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Growth process of gas bubble in ruby single crystals by floating zone method
Journal of Crystal Growth 74 (1986) 385-390 North-Holland, Amsterdam
385
GROWTH PROCESS OF GAS BUBBLE IN RUBY SINGLE CRYSTALS BY FLOATING ZONE METHOD M. SAITO Seiko Epson Corporation, Suwa-shi, Nagano, Japan
Received 3 April 1985; manuscript received in final form 30 November 1985
Bubbles in ruby single crystals grown by the floating zone method were investigated. It was found that though bubbles had facets, they were not crystals but probably a gas. The bubble growth process near the interface was made visible by quenching the crystal during growth and by subsequent microphotography of bubbles near the interface. The bubble growth process was explained by the model that the gas contituent which was rejected from the solidified melt formed bubles at the interface and the bubbles were incorporated into the crystal.
I. Introduction Gas bubbles have often been found in single crystals, for example. ruby [1], neodymium gallium garnet [2], [NT [3] and Pb5Ge5O11 [4]. The origin of the bubbles has been discussed by Nassau and Groyer [5], Cockayne [6] and Kobayashi [7]. These crystals were grown by the floating zone method with an infrared radiation convergence type heater or by the Czochralski method. In order to grow bubble-free single crystals for use as laser rods or for many kinds of optelectronic devices, it is important to make clear the mechanism of bubble formation. We proposed in our previous paper [11 the model that as the melt solidified and the gas constituent (probably air) was rejected into the melt, bubbles were formed near the interface between the melt and the crystal. This occurred when the limit concentration necessary for bubbles to nucleate was exceeded. In the next step bubbles were incorporated into the crystal under growth. The model was consistent with the previous experimental results [1] and explains well the cur rent experiments. However, we were unable to present photographs in the previous paper which directly showed the process of bubble formation, bubble growth near the interface and bubble incorporation into the crystal. In this work the melt and crystal were quenched during growth by turn-
ing off the floating zone furnace power source to freeze the interface, melt and bubbles in position. The aim of the present paper is (1) to research the bubble content, (2) to investigate the bubble formation process and (3) to present microphotographs of bubbles formed near the interface and of the bubble growth process.
2. Experiment 2.1. Preparation of sintered rods
Two kinds of sintered rods were prepared in this study. One was sintered at 1450°C so that bubbles could be formed easily in the crystals, and the other was at 1700°C.The procedure is shown schematically in fig. 1. The alumina used was a crystal type of ~-Al2O3 with a purity of 99.99%. The concentration of chromium was 2.Owt%. The well-mixed powder, i.e. s-Al,01, Cr7O~, were -____________
Materials
I
_______
IMain—Al 0 0 t-C
L±~
r2
______
________
WEIGHINGF—fXrNGH{PRESS~i~]
0 ~
~FULLINGj_~~_jSETTING~_HSINTERING! FURNACE
Fig. I. Flow chart of the procedure in ruby single crystals.
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
356
,%/..S’aito
Groicilt
of
go. huh/ui’
ut
rtuht hi /lowiutu,’ :oiti’ unit/ui!
pressed at 1.0 ton/crni and sintered for ten hours. The density of sintered rods at 1450°C was about 33 g/cmi and that at 1700°C about 3.9 g/cmi.
3. Results and Discussion
2.2. Growth condition
Microphotographs of the polished samples under bright and dark field illumination are shown in fig.3. The sintering temperature of this material was 1700°C. The solidified melt consisting of polycrystals with very small grains was not only convex toward the grown crystal, hut was also remarkably rough in the center of the interface. In the interface represented the isothermal line. it was found that the temperature distribution was disturbed in the center part of the interface. probably because the concentration of gas constituents in the center of the melt was high [1] and the gas constituents result in constitutional supercooling or non-equilibrium conditions. In fact, bubbles in this crystal were observed only in the center of’ the crystal, not at the periphery of the crystal [NJ.
An apparatus made by the Nihon Electric Company Ltd. was used for this study. It had two halogen lamps as heat sources with output of up to 3.5 kW. A diagram of the arrangement is shown in fig. 2. The crystal growth rate was 1.5 mm/h and the rotation rate ratio was 2 1 in the same direction between the rotation speed of a axis holding a sintered rod and that holding a growing crystal. In the previous work the rotation ratio was insensitive to the bubble formation [I]. The atmosphere in the growth chamber was air, 2.3. Quench conditwns Quenching of ruby single crystals was done by turning off the floating zone furnace power source when the crystals were growing. The time necessary for the melt to he fully solidified was not measured: however, the melt appeared to be solidified in an instant when observed on the screen of the floating zone (FZ) furnace. The grown ruby single crystals were cut perpendicular or parallel to the growth direction (100) and polished to provide a cross section.
