Creat membership Creat membership
Sign in

Forgot password?

Confirm
  • Forgot password?
    Sign Up
  • Confirm
    Sign In
home > search

Now showing items 113 - 128 of 144

  • Length-controlled few-walled carbon nanotubes and their effect on the electrical property of flexible transparent conductive films

    Lee, Byeong-Joo   Shin, Eui-Chul   Jeong, Goo-Hwan  

    We investigated the effect of the length of carbon nanotubes (CNTs) on the electrical property of CNT-based flexible, transparent, and conductive films (TCFs). We grew vertically aligned CNTs with controlled lengths, dispersed them in ethanol by ultrasonication, and spray coated them onto polyethylene terephthalate (PET) sheets. We focused on the variation in the sheet resistance and transmittance of the above-mentioned films as a function of the CNT length, and we found that the optimum CNT length was 200 mu m. We concluded that the CNT length should be carefully optimized because a shorter tube affords the advantage of efficient dispersion, while a longer tube helps in reducing the number of contact points between tubes along the electrical conduction path.
    Download Collect
  • Ferritin-mixed solution plasma system yielding low-dimensional carbon nanomaterials and their application to flexible conductive paper

    Lee, Byeong-Joo   Jeong, Goo-Hwan  

    We demonstrate the production of low-dimensional carbon nanomaterials using a solution plasma system and their application to flexible conductive paper. The solution plasma system consists of two graphite electrodes and a beaker filled with ferritin-mixed deionized water. Ferritin molecules are used as the growth catalyst of the carbon nanomaterials. A high voltage of 15 kV at a frequency of 25 kHz is supplied to the electrodes using an alternating-current power source. The effects of the graphite rod diameters and the concentration of ferritin molecules are comparatively investigated. The produced carbon nanomaterials are characterized using Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. We confirmed the synthesis of graphitic platelets, onion-like structures, and carbon nanotubes. Finally, we fabricated flexible conductive papers using the produced materials with a good electrical conductance. (C) 2015 Elsevier B.V. All rights reserved.
    Download Collect
  • Ferritin-mixed solution plasma system yielding low-dimensional carbon nanomaterials and their application to flexible conductive paper

    Lee, Byeong-Joo   Jeong, Goo-Hwan  

    Highlights • We construct a ferritin-mixed solution plasma system for low-dimensional carbon nanomaterial synthesis. • We confirm the formation of graphitic plates, carbon nanotubes, and onion-like structures. • The structural quality of the products using the ferritin-mixed solution is greater than that using DI water only. • We also fabricate flexible conductive paper using the produced nanomaterials. Abstract We demonstrate the production of low-dimensional carbon nanomaterials using a solution plasma system and their application to flexible conductive paper. The solution plasma system consists of two graphite electrodes and a beaker filled with ferritin-mixed deionized water. Ferritin molecules are used as the growth catalyst of the carbon nanomaterials. A high voltage of 15 kV at a frequency of 25 kHz is supplied to the electrodes using an alternating-current power source. The effects of the graphite rod diameters and the concentration of ferritin molecules are comparatively investigated. The produced carbon nanomaterials are characterized using Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. We confirmed the synthesis of graphitic platelets, onion-like structures, and carbon nanotubes. Finally, we fabricated flexible conductive papers using the produced materials with a good electrical conductance.
    Download Collect
  • Effects of Cr Reduction on High-Temperature Strength of High-Ni Austenitic Cast Steels Used for High-Performance Turbo-chargers

    Yoo, Jisung   Choi, Won-Mi   Sohn, Seok Su   Lee, Byeong-Joo   Kim, Gi-Yong   Oh, Yong-Jun   Lee, Sunghak  

    Since greater high-temperature strength is required for maintaining high-performance turbo-chargers at higher exhaust gas temperatures, e.g., 1323 K (1050 degrees C), high-Ni (20 wt pct) austenitic steel (ASTM HK40 steel) is presented as an excellent turbo-charger candidate material. To enhance the strength, three types of austenitic cast steel were fabricated in this study by controlling the Cr content in HK40 steel, and high-temperature strength improvement was achieved by detailed microstructural evolution including carbide formation and matrix strengthening. Room temperature and high-temperature strengths were expected to be proportional to the carbide volume fraction, but revealed an opposite trend because the steel containing more Cr (having more carbides) revealed lower strength than the steel containing less Cr (having fewer carbides). This result was associated mainly with the M7C3 to M23C6 decomposition occurring at high temperatures in the less-Cr-steel that beneficially strengthened the austenite matrix and reduced the hardness difference between the carbide and matrix, consequently improving the high-temperature strength. In considering the alloying prices (14 pct cost saving of alloying elements) as well as the high-temperature strength, the steel containing less Cr is promising for new high-performance turbo-charger applications. (C) The Minerals, Metals & Materials Society and ASM International 2018
    Download Collect
  • Effects of strain rate on room- and cryogenic-temperature compressive properties in metastable V10Cr10Fe45Co35 high-entropy alloy

