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Now showing items 1 - 16 of 52

  • Emission properties of Zinc Gallate nanophosphors

    Lee, Mun Keun   Kang, Shinhoo  

    Nano-sized ZnGa2O4, ZnGa2O4:Cr and ZnGa2O4:Ti phosphors were synthesized by low-temperature combustion method and the luminescent properties were studied in comparison with those from conventional solid-state reaction. The combustion reaction resulted in a single-phase spinel structure and the particles were 5-10nm in size. Of all samples, undoped nano ZnGa2O4 exhibited an intrinsic blue emission and Ti-doped nano ZnGa2O4 had a tendency of white emission whereas Cr-doped nano ZnGa2O4 showed red emissions. Titanium ions in spinel structure was found to exist as in Ti3+ and Ti4+, but favouring trivalent state. The oxidation state of Cr ions was confirmed as 3(+) in nano ZnGa2O4. (C) 2018 The Ceramic Society of Japan. All rights reserved.
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  • Extremely toughened Ti-based solid-solution carbide cermets

    Kwak, Nojun   Kang, Shinhoo   Park, Choongkwon   Han, Heung Nam  

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  • Defects on the Surface of Ti-Doped MgAl2O4 Nanophosphor

    Lim, Jaehyuk   Kim, Yongseon   Kim, Sungdae   Kim, Youngwoon   Kang, Shinhoo  

    Ti-doped nano MgAl2O4 for white emission was synthesized by combustion method. Extrinsic Schottky defects, Al vacancies and Ti4+ dopant in Al sites, which are considered to be responsible for bluish-white emission, were observed by STEM on the surface of Ti-doped nano MgAl2O4 powder. The stabilities of the Schottky defect associates, (Ti-Al-V'''(Al))'', were demonstrated by DFT calculation. The emission behavior was interpreted with these results.
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  • Effect of two-step reduction on ZrC size and dispersion

    Kim, Jae-Hee   Seo, Moonsu   Park, Choongkwon   Kang, Shinhoo  

    A two-step carbothermal reduction process produced zirconium carbide particles with less aggregation and smaller ZrC grains than those produced by one-step reduction. Reduction at 1500 degrees C for 10 min (1st step) and then at 1300 degrees C for 30 min (2nd step) produced significantly smaller agglomerations (1-8 mu m) and particles (33 nm) than those produced using other reduction processes, which were 2200 mu m and >1 mu m, respectively. The first high-temperature step allowed effective reduction, while the subsequent cooler step inhibited grain growth and agglomeration. The resulting ZrC particles showed a stoichiometric composition of ZrC0.97 through control of the carbon content. A sintered W-ZrC composite made using the two-step-reduced ZrC was harder and showed a more homogeneous dispersion of ZrC in the tungsten matrix than a composite made using one-step sintered ZrC. (C) 2015 Elsevier B.V. All rights reserved.
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  • Microstructures and properties of ultrafine grained W-ZrC composites

    Kim, Jae-Hee   Park, Choongkwon   Lim, Jaehyuk   Kang, Shinhoo  

    The microstructures and properties of W-ZrC composites prepared in situ were compared with those of conventionally prepared W-ZrC and W-(Zr, W) C. In situ preparation led to an ultrafine microstructure with a homogeneous dispersion of ZrC, while the other composites showed microstructures similar to each other. The composite of W containing 30 vol.% (Zr-0.88 W-0.12) C showed an excellent flexural strength of similar to 1425 MPa at 1000 degrees C. It also showed an excellent flexural strain of 0.051 at 1400 degrees C, which was obtained by using a maximum displacement of 2.41 mm. Those values have never been reported previously. A similar W composite, in situ W-10 vol.% ZrC, demonstrated a flexural strength of 1324 MPa and a displacement of 0.9 mm under similar conditions. The results of this study are discussed in terms of microstructure and phase stability. (C) 2014 Elsevier B.V. All rights reserved.
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  • Thermodynamic stability of in situ W-ZrC and W-Zr(CN) composites

