Effect of Pulling Rate on Bubbles Distribution in Sapphire Crystals

Article pubs.acs.org/crystal

Effect of Pulling Rate on Bubbles Distribution in Sapphire Crystals Grown by the Micropulling Down (μ-PD) Technique E. A. Ghezal,† H. Li,† A. Nehari,‡ G. Alombert-Goget,† A. Brenier,† K. Lebbou,*,† M. F. Joubert,† and M. T. Soltani# †

Université de Lyon, Université Lyon 1, CNRS, UMR5620, Laboratoire de Physico-Chimie des Matériaux Luminescents, F-69622 Villeurbanne Cedex, France ‡ RSA le rubis SA, BP 16, 38560 Jarrie/Grenoble, France # Laboratoire de Chimie Appliquée, Equipe: Verres et Matériaux Photoniques, Université Mohamed Khider de Biskra, Biskra, Algeria ABSTRACT: 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.

I. INTRODUCTION Because of their excellent properties, sapphire crystals (αAl2O3) remain a strategic material for a large range of civil and military applications. They have a high refractive index, transmit light from the ultraviolet to 3−5 μm infrared region, and have very good thermal conductivity, tensile strength, and thermal shock resistance.1−4 Because of their chemical stability and resistance to corrosion at high temperature, sapphire crystals are applied as an envelope in high pressure gas-discharge lamps that use a discharge in alkali-metal vapor.5 Sapphire crystals doped by Ti3+ and Cr3+ are excellent laser materials.6−8 They are used as substrate materials in blue, green, ultraviolet, and white light-emitting diodes (LED).9 Chemical simplicity and congruent melting allowed sapphire crystal growth by different technologies including Czochralski (Cz), Kyropoulos (KY), heat exchanger method (HEM), edge defined film fed growth (EFG), and micropulling down (μ-PD).10−14 In spite of sapphire properties and the progress in crystal growth technology, it is difficult to shape sapphire because of its high hardness and crystallographic anisotropy related to its hexagonal crystal structure. Therefore, the optimization of controlled crystal growth process and development of new technologies are necessary in order to increase the crystal sizes (production yield), improve the quality, and reduce the cost (crystals and alumina raw materials). Regarding shaped sapphire crystals grown from the melt, bubbles are one of the major defects.15−17 One of the reasons for bubble formation is the thermal dissociation of the liquid allowing liberation of diluted gases. This problem is not yet resolved and not completely understood. The main aim of this paper is to study the bubbles' propagation in shaped sapphire rod crystals grown by the micropulling down (μ-PD) method © 2012 American Chemical Society

as a function of the pulling rate. The bubbles' dimensions, behaviors, and direct effect on the optical properties are discussed here in some detail. In addition, growth phenomena observed in sapphire crystal grown by the μ-PD technique and the crystals' characterization are reported.

II. EXPERIMENTAL PROCEDURES Sapphire-shaped crystals have been grown by the μ-PD technique.18 The initial raw material was alumina (α-Al2O3) powder of spherical geometry, micrometer-scale dimensions, and the structure corresponding to JSPDS File No. 46-1212. A radio frequency (RF) heated μ-PD system has been used to pull the crystals. The alumina powder was melted in an iridium crucible, and thereafter the melt was pulled down continuously through a capillary channel made at the bottom of the crucible. The die used had cylindrical geometry of 3 mm in diameter. The liquid flowed through the channel of 1 mm in diameter. The diameter (ϕ) and the height (h) of the capillary die were strongly dependent on the liquid proprieties following the Jurin law (h = (4σ)/ (ϕρg)), where σ is superficial tension, ρ is liquid density, and g is gravitational acceleration. Iridium after heater and alumina ceramic insulation installed around the hot zone have been used to control thermal gradients and to visualize the melt meniscus and the growing crystal with a CCD camera. Table 1 summarizes the experimental parameters used in this study. The crystals were grown in argon atmosphere (2 L/min) to protect the crucible from oxidation and damage. The temperature distribution in the melt and the capillary die was measured by an Ircon pyrometer. Seed of orientation ⟨0001⟩ has been dipped into the melt at the bottom of the crucible in the capillary channel, and the melt temperature was adjusted to obtain the desired Received: May 1, 2012 Revised: June 18, 2012 Published: June 21, 2012 4098

dx.doi.org/10.1021/cg300589h | Cryst. Growth Des. 2012, 12, 4098−4103

Crystal Growth & Design

Article

Table 1 properties

parameters

alumina powder (α)

