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AEROGEL IN THE SYSTEM Nb-Si
A. Sivade, D. Bourret, J. Bouaziz, R. Sempere
To cite this version:
A. Sivade, D. Bourret, J. Bouaziz, R. Sempere. AEROGEL IN THE SYSTEM Nb-Si. Journal de
Physique Colloques, 1989, 50 (C4), pp.C4-79-C4-84. �10.1051/jphyscol:1989413�. �jpa-00229488�
R E W E DE PHYSIQUE APPLIQUBE
Colloque
C4,Supplement au n04, Tome
24,Avril
1989AEROGEL I N THE SYSTEM N b - S i
A. SIVADE, D. BOURRET,
3 .BOUAZIZ and
R.SEMPERE
Laboratoire de Science des Materiaux Vitreux,
USTL,Place
E.Bataillon, Fa34060 Montpellier Cedex
2,France
R6sum6 - Des aerogels mixtes ont Ote obtenus dans le systeme Nb,O,-SiO, pour un rapport molaire Nb/Si allant de
1 A50%. La synthgse est realisbe
&partir de solutions de NbC1, dans le methanol et de TMOS pr6hydrolys6. L'hydrolyse est conduite avec une quantite stoechiometrique de H20 (pH
7).L'influence de divers parametres sur la tranparence et la monolithicite des aerogels a Bt6 etudi6e. Par s6cQpge hypercritique on obtient des aerogels monolithiques bleus
(Nb
) .Une etude particuliere menee dans le cas d'un rapport molaire
de
10%a permis de montrer
1' influence determinante de la proportion initiale de m6thanol' sur 1 'apparition d'une phase Nb206 cristall isee sous la forme "haute pression".
Abstract - The synthesis of mixed aerogels in the system Nb205-Si02 is
- . -
presented. Aerogels have been obtained ranging from
1to 50% for the molar ratio Nb/Si: The synthesis is realized from NbC15 in CH30H and prehydrolysed TMOS solution, followed by hydrolysis with a stoechiometric amount of H,O. The influence of various parameters on
L
the transparency and monolithicity of the aerogels,+ is studied.
Supercri tical drying in autoclave conducts to blue (Nb
)mono1 ithic aerogels. In the case of 10% molar ratio Nb/Si we report the drastic influence of the ROH initial dilution on the crystallization of a Nb205 "high pressure" phase.
1
- INTRODUCTION
Gels can be obtained from organometallic precursors, by hydrolysis and polycondensation of metal alkoxides. Multicomponent systems can be mixed leading to new mixed materials or glasses. However due to rarity and difficulties in utilization of some metal alkoxide, Nb(OR), for example, this process is limited to a small number of oxide, namely SiO,, TiO, and ZrO,.
Recently Alquier et al. /I/ have proposed the synthesis of Nb20, gels from chloroalkoxides NbCl,(OR),. Thus, this paper deals with the synthesis of SiO, - Nb206 aerogels and the study of the 10% molar ratio Nb
/Si sample using
Xrays analysis, D. T. A. and di latation experiments. Conclusions about mixed Nb206 - SiO, glasses elaboration are drawn.
2
- EXPERIMENTAL
2-1
-
Nb205 ge 7 e l a b o r a t i o nNiobium pentachloride (NbCl,) and methyl or isopropyl alcohols were used as starting materials. Some authors /I-,/ have already studied the reaction between NbC1, and alcohols. This reaction .leads to niobium chloroalkoxides:
NbC1, +
3ROH --> N~c~,(oR), +
3H C I ~
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989413
I t appears t o be v e r y e x o t h e r m i c and HC1 gas i s e v o l v e d . As p r e v i o u s l y shown /I/ t h e b e h a v i o u r o f n i o b i u m c h l o r o a l k o x i d e h y d r o l y s i s depends on t h e a l c o h o l used: w i t h s t o e c h i o m e t r i c c o n d i t i o n s ( 1 Nb / 5 H,O), g e l was o b t a i n e d o n l y w i t h i s o p r o p y l a l c o h o l . N e v e r t h e l e s s , n i o b i u m c h l o r o a l k o x i d e h y d r o l y s i s w i t h water added i n excess l e a d s t o a g e l whatever t h e a l c o h o l used. B u t t h e moment a t which s o l - g e l t r a n s i t i o n i s performed i s depending on t h e k i n d o f a l c o h o l used / a / . For example, i t i s i n s t a n t a n e o u s w i t h i s o p r o p y l a l c o h o l , r a t h e r s l o w w i t h e t h y l a l c o h o l and i t w i l l t a k e many days w i t h methyl a l c o h o l .
