• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.14 no.1
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• pp.8-14
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• 2018

Printed circuit heat exchangers (PCHEs) are widely used with an increasing demand for industrial applications. PCHEs are capable of operating at high temperatures and pressure. We consider a PCHE as a candidate intermediate heat exchanger type for a high temperature gas-cooled reactor (HTGR). For conventional application using stainless steels, design and manufacturing of PCHEs are well established. For applications to HTGR, knowledge of longitudinal conduction and deformation of channel is required to estimate design margin. This paper analyzes the effects of longitudinal conduction and deformation of channel on thermal performance using a code internally developed for design and analysis of PCHEs. The code has a capability of two dimensional simulations. Longitudinal conduction is estimated using the code. In HTGR operating condition, about ten percent of design margin is required to compensate thermal performance. The cross-sectional images of PCHE channels are obtained using an optical microscope. The images are processed with computer image process technique. We quantify the deformation of channel with dimensional parameters. It is found that the deformation has negative effect on structural integrity. The deformation enhances thermal performance when the shape of channel is straight in laminar flow regime. It reduces thermal performance in cases of a zigzag channel and turbulent flow regime.

• Journal of the Korean Society of Visualization
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• v.19 no.3
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• pp.69-76
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• 2021

Printed circuit heat exchanger (PCHE) is a compact heat exchanger with good heat transfer performance, high structure integrity, and reliability over a wide range of temperatures and pressures. Instead of the traditional zigzag and straight shape channel, the sinusoidal shape channel was adopted in this study to investigate the relation of thermal-hydraulic performance and waviness factors (period and amplitude). The local flow characteristics and the heat flux distribution were compared to verify the effects of period and amplitude on heat transfer performance. As the period of channel becomes shorter, the rapid change of the flow direction can produce high flow separation around the corner leading to the disturbance of the boundary layer opposite wall. The nonuniform distribution of flow velocity appeared around the corner positions can promote fluid mixing and lead to higher thermal performance. An evaluation index was used to compare the comprehensive performance of PCHE considering the Nusselt number and Fanning factor. Based on the simulation results, the optimal design parameters of PCHE channel shape were found that the channel with an equivalent bending angle of 15° offers the highest heat flux capacity.

• Nuclear Engineering and Technology
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• v.52 no.12
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• pp.2771-2788
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• 2020

Experiments are conducted to study bubble flow behavior during the instability of subcooled boiling under uniform and non-uniform transverse heating. The non-uniform heat distribution introduces nonuniform bubble generation and condensation rates on the heated surface, which is different from the uniform heating. These bubble generation and condensation characteristics introduce a non-uniform local pressure distribution in the transverse direction, which creates an extra non-uniform pressure on the flowing bubbles. Therefore, different bubble flow behavior can be observed between uniform and non-uniform heating conditions. In the uniform heating, bubble velocity fluctuations are low, and the bubbles travel straight along the axial direction. In the non-uniform heating, more fluctuation in the bubble velocity occurs at low mass flow rate and high subcooled inlet temperatures, and reverse flow is observed. Additionally, the bubbles show a zigzag trajectory when they pass through the channel, which indicates the existence of cross flow in the transverse direction.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.230-230
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• 2012

Graphene have showed promising performance as electrodes of organic devices such as organic transistors, light-emitting diodes, and photovoltaic solar cells. In particular, among various organic materials of graphene-based organic devices, pentacene has been regarded as one of the promising organic material because of its high mobility, chemical stability. In the bottom-contact device configuration generally used as graphene based pentacene devices, the morphology of the organic semiconductors at the interface between a channel and electrode is crucial to efficient charge transport from the electrode to the channel. For the high quality morphology, understanding of initial stages of pentacene growth is essential. In this study, we investigate self-assembly of pentacene molecules on graphene formed on a 6H-SiC (0001) substrate by scanning tunneling microscopy. At sub-monolayer coverage, adsorption of pentacene molecules on epitaxial graphene is affected by $6{\times}6$ pattern originates from the underlying buffer layer. And the orientation of pentacene in the ordered structure is aligned with the zigzag direction of the edge structure of single layer graphene. As coverage increased, intermolecular interactions become stronger than molecule-substrate interaction. As a result, herringbone structures the consequence of higher intermolecular interaction are observed.

• Proceedings of the KIEE Conference
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• 2005.07c
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• pp.2140-2142
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• 2005

This paper presents the experimental results on the preliminary breakdown characteristics under a highly non-uniform electric field in $SF_6/CO_2$ gas mixtures. The impulse preliminary breakdown developments are investigated by the measurements of corona current and light emission images. As a result, the preliminary breakdown development mechanisms for both positive and negative polarities are same. The first streamer corona is initiated at the tip of needle electrode, and the leaders develop with a stepwise propagation and bridge the test gap. The pause time of leader pulses in the positive polarity is significantly shorter than that in the negative polarity. Also, the time interval between the first streamer corona onset and breakdown in the negative polarity was much longer than that in the positive polarity The discharge channel paths in the positive polarity were zigzag. On the other hands, the leader channel in the negative polarity was thicker than that in the positive polarity.

