• Applied Chemistry for Engineering
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• v.32 no.6
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• pp.632-639
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• 2021

In this research, fluorine doping was performed to enhance the photocatalytic activities of WO₃ which were measured using methylene blue dye. WO_3-xF_x photocatalyts were prepared by a vaper phase fluorination during a sintering for preparing WO₃ photocatalysts from a WCl₆ precursor. The bandgap energy of WO₃ photocatalysts decreased from 2.95 eV to 2.54 eV, and the oxygen vacancies site increased by about 55% after fluorine doping. In addition, the initial degradation efficiency of methylene blue showed that the fluorine doped sample showed a 6-fold increase in photocatalytic activities from 10% to 60% compared to that of the untreated sample. It is believed that fluorine is doped to reduce the band gap of photocatalysts, enabling the catalytic activity with low energy, and that oxygen vacancies-generated surface defects increase the visible light absorption region of WO₃ photocatalysts, thereby increasing photocatalytic activity. In this study, it was confirmed that fluorine-doped WO_3-xF_x photocatalysts with an excellent photocatalytic activity can be manufactured easily using a one-step vaper phase fluorination that does not require a post-treatment process.

• Korean Journal of Materials Research
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• v.23 no.9
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• pp.537-542
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• 2013

Two types of Tb- and Na-substituted green phosphors $Ca_{(1-1.5x)}WO_4:Tb_x^{3+}$: and $Ca_{(1-2x)}WO_4:Tb_x^{3+},Na_x^+$ were synthesized with various x values, using a solid-state reaction. The former phosphors contained both substitutional and vacancy point defects, while the later had only substitutional defects. X-ray diffraction results showed that the main diffraction peak, (112), was centered at $2{\theta}=28.72^{\circ}$ and indicated that there was no basic structural deformation caused by substitutions or vacancies. The photoluminescence emission and photoluminescence excitation spectra revealed the optical properties of trivalent terbium ions, $Tb^{3+}$. Typical transitions, $^5D_3{\rightarrow}^7F_6,\;^7F_5,\;^7F_4$ and $^5D_4{\rightarrow}^7F_6,\;^7F_5,\;^7F_4,\;^7F_3$, and cross relaxations were observed. Subtle differences in the photoluminescence of green phosphors were observed as a result of the point defects. The FT-IR spectra indicated that some of the ungerade vibrational modes had shifted positions and changed shapes, spreading out over a wide range of frequencies. This change can be attributed to the different masses of $Tb^{3+}$ and $Na^+$ ions and $V_{Ca}$" vacancies compared to $Ca^{2+}$ ions. The gerade normal modes of the Raman spectra exhibited subtle differences resulting from point defects in $Ca_{(1-1.5x)}Tb_xWO_4$ and $Ca_{(1-2x)}Tb_xNa_xWO_4$.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.16 no.5
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• pp.397-403
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• 2003

Microwave dielectric properties of (1-x)ZnW $O_4$-xTi $O_2$ ceramic systems were investigated with calcination temperatures and Ti $O_2$ contents. The ZnW $O_4$ ceramic could be suitably sintered at 1075$^{\circ}C$ and showed the dielectric constant of 13.6, quality factor(Q$\times$ $f_{O}$value) of 22,000 and temperature coefficient of resonant frequency($\tau$$_{f}$) of -65$\pm$2ppm/$^{\circ}C$. Increasing the amount of Ti $O_2$ in the range of 0.25 to 0.45 mol, the dielectric constant and $\tau$$_{f}$ increased due to the role of Ti $O_2$ but the quality factor decreased due to the increase of phase boundaries. The 0.7ZnW $O_4$-0.3Ti $O_2$ ceramic showed the dielectric constant of 19.8, qualify factor(Q$\times$ $f_{0}$) of 20,000 and $\tau$$_{f}$ of -3$\pm$1ppm/$^{\circ}C$.>.EX>.>.>.

