• Nuclear Engineering and Technology
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• 제38권8호
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• pp.753-762
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• 2006

In this paper, works related to enhancement of the CHF are reviewed in terms of fundamental mechanisms and practical applications. Studies on CHF enhancement in forced convection are divided into two categories, CHF enhancement of internal flow in tubes and enhancement of CHF in the nuclear fuel bundle. Methods of enhancing the CHF of internal flows in tubes include enhancement of the swirl flow using twisted tapes, a helical coil, and a grooved surface; promotion of flow mixing using a hypervapotron; altering the characteristics of the heated surface using porous coatings and nano-fluids; and changing the surface tension of the fluid using additives such as surfactants. In the fuel bundle, mixing vanes or wire wrapped rods can be employed to enhance the CHF by changing the flow distributions. These methods can be applied to practical heat exchange systems such as nuclear reactors, fossil boilers, fusion reactors, etc.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2007년도 춘계학술발표회 초록집
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• pp.155-156
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• 2007

본 연구에서는 태양전지 표면에 입사된 빛의 반사율을 최소화하기 위해서 단결정 실리콘 기판 표면에 다공성 실리콘층을 적용하여 반사방지막(Anti-Reflection Coating, ARC)을 형성하는 실험을 하였다. 다공성 실리콘(Porous silicon, PSi)은 실온에서, 기판 성질에 따라 일정 비율로 만든 전해질 용액($HF-C_2H_5OH-H_2O$)을 사용하여 실리콘 표면에 양극산화처리 함으로써 단순 공정만으로 실리콘 기판의 반사율을 낮출 수 있다. 본 연구는 일정한 면저항을 가지는 단결정 실리콘 기판에 다공성 실리콘층을 여러 조건으로 형성하여 반사방지막으로써의 특성을 비교 분석하였다.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제15권3호
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• pp.223-231
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• 2011

During spray coating, especially in an air plasma spray (APS), pores, cracks, and splat boundaries are developed and those factors exert influence on thermomechanical properties such as elastic modulus, thermal conductivity, and coefficient of thermal expansion. Moreover, the thermo mechanical properties are crucial elements to determine the thermoelastic characteristics, for instance, temperature distribution, displacements, and stresses. Two types of thermal barrier coating (TBC) model, the dense and porous microstructures, are taken into account for the analysis of microstructural characterizations. $TriplexPro^{TM}$-200 system was applied to prepare TBC samples, and the METECO 204 C-NS powder is adopted for the relatively porous microstructure and METECO 204 NS powder for the dense microstructure in the top coat of TBCs. Governing partial differential equations were derived based on the thermoelastic theory and approximate estimates for the thermoelastic characteristics were obtained using a finite volume method for the governing equations.

• 한국정밀공학회지
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• 제31권12호
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• pp.1171-1176
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• 2014

Oil retention experiments were conducted and the performance was characterized for electrochemically etched stainless steels. The 304 stainless steels were electrochemically etched in dilute Aqua Regia to form porous structures. The structures were also hierarchical, which provides very large area for oils to adhere. Also the structures had deep valleys, which act as reservoir and are able to resist against oil-detaching forces. Several commercial oils were dispensed to characterize oil retention properties via rotating disk experiment method. The results showed that the etched surfaces have superior oil retention performance in every conditions. Also the retention enhancement ratio went particularly higher as the environments became more severe. This surface modification technique could be applied to other steel products for pretreatments of various kinds of coatings.

• Nuclear Engineering and Technology
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• 제48권4호
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• pp.932-940
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• 2016

Pool boiling heat transfer of water saturated at atmospheric pressure was investigated experimentally on Cu surfaces with high-temperature, thermally-conductive, microporous coatings (HTCMC). The coatings were created by sintering Cu powders on Cu surfaces in a nitrogen gas environment. A parametric study of the effects of particle size and coating thickness was conducted using three average particle sizes (APSs) of $10{\mu}m$, $25{\mu}m$, and $67{\mu}m$ and various coating thicknesses. It was found that nucleate boiling heat transfer (NBHT) and critical heat flux (CHF) were enhanced significantly for sintered microporous coatings. This is believed to have resulted from the random porous structures that appear to include reentrant type cavities. The maximum NBHT coefficient was measured to be approximately $400kW/m^2k$ with APS $67{\mu}m$ and $296{\mu}m$ coating thicknesses. This value is approximately eight times higher than that of a plain Cu surface. The maximum CHF observed was $2.1MW/m^2$ at APS $67{\mu}m$ and $428{\mu}m$ coating thicknesses, which is approximately double the CHF of a plain Cu surface. The enhancement of NBHT and CHF appeared to increase as the particle size increased in the tested range. However, two larger particle sizes ($25{\mu}m$ and $67{\mu}m$) showed a similar level of enhancement.

• 한국세라믹학회지
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• 제45권10호
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• pp.579-583
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• 2008

A porous $Al_2O_3$, scaffold coated with tricalcium phosphate(TCP) was fabricated by replica method using polyurethane(PU) foam as a fugitive material. Successive coatings of $Al_2O_3$ and hydroxyapatite(HAp) were applied via dip coating onto polyurethane foam, which has a slender and well interconnected network. A porous structure was obtained after sequentially burning out the foam and then sintering at $1500^{\circ}C$. The HAp phase was changed to TCP phase at high temperature. The scaffold showed excellent interconnected porosity with pore sizes ranging from $300{\sim}700{\mu}m$ in diameter. The inherent well interconnected structural feature of PU foam remained intact in the fabricated porous scaffold, where the PU foam material was entirely replaced by $Al_2O_3$ and TCP through a consecutive layering process. Thickness of the $Al_2O_3$ base and the TCP coating was about $7{\sim}10{\mu}m$ each. The TCP coating was homogeneously dispersed on the surface of the $Al_2O_3$ scaffold.

