• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.6
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• pp.356-360
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• 2017

We investigated the phase evolution, microstructure, and microwave dielectric properties of Na- and Zr-doped $Ba(Mg_{0.5}W_{0.5})O_3$ [i.e., ($Ba_{1-2x}Na_{2x})(Mg_{0.5-x}Zr_xW_{0.5})O_3$] ceramics. $BaWO_4$ as a secondary phase was observed in all compositions, and it increased as the dopant concentration increased. All specimens revealed a dense microstructure. For the composition of x=0.01, polyhedral grains were observed. As the dopant concentration increased, the densification and the grain growth were promoted by a liquid phase. The quality factor($Q{\times}f_0$) decreased remarkably, whereas the dielectric constant (${\varepsilon}_r$) tended to decrease as the dopant concentration increased. The dielectric constant, quality factor, and temperature coefficient of the resonant frequency of the composition of x=0.01 sintered at $1,700^{\circ}C$ for 1 h were 18.6, 216,275 GHz, and $-22.0ppm/^{\circ}C$, respectively.

• Journal of the Korean Ceramic Society
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• v.44 no.3 s.298
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• pp.152-156
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• 2007

Tungsten carbide powder was synthesized by SHS (self-propagating high-temperature synthesis). Except $WO_{3}$, each concentration of raw material ($WO_{3},\;Mg,\;NaCl,\;Na_{2}CO_{3},\;C$) was investigated. Final product was characterized by XRD and SEM. X-ray data demonstrated that the $NaCl+Na_{2}CO_{3}$ combined mixture has superiority in the WC formation process. Single phase and submicrometer WC powder was synthesized at the temperature below $1600^{\circ}C$. The role of sodium salts in combustion process was discussed, and chemical mechanism of WC formation was proposed. WC powder prepared by salt-assisted combustion synthesis has a size $0.2{\sim}3\;{\mu}m$ and low agglomeration degree.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.643-643
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• 2013

Recently, synthesis of low-dimensional nanostructures is gaining more importance due to their structural properties and growing potential applications. On the other hand, luminescent materials doped with rare earth ions have drawn immense attention. The commercial phosphors are based on many host materials. Among them, tungstates are being currently investigated by many research groups owing to a wide range of applications. Tungstates are formed by different metal cations (e.g., SrWO4, Na2WO4, NiWO4, Cr2WO6, and ZrW2O8) and their structure depends on the size of the metal cation. Tungstates with large bivalent cations (${\gg}0.1\;nm$) have the scheelite structure and the wolframite structure with smaller ions (＜0.1 nm). Strontium tungstate has the scheelite structure which is tetragonal with space group I41/a. The luminescent properties of the tungstate have been extensively explored in application fields such as sensors, detectors, lasers, photoluminiscent devices, photo catalysts, etc. In this work, we synthesized SrWO4 phosphors with different Eu3+ concentrations by using a facile route. The morphology was analyzed by using a field-emission scanning electron microscope, which exhibits the spherical shape. Transmission electron microscope image revealed the spheres composed of nanoparticles. X-ray diffraction patterns confirmed their tetragonal shape. The photoluminescence excitation and emission spectra were analyzed by varying the Eu3+ concentration, which shows a dominant red emission.

• Korean Journal of Materials Research
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• v.29 no.12
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• pp.741-746
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• 2019

New triple tungstate phosphors NaPbLa(WO₄)₃:Yb³⁺/Ho³⁺ (x = Yb³⁺/Ho³⁺ = 7, 8, 9, 10) are successfully fabricated by microwave assisted sol-gel synthesis and their structural and frequency upconversion (UC) characteristics are investigated. The compounds crystallized in the tetragonal space group I4₁/a and the NaPbLa(WO₄)₃ host have unit cell parameters a = 5.3927(1) and c = 11.7961(3) Å, V = 343.05(2) ų, Z = 4. Under excitation at 980 nm, the phosphors have yellowish green emissions, which are derived from the intense ⁵S₂/⁵F₄→⁵I₈ transitions of Ho³⁺ ions in the green spectral range and strong ⁵F₅→⁵I₈ transitions in the red spectral range. The optimal Yb³⁺:Ho³⁺ ratio is revealed to be x = 9, which is attributed to the quenching effect of Ho³⁺ ions, as indicated by the composition dependence. The UC characteristics are evaluated in detail under consideration of the pump power dependence and Commission Internationale de L'Eclairage chromaticity. The spectroscopic features of Raman spectra are discussed in terms of the superposition of Ho³⁺ luminescence and vibrational lines. The possibility of controlling the spectral distribution of UC luminescence by the chemical content of tungstate hosts is demonstrated.

• Resources Recycling
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• v.27 no.4
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• pp.57-64
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• 2018

Water leaching of tungsten(W) and vanadium(V) was researched from their oxides through mechanochemical (MC) reaction with alkali compounds. Intensive grinding for the mixture of tungsten/vanadium oxide and alkali compounds (NaOH, $Na2CO_3$) was carried out with change of their mixing ratios and grinding duration. Water soluble compounds, $Na_2WO_4$ and $NaVO_3$, were synthesized through MC reaction and their solubilities increased in proportion to the mixing ratio of sodium compound and grinding times. Whereas vanadium leachability was less affected by the mixting ratio and grinding times. The leachabilities of 99.0% were accomplished by a short period of MC treatment, W (30 min.) and V (5 min.). This process enable us to extract W and V from their oxides via a water leaching, and can be applied to the selective recovery of W and V from $DeNO_x$ spent catalysts.

