• Korean Journal of Materials Research
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• v.8 no.5
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• pp.424-428
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• 1998

Mn-Ni oxide thin films for NTC thermistor application were deposited on alumina substrates by using rf magnetron sputter. Effects of various substrate temperatures and annealing temperatures on the microstructure. crystal phase, resistivity and B constant were investigated. Microstructure of the films deposited below 178$^{\circ}C$ was fibrous microcrystalline and at 32$0^{\circ}C$and 40$0^{\circ}C$their microstructure was changed to columnar grain structure. After annealing at 90$0^{\circ}C$, the microstructure was transformed to equiaxed grain structure. Most of the phases were mixture of cubic spinel and $Mn_2O_2$ The crystal phase of the film deposited at 40$0^{\circ}C$ was changed to cubic spinel after annealing above 700"c. As the substrate temperature increased, the resistivity and B constant were greatly decreased, and these values become low and stable after annealing between $600^{\circ}C$and $700^{\circ}C$, All thin films deposited in the present study showed NTC thermistor characteristicsstics.

• Korean Chemical Engineering Research
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• v.50 no.3
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• pp.562-566
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• 2012

A novel design of tubular segmented-in-series(SIS) solid oxide fuel cell (SOFC) sub module was presented in this paper. The tubular ceramic support was fabricated by the extrusion technique. The NiO-YSZ anode and the yttria-stabilized zirconia (YSZ) electrolyte were deposited onto the ceramic support by dip coating method. After sintering at $1350^{\circ}C$ for 5 h, a dense and crack-free YSZ film was successfully fabricated. Also, the multi-layered cathode composed of LSM-YSZ composite, LSM and LSCF were coated onto the sintered ceramic support by dip coating method and sintered at $1150^{\circ}C$. The performance of the tubular SIS SOFC cell and sub module electrically connected by the Ag-glass interconnect was measured and analysed with different fuel flow and operating temperature.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.211-212
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• 2010

GaN 기반의 상부발광형 LED는 동작되는 동안 생기는 전기적 단락, 그리고 칩 위의 p-형 전극과 n-형 전극 사이에 생기는 누설전류 및 신뢰성 확보를 위하여 칩 표면에 passivation 층을 형성하게 된다. SiO2, Si3N4와 같은 passivation layers는 일반적으로 PECVD (Plasma Enhanced Chemical Vapor Deposition)공정을 이용한다, 하지만 이는 공정 특성상 plasma로 인한 damage가 유발되기 때문에 표면 누설 전류가 증가 한다. 이로 인해 forward voltage와 reverse leakage current의 특성이 저하된다. 본 실험에서는 원자층 단위의 박막 증착으로 인해 PECVD보다 단차 피복성이 매우 우수한 PEALD(Plasma Enhanced Atomic Layer Deposition)공정을 이용하여 Al2O3 passivation layer를 증착한 후, 표면 누설전류와 빛의 출력 특성에 대해서 조사해 보았다. PSS (patterned sapphire substrate) 위에 성장된 LED 에피구조를 사용하였고, TCP(Trancformer Copled Plasma)장비를 사용하여 에칭 공정을 진행하였다. 이때 투명전극을 증착하기 위해 e-beam evaporator를 사용하여 Ni/Au를 각각 $50\;{\AA}$씩 증착한 후 오믹 특성을 향상시키기 위하여 $500^{\circ}C$에서 열처리를 해주었다. 그리고 Ti/Au($300/4000{\AA}$) 메탈을 사용하여 p-전극과 n-전극을 형성하였다. Passivation을 하지 않은 경우에는 reverse leakage current가 -5V 에서 $-1.9{\times}10-8$ A 로 측정되었고, SiO2와 Si3N4을 passivation으로 이용한 경우에는 각각 $8.7{\times}10-9$과 $-2.2{\times}10-9$로 측정되었다. Fig. 1 에서 보면 알 수 있듯이 5 nm의 Al2O3 film을 passivation layer로 이용할 경우 passivation을 하지 않은 경우를 제외한 다른 passivation 경우보다 reverse leakage current가 약 2 order ($-3.46{\times}10-11$ A) 정도 낮게 측정되었다. 그 이유는 CVD 공정보다 짧은 ALD의 공정시간과 더 낮은 RF Power로 인해 plasma damage를 덜 입게 되어 나타난 것으로 생각된다. Fig. 2 에서는 Al2O3로 passivation을 한 소자의 forward voltage가 SiO2와 Si3N4로 passivation을 한 소자보다 각각 0.07 V와 0.25 V씩 낮아지는 것을 확인할 수 있었다. 또한 Fig. 3 에서는 Al2O3로 passivation을 한 소자의 output power가 SiO2와 Si3N4로 passivation을 한 소자보다 각각 2.7%와 24.6%씩 증가한 것을 볼 수 있다. Output power가 증가된 원인으로는 향상된 forward voltage 및 reverse에서의 leakage 특성과 공기보다 높은 Al2O3의 굴절률이 광출력 효율을 증가시켰기 때문인 것으로 판단된다.

