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Effect of dissolved water on the internal friction of glass
Journal of Non-Crystalline Solids, 14 (1974) 165-177. © North-Holland Publishing Company
EFFEC'I" OF DISSOLVED WATER ON THE INTERNAL FRICTION OF GLASS Delbert E. DAY Ceramic Engineering Department, University of M~souri.Rolla~ Rollq, Mo. 65401, USA
and J.M. STEVELS Eindhoven University of Technology, Eindhoven, The Netherlands
The two internal friction peaks in (1-X) Na2 P206" X AI203 glasses showed a pronounced d~pendence on the water content. With increasing water content, the alk',di peak became 10-20% smaller and a slight increase in the activation energy was indicated. Similarly, the second peak became 2-4 times larger and shifted to lower temperatures. From *he close correlation between the magnitude of the second peak and the water content, the mechaaism got Lhispeak was concluded to consist of the cooperative movement of a sodium ion and a neighboring proton. Comparison of aluminophosphate and alkali silicate ghsse~ showed that the internal friction peaks have a similar dependence upon the water content in both types of glasses, It is concluded that the water content of a glass should be closely controlled when the effect of other compositional changes on the intema! friction is being studied.
1. Introduction Glasses normally melted in air always contain a small amount of dissolved water; depending upon melt composition, melting temperature and time, and the water content of the furnace atmosphere. The water content of a glass is known to affect many properties, but there have been relatively few investigations of the effect of water content on the internal friction. In vitreous SiO 2 and B20 3, the acoustic loss peak at 50 and 300K (f-- 20MHz), respectively, has been reported to become smaller with decreasing water content
11,21. A dependence on water content has also been reported for the two internal friction peaks commonly observed in alkali silicate glasses [3, 4]. In general, the peak Oue to the stress induced diffusion of the alkali ions (low temperature or alkali p~ak) becomes smaller and moves to higher temperatures with increasing water content, Conversely, the second and generally smaller peak, located at higher temperatures,
166
D.E. Day, ZM. Stevels, Internal friction of glass
becomes larger and moves to lower temperatures with increasing water content. Some of the mechanisms proposed for the second peak in alkali silicate glasses relate this peak to protons. It has been called an 'interaction peak' by Coenen [5] and his suggested mechanism involved alkali ions interacting ,,ith those protons in hydrogen bonded bridging positions. Doremus [6] associated this peak with the chemical durability of a glass and noted that glasses that weather rapidly were more likely to exhibit this peak than those with a higher stability in air. He attributed this peak to the motion of hydrogen ions (protons) that entered the glass by exchanging with the alkali ions on the glass surface when the glass was in contact with moisture in the air. Protons introduced into a glass by ion-exchanging the alkali ions, using molten NH 4HSO4, also seem to produce essentially the same changes in internal friction that are observed when a glass is bubbled with water vapor, i.e., the second peak becomes larger and moves to lower temperatures in sodium silicate glasses [7]. In a Li20" A1203 • 2SiO 2 glass, ion-exchanged in NH 4 HSO4, a peak appeared at about 220°C that was absent in the un-exchanged glass [8]. The mechanism for the second peak in alkali silicate glasses has also been attributed to the presence of non-bridging oxygen ions [9-11 ]. Measurements on a NaPO 3 glass [ 12], which consists ofPO 4 chains with two nonbridging oxygen ions per tetrahedron, showed that the second internal friction peak nominally located at 100°C, f = 0.5 Hz, was especially sensitive to the water content. It was concluded that this peak would be absent in a NaPO 3 glass totally free of water, but containing a significant concentration of non-bridging oxygen ions. The mechanism for this peak was attributed to a cooperative movement of sodium ions and neighboring protons rather than to a motion of the non.bridging oxygen ions. This paper describes internal friction measurerrlents made on ( I-,Y)Na2P206 • XAI203 glasses in order to obtain further information about how the water content affects the internal friction and the mechanism for the second internal friction peak observed in these glasses. Pre'dously suggested mechanisms for the second internal friction peak observed in many alkali containing glasses are also reconsidered.
2. Experimental procedure The glasses were prepared from reagent grade NaH2PO4.H20 and AI(OH)3 and melted in platinum crucibles. Prior to use, the raw materials were calcined to remove tile combined water. The melt was stirred several times with a platinum rod and then bubbled with either dry oxygen or oxygen previously saturated with water vapor by bubbling the oxygen through a water bath at 100°C. Fiber and rectangular bar specimens, for the internal friction measurements, were obtained from the same melt at periodic intervals. These specimens were stored in containers filled with a water-free oil. The methods of measuring the water content and the internal friction were the
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Glass composition and description
Table I Internal friction of X Al2O3"( 1 -X)NazP206 glasses of varying water coment.
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D.If. Day, ZM. $tevels, Internal friction of glass
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Fig, 1. Comparison of percent weight loss and extinction for (1-X) Na2P20 6"XAI203 glasses with that previously determined [12] for NaPe3 glassescontaining water. same as described previously [12l. Briefly, the water content was determined from the measured weight loss when a portion of the glass was reheated (to 700-770°C) for three hours in a vacuum thermobalance and from the intensity of the infrared absorption at approximately 2910 cm -I . The weight loss and extinction at 2910 cm-1 are given in table 1 and were in good agreement, fig. 1. The straight line in fig. 1 was determined from the previous measurements [121 on NaPe 3 glass.
