Journal of Non-Crystalline Solids 140 (1992) 112-118 North-Holland

]OURNA L OF

NON-CRYS LLINE SOLIDS

Influence of sulphate anions on the chemical durability of the BATY glass

M o h a m m e d Saad and M o h a m e d E1 Farissi Ddpartement de Chimie, Facult~ des Sciences, Universitd Mohamed 1, Oujda, Morocco

Marce l Pou la in Centre d'Etude des Matdriaux Avancds, Universitd de Rennes I, Campus de Beaulieu, 35042 Rennes, France

Variable amounts of sulphate anions were introduced in the s tandard BATY fluoroaluminate glass. Up to 3% BaSO 4 was substi tuted for BaF 2. Sulphate-free BATY glass shows neither weight loss nor water-induced IR absorption band after 72 h leaching at 88 ° C. In the same conditions, sulphate-doped samples are corroded and the leaching rate appears correlated to sulphate concentration. Doping BATY glass with a luminium oxide decreases its chemical durability even more dramatically.

1. Introduction

The practical development of optical compo- nents depends on their behaviour in the real conditions of use. Fluoride glasses suffer from their sensitivity to water, which has been exten- sively studied by several authors [1-9]. However, it appears that chemical durability may be de- fined in several ways: in wet atmosphere, in deionized or buffered solutions, in cycling water, by measuring weight loss or optical absorption. In practice, the surface quality is essential: a glass which exhibits a small leaching rate with the formation of a corrosion layer cannot be used in a wet environment, while a more soluble glass may retain good optical qualities under the same conditions.

While fluorozirconate glasses are fairly stable in neutral or slightly basic buffered solutions [6], they are rather sensitive to uncontrolled concen- trations changes, which results in a pH decrease due to corrosion and enhances surface attack. Some fluoroaluminate glasses appear more resis- tant to aqueous solutions. Usually a A1F 3 content lower than 30-40 mol% seems necessary. The quaternary BATY glass demonstrates a very good

chemical durability, with very limited surface al- teration and weight loss [10].

As the corrosion process originates at surface, the role of impurities may be significant. This paper investigates the influence of sulphate an- ions on water attack in this glass.

2. Experimental

The starting materials are KHSO4, BaSO 4 and A1203 from Merck, ThOz 4N and "Y203 3N op- tique from Rhone Poulenc, BaF 2 rectapur from Prolabo. Ammonium bifluoride NHsF 2 technical grade is provided by Kali Chemie.

Basic BATY glass composition, expressed in tool%, is 28.8AIF3, 28.8YF3, 22.4ThF 4, 20BaF 2. Glass is made from the mixture of fluorides and oxides, using the ammonium bifluoride process as described elsewhere [11]. Dry processing was not used, so that a residual OH content was generally observed as a result of a limited corrosion by atmospheric moisture.

The preparation of sulphate-doped glass sam- ples required two steps: the first was fluorination and heating to 700 °C of the basic components

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

M. Saad et al. / Chemical durability of the BATY glass 113

followed by cooling and mixing with the calcu- lated amount of sulphate, then melting and cast- ing. In this way, decomposition of sulphate anions was minimized.

Final samples were annealed and polished. Typical plate size was 15 × 10 × 3 mm. Samples were exposed to cycling water at 88°C in a Soxhlet-type apparatus for 72 hours. Infrared transmission spectra were recorded before and after treatment using a Bomem Michelson 200 set-up.

3. Results

Preliminary investigations showed that sul- phate containing BATY glass samples undergo a visible surface attack. A first set of experiments has been carried out using potassium hydrogeno- sulphate KHSO 4 as a doping reagent. While a close correlation is observed between dopant con- centration and estimated corrosion, potassium in- corporation introduces an additional factor which makes conclusions more difficult.

Consequently, another set of preparations has been implemented using BaSO 4 as a substitute for BaF 2 in BATY glass. General glass composi- tion is written as

28.8A1F3, 28.8YF3, 22.4ThF 4 ( 2 0 - x ) B a F 2 ,

xBaSO4, with 0 < x < 3,

where x is 0, 0.25, 0.5, 1, 2 and 3%. At first glance, there is no significant differ-

ence in the behaviour of BaSO 4 and KI-ISO 4 doped samples with respect to chemical durabil-

ity. The major factor appears to be SO 4 anion concentration.

