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Thermal stability and heat capacity changes at the glass transition in K2O–WO3–TeO2 glasses
Journal of Non-Crystalline Solids 242 (1998) 154±164
Thermal stability and heat capacity changes at the glass transition in K2O±WO3±TeO2 glasses T. Kosuge a, Y. Benino a, V. Dimitrov a, R. Sato b, T. Komatsu b
a,*
a Department of Chemistry, Nagaoka University of Technology, Kamitomioka-cho, Nagaoka 940-2188, Japan Department of Material Engineering, Tsuruoka National College of Technology, Oaza-Ioka, Tsuruoka 997, Japan
Received 9 April 1998; received in revised form 10 July 1998
Abstract The thermal stability and heat capacity changes in the glass transition region of K2 O±WO3 ±TeO2 glasses (glass formation range 20±90 mol% TeO2 ) have been studied to examine the structural relaxation behavior. The glasses with 60±70 mol% TeO2 and with both K2 O and WO3 are thermally stable against crystallization. It is proposed from Raman spectral analyses that TeO4 trigonal bipyramids change to TeO3 trigonal pyramids with the addition of K2 O and that Te±O±W bonds are formed in the substitution of WO3 for TeO2 . Heat capacity changes of DCp 48±58 J molÿ1 Kÿ1 (DCp Cpl ÿ Cpg , Cpg and Cpl are the heat capacities of the glasses and supercooled liquids, respectively), and ratios Cpl =Cpg 1.6±1.8 are obtained for xK2 O xWO3 (100 ) 2x)TeO2 glasses. The DCp and Cpl =Cpg increase with decreasing TeO2 content, indicating an increase in thermodynamic fragility with decreasing TeO2 content. But, the kinetic fragility estimated from the activation energy for viscous ¯ow is almost constant irrespective of TeO2 content. These behaviors have been analyzed using the con®gurational entropy model proposed by Adam and Gibbs. The results indicate that in K2 O±WO3 ±TeO2 glasses, Te±O±Te bonds are weak and bond breakings occur easily in the glass transition region, leading to large con®gurational entropy changes and thus large DCp . Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Tellurium oxide (TeO2 )-based glasses are of scienti®c and technical interest on account of their various unique properties, and have been considered as promising materials for use in optical ampli®ers because of their low phonon energy or nonlinear optical devices because of their large third-order non-linear susceptibility [1,2]. Recently, optically transparent TeO2 -based glass-ceramics showing second harmonic generation have been
* Corresponding author. Tel.: +81-258 47 9313; fax: +81-258 47 9300; e-mail: [email protected]
discovered [3,4]. The structure of TeO2 -based glasses is also of interest, because there are two types of the basic structural units, i.e. TeO4 trigonal bipyramid (tbp) and TeO3 trigonal pyramid (tp). In order to develop TeO2 -based glasses as new optical functional materials, an understanding of the thermal stability against crystallization and the structural relaxation behavior in the glass transition region is necessary, but such information is scarce. It is known that a pure TeO2 does not become a glass under usual quenching rates and the addition of other elements is needed to form TeO2 -based bulk glasses. Imaoka and Yamazaki [5] examined the glass-forming regions in various TeO2 -based
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 0 0 - X
T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
systems. Among the glass-forming regions reported in their paper, it is noted that the systems of R2 O±WO3 ±TeO2 (R: Li, Na, K) have extremely wide glass-forming regions. These results suggest that R2 O±WO3 ±TeO2 glasses, which are available with a wide range in TeO2 content, are suitable for the study of the structure, properties, thermal stability, structural relaxation and crystallization behaviors of TeO2 -containing glasses. In this paper, the thermal stability against crystallization and the heat capacity changes in the glass transition region of various K2 O±WO3 ±TeO2 glasses have been examined. The structural changes due to the substitution of K2 O and WO3 for TeO2 have been studied using Raman scattering spectroscopy. To the best of our knowledge, the
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study of the structural relaxation behavior of TeO2 -based glasses by measurements of heat capacity is limited to a few systems of Li2 O±TeO2 , AgI±T1O0:5 ±TeO2 , Na2 O±TeO2 and Li2 O±Na2 O± TeO2 [6±10]. 2. Experimental procedure The nominal compositions examined in the present study are shown in Table 1. Commercial powders of reagent K2 CO3 (99.5%), WO3 (99%) and TeO2 (99%) were mixed and melted in a platinum crucible at around 800°C for 30 min in an electric furnace. The batch weight was 20 g. The quenched glasses were prepared by pouring the
Table 1 Values of glass transition, Tg , crystallization onset, Tx , and melting, Tm , temperatures, density, q, and refractive index, n, for K2 O±WO3 ±TeO2 glasses Glasses
Tg (°C)
Tx (°C)
Tm (°C)
q (g/cm3 )
n
K2 O
WO3
TeO2
(2)
(2)
(2)
(0.01)
(0.02)
0 5 10 0 10 15 20 0 10 15 20 5 10 15 20 25 22.5 23.75 15 20 25 27.5 15 20 25 20 25 30 25 30
10 5 0 20 10 5 0 30 20 15 10 35 30 25 20 15 22.5 23.75 35 30 25 27.5 45 40 35 50 45 40 55 50
90 90 90 80 80 80 80 70 70 70 70 60 60 60 60 60 55 52.5 50 50 50 45 40 40 40 30 30 30 20 20
330 306 281 353 308 275 235 368 344 307 269 388 361 331 300 252 296 294 370 331 281 280 396 363 314 380 345 290 369 325
427 416 416 506 456 370 320 536 509
628 687 459 630 619 487 630 604
416
454
5.79 5.60 5.08 5.97 5.31 4.98 4.57 6.11 5.50 5.13 4.79 5.94 5.61 5.35 5.02 4.62 4.96
2.17 2.11 2.05 2.17 2.04 1.99 1.92 2.16 2.05 1.99 1.92 2.11 2.05 2.00 1.92 1.86 1.90
538
636
419 375 511 486 452 458 439 377 439 422
482 477 665 630 504 684 544 511 550 550
5.51 5.13 4.83 4.82
1.99 1.92 1.87 1.84
5.32 5.08
1.92 1.86
5.21
1.85
5.02
1.62
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T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
melts onto an iron plate and by pressing the sample rapidly to a thickness of 1±2 mm using another iron plate. The glassy state in the quenched samples was con®rmed by X-ray diraction (XRD) analyses at room temperature using CuK a radiation. Glass transition, Tg and crystallization onset, Tx , temperatures were determined using dierential thermal analyses (DTA) at a heating rate of 10 K minÿ1 . Densities of the glasses were determined by the Archimedes method using distilled water as the immersion liquid. Refractive indices at a wavelength of 632.8 nm (He±Ne laser) were measured at room temperature using an ellipsometer (Mizojiri Optical, DVA-36 model). The relative permittivity, er , at room temperature in the frequency range from 0.1 to 100 kHz was measured by an LCR meter. Ion-sputtered gold ®lms were used as electrodes for the measurements. Viscosities in the glass transition region (107 ±1011 Pa) were measured by a penetration method within an accuracy of 5%. Raman scattering spectra at room temperature for the glasses were measured in the wave number range from 200 to 1000 cmÿ1 using an Ar laser (wavelength 514.5 nm, laser power 100 mW) and with a JASCO NR-1100 type Laser Raman Spectrometer. The annealed glasses, which were obtained by annealing the quenched glasses at a temperature Tg +10°C for 15 min and by cooling to a temperature below Tg at a rate of 1 K minÿ1 , were used for the measurements of heat capacity changes. Heat capacities, Cp , in the glass transition region in a heating process were measured using dierential scanning calorimetry, DSC, (Rigaku Thermo¯ex TAS 200, DSC8230D). The heating rate was 10 K minÿ1 . The sample weight was around 30 mg. Standard specimens of alumina (sapphire) were used as a heat capacity standard. The molecular weight of glasses was calculated based on batch composition. 3. Results 3.1. Some properties and thermal stability The glass-forming region in the K2 O±WO3 ± TeO2 system obtained in the present study is
Fig. 1. Glass-forming region in the K2 O±WO3 ±TeO2 system. (s) glass, (g) partially crystallized, (d) crystallized.
