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Sundara Ramaprabhu
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Sundara Ramaprabhu
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Sundara Ramaprabhu
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Sundara, Ramaprabhu
Ramaprabhu, Sundara
Ramaprabhu, S.
Ramaprabhu, Sundar
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18 results
Now showing 1 - 10 of 18
- PublicationMagnetic and transport properties of Laves phase Dy1-xMmxCo2 (x = 0-0.5) alloys(01-03-2009)
;Srinivas, G.; The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy1-xMmx)Co2 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (TC) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in TC and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed. © 2008 Elsevier Ltd. All rights reserved. - PublicationOptical switching properties of RCo2 -type alloy hydride based solid state device(10-10-2008)
;Srinivas, G.; The optical switching properties of a solid state device based on Ho0.6 Mm0.4 Co2 (HMC) alloy thin film as a switching active layer, water pretreated Nafion membrane as a solid electrolyte, and a transparent conducting indium tin oxide (ITO) as a counterelectrode are investigated. The device is simple and has a reduced layer sequence of HMC/Pd/Nafion/ITO. The reversible optical switching of this device has been studied during electrochemical galvanostatic charging-discharging as well as cyclic voltammetric measurements. Further, the optical switching durability of the device has been tested by repeated electrochemical hydrogenation-dehydrogenation, and the variations in the optical switching properties are discussed. The special characteristic of the device is that it can reversibly switch between a metallic reflecting state and a semiconducting transparent state by a small reversible applied current/voltage indicating the potential substitution for conventional electrochromic devices. © 2008 American Institute of Physics. - PublicationFunctionalized graphene-PVDF foam composites for EMI shielding(12-10-2011)
;Eswaraiah, Varrla; Novel foam composites comprising functionalized graphene (f-G) and polyvinylidene fluoride (PVDF) were prepared and electrical conductivity and electromagnetic interference (EMI) shielding efficiency of the composites with different mass fractions of f-G have been investigated. The electrical conductivity increases with the increase in concentration of f-G in insulating PVDF matrix. A dramatic change in the conductivity is observed from 10 -16 S · m-1 for insulating PVDF to 10-4 S · m-1 for 0.5 wt.% f-G reinforced PVDF composite, which can be attributed to high-aspect-ratio and highly conducting nature of f-G nanofiller, which forms a conductive network in the polymer. An EMI shielding effectiveness of ≈20 dB is obtained in X-band (8-12 GHz) region and 18 dB in broadband (1-8 GHz) region for 5 wt.% of f-G in foam composite. The application of conductive graphene foam composites as lightweight EMI shielding materials for X-band and broadband shielding has been demonstrated and the mechanism of EMI shielding in f-G/PVDF foam composites has been discussed. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. - PublicationInorganic nanotubes reinforced polyvinylidene fluoride composites as low-cost electromagnetic interference shielding materials(01-01-2011)
;Eswaraiah, Varrla; Novel polymer nanocomposites comprising of MnO2 nanotubes (MNTs), functionalized multiwalled carbon nanotubes (f-MWCNTs), and polyvinylidene fluoride (PVDF) were synthesized. Homogeneous distribution of f- MWCNTs and MNTs in PVDF matrix were confirmed by field emission scanning electron microscopy. Electrical conductivity measurements were performed on these polymer composites using four probe technique. The addition of 2 wt.% of MNTs (2 wt.%, f-MWCNTs) to PVDF matrix results in an increase in the electrical conductivity from 10-16S/m to 4.5 × 10-5S/m (3.2 × 10-1S/m). Electromagnetic interference shielding effectiveness (EMI SE) was measured with vector network analyzer using waveguide sample holder in X-band frequency range. EMI SE of approximately 20 dB has been obtained with the addition of 5 wt.% MNTs-1 wt.% f-MWCNTs to PVDF in comparison with EMI SE of approximately 18 dB for 7 wt.% of f-MWCNTs indicating the potential use of the present MNT/f-MWCNT/PVDF composite as low-cost EMI shielding materials in X-band region. © 2011 Eswaraiah et al. - PublicationTemperature and concentration dependent electrical resistivity of Ho1-xTixCo2-hydrogen system(01-10-2007)
;Srinivas, G.; The structure and electrical resistivity of Ho1-xTixCo2-hydrides (x = 0.1-0.6) have been determined through the powder X-ray diffraction (XRD) and temperature dependence of electrical resistivity (ρ(T)) at different hydrogen concentrations. The variations in the lattice parameters in different phase regions and the lattice expansion with respect to the hydrogen concentration have been studied. The temperature dependent electrical resistivity of hydrides has been discussed based on the conduction electron scattering and spin fluctuation scattering mechanisms. The changes in ρ(T) upon Ti substitution and increasing concentration of hydrogen have been discussed and the results have been correlated with their structural properties. © 2007. - PublicationGraphene-based engine oil nanofluids for tribological applications(23-11-2011)
