• Corrosion Science and Technology
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• 제18권5호
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• pp.196-205
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• 2019

Surface modification techniques are known to improve SCC by adding large compressive residual stresses to metal surfaces. This surface modification technology is attracting attention because it is an economical and practical technology compared to the maintenance method of existing nuclear power plants. Surface modification techniques include laser, water jet and ultrasonic peening, pinning and ultrasonic Nano-crystal surface modification (UNSM). The focus of this study was on the effect of ultrasonic amplitude in UNSM treatment on the corrosion properties of Alloy 600. A microstructure analysis was conducted using an optical microscope (OM), scanning electron microscope (SEM) and electron backscattering diffraction (EBSD). A cyclic polarization test and AC-impedance measurement were both used to analyze the corrosion properties. UNSM treatment influences the corrosion resistance of Alloy 600 depending on its amplitude. Below the critical amplitude value, the pitting corrosion properties are improved by grain refinement and compressive residual stress, but above the critical amplitude value, crevices are formed by the formation of overlapped waves. These crevices act as corrosion initiators, reducing pitting corrosion resistance.

• 한국산학기술학회논문지
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• 제19권4호
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• pp.517-524
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• 2018

본 연구의 목표는 수평 관외 초소수성 튜브에서 비응축가스(NCG)의 영향을 관찰하고, 이를 일반 알루미늄 튜브의 응축 현상과 비교하는 것이다. 초소수성 튜브 제작을 위해 Self-Assembled Monolayer(SAM) 코팅으로 알루미늄 튜브의 외부를 표면개질 했다. 응축 성능을 나타내기 위해 총합 열전달계수를 사용하였고, 이 값으로 응축 성능을 비교 검토하였다. 본 연구의 주요 변수는 비응축가스 질량 분율로, 0.08에서 0.45의 범위에서 실험을 진행하였다. 응축 실험을 통해 비응축가스 질량 분율이 낮아질수록 응축 성능이 SAM 튜브와 일반 튜브에서 모두 향상되는 것을 확인했다. SAM 튜브에서 적응축 열전달 성능은 일반 튜브 대비하여 평균 약 1.9배에서 2.5배 정도 큰 것을 관찰하였다. SAM 튜브에서 비응축가스 질량 분율이 낮아지면서 응축 성능 상승폭이 감소하게 되는데, 이는 Flooded 응축 현상이 발현되었기 때문이다. 응축이 더 활발하게 진행되면 SAM 튜브에서 막응축이 일어나는 것을 관찰하였고, 이 때 성능은 일반 알루미늄 튜브보다도 저하된 성능을 보였다. SAM 알루미늄 튜브에서 Flooded 응축과 막응축이 일어나는 원인으로 표면에서의 Pinning 효과를 이용하여 설명하였다. 결론적으로, SAM 튜브를 실제 응축기에 적용해 표면개질로 인한 응축 성능 개선 효과를 얻기 위해서는 적응축 또는 Flooded 응축이 일어나는 조건으로 응축기 내 환경 조성을 해야 한다.

• 한국자기학회지
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• 제1권2호
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• pp.60-68
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• 1991

급속냉각기술로 제조된 $Nd_{14}Fe_{76}Co_{4}B_{6}$ 및 $Nd_{10.5}Fe_{79}Co_{2}Zr_{15}B_{7}$ 자성분말을 사용하여 성형체와 본드자석을 제조하였다. 자기특성, 최적충진률 등의 분말입도 및 형상비에 따른 거동은 합금조성에 따라 달리 나타난다. Co/Fe 비가 작은 저밀도의 $Nd_{10.5}Fe_{79}Co_{2}Zr_{1.5}B_{7}$ 합금이 같은 성형압력에서 성형체의 성형밀도 및 최적충진률이 높아 자기특성이 우수하였으나 반면에 분말입도에 따라 자기특성이 변화하였다. 성형밀도(p)와 성형압력(P) 사이 에는 ${\phi}(g/cm^{2})=5.2~5.6{\times}P^{0.045~0.065}(ton/cm^{2})$의 관계식이 성립된다. 높은 Nd/Fe 비를 갖는 급속냉각된 $Nd_{14}Fe_{76}Co_{4}B_{6}$ 합금의 초미세립(50~60 nm)으로써 높은 보자력($iH_{c}=14~15kOe$)을 보유하나, Nd/Fe비가 낮은 $Nd_{10.5}Fe_{79}Co_{2}Zr_{1.5}B_{7}$ 합금은 결정립이 조대하고(150~400 nm)Nd-rich한 입경계상의 발달이 부진하여 자벽고착기구가 주 보자력 기구 로 판명된 두 합금에서 고착지점으로서의 역할이 불충분하여 보자력이 낮은 것으로 판명되었다.

