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Viscosity of fluoride glasses near the fiber drawing temperature region
Journal of Non-Crystalline Solids 256&257 (1999) 135±142
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Viscosity of ¯uoride glasses near the ®ber drawing temperature region G. Zhang
a,b
, J. Jiang
a,b
, M. Poulain
a,*
, A.S. Delben b, J.R. Delben
b
a b
Laboratoire des Mat eriaux Photoniques, Universit e de Rennes 1, Rennes 35042, France Dept. de Fisica, Univ. Federal de Mato Grosso do Sul, 79070-900, Campo Grande, Brazil
Abstract The temperature±viscosity dependence of ZrF4 -, InF3 - and CdF2 -based ¯uoride glasses was obtained by two methods. The ®rst method uses a predictive relation based on dierential scanning calorimetric (DSC) data based on the width of the glass transition region i.e. Tg and Tg0 , the beginning and end temperatures of the glass transition. We de®ned the factor, G Tg0 =Tg Tg0 ÿ Tg ; in the relation which determines the viscosity at the temperature (T ÿ Tg0 ). The slope of the calculated log g versus T decreased in the order: ¯uorozirconate glass (0.085 Kÿ1 ) > ¯uoroindate glasses (0.067 Kÿ1 ) > cadmium ¯uorochloride glass (0.047 Kÿ1 ). The second method was by measurement using the parallel-plate technique. The same order of (log g(T)) slope was observed in the measured results, e.g. 0:12 0:02; 0:09 0:02 and 0:08 0:02 Kÿ1 , respectively, for the tetravalent, the trivalent, and the divalent ¯uoride glasses. The Arrhenius activation energies of the shear viscosity are 115 10 kJ/mol for PGICdZn and 64 10 kJ/mol for CdFCl. These dierences in activation energy aect the ®ber drawing ability, because a larger activation energy corresponds to a drawing temperature closer to Tg . Ó 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Fluoride glasses are promising materials for many applications, such as infrared optical signal and energy transmission, ampli®cation for telecommunications, host materials for laser optics, due to their wide transmission range, lower phonon energy of glass network and potentially smallest intrinsic losses. However most of the applications based on their optical properties requires a ®ber form [1]. Unlike silica, where ®bers are fabricated by chemical vapor deposition (CVD) or modi®ed CVD (MCVD) techniques,
* Corresponding author. Tel.: +33-2 99 28 62 63; fax: +33-2 99 28 69 72; e-mail: [email protected]
¯uoride optical ®ber is mainly obtained by drawing a preform at a temperature greater than the glass transition temperature at which the viscosity should be in the range 103 ±106 Pa s [2,3]. The rate of crystallization at this temperature is the key to getting optical quality ®ber [3]. Thus the log g(T ) (T temperature) relation and thermal stability range (Tx ÿ Tg ) calculated from the temperatures for glass transition temperature (Tg ) and crystallization onset temperature (Tx ) are the most important factors for ®ber drawing. Hence an understanding of the viscosity in the ®ber drawing range is essential. Methods, such as beam bending (109 ±1011 , viscosity in Pa s), penetration (107 ±1011 Pa s) and parallel plate (105 ±1010 ) [4], have been applied to measure viscosity at temperatures greater than Tg . Among these techniques, parallel-plate has
0022-3093/99/$ - see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 5 5 - X
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G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
advantages in that a small and simple sample is required and the technique has the capability of measuring viscosity in the range near the ®ber drawing temperature. However the tendency for crystallization of some ¯uoride glasses often restricts the measurement to the smaller viscosity range. Based on dierential scanning calorimetry (DSC) data, the width of the glass transition region, i.e. the beginning and end temperatures, Tg and Tg0 , Moynihan [5] proposed an equation to estimate the temperature dependence of viscosity for predicting ®ber drawing temperature. The viscosities of ¯uorozirconate and ¯uoroaluminate glasses have been studied [6±13]. Their viscosity±temperature dependencies are not ®t by an Arrhenian function or by a Fulcher relation as are classic covalent silicate glass liquids; the ¯uoride liquids are more ionic and they are `fragile glasses' according to the concept introduced by Angell [14]. Similar log g(T) dependencies were observed for ZBLA, ZBLAN [7,9,11], ABCSMY [8,12], ZBLALi [9,13] and ZBLALiPb [6,10] liquids (where Z ZrF4 , B BaF2 , L LaF3 , A AlF3 , C CaF2 , M MgF2 and Y YF3 ). Some of these have been successfully drawn in ®ber. However new heavy halide and chalcogenide glasses, which have phonon energies smaller than current ZrF4 - and AlF3 -based systems, are required for applications as optical ®ber ampli®ers and ®ber lasers [15]. Some ¯uoroindate [16,17] and cadmium ¯uorochloride [18,19] glasses have thermal stability ranges similar to standard ZBLAN ®ber glasses, but ®ber drawing the lower phonon energy glass systems is dicult. The aim of this work is to investigate the composition dependence of viscosity in various ¯uoride glasses, and also to discuss the possibility of ®ber drawing glasses with those phonon energies of interest for active applications.
