• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.286-286
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• 2016

Hexagonal boron nitride (hBN) is a dielectric insulator with a two-dimensional (2D) layered structure. It is an appealing substrate dielectric for many applications due to its favorable properties, such as a wide band gap energy, chemical inertness and high thermal conductivity[1]. Furthermore, its remarkable mechanical strength renders few-layered hBN a flexible and transparent substrate, ideal for next-generation electronics and optoelectronics in applications. However, the difficulty of preparing high quality large-area hBN films has hindered their widespread use. Generally, large-area hBN layers prepared by chemical vapor deposition (CVD) usually exhibit polycrystalline structures with a typical average grain size of several microns. It has been reported that grain boundaries or dislocations in hBN can degrade its electronic or mechanical properties. Accordingly, large-area single crystalline hBN layers are desired to fully realize the potential advantages of hBN in device applications. In this presentation, we report the growth and transfer of centimeter-sized, nearly single crystal hexagonal boron nitride (hBN) few-layer films using Ni(111) single crystal substrates. The hBN films were grown on Ni(111) substrates using atmospheric pressure chemical vapor deposition (APCVD). The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and remaining Ni(111) substrates were repeatedly re-used. The crystallinity of the grown films from the atomic to centimeter scale was confirmed based on transmission electron microscopy (TEM) and reflection high energy electron diffraction (RHEED). Careful study of the growth parameters was also carried out. Moreover, various characterizations confirmed that the grown films exhibited typical characteristics of hexagonal boron nitride layers over the entire area. Our results suggest that hBN can be widely used in various applications where large-area, high quality, and single crystalline 2D insulating layers are required.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.362.1-362.1
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• 2016

InAs nanowires were synthesized by a vapor-liquid-solid method with InAs powder. The composition and crystalline structure of nanowires were confirmed by energy-dispersive spectroscopy (EDS) and high resolution transmission electron microscopy (HRTEM), respectively. The thermal conduction of nanowires was investigated by the optical method using Raman spectroscopy: i.e., the local temperature on nanowire was determined by laser heating. As temperature increased, the Raman peaks are shifted to low frequency and broadened. The temperature dependent Raman scattering experiments was realized on InAs nanowires with different percentages of zinc-blende and wurtzite structure. The temperature dependence on the nanowire structure has been successfully obtained: the phonon scattering was more increased in InAs heretostructure nanowires, compared to the InAs nanowires with homostructure. The result strongly suggests that the thermal conduction can be effectively controlled by ordered interface without any decrease in electrical conduction.

• Nuclear Engineering and Technology
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• v.55 no.6
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• pp.2298-2304
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• 2023

Titanium alloys are expected to become one of the candidate materials for nuclear-powered spacecraft due to their excellent overall performance. Nevertheless, atomistic mechanisms of the defect accumulation and evolution of the materials due to long-term exposure to irradiation remain scarcely understood by far. Here we investigate the heavy irradiation damage in a-titanium with a dose as high as 4.0 canonical displacements per atom (cDPA) using atomistic simulations of Frenkel pair accumulation. Results show that the content of surviving defects increases sharply before 0.04 cDPA and then decreases slowly to stabilize, exhibiting a strong correlation with the system energy. Under the current simulation conditions, the defect clustering fraction may be not directly dependent on the irradiation dose. Compared to vacancies, interstitials are more likely to form clusters, which may further cause the formation of 1/3<1210> interstitial-type dislocation loops extended along the (1010) plane. This study provides an important insight into the understanding of the irradiation damage behaviors for titanium.

• East Asian mathematical journal
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• v.37 no.1
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• pp.141-147
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• 2021

In [1], we studied the PDE system with time-varing delay. Time delay occurs due to loosening in a high-speed moving axially directed membrane (string, belt, or plate) at production. Our purpose in this work derives a mathematical model with internal time delay. First, we consider the physical phenomenon of axially moving membrane with respect to kinetic energy, potential energy and work done. By the energy conservation law in physics, we get the second order nonlinear PDE system with internal time delay.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.36.3-36.3
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• 2017

The Telescope Array (TA) experiment reported the arrival direction distribution of ultra-high-energy cosmic rays (UHECRs) with energies above $5.7{\times}10^{19}eV$ in the northern sky. A clustering of TA events, the so-called hotspot, was found; however, its nature has not yet been understood. To understand the origin of the TA hotspot, we examine the sky distributions of the TA UHECR arrival direction and filamentary structures of galaxies in the local universe. By statistical tests for anisotropy, we find a close correlation of the TA events with the filaments of galaxies connected to the Virgo cluster. We discuss our finding and its implications.

