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Growth of high-quality Y2O3–ZrO2 single-crystal optical fibers for ultra-high-temperature fiber-optic sensors
Journal of Crystal Growth 217 (2000) 281 } 286
Growth of high-quality Y O }ZrO single-crystal optical "bers for ultra-high-temperature "ber-optic sensors Limin Tong Department of Physics, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China Received 7 December 1999; accepted 7 March 2000 Communicated by R.S. Feigelson
Abstract Y O }ZrO (Y O stabilized ZrO ) single-crystal "bers are promising candidates for developing ultra-high-temper ature "ber-optic sensors; however, grown by conventional laser heated pedestal growth (LHPG) system, these "bers showed relatively low optical and mechanical qualities due to thermal stress caused by large temperature gradient during the growth, preventing the sensors from achieving high performance in our previous work. In order to grow high-quality Y O }ZrO single-crystal "bers with less stress, in this paper, according to the theoretical analysis, we have improved the growth system by adding an additional heating system, and the temperature gradient during growth has been reduced to an acceptable value. Five Y O }ZrO single-crystal "bers have been grown by the improved system; experimental test results showed that the optical and mechanical qualities of these "bers have been greatly enhanced, with which the ultra-high-temperature "ber-optic sensors with better performance could be developed. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 81.10; 42.81; 78.66 Keywords: Crystal growth; Y O }ZrO single-crystal "ber; Optical properties
1. Introduction Radiation-based high-temperature "ber-optic sensors, especially the sapphire "ber-optic sensor, have been widely used due to their special advantages of high accuracy, fast response, intrinsic immunity to electromagnetic interference and long life [1,2]. However, the melting point, 20453C, of the sapphire crystal "ber limits the sensor to be used beyond 20003C. In order to extend the working temperature of "ber-optic sensors above 20003C, a new kind of crystal "ber with higher melting point must be used to substitute for the sapphire "ber.
Y O }ZrO (Y O -stabilized ZrO ) is a very promising material for this purpose due to its very high melting point (about 27003C) and relatively stable physical and chemical properties within visible and near-infrared band. Several years ago, we had grown Y O }ZrO single-crystal "bers by means of laser-heated pedestal growth (LHPG) method [3]; however, the optical and mechanical properties of those "bers were not as good as we expected due to the large residual thermal stress. Recently, a zirconia (Y O }ZrO ) single-crystal "ber-optic sensor was developed [4]. Although it had been successfully operated upto 23003C, its
0022-0248/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 4 8 2 - 6
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L. Tong / Journal of Crystal Growth 217 (2000) 281}286
accuracy was much lower than the sapphire "beroptic sensor because of the low transmissivity of the Y O }ZrO "ber. In order to improve the proper ties of Y O }ZrO single-crystal "bers and then improve the properties of zirconia "ber-optic sensors, it was necessary to improve our original LHPG system and growth conditions. This paper describes how a reduction in temperature gradients in the molten zone during the growth, reduces the residual thermal stress inside the "ber after the growth, and, yields Y O }ZrO single-crystal "bers with better optical and mechanical properties.
2. Theoretical analysis The LHPG method is derived from Czochralski-growth method; one of its main features is high growth rate, which requires large temperature gradient during the growth, resulting in large residual thermal stress inside the Y O }ZrO "ber after growth with concomitant cracks, which prevents Y O }ZrO "bers from achieving high optical and mechanical properties in previous studies [3,5]. In order to avoid the cracking, the temperature gradient during the growth must be reduced to a certain degree. The maximum acceptable axial temperature gradient during the growth and maximum acceptable cooling rate after the growth for growing an uncracked crystal by Czochralski-growth method has been obtained as [6]
d¹ dz
"
4e aR
1 1 1! hR , h 2
(1)
Ke , "2(2 (2) aC R
T where (d¹/dz) is the maximum acceptable axial
temperature gradient during the growth, Z represents the axial direction of the "ber, (d¹/dt) the
maximum acceptable cooling rate after the growth, h the cooling constant, R the radius of the "ber, e the breaking strain of the crystal, a the expansion coe$cient, K the thermal conductivity and C 4 is the speci"c heat of the crystal. For cubic Y O }ZrO crystal, the relevant parameters are given in Table 1, for comparison, data of sapphire are also listed in the table. From Table 1, we can see that for the same radius R"0.25 mm, (d¹/dz)
and (d¹/dt) of sapphire are much larger than
that of Y O }ZrO , so it is easier to grow high quality sapphire "bers than Y O }ZrO "bers un der similar growth conditions. During the LHPG growth, the temperature distribution inside a long "ber can be expressed as [6] d¹ dt
(1!hr/2R) 1!hR 2h ;exp ! Z , R
¹(r, z)"¹ #(¹ !¹ )
(3)
where ¹ is the environmental temperature sur rounding the molten zone, ¹ the melting point of
Table 1 Properties of cubic Y O }ZrO and sapphire single crystals [7}10] Property
Y O }ZrO crystal
Sapphire (a-Al O )
Thermal expansion a (K\) Speci"c heat C (J/cm K) T Thermal conductivity K (J/cm s K) Breaking strain e Cooling constant h (cm\) Melting point ¹ (K)
For "bers with radius R"0.025 cm (d¹/dz) (K/cm)
(d¹/dt) (K/s)
8.8;10\ (293 K) 1.25 (2755 K) 0.105 (2523 K) 2.0;10\ 1.1 2963
5.0;10\ (c-axial) (293 K) 1.31 (2273 K) 0.33 (2073 K) 2.8;10\ 0.65 2318
2.2;10 8.6;10
6.9;10 6.4;10
Experimental results.
