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THE INVESTIGATION OF FIELD ADSORPTION AND DIFFUSION OF HELIUM ON THE SURFACE
OF TUNGSTEN BY AP-FIM
Ren Gang
To cite this version:
Ren Gang. THE INVESTIGATION OF FIELD ADSORPTION AND DIFFUSION OF HELIUM ON THE SURFACE OF TUNGSTEN BY AP-FIM. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-157-C7-160. �10.1051/jphyscol:1986728�. �jpa-00225921�
JOURNAL DE PHYSIQUE
C o l l o q u e C7, s u p p l e m e n t a u n o 11, Tome 47, Novembre 1 9 8 6
THE INVESTIGATION OF FIELD ADSORPTION AND DIFFUSION OF HELIUM ON THE SURFACE OF TUNGSTEN BY AP-FIM
REN DA GANG
Institute of Metal Research, Academia Sinica, Shenyang, China
A b s t r a c t - T h i s work was c a r r i e d o u t w i t h s t u d i e s o n t h e a d s o r p - t i o n a n d d i f f u s i o n o f h e l i u m on t h e s u r f a c e o f t u n g s t e n by AP-FIM m e a s u r e m e n t s and showed t h a t h e l i u m i s o n l y a d s o r b e d o n t h e s u r f a c e t h e W ( 0 0 1 ) and W ( 1 1 1 ) s u r f a c e a n 3 t h a t t h e b i n d i n a e n e r q y o f t h e f i e l d a d s o r ~ t i o n b e t w e e n . 5 e l i u m and t u n a s t e n i s 0.16 e V . The d e p t h ~ r o f i l e a l o n g t h e W[001] shows t h e d e ~ t h o f h e l i u m t o b e 40-50 a t o m i c l a y e r s a n d t h e d i f f u s i o n a c t i v a t i o n e n e r q y a s 0.22 e V .
1 - INTRODUCTION
N o b l e q a s a t o m s c a n n o t b e a d s o r b e d o n t h e s u r f a c e o f m e t a l s a t low t e m ~ e r a t u r e b e c a u s e t h e i r v a n d e r Waals b i n d i n g e n e r g y w i t h t h e s u r f a c e i s p a r t i c u l a r l y s m a l l (200-600 c a l / m o l ) . However, a n o b l e g a s atom c a n b e a d s o r b e d o n t h e s u r f a c e o f m e t a l s i n t h e FIM d u e t o t h e c r e a t i o n o f a s h o r t r a f g e d d i p o l e - d i p o l e i n t e r a c t i o n i n d u c e d b y t h e a p p l i e d e l e c t r i c f i e l d . The b i n d i n g e n e r g y o f a n o b l e g a s atom a d s o r b e d o n t h e s u r f a c e o f a m e t a l is a b o u t 0 . 2 3 - 0 . 2 8 e V a n d i s p r o p o r t i o n a l t o s q u a r e o f t h e f i e l d s t r e n q t h ' f . I t i s a n i n t e r e s t i n g p r o b l e m t o s t u d y t h e b e h a v i o r o f h e l i u m i n m e t a l s f o r n u c l e a r
e n e r g y m a t e r i a l s . The p r o b l e m o f d e f e c t f o r m a t i o n a n d t h e s w e l l i n g o f t h e m a t e r i a l s is a f f e c t e d b y h e l i u m p r o d u c e d b y c o l l i s i o n s . The p r e s e n t p a p e r s t u d i e s a d s o r p t i o n a n d d i f f u s i o n o f h e l i u m o n p a r - t i c u l a r c r y s t a l p l a n e s a n d h e l i u m was a d s o r b e d on s u r f a c e o f t h e t u n g s t e n b y t h e f i e l d .
