• Journal of the Korean Crystal Growth and Crystal Technology
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• v.27 no.6
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• pp.275-281
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• 2017

AlN is a promising material for wide band gap and high-frequency electronics device due to its wide bandgap and high thermal conductivity. AlN has advantages as materials for power semiconductors with a larger breakdown field, and a smaller specific on-resistance at high voltage. The growth of a p-type AlN epilayer with high conductivity is important for a manufacturing an AlN-based applications. In this paper, Mg doped AlN epilayers were grown by a mixed-source HVPE. Al and Mg mixture were used as source materials for the growth of Mg-doped AlN epilayers. Mg concentration in the AlN was controlled by modulating the quantity of Mg source in the mixed-source. Surface morphology and crystalline structure of AlN epilayers with different Mg concentrations were characterized by FE-SEM and HR-XRD. XPS spectra of the Mg-doped AlN epilayers demonstrated that Mg was doped successfully into the AlN epilayer by the mixed-source HVPE.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.9
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• pp.947-954
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• 2011

Drive-train friction loss in a vehicle may account for 4% of its total fuel consumption loss. An ATF W/C (auto-transmission fluid warmer/cooler) plate-fin heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between the auto-transmission fluid and coolant. The use of an ATF W/C heat exchanger can result in a fuel economy improvement of about 1% in vehicles. An experimental setup for testing the thermal performance of an ATF W/C plate-fin heat exchanger is developed. In this study, the influence of the ATF and coolant, flow rates, and temperature on the efficiency of an ATF W/C heat exchanger are investigated experimentally. From the experimental data, a simple correlation for predicting the efficiency of an ATF W/C heat exchanger is proposed. On the basis of this correlation, the fuel economy of a vehicle with and without an ATF W/C heat exchanger is compared by using Simulink. Finally, it is shown that the fuel economy is improved by 0.992% when an ATF W/C heat exchanger is installed in the vehicle.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.11
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• pp.927-933
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• 2016

In a solid propulsion system, the rocket nozzle is exposed to high temperature combustion gas. Hence, choosing an appropriate material that could demonstrate adequate performance at high temperature is important. As advanced materials, carbon/silicon carbide composites (C/SiC) have been studied with the aim of using them for the rocket nozzle throat. However, when compared with typical structural materials, C/SiC composites are relatively weak in terms of both strength and toughness, owing to their quasi-brittle behavior and oxidation at high temperatures. Therefore, it is important to evaluate the thermal and mechanical properties of this material before using it in this application. This study presents an experimental method to investigate the fracture behavior of C/SiC composite material manufactured using liquid silicon infiltration (LSI) method at elevated temperatures. In particular, the effects of major parameters, such as temperature, loading, oxidation conditions, and fiber direction on strength and fracture characteristics were investigated. Fractography analysis of the fractured specimens was performed using an SEM.

• Journal of Acupuncture Research
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• v.19 no.2
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• pp.114-127
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• 2002

Objective : The purpose of this study is to investigate the mechanism and effect of moxibustion objectively and to be used as the quantitative data for developing the new thermal stimulating treatment by observing the combustion time and temperature of commercial moxaes. Methods : We have selected two types(large-size moxa A(LMA), large-size moxa B (LMB)) among large moxaes used widely in the clinic. We examined combustion times, temperatures in each period during a combustion of moxa. Results : 1. The combustion time in the preheating period was about 30sec in both moxaes on the non-contact heated surface. 2. The combustion time in the heating period was about 345sec in LMA and about 1391 sec in LMB, about 4 times longer in LMB on the non-contact surface. 3. The maximum temperature in the heating period was $44.5^{\circ}C$ in LMA and $45.4^{\circ}C$ in LMB respectively, higher by $0.9^{\circ}C$ in LMB. The average temperature in the heating period was $35.5{\sim}37.6^{\circ}C$ in LMA and $36.0{\sim}39.8^{\circ}C$ in LMB, a little higher in LMB. 4. The combustion time in the retaining period in LMA was 45.4sec and 13% of that in the heating period, and in LMB 594.7sec and 43% of that in the heating period on the non-contact surface. 5. On the point(PH) measured maximum temperature, the average temperature during the retaining period was $44.0^{\circ}C$, $42.9^{\circ}C$ respectively and the temperature at an end of the retaining period was $43.0^{\circ}C$, $40.2^{\circ}C$ respectively. 6. The time at a beginning of the cooling period was about 418 sec from ignition in LMA and 2021sec in LMB, and the temperature at that time was $36.9{\sim}39.1^{\circ}C$ on the non-contact surface. Conclusion : It was thought that not only the figure of moxicombustion device, but also the form and size of moxa had influence on the combustion characteristics deciding the performance of stimulus seriously.

• Polymer(Korea)
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• v.25 no.1
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• pp.133-141
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• 2001

The friction and wear properties of carbon-carbon composites made with different weight percent of $MoSi_2$ as an oxidation inhibitor were investigated using a constant speed wear test apparatus in an oxidation environment. The results indicated the carbon-carbon composites undergoing an abrupt transition of friction coefficient, from low-friction behavior(${\mu}$=0.15~0.2) during normal wear regime to the high-friction behavior(${\mu}$=0.5~0.6) during dusting wear regime at the frictional temperature range of 150~180${\circ}C$. The existence of temperature-dependent friction and wear regimes implied that the performance of specimen made with carbon-carbon composites was markedly affected by the thermal properties of the composites. The carbon-carbon composites filled with MoSi2 exhibited two times lower coefficient of friction and wear rate in comparison with the composites without $MoSi_2$. Especially, the composites containing 4wt% $MoSi_2$ filler showed a significantly improved activation energy for wear due to the reduction of both the porosity and powdery debris film formation on sliding surface when compared to those without $MoSi_2$.

