Jonghwan Suhr

Sungkyunkwan University, Sŏul, Seoul, South Korea

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Publications (62)264.49 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Graphene films fabricated by chemical vapor deposition are promising electrode materials for stretchable energy storage devices. The buckled four-layer graphene on a polydimethylsiloxane film substrate subject to various applied tensile strains has been characterized by atomic force microscopy and micro-Raman mapping. The small redshift of 2D band and the indiscernible D band demonstrated that the tensile strains of up to 40% only induced a strain variation of 0.2% and did not cause any observable damage in the graphene film. This study has confirmed that the graphene film in the buckled state is suitable for its application as a stretchable electrode.
    Carbon 11/2015; 93:620-624. DOI:10.1016/j.carbon.2015.05.096 · 6.16 Impact Factor
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    ABSTRACT: The local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as lateral support effect during buckling process. The local buckling propagation and developing of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interaction among nanotubes. Both experimental and theoretical analysis show that VACNTs buckled in the bottom region with many short waves and almost identical wave length, indicating a high mode buckling. Furthermore, the propagation and developing mechanism of buckling waves follow the wave damping effect.
    Nanoscale 07/2015; DOI:10.1039/C5NR03581C · 6.74 Impact Factor
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    ABSTRACT: Different chemical vapour deposition (CVD) fabrication conditions lead to a wide range of variation in the microstructure and morphologies of carbon nanotubes (CNTs), which actually determine the compressive mechanical properties of CNTs. However, the underlying relationship between the structure/morphology and mechanical properties of CNTs is not fully understood. In this study, we characterized and compared the structural and morphological properties of three kinds of vertically aligned carbon nanotube (VACNT) arrays from different CVD fabrication methods and performed monotonic compressive tests for each VACNT array. The compressive stress–strain responses and plastic deformation were first compared and analyzed with nanotube buckling behaviours. To quantify the compressive properties of the VACNT arrays, a strain density energy function was used to determine their intrinsic material constants. Then, the structural and morphological effects on the quantified material constants of the VACNTs were statistically investigated and analogized to cellular materials with an open-cell model. The statistical analysis shows that density, defect degree, and the moment of inertia of the CNTs are key factors in the improvement of the compressive mechanical properties of VACNT arrays. This approach could allow a model-driven CNT synthesis for engineering their mechanical behaviours.
    Nanotechnology 06/2015; 26(24). DOI:10.1088/0957-4484/26/24/245701 · 3.67 Impact Factor
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    Materials Today 05/2015; 255. DOI:10.1016/j.mattod.2015.05.001 · 10.85 Impact Factor
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    ABSTRACT: Recently, carbon materials such as carbon nanotubes and graphene have been proposed as alternatives to indium tin oxide (ITO) for fabricating transparent conducting materials. However, the low sheet resistance and high transmittance of these carbon materials have been challenged due to the intrinsic properties of the materials. In this paper, we introduce highly transparent and flexible conductive films based on a hybrid structure of graphene and an Ag-grid. Electrohydrodynamic (EHD) jet printing was used to produce a micro-scale grid consisting of Ag lines less than 10 μm wide. We were able to directly write the Ag-grid on a large-area graphene/flexible substrate due to the high conductivity of graphene. The hybrid electrode could be fabricated using hot pressing transfer and EHD jet printing under a non-vacuum, maskless, and low-temperature environment. The hybrid electrode offers an effective and simple route to achieving a sheet resistance as low as ~4 Ω/square with ~78% optical transmittance. Finally, we demonstrate that transparent flexible heaters based on the hybrid conductive films could be used in a vehicle or smart window system.
