The Characterization of Carbon Deposit Morphologies Using In Situ

S h e l l 1 thicknesses are estimated to be ^0.1 um. As deposition continues, the laminar ... localised cooling, which would move the precipitation zo...
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10 The Characterization of Carbon Deposit Morphologies Using In Situ Scanning Electron Microscopy Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch010

A. M . BROWN and M . P. HILL Central Electricity Research Laboratories, Surrey, United Kingdom

Carbon deposits have been grown on heated polycrystalline iron specimens i n a gas reaction c e l l i n a scanning electron microscope. The complete range of deposits formed from methane at temperatures between 620°C and 850°C have been surveyed, from the f i r s t appearance of solid carbon on the surface to the f i n a l structural breakdown of the metal. Four characteristic morphologies have been identified. There are indications that solution-precipitation processes are involved i n the growth mechanisms i n each case. The importance of structural disorder i n the metal is considered and the behaviour of well annealed material i s compared to structurally less stable but clean metal surfaces. The processes that occur i n the development of particulate deposits have been followed. Auger analysis of individual morphologies has highlighted particular problems i n the analysis of carbon deposits after ion etching. The implications of these observations to deposition on less well-defined metal surfaces and gases containing trace impurities i s discussed. The f o r m a t i o n o f c a r b o n d e p o s i t s i s a n u n d e s i r a b l e f e a t u r e o f a number o f i n d u s t r i a l p r o c e s s e s , f o r e x a m p l e o n t h e f u e l p i n s i n n u c l e a r r e a c t o r s , on t h e c a t a l y s t s and s t r u c t u r a l m a t e r i a l s o f r e a c t o r s i n p e t r o c h e m i c a l p l a n t , and on f u r n a c e w a l l l i n i n g s . A g r e a t e r u n d e r s t a n d i n g o f t h e mechanism o f d e p o s i t i o n i s r e q u i r e d i n o r d e r t o d e v e l o p i m p r o v e d methods o f c o n t r o l . M e t a l - c a r b o n i n t e r a c t i o n s a r e a l s o important i n the c a t a l y s i s o f carbon g a s i f i c a t i o n f o r the manufacture o f s y n t h e t i c f u e l s . The d e p o s i t s f o r m e d o n t r a n s i t i o n m e t a l s f r o m h y d r o c a r b o n s o r carbon monoxide e x h i b i t a v a r i e t y o f m o r p h o l o g i e s i n c l u d i n g g r a p h i t i c l a m i n a r f i l m s , mound ( m o n t i c u l a r ) g r o w t h s , c o l u m n a r , p a r t i c u l a t e , a n d numerous f i l a m e n t a r y t y p e s o f c a r b o n , (1-4). The

