• 대한기계학회논문집B
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• 제25권3호
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• pp.296-304
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• 2001

A new method of measuring the thermal diffusivity of solid material at room temperature using photothermal displacement is proposed. The influence of the parameters, such as radius and modulation frequency of the pump beam and the sample thickness, was studied. From the minimum position of phase of measured deflection with respect to the pump beam, the thermal diffusivity of the materials can be obtained. The position where phase has the minimum value is determined using multiparameter least-square regression fitting. The experimental values for different samples obtained by applying the new method are in good agreement with the literature values.

• 천문학회지
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• 제37권5호
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• pp.405-412
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• 2004

Cosmological shocks form as an inevitable consequence of gravitational collapse during the large scale structure formation and cosmic-rays (CRs) are known to be accelerated at collisionless shocks via diffusive shock acceleration (DSA). We have calculated the evolution of CR modified shocks for a wide range of shock Mach numbers and shock speeds through numerical simulations of DSA in 1D quasi-parallel plane shocks. The simulations include thermal leakage injection of seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion is assumed. We show that CR modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies (p/mc $\ge$ 1), and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. We find that $10^{-4} - 10^{-3}$ of the particles passed through the shock are accelerated to form the CR population, and the injection rate is higher for shocks with higher Mach number. The CR acceleration efficiency increases with shock Mach number, but it asymptotes to ${\~}50\%$ in high Mach number shocks, regardless of the injection rate and upstream CR pressure. On the other hand, in moderate strength shocks ($M_s {\le} 5$), the pre-existing CRs increase the overall CR energy. We conclude that the CR acceleration at cosmological shocks is efficient enough to lead to significant nonlinear modifications to the shock structures.

• 한국전기전자재료학회논문지
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• 제20권5호
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• pp.394-398
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• 2007

A simple doping method to fabricate a very thin channel body of the nano-scaled n-type fin field-effect-transistor (FinFET) by arsenic solid-Phase-diffusion (SPD) process is presented. Using the As-doped spin-on-glass films and the rapid thermal annealing for shallow junction, the n-type source-drain extensions with a three-dimensional structure of the FinFET devices were doped. The junction properties of arsenic doped regions were investigated by using the $n^+$-p junction diodes which showed excellent electrical characteristics. The n-type FinFET devices with a gate length of 20-100 nm were fabricated by As-SPD and revealed superior device scalability.

• 대한기계학회논문집B
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• 제28권2호
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• pp.160-166
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• 2004

Combustion using oxygen enriched air is known as a technology which can increase flame stability as well as thermal efficiency due to improving the burning rate. Lift-off, blowout limit and flame length were examined as a function of jet velocity, coflow velocity and OEC(Oxygen Enriched Concentration). Blowout limit of the flame below OEC 25% decreased with increase of coflow velocity, but the limit above OEC 25% increased inversely. Lift-off height decreased with increase of OEC. In particular, lift-off hardly occurred in the condition above OEC 40%. Flame length of the flames above OEC 40% was increased until the blowout occurred. Great flame stability was obtained since lift-off and blowout limit significantly increased with increase of OEC.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.326-331
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• 2003

Combustion using oxygen enriched air is known as a technology which can increase flame stability as well as thermal efficiency due to improvement of the burning rate. Lift-off, blowout limit and flame length were examined as a function of jet velocity, coflow velocity and OEC(Oxygen Enriched Concentration). Blowout limit of the flame below OEC 25% decreased with coflow velocity, but the limit above OEC 25% increased inversely. Lift-off height decreased with increase of OEC. Especially lift-off hardly occurred in the condition above OEC 40%. Flame length of the flames above OEC 40% was increased until the blowout occurred. Flame stability became improved since lift-off and blowout limit increased much with increase of OEC.

• 설비공학논문집
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• 제17권12호
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• pp.1139-1144
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• 2005

This study has been performed to investigate the thermal performance of variable conductance heat pipe (VCHP) with meshed wick. The length of condenser portion in a VCHP is varied by the expansion of inert gas with the operation temperature, and the heat transport capacity is thus varied with the operating temperature. In this study, numerical evaluation of the VCHP is made for the thermal performance of VCHP, based on the diffusion model of inert gas. Water is used as a working fluid and nitrogen as a control inert gas in the copper tube. As a result, the thermal performance of VCHP has been compared with that of constant conductance heat pipe (CCHP) according to the variation of operation temperature. Maximum heat transport capacity of VCHP is mainly presented for operation temperature and the variation of operation temperature is also presented for heat transfer rate of VCHP.

