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.

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.

$HfCl_4$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.

비화학양론적 Na⁺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.

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.

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".

Artsim'을 이용한 모음의 조음점 추정에 관한 연구

김대현,조철우,Kim Dae-Ryun,Cho Cheol-Woo 대한음성학회 1998 말소리 Vol.35 No.-

In this paper, articulatory simulator 'Artsim' is used as a tool for the experiments to examine the articulatory characteristics of 6 different vowels. Each vowels are defined by some articulatory points from their vocal tract area functions and shapes of tongues. Each points are varied systematically to synthesize vowels and the synthesized sound is evaluated by human listners. Finally distributions of each vowels within vowel space is obtained. From the experimental results it is verified that our articulatory simulator can be used effectively to investigate the articulatory characteristics of speech.

송엽(松葉)이 고지방식이(高脂肪食餌)로 유발된 백서(白鼠)의 지방(脂肪)과 혈청지질(血淸脂質)에 미치는 영향(影響)

김대현,소경순,Kim, Dae-Hyun,Soh, Kyung-Sun 대한약침학회 2007 Journal of pharmacopuncture Vol.10 No.1

In order to study the effects of Pinus densiflora on hyperlipidemia and lipid in rats, we divided the rats into groups(Normal group, Control group and Sample group) and perfomed the experimental research. Hyperlipidemia and lipid in rats were induced by high fat diets for 8weeks. The sample group was administerd the extract of Pinus densiflora for 14 days and control group was administerd equal dose of oral. And then we measured the amount of serum triglyceride, Total cholesterol, LDL-cholesterol, HDL-cholesterol, Free Fatty Acid, phospholipid, Insuline, Laptin, Body weight, epididymis fat weight & rate, epididymis fat cell, Cardiac Risk Factor(CRF). The results were as followers : 1. Sample Group showed decreasing effects on Total cholesterol, Trigylceride, LDL-cholesterol, and Phospholipid levels in serum and CRF significantly. 2. Sample Group showed increasing effects on HDL-cholesterol level in serum significantly. 3. Sample Group showed decreasing effects on Insuline in serum significantly. 4. Sample Group showed increasing effects on Laptin in serum significantly. 5. Sample Group showed decreasing effects on Body weight, epididymis fat weight & rate, epididymis fat cell significantly. According to the above results, Pinus densiflora showed significant decreasing effects on hyperlipidemia and lipid in rats, it is considered that it is appropriate to apply for hyperlipidemia, obesity.

쉴드 TBM 뒤채움용 무기계 가소성 그라우트의 개발 및 적합성 평가

김대현,정두회,정경환,Kim, Dae-Hyun,Jung, Du-Hwoe,Jeong, Gyeong-Hwan 한국터널지하공간학회 2009 한국터널지하공간학회논문집 Vol.11 No.3

본 연구는 쉴드 TBM 테일 보이드 동시주입용 가소성 그라우트의 적합성을 규명하기 위하여 강도특성, 수중 재료분리현상 및 점도특성 실험을 실시하였다. 무기계 가소성 그라우트는 쉴드 TBM 뒤채움 그라우트 주입 시 시멘트, 물, MG-A로 구성된 A액과 스칼렛, 물, MG-B로 구성된 B액을 주입하는 2액성 그라우트 주입공법이다. 적정 배합비로 제작된 무기계 가소성 그라우트의 일축압축강도는 초기강도 및 장기강도 기준을 만족하는 결과를 보였다. 지하수가 존재하는 현장에서 그라우트 주입 시 발생하는 재료분리현상을 관찰하기 위하여 기존 현장에서 적용된 규산계 그라우트와 무기계 가소성 그라우트를 비교하였으며, 그 결과 규산계 그라우트에서 발생하는 재료분리 현상이 무기계 가소성 그라우트에서는 관찰되지 않았다. 또한 무기계 가소성 그라우트의 장거리 이송에 대한 현장 적용성을 확인하기 위하여 A, B액의 점도를 측정한 결과 A, B액 모두 120분 이상 2000 cP 이하가 측정되었다. 이러한 결과는 추후 현장적용 시 A, B액의 원활한 이송이 가능한 점도를 확보하고 있음을 확인하였다. A suitability of a thixotropic grout developed in this study has been examined through laboratory tests on strength, segregation, and viscosity. The thixotropic grout is a mixture of two types of liquid components. The A-liquid component consists of cement, water, and MG-A and the B-liquid component consists of scarlet, water, and MG-B. Unconfined compressive strength of specimens prepared with a prefer mix-proportion satisfied a design criteria for the backfilling of tail voids. A material segregation phenomenon under water condition was not observed in the thixotropic grout whereas it was observed in the existing silica-type grout. In addition, viscosity tests have been rallied out on the thixotropic grout to verify the capability of a long-distance delivery in the field. Both the A-liquid component and the B-liquid component maintained a viscosity of below 2,000 cP for 120 minutes. This experimental result confirms that two liquid components guarantees a long-distance delivery in tile field application.

교수강의평가제 실시 현황

김대현,Kim, Dae-Hyeon 한국대학교육협의회 2005 大學敎育 Vol.137 No.-