• Korean Journal of Materials Research
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• v.4 no.6
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• pp.638-643
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• 1994

The Si(ll1) surface structures induced by deposition of Au atoms were investigated by RHEED system. When Au atoms were deposited on the Si(ll1) $7\times7$ surfade, the dependence of structures and phases on the substrate temperatures and coverages was drastic. For O.1ML to 0.4ML of coverage the $7\times7$ structure changes to $7\times7$ + $5\times2$ structure as temperature increases to $350^{\circ}C$-$750^{\circ}C$. Between 0.4M1 to 1.OML the phase changed to $5 \times 2,\alpha- \sqrt{3} \times \sqrt{3},\beta- \sqrt{3} \times \sqrt{3}$ structure according to the substrate temperature and coverages. When the coverages exceeds O.SML, the 6 x 6 structure appears at the substrate temperature range between $270^{\circ}C$-$370^{\circ}C$ and compeletely transforms to 6 x6 at 1,OML. The isothermal desorption of Au on Si(ll1) surface investigated by using AES in the $\alpha- \sqrt{3} \times \sqrt{3},5 \times 2$ structures shows that the desorption energys of $\alpha- \sqrt{3} \times \sqrt{3}$ and 5 x 2 were 79Kcal/mol and 82 Kcal/mol respectively.

• Korean Journal of Materials Research
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• v.9 no.10
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• pp.1012-1017
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• 1999

Low resistance ohmic contacts to the Si-doped $\textrm{In}_{0.17}\textrm{Ga}_{0.83}\textrm{N}$(~$\times10^{19}\textrm{cm}^{-3}$) were obtained using the W metallization schemes. Specific contact resistance decreased with increasing annealing temperature. The lowest resistance is obtained after a nitrogen ambient annealing at $950^{\circ}C$ for 90 s, which results in a specific contact resistance of $2.75\times10^{-8}\Omega\textrm{cm}^{-3}$. Interfacial reactions and surface are analyzed using x-ray diffraction and scanning electron microscopy (SEM). The X-ray diffraction results show that the reactions between the W film and the $\textrm{In}_{0.17}\textrm{Ga}_{0.83}\textrm{N}$ produce a $\beta$-$W_2N$ phase at the interface. The SEM result shows that the morphology of the contacts is stable up to a temperature as high as $850^{\circ}C$. Possible mechanisms are proposed to describe the annealing temperature dependence of the specific contact resistance.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.1
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• pp.169-178
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• 1998

The crystal phases precipitated in various compositions glass of CaO-$SiO_2-P_2O_5$ system, were identified by XRD. E composition (CaO 49.4, $SiO_2\;36.8,\;P_2O_5$ 8.8 wt%) glass in which both apatite(($Ca_{10}(PO_4)_6O$ and $\beta$-wollastonite($CaSiO_3$) crystals would precipitate by heating, was selected as an experimental composition to prepare the glass ceramics with high bending strength and good bioactivity to the living bone. Glass powders of E composition were unidirectionally crystallized at $1050^{\circ}$C in the temperature-gadient furnace and the resultant glass ceramics were characterized. Bending strength of the glass ceramics was also measured. To investigate the bond forming ability between the glass ceramics and living bone tissue, soaking test of glass ceramics in simulated body fluid was carried out. Densed glass ceramics composed of apatite and $\beta$-wollastonite crystal were prepared by unidirectional crystallization under the optimum conditions. (2 0 2) plane of $\beta$-wollastonite crystals tended to grow perpendicularly to the crystallization direction. Average bending strength of this glass ceramics was 186.9 MPa, higher than that of the glass ceramics prepared by isothermal (not directional) crystallization In soaking test, a thin layer of apatite crystallite was formed on surface of the glass ceramics in 3 days. Apatite crystals formed on the glass ceramics could be act a role to make the chemical bond between the glass ceramics and living bone tissue.

• Journal of Magnetics
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• v.18 no.4
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• pp.400-404
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• 2013

Grain boundary diffusion technique with $DyH_3$ nanoparticles was applied to fabricate Dy-less sintered Nd-Fe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet were improved by 60% and reduced by 7% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient (${\alpha}$) and the coercivity temperature coefficient (${\beta}$) of the magnets were improved after diffusion treatment. Investigation shows that Dy is preferentially enriched as (Nd, Dy)$_2Fe_{14}B$ phase in the surface region of the $Nd_2Fe_{14}B$ matrix grains indicated by the remarkable enhancement of the magneto-crystalline anisotropy field of the magnet. As a result, the magnet diffused with a small amount of Dy nanoparticles possesses enhanced coercivity without remarkably sacrificing its magnetization.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.34 no.1
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• pp.1-7
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• 2024