    Song, Hyejin   Kim, Dong Geun   Kim, Dae Woong   Jo, Min Cheol   Jo, Yong Hee   Kim, Wooyeol   Kim, Hyoung Seop   Lee, Byeong-Joo   Lee, Sunghak  

    Quasi-static and dynamic compressive properties of an FCC-based metastable HEA (composition; V10Cr10Fe45Co35 (at.%)) showing both Transformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) were investigated at room and cryogenic temperatures. During the quasistatic and dynamic compression at room temperature, the FCC to BCC TRIP occurred inside FCC grains, and resulted in very high strain-hardening rate and consequently maximum compressive strength over 1.6 GPa. The dynamic compressive strength was higher by 240 MPa than the quasi-static strength because of strain-rate-hardening effect, and kept increasing with a high strain-hardening rate as the twinning became activated. The cryogenic-temperature strength was higher than the room-temperature strength as the FCC to BCC TRIP amount increased by the decrease in stability of FCC phase with decreasing temperature. Under dynamic loading at cryogenic temperature, twins were not formed because the increase in SFE due to adiabatic heating might not be enough to reach the TWIP regime. However, the dynamically compressed specimen showed the higher strength than the quasistatically compressed specimen as the strain-rate-hardening effect was added with the TRIP.
    Download Collect
  • Effects of strain rate on room- and cryogenic-temperature compressive properties in metastable V10Cr10Fe45Co35 high-entropy alloy

    Song, Hyejin   Kim, Dong Geun   Kim, Dae Woong   Jo, Min Cheol   Jo, Yong Hee   Kim, Wooyeol   Kim, Hyoung Seop   Lee, Byeong-Joo   Lee, Sunghak  

    Download Collect
  • Effect of Austenite Stability on Microstructural Evolution and Tensile Properties in Intercritically Annealed Medium-Mn Lightweight Steels

    Song, Hyejin   Sohn, Seok Su   Kwak, Jai-Hyun   Lee, Byeong-Joo   Lee, Sunghak  

    The microstructural evolution with varying intercritical-annealing temperatures of medium-Mn (alpha + gamma) duplex lightweight steels and its effects on tensile properties were investigated in relation to the stability of austenite. The size and volume fraction of austenite grains increased as the annealing temperature increased from 1123 K to 1173 K (850 A degrees C to 900 A degrees C), which corresponded with the thermodynamic calculation data. When the annealing temperature increased further to 1223 K (950 A degrees C), the size and volume fraction were reduced by the formation of athermal alpha'-martensite during the cooling because the thermal stability of austenite deteriorated as a result of the decrease in C and Mn contents. In order to obtain the best combination of strength and ductility by a transformation-induced plasticity (TRIP) mechanism, an appropriate mechanical stability of austenite was needed and could be achieved when fine austenite grains (size: 1.4 mu m, volume fraction: 0.26) were homogenously distributed in the ferrite matrix, as in the 1123 K (850 A degrees C)-annealed steel. This best combination was attributed to the requirement of sufficient deformation for TRIP and the formation of many deformation bands at ferrite grains in both austenite and ferrite bands. Since this medium-Mn lightweight steel has excellent tensile properties as well as reduced alloying costs and weight savings, it holds promise for new automotive applications. (C) The Minerals, Metals & Materials Society and ASM International 2016
    Download Collect
  • Effects of transformation-induced plasticity (TRIP) on tensile property improvement of Fe45Co30Cr10V10Ni5-xMnx high-entropy alloys

    Yang, Junha   Jo, Yong Hee   Kim, Dae Woong   Choi, Won-Mi   Kim, Hyoung Seop   Lee, Byeong-Joo   Sohn, Seok Su   Lee, Sunghak  

    A new metastable high-entropy alloy (HEA) system was suggested by thermodynamic calculations based on the Gibbs free energies of FCC and HCP and the associated stacking fault energy (SFE). The Fe45Co30Cr10V10Ni5-xMnx (x =3D 0, 2.5, and 5 at.%) alloys were fabricated, and their tensile properties were evaluated at room and cryogenic temperatures. The relationship between the deformation mechanism and strain hardening behavior was investigated to reveal the role of deformation-induced martensitic transformation on tensile properties. The difference in Gibbs energy decreases with increasing Mn content, leading to the decreased SFE in sequence. At room temperature, similar to 60% of BCC martensite in the SUM HEA contributes effectively to the steady strain hardening, suppressing the plastic instability. This TRIP effect achieves much eminence in the cryogenic deformation, enabling the tensile strength to reach over 1.6 GPa due to 100% of BCC and HCP martensite. In addition to the fraction of martensite, the increased Mn content reduces a critical strain required to trigger the martensitic transformation and then raises the transformation rate. The present findings may provide a guide for the design of metastable HEAs to enhance tensile properties for cryogenic applications through adjusting SFE and TRIP effect.
    Download Collect
  • Formulation of the aL/L12 atomic ordering energy and a thermodynamic analysis of the Fe-Ni System

