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Diffusion of water in vitreous silica
~,URNAL
ELSEVIER
OF
Journal of Non-Crystalline Solids 179 (199~,) 185-193
D i f f u s i o n of w a t e r in vitreous silica " P.B. McGinnis *, J.E. Shelby Glass Science Laboratory, NYS College of Ceramics, Alfred University, Alfred, NY 14802, USA
Abstract Hydroxyl has been removed from three different commercial vitreous silicas at temperatures between 500 and 1100°C. The results indicate a change in activation energy with temperature. This change may be due to a change in the dehydroxylation mechanism with temperature or due to a change in the fictive temperature during treatment. The entry of water into five commercial silica glasses has been studied between 800 and 1000°C. A change in the solubility as a function of time at 1000°C is evident. A comparison of the results for the entry and removal of water is made.
I. Introduction A n u m b e r of papers [1-9] have discussed the diffusion of water in vitreous silica below the softening temperature. T o d d [1] was the first to determine diffusion coefficients from vacuum heat t r e a t m e n t s of glass below the transition range. Evolved gases were c o n d e n s e d in a liquid nitrogen trap and volumes were m e a s u r e d as a function of time. Results were r e p o r t e d for a n u m b e r of commercial glasses including two Vycor ® #1 type pseudo-vitreous silicas. Both Vycor ® glasses were 96% silica and had a water concentration gradient, but the activation energies found for dehydroxylation were significantly different. O n e glass was a vacuum-fired product and had an activation energy for dehydroxylation of 57
~ This work was supported by ~. grant from the Center for Glass Research at the New York State College of Ceramics at Alfred University, Alfred, NY 14802, USA. * Corresponding author. Tel: + 1-607 871 2120. Telefax: + 1-607 871 3469.
k c a l / m o l . The other glass was not vacuum-fired and had an activation energy of 75 k c a l / m o l . Moulson and Roberts [2,3] determined the diffusion coefficients and activation energies for both the entry and removal of hydroxyl from vitreous silica. Infrared absorption spectra were measured as a function of t r e a t m e n t time for samples heated in an alumina tube placed in a furnace. The tube could be evacuated in order to remove water, or opened to a constant water vapor partial pressure for the entry of water. The diffusion coefficients for the entry of water were approximately 3.5 times greater than those reported for the removal of water. The activation energies for diffusion into and out of vitreous silica were equal within the limits of experimental error. Different grades of vitreous silica were used in the same experiment. Others [4,5] have shown that the out-diffusion of water in vitreous silica is d e p e n d e n t on the method of production.
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186
P.B. McGinnis, Z E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
The diffusion of water in silica glass has also been investigated through the use of tritiated water vapor [6-9]. Tritium (3H) is a radioactive isotope of hydrogen that emits 13-particles upon decay. The 13-decay energy is estimated to be approximately 18.6 keV [10] with a range of only about 1.3 ixm in silica glass [8]. Thus, measurement of 13-emission as a function of distance into the sample reveals a diffusion profile. Roberts and Roberts [6] studied the effect of fictive temperature on both the solubility and diffusion coefficient of tritiated water vapor into vitreous silica. Isothermal in vacuo heat treatments were used to change the fictive temperature of the silica samples. Samples which had been treated to obtain a fictive temperature of l l00°C and as-received samples (fictive temperature = 1200°C) were treated together at temperatures between 700 and 1200°C. The diffusion coefficient of water into heat-treated samples was greater than that of as-received samples in most instances. No explanation was given for the observed behavior. In a similar experiment, samples with fictive temperatures between 1000 and 1300°C were treated in tritiated water vapor at 750°C. The log of the solubility was a linear function of reciprocal absolute fictive temperature. The solubility increases with fictive temperature. This observation was explained as due to an increase in thermally produced defects, such as elongated or broken oxygen bonds, with increasing fictive temperature. The increased defect concentration provides a higher concentration of favorable sites for hydroxyl formation. Roberts and Roberts [7] also treated vitreous silica samples at 1000 and l l00°C for different times in v a c u o until further treatment resulted in no additional structural relaxation (the fictive temperature equaled the treatment temperature). Subsequent treatments in tritiated water vapor allowed measurement of the solubility as a function of vacuum treatment time. The solubility decreased as the treatment time increased. Thus as the fictive temperature of the as-received sampie decreased, the solubility decreased. The decrease in solubility with time follows a bimolecular reaction law of kinetics. Bimolecular kinetics are obeyed when defects composed of two parts
are annihilated when they react. The decrease in solubility was attributed to a reduction in defect concentration which they proposed to be broken or elongated silicon-oxygen bonds. Tracer work using tritiated water was used to determine the diffusion coefficient of 'water' into silica glass as a function of the tritiated water concentration in the glass [8]. Plots of the diffusion coefficient as a function of inverse absolute temperature showed an obvious change in activation energy between 1000 and l l00°C. Activation energies were shown to be relatively constant with concentration, but the diffusion coefficients increased with concentration. Burn and Roberts [9] studied the effect of initial hydroxyl concentration on the diffusion ot ~ water into silica glass. A synthetic silica glass (Spectrosil ® #2) had a significantly greater hydroxyl concentration than a flame fused quartz crystal (Optical Grade Vitreosil®#2). Samples were isothermally heat-treated in vacuum in order to produce known fictivc temperatures. After heat treatment, samples were taken from the middle of blocks in the hope of minimizing any hydroxyl gradient formed during vacuum treatment. Results of tritiated water vapor treatments at 700°C showed that the diffusion penetration of water into silica glass decreases with initial hydroxyl concentration for samples with the same fictive temperature. As the fictive temperature is increased, the effect is reduced. The results indicated a significant decrease in activation energy as the treatment temperature is increased in the 900-1000°C range. The quantitative effect of 13-particles on the silica network is unknown, but the formation of defects is known to occur. The 18.6 keV 13-decay energy of tritium [10] has been mimicked by irradiating vitreous silica with 18 keV electrons [11]. Low energy electron radiation is known to cause ionization, but is incapable of causing atomic displacement. Tritiated water (a mixture of T20, HTO and H 2O) will decay to yield a 13-particle + ( O H ) - + 3 H e . The ionization damage caused by
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P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
the 13-emission creates favorable sites for hydroxyl or OT formation which is greater than if these sites were not available. Therefore the measured tritium penetration profile is artificially inflated because of the ionization created by the 13-decay. The effect is increased as the T / H ratio is increased. It follows that, while some uncharacterized effect of tritium on the results presented in previous tracer studies [6-9] must be present, that work is still valuable in understanding hydroxyl formation and water diffusion in vitreous silica. The present study investigated the diffusion of water (hydroxyl) into and out of several commercial silica glasses. The results verify previous reports of varying activation energies. They also show that diffusion coefficients differ among glasses and with direction of diffusion. These observations are discussed in terms of changes in mechanisms a n d / o r fictive temperatures.