3.1. (on/~gu~’cztio~t of the inter/ace
3.2. Bubble content Bubble distribution is shown under a polarizing microscope with crossed Nicols in fig. 4. The sintering temperature for this sample was 1700°C’. The black dots in the photographs appeared to he precipitates because of their facets. If the black dots in the photograph had been uniaxial or hiaxial crystal precipitates, the brightness of the black dots should have changed with the rotation of a polished specimen. i.e.. from dark to light dots. No change was observed and X-ray diffractornetry gave only sharp lines of Al ~ Furthermore, specimens sintered at 1450°C’were
Melted zone
El lip so ~t mirror
prepared in order to include more air and more readily in the crystal. This permitted a comparison
Siritered material
of bubble concentration in crystals grown under the same conditions hut from sintered rods having different air content. Microphotographs from this crystal are shown in fig. 5. Comparing fig. 5 with fig. 4, it can he seen that the bubble diameter in fig. 5 was remarkably larger than in fig. 4. In fig. 5 the largest was about 1 mm in diameter whereas the largest bubble diameter shown in fig. 4 was about 20 1im. The bubbles were empty and no
s i
/ 2
~wrng
/
tialogeo lamp Lens
tat mg a id moving
axis
Screen
Fig. 2. Schematic illustration of the floating zone susiem cross section.
kinds of crystals were detected. Therefore. although the bubbles had facets, it was concluded
M. Sam
/
Growth of gas bubble in ,‘ubr In 1/noting zone method
387
Solidified
A melt
Cr y s ta 1 ~ 1mm (a)~
(b)
E—’ig. 3. Phoiographs of quenched ruh~under growth in .i cross seciion parallel to the gros~n direciion: I a) bright—field illumination method; (b) dark-field illumination method.
that they were not precipitates but were the results of pores filled with air incorporated in the sintered rods. Our previous work [1] showed that the hub-
hle content was probably air related to the atmosphere during crystal growth.
3.3. (c’) Bubble growth process
Bubble growth in a line near the interface is shown in fig. 6. The bubble growth process near the interface is shown in fig. 7. The micorphoto-
5
4Opm (a)
(b)
Fig. 4. Microphotographs of bubbles in ruby single crystal in a cross section perpendicular to the grown direction under a polarizing microscope: (a) bubble distribution; (h) a bubble with facets,
355
t/ Sau/i
(,u au i/i if gut u /uiu/u/i/c
iii
1mm
utuhi hi f/ouuiiuug zoutu uuuu’//u,iu/
I ‘u I3uh’uhle dusiruhuiiuun uul tubs .,ttugle crusts) bus ulue d,uik held tiietluuud
Solidified melt
C~1
W *
(a) us N \lucuuipltuutuiutr.iplus oh hobbles
it
.u butte ltuuilt It) intl I hiu sueuu.
Ctiun
it iuuudeu
100pm (b)
lie s,uitue tiiuudituiiuu
4 _~ 100pm Crystal
~ (b)
(c)
7. Mieruuphiotiigrapht uif the bubble grontht pruueess: ).ut .u bubble )~irtu.uhls uuueiurpuir:uted uultuu the cr~st:uh: hi .u huuubhhe ,utt,iclued to
the iiuuerf:uce: he) .u bubble ,una\ bruuni thue interlace.
M. So/to
/
Growth ofgas bubble in ruby by floating zone method
389
Material Melt
0
Crystal (a)
(b)
(c)
(d)
(e)
Fig. 8. Schematic illustration of the bubble growth process: (a) a bubble formed near the interface; (b) a bubble partially incorporated into the crystal; (c) a bubble attached to the interface; (d) a bubble in the crystal: (e) a new bubble nucleation.
graph in fig. 7a dipicts the bubble partially incorporated into the crystal. In fig. 7b the bubble is partially attached to the interface. In fig. 7c the bubble exists away from the interface. This process (a) (b) (c) keeps repeating and a continuous —~
‘-~
~
8Opm
Fig. 9. Microphotographs of a bubble like a tube.
