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PAPERS

Water in vitreous silicaPart 2: Some aspects of hydrogen-water-silica equilibriaT. Bell, G. Hetherington & K. H. JackThe Thermal Syndicate Linlited, Wallsend

Whe n 'w a te r ' fi e vitreous silica is heated irz dry hydro- Although the equilibrium solubility of hydroxyl ingen, silicon-hj~droxyl (-Si-OH ) grozps are produced in vitreous silica depends at a given temperature upon thethe solid and the hydroxyl concentration gradient is partial pressure of water vapour in the surroundingrelated to the rate of diffusion of hydrogen in silica. The atmosphere,(l* ) it seemed possible that 'water' solu-equilibrium concentration of hydroxyl introduced by bility might also depend on the nature of the silicon-this reaction is proportional to the square root of the oxygen lattice, and particularly on its state of oxidationpartial pressure of molecular hydrogen and is of the or reduction.same order of magnitude as previously published valuesfor the so-called's ~l~~bi1 ityf hydrogen9 n vitreous silica.Wh en hydroxyl-containing vitreous silica is heated in aclosed evacuated system, hydrogen as well as water isevolved.These observations suggest the existence of theequilibrium :

Si4+ 202- + + Hz + Si3+ 02 - H 1-stoichiometric reduced silicasilica with hydro xyl'W ater y-fre e itreous silica is normally produced in apartially reduced state. It absorbs in the ultm -violet at2425 A and it exhibitsJIuorescence. W hen heated in anoxidizing atmosphere the absorption band and thefluorescence disappear slowly at a rate which is deter-mined by the d~ffusionof oxygen through the solid.However, non-stoichiometric vitreous silica which alsocontains hydrox yl is oxidized much mo re rapidly and ata rate which is independent of the nature of the szlr-rounding atmosphere. In this case the oxidation mechan-ism is that shown by the reverse reaction of the aboveequilibrium and in which the oxidizing species (hydr oxy lgroups) are present throughout the solid. Th e rate ofoxidation is then governed not by dzffusion of ox ygen butby diffusion and removal of hydrogen.Th e existen ce of a hydrogen-water-silica equilibriumaccounts for other previously unexplained feature s of thebehaviour of vitreous silica at elevated temperatures.

Reaction of hydrogen with hydroxyl-free vitreous silicaExperiments were first made with I.R. Vitreosil, that is,with the material containing almost zero hydroxyl andmade by electrical fusing of quartz crystal. Specimensof different thicknesses were heated in pure dry hydro-gen at 800C and at 1050C for varying times andwithout exposing them to any water vapour it wasfound that the hydroxyl concentration, as measuredby the optical density at 2-73 y, steadily increased.Figure 1 shows that at each temperature the opticaldensity per mm reaches a constant value independentof the specimen thickness. This corresponds to anequilibrium hydroxyl solubility. These solubilities aresmall; for example at 800C the observed optical dens-ity of 0.018 mm-I is equivalent to only 0.0018 wt. %(-OH) and at 1050C the hydroxyl content is only0.00165 wt. %. Nevertheless, such concentrations canbe measured accurately by allowing thick specimens tocome to equilibrium over a long time since the finaltotal absorption at 2-7y is then quite large.By removing successive slices from a thick specimenafter treatment with hydrogen for only a relativelyshort time, the hydroxyl concentration gradient shownin Figure 2 was obtained. Although the curve is sig-moidal and thus suggests that the diffusivity varieswith concentration, the total hydroxyl content aftershort-time hydrogen treatment is small and can not be

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T . B E L L , G . H E T H E R I N G T O N A N D K . H . J A C K : W A T ER I N V I T R E O U S S I L I C A . P A R T 20.0200.015

Specimen heated in dry(0 2ree) hydrogen at 800C

Specimen heated in dr y( 0 2 free) hydrogen at 1050C

. o l I L I0 10 20 30 40 50 60 70 80 90 106Time (h)

