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THE WETTABILITY OF LIQUID METAL ION SOURCES
M. Bozack, L. Swanson, A. Bell
To cite this version:
M. Bozack, L. Swanson, A. Bell. THE WETTABILITY OF LIQUID METAL ION SOURCES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-115-C2-120. �10.1051/jphyscol:1986217�. �jpa-00225650�
JOURNAL D E PHYSIQUE
Colloque C2, suppl6ment au n 0 3 , T o m e 47, m a r s 1986 page ~ 2 - 1 1 5
THE WETTABILITY OF LIQUID METAL ION SOURCES
M.J. BOZACK*, L.W. SWANSON and A.E. BELL
Oregon Graduate Center, 19600 NW von Neumann Drive, Beaverton, O R 97006, U.S.A.
Abstract - Wetting of liquid metal alloys to solid substrates is governed by Young's equation, which relates the degree of wetting to the combined influence of solid-liquid, solid-vapor, and liquid-vapor surface tensions.
It is important that a liquid metal alloy source exhibit good wetting with a near-zero contact angle. The wettability of several B-containing alloy systems is considered, with emphasis on the role of surface segregation on contact angle.
I - INTRODUCTION
In order to achieve a successful liquid metal ion source (LMIS) of high melting nonmetals, it is necessary that the MIS be formed from an alloy that provides both an electrically conductive medium and a low melting eutectic. Once this is
accomplished, a suitable substrate material must be selected that allows for good wetting and flow of the alloy to the needle apex (as in the case of the wetted needle type), but not excessive chemical interaction with the solid substrate so that dissolution by the alloy occurs. In this paper we provide evidence from stu- dies of liquid alloy surfaces which shows that wetting of B-containing alloys to nonmetallic substrates is governed by surface segregation of low-level bulk impuri- ties in the alloys. During melting, the impurities segregate to the surface of the alloy and inhibit reaction between alloy and substrate. A poorly wetted droplet with a high contact angle results. Employing surface coatings composed of materials reactive to the impurities acts to suppress segregation and promote wetting.
I1 - EXPERIMENTAL METHODS
Wetting studies were carried out in an ULTEK TNB-X 250 l/s ultra-high vacuum system c0ntaining.a Physical Electronics CMA model 10-155 Auger electron spectrometer.
The substrate and support mechanism were mounted in a vertical orientation inside the vacuum chamber, and a small fragment of solid alloy was placed in contact with the substrate. Contact was maintained by gravity and no adhesive was used. The system was subsequently evacuated to below 10-7 torr. Wetting was initiated by resistively heating the substrate with a D.C. power supply until alloy melting and wetting was observed. Temperatures were measured by means of a single wavelength optical pyrometer by comparison to the substrate brightness, whose emissivity characteristics were well-known 111. Auger surface compositions reported are
"Present address : Surface Science Center, University o f Pittsburgh, Pittsburgh, P A 15260, U.S.A.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986217
~ 2 - 1 1 6 J O U R N A L DE PHYSIQUE
c a l c u l a t e d on t h e b a s i s of s e n s i t i v i t y f a c t o r s g i v e n by Palmberg / 2 / . 111 - WETTING OF NICKEL AND PLATINUM BORIDE ALLOYS
The s t r o n g r e a c t i o n between r e f r a c t o r y m e t a l s and l i q u i d m e t a l a l l o y s r e s u l t s i n s e v e r e l i m i t a t i o n s on t h e l i f e t i m e of high-melting LMIS employing m e t a l l i c sub- s t r a t e s . For example, a l t h o u g h Re i s w e l l - w e t t e d by B - c o n t a i n i n g a l l o y s , t h e s u s c e p t i b i l i t y of Re t o a t t a c k by t h e l i q u i d a l l o y r e s u l t s i n l i f e t i m e s o f t h e o r d e r of a few h o u r s f o r M I S u s i n g Re n e e d l e s u b s t r a t e s . Because of t h i s drawback, c o n t a c t s y s t e m s u s i n g n o n m e t a l l i c s u b s t r a t e s have been examined.
