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Upadhyayula V Varadaraju
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Upadhyayula V Varadaraju
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Upadhyayula V Varadaraju
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Varadaraju, Upadhyayula V.
Varadaraju, Upadhyayula
Varadaraju, U. V.
Varadaraju, Upadhyayula Venkata
Varadaraju, Upadhayayula V.
Varadaraju, U.
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45 results
Now showing 1 - 10 of 45
- PublicationA new wide band gap thermoelectric quaternary selenide Cu2MgSnSe4(21-10-2015)
;Pavan Kumar, V. ;Guilmeau, Emmanuel ;Raveau, Bernard ;Caignaert, VincentCu2MgSnSe4 based compounds composed of high earth abundant elements have been identified to exhibit good thermoelectric performance in the mid-temperature range. The pristine phase shows a band gap of 1.7 eV, which is slightly higher than similar ternary and quaternary copper based stannite compounds. Cu2MgSnSe4 crystallizes in the tetragonal I4�2m space group. Substitution of In at Sn site tends to decrease the tetragonal distortion toward the cubic symmetry. The electrical and thermal transport properties of Cu and In-doped Cu2MgSnSe4 in the temperature range of 300 K-700 K are studied. The substitution of In3+ for Sn4+ and Cu2+ for Mg2+ induces charge carriers as holes, which in turn lead to improvement in thermoelectric efficiency. The role of mass fluctuations and structural disorder in the evolution of the thermal conductivity of the doped selenides is discussed. A maximum ZT of 0.42 is attained for Cu2MgSn0.925In0.075Se4 around 700 K, and this value is comparable to that of Cu2ZnSnSe4. - PublicationReversible lithium storage behaviour of aromatic diimide dilithium carboxylates(01-03-2016)
;Veerababu, M.; Kothandaraman, R.Electrochemical lithiation/delithiation studies are carried out on three different aromatic diimide lithium carboxylates. Among all, naphthalene diimide based dilithium carboxylate delivers a remarkably stable capacity of 134 mAh g-1 at 2.24 V vs. Li/Li+ over 125 cycles with a low polarization of 50 mV. A striking feature is that, the lithiation/delithiation process is biphasic with excellent plateau behaviour in the voltage-composition profile. DFT calculations confirm the lower band gap and higher electron affinity for this compound. - PublicationReversible Li insertion studies on V4O3(PO4)3 as high energy storage material for Li-Ion battery applications(01-01-2017)
;Satyanarayana, M. ;Rao, R. S. ;Pralong, V.Li-insertion studies were performed on V4O3(PO4)3 that belongs to the libscombite/lazulite family. Availability of multiple oxidation states and vacancies in crystal structure allows for the insertion of more than 7 lithium ions per formula unit. We will show that in the voltage window of 1-4 V vs. Li+/Li, 6.0 Li-ions could be inserted leading to a reversible capacity of 195 mAh/g at a C/5 rate. A structural transformation is observed from ex-situ XRD patterns after the insertion of 2 lithium at 2.4 V vs. Li+/Li, consistent with the available crystallographic sites in the structure. Interestingly we show that from this phase Li2V4O3(PO4)3, further lithium insertion lead to an amorphous material but the structure is completely recovered on charge. - PublicationImproved electrochemical performance of lithium/sodium perylene-3,4,9,10-tetracarboxylate as an anode material for secondary rechargeable batteries(16-11-2015)
