• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.2
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• pp.141-148
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• 1999

High quality pure and Nb-doped $PbWO_{4}$ Single Crystal were grown from a 50 %~50 % mixture of Lead oxide (PbO) and Tungsten oxide $(WO_{3})$ by Czochralski method in Iridium crucible. The stoichiometric deviation correspond to the selective loss of the crystal constituents is found to be responsible for the yellowish coloration of $PbWO_{4}$. Through the X-ray powder diffraction experiment, we have investigated the lattice constant variations of each $PbWO_{4}$ crystals. We also present information on their photoluminescence (PL), optical absoption properties and Raman spectra. The temperature dependence of PL intensity and FWHM (Full Width Half Maximum) were measured in the temperature range 10 K~300 K. One observes a slight temperature dependence in the low temperature region and PL intensity decreases over 200 K by thermal quenching. The activation energy, Huang-Rhys coupling constant and inhomogenious brodenning acquired from their temperature dependence.

• Bulletin of the Korean Chemical Society
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• v.28 no.7
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• pp.1097-1103
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• 2007

We report a novel method to synthesize nanocomposites composed of titania nanoparticles and phosphotungstate ions with various composition ratios ranging from W/Ti = 12/10 to 12/500 by inducing the electrostatic interaction between the positively charged protonated titania sol-particles and the negatively charged phosphotungstate anions to flocculate and precipitate. The precipitates showed varied features depending on the composition. The precipitate from the tungsten-richest W/Ti = 12/10 reaction is amorphous in its powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy data. This material shows the Type II adsorption characteristics in its N2-adsorption isotherm, but with quite low surface area of 34 m2/g. To the contrary, the precipitates from the titanium-richer reactions (W/Ti = 12/50- 12/500) are composed of anatase nanoparticles of 2-6 nm by XRD, TEM and Raman and show the Type I adsorption characteristics. The surface area linearly increases with the titanium content from 131 m2/g for W/ Ti = 12/50 to 228 m2/g for 12/500. The precipitate from the reaction with the intermediate composition W/Ti = 12/20 is composed of anatase nanoparticles and does not have any pore accessible to N2. With the wide variety of the physical properties of the precipitates, the present method can be a novel, viable means to tailor synthesis of nanocomposite materials. A formation mechanism of the precipitates is based on the electrostatic interactions between the titania nanoparticles and phosphotungstate ions.

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.1
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• pp.14-20
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• 2003

Piezoelectric ultrasonic transducers for high temperature contact measurement were developed. These high temperature ultrasonic transducers (HTUT) consisted of bismuth titanate piezoceramic element whose Curie temperature is higher than $600^{\circ}C$, a backing material of the mixture of tungsten powder and inorganic binder, an inner alumina tube, a wear Plate and a housing. The operational frequencies or the HTUT were 1.04 and 2.08 MHz, respectively. Various commercially available couplants for high temperature were evaluated and compared. As a couplant for high temperature ultrasonic testing between HTUT and test specimen, gold epoxy was selected. The peak amplitude of pulse-echo signals from steel test specimen decreased with increasing temperature. The operational temperature of the HTUT reached up to $500^{\circ}C$ at which the continuous contact measurement was possible.

• Korean Journal of Materials Research
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• v.12 no.12
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• pp.930-934
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• 2002

Nanosized $WO_3$ powders were synthesized by the sol-precipitation process using $WCl_{6}$ as the starting material, ethanol as a solvent and $NH_4$OH solution as a precipitant, followed by a washing-drying treatment and calcination. The effects on the powder crystallinity and microstructure of calcination temperature were investigated with XRD and FE-SEM. The $WO_3$ powders calcined at $500^{\circ}C$ and $700^{\circ}C$ showed good crystallinity and their mean particle size was 30nm and 70nm, respectively. These powders were used for the preparation of pastes which were printed as thick films on alumina substrates with comb-type Pt electrodes. The particle size strongly influenced the $NO_2$ gas sensing property of the thick films. A significant reduction in the $NO_2$ sensitivity was observed for the film prepared from larger particle size, having thus a larger grain size. For the film having a smaller grain size, on the other hand, the higher $NO_2$ sensitivity was observed and the sensitivity increased with $NO_2$ concentration.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.25 no.8
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• pp.602-610
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• 2012