0097-6156/82/0202-0193$08.50/0 © 1982 American Chemical Society

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch010

194

COKE FORMATION

o b s e r v a t i o n s suggest t h a t s e v e r a l d i f f e r e n t c h e m i c a l and/or p h y s i c a l p r o c e s s e s o c c u r b o t h s i m u l t a n e o u s l y and s e q u e n t i a l l y during the r e a c t i o n . A l t h o u g h one p a r t i c u l a r m o r p h o l o g y may have b e e n i d e n t i f i e d as a p r o d u c t o f s e v e r a l h y d r o c a r b o n - m e t a l r e a c t i o n s , t h e r e have b e e n few s y s t e m a t i c s t u d i e s o f t h e g r o w t h mechanisms. However, c o n s i d e r a b l e p r o g r e s s has b e e n made in u n d e r s t a n d i n g the growth of f i l a m e n t a r y carbon ( 4 ) , u s i n g cont r o l l e d a t m o s p h e r e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y w h i c h has demonstrated the v a l u e of in-situ observations (5). Transmission e l e c t r o n m i c r o s c o p y has t h e b e n e f i t s o f h i g h r e s o l u t i o n and t h e a v a i l a b i l i t y of e l e c t r o n d i f f r a c t i o n , but i s l i m i t e d t o the e x a m i n a t i o n o f s m a l l p a r t i c l e s , t h i n f o i l s , and p r o d u c t s formed a t the edges o f s p e c i m e n s t o o t h i c k f o r t r a n s m i s s i o n o f t h e e l e c t r o n beam. I n o r d e r t o s t u d y c a r b o n d e p o s i t i o n on b u l k m e t a l s in-situ s c a n n i n g e l e c t r o n m i c r o s c o p y i s more s u i t a b l e , o f f e r i n g t h e f a c i l i t i e s o f r e s o l u t i o n down t o ^50 X, good d e p t h o f f i e l d , s t e r e o s c o p i c v i e w i n g o f d e p o s i t s , and t h e use o f v a r i o u s m i c r o a n a l y t i c a l techniques. The m e t a l most a c t i v e in s u p p o r t i n g c a r b o n d e p o s i t i o n i s i r o n , c l o s e l y f o l l o w e d by n i c k e l , and i s t h e one s e l e c t e d f o r t h i s w o r k . The i r o n - c a r b o n s y s t e m i s c o m p l e x b e c a u s e b o t h g r a p h i t e and i r o n c a r b i d e s a r e p o s s i b l e r e a c t i o n p r o d u c t s . B e l o w t h e Fe-C e u t e c t o i d t e m p e r a t u r e (721°C o r 738°C) a d d i t i o n a l s o l i d - s t a t e t r a n s f o r m a t i o n s are known l e a d i n g t o t h e s t r u c t u r e s f o u n d in f e r r i t i c s t e e l s , i . e . p e a r l i t e , m a r t e n s i t e and b a i n i t e e t c . C o n s e q u e n t l y , c a r e must be t a k e n o v e r t h e t e m p e r a t u r e c y c l i n g e x p e r i e n c e d by t h e s p e c i m e n t o avoid introducing a d d i t i o n a l complications. In p a r t i c u l a r , cooling a specimen t o ambient temperature f o r e x a m i n a t i o n i n c r e a s e s cons i d e r a b l y the r i s k of m o d i f y i n g e x i s t i n g s t r u c t u r e s or i n t r o d u c i n g new p r e c i p i t a t i o n p r o d u c t s . A m b i g u i t i e s a r i s i n g f r o m t h e s e s o u r c e s are l a r g e l y a v o i d e d by use o f t h e in-situ SEM t e c h n i q u e . S e v e r a l mechanisms o f c a r b o n d e p o s i t i o n have b e e n p r o p o s e d (4, 6-8) i n c l u d i n g : (i) carbon d i s s o l u t i o n , d i f f u s i o n to s u i t a b l e s i t e s , f o l l o w e d by p r e c i p i t a t i o n , (ii) f o r m a t i o n o f m e t a l c a r b i d e on t h e s u r f a c e f o l l o w e d by t h e r m a l d e c o m p o s i t i o n , (iii) s u r f a c e d i f f u s i o n o f c a r b o n atoms o r a C/H/metal o r g a n o - m e t a l l i c type complex s p e c i e s , (iv) b u l k carbon d i f f u s i o n . Under d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s one o r o t h e r o f t h e s e mechanisms may p r e d o m i n a t e . However, a t t h e p r e s e n t t i m e t h e growth mechanism f o r t h e m a j o r i t y o f c a r b o n s t r u c t u r e s i s u n c e r t a i n . I n c a r b o n d e p o s i t i o n on a b u l k m e t a l s p e c i m e n , t h e c a r b o n u p t a k e f o l l o w s a s i g m o i d a l c u r v e as a f u n c t i o n o f t i m e t y p i c a l o f an a u t o c a t a l y t i c r e a c t i o n . I n i t i a l l y c a r b o n d i s s o l v e s in t h e m e t a l u n t i l t h e s a t u r a t i o n s o l u b i l i t y has b e e n e x c e e d e d . This step i s o b s e r v e d as an i n c u b a t i o n p e r i o d . The i n i t i a l s o l i d p r o d u c t formed c a u s e s no d i s r u p t i o n o f t h e m e t a l s u r f a c e and t h e g r o w t h r a t e remains low. I t s t a r t s t o r i s e r a p i d l y as p a r t i c u l a t e

