Jaegeun Lee

Shape memory polyurethane containing mesogenic moiety

The mechanical and thermal properties, in particular the shape memory effect, of TPUs synthesized from diol-terminated poly(caprolactone) (PCL diol), 4,4′-diphenylmethane diisocyanate (MDI), and mesogenic chain extenders, 4,4′-bis-(2-hydroxyethoxy)biphenyl (BEBP) or 4,4′-bis-(6-hydroxyhexoxy)biphenyl (BHBP) were examined and compared with results for PCL diol/MDI/1,4-butanediol (BD) based TPUs in a previous study. Some results related to the rigid structure of BEBP or BHBP were observed in the thermomechanical properties. Shape fixity was related to the crystallization of the PCL phase.

Microwave heating of carbon-based solid materials

As a part of the electromagnetic spectrum, microwaves heat materials fast and efficiently via direct energy transfer, while conventional heating methods rely on conduction and convection. To date, the use of microwave heating in the research of carbon-based materials has been mainly limited to liquid solutions. However, more rapid and efficient heating is possible in electron-rich solid materials, because the target materials absorb the energy of microwaves effectively and exclusively. Carbon-based solid materials are suitable for microwave- heating due to the delocalized pi electrons from sp2-hybridized carbon networks. In this perspective review, research on the microwave heating of carbon-based solid materials is extensively investigated. This review includes basic theories of microwave heating, and applications in carbon nanotubes, graphite and other carbon-based materials. Finally, priority issues are discussed for the advanced use of microwave heating, which have been poorly understood so far: heating mechanism, temperature control, and penetration depth.

Synthesis of high-quality carbon nanotube fibers by controlling the effects of sulfur on the catalyst agglomeration during the direct spinning process

The effects of sulfur on the size of iron catalyst particles and synthesized carbon nanotubes (CNTs) were investigated during the direct spinning of CNT fibers. CNT fibers containing mainly double-walled CNTs (DWCNTs) 5–10 nm in diameter were synthesized from acetone, ferrocene, and thiophene, whereas CNT fibers containing mainly single-walled CNTs (SWCNTs) 1–1.5 nm in diameter were obtained from methane, ferrocene, and sulfur. The differences in the products arose from the anti-agglomeration effects of the sulfur atoms, which were adsorbed onto the surfaces of the iron catalyst particles, as indicated by direct experimental evidence. A model for the interplay between sulfur atoms and the iron catalyst particles was proposed based on these results, and experimental confirmation of this model was sought by modulating the sulfur injection time into the reactor at different temperatures. This simple experimental modification was used to control the majority of CNTs in the CNT fibers synthesized from SWCNTs, through DWCNTs, and finally to multi-walled CNTs (MWCNTs), with corresponding IG/ID ratios that varied from 26.9 to 1.5.

Full graphitization of amorphous carbon by microwave heating

Natural graphite is labelled as a supply risk material due to rapidly increasing demand and limited reserves. The conventional method for the production of synthetic graphite has relied on the thermal heating at an extremely high temperature, 3000 °C, and long processing time, typically 2 weeks. Here, we report a novel and efficient method of graphitization using microwave heating with metal catalysts. The amorphous carbon powders turned into crystalline graphite in 5 minutes. Ideas for the scale-up of this work were proposed. In addition, numerical analysis revealed that the Maxwell–Wagner–Sillars polarization is inadequate for the mechanism underlying the microwave heating of solid carbon materials.

Improving the tensile strength of carbon nanotube yarn via one-step double [2+1] cycloadditions

The tensile strength of a CNT yarn was improved through simple one-step double [2+1] cycloaddition reactions that crosslinked the constituent CNTs using a polyethylene glycol (PEG)-diazide crosslinker. The FT-IR spectrum confirmed that the azide groups in the PEG-diazide were converted into aziridine rings, indicating that the cycloaddition reaction was successful. The generation of crosslinked CNTs was also supported by the observation of N1s peak in the XPS spectrum and the increased thermal stability of the material, as observed by TGA. The tensile strength of the CNT yarn was increased from 0.2GPa to 1.4GPa after the crosslinking reaction when twisted at 4000 twists/ meter. The appropriate selection of the crosslinker may further optimize the CNT yarn crosslinking reaction. The simplicity of this one-step crosslinking reaction provides an economical approach to the mass production of high-strength CNT yarns.

