7 research outputs found

    Etude expérimentale d'une flamme de diffusion oxygÚne-hydrogÚne ensemencée en particules solides d'alumine

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    Le processus industriel auquel nous nous sommes intéressés permet, par le transit de particules d'alumine (AL203) dans une flamme, la génération de microsphÚres de saphir. La flamme de diffusion confinée O2/H2 est dirigée verticalement vers le bas et le jet central d'oxygÚne est ensemencé. Des études expérimentale (ADL, PIV) et numérique (N3S-Natur) ont été réalisées. (1) L'analyse du comportement a montré que la nature laminaire était conservée en écoulement réactif. Seuls les gradients de vitesse auxquels sont soumises les particules génÚreront des collisions. (2) Deux trajectoires caractéristiques, possédant chacune un temps de passage différent dans la zone de haute température, ont été mises en évidence. (3) L'analyse des mesures PIV a permis de montrer que la densité des particules était inhomogÚne. (4) Contrairement à l'écoulement d'hydrogÚne, une variation de débit sur le jet central d'oxygÚne va causer une modification des vitesses de la flamme ainsi que de sa thermique.Monocristalline sapphire microspheres are generated through the melting of alumine (AL203) particles in a flamme. The alumina particles are injected in a very peculiar 02/H2 confinened diffusion flame as it is a downwards vertical flame having fuel in periphery of a central powdered oxygen jet. Quantitative measurements were carried out (ADL, PIV) and supplemented by a numerical study (N3S-Natur). (1) The laminar behavior of the isothermal conditions is kept through reactive flow. Therefore, particles will mainly collide due to spedd gradients. (2) It has been shown that an axial particle will have a transit time int the high temperature zone very different to that of an off-line one. (3) The PIV date proved that the particle density was not homogeneous. (4) The hydrogen jet hardly influences the flame aerodynamic structure. Conversly, the central oxygen jet is at premium due to its effect on both the flame speed and temperature distribution.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

    Numerical Analysis of Thermal Stress in Semi-Transparent Oxide Crystals Grown by Czochralski and EFG Methods

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    International audienceCrystal growth of oxides is generally difficult since large curvatures of the growth interface in these systems generate high thermal stress, dislocations and crystal cracking. Three‐dimensional numerical modeling is applied to investigate thermal stress distribution in sapphire and langatate La3Ta0.5Ga5.5O14 (LGT) semi‐transparent crystals grown by Czochralski (Cz) and Edge‐defined Film‐fed Growth (EFG) techniques. The analysis of thermal stress distribution in a sapphire ingot grown in a Czochralski furnace shows high von Mises stresses distributed almost symmetrically on large areas in the crystal. Thermal stress computations for piezoelectric langatate crystals grown in a Czochralski configuration show non‐symmetrical von Mises distribution with higher stress on one side of the ingot. These numerical results are in agreement with experimental results showing non‐symmetrical cracking at the outer surface of the crystal. 3D modeling of multi‐die EFG growth of white sapphire ribbons shows that the von Mises stress is almost constant when the number of ribbons is increased from two to ten. Two models are applied to simulate the internal radiative heat transfer in the sapphire crystals: P1 approximation and the Rosseland radiation model. Numerical results show that applying Rosseland formula introduces significant errors in temperature field calculations especially in the case of the EFG configuration

    Influence of growth process and crystal defects on sapphire brittleness

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    International audienceBrittleness of sapphire has been studied by four point bending and ball on three ball tests. A significantly largerflexural stress was observed for crystals grown by the Verneuil process compared to EFG (Edge-defined Film-fedGrowth) crystals. Crystallographic defects were characterized by X-ray topography and it was shown that sub­grain boundaries found in Verneuil crystals do not impact the fracture behavior. Larger amounts of basal dis­locations in Verneuil boules appeared to account for the differences in brittleness. The dislocation densities havebeen related to the temperature fields experienced by the crystals during the growth processes. Characterizationof point defects by thermo-stimulated luminescence revealed that they are more numerous in Verneuil crystals,especially after annealing, and that they pin dislocations, contributing to the mechanical response of the crystal

    Ti-Doped Sapphire (Al(2)O(3)) Single Crystals Grown by the Kyropoulos Technique and Optical Characterizations.

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    International audienceTransparent high optical quality and large Ti-sapphire (Ti(3+)-doped Al(2)O(3)) single crystals have been grown by the Kyropoulos technique (KT) for optical amplification. The present work shows that by the utilization of KT growth technology and the optimization of the growth conditions it is possible to grow Ti-doped Al(2)O(3), 100 mm in diameter and 5 kg in weight. We have demonstrated that large Ti(0.25 atom %)-doped Al(2)O(3) crystals show high chemical homogeneities and good optical properties and amplify the energy without any special annealing. Ti-doped sapphire crystals are for high power laser applications and particularly for the shortest pulses ever produced from a laser oscillator

    New and forthcoming reference books from Gale Research company

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