Faculty Publications
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Item Development of nano-structured cyclic multilayer Zn-Ni alloy coatings using triangular current pulses(2011) Bhat, R.S.; Hegde, A.C.Cyclic multilayer alloy (CMA) deposits of Zn-Ni were developed on mild steel from sulphate bath having thiamine hydrochloride (THC) and citric acid (CA) as additives. CMA coatings were developed galvanostatically using triangular current pulses, under different conditions of cyclic cathode current density (CCCD's) and number of layers. The corrosion behaviors of the coatings were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy methods, and were compared with that of monolayer Zn-Ni alloy of same thickness. At optimal configuration, CMA coating represented as, (Zn-Ni)2.0/5.0/300 was found to exhibit ?40 times better corrosion resistance compared to monolayer alloy, (Zn-Ni)3.0. Cyclic voltammetry study demonstrated that THC and CA have improved the appearance of the deposit by complexation with metal ions. The corrosion protection efficacy of CMA coatings was attributed to the difference in phase structure of the alloy in successive layers, evidenced by XRD analysis. The formation of multilayer and corrosion mechanism was analyzed by Scanning Electron Microscopy (SEM) study. © 2011 Allerton Press, Inc.Item Corrosion protection of electrodeposited multilayer nanocomposite Zn-Ni-SiO2 coatings(2013) Ullal, Y.; Hegde, A.C.Multilayer nanocomposite coatings of Zn-Ni-SiO2 were deposited galvanostatically on mild steel (MS) from Zn-Ni bath, having Zn+2 and Ni+2 ions and uniformly dispersed nano-SiO2 particles. The corrosion characteristics and properties of multilayered nanocomposite (MNC) coatings were evaluated by electrochemical polarization and impedance methods. Such deposition conditions as, bath composition, cyclic cathode current densities (CCCD's) and number of layers were optimized for peak performance of coatings against corrosion. A significant improvement in the corrosion performance of MNC coatings was observed when a coating was changed from a monolayer to multilayer type. Corrosion rate (CR) of MNC coating decreased progressively with number of layers up to an optimal level, and then started increasing. The increase of CR at a higher degree of layering is attributed to diffusion of layers due to a very short deposition time, failing to give the enhanced corrosion protection. The formation of layers, inclusion of silica particle in MNC coating matrix were confirmed by SEM and XRD study. At optimal current densities, i.e. at 3.0-5.0 A/cm2, the Zn-Ni-SiO2 coating having 300 layers, represented as (Zn-Ni-SiO2)30/5.0/300 is found to be about 107 times more corrosion resistant than a monolayer Zn-Ni-SiO2 coating, developed from the same bath for the same time. The reasons responsible for the extended corrosion protection of MNC Zn-Ni-SiO2 coatings, compared to corresponding monolayer Zn-Ni and (Zn-Ni-SiO2) coatings were analyzed, and results were discussed. © 2013 Allerton Press, Inc.Item Nanofabricated multilayer coatings of Zn-Ni alloy for better corrosion protection(2013) Rao, V.R.; Hegde, A.C.As an effort to increase the corrosion resistance of conventional monolayer Zn-Ni alloy coating, the multilayer Zn-Ni alloy coating have been done electrolytically on mild steel (MS), using gelatin and glycerol as additives. Multilayered, or more correctly composition modulated multilayer alloy (CMMA) coatings have been developed using square current pulse. Successive layers of alloys, in nanometric scale having alternately changing composition were fabricated by making the cathode current to cycle between two values, called cyclic cathode current densities (CCCD's). The coatings having different configuration, in terms of composition and thicknesses of individual layers were developed and their corrosion performances were evaluated by electrochemical methods. The corrosion rate (CR)'s were found to decrease drastically with progressive increase in number of layers (up to 300 layers), and then increased. The coating configurations have been optimized for best protection against corrosion. The CMMA Zn-Ni coating having 300 layers was found to be about 37 times more corrosion resistant than corresponding monolayer alloy, developed from same bath for same time. High protection efficacy of the coatings were attributed to alternate layers of alloys having different surface structure and composition, supported by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) study, respectively. Optimization procedure has been explained, and results are discussed. © 2013 Pleiades Publishing, Ltd.Item Electrodeposition of laminar coatings of Ni-W alloy and their corrosion behaviour(Elsevier, 2015) Elias, L.; Hegde, A.The attractiveness of electroplating for the synthesis of advanced materials is linked to large selection of plating conditions coupled with