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pISSN : 1225-1429 / eISSN : 2234-5078

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2014, Vol.24, No.4

  • 1.

    The molten KOH/NaOH wet chemical etching of HVPE-grown GaN

    박재화 | 홍윤표 | Cheol Woo Park and 5other persons | 2014, 24(4) | pp.135~139 | number of Cited : 4
    Abstract PDF
    The hydride vapor phase epitaxy (HVPE) grown GaN samples to precisely measure the surface characteristicswas applied to a molten KOH/NaOH wet chemical etching. The etching rate by molten KOH/NaOH wet chemical etchingmethod was slower than that by conventional etching methods, such as phosphoric and sulfuric acid etching, which may bedue to the formation of insoluble coating layer. Therefore, the molten KOH/NaOH wet chemical etching is a betterefficient method for the evaluation of etch pits density. The grown GaN single crystals were characterized by using X-raydiffraction (XRD) and X-ray rocking curve (XRC). The etching characteristics and surface morphologies were studied byscanning electron microscopy (SEM). From etching results, the optimum etching condition that the etch pits were wellindependently separated in space and clearly showed their shape, was 410oC and 25 min. The etch pits density obtained bymolten KOH/NaOH wet chemical etching under optimum etching condition was around 2.45 × 106cm−2, which iscommercially an available materials.
  • 2.

    Investigation of Co- and Pr-doped yttria-stabilized cubic zirconia (YSZ) single crystal grown by skull melting method

    뮨소이 | SEOK, JEONG-WON | 2014, 24(4) | pp.140~144 | number of Cited : 0
    Abstract PDF
    Co-(0.7 wt%) and Pr-(2.0, 3.5 or 5.0 wt%) doped cubic zirconia (ZrO2 : Y2O3 = 50 : 50 wt%) single crystalsgrown by a skull melting method were heat-treated in N2 at 1150oC for 5 hrs. The brown colored as-grown single crystalswere changed into either dark brownish green, greenish blue and light green color after the heat treatment. Before and afterthe heat treatment, the YSZ (yttria-stabilized zirconia) single crystals were cut for wafer form (ø7.5 mm × t3 mm). The opticaland structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Co2+(≒589 nm:Γ8[4A2(4F)]→Γ8 +Γ7[4T1(4F)], ≒610 nm: Γ8[4A2(4F)]→Γ8[4T1(4F)], ≒661 nm: Γ8[4A2(4F)]→Γ6[4T1(4F)]) and Pr3+(≒450 nm:3H4 −3P2, ≒473 nm:3H4→3P1, ≒484 nm:3H4→3P0), change of ionization energy and lattice parameter were confirmed.
  • 3.

    Selective growth of GaN nanorods on the top of GaN stripes

    유연수 | Jun Hyeong Lee | Hyung Soo Ahn and 3other persons | 2014, 24(4) | pp.145~150 | number of Cited : 1
    Abstract PDF
    GaN nanorods were grown on the apex of GaN stripes by three dimensional selective growth method. SiO2mask was partially removed only on the apex area of the GaN stripes by an optimized photolithography for the selectivegrowth. Metallic Au was deposited only on the apex of the GaN stripes and a selective growth of GaN nanorods wasfollowed by a metal organic vapor phase epitaxy (MOVPE). We confirmed that the shape and size of the GaN nanorodsdepend on growth temperature and flow rates of group III precursor. GaN nanorods were grown having a taper shapewhich have sharp tip and triangle-shaped cross section. From the TEM result, we confirmed that threading dislocationswere rarely observed in GaN nanorods because of the very small contact area for the selective growth. Stacking faultswhich might be originated from a difference of the crystal facet directions between the GaN stripe and the GaN nanorodswere observed in the center area of the GaN nanorods.
  • 4.

    Interfacial degradation of thermal barrier coatings in isothermal and cyclic oxidation tests

    전설 | Lee, Hee-Soo | Choi youngkue and 2other persons | 2014, 24(4) | pp.151~157 | number of Cited : 0
    Abstract PDF
    The degradation mechanisms of thermal barrier coatings (TBCs) were investigated in different thermal fatiguecondition in terms of microstructural analyses. The isothermal and cyclic oxidation tests were conducted to atmosphericplasma sprayed-TBCs on NIMONIC 263 substrates. The delamination occurred by the oxide layer formation at theinterface, the Ni/Cr-based oxide was formed after Al-based oxide layer grew up to ~10 μm in the isothermal condition. Inthe cyclic oxidation with dwell time, the failure occurred earlier (500 hr) than in the isothermal oxidation (900 hr) at sametemperature. The thickness of Al-based oxide layer of the delaminated specimen in the cyclic condition was ~4 μm and theinterfacial cracks were observed. The acoustic emission method revealed that the cracks generated during the cooling step. It was considered that the specimens were prevented from the formation of the Al-based oxide by cooling treatment, andthe degradation mode in the cyclic test was dominantly interfacial cracking by the difference of thermal expansioncoefficients of the coating layers.
  • 5.