;Eswaraiah, Varrla; Ultrathin graphene (UG) has been prepared by exfoliation of graphite oxide by a novel technique based on focused solar radiation. Graphene based engine oil nanofluids have been prepared and their frictional characteristics (FC), antiwear (AW), and extreme pressure (EP) properties have been evaluated. The improvement in FC, AW, and EP properties of nanofluids is respectively by 80, 33, and 40% compared with base oil. The enhancement can be attributed to the nanobearing mechanism of graphene in engine oil and ultimate mechanical strength of graphene. © 2011 American Chemical Society. - PublicationDiffusion of hydrogen in cubic Laves phase Ho1 - x Mmx Co2 (x = 0, 0.2 and 0.4) alloys(01-09-2007)
;Srinivas, G.; The diffusion of hydrogen in cubic (C15-type) Laves phase Ho1 - x Mmx Co2 (x = 0, 0.2 and 0.4, Mm = mischmetal) alloys was measured in the α-phase (solid solution) region over the temperature range of 500 - 800 {ring operator} C using Sieverts-type apparatus. The alloys and its hydrides have been characterized by powder X-ray diffraction (XRD) Rietveld method. The diffusion constants have been determined from the gas-solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy is obtained from the temperature dependence of diffusion using Arrhenius relation. © 2007 International Association for Hydrogen Energy. - PublicationThermodynamic and hydrogen-induced structural properties of Ho 1-xMmxCo2-hydrides(21-02-2007)
;Srinivas, G.; Thermodynamic and hydrogen-induced structural properties of hydrogenated Ho1-xMmxCo2 (x ≤ 0, 0.1, 0.2, 0.3 and 0.4, Mm ≤ mischmetal) alloys have been investigated. The hydrogen absorption pressure-composition isotherms were studied in the temperature range 50-200 °C and pressure range 0.001-1 bar using a Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydride stability has been discussed. Structural analyses and hydrogen-induced amorphization of Ho 1-xMmxCo2hydrides were performed using powder x-ray diffraction. The structural stability of these alloys upon repeated hydrogenation and dehydrogenation cycles has been investigated. At lower hydriding pressure and temperatures, Ho1-xMmxCo 2-hydrides retain their Laves C15 structure; however, the large lattice volume expansion about 24% is observed in the β-phase. The effect of hydrogenation pressure and temperature and Mm concentration on the hydrogen-induced transformation from crystalline Ho1-xMm xCo2-H to amorphous Ho1-xMmxCo 2-H and recrystallization into elemental hydrides have been studied and discussed. © 2007 IOP Publishing Ltd. - PublicationInfluence of hydrogen absorption-desorption on structural properties of Dy1-xMmxCo2 alloys(25-06-2008)
;Srinivas, G.; The effect of hydrogen absorption-desorption on the structural properties of Laves phase Dy1-xMmxCo2 (x = 0.1, 0.3 and 0.5; Mm = mischmetal, a natural mixture of the light rare earth metals containing 50 wt% Ce, 35 wt% La, 8 wt% Pr, 5 wt% Nd and 1.5 wt% of other rare earth elements and 0.5 wt% Fe) alloys has been investigated by means of hydrogen absorption-desorption pressure-composition (PC) isotherms, kinetics of hydrogen absorption and powder x-ray diffraction (XRD). The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.001-1 bar and temperature range 50-200 °C using Sieverts-type apparatus. The experimental results of the kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions have been identified from the different slope regions of the PC isotherms and first-order type kinetic plots. The dependence of the reaction rate parameter upon hydriding pressure and temperature in the (α+β)-phase region has been discussed. The effect of hydrogenation pressure, temperature and Mm concentration on the hydrogen-induced transformation from crystalline Dy1-xMm xCo2-H to amorphous Dy1-xMmxCo 2-H and decomposition into crystalline (Dy, Mm)H2 and Co have been discussed in detail. Further, the effect of dehydrogenation on the recovery of the crystalline Laves phase structure of Dy1-xMm xCo2 from its decomposed state is presented. This hydrogenation-disproportionation-desorption-recombination (HDDR) process can be conveniently used in powder metallurgy. © 2008 IOP Publishing Ltd. - PublicationInfluence of hydrogen absorption on structural and electrical transport properties of Ho1-xMmxCo2 alloys(08-10-2007)
;Srinivas, G.; The structural and electrical transport properties of Ho1-x Mmx Co2 (x=0, 0.1, 0.2, 0.3, and 0.4 and Mm=mischmetal) alloys and their hydrides in the hydrogen concentration (y) range of 0≤y≤3.6 have been determined through the powder x-ray diffraction (XRD) and temperature dependence of electrical resistivity [ρ (T)] measurements. At room temperature, these compounds crystallize in Mg Cu2 -type (C15) structure. The crystalline nature and lattice expansion of hydrogenated alloys have been studied using the hydrogen concentration dependence of XRD peak analysis indicating the coexistence of two hydride phases at intermediate hydrogen concentrations. The temperature dependence of the electrical resistivity of alloys has been discussed based on the conduction electron scattering and spin fluctuation scattering mechanisms. The changes in magnetic ordering and transition temperature upon Mm substitution and at different concentrations of hydrogen loadings have been discussed. Furthermore, the transformation from metalliclike conductivity to thermally activated conduction mechanism and different conduction regimes of temperature dependent resistivity upon increasing H concentration have been well described by Kondo-type and electron-electron scattering effects. © 2007 American Institute of Physics.