Internal friction of NaPO3 glasses containing water

JOURNALOF NON-CRYSTALLINESOLIDS11 (1973) 459-470 © North-Holland Publishing Co.

INTERNAL FRICTION

OF NaPO 3 GLASSES

CONTAINING

WATER

DELBERT E. D A Y * and J. M. STEVELS Laboratory o f Inorganic Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands Received 10 October 1972 The internal friction of sodium metaphosphate glasses containing from 0.016 to 0.330 wt water has been investigated. Weight loss and infrared absorption measurements were used to determine the water content. Of the two internal friction peaks observed between -- 100°C and ~250°C, the second peak occurring above room temperature had a pronounced dependence upon the water content; increasing water content causing the activation energy to decrease as the peak increased in size. A mechanism consisting of the cooperative motion of sodium ions and protons has been proposed for this peak. It is concluded that the second peak in the NaPOa glasses and the similar peak in alkali silicate glasses is not associated with the movement of the non-bridging oxygen ions. I. Introduction

The internal friction versus t e m p e r a t u r e curves for s o d i u m p h o s p h a t e glasses 1) are similar to those for s o d i u m silicate glasses in t h a t two p e a k s are o b s e r v e d between - 1 0 0 ° C a n d the a n n e a l i n g range, at 1 Hz. There is c o n s i d e r a b l e evidence for the silicate glasses z-4) t h a t the low t e m p e r a t u r e p e a k , l o c a t e d between - 4 0 a n d 0 ° C at 1 Hz, is due to the m o v e m e n t o f the s o d i u m ions. Several m e c h a n i s m s have been suggested for the second, a n d generally smaller, p e a k l o c a t e d between ,-~ 150 a n d ~ 2 0 0 ° C at 1 Hz. The m e c h a n i s m s for the second p e a k in alkali silicate glasses have generally been r e l a t e d to the presence o f n o n - b r i d g i n g oxygen ions 5-7). Recently, this p e a k has been a t t r i b u t e d to the n o n - b r i d g i n g oxygen ions a s s o c i a t e d with an alkali ion a n d p r o t o n 8, 9) or to the m o v e m e n t o f h y d r o g e n ions1°). On the a s s u m p t i o n t h a t the second internal friction p e a k in s o d i u m silicate a n d p h o s p h a t e glasses is due to essentially the same m e c h a n i s m , an investigation o f vitreous s o d i u m m e t a p h o s p h a t e was u n d e r t a k e n . Since previous investigations on silicate glasses have e m p h a s i z e d the i m p o r t a n c e o f the n o n - b r i d g i n g oxygen ions to the second internal friction peak, the m e t a * On sabbatical leave from the Ceramic Engineering Department, University of MissouriRolla, Rolla~ Missouri 65401, U.S.A. at the time of this work. 459

460

D.E. DAY AND J. M. STEVELS

phosphate glass has the advantage of containing a much higher concentration of non-bridging oxygen ions. X-ray diffraction analysis of vitreous sodium metaphosphate n,12) has shown that the structure consists of infinitely long chains of P04 tetrahedra which are essentially cross linked by the sodium ions. Each P04 group ideally contains two non-bridging oxygen ions. Preliminary measurements of the internal friction showed that the second internal friction peak in vitreous sodium metaphosphate had a particularly significant dependence on the time and temperature of melting. In view of the previously reported effects of dissolved water on the transformation temperaturela), microhardness14), and degree of cross linking 15) in phosphate glasses and the changes in the second internal friction peak of sodium silicate glasses of different water contentg), it was decided to determine the extent to which water dissolved in vitreous sodium metaphosphate affected the internal friction, especially the second internal friction peak.

2. Experimental procedure 2.1.

SAMPLE PREPARATION

The NaPO3 glass was prepared by melting reagent grade NaH2PO 4- H20 in a platinum crucible in an electric furnace at 900 to 1000°C. The conditions used to obtain samples of different water content, table 1, consisted of melting in a vacuum, bubbling the melt with dry oxygen, and bubbling the melt with oxygen which had been bubbled through a water bath at 100°C. The first five samples in table I were obtained from the same melt with the remaining four samples (bubbled with dry oxygen for eight hours, followed by exposure to air) being obtained from a second melt. After the melt had been stirred several times and was free of bubbles, fiber (0.5 mm. dia.) and bar (6 x 10 x 100 ram) specimens were obtained after holding in an air atmosphere for 1 hr at 900°C. The melt was then bubbled with oxygen which had been dried with Drierite and soda asbestos. Fiber and bar specimens were again obtained after bubbling with dry oxygen for ½ and 1½ hr. A portion of the melt was transferred to another platinum crucible for melting under vacuum. Water was then introduced into the remaining melt by bubbling for 2½ hr with oxygen saturated with water vapor. The pulling of fibers and casting of bars was performed in the laboratory atmosphere. The fibers were immediately placed in glass tubes containing a water-free oil. The bars were transferred to another furnace (air atmosphere), annealed for 30--45 min at 290°C, and slowly cooled to room temperature. The vacuum melted sample was heated at a rate such that the pressure did not exceed 50 x 10- ~ mm Hg. Visible gas evolution from the melt started at

