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METAL-OXIDE INTERFACES OBTAINED BY SELECTIVE CHEMICAL REDUCTION OF
LAMELLAR OXIDE-OXIDE EUTECTIC STRUCTURES
B. Bonvalot, G. Dhalenne, Florence Millot, A. Revcolevschi
To cite this version:
B. Bonvalot, G. Dhalenne, Florence Millot, A. Revcolevschi. METAL-OXIDE INTERFACES OBTAINED BY SELECTIVE CHEMICAL REDUCTION OF LAMELLAR OXIDE-OXIDE EU- TECTIC STRUCTURES. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-781-C1-787.
�10.1051/jphyscol:19901122�. �jpa-00230031�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl6ment au nOl, Tome 51, janvier 1990
METAL-OXIDE INTERFACES OBTAINED BY SELECTIVE CHEMICAL REDUCTION OF LAMELLAR OXIDE-OXIDE EUTECTIC STRUCTURES
B. BONVALOT, G. DHALENNE, F. MILLOT and A. REVCOLEVSCHI
Laboratoire de Chirnie
des
Solides, UA CNRS 446, Universite Paris Sud,Bdt.
414, F-91405 Orsay Cedex, FranceR-esgm.5,. -
Des i n t e r f a c e s o x y d e - m e t a l o n t e t 6 o b t e n u s p a r r e d u c t i o n c h i m i - que de m l c r o s t r u c t u r e s e u t e c t i q u e s oxyde-oxyde dans des c o n d i t i o n s de t e m p e r a t u r e e t p r e s s i o n p a r t i e l l e d ' o x y g e n e p o u r l e s q u e l l e s s e u l l ' u n d e s deux oxydes p o u v a i t & t r e r e d u i t a l ' e t a t m e t a l l i q u e . L e s systgmes e t u d i e s s o n t NiO-ZrOz(Ca0) ; Coo-ZrOz(Ca0) ; NiO-ZrOz p u r e e t NiO-Y203. Dans l e s q u a t r e syst&mes e t u d i d s une r e d u c t i o n r a p i d e de N10 e n N i a e t e o b s e r v e e , t o u t e f o i s deux t y p e s d i s t i n c t s de m i c r o s t r u c t u r e s o x y d e - m & t a l s o n t o b s e r v e s . Deux modeles d i f f e r e n t s a o n t p r o p o s e s p o u r r e n d r e compte des r e s u l t a t s o b t e n u s e t c o n f r o n t e s aux r e s u l t a t s e x p e r i m e n t a u x .AQ,~,t.r,.g,c,$
-
M e t a l - o x i d e i n t e r f a c e s were p r e p a r e d b y c h e m i c a l r e d u c t i o n o-f r e g u l a r l y a l i g n e d l a m e l l a r o x i d e - o x i d e e u t e c t i c m i c r o s t r u c t u r e s i n c o n d i - t i o n s o f t e m p e r a t u r e and p a r t i a l p r e s s u r e o f oxygen where o n l y one o f t h e two o x i d e phases i s t r a n s f o r m e d t o t h e c o r r e s p o n d i n g m e t a l . The s y s t e m s w h l c h were s t u d i e d a r e N i O - c a l c i a s t a b i l i z e d z i r - o o n i a ; C O O - c a l c i a s t a b i l i z e d z i r c o n i a ; N i O - p u r e z i r c o n i a and N i C J - Y t t r i a . R a p i d r e d u c t i o n o f NiO i n t o N i was o b s e r v e d i n a l l f o u r s y s t e m s and two t y p e s o f m e t a l - o x i d e m i c r o s t r u c t u r e s were o b t a i n e d - T w o d i s t i n c t models a r e p r o p o s e d t o a c c o u n t f o r t h e r e d u c t i o n p r o c e s s and d i s c u s s e d i n v i e w o f e x p e r i m e n t a l d a t a .1 -
INTRODUCTION