2 - EXPERIMENTAL RESULTS A N D PROCEDURES
T h e e x p e r i m e n t s were c a r r i e d o u t i n a n atom p r o b e f i e l d i o n m i c r o - s c o p e . The i n s t r u m e n t h a s a s t r a i g h t f l i g h t t u b e a n d i s c o m p u t e r c o n t r o l l e d . The c o l d f i n g e r o f t h e m i c r o s c o p e was c o o l e d c o n t i n u o u s l y w i t h l i q u i d n i t r o g e n . S ~ e c i m e n s a r e ~ o l y c r y s t a l l i n e D u r e t u n g s t e n wires w i t h t h e w i r e a x i s p a r a l l e l t o t h e [ 0111 d i r e c t i o n and were a n n e a l e d a t 1600°C f o r 30 m i n u t e s i n t h e vacuum s y s t e m . The t i ~ s w e r e p r e p a r e d b y elec- t r o p o l i s h i n g them i n NaOH & NH40H s o l u t i o n . The b a c k g r o u n d p r e s s u r e i n t h e vacuum s y s t e m i s 5 ~ 1 0 - ~ t o r r . The h e l i u m a a s is i n t r o d u c e d a t a D r e s s u r e o f 5 x 1 0 - 5 t o r r i n o r d e r t o o b s e r v e t h e i m a a e . The s ~ e c i m e n t e m p e r a t u r e i s a b o u t 85K.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986728
C7-158 JOURNAL DE PHYSIQUE
I n o u r e x p e r i m e n t s , t h e f i e l d i o n i m a g e s w e r e o r o d u c e d a f t e r r o t a t i n g t h e FIM s p e c i m e n a l o n q c r y s t a l d i r e c t i o n s . I n o r d e r t o d e t e c t t h e c o m p l e t e f i e l d e v a p o r a t i o n b e h a v i o r o f a n i n d i v i d u a l c r y s t a l p l a n e , t i p o r i e n t a t i o n i s c o n t r o l l e d by t h e m a n i p u l a t o r . The p r o b e h o l e i s aimed a t v a r i o u s c r y s t a l p l a n e s o f t u n g s t e n : ( a ) W(001) p l a n e , ( b ) W(111) p l a n e and ( c ) W(011) p l a n e i n f i g . 1 . A s f i g s . l a and b show, t h e s u r f a c e o f W(001) and W(111) h a v e a d s o r p t i o n of h e l i u m . The AP mass h i s t o g r a m s (Vp/Vdc=O.l) f o r t h e v a r i o u s c r y s t a l p l a n e s o f t u n g s t e n a r e shown i n f i g . 2 . C o r r e s p o n d i n g image i s shown i n f i g . 1 . I t i s o b s e r v e d t h a t t h e W(001) h a s a n a b u n d a n t s p e c t r u m o f h e l i u m , t h e W ( 1 11 ) h a s less and t h e W ( 01 1 ) h a s none. Then t h e p r o b e h o l e i s aimed a t W(001) and i s e v a p o r a t e d c o n t i n u o u s l y .
The c u m u l a t i v e number o f H e and W atoms was r e c o r d e d e v e r y t i m e . The c o n c e n t r a t i o n of He on W(001) by l a y e r i s shown i n f i g . 3. I t is con- c l u d e d t h a t h e l i u m d i f f u s e d i n t o t h e t u n g s t e n c r y s t a l .
3 - DISCUSSION
1 . A n o b l e g a s c a n b e a d s o r b e d on t h e s u r f a c e o f m e t a l s i n FIM d u e t o t h e c r e a t i o n o f a s h o r t r a n g e d d i p o l e - d i p o l e i n t e r a c t i o n i n d u c e d b y t h e a p p l i e d e l e c t r i c f i e l d when h i g h v o l t a g e was a p p l i e d t o t h e t i p t o r e a c h t h e b e s t imaqe v o l t a g e VBIV, t h e f i e l d s t r e n g t h o f t h e s u r f a c e o f t h e t i p i s F = V / K r , where K i s a g e o m e t r i c a l f a c t o r o f e q u a l t o 4.5 and r t h e ? i p $&Xius o f e q u a l t o 335 8 . C o r r e s p o n d i n g t o t h e imaqe v o l t a g e i n f i g . 1 o f 7.0 kV, w e s e e F = 4.6 v / R . So t h a t a m o b i l e g a s atom was p o l a r i z e d i n t h e p r o t r u d i n 8 s u r f a c e o f metah atom a t a k i n k s i t e . The s h o r t - r a n q e d i p o l e - d i p o l e b i n d i n q e n e r q y i s
where a A i s t h e ~ o l a r i z a b i l i t y o f t h e g a s atom and f A an enhancement f a c t o r c a l c u l a t e d form t h e d i p o l e - d i p o l e i n t e r a c t i o n a s
( 1 + 2 a M / d 3 2 f A =
I
-
4aAcc,/a'5w h e r e d r e p r e s e n t s t h e d i s t a n c e b e t w e e n t h e two i n t e r a c t i n q atoms and a t h e p o l a r i z a b i l i t y o f t h e s u r f a c e m e t a l atom a t i t s k i n k s i t e . 3 C o r r e s ~ o n d ' n t o o u r e x p e r i m e n t a l c o n d i t i o n F = 4.6 ~ / 8 , a A = 0 . 2 8 ,
a M = 3 . 4 4 ~ 3 , q d = r + r =2.58 8 , t h e f i e l d b i n d i n e e n e r a y h a s b e e n c a l - c u l a t e d t o b e o n M t h % o r d e r o f 0.16 eV a t a h o l d i n q f i e l d o f 4.6 v/X.
T h i s v a l u e is more t h a n t h e van d e r Waals b i n d i n q e n e r q i e s and l e a d s t o t h e H e atom a d s o r p t i o n on t h e s u r f a c e o f t u n q s t e n . But we h a v e n o t c o n s i d e r e d t h e d i f f e r e n c e o f a r r a n g e m e n t and r a d i u s o f c u r v a t u r e f o r v a r i o u s r e q i o n o r endform o f t i p .