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approximately 450 °C and continued for approximately one hour as the pressure gradually decreased to < 2 x 10 -5 m m Hg. As the temperature was increased to 750°C, the pressure remained steady at < 2 x 10 -5 m m Hg, although intermittent bubbling of the melt continued. The melt was crystallized by slow cooling and then reheated to 750 °C for 3 hr. No gas evolution was visible upon reheating and the final pressure was < I x 10 -5 m m Hg. In order to obtain fiber and bar specimens, it was necessary to remelt the vacuum melted sample in a dry oxygen atmosphere in an electric furnace for approximately 35 min. The fiber and bar specimens were exposed to the laboratory atmosphere for a few minutes during preparation. The second group of samples were prepared from a melt initially bubbled with dry oxygen for 8 hr. This melt was held at 1000°C while exposed to the laboratory atmosphere, stirred at periodic intervals, and fibers drawn at the times shown in table 1 (except 4 hr) 2.2. DETERMINATIONOF WATER CONTENT The water content was determined from weight loss measurements, in vacuum, and the O H infrared absorptions. A thermobalance*, sensitivity of 2 × 10 -4 g, was used to measure the weight loss occurring between 200°C and after the sample had been held at 700 °C for 3 hr at a pressure ranging from 1 × 10 -5 to 5 × 10 - s m m Hg. Experiments showed that no significant volatilization of the melt occurred below 800°C and that the samples achieved a practically constant weight after 3 hr at 700°C. The original sample weights were between 3 and 6 g. Duplicate measurements of the weight loss, table 1, were within about _+0.01~o. The intensity of the O H absorption at approximately 2910cm -~, as measured** on thin plates, was also used to determine the relative water content of the glasses. Three absorptions at 3400, 2910, and 2355 cm-1 were observed. The 2910 cm -1 absorption was used in the present investigation, since this absorption band has been used previously to determine the water content in phosphate glasses 16) and the absorption at 3400cm -1 was generally poorly resolved as a shoulder on the 2910cm -a absorption. Portions of the bar specimens were used for the infra-red measurements after grinding to thicknesses ranging from 0.9 to 0.1 ram. During grinding the samples were continually protected with dry oil to prevent attack of the specimens by atmospheric moisture. The error in the calculated extinction at 2910 cm -~, table 1, is based on a ___ 0.005 m m uncertainity in the thickness of the specimens. This error is obviously more significant for the speci* Massflow Vacuum and Gas Atmosphere Thermobalance, Model HF-H5, Stanton Instruments Ltd. ** Grubb Parsons, MK III Spectromaster.

INTERNAL FRICTION OF

NaPOa

GLASSES CONTAINING WATER

463

mens of high water content, i.e., smaller thickness. The absorption of each specimen was measured in triplicate. 2.3.

INTERNAL FRICTION MEASUREMENTS

All measurements were made using two different techniques, on specimens annealed at 280-290°C for 30 rain. The fiber specimens were annealed and measured in a torsion p e n d u l u m , f = 1 to 2 Hz, pressure < 5 × 10 -3 mm Hg. This apparatus has been described previouslyXT). The internal friction of the bar specimens vibrating in flexure was measured* in air using a resonance technique 18) at frequencies ranging from 2 to 3 kHz. The activation energy, Ea, for the internal friction peaks was calculated from the temperature shift of the peak maximum at different frequencies using the equation R(Infl - In f2) Ea ~

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(1)

where f l and f2 = frequency at peak maximum (Hz), T I and T 2 = temperature (°K) of peak maximum, E a = apparent activation energy, kcal/mole. 3. Results and discussion