2. Procedure
Bruckner [12] has categorized commercial silica glasses into four types by manufacturing method. Suprasil-I ® #3 and CGW-7940 ® #1 are synthetic vitreous silicas produced by hydrolyzation of SiCI 4 in an oxygen-hydrogen flame (typeIII) [12]. T-08 ® ~3 (type-ll) is produced by the fusion of quartz powder in an oxygen-hydrogen flame (Verneuille process). Suprasil-W ® #3 (typeIV), is a synthetic vitreous silica produced by hydrolyzation of SiCI 4 in a water-vapor-free plasma flame. Infrasil ® #3 (type-I) is produced by electrical fusion of natural quartz under vacuum or inert atmosphere. The as-received water concentrations + one standard deviation for five commercial silica glasses are given in Table 1. These error limits are typical for the data reported in this paper. The concentration given in terms of ppm of water (weight basis) were calculated using an extinction coefficient for H 2 0 equal to 181.0 l / m o l cm [13].
187
Table 1 Concentration of water in five commercial silica glasses Commercial silica
A b s / m m at 3660 c m - I
ppm (gH2o/gsio2 )
Suprasil-I ® CGW-7940 ® T-08 ® Suprasii-W ® Infrasil ®
1.182 _+0.030 0.814+_0.047 0.187+_0.031 undetectable undetectable
534 + 14 452+_ 17 85+_ 14 undetectable undetectable
Infrared spectra were measured for SuprasilI®, CGW-7940 ® and T-08 ® and peak heights of the free hydroxyl band were recorded. The facility used to remove hydroxyl from the samples has been described previously [14]. Samples were isothermally heated under vacuum and periodically removed from the furnace to measure the infrared spectra. The percent hydroxyl removed was calculated by subtracting the peak height for the treated sample from that of the untreated sample and dividing the result by the initial peak height. Samples werc then returned to the vacuum chamber for subsequent treatments. The facility used to diffuse water into vitreous silica is shown schematically in Fig. 1. The five Steam Out
Platinum Sample I
imace
Refractory Stage (Inside MuUiteTube) ry Plug
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Fig. 1. Schematic of the apparatus used for the entry of water (hydroxyl).
188
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
commercial silicas listed in Table 1 were isothermally treated in one atmosphere of water vapor. Samples were removed periodically and analyzed using infrared measurements. The percent hydroxyl added was calculated by subtracting the initial peak height from that of the treated sample and dividing by the solubility. The solubility was defined as the concentration level at which further treatment resulted in no additional increase in concentration.
0.6
0.4
0.2
3. Results
The removal of hydroxyl from vitreous silica clearly follows a square-root of time dependence as shown in Fig. 2. Although. a square-root of time dependence is often associated with a diffusion-controlled process, it does not have to be the case. The initial non-linearity in the CGW-7940 ® curve is proposed to be due to a metastable hydroxyl concentration that is easily and entirely removed within the first few minutes of treatment [13]. After the initial treatment, the CGW-7940 ® results obey a square-root of time dependence. The extrapolated concentration at 0 h treatment,
I 1
Eocow7940[
-- ~
[ [] Suprasil-I
0
~
0.8 Z 0 rfl ~ <
0.6
0.40
5'
, 10
15
Fig. 2. Hydroxyl concentration as a function of the square-root of the treatment time in vacuum at 950°C. The CGW-7940® vitreous silica shows linear behavior only after the initial treatment. The data are fitted to a linear function of (/'.
o
0
25 50 75 ~/L (~h-rlmm) Fig. 3. Dehydroxylation of Suprasil-I ® vitreous silica at four different heat treatment temperatures.
given as absorbance per millimeter in Fig. 2, is defined as the initial concentration rather than the value actually measured. The extrapolated value correlates to the level of water in the form of stable hydroxyl in the as-received sample. Therefore, the fraction of the total hydroxyl concentration removed from CGW-7940 ® does not include the unstable hydroxyl removed in the initial minutes of treatment. The fraction of total hydroxyl removed is plotted as a function of the square-root of treatment time divided by the thickness of the sample in Figs. 3-5 for Suprasil-I ®, CGW-7940 ® and T08 ®, respectively. The removal of water is linear with the square-root of the treatment time within the error limits of the data for all three glasses at all temperatures investigated. The initial slopes of these curves, R, are used in calculating the average diffusion coefficient for the removal of water. The determination of the diffusion coefficients from the initial slope is calculated using the following equations for adsorption and desorption from two parallel surfaces of a semi-infinite plate of thickness, L [15]: M t / M ~ = 4/.tr [/2 ( D t / L 2 ) !/2,
(I)
P.B. McGinnis, .I.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
r~ ©
0.6
!050 °C. . . . o~ ~3u t.:
189
It follows that the average diffusion coefficient is given by
750oC
0.5
D = ( rr/16)R 2.
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0.4
>' =
0.3
< [., O [..,
0.2
The five commercial silica glasses were treated in one atmosphere of water vapor at 800, 900 and 1000°C. Crystallization became a problem when treating in the wet atmosphere, whereas it was not a problem for similar treatments in vacuum. Heslin [16] has shown that surface crystallization of vitreous silica is significantly affected by the hydroxyl concentration as well as surface contamination. The initial samples were cleaned only with ethyl alcohol and rinsed thoroughly with distilled water. Upon subsequently treating the samples in the we~ atmosphere, crystallization occurred on some samples after different time periods. The only samples that resisted crystallization at all three treatment temperatures were CGW-7940 ® and Suprasi!.W ':~ In an attempt to limit surface crystallization, one batch of samples was cleaned in 36N chromic acid, which is known to dissolve organics extremely well. These samples were treated in one atmosphere partial pressure of water vapor at 1000°C for 154 h without any noticeable crystallization. The hydroxyl concentration, given as absorbance per millimeter of the free hydroxyl band, is plotted as a function of the square-root of treatment time in Fig. 6. Suprasil-W ':'~ results were very similar to Infrasil ~ results and have been omitted in the interest in clarity. The loss of metastable hydroxyl in CGW-7940 ® after an initial treatment is evident. The extrapolated concentration at 0 h treatment is once again defined as the initial concentration for the calculation of diffusion coefficients in CGW-7940 ':~. The hydroxyl concentration in Suprasil-I ® is reduced with treatment time in one atmosphere of water vapor because the solubility at one atmosphere is less than the initial concentration. Treatments of Suprasil-I ® at 800 and 900°C resulted in an increase of hydroxyl concentration, presumably due to an increase in solubility at lower temperatures. Structural relaxation causes a reduction in the solubility as shown in Fig. 6. This effect is most clearly shown by data for the T-08 '~ sample. The
© Z e E-U < LI.
