Europium-Doped Tellurite Glasses: The Eu2+ Emission in Tellurite, Adjusting Eu2+ and Eu3+ Emissions toward White Light Emission

Europium-doped magnesium tellurite glasses were prepared using melt quenching techniques and attenuated total reflection (ATR) spectroscopy was used to study the glass structure. The glass transition temperature increased with increasing MgO content. Eu2+ and Eu3+ emissions were studied using photoluminescence spectroscopy (PL). The broad emission of Eu2+ ions centered at approximately 485 nm was found to decrease in intensity with increasing MgO content, while the Eu3+ emission was enhanced. The Eu3+ emission lay within the red orange range and its decay time was found to increase with increasing MgO content. Different excitation wavelengths were used to adjust Eu2+ to Eu3+ emissions to reach white light emission. The white light emission was obtained for the sample with the lowest MgO content under excitation in the near-UV range

Tuning bandit algorithms in stochastic environments

International audienceAlgorithms based on upper-confidence bounds for balancing exploration and exploitation are gaining popularity since they are easy to implement, efficient and effective. In this paper we consider a variant of the basic algorithm for the stochastic, multi-armed bandit problem that takes into account the empirical variance of the different arms. In earlier experimental works, such algorithms were found to outperform the competing algorithms. The purpose of this paper is to provide a theoretical explanation of these findings and provide theoretical guidelines for the tuning of the parameters of these algorithms. For this we analyze the expected regret and for the first time the concentration of the regret. The analysis of the expected regret shows that variance estimates can be especially advantageous when the payoffs of suboptimal arms have low variance. The risk analysis, rather unexpectedly, reveals that except for some very special bandit problems, the regret, for upper confidence bounds based algorithms with standard bias sequences, concentrates only at a polynomial rate. Hence, although these algorithms achieve logarithmic expected regret rates, they seem less attractive when the risk of suffering much worse than logarithmic regret is also taken into account

Devitrification Behavior of Sol-Gel Derived ZrO2-SiO2 Rare-Earth Doped Glasses: Correlation between Structural and Optical Properties

Optical and structural properties of glasses and glass-ceramics (GC) obtained by different heat-treatment of Tb and Tb-Yb doped sol-gel derived 30ZrO2-70SiO2 materials were investigated. A glass was formed after treatment at 700 °C whereas devitrification of the media after the treatment at 1000 and 1100 °C, led to the formation of GC containing up to three different crystalline phases, namely, tetragonal ZrO2, Yb-disilicate and cristobalite. The modification of the optical properties through the heat treatment was caused by redistribution of the rare earth elements (REE) among the different phases: both Tb and Yb entered the t-ZrO2 lattice, Yb can also be present in the form of a Yb2Si2O7 crystal. Devitrification led to an increase in Tb→Yb energy transfer efficiency as compared to the glass, though it was higher in the samples heat-treated at 1000 °C than in those treated at 1100 °C. The most intensive Yb3+ luminescence, induced by the energy transfer from the Tb3+ ion, was observed at the interface between t-ZrO2 and the glassy phases, due to the high concentration of REE in this area caused by the inability of ZrO2 to accept larger amounts of the REE. The mechanisms of the Tb→Yb energy transfer vary between different phases of the GC. The results obtained in this study are important for the development of spectral down-converters for potential solar energy applications based on Tb-Yb co-doped glass-ceramics