JSPDS File No. 46-1212 (a = 4.7570 Å, c = 12.9898 Å) melting temperature (°C) 2050 ± 10 outer radius of the die (mm) 1.5 capillary channel radius 0.5 (mm) shape factor (Rhole/ 0.33 Rcapillary die) meniscus length (h) (μm) 60−1000 (depending on pulling rate) pulling rate (mm/min) 0.25−3 solid density (g/cm3) 3.97 melt density (g/cm3) 3.06 Reynolds (Recap) capillary Recap = (ρRcap)Vcap/μ die Reynolds (Remen) meniscus Remen = (ρhmeniscus)Vh/μ μ dynamic viscosity (kg m−1 3 × 10−2 s−1)

meniscus shape. The seed was pulled down continuously with a pulling rate ranging from 0.25 to 3 mm/min.

III. RESULTS III-1. Crystal Growth. Different pulling rates have been used, and their effect on the crystal quality and meniscus length has been inspected. In all growths, 100% of the melt was solidified into the rods. The meniscus shape and length as well as the grown sapphire rod diameter were affected by the melt high and the pulling rate. The shape observed by the CCD camera was symmetric, but for low pulling rate (v ≤ 1 mm/ min), the wetting line at the melt die junction was not clearly detected which made it difficult to resolve the CCD image. Figure 1 shows the relationship between the meniscus length and the pulling rate. With increasing the pulling rate, the meniscus length also increased due to greater release of the heat of solidification that resulted in smaller rod diameter. According to Figure 1, the measured meniscus height varied between 100 μm ± 10 (v = 0.5 mm/min) and 1000 μm ± 10 μm (v = 2.5 mm/min). When the solid rod started to grow, the solidification interface was fed with the melt passed through the central channel of the capillary die. Behavior of the capillary die is characterized by the Reynolds numbers (Recap) in the capillary channel and Reynolds number in the meniscus (Remen). The flow Reynolds number ranged from 10.6 × 10−12 to 1.02 × 10−6 depending on the pulling rate. Because of the low fluid speed and the low growth rate, the liquid flow was characterized by a low Reynolds number giving a laminar liquid flow. Figure 2 shows the as-grown rods as a function of pulling rate. The crystals were transparent and colorless. No modification of the melt composition resulting in a secondphase formation was detected during the growth experiments. The best rod cross-section uniformity was obtained in crystal grown with the diameter close to the diameter of the capillary die at low pulling rate (V ≤ 1.5 mm/min). The length of the crystals was up to 500 mm depending on the quantity of the starting raw materials. The best reproducibility of the growth results was found at a pulling rate ≤ 1 mm/min. The transparency of the crystals slightly decreased when the pulling rate was greater than 2 mm/min. At the rates above 3.5 mm/ min, it was difficult to produce crystals under stationary stable regime, and the rods are opaque (Figure 3). In a such case, the crystallization interface was strongly perturbed, and instability occurred preferentially at a regions of maximum curvature of

Figure 1. Meniscus length evolution as a function of the pulling rate (V). (a) v = 0.75 mm/min, (b) v = 1.5 mm/min, and (c) v = 2 mm/ min.

Figure 2. View of the as-grown sapphire rods as a function of the pulling rate.

Figure 3. Sapphire rod grown with high pulling rate (v = 3 mm/min). 4099



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the solidification front. In addition, the crystallization front became faceted resulting in a new interface shape. The rods grown in the [0001] direction with high pulling rates (V ≥ 2.5 mm/min) were faceted by {112̅0} and {101̅0} planes (Figure 4) that led to the capture of the bubbles.19 The faceting was reduced if the pulling rate decreased.

Figure 4. Faceting of the sapphire rod grown along the c-axis using a high pulling rate (2.5 mm/min).