shown in Fig. 1. It should be pointed out that glass formation is observed in compositions with 20±90 mol% TeO2 . The region shown in Fig. 1 is slightly narrower at low TeO2 content than that reported by Imaoka and Yamazaki [5]. The DTA curves of xK2 O xWO3 (100 ) 2x)TeO2 glasses with x 10, 15 and 25 are shown in Fig. 2 as an example. In the glass with x 10, the glass transition of Tg 308 C and a crystallization onset of Tx 456 C are observed. The dierence between Tg and Tx , i.e. DT Tx ÿ Tg , in this glass is 148 C, indicating that the thermal stability against crystallization is considerably high. In the glass with x 15, a glass transition of Tg 307 C, is observed, but no crystallization peak is detected. A similar DTA pattern showing no clear crystallization peak was obtained for the glass with x 20. In the glass with x 25, a crystallization peak is clearly observed at around 419 C. The values of Tg , Tx and the melting temperature, Tm , for the other glasses are given in Table 1. The thermally stable glasses, in which crystallization peaks were not observed in the DTA curves taken at a heating rate of 10 K minÿ1 , are obtained at compositions with 60±70 mol% TeO2 and with both K2 O and WO3 . As seen in Table 1, the substitution of K2 O for TeO2 or WO3 gives a rapid decrease in Tg . On
T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
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larizability of the glasses is comparable to that of TeO2 . It is found from Table 1 that the refractive index increases almost linearly with increasing density. The frequency dependence of dielectric constant, er , at room temperature for xK2 O xWO3 (100 ) 2x)TeO2 glasses is shown in Fig. 3. The values of er decrease gradually with decreasing TeO2 content. But, the glass with x 25 still has a large value of er 19, implying again that the polarizability of WO3 is large. The activation energies for viscous ¯ow, Eg, in the glass transition region for xK2 O xWO3 (100 ) 2x)TeO2 glasses were evaluated by using the Arrhenius equation, g A exp Eg=RT , where g is a viscosity, A is a constant, R is the gas constant and T is a temperature. The evaluated values of Eg for these glasses are given in Table 2. The experimental error in Eg is around 3%. It is seen that Eg is in the range of 500±552 kJ molÿ1 and decreases slightly with decreasing TeO2 content. These values are comparable to those (418±595 kJ molÿ1 ) of (20 ) x)Li2 O xNa2 O 80TeO2 glasses [11], but are smaller than those (730±870 kJ molÿ1 ) of xLi2 O (100 ) x)TeO2 glasses [6].
Fig. 2. DTA patterns for some xK2 O xWO3 (100 ) 2x) TeO2 glasses. Tg and Tx are glass trasition and crystallization onset temperatures, respectively. Heating rate was 10 K minÿ1 .
the other hand, Tg increases due to the substitution of WO3 for TeO2 . These results suggest that K2 O acts as network modi®er and breaks the network structure and WO3 would take part in the network structure as network former. The values of density, q, and refractive index, n, for the K2 O±WO3 ±TeO2 glasses are given in Table 1. For xK2 O xWO3 (100 ) 2x)TeO2 glasses, they decrease almost linearly with decreasing TeO2 content. For the (30 ) x)K2 O xWO3 70TeO2 glasses, they increase with increasing WO3 content. As seen in Table 1, for example, the value of n 2:05 for the 10K2 O 90TeO2 glass is the same as that for the 10K2 O 20WO3 70TeO2 glass, implying that the contribution of WO3 to the po-
Fig. 3. Relative permittivity, er , at room temperature in the frequency range of 0.1±100 kHz for xK2 O xWO3 (100 ) 2x)TeO2 glasses. (X) x 5, (s) x 10, (m) x 15, (d) x 20, (n) x 25.
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Table 2 Values of the glass transition temperature, Tg , the activation energy for viscous ¯ow near the glass transition temperature, Eg, the Eg/ Tg ratio, the degree of fragility estimated from Eq. (1), m, the heat capacities of glasses, Cpg , and supercooled liquids, Cpl , the heat capacity change in the glass transition region, D Cp Cpl ÿ Cpg , and the product of Tg and D Cp for xK2 O á xWO3 á (100 ) 2x)TeO2 glasses Sample
Tg (K)
Eg (kJ molÿ1 )
Eg/Tg (kJ molÿ1 Kÿ1 )
m
Cpg
Cpl (J molÿ1 Kÿ1 )
DCp
Tg D Cp (J molÿ1 )
x5 x 10 x 15 x 20 x 25 x 27.5
579 581 580 573 554 553
552 534 511 500
0.95 0.92 0.89 0.90
50 48 47 47
72 73 74 75 73 72
120 121 122 126 130 130
48 48 48 51 57 58
)28 )28 )28 )29 )31 )32
Fig. 4. Raman scattering spectra at room temperature for xK2 O xWO3 (100 ) 2x)TeO2 glasses.
3.2. Raman scattering spectra The Raman scattering spectra for xK2 O xWO3 (100 ) 2x)TeO2 glasses are shown in Fig. 4. Similar Raman spectra are obtained for 20K2 O xWO3 (100 ) x)TeO2 glasses. Each spectrum was deconvoluted by using seven symmetrical Gaussian functions, considering peak assignments in Raman scattering spectra for K2 O±TeO2 and WO3 ±TeO2 glasses reported so far [12±14]. As an example, the ®tting result for 25K2 O 25WO3 50TeO2 glass is shown in Fig. 5. The peaks at around 470, 610, 670, 730, 790, 860 and 920 cmÿ1 are designated here as peak A, B, C, D, E, F and G, respectively. The relative peak intensity of seven peaks obtained by peak deconvolutions in xK2 O xWO3 (100 ) 2x)TeO2 and 20K2 O
000 000 000 000 000 000
Fig. 5. Raman scattering spectrum at room temperature 25K2 O 25WO3 50TeO2 glass. The spectrum was deconvoluted by using symmetrical Gaussian functions.
xWO3 (80 ) x)TeO2 glasses are shown in Fig. 6. The present peak assignments are performed in accordance with Raman spectra data cited above [12±14]. Peak A at around 460 cmÿ1 is assigned to symmetrical stretching vibrations of Te±O±Te linkages, Peak B at around 615 cmÿ1 is assigned to a vibration of the continuous network composed of TeO4 tbp, Peak C at around 670 cmÿ1 is assigned to antisymmetric vibrations of Te±O±Te linkages constructed by two unequivalent Te±O bonds, Peak D at around 720 cmÿ1 is assigned to stretching vibrations between Te and non-bridging oxygen (NBO) of TeO31 polyhedra and TeO3 tp, Peak E at around 780 cmÿ1 is assigned to Te±Oÿ stretching vibrations of TeO31 polyhedra and TeO3 tp. The Peaks F and G at around 840 cmÿ1 and 930 cmÿ1 , respectively, are connected mainly
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159
obtained. Since the intensity of the peaks A and C decreases rapidly with increasing K2 O or WO3 content, the number of Te±O±Te linkage decreases, indicating a decrease in network connectivity. The intensity of Peaks D and E in xK2 O xWO3 (100 ) 2x)TeO2 glasses increases with increasing K2 O and WO3 content, but that in 20K2 O xWO3 (80 ) x)TeO2 glasses is almost constant with WO3 content. This means that TeO4 tbp structures change to TeO31 or TeO3 tp with increasing K2 O content, but such a change is not generated from the addition of only WO3 . Since the intensity of Peak A, attributable to symmetrical stretching vibrations of Te±O±Te linkages in 20K2 O xWO3 (80 ) x)TeO2 glasses, decreases with increasing WO3 content, but the intensity of Peaks D and E, attributable to stretching vibrations related to TeO31 polyhedra and TeO3 tp, does not change with increasing WO3 content, the formation of Te±O±W bonds is strongly suggested. Indeed, the formation of W±O±Te linkages in WO3 ±TeO2 glasses has been proposed [12,14±17]. The position of Peak G, which is distinct from the other peaks, is shown in Fig. 7. It is seen that the vibration frequency of this peak xK2 O xWO3
Fig. 6. Relative intensities of seven peaks in Raman scattering spectra for (a) xK2 O xWO3 (100 ) 2x)TeO2 glasses and for (b) 20K2 O xWO3 80 ÿ xTeO2 glasses. (n) peak A at around 470 cmÿ1 , (d) peak B at around 610 cmÿ1 (m) peak C at around 670 cmÿ1 , (¨) Peak D at around 730 cmÿ1 , (h) Peak E at around 790 cmÿ1 , (s), peak F at around 860 cmÿ1 and (n) peak G at around 920 cmÿ1 . The lines are drawn as a guide for the eye.
with tungsten-oxygen stretching vibration of both WO4 and WO6 groups [13±16]. It is dicult to separate them because the neighbouring frequency ranges of the dierent vibrating species. In this relation, Peak G, which is not overlapped with other peaks, can be assigned to the stretching vibrations of W±Oÿ and W@O terminal bonds associated with WO4 and WO6 polyhedra, respectively [13±16]. From Fig. 6, the following information on the structural changes in K2 O±WO3 ±TeO2 glasses is
Fig. 7. Positions of the peak at around 920 cmÿ1 in Raman spectra for (s) xK2 O xWO3 (100 ) 2x)TeO2 glasses and for (d) 20K2 O xWO3 80 ÿ xTeO2 glasses. The lines are drawn as a guide for the eye.
