SUPERCONDUCTING MATERIALS BY AEROGEL PROCESS

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SUPERCONDUCTING MATERIALS BY AEROGEL PROCESS

B. Pommier, S. Teichner, P. Lejay, André Sulpice, R. Tournier

To cite this version:

B. Pommier, S. Teichner, P. Lejay, André Sulpice, R. Tournier. SUPERCONDUCTING MATE- RIALS BY AEROGEL PROCESS. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-41-C4-44.

�10.1051/jphyscol:1989407�. �jpa-00229482�

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REVUE DE PHYSIQUE APPLIQUÉE

Colloque C4, Supplément au n°4, Tome 24, Avril 1989 C4-41

SUPERCONDUCTING MATERIALS B Y AEROGEL PROCESS

B . POMMIER, S.J. TEICHNER, P. LEJAY*, A . SULPICE* and R. TOURNIER*

Université Claude Bernard, Lyon I, 43 bd du 11 novembre 1918. ¥-69622 Villeurbanne Cedex, France

* C.R.T.B.T. (CNRS), 38 avenue des Martyrs, BP 166X, F-38042 Grenoble Cedex, France

Résume - Lorsque les oxydes précurseurs du supraconducteur Y Ba. Cu, 0, sont sous forme d'aérogel mixte ternaire et non pas d'un mélange mécanique conventionnel, la conversion en céramique supraconductrice, par chauffage sous oxygène, se produit beaucoup plus rapidement, sans l'apparition de la phase eutectique liquide.

Abstract - When oxide precursors of the superconductor Y Ba Cu, 07_ are under the form of a ternary mixed aerogel and not as a conventional mechanical mixture, their conversion into the superconducting ceramics, by heating under oxygen, is produced much faster, without the formation of the liquid eutectic.

I - INTRODUCTION

The achievement of homogeneous mixing of reactant components in a precursor state to avoid conventional solid state reaction (undesirable) between large and chemically dissimilar particles is an important and fundamental problem in solid-state chemistry and in particular in the generation of high temperature superconducting solids which are' formed by chemical interaction between three or more inorganic oxides.

The sol-gel approach in a solvent like water is used successfully to prepare small reactive particles (20 nm or less) of a given reactant (like Al 0,). This procedure is difficult to apply for obtaining a chemically homogeneous particle where two or more • reactants are present. The aerogel process in an organic solvent for the preparation of mixed oxides /l/

allows to avoid solid state reaction between chemically dissimilar particles. Moreover, the aerogel process leads to amorphous oxides in a highly divided state. These two properties are those required for a high reactivity of the precursors in the dry solid state, at temperatures much lower than conventional firing temperatures used for crystallized precursors.

In the case of YBa. Cu 0, superconductor the initial precursor is obtained by controled hydrolysis of organic derivatives of the corresponding elements, dissolved in methanol. The evacuation of methanol in supercritical conditions leads /2/ to a very homogeneous amorphous, highly divided solid, in the form of aerogel.

II - EXPERIMENTAL 1 - Materials

The aerogel containing the oxides of Y, Ba and Cu in the proportions given above is prepared according to the following procedure. In 60 g of methanol containing 9.82 g of ethylacetoacetate are introduced 7.53 g (3 x 0.012586 moles) of Copper(II) acetate monohydrate. After 48 h stirring the initial blue dispersion of powdered copper acetate is converted into a green dispersion of colloidal flakes. In 50 g of methanol, on the other hand, are dissolved 4.255 g (1 x 0.012586 moles) of Yttrium acetate tetrahydrate. Finally, in 85 g of methanol containing 1 g of water are introduced 6.43 g (2 x 0.012586 moles) of Baryum acetate. This reactant is not entirely dissolved even after prolonged stirring. However by addition of the methanolic solution of Yttrium acetate a homogeneous solution is obtained.

The methanolic dispersion obtained with Copper(II) acetate is now added to this solution of Yttrium and Barium acetates and the system is stirred for 48 h. A green homogeneous solution is then obtained containing the elements in the required proportions, i.e. YBa, Cu 0 . This methanolic solution is introduced into an autoclave with an extra amount of metnanoX, In a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989407

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s e p a r a t e v e s s e l , r e q u i r e d t o achieve t h e s u p e r c r i t i c a l c o n d i t i o n s d u r i n g t h e heating. T h i s amount of e x t r a methanol depends on t h e volume of t h e autoclave. The a u t o c l a v e i s heated t o 270-280°C (above t h e c r i t i c a l temperature of methanol) which t a k e s 2-3 hours. The p r e s s u r e i s t h e n of t h e o r d e r of 80-100 b a r s (above t h e c r i t i c a l p r e s s u r e of methanol). This p r e s s u r e i s f i n a l l y g e n t l y r e l e a s e d ( i n about 1 hour) and t h e atmospheric p r e s s u r e i s e s t a b l i s h e d . A flow of d r y n i t r o g e n d u r i n g 15 minutes, through t h e a u t o c l a v e maintained a t 270°C, removes t h e l a s t t r a c e s of methanol. The temperature i s t h e n lowered t o room temperature ( i n about 2 hours) and t h e a u t o c l a v 5 i s open. The a e r o g e l obtained i n t h i s way a s a homogeneous, low d e n s i t y powder (S = 52 m / g ) , i s amorphous t o X-Rays w i t h a few l i n e s due t o m e t a l l i c copper ( f i g . 1).