• Korean Journal of Materials Research
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• v.16 no.4
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• pp.225-230
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• 2006

Nanoimprint lithography (NIL) is a novel method of fabricating nanometer scale patterns. It is a simple process with low cost, high throughput and resolution. NIL creates patterns by mechanical deformation of an imprint resist and physical contact process. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting process. Stiction between the resist and the stamp is resulted from this physical contact process. Stiction issue is more important in the stamps including narrow pattern size and wide area. Therefore, the antistiction layer coating is very effective to prevent this problem and ensure successful NIL. In this paper, an antistiction layer was deposited and characterized by PECVD (plasma enhanced chemical vapor deposition) method for metal stamps. Deposition rates of an antistiction layer on Si and Ni substrates were in proportion to deposited time and 3.4 nm/min and 2.5 nm/min, respectively. A 50 nm thick antistiction layer showed 90% relative transmittance at 365 nm wavelength. Contact angle result showed good hydrophobicity over 105 degree. $CF_2$ and $CF_3$ peaks were founded in ATR-FTIR analysis. The thicknesses and the contact angle of a 50 nm thick antistiction film were slightly changed during chemical resistance test using acetone and sulfuric acid. To evaluate the deposited antistiction layer, a 50 nm thick film was coated on a stainless steel stamp made by wet etching process. A PMMA substrate was successfully imprinting without pattern degradations by the stainless steel stamp with an antistiction layer. The test result shows that antistiction layer coating is very effective for NIL.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.181-181
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• 2012

Direct synthesis of graphene using a chemical vapor deposition (CVD) has been considered a facile way to produce large-area and uniform graphene film, which is an accessible method from an application standpoint. Hence, their fundamental understanding is highly required. Unfortunately, the CVD growth mechanism of graphene on Cu remains elusive and controversial. Here, we present the evidences for two different growth modes of graphene on Cu investigated by varying carbon feedstock (C2H2 and CH4) and working pressure. The number of uniform graphene layer grown by C2H2 increased with increasing its injection time. A combined secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) study revealed a carbon-diffused Cu layer created below surface region of Cu substrate with the expansion of Cu lattice. The graphene on Cu was grown by the diffusion and precipitation mode not by the surface adsorption mode, because similar results were observed in graphene/Ni system. The carbon-diffused Cu layer was also observed after graphene growth under high CH4 pressure. Based on various previous results and ours, we have successfully found that there are two selective growth modes for graphene on Cu substrate, and a desired mode can be chosen by tuning working pressure corresponding to the kind of carbon feedstock. We believe that this finding will shed light on high quality graphene growth and its multifaceted applications.

• Journal of Powder Materials
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• v.22 no.1
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• pp.52-59
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• 2015

As well-known wrought stainless steel, sintered stainless steel (STS) has excellent high-temperature anti-corrosion even at high temperature of $800^{\circ}C$, and exhibits good corrosion resistance in air. However, when temperature increases above $900^{\circ}C$, the corrosion resistance of STS begins to deteriorate and dramatically decreases. In this study, the effects of phase and composition of STS on high-temperature corrosion resistances are investigated for STS 316L, STS 304 and STS 434L above $800^{\circ}C$. The morphology of the oxide layers are observed. The oxides phase and composition are identified using X-ray diffractometer and energy dispersive spectroscopy. The results demonstrate that the best corrosion resistance of STS could be improved to that of 434L. The poor corrosion resistance of the austenitic stainless steels is due to the fact that $NiFe_2O_4$ oxides forming poor adhesion between the matrix and oxide film increase the oxidation susceptibility of the material at high temperature.

