Upadhyayula V Varadaraju

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.

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.

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.

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.

Many Zr-containing phases of the sodium zirconium phosphate (NZP) type structure have been evaluated as host materials for the immobilization of radionuclides. Phosphates crystallizing in eulytine type structure are identified as promising crystalline hosts matrices for the immobilization of rare earth cations and are tested for the first time. The soxhlet tests performed on these two kinds of phosphates with different crystal structure show that the Ln3+ in eulytine and Zr4+ in the NZP phases show very low leach rates of the order of 10-5 to 10-6gm2 day even in powdered samples. The leach rates attain levelling after 4-5 days. It is found that the leaching process is controlled by both diffusion and dissolution, The leaching process of the individual species is found to depend on the charge density of the respective ions present in the matrix.

Synthesis, characterization and superconducting behavior of the phases (i) Y2-xBaxCu2O5 (0\leqslantx\leqslant1.6) and (ii) YBa2Cu3CxO7Cy (0\leqslantx\leqslant2.0) are reported. The composition YBa2Cu4O7Cy shows a Tc (zero resistance) of 94K. © 1987 The Japan Society of Applied Physics.

Cu2MgSnSe4 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.

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.

Magnetization measurements have been performed on superconducting YNi2B2C at different temperatures below Tc with maximum applied fields ranging from 200 Oe to 15 kOe. The experimental data were analyzed using Kim and exponential field dependent critical state models. It is found that Kim model provides a better estimation of Jc.

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.