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YAG:Ce melt growth composite
Journal of Crystal Growth 416 (2015) 100–105
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Vertical Bridgman growth of Al2O3/YAG:Ce melt growth composite Masafumi Yoshimura a,b,n, Shin-ichi Sakata a, Hisayoshi Iba a, Takafumi Kawano a, Keigo Hoshikawa b a b
Inorganic Specialty Product Research Laboratory, Ube Industries Ltd., Japan Faculty of Engineering, Shinshu University, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 14 July 2014 Received in revised form 8 January 2015 Accepted 11 January 2015 Communicated by M. Tischler Available online 19 January 2015
The vertical Bridgman (VB) method was investigated as a way to grow Al2O3/YAG:Ce melt growth composite (Ce-doped MGC), an attractive candidate material for blue-to-yellow light conversion. Crucibles made from molybdenum (Mo) and iridium (Ir), whose linear thermal expansion coefficients are significantly different, were examined as potential solutions to the problem of crucible reuse in the growth of large Ce-doped MGC ingots. It was found that Ce-doped MGC ingots grown in Mo crucibles could easily be released nondestructively, while ingots grown in Ir crucible could not. We confirmed that the inside diameter of a Mo crucible is always larger than the diameter of the Ce-doped MGC ingots formed in it, at all temperatures from the melting point to room temperature during the entire cooling process. We also confirmed that larger diameter, 2- and 3-in., Ce-doped MGC ingots with the same orientation as the seed could be grown using Mo crucibles with the VB method. We conclude that Mo crucibles, with a smaller linear thermal expansion coefficient than that of Ce-doped MGC at all temperatures from the melting point to room temperature are useful for growing large ingots and allowing crucible reuse. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Eutectics A2. Bridgman technique B1. Oxides B3. Light Emitting diodes
1. Introduction Unidirectionally solidified Al2O3/YAG composites have been widely studied as materials for high-efficiency power-generation gas turbines because of their characteristics of high-temperature strength and thermal stability [1–3]. The Al2O3/YAG composites, in which continuous networks of single-crystal Al2O3 phases and single-crystal YAG (Y3Al5O12) phases interpenetrate without grain boundaries, is called Al2O3/YAG melt growth composite (Al2O3/ YAG-MGC) [3,4]. This MGC is one of a number of eutectics made by unidirectional solidification. Recently, Sakata et al. proposed the new concept that the Al2O3/ Ce0.09Y2.91Al5O12 binary crystalline system, in which some of the Y-atoms in the YAG are replaced by Ce-atoms, could display ultraluminous behavior in high-power white LEDs [5,6]. In Ref. 6 Ishikawa et al. reported that the combination of a blue-LED chip and the Al2O3/Ce0.09Y2.91Al5O12 binary crystalline system can radiate yellow light by irradiation of blue light. They also reported that the binary crystalline system showed excellent emission
n
Corresponding author. E-mail addresses: [email protected] (M. Yoshimura), [email protected] (K. Hoshikawa). http://dx.doi.org/10.1016/j.jcrysgro.2015.01.008 0022-0248/& 2015 Elsevier B.V. All rights reserved.
intensity in photoluminescence properties. Since then, the Al2O3/ CexY3 xAl5O12 binary crystalline system has been developed to yield blue-to-yellow light conversion materials for white light emitting diodes (LEDs) with high power, low power consumption and high durability [7,8]. We consider the Al2O3/CexY3 xAl5O12 binary crystalline system to be an Al2O3/YAG:Ce melt growth composite (Ce-doped MGC) composed of an Al2O3 single-crystal phase and a CexY3 xAl5O12 (YAG:Ce) single-crystal phase. Both the Al2O3/YAG melt growth composite (MGC) developed as a high-temperature strength material and the Ce-doped MGC developed as a light-conversion material have been reported to be grown experimentally by a unidirectional solidification method using a Bridgman-type apparatus [1–10]. However, the details of the apparatus and the parameters essential for the growth techniques were not included in those papers, and both the MGC and Ce-doped MGC samples grown were small, at less than 53 mm in diameter. More recently, Sai et al. reported on the Ce-doped Al2O3–YAG eutectic and its application in white LEDs [11]. Their Ce-doped Al2O3–YAG eutectic is the same as the Al2O3/CexY3 xAl5O12 binary crystalline system previously reported by Sakata et al. [5,6], although they did not discuss the Sakata report. However, there has been considerable interest in the fact that their 25 mm diameter, 50 mm long Ce-doped Al2O3–YAG eutectic was grown by the Czochralski method. But they did not compare their growth
M. Yoshimura et al. / Journal of Crystal Growth 416 (2015) 100–105
techniques and results using the Czochralski method with the conventional unidirectional solidification method, nor did they deal with considerations relevant to growing larger crystals. We could expect some advantages from growing Ce-doped MGC by the vertical Bridgman method [12,13], which would allow the production of large Ce-doped MGC ingots with high reproducibility because no diameter control would be required. It is essential for the industrial production of Ce-doped MGC that the expensive crucibles are reusable for many growth runs. Miyagawa et al. demonstrated crack-free c-axis sapphire crystal growth using the vertical Bridgman (VB) method in which as-grown ingots in Mo and W crucibles were easily and nondestructively released from the crucibles [13]. We could also expect to reuse crucibles used for Ce-doped MGC growth, just as for sapphire crystal growth under similar conditions, considering that Ce-doped MGC is composed of an Al2O3 single-crystal phase and a Ce-doped YAG single-crystal phase. There are several reports on the growth of MGC and Ce-doped MGC using Mo crucibles in a Bridgman-type furnace [1,4,6], but no information has been reported on the release of ingots or the reuse of these crucibles. In this paper, we study the VB growth of Ce-doped MGC in Mo and Ir crucibles and examine whether Ce-doped MGC ingots grown in such crucibles can be nondestructively released. We decided to use Mo and Ir crucibles in the present study because their linear thermal expansion coefficients differ significantly from each other [14,15]. We measure the linear thermal expansion coefficient of Ce-doped MGC grown by our VB method for comparison with those of Mo and Ir and we discuss the difference in thermal constriction between the grown Ce-doped MGC ingots and their crucibles during the cooling process. It was found that Ce-doped MGC ingots grown in Mo crucibles could easily be released nondestructively, while ingots grown in Ir crucible could not. It was also found that the difficulty of release of the ingot from the crucible was related to the linear thermal expansion coefficients of the crucible material and Ce-doped MGC. We also present the crystalline properties of the Ce-doped MGC ingots released from the crucibles.
