• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.7
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• pp.587-593
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• 2007

Oil-based nanofluids were prepared by dispersing spherical and fiber shaped $Al_2O_3$ and AlN nanoparticles in transformer oil. Two hydrophobic surface modification processes using oleic acid (OA) and polyoxyethylene alkyl acid ester (PAAE) were compared in this study. The dispersion stability, viscosity and breakdown voltage of the nanofluids were also characterized. $(Al_2O_3+AlN)$ mixed nanofluid was prepared to take an advantage of the excellent thermal conductivity of AlN and a good convective heat transfer property of fiber shaped $Al_2O_3$. For $(Al_2O_3+AlN)$ particles with 1 % volume fraction in oil, the enhancement of thermal conductivity and convective heat transfer coefficient was nearly 11 % and 30 %, respectively, compared to pure transformer oil. The nanofluid, containing $Al_2O_3+AlN$, successfully lowered the temperature of the heating element and oil itself during a natural convection test using a prototype transformer.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1207-1212
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• 2004

A model for the phonon dispersion relationship for cubic zinc sulfide structure, for example SiC, is developed in terms of two unknown force constants. Born model that incorporates bond bending and bond stretching, is used for the force constants. The force constants are determined by fitting to experimental data. Using only the nearest-neighbor coupling results in $6{\times}6$ sized dynamic matrix. The eigenvalues of dynamics matrix for each wavenumber in 3-D ${\kappa}$ space correspond to frequencies, 3 for optical phonon and 3 for acoustic phonon, which is so-called dispersion relation (${\kappa}$-${\omega}$). The density of state is determined by counting the states for each frequency bin, and the properties such as specific heat and thermal conductivity can be obtained. The specific heat is estimated on this model and compared with experiment and other models, i.e. Debye model, Einstein model and combined Debye-Einstein model. In spite of the simple bond potential model, reasonable agreements are found.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.03b
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• pp.41-41
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• 2010

We present facile chemical route stabilizing dispersion of carboxylated single-walled carbon nanotubes (SWCNTs) in ZnOsol prepared by using diethanolamine as a stabilizer. The dispersion was stabilized via capping of carboxyl groups on the SWCNT surface by a titania layer. We also demonstrated that the conductivity of the films prepared P3/$TiO_2$/ZnO as enhanced by therml treatment, and thethermal stbility of the film improved hybridization with ZnO sol pristine P3, P3/$SiO_2$ and P3/$TiO_2$ hybrid films.

• Korean Chemical Engineering Research
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• v.58 no.1
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• pp.64-68
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• 2020

In this study, metal copper clad laminate can be prepared using epoxy composite filled with thermally conductive fillers. In order to improve the thermal conductivity of epoxy composites, it is important factor to form conductive networks through appropriate packing of conductive fillers in epoxy composite matrix and to decrease the amount of thermally resistant junctions involving a epoxy composite matrix layer between adjacent filler units. This is because epoxy has a thermal conductivity of only 0.2-0.3W, so in order to maintain high thermal conductivity, thermally conductive fillers are connected to each other, so that the gap between particles can be reduced to reduce thermal resistance. The purpose of this study is to find way to achieve highly thermally conductive in the epoxy composite matrix filled with Al₂O₃ and Boron Nitride(BN) filler by filler loading and uniform dispersion. As a results, the use of Al₂O₃/BN hybrid filler in epoxy matrix was found to be effective in increasing thermal conductivity of epoxy composite matrix due to the enhanced connectivity offered by more continuous thermally conductive pathways and uniform dispersion without interfacial voids in epoxy composite matrix. In addition, surface treatmented s-BN improves the filler dispersion and adhesion between the filler and the epoxy matrix, which can significantly decrease the interfacial thermal resistance and increase the thermal conductivity of epoxy composite matrix.

• Environmental Sciences Bulletin of The Korean Environmental Sciences Society
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• v.1 no.1
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• pp.35-43
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• 1997

Handling the emergency problems such as Chemobyl accident require real time prediction of pollutants dispersion. One-point real time sounding at pollutant source and simple model including turbulent-radiation process are very important to predict dispersion at real time. The stability categories obtained by one-dimensional numerical model (including PBL dynamics and radiative process) are good agreement with observational data (Golder, 1972). Therefore, the meteorological parameters (thermal, moisture and momentum fluxes; sensible and latent heat; Monin-Obukhov length and bulk Richardson number; vertical diffusion coefficient and TKE; mixing height) calculated by this model will be useful to understand the structure of stable boundary layer and to handling the emergency problems such as dangerous gasses accident. Especially, this simple model has strong merit for practical dispersion models which require turbulence process but does not takes long time to real predictions. According to the results of this model, the urban area has stronger vertical dispersion and weaker horizontal dispersion than rural area during daytime in summer season. The maximum stability class of urban area and rural area are 'A' and 'B' at 14 LST, respectively. After 20 LST, both urban and rural area have weak vertical dispersion, but they have strong horizontal dispersion. Generally, the urban area have larger radius of horizontal dispersion than rural area. Considering the resolution and time consuming problems of three dimensional grid model, one-dimensional model with one-point real sounding have strong merit for practical dispersion model.