ZBLAN [20]: 53ZrF4 ±20BaF2 ±4LaF3 ±3AlF3 ± 20NaF ZBLA [20]: 57ZrF4 ±36BaF2 ±3LaF3 ±4AlF3 IZnBS[16]: 34InF3 ±6GaF3 ±20ZnF2 ±16BaF2 ± 20SrF2 ±2GdF3 ±2NaF BIZnYbT [5]: 30BaF2 ±30InF3±20ZnF2 ± 10YbF3 ±10ThF4 GIPCdZn [17]: 22GaF3 ±13InF3 ±30PbF2 ± 18CdF2 ±13ZnF2 ±2GdF3 ±2NaF CdFCl47 [19]: 30CdF2 ±18.5CdCl2 ±8NaF± 25NaCl±12.5BaF2 ±2KF±4LiF The ZBLAN, GIPCdZn and IZnBS glasses were prepared by a typical NH4 HF2 reactive process [20]. Reagent grade oxides and/or ¯uorides were used as raw materials. Batches, 10±20 g in weight, were placed in a platinum tube for ¯uorination at 350°C and 500°C for a few hours. The ¯uorochloride glasses were melted from anhydrous ¯uorides and chlorides. After the normal melting, casting, and annealing steps homogenous bodies or rods were obtained. The glass characteristic temperatures of samples were measured under N2 ¯ow using dierential scanning colorimetry technique (Seiko DSC-220). The standard heating rate was 10°C/min. The errors in the characteristic temperatures were 1 C for the glass transition (Tg ), the end of glass transition (Tg0 ), the onset and peak temperatures of crystallization (Tx and Tp ), and 2 C for melting point (Tm ) and liquidus temperature (Tl ). 2.2. Estimation of viscosity from DSC data By Moynihan's relation [5], we can simulate the viscosity near the ®ber drawing range from DSC data, Tg and Tg0 : log g ÿ5
2. Experimental 2.1. Sample preparation and characteristic temperatures The glass compositions (mol%) used in this work were:
14:2 0:147 T ÿ Tg0 =Tg02 1 ÿ Tg ÿ 1 ÿ Tg0 1
ÿ5
14:2 : 0:147 T ÿ Tg1 =G 1
1
We de®ne a factor G Tg0 =Tg Tg0 ÿ Tg in the relation determining the log g(T) curve at the temperature (T ÿ Tg0 ). For the glasses with
G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
the higher Tg s, the ®ber drawing temperature, Td , can be obtained by Td T ÿ Tg0 log Tg0 T ÿ Tg0 log Tg G:
2
This method yields information about the possibility of ®ber drawing based on a minimal experimental input (Tg and Tg0 only). One of the problems of this approach lies in the dependence of the characteristic temperatures on thermal history. Ideally, the heating rate of the DSC scan should be the same as the cooling rate of the sample through the glass transition region. This condition may be achieved for small laboratory samples. However real preforms used for ®ber drawing are not processed this way. They are usually annealed below Tg to remove thermal stresses and improve sample homogeneity. As a result ®ctive temperature is less, which corresponds to a smaller cooling rate, probably <1°C/ min. A second problem relates to the uncertainty of the Tg s and Tg0 s. Because they are close, the relative error on G may be large: a random error of 1°C for Tg and Tg0 corresponds to an uncertainty of 2 K on G. As will be seen, the observed dierences are large enough to be unambiguous. 2.3. Determination of T±g parallel-plate method The cylindrical samples were slices cut from annealed glass rods and polished. The sample dimensions were typically 7 mm in diameter and 4 mm in thickness. The parallelism of the two surfaces of the sample was better than 3 ´ 10ÿ3 rad. The samples were sandwiched between two pieces of platinum foil, or between two silica plates, and then put into two viscometers (Theta Industries and a modi®ed commercial TMA/SS-220 Seiko). The applied load ranged from 10 to 200 g. By measuring the deformation rate with temperature, the temperature±viscosity relationship was found by using GentÕs equation [21]: g 2Mgh5 =3V dh=dt 2h3 V :
3
It has been reported that the measured viscosity may be aected by the residual stress for a heating rate of about 10 K/min during dynamic measurements [22]. To reduce this eect and also to get the temperature dependence of viscosity closer to that
137
of the real ®ber in its drawing range, we measured viscosity at a 2 K/min heating rate. This rate was chosen empirically by considering the parameters used in normal ¯uoride ®ber drawing and should be not far from a real case. 3. Results 3.1. Calculated temperature dependence of viscosity As the slope of the log g(T ) plot depends on G, the determination of G Tg0 =Tg Tg0 ÿ Tg is important. Tg and Tg0 could vary with the glass annealing and heating rates [22]. Ideally, cooling rate for sample preparation and heating rate for the DSC scan should be identical. In practice, this factor has only a limited eect on the experimental Tg and Tg0 because relaxation times of ¯uoride glasses vary with temperature. In addition, the error which arises from this discrepancy is similar for the two temperatures and close to the uncertainty of the measurement method, i.e., tangent intersection. The Tg s and Tg0 s and the resulting Gs for dierent glasses are reported in Table 1. The eect of the cooling rate, qc , and heating rate, qh , on G appears from this table. Only Gs obtained with rates satisfying the relation 0:2 < qc =qh < 5 have been used for viscosity calculations [5]. For this reason we do not report viscosity for the PGICZ samples as qc /qh < 0:2. The G dierence between the three groups of glasses exceeds the experimental error. The G dierence is due to the dierent temperature dependencies of viscosity. Table 1 also lists the calculated log g±T slopes in the viscosity range 107 ±109 Pa s. based on the selected Gs. The slope of the calculated log g(T ) decreased in the order: ZBLAN 0:085 0:015 > IZnBS 0:067 0:015 > CdFCl47 0:047 0:012:
We assume that the dierence in slopes of the log g(T ) plots is related to the ionicity of the bonding [14]. By comparison with silicate glasses, ¯uoride glasses are ionic and have a larger dlog g/ dT. Among the systems studied in this work, ZrF4 based glasses are more ionic and therefore have the
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G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
Table 1 Determination of viscosity characteristics of some ¯uoride glasses by DSC data Glass
Ratio qc /qh (K/mn)/(K/mn)
Tg (K) 1
Tg0 (K) 1
G (K) 2
Selected G
Dlog g/DT (at Tg0 )
Dlog g/DT ( log g 7±9)
ZBLAN
As made/10 1/10 2/10 6.3/10
537 537 536 535
555 553 554 555
18.2 16.0 18.4 20.4
20.4
0.102
0.085
GIPCdZn
As made/10 1/10
521 520
541 542
21.0 22.3
IZBS
As made 1/10 4.9/10
565 565 563
589 587 588
24.8 22.7 25.9
25.9
0.080
0.067
CdFCl47
As made 1/10 in DSC 4.9/10
399 399 399
431 432 433
34.6 35.3 36.8
36.8
0.056
0.047
Tg and Tg0 are the temperatures for beginning and end of glass transition region. qc is the cooling rate in glass preparation and qh is the heating rate in the DSC scan. G factor is calculated as G Tg0 =Tg Tg0 ÿ Tg . Random errors are 1 K for temperatures, 2 K for G and 0.015 Kÿ1 for the averages slopes Dlog g/DT.
