W.N. Kim’s research while affiliated with Yonsei University and other places

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Publications (34)


White-Light Emitting Diode through Ultraviolet InGaN-pumped Sr2Si1−xGexO4 : Eu2+ phosphors
  • Article

January 2011

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5 Reads

Materials Research Society symposia proceedings. Materials Research Society

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P.E. Jeon

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W.N. Kim

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[...]

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The Sr2Si1−xGexO4 : Eu2+ phosphors are formed by means of a new synthesis method. The Sr2Si1−xGexO4 : Eu2+ have the mean particle size of 200 nm and the spherical shape. The Sr2Si1−xGexO4 : Eu2+ show two emission colors : the blue color of 480 nm and the green color of 525 nm. The intensity of green band with respect to blue band is significantly decreased in comparison with that of solid-reacted bulk sample, described by the hindrance of the energy transfer between the blue and green band by numerous nanoparticle boundaries. As the increase of Ge4+ ions, the 480 nm emission intensity of the Sr2Si1−xGexO4 : Eu2+ are more dominant. This behavior can be understood in terms of the effect of Ge4+ ions on the energy transfer from 480 nm band to 525 nm band. The fabricated white-light emitting diode using ultraviolet InGaN chip with the blue and green emitting Sr2Si1−xGexO4 : Eu2+ phosphor shows warm white light of 4300 K and higher color stability against input power in comparison with a commercial blue GaN-pumped YAG:Ce3+.


Effects of clay on the morphology of poly(acrylonitrile‐butadiene‐styrene) and polypropylene nanocomposites

October 2007

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23 Reads

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40 Citations

Polymer Engineering and Science

The effects of clay on the morphology of the poly(acrylonitrile-butadiene-styrene) (ABS)/polypropylene (PP) nanocomposites were investigated. From the studies of wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) analysis of the ABS/PP (70/30) nanocomposites with clay, it was observed that most of the clay existed in the ABS matrix because of the good affinity between the ABS and clay which contained benzyl group. From the studies of the scanning electron microscopy (SEM) analysis, the size of the PP droplet decreased with clay from 16.7 to 5.6 μm when the clay was added up to 4.0 phr. When the clay content was increased, the droplet of the dispersed phase (PP) was shown to elongated structure. The elongated structure could be explained by the decrease of the viscosity ratio of the PP and continuous phase (ABS) with clay, which corresponded to the increase of the viscosity of the continuous phase (ABS/clay). From the results of the SEM, TEM, and WAXD analysis, it is suggested that the increase of the viscosity of the continuous phase (ABS/clay) is mainly due to the location of clay in the ABS matrix. POLYM. ENG. SCI., 47:1671–1677, 2007. © 2007 Society of Plastics Engineers



Effects of ultrasound on the synthesis and properties of polyurethane foam/clay nanocomposites

November 2006

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127 Reads

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62 Citations

Polyurethane foam (PUF)/clay nanocomposites were synthesized with clay modified by polymeric 4,4′-diphenylmethane diisocyanate (PMDI) with the application of ultrasound. Transmission electron micrographs showed that the interlayer distance increased for the polyurethane (PU)/clay nanocomposites where ultrasound was applied. The results of the transmission electron microscopy and X-ray measurements suggest that the application of ultrasound to the clay modification with PMDI improved the efficiency of the clay modification by the effective breakup of the clay agglomerates and intercalation of the silicate layers. In the mechanical tests of the PUF/clay nanocomposites, the flexural and tensile strengths of the PUF/clay nanocomposites showed the maximum value at 3.0 wt % clay content based on PMDI. These results suggest that the increases in the flexural and tensile strengths were perhaps due to the uniform dispersion of the clay by the application of ultrasound. At the same modified clay content, the fire resistance properties were increased for the PUF/clay nanocomposites with the application of ultrasound compared to the PUF/clay nanocomposites without the application of ultrasound. The cell size and thermal conductivity were decreased for the PUF/clay nanocomposites with the application of ultrasound compared to the PUF/clay nanocomposite without the application of ultrasound. Because of these results, we suggest that the smaller cell size and lower thermal conductivity of the PUF/clay nanocomposites were mainly due to the enhanced dispersion of the clay by the application of ultrasound to the mixture of PMDI and clay. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3764–3773, 2006


