CERIUM AND GADOLINIUM OXIDES PRECURSORS OBTAINED VIA UREA HYDROLYSIS

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CERIUM AND GADOLINIUM OXIDES

PRECURSORS OBTAINED VIA UREA HYDROLYSIS

S. Meriani, E. Lucchini, O. Sbaizero

To cite this version:

S. Meriani, E. Lucchini, O. Sbaizero. CERIUM AND GADOLINIUM OXIDES PRECURSORS OB- TAINED VIA UREA HYDROLYSIS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-19-C1-23.

�10.1051/jphyscol:1986103�. �jpa-00225487�

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JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n"2, Tome 47, f 6 v r i e r 1986 page cl-19

CERIUM AND GADOLINIUM OXIDES PRECURSORS OBTAINED VIA UREA HYDROLYSIS

S. M E R I A N I , E . L U C C H I N I and 0 . SBAIZERO

I s t i t u t o d i Chimica A p p l i c a t a e I n d u s t r i a l e , Via V a l e r i o , 2 , I-34127 T r i e s t e , I t a l y

R6sum6

-

On a obtenu par hydrolyse i l f u r 6 e de solutions aqueuses de nitrates, la coprkcipitation des carbonates de cgrium et de gadolinium utilisables pour la prkparation de solutions solides entre les oxydes correspondants.

A b s t r c c t

-

Mixed cerium and gadolinium " c a r b o n a t e s " , a s p r e c u r s o r s

--

of t h e i r o x i d e s ' s o l i d s o l u t i o n s , have been o b t a i n e d by t h e a c t i o n of homogeneous h y d r o l y s i s of u r e a on t h e i r n i - t r a t e s aqueous s o l u t i o n s .

I

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I N T R O D U C T I O N

Ceramic m a t e r i a l s w i t h t h e " f l u o r i t e " t y p e of s t r u c t u r e based on "doped c e r i a " e x h i b i t i n t e r e s t i n g e l e c t r i c a l p r o p e r t i e s a t high temperatures which make them c a n d i d a t e s f o r oxygen monitoring d e v i c e s and f u e l c e l l s systems / I , 2 / .

The method of p r e p a r i n g t h e s t a r t i n g powders g r e a t l y i n f l u e n c e s t h e f i - n a l p r o p e r t i e s of t h e s i n t e r e d samples. The i m p u r i t i e s i n t e r f e r e with t h e g r a i n boundaries and a f f e c t t h e i r m i c r o s t r u c t u r e and t h u s t h e e l e c - t r i c a l c o n d u c t i v i t y of p o l y c r i s t a l l i n e m a t e r i a l s / 3 / .

Chemical methods, o r i e n t e d t o powders' p r e p a r a t i o n , have gained i n c r e a s - i n g i n t e r e s t among m a t e r i a l s s c i e n t i s t s and t e c h n o l o g i s t s . The chemical s y n t h e s i s of t h e ceramic m a t e r i a l s i s expected t o p l a y a major impact r o l e on t h e ceramic and g l a s s f a b r i c a t i o n p r o c e s s e s .

"Chemical-ceramics" o r "Synthetic-ceramics" may become a f i e l d of com- mon i n t e r e s t between chemists and c e r a m i s t s i n view of promising mutual b e n e f i t s .

The work h e r e i n r e p o r t e d i s p a r t of a r e s e a r c h program undertaken t o provide s i n t e r - r e a c t i v e powders of ceramic i n t e r e s t which should meet higher m a t e r i a l s requirements. These a r e some p r e l i m i n a r y r e s u l t s about t h e thermal e v o l u t i o n of t h e "carbonates" of cerium and gadolinium.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986103

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C 1 - 2 0 JOURNAL DE PHYSIQUE

These " c a r b o n a t e s " have been o b t a i n e d a s p r e c i p i t a t e G . c r y s t a l l i n e pro- d u c t s by t h e a c t i o n o f t h e homogeneous h y d r o l y s i s o f u r e a on t h e i r b o i l - i n g n i t r a t e s s o l u t i o n s .