• Clean Technology
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• v.12 no.1
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• pp.11-18
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• 2006

In this study, aqueous polyurethane dispersion(PUD) was synthesized using polyhexamethylene carbonate glycol (PHMCG) as soft segment, isophorone diisocyanate (IPDI) and dimethylol propionic acid (DMPA) as hard segment. Also, polyurethane/clay nanocomposites were prepared by adding pristine montmorillonite (PM) and organically modified clays, C15A and C30B into PUD. The degree of clay dispersion in the nanocomposites was investigated using XRD and the physical and thermal properties were examined through UTM and TGA. These results showed that nanocomposites with C15A gave higher physical and thermal properties than those with C30B or PM. As a result, the properties of nanocomposites were observed to vary depending on the types of clay modifiers and clay contents.

• Advances in nano research
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• v.6 no.1
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• pp.21-37
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• 2018

In the present article, wave dispersion behavior of a temperature-dependent functionally graded (FG) nanobeam undergoing rotation subjected to thermal loading is investigated according to nonlocal strain gradient theory, in which the stress numerates for both nonlocal stress field and the strain gradient stress field. The small size effects are taken into account by using the nonlocal strain gradient theory which contains two scale parameters. Mori-Tanaka distribution model is considered to express the gradually variation of material properties across the thickness. The governing equations are derived as a function of axial force due to centrifugal stiffening and displacements by applying Hamilton's principle according to Euler-Bernoulli beam theory. By applying an analytical solution, the dispersion relations of rotating FG nanobeam are obtained by solving an eigenvalue problem. Obviously, numerical results indicate that various parameters such as angular velocity, gradient index, temperature change, wave number and nonlocality parameter have significant influences on the wave characteristics of rotating FG nanobeams. Hence, the results of this research can provide useful information for the next generation studies and accurate deigns of nanomachines including nanoscale molecular bearings and nanogears, etc.

• Korean Journal of Materials Research
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• v.19 no.3
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• pp.156-162
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• 2009

Lubricant-based nanofluids were prepared by dispersing carbon nanoparticles in gear oil. In this study, the effects of the particle size, shape and dispersity of the particles on the tribological properties of nanofluids were investigated. Dispersion experiments were conducted with a high-speed bead mill and an ultrasonic homogenizer, and the surfaces of the nanoparticles were simultaneously modified with several dispersants. The effective thermal conductivity of the nanofluids was measured by the transient hot-wire method, and the tribological behaviors of the nanofluids were also investigated with a disk-on-disk tribo-tester. The results of this study clearly showed that the combination of the nanoparticles, the deagglomeration process, the dispersant and the dispersion solvent is very important for the dispersity and tribological properties of nanofluids. Lubricant-based nanofluids showed relatively low thermal conductivity enhancement, but they were highly effective in decreasing the frictional heat that was generated. For nanofluids containing 0.1vol.% graphite particles in an oil lubricant, The friction coefficient in the boundary and fluid lubrication range was reduced to approximately 70% of the original value of pure lubricant.

• International Journal of Ocean System Engineering
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• v.3 no.2
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• pp.102-110
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• 2013

The purpose of this study was to improve the properties of epoxy resin using titanium oxide nanoparticles. The effects of particle weight fraction, dispersion agent, and curing agents with different molecular weights on the thermal and mechanical properties of titanium-oxide-reinforced epoxy resin were investigated. In addition, the effect of the particle dispersion condition on the mechanical properties of nanocomposites was studied. As a result, it was found that the glass transition temperature of film-shaped nanocomposites decreased with an in-crease in the nanoparticle content. Because nanoparticles interrupted the cross linkage between the epoxy resin and the amine curing agent, the cross-link density of the epoxy became lower and led to a decrease in $T_g$ in the nanocompo-sites. The tensile strength and modulus in film-shaped nanocomposites also increased with the particles content. But in the case of dog-bone-shaped nanocomposites, the values were not similar to the trend for the film-shaped nanocompo-sites. This was probably a result of the different nanoparticles dispersions in the epoxy resins resulting from the respective-thicknesses of the film and dog-bone-shaped samples.

• Journal of Welding and Joining
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• v.12 no.2
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• pp.39-48
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• 1994

In comparison to the conventional brazing, laser welding of ceramics has advantages of direct bonding without filler material, which causes the thermal stress due to the differences of thermal expansion coefficients. In pulse-mode, laser welding of dispersion ceramic having high thermal resistance is possible at relatively low preheating temperature of $1300^{\circ}C$ In CW-mode, alumina can be welded at high preheating temperature $1500^{\circ}C$ under the condition of low feed rate of 500 mm/min, respectively. Further studies on developing mechanism of pores in the bead during laser welding of ceramics is required.