    Nanoscale 03/2015; 7(15). DOI:10.1039/C4NR06984F · 6.74 Impact Factor
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    ABSTRACT: Thermal management to prevent extreme heat surge in integrated electronic systems and nuclear reactors is a critical issue. To delay the thermal surge on the heater effectively, we report the benefit of a three dimensional nanotubular porous layer via noncovalent interactions (hydrophobic forces and hydrogen bonds). To observe the contribution of individual noncovalent interactions in a porous network formation, pristine carbon nanotubes (PCNTs) and oxidatively functionalized carbon nanotubes (FCNTs) were compared. Hydrogen-bonded interwoven nanotubular porous layer showed approximately two times critical heat flux (CHF) increase compared to that of a plain surface. It is assumed that the hydrophilic group-tethered nanotubular porous wicks and enhanced fluidity are the main causes for promoting the CHF increase. Reinforced hydrophilicity assists liquid spreading and capillarity-induced liquid pumping, which are estimated by using Electrochemical Impedance Spectroscopy. Also, shear induced thermal conduction, thermal boundary reduction, and rheology of nanoparticles could attribute to CHF enhancement phenomena.
    Scientific Reports 10/2014; 4:6817. DOI:10.1038/srep06817 · 5.58 Impact Factor
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    ABSTRACT: This article describes the use of a combination of experimental nanoindentation and finite element numerical simulations to indirectly determine the elastic modulus of individual porous, micron-sized silica (SiO2) particles. Two independent nanoindentation experiments on individual silica particles were employed, one with a Berkovich pyramidal nanoindenter tip, the other with a flat punch nanoindenter tip. In both cases, 3D finite element simulations were used to generate nanoindenter load–displacement curves for comparison with the corresponding experimental data, using the elastic modulus of the particle as a curve-fitting parameter. The resulting indirectly determined modulus values from the two independent experiments were found to be in good agreement, and were considerably lower than the published values for bulk or particulate solid silica. The results are also consistent with previously reported modulus values for nanoindentation of porous thin film SiO2. Based on a review of the literature, the authors believe that this is the first article to report on the use of nanoindentation and numerical simulations in a combined experimental/numerical approach to determine the elastic modulus of individual porous silica particles.
    Particulate Science And Technology 10/2014; 33(2):141217135346009. DOI:10.1080/02726351.2014.950396 · 0.48 Impact Factor
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    ABSTRACT: The focus of this study is to experimentally investigate the mechanical properties of fiberglass reinforced composite with various aspect ratios and loading fractions in the quasi-static and low-velocity impact loading conditions. In this study, short fiberglass reinforced polycarbonate composite materials were fabricated via a solution mixing method and characterized for their tensile properties by varying both fiberglass loading fraction and aspect ratio. The tensile properties including tensile toughness of the fiberglass reinforced composites were characterized and compared. It was observed in this study that the toughness of the composite was dramatically improved whereas the tensile strength and Young's modulus were moderately enhanced over the neat polymer, which were measured to be only up to 15% and 70% increase, respectively. The low-velocity impact behaviors of the fiberglass composites were also investigated and compared to the tensile toughness of the corresponding composites. Besides, the effect of thickness on their low-velocity impact properties was investigated. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40821.
    Journal of Applied Polymer Science 10/2014; 131(19). DOI:10.1002/app.40821 · 1.64 Impact Factor
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    ABSTRACT: Interfacial shear strength (IFSS) between particle and matrix in particulate polymer composites is a critical property in determining the mechanical behaviors since it is directly related to not only their Young’s modulus or specific strength, but also energy absorbing capability. However, the conventional techniques often present a technical challenge to accurately measure the IFSS between fillers and matrix in the composites. This is more apparent in graphene particulate composites due to their nano-scale dimensions as well as the platelet-shaped geometry. Here, the focus of this study is to use a semi-empirical approach to determine the IFSS of graphene particulate composites by combining experiments with finite element (FE) modeling. The materials of interest are reduced graphene oxide (RGO) and polycarbonate (PC). The tensile testing was performed to characterize the mechanical properties, while simultaneously monitoring the acoustic emission events in order to measure the global debonding stress (GDS) in the composites. By coupling thermal stress analysis and deformation analysis with the GDS as input to a FE model, the IFSS of the RGO particulate PC composites was successfully estimated by about 136 MPa, avoiding unnecessary assumptions and uncertainties which are seem to be inevitable with the conventional techniques for the IFSS measurement.