3. Results
The curves in fig. 2a and b are typical examples Gf how the internal friction changed as tile water content varied in the compositions studied. The second peak at higher temperatures was especially sensitive to the water content, and generally, the magnitude of this peak depended more upon the water than alumina content. With increasing water content, the general effect on the internal friction was for the low temperature peak to become smaller and shift to slightly higher temperatures while the second peak became considerably larger and shifted to lower temperatures. The large variation in magnitude of the two internal friction peaks in the aluminophospilate glasses emphasizes the importance of controlling the water content when the internal friction of different glass compositions is being studied.
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Since the activation energy for the low temperature peak, table l, is essentially the same as that for sodium diffusion and electrical conductivity [ 13] in sodium aluminophosphate glasses, this peak is attributed to the movement of the sodium ions and
D.E. Day, ZM. Stevels, Internal friction oi" glass
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is hereafter referred to as an alkali peak. Compared with the experimental error, the activation energy for this peak did not vary significantly with either the water or alumina content. Based on the measured activation energies and the shift of this peak to slightly higher temperatures (at constant measuring frequency), however, a small increase of I or 2 kcal/mol in the activation energy is considered likely over the range of water contents investigated. Fig. 3 shows how the magnitude of both peaks varied with alu,-~:na content for 'as melted' glasses and those bubbled with dry oxygen. These curves reflect the combined effects of the alumina and water content since for a given melting condition, e.g., bubbled with dry oxygen, the water content did not remain constant. The decrease in magnitude of the alkali peak with increasing alumina content is considered reasonably reliable since this peak was less sensitive to the water content and a decrease in magnitude with decreasing sodium oxide content is consistent with this peak being due to the movement of the sodium ions. The curves for the second peak are
D.E. Day, J.M. Stevels, Internal friction of glass
171
considered less reliable in showing the change in magnitude with alumina content since this peak was considerably more sensitive to the water content. When the magnitude of the second peak is compared with the weight loss, i.e., water content, the alumina content does not appear to be especially important. In fig. 4 the magnitude of the second peak is seen to increase smoothly with increasing water content, with the points for the alum~nophosphate glasses being independent of the alumina content. The numbers at each point correspond to the values of X in table 1. For water contents below 0.10 wt% the magnitude of the second peak varied almost linearly with the alumina content. While the concentration of alumina in the aluminophosphate glasses seems to have a minor effect upon the magnitude of the second peak, it should be noted that a given water content results in a larger second peak in the aluminophosphate glasses than in the NaPe 3 glass. The activation energies for the second peak in table 1 do not seem to vary in any definite or consistent manner with alumina content. For any given composition, the second peak always moved to lower temperatures with increasing water content, fig. 2, indicating a reduction in activation energy. In the glasses X = 0 and X = 0.I0, this reduction was experimentally verified, however, a decrease was not verified in the X = 0.33 glass, and the measured activation energies were the same within experimental error. The lack of any consistent trend in the activation energy is attributed to the combined effect of differences in the water and alumina content since the activation energy depends upon both compositional factors, and to some extent, to the experimental error in the measurements.
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172
D.E. Day, J.M. Stevels, Internal friction of glass
4. Discussion
4.1. Aluminophosphate glasses The two internal friction peaks in the aluminophosphate glasses showed the same general dependency upon water content which has been reported previously for alkali silicate glasses [3, 41. Compared to silicate glasses, the two peaks in the aluminophosphate glasses varied considerably more in magnitude, thus allowing the dependency of the internal friction on water content to be seen more clearly. This is particularly true for the second peak, which showed pronounced changes with water content, but was relatively independent of the overall composition. While additions of alumina affected the magnitude of the second peak to some degree, these changes were minor :ompared to those resulting from a change in water content of only 0.10 wt%. For example, in glasses of nearly equal water content., the magnitude of this peak in the glass of highest alumina content X = 0.33 (weight loss of 0.036 wt%) was almost the same as that in the vacuum melted NaPO 3 glass (weight loss of 0.016 wt%). The clear dependence of the second peak on water content is taken as evidence that protons are important to the mechanism for this peak. This importance is shown by the curves in fig. 4 which indicate that the peak would be quite small, if non totally absent, in water.free phosphate glasses. Furthermore, the close correlation between the magnitude of the second peak and the water content, rather than the alumina content, can be accounted for by a mechanism dependent upon the proton concentration. The second peak in the aluminophosphate glasses containing water, therefore, is attributed to a mechanism based on the cooperative movement of a sodium ion and proton. With increasing water content, a larger percentage of the sodium ions have nearby protons and the magnitude of the second peak would be expected to increase. Another reason for suggesting that the mechanism consists of the cooperative movement of both ions, rather than just the protons, is the close similarity in the change in the internal friction observed in the present study with increasing water content and that reported for tile two internal friction peaks in single alkali silicate [14] and phosphate glasses [15] with the addition of a second alkali. In mixed alkali glasses, the alkali peak becomes smaller as a second alkali is added and a new larger peak appears whose magnitude is especially sensitive to the first small amounts of the second alkali. A cooperative movement of both alkali ions has been proposed a., the mechanism for this new peak in mixed alkali glasses and a similar mechanism is considered possible involving an alkali ion and a nearby proton in glasses containing water. The aluminophosphate glasses provide little evidence supporting a mechanism based on the movement of the non-bridging oxygen ions as has been proposed for the second peak in alkali silicate glasses [10, 11]. Depending upon how the aluminum ions are present in the glass structure, a significant change in the concentration of non-bridging oxygen ions would be expected as X varied from 0 to 0.33, but this