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(100 ) 2x)TeO2 glasses is almost constant, while in the spectra of 20K2 O xWO3 (80 ) x)TeO2 glasses it increases with increasing WO3 content. A similar shift has been reported in infrared spectra of WO3 ±TeO2 glasses by Dimitrov et al. [15]. They suggested that when the WO3 concentration is small, the tungsten enters the glass structure in WO4 groups to form Te±O±W bonds, and, with the increase in WO3 content, WO6 groups are also formed. Brie¯y, the analysis of the Raman spectra presented here for K2 O±WO3 ±TeO2 glasses shows that a breaking of Te±O±Te network bonds and the formation of TeO3 groups occur due to addition of K2 O and that Te±O±W bonds appear in the structure due to the substitution of WO3 for TeO2 . 3.3. Heat capacity changes at Tg The heat capacity, Cp , in the glass transition region for the 10K2 O 10WO3 80TeO2 sample is shown in Fig. 8, as an example. A jump in Cp is clearly observed at the transformation from the glassy state to the supercooled liquid state. The relaxational overshoot in the heat capacity is also clearly observed. Similar patterns in Cp vs. temperature curves are observed in other xK2 O xWO3 (100 ) 2x)TeO2 samples. From Cp vs.
Fig. 8. Heat Capacity, Cp , in the glass transition region for the 10K2 O 10WO3 80TeO2 sample. Cpg and Cpl are the heat capacities of the glasses and supercooled liquids, respectively. DCp is equal to dierence between Cpl and Cpg at the glass transition temperature Tg . Heating rate was 10 K minÿ1 .
temperature curves, we determined the heat capacities of glasses, Cpg , and of supercooled liquids, Cpl , and the heat capacity changes at the glass transition, DCp Cpl ÿ Cpg . The values of Cpg (200°C), Cpl (350°C) and DCp for 10K2 O 10WO3 80TeO2 sample are 73, 121 and 48 J molÿ1 Kÿ1 respectively. These are the average values of three measurements. The values of Cpg and Cpl for xK2 O xWO3 (100 ) 2x)TeO2 samples are 72±75 and 120±130 J molÿ1 Kÿ1 , respectively. The values of DCp and Cpl =Cpg for xK2 O xWO3 (100 ) 2x)TeO2 samples are shown in Fig. 9. It is seen that these values in the samples with x 5, 10 and 20 are almost the same, but they increase gradually in samples with x 20±30. It should be pointed out that the value of Cpl =Cpg 1.65±1.81 obtained in the xK2 O xWO3 (100 ) 2x)TeO2 samples is extremely large compared with Cpl =Cpg 1:1 in SiO2 or GeO2 sample. The values of Cpl =Cpg in various glass-forming samples are given in Fig. 10 [18]. It is clear that the value of Cpl =Cpg in K2 O±WO3 ±TeO2 glasses is comparable to those in KNO3 ±Ca(NO3 2 having
Fig. 9. Heat capacity changes, DCp , at the glass transition temperature and Cpl =Cpg ratios for xK2 O xWO3 (100 ) 2x)TeO2 samples. The lines are drawn as a guide for the eye.
T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
Fig. 10. Values of Cpl =Cpg ratio for various glass-forming liquids. The data except for the K2 O ÿ WO3 ÿ TeO2 samples are taken from Ref. [18].
ionic bonding character and in gycerol, a molecular-type liquid. 4. Discussion 4.1. Thermal stability and structure In various binary MO±TeO2 and ternary R2 O± Nb2 O5 ±TeO2 systems (R Li, K), glasses thermally stable against crystallization are obtained at a composition around 80 mol% TeO2 , where MO is a modi®er [11,19±22]. It has been proposed that the rate of homogeneous nucleation becomes low in glasses with around 80 mol% TeO2 [22]. In the K2 O±WO3 ±TeO2 system, however, the most thermally stable glasses are obtained in the composition of 60±70 mol% TeO2 , as demonstrated in the present study. This means that WO3 contributes signi®cantly to the thermal stability against crystallization. Based on the Raman scattering spectra obtained in the present study, it is concluded that in the ternary K2 O±WO3 ±TeO2 glasses the network
161
structure is basically composed of TeO4 , TeO3 and WO4 (and WO6 ) units, and the breaking of Te±O±Te bonds occurs due to the addition of K2 O and further Te±O±W bonds are formed due to the substitution of W6 for Te4 . The melting temperatures of TeO2 and WO3 crystals are 452°C and 1473°C, respectively, implying that W±O bonds in K2 O±WO3 ±TeO2 glasses are much stronger than Te±O bonds [23]. These structural and bonding features would cause the compositional dependence of Tg in K2 O±WO3 ±TeO2 glasses, i.e. the substitution of K2 O for TeO2 gives a rapid decrease in Tg , but the substitution of WO3 for TeO2 gives an increase in Tg . Further, the formation of Te±O±W bonds might retard the rearrangements of Te4 or W6 structures necessary for crystallization. Uchino and Yoko [24] reported that the Te±O (axial) bonds in TeO4 are much weaker than the Te±O (equatorial) bonds. Himei et al. [25] reported that only a single component was observed in O1s photoelectron spectra in R2 O±TeO2 glasses (R: Li, Na, Rb) and non-bridging oxygen (NBO) atoms could not be resolved from bridging oxygen (BO) atom. They suggest that the electronic density of the valence shell on an NBO is almost equal to that of a BO atom. Further, Tatsumisago et al. [26] reported from the high-temperature Raman spectra for TeO2 -based glasses that TeO4 units change to TeO3 as the temperature is increased above Tg . These studies strongly suggested that the bond strength in the network consisting of TeO4 in TeO2 -based glasses is substantially weak, almost irrespective of the TeO4 /TeO3 ratio. But, it is obvious from the present study that the characteristic of weak network bonds does not mean low thermal stability against crystallization in TeO2 -based glasses. 4.2. Heat capacity changes in the glass transition region As proposed by Angell [18], glass forming liquids having small DCp or small Cpl =Cpg are called strong liquids, while those showing large DCp or large Cpl =Cpg are called fragile liquids. It is well recognized that SiO2 and GeO2 having small DCp and Cpl =Cpg ; 1:1 are strong glass-forming liquids.
