The sensitivity of the PdQ thin films was found to be as high as 90%. After the thin films were exposed to hydrogen gas, the nano-sized cracks were discovered to have formed on the surface of the PdQ thin films. These types of nano-cracks that formed onthe deoxidized PdQ thin films are known toplay a key role incausing a four-fold reduction of the response time of the absorption process. The results of this study demonstrate that deoxidized PdQ thin films can be applied for use in the creation of high-sensitivity hydrogen sensors. (Received November 16, 2009)

keyword : Hydrogen absorbing materials, sputtring, hydrogen, scanning electron microscopy, SEM, microstructure