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
the crystal, and r represents the radial direction. From Eq. (3) we can obtain
2h Z . R
(4)
Therefore, the largest temperature gradient exists at the point r"0 and z"0, which is
d¹ dz
(2h/R) . "(¹ !¹ )
1!hR
(5)
In our previous work, ¹ +203C (the room tem perature), assume the "ber radius R"0.25 mm; then for Y O }ZrO "ber, "d¹/dz" "
2.5;10 (K/cm), which is larger than the maximum acceptable temperature gradient obtained by Table 1 (about 2.2;10 K/cm), while for sapphire "ber with the same radius, "d¹/dz" "1.7;
10 (K/cm), which is much smaller than the maximum acceptable temperature gradient obtained by Table 1 (about 6.9;10 K/cm), so the Y O }ZrO "ber is very likely to crack after growth while the sapphire "bers can be easily grown without cracking by the conventional LHPG system. These results indicate that, in order to grow Y O }ZrO "bers without cracking, the large temperature gradient must be reduced. In addition, in our previous work, the "ber was directly cooled by the surrounding air during the growth, so the largest cooling rate can be estimated as
d¹ dt
"
d¹ dz dz dt
In order to grow high-quality Y O }ZrO "ber without crack, the temperature gradient must be reduced; for this purpose, in our work, an additional heating system was added to the conventional LHPG system as shown in Fig. 1. The Y O }ZrO seed "ber was enclosed by a heating cylinder that consisted of a 6-mm-diameter 120mm-length ceramic tube and electric heating coils. The heating cylinder was wrapped by asbestos insulating wall to maintain high temperature inside the ceramic tube. A stream of nitrogen gas was introduced into the ceramic tube from the top by a corundum tube, which was employed for the following reasons: (1) During melting, Y O }ZrO shows a percep tible volatility at these high temperatures [3,5]. The surface of the cooling "ber therefore becomes coated with a layer of condensed material during the growth process and thus causes large optical
"
3. Experimental details 3.1. Improved LHPG system
d¹ 2h (1!hr/2R) "(¹ !¹ )
R dz 1!hR ;exp !
283
d¹ dz
dz dt
(2h/R) "(¹ !¹ ) v ,
1!hR D
(6)
where the growth rate v ""dz/dt" is about D 2 mm/min, then for a 0.25 mm-radius Y O }ZrO "ber, the largest cooling rate "dz/dt" obtained by
Eq. (6) is about 100 K/s, which is much smaller than the maximum acceptable cooling rate (about 8.6;10 K/s) listed in Table 1, so the cooling rate is not the reason for cracking.
Fig. 1. Improved LHPG system for growing Y O }ZrO single-crystal "bers.
