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Crystallization of fluoroaluminate glasses
306
Journal of Non-Crystalline Solids 112 (1989) 306-308 North-Holland, A m s t e r d a m
CRYSTALLIZATION OF FLUOROALUMINATE GLASSES HU Hefang, LIN Fengying, YUAN Yibo and F E N G Jitian Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, PR China
The crystallization kinetics of some fluoroaluminate glasses during the melt-cooling and glass-reheating processes were studied under non-isothermal conditions. The critical cooling rate, Re, for glass formation was estimated, based on the relationship between the undercooling and the cooling rate, using the differential thermal analysis technique. The activation energy for crystalfization was calculated using the J o h n s o n - M e h l - A v r a m i equation. It was found that A I F 3 - Y F 3 - M g F 2 C a F 2 - S r F 2 - B a F 2 glasses have a small value of R c. The crystalline phases in the devitrified fluoroaluminate glasses were also investigated.
1. Introduction
aluminate glasses has not been extensively investigated. In this paper the crystallization kinetics of fluoroaluminate glasses during the melt-cooling and glass-reheating processes are studied under nonisothermal conditions using the differential thermal analysis (DTA) technique. It is found that AIF3-YF3-MgFz-CaFz-SrF2-BaF 2 glasses have a small critical cooling rate R c, which is close to that of fluorozirconate glasses. The crystalline phases in the devitrified glasses are also investigated.
In the past decade fluorozirconate glasses have received much attention as potential ultralow-loss optical fiber materials. The optical loss of fluorozirconate glass fibers have been greatly reduced to less than 1 d b / k m [1]. However, the poor chemical durability and the low mechanical strength of fluorozirconate glasses must be improved for practical applications. It has been reported [2] that the fluoroaluminate glasses have many attractive properties for optical communications. These are a wide transparent region from 0.2 to 7 ~m, a low refractive index, a high Abb6 number, and good chemical durability and mechanical properties. However, the tendency to crystallization during the melt-cooling and glass-reheating processes is still large, and the crystallization of fluoro-
2. Experimental The glasses were prepared from anhydrous, chemically pure (NH4)3A1F6, YF 3, MgF2, CaF 2,
Table 1 Chemical composition and properties of the glasses Glass
Nominal composition (mol%) A1F3
ACB Y-0 Y-5 Y-10 Y-15 Y-20 Y-25
40 40 40 40 40 40 40
YF 3
5 10 15 20 25
MgF2
CaF2
10 10 10 10 10 10
39 30 30 30 30 30 30
SrF 2
BaF 2
10 10 10 10 10 10
21 10 10 10 10 10 10
0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Tg
Tx
Tx- Tg
R~
E
(K)
(K)
(K)
(K/min)
(kcal/mol)
In ~' (~, 1/min)
692 693 697 699 701 704 704
757 787 797 808 826 815 810
65 94 100 109 125 111 106
70 35 23 19 17 20 22
88 65 55 53 49 52 65
57.0 40.4 32.9 31.1 28.1 30.6 38.8
Hu Hefang et al. / Crystallization of fluoroaluminate glasses
SrF2 and BaF2 by melting 20-50 g batches in a Pt crucible at 1200-1300 K under a protective atmosphere, and quenching the melts on a cold a l u m i n u m plate. The chemical composition of the glass studied is listed in table 1. The melt-to-crystal transformation was studied by measuring the first crystallization temperature at different cooling rates using DTA. The specimens were put in capped Pt crucibles, heated to 1200 K, held at that temperature for 5 min and then cooled at a rate of 1 to 40 K / m i n . The crystallization kinetics during the glass-reheating process were investigated by measuring the peak temperature of crystallization at different heating rates using the DTA apparatus. Bulk glass specimens were used to avoid surface effects.
307
"~~o
o
~:3 _c
1
I 2
r 4
I 6
i 8
110
ATc2XlO4
Fig. 1. Plot of In R versus 1/ATc2 for the fluoroaluminate glasses indicated.
