i

Mat. Res. Bu l l . , Vol. 25, pp . 699-703, 1990. Pr in ted in the USA. 0025-5408/90 $3.00 + .00 Copyr igh t (e) 1990 Pergamon Press plc .

PREPARATION OF SUPERFINE POWDERS AND ORIENTED SUPERCONDUCTING FIL~S OF Y-Ba-Cu-O SYSTE~

Z.E.Liu Department of Inorganic ~aterials,

East China University of Chemical Technology Shanghai 200237,P.R.China

(Received October 23, 1989; Refereed)

ABSTRACT The superfine powders of Y-Ba-Cu-O system were prepared by sol-gel method using inorganic salts of yttrium, barium and copper as raw materials, alcohol and chela- ting agent as solvents, and the sintering temperature of YBa2Cu307_ x superconducting ceramics decreased by

50°C by using the superfine powders prepared in this paper as raw materials. The superconducting films were also fabricated by dip-coating method on ZrO 2 substra-

te, and the thickness of the film of Tc,zero=84K with c-axis orientation was about 1.4~m. This preparing pro- cess for superconducting film is very simple, and the cost low. MATERIALS INDEX: yttrium, barium, copper, oxides,

superconductors

Introduction

Usually the high Tc superconducting oxides reported in La-Ba-Cu-O and Y-Ba-Cu-O system were prepared by the conven- tional powder mixing process and oxalate route(l,2), in order to improve their properties, scientists are trying to use su- perfine powders of Y-Ba-Cu-O system to prepare high quallity superconducting bulk ceramics(3,4). Because of their potential application, superconducting fibers and films were also widely investigated(4,6). It is noticed that the sol-gel method is a promising one among so many preparing processes(7).

This paper describes an application of the sol-gel method

699

700 Z.E. LIU Vol. 25, No. 6

for preparing high Tc superconducting oxides and films of Y-Ba- Cu-O system on the basis of previous work(8).

Experimental

Inorganic salts of Y, Ba and Cu in the mole ratio Y:Ba:Cu = 1:2:3 were added into alcohol, then mixed with proper amount of

'inorganic salts I of Y,Ba and Cu I

alcohol +

chelating agent

I stirring for 2 to 3h

l homogeneous solution con- I taining Y, Ba and Cu ions I

l dip-coat on 1 ZrO 2 substrate

$ Ipreheat at 600°C I for I0 min

|

I repeat above Pr-1 ocess for 8times I

$ l heat-treat at ] 90000 for lh

I keep lOh at IO0°C I

Ikeep lOh at 20000 I

I calcine for 6h 1 at 400@0

I superfine 'l powders

FIG. i

Preparing process of superfine powders and superconducting films

chelating agent, after 2 to 3 hours stirring, a stable solution of the final concentration y3+; O.lmol/L, Ba2*; 0.2mol/L,Cu2+;O.3mol/L, was obtained. Fig. i shows the preparing process of superfine powders and supercondu- ctin~ films of Y-Ba-Cu- 0 system.

All the powder sam- ples were obtained by sintering the superfine powders in air at di- fferent temperatures for 2 hours, then slow- ly cooling to room tem- perature. For the su- perconducting films,the final heat treatment was made in an oxygen flow by heating the specimen at 750°C for 3h, and sintering at 900°C for lh, then slowly cooling to 450°C in 5h, holding for 4h and finally cooling to room temperature in 5h.

The resistance of the prepared films was measured by a conventi- onal dc four-probe te- chnique. Indium elec- trode was used. the crystal structure of the superfine powders and superconducting

films were analyzed by X-ray diffractometer(DMAX-~). The diffe- rential thermal analysis was also made by CRY-I DTA equi- pment.

Vol. 25, No. 6 SUPERFINE POWDERS 701

Results and Discussion

Fig. 2 is the X-ray diffraction patterns for sintered at different temperatures using the superfine

° QSO o C

I 0 0

0

QO0°C 0 0

8 5 0 °C 0

0 0

o o 8 0 0 ° C o

I I J

2 0 ,30 4 0 ,50 60

28

FIG. 2

X-ray diffraction patterns for samples sintered at different temperatures: (o) YBa2Cu307_ x

samples powders

as starting materials. The superfine powders were main- ly composed of BaC0~, Cu0

and Y203. Above 800 C, the

concentration of the carbo- nate began to decrease, and the YBa2Cu307_ x phase began

to form. When sintering at 900°C, single phase YBa2Cu307_ x could be obtai-

ned. Small amount of nonsu- perconducting phases were formed when the sample was sintered at 950ec. Fig. 2 indicated that the superfine powders were very active due to their small particle size, and the sintering tem- perature decreased from 950@C to 900°C compared with the conventional powder mi- xing process.

The DTA and GC( gas chro- matography ) curves for the superfine powders are shown in Fig.3. The GC curve shows the CO o peak from 455@C to 950°C.~It could be assigned to the decomposition of BaCO 3. There were two ther-

monegative peak at 850°C and 970"C on DTA curve due to the crystallographic trans- formation of BaCO 3. Above

850@C, the decomposing rate of BaCO 3 increased.The small

one beside CO 2 peak from

850°C to 970°C on GC curve was arised from oxygen.

The resistance vs temper- ature curve for the superco- nducting film is shown in Fig. 4.The Tc,onset, Tcrzero

were found to be 95K and 84K respectively. The film exibited c-axis orientation perpendicular to the substrate surface (Fig. 5). The thickness of the film was about 1.4pm.This result

702 Z . E . LIU Vol . 25 , No. 6

--t~-r

GC , r-r--~-~---~-~-r.--~-t~-~.~ -*

I I I I I

200 400 600 800 I000

Temperature ( °g )

FIG. 3

DTA and GC curves for superfine powders

FIG. 4

Dependence of resistance on temperature for Y-Ba- Cu-0 film sintered at 900°C for lh.

2.0

/.o

0 0

0 0 0 0 0 0 0 0 ~ o

0

0

0

8 1 t J

I00 200 Tempo~fure ( K )

SO0

FIG. 5

X-ray diffraction pattern for Y-Ba-Cu-O supercondu- cting film sintered at 900°C for lb.

K3 Ub Cb

Cb Cb Cb

I0 20 40

2e

59

Vol. 25, No. 6 SUPERFINE POWDERS 703

was reproduceable, and the samples were stable under test and could be kept for several weeks in exsiccator. Beyond that, the transition temperature of the samples will obviously deteriora- te. On the other hand, by means of controlling the dip-coating times, high Tc superconducting films with different thicknesses can also be obtained. The problem is that the critical current density was low.

Conclusion

Compared with the conventional powder mixing process, the sintering temperature of YBa2Cu307_ x superconducting ceramics

fabricated in this paper decreased by 50°C, moreover, there is no need of the grinding process. If the sintering time is pro