D.E. Day, J.M. Stevels, lnternal friction of glass
173
change in alumina content affected the magnitude of the second peak only slightly. If the aluminum ions are assumed to be present as AIO4 tetrahedra, then a 50% decrease in the concentration of non-bridging oxygens would be expected as the O/(P + AI) ratio changed from 3 in the NaPO 3 glass to 2.5 when X = 0.33. Conversely, the aluminum ions could be assumed to introduce more, rather than less non-bridging oxygen ions, but this is not supported by the increasing rigidity of the network as indicated by the temperature where the background internal friction equals 10 -2, table 1. If the effect of the differences in water content on the shape of the curves in fig. 3 are totally ignored, then the change in magnitude for the second peak with increasing alumina content can only be accounted for by assuming some complex structural role for the aluminum ions. In this case the magnitude of this peak would not be expected to be essentially independent of the alumina content as shown in fig. 4. To a limited extent the concentration of non-bridging oxygen ions in a phosphate glass will be determined by the water content, assuming the chemical dissolution of water proceeds according to the equation O
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There has been considerable discussion about the mechanism for the second peak observed in alkali silicate glasses, but there is evidence that this peak depends upon the water content in a manner similar to that for the second ~ a k in alkali phosphate and atuminophosphate glasses. Data by Maklad and KreidL [4] show that this peak becomes larger when a 18 Na20.82SiO 2 glass, mol%, is bubbled with steam, Although no experimental data for glasses of varying water content were published, Coenen [5] also reported that this peak depended upon the water content and he proposed a mechanism involving alkai ions and protons in hydrogen bridging posi-, tions. In view of the indicated similarity between the effect of water on the second peak in the alkali phosphate, aluminophosphate and silicate glasses, the data for alkali silicate glasses have been reviewed to determine whether the behavior of this peak could be accounted for by differences in water content. One of the characteristic features of the second peak in alkali silicate glasses is that it becomes larger with increasing alkali content. This is illustrated for sodium
D.E. Day, J.M. Stevels, Internal [riction of gbss
174
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silicate glasses in fig. 5 which summarizes the results of numerous investigations. A portion of the scatter in the internal friction data is due to the difficulty in estimating the background in those studies where the height above background was not given. While the peak magnitude has generally been related to the change in alkali content, :it is seen from fig. 5 that the change in peak magnitude closely follows the change in water content as reported by Franz and Scholze [ 16]. Whether this difference in water content can account completely for the change in peak magnitude is not known, but it must be considered a contributing factor. Differences in water content, res~dting from melting under ambient conditions, may also partially explain why different peak magnitudes have been reported for the same glass by different investiga. tors. The differences in the magnitude of the second peak for silicate glasses containing different alkali ions, but having the same total alkali content, also correlate reason. ably well with the water content. This is illustrated in table 2 for 25 tool% alkali silicate glasses where the peak magnitude is seen to increase from Li to Rb. The water content also increases in the same order for melts bubbled with water vapor. Assuming that glasses melted in air, while containing less water, would contain approximately the same relative amounts of wator as those bubbled with steam, then a change in peak magnitude is again observed which is consistent with and nearly proportional to the difference in water content.
D.E. Day, J.M. Stevels, Internal friction of glass
175
Table 2 Height of second internal frictit, t peak and water content for silicate glasses containing 25 mole percent alkali oxide. Alkafi
Peak height above background a) Q-I X 103
Mol percent water b) at 1480°C
Li Na K Rb
1.6 3.4 4.5 4.5
0.398 (27.5) 0.513 (25.6) 0.804 (26.7)
a) Ref. [ 17]; peak heights measured with torsion pendulum. b) Ref. [ 16]; numbers in parentheses are analyzed alkali content.
Previously, the reduction in magnitude for the second peak when alkaline earth oxides or alumina are added to an alkali silicate glass has been attributed to a change in the concentration of the non-bridging oxygen ions [ I 0, 11 ]. However, the water content also generally decreases with increasing additions of either alkaline earth oxides or alumina to an alkali silicate glass [6]. The lower proton concentration result ing from the compositional changes is an alternative explanation for the reduction ia peak magnitude. Wherever comparisons have been made the magnitude of the second peak in alkali silicate glasses is found to vary consistently with the water content, in certain instances, the proton exchanged glasses [71 and glasses conlaining equal amounts o|" different alkalis, a mechanism based on the active participation of the protons is considered a more satisfactory explanation than a mechanism based on the movenient or" the non-bridging oxygen ions. Even though the protons are associated with what can be considered non.bridging oxygen ions, the oxygen ion is considered of secondary importance to the mechanism, in agreement with Coenen's previous interpretation [5], alkali ions with neighboring protons are considered the principal structural condition required for the relaxation mechanism. It is concluded, therefore, that a mechanism consisting of the cooperative movement of an alkali ion and neighboring proton provides a satisfactory explanation for the behavior of the second peak observed in alkali silicate glasses. In this regard the second peak in single alkali silicate glasses is considered similar to the large peak in mixed alkali glasses which has been attributed to a cooperative movement of dissimilar alkali ions, except that in the single alkali glasses the mechanism involves an alkali ion and proton.