284
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
scattering losses. The nitrogen gas blows volatile material away from the molten zone and thus prevents the "ber surface from being coated. (2) Since the molten zone is mainly heated by the CO laser beams input from outside, the additional heater could not be made long enough to encircle the zone or the laser beams would be blocked. Therefore, the heater could not directly provide a high-temperature ambience close enough to the zone. The small gas stream which is heated by the heating cylinder is able to maintain higher temperatures near the zone (at the bottom of the heating tube). (3) The nitrogen gas can also protect the tungsten heating coil from being oxidized under high temperature for long working times. In this work, the #ux of the nitrogen gas was 6.5 ml/s, the temperature distribution (begins from the molten zone) inside the heating tube measured by a thermocouple is shown in Fig. 2, which shows that the environmental temperature around the molten zone is about 9503C, and the temperature inside the heating cylinder is about 11003C. Inserting ¹ "9503C (1223 K) into Eq. (5), and assuming the "ber radius R"0.25 mm, the largest temperature gradient "d¹/dz" is calculated to be
1.66;10 (K/cm). This is much smaller than the largest temperature gradient in our previous work (2.5;10 K/cm), and also smaller than the maximum acceptable temperature gradient
(2.2;10 K/cm). So the additional heating system had reduced the temperature gradient to an acceptable value and made it possible to stably grow Y O }ZrO "bers without cracking. Meanwhile, it also o!ered a high-temperature ambience for annealing the "ber just after the growth. The growth rate was 0.55}0.85 mm/min, which was slower than the rate used in our previous work (which was greater than 2 mm/min). The low growth rate could o!er the "ber a su$cient annealing time (about 2.4}3.6 h) inside the heating tube and thus reduce the residual stress inside the "ber. The source rods used here were originally cut from a bulky cubic 21.2 at% Y O }stabilized ZrO crystal and then ground into 1.30-mm-dia meter 50-mm-length cylinders. 3.2. Properties of Y2 O3 }ZrO2 single-crystal xbers Five Y O }ZrO single-crystal "bers were grown by the improved LHPG system, the growth conditions and optical losses of these "bers are listed in Table 2. In comparison, Fig. 3 gives transmission spectra of "ber no. 3 and a Y O }ZrO "ber grown in our previous work with similar diameter and length. The optical losses and transmission spectra were measured by our FM-1 singlecrystal "ber measurement system [11]. Experimental results show that optical losses of these "bers approximately decrease with the decreasing of their growth rates, which can be explained as less defects exist in "bers with lower growth rates, the exceptional result of "ber no. 5 in Table 2 may be caused by the larger temperature gradient existing in the growth due to its relatively larger "ber diameter. The average loss of these Table 2 Growth conditions and optical losses of Y O }ZrO single crystal "bers grown by improved LHPG system
Fig. 2. Temperature distribution along the axial direction beginning from the molten zone.
No.
Diameter (mm)
Length (mm)
Growth rate (mm/min)
Optical loss at 900 nm (dB/cm)
1 2 3 4 5
0.32$0.02 0.39$0.02 0.45$0.01 0.48$0.01 0.53$0.02
180 285 200 155 120
0.78 0.85 0.75 0.55 0.60
0.39 0.43 0.38 0.35 0.41
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
285
Table 3 Strain to failure (STF) test results of Y O }ZrO single-crystal "bers grown separately by the improved LHPG system and the conventional LHPG system No.
1 2 3 4 5 6 Fig. 3. Transmission spectrum of two 0.45-mm-diameter and 200-mm-length Y O }ZrO single-crystal "bers grown separ ately by conventional LHPG system and improved LHPG system.
Y O }ZrO "bers is about 0.39 dB/cm at wavelength of 900 nm, although it is still much larger than that of sapphire "bers (which is about 0.03 dB/cm at 900 nm), it is much more transparent than those "bers grown in our previous work [3] (which was about 0.60 dB/cm at 900 nm) and more suitable for radiation-based high-temperature "ber-optic sensors. The mechanical strength of these Y O }ZrO "bers was also enhanced. Our previously grown Y O }ZrO "bers were very weak to mechanical impact due to inner micro-cracks produced by the large temperature gradients, they often broke in the cutting, grinding and polishing processes. For this reason, they were always used in short lengths and it was quite di$cult to produce a ready-to-use Y O }ZrO "bers. In this work, the Y O }ZrO "bers listed in Table 1 have been safely ground by an electric grinder and polished by diamond "lms before optical test. The strain to failure (STF) of "ber nos. 1, 3 and 5 have been tested by a threepoint bending method and the test results are listed in Table 3, along with three Y O }ZrO "bers from our previous experiment (nos. 2, 4 and 6) for comparison, they all have similar diameters. All tests were performed at 203C and 65% relative humidity with the gauge length of 50 mm. The mechanical test results in Table 3 shows that the
Diameter (mm)
0.32$0.02 0.35$0.02 0.45$0.01 0.44$0.01 0.53$0.02 0.51$0.03
STF (%)
0.31 0.16 0.26 0.10 0.18 0.07
Growth conditions Growth rate (mm/min)
System used
0.78 3.50 0.75 2.34 0.60 3.15
Improved Conventional Improved Conventional Improved Conventional
average STF (about 0.25%) of Y O }ZrO "bers grown in this work was much higher than previous result (0.11%), and they are more resistive to mechanical shock and thus can be used in longer lengths.