2. Experimental As the eutectic temperature of Al2O3 and YAG is 2083 K [16], the refractory metals Ir, Mo, W and a Mo–W alloy can be used as the crucible material. We used the Mo crucible that was used in the previous reports [6] and an Ir crucible with the largest linear thermal expansion coefficient of the four listed refractory metals [14,15]. By comparing crucibles made of materials with strikingly different thermal expansion, we intended to quantitatively examine the differences produced by differing thermal constriction between the inside of the crucible and the surface of the Ce-doped MGC ingot grown therein during its cooling to room temperature from melting point. To do this, we used Mo and Ir crucibles with the same shape of 1 in. in inside diameter, 3 mm thick and 120 mm long. We also prepared two Mo crucibles with different sizes: 1- and 2-in. in inside diameter, to examine the effect of different ingot sizes on growth processes and on the release of ingots from the crucibles. These crucibles were tapered inside by a few degrees to make it easy to release the ingots. The seed orientations of Al2O3 and YAG single crystal phases in Ce-doped MGC were [11 20] (a-axis) and [011] respectively. The [0001] (c-axis) and the [10 10] (m-axis) of the Al2O3 phase and the [ 112] of the YAG phase are perpendicular to the growth direction of the seed [17], as shown in Fig. 1. The seeds were about 25 and 50 mm in diameter, the same as the inside diameter of their crucibles, and 25 mm long.
101
Al2O3[11-20](a-axis) YAG[011]
Al2O3[0001](c-axis) YAG[-112]
Al2O3[10-10](m-axis)
Fig. 1. Diagram of seed orientation.
Airtight Chamber Heat insulator Carbon suscepter Induction coil
Crucible Thermocouple Crucible rod
Fig. 2. Schematic diagram of VB furnace for Ce-doped MGC growth.
Commercially available Al2O3 powder (AKP-30, produced by Sumitomo Chemical Co. Ltd.) and Y2O3 and CeO2 powders (submicron type, produced by Shin-Etsu Chemical Co. Ltd.) were mixed to produce raw material with the molar ratio (Y0.99Ce0.01)3Al5O12: 20 mol% and Al2O3: 80 mol%. The mixed powder was treated with wet ball milling to produce a uniform composite powder slurry, which was then dried in a rotary evaporator to remove the ethanol. The slurry was calcinated at 1273 K for 15 h. Blocks of calcinated slurry were prepared by cold isostatic pressing (CIP) at 150 MPa, and the blocks were then sintered at 1973 K for three hours, reaching a density of about 4.0 g/cm3. Fig. 2 shows a schematic diagram of the VB furnace used for Ce-doped MGC growth. The crucible was mounted on a Mo crucible rod that can rotate and also translate vertically. A W/Re thermocouple was inserted in the crucible rod to measure the temperature of the crucible bottom [18]. The carbon heater was powered by high-frequency induction coils. An argon atmosphere was maintained in the airtight chamber at just over 100 kPa. Fig. 3 shows a schematic diagram of the Ce-doped MGC growth process using the VB method. The crucible in which the seed crystals and the raw materials were charged was set up on the crucible rod. Before beginning the growth process, the temperature distribution in the furnace was maintained as shown in Fig. 3. In the seeding process, the crucible was translated up to the seeding position, where the entire raw material block was melted and the upper portion of the seed was partly melted. Then, the crucible was slowly translated down at the rate of 2 mm/h to drive the growth process. After all of the melt had been consumed by the growth from the seed, the crucible was cooled to room temperature at a cooling rate of about 100 K/h. The Ce-doped MGC ingots, after the finish of cooling process, were released from the crucibles by turning them upside down. The released ingots were annealed in air at 1773 K for 3 h.
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The orientation of the Ce-doped MGC ingot was determined as follows. A wafer sample was cut from the ingot vertically to the growth direction. The pole figure measurement was carried out on the central part of the wafer. The measured planes of the wafer were (30 30) for the Al2O3 phase, and (400) for the YAG:Ce phase. We used Rigaku Advanced Thin film X-ray system-Grazing (ATX-G) for the measurement. After the measurement, we made a stereographic projection of the growth direction of the Ce-doped MGC ingot. The linear thermal expansion coefficient of Ce-doped MGC grown in the present study was measured with a Shinagawa Refractories SL-2000M by a method based on JIS Z 2285:2003. The measured orientation was m-axis and c-axis of the Al2O3 phase in Ce-doped MGC. The measurement was carried out in an Ar atmosphere with a flow rate of 100 cc/min at temperatures ranging from 373 K to 1773 K. The value of the linear thermal expansion coefficient of Ce-doped MGC in the range from room temperature to the eutectic temperature of Al2O3 and YAG was calculated by using measured data and applying Eq. (1). In the calculation, we consider that the eutectic temperature of Al2O3 and YAG is not very different from the melting temperature of our Al2O3/Ce0.09Y2.91Al5O12. α¼
dL 1 dT L0
where α is the liner thermal expansion coefficient, L is the length, T is temperature and L0 is the length at room temperature.
3. Results and discussion Fig. 4 shows a photograph of Mo and Ir crucibles with 1 in. inside diameters, and Ce-doped MGC ingots when the growth processes were terminated. The ingot grown in the Mo crucible could easily be released, as shown in Fig. 4(a), but that grown in the Ir crucible could not be released, as shown in Fig. 4(b). We measured the outside diameter of the Ir crucible because it seemed to be slightly distorted in some places. It was found that the outside diameter of the outer portions increased by up to about 100 μm from its initial diameter before the growth process. We considered from this that some parts of the crucible were expanded by the force generated between the ingot and the crucible's interior during cooling. Fig. 5 shows the temperature dependence of the linear thermal expansion coefficient of Ce-doped MGC and those of the crucible
ð1Þ
Fig. 3. Ce-doped MGC growth process by the VB method.
