• 대한기계학회논문집B
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• 제40권9호
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• pp.621-627
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• 2016

본 연구에서는 실험적 데이터를 근간으로 박막재료의 스펙트럼(spectrum) 분포 별 포논-표면 산란율을 직접 계산할 수 있는 모델을 제시했다. 실험 측정결과인 포논 평균자유행로(mean free path, MFP) 스펙트럼 분포 별 열전달 기여도로부터 스펙트럼 의존적 포논-표면 산란율을 직접 도출하는 모델을 개발했고, 이 모델을 아직 실험적 방법으로 포논-표면 산란율을 측정하지 못한 $Si_{0.9}-Ge_{0.1}$ 나노선(Nanowire, NW)에 적용하여, $Si_{0.9}-Ge_{0.1}$ NW 내 포논 MFP 스펙트럼 분포를 구하고, 주파수에 따른 포논 전달특성을 살폈다. 이를 바탕으로 $Si_{0.9}-Ge_{0.1}$ NW 단위길이당 포논-표면 산란율을 제시하여, 가로갈래 포논 주파수 의존성을 살폈다. 본 연구에서 제시한 모델은 향후 나노재료의 공학적 응용을 위한 나노구조물 열전달 해석모델 개발 및 나노재료 열전달 특성 조정(tailoring)을 위한 나노재료 최적설계에 활용될 수 있다.

• 대한기계학회논문집B
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• 제40권2호
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• pp.111-117
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• 2016

본 연구의 목적은 실리콘 열전달 조절을 위한 포논의 평균자유행로(Mean free path, MFP) 스펙트럼 열전달 기여도 예측이다. 열전달의 크기 효과는 포논의 MFP 와 재료의 특성길이가 비슷할 때 나타나는데, 나노시스템 응용을 위한 재료의 열전달 증감을 위해 포논 MFP 스펙트럼에 대한 열전달 기여도 예측이 중요하다. 이를 위해 포논의 주파수 의존성이 고려된 볼츠만 수송방정식(Boltzmann transport equation) 근간의 full phonon dispersion 모델을 통해 실리콘 박막(Silicon-on-Insulator) 트랜지스터의 실리콘 박막 두께 변화(41-177 nm)에 따른 포논 MFP 스펙트럼 열전달 특성 및 비등방성을 해석함으로써, 본 연구 결과는 향후 박막 트랜지스터에 대한 고효율 열소산(heat dissipation) 설계전략에 활용될 수 있다.

• 대한기계학회논문집B
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• 제41권5호
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• pp.341-346
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• 2017

본 연구는 해석적 접근이 용이한 실리콘 내 포논 평균자유행로(mean free path, MFP) 스펙트럼(spectrum) 열전도 특성 예측 모델을 제시했다. 해석이 용이한 포논 기체 운동론(kinetic theory)을 적용하기 위해, 나노구조물의 현상학적 접근으로 열전도에 관여하는 포논 모드(mode)들만 추출하고, 300 K의 실리콘에 대한 포논의 분산관계(dispersion relations) 및 분극(polarization) 효과가 고려된 포논의 주파수 변화에 따른 비열(specific heat)과 군속도(group velocity) 및 MFP 정보를 사용했다. 300 K의 실리콘 내 포논의 MFP 스펙트럼 열전도율 기여를 계산하고, 기존 실험결과 및 제1원리 기법 결과와 비교하여, 본 방법의 타당함을 보였다. 본 연구를 통해, 나노구조물 열전달 해석모델 개발 및 나노재료 열전달 특성 조정(tailoring) 전략 설계에 필요한 포논 MFP 스펙트럼 열전도 특성 정보를 해석이 용이한 방법으로 구할 수 있는 방법을 제공했다.

• Current Optics and Photonics
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• 제5권6호
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• pp.617-626
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• 2021

A parallel Monte Carlo photon migration algorithm for graphics processing units that implements an improved load-balancing strategy is presented. Conventional parallel Monte Carlo photon migration algorithms suffer from a computational bottleneck due to their reliance on a simple load-balancing strategy that does not take into account the different length of the mean free paths of the photons. In this paper, path-partition load balancing is proposed to eliminate this computational bottleneck based on a mathematical formula that parallelizes the photon path tracing process, which has previously been considered non-parallelizable. The performance of the proposed algorithm is tested using three-dimensional photon migration simulations of a human skin model.

• Journal of Magnetics
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• 제10권3호
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• pp.77-79
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• 2005

In this work, we present a comprehensive and systematic approach for the derivation of the dc voltage generated by a magnetic multilayer undergoing ferromagnetic resonance, originally derived by Berger. Our alternative derivation applies especially in the limit of the spin diffusion length much longer than the carrier mean free path.

