Optical Materials 69 (2017) 73e80 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Interface effect on titanium distribution during Ti-doped sapphire crystals grown by the Kyropoulos method * Carmen Stelian a, c, Guillaume Alombert-Goget b, , Gourav Sen a, Nicolas Barthalay c, Kheirreddine Lebbou b, Thierry Duffar a a University Grenoble-Alpes, CNAS, SIMAP-EPM, 1340 Rue de la Piscine, BP 75, F-38402, Saint Martin d’Heres, France b Univ Lyon, Universite Claude Bernard Lyon 1, CNRS, Institut Lumiere Matiere, F-69622, Villeurbanne, France c Le Rubis SA, BP 16, 38560, Jarrie, Grenoble, France article info abstract Article history: Large ingot Ti-doped sapphire crystals were successfully grown by Kyropoulos method. Optical charac- Received 16 January 2017 terization of 10 cm diameter crystals shows non-uniform radial distribution of titanium. The measure- þ Received in revised form ments of Ti3 ion distribution in several slices cut perpendicular to the growth direction show that the 5 April 2017 concentration is higher at the periphery of the crystal as compared to the central part of the ingot. Accepted 6 April 2017 Numerical modeling is applied to investigate heat transfer, melt convection and species transport during the Kyropoulos growth process. The transient simulation shows an unsteady convection generated by the strong interaction between the flow and the thermal field. The distribution of Ti in the melt is nearly Keywords: Ti-doped sapphire uniform due to the intense convective mixing. The radial distribution of titanium depends mainly on the fi Solute segregation shape of the crystal-melt interface. The measured U-shaped concentration pro le is explained by ac- Luminescence counting the conical shape of the interface. High curvatures of the growth interface observed in ex- Numerical simulation periments are explained by the increased thermal conductivity of the sapphire crystal, which is Kyropoulos method participating to the radiative heat transfer. The interface curvature depends on the absorption coefficient, which is higher for Ti-doped sapphire crystals than for undoped crystals. Therefore, the initial Ti con- centration in the melt should be decreased in order to reduce the absorption coefficient and the interface curvature. The comparison of numerically computed titanium distribution in the crystals to experimental measurements shows a good agreement. © 2017 Elsevier B.V. All rights reserved. 1. Introduction crystals has been experimentally observed [6]. Since the absorption coefficient of the crystal is strongly dependent on Ti concentration, During the last decade, titanium doped sapphire has become the the input laser beam is non-uniformly absorbed in the transversal reference crystal for the development of ultrashort laser systems cross-section, giving rise to variations in the intensity of emitted producing very intense and short pulses up to petawatt level [1,2]. laser beam. The objective of the present work is to use numerical Various crystal growth methods are presently used to grow modeling and experiments in order to investigate the factors large size sapphire crystals: Czochralski [3] (Cz), heat exchanger affecting the distribution of titanium in sapphire crystals grown by method [4] (HEM), controlled heat extraction system [5] (CHES) the Kyropoulos method. and Kyropoulos [6,7] (Ky). Large size Ti doped sapphire boules have Several papers have been dedicated to numerical simulation of been successfully grown by using the Kyropoulos technique [6,7]. sapphire Kyropoulos growth [9e16]. Existing models are still The Kyropoulos furnace is designed to achieve low temperature limited to steady-state analysis of the thermal field and melt con- gradients and small growth rates. The ingots grown in these con- vection at different stages of the crystallization process. In our ditions have a good structural quality with low dislocation density previous work [16], we performed transient computations which [8]. Some non-uniformity in the radial titanium distribution in the use a deformed mesh, in order to investigate the factors affecting the shape of the crystal-melt interface during the growth process. The analysis is extended in the present work to simulate the * Corresponding author. coupling between heat transfer, melt convection and species E-mail address: [email protected] (G. Alombert-Goget). http://dx.doi.org/10.1016/j.optmat.2017.04.020 0925-3467/© 2017 Elsevier B.V. All rights reserved. 