3. Results and discussion
Some characteristic temperatures, Tg and Tx, of glasses studied are listed in table 1. These characteristic temperatures were measured at a heating rate of 6 K / m i n using the DTA apparatus. The experimental error of these values is less than + 2 K. The critical cooling rates, R c, for glass formation were estimated using eq. [3] In R = In R c - B / A T c 2,
(1)
where R is the cooling rate, AT~ is the undercooling (the difference between the liquidus temperature T~ and the first crystallization temperature T~ for melt cooling) and B is a constant. T l was obtained as the temperature of the first crystallization peak at a cooling rate of 1 K / m i n . Plots of lnR v e r s u s I/ATc 2 are shown in fig. 1. The Rc value for glasses studied, calculated by computer fit, are listed in table 1. This shows that the glass ACB has the largest R~ value and that a moderate amount of YF 3 enhances the glass-forming ability of those fhioroaluminate glasses. The Johnson-Mehl-Avrami equation [4,5], y=l-exp[-(kt)n],
(2)
for an isothermal solid state transformation, was used to study the crystallization kinetics of the fluoroaluminate glasses during the reheating pro-
cess. In eq. (2) y is the volume fraction transformed at time t when heated at constant temperature, n is a parameter known as the Avrami exponent, which depends upon the nucleation a n d / o r growth morphology; k is the reaction rate constant and is expressed as k = v exp(-E/RT),
(3)
where E and u are the activation energy and the frequency factor for the overall transformation process, respectively, and R is the universal gas constant. E and u can be determined by plotting ln(Tff/a) versus 1/Tp, according to the Kissinger type equation [6] l n ( T ~ / a ) = I n ( E / R ) - In u + E / R T p ,
(4)
where Tp is the peak temperature of the exothermic peak and a is the heating rate. Plots of ln(Tp2/a) versus 1 / T p for some of the glasses studied are shown in fig. 2. The activation energies and frequency factors, calculated from the experimental data by computer fit, are listed in table 1. The glass ACB is characterized by the highest activation energy and the largest value of the frequency factor. For the 40A1F3-10MgF2 ± 30CaF2-10SrF2-10BaF2-xYF 3 glasses, the activation energy and the frequency factor decrease until x = 15 and then increase slightly as the Y ~ content increases.
Hu Hefang et al. / Crystallization of fluoroaluminate glasses
308
/
12
c
/// 11
/ 12
/ 13
T~IxIo 4
Fig. 2. Plot of ln(T2/a)__ versus 1 / T o for the fluoroaluminate glasses indicated.
X-ray diffraction (XRD) curves for the devitrified Y-0 to Y-20 samples are shown in fig. 3. The main crystalline phases in the devitrified Y-0 and Y-5 samples are CaSrA1F 7 and fl-CaA1F5. With increasing YF 3 content, the XRD intensity of these two phases decreases, and a new unidentified phase, which may contain YF3, appears in the devitrified Y-10 sample. The XRD peaks of the unidentified phase are located at 3.23, 3.11, 1.98, 1.68, 3.48, 3.58 and 2.80 ,~ etc. In the devitrified Y-15 and Y-20 samples, both the CaSrA1F7 and the fl-CaA1F5 phases disappear and the unidentified phase becomes the major crystalline phase. It is clear that the introduction of a suitable amount of YF 3 restrains the formation of the CaSrA1F7 and fl-CaA1F5 crystalline phases in these fluoroaluminate glasses during the reheating process and hence enhances glass formation.
4. Conclusions The introduction of a suitable amount of YF 3 restrains the formation of CaSrA1F7 and fl-CaA1F5, the main crystalline phases in the present devitrifled fluoroaluminate glasses, and thus enhances the glass-forming ability. The optimum amount of YF 3 is in the vicinity of 15 mol%. The critical cooling rates for some of the fluoroaluminate glass studied are close to those for typical fluorozirconate glasses.
References
°**|j|t,.,**|jlt,|a,i*ttlltJat**l*l*.ll*.*ml|
20 30 4o 50 60 20 Fig. 3. X-ray diffraction curves for the devitfified glasses.
[1] T. Kanamori and S. Sakaguchi, Jap. J. Appl. Phys. 25 (1986) L468. [2] Hefang Hu, Fengying Lin et al., Chinese J. Infr. Mill. Waves 6 (1987) 181. [3] J.M. Barandiaran and J. Colmenero, J. Non-Cryst. Solids 46 (1981) 277. [4] W.A. Johnson and R.F. Mehl, Trans. Amer. Inst. Min. (Metal.). Engs. 135 (1939) 416. [5] M. Avrami, J. Chem. Phys. 9 (1941) 177. [6] E. Kissinger, Anal. Chem. 29 (1957) 1702.
