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Dimensional dependence of phonon transport in freestanding atomic layer systems.
Kim, Duckjong,Hwangbo, Yun,Zhu, Lijing,Mag-Isa, Alexander E,Kim, Kwang-Seop,Kim, Jae-Hyun RSC Pub 2013 Nanoscale Vol.5 No.23
<P>Due to the fast development of nanotechnology, we have the capability of manipulating atomic layer systems such as graphene, hexagonal boron nitride and dichalcogenides. The major concern in the 2-dimensional nanostructures is how to preserve their exceptional single-layer properties in 3-dimensional bulk structures. In this study, we report that the extreme phonon transport in graphene is highly affected by the graphitic layer stacking based on experimental investigation of the thermal conduction in few-layer graphene, 1-7 layers thick, suspended over holes of various diameters. We fabricate freestanding axisymmetric graphene structures without any perturbing substrate, and measure the in-plane transport property in terms of thermal conduction by using Raman spectroscopy. From the difference in susceptibility to substrate effect, size effect on hot-spot temperature variation and layer number dependence of thermal conductivity, we show that the graphitic membranes with 2 or more layers have characteristics similar to 3-dimensional graphite, which are very different from those of 2-dimensional graphene membranes. This implies that the scattering of out-of-plane phonons by interlayer atomic coupling could be a key mechanism governing the intrinsic thermal property.</P>
Kim, Duckjong,Kim, Chihyun,Park, Jinsung,Kim, Tae Young Elsevier 2019 Energy conversion and management Vol.183 No.-
<P><B>Abstract</B></P> <P>Thermoelectricity is regarded as one of the promising waste heat recovery candidates. A fundamental but effective method for the best use of thermoelectric generators (TEGs) is to maximize the heat flow crossing thermoelectric materials. The main focus of the present study was to develop a thermal interface material (TIM) with high thermal conductivity and temperature resistance as the key to minimizing the overall thermal resistance of the heat flow path of a TEG module operating under high-temperature conditions. In combination with a polyimide matrix and a multi-dimensional filler compound, a new TIM having stable heat conduction behavior at high temperatures was produced. The developed TIM was stable without losing mass up to ∼500 °C, and its thermal conductivity reached 81.4 W/m·K. It was applied to the interface between a TEG and a heat source whose temperature ranged from 100 to 300 °C and, the effect of the thermal conductivity and interface thermal resistance of the TIM on thermoelectric power generation performance during onsite curing of the TIM was investigated. Reduction of the interface thermal resistance by the new TIM improved the power generation by, at most, 132.3 and 38.6% compared to cases without a TIM and with a conventional graphite foil TIM, respectively. In terms of energy conversion efficiency, the TEG with the new TIM showed maximum improvements of 73.2 and 20.9% over the cases without a TIM and with a graphite TIM for the same temperature difference across the TEG, respectively. In addition, thermal cyclic testing confirmed the long-lasting heat-conducting feature of the developed TIM. The present work clearly shows the potentially significant influence that TIMs have on the waste heat recovery performance of TEGs.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We developed a TIM paste with high thermal conductivity and temperature resistance. </LI> <LI> Compared to graphite foil, we reduced interface thermal resistance by at most 80.9%. </LI> <LI> The reduced thermal resistance enhanced the thermoelectric power by at most 38.6%. </LI> <LI> Thermal cyclic test confirmed long-lasting heat conducting feature of the TIM. </LI> <LI> This study shows significant influence that TIMs have on the waste heat recovery. </LI> </UL> </P>
Paved phonon transport route in graphene by vapor phase process
Kim, Duckjong,Kim, Woochang,Lee, Seung-Mo,Luo, Tengfei,Moon, Choongman,Kim, Jae-Hyun,Lee, Hak-Joo,Park, Jinsung,Lee, Joo-Hyoung Elsevier 2018 INTERNATIONAL JOURNAL OF THERMAL SCIENCES Vol.133 No.-
