• Transactions on Electrical and Electronic Materials
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• v.7 no.1
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• pp.36-41
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• 2006

In this paper, the time evolution of undoped GaN epilayers on a low-temperature GaN buffer layer grown on c-plane sapphire at a low pressure of 300 Torr was studied via a two-step growth condition in a horizontal MOCVD reactor. As a function of the growth time at a high-temperature, the surface morphology, structural quality, and optical and electrical properties were investigated using atomic force microscopy, high-resolution x-ray diffraction, photoluminescence, and Hall effect measurement, respectively. The root-mean-square roughness showed a drastic decrease after a certain period of surface roughening probably due to the initial island growth. The surface morphology also showed the island coalescence and the subsequent suppression of three-dimensional island nucleation. The structural quality of the GaN epilayer was improved with increasing growth time considering the symmetrical (002) and asymmetrical (102) rocking curves. The variations of room-temperature photoluminescence, background carrier concentration, and Hall mobility were measured and discussed.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.11
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• pp.859-863
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• 2010

Evolution of surface morphology of ${\alpha}-Al_2O_3$ substrate was investigated as a function of annealing temperature and time. Commercial (0001) ${\alpha}-Al_2O_3$ single crystal substrates were annealed in the range of $600-1000^{\circ}C$ in air. At $600^{\circ}C$, step-terrace structure started to be formed on the substrate. However, the surface roughness on the terrace was still considerable and a number of islands were observed on the step edges as well as the terraces. As the annealing temperature increased, the islands were absorbed into the step edges. Thus the terraces were smoother and the step edges were more straightened. Well-defined surface with a step height of 0.2 nm was formed above $900^{\circ}C$. On the other hand, when the substrate was annealed at a fixed temperature of $1000^{\circ}C$, the change of surface morphology was observed for the substrate annealed for 10 min. After the annealing for 30 min, the surface on which any islands could not survive was observed.

• Journal of Electrochemical Science and Technology
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• v.11 no.3
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• pp.220-237
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• 2020

Modern electrochemical energy devices involve generation and reduction of fuel gases through electrochemical reactions of water splitting, alcohol oxidation, oxygen reduction, etc. Initially, these processes were executed in the presence of noble metal-based catalyst that showed low overpotential and high current density. However, its high cost, unavailability, corrosion and related toxicity limited its application. The search for alternative with high stability, durability, and efficiency led scientists towards carbon nanoparticles supported catalysts which has high surface area, good electrical conductivity, tunable morphology, low cost, ease of synthesis and stability. Carbon nanoparticles are classified into two groups based on morphology, one and zero dimensional particles. Carbon nanoparticles at zero dimension, denoted as carbon dots, are less used carbon support compared to other forms. However, recently carbon dots with improved electronic properties have become popular as catalyst as well as catalyst support. This review focused on the recent advances in electrocatalytic activities of carbon dots. The mechanisms of common electrocatalytic reactions and the role of the catalysts are also discussed. The review also proposed future developments and other research directions to overcome current limitations.

• International Journal of Precision Engineering and Manufacturing
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• v.9 no.4
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• pp.57-62
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• 2008

The aim of this investigation was to extract the fatigue properties at the designated fatigue life of copper foil and observe the mean stress and stress amplitude effects on both the fatigue life and the corresponding surface morphology. Tensile tests were performed to determine the baseline monotonic material properties of the proportional limit and ultimate tensile strength. Constant amplitude fatigue tests were carried out using a feedback-controlled fatigue testing machine. The mean stress and the stress amplitude were changed to obtain the complete nominal stress-life curves. An atomic force microscope was utilized to observe the relationship between the fatigue damage and the corresponding changes in surface morphology. A Basquin's exponent of-0.071 was obtained through the fatigue tests. An endurance limit of 122 MPa was inferred from a Haigh diagram. The specimen surface became rougher as the number of fatigue cycles increased, and there was a close relationship between the fatigue damage and the surface roughness evolution.

• Proceedings of the Korean Society of Rheology Conference
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• 2001.06a
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• pp.11-14
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• 2001

Our study have aimed to identify the deformation and breakup mechanism of minor phase in polymer blends under uniaxial enlongational flow. Experimentally, we measured the transient elongational viscosity of PS/HDPE blends using the uniaxial elongational rheometer at two temperatures. And we observed the evolution of blend morphology with elongation time. Morphological change was observed by quenching the specimen after deformation. If the viscosity variation of PS was compared with that of HDPE at each temperature, PS showed larger temperature dependence than HDPE. At 155$^{\circ}C$, the dispersed phase of larger size were easily affected by affine deformation. The initial spherical shape changed to flat ellipsoid at first, then flat ellipsoid to bulbous shape, and bulbous to thin thread and its satellites. But dispersed phase of smaller size showed the change from sphere to ellipsoid. At 175$^{\circ}C$, the dispersed phase were mostly deformed from spherical shape to ellipsoid. As a result, the morphological change of dispersed phase in elongational deformation is affected by chain flexibility and viscosity ratio. We need to further study to make sure the mechanism of elongation of viscoelastic polymer blends.

• Journal of the Korean Institute of Intelligent Systems
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• v.20 no.4
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• pp.522-527
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• 2010

In this paper, a new MBHF(Morphology-Based Homomorphic filter) is presented to enhance contrast in mammographic images. The MBH filtering is performed based on the morphological sub-bands, in which an image is morphologically decomposed. The filter is designed to have optimal gain and structuring element in each sub-band through differential evolution. Experimental results show that the proposed method improves the contrast in mammographic images such that an evaluation criterion, WPSNR(Weighted Peak Signal to Noise Ratio) which takes into account human visual system is increased compared with a wavelet-based Homomorphic filter.