Weight losses after 72 h treatment are re- ported in table 1. No mass change is observed for basic glass. Previous measurements even indi- cated a very small weight increase, which is within the experimental error range. Real solubility, which appears limited, should be estimated from more accurate weighing and chemical analysis of leaching solution.

Figures 1 and 2 compare the various infrared transmission spectra for each sample before and after treatment. The appearance of the surface of the basic BATY glass remains unchanged, while IR transmission shows no difference: the two plots have been shifted for clarification on fig. 1, but can be superimposed within the experimental error margin.

As sulphate concentration increases, an ab- sorption band appears in the 2000-2400 cm -~ range. The magnitude of this band is related to the actual sulphate concentration, taking sample thickness into account. Average absorption at 2.9 p.m is increased in comparison with basic glass.

Water attack after 72 h treatment in a Soxhlet-type apparatus results in the following observations:

(1) the formation of a white layer on glass surface;

(2) the variable weight loss; (3) the evolution of IR transmission spectrum

including (i) the shift of the base line because of scat-

tering losses induced by corrosion layer, (ii) an intense and broad OH absorption

band around 2.9 ~xm, and

T a b l e 1

W e i g h t losses o f B a S O 4 - d o p e d glass s a m p l e s a f t e r l e a c h i n g

mass x = 0 x = 0.25 x = 0.5 x = 1 x = 2 x = 3

m 0 (g) 2 .7226 2 .9407 3 .3259 2 .8302 2 .4544 2 .4278

m~ (g) 2 .7226 2 .9393 3 .3229 2 .2892 2 .4532 2 .4244

( m 0 - m 1) (g) 0 0 .0014 0 .0030 0 .0010 0 .0012 0 .0034

m 0 a n d m I a r e the we igh t s in g r a m s b e f o r e and. a f t e r t r e a t m e n t respect ive ly . G las s compoS i t i on is 28.8AIF3, 28 .8YF3, 2 2 . 4 T h F 4 ( 2 0 - x ) B a F 2 , x B a S O 4.

114 M. Saad et al. / Chemical durability of the BA TY glass

2.5 100

o~ 80

6O i,° 20

( micrometers )

3 3.5 4 5 6 7 I I I I I

i ................ ...... iii' ...... I

4000 3600 3200 2800 2400 2000 1600 1200

2.5 100

o~ 80

60

i,° 20

V"( cm -1 )

~, (m ic rometers )

3 3.5 4 5 6 7 I I I I I

................ i ............. i .................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4000 3600 3200 2800 2400 2000 1600 1200

"~( cm-1 ) Fig. 1. Infrared transmission spectra of BATY glass samples doped with variable amounts of BaSO4, before treatment (continuous plot) and after 72 h soxhlet leaching (dashed line). For basic BATY glass (x = 0), the two curves have been arbitrarily shifted for

clarification.

(iii) a narrower H 2 0 absorption band at 6.2 p,m.

It must be emphasized that the sample corre- sponding to x = 0,5 has been more severely cor- roded than samples at 1 and 2% BaSO 4.

4. Discuss ion

This set of experiments illustrates the influ- ence of sulphate anion on the chemical durability of BATY glass. This observation would have been

M. Saad et al. / Chemical durability o f the B A T Y glass 115

o~ Z o m O3

=Z O3 Z

n- l--

2.5 100

80

60

40

20

(micrometers)

3 3.5 4 5 6 7 I I I I I

. . . . . . . i -i . . . . . . . . . . . . . . . . .

................ i ................ i ................. i ................ i ...... ...... i ...............

4000 3600 3200 2800 2400 2000 1600 1200

V-'( cm -1 )

Fig. 1. (continued).

difficult in a more reactive vitreous matrix, such as fluorozirconates.

It is assumed that water molecules react first with SO 2- anions by enhancing hydrogen bond- ing and O H - diffusion through glass surface. Because of the large size of sulphate anions, their dissolution creates large sites where water molecules can attack the glass.

While the real process is complex, the rate of attack depends on the surface concentration of sulphate anions. When the sulphate anions are isolated and separated by a distance larger than some minimum value, the corrosion mechanism is slow. When the surface density of the sulphate anions exceeds some percolation threshold, chemical attack may develop from the glass sur- face. Several observations are consistent with this argument.

