• Journal of Mechanical Science and Technology
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• v.20 no.8
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• pp.1292-1301
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• 2006

This article investigates numerically the carrier-phonon interactions in thin gallium arsenide (GaAs) film structures irradiated by subpicosecond laser pulses to figure out the role of several recombination processes on the energy transport during laser pulses and to examine the effects of laser fluences and pulses on non-equilibrium energy transfer characteristics in thin film structures. The self-consistent hydrodynamic equations derived from the Boltzmann transport equations are established for carriers and two different types of phonons, i.e., acoustic phonons and longitudinal optical (LO) phonons. From the results, it is found that the two-peak structure of carrier temperatures depends mainly on the pulse durations, laser fluences, and nonradiative recombination processes, two different phonons are in nonequilibrium state within such lagging times, and this lagging effect can be neglected for longer pulses. Finally, at the initial stage of laser irradiation, SRH recombination rates increases sufficiently because the abrupt increase in carrier number density no longer permits Auger recombination to be activated. For thin GaAs film structures, it is thus seen that Auger recombination is negligible even at high temperature during laser irradiation.

• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.2
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• pp.150-156
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• 2011

In this paper, infrared thermography measurement technique has been used to develop standard measurement technique for nondestructive testing of composite materials which is widely used in aerospace industries. To increase the defect detection rate, the related experiment used the lock-in IR-thermographiy method. Therefore it is of considerable interest in the field of non-destructive testing for fast discontinuity detection by using ultrasonic lock-in infrared thermography. The result also shows that as the investigation period of light source is lengthened according to the thickness of specimen, the possibility of detecting defects gets higher as well. However, the reason why the result values were not favorable when less than 50 mHz of light source was provided is because it was difficult to detect defects as the defect parts became a state of thermal equilibrium in general when thermal diffusivity affects the entire materials.

• Korean Journal of Food Science and Technology
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• v.17 no.3
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• pp.141-145
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• 1985

In the present study, an attempt was made to investigate the characteristics of rheological properties of a themally oxidized soybean oil. Various flow curves of the oxidized oils were obtained using a broad range of shear rate (452-904 rpm). The flow curves showed the characteristics of thixotropic flow, and as temperature decreased (range : 13-$37^{\circ}C$) they demonstrated increasingly stronger thixotropic properties. A thixotropic slope similar to the coefficient of thixotropy as applied to the rheological characteristics of the thermally oxidized soybean oil to predict its rheological properties quantitatively at an equilibrium state. The empirical formula with the thixotropic slope was found satisfactory in predicting the Non-Newtonian behavior of the thermally oxidized soybean oil.

• Nuclear Engineering and Technology
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• v.26 no.2
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• pp.225-236
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• 1994

The sudden discharge of the loop seal water, which is present upstream of the SEBIM POSRV, creates large momentum and inertia forces on the downstream of the discharge piping system. This study provides the procedures and results of analysis of the thermal-hydraulic transient in the SEBIM POSRV discharge piping during the valve opening. The analysis is peformed by RELAP5/MOD3. The appropriate modeling of the discharge piping system, SEBIM POSRV opening characteristics, and loop seal water discharge for the RELAP5/MOD3 analysis is suggested. Also performed is the sensitivity study for the selection of proper options for the junction and volume control. flags. The analysis results demonstrate the adequacy of the RELAP5/HOD3 for the thermal-hydraulic transient analysis of the loop seal water discharge of the SEBIM POSRV discharge piping system. From the sensitivity analysis results, it is shown that the smooth area change option with reasonable geometric pressure drop distribution, non-equilibrium option, and proper time step should be selected for loop seal water discharge analysis.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.102-102
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• 2018