    Lee, Byeong-Joo   Nyung Lee, Dong  

    Download Collect
  • Effects of Mn Addition on Tensile and Charpy Impact Properties in Austenitic Fe-Mn-C-Al-Based Steels for Cryogenic Applications

    Lee, Junghoon   Sohn, Seok Su   Hong, Seokmin   Suh, Byeong-Chan   Kim, Sung-Kyu   Lee, Byeong-Joo   Kim, Nack J.   Lee, Sunghak  

    Effects of Mn addition (17, 19, and 22 wt pct) on tensile and Charpy impact properties in three austenitic Fe-Mn-C-Al-based steels were investigated at room and cryogenic temperatures in relation with deformation mechanisms. Tensile strength and elongation were not varied much with Mn content at room temperature, but abruptly decreased with decreasing Mn content at 77 K (-196 A degrees C). Charpy impact energies at 273 K (0 A degrees C) were higher than 200 J in the three steels, but rapidly dropped to 44 J at 77 K (-196 A degrees C) in the 17Mn steel, while they were higher than 120 J in the 19Mn and 22Mn steels. Although the cryogenic-temperature stacking fault energies (SFEs) were lower by 30 to 50 pct than the room-temperature SFEs, the SFE of the 22Mn steel was situated in the TWinning-induced plasticity regime. In the 17Mn and 19Mn steels, however, alpha'-martensites were formed by the TRansformation-induced plasticity mechanism because of the low SFEs. EBSD analyses along with interrupted tensile tests at cryogenic temperature showed that the austenite was sufficiently deformed in the 19Mn steel even after the formation of alpha'-martensite, thereby leading to the high impact energy over 120 J.
    Download Collect
  • Role of brittle sigma phase in cryogenic-temperature-strength improvement of non-equi-atomic Fe-rich VCrMnFeCoNi high entropy alloys

    Jo, Yong Hee   Choi, Won-Mi   Sohn, Seok Su   Kim, Hyoung Seop   Lee, Byeong-Joo   Lee, Sunghak  

    An equi-atomic single-fcc-phase CrMnFeCoNi high entropy alloy (HEA) shows much higher tensile properties at cryogenic temperature than at room temperature because of its fee characteristics and abundant twinning at cryogenic temperature. In order to further improve the cryogenic-temperature tensile properties of single-fcc-phase HEAs, we propose non-equi-atomic Fe-rich VCrMnFeCoNi HEAs, and analyze the strengthening effects of the brittle intermetallic sigma (sigma) phase. The sigma phase is unintentionally obtained, but favorably shows a pronounced strengthening by its hardness and grain refinement effect due to grain-boundary pinning, which leads to high yield and tensile strengths of 0.76 GPa and 1.23 GPa, respectively, together with good ductility of 54%. This positive utilization of the sigma phase is unexpected because its formation has been suppressed in typical HEAs. Our results demonstrate that the present Fe-rich VCrMnFeCoNi design and o-phase strengthening has potential in high-strength HEA studies.
    Download Collect
  • Effects of Phosphorus and Carbon Contents on Amorphous Forming Ability in Fe-based Amorphous Alloys Used for Thermal Spray Coatings

    Do, Jeonghyeon   Jung, Seungmun   Lee, Hyuk-Joong   Lee, Byeong-Joo   Cha, Gil-Up   Jo, Chang Yong   Lee, Sunghak  

