Emission decay and energy transfer in Yb/Tm Y-codoped fibers based on nano-modified glass

We report the results of an experimental investigation and theoretical analysis of luminescence decay in Yb/Tm Y-codoped fibers based on nano-modified glass. Based on the experimental results, numerical simulations allowed us to estimate the energy transfer efficiency between Yb3+ and Tm3+ ions. It was shown that yttria enhances the Yb/Tm energy transfer making fibers with Y-codoping a promising candidate for the development of light sources for laser applications and up-conversion emitters for visualization applications. These fibers demonstrate energy transfer efficiency of ∼50%, which makes them attractive for diode-pumping of Yb-ions at a wavelength of 975 nm

Evaluation of the performance of high phosphorous with germanium codoped multimode optical fiber for use as a radiation sensor at low dose rates

We propose a GeO(2)-P(2)O(5)-codoped step index multimode (SIMM) fiber having a core diameter of around 50 mu m with numerical aperture of around 0.21-0.22. The proposed SIMM fiber shows excellent linear radiation response behavior with sensitivity of around 0.69-0.97 dB/m/100 rad at a 505 nm wavelength within the dose rate range of 10-100 rad/h, as well as very low recovery at room temperature using a (60)Co gamma radiation source. This enables its practical application in fiber optic personal dosimeters for measurement of low dose gamma radiation. (C) 2011 Optical Society of Americ

Electrochemical performance of mixed crystallographic phase nanotubes and nanosheets of titania and titania–carbon/silver composites for lithium-ion batteries

The role of homogeneity in ex situ grown conductive coatings and dimensionality in the lithium storage properties of TiO₂ is discussed here. TiO₂ nanotube and nanosheet comprising of mixed crystallographic phases of anatase and TiO₂ (B) have been synthesized by an optimized hydrothermal method. Surface modifications of TiO₂ nanotube are realized via coating the nanotube with Ag nanoparticles and amorphous carbon. The first discharge cycle capacity (at current rate = 10 mA g⁻¹) for TiO₂ nanotube and nanosheet were 355 mAh g⁻¹ and 225 mAh g⁻¹, respectively. The conductive surface coating stabilized the titania crystallographic structure during lithium insertion–deinsertion processes via reduction in the accessibility of lithium ions to the trapping sites. The irreversible capacity is beneficially minimized from 110 mAh g⁻¹ for TiO₂ nanotubes to 96 mAh g⁻¹ and 57 mAh g⁻¹ respectively for Ag and carbon modified TiO₂ nanotubes. The homogeneously coated amorphous carbon over TiO₂ renders better lithium battery performance than randomly distributed Ag nanoparticles coated TiO₂ due to efficient hopping of electrons

Oxide particle surface chemistry and ion transport in “soggy sand” electrolytes

The crucial role of oxide surface chemical composition on ion transport in “soggy sand” electrolytes is discussed in a systematic manner. A prototype soggy sand electrolytic system comprising aerosil silica functionalized with various hydrophilic and hydrophobic moieties dispersed in lithium perchlorate−ethylene glycol solution was used for the study. Detailed rheology studies show that the attractive particle network in the case of the composite with unmodified aerosil silica (with surface silanol groups) is most favorable for percolation in ionic conductivity, as well as rendering the composite with beneficial elastic mechanical properties. Though weaker in strength compared to the composite with unmodified aerosil particles, attractive particle networks are also observed in composites of aerosil particles with surfaces partially substituted with hydrophobic groups. The percolation in ionic conductivity is, however, dependent on the size of the hydrophobic moiety. No spanning attractive particle network was formed for aerosil particles with surfaces modified with stronger hydrophilic groups (than silanol), and as a result, no percolation in ionic conductivity was observed. The composite with hydrophilic particles was a sol, contrary to gels obtained in the case of unmodified aerosil, and partially substituted with hydrophobic groups

High Lithium Storage in Mixed Crystallographic Phase Nanotubes of Titania and Carbon-Titania