The s t u d y o f SiO,
-
Nb,O, g e l s depends upon t h e process, t h e t e m p e r a t u r e and t h e c o n c e n t r a t i o n o f v a r i o u s c h e m i c a l s used.I n o r d e r t o o b t a i n n i o b i u m c h l o r o a l k o x i d e , NbC1, was d i s s o l v e d i n V ' + V, c c o f m e t h y l a l c o h o l ( V ' i s t h e r e a g e n t f o r t o t a l n i o b i u m a l k o x i d e e l a b o r a t i o n , V, i s added as s o l v e n t ) . The s o l u t i o n i s , then, added t , V, c c o f t e t r a m e t h o x y s i l a n e (TMOS) d i s s o l v e d i n V, cc o f m e t h y l a l c o h o l . H y d r o l y s i s i n s t o e c h i o m e t r i c c o n d i t i o n s o f t h e s o l u t i o n o b t a i n e d a f t e r s t i r r i n g was, t h e n , performed by a d d i n g water drop by d r o p w h i l e s t i r r i n g . The r e s u l t i n g s o l u t i o n ( S o l ) was poured i n t o c l o s e d c o n t a i n e r s and m a i n t a i n e d a t a c o n s t a n t t e m p e r a t u r e o f 50°C u n t i l g e l f o r m a t i o n . G e l s were aged a t t h e same t e m p e r a t u r e f o r , a t l e a s t , a week. Coding was used f o r g e l s and a e r o g e l s u s i n g t h e v a l u e 100.V2/(2Vl+V,)=x w i t h 0 < x < 1 0 0 f o r s o l v e n t e f f e c t and mol% o f S i / molX o f Nb f o r c o m p o s i t i o n .
I n a t y p i c a l e x p e r i m e n t , 0.259 o f NbC1, was d i s s o l v e d i n 0.6ml ( V , = 0.41 o f m e t h y l a l c o h o l i n p y r e x beaker and s t i r r e d d u r i n g 15 min. t o a l l o w t h e r e l e a s e o f most o f t h e HC1. T h i s s o l u t i o n was added t o a s t i r r e d s o l u t i o n made by m i x i n g 1.23ml (V2) o f TMOS and 0.41ml ( V i ) o f m e t h y l a l c o h o l . H y d r o l y s i s i s t h e n p e r f o r m e d by a d d i n g 0.683m1 o f water d r o p by d r o p w h i l e s t i r r i n g . These m a n i p u l a t i o n s l e a d t o a A60S, 90/10 Si/Nb g e l ( x
=
6 0 ) .The parameters used i n t h i s e x p e r i m e n t were p r o g r e s s i v e 1 y m o d i f i e d , and t h e r e s u l t s o b t a i n e d a r e g i v e n on t a b l e 1.
