• 한국초전도학회:학술대회논문집
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• v.9
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• pp.280-290
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• 1999

The microstructures of a Tl$_{0.8}Pb_{0.2}Bi_{0.2}Sr_{1.6}Ca_2Cu_3O_{9+{\delta}}$/Ag tape (tape I ) with J$_c$ of 17,600 A/cm$^2$ at 77 K and 0 T and three Tl$_{0.8}Pb_{0.2}Bi_{0.2}Sr_{1.8}Ba_{0.2}Ca_{2.2}Cu_3O_{9+{\delta}}$/Ag tapes with J$_c$'s of 9,300 (tape II), 16,700 (tape III) and 25,200 A/cm$^2$ (tape IV)prepared using the powder-in-tube method and an in-situ reaction method, were investigated using scanning electron microscopy and high-resolution transmission electron microscopy, and compared each other. ln the tape preparation, an intermediate rolling process was incorporated during final heat-treatment for the last tape, but not for the rest of the tapes. The microstructural analysis revealed clear differences in grain-texturing, crystallographic defects and impurity phases, depending on the chemical composition of the tape. Tendency of directional grain-alignment increased in an order of tapes I, II III and IV. In tape IV, T1-1223 grains are textured, at least in local regions. In crystallographic defects, while stacking faults were prevalent in the former composition, dislocations and voids were frequently observed in the latter. Also impurity phases were appeared to be more abundant in the former than in the latter. The relationship between 1,and the microstructure in the tapes was attempted to explain in a term of grain-linking.

• Korean Journal of Materials Research
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• v.13 no.7
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• pp.478-483
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• 2003

The crystallographic characteristics of the $(Bi, La)_4$$Ti_3$$O_{12}$ thin film, which is considered as an applicable dielectrics in the ferroelectric RAM device due to a low crystallization temperature and a good fatigue property, were investigated at the atomic scale by high resolution transmission electron microscopy and the high resolution Z-contrast technique. The analysis showed that a (00c) preferred orientation and a crystallization of the film were enhanced with the diffraction intensity increase of the (006) and (008) plane as the annealing temperature increased. It indicated a change of the atomic arrangement in the (00c) plane. Stacking faults on the (00c) plane were also observed. Through the comparison of the high-resolution Z-contrast image and the $Bi_4$$Ti_3$$O_{12}$ atomic model, it was evaluated that the intensity of the Bi atom was different according to the atomic plane, and it was attributed to a substitution of La atom for Bi at the specific atom position.

• Journal of the Korean Ceramic Society
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• v.52 no.6
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• pp.441-448
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• 2015

Silicon carbide (SiC) coatings for tri-isotropic (TRISO) nuclear fuel particles were fabricated using a chemical vapor deposition (CVD) process onto graphite. A micro-tensile-testing system was developed for the mechanical characterization of SiC coatings at high temperatures. The fracture strength of the SiC coatings was characterized by the developed micro-tensile test in the range of $25^{\circ}C$ to $1000^{\circ}C$. Two types of CVD-SiC films were prepared for the micro-tensile test. SiC-A exhibited a large grain size (0.4 ~ 0.6 m) and the [111] preferred orientation, while SiC-B had a small grain size (0.2 ~ 0.3 mm) and the [220] preferred orientation. Free silicon (Si) was co-deposited onto SiC-B, and stacking faults also existed in the SiC-B structure. The fracture strengths of the CVD-SiC coatings, as measured by the high-temperature micro-tensile test, decreased with the testing temperature. The high-temperature fracture strengths of CVD-SiC coatings were related to the microstructure and defects of the CVD-SiC coatings.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1999.06a
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• pp.187-207
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• 1999

The thermal distributions near the growth interface of 150mm CZ crystals were measured by three thermocouples installed at the center, middle (half radius) and edge (10m from surface) of the crystals. The results show that larger growth rates produced smaller thermal gradients. This contradicts the widely used heat flux balance equation. Using this fact, it si confirmed in CZ crystals that the type of point defects created is determined by the value of the thermal gradient (G) near the interface during growth, as already reported for FZ crystals. Although depending on the growth systems the effective lengths of the thermal gradient for defect generation are varied, were defined the effective length as 10mm from the interface in this experiment. If the G is roughly smaller than 20C/cm, vacancy rich CZ crystals are produced. If G is larger than 25C/cm, the species of point defects changes dramatically from vacancies to interstitial. The experimental results which FZ and CZ crystals are detached from the melt show that growth interfaces are filled with vacancy. We propose that large G produces shrunk lattice spacing and in order to relax such lattice excess interstitial are necessary. Such interstitial recombine with vacancies which were generated at the growth interface, next occupy interstitial sites and residuals aggregate themselves to make stacking faults and dislocation loops during cooling. The shape of the growth interface is also determined by the distributions of G across the interface. That is, the small G and the large G in the center induce concave and convex interfaces to the melt, respectively.

