Department of Physics

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IIT Jodhpur

We use all-atom molecular dynamics simulations to study the effect of polymer polarity, as quantified by the dielectric constant, on the transport properties of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) doped polyethers. Our results indicate that increasing the host dielectric constant leads to a decrease in ionic cluster sizes and reduction in correlated motion of oppositely charged ions. This causes the ionic conductivity to more closely approach the Nernst-Einstein limit in which ionic conductivity is only limited by the diffusivities of Li+ and TFSI-. We compare our results to recent experimental observations which demonstrate similar qualitative trends in host polarity. © 2017 American Chemical Society.

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ACS Macro Letters

Influence of Dielectric Constant on Ionic Transport in Polyether-Based Electrolytes

We report the results of atomistic simulations of the structural equilibrium properties of PS-PEO block copolymer (BCP) melt in the ordered lamellar phase doped with LiPF6 salt. A hybrid simulation strategy, consisting of steps of coarse-graining and inverse coarse-graining, was employed to equilibrate the melt at an atomistic resolution in the ordered phase. We characterize the structural distributions between different atoms/ions and compare the features arising in BCPs against the corresponding behavior in PEO homopolymers for different salt concentrations. In addition, the local structural distributions are characterized in the lamellar phase as a function of distance from the interface. The cation-anion radial distribution functions (RDF) display stronger coordination in the block copolymer melts at high salt concentrations, whereas the trends are reversed for low salt concentrations. Radial distribution functions isolated in the PEO and PS domains demonstrate that the stronger coordination seen in BCPs arises from the influence of both the higher fraction of ions segregated in the PS phase and the influence of interactions in the PS domain. Such a behavior also manifests in the cation-anion clusters, which show a larger fraction of free ions in the BCP. While the average number of free anions (cations) decreases with increasing salt concentration, higher order aggregates of LiPF6 increase with increasing salt concentration. Further, the cation-anion RDFs display spatial heterogeneity, with a stronger cation-anion binding in the interfacial region compared to bulk of the PEO domain. © 2017 American Chemical Society.

Macromolecules

Multiscale Simulations of Lamellar PS-PEO Block Copolymers Doped with LiPF6 Ions

We use all atom molecular dynamics simulations to investigate the influence of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) ionic liquid on the structure and transport properties of poly(ethylene oxide) (PEO) polymer electrolytes doped with LiPF6 salt. We observe enhanced diffusivities of the Li+, PF6−, and BMIM+ ions with increasing loading of the ionic liquid. Interplay between the different ion-ion and ion-polymer interactions is seen to lead to a destabilization of the Li-PF6 coordination and increase in the strength of association between the Li+ cations and the polymer backbone. As a consequence, the polymer segmental relaxation times are shown to be only moderately affected by the addition of ionic liquids. The ionic-liquid induced changes in the mobilities of Li+ ions are seen to be correlated to polymer segmental relaxation times. However, the mobilities of BMIM+ ions are seen to be more strongly correlated to the BMIM-PF6 ion-pair relaxation times. © 2017 Author(s).

Fulltext

Journal of Chemical Physics

Structure and mechanisms underlying ion transport in ternary polymer electrolytes containing ionic liquids

Four different configurations of plasma cathode electron (PCE) guns have been designed and developed in which pseudospark discharge concept has been used. Experimental studies have been performed to identify the dependence of geometrical design parameters like shape of aperture, distribution of apertures, and the number of gaps between hollow cathode and anode on high-energy and high-current-density electron beam emissions. The beam current density has been increased around five times by changing the shape of aperture from circular to sheet keeping the aperture area and other geometrical parameters identical. The perveance of the electron beam has also been increased by increasing the number of apertures on hollow cathode and anode surface. The beam current density from single aperture to multiple apertures has increased from 225 to 900 A/cm2 with low beam energy for the multiple-aperture case. The multiple gaps between hollow cathode and anode increase the electron beam energy. The maximum beam energy is found to be 16-14.5 keV at an applied gap voltage of 16 kV from the developed multigap PCE gun, which is much higher than that from the single gap configuration. © 1963-2012 IEEE.

IEEE Transactions on Electron Devices

Analysis of Geometrical Design Parameters for High-Energy and High-Current-Density Pseudospark-Sourced Electron Beam Emission

In this work, we take a step toward nanometer resolution magnetic tomography, demonstrating the use of resonant magnetic x-ray scattering for three-dimensional (3D) characterization of Co/Pd multilayers. Trading coherence for x-ray flux, we measured magnetic scattering out to full period resolutions of 20 nm for incidence angles up to 75 from normal incidence, which was sufficient for a semiquantitative analysis of the 3D magnetic domain structure of such films. For the analysis of such patterns, we developed a scattering model based upon multiple plane propagation of the beam through the sample, taking into account the effects of multiple scattering not accounted for with Born or Rytov approximation based models. Our results demonstrate that 3D statistical characterization of nanoscale magnetic features is currently possible, and it is expected that the advent of high brilliance sources such as MAX IV in Sweden and SIRIUS in Brazil will provide the coherent flux necessary to permit the generalization of this work to nanometer resolution magnetic tomography. © 2017 American Physical Society.