Synthesis and properties of polyurethane/clay nanocomposite by clay modified with polymeric methane diisocyanate

September 2006

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53 Reads

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54 Citations

A polyurethane (PU)/clay nanocomposite was synthesized from polyol, polymeric 4,4′-diphenyl methane diisocyanate (PMDI), and modified clay with PMDI. To achieve the modified clay with PMDI, the silanol group of the clay and the NCO group of the PMDI were reacted for 24 h at 50°C to form urethane linkage. Fourier transform IR analysis of the clay modified with the PMDI demonstrated that the NCO characteristic peak was observed in the clay after a modification reaction with PMDI. The results of the X-ray pattern suggested that the clay layers were exfoliated from the PU/clay nanocomposite. From the results of the mechanical properties, the maximum values of the flexural and tensile strength were observed when 3 wt % clay based on PMDI was added into the PU/clay nanocomposites. The glass-transition temperature and change in the heat capacity at glass transition temperature (ΔCp) of the PU/clay nanocomposite decreased with an increase in the modified clay content. We suggested that the decrease in the ΔCp with the modified clay content might be due to the increase of steric hindrance by the exfoliated clay layers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2879–2883, 2006


Rheological and electrical properties of polycarbonate/multi-walled carbon nanotube composites

May 2006

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143 Reads

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165 Citations

Polymer

Rheological and electrical properties of the polycarbonate (PC)/multi-walled carbon nanotube (MWNT) were studied. The MWNT was funtoinalized by treating with the hydrogen peroxide (H2O2). The H2O2 treated MWNT was dried by thermal and freeze drying methods. From the morphological studies, the degree of entanglement of the MWNT was decreased after treating with the H2O2. For the H2O2 treated MWNT (thermal drying), the length of the MWNT was shortened compared that of the H2O2 treated MWNT (freeze drying). The rheological and electrical properties of the PC/MWNT (H2O2 treated) composites increased compared that of the PC/MWNT (untreated) composites. Also, the electrical conductivity showed higher value for the PC/MWNT (H2O2 treated, freeze drying) composites compared that of the PC/MWNT (H2O2 treated, thermal drying) composites. From the results of the morphological, rheological, and electrical properties of the PC/MWNT composites, it is suggested that the electrical and rheological properties of the PC/MWNT composites are affected by the MWNT–MWNT network structure, which is related with the MWNT morphologies such as the degree of aggregation and aspect ratio of the MWNT.


Modification of natural zeolite powder and its application to interior non-woven textile for indoor air quality control

January 2006

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37 Reads

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3 Citations

Materials Science Forum

The application of VOC-free materials for building interior is the first step to reduce the contamination level, however, it is practically unavoidable to find a way to eliminate existing pollutants from furniture, adhesives, etc. As a positive way of controlling the air indooors, the air conditioner may be applied with high functional filter, which uses electricity for compulsory air circulation. As an alternative, we modified natural zeolite powder by ion exchange and also by addition of TiO2 photo-catalyst for multifunctional powder to contribute to controlling the indoor air quality. The modified zeolites were applied on the interior non-woven polypropylene sheet to reduce indoor air pollutants not only by adsorption mechanism but also by decomposition mechanism. We studied deodorization of the multi-functional powders and also their coating on non-woven textiles by a Gastec method.


Effects of crystallinity and crosslinking on the thermal and rheological properties of ethylene vinyl acetate copolymer

November 2005

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125 Reads

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112 Citations

Polymer

Effects of crosslinking and crystallinity on the properties of the thermal and rheological properties of the EVA were studied. From the studies of storage modulus of the EVA with VA content in the solid temperature range (about −70 to 50 °C), the storage modulus decreased with increasing the VA content. This result suggested that the crystallinity of the EVA affected the storage modulus of the EVA because of the weak crosslinking of the EVA by DCP. From the studies of complex viscosity of the EVA with and without DCP in the melt state, the values of the power law parameter of the EVA without DCP ranged from 0.39 to 0.50 and the EVA with DCP ranged from 0.03 to 0.12. In the measurement of the complex viscosity of the EVA in the melt state, the crosslinking affected the complex viscosity of the EVA with DCP.


