FARADAY EFFECT IN DOPED AEROGELS

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FARADAY EFFECT IN DOPED AEROGELS

R. Sempere, D. Bourret, J. Bouaziz, A. Sivade

To cite this version:

R. Sempere, D. Bourret, J. Bouaziz, A. Sivade. FARADAY EFFECT IN DOPED AEROGELS.

Journal de Physique Colloques, 1989, 50 (C4), pp.C4-227-C4-232. �10.1051/jphyscol:1989437�. �jpa-

00229520�

REWE DE PHYSIQUE APPLIQU~E

C o l l o q u e C4, Suppl6ment au n 0 4 , Tome 24, a v r i l 1989

FARADAY EFFECT IN DOPED AEROGELS

R. SEMPERE, D. BOURRET, J. BOUAZIZ and A. SIVADE

L a b o r a t o i r e d e S c i e n c e des M a t e r i a u x V i t r e u x , USTL, P l a c e E . B a t a i l l o n , F-34060 M o n t p e l l i e r Cedex 2, F r a n c e

ROsum6

-

Des a e r o g e l s b i n a i r e s de t e r r e s r a r e s e t de s i 1 ic e o n t 6 t 6 p r e p a r e s p a r l a m6thode s o l - g e l &I p a r t i r des n i t r a t e s de dysprosium ou d ' e r b i u m e t de t B t r a m 6 t h o x y s i l a n e (TMOS). Des a e r o g e l s avec des c o n c e n t r a t i o n s 4 l e v 6 e s en 1 a n t h a n i des ( 14% mol a i r e ) o n t 6 t 6 obtenus.

Les c o n s t a n t e s de Verdet de ces a e r o g e l s o n t B t 6 determinees.

A b s t r a c t

-

R a r e - e a r t h s i l i c a t e a e r o g e l s have been o b t a i n e d by t h e s o l - g e l method s t a r t i n g w i t h Dy o r E r n i t r a t e s and TMOS. The a e r o g e l s h a v i n g h i g h r a r e - e a r t h c o m p o s i t i o n ( 1 4 mol.%) were o b t a i n e d . Verdet c o n s t a n t s o f t h e s e a e r o g e l s have been determined.

1

-

INTRODUCTION

B i n a r y s i l i c a r a r e - e a r t h g l a s s e s a r e o f g r e a t i n t e r e s t when m a t e r i a l w i t h l a r g e V e r d e t c o n s t a n t s o r l a s i n g a b i l i t y a r e r e q u i r e d . E f f e c t i v e l y , r a r e - e a r t h s i 1 i c a g l a s s e s a r e needed t o manufacture d e v i c e s such as i s o l a t o r s , c i r c u l a t o r s / I / , magnetic f i e l d s e n s o r s / 2 / , f a s t o p t i c a l s w i t c h e s and m o d u l a t o r s / 3 / .

R a r e - e a r t h o x i d e s and SiO, have v e r y h i g h m e l t i n g p o i n t s , t h u s , the p r e p a r a r a t i o n o f l a n t h a n i d e s i l i c a t e g l a s s e s f r o m m e l t r e q u i r e s t e m p e r a t u r e s up t o 1800°C. T h i s method i s a l s o l i m i t e d by phase s e p a r a t i o n phenomena which u s u a l l y o c c u r s i n SiO,

-

^{Ln,O, systems.} W i t h t h i s r e s p e c t u l t r a q u e n c h t e c h n i q u e s a r e necessary t o keep t h e g l a s s homogeneous and t h e s e t e c h n i q u e s r e a l l y a p p l y o n l y on a few systems. Moreover, p l a t i n i u m i n c l u s i o n s coming f r o m t h e c r u c i b l e d i s s o l u t i o n a r e always p r e s e n t i n such g l a s s e s / 4 / . These e x p e r i m e n t a l d i f f i c u l t i e s o f t e n l e a d t o poor o p t i c a l q u a l i t y g l a s s e s / 6 / .