Figure 1. Hydro xyl concentration for different thicknesses of 1.R.Vitreosil after heating at 800 and 1050"C in hydrogenmeasured, particularly after removing successive thinslices, with sufficient accuracy to make unequivocaldeductions about the diffusion profile. It is assumedthroughout the work that the diffusion constant isindependent of concentration and so all calculateddiffusivities represent average values.A plot of the total optical density against J t gives astraight line (see Figure 3) and again suggests that theproduction of hydroxyl in the silica must be diffusion-controlled. It will be noticed from Figure 3, however,that had this same specimen been exposed to a partialpressure of water vapour at 1050C to give the same

equilibrium concentration of hydroxyl(0.00165 wt. %)then the rate of hydroxyl formation, as calculated fromMoulson's & Roberts' results,(1) would have beennegligible. In other words, in the hydrogen reaction the

Thickness (mm)Figure 2. Hy drox yl diffusion profile for hydrogen treatm ent of I.R .Vitreo sil for 15 h at 1050"C

Table 1. DzfSusivities of 'hydrogen' and 'water', bothproducing hydroxyl in vitreous silica1. Diffusivity of 'hydrogen', prod ucing hydroxyl:

-15 800D = 9. 5 x ex p2m2 s-1R T2. Diffusivity of 'water' in silica:

-18 300D = 1.0 x ex pA m2 s-IR Tdiffusing species is not the same as for the reaction ofwater with silica although both reactions producesilicon-hydroxyl groups.From the large number of data on thin and thickspecimens the diffusion constant at each temperatureand its activation energy were calculated. The resultsgiven in Table 1 show that in the temperature rangefrom 800 to 1050C the diffusivities are almost tenthousand times greater than those for water' in vitreoussilica.The dependence of the equilibrium hydroxyl con-centration on hydrogen pressure was investigated byexposing thick specimens of I.R. Vitreosil to hydrogenat different pressures from 28 mm to 760 mm and atthe two temperatures 800 and 1050"C . Figure 4 showsthat the hydroxyl solubility at each temperature isproportional to the square root of the hydrogenpressure and so suggests the existence of the reactionequilibrium:$Hz ?+ +Hz + -0- + -OHgas dissolved silica hydroxyl

Since infra-red methods allow the determination ofonly the concentration of hydroxyl hydrogen it is im-possible to estimate the amount of dissolved molecular

0 line represents curvelevel of hydroxyl at 1050C(Moulson & Roberts('))- - J=-d.-.-,1 2 3 4 5 6 7 8 9 1 0

(Time in h)fFigure 3. Variation of total hydroxyl content (optical density at2.73 p) with Jt for thick I.R . Vitreosil specimens heated at 800 and1050"C in hydrogen at atmospheric pressure

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T . B E L L , G . H E T H E R I N G T O N A N D K . H . J A CK :W A T ER I N V I T R E O U S S I L I C A . P A R T 2Table 2. DlfSusivities of 'hydrogen' a t dlfSerent pressuresproducing hydroxyl by reaction with I. R . Vitreosil at800" and 1050"CHydro gen pressure Diffu sivity , en12 s-lmm Hg 800"C 1050"C

The heat of solution of 'hydrogen' in silica to givehydroxyl is very similar to the heat of solution of'water'. Respective values derived from solubility da taare -2 and -6 kcal/mole; see Table 3. This is no t to osurprising since the difference between the bond ener-gies hydrogen-hy drogen (103 kcal) and hyd rogen-hydroxyl (1 10 kcal) is only 7 kcal/mole.If the very fast diffusing species which controls therate of formation of hydroxyl in hydrogen-treatedsilica is molecular hydrogen, then the values given inTable 1 should be comparable with previously pub-lished results for the diffusion of hydrogen in silica.Moreover, it seems possible that results of previousdeterminations of the solubility of hydrogen in silicaare, in fact, solubilities of hydroxyl.There a re no extensive da ta fo r either the diffusivityor the solubility of hydrogen in silica at high tempera-tures. Fo r example Barrer's resultd3) are extrapolatedfrom observations in the temperature range 200-500C. However, it can be seen from Table 4 that thepresent diffusivity results agree within an order ofmagn itude with his. Furthe rmo re, the present solubili-ties for 'hydrogen' which is now shown to be present

