New Elastoplastic Analysis of Two-Dimensional Frames When Some Plastic Hinges Unload Elastically

Fethullah Uslu,Mehmet Tevfik Bayer,Mustafa Haluk Saraçoğlu 한국강구조학회 2021 International Journal of Steel Structures Vol.21 No.2

Two-dimensional frames are made of elastic and perfectly plastic materials. Frames are analized by using the step by step elastoplastic analysis method where the applied loads are gradually increased at each step until a plastic hinge is developed. Then an equivalent frame model is generated by placing a mechanical hinge at the location of the plastic hinge and the increased external loads and the reduced plastic moments acting at the plastic hinge sections are applied to this model and elastoplastic analysis is performed. This elastoplastic analysis is repeated by incrementally increasing the external loads and applying the new reduced plastic moments, until the next plastic hinge is developed. Then a new equivalent frame model is generated to search for the following plastic hinge. This step by step elastoplastic analysis continues until the frame partially or totally collapses. When a plastic hinge is developed, it means that the section is at a fully plastic stress state. In the following small load increment: if this fully plastic stress state moves to another fully plastic stress state then this movement is called plastic fl ow. But if this fully plastic stress state moves to an elastic–plastic stress state then this movement is called elastic unloading. During the search of the next plastic hinge if one of the plastic hinges begins to unload elastically then the current equivalent frame model must be revised. For this the frame member which contains the elastically unloading plastic hinge is replaced by the equivalent frame member carrying balancing loads.

UV-LIGHT-ASSISTED OXIDATIVE sp^3 HYBRIDIZATION OF GRAPHENE

FETHULLAH GÜNEŞ,한강희,신현진,이시영,MEIHUA JIN,DINH LOC DUONG,채승진,김은성,FEI YAO,ANASS BENAYAD,최재영,이영희 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2011 NANO Vol.6 No.5

We report the transformation of electronic structures of sp^2 graphene to sp^3 graphene by UV-light-assisted oxidation. Two distinctive oxidation mechanisms were observed during this metal–insulator transition: (i) At low-oxidation regime, p-doping behavior by oxygen species extracting electrons from graphene and (ii) at high-oxidation regime, n-doping behavior by electron-hopping via strongly localized oxide states. We also found that the dominant oxygen-related functional group by UV-light-assisted oxidation was an epoxide group rather than hydroxyl group, which differed from conventional graphite oxide.

True Collapse Mechanisms of Two Dimensional Frames Determined from True Nonlinear Yield Surfaces

Fethullah Uslu,Mehmet Tevfik Bayer,Mustafa Haluk Saraçoğlu 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.1

In this work, collapse mechanisms and collapse load factors of two-dimensional frames which are subject to point loads and uniformly distributed loads are determined by employing new elastoplastic analysis method. In this new iterative method as the applied loads on frames are gradually increased, plastic hinges begin to develop at frame member sections and this continues until the partial or total collapse occurrence. If a plastic hinge develops at a section of a frame member, then a mechanical hinge is defined at this location and at each iteration reduced plastic moments are also applied as external loads. This is a new approach to search for the next plastic hinge where a series of linear elastoplastic analyses are executed. For each analysis the revised reduced plastic moments are used in the calculations. For elastoplastic calculations of two-dimensional frames, yield surface definitions of frame member sections are needed, and for I sections, yield surfaces are defined by two curves. In order to simplify collapse load factor calculations these yield surfaces are generally approximated by two lines. In this work the influences of the approximations on the elastoplastic behaviour of two-dimensional frames are examined by comparing the solutions of four example frames. When their solutions are compared, it is observed that almost equal collapse load factors are determined when true and approximate yield surfaces are used in the elastoplastic calculations but true collapse mechanisms are obtained only when true nonlinear yield surfaces are used.

LARGE-AREA GRAPHENE-BASED FLEXIBLE TRANSPARENT CONDUCTING FILMS

이영희,FETHULLAH G¨UNES,GANG HEE HAN,김기강,EUN SUNG KIM,SEUNG JIN CHAE,MIN HO PARK,HAE-KYUNG JEONG,임성주 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2009 NANO Vol.4 No.2

A simple approach by direct synthesis of few-layer graphene sheets and transferring them onto flexible substrate is demonstrated. The hydrogen effect on the D band intensity of Raman spectra for few-layer graphene sheets synthesized by chemical vapor deposition (CVD) on Ni-evaporated silicon substrate is investigated by optimizing the mixing ratio of C2H4/H2. While the Ni etchant is used to melt away the squeezed Ni layers between graphene sheet and silicon substrate, the graphene sheet is transferred onto polyethylene terephthalate film by a fishing method. It is found that the condition of graphene transfer strongly relies on the cooling rate of the film during CVD synthesis. The sheet resistance of the film decreases as the film thickness increases. A sheet resistance of 233 Ω/sq is obtained at a transmittance of 62%.

