• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.123-123
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• 2010

3족 질화물계 물질은 발광다이오드와 같은 광전자소자적용에 있어서 매우 우수한물 질이다.일반적으로, GaN 에피 성장에 있어서 저온 중간층을 삽입한 2 단계 성장 방법은 낮은 결함밀도와 균일한 표면을 얻기 위해 도입된 기술이다. 특히 AlN 중간층은 GaN 중간층과 비교하였을 때 결정성뿐만 아니라 높은 온도에서의 열적안정성, GaN 기반의 자외선 검출기서의빛 흡수 감소 등의 장점을 가지고 있다. 또한 패턴 사파이어 기판위 GaN 에피 성장은 측면성장 효과를 통해 결함 밀도 감소와 광 추출 효율을 향상시키는 것으로 알려져 있다.또한 열응력으로 인한 기판의 휨 현상은 박막성장중 기판의 온도 분포를 불균일하게 만드는 원인이 되며 이는 결국 박막 조성 및 결정성의 열화를 유도하게 되고 최종적으로 소자특성을 떨어 뜨리는 원인이 되는데 AlN 중간층의 도입으로 이것을 완화시킬 수 있는 효과가 있다. 하지만, AlN 중간층이 패턴된 기판 위에 성장시킨 GaN 에피층에 미치는 영향은 명확하지 않다. 본 연구팀은 일반적인 c-plane 사파이어 기판과 플라즈마 건식 에칭을 통한 렌즈 모양의 패턴된 사파이어 기판을 이용해서 AlN 중간층과 GaN 에피층을 유기금속 화학기상증착법으로 성장하였다. 특히, 렌즈 모양의 패턴된 사파이어 기판은 패턴 모양과 패턴 밀도가 성장에 미치는 영향을 연구하기 위해 두가지 패턴의 사파이어 기판을 이용하였다. AlN 중간층 두께를 조절함으로써 최적화된 GaN 에피층을 90분까지 4단계로 시간 변화를 주어 성장 양상을 관찰한 결과, GaN 에피박막의 성장은 패턴 기판의 trench 부분에서 시작하여 기판의 패턴부분을 덮는 측면 성장을 보이고있다. 또한 TEM과 CL을 통해 GaN 에피박막의 관통 전위를 분석해 본 결과 측면 성장과정에서 성장 방향을 따라 옆으로 휘게 됨으로 표면까지 도달하는 결정결함의 수가 획기적으로 줄어드는 것을 확인함으로써 고품질의 GaN 에피층을 성장시킬 수 있었다. 그리고 패턴밀도가 높고 모양이 볼록할수록 측면 성장 효과로 인한 결정성 향상과 난반사 증가를 통한 임계각 증가로 광추출 효율이 향상 되는 것을 확인할 수 있었다. 이러한 결과를 바탕으로 최적화된 AlN 중간층을 이용하여 패턴 기판위에서 고품질의 GaN 에피층을 성장시킬 수 있었다.

• Korean Journal of Materials Research
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• v.7 no.3
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• pp.218-223
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• 1997

At the aim of finding a Fehased amorphous alloy with a wide supercooled liquid region (${\Delta}T_{x}=T_{x}-T_{g}$) before crystallization, the changes in glass transition temperatudfI$T_{g}$ and crystallization temperature ($T_{x}$) by the dissolution of additional M elements were examined for the $Fe_{80}P_{10}C_{6}B_{4}$(x~6at%. M= transition metals) amorphous alloys. The ${\Delta}T_{x}$ value is 27K for the Fe,,,P,,,C,,R, alloy and increases to 40K for the addition of M=4at%Hf, 4at%Ta or 4at%Mo. The increase in ${\Delta}T_{x}$ is due to the increase of $T_{x}$ exceeding the degree in the increase in $T_{g}$. The $T_{g}$ and $T_{x}$ increase with decreasing electron concentration (e/a) from about 7 38 to 7.05. The decrease of e/a also implies the increase in the attractive bonding state between the M elements and other constitutent elements. It is therefore said that $T_{g}$ and $T_{x}$ increase kith increasing attractive bonding force.