• Journal of Welding and Joining
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• 제25권1호
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• pp.57-62
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• 2007

In the coarse grained HAZ of conventional TiN steel, most TiN particles are dissolved and austenite grain growth easily occurrs during high heat input welding. To avoid this difficulty, thermal stability of TiN particles is improved by increasing nitrogen content in EH36-TM steel. Increased thermal stability of TiN particle is helpful for preventing austenite grain growth by the pinning effect. In this study, the mechanical properties and microstructures of high heat input welded Tandem EGW joint in EH36-TM steel with high nitrogen content were investigated. The austenite grain size in simulated HAZ of the steel at $1400^{\circ}C$ was much smaller than that of conventional TiN steel. Even for high heat input welding, the microstructure of coarse grained HAZ consisted of fine ferrite and pearlite and the mechanical properties of the joint were sufficient to meet all the requirements specified in classification rule.

• 한국전기전자재료학회논문지
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• 제11권6호
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• pp.493-501
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• 1998

$YBa_2Cu_3O_X$ samples were fabricated by MPMG(Melt Powdered Melt Growth). Intitial composition of the mixed powders were prepared as follow ; $YBa_2Cu_3O_X$ : $Y_2BaCuO_5$ = 1:0, 1:0.2, 1:0.3, 1:0.4. After the first melt and cooling, 5wt% to 20wt% fo Ag power was added to the powder. Effects of the different initial $Y_2BaCuO_5$ and Ag addition amount on $J_c$ and magnetization of $YBa_2Cu_3O_X$ fabricated by MPMG method were investigated. The critical current density increased with the amount of $Y_2BaCuO_5$ and Ag. It was also observed that the difference between negative and positive magnetization in the magnetization hysteresis measurement at 77K was larger than the case where $Y_2BaCuO_5$and Ag were not added to the $YBa_2Cu_3O_X$ powders. It is concluded that $Y_2BaCuO_5$ sample with 40wt% amount of $Y_2BaCuO_5$ and 20wt% amount of Ag has not only the largest $J_c$ but also the improved pinning effect.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2010년도 하계학술대회 논문집
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• pp.191-191
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• 2010

초전도 전력 기기를 안정적으로 운용하기 위해서는 고자장하에서 높은 임계 전류 밀도($J_c$)를 지닌 초전도체 개발이 필수적이다. 최근 고자기장에서 전기적 특성을 향상시키는 방법으로는 YBCO 박막선재에 인위적 피닝센터로 고자장하에서도 $J_c$가 크게 증가 되었다고 보고되고 있다. 본 연구에서는 STO(100) 기판 위에 SAM 방법을 이용하여 금 나노분말을 분산시킨 후 PLD로 YBCO 박막을 증착하여 미세구조와 전기적인 특성을 분석하였다. 분산된 금 나노분말은 열처리전 나노입자의 높이는 29~32 nm, 지름은 41~49 nm툴 나타내었고 $800^{\circ}C$에서 진공 열처리 후에는 높이는 25~30 nm, 지름은 52~60 nm로 변형되었다. 임계온도는 순수 YBCO에서 85 K을 나타냈지만 금 나노입자를 적용한 YBCO의 경우는 80K으로 낮아진 것을 확인하였다. 임계전류밀도는 4T에서 측정된 경우 65 K에서는 순수한 YBCO는 141 KA/$cm^2$에서 금 나노입자가 형성된 기판에 증착한 YBCO는 42 KA/$cm^2$로 낮아졌다.