(Hydrogen pressure in mm Hg)fFigure 4. The variation w ith hydrogen pressure of the equilibriumoptical density per mm at 2.73 p for. I . R . Vitreosil heated inhydrogen at 800 an d 10.50"Chydrogen. It is hoped that future work using tritiumtechniques in combination with infra-red methods willsolve this problem.At all hydrogen pressures so far examined thehydroxyl concentration increases linearly with , / t (seeFigu re 5) again indicating tha t the proce ss is diffusion-controlled. Th e diffusivities for e ntry of hydrogen atdifferent pressures to give hydroxyl, and calculatedfrom these observations, are listed in Table 2 a nd arein remarkable agreement with one another at each ofthe tw o temperatures. It seems that the same diffusionmechanism must operate at all pressures up toatmospheric.

Table 3. Heats of solution per mole of hydrogen and ofwater at 800-1050" C in vitreous silica, each producinghydroxy l and each at one atmosphere pressure; valuesderived from solubility d ataH~ -t 2 Si4+..,022-+ 2 Si3+ ..02-.. 0H)l-(H z -+2 OH)( A H ) # = -2 kcal/mole-lH 2 0 + 2 Si4+...022-+ S ~ Z ~ + . . . O ~ ~ - . . . ( O H ) Z ~ -(Hz0+ OH)( A H ) , = - k~a l /mole -~

Table 4. DifSusivity and solubility of 'hydrogen' invitreous silicaDiffusivity of 'hydrogen ', cm2 s-1700"C 800" 900C 1000" C

Barrer@) 0.6 1.0 1.5 2.1 x 10-7Present work 2.7 5.3 10.8 18.4 x 10-7(Time in h)f Solubility of 'hydrogen', cm3 (N.T.P.)em-3 SiOz700" 800" 900" 1000" CFigure 5. The variation with time of the total optical density at2.73 p or thick specimens of I . R . Vitreosil heated in hydrogen a tdifferent pressures Wiistnerc4) 10 11 10 l o x lo4Present work 27 26 25 24 x 10-3

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T . BELL, G . HETHERINGTON AND K . H . J A C K : WA TE R IN VITRE OUS SILICA. PART 2Table 5. Permeability of 'hydrogen' in vitreous silica, 6x hydroxyls formed by complete reaction ofcm3 (N.T.P.) .mm sl ~ r n - ~m Hg-I hydrogen with I.R. Vitreosil at the same temperature.

600"C 700"C 800"C 900"C 1000C In the first case the amount of hydrogen evolved and inthe second case the amount of hydrogen dissolved are-Barrer (1941)(3) 14 25 43 64 10 0x 10-10- determined ultimately by the extent to which silica canNorton (1953)(5) 13 21 - - x 10-10Altemose (1961)@) 23 (39) (71) - - 10-lo be reduced. This chemical reduction can be repre-Present work 31 73 138 271 442x10-'0 sented as a departure from the stoichiometric com-position S~OZ,o that the above results correspondas hydroxyl, are in reasonable agreement with theaccepted solubilities(4) for what was previouslyassumed to be dissolved molecular hydrogen.There are more recent results for the permeability ofhydrogen in silica and these are compared in Table 5with values calculated from diffusivities and solubili-ties observed in the present work. The agreement issufficient to conclude that dissolved molecular hydro-gen in silica exists substantially as silicon-hydroxylgroups and that the diffusing species, whatever theyare, are very much more mobile than those involved inthe diffusion of 'water' through silica.The reaction between hydrogen and silica can berepresented as a chemical reduction in which, for everyhydroxyl produced, one silicon atom changes itsvalency state from 4+ to 3+ ; see Figure 6, which isintended merely as a diagrammatic representation ofthe reaction of hydrogen with the silica structure. It isclear that an equilibrium must exist and that the re-verse reaction must also occur.Hydrogen produced from hydroxyl-containing vitreoussilicaTo explore this latter possibility a known amount ofO.G. Vitreosil was heated in a vacuum. It will be re-membered that this type of vitreous silica contains arelatively high hydroxyl concentration, 0-04wt. %, andis also in a chemically reduced state. At 1050"C bothhydrogen and water were evolved, but by condensingout the water in a liquid-oxygen-cooled trap thehydrogen was allowed to build up in the closed systemand the final hydrogen pressure corresponded to about2x 10-4 atoms/mole of SiOz. This agrees as closely ascan be expected with the observed concentration ofSiJ+ 202- ++Hz- i3f 02-H-- -toichiornetric hydrogen reduced silica