0.1
,K)~
0.0
I
i
I
I
10
20
30
40
50
~/L ( ~ / ~ ) Fig. 4. Dehydroxylation of CGW-7940 ~ vitreous silica at four different heat treatment temperatures.
where Mt/M~ is the fraction formed/removed at time, t. The initial gradient, R, is defined as
R=d(M,/M~)/d(t/L2)l/2=(16D/rt)
~m
~/2. (2)
0.5 1100°C 0.4 /
lO00oc
900°C
0.3
0.2 ~> 800°C
O. 1
0
50
100
150
200
250
~ / L (~f-~-/mm) Fig. 5, Dehydroxylation of T-08 ~ vitreous silica at four different heat treatment temperatures.
(3)
P.B. McGinn~, ZE. Shelby/'Journal of Non-Crystalline Solids 179 (1994) 185-193
190
initial hydroxyl concentration increases linearly with the square-root of time and continues to increase until a solubility limit is reached. Subsequent treatments result in further structural relaxation and a reduction in the solubility. There is no statistical difference among the final solubilities for the various vitreous silicas (error limits are of the order of the size of the symbols). Diffusion coefficients were calculated using a solubility limit defined as the highest concentration formed during treatment. The CGW-7940 ® sample had a significantly greater diffusion coefficient for the entry of water than the other four samples, which were equal within acceptable limits of experimental error at 1000°C. The diffusion coefficients for the entry of water into CGW7940 ® are significantly greater than those for removal. A certain amount of error is inherent to the diffusion coefficient calculation for the entry of water since it is impossible to know the solubility as a function of treatment time and what effect a change in fictive temperature during treatment
1.2
0
A
<>
o
09
0
g
[]
0.6
O.3
[]
0
Suprasil-I
[]
Infrasil
0
CGW-7940
A T-08 0.0 0
I
I
4
8
12
(hoV~-~u~) Fig. 6. Hydroxylation at 1000°C of four different commercial silica glasses. Data for Suprasil-W ® have been omitted for clarity; the results are similar to those for Infrasii ®. CGW7940 ® shows linear behavior only after the initial treatment. The hydroxyl concentration of Suprasil-I ® decreases due to a solubility lower than that corresponding to one atmosphere of water vapor.
1.00
~
to
O
O
0.75
0.50
i
0.25
O
800°C
[]
900°C
O I
0.00 0
25
1000°C
I
I
50
75
100
~n.. (~/-~-/mm) Fig. 7. Hydroxylation of Suprasil-W ® vitreous silica at three different heat treatment temperatures. The data are fitted to functions of ¢[/L.
has on the diffusion coefficient. The amount of relaxation cannot be estimated since the initial fictive temperatures are not known for any of the five samples. Douglas and Isard [17] reported that a silica sample heated at 993°C requires 600 h treatment to attain equilibrium. Their results may only be used as a guide since they too did not know their initial fictive temperature and their hydroxyl concentration was probably much lower than in the present study (they only defined their glass as a 'commercially available clear fused quartz rod'). The greater hydroxyl concentration of the wetter vitreous silicas probably lowers the transition range. Since relaxation follows an exponential time dependence, the rate of structural rearrangement decreases with time, thus the greatest reduction in solubility occurs during the period where the initial slope, R, is measured for the diffusion coefficient calculation. Figs. 7 and 8 show the fractional increase in hydroxyl content at 800, 900 and 1000°C for Suprasil-W ® and CGW-7940 ®, respectively. The initial slopes are used in the calculation of diffusion coefficients for the entry of water into silica glass. The slopes of the 900 and 1000°C curves in Fig. 8 are identical within experimental error.
P.B. McGinnis, J.E. Shelby ~Journal of Non-Crystalline Solids 179 (1994) 185-193
ation curves and possibly in the hydroxyiatfon curves as well. The diffusion coefficients for the entry of water are zignificantly greater than those for the removal of water in CGW-7940 ®.
1.00
c~o o <> [] 0
0.75
191
4. Discussion 0.50
0.25
0 800°C [] 900°C O 1000°C
0.00
' 10
0
'. 20
'
30
40
~/itL (f~-tmm) Fig. 8. Hydroxylation of CGW-7940 ® vitreous silica at three different heat treatment temperatures.
Fig. 9 contains diffusion data for the entry and removal of water from silica glass. The curves for diffusion into Suprasil-W ® and CGW-7940 ® are marked with filled symbols. The removal curves are marked by unfilled symbols. It is easily seen that a change in slope occurs in the dehydroxyl-
-81
0
/
~
-9 I r~ Z @
~
/
Suprasil-I Removal
[] T-08 Removal ~
O
CGW-7940 Removal
",eS ~->.,X~ i
i=,,t
r.)
tm o
-13
"
A
• -14 0.7
,
Suprasil-W Entry
~
CGW-7940 Entry I t 0.8 0.9 1.0
I 1.1
t 1.2
1.3
1000fr (l/K) Fig. 9. Log effective diffusion coefficient for the removal and entry of hydroxyl in different commercial silica glasses. The lines are drawn as a guide for the eye.
