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최중환 대한신생아학회 2013 Neonatal medicine Vol.20 No.3
Neonatal intensive care was introduced much later in Korea than in Europe and the United States, where it was introduced during the last decade of the 19th century. During the 1950s–60s, general supportive neonatal care could be provided in nurseries, and during the 1960s–70s, incubator care could be given to preterm infants in premature rooms. In the 1980s, neonatal intensive care could be provided to premature and sick full-term infants in neonatal intensive care units (NICUs)of some major medical centers; the quality of neonatal intensive care improved and the number of NICUs rapidly expanded during the 1990s and 2000s. It can be assumed that the rapid development of neonatal intensive care in Korea may have been induced by the improvement in the Korean socio-economical environment,public demand, numerous study visits and lectures in Korea by distinguished foreign neonatologists, return of native Korean neonatologists trained at famous foreign institutions, and establishment of Korean medical societies (e.g., the Korean Society of Perinatology and the Korean Society of Neonatology). As a result,neonatal and infant mortality rates have dramatically decreased in Korea, especially the mortality rates of low birth weight, very low birth weight, and extremely low birth weight infants. However, despite present and future difficulties in korea,it is essential to develop and promote the more advanced and institutionalized neonatal intensive care with the set-up of regionalization and transportation system at the perinatal centers in cooperation with primary and secondary hospitals, the government, city, and communities.
7050 Al합금의 응력부식저항성에 미치는 2단 시효처리의 영향
최중환,김종기,김헌규,이상래,김인배,Choe, Jung-Hwan,Kim, Jong-Gi,Kim, Heon-Gyu,Lee, Sang-Rae,Kim, In-Bae 한국재료학회 1999 한국재료학회지 Vol.9 No.7
7050 Al합금의 응력부식저항성에 미치는 2단 시효처리의 영향을 미세조직관찰, 전기전도도시험 및 SCF값을 평가함으로써 조사하였다. 최대 경도 이상 과시효에 의해 주 강화상인 η'상이 η상으로 변태되었고, 입내 및 입계에 존재하는 η상의 크기와 간격이 커졌으며 그 결과 응력부식저항성이 증대되었다. 2차 시효시간의 증가에 따라 전기전도도값은 증가되었으나 항복강도의 감소로 인하여 SCF값은 감소되었는데, 이러한 결과는 응력부식저항성의 증가를 의미한다. AMS 4107규격을 기준으로 하였을 때 7050 Al합금 단조재의 적정시효조건은 1차시효가 12$0^{\circ}C$에서 6시간, 2차시효는 $175^{\circ}C$에서 12시간이었다. Effects of two-step aging treatment on the stress corrosion cracking(SCC) resistance of 7050 Al alloy were investigated by transmission electron microscopy, electrical conductivity measurement and stress corrosion facter(SCF) evaluation. It was found that η', principal hardening phase, transformed to η during over aging above maximum hardness, and SCC resistance was improved by increasing of the size and interspacing of η particles in matrix and grain boundary. The electrical conductivity increased with aging time, but SCF decreased due to the decrease of yield strength. This results mean increase of SCC resistance The optimum two-step aging condition in forged 7050 Al alloy was to be first aged at $120^{\circ}C$ for 6h and then finally aged at $175^{\circ}C$ for 12h.
INCONEL 718 초내열 합금의 열처리에 따른 미세조직 변화
최중환,이기룡,조창용,김인배 ( J . H . Choi,K . R . Lee,C . Y . Jo,I . B . Kim ) 한국열처리공학회 1992 熱處理工學會誌 Vol.5 No.2
Microstructural evolution of wrought Inconel 718 superalloy with different heat treatment conditions was studied. Heat treatment was performed via conventional(CHT), modified(MHT), Merrick(MeHT) and modified Merrick(MMeHT) methods. The size of γ` and γ precipitates which are principal strengthening phases in Inconel 718 superalloy increase in order of CHT, MHT, MeHT. For the case of MMeHT, a coexistence of fine γ` precipitate and very coarse particles due to exess growth of γ which is called bimodal distribution, was observed. CHT gave the finest grain size. (Ti, Nb)C carbide and needle-like δ phase were formed together at grain boundaries for CHT, and were formed both inside and at boundaries of grains for MHT, MeHT and MMeHT. Morphology of partially serrated grain boundaries was developed in all heat treatment conditions except CHT.
동적 크리프 컴플라이언스를 이용한 완화시간분포의 근사계산법
최중환,Choi, Joong Hwan 한국섬유공학회 2014 한국섬유공학회지 Vol.51 No.5
The relaxation time spectrum is a very effective parameter for transforming material functions even with a limited amount of data. The spectrum also helps in understanding the relaxation behavior of polymers at the molecular scale. In this study, a relaxation time spectrum based on Marin-Graessley's dynamic compliance model was calculated using Fuosso-Kirkwood's complex analysis. The resulting model could successfully describe the viscoelastic behavior of mono-disperse polystyrene. However, it was clear that a more effective numerical analysis method was needed to obtain more accurate parameter values.
단분산성 고분자 용융체의 완화시간분포의 해석적인 근사계산법
최중환,Choi, Joong Hwan 한국섬유공학회 2014 한국섬유공학회지 Vol.51 No.4
The relaxation time spectrum is a versatile parameter to calculate various polymer viscoelastic properties, such as the relaxation modulus (G(t)), dynamic modulus (G'(${\omega}$)) and loss modulus (G"(${\omega}$)). However, it cannot be measured experimentally. This study calculated the relaxation time spectrum using a calculation method based on Fusso-Kirkwood's theoretical research and using the empirical loss modulus formula. The calculated relaxation time spectrum and Rouse's model could describe the viscoelastic behavior of monodisperse polystyrene successfully. This analytical approximation of the relaxation time spectrum differed from experimental data in the intermediate region between the reputation and Rouse region. This may have resulted from a limitation of the molecular theory in the Rouse region.