• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1415-1420
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• 2014

Super-critical $CO_2$ ($S-CO_2$) Brayton cycle has been considered to replace the current steam Rankine cycle in Sodium-cooled Fast Reactor (SFR) in order to improve the inherent safety and thermal efficiency. Several austenitic alloys are considered as the structural materials for high temperature $S-CO_2$ environment.. Microstructural change after long-term exposure to high temperature $S-CO_2$ environment could affect to the mechanical properties. In this study, candidate materials (austenitic stainless steels and Alloy 800HT) were exposed to $S-CO_2$ to assess oxidation resistance and the change in tensile properties. Loss of ductility was observed for some austenitic stainless steels even after 250 h exposure. The contribution of $S-CO_2$ environment on such changes was analyzed based on the characterization of the surface oxide and carburization of the materials in which 316H and 800H showed different oxidation behaviors.

• Korean Journal of Materials Research
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• v.9 no.5
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• pp.484-490
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• 1999

$(Pb,La)TiO_3$(PLT) thin films were prepared on Pt/SiO$_2$/Si substrates by the sol-gel method and investigated the crystalline and electrical properties according to La concentration and post-annealing temperatures. The PLT films annealed at above $600^{\circ}C$ were exhibited the typical perovskite structures regardless of La contents. When the $(Pb,La)TiO_3$(PT) films were doped with La concentration up to 10mol%(PLT-10), the degree of z-axis orientation was greatly decreased from 63% to 26%. From AES depth profiles for the PLT-10 samples, no remarkable inter-reaction between PLT film and lower Pt electrode was found. The remanent polarization$(2Pr,Pr_+-Pr_-)$ were increased from $4\muC\textrm{cm}^2 to 16\muC\textrm{cm}^2$ as the annealing temperature increased from $600^{\circ}C to 700^{\circ}C$. This result may be ascribed to the improvement of crystallinity by the high temperature post-annealing. The dielectric constant$({\varepsilon}r)$ and tangent loss(tan$\delta$) of the PLT-10 films annealed at $650^{\circ}C$ were about 193 and 0.02, respectively with the pyroelectric coefficient($\gamma$) of around $4.0nC/\textrm{cm}^2{\cdot}^{\circ}C at 30^{\circ}C$.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2017.05a
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• pp.83-83
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• 2017

