• 한국주조공학회지
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• 제13권5호
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• pp.462-475
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• 1993

A numerical analysis on the microstructural evolutions of microcellular and cellular ${\alpha}-aluminum$ phase in the gas-atomized Al-8wt. pct droplets was represented. The 2-dimensional non-Newtonian heat transfer and the dendritic growth theory in the undercooled melt were combined under the assumptions of a point nucleation on droplet surface and the macroscopically smooth solid-liquid interface enveloping the cell tips. It reproduced the main characteristic features of the reported microstructures quite well. It predicted a considerable volume fraction of segregation-free region in a droplet smaller than $l0{\mu}m$ if an initial undercooling larger than 100K is given. The volume fractions of the microcellular region($g_A$) and the sum of the microcellular and cellular region($g_a$) were predicted as functions of the heat transfer coefficient, h and initial undercooling, ${\triangle}T$. It was shown that $g_A$ and $g_a$, in the typical gas-atomization processes with $h=0.1-1.0W/cm^2K$, are dominated by ${\triangle}T$ and h, respectively, but for h larger than $4.0W/cm^2K$, a fully microcellular structure can be obtained irrespective of the initial undercooling.

• 한국세라믹학회지
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• 제41권4호
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• pp.297-301
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• 2004

분무연소합성법을 이용하여 나노크기의 물라이트(3Al$_2$O$_3$$.$2SiO$_2$) 콜로이드를 제조하였다. 연소반응을 위한 산화제로서 Al(NO$_3$)$_3$$.$9$H_2O$와 환원제(연료)로서 CH$_{6}$N$_4$O를 사용하였으며, 실리카 소스로서 콜로이드 실리카를 첨가하였다. 분무된 액적들의 착화를 위해 연소반응기의 온도를 80$0^{\circ}C$로 유지하였다. 액적의 응고에 의한 액적크기 성장을 억제하기 위하여 금속 스크린 필터를 사용함으로써 액적의 개수 농도를 감소시켰으며, 에어로졸 입자의 체류시간을 2.5초로 유지하여 열 유체의 흐름을 층류로 유도하였다. 제조된 입자들의 모양은 모두 구형이었으며, 평균입자크기는 130nm이었다 XRD와 TEM 분석 결과 각각의 초미립자들은 정량화합물의 물라이트 결정성을 나타내었다.

• Corrosion Science and Technology
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• 제12권4호
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• pp.179-184
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• 2013

Pipe wall thinning caused by erosion and corrosion can adversely affect the operation of aged nuclear power plants. Some injured workers owing to pipe rupture has been reported and power reduction caused by unexpected pipe damage has been occurred consistently. Therefore, it is important to develop erosion-corrosion damage prediction model and investigate its mechanisms. Especially, liquid droplet impingement erosion(LDIE) is regarded as the main issue of pipe wall thinning management. To investigate LDIE mechanism with corrosion environment, we developed erosion-corrosion damage simulation apparatus and its capability has been verified through the preliminary damage experiment of 6061-Al alloy. The apparatus design has been based on ASTM standard test method, G73-10, that use high-speed rotator and enable to simulate water hammering and droplet impingement. The preliminary test results showed mass loss of 3.2% in conditions of peripheral speed of 110m/s, droplet size of 1mm-diameter, and accumulated time of 3 hours. In this study, the apparatus design revealed feasibility of LDIE damage simulation and provided possibility of accelerated erosion-corrosion damage test by controlling water chemistry.

• Asian Journal of Atmospheric Environment
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• 제8권4호
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• pp.175-183
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• 2014

To visually and chemically verify the rainout of soot particles, a model experiment was carried out with the cylindrical chamber (0.2 m (D) and 4 m (H)) installing a cloud drop generator, a hydrotherometer, a particle counter, a drop collector, a diffusing drier, and an artificial soot particle distributer. The processes of the model experiment were as follows; generating artificial cloud droplets (major drop size : $12-14{\mu}m$) until supersaturation reach at 0.52%-nebulizing of soot particles (JIS Z 8901) with an average size of $0.5{\mu}m$-counting cloud condensation nuclei (CCN) particles and droplets by OPC and the fixation method (Ma et al., 2011; Carter and Hasegawa, 1975), respectively - collecting of individual cloud drops - observation of individual cloud drops by SEM - chemical identifying of residual particle in each individual droplet by SEM-EDX. After 10 minutes of the completion of soot particle inject, the number concentrations of PM of all sizes (> $0.3{\mu}m$) dramatically decreased. The time required to return to the initial conditions, i.e., the time needed to CCN activation for the fed soot particles was about 40 minutes for the PM sized from $0.3-2.0{\mu}m$. The EDX spectra of residual particles left at the center of individual droplet after evaporation suggest that the soot particles seeded into our experimental chamber obviously acted as CCN. The coexistence of soot and mineral particle in single droplet was probably due to the coalescence of droplets (i.e., two droplets embodying different particles (in here, soot and background mineral particles) were coalesced) or the particle capture by a droplet in our CCN chamber.