    Fabrication of mineral fiber via melt spinning method from blast furnace slag

    Xiao-Song Wang | BoYoungHur | 2014, 24(4) | pp.158~163 | number of Cited : 0
    Abstract PDF
    Mineral fiber, or be called mineral wool when it assembles in large amounts, is a kind of wide applied manmadematerial with excellent thermal and acoustic insulation properties. In this work, mineral fiber was produced via meltspinning method by using iron blast furnace slag as raw material. Two critical experimental parameters for fabrication wereinvestigated: melt pouring temperature and rotating speed of spinning wheels. The mineral fiber produced under thecondition of melt pouring temperature 1500oC and spinning speed 4000 rpm, showed the smoother surface and most quality,while the others had rough surfaces or with heavy shots. In general, mineral fibers with the size in the range of 12~49 μmin diameter and 8~130 mm in length can be fabricated by this method, and the production rate is more than 34 wt.%,which could be up to 57 wt.% at maximum.
  • 6.

    Changes of discharge voltage of plate-type sodium sulfur batteries in the early charge/discharge cycles

    김성인 | 김헌태 | Choi Hee Lack and 4other persons | 2014, 24(4) | pp.164~168 | number of Cited : 1
    Abstract PDF
    The sodium-sulfur batteries which operate at 350oC have been mainly used in the field of energy storagesystem. This batteries consist of liquid sodium anode, sulfur cathode and β''-alumina solid electrolyte. The conditioningprocess for stabilization of the batteries is essential since the cells show considerable fluctuation of discharge voltage at thebeginning of discharge/charge cycles. It is found that one of the reasons of the fluctuation is the gradual change of contactarea between molten sodium and solid electrolyte.
  • 7.

    Synthesis and characterization of thermally stable pink-red inorganic pigment for digital color

    이원준 | HAE JIN HWANG | Kim Jinho and 2other persons | 2014, 24(4) | pp.169~175 | number of Cited : 3
    Abstract PDF
    Digital ink-jet printing system has many advantages such as fast and fine printing of various images, highefficiency and low cost process. Generally digital ink-jet printing requires ceramic pigments of cyan, magenta, yellow andblack with thermal and glaze stability above 1000oC for the application of porcelain product design. In this study, pink-redcolored CaO-SnO2-Cr2O3-SiO2 pigment was synthesized using solid state reaction. The synthesis conditions of Ca(Cr,Sn)SiO5pigment such as annealing temperature, amount of mineralizer and non-stoichiometric composition were optimized. Crystalstructure and morphology of the obtained Ca(Cr,Sn)SiO5 pigment were analyzed using XRD, SEM, PSA, FT-IR and effectof Cr substitution on the pigment color was analyzed using Uv-vis. spectrophotometer and CIE L*a*b* measurement.
  • 8.

    Solid state reactive sintering of cold pressed thermoelectric Mg3Sb2

    Kim In Ki | Kyung-Wook Jang | Han-Jun Oh | 2014, 24(4) | pp.176~182 | number of Cited : 1
    Abstract PDF
    We intended to prepare Mg3Sb2 compound bodies through solid state reactive sintering after cold-pressingmixtures of elementary Mg and Sb powders and investigated the crystal phases of the sintered bodies according to Mg/Sbmole ratios and reaction temperatures. The Mg3Sb2 bodies sintered at the temperatures of 773~843 K showed typicalcrystalline phases of Mg3Sb2 compounds, but their diffraction angles in XRD patterns were slightly different along with thevertical axis of the bodies obtained. All the bottom parts of the sintered Mg3Sb2 bodies were composed of the typicalcrystalline phases of Mg3Sb2 compounds and their diffraction angles were completely in accord with those of the α-Mg3Sb2phase, when Mg : Sb = 3.15 : 1.85 at 823 K, or when the Mg moles were greater than or equal to 3.10 at 843 K. It wasconsidered that the slightly remaining Mg phases were formed by precipitation from α-Mg3Sb2 phases during thesolidification process of liquid phase.