5. Conclusions Generally, the internal friction of alkali containing aluminophosphate and silicate glasses should be expected to depend upon the water content. It is important, there-
176
D.E. Day, ZM. Stevels, Internal friction of glass
fore, to control the water content when the effects of other compositional changes on the internal friction are being svadied. The pronounced sensitivity of the second peak in phosphate and aluminophosphate glasses to the water content is taken as evidence that protons are necessary for this peak to be observed. Consequently, the mechanism for this peak in the aluminophosphate glasses of varying water content is attributed to a cooperative motion of the alkali ions and nearby protons. The non-bridging oxygen ion, with which the proton is associated, is considered a passive feature of the mechanism. This same mechanism is also considered applicable to the second peak observed in alkali silicate glasses. This opinion is based on the essentially identical dependence of the second peak in both the aluminophosphate and silicate glasses upon water con-. tent and the correlation of the second peak in various alkali silicate glasses with their estimated water content. By considering the second peak in alkali glasses to be a type of mixed peak involving both alkali ions and protons, all of the internal friction peaks previously reported for alkali glasses can be attributed to one general mechanism associated with alkali ion movement; the activation energy and relaxation strength of which depends on the near neighbor environment of the alkali ions. With this interpretation, the alkali peaks are associated with the movement of those alkali ions surrounded by alkali ions of the same type. Similarly, the second peak in alkali glasses melted in air and containing water, or the mixed peaks in mixed alkali glasses are associated with the structural condition wl~ere an alkali ion has a dissimilar ion as a neighbor; i.e., a proton or different alkali ion, respectively. Viewed in this way, the various internal friction peaks observed in glass are not very different.
References [1] J.T. Krause, J. Appl. Physics 42 (1971) 3035. [2l C.R, Kurkjian and LT. Krause, J. Amer. Ceram. Soc. 49 (1966) 171. [3] R. Briickner, Glastechn. Ber. 37 (1964) 536.
[41 M.S. Maklad and N.J. Kreidl, Sci. and Tech. Comm. of the 9th Intl. Congress on Glass, vol. I, (Inst. du Verre, Paris, 1971) p. 75.
[51 M. Coenen, Zeit. ff/r Electrochemie 65 (196 I) 903. [61 R,H. Doremus, J. Non.Crystalline Solids 3 (1970) 369. 17] H. de Waal, J. Amer. Ceram. Soc. 52 (1969) 165.. 18] A.I.A. Abdel-Latif and D.E. Day, J. Amer. Ceram. Soc. 55 (1972) 254.
19] H. R~tger, Glastech. Ber. 31 (1958)54. [1o] R.J. Ryder and G.E. Rindone, J. ~mer. Ceram. Soc. 44 (1961) 532. [Ill D.E. Day and G.E. Rindone, J. Amer. Ceram. Soc. 45 (1962) 495. [121 D.E. Day and J.M. Stevels, J. Non-Crystalline Solids 11 (1973) 459. f13] R. Terai, J. Ceram. Assoc. Japan 72 (1964) 7. J.E. Shelby, Jr. and D.E. Day, J. Amer. Ceram. Soc. 33 (1970) 182. [151 H,M. Van Ass and J.M. Stevels, Technical University of Eindhoven, The Netherland, personal communication. [16] H. Franz and H. Scholze, Glastech. Ber. 36 (1963) 347.
D.E. Day, J.M. Stevels, Internal friction of glass [17] J.E. Shelby, Jr. and D.E. Day, J. Amer. Ceram. Soc. 52 (1969) 169. [18] R. Jagdt, Glastechn. Bet. 33 (1960) lO. [19] R.H. Redwine and M.B. Field, J. Mat. Sci. 4 (1969) 713.
177
EFFEC'I" OF DISSOLVED WATER ON THE INTERNAL FRICTION OF GLASS Delbert E. DAY Ceramic Engineering Department, University of M~souri.Rolla~ Rollq, Mo. 65401, USA
and J.M. STEVELS Eindhoven University of Technology, Eindhoven, The Netherlands
The two internal friction peaks in (1-X) Na2 P206" X AI203 glasses showed a pronounced d~pendence on the water content. With increasing water content, the alk',di peak became 10-20% smaller and a slight increase in the activation energy was indicated. Similarly, the second peak became 2-4 times larger and shifted to lower temperatures. From *he close correlation between the magnitude of the second peak and the water content, the mechaaism got Lhispeak was concluded to consist of the cooperative movement of a sodium ion and a neighboring proton. Comparison of aluminophosphate and alkali silicate ghsse~ showed that the internal friction peaks have a similar dependence upon the water content in both types of glasses, It is concluded that the water content of a glass should be closely controlled when the effect of other compositional changes on the intema! friction is being studied.
