• Nuclear Engineering and Technology
• /
• 제32권6호
• /
• pp.605-617
• /
• 2000

Nodal transport methods are studied for the solution of two dimensional discrete-ordinates, simplified even-parity transport equation(SEP) which is known to be an approximation to the true transport equation. The polynomial expansion nodal method(PEN) and the analytic function expansion nodal method(AFEN）which have been developed for the diffusion theory are used for the solution of the discrete-ordinates form of SEP equation. Our study shows that while the PEN method in diffusion theory can directly be converted without complication, the AFEN method requires a theoretical modification due to the nonhomogeneous property of the transport equation. The numerical results show that the proposed two methods work well with the SEP transport equation with higher accuracies compared with the conventional finite difference method.

• 한국전기전자재료학회논문지
• /
• 제34권6호
• /
• pp.461-465
• /
• 2021

A diffusion heat treatment process for YBa₂Cu₃O_7-y bulk superconductor in a Gd₂O₃ powder was attempted. As a result of measuring the critical temperature of the superconducting bulk, there was no change in the superconducting transition temperature as the Gd particles diffused into the YBa₂Cu₃O_7-y lattice, resulting in dense microstructure. As a result of measuring the critical current, the critical current density (J_c) of the superconducting bulk having treated by the Gd thermal diffusion treatment at 0 T increased to 3×10⁴ A/cm² at 0 T, which was higher than that of the superconducting bulk without thermal diffusion treatment. The surface magnetic force of the superconducting bulk with Gd thermal diffusion treatment was observed at the center of the superconducting bulk with the maximum trapped magnetic force (Hmax) of 1.51 kG. This result means that the Gd thermal diffusion treatment contributes to improving the critical current density Jc of YBa₂Cu₃O_7-y, and it is believed that Gd particles migrating into the superconducting bulk through thermal diffusion either fill the surface pores of YBa₂Cu₃O_7-y superconductors or act as a flux pinning center.

• Journal of Pharmaceutical Investigation
• /
• 제41권6호
• /
• pp.337-345
• /
• 2011

The objective of this work is to investigate the effect of chemical enhancer and current on the flux of donepezil hydrochloride (DH) through skin. Ethanol and N-methyl pyrrolidone (NMP) were used as chemical enhancers in combination with iontophoresis. We also have studied the effect of pH on flux and evaluated the role of electroosmosis. In vitro flux study was performed at $33^{\circ}C$, using side-by-side diffusion cell and full thickness hairless mouse skin. Passive flux of DH without enhancer was very small. As the concentration of enhancer increased, passive flux increased. After current application, flux increased markedly and the time to reach maximum decreased. Without enhancer, maximum flux was about 50 fold larger than that obtained without current. These results indicate that electromigration is playing a major role for the transport. As the enhancer concentration increased, flux also increased. NMP and ethanol increased not only the passive delivery, but also the iontophoretic delivery. Flux results indicate that ethanol has better ability than NMP in enhancing the transport of DH. The magnitudes of increase in flux by these enhancers indicate that there is a large synergistic effect in flux enhancement. Flux results from pH study showed that electroosmotic flow is reversed at low pH and the flux is hindered. These results provided some information on the flux enhancing ability of ethanol and NMP in combination with iontophoresis. The data also provided some mechanistic insights into the role of electromigration and electroosmosis on flux through skin.

• JSTS:Journal of Semiconductor Technology and Science
• /
• 제7권3호
• /
• pp.196-208
• /
• 2007

