• 한국진공학회:학술대회논문집
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• 한국진공학회 2000년도 제18회 학술발표회 논문개요집
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• pp.140-140
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• 2000

The initial growth mode of Nb on Ag(11) in sub-monolayer regime and the influence of subsequent 520K annealing are studied using UHV Scanning Tunneling Microscopy. E-beam evaporated Nb is deposited onto the substrate at RT, and STM measurements are carried out at RT and 78 K. With Nb being immiscible in bulk Ag, 3D islands formation begins at early stage and no particular ordered structure is found. After annealing to 520K, most of islands are disappeared from terrace. There exist 2 possibilities. : (1) Diffusion of Nb into the 2nd or 3rd layer of Ag substrate or (2) agglomeration of Nb on Ag at higher temperature. A model will be given to explain the evidence. In addition, we investigated the change of STM image according to bias voltage depending on island size. Possible physical mechanism responsible for such behavior together with interaction between Nb islands and reactive gases will be also discussed.

• Bulletin of the Korean Chemical Society
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• 제19권4호
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• pp.439-444
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• 1998

The protonation of tetraphenylporphyrin (TPP) in acidic organic solutions was analyzed by acid titrimetric and temperature-dependent absorption measurements. Competition between the protonation of free base TPP $(TPPH_2)$ and the solvation of proton by near solvent molecules determines the equilibrium of the diprotonated TPP $(TPPH_4^{2+})$ formation. The diprotonated TPP exists as an ion pair complex with the acid counterions, which are found to affect the degree of red shift of the Soret band. The rotation of the phenyl rings also plays an important role in the diprotonation, as suggested by the decrease in the degree of diprotonation for the fluorophenyl TPP derivatives whose phenyl ring rotation is significantly hindered relative to normal TPP. The difference of fluorescence lifetime between $TPPH_2 \;({\pi}_{FL}=19.6\;ns)\; and\; TPPH_4^{2+} \;({\pi}_{FL}=2.1 \;ns)$ was used advantageously to measure the rate of protonation in the excited state. The protonation of TPPH2 are found to occur much slower than the diffusion of protons from bulk solution to the porphyrin ring. The monoprotonated TPP is suggested to be the transient species for the diprotonation process.

• 천문학회지
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• 제52권5호
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• pp.173-180
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• 2019

We present IR flux density measurements, models of the broadband SED, and results of SED modeling for the Pulsar Wind Nebula (PWN) 3C 58. We find that the Herschel flux density seems to be slightly lower than suggested by interpolation of previous measurements in nearby wavebands, implying that there may be multiple electron populations in 3C 58. We model the SED using a simple stationary one-zone and a more realistic time-evolving multi-zone scenario. The latter includes variations of flow properties in the PWN (injected energy, magnetic field, and bulk speed), radiative energy losses, adiabatic expansion, and diffusion, similar to previous PWN models. From the modeling, we find that a PWN age of 2900-5400 yrs is preferred and that there may be excess emission at ${\sim}10^{11}Hz$. The latter may imply multiple populations of electrons in the PWN.

• 한국자기학회지
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• 제5권5호
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• pp.745-748
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• 1995

Magnetic nanoparticles of iron and iron oxide have been prepared in a modified upward thermal diffusion cloud chamber using pulsed laser evaporation. SEM/TEM studies of these particles reveal a size distribution with a mean diameter of about $60\;{\AA}$. FTIR spectrum measurements are used to investigate the difference in oxidation level between nanoparticles prepared at different partial oxygen pressures. The complex magnetic behaviour of these particles was studied using DC- and AC-susceptibility measurements. All samples exhibit superparamagnetism with blocking temperatures ranging from 50 K to above room temperature. The coercivity fields as well as the dependence of the blocking temperature on measuring frequency have been studied. magnetic anisotropy constants are found to be one order of magnitude higher than is known for the bulk values. The mean particle size estimated from the magnetic data is found to be in perfect agreement with the TEM observations.