• Korean Journal of Materials Research
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• v.9 no.12
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• pp.1245-1251
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• 1999

Phase-change optical disk very rapid recording, high densification of data, resulting in high feedback rate and good C/N(carrier to noise) ratio of a feedback signal. However, repetitive thermal energy may cause the deformation of a disk or the lowering of an eliminability and a cyclability of the recording. The lowering of the cyclability can be reduced by insertion of thin layer of ZnS-$SiO_2$ dielectric thin film in appropriate disk structure between the upper and lower part of the recording film. Using the Taguchi method, optimum conditions satisfying both the optimized quality characteristic values and the scattering values for film formation were found to be the target R.F. power of 200W, the substrate R.F. power of 20W, the Ar pressure of 6mTorr, and the electrode distance of 6cm. From the refractive index data, the existence of the strong interaction between the electrode distance and Ar pressure was confirmed, and so was the large effect of the electrode distance on transmittance. According to the analysis of TEM and XRD, the closer the electrode distance was, the finer was the grain size due to the high deposition rate. However, the closer electrode distance brought the negative effect on the morphology of the film and caused the reduction of transmittance. AFM and SEM analyses showed that the closer the electrode distance was, the worse was the morphology due to the high rate of the deposition. Under optimum condition, the deposited thin film showed a good morphology and dense microstructure with less defects.

• Applied Chemistry for Engineering
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• v.16 no.6
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• pp.809-814
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• 2005

In this work, the thermal properties of epoxy coating system on the liner plate in the containment structure of nuclear power plants had been examined by irradiation and design basis accident (DBA) conditions. The effect of immersion in hot water on adhesion strength of the coating system had been also studied. The glass transition temperature ($T_g$) and thermal stability of ET-5290/carbon steel A 32 epoxy coating systems were measured by DSC and TGA analyses, respectively. Contact angle measurements were used to determine the effect of immersion on the surface energetics of epoxy coating system, with a viewpoint of surface free energy. Adhesion tests were also executed to evaluate the adhesion strength at interfaces between carbon steel plate and epoxy resins. As a result, it was found that the irradiation led to an improvement of internal crosslinked structure in cured epoxy systems, resulting in significantly increasing the thermal stability, as well as the $T_g$. Also, the immersion in hot water made a role in the post-curing of epoxy resins and increased the mechanical interlocking of the network system, resulting in increasing the adhesion strength of the epoxy coating system.

• Applied Chemistry for Engineering
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• v.9 no.5
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• pp.704-709
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• 1998

Non-linear optical polymer based on poly (${\alpha}$-methylstyrene-co-maleic anhydride) (MSMA) substrate polymer was prepared by polymer reaction method and its thermal and electro-optic properties were examined. In the polymer reaction between MSMA substrate polymer and 2-[4-(4-nitrophenylazo)-N-ethylphenylamino]ethanol (DR1) chromophore, the degree of substitution of DR1 into MSMA was higher with the 4-dimethylaminopyridine (DMAP) as catalyst and 3-dicyclohexyl carbodiimide (DCC) as dehydrating agent (sample, MSMA-DC) than the one with just 4-dimethylaminopyridine as catalyst (sample, MSMA-D). The synthesized NLO polymer (MSMA-DC) exhibited electro-optic coefficient of 18 pm/V (632.8 nm) and glass transition temperature ($T_g$) of about $175^{\circ}C$.

• Applied Chemistry for Engineering
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• v.25 no.1
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• pp.90-95
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• 2014

Nylon66 extrudates as a function of the extrusion number were prepared by a twin screw extruder. Chemical structures, thermal properties, melt index, crystal structures, mechanical properties such as the tensile strength, elongation at break and impact strength, and rheological property were measured by FT-IR, $^1H$-NMR, melt indexer, DSC, TGA, XRD, universal tensile tester, Izod impact tester, and rheometer. FT-IR and $^1H$-NMR characterizations indicated that the number of extrusions did not affect the chemical structure. The decrease in the molecular weight was checked by the melt index of extrudates. There were no effects of the thermal history on the melting and degradation temperature. The tensile and impact strength and modulus were found to be similar, regardless of the number of extrusions, but the elongation decreased significantly. The complex viscosity of extrudates at low frequencies decreased with the extrusion number. No structural changes after extrusion were confirmed from the fact that there was no change in the slope and shape of G'-G" plot.

