• 한국재료학회지
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• 제25권11호
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• pp.602-606
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• 2015

Porous W with spherical and directionally aligned pores was fabricated by the combination of sacrificial fugitives and a freeze-drying process. Camphene slurries with powder mixtures of $WO_3$ and spherical PMMA of 20 vol% were frozen at $-25^{\circ}C$ and dried for the sublimation of the camphene. The green bodies were heat-treated at $400^{\circ}C$ for 2 h to decompose the PMMA; then, sintering was carried out at $1200^{\circ}C$ in a hydrogen atmosphere for 2 h. TGA and XRD analysis showed that the PMMA decomposed at about $400^{\circ}C$, and $WO_3$ was reduced to metallic W at $800^{\circ}C$ without any reaction phases. The sintered bodies with $WO_3$-PMMA contents of 15 and 20 vol% showed large pores with aligned direction and small pores in the internal walls of the large pores. The pore formation was discussed in terms of the solidication behavior of liquid camphene with solid particles. Spherical pores, formed by decomposition of PMMA, were observed in the sintered specimens. Also, microstructural observation revealed that struts between the small pores consisted of very fine particles with size of about 300 nm.

• 한국재료학회지
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• 제25권6호
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• pp.274-278
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• 2015

The possibility of using the chemical precipitation method of up-cycled ammonium paratungstate (APT) was studied and compared with the thermal decomposition method. $WO_3$ particles were synthesized by chemical precipitation method using a 1:2 weight ratio of APT: Di-water. For thermal decomposition, APT powder was heated for 4h at $600^{\circ}C$ in air atmosphere. The reaction products were characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF), particle size analyzer (PSA), and field emission-scanning electron microscopy (FE-SEM). Thermogravimetric analysis (TGA) of the up-cycled APT allowed for the identification of the sequence of decomposition and reduction reactions that occurred during the heat treatment. TGA data indicated a total weight loss of 10.78% with the reactions completed in $658^{\circ}C$. The XRD results showed that APT completely decomposed to $WO_3$ by thermal decomposition and chemical precipitation. The particle size of the synthesized $WO_3$ powders by thermal decomposition with 2 h of planetary milling was around $2{\mu}m$ During the chemical precipitation process, the particle size of the synthesized $WO_3$ powders showed a round-shape with ${\sim}0.6{\mu}m$ size.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2016년도 추계학술대회 논문집
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• pp.173.1-173.1
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• 2016

전이금속 디칼코게나이드는 서로 다른 전이 금속원소와 칼코겐 원소의 결합으로 이루어진 층상 구조의 물질로서, 그래핀과 비슷한 2D 결정성 구조를 지니면서도, 그래핀과는 달리 밴드갭을 가지는 반도체적 성질 때문에 최근 많은 연구가 진행되고 있다. 특히 $WS_2$는 촉매, 전자, 광전자, 센서와 같은 반도체등 다양한 소자에 적용된다. $WS_2$ 합성 방법에는 기계적 박리법, 화학기상증착법, 용액법 등이 있다. 기계적 박리법은 방법이 간단하나 수율이 낮고 균일하게 얻어지지 않으며, 화학기상증착법은 고가의 고온공정이라는 한계점을 가지고 있다. 반면에 용액법은 제조공정이 쉬우며, 저가 대량생산이 가능하다는 이점이 있다. 더욱이 본래 용액법에서는 $WS_2$를 합성하기 위해 $WO_3$를 추가적으로 합성 후 진행하였지만, 쉽게 제조 가능한 $WO_3$ colloidal 용액을 이용하면 sulfurization을 진행하여 $WS_2$를 합성할 수 있다. colloidal 용액을 이용한 합성법은 입자크기 조절이 가능하기 때문에 균일한 나노입자를 uniform 하게 형성할 수 있는 장점이 있다. 본 연구에서는 $WO_3$ colloidal 용액을 spin coating 과 sulfurzation 공정을 거쳐 2D triangle $WS_2$의 합성 및 특성을 분석하였다. 2D $WS_2$의 나노결정구조, 입자 형상 및 광학 특성을 주사전자현미경, 라만 분광기, x-ray 회절분석기 등을 통해 확인하였다. 또한, 합성된 $WS_2$를 이용하여 트랜지스터를 제작하여 전기적 특성을 확인하였다.

