• Journal of ILASS-Korea
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• v.19 no.4
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• pp.174-181
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• 2014

This study investigates the effects of nanoparticle size and temperature on the thermal conductivity enhancement of water-based alumina ($Al_2O_3$) nanofluids, using the centrifuging method and relative centrifugal forces of differing magnitude to produce nanofluids of three different particles without involving any dispersants or surfactants. We determined the coupling dependency in thermal conductivity enhancement relative to nanoparticle size and temperature of the alumina nanofluids and also experimentally showed that the effect of temperature on thermal conductivity is strongly dependent on nanoparticle size. Also, our experimental data presented that the effective medium theory models such as the Maxwell model and Hasselman and Johnson model are not sufficient to explain the thermal conductivity of nanofluids since they cannot account for the temperature- and size-dependent nature of water-based alumina nanofluids.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.9 s.240
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• pp.1065-1073
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• 2005

The thermal conductivity of water- and ethylene glycol-based nanofluids containing alumina $(Al_2O_3)$, zinc oxide (ZnO) and titanium dioxide $(TiO_2)$ nanoparticles is measured by varying the particle diameter and volume fraction. The transient hot-wire method using an anodized tantalum wire for electrical insulation is employed for the measurement. The experimental results show that nanofluids have substantially higher thermal conductivities than those of the base fluid and the ratio of thermal conductivity enhancement increases linearly with the volume fraction. It has been found that the ratio of thermal conductivity enhancement increases with decreasing particle size but no empirical or theoretical correlation can explain the particle-size dependence of the thermal conductivity. This work provides, for the first time to our knowledge, a set of consistent experimental data over a wide range of nanofluid conditions and can therefore serve as a basis for developing theoretical models to predict thermal conduction phenomena in nanofluids.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.243.2-243.2
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• 2011

ZnO materials with a wide band gap of approximately 3.3 eV has been used in transparent conducting oxides (TCO) due to exhibitinga high optical transmission, but its low conductivity acts as role of a limitation for conducting applications. Recently, Ga or Al-doped ZnO (GZO, AZO) becomes transparent conducting materials because of high optical transmission and excellent conductivity. However, the fundamental mechanism underlying the improvement of electrical conductivity of the GZO is still the subject of debate. In this study, we have fully investigated the reasons of high conductivity through the characterization of plane defects, crystal orientation, doping contents, crystal structure in Zn1-xGaxO (x=0, 3, 5.1, 5.6, 6.6 wt%). We manufactured Zn1-xGaxO by sintering ZnO and Ga2O3 powers, having a theoretical density of 99.9% and homogeneous Ga-dopant distribution in ZnO grains. The GZO containing 5.6 wt% Ga represents the highest electrical conductivity of $7.5{\times}10^{-4}{\Omega}{\cdot}m$. In particular, many twins and superlattices were induced by doping Ga in ZnO, revealed by X-ray diffraction measurements and TEM (transmission electron microscopy) observations. Twins developed in conventional ZnO crystal are generally formed at (110) and (112) planes, but we have observed the twins at (113) plane only, which is the first report in ZnO material. Interestingly, the superlattice structure was not observed at the grains in which twins are developed and the opposite case was true. This structural change in the GZO resulted in the difference of electrical conductivity. Enhancement of the conductivity was closely related to the extent of Ga ordering in the GZO lattice. Maximum conductivity was obtained at the GZO with a superlattice structure formed ideal ordering of Ga atoms.

• Proceedings of the SAREK Conference
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• 2006.06a
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• pp.713-718
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• 2006

This paper concerns the measurement of thermal conductivity of grouting materials for ground loop heat exchanger. A thermal conductivity meter, QTM-500 based on modified transient hot wire method was used to measure the thermal conductivity of neat bentonite and mixtures of bentonite and various additives. Relative to the total mixture mass, as the percent additive was increased the mixture thermal conductivity increased. For the bentonite-silica sand mixtures, the higher density of the sand particles resulted in much higher mixture thermal conductivity.