• Korean Journal of Materials Research
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• v.26 no.12
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• pp.757-763
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• 2016

$Ho^{3+}/Yb^{3+}/Tm^{3+}$ tri-doped $NaY_{1-x}(WO_4)_2$ phosphors with proper doping concentrations of $Ho^{3+}$, $Yb^{3+}$ and $Tm^{3+}$ ($x=Ho^{3+}+Yb^{3+}+Tm^{3+}$, $Ho^{3+}$=0.04, 0.03, 0.02, 0.01, $Yb^{3+}$=0.35, 0.40, 0.45, 0.50 and $Tm^{3+}$=0.01, 0.02, 0.03, 0.04) were successfully synthesized via the microwave sol-gel route, and their upconversion properties were investigated. Well-crystallized microcrystalline particles showed fine and homogeneous microcrystalline morphology with particle sizes of $1-2{\mu}m$. The optical properties were comparatively examined using photoluminescence emission and Raman spectroscopy. Under excitation at 980 nm, the doped particles exhibited white emissions based on blue, green and red emission bands, which correspond to the $^1G_4{\rightarrow}^3H_6$ transitions of $Tm^{3+}$ in the blue region, the $^5S_2/^5F_4{\rightarrow}^5I_8$ transitions of $Ho^{3+}$ in the green region, the $^5F_5{\rightarrow}^5I_8$ transitions of $Ho^{3+}$, and the $^1G_4{\rightarrow}^3F_4$ and $^3H_4{\rightarrow}^3H_6$ transitions of $Tm^{3+}$ in the red region. The pump power dependence of the upconversion emission intensity and the Commission Internationale de L'Eclairage chromaticity coordinates of the phosphors were evaluated in detail.

• Resources Recycling
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• v.4 no.1
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• pp.31-37
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• 1995

This study was to investigate the effects oi agitation speed, pulp concentration, ion concentration ill the slurry solution, printed area of the waste newspaper, immerwng time and the temperature on the defiberation process for the waste newspaper. The defikration efficiency at 50% is twice that of 16%. The efficiency is improved as two times according to elevate the agtation speed as two times in the range of 200-2WO r.p.m.. Defiberation with NaOH 1X10-'M solution has higher efficiency than that of NaOH 1 x 1 0 - M solution as 3 times at the conditions of 16%, 1200 r.p.m, and 1% pulp concentration The temperature of immersing salut~on aifects on the efficiency more than immersing time does. Increasing the printed area of newspaper decreases the velocity of defiberation. The alkaline solution is effective to defiberate and the defiberation efficiency at the same dosage of alkalinity is in the order a1 NaOH) KOH) Na,SiO, ) Na,CO, ) Ca(OH)2.

• Bulletin of the Korean Chemical Society
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• v.33 no.1
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• pp.65-68
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• 2012

Novel type $Li_{1.1}V_{0.9-2x}W_xMo_xO_2$ powders were prepared by a solid-state reaction of $Li_2CO_3$, $V_2O_3$, $WO_2$ and $MoO_2$ precursors in a nitrogen atmosphere containing 10 mol % hydrogen gas, and assessed as anode materials in lithium-ion batteries. The specific charge and discharge capacities of the $Li_{1.1}V_{0.9-2x}W_xMo_xO_2$ anodes were higher than those of the bare $Li_{1.1}V_{0.9}O_2$ anode. The cyclic efficiency of these anodes was approximately 73.3% at the first cycle, regardless of the presence of W and Mo doping. The composite anode, which was composed of $Li_{1.1}V_{0.75}W_{0.075}Mo_{0.075}O_2$ (20 wt %) and natural graphite (80 wt %), demonstrated reasonable specific capacity, high cyclic efficiency, and good cycling performance, even at high rates without capacity fading.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2014.11a
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• pp.178-179
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• 2014

In this study, $Ni-W-Si_3N_4$ alloy composite coatings were prepared by pulse electrodeposition method using nickel sulfate bath with different contents of tungsten source, $Na_2WO_4.2H_2O$, and dispersed $Si_3N_4$ nano particles. The structure and microstructure ofcoatings was separately analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results indicated that nano $Si_3N_4$ and W content in alloy had remarkable effect on microstructure, microhardness and scratch resistant properties. Tungsten content in Ni-W and $Ni-W-Si_3N_4$ alloy ranged from 7 to 14 at.%. Scratch test results suggest that as compared to Ni-W only, $Ni-W-Si_3N_4$ prepared from Ni/W molar ratio of 1:1.5 dispersed with 20 g/L $Si_3N_4$ has shown the best result among different samples.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2015.05a
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• pp.171-172
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• 2015

$Ni-W-Si_3N_4$ alloy composite coatings were prepared by pulse electro-deposition method using nickel sulfate bath with different contents of tungsten source, $Na_2WO_4.2H_2O$, and dispersed $Si_3N_4$ nano-particles. The structure and micro-structure of coatings was separately analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results indicated that nano $Si_3N_4$ and W content in alloy had remarkable effect on micro-structure, micro-hardness and scratch resistant properties. Tungsten content in Ni-W and $Ni-W-Si_3N_4$ alloy ranged from 7 to 14 at.%. Scratch test results suggest that as compared to Ni-W only, $Ni-W-Si_3N_4$ prepared from Ni/W molar ratio of 1:1.5 dispersed with 20 g/L $Si_3N_4$ has shown the best result among different samples.