6 0 50 4 0 3 0 2 0 10

'7- 2 0 Fig. 1

-

Cu & d i f f r a c t i o n p a t t e r n of t h e a e r o g e l p r e c u r s o r .

T h i s a e r o g e l i s converted i n t o a c r y s t a l l i n e compound of t h e composition Y Ba Cu 0 f o l l o w i n g a procedure, e s t a b l i s h e d by t h e c o n s i d e r a t i o n s given below. I t i s known f h a t ' d u r k ~ t h e s o l i d s t a t e r e a c t i o n between conventional mixture of c r y s t a l l i z e d r e a c t a n t s i t i s formed i n each g r a i n a l a r g e range of v a r i o u s chemical compositions. The i n t e r a c t i o n k i n e t i c s i s c o n t r o l l e d by d i f f u s i o n i n t h e s o l i d s t a t e . The v a r i o u s homogenizations between t h e s e compositions a r e a l s o d i f f u s i o n c o n t r o l l e d . Moreover, i n t h e t e r n a r y diagram Y 0 5-BaO-Cu0 an e u t e c t i c composition i s observed around 900°C 1 3 1 , which i s c l o s e t o tAE! nominal composition of t h e supraconductor given p r e v i o u s l y , which corresponds t o t h e so-called 1-2-3 formula. T h i s l i q u i d e u t e c t i c i n t r o d u c e s t h e h e t e r o g e n e i t y of composition i n t h e system. The temperatures r e q u i r e d f o r t h e i n t e r a c t i o n between t h e s o l i d r e a c t a n t s should be t h e r e f o r e maintained below t h e temperature of t h e e u t e c t i c t i l l t h e s t a t e of a p e r f e c t homogeneity of t h e m a t e r i a l i s achieved. I n g e n e r a l , i n t h i s type of r e a c t i o n s t h e p r e h e a t i n g r e q u i r e s 12-48 hours w i t h i n t e r m e d i a t e g r i n d i n g s i n o r d e r t o achieve homogenization of t h e s o l i d 141.

I n t h e o p p o s i t e way, f o r t h e r e a c t a n t s i n t h e a e r o g e l form, t h e sub-micronic dimensions of tlie p a r t i c l e s , t h e i r amorphous s t a t e and t h e i n i t i a l homogeneity of t h e mixture allow t h e s y n t h e s i s of t h e f i n a l s o l i d without v a r i o u s p r e h e a t i n g s and t h e k i n e t i c s of t h e i n t e r a c t i o n s seems t o be c o n s i d e r a b l y i n c r e a s e d .

The i n i t i a l amorphous a e r o g e l , of t h e composition Y Ba2 Cu 0 s t a r t s , t o c r y s t a l l i z e a t 3 n

750°C. A t 950°C, under pure oxygen, without any previous homogenization by p r e h e a t i n g and g r i n d i n g , i t g i v e s t h e f i n a l superconductor a f t e r 2 hours heating. The s o l i d obtained corresponds t o t h e r e q u i r e d c r y s t a l l i n e s t a t e ( s e e below) w i t h no t r a c e s of any u n d e s i r a b l e phase, e i t h e r i n t h e o r i g i n a l powder form o r a s a compressed p e l l e t . It has been observed t h a t t h e l i q u i d ( e u t e c t i c ) phase i s not formed d u r i n g t h e h e a t i n g a t 950°C. This i s probably due t o t h e homogeneity of t h e i n i t i a l m a t e r i a l which a l l o w s a d i r e c t formation (by s o l i d s t a t e d i f f u s i o n ) of t h e d e s i r e d compound, without t h e s e p a r a t i o n of t h e e u t e c t i c . The w e l l known i n t e r a c t i o n s between t h e l i q u i d e u t e c t i c and t h e m a t e r i a l of t h e v e s s e l a r e t h e r e f o r e minimized. The c o o l i n g i s c a r r i e d out under oxygen w i t h t h e r a t e 20°/hour.

2

-

^Uethods

The v a r i o u s p h y s i c a l methods employed i n t h e i n v e s t i g a t i o n s on superconductors a r e w e l l d e s c r i b e d and a r e only b r i e f l y mentioned below.

