Kyoung-Seok Moon

Metallic conduction induced by direct anion site doping in layered SnSe2.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe2 by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~1020u2009cm−3 is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170u2009S·cm−1 from ~1.7u2009S·cm−1 for non-doped SnSe2. When the carrier concentration exceeds ~1019u2009cm−3, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe2.

Pack aluminization of nickel anode for molten carbonate fuel cells

The aluminium pack cementation (pack aluminization) process on a porous nickel anode for molten carbonate fuel cells has been studied to improve anode creep resistance. The porous nickel substrates used in this study were fabricated by doctor blade equipment followed by sintering (850 °C). Packs surrounding the Ni anode were made by mixing Al2O3 powder, Al powder, and NaCl as activator. The pack aluminization was performed at 700 to 850 °C for 0.5–5.0 h. After pack aluminization, the principal Niue5f8Al intermetallic compounds detected were Ni3Al at 700 °C, NiAl at 750 °C and Ni3Al2 at 800 °C. The aluminum content in the aluminized Ni anode was proportional to the square root of pack aluminizing time. With increasing the Al content in the anode, the creep of the anode decreased. It was nearly constant (2.0%) when the Al content was above 5.0%. Although the exchange current density (24 mA/cm2) for the aluminized (2.5 wt.%) Ni anode was somewhat lower than that of the pure Ni anode (40 mA/cm2), the performance of a single cell using an aluminized Ni anode was similar to that of the one with pure Ni anode.

Fabrication of flexible magnetic papers based on bacterial cellulose and barium hexaferrite with improved mechanical properties

We report on a simple approach to fabricate mechanically robust magnetic cellulose papers containing M-type barium hexaferrite (BaFe12O19) nanoplates. BaFe12O19 nanoplates were synthesized by a hydrothermal method and then chemically functionalized by using a silane coupling agent. The magnetic cellulose papers prepared with the silane-treated BaFe12O19 nanoplates exhibited improved mechanical properties with tensile strength of 58.5 MPa and Young’s modulus of 2.95 GPa.

Temperature dependence of contact resistance at metal/MWNT interface

Although contact resistance of carbon nanotube (CNT) is one of the most important factors for practical application of electronic devices, a study regarding temperature dependence on contact resistance of CNTs with metal electrodes has not been found. Here, we report an investigation of contact resistance at multiwalled nanotube (MWNT)/Ag interface as a function of temperature, using MWNT/polydimethylsiloxane (PDMS) composite. Electrical resistance of MWNT/PDMS composite revealed negative temperature coefficient (NTC). Excluding the contact resistance with Ag electrode, the NTC effect became less pronounced, showing lower intrinsic resistivity with the activation energy of 0.019u2009eV. Activation energy of the contact resistance of MWNT/Ag interface was determined to be 0.04u2009eV, two times larger than that of MWNT-MWNT network. The increase in the thermal fluctuation assisted electron tunneling is attributed to conductivity enhancement at both MWNT/MWNT and MWNT/Ag interfaces with increasing temperature.

Novel Flexible Transparent Conductive Films with Enhanced Chemical and Electromechanical Sustainability: TiO2 Nanosheet–Ag Nanowire Hybrid

Flexible transparent conductive films (TCFs) of TiO2 nanosheet (TiO2 NS) and silver nanowire (Ag NW) network hybrid were prepared through a simple and scalable solution-based process. The as-formed TiO2 NS-Ag NW hybrid TCF shows a high optical transmittance (TT: 97% (90.2% including plastic substrate)) and low sheet resistance (Rs: 40 Ω/sq). In addition, the TiO2 NS-Ag NW hybrid TCF exhibits a long-time chemical/aging and electromechanical stability. As for the chemical/aging stability, the hybrid TCF of Ag NW and TiO2 NS reveals a retained initial conductivity (ΔRs/Rs < 1%) under ambient oxidant gas over a month, superior to that of bare Ag NW (ΔRs/Rs > 4000%) or RuO2 NS-Ag NW hybrid (ΔRs/Rs > 200%). As corroborated by the density functional theory simulation, the superb chemical stability of TiO2 NS-Ag NW hybrid is attributable to the unique role of TiO2 NS as a barrier, which prevents Ag NWs chemical corrosion via the attenuated adsorption of sulfidation molecules (H2S) on TiO2 NS. With respect to the electromechanical stability, in contrast to Ag NWs (ΔR/R0 ∼ 152.9%), our hybrid TCF shows a limited increment of fractional resistivity (ΔR/R0 ∼ 14.4%) after 200u2009000 cycles of the 1R bending test (strain: 6.7%) owing to mechanically welded Ag NW networks by TiO2 NS. Overall, our unique hybrid of TiO2 NS and Ag NW exhibits excellent electrical/optical properties and reliable chemical/electromechanical stabilities.

Mechanically Robust Magnetic Carbon Nanotube Papers Prepared with CoFe2O4 Nanoparticles for Electromagnetic Interference Shielding and Magnetomechanical Actuation

The introduction of inorganic nanoparticles into carbon nanotube (CNT) papers can provide a versatile route to the fabrication of CNT papers with diverse functionalities, but it may lead to a reduction in their mechanical properties. Here, we describe a simple and effective strategy for the fabrication of mechanically robust magnetic CNT papers for electromagnetic interference (EMI) shielding and magnetomechanical actuation applications. The magnetic CNT papers were produced by vacuum filtration of an aqueous suspension of CNTs, CoFe2O4 nanoparticles, and poly(vinyl alcohol) (PVA). PVA plays a critical role in enhancing the mechanical strength of CNT papers. The magnetic CNT papers containing 73 wt % of CoFe2O4 nanoparticles exhibited high mechanical properties with Youngs modulus of 3.2 GPa and tensile strength of 30.0 MPa. This magnetic CNT paper was successfully demonstrated as EMI shielding paper with shielding effectiveness of ∼30 dB (99.9%) in 0.5-1.0 GHz, and also as a magnetomechanical actuator in an audible frequency range from 200 to 20u2009000 Hz.

Grain growth behavior of Ba1.5Sr1.5Co2Fe24O41 flakes in molten salt synthesis and the magnetic properties of flake/polymer composites

Single-phase Ba1.5Sr1.5Co2Fe24O41 (Ba1.5Sr1.5Z) hexaferrite flakes were synthesized using a two-step grain growth process, involving a calcination process and molten salt synthesis. Geometric parameters such as aspect ratio and the degree of agglomeration can be controlled by tuning this calcination-molten salt method. The morphological evolution of the flakes was explained using the concept of mixed-control grain growth, i.e., a combination of diffusion for growth and interface reactions, which is a growth mechanism for a faceted interface. The single-phase Ba1.5Sr1.5Z flake particle with high aspect ratio turned out to be a good candidate of soft magnetic inclusion, through an investigation of the correlation between material composition, magnetic behavior, and particle morphology.