• 한국재료학회지
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• 제21권2호
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• pp.106-110
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• 2011

A high thermal conductive AlN composite coating is attractive in thermal management applications. In this study, AlN-YAG composite coatings were manufactured by atmospheric plasma spraying from two different powders: spray-dried and plasma-treated. The mixture of both AlN and YAG was first mechanically alloyed and then spray-dried to obtain an agglomerated powder. The spray-dried powder was primarily spherical in shape and composed of an agglomerate of primary particles. The decomposition of AlN was pronounced at elevated temperatures due to the porous nature of the spray-dried powder, and was completely eliminated in nitrogen environment. A highly spherical, dense AlN-YAG composite powder was synthesized by plasma alloying and spheroidization (PAS) in an inert gas environment. The AlN-YAG coatings consisted of irregular-shaped, crystalline AlN particles embedded in amorphous YAG phase, indicating solid deposition of AlN and liquid deposition of YAG. The PAS-processed powder produced a lower-porosity and higher-hardness AlN-YAG coating due to a greater degree of melting in the plasma jet, compared to that of the spray-dried powder. The amorphization of the YAG matrix was evidence of melting degree of feedstock powder in flight because a fully molten YAG droplet formed an amorphous phase during splat quenching.

• 한국주조공학회지
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• 제5권2호
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• pp.118-124
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• 1985

The thickness ranges and conditions to form the supersaturated solid solution in Al-Cr alloys were investigated with various rapid solidification conditions. Al-Cr alloys, rapidly solidified by using the small droplet chill quenching method, were examined by means of micro-vickers hardness, lattice parameter, thermal analysis and microscopic observation. The results obtained were as follows; 1. With the increase of solidification rate, the solidified structures were changed to intermetallic compound + solid solution, incompletely supersaturated solid solution, completely supersaturated solid solution, in turn. 2. The minimum solidification rate required to form completely supersaturated solid solution was $2.5{\times}10^{-2}cm/sec$, $3.6{\times}10^{-2}cm/sec$ and $6.0{\times}10^{-2}cm/sec$ for Al-1.0wt%Cr, Al-1,2wt%Cr and Al-1.5wt%Cr, respectively. 3. The maximum distance from the chill surface required to form completely supersaturated solid solution was 5mm, 1.3mm and 0.3mm for Al-1.0wt%Cr, Al-1.2wt%Cr and Al-1.5wt% Cr, respectively.

• Journal of Welding and Joining
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• 제21권2호
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• pp.57-63
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• 2003

Since new types of vehicles or structures made from thin aluminum alloy are under rapid development and some products are already on the market, welding of aluminium sheet is increasing. MIG(Metal Inert Gas), MIG-Pulse, TIG(Tungsten Inert Gas) welding are the typical Ai welding. MIG welding has the advantage of high speed, but it is difficult to apply to the thin plate, because of bum-through by the high heat input and spatter. MIG-Pulse welding can weld without spatter and burn-through, but when the gap exists at the welding joint, there is quite a possibility of bum-through. TIG welding is difficult to weld at a high speed. AC Pulse welding alternates between DCEP(Direct Current Electrode Positive) and DCEN(Direct Current Electrode Negative). DCEN is higher wire melting rate than DCEP, while lower temperature of droplet than DCEP. In AC Pulse welding, far fixed welding current, wire melting rate increases as the EN ratio increases. For fixed wire feed rate, welding current decreases as the EN ratio increases. Because of these features, the temperature of droplet, the depth of penetration, the width of bead decrease and the reinforcement height increases as EN ratio increases, and these are able to weld at a high speed, lower heat input. It is the purpose of this study that design of AC pulse current waveform for MIG welding of Al sheet and estimation of output characteristic.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.329-329
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• 2013