1. Introduction Glasses normally melted in air always contain a small amount of dissolved water; depending upon melt composition, melting temperature and time, and the water content of the furnace atmosphere. The water content of a glass is known to affect many properties, but there have been relatively few investigations of the effect of water content on the internal friction. In vitreous SiO 2 and B20 3, the acoustic loss peak at 50 and 300K (f-- 20MHz), respectively, has been reported to become smaller with decreasing water content
11,21. A dependence on water content has also been reported for the two internal friction peaks commonly observed in alkali silicate glasses [3, 4]. In general, the peak Oue to the stress induced diffusion of the alkali ions (low temperature or alkali p~ak) becomes smaller and moves to higher temperatures with increasing water content, Conversely, the second and generally smaller peak, located at higher temperatures,
166
D.E. Day, ZM. Stevels, Internal friction of glass
becomes larger and moves to lower temperatures with increasing water content. Some of the mechanisms proposed for the second peak in alkali silicate glasses relate this peak to protons. It has been called an 'interaction peak' by Coenen [5] and his suggested mechanism involved alkali ions interacting ,,ith those protons in hydrogen bonded bridging positions. Doremus [6] associated this peak with the chemical durability of a glass and noted that glasses that weather rapidly were more likely to exhibit this peak than those with a higher stability in air. He attributed this peak to the motion of hydrogen ions (protons) that entered the glass by exchanging with the alkali ions on the glass surface when the glass was in contact with moisture in the air. Protons introduced into a glass by ion-exchanging the alkali ions, using molten NH 4HSO4, also seem to produce essentially the same changes in internal friction that are observed when a glass is bubbled with water vapor, i.e., the second peak becomes larger and moves to lower temperatures in sodium silicate glasses [7]. In a Li20" A1203 • 2SiO 2 glass, ion-exchanged in NH 4 HSO4, a peak appeared at about 220°C that was absent in the un-exchanged glass [8]. The mechanism for the second peak in alkali silicate glasses has also been attributed to the presence of non-bridging oxygen ions [9-11 ]. Measurements on a NaPO 3 glass [ 12], which consists ofPO 4 chains with two nonbridging oxygen ions per tetrahedron, showed that the second internal friction peak nominally located at 100°C, f = 0.5 Hz, was especially sensitive to the water content. It was concluded that this peak would be absent in a NaPO 3 glass totally free of water, but containing a significant concentration of non-bridging oxygen ions. The mechanism for this peak was attributed to a cooperative movement of sodium ions and neighboring protons rather than to a motion of the non.bridging oxygen ions. This paper describes internal friction measurerrlents made on ( I-,Y)Na2P206 • XAI203 glasses in order to obtain further information about how the water content affects the internal friction and the mechanism for the second internal friction peak observed in these glasses. Pre'dously suggested mechanisms for the second internal friction peak observed in many alkali containing glasses are also reconsidered.
2. Experimental procedure The glasses were prepared from reagent grade NaH2PO4.H20 and AI(OH)3 and melted in platinum crucibles. Prior to use, the raw materials were calcined to remove tile combined water. The melt was stirred several times with a platinum rod and then bubbled with either dry oxygen or oxygen previously saturated with water vapor by bubbling the oxygen through a water bath at 100°C. Fiber and rectangular bar specimens, for the internal friction measurements, were obtained from the same melt at periodic intervals. These specimens were stored in containers filled with a water-free oil. The methods of measuring the water content and the internal friction were the
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Glass composition and description
Table I Internal friction of X Al2O3"( 1 -X)NazP206 glasses of varying water coment.
255 226
23.3 22.8
26.1
24.6
25.0
31.6
24.6
26.4
355
364
369
327
348
298
298
280
288
275
26.8
29.4
272
Temp where backgound Q-t = 10=2 (°C)
~28±2
Act. energy (kcal/mole) (± 1.0)
D.If. Day, ZM. $tevels, Internal friction of glass
168
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Fig, 1. Comparison of percent weight loss and extinction for (1-X) Na2P20 6"XAI203 glasses with that previously determined [12] for NaPe3 glassescontaining water. same as described previously [12l. Briefly, the water content was determined from the measured weight loss when a portion of the glass was reheated (to 700-770°C) for three hours in a vacuum thermobalance and from the intensity of the infrared absorption at approximately 2910 cm -I . The weight loss and extinction at 2910 cm-1 are given in table 1 and were in good agreement, fig. 1. The straight line in fig. 1 was determined from the previous measurements [121 on NaPe 3 glass.
3. Results
The curves in fig. 2a and b are typical examples Gf how the internal friction changed as tile water content varied in the compositions studied. The second peak at higher temperatures was especially sensitive to the water content, and generally, the magnitude of this peak depended more upon the water than alumina content. With increasing water content, the general effect on the internal friction was for the low temperature peak to become smaller and shift to slightly higher temperatures while the second peak became considerably larger and shifted to lower temperatures. The large variation in magnitude of the two internal friction peaks in the aluminophospilate glasses emphasizes the importance of controlling the water content when the internal friction of different glass compositions is being studied.