An analytical model is proposed for an optically controlled Metal Semiconductor Field Effect Transistor (MESFET), known as Optical Field Effect Transistor (OPFET) considering the diffusion fabrication process. The electrical parameters such as threshold voltage, drain-source current, gate capacitances and switching response have been determined for the dark and various illuminated conditions. The Photovoltaic effect due to photogenerated carriers under illumination is shown to modulate the channel cross-section, which in turn significantly changes the threshold voltage, drainsource current, the gate capacitances and the device switching speed. The threshold voltage $V_T$ is reduced under optical illumination condition, which leads the device to change the device property from enhancement mode to depletion mode depending on photon impurity flux density. The resulting I-V characteristics show that the drain-source current IDS for different gate-source voltage $V_{gs}$ is significantly increased with optical illumination for photon flux densities of ${\Phi}=10^{15}\;and\;10^{17}/cm^2s$ compared to the dark condition. Further more, the drain-source current as a function of drain-source voltage $V_{DS}$ is evaluated to find the I-V characteristics for various pinch-off voltages $V_P$ for optimization of impurity flux density $Q_{Diff}$ by diffusion process. The resulting I-V characteristics also show that the diffusion process introduces less process-induced damage compared to ion implantation, which suffers from current reduction due to a large number of defects introduced by the ion implantation process. Further the results show significant increase in gate-source capacitance $C_{gs}$ and gate-drain capacitance $C_{gd}$ for optical illuminations, where the photo-induced voltage has a significant role on gate capacitances. The switching time ${\tau}$ of the OPFET device is computed for dark and illumination conditions. The switching time ${\tau}$ is greatly reduced by optical illumination and is also a function of device active layer thickness and corresponding impurity flux density $Q_{Diff}$. Thus it is shown that the diffusion process shows great potential for improvement of optoelectronic devices in quantum efficiency and other performance areas.

• Nuclear Engineering and Technology
• /
• 제53권2호
• /
• pp.399-413
• /
• 2021

The solution of the time-dependent neutron diffusion equation can be approximated using quasi-static methods that factorise the neutronic flux as the product of a time dependent function times a shape function that depends both on space and time. A generalization of this technique is the updated modal method. This strategy assumes that the neutron flux can be decomposed into a sum of amplitudes multiplied by some shape functions. These functions, known as modes, come from the solution of the eigenvalue problems associated with the static neutron diffusion equation that are being updated along the transient. In previous works, the time step used to update the modes is set to a fixed value and this implies the need of using small time-steps to obtain accurate results and, consequently, a high computational cost. In this work, we propose the use of an adaptive control time-step that reduces automatically the time-step when the algorithm detects large errors and increases this value when it is not necessary to use small steps. Several strategies to compute the modes updating time step are proposed and their performance is tested for different transients in benchmark reactors with rectangular and hexagonal geometry.

• 대한약학회:학술대회논문집
• /
• 대한약학회 2002년도 Proceedings of the Convention of the Pharmaceutical Society of Korea Vol.2
• /
• pp.411.2-411.2
• /
• 2002

In order to develope an optimum formulation for iontophoretic delivery of vitamine-C-2 phosphate. we have prepared 3 different formulations using hydrophilic polymers, such as poloxamer, carbo pol and HPMC and iontophoretic flux through skin from these hydrogel formulations was carried out. The effect of current density, drug concentration and current profile on flux was investigated. In-vitro flux study was performed at 36.5$^{\circ}C$, using side-by-side diffusion cell. Full-thickness hairless mouse skin was used for this work. (omitted)

• 공업화학
• /
• 제10권4호
• /
• pp.608-614
• /
• 1999

촉매 담체로 사용되는 알루미나와 실리카-알루미나에 흡착된 tetra-phenylprophine과 coronene이 아세톤의 주입에 의해 탈착되는 역확산 과정과, coronene이 tetra-phenylporphine 용액에 의한 역확산 과정을 조사하였다. 이상흡착용액 이론을 적용한 역확산 모델을 이용하여 역확산 과정을 해석하고 모사한 결과로부터 역확산 계수를 결정하였다. 과량의 아세톤 주입에 의한 탈착에서 tetra-phenylporphine과 coronene의 역확산 계수는 ${\sim}10^{-15}m^2/sec$ 범위로 얻어졌다. 흡착용액과 비슷한 농도를 갖는 tetra-phenylprophine 용액에 의한 역확산의 경우 탈착되는 coronene의 역확산 계수의 값도 ${\sim}10^{-15}m^2/sec$ 범위에 있으며, 세공내로 들어가는 tetra-phenylporphine의 확산 계수는 ${\sim}10^{-11}m^2/sec$ 이었다. 흡착제 밖으로 탈착되어 나오는 coronene의 역확산 계수는 아세톤이나 tetra-phenylporphine 용액의 주입에 의한 흡착-탈착의 경우 탈착제는 종류와 양에 관계없이 모두 비슷한 범위의 값을 보여 주었다. 역확산계수는 비어 있는 세공내로의 확산에 대한 유효확산계수에 비해 현저히 감소하였다. 이러한 현상은 역확산 과정에 나타나는 확산 흐름의 교차에 기인하는 것으로 설명되었다.

