• Korean Journal of Materials Research
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• v.16 no.8
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• pp.516-523
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• 2006

In recent years, Magnesium (Mg) and its alloys have become a center of special interest in the automobile industry. Due to their high specific mechanical properties, they offer a significant weight saving potential in modern vehicle constructions. Most Mg alloys show very good machinability and processability, and even the most complicated die-casting parts can be easily produced. The die casting process is a fast production method capable of a high degree of automation for which certain Mg alloys are ideally suited. In this study, step-dies and flowability tests for AM50 were performed by die-casting process according to various combination of casting pressure and plunger velocity. We were discussed to velocity effect of forming conditions followed by results of microstructure, FESEM-EDX, hardness and tensile strength. Experimental results represented that the conditions of complete filling measured die-casting pressure 400 bar, 1st plunger velocity 1.0 m/s and 2nd plunger velocity 6.0 m/s. The increasing of 2nd plunger velocity 4.0 to 7.0 m/s decreased average grain size of $\alpha$ phase and pore. It was due to rapid filling of molten metal, increasing of cooling rate and pressure followed by increased 2nd plunger velocity. The pressure should maintain until complete solidification to make castings of good quality, however, the cracks were appeared at pressure 800bar over.

• Korean Journal of Food Science and Technology
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• v.28 no.3
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• pp.514-520
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• 1996

The purpose of this study was to develop the production technology of modified starch. Corn starches were gamma irradiated at 0-110 kGy and the effect of irradiation dose levels on the physicochemical properties of corn starches were investigated. Blue value linearly decreased, while alkali number and solubility markedly increased as irradiation dose levels were increased. The optical transmittance increased as applied irradiation dose levels were increased in the temperature range of $65-95^{\circ}C.$ Water binding capacity and swelling power showed maximum value at 30 and 10 kGy, respectively and they tended to decrease thereafter. Gelatinization viscosity of the gamma irradiated starch considerably decreased as compared to that of the non-irradiated starch. Irradiation at 110kGy resulted in a marked reduction of peak viscosity and cooling viscosity at $30^{\circ}C$ by 100 and 300 times, respectively. The physicochemical properties of corn starch irradiated at 30 kGy were similar to those of commercial acid-modified starch, while those of corn starch irradiated at 100 kGy were similar to those if oxidized starch.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.4
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• pp.303-311
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• 2011

This study presents a new idea of liquid desiccant dehumidifier with extended surface to improve the compactness. Extended surface is inserted between vertical cooling tubes, and the liquid desiccant flows down along the tube walls and the extended surface as well. Though the extended surface contributes to the increase in the mass transfer area, the effect tends to be limited because less conductive non-metallic materials need to be applied due to the high corrosiveness of liquid desiccant. To analyze the effects of the extended surface insertion, mathematical modelling and numerical integration are performed for the heat and mass transfer in the liquid desiccant dehumidifier. The results show that, though the liquid desiccant on the extended surface is heated due to the moisture absorption, the temperature can be maintained by periodic mixing at the contact points between the tube and the extended surface with the liquid desiccant stream from the tube side at a relatively low temperature. This implies the absorption heat from the extended surface side can be removed effectively by mixing, which leads to a substantial improvement of the dehumidification in the liquid desiccant dehumidifier with extended surface. When the interval of the extended surface, $p_e/L$, is less than 0.1, the dehumidification is shown to increase by more than two times compared with that without extended surface.

• Elastomers and Composites
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• v.40 no.3
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• pp.212-221
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• 2005

Polyurethanes containing mesogen unit (MPU-6) were prepared from 4,4'-diphenylmethane diisocyanate (MDI) and 4,4'-bis(6-hydroxy hexoxy)biphenyl (BP-6) having flexible chain composed of 6 carbons. Intrinsic viscosities of MPU-6s were in the range of $0.23{\sim}0.56 dL/g$. The mesomorphic behaviors of MPU-6 were observed in X-ray and polarizing microscopy analysis. However, MPU-6s demonstrated a 'virtual liquid crystal' behavior, which did not exhibit mesophase on slow heating and slow cooling. MPU-6 having lower molecular weight exhibited higher crystallization rate and melting crystallization temperature due to increased mobility of polymer chains. The increased mobility of polymer chains facilitate the orientation of mesogen units that may act as a nucleating agent.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.5
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• pp.262-266
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• 2013

A new concept of periapical radiography, intra oral x-ray tube and detector system is introduced. It is new system that a miniature x-ray tube is inserted into mouse and it acquired digital image using external detector. In this study, we have investigated temperature and dose distribution of insertional x-ray tube for periapical radiography. To analyze temperature characteristic of x-ray tube, we attached the thermocouple to surface of x-ray tube and we measured the temperature according to distance. Also, we measured the dose distribution of a miniature x-ray tube according to distance. As a result, temperature was constant to $27^{\circ}C$ over 2mm without cooling system, dose distribution of x-ray tube was 3.14 and 1.84mGy in 3 and 5cm, respectively. Therefore, the proposed x-ray system works in lower dose than conventional dental x-ray system. Thus, it is considered that new concept of system will have a significant effect on medical imaging technology.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.869-872
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• 2008