• 한국표면공학회지
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• 제51권6호
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• pp.415-420
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• 2018

The possibility of an improvement in heat dissipation performance of aluminum alloy heat sink for shipboard LED luminaries through plasma electrolytic oxidation (PEO) was investigated. Four different PEO coatings were produced on aluminum alloy 5052 in silicate based alkaline solution by varying current density ($50{\sim}200mA/cm^2$). On voltage-time response curves, three stages were clearly distinguished at all current densities, namely an initial linear increase, slowdown of increase rate, and steady state(constant voltage). It was found that the increase in current density caused the breakdown voltage to increase. Two different surface morphologies - coralline porous structure and pancake structure - were confirmed by SEM examination. The coralline porous structure was predominant in the coatings produced at lower current densities (50 and $100mA/cm^2$) while under high current densities(150 and $200mA/cm^2$) the pancake structure became dominant. The coating thickness was measured and found to be in a range between about $13{\mu}m$ and $44{\mu}m$, showing increasing thickness with increasing current density. As a result, $100mA/cm^2$ was proposed as an effective process parameter to improve the heat dissipation performance of aluminum alloy heat sink, which could lower the LED operating temperature by about 30%.

• 한국물환경학회지
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• 제38권3호
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• pp.143-149
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• 2022

Membrane distillation (MD) has emerged as a sustainable desalination technology to solve the water and energy problems faced by the modern society. In particular, the surface wetting properties of the membrane have been recognized as a key parameter to determine the performance of the MD system. In this study, a novel surface modification technique was developed to induce a Janus-type hydrophilic/hydrophobic layer on the membrane surface. The hydrophilic layer was created on a porous PVDF membrane by vapor phase polymerization of the pyrrole monomer, forming a thin coating of polypyrrole on the membrane walls. A rigid polymeric coating layer was created without compromising the membrane porosity. The hydrophilic coating was then followed by the in-situ growth of siloxane nanoparticles, where the condensation of organosilane provided quick loading of hydrophobic layers on the membrane surface. The composite layers of dual coatings allowed systematic control of the surface wettability of porous membranes. By the virtue of the photothermal property of the hydrophilic polypyrrole layer, the desalination performance of the coated membrane was tested in a solar MD system. The wetting properties of the dual-layer were further evaluated in a direct-contact MD module, exploring the potential of the Janus membrane structure for effective and low-energy desalination.

• 대한치과보철학회지
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• 제44권5호
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• pp.617-627
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• 2006

Statement of problem: HA has been used as a coating material on Ti implants to improve osteoconductivity. However. it is difficult to form uniform HA coatings on implants with complex surface geometries using a plasma spraying technique. Purpose : To determine if Ti6Al4V sintered porous-surfaced implants coated with HA sol-gel coated and hydrothermal treated would accelerate osseointegration. Materials and Methods : Porous implants which were made by electric discharge were used in this study. Implants were anodized and hydrothermal treatment or HA sol-gel coating was performed. Hydrothermal treatment was conducted by high pressure steam at $300^{\circ}C$ for 2 hours using a autoclave. To make a HA sol, triethyl phosphite and calcium nitrate were diluted and dissolved in anhydrous ethanol and mixed. Then anodized implant were spin-coated with the prepared HA sols and heat treated. Samples were soaked in the Hanks solution with pH 7.4 at $37^{\circ}C$ for 6 weeks. The microstructure of the specimens was observed with a scanning electron microscope (SEM), and the composition of the surface layer was analyzed with an energy dispersive spectroscope (EDS). Results : The scanning electron micrographs of HA sol-gel coated and hydrothermal treated surface did not show any significant change in the size or shape of the pores. After immersion in Hanks' solution the precipitated HA crystals covered macro- and micro-pores The precipitated Ca and P increased in Hanks' solution that surface treatment caused increased activity. Conclusion : This study shows that sol-gel coated HA and hydrothermal treatment significantly enhance the rate of HA formation due to the altered surface chemistry.

• 한국재료학회지
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• 제26권5호
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• pp.229-234
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• 2016

In this study, the degree of particle melting in $Y_2O_3$ plasma spraying and its effects on coating characteristics have been investigated in terms of microstructural features, microhardness and scratch resistance. Plasma sprayed $Y_2O_3$ coatings were formed using two different powder feeding systems: a system in which the powder is fed inside the plasma gun and a system in which the powder is fed externally. The internal powder spraying method generated a well-defined lamellae structure that was characterized by a thin porous layer at the splat boundary and microcracks within individual splats. Such micro-defects were generated by the large thermal contraction of splats from fully-molten droplets. The external powder spraying method formed a relatively dense coating with a particulate deposition mode, and the deposition of a higher fraction of partially-melted droplets led to a much reduced number of inter-splat pores and intra-splat microcracks. The microhardness and scratch resistance of the $Y_2O_3$ coatings were improved by external powder spraying; this result was mainly attributed to the reduced number of micro-defects.