I1

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EXPERIMENTAL METHODS

The C e ( + 3 ) and t h e Gd(+3) n i t r a t e s s o l u t i o n s , mixed w i t h t h e p r o p e r a- mount o f u r e a and b u f f e r e d w i t h ammonium n i t r a t e , upon b o i l i n g were e x p e c t e d t o d e l i v e r t h e c a r b o n a t e s a c c o r d i n g t o t h e t h e r e a c t i o n :

The washed and d r i e d samples have been i n v e s t i g a t e d w i t h t h e Simultan- eous Thermal A n a l y s e r (STA) Netzsch t y p e 4 0 9 , c o u p l e d w i t h a B a l z e r s Q u a d r u p o l e Mass S p e c t r o m e t e r (QMS)-311, under a p u r i f i e d A r atmosphere f o r b e t t e r d e t e c t i o n o f t h e evolved g a s e s .

Scanning E l e c t r o n Microscopy images a r e r e p o r t e d t o g i v e e v i d e n c e o f powders' morphology and s i z e b e f o r e and a f t e r t h e decomposition s t e p s . 111

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RESULTS AND DISCUSSION

The t h e r m a l e v o l u t i o n o f t h e C e ( + 3 ) and Gd(+3) c a r b o n a t e s a r e r e p o r t e d i n F i g u r e s 1 and 2 , r e s p e c t i v e l y . On t h e b a s i s of t h e STA

-

^M^S r e s u l t s t a k i n g i n t o a c c o u n t t h e r e p o r t e d l i t e r a t u r e d a t a /5,6/, we a r e o f t h e o p i n i o n t h a t t h e most p r o b a b l e compound o b t a i n e d i n t h i s p r e c i p i t a t i o n p r o c e d u r e i s t h e mono-0x0-carbonate-hydrate of C e ( + 3 ) . Accordingly t h e proposed decomposition mechanism i s t h e f o l l o w i n g .

Ce20(C03)2.2H 0 = Ce 0 (C03) + 2 H20 + C02

2 2 2 ( t h . w . 1 . 1 7 . 7 % )

Ce2O2(Co3) = 2 C e O + CO ( t h . w . 1 . 6 . 2 % )

2

The t h e r m a l decomposition of t h e Gd-carbonate i s r e p o r t e d i n F i g . 2.

I t i s c h a r a c t e r i z e d by t h r e e s t e p s . It i s b e l i e v e d t h a t t h e most p r o - b a b l e f o r m u l a , b a s e d on t h e s e r e s u l t s and t h e l i t e r a t u r e d a t a / 7 , 8 / , i s t h a t of t h e b a s i c c a r b o n a t e w i t h t h i s decomposition mechanism:

Gd2 ( O H ) ( C O ) = Gd 0 (C03) + 0 . 5 H 0 + 1 . 5 C02

3 2.5 2 2 2 ( t h . w . 1 . 1 5 . 6 % )

Gd202 ( C03 ) = Gd203 + C02 i n two s t e p s ( t h . w . 1 . 5 . 5 + 3 . 5 % ) I t must be p o i n t e d o u t t h a t t h e two s t e p s can d e r i v e from " b r i d g i n g "

(O=Gd-CO -Gd=O) o r " b i d e n t a t e " ( C O 3 = ~ d - 0 - ~ d = o ) c a r b o n a t e i o n s , a s it h a s been proposed by s p e c t r o s c o p y methods 3

/ a / .

The c o p r e c i p i t a t e d Ce-Gd-carbonate mixed c r y s t a l s e x h i b i t a t h e r m a l decomposition r e p o r t e d i n F i g . 3 which i s q u i t e s i m i l a r t o t h a t o f t h e s i n g l e Ce compound. The w e i g h t l o s s e s and t h e i r sequence w e l l a- g r e e w i t h t h e mono-0x0-carbonate-hydrate formula a c c o r d i n g t o t h e f o l - lowing decomposition mechanism:

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with a th.w.1. of 17.5 in the first and of 6.6% in the second. Found weight losses are reported in Fig. 3, as they were in previous ones.

Cerium Catbonate Gadol~n~um Carbonate

Argon Atmosphere T.G. Argon Atmosphere

T.G.

-

M.S. 18 ClO)

D.T.A.