The present study appears to be the first to directly investigate the kinetics of formation and removal of hydroxyl from different commercial vitreous silica glasses. Previous papers [2,3] have studied the entry and removal of water from vitreous silica by measuring the water content directly through infrared analysis. These studies, however, made no distinction between silica glass identity. Others [6-9] have studied the kinetics of formation and removal of tritiated water in various silica glasses. The 13.-particle emission used to produce a tracer penetration profile creates an inherent error in the measurement. The present study agrees with previous work [8,9] in that a change in activation energy occurs for the removal of hydroxyl between approximately 000 and 1000°C. Dtury aad R.oberts [8] report an activation energy of 10-20 kcal/mol between 1100 and 1300°C. Burn and Roberts [9] report an activation energy equal to 14.0 + 0.5 kcal/mol between 1000 and 1200°C and 25 _+ 1 kcal/mol between 600 and 900°C. The activation energies calculated in the present study for the removal of hydroxyl between 500°C and 900°C are equal to 38.7 kcal/mol and 34.6 kcal/mol for CGW-7940 ® and Suprasil-I ®, respectively. The activation energy for the removal of hydroxyl from T-08 ® is 68.4 kcal/mol between 800 and 900°C. The number of high-temperature measurements is insufficient to accurately determine an activation energy above 1000°C, but an approximate value for Suprasil-I ® is 14 kcal/mol. The change in activation energy between 900 and 1000°C shown in this study has been reported by others [8,9]. Burn and Roberts [9] concluded that a change in the diffusion mechanism was brought on by the reduction of the 'jump' distance to the nearest neighbor level at higher temperatures. They suggested that water diffusion occurs by the continual creation and destruc-
192
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
tion of reactive sites. At lower temperatures, the H ÷ and O H - ions must condense to form water molecules that subsequently diffuse to new reactive sites. The reactive site concentration increases with temperature. At higher temperatures, the need for condensation reactions is eliminated as the distance between reactive sites approaches the distance between nearest neighbors. The diffusion of water at higher temperatures occurs as associated proton and hydroxyl pairs. There is still much speculation on the mechanism of diffusion of water in glass. It is possible to explain the change in activation energy observed in this study through a change in the controlling process from a reaction-controlled process at lower temperatures to a diffusion-controlled process at higher temperatures. This behavior has been shown before by Van der Steen [18] for the removal of H 2 form SiO 2 by the following reaction: =SiOH + - S i l l ~ 2SiO 2 + H2(phys. diss)'
(4)
Van der Steen showed that below 800°C the hydroxyl concentration in Eq. (4) is controllcd by reaction kinetics. Above 1150°C, the removal of H 2 is a diffusion-controlled process where the H 2 concentration is a function of both the diffusion coefficient and the solubility. It is possible that similar arguments apply for water diffusion. Another possible explanation for the change in activation energy is based on structural relaxation. The change in activation energy for the removal of hydroxyl occurs approximately ever the same temperatures as the transition range. It is unclear whether this is simply a coincidence or whether the two phenomena are related. It must be remembered that an increase in the fictive temperature of vitreous silica in this temperature range, unlike that of other glasses, actually results in a decrease in density [12]. The expected effect would be a decrease in activation energy, as the openness of the structure increases, just as is found in the present and previous studies [8,9]. Further, the viscosity of vitreous silica approaches infinity below the transition range. Within the transition range, viscous flow becomes possible and may account for the decrease in activation energy as the structure becomes less rigid.
The differences in diffusion coefficients as well as activation energies between different commercial silica glasses may be due to differences in fictive temperatures, impurity levels or initial hydroxyl concentrations. Since the initial fictive temperatures and impurity levels are unknown, it is difficult to distinguish between possible causes for the differences in diffusion data. It is clear, however, that the manufacturing method affects hydroxyl diffusion. The entry of water into vitreous silica at 1000°C shows a change in solubility which is probably due to a change in fictive temperature. Diffusion coefficients for the entry of water at 1000°C were equal within the limits of experimental error for all samples except the CGW-7940 ®, which has a noticeably higher value. The diffusion coefficients for the entry of water into CGW-7940 ® between 800 and 1000°C were greater than corresponding values for the removal of water. Although this behavior has been shown previously [2], an adequate explanation was not presented. Further work is necessary to adequately compare data for the entry and removal of hydroxyl.
5. Summary and conclusions The change in activation energy for the removal of hydroxyl from three different commercial silica glasses found in the present study is consistent with others [8,9]. The change in activation energy is believed to be due to either structural relaxation, or to a change in the controlling mechanism. A change in solubility is apparent for samples treated at 1000°C in one atmosphere of water vapor. The decrease in solubility is due to a decrease in fictive temperature with heat treatment time. Structural relaxation causes a reduction in defect concentration, thus lowering the number of reactive sites and consequently the solubility. The magnitude of change is unknown. The diffusion coefficients for the entry of water at 1000°C were equal for all but one of the samples. The diffusion coefficients for the entry of water into CGW-7940 ® are greater than those for removal from 800 to 1000°C.
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
Additional work is underway in determining diffusion coefficients for the entry of water into the five commercial silica glasses at temperatures below 1000°C. Further study on the effect of fictive temperature will be needed to adequately explain the observed change in activation energy. Determination of diffusion profiles for samples treated at temperatures above, below and in the transition range may answer questions about a possible change in the controlling mechanism resuiting in the change in activation energy since totally reaction-controlled processes should have a constant concentration profile.
References [1] B.J. Todd, J. Appl. Phys. 26 (1955) 1238. [2] A.J. Moulson and J.P. Roberts, Trans. Faraday Soc. 57 (1960) 1208. [3] A.J. Moulson and J.P. Roberts, Trans. Br. Ceram. Soc. 59 (1960) 389. [4] P.B. McGinnis and J.E. Shelby, in: Proc. 16th Int. Congr. on Glass, Vol. 3 (Bol. Esp. Ceram. Vid., Madrid, 1992) p. 239.
~93
[5] H.D. Witzke, in: Proc. 2nd Int. Conf. on Advances in Fusion and Processing of Glass, Glastech. Ber. 63K (1990) 333. [6] G.J. Roberts and J.P. Roberts, Phys. Chem. Glasses 5 (1964) 26. [7] G.J. Roberts and J.P. Roberts, in: Proc. 7th Int. Congr. on Glass, Brussels (Federation de L'lndustrie du Verre, Brussels, 1965) p. 31. [8] T. Drury and J.P. Roberts, Phys. Chem. Glasses 4 (1963) 79. [9] I. Burn and J.P. Roberts, Phys. Chem. Glasses 11 (1970) 106. [10] F.T. Porter, Phys. Rev. 115 (1959) 450. [11] T.A. Deilin, D.A. Tichenor and E.H. Barsis, J. Appl. Phys. 48 (1977) 1131. [12] R. Bruckner, J. Non-Cryst. Solids 5 (1970) 123. [13] J.E. Shelby, J. Vitko Jr. and R.E. Brenner, J. Am. Ceram. Soc. 65 (1982) C-6~. [14] J.E. Shelby, in: Experimental Techniques of Glass Science, ed. C.J. Simmons an~_;O.H. EI-Bayoumi (American Ceramic Society, Westervill,:, OH, 1993) ch. 10. [15] J. Crank, The Mathematics of Diffusion, 2nd Ed. (Clarendon, Oxford, 1989). [16] M.R. Heslin, PhD Thesis, Alfred University (1993). [17] R.W. Douglas and J.O. Isard, J. Soc. Glass Technoi. 35 (1951) 206T. [18] G.H.A.M. Van der Steen, PhD thesis, University of Eindhoven (1976).
