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      • KCI등재

        리튬 이온 전지용 리튬 코발트 산화물 양극에서의 삽입 전압과 리튬 이온 전도

        김대현,김대희,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Seo, Hwa-Il,Kim, Yeong-Cheol 한국전기화학회 2010 한국전기화학회지 Vol.13 No.4

        본 연구는 밀도 범함수 이론을 이용하여 Li이온전지에 사용되는 Li코발트 산화물에서의 Li이온 삽입 전압과 전도에 관한 것이다. Li이온은 Li코발트 산화물 원자구조의 각 층을 1개씩 채우거나 한 층을 다 채우고 다음 층을 채울 수 있다. 평균 삽입 전압은 3.48V로 동일하나, 전자가 후자보다 더 유리하였다. 격자상수 c는 Li농도가 0.25보다 작을 때는 증가하였으나, 0.25보다 클 때는 감소하였다. Li농도가 증가하면, Li코발트 산화물에서의 Li이온 전도를 위한 에너지 장벽은 증가하였다. Li이온전지가 방전 중 출력 전압이 낮아지는 현상은 Li농도 증가에 따른 삽입 전압의 감소와 전도 에너지 장벽의 증가로 설명할 수 있었다. We performed a density functional theory study to investigate the intercalation voltage and lithium ion conduction in lithium cobalt oxide for lithium ion battery as a function of the lithium concentration. There were two methods for the intercalation of lithium ions; the intercalation of a lithium ion at a time in the individual layer and the intercalation of lithium ions in all the sites of one layer after all the sites of another layer. The average intercalation voltage was the same value, 3.48 V. However, we found the former method was more favorable than the latter method. The lattice parameter c was increased as the increase of the lithium concentration in the range of x < 0.25 while it was decreased as increase of the lithium concentration in the range of x > 0.25. The energy barrier for the conduction of lithium ion in lithium cobalt oxide was increased as the lithium concentration was increased. We demonstrated that the decrease of the intercalation voltage and increase of the energy barrier as the increase of the lithium concentration caused lower output voltage during the discharge of the lithium ion battery.

      • KCI등재

        DEMS와 H-terminated Si (001) 표면의 상호작용: 제일원리연구

        김대현,김대희,박소연,서화일,이도형,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Park, So-Yeon,Seo, Hwa-Il,Lee, Do-Hyoung,Kim, Yeong-Cheol 한국세라믹학회 2009 한국세라믹학회지 Vol.46 No.4

        We performed a density functional theory study to investigate the interaction of DEMS (diethoxymethylsilane) with the H-terminated Si (001) surface. The optimum structure of DEMS was first calculated by a first principles study. The dissociation probability of the O-C bond of DEMS was higher than the other seven bonds based on the bond energy calculation. When the fragmented DEMS groups reacted with the H-terminated Si (001) surface, it was the most favorable among the eight reactions to form a bond between the Si atom on the surface and the O atom of a fragmented DEMS group (($C_2H_5O$)Si($CH_3$)(H)-O-) by forming a $C_2H_6$ as by-product.

      • KCI등재

        아파트 단지 옥외공간 변천 및 요인 연구

        김대현,김대수,신지훈,김순분,Kim, Dae-Hyun,Kim, Dae-Soo,Sin, Ji-Hoon,Kim, Soon-Boon 한국조경학회 2005 韓國造景學會誌 Vol.32 No.6

        The purposes of this study were to classify the changing periods of apartment outdoor space in Korea and to clearly understand the specific characteristics of each period. The study has been done through on-site observation of apartment complexes, and through the analysis of housing estate planning techniques, rules and regulations affecting outdoor spaces during the rapidly developing era. The results of this study shows that three major periods with distinct characteristics were found, which were as follows: 1) The first well defined period spans from 1960 to 1990. In this period, apartment construction was rapid, and as a result, quality of housing environments were sacrificed. As little attention was given to the landscapes in apartment complex, they were constructed very simple and identical in outdoor space of apartment complex. This period can be referred to as the 'commonly universal period in apartment outdoor space'. 2) The second period spans from 1991 to 1996, during which time a housing construction project for two million residences was completed in order to significantly stimulate the provision of houses to the general public. As a result, regulations were relaxed causing housing environments to deteriorate. Although apartments were constructed very close together, there was some diversification and experimentation in outdoor styles. This period can be referred to as the 'congestion period of apartment outdoor space'. 3) The third period spans from 1997 to the present. During this time the number of unsold apartments has grown considerably throughout the country. Many different apartment complex marketing strategies have been employed to reduced the glut of unsold apartments. This experimental trend has continued and been well-received since the IMF period. The distinguishing characteristics of this period are the introduction of a rich natural landscape, the increase in communications among residents, the human oriented design, and the refined and elegant design for the outdoor street furnitures. As a result, this period can be named the "differentiation period of apartment outdoor space".