• 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 Institute of Electrical and Electronic Material Engineers
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• v.19 no.9
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• pp.855-861
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• 2006

Sinterability and microwave dielectric properties of $xZWO_{4}-(1-x)TiO_{2}$ ceramic systems with zinc-borosilicate glass and $TiO_{2}$ contents for LTCC(Low Temperature Co-fired Ceramics) were investigated. The addition of $3{\sim}10\;wt%$ ZBS glass ensured the sinterability below $900^{\circ}C$. In general, increasing ZBS glass content seemed to enhance the sinterability, but the quality factor($Qxf_{0}$) significantly decreased due to the formation of an excessive liquid and second phases. As for the addition of $TiO_{2}$, the dielectric constant(${\varepsilon}_{r}$) and temperature coefficient of resonant frequency(${\tau}_{f}$) showed to increase, while the quality factor($Qxf_{0}$) did not show an apparent change. The composition of $0.7xZnWO_{4}-0.3TiO_{2}$ ceramics sintered at $900^{\circ}C$ with 5 wt% ZBS glass demonstrated 21.6 in dielectric constant(${\varepsilon}_{r}$), 14,800 in quality factor($Qxf_{0}$), and $+5\;ppm/^{\circ}C$ in temperature coefficient of resonant frequency(${\tau}_{f}$).

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.33 no.5
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• pp.393-399
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• 2020

A series of phosphors, SrWO₄:5 mol% Dy³⁺, SrWO₄:5 mol% Sm³⁺, and SrWO₄:5 mol% Dy³⁺:x Sm³⁺ (x＝1~15 mol%), were prepared using a facile co-precipitation. The crystal structure, morphology, photoluminescence properties, and application in anti-counterfeiting fields were investigated. The crystalline structures of the prepared phosphors were found to be tetragonal systems with the dominant peak occurring at the (112) plane. The excitation spectra of the Dy³⁺ singly-doped SrWO₄ phosphors were composed of an intense charge-transfer band centered at 246 nm in the range of 210~270 nm and two weak peaks at 351 nm and 387 nm due to the ⁶H_15/2→⁶P_7/2 and ⁶H_15/2→⁴I_13/2 transitions of Dy³⁺ ions, respectively. The wavelength of 246 nm was optimum for exciting the luminescence of Dy³⁺ and Sm³⁺ co-doped SrWO₄ phosphors. The emission spectra consisted of two intense blue and yellow emission bands at 480 nm and 573 nm corresponding to the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺, and two strong emission peaks at 599 nm and 643 nm originating from the ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 transitions of Sm³⁺, respectively. As the concentration of Sm³⁺ ions increased, the emission intensities of Dy³⁺ rapidly decreased, while the emission intensities of Sm³⁺ gradually increased. These results suggest that the color of the emission light can be tuned from yellow to white by changing the concentration of Sm³⁺ ions at a fixed 5 mol% Dy³⁺. Furthermore, the fluorescent security inks were synthesized for use in anti-counterfeiting applications.

• 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.

• Journal of the Korean institute of surface engineering
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• v.54 no.3
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• pp.112-118
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• 2021

Dy³⁺- and Eu³⁺-codoped BaWO₄ phosphors for white light-emitting diode were synthesized with different activator ions via a solid-state reaction process. The structural, morphological, and optical properties of the BaWO₄:Dy³⁺,Eu³⁺ phosphors were investigated as a function of Eu³⁺ concentration at a fixed concentration of Dy³⁺ ions. XRD patterns exhibited that all the synthesized phosphors had a tetragonal system, irrespective of the concentrations of Dy³⁺ and Eu³⁺ ions. The excitation spectra of the synthesized phosphors were composed of three intense bands centered at 251, 355, and 393 nm and several weak peaks. For the BaWO₄:Dy³⁺,Eu³⁺ phosphors synthesized with 1 mol% of Eu³⁺, the emission spectra under ultraviolet excitation at 393 nm showed two strong blue and yellow bands at 485 and 577 nm corresponding to the ⁴F_9/2⁶H_15/2 and ⁴F_9/2⁶H_13/2 transitions of Dy³⁺ ions, respectively and several weak bands in the range of 600-700 nm resulting from the 4f transitions of Eu³⁺ ions. As the concentration of Eu³⁺ ions increased, intensities of the blue and yellow emission bands gradually decreased while those of the red emissions increased rapidly and the energy transfer efficiency from Dy³⁺ to Eu³⁺ ions was 95.3% at 20 mol% of Eu³⁺. The optimum white light emission with x=0.363, y=0.357 CIE 1931 chromaticity coordinates was obtained for the sample doped with 5 mol% Dy³⁺ and 1 mol% of Eu³⁺.