I11

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^RESULTS^ANLIDISCUSSION

The f i n a l s o l i d has t h e composition Y Ba2 Cu 07-x w i t h orthorhombic symmetry (a = 3.821 ; b = 3.885

%

; c = 11.676

x)

whose X-Rays d?agram i s given on Fig. 2. The s p l i t t i n g of t h e l i n e s 013 and 103 around 2 9 = 33" i s c h a r a c t e r i s t i c of t h e formula given above. When x i n c r e a s e s t h e symmetry changes from orthorhombic t o q u a d r a t i c and t h e s p l i t t i n g of t h e p r e v i o u s l i n e s vanishes.

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Fig. 2

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Cu K a pattern of the orthorhombic Y Ba2 Cu 7 superconductor.

3 0-x

Magnetic measurements were performed on a rod cut from a compressed pellet. Figure 3 gives the variation of the magnetization with the magnetic-Sield applied at 4 K. The diamagnetic susceptibility derived from this diagram is 1.43 x 10 uem/g. This value corresponds, after the correction of the coefficient of demagnetizing field, to a susceptibility

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1 / 4 T which corresponds to a superconducting state of the whole sample (no undesirable phases present).

Fig. 3

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Magnetization versus magnetic field at 4 K.

Figure 4 represents the variation of the diamagnetic susceptibility with the temperature in the magnetic field of 10 ce

.

^Curve⁵(crosses) is obtained for the sample cooled in the absence of the magnetic field which is applied once the temperature of 4 K is obtained. The susceptibility is then recorded at increasing temperatures. A large screen effect is observed until 80 K and is followed by an abrupt transition achieved around 92 K. Curve

>

(squares) is obtained for the sample cooled in the magnetic field of 10

.

The susceptibility is therefore recorded for decreasing temperatures. The Meissner effect observed represents 10 % of the screen effect. This low value is characteristic of this type of high-pinning superconductors.

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Fig. 4

-

Diamagnetic s u s c e p t i b i l i t y v e r s u s t e m p e r a t u r e .

The s u p e r c o n d u c t i n g p r o p e r t i e s of t h e samples d e r i v e d from t h e a e r o g e l p r e c u r s o r a r e t h u s v e r y comparable t o t h o s e p u b l i s h e d e a r l i e r i n t h e l i t t e r a t u r e concerning t h e samples o b t a i n e d by t h e c o n v e n t i o n a l s o l i d s t a t e r e a c t i o n w i t h c r y s t a l l i z e d p r e c u r s o r s

151.

C r i t i c a l c u r r e n t measurements under h i g h magnetic f i e l d a r e now i n p r o g r e s s .

I t h a s been shown t h a t f o r t h e amorphous, h i g h l y d i v i d e d , a e r o g e l p r e c u r s o r , t h e k i n e t i c s of t h e s o l i d s t a t e r e a c t i o n under oxygen, g i v i n g orthorhombic Y Ba2 Cu 0 a s a v e r y p u r e phase, i s c o n s i d e r a b l y i n c r e a s e d and t h e p a r a l l e l u n d e s i r a b l e re?ct!zx?s a r e minimized. The s u p e r c o n d u c t i n g p r o p e r t i e s of t h e samples d e r i v e d by t h i s method a r e comparable t o t h o s e of samples produced by t h e c o n v e n t i o n a l p r o c e s s i n v o l v i n g c r y s t a l l i z e d p r e c u r s o r s .

The s i n t e r e d p e l l e t s of a e r o g e l a r e

bulk

s u p e r c o n d u c t o r s w i t h an a b r u p t t r a n s i t i o n a t 92 K , a s determined by D.C. s u s c e p t i b i l i t y measurements. The h i g h l y d i v i d e d , powdered, a e r o g e l p r e c u r s o r i s a v e r y s u i t a b l e m a t e r i a l f o r t h e p r o d u c t i o n of v e r y d e n s e ceramics and t h i c k f i l m s on c e r t a i n s u b s t r a t e s i n o r d e r t o improve t h e c r i t i c a l c u r r e n t d e n s i t y i n t h e s u p e r c o n d u c t i n g s t a t e a t l i q u i d n i t r o g e n temperature.

REFERENCES

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1968

(1906) 1900.

/ 2 / T e i c h n e r , S.J., Nicolaon, G.A., V i c a r i n i , M.A. and Gardes, G.E.E., Adv. Coll. I n t e r f . S c i . ,

5

(1976) 245.

/ 3 / Poeppel, R.B., Flandermayer, B.K., Dusek, J.T. and Bloom, I.D., "Chemistry of High-Temperature Superconductors" ACS Symp. S e r i e s 351 (1987) 261.

/ 4 / Norton, M.L., "Chemistry of High-Temperature Superconductors" ACS Symp. S e r i e s 351, (1987) 56.

/ 5 / Laborde, O . , Tholence, J . L . , L e j a y , P . , S u l p i c e , A., T o u r n i e r , R., Capponi, J . J . , Michel, C. and P r o v o s t , J., S o l i d s t a t e Comm.,

63

(1987) 877.