• Korean Chemical Engineering Research
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• 제57권3호
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• pp.368-371
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• 2019

Using first principles calculations we investigated the thermomechanical stability of spent nuclear fuels (SNF), especially how mechanical properties of $UO_2$, such as, bulk, shear and Young's moduli and Poisson's ratio vary through alpha-decay of U into Th with generation of He gas. Our results indicate that substitution of U by Th through alpha decay ($U_{1-x}Th_xO_2$) does not significantly affect the stability of the grain in a fuel matrix. In addition, we studied the transport properties of He in and boundaries of the $U_{1-x}Th_xO_2$ grain. Helium preferentially resides at the grain boundaries through diffusion. Our study can contribute to substantial reduction of environmentally risk and enhancement of our sustainability by safe control of radioactive materials.

• 한국항만학회지
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• 제11권2호
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• pp.203-214
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• 1997

Hydraulic dredgers(Hopper dredger) are the most important piece of equipment in the entire harbor engineering field, and most suitable for the removal of sand and weakly consloidated sediment such as silt. In maintenance dredging, specially confined harbor or congested passage area, Hopper dredger is user most popularly because less obstruction and danger to navigation than other mostly stationary dredgers. Investigation of the physical behave of dredged material disposal in coastal water from the Hopper dredger includes estimations of pattern as well as thickness of material on the bottom. Calculation based on vertical settling and horizontal advection of single particles ignore the effects of bulk properties of the disposed marterial, vertical and horizontal diffusion. and material dilution due to the entrainment of ambient water during descent. This paper focuses on the analysis of dredging and dumping characteristics and the spatial and temporal changes in the dumping fields for the water column and bottom at a hypothetically confined coastal water. This model accounts the behavior of material after release from the hopper dredger. It is shown that the model describes the qualitative feature of prototype dumping process and its response.

• 한국진공학회지
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• 제18권2호
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• pp.127-132
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• 2009

Sb에 기초한 응력 초격자 적외선검출소자의 구성 물질인 도핑하지 않은 기판 GaSb 결정과 GaSb/SI-GaAs 박막에 잔존하고 있는 진성결함 (intrinsic defect)을 비교 조사하였다. 상온 근처 (250 K)까지 광여기 발광 (PL)을 보이는 GaSb 결정에서의 발광 에너지의 온도의존성으로부터, 밴드갭 에너지에 관한 경험식인 Varshni 함수의 파라미터 ($E_o$, $\alpha$, $\beta$)를 결정하였다. GaAs 기판 위에 성장된 이종 GaSb 박막에서는 GaSb 주요 진성결함으로 알려져 있는 29 meV의 이온화 에너지를 가지는 위치반전 (antisite) Ga ([$Ga_{Sb}$]) 결함과 함께 위치반전 Sb ([$Sb_{Ga}$])와의 복합결함 ([$Ga_{Sb}-Sb_{Ga}$])과 관련된 것으로 분석된 732/711 meV의 한 쌍의 깊은준위 (deep level)가 관측되었다. PL의 온도 및 여기출력 의존성을 분석하여, Sb-rich상태에서 성장된 GaSb 박막에서는 잉여 Sb의 자발확산 (self-diffusion)에 의하여 치환된 위치전도 [$Ga_{Sb}$] 및 [$Sb_{Ga}$]가 결합하여 [$Ga_{Sb}-Sb_{Ga}$]의 깊은준위를 형성하는 것으로 해석되었다.