4. Conclusions To grow higher quality Y O }ZrO single-crys tal "bers by LHPG system with smaller thermal stress, theoretical analysis suggested that an additional heating system should be added to our conventional LHPG system to reduce the thermal gradient. Five Y O }ZrO single-crystal "bers have been grown in the improved system. Better optical and mechanical properties were achieved compared to previous studies. These "bers are very promising for ultra-high-temperature "ber-optic sensor applications.
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 59906011), the author thanks Ms. Jingyi Lou in our lab for helping with the con"guration and temperature measurement of the heating system. Thanks are also given to Prof. Jiqin Chen in MSE Department for helpful discussion on the growth conditions of Y O }ZrO "bers.
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L. Tong / Journal of Crystal Growth 217 (2000) 281}286
References [1] R.R. Dils, High temperature optical "ber thermometer, J. Appl. Phys. 54 (1983) 1198. [2] H. Amick, Optical "ber sensors broaden temperature measurement limits, Research & Development, Report, Accu"ber Inc, Vancouver, 1986, pp. 64}66. [3] Limin Tong, Yanqi Wang, Zuchang Ding, Growth and characteristics of Y}ZrO SCF for high-temperature optic sensors, Proceedings of SPIE, Vol. 2292, 1994, pp. 429}438. [4] Limin Tong, Yonghang Shen, Linhua Ye, Zuchang Ding, A zirconia single-crystal "bre-optic sensor for contact measurement of temperatures above 20003C, Measurement Sci. Technol. 10 (1999) 607}611. [5] Limin Tong, Study on growth and properties of Y O }ZrO single-crystal "bers for "ber-optic sensors, M.S. Thesis, Zhejiang University, 1994.
[6] J.C. Brice, The cracking of Czochralski-grown crystals, J. Crystal Growth 42 (1977) 427. [7] Paul Klocek, Handbook of Infrared Optical Materials, Marcel Dekker Inc., New York, 1991. [8] Samuel J. Schneider, Engineered Materials Handbook, Vol. 4, Ceramics & Glasses, ASM International, OH, Handbook Committee. [9] Y.S. Touloukian, E.H. Buyco, Thermophysical Properties of Matter, Vol. 5, Speci"c Heat, Nonmetallic Solids, IFI/Plenum, New York, 1970. [10] Y.S. Touloukian, P.W. Powell, C.Y. Ho, P.G. Klemens, Thermophysical Properties of Matter, Vol. 2, Thermal Conductivity, Nonmetallic Solids, IFI/Plenum, New York, 1970. [11] Mianyu Dong, Limin Tong, Zuchang Ding, SPIE 2290 (1994) 378.
Growth of high-quality Y O }ZrO single-crystal optical "bers for ultra-high-temperature "ber-optic sensors Limin Tong Department of Physics, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China Received 7 December 1999; accepted 7 March 2000 Communicated by R.S. Feigelson
Abstract Y O }ZrO (Y O stabilized ZrO ) single-crystal "bers are promising candidates for developing ultra-high-temper ature "ber-optic sensors; however, grown by conventional laser heated pedestal growth (LHPG) system, these "bers showed relatively low optical and mechanical qualities due to thermal stress caused by large temperature gradient during the growth, preventing the sensors from achieving high performance in our previous work. In order to grow high-quality Y O }ZrO single-crystal "bers with less stress, in this paper, according to the theoretical analysis, we have improved the growth system by adding an additional heating system, and the temperature gradient during growth has been reduced to an acceptable value. Five Y O }ZrO single-crystal "bers have been grown by the improved system; experimental test results showed that the optical and mechanical qualities of these "bers have been greatly enhanced, with which the ultra-high-temperature "ber-optic sensors with better performance could be developed. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 81.10; 42.81; 78.66 Keywords: Crystal growth; Y O }ZrO single-crystal "ber; Optical properties
1. Introduction Radiation-based high-temperature "ber-optic sensors, especially the sapphire "ber-optic sensor, have been widely used due to their special advantages of high accuracy, fast response, intrinsic immunity to electromagnetic interference and long life [1,2]. However, the melting point, 20453C, of the sapphire crystal "ber limits the sensor to be used beyond 20003C. In order to extend the working temperature of "ber-optic sensors above 20003C, a new kind of crystal "ber with higher melting point must be used to substitute for the sapphire "ber.