• 한국화재소방학회논문지
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• 제24권6호
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• pp.170-175
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• 2010

동축류버너를 이용한 제트확산화염에서 이산화탄소의 첨가가 매연 생성에 미치는 영향을 조사하였다. R-타입 열전대를 이용하여 화염대 및 황염과 청염의 경계온도를 측정하였다. 광감쇄법을 이용하여 화염이 존재하는 국소부분에서의 상대적인 매연 농도(1-I/$I_0$)를 측정하였다. 광원으로는 파장이 632.8nm인 He-Ne 레이저가 사용되었고 디텍터를 이용하여 매연입자에 의해 산란과 흡수를 겪은 후의 감쇄된 신호를 직접 측정하였다. 또한, 매연 생성에 있어서의 열적 효과를 알아보기 위해 산화제의 유속을 변화시켜 유속에 의한 영항을 알아보았다. 실험 결과로써, 황염과 청염 각각의 온도는 이산화탄소의 첨가에 따라 점차 낮아졌지만 황염과 청염의 경계온도는 거의 일정하였다. 산화제 측에 이산화탄소를 첨가함에 따라 상대적인 매연 농도는 낮아졌고 이는 산화제의 유속을 증가시켰을 때의 효과와 유사했다. 이것은 화염온도의 저하와 매연입자의 체류시간 감소에 기인하는 것으로 생각된다. 또한 이산화탄소의 첨가가 화염의 불안정성을 야기하여 순수 에틸렌/공기 화염에 비해 화염의 길이가 다소 증가하는 것으로 나타났다.

• 대한전자공학회논문지SD
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• 제47권10호
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• pp.14-22
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• 2010

기존의 n-type metal-oxide-semiconductor field effect transistor(NMOSFET)은 $n^+/p^{(+)}/n^+$ type의 이온 주입을 통하여 소스/채널/드레인 영역을 형성하게 된다. 30 nm 이하의 채널 길이를 갖는 초미세 소자를 제작함에 있어서 설계한 유효 채널 길이를 정확하게 얻기 위해서는 주입된 이온들을 완전히 activation하여 전류 수준을 향상시키면서도 diffusion을 최소화하기 위해 낮은 thermal budget을 갖도록 공정을 설계해야 한다. 실제 공정에서의 process margin을 완화할 수 있도록 오히려 p-type 채널을 형성하져 않으면서도 기존의 NMOSFET의 동작을 온전히 구현할 수 있는 junctionless(JL) MOSFET이 연구중이다. 본 논문에서는 3차원 소자 시뮬레이션을 통하여 silicon nanowire(SNW) 구조에 접목시킨 JL MOSFET을 최적 설계하고 그러한 조건의 소자에 대하여 conductance, maximum oscillation frequency($f_{max}$), current gain cut-off frequency($f_T$) 등의 기본적인 고주파 특성을 분석한다. 채널 길이는 30 nm이며 설계 변수는 채널 도핑 농도와 채널 SNW의 반지름이다. 최적 설계된 JL SNW NMOSFET에 대하여 동작 조건($V_{GS}$ = $V_{DS}$ = 1.0 V)에서 각각 367.5 GHz, 602.5 GHz의 $f_T$, $f_{max}$를 얻을 수 있었다.

• Nuclear Engineering and Technology
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• 제54권7호
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• pp.2367-2375
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• 2022

In this work, we propose a new mechanistic model for the treatment of helium behaviour at the grain boundaries in oxide nuclear fuel. The model provides a rate-theory description of helium inter-granular behaviour, considering diffusion towards grain edges, trapping in lenticular bubbles, and thermal resolution. It is paired with a rate-theory description of helium intra-granular behaviour that includes diffusion towards grain boundaries, trapping in spherical bubbles, and thermal re-solution. The proposed model has been implemented in the meso-scale software designed for coupling with fuel performance codes SCIANTIX. It is validated against thermal desorption experiments performed on doped UO₂ samples annealed at different temperatures. The overall agreement of the new model with the experimental data is improved, both in terms of integral helium release and of the helium release rate. By considering the contribution of helium at the grain boundaries in the new model, it is possible to represent the kinetics of helium release rate at high temperature. Given the uncertainties involved in the initial conditions for the inter-granular part of the model and the uncertainties associated to some model parameters for which limited lower-length scale information is available, such as the helium diffusivity at the grain boundaries, the results are complemented by a dedicated uncertainty analysis. This assessment demonstrates that the initial conditions, chosen in a reasonable range, have limited impact on the results, and confirms that it is possible to achieve satisfying results using sound values for the uncertain physical parameters.

• 대한전자공학회논문지SD
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• 제45권9호
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• pp.8-13
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• 2008

균일하게 도핑된 반도체에서, 분포된 확산 잡음원에 의해서 발생하는 단자잡음전류의 전력주파수밀도를 계산하였다. 고정된 전압에서 반도체의 길이가 작아짐에 따라, 또는 주어진 반도체에서 전류레벨이 증가함에 따라, AC 단락잡음전류는 열잡음 뿐만 아니라 과잉잡음을 보인다. 이 과잉잡음은 채널길이가 외인성 Debye 길이에 비해 매우 작은 경우에는 산탄잡음의 스펙트럼과 같은 모습을 보인다. 유한한 주파수에서 속도요동 잡음원에 의한 외인성 반도체에서 발생하는 과잉잡음을 최초로 유도하였다. 유도된 과잉잡음 공식은 반도체 채널의 통과 시간, 유전 이완 시간, 속도 이완 사이의 상호 작용에 따라 단자잡음 전류와 캐리어 농도 요동이 결정됨을 명시적으로 보여준다. 또한 유도된 해석적 식을 사용하여 여러 가지 반도체 샘플 길이와 바이어스, 주파수에 따른 잡음 스펙트럼의 변화도를 계산하였다. 유도된 공식은 quasi-ballistic 수송현상이 중요한 역할을 하는 나노스케일 MOSFET의 잡음 발생 기제를 이해할 수 있는 기반이 된다.