Dielectric properties of (100-x)Li2B4O7-x(Ba5Li2Ti2Nb8O30) glasses and glass nanocrystal composites

Transparent glasses of various compositions in the system (100 -x)(Li2B4O7)-x(Ba5Li2Ti2Nb8O30) (5 <= x <= 20, in molar ratio) were fabricated by splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses (DTA). X-ray powder diffraction studies confirmed the as-quenched glasses to be amorphous and the heat-treated to be nanocrystalline. Controlled heat-treatment of the as-quenched glasses at 500 degrees C for 8 h yielded nanocrystallites embedded in the glass matrix. High Resolution Transmission Electron Microscopy (HRTEM) of these samples established the size of the crystallites to be in the nano-range and confirmed the phase to be that of Ba5Li2Ti2Nb8O30 (BLTN) which was, initially, identified by X-ray powder diffraction. The frequency, temperature and compositional dependence of the dielectric constant and the electrical conductivity of the glasses and glass nanocrystal composites were investigated in the 100 Hz to 10 MHz frequency range. Electrical relaxations were analyzed using the electric modulus formalisms. The imaginary part of electric modulus spectra was modeled using an approximate solution of Kohlrausch-Williams-Watts relation. The frequency dependent electrical conductivity was rationalized using Jonscher's power law. The activation energy associated with the dc conductivity was ascribed to the motion of Li+ ions in the glass matrix. The activation energy associated with dielectric relaxation was almost equal to that of the dc conductivity, indicating that the same species took part in both the processes. (C) 2010 Elsevier B.V. All rights reserved.

Recombination and trapping in n-type cobalt-doped germanium

The role of cobalt centers in promoting the recombination and trapping processes in n-type germanium has been investigated. Data on lifetime measurements carried out by the steadystate photoconductivity and photo-magneto-electric methods in the temperature range 145 to 300°K on n-type germanium samples containing cobalt in the concentration range 1·1013 to 5.·014/cm3 are presented. The results are analysed on the basis of Sah-Shockley's multi-level formula to yield the capture cross-sections Sp= (hole capture cross-section at doubly negatively charged center) and Sn-(electron capture cross-section at singly negatively charged center) and temperature dependence. It is found that Sp= is (22 ± 6). 10-16 cm2 and Sn- is ∼ 0·1. 10-16 cm2 at 145°K. Sp= varies (n = 3·5 to 4·5) in the range 145-220°K; above 225°K the index 'n' tends to a smaller value. Sn- is practically temperature independent below 180°K and increases with increase of temperature above 180°K. The value of Sp= and its temperature variation lead one to the conclusion that during capture at attractive centers, the phonon cascade mechanism is responsible for the dissipation of the recombination energy.

Space-charge limited conduction in doped polypyrrole devices

Doping dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were carried out on polypyrrole devices in metal-polymer-metal sandwich structure. Temperature dependent I-V measurements infer that space-charge limited conduction (SCLC) with exponential trap distribution is appropriate for the moderately doped samples, whereas trap-free SCLC is observed in lightly doped samples. Trap densities and energies are estimated, the effective mobility is calculated using the Poole-Frenkel model, and the mobility exhibits thermally activated behavior. Frequency dependent capacitance-voltage characteristics show a peak near zero bias voltage, which implies that these devices are symmetric with a negligible barrier height at the metal-polymer interface. Low frequency capacitance measurements have revealed a negative capacitance at higher voltages due to the processes associated with the injection and redistribution of space-charges. (C) 2010 American Institute of Physics.

Electron-hole recombination in cobalt-doped p-type germanium

The recombination properties of cobalt centers in p-type germanium containing cobalt in the concentration range 1014 to 1016 atoms/cm3 have been investigated. The measurement of lifetime has been carried out by steady-state photoconductivity and photo-magneto-electric methods in the temperature range 145 to 300°K. The cross-sections Sno (electron capture cross-section at neutral centers). Sn- (electron capture cross-section at singly negatively charged centers) and their temperature variations have been estimated by the analysis of the lifetime data on the basis of Sah-Shockley's multi-level formula. The value of Sno is (15±5).10-16 cm2 and is temperature independent. The value of Sn- is ≈4·10-16 cm2 around 225°K and it increases with increase of temperature. The possible mechanisms for capture at neutral and repulsive centers are discussed and a summary of the capture cross-sections for cobalt centers is given. A comparison of the cross-section values of cobalt and their temperature variations with those of the related impurities-manganese, iron and nickel-in germanium has been made.

An inexpensive all glass sealed stirrer

An inexpensive all glass sealed stirrer may be constructed using a surgical syringe of about 10-ml capacity.

Seawater Durability of Epoxy/Vinyl Ester Reinforced with Glass/Carbon Composites

Seawater aging response was investigated in marine-grade glass/epoxy, glass/vinyl ester, carbon/epoxy and carbon/vinyl ester composites with respect to water uptake, interlaminar shear strength, flexural strength, tensile strength, and tensile fracture surface observations. The reduction of mechanical properties was found to be higher in them initial stages which showed saturation in the longer durations of seawater immersion. The flexural strength and ultimate tensile strength (UTS) dropped by about 35% and 27% for glass/epoxy, 22% and 15% for glass/vinyl ester, 48% and 34% for carbon/epoxy 28%, and 21% carbon/vinyl ester composites respectively. The water uptake behavior of epoxy-based composites was inferior to that of the vinyl system.