    Carbon 10/2014; 77:390–397. DOI:10.1016/j.carbon.2014.05.042 · 6.16 Impact Factor
  • Jong-Hun Choi, Jonghwan Suhr, Bong-Hwan Koh
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    ABSTRACT: This comparative study investigates the mechanical properties of polycarbonate in manufacturing conditions of different cooling speed. All experiments were conducted using 0.8 mm thick specimens made of commercial Polycarbonate granule (3 mm), according to the ASTM standard. The test results illustrate that polycarbonate specimens manufactured in fast-cooling (FC) condition exhibit at least five times higher resilience in ambient temperature than those of slow-cooling (SC) condition. However, the resilience of FC polycarbonate specimen quickly deteriorates, as the test temperature reduces to negative 40 °C. On the other hand, SC specimens barely changed their tensile properties. Thus, the test reveals that tensile properties of polycarbonate are significantly affected by the cooling speed in the manufacturing stage, and exposed temperature conditions. In this manuscript, the correlations between toughness and yield strength of polycarbonate specimen are summarized and discussed in terms of the cooling conditions and environmental temperature.
    Journal of Nanoscience and Nanotechnology 10/2014; 14(10). DOI:10.1166/jnn.2014.9563 · 1.34 Impact Factor
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    ABSTRACT: Due to their exceptional flexibility and transparency, CVD graphene films have been regarded as an ideal replacement of indium tin oxide for transparent electrodes, especially in applications where electronic devices may be subjected to large tensile strain. However, the search for a desirable combination of stretchability and electrochemical performance of such devices remains a huge challenge. Here, we demonstrate the implementation of a laminated ultrathin CVD graphene film as a stretchable and transparent electrode for supercapacitors. Transferred and buckled on PDMS substrates by a prestraininig-then-buckling strategy, the 4-layer graphene films maintained its outstanding quality, as evidenced by Raman spectra. Optical transmittance of up to 72.9% at a wavelength of 550 nm and stretchability of 40% were achieved. As the tensile strain increased up to 40%, the specific capacitance showed no degradation and even increased slightly. Furthermore, the supercapacitor demonstrated excellent frequency capability with small time constants under stretching.
    ACS Nano 08/2014; 8(9). DOI:10.1021/nn503570j · 12.03 Impact Factor
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    ABSTRACT: Graphene oxide (GO) has recently become an attractive building block for fabricating graphene-based functional materials. GO films and fibers have been prepared mainly by vacuum filtration and wet spinning. These materials exhibit relatively high Young's moduli but low toughness and a high tendency to tear or break. Here, we report an alternative method, using bar coating and drying of water/GO dispersions, for preparing large area GO thin films (e.g. 800-1200 cm(2) or larger) with an outstanding mechanical behavior and excellent tear resistance. These dried films were subsequently scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76 %), high toughness (up to 17 J/m(3)) and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability. This method is simple and after thermal reduction of the GO material, it can render highly electrically conducting graphene-based fibers with values up to 416 S/cm at room temperature. In this context, GO fibers annealed at 2000 °C were also successfully used as electron field emitters operating at low turn on voltages of ca. 0.48 V/μm and high current densities (5.3 A/cm(2)). Robust GO fibers and large-area films with fascinating architectures and outstanding mechanical and electrical properties were prepared bar coating followed by dry film scrolling.