• Journal of Radiation Protection and Research
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• 제20권4호
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• pp.265-274
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• 1995

Co-60은 원자력발전소에서 발생하는 부식생성물로서 중저준위 방사성폐기물에 함유된 가장 중요한 핵종중 하나다 방사성폐기물처분장 충전물로 많이 사용되는 점토층을 통한 Co-60 확산실험을 하여 밀도변화에 따른 확산계수를 구하였다. Co-60의 확산실험 기간은 점토 밀도에 따라 최소 9시간에서 최대 120일이 걸렸으며, 확산계수는 저밀도인 $0.41g/cm^3$에서 $8.79{\times}10^{11}m^2/s$로부터 고밀도인 $2.03g/cm^3$에서 $6.82{\times}10^{14}m^2/s$까지 급격히 감소하는 현상을 보여주었다. 그리고, 기존에 수행한 연구와 비친 결과 다음과 같은 사실을 확인할 수 있었다. 첫째, 2가 이온인 Sr나 Co이온은 저밀도 점토층에서 1가 이온인 Cs 보다 큰 확산속도를 갖는다. 이러한 현상은 점토표면에 흡착되는 양이온들의 수화상태로 부터 설명할 수 있었다. 둘째, Co 이온의 경우 저밀도에서는 Sr이온보다도 큰 확산계수를 보여준다. 이러한 현상은 Co이온의 수화반경이 Sr이온보다 크다는 사실로 해석할 수 있었다. 셋째, 밀도가 증가함에 따라 Co 이온의 확산계수는 Cs 이온보다도 작은 값으로 급격히 감소한다. 이러한 현상은 점토층의 밀도가 증가함에 따라 점토표면과 화학결합을 하는 Co 이온들이 급격히 증가하고, 점토의 결정속으로 잠적하여 점토의 일부가 되는 이온들이 급격히 증가하기 때문인 것으로 생각된다. 이와 같은 현상들은 표면확산이론으로 설명이 가능하며 특히 저밀도 점토층에서 Co-60의 확산속도가 크다는 것은 중 저준위 방사성 폐기물 처분장 설계나 안전성 평가에 매우 중요한 자료가 될 것이다.

• 대한전자공학회논문지SD
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• 제39권3호
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• pp.9-19
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• 2002

본 논문에서는 thermionic emission 모델을 이용하여 SCH 양자우물 레이저 다이오드에 대한 L-I-V특성을 해석적으로 도출하였다. SCH의 bulk 캐리어와 양자우물 속박 캐리어의 관계를 도출하였고, 주입된 전류를 각 영역에서의 캐리어 재결합을 고려한 전류 연속 방정식을 만족하도록 하였다. 또한, high level injection과 전하 중성 조건하에 ambipolar 확산 방정식을 이용하여 캐리어 분포를 고찰하였다. 위 해석적인 모델을 이용하여 계산한 결과, 클래딩 영역의 전위장벽 변화가 전류 전압 특성 변화의 주요 원인으로 나타났다. 또한 thermionic emission에 의한 주입 전류의 forward flux 증가가 캐리어 주입을 증가시키고, 레이저 다이오드의 직렬 저항을 감소시키는 것을 보였다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2004년도 추계학술대회 논문집 Vol.17
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• pp.276-279
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• 2004

Plasma enhanced chemical vapor deposition (PECVD) of silicon nitride (SiN) is a proven technique for obtaining layers that meet the needs of surface passivation and anti-reflection coating. In addition, the deposition process appears to provoke bulk passivation as well due to diffusion of atomic hydrogen. This bulk passivation is an important advantage of PECVD deposition when compared to the conventional CVD techniques. A further advantage of PECVD is that the process takes place at a relatively low temperature of 300t, keeping the total thermal budget of the cell processing to a minimum. In this work SiN deposition was performed using a horizontal PECVD reactor system consisting of a long horizontal quartz tube that was radiantly heated. Special and long rectangular graphite plates served as both the electrodes to establish the plasma and holders of the wafers. The electrode configuration was designed to provide a uniform plasma environment for each wafer and to ensure the film uniformity. These horizontally oriented graphite electrodes were stacked parallel to one another, side by side, with alternating plates serving as power and ground electrodes for the RF power supply. The plasma was formed in the space between each pair of plates. Also this paper deals with the fabrication of multicrystalline silicon solar cells with PECVD SiN layers combined with high-throughput screen printing and RTP firing. Using this sequence we were able to obtain solar cells with an efficiency of 14% for polished multi crystalline Si wafers of size 125 m square.