Carbon has been u s e d s u c c e s s f u l l y i n many l i q u i d m e t a l environments. It i s f r e - q u e n t l y employed a s a c r u c i b l e m a t e r i a l and a s an e l e c t r o d e i n e l e c t r i c a r c m e l t i n g of m e t a l s . P o l y c r y s t a l l i n e g r a p h i t e h a s been s u c c e s s f u l l y used a s a s u b s t r a t e f o r a n A l LMIS / 3 / . T r a n s i t i o n m e t a l b o r i d e s of N i and P t a r e e x c e l l e n t c a n d i d a t e s f o r B s o u r c e m a t e r i a l due t o t h e i r r e l a t i v e l y low m e l t i n g p o i n t (-1000 C) o v e r a f a i r l y wide r a n g e of c o m p o s i t i o n s around 50% a t o m i c B. The e x p e c t a t i o n t h a t b o r i d e s of t h e s e m e t a l s are e x p e c t e d t o wet c a r b o n i s based upon r e p o r t e d e v i d e n c e / 4 / t h a t m e t a l l i c N i by i t s e l f w e t s g r a p h i t e w i t h a c o n t a c t a n g l e o f 45 d e g r e e s . F u r t h e r , B i s o f t e n used a s a n a d d i t i v e t o m e t a l s t o promote w e t t i n g on g r a p h i t e . Pure Cu, f o r example, h a s been r e p o r t e d t o e x h i b i t a c o n t a c t a n g l e of 140 d e g r e e s on g r a p h i t e , b u t Cu w i t h 5% added B h a s a c o n t a c t a n g l e of 36 d e g r e e s / 5 / . There i s t h u s good r e a s o n t o b e l i e v e t h a t e u t e c t i c s of NiB and PtB s h o u l d wet carbon. That e x p e r i m e n t a l o b s e r v a t i o n s of w e t t i n g b e h a v i o r prove o t h e r w i s e w i l l be e x p l a i n e d below.
A. Wetting of N i c k e l and P l a t i n u m Boride A l l o y s t o Carbon
N i c k e l b o r i d e a l l o y p r e p a r e d by t h e a r c m e l t i n g t e c h n i q u e was a p p l i e d t o a f l a t r i b b o n of g r a p h i t e and h e a t e d t o m e l t i n g . W e t t i n g proved t o be u n s u c c e s s f u l . The
1 0 0 p r n 1 0 0 p m
- H H
* Position @ ori it ion@ position@ ~ o s i t i o n a
F i g . 1 - Wetting of Ni62B38 a l l o y on g r a p h i t e . (Top l e f t ) View of poorly-wetted d r o p l e t a t room t e m p e r a t u r e . (Top r i g h t ) C r o s s - s e c t i o n a l s l i c e t h r o u g h d r o p l e t .
(Bottom) Auger s u r f a c e e l e m e n t a l c o m p o s i t i o n of 4 p o s i t i o n s on t h e m o l t e n d r o p l e t . High c o n c e n t r a t i o n s o f C a r e e v i d e n t .
melted alloy formed a poorly-wetted droplet with a contact angle greater than 90 degrees. Auger analysis of the molten droplet surface at temperatures near the alloy melting point revealed large surface concentrations of C (and often N). A representative case is given in Figure 1. It was frequently possible to elicit wetting by increases in temperature. When this was done, a wetting front could be observed to expand from a side of the droplet which wet the substrate with a con- tact angle of near-zero. Auger analysis of this situation showed that the wetting front consisted of material composed solely of alloy components, while the droplet continued to exhibit large concentrations of C and N.
Wetting of platinum boride eutectics to carbon exhibited behavior similar to that described above for nickel boride alloys. At melting, the alloy formed a poorly- wetted droplet with a contact angle of near 180 degrees. In many cases, the affi- nity of the alloy for the substrate was so poor that the droplet rolled off the substrate into the vacuum chamber. Auger analysis of the molten surface near the melting point again revealed large concentrations of carbon and nitrogen.
Our conclusions from studies of numerous B-containing alloy systems and stoichiometries are:
1) Pure nickel boride and platinum boride alloys wet and spread over virgin polycrystalline graphite.
21 Nickel boride and platinum boride alloys containing high surface concentrations of carbon and/or nitrogen wet virgin polycrystalline graphite reluctantly or not at all.
B. Wetting of Nickel and Platinum Borides to Other Substrates
In order to isolate the source of the carbon and nitrogen found in the molten alloy surfaces, wetting of eutectic compositions of nickel and platinum borides to non- carbon substrates was studied. This procedure allowed elimination of thermal dif- fusion between substrate and alloy as the origin of these elements.
Wetting of nickel boride to aluminum oxide is shown in Figure 2. Wetting of NiB to A1203 was attempted due to evidence from Lugscheider / 6 / indicating that some alloys of nickel and boron were found to wet aluminum oxide. The wetting was poor, consisting of a contact angle of about 130 degrees. More significantly, however, were the large concentrations of C and N at the surface of the poorly-wetted droplet. This material could only originate from the alloy itself, as the experi- ment was performed under conditions of ultra-high vacuum and the alloy is totally isolated from outside sources of these elements.
j; -
z
9 Position @ Position @ Position @ position @
k g so- a 1 4 0 -
Q 30 33;.