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch010

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m a t e r i a l i s e x t r a c t e d f r o m t h e m e t a l and becomes i n c o r p o r a t e d in the d e p o s i t , l e a d i n g e v e n t u a l l y t o complete breakdown o f t h e m e t a l structure. Most o f t h e c a r b o n d e p o s i t i s formed d u r i n g t h i s p e r i o d as i n c r e a s i n g amounts o f a c t i v e c a t a l y s t become a v a i l a b l e , and t h e g r e a t e s t v a r i e t y o f d e p o s i t m o r p h o l o g i e s a r e o b s e r v e d . The r e a c t i o n e v e n t u a l l y s l o w s down e i t h e r b y f o r m a t i o n o f i n e r t p r o d u c t s , e . g . Fe + CO ( 9 ) o r b y e n c a p s u l a t i o n o f a c t i v e m e t a l p a r t i c l e s in c a r b o n Q 0) . I n t h e w o r k r e p o r t e d in t h i s p a p e r , t h e in-situ h o t s t a g e SEM t e c h n i q u e h a s b e e n u s e d t o e x a m i n e t h e c a r b o n d e p o s i t s f o r m e d in the i r o n - m e t h a n e r e a c t i o n , w i t h t h e o b j e c t i v e o f s e p a r a t i n g t h e d i f f e r e n t stages by i d e n t i f y i n g t h e c h a r a c t e r i s t i c morphologies. I t i s p r e s e n t e d as a s u r v e y o f t h e r a n g e o f p r o d u c t m o r p h o l o g i e s s t a r t i n g w i t h t h e c a r b o n d i s s o l u t i o n s t a g e and e n d i n g w i t h t h e complete s t r u c t u r a l breakdown o f t h e e x p e r i m e n t a l f o i l . Experimental The SEM u s e d f o r t h e w o r k i s a s p e c i a l l y a d a p t e d u l t r a - h i g h vacuum f i e l d e m i s s i o n s o u r c e i n s t r u m e n t (V.G. M i c r o s c o p e s L t d . ) . I t has a gas r e a c t i o n c e l l w h i c h s u r r o u n d s t h e specimen w h i l s t p e r m i t t i n g most m i c r o s c o p e s t a g e movements. R e a c t i o n s c a n be c a r r i e d o u t a t p r e s s u r e s up t o ^50 T o r r , b u t above ^0.2 T o r r t h e e l e c t r o n gun chamber must be i s o l a t e d and s o c o n t i n u o u s s p e c i m e n observation i snot p o s s i b l e . I n t h e n o r m a l m i c r o s c o p e mode t h e r e s o l u t i o n i s b e t t e r t h a n 50 8. a t s p e c i m e n t e m p e r a t u r e s up t o 800°C, b u t i s r e d u c e d t o ^250 8. in t h e r e a c t i o n c e l l b e c a u s e o f a requirement t o i n c r e a s e t h e probe c u r r e n t t o a c h i e v e s u f f i c i e n t signal(11). The m i c r o s c o p e i s e q u i p p e d w i t h a n e n e r g y d i s p e r s i v e X - r a y a n a l y s e r , a c y l i n d r i c a l m i r r o r A u g e r a n a l y s e r s (CMA) and a n a r g o n i o n gun. As i n d i c a t e d in F i g u r e 1, t h e r e a c t i o n c e l l must be opened t o p e r m i t m i c r o a n a l y s i s and i o n e t c h i n g . Full operation o f t h e m i c r o s c o p e i s a v a i l a b l e o n t h e h e a t e d s p e c i m e n s and a l l t h e m i c r o g r a p h s and a n a l y t i c a l d a t a p r e s e n t e d were t a k e n w i t h t h e specimens a t t h e a p p r o p r i a t e r e a c t i o n t e m p e r a t u r e . The s p e c i m e n i s in t h e f o r m o f a 25 um t h i c k f o i l . Three m o u n t i n g s y s t e m s have b e e n u s e d : (i) an e l e c t r i c a l l y heated p l a t e , (ii) direct resistive heating, (iii) i n d i r e c t e l e c t r i c a l h e a t i n g in w h i c h t h e f o i l i s s p o t - w e l d e d t o two D.C. h e a t e d c o n d u c t i n g w i r e s ( 1 1 ) . Methods ( i ) and ( i i i ) g i v e u n i f o r m t e m p e r a t u r e s , b u t t h e h o t plate i s more s u s c e p t i b l e t o t r a n s f e r o f i m p u r i t i e s . Method ( i i ) i n t r o d u c e s s t e e p t e m p e r a t u r e g r a d i e n t s b e t w e e n t h e c e n t r e and ends of t h e f o i l , b u t p e r m i t s b o t h c a r b o n d i s s o l u t i o n ( i n t h e c e n t r e ) and c a r b o n p r e c i p i t a t i o n ( i n t h e c o o l e r r e g i o n s ) t o t a k e p l a c e simultaneously. The c a r b o n d e p o s i t s t r u c t u r e i s i n d e p e n d e n t o f t h e method u s e d . Q u a n t i f i c a t i o n o f Auger d a t a c a n p r e s e n t problems on rough s u r f a c e s , f o r example, a f t e r i o n e t c h i n g . To m i n i m i s e r o u g h n e s s f

1

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

COKE FORMATION

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch010

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