• 열처리공학회지
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• 제17권4호
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• pp.230-235
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• 2004

Present work was carried out to investigate the influence of welded conditions, such as welding current, diameter of welding wire on the microstructural change and mechanical properties of TIG welded joint in AZ31B Mg alloy. It was found that good and sound welded joint was achieved in all welding conditions. The grain size decreased with increasing welding current and decreasing diameter of welding wire. Also, the second phases were homogeneously distributed in the grain and grain boundary as decreasing welding current and diameter of welding wire. The ${\beta}$ discontinuos precipitates were observed in the welded joint, but this microstructure has not been reported by previous researchs in AZ31B Mg alloy. The hardness value is affected by the existence state of the second phase and the hardness of the welded joint region is lower than the other regions in welded AZ31B Mg alloy. The strength of the welded joint region was influenced by the grain size and has more than 90%, compared to that of ASTM standard specification.

• 한국자기학회지
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• 제5권5호
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• pp.716-719
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• 1995

Pd/(Pt/Co/Pt) modulated multilayer films have been deposited on various substrates with Pd/Pt buffer layers. Films grown at different temperatures have very distinct magnetic properties and surface microstructures. Atomic force(AFM) and scanning tunneling (STM) microscopies studies of these films reveal that films deposited at room temperature have small grain structures with an average grain size of about $140\;{\AA}$. However, much larger grains of about $1200\;{\AA}$ in size are observed in the films grown on buffer layers which were deposited at $500^{\circ}C$. These large grains are found to actually consist of smaller grains of about $170{\AA}$ in diameter. SQUID magnetic and Kerr hysteresis loop measurements indicate that multilayer films with large grains exhibit high magnetic coercivities of around 5 kOe. A subgrain growth model is proposed to understand the observed grain structures in the multilayers.

• Journal of Ceramic Processing Research
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• 제20권1호
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• pp.59-62
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• 2019

Y2O3-MgO nanocomposites are promising materials for hypersonic infrared windows and domes due to their excellent midIR transmittance and mechanical properties. In this work, influence of SPS heating rate on the microstructure, IR transmittance, and mechanical properties of Y2O3-MgO nanocomposites was investigated. It was found that the average grain size decreases with a decreasing heating rate, which can be attributed to high defect concentration by rapid heating and deformation during densification. Also, the residual porosity decreases with a decreasing heating rate, which is ascribed to the enhancement of grain boundary diffusion by a large grain-boundary area (a small grain size). Consequently, high transmittance and hardness were attained by the low heating rate. On the other hand, the mechanical strength showed little difference with the heating rate change, which is somewhat different from the general knowledge on ceramics and will be discussed in this letter.

• 한국재료학회지
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• 제33권4호
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• pp.135-141
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• 2023

The volatilization of alkali ions in (K,Na)NbO₃ (KNN) ceramics was inhibited by doping them with alkaline earth metal ions. In addition, the grain growth behavior changed significantly as the sintering duration (t_s) increased. At 1,100 ℃, the volatilization of alkali ions in KNN ceramics was more suppressed when doped with alkaline earth metal ions with smaller ionic size. A Ca²⁺-doped KNN specimen with the least alkali ion volatilization exhibited a microstructure in which grain growth was completely suppressed, even under long-term sintering for t_s = 30 h. The grain growth in Sr²⁺-doped and Ba²⁺-doped KNN specimens was suppressed until t_s = 10 h. However, at t_s = 30 h, a heterogeneous microstructure with abnormal grains and small-sized matrix grains was observed. The size and number of abnormal grains and size distribution of matrix grains were considerably different between the Sr²⁺-doped and Ba²⁺-doped specimens. This microstructural diversity in KNN ceramics could be explained in terms of the crystal growth driving force required for two-dimensional nucleation, which was directly related to the number of vacancies in the material.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2007년도 제28회 춘계학술대회논문집
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• pp.374-377
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• 2007

본 보고서는 소형 로켓 제작에 앞서 행해지는 설계 단계를 기술하였다. 각 요구 조건에 맞춰 사용되어질 추력, 추진제 특성, mass distribution, 동체 형상 설계, Motor 설계에 대해 다뤘다. 로켓 설계에 있어서 가장 중요한 추진제 선정과 노즐형상 설계를 중점적으로 다루었으며, 차후 본 과정에 의하여 소형로켓을 제작하겠다.

