• Korean Journal of Materials Research
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• v.9 no.7
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• pp.701-706
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• 1999

The crystal structure and magnetic properties of magnetic oxide system (F $e_2$ $O_3$)$_{5}$(A $l_2$ $O_3$)$_{4-x}$(G $a_2$ $O_3$)$_{x}$)SiO has been studied using X-ray diffraction and Mossbauer spectroscopy The changes of magnetic structure by the Ga ion substitution and the temperature variation have been investigated using Mossbauer spectroscopy, and the results are compared with those of the SQUIB measurements. Results of X-ray diffraction indicated that the crystal structures of the system change from a cubic spinel type to an orthorhombic via the intermediate region. This magnetic oxide system seems to be new kind of spinel type ferrites containing high concentration of cation vacancies. Various and complicated Mossbauer spectra were observed in the samples (x>0.2) at temperatures lower than room temperature. This result could be explained by freezing of the superparamagnetic dusters. On cooling and substitution, magnetic states of the system show various and multicritical properties. Unexpected dip in magnetization curves below 50K was observed in SQUID measurements. It was interpreted as an effect of spin canting including spin freezing or collective spin behavior.ior.r.

• New Physics: Sae Mulli
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• v.68 no.12
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• pp.1308-1314
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• 2018

$Er^{3+}$ and $Yb^{3+}$ co-doped $Y_3Ga_5O_{12}$ polycrystalline powders were prepared by using a solid-state reaction method, and their crystallinities were measured using X-ray diffraction. According to the results of X-ray diffraction, the powders showed a polycrystalline tetragonal structure. The photoluminescence and the upconversion luminescence properties of the $(Y_{0.85-x}Yb_{0.15})_3Ga_5O_{12}:Er^{3+}_x$ (x = 0.03, 0.06, 0.09, 0.12 and 0.15) phosphors were investigated in detail. Green and red upconversion emissions were observed for the phosphors excited by 980 nm radiation from a semiconductor laser. The powders exhibited strong green and weak red upconversion emission peaks at 553 and 660 nm, respectively. Also, their upconversion processes were explained using an energy-diagram analysis and the strongest upconversion intensity was emitted by the powder with a 0.12 mol $Er^{3+}$ ion concentration.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.411.2-411.2
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• 2016

직접 메탄올 연료전지 (DMFCs)는 친환경적이고 낮은 작동 온도로 인한 빠른 구동, 높은 에너지 밀도 등 다양한 장점을 가지고 있어 차세대 에너지 변환소자로 많은 관심을 받고 있다. 직접 메탄올 연료전지는 메탄올을 연료로 사용하며, 메탄올이 보유하고 있는 화학적 에너지를 전기 에너지로 변환하는 장치로써 음극에서는 백금 촉매로 인한 메탄올 산화반응, 양극에서는 환원 반응이 일어나며 전기화학적 구동을 하게 된다. 하지만 일산화탄소 피독으로 인한 촉매 활성 저하, 메탄올의 cross over, 백금 촉매 사용으로 인한 고비용 등의 문제점을 가지고 있다. 따라서 많은 연구자들이 백금 사용량을 줄이고 백금 촉매를 고르게 분포하기 위해 값이 저렴하고 넓은 비표면적을 갖는 탄소계 (graphite, graphene, carbon nanotube, carbon nanofiber 등) 지지체 재료를 도입하고 있다. 이 중 탄소나노섬유 (carbon nanofibers, CNFs)는 우수한 전기전도도와 열적/화학적 안정성을 가지고 있으며, 특히 넓은 비표면적을 가지고 있어 백금 촉매의 지지체로서 많은 연구가 진행되고 있다[1]. 따라서 우리는 전기방사법을 활용하여 넓은 비표면적을 보유하는 다공성 탄소나노섬유를 성공적으로 합성하였다. 또한, 이를 백금 촉매의 지지체로 도입하여 직접 메탄올 연료전지를 위한 다공성 탄소나노섬유에 담지된 고분산성 백금 촉매를 제조하였다. 제조한 다공성 탄소나노섬유의 형상 및 구조 분석은 주사전자 현미경 (field-emission scanning electron microscopy)와 투과전자 현미경 (transmission electron microscopy)를 이용하여 분석하였고, 결정구조와 화학적 결합상태는 X-선 회절분석 (X-ray diffraction) 및 X-선 광전자 분광법 (X-ray photoelectron spectroscopy)를 이용하여 규명하였다. 전기화학적 특성은 순환 전압 전류법 (cyclic voltammetry)를 이용하였다. 이러한 실험 결과들을 바탕으로 다공성 탄소나노섬유에 담지된 고분산성 백금 촉매의 자세한 특성을 본 학회에서 다루도록 하겠다.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.10 no.1
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• pp.7-13
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• 2010