From t h e s o l - g e l method, p u r e and homogeneous b i n a r y r a r e - e a r t h g l a s s e s can be p r e p a r e d a t lower t e m p e r a t u r e s t h a n r e q u i r e d by c o n v e n t i o n a l m e l t quench t e c h n i q u e s . R e c e n t l y K . Sun e t a l . / 6 / d e s c r i b e d t h e p r e p a r a t i o n o f r a r e - e a r t h s i l i c a x e r o g e l s and g l a s s e s f r o m r a r e - e a r t h c a r b o n a t e s and t e t r a e t h o x y s i l a n e (TEOS). N e v e r t h e l e s s g l a s s e s w i t h b e t t e r homogeneity and good o p t i c a l p r o p e r t i e s can be p r e p a r e d f r o m a e r o g e l way. E f f e c t i v e l y u s u a l s o l - g e l methods use powder which must be h o t - p r e s s e d o r m e l t e d .

I n o t h e r hand a b e t t e r m o n o l i t h i c i t y and c l a s s i c a l i s o t h e r m a l s i n t e r i n g i s o b t a i n e d w i t h a e r o g e l s . Now, t h e b i n a r y r a r e - e a r t h a e r o g e l s p r e p a r a t i o n has n o t y e t been r e p o r t e d . As s i l i c a based r a r e - e a r t h a e r o g e l s can a l s o be d i r e c t l y used, we r e p o r t i n t h i s paper t h e i r p r e p a r a t i o n s t a r t i n g f r o m r a r e - e a r t h n i t r a t e s and t e t r a m e t h o x y s i l a n e (TMOS). A e r o g e l s containYng dysprosium and erbium up t o 14 mol.% have been p r e p a r e d and analysed.

2

-

EXPERIMENTAL

Tetramethoxysi l a n e S i (OCH, ), and r a r e - e a r t h n i t r a t e s ~ n ( N 0 , ), .5H,O w i t h Ln = Dy o r E r were used as s t a r t i n g m a t e r i a l s . The amount o f w a t e r , i n c l u d i n g h y d r a t i o n water o f r a r e - e a r t h n i t r a t e s , was a d j u s t e d i n o r d e r t o p e r f o r m h y d r o l y s i s o f TMOS under s t o e c h i o m e t r i c c o n d i t i o n s . I s o p r o p y l a l c o h o l was used t o d i l u t e b o t h TMOS and r a r e - e a r t h n i t r a t e s .

TMOS and r a r e - e a r t h n i t r a t e s were d i s s o l v e d i n t h e same volume o f i s o p r o p y l a l c o h o l ( 1 v o l . TMOS i n 1 v o l . o f i s o p r o p y l a l c o h o l and r a r e - e a r t h n i t r a t e i n 1 v o l . o f i s o p r o p y l a l c o h o l ) .

F i r s t , t h e TMOS s o l u t i o n was p a r t i a l l y h y d r o l y z e d w i t h a water amount c o r r e s p o n d i n g t o 1 mole p e r TMOS m o l e c u l e . The water ( 0 . 2 M HNO, ) was added

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989437

C4-228 REVUE DE PHYSIQUE APPLIQUEE

dropwise t o t h e TMOS s o l u t i o n . The s o l u t i o n was, then, s t i r r e d under r e f l u x f o r 15 minutes.

The l a n t h a n i de n i t r a t e pentahydrate s o l u t i o n was a l s o s t i r r e d under r e f l u x f o r 1 hour and then poured i n t o t h e p a r t i a l l y hydrolyzed TMOS s o l u t i o n .

Refluxes were c a r r i e d on f o r 30 min. before and a f t e r a c i d i c h y d r o l y s i s w i t h a water amount corresponding t o 3 moles per TMOS molecule (here we t a k e t h e h y d r a t i o n water o f lanthanide n i t r a t e i n t o account). The r e s u l t i n g s o l was then poured i n t o c o n t a i n e r s which a r e then closed and kept a t 5 0 ' C u n t i l g e l a t i o n . Alcogels were aged a t the same temperature f o r a t l e a s t a week.

S u p e r c r i t i c a l d r y i n g i n an autoclave was then conducted i n order t o o b t a i n aerogels.

