Sang-eui Lee

Highly Effective Electromagnetic Interference Shielding Materials based on Silver Nanowire/Cellulose Papers

We fabricated silver nanowire (AgNW)-coated cellulose papers with a hierarchical structure by an efficient and facile dip-coating process, and investigated their microstructures, electrical conductivity and electromagnetic interference (EMI) shielding effectiveness. SEM images confirm that AgNWs are coated dominantly on the paper surfaces, although they exist partially in the inner parts of the cellulose papers, which demonstrates that the AgNW density gradually decreases in thickness direction of the AgNW/cellulose papers. This result is supported by the anisotropic apparent electrical conductivity of the AgNW/cellulose papers depending on in-plane or thickness direction. Even for a AgNW/cellulose paper obtained by a single dip-coating cycle, the apparent electrical conductivity in the in-plane direction of 0.34 S/cm is achieved, which is far higher than the neat cellulose paper with ∼10(-11) S/cm. In addition, the apparent electrical conductivity of the papers in the in-plane direction increases significantly from 0.34 to 67.51 S/cm with increasing the number of dip-coating cycle. Moreover, although the AgNW/cellulose paper with 67.51 S/cm possesses 0.53 vol % AgNW only, it exhibits high EMI shielding performance of ∼48.6 dB at 1 GHz. This indicates that the cellulose paper structure is highly effective to form a conductive AgNW network. Overall, it can be concluded that the AgNW/cellulose papers with high flexibility and low density can be used as electrically conductive components and EMI shielding elements in advanced application areas.

Anti-frost coatings containing carbon nanotube composite with reliable thermal cyclic property

One of the most important applications for superhydrophobic coatings is anti-frosting for safety and energy conservation. Safety concerns are especially critical in cold-climate regions where the daily temperature fluctuation is large. However, superhydrophobic coatings have not been studied in terms of their thermomechanical reliability. In this study, wetting characteristics and stress relaxation behavior were quantitatively investigated with multi-walled carbon nanotube (MWNT)–silicone composite films under thermal cycling conditions. It is concluded that an open structure with numerous nanopores among the fillers, constituting air pockets described as the “Cassie structure,” is of great importance not only for developing a films superhydrophobic nature but also for accommodation of thermal stress that results from a difference in coefficient of thermal expansion between the coating and the substrate. The amount of stress relaxation for a 30 vol% MWNT–silicone composite film with open structure reaches ∼80% of the value for its counterpart with a closed structure and no pores. A superhydrophobic MWNT–silicone composite film that can endure over 4000 thermal cycles (−30 °C to room temperature) is fabricated by controlling the composition and microstructure of the composite. In addition, the importance of the size and shape of the nanofillers in delaying nucleation and growth of frost on superhydrophobic coatings is also discussed.

Bioinspired superhydrophobic surfaces, fabricated through simple and scalable roll-to-roll processing.

A simple, scalable, non-lithographic, technique for fabricating durable superhydrophobic (SH) surfaces, based on the fingering instabilities associated with non-Newtonian flow and shear tearing, has been developed. The high viscosity of the nanotube/elastomer paste has been exploited for the fabrication. The fabricated SH surfaces had the appearance of bristled shark skin and were robust with respect to mechanical forces. While flow instability is regarded as adverse to roll-coating processes for fabricating uniform films, we especially use the effect to create the SH surface. Along with their durability and self-cleaning capabilities, we have demonstrated drag reduction effects of the fabricated films through dynamic flow measurements.

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.

Dynamic superhydrophobic behavior in scalable random textured polymeric surfaces

Superhydrophobic (SH) surfaces, created from hydrophobic materials with micro- or nano- roughness, trap air pockets in the interstices of the roughness, leading, in fluid flow conditions, to shear-free regions with finite interfacial fluid velocity and reduced resistance to flow. Significant attention has been given to SH conditions on ordered, periodic surfaces. However, in practical terms, random surfaces are more applicable due to their relative ease of fabrication. We investigate SH behavior on a novel durable polymericrough surface created through a scalable roll-coating process with varying micro-scale roughness through velocity and pressure drop measurements. We introduce a new method to construct the velocity profile over SH surfaces with significant roughness in microchannels. Slip length was measured as a function of differing roughness and interstitial air conditions, with roughness and air fraction parameters obtained through direct visualization. The slip length was matched to scaling laws with good agreement. Roughness at high air fractions led to a reduced pressure drop and higher velocities, demonstrating the effectiveness of the considered surface in terms of reduced resistance to flow. We conclude that the observed air fraction under flow conditions is the primary factor determining the response in fluid flow. Such behavior correlated well with the hydrophobic or superhydrophobic response, indicating significant potential for practical use in enhancing fluid flow efficiency.

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 0.019 eV. Activation energy of the contact resistance of MWNT/Ag interface was determined to be 0.04 eV, 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.

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.

Advanced catalyst design induced enhancement of multi-walled nanotube debundling and electrical conductivity of multi-walled nanotube/silicone composites

Multi-walled nanotube (MWNT)/silicone composites were fabricated with two different kinds of MWNT bundles grown by catalysts with different morphology. The order of agglomeration of MWNTs turned out to be closely related to the shape of the catalyst particles. Though the same composition of precursors was used, catalyst particles made from gelation of the precursors followed by flame synthesis (FS) consisted of chunk-type particles, while those from spraying of the precursor solution followed by thermal decomposition (STD) were fabricated with the shape of thin sheets. After CVD growth, the MWNT bundles were entangled to form large masses for FS-catalysts but they maintained rod-like morphology for STD-catalysts. Individual bundles of the STD-MWNTs also contained a smaller population of MWNTs with more room inside, which finally resulted in highly conductive MWNT/silicone composite due to effective dispersion of the MWNTs. In this study, for the first time, direct correlation between morphology of MWNT catalysts and electrical conductivity of MWNT/polymer composites was experimentally demonstrated and a high electrical conductivity of 1407 S m−1 was acquired using a mass production compatible three roll milling process.

Carbon Nanotube Nanocomposite Having Segregated Network Structure for Wearable Thermotherapy Application

Multiwalled nanotube (MWNT)/silicone composites having segregated MWNT network and micro-void structure were developed for wearable thermotherapy application. The nano-composites could be quickly fabricated from instant evaporation of aqueous medium during spray coating process. Fast electric heating behavior (4.8 °C/s) was demonstrated, in comparison with that (1.4 °C/s) of conventional silicone rubber composites having no void. Long-term stability was also verified with thermal aging and thermal cycling tests up to 100 h.