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GROWTH OF NICKEL OXIDE - BASED ORIENTEDEUTECTIC STRUCTURES AND

CHARACTERIZATION OF THE INTERPHASEBOUNDARIES

A. Revcolevschi, G. Dhalenne, F. d’Yvoire

To cite this version:A. Revcolevschi, G. Dhalenne, F. d’Yvoire. GROWTH OF NICKEL OXIDE - BASED ORIENTEDEUTECTIC STRUCTURES AND CHARACTERIZATION OF THE INTERPHASE BOUND-ARIES. Journal de Physique Colloques, 1985, 46 (C4), pp.C4-441-C4-447. �10.1051/jphyscol:1985449�.�jpa-00224700�

JOURNAL DE PHYSIQUE

Colloque C4, supplement au n°», Tome 46, avri l 1985 page C4-W1

GROWTH OF NICKEL OXIDE - BASED ORIENTED EUTECTIC STRUCTURES AND

CHARACTERIZATION OF THE INTERPHASE BOUNDARIES

A. Revco levsch i , G. Dhalenne and F . d 'Yvoi re

Laboratoire de Chimie Appliquée, Université Paris-Sud, 91405 Orsay Cedex, France

Résumé - Des structures eutectiques orientées ont été élaborées par solidifi­cation dirigée dans plusieurs systèmes binaires à base d'oxyde NiO. Les directions de croissance, les relations cristallographiques entre phases eutectiques et la nature des plans d'interface ont été déterminées. Les résul­tats sont comparés à ceux relatifs à d'autres systèmes oxyde-oxyde.

Abstract - Well a l igned eutec t ic s t ruc tures have been grown by d i rec t i ona l s o l i f i c a t i o n in several n ickel oxide-based binary systems. Growth d i rec t ions of the al igned s t r uc tu res , c rys ta l !ograph ic re la t i ons between the two phases of each eu tec t i c and the M i l l e r indices of the in te r face planes were determi­ned. The resu l ts are compared wi th data r e l a t i v e to other oxide-oxide eu tec t i cs .

I - INTRODUCTION

Within the framework of general studies of the structure and properties of oxide boundaries, carried out particularly on bicrystalline samples of nickel oxide /1,2,3/ we have undertaken the examination of the structure of interfaces separating two dif­ferent oxide phases. Eutectic interfaces which are a good example of such phase boun­daries have been considered. Directional solidification experiments were performed in several nickel oxide-based binary systems, with the object of yielding interfaces associating a sodium chloride type structure (NiO) with other phases of cubic sym­metry .

The systems which have been considered were :

. NiO-CaO (sodium chloride)

. NiO-NiAl204 (spinel)

. N iO-s tab i l ized cubic ZrO„ ( f l u o r i t e )

. NiO-Y203 ( f l u o r i t e - r e l a t e d b i x b y i t e ) .

The present paper reports observations on the crysta l lography of the al igned eutec t ic s t ructures and in ter faces which were obta ined, and compares the resu l ts w i th data r e l a t i v e to other oxide-oxide systems.

I I - CRYSTAL GROWTH

The directional solidification experiments were carried out by two methods.

One involved a floating zone technique associated with a double ellipsoid image fur­nace /4,5/, in which a molten zone was established between two cylindrical rods pre­pared by compression and sintering : rods were rotated in opposite directions at about 60 rpm and translated at a rate of 1.5 cm.h"1.

The other technique is the "skull method" /6/, consisting of r.f. direct induction

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985449

C4-442 JOURNAL DE PHYSIQUE

heat ing o f batches o f about 500 grams contained i n a copper-cooled c r u c i b l e about 5 cm i n diameter and 6 cm high. Preheating o f the powder mixtures was achieved by using chunks o f metal corresponding t o one o f the oxides. A f t e r mel t ing, the c r u c i b l e content was s lowly c ~ o l e d ~ y i e l d i n g 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 a t a r a t e o f about 6 mm.h-'.

S t a r t i n g powders were h igh p u r i t y oxides (> 99.9 %) mixed i n the propor t ions ind ica - ted i n Table I. These propor t ions are s l i g h t l y d i f f e r e n t from those ind ica ted by the phase diagrams/7,8,9/'and were determined by meta l lographic examination. I n the par- t i c u l a r case o f NiO-calcium s t a b i l i z e d Zr02, no phase diagram was ava i lab le . There- f o r e a systematic study o f var ious mixtures o f NiO, Zr02 and CaO was c a r r i e d out, the propor t ions o f the l a s t two oxides being chosen i n a manner t o correspond t o a Zr02-15 mol % CaO-stabi l ized cubic z i r c o n i a : the bes t r e s u l t s i .e. regu la r e u t e c t i c s t ruc tu res and absence o f pr imary phases were obtained f o r the composit ion 70 mol % Ni0-25.5 % Zr02-4.5 % CaO.

