A.E.S. STUDIES OF SURFACE COMPOSITION OF Al-Sn AND Al-Cu LIQUID ALLOYS

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A.E.S. STUDIES OF SURFACE COMPOSITION OF

Al-Sn AND Al-Cu LIQUID ALLOYS

L. Goumiri, P. Laty, J. Joud, P. Desre

To cite this version:

JOURNAL DE PHYSIQUE Colloque C8, suppZ6ment au n08, Tome 41, aoct 1980, page 03-787

A 8 E , S , STUDIES OF SURFACE COMPOSITION OF A1-Sn AND A1-Cu L I Q U I D ALLOYS

L. Goumiri, P . L a t y , J.C. J o u d a n d P . Desre

Laboratoire de Thermodynamique

e t

Physico-Chimie M t a l l u r g i q u e s (associd au CNRS L.A. 29)

E. N. S. E . E . G., Domaine Uniuersitaire, B. P. 4 4 , 38401 S a i n t Martin d 'H&res, France. I

-

INTRODUCTION.-

The a p p l i c a t i o n o f surface thermodyna- v i o u s l y ( 6 ) which c o r r e l a t e t h i s thermodynamic beha- mic p r i n c i p l e s t o mu1 ticomponent systems r e a d i l y v i o u r w i t h s t r o n g surface segregation. The s o l u t e -

shows t h a t i n e q u i l i b r i u m t h e s u r f a c e composition s o l v e n t i n t e r a c t i o n s and t h e s i z e e f f e c t ( t h e d i f- may be markedly d i f f e r e n t from t h e b u l k composition. ference between t h e s u r f a c e average area occupied I n t h i s way, s t a t i s t i c a l approaches have been deve- per atom A and

B)

a r e r e s p o n s i b l e f o r t h e s t r o n g

t o e v a l u a t e surface compositions o f d i f - f a l l i n t h e surface t e n s i o n isotherm and t h e surfa- f e r e n t types o f systems. Auger e l e c t r ~ n Spectrosco- ce s a t u r a t i o n by Sn species.

py (AES) p r o v i d e s a means t o measure d i r e c t l y t h e I n t h e o p p o s i t e way t h e A1-CU system has s t r o n g composition of t h e topmost atomic l a y e r s . The pur- chemical heteroatomic i n t e r a c t i o n s c o n s i s t e n t w i t h pose o f t h i s work i s f i r s t t o determine surface t h e existence o f numerous s t a b l e i n t e r m e d i a t e pha- compositions of A1-Sn and A1-Cu a l l o y s a t r e s p e c t i - ses observed i n phase diagram. Indeed t h e l i q u i d s v e l y 973K and 1048K by t h i s technique. I n another s u r f a c e t e n s i o n i n such systems e x h i b i t i n f l e c t i o n hand, we compare them w i t h those p r e d i c t e d by t h e p o i n t s ( 7 ) and s t r o n g i n d i c a t i o n s make t h e existence monolayer models and t h e surface composition o b t a i - of c l u s t e r s i n t h e l i q u i d and i t s surface l a y e r ned from Gi bbs a d s o r p t i o n formula u s i n g experimen- h i g h l y probable. Recently t h e o r e t i c a l works have

t a l data o f surface tension. been i n an attempt t o analyse these

I

I 1

-

THERMODYNAMIC BASIS AND STATISTICAL APPROACHES phenomena. Selected p r o p e r t i e s and parameters of The Gibbs adsorption equation which r e -

A1-Sn and A1-CU systems are shown in Table s u l t s from a thermodynamic treatment o f t h e v a r i a -

t i o n o f s u r f a c e t e n s i o n w i t h composition, s t a t e s t h a t do =

-

z

ridpi where ui and Ti a r e r e s p e c t i v e -

1

l y the chemical p o t e n t i a l and t h e surface adsorp- t i o n ( d e f i n e d as t h e excess surface c o n c e n t r a t i o n

2

i n moles/cm ) of the component i.. Thus u s i n g mono-

l a y e r r e p r e s e n t a t i o n o f t h e surface and Gibbs-Duhem Table 1 : Selected p r o p e r t i e s and thermodynamic formula we deduce t h e s u r f a c e composition from t h e data f o r A1-Sn and A1-CU systems.