3

-

^RESULTS

As shown on t a b l e 1 t h e lanthanide content i s lower than expected from t h e s t a r t i n g composition. Figure 1 shows t h e y i e l d o f t h e process: i t i s r a t h e r good f o r erbium and f o r dysprosium a t a l l concentrations.

Table 1: Lanthanide concentrations (mol.% Ln20,) i n s o l and i n aerogels.

Concentration

Figure 7 : Y i e l d (aerogel mol.%

L

s o l .mol

.%l

o f t h e process ( Er20, ;+ Dy20,) a g a i n s t r a r e - e a r t h oxide concentration mol.% ( Ln20,).

Aerogels c , d, e f o r b o t h l a n t h a n i d e s a r e m o n o l i t h i c , b u t a and b a r e broken i n s e v e r a l p i e c e s . I t can be seen f r o m X r a y s p a t t e r n s t h a t t h e samples were always amorphous. On t h e o t h e r hand a l i t t l e powder was found i n each c o n t a i n e r . T h i s powder l i e s on t h e w a l l a l o n g t h e aerogel and on t h e bottom o f t h e c o n t a i n e r . Chemical a n a l y s i s o f powders shows t h a t c o n c e n t r a t i o n s i n r a r e - e a r t h a r e always h i g h e r t h a n s i l i c a ones. F i n a l l y , i n a l l cases, X r a y s p a t t e r n s e x h i b i t c r y s t a l l i z a t i o n o f Er20, and Dy,03 (see f i g u r e 2 ) . These r e s u l t s a r e l i k e l y due t o r a r e - e a r t h m i g r a t i o n f r o m t h e a l c o g e l t o t h e s o l v a n t d u r i n g t h e s u p e r c r i t i c a l d r y i n g process. T h i s was confirmed by immersing a4cogels o f same s i z e ( 4 cm ) i n a g r e a t amount o f

i s o p r o p y l a l c o h o l (15 cm ) : from t h e r e s u l t s g i v e n i n t a b l e 2, r a r e - e a r t h m i g r a t i o n can be proved.

Table 2 : Rare-earth c o n c e n t r a t i o n s i n mol

.

^{1-' (deduced}

^from

v i s i b l e s ~ e c t r a ) i n a l c o n e l s b e f o r e and a f t e r immersing jr~ i s o p r o ~ v l a l c o h o l f o r two davs.

-

F i g u r e & X r a y s a n a l v s i s

of

powders found i n c o n t a i n e r s a f t e r s u p e r c r i t i c a l d r v i n g .

Thus, d u r i n g s u p e r c r i t i c a l d r y i n g , r a r e - e a r t h and some s i 1 ic a a r e drawn o u t o f t h e a l c o g e l g i v i n g t h e powder observed a f t e r t h e d r y i n g process. T h i s can be e x p l a i n e d by a l a c k o f

$ s ~ - o - L ~ <

bonds i n t h e a l c o g e l l a t t i c e , c e r t a i n l y due t o r a t h e r s t r o n g bonds i n t h e Ln.5H20 complex. E f f e c t i v e l y , NMR measurements / 7 / make us t h i n k t h a t i f Ln-OR bonds can be a by-product o f t h e r e a c t i o n between Ln.5H20 and ROH, o n l y a v e r y s m a l l amount o f these bounds

l a b e l

a 1 bl

1 1 1

a r e r e a l l y o b t a i n e d . By another way, as p o i n t e d o u t by T. Woignier / 8 / i n Si02-B,03 b i n a r y a e r o g e l s , some

;s~-o-s~:

l a t t i c e bonds a r e c e r t a i n l y broken d u r i n g s u p e r c r i t i c a l d r y i n g . A1 1 t h e s e f a c t s e x p l a i n t h e powder compositions observed.

