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SURFACE EXTENDED X-RAY ABSORPTION FINE STRUCTURE APPLIED TO THE
POLYCRYSTALLINE SURFACES OF REAL THERMIONIC CATHODES
D. Norman, R. Tuck, H. Skinner, P. Wadsworth, T. Gardiner, I. Owen, C.
Richardson, G. Thornton
To cite this version:
D. Norman, R. Tuck, H. Skinner, P. Wadsworth, T. Gardiner, et al.. SURFACE EXTENDED X- RAY ABSORPTION FINE STRUCTURE APPLIED TO THE POLYCRYSTALLINE SURFACES OF REAL THERMIONIC CATHODES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-529- C8-532. �10.1051/jphyscol:1986899�. �jpa-00226230�
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment au n o 12, Tome 47, d6cembre 1986
SURFACE EXTENDED X-RAY ABSORPTION FINE STRUCTURE APPLIED TO THE POLYCRYSTALLINE SURFACES OF REAL THERMIONIC CATHODES
D. NORMAN, R.A. TUCK", H.B. SKINNER*, P.J. WADSWORTH*, T.M. GARDINER*, I.W. C.H. RICHARDS ON'*(^) and G. THORNTON*" ( 3 )
SERC Daresbury Laboratory, GB-Warrington WA4 4AD, Great-Britain
horn-~~r-~arian Ltd, 248 Blyth Road, GB-Hayes UB3 IHR, Middlessex, Great-Britain
' " ~ e p a r t m e n t of Chemistry, University of Manchester, GB-Manchester M13 9PL, Great-Britain
A b s t r a c t
We have used Surface Extended X-ray Absorption Fine S t r u c t u r e (SEXAFS) t o study t h e l o c a l geometry around barium atoms on t h e r m i o n i c e m i t t i n g cathodes a t t h e i r normal o p e r a t i n g temperatures. On t h e surfaces o f tungsten and tungsten/osmium a l l o y dispenser cathodes Ba i s bonded t o oxygen w i t h a w e l l - d e f i n e d short-range order. The Ba-0 distance i s almost t h e same f o r both cathodes, b u t t h e a l l o y cathode has Ba bonded t o two oxygen near neighbours (compared t o one f o r t h e tungsten cathode), which l i e i n hollow s i t e s o f t h e substrate. This provides an e x p l a n a t i o n o f t h e lower work f u n c t i o n observed f o r t h e a l l o y cathode.
I n t r o d u c t i o n
SEXAFS has proved t o be t h e best method f o r p r e c i s e determination o f t h e s t r u c t u r e of adsorbates on surfaces [I I . Here we describe an extension o f t h e technique i n t o a new area o f a p p l i c a t i o n : t h e geometry o f a surface complex on a p o l y c r y s t a l l i n e substrate. I n t h i s study t h e bond l e n g t h s and o r i e n t a t i o n have been determined f o r t h e barium-oxygen-substrate complex on t h e surfaces o f r e a l thermionic dispenser cathodes 121.
Thermionic cathodes are important p r a c t i c a l devices, being w i d e l y used i n a p p l i c a t i o n s needing h i g h e l e c t r o n c u r r e n t d e n s i t i e s , such as k l y s t r o n s and cathode r a y tubes. The dispenser cathode i s constructed f r o m a porous m a t r i x o f p o l y c r y s t a l l i ne tungsten, impregnated w i t h b a r i um c a l c i um a1 umi nate, surrounding an i n t e r n a l f i l a m e n t . I n t h e o p e r a t i n g temperature range o f 1100 t o 1400 K, t h e impregnant d i s s o c i a t e s and a m i x t u r e o f Ba and BaO d i f f u s e s through t h e pores between t h e tungsten c r y s t a l l i t e s and across t h e surface. This forms a low work f u n c t i o n c o a t i n g which i s i n dynamic e q u i l i b r i u m w i t h t h e barium-containing evaporants being t h e r m a l l y desorbed from t h e s u r f ace. The performance o f dispenser cathodes has been improved by c o a t i n g t h e surface w i t h t h i r d - r o w t r a n s i t i o n metals o r t h e i r tungsten a l l o y s [2]. Re, 0s and Ir are t h e best such metals a t reducing t h e work f u n c t i o n ( 4 ) o f t h e cathode, a r e d u c t i o n o f up t o 0.2eV being achieved; P t and Au are n o t a b l y bad, w i t h W - P t coated 'cathodes' having a work f u n c t i o n 0.2 eV h i g h e r than a pure !I! m a t r i x cathode.