The weight loss and extinction at 2910 c m - 1 for the sodium metaphosphate glass prepared under different conditions are given in table 1. Both methods of determining the relative water content were in good general agreement, fig 1. After bubbling with dry oxygen for approximately 1 hr at 1000°C, the water content reached a relatively constant value that was probably determined by the partial pressure of water vapor in the furnace atmosphere. With the melting conditions employed, the water content varied by a factor of about 15. The amount of dissolved oxygen is apparently insignificant compared to that of water, since the samples bubbled with oxygen for prolonged periods showed no additional weight loss which could be attributed to dissolved oxygen. The diffusion of water vapor from the atmosphere into an initially " d r y " melt is apparent from the weight loss and extinction data, table 1, for the samples bubbled with dry oxygen for 8 hr and then exposed to air for a maximum of 95 hr. These samples provided an independent check of the effect of dissolved water on the internal friction, since they all received the same drying treatment. * Elastomat, Type 1.015, Institut Dr. F6rster, Reutlingen, Germany.

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Fig. 2 shows that the internal friction for vitreous sodium metaphosphate varies significantly for the melting conditions employed in the present investigation. The most pronounced changes are associated with the second peak in the region of 100°C, and by comparison, the peak at - 1 0 ° C is affected only slightly. With increasing concentration of dissolved water, the second peak became larger while the activation energy became smaller, table 1. The larger error in the activation energy for the second peak of the vacuum melted sample is due to the small size of this peak and the fact that it is partially obscured by the background in the high frequency measurements. The activation energy for the low temperature peak remained essentially constant, but there is evidence of a small reduction in magnitude with increasing water content. The change in the background internal friction at higher temperatures, fig. 2, is also consistent with the water contents as determined from the weight loss and infra-red measurements. Reducing the water content from approximately 0.330 to 0.016~ caused the background to shift from 226 to 272°C for an arbitrarily selected value of Q - 1 = 10-2, table 1. Since the background is usually associated with the characteristics of the glass network, a reduction in the water content which would reduce the number of

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TEMPERATURE °C Fig. 2. Internal friction of NaPOa glasses containing water. Portions of the low temperature alkali peak have been omitted for clarity since the differences are small, see table 1. A=bubbled with wet oxygen for 2athr at 1000°C; B = a s melted, 1 hr at 900°C; C-bubbled with dry oxygen for 1½hr at 1000°C; D ~ vacuum melted for 3 hr at 750°C. breaks in the P O 4 chains would be expected to cause the background to shift to higher temperatures. The low temperature peak in figure 2 is believed to be due to the movement of the sodium ions, as in silicate 2-5) and borate 17) glasses, and has been designated an alkali peak in table 1. This assignment is based on limited data, but the dependence of this peak on sodium content 1) is similar to that for the silicate and borate glasses. Also, the activation energy of 17-18 kcal/mote is in adequate agreement with the ,-~20 kcal/mole reported 19) for sodium diffusion in vitreous NaPO3. Of more interest in the present study, was the behavior of the second peak. The changes in the internal friction shown in fig 2 are believed due to the difference in the water content of the samples rather than to some other factor. Chemical analysis (flame photometry) showed no systematic variations in the sodium content for the different melting conditions: the analyzed NazO content ranged from 49.9 m o l e ~ for the sample bubbled with wet oxygen to 48.6 m o l e ~ for the sample bubbled with dry oxygen for 1½ hr. Furthermore, within experimental error, the height of the second peak varied linearly with the weight loss (or water content), fig. 3, except at the low concentration of dissolved water.

466

D . E . D A Y A N D J. M. STEVELS

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Several attempts were made to prepare samples with a water content lower than that of the vacuum melted sample, since it was desired to determine whether the second peak would be absent in a completely water-free glass. However, the second internal friction peak was always observed with the torsion pendulum measurements. One reason for this is that ~t was impossible to prevent exposing the fiber samples to the atmosphere (for only a few minutes) while pulling the fibers and mounting them in the torsion pendulum. Water vapor may have diffused from the laboratory atmosphere into the outer surface of the fibers in sufficient quantities to account for the small peak that was observed. Ion exchange experiments 2o, ~t) have shown that a low concentration of protons on the outer surface of a fiber specimen can produce large and easily detectable changes in the internal friction of a fiber specimen in the temperature region where the second peak is located. Nevertheless, it is felt that the second peak would be absent in an absolutely " d r y " sodium metaphosphate glass. Based on the indication that the second peak would be absent in a waterfree glass, it is concluded that this peak is not solely dependent upon the non-bridging oxygen ions. If the mechanism for this peak directly involved