Y O }ZrO (Y O -stabilized ZrO ) is a very promising material for this purpose due to its very high melting point (about 27003C) and relatively stable physical and chemical properties within visible and near-infrared band. Several years ago, we had grown Y O }ZrO single-crystal "bers by means of laser-heated pedestal growth (LHPG) method [3]; however, the optical and mechanical properties of those "bers were not as good as we expected due to the large residual thermal stress. Recently, a zirconia (Y O }ZrO ) single-crystal "ber-optic sensor was developed [4]. Although it had been successfully operated upto 23003C, its
0022-0248/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 4 8 2 - 6
282
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
accuracy was much lower than the sapphire "beroptic sensor because of the low transmissivity of the Y O }ZrO "ber. In order to improve the proper ties of Y O }ZrO single-crystal "bers and then improve the properties of zirconia "ber-optic sensors, it was necessary to improve our original LHPG system and growth conditions. This paper describes how a reduction in temperature gradients in the molten zone during the growth, reduces the residual thermal stress inside the "ber after the growth, and, yields Y O }ZrO single-crystal "bers with better optical and mechanical properties.
2. Theoretical analysis The LHPG method is derived from Czochralski-growth method; one of its main features is high growth rate, which requires large temperature gradient during the growth, resulting in large residual thermal stress inside the Y O }ZrO "ber after growth with concomitant cracks, which prevents Y O }ZrO "bers from achieving high optical and mechanical properties in previous studies [3,5]. In order to avoid the cracking, the temperature gradient during the growth must be reduced to a certain degree. The maximum acceptable axial temperature gradient during the growth and maximum acceptable cooling rate after the growth for growing an uncracked crystal by Czochralski-growth method has been obtained as [6]
d¹ dz
"
4e aR
1 1 1! hR , h 2
(1)
Ke , "2(2 (2) aC R
T where (d¹/dz) is the maximum acceptable axial
temperature gradient during the growth, Z represents the axial direction of the "ber, (d¹/dt) the
maximum acceptable cooling rate after the growth, h the cooling constant, R the radius of the "ber, e the breaking strain of the crystal, a the expansion coe$cient, K the thermal conductivity and C 4 is the speci"c heat of the crystal. For cubic Y O }ZrO crystal, the relevant parameters are given in Table 1, for comparison, data of sapphire are also listed in the table. From Table 1, we can see that for the same radius R"0.25 mm, (d¹/dz)
and (d¹/dt) of sapphire are much larger than
that of Y O }ZrO , so it is easier to grow high quality sapphire "bers than Y O }ZrO "bers un der similar growth conditions. During the LHPG growth, the temperature distribution inside a long "ber can be expressed as [6] d¹ dt
(1!hr/2R) 1!hR 2h ;exp ! Z , R
¹(r, z)"¹ #(¹ !¹ )
(3)
where ¹ is the environmental temperature sur rounding the molten zone, ¹ the melting point of
Table 1 Properties of cubic Y O }ZrO and sapphire single crystals [7}10] Property
Y O }ZrO crystal
Sapphire (a-Al O )
Thermal expansion a (K\) Speci"c heat C (J/cm K) T Thermal conductivity K (J/cm s K) Breaking strain e Cooling constant h (cm\) Melting point ¹ (K)
For "bers with radius R"0.025 cm (d¹/dz) (K/cm)
(d¹/dt) (K/s)
8.8;10\ (293 K) 1.25 (2755 K) 0.105 (2523 K) 2.0;10\ 1.1 2963
5.0;10\ (c-axial) (293 K) 1.31 (2273 K) 0.33 (2073 K) 2.8;10\ 0.65 2318
2.2;10 8.6;10
6.9;10 6.4;10
Experimental results.