Material
___________
I
bubble line is formed, as shown in fig. 6. These facts were well explained by the model we had proposed. The solidified melt rejected gas constituents into the melt during growth and resulted in an increase of the concentration of gas constituent in the melt. The concentration of gas was especially high near the interface. When the concentration exceeded the limit necessary for bubbles to nucleate bubbles nucleated near the interface After that the formed bubbles were in corporated into the growing crystal. This explanation is consistent with the microphotographs in fig. 7. This phenomenon is schematically illustrated in fig8 detailed observatton the conftgurations of the bubbles are of two types One is spherical as described above and the other is tube-like as shown in fig. 9. As we take the assumptions that (1) the bubble itself acts as a sink for the gas constituent and (2) the gas constituent existing near the bubble is sufficiently concentrated to act as a source at (b) in fig. 8, the bubble will not grow like a sphere constituent in fig. 9 is ofhigher than that in fig. 8. A schematic illustration a tube-like growth process
___________
Melt
~ /
___________
butdepicted is as a tube. in fig. Therefore, 10. The difference it is foundbetween that thefig.gas8 and fig. 9 is the concentration of gas constituent in the melt near the interface.
Crystal (a)
(b)
(c)
Fig. 10. Schematic illustration of the growth process in case of a bubble like a tube: (a) a bubble formed and enriched gas solute; (h) a bubble incorporated and enriched gas solute; (c) a bubble grown like a tube.
Acknowledgement
The author wishes to thank Mr. Masanao Kunugi for preparing the specimens in this study.
39))
.51. Suuutuu
//
Gruuuut/t uuf gas /uti/u/u/u’ uut ruby hi’ /iuuating :iusut’ uiuc’r/uuuu/
References
f4J S. Misas:uma. J. C natal (iruuwiht 49 198th) 515. [5] K. Nassau and AM. Gros.er.3. AppI. lThvs. 33 11962) 31)64.
[If
M. Saito. J. Crystal Growth 71)1985) 664. [21 K. Kitamura. M. Tsutsumi. S. Kimunu and H. Kuumatsu, j,
[6] II. C’uuckayne. J. Crystal Growth 42 II981) 413. 7] ~ Kuuhanashi. 3. C rystab Growth 54 11981) 414.
Crystal Growih 67 11984) 656 [3] F. Shuiinara and Y. Fujinuu. J. (‘natal (iruuwtbu 38 11977) 293.
7]
JR. (arruihers antI K. Nassau, 3. (natal Gruuwth 39 11968) 52h5
385
GROWTH PROCESS OF GAS BUBBLE IN RUBY SINGLE CRYSTALS BY FLOATING ZONE METHOD M. SAITO Seiko Epson Corporation, Suwa-shi, Nagano, Japan
Received 3 April 1985; manuscript received in final form 30 November 1985
Bubbles in ruby single crystals grown by the floating zone method were investigated. It was found that though bubbles had facets, they were not crystals but probably a gas. The bubble growth process near the interface was made visible by quenching the crystal during growth and by subsequent microphotography of bubbles near the interface. The bubble growth process was explained by the model that the gas contituent which was rejected from the solidified melt formed bubles at the interface and the bubbles were incorporated into the crystal.
I. Introduction Gas bubbles have often been found in single crystals, for example. ruby [1], neodymium gallium garnet [2], [NT [3] and Pb5Ge5O11 [4]. The origin of the bubbles has been discussed by Nassau and Groyer [5], Cockayne [6] and Kobayashi [7]. These crystals were grown by the floating zone method with an infrared radiation convergence type heater or by the Czochralski method. In order to grow bubble-free single crystals for use as laser rods or for many kinds of optelectronic devices, it is important to make clear the mechanism of bubble formation. We proposed in our previous paper [11 the model that as the melt solidified and the gas constituent (probably air) was rejected into the melt, bubbles were formed near the interface between the melt and the crystal. This occurred when the limit concentration necessary for bubbles to nucleate was exceeded. In the next step bubbles were incorporated into the crystal under growth. The model was consistent with the previous experimental results [1] and explains well the cur rent experiments. However, we were unable to present photographs in the previous paper which directly showed the process of bubble formation, bubble growth near the interface and bubble incorporation into the crystal. In this work the melt and crystal were quenched during growth by turn-
ing off the floating zone furnace power source to freeze the interface, melt and bubbles in position. The aim of the present paper is (1) to research the bubble content, (2) to investigate the bubble formation process and (3) to present microphotographs of bubbles formed near the interface and of the bubble growth process.