Recently, the effects of high temperature on compressive strength, elastic modulus and strain at peak stress of high strength concrete were experimentally investigated. The present study is aimed to study the effect of elevated temperatures ranging from 20 to $700^{\circ}C$ on the material mechanical properties of high-strength concrete of 40, 60, 80MPa grade. In this study, the types of test were the stressed test and stressed residual test that the specimens are subjected to a 25% of ultimate compressive strength at room temperature and sustained during heating and when target temperature is reached, the specimens are loaded to failure. Or specimens are loaded to failure after 24hour cooling time. tests were conducted at various temperatures ($20{\sim}700^{\circ}C$) for concretes made with W/B ratios 46%, 32% and 25%. Test results showed that the relative values of compressive strength and elastic modulus decreased with increasing compressive strength grade of specimen.

• Journal of the Korean earth science society
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• v.24 no.4
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• pp.325-336
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• 2003

The onset time of sea breeze at Haeundae is faster than that at Suyoung in spite of the observation site at Suyoung being 5m and that of Haeundae being 1 km away from the coastline. We therefore simulate the effects of terrain on the onset time of sea breeze at Suyoung and Haeundae districts by using the LCM(Local Circulation Model). This phenomenon is due to the nighttime density flow, which is created by nighttime radiative cooling. It follows the slope of the highlands surrounding the urban area, gathers at a central area of Busan, and then flows out to a lower area like Suyoung river. This process continues after sunrise. In researching the AWS wind speed, we find an important thing. That is to say, the nighttime mean wind speed at Suyoung is three times greater than that at Haeundae. This property shows that Suyoung is an outflow channel of nighttime air mass. The above observed data shows that terrain effect has a important role on the onset of sea breeze.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.761-764
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• 2008

The present study is aimed to study the effect of elevated temperatures ranging from 20 to $700^{\circ}C$ on the strain properties of high-strength concrete of 40, 60, 80MPa grade. In this study, the types of test were the stressed test and stressed residual test that the specimens are subjected to a 25% of ultimate compressive strength at room temperature and sustained during heating and when target temperature is reached, the specimens are loaded to failure. Or specimens are loaded to failure after 24hour cooling time. tests were conducted at various temperatures ($20{\sim}700^{\circ}C$) for concretes made with W/B ratios 46%, 32% and 25%. Test results showed that the relative values of elastic modulus decreased with increasing compressive strength grade of specimen and the axial strain at peak stress were influenced by the load before heating. thermal strain of concrete at high temperature was affected by the preload as well as the compressive strength.

• Proceedings of the KIEE Conference
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• 2000.07c
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• pp.2101-2103
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• 2000

In this paper, it is purpose to develop a pulsed $CO_2$ laser with stable output at pulse repetition rate range of 2 kHz. We used a IGBT as a switching device. The laser cavity was fabricated as an axial and water cooled type. It was used a ring blower to increase a cooling effect. The laser performance characteristics as parameters, such as pulse repetition rate, gas pressure have been investigated. The experiment was done under 3 electrode-type instead of 2 electrode-type. To achieve 3 electrode-type, we used two pulse-transformers which is operated parallel. As a result. the maximum output was about 28 W at the total pressure, of 20 Torr(the gas mixture $CO_2$:$N_2$:He=1:9:15 and the pulse repetition rate of 1300 Hz).

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.4
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• pp.432-436
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• 2015

This study computationally explored the thermal performance of passively-cooled polymer heat sinks utilizing seawater. Polymer heat sinks are proposed as cooling modules of the cold sides of thermoelectric generators for waste heat recovery. 3-D Computational Fluid Dynamics (CFD) modelling was conducted for a detailed numerical study. Polyphenylene sulfide (PPS) and pyrolytic graphite (PG) were selected for the base materials of polymer heat sinks. The computational study evaluated the performance of the PPS and PG heat sinks at various fin numbers and fin thicknesses. Their performances were compared with those of aluminum (Al) and titanium (Ti) heat sinks. The study results showed that the thermal performance of the PG heat sink was 3~4 times better than that of the Ti heat sink. This might be due mainly to the better heat spreading of the PG heat sink than the Ti heat sink. The effect of the number of fins on the performance of the PG heat sink was dissimilar to the cases of the PPS and Ti heat sinks. This result can be explained by the interrelationships among heat spreading, surface area enhancement, and fluidic resistance incorporating with an increase in the number of fins.