Mechanical Engineering Publications

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https://hdl.handle.net/1911/64878

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Browsing Mechanical Engineering Publications by Author "Ajayan, P.M."

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    Hybrid supercapacitor-battery materials for fast electrochemical charge storage
    (Nature, 2014) Vlad, A.; Singh, N.; Rolland, J.; Melinte, S.; Ajayan, P.M.; Gohy, J.-F.
    High energy and high power electrochemical energy storage devices rely on different fundamental working principles - bulk vs. surface ion diffusion and electron conduction. Meeting both characteristics within a single or a pair of materials has been under intense investigations yet, severely hindered by intrinsic materials limitations. Here, we provide a solution to this issue and present an approach to design high energy and high power battery electrodes by hybridizing a nitroxide-polymer redox supercapacitor (PTMA) with a Li-ion battery material (LiFePO4). The PTMA constituent dominates the hybrid battery charge process and postpones the LiFePO4 voltage rise by virtue of its ultra-fast electrochemical response and higher working potential. We detail on a unique sequential charging mechanism in the hybrid electrode: PTMA undergoes oxidation to form high-potential redox species, which subsequently relax and charge the LiFePO4 by an internal charge transfer process. A rate capability equivalent to full battery recharge in less than 5 minutes is demonstrated. As a result of hybrid?s components synergy, enhanced power and energy density as well as superior cycling stability are obtained, otherwise difficult to achieve from separate constituents.
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    Large enhanced dielectric permittivity in polyaniline passivated core-shell nano magnetic iron oxide by plasma polymerization
    (AIP Publishing, 2014) Joy, Lija K.; Sooraj, V.; Sajeev, U.S.; Nair, Swapna S.; Narayanan, T.N.; Sethulakshmi, N.; Ajayan, P.M.; Anantharaman, M.R.
    Commercial samples of Magnetite with size ranging from 25-30nm were coated with polyaniline by using radio frequency plasma polymerization to achieve a core shell structure of magnetic nanoparticle (core)-Polyaniline (shell). High resolution transmission electron microscopy images confirm the core shell architecture of polyaniline coated iron oxide. The dielectric properties of the material were studied before and after plasma treatment. The polymer coated magnetite particles exhibited a large dielectric permittivity with respect to uncoated samples. The dielectric behavior was modeled using a Maxwell-Wagner capacitor model. A plausible mechanism for the enhancement of dielectric permittivity is proposed.
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    Universal ac conduction in large area atomic layers of CVD-grown MoS2
    (American Physical Society, 2014) Ghosh, S.; Najmaei, S.; Kar, S.; Vajtai, R.; Lou, J.; Pradhan, N.R.; Balicas, L.; Ajayan, P.M.; Talapatra, S.
    Here, we report on the ac conductivity [σ’(ω); 10 mHz < ω < 0.1 MHz] measurements performed on atomically thin, two-dimensional layers of MoS2 grown by chemical vapor deposition (CVD). Σ’(ω) is observed to display a “universal” power law, i.e., σ’(ω) ∼ ωs measured within a broad range of temperatures, 10 K< T <340 K. The temperature dependence of ‘‘s” indicates that the dominant ac transport conduction mechanism in CVD-grown MoS2 is due to electron hopping through a quantum mechanical tunneling process. The ac conductivity also displays scaling behavior, which leads to the collapse of the ac conductivity curves obtained at various temperatures into a single master curve. These findings establish a basis for our understanding of the transport mechanism in atomically thin, CVD-grown MoS2 layers.