Fig. 5. Temperature dependence of the liner thermal expansion coefficient of Ce-doped MGC and. those of crucible materials of Mo and Ir.
Fig. 4. Photograph of Mo (a) and Ir (b) crucibles and Ce-doped MGC ingots.
M. Yoshimura et al. / Journal of Crystal Growth 416 (2015) 100–105
materials, Mo and Ir. The measured orientations of Ce-doped MGC grown in the present experiment were m-axis and c-axis for the Al2O3 phase, which lie in the plane perpendicular to the growth direction. The temperature dependences of the linear thermal expansion coefficients of Mo and Ir are from Refs. [14,15] respectively. No significant difference was found between the temperature dependence of the linear thermal expansion coefficient shown in Fig. 5 of the Ce-doped MGC obtained in the present study and that of the Al2O3/YAG-MGC reported by Ochiai et al. [19]. This leads us to consider that the doping of Al2O3/YAG-MGC with small amounts of Ce has little effect on the linear thermal expansion coefficient. We therefore concluded that the temperature dependence of the linear thermal expansion coefficients of Ce-doped MGC obtained in our experiment could be useful in the discussion below. It was also found in Fig. 5 that the linear thermal expansion coefficient of Mo is smaller than that of Ce-doped MGC at all temperatures from room temperature to the melting point. In contrast, the linear thermal expansion coefficient of Ir is larger than that of the m-axis of the Al2O3 phase in Ce-doped MGC in the temperature range from 1457 K to the melting point and also larger that of the c-axis of the Al2O3 phase in the temperature range from 1882 K to the melting point. Fig. 6 shows a plot of temperature from the melting point of the Ce-doped MGC to room temperature vs. diameter variations of a Ce-doped MGC ingot in the directions of the m-axis and c-axis of the Al2O3 phases. These were calculated by using the temperature dependence data of the linear thermal expansion coefficients shown in Fig. 5. The temperature dependent variations of the inside diameters of Mo and Ir crucibles, calculated in the same way, are also shown in Fig. 6. In the calculations for Fig. 6, we
Fig. 6. Temperature vs. diameter variations of Ce-doped MGC ingot for two directions of a-axis. and c-axis of Al2O3 phases and inside diameters of Mo and Ir crucibles.
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normalized to 1.0 the values of the diameters of the m-axis and c-axis orientations of the Al2O3 phases in Ce-doped MGC and the inside diameters of the Mo and Ir crucibles at the melting point just after the Ce-doped MGC ingots were grown. The inside diameter of the Mo crucible was always larger than the diameters of both m- and c-axes of the Al2O3 phase grown in the crucibles at all temperatures from the melting point to room temperature, as shown in Fig. 6. The result confirmed in Fig. 6 is consistent with the fact that the Ce-doped MGC ingots grown in Mo crucibles could easily be released from them, as shown in Fig. 4(a). In contrast, the inside diameter of the Ir crucible was smaller than the diameter of the c-axis orientation of the Al2O3 phase in the Ce-doped MGC ingot over the range from the melting point to 1443 K, as shown in Fig. 7(a), and the m-axis orientation of the Al2O3 phase in the Ce-doped MGC ingot over the range from the melting point to 531 K, as shown in Fig. 7(b). It is considered from the above results that a strong compressive stress occurs at the interface between the inside of the Ir crucible and the periphery of the c-axis orientation of the Al2O3 phase in a Ce-doped MGC ingot over the temperature range from melting point to 1443 K and the m-axis orientation of the Al2O3 phase over the temperature range from melting temperature to 531 K. These results, shown in Fig. 7, are consistent with the fact that the Ce-doped MGC ingot grown in the Ir crucible could not be released from it, as shown in Fig. 4(b). It seems likely that the increase of about 100 μm in the outer diameter of the Ir crucible was caused by the crucible expanding with a plastic deformation in the high temperature region from the strong compressive stress at the interface of the crucible's inside surface and the periphery of the Ce-doped MGC ingot, due to the contraction difference between the crucible and the ingot during the cooling process. It also seems likely that these compressive stresses have an effect on the Ce-doped MGC ingots, such as the generation of plastic deformations at high temperatures and/or cracks at lower temperatures during the cooling process. Fig. 8 shows the pole figures and the stereographic projections in the plane perpendicular to the growth direction of Ce-doped MGC ingots grown in Mo and Ir crucibles. In Ce-doped MGC grown in an Ir crucible, the experimental samples were prepared from the ingot after its release by breaking the crucible. We could confirm in Fig. 7 that just as the stereographic projections of Al2O3 [11–20], [0001], and YAG [001], [ 112] show the same orientations as the seed, the Ce-doped MGC grown in the Mo crucible also retained the orientation of the seed. In contrast, we found that in Ce-doped MGC grown in an Ir crucible, while the orientation of the Al2O3 phase was the same as the seed, however, the orientation of the YAG phase was inclined about 131 from that of the seed. Furthermore, some other different orientations were found in the YAG phase, as shown in the pole figure of Fig. 8. We concluded from the above results that the YAG phase grown
Fig. 7. Enlarged figures in the temperature from 1400 to 1500 K (a) and from 480 to 580 K (b) of Fig. 6.
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Fig. 8. The pole figures and the stereographic projections in the plane perpendicular to the growth orientation of Ce-doped MGC ingots grown by using Mo and Ir crucibles.