    Kim, Jae-Hee   Zhe, Gao   Lim, Jaehyuk   Park, Choongkwon   Kang, Shinhoo  

    Powders of W-ZrC and W-Zr(CN) were carbothermally synthesized in situ from milled mixtures of graphite, WO3 and ZrO2. The thermal stability of Zr(CN) in a W matrix was simulated and compared with that of ZrC in W in terms of free energy change and carbide coarsening. Carbon and nitrogen had high mutual affinity in Zr(CN) of B1 crystal structure, which led their activity curves to exhibit strong negative deviation from ideal mixing behavior. Zr(CN) was more stable than ZrC up to 2075 K; however, a microstructural study showed that it became less stable than ZrC at around 1975 K. This result is attributed to the decreasing thermodynamic stability of ZrN with increasing temperature. Other transition metal carbonitrides containing group 4-6 elements are expected to show similar coarsening behaviors at high temperatures. (C) 2015 Elsevier B.V. All rights reserved.
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  • Effect of nano-sized SiO2 on the optical properties of YVO4:Eu3+ phosphors

    Rafiaei, Seyed Mahdi   Kang, Shinhoo  

    In this paper, YVO4: 1%Eu3+ phosphor was synthesized via solid state method at 1100 degrees C. Then TEOS was used as the source of silica, to coat the phosphors, using sol-gel approach. HRTEM analysis confirmed the formation of adhered and smooth coating layer with the thickness of 40-50nm. From the experiments and characterizations, we found that although the amounts of added SiO2 to the phosphors were not remarkable, but it resulted in enhancement of photoluminescence properties. Interestingly, under the excitation wavelength of 310nm, the efficiency of the phosphors increased by about 20%. Also, a considerable effect of coating layer on decrease in surface oxygen vacancies was studied using ESR technique. Finally it was found that SiO2 coating of YVO4:Eu3+ phosphors, improves both chemical stability and thermal quenching, effectively.
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  • Comprehensive Hard Materials || Cermets

    Kang, Shinhoo  

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  • Morphology controlled Co-precipitation method for nano structured transparent MgAl2O4

    Nam, Sangwoo   Lee, Munkeun   Kim, Byung-Nam   Lee, Younghwan   Kang, Shinhoo  

    MgAl2O4 spinel nano powders with various morphologies were synthesized with ammonium aluminum carbonate hydroxide by carbonate precipitation. The powders with rod, corn, and spherical morphologies were synthesized, varying the concentration of ammonium ions, by growth and aggregation of nano precipitates. The powders with spherical morphology formed spinel phase at a lower calcination temperature than those with other shapes. The morphology provided a superb sinterability due to improved particle packing. Although the synthetic conditions resulted in a composition slightly off the stoichiometric composition with Mg loss, this provided a proper condition for inhibiting grain growth. Finally, Mg-spinel which was obtained from spherical particles and fabricated by spark plasma sintering (SPS) showed much improved transparency with an average grain size of 200 nm (MT and TFT are 55.92% and 63.75% at 550 nm 75.35% and 78.91% at 1500 nm, respectively) than those from other shaped particles.
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  • Stability domain of Ti(CN) in Ti-TiC-TiN

    Kwon, Hanjung   Kang, Shinhoo  

    Phase stability diagrams of Ti-TiC-TiN-Ti(CN) system were constructed at 1673 K as a function of carbon activity, nitrogen pressure, and characteristics of solution formation. The domains for Ti and Ti(CN) exist in real systems. It was also found that the domain for Ti(CN) is much larger than that of ideal mixing of TiC and TiN. This is because the mixing reaction of TiC and TiN or the nitriding of TiC occurs exothermically. The stable area of nano-sized Ti(CN) was smaller than that of micron-sized Ti(CN) due to the instability of nano-sized Ti(CN). (C) 2011 Elsevier B.V. All rights reserved.
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  • Tool performance of new wear-resistant cermets

    Kim, Yong-Sun   Kwon, Won Tae   Seo, Moonsu   Kang, Shinhoo  

    Ti(C,N)-based cermets were recently designed for tool applications as they offer enhanced interfacial strength. Such a cermet exhibits a typical core/rim microstructure as found in conventional cermets. To determine the possibility of using new cermet as a cutting tool, Ti(C,N) along with various carbides, nitrides, solid-solution carbonitride such as (Ti,W)(C,N), and Ni were milled together, sintered into the size of SNGN120408 and used in turning the SM45C (AISI45). Commercial WC and cermet insert of an identical size were selected to compare its cutting capability with that of the home-made cermet tool. To ensure a fair comparison of the cutting performance, the optimal machining conditions for commercial WC, cermet and home-made cermet tools were obtained using the Taguchi method. Experiments under both optimal conditions showed the excellent cutting performance of the new home-made cermet tool. It is believed that the interface between core Ti(C,N) and rim phases has been improved significantly in the new cermets.
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  • Carbide/binder interfaces in Ti(CN)-(Ti, W)C/(Ti, W)(CN)-based cermets

