Chang-Sup Oh

Korea Institute of Science and Technology Information (KISTI), Daiden, Daejeon, South Korea

Are you Chang-Sup Oh?

Claim your profile

Publications (18)14.98 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The present study prepared molybdenum trioxide (MoO3), the most important intermediate of molybdenum metal, by using a fluidized bed reactor for the thermal decomposition of ammonium molybdate (AM) in the presence of an air flow. During the process of fluidizing the sample inside the reactor, the reaction time and temperature were optimized with a close analysis of the X-ray diffraction (XRD) data and with thermogravimetric analysis (TGA). In particular, the temperature level, at which the AM decomposition is completed, is very important as a primary operating parameter. The analysis of the XRD and TGA data showed that the AM decomposition is almost completed at ~350 °C with a reaction time of 30 min. A shorter reaction time of 10 min. required a higher reaction temperature of ~500 °C with the same air flow rate to complete the AM decomposition. A sharp rise in the decomposition efficiency at a temperature ranging between 320 and 350 °C indicated a threshold for the AM decomposition. The operating conditions determined in this study can be used for future scale-ups of the process.
    No preview · Article · Oct 2015 · Korean Journal of Materials Research
  • Chang-Sup Oh · Yong-Ha Kim · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Al-N-Co-(Zr/Fe/Zr-Fe) films were prepared by using a two-facing target type dc sputtering (TFTS) system. The deposited films were annealed isothermally at 773 K and their magnetic properties and resistivity were investigated. This TFTS method is suitable for preparing Al-N-Co-(Zr/Fe/Zr-Fe) films. The X-ray diffraction profile for Al-N-Co-Zr film shows that it crystallized into the phases of AlN and-Co, while the profiles for Al-N-Co-Fe and Al-N-Co-Zr-Fe films show that they crystallize into three phases of AlN,-Co and-Co. Resistivity shows monotonous increase with increasing annealing time for all kinds of films. The highest resistivity was found for Al-N-Co-Fe-Zr film while lowest was obtained for Al-N-Co-Zr films.
    No preview · Article · Jan 2015
  • Source

