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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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54 results
Now showing 1 - 10 of 54
- PublicationAn orange-red phosphor under near-UV excitation for white light emitting diodes(01-01-2007)
;Sivakumar, V.A series of orange-red emitting phosphor compositions Sr1.9-x Bax Eu0.05 Li0.05 MoO6 (x=0-1.9) have been synthesized and characterized by powder X-ray diffraction (XRD) and photoluminescence. XRD results reveal a phase transition from orthorhombic to pseudocubic structure for x0.2. All the compositions show broad charge-transfer band absorption in the near-ultraviolet (UV) region. Orange-red emission is observed for compositions with lower x values (x0.6), whereas orange emission is observed for compositions with higher x values. Select compositions in this system of compounds could find potential applications as orange-red phosphors for white light generation using near-UV/blue GaN -based light emitting diodes. © 2006 The Electrochemical Society. - PublicationSynthesis, phase transition and photoluminescence studies on Eu3+-substituted double perovskites-A novel orange-red phosphor for solid-state lighting(01-12-2008)
;Sivakumar, V.Photoluminescence studies on Eu3+-doped double perovskites with the formula A2CaWO6 (A=Sr, Ba) revealed that the forced electric dipole (ED) transition is present when Eu3+ is substituted at the non-centrosymmetric Sr-site vis-a-vis substitution at the centrosymmetric Ca-site shows both ED and magnetic dipole (MD) transition. A series of novel orange-red-emitting phosphor compositions Sr1.9-xBaxEu0.05Li0.05CaWO6 (x=0-1.9) have also been synthesized and characterized by powder X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and photoluminescence. XRD results reveal a phase transition from monoclinic to pseudo-cubic structure for x≤0.2. All the compositions show broad charge transfer band and orange-red (MD and ED) emission. However, the relative intensity of the MD and ED depends on the Ba content present in the host lattice. Select compositions in this system of compounds could find potential application as orange-red phosphors for white light generation using blue/near-UV GaN-based light-emitting diodes (LEDs). © 2008 Elsevier Inc. All rights reserved. - PublicationIntense red-emitting phosphors for white light emitting diodes(11-11-2005)
;Sivakumar, V.Intense red phosphors, AgGd0.95Eu0.05(WO 4)2-x(MoO4)x (x = 0-2) have been synthesized and characterized by powder X-ray diffraction (XRD) and photoluminescence. Powder XRD results reveal a phase transition from monoclinic to tetragonal structure at x ≥ 0.5. All compositions with Eu3+ show red emission on excitation either in the charge-transfer band or Eu3+ levels. Intense red emission is obtained in the molybdates, AgGd0.95Eu 0.05(MoO4)2 under 465-nm excitation. Studies on AgGd1-yEuy(WO4)2 and AgGd 1-yEuy(WO4)2 (y = 0.1 - 1 in steps of 0. 1) show that the emission intensity is maximum for compositions with y = 0.2 and 0.3, respectively, and a decrease in emission intensity is observed for higher y values. The intense red emission of the tungstate and molybdate phosphors under 394 and 465 nm excitation, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based light-emitting diodes for white light generation. © 2005 The Electrochemical Society. All rights reserved. - PublicationEu2+, Ce3+ luminescence and Ce3+ → Eu2+ energy-transfer studies on Sr2 LiSiO4 F: A white light-emitting phosphor(01-06-2009)
;Sivakumar, V.Eu2+ luminescence is studied in Sr2 LiSiO4 F, and the emission spectrum shows an intense and broad green emission band with two peak maxima (475 and 505 nm). The two emission bands are due to Eu2+ present in the two crystallographic distinct Sr sites in the host lattice. Ce3+ luminescence is studied in Sr2 LiSiO4 F and shows a broad and bright blue emission band at around 430 nm under 350 nm excitation. The effect of Ce3+ co-doping is studied in Sr2 LiSiO4 F: Eu2+ phosphor. Thus, the co-doping of Ce3+ also enhances the absorption of Eu2+ in the near-UV region, where the light-emitting diode emission occurs. Ce3+ co-doped samples show broad-band emission ranging from 360 to 620 nm (blue to green-yellow), which results in white emission. This indicates partial energy transfer from Ce3+ to Eu2+. Selected compositions of this phosphor material can find potential application in solid-state lighting. © 2009 The Electrochemical Society. - PublicationElectrochemical reaction of lithium with Zn3P2(01-06-2005)