• Korean Journal of Materials Research
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• v.22 no.5
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• pp.215-219
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• 2012

Red phosphors $Ca_{1-1.5x}WO_4:{Eu_x}^{3+}$ were synthesized with different concentrations of $Eu^{3+}$ ions by using a solid-state reaction method. The crystal structure of the red phosphors was found to be a tetragonal system. X-ray diffraction (XRD) results showed the (112) main diffraction peak centered at $2{\theta}=28.71^{\circ}$, and the size of crystalline particles exhibited an overall decreasing tendency according to the concentration of $Eu^{3+}$ ions. The excitation spectra of all the phosphors were composed of a broad band centered at 275 nm in the range of 230-310 nm due to $O^{2-}{\rightarrow}W^{6+}$ and a narrow band having a peak at 307 nm caused by $O^{2-}{\rightarrow}Eu^{3+}$. Also, the excitation spectrum presents several strong lines in the range of 305-420 nm, which are assigned to the 4f-4f transitions of the $Eu^{3+}$ ion. In the case of the emission spectrum, all the phosphor powders, irrespective of $Eu^{3+}$ ion concentration, indicated an orange emission peak at 594 nm and a strong red emission spectrum centered at 615 nm, with two weak lines at 648 and 700 nm. The highest red emission intensity occurred at x = 0.10 mol of Eu3+ ion concentration with an asymmetry ratio of 12.5. Especially, the presence of $Eu^{3+}$ in the $Ca_{1-1.5x}WO_4:{Eu_x}^{3+}$ shows very effective use of excitation energy in the range of 305-420 nm, and finally yields a strong emission of red light.

• Journal of the Korean institute of surface engineering
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• v.53 no.4
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• pp.131-137
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• 2020

A series of Dy³⁺, Sm³⁺, and Dy³⁺/Sm³⁺ doped Gd₂WO₆ phosphors were synthesized by the conventional solid-state reaction. The X-ray diffraction patterns revealed that all of the diffraction peaks could be attributed to the monoclinic Gd₂WO₆ crystal structure, irrespective of the type and the concentration of activator ions. The photoluminescence (PL) excitation spectra of Dy³⁺-doped Gd₂WO₆ phosphors contained an intense charge transfer band centered at 302 nm in the range of 240-340 nm and two weak peaks at 351 and 386 nm. Under an excitation wavelength of 302 nm, the PL emission spectra consisted of two strong blue and yellow bands centered at 482 nm and 577 nm. The PL emission spectra of the Sm³⁺-doped Gd₂WO₆ phosphors had a series of three peaks centered at 568 nm, 613 nm, and 649 nm, corresponding to the ⁶G_5/2 → ⁶H_5/2, ⁶G_5/2 → ⁶H_9/2, and ⁶G_5/2 → ⁶H_11/2 transitions of Sm³⁺, respectively. The PL emission spectra of the Dy³⁺- and Sm³⁺-codoped Gd₂WO₆ phosphors showed the blue and yellow emission lines originating from the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁴H_13/2 transitions of Dy³⁺ and reddish-orange and red emission bands due to the ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 transitions of Sm³⁺. As the concentration of Sm³⁺ increased from 1 to 15 mol%, the intensities of two PL spectra emitted by the Dy³⁺ ions gradually decreased, while those of the three emission bands due to the Sm³⁺ ions slowly increased, thus producing the color change from white to orange. The CIE color coordinates of Gd₂WO₆:5 mol% Dy³⁺, 1 mol% Sm³⁺ phosphors were (0.406, 0.407), which was located in the warm white light region.