(''present address : Vacuum Generators Ltd, Menzies Road. GB-Hastings. TN34 1QY East Sussex Great-Britain
(alpresent address : V.G. Scientific Ltd., The Birches Industrial Estate. Imberhorne Lane.
GB-East Grinstead, RH19 1UB West Sussex, Great-Britain
( 3 ) ~ l s o at SERC Daresbury Laboratory, GB-Warrington WA4 4AD. Great-Britain
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986899
C8-530 JOURNAL DE PHYSIQUE
I t i s known [21 t h a t t h e low- $ c o a t i n g i s l e s s t h a n a monolayer, and t h a t t h e p r i n c i p a l c o n s t i t u e n t s o f t h e a c t i v e l a y e r a r e barium, oxygen and t u n g s t e n . However, t h e d e t a i l s o f t h e b o n d i n g and t h e o r i e n t a t i o n o f t h e Ba-0-W complex have n o t been f u l l y understood, l a r g e l y because t h e s u r f a c e s t r u c t u r e has no l o n g range o r d e r , t h u s p r e c l u d i n g s t u d y b y l o w energy e l e c t r o n d i f f r a c t i o n (LEED) o r o t h e r d i f f r a c t i o n techniques. T h i s i s where SEXAFS h o l d s a m a j o r advantage, s i n c e i t probes o n l y t h e l o c a l s t r u c t u r e around adsorbate atoms, and i s t h u s t h e t e c h n i q u e o f c h o i c e f o r s t u d y i n g d i s o r d e r e d and p o l y c r y s t a l l i n e s u r f a c e phases.
Experiment and R e s u l t s
Our samples were r e a l d i s p e n s e r cathodes, manufactured i n t h e u s u a l way, and d i f f e r i n g f r o m commerical d e v i c e s o n l y i n b e i n g p o l i s h e d t o g i v e a f l a t t e r s u r f a c e . They were chosen t o be r e p r e s e n t a t i v e of t h e b e s t uncoated W m a t r i x cathodes (B-type), an improved a1 1 oy-coated cathode (CD-type), and a ' c a t h o d e ' w i t h p o o r e m i s s i o n c h a r a c t e r i s t i c s (W-Pt a1 1 o y ) . The e m i s s i o n c h a r a c t e r i s t i c s and Auger e l e c t r o n s p e c t r a (AES) o f t h e cathodes were checked b e f o r e and a f t e r t h e SEXAFS measurements, and f o u n d t o be i n d i s t i n g u i s h a b l e f r o m t h o s e o b t a i n e d f r o m batches o f normal cathodes.
S u r f a c e EXAFS s p e c t r a were measured u s i n g apparatus [31 a t t h e Daresbury L a b o r a t o r y S y n c h r o t r o n R a d i a t i o n Source, w i t h t h e a b s o r p t i o n c o e f f i c i e n t above t h e Ba L -edge a t around 5247 eV b e i n g r e c o r d e d b y c o l l e c t i n g t h e t o t a l e l e c t r o n y i e l d i n a Faraday cup d e t e c t o r p l a c e d c l o s e t o t h e sample. 3 The b e s t q u a l i t y SEXAFS d a t a were c o l l e c t e d w i t h t h e cathodes a t room temperature, b u t a s i g n i f i c a n t number o f t h e SEXAFS measurements were p e r f o r m e d w i t h t h e samples a t temperatures between 825 and 880°C, where t h e y a r e a c t u a l l y w o r k i n g as good cathodes, e m i t t i n g about 1 Amp o f e l e c t r o n s . The cathodes were b i a s s e d t o suppress t h i s , so t h a t t h e d e t e c t o r r e c e i v e d o n l y t h e c u r r e n t due t o photon a b s o r p t i o n . C a r e f u l comparison o f many s p e c t r a from a l l o f t h e samples i n o u r s t u d y showed t h a t t h e same f r e q u e n c y components were r e p r o d u c i b l y p r e s e n t i n t h e SEXAFS d a t a f r o m h o t and c o l d cathodes, i n d i c a t i n g t h a t t h e s u r f a c e complex i n v o l v i n g t h e b a r i u m atoms has t h e same b a s i c l o c a l s t r u c t u r e a t b o t h temperatures.
