Jonghwan Suhr

Superb electromagnetic wave-absorbing composites based on large-scale graphene and carbon nanotube films

AbstarctGraphene has sparked extensive research interest for its excellent physical properties and its unique potential for application in absorption of electromagnetic waves. However, the processing of stable large-scale graphene and magnetic particles on a micrometer-thick conductive support is a formidable challenge for achieving high reflection loss and impedance matching between the absorber and free space. Herein, a novel and simple approach for the processing of a CNT film-Fe3O4-large scale graphene composite is studied. The Fe3O4 particles with size in the range of 20–200u2009nm are uniformly aligned along the axial direction of the CNTs. The composite exhibits exceptionally high wave absorption capacity even at a very low thickness. Minimum reflection loss of −44.7u2009dB and absorbing bandwidth of 4.7u2009GHz at −10u2009dB are achieved in composites with one-layer graphene in six-layer CNT film-Fe3O4 prepared from 0.04u2009M FeCl3. Microstructural and theoretical studies of the wave-absorbing mechanism reveal a unique Debye dipolar relaxation with an Eddy current effect in the absorbing bandwidth.

A flexible supercapacitor based on vertically oriented ‘Graphene Forest’ electrodes

Vertically-grown graphene electrodes are developed for flexible electric double-layer capacitors (EDLC). Solid-state electrolytes and large area flexible electrodes are essential parts for wearable applications. In this work, the vertically-grown graphene electrodes were fabricated by a plasma-enhanced chemical vapor deposition process and applied to flexible electrodes of EDLC. The areal capacitance of the capacitor based on vertical graphene is 2.45 mF cm−2, which is much better even with solid electrolytes when compared to other reported vertical graphene capacitors. The capacitance also shows good flexibility and it remains unaltered even after 100u2006000 times of bending or 180 degree folding. These unique features of the capacitor could be ascribed to a discrete ‘tree-like’ morphology of the vertical graphene, which has not been known before.

Determination of material constants of vertically aligned carbon nanotube structures in compressions.

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.

Tensile properties of millimeter-long multi-walled carbon nanotubes

There have been a number of theoretical and experimental studies on tensile properties of carbon nanotubes (CNT), reporting the Young’s modulus of the individual CNT up to 1 TPa. Although CNT shows the promise to be used as reinforcement in a high modulus/strength composite material, it exhibits quite disappointing in terms of modulus or strength. Along with recent advance in CNT growth technique, we will be able to directly measure tensile properties of millimeter-long MWCNTs. This study firstly tackles the direct measurement of the tensile properties of millimeter-long MWCNTs that can be used as reinforcement in a composite system. A carefully designed tensile testing technique for the MWCNTs is developed, which allows us to obtain more accurate and reliable measured values. The average tensile strength and Young’s modulus of the CNTs investigated in this study are measured to be 0.85u2009GPa and 34.65u2009GPa, respectively. Also, this work statistically investigates the effect of the CNT dimensions including length, diameter and volume on the tensile properties. To the best of our knowledge, this is the very first report on the tensile properties of macroscopically long and continuous CNTs.

Use of Nanoindentation, Finite Element Simulations, and a Combined Experimental/Numerical Approach to Characterize Elastic Moduli of Individual Porous Silica Particles

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.

Facilitating Interfacial Slip in Carbon Nanotube Polycarbonate Composites

*† In this paper we examine the effect of pre-strain (or biased strain) and temperature on interfacial slip in singlewalled carbon nanotube polycarbonate composites. The nanotube composite is tested under uni-axial sinusoidal loading to characterize the baseline response (storage, loss modulus) of the material. Next a biased strain (0 to 0.85%) is applied and then the sinusoidal load is superposed on to the pre-strain. The results indicate a significant damping enhancement which suggests that pre-strain facilitates the activation of nanotube polymer interfacial slip. Next we tested the nano-composite under sinusoidal loading with zero pre-strain, while changing the temperature in the 25-90 °C range. The results indicate that temperature also facilitates the activation of interfacial slip leading to a significant increase in energy dissipation.