ELSEVIER
OF
Journal of Non-Crystalline Solids 179 (199~,) 185-193
D i f f u s i o n of w a t e r in vitreous silica " P.B. McGinnis *, J.E. Shelby Glass Science Laboratory, NYS College of Ceramics, Alfred University, Alfred, NY 14802, USA
Abstract Hydroxyl has been removed from three different commercial vitreous silicas at temperatures between 500 and 1100°C. The results indicate a change in activation energy with temperature. This change may be due to a change in the dehydroxylation mechanism with temperature or due to a change in the fictive temperature during treatment. The entry of water into five commercial silica glasses has been studied between 800 and 1000°C. A change in the solubility as a function of time at 1000°C is evident. A comparison of the results for the entry and removal of water is made.
I. Introduction A n u m b e r of papers [1-9] have discussed the diffusion of water in vitreous silica below the softening temperature. T o d d [1] was the first to determine diffusion coefficients from vacuum heat t r e a t m e n t s of glass below the transition range. Evolved gases were c o n d e n s e d in a liquid nitrogen trap and volumes were m e a s u r e d as a function of time. Results were r e p o r t e d for a n u m b e r of commercial glasses including two Vycor ® #1 type pseudo-vitreous silicas. Both Vycor ® glasses were 96% silica and had a water concentration gradient, but the activation energies found for dehydroxylation were significantly different. O n e glass was a vacuum-fired product and had an activation energy for dehydroxylation of 57
~ This work was supported by ~. grant from the Center for Glass Research at the New York State College of Ceramics at Alfred University, Alfred, NY 14802, USA. * Corresponding author. Tel: + 1-607 871 2120. Telefax: + 1-607 871 3469.
k c a l / m o l . The other glass was not vacuum-fired and had an activation energy of 75 k c a l / m o l . Moulson and Roberts [2,3] determined the diffusion coefficients and activation energies for both the entry and removal of hydroxyl from vitreous silica. Infrared absorption spectra were measured as a function of t r e a t m e n t time for samples heated in an alumina tube placed in a furnace. The tube could be evacuated in order to remove water, or opened to a constant water vapor partial pressure for the entry of water. The diffusion coefficients for the entry of water were approximately 3.5 times greater than those reported for the removal of water. The activation energies for diffusion into and out of vitreous silica were equal within the limits of experimental error. Different grades of vitreous silica were used in the same experiment. Others [4,5] have shown that the out-diffusion of water in vitreous silica is d e p e n d e n t on the method of production.
t Registered Trade Mark of Corning, Inc., Corning NY, USA.
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00268-R
186
P.B. McGinnis, Z E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
The diffusion of water in silica glass has also been investigated through the use of tritiated water vapor [6-9]. Tritium (3H) is a radioactive isotope of hydrogen that emits 13-particles upon decay. The 13-decay energy is estimated to be approximately 18.6 keV [10] with a range of only about 1.3 ixm in silica glass [8]. Thus, measurement of 13-emission as a function of distance into the sample reveals a diffusion profile. Roberts and Roberts [6] studied the effect of fictive temperature on both the solubility and diffusion coefficient of tritiated water vapor into vitreous silica. Isothermal in vacuo heat treatments were used to change the fictive temperature of the silica samples. Samples which had been treated to obtain a fictive temperature of l l00°C and as-received samples (fictive temperature = 1200°C) were treated together at temperatures between 700 and 1200°C. The diffusion coefficient of water into heat-treated samples was greater than that of as-received samples in most instances. No explanation was given for the observed behavior. In a similar experiment, samples with fictive temperatures between 1000 and 1300°C were treated in tritiated water vapor at 750°C. The log of the solubility was a linear function of reciprocal absolute fictive temperature. The solubility increases with fictive temperature. This observation was explained as due to an increase in thermally produced defects, such as elongated or broken oxygen bonds, with increasing fictive temperature. The increased defect concentration provides a higher concentration of favorable sites for hydroxyl formation. Roberts and Roberts [7] also treated vitreous silica samples at 1000 and l l00°C for different times in v a c u o until further treatment resulted in no additional structural relaxation (the fictive temperature equaled the treatment temperature). Subsequent treatments in tritiated water vapor allowed measurement of the solubility as a function of vacuum treatment time. The solubility decreased as the treatment time increased. Thus as the fictive temperature of the as-received sampie decreased, the solubility decreased. The decrease in solubility with time follows a bimolecular reaction law of kinetics. Bimolecular kinetics are obeyed when defects composed of two parts
are annihilated when they react. The decrease in solubility was attributed to a reduction in defect concentration which they proposed to be broken or elongated silicon-oxygen bonds. Tracer work using tritiated water was used to determine the diffusion coefficient of 'water' into silica glass as a function of the tritiated water concentration in the glass [8]. Plots of the diffusion coefficient as a function of inverse absolute temperature showed an obvious change in activation energy between 1000 and l l00°C. Activation energies were shown to be relatively constant with concentration, but the diffusion coefficients increased with concentration. Burn and Roberts [9] studied the effect of initial hydroxyl concentration on the diffusion ot ~ water into silica glass. A synthetic silica glass (Spectrosil ® #2) had a significantly greater hydroxyl concentration than a flame fused quartz crystal (Optical Grade Vitreosil®#2). Samples were isothermally heat-treated in vacuum in order to produce known fictivc temperatures. After heat treatment, samples were taken from the middle of blocks in the hope of minimizing any hydroxyl gradient formed during vacuum treatment. Results of tritiated water vapor treatments at 700°C showed that the diffusion penetration of water into silica glass decreases with initial hydroxyl concentration for samples with the same fictive temperature. As the fictive temperature is increased, the effect is reduced. The results indicated a significant decrease in activation energy as the treatment temperature is increased in the 900-1000°C range. The quantitative effect of 13-particles on the silica network is unknown, but the formation of defects is known to occur. The 18.6 keV 13-decay energy of tritium [10] has been mimicked by irradiating vitreous silica with 18 keV electrons [11]. Low energy electron radiation is known to cause ionization, but is incapable of causing atomic displacement. Tritiated water (a mixture of T20, HTO and H 2O) will decay to yield a 13-particle + ( O H ) - + 3 H e . The ionization damage caused by
2 Registered Trade Mark of Thermal Syndicate Ltd., UK.
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
the 13-emission creates favorable sites for hydroxyl or OT formation which is greater than if these sites were not available. Therefore the measured tritium penetration profile is artificially inflated because of the ionization created by the 13-decay. The effect is increased as the T / H ratio is increased. It follows that, while some uncharacterized effect of tritium on the results presented in previous tracer studies [6-9] must be present, that work is still valuable in understanding hydroxyl formation and water diffusion in vitreous silica. The present study investigated the diffusion of water (hydroxyl) into and out of several commercial silica glasses. The results verify previous reports of varying activation energies. They also show that diffusion coefficients differ among glasses and with direction of diffusion. These observations are discussed in terms of changes in mechanisms a n d / o r fictive temperatures.
