S.A.X.S. STUDY OF NASIGEL AND NASIGLAS (SODIUM SUPERIONIC GELS AND GLASSES)

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S.A.X.S. STUDY OF NASIGEL AND NASIGLAS

(SODIUM SUPERIONIC GELS AND GLASSES)

A. Dauger, F. Chaput, J. Pouxviel, J. Boilot

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, supplément au n°12t Tome ^6, décembre 1985 psg€ CS-455

S.A.X.S. STUDY OF NASIGEL AND NASIGLAS (SODIUM SUPERIONIC GELS AND GLASSES)

A. Dauger, F. Chaput , J.C. Pouxviel and J.P. Boilot

Ecole Nationale Supérieure de Céramique Industrielle, 47 - 73 avenue A. Thomas, 87065 Limoges, France

Groupe de Chimie du Solide, Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique, 91128 Palaiseau, France

*SAINT-GOBAIN-RECHERCHE, 39, Quai Lucien Lefranc, 93300 Aubervilliers, France

Résumé - Des gels monolithiques ont été préparés dans le Systems Na-O-ZrO--SiCL-PpGj-, par la méthode sol/gel, (hydrolyse - polycondensation d'alkoxydes) . L'évolutxon structurale pendant la gélification et la transition gel/verre a été étudiée par diffusion centrale de rayons X pour deux compositions. Des in-homogénéités peuvent exister dans le gel, elles correspondent probablement à la présence de clusters de zircone cubique qui se dissolvent progressivement dans la matrice silicâtée lors de la transition gel/verre.

Abstract - Monolithic gels have been prepared in the NaJD-ZrOj-SiCu-P-CV sys-tem by a low sys-temperature chemical polymerization from metal or non metal al-koxide hydrolysis. We present here, for two typical compositions, structural results from a small angle X-ray scattering study (S.A.X.S.), concerning sol/ gel, gel/xerogel and xerogel/glass transitions. Gels can exhibit inhomogenei-ties, probably due to cubic zirconia clusters, which progressively disappear during the gel/glass conversion.

I - INTRODUCTION

Multicomponent oxides (glasses, glass-ceramics or crystalline substances) can be pre-pared using the alkoxides of various elements (M(OR) , R = alkyl group) by the sol gel technique. It is customary to divide the process into three stages /1,3/ : i) The complex formation of several metal or non metal alkoxides with bonding of the glass constituents to each other (alkoxides are mixed into a transparent solu-tion) .

ii) Partial hydrolysis (by exposure to air or by addition of an alcoholic solution of water) of the OR substituents into OH and condensation reactions which build three dimensionally cross-linked polymers

RO ^ ^ OR RO N ^ OR RO - M - OH + HO - M' - OR •* R O - M - O - M ' - O R + H.O

RO -OR RO ^ N OR

iii) Polycondensation with removal of the OR and OH substituents. Conversion from gel to glass by thermal treatment below Tg.

The chemical polymerization technique has the advantage over the conventional melting method of supplying glasses at low temperature, with high homogeneity and purity. Consequently, high temperature reactions such as crystallization, phase separation.., which restrict glass formation in certain systems can be largely avoided. For ins-tance, we have recently prepared /4,5/, by the sol-gel precursor technique, new non-crystalline solids, outside the usual range in the Na20-Zr02-Si02-P20,- system. Some

of these amorphous compounds exhibit high ionic conductivity similar to the one of well known NaSiCON type crystalline compounds and are probably well adapted to be used in low power electrochemical devices such as sensors, microbatteries, displays or capacitors.

Fpr multicomponent oxides, it's generally admitted that the sol-gel process leads to a great homogeneity /1,3/. This is rather unexpected, because alkoxides exhibit very different rate of hydrolysis. For instance, the rate of reaction is much slower for the tetraethylorthosilicate (THOS) than for alkoxides of Zr and Na. Therefore, one

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C8-456

JOURNAL DE PHYSIQUE

can expect that self p l p r i z i n g s , ~ c i e s are i n i t i a l l y fonned from a f a s t hydroly- sinq alkoxide, inplying inhomqeneities i n the gel.