Comparing Damping Properties of Singlewalled and Multiwalled Carbon Nanotube Polymer Composites

In this paper we compare the damping properties of polymer nano-composites filled with singlewalled and multiwalled carbon nanotube fillers. The polymer material chosen for this study is polycarbonate (Lexan 121, General Electric). The nanotube fillers are dispersed in the matrix using a novel solution mixing technique with tetra-hydro-furan as the solvent. Both the multiwalled and the singlewalled nanotube filled samples show higher damping level compared to the pristine (or unfilled) polycarbonate. However, the loss modulus of nano-composite samples with singlewalled nanotubes is significantly greater than with multiwalled tubes. This suggests that since the inner shells of the multiwalled tubes are not in contact with the polymer, they do not contribute to interfacial frictional sliding; this reduces the damping efficiency of the multiwalled tubes relative to the singlewalled tubes.

Hierarchical Porous Chitosan Sponges as Robust and Recyclable Adsorbents for Anionic Dye Adsorption

Biomass waste treatment and detrimental dye adsorption are two of the crucial environmental issues nowadays. In this study, we investigate to simultaneously resolve the aforementioned issues by synthesizing chitosan sponges as adsorbents toward rose bengal (RB) dye adsorption. Through a temperature-controlled freeze-casting process, robust and recyclable chitosan sponges are fabricated with hierarchical porosities resulted from the control of concentrations of chitosan solutions. Tested as the adsorbents for RB, to the best of our knowledge, the as-prepared chitosan sponge in this work reports the highest adsorption capacity of RB (601.5u2009mg/g) ever. The adsorption mechanism, isotherm, kinetics, and thermodynamics are comprehensively studied by employing statistical analysis. Importantly and desirably, the sponge type of chitosan adsorbents exceedingly facilitates the retrieving and elution of chitosan sponges for recyclable uses. Therefore, the chitosan sponge adsorbent is demonstrated to possess dramatically squeezable capability with durability for 10,000 cycles and recyclable adsorption for at least 10 cycles, which provides an efficient and economical way for both biomass treatment and water purification.

Multi-Step Cure Kinetic Model of Ultra-Thin Glass Fiber Epoxy Prepreg Exhibiting both Autocatalytic and Diffusion-Controlled Regimes under Isothermal and Dynamic-Heating Conditions

As packaging technologies are demanded that reduce the assembly area of substrate, thin composite laminate substrates require the utmost high performance in such material properties as the coefficient of thermal expansion (CTE), and stiffness. Accordingly, thermosetting resin systems, which consist of multiple fillers, monomers and/or catalysts in thermoset-based glass fiber prepregs, are extremely complicated and closely associated with rheological properties, which depend on the temperature cycles for cure. For the process control of these complex systems, it is usually required to obtain a reliable kinetic model that could be used for the complex thermal cycles, which usually includes both the isothermal and dynamic-heating segments. In this study, an ultra-thin prepreg with highly loaded silica beads and glass fibers in the epoxy/amine resin system was investigated as a model system by isothermal/dynamic heating experiments. The maximum degree of cure was obtained as a function of temperature. The curing kinetics of the model prepreg system exhibited a multi-step reaction and a limited conversion as a function of isothermal curing temperatures, which are often observed in epoxy cure system because of the rate-determining diffusion of polymer chain growth. The modified kinetic equation accurately described the isothermal behavior and the beginning of the dynamic-heating behavior by integrating the obtained maximum degree of cure into the kinetic model development.

Accelerated Aging and Lifetime Prediction of Graphene-Reinforced Natural Rubber Composites

Accelerated lifetime prediction is the key technology to assure the safety and reliability of rubber products in automotive components. In the present study, carbon black (CB), which has been a conventional reinforcing filler in the rubber industry, was partially replaced by graphene in natural rubber (NR) composite system. Due to the superior intrinsic thermal conductivity of graphene, it is getting much attention to reduce heat build-up problem of vibration mount for longer lifetime in its demanding performance. Therefore this study not only focuses on the effect of graphene loading on mechanical properties of NR composites but also on the accelerated lifetime prediction of graphene-reinforced NR composites. Hardness and compressive properties were linearly increased as CB was partially replaced by graphene (1-5 phr; parts per hundred rubber) showing better reinforcing effect. In accelerated lifetime experiments, hardness and compression set were significantly changed after 24 h when the samples were exposed to 85 °C and 70 °C, respectively. From the relationship between thermal aging temperature and lifetime, Arrhenius plot using least square method was drawn to predict the lifetime of NR composites resulting that the activation energy reached at a maximum when 3 phr of graphene was filled in NR composite. Through the accelerated thermal aging test, lifetime prediction of graphene-reinforced NR composites were successfully drawn particularly focusing on compression set property which is one of the most dominant factors for the aging of vibration mount.