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Magnetoresistance in Post annealed Bi Thin Films on PbTe buffered CdTe(111)B and on Mica Substrates
Yunki Kim(김윤기),Jin Sung Choi(최진성),Hai Bo Li(이해파),Sunglae Cho(조성래) 한국진공학회(ASCT) 2006 Applied Science and Convergence Technology Vol.15 No.4
비스무스의 녹는점보다 3도 낮은 온도인 268℃ 에서 후열처리를 하여 비스무스 박막에서 자기저항의 큰 증가를 관측하였다. 레드텔러라이드/케드뮴텔러라이드 기판 위에서는 온도 5 K, 자기장 5 T 하에서 190에서 260으로, 마이카 기판위에서는 620에서 120으로 자기저항의 큰 증가를 나타내었다. 이러한 자기저항의 큰 증가는, 열처리에 따른 결정도의 향상에 기인한 것으로 보인다. 하지만 일정 시간 이상의 오랜 시간의 열처리는 자기저항을 감소시키는 것으로 관측되었다. We have observed a large increase in the magnetoresistance (MR) of Bi thin films, which were subjected to a post annealing procedure at 268°C, 3°C below the Bi melting point. We have achieved an increase in the MR by 260 fold and 1200 fold at 5 K and 5 T after post annealing, as compared with 190 and 620 for an as deposited Bi film on PbTe/CdTe(111) and on mica, respectively. The large MR increase by post annealing might be due to the improvement of crystallinity according to the x ray analysis. However, post annealing over a certain amount time showed the reduction in MR values.
Yunki Kim,Kyu-Jin Jo,Jin-Su Oh,Cheol-Woong Yang 대한금속·재료학회 2024 ELECTRONIC MATERIALS LETTERS Vol.20 No.1
In this study, a resistive random-access memory device based on a Ta/Ta x Mn y O z /Pt metal–insulator–metal structure wasfabricated and examined. The test device exhibited stable bipolar resistive switching characteristics with DC endurance ofmore than 300 cycles and robust retention up to 10 4 s at room temperature. Moreover, the device had a low forming voltageand a resistance window of ~ 10 3 . The conduction mechanism in each resistance state of the device was analyzed throughcurrent–voltage curve fi tting. It was confi rmed that the primary conduction mechanisms were ohmic and Poole–Frenkelconduction in the low- and high-resistance states, respectively. By analyzing the cross section of the fabricated device throughtransmission electron microscopy, it was found that the Ta x Mn y O z layer was deposited in amorphous form. The compositionand chemical bonding state of the Ta x Mn y O z layer were also analyzed using X-ray photoelectron spectroscopy. With thesecharacteristics, the amorphous Ta x Mn y O z layer has strong potential for nonvolatile memory applications.
차세대 위성탑재 컴퓨터의 표준 구조 설계에 관한 개념 연구
이윤기(Yunki Lee),김지훈(Jihoon Kim) 한국항공우주학회 2013 韓國航空宇宙學會誌 Vol.41 No.12
저궤도와 정지궤도 위성을 위한 위성 탑재 컴퓨터 (OBC)는 서로 유사한 기능을 가짐에도 불구하고, 지금까지 별도의 구조로 개발되어왔다. 본 논문에서는 OBC의 표준 구조에 대한 개념 연구 결과를 제시하며, 차세대 위성용 OBC를 개발하기 위한 방안을 제안한다. 제안된 구조는 가변성이 매우 높은 구조이며, 저/중/정지궤도 그리고 달 탐사선/심 우주탐사선에 모두 사용되도록 하는 것을 목표로 하고 있다. 또한 현재 개발 진행 중인 표준형 OBC의 개발 현황을 소개한다. On-Board computers (OBC) for LEO & GEO satellites have been developed with their own dedicated architecture so far even though they have many similar functionalities. In this paper, we present a conceptual study results of standard OBC architecture design and propose the domestic development plan for the next generation satellite OBC. Proposed architecture is highly flexible and can be used at LEO/MEO/GEO and Moon Explorer/Deep Space Probe. Also, we introduce current status of standard OBC which is under development.