Fig. 1 - Transverse sec t ion of NiO-Zr02 e u t e c t i c d i r e c t i o n a l l y s o l i d i f i e d a t 1.5 cm.h-I.

Well a l i gned e u t e c t i c s t ruc tu res were observed i n a l l f o u r systems under study(Fig.1) the e u t e c t i c phases and t h e i r l a t t i c e parameters being those repor ted i n Table I. Structures are lame l la r f o r NiO-CaO, NiO-Zr02 and NiO-Y203 ; t h i s observat ion i s i n good agreement w i t h ca lcu la t ions /10,11/ r e l a t i v e t o lamel lar - to- rod t r a n s i t i o n s i n a eu tec t i c , which establ ished t h a t lame l la r s t ruc tu res should be s tab le when the minor component has a volume f r a c t i o n h igher than 0.3. I n the case o f Ni0-NiA1204, i n s p i t e o f volume f r a c t i o n s o f 0.42 f o r N i O and 0.58 f o r NiA1204, f i b e r type a l igned structures were obtained. Such anomalies have been observed i n other systems e.g. MgO-CaO /12/ b u t have n o t been w e l l explained so f a r .

111 - CRYSTALLOGRAPHIC RELATIONS - Microfragments o f the 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 ien ted along the growth d i - r e c t i o n were s tud ied by r o t a t i n g c r y s t a l , Weissenberg (F ig . 2) and Buerger precession methods t o determine p re fe r red growth d i r e c t i o n s and e p i t a x i a l r e l a t i o n s . The pat- te rns revealed t h a t growth d i r e c t i o n s have low M i l l e r i nd ices and t h a t p rec ise and simple o r i e n t a t i o n r e l a t i o n s e x i s t between the two phases (Table I ) . The in fo rmat ion concerning growth d i r e c t i o n s o f the e u t e c t i c phases obta ined from these microf rag- ments was confirmed by X-ray d i f f r a c t o m e t r y experiments c a r r i e d o u t on l a r g e t rans- verse sect ions o f s o l i d i f i e d ingo ts : h igh i n t e n s i t y peaks corresponding t o planes normal t o growth d i r e c t i o n s were observed.

Fig. 2 - Weissenberg zero l e v e l photograph normal t o [001] o f Zr02 and [110] o f NiO showing the para1 1 e l ism o f [100]Tro2 and [lll]\iO.

To spec i f y the M i l l e r i nd ices of the interphase planes i n the case o f l ame l la r styuc- tures a specia l procedure was app l ied f o r the NiO-CaO and NiO-Y20 eu tec t i cs 113,141. Fragments o f the d i r e c t i o n a l l y s o l i d i f i e d samples having a morpho?ogy which was pre- v i o u s l y i d e n t i f i e d by o p t i c a l observat ion and X-ray d i f f r a c t i o n , were placed on a

Table I - Crysta l lographic r e l a t i o n s a t the lame l la r i n te r faces .

Eu tec t i c composit ion

Eu tec t i c phases

Structure type --

Lattice parameter of the eutectic phases(A)

Growth d i r e c t i o n

E p i t a x i a l r e l a t i o n s h i p s

I n t e r f a c e plane

NiO - CaO

40 mol % CaO

NiO

NaC1 ~

4.20

<110>

CaO

NaCl

4.75

<110>

NiO - Y203

31 mol % Y203

NiO

NaC1

4.18

<110>

Y203

P;::iO -

10.60

<001>

NiO - A1203

14 mol % A1203

(hk l ) / / ( h k l )

NiO

NaCl

4.19

< I l l >

NiO - Zr02(CaO)

70 mol % NiO

(111)

NiA1204

MgA1204 ------

8.07

< I l l >

NiO

NaCl

4.18

<110>

(111)

[lie] I/ [ o o ~ ]

(111) I/ (100) ( h k l ) I/ ( h k l )

f i b e r s

Zr02

CaF2

5.12

<001>

(111) (100)

[lie] /I [ o o ~ ]

(111) 11 (100)

(111) (100)

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Fig. 3 - Computer simulation of the atomic arrangement a t the NiO-Y203 in- terface, viewed perpendicularly to the growth direction and showing the mating of three-fold and four-fold symmetry surfaces. The same crystallographic relationship i s observed fo r the NiO-Zr02 eutectic.

goniometer head and observed by S.E.M. The orientation of the lamellae was estimated micrographically from thei r intersection with the surfaces of the fragments : in ter - faces are (111) fo r both NiO and CaO phases in the NiO-CaO system and ( 1 1 1 ) ~ i o and (100)y203for the NiO-Y203 system. The method could not be applied so f a r to the NiO-

Zr02 eutectic but considering the identical epitaxial relat ions found fo r both NiO- Y2O3 and NiO-ZrOp systems, and the very close crystallographic structures of Zr02 and Y2O3 (cubic zirconia has a f luo r i t e type structure, and yttrium oxide has a bixbyite type structure which may be described as a f luo r i t e superstructure obtained by orde- ring of oxygen vacancies on the anionic subla t t ice) i t i s l ikely tha t the interface planes are and (100)zro2 in NiO-Zr02. Additional arguments tending to confirm

th i s will be discussed l a t e r in th i s paper. Fig. 3 represents a computer simulation of the atomic arrangements corresponding to such an epitaxy between four-fold and three-fold symmetry surfaces in the part icular case of NiO-Y203.