a l c o g e l 1.05 0.53 0.24 0.125 0.025

a l c o g e l immersed f o r two days

0.34 0.125 0.075 0 ..04 0.004

a l c o g e l 0.97 0.55 0.24 0.12 0.025 l a b e l

a2 b2 C2

2 e 2

a l c o g e l immersed f o r two days

0.34 0.12 0.06 0.04 0.008

REVUE DE PHYSIQUE APPLIQUEE

4

-

FARADAY E F F E C T

L i g h t a t t e n u a t i o n due t o s c a t t e r i n g i s r a t h e r s m a l l (see f i g u r e 31, so, Faraday measurements u s i n g a He-Ne l a s e r (632.8 nm) a r e p o s s i b l e . The Faraday apparatus has been p r e v i o u s l y d e s c r i b e d / 9 / ; however we have r e c e n t l y

improuved t h e e x p e r i m e n t a l set-up (see f i g u r e 4 ) .

F i g u r e & L i g h t t r a n s m i s s i o n

vs.

wavelennth (nm)

for

3.75 mol.% Er,O, aerogel ( 0 . 3 cm l o n g ) .

magnet

hall probe

riJ

U

rotating second

analyzer analyzer I

,

divider

m w r input /vdnoa inwl

I

high speed voltmeter

I

printer P.C. computer

When a second f i x e d a n a l y z e r i s p l a c e d behind t h e r o t a t i n g a n a l y z e r , t h e l i g h t i n t e n s i t y , I , which i l l u m i n a t e s t h e photo d e t e c t o r obeys t h e f o l l o w i n g r e l a t i o n :

I

=

I, cos2 (6

+

*F ) c0sa (

+ , ⁺

^R

^-

^{9 )}

where 6 i s t h e a n g l e between p o l a r i z a t i o n phase o f t h e i n c i d e n t beam and t h e r o t a t i n g a n a l y z e r and B t h e angle between t h e crossed p o s i t i o n and second a n a l y z e r d i r e c t i o n ( h e r e R was n / 4 ) . Thus t h e Faraday r o t a t i o n i , i s deduced

from t h e measurements o f minima w i t h and w i t h o u t magnetic f i e l d . Then, we compute t h e Verdet constant o f t h e m a t e r i a l :

*F

vo

-

_.t

IHJ

^{cos a}

where a i s t h e angle between t h e f i e l d H and t h e beam-propagation d i r e c t i o n s , 4 i s t h e sample l e n g t h .

R e s u l t s o b t a i n e d f o r r a r e e a r t h aerogels a r e g i v e n i n t a b l e 3.

- 1 - 1

Table 3:Verdet constants degree.gauss

.cm 1

f o r erbium dysprosium

aerogels.

l a b e l l a b e l

3.5 - 2.63

3.3 2

-

^{5.7 lo-?}

3.5 2

When paramagnetic i o n s such as

jab or

~ r a r e ~ +present i n a m a t e r i a l , t h e i r c o n t r i b u t i o n t o t h e Verdet c o n s t a n t i s g i v e n by / l o / :

' p a r a

=

A . N / ( 1 2

-

k 2 )

=

K / ( A:

-

k 2 )

where A i s a constant, N t h e number o f r a r e - e a r t h i o n s per cm3, A t h e i n c i d e n t wavelength and 1, t h e weighted average o f t h e t r a n s i t i o n wavelength.

Away from resonance t h a t i s t o say i n t h e long-wavelength l i m i t V p a r a i s c l o s e d t o

-

^K / k 2 . Thus t h e paramagnetic Verdet c o n s t a n t has t h e same wavelength dependence b u t i t s s i g n i s o p p o s i t e t o t h a t o f t h e diamagnetic

l a t t i c e :

=

" l a t t i c e

-

V ~ n 3 +

T h i s e x p l a i n s t h a t sample e, has a p o s i t i v e V, and i s diamagnetic because Vlpttice> V L n 3 + ; on t h e o t h e r hand, w i t h V l a t t i c e

<

V L n 3 +

,

samples d, and c, have a n e g a t i v e Vo and a r e paramagnetic.