III-2. Bubbles Characterization. This subsection is focused on the characterization of the bubbles from the point of view of their distribution, density, and diameter. To determine the effect of the growth rate, the crystals were grown at rates of 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, and 3 mm/min. The quality of the lateral (side) surfaces of the grown crystals was acceptable for the bubbles detection. The average roughness was about 0.03 μm. Therefore, the bubbles oriented parallel to the rod axis were observed without any special surface preparation. The bubbles have been observed on the lateral surface of the rods and exhibited almost uniform distribution along the growth direction (Figure 5) in all the crystals independently on growth rate, even when it was as low as 0.5 mm/min. The most bubbles had a spherical shape, and they were located primarily on the planes making an angle ≤90° with crystallization interface. But for the higher pulling rate (V ≥ 1.5 mm/min) about 5% of the bubbles were elongated along the growth direction (Figure 6) and their appearance was strongly dependent on the pulling rates. The secondary phases related to the precipitation of unknown compounds were not observed in the crystal. Optical microscopy demonstrated that the bubbles were well aligned along the rod axis (Figure 5). In the range of the lower pulling rate (0.25 mm/min < V ≤ 0.5 mm/min), the crystallization interface was flat and there was only one lateral layer of the bubbles. At low crystallization rates, the interface is flat and bubbles are generally not trapped by the crystal. Chernov et al.20 have calculated the critical growth rate at which foreign impurities and bubbles are not trapped by the grown crystal. The critical growth rate for the sapphire is approximately 0.5 mm/min. With increasing the pulling rate (V > 1 mm/min), the shape of the crystallization interface becomes convex. Then the number of the bubble layers increase and the sizes of the bubbles decreases (Figure 7). This behavior is similar for the different parts of the rods (beginning, middle, and end). The rods grown at the pulling rate of 0.25 mm/min were bubbles free (Figure 8a). However, at greater pulling rate, the bubbles were localized close to the lateral periphery of the rods forming a crown around the transversal section (Figure 8b). When the pulling rate was increased further the crowns contain several

Figure 5. Bubbles' distribution along the rods axis as a function the pulling rate (V). Observation on the lateral periphery of the grown rod. (a) v = 0.5 mm/min, (b) v = 0.75 mm/min, (c) v = 1 mm/min, (d) v = 1.5 mm/min, (e) v = 2 mm/min, and (f) v = 2.5 mm/min.

layers of the bubbles that were irregularly distributed close to the periphery of the rod (Figure 8c). At a pulling rate greater than 2.5 mm/min, the smaller bubbles (ϕ ≈ 1−6 μm) were observed in the volume and the core of the rod (Figure 8d). 4100



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Figure 6. Bubble elongated along the growth direction (growth rate v = 1.5 mm/min).

Figure 9. Bubbles' density variation as a function of the pulling rate.

distributed in the direction of the periphery of the meniscus where the vortex due to the thermocapillary convection will drive them out toward the external atmosphere.17 The shape of the solidification interface depends on the pulling rate. For low pulling rate (v ≤ 1 mm/min), the crystallization interface was flat and a small concentration of the bubbles was trapped in the rod, but for a higher pulling rate (>1 mm/min), the solidification interface shape became convex and more bubbles were trapped in the crystal rod and their number became larger. We have extended the study to the effect of the starting raw material by using sapphire crackle and γ-Al2O3 powder as charge. As a function of the initial charge, the bubbles' morphology, size, density, and distribution exhibit the same results which has already been observed by Bunoiu et al.22 In the case of the utilization of a Mo crucible, the gases contained in the bubbles can be related to the chemical reaction between the molten alumina, the charge container (Mo crucible), and the graphite devices according to the following reactions:

Figure 7. Variation of the bubbles' diameter as a function of the pulling rate.