    Park, Choongkwon   Nam, Sangwoo   Kang, Shinhoo  

    Partially solutionized carbide cermets (PSCs) were prepared for cutting tool applications by replacing a portion of Ti(CN) with (Ti0.88W0.12) C or (Ti0.88W0.12)(C0.7N0.3) in a conventional Ti(CN)-based cermet. The PSC containing (Ti0.88W0.12) C exhibited high fracture toughness with no loss of hardness. The elimination of Ti(C0.7N0.3) cores in the microstructure reduced the strain at the core/rim interfaces, leading to the increase in fracture toughness. The PSC containing (Ti0.88W0.12) C showed coherent relationships at the core/rim and rim/binder interfaces, contributing to the improved mechanical properties. In contrast, nitrogen in (Ti0.88W0.12)(C0.7N0.3) resulted in a different microstructure and properties. The major factors determining the mechanical properties of the cermets are discussed in terms of the carbide/binder interfaces and the thermal stability of the added carbides. (C) 2015 Elsevier B.V. All rights reserved.
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  • Solid-solution red phosphors for white LEDs

    Lee, Gwan-Hyoung   Kang, Shinhoo  

    Solid solutions of Eu3+-doped metal tungstate and molybdate phosphors were synthesized by solid-state reactions. The crystal structure of the solid-solution phosphors was determined to be tetragonal with a space group of I4(1)/c. The red-emitting solid-solution phosphors exhibited a broad absorption band in the range 220-340 nm and sharp excitation peaks in the near UV to green region. The emission intensities of the solid-solution phosphors were enhanced due to the stiff lattices, as a result of the ordered distribution of cations and anions according to differences in ionic size. The increase in the energy transfer is discussed in terms of metal-ligand distances, ionization potential of Mo and the relatively large overlap between the excitation and emission spectra. Such solid-solution phosphors with a bright red emission intensity, relatively short decay time and appropriate color chromaticity have potential for use as red-emitting materials for white LEDs. (C) 2011 Elsevier B.V. All rights reserved.
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  • High-surface area ceramic-derived boron-nitride and its hydrogen uptake properties

    Kim, Jinhong   Han, Jisun   Seo, Moonsu   Kang, Shinhoo   Kim, Dongok   Ihm, Jisoon  

    A chlorine-assisted method for synthesizing porous boron nitride (BN) is developed. Large scale fabrication is possible by this method under moderate conditions. B and N sources, TiB2 and TiN, respectively, are reacted under a chlorine atmosphere to yield a porous turbostratic BN that displayed a higher surface area (960m(2) g(-1)) and a larger micropore volume (0.38 cm(3) g(-1)) than have been reported by others. The products exhibit a high hydrogen uptake, as high as 1.01 wt% at 77 K and 1 bar. These results also indicate that the porous BN is a good candidate for testing as a catalyst support and gas sorption material.
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  • Microstructure of (W,Ti)C-Co system containing platelet WC

    Nam, Hyomoon   Lim, Jaehyuk   Kang, Shinhoo  

    (W(1-x),Ti(x))C-10Co cermets consist of two phases. hcp (W,Ti)C and fcc (Ti,W)C. The hcp (W,Ti)C exhibits thin platelet morphology. The (101) prism plane was developed preferentially in (W,Ti)C powder and the platelet grew via dissolution/precipitation during sintering. The morphology is found effective in enhancing the toughness of the system. (C) 2010 Published by Elsevier B.V.
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  • WC-reinforced (Ti,W)(CN)

    Kang, Youngjae   Kang, Shinhoo  

    A ceramic microstructure of WC-reinforced (Ti,W)(CN) was designed using thermodynamic instability among the constituent elements and gas species involved in the sintering process. During sintering WC in the shape of platelets formed in (Ti,W)(CN) solid-solution ceramics, which were synthesized from milled Mixtures of oxides with carbon. The WC platelets separated from the (Ti,W)(CN) ceramics due to the low chemical affinity of tungsten for nitrogen. The shape of WC varies depending on the composition of the binder phase and the sintering conditions. With increasing binder content, the WC platelet shape became more irregular. This change was attributed not only to surface energy but also to the formation mechanism. The presence of WC platelets toughened (Ti,W)(CN) significantly at excellent hardness value (H(v): 18.5-20.0 GPa, K(IC): 6.0-6.8 MPa m(1/2)) as compared to those of recently reported advanced structural ceramics. (C) 2009 Elsevier Ltd. All rights reserved.
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