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SiO2 and GeO2 have generally tetrahedrally coordinated network structures with strong covalent bonds that are expected to experience relatively little disruption during heating. It is obvious that the xK2 O xWO3 100 ÿ 2xTeO2 samples having the values of DCp 48±58 J molÿ1 Kÿ1 and Cpl =Cpg 1.6±1.8 are included in the category of fragile liquids, implying that an inferred rapid breakdown of their con®gurational structure occurs with increasing temperature. Lee et al. [6] reported that two measures of fragility, DCp and Eg=Tg , were increased with increasing Li2 O content in the Li2 O±TeO2 system. They observed a good correlation between DCp and Eg=Tg , indicating that the glasses became more fragile as the Li2 O content increased. On the other hand, the present authors [8,10,11] found that DCp at Tg in mixed-alkali tellurite glasses of 20 ÿ xLi2 O xNa2 O 80TeO2 is almost the same irrespective of the Na2 O=Li2 O ratio, but the degree of fragility, m, estimated from Eq. (1), depends on the Na2 O=Li2 O ratio, i.e. the mixed-alkali tellurite glasses are more `strong' in comparison with single alkali tellurite glasses: dlogg Eg : 1 m d T =T 2:303 RT g
T Tg
g
Angell [18] has classi®ed alcohols as thermodynamically fragile but kinetically strong liquids, because they show low fragility (temperature dependence of viscosity) despite large DCp . Recently, Senapati and Varshneya [27] have reported that there is a lack of correlation between viscositybased and heat capacity-based classi®cation of strong and fragile liquids in the overconstrained region of Ge±Se and Ge±Sb±Se glass-forming systems. As proposed by Angell [18,28], therefore, one should consider the strong/fragile concept in supercooled liquids from kinetic and thermodynamic behaviors. In other words, for example, one should use the terms `kinetic fragility', estimated from the temperature dependence of viscosity at Tg , and `thermodynamic fragility', estimated from DCp , to avoid any confusion with the use of the term `fragile'. We estimated the degree of fragility of xK2 O xWO3 (100 ) 2x)TeO2 samples using Eq. (1), and the obtained values are given in Ta-
ble 2. The value of m are 47±50, being almost the same irrespective of TeO2 content. That is, the kinetic fragility of, for example, 10K2 O 10WO3 80TeO2 is almost the same as that in 25K2 O 25WO3 50TeO2 . We can say, therefore, that the thermodynamic fragility (i.e DCp ; Cpl =Cpg in Fig. 9) in these samples changes with TeO2 content, but the kinetic fragility (i.e. m in Table 2) does not appear to change. We consider these phenomena using the structural relaxation model (the con®gurational entropy model) proposed by Adam and Gibbs [29], which has been successfully applied to the structural relaxation behavior of various glasses [18,27,29±33]. According to Adam and Gibbs [29], the structural relaxation time, s, involving the con®gurational entropy Sc is expressed by Eq. (2), DlSc ; 2 s s0 exp kTSc where s0 is constant, Dl is the potential barrier against rearrangement and k is Boltzmann's constant Sc is the con®gurational entropy of the smallest cooperatively group of molecules (cluster) that can undergo a rearrangement; generally Sc ; k ln2 is assumed [30]. In their model, the heat capacity dierence between supercooled liquids and glasses, DCp , is considered to be the con®gurational heat capacity, Cpconf , i.e. DCp Cpconf Cpl ÿ Cpg . That is, DCp is the thermodynamic measure of temperature-induced structural changes such as the change in distribution of diering chemical elements onto analogous sites of the structure and the change in the topology of the structure. Richet et al. [31±33] have analyzed the heat capacity changes in various silicate glasses by assuming that DCp is considered to be the con®gurational heat capacity, because the con®gurational entropy in silicate liquids is usually twice or three times as great as the entropy frozen-in at the glass transition. The con®gurational entropy is given by Eq. (3): ZT Sc T T2
DCp dT ; T
3
where DCp is equal to Cpl ÿ Cpg , and T2 is the temperature at which Sc 0. The gradual increase
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in DCp (Fig. 9) in the xK2 O xWO3 (100 ) 2x)TeO2 samples suggests that the con®gurational entropy increases gradually with decreasing TeO2 content, although the value of Sc has not been determined in this study. This would be supported by the structural information obtained from Raman spectra, i.e. the breaking of Te±O±Te network bonds occurs due to the addition of K2 O, because the increase in the con®gurational entropy means that cooperative rearrangements of the structure can take place independently in smaller and smaller regions of the liquid [29,33]. Since the viscosity is proportional to the structural relaxation time, i.e. g sG1 , where G1 is the high frequency shear modulus of the liquid, one can obtain the viscosity equation from Eq. (2) [30,32], log g A
DlSc ; kTSc
4
where A is a pre-exponential term. Applying the Adam and Gibbs model to the concept of fragility, the degree of fragility, m, is expressed by Eq. (5), because the activation energy for viscous ¯ow corresponds to DlSc =Sc ; 1 DlSc : 5 m Sc 2:303 RTg In the structural relaxation model proposed by Adam and Gibbs, therefore, the kinetic fragility, m, is determined from the value of DlSc =Sc =Tg , meaning that the degree of fragility is a function of Dl; Sc and Tg (here we assume Sc ; K ln2). As can be seen in Table 2, the value of Eg in the xK2 O xWO3 (100 ) 2x)TeO2 samples decreases with decreasing TeO2 content. This means that the value of (DlSc =Sc ) decreases with decreasing TeO2 content. But, since the glass transition temperature decreases steeply with decreasing TeO2 content, the value of Eg=Tg is almost constant irrespective of TeO2 content as given in Table 2, meaning that the value of DlSc =Sc Tg is also almost constant. The product of DCp and Tg increases slightly with decreasing TeO2 content (see in Table 2). It is, therefore, considered that the value of Dl would increase slightly, but would not change much with decreasing TeO2 content in the xK2 O xWO3 (100 ) 2x)TeO2 samples.
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The present results indicate that the kinetic fragility estimated from the activation energy for viscous ¯ow at and near the glass transition temperature must be carefully analyzed and the appearance of the dierence between the thermodynamic fragility estimated from DCp and the kinetic fragility estimated from Eg=Tg is realistic even for oxide glass forming liquids. It is known that there are some liquids (such as glycerol) with large DCp (i.e. thermodynamically fragile), but relative strong viscosity behavior (i.e. kinetically strong) [18,27,29]. In the previous papers [8,10,11], it was found that the mixed-alkali tellurite glasses of (20 ) x)Li2 O xNa2 O 80TeO2 are more stronger kinetically in comparison with single alkali telurite glasses, although the values of DCp are almost constant irrespective of the Na2 O=Li2 O ratio. It is desired to examine the relationship between DCp and Eg=Tg in other various TeO2 -based glasses, giving a clearer understanding of the unique features of structure, bonding, thermal stability against crystallization and structural relaxation behaviors in TeO2 -based glasses. 5. Conclusion As described in the introduction, information on thermal stability against crystallization and structural relaxation behavior in TeO2 -based glasses is lacking compared to the study of structure and optical properties. The present study clearly demonstrated that TeO2 -based glasses show unique behaviors even in thermal stability and structural relaxation. For example, some glasses such as 15K2 O 15WO3 70TeO2 show large heat capacity changes, DCp , in the glass transition region and at the same time show very high thermal stability against crystallization. The compositional dependence of the thermodynamic fragility estimated from DCp and the kinetic fragility estimated from the activation energy for viscous ¯ow at Tg in xK2 O xWO3 (100 ) 2x)TeO2 samples is largely dierent. These behaviors have been analyzed using the structural relaxation model proposed by Adam and Gibbs. In order to get more detailed information on the structural relaxation in TeO2 based glasses, the evaluation of con®gurational
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entropy and the kinetics of heat capacity changes are strongly desired. Acknowledgements This work was supported from a Grant of the Asahi Glass Foundation and a Grant-in-Aid for Scienti®c Research of the Ministry of Education, Sports and Culture in Japan. References [1] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Mater. 3 (1994) 187. [2] S.H. Kim, T. Yoko, J. Am. Ceram. Soc. 78 (1995) 1061. [3] K. Shioya, T. Komatsu, H.G. Kim, R. Sato, K. Matusita, J. Non-Cryst. Solids 189 (1995) 16. [4] H.G. Kim, T. Komatsu, K. Shioya, K. Matusita, K. Tanaka, K. Hirao, J. Non-Cryst. Solids 208 (1996) 303. [5] M. Imaoka, T. Yamazaki, J. Ceram. Soc. Jpn. 76 (1968) 160. [6] S.K. Lee, M. Tatsumisago, T. Minami, Phys. Chem. Glasses 35 (1994) 226. [7] C.Y. Zahra, A.M. Zahra, J. Non-Cryst. Solids 190 (1995) 251. [8] T. Komatsu, T. Noguchi, R. Sato, J. Am. Ceram. Soc. 80 (1997) 1327. [9] K. Tanaka, K. Hirao, K. Kashima, Y. Benino, N. Soga, Phys. Chem. Glasses 38 (1997) 87. [10] T. Komatsu, T. Noguchi, Y. Benino, J. Non-Cryst. Solids 222 (1997) 206. [11] T. Komatsu, R. Ike, R. Sato, K. Matusita, Phys. Chem. Glasses 36 (1995) 216. [12] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Non-Cryst. Solids 144 (1992) 128.