2. Procedure
Bruckner [12] has categorized commercial silica glasses into four types by manufacturing method. Suprasil-I ® #3 and CGW-7940 ® #1 are synthetic vitreous silicas produced by hydrolyzation of SiCI 4 in an oxygen-hydrogen flame (typeIII) [12]. T-08 ® ~3 (type-ll) is produced by the fusion of quartz powder in an oxygen-hydrogen flame (Verneuille process). Suprasil-W ® #3 (typeIV), is a synthetic vitreous silica produced by hydrolyzation of SiCI 4 in a water-vapor-free plasma flame. Infrasil ® #3 (type-I) is produced by electrical fusion of natural quartz under vacuum or inert atmosphere. The as-received water concentrations + one standard deviation for five commercial silica glasses are given in Table 1. These error limits are typical for the data reported in this paper. The concentration given in terms of ppm of water (weight basis) were calculated using an extinction coefficient for H 2 0 equal to 181.0 l / m o l cm [13].
187
Table 1 Concentration of water in five commercial silica glasses Commercial silica
A b s / m m at 3660 c m - I
ppm (gH2o/gsio2 )
Suprasil-I ® CGW-7940 ® T-08 ® Suprasii-W ® Infrasil ®
1.182 _+0.030 0.814+_0.047 0.187+_0.031 undetectable undetectable
534 + 14 452+_ 17 85+_ 14 undetectable undetectable
Infrared spectra were measured for SuprasilI®, CGW-7940 ® and T-08 ® and peak heights of the free hydroxyl band were recorded. The facility used to remove hydroxyl from the samples has been described previously [14]. Samples were isothermally heated under vacuum and periodically removed from the furnace to measure the infrared spectra. The percent hydroxyl removed was calculated by subtracting the peak height for the treated sample from that of the untreated sample and dividing the result by the initial peak height. Samples werc then returned to the vacuum chamber for subsequent treatments. The facility used to diffuse water into vitreous silica is shown schematically in Fig. 1. The five Steam Out
Platinum Sample I
imace
Refractory Stage (Inside MuUiteTube) ry Plug
3 Registered Trade Mark of Heraeus Amersil, Germany.
Fig. 1. Schematic of the apparatus used for the entry of water (hydroxyl).
188
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
commercial silicas listed in Table 1 were isothermally treated in one atmosphere of water vapor. Samples were removed periodically and analyzed using infrared measurements. The percent hydroxyl added was calculated by subtracting the initial peak height from that of the treated sample and dividing by the solubility. The solubility was defined as the concentration level at which further treatment resulted in no additional increase in concentration.
0.6
0.4
0.2
3. Results
The removal of hydroxyl from vitreous silica clearly follows a square-root of time dependence as shown in Fig. 2. Although. a square-root of time dependence is often associated with a diffusion-controlled process, it does not have to be the case. The initial non-linearity in the CGW-7940 ® curve is proposed to be due to a metastable hydroxyl concentration that is easily and entirely removed within the first few minutes of treatment [13]. After the initial treatment, the CGW-7940 ® results obey a square-root of time dependence. The extrapolated concentration at 0 h treatment,
I 1
Eocow7940[
-- ~
[ [] Suprasil-I
0
~
0.8 Z 0 rfl ~ <
0.6
0.40
5'
, 10
15
Fig. 2. Hydroxyl concentration as a function of the square-root of the treatment time in vacuum at 950°C. The CGW-7940® vitreous silica shows linear behavior only after the initial treatment. The data are fitted to a linear function of (/'.
o
0
25 50 75 ~/L (~h-rlmm) Fig. 3. Dehydroxylation of Suprasil-I ® vitreous silica at four different heat treatment temperatures.
given as absorbance per millimeter in Fig. 2, is defined as the initial concentration rather than the value actually measured. The extrapolated value correlates to the level of water in the form of stable hydroxyl in the as-received sample. Therefore, the fraction of the total hydroxyl concentration removed from CGW-7940 ® does not include the unstable hydroxyl removed in the initial minutes of treatment. The fraction of total hydroxyl removed is plotted as a function of the square-root of treatment time divided by the thickness of the sample in Figs. 3-5 for Suprasil-I ®, CGW-7940 ® and T08 ®, respectively. The removal of water is linear with the square-root of the treatment time within the error limits of the data for all three glasses at all temperatures investigated. The initial slopes of these curves, R, are used in calculating the average diffusion coefficient for the removal of water. The determination of the diffusion coefficients from the initial slope is calculated using the following equations for adsorption and desorption from two parallel surfaces of a semi-infinite plate of thickness, L [15]: M t / M ~ = 4/.tr [/2 ( D t / L 2 ) !/2,
(I)
P.B. McGinnis, .I.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
r~ ©
0.6
!050 °C. . . . o~ ~3u t.:
189
It follows that the average diffusion coefficient is given by
750oC
0.5
D = ( rr/16)R 2.
~" X ©
0.4
>' =
0.3
< [., O [..,
0.2
The five commercial silica glasses were treated in one atmosphere of water vapor at 800, 900 and 1000°C. Crystallization became a problem when treating in the wet atmosphere, whereas it was not a problem for similar treatments in vacuum. Heslin [16] has shown that surface crystallization of vitreous silica is significantly affected by the hydroxyl concentration as well as surface contamination. The initial samples were cleaned only with ethyl alcohol and rinsed thoroughly with distilled water. Upon subsequently treating the samples in the we~ atmosphere, crystallization occurred on some samples after different time periods. The only samples that resisted crystallization at all three treatment temperatures were CGW-7940 ® and Suprasi!.W ':~ In an attempt to limit surface crystallization, one batch of samples was cleaned in 36N chromic acid, which is known to dissolve organics extremely well. These samples were treated in one atmosphere partial pressure of water vapor at 1000°C for 154 h without any noticeable crystallization. The hydroxyl concentration, given as absorbance per millimeter of the free hydroxyl band, is plotted as a function of the square-root of treatment time in Fig. 6. Suprasil-W ':'~ results were very similar to Infrasil ~ results and have been omitted in the interest in clarity. The loss of metastable hydroxyl in CGW-7940 ® after an initial treatment is evident. The extrapolated concentration at 0 h treatment is once again defined as the initial concentration for the calculation of diffusion coefficients in CGW-7940 ':~. The hydroxyl concentration in Suprasil-I ® is reduced with treatment time in one atmosphere of water vapor because the solubility at one atmosphere is less than the initial concentration. Treatments of Suprasil-I ® at 800 and 900°C resulted in an increase of hydroxyl concentration, presumably due to an increase in solubility at lower temperatures. Structural relaxation causes a reduction in the solubility as shown in Fig. 6. This effect is most clearly shown by data for the T-08 '~ sample. The
© Z e E-U < LI.