Lithium solubility limit in ZnGa2O4:Bi0.0013+,Li+ phosphor

August 2005

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29 Reads

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11 Citations

Materials Letters

The lithium solubility limit, photoluminescence (PL) and photoluminescence excitation (PLE) properties of lithium ion co-activated ZnGa2O4:Bi3+,Li+ phosphor have been investigated. A LiGaO2 second phase began to appear from 3 mol% Li+ ion co-activated ZnGa2O4:Bi3+,Li+ phosphor. The enhanced brightness of blue (λex = 254 nm) and white (λex = 315 nm) colors of bismuth ions doped ZnGa2O4:Bi3+,Li+ phosphor was assigned to the formation of LiGaO2. Bi3+ activated lithium zinc gallate phosphor showed a more enhanced PLE peak around 315 nm than that of lithium zinc gallate phosphor when λem = 520 nm. Thus, we observed that the PL intensity of ZnGa2O4:Bi3+,Li+ phosphor with λem = 520 nm was much greater than that of ZnGa2O4:Li+ phosphor. Also, ZnGa2O4:Bi3+,Li+ phosphor exhibited a shorter decay time than that of ZnGa2O4:Li+ phosphor by about a factor of about 2.


Dynamic mechanical and morphological properties of polycarbonate/multi-walled carbon nanotube composites

July 2005

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93 Reads

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93 Citations

Polymer

Dynamic mechanical and morphological properties of the polycarbonate (PC)/multi-walled carbon nanotube (MWNT) composites were studied by dynamic mechanical thermal analysis (DMTA) and X-ray diffractometry, respectively. For the without annealed PC/MWNT composites containing the higher content of the MWNT (≥7.0 wt%), double tan δ peaks were observed, which could be explained by the phase separation morphology model. For the annealed PC/MWNT composites, a broad single tan δ peak was observed. From the X-ray diffraction of the annealed PC/MWNT composites, it was observed that more regular structure of the PC was obtained, which was consistent with the result of the thermal analysis of the annealed PC/MWNT composites. From the dynamic mechanical properties, thermal analysis, and X-ray diffraction of the annealed PC/MWNT composites, it is suggested that PC/MWNT composites show a broad single tan δ peak and partially crystalline structure of the PC in the PC/MWNT composites by annealing.


Citations (23)


... However, these are less costly but need more material to achieve the same insulating efficiency as RPUF. This attribute is credited mostly to the cell morphology and cell size of RPUF, trapping the small thermal conductive gases (17,18). Against the good features of RPUF, it exhibits a low limiting oxygen index (LOI) value between 16-18% (19,20), making it highly flammable and exhibits low thermal resistance, a major drawback undermining its use (21,22) in its stronghold; insulation (building and appliance) and packaging (23,24). ...

Reference:

Intumescent flame retardant based on sepiolite filled rigid polyurethane foam
Properties of rigid polyurethane foams blown by HFC-365 mfc and distilled water
  • Citing Article
  • January 2007

Journal of Industrial and Engineering Chemistry

... Additionally, the use of mesoporous materials, like zeolite as a support for these series of semiconductors, has recently become the focus of intensive research on vat dye photodecolorization, due to the fact that the semiconductor support influences the photocatalytic efficiency through structural features, and the interaction between the vat dyes leads to the enhancement of contact between the surface, and irradiation likewise decreases with the amount of semiconductor required [179]. Thus, there are some studies focused on the importance of semiconductor supported zeolite for vat dye photodegradation, including Co-ZSM-5, TiO 2 -HZSM-5, Fe-exchange zeolite, and CuO-X zeolite [180, 181]. TiO 2 has been the most studied material for the photocatalysis of vat dyes182183184185186. ...

Modification of natural zeolite powder and its application to interior non-woven textile for indoor air quality control
  • Citing Conference Paper
  • January 2006

Materials Science Forum

... Taking into account the comparative advantages of each thermoplastic (e.g., the thermal stability of PEI and the chemical resistance of PEEK), it is of great interest to combine their properties and minimize their respective drawbacks by means of a PEEK/PEI blend, whose molecular miscibility is a proven fact [13][14][15][16]. Until now, the melt blending process, which commonly uses a twin-screw extruder, has been the most widely used technique in manufacturing PEEK/PEI polymer blends [17][18][19][20][21][22]. ...