:3.1 33 2 32 8 3 3 3 33.4 32.5 352
1 25 0 23 1 23 0
2 20-
Z w 10- 8 4 7 7
Fig. 2 - Auger surface elemental composition vs position for wetting of Ni55B45 on Al2O3. Surface segregation of C and N are evident. The temperature is Tm + 60° =
1355 K.
JOURNAL DE PHYSIQUE
Similar behavior was found when wetting platinum boride alloy to noncarbon sub- strates. In particular, when platinum boride was wetted to Re, although the wetting was excellent, the surface was found to possess large concentrations of C and N.
IV - EVIDENCE FOR SURFACE SEGREGATION OF LOW-LEVEL IMPURITIES IN LIQUID METAL ALLOYS
The wettability of solids by liquid metal alloys appears to be governed by surface segregation of low-level impurities inherent to the alloys. Segregation of these impurities has been found in every alloy system we have studied, and allows an integration of a number of diverse results into a coherent picture of wetting beha- vior. Further, the existence of the surface segregation casts doubt on many pre- vious studies of wetting, as the existence of ppm impurities in the wetting materials can radically affect the contact angle. The evidence for segregation is as follows:
1) Direct experimental observation of large relative concentrations of low-level impurities at molten alloy surfaces. In the case of wetting to nonmetallic sub- strates such as carbon, the segregated materials act to inhibit wetting. In cases of wetting to metals such as rhenium, wetting is able to occur despite the presence of the surface impurities.
2 ) Theoretical calculations of surface enrichment. In particular, emission
spectroscopic analyses of the alloy materials showed typical levels of carbon impurity near 0.02 w%. Assuming the entire weight percent of C to segregate to the surface of a spherical droplet 0.5 mm in radius, the thickness L of the C layer formed may be calculated to be
where D(NiB) and D(C) are the densities of nickel boride alloy and graphite respec- tively. We have assumed the density of graphite to be 2.0 gm/cm3, and have calcu- lated the density of NiB (7.2 grn/cm3) from values given in the literature /7,8/.
Such analyses demonstrate that it is feasible for the high concentrations of C observed at the molten surfaces to orignate from surface segregated material.
Since the alloy fragments used in the wetting studies are fractured from the interior of the melt, the impurities are characteristic of bulk material.
3) Microscopic examinations of poorly-wetted droplets. Figure 1 above shows SEM photos of a nickel boride eutectic wetted above a graphite substrate. Auger analy- sis of the molten droplet surface showed large percentages of C and N, shown in the accompanying bar charts. The view at room temperature, shown in the left photo of the figure, shows that the droplet surface is covered with recrystallized precipi- tates of hexagonal material that scanning Auger analysis proved to be graphite.
Similar examinations of cross-sectional slices through such droplets showed no evi- dence of second phase graphite in the interior of the droplet. The interior appears uniform and homogeneous, characteristic of pure alloy material.
4 ) The composition of alloy material flowing from the interior of poorly-wetted
droplets. In several instances it has been possible to fracture poorly-wetted droplets of alloy by heat treatment. The efflux of material originating from the interior of the droplets was found to consist of pure alloy material that exhibited excellent wetting of the graphite substrate. That impurities are found on the sur- face of the droplet but not in the droplet interior is further evidence for surface segregation of low-level impurities in the alloys which occurs on melting.
V - THE EFFECT OF SURFACE COATINGS ON WETTING
Thermodynamic considerations are key to understanding the wettability of solids by liquid metal alloys. Wetting is governed by rapid surface segregation of low- level bulk impurities which arise during alloy manufacture. The segregated
m a t e r i a l forms a r e a c t i o n b a r r i e r which i n h i b i t s c o n t a c t between a l l o y and s u b s t r a t e and r e s u l t s i n poor c o n t a c t a n g l e s . There a r e t h r e e p o s s i b l e s o l u t i o n s t o t h i s problem. The f i r s t i s t o e l i m i n a t e t h e a l l o y i m p u r i t i e s by employing u l t r a - pure s t a r t i n g m a t e r i a l s and s t r i c t c o n t r o l of t h e a r c m e l t i n g p r o c e s s . The second i s t o skim away t h e a l l o y i m p u r i t i e s a f t e r t h e i m p u r i t i e s have s e g r e g a t e d . The t h i r d i s t o i n t r o d u c e m a t e r i a l s i n t o t h e c o n t a c t system which p o s s e s s h i g h chemical a f f i n i t y t o t h e major s e g r e g a t i n g i m p u r i t i e s , e i t h e r a s a s u r f a c e c o a t i n g o r by i n c o r p o r a t i o n i n t o t h e a l l o y d u r i n g t h e a r c m e l t i n g process. The m a t e r i a l a c t s t o s u p p r e s s s e g r e g a t i o n by t y i n g up t h e a l l o y i m p u r i t i e s d u r i n g compound formation.