[13] T. Sekiya, N. Mochida, S. Ogawa, J. Non-Cryst. Solids 176 (1994) 105. [14] I. Shaltout, Y. Tang, R. Braunstein, A.M. Abu-Elazm, J. Phys. Chem. Solids 56 (1995) 141. [15] V. Dimitrov, M. Arnaudov, Y. Dimitriev, Monatsh. Chem. 115 (1984) 987. [16] F. Knee, R.A. Condrate, J. Phys. Chem. Solids 40 (1979) 1145. [17] V. Kozhukharov, S. Neov, I. Gerasimova, P. Mikula, J. Mater. Sci. 21 (1986) 1707. [18] C.A. Angell, J. Non-Cryst. Solids 131±133 (1991) 13. [19] T. Komatsu, H. Tawarayama, H. Mohri, K. Matusita, J. Non-Cryst. Solids 135 (1991) 105. [20] M. Zhang, S. Mancini, W. Bresser, P. Boolchand, J. NonCryst. Solids 151 (1992) 149. [21] J. Heo, D. Lam, G.H. Sigel, E.A. Endoza, D.A. Hensley, J. Am. Ceram. Soc. 75 (1992) 277. [22] T. Komatsu, K. Shioya, H.G. Kim, Phys. Chem. Glasses 38 (1997) 188. [23] L.G. Van Uitert, H.M. O'Bryan, M.E. Lines, H.J. Guggenheim, G. Zydzig, Mater. Res. Bull. 12 (1977) 261. [24] T. Uchino, T. Yoko, J. Non-Cryst. Solids 204 (1996) 243. [25] Y. Himei, Y. Miura, T. Nanba, A. Osaka, J. Non-Cryst. Solids 211 (1997) 64. [26] M. Tatsumisago, T. Minami, Y. Kowada, H. Adachi, Phys. Chem. Glasses 35 (1994) 89. [27] U. Senapati, A.K. Varshneya, J. Non-Cryst. Solids 197 (1996) 210. [28] R. Richet, A. Blumen, Disorder Eects on Relaxational Processes, Springer, Berlin, 1994, p. 22. [29] G. Adam, J.H. Gibbs, J. Chem. Phys. 43 (1965) 139. [30] G.W. Sherer, J. Non-Cryst. Solids 123 (1990) 75. [31] P. Richet, D.R. Neuville, Adv. Phys. Geochem. 10 (1992) 132. [32] P. Richet, M.A. Bouhifd, P. Courtial, C. Tequi, J. NonCryst. Solids 211 (1997) 271. [33] A. Sipp, D.R. Neuville, P. Richet, J. Non-Cryst. Solids 211 (1997) 281.
Thermal stability and heat capacity changes at the glass transition in K2O±WO3±TeO2 glasses T. Kosuge a, Y. Benino a, V. Dimitrov a, R. Sato b, T. Komatsu b
a,*
a Department of Chemistry, Nagaoka University of Technology, Kamitomioka-cho, Nagaoka 940-2188, Japan Department of Material Engineering, Tsuruoka National College of Technology, Oaza-Ioka, Tsuruoka 997, Japan
Received 9 April 1998; received in revised form 10 July 1998
Abstract The thermal stability and heat capacity changes in the glass transition region of K2 O±WO3 ±TeO2 glasses (glass formation range 20±90 mol% TeO2 ) have been studied to examine the structural relaxation behavior. The glasses with 60±70 mol% TeO2 and with both K2 O and WO3 are thermally stable against crystallization. It is proposed from Raman spectral analyses that TeO4 trigonal bipyramids change to TeO3 trigonal pyramids with the addition of K2 O and that Te±O±W bonds are formed in the substitution of WO3 for TeO2 . Heat capacity changes of DCp 48±58 J molÿ1 Kÿ1 (DCp Cpl ÿ Cpg , Cpg and Cpl are the heat capacities of the glasses and supercooled liquids, respectively), and ratios Cpl =Cpg 1.6±1.8 are obtained for xK2 O xWO3 (100 ) 2x)TeO2 glasses. The DCp and Cpl =Cpg increase with decreasing TeO2 content, indicating an increase in thermodynamic fragility with decreasing TeO2 content. But, the kinetic fragility estimated from the activation energy for viscous ¯ow is almost constant irrespective of TeO2 content. These behaviors have been analyzed using the con®gurational entropy model proposed by Adam and Gibbs. The results indicate that in K2 O±WO3 ±TeO2 glasses, Te±O±Te bonds are weak and bond breakings occur easily in the glass transition region, leading to large con®gurational entropy changes and thus large DCp . Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Tellurium oxide (TeO2 )-based glasses are of scienti®c and technical interest on account of their various unique properties, and have been considered as promising materials for use in optical ampli®ers because of their low phonon energy or nonlinear optical devices because of their large third-order non-linear susceptibility [1,2]. Recently, optically transparent TeO2 -based glass-ceramics showing second harmonic generation have been
* Corresponding author. Tel.: +81-258 47 9313; fax: +81-258 47 9300; e-mail: [email protected]
discovered [3,4]. The structure of TeO2 -based glasses is also of interest, because there are two types of the basic structural units, i.e. TeO4 trigonal bipyramid (tbp) and TeO3 trigonal pyramid (tp). In order to develop TeO2 -based glasses as new optical functional materials, an understanding of the thermal stability against crystallization and the structural relaxation behavior in the glass transition region is necessary, but such information is scarce. It is known that a pure TeO2 does not become a glass under usual quenching rates and the addition of other elements is needed to form TeO2 -based bulk glasses. Imaoka and Yamazaki [5] examined the glass-forming regions in various TeO2 -based
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 0 0 - X
T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
systems. Among the glass-forming regions reported in their paper, it is noted that the systems of R2 O±WO3 ±TeO2 (R: Li, Na, K) have extremely wide glass-forming regions. These results suggest that R2 O±WO3 ±TeO2 glasses, which are available with a wide range in TeO2 content, are suitable for the study of the structure, properties, thermal stability, structural relaxation and crystallization behaviors of TeO2 -containing glasses. In this paper, the thermal stability against crystallization and the heat capacity changes in the glass transition region of various K2 O±WO3 ±TeO2 glasses have been examined. The structural changes due to the substitution of K2 O and WO3 for TeO2 have been studied using Raman scattering spectroscopy. To the best of our knowledge, the
155
study of the structural relaxation behavior of TeO2 -based glasses by measurements of heat capacity is limited to a few systems of Li2 O±TeO2 , AgI±T1O0:5 ±TeO2 , Na2 O±TeO2 and Li2 O±Na2 O± TeO2 [6±10]. 2. Experimental procedure The nominal compositions examined in the present study are shown in Table 1. Commercial powders of reagent K2 CO3 (99.5%), WO3 (99%) and TeO2 (99%) were mixed and melted in a platinum crucible at around 800°C for 30 min in an electric furnace. The batch weight was 20 g. The quenched glasses were prepared by pouring the
Table 1 Values of glass transition, Tg , crystallization onset, Tx , and melting, Tm , temperatures, density, q, and refractive index, n, for K2 O±WO3 ±TeO2 glasses Glasses
Tg (°C)
Tx (°C)
Tm (°C)
q (g/cm3 )
n
K2 O
WO3
TeO2
(2)
(2)
(2)
(0.01)
(0.02)
0 5 10 0 10 15 20 0 10 15 20 5 10 15 20 25 22.5 23.75 15 20 25 27.5 15 20 25 20 25 30 25 30
10 5 0 20 10 5 0 30 20 15 10 35 30 25 20 15 22.5 23.75 35 30 25 27.5 45 40 35 50 45 40 55 50
90 90 90 80 80 80 80 70 70 70 70 60 60 60 60 60 55 52.5 50 50 50 45 40 40 40 30 30 30 20 20
330 306 281 353 308 275 235 368 344 307 269 388 361 331 300 252 296 294 370 331 281 280 396 363 314 380 345 290 369 325
427 416 416 506 456 370 320 536 509
628 687 459 630 619 487 630 604
416
454
5.79 5.60 5.08 5.97 5.31 4.98 4.57 6.11 5.50 5.13 4.79 5.94 5.61 5.35 5.02 4.62 4.96
2.17 2.11 2.05 2.17 2.04 1.99 1.92 2.16 2.05 1.99 1.92 2.11 2.05 2.00 1.92 1.86 1.90
538
636
419 375 511 486 452 458 439 377 439 422
482 477 665 630 504 684 544 511 550 550
5.51 5.13 4.83 4.82
1.99 1.92 1.87 1.84
5.32 5.08
1.92 1.86
5.21
1.85
5.02
1.62
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melts onto an iron plate and by pressing the sample rapidly to a thickness of 1±2 mm using another iron plate. The glassy state in the quenched samples was con®rmed by X-ray diraction (XRD) analyses at room temperature using CuK a radiation. Glass transition, Tg and crystallization onset, Tx , temperatures were determined using dierential thermal analyses (DTA) at a heating rate of 10 K minÿ1 . Densities of the glasses were determined by the Archimedes method using distilled water as the immersion liquid. Refractive indices at a wavelength of 632.8 nm (He±Ne laser) were measured at room temperature using an ellipsometer (Mizojiri Optical, DVA-36 model). The relative permittivity, er , at room temperature in the frequency range from 0.1 to 100 kHz was measured by an LCR meter. Ion-sputtered gold ®lms were used as electrodes for the measurements. Viscosities in the glass transition region (107 ±1011 Pa) were measured by a penetration method within an accuracy of 5%. Raman scattering spectra at room temperature for the glasses were measured in the wave number range from 200 to 1000 cmÿ1 using an Ar laser (wavelength 514.5 nm, laser power 100 mW) and with a JASCO NR-1100 type Laser Raman Spectrometer. The annealed glasses, which were obtained by annealing the quenched glasses at a temperature Tg +10°C for 15 min and by cooling to a temperature below Tg at a rate of 1 K minÿ1 , were used for the measurements of heat capacity changes. Heat capacities, Cp , in the glass transition region in a heating process were measured using dierential scanning calorimetry, DSC, (Rigaku Thermo¯ex TAS 200, DSC8230D). The heating rate was 10 K minÿ1 . The sample weight was around 30 mg. Standard specimens of alumina (sapphire) were used as a heat capacity standard. The molecular weight of glasses was calculated based on batch composition. 3. Results 3.1. Some properties and thermal stability The glass-forming region in the K2 O±WO3 ± TeO2 system obtained in the present study is
Fig. 1. Glass-forming region in the K2 O±WO3 ±TeO2 system. (s) glass, (g) partially crystallized, (d) crystallized.