• Applied Chemistry for Engineering
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• v.25 no.1
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• pp.72-77
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• 2014

Microstructure and thermal stability of mechanically ball milled $FeS_2$ were investigated. The average particle size and distribution of $FeS_2$ powder were changed in two steps with the increased ball milling time. The average particle size drastically decreased from $98.4{\mu}m$ to 1.01 and $0.89{\mu}m$ after ball milling of 10 h and 30 h, respectively. However, the distribution was broad and a shoulder appeared at $2{\mu}m$ because the pulverization was still in process at 10 h ball milling. After 60 h ball milling, the distribution became narrower. After ball milling of 120 h, the average particle size increased because of $FeS_2$ particle agglomeration. Therefore, the particle size distribution became broaden again. Finally, after ball milling of 170 h, $FeS_2$ with the narrowest size distribution can be obtained. Thermal stability of $FeS_2$ was unstable as the $FeS_2$ particle was pulverized. Therefore, the activation energy of the fine size particles is 27% lower than that of the as-received $FeS_2$.

• Applied Chemistry for Engineering
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• v.23 no.2
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• pp.217-221
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• 2012

Blending films having enhanced water-resistance and barrier properties were prepared using the mixtures of poly(vinyl alcohol) (PVA) aqueous solution and poly(ethylene-co-acrylic acid) (EAA) dispersed in water. Thermal-mechanical properties, contact angles, water-vapor transmission rates (WVTR) and oxygen transmission rates $(O_2TR)$ were measured with the content of EAA of blending films, and their water-resistance was evaluated. The tensile strength of the films was found to be $9.16{\sim}11.75\;kg/mm^2$ which showed no significant difference compared with that of PVA, and the hardness increased with the content of EAA. The glass transition temperature and melting temperature of the blending films were slightly improved. The film prepared with PVA/EAA (= 90/10), of which the swelling and solubility were measured to be 109 and 0%, respectively, showed improved water-resistance. The WVTR and $O_2TR$ for the PET film (thickness $50\;{\mu}m$) coated with PVA/EAA (= 90/10) film (thickness $2.5\;{\mu}m$) were measured to be $9.1\;g/m^2/day$ and $2.0\;cc/m^2/day$, respectively.

• Applied Chemistry for Engineering
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• v.25 no.3
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• pp.318-323
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• 2014

In order to analyze the effect of maleic anhydride (MAH) content and styrene monomer (SM)/MAH mole ratio on reactive extrusion of maleic anhydride grafted random polypropylenes (MAH-g-rPP), MAH-g-rPPs were prepared by using a twin screw extruder. MAH contents were 0.5, 1.0, 3.0, and 5.0 phr and SM/initiator mole ratio was 0.0, 1.0, and 2.0. Dicumyl peroxide (DCP) was used as an initiator. The graft degree of MAH was confirmed by the existence of carbonyl group (C = O) stretching peak at $1700cm^{-1}$ from FT-IR spectrum. The degree of graft reaction increased up to 3.0 phr MAH and showed the optimum value at 1.0 SM/MAH mole ratio from the area ratio of C = O and C-H stretching peak. Thermal and crystallization properties of MAH-g-rPP and PP/MAH-g-rPP/pulp composites were investigated by DSC, TGA, XRD, and POM. There was a decrease in non-isothermal crystallization temperature of PP/MAH-g-PP/pulp composites. Based on tensile properties and SEM pictures for the fractured surface of PP/MAH-g-PP/pulp composites, MAH content of 1.0 wt% and SM/MAH mole ratio of 1.0 were the optimum formulation as the compatibilizer. The rheological properties of the composites were measured by dynamic Rheometer to compare the processability of the composites with and without compatibilizer. The power law index showed slightly low value at the composites with compatibilizer.

• Applied Chemistry for Engineering
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• v.25 no.3
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• pp.254-261
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• 2014

The transformer is a static electrical device that transfers energy by inductive coupling. Then, heat is occurred at coils, inner transformer was filled with insulating oils for cooling and insulation. Although mineral oil as insulating oil has been widely used, it does not meet health and current environmental laws because it is not biodegradable. Therefore, in this study, the diglycerol ester was synthesized with diglycerol and fatty acids (oleic acid and caprylic acid) over various catalysts for insulating oil having biodegradability, high flash points and low pour points. The sulfated zirconia ($SO_4{^{2-}}/ZrO_2$) catalyst prepared at different calcination temperature shows the highest conversion of fatty acids at $600^{\circ}C$ due to crystallinity and high density of acid sites per surface area. When the molar ratio of oleic acid and caprylic acid is 1:3, the diglycerol ester shows superior insulation properties that are the flash point of $306^{\circ}C$ and pour point of $-50^{\circ}C$. The insulation properties of synthesized diglycerol ester shows the pour point of $-50^{\circ}C$ and the flash point of over $300^{\circ}C$. Therefore, diglycerol ester is superior to the vegetable oils in insulation properties.