F i g . 2 shows t h a t h e l i u m atoms w e r e a d s o r b e d c o n s i d e r a b l y on t h e W(OOl), t o a less d e g r e e o n t h e W ( 1 1 1 ) , and n o t a d s o r b e d on t h e W(Ol1). T h i s c a n b e s e e n a l s o i n £ i s . 1 , t h e s e a r e a t t r i b u t e d t o t h e d i f f e r e n c e o f r a d i u s o f c u r v a t u r e and c l o s e packed a r r a n g e m e n t f o r v a r i o u s c r y s t a l p l a n e s . T h e s e a s s u m p t i o n s w e r e c o n f i r m e d b y a p i c t u r e o f t h e t i p by e l e c t r o n m i c r o s c o p y . The W(011) p l a n e s i t s on t o p o f t h e t i p e n d f o r m , i t h a s a l a r g e r a d i u s o f c u r v a t u r e c o r r e s p o n d i n g t o a r e a s o f low b i n d i n g e n e r q y o f p o l a r i z a t i o n . The W(001) and W(111) p l a n e s s i t on t h e s i d e o f t h e t i p e n d f o r m , h a v i n g a s m a l l l o c a l r a d i u s o f c u r v a t u r e and l e a d i n g t o a r e a s o f h i q h b i n d i n q e n e r q y o f p o l a r i z a t i o n . The W(O1l) i s a close uacked u l a n e , W(001) o r W(111) i s a l o o s e l y p a c k e d p l a n e f o r t h e b c c t u n q s t e n c r y s t a l . T h e s e atoms o f d i f f e r e n t c o s e packed a r r a n g e m e n t s c o n t r i b u t e t o t h e p o l a r i z a t i o n e n e r g y p = - .) ( a - a i ) F d i f f e r e n t l y .
2 a
Thus t h e s u r f a c e o f W(001) and W(111) h a v e h i g h v a l u e s o f p o l a r - i z a t i o n e n e r g y and t h e f i e l d a d s o r p t i o n r e a d i l y t a k e s p l a c e .
2 . The p r o f i l e o f AP w i t h He i s shown i n f i g . 3. The He atoms h a v e e n t e r e d i n t o t h e c r y s t a l o f t h e t u n g s t e n by d i f f u s i o n . The d e p t h i s Ca. a few t e n l a y e r s . A s c h e m a t i c d i a g r a m o f h e l i u m a d s o r b e d on t h e s u r f a c e t o d i f f u s e i n t o t h e c r y s t a l i s shown i n f i g . 4 . The endform of t h e t i p i s assumed t o b e a h e m i s p h e r e model. The r a d i u s o f c u r v a t u r e i s r . According t o t h e TLK ~ o t e n t i a l b a r r i e r model o f a t o m i c d i f - f u s i o n , t h e r a d i a l c o n c e n t r a t i o n p r o f i l e t h r o u g h t h e s p h a r e i s a f u n c t i o n of r , g i v e n by t h e E i n s t e i n e x p r e s s i o n .
where D i s t h e h e l i u m d i f f u s i v i t y and t i s t h e d i f f u s i o n t i m e a t tem- p e r a t u r e T. S i n c e
- A H D = D e x p (- )
0 kT
t h e c h a n g e o f m i g r a t i o n e n t h a l p y A H i s g i v e n by
where k is B o l t z m a n n t s c o n s t a n t and D is t h e p r e - e x p o n e n t i a l f a c t o r . I n o u r e x p e r i m e n t c o n d i t i o n , D = 1 .I O x lo-' cm* / s e c . , t = 3 6 0 0 s e c . r=120 8 , T=85K. A H was c a l c u l a v e d t o b e 0.23eV. The m i g r a t i o n e n e r q y
( A E ) w i t h He i n t u n q s t e n i s d e f i n e d by e x p r e s s i o n
w h e r e P i s t h e p r e s s u r e and A V is t h e volume c h a n g e o f m i g r a t i o n . S i n c e e s s e n t i a l l y no e x t e r n a l p r e s s u r e was a p p l i e d t o t h e s p e c i m e n d u r i n g t h e d i f f u s i o n i n t e r v a l a t T t h e n AH " 4 E . I t i s shown t h a t m i g r a t i o n e n e r u y of d i f f u s i o n w i t h He i n t h e t u n g s t e n h a s same o r d e r o f m a g n i t u d e a s t h e f i e l d a d s o r b e d b i n d i n g e n e r g y .
REFERENCES
1 . E.W. M u l l e r , P h y s i c a l R e w . 102 ( 1 9 5 6 ) 624.
2. T.T. T s o n g , P h y s i c a l Rew. L e t t e r s , 25 (1970! 911.
F i g . 1 The p r o b e h o l e i s aimed a t v a r i o u s c r y s t a l p l a n e o f t u n g s t e n . ( a ) W(001) c a . ( b ) W(111) c a . ( c ) W(011 ) c a .
JOURNAL DE PHYSIQUE
Fig. 2 The mass histogram of AP for the various crystal planes of tungsten ( a ) W(001) Ca. ( b ) W(111) Ca. ( c ) W(Ol1) Ca.
Fig. 3 The concentration profile Fig.4 The illustration of diffusion of He on the W(001) of of He in tip of tungsten.
tungsten by AP.
