Won Suk Chang

3D Printing of Reduced Graphene Oxide Nanowires

3D printing of reduced graphene oxide (rGO) nanowires is realized at room temperature by local growth of GO at the meniscus formed at a micropipette tip followed by reduction of GO by thermal or chemical treatment. 3D rGO nanowires with diverse and complicated forms are successfully printed, demonstrating their ability to grow in any direction and at the selected sites.

Three-Dimensional Printing of Highly Conductive Carbon Nanotube Microarchitectures with Fluid Ink

Moving printed electronics to three dimensions essentially requires advanced additive manufacturing techniques yielding multifunctionality materials and high spatial resolution. Here, we report the meniscus-guided 3D printing of highly conductive multiwall carbon nanotube (MWNT) microarchitectures that exploit rapid solidification of a fluid ink meniscus formed by pulling a micronozzle. To achieve high-quality printing with continuous ink flow through a confined nozzle geometry, that is, without agglomeration and nozzle clogging, we design a polyvinylpyrrolidone-wrapped MWNT ink with uniform dispersion and appropriate rheological properties. The developed technique can produce various desired 3D microstructures, with a high MWNT concentration of up to 75 wt % being obtained via post-thermal treatment. Successful demonstrations of electronic components such as sensing transducers, emitters, and radio frequency inductors are also described herein. We expect that the technique presented in this study will facilitate selection of diverse materials in 3D printing and enhance the freedom of integration for advanced conceptual devices.

One-Step Synthesis of PEG-Coated Gold Nanoparticles by Rapid Microwave Heating

Polyethylene Glycol- (PEG-) coated gold nanoparticles (PEG-AuNPs) are synthesized by a one-step route with rapid microwave heating. Homogeneous nucleation of the primary gold particles is enhanced by increasing the applied microwave power during the initial stage of the synthesis, increasing the temperature ramping rate () and resulting in decreased size and improved uniformity of the synthesized PEG-AuNPs. Using rapid microwave heating, we successfully produce uniform colloidal PEG-AuNPs with an average diameter of  nm within a few minutes. By appropriate tuning of the growth parameters, microwave synthesis can produce largely colloidal PEG-AuNPs with high uniformity.

Self-passivation of transparent single-walled carbon nanotube films on plastic substrates by microwave-induced rapid nanowelding

We developed a straightforward method for enhancing the environmental stability of transparent single-walled carbon nanotube (SWCNT) network films on plastic substrates using a rapid microwave heating to produce SWCNT film–substrate nanowelding without any chemicals. The selective heating of SWCNTs induced by microwave irradiation leads to embedding the SWCNTs in the substrate, even within 10 s, without distortion of the substrate. The SWCNTs-embedded surface of the substrate played the role of a self-passivation layer that protected the SWCNTs from water molecules. The sheet resistance values of the nanowelded films had not increased more than 10%.

Carbon nanotube-conducting polymer composite wires formed by fountain pen growth (FPG) route

We demonstrate the three dimensional (3D) growth of multiwalled carbon nanotube (MWNT)–PEDOT : PSS composite microwires by a fountain pen growth (FPG) route. The electrical conductivity of the grown composite wire is improved by enhancing the electrical network of entrapped MWNTs formed in the PEDOT : PSS matrix.

Energy efficiency of a scaled-up microwave-assisted transesterification for biodiesel production

We propose a scalable and energy-efficient microwave-assisted chemical reactor for biodiesel production, which is composed of a partially modified conventional 10-L stainless steel vessel and a microwave coupler to enable an optimized microwave injection of 99% power efficiency. The microwave power applied via a waveguide can be directly injected into the reaction vessel using a coupling rod clamped to a pressured microwave window, giving convenience of scale-up of the reactor volume because a conventional microwave transparent vessel like glass is not need. Microwave-assisted transesterification of triglycerides with potassium hydroxide catalyst achieved an accelerated conversion of 95% in 5 min. The precisely measured microwave energy consumption was only 87% of the calculated heat requirement for both the reactant and the vessel. Computer simulation studies indicated that the cause of the energy efficiency for microwave heating was the relatively low temperature of the vessel due to a reverse temperature gradient, in contrast to those done with conventional hot wall heating.

Three-dimensional Printing of Silver Microarchitectures Using Newtonian Nanoparticle Inks

Although three-dimensional (3D) printing has recently emerged as a technology to potentially bring about the next industrial revolution, the limited selection of usable materials restricts its use to simple prototyping. In particular, metallic 3D printing with submicrometer spatial resolution is essential for the realization of 3D-printed electronics. Herein, a meniscus-guided 3D printing method that exploits a low-viscosity (∼7 mPa·s) silver nanoparticle (AgNP) ink meniscus with Newtonian fluid characteristics (which is compatible with conventional inkjet printers) to fabricate 3D silver microarchitectures is reported. Poly(acrylic acid)-capped AgNP ink that exhibits a continuous ink flow through a confined nozzle without aggregation is designed in this study. Guiding the ink meniscus with controlled direction and speed enables both vertical pulling and layer-by-layer processing, resulting in the creation of 3D microobjects with designed shapes other than those for simple wiring. Various highly conductive (>104 S·cm-1) 3D metallic patterns are demonstrated for applications in electronic devices. This research is expected to widen the range of materials that can be employed in 3D printing technology, with the aim of moving 3D printing beyond prototyping and into real manufacturing platforms for future electronics.

Field-emission X-ray sources with an anisotropic focusing lens for isotropic X-ray focal spots

Hermetic-sealed field-emission X-ray sources with an anode voltage and current of 70 kV and 0.5 mA have been developed using an anisotropic focusing electron lens structure for an isotropic focal spot. The anisotropic focusing of an electron beam was achieved using an electrostatic electron lens with anisotropic aperture geometry integrated in a gate (or grid) electrode. The anisotropic geometry has an influence on the electron beam with different horizontal and vertical electric fields, thereby anisotropically focusing the beam on an anode target.

Effects of grid parameters on field emission characteristics in triode type CNT X-ray source

Summary form only given. Field emission X-ray sources have been developed by using carbon nano-tubes (CNT) for diverse applications in fields of biological, medical and material diagnostic technologies. For such applications, it is required to improve the efficiency of field emission for obtaining higher X-ray flux. Previous studies have reported that several parameters, including the aspect ratio of CNT, the inter-tube distance and so on, can influence on the field emission properties of CNT emitter. One of the important factors in field emission of CNT is the field enhancement factor (FEF) which is defined as the ratio of the actual electric field at the apex of CNT and the macroscopic electric field. The FEF of CNT emitter in diode structure depends dominantly on the shape of individual CNT and the pattern of CNT array, whereas the FEF in the triode structure, especially in case of planar type emitter, is modulated by the grid structure, consisting of grid mesh size (i.e., grid mesh pitch, opening area rate, grid mesh line width) and gap distances between grid and CNT emitter. Here, we demonstrate that the grid parameter significantly affects the FEF of the planar type CNT emitter in triode structure. The experimental measurements as well as computer simulation works are performed by adjusting the gap distance and the grid mesh size to interpret characteristics of the field emission from CNT emitters. The results clearly show that the variation of grid structure - a decrease of the grid mesh size and an increase of the gap distance - induces modification of local electric field on individual CNT, resulting in an effective increase of the FEF.