Coextrusion of Reaction‐Bonded Carbides by Robocasting

Coextrusion by robocasting is a suitable process for fabricating multimaterial ceramic structures. Herein, the robocasting process is used to fabricate core–shell structures, combined with subsequent liquid silicon infiltration (LSI). Thus, reaction‐bonded silicon carbide (RBSC), reaction‐bonded boron carbide (RBBC), and reaction‐bonded silicon–boron carbide composites are produced. The LSI process offers the possibility to circumvent high temperatures and pressures used in traditional fabrication. Pastes with high solid loading and necessary carbon content are used in order to combine the robocasting with the subsequent LSI process. The influence of the paste rheology on the sample fabrication of multimaterial core–shell structures of reaction‐bonded carbides is investigated. The key rheological data, such as the viscosities of the combined pastes, are correlated with the observations from the microstructural investigation using scanning electron microscopy. A correlation between the difference in viscosity and the core geometry can be established. Crack formation in the material combination of RBSC and RBBC is found and compared with layered multimaterial structures of reaction‐bonded carbides. Residual stresses, which can be used to explain the crack formation, are investigated using Raman spectroscopy.For the first time, the fabrication of core–shell structures from reaction‐bonded carbides is demonstrated by a combination of additive manufacturing and postinfiltration with silicon. Various structures are printed by robocasting and then successfully infiltrated. In some cases, different crack patterns are observed due to the formation of residual microstresses in each material. These stresses are investigated by Raman spectroscopy. image © 2023 WILEY‐VCH GmbH Projektträger Jülich (PtJ, Germany)Deutsche Forschungsgemeinschaf

Utilizing Rare-Earth-Elements Luminescence and Vibrational-Spectroscopies to Follow High Pressure Densification of Soda−Lime Glass

A new series of soda–lime glass naturally doped with Nd and doped with 0.2 wt% of Eu2O3 was densified in a multi-anvil press up to 21 GPa. The densities of the millimetric samples were precisely measured using a floatation method in a heavy liquid made with sodium polytungstate. The obtained densification curve is significantly different from the calibration previously reported, reaching a maximum densification saturation of 3.55 ± 0.14%. This difference could be due to better hydrostatic conditions realized in this new study. The densified samples were characterized using Raman and Brillouin spectroscopy, as well as the emission of both Eu3+ and Nd3+. The evolution of the spectra was evaluated using integration methods to reduce error bars. The relative precision of the calibration curves is discussed. The evolution of Nd3+ transition was found to be the most sensitive calibration. Linear dependence with the density was found for all observables, with exception for Brillouin spectroscopy showing a divergent behavior. The Brillouin shift shows an unreported minimum for a densification ~0.4%

Tailoring the Mechanical Properties of Metaluminous Aluminosilicate Glasses by Phosphate Incorporation

The characterization of aluminosilicate glasses is highly relevant in geosciences and for engineering applications such as reinforcement fibers or touchscreen covers. The incorporation of phosphate as a third network-forming species into these glasses offers unique opportunities for fine-tuning glass properties via changes in glass structure and polymerization. In this work, we studied melt-quenched aluminosilicate glasses within the system SiO2-Al2O3-Na2O-P2O5 with 50–70 mol% SiO2 and up to 7.5 mol% P2O5. All glasses were metaluminous (Al:Na = 1) in order to maximize the degree of polymerization. Increasing the phosphate content at the expense of NaAlO2 led to reduced glass polymerization and density, resulting in a decrease in elastic moduli and hardness and an increase in strain-rate sensitivity. When increasing the silica content by substituting SiO4 for AlO4 tetrahedra, network polymerization remained mostly unchanged, as confirmed by nearly constant hardness. Densification upon indentation was analyzed by Raman spectroscopy and finite element analysis. We find that the elastic properties and hardness of metaluminous phospho-aluminosilicate glasses are governed by changes in density and network polymerization. Other mechanical properties underlie more complex changes in glass structure

Electronic and Structural Properties of a 4d-Perovskite: Cubic Phase of SrZrO3_3

First-principles density functional calculations are performed within the local density approximation to study the electronic properties of SrZrO$_3$, an insulating 4d-perovskite, in its high-temperature cubic phase, above 1400 K, as well as the generic 3d-perovskite SrTiO$_3$, which is also a d^0-insulator and cubic above 105 K, for comparison reasons. The energy bands, density of states and charge density distributions are obtained and a detailed comparison between their band structures is presented. The results are discussed also in terms of the existing data in the literature for both oxides.Comment: 5 pages, 2 figure