• 한국자기학회지
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• 제17권2호
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• pp.76-80
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• 2007

화학기상응축공정법으로 세 가지 분해온도에서 제조된 나노 Fe-N 시료들을 뫼스바우어 분광기, XRD와 BET를 이용하여 자기적 특성의 변화를 연구하였다. 분해온도가 낮을수록 ${\gamma}'-Fe_4N$의 형성이 용이하였으며, 중간온도에서의 ${\epsilon}-Fe_{2.12}N$을 거쳐 높은 분해온도에서는 ${\gamma}-Fe$가 주로 형성되었다. 높은 분해온도에서는 Fe와 N이 서로 잘 결합되지 못하였는데, 이는 Fe와 N을 결합시키기 위해서는 분해온도를 낮게 하는 것이 바람직하다는 것을 의미한다.

• 한국입자에어로졸학회지
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• 제5권4호
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• pp.171-178
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• 2009

$PM_{2.5}$ is one of critical air pollutants due to its high absorbability of heavy metallic fumes, PAH and bacillary micro organisms. Such a fine particulate matter is often formed through various nucleation processes including condensation. This study attempts to find the nucleation behaviors of $PM_{2.5}$ arisen from coal power stations using a classical heterogeneous Fletcher's theory. The numerical simulation by C-language could approximate the nucleation process of $PM_{2.5}$ from water vapor, of which approach revealed the required energy for embryo formation and embryo size and nucleation rate. As a result of the calculation, it was found that wetting agents could affect the particle nucleation in vapor condensation. In particular, critical contact angle relates closely with the vapor saturation. Particle condensation could be reduced by lowering the angles. The wetting agents aid to decrease the contact angle and surface tensions, thereby may contribute to save the formation energy.

• 한국분말재료학회지
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• 제12권3호
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• pp.225-230
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• 2005

In order to prevent the oxide formation on the surface of nano-size iron particles and thereby to improve the oxidation resistance, iron nanoparticles synthesized by a chemical vapor condensation method were directly soaked in hexadecanethiol solution to coat them with a polymer layer. Oxygen content in the polymer-coated iron nanoparticles was significantly lower than that in air-passivated particles possessing iron-core/oxide-shell structure. Accordingly, oxidation resistance of the polymer-coated particles at an elevated temperature below $130^{\circ}C$ in air was $10\~40$ times higher than that of the air- passivated particles.

• 한국분말재료학회지
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• 제17권3호
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• pp.223-229
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• 2010

Iron(Fe)-Molybdenum(Mo) alloyed nanoparticles and nanowires were produced by the chemical vapor condensation(CVC) process using the pyrolysis of iron pentacarbonyl($Fe(CO)_5$) and Molybdenum hexacarbonyl($Mo(CO)_6$). The influence of CVC parameter on the formation of nanoparticle, nanowire and size control was studied. The size of Fe-Mo alloyed nanoparticles can be controlled by quantity of gas flow. Also, Fe-Mo alloyed nanowires were produced by control of the work chamber pressure. Moreover, we investigated close correlation of size and morphology of Fe-Mo nanoparticles and nanowires with atomic quantity of inflow precursor into the electric furnace as the quantitative analysis. Obtained nanoparticles and nanowires were investigated by field emission scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction.

• 한국분말재료학회지
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• 제11권2호
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• pp.124-129
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• 2004

Nanosized WC and WC-Co powders were synthesised by chemical vapor condensation(CVC) process using the pyrolysis of tungsten hexacarbonyl(W(CO)$_6$) and cobalt octacarbonyl(Co$_2$(CO)$_8$). The microstructural changes and phase evolution of the CVC powders during post heat-treatment were studied using the XRD, FE-SEM, TEM, and ICP-MS. CVC powders were consisted of the loosely agglomerated sub-stoichimetric WC$_{1-x}$ and the long-chain Co nanopowders. The sub-stochiometric CVC WC and WC-Co powders were carburized using the mixture gas of CH$_4$-H$_2$ in the temperature range of 730-85$0^{\circ}C$. Carbon content of CVC powder controlled by the gas phase carburization at 85$0^{\circ}C$ was well matched with the theoretical carbon sioichiometry of WC, 6.13 wt％. During the gas phase carburization, the particle size of WC increased from 20 nm to 40 nm and the long chain structure of Co powders disappeared.