Morphology and electrochemical performance of mixed crystallographic phase titania nanotubes for prospective application as anode in rechargeable lithium ion batteries are discussed. Hydrothermally grown nanotubes of titania (TiO2) and carbon-titania (C-TiO2) comprise a mixture of both anatase and TiO2 (B) crystallographic phases. The first cycle capacity (at Current rate = 10 mAg(-1)) for bare TiO2 nanotubes was 355 mAhg(-1) (approximately 1.06 Li), which is higher than both the theoretical capacity (335 mAhg(-1)) and the reported values for pure anatase and TiO2 (B) nanotubes. Higher capacity is attributed to it combination of the presence of mixed crystallographic phases of titania and trivial size effects. The surface area of bare TiO2 nanotubes was very high at 340 m(2) g(-1). C-TiO2 nanotubes showed a slightly lower first-cycle specific capacity of 307 mAhg(-1), but the irreversible capacity loss in the first cycle decreased by half compared to bare TiO2 nanotubes. The C-TiO2 nanotubes also showed a better rate capability, that is, higher capacities compared to bare TiO2 nanotubes in the Current range 0.1-2 Ag-1. Enhanced rate capability in the case of C-TiO2 is attributed to the efficient percolation of electrons as well its to the decrease in the anatase phase

Influence of oxide particle network morphology on ion solvation and transport in “soggy sand” electrolytes

The role of oxide surface chemical composition and solvent on ion solvation and ion transport of “soggy sand” electrolytes are discussed here. A “soggy sand” electrolyte system comprising dispersions of hydrophilic/hydrophobic functionalized aerosil silica in lithium perchlorate−methoxy polyethylene glycol solution was employed for the study. Static and dynamic rheology measurements show formation of an attractive particle network in the case of the composite with unmodified aerosil silica (i.e., with surface silanol groups) as well as composites with hydrophobic alkane groups. While particle network in the composite with hydrophilic aerosil silica (unmodified) were due to hydrogen bonding, hydrophobic aerosil silica particles were held together via van der Waals forces. The network strength in the latter case (i.e., for hydrophobic composites) were weaker compared with the composite with unmodified aerosil silica. Both unmodified silica as well as hydrophobic silica composites displayed solid-like mechanical strength. No enhancement in ionic conductivity compared to the liquid electrolyte was observed in the case of the unmodified silica. This was attributed to the existence of a very strong particle network, which led to the “expulsion” of all conducting entities from the interfacial region between adjacent particles. The ionic conductivity for composites with hydrophobic aerosil particles displayed ionic conductivity dependent on the size of the hydrophobic chemical moiety. No spanning attractive particle network was observed for aerosil particles with surfaces modified with stronger hydrophilic groups (than silanol). The composite resembled a sol, and no percolation in ionic conductivity was observed

Influence of mesoporosity and carbon electronic wiring on electrochemical performance of anatase titania

The combined benefits of mesoporosity and in situ grown conductive amorphous carbon on lithium storage capacity of anatase TiO2 are discussed here. Micron-sized anatase TiO2 spheres with equal amount of carbon but different degree of mesoporosity and normal mesoporous TiO2 are synthesized by solvothermal and sol-gel methods respectively. While the specific surface areas were 80, 35 and 85 m2 g-1 respectively for mesoporous carbon-TiO2 sphere, carbon-TiO2 sphere and mesoporous TiO2, the primary size of the grains constituting the micrometer sized spheres was ∼11 nm. The electrochemical lithium stored by all synthesized TiO2 materials show strong dependency on mesoporosity and carbon. Mesoporous carbon-TiO2 sphere exhibited a first discharge cycle capacity of 334 mAh g-1 at current rate of 66 mAg-1, whereas carbon-TiO2 sphere and mesoporous TiO2 stored 120 and 270 mAh g-1 respectively. Demonstration of high lithium storage capacity and good cyclability in mesoporous carbon-TiO2 sphere is attributed to the synergy of mesoporosity and in situ grown carbon in the formation of an effective percolation network for the conducting species (Li+/e−) around TiO2 nanoparticles