D.E. Day, J.M. Stevels, Internal friction of glass e
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Since the activation energy for the low temperature peak, table l, is essentially the same as that for sodium diffusion and electrical conductivity [ 13] in sodium aluminophosphate glasses, this peak is attributed to the movement of the sodium ions and
D.E. Day, ZM. Stevels, Internal friction oi" glass
170
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is hereafter referred to as an alkali peak. Compared with the experimental error, the activation energy for this peak did not vary significantly with either the water or alumina content. Based on the measured activation energies and the shift of this peak to slightly higher temperatures (at constant measuring frequency), however, a small increase of I or 2 kcal/mol in the activation energy is considered likely over the range of water contents investigated. Fig. 3 shows how the magnitude of both peaks varied with alu,-~:na content for 'as melted' glasses and those bubbled with dry oxygen. These curves reflect the combined effects of the alumina and water content since for a given melting condition, e.g., bubbled with dry oxygen, the water content did not remain constant. The decrease in magnitude of the alkali peak with increasing alumina content is considered reasonably reliable since this peak was less sensitive to the water content and a decrease in magnitude with decreasing sodium oxide content is consistent with this peak being due to the movement of the sodium ions. The curves for the second peak are
D.E. Day, J.M. Stevels, Internal friction of glass
171
considered less reliable in showing the change in magnitude with alumina content since this peak was considerably more sensitive to the water content. When the magnitude of the second peak is compared with the weight loss, i.e., water content, the alumina content does not appear to be especially important. In fig. 4 the magnitude of the second peak is seen to increase smoothly with increasing water content, with the points for the alum~nophosphate glasses being independent of the alumina content. The numbers at each point correspond to the values of X in table 1. For water contents below 0.10 wt% the magnitude of the second peak varied almost linearly with the alumina content. While the concentration of alumina in the aluminophosphate glasses seems to have a minor effect upon the magnitude of the second peak, it should be noted that a given water content results in a larger second peak in the aluminophosphate glasses than in the NaPe 3 glass. The activation energies for the second peak in table 1 do not seem to vary in any definite or consistent manner with alumina content. For any given composition, the second peak always moved to lower temperatures with increasing water content, fig. 2, indicating a reduction in activation energy. In the glasses X = 0 and X = 0.I0, this reduction was experimentally verified, however, a decrease was not verified in the X = 0.33 glass, and the measured activation energies were the same within experimental error. The lack of any consistent trend in the activation energy is attributed to the combined effect of differences in the water and alumina content since the activation energy depends upon both compositional factors, and to some extent, to the experimental error in the measurements.
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0.40
Fig. 4. Height, above background, of second internal friction peak in ( l - X ) I Ia2PiO6"XAl203 glasses of varying water content. Data for NaPe3 glasses from ref. [12].
172
D.E. Day, J.M. Stevels, Internal friction of glass
4. Discussion
4.1. Aluminophosphate glasses The two internal friction peaks in the aluminophosphate glasses showed the same general dependency upon water content which has been reported previously for alkali silicate glasses [3, 41. Compared to silicate glasses, the two peaks in the aluminophosphate glasses varied considerably more in magnitude, thus allowing the dependency of the internal friction on water content to be seen more clearly. This is particularly true for the second peak, which showed pronounced changes with water content, but was relatively independent of the overall composition. While additions of alumina affected the magnitude of the second peak to some degree, these changes were minor :ompared to those resulting from a change in water content of only 0.10 wt%. For example, in glasses of nearly equal water content., the magnitude of this peak in the glass of highest alumina content X = 0.33 (weight loss of 0.036 wt%) was almost the same as that in the vacuum melted NaPO 3 glass (weight loss of 0.016 wt%). The clear dependence of the second peak on water content is taken as evidence that protons are important to the mechanism for this peak. This importance is shown by the curves in fig. 4 which indicate that the peak would be quite small, if non totally absent, in water.free phosphate glasses. Furthermore, the close correlation between the magnitude of the second peak and the water content, rather than the alumina content, can be accounted for by a mechanism dependent upon the proton concentration. The second peak in the aluminophosphate glasses containing water, therefore, is attributed to a mechanism based on the cooperative movement of a sodium ion and proton. With increasing water content, a larger percentage of the sodium ions have nearby protons and the magnitude of the second peak would be expected to increase. Another reason for suggesting that the mechanism consists of the cooperative movement of both ions, rather than just the protons, is the close similarity in the change in the internal friction observed in the present study with increasing water content and that reported for tile two internal friction peaks in single alkali silicate [14] and phosphate glasses [15] with the addition of a second alkali. In mixed alkali glasses, the alkali peak becomes smaller as a second alkali is added and a new larger peak appears whose magnitude is especially sensitive to the first small amounts of the second alkali. A cooperative movement of both alkali ions has been proposed a., the mechanism for this new peak in mixed alkali glasses and a similar mechanism is considered possible involving an alkali ion and a nearby proton in glasses containing water. The aluminophosphate glasses provide little evidence supporting a mechanism based on the movement of the non-bridging oxygen ions as has been proposed for the second peak in alkali silicate glasses [10, 11]. Depending upon how the aluminum ions are present in the glass structure, a significant change in the concentration of non-bridging oxygen ions would be expected as X varied from 0 to 0.33, but this
D.E. Day, J.M. Stevels, lnternal friction of glass
173
change in alumina content affected the magnitude of the second peak only slightly. If the aluminum ions are assumed to be present as AIO4 tetrahedra, then a 50% decrease in the concentration of non-bridging oxygens would be expected as the O/(P + AI) ratio changed from 3 in the NaPO 3 glass to 2.5 when X = 0.33. Conversely, the aluminum ions could be assumed to introduce more, rather than less non-bridging oxygen ions, but this is not supported by the increasing rigidity of the network as indicated by the temperature where the background internal friction equals 10 -2, table 1. If the effect of the differences in water content on the shape of the curves in fig. 3 are totally ignored, then the change in magnitude for the second peak with increasing alumina content can only be accounted for by assuming some complex structural role for the aluminum ions. In this case the magnitude of this peak would not be expected to be essentially independent of the alumina content as shown in fig. 4. To a limited extent the concentration of non-bridging oxygen ions in a phosphate glass will be determined by the water content, assuming the chemical dissolution of water proceeds according to the equation O
I
O
I
-O-P-O-P-O-
I
O
I
O
I
O
1
+ H20 ~ - - O - P - O H + HO-P-O.
