• Applied Chemistry for Engineering
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• v.34 no.4
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• pp.434-443
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• 2023

We have investigated the performance improvement of grease by synthesizing calcium sulfonate grease as an alternative to lithium grease, which is widely used globally. Since the composition ratio of the grease, when manufactured, is usually 50% base oil and 50% thickener, using grease as a lubricant in a cryogenic environment is not encouraged due to its inferior low-temperature performance. In this study, we have synthesized three types of calcium sulfonate grease with paraffin oil and PAO-based base oil and thickener. Furthermore, lithium grease was synthesized via saponification with PAO-based base oil, lithium hydroxide, 12-hydroxystearic acid, and sebacic acid. We have measured low-temperature characteristics using a rheometer and low-temperature torque meter, and tribology characteristics were obtained using a four-ball lubricant tester and schwingung reibung verschleiß (SRV). As a result, the flow point of the calcium sulfonate grease synthesized with a PAO-based base oil and thickener was found to be -40℃, overcoming the existing calcium sulfonate grease's low-temperature limitation. Moreover, the synthesized calcium sulfonate grease showed low-temperature performance similar to that of lithium grease synthesized with PAO base oil, but superior anti-wear, extreme pressure, coefficient of friction, heat resistance, adhesion, and corrosion resistance. It is thus expected to commercially replace the existing lithium grease.

• Journal of the Korea Institute of Military Science and Technology
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• v.17 no.4
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• pp.413-421
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• 2014

Since the structural temperature of a flight vehicle flying at high speed rises rapidly due to aerodynamic heating, it is necessary for optimum structural design to obtain proper material properties at high temperature by taking into account of its operational environment. For a special alloy, analysis data on strength change due to exposure time to high temperature are very limited, and most of them are for an exposure time longer than 30 minutes for long term operations. In this study, base and weld metal samples of Ti-6Al-4V alloy had been prepared and high temperature tensile tests with induction heating were performed, and then high temperature strength characteristics and strength recovery characteristics through cooling have been analyzed. Pre-tests to determine maximum heating rate were performed, and response characteristics for temperature control were confirmed. As a result, high temperature tensile strength appeared to be lower than that of room temperature, but it was higher than that of high temperature of 30 minite exposure listed in MMPDS. In strength recovery through cooling Ti-6Al-4V alloy has shown higher recovery rate compared with other alloys.

• Journal of the Korean Society of Clothing and Textiles
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• v.41 no.4
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• pp.687-697
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• 2017

To figure out an appropriate pressure level for a body warming base layer, human responses were observed when the pattern reduction of base layers varied. Under the condition of $2^{\circ}C$, 60% RH, 0.1m/s, ten male subjects participated in the experiment with four sizes of experimental vests where heating pads were attached. The subjective evaluations of the heating vests with different sizes were reported using 7 or 9 point scales. We simultaneously observed chest, abdomen and scapula skin temperatures and microclimate humidity. It was found that the tight pattern as in the case of A or B provided a warmer subjective sensation and skin temperature than C or D; however, there were no differences in skin temperature at the chest. Eventually, the chest temperature decreased after about 30 mins of heating; however, temperature of abdomen increased and indicated that heating with two commercial pads used was inadequate for whole body warming. The pressure sensation of 'tight' was improved after warming the abdomen in a cold environment. Overall, the gaps beyond the original circumference of the abdomen, as in C or D, were not desirable for the local heating of abdomen under the conditions of this experiment where walking was included in the protocol. The experiment garment B with nude waist circumference was the best, and D with the largest ease, was the worst for a comfortable warming vest.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.6 s.249
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• pp.531-537
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• 2006

A lever type $NO_2$ micro gas sensor was fabricated by MEMS technology. In order to heat up the gas sensing material to a target temperature, a micro heater was built on the gas sensor. The sensing material laid on the heater and electrodes and did not contact with the silicon base to minimize the heat loss to the silicon base. The electric power to heat up the gas sensor to a target temperature was measured. The temperature distribution of micro gas sensor was analyzed by a CFD program. The predicted electric power of micro heater to heat up the sensing material to the target temperature showed a good agreement with the measured data. The design of micro gas sensor could be modified to show more uniform temperature distribution and to consume less electric power by optimizing the layout of micro heater and electrodes.