Al 2 O3
YAG:Ce
Fig. 9. Photograph of Ce-doped MGC ingots grown with 1- and 2-in. inside diameter Mo crucibles.
in an Ir crucible does not retain the orientation of the seed. We speculate that one reason for this is that the YAG phase was subject to plastic deformation by the compressive stress experienced during the cooling process in the relatively high temperature region. Confirmation, however, will require further investigation. We could not find any cracks in the Ce-doped MGC ingot grown in the 1-in. diameter Ir crucible and released by breaking the crucible. However, we must conclude that Ir crucibles will not be useful for the growth of larger Ce-doped MGC ingots by the VB method because the amount of compressive stress between the ingot and the crucible must increase as the ingot diameter increases. The growth of 2-in. diameter Ce-doped MGC in an Ir crucible was not carried out in the present study because Ir crucibles are expensive. In contrast, we can predict that in the growth of Ce-doped MGC in Mo crucibles, a gap between the periphery of grown ingots and the inside surface of the crucibles must increase as their diameter increases. We can also expect that there could be many advantages in the growth of large Ce-doped MGC ingots beyond easy ingot release, due to reduced stress at the interface between ingot and crucible. Fig. 9 shows Ce-doped MGC ingots grown in 1- and 2-in. inside diameter Mo crucibles. As we predicted above, the 2-in. diameter ingot was as easily released from the Mo crucible as the 1-in. ingots were. We conclude that Mo crucibles, with a smaller linear thermal expansion coefficient than that of Ce-doped MGC over the entire relevant temperature range, are quite useful for growing Ce-doped MGC. In addition, we confirmed that these Mo crucibles were reusable by growing several ingots in a single crucible in a sequential series of growth experiments. Crucibles made of W and W–Mo alloy are also useful for growing Ce-doped MGC because their linear thermal expansion coefficients are
Fig. 10. SEM image of microstructure of cross-section perpendicular to the growth direction.
Fig. 11. The X-ray diffraction patterns of Ce-doped MGC.
smaller than that of Mo. We have confirmed this by growing Ce-doped MGCs in 1-in. diameter crucibles made from W and from an alloy of 49 wt% W and 51 wt% Mo. These crucibles, however, are more expensive than Mo crucibles. In addition, we have shown in Fig. 5 that the linear thermal expansion coefficient of Ce-doped MGC (Al2O3/
M. Yoshimura et al. / Journal of Crystal Growth 416 (2015) 100–105
Al 2 O 3 [11-20] YAG[011]
105
must increase as the diameter increases. We confirmed this expectation by growing larger diameter, 2- and 3-in. ingots with the same growth orientation as that of the seed, by using Mo crucibles. We conclude that the VB growth of Ce-doped MGC in a Mo crucible is very useful for the industrial production of large-size, low-cost ingots with a controlled growth orientation, due to the linear thermal expansion coefficient of Mo being smaller than that of Ce-doped MGC over the entire temperature range from the melting point to room temperature. References
Fig. 12. Photograph of Ce-doped MGC ingot grown with 3-in. inside diameter Mo crucible.
Ce0.09Y2.91Al5O12) is the same as that of Al2O3/YAG-MGC. So it should be understood that a Mo crucible is useful for growing not only Ce-doped MGC for light-conversion, but also conventional MGC for high-temperature strength materials. Fig. 10 shows an SEM image of the microstructure of a crosssection perpendicular to the growth direction of a 2-in. diameter Ce-doped MGC ingot. The microstructure of Ce-doped MGC is lamellar, with a three-dimensional and complexly entangled configuration similar to that observed in the MGC reported in Ref. [6]. It was also found by X-ray diffraction analysis that Ce-doped MGC is composed of an Al2O3 single-crystal phase and a YAG single-crystal phase, as shown in Fig. 11. Finally, we note the successful growth of 3-in. diameter Ce-doped MGC ingots with large Mo crucibles in another resistance heating furnace, as shown in Fig. 12. 4. Summary and conclusion We studied the VB growth of Ce-doped MGC in Mo and Ir crucibles, which have significantly different linear thermal expansion coefficients. It was found that Ce-doped MGC ingots could easily and nondestructively be released from Mo but not from Ir crucibles. It was also found that the difference in ingot release between Mo and Ir crucibles was related to their different coefficients of linear thermal expansion. During the cooling process, the inside diameter of a Mo crucible was always larger than the diameter of the grown Ce-doped MGC ingot over all temperatures from the melting point to room temperature. This suggested that a Mo crucible has a great advantage when growing large-size ingots because the gap between the ingots and the inside of Mo crucibles