I n t e r n a l i n t e r f a c e s v e r y o f t e n g o v e r n t h e p h y s l c a l p r o p e r t i e s o f m a t e r i a l s . Among t h e d i f f e r e n t t y p e s o f homophase o r h e t e r o p h a s e i n t e r f a c e s s t u d i e d 1.2 r e c e n t y e a r s , t h e b o u n d a r i e s s e p a r a t i n g t h e two phases o f a n e u t e c t i c s t r u c t u r e , i n p a i - t i c u l - a t - i n c e r a m l c m a t e r i a l s , have g i v e n r i s e t o a n i n c r e a s i n g i n t e r e s t
/ l -
I n t h i s c o n t e x t , s e v e r a l . s t u d i e s were c a r r i e d o u t i n o u r l a b o r a t o r y o n s e v e r a l o x i d e - o x i d e e u t e c t i c s t r u c t u r e s , xn p a r t i c u l a r n i c k e l o x i d e - b a s e d a l i g n e d e u t e c t i c s /5-6/.'The most w i d e s p r e a d method f o r t h e f a b r l c a t l o n o f a l i g n e d c e r a m i c e u t e c t i c s i s d i r e c . t i o n a 1 s o l i d i f i c a t i o n o f m e l t s o f e u t e c t i c c o m p o s i t i o n . T h i s method has been f o u n d t o y i e l d l a m e l l a r or- f i b r o u s c o m p o s i t e m i c r o s t r u c t u r e s exhibiting c o h e r e n t i n t e r p h a s e b o u n d a r i e s w h i c h c a n n o t b e s y s t e m a t i c a l l y o b t a s n e d i n m a t e r - i a l s p r e p a r e d by methods s u c h a s s i n t e r z n g .
H t t e m p t s t o grow m e t a l - o x l d e a l l g n e d s t r u c t u r e s b y d ~ r e c t s o n a l s o l s d l f l c a t ~ c n f r o m t h e m e l t have a l s o been r e p o r t e d / 7 - 9 / , b u t t h e t e c h n i q u e s s o n l y applicable t o a v e r y l l m s t e d number o f systernc,.
The o b j e c t o f t h e s t u d y r e p o r t e d h e r e , was t o e x p l o r e t h e p o s s l b l l i t y o f f a b r i - c a t l n g a l l g n e d m e t a l - o x r d e l a m e l l a r s t r u c t u r e s b y selective c h e m l c a l reduction o f one o f t h e phases o f a n o x l d e - o x l d e e u t e c t l c .
R e d u c t i o n experiments were c a r r l e d o u t o n s e v e r a l o f t h e l a ~ e l l a r NIO-based e u t e c t i c o x i d e - o x i d e s t r u c t u r e s a l r e a d y s t u d l e d i n t h e l a b o r a t o r y , name], N10-ZrO2(CaU), Coo-ZrOz(CaO), NxO-Y~OJ and NIO-pure ZrOz. Two d i f f e r e n t t y p e ? o f m i c t - o s t r u c t u r e s were o b s e r v e d f o r t h e m e t a l - o x r c l e composites r e s u l t l n g f r o m transformation f r o m NIO I n t o NI.. Models f o r t h e k l n e t l c s o f t h e r e d u c t i o n p r o c e s s accounting f o r t h e two t y p e s o f m l c r o s t r u c t u r e s w h l c h were o b t a l n e d have been e s t a b l s s h e d .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901122
C 1-782 COLLOQUE
DE
PHYSIQUE2 - EXPERIMENTAL PROCEDURE
A l i g n e d o x i d e - o x i d e e u t e c t i c s t r u c t u r e s were grown i n t h e NiO-Zr02(CaO), Coo-Zr02(CaO), N i O - Y 2 0 ~ and NiO-pure Z r O z systems by d i r e c t i o n a l s o l i d i f i c a t i o n i n a d o u b l e e l l i p s o i d image f u r n a c e /10/.