본 연구에서는 PEDOT:PSS와 crystalline-ITO (c-ITO) 박막 계면에서의 화학적 반응을(박리 및 용해 특성)을 관찰하기위해 spin-coating 및 droplet dropping을 통하여 PEDOT:PSS 용액을 코팅하고 이후 화학적 거동에 따른 전기적, 광학적 및 구조적 특성 변화를 관찰하였다. 강산성을 띄는 PEDOT:PSS (Al 4083) 박막의 코팅 전, 0.4T sodalime glass 위에 열처리를 통하여 성막된 c-ITO 투명전극을 15분 동안 상압 오존 공정을 통하여 계면처리함으로써 다른 변수의 영향을 배제하였으며, 표면 처리 후 spin-coating 및 droplet dropping method를 통하여 PEDOT:PSS를 코팅하여 c-ITO와 PEDOT:PSS 계면사이의 화학적 반응의 영향을 시간 경과에 따라 분석하였다. PEDOT:PSS 코팅 후 솔밴트 제거를 위해 hot plate를 이용하여 $110^{\circ}C$로 열처리되었다. Spin-coating 방법과는 달리 droplet dropping 방법을 통해 형성된 c-ITO 투명전극/PEDOT:PSS 계면에서는 spin coating에서 적용된 동일한 공정변수적용에도 불구하고 PEDOT:PSS의 산성으로 인한 ITO 투명전극 표면에서의 화학적 조성변화가(In, Sn, O의 조성의 변화) 발생됨을 x-ray photoelectron spectroscopy 결과를 통해 확인하였다. 뿐만 아니라 계면 조성반응 변화에 따른 전기적 특성 및 광학적 투과율의 열화가 발생됨을 Hall measurement 측정과 UV/Vis spectrometer 결과를 통하여 도출하였다. 본 결과를 통해 c-ITO/PEDOT:PSS 사이에서 발생되는 내산특성/계면 화학변화가 유기태양전지에서의 산화물 투명전극과 유기물 계면 열화현상에 영향을 받을 수 있음을 나타낸다.

• 한국재료학회지
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• 제22권10호
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• pp.539-544
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• 2012

We have grown AlN nanorods and AlN films using plasma-assisted molecular beam epitaxy by changing the Al source flux. Plasma-assisted molecular beam epitaxy of AlN was performed on c-plane $Al_2O_3$ substrates with different levels of aluminum (Al) flux but with the same nitrogen flux. Growth behavior of AlN was strongly affected by Al flux, as determined by in-situ reflection high energy electron diffraction. Prior to the growth, nitridation of the $Al_2O_3$ substrate was performed and a two-dimensionally grown AlN layer was formed by the nitridation process, in which the epitaxial relationship was determined to be [11-20]AlN//[10-10]$Al_2O_3$, and [10-10]AlN//[11-20]$Al_2O_3$. In the growth of AlN films after nitridation, vertically aligned nanorod-structured AlN was grown with a growth rate of $1.6{\mu}m/h$, in which the growth direction was <0001>, for low Al flux. However, with high Al flux, Al droplets with diameters of about $8{\mu}m$ were found, which implies an Al-rich growth environment. With moderate Al flux conditions, epitaxial AlN films were grown. Growth was maintained in two-dimensional or three-dimensional growth mode depending on the Al flux during the growth; however, final growth occurred in three-dimensional growth mode. A lowest root mean square roughness of 0.6 nm (for $2{\mu}m{\times}2{\mu}m$ area) was obtained, which indicates a very flat surface.

• 한국주조공학회지
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• 제17권5호
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• pp.480-487
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• 1997

Particle reinforced metal matrix composites(MMCs) via a centrifugal spray-cast deposition(CSD) process were fabricated by injecting second phase particles($Al_2O_3$<40${\mu}m$, W<17.3${\mu}m$) into copper melt on the atomizing disc. Compositing modes were investigated by combining microstructures and mathematical modeling between Cu droplets and the reinforced particles injected. The $Cu/W_P$ powders were shown that the W particles penetrate and get embedded in the Cu droplets. It is considered that the W particles composite preferentially in Cu melt on the atomizing disc. On the other hand, the $Al_2O_3$, particles did not penetrate into the Cu droplets on the atomizing disc but get attached in surface of Cu droplets during the flight. It is considered that the compositing may be attained in the flight distance which the relative velocity between Cu droplet and $Al_2O_3$, particle is maximum. The microstructure of the $Cu/W_P$ and the $Cu/(Al_2O_3)_p$ composite preform was strongly influenced by compositing modes of droplets, and after subsequent deposition it was comprised as it is called the dispersed type and the cell type of microstructure, respectively.