β-Ga₂O₃ is a material with a wide band gap of ~4.8 eV and a high breakdown-voltage of 8 MV/cm, and is attracting much attention in the field of power device applications. In addition, compared to representative WBG semiconductor materials such as SiC, GaN and Diamond, it has the advantage of enabling single crystal growth with high growth rate and low manufacturing cost [1-4]. In this study, we succeeded in growing a 10 mm thick β-Ga₂O₃ single crystal doped with 0.3 mol% SnO₂ through the EFG (Edge-defined Film-fed Growth) method using multi-slit structure. The growth direction and growth plane were set to [010]/(010), respectively, and the growth speed was about 12 mm/h. The grown β-Ga₂O₃ single crystal was cut into various crystal planes (010, 001, 100, ${\bar{2}}01$) and surface processed. The processed samples were compared for characteristics according to crystal plane through analysis such as XRD, UV/VIS/NIR/Spec., Mercury Probe, AFM and Etching. This research is expected to contribute to the development of power semiconductor technology in high-voltage and high-temperature applications, and selecting a substrate with better characteristics will play an important role in improving device performance and reliability.

• Journal of Surface Science and Engineering
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• v.52 no.6
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• pp.283-288
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• 2019

MCrAlY overaly coatings are used as oxidation barrier coatings to prevent degradation of the underlying substrate in high temperature and oxidizing environment of the hot section of gas turbines. Therefore, oxidation resistance in high temperature is important property of MCrAlY coatings. Also, coefficients of thermal expansion (CTE) of MCrAlY have middle value of that of Ni-based superalloys and oxides, which have the effect of preventing the delamination of the surface oxides. Cyclic oxidation test is one of the most useful methods for evaluating the high temperature durability of coatings used in gas turbines. In this study, NiCoCrAlY overlay coatings were formed on Inconel 792(IN 792) substrates by vacuum plasma spraying process. Vacuum plasma sprayed NiCoCrAlY coatings and IN 792 susbstrates were exposed to 1000℃ one-hour cyclic oxidation environment. NiCoCrAlY coatings showed lower weight gain in short-term oxidation. In long-term oxidation, IN 792 substrates showed higher weight loss due to delamination of surface oxide but NiCoCrAlY coatings showed lower weight loss. X-ray diffraction (XRD) analysis showed α-Al2O3 and NiCr₂O₄ was formed during the cyclic oxidation test. Through cross-section observation using scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD) analysis, thermally grown oxide (TGO) layer composed of α-Al₂O₃ and NiCr₂O₄ was formed and the thickness of TGO increased during 1000℃ cyclic oxidation test. β phase in upper side of NiCoCrAlY coating was depleted due to oxidation of Al and outer beta depletion zone thickness also increased as the cyclic oxidation time increased.

• Korean Journal of Materials Research
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• v.21 no.7
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• pp.357-363
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• 2011

In this paper, high temperature oxidation behavior of newly developed alloys, Ti-6Al-4Fe and Ti-6Al-1Fe, is examined. To understand the effect of Fe on the air oxidation behavior of the Ti-Al-Fe alloy system, thermal oxidation tests are carried out at $700^{\circ}C$ and $800^{\circ}C$ for 96 hours. Ti-6Al-4V alloy is also prepared and tested under the same conditions for comparison with the developed alloys. The oxidation resistance of the Ti-Al-Fe alloy system is superior to that of Ti-6Al-4V alloy. Ti-6Al-4V shows the worst oxidation resistance for all test conditions. This is not a result of the addition of Fe, but rather it is due to the elimination of V, which has deleterious effects on high temperature oxidation. The oxidation of the Ti-Al-Fe alloy system follows the parabolic rate law. At $700^{\circ}C$, Fe addition does not have a noticeable influence on the amount of weight gain of all specimens. However, at $800^{\circ}C$, Ti-6Al-4Fe alloy shows remarkable degradation compared to Ti-6Al-1Fe and Ti-6Al. It is discovered that the formation of $Al_2O_3$, a diffusion resistance layer, is remarkably hindered by a relative decrease of the ${\alpha}$ volume fraction. This is because Fe addition increases the volume fraction of ${\beta}$ phase within the Ti-6Al-xFe alloy system. Activities of Al, Ti, and Fe with respect to the formation of oxide layers are calculated and analyzed to explore the oxidation mechanism.