생체재료의 표면은 이식과 동시에 생체계면의 역할을 하게 되어, 일련의 생물학적 반응이 시작되고 진행되는 중요한 장소가 된다. 초기에 생체계면에서 일어나는 단백질 흡착이나 염증반응을 비롯한 생물학적 반응들은 궁극적으로 임플란트의 성패를 좌우할 만큼 중요하다. 골융합을 개선하기 위한 다른 방법으로 생체불활성의 타이타늄 (Ti)과 골조직의 능동적인 반응을 이루기 위해 생체활성 표면을 부여함으로서 계면에서의 골형성 반응을 증진시키는 방법이 이용된다. 생체불활성의 Ti과 Ti합금은 골조직과 직접적인 결합을 이루지 못하므로, 골조직과의 반응을 향상하기 위해 여러 종류의 생체활성 재료를 코팅하는 방법이 연구되어 왔고, 이 중 생체의 변화와 가장 유사한 하이드록시아파타이트 코팅이 가장 대중적인 방법으로 사용되었으며 이는 초기 골형성을 촉진하는 것으로 알려졌다. 치과용 임플란트의 표면형상과 화학조성이 골 융합에 영향을 미치는 가장 중요한 인자이므로 최근의 연구동향은 이들 두 가지 표면특성을 결합함으로서 결과적으로 최적의 골세포반응을 유도하고, 골융합 후 골조직과의 micromechanical interlocking에 의해 임플란트의 안정성에 중요한 역할을 하는 마이크론 단위의 표면조도와 표면 구조를 유지하면서, 부가적으로 골 조직 반응을 능동적으로 개선할 수 있는 생체활성 성분을 부여하여 골 융합에 상승효과를 이루기 위한 표면처리법에 관해 많은 연구가 요구되어지고 있다. 따라서 골을 구하는 원소인 망간과 실리콘으로 치환된 하이드록시아파타이트를 플라즈마 전해 산화법으로 코팅하여 세포와 잘 결합할 수 있는 표면을 제공함으로써 골 융합과 치유기간을 단축시킬 수 있을 것으로 사료된다. 실험방법은 시편은 치과 임플란트 제작 합금인 Ti-6Al-4V ELI disk (grade 5, Timet Co., USA; diameter, 10 mm, thickness, 3 mm)이며, calcium acetate monohydrate, calcium glycerophosphate, manganese(II) acetate tetrahydrate, sodium metasilicate을 설계조건에 따라 혼합 제조된 전해질 용액을 이용하여 플라즈마 전해 산화법으로 표면 코팅을 실시하였다. 각 시편의 플라즈마 전해시 전압은 280V로 인가하였고, 전류밀도는 70mA로 정전류를 공급하여 해당 인가전압 도달 후 3분 동안 정전압 방식을 유지하였다. 코팅된 피막 표면을 주사전자현미경과 X-선 회절분석을 통하여 미세구조 및 결정상을 관찰하였다. 또한 코팅된 표면의 생체활성 평가는 정량적으로 평가하기 위해 동전위시험과 AC 임피던스를 통하여 시행하였다. 분극거동을 확인하기 위해 potentiostat (Model PARSTAT 2273, EG&G, USA)을 이용하여 구강 내 환경과 유사한 $36.5{\pm}1^{\circ}C$의 0.9 wt.% NaCl에서 실시하였다. 전기화학적 부식 거동은 potentiodynamic 방법으로 조사하였고 인가전위는 -1500 mV에서 2000 mV까지 분당 1.67 mV/min 의 주사속도로 인가하여 시험을 수행하였다. 임피던스 측정은 potentiostat (Model PARSTAT 2273, EG&G, USA)을 이용하였으며, 측정에 사용한 주파수 영역은 10mHz ~ 100kHz 까지의 범위로 하여 조사하였고 ZSimWin(Princeton applied Research, USA) 소프트웨어를 사용하여 용액의 저항, 분극 저항 값을 산출하였다. 망간의 함량이 증가할수록 불규칙한 기공을 보였으며, 실리콘은 $TiO_2$ 산화막 형성을 저해하는 경향을 확인할 수 있었다. 단독으로 표면을 처리한 경우보다 두 가지 원소를 이용해 복합 표면처리를 시행한 경우가 내식성이 좋아 임플란트과의 골 유착에 긍정적인 영향을 미칠 것으로 사료된다.

• Korean Journal of Materials Research
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• v.2 no.2
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• pp.110-118
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• 1992

When CVD-W films deposited on the reactively sputter-deposited TiN(${\circled1}$), the $NH_3$-RTP (rapid themal processed) TiN(${\circled2}$), and the furnace-annealed TiN submitate (${\circled3}$) by $SiH_4$, reduction, deposition rate is in the order of ${\circled1}>{\circled2}>{\circled3}$ and incubation period of W nucleation is in the order of ${\circled1}{\leq}{\circled2}<{\circled3}$. The longest incubation period of nucleation and lowest deposition rate for the CVD-W on the annealed TiN is due to the incorporation of oxygen from the nitrogen ambient containing some oxygen as contaminant into the TiN film. The higher W deposition rate and the lower incubation period of W nucleation on the RTP-TiN substrate in comparison with those on the sputtered TiN substrate seem to be due to a negative effect of the high compressive stress of the RTP-TiN on the nucleation and growth of W. Also the thickness uniformity of the W film deposited on the TiN substrate by $SiH_4$ reduction turns out to be better than that by $H_2$ reduction.

• Journal of Sensor Science and Technology
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• v.9 no.1
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• pp.70-75
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• 2000

The recycled single crystal silicon wafers have been fabricated into solar cells. It can be a solution for the high cost in materials for solar cells and recycling of materials. So, p-type (100) single crystal silicon wafers with high resistivity of $10-14\;{\Omega}cm$ and the thickness of $650\;{\mu}m$ were used for the fabrication of solar cells. Optimistic conditions of formation of back surface field, surface texturing and anti-reflection coating were studied for getting high efficiency. In addition, thickness variation of solar cell was also studied for increase of efficiency. As a result, the solar cell with efficiency of 10% with a curve fill factor of 0.53 was fabricated with the wafers which have the area of $4\;cm^2$ and thickness of $300\;{\mu}m$. According to above results, recycling possibility of wasted wafers to single crystal silicon solar cells was confirmed.