largest viscosity±temperature slope. In the same way the trivalent ¯uoride (GaF3 and InF3 ) based glasses have a have a larger dlog g/dT than the divalent ¯uoride glasses (CdFCl47). These expected trends will be tested by the experimental data from parallel-plate viscosimetry presented in the next section. 3.2. Determination of log g(T) by parallel-plate method Process parameters such as heating rate and applied load were adjusted to give a viscosity range from 107 to 1010 Pa s to be measured by parallelplate method. For the ®rst viscometer (Theta Industries) a 100 g load could be used with a silica pressing probe, 12 mm in diameter. The maximum viscosity (1010 Pa s) is observed at a temperature close to Tg . Smaller samples and loads were used with the modi®ed viscometer (Seiko TMA/SS220), because the silica probe is small (2 mm in diameter). Another limit of the modi®ed viscometer is its downward travel distance for measuring sample deformation, less than 1.5 mm. Deformation curves depend on the applied load and measurements can be implemented at lower temperature with a heavier load while a light load
will be bene®cial to obtain viscosity at a higher temperature, e.g., nearer the ®ber drawing range. Classically, the evolution of log g(1/T ) is linear insofar as the Arrhenian approximation applies. But the plot of log g vs T is also approximately linear if the viscosity range is limited, as in the present case (107 ±1010 Pa s). However, the plot obtained under a 10 g load (Fig. 1) deviates more from linearity than that obtained under larger loads (50 and 100 g). This linearity of the log g(T ) curve in the larger viscosity range is probably due to the contact between the silica plate and the sample under a light load, which could aect deformation rate especially at the lower temperature before the glass is `soft'. As the applied load was increased (50±200 g), the linearity of the log g vs T ÿ1 plot improved in the viscosity range 107 ±109 Pa s in both viscometers. The same results were observed during the calibration using the standard glass 711 from the National Institute of Standards and Technology (NIST). Table 2 reports the log g(T ) slope between 107 and 109 Pa s. Just as the viscosity±temperature dependence estimated in Section 3.1, the measured data show again that the log g(T ) slope decreased with decreasing ionicity from ZBLAN to GIPCdZn and then to CdFCl47 samples.
G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
Fig. 1. log g(T) curves of GIPCdZn glass determined under dierent applied loads. Line is drawn as a guide to the eye.
The activation energies of viscous ¯ow of GIPCdZn and CdFCl47 samples were obtained by plotting log g vs 1000/T (Fig. 2) and were 115 10 kJ/mol for GIPCdZn and 64 kJ/mol for CdFCl47, respectively [23]. These energies could be related to the average bonding energy, and possibly to ionicity. 3.3. Possibility of ®ber drawing The possibility of ®ber drawing may be evaluated from the thermal stability range and the viscosity. We de®ne T0 here as the onset crystallization temperature at which an exothermic deviation from the baseline could be observed. It is evident that the ®ber drawing temperature should be less than T0 instead of Tx because the crystal-
139
lization rate at Tx is too large for an optical ®ber. Table 3 lists T0 from DSC for 40 mg samples. The heating rate was 2 K/min, i.e., the same heating rate used in the viscosity measurement. The temperatures, Td (calc) and Td (meas), are ®ber drawing temperatures at which log g 5 (Pa s), respectively, from the calculated and the measured viscosities. Td (calc) was obtained by calculation from Eq. (2) and Td (meas) by extrapolating the measured viscosity curve to log g 5 (Pa s). The conclusion we deduce from Table 3 is that ®ber drawing from a preform is easy for ZBLAN, but more dicult for the GIPCdZn and IZnBS glasses and maybe impossible for a CdFCl glass. This evaluation agrees with practical ®ber drawing experiences: ZBLAN ®ber has been fabricated with a loss less than 1 dB/km, whilst GIPCdZn and other Pb-containing GaF3 /InF3 based glasses drawn into experimental ®bers have a loss two orders of magnitude higher than that of ZBLAN [24]. Today, we have no report of a CdFCl ®ber. 4. Discussion For ¯uoride glass forming liquids, a smaller activation energy of crystallization and/or of viscous ¯ow is usually associated with greater thermal stability. Sometimes this assumption is not valid. The crystallization activation energy of ZBLAN and ZBLALi glasses have been reported as 196 [25] and 168 [26] kJ/mol, respectively, indicating that ZBLALi has better resistance to devitri®cation. In fact ZBLAN is usually more stable than ZBLALi both in glass forming and ®ber drawing. Although the crystallization activation energy (Ea ) and viscous ¯ow activation energy (Eg )
Table 2 Determination of viscosity characteristics by parallel-plate method Glass
dlog g/dT ( log g 7±9) (by TMA-220 Seiko)
dlog g/dT ( log g 7±9) (by Viscometer Theta Industries)
ZBLAN20
0:12 0:02 (50 g, 2 K/min)
0:14 0:02 (200 g, 2 K/min)
GIPCdZn
0:093 0:02 (100 g, 2 K/min) 0:090 0:02 (50 g, 2 K/min) 0:085 0:02 (50 g, 1 K/min)
0:12 0:02 (200 g, 2 K/min)
CdFCl47
0:080 0:02 (50 g, 2 K/min)
0:11 0:02 (200 g, 2 K/min)
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G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
of the base line even suggests that ZBLAN has less tendency to devitri®cation than ZBLANI. So the relation between glass stability and activation energy is not always valid. The basic equations often used to calculate Ea and E are n
x 1 ÿ exp ÿ kt Avrami equation; nk ln k0 ÿ Ea =RT crystallization; log g A Eg=T viscous flow:
Fig. 2. Evolution of viscous ¯ow vs reciprocal temperature for GIPCdZn and CdFCl47 glasses. Activation energies are calculated from the slope of the straight line A + B.
