Seungae Lee

Enhanced Electroresponsive Performance of Double-Shell SiO2/TiO2 Hollow Nanoparticles

The double-shell SiO2/TiO2 hollow nanoparticles (DS HNPs) are successfully fabricated and adopted as dispersing materials for electrorheological (ER) fluids to investigate an influence of shell structure on ER properties. The DS HNPs-based ER fluid exhibits outstanding ER performance which is 4.1-fold higher compared to that of single shell SiO2/TiO2 hollow nanoparticles (SS HNPs)-based ER fluid. The significantly improved ER property of DS HNPs-based ER fluid is ascribed to the enhanced interfacial polarization. In addition, the ER activities of DS HNPs-based ER fluids are examined depending on the particle diameter. The yield stress of DS HNPs-based ER fluids increases up to 302.4 kPa under an electric field of 3 kV mm(-1) by reducing the particle size, which is remarkable performance enough to promise sufficient probability for practical and industrial applications. The enhanced ER performance of the smaller DS HNPs is attributed to the increased surface area of large pores (30-35 nm) within the shells, resulting in a large achievable polarizability determined by dielectric constants. Furthermore, the antisedimentation property is analyzed in order to offer an additional insight into the effect of particle size on the ER fluids.

Multifunctional Graphene Sheets Embedded in Silicone Encapsulant for Superior Performance of Light-Emitting Diodes

Graphene nanosheets with uniform shape are successfully incorporated into a silicone encapsulant of a light-emitting diode (LED) using a solvent-exchange approach which is a facile and straightforward method. The graphene nanosheets embedded in the silicone encapsulant have a multifunctional role which improves the performance of light-emitting diodes. The presence of graphene gives rise to effective heat dissipation, improvement of protection ability from external stimuli, such as moisture and hazardous gas, and enhancement of mechanical properties such as elastic modulus and fracture toughness. Consequently, the LEDs composed of a graphene-embedded silicone encapsulant exhibit long-term stability without loss of luminous efficiency by addition of relatively small amounts of graphene. This novel strategy offers a feasible candidate for their practical or industrial applications.

Enhanced electrorheological performance of a graphene oxide-wrapped silica rod with a high aspect ratio

In this study, the influence of particle geometry on electrorheological (ER) activity is examined by varying the aspect ratio of graphene oxide (GO)-wrapped silica materials. The GO-wrapped silica material-based ER fluid exhibits a high shear stress as the aspect ratio of the particle increases; this is attributed to the flow resistance and mechanical stability of the fluid. Additionally, the dielectric loss model is used to investigate the dielectric properties of ER fluids, which have been shown to be associated with the enhancement of ER activity. The GO-wrapped silica material with a higher aspect ratio exhibits a higher dielectric constant and shorter relaxation times for interfacial polarization, due to greater polarizability. Thus, the aspect ratio of GO-wrapped silica materials plays a prominent role in the enhancement of ER performance.

Evaluation of anti-scratch properties of graphene oxide/polypropylene nanocomposites

Graphene oxide/polypropylene (GO/PP) nanocomposites were fabricated by varying the concentration of GO in the presence of octyltriethoxysilane (OTES) using an internal batch mixer to enhance the anti-scratch properties of pristine PP. Mechanical properties, including hardness, elastic modulus, and fracture toughness, of the prepared GO/PP nanocomposites were analyzed by nanoindentation. As a compatibilizer, OTES increased dispersibility, interfacial interactions, and mechanical interlocking between the GO nanofiller and the PP matrix, improving mechanical strength. A 286% increase in hardness, 127% improvement of elastic modulus and 117% enhancement on fracture toughness are achieved by addition of only 1.0 wt% of GO and OTES. For scratch tests, several GO/PP nanocomposite samples were analyzed according to various scratch parameters. Furthermore, scratch hardness, elastic recovery, and tangential forces acting on the scratch probe were investigated to provide insight into scratch behavior. The outstanding resistance to scratch deformation such as a maximum increase in scratch hardness of 189% was likely due to the excellent mechanical properties of the GO/PP nanocomposites.

Graphene size control via a mechanochemical method and electroresponsive properties.

Highly dispersible graphene oxide (GO) sheets of uniform submicrometer size were successfully fabricated from pristine graphite using a simple mechanochemical process. The GO flake morphology was transformed into a spherical form, and the density was decreased slightly via the ball-milling process. Ball-milled GO can be used as an electrorheological (ER) material because of its small particle size, low conductivity, and outstanding dispersibility in silicone oil. We found that the 2-h ball-milled GO-based ER fluid had the best ER performance (shear stress of 78.5 Pa and 630% ER efficiency), which was double that of the nonmilled GO-based ER fluid. The response time to form a fibrillar structure along the applied electric field direction and the recovery time to the starting level decreased with increasing ball-milling time. Additionally, the retarded settling velocity of isolated GO sheets and the electrostatic repulsion between oxygen functional groups on the GO sheets combined to improve the antisedimentation property. The ability to control the size of graphene sheets is a great opportunity to advance graphene commercialization in a high-quality, scalable production setting.

