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STABLE NECKS ON METAL TIPS
M. Drechsler, S. Ramdani, Alain Claverie, A. Maas
To cite this version:
M. Drechsler, S. Ramdani, Alain Claverie, A. Maas. STABLE NECKS ON METAL TIPS. Journal
de Physique Colloques, 1987, 48 (C6), pp.C6-209-C6-214. �10.1051/jphyscol:1987634�. �jpa-00226838�
JOURNAL DE PHYSIQUE
Colloque C6, suppl6ment a u n O 1 l , Tome 48, novembre 1987
STABLE NECKS ON METAL TIPS
M. Drechsler, S. Ramdani, A. ~ l a v e r i e * a n d A. ~ a a s * *
CRMC2-CNRS, Campus d e Luminy c a s e 913, 13288 Marseille c e d e x 09, F r a n c e
*CNRS, Laboratoire d1Optique Electronique, 31055 Toulouse, F r a n c e
**
A.G. Festkorper-Oberflachen, Universitlt Bonn, 5300 Bonn, F.R.G.ABsTRACr I On a Patal t i p (01, N i , Au, Cu) heated i n an e l e c t r o n mlcroecope
(m)
a forpation o f stable &ce ie abeemed. M thie d o e m not agree with the theory, experlmente are me& f o r c l a r i f i c a t i o n . I n e i t u e l e c t r o n micron- (TW,-,
XEXS) ehcrwe the existence o f graphit- aurface layem, which eurprieingly ramah stable up t o near the m l t i n g p o i n t . Theme layera hinder a -1ete meparation o f a e o l i d m e t a l drop from t h e tip end. On t h e baeie o f t h i s r e s u l t epectacular t i p and neck ehapc changee are explainable an an ot3twald ripening.n u n a m a r l y c y l i n d r i c a l metal t i p I n heated i n the vacuum of an e l e c t r o n mlcmxmope.
apbctacular r e l a t i v e l y m l e neck3 are £0- /1//2/. M t h l n phenrPenon ie not understood, w h i c h hindere tha uee o f t i p a f o r eurface and materiala science s t u d i e s , ws have now atudied the^ stable neck phenomenon in ecme d e t a i l and deecriba the r e s u l t in thi. paper.
2. ~2.1. Theo7xkical basis. m I OF mTRE
-
Which ie the morphological evolution o f a heated eol5.d laeta1 in vacuun ? An a n w r is given on the b a e i s of tha NERNST-EINSTEIN equation o f antter t r a m p o r t i n the came of ostal t i p a by the c a l c u l a t e d r e s u l t 8 o f NICBOIS and WLLINs /1/. Depending on the t i p cone angle a two damaine o f t i p evolution e x i s t r ( 1 ) f o r cone anglee a > 30 the t i p end radiu r increaeee c o n t l n u o u l y and a measurement of r as a function of tipe has been o f t e n uead to deterPine s u r f a c e r r e l f d i f f u i o n c o e f f i c i e n t e o f metale /2/. ( 2 )
For aaaller cone anglee (a < 30) the c a l c u l a t i o n p r e d i c t e a formation of a t i p neck of decreruing dlamter u n t i l a clol5.d drop i e separated frcm~ the t i p end. An example of euch an evolution is ahawn in f i a . 1. Thie eecond evolution i e the baeie o f t h e p h e m n o n dellcrlbed in t h i e paper.
0 min
9 min
60 min
135 min Pig. 1
-
calculated ahape evolution Pig. 2-
Shape evolution o f a mingle of a heated t i p c a p i l l a r i t y orystal tungoten tip v i c l u a l ~ ly a Muce4 arcfaca w l f d l f A u i o n ecannlng e l e c t r o n micmocoge /3/.t r a ~ p o r t / ~ / 3 / . *p corn angle 20. T = 2300 K I a
-
30.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987634
C6-210 JOURNAL DE PHYSIQUE
2.2. Stable neck on a e i n g l e c r y e t a l t i p .
The experimental evolution of a e i n g l e c r y e t a l t i p is ehown i n f i g . 2. A t first view c a l c u l a t e d and experimental evolutions eeema t o agree. However there i e an important discrepancy i n the time dependence of t h i s evolution. lbe c a l c u l a t i o n p r e d i c t s that the time of the existence of a emall neck i e emall caapamd t o the time of the formation of the neck b u t t h i e is opposite t o the experimental r e s u l t . So t h e fundamental problem exist t why are t h i n necka so stable ?
2.3. stable neck8 on a p o l y c r i e t a l l i n e t i p .