We have investigated effects of the rapid thermal annealing of GaN epilayers by molecular beam epitaxy in nitrogen atmosphere. The improvement of structural properties of the samples was observed after rapid thermal annealing under optimum conditions. This improvement in crystal quality is due to a reduction of the spread in the lattice parameter in epilayers. The annealing has been performed in a rapid thermal annealing furnace at $950^{\circ}C$. The effect of rapid thermal annealing on the structural properties of GaN was studied by x-ray diffraction. The Bragg peak shifts toward larger angle as the annealing time increases. As the thermal treatment time increases, FWHM(full width at half maximum) of the peak slightly increase with its decreases followed and it increases again. Results demonstrate that rapid thermal annealing did not always promote qualities of GaN epilayers. However, rapid thermal annealing under optimum conditions improve structural properties of the samples, elevating their crystal quality with a reduction of inaccuracy in the lattice parameter of the epilayers.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.6
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• pp.246-252
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• 2011

The semiconducting $MnSi_{1.73}$ compound has been recognized as a thermoelectric material with excellent oxidation resistance and stable characteristics at elevated temperature. In the present work, we applied mechanical alloying (MA) technique to produce $MnSi_{1.73}$ compound using a mixture of elemental manganese and silicon powders. The mechanical alloying was carried out using a Fritsch P-5 planetary mill under Ar gas atmosphere. The MA powders were characterized by the X-ray diffraction with Cu-$K{\alpha}$ radiation, thermal analysis and scanning electron microscopy. Due to the observed larger loss of Si relative to Mn during mechanical alloying of $MnSi_{1.73}$, the starting composition of a mixture Mn-Si was modified to $MnSi_{1.83}$ and then $MnSi_{1.88}$. The single $MnSi_{1.73}$ phase has been obtained by mechanical alloying of $MnSi_{1.88}$ mixture powders for 200 hours. It is also found that the grain size of $MnSi_{1.73}$ compound powders analyzed by Hall plot method is reduced to 40 nm after 200 hours of milling.

• Korean Journal of Materials Research
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• v.9 no.7
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• pp.688-694
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• 1999

Ti-6Al-4V-N films have been grown onto glass substrates by dc reactive magnetron sputtering from a Ti-6Al-4V-N alloy target at different nitrogen partial pressure, input powers and sputtering times. The influence of various sputtering conditions on structural properties of Ti-6Al-4V-N films was investigated by measuring their X-ray diffraction. The quaternary Ti-6Al-4V-N film is crystallizing in a face centered cubic TiN structure, the lattice parameter is smaller than the TiN parameter as titanium atoms of the TiN lattice are replaced by aluminum and vanadium atoms. The films show the (111) preferred orientation and the (111) peak intensity decreases as the nitrogen partial pressure is increased, but the intensity increases as the sputtering time is increased. The deposition rate and the grain size are alto related with the variation of various sputtering conditions.