Electronic industry had required the finer size and the higher performance of the device. Therefore, 3-D die stacking technology such as TSV (through silicon via) and micro-bump had been used. Moreover, by the development of the 3-D die stacking technology, 3-D structure such as chip to chip (c2c) and chip to wafer (c2w) had become practicable. These technologies led to the appearance of HBM (high bandwidth memory). HBM was type of the memory, which is composed of several stacked layers of the memory chips. Each memory chips were connected by TSV and micro-bump. Thus, HBM had lower RC delay and higher performance of data processing than the conventional memory. Moreover, due to the development of the IT industry such as, AI (artificial intelligence), IOT (internet of things), and VR (virtual reality), the lower pitch size and the higher density were required to micro-electronics. Particularly, to obtain the fine pitch, some of the method such as copper pillar, nickel diffusion barrier, and tin-silver or tin-silver-copper based bump had been utillized. TCB (thermal compression bonding) and reflow process (thermal aging) were conventional method to bond between tin-silver or tin-silver-copper caps in the temperature range of 200 to 300 degrees. However, because of tin overflow which caused by higher operating temperature than melting point of Tin ($232^{\circ}C$), there would be the danger of bump bridge failure in fine-pitch bonding. Furthermore, regulating the phase of IMC (intermetallic compound) which was located between nickel diffusion barrier and bump, had a lot of problems. For example, an excess of kirkendall void which provides site of brittle fracture occurs at IMC layer after reflow process. The essential solution to reduce the difficulty of bump bonding process is copper to copper direct bonding below $300^{\circ}C$. In this study, in order to improve the problem of bump bonding process, copper to copper direct bonding was performed below $300^{\circ}C$. The driving force of bonding was the self-annealing properties of electrodeposited Cu with high defect density. The self-annealing property originated in high defect density and non-equilibrium grain boundaries at the triple junction. The electrodeposited Cu at high current density and low bath temperature was fabricated by electroplating on copper deposited silicon wafer. The copper-copper bonding experiments was conducted using thermal pressing machine. The condition of investigation such as thermal parameter and pressure parameter were varied to acquire proper bonded specimens. The bonded interface was characterized by SEM (scanning electron microscope) and OM (optical microscope). The density of grain boundary and defects were examined by TEM (transmission electron microscopy).

• Nuclear Engineering and Technology
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• v.54 no.8
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• pp.3117-3129
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• 2022

Deeply understanding the phase change of thin liquid sodium film inside wick pore is very important for further studying high-temperature sodium heat pipe's heat transfer. For the first time, the evaporation and condensation of thin liquid sodium film are investigated by the Lennard-Jones potential of molecular dynamics. Based on the startup and normal operation of the sodium heat pipe, three different cases are simulated. First, the equilibrium is achieved and the Mass Accommodation Coefficients of the three cases are 0.3886, 0.2119, 0.2615 respectively. Secondly, the non-equilibrium is built. The change of liquid film thickness, the number of gas atoms, the net evaporation flux (J_net), the heat transfer coefficient (h) at the liquid-gas interface are acquired. Results indicate that the magnitude of the J_net and the h increase with the basic equilibrium temperature. In 520-600 K (the startup of the heat pipe), the h has approached 5-6 W m^-2 K^-1 while liquid film thickness is in 11-13 nm. The fact shows that during the initial startup of the sodium heat pipe, the thermal resistance at the liquid-gas interface can't be negligible. This work is the complement and extension for macroscopic investigation of heat transfer inside sodium heat pipe. It can provide a reference for further numerical simulation and optimal design of the sodium heat pipe in the future.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.2
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• pp.103-108
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• 2019

Non-equilibrium molecular dynamics simulation on the thermal boundary resistance(TBR) of an aluminum(Al)/silicon(Si) interface was performed in the present study. The constant heat flux across the Si/Al interface was simulated by adding the kinetic energy in hot Si region and removing the same amount of the energy from the cold Al region. The TBR estimated from the sharp temperature drop at the interface was independent of heat flux and equal to $5.13{\pm}0.17K{\cdot}m^2/GW$ at 300K. The simulation result was experimentally confirmed by the time-domain thermoreflectance technique. A 90nm thick Al film was deposited on a Si(100) wafer using an e-beam evaporator and the TBR on the film/substrate interface was measured using the time-domain thermoreflectance technique based on a femtosecond laser system. A numerical solution of the transient heat conduction equation was obtained using the finite difference method to estimate the TBR value. Experimental results were compared to the prediction and discussions on the nanoscale thermal transport phenomena were made.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.265-267
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• 2011

