• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.17 no.6
• /
• pp.580-588
• /
• 1992

Plasma-spraying was conducted to produced the microwave absorbing surface films on the alumi-num-alloy used for the fuselage to protect the aircraft against the RADAR detction. The surface films were produced by plasma-splaying the mecharucally mixed composite powders of the silicon carblde and Ni-Zn ferrite. This M /W absorbers were designed experimentally and fabricated trialty, as a result of which the rolative frequency bandwidth of 7.6 to 8.4% were obtained under the tolerance limits of the re-flection coefficients lower than -6dB(absorption ratio 75%), and the maximum absorption thickness becomes 0.5 to 0.5.imm, which Is much thinner than that of the conventional ones.

• Korean Journal of Materials Research
• /
• v.27 no.8
• /
• pp.438-444
• /
• 2017

In this work, the effects of hydrogen reduction on the microstructure and thermoelectric properties of $(GeTe)_{0.85}(AgSbTe_2)_{0.15}$ (TAGS-85) were studied by a combination of gas atomization and spark plasma sintering. The crystal structure and microstructure of TAGS-85 were characterized by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The oxygen content of both powders and bulk samples were found to decrease with increasing reduction temperature. The grain size gradually increased with increasing reduction temperature due to adhesion of fine grains in a temperature range of 350 to $450^{\circ}C$. The electrical resistivity was found to increase with reduction temperature due to a decrease in carrier concentration. The Seebeck coefficient decreased with increasing reduction temperature and was in good agreement with the carrier concentration and carrier mobility. The maximum power factor, $3.3{\times}10^{-3}W/mK^2$, was measured for the non-reduction bulk TAGS-85 at $450^{\circ}C$.

• Proceedings of the Korean Institute of Building Construction Conference
• /
• 2013.11a
• /
• pp.68-71
• /
• 2013

Reactive Powder Concrete (RPC) is an ultra high strength and high ductility cement-based composite material and has shown some promise as a new generation concrete in construction field. It is characterized by a silica fume-cement mixture with very low water-binder (w/b) ratio and very dense microstructure, which is formed using various powders such as cement, silica fume and very fine quartz sand (0.15~0.4mm) instead of ordinary coarse aggregate. However, the unit weight of cement in RPC is as high as 900~1,000 kg/㎥ due to the use of very fine sand instead of coarse aggregate, and a large volume of relatively expensive silica fume as a high reactivity pozzolan is also used, which is not produced in Korea and thus must be imported. Since the density of RPC has a heavy weight at 2.5~3.0 g/㎤. In this study, the modified RPC was made by the combination of ternary pozzolanic materials such as blast furnace slag and fly ash, silica fume in order to economically and practically feasible for Korea's situation. The fire resistance and structural behavior of the modified RPC exposed to high temperature were investigated.

• Korean journal of food and cookery science
• /
• v.32 no.1
• /
• pp.9-15
• /
• 2016

The changes of resistant starch (RS) contents of cooked rice with soybean and coconut oils under different storage conditions were investigated and RS contents were compared between the rice and cooker types. The japonica (Hopyeong) and the indica (Thailand) type rice were cooked (washed rice: water = 100: 130) using an electric cooker and a saucepan. The coconut oil and soybean oil (3%, based on rice, w/w) were added into cooking water before heating. The RS contents of freeze-dried cooked rice powders (newly-cooked rice, stored for 12 h in the refrigerator, microwave heating after storage for 12 h in the refrigerator) were measured by the AOAC method. The RS contents of cooked rice using a saucepan were higher than those using an electric cooker. The indica type cooked rice had a higher RS content than the japonica type cooked rice, regardless of storage conditions. However, addition of oil before cooking rice resulted in increased RS content on storage in the refrigerator. The highest RS content of the cooked indica type rice with soybean oil ($5.89{\pm}0.22%$) that was stored for 12 h in the refrigerator was analyzed. The results suggested that the cooked rice formed retrograded (RS3) and amylose-lipid complex (RS5) type RS; furthermore, the RS content is affected by storage conditions, rice, cooker and oil types.

• Korean journal of food and cookery science
• /
• v.22 no.6 s.96
• /
• pp.905-913
• /
• 2006

The properties of water soluble powder of propolis(WSP), made with different levels(0, 20, 40, 60, 80%) of ethanol extract of propolis(EEP) and hydrocolloid were investigated, along with the quality changes of its bread after 7 days' of storage at $30^{\circ}C$ The yield of WSP containing 40% EEP treated at $160^{\circ}C$ was the highest at 59.3% and the brown color of all the powders tended to be darkened with increasing EEP content. The turbidity of WSP treated at higher temperature was decreased in its aqueous solution (10%, w/w), and this was considered to be due to the presence of minute nonsoluble particles. Antioxidative activities determined by DPPH(1,1-diphenyl-2-picrylhydrazyl) were the lowest in WSP treated at $140^{\circ}C$, while those of the WSP samples prepared at 160 and $180^{\circ}C$ were as high as that of WSP containing more than 40% EEP, regardless of EEP concentration. The propolis breads with added WSP made at $160^{\circ}C$ were selected as the most desirable powder for subsequent study. Bread with WSP40 was the heaviest while the volume loss of WSP80 was the greast after baking. The moisture contents of the propolis bread were drastically decreased until 3 days' of storage, but it was thought that WSP might be ineffective for the prevention of moisture loss. The pH of breads without EEP was decreased after 3 days' of storage, while that of the WSP breads remained almost unchanged until 5 days' of storage. Total bacterial counts also exhibited decay levels during the storage. In conclusion, water soluble powder of propolis is useful as a natural antioxidative and antibacterial material in various types of food.