I n a l l f o u r systems, t h e v e r y r e g u l a r e u t e c t i c m i c r o s t r u c t u r e s which were o b t a i n e d were l a m e l l a r ; t h e y were made o f a l t e r n a t e p l a t e s , p a r a l l e l t o t h e g r o w t h d i r e c t i o n , w i t h t h i c k n e s s e s o f a p p r o x i m a t e l y 1 pm f o r each phase f o r a s o l i d i f i c a t i o n r a t e o f 15 mm h - 1 .
M i c r o f r a g m e n t s o f t h e d i r e c t i o n a l l y s o l i d i f i e d samples o r i e n t e d a l o n g t h e g r o w t h d i r e c t i o n were s t u d i e d by X-ray d i f f r a c t i o n methods t o d e t e r m i n e p r e f e r r e d g r o w t h d i r e c t i o n s and o r i e n t a t i o n relationships between phases /ll/.
P a r a l l e l e p i p e d - s h a p e d e u t e c t i c specimens e l o n g a t e d i n t h e solidification d i r e c t i o n and s i n g l e c r y s t a l NiO were c h e m i c a l l y reduced by a n n e a l i n g a t 1075°C under CO/C02 gas m i x t u r e s i n c o n d i t i o n s where NiO s h o u l d be reduced t o n i c k e l : e x p e r i m e n t s were conducted under oxygen a c t i v i t i e s e q u a l t o 1.6 10-11 and 10-10 atm.
3
-
RESULTSa
> La.m.el..l.ar
....C?.~g-~Zzr.r?~,~,.CaO~ and
.,.Co.:.Z ,.r .PIL..CC.~.~.>.>..>...>.>.?.~ ructu r.ec,,
The two systems e x h i b i t s i m i l a r m i c r o s t r u c t u r e s . F i g u r e 1 shows t h e morphology o f a p a r t i a l l y reduced Ni0-ZrOz(Ca0) composite specimen. The m i c r o s t r u c t u r e r e s u l t i n g f r o m r e d u c t i o n i n d i c a t e s t h a t t h e a l i g n e d n a t u r e o f t h e o r i g i n a l e u t e c t i c s t r u c t u r e and t h e c o h e s i o n between p l a t e s were m a i n t a i n e d . I t can a l s o be seen t h a t r e d u c t i o n has c r e a t e d a r a t h e r r e g u l a r a r r a y o f c r a c k s p a r a l l e l t o t h e l a m e l l a e w i t h a s p a c i n g o f about 40 pm. The c r a c k s p r o v i d e a c c u r a t e l y t h e 40% r e d u c t i o n i n volume r e q u l r e d f o r t h e t r a n s f o r m a t i o n o f NiO t o N i .
F i g . 1- SEM o b s e r v a t i o n o f t h e r e d u c t i o n F i g . 2- SEM m i c r o g r a p h o f a Ni/NiO f r o n t r n t h e l o n g i t u d i n a l direction o f i n t e r f a c e showing l o n g f o r k s o f N i an e u t e c t i c NlO-Zr02(CaO) showing ~ n t h e i n a a l i g n e d NiO-ZrOz(Ca0) e u t e c t i c upper p a r t o f t h e p i c t u r e t h e o x i d e - a f t e r exposure i n a CO/C02 gaseous o x i d e e u t e c t i c and b e l o w , t h e Ni-ZrQz(Ca0) m i x t u r e .
composite
.
I n t h e reduced p a r t o f t h e specimen, t h e n l c k e l p r o d u c t LS dense and x n s l d e t h e o x l d e l a m e l l a e one can observe l o n g f o r k s o f n i c k e l a t t h e N l / N l O i n t e r f a c e . Such f o r k s a r e about 30 pm l o n g a f t e r 1 hour r e d u c t i o n a t 107S°C and c o r r e s p o n d t o a n g l e s , between t h e Ni/NlO and N10/ZrOz ~ n t e r f a c e s , o f about 2 " . The e x t e n - s i v e l e n g t h o f t h e N i f o r k s s t r e t c h e s o v e r t h e zone where t h e volume change occurs. The r e s u l t i n g N i p l a t e l e t s remaln r n t a c t o v e r t h l s d ~ s t a n c e , w h l l e t h e Z r O 2 p l a t e s experience accomodating p l a s t l c c r e e p d o f o r m a t l o n (Fxg. 2 ) .