• Journal of the Korean Magnetics Society
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• v.6 no.6
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• pp.411-416
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• 1996

We have studied the effect of a spin-dependence interface electron scattering on the giant magnetoresistance by adding a Fe magnetic material to the Co/Cu interfaces. The $Fe(50\;{\AA})/[Co(17\;{\AA})/Fe(t\;{\AA})/Cu(24\;{\AA})]_{20}$ multilayers are deposited on the Corning glass 2948 and 7059 substrates in a dc magnetron sputtering system. The magnetoresistance ratio is 22 % in the only Co/Cu multilayer, while it is increased to 26 % with inserted ultra thin Fe interface layer and reduced with increasing thickness of the Fe interface layer. It was investigated to the dependence of the magnetoresistance behaviors on annealing temperature. The magnetic properties of the multilayers were measured by vibrating sample magnetometer. Also, the structures and the surface roughness of samples were characterized by X-ray diffraction and atomic force microscope, respectively. The magnetoresistance ratio was increased to annealing temperature $300^{\circ}C$, but reduced at the temperature higher than $300^{\circ}C$ due to the interfacial diffuse.

• Journal of the Korean Vacuum Society
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• v.19 no.4
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• pp.287-292
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• 2010

We have investigated the effects of oxygen flow ratios on the structural, morphological, and optical properties of AlN thin films grown by using radio-frequency reactive magnetron sputtering. The AlN thin films were deposited at $300^{\circ}C$ of substrate temperature, and the reactive gas were supplied with both nitrogen and oxygen. The oxygen flow ratio was varied by controlling the amount of oxygen with respect to the total mixed gases, 0%, 10%, 15%, 20%, 25%, and 30%. The structural, morphological, and optical properties of the deposited AlN thin films were examined by using X-ray diffractometer, scanning electron microscopy, and ultraviolet-visible spectrophotometer. The AlN thin film grown at 10% of oxygen flow ratio indicated an average transmittance of 91.3% in the wavelength range of 350~1,100 nm and an optical band gap of 4.30 eV. The experimental results suggest that AlN thin films can be deposited optionally by varying the oxygen flow ratio.

• Journal of the Korean Ceramic Society
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• v.40 no.10
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• pp.998-1004
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• 2003

With starting materials of two different powder mixtures, $Al_2$(S $O_4$)$_3$+xNa$_2$B$_4$ $O_{7}$$.$10$H_2O$(㏖ ratio; x=0.1, 0.7) and ${\gamma}$-Al$_2$ $O_3$+xNa$_2$B$_4$ $O_{7}$$.$10$H_2O$(㏖ ratio; x=0.1, 0.7), whisker-type $Al_{18}$B$_4$ $O_{33}$ particles were synthesized by using conventional and microwave heat-treatment. The effects of microwave, amount of flux and temperature on the growth of whisker-type $Al_{18}$B$_4$ $O_{33}$ particles were investigated by using X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM). With increase of heat-treatment temperature and amount of flux, the size of whisker-type $Al_{18}$B$_4$ $O_{33}$ particles increased in both conventional and microwave heat-treated samples. However, compared to the conventional heat-treated samples, whisker-type $Al_{18}$B$_4$ $O_{33}$ particles were well grown for the microwave heat-treated samples.ted samples.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.3
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• pp.273-279
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• 1999

The home-made hot wall epitaxy (HWE) system was utilized for GaN epitaxial layer growth on the Si(001) substrate. It was appeared that GaN epilayer grow with mixed phase of Zinc blende and Wurtzite structure from photoluminescence (PL) and x-ray diffraction (XRD) analysis at the room temperature. We found that intial growth layer has Wurtzite structure from photoluminescence (PL) and x-ray diffractio (XRD) analyses at the room temperature. Wefound that initial growth layer has Wurtzite structure when initial deposition time, the temperature of substrate and source are 4 min, $720^{\circ}C$ and $860^{\circ}C$ respectively, and at the epi growth process GaN, epilayer was grown with relatively stable Wurtzite structure when the temperature of substrate and source are $1020^{\circ}C$ and $910^{\circ}C$ respectively.