The bubbles' density increased with increasing the pulling rate (Figure 9). The variation of the diameter of the bubbles as a function of the pulling rate can be described by Vrk = constant, where V is pulling rate, r is the bubble radii, and k is constant (k = −2.4 for the characterized crystals).21 At any pulling rate, the distance between the bubbles demonstrated linear dependence, and the spacing decreased when the pulling rate increased in good agreement with density evolution. The bubbles were formed either in the capillary channel or in the molten zone. Their propagation in the crystal rod depends on the fluid flow in the meniscus. The Marangoni convection in the free meniscus surface area affects the molten zone, and a convective vortex can probably formed and influence the meniscus height. For low pulling rate, bubbles were trapped only in a layer at the surface periphery of the rod, and they were

Mo(s) + Al 2O3(l) → MoO(g) + Al 2O(g) + (O)g Mo(s) + O(g) → MoO(g) Mo(s) + Al 2O(g) → MoO(g) + 2Al(g) Al 2O(g) + C(s) → CO(g) + 2Al(g)

3MoO + 5C → Mo3C2 + 3CO

Using the Mo crucible, the formed gaseous molybdenum oxide plays the role of catalyst agent between the molten alumina and the graphite. It is important to prevent any contact of the gas in equilibrium with the crucible and the molten zone with hot graphite material (thermal insulation). In this research program,

Figure 8. . Bubbles' distribution in the cross section of the sapphire rod crystal as a function of the pulling rate (V) (a) v = 0.25 mm/min, (b) v = 1 mm/min, (c) v = 1.5 mm/min, and (d) v = 2.5 mm/min. 4101



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effect of the bubbles on the intrinsic quality of the grown rods, we have used a nondestructive analysis method based on the utilization of He/Ne laser emitting at 633 nm.24 A focused beam from the laser was sent through the rod (length 60 mm, ϕ ≈ 3 mm). The output intensity was recorded with a CCD digital camera connected to a computer. The region without bubbles shows a Gaussian profile of the transmitted laser beam (Figure 11). On the other hand, for the crystals containing

we have used an iridium crucible and high purity argon gas atmosphere to overcome the reaction cited below. But the composition of aluminum oxide melt strongly depends on the temperature in the crucible and the molten zone. At low melt overheating (T < 2100 °C),23,24 the following dissociation reactions were observed: Al 2O3 → AlO2 + AlO

2Al 2O3 → Al + 3AlO2

At more significant overheating (T ≈ 2100 °C) the following reactions may be possible: AlO2 → AlO + 0.5O2

Al 2O3 → Al 2O2 + 0.5O2 Figure 11. Intensity distribution in the He/Ne laser beam (P = 0.95 mW) in air (a) and after light passing through the crystals grown at a pulling rate of 1 mm/min (b) and 2 mm/min (c).

Al 2O3 → Al 2O + O2 Al 2O → AlO + Al

But at very high overheating (T > 2150 °C) the following reactions can be also possible bubbles, the beam profile was non-Gaussian and disturbed. The bubble defects in the sapphire crystals cause losses in the intensity profile as well as phase deformations. Figure 12 shows wavefronts recorded using a 60 mm long and 3 mm diameter sapphire crystal grown under a stationary regime (V = 1 mm/ min). A focused beam from a He−Ne laser was sent through the rod. The diameter of the laser waist without sample was measured to be 140 μm. A focal imaging system was used to image the end face of the sample into the beam analyzer Beamwave 500 from Phaseview. The analyzer recorded the beam and the wavefront profiles. The Getlase software was used to analyze the beam profile and to calculate the coefficients of the wavefront decomposition using Zernike polynomials. The wavefront profile measured without sample was perfectly plane (rms = 16 nm). Figure 12A illustrates the wavefront profile for the laser beam propagation in the center of the rod. 174 nm rms phase deformation was measured in the wavefront profile after propagation through the rod. Figure 12C shows the histograms featuring pertinent terms in the Zernike decomposition. The rod induces principally astigmatism and defocus aberrations. In Figure 12B, the wavefront profile had 292 nm rms phase deformation. The wavefront profile measured in the periphery region (Figure 12B) confirmed the results obtained previously. Figure 12D illustrates an increase in the different aberrations with a shift of the laser beam to the edge of the rod.

