Now showing 1 - 10 of 29
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    Self-aggregation, H-bonding, and photoresponse in film and solution states of azobenzene containing polyurea
    (06-09-2022)
    Erekath, Swathi
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    Chordiya, Kalyani
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    Vidhya, K. V.
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    Kahaly, Mousumi Upadhyay
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    We critically understand the hydrogen bonding interactions and electronic transitions occurring in a thin film as well as in solution of a photo-responsive polymer, azo-polyurea (azo-PU). We synthesize azo-PU by covalent attachment of the azobenzene chromophore to the main chain of polyurea. Azo-PU shows reversible photoisomerization between trans and cis states upon light exposure, the occurrence of which is typically analysed using the π-π* and n-π* electronic transition peaks in the UV-visible absorption spectrum. We find that the π-π* and n-π* bands undergo a redshift and blueshift respectively on dissolving azo-PU in DMF solvent, resulting in a single overlapped peak in the spectrum. However, upon UV irradiation, these bands split into two independent transitions that are characteristic of azo-PU solid films. These observations are explained based on the changes in polymer-polymer and polymer-solvent interactions through hydrogen bonding and self-aggregation tendency. The experimental findings are corroborated using DFT simulations which provide useful insights into electronic orbital transitions, electron distribution, and hydrogen bonding interaction through IR vibrational modes.
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    Investigation on the corrosion behavior of lanthanum phosphate coatings on AZ31 Mg alloy obtained through chemical conversion technique
    (05-05-2019)
    Jayaraj, Jithu
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    Rajesh, K. R.
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    Amruth Raj, S.
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    Srinivasan, A.
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    Ananthakumar, S.
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    Dhaipule, Nanda Gopala Krishna
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    Pillai, U. T.S.
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    Mudali, U. Kamachi
    A lanthanum based phosphate coating on magnesium AZ31 alloy was prepared through chemical conversion coating technique and characterized their corrosion performance. The coating was obtained in a two stage procedure using two different solvents, water or ethanol for preparing the coating nitrate bath. The chemical compositions and structures were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Corrosion performance of the developed coatings in 3.5 wt% NaCl was evaluated using electrochemical polarization and immersion tests. The results indicated that lanthanum hydroxide formation rather than lanthanum phosphate was predominant in the coating prepared using water as solvent for the nitrate bath whereas using ethanol as solvent produced lanthanum phosphate rich and defect less coating. An excellent corrosion resistance (in immersion test) was noticed with coatings prepared using ethanol as solvent (0.046 mm/y) over the bare sample (1.30 mm/y) as well as samples coated using water as solvent (0.278 mm/y).
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    Thickness regimes of power law liquids dip coated onto permeable substrates
    (01-04-2021)
    Sathyanath, Rahul
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    Coating thin liquid films with complex rheological behaviour on permeable substrates is often an important requirement in several applications such as contact lenses, textiles, and paper-based electronics. Here, we extend the classical Landau-Levich problem of dip coating of Newtonian liquids on rigid substrates to liquids of power-law rheology on permeable substrates. Our results suggest distinct deviation from the classical Landau-Levich relation through exhibition of different regimes of varying dependence of coating film thickness on withdrawal speed. A process map is presented depicting these coating thickness regimes for a wide range of operating parameters such as the substrate permeability factor, power-law exponent of the liquid, and a rescaled capillary number.
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    Electrocapillary effect in liquid films with an electrically charged interface
    (01-01-2021)
    Shantharama,
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    The electrocapillary effect is the change in interfacial tension between two immiscible liquids when subjected to an electric field across the interface. The change in interfacial tension occurs due to variations in the surface charge density of adsorbed ions or polar species at the interface. In the present work, we provide fluid mechanical insights into electrocapillarity-based dynamics of an interface between a passive air layer and a thin liquid film that has small, but finite electrical conductivity. We formulate and solve mathematical equations that model the situation in which a liquid film bounded by an air layer of controllable thickness is sandwiched between two electrode plates, and an electric field is applied across the layers. As the liquid and air have different conductivities, free charges would accumulate at the liquid-air interface to ensure that current is conserved across the layers. In our model, the interfacial tension is assumed to vary as per the classical Lippmann's equation. The present mathematical model describes spatiotemporal variation of the film height and interfacial charge density as a function of the applied potential, air layer width, and the sensitivity of interfacial tension to the electric potential. Our results show that ultrathin liquid films (<100 nm) in presence of electrocapillary Marangoni effect are destabilized and break up faster than in its absence. Furthermore, by controlling the air gap width, different morphological patterns can be generated. Finally, we illustrate that the dynamics is profoundly affected if the electrode plate bounding the liquid film possesses patterned wettability.