Tab1 e qua1 i t y o f g e l s o b t a i n e d as
a
f u n c t i o n o f t h e p a r a m e t e r s used i n e x ~ e r i m e n t (amount-& s o l v e n t and Nb c o n c e n t r a t i o n ) .c o d i n g A60S
A50S
A40S
A30S
A20S
90/10 i n s t a n t a n e o u s g e l a t i o n ( f e w seconds)
f a s t g e l a t i o n ( 5 mn) r a t h e r f a s t g e l a t i o n
(30 mn)
tenuous g e l h i g h f l u i d i t y (30 mn)
v e r y h i g h f l u i d i t y l i g h t w h i t e p r e c i p i t a t e ( > 2 h o u r s )
25/75 homogeneous g e l a t i o n a f t e r one week
homogeneous g e l a t i o n a f t e r 2 weeks v e r y slow g e l a t i o n ( a b o u t 1 month)
no g e l a t i o n
no g e l a t i o n 75/25
i n s t a n t a n e o u s g e l a t i o n w i t h b u b b l e s
i n g e l same as A60S
homogeneous g e l a t i o n a f t e r 5mn
homogeneous g e l a t i o n a f t e r 30 mn homogeneous g e l a t i o n a f t e r 4 h o u r s
50/50 i n s t a n t a n e o u s g e l a t i o n
-
non homogeneous-
homogenization a f t e r few days same as A60S
i n s t a n t a n e o u s p a r t i a1 g e l a t i o n d i s s o l u t i o n a f t e r 1 day;
homogeneous g e l a t i o n a f t e r 2 days g e l a t i o n a f t e r 5 days
-
y e l l o w g e l no g e l a t i o n
We can see t h a t :
-
f o r 90/10 g e l , t h e p e r i o d o f t i m e needed t o o b t a i n g e l a t i o n i n c r e a s e as t h e c o n c e n t r a t i o n o f a l c o h o l i n c r e a s e s ;-
f o r 50/50 and 25/75 g e l s , t h e t r a n s p a r e n c y i n c r e a s e s w i t h t h e a l c o h o l c o n c e n t r a t i o n ;- f o r A40S, 50/50 g e l , j u s t a f t e r water a d d i t i o n a g e l i s o b t a i n e d i n a l i q u i d phase. A few h o u r s l a t e r , t h e g e l d i s s o l v e s and t h e s o l u t i o n becomes homogeneous. A f t e r a p e r i o d o f 2 days, t h e r e i s a t o t a l and homogeneous g e l a t i o n . T h i s phenomenon can be reproduced i f t h e same e x p e r i m e n t a l c o n d i t i o n s a r e used;
- f o r a 90/10 a t 50"C, m i x t u r e s o b t a i n e d w i t h c o n c e n t r a t i o n s between A60S and A40S l e a d t o g e l s w i t h o u t any c r a c k s a f t e r a g i n g .
As t h e h y d r o l y s i s o f n i o b i u m c h l o r o a l k o x i d e i s f a s t e r t h a n TMOS one, p a r t i a l h y d r o l y s i s ( t mol.H,O/ 1 mol .TMOS i n a l c o h o l ) under s t i r r i n g and r e f l u x f o r 30 mn was performed. T h i s method g i v e s homogeneous, c l e a r and b l u e g e l s . Thus p a r t i a l h y d r o l y s i s o f TMOS makes t h e f o r m a t i o n o f S i - 0 - N b bonds e a s i e r .
2 - 3
-
Si02-Nb205 aeroge 1s e l a b o r a t i o nS u p e r c r i t i c a l d r y i n g i n an a u t o c l a v e was used i n o r d e r t o o b t a i n a e r o g e l s . I n f a c t m o n o l i t h i c a e r o g e l s were o b t a i n e d f r o m SiO, t o 50/50 Si/Nb c o n c e n t r a t i o n s . Thus o n l y t h e b l u e ( ~ b ~ * , 90/10 a e r o g e l was s t u d i e d i n t h e n e x t p a r t o f t h i s paper.
3- RESULTS AND DISCUSSION
X r a y s a n a l y s i s shows a b e g i n n i n g o f c r y s t a l l i z a t i o n ( s e e f,ig 1 ) f o r A20S and A30S samples. T h i s c r y s t a l l i z a t i o n o f Nb,O, " h i g h p r e s s u r e "
s t r u c t u r e i s c o n f i r m e d by X r a y s a n a l y s i s o f samples,,used f o r d i l a t o m e t r y ( f i g . 2 ) . The c r y s t a l l i z a t i o n o f Nb,O, " h i g h p r e s s u r e phase i s due t o t h e s u p e r c r i t i c a l d r y i n g . E f f e c t i v e 1 y X r a y s a n a l y s i s o f a 90/10, A40S x e r o g e l a f t e r an i s o t h e r m a l t r e a t m e n t a t 1000'C d u r i n g 24 h o u r s e x h i b i t s a c r y s t l l i z a t i o n o f t h e " h i g h t e m p e r a t u r e " Nb,O, phase.
, .
N
N U,
Intensity
1
m N v. LDU3 C) U3 e
- i
F i g u r e & rays a n a l y s e o f 90/10 a e r o g e l s (A60S A20S). arrows
&rs for
" h m p r e s s u r e " c r y s t a l l i z e d Nb,O, peaks.