;Veerababu, M.; Kothandaraman, R.Lithium perylene-3,4,9,10-tetracarboxylate (Li-PTCA) is synthesized starting from perylene-3,4,9,10-tetracaboxylicacid-dianhydride (PTCDA). In Li-PTCA, the carbonyl group of the carboxylate redox centers exhibits excellent electrochemical reversibility at an average voltage of 1.2 V with respect to Li+/Li. We have improved the rate capability and capacity of Li-PTCA based electrode by in-situ coating of Li-PTCA with conducting acetylene black carbon. We have demonstrated enhanced rate capability, improved capacity and reduced charge transfer resistance in the in-situ carbon coated electrode vis a vis electrode fabricated by conventional method. A capacity of 120 mAh g-1 is observed at the end of the 50 cycles, at 240 mA g-1. Ex-situ XRD studies have revealed that the crystal structure is robust and is stable for reversible insertion of Li. In-situ carbon coated Na-PTCA shows better reversibility and high capacity retention even after 100 cycles when cycled using current densities as high as 500 mA g-1. - PublicationHost-sensitized emission of LiInW2O8 wolframites: From red-Eu3 to white-Dy3 phosphors(01-09-2011)
;Asiri Naidu, S. ;Boudin, S.; Raveau, B.The LiInW2O8:Eu3, LiInW2O 8:Dy3 and LiInW2O8:Eu 3/Dy3 phosphors were synthesized by solid-state reaction and their photoluminescence properties were studied. Under UV excitation, the LiInW2O8:Eu3 phosphor exhibits an intense red emission whereas the LiInW2O8:Dy3 and LiInW2O8:Dy3/Eu3 phosphors show a white emission. The WO6 octahedra play a major role in the luminescence of the host lattice, characterized by a blue emission under UV excitation. The emission of activator ion results from an efficient energy transfer from the LiInW2O8 host lattice to the Eu 3 and Dy3 ions. The LiIn0.97Dy30.03W2O8 and LiIn0.965 Dy 30.03Eu30.005W2O 8 samples, optimized for white emission, are interesting candidates for solid-state lighting applications. © 2011 Elsevier Inc. - PublicationPb3O4 type antimony oxides MSb2O 4 (M = Co, Ni) as anode for Li-ion batteries(01-06-2012)
;Jibin, A. K. ;Reddy, M. V. ;Subba Rao, G. V.; Chowdari, B. V.R.Polycrystalline samples of isostructural MSb2O4 (M = Co, Ni) have been prepared by solid state synthesis and lithium-storage is investigated as possible anode materials for lithium-ion batteries. The reaction mechanism of lithium with MSb2O4 (M = Co, Ni) is explored by galvanostatic cycling, cyclic voltammo-gram and ex situ studies. Both CoSb2O4 and NiSb2O4 exhibit similar electrochemical behavior and show reversible capacity of 490 and 412 mAhg -1 respectively in the first cycle. Reversible alloying de-alloying of Lix Sb takes place in an amorphous matrix of M (Co, Ni) and Li2O during electrochemical cycling. © 2012 Elsevier Ltd. All rights reserved. - PublicationIntense Photoluminescence Emission in Eu3+- and Dy3+- Doped Low-Band Gap Perovskite Titanate, Na0.5Gd0.5TiO3(22-06-2018)
;Tiwari, Hansnath ;Naidu, Salla AsiriNa0.5Gd0.5-xRExTiO3 (RE=Eu and Dy) phases with orthorhombic perovskite structure were synthesized by high temperature solid state reaction and the photoluminescence properties were investigated. The phase purity was confirmed by Rietveld refinement of the powder XRD patterns in Pnma space group. The band gap of the host was found to be 3.41 eV from UV/Vis diffuse reflectance spectrum. In the Eu3+ doped phases, the 5D0→7F2 electric dipole (ED) transition is predominant in the emission spectrum of Eu3+, under 397 nm excitation, and is in agreement with the non-centrosymmetric Cs point group of the EuO8 polyhedron. The critical concentration of the Eu3+ was found to be x=0.075 above which, concentration quenching occurs. Dy3+ doping resulted in intense white light emission under 389 nm excitation. - PublicationTernary lithium molybdenum oxide, Li2Mo4O13: A new potential anode material for high-performance rechargeable lithium-ion batteries(20-12-2017)