• Journal of Chest Surgery
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• v.36 no.7
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• pp.472-482
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• 2003

Recent studies have demonstrated that cerebral desaturation during rewarming period of CPB was associated with postoperative neurologic dysfunction. The prevention of cerebral desaturation during CPB may reduce the incidences of neurologic and neuropsychological complications. The present study was prospectively undertaken to compare the clinical effects between two strategies (hypercapnic CPB and high flow CPB) to prevent cerebral desaturation for establishing a proper CPB technique. Material and Method: Thirty-six adult patients scheduled for elective cardiac surgery were randomized into either hypercapnic (Pa$CO_2$ 45~50mmHg, n＝18) or high flow group (flow rate 2.75 L/ $m^2$/min and Pa$CO_2$ 35~40mmHg, n＝18) during rewarming period of CPB. In each patient, middle cerebral artery blood flow velocity ( $V_{MCA}$), cerebral arteriovenous oxygen content difference (C(a-v) $O_2$), modified cerebral metabolic rate for oxygen (MCMR $O_2$), cerebral oxygen transport rate ( $T_{E}$ $O_2$), incidence of cerebral desaturation (internal jugular bulb blood oxygen saturation $\leq$ 50%), increased rate of S-100 $\beta$ concentration, and arterial and internal jugular bulb blood gas were measured during the five phases of the operation; Pre-CPB, CPB-10 min (steady-state CPB, nasopharyngeal temperature 29~3$0^{\circ}C$), Rewarm-1 (rewarming phase, nasopharyngeal temperature 33$^{\circ}C$), Rewarm-2 (nasopharyngeal temperature 37$^{\circ}C$), and CPB-off. Incidence of postoperative delirium and duration were assessed in all patients. All variables were compared between the two groups. Result: $V_{MCA}$ (157.88$\pm$10.87 vs 120.00$\pm$6.18%, p＝0.006), internal jugular bulb $O_2$ saturation (68.01$\pm$2.75 vs 61.28$\pm$2.87%, p＝0.03) and $O_2$ tension (41.01$\pm$2.25 vs 32.02$\pm$ 1,67 mmHg, p＝0.03), and $T_{E}$ $O_2$(110.84$\pm$7.41 vs 81.15$\pm$8.11%, p＝0.003) at rewarming periods were higher in the hypercapnic group than in the high flow group. C(a-v) $O_2$ (4.0$\pm$0.30 vs 4.84$\pm$0.38 mg/dL, p＝0.04), COE (0.36$\pm$0.03 vs 0.42$\pm$0.03, p＝0.04), increased rate of S- 100$\beta$ (391.67$\pm$23.40 vs 940.0$\pm$17.02%, p＝0.003), and incidence of cerebral desaturation (2 vs 4 patients, p＝0.04) at rewarming periods, and duration of postoperative delirium (18 vs 34 hr, p＝0.02) were low in the hypercapnic group compared to the high flow group. Conclusion: These results indicate that hypercapnic CPB may provide relatively diminished cerebral injury and beneficial effects for cerebral metabolism relatively compared to high flow CPB.low CPB.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.19 no.2
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• pp.79-83
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• 2009

The glass-ceramic of the $Li_{2}O-Al_{2}O_{3}-SiO_{2}$ system was investigated to develop the low thermal expansion materials. The glass of this system was heat treated at $775^{\circ}C$ for 2 h for nucleation and subsequently at $825{\sim}900^{\circ}C$ for 2 h for crystallization. The crystal structure of the glass-ceramic of this system was a single phase of $\beta$-quartz solid solution($Li_{x}Al_{x}Si_{1-x}O_{2}$). The thermal expansion of the glass-ceramic showed $4.40{\times}10^{-7}{\sim}1.33{\times}10^{-6}K^{-1}$ between $25{\sim}300^{\circ}C$ and $1.56{\times}10^{-6}{\sim}2.53{\times}10^{-6}K^{-1}$ between $25{\sim}800^{\circ}C$, higher than lower temperature range. The mechanical strength remained almost same at around high 110 MPa with heating temperature changes.

• Korean Journal of Materials Research
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• v.4 no.6
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• pp.626-637
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• 1994

The relationship between microstructures and tensile properties depending on various solution treatment temperature and cooling rate of Ti-6A1-4V alloy have been investigated. The complex and random edge shaped $\alpha$ phases were formed after the 1st solution treatment at $\beta$ region and the 2nd solution treatment at $900^{\circ}C$, which was followed by furnace cooled. When the specimen was subjected to the 2nd solution treated at $950^{\circ}C$, and furnace cooled, $\alpha$ phase changed its morphology to equiaxed structure. The aspect ratio showing the appreciation basis of microstructual refinement decreases with the temperature of 1st and 2nd solution treatment. The slightly decrease in strength were observed in the Widmanstltten structures than in the bimodal structures. Also, ductility of the Widmanstatten structures was considerable lower than that of bimodal structures. The tensile-fractured surface of the Widmanstatten structures appears to be quasi-cleavage and dimple fracture, while that of the bimodal structures was the type of ductile fracture. The tensile fracture surface of the bimodal structures can easily be separated into cental crack areas lying generally perpendicular to the tensile axis and shear lip areas lying at angles of high shear(around 45 deg.) to the tensile axis.