• Bulletin of the Korean Chemical Society
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• v.31 no.9
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• pp.2547-2552
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• 2010

CdS nano-particulate films were prepared at the air/water interface under Langmuir monolayers of arachidic acid (AA) via interfacial reaction between $Cd^{2+}$ ions in the subphase and $H_2S$ molecules in the gaseous phase. The films were made up of fine CdS nanoparticles with hexagonal Wurtzite crystal structure after reaction. It was revealed that the formation of CdS nano-particulate films depends largely on the experimental conditions. When the films were ripened at room temperature or an increased temperature ($60^{\circ}C$) for one day, numerous holes were appeared due to the dissolution of smaller nanoparticles and the growth of bigger nanoparticles with an improved crystallinity. When the films were ripened further, CdS rodlike nanoparticles with cubic zinc blende crystal structure appeared due to the re-nucleation and growth of CdS nanoparticles at the stacking faults and defect structures of the hexagonal CdS grains. These structures were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and X-ray diffraction (XRD). These results declare that CdS semiconductor nanoparticles formed at the air/water interface change their morphologies and crystal structures during the ripening process due to dissolution and recrystallization of the particles.

• Korean Journal of Materials Research
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• v.25 no.11
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• pp.630-635
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• 2015

The fatigue strength of a nickel-base superalloy was studied. Stress-controlled fatigue tests were carried out at $700^{\circ}C$ and 5 Hz using triangular wave forms. In this study, two kinds of testing procedures were adopted. One is the conventional tension-zero fatigue test(R = 0). The other was a procedure in which the maximum stress was held at 1000 MPa and the minimum stress was diverse from zero to 1000 MPa at 24 and $700^{\circ}C$. The results of the fatigue tests at $700^{\circ}C$ indicate that the fracture mechanism changed according to both the mean stress and the stress range. At a higher stress range, ${\gamma}^{\prime}$ precipitates are sheared by a/2<110> dislocation pairs coupled by APB. Therefore, in a large stress range, the deformation occurred by shearing of ${\gamma}^{\prime}$ by a/2<110> dislocations, which brought about crystallographic shear fracture. As the stress range was decreased, the fracture mode gradually changed from crystallographic shear fracture to gradual growth of fatigue cracks. At an intermediate stress range, as it became more difficult for a/2<110> dislocation pairs to shear ${\gamma}^{\prime}$ particles, cracks started to propagate in the matrix, avoiding the harder ${\gamma}^{\prime}$ particles. High mean stress induced creep deformation, that is, ${\gamma}^{\prime}$ particles were sheared by {111}<112> slip systems, which led to the formation of stacking faults in the precipitates. Thus, the change in fracture mechanism brought about the inversion of the S-N curves.

• Journal of the Korean Society for Heat Treatment
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• v.9 no.1
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• pp.12-18
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• 1996

Austenite(${\gamma}$) grain size, ${\varepsilon}$ martensite volume fraction and damping capacity of Fe-17%Mn alloy have been investigated as a function of solution treatment temperature of $600^{\circ}C$ to $1100^{\circ}C$. With increasing the solution temperature, ${\gamma}$ grain size, ${\varepsilon}$ martensite content and damping capacity are increased, while the hardness is decreased. When ${\gamma}$ grains are small, ${\varepsilon}$ plates grow in only one direction in each ${\gamma}$ grain. However, if the ${\gamma}$ grains are large in accordance with high solution treating temperature, several ${\varepsilon}$ variants with different orientations are formed and intersected each other in each ${\gamma}$ grain. In spite of small ${\varepsilon}$ martensite content, the damping capacity of the specimen which was annealed at $700^{\circ}C$, followed by subzero treatment at $-196^{\circ}C$, is almost equal to that of the specimen annealed at $1000^{\circ}C$ and subsequently quenched to room temperature. From this result it is suggested that the damping capacity of Fe-17%Mn alloy having fine ${\gamma}$ grains is mainly attributed to the movement of ${\gamma}/{\varepsilon}$ interface without the operation of other damping sources such as ${\varepsilon}/{\varepsilon}$ boundaries and stacking faults in ${\varepsilon}$ reported previously.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.291-291
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• 2011

