Junbeom Park

Carbon nanotube yarns

A CNT yarn is a collection of interlocked CNTs which form a long and continuous fiber of macroscopic scale. CNT yarns of more than a kilometer are now available so that they have been drawing ever-growing attention from the scientific community. In principle, CNT yarns can inherit the excellent electrical, mechanical and chemical properties of CNTs provided they are produced perfectly. In this perspective review, the production methods of CNT yarns are extensively investigated and reported in detail. Although CNT yarns have a great potential to revolutionize our future, it can only be possible by improving their essential material properties such as tensile strength and edectrical conductivity.

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.

Effect of order-disorder transformation modes on the anomalous yield behaviour of Fe3Al intermetallic compounds

Compression tests were performed to clarify the effects of transformation modes on the anomalous yield behaviour of hypo- and hyper-stoichiometric Fe3Al alloys which show a first- and a second-order transition, respectively. There were great differences in the anomalous yield behaviour depending on the transformation modes. In the first-order transformation alloy, changes in the degree of order played an important role before phase separation, while precipitation of α phase had a great influence on the anomalous behaviour after phase separation. In contrast, only the change in the degree of order was a dominant factor in the second-order transformation alloy.

The effect of systemically administered bisphosphonates on bony healing after tooth extraction and osseointegration of dental implants in the rabbit maxilla.

PURPOSE To evaluate the effects of bisphosphonates on bone healing after tooth extraction and osseointegration of dental implants in a rabbit model. MATERIALS AND METHODS Twenty-four rabbits were divided into four groups; one control and three experimental. The experimental were treated with intravenous zoledronic acid (ZA, 0.1 mg/kg) twice per week starting 4 (Z4 group) and 8 (Z8 group) weeks before surgery until the end of the experiments. The experimental ZD4 group was treated with intravenous ZA (0.01 mg/kg) and intramuscular dexamethasone (1 mg/kg) twice per week starting 4 weeks before surgery until the end of the experiments. The maxillary first premolar was extracted, and an implant with a diameter of 1.5 mm was placed between the incisor and the premolar of each maxilla. Healing of the extraction socket was evaluated and histomorphometric analysis around the implant was performed, using the bone-to-implant contact ratio (BIC) and bone area ratio (BA) 4 and 8 weeks after the surgery. RESULTS The control group underwent a normal healing process, but all experimental groups showed necrotic bone with hollow lacunae. BIC and BA in the control group increased from the 4- to 8-week evaluations, but decreased in the experimental groups from 4 to 8 weeks. BIC and BA of the Z8 and ZD4 groups were higher than those of the control group at the 4-week evaluation, but were lower than the control at the the 8-week evaluation. CONCLUSIONS This study showed that administration of bisphosphonates interferes with normal bone remodeling after tooth extraction. The experimental groups showed good initial stability, but long-term healing around the implants was impaired. Within the limits of this study, it may be suggested that patients taking bisphosphonates should be treated with caution when performing tooth extraction or placing dental implants.

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.

Triazenyl Radicals Stabilized by N-Heterocyclic Carbenes

Notwithstanding the notable progress in the synthesis of N-heterocyclic carbene-stabilized radicals, aminyl radicals, supported by NHCs or otherwise, have been scarcely studied due to synthetic challenges. Triazenyl radical is a particular form of aminyl radical that contains three adjacent nitrogen atoms, and offers intriguing possibilities for unique reactivity and physical properties stemming from expected delocalization of the spin density over the NNN moiety and its conjugated substituents. Here, we report the synthesis and full characterization of the first NHC-stabilized triazenyl radicals, obtained by one-electron reduction of the corresponding triazenyl cations with potassium metal. These radicals reversibly oxidize back to the cations upon treatment with transition metal sources or electrophiles, and abstract H atom from xanthene to form a new N-H bond at the center nitrogen atom. Potential application of the redox couple between triazenyl cation and triazenyl radical was demonstrated as cathode active materials in lithium ion batteries.

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.

Association between oral health behavior and periodontal disease among Korean adults: The Korea national health and nutrition examination survey.

Abstract This study was performed to assess the association between oral health behavior and periodontal disease using nationally representative data. This study involved a cross-sectional analysis and multivariable logistic regression analysis models using the data from the Korean National Health and Nutrition Examination Survey. A community periodontal index greater than or equal to code 3 was used to define periodontal disease. Adjusted odds ratios and their 95% confidence intervals of periodontitis for the toothbrushing after lunch group and the toothbrushing before bedtime group were 0.842 (0.758, 0.936) and 0.814 (0.728, 0.911), respectively, after adjustments for age, sex, body mass index, drinking, exercise, education, income, white blood cell count, and metabolic syndrome. Adjusted odds ratios and their 95% confidence intervals of periodontitis for the floss group and the powered toothbrush group after adjustment were 0.678 (0.588, 0.781) and 0.771 (0.610, 0.974), respectively. The association between oral health behavior and periodontitis was proven by multiple logistic regression analyses after adjusting for confounding factors among Korean adults. Brushing after lunch and before bedtime as well as the use of floss and a powered toothbrush may be considered independent risk indicators of periodontal disease among Korean adults.