----

(c 01 M.S. 28

M.S. 44

(cod

I

200 300 400 500 600 T 'C 300 400 500 600 700 800 T'C

Temperature

Fig. 1 Fig. 2

Thermal decomposition (TG-DTA) and MS curves for Ce-mono-0x0-carbonate hydrate (left) and Gd-basic-carbonate (right) performed under Ar atmosphere flow 150 ml/min. Heating rate 10°K/nin. TG sens. 12.5 mg / full scale. DTA sens. 0.05 mV ( f .s. ) ; log ion currents (A) are: -5 (ms 44)

,

-6 (ms 28), -6 (ms 18). Samples 20 mg vs 20 mg Alumina in Pt holders.

Thermal hydrate

Cer~um-Gadol~n~um Carbonate Coprecrp~taled

D.T.A. Argon Atmosphere

w t L O S S 24.3%

(C02)

I I I I I I

200 300 400 500 600 700 T 'C

Fig. 3

decomposition of the coprecipitated mixed mono-0x0-carbonate of Ce and Gd according to the same experimental conditions.

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~ 1 - 2 2 JOURNAL DE PHYSIQUE

F i g . 4 F i g . 5

SEM image (5000 x ) of t h e s i n g l e SEM image (5000 x) o f t h e s i n g l e Cerium "carbonate" p r e c i p i t a t e d Gadolinium "carbonate" p r e c i p i t a t e d

Ce O(C0 ) -2H20

2 3 2 Gd2 ( O H ) KO3) 2 .

F i g . 6 F i g . 7

SEM image (5000 x) of t h e mixed SEM image (5000 x) of t h e Ce-Gd co-- Ce-Gd mono-0x0-carbonate-hydrate p r e c i p i t a t e a f t e r decomposition.

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F i g u r e s 4,5 and 6 r e p o r t t h e powders o f t h e s i n g l e and c o p r e c i p i t a t e d m a t e r i a l s . T h i s l a t t e r , a f t e r t h e decomposition s t e p s , i s p r e s e n t e d i n F i g . 7. The s h a p e o f t h e c r y s t a l s i s m a i n t a i n e d a l t h o u g h a t a c l o s e r e - x a m i n a t i o n , t h e s u r f a c e a p p e a r s porous. F u r t h e r m o r e , i n o r d e r t o com- p a r e t h e s h r i n k i n g t r e n d s of t h e c o p r e c i p i t a t e d powders w i t h t h e "mixed and ground" c r y s t a l l i n e o x i d e s of t h e same c o m p o s i t i o n , some p r e l i m i n a - r y d i l a t o m e t r i c r u n s were u n d e r t a k e n w i t h t h e r e s u l t s r e p o r t e d i n Fig.8 I n v e s t i g a t i o n s a b o u t t h e c e r a m i c b e h a v i o u r o f t h e s e m a t e r i a l s a r e i n p r o g r e s s .

A l

A = 7.4 % (480 pm) E = 1.6 % (130 urn)

(Ce.85Gd.15)01 -9.25

T

t i m e 1 2 3 4 5 6 7 E ' ( h ) t F i g . 8

-

D i l a t o m e t r i c b e h a v i o u r s I V

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CONCLUSIONS

From t h i s p r e l i m i n a r y i n v e s t i g a t i o n it c a n b e r e c o g n i z e d t h a t t h e homogeneous h y d r o l y s i s of u r e a i n b o i l i n g aqueous s o l u t i o n s of Ce

-

^Gd ^{n i -}

t r a t e s p r o d u c e s a q u a n t i t a t i v e p r e c i p i t a t i o n of e i t h e r "oxo" o r "hydr- 0x0" c a r b o n a t e s , which upon t h e r m a l t r e a t m e n t d e l i v e r t h e o x i d e s s u i t a - b l e f o r c e r a m i c p r o c e s s i n g s .

V

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REFERENCES

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3 1 4 (1983) 398.

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/7/ C h a i , B . H . T . and Mroczkowski, S.

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/8/ Goldsmith, S.A. and Ross, S.D., S p e c t r o c h i m i c a Acta (1966)1909.

T h i s work was u n d e r t a k e n under t h e f i n a n c i a l s u p p o r t of t h e CNR-Italy.

Authors t h a n k D r . E. K a i s e r b e r g e r , Netzsch-Selb (FRG) which made t h e s e measurements p o s s i b l e .