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Defect-induced Visible-light Emission in GaN Nanocrystals Synthesized through a Solution-based Route
Yinsheng He,Zhixin Wan,신기삼,Jiling Dong,Youngmoom Yu 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.9
In this study, nanocrystalline GaN was synthesized through a metathesis reaction at room temperature. The microstructure of the GaN examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the single nature with würtzite structure. High resolution TEM (HRTEM) images showed that the defects were formed in the as-prepared GaN nanocrystals. Room temperature photoluminescence (PL) spectra showed the near band-edge emission at 356 nm (3.48 eV) as well as visible light emission at 420 ~ 460 nm (2.70 ~ 2.95 eV). The increase in the intensity of visible light peak was observed by annealing the as-prepared specimen at 850 ℃ for 3 hrs, which was due to the increase in the numeric density of defects caused by de-passivation of impurity during annealing. Therefore, it can be concluded that the visible light emission in the GaN nanocrystals was induced by the defects (possibly N vacancies) in the structure.
Yinsheng Song,Guanqi Wang,Feng Li,Hao Shen,Jing Wang 제어·로봇·시스템학회 2023 International Journal of Control, Automation, and Vol.21 No.10
This paper is concerned with the dynamic event-triggered H∞ control issue of singularly perturbed switched systems, in which a persistent dwell-time switching regularity is embraced. The dynamic event-triggered mechanism is introduced to reduce some unnecessary data transmitted in the communication network so as to that the system can maintain the desired performance under better resource efficiency. Under this communication mechanism, a singular perturbation parameter independent criterion is derived to guarantee the globally uniform exponential stability with an H∞ performance for the considered persistent dwell-time switched singularly perturbed systems. Based on the criterion, a method is presented for calculating the gains of the desired controller. Finally, a numerical example is provided to verify the rationality and validity of the proposed method.
Half-metallicity and spin transport studies on quaternary CoCuMnSb Heusler alloy
Yinsheng Huang,Chao Jing,Yanning Wu,Yuanlei Zhang,Zhe Li,Xiaodong Sun,Miaofu Ye 한국물리학회 2019 Current Applied Physics Vol.19 No.11
Herein, the crystalline structure and spin transport properties of CoCuMnSb (CCMS) half-metallic Heusler alloy have been investigated with ab-initio calculations and nonequilibrium Green's function. Experimental and theoretical results reveal that CCMS alloy stabilizes in a highly ordered LiMgPdSn-type structure with lattice parameters of a=6.113 Å and α=90°, which match well with the reported lattice parameters of GaSb. Furthermore, the theoretical calculations indicate that CCMS exhibits perfect polarized spin at the Fermi level. Therefore, we have designed CCMS/GaSb heterostructure and CCMS/GaSb/CCMS magnetic tunnel junction (MTJ). Our architecture exhibits effective spin filtering and the MTJ produces a dramatic magnetoresistance ratio (~445). These outstanding properties marks CCMS as a strong system for applications in spintronics.
He, Yinsheng,Dong, Jiling,Choi, Woosung,Jung, Jinesung,Shin, Keesam John Wiley Sons, Ltd 2012 Surface and interface analysis Vol.44 No.11
<P>An improved non‐destructive replication (NDR) metallography method was developed and applied successfully for the precipitates study in 1Cr‐1Mo‐0.25V turbine rotor steel. The preparation of the replicas for electron microscopy was carried out with a combination of electropolishing, etching, and acetate film setting for extraction medium. The exposure of the precipitates was improved by electropolishing followed by chemical etching. The electropolishing after application of acetate film, instead of stripping, resulted in a quality improvement of the extraction replica. Three types of precipitates (M<SUB>23</SUB>C<SUB>6</SUB>, Mo<SUB>2</SUB>C, and VC) were identified by TEM analysis of thin foil. The types, sizes, and chemical compositions of the precipitates from the bulk SEM observation and TEM analyses of thin foil specimen and the NDR extraction replica matched very well, indicating NDR extracted all types of the precipitates with high efficiency. Copyright © 2012 John Wiley & Sons, Ltd.</P>
Microstructural Evolution of Grade 91 Steel upon Heating at 760~1000℃
He, Yinsheng,Chang, Jungchel,Lee, Je-Hyun,Shin, Keesam Materials Research Society of Korea 2015 한국재료학회지 Vol.25 No.11
The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at $760{\sim}1000^{\circ}C$ for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite($760{\sim}850^{\circ}C$), and to fresh martensite($900{\sim}1000^{\circ}C$), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at $925^{\circ}C$), indicating grain recovery upon intercritical heating. The main precipitate, $M_{23}C_6$, was found to be coarsened slightly at $760{\sim}850^{\circ}C$; it then dissolved at $850{\sim}1000^{\circ}C$. Besides this, $M_3C$ cementite was formed at $900{\sim}1000^{\circ}C$. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.
He, Yinsheng,Li, Kejian,Cho, In Shik,Park, In Gyu,Shin, Keesam American Scientific Publishers 2014 Journal of nanoscience and nanotechnology Vol.14 No.11
<P>In this work, an Al-7 wt.% Si alloy, which is widely used as the structural materials in the automotive and aerospace industries for their high specific strength, was subjected to ultrasonic nanocrystalline surface modification (UNSM) treatment. After UNSM treatment, the effect of UNSM on the microstructural evolution of both Al grain and the dispersed Si particles was studied by using scanning electron microscope (SEM) and transmission electron microscope (TEM). Experimental results show that the ultra-fine grain (UFG, - 400 nm in size) structure is developed in the top surface layer (up to - 15 μm in depth). The coarse Si particles were refined and well dispersed in the UFG Al matrix. Cross-sectional TEM observation revealed that the grain refinement mechanism involved the formation of new grain boundaries dividing the coarse grain into UFG structure. Nanotwin and nanosize Si were formed within the original coarse Si particles. The presence of dispersed Si particles in the Al matrix accelerated the Al grain refinement process.</P>
On-Site Corrosion Behavior of T91 Steel after Long-Term Service in Power Plant
He, Yinsheng,Chang, Jungchel,Lee, Je-Hyun,Shin, Keesam Materials Research Society of Korea 2015 한국재료학회지 Vol.25 No.11
In this work, on-site corrosion behavior of heat resistant tubes of T91, used as components of a superheater in a power plant for up to 25,762 h, has been investigated using scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy (EDS), and electron backscattered diffraction(EBSD), with the objectives of studying the composition, phase distribution, and evolution during service. A multi-layer structure of oxide scale was found on both the steamside and the fireside of the tube surface; the phase distribution was in the order of hematite/magnetite/spinel from the outer to the inner matrix on the steamside, and in the order of slag/magnetite/spinel from the outer to the inner matrix on the fireside. The magnetite layer was found to be rich in pores and cracks. The absence of a hematite layer on the fireside was considered to be due to the low oxygen partial pressure in the corrosion environment. The thicknesses of the hematite and of the slag-deposit layer were found to exhibit no significant change with the increase of the service time.