    Preview · Article · Nov 2014 · Metals and Materials International
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Metal films (i.e., Ti, Al and SUH310S) were prepared in a magnetron sputtering apparatus, and their crosssectional structures were investigated using scanning electron microscopy. The apparatus used consisted of a cylindrical metal target which was electrically grounded, and two anode rings attached to the top and to the bottom of the target. A wire was placed along the center-line of the cylindrical target to provide a substrate. When the electrical potential of the substrate was varied, the metal-film formation rate depended on both the discharge voltage and the electrical potential of the substrate. As we made the magnetic field stronger, the plasma which appeared near the target collected on the plasma wall surface and thereby decreased the bias current. The bias current on the conducting wire was different from that for cation collection. The bias current decreased because the collection of cations decreased when we increased the magneticcoil current. When the substrate was electrically isolated, the films deposited showed a slightly coarse columnar structure with thin voids between adjacent columns. In contrast, in the case of the grounded substrate, the deposited film did not show any clear columns but instead, showed a densely-packed granular structure. No peeling region was observed between the film and substrate, indicating good adhesion.
    No preview · Article · Jul 2014 · Korean Journal of Materials Research
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Structural studies have been performed on precipitation hardening found in Ni3Al-base ordered alloys using transmission electron microscopy. The γ′ phase hardens appreciably by the fine precipitation of disordered γ. The strength of γ′ increases over the temperature range of experiment by the precipitation of fine γ particles. The peak temperature where a maximum strength was obtained shifted to higher temperature. Superlattice dislocations dissociate into fourfold Shockley partial dislocations in a uniform supersaturated solid solution of the γ′ phase. Dislocations are attracted into the disordered γ phase and dissociate further in the particles. At any stage of aging, dislocations cut through the particles and the Orowan bypassing process does not occur even in the overaged stage of this alloy system. When the applied stress is removed, the dislocations make cross slip into (010) plane, while those in γ precipitates remain on the (111) primary slip plane. The increase of high temperature strength in γ′ containing γ precipitates is due to the restraint of cross slip of dislocations from (111) to (010) by the dispersion of disordered γ particles. The orientation dependence of strength is decreased by the fine precipitation of a disordered γ phase.
    No preview · Article · Sep 2013 · Metals and Materials International
  • Source
    Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: The mechanical properties and microstructures of aluminum-matrix composites fabricated by the dispersion of fine alumina particles less than in size into 6061 aluminum alloys are investigated in this study. In the as-quenched state, the yield stress of the composite is 40~85 MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix introduced due to the difference in the thermal expansion coefficients between the matrix and the reinforcement. The difference in the yield stress between the composite and the 6061 alloy decreases with the aging time and the age-hardening curves of both materials show a similar trend. At room temperature, the strain-hardening rate of the composite is higher than that of the 6061 alloy, most likely because the distribution of reinforcements enhances the dislocation density during deformation. Both the yield stress and the strain-hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.
    Preview · Article · Mar 2013 · Korean Journal of Materials Research
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)2Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix was (0001)p//(111)B2 and $[\bar 12\bar 10]_p //[\bar 110]_{B2}$ , where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)2Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.
    No preview · Article · Mar 2013 · Metals and Materials International
  • Source
    Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: The B2-ordered NiAl has attracted much attention as one of the candidates as a next generation high temperature material, because it has a high melting temperature, a low specific gravity and an excellent high temperature oxidation resistance. However, the application of NiAl to structural materials needs the improvement of its brittleness at room temperature. The study was carried out on the relation between several properties of NiAl and some variation of Ni content within NiAl phase, which means deviations from the stoichiometric composition. The main results were as follows; (i) Good ductility was obtained at the testing temperature more than 1073 K irrespective of Ni content. (ii) Increasing Ni content offered preferable tensile properties. (iii) Every NiAl with varying Ni contents showed the superior oxidation resistance.
    Preview · Article · Jan 2013
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Transmission electron microscopy (TEM) examinations of Al3Ti with an L12-ordered structure have revealed the precipitation of D023-Al11Ti5 and L10-TiAl upon aging after quenching from higher temperatures. TEM observations revealed that fine uniform precipitation of Al2Ti occurs when the supersaturation is sufficiently high, and, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L12-Al3Ti is supersaturated with DO22-Al3Ti, DO23-Al11Ti5 with a multidomain structure is formed during aging. On the other hand, plate-like L10-TiAl precipitates lie on the {001} planes of (Al,Ag)3Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. The Ll2 phase field in the Al-Ti-Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.
    No preview · Article · Jun 2012 · Metals and Materials International
  • Seong-Hoon Hong · Dong-Kyu Lee · Chang-Sup Oh · Yong-Ha Kim