• Journal of Powder Materials
• /
• v.24 no.3
• /
• pp.195-201
• /
• 2017

In this study, H13 tool steel sculptures are built by a metal 3D printing process at various laser scan speeds. The properties of commercial H13 tool steel powders are confirmed for the metal 3D printing process used: powder bed fusion (PBF), which is a selective laser melting (SLM) process. Commercial H13 powder has an excellent flowability of 16.68 s/50 g with a Hausner ratio of 1.25 and a density of $7.68g/cm^3$. The sculptures are built with dimensions of $10{\times}10{\times}10mm^3$ in size using commercial H13 tool steel powder. The density measured by the Archimedes method is $7.64g/cm^3$, similar to the powder density of $7.68g/cm^3$. The hardness is measured by Rockwell hardness equipment 5 times to obtain a mean value of 54.28 HRC. The optimum process conditions in order to build the sculptures are a laser power of 90 W, a layer thickness of $25{\mu}m$, an overlap of 30%, and a laser scan speed of 200 mm/s.

• Transactions of Materials Processing
• /
• v.27 no.5
• /
• pp.314-322
• /
• 2018

AlSi12 is a heat-resistant aluminum alloy that is lightweight, corrosion-resistant, machinable and attracting attention as a functional material in aerospace and automotive industries. For that reason, AlSi12 powder has been used for high performance parts through 3D printing technology. The purpose of this study is to observe deposition characteristics of AlSi12 powder in a direct energy deposition (DED) process (one of the metal 3D printing technologies). In this study, deposition characteristics were investigated according to various process parameters such as laser power, powder feed rate, scan speed, and slicing layer thickness. In the single track deposition experiment, an irregular bead shape and balling or humping of molten metal were formed below a laser power of 1,000 W, and the good-shaped bead was obtained at 1.0 g/min powder feed rate. Similar results were observed in multi-layer deposition. Observation of deposited height after multi-layer deposition revealed that over-deposition occurred at all conditions. To prevent over-deposition, slicing layer thickness was experimentally determined at given conditions. From these results, this study presented practical conditions for good surface quality and accurate geometry of deposits.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2002.04a
• /
• pp.17-20
• /
• 2002

The effects of relatively low lead content (0.2~0.3) on the 2223 phase formation and transport property of Bi-2223/Ag tapes have been investigated. The results show that lead contents have great impact on the phase assemblage of precursors, subsequently, on the phase formation and transport property of silver sheathed BSCCO-2223 tapes. Powders containing Pb=0.25 and Pb=0.3 resulted in the nearly identical $J_c$ values in fully processed tapes, but leaded to significant difference on the phase formation process. For the case of Pb=0.2, both low conversion fraction of 2212 to 2223 and low $J_c$ value were obtained in final reacted tape, which was probably due to lack of enough liquid phase to facilitate the phase transformation.

• Preventive Nutrition and Food Science
• /
• v.20 no.3
• /
• pp.215-220
• /
• 2015

The moisture sorption isotherms of vacuum-dried edible green alga (Capsosiphon fulvescens) powders were determined at 25, 35, and $45^{\circ}C$ and water activity ($a_w$) in the range of 0.11~0.94. An inversion effect of temperature was found at high water activity (>0.75). Various mathematical models were fitted to the experimental data, and Brunauer, Emmett, and Teller model was found to be the most suitable model describing the relationship between equilibrium moisture content and water activity (<0.45). Henderson model could also provide excellent agreement between the experimental and predicted values despite of the intersection point. Net isosteric heat of adsorption decreased from 15.77 to 9.08 kJ/mol with an increase in equilibrium moisture content from 0.055 to $0.090kg\;H_2O/kg$ solids. The isokinetic temperature ($T_{\beta}$) was 434.79 K, at which all the adsorption reactions took place at the same rate. The enthalpy-entropy compensation suggested that the mechanism of the adsorption process was shown to be enthalpy-driven.

• Journal of Powder Materials
• /
• v.23 no.5
• /
• pp.364-371
• /
• 2016

This study investigates the thermal shock property of a polycrystalline diamond compact (PDC) produced by a high-pressure, high-temperature (HPHT) sintering process. Three kinds of PDCs are manufactured by the HPHT sintering process using different particle sizes of the initial diamond powders: $8-16{\mu}m$ ($D50=4.3{\mu}m$), $10-20{\mu}m$ ($D50=6.92{\mu}m$), and $12-22{\mu}m$ ($D50=8.94{\mu}m$). The microstructure observation results for the manufactured PDCs reveal that elemental Co and W are present along the interface of the diamond particles. The fractions of Co and WC in the PDC increase as the initial particle size decreases. The manufactured PDCs are subjected to thermal shock tests at two temperatures of $780^{\circ}C$ and $830^{\circ}C$. The results reveal that the PDC with a smaller particle size of diamond easily produces microscale thermal cracks. This is mainly because of the abundant presence of Co and WC phases along the diamond interface and the easy formation of Co-based (CoO, $Co_3O_4$) and W-based ($WO_2$) oxides in the PDC using smaller diamond particles. The microstructural factors for controlling the thermal shock property of PDC material are also discussed.