• Progress in Superconductivity and Cryogenics
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• v.6 no.3
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• pp.50-55
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• 2004

We fabricated and tested a resistive type superconducting fault current limiter (SFCL) of three-phase 6.6 $kV_{rms}/200 A_{rms}$ rating based on YBCO thin films grown on sapphire substrates with a diameter of 4 inches, Short circuit tests were carried out at a accredited test facility for single line-to- ground faults, phase-to-phase faults and three-phase faults, Each phase of the SFCL was composed of 8${\times}$6 elements connected in series and parallel respectively. Each element was designed to have the rated voltage of 600 $V_{rms}$. A NiCr shunt resistor of 23 Ω was connected to each element for simultaneous quenches. Firstly, single phase-to-ground fault tests were carried out. The SFCL successfully developed the impedance in the circuit within 0.12 msec after fault and controlled the fault current of 10 $kA_{rms} below 816 A_{peak}$ at the first half cycle. In addition, in case of phase-to-phase fault and three- phase fault test. simultaneous quenches among the SFCLs of the phases successfully accomplished. In conclusion. the SFCL showed excellent performance of current limitation upon fault and stable operation regardless of the amplitude of fault currents.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.25 no.8
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• pp.646-650
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• 2012

Multifunctional structures with two kinds of materials have been intensively investigated in order to improve their electrical characteristic with two functions simultaneously. However, the research regarding of multifunctional ceramic sensor is still in a preliminary stage and how to integrate them with low-cost and high-yield mass production process remains a challenge issue. In this study, we fabricated the multifunctional ceramic sensor composed of temperature and gas sensors. Moreover, we investigated the CO sensing properties of three dimensional nanostuctured $Nb_2O_5$ thin film gas sensors fabricated with silica ($SiO_2$ nanosphere (${\O}$= 750 nm). Compared to plain films, the nanostructured films show enhanced gas sensing of greater sensitivity and a faster response. This result reveals that significantly increased sensitivity is an increase in the effective surface area for the adsorption of gas molecules.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.4
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• pp.409-416
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• 2012

Recently, surface modification technology is of importance to improve the wear resistance and the corrosive resistance for high accurate mechanical parts such as LM guide, Ball Screw and Roller Bearing etc., Those has generally featured on rolling contact mechanism to improve not only the wear and the friction, but also the accuracy and the corrosion performances. For surface modifications of high accurate mechanical parts, normally thermal spray, PVD, CVD and E.P. processes have been used with many materials such as DLC, raydent, W, Ni, Ti etc. Diamondlike carbon (DLC) films possess a combination of attractive properties and have been largely employed to modify the tribological behaviors such as friction, wear, corrosion, fretting fatigue, biocompatibility, etc. However, for rolling contact mechanism mechanical parts DLC films are needed to study for commercial benefit. Rolling contact mechanism has features on effects of cyclic motions and stresses, and also not simply sliding motions. The papers focused on the performance test of wear and corrosive resistance according to Me-DLC film thickness. And also, its thickness effect of wear analysis was carried out through the simulation of the maximum shear stress under the rolling contact surface. As the results, Me-DLC films have more potential to improve the wear resistance for high precision mechanical parts than raydent films.

• 한국신재생에너지학회:학술대회논문집
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• 2007.06a
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• pp.141-145
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• 2007

Due to their excellent catalytic activity with respect to methanol oxidation on platinum at low temperature, platinum nanosized catalysts have been a topic of great interest for use in direct methanol fuel cells (DMFCs). Since pure platinum is readily poisoned by CO, a by-product of methanol electrooxidation, and is extremely expensive, a number of efforts to design and characterize Pt-based alloy nanosized catalysts or Pt nanophase-support composites have been attempted in order to reduce or relieve the CO poisoning effect. In this review paper, we summarize these efforts based upon our recent research results. The Pt-based nanocatalysts were designed by chemical synthesis and thin-film technology, and were characterized by a variety of analyses. According to bifunctional mechanism, it was concluded that good alloy formation with $2^{nd}$ metal (e.g., Ru) as well as the metallic state and optimum portion of Ru element in the anode catalyst contribute to an enhanced catalytic activity for methanol electrooxidation. In addition, we found that the modified electronic properties of platinum in Pt alloy electrodes as well as the surface and bulk structure of Pt alloys with a proper composition could be attributed to a higher catalytic activity for methanol electooxdation. Proton conducting contribution of nanosized electrocatalysts should also be considered to be excellent in methanol electrooxidation (Spillover effect). Finally, we confirmed the ensemble effect, which combined all above effects, in Pt-based nanocatalsyts especially, such as PtRuRhNi and $PtRuWO_{3}$, contribute to an enhanced catalytic activity.