저온 대기압 플라즈마는 21세기에 들어 생의학 분야에 이용될 수 있는 새로운 도구로서 많은 관심을 받고 있다. 대기압 플라즈마는 고가의 진공 장비를 필요로 하지 않고 저전력 구동이 가능하기 때문에 저비용 구동이 가능하고 방전 장치와 전력공급 장치의 소형화에 매우 유리하다. 특히 저온 대기압 플라즈마는 고온의 전자와 저온의 이온 입자가 공존하는 열적 불평형(thermal non-equilibrium) 상태에 있기 때문에 플라즈마의 저온 특성은 유지하면서도 (${\sim}30^{\circ}C$) 물리/화학적 반응성은 매우 높아 그 응용 분야가 매우 넓다. 플라즈마의 다양한 생의학 분야 응용 가운데 세포의 사멸 유발 또는 생장 촉진, 살균/멸균, 지혈, 상처 치유 등에 저온 대기압 플라즈마가 매우 뛰어난 효능을 보인다는 것이 국내외의 다양한 연구를 통해 밝혀지고 있다 [1]. 20 kHz 정현파로 구동되는 플라즈마 장치를 이용한 암 세포 제거 실험에서 플라즈마 처리 효과를 증대시키기 위해 항체-금나노입자 중합체를 암 세포에 주입시켰다 (그림 1(a)). 그 결과 세포의 사멸율은 74%로서 플라즈마 또는 플라즈마-금나노입자만을 처리한 경우에 비하여 사멸율이 매우 높게 나타났다 (그림 1(b)). 이를 통해 암세포 선택성을 가진 항체-금나노입자 중합체와 플라즈마 처리 기술을 융합한 암 세포의 선택적 사멸 유발 기술의 개발 가능성이 열렸다. 또한 플라즈마 처리를 통해 일어나는 세포의 자멸사 기작이 Cytochrome C의 방출 이후 이어지는 Caspase-3의 활성화 경로와 관계가 있음이 밝혀졌다 (그림 1(c)). 치아 미백은 최근 부상하고 있는 저온 대기압 플라즈마의 새로운 응용 분야이다 [5-6]. 대기압에서 동작하는 헬륨 플라즈마 제트를 미백제(과산화수소)와 함께 발치된 치아에 적용하였을 때 (그림 2(a)) 미백제만을 사용하였을 경우에 비해 치아의 색상 변화가 2배 이상 크게 나타나는 것을 확인하였다 (그림 2(b)). 이처럼 최근 그 범위가 크게 넓어지고 있는 저온 대기압 플라즈마의 생의학 응용 기술의 최적화를 위해서는, 다양한 생의학 응용 분야에 따라 요구되는 플라즈마의 특성 및 응용별 기저 기 작에 대한 이해와 연구가 필요하다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.1135-1139
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• 2017

Spatial distribution of chemical species in flame is a important indicator understanding the flame structure and combustion characteristics, and optical emission spectroscopy has been widely used for the measurement because of its simple and non-intrusive methodology. In this study, we suggest the feasibility of the measurement of chemical species (OH radical) distribution in rocket plume using optical emission spectrometer which was developed for the spatially resolved measurement along the line-of-sight. In order to predict the ground state concentration of species from the measured emission intensity by optical emission spectrometer, we consider thermal and chemical excitation mechanisms in flame, and assume thermodynamic equilibrium for the thermally excited species. We also present the spatial resolution and the correction of collection characteristics of the optical emission spectrometer depending on object distance.

• Korean Journal of Materials Research
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• v.30 no.7
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• pp.350-358
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• 2020

A variety of composite powders having different aluminum and carbon contents are prepared using various organic solvents having different amounts of carbon atoms in unit volume as ball milling agents for titanium and aluminum ball milling. The effects of substrate temperature and post-heat treatment on the texture and hardness of the coating are investigated by spraying with this reduced pressure plasma spray. The aluminum part of the composite powder evaporates during spraying, so that the film aluminum content is 30.9 mass%~37.4 mass% and the carbon content is 0.64 mass%~1.69 mass%. The main constituent phase of the coating formed on the water-cooled substrate is a non-planar α₂ phase, obtained by supersaturated carbon regardless of the alloy composition. When these films are heat-treated at 1123 K, the main constituent phase becomes γ phase, and fine Ti₂AlC precipitates to increase the film hardness. However, when heat treatment is performed at a higher temperature, the hardness is lowered. The main constitutional phase of the coating formed on the preheated substrate is an equilibrium gamma phase, and fine Ti₂AlC precipitates. The hardness of this coating is much higher than the hardness of the coating in the sprayed state formed on the water-cooled substrate. When hot pressing is applied to the coating, the porosity decreases but hardness also decreases because Ti₂AlC grows. The amount of Ti₂AlC in the hot-pressed film is 4.9 vol% to 15.3 vol%, depending on the carbon content of the film.