[1] T. Mah, T.A. Parthasarathy, Processing and mechanical properties of Al2O3/ Y3Al5O12 (YAG) eutectic composite, Ceram. Eng. Sci. Proc. 11 (1990) 1617–1627. [2] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, A ductile ceramic eutectic composite with high strength at 1873 K, Nature 389 (1997) 49–52. [3] Y. Waku, Oxide Eutectic Ceramic Composites, Metal and Ceramic Matrix Composites, in: B. Cantor, F.P.E. Dunne, I.C. Stone (Eds.), IOP Publishing Ltd., UK, 2004, pp. 407–419. [4] N. Nakagawa, H. Ohtsubo, A. Mitani, K. Shimizu, Y. Waku, High temperature strength and thermal stability for melt growth composite, J. Eur. Ceram. Soc. 25 (2005) 1251–1257. [5] S. Sakata, Y. Waku, Ceramic Composite Material for Optical Conversion and Use Thereof, PTC WO 2004/065324 A1 (05.08.04). [6] T. Ishikawa, S. Sakata, A. Mitani, Durable, ultraluminous structure for incandescent, high-power white-LED, Int. J. Appl. Ceram. Technol. 3 (2) (2006) 144–149. [7] S. Sakata, A. Mitani, T. Ishikawa, Light converting parts made from a melt growth composite, Phosphor Research Society, in: Proceedings of the 314th Meeting Technical Digest, September 29th 2006, pp. 1–7 (in Japanese). [8] S. Sakata, A. Solid, Light conversion member for white LEDs using eutectics, Ceram. Jpn. 47 (3) (2012) 139–144 (in Japanese). [9] Y. Waku, H. Ohtsubo, N. Nakagawa, Y. Kohtoku, Sapphire matrix composites reinforced with single-crystal YAG phases, J. Mater. Sci. 31 (1996) 4663–4670. [10] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite, J. Mater. Sci. 33 (1998) 1217–1225. [11] Q. Sai, Z. Zhao, C. Xia, X. Xu, F. Wu, J. Di, L. Wang, Ce-doped Al2O3–YAG eutectic and its application for white LEDs, Opt. Mater. 35 (2013) 2155–2159. [12] S. Uda, H. Inaba, J. Harada, K. Hoshikawa, Growth of langasite via Bridgman technique along [0 0 0 1], [2 1 10] and [0 1 1 1] for piezoelectric applications, J. Cryst. Growth 271 (2004) 229–237. [13] C. Miyagawa, T. Kobayashi, T. Taishi, K. Hoshikawa, Demonstration of crackfree c-axis sapphire crystal growth using the vertical Bridgman method, J. Cryst. Growth 372 (2013) 95–99. [14] K. Wang, R.R. Reeber, The role of defects on thermophysical properties: thermal expansion of V, Nb, Ta, Mo and W, Mater. Sci. Eng. R23 (1998) 101–137. [15] G.K. White, A.T. Pawlowicz, Thermal expansion of rhodium, iridium, and palladium at low temperatures, J. Low Temp. Phys. 2 (1970) 631–639. [16] O. Fabrichnaya, H.J. Seifert, T. Ludwig, F. Aldinger, A. Navrotsky, The assessment of thermodynamic parameters in the Al2O3–Y2O3 system and phase relations in the Y–Al–O system, Scand. J. Metall. 30 (3) (2001) 175–183. [17] S. Sakata, A. Mitani, K. Shimizu, Y. Waku, Crystallographic orientation analysis and high temperature strength of melt growth composite, J. Eur. Ceram. Soc. 25 (2005) 1441–1445. [18] K. Hoshikawa, J. Osada, Y. Saitou, E. Ohba, C. Miyagawa, T. Kobayashi, J. Yanagisawa, M. Shinozuka, K. Kanno, Vertical Bridgman growth of Sapphire – seed crystal shapes and seeding characteristics, J. Cryst. Growth 395 (2014) 80–89. [19] S. Ochiai, T. Ueda, K. Sato, M. Hojo, Y. Waku, S. Sakata, A. Mitani, T. Takahashi, N. Nakagawa, Elastic modulus and coefficient of thermal expansion of Al2O3/ YAG composite at ultra high temperatures, Mater. Sci. Res. Int. Spec. Tech. Publ. 2 (2001) 281–285.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Vertical Bridgman growth of Al2O3/YAG:Ce melt growth composite Masafumi Yoshimura a,b,n, Shin-ichi Sakata a, Hisayoshi Iba a, Takafumi Kawano a, Keigo Hoshikawa b a b
Inorganic Specialty Product Research Laboratory, Ube Industries Ltd., Japan Faculty of Engineering, Shinshu University, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 14 July 2014 Received in revised form 8 January 2015 Accepted 11 January 2015 Communicated by M. Tischler Available online 19 January 2015
The vertical Bridgman (VB) method was investigated as a way to grow Al2O3/YAG:Ce melt growth composite (Ce-doped MGC), an attractive candidate material for blue-to-yellow light conversion. Crucibles made from molybdenum (Mo) and iridium (Ir), whose linear thermal expansion coefficients are significantly different, were examined as potential solutions to the problem of crucible reuse in the growth of large Ce-doped MGC ingots. It was found that Ce-doped MGC ingots grown in Mo crucibles could easily be released nondestructively, while ingots grown in Ir crucible could not. We confirmed that the inside diameter of a Mo crucible is always larger than the diameter of the Ce-doped MGC ingots formed in it, at all temperatures from the melting point to room temperature during the entire cooling process. We also confirmed that larger diameter, 2- and 3-in., Ce-doped MGC ingots with the same orientation as the seed could be grown using Mo crucibles with the VB method. We conclude that Mo crucibles, with a smaller linear thermal expansion coefficient than that of Ce-doped MGC at all temperatures from the melting point to room temperature are useful for growing large ingots and allowing crucible reuse. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Directional solidification A1. Eutectics A2. Bridgman technique B1. Oxides B3. Light Emitting diodes
1. Introduction Unidirectionally solidified Al2O3/YAG composites have been widely studied as materials for high-efficiency power-generation gas turbines because of their characteristics of high-temperature strength and thermal stability [1–3]. The Al2O3/YAG composites, in which continuous networks of single-crystal Al2O3 phases and single-crystal YAG (Y3Al5O12) phases interpenetrate without grain boundaries, is called Al2O3/YAG melt growth composite (Al2O3/ YAG-MGC) [3,4]. This MGC is one of a number of eutectics made by unidirectional solidification. Recently, Sakata et al. proposed the new concept that the Al2O3/ Ce0.09Y2.91Al5O12 binary crystalline system, in which some of the Y-atoms in the YAG are replaced by Ce-atoms, could display ultraluminous behavior in high-power white LEDs [5,6]. In Ref. 6 Ishikawa et al. reported that the combination of a blue-LED chip and the Al2O3/Ce0.09Y2.91Al5O12 binary crystalline system can radiate yellow light by irradiation of blue light. They also reported that the binary crystalline system showed excellent emission
n
Corresponding author. E-mail addresses: [email protected] (M. Yoshimura), [email protected] (K. Hoshikawa). http://dx.doi.org/10.1016/j.jcrysgro.2015.01.008 0022-0248/& 2015 Elsevier B.V. All rights reserved.
