Study and characterization of alpha alumina milli-billes synthesis and undoped and titanium (Ti3+) sapphire crystal growth

Ce travail est consacré à l’étude de la croissance cristalline du saphir non dopé et dopé titane (Ti3+). Pour faire croître des cristaux ayant des performances optimisées, il est indispensable de maitriser l’alumine (Al2O3) de départ. Toute contamination de la charge initiale à fondre, dégrade l’état stationnaire de croissance, entraîne la propagation de défauts cristallins, provoque la corrosion des creusets, en particulier en molybdène, et affecte les propriétés physico-chimiques, optiques et laser du cristal. Dans la première partie de ce travail, un nouveau procédé industriel de synthèse de millibilles d’alumine alpha (Al2O3-α) de géométrie sphérique, de haute pureté, a été mis au point. Le procédé est basé sur l’écoulement goutte à goutte de l’alumine fondue à travers des filières situées sous le creuset. Le procédé est particulièrement complexe, les études menées ont néanmoins visé à des descriptions qualitatives et quantitatives des phénomènes. La simulation et la modélisation de la thermique autour du creuset en iridium ont permis de maitriser le système et le contrôle continu du procédé. Dans le but de valider la qualité des millibilles d’alumine obtenues, des cristaux de saphir ont été tirés en utilisant des installations de croissance du LPCML (Czochralski, micro-pulling down) et de la société RSA le Rubis (EFG) qui ont des paramètres de fonctionnement différents (chauffage, atmosphère, creuset). La deuxième partie de la thèse, concerne la croissance cristalline du saphir dopé titane (Ti3+) qui est un matériau de très haute technologie. La demande actuelle porte sur des cristaux de 200 mm de diamètre, leur contrôle et caractérisation, le contrôle de la matière première, la mise en œuvre de différents procédés de croissance, le développement de mesures spécifiques des propriétés structurales, optiques et laser des cristaux. L’incorporation de l’ion Ti3+ dans la matrice du saphir se fait par substitution de l’ion Al3+. La différence de rayons ioniques rend difficile l’incorporation du Ti3+, conduisant à une concentration réelle en titane inférieure à celle souhaitée. Il s’ensuit une ségrégation du titane entre le bain fondu et le cristal lors de la croissance cristalline, ce qui provoque des hétérogénéités qui dégradent la qualité des cristaux. Dans cette partie de thèse, nous avons travaillé sur :1-La croissance stable en bain fondu de cristaux de Ti: saphir de 100 mm. 2-L’amélioration des procédés de croissance pour produire de manière reproductible des cristaux plus homogènes (concentration constante en ions titane), de bonne qualité optique (exempt de défauts) et offrant de bonnes performances laser. 3-La caractérisation des propriétés structuralesThis PhD thesis is established to study undoped and titanium (Ti3+) doped sapphire single crystal growth. To grow crystals with optimized performances, it is necessary to control starting Alumina (Al2O3). Any contamination of initial charge to be melted, will affect the stationary crystal growth, causes defects propagation in the crystal, crucibles corrosion, particularly Molybdenum and affect the physical-chemistry, optics and laser properties of the crystal. In the first part of this work, a new industrial process to synthesize (Al2O3-) alpha alumina millibilles of spherical geometry, high purity has been developed. The process is based on drop flow from shaped crucible. The process is complex, the study involve description of qualitative and quantitative phenomena during the synthesis stage. The thermal modeling and simulation around iridium crucible have allowed controlling the system and continued process. In order to validate the quality of the obtained Alumina (millibilles), sapphire crystals have been pulled by crystal growth equipments (Czochralski, micro—pulling down) available in LPCML laboratory and (EFG) in RSA Le Rubis company which have different operating parameters. The second part concern titanium (Ti3+) doped sapphire single crystal growth, which is material for high technology. Current demand is for controlled 200 mm diameter crystals, characterization, starting material control, adjustment of crystal growth process, development of specific measurement of structure properties, optics and lasers of the crystals. The incorporation of Ti3+ in sapphire host, is realised by the substitution of Al3+ ion. Because of the difference of the ionic radii, the incorporation of Ti3+ is difficult and the real titanium concentration will be lower than the wishes one. Segregation of titanium during crystal growth causes heterogeneities which damage the crystal quality. In this PhD part we have worked on: 1-Stable melt crystal growth of Ti-sapphire of 100 mm in diameter. 2- Improvement of crystal growth process to produce reproducible homogeneous crystals ( constant ions titanium concentration), good optical quality ( defects free) and good laser performances. 3- Characterisation of structural, optics and laser crystals propertie

Drop model for continuous milli-beads production

International audienceProduction of solid spheres from droplet solidification is gaining more and more interest, but needs a careful control of the drop diameter. The technique of dropping liquid alumina from a nozzle is used for processing the starting materials which are filling the crucibles in sapphire crystal growth technology. In the present work, experimental, theoretical and numerical analysis is carried out in order to describe the time evolution of the drop formation from a capillary tube. The main goal of this study is to develop a physical model able to explain the dynamics of drop formation, with application in drop weight method and production of small alumina balls. A new formula predicting the volume of falling drops is derived from the force balance at the instant of drop detachment. The theoretical predictions are compared to present numerical computations and experimental results of alumina milli-beads production. Based on this model, the crucible and capillary tube geometries and the process parameters can be designed in order to get continuous production of reproducible droplets for mass production

Revisiting the Luminescence Properties of Er3+, Tm3+ and Ho3+ Doped Bi4ge3o12 Rod-Type Crystal Fibers for the Laser Application

International audienc

Color Origins in Langatate Crystals

Langatate La3Ga5.5Ta0.5O14 is piezoelectric crystal from langasite family, commonly grown by Czochralski method from Ir crucible. Langatate crystals of different colors (colorless, orange, green) have been studied by optical spectroscopy in UV-Visible (200 – 800 nm) and IR (7000 – 1000 cm-1) ranges. Furthermore, the effects of irradiation by ultraviolet laser source (λ=266 nm) and post-growth annealing in N2+O2 atmosphere have been investigated. The yellow-orange is mainly due to an absorption centered in the ultraviolet that extends into the blue of the visible spectrum (250-500 nm). The IR optical absorption spectra of Langatate crystals exhibit an absorption band at 5370 cm-1. It seems linked to a point defect responsible for color. The intensity of the absorption band at 3430 cm-1 increases after annealing in oxygen containing atmosphere. We have discussed phenomenon that can occur simultaneously in langatate crystals and produce very similar colors which are related to structural defects. First, metal ions impurities (as Iron, Titanium…), whose presence is previously confirmed by femtosecond laser ablation coupled with ICP-MS spectroscopy, can contribute to langatate color. Second, ultraviolet absorption leads us to think about charge transfer phenomenon such as O2- → Fe3+ and/or Fe3+-Fe3+ pair transitions. Third, the irradiation by ultraviolet (λ=266 nm) laser source locally color the langatate sample by the creation of color centers. Origins of color centers, particularly those related to oxygen vacancies, ( , 2e′)x, are discussed. And, finally, point defects changes the band gap of langatate, leading to extend the absorption to visible light regions

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

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