We r e p r t here structural investigation by mall angle X-ray scattering (S.A.X.S.) of the gelling process and of the gel/glass transition for two typical coqositions i n the Na 0-Z-f12-SiO -P205 system : a low alkaline composition (1/4 Na O-ZIO~-P~F)~

-

4 SiO

f

and a hig& one (Na20-2 Z10 -P 0 -4 SiO )

,

here-after respc&vely desslgnated

2

sample 1 and sample 2. We principa~ly2s~ow tha? inhomgeneities can appear i n gels, with probable formation of cubic zirconia clusters, and can *st conpletely disap-

pear during the gel/glass conversion.

I1

-

EXPEzmENTAL

2.1. Sample preparation

Raw m t e r i a l s , Z r (C3H70) 4 , S i (C H 0) 4 , PO (C H 0) PJa (C4H90)

,

are mixed i n dry a t m s phere. Propanol was used for d i f u h g the 1n&?ur2~which i s s t i r r e d several hours a t 40°C. The ageing of the solution was carried out i n a cylindrical glass container, a t room temperature, and insmatnosphere of 50 % relative humidity. During the course of hydrolysis and gelation, aliquot5 of solution were extracted and cooled to quench the reaction. The uniform mixture gelatinizes and becomes viscous within a few t o several

ten of hours.

The ageing of the gel was carried out by exposure t o a i r and the plycondensation substep leads t o a contraction which proceeds with the vaporization of the alcohol u n t i l the qel is completely dry and solid.

After drying (60°C), the xerogel was converted to glass by heat t r e a t m a t i n a i r . During t h i s conversion, ?,eight loss is observed due to the remval of organic matter and water.

For the sample 1, near the gel p i n t (gelation time : 15 days), 29 Si N M R study shows the presence of an i n t e r s t i t i a l liquid phase, consisting i n a distribution of unhydrolyzed oligomsric (Si-O-Si) s z c i e s ( i - e . without bridging oxygen between dissimi-

l a r constituents, such a s Zr-0-Si o r P-0-Si bondings). On the contrary, no liquid phase has

been

displayed, by NMR, in the sample 2 which moreover exhibits a shorter gelation tin^ (2 days)

.

2.2. SAXS Instrumatation

The scattering data a r e obtained with a s l i t

type

small angle c m r a ; the mnochro- m t i c CU-K a radiation i s selected through a quartz mnochromtor i n the incident beam. The d e b c t o r is a p s i t i o n sensitive proportional counter with an effective length of 55 mn.

ihen the s p e c k is a thin solid sample of xerogel o r glass, the s q l e t o detector distance is 287.5 nnn. Since

the

spcirnen is a liquid o r a gel v e q sensitive to va-

cuum, a sample c e l l is used with 0.025

m thick "!5ylar1' windows and 0 -5 t o 1

m

path length. In sorre cases, a heating sample holder operating up t o 900°C, directly allows t o observe qualitative msdifications of the scattering pattern with temperature, thanks t o the rather short -sure tim needed by the position sensitive detector.

(187.5

m

saqle to detector distance)

.

The experimental r e s u l t s are corrected for parasitic scattering and n o ~ i z e d to

equivalent sample thickness and incident beam intensity. The cross section of the

x ray beam m y be considered a s "linear and infinite".

I11

-

WSULTS

3.1. L m alkaline composition (sample 1)

Fig.. l a shows the evolution of the scattering curve during the gel processing. One

can note t h a t near the gel p i n t , the scattered intensity does not decrease continu- ously and that, a f t e r a few days, curves exhibit a well pronounced maximum. With increasing t i ~ ~ , the maxirmrm occurs a t larger angles (H = 4

II

sin @/A). The analysis of

the intermediate angle region (fig. lb) displays the cross-over from a p l y m r i c state with randomly branched chains (slope P = -1.5 near the gel pint) to a s t a t e for-

med by particles with w11 defined boundaries (slope P = -3) /6,7/.