The necke on p o l y c r y e t a l l i n e t i p s are ecmejhat d i f f e r e n t t o tho- on e i n g l e c r y s t a l t i p a . A t y p i c a l experimental r e e u l t i e echematized i n f i g . 3. (aee also f i g . 1 0 ) . P i r e t l y t h e ueual system o f i r r e g u l a r g r a i n boundaries is tramformed by boundary migratione i n a eystem of "bamboon boundaries ( f i g . 3 a and b ) . Boundary grooves and necke are formed a t theee bountiaries ( f i g . 3 b and c ) /4/.
Fig. 36 I l l u e t r a t e s again the main problem etudied i n t h i e paper r While the c a l c u l a t i o n p r e d i c t e a g r a i n o r s o l i d drop eeparation from t h e t i p , the experiment shows i n nearly a l l cases an appearence o f a e t a b l e neck. What is the reason f o r t h i s diecrepancy ?
tip with irregular boundaries
b)
"bamboo" boundaries are formed
Pig. 3
-
Scheme o f the fonaation o fs t a b l e neck8 on a p l y c r y s t a l l i n e t i p C,
( t i p radiua increase i e neglected). grain boundary grooving
theory
/
/ \
experiment2.4. C y l i n d r i c a l neclur.
solid :rap stable necks sepdrstion
[wHy?I
A e p e c i a l type o f ( r e l a t i v e l y ) e t a b l e necke are cyJindrica1 necke. An exemple i e e h m in f i g . 4. 'Pha diameter of the neck i e about 400 A. It is e u r p r i s i n g t h a t a neck with euch a g r e a t eurface t o volume r a t i o is few.
0 min
3 min Pig. 4
-
PorPsatlon o f a c y l i n d r i c a l stableneck on a N i t i p /3/ heated (1250 I) i n the abeence o f e l e c t r o n impact.
6 min
3. lTlE PINDING THAT STAB= NECKS NUST BE RELATED l 0 SUWACE LRYERS
I n order t o o b t a i n b e t t e r information on stable nedcs w e s t a r t e d t o study the problem by transmission ( i n s t e a d of scanning) e l e c t r o n microscopy. Used wae a 100 kV 'FEU ( P h i l i p e ) with a heating chamber up t o 1500 K /I/ and t h e 3 MU microscope o f lbU10~Se w i t h a heating chamber up t o 1000 K. I n s i t u video r e g i s t r a t i o n wae ueed i n Both canes.
The advantages o f w i n g a TEn are r ( 1 ) The much better r e s o l u t i o n ( - 5 A). ( 2 ) The v i s u a l i z a t i o n o f s u r f a c e layers /6/ ( f i g . 8 ) . ( 3 ) The p o s s i b i l i t y t o obtain i n f o l l ~ a t i o n on impurities by dark field d i f f r a c t i o n hmging. ImpuritiefY Can o r i g i n a t e fmm the r e s i d u a l gae i n the microscope o f
-
lod Torr ( t y p i c a l l y H20, 02, N2, H2, hydmmwbone aB CE4,. . .
). I n f a c t the dark f i e l d study i n d i c a t e s and l o c a l i z e s an impurity ( f i g . 5 an4 6 ) . The surpri8ing r e s u l t is t h a t the neck region is sametimes completely mtal (Cu) free ( f i g . 5 and 6 ).