Thermal properties of melt-blended poly(ether ether ketone) and poly(ether imide)
  • Citing Article
  • May 1999

Journal of Applied Polymer Science

... Tg, relaxation enthalpy and peak temperature (Tp) before and after seawater exposure are listed in Table 4. It has been widely stated that the Tg for Ultem with over 10% PC blended with PEI will lead to phase separation that is evident in the material [50][51][52][53]. As a result, there are often two distinct Tgs observed in the PEI/PC blend product, which are produced by the PC-rich and PEI-rich regions. ...

Properties of blends of polycarbonate and polypropylene (I): Crystallization behavior
  • Citing Article
  • January 1996

... Helfand and Tagami suggested a self-consistent mean-field theory for symmetric polymers assuming infinite molecular weight for polymers and an incompressible interface. Later, they corrected their idea by considering the effect of statistical segmental length differences of the components via symmetric interface assumption, as follows [25]: ...

Evaluation of interfacial tension for poly(methyl methacrylate) and polystyrene by rheological measurements and interaction parameter of the two polymers
  • Citing Article
  • September 2004

Korea-Australia Rheology Journal

... However, the limitation of PA6 was also remarked, it has high moisture absorption. In order to create new and useful materials, blends of polyamide (PA) and polybutylene terephthalate (PBT) were successfully determined by many researchers since both polymers are among the most frequently used [6][7][8][9]. Unfortunately, blend of the two different materials cannot be compatible with each other leading to phase separation and reduce mechanical properties. ...

Reactive blends of poly(butylene terephthalate)/ polyamide-6 with ethylene glycidyl methacrylate
  • Citing Article
  • January 2001

... Tabaza et al. reported the effect of bismuth doping on the luminescent properties of magnesium aluminate [37]. Kim et al. illustrated on the phosphorescence activities of bismuth doped zinc gallate [38]. Bismuth-based nanoparticles are reported for good photocatalytic activity [39]. ...

Lithium solubility limit in ZnGa2O4:Bi0.0013+,Li+ phosphor
  • Citing Article
  • August 2005

Materials Letters

... Generally, it is expected that the relaxation times increase when the compatibility of a blend improves. [36] Therefore, it seems that the finer distribution of the PHB droplets is probably mainly due to high mechanical shear forces. ...

Rheological properties and interfacial tension of polypropylene - Poly(styrene-co-acrylonitrile) blend containing compatibilizer
  • Citing Article
  • March 2003

Polymer

... Although several ternary polymer blend systems have been developed [12][13][14][15][16][17][18] and even reinforcement has been observed in some of these ternary blend systems containing a rigid polymer or a thermotropic liquid crystalline polymer, no miscible ternary system of a rigid polymer / flexible polymer / flexible polymer with molecular reinforcement at nanometre scale has been reported. This study aims to develop the first ternary miscible polymer system of two flexible polymers and a rigid-rod macromolecule in the form of a tercet molecular nanocomposite of PPTA/ [PA6/PA66] of a miscible polyamide-6/ polyamide-6,6 blend molecularly reinforced at the nanometre level by a rigid-rod poly(p-phenylene terephthalamide). ...

Compatibility studies of blends of a thermotropic liquid crystalline polymer and flexible chain polymers by application of Flory's lattice theory
  • Citing Article
  • March 2001

Polymer

... Even in A/B polymer blends, a three-phase model, comprising crystalline fraction (C), mobile amorphous fraction (MAF), and rigid amorphous fraction (RAF), is proposed to describe the phase structure, instead of using a conventional two-phase model comprising only crystalline and amorphous fractions. RAF has been found in many well-crystallized polymer blends such as isotactic polystyrene/atactic polystyrene blends [19], polyurethane/ poly(methyl methacrylate) blends [20], PET/polystyrene blends [21], polycarbonate/poly(-caprolactone) blends [22], poly(ether ether ketone)/polyarylate blends [23], and poly(ether ether ketone)/poly(ether imide) blends [24] . The rigid amorphous fraction (RAF) is the intermediate phase connecting the crystalline phase (C) to the mobile amorphous fraction (MAF); MAF is also called the " conventional " amorphous phase25262728293031. ...

Glass transition temperatures and rigid amorphous fraction of poly(ether ether ketone) and polyarylate blends
  • Citing Article
  • November 2000

Polymer