    Cost-effective Fe-based amorphous alloys used for thermal spray coatings were developed by varying contents of P and C, and their microstructure, hardness, and corrosion resistance were analyzed. In order to achieve chemical compositions having high amorphous forming ability, thermodynamically calculated phase diagrams of Fe-Al-P-C-B five-component system were used, from which compositions of super-cooled liquid having the lowest driving force of formation of crystalline phases were obtained. The thermodynamic calculation results showed that only phases of Fe3P and Fe3C were formed in the Fe78Al2P(18.3-x)C (x) B-1.7 alloy system. Considering driving force curves of Fe3P and Fe3C, the carbon contents were selected to be 6.90 and 7.47 at. pct, when the thermodynamic calculation temperatures were 697 K (414 A degrees C) and 715 K (442 A degrees C), respectively. According to the microstructural analysis of suction-cast alloys, the Fe78Al2P10.83C7.47B1.7 alloy showed a fully amorphous microstructure, whereas the Fe78Al2P11.40C6.9B1.7 and Fe78Al2P10.3C8.0B1.7 alloys contained Fe3P and Fe3C phases. This Fe78Al2P10.83C7.47B1.7 alloy showed the better hardness and corrosion resistance than those of conventional thermal spray coating alloys, and its production cost could be lowered using cheaper alloying elements, thereby leading to the practical application to amorphous thermal spray coatings. DOI: 10.1007/s11661-013-1615-0 (C) The Minerals, Metals & Materials Society and ASM International 2013
    Download Collect
  • Formulation of the A2/B2/D03 atomic ordering energy and a thermodynamic analysis of the Fe-Si system

    Lee, Byeong-joo   Lee, Seh Kwang   Lee, Dong Nyung  

    Download Collect
  • Size engineering of metal nanoparticles to diameter-specified growth of single-walled carbon nanotubes with horizontal alignment on quartz

    Kim, Jin-Ju   Lee, Byeong-Joo   Lee, Seung-Hwan   Jeong, Goo-Hwan  

    The electronic, physical and optical properties of single-walled carbon nanotubes (SWNTs) are governed by their diameter and chirality, and thus much research has been focused on controlling the diameter and chirality of SWNTs. To date, control of the catalyst particle size has been thought to be one of the most promising approaches to control the diameter or chirality of SWNTs owing to the correlation between catalyst particle size and tube diameter. In this study, we demonstrate the size engineering of catalytic nanoparticles for the controlled growth of diameter-specified and horizontally aligned SWNTs on quartz substrates. Uniformly sized iron nanoparticles derived from ferritin molecules were used as a catalyst, and their size was intentionally decreased via thermal heat treatment at 900 degrees C under atmospheric Ar ambient. ST-cut quartz wafers were used as growth substrates in order to elucidate the effect of the size of the nanoparticles on the tube diameter and the effect of catalyst size on the degree of parallel alignment on the quartz substrates. SWNTs grown by chemical vapor deposition using methane as feedstock exhibited a high degree of horizontal alignment when the particle density was low enough to produce individual SWNTs without bundling. Annealing for 60 min at 900 degrees C produced a reduction of nanoparticle diameter from 2.6 to 1.8 nm and a decrease in the mean tube diameter from 1.2 to 0.8 nm, respectively. Raman spectroscopy results corroborated the observation that prolonged heat treatment of nanoparticles yields thinner tubes with narrower size distributions. The results of this work suggest that straightforward thermal annealing can be a facile way to obtain uniform-sized SWNTs as well as catalytic nanoparticles.
    Download Collect
  • Molecular dynamics simulation study of the effect of grain size on the deformation behavior of nanocrystalline body-centered cubic iron

    Jeon, Jong Bae   Lee, Byeong-Joo   Chang, Young Won  

    A molecular dynamics simulation study has been carried out to clarify the effect of grain size on the deformation behavior of nanocrystalline body-centered cubic Fe. Average flow stresses were found to decrease with grain refinement below 14.7 nm, exhibiting a breakdown in the Hall-Petch relation. A change in the dominant deformation mechanism from dislocation glide to grain boundary sliding appeared to be the direct cause of the breakdown in the Hall-Petch relation observed in the present nanocrystalline pure Fe. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    Download Collect
  • Fabrication of graphene-carbon nanotubes composite-based flexible transparent conductive films and their improved durability on repetitive strain

    Lee, Byeong-Joo   Jeong, Goo-Hwan  

    We demonstrate the fabrication of graphene-carbon nanotubes (CNTs) composite-based flexible transparent conductive films (GC-TCFs) and their improved durability on repetitive strain. The graphene and CNTs are synthesized using thermal chemical vapor deposition. To fabricate GC-TCFs, the graphenes are transferred and the CNTs are successively spray-deposited on polymer substrates, respectively. The change of electrical property of the TCFs is investigated as the response of repetitive strain loading and unloading. The sheet resistance of the GC-TCFs is much lower than CNT-based TCFs, owing to the lower contact resistance. In addition, when the cyclic strain is applied on the GC-TCFs, the films show improved durability in electrical property compared to graphene-based TCFs. Finally, the coated CNTs act as one dimensional conductive path across the cracks, which prevent electrical degradation during the repetitive strain application.
    Download Collect
1 2 3 4 5 6 7 8 9

Contact

If you have any feedback, Please follow the official account to submit feedback.

Turn on your phone and scan

Submit Feedback