of ZBLAN are much larger than that of InF3 containing ZBLANI [13], there is no dierence in isothermal DSC scans at 20°C and 30°C below Tx . Even from the DSC curves at Tx ±10°C the raising
In these equations, x is the crystalline fraction at the temperature T for a time t, k is the parameter of the Avrami equation, k0 is the frequency factor and A is a constant. Thus a larger Ea corresponds to larger slope of the ln k±T ÿ1 and ln k±T plots. As a consequence, k becomes large at some temperature between glass transition and melting temperatures. As long as kt 1, exp ÿ(kt)n in the Avrami equation is close to 0, and crystalline fraction, x, is negligible. When k is larger, x > 10ÿ3 for small times (t < 1 min). Similarly a larger Eg (or smaller G) corresponds to a larger log g(T) slope, which is often correlated to smaller viscosities at liquidus temperature, according to the concepts of the `fragile' glasses theory [14]. The correlation between Ea and Eg and thermal stability may dier in the `melt ® glass' cooling process and the `glass ® ®ber' heating process. As pointed out in the previous part, ®ber drawing should take place at a temperature for which the viscosity is close to 105 Pa s and the crystallization rate is negligible. When Eg is larger, viscosity decreases more rapidly with temperature, and the ®bre drawing temperature is closer to Tg . Then, there is a greater probability that devitri®cation rate is smaller during drawing, which is
Table 3 Prediction of ®ber drawing possibility in ¯uoride glasses Glass
T0 (°C) 1
Td calculated (°C) 1
Td measured (°C) 1
T0 ±Td cal (°C) 2
T0 ±Td
ZBLAN GIPCdZn IZBS CdFCl
344 322 345 205
326 331 384 258
320 321 353 235
+18 ÿ9 ÿ39 ÿ53
+24 +1 ÿ8 ÿ30
Td is drawing temperature and T0 is the temperature at which the DSC curve deviates from base line.
meas
(°C) 2
G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
desirable. On the other hand, it is usually observed that glass stability and thermal stability range, Tx ÿ Tg , are larger when Ea is smaller. As a consequence, the ideal situation for ®ber drawing should be smaller Ea with larger Eg . Unfortunately this makes a contradictory requirement as it is often assumed that both are the same at the same temperature. However, thermal stability depends also on k0 , and the Avrami equation shows that k determines crystallization frequency while Ea simply expresses its dependence as a function of temperature. Thus, depending on k0 s and Avrami exponent ns, glass stability may be increased by composition changes with minor eect on the viscosity±temperature pro®le. This condition may correspond to ZBLAN and related ¯uorozirconate glasses. The comparison between the three glasses of this study describes the dierence in the slope of the viscosity vs temperature plot. The direct consequence is that the ®ber drawing temperature may be too high to keep nucleation and crystal growth negligible, especially for cadmium ¯uorochloride glass, which accounts for the observed problems in ®ber drawing. From Tables 1 and 2, there is some discrepancy between the dlog g/dT slopes obtained from DSC data and from measurements implemented with the two dierent viscometers. This does not jeopardize the above conclusion because the evolution of the viscosity pro®le from one glass to another is not ambiguous. However, it raises questions about the required conditions for the application of Moynihan's method [5] and the viscosity measurements. As time, load and sample size dier, surface phenomena may aect a measurement, and a more systematic study could help to assess their real in¯uence. 5. Conclusions The temperature±viscosity dependence of ZrF4 -, InF3 - and CdF2 -based ¯uoride glasses was obtained both from Moynihan's relation, by using the DSC data considering the DT of the glass transition region, and from the parallel-plate measurement. The slope of the calculated log g±T (in 107 ±109 Pa s) decreased as follows: ¯uorozir-
141
conate glass (0:085 0:015 Kÿ1 ) > ¯uoroindate glasses (0.067 0.015 Kÿ1 ) > cadmium ¯uorochloride glass (0:047 0:012 Kÿ1 ). The same order of (log g±T ) slope was observed in the measured results, e.g., 0:12 0:02, 0:09 0:02 and 0:08 0:02 Kÿ1 , respectively, for the tetravalent, the trivalent and the divalent ¯uoride glasses. This log g±T dierence in slope between the glasses studied is related to their ionicity. The Arrhenian activation energies of the shear viscosity are 115 kJ/mol for PGICdZn and 64 kJ/mol for CdFCl. The activation energies of viscous ¯ow and of crystallization in¯uence glass formation and ®ber drawing in dierent ways. Acknowledgements The authors gratefully acknowledge Dr Marc Matecki at Laboratoire des Verres et Ceramiques, Universite de Rennes 1 (France), for his help in the viscosity measurements using the Theta Industries Inc. viscometer and for useful discussions.
References [1] M. Poulain, Proceedings of the XVIIIth International Congress on Glass, and Proceedings of the SPIE 3416, 1998, p. 2. [2] S. Takahashi, in: I.D. Aggarwal, G. Lu (Eds.), Fluoride Glass Fiber Optics, Academic Press, Boston, 1991, p. 213. [3] H.W. Schneider, in: A. Comyns (Ed.), Fluoride Glasses, Wiley, Chichester, 1989, p. 185. [4] E.H. Fontana, Am. Ceram. Soc. Bull. 49 (1970) 594. [5] C.T. Moynihan, J. Am. Ceram. Soc. 76 (1993) 1081. [6] S.J. Wilson, D. Poole, Mater. Sci. Forum 6 (1985) 665. [7] S.N. Crichton, R. Mossadegh, C.T. Moynihan, Mater. Sci. Forum 19&20 (1987) 435. [8] A. Jha, J.M. Parker, Phys. Chem. Glasses 30 (1989) 220. [9] T. Yano, S. Inoue, H. Kawazoe, M. Yamane, Mater. Sci. Forum 32&33 (1988) 583. [10] M. Braglia, C. Bruschi, D. Cavailli, G. Cocito, D. Guojun, J. Krauss, S. Mosso, Non-Cryst. Solids 213&214 (1997) 325. [11] W.C. Hasz, S.N. Crichton, C.T. Moynihan, Mater. Sci. Forum 32&33 (1988) 589. [12] F. Lin, H. Zhang, H. Hu, F. Gan, Mater. Sci. Forum 32&33 (1988) 595. [13] P. Baniel, C. Jacoboni, U. Unrau, Mater. Sci. Forum 32&33 (1988) 601. [14] C.A. Angell, J. Non-Cryst. Solids 112&113 (1991) 13.
