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손창현,KIHM KENNETH DAVID 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.5
In this paper, heat transfer in a laminar convective flow of nano-fluids through a circular duct is investigated, and the results are presented. The study has been conducted under uniform heating and cooling conditions. A numerical analysis for water-alumina (Al2O3) nanofluids has clearly shown that the conventional homogeneous properties of nanofluids cannot explain the enhancement of the Nusselt number observed in experiments. A nonhomogeneous nano-fluid model using Brownian diffusion and thermophoresis effects has been incorporated in the present numerical analysis. Results indicate that the Darcy friction factor decreases under heating and increases under cooling condition than predicted by a pure-fluid correlation. The Nusselt number shows an increasing trend with increasing nanoparticle volume fraction for heating, but the values are lower than predicted by using a pure-fluid correlation for cooling. Results also indicate that nanofluids might act as good coolants under heating condition, but not under cooling condition.
Lee, Woomin,Kihm, Kenneth David,Kim, Hong Goo,Shin, Seungha,Lee, Changhyuk,Park, Jae Sung,Cheon, Sosan,Kwon, Oh Myoung,Lim, Gyumin,Lee, Woorim American Chemical Society 2017 Nano letters Vol.17 No.4
<P>Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H-2), allowed for synthesis of polycrystalline single layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 mu m diameter patterned holes on a silicon-nitride (Si3N4) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m center dot K for average grain sizes of 4.1, 2.2, and 0.5 mu m, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m center dot K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 mu m. Furthermore, detailed elaborations were performed to assess the measurement reliability of k by addressing the hole-edge boundary condition, and the airconvection/radiation losses from the graphene surface.</P>
How to optically count graphene layers.
Cheon, Sosan,Kihm, Kenneth David,Park, Jae Sung,Lee, Joon Sik,Lee, Byeong Jun,Kim, Hyeoungkeun,Hong, Byung Hee Optical Society of America 2012 Optics letters Vol.37 No.18
<P>The total thickness of a graphene sample depends upon the number of individually stacked graphene layers. The corresponding surface plasmon resonance (SPR) reflectance alters the SPR angle, depending on the number of graphene layers. Thus, the correlation between the SPR angle shift and the number of graphene layers allows for a nonintrusive, real-time, and reliable counting of graphene layers. A single-layer graphene (SLG) is synthesized by means of chemical vapor deposition, followed by physical transfer to a thin gold film (48 nm) repeatedly, so that multilayer graphene samples with one, three, and five layers can be prepared. Both the measured SPR angles and the entire reflectance curve profiles successfully distinguish the number of graphene layers.</P>
Boosted thermal conductance of polycrystalline graphene by spin-coated silver nanowires
Lee, Woorim,Kihm, Kenneth David,Lee, Woomin,Won, Phillip,Han, Seonggeun,Lim, Gyumin,Pyun, Kyung Rok,Ko, Seung Hwan Elsevier 2019 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.134 No.-
<P><B>Abstract</B></P> <P>Spin-coated silver nanowires (AgNWs) on graphene show a significantly improved thermal conductance of the composite in comparison with pristine graphene with no nanowires. CVD-synthesized graphene is transferred onto an 8-nm thin TEM grid substrate, and AgNWs (average diameter 150-nm and average length 30-μm) are chemically grown from an AgNO<SUB>3</SUB> reagent solution. The AgNW bridging overrides the negative effect of the grain boundary scattering of the electron/phonon energy carriers propagating in the polycrystalline CVD graphene and ultimately enhances the grain-to-grain heat transport by widening their passages. This boosting contribution of AgNWs is quantitatively assessed by measurement of thermal conductance for synthesized AgNW/graphene composite samples. The Raman thermometry measurement locations are selected to be beside a single AgNW (G-1), two AgNWs (G-2), and three or more AgNWs (G-3), so that the effect of AgNW density can be examined. The average enhanced thermal conductance values for the three AgNW-laid graphene samples are 319.27 nW/K, 343.66 nW/K, and 455.26 nW/K, respectively.</P> <P><B>Highlights</B></P> <P> <UL> <LI> CVD synthesized graphene composites with spin-coated silver nanowires (AgNWs). </LI> <LI> Raman thermometry to measure the thermal sheet conductance <SUB> G S </SUB> of the composite. </LI> <LI> AgNWs widen the transport passages and for both phonons and electrons. </LI> <LI> AgNWs composites show the 113% enhancement in <SUB> G S </SUB> compared to pristine graphene. </LI> </UL> </P>
Two orders of magnitude suppression of graphene's thermal conductivity by heavy dopants (Si)
Lee, Woorim,Kihm, Kenneth David,Kim, Hong Goo,Lee, Woomin,Cheon, Sosan,Yeom, Sinchul,Lim, Gyumin,Pyun, Kyung Rok,Ko, Seung Hwan,Shin, Seungha Elsevier 2018 Carbon Vol.138 No.-
<P><B>Abstract</B></P> <P>The in-plane thermal conductivity ( <SUB> k S i G </SUB> ) of silicon-doped graphene (SiG) was greatly suppressed primarily due to increased phonon scattering associated with the large mass difference of Si from its host C atoms. For SiG as supported on an 8 nm-thick SiO<SUB>2</SUB> substrate, the measured <SUB> k S i G </SUB> represents progressive decrease and saturation with the increase of Si dopants concentration, showing more than an order-of-magnitude reduction from that of supported pristine graphene (PG) and nearly two order-of-magnitude reductions when compared with suspended PG at about 2% doping concentration. The enhanced graphene-substrate conformity through thermal annealing in a vacuum additionally lowers <SUB> k S i G </SUB> from that of ambient annealing. The substitutional Si dopants tend to suppress the contribution of temperature-sensitive phonons with long mean free paths and weaken the temperature dependence of <SUB> k S i G </SUB> . The presence of Si dopants seems to allow for faster attainment of thermal equilibrium between different heat carriers due to the reduced phonon mean free paths. We believe that SiG holds the possibility of exclusively controlling the thermal properties of graphene, since the substitutional dopants do not violently destruct the hexagonal lattice structure of graphene and may possibly have minimal effects on graphene's electrical properties.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
천소산(Sosan Cheon),Kenneth David Kihm,김홍구(Hong goo Kim),임규민(Gyumin Lim),박재성(Jae Sung Park),이준식(Joon Sik Lee) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11
Starting from graphene, “atomically thin 2-D materials” has become one of the most popular research areas for the next industrial revolution. From electronic charge carrier mobility [1] to thermal conductivity [2], the physical properties of these 2-D materials are completely different from those of bulk materials, even though made from the same atomic element. One of the most promising and feasible applications of 2-D materials is that for transparent electrodes to replace the currently adopted but relatively expensive indium tin oxide (ITO), and still the most competent candidate is graphene and its derivatives. For these applications, not only electrical but also optical characterization is critical. However, the methods to measure optical properties of graphene [3-12] are imperfect yet, in terms of the availability, robustness, and accuracy. Moreover, there is no report on the proper optical properties measurement method for doped graphene by foreign atoms. In this work, two optical measurements, surface plasmon resonance (SPR) and attenuated total reflection (ATR), have been conducted in parallel, to measure the complex refractive index (RI) of graphene layers. The graphene layers are synthesized by chemical vapor deposition (CVD) and transferred onto glass or gold substrates. Comparison of theoretical calculation results with the experimental data can determine the optical properties, i.e. complex refractive indices, of the graphene layers uniquely.