intensity in photoluminescence properties. Since then, the Al2O3/ CexY3 xAl5O12 binary crystalline system has been developed to yield blue-to-yellow light conversion materials for white light emitting diodes (LEDs) with high power, low power consumption and high durability [7,8]. We consider the Al2O3/CexY3 xAl5O12 binary crystalline system to be an Al2O3/YAG:Ce melt growth composite (Ce-doped MGC) composed of an Al2O3 single-crystal phase and a CexY3 xAl5O12 (YAG:Ce) single-crystal phase. Both the Al2O3/YAG melt growth composite (MGC) developed as a high-temperature strength material and the Ce-doped MGC developed as a light-conversion material have been reported to be grown experimentally by a unidirectional solidification method using a Bridgman-type apparatus [1–10]. However, the details of the apparatus and the parameters essential for the growth techniques were not included in those papers, and both the MGC and Ce-doped MGC samples grown were small, at less than 53 mm in diameter. More recently, Sai et al. reported on the Ce-doped Al2O3–YAG eutectic and its application in white LEDs [11]. Their Ce-doped Al2O3–YAG eutectic is the same as the Al2O3/CexY3 xAl5O12 binary crystalline system previously reported by Sakata et al. [5,6], although they did not discuss the Sakata report. However, there has been considerable interest in the fact that their 25 mm diameter, 50 mm long Ce-doped Al2O3–YAG eutectic was grown by the Czochralski method. But they did not compare their growth
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techniques and results using the Czochralski method with the conventional unidirectional solidification method, nor did they deal with considerations relevant to growing larger crystals. We could expect some advantages from growing Ce-doped MGC by the vertical Bridgman method [12,13], which would allow the production of large Ce-doped MGC ingots with high reproducibility because no diameter control would be required. It is essential for the industrial production of Ce-doped MGC that the expensive crucibles are reusable for many growth runs. Miyagawa et al. demonstrated crack-free c-axis sapphire crystal growth using the vertical Bridgman (VB) method in which as-grown ingots in Mo and W crucibles were easily and nondestructively released from the crucibles [13]. We could also expect to reuse crucibles used for Ce-doped MGC growth, just as for sapphire crystal growth under similar conditions, considering that Ce-doped MGC is composed of an Al2O3 single-crystal phase and a Ce-doped YAG single-crystal phase. There are several reports on the growth of MGC and Ce-doped MGC using Mo crucibles in a Bridgman-type furnace [1,4,6], but no information has been reported on the release of ingots or the reuse of these crucibles. In this paper, we study the VB growth of Ce-doped MGC in Mo and Ir crucibles and examine whether Ce-doped MGC ingots grown in such crucibles can be nondestructively released. We decided to use Mo and Ir crucibles in the present study because their linear thermal expansion coefficients differ significantly from each other [14,15]. We measure the linear thermal expansion coefficient of Ce-doped MGC grown by our VB method for comparison with those of Mo and Ir and we discuss the difference in thermal constriction between the grown Ce-doped MGC ingots and their crucibles during the cooling process. It was found that Ce-doped MGC ingots grown in Mo crucibles could easily be released nondestructively, while ingots grown in Ir crucible could not. It was also found that the difficulty of release of the ingot from the crucible was related to the linear thermal expansion coefficients of the crucible material and Ce-doped MGC. We also present the crystalline properties of the Ce-doped MGC ingots released from the crucibles.
2. Experimental As the eutectic temperature of Al2O3 and YAG is 2083 K [16], the refractory metals Ir, Mo, W and a Mo–W alloy can be used as the crucible material. We used the Mo crucible that was used in the previous reports [6] and an Ir crucible with the largest linear thermal expansion coefficient of the four listed refractory metals [14,15]. By comparing crucibles made of materials with strikingly different thermal expansion, we intended to quantitatively examine the differences produced by differing thermal constriction between the inside of the crucible and the surface of the Ce-doped MGC ingot grown therein during its cooling to room temperature from melting point. To do this, we used Mo and Ir crucibles with the same shape of 1 in. in inside diameter, 3 mm thick and 120 mm long. We also prepared two Mo crucibles with different sizes: 1- and 2-in. in inside diameter, to examine the effect of different ingot sizes on growth processes and on the release of ingots from the crucibles. These crucibles were tapered inside by a few degrees to make it easy to release the ingots. The seed orientations of Al2O3 and YAG single crystal phases in Ce-doped MGC were [11 20] (a-axis) and [011] respectively. The [0001] (c-axis) and the [10 10] (m-axis) of the Al2O3 phase and the [ 112] of the YAG phase are perpendicular to the growth direction of the seed [17], as shown in Fig. 1. The seeds were about 25 and 50 mm in diameter, the same as the inside diameter of their crucibles, and 25 mm long.
101
Al2O3[11-20](a-axis) YAG[011]
Al2O3[0001](c-axis) YAG[-112]
Al2O3[10-10](m-axis)
Fig. 1. Diagram of seed orientation.
Airtight Chamber Heat insulator Carbon suscepter Induction coil
Crucible Thermocouple Crucible rod
Fig. 2. Schematic diagram of VB furnace for Ce-doped MGC growth.
Commercially available Al2O3 powder (AKP-30, produced by Sumitomo Chemical Co. Ltd.) and Y2O3 and CeO2 powders (submicron type, produced by Shin-Etsu Chemical Co. Ltd.) were mixed to produce raw material with the molar ratio (Y0.99Ce0.01)3Al5O12: 20 mol% and Al2O3: 80 mol%. The mixed powder was treated with wet ball milling to produce a uniform composite powder slurry, which was then dried in a rotary evaporator to remove the ethanol. The slurry was calcinated at 1273 K for 15 h. Blocks of calcinated slurry were prepared by cold isostatic pressing (CIP) at 150 MPa, and the blocks were then sintered at 1973 K for three hours, reaching a density of about 4.0 g/cm3. Fig. 2 shows a schematic diagram of the VB furnace used for Ce-doped MGC growth. The crucible was mounted on a Mo crucible rod that can rotate and also translate vertically. A W/Re thermocouple was inserted in the crucible rod to measure the temperature of the crucible bottom [18]. The carbon heater was powered by high-frequency induction coils. An argon atmosphere was maintained in the airtight chamber at just over 100 kPa. Fig. 3 shows a schematic diagram of the Ce-doped MGC growth process using the VB method. The crucible in which the seed crystals and the raw materials were charged was set up on the crucible rod. Before beginning the growth process, the temperature distribution in the furnace was maintained as shown in Fig. 3. In the seeding process, the crucible was translated up to the seeding position, where the entire raw material block was melted and the upper portion of the seed was partly melted. Then, the crucible was slowly translated down at the rate of 2 mm/h to drive the growth process. After all of the melt had been consumed by the growth from the seed, the crucible was cooled to room temperature at a cooling rate of about 100 K/h. The Ce-doped MGC ingots, after the finish of cooling process, were released from the crucibles by turning them upside down. The released ingots were annealed in air at 1773 K for 3 h.