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[15] D.W. Hewak, R.S. Deol, J. Wang, G. Wylangowski, J.A. Mederios Neto, B.N. Samson, R.I. Laming, W.S. Brocklesby, D.N. Payne, A. Jha, M. Poulain, S. Otero, S. Surinach, M.D. Baro, Electron. Lett. 29 (2) (1993) 237. [16] Y. Messaddeq, A. Delben, M.A. Aegerter, A. Sou®ane, M. Poulain, J. Mater. Res. 8 (1993) 885. [17] G. Zhang, J. Jiang, M. Poulain, J. Non-Cryst. Solids 221 (1997) 78. [18] M. Matecki, M. Poulain, J. Non-Cryst. Solids 140 (1992) 82. [19] G. Zhang, M. Poulain, A. Jha, J. Non-Cryst. Solids 184 (1995) 72.
[20] M. Poulain, in: I.D. Aggarwal, G. Lu (Eds.), Fluoride Glass Fiber Optics, Academic Press, Boston, 1991, p. 1. [21] A.N. Gent, Brit. J. Appl. Phys. 11 (1960) 85. [22] A.V. Cardoso, A.B. Seddon, Glass Technol. 32 (1991) 174. [23] G. Zhang, these de doctorat d'Universite, Rennes, 1996. [24] See for example the reports of the European Research Contract RACE 2038 Fluor II. [25] P. Baniel, A. Kober, Mater. Sci. Forum 19 (1987) 33. [26] M.P. Bansal, R.H. Doremus, C.T. Moynihan, Mater. Sci. Forum. 5 (1985) 211.
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Viscosity of ¯uoride glasses near the ®ber drawing temperature region G. Zhang
a,b
, J. Jiang
a,b
, M. Poulain
a,*
, A.S. Delben b, J.R. Delben
b
a b
Laboratoire des Mat eriaux Photoniques, Universit e de Rennes 1, Rennes 35042, France Dept. de Fisica, Univ. Federal de Mato Grosso do Sul, 79070-900, Campo Grande, Brazil
Abstract The temperature±viscosity dependence of ZrF4 -, InF3 - and CdF2 -based ¯uoride glasses was obtained by two methods. The ®rst method uses a predictive relation based on dierential scanning calorimetric (DSC) data based on the width of the glass transition region i.e. Tg and Tg0 , the beginning and end temperatures of the glass transition. We de®ned the factor, G Tg0 =Tg Tg0 ÿ Tg ; in the relation which determines the viscosity at the temperature (T ÿ Tg0 ). The slope of the calculated log g versus T decreased in the order: ¯uorozirconate glass (0.085 Kÿ1 ) > ¯uoroindate glasses (0.067 Kÿ1 ) > cadmium ¯uorochloride glass (0.047 Kÿ1 ). The second method was by measurement using the parallel-plate technique. The same order of (log g(T)) slope was observed in the measured results, e.g. 0:12 0:02; 0:09 0:02 and 0:08 0:02 Kÿ1 , respectively, for the tetravalent, the trivalent, and the divalent ¯uoride glasses. The Arrhenius activation energies of the shear viscosity are 115 10 kJ/mol for PGICdZn and 64 10 kJ/mol for CdFCl. These dierences in activation energy aect the ®ber drawing ability, because a larger activation energy corresponds to a drawing temperature closer to Tg . Ó 1999 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Fluoride glasses are promising materials for many applications, such as infrared optical signal and energy transmission, ampli®cation for telecommunications, host materials for laser optics, due to their wide transmission range, lower phonon energy of glass network and potentially smallest intrinsic losses. However most of the applications based on their optical properties requires a ®ber form [1]. Unlike silica, where ®bers are fabricated by chemical vapor deposition (CVD) or modi®ed CVD (MCVD) techniques,
* Corresponding author. Tel.: +33-2 99 28 62 63; fax: +33-2 99 28 69 72; e-mail: [email protected]
¯uoride optical ®ber is mainly obtained by drawing a preform at a temperature greater than the glass transition temperature at which the viscosity should be in the range 103 ±106 Pa s [2,3]. The rate of crystallization at this temperature is the key to getting optical quality ®ber [3]. Thus the log g(T ) (T temperature) relation and thermal stability range (Tx ÿ Tg ) calculated from the temperatures for glass transition temperature (Tg ) and crystallization onset temperature (Tx ) are the most important factors for ®ber drawing. Hence an understanding of the viscosity in the ®ber drawing range is essential. Methods, such as beam bending (109 ±1011 , viscosity in Pa s), penetration (107 ±1011 Pa s) and parallel plate (105 ±1010 ) [4], have been applied to measure viscosity at temperatures greater than Tg . Among these techniques, parallel-plate has
0022-3093/99/$ - see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 5 5 - X
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advantages in that a small and simple sample is required and the technique has the capability of measuring viscosity in the range near the ®ber drawing temperature. However the tendency for crystallization of some ¯uoride glasses often restricts the measurement to the smaller viscosity range. Based on dierential scanning calorimetry (DSC) data, the width of the glass transition region, i.e. the beginning and end temperatures, Tg and Tg0 , Moynihan [5] proposed an equation to estimate the temperature dependence of viscosity for predicting ®ber drawing temperature. The viscosities of ¯uorozirconate and ¯uoroaluminate glasses have been studied [6±13]. Their viscosity±temperature dependencies are not ®t by an Arrhenian function or by a Fulcher relation as are classic covalent silicate glass liquids; the ¯uoride liquids are more ionic and they are `fragile glasses' according to the concept introduced by Angell [14]. Similar log g(T) dependencies were observed for ZBLA, ZBLAN [7,9,11], ABCSMY [8,12], ZBLALi [9,13] and ZBLALiPb [6,10] liquids (where Z ZrF4 , B BaF2 , L LaF3 , A AlF3 , C CaF2 , M MgF2 and Y YF3 ). Some of these have been successfully drawn in ®ber. However new heavy halide and chalcogenide glasses, which have phonon energies smaller than current ZrF4 - and AlF3 -based systems, are required for applications as optical ®ber ampli®ers and ®ber lasers [15]. Some ¯uoroindate [16,17] and cadmium ¯uorochloride [18,19] glasses have thermal stability ranges similar to standard ZBLAN ®ber glasses, but ®ber drawing the lower phonon energy glass systems is dicult. The aim of this work is to investigate the composition dependence of viscosity in various ¯uoride glasses, and also to discuss the possibility of ®ber drawing glasses with those phonon energies of interest for active applications.