-
F i g u r e 4 shows D . T . A f o r a A40S 90/10 sample. The f i r s t peak i s due t o water r e l e a s e f r o m t h e l a t t i c e . Exothermic e f f e c t s observed f o r s i l i c a a e r o g e l s 0-R groups o x i d i z i n g a r e n o t observed f o r Si-Nb a e r o g e l . T h i s r e s u l t i s i n good agreement w i t h Sanchez one / 2 / , e f f e c t i v e l y t h e y have shown by chemical a n a l y s i s t h a t c h l o r i n e s t i l l remain ( 1 C1 f o r 3 Nb) a f t e r o x ~ d a t i o n o f a l l 0-R groups.
I s o t h e r m a l d a t a (1010 " C f o r A40S i n f i g . 6 ) can be d e s c r i b e d b y t h e r e l a t i o n s h i p :
A 8
=
K . t nwhere A.e/P, i s t h e r e l a t i v e s h r i n k a g e , t t h e t i m e i n minutes, K i s a c o n s t a n t which depends on t h e temperature and t h e n a t u r e o f m a t e r i a l , n i s a c o e f f i c i e n t r e l a t e d t o t h e s i n t e r i n g mechanism. For a v i s c o u s f l o w mechanism n = 1; whereas f o r a d i f f u s i o n a l process n i s between 0.3 and 0.5 /a-6/. Here t h e s l o p e i s 0.8, t h u s we can conclude t o mixed v i s c o u s f l o w and d i f f u s i o n mechanisms f o r s i n t e r i n g o f 90/10 Si/Nb samples.
Intensity N
-
& LDi
CO NI/ I
rw m .1 I
I i
II
".+I
i\
4-11
I -'-'---A---d8 \---
/ i i i A
---*-a,
F i g u r e &
X r a ~ s
a n a l y s i sof
90/10 samples a f t e r d i l a t o m e t r y s t u d i e s ( t e m p e r a t u r e r i s e a t 3 "C/mn, 30 mn a t 1000 "C and decayat 10
Arrows a r e f o r " h i g h p r e s s u r e " c r y s t a l l i z e d Nb,O, peaks.- -
F i a u r e 2 X r a y s a n a l y s i s o f 90/10 A40S x e r o a e l a f t e r i s o t h e r m a l t r e a t m e n t .
Endothermic effect
Exothermic effect
Temperature ( O C )
Figure 4: D.T.A. f o r SiO, and 90/10 A40S ( 2 ) samoles.
0 0 2 0 4 0.6 0 8 1
Temperature
("C
x 103)Figure S a m ~ l e s l e n g t h e v o l u t i o n ( A t / t , , )
vs.
temperature.I n t h i s paper, synthesis Nb,O,-SiO, b i n a r y g e l s have been reported. The use o f niobium p e n t a c h l o r i d e and a l c o h o l leads t o c h l o r o a l k o x i d e s which can r e a c t w i t h p a r t i a l l y hydrolyzed TMOS before h y d r o l y s i s .
Aerogels obtained by s u p e r c r i t i c a l d r y i n g i n an autoclave a r e p a r t i a l 1 y c r y s t a l l i z e d . Nevertheless A60S sample remains amorphous a f t e r isothermal treatment. Thus glasses may be c e r t a i n l y prepared w i t h o u t c r y s t a l l i z a t i o n when s t a r t i n g from A60S aerogels.
REVUE DE PHYSIQUE APPLIQUEE
F i g u r e & ~ n ( A t / t ~ ) a g a i n s t L n ( t ) f o r A40S 90/10 ~ i / N b sample
'C.
The a u t h o r s would l i k e t o t h a n k D r J. P h a l l i p o u as w e l l as Dr T . Woignier f o r v a l u a b l e d i s c u s s i o n s about t h i s work.
REFERENCES
/ I / - A l q u i e r C . A . , Vandenborre N.T. and Henry M . , J. Non-Cryst. S o l i d s 79
(1986) 383.
/ 2 / - Sanchez C . , E c o l e d ' B t 6 CNRS, (Bombannes), 28 sept.-2 o c t ( 1 9 8 7 ) .
/ 3 / - S o l o v ' e v S . I . and K r y l o v E . I . , Russ. J. I n o r g . Chem. 3 (1958) 2487.
/ 4 / - F r e n k e l J . , J. Phys. USSR
9
( 1 9 4 5 ) 385./6/- Johnson D.L. and C u t l e r I . B . , J. Am. Ceram. Soc.
