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COMPOSITION DEPENDENT HIGH
TEMPERATURE INTERNAL FRICTION IN MgO
(Al2O3)n SPINELS
J. Woirgard, P. Mazot, M. Halbwacks
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C10, supplement au n012, Tome 46, decembre 1985 page C10-553
COMPOSITION DEPENDENT HIGH TEMPERATURE INTERNAL FRICTION IN MgO
( A l , 0, ), SPINELS
J. WOIRGARD,P. MAZOT AND M. HALBWACKSf
Laboratoire de Mecanique et Physique des Materiaux, U A CNRS 863, ENSMA, 86034 Poitiers Cedex, France
'Universitk de Savoie, BP 1104, 73011 Chambery Cedex, France
ABSTRACT
Isothermal damping spectra obtained between 800 K and 1100 K i n non s t o i c h i o m e t r i c s p i n e l specimens revealed an important r e l a x a t i o n peak f o r t o r s i o n a l v i b r a t i o n frequencies ranging between 10-4 Hz and 10 Hz.
The corresponding a c t i v a t j o n energy (H = 3.84 eV) i s i n good agreement w i t h c a t i o n i c d i f f u s i o n , i n d i c a t i n g t h a t t h e r e l a x a t i o n probably a r i s e s from s t r e s s induced r e o r i e n t a t i o n o f e l a s t i c d i p o l e s formed by t h e a s s o c i a t i o n o f M~~~ c a t i o n s and s t r u c t u r a l c a t i o n i c vacancies. I n s i n g l e c r y s t a l s t h e o r i e n t a t i o n dependence o f t h e r e l a x a t i o n s t r e n g t h has been i n v e s t i g a t e d leading t o a <110> p o s s i b l e o r i e n t a t i o n f o r t h e e l a s t i c d i p o l e s .
The composition dependence o f t h e r e l a x a t i o n s t r e n g t h has a l s o been i n v e s t i g a t e d i n b o t h p o l y and s i n g l e c r y s t a l s f o r n ranging between 1 and 10. The maximum r e l a x a t i o n s t r e n g t h corresponds t o n values between 2 and 3. I
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INTRODUCTIONSpinels s t r u c t u r e s can be c h a r a c t e r i z e d by t h e formula :
M2+ M3+ 0 M ~ + and M3+ being r e s p e c t i v e l y d i v a l e n t and t r i v a l e n t c a t i o n s . The u n i t c e l l i s &scribed as an almost p e r f e c t close-packed oxygen s t r u c t u r e w i t h 64 t e t r a h e d r a l s i t e s and 32 octahedrp+l o n e ~ . ~ $ r o m theses s i t e s 8 t e t r a h e d r a l and 16 o$fa;$$ral ones can be occupied by M and M cations, g i v i n g f o r t h e u n i t c e l l .
M '32.
This paper deals w i t h s y n t h e t i c n o n s t o i c h i o m e t r i c s p i n e l s z
MgO(A1203)ncorresponding
t o a s o l i d s o l u t i o n o f alumina A1203 i n t h e Mg2A1204 s t o c h i o m e t r i c compound.Some d i s o r d e r i n t h e c a t i o n i c d i r e c t d i s t r i b u t i o n a r i s e s from an i n v e r s i o n phenomenon, t h e t r i v a l e n t c a t i o n s normally l y i n g i n octahedral s i t e s , being interchanged w i t h d i v a l e n t ones normally l y i n g i n t e t r a h e d r a l s i t e s . The degree o f i n v e r s i o n amounts between 10 t o 15 % (1,2,3).
A d d i t i o n a l d i s o r d e r a r i s e s from t h e presence o f a l a r g e amount o f s t r u c t u r a l vacancies introduced t o preserve e l e c t r o n e u t r a l i t y :
3 2 A I ~ +
+ o
F o u r i e r a n a l y s i s o f sharp X-ray i n t e n s i t i e s have shown t h a t t h e s t r u c t u r a l vacancies occur i n octahedral s i t e s ( 4 ) . Furthermore Jodgatzinski and Haefner (5)
suggested t h a t t h e most probable arrangement f o r vacancies i s p a i r s formation i n octahedral s i t e s , along <I 10) d i r e c t i o n s i n dense (1 11) planes.
Such defects are s u s c e p t i b l e t o g i v e r i s e t o t h e l a r g e a n e l a s t i c r e l a x a t i o n phenomenon p r e v i o u s l y r e p o r t e d /6,7,8/, b u t o t h e r d e f e c t s a r e p o s s i b l e :
Mg 2t-~g2tor M ~ ~p a i r s , a l s o l y i n g along (1 10) ~ - d i r e c t i o n s .