Due t o t h e f a v o r a b l e Gibbs f r e e e n e r g i e s of f o r m a t i o n of S i c and BqC, c o a t i n g s of B and S i a r e e x p e c t e d t o combine w i t h C and promote w e t t i n g .
The e f f e c t of a B s u r f a c e c o a t i n g on t h e w e t t i n g of n i c k e l b o r i d e a l l o y t o a g r a p h i t e s u b s t r a t e whose f r o n t s u r f a c e was c o a t e d w i t h 325 mesh red boron i s shown i n F i g u r e 3. The boron l a y e r was a p p l i e d a s a s l u r r y w i t h a c e t o n e w i t h a
p a i n t b r u s h . The w e t t i n g is e x c e l l e n t , and flow i s observed t o o c c u r beyond t h e boron u n d e r l a y e r and o n t o t h e nonboron coated b a c k s i d e of t h e ribbon. Auger a n a l y - s i s of t h e molten s u r f a c e show r a d i c a l l y depressed p e r c e n t a g e s of C r e l a t i v e t o t h e c a s e f o r w e t t i n g on a v i r g i n g r a p h i t e s u b s t r a t e . F u r t h e r , t h e s u r f a c e i s e n r i c h e d w i t h boron, and has a much d i f f e r e n t s t o i c h i o m e t r y t h a n t h e bulk. This is due t o s u r f a c e s e g r e g a t i o n of B, which a r i s e s because ( n o n m e t a l l i c ) B p o s s e s s e s a lower s u r f a c e t e n s i o n t h a n ( m e t a l l i c ) N i .
Fig. 3 - Wetting of Ni55B45 a l l o y t o boron-coated g r a p h i t e . ( L e f t ) View of t h e f r o n t s i d e w e t t i n g . ( R i g h t ) Auger s u r f a c e e l e m e n t a l composition of t h e s u b s t r a t e and a l l o y a t i t s m e l t i n g p o i n t .
100 - 7 90- 91 2
- 80 - B o r o n i z e d A l l o y a t
z Substrata W e t t i n g
5 7 0 -
k 63.1
Fig. 4 - Wetting of Pd72828 t o a s i l i c o n - c o a t e d g r a p h i t e e m i t t e r s t r u c t u r e a f t e r 100 hours of h e a t i n g a t 1200 K.
S u r f a c e c o a t i n g s of S i a f f e c t w e t t i n g s i m i l a r l y . F i g u r e 4 shows t h e w e t t i n g of a
3 60 -
: 5 0 -
8 4 0 - 3 3 0 -
z w 20 - 17 9
15.1 4 100 0 B N I C O N
C 2 - 1 2 0 JOURNAL DE PHYSIQUE
palladium b o r i d e a l l o y t o a carbon t i p w i t h an u n d e r l a y e r of s i l i c o n . The photos were t a k e n a f t e r 100 hours of h e a t i n g a t a t e m p e r a t u r e of 30 (iegrees above t h e m e l t i n g p o i n t . The w e t t i n g procedure c o n s i s t e d of c o a t i n g a g r a p h i t e e m i t t e r w i t h a s l u r r y of powdered S i and propanol, which was then h e a t e d t o t h e m e l t i n g p o i n t of S i . At m e l t i n g , t h e S i wets and s p r e a d s s u p e r b l y over t h e g r a p h i t e . The i n t e r - f a c i a l compound which forms i s S i c , which was v e r i f i e d by Auger s p e c t r o s c o p y a f t e r f l a s h i n g away t h e f r e e S i . The w e t t i n g l a y e r t h e r e f o r e c o n s i s t s o f a n i n t e r f a c i a l compound of S i c covered by a t h i c k l a y e r of f r e e Si. A t t h i s p o i n t , t h e PdB a l l o y was added i n powdered form w i t h propanol, and t h e n e l e c t r o n bombarded u n t i l t h e a l l o y melted.