F i g u r e 3 shows a l o n g i t u d i n a l s e c t i o n o f a p a r t i a l l y reduced NiO-pure Z r O z specimen.
I n t h e reduced p a r t o f t h e sample, t h e a l i g n e d s t r u c t u r e i s made o f a l t e r n a t e p l a t e s o f m e t a l l i c n i c k e l and non-reacted z i r c o n i a . Here, i n c o n t r a s t w i t h what was observed i n Ni0-ZrOz(Ca0) s t r u c t u r e s , t h e t h i c k n e s s o f n i c k e l p l a t e l e t s remains e q u a l t o t h a t o f n i c k e l o x i d e . One can a l s o observe, i n s i d e t h e n i c k e l phase, t h e f o r m a t i o n o f v o i d s which accomodate t h e r e q u i r e d s h r i n k a g e from NiO t o N i .
F i g . 3
-
R e d u c t i o n f r o n t i n t h e l o n - g i t u d i n a l d i r e c t i o n o f an e u t e c t i c NiO-ZrO2 p a r t i a l l y reduced a f t e r exposure i n a CO/C02 gaseous m i x t u r e .The shape o f t h e r e d u c t i o n f r o n t ( i n t e r f a c e Ni/NiO) i s shown i n f i g u r e 4 , where t h e presence o f s m a l l n i c k e l f o r k s I n t h e N i 0 phase can be n o t e d .
F i g . 4
-
SEM o b s e r v a t i o n o f a Ni/NiO i n t e r f a c e showing s m a l l f o r k s o f N i i n an o r i e n t e d NiO-ZrOz e u t e c t i c a f t e r exposure i n a CO/C02 gaseous m i x t u r e .The morphology o f p a r t i a l l y reduced specimens o f NiO-Ya03 is v e r y s i m i l a r t o t h a t observed i n t h e NiO-ZrOz system.
P l o t s o f t h e measured d e p t h s o f NiO r e d u c t i o n i n t h e l o n g i t u d i n a l d i r e c , t i o n o f t h e a l i g n e d s t r u c t u r e s which were s t u d i e d a r e p r e s e n t e d on f i g u r e s 6 and 9.
One can o b s e r v e , i n comparison w i t h t h e r e d u c t i o n o f s i n g l e c r y s t a l NiO i n s i m i - l a r c o n d i t i o n s , t h a t t h e r e d u c t i o n d e p t h i n t h e composites i s much 1 a r g e r . I n t h e case o f s i n g l e c r y s t a l s , t h e o x i d e s u r f a c e i s c o v e r e d b y a l a y e r o f n i c k e l which i s o l a t e s t h e NiO p l a t e s from t h e gas phase and slows down f u r t h e r r e d u c t i o n . C o n s i d e r i n g t h e two d i f f e r e n t t y p e s o f morphologies o f t h e p a r t i a l l y reduced samples under s t u d y , we have developed two models a c c o u n t i n g f o r t h e r e d u c t i o n o f NiO.
Cl-784 COLLOQUE DE PHYSIQUE
r ) Reduction
ot nlckel oxlde platelets In N1O-ZrO2(CaO).
If orie considers that the ionic conducti\/ity of oxygen ions of calcia-stabili- zed ZrOz is very high, as is the electronic conductivity in metallic nicke1,the rapid longitudinal reduction of oxide plates in the composite may be described as resulting from an electrochemical local cell actlon in which the diffusion of ionic oxygen from the metal-oxide interface to the external surface occurs through the cublc zirconia plates, while the electrons reach the reduction site in the interior of the sample through the metal phase.
Figure
5shows the
reductionscheme and the notation used for the formulation of the kinetlcs equations for the internal reduction of NiO to Ni.
G A S P H A S E
Fig.