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I
O
(l)
I
O
The change in concentration of non-bridging oxygen ions. i.e., hydroxyl ions, resulting frt~m 0.10 wt% water in a given glass, however, would be negligible compared to the concentration originally present in a NaPO 3 glass. It is considered unlikely that the sn~all change in the concentration of non-bridging oxygen ions can account for the large changes in tile size of tl:e second peak. 4.2. Alkali silicate glasses
There has been considerable discussion about the mechanism for the second peak observed in alkali silicate glasses, but there is evidence that this peak depends upon the water content in a manner similar to that for the second ~ a k in alkali phosphate and atuminophosphate glasses. Data by Maklad and KreidL [4] show that this peak becomes larger when a 18 Na20.82SiO 2 glass, mol%, is bubbled with steam, Although no experimental data for glasses of varying water content were published, Coenen [5] also reported that this peak depended upon the water content and he proposed a mechanism involving alkai ions and protons in hydrogen bridging posi-, tions. In view of the indicated similarity between the effect of water on the second peak in the alkali phosphate, aluminophosphate and silicate glasses, the data for alkali silicate glasses have been reviewed to determine whether the behavior of this peak could be accounted for by differences in water content. One of the characteristic features of the second peak in alkali silicate glasses is that it becomes larger with increasing alkali content. This is illustrated for sodium
D.E. Day, J.M. Stevels, Internal [riction of gbss
174
0 ref.[16] 0.70
r.f.[9]
0.60
• ref. [ 7 ] ° ,ef.[lS]
£.
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I
4.> L-
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O ,tl,,. 40 U ,,is=,
ref. [17]
5.0
x ref. [11]
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t~ ci)
. ,el.[19]
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7.0
x
4.0 40 Qr-
0.30
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!
O :g !
Cr 0 U
2.0
0.20
q0
0.I()
o
•
1.0
fo
40
O h
o
10
20 mole percent, Na20
30
40
Fig. 5. Height, above background, of second internal friction peak and water content [ 16] for sodium silicate glasses. Water content determined for glasses bubbled with water vapor at 1480°C a.d higher. Upper line shows water content.
silicate glasses in fig. 5 which summarizes the results of numerous investigations. A portion of the scatter in the internal friction data is due to the difficulty in estimating the background in those studies where the height above background was not given. While the peak magnitude has generally been related to the change in alkali content, :it is seen from fig. 5 that the change in peak magnitude closely follows the change in water content as reported by Franz and Scholze [ 16]. Whether this difference in water content can account completely for the change in peak magnitude is not known, but it must be considered a contributing factor. Differences in water content, res~dting from melting under ambient conditions, may also partially explain why different peak magnitudes have been reported for the same glass by different investiga. tors. The differences in the magnitude of the second peak for silicate glasses containing different alkali ions, but having the same total alkali content, also correlate reason. ably well with the water content. This is illustrated in table 2 for 25 tool% alkali silicate glasses where the peak magnitude is seen to increase from Li to Rb. The water content also increases in the same order for melts bubbled with water vapor. Assuming that glasses melted in air, while containing less water, would contain approximately the same relative amounts of wator as those bubbled with steam, then a change in peak magnitude is again observed which is consistent with and nearly proportional to the difference in water content.
D.E. Day, J.M. Stevels, Internal friction of glass
175
Table 2 Height of second internal frictit, t peak and water content for silicate glasses containing 25 mole percent alkali oxide. Alkafi
Peak height above background a) Q-I X 103
Mol percent water b) at 1480°C
Li Na K Rb
1.6 3.4 4.5 4.5
0.398 (27.5) 0.513 (25.6) 0.804 (26.7)
a) Ref. [ 17]; peak heights measured with torsion pendulum. b) Ref. [ 16]; numbers in parentheses are analyzed alkali content.