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The orientation of the Ce-doped MGC ingot was determined as follows. A wafer sample was cut from the ingot vertically to the growth direction. The pole figure measurement was carried out on the central part of the wafer. The measured planes of the wafer were (30 30) for the Al2O3 phase, and (400) for the YAG:Ce phase. We used Rigaku Advanced Thin film X-ray system-Grazing (ATX-G) for the measurement. After the measurement, we made a stereographic projection of the growth direction of the Ce-doped MGC ingot. The linear thermal expansion coefficient of Ce-doped MGC grown in the present study was measured with a Shinagawa Refractories SL-2000M by a method based on JIS Z 2285:2003. The measured orientation was m-axis and c-axis of the Al2O3 phase in Ce-doped MGC. The measurement was carried out in an Ar atmosphere with a flow rate of 100 cc/min at temperatures ranging from 373 K to 1773 K. The value of the linear thermal expansion coefficient of Ce-doped MGC in the range from room temperature to the eutectic temperature of Al2O3 and YAG was calculated by using measured data and applying Eq. (1). In the calculation, we consider that the eutectic temperature of Al2O3 and YAG is not very different from the melting temperature of our Al2O3/Ce0.09Y2.91Al5O12. α¼
dL 1 dT L0
where α is the liner thermal expansion coefficient, L is the length, T is temperature and L0 is the length at room temperature.
3. Results and discussion Fig. 4 shows a photograph of Mo and Ir crucibles with 1 in. inside diameters, and Ce-doped MGC ingots when the growth processes were terminated. The ingot grown in the Mo crucible could easily be released, as shown in Fig. 4(a), but that grown in the Ir crucible could not be released, as shown in Fig. 4(b). We measured the outside diameter of the Ir crucible because it seemed to be slightly distorted in some places. It was found that the outside diameter of the outer portions increased by up to about 100 μm from its initial diameter before the growth process. We considered from this that some parts of the crucible were expanded by the force generated between the ingot and the crucible's interior during cooling. Fig. 5 shows the temperature dependence of the linear thermal expansion coefficient of Ce-doped MGC and those of the crucible
ð1Þ
Fig. 3. Ce-doped MGC growth process by the VB method.
Fig. 5. Temperature dependence of the liner thermal expansion coefficient of Ce-doped MGC and. those of crucible materials of Mo and Ir.
Fig. 4. Photograph of Mo (a) and Ir (b) crucibles and Ce-doped MGC ingots.
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materials, Mo and Ir. The measured orientations of Ce-doped MGC grown in the present experiment were m-axis and c-axis for the Al2O3 phase, which lie in the plane perpendicular to the growth direction. The temperature dependences of the linear thermal expansion coefficients of Mo and Ir are from Refs. [14,15] respectively. No significant difference was found between the temperature dependence of the linear thermal expansion coefficient shown in Fig. 5 of the Ce-doped MGC obtained in the present study and that of the Al2O3/YAG-MGC reported by Ochiai et al. [19]. This leads us to consider that the doping of Al2O3/YAG-MGC with small amounts of Ce has little effect on the linear thermal expansion coefficient. We therefore concluded that the temperature dependence of the linear thermal expansion coefficients of Ce-doped MGC obtained in our experiment could be useful in the discussion below. It was also found in Fig. 5 that the linear thermal expansion coefficient of Mo is smaller than that of Ce-doped MGC at all temperatures from room temperature to the melting point. In contrast, the linear thermal expansion coefficient of Ir is larger than that of the m-axis of the Al2O3 phase in Ce-doped MGC in the temperature range from 1457 K to the melting point and also larger that of the c-axis of the Al2O3 phase in the temperature range from 1882 K to the melting point. Fig. 6 shows a plot of temperature from the melting point of the Ce-doped MGC to room temperature vs. diameter variations of a Ce-doped MGC ingot in the directions of the m-axis and c-axis of the Al2O3 phases. These were calculated by using the temperature dependence data of the linear thermal expansion coefficients shown in Fig. 5. The temperature dependent variations of the inside diameters of Mo and Ir crucibles, calculated in the same way, are also shown in Fig. 6. In the calculations for Fig. 6, we
Fig. 6. Temperature vs. diameter variations of Ce-doped MGC ingot for two directions of a-axis. and c-axis of Al2O3 phases and inside diameters of Mo and Ir crucibles.
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normalized to 1.0 the values of the diameters of the m-axis and c-axis orientations of the Al2O3 phases in Ce-doped MGC and the inside diameters of the Mo and Ir crucibles at the melting point just after the Ce-doped MGC ingots were grown. The inside diameter of the Mo crucible was always larger than the diameters of both m- and c-axes of the Al2O3 phase grown in the crucibles at all temperatures from the melting point to room temperature, as shown in Fig. 6. The result confirmed in Fig. 6 is consistent with the fact that the Ce-doped MGC ingots grown in Mo crucibles could easily be released from them, as shown in Fig. 4(a). In contrast, the inside diameter of the Ir crucible was smaller than the diameter of the c-axis orientation of the Al2O3 phase in the Ce-doped MGC ingot over the range from the melting point to 1443 K, as shown in Fig. 7(a), and the m-axis orientation of the Al2O3 phase in the Ce-doped MGC ingot over the range from the melting point to 531 K, as shown in Fig. 7(b). It is considered from the above results that a strong compressive stress occurs at the interface between the inside of the Ir crucible and the periphery of the c-axis orientation of the Al2O3 phase in a Ce-doped MGC ingot over the temperature range from melting point to 1443 K and the m-axis orientation of the Al2O3 phase over the temperature range from melting temperature to 531 K. These results, shown in Fig. 7, are consistent with the fact that the Ce-doped MGC ingot grown in the Ir crucible could not be released from it, as shown in Fig. 4(b). It seems likely that the increase of about 100 μm in the outer diameter of the Ir crucible was caused by the crucible expanding with a plastic deformation in the high temperature region from the strong compressive stress at the interface of the crucible's inside surface and the periphery of the Ce-doped MGC ingot, due to the contraction difference between the crucible and the ingot during the cooling process. It also seems likely that these compressive stresses have an effect on the Ce-doped MGC ingots, such as the generation of plastic deformations at high temperatures and/or cracks at lower temperatures during the cooling process. Fig. 8 shows the pole figures and the stereographic projections in the plane perpendicular to the growth direction of Ce-doped MGC ingots grown in Mo and Ir crucibles. In Ce-doped MGC grown in an Ir crucible, the experimental samples were prepared from the ingot after its release by breaking the crucible. We could confirm in Fig. 7 that just as the stereographic projections of Al2O3 [11–20], [0001], and YAG [001], [ 112] show the same orientations as the seed, the Ce-doped MGC grown in the Mo crucible also retained the orientation of the seed. In contrast, we found that in Ce-doped MGC grown in an Ir crucible, while the orientation of the Al2O3 phase was the same as the seed, however, the orientation of the YAG phase was inclined about 131 from that of the seed. Furthermore, some other different orientations were found in the YAG phase, as shown in the pole figure of Fig. 8. We concluded from the above results that the YAG phase grown
Fig. 7. Enlarged figures in the temperature from 1400 to 1500 K (a) and from 480 to 580 K (b) of Fig. 6.