INTERNAL FRICTION OF

NaPOa GLASSES CONTAINING WATER

467

the non-bridging oxygen ions it is believed that the peak would reach some low limiting value as the water content is reduced. Compared to the total concentration of non-bridging oxygen ions in the glass, two per P04 tetrahedron, the changes in the height of the second peak are considered too large to be accounted for by the relatively small changes in concentration of non-bridging oxygen ions resulting from the change in water content. The mechanism for this peak is more sensitive to the water content than to the non-bridging oxygen ion concentration. In several respects the effect of dissolved water on the internal friction of vitreous sodium metaphosphate is very similar to that observed in alkali silicate glasses. Previous studies show that introducing protons into a glass, either by bubbling the melt with water vapor 9) or with ion exchange techniques zl), causes a decrease in the magnitude and an increase in the activation energy (shift to higher temperatures) of the alkali peak and an increase in magnitude and a decrease in activation energy for the second peak. Even when alkali silicate glasses are bubbled with a dry gas to reduce the water content, a second peak is always observed. In silicate glasses it is difficult to conclude whether the increase in size of the second peak, brought about by a higher water content, is due to the growth of the original peak or to the appearance of a new peak in the same temparature region. A peak at 220°C, f = 0.65 Hz., has been reported 21) when Li ions are ion exchanged by protons in a Li20.A1203-2SIO2 glass. The mechanism suggested for this new peak is the cooperative movement of Li ÷ and H ÷. The relative importance of the protons, as contrasted to the non-bridging oxygen ion concentration, is indicated by the similar changes in the internal friction when protons are introduced in either a Li20'AI203"2SIO2 glass, which ideally contains no non-bridging oxygen ions, or vitreous sodium metaphosphate which contains a considerably higher concentration of nonbridging oxygen ions. In both glasses a peak is observed when the proton concentration exceeds some currently unknown value; and which is absent at lower proton concentrations; its complete absence in vitreous NaPO 3 not being totally established at this point. Increasing the water content of vitreous sodium metaphosphate also produces the same general changes in the internal friction that are observed when a second alkali is added to silicate ~) or aluminosilicate z3) glasses; namely, the appearance of a new peak, the activation energy of which decreases while the magnitude increases with increasing second alkali additions. It is concluded, therefore, that the mechanism for the second peak in vitreous sodium metaphosphate involves the cooperative movement of Na + and H + in a manner similar to that for the cooperative movement of two alkalis in mixed alkali glasses. Thus, the second peak in vitreous

468

D.E. DAY AND J. M. STEVELS

sodium metaphosphate is considered a "mixed alkali" peak similar to those observed in glasses containing two types of alkali ions. In this case the proton is considered to behave as an alkali ion. The interpretation that the second peak in vitreous sodium metaphosphate is a type of mixed alkali peak involving Na ÷ and H ÷ is in some respects similar to the previous suggestion 10) that the second peak in silicate glasses is due to the movement of hydrogen ions. Rather than attributing this peak to the movement of hydrogen ions as a distinct species, it is preferred to attribute this peak to a cooperative movement of both Na + and H +. This preference is based on two factors. First, the interaction between two different alkali ions produces strong internal friction effects ~) and it is believed that similar interactions are as likely between Na ÷ and H ÷ as between Na ÷ and some other alkali ion. Secondly, the pronounced dependence of the magnitude of the second peak in vitreous NaPO 3 on the proton concentration is quite like the dependence of the mixed alkali peaks on the concentration of second alkali, i.e., a few hundreths of a tool percent of second alkali are sufficient to produce a mixed alkali peak as large as the second peak in the "wet" sodium metaphosphate glass. If the hydrogen ions in a glass simply moved in a manner similar to that of the alkali ions, it is difficult to understand why a low concentration of hydrogen ions, such as that in the sodium metaphosphate glasses, could cause a peak nearly as large as the alkali peak which is due to a considerably larger concentration of sodium ions. Although a motion of hydrogen ions independent of the sodium ions cannot be completely excluded at this time, it is concluded that the second peak in vitreous sodium metaphosphate involves the stress induced movement of the hydrogen ions coupled with the sodium ions rather than the motion of the non-bridging oxygen ions. On the basis of the present investigation it is suggested that the second internal friction peak in alkali silicate and other glasses, which has been called the non-bridging oxygen ion peak, is most likely due to the cooperative movement of the alkali and hydrogen ions and is a type of mixed alkali peak similar to the large internal friction peaks observed in mixed alkali glasses. Its small size in normally melted glasses is considered to result from the low concentration of protons. In general, the magnitude of this peak in alkali silicate glasses is in reasonable agreement with what would be expected from the water content. In silicate glasses containing 25 mole~ alkali the peak magnitude varies from 1.6 to 3.4 to 4.5 × 10 - 3 for Li, Na, and K, respectivelyZ4). Similarly, the proton concentration for 20 mole~o alkali glasses 16) bubbled with water vapor under identical conditions is stated to be 3.5, 12.1, and 16.0 × 1019 protons/cm a for Li, Na, and K silicate glasses, respectively. It is suggested that the preparation of sufficiently