shown in Fig. 1. It should be pointed out that glass formation is observed in compositions with 20±90 mol% TeO2 . The region shown in Fig. 1 is slightly narrower at low TeO2 content than that reported by Imaoka and Yamazaki [5]. The DTA curves of xK2 O xWO3 (100 ) 2x)TeO2 glasses with x 10, 15 and 25 are shown in Fig. 2 as an example. In the glass with x 10, the glass transition of Tg 308 C and a crystallization onset of Tx 456 C are observed. The dierence between Tg and Tx , i.e. DT Tx ÿ Tg , in this glass is 148 C, indicating that the thermal stability against crystallization is considerably high. In the glass with x 15, a glass transition of Tg 307 C, is observed, but no crystallization peak is detected. A similar DTA pattern showing no clear crystallization peak was obtained for the glass with x 20. In the glass with x 25, a crystallization peak is clearly observed at around 419 C. The values of Tg , Tx and the melting temperature, Tm , for the other glasses are given in Table 1. The thermally stable glasses, in which crystallization peaks were not observed in the DTA curves taken at a heating rate of 10 K minÿ1 , are obtained at compositions with 60±70 mol% TeO2 and with both K2 O and WO3 . As seen in Table 1, the substitution of K2 O for TeO2 or WO3 gives a rapid decrease in Tg . On
T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
157
larizability of the glasses is comparable to that of TeO2 . It is found from Table 1 that the refractive index increases almost linearly with increasing density. The frequency dependence of dielectric constant, er , at room temperature for xK2 O xWO3 (100 ) 2x)TeO2 glasses is shown in Fig. 3. The values of er decrease gradually with decreasing TeO2 content. But, the glass with x 25 still has a large value of er 19, implying again that the polarizability of WO3 is large. The activation energies for viscous ¯ow, Eg, in the glass transition region for xK2 O xWO3 (100 ) 2x)TeO2 glasses were evaluated by using the Arrhenius equation, g A exp Eg=RT , where g is a viscosity, A is a constant, R is the gas constant and T is a temperature. The evaluated values of Eg for these glasses are given in Table 2. The experimental error in Eg is around 3%. It is seen that Eg is in the range of 500±552 kJ molÿ1 and decreases slightly with decreasing TeO2 content. These values are comparable to those (418±595 kJ molÿ1 ) of (20 ) x)Li2 O xNa2 O 80TeO2 glasses [11], but are smaller than those (730±870 kJ molÿ1 ) of xLi2 O (100 ) x)TeO2 glasses [6].
Fig. 2. DTA patterns for some xK2 O xWO3 (100 ) 2x) TeO2 glasses. Tg and Tx are glass trasition and crystallization onset temperatures, respectively. Heating rate was 10 K minÿ1 .
the other hand, Tg increases due to the substitution of WO3 for TeO2 . These results suggest that K2 O acts as network modi®er and breaks the network structure and WO3 would take part in the network structure as network former. The values of density, q, and refractive index, n, for the K2 O±WO3 ±TeO2 glasses are given in Table 1. For xK2 O xWO3 (100 ) 2x)TeO2 glasses, they decrease almost linearly with decreasing TeO2 content. For the (30 ) x)K2 O xWO3 70TeO2 glasses, they increase with increasing WO3 content. As seen in Table 1, for example, the value of n 2:05 for the 10K2 O 90TeO2 glass is the same as that for the 10K2 O 20WO3 70TeO2 glass, implying that the contribution of WO3 to the po-
Fig. 3. Relative permittivity, er , at room temperature in the frequency range of 0.1±100 kHz for xK2 O xWO3 (100 ) 2x)TeO2 glasses. (X) x 5, (s) x 10, (m) x 15, (d) x 20, (n) x 25.
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Table 2 Values of the glass transition temperature, Tg , the activation energy for viscous ¯ow near the glass transition temperature, Eg, the Eg/ Tg ratio, the degree of fragility estimated from Eq. (1), m, the heat capacities of glasses, Cpg , and supercooled liquids, Cpl , the heat capacity change in the glass transition region, D Cp Cpl ÿ Cpg , and the product of Tg and D Cp for xK2 O á xWO3 á (100 ) 2x)TeO2 glasses Sample
Tg (K)
Eg (kJ molÿ1 )
Eg/Tg (kJ molÿ1 Kÿ1 )
m
Cpg
Cpl (J molÿ1 Kÿ1 )
DCp
Tg D Cp (J molÿ1 )
x5 x 10 x 15 x 20 x 25 x 27.5
579 581 580 573 554 553
552 534 511 500
0.95 0.92 0.89 0.90
50 48 47 47
72 73 74 75 73 72
120 121 122 126 130 130
48 48 48 51 57 58
)28 )28 )28 )29 )31 )32
Fig. 4. Raman scattering spectra at room temperature for xK2 O xWO3 (100 ) 2x)TeO2 glasses.
3.2. Raman scattering spectra The Raman scattering spectra for xK2 O xWO3 (100 ) 2x)TeO2 glasses are shown in Fig. 4. Similar Raman spectra are obtained for 20K2 O xWO3 (100 ) x)TeO2 glasses. Each spectrum was deconvoluted by using seven symmetrical Gaussian functions, considering peak assignments in Raman scattering spectra for K2 O±TeO2 and WO3 ±TeO2 glasses reported so far [12±14]. As an example, the ®tting result for 25K2 O 25WO3 50TeO2 glass is shown in Fig. 5. The peaks at around 470, 610, 670, 730, 790, 860 and 920 cmÿ1 are designated here as peak A, B, C, D, E, F and G, respectively. The relative peak intensity of seven peaks obtained by peak deconvolutions in xK2 O xWO3 (100 ) 2x)TeO2 and 20K2 O
000 000 000 000 000 000
Fig. 5. Raman scattering spectrum at room temperature 25K2 O 25WO3 50TeO2 glass. The spectrum was deconvoluted by using symmetrical Gaussian functions.