    ACS Nano 05/2014; 8(6). DOI:10.1021/nn501098d · 12.03 Impact Factor
  • SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring; 03/2014
  • Source
    Jonghwan Suhr
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    ABSTRACT: The high Young's modulus and tensile strength of carbon nanotubes has attracted great attention from the research community given the potential for developing super-strong, super-stiff composites with carbon nanotube reinforcements. Over the decades, the strength and stiffness of carbon nanotube-reinforced polymer nanocomposites have been researched extensively. However, unfortunately, such strong composite materials have not been developed yet. It has been reported that the efficiency of load transfer in such systems is critically dependent on the quality of adhesion between the nanotubes and the polymer chains. In addition, the waviness and orientation of the nanotubes embedded in a matrix reduce the reinforcement effectiveness. In this study, we carried out performed micromechanics-based numerical modeling and analysis by varying the geometry of carbon nanotubes including their aspect ratio, orientation, and waviness. The results of this analysis allow for a better understanding of the load transfer capabilities of carbon nanotube-reinforced polymer composites.
    Transactions of the Korean Society of Mechanical Engineers B 01/2014; 38(1). DOI:10.3795/KSME-B.2014.38.1.089
  • 11/2013; 21(6):49-57. DOI:10.7467/KSAE.2013.21.6.049
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    ABSTRACT: A three-dimensional (3D) nitrogen-doped multiwall carbon nanotube (N-MWCNT) sponge possessing junctions induced by both nitrogen and sulfur was synthesized by chemical vapor deposition (CVD). The formation of "elbow" junctions as well as "welded" junctions, which are attributed to the synergistic effect of the nitrogen dopant and the sulfur promoter, plays a critically important role in the formation of 3D nanotube sponges. To the best of our knowledge, this is the first report showing the synthesis of macroscale 3D N-MWCNT sponges. Most importantly, the diameter of N-MWCNT can be simply controlled by varying the concentration of sulfur, which in turn controls both the sponge's mechanical and its electrical properties. It was experimentally shown that, with increasing diameter of N-MWCNT, the elastic modulus of the sponge increased while the electrical conductivity decreased. The mechanical behaviors of the sponges have also been quantitatively analyzed by employing strain energy function modeling.
    Nano Letters 10/2013; 13(11). DOI:10.1021/nl403109g · 13.59 Impact Factor
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    ABSTRACT: Several analytical models exist for determination of the Young's modulus and coefficient of thermal expansion (CTE) of particulate composites. However, it is necessary to provide accurate material properties of the particles as input data to such analytical models in order to precisely predict the composite's properties, particularly at high particle loading fractions. In fact, the constituent's size scale often presents a technical challenge to accurately measure the particles' properties such as Young's modulus or CTE. Moreover, the in situ material properties of particles may not be the same as the corresponding bulk properties when the particles are embedded in a polymer matrix. To have a better understanding of the material properties and provide useful insight and design guidelines for particulate composites, the concept of "effective in situ constituent properties'' and an indirect method were employed in this study. This approach allows for the indirect determination of the particle's in situ material properties by combining the experimentally determined composite and matrix properties and finite element (FE) models for predicting the corresponding composite properties, then backing out the effective in situ particle properties. The proposed approach was demonstrated with micron-size SiO2 particle reinforced epoxy composites over a range of particle loading fractions up to 35 vol.% by indirectly determining both the effective Young's modulus and the effective CTE of the particles. To the best of our knowledge, this study is the first published report on the indirect determination of both the Young's modulus and the CTE of micron size particles in particulate composites. Similar results on Young's modulus of micron-size SiO2 particles measured from nano-indentation testing are encouraging. (c) 2013 Elsevier Ltd. All rights reserved.
    Materials and Design 10/2013; 51:219-224. DOI:10.1016/j.matdes.2013.04.025 · 3.50 Impact Factor
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    ABSTRACT: The focus of this study is to experimentally investigate the effect of debonding stress, the interface between the fibers and the polymer matrix, on the damping properties of the short fiberglass reinforced polymer composites. In this study, short fiberglass reinforced polycarbonate composite materials were fabricated and characterized for their tensile properties by varying the fiberglass loading fraction. The debonding stress was evaluated by coupling the acoustic emission technique with the tensile testing. After the determination of the debonding stress was completed, dynamic cyclic testing was performed in order to investigate the effect of debonding on the damping properties of the polymer composites. It was experimentally observed in this study that the debonding can facilitate the stick-slip friction under cyclic loadings, which then gives rise to better damping performance in the fiberglass composites.