Fig. 2 shows the evolution of the scattering curves during the thermal trea-t of the gel. Between 20 and 300°C, the scattered intensity decreases i n the lower angle

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decreases corresponding t o the progressive disamaring of the scattering particles.

i

I (RRB.UNIT)

I

5000

f

2

SAMPLE

-

1

- " 1 DAY-ROOM TEMPERATURE-

'

1 0 DAYS 30 DRYS 0 fi

H

(nm-1)

,,,,

..

1 0 D k Y S ( R , T . ) P = - 1 . 5

.

30 DAYS (R.T.) P=-2.5 S O D A Y S ( R . T . ) P = - 3 . 1 Fig. l a Fia. lb

F&g. 1 Gelling process : ageing in ambient atmosphere after the gel point. a) Small angle X-ray scattering curves versus tire.

b) Porod plot in the int-iate r e g h where power law behaviour is

expected.

Fig. 2 Gel to glass conversion. S.A.X.S. curves a t different temperatures. 3.2. High alkaline corrrposition (sz'mle 2)

Fig. 3 shms the evolution of the scattering curves during the heatin9 of the gel. For tkis sample, a t a l l tempratures,scattering decreases continuousl~r from a paxi-

nnnn

a t zero angle and the low angle Guinier region leads to a radius of gyration of 36

z.

For the xerogel, a t R.T., power law analysis gives a slope of -2 (instead of -3 for the xerogel l), consistant w i t h the formation of rather w ~ A l y cross-linked po1:mric mcromlecules. Between 20 and 300°C, the scattered intensity increases and we ober- ve the crossover t o a state with w e l l defined dense particles (slope P = -3) .Above

300°C, scattering decreases while the P o d slope, , d sconstant.

IV

-

DISCUSSION

The behaviour of the sample 1 can be explained either i n terms of interparticle in- terferences, resulting from very strong electrostatic interactions between particles or in t e r n of indepndent particles with a complex internal structure, i-e regions of different electronic density inside each particle /8,9/.

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C8-45

8

J O U R N A L

DE

PHYSIQUE

Big. 3a Fig. 3b

Fig. 3 Gel t o $ass conversion (high alkaline composition) a ) S.A.X.S. curves a t different t e q e r a t u r e s

b) corresponding Log-Log plot i n the i n t e m d i a t e reg&

can't be c o q l e t e l y dismissed, does not drastically change the physical interpreta- tion concerning the evolving gel structure.

The structural a d v a n c e n t can be summarized a s follows :

i) aelation : f o m t i o n of the network and of scattering particles.

In the liquid phase, i . e . before the gel pint, 29S6 N M R and power law analysis of S.A.X.S. curve (slope P = -1.5) show the existence of a distribution of weakly branched (SiQ-Si) oligomers (Fig. 4a). Subsequent hydrolysis steps lead to a weakly ramified p o l w r i c structure and, a t the gel point, the netmrk is l e s s crosslinked

than i n a corresponding m l t e d s i l i c a glass.

mreover, near the gel point, a liquid phase r a i n s in the interstices of the s i l i c a network structure (Fig. 4b). This phase contains unhydrolyzed (Si-O-Si) o l i g m s , a s shown by 29s NMR and hydrolysis products of f a s t hydrolyzing alkoxides ( Z r , Na). Therefore, aktive p o l p r i z i n g species, such a s Z r (OH) (OR)q-x can give by self mlymrization, -1 particles of cubic zirconia /lo/

7 ~ x 0 ~

gr6ups have been pre- viously detected by IR snectroscopy i n gels of the SO2-Zr02 system /11/).

Fig. 4

Gelling process in sample 1 a ) Liquia phase with weaklv branched SiQ-Si o l i q m r s . b) Polymric structure with i n t e r s t i t i a l liquid phase.

C ) Isolated q l e x particles with zirconia clusters surrounded by a depleted shell.