0.1
,K)~
0.0
I
i
I
I
10
20
30
40
50
~/L ( ~ / ~ ) Fig. 4. Dehydroxylation of CGW-7940 ~ vitreous silica at four different heat treatment temperatures.
where Mt/M~ is the fraction formed/removed at time, t. The initial gradient, R, is defined as
R=d(M,/M~)/d(t/L2)l/2=(16D/rt)
~m
~/2. (2)
0.5 1100°C 0.4 /
lO00oc
900°C
0.3
0.2 ~> 800°C
O. 1
0
50
100
150
200
250
~ / L (~f-~-/mm) Fig. 5, Dehydroxylation of T-08 ~ vitreous silica at four different heat treatment temperatures.
(3)
P.B. McGinn~, ZE. Shelby/'Journal of Non-Crystalline Solids 179 (1994) 185-193
190
initial hydroxyl concentration increases linearly with the square-root of time and continues to increase until a solubility limit is reached. Subsequent treatments result in further structural relaxation and a reduction in the solubility. There is no statistical difference among the final solubilities for the various vitreous silicas (error limits are of the order of the size of the symbols). Diffusion coefficients were calculated using a solubility limit defined as the highest concentration formed during treatment. The CGW-7940 ® sample had a significantly greater diffusion coefficient for the entry of water than the other four samples, which were equal within acceptable limits of experimental error at 1000°C. The diffusion coefficients for the entry of water into CGW7940 ® are significantly greater than those for removal. A certain amount of error is inherent to the diffusion coefficient calculation for the entry of water since it is impossible to know the solubility as a function of treatment time and what effect a change in fictive temperature during treatment
1.2
0
A
<>
o
09
0
g
[]
0.6
O.3
[]
0
Suprasil-I
[]
Infrasil
0
CGW-7940
A T-08 0.0 0
I
I
4
8
12
(hoV~-~u~) Fig. 6. Hydroxylation at 1000°C of four different commercial silica glasses. Data for Suprasil-W ® have been omitted for clarity; the results are similar to those for Infrasii ®. CGW7940 ® shows linear behavior only after the initial treatment. The hydroxyl concentration of Suprasil-I ® decreases due to a solubility lower than that corresponding to one atmosphere of water vapor.
1.00
~
to
O
O
0.75
0.50
i
0.25
O
800°C
[]
900°C
O I
0.00 0
25
1000°C
I
I
50
75
100
~n.. (~/-~-/mm) Fig. 7. Hydroxylation of Suprasil-W ® vitreous silica at three different heat treatment temperatures. The data are fitted to functions of ¢[/L.
has on the diffusion coefficient. The amount of relaxation cannot be estimated since the initial fictive temperatures are not known for any of the five samples. Douglas and Isard [17] reported that a silica sample heated at 993°C requires 600 h treatment to attain equilibrium. Their results may only be used as a guide since they too did not know their initial fictive temperature and their hydroxyl concentration was probably much lower than in the present study (they only defined their glass as a 'commercially available clear fused quartz rod'). The greater hydroxyl concentration of the wetter vitreous silicas probably lowers the transition range. Since relaxation follows an exponential time dependence, the rate of structural rearrangement decreases with time, thus the greatest reduction in solubility occurs during the period where the initial slope, R, is measured for the diffusion coefficient calculation. Figs. 7 and 8 show the fractional increase in hydroxyl content at 800, 900 and 1000°C for Suprasil-W ® and CGW-7940 ®, respectively. The initial slopes are used in the calculation of diffusion coefficients for the entry of water into silica glass. The slopes of the 900 and 1000°C curves in Fig. 8 are identical within experimental error.
P.B. McGinnis, J.E. Shelby ~Journal of Non-Crystalline Solids 179 (1994) 185-193
ation curves and possibly in the hydroxyiatfon curves as well. The diffusion coefficients for the entry of water are zignificantly greater than those for the removal of water in CGW-7940 ®.
1.00
c~o o <> [] 0
0.75
191
4. Discussion 0.50
0.25
0 800°C [] 900°C O 1000°C
0.00
' 10
0
'. 20
'
30
40
~/itL (f~-tmm) Fig. 8. Hydroxylation of CGW-7940 ® vitreous silica at three different heat treatment temperatures.
Fig. 9 contains diffusion data for the entry and removal of water from silica glass. The curves for diffusion into Suprasil-W ® and CGW-7940 ® are marked with filled symbols. The removal curves are marked by unfilled symbols. It is easily seen that a change in slope occurs in the dehydroxyl-
-81
0
/
~
-9 I r~ Z @
~
/
Suprasil-I Removal
[] T-08 Removal ~
O
CGW-7940 Removal
",eS ~->.,X~ i
i=,,t
r.)
tm o
-13
"
A
• -14 0.7
,
Suprasil-W Entry
~
CGW-7940 Entry I t 0.8 0.9 1.0
I 1.1
t 1.2
1.3
1000fr (l/K) Fig. 9. Log effective diffusion coefficient for the removal and entry of hydroxyl in different commercial silica glasses. The lines are drawn as a guide for the eye.