IV - INTERFACE STRUCTURES

Considering the data presented in Table I , i t i s interesting to represent the struc- tures of the oxides involved in lamellar eutectics, perpendicularly to growth direc- t ions , in order to describe atomic arrangements near the interfaces (Fig. 4 ) .

In the case of the NiO-CaO eutectic made of nickel oxide and calcium oxide solid so- lutions, the interface can be easily described by (111) al ternate planes of metal and close-packed oxygen : the metal planes are r icher in nickel on one side of the inter- face and in calcium on the other side since we deal with solid solutions (Fig. 4a). The distances between anionic planes in each phase d i f f e r only s l ight ly .

In the case of the NiO-Y203 eutectic where there i s no detectable solubi l i ty of one oxide in the other, a description of the interface in terms of oxygen and metal planes

NiO -CaO NiO- Z r 0 2

I r NiO-Y203 /-, * \

Fig.4-Representat ion o f the NiO-CaO in te r face and o f the atomic layers o f YpO3, N i O and Zr02, viewed perpendicu lar ly t o growth d i r e c t i o n s ( i .e . p a r a l l e l t o i n t e r f a c e s ) .

may a lso be considered (F ig . 4 ) . Th is d e s c r i p t i o n i s complicated by the f a c t t h a t i n b i x b y i t e the oxygen vacancy d i s t r i b u t i o n def ines two d i f f e r e n t c rys ta l log raph ic pos i - t i o n s f o r y t t r i u m atoms and one s e t o f oxygen s i t e s i n the cubic s t r u c t u r e /15/ : along a <loo> d i r e c t i o n , atoms are n o t s t r i c t l y coplanar and de f ine e i t h e r planes o r "sheets" (F ig. 4b) /14/. Because o f the existence o f two d i f f e r e n t types o f c a t i o n i c layers i n Y203, three d i f f e r e n t a l t e r n a t i v e forms o f ep i taxy can be considered i n order t o s a t i s f y the observed (100)y203 / / r e l a t i o n a t the in te r face . We

have shown t h a t the most probable i n t e r f a c e s t r u c t u r e would be t h a t i n which a n icke l oxide oxygen plane would be fo l lowed by an y t t r i u m plane, i t s e l f fo l lowed by an y t t r i a type oxygen sheet /14/ : t h i s s t r u c t u r e i s the on ly one i n which the s i x - f o l d coord i - n a t i o n observed f o r Y i n YpO3 i s maintained. We s h a l l a l so note t h a t i n both NiO and YpO3 phases, the distances between two consecutive metal-oxygen layers, a long. the d i r e c t i o n perpendicular t o the growth d i r e c t i o n , are very near one another.

The analys is o f the NiO-Zr02 i n t e r f a c e seems easier because o f the s impler f l u o r i t e s t r u c t u r e o f Zr02,viewed perpendicu lar ly t o <loo> i n F ig . 4d : one would have a t the i n t e r f a c e successively, zirconium, oxygen and n i c k e l planes. O f course t h i s rough descr ip t ion does no t take i n t o account the small d i s t o r t i o n s associated w i t h t h e va- cancies in t roduced i n the Zr02 oxygen s u b l a t t i c e by the calcium oxide wh ichs tab i l i zes the cubic mod i f i ca t ion o f z i r con ia . Here again we s h a l l note t h a t i n both N i O and ZrOp phases the distances between two consecutive metal-oxygen layers along the d i r e c t i o n perpendicular t o the growth d i r e c t i o n , are not very d i f f e r e n t .

V - DISCUSSION

Our r e s u l t s b r i n g add i t i ona l data t o the already l a r g e v a r i e t y o f c rys ta l log raph ic c h a r a c t e r i s t i c s determined f o r non-meta l l ic eu tec t i cs . Obviously i t appears t h a t i n b inary s t ruc tu res i n v o l v i n g phases o f cubic symmetry, the <110>, < I l l > and <loo> growth d i r e c t i o n s p r e v a i l . Th is i s t r u e f o r the systems mentioned i n t h i s study b u t a lso f o r Mg0-MgA1204, MgO-cubic Zr02 and MgO-CaO where growth d i r e c t i o n s are < I l l > f o r a l l phases /12/. This was a lso observed i n sodium chlor ide- type h a l i d e eu tec t i cs such as NaCl-NaF and LiF-NaF /16,17/. Th is holds a lso f o r the f l u o r i t e - t y p e phases o f the ZrO Ln203 (Ln = Nd, Sm, Dy) eu tec t i cs /18/ and f o r the cubic Zr02 phase o f the z ~ o ~ - c ~ Z ; O ~ e u t e c t i c /12/, which a l l grow along <110>.