Accurate measurement of specific tensile strength of carbon nanotube fibers with hierarchical structures by vibroscopic method

The specific strength of a carbon nanotube (CNT) fiber can be estimated to be much higher than its real value when the linear density of the fiber is measured using the vibroscopic method. This is because CNT fibers are not made of a single fiber, as assumed in the standard ASTM procedure, but rather have a hierarchical structure composed of CNTs and CNT bundles. Based on careful investigation, a new procedure using the vibroscopic method is proposed to drastically reduce the probability of erroneous results and provide a more reliable tool to investigate the mechanical properties of one-dimensional nanostructured fibers.

High-strength carbon nanotube/carbon composite fibers via chemical vapor infiltration

In this study, we have developed an efficient and scalable method for improving the mechanical properties of carbon nanotube (CNT) fibers. The mechanical properties of as-synthesized CNT fibers are primarily limited by their porous structures and the weak bonding between adjacent CNTs. These result in inefficient load transfer, leading to low tensile strength and modulus. In order to overcome these limitations, we have adopted chemical vapor infiltration (CVI) to efficiently fill the internal voids of the CNT fibers with carbon species which are thermally decomposed from gas phase hydrocarbon. Through the optimization of the processing time, temperature, and gas flow velocity, we have confirmed that carbon species formed by the thermal decomposition of acetylene (C2H2) gas successfully infiltrated into porous CNT fibers and densified them at relatively low temperatures (650-750 °C). As a result, after CVI processing of the as-synthesized CNT fibers under optimum conditions, the tensile strength and modulus increased from 0.6 GPa to 1.7 GPa and from 25 GPa to 127 GPa, respectively. The CVI technique, combined with the direct spinning of CNT fibers, can open up a route to the fast and scalable fabrication of high performance CNT/C composite fibers. In addition, the CVI technique is a platform technology that can be easily adapted into other nano-carbon based yarn-like fibers such as graphene fibers.

Turning refuse plastic into multi-walled carbon nanotube forest

Abstract A novel and effective method was devised for synthesizing a vertically aligned carbon nanotube (CNT) forest on a substrate using waste plastic obtained from commercially available water bottles. The advantages of the proposed method are the speed of processing and the use of waste as a raw material. A mechanism for the CNT growth was also proposed. The growth rate of the CNT forest was ∼2.5 μm min−1. Transmission electron microscopy images indicated that the outer diameters of the CNTs were 20–30 nm on average. The intensity ratio of the G and D Raman bands was 1.27 for the vertically aligned CNT forest. The Raman spectrum showed that the wall graphitization of the CNTs, synthesized via the proposed method was slightly higher than that of commercially available multi-walled carbon nanotubes (MWCNTs). We expect that the proposed method can be easily adapted to the disposal of other refuse materials and applied to MWCNT production industries.

Effects of a SiO2 sub-supporting layer on the structure of a Al2O3 supporting layer, formation of Fe catalyst particles, and growth of carbon nanotube forests

We investigate the effects of a SiO2 sub-supporting layer on the growth of carbon nanotube (CNT) forests, especially on the spinnability of the CNT forest into a CNT yarn, which is one of the most promising application areas based on CNTs as a light-weight, strong, and electrically and thermally conductive macro-scale material. So far, most spinnable CNT forest growths have been performed using Fe/Al2O3 deposited on a Si wafer with a thermally grown SiO2 layer. However, only a few studies have focused on examining the effects of the SiO2 sub-supporting layer on the growth and spinnability of CNT forests by microscopic analyses. Herein, using atomic force microscopy (AFM), transmission electron microscopy (TEM), liquid contact angle, and ellipsometry measurements, we demonstrate that the presence of a SiO2 sub-supporting layer significantly affects the structure of Al2O3, adhesion between Al2O3 and Fe layers, and the number density of Fe catalyst particles, thereby strongly affecting the growth and spinnability of CNT forests. This study opens up new possibilities for accurately controlling the growth of CNT forests by proper designing of the sub-supporting layers.

Significantly Increased Solubility of Carbon Nanotubes in Superacid by Oxidation and Their Assembly into High-Performance Fibers

This study demonstrates that small amount of oxygen incorporated into carbon nanotubes (CNTs) during the purification process greatly increases their solubility in chlorosulfonic acid (CSA). Using as-purchased and unpurified CNT powders, the optimal purification process is established to significantly increase the solubility of CNTs in CSA, and spin CNT fibers with high mechanical strength (0.84 N tex-1 ) and electrical conductivity (1.4 MS m-1 ) from the CNT liquid crystal dope with high concentration of CNTs in CSA. The knowledge obtained here may guide development of a way to dissolve extremely long CNTs at high concentration and thereby to enable production of CNT fibers with ultimate properties.