• Proceedings of the Korean Society of Medical Physics Conference
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• 1999.11a
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• pp.320-322
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• 1999

• Proceedings of the Korean Society of Medical Physics Conference
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• 1999.11a
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• pp.327-329
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• 1999

• Progress in Medical Physics
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• v.27 no.3
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• pp.125-130
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• 2016

Dual energy computed tomography (DECT) is used to classify two materials and quantify the mass density of each material in the human body. An energy modulation filter based DECT could acquire two images, which are generated by the low- and high-energy photon spectra, in one scan, with one tube and detector. In the case of DECT using the energy modulation filter, the filter should perform the optimization process for the type of materials and thicknesses for generating two photon spectra. In this study, Geant4 Monte-Carlo simulation toolkit was used to execute the optimization process for determining the property of the energy modulation filter. In the process, various materials used for the energy modulation filter are copper (Cu, $8.96g/cm^3$), niobium (Nb, $8.57g/cm^3$), stannum (Sn, $7.31g/cm^3$), gold (Au, $19.32g/cm^3$), and lead (Pb, $11.34g/cm^3$). The thickness of the modulation filter varied from 0.1 mm to 1.0 mm. To evaluate the overlap region of the low- and high-energy spectrum, Geant4 Monte-Carlo simulation is used. The variation of the photon flux and the mean energy of photon spectrum that passes through the energy modulation filter are evaluated. In the primary photon spectrum of 80 kVp, the optimal modulation filter is a 0.1 mm lead filter that can acquire the same mean energy of 140 kVp photon spectrum. The lead filter of 0.1 mm based dual energy CBCT is required to increase the tube current 4.37 times than the original tube current owing to the 77.1% attenuation in the filter.

• Progress in Medical Physics
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• v.28 no.2
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• pp.45-48
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• 2017

This paper evaluated the amount of radiation generated by wedge filters during radiation therapy using a high-energy linear accelerator, and the dose to the worker during wedge replacement. After 10-MV photon beam was irradiated with wedge filter, the wedge was removed from the linear accelerator, and the dose rate and energy spectrum were measured. The initial measurement was approximately 1 uSv/h, and the radiation level was reduced to 0.3 uSv/h after 6 min. The effective half-life derived from the dose rate measurement was approximately 3.5 min, and the influence of AI-28 was about 53%. From the energy spectrum measurements, a peak of 1,799 keV was measured for AI-28, while the peak for Co-58 was not measured in the control room. The peaks for Au-106 and Cd-105 were found only measurement was done without wedge removement from the linear accelerator. The additional doses received by the radiation worker during wedge replacement were estimated to be 0.08-0.4 mSv per year.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.38.3-38.3
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• 2015

Recently, the Pierre Auger Observatory (PAO), the largest ground-based project for detecting ultra-high-energy cosmic rays (UHECRs), published their 10-year data. We can access an unprecedented number of UHECR data observed by the project, which give us a possibility to get an accurate statistical test result. In this work, we investigate the influence of the galactic magnetic field (GMF) on the distribution of UHECRs by searching the correlation with the large-scale structure (LSS) of the universe. We simulate the mock UHECR events whose trajectories from the sources would be deflected by the Gaussian smearing angle which reflects the influence by the GMF. By the statistical test, we compare the correlation between the expected/observed distribution of UHECRs and the LSS of the universe in the regions of sky divided by the galactic latitude, varying the smearing angle. Here, we assume the deflections by the GMF are mainly dependent on the galactic latitude. Using the maximum likelihood estimation, we find the best-fit smearing angle in each region. If we get a trend that best-fit smearing angles differ from each region, the influence of GMF may be stronger than that of intergalactic magnetic fields (IGMF) because it is known that the distribution of IGMF follows the LSS of the universe. Also, we can estimate the strength of the GMF using the best-fit parameter by the maximum likelihood.