Potassium phosphate as a reagent for the gravimetric estimation of lithium

Previous attempts for the quantitative estimation of lithium as orthophosphate, employing an alkali metal phosphate, have not been successful. A method, is described for the estimation of lithium as trilithium phosphate from 60% ethyl alcohol solution at 65° to 70° C., employing potassium phosphate reagent, at pH 9.5. The method is applicable in the presence of varying amounts of sodium and/or potassium cations and chloride, sulfate, nitrate, and phosphate anions.

Investigation of activated transport in hole doped rare earth manganites in the high temperature paramagnetic regime

Electronic transport in the high temperature paramagnetic regime of the colossal magnetoresistive oxides, La(1-x)A(x)MnO(3), A=Ca, Sr, Ba, x similar or equal to 0.1-0.3, has been investigated using resistivity measurements. The main motivation for this work is to relook into the actual magnitude of the activation energy for transport in a number of manganites and study its variation as a function of hole doping (x), average A-site cation radius (< r(A)>), cationic disorder (sigma(2)) and strain (epsilon(zz)). We show that contrary to current practice, the description of a single activation energy in this phase is not entirely accurate. Our results clearly reveal a strong dependence of the activation energy on the hole doping as well as disorder. Comparing the results across different substituent species with different < r(A)> reveals the importance of sigma(2) as a metric to qualify any analysis based on (r(A)). (c) 2006 Elsevier Ltd. All rights reserved.

An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals

We report here a synthetic route for high-quality Mn-doped ZnSe nanocrystals using selenourea as a selenium source, avoiding the more conventional route-using tributylphosphine (TBP) that restricts the growth of spherical ZnSe nanocrystals below 5 nm in size, besides being highly toxic and pyrophoric. Spherical ZnSe nanocrystals with unprecendented sizes (up to 12 nm) are synthesized, the large size of the host helps to keep dopant ions well inside the nanocrystal leading to intense and stable dopant emission. Mn-doped ZnSe nanocrystals with more than 50% quantum yield (QY) are synthesized in this method and found to be stable both in aqueous and nonaqueous dispersions for months.

Intrinsic Electron localization in manganites

We mention here an unusual disorder effect in manganites, namely the ubiquitous hopping behavior for electron transport observed in them over a wide range of doping. We argue that the implied Anderson localization is intrinsic to manganites, because of the existence of polarons in them which are spatially localized, generally at random sites (unless there is polaron ordering). We have developed a microscopic two fluid lb model for manganites, where l denotes lattice site localized l polarons, and b denotes band electrons. Using this, and the self-consistent theory of localization, we show that the occupied b states are Anderson localized in a large range of doping due to the scattering of b electrons from l polarons. Numerical simulations which further include the effect of long range Coulomb interactions support this, as well the existence of a novel polaronic Coulomb glass. A consequence is the inevitable hopping behaviour for electron transport observed in doped insulating manganites.

Sol-Gel Synthesis, characterization and optical properties of Tio2 thin films deposited on ITO/Glass substrates

TiO2 thin films have been deposited on glass and indium tin oxide (ITO) coated glass substrates by sol-gel technique. the influence of annealing temperature on the structural , morphological and optical properties has been examined. X-ray diffraction (XRD) results reveal the amorphous nature of the as-deposited film whereas the annealed films are found to be in the crystalline anatase phase. The surface morphology of the films at different annealing temperatures has been examined by atomic force microscopy (AFM). The in situ surface morphology of the as-deposited and annealed TiO2 films has also been examined by optical polaromicrograph (OPM). TiO2 films infatuated different structural and surface features with variation of annealing temperature. The optical studies on these films suggest their possible usage in sun-shielding applications.

Structural and optical properties of (100-x)Li2B4O7 center dot x(Ba5Li2Ti2Nb8O30) glasses and glass nanocrystal composites

Optically clear glasses of various compositions in the system (100-x)Li2B4O7 center dot x(Ba5Li2Ti2Nb8O30) (5 <= x <= 20, in molar ratio) were fabricated by splat quenching technique. Controlled heat-treatment of the as-quenched glasses at 500 degrees C for 8 h yielded nanocrystallites embedded in the glass matrix. High Resolution Transmission Electron Microscopy (HRTEM) of these samples established the composition of the nano-crystallites to be that of Ba5Li2Ti2Nb8O30. B-11 NMR studies revealed the transformation of BO4 structural units into BO3 units owing to the increase in TiO6 and NbO6 structural units as the composition of Ba5Li2Ti2Nb8O30 increased in the glass. This, in turn, resulted in an increase in the density of the glasses. The influence of the nominal composition of the glasses and glass nanocrystal composites on optical band gap (E-opt), Urbach energy (Delta E), refractive index (n), molar refraction (R-m), optical polarizability (alpha(m)) and third order non-linear optical susceptibility (chi(3)) were studied.

Analysis of Dynamic Heterogeneity in a Glass Former from the Spatial Correlations of Mobility

The growth of characteristic length scales associated with dynamic heterogeneity in glass-forming liquids is investigated in an extensive computational study of a four-point, time-dependent structure factor defined from spatial correlations of mobility, for a model liquid for system sizes extending up to 351 232 particles, in constant-energy and constant-temperature ensembles. Our estimates for dynamic correlation lengths and susceptibilities are consistent with previous results from finite size scaling. We find scaling exponents that are inconsistent with predictions from inhomogeneous mode coupling theory and a recent simulation confirmation of these predictions.