different mass transfer processes towards the cathode, and this allows the tailoring of different properties of many electrodeposited coatings. This theme has been exploited effectively in the development of a new class of coatings; called composition modulated multilayered (CMM), or in short laminar coatings. The work embodied in this paper is to demonstrate how the corrosion resistance of monolayer Ni-W alloys can be increased to many fold of its magnitude by multilayer deposition. Ni-W coatings have been deposited on mild steel (MS) in a laminar multilayer pattern from a citrate bath using single bath technique (SBT). Electrodeposits having alternate layers of alloys, having different compositions were developed by modulating the direct current (DC). CMM coating configurations have been optimized from a newly developed bath, in terms of current pulse height and thickness of each layer to maximize its corrosion protection ability, in relation to its monolayer coating, developed from same bath for same time. The process and product of the Ni-W coatings have been characterized using different instrumental methods, such as cyclic voltammetry (CV), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) study. The better corrosion resistance behaviour of CMM Ni-W coatings has been analysed in the light of increased surface areas of the coatings due to layering, and results are discussed. © 2015 Elsevier B.V.Item Development of multilayer Sn-Ni alloy coating by pulsed sonoelectrolysis for enhanced corrosion protection(Royal Society of Chemistry, 2016) Shetty, S.; Mohamed, M.J.; Bhat, D.; Hegde, A.C.Multilayer Sn-Ni alloy coating has been developed electrochemically on mild steel using an ultrasound effect, as a tool to modulate mass transfer process at electrical double layer, during deposition. Sn-Ni coatings having alternate layers of alloys of different compositions were developed on a nano/micrometric scale by pulsing sonicator ON (tON) and OFF (tOFF), periodically. The composition modulated multilayer alloy (CMMA) Sn-Ni coatings have been deposited by inducing the ultrasound field periodically at optimal current density. Corrosion performances of ultrasound-assisted multilayer Sn-Ni alloy coatings have been evaluated by electrochemical methods. Corrosion data revealed that CMMA Sn-Ni coating, developed using pulsed ultrasonic field and having 150 layers, represented as (Sn-Ni)2/2/150, is the most corrosion resistant, compared to its monolayer alloy coatings developed by both with/without ultrasound effect. Corrosion protection efficacy of multilayer coatings was found to be decreased at high degree of layering due to diffusion of layers. Improved corrosion resistance of multilayer Sn-Ni coatings is attributed to an increase in the number of layers, or interfaces separating alloys of the same metals, but of different composition, surface morphologies and phase structures, supported by energy dispersive spectroscopy, field emission scanning electron microscopy and X-ray diffraction study, respectively. The better corrosion protection of CMMA Sn-Ni coatings, compared to monolayer counterparts, is attributed to an increase in the number of layers, hence phase boundaries between layers, and experimental results are discussed. © 2018 The Royal Society of Chemistry.Item Scanning and resetting the phase of a pinned spiral wave using periodic far field pulses(Institute of Physics Publishing helen.craven@iop.org, 2016) Shajahan, T.K.; Berg, S.; Luther, S.; Krinski, V.; Bittihn, P.Spiral waves in cardiac tissue can pin to tissue heterogeneities and form stable pinned waves. These waves can be unpinned by electric stimuli applied close to the pinning center during the vulnerable window of the spiral. Using a phase transition curve (PTC), we quantify the response of a pinned wave in a cardiac monolayer to secondary excitations generated electric field pulses. The PTC can be used to construct a one-dimensional map that faithfully predicts the pinned wave's response to periodic field stimuli. Based on this 1D map, we predict that pacing at a frequency greater than the spiral frequency, over drive pacing, leads to phase locking of the spiral to the stimulus, which hinders unpinning. In contrast, under drive pacing can lead to scanning of the phase window of the spiral, which facilitates unpinning. The predicted mechanisms of phase scanning and phase locking are experimentally tested and confirmed in the same monolayers that were used to obtain the PTC. Our results have the potential to help choose optimal parameters for low energy antifibrillation pacing schemes. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.Item Valley-Coherent Hot Carriers and Thermal Relaxation in Monolayer Transition Metal Dichalcogenides(American Chemical Society service@acs.org, 2016) Kallatt, S.; Umesh, G.; Majumdar, K.We show room-temperature valley coherence in MoS2, MoSe2, WS2, and WSe2 monolayers using linear polarization-resolved hot photoluminescence (PL) at energies close to the excitation, demonstrating preservation of valley coherence before sufficient scattering events. The features of the copolarized hot luminescence allow us to extract the lower bound of the binding energy of the A exciton in monolayer MoS2 as 0.42 (±0.02) eV. The broadening of the PL peak is found to be dominated by a Boltzmann-type hot luminescence tail, and using the slope of the exponential decay, the carrier temperature is extracted in situ at different stages of energy relaxation. The temperature of the emitted optical phonons during the relaxation process is probed by exploiting the corresponding broadening of the Raman peaks due to temperature-induced anharmonic effects. The findings provide a physical picture of photogeneration of valley-coherent hot carriers and their subsequent energy relaxation pathways. © 2016 American Chemical Society.Item Photoresponse of atomically thin MoS2 layers and their planar heterojunctions(Royal Society of Chemistry, 2016) Kallatt, S.; Umesh, G.; Bhat, N.; Majumdar, K.MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied by a very strong exciton binding energy-resulting in complex photoresponse characteristics. We study the electrical response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. A strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) the photoelectric effect (PE), (ii) the photothermoelectric effect (PTE), (iii) the excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show a peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/few-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/few-layer heterojunction. Finally, we investigate the laser position dependent transient response of the photocurrent to reveal that trapping of carriers in SiO2 at the source junction is a critical factor to determine the transient response in 2D photodetectors, and also show that, by a systematic device design, such trapping can be avoided in the heterojunction devices, resulting in a fast transient response. The insights obtained will play an important role in designing a fast 2D TMD based photodetector and related optoelectronic and thermoelectric devices. © 2016 The Royal Society of Chemistry.Item Assessment of the mechanical properties of monolayer graphene using the energy and strain-fluctuation methods(Royal Society of Chemistry, 2018) Thomas, S.; Ajith, K.M.; Lee, S.U.; Valsakumar, M.C.Molecular statics and dynamics simulations were performed to investigate the mechanical properties of a monolayer graphene sheet using an efficient energy method and strain-fluctuation method. Using the energy method, we observed that the mechanical properties of an infinite graphene sheet are isotropic, whereas for a finite sheet, they are anisotropic. This work is the first to report the temperature-dependent elastic constants of graphene between 100 and 1000 K using the strain-fluctuation method. We found that the out-of-plane thermal excursions in a graphene membrane lead to strong anharmonic behavior, which allows large deviations from isotropic elasticity. The computed Young's modulus and Poisson's ratio of a sheet with an infinite spatial extent are 0.939 TPa and 0.223, respectively. We also found that graphene sheets with both finite and infinite spatial extent satisfy the Born elastic stability conditions. We extracted the variation in bending modulus with the system size at zero kelvin (0.83 eV) using a formula derived from the Foppl-von Karman approach. When the temperature increases, the Young's modulus of the sample decreases, which effectively reduces the longitudinal and shear wave velocities. © 2018 The Royal Society of Chemistry.Item Spiral wave unpinning facilitated by wave emitting sites in cardiac monolayers(Royal Society Publishing, 2019) Punacha, S.; Berg, S.; Sebastian, A.; Krinski, V.I.; Luther, S.; Shajahan, T.K.Rotating spiral waves of electrical activity in the heart can anchor to unexcitable tissue (an obstacle) and become stable pinned waves. A pinned rotating wave can be unpinned either by a local electrical stimulus applied close to the spiral core, or by an electric field pulse that excites the core of a pinned wave independently of its localization. The wave will be unpinned only when the pulse is delivered inside a narrow time interval called the unpinning window (UW) of the spiral. In experiments with cardiac monolayers, we found that other obstacles situated near the pinning centre of the spiral can facilitate unpinning. In numerical simulations, we found increasing or decreasing of the UW depending on the location, orientation and distance between the pinning centre and an obstacle. Our study indicates that multiple obstacles could contribute to unpinning in experiments with intact hearts. © 2019 The Author(s) Published by the Royal Society. All rights reserved.