Journal of Non-Crystalline Solids 112 (1989) 306-308 North-Holland, A m s t e r d a m
CRYSTALLIZATION OF FLUOROALUMINATE GLASSES HU Hefang, LIN Fengying, YUAN Yibo and F E N G Jitian Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, PR China
The crystallization kinetics of some fluoroaluminate glasses during the melt-cooling and glass-reheating processes were studied under non-isothermal conditions. The critical cooling rate, Re, for glass formation was estimated, based on the relationship between the undercooling and the cooling rate, using the differential thermal analysis technique. The activation energy for crystalfization was calculated using the J o h n s o n - M e h l - A v r a m i equation. It was found that A I F 3 - Y F 3 - M g F 2 C a F 2 - S r F 2 - B a F 2 glasses have a small value of R c. The crystalline phases in the devitrified fluoroaluminate glasses were also investigated.
1. Introduction
aluminate glasses has not been extensively investigated. In this paper the crystallization kinetics of fluoroaluminate glasses during the melt-cooling and glass-reheating processes are studied under nonisothermal conditions using the differential thermal analysis (DTA) technique. It is found that AIF3-YF3-MgFz-CaFz-SrF2-BaF 2 glasses have a small critical cooling rate R c, which is close to that of fluorozirconate glasses. The crystalline phases in the devitrified glasses are also investigated.
In the past decade fluorozirconate glasses have received much attention as potential ultralow-loss optical fiber materials. The optical loss of fluorozirconate glass fibers have been greatly reduced to less than 1 d b / k m [1]. However, the poor chemical durability and the low mechanical strength of fluorozirconate glasses must be improved for practical applications. It has been reported [2] that the fluoroaluminate glasses have many attractive properties for optical communications. These are a wide transparent region from 0.2 to 7 ~m, a low refractive index, a high Abb6 number, and good chemical durability and mechanical properties. However, the tendency to crystallization during the melt-cooling and glass-reheating processes is still large, and the crystallization of fluoro-
2. Experimental The glasses were prepared from anhydrous, chemically pure (NH4)3A1F6, YF 3, MgF2, CaF 2,
Table 1 Chemical composition and properties of the glasses Glass
Nominal composition (mol%) A1F3
ACB Y-0 Y-5 Y-10 Y-15 Y-20 Y-25
40 40 40 40 40 40 40
YF 3
5 10 15 20 25
MgF2
CaF2
10 10 10 10 10 10
39 30 30 30 30 30 30
SrF 2
BaF 2
10 10 10 10 10 10
21 10 10 10 10 10 10
0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Tg
Tx
Tx- Tg
R~
E
(K)
(K)
(K)
(K/min)
(kcal/mol)
In ~' (~, 1/min)
692 693 697 699 701 704 704
757 787 797 808 826 815 810
65 94 100 109 125 111 106
70 35 23 19 17 20 22
88 65 55 53 49 52 65
57.0 40.4 32.9 31.1 28.1 30.6 38.8
Hu Hefang et al. / Crystallization of fluoroaluminate glasses
SrF2 and BaF2 by melting 20-50 g batches in a Pt crucible at 1200-1300 K under a protective atmosphere, and quenching the melts on a cold a l u m i n u m plate. The chemical composition of the glass studied is listed in table 1. The melt-to-crystal transformation was studied by measuring the first crystallization temperature at different cooling rates using DTA. The specimens were put in capped Pt crucibles, heated to 1200 K, held at that temperature for 5 min and then cooled at a rate of 1 to 40 K / m i n . The crystallization kinetics during the glass-reheating process were investigated by measuring the peak temperature of crystallization at different heating rates using the DTA apparatus. Bulk glass specimens were used to avoid surface effects.
307
"~~o
o
~:3 _c
1
I 2
r 4
I 6
i 8
110
ATc2XlO4
Fig. 1. Plot of In R versus 1/ATc2 for the fluoroaluminate glasses indicated.