xWO3 (80 ) x)TeO2 glasses are shown in Fig. 6. The present peak assignments are performed in accordance with Raman spectra data cited above [12±14]. Peak A at around 460 cmÿ1 is assigned to symmetrical stretching vibrations of Te±O±Te linkages, Peak B at around 615 cmÿ1 is assigned to a vibration of the continuous network composed of TeO4 tbp, Peak C at around 670 cmÿ1 is assigned to antisymmetric vibrations of Te±O±Te linkages constructed by two unequivalent Te±O bonds, Peak D at around 720 cmÿ1 is assigned to stretching vibrations between Te and non-bridging oxygen (NBO) of TeO31 polyhedra and TeO3 tp, Peak E at around 780 cmÿ1 is assigned to Te±Oÿ stretching vibrations of TeO31 polyhedra and TeO3 tp. The Peaks F and G at around 840 cmÿ1 and 930 cmÿ1 , respectively, are connected mainly
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159
obtained. Since the intensity of the peaks A and C decreases rapidly with increasing K2 O or WO3 content, the number of Te±O±Te linkage decreases, indicating a decrease in network connectivity. The intensity of Peaks D and E in xK2 O xWO3 (100 ) 2x)TeO2 glasses increases with increasing K2 O and WO3 content, but that in 20K2 O xWO3 (80 ) x)TeO2 glasses is almost constant with WO3 content. This means that TeO4 tbp structures change to TeO31 or TeO3 tp with increasing K2 O content, but such a change is not generated from the addition of only WO3 . Since the intensity of Peak A, attributable to symmetrical stretching vibrations of Te±O±Te linkages in 20K2 O xWO3 (80 ) x)TeO2 glasses, decreases with increasing WO3 content, but the intensity of Peaks D and E, attributable to stretching vibrations related to TeO31 polyhedra and TeO3 tp, does not change with increasing WO3 content, the formation of Te±O±W bonds is strongly suggested. Indeed, the formation of W±O±Te linkages in WO3 ±TeO2 glasses has been proposed [12,14±17]. The position of Peak G, which is distinct from the other peaks, is shown in Fig. 7. It is seen that the vibration frequency of this peak xK2 O xWO3
Fig. 6. Relative intensities of seven peaks in Raman scattering spectra for (a) xK2 O xWO3 (100 ) 2x)TeO2 glasses and for (b) 20K2 O xWO3 80 ÿ xTeO2 glasses. (n) peak A at around 470 cmÿ1 , (d) peak B at around 610 cmÿ1 (m) peak C at around 670 cmÿ1 , (¨) Peak D at around 730 cmÿ1 , (h) Peak E at around 790 cmÿ1 , (s), peak F at around 860 cmÿ1 and (n) peak G at around 920 cmÿ1 . The lines are drawn as a guide for the eye.
with tungsten-oxygen stretching vibration of both WO4 and WO6 groups [13±16]. It is dicult to separate them because the neighbouring frequency ranges of the dierent vibrating species. In this relation, Peak G, which is not overlapped with other peaks, can be assigned to the stretching vibrations of W±Oÿ and W@O terminal bonds associated with WO4 and WO6 polyhedra, respectively [13±16]. From Fig. 6, the following information on the structural changes in K2 O±WO3 ±TeO2 glasses is
Fig. 7. Positions of the peak at around 920 cmÿ1 in Raman spectra for (s) xK2 O xWO3 (100 ) 2x)TeO2 glasses and for (d) 20K2 O xWO3 80 ÿ xTeO2 glasses. The lines are drawn as a guide for the eye.
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(100 ) 2x)TeO2 glasses is almost constant, while in the spectra of 20K2 O xWO3 (80 ) x)TeO2 glasses it increases with increasing WO3 content. A similar shift has been reported in infrared spectra of WO3 ±TeO2 glasses by Dimitrov et al. [15]. They suggested that when the WO3 concentration is small, the tungsten enters the glass structure in WO4 groups to form Te±O±W bonds, and, with the increase in WO3 content, WO6 groups are also formed. Brie¯y, the analysis of the Raman spectra presented here for K2 O±WO3 ±TeO2 glasses shows that a breaking of Te±O±Te network bonds and the formation of TeO3 groups occur due to addition of K2 O and that Te±O±W bonds appear in the structure due to the substitution of WO3 for TeO2 . 3.3. Heat capacity changes at Tg The heat capacity, Cp , in the glass transition region for the 10K2 O 10WO3 80TeO2 sample is shown in Fig. 8, as an example. A jump in Cp is clearly observed at the transformation from the glassy state to the supercooled liquid state. The relaxational overshoot in the heat capacity is also clearly observed. Similar patterns in Cp vs. temperature curves are observed in other xK2 O xWO3 (100 ) 2x)TeO2 samples. From Cp vs.
Fig. 8. Heat Capacity, Cp , in the glass transition region for the 10K2 O 10WO3 80TeO2 sample. Cpg and Cpl are the heat capacities of the glasses and supercooled liquids, respectively. DCp is equal to dierence between Cpl and Cpg at the glass transition temperature Tg . Heating rate was 10 K minÿ1 .
temperature curves, we determined the heat capacities of glasses, Cpg , and of supercooled liquids, Cpl , and the heat capacity changes at the glass transition, DCp Cpl ÿ Cpg . The values of Cpg (200°C), Cpl (350°C) and DCp for 10K2 O 10WO3 80TeO2 sample are 73, 121 and 48 J molÿ1 Kÿ1 respectively. These are the average values of three measurements. The values of Cpg and Cpl for xK2 O xWO3 (100 ) 2x)TeO2 samples are 72±75 and 120±130 J molÿ1 Kÿ1 , respectively. The values of DCp and Cpl =Cpg for xK2 O xWO3 (100 ) 2x)TeO2 samples are shown in Fig. 9. It is seen that these values in the samples with x 5, 10 and 20 are almost the same, but they increase gradually in samples with x 20±30. It should be pointed out that the value of Cpl =Cpg 1.65±1.81 obtained in the xK2 O xWO3 (100 ) 2x)TeO2 samples is extremely large compared with Cpl =Cpg 1:1 in SiO2 or GeO2 sample. The values of Cpl =Cpg in various glass-forming samples are given in Fig. 10 [18]. It is clear that the value of Cpl =Cpg in K2 O±WO3 ±TeO2 glasses is comparable to those in KNO3 ±Ca(NO3 2 having
Fig. 9. Heat capacity changes, DCp , at the glass transition temperature and Cpl =Cpg ratios for xK2 O xWO3 (100 ) 2x)TeO2 samples. The lines are drawn as a guide for the eye.
T. Kosuge et al. / Journal of Non-Crystalline Solids 242 (1998) 154±164
Fig. 10. Values of Cpl =Cpg ratio for various glass-forming liquids. The data except for the K2 O ÿ WO3 ÿ TeO2 samples are taken from Ref. [18].
ionic bonding character and in gycerol, a molecular-type liquid. 4. Discussion 4.1. Thermal stability and structure In various binary MO±TeO2 and ternary R2 O± Nb2 O5 ±TeO2 systems (R Li, K), glasses thermally stable against crystallization are obtained at a composition around 80 mol% TeO2 , where MO is a modi®er [11,19±22]. It has been proposed that the rate of homogeneous nucleation becomes low in glasses with around 80 mol% TeO2 [22]. In the K2 O±WO3 ±TeO2 system, however, the most thermally stable glasses are obtained in the composition of 60±70 mol% TeO2 , as demonstrated in the present study. This means that WO3 contributes signi®cantly to the thermal stability against crystallization. Based on the Raman scattering spectra obtained in the present study, it is concluded that in the ternary K2 O±WO3 ±TeO2 glasses the network
161
structure is basically composed of TeO4 , TeO3 and WO4 (and WO6 ) units, and the breaking of Te±O±Te bonds occurs due to the addition of K2 O and further Te±O±W bonds are formed due to the substitution of W6 for Te4 . The melting temperatures of TeO2 and WO3 crystals are 452°C and 1473°C, respectively, implying that W±O bonds in K2 O±WO3 ±TeO2 glasses are much stronger than Te±O bonds [23]. These structural and bonding features would cause the compositional dependence of Tg in K2 O±WO3 ±TeO2 glasses, i.e. the substitution of K2 O for TeO2 gives a rapid decrease in Tg , but the substitution of WO3 for TeO2 gives an increase in Tg . Further, the formation of Te±O±W bonds might retard the rearrangements of Te4 or W6 structures necessary for crystallization. Uchino and Yoko [24] reported that the Te±O (axial) bonds in TeO4 are much weaker than the Te±O (equatorial) bonds. Himei et al. [25] reported that only a single component was observed in O1s photoelectron spectra in R2 O±TeO2 glasses (R: Li, Na, Rb) and non-bridging oxygen (NBO) atoms could not be resolved from bridging oxygen (BO) atom. They suggest that the electronic density of the valence shell on an NBO is almost equal to that of a BO atom. Further, Tatsumisago et al. [26] reported from the high-temperature Raman spectra for TeO2 -based glasses that TeO4 units change to TeO3 as the temperature is increased above Tg . These studies strongly suggested that the bond strength in the network consisting of TeO4 in TeO2 -based glasses is substantially weak, almost irrespective of the TeO4 /TeO3 ratio. But, it is obvious from the present study that the characteristic of weak network bonds does not mean low thermal stability against crystallization in TeO2 -based glasses. 4.2. Heat capacity changes in the glass transition region As proposed by Angell [18], glass forming liquids having small DCp or small Cpl =Cpg are called strong liquids, while those showing large DCp or large Cpl =Cpg are called fragile liquids. It is well recognized that SiO2 and GeO2 having small DCp and Cpl =Cpg ; 1:1 are strong glass-forming liquids.