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
the crystal, and r represents the radial direction. From Eq. (3) we can obtain
2h Z . R
(4)
Therefore, the largest temperature gradient exists at the point r"0 and z"0, which is
d¹ dz
(2h/R) . "(¹ !¹ )
1!hR
(5)
In our previous work, ¹ +203C (the room tem perature), assume the "ber radius R"0.25 mm; then for Y O }ZrO "ber, "d¹/dz" "
2.5;10 (K/cm), which is larger than the maximum acceptable temperature gradient obtained by Table 1 (about 2.2;10 K/cm), while for sapphire "ber with the same radius, "d¹/dz" "1.7;
10 (K/cm), which is much smaller than the maximum acceptable temperature gradient obtained by Table 1 (about 6.9;10 K/cm), so the Y O }ZrO "ber is very likely to crack after growth while the sapphire "bers can be easily grown without cracking by the conventional LHPG system. These results indicate that, in order to grow Y O }ZrO "bers without cracking, the large temperature gradient must be reduced. In addition, in our previous work, the "ber was directly cooled by the surrounding air during the growth, so the largest cooling rate can be estimated as
d¹ dt
"
d¹ dz dz dt
In order to grow high-quality Y O }ZrO "ber without crack, the temperature gradient must be reduced; for this purpose, in our work, an additional heating system was added to the conventional LHPG system as shown in Fig. 1. The Y O }ZrO seed "ber was enclosed by a heating cylinder that consisted of a 6-mm-diameter 120mm-length ceramic tube and electric heating coils. The heating cylinder was wrapped by asbestos insulating wall to maintain high temperature inside the ceramic tube. A stream of nitrogen gas was introduced into the ceramic tube from the top by a corundum tube, which was employed for the following reasons: (1) During melting, Y O }ZrO shows a percep tible volatility at these high temperatures [3,5]. The surface of the cooling "ber therefore becomes coated with a layer of condensed material during the growth process and thus causes large optical
"
3. Experimental details 3.1. Improved LHPG system
d¹ 2h (1!hr/2R) "(¹ !¹ )
R dz 1!hR ;exp !
283
d¹ dz
dz dt
(2h/R) "(¹ !¹ ) v ,
1!hR D
(6)
where the growth rate v ""dz/dt" is about D 2 mm/min, then for a 0.25 mm-radius Y O }ZrO "ber, the largest cooling rate "dz/dt" obtained by
Eq. (6) is about 100 K/s, which is much smaller than the maximum acceptable cooling rate (about 8.6;10 K/s) listed in Table 1, so the cooling rate is not the reason for cracking.
Fig. 1. Improved LHPG system for growing Y O }ZrO single-crystal "bers.
284
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
scattering losses. The nitrogen gas blows volatile material away from the molten zone and thus prevents the "ber surface from being coated. (2) Since the molten zone is mainly heated by the CO laser beams input from outside, the additional heater could not be made long enough to encircle the zone or the laser beams would be blocked. Therefore, the heater could not directly provide a high-temperature ambience close enough to the zone. The small gas stream which is heated by the heating cylinder is able to maintain higher temperatures near the zone (at the bottom of the heating tube). (3) The nitrogen gas can also protect the tungsten heating coil from being oxidized under high temperature for long working times. In this work, the #ux of the nitrogen gas was 6.5 ml/s, the temperature distribution (begins from the molten zone) inside the heating tube measured by a thermocouple is shown in Fig. 2, which shows that the environmental temperature around the molten zone is about 9503C, and the temperature inside the heating cylinder is about 11003C. Inserting ¹ "9503C (1223 K) into Eq. (5), and assuming the "ber radius R"0.25 mm, the largest temperature gradient "d¹/dz" is calculated to be
1.66;10 (K/cm). This is much smaller than the largest temperature gradient in our previous work (2.5;10 K/cm), and also smaller than the maximum acceptable temperature gradient
(2.2;10 K/cm). So the additional heating system had reduced the temperature gradient to an acceptable value and made it possible to stably grow Y O }ZrO "bers without cracking. Meanwhile, it also o!ered a high-temperature ambience for annealing the "ber just after the growth. The growth rate was 0.55}0.85 mm/min, which was slower than the rate used in our previous work (which was greater than 2 mm/min). The low growth rate could o!er the "ber a su$cient annealing time (about 2.4}3.6 h) inside the heating tube and thus reduce the residual stress inside the "ber. The source rods used here were originally cut from a bulky cubic 21.2 at% Y O }stabilized ZrO crystal and then ground into 1.30-mm-dia meter 50-mm-length cylinders. 3.2. Properties of Y2 O3 }ZrO2 single-crystal xbers Five Y O }ZrO single-crystal "bers were grown by the improved LHPG system, the growth conditions and optical losses of these "bers are listed in Table 2. In comparison, Fig. 3 gives transmission spectra of "ber no. 3 and a Y O }ZrO "ber grown in our previous work with similar diameter and length. The optical losses and transmission spectra were measured by our FM-1 singlecrystal "ber measurement system [11]. Experimental results show that optical losses of these "bers approximately decrease with the decreasing of their growth rates, which can be explained as less defects exist in "bers with lower growth rates, the exceptional result of "ber no. 5 in Table 2 may be caused by the larger temperature gradient existing in the growth due to its relatively larger "ber diameter. The average loss of these Table 2 Growth conditions and optical losses of Y O }ZrO single crystal "bers grown by improved LHPG system
Fig. 2. Temperature distribution along the axial direction beginning from the molten zone.