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Fig. 8. The pole figures and the stereographic projections in the plane perpendicular to the growth orientation of Ce-doped MGC ingots grown by using Mo and Ir crucibles.
Al 2 O3
YAG:Ce
Fig. 9. Photograph of Ce-doped MGC ingots grown with 1- and 2-in. inside diameter Mo crucibles.
in an Ir crucible does not retain the orientation of the seed. We speculate that one reason for this is that the YAG phase was subject to plastic deformation by the compressive stress experienced during the cooling process in the relatively high temperature region. Confirmation, however, will require further investigation. We could not find any cracks in the Ce-doped MGC ingot grown in the 1-in. diameter Ir crucible and released by breaking the crucible. However, we must conclude that Ir crucibles will not be useful for the growth of larger Ce-doped MGC ingots by the VB method because the amount of compressive stress between the ingot and the crucible must increase as the ingot diameter increases. The growth of 2-in. diameter Ce-doped MGC in an Ir crucible was not carried out in the present study because Ir crucibles are expensive. In contrast, we can predict that in the growth of Ce-doped MGC in Mo crucibles, a gap between the periphery of grown ingots and the inside surface of the crucibles must increase as their diameter increases. We can also expect that there could be many advantages in the growth of large Ce-doped MGC ingots beyond easy ingot release, due to reduced stress at the interface between ingot and crucible. Fig. 9 shows Ce-doped MGC ingots grown in 1- and 2-in. inside diameter Mo crucibles. As we predicted above, the 2-in. diameter ingot was as easily released from the Mo crucible as the 1-in. ingots were. We conclude that Mo crucibles, with a smaller linear thermal expansion coefficient than that of Ce-doped MGC over the entire relevant temperature range, are quite useful for growing Ce-doped MGC. In addition, we confirmed that these Mo crucibles were reusable by growing several ingots in a single crucible in a sequential series of growth experiments. Crucibles made of W and W–Mo alloy are also useful for growing Ce-doped MGC because their linear thermal expansion coefficients are
Fig. 10. SEM image of microstructure of cross-section perpendicular to the growth direction.
Fig. 11. The X-ray diffraction patterns of Ce-doped MGC.
smaller than that of Mo. We have confirmed this by growing Ce-doped MGCs in 1-in. diameter crucibles made from W and from an alloy of 49 wt% W and 51 wt% Mo. These crucibles, however, are more expensive than Mo crucibles. In addition, we have shown in Fig. 5 that the linear thermal expansion coefficient of Ce-doped MGC (Al2O3/
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Al 2 O 3 [11-20] YAG[011]
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must increase as the diameter increases. We confirmed this expectation by growing larger diameter, 2- and 3-in. ingots with the same growth orientation as that of the seed, by using Mo crucibles. We conclude that the VB growth of Ce-doped MGC in a Mo crucible is very useful for the industrial production of large-size, low-cost ingots with a controlled growth orientation, due to the linear thermal expansion coefficient of Mo being smaller than that of Ce-doped MGC over the entire temperature range from the melting point to room temperature. References
Fig. 12. Photograph of Ce-doped MGC ingot grown with 3-in. inside diameter Mo crucible.
Ce0.09Y2.91Al5O12) is the same as that of Al2O3/YAG-MGC. So it should be understood that a Mo crucible is useful for growing not only Ce-doped MGC for light-conversion, but also conventional MGC for high-temperature strength materials. Fig. 10 shows an SEM image of the microstructure of a crosssection perpendicular to the growth direction of a 2-in. diameter Ce-doped MGC ingot. The microstructure of Ce-doped MGC is lamellar, with a three-dimensional and complexly entangled configuration similar to that observed in the MGC reported in Ref. [6]. It was also found by X-ray diffraction analysis that Ce-doped MGC is composed of an Al2O3 single-crystal phase and a YAG single-crystal phase, as shown in Fig. 11. Finally, we note the successful growth of 3-in. diameter Ce-doped MGC ingots with large Mo crucibles in another resistance heating furnace, as shown in Fig. 12. 4. Summary and conclusion We studied the VB growth of Ce-doped MGC in Mo and Ir crucibles, which have significantly different linear thermal expansion coefficients. It was found that Ce-doped MGC ingots could easily and nondestructively be released from Mo but not from Ir crucibles. It was also found that the difference in ingot release between Mo and Ir crucibles was related to their different coefficients of linear thermal expansion. During the cooling process, the inside diameter of a Mo crucible was always larger than the diameter of the grown Ce-doped MGC ingot over all temperatures from the melting point to room temperature. This suggested that a Mo crucible has a great advantage when growing large-size ingots because the gap between the ingots and the inside of Mo crucibles
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