On t h e o t h e r hand, V l a t t i c e depends on t h e number, N, o f diamagnetic species per cm3, so we can deduce:

" l a t t i c e = 'sio2 ' d a e r o g e l / d,io2

where daerogel i s t h e d e n s i t y o f t h e aerogel, Vsio and dsio2 t h e Verdet

2

c o n s t a n t and t h e d e n s i t y o f s i l i c a glass. Thus t h e c o n t r i b u t i o n o f r a r e - e a r t h i o n s ~ n t o V,~ + can be o b t a i n e d from:

R e s u l t s a r e g i v e n on f i g u r e 5. Values deduced f o r t h e glasses prepared w i t h e and d samples a r e a l s o r e p o r t e d a f t e r c o r r e c t i o n from d e n s i t y e f f e c t :

vo

= v,

a e r o g e l g l a s s d o e P o g e l / d g l a s s

The val!ue c a l c u l a t e d from p a r t i a l l y d e n s i f i e d c, samples ( f r o m d= 0.3 t o 1.2) u s i n g t h e s a m e . c o r r e c t i o n f o r d e n s i t y e f f e c t i s a l s o r e p o r t e d on f i g u r e 5.

For dysprosium V,

- dlattice

a g a i n s t t h e c o n c e n t r a t i o n i s l i n e a r w i t h a s l o p e o f -0.002 f o r a l l r e s u l t s . B u t , f o r erbium aerogels V,

-

V l o t t i c e V S .

t h e c o n c e n t r a t i o n i s a s t r a i g h t l i n e w i t h a s l o p e of -0.0007 o n l y f o r values deduced from t h e g l a s s ones. The r a t i o , 2.9, between these slopes i s v e r y c l o s e t o t h e v a l u e o b t a i n e d f o r D y and E r n i t r a t e s i n isopropanol ( 3 . 0 ) .

C4-232 REVUE DE PHYSIQUE APPLIQUEE

U n f o r t u n a t e l y measurements f o r Erbium a e r o g e l s g i v e a c o n s t a n t v a l u e near V,a,+,ioe and we a r e unable t o e x p l a i n such a discrepancy.

concentration

F i g u r e 5: Verdet c o n s t a n t s

of

r a r e - e a r t h a e r o g e l s (Dy

+,

E r CI )

and

c a l c u l a - t e d from g l a s s e s (Dy A , E r O )

or

p a r t i a l l y d e n s i f i e d (Dy

A ) vs.

1 / 2

[L~,o,].

- -

5

-

CONCLUSION

B i n a r y r a r e - e a r t h s i l i c a a e r o g e l s have been prepared w i t h Erbium o r Dysprosium n i t r a t e s . Lanthanide c o n c e n t r a t i o n s i n a e r o g e l s were s m a l l e r t h a n expected. T h i s r e s u l t i s e x p l a i n e d by r a r e - e a r t h m i g r a t i o n f r o m a l c o g e l t o s o l v a n t d u r i n g s u p e r c r i t i c a l d r y i n g process.

The a e r o g e l s a r e amorphous and t r a n s p a r e n t enough a t 632.8 nm t o a l l o w Faraday e f f e c t s t u d i e s . As expected, f o r dysprosium, Verdet c o n s t a n t s c o r r e c t e d f r o m l a t t i c e c o n t r i b u t i o n o r from d e n s i t y e f f e c t have a l i n e a r e v o l u t i o n a g a i n s t r a r e - e a r t h c o n c e n t r a t i o n .

U n f o r t u n a t e l y t h e discrepency observed f o r Erbium a e r o g e l s cannot be e x p l a i n e d by e x p e r i m e n t a l e r r o r s o n l y . N e v e r t h e l e s s s t r e s s e s were d e t e c t e d i n t h e samples and Faraday measurements a r e q u i t e d i f f i c u l t i n such a case.

We thank D r . J. P h a l i p p o u and D r . T. Woignier f o r h e l p f u l d i s c u s s i o n s . REFERENCES

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/ 2 / a

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MASSEY, G . A . , ERICKSON, D.C., KADTEC, R . A . , Appl. O p t i c s ,

14

(1975)

2712.

b

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^MASSEY, G.A., JOHNSON, J.C., ERICKSON, D.C., Soc. Photo-Opt. I n s t . Engin.,

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