<P><B>Abstract</B></P> <P>Various atomistic defects ever-present in graphene have triggered unwanted phonon scattering, thereby significantly deteriorating the heat transport property of the graphene. Consequently, continuous endeavors have been made to minimize these atomistic defects. Here, we first experimentally show that defect healing based on atomic layer deposition (ALD) can significantly mitigate the degradation in the thermal conductivity of the graphene. After defect healing with Pt and Al<SUB>2</SUB>O<SUB>3</SUB> ALD, the thermal conductivities of graphene synthesized by conventional chemical vapor deposition were improved by ∼83% and ∼64%, respectively. It is speculated that the healing atoms facilitate heat transfer across the defects, by eliminating the influence of vacancy in the graphene layer, which likely increases the phonon lifetime (leading to a noticeable improvement in thermal conductivity). This study provides an understanding of how defect engineering could affect phonon transport in 2D materials and sheds light on affordable graphene with excellent thermal conductivity.</P>
An electrokinetic pressure sensor
Kim, Dong-Kwon,Kim, Duckjong,Kim, Sung Jin IOP 2008 JOURNAL OF MICROMECHANICS AND MICROENGINEERING - Vol.18 No.5
<P>A new concept for a micro pressure sensor is demonstrated. The pressure difference between the inlet and the outlet of glass nanochannels is obtained by measuring the electrokinetically generated electric potential. To demonstrate the proposed concept, experimental investigations are performed for 100 nm wide nanochannels with sodium chloride solutions having various concentrations. The proposed pressure sensor is able to measure the pressure difference within a 10% deviation from linearity. The sensitivity of the electrokinetic pressure sensor with 10<SUP>−5</SUP> M sodium chloride solution is 18.5 µV Pa<SUP>−1</SUP>, which is one order of magnitude higher than that of typical diaphragm-based pressure sensors. A numerical model is presented for investigating the effects of the concentration and the channel width on the sensitivity of the electrokinetic pressure sensor. Numerical results show that the sensitivity increases as the concentration decreases and the channel width increases.</P>
Manifold Design for Uniform Flow Distribution to Parallel Microchannels
duckjong Kim(김덕종),Seok Ho Yoon(윤석호),Jun Seok Choi(최준석) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10
For efficient scale up of microchannel devices, uniform flow distribution to parallel microchannels is very important. In the present work, a dimensionless geometric parameter indicating the optimal manifold configuration is derived through a numerical parametric study and validated by micro particle image velocimetry. It is clearly shown that the flow uniformity is maximized when the dimensionless parameter is about 0.8. This study presents a simple and systematic design methodology for a uniform flow distribution to multiple microchannels operating in parallel.
Transparent Flexible Heater Using Conductive Nanomaterials
Duckjong Kim(김덕종),Seung-Ryeol Kim(김승렬) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Thin tin-doped indium-oxide (ITO) films have been used as transparent film heaters in various applications requiring fast securement of a clear view. However, Indium is rare and its price fluctuates due to market demand. To make a high quality ITO film, relatively high temperature process is required. In addition, due to brittleness of ITO, ITO cannot be used in any applications requiring mechanical flexibility. To overcome the limit of the ITO film heater, several electrically conductive nanomaterials have been tried. However, since each nanomaterial has its own advantages and disadvantages, satisfying various requirements from industry has so far been almost impossible. Here, we present a high-performance transparent film heater based on a hybrid of carbon nanotubes and silver nanowires. The new hybrid film shows a marked advance in haze reduction and an improvement in the flexibility of the silver nanowire film while maintaining excellent electrical conductivity. We demonstrate sufficient temperature rise even at low voltages and long-term stability of the new film heater. This study shows possibility of new high performance transparent film heater based on hybrid of nanomaterials and the film heater could be widely used in various fields requiring both high electrical conductivity and excellent transparency.
Interlayer Coupling of Heat Transport in Suspended Graphene
Duckjong Kim(김덕종),Yun Hwangbo(황보윤),Jae-Hyun Kim(김재현) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Graphene is now in spotlight due to excellent physical properties. Due to the extremely small thickness of the monolayer graphene, if the excellent physical properties are maintained regardless of the system size in the field has been a key question for practical applications. Here, we investigate the thermal conduction in few-layer graphene suspended over holes with specified diameter and try to answer the question in terms of the phonon transport. Our results show that the graphene with 2 or more layers shows characteristics of bulk materials in terms of thermal conduction and that the interlayer coupling must be the key mechanism yielding the intrinsic property.