    Modern Physics Letters B 06/2013; 27(15):50108-. DOI:10.1142/S021798491350108X · 0.69 Impact Factor
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    ABSTRACT: Recently, considerable attention has been paid on the utilization of natural materials in structures. Utilizing natural materials over traditional, synthetic structures results in a reduction of carbon emissions from material synthesis; such a source of materials could also be renewable and recyclable. Currently, few reports exist on sound and vibrational properties in sandwich composites with the use of natural materials. Sandwich composites are commonly used in structures for their superior strength and stiffness-to-weight ratios, but from these same properties, they radiate noise efficiently. Here, in this study, the sound and vibration damping properties of natural material based sandwich composites were explored and characterized. It was experimentally observed that utilizing a balsa wood core with a natural fiber based face sheet has a 100% improvement in coincidence frequency, a metric of acoustic performance, and the combination of a natural fiber based face sheet with a Rohacell 51 WF synthetic core exhibits a 233% increase over a fully synthetic sandwich composite. As these improvements in acoustic performance are achieved with only small sacrifices in bending stiffness, these results suggest that, if optimized, natural material based sandwich composites could be an environmentally friendly solution to the sandwich structure-noise radiation problem.
    Composite Structures 02/2013; 96:538–544. DOI:10.1016/j.compstruct.2012.09.006 · 3.32 Impact Factor
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    ABSTRACT: An ever increasing demand for material performance coupled with recent advances in the production and availability of nanoscale materials has led to a significant interest in the use of nanoscale fillers to augment and tailor material performance in nanocomposites. Specifically, the use of high aspect ratio fillers, such as carbon nanotubes and carbon nanofibers (CNF) to augment the viscoelastic performance of nanocomposites has been the focus of many studies. Previous study has shown the use of high aspect ratio fillers to significantly enhance the damping capacity at low frequencies by more than 100 %, relative to the neat epoxy. In light of the promise, this technology holds for use in engineered applications, requiring specific damping performance, there remains a fundamental lack in understanding of the precise mechanisms and thereby a lack of ability to accurately predict material performance, which is limiting application of the technology. This study looks at both the effect of the random filler orientation and the effect of filler waviness in examining the viscoelastic response of CNF-reinforced nanocomposites. Using a fundamental approach, this study employs experimental, analytical, and numerical modeling techniques to characterize the amount of strain energy transferred to the filler and the matrix, and to indirectly estimate the effective loss factor of the filler. Utilizing experimental investigation coupled with parametric inquiries using strain energy methods relative to both filler orientation and waviness, this study provides fundamental insight into the effect of imperfect geometries and random filler distributions seen in nanocomposites utilizing high aspect ratio fillers, such as CNF.
    Journal of Materials Science 01/2013; 48(2):832-840. DOI:10.1007/s10853-012-6803-6 · 2.37 Impact Factor

Publication Stats

1k Citations
264.49 Total Impact Points

Institutions

  • 2013–2015
    • Sungkyunkwan University
      • Department of Polymer Science and Engineering
      Sŏul, Seoul, South Korea
  • 2011–2014
    • University of Delaware
      • Department of Mechanical Engineering
      Ньюарк, Delaware, United States
  • 2007–2010
    • University of Nevada, Reno
      • Department of Mechanical Engineering
      Reno, Nevada, United States
  • 2004–2007
    • Rensselaer Polytechnic Institute
      • Department of Mechanical, Aerospace and Nuclear Engineering
      Troy, New York, United States