First, the sulphate impurities in the basic glass, at tens of ppm level, have no significant influence on leaching rate. Also, it has been observed that some sulphate-doped samples show no visible signs of corrosion after leaching. However, after surface polishing, corrosion occurred in a second leaching test. In the first case, surface concentra- tion is too low. In the second case, glass inhomo-

geneity - in probable relation with processing - may be responsible for a low SO 4 surface con- centration in the starting sample, while a subse- quent polishing creates a surface with a higher density in sulphate anions. Conversely, the high weight loss of the 0.5% BaSO 4 sample from table 1 may result from such variations.

However, it should be surprising that composi- tion fluctuations are very large and one may wonder whether other chemical factors influence water attack. In particular, anionic oxygen is al- ways present as a residual impurity at an uncon- trolled level.

4.1. Corrosion of an oxygen-doped BATY glass

In order to assess a possible correlation be- tween chemical durability and oxygen, we have made an AlzO3/AIF 3 substitution in a BATY glass, resulting in the composition

27.8A1F3,: 28.8YF 3, 2.4ThF 4, 20BaF z, 1A120 3

or alternatively

A10.z9Y0.29 Th0.22Ba0.2F2.9400.03.

116 M. Saad et al. / Chemical durability of the BATY glass

Z _o ( / )

Z

cc i.-

~. ( micrometers )

2+5 lO0

80

60

40

20

3 3.5 4 5 6 7 I I I I I i

i : i !

............... i ................. i ............... + ............... + ................. ; ......... i ................. . . . . . . . . . . . . . . . . . .

+ i i ~ 4000 3600 3200 2800 2400 2000 1600 1200

~ ' ( cm -1 )

~, ( m i c r o m e t e r s )

2.5 3 3.5 4 5 6 7 100 i l i I t

' I

+4oo ............... 20 ................. i ................. , ................

4000 3600 3200 2800 2400 2000 1600 1200

V"( c m ' l )

Fig. 2. Infrared transmission of BaSO4-doped BATY glass samples before continuous plot and after (dashed line) leaching.

Gross formula is MF 3 and oxygen represents 1% of all anions.

The infrared transmission of this glass before and after 72 h leaching at 88 ° C is reported in fig. 3 . Starting sample exhibits an unusually strong OH absorption band and no shoulder at 1350 cm-~. This observation suggests that most oxygen

is incorporated as hydroxyl ions, and conse- quently that incomplete fluorination enhances OH formation. This OH band may be decreased by thermal decomposition at high temperature [12]. After such a treatment, the 1350 cm -1 shoulder typical of anionic oxygen in fluoride glasses may be observed. This indicates that OH

117

lOO

o~ 80

60

.~ 40

2O

4000 3600 3200 2800 2400 2000 1600 1200

V ( cm ) Fig. 2. (continued).

2.5

anions are rather equivalent to F - anions in their contribution to the multiphonon absorption edge.

For leaching, a surface corrosion layer appears in which typical OH and H 2 0 absorption bands

are observed. Taking concentration into account, anionic oxygens seem more active than sulphate anions for promoting chemical attack of water. This conclusion is an additional reason to reduce

)~ ( micrometers )

3 3.5 4 5 6 7

- i ! " ; i i . . . . . . . . . . . . ) . . . . . . . . . . . " , , i . . . . . . . . . . . .

100

80

60 i4o 20

M. Saad et al. / Chemical durability of the BA TY glass

~, ( micrometers )

2.5 3 3.5 4 5 6 7

4000 3600 3200 2800 2400 2000 1600 1200

( c m ' l )

Fig. 3. Infrared transmission of a BATY glass sampled doped with 1 mol% A1203.

118 M. Saad et al. / Chemical durability of the BATY glass

the res idua l con ten t of an ion ic oxygen in f luor ide glass op t ica l f ibres.

5. Conclusion

This s tudy of s u l p h a t e - d o p e d f l uo roa lumina t e glasses conf i rms tha t the s t a n d a r d B A T Y glass is s table in hot aqueous solut ions. T h e inco rpora - t ion of su lpha te anions enhances s ignif icant ly the ra te of a t tack, resu l t ing in the fo rma t ion of a whi te cor ros ion layer and I R abso rp t ion or ig inat - ing f rom O H and H 2 0 groups . Fu r the r , an ionic oxygen is found to enhance hydroxyl f o rma t ion and d rama t i ca l ly to dec rea se chemica l durabi l i ty . Consequent ly , it may be expec ted tha t the sur- faces of very pu re f luor ide glasses will be more res i s tan t to the chemica l ac t ion of water . This is of pa r t i cu l a r i m p o r t a n c e for making high s t reng th and slow ageing f luor ide glass f ibres.

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