;Satya Kishore, M. V.V.M.Zn3P2 has been studied as an anode material for lithium-ion batteries. Electrochemical studies demonstrate that the initial discharge and charge capacities are 1056 and 710 mAh g-1, respectively. The discharge-charge reaction mechanism of lithium with Zn 3P2 is analyzed by ex situ X-ray diffraction. On initial discharge, LiZn alloy is formed in a matrix of Li3P. Upon charge, LiZn alloy is transformed completely into Zn metal and Li3P is converted partially to P, which reacts with Zn to form the original Zn 3P2 phase. The reversible capacity of Zn3P 2 is improved when cycled in the limited voltage window. © 2005 Elsevier B.V. All rights reserved. - PublicationFacile insertion of lithium into nanocrystalline AlNbO4 at room temperature(22-07-2008)
;Anji Reddy, M.A significant Li uptake in the nanocrystalline AlNbO4 form vis a vis the bulk form was investigated. Micrometer-sized and nanometer-sized samples of AlNbO4 were prepared by the conventional solid state reaction (SSR) and polymerizable complex (PC) method respectively. All the peaks in both the XRD patterns were indexed according to the JCPDS card no. 41-0347. The peaks in the XRD patterns were broad, due to the nanocrystalline nature of the sample and are further broadened by the RT insertion reaction. Shifts in peak positions of the lithiated phases vis a vis the parent phase were not discernible. It was observed that the high theoretical specific capacity of 291 mA h g-1 can be realized in nanocrystalline form. - PublicationThermal expansion behaviour of sodium zirconium phosphate structure type phosphates containing tin(01-03-2004)
;Buvaneswari, G. ;Govindan Kutty, K. V.Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates of the formula AM3+SnP3O12 (A = Ca, Sr and Ba; M3+ = Cr and Fe) was studied by high temperature X-ray diffraction and dilatometry in the temperature range 298-1073K. The variation in the hexagonal lattice parameters of the Ca-containing compounds is in line with the 'sodium zirconium phosphate behaviour'. However, the strontium- and barium-containing compounds display an altogether different behaviour of axial expansion. The results are explained based on the crystal chemistry of these compounds. © 2003 Elsevier Ltd. All rights reserved. - PublicationSynthesis and characterization of new apatite-related phosphates(01-01-2000)
;Buvaneswari, G.New phosphates of the formula M3+M4/2+(PO4)3O (M3+ = Bi and La; M2+ = Ca and Sr) are synthesized. The compounds are isostructural with bismuth calcium vanadate, BiCa4(VO4)3O. A nonlinear increase in both the a and c lattice parameters with an increase in strontium content in the solid solution BiCa(4-x)Sr(x)(PO4)3O (x = 1-4.) is observed. The compounds are characterized by infrared spectroscopy. (C) 2000 Academic Press. - PublicationElectrochemical performance of LiMSnO 4 (M=Fe, In) phases with ramsdellite structure as anodes for lithium batteries(01-11-2004)
;Satya Kishore, M. V.V.M.; Raveau, B.LiMSnO 4 (M=Fe, In) compounds were synthesized by high temperature solid-state reaction method and the electrochemical studies were carried out vs. lithium metal. Lithium is reversibly intercalated and deintercalated in LiFeSnO 4 with a constant capacity of ∼90 mAh/g. In situ X-ray diffraction data show that ramsdellite structure is stable for lithium intercalation and deintercalation in LiFeSnO 4. Galvanostatic discharge/charge of LiFeSnO 4 in the voltage window 0.05-2.0 V shows a reversible capacity of ∼100 mAh/g. The observed capacity in LiFeSnO 4 is due to the two processes involving alloying/dealloying of Li 4.4Sn and formation/decomposition of Li 2O. In contrast, the new isotypic oxide LiInSnO 4 does not exhibit any lithium intercalation due to the absence of mixed valence for indium. Its reversible capacity is strongly dependent on the voltage window. LiInSnO 4 exhibits severe capacity fading on cycling in the voltage window 0.05-2.0 V, but shows a stable capacity of ∼90 mAh/g in the voltage range 0.75-2.0 V. In situ XRD patterns of Li xFeSnO 4 (1≤x≤2) at various Li contents during initial lithium intercalation and deintercalation. © 2004 Elsevier Inc. All rights reserved. - PublicationFabrication and characterization of uniform TiO2 nanotube arrays by sol-gel template method(01-12-2006)
;Maiyalagan, T. ;Viswanathan, B.TiO2 nanotubes have been synthesized by sol-gel template method using alumina membrane. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, UV absorption spectrum and X-ray diffraction techniques have been used to investigate the structure, morphology and optical properties of TiO2 nanotubes. SEM image showed that TiO2 nanotubes obtained were ordered and uniform. The diameter and length of the nanotubes were decided by the pore size and thickness of alumina template. Raman and XRD measurements confirmed the crystallinity and anatase phase of the TiO2 nanotubes. The optical absorption measurement of TiO2 nanotubes exhibits a blue shift with respect to that of the bulk TiO2 owing to the quantum size effect. © Indian Academy of Sciences.