When comparing growth d i r e c t i o n s i n s t r u c t u r a l l y homologous systems, we f i n d i t hard

C4-446 JOURNAL DE PHYSIQUE

t o make genera l i za t ions : if we note a good agreement between data r e l a t i v e t o the NaC1-spinel type systems Ni0-NiA1204 and Mg0-MgA1204 /19/, on the o ther hand, i n the case o f NaC1-f luor i te type systems, we f i n d t h a t the s i m i l a r r e s u l t s concerning NiO- Y 0 and NiO-Zr02 d i f f e r from those proposed by Minford e t a1 /20/ f o r MgO-Zr02 where < f l j>MgO / / <111>Zr~2. Discrepancies e x i s t a l so among the NaC1-NaCl type systems

where <100>1 // <100>1~, <111>1 // <111>11 and <110>1 // <110>11 r e l a t i o n s have been found /12,17/.

General izat ions seem a l s o d i f f i c u l t when consider ing the epi taxy r e l a t i o n s repor ted SO far fo r non-metal1 i c systems, p a r t i c u l a r l y f o r l ame l la r s t ruc tu res . It seems however t h a t a l a r g e p a r t of the data f i t w i t h the w e l l accepted concepts o f min imizat ion o f l a t t i c e m i s f i t s and balance o f charge dens i t i es a t the in te r faces . These f a c t o r s would p lay a major p a r t i n the establ ishment o f i n t e r f a c e planes and r e l a t i v e o r ien - t a t i o n s o f the l a t t i c e s o f the two phases. Considering t h a t i n l a m e l l a r o r ien ted s o l i d i f i c a t i o n , i n t e r f a c e s are para1 l e l t o growth d i rec t ions , growth axes should be d i r e c t i o n s o f s t rong atomic bonding o r more p r e c i s e l y o f "per iod ic bond chains" 1211 contained i n the i n t e r f a c e plane. Hence i n one such plane a very l i m i t e d number of growth d i r e c t i o n s might be poss ib le , among which, as seen so f a r i n most systems, on ly one would p r e v a i l ; other would be poss ib ly s t a b i l i z e d by l o c a l per turbat ions dur ing the growth process, o r by i m p u r i t i e s . This seems t o be confirmed by observa- t i o n s made on the NiO-Zr02 e u t e c t i c f o r which,besides the major growth axes ind ica ted i n Table I we have found on microfragments o r ien ted a lon the growth d i r e c t i o n and studied by the Weissenberg method, a growth d i r e c t i o n [2Q11Ni0 // [ 0 1 3 ] ~ ~ ~ ~ . This

r e l a t i o n s h i p i s equivalent,wi t h i n a few minutes precision, t o [IT01 NiO // [001] zro2 proposed i n Table I and ind ica tes t h a t r e l a t i v e l a t t i c e o r i e n t a t i o n s are the same i n t h i s fragment and i n the bu lk ; i t confirms a l s o the above-made assump- t i o n concerning the i n t e r f a c e plane i n NiO-Zr02 which i s c l e a r l y def ined i n each phase by two growth d i r e c t i o n s : t h a t o f the fragment and t h a t o f the bu lk e u t e c t i c mate r ia l . We s h a l l a l so note t h a t the r a t h e r unusual [103]zr02 growth a x i s we have

determined, has been q u i t e sys temat i ca l l y observed on po lyc rys ta l l i n e cubic s t a b i l i - zed z i r c o n i a i n the course o f a recent study o f the r e l a t i o n between morphology and s t r u c t u r e i n s t a b i l i z e d Zr02 /22/.

V I - CONCLUSION

Crysta l lographic r e l a t i o n s and i n t e r f a c e planes have been determined f o r several oxide-oxide e u t e c t i c systems and add t o the in fo rmat ion from which i n a near f u t u r e genera l izat ions should be made. To improve the present d e s c r i p t i o n o f e u t e c t i c crys- t a l l og raph ic r e l a t i o n s h i p s and i n t e r f a c e s i t w i l l be i n t e r e s t i n g t o apply t o these b inary systems the co inc iden t s i t e l a t t i c e theory. High r e s o l u t i o n e l e c t r o n micros- copy, 1 argel y used t o day for g r a i n boundary studies, should a1 so improve our under- standing o f the two-phase type i n t e r f a c e s discussed i n t h i s paper.

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