The present study appears to be the first to directly investigate the kinetics of formation and removal of hydroxyl from different commercial vitreous silica glasses. Previous papers [2,3] have studied the entry and removal of water from vitreous silica by measuring the water content directly through infrared analysis. These studies, however, made no distinction between silica glass identity. Others [6-9] have studied the kinetics of formation and removal of tritiated water in various silica glasses. The 13.-particle emission used to produce a tracer penetration profile creates an inherent error in the measurement. The present study agrees with previous work [8,9] in that a change in activation energy occurs for the removal of hydroxyl between approximately 000 and 1000°C. Dtury aad R.oberts [8] report an activation energy of 10-20 kcal/mol between 1100 and 1300°C. Burn and Roberts [9] report an activation energy equal to 14.0 + 0.5 kcal/mol between 1000 and 1200°C and 25 _+ 1 kcal/mol between 600 and 900°C. The activation energies calculated in the present study for the removal of hydroxyl between 500°C and 900°C are equal to 38.7 kcal/mol and 34.6 kcal/mol for CGW-7940 ® and Suprasil-I ®, respectively. The activation energy for the removal of hydroxyl from T-08 ® is 68.4 kcal/mol between 800 and 900°C. The number of high-temperature measurements is insufficient to accurately determine an activation energy above 1000°C, but an approximate value for Suprasil-I ® is 14 kcal/mol. The change in activation energy between 900 and 1000°C shown in this study has been reported by others [8,9]. Burn and Roberts [9] concluded that a change in the diffusion mechanism was brought on by the reduction of the 'jump' distance to the nearest neighbor level at higher temperatures. They suggested that water diffusion occurs by the continual creation and destruc-
192
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
tion of reactive sites. At lower temperatures, the H ÷ and O H - ions must condense to form water molecules that subsequently diffuse to new reactive sites. The reactive site concentration increases with temperature. At higher temperatures, the need for condensation reactions is eliminated as the distance between reactive sites approaches the distance between nearest neighbors. The diffusion of water at higher temperatures occurs as associated proton and hydroxyl pairs. There is still much speculation on the mechanism of diffusion of water in glass. It is possible to explain the change in activation energy observed in this study through a change in the controlling process from a reaction-controlled process at lower temperatures to a diffusion-controlled process at higher temperatures. This behavior has been shown before by Van der Steen [18] for the removal of H 2 form SiO 2 by the following reaction: =SiOH + - S i l l ~ 2SiO 2 + H2(phys. diss)'
(4)
Van der Steen showed that below 800°C the hydroxyl concentration in Eq. (4) is controllcd by reaction kinetics. Above 1150°C, the removal of H 2 is a diffusion-controlled process where the H 2 concentration is a function of both the diffusion coefficient and the solubility. It is possible that similar arguments apply for water diffusion. Another possible explanation for the change in activation energy is based on structural relaxation. The change in activation energy for the removal of hydroxyl occurs approximately ever the same temperatures as the transition range. It is unclear whether this is simply a coincidence or whether the two phenomena are related. It must be remembered that an increase in the fictive temperature of vitreous silica in this temperature range, unlike that of other glasses, actually results in a decrease in density [12]. The expected effect would be a decrease in activation energy, as the openness of the structure increases, just as is found in the present and previous studies [8,9]. Further, the viscosity of vitreous silica approaches infinity below the transition range. Within the transition range, viscous flow becomes possible and may account for the decrease in activation energy as the structure becomes less rigid.
The differences in diffusion coefficients as well as activation energies between different commercial silica glasses may be due to differences in fictive temperatures, impurity levels or initial hydroxyl concentrations. Since the initial fictive temperatures and impurity levels are unknown, it is difficult to distinguish between possible causes for the differences in diffusion data. It is clear, however, that the manufacturing method affects hydroxyl diffusion. The entry of water into vitreous silica at 1000°C shows a change in solubility which is probably due to a change in fictive temperature. Diffusion coefficients for the entry of water at 1000°C were equal within the limits of experimental error for all samples except the CGW-7940 ®, which has a noticeably higher value. The diffusion coefficients for the entry of water into CGW-7940 ® between 800 and 1000°C were greater than corresponding values for the removal of water. Although this behavior has been shown previously [2], an adequate explanation was not presented. Further work is necessary to adequately compare data for the entry and removal of hydroxyl.
5. Summary and conclusions The change in activation energy for the removal of hydroxyl from three different commercial silica glasses found in the present study is consistent with others [8,9]. The change in activation energy is believed to be due to either structural relaxation, or to a change in the controlling mechanism. A change in solubility is apparent for samples treated at 1000°C in one atmosphere of water vapor. The decrease in solubility is due to a decrease in fictive temperature with heat treatment time. Structural relaxation causes a reduction in defect concentration, thus lowering the number of reactive sites and consequently the solubility. The magnitude of change is unknown. The diffusion coefficients for the entry of water at 1000°C were equal for all but one of the samples. The diffusion coefficients for the entry of water into CGW-7940 ® are greater than those for removal from 800 to 1000°C.
P.B. McGinnis, J.E. Shelby/Journal of Non-Crystalline Solids 179 (1994) 185-193
Additional work is underway in determining diffusion coefficients for the entry of water into the five commercial silica glasses at temperatures below 1000°C. Further study on the effect of fictive temperature will be needed to adequately explain the observed change in activation energy. Determination of diffusion profiles for samples treated at temperatures above, below and in the transition range may answer questions about a possible change in the controlling mechanism resuiting in the change in activation energy since totally reaction-controlled processes should have a constant concentration profile.
References [1] B.J. Todd, J. Appl. Phys. 26 (1955) 1238. [2] A.J. Moulson and J.P. Roberts, Trans. Faraday Soc. 57 (1960) 1208. [3] A.J. Moulson and J.P. Roberts, Trans. Br. Ceram. Soc. 59 (1960) 389. [4] P.B. McGinnis and J.E. Shelby, in: Proc. 16th Int. Congr. on Glass, Vol. 3 (Bol. Esp. Ceram. Vid., Madrid, 1992) p. 239.
~93
[5] H.D. Witzke, in: Proc. 2nd Int. Conf. on Advances in Fusion and Processing of Glass, Glastech. Ber. 63K (1990) 333. [6] G.J. Roberts and J.P. Roberts, Phys. Chem. Glasses 5 (1964) 26. [7] G.J. Roberts and J.P. Roberts, in: Proc. 7th Int. Congr. on Glass, Brussels (Federation de L'lndustrie du Verre, Brussels, 1965) p. 31. [8] T. Drury and J.P. Roberts, Phys. Chem. Glasses 4 (1963) 79. [9] I. Burn and J.P. Roberts, Phys. Chem. Glasses 11 (1970) 106. [10] F.T. Porter, Phys. Rev. 115 (1959) 450. [11] T.A. Deilin, D.A. Tichenor and E.H. Barsis, J. Appl. Phys. 48 (1977) 1131. [12] R. Bruckner, J. Non-Cryst. Solids 5 (1970) 123. [13] J.E. Shelby, J. Vitko Jr. and R.E. Brenner, J. Am. Ceram. Soc. 65 (1982) C-6~. [14] J.E. Shelby, in: Experimental Techniques of Glass Science, ed. C.J. Simmons an~_;O.H. EI-Bayoumi (American Ceramic Society, Westervill,:, OH, 1993) ch. 10. [15] J. Crank, The Mathematics of Diffusion, 2nd Ed. (Clarendon, Oxford, 1989). [16] M.R. Heslin, PhD Thesis, Alfred University (1993). [17] R.W. Douglas and J.O. Isard, J. Soc. Glass Technoi. 35 (1951) 206T. [18] G.H.A.M. Van der Steen, PhD thesis, University of Eindhoven (1976).