• Journal of The Korean Astronomical Society
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• v.54 no.2
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• pp.71-88
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• 2021

We use N-body/hydrodynamic simulations to study the evolution of the spin of a Milky Way-like galaxy through interactions. We perform a controlled experiment of co-planar galaxy-galaxy encounters and study the evolution of disk spins of interacting galaxies. Specifically, we consider cases where the late-type target galaxy encounters an equally massive companion galaxy, which has either a late or an early-type morphology, with a closest approach distance of about 50 kpc, in prograde or retrograde sense. By examining the time change of the circular velocity of the disk material of the target galaxy from each case, we find that the target galaxy tends to lose the spin through prograde collisions but hardly through retrograde collisions, regardless of the companion galaxy type. The decrease of the spin results mainly from the deflection of the orbit of the disk material by tidal disruption. Although there is some disk material which gains the circular velocity through hydrodynamic as well as gravitational interactions or by transferring material from the companion galaxy, it turns out that the amount of the material is generally insufficient to increase the overall galactic spin under the conditions we set. We find that the spin angular momentum of the target galaxy disk decreases by 15-20% after a prograde collision. We conclude that the accumulated effects of galaxy-galaxy interactions will play an important role in determining the total angular momentum of late-type galaxies.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.32 no.1B
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• pp.53-57
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• 2012

This study presents a numerical model that is capable of simulating the evolution of mining pit in a stream. The numerical model is based on the quasi-steady assumption that the flow is steady with time-dependent morphological change. This hypothesis is valid due to the fact that the stream morphology changes over a long period compared with the time of flow change. Before applications, numerical experiments are carried out with two total load formulas such as Engelund and Hansen's (1967) and Ackers and White's (1973). It is found that the use of Engelund and Hansen's formula reproduces evolution of mining pit best compared with simulated profiles in Parker (2004). Then, the model is applied to two laboratory experiments in the literature. In general, the numerical model simulates properly the evolution of mining pit in laboratory open-channels. However, it is found that the model does not reproduce head-cutting, propagating upstream, and under-estimates the wave of the bed, propagating downstream, after finishing the re-fill of the mining pit.

• Steel and Composite Structures
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• v.46 no.3
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• pp.385-401
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• 2023

Steel is becoming increasingly popular due to its high strength, excellent ductility, great assembly performance, and recyclability. In reality, steel structures serving for a long time in atmospheric, industrial, and marine environments inevitably suffer from corrosion, which significantly decreases the durability and the service life with the exposure time. For the mechanical properties of corroded steel, experimental studies are mainly conducted. The existing numerical analyses only evaluate the mechanical properties based on corroded morphology at the isolated time-in-point, ignoring that this morphology varies continuously with corrosion time. To solve this problem, the relationships between pit depth expectation, standard deviation, and corrosion time are initially constructed based on a large amount of wet-dry cyclic accelerated test data. Successively, based on that, an in-situ pitting evolution method for evaluating the residual tensile strength of corroded steel is proposed. To verify the method, 20 repeated simulations of mass loss rates and mechanical properties are adopted against the test results. Then, numerical analyses are conducted on 135 models of corrosion pits with different aspect ratios and uneven corrosion degree on two corroded surfaces. Results show that the power function with exponents of 1.483 and 1.091 can well describe the increase in pit depth expectation and standard deviation with corrosion time, respectively. The effect of the commonly used pit aspect ratios of 0.10-0.25 on yield strength and ultimate strength is negligible. Besides, pit number ratio α equating to 0.6 is the critical value for the strength degradation. When α is less than 0.6, the pit number increases with α, accelerating the degradation of strength. Otherwise, the strength degradation is weakened. In addition, a power function model is adopted to characterize the degradation of yield strength and ultimate strength with corrosion time, which is revised by initial steel plate thickness.

• Korean Institute of Interior Design Journal
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• v.19 no.4
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• pp.90-98
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• 2010

This study intends to understand the change of the library's bookshelves shape, composition and arrangement based on the social-culture in each period. For this purpose, this study analyzes the morphological shift of the library's bookshelves and knowledge information focused on the history of western libraries from the ancient libraries to the modern public libraries before the 20th century. And the results are follows. 1) We can make sure that the changes of the library's bookshelves are based on the environmental factors like social cultural political factors and also understand that the knowledge information are developed based on the function which is decided by the needs of the times and the serious necessities of users. With this reason, the morphology and the function of the library's bookshelves have been changed. 2) The characters and meanings of the library's bookshelves are changed with the times from movable type to fixed type, from closing type to opening type, from privileged use to public use, from religious contents to general scientific contents. 3) The morphology of the library's bookshelves are shifted from cylindrical box to Chest type Press type and Lectern system, Stall system, Wall system and advanced to Mixed(Stall+Wall) system. 4) The construction and arrangement methods of the library's bookshelves are determined by the convenience and efficiency of the installation for the perusal environments as well as the regarding to the social-cultural factors. The arrangement criteria are determined by the security topics in the earlier stage, after that determined by the lightening topics and nowadays the access topics to the information. These analysis results are very useful to research the technology of the library's bookshelves to be changed continuously and also to forecast the morphology of the library's bookshelves can be planned in the spaces of the future libraries.