• Applied Chemistry for Engineering
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• v.8 no.2
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• pp.179-190
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• 1997

To study the effects of chemical composition on the fusion temperatures of coal ashes, the chemical composition, mineral matter, and fusion temperature were studied with 54 kinds of coal ash samples including Korean anthracite coals. CaO, MgO and $Fe_2O_3$ were observed to be major fluxing elements in reducing and oxidizing atmosphere. The fluxing effect of $Fe_2O_3$ was increased more in reducing atmosphere. In a base/acid ratio, the fusion temperature decreased with increasing amounts of basic components. Nevertheless, the correlation between a fusion temperature and base/acid ratio was not shown well in a higher ratio of $Fe_2O_3/CaO$. The differences of fusion temperatures between oxidizing and reducing atmosphere showed close relationship with $SiO_2/Al_2O_3$ ratio rather than with $Fe_2O_3$ contents. Multiple regression was used to predict the fusion temperature of coal ashes, and it was established that the major predictors in oxidizing atmosphere were Base/Acid, $Fe_2O_3/CaO$, $SiO_2/Al_2O_3$, and $(SiO_2/A1_2O_3){\cdot}(Base/Acid)$ and Base/Acid, $Fe_2O_3/CaO$, $SiO_2$, and $TiO_2$ were major ones in reducing atmosphere.

• New & Renewable Energy
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• v.3 no.1 s.9
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• pp.13-19
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• 2007

A triangular fin with variable fin base thickness and base height is analyzed and optimized for the fixed fin base height using a two-dimensional analytical method. At the middle of the fin length, the variation of the temperature along the fin height is listed. The influences of the fin length, base thickness and base height on the heat loss and fin efficiency are analyzed, The optimum heat loss, corresponding optimum efficiency and optimum fin length as a function of the fin base thickness are presented. The optimum heat loss and optimum fin tip length as a function of the convection characteristic number are represented.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.33A no.12
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• pp.39-46
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• 1996

In AlGaAs/GaAs HBT the temperature dependence of DC parameters was investigated over the temperature range between 95K and 580K. The temperature dependence of DC parameters depends on the relative contribution of each of the current components suc as emitter-injection-current, base-injection-current, bulk recombination current, interface recombination curretn, thermal generation ecurrent and avalanche current due to impact ionization within the collector space charge layer in a specific temperature. In this paper we investigated the temperature effects on DC parameters such as V$_{BE,ON}$ current gain, input and output characteristics, V$_{CE, OFF}$, R$_{E}$, R$_{C}$ and analyzed the origins, and extracted the qualitativ econditions for a stable HBTs against the temperature variation. Finally, in order to keep HBTs stable with respect to the variation of temperature, the valance-band-energy-discontinuity at emitter-base heterojunction should be large enough to enhance the effect of carrier suppression at a relatively high temperature. In addition the recombination centers, especially around collector junction, should be removed and the area of emitter and collector junction should be identical as well.

• Korea-Australia Rheology Journal
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• v.21 no.1
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• pp.13-16
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• 2009

We investigate time-temperature superposition from the viewpoint of geometry. The arc length of viscoelastic plots provides powerful resolution for check of the validity of time-temperature superposition. We also suggest a new algorithm for determination of shift factor which is base on the minimization of the total arc length and does not assume any functional form of viscoelastic function.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.8
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• pp.4749-4755
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• 2014

In this study, to maximize the heat release from the heat generating environment, such as a high-capacity inverter, the heat release performance of the extruded-type heat sinks with the variation of the base thickness were investigated using the experimental and numerical methods. The base thickness was varied from 5 to 14 mm. The heat release was characterized by the amount of heat released through the heat sink, the surface temperature of heat sink base between the heat sources, and temperature of heat sources. The surface temperatures between heat sources and the amounts of heat release were improved more as the base thickness was decreased. In contrast, the temperatures of the heat sources decreased with increasing base thickness. Based on the case study of these heat sinks, it is believed that a heat sink with a 9.5mm-thick base was optimized for the heat release.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.20 no.7
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• pp.455-461
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• 2008

A reversed trapezoidal fin with variable fin base thickness is optimized using a two-dimensional analytical method. For the fin base boundary condition, instead of a constant temperature, heat transfer from the inside fluid to the fin base is considered. Heat loss from the fin tip is not ignored. The maximum heat loss, corresponding optimum fin effectiveness, fin length and base height are presented as a function of the fin base thickness, shape factor and volume.