• Korean Chemical Engineering Research
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• 제49권4호
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• pp.405-410
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• 2011

본 연구에서는 화학 증착법을 이용하여 텅스텐 산화물을 제조한 후 기판 온도에 따른 성막 특성을 분석하고, 성막 된 텅스텐 산화물을 전기 변색 소자 제조에 응용하여 소자 특성을 살펴보았다. 증착 온도 $300^{\circ}C$ 이상에서 최대 성막속도($8{\mu}m/min$)를 얻을 수 있었으며, $275^{\circ}C$ 이하에서는 표면 반응 율속 특성을 보였고 이때 겉보기 활성화 에너지 값은 45.9 kJ/mol이였다. 성막 된 텅스텐 산화물은 $275^{\circ}C$ 이하에서는 비정질막이, 그 이상 온도에서는 결정질 막이 형성되었다. 전기 변색 소자 적용시 유리한 비정질막이 성막되는 조건에서 증착 온도 및 두께 변화에 따른 전기 변색 특성을 평가하였다. 증착 온도가 동일한 경우 두께가 두꺼울수록 그리고 두께가 일정한 경우는 증착 온도가 낮을수록 변색 효율 측면에서 유리한 결과를 얻었다.

• Bulletin of the Korean Chemical Society
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• 제25권12호
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• pp.1881-1888
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• 2004

A series of $NiO-TiO_2$/$WO_3$ catalysts was prepared by drying powdered $Ni(OH)_2-Ti(OH)_4$ with ammonium metatungstate aqueous solution, followed by calcining in air at high temperature. Characterization of prepared catalysts was performed by using FTIR, Raman, XPS, XRD, and DSC and by measuring surface area. Upon the addition of tungsten oxide to titania up to 25 wt%, the specific surface area and acidity of catalysts increased in proportion to the tungsten oxide content due to the interaction between tungsten oxide and titania. Since the -$TiO_2$/stabilizes the tungsten oxide species, for the samples equal to or less than 25 wt%, tungsten oxide was well dispersed on the surface of titania, but for the samples containing above 25 wt%, the triclinic phase of $WO_3$ was observed at calcination temperature above 400 $^{\circ}C$. The catalytic activities of 10-NiO-$TiO_2$/$WO_3$ for 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. NiO may attract reactants and enhance the local concentration of reactants near the acid sites, consequently showing the increased catalytic activities.

• 한국분말재료학회지
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• 제28권4호
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• pp.331-335
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• 2021

The effects of drying temperature on the microstructure of porous W fabricated by the freeze-casting process of tert-butyl alcohol slurry with WO₃ powder was investigated. Green bodies were hydrogen-reduced at 800℃ for 1 h and sintered at 1000℃ for 6 h. X-ray diffraction analysis revealed that WO₃ powders were completely converted to W without any reaction phases by hydrogen reduction. The sintered body showed pores aligned in the direction of tert-butyl alcohol growth, and the porosity and pore size decreased as the amount of WO₃ increased from 5 to 10vol%. As the drying temperature of the frozen body increased from -25℃ to -10℃, the pore size and thickness of the struts increased. The change in microstructural characteristics based on the amount of powder added and the drying temperature was explained by the growth behavior of the freezing agent and the degree of rearrangement of the solid powder during the solidification of the slurry.