      • KCI등재

        $HfCl_4$와 Si (001) 표면에 결합된 두 개의 수산화기와의 상호작용: 제일원리 연구

        김대현,김대희,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Seo, Hwa-Il,Kim, Yeong-Cheol 한국반도체디스플레이기술학회 2009 반도체디스플레이기술학회지 Vol.8 No.2

        Density functional theory was used to investigate the adsorption and reaction of $HfCl_4$ with two hydroxyls on Si (001)-$2{\times}1$ surface in atomic layer deposition (ALD) process. We prepared a reasonable Si substrate which consisted of six inter-dimer dissociated $H_2O$ molecules and two intra-dimer dissociated $H_2O$ molecules. The $HfCl_4$must react with two hydroxyls to be a bulk-like structure. When $HfCl_4$ was adsorbed on a hydroxyl, there was energy benefit of -0.55 eV. Though there was energy loss for $HfCl_4$ to react with H of hydroxyl, thermal energy of ALD chamber would be enough to pass the energy barriers. There were five reaction pathways for $HfCl_4$ to react with two hydroxyls; inter-dimer, intra-dimer, cross-dimer, inter-row, and cross-row. Inter-row, inter-dimer and intra-dimer were relatively favorable among the five reaction pathways based on the energy difference. The electron densities between O and Hf in these three reactions were higher than the others and they had shorter Hf-O and O-O bond lengths than the other two reaction pathways.

      • SCOPUSKCI등재

        비화학양론적 Na<sup>+</sup>β-alumina를 위한 Mg 원자의 치환: 제일원리 계산

        김대현,김대희,정용찬,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Jeong, Yong-Chan,Seo, Hwa-Il,Kim, Yeong-Cheol 한국재료학회 2010 한국재료학회지 Vol.20 No.2

        $Na^+$ ion conductivity can be improved by the substitution of an Mg atom for an Al atom to form a nonstoichiometric $Na^+$ $\beta$-alumina. We performed a first principles study to investigate the most stable substitution site of an Mg atom and the resulting structural change of the nonstoichiometric $Na^+$ $\beta$-alumina. Al atoms were classified as four different layers in the spinel block that are separated by conduction planes in the nonstoichiometric $Na^+$ $\beta$-alumina. The substitution of an Mg atom for an Al atom at a tetragonal site was more favorable than that at an octahedral site. The substitution in the spinel block was more favorable than that close to the conduction plane. This result was well explained by the volume changes of the polyhedrons, by the standard deviation of the Mg-O distance, and by the comparison with bulk MgO structure. Our result indicates that the most preferable site for the Mg atom was the tetrahedral site at the spinel block in the nonstoichiometric $Na^+$ $\beta$-alumina.

      • KCI등재

        Density Functional Theory를 이용한 Si (001) 표면 위의 In 나노선 성장 연구

        김대현,김대희,서화일,김영철,Kim, Dae-Hyun,Kim, Dae-Hee,Seo, Hwa-Il,Kim, Yeong-Cheol 한국재료학회 2009 한국재료학회지 Vol.19 No.3

        Density functional theory was utilized to investigate the growth of an indium nanowire on a Si (001) buckled surface. A site between the edge of two Si dimers is most favorable when the first In atom is adsorbed on the surface at an adsorption energy level of 2.26 eV. The energy barriers for migration from other sites to the most favorable site are low. When the second In atom is adsorbed next to the first In atom to form an In dimer perpendicular to the Si dimer row, the adsorption energy is the highest among all adsorption sites. The third In atom prefers either of the sites next to the In dimer along the In dimer direction. The fourth In atom exhibited the same tendency showed by the second atom. The second and fourth In adsorption energy levels are higher than the first and third levels as the In atoms consume the third valence electron by forming In dimers. Therefore, the In nanowire grows perpendicular to the Si dimer row on the Si (001) surface, as it satisfies the bonding of the three valence electrons of the In atoms.

      • KCI등재후보

        6.25 전쟁 당시 외상을 경험한 노인의 후유증상에 관한 예비연구

        김대현,류성곤,김호찬,연병길,한창환,Kim, Dae-Hyun,Ryu, Seong-Gon,Kim, Ho-Chan,Yeun, Byung-Kil,Han, Chang-Whan 대한불안의학회 2009 대한불안의학회지 Vol.5 No.2

        Objectives : This pilot study examined the physical and psychological sequela of Korean War victims. Methods : Of 255 elderly who completed the Mini International Neuropsychiatric Interview (MINI), we selected 16 subjects who suffered severe traumatic experiences during the Korean War and met more than 2 specified symptoms of PTSD in DSM-IV-TR and 16 controls. Demographic characteristics, medical history including hypertension, diabetes mellitus, hypercholesterolemia and brain infarction, Geriatric Depression Scale, State-Trait Anxiety Inventory, suicide scale in MINI, and a scale for the assessment of somatic symptoms were compared between subjects with trauma experience and controls. Results : Subjects with trauma experience were more likely to have brain infarction and diabetes mellitus compared to control group. Depressive symptoms and suicidal risk were significantly higher in subjects with trauma experience than controls. Subjects with trauma experience showed significantly higher scores in Trait Anxiety scale and more complained of pain than controls. Conclusion : This study suggests that traumatic experience probably induce physical and psychological problem even 60 years later. Further studies are needed to confirm the results of this pilot study.