ZBLAN [20]: 53ZrF4 ±20BaF2 ±4LaF3 ±3AlF3 ± 20NaF ZBLA [20]: 57ZrF4 ±36BaF2 ±3LaF3 ±4AlF3 IZnBS[16]: 34InF3 ±6GaF3 ±20ZnF2 ±16BaF2 ± 20SrF2 ±2GdF3 ±2NaF BIZnYbT [5]: 30BaF2 ±30InF3±20ZnF2 ± 10YbF3 ±10ThF4 GIPCdZn [17]: 22GaF3 ±13InF3 ±30PbF2 ± 18CdF2 ±13ZnF2 ±2GdF3 ±2NaF CdFCl47 [19]: 30CdF2 ±18.5CdCl2 ±8NaF± 25NaCl±12.5BaF2 ±2KF±4LiF The ZBLAN, GIPCdZn and IZnBS glasses were prepared by a typical NH4 HF2 reactive process [20]. Reagent grade oxides and/or ¯uorides were used as raw materials. Batches, 10±20 g in weight, were placed in a platinum tube for ¯uorination at 350°C and 500°C for a few hours. The ¯uorochloride glasses were melted from anhydrous ¯uorides and chlorides. After the normal melting, casting, and annealing steps homogenous bodies or rods were obtained. The glass characteristic temperatures of samples were measured under N2 ¯ow using dierential scanning colorimetry technique (Seiko DSC-220). The standard heating rate was 10°C/min. The errors in the characteristic temperatures were 1 C for the glass transition (Tg ), the end of glass transition (Tg0 ), the onset and peak temperatures of crystallization (Tx and Tp ), and 2 C for melting point (Tm ) and liquidus temperature (Tl ). 2.2. Estimation of viscosity from DSC data By Moynihan's relation [5], we can simulate the viscosity near the ®ber drawing range from DSC data, Tg and Tg0 : log g ÿ5
2. Experimental 2.1. Sample preparation and characteristic temperatures The glass compositions (mol%) used in this work were:
14:2 0:147 T ÿ Tg0 =Tg02 1 ÿ Tg ÿ 1 ÿ Tg0 1
ÿ5
14:2 : 0:147 T ÿ Tg1 =G 1
1
We de®ne a factor G Tg0 =Tg Tg0 ÿ Tg in the relation determining the log g(T) curve at the temperature (T ÿ Tg0 ). For the glasses with
G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
the higher Tg s, the ®ber drawing temperature, Td , can be obtained by Td T ÿ Tg0 log Tg0 T ÿ Tg0 log Tg G:
2
This method yields information about the possibility of ®ber drawing based on a minimal experimental input (Tg and Tg0 only). One of the problems of this approach lies in the dependence of the characteristic temperatures on thermal history. Ideally, the heating rate of the DSC scan should be the same as the cooling rate of the sample through the glass transition region. This condition may be achieved for small laboratory samples. However real preforms used for ®ber drawing are not processed this way. They are usually annealed below Tg to remove thermal stresses and improve sample homogeneity. As a result ®ctive temperature is less, which corresponds to a smaller cooling rate, probably <1°C/ min. A second problem relates to the uncertainty of the Tg s and Tg0 s. Because they are close, the relative error on G may be large: a random error of 1°C for Tg and Tg0 corresponds to an uncertainty of 2 K on G. As will be seen, the observed dierences are large enough to be unambiguous. 2.3. Determination of T±g parallel-plate method The cylindrical samples were slices cut from annealed glass rods and polished. The sample dimensions were typically 7 mm in diameter and 4 mm in thickness. The parallelism of the two surfaces of the sample was better than 3 ´ 10ÿ3 rad. The samples were sandwiched between two pieces of platinum foil, or between two silica plates, and then put into two viscometers (Theta Industries and a modi®ed commercial TMA/SS-220 Seiko). The applied load ranged from 10 to 200 g. By measuring the deformation rate with temperature, the temperature±viscosity relationship was found by using GentÕs equation [21]: g 2Mgh5 =3V dh=dt 2h3 V :
3
It has been reported that the measured viscosity may be aected by the residual stress for a heating rate of about 10 K/min during dynamic measurements [22]. To reduce this eect and also to get the temperature dependence of viscosity closer to that
137
of the real ®ber in its drawing range, we measured viscosity at a 2 K/min heating rate. This rate was chosen empirically by considering the parameters used in normal ¯uoride ®ber drawing and should be not far from a real case. 3. Results 3.1. Calculated temperature dependence of viscosity As the slope of the log g(T ) plot depends on G, the determination of G Tg0 =Tg Tg0 ÿ Tg is important. Tg and Tg0 could vary with the glass annealing and heating rates [22]. Ideally, cooling rate for sample preparation and heating rate for the DSC scan should be identical. In practice, this factor has only a limited eect on the experimental Tg and Tg0 because relaxation times of ¯uoride glasses vary with temperature. In addition, the error which arises from this discrepancy is similar for the two temperatures and close to the uncertainty of the measurement method, i.e., tangent intersection. The Tg s and Tg0 s and the resulting Gs for dierent glasses are reported in Table 1. The eect of the cooling rate, qc , and heating rate, qh , on G appears from this table. Only Gs obtained with rates satisfying the relation 0:2 < qc =qh < 5 have been used for viscosity calculations [5]. For this reason we do not report viscosity for the PGICZ samples as qc /qh < 0:2. The G dierence between the three groups of glasses exceeds the experimental error. The G dierence is due to the dierent temperature dependencies of viscosity. Table 1 also lists the calculated log g±T slopes in the viscosity range 107 ±109 Pa s. based on the selected Gs. The slope of the calculated log g(T ) decreased in the order: ZBLAN 0:085 0:015 > IZnBS 0:067 0:015 > CdFCl47 0:047 0:012:
We assume that the dierence in slopes of the log g(T ) plots is related to the ionicity of the bonding [14]. By comparison with silicate glasses, ¯uoride glasses are ionic and have a larger dlog g/ dT. Among the systems studied in this work, ZrF4 based glasses are more ionic and therefore have the
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G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
Table 1 Determination of viscosity characteristics of some ¯uoride glasses by DSC data Glass
Ratio qc /qh (K/mn)/(K/mn)
Tg (K) 1
Tg0 (K) 1
G (K) 2
Selected G
Dlog g/DT (at Tg0 )
Dlog g/DT ( log g 7±9)
ZBLAN
As made/10 1/10 2/10 6.3/10
537 537 536 535
555 553 554 555
18.2 16.0 18.4 20.4
20.4
0.102
0.085
GIPCdZn
As made/10 1/10
521 520
541 542
21.0 22.3
IZBS
As made 1/10 4.9/10
565 565 563
589 587 588
24.8 22.7 25.9
25.9
0.080
0.067
CdFCl47
As made 1/10 in DSC 4.9/10
399 399 399
431 432 433
34.6 35.3 36.8
36.8
0.056
0.047
Tg and Tg0 are the temperatures for beginning and end of glass transition region. qc is the cooling rate in glass preparation and qh is the heating rate in the DSC scan. G factor is calculated as G Tg0 =Tg Tg0 ÿ Tg . Random errors are 1 K for temperatures, 2 K for G and 0.015 Kÿ1 for the averages slopes Dlog g/DT.