Assuming

that

zirconia clusters contribute in a larae part t o the scattered intensity, the increasing of this and the s h i f t of the maximum tmards higher angles, durinq the gelification, can be explained by the g r a d of complex scattering particles. Each particle i s constituted by a spherical nucleus of high electronic density (zirconia clusters where surface oxyqens are associated with protons and al?ql groups), surrounded by a depleted shell (H20, alcohol and r m i f i e d SiQ-Si o l i g o ~ r s ) of den- s i t y t h a t is l e s s than the average electronic density ofrthe structure "frozen in" a t

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ii) Conversion from gel to glass.

Between 20 and 300°C, weight l o s s is mainllr related to "OR" liberation and, i n the ramified Si-O-Si structure, continued cross-linking occurs, resulting from condensation reactions. As shown i n figure 2, the progressive densification of the gel i s associated with a M f i c a t i o n i n the low angle part of the S.A.X.S. curve and with the s h i f t of the IMxirmrm towards higher H. Probably, these changes only correspond t o the decreasing s i z e of the low electronic density areas by r m v a l of the v o l a t i l s and by condensation between Si-O-Si oligomrs.

Between 300 and 700°C, s m l l e r weight losses, related to "OH" liberation, occur throughout the heat t r e a m t and the gel evolves towards f u l l y cross-linked glass via condensation reactions. The decreasing of the scattered intensity d m n s t r a t e s t h a t conplex particles a r e progressively dissolved in the s i l i c a network. This im- p r t a n t interaction between zirconia clusters and the network is in a g r e e r a t with a recent study of s i l i c a gels which has shown t h a t the network structure resulting from gelation o r from dehydration is considerably different from that of a conven- tionally melted glass. It contains greater free volme and has enhanced atomic mbi- l i t y /12/.

Concerning sample 2, X-ray scattering curve from xerogel, and power law analysis (slope P = -2) suggest t h a t zirconium atoms are homogeneously distributed i n the gel. In f a c t , for t h i s high alkaline composition, the f a s t hydrolyzing sodium alkoxide leads t o a high OH concentration which probably active the slow hydrolyzing P and S i alkoxides ( i n agreerent with the short gelation ti^^ for t h i s sample). Active w l p r i z i n g species, such a s S i (OR)

then

reacted with unaltered f a s t hydrolyzing Z r alkoxide, and dissinil& constituents tend to becom neighburs, i .e.

M -0 bonds rather than M -O-Pfl, and thus greater homgeneity is attained. By hea- t ? n g x e scattered i n t n 9 i k y decreases, correspzxkiing t o the crossover towards a dense homgeneous amorphous s t a t e .

v

-

CONCLUSION

The r e s u l t s of our experiments shm t h a t the f o m t i o n of multicomponent oxide gels is generally not straightforward, because alkoxides exhibit different r a t e s of hydrolysis. Therefore, s e l f - p l y m r i z a t i o n of active polymerizing species, can lead t o

inhomgeneities i n gels. Nevertheless, inhomgeneities can be progressively dissolved during the conversion from gel to glass, a s a consequence of the high reactivity of the ramified s i l i c a structure which i s m r e open than i n a mlt-prepared glass, and thus m y be described by an effective high f i c t i v e temperature.

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(1979) 1843. /3/ Mukherjee, S.P., J. Non-Cryst. Solids

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(19831639 /5/ Boilot, J.P., Colorban, Ph., J. .Flat. Sci. Lett.

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/6/ Porod, G., Kolloid 2 .

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/7/ Brinker, C . J . , Keefer, K.D., Schaefer, D.W., Assink, R.A., Kay, B.D. and Aschley, C.S., J. bJon-Cryst. Solids

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(1984) 45.

/8/ Guinier, A., Foumet, G., Small Anqle Scattering of X ravs, John Wiley (1955). /9/ Glatter, O., Kratky, O., Small Angle X ray Scattering, Academic Press (1982).

/lo/

Fryer, J . R . , Hutchinson, J.L., and Paterson, R., J. Colloid Interface Sci.

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/11/ Nogami M., J. of Non-Cxyst. Solids

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