• 한국결정성장학회지
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• 제24권3호
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• pp.111-119
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• 2014

A novel pulsed laser ablation process in liquid was investigated to prepare scheelite-type ceramic [calcium tungstate ($CaWO_4$) and calcium molybdate ($CaMoO_4$)] nanocolloidal particles. The crystalline phase, particle morphology, particle size distribution, absorbance and optical band-gap were investigated. Stable colloidal suspensions consisting of well-dispersed $CaWO_4$ and $CaMoO_4$ nanoparticles with narrow size distribution could be obtained without any surfactant. Particle tracking analysis using optical microscope combined with image analysis was applied for a fast determination of particle size distribution in the prepared nanocolloidal suspensions. The mean nanoparticle size of $CaWO_4$ and $CaMoO_4$ colloidal nanoparticles were 16 nm and 30 nm, with the standard deviations of 2.1 and 5.2 nm, respectively. The optical absorption edges showed blue-shifted values about 60~70 nm than those of reported in bulk crystals. And also, the estimated optical energy band-gaps of $CaWO_4$ and $CaMoO_4$ colloidal particles were 5.2 and 4.7 eV. The observed band-gap widening and blue-shift of the optical absorbance could be ascribed to the quantum confinement effect due to the very small size of the $CaWO_4$ and $CaMoO_4$ nanocolloidal particles prepared by pulsed laser ablation in liquid.

• Journal of Information Science Theory and Practice
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• 제9권3호
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• pp.76-106
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• 2021

The study analyzed bibliometric data of papers published in Korea Citation Index (KCI) and Web of Science (WoS) journals from 2002 to 2021. After examining size differences of KCI and WoS domains in the number of authors, institutions, and journals to put publication and citations counts in perspective, the study investigated co-authorship patterns over time to compare collaboration trends of Korean and international scholars and analyzed the data at author, institution, and journal levels to explore how the influences of authors, institutions, and journals on research output differ across domains. The study also conducted frequency-based analysis of keywords to identify key topics and visualized keyword clusters to examine topic trends. The result showed Korean LIS authors to be twice as productive as international authors but much less impactful and Korean institutions to be at comparable levels of productivity and impact in contrast to much of productivity and impact concentrated in top international institutions. Citations to journals exhibited initially increasing pattern followed by a decreasing trend though WoS journals showed far more variance than KCI journals. Co-authorship trends were much more pronounced among international publication, where larger collaboration groups suggested multi-disciplinary and complex nature of international LIS research. Keyword analysis found continuing diversification of topics in international research compared to relatively static topic trend in Korea. Keyword visualization showed WoS keyword clusters to be much denser and diverse than KCI clusters. In addition, key keyword clusters of WoS were quite different from each other unlike KCI clusters which were similar.

• 마이크로전자및패키징학회지
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• 제2권2호
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• pp.31-40
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• 1995

Donor dopant로 WO3, Ta2O5 및 Nb2O5를 첨가한 Zr0.8Sn0.2TiO4 세라믹스의 유전상수 $\varepsilon$r 품 질계수 Q 및 공진주파수의 온도계수 rf에 대하여 연구하였다. 139$0^{\circ}C$에서 32시간 소결시 donor dopant 첨가량에 따른 ZST의 유전상수는 소결밀도의 변화 거동과 잘 일치하였다. 5.5 GHz에서 측정 한 ZST의 품질계수 Q는 ~0.5 mol% WO3 Ta2O5 및 Nb2O5 첨가에 의해 6800에서 8500 정도로 증가 하였다. ZST의 $\tau$f는 0.3 mol%까지의 WO3 첨가량 증가에 따라 0 ppm/$^{\circ}C$에서 -4.6 ppm/$^{\circ}C$까지 음 의 값으로 직선적으로 감소하였으며 0.4 mol% 범위의 Ta2O5 및 Nb2O5 첨가에 의해 -7 ppm/$^{\circ}C$ 까지 직선적으로 감소하였다.

• 센서학회지
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• 제5권1호
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• pp.9-14
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• 1996

$WO_{3}$가 첨가된 $BaTiO_{3}$를 고상법으로 제작하여 상온에서 $160^{\circ}C$의 온도 범위와 300 kHz - 300 MHz의 주파수 범위에서 회로망분석기를 이용하여 2포트 S-파라메터 방법으로 고주파 저항, 리액턴스 및 임피던스를 측정하여 $130^{\circ}C$ 이하에서 주파수 및 온도센서로서의 가능성을 진단하였다.