largest viscosity±temperature slope. In the same way the trivalent ¯uoride (GaF3 and InF3 ) based glasses have a have a larger dlog g/dT than the divalent ¯uoride glasses (CdFCl47). These expected trends will be tested by the experimental data from parallel-plate viscosimetry presented in the next section. 3.2. Determination of log g(T) by parallel-plate method Process parameters such as heating rate and applied load were adjusted to give a viscosity range from 107 to 1010 Pa s to be measured by parallelplate method. For the ®rst viscometer (Theta Industries) a 100 g load could be used with a silica pressing probe, 12 mm in diameter. The maximum viscosity (1010 Pa s) is observed at a temperature close to Tg . Smaller samples and loads were used with the modi®ed viscometer (Seiko TMA/SS220), because the silica probe is small (2 mm in diameter). Another limit of the modi®ed viscometer is its downward travel distance for measuring sample deformation, less than 1.5 mm. Deformation curves depend on the applied load and measurements can be implemented at lower temperature with a heavier load while a light load
will be bene®cial to obtain viscosity at a higher temperature, e.g., nearer the ®ber drawing range. Classically, the evolution of log g(1/T ) is linear insofar as the Arrhenian approximation applies. But the plot of log g vs T is also approximately linear if the viscosity range is limited, as in the present case (107 ±1010 Pa s). However, the plot obtained under a 10 g load (Fig. 1) deviates more from linearity than that obtained under larger loads (50 and 100 g). This linearity of the log g(T ) curve in the larger viscosity range is probably due to the contact between the silica plate and the sample under a light load, which could aect deformation rate especially at the lower temperature before the glass is `soft'. As the applied load was increased (50±200 g), the linearity of the log g vs T ÿ1 plot improved in the viscosity range 107 ±109 Pa s in both viscometers. The same results were observed during the calibration using the standard glass 711 from the National Institute of Standards and Technology (NIST). Table 2 reports the log g(T ) slope between 107 and 109 Pa s. Just as the viscosity±temperature dependence estimated in Section 3.1, the measured data show again that the log g(T ) slope decreased with decreasing ionicity from ZBLAN to GIPCdZn and then to CdFCl47 samples.
G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
Fig. 1. log g(T) curves of GIPCdZn glass determined under dierent applied loads. Line is drawn as a guide to the eye.
The activation energies of viscous ¯ow of GIPCdZn and CdFCl47 samples were obtained by plotting log g vs 1000/T (Fig. 2) and were 115 10 kJ/mol for GIPCdZn and 64 kJ/mol for CdFCl47, respectively [23]. These energies could be related to the average bonding energy, and possibly to ionicity. 3.3. Possibility of ®ber drawing The possibility of ®ber drawing may be evaluated from the thermal stability range and the viscosity. We de®ne T0 here as the onset crystallization temperature at which an exothermic deviation from the baseline could be observed. It is evident that the ®ber drawing temperature should be less than T0 instead of Tx because the crystal-
139
lization rate at Tx is too large for an optical ®ber. Table 3 lists T0 from DSC for 40 mg samples. The heating rate was 2 K/min, i.e., the same heating rate used in the viscosity measurement. The temperatures, Td (calc) and Td (meas), are ®ber drawing temperatures at which log g 5 (Pa s), respectively, from the calculated and the measured viscosities. Td (calc) was obtained by calculation from Eq. (2) and Td (meas) by extrapolating the measured viscosity curve to log g 5 (Pa s). The conclusion we deduce from Table 3 is that ®ber drawing from a preform is easy for ZBLAN, but more dicult for the GIPCdZn and IZnBS glasses and maybe impossible for a CdFCl glass. This evaluation agrees with practical ®ber drawing experiences: ZBLAN ®ber has been fabricated with a loss less than 1 dB/km, whilst GIPCdZn and other Pb-containing GaF3 /InF3 based glasses drawn into experimental ®bers have a loss two orders of magnitude higher than that of ZBLAN [24]. Today, we have no report of a CdFCl ®ber. 4. Discussion For ¯uoride glass forming liquids, a smaller activation energy of crystallization and/or of viscous ¯ow is usually associated with greater thermal stability. Sometimes this assumption is not valid. The crystallization activation energy of ZBLAN and ZBLALi glasses have been reported as 196 [25] and 168 [26] kJ/mol, respectively, indicating that ZBLALi has better resistance to devitri®cation. In fact ZBLAN is usually more stable than ZBLALi both in glass forming and ®ber drawing. Although the crystallization activation energy (Ea ) and viscous ¯ow activation energy (Eg )
Table 2 Determination of viscosity characteristics by parallel-plate method Glass
dlog g/dT ( log g 7±9) (by TMA-220 Seiko)
dlog g/dT ( log g 7±9) (by Viscometer Theta Industries)
ZBLAN20
0:12 0:02 (50 g, 2 K/min)
0:14 0:02 (200 g, 2 K/min)
GIPCdZn
0:093 0:02 (100 g, 2 K/min) 0:090 0:02 (50 g, 2 K/min) 0:085 0:02 (50 g, 1 K/min)
0:12 0:02 (200 g, 2 K/min)
CdFCl47
0:080 0:02 (50 g, 2 K/min)
0:11 0:02 (200 g, 2 K/min)
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G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
of the base line even suggests that ZBLAN has less tendency to devitri®cation than ZBLANI. So the relation between glass stability and activation energy is not always valid. The basic equations often used to calculate Ea and E are n
x 1 ÿ exp ÿ kt Avrami equation; nk ln k0 ÿ Ea =RT crystallization; log g A Eg=T viscous flow:
Fig. 2. Evolution of viscous ¯ow vs reciprocal temperature for GIPCdZn and CdFCl47 glasses. Activation energies are calculated from the slope of the straight line A + B.