• 한국주조공학회지
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• 제37권2호
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• pp.31-37
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• 2017

A new form of austenitic heat-resistant ductile iron was developed and its microstructures and elevated temperature properties were compared to those of Ductile Ni-Resist D5S. According to JMatPro calculations, it was predicted that Mo-rich carbides would be crystallized before the eutectic reaction starts in the developed alloy. At the austenite cell boundaries of the developed alloy, both Mo-rich carbides and Cr-rich carbides were found. In addition, Ni-silicides were found adjacent to Cr-rich carbides in D5S specimen and were identified as $Ni_2Si$. The developed alloy also had greater yield strength and lower tensile strength levels with less elongation due to the dissolution of Mo atoms into the austenite matrix and the precipitation of Mo-rich carbides. From the results of elevated temperature tensile tests and stress-rupture tests, it was found that the developed alloy had elevated temperature properties superior to those of D5S. This was due to the pinning effect of the dissolved Mo atoms in the austenite matrix.

• 한국재료학회지
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• 제26권11호
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• pp.590-597
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• 2016

In this study, six kinds of low-carbon steel specimens with different ferrite-pearlite microstructures were fabricated by varying the Nb content and the transformation temperature. The microstructural factors of ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured based on optical and scanning electron micrographs; then, Charpy impact tests were conducted in order to investigate the correlation of the microstructural factors with the impact toughness and the ductile-brittle transition temperature (DBTT). The microstructural analysis results showed that the Nb4 specimens had ferrite grain size smaller than that of the Nb0 specimens due to the pinning effect resulting from the formation of carbonitrides. The pearlite interlamellar spacing and the cementite thickness also decreased as the transformation temperature decreased. The Charpy impact test results indicated that the impact-absorbed energy increased and the ductile-brittle transition temperature decreased with addition of Nb content and decreasing transformation temperature, although all specimens showed ductile-brittle transition behaviour.

• 한국주조공학회지
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• 제29권5호
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• pp.213-219
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• 2009

Optimum conditions for production of semi-solid Al-Zn-Mg alloy billets was carried out by the Taguchi design method. And, Al-Zn-Mg alloy billets contained Sc (free, 0.1 and 0.3 mass %) were fabricated at optimum conditions. Evolution of microstructure in semi-solid state was investigated through various liquid fractions, holding times and holding temperatures. The Al-Zn-Mg alloy billets reheated at $615^{\circ}C$ during 30min are grain growth and it was fractured due to increasing liquid fraction before quenching. And, during reheating up to $600^{\circ}C$, grain growth of Al-Zn-Mg alloy billets contained Sc (0.1 and 0.3 mass %) was not occurred in comparison with those of Al-Zn-Mg alloy without Sc. It was thought that $Al_3Sc$ phases have a pinning effect in grain boundary and Sc content of 0.1 mass% is able to inhibit grain growth effectively through reheating process.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1997년도 하계학술대회 논문집 C
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• pp.1361-1363
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• 1997

Zinc oxide-based MOV was fabricated with $SiO_2$ additive ranging from 0.5 to 4.0 mol%, and the microstructure and electrical characteristics were investigated. $Zn_2SiO_4$ phase formed by $SiO_2$ additive was distributed at ZnO grains, grain boundaries, and multiple grain junctions. As the content of $SiO_2$ additive increases, average grain size decreased from 40.6 to $26.9{\mu}m$ due to the Pinning effect by $Zn_2SiO_4$ at grain boundaries Breakdown voltage and nonlinear exponent increased, and leakage current decreased in the range of $11.2{\sim}6.14{\mu}A$ with an increasing $SiO_2$. Donor concentration and interface state density decreased, and barrier height increased in the range of $0.71{\sim}1.04eV$ with an increasing $SiO_2$. While, as the content of $SiO_2$ additive, apparent dielectric constant decreased, peak frequency of dissipation factor decreased in the range of $6.45{\times}10^5{\sim}3.00{\times}10^5Hz$, and dissipation peak was $0.31{\sim}0.22$ at Peak frequency.