silica with hydroxyl

Figure 6. The reaction of hydrogen with vitreous silica

with a non-stoichiometry SiOz-, where the values ofx are respectively 1x and 3 x oxygen atoms.The oxidation of vitreous silicaThere is even more convincing evidence of the occur-rence of the reaction by which hydroxyl reacts withinsilica to give hydrogen.As normally produced, vitreous silica containing nohydroxyl is in a chemically reduced state. It absorbsstrongly at about 2400 A in the ultra-violet and re-emits this absorbed energy as a visible fluorescence.The absorption and the fluorescence can be regardedas measures of the degree of reduction, and on heatingthe silica in an oxidizing atmosphere they are removedslowly at a rate which is probably determined by thediffusion of oxygen through the solid.A non-stoichiometric silica which also containsa high hydroxyl concentration, for example O.G.Vitreosil, is oxidized much more rapidly than one con-taining no water, for example I.R. Vitreosil.Figure 7 compares the rates of disappearance of theabsorption bands of O.G. and I.R. Vitreosils whenheated at 1050C in a vacuum. T is observed that forO.G. material the oxidation is complete after 85 h

2000 2500 3000 2000 2500 3000Wavelength (A) Wavelength (A)

Figure 7. Oxidation of vitreous silica144 Physics and Chemistry of Glasses Vol. 3 No. 5 October I962

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T. BELL, G. HETHERINGTON AND K . H. JACK :WATER IN VITREOUS SILICA. PART 2

0- - - - -0 VacuumX X Ai rA ........... .A Nitrosell

Time (h )Figure 8. Rate o f oxidatiorz on heat-treatnfent oJ O .G . and I.R .Vitreosils at 10.50 C in various atnlosphereswhereas there is scarcely any change in the I.R.Vitreosil even after 562 h.The curves of Figure 8 show rates of oxidation forO.G. Vitreosil heated in a variety of atmospheres(oxygen, nitrogen, steam, and vacuum) and withinexperinlental error they are nearly identical. Fromthese curves the calculated diffusivities at 1050 C areall within the range 2.2 x to 2-7x 10-6 cm2 s-1which is exactly the same as the diffusivity of 'hydro-gen' to p roduc e hydroxyl (2.3 5x 10-6 cm 2 s-1, seeTable I). It is concluded tha t the oxidation of a reducedsilica which also con tains a high hydroxyl conce ntra-tion takes place by reaction of the hydroxyl to givehydrogen,OH+0 + +Hz,and that the rate is governed by the outw ard diffusionand removal of hydrogen.Oxida tion by diffusion and removal of hydrogen presentinitidly as hydroxylThe most significant experiment was that carried outo n an I.R. Vitreosil specimen which had previouslybeen treated, almost to completion, with hydrogen at1050C to give a tota l optical density, due to hydroxyl,of 0.261 for a pathlength of abo ut 18 mm. This speci-men was then heated in air at 1050"C and after 74 h theoptical density was found to be reduced to 0.1 15. Thisrate of disappearance of hydroxyl corresponds to adiffusivity of 0.8 x 10-6 cn12 s-1 and is in fair agree-ment with the value of 2 . 3 5 ~0-6 cm2 s-1 for the

form hydroxyl. At this temperature the removal ofhydroxyl as water from vitreous silica with a high'water' content is very much slower an d gives anapparent diffusivity of 3.8 x 10-lo cm 2 s-l.A numbe r of specimens of I.R. Vitreosil of differentthicknesses varying from 2 to 20 mm were saturatedwith hydroxyl (0.00165 wt. %-OH) b y reaction withhydrogen at 760 mm and 1050C. These were then