INTERNAL FRICTION OF

NaPOa

GLASSES CONTAINING WATER

469

" d r y " alkali silicate glasses will show that the second internal friction peak is absent in these glasses.

4. Summary and conclusions Sodium metaphosphate glasses containing dissolved water exhibit two internal friction peaks. The peak below room temperature at 1 Hz is attributed to the stress induced motion of the sodium ions. The second peak above room temperature had a pronounced dependence upon the water content and with increasing water content this peak became larger as the activation energy became smaller. It is concluded that the mechanism for the second peak is a cooperative movement of sodium ions and protons, and that the mechanism is similar to that proposed for the mixed alkali peaks observed in mixed alkali glasses. The mechanism is not considered to depend on the non-bridging oxygen ion concentration and it is proposed that this peak would be absent in completely " d r y " sodium rnetaphosphate glasses. From the limited data available for the effect of water on the internal friction of alkali silicate glasses, it is suggested that the internal friction peak in these glasses, previously ascribed to the non-bridging oxygen ions, is also due to a cooperative movement of the alkali ions and protons. It is believed that this peak would be absent in glasses containing non-bridging oxygen ions, but free of water.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 12) 14) 15)

R. L. Myerson, Mass. Inst. of Technology, M. S. Thesis, June 1961. R. Jadgt, Glastechn. Ber. 33 (1960) 10. K. E. Forty, J. Am. Ceram. Soc. 40 (1957) 90. G. L. McVay and D. E. Day, J. Am. Ceram. Soc. 53 (1970) 284. H. R6tger, Glastechn. Bet. 31 (1958) 54. R. J. Ryder and G. E. Rindone, J. Am. Ceram. Soc. 44 (1961) 532. D. E. Day and G. E. Rindone, J. Am. Ceram. Soc. 45 (1962) 495. M. Coenen, in: Physics of Non-Crystalline Solid~, Delft, 1964, Ed. J. A. Prins (NorthHolland, Amsterdam, 1965) pp. 444-460. M. S. Maklad and N. J. Kreidl, in: Sci. and Techn. Commun. of the 9th Intern. Congr. on Glass, Vol. I (Inst. du Verre, Paris, 1971). R. H. Doremus, J. Non-Crystalline Solids 3 (1970) 369. G. W. Brady, J. Chem. Phys. 28 (1958) 48. M. E. Milberg and M. C. Daly, J. Chem. Phys. 39 (1963) 2966. N. H. Ray and C. J. Lewis, J. Mater. Sci. 1 (1972) 47. V. V. Gerasimov, L. I. Kuznetsov-Fetesov, E.V. Kusnetzov and T. B. Shakhmina, Polymer Structure of Phosphate Glasses, USSR Inorgan. Mater. Consultants Bureau Transl. 5 (1969) 903. A. Eisenberg and T. Sasada, in: Physics of Non-Crystalline Solids, Delft, 1964, Ed. J. A. Prins (North-Holland, Amsterdam, 1965) pp. 99-114.

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D . E . DAY AND J. M. STEVELS

16) W. Muller-Warmuth ,G. W. Schultz, N. Neuroth, F. Meyer and E. Deeg, Z, Natur. forsch. 20a (1965) 902. 17) H. de Waal, Thesis, Delft, The Netherlands (1967), 18) R. Forster, Z. Metallk, 29 (1937) 109. 19) R. Terai, J. Ceram. Assoc. Japan 72 0964) 817. 20) H. de Waal, J. Am. Ceram. Soc. 52 (1968) 165. 21) A. I. A. Abdel-Latif and D. E. Day, J. Am. Ceram. Soc. 55 (1972) 254. 22) J. E. Shelby Jr. and D, E. Day, J. Am. Ceram, Soc. 53 (1970) 182. 23) D. E. Day and W. E. Steinkamp, J. Am. Ceram. Soc. 52 (1969) 571. 24) J. E. Shelby Jr. and D. E. Day, J. Am, Ceramic. Soc. 52 (1969) 169.