5- Schematic diagram of a cross- sectlon of the Ni/NiO interface in a lamellar eutectic NiO-ZrO2(CaO) parti- ally reduced at high temperature under a CO/C02 mixture.
I f
we call Y the aepth of reduction, i.e. the distance from the Ni/NiO interfa- ce to the sample surface, we can establish the following kinetics equation /12/
where
oand are the half thicknesses of the Ni0 and ZrOz plates,
V ~ i othe molar vol.ume of NiO, o the electrical conductivity of the oxygen ions in stabilized ZrOz and F =
96,490coulombs.
Figure
6represents plots of the measured depths of NiO reduction
i nthe longitudinal direction and the corresponding theoretical curve of equation
[l].Fig.
6- Comparison of experimental results and calculated values corresponding
to the predictions based on electrochemical local cell actlon model, for the
depth of Ni for Ni0-ZrOz(Ca0) eutectic.
To t e s t t h e dependence o f t h e reduction r a t e on the c o m p o s i t i o n o f t h e r e d u c i n g gas, as p r e d i c t e d by e q u a t i o n [l], a few experiments were conducted w i t h a C O z / C O r a t 1 0 e q u a l t o 24.75 c o r r e s p o n d i n g t o P02
=
10-10 atm. The r e s u l t i n g d a t a a r e a l s o p r e s e n t e d f i g u r e 6. A r e l a t i v e l y good agreement between experiment and t h e o r y i s observed.An i d e n t i c a l model was a p p l i e d t o account f o r t h e r e d u c t i o n o f COO i n t o CO i n t h e COO-ZrUz(CaO) a l i g n e d e u t e c t i c 3 Here a l s o a good agreement between e x p e r i m e n t a l d a t a and those c a l c u l a t e d t h r o u g h t h i s model seems t o c o n f i r m t h e v a l i d i t y o f t h e e l e c t r o c h e m i c a l p r o c e s s .
F o r these two systems, i n view o f t h e m i c r o s t r u c t u r a l f e a t u r e s mentioned above and g i v e n t h e i o n i c conductivities o f m o n o c l i n i c Z r O z and Y Z Q J a t 1075°C which a r e l o w e r by s e v e r a l o r d e r s o f magnitude t h a n t h a t o f c u b i c z i r c o n i a , t h e observed r e d u c t i o n mechanism cannot be e x p l a i n e d by t h e p r e v i o u s l y d e s c r i b e d e l e c t r o c h e m i c a l l o c a l c e l l model.
The s t r u c t u r a l f e a t u r e s suggest r a t h e r a gas d i f f u s i o n c o n t r o l l e d r e d u c t i o n . A p o s s i b l e p a t h f o r d i f f u s i o n i s t h e gas phase c o n s t i t u t e d by t h e channels l e f t between n i c k e l g r a i n s a f t e r NiO r e d u c t i o n .
F i g u r e 7 i s a schematic d r a w i n g o f t h e n i c k e l p r o d u c t l a m e l l a r phase r e s u l t i n g from r e d u c t i o n , made o f i d e n t i c a l s i z e n i c k e l p a r t i c l e s h a v i n g a s e c t i o n o f
" a X a" and a t h i c k n e s s 6, e q u a l t o t h a t o f t h e i n i t i a l l a m e l l a , and s e p a r a t e d by a d i s t a n c e b.
F i g . 7
-
Schematic diagram o f t h e morphology r e s u l t i n g f r o m t h e r e d u c t i o n o f t h e NiO-ZrOz l a m e l l a r e u t e c t i c : t h e m i c r o s t r u c t u r e i s mada o f u n a l t e r e d Z r O z l a m e l l a e s e p a r a t e d b y porous n i c k e l ( l ) , f o r which a model i s r e p r e s e n t e d i n ( 2 J .CO gas f l o w s , between n i c k e l particles, towards t h e Ni-NiO r e d u c t i o n f r o n t where i t r e a c t s t o form C O 2 which f l o w s back t o t h e e x t e r n a l s u r f a c e .