• Proceedings of the IEEK Conference
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• 2007.07a
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• pp.433-434
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• 2007

This paper describes a low-voltage dynamic random-access memory (DRAM) focusing on subthreshold leakage reduction during self-refresh (sleep) mode. By sharing a power switch, multiple iterative circuits such as row and column decoders have a significantly reduced subthreshold leakage current. To reduce the leakage current of complex logic gates, dual channel length scheme and input vector control method are used. Because all node voltages during the standby mode are deterministic, zigzag super-cutoff CMOS is used, allowing to Preserve internal data. MTCMOS technique Is also used in the circuits having no need to preserve internal data. Sub-1.2-V 1-Gb mobile DDR DRAM employing all these low-power techniques was designed in a 60 nm CMOS technology and achieved over 77% reduction of overall leakage current during the self-refresh mode.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.20 no.1
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• pp.85-90
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• 2006

This paper presents experimental results relating to the preliminary breakdown characteristics in $SF_6/CO_2$ gas mixtures under a highly non-uniform electric field. The impulse pre-breakdown developments are investigated by the measurements of corona current and light emission images. As a result, the preliminary breakdown development mechanisms for both the positive and negative polarities were fundamentally same. The first streamer corona was initiated at the tip of needle electrode, and the leaders developed with a stepwise propagation and bridged the test gap. The pause time of leader pulses in the positive polarity was significantly shorter than that in the negative polarity. Also, the time interval between the first streamer corona onset and breakdown in the negative polarity was much longer than that in the positive polarity. The discharge channel path in the positive polarity was zigzag, and the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2005.05a
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• pp.57-60
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• 2005

This paper describes the experimental results of the pre-breakdown phenomena in $SF_6/CO_2$ mixtures under non-uniform electric fields caused by positive and negative lightning negative voltages. $SF_6/CO_2$ mixtures have an advantage of an environmental aspect and cost reduction, and safety aspects. In order to analyze the pre-breakdown processes in $SF_6/CO_2$ mixtures stressed by impulse voltages, pre-breakdown current and luminous signals were measured by a shunt and a photo-multiplier tube, respectively. Dielectric strengthes of $SF_6/CO_2$ mixtures were investigated. Additionally, characteristics of discharge channels were observed by high speed cameras and the physical properties were discussed. The pre-breakdown propagates with a stepwise process. The in to breakdown from the corona onset point in positive polarity was shorter than that in negative polarity. The time intervals of positive leaders are shorter than those of negative leaders, and the path of positive leader channel is zigzag.

• Carbon letters
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• v.21
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• pp.8-15
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• 2017

The thermoelectric Seebeck and Peltier effects of a single walled carbon nanotube (SWCNT) quantum dot nanodevice are investigated, taking into consideration a certain value of applied tensile strain and induced ac-field with frequency in the terahertz (THz) range. This device is modeled as a SWCNT quantum dot connected to metallic leads. These two metallic leads operate as a source and a drain. In this three-terminal device, the conducting substance is the gate electrode. Another metallic gate is used to govern the electrostatics and the switching of the carbon nanotube channel. The substances at the carbon nanotube quantum dot/metal contact are controlled by the back gate. Results show that both the Seebeck and Peltier coefficients have random oscillation as a function of gate voltage in the Coulomb blockade regime for all types of SWCNT quantum dots. Also, the values of both the Seebeck and Peltier coefficients are enhanced, mainly due to the induced tensile strain. Results show that the three types of SWCNT quantum dot are good thermoelectric nanodevices for energy harvesting (Seebeck effect) and good coolers for nanoelectronic devices (Peltier effect).

• Bulletin of the Korean Chemical Society
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• v.26 no.10
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• pp.1543-1548
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• 2005

Two isostructural new alkaline earth germanium nitrides, $Sr_6Ge_5N_2\;and\;Ba_6Ge_5N_2$, were obtained as single crystals from constituent elements in molten Na. They both crystallize in space group $P_{mmn}$ (No. 59) with a = 4.0007(8), b = 17.954(3), c = 9.089(2) $\AA$, Z = 2, and a = 4.1620(2), b = 18.841(1), c = 9.6116(5) $\AA$, Z = 2, for $Sr_6Ge_5N_2\;and\;Ba_6Ge_5N_2$, respectively. Their crystal structure contains features for both Zintl and nitride phases: zigzag anionic chain of $_{\infty}Ge^{2-}$, and dumbbell-shaped bent anion of ${GeN_2}^{4-}$. Counter cations of Sr or Ba wrap these anionic units in a channel-like arrangement. Unlike in other germanium nitrides, bond lengths of both Ge-N arms of the ${GeN_2}^{4-}$, are same in $Sr_6Ge_5N_2\;and\;Ba_6Ge_5N_2$.