Al 2O3 → 2Al + 3/2O2 AlO → Al + 0.5O2 AlO2 → Al + O2

Thermal dissociation of alumina and its dissociative evaporation during shaped sapphire crystal growth is important during the growth process. To surmount alumina dissociation, it was necessary to control temperature in the molten zone and minimize the melt overheating in order to be as near as possible to the melting temperature of sapphire. III-3. Optical Characterization. Figure 10 illustrates transmission of the sapphire crystals as a function of the

IV. CONCLUSION It was demonstrated experimentally that the pulling rate in the crystal growth by the micropulling down method has a significant effect on the quality of the grown crystals. Considering economic aspects of the mass crystal production, increasing the pulling rate is always desirable. However, a shortcoming of this strategy is a considerable decrease of the crystal quality. If the pulling rate increases the diameter of the bubbles decreases, but their number increases resulting in low optical performance. At a low pulling rate (v < 1 mm/min), the crystallization interface is flat, the bubble concentration is low, and they are regularly distributed in the lateral periphery of the crystals. Thus, low pulling rate sapphire growth is preferable to avoid formation of the bubbles and to produce high quality crystals.

Figure 10. Room temperature transmission (statistic distribution) of the as-grown rods as a function of the pulling rate.

pulling rate. For low pulling rate (v ≤ 1 mm/min), the transmissions exceeded 80% at room temperature. However, at a greater pulling rate the transmission decreased. The decreasing of the transmission at a pulling rate >1 mm/min is most probably related to the increasing of the density of the bubbles that absorb the light in the measurement range and strongly affect the optical properties. In order to estimate the 4102



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Figure 12. Wavefronts measured using 60 mm long and 3 mm diameter sapphire crystal grown under stationary regime (V = 1 mm/min). (A) Wavefront profile measurement for the laser propagation in the center of the rod . (B) Wavefront profile measurement for the laser propagation in the rod at the region containing bubbles (close periphery). (C) Zernike coefficient histogram obtained from the A wavefront profile. (D) Zernike coefficient histogram obtained from the B wavefront profile.

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(14) Nehari, A.; Mehnaoui, M.; Breton, G.; Gullin, Y.; Orival, S.; Panczer, G.; Brenier, A.; Lebbou, K.; Godfroy, J.; Labor, S.; Legal, H. Opt. Mater. 2010, 32, 731. (15) Borodin, V.; Steriopolo, T.; Tatarchenko, V.; Yavolets, T. Izvestiya Akademii Nauk SSSR, Seriya Fizicheskaya 1983, 47 (2), 368. (16) Nicoara, I.; Bunoiu, O. M.; Vizman, D. J. Cryst. Growth 2006, 287, 291. (17) Bunoiu, O. M.; Duffar, Th.; Nicoara, I. Prog. Cryst. Growth Charact. Mater. 2010, 56, 123. (18) Lebbou, K.; Perrodin, D. Advances in Materials Research, Shaped Crystal Growth by Micro-Pulling-Down Technique; .Fukuda, T. .Chani, V., Eds.; Springer-Verlag: New York, 2007; p 173. (19) Kurlov, V. N.; Rossolenko, S. N.; Belenko, S. V. J. Cryst. Growth 1998, 191, 520. (20) Chernov, A. A.; Temkin, D. T.; Melinkova, A. M. Kristallografia 1976, 21, 652. (21) Bunoiu, O. M.; Duffar, Th.; Nicoara, I. Prog. Cryst. Growth Charact. Mater. 2010, 56, 123. (22) Bunoiu, O. M.; Nicoara, I.; Santailler, J. L.; Theodore, F.; Duffar, T. Cryst. Res. Technol. 2005, 40, 852. (23) A.Nehari, Ph.D. thesis, December 15,2011, Claude Bernard Lyon 1 University (in French). (24) Bagdasarov, Kh. S. ISCGT-1, Beatenberg, Switzerland, September 5−16, 1998; p 608. (25) Lebbou, K.; Perrodin, V. I.; Chani, O.; Aloui, A.; Brenier, J. M.; Fourmigue, J.; Didierjean, F.; Balembois, P. G. J. Am. Ceram. Soc. 2006, 89 (1), 75.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Corning Avon (Fontianebleau), Cyberstar, RSA Le Rubis companies and Cristal innov platform for their great cooperation and collaboration.

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REFERENCES

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Effect of Pulling Rate on Bubbles Distribution in Sapphire Crystals

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