Layer-by-Layer Doping of Few-Layer Graphene Film

Gü,neş,, Fethullah,Shin, Hyeon-Jin,Biswas, Chandan,Han, Gang Hee,Kim, Eun Sung,Chae, Seung Jin,Choi, Jae-Young,Lee, Young Hee American Chemical Society 2010 ACS NANO Vol.4 No.8

<P>We propose a new method of layer-by-layer (LbL) doping of thin graphene films. Large area monolayer graphene was synthesized on Cu foil by using the chemical vapor deposition method. Each layer was transferred on a polyethylene terephthalate substrate followed by a salt-solution casting, where the whole process was repeated several times to get LbL-doped thin layers. With this method, sheet resistance was significantly decreased up to ∼80% with little sacrifice in transmittance. Unlike samples fabricated by topmost layer doping, our sample shows better environmental stability due to the presence of dominant neutral Au atoms on the surface which was confirmed by angle-resolved X-ray photoelectron spectroscopy. The sheet resistance of the LbL-doped four-layer graphene (11 × 11 cm<SUP>2</SUP>) was 54 Ω/sq at 85% transmittance, which meets the technical target for industrial applications.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2010/ancac3.2010.4.issue-8/nn1008808/production/images/medium/nn-2010-008808_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn1008808'>ACS Electronic Supporting Info</A></P>

Influence of Copper Morphology in Forming Nucleation Seeds for Graphene Growth

Han, Gang Hee,Gü,neş,, Fethullah,Bae, Jung Jun,Kim, Eun Sung,Chae, Seung Jin,Shin, Hyeon-Jin,Choi, Jae-Young,Pribat, Didier,Lee, Young Hee American Chemical Society 2011 NANO LETTERS Vol.11 No.10

<P>We report that highly crystalline graphene can be obtained from well-controlled surface morphology of the copper substrate. Flat copper surface was prepared by using a chemical mechanical polishing method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. We also found that grain boundaries and intentionally formed scratched area play an important role for nucleation seeds. Although the best monolayer graphene was grown from polished Cu with a low sheet resistance of 260 Ω/sq, a small portion of multilayers were also formed near the impurity particles or locally protruded parts.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-10/nl201980p/production/images/medium/nl-2011-01980p_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl201980p'>ACS Electronic Supporting Info</A></P>

Diffusion Mechanism of Lithium Ion through Basal Plane of Layered Graphene

Yao, Fei,Gü,neş,, Fethullah,Ta, Huy Quang,Lee, Seung Mi,Chae, Seung Jin,Sheem, Kyeu Yoon,Cojocaru, Costel Sorin,Xie, Si Shen,Lee, Young Hee American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.20

<P>Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lithium ion diffusion mechanism. In this report, two types of graphene samples were prepared by chemical vapor deposition (CVD): (i) well-defined basal plane graphene grown on Cu foil and (ii) edge plane-enriched graphene layers grown on Ni film. Electrochemical performance of the graphene electrode can be split into two regimes depending on the number of graphene layers: (i) the corrosion-dominant regime and (ii) the lithiation-dominant regime. Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites. The critical layer thickness (<I>l</I><SUB>c</SUB>) to effectively prohibit substrate reaction using CVD-grown graphene layers was predicted to be ∼6 layers, independent of defect population. Our density functional theory calculations demonstrate that divacancies and higher order defects have reasonable diffusion barrier heights allowing lithium diffusion through the basal plane but neither monovacancies nor Stone-Wales defect.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-20/ja301586m/production/images/medium/ja-2012-01586m_0003.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja301586m'>ACS Electronic Supporting Info</A></P>

Negative and Positive Persistent Photoconductance in Graphene

Biswas, Chandan,Gü,neş,, Fethullah,Loc, Duong Dinh,Lim, Seong Chu,Jeong, Mun Seok,Pribat, Didier,Lee, Young Hee American Chemical Society 2011 NANO LETTERS Vol.11 No.11

<P>Persistent photoconductance, a prolonged light-induced conducting behavior that lasts several hundred seconds, has been observed in semiconductors. Here we report persistent negative photoconductance and consecutive prominent persistent positive photoconductance in graphene. Unusually large yields of negative PC (34%) and positive PC (1652%) and remarkably long negative transient response time (several hours) were observed. Such high yields were reduced in multilayer graphene and were quenched under vacuum conditions. Two-dimensional metallic graphene strongly interacts with environment and/or substrate, causing this phenomenon, which is markedly different from that in three-dimensional semiconductors and nanoparticles.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-11/nl202266h/production/images/medium/nl-2011-02266h_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl202266h'>ACS Electronic Supporting Info</A></P>

Synthesis of Large-Area Graphene Layers on Poly-Nickel Substrate by Chemical Vapor Deposition: Wrinkle Formation

Chae, Seung Jin,Gü,neş,, Fethullah,Kim, Ki Kang,Kim, Eun Sung,Han, Gang Hee,Kim, Soo Min,Shin, Hyeon-Jin,Yoon, Seon-Mi,Choi, Jae-Young,Park, Min Ho,Yang, Cheol Woong,Pribat, Didier,Lee, Young WILEY-VCH Verlag 2009 Advanced Materials Vol.21 No.22

<B>Graphic Abstract</B> <P>Large-area, few-layer graphene is grown on a poly-nickel substrate using optimized CVD conditions. High temperature, short growth time, and an optimal gas mixing ratio (C<SUB>2</SUB>H<SUB>2</SUB>/H<SUB>2</SUB> = 2/45) are found to be necessary to synthesize highly crystalline few-layer grapheme, which may find applications in electronic devices. The wrinkles that are observed under all growth conditions are proposed to be formed by two processes. <img src='wiley_img/09359648-2009-21-22-ADMA200803016-content.gif' alt='wiley_img/09359648-2009-21-22-ADMA200803016-content'> </P>

Probing graphene grain boundaries with optical microscopy

Duong, Dinh Loc,Han, Gang Hee,Lee, Seung Mi,Gunes, Fethullah,Kim, Eun Sung,Kim, Sung Tae,Kim, Heetae,Ta, Quang Huy,So, Kang Pyo,Yoon, Seok Jun,Chae, Seung Jin,Jo, Young Woo,Park, Min Ho,Chae, Sang Hoo Nature Publishing Group, a division of Macmillan P 2012 Nature Vol.490 No.7419

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy and scanning tunnelling microscopy, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.