• 설비공학논문집
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• 제10권5호
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• pp.569-580
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• 1998

In this study, condensation heat transfer coefficients(HTCs) of a plain tube, low fin tube, and Turbo-C enhanced tube for CFC-11, HCFC-123, CFC-12, HFC-l34a are measured and compared against each other. All data are taken at the vapor temperature of 39$^{\circ}C$ with a wall subcooling temperature 3~8$^{\circ}C$. Test results show that HTCs of a low vapor pressure refrigerant, HFC-123, for a plain, low fin, and Turbo-C tubes are 10.5~20.5%, 8.2~12.2%, 16.5~19.2% lower than those of CFC-11, respectively. On the other hand, HTCs of a medium vapor refrigerant, HFC-l34a, for a plain, low fin, and Turbo-C tubes are 20.6~31.8%, 0.0~8.0%, 13.2~20.9% higher than those of CFC-12, respectively. For all refrigerants tested, HTCs of Turbo-C tube are the highest among the three tubes showing almost 8 times increase in HTCs as compared to those of a plain tube. Nusselt's prediction equation for a plain tube yielded 12% deviation for all plain tube data while Realty and Katz's prediction equation for a low fin tube yielded 20% deviation for all low tube data.

• 설비공학논문집
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• 제14권2호
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• pp.161-167
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• 2002

In this study, external condensation heat transfer coefficients (HTCs) of two non-azeotropic refrigerant mixtures of HFC32/HFC134a and HF0134a/HCF0123 at various compositions were measured on both low fin and Turbo-C enhanced tubes of 19.0 mm outside diameter All data were taken at the vapor temperature of 39$^{\circ}C$ with a wall subcooling of 3- 8 K. Test results showed that HTCs of the tested mixtures on the enhanced tubes were much lower than the ideal values calculated by the mass fraction weighting of the pure compo- nents'HTCs. Also the reduction of HTCs due to the diffusion vapor film was much larger than that of a plain tube. Unlike HTCs of pure fluids, HTCs of the mixtures measured on enhanced tubes increased as the wall subcooling increased, which was due to the sudden break up of the vapor diffusion film with an increase in wall subcooling. Finally, heat transfer enhancement ratios for mixtures were found to be much lower than those of pure fluids.

• Nuclear Engineering and Technology
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• 제54권12호
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• pp.4776-4797
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• 2022

In the one-dimensional thermal-hydraulic (TH) codes, a subcooled boiling model to predict the void fraction profiles in a vertical channel consists of wall heat flux partitioning, the vapor condensation rate, the bubbly-to-slug flow transition criterion, and drift-flux models. Model performance has been investigated in detail, and necessary refinements have been incorporated into the Safety and Performance Analysis Code (SPACE) developed by the Korean nuclear industry for the safety analysis of pressurized water reactors (PWRs). The necessary refinements to models related to pumping factor, net vapor generation (NVG), vapor condensation, and drift-flux velocity were investigated in this study. In particular, a new NVG empirical correlation was also developed using artificial neural network (ANN) techniques. Simulations of a series of subcooled flow boiling experiments at pressures ranging from 1 to 149.9 bar were performed with the refined SPACE code, and reasonable agreement with the experimental data for the void fraction in the vertical channel was obtained. From the root-mean-square (RMS) error analysis for the predicted void fraction in the subcooled boiling region, the results with the refined SPACE code produce the best predictions for the entire pressure range compared to those using the original SPACE and RELAP5 codes.

• 한국분말재료학회지
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• 제9권5호
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• pp.322-328
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• 2002

Fe-Co nanocomposite powders with different composition were prepared by chemical vapor condensation (CVC) process and their characterizations were studied by means of X-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. The particles having the mean size of 5~25 nm consisted of metallic cores and oxide shells. The Co contents and particle size increased with increasing the carrier gas flow rate of Co precursor. The saturation magnetization and coercivity increased with increasing Co content. and the saturation magnetization maximized at the 40 wt.%Co. The Fe-Co nanocomposite powder oxidized at $400^{\circ}C$ showed the maximum coercivity of 1739 Oe.

• 설비공학논문집
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• 제14권6호
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• pp.468-476
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• 2002

This paper deals with heat and mass transfer characteristics and performance evaluation of a counter flow double-tube condenser for a multi-component refrigerant mixture. The local heat and mass transfer characteristics of ternary zeotropic refrigerant mixtures composed of HFC32/HFC125/HFC134a are evaluated for a counter flow double-tube condenser cooled by water. Then, the local values of vapor quality, thermodynamic states at bulk vapor, vapor-liquid interface and bulk liquid, heat flux and condensation mass flux are obtained. The heat exchange performance for ternary zeotropic refrigerant mixtures composed of HFC32/HFC125/HFC134a on the total pressure drop and the heat transfer characteristics are also compared with those for R404A, R410A, R502, R22, R32, Rl23 and R134a.