V I - THE INFLUENCE OF SURFACE SEGREGATION ON WETTING
S u r f a c e s e g r e g a t i o n of low-level, low s u r f a c e t e n s i o n carbon and n i t r o g e n i m p u r i t y i n t h e form of C , BqC, and BN i s t h e primary cause f o r w e t t i n g f a i l u r e i n c o n t a c t systems of g r a p h i t e and B-containing b i n a r y a l l o y s . We s a y t h a t n i t r o g e n i n t h e form of BN s e g r e g a t e s because i t i s u n l i k e l y t h a t N e x i s t s i n t h e a l l o y s i n elemen- t a l form. F u r t h e r , BN has been d i r e c t l y i d e n t i f i e d i n t h e a l l o y s u r f a c e s by eva- l u a t i o n of Auger chemical s h i f t s . A t m e l t i n g , carbon and n i t r o g e n p r e s e n t as a l l o y i m p u r i t i e s s e g r e g a t e t o t h e s u r f a c e . The r e s u l t i n g l a y e r of carbon, boron n i t r i d e , and boron c a r b i d e i n h i b i t s i n t e r a c t i o n between t h e a l l o y components and t h e
s u b s t r a t e and r e s u l t s i n an u n f a v o r a b l e combination of s u r f a c e t e n s i o n s . Boron n i t r i d e and boron c a r b i d e p o s s e s s f a v o r a b l e Gibbs f r e e e n e r g i e s of f o r m a t i o n and may be expected t o form d u r i n g t h e a r c m e l t i n g p r o c e s s and a t t e m p e r a t u r e s n e a r t h e m e l t i n g p o i n t s of t h e e u t e c t i c compositions of t h e boron a l l o y s we have s t u d i e d . Boron and s i l i c o n s u r f a c e c o a t i n g s f a c i l i t a t e w e t t i n g by t y i n g up t h e carbon t h a t would o r d i n a r i l y s e g r e g a t e by f o r m a t i o n of i n t e r f a c i a l boron c a r b i d e and s i l i c o n c a r
bide. I n e f f e c t , t h e s u r f a c e c o a t i n g a c t s a s a g e t t e r f o r s e g r e g a t e d m a t e r i a l . The r e s u l t a n t chemical r e a c t i o n i s p r e c i s e l y what i s n e c e s s a r y f o r w e t t i n g . The c o a t i n g s work because of what t h e a l l o y s e e s a t t h e m e l t i n g p o i n t . A C-C nonreac- t i o n i s r e p l a c e d by a C-B o r C-Si r e a c t i o n . When t h e a l l o y i s p u r i f i e d i n t h i s way, t h e a l l o y may t h e n wet v i r g i n m a t e r i a l because t h e r e i s no more s e g r e g a t e d m a t e r i a l b l o c k i n g t h e i n t e r a c t i o n of a l l o y and s u b s t r a t e . T h i s e x p l a i n s why l i q u i d a l l o y m a t e r i a l i n i t i a l l y needs a s u r f a c e c o a t i n g , b u t t h e n can wet and s p r e a d o v e r v i r g i n g r a p h i t e .
The f u l l e s t impact of s u r f a c e c o a t i n g s on w e t t i n g i s r e a l i z e d when t h e c o a t i n g con- s i t s of a t h i c k ( i . e . , > 5 micron) l a y e r of f r e e B o r S i . That i s , s u r f a c e s where t h e s e e l e m e n t s a r e l a r g e l y f l a s h e d away, o r s u r f a c e s c o n s i s t i n g of c a r b i d e s of B and S i r e s u l t i n poor w e t t i n g . T h i s may be e x p l a i n e d by c o n s i d e r a t i o n of t h e r e l a - t i v e magnitudes of cohesion and adhesion i n t h e c o n t a c t system. Good w e t t i n g r e s u l t s when t h e a d h e s i o n between t h e s u b s t r a t e and a l l o y i s l a r g e compared t o t h e c o h e s i o n of e i t h e r m a t e r i a l . For w e t t i n g of c a r b i d e s u r f a c e s , a d d i t i o n a l energy i s n e c e s s a r y t o overcome t h e chemical bonding of S i t o B and C. For w e t t i n g t o f r e e S i and B, no a d d i t i o n a l energy i s necessary. I n o t h e r words, f r e e S i and B a r e r e q u i r e d t o t i e up t h e s e g r e g a t e d carbon i n t h e system, and t h i s i s f a c i l i t a t e d i f t h e s e e l e m e n t s a r e n o t a l r e a d y t i e d up t o begin with.
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