Ai r and steam II.R. 1

60 80 100

penetration of hydrogen at the same temperature to Figure 9. Growth of bubbles in SpectrosilPhysics and Chemistry of Glasses Vol. 3 No. 5 October 1962

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T . B E L L , G . H E T H E R I N G T O N A N D K . H . J A C K :W A T E R I N V I T R E O U S S I L I C A . P A R T 2Table 6. Di ~us i v i t i e sor the removal of 'hydrogen 7 romI. R. Vitreos il specimens previously saturated withhydroxyl b y complete reaction with hydrogen at 760 mmand 1050" CSpecimen thicknessmm

DiJJusivity,em2 s-lin air in vacuo0.3 x 10-6 0.6 x 10-60.6 x 10-6 0.9 x 10-60.6 x 10-6 1.3 x 10-61.1 x 10-6 1.5x 10-60.8 x 10-6 -1-5x 10-6 2-0x 10-6

heat-treated at 1050C in air and also in vacuum andthe reduction in hydroxyl concentration was followedby infra-red absorption measurements at 2.73 p . Asshown in Table 6 , the calculated diffusivities were in-dependent within experimental error of thickness andof the atmosphere and had values all within the range0.3-2.1 x 10-6 cm2 s-1.The results of the present investigation show, there-fore, that the reaction of hydrogen with silica isreversible and that it occurs in the solid at rates which,for both directions, are governed by the diffusivity ofhydrogen.Practical aspects of the hydrogen-hydroxyl-silicaequilibriumThe existence of a hydrogen-hydroxyl-silica equili-brium accounts for several previously unexplainedfeatures in the behaviour of vitreous silica. Forexample in the manufacture of Spectrosil, the materialwhich has the highest hydroxyl concentration, bubblesare occasionally produced. In the plastic range thesebubbles continue to grow and internal pressures ofseveral atmospheres are developed (see Figure 9) . Massspectrographic analysis shows that these bubbles con-sist of pure hydrogen. By reaction of hydroxyl with

reduced silica the hydrogen which is produced nor-mally diffuses to the solid surface.A nucleus within thesolid acts as an internal surface and so a hydrogenbubble, when once it is nucleated, continues to grow.Spectrosil shows no absorption at 2400 A and nofl~o~escenceecause it is self-oxidized as rapidly as it isformed. However, by creating a very steep hydroxylconcentration gradient near the surface of Spectrosil,for example by heating it in a vacuum, fluorescencecan be induced.The rapid removal of hydroxylSi4++202-+*HZ-+ Si3++02-OH1-shifts the equilibrium in such a way as to increase theconcentration of 'reduced' silica.Added noteWe are obliged to the Referee for his suggestion thatreaction of hydrogen with vitreous silica might pro-duce equal numbers of Si-OH and Si-H groups bydisruption of one silicon-oxygen link per hydrogenmolecule, i.e.Si-0-Si+Hz -+ Si-OH+ H-Si

This would also result in the observed linear relation-ship between the equilibrium hydroxyl concentrationand the square root of the pressure of molecularhydrogen. There is no direct evidence for the existenceof Si-H links in vitreous silica.References1. Moulson, A . J. & Roberts, J. P. (1960). Trans. Brit. Cerarn. Soc.59, 388.2. Hetherington, G. & Jack, K. H. (1961). To be published inBull . Soc. Fra n~ . dram. Papers presented at the InternationalColloquium, Paris, 7 September 1961.3. Barrer, R. M. (1941). Diffusion in and through Solids. CambridgeUniversity Press.4. Wiistner, H. (1915). Ann. Phys. 46, 1095.5. Norton, F. J. (1953).J . Amer. Ceram. Soc. 36.90.6. Altemose, V. 0 . (1961). J . App. Phys. 32 , 1309.

146 Pltysics and Chenlistry of Glasses Vol. 3 N o. 5 October 1962