The d a t a were analysed, a f t e r t h e u s u a l background s u b t r a c t i o n and c o n v e r s i o n t o k-space, by a c u r v e f i t t i n g r o u t i n e based on t h e r a p i d c u r v e d wave c o m p u t a t i o n a l scheme [4], i n c l u d i n g a l e a s t squares i t e r a t i o n t o g i v e t h e b e s t t h e o r e t i c a l f i t t o t h e e x p e r i m e n t a l d a t a . E l e c t r o n s c a t t e r i n g phase s h i f t s f o r Ba, 0 and W were c a l c u l a t e d u s i n g a " m u f f i n t i n " p o t e n t i a1 -based method, and checked a g a i n s t o u r measured EXAFS s p e c t r a f r o m BaO ( p r e p a r e d i n vacuo by t h e r m a l decomposition o f BaC03 on a P t s u b s t r a t e ) , BaWO , BaMoO and samples o f t h e impregnant compound.
The c o o r d i n a t i o n nfimbers a8d bond l e n g t h s d e r i v e d f r o m o u r SEXAFS a n a l y s i s a r e p r e s e n t e d i n Table 1 . The e x p e r i m e n t a l n o i s e l e v e l p r e v e n t s o b s e r v a t i o n o f s h e l l s a t g r e a t e r d i s t a n c e s t h a n about 4.5 A. The e r r o r b a r s on t h e d i s t a n c e s ( a r i s i n g m a i n l y form random s t a t i s t i c a l f l u c t u a t i o n s between s u c c e s s i v e r u n s ) a r e somewhat worse t h a n t h o s e a t t a i n a b l e w i t h s t a t e - o f - t h e - a r t SEXAFS on we1 1 - d e f i n e d s i n g l e c r y s t a l s u r f a c e s . A l t h o u g h t h e EXAFS t e c h n i q u e i s n o t p a r t i c u l a r l y good a t d i s c r i m i n a t i n g between d i f f e r e n t t y p e s o f n e i g h b o u r atoms, i n t h i s case t h e d i s t i n c t l y d i f f e r e n t b a c k s c a t t e r i n g s i g n a t u r e s f o r Ba, oxygen and t h e s u b s t r a t e metal r e a d i l y a l l o w us t o d e t e r m i n e which atoms a r e i n v o l v e d i n each s h e l l o f neighbours ( a l t h o u g h t h e s m a l l d i f f e r e n c e s between W, 0 s and P t cannot be d i s t i n g u i s h e d ) . I t can t h u s be s t a t e d t h a t t h e Ba atoms a r e c o o r d i n a t e d d i r e c t l y t o oxygen, w i t h s u b s t r a t e atoms as n e x t n e a r e s t neighbours, f o r b o t h t y p e s o f good cathode s t u d i e d h e r e . Such s u r f ace s t r u c t u r a l i n f o r m a t i o n has n o t p r e v i o u s l y been o b t a i n e d on r e a l cathodes, and i n d e e d i t was n o t even known whether t h e r e was a unique, w e l l - d e f i n e d Ba-0-W s u r f a c e complex.
The same samples used i n t h e s e measurements were a l s o analysed b y X-ray d i f f r a c t i o n (XRD) i n a g r a z i n g i n c i d e n c e geometry t o emphasise t h e c o n t r i b u t i o n o f s u b s t r a t e p l a n e s near t o t h e surface. F o r t h e B-type cathode ( p u r e W m a t r i x ) , a b.c.c. d i f f r a c t i o n p a t t e r n was o b t a i n e d which i n d i c a t e d a s l i g h t prepronderance o f a number o f c r y s t a l p l a n e s p a r a l l e l t o t h e s u r f a c e . The CD-type (W/Os a l l o y ) cathode was f o u n d - t o be m a i n l y h.c.p., as m i g h t b e expected f r o m t h e b u l k phase diagram f o r W/OS, { l o 1 0 3 p l a n e s d o m i n a t i n g t h e s u r f a c e c o n t r i b u t i o n t o t h e XRD p a t t e r n . F o r t h e P ~ / W a l l o y cathode, t h e surface was f o u n d t o almost e n t i r e l y c o n s i s t o f p l a t i n u m (f.c.c.1 I 1 1 1 3 p l a n e s .
Table 1. Nearest and next-nearest neighbour distances (R, A ) and c o o r d i n a t i o n numbers (N) d e r i v e d b y SEXAFS f o r Ba atoms on t h e surfaces o f t h r e e types o f thermionic dispenser cathode.