The powders for the $Na_xWO_3$ (x= 1 and 0.75) sputtering targets were synthesized by the calcination in reductive atmosphere. Near single-phase $NaWO_3$ and single-phase $Na_{0.75}WO_3$ powder targets were prepared. By using the targets, thin films of each composition were deposited by rf magnetron sputtering on the $SiO_2$ (100 nm)/Si substrates and annealed by RTP (rapid thermal processing) for crystallization. In the case of the $NaWO_3$ composition, single-phase $Na_xWO_3$ thin films, where x was believed to be slightly less than 1, were fabricated accompanying the Na-diffusion into the substrates during RTP. However, in the case of the $Na_{0.75}WO_3$ thin film preparation, it was unable to make single-phase thin films. From the phase formation behaviors of both powders and thin films, it was revealed that $Na_xWO_3$ with nonstoichiometric composition of x, which was slightly less than 1, was favorable. The good electrical conduction properties were obtained from the single-phase $Na_xWO_3$ thin films. Their electrical resistivities were as low as $7.5{\times}10^{-4}{\Omega}{\cdot}cm$.

• Nuclear Engineering and Technology
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• v.40 no.1
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• pp.21-36
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• 2008

High bum-up fuel technology has been developed through a national R&D program, which covers key technology areas such as claddings, $UO_2$ pellets, spacer grids, performance code, and fuel assembly tests. New cladding alloys were developed through alloy designs, tube fabrication, out-of-pile test and in-reactor test. The new Zr-Nb tubes are found to be much better in their corrosion resistance and creep strength than the Zircaloy-4 tube, owing to an optimized composition and heat treatment of the new Zr-Nb alloys. A new fabrication technology for large grain $UO_2$ pellets was developed using various uranium oxide seeds and a micro-doping of Al. The uranium oxide seeds, which were added to $UO_2$ powder, were prepared by oxidizing and heat-treating scrap $UO_2$ pellets. A $UO_2$ pellet containing tungsten channels was fabricated for a thermal conductivity enhancement. For the fuel performance analysis, new high burnup models were developed and implemented in a code. This code was verified by an international database and our own database. The developed spacer grid has two features of contoured contact spring and hybrid mixing vanes. Mechanical and hydraulic tests showed that the spacer grid is superior in its rodsupporting, wear resistance and CHF performance. Finally, fuel assembly test technology was also developed. Facilities for mechanical and thermal hydraulic tests were constructed and are now in operation. Several achievements are to be utilized soon by the Korea Nuclear Fuel and thereby contribute to the economy and safety of PWR fuel in Korea

• Korean Journal of Materials Research
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• v.32 no.2
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• pp.57-65
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• 2022

Herein a rich, Se-nanoparticle modified Mo-W₁₈O₄₉ nanocomposite as efficient hydrogen evolution reaction catalyst is reported via hydrothermal synthesized process. In this work, Na₂SeSO₃ solution and selenium powder are used as Se precursor material. The structure and composition of the nanocomposites are characterized by X-ray diffraction (XRD), high-resolution field emission scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), EDX spectrum analysis and the corresponding element mapping. The improved electrochemical properties are studied by current density, and EIS analysis. The as-prepared Se modified Mo-W₁₈O₄₉ synthesized via Na₂SeSO₃ is investigated by FE-SEM analysis and found to exhibit spherical particles combined with nanosheets. This special morphology effectively improves the charge separation and transfer efficiency, resulting in enhanced photoelectric behavior compared with that of pure Mo-W₁₈O₄₉. The nanomaterial obtained via Na₂SeSO₃ solution demonstrates a high HER activity and low overpotential of -0.34 V, allowing it to deliver a current density of 10 mA cm^-2.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.2
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• pp.60-64
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• 2011