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    Polymer Coatings and Patterning Techniques
    (01-01-2016)
    In the context of smaller length scales being sought after everywhere by the present day scientific community, this chapter attempts to describe some of the latest developments in polymer coatings in the last decade or so, which have in turn strengthened tremendously the fields of nanotechnology and interfacial engineering. In Section 8.1 a review of materials, chemistry, and techniques used for polymer coatings is provided. Typical material choices based on suitability for industrial use are discussed here, followed by the preferred means of their application. With miniaturization of latest devices and displays being an endless effort, creation of small-scale patterns of polymeric surfaces have assumed importance, considering the flexible nature of such surfaces. Section 8.2 looks at the various patterning techniques for polymers, and Section 8.3 deals with a closely associated technique of liquid dewetting, which has also gained attention as a fabrication strategy for creating nanostructures. The focus in Section 8.3 will be on the theoretical framework and supporting mechanisms for such dewetting and pattern creation. The key features of all these latest developments are summarized in Section 8.4 and possible directions for future research in polymer coatings are suggested.
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    A Mechanistic Study on the Structure Formation of NiCo2O4 Nanofibers Decorated with In Situ Formed Graphene-Like Structures
    (01-09-2018)
    Sachin Kumar, B.
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    Gudla, Visweswara C.
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    Ambat, Rajan
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    Anandhan, S.
    Nickel cobaltite (NCO) nanofibers were synthesized using poly(styrene-co-acrylonitrile) (SAN) as the polymeric binder through sol–gel assisted electrospinning. Defect-free precursor nanofiber mats were pyrolyzed at 773 K at three different pyrolysis soaking times t = 2, 4, and 6 h. The SAN present in the precursor nanofibers caused morphological changes in the NCO nanofibers during their thermochemical degradation. Consequently, fractal aggregates of NCO nanoparticles were formed along the length of the nanofibers. X-ray photoelectron spectroscopy (XPS) revealed both + 2 and + 3 oxidation states for Ni and Co, with spinel crystal defects due to oxygen rich atmosphere. XPS, high-resolution transmission microscopy, and optical analysis showed graphene-like structures embedded within the NCO nanofibers. With increase in pyrolysis soaking time, the morphology of the NCO particles markedly changed from spherical to rod-like. We propose a mechanism for the morphological change of NCO nanoparticles on the basis of crystallite splitting accompanied by particle splitting and reordering.
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    Some new observations on the structural and phase evolution of nickel titanate nanofibers
    (15-06-2017)
    Kumar, B. Sachin
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    Shanmugharaj, A. M.
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    Anandhan, S.
    In this study, we report for the first time the synthesis of nickel titanate (NTO) nanofibers containing a mixture of ilmenite and spinel phases of NTO, at an atypical low temperature. Precursor nanofibers produced by sol-gel electrospinning were calcined at three different temperatures to produce the NTO nanofibers. Thermal analysis along with X-ray photoelectron spectroscopy confirmed the formation of non-crystalline stable phases of TiN and Ti-O-N that restrained the formation of ilmenite NTO, and the Ni-rich environment pushed the Ti atoms to tetrahedral sites to form a defective spinel structure. The crystallite size of spinel NTO was observed to increase as a function of the calcination temperature above 700 °C, as the activation energy for coalescence and growth of spinel NTO was favorable. NTO nanofibers obtained above the calcination temperature of 700 °C exhibited new band gap energy around 2.5 eV in Tauc plot. Oxygen vacancies in these ceramic nanofibers decreased as the calcination temperature was increased. A hypsochromic shift of 20 nm in the photoluminescence spectra suggested that the material had a Ni2+ rich NTO (spinel).
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    Liquid film entrainment during dip coating on a saturated porous substrate
    (08-06-2020)
    Sathyanath, Rahul
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    Aarthi, A.
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    The present work examines the effect of substrate surface porosity on the coating thickness and meniscus profile during dip coating under saturated porous media conditions. The classical Landau-Levich formulation is modified by encoding the influence of porosity in an effective Navier slip boundary condition at the porous substrate surface. It is shown that simplified Navier slip-based model works well for creeping flow through the porous medium. The film height profile equation is derived as a function of a rescaled capillary number (Ca‾) and a substrate permeability factor, with inertial effects neglected. Numerical solutions show that the classical 2/3rd power dependence of film thickness on capillary number is recovered only at sufficiently high Ca‾ values. As Ca‾ is decreased, a marked deviation is seen. The shrinking of the entrainment meniscus and the change in meniscus curvature are analyzed in detail. The theoretical results are also validated with a suitable experimental system.
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    Effect of heat treatment on microstructure, corrosion, and shape memory characteristics of laser deposited NiTi alloy
    (05-05-2018)
    Marattukalam, Jithin J.
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    Balla, Vamsi K.
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    Das, Mitun
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    Bontha, Srikanth
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    The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm2, and lenticular or plate-like martensite in the samples processed at 80 J/mm2. The XRD results revealed relatively high concentration of martensite (B19′) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing.
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    Photochemically assisted patterning: An interfacial hydrodynamic model perspective
    (01-05-2022)
    Erekath, Swathi
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    Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.