리튬이온 전지 설계를 위한 실리콘 음극재료의 이상반응 메커니즘 해석
곽윤기(Yunki Gwak),문장혁(Janghyuk Moon),조맹효(Maenghyo Cho) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
Rechargeable lithium-ion batteries are widely used as electric power source for portable electronics and electric vehicle (EV) due to its excellent characteristics such as high voltage, and high energy density. Silicon is a promising anode material for high performance lithium ion batteries, as it has very high theoretical reversible specific capacity (3579 mAh/g for Li15Si4) at room temperature. However, the implementations of siliconbased materials have some challenges. Active particle pulverization caused by the large volume expansion leads to the loss of electrical contacts within the electrode. It is recognized as one of the major capacity fade with repeated charges?discharge cycling. Therefore, it is necessary to understand volumetric changes, stress evolution and fracture behavior of silicon anodes for development of high performance Lithium ion batteries. Recent experimental results reveal that the first lithiation of crystalline Si is a two-phase reaction mechanism, in which the crystalline Si is consumed to form highly lithiated amorphous LixSi. The kinetics of two phase lithiation has been found that it is limited by the reaction at the phase boundary to break Si?Si bonds in crystalline structure. This two phase reaction mechanism can induce large mechanical stress at the phase boundary and eventually lead to the fracture of silicon. Therefore, we provide an intercalation-induced stress model for crystalline silicon to understand the degradation mechanisms in silicon anode materials.
Lee, Yunki,Bae, Jin Woo,Hoang Thi, Thai Thanh,Park, Kyung Min,Park, Ki Dong The Royal Society of Chemistry 2015 Chemical communications Vol.51 No.42
<P>Graphene-based nanomaterials with different oxidation degrees were incorporated into Tetronic–tyramine (Tet–TA) hydrogels <I>via</I> enzymatic cross-linking. The molecular oxidation of graphene in combination with amphiphilic Tet–TA significantly improved the water dispersibility of graphene oxide (GO), resulting in a significant reinforcement of Tet–TA/GO composite hydrogels that can be used as an injectable biomaterial platform.</P> <P>Graphic Abstract</P><P>Graphene-based nanomaterials with different oxidation degrees were incorporated into Tetronic–tyramine (Tet–TA) hydrogels <I>via</I> enzymatic cross-linking. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5cc02511g'> </P>
Lee, Yunki,Balikov, Daniel A.,Lee, Jung Bok,Lee, Sue Hyun,Lee, Seung Hwan,Lee, Jong Hun,Park, Ki Dong,Sung, Hak-Joon MDPI 2017 INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES Vol.18 No.8
<P>Directing angiogenic differentiation of mesenchymal stem cells (MSCs) still remains challenging for successful tissue engineering. Without blood vessel formation, stem cell-based approaches are unable to fully regenerate damaged tissues due to limited support for cell viability and desired tissue/organ functionality. Herein, we report in situ cross-linkable gelatin−hydroxyphenyl propionic acid (GH) hydrogels that can induce pro-angiogenic profiles of MSCs via purely material-driven effects. This hydrogel directed endothelial differentiation of mouse and human patient-derived MSCs through integrin-mediated interactions at the cell-material interface, thereby promoting perfusable blood vessel formation in vitro and in vivo. The causative roles of specific integrin types (α<SUB>1</SUB> and α<SUB>v</SUB>β<SUB>3</SUB>) in directing endothelial differentiation were verified by blocking the integrin functions with chemical inhibitors. In addition, to verify the material-driven effect is not species-specific, we confirmed in vitro endothelial differentiation and in vivo blood vessel formation of patient-derived human MSCs by this hydrogel. These findings provide new insight into how purely material-driven effects can direct endothelial differentiation of MSCs, thereby promoting vascularization of scaffolds towards tissue engineering and regenerative medicine applications in humans.</P>