    No preview · Article · Mar 2012
  • Source
    Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: The purpose of this study is to prepare a high strength fiberglass reinforced metal. Aluminum covering was carried out over carbon materials such as carbon fiber in order to increase their wettability to molten metals such as aluminum. A sputtering apparatus with a cylindrical target was fabricated to carry out the covering. Sputtering was caused by glow discharge between the target and the two anode plates attached to its top and bottom. As the substrate for preliminary test, a thin carbon wire was used instead of carbon fiber, and the wire was placed at the central axis of the target. Aluminium coating was formed on the whole surface of the substrate. The formation rate and structure of coating were varied by controlling the electrical potential of substrate. When the substrate was electrically isolated, coating with columnar structure was formed with a formation rate of . In case of grounded substrate, coating with amorphous structure was formed with a formation rate of .
    Preview · Article · Jan 2012
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: The mechanical behavior and microstructural evolution during high temperature tensile deformation of recrystallized Ni 3Al polycrystals doped with boron were investigated as functions of initial grain size, tensile strain rate and temperature. In order to obtain more precise information on the deformation mechanism, tensile specimens were rapidly quenched immediately after deformation at a cooling rate of more than 2000k s-1 and were then observed by transmission electron microscopy (TEM). Mechanical tests in the range of 923 K to 1012 K were carried out in a vacuum of less than 3×10 -4 Pa using an Instron-type machine with various but constant cross head speeds corresponding to the initial strain rates from 1.0×1.0 -4 to 3.1×1.0 -5s-1 After heating to deformation temperature, the specimen was kept for more than 1.8 ks before testing. The following results were obtained: (1) Flow behavior was affected by initial strain size; with decreasing initial grain size, the level of a stress peak in the true stress-true strain curve decreased, the steady state region was enlarged and elongation increased. (2) On the basis of TEM observation of rapidly quenched specimens, it was confirmed that dynamic recrystallization certainly occurred on deformation of fine-grained (3.3μm) and intermediate-grained (5.0μm) specimens at an initial strain rate of 3.1×1.0 -5s-1 and at 973 K. (3) There were some dislocation-free grains among the new recrystallized grains. The obtained results suggest that both dynamic recrystallization and grain boundary sliding are operative during high temperature deformation.
    No preview · Article · Jan 2012 · Korean Journal of Materials Research
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Aluminum nitride(AlN) is a compound (III-V group) of hexagonal system with a crystal structure. Its Wurzite phase is a very wide band gap semiconductor material. It has not only a high thermal conductivity, a high electrical resistance, a high electrical insulating constant, a high breakdown voltage and an excellent mechanical strength but also stable thermal and chemical characteristics. This study is on the preferred orientation characteristics of A1N thin films by reactive evaporation using NH 3. We have manufactured an A1N thin film and then have checked the crystal structure and the preferred orientation by using an X-ray diffractometer and have also observed the microstructure with TEM and A1N chemical structure with FT-IR. We can manufacture an excellent A1N thin film by reactive evaporation using NH 3 under 873 K of substrate temperature. The A1N thin film growth is dependent on Al supplying and NH 3 has been found to be effective as a source of N 2. However, the nuclear structure of A1N did not occur randomly around the substrate a particle of the a-axis orientation in fast growth speed becomes an earlier crystal structure and is shown to have an a-axis preferred orientation. Therefore, reactive evaporation using NH 3 is not affected by provided H 2 amount and this can be an easy a-axis orientation method.
    No preview · Article · Jan 2012 · Taehan-Kŭmsok-Hakhoe-chi = Journal of the Korean Institute of Metals and Materials
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)2Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [[`1]\bar 12[`1]\bar 10]p//[[`1]\bar 110]B2, where the suffixes p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)2Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles. Keywordsintermetallics–aging–mechanical properties–transmission electron microscopy (TEM)–fine precipitation
    No preview · Article · Apr 2011 · Metals and Materials International
  • Chang-Sup Oh · Chang-Suk Han
    [Show abstract] [Hide abstract]
    ABSTRACT: When alloys are vacuum-deposited on cooled substrates, super-rapidly cooled alloy films in the unequilibrium state can be obtained. As an application of this method, Ag-Cu, Ag-Ni and Ag-C alloys were successfully produced, and their mechanical properties with tempering temperature were investigated. The following results were obtained : (1) In case of Ag-Cu alloys, the solid solution was hardened by tempering at . The hardening is considered to occur when the solid solution begins to decompose into and phases. The Knoop hardness number of a 40 at.%Ag-Cu alloy film deposited on a cooled glass substrate was 390 . The as-deposited films were generally very hard but fractured under stresses below their elastic limits. (2) In case of Ag-Ni and Ag-C alloys, after the tempering of 4 at.%Ni-Ag alloy at and of 1 and 2 at.%C-Ag alloys at , they were hardened by the precipitation of fine nickel and carbon particles. The linear relationship between proof stress vs. for bulk silver polycrystals can be applied to vacuum-deposited films up to about 0.1 grain diameter, but the proof stress of ultra-fine grained silver with grain diameters of less than 0.1 was smaller than the value expected from the Petch's relation.
    No preview · Article · Jan 2011
  • [Show abstract] [Hide abstract]
    ABSTRACT: A transmission electron microscopy investigation on the phase decomposition of B2-ordered (Ni,Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni,Co)2Al which has not been expected from the reported equilibrium phase diagram. The (Ni,Co)2Al phase has a hexagonal structure and takes a rodlike shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [$$ \bar 1 $$2$$ \bar 1 $$0]p//[$$ \bar 1 $$10]B2, where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by the fine precipitation of the (Ni,Co)2Al phase.
    No preview · Article · Apr 2010 · Metals and Materials International
  • [Show abstract] [Hide abstract]
    ABSTRACT: Because of their extremely large surface art a, carbon nano tubes (CNTs) and carbon nano fibers (CNFs) have been widely used to remove heavy metals from water. But the removal of such CNTs and CNFs from water is another important aspect in water purification process. In the present work, we discuss CNT and CNF separation from water using a magnetic separation technique. In order to compare the separation efficiencies, CNTs and CNFs having wide variation in their magnetic properties were selected. The effects of CNT and CNF concentrations in untreated water and water now velocity on the separation efficiency were investigated systematically.
    No preview · Article · Jul 2008 · Journal of chemical engineering of Japan
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm3 and 520 m2/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.
    No preview · Article · Mar 2007 · Microporous and Mesoporous Materials