High electrothermal performance of expanded graphite nanoplatelet-based patch heater

A sub-kilogram scale (∼500 g) of expanded graphite nanoplatelet (EGnP) with multi-layered graphene sheets were successfully fabricated using a simple mild-oxidation of pristine graphite. In particular, a substantial amount of trapped water molecules in EGnP make it a good ionic conductor, while simultaneously allowing it to serve as an electrolyte with ion transport characteristics. Due to its high electrical and thermal conductivity, micro-patterned EGnP can be used to produce electro-heating elements for line heaters. We found that the surface resistance of EGnP-based films was two orders of magnitude smaller than that of graphene-based thin films. The EGnP-based line heater demonstrated efficient heat propagation with uniform temperature distribution, resulting in an energy savings of up to ca. 37% in comparison to the graphene-based flexible heater. Especially, the steady-state temperature increased as the applied voltage increased and it reached to 172.3 °C at a driving voltage of 14 V. In addition, the EGnP-based line heaters under a bending radius of 4 cm had a 25 °C higher temperature as compared with heaters under flat conditions. Most of all, screen printing technique provides the facile formation of shape and size, and makes it possible to be used as a cheap and lightweight patch heater for industrial applications.

Molecular characterization of Mycobacterium intracellulare-related strains based on the sequence analysis of hsp65, internal transcribed spacer and 16S rRNA genes

We investigated the molecular epidemiological features of 94 Mycobacterium intracellulare-related strains, isolated from Korean patients, using sequence analysis targeting 3 independent chronometer molecules, hsp65, the internal transcribed spacer 1 region and the 16S rRNA gene. By collective consideration of these three gene-based approaches, the 94 strains were divided into 5 groups (INT1, INT2, INT3, INT4 and INT5). The frequencies of genotype INT1, 2, 3, 4 and 5 in the 94 isolates were 57.4 % (54), 27.7 % (26), 6.4 % (6), 5.3 % (5) and 3.2 % (3), respectively. When correlations between genotypes and clinical parameters (age, sex, radiological type and the presence of a cavity) were analysed in 78 patients with non-tuberculous mycobacteria pulmonary diseases, no relationships were observed with respect to age, sex and radiological type, but genotype and the presence of a cavity tended to be related (P=0.051).

Synthesis and electrical response of polyaniline/poly(styrene sulfonate)-coated silica spheres prepared by seed-coating method

The polyaniline/poly(styrene sulfonate) (PANI/PSS)-coated silica spheres with three different sizes (50, 100, and 250 nm) are fabricated through seed-coating method and adopted as dispersing materials for electrorheological (ER) fluids to examine the influence of particle diameter on ER activity. Interestingly, the ER properties of PANI/PSS-coated silica spheres exhibit a dependence on their size. Performances of PANI/PSS-coated silica spheres-based ER fluids enhanced with decreasing the diameter of particle. It is believed that the size effect played a dominant role in enhancing the performance of ER fluid. Furthermore, the fibrillation phenomenon of prepared PANI/PSS-coated silica spheres-based ER fluid was observed via an optical microscope in the applied electric field. Sedimentation properties were also analyzed to provide additional insight into the size effect of ER fluids.

Electrorheological performance of multigram-scale mesoporous silica particles with different aspect ratios

Mesoporous silica (mSiO2) particle-based electrorheological (ER) fluids were examined to determine the effect of the particles’ aspect ratio on ER performance. Multigram-scale mSiO2 particles were fabricated with different aspect ratios using the sol–gel method. The ER performance of various mSiO2 particle-based ER fluids improved with an increasing aspect ratio due to the better flow resistance and mechanical stability. Moreover, an incremental increase in the aspect ratio enhanced the interfacial polarization of the material. Thus, mSiO2 materials with a high aspect ratio exhibited the highest ER performance due to combined contributions from geometrical effects and their dielectric properties. In addition, various mSiO2-based ER fluids showed potential for large-scale production and a wide applicable range of electric field strengths (up to 10.0 kV mm−1).

Enhanced Electroresponse of Alkaline Earth Metal-Doped Silica/Titania Spheres by Synergetic Effect of Dispersion Stability and Dielectric Property

A series of alkaline earth metal-doped hollow SiO2/TiO2 spheres (EM-HST) are prepared as electrorheological (ER) materials via sonication-mediated etching method with various alkaline earth metal hydroxides as the etchant. The EM-HST spheres are assessed to determine how their hollow interior and metal-doping affects the ER activity. Both the dispersion stability and the dielectric properties of these materials are greatly enhanced by the proposed one-step etching method, which results in significant enhancement of ER activity. These improvements are attributed to increased particle mobility and interfacial polarization originating from the hollow nature of the EM-HST spheres and the effects of EM metal-doping. In particular, Ca-HST-based ER fluid exhibits ER performance which is 7.1-fold and 3.1-fold higher than those of nonhollow core/shell silica/titania (CS/ST) and undoped hollow silica/titania (HST)-based ER fluids, respectively. This study develops a versatile and simple approach to enhancing ER activity through synergetic effects arising from the combination of dispersion stability and the unique dielectric properties of hollow EM-HST spheres. In addition, the multigram scale production described in this experiment can be an excellent advantage for practical and commercial ER application.