Cathode t y p e Nearest neighbour (Ba-0) Next nearest neighbour (Ba-W) ( a )
R (f0.044) N (f0.5) R (f0.12 A ) N (f1.5)
B ( W m a t r i x ) 2.62 1 3.59 4
CD(W/Os a l l o y ) 2.61 2 3.75 4
d\l/Pt a l l o y 2.72 1 - ( b ) -
( a ) t h e technique cannot d i s t i n g u i s h between W, '0s and Pt neighbours ( b ) No w e l l - d e f i n e d Ba-metal d i s t a n c e was observed f o r t h i s surface.
Discussion
To deduce surface s t r u c t u r a l models f o r t h e cathodes, we combine i n f o r m a t i o n on l o c a l s t r u c t u r e (SEXAFS) , t h e predominant morphology (XRD) , surface composi t i d n (AES) and work f u n c t i o n . @ can be r e l a t e d s e m i - q u a n t i t a t i v e l y t o t h e d i p o l e s t r e n g t h o f t h e o r i e n t e d Ba-0 surface l a y e r , t h i s d i p o l e s t r e n g t h depending on t h e i o n i c i t y of t h e Ba and 0 v i a t h e well-known Helmholtz r e l a t i o n s h i p . The observed work functions can t h e r e f o r e be used t o estimate t h e a p p r o p r i a t e Ba and 0 i o n i c i t i e s and hence t h e i r atomic r a d i i 151.
Numerous models o f Ba-0-W surface complexes on t h e exposed c r y s t a l planes o f t h e cathodes were considered. For t h e B-type and CD-type cathodes, t h e r e s u l t s o f our modelling e x e r c i s e s t r o n g l y favour surface complexes o f t h e type shown i n Fig.
1. S i m i l a r s t r u c t u r e s can be derived f o r a l l c r y s t a l planes present i n s i g n i f i c a n t p r o p o r t i o n s a t t h e B-type cathode surface.
(a I ( b )
Fig. 1 The surface complexes d e r i v e d f o r ( a ) a { 1 0 i 0 ) f a c e o f t h e W/Os a l l o y (CD-type) and ( b ) a (2111 f a c e o f t h e W m a t r i x (B-type) cathodes. The Ba atoms are depicted as white, 0 atoms grey and s u b s t r a t e atoms black, w i t h no d i s t i n c t i o n being made i n t h i s model between t h e W, 0s and Pt s u b s t r a t e atoms. The choice o f atomic r a d i i i s described i n t h e t e x t .
We propose t h a t t h e morphology o f t h e surface p l a y s an important r o l e i n determining t h e e f f i c i e n c y o f cathodes. The more open s t r u c t u r e o f t h e CD-type surface planes has a Ba atom b r i d g i n g two 0 atoms. This w i l l r e s u l t i n g r e a t e r e l e c t r o n t r a n s f e r towards t h e surface, a l a r g e r surface d i p o l e , and a smaller work work f u n c t i o n . I n a d d i t i o n , an open s t r u c t u r e a l l o w s t h e 0 atoms t o s i t i n t h e surface l a y e r , below t h e Ba atom ( t h i s o r i e n t a t i o n i s a l s o c o n s i s t e n t w i t h our angul a r l y - r e s o l v e d AES measurements). I n t h i s c o n f i g u r a t i o n , e l e c t r o n t r a n s f e r f r o m t h e oxygen end o f t h e Ba-Ox adsorbate i n t o t h e surface should be maximised,
CS-532 JOURNAL DE PHYSIQUE
i n c r e a s i n g t h e s u r f a c e d i p o l e . By t h e same argument, charge t r a n s f e r f r o m Ba-0 t o t h e c l o s e packed s u r f a c e o f t h e W/Pt a l l o y w i l l be minimised. There may t h u s be an optimum s u r f a c e c r y s t a l l o g r a p h y which c o u l d h o l d t h e key t o development o f even more e f f i c i e n t cathodes.
References
1. f o r a review, see, e.g. D. Norman, 3. Phys. C: S o l i d S t a t e Phys., i n press.
2. r e v i e w e d by R.A. Tuck, Vacuum 33, 715 (1983).
3. A.A. MacDowell, D. Norman and J.B. West, Daresbury L a b o r a t o r y T e c h n i c a l Memorandum DL/SCI/TM48E (1986).
4. S.J. Gurman, N. B i n s t e d and I . Ross, J. Phys. C : S o l i d S t a t e Phys. 17, 143 (1984).
5. L . P a u l i n g , "The N a t u r e o f t h e Chemical Bond", p u b l . C o r n e l l U n i v e r s i t y Press, I t h a c a , New York, 1960