2. Experiment 2.1. Preparation of sintered rods
Two kinds of sintered rods were prepared in this study. One was sintered at 1450°C so that bubbles could be formed easily in the crystals, and the other was at 1700°C.The procedure is shown schematically in fig. 1. The alumina used was a crystal type of ~-Al2O3 with a purity of 99.99%. The concentration of chromium was 2.Owt%. The well-mixed powder, i.e. s-Al,01, Cr7O~, were -____________
Materials
I
_______
IMain—Al 0 0 t-C
L±~
r2
______
________
WEIGHINGF—fXrNGH{PRESS~i~]
0 ~
~FULLINGj_~~_jSETTING~_HSINTERING! FURNACE
Fig. I. Flow chart of the procedure in ruby single crystals.
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
356
,%/..S’aito
Groicilt
of
go. huh/ui’
ut
rtuht hi /lowiutu,’ :oiti’ unit/ui!
pressed at 1.0 ton/crni and sintered for ten hours. The density of sintered rods at 1450°C was about 33 g/cmi and that at 1700°C about 3.9 g/cmi.
3. Results and Discussion
2.2. Growth condition
Microphotographs of the polished samples under bright and dark field illumination are shown in fig.3. The sintering temperature of this material was 1700°C. The solidified melt consisting of polycrystals with very small grains was not only convex toward the grown crystal, hut was also remarkably rough in the center of the interface. In the interface represented the isothermal line. it was found that the temperature distribution was disturbed in the center part of the interface. probably because the concentration of gas constituents in the center of the melt was high [1] and the gas constituents result in constitutional supercooling or non-equilibrium conditions. In fact, bubbles in this crystal were observed only in the center of’ the crystal, not at the periphery of the crystal [NJ.
An apparatus made by the Nihon Electric Company Ltd. was used for this study. It had two halogen lamps as heat sources with output of up to 3.5 kW. A diagram of the arrangement is shown in fig. 2. The crystal growth rate was 1.5 mm/h and the rotation rate ratio was 2 1 in the same direction between the rotation speed of a axis holding a sintered rod and that holding a growing crystal. In the previous work the rotation ratio was insensitive to the bubble formation [I]. The atmosphere in the growth chamber was air, 2.3. Quench conditwns Quenching of ruby single crystals was done by turning off the floating zone furnace power source when the crystals were growing. The time necessary for the melt to he fully solidified was not measured: however, the melt appeared to be solidified in an instant when observed on the screen of the floating zone (FZ) furnace. The grown ruby single crystals were cut perpendicular or parallel to the growth direction (100) and polished to provide a cross section.
3.1. (on/~gu~’cztio~t of the inter/ace
3.2. Bubble content Bubble distribution is shown under a polarizing microscope with crossed Nicols in fig. 4. The sintering temperature for this sample was 1700°C’. The black dots in the photographs appeared to he precipitates because of their facets. If the black dots in the photograph had been uniaxial or hiaxial crystal precipitates, the brightness of the black dots should have changed with the rotation of a polished specimen. i.e.. from dark to light dots. No change was observed and X-ray diffractornetry gave only sharp lines of Al ~ Furthermore, specimens sintered at 1450°C’were
Melted zone
El lip so ~t mirror
prepared in order to include more air and more readily in the crystal. This permitted a comparison
Siritered material
of bubble concentration in crystals grown under the same conditions hut from sintered rods having different air content. Microphotographs from this crystal are shown in fig. 5. Comparing fig. 5 with fig. 4, it can he seen that the bubble diameter in fig. 5 was remarkably larger than in fig. 4. In fig. 5 the largest was about 1 mm in diameter whereas the largest bubble diameter shown in fig. 4 was about 20 1im. The bubbles were empty and no
s i
/ 2
~wrng
/
tialogeo lamp Lens
tat mg a id moving
axis
Screen
Fig. 2. Schematic illustration of the floating zone susiem cross section.
kinds of crystals were detected. Therefore. although the bubbles had facets, it was concluded
M. Sam
/
Growth of gas bubble in ,‘ubr In 1/noting zone method
387
Solidified
A melt
Cr y s ta 1 ~ 1mm (a)~
(b)
E—’ig. 3. Phoiographs of quenched ruh~under growth in .i cross seciion parallel to the gros~n direciion: I a) bright—field illumination method; (b) dark-field illumination method.
that they were not precipitates but were the results of pores filled with air incorporated in the sintered rods. Our previous work [1] showed that the hub-
hle content was probably air related to the atmosphere during crystal growth.