A d e t a i l e d a n a l y s i s o f t h e k i n e t i c s o f such a r e d u c t i o n p r o c e s s i s g i v e n i n /14/ where i t i s shown t h a t t h e m e t a l t h i c k n e s s , Y , can be expressed as a f u n c t i o n o f t l m e t , by t h e e q u a t i o n :
D12 V N ~ O (Cext
-
C i n t )y 2 (cmz )
=
2 - - - m -----
- - - m - - --- .
t (S) 1.5 ( l+
a / b )where D12 i s t h e a p p a r e n t i n t e r d i f f u s i o n c o e f f i c i e n t o f CO i n t h e NZ-CO-CO2 m i x t u r e which i s e v a l u a t e d f r o m t h e k i n e t i c t h e o r y o f gases ; t h e v a l u e s a and b a r e r e l a t e d t o V ~ i o and V N ~ by ( a f b)2
/
a2=
V N ~ D / V N i=
1.7 and C e x t and C i n t a r e t h e CO c o n c e n t r a t i o n s i n t h e gas phase and a t t h e r e d u c t i o n f r o n t , r e s p e c t i v e l y .Cl-786 COLLOQUE DE PHYSIQUE
We can evaluate the numerical expression of equation
[ Z ]at 1075°C taking Cext-Cint = 7.5 10-8 mol cm-3 [Cext is deduced from the composition of the gas phase flowing past the sample (P02 = 1.6 10-11 atm with Pcoz/Pco =
9.9and Pc02
+ Pco = 0.109 atm) and Cint is calculated from the kinetic theory of gases and assuming that thermodynamic equilibrium between CO-C02-Ni-NiO is achieved at the reduction front
(P02 =
5.810-10 atm and Pco2/Pco =
6 2 : 4 ) ] .One gets
:This expression predicts values of the reduction depth which are larger than those experimentally observed (Fig. 8).This discrepancy between theoretical and experimental results may arise from the uncertainty attached to various kinetic parameters, such a s the effective length for diffusion,for which an approximate value of 1.5 Y is chosen, the non-uniformity of the grain size of nickel parti- cles and the dimension of the channels. In the light of the uncertainties, the agreement between the experimental and the expected results is considered satisfactory .
Fig.
8- Variation of the depth of nickel in the longitudinal direction, a s a functlon of the annealing time for oriented lamellar NiO-ZrO2 and Ni0-Yz05 eutectics and single crystals NiO.
The validity of the proposed model can be assessed by changing the total pres- sure Pc02 + PCO of the mixture without changing the ratio Pco2/Pco.This permits a differentiation between solid state diffusion, where the driving force of reduction is the gradient of chemical potential of oxygen a s in the case of NiO-Zr02(CaO) and the present model where the force is the gradient of concen- tration of CO.In this latter case,a linear relation between the reduction depth Y and (Pcoext - Pcoint)l/2 is expected. Figure
9shows that such a relation is approximately verified for two reduction times.
Fig.
9- Effect of CO/COz
dilution on the reduction
rate of NiO-Zr02 eutectic
at 1075°C and for P02 =
1.6 10-11 atm.
4
-
CONCLUSIONI n t h e p r e s e n t s t u d y , w e have s h o w n t h a t t h e r a p i d r e d u c t i o n o f N i O - b a s e d e u t e c t i c o x i d e - o x i d e s t r u c t u r e s i n CO/COz gas m i x t u r e s c o u l d be d e s c r i b e d b y t w o d i f f e r e n t m o d e l s i n v i e w o f t h e m o r p h o l o g y o f t h e n i c k e l phase r e s u l t i n g f r o m r e d u c t i o n .
I t i s f e l t t h a t t h e m o d e l s g i v e n h e r e w i l l a p p l y t o a n u m b e r o f p r a c t i c a l s y r - t e m s and s h o u l d p r o v i d e a u s e f u l s t a r t i n g p o i n t f o r f u r t h e r w o r k i n t h i s a r e a . REFERENCES
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