3. Results and discussion
Some characteristic temperatures, Tg and Tx, of glasses studied are listed in table 1. These characteristic temperatures were measured at a heating rate of 6 K / m i n using the DTA apparatus. The experimental error of these values is less than + 2 K. The critical cooling rates, R c, for glass formation were estimated using eq. [3] In R = In R c - B / A T c 2,
(1)
where R is the cooling rate, AT~ is the undercooling (the difference between the liquidus temperature T~ and the first crystallization temperature T~ for melt cooling) and B is a constant. T l was obtained as the temperature of the first crystallization peak at a cooling rate of 1 K / m i n . Plots of lnR v e r s u s I/ATc 2 are shown in fig. 1. The Rc value for glasses studied, calculated by computer fit, are listed in table 1. This shows that the glass ACB has the largest R~ value and that a moderate amount of YF 3 enhances the glass-forming ability of those fhioroaluminate glasses. The Johnson-Mehl-Avrami equation [4,5], y=l-exp[-(kt)n],
(2)
for an isothermal solid state transformation, was used to study the crystallization kinetics of the fluoroaluminate glasses during the reheating pro-
cess. In eq. (2) y is the volume fraction transformed at time t when heated at constant temperature, n is a parameter known as the Avrami exponent, which depends upon the nucleation a n d / o r growth morphology; k is the reaction rate constant and is expressed as k = v exp(-E/RT),
(3)
where E and u are the activation energy and the frequency factor for the overall transformation process, respectively, and R is the universal gas constant. E and u can be determined by plotting ln(Tff/a) versus 1/Tp, according to the Kissinger type equation [6] l n ( T ~ / a ) = I n ( E / R ) - In u + E / R T p ,
(4)
where Tp is the peak temperature of the exothermic peak and a is the heating rate. Plots of ln(Tp2/a) versus 1 / T p for some of the glasses studied are shown in fig. 2. The activation energies and frequency factors, calculated from the experimental data by computer fit, are listed in table 1. The glass ACB is characterized by the highest activation energy and the largest value of the frequency factor. For the 40A1F3-10MgF2 ± 30CaF2-10SrF2-10BaF2-xYF 3 glasses, the activation energy and the frequency factor decrease until x = 15 and then increase slightly as the Y ~ content increases.
Hu Hefang et al. / Crystallization of fluoroaluminate glasses
308
/
12
c
/// 11
/ 12
/ 13
T~IxIo 4
Fig. 2. Plot of ln(T2/a)__ versus 1 / T o for the fluoroaluminate glasses indicated.
X-ray diffraction (XRD) curves for the devitrified Y-0 to Y-20 samples are shown in fig. 3. The main crystalline phases in the devitrified Y-0 and Y-5 samples are CaSrA1F 7 and fl-CaA1F5. With increasing YF 3 content, the XRD intensity of these two phases decreases, and a new unidentified phase, which may contain YF3, appears in the devitrified Y-10 sample. The XRD peaks of the unidentified phase are located at 3.23, 3.11, 1.98, 1.68, 3.48, 3.58 and 2.80 ,~ etc. In the devitrified Y-15 and Y-20 samples, both the CaSrA1F7 and the fl-CaA1F5 phases disappear and the unidentified phase becomes the major crystalline phase. It is clear that the introduction of a suitable amount of YF 3 restrains the formation of the CaSrA1F7 and fl-CaA1F5 crystalline phases in these fluoroaluminate glasses during the reheating process and hence enhances glass formation.
4. Conclusions The introduction of a suitable amount of YF 3 restrains the formation of CaSrA1F7 and fl-CaA1F5, the main crystalline phases in the present devitrifled fluoroaluminate glasses, and thus enhances the glass-forming ability. The optimum amount of YF 3 is in the vicinity of 15 mol%. The critical cooling rates for some of the fluoroaluminate glass studied are close to those for typical fluorozirconate glasses.
References
°**|j|t,.,**|jlt,|a,i*ttlltJat**l*l*.ll*.*ml|
20 30 4o 50 60 20 Fig. 3. X-ray diffraction curves for the devitfified glasses.
[1] T. Kanamori and S. Sakaguchi, Jap. J. Appl. Phys. 25 (1986) L468. [2] Hefang Hu, Fengying Lin et al., Chinese J. Infr. Mill. Waves 6 (1987) 181. [3] J.M. Barandiaran and J. Colmenero, J. Non-Cryst. Solids 46 (1981) 277. [4] W.A. Johnson and R.F. Mehl, Trans. Amer. Inst. Min. (Metal.). Engs. 135 (1939) 416. [5] M. Avrami, J. Chem. Phys. 9 (1941) 177. [6] E. Kissinger, Anal. Chem. 29 (1957) 1702.