S

corresponding equation XA = XA-w(1-XA) da/duA. For I11

-

EXPERIMENTAL.-

t h e system A1-Sn which i s c h a r a c t e r i z e d by associa- _111.1

_-

_{Apearatu~-and-sam~les~~reeaca-}

319"

t i o n s between atoms of t h e same type, c o n s i s t e n t

c8-788

JOURNAL DE PHYSIQUE t r a were recorded u s i n g a c y l i n d r i c a l m i r r o r analy-

s e r i n t h e f o l l o w i n g c o n d i t i o n s : Ep-2kv I t a r g e t

=

10uA, modulating t e n s i o n

=

2v.

S i x a l l o y s c o n s i s t i n g o f 99.999 % A1 and Sn b o t h from JMC ( c h e m i c a l l y analyzed as 0.62, 0.88, 1.34, 2.52, 8.62, and 10.53 weight % Sn) and f o u r a1 l o y s o f 99.999 % A1 (JMC) and 99.999 % Cu ( A s a m )

( c h e m i c a l l y analyzed as 19.7 %, 33.3 %, 50.1 %, 60.9 % weight % Cu) were used i n t h e present study. The c l e a n i n g treatment and t h e experimental appara- t u s used f o r t h e A1-CU system has been described p r e v i o u s l y ( 9 ) . For Al-Sn system, f l a s h - h e a t i n g t o 873K f o l l o w i n g by 30 mn Ar bombardments (Zkv,

2

15pA/cm ) are necessary t o reduce C, 0, S Auger peaks t o t h e noise l e v e l . The u l t i m a t e aim of e l i - m i n a t i n g a l l contaminents was never q u i t e achieved. Data were c o l l e c t e d o n l y a f t e r measurements showed t h a t C and S Auger peaks were s u f f i c i e n t l y reduced.

111.2

-

N o y l

jz~tjon-gf-Ayge~-~pec_t_rg

:

A c a l i b r a t i o n of Auger measurements i s necessary f o r q u a n t i t a t i v e i n t e r p r e t a t i o n s . The s p e c t r a o f t h e pure elements Sn and A1 were f i r s t examined a t 973K under i d e n t i c a l experimental c o n d i t i o n s i n o r d e r t o estimate t h e r e l a t i v e sensi- t i v i t y f a c t o r . The Auger spectra o f pure l i q u i d A1 and Sn were found t o be i d e n t i c a l w i t h those o b t a i - ned i n s o l i d s t a t e . Thus, aluminium e x h i b i t s o n l y a s t r o n g peak, l o c a t e d a t 68 ev, which i s associa- t e d w i t h an LZ3VV Auger t r a n s i t i o n . For Sn s p e c i - men t h e i m p o r t a n t d o u b l e t l o c a t e d a t 430-437 ev i s obtained. So i t was decided t o s e l e c t f o r c a l i b r a - t i o n t h e peak t o peak i n t e n s i t y r a t i o o f t h e a l u - minium 68 ev and the t i n 430 ev

(M5

NqY5 N4,5) t r a n s i t i o n s . Then changes i n t h i s i n t e n s i t y r a t i o are used t o estimate surface composition o f l i q u i d a l l o y s whose t y p i c a l spectrum i s g i v e n i n F i g . 1.

F o r A?-Cu system, such experimental measurements o f pure standards were impossible (due t o t h e vapor

pressure o f Cu a t m e l t i n g p o i n t ) . So as a rough estimate of t h e s e n s i t i v i t y f a c t o r ; peak t o peak i n t e n s i t y r a t i o o f aluminium 68 ev and copper 920 ev t r a n s i t i o n s i s s e l e c t e d from the Handbook o f s o l i d standard Auger spectra.

F i g . 1 T y p i c a l spectrum of A1-Sn a l l o y a t 973K.