• Journal of the Korea Institute of Building Construction
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• v.12 no.2
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• pp.203-210
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• 2012

Recently, the interest and demand for large-scale buildings and skyscrapers have been on the rise, and the performance of concrete is an area of high priority. Securing 'mass concrete and high strength concrete' is very important as a key construction technology. For high strength concrete, the high heat of hydration takes place inside the concrete because of the vitality of hydration in cement due to the large amount of powder, and leads to problems such as an increase of thermal stress due to the temperature difference with the outside, which results in cracks and slump loss. For this reason, measures to solve these problems are needed. This study aims to reduce the hydration heat of high strength concrete to control the hydration heat of mass concrete and high strength concrete, by replacing the type of admixture, The purpose of this study is to control the hydration heat of high strength concrete and mass concrete. Our idea for this purpose is to apply not only the types and contents of admixture but also incorporation mixing water to ice-flake. As a result of the test, the use of blast furnace slag and fly ash as admixture, and the use of ice-flake as mixing water can improve the liquidity of concrete and reduce slump loss. Significantly dropping the maximum temperature will contribute greatly to reducing cracks due to hydration heat in mass concrete and high strength concrete, and improve quality.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.8
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• pp.23-30
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• 2016

In this study, the heat transfer and pressure drop characteristics were evaluated for multi-tube $CO_2$ water heaters with lengths of 4.5 m and 7.5 m. The evaluation was done using the -NTU method, and the results were compared with experimental data. Water flows through the shell side of the water heater, while $CO_2$ flows through 8 inner tubes. The heater uses a counter-current design to maximize the heat transfer efficiency. The energy balance equation describing the flows of $CO_2$ and water for each node is set up using the section-by-section method. The calculated heat transfer rates agree well with the experimental data within ${\pm}5%$ error. The outlet water temperature decreased linearly with the increase of the water flow rate. The calculated heat transfer rates agreed well with the experimental data within ${\pm}3%$ error. The results show that the heat transfer rate increases almost linearly with the increase of water flow rate or $CO_2$ inlet temperature in both the 4.5-m and 7.5-m water heaters, whereas the water outlet temperature linearly decreases with the increase of the water flow rate. The comparison of the $CO_2$ pressure drop between the calculation and experiment results shows good agreement at the high $CO_2$ flow rate within 5 % error, but the value is about 20 % higher in the experimental pressure drop at the low $CO_2$ flow rate.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.12
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• pp.608-613
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• 2019

NOx (Nitrogen oxide) in the exhaust gas from vehicle engines is considered one of the most harmful substances in air pollution problems. NOx is made when combustion occurs under high temperature conditions and EGR (exhaust gas recirculation) is normally used to lower the combustion temperature. As the EGR ratio increases, the NOx level becomes low, but a high EGR ratio makes the combustion unstable and causes further air pollution problems, such as CO and unburned hydrocarbon level increase. This study showed that fuel droplets could move more freely by the radiation of sonic wave for the stable combustion. In addition, the engine performance improved with increasing EGR ratio. As a basic study, the effect of sonic wave radiation on the velocity of fuel droplets was studied using CFD software. The results showed that the velocity of small droplets increased more under high frequency sonic wave conditions and the velocity of the large droplets increased at low frequency sonic wave conditions. In addition, an engine analysis model was used to study the effects of the increased combustion stability. These results showed that a 15% increase in EGR ratio in combustion resulted in a 45% decrease in NOx and a 10% increase in thermal efficiency.

• Journal of the Korean Electrochemical Society
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• v.12 no.2
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• pp.155-161
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• 2009

Lithium secondary batteries have been widely used in the portable electric devices as power source. Recently it is expected that the realm of its applications expands to the markets such as energy storage medium of hybrid electric vehicle(HEV), electric vehicle(EV). Cathode active material is crucial in terms of performance, durability, capacity of lithium secondary batteries. It is urgent to develope the technology for mass production of cathode material to cope with the markets' demands in the near future. In this study, a calcination furnace running in real production line is modelled in 3D, and the thermal flow and gas flow after chemical reaction in the furnace is analyzed through numerical computations. Based on the results, it is shown that large volume of $CO_2$ gas is generated from chemical reaction. High concentration of $CO_2$ gas and it's stagnation is clearly found from the reactant containers in which the reaction occur to the bottom area of the furnace. It is also studied that 15% or more $CO_2$ mol fraction could affect to proper formation of $LiCoO_2$ through TGA-DSC analysis. The solutions to evacuate carbon dioxide from the furnace are suggested through the change of furnace design and operating condition as well.

• Journal of the Korean Wood Science and Technology
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• v.42 no.6
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• pp.691-699
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• 2014

In order to improve the dimensional stability and durability of wood, this study attempted to impregnate bismuth (Bi) - tin (Sn) alloy metal with low melting temperature into solid woods of three species such as radiata pine, red oak and white oak, and investigated to determine an optimum condition of manufacturing the metal alloy-wood composites with natural wood grains. These Bi-Sn alloys were chosen for this study because they were harmless to human and melting at low temperatures. The composites resulted in high dimensional stability and low thickness swelling, and also showed much improved performance such as high bending strength, high hardness, high electric conductivity, and high thermal conductivity as floor materials. A proper impregnating condition of all specimens was determined as 10 minutes of the preliminary vacuum time, and $185^{\circ}C$ of the heating temperature. The proper processing condition for radiata pine wood was 2.5 minutes of the pressuring time at the pressure of $10kgf/cm^2$. For red oak wood, 10 minutes of the pressuring time at the pressure of $30kgf/cm^2$ were the proper condition. The proper manufacture conditions for white oak wood was determined as 10 minutes of the pressuring time at the pressure of $50kgf/cm^2$.