74 C. Stelian et al. / Optical Materials 69 (2017) 73e80 transport during Kyropoulos growth of Ti-doped sapphire crystals. around 1 mm. The polarization of the laser beam is linear and set Numerical results are compared to experimental measurements of parallel to the c-axis of the crystal (p polarization) by a Glan-Taylor titanium distribution in several slices cut perpendicular to the polarizer. Sample positioning (horizontal and vertical) is performed growth direction. by linear translation stages with micrometric precision. A detailed description of the optical characterization of Ti-doped sapphire 2. Experimental procedure crystals is provided in previous experimental works [6,17]. Fig. 3 shows the radial profiles of Ti3þ concentration measured The Kyropoulos method is very similar to a Czochralski growth in three slices cut from the top, middle and bottom parts of a process. In this method, the seed is introduced in the melted sapphire ingot grown in a crucible of 15 cm in diameter. The alumina contained in a crucible, then the power in the furnace is recorded luminescence profiles in the top and middle samples are þ decreased (see Fig. 1). After seeding, the crystal growth inside the U-shaped, featuring higher Ti3 concentration at the periphery of melt is promoted by the slow decrease of the temperature. the crystal as compared to the central part of the ingot. These U- Titanium doped sapphire crystals of various dimensions shaped profiles will be investigated in the following paragraphs. (5e33 kg) have been grown. Several experiments were carried out The profile measured at the bottom of the ingot is W-shaped, with in order to visualize the shape of the crystal-melt interface in the higher concentrations at the center of the sample. This behaviour case of ingots grown in crucibles of 15cm in diameter (see Fig. 2). In could be explained by different growth conditions at the end of the these experiments, the crystallization process was interrupted by crystallization process, when the interface is less curved and the quickly pulling out the crystal from the melt. The growth time for convective intensity decreases, as shown in other numerical works the full crystallization process is t ¼ 150h. Fig. 2 shows the photos [11,14]. The last stage of the growth process was not simulated in of two sapphire ingots obtained by stopping the growth process at the present work. times t ¼ 67h and t ¼ 71h. The crystal-melt interface has a convex- concave shape with a large curvature. The dimensionless interface deflection t ¼ f =rc (f - interface deflection and rc- crystal radius) 3. Numerical modeling increases during the crystallization process from t ¼ 9cm=4:5cm ¼ 2(Fig. 2a) to t ¼ 11cm=5cm ¼ 2:2(Fig. 2b). 3.1. Model description Qualitative measurements of Ti3þ ion concentration in several slices cut perpendicular to the growth direction have been per- Numerical modeling is performed by using the finite element formed by microluminescence analysis and transmittance mea- software COMSOL Multiphysics. The time-dependent equations surements. The laser excitation used for the luminescence analysis governing the heat transfer, melt convection and species transport was performed with an Ekspla NT342 optical parametric oscillator are: (OPO) pumped by a pulsed frequency-tripled Nd:YAG laser. The luminescence of Ti3þ was collected and transmitted via an optical vT ! rc þ u VT ¼ kV2T þ Q (1) fiber to an ANDOR SHAMROCK SR303i monochromator equipped P vt R with a grating blazed at 500 nm, with a resolution of 0.2 nm using ! 1200 lines/mm, and analyzed with an iSTAR CCD. The absorption v u ! ! 2! ! coefficient profiles are obtained after transmittance measurements r þðu VÞ u ¼Vp þ mV u þ r g ½1 À b ðT À T Þ (2) vt T 0 by the propagation of laser diode emission at 532 nm through the sample. The laser beam diameter on the surface of the sample is Fig. 1. Sketch of the Kyropoulos growth process. Download English Version: https://daneshyari.com/en/article/5442605 Download Persian Version: https://daneshyari.com/article/5442605 Daneshyari.com.