of ZBLAN are much larger than that of InF3 containing ZBLANI [13], there is no dierence in isothermal DSC scans at 20°C and 30°C below Tx . Even from the DSC curves at Tx ±10°C the raising
In these equations, x is the crystalline fraction at the temperature T for a time t, k is the parameter of the Avrami equation, k0 is the frequency factor and A is a constant. Thus a larger Ea corresponds to larger slope of the ln k±T ÿ1 and ln k±T plots. As a consequence, k becomes large at some temperature between glass transition and melting temperatures. As long as kt 1, exp ÿ(kt)n in the Avrami equation is close to 0, and crystalline fraction, x, is negligible. When k is larger, x > 10ÿ3 for small times (t < 1 min). Similarly a larger Eg (or smaller G) corresponds to a larger log g(T) slope, which is often correlated to smaller viscosities at liquidus temperature, according to the concepts of the `fragile' glasses theory [14]. The correlation between Ea and Eg and thermal stability may dier in the `melt ® glass' cooling process and the `glass ® ®ber' heating process. As pointed out in the previous part, ®ber drawing should take place at a temperature for which the viscosity is close to 105 Pa s and the crystallization rate is negligible. When Eg is larger, viscosity decreases more rapidly with temperature, and the ®bre drawing temperature is closer to Tg . Then, there is a greater probability that devitri®cation rate is smaller during drawing, which is
Table 3 Prediction of ®ber drawing possibility in ¯uoride glasses Glass
T0 (°C) 1
Td calculated (°C) 1
Td measured (°C) 1
T0 ±Td cal (°C) 2
T0 ±Td
ZBLAN GIPCdZn IZBS CdFCl
344 322 345 205
326 331 384 258
320 321 353 235
+18 ÿ9 ÿ39 ÿ53
+24 +1 ÿ8 ÿ30
Td is drawing temperature and T0 is the temperature at which the DSC curve deviates from base line.
meas
(°C) 2
G. Zhang et al. / Journal of Non-Crystalline Solids 256&257 (1999) 135±142
desirable. On the other hand, it is usually observed that glass stability and thermal stability range, Tx ÿ Tg , are larger when Ea is smaller. As a consequence, the ideal situation for ®ber drawing should be smaller Ea with larger Eg . Unfortunately this makes a contradictory requirement as it is often assumed that both are the same at the same temperature. However, thermal stability depends also on k0 , and the Avrami equation shows that k determines crystallization frequency while Ea simply expresses its dependence as a function of temperature. Thus, depending on k0 s and Avrami exponent ns, glass stability may be increased by composition changes with minor eect on the viscosity±temperature pro®le. This condition may correspond to ZBLAN and related ¯uorozirconate glasses. The comparison between the three glasses of this study describes the dierence in the slope of the viscosity vs temperature plot. The direct consequence is that the ®ber drawing temperature may be too high to keep nucleation and crystal growth negligible, especially for cadmium ¯uorochloride glass, which accounts for the observed problems in ®ber drawing. From Tables 1 and 2, there is some discrepancy between the dlog g/dT slopes obtained from DSC data and from measurements implemented with the two dierent viscometers. This does not jeopardize the above conclusion because the evolution of the viscosity pro®le from one glass to another is not ambiguous. However, it raises questions about the required conditions for the application of Moynihan's method [5] and the viscosity measurements. As time, load and sample size dier, surface phenomena may aect a measurement, and a more systematic study could help to assess their real in¯uence. 5. Conclusions The temperature±viscosity dependence of ZrF4 -, InF3 - and CdF2 -based ¯uoride glasses was obtained both from Moynihan's relation, by using the DSC data considering the DT of the glass transition region, and from the parallel-plate measurement. The slope of the calculated log g±T (in 107 ±109 Pa s) decreased as follows: ¯uorozir-
141
conate glass (0:085 0:015 Kÿ1 ) > ¯uoroindate glasses (0.067 0.015 Kÿ1 ) > cadmium ¯uorochloride glass (0:047 0:012 Kÿ1 ). The same order of (log g±T ) slope was observed in the measured results, e.g., 0:12 0:02, 0:09 0:02 and 0:08 0:02 Kÿ1 , respectively, for the tetravalent, the trivalent and the divalent ¯uoride glasses. This log g±T dierence in slope between the glasses studied is related to their ionicity. The Arrhenian activation energies of the shear viscosity are 115 kJ/mol for PGICdZn and 64 kJ/mol for CdFCl. The activation energies of viscous ¯ow and of crystallization in¯uence glass formation and ®ber drawing in dierent ways. Acknowledgements The authors gratefully acknowledge Dr Marc Matecki at Laboratoire des Verres et Ceramiques, Universite de Rennes 1 (France), for his help in the viscosity measurements using the Theta Industries Inc. viscometer and for useful discussions.
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[20] M. Poulain, in: I.D. Aggarwal, G. Lu (Eds.), Fluoride Glass Fiber Optics, Academic Press, Boston, 1991, p. 1. [21] A.N. Gent, Brit. J. Appl. Phys. 11 (1960) 85. [22] A.V. Cardoso, A.B. Seddon, Glass Technol. 32 (1991) 174. [23] G. Zhang, these de doctorat d'Universite, Rennes, 1996. [24] See for example the reports of the European Research Contract RACE 2038 Fluor II. [25] P. Baniel, A. Kober, Mater. Sci. Forum 19 (1987) 33. [26] M.P. Bansal, R.H. Doremus, C.T. Moynihan, Mater. Sci. Forum. 5 (1985) 211.