3.3. (c’) Bubble growth process
Bubble growth in a line near the interface is shown in fig. 6. The bubble growth process near the interface is shown in fig. 7. The micorphoto-
5
4Opm (a)
(b)
Fig. 4. Microphotographs of bubbles in ruby single crystal in a cross section perpendicular to the grown direction under a polarizing microscope: (a) bubble distribution; (h) a bubble with facets,
355
t/ Sau/i
(,u au i/i if gut u /uiu/u/i/c
iii
1mm
utuhi hi f/ouuiiuug zoutu uuuu’//u,iu/
I ‘u I3uh’uhle dusiruhuiiuun uul tubs .,ttugle crusts) bus ulue d,uik held tiietluuud
Solidified melt
C~1
W *
(a) us N \lucuuipltuutuiutr.iplus oh hobbles
it
.u butte ltuuilt It) intl I hiu sueuu.
Ctiun
it iuuudeu
100pm (b)
lie s,uitue tiiuudituiiuu
4 _~ 100pm Crystal
~ (b)
(c)
7. Mieruuphiotiigrapht uif the bubble grontht pruueess: ).ut .u bubble )~irtu.uhls uuueiurpuir:uted uultuu the cr~st:uh: hi .u huuubhhe ,utt,iclued to
the iiuuerf:uce: he) .u bubble ,una\ bruuni thue interlace.
M. So/to
/
Growth ofgas bubble in ruby by floating zone method
389
Material Melt
0
Crystal (a)
(b)
(c)
(d)
(e)
Fig. 8. Schematic illustration of the bubble growth process: (a) a bubble formed near the interface; (b) a bubble partially incorporated into the crystal; (c) a bubble attached to the interface; (d) a bubble in the crystal: (e) a new bubble nucleation.
graph in fig. 7a dipicts the bubble partially incorporated into the crystal. In fig. 7b the bubble is partially attached to the interface. In fig. 7c the bubble exists away from the interface. This process (a) (b) (c) keeps repeating and a continuous —~
‘-~
~
8Opm
Fig. 9. Microphotographs of a bubble like a tube.
Material
___________
I
bubble line is formed, as shown in fig. 6. These facts were well explained by the model we had proposed. The solidified melt rejected gas constituents into the melt during growth and resulted in an increase of the concentration of gas constituent in the melt. The concentration of gas was especially high near the interface. When the concentration exceeded the limit necessary for bubbles to nucleate bubbles nucleated near the interface After that the formed bubbles were in corporated into the growing crystal. This explanation is consistent with the microphotographs in fig. 7. This phenomenon is schematically illustrated in fig8 detailed observatton the conftgurations of the bubbles are of two types One is spherical as described above and the other is tube-like as shown in fig. 9. As we take the assumptions that (1) the bubble itself acts as a sink for the gas constituent and (2) the gas constituent existing near the bubble is sufficiently concentrated to act as a source at (b) in fig. 8, the bubble will not grow like a sphere constituent in fig. 9 is ofhigher than that in fig. 8. A schematic illustration a tube-like growth process
___________
Melt
~ /
___________
butdepicted is as a tube. in fig. Therefore, 10. The difference it is foundbetween that thefig.gas8 and fig. 9 is the concentration of gas constituent in the melt near the interface.
Crystal (a)
(b)
(c)
Fig. 10. Schematic illustration of the growth process in case of a bubble like a tube: (a) a bubble formed and enriched gas solute; (h) a bubble incorporated and enriched gas solute; (c) a bubble grown like a tube.
Acknowledgement
The author wishes to thank Mr. Masanao Kunugi for preparing the specimens in this study.
39))
.51. Suuutuu
//
Gruuuut/t uuf gas /uti/u/u/u’ uut ruby hi’ /iuuating :iusut’ uiuc’r/uuuu/
References
f4J S. Misas:uma. J. C natal (iruuwiht 49 198th) 515. [5] K. Nassau and AM. Gros.er.3. AppI. lThvs. 33 11962) 31)64.
[If
M. Saito. J. Crystal Growth 71)1985) 664. [21 K. Kitamura. M. Tsutsumi. S. Kimunu and H. Kuumatsu, j,
[6] II. C’uuckayne. J. Crystal Growth 42 II981) 413. 7] ~ Kuuhanashi. 3. C rystab Growth 54 11981) 414.
Crystal Growih 67 11984) 656 [3] F. Shuiinara and Y. Fujinuu. J. (‘natal (iruuwtbu 38 11977) 293.
7]
JR. (arruihers antI K. Nassau, 3. (natal Gruuwth 39 11968) 52h5