No.
Diameter (mm)
Length (mm)
Growth rate (mm/min)
Optical loss at 900 nm (dB/cm)
1 2 3 4 5
0.32$0.02 0.39$0.02 0.45$0.01 0.48$0.01 0.53$0.02
180 285 200 155 120
0.78 0.85 0.75 0.55 0.60
0.39 0.43 0.38 0.35 0.41
L. Tong / Journal of Crystal Growth 217 (2000) 281}286
285
Table 3 Strain to failure (STF) test results of Y O }ZrO single-crystal "bers grown separately by the improved LHPG system and the conventional LHPG system No.
1 2 3 4 5 6 Fig. 3. Transmission spectrum of two 0.45-mm-diameter and 200-mm-length Y O }ZrO single-crystal "bers grown separ ately by conventional LHPG system and improved LHPG system.
Y O }ZrO "bers is about 0.39 dB/cm at wavelength of 900 nm, although it is still much larger than that of sapphire "bers (which is about 0.03 dB/cm at 900 nm), it is much more transparent than those "bers grown in our previous work [3] (which was about 0.60 dB/cm at 900 nm) and more suitable for radiation-based high-temperature "ber-optic sensors. The mechanical strength of these Y O }ZrO "bers was also enhanced. Our previously grown Y O }ZrO "bers were very weak to mechanical impact due to inner micro-cracks produced by the large temperature gradients, they often broke in the cutting, grinding and polishing processes. For this reason, they were always used in short lengths and it was quite di$cult to produce a ready-to-use Y O }ZrO "bers. In this work, the Y O }ZrO "bers listed in Table 1 have been safely ground by an electric grinder and polished by diamond "lms before optical test. The strain to failure (STF) of "ber nos. 1, 3 and 5 have been tested by a threepoint bending method and the test results are listed in Table 3, along with three Y O }ZrO "bers from our previous experiment (nos. 2, 4 and 6) for comparison, they all have similar diameters. All tests were performed at 203C and 65% relative humidity with the gauge length of 50 mm. The mechanical test results in Table 3 shows that the
Diameter (mm)
0.32$0.02 0.35$0.02 0.45$0.01 0.44$0.01 0.53$0.02 0.51$0.03
STF (%)
0.31 0.16 0.26 0.10 0.18 0.07
Growth conditions Growth rate (mm/min)
System used
0.78 3.50 0.75 2.34 0.60 3.15
Improved Conventional Improved Conventional Improved Conventional
average STF (about 0.25%) of Y O }ZrO "bers grown in this work was much higher than previous result (0.11%), and they are more resistive to mechanical shock and thus can be used in longer lengths.
4. Conclusions To grow higher quality Y O }ZrO single-crys tal "bers by LHPG system with smaller thermal stress, theoretical analysis suggested that an additional heating system should be added to our conventional LHPG system to reduce the thermal gradient. Five Y O }ZrO single-crystal "bers have been grown in the improved system. Better optical and mechanical properties were achieved compared to previous studies. These "bers are very promising for ultra-high-temperature "ber-optic sensor applications.
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 59906011), the author thanks Ms. Jingyi Lou in our lab for helping with the con"guration and temperature measurement of the heating system. Thanks are also given to Prof. Jiqin Chen in MSE Department for helpful discussion on the growth conditions of Y O }ZrO "bers.
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L. Tong / Journal of Crystal Growth 217 (2000) 281}286
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