Youngjae Yoo

Capacitance behavior of composites for supercapacitor applications prepared with different durations of graphene/nanoneedle MnO2 reduction

Abstract Graphene/MnO 2 composites were prepared by hydrazine hydrate-mediated reduction of graphene oxide (GO)/MnO 2 at various reduction times to determine the optimal conditions for obtaining materials with excellent electrochemical performance. Variations in the oxygen-containing surface functional groups were observed as the reduction time was varied. These changes were found to affect the electrical conductivity and density of nanoneedle MnO 2 , which influence the surface area and significantly affect the supercapacitive performance of the composites. Morphological and microstructural characterizations of the as-prepared composites demonstrated that MnO 2 was successfully formed on the GO surface and indicated the efficacy of hydrazine hydrate as a reducing agent for GO. The capacitive properties of the graphene/MnO 2 electrodes prepared at a reduction time of 28xa0h (rGO(28)/MnO 2 ) exhibited a low sheet-resistance value as well as a high surface area, resulting in a GO/MnO 2 composite with excellent electrochemical performance (371.74xa0Fxa0g −1 at a scan rate of 10xa0mVxa0s −1 ). It is anticipated that the formation of MnO 2 -based nanoneedles on GO surfaces by the demonstrated 28-h hydrazine-reduction protocol is a promising method for supercapacitor electrode fabrication.

Synthesis and characterization of polyethersulfone with intrinsic microporosity

Polymers with intrinsic microporosity (PIMs) are usually ladder-type polymers that adopt a spiro structure. In the present study, a new non-ladder-type polyethersulfone-type PIM that incorporates a linear sulfone moiety was synthesized. Brunauer–Emmett–Teller measurements confirmed that the synthesized polymer had a high specific surface area (430–590 m2 g−1) due to the presence of micropores of size 0.54–0.66 nm. The intrinsic porosity characteristics of the new PIM are attributed to the resonance structures of sulfone and benzene moieties. Syntheses of new gas-separation membranes using PIMs containing sulfone or similar functional groups are expected in the future.

Anisotropy-Driven High Thermal Conductivity in Stretchable Poly(vinyl alcohol)/Hexagonal Boron Nitride Nanohybrid Films

Controlling the anisotropy of two-dimensional materials with orientation-dependent heat transfer characteristics is a possible solution to resolve severe thermal issues in future electronic devices. We demonstrate a dramatic enhancement in the in-plane thermal conductivity of stretchable poly(vinyl alcohol) (PVA) nanohybrid films containing small amounts (below 10 wt %) of hexagonal boron nitride ( h-BN) nanoplatelets. The h-BN nanoplatelets were homogeneously dispersed in the PVA polymer solution by ultrasonication without additional surface modification. The mixture was used to prepare thermally conductive nanocomposite films. The in-plane thermal conductivity of the resulting PVA/ h-BN nanocomposite films increased to 6.4 W/mK when the strain was increased from 0 to 100% in the horizontal direction. More specifically, the thermal conductivity of a PVA/ h-BN composite film with 10 wt % filler loading can be improved by up to 32 times as compared to pristine PVA. This outstanding thermal conductivity value is significantly larger than that of materials currently used in in-plane thermal management systems. This result is attributed to the anisotropic alignment of h-BN particles in the PVA chain matrix during stretching, enhancing phonon conductive paths and hence improving the thermal conductivity and thermal properties of PVA/ h-BN nanocomposite films. These polymer nanocomposites have low cost as the amount of expensive conductive fillers is reduced and can be potentially used as high-performance materials for thermal management systems such as heat sink and thermal interface materials, for future electronic and electrical devices.

Synthesis and Analysis of Flow Modifiers for PPS Flowability Enhancement

Polyphenylene sulfide (PPS) is used for electronic devices, precision equipments, and various fields because of its outstanding properties such as thermal stability, mechanical properties, chemical stability and dimensional stability. Also, it is lightweight and has better mechanical properties than other engineering plastics so it can substitute metals in many industries. However, there are a few studies on the improvement of flowability of PPS and it is extremely difficult to understand how the additives affect the fluidity. In this study, several flow modifiers were designed and synthesized. Their core structure was diphenyl thioether, which was flanked by ether or amide bonds. Flowability was confirmed by preparing spiral specimens after mixing PPS and additives for 3 min using a microcompounder. Torque was measured by v 1.5 microcompounder program. Disc specimens were prepared and their viscosity was measured. The relationships between the flowability and the structure of flow modifiers were analyzed.