• 연구논문집
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• s.33
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• pp.147-155
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• 2003

In order to fabricate porous materials having an anisotropic thermal conductivity by aligning plate-shaped pores structure, alumina powder (AM-21, mean particle size $4\mum$) and flake crystalline graphite was used. The aligned pore structure was realized using multi-pressing process. Degree of pore orientation increased with the number of pressing and thermal conductivity, parallel to the pressing direction, decreased with the number of pressing. Thermal conductivity decreased significantly to the addition of 30vol% crystalline graphite, however, in the case of 60vol%, thermal conductivity did not decrease significantly due to the breakage of crystalline graphite. An anisotropy of the thermal conductivity increased with the content of crystalline graphite up to 30vol%. Graded pore structure was fabricated by controlling the content and size of crystalline graphite, which provides, possibly, the enhancement in mechanical strength and thermal insulation properties of the insulating bricks.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.6
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• pp.862-868
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• 2008

Many studies have been conducted to increase heat transfer in fluid. One of the various heat transfer enhancement techniques is suspending fine metallic or nonmetallic solid powder in traditional fluid. Nanofluid is defined as a new kind of heat transfer fluid containing a very small quantity of nanometer particles that are uniformly and stably suspended in a liquid. This study investigates the effect of nanofluid containing diamond, CuNi and CuAg nanometer particles, and proposes the heat transport mechanism of nanofluid. The test result shows that the thermal conductivity of nanofluid is much higher than that of traditional fluid, and the increasing rate of the conductivity is dependent on the conductivity of the solid metal.

• Journal of the Korean Ceramic Society
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• v.47 no.1
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• pp.71-74
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• 2010

We have investigated protonic conduction in highly dense (>98%) polycrystalline Y-doepd $BaZrO_3$ (BYZ) ceramic with an average grain size of ~85 nm. It is observed that the protonic conductivity across the grain boundaries in this nano-crystallilne BYZ (n-BYZ) is significantly higher than the microcrystalline counterpart. Such a remarkable enhancement in grain boundary conductivity results in high overall conductivity that may allow this chemically stable protonic conductor to serve as a solid electrolyte for low-temperature solid oxide fuel cell applications.

• Plant Biotechnology Reports
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• v.5 no.1
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• pp.71-77
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• 2011

Drought and salinity are the most important abiotic stresses that affect the normal growth and development of plants. Glycine betaine is one of the most important osmolytes present in higher plants that enable them to cope with environmental stresses through osmotic adjustment. In this study, a betaine aldehyde dehydrogenase (BADH) gene from spinach under the control of the stress-induced promoter rd29A from Arabidopsis thaliana was introduced into potato cultivar Gannongshu 2 by the Agrobacterium tumefaciens system. Putative transgenic plants were confirmed by Southern blot analysis. Northern hybridization analysis demonstrated that expression of BADH gene was induced by drought and NaCl stress in the transgenic potato plants. The BADH activity in the transgenic potato plants was between 10.8 and 11.7 U. There was a negative relationship (y = -2.2083x + 43.329, r = 0.9495) between BADH activity and the relative electrical conductivity of the transgenic potato plant leaves. Plant height increased by 0.4-0.9 cm and fresh weight per plant increased by 17-29% for the transgenic potato plants under NaCl and polyethylene glycol stresses compared with the control potato plants. These results indicated that the ability of transgenic plants to tolerate drought and salt was increased when their BADH activity was increased.

• Korean Journal of Environmental Agriculture
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• v.42 no.1
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• pp.1-7
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• 2023

Super absorbent polymers (SAPs) are hydrophilic molecules that can absorb large amounts of water. This study was conducted to investigate the enhancement of the physicochemical properties of sand soil blended with three SAPs imbibed with 100, 150, and 200-fold water. Three treatments were applied, namely, 100SAP, 150SAP, and 200SAP. The three SAPs were blended at concentrations of 0% (control), 3%, 5%, 7%, and 10% with sand. The pH, electrical conductivity, and cation exchangeable capacity (CEC) of soil blended with the three SAPs were pH 6.35-6.46, 0.09-0.65 dS/m, and 1.42-1.92 cmol_c/kg, respectively, and their capillary porosity, total porosity, and saturated hydraulic conductivity were 21.0-29.3%, 39.2-48.7%, and 272-470 mm/hr. CEC, capillary porosity, total porosity, and saturated hydraulic conductivity of soil were positively correlated with the ratio of the SAPs (p<0.01). These results indicate that blending sand soil with SAPs increased CEC, capillary porosity, and saturated hydraulic conductivity, thus improving the nutrient-retention capacity, water-retention capacity, and permeability of the soil.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2005.06a
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• pp.177-182
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• 2005

Many studies have been conducted to increase heat transfer in fluid. One of the various heat transfer enhancement techniques is to suspend fine metallic or nonmetallic solid powder in traditional fluid. Nanofluid is defined at a new kind of heat transfer fluid containing a very small quantity of nanometer particles that are uniformly and stably suspended in a liquid. In this study CuNi or CuAg nano particles are used to investigate heat transfer enhancement. The result shows the thermal conductivity of nanofluid is much higher than that of traditional fluid.