I V

-

RESULTS AND DISCUSSION.-

1 V . l

-

Atlln-y~sgm

:

We suppose t h a t t i n atoms ( s u r f a c e a c t i v e species) cover a f r a c t i o n X o f a p r a c t i c a l - l y pure aluminium substrate. The q u a n t i f i c a t i o n i s considered w i t h reference t o s p e c t r a from b u l k standards. Data on I n e l a s t i c mean f r e e p a t h (IYFP) i n s o l i d s ( l O ) and back s c a t t e r i n g correction('') have been used t o determine t h e composition o f t h e s u r f a c e atom l a y e r . Hence according t o Seah and c o ( l O ) the i n t e n s i t i e s o f t h e c h a r a c t e r i s t i c t r a n - s i t i o n a r e g i v e n by :

l+r~l m

'sn =

'(5)

(

1

-

exp

-

-) and Sn

IAl =

lil

( 1

-

X

+

X exp

(s))

where

I;,

and

A 1

ming t h e surface phase.

Because o f t h e r e p u l s i o n o f nearest neighbour i n t e r a c t i o n s (Al-Sn bonds n o t p r e f e r e d ) and t h e excess s i z e s o l u t e atoms Sn, t h e s u r f a c e i s consi- d e r a b l y r i c h e r i n t h i s element than t h e b u l k as shown i n Fig. 2. Furthermore t h e p l o t t e d curves ( s t a t i s t i c a l model and Gibbs treatment) are i n good q u a l i t a t i v e agreement w i t h t h e Auger data ob- t a i n e d from a monolayer model (m = 1 ) .

tin atom.% in bulk

F i g . 2 : T i n surface enrichment deduced from s t a - t i s t i c a l model, Gi bbs treatment and Auger measurements assuming m = 1, 2 o r 5 layers f o r A1-Sn system.

IV.2

-

AIzCusystem

:

Here we t r e a t o r d e r i n g a l l o y s w i t h t h e same method and w i t h i n t h e same approximations. Results presented i n t a b l e 11, r e v e a l s u r f a c e e n r i - chment i n A1 obtained from s t a t i s t i c a l model, Gibbs a d s o r p t i o n r e l a t i o n and Auger measurements assuming m = 2, 3 and 10 l a y e r s . We observe a q u a l i t a t i v e

C8-789

agreement between t h e d i f f e r e n t e s t i m a t i o n s . N e v e r t h e l e s s a weak v a r i a t i o n o f s u r f a c e composition w i t h number o f monolayers i s deduced f o r A1-Cu Auger e s t i m a t i o n comparatively t o A1-Sn system. This l a s t p o i n t i s m a i n l y due t o t h e f a c t t h a t i n A1-Cu system t h e s o l v e n t i s a l s o t h e segregating element.

V

-

CONCLUSION.-

I n t h i s work A.E.S. measurements have been performed on l i q u i d A1-Sn and A1-CU a l l o y s a t r e s p e c t i v e l y 973K and 1048K. The corresponding sur- face compositions a r e deduced from c l a s s i c a l expres- s i o n o f Auger i n t e n s i t y . Compositions a r e c a l c u l a - t e d assuming v a r i o u s surface l a y e r t h i c k n e s s /and compared w i t h thermodynamical e s t i m a t i o n s ( s t a t i s -

4

t i c a l models o r Gibbs treatments o f s u r f a c e ' t e n s i m data). F o r b o t h systems which e x h i b i t o p p o s i t e thermodynamic behaviours we obtained a f a i r l y good agreement between t h e d i f f e r e n t e s t i m a t i o n s o f surface enrichment. I n p a r t i c u l a r we can emphasize t h e d i r e c t v e r i f i c a t i o n by A.E.S. o f Gibbs adsorp- t i o n equation. I n a d d i t i o n r e s u l t s i n d i c a t e a l o - c a l i z e d segregation f o r s t u d i e d systems a t t h e r - modynamical e q u i l i b r i u m .

-

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C8-790

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