;Verma, Rakesh ;Park, Chan Jin ;Kothandaraman, R.The need to identify lithium ion battery anodes consisting of new materials that display high energy density and good cycling stability has interested the research on reversible so-called conversion reaction between lithium and molybdenum oxides such as ternary metal oxide (Li2Mo4O13). Polycrystalline Li2Mo4O13 was synthesized by conventional solid-state reaction route and explored as new potential anode material for secondary lithium ion battery applications vs. Li+/Li in half-cell mode. The electrochemical performance of the Li2Mo4O13 electrode was studied by cyclic voltammograms and galvanostatic discharge-charge cycling under different rates. In the working voltage between 2.5 V and 0.1 V, Li2Mo4O13 shows a high first charge capacity of 1062 mAh g−1 at current rate of C/10 (24 Li react with 10 h) and a superior rate capability with capacity retention of 1008, 842, 713 and 640 mAh g−1 under current rates of C/10, C/5, C/3 and C/2 (24Li react with in 2 h), respectively. Further, the cycling performance was evaluated at C/3 rate (424 mA g−1) and after 100 cycles a reversible capacity of 550 mAh g−1 was obtained with columbic efficiency of ∼100%. Ex-situ XRD studies confirmed that the electrochemical reaction involves insertion of 5Li/f.u vs. Li+/Li during discharge to 1.3 V and the crystal structure was retained when charged to 2.5 V. Below 0.8 V, conversion reaction occurs leading to amorphization of the phase. When discharged to 0.1 V, Mo+6 is reduced to Mo0 state on the basis of the conversion reaction. - PublicationDisodium dimolybdate: a potential high-performance anode material for rechargeable sodium ion battery applications(01-05-2016)
;Verma, Rakesh ;Raman, R. K.Na2Mo2O7 was synthesized by solid-state reaction route and explored as possible anode material for sodium ion battery for the first time. The electrochemical reaction with sodium involves an initial insertion of 0.33 Na/f.u into the lattice followed by conversion reaction. The material shows good reversibility and high rate capability. A reversible capacity of ∼200 mAh g−1 is obtained after 50 cycles. The presence of lattice sodium facilitates reversible sodiation/de-sodiation. - PublicationSimultaneous increase in thermopower and electrical conductivity through Ta-doping and nanostructuring in half-Heusler TiNiSn alloys(01-09-2019)
;Karati, Anirudha ;Mukherjee, Shriparna ;Mallik, Ramesh Chandra ;Shabadi, Rajashekhara ;Murty, B. S.A new series of Ta-doped Ti1−xTaxNiSn (x = 0–0.05) were synthesized through mechanical alloying (MA) followed by spark plasma sintering (SPS) for the first time. MA for 5 h gave rise to Ti/Ti+Ta and Ni3Sn4 phases. With increasing milling time and subsequent SPS, half-Heusler phase formed along with Ni3Sn4 and TiC as secondary phases. To see the effect of temperature on the density of TiNiSn pellets, SPS was carried out at 1073, 1173 and 1273 K. Sintering of TiNiSn at 1073 K produced full-Heusler (TiNi2Sn) and Ni3Sn4as secondary phases and the pellets had long continuous pores. In contrast, SPS at 1273 K gave rise to increased volume fraction of TiNi2Sn impurity phase with almost no porosity. Thus, SPS conditions were set with respect to optimized densification (95 %) and volume fraction of secondary phases (19 %) at 1173 K for 5 h MA of TiNiSn. Ta-doping is expected to decrease the thermal conductivity and improve the ZT of the alloys. The Ta-doped TiNiSn samples were subsequently milled for 5 h and SPS were carried out for them at 1173 K. Increase in Thermopower and electrical conductivity were observed with increasing temperature for all the samples. Due to the simultaneous improvement in electrical conductivity and Thermopower, a maximum power factor of 1.12 and 2.57 mWm−1K−2 at 830 K were observed for TiNiSn and Ti0.95Ta0.05NiSn sintered at 1173 K, respectively. Due to reduced lattice thermal conductivity of 1.8 and 3 Wm−1K−1 for TiNiSn and Ti0.95Ta0.05NiSn sintered at 1173 K, respectively, maximum ZT of 0.17 and 0.31 were obtained at 823 K.