Previously, the reduction in magnitude for the second peak when alkaline earth oxides or alumina are added to an alkali silicate glass has been attributed to a change in the concentration of the non-bridging oxygen ions [ I 0, 11 ]. However, the water content also generally decreases with increasing additions of either alkaline earth oxides or alumina to an alkali silicate glass [6]. The lower proton concentration result ing from the compositional changes is an alternative explanation for the reduction ia peak magnitude. Wherever comparisons have been made the magnitude of the second peak in alkali silicate glasses is found to vary consistently with the water content, in certain instances, the proton exchanged glasses [71 and glasses conlaining equal amounts o|" different alkalis, a mechanism based on the active participation of the protons is considered a more satisfactory explanation than a mechanism based on the movenient or" the non-bridging oxygen ions. Even though the protons are associated with what can be considered non.bridging oxygen ions, the oxygen ion is considered of secondary importance to the mechanism, in agreement with Coenen's previous interpretation [5], alkali ions with neighboring protons are considered the principal structural condition required for the relaxation mechanism. It is concluded, therefore, that a mechanism consisting of the cooperative movement of an alkali ion and neighboring proton provides a satisfactory explanation for the behavior of the second peak observed in alkali silicate glasses. In this regard the second peak in single alkali silicate glasses is considered similar to the large peak in mixed alkali glasses which has been attributed to a cooperative movement of dissimilar alkali ions, except that in the single alkali glasses the mechanism involves an alkali ion and proton.
5. Conclusions Generally, the internal friction of alkali containing aluminophosphate and silicate glasses should be expected to depend upon the water content. It is important, there-
176
D.E. Day, ZM. Stevels, Internal friction of glass
fore, to control the water content when the effects of other compositional changes on the internal friction are being svadied. The pronounced sensitivity of the second peak in phosphate and aluminophosphate glasses to the water content is taken as evidence that protons are necessary for this peak to be observed. Consequently, the mechanism for this peak in the aluminophosphate glasses of varying water content is attributed to a cooperative motion of the alkali ions and nearby protons. The non-bridging oxygen ion, with which the proton is associated, is considered a passive feature of the mechanism. This same mechanism is also considered applicable to the second peak observed in alkali silicate glasses. This opinion is based on the essentially identical dependence of the second peak in both the aluminophosphate and silicate glasses upon water con-. tent and the correlation of the second peak in various alkali silicate glasses with their estimated water content. By considering the second peak in alkali glasses to be a type of mixed peak involving both alkali ions and protons, all of the internal friction peaks previously reported for alkali glasses can be attributed to one general mechanism associated with alkali ion movement; the activation energy and relaxation strength of which depends on the near neighbor environment of the alkali ions. With this interpretation, the alkali peaks are associated with the movement of those alkali ions surrounded by alkali ions of the same type. Similarly, the second peak in alkali glasses melted in air and containing water, or the mixed peaks in mixed alkali glasses are associated with the structural condition wl~ere an alkali ion has a dissimilar ion as a neighbor; i.e., a proton or different alkali ion, respectively. Viewed in this way, the various internal friction peaks observed in glass are not very different.
References [1] J.T. Krause, J. Appl. Physics 42 (1971) 3035. [2l C.R, Kurkjian and LT. Krause, J. Amer. Ceram. Soc. 49 (1966) 171. [3] R. Briickner, Glastechn. Ber. 37 (1964) 536.
[41 M.S. Maklad and N.J. Kreidl, Sci. and Tech. Comm. of the 9th Intl. Congress on Glass, vol. I, (Inst. du Verre, Paris, 1971) p. 75.
[51 M. Coenen, Zeit. ff/r Electrochemie 65 (196 I) 903. [61 R,H. Doremus, J. Non.Crystalline Solids 3 (1970) 369. 17] H. de Waal, J. Amer. Ceram. Soc. 52 (1969) 165.. 18] A.I.A. Abdel-Latif and D.E. Day, J. Amer. Ceram. Soc. 55 (1972) 254.
19] H. R~tger, Glastech. Ber. 31 (1958)54. [1o] R.J. Ryder and G.E. Rindone, J. ~mer. Ceram. Soc. 44 (1961) 532. [Ill D.E. Day and G.E. Rindone, J. Amer. Ceram. Soc. 45 (1962) 495. [121 D.E. Day and J.M. Stevels, J. Non-Crystalline Solids 11 (1973) 459. f13] R. Terai, J. Ceram. Assoc. Japan 72 (1964) 7. J.E. Shelby, Jr. and D.E. Day, J. Amer. Ceram. Soc. 33 (1970) 182. [151 H,M. Van Ass and J.M. Stevels, Technical University of Eindhoven, The Netherland, personal communication. [16] H. Franz and H. Scholze, Glastech. Ber. 36 (1963) 347.
D.E. Day, J.M. Stevels, Internal friction of glass [17] J.E. Shelby, Jr. and D.E. Day, J. Amer. Ceram. Soc. 52 (1969) 169. [18] R. Jagdt, Glastechn. Bet. 33 (1960) lO. [19] R.H. Redwine and M.B. Field, J. Mat. Sci. 4 (1969) 713.
177