• 한국농공학회지
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• 제16권1호
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• pp.3263-3291
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• 1974

Experimental work of grain bin was carried out to develop the methods of natural air in-bin drying and storage. The method is considered to be more economical, labour saving, and an effective countermeasure to grain loss. To examine the possibility of farm use of the grain bin and to analyze the related factors concerned with in-bin grain drying and storage, ambient air conditions (especially the change of air temperature and relative humidity) and grain quality during drying and storage periods were investigated. A laboratory model bin was constructed to investigate the effect of different forced air conditions on the drying characteristics of rice. In addition, a grain bin with 2.2m diameter and 1.8m height, considered to be the optimum size for the average Korean farm, was constructed and tested to examine the drying and storing characteristics of rice. The weather data analyzed in this study was the nine-year (from 1964 to 1972) record of air temperature and relative humidity in the Suweon area, and the thirty-year (from 1931 to 1960) record of pentad normal relative humidity and air temperature in the Seoul area. From the results of the weather data analyses, the adequate air delivery hours (which was arbitrary defined as the condition to give less than 75% relative humidity) to dry the rice during October were about nine hours (from approximately 10 A.M. to 7 P.M, ) a day, in which the average air temperature was about 15.9$^{\circ}C$ and average relative humidity was 66%. The occurence of days having three hours of such conditions was 1, 2, and 1-day within the 1st, 2nd add last 10-day periods for the month of October, respectively. Therefore, it may be considered that the weather condition in October was satisfactory for the forced natural air drying. The results of the laboratory model bin test were analyzed to obtain the drying curve and drying rate for different drying stages and grain layers in the bin corresponding to various conditions of forced natural air. A drying experiment with a prototype grain bin showed that an approximate 5 percent grain moisture gradient through a 1.6 meter grain deposit was observed after 80 hours of intermittent drying, giving an over dried zone in the lower grain layers and an extremely high grain moisture zone in the upper layers. This indicates that an effective measure should be taken to reduce this high moisture gradient. In order to investigate the drying characteristics of bulk grain in a layerturning operation a grain bin test was performed. This showed a significant improvement of uniform drying. In this test, approximate 107 hours were required to dry a depth of 1.6 meter of grain from an initial moisture content of 22.2 percent to a moisture content of 16.7 percent using an air delivery rate of 2.8 cubic meter per a minute per every cubic meter of grain. This resulted in a 2 percent moisture gradient from the top to the bottom of the bin. During storage period, till the end of June the average temperature of grain was 2~3$^{\circ}C$ higher than ambient air temperature. But during July when the grain moisture content went up slightly (less than 1 percent), the average temperature of the grain also increased to 3~5$^{\circ}C$ higher than ambient air temperature. It is therefore recommended that for safe grain storage, grain should not be stored in sheet metal bins after mid May. From the above results, in-bin rice drying and storage can be used effectively on Korean farms. It is strongly recommended that the use of grain-bin system should be implemented for farm use to improve farm drying and storage of rice.

• 한국지반환경공학회 논문집
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• 제2권3호
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• pp.5-15
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• 2001

새만금방조제 축조 전 후 금강하구의 조류 변화와 잔차류특성, 금강의 부유사 유입에 의한 퇴적 과정을 알아보았다. 잔차류는 새만금방조제 축조 전후 금강 하구역의 조석잔차류와 관측된 수온 염분, 바람자료를 이용하여 계산하였다. 계산에 의한 퇴적과정은 잔차류와 입자의 침강속도를 고려하여 Euler Lagrange 방법으로 부유사의 분산 퇴적과정을 3차원 수치 실험에 의하여 재현하였다. 토립자(土粒子)의 중앙입경은 $150{\mu}m$, $300{\mu}m$, $400{\mu}m$의 경우에 대하여 적용하였다. 토립자(土粒子)의 분포사항은 금강하구둑에서 토립자(土粒子) 방류후 24시간과 72시간에 대하여 알아보았다. 그 결과 봄에 관측한 수온 염분, 바람, 조석응력을 이용하여 계산한 잔차류의 특성은 상층에서 만경강과 동진강 하구역에서 시계방향의 흐름분포를 나타내었다. 그리고 하구특성의 하나인 밀도류에 의한 흐름은 하구 쪽 방향으로 아주 작은 분포를 하고 있었다. 부유사의 분산 퇴적과정을 수치실험 한 결과 24시간후의 퇴적분포는 대부분 금강하구와 장항해역에서 퇴적분포를 나타내고 있었으나 72시간 후에는 금강하구와 장항해역, 만경강과 동진강하구까지 퇴적분포를 나타내었다.