• 대한토목학회논문집
• /
• 제26권3B호
• /
• pp.311-319
• /
• 2006

저면경계층에서 부유사와 소류사 flux를 포함하는 실제적인 표사이동에 기초한 지형변화모델(SED-FLUX)이 개발되었다. SED-FLUX는 파랑모듈, 동수역학 모듈 및 부유사농도, 순부상 flux($Q_s$)와 소류사 flux를 계산하는 표사수송 및 확산모듈을 포함하며, 소류사 flux는 파-흐름 공존장에서 검증된 van Rijn의 TRANSPOR 프로그램에 의해 평가되어진다. 저면에서 순부상 flux $Q_s$는 표사확산모듈에서 source/sink 항으로서 평가되어지며, 수심변화모듈은 수심변화량을 계산하고 시간에 따른 bed level의 변화를 계산한다. 모델의 검증을 위하여 소류사이동의 이동한계수심은 방사성 동위원소 추적자를 사용한 현장 실험자료와 파와 흐름에 의한 표사이동한계수심에 대한 몇몇 경험식과 비교되었다. 본 모델을 파에 의한 해빈 단면변화에 적용한 결과 입사파의 특성에 따른 명확한 침식과 퇴적분포를 나타내었다. 끝으로, 이안제 배후에서 파와 해빈류에 의한 수심변화를 계산한 결과 이안제의 배후에서 초기 tombolo의 형성을 보여주었다.

• 천문학회보
• /
• 제41권1호
• /
• pp.45.1-45.1
• /
• 2016

Spatially rotating magnetic fields have been observed in the solar wind and in the Earth's magnetopause as well as in reversed field pinch (RFP) devices. Such field configurations have a similarity with extended current layers having a spatially varying plasma pressure instead of the spatially varying guide field. It is thus expected that magnetic reconnection may take place in a rotating magnetic field no less than in an extended current layer. We have investigated the spontaneous evolution of a collisionless plasma system embedding a rotating magnetic field with a two-and-a-half-dimensional electromagnetic particle-in-cell (PIC) simulation. In magnetohydrodynamics, magnetic flux can be decreased by diffusion in O-lines. In kinetic physics, however, an asymmetry of the velocity distribution function can generate new magnetic flux near O- and X-lines, hence a dynamo effect. We have found that a magnetic-flux-reducing diffusion phase and a magnetic-flux-increasing dynamo phase are alternating with a certain period. The temperature of the system also varies with the same period, showing a similarity to sawtooth oscillations in tokamaks. We have shown that a modified theory of sawtooth oscillations can explain the periodic behavior observed in the simulation. A strong guide field distorts the current layer as was observed in laboratory experiments. This distortion is smoothed out as magnetic islands fade away by the O-line diffusion, but is soon strengthened by the growth of magnetic islands. These processes are all repeating with a fixed period. Our results suggest that a rotating magnetic field configuration continuously undergoes deformation and relaxation in a short time-scale although it might look rather steady in a long-term view.

• 멤브레인
• /
• 제8권3호
• /
• pp.148-156
• /
• 1998

이온교환막을 이용하는 확산투석은 농도차가 구동력인 막공정으로서 철강 산업, 금속 제련 산업, 전기 도금 산업 등으로부터 발생되어지는 폐산으로부터 염산 및 기타 산들을 회수하는데 사용된다. 이론적으로는 초기산 농도가 증가함에 따라 구동력인 농도차가 커지므로 선형적으로 산의 투과속도가 증가하게 된다. 그러나 염산의 경우 약 3 N 이상의 고농도 범위에서는 투과속도 증가율이 농도차가 커짐에 따라 감소하였다. 본 연구에서는 막흡수 실험과 확산투석조 실험을 통해 염산을 회수하기 위한 확산투석에 산의 농도가 미치는 영향과 농도 범위에 따른 음이온 교환막 내부에서의 산의 이동 기작을 고찰하였다. 실험 결과 저농도(<2 N)산의 경우에는 산의 분자확산, 그리고 고농도 (>2 N)산의 경우에는 수소이온 투과현상 (proton leakage)이 막에서의 지배적인 이동 기작임을 알 수 있었다. 또한 고농도 범위에서는 삼투압에 의한 물분자의 이동과 막의 탈수현상이 수소이온의 이동을 방해하므로 막에서의 산 플럭스 증가율이 감소하였다.