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SiO2 and GeO2 have generally tetrahedrally coordinated network structures with strong covalent bonds that are expected to experience relatively little disruption during heating. It is obvious that the xK2 O xWO3 100 ÿ 2xTeO2 samples having the values of DCp 48±58 J molÿ1 Kÿ1 and Cpl =Cpg 1.6±1.8 are included in the category of fragile liquids, implying that an inferred rapid breakdown of their con®gurational structure occurs with increasing temperature. Lee et al. [6] reported that two measures of fragility, DCp and Eg=Tg , were increased with increasing Li2 O content in the Li2 O±TeO2 system. They observed a good correlation between DCp and Eg=Tg , indicating that the glasses became more fragile as the Li2 O content increased. On the other hand, the present authors [8,10,11] found that DCp at Tg in mixed-alkali tellurite glasses of 20 ÿ xLi2 O xNa2 O 80TeO2 is almost the same irrespective of the Na2 O=Li2 O ratio, but the degree of fragility, m, estimated from Eq. (1), depends on the Na2 O=Li2 O ratio, i.e. the mixed-alkali tellurite glasses are more `strong' in comparison with single alkali tellurite glasses: dlogg Eg : 1 m d T =T 2:303 RT g
T Tg
g
Angell [18] has classi®ed alcohols as thermodynamically fragile but kinetically strong liquids, because they show low fragility (temperature dependence of viscosity) despite large DCp . Recently, Senapati and Varshneya [27] have reported that there is a lack of correlation between viscositybased and heat capacity-based classi®cation of strong and fragile liquids in the overconstrained region of Ge±Se and Ge±Sb±Se glass-forming systems. As proposed by Angell [18,28], therefore, one should consider the strong/fragile concept in supercooled liquids from kinetic and thermodynamic behaviors. In other words, for example, one should use the terms `kinetic fragility', estimated from the temperature dependence of viscosity at Tg , and `thermodynamic fragility', estimated from DCp , to avoid any confusion with the use of the term `fragile'. We estimated the degree of fragility of xK2 O xWO3 (100 ) 2x)TeO2 samples using Eq. (1), and the obtained values are given in Ta-
ble 2. The value of m are 47±50, being almost the same irrespective of TeO2 content. That is, the kinetic fragility of, for example, 10K2 O 10WO3 80TeO2 is almost the same as that in 25K2 O 25WO3 50TeO2 . We can say, therefore, that the thermodynamic fragility (i.e DCp ; Cpl =Cpg in Fig. 9) in these samples changes with TeO2 content, but the kinetic fragility (i.e. m in Table 2) does not appear to change. We consider these phenomena using the structural relaxation model (the con®gurational entropy model) proposed by Adam and Gibbs [29], which has been successfully applied to the structural relaxation behavior of various glasses [18,27,29±33]. According to Adam and Gibbs [29], the structural relaxation time, s, involving the con®gurational entropy Sc is expressed by Eq. (2), DlSc ; 2 s s0 exp kTSc where s0 is constant, Dl is the potential barrier against rearrangement and k is Boltzmann's constant Sc is the con®gurational entropy of the smallest cooperatively group of molecules (cluster) that can undergo a rearrangement; generally Sc ; k ln2 is assumed [30]. In their model, the heat capacity dierence between supercooled liquids and glasses, DCp , is considered to be the con®gurational heat capacity, Cpconf , i.e. DCp Cpconf Cpl ÿ Cpg . That is, DCp is the thermodynamic measure of temperature-induced structural changes such as the change in distribution of diering chemical elements onto analogous sites of the structure and the change in the topology of the structure. Richet et al. [31±33] have analyzed the heat capacity changes in various silicate glasses by assuming that DCp is considered to be the con®gurational heat capacity, because the con®gurational entropy in silicate liquids is usually twice or three times as great as the entropy frozen-in at the glass transition. The con®gurational entropy is given by Eq. (3): ZT Sc T T2
DCp dT ; T
3
where DCp is equal to Cpl ÿ Cpg , and T2 is the temperature at which Sc 0. The gradual increase
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in DCp (Fig. 9) in the xK2 O xWO3 (100 ) 2x)TeO2 samples suggests that the con®gurational entropy increases gradually with decreasing TeO2 content, although the value of Sc has not been determined in this study. This would be supported by the structural information obtained from Raman spectra, i.e. the breaking of Te±O±Te network bonds occurs due to the addition of K2 O, because the increase in the con®gurational entropy means that cooperative rearrangements of the structure can take place independently in smaller and smaller regions of the liquid [29,33]. Since the viscosity is proportional to the structural relaxation time, i.e. g sG1 , where G1 is the high frequency shear modulus of the liquid, one can obtain the viscosity equation from Eq. (2) [30,32], log g A
DlSc ; kTSc
4
where A is a pre-exponential term. Applying the Adam and Gibbs model to the concept of fragility, the degree of fragility, m, is expressed by Eq. (5), because the activation energy for viscous ¯ow corresponds to DlSc =Sc ; 1 DlSc : 5 m Sc 2:303 RTg In the structural relaxation model proposed by Adam and Gibbs, therefore, the kinetic fragility, m, is determined from the value of DlSc =Sc =Tg , meaning that the degree of fragility is a function of Dl; Sc and Tg (here we assume Sc ; K ln2). As can be seen in Table 2, the value of Eg in the xK2 O xWO3 (100 ) 2x)TeO2 samples decreases with decreasing TeO2 content. This means that the value of (DlSc =Sc ) decreases with decreasing TeO2 content. But, since the glass transition temperature decreases steeply with decreasing TeO2 content, the value of Eg=Tg is almost constant irrespective of TeO2 content as given in Table 2, meaning that the value of DlSc =Sc Tg is also almost constant. The product of DCp and Tg increases slightly with decreasing TeO2 content (see in Table 2). It is, therefore, considered that the value of Dl would increase slightly, but would not change much with decreasing TeO2 content in the xK2 O xWO3 (100 ) 2x)TeO2 samples.
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The present results indicate that the kinetic fragility estimated from the activation energy for viscous ¯ow at and near the glass transition temperature must be carefully analyzed and the appearance of the dierence between the thermodynamic fragility estimated from DCp and the kinetic fragility estimated from Eg=Tg is realistic even for oxide glass forming liquids. It is known that there are some liquids (such as glycerol) with large DCp (i.e. thermodynamically fragile), but relative strong viscosity behavior (i.e. kinetically strong) [18,27,29]. In the previous papers [8,10,11], it was found that the mixed-alkali tellurite glasses of (20 ) x)Li2 O xNa2 O 80TeO2 are more stronger kinetically in comparison with single alkali telurite glasses, although the values of DCp are almost constant irrespective of the Na2 O=Li2 O ratio. It is desired to examine the relationship between DCp and Eg=Tg in other various TeO2 -based glasses, giving a clearer understanding of the unique features of structure, bonding, thermal stability against crystallization and structural relaxation behaviors in TeO2 -based glasses. 5. Conclusion As described in the introduction, information on thermal stability against crystallization and structural relaxation behavior in TeO2 -based glasses is lacking compared to the study of structure and optical properties. The present study clearly demonstrated that TeO2 -based glasses show unique behaviors even in thermal stability and structural relaxation. For example, some glasses such as 15K2 O 15WO3 70TeO2 show large heat capacity changes, DCp , in the glass transition region and at the same time show very high thermal stability against crystallization. The compositional dependence of the thermodynamic fragility estimated from DCp and the kinetic fragility estimated from the activation energy for viscous ¯ow at Tg in xK2 O xWO3 (100 ) 2x)TeO2 samples is largely dierent. These behaviors have been analyzed using the structural relaxation model proposed by Adam and Gibbs. In order to get more detailed information on the structural relaxation in TeO2 based glasses, the evaluation of con®gurational
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