      • Copy Milling(Celay) System을 이용한 All Ceramic Bridge 임상 증례

        김대현,Kim, Dae-Hyun 대한심미치과학회 2000 Journal of the Korean Academy of Esthetic Dentistr Vol.9 No.1

        치과 재료분야는 그 동안 많은 발전을 해왔음에도 불구하고 자연치아의 색조 재현 및, 인접 치와의 조화에 따른 심미성과 지대치와의 적합성 등에 많은 문제가 야기되어왔다. 기존 all ceramic 은 단관 제작 시 심미적인 요구가 다소 해결되었으나 bridge work 에 있어서는 그 강도 및, 작업의 효율성은 여전히 문제시되어 오고 있다. 최근 기존 Inceram의 단점을 보안한 Celay Inceram system은 spinell, alumina, zirconia Blanks를 이용하여 더욱더 기능적인 자연스러움으로 접근되어진다. 이에 그 system의 alumina blank를 이용한 bridge work 의 임상 증례를 살펴보기로 한다. In esthetic dentistry, color and strength are basic requirements for the long-term success of the restorative materials. Several all ceramic systems have been introduced to esthetic dentistry recently. However, the inherent natures of ceramic material, the application of all ceramic system is mainly limited to single tooth restorations. With the improvement of material science, the alumina and zirconia/alumina composite power and block can be applied to fabrication of all ceramic bridges. The conventional inceram core fabrication takes time for sintering however, the shaping of block with a copy milling machine can reduce great amount of time. The block is easy to manipulate and prepare in any shape accurately. This clinical report demonstrates the application of all ceramic ante rior 3 unit bridge with a alumina block in CELAY system.

      • 공간정보를 활용한 격자체계 개선방안

        김대현,김재명,윤병찬,장은미,최윤수,Kim, Dae Hyun,Kim, Jae Myeong,Yoon, Byung Chan,Chang, Eun Mi,Choi, Yun Soo 한국공간정보학회 2015 한국공간정보학회지 Vol.23 No.6

        현재 우리나라는 기관별 업무활용을 위해 각자의 격자체계에 따라 자료가 구축되어 있어 상호 연계가 되지 않고, 자료 활용 및 접근성의 한계 등의 문제점이 발생하고 있다. 본 연구에서는 이러한 문제점을 해결하기 위해 국가적인 차원에서의 국가격자체계와 격자데이터를 표준안을 검토하고, 장단점을 파악하고자 하였다. 국내외 격자체계 활용현황을 조사하고 격자데이터 활용문제점 개선을 위해 관계기관 및 관련분야 연구자들을 대상으로 수요조사를 실시하였다. 그 결과 격자데이터를 필요로 하는 사용자가 쉽게 접근가능하고, 격자데이터의 공유방안과 상호호환을 위한 데이터 제공형태 및 방법도출 등 국가격자데이터 통합관리체계를 위한 국가격자체계안을 제시하였다. Currently, each organization has developed its gridded data in the form of respective grid system for its own purpose. Interoperability among the organization had been limited and resulted in inconvenient data access and application across domains. In this study, we investigated potential standards for National grid system and their strength and weakness. We also reviewed existing gridding schemes and had a survey of demand about grid system to those who have used or would plan to use gridded data in academic and business sectors. As the result of survey of demand, we suggested national grid system for national grid data integration management system which has the mutual compatibility and also proposed sharing scheme of the grid type for users who need to grid data.

      • KCI등재

        Cu-Fe계 합금의 강도 및 전기전도도에 미치는 Cr 원소첨가의 영향

        김대현,이광학,Kim, Dae-Hyun,Lee, Kwang-Hak 한국재료학회 2010 한국재료학회지 Vol.20 No.2

        This study looked at high performance copper-based alloys as LED lead frame materials with higher electrical-conductivity and the maintenance of superior tensile strength. This study investigated the effects on the tensile strength, electrical conductivity, thermal softening, size and distribution of the precipitation phases when Cr was added in Cu-Fe alloy in order to satisfy characteristics for LED Lead Frame material. Strips of the alloys were produced by casting and then properly treated to achieve a thickness of 0.25 mm by hot-rolling, scalping, and cold-rolling; mechanical properties such as tensile strength, hardness and electrical-conductivity were determined and compared. To determine precipitates in alloy that affect hardness and electrical-conductivity, electron microscope testing was also performed. Cr showed the effect of precipitation hardened with a $Cr_3Si$ precipitation phase. As a result of this experiment, appropriate aging temperature and time have been determined and we have developed a copper-based alloy with high tensile strength and electrical-conductivity. This alloy has the possibility for use as a substitution material for the LED Lead Frame of Cu alloy.

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