• 대한방사선치료학회지
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• 제13권1호
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• pp.14-22
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• 2001

Purpose: The aim of this study is to investigate geometrical and volumetrical changes of liver due to breathing and its impact to NTCP. In order to attain better treatment results it should be considered deliberately during planning session. Mehtods and Materials : Seven patients were examined in this study who have done TACE for accurate tumor margin drawing. After contrast media injection, C-T scan data were obtained in supine position during breathing free, inhalation and exhalation, respectively. For all patients C-T scan were done with same scanning parameters- 5 mm index, 5 mm thickness and pitch 1. Based on C-T data we have measured differences of each variables between breathing status such as changes of total and remained liver volumes, GTV, beam path length and superior to inferior shift. NTCP were calculated using Lyman's effective volume DVH reduction scheme and for this NTCP calculation, the V50 was computed from DVH and each m, n value were referred from Burmans data. Results : The measured total tilter volume and the remained liver volume changed between inspiration and expiration about $1.2-7.7\%(mean+2.7\%)$ and $2.5-13.23\%(mean=5.8\%)$ respectively, and these results were statistically significant(p>0.1). The GTV difference in each patient varied widely from $1.17\%\;to\;30.69\%$, but this result was not statistically significant. Depending on the breathing status, the beam path length was changed from 0.5 cm to 1.1 cm with the average of 0.7 cm, and it was statistically significant(p=0.006). The measured superior to inferior shifts were ranged from 0.5 cm to 3.74 cm. The NTCPs were changed relatively small in each patient, but the variation was large between the patients. The mean NTCP difference was $10.5\%$, with the variation ranged from $7\%\;to\;23.5\%$. Conclusion : Variations of liver volume and of beam path length were changed significantly depending on the breathing statues and the range of variation itself was very different between the patients. Since this variance could seriously affect the clinical outcomes of radiation treatments, the breathing of patients need to be accounted when a final treatment planning is derided.

• 대한전자공학회논문지SD
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• 제45권4호
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• pp.57-63
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• 2008

본 논문에서는 Contact Etch Stop Layer (CESL)인 nitride film의 mechanical stress에 의해 인가되는 channel stress가 소자 특성에 미치는 영향에 대해 분석하였다. 잘 알려진 바와 같이 NMOS는 tensile stress와 PMOS에서는 compressive stress가 인가되었을 경우 drain current가 증가하였으며 그 원인을 체계적으로 분석하였다. NMOS의 경우 tensile stress가 인가됨으로써 back scattering ratio ($\tau_{sat}$)의 감소와 thermal injection velocity ($V_{inj}$)의 증가로 인해 mobility가 개선됨을 확인하였다. 또한 $\tau_{sat}$, 의 감소는 온도에 따른 mobility의 감소율이 작고, 그에 따른 mean free path ($\lambda_O$)의 감소율이 작기 때문인 것으로 확인되었다. 한편 PMOS의 compressive stress 경우에는 tensile stress에 비해 온도에 따른 mobility의 감소율이 크기 때문에 channel back scattering 현상은 심해지지만 source에서의 $V_{inj}$가 큰 폭으로 증가함으로써 mobility가 개선됨을 확인 할 수 있었다. 따라서 CES-Layer에 의해 인가된 channel stress에 따른 소자 특성의 변화는 inversion layer에서의 channel back scattering 현상과 source에서의 thermal injection velocity에 매우 의존함을 알 수 있다.

• 한국초전도ㆍ저온공학회논문지
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• 제21권3호
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• pp.18-21
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• 2019

We studied the effect of Fe ion irradiation on the upper critical field ($H_{c2}$) of 410 nm single-crystalline $MgB_2$ thin films. The irradiation energy was fixed at 140 keV when we increased the irradiation doses from $1{\times}10^{14}ion/cm^2$ to $4{\times}10^{14}ion/cm^2$. We found that $H_{c2}$ significantly increase with increasing irradiation dose, despite the low irradiation energy. The enhancement of $H_{c2}$ could be explained by the reduction of electron mean free path caused by defects induced from irradiation, leading to a decrease of coherence length (${\xi}$). We also discussed the effect of irradiation on temperature-dependent resistivity in details.

• 한국재료학회지
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• 제14권7호
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• pp.489-493
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• 2004

The electrical property of MMC is essentially important to some applications such as power transmission lines and cables, electronic and electrical components as well as electromagnetic shielding equipments. The behavior of electrical resistivity of $SiC_{w}/Al$ alloy composites under as-extruded and annealed conditions has been investigated within the temperature range from room temperature to $450^{\circ}C$. It can be seen that within entire temperature range, the electrical resistivity of composites was higher than that of an unreinforced matrix alloy under the same condition of either as-extrusion or annealing. The temperature dependence of both exhibited positive incline like a typical metal. The variation of electrical resistivity of an unreinforced matrix alloy with temperature from ambient temperature to $450^{\circ}C$ was nearly monotonous, while those of composites increased monotonously at low temperature and rose to a high level after about $250^{\circ}C or 275^{\circ}C$. The difference of these temperature dependences on electrical resistivity can be interpreted as qualitatively the interfaces of $SiC_{w}$ fibers and matrix, where act as nucleation sites.

• 대한기계학회논문집B
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• 제33권10호
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• pp.820-826
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• 2009

The anisotropic phonon conductions with varying Joule heating rate of the silicon film in Silicon-on-Insulator devices are examined using the electron-phonon interaction model. It is found that the phonon heat transfer rate at each boundary of Si-layer has a strong dependence on the heating power rate. And the phonon flow decreases when the temperature gradient has a sharp change within extremely short length scales such as phonon mean free path. Thus the heat generated in the hot spot region is removed primarily by heat conduction through Si-layer at the higher Joule heating level and the phonon nonlocality is mainly attributed to lower group velocity phonons as remarkably dissimilar to the case of electrons in laser heated plasmas. To validate these observations the modified phonon nonlocal model considering complete phonon dispersion relations is introduced as a correct form of the conventional theory. We also reveal that the relation between the phonon heat deposition time from the hot spot region and the relaxation time in Si-layer can be used to estimate the intrinsic thermal resistance in the parallel heat flow direction as Joule heating level varies.