Al2O3 shaped crystals grown under stationary stable state and optical characterizations

Nehari³

et al. 2012

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Bubbles defects have been always observed in sapphire crystals. Their distribution and size are strongly dependent on the growth conditions. We have studied the effect of the pulling rate on the bubbles' size and their distribution in sapphire rods grown by the micropulling down (μ-PD) technique. Using a central circular capillary die of shape factor 0.33, the bubbles' diameter decreased as a function of the pulling rate. The transmission decreased at a pulling rate greater than 1 mm/min.

Section: Introductionmentioning

confidence: 99%

Effect of Pulling Rate on Bubbles Distribution in Sapphire Crystals Grown by the Micropulling Down (μ-PD) Technique

Nehari³

et al. 2012

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“…The chemical simplicity (Al 2 O 3 ) and congruent melt behavior allow for growing bulk single crystals by several different growth methods . Nowadays, it is possible to grow bulk crystals of diameter higher than 100 mm. − Shaped crystals such as plates, rods, tubes, and complex geometries as well as thin fibers of diameter less than 300 μm − can be obtained by the edge-defined film-fed growth (EFG) and the micropulling-down (μ-PD) techniques. − Regardless of the growth process, some defects are very often observed in the grown crystals. In spite of the improvement of crystal growth technology and the deep knowledge of the physical and physicochemical properties of this simple oxide, it is presently difficult to grow defect-free sapphire crystals.…”

Section: Introductionmentioning

confidence: 99%

Observation of Gas Bubble Incorporation during Micropulling-Down Growth of Sapphire

Nehari²,

Lebbou

et al. 2012

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Quite often sapphire shaped crystals contain specific defects called bubbles of average diameter higher than 100 μm or microbubbles of diameter smaller than 10 μm. These defects strongly affect the crystal properties. Bubbles of 100 μm in diameter have been observed in the molten zone during micropulling-down of sapphire fibers. Before their incorporation inside the crystal, they show a periodic oscillation and consequently deform the crystallization interface. These observations are discussed with reference to the available literature.

“…9,10 The sapphire seed crystal is connected to the melt at the bottom of the Ir capillary die, and the heating power is adjusted until a stable meniscus is formed. 11 Then the molten zone volume and meniscus length are controlled by careful adjustment of the heater power.…”

Section: Methodsmentioning

confidence: 99%

Chemical Segregation of Titanium in Sapphire Single Crystals Grown by Micro-Pulling-Down Technique: Analytical Model and Experiments

Nehari

Duffar

et al. 2014

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International audienceThe micro-pulling-down (mu-PD) process consists in pulling a crystal under a capillary channel placed at the bottom of a crucible. Despite it being limited to rather small liquid volumes, it is used to grow single crystal fibers and shaped crystals of various cross sections, mainly applied industrially for optical applications, such as lasers, optics, or scintillators. Consequently, those crystals should be doped with active elements to fit the target application. Unfortunately, whatever the growth parameters and the dopant type, quite often segregation problems are observed. It is generally believed that chemical partition in mu-PD technique is restricted to the first grown millimeters, but some experiments show that it is not always the case. An analytical one-dimensional model is presented, aiming to predict the longitudinal segregation along the growth direction. It is shown that it depends in practice on growth parameters such as capillary length, meniscus height, capillary section, and pulling rate. The characteristic numbers controlling the segregation profile are derived and a parametric study is performed in the case of Ti-doped sapphire single crystal fibers. Ti3+:Al2O3 single crystal fibers oriented along c-axis have been grown under stationary stable growth regime using different pulling rates and the longitudinal chemical segregation has been characterized by photoluminescence. Results are in agreement with the model predictions