1차원 반도체인 nanowires (NWs)는 전기적, 광학적으로 일반 bulk구조와 다른 특성을 가지고 있어서 현재 많은 연구가 되고 있다. 일반적으로 NWs는 Au 등의 금속 촉매를 이용하여 성장을 하게 되는데 이때 촉매가 오염물로 작용을 해서 결함을 만들어서 bandgap내에 defect level을 형성하게 된다. 본 연구는 Si(111) 기판 위에 Ga-droplet을 촉매로 사용을 하여 molecular beam epitaxy로 성장을 하였다. 성장온도는 600$^{\circ}C$로 고정을 하였고 growth rate은 GaAs(100) substrate에서 2.5 A/s로 Ga의 양을 고정하고 V/III ratio를 1부터 8까지 변화를 시켰다. As의 양에 따라서 생성되는 NWs의 개수가 증가하고 growth rate이 빨라지는 것을 확인할 수 있었다. Transmission Electron Microscopy 분석 결과 낮은 V/III ratio에서는 zincblende, wurtzite 그리고 stacking faults 가 혼재 되어 있는 것을 확인 할 수 있었다. 이러한 결함은 소자를 만드는데 한계가 있기 때문에 pure zincblende나 pure wurtzite를 가져야 하는데 V/III ratio : 8 에서 pure zincblende구조가 되었다. Gibbs-Thomson effect에 따르면 구조적 변화는 Ga droplet과 NWs의 접면에서 크기가 중요한 역할을 한다[1]. 연구 결과 V/III ratio : 8일 때 Ga droplet의 크기가 zincblende성장에 알맞다는 것을 예상할 수 있었다. laser confocal photoluminescence 결과 상온에서 1.43 eV의 bandgap을 가지는 bulk구조와는 다른 와 1.49eV의 bandgap을 가지는 것을 확인하였다.

• Transactions on Electrical and Electronic Materials
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• v.4 no.6
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• pp.17-20
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• 2003

This paper describes the effect of halogenic gettering during oxide passivation of commercial solar cell with the $N^{+}$-P-$N^{+}$ structure. In order to study the effect of halogenic gettering on $N^{+}$-P-$N^{+}$ structure mono-crystalline silicon solar cell, we performed conventional POCl$_3$ diffusion for emitter formation and oxide passivation in the presence of HCl vapors. The $N^{+}$-P-$N^{+}$ structure based silicon solar cells were found to have higher short circuit current and minority carrier lifetime. Their performance was also found to be superior than the conventional $N^{+}$-P-$N^{+}$ structure based mono-crystalline silicon solar cell. The cell parameters of the $n^{+}$-p-$p^{+}$ and $n^{+}$-p-$n^{+}$ structure based cells, passivated by HCl assisted oxidation were measured. The improvement in $I_{sc}$ was attributed to the effect of the increased diffusion length of minority carriers, which came from the halogenic gettering effect during the growth of passivating oxide. The presence of chlorine caused gettering of the cells by removing the heavy metals, if any. The other advantage of the presence of chlorine was the removal of the diffusion induced (in oxygen environment) stacking faults and line defects from the surfaces of the silicon wafers. All these effects caused the improvement of the minority carrier lifetime, which in-turn helped to improve the quality of the solar cells.

• Journal of Korea Foundry Society
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• v.40 no.4
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• pp.118-127
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• 2020

Microstructural evolution during a heat treatment and high-temperature tensile properties have been investigated in conventionally cast CM247LC. In as-cast specimens, MC carbides with high amounts of Ta, Ti, Hf, and W were found to exist in the interdendritic regions, and γ' was observed in the form of cubes and octocubes prior to decomposition into cubes. In the heat-treated condition, some portion of eutectic γ-γ' remained, and uniform cubic γ' was observed in both interdendritic regions and dendrite core. Three types of carbides with different stoichiometries and compositions were found at the grain boundaries. MC carbides with high Hf contents were observed in the vicinity of eutectic γ-γ'. The highest tensile strength value was found at 750℃, whereas the greatest ductility appeared at 649℃. The effect of the temperature on the tensile properties was closely related to the dislocation structure. With increase in the test temperature, the density of dislocations inside γ' decreased, whereas that in the γ matrix increased. Stacking faults generated in γ' at 750℃ had a strengthening effect, whereas thermally activated dislocation motion at a high temperature was considered to have the opposite effect.