Physical Review B

Three-dimensional characterization of Co/Pd multilayer thin films using resonant soft x-ray scattering

Here, we report a facile hydrothermal synthesis method to prepare BiFeO3 nanowire-reduced graphene oxide (BFO-RGO) nanocomposites. The unique properties of 2-D reduced graphene oxide (RGO) and 1-D BiFeO3 nanowires (BFO) were exploited to design nanocomposites to obtain high performing microwave absorber materials. The composite with 97 wt % BFO and 3 wt % RGO exhibited minimum reflection loss value of -28.68 dB at 10.68 GHz along with the effective absorption bandwidth (≥ -10 dB) ranging from 9.6 to 11.7 GHz when the absorber thickness was only 1.55 mm. Firstprinciples calculations based on density functional theory (DFT) of BFO, graphene, and BFO-RGO nanocomposites were performed to obtain information about their electronic structures to interpret their complex permittivity and its derived properties. To the best of our knowledge, this is the first time investigations on microwave absorption properties of the BiFeO3 nanowire and BFO-RGO nanocomposites have been reported, and this nanocomposite shows its potential to be used as a lightweight, high performing microwave absorber in the X-band region. © 2017 American Chemical Society.

Journal of Physical Chemistry C

Synthesis and microwave absorption properties of BiFeO3 nanowire-RGO nanocomposite and first-principles calculations for insight of electromagnetic properties and electronic structures

We report on the structural, magnetic, optical, and electrical properties of iron titanate thin films prepared using a spin-coating technique having nominal compositions of FeTiO3 and Fe1.4Ti0.6O3. X-ray diffraction measurements show clear evidence for the presence of both ilmenite and pseudobrookite crystal structures, which is confirmed using Raman spectroscopy. X-ray photoemission spectroscopy studies indicate that Fe is present mainly in the 2+ valence state, with some Fe3+ present on the more heavily oxidized surface of the films. The as-prepared samples exhibit weak ferromagnetism at room temperature. While the valence state of Fe is not significantly affected by vacuum annealing, the saturation magnetization is increased dramatically, reaching nearly 220 emu mole-1 for the Fe-rich film. The optical band gap was found to be roughly 2.0 eV for all samples, with negligible changes on vacuum annealing. © 2017 Author(s).

Journal of Applied Physics

Structural, magnetic, and electrical properties of spin coated ilmenite-pseudobrookite xFeTiO3-(1-x)Fe2O3 thin films

A three-party scheme for secure quantum communication, namely controlled quantum dialogue (CQD), is analyzed under the influence of non-Markovian channels. By comparing with the corresponding Markovian cases, it is seen that the average fidelity can be maintained for relatively longer periods of time. Interestingly, a number of facets of quantum cryptography, such as quantum secure direct communication, deterministic secure quantum communication and their controlled counterparts, quantum dialogue, quantum key distribution, quantum key agreement, can be reduced from the CQD scheme. Therefore, the CQD scheme is analyzed under the influence of damping, dephasing and depolarizing non-Markovian channels, and subsequently, the effect of these non-Markovian channels on the other schemes of secure quantum communication is deduced from the results obtained for CQD. The damped non-Markovian channel causes a periodic revival in the fidelity, while fidelity is observed to be sustained under the influence of the dephasing non-Markovian channel. © 2017, Springer Science+Business Media New York.

Quantum Information Processing

Quantum cryptography over non-Markovian channels

The heat packs using phase change materials (PCMs) are designed for possible applications such as body comfort and medical applications under adverse situations. The development and performance of such heat packs rely on thermophysical properties of PCMs such as latent heat, suitable heat releasing temperature, degree of supercooling, effective heat releasing time, crystallite size, stability against spontaneous nucleation in metastable supercooled liquid state and thermal stability during heating and cooling cycles. Such PCMs are rare and the available PCMs do not exhibit such properties simultaneously to meet the desired requirements. The present work reports a facile approach for the design and development of ethylene glycol (EG) and aqueous sodium acetate trihydrate (SAT) based composite phase change materials, showing these properties simultaneously. The addition of 2-3 wt% EG in aqueous SAT enhances the softness of SAT crystallites, its degree of supercooling and most importantly the effective heat releasing time by ∼10% with respect to aqueous SAT material. In addition, the maximum heat releasing temperature of aqueous SAT has been tailored from 56.5 °C to 55 °C, 54.9 °C, 53.5 °C, 51.8 °C and 43.2 °C using 2%, 3%, 5%, 7% and 10 wt% EG respectively, making the aqueous SAT-EG composite PCMs suitable for desired thermal applications. © 2017 The Author(s).

Scientific Reports

Development of sodium acetate trihydrate-ethylene glycol composite phase change materials with enhanced thermophysical properties for thermal comfort and therapeutic applications