• 한국주조공학회지
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• 제29권6호
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• pp.270-276
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• 2009

After casting button-type small ingots of ternary Fe-Mn-S alloys which had three different Mn/S ratios (1, 5 and 70) in a vacuum arc furnace, the effect of the ratio on the sulfide formation was investigated. In case of the Mn/S ratio of 1, if alloy composition was located in an iron-rich corner on a Fe-Mn-S ternary phase diagram, only duplex MnS-FeS sulfide films were observed in the grain boundary. If the alloy composition was located in the miscibility gap area of the phase diagram, primary globular dendritic sulfides and dendritic sulfide slags were generated within the grain and tubular monotectic sulfides were also detected in the grain boundary. When the Mn/S ratio was 5, if the alloy composition was in the iron-rich corner, only bead-like sulfides were generated. On the other hand, if the composition was in the miscibility gap area, globular dendritic sulfides and dendritic sulfide slags were generated in the form of primary sulfide inclusions and rod-like eutectic sulfides were observed in the grain boundary. Especially, if the contents of Mn and S increased more in the miscibility gap area of the phase diagram, primary globular sulfides containing iron intrusions were observed. In case of Mn/S ratio of 70, if the contents of Mn and S was decreased in the Fe corner of the phase diagram, only bead-like sulfides were observed in the grain boundary. Despite the composition was outside the miscibility gap area of the phase diagram, if the contents of Mn and S increased, clusters of fine sulfide particles as well as fine spherical primary monophase sulfides were observed in the grain boundary.

• 한국작물학회지
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• 제28권2호
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• pp.211-215
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• 1983

수집된 재래의 유색대두종을 일률적인 재배조건하에서 종실특성 변이와 종실종군별 생육진전 및 수량형질 특성을 조사한 결과 다음의 결론을 얻었다. 1. 유색대두 수집종들은 100립중을 기준으로 하여 20gr 이하의 소립중, 20-25gr의 중립중, 25-30gr의 대립종 및 30gr 이상의 특대립종 등 4개군으로 나눌 수 있었다. 2. 종실크기에 관련된 요인중 종실장과 종실폭의 변이는 종실후보다 크게 나타났다. 3. 종실중과 경중 및 주당협수 등의 형질들은 변이계수에서 생육일수나 결과일수 등의 형질보다 큰 경향이었다. 4. 생육일수와 성숙일수 및 개화시기는 립중군별간에 유의한 차이를 나타내지 않았다. 5. 개화기와 성숙일수는 경장 및 경중과 정의 상관관계를, 경장과 경중은 수량형질과 정의 상관을 나타내었으며, 주당협수는 100립중과 부의 상관을, 또한 수량과는 정의 상관관계를 나타내었다.

• Journal of Biosystems Engineering
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• 제9권2호
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• pp.65-73
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• 1984

While most of researches on the performance of high temperature grain dryer have dealt mainly with improving dryer capacity and drying speed during the last twenty years, energy efficiency, in fact, has not been emphasized. Current fuel supplies and energy cost have shifted the emphasis to reducing the energy consumption for grain drying while maintaining dryer capacity and grain quality. Since the energy input for drying is relatively large, the recovery and reuse of at least part of the exhaust energy can significantly reduce the total energy consumption in existing drying systems. Unilization of exhaust heat in grain dryer either through direct recycling or by a thermal coupling in heat exchanger have been subject of a number of investigators. However, very seldom research in Korea has been done in this area. Three drying tests(non-recycling, 0.22 recycle ratio, and 0.76 recycle ratio)were performed to investigate the thermal efficiency and heat loss factors of continuous flow type dryer, and to analyze the effect of recycle ratio (weight of exhaust air recycled/total weight of input air) on the energy requriements for rough rice drying. The test results showed that when the exhaust air was not recycled, the energy lost from furnace was 15.3 percent of input fuel energy, and latent and sensible heat of exhaust air were 61.4 percent and 11.2 percent respectively. The heat which was required in raising grain temperature and stored in dryer was relatively small. As the recycle ratio of exhaust air was increased, the drying rate was suddenly decreased, and thermal efficiency of the kerosene burner was also decreased. Drying test with 0.76 recycle ratio resulted in 12.4% increase in fuel consumption, and 38.4% increase in electric power consumption as compared to the non-recycled drying test. Drying test of 0.22 recycle ratio resulted in 6.8% saving in total energy consumption, 8.0% reduction in fuel consumption, and 2.5% increase in electric power consumption as compared to the non-recycled drying test.