Bright field imaging show t h a t the *ole t i p SurfaCe is o f t e n rovered with a surface l a y e r ( g i g . 8 ) /6/. The measured layer thicknese i e in t h e range between zero and -300 A /6/. A c o r r e c t layer eubetance a n a l y s i s vae made by SlTM e l e c t r o n enezqy l o s e spectroscopy ( w i t h t h e h e l p o f W . FoWmeaUX, Toulowe). The generalized r e s u l t is t h a t t h e substance of the l a y e r s is graphitized carbon. This is s u r p r i e i n g because the w e l l knam g r a p h i t i d layera i n TELI and SEbI microscopy are known t o be detached from the specimen eurface a t temperatures i n t h e o r d e r of 300 C ( a l s o confirmed by our observations) while o t h e r layere were e t a b l e up t o about 1000 C and eceae probably up t o 20000C. A general c o n c l w i o n o f t h e s e r e e u l t a is t h a t graphitized l a y e r s of very d i f f e r e n t s t a b i l i t y exist. If a l a y e r i e mom o r l e e s stable eeame t o depend ( 1 ) on t h e l a y e r preparation and ( 2 ) on the specimen geometry (plane film, t i p , supported particles ).Pig. 5
-
S t a b l e neck on a Cu t i p during i n e i t u heating (1200 IC) and video r e g i s t r a t i o n . TEM darkf i e l d Image. The b r i g h t l i n e s i n d i c a t e a copper surface layer. N e c k diameter
- -2:-
4. EXPLANATICm OP THE STA8LE NECK PmRoHmcm
The stable neck model which agrees b e e t with M e experimental r e s u l t 0 is echeaatieed i n f i g . 7 . Pig. 7a shawe a low angle t i p a f t e r some s u r f a c e s e l f - d i f f w i o n . A t
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lod'Ibrr and elevated temperature a graphitized s u r f a c e l a y e r o f increasing thicknees is formed. I n d e t a i l , the hydrocarbons o f the gas are adsorbed and demrbed on the t i p surface, w h e r e a eubslonolayer of adsorbed hydrocarbons is formed. These are cracked by the high temperature, by the impinging e l e c t r o n s (100 kV) o r by both. !me hydrogen denorbe while the carbon effect6 the gruwth of the graphitized l a y e r . The experience show that the preeence of the graphitized l a y e r hae o f t e n ( s u r p r i s i n g l y ) l i t t l e influence on the Cu s u r f a c e d i f f u s i o n t r a n s p o r t along the Cu surface. I n former i n t e r p r e t a t i o n o f s t a b l e neck micrographs (ae i n f i g . 1 0 ) it w a e assumed that the m l i d metal drop is not eeparated frwa t h e t i p shank. This i n t e r p r e t a t i o n must be
C6-212 J O U R N A L DE PHYSIQUE
revised. The drop is mostly separated from t h e t i p metal but s t i l l hold by t h e non- m e t a l l i c l a y e r ( f i g . 7c, f i g . 6 and e ) .
-- .\
a )
initial shape
Pig. 7
-
Explanation of t h e formation of as t a b l e neck on a s i n g l e c r y s t a l t i p ( s e e t e x t ) . b' a neck and a graphitized layer is formed metal drop seperated from the
tip shank
residual 'metal tip shank graphiiizad layer of tubule shape
pig. 8
-
TEM (3x106 V) micrograph o f a Cu t i p a f t e r h e a t i n g during observation( - 800 C f o r
-
20 min). The - l i d Cu drop(- 1 , 5 p) is separated from t h e metal shank and is only hold by t h e l a y e r ( -
l o 3 A t h i c k ) which h a s a t u b u l e shape a t t h e neck. STEM-EELS a n a l y s i s h a s shorn t h a t t h e b r i g h t appearing l a y e r is g r a p h i t i z e d carbon. The few wlall dark p o i n t i n t h e l a y e r are i s o l a t e d very small Cu c r y a t a l l i t e s .
5 . A HDDEL FOR THE PORMRTION OF CYLINDRICAL NECKS
Why stable necks e v o l u t e sometimes t o f i l i f o r m necks ( f i g . 3 ) must be explained t o o . When t h e metal i n s i d e a neck ( f i g . 9a) is transported t o t h e metal drop o r t h e t i p shank a s p e c i a l vacuum space is fonned ( f i g . 9b). The r e s i d u a l layer ha6 a shape similar t o a hollaw double cone. Such a cone seems t o be submitted t o a d r i v i n g f o r c e which t e n d s t o reduce t h e cone diameters ( r e d u c t i o n of t h e s u r f a c e a r e a ) . The hypothesis is t h a t t h i s cone can not resist t o t h i s force because ( 1 ) experiments show t h a t such l a y e r s are very f l e x i b l e ( a d a p t a t i o n t o support shape changes) and ( 2 ) t h e t i n y l a w l l a s of a g r a p h i t i z e d l a y e r should be e a e i l y d i s p l a c a b l e at high ternparaturea i n lamella d i r e c t i o n s . Consequently t h e cone diameters should decrease, t h e larnellae m y become more s l i g h t l y curved and t h e l a y e r t h i c k n e s s may i n c r e a s e sanewhat. B u t t h i s process must be l i m i t e d because t k e increaaing curvature causes an increasing counter f o r c e . The state of minimum f r e e energy formed should be a t u b u l e of a l i m i t diameter.
6. ONSPABILITIES OF STABLE NECKS
S t a b l e necks are only r e l a t i v e l y s t a b l e . I n f a c t necks and tip8 shaw slow b u t s p e c t a c u l a r shape v a r i a t i o n s . An e-le /3/ is shown i n f i g . 10. Formerly it waa auppoeed t h a t such shape changes a r e perhaps caused by t h e n a o d i f f u s i o n . Though thenuo-diffuaion may play a r o l e , it seem not be a dominant r o l e . The f i n d i n g of t h e e x i s t e n c e of t h e g r a p h i t i z e d l a y e r s opens a new way t o explain t h e myeterioua u n s t a b i l i t i e s .