• 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 Journal of Materials Research
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• v.9 no.10
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• pp.1006-1011
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• 1999

B-doped hydrogenated amorphous silicon carbide (a-SiC:H) thin films were prepared by plasma-enhanced chemical-vapor deposition in a gas mixture of $SiH_4$, $CH_4$ and $B_2H_6$. Microstructures and chemical properties of a-SiC:H films grown with varing the volume ratio of $CH_4$ to $SiH_4$ were characterized with various analysis methods including scanning electron microscopy(SEM), X-ray diffractometry(XRD), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy. X-ray photoelectron spectroscopy(XPS), UV absorption spectroscopy and photoconductivity measurements. While Si:H films grown without $CH_4$ showed amorphous state, the addition of $CH_4$ during deposition enhanced the development of a microcrystalline phase. By introducing C atoms into the film, Si-Si and Si--$\textrm{H}_{n}$ bonds of a -Si:H films were gradually replaced by Si-C, C-C, and Si--$\textrm{C}_{n}\textrm{H}_{m}$ bonds. Consequently, the electrical resistivity and optical bandgap of a-SiC:H films were increased with the C concentration in the film.

• Korean Journal of Materials Research
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• v.11 no.5
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• pp.405-412
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• 2001

Corrosion tests were carried out in $360^{\circ}C$ water and $360^{\circ}C$ 70ppm LiOH solution to investigate the corrosion behavior of new zirconium alloys (Zr-0.4Nb-0.8Sn-xFeCrMn, Zr-0.2Nb-1.1Sn-xFeCrMn, Zr-1.0Nb-xFeCu). Microstructures of tested alloys were analyzed by optical microscope and TEM. The cross-sectional surface and crystalline structure of the oxide layer were analyzed by SEM and XRD. From the results of corrosion test, all the alloys showed higher corrosion rates in $360^{\circ}C$ 70ppm LiOH aqueous solution thats in $360^{\circ}C$ water. Especially, high Nb-containing alloy exhibited the acceleration of corrosion rate in LiOH solution. The low Nb- and Sn-added alloys showed better corrosion resistance than the Sn- free high Nb alloy. from the effect of final annealing on the corrosion, it was observed that the partially recrystallized alloys showed better corrosion resistance than fully recrystallized alloys. This would be related to the size and distribution of the second phase particles.

• Korean Journal of Materials Research
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• v.8 no.12
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• pp.1099-1109
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• 1998

To investigate the effect of V and Sb on the corrosion behavior of Zr- based alloys, corrosion tests were performed on 6 kinds of Zr alloys in an autoclave at $360^{\circ}C$ for 100 days. The transition of the corrosion rate occurred in the sample containing 0.1wt.%V after 10 days but did not occur in the samples containing 0.2wt.%V and 0.4wt.%V. The corrosion resistance of V containing alloys increased with increasing V contents from 0.1 to 0.4wt.% and the alloys containing 0.4wt.%V showed the best corrosion resistance. In the ternary alloys containing 0.1wt.%Sb and 0.4wt.%Sb, the corrosion rate increased significantly from the short exposure time. It was observed that the optimal Sb content for corrosion resistance was 0.2wt.%. The size and volume fraction of precipitates increased with increasing V and Sb contents. The superior corrosion resistance was observed in the Zr alloy having precipitate size of 0.11-0.13$\mu\textrm{m}$. From the result of corrosion behavior and the obserbation of precipitates, the optimal size of the precipitate appear to control the electron conduction in the cathodic reaction and play an important role in maintaining a stable oxide microstructure.

• Korean Journal of Materials Research
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• v.11 no.3
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• pp.236-242
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• 2001

HfN/Si$_3$N$_4$and NbN/Si$_3$N$_4$ multilayer coatings were deposited onto a high speed tool steel substrate by reactive sputtering and their mechanical properties were evaluated in terms of the dependence of hardness and adhesion strength on the sputter deposition process parameters. The hardnesses of both HfN/Si$_3$N$_4$and NbN/Si$_3$N$_4$ multiplayer coatings increase up to the flux ratio of 0.4 but nearly do not change after that as the $N_2$/Ar flux ratio in nitride sputter deposition increases. The hardnesses of both multiplayer coatings nearly do not change with annealing at low temperatures but decrease owing to oxidation with annealing at a high temperature like 80$0^{\circ}C$ after depositing the layers by sputtering. Post-annealing at low temperatures increases the adhesion strength of the multilayers. but high temperature annealing is not desirable since it decreases the adhesion strength besides the hardness deterioration.