C10-554 JOURNAL
DE
PHYSIQUETherefore t h e aim o f t h i s paper i s t o study more p r e c i s e l y t h e composition dependence o f t h e r e l a x a t i o n s t r e n g t h i n order t o o b t a i n more p r e c i s e i n f o r m a t i o n about t h e n a t u r e o f t h e d e f e c t r e s p o n s i b l e f o r t h e r e l a x a t i o n .
111- EXPERIMENTAL PROCEDURE
The i n t e r n a l t r i c t i o n pendul um used i n these experiments has been p r e v i o u s l y described (9.10). We s h a l l o n l y r e c a l l t h a t isothermal measurements can be ~ e r f o r m e d i n a wide frequency range, between room temperature and up t o 2000 K.
Specimens were e l a b o r a t e d from powders s u p p l i e d by Baikowsky. For n r a t i o between 1 and 3.5 s i n g l e c r y s t a l s were grown by t h e Verneuil method.
For l a r g e r n values t h e plasma spraying technique was used t o r e t a i n t h e metastable n o n - s t o i c h i o m e t r i c s o l i d s o l u t i o n /11/. P o l y c r y s t a l l i n e specimens were obtained w i t h a p o r o s i t y n o t exceeding 15 %. Due t o t h e l o s s o f magnesia occuring d u r i n g t h e plasma spraying, t h e composition o f t h e f i n a l d e p o s i t was determined by atomic absorption spectrometry and X-ray u n i t c e l l parameter measurement.
I V - EXPERIMENTAL RESULTS
Some i n t e r n a l f r i c t i o n peaks obtained i n s i n g l e c r y s t a l s f o r n values r a n g i n g between 1.5 and 3.5, are shown i n F i g u r e 1. I n t h i s composition domain t h e r e l a x a t i o n s t r e n g t h i s observed t o increase w i t h t h e alumina content up t o n = 3.
For n = 1, which correspond t o t h e equimolar compound, no peak c o u l d be observed. S i m i l a r r e s u l t s were obtained i n polycrystals,. f o r n values ranging between 1 and 10. The r e s u l t s corresponding are p l o t t e d i n F i g u r e 2 ; t h e neat i n t e r n a l f r i c t i o n peaks have been obtained a f t e r removing a low frequency background probably due t o t h e r a t h e r h i g h p o r o s i t y o f t h e specimens. No peak was observed i n pure y alumina. The r e l a x a t i o n appears v e r y a n i s o t r o p i c as can be observed i n F i g u r e 3 i n which a r e p l o t t e d i n t e r n a l f r i c t i o n peaks obtained f o r t h e same composition i n < I 0 0 > < 1 1 0 >
<Ill> s i n g l e c r y s t a l s and a f i n e grained p o l y c r y s t a l i n e specimen.
It can be seen t h a t t h e r e l a x a t i o n s t r e n g t h never vanishes, i n d i c a t i n g t h a t t h e e l a s t i c d i p o l e can be an orthorhombic o r a monoclinic d e f e c t l y i n g along a <110> d i r e c t i o n /12/. I n F i g u r e 4 a r e p l o t t e d t h e peak h e i g h t obtained i n specimens w i t h various compositions. The r e l a x a t i o n s t r e n g t h i s zero i n equimolar m a t e r i a l s , goes through a maximum f o r n between 2 and 3 and tends t o vanish f o r l a r g e n values
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no peak i s observed i nr
alumina. The o n l y mechanism a b l e t o e x p l a i n such a behavior i s t h e r e l a x a t i o n o f Mg2+- D p a i r s . I n t h a t case t h e r e l a x a t i o n s t r e n g t h w i l l vary as t h e product o f t h e concentrations, which i s p l o t t e d on t h e same p i c t u r e :I t can be observed t h a t t h e agreement i s b e t t e r f o r p o l y c r y s t a l s than f o r s i n g l e c r y s t a l s . The f a c t t h a t t h e r e l a x a t i o n process i n v o l v e s t h e motion o f M ~ cations, ~ +
through a vacancy mechanism, i s f u r t h e r confirmed by t h e apparent a c t i v a t i o n energy deduced from t h e frequency s h i f t o f t h e peak w i t h temperature :
V- CONCLUSION
A c a t i o n i c m o b i l i t y mechanism has been proved t o be r e s p o n s i b l e f o r t h e l a r g e r e l a x a t i o n peak observed i n t h e 900
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1200 K temperature range i n non s t o i c h i o m e t r i c MgO(A1 203) s p i n e l s . From t h e observed v a r i a t i o n s i n t h e r e l a x a t i o n s t r e n g t h w i t h t h e amount o f solved A1 03, i t has been e s t a b l i s h e d t h a t t h e i n v o l v e d d e f e c t s were ~ g 2 + - p a i r s l y i n g $ong <110> d i r e c t i o n s .V I
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