The new hypothesis is t h a t t h e disappearence of stable necks is a e p e c i a l type of Ostwald ripening. The r i p e n i n g of s p h e r i c a l metal c r y s t a l s is schematized i n f i g . l l a and b. Surface d i f f u s i o n e f f e c t s a t r a n s p o r t of metal a t o m from t h e m a l l t o t h e
greater cryetal; which reduces the t o t a l metal eurface area and the t o t a l fzee energy of the system. In the caee of a t i p with a atable neck ( f i g . 11 c and d ) the metal atom mlarate also t o surface reaiom - -
02
- - araater surface radii--
- ~- - - - - - - - - that the free e n e r w of - ~ ~--
the myatem l a reduced (the eurface free energy of the graphitbed layer ie very mall and can be neglected i n the energy balance for eimplification).I
graphitized layer solid metal
Pig. 9
-
Explanation of the formation of a f ilifozm neck.b)
terface diffusion of
diffusion of metal atoms
An analveia of micmarac?m eerie8 a8
-
- --- -
that of f i a . - 10 ahow that the total -1 eurface area ir, continuously decreasing with time i n a l l case8 studied eo far. T h i e i e a atrong argument t o aseunm that the described Oatwild ripening hypotheeia ie correct. Residual part8 of graphit- layer are not visible i n the SW micrograph8 of fig. 10, but found in analogous T m experimante with Cu tipe.I
0 mln
Pig. 10
-
Diaappearence of a =lid dropwith a etable neck (W, 2600 11). 20 mln
c 2 4 mln
0 . 2 p
2 8 mln 7 . D I ~ I O N
Tha explanation of the etable nedt phenomena presented here ie preliminary and need t o be confirmed. I t may be aleo desirable t o study the etable neck8 i n inre detail i n a quantitative manner.
d ~ f f u s ~ o n transport
JOURNAL DE PHYSIQUE
d~ffusion transport
( k n o w n case ) graphitized ' layer
Pig. 11
-
Scheme of the dieappearen- of a c r y s t a l by Oehrsld r i p e n i n g on a plane support ( a and b ) awl on a t i p with a g r a g h i t i e e d layer ( c and d ) . Ihe arrcrns indicate tha d i f f u s i o n t r a m p o r t d i r e c t i o n .( 1 ) Ihe f o w t i o n o f a t a b l e necks on metal tip ie a conemquence o f an e x i s t e n c a of g r a p h i t i z e d s u r f a c e layers.
( 2 ) Besides the knawn q r a p h l t l m d layera fora*td on electron laicroecope specIrene (and aetadhea above
-
3OOoC) exiet e r e *miCh are s t a b l e up t o-
2000oC).( 3 ) A stable necR is formed by a c a p l l l a r i t p inducecl a e p e r a t i o n o f a mlid metal drop f n a the tip but both mmaLn hold t o g e t h e r by t h e tubule g r a p h i t i z e 6 layer.
( 4) m e t a l at- d l f f u s a e a e i l y along ( 1 ) t h e I n t e r f a c e metal-graphitized l a y e r and ( 2 )
the i n n e r e i d a o f the t u b u l e l a y e r .
( 6 ) S o l i d drope and s t a b l e mtckr, d- on long term by Olrtwald ripening.
The a u t h o r s llke t o thank Mr. R. -, 'I'oulo~ea, Labomtoire d 9 0 p t i ~ Xlectronique du OIRS, f o r the analyola of the layer eubeance.
/l/ P.A. Widhole and W.W. m l l i n a r J. Appl. Physic0 36 ( l % 5 ) 1826
/2/ V.T. ~ h h , A. Piquet, 8 . Iliwrr, R. U m n , M. Dmchaler r Surface S c i .
2
(1971) 340 /3/ M. Drechsler, A. Piquet, V.T. ~ l n h and R. U M ~ , in S t r u c t u r e et P r o p r i e t e a den Surfaces d e s Solidea, Coll. Int. dum,
Par- 1970, p. 193 and Surface S c i . ( 1969) 467/4/ V.T. Binh, M. Chaudier, J.C. Couturier, R. Uuan and M. Dredheler r Surface S c l .
7
(1976) 184
/6/ A. Maam r lIheinlnch WestfiillaQw, Akad. d. W l a m e f m c h . , vortriiga N 301.
Weetdeutmcher Verlag, Diieaeldorf 1981, 51-124
/6/ M. Dmdmler, 9. RaPdani and A. Haan r paper ZtCCepted by Surface S c i .