High sensitivity IR image sensors require materials characteristics with temperature coefficient of resistance (TCR) and IR range absorption. In this study, the metal-dielectric thermo sensitive films (MDTF) based on $(SiO_2)_x-(Ti)_y$ composition were deposited on substrates of germanium and glass by thermal evaporator. The $SiO_2$ : Ti mixture was made from the ratio of 9 : 1, 8 : 2, 7 : 3, 6 : 4, respectively. $(SiO_2)_x-(Ti)_y$ mixture powder was loaded on tungsten boat in evaporator and was 15.5 cm from the substrate. Resistance of $(SiO_2)_x-(Ti)_y$ in the range of 273~333K were measured as a function of temperature. Temperature coefficient of resistance (TCR) was calculated by the resistance variation. Under the various mixture ratios condition, it is possible to obtain $SiO_2$-Ti layers with resistance from units kilo-ohm to hundreds kilo-ohm. Finally, our results showed that Temperature coefficient of resistance (TCR) of these films varies from -1.4 to $-2.6%K^{-1}$.

• Journal of Advanced Marine Engineering and Technology
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• v.36 no.2
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• pp.276-285
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• 2012

The objective of this research is to develop the coating technique by a porous self-fluxing alloy for improving the mechanical properties of run-out table roller surface with the hot rolling. To enhance the durability of run-out table roller with the hot rolling, the high hardness of roller surface should be maintained at high temperatures, and the improvement of wear resistance, corrosion resistance, heat resistance, burn resistance and adhesion resistance should be maintained. In order to be able to transport reliably a hot rolled steel sheet, also, the appropriate friction coefficient on the roller surface should be maintained and the slip between roller and steel should not occur. In this study, the wear resistance of roller increases after the self-fluxing alloy is changed to a cermet by adding the tungsten carbide(WC), and the coefficient of friction increases and the ability of grip is improved because the porosities are made by coating with fine iron powder on the roller surface. As a result, it is found that the ability of grip between the steel and the roller coated by a porous self-fluxing alloy contained to 5 ~ 10 wt% of Fe in the coating layer is improved compared to the roller coated by Ni-Cr. This is because the porosities are made after Fe contained in the roller is partially alloyed by heating with a furnace in the fusing process and the rest is eliminated by oxidation and dissolution.

• Journal of the Korean institute of surface engineering
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• v.46 no.4
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• pp.145-152
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• 2013

The growth behavior of nanocrystalline diamond (NCD) film has been studied for three different substrates, i.e. bare Si wafer, 1 ${\mu}m$ thick W and Ti films deposited on Si wafer by DC sputter. The surface roughness values of the substrates measured by AFM were Si < W < Ti. After ultrasonic seeding treatment using nanometer sized diamond powder, surface roughness remained as Si < W < Ti. The contact angles of the substrates were Si ($56^{\circ}$) > W ($31^{\circ}$) > Ti ($0^{\circ}$). During deposition in the microwave plasma CVD system, NCD particles were formed and evolved to film. For the first 0.5h, the values of NCD particle density were measured as Si < W < Ti. Since the energy barrier for heterogeneous nucleation is proportional to the contact angle of the substrate, the initial nucleus or particle densities are believed to be Si < W < Ti. Meanwhile, the NCD growth rate up to 2 h was W > Si > Ti. In the case of W substrate, NCD particles were coalesced and evolved to the film in the short time of 0.5 h, which could be attributed to the fact that the diffusion of carbon species on W substrate was fast. The slower diffusion of carbon on Si substrate is believed to be the reason for slower film growth than on W substrate. The surface of Ti substrate was observed as a vertically aligned needle shape. The NCD particle formed on the top of a Ti needle should be coalesced with the particle on the nearby needle by carbon diffusion. In this case, the diffusion length is longer than that of Si or W substrate which shows a relatively flat surface. This results in a slow growth rate of NCD on Ti substrate. As deposition time is prolonged, NCD particles grow with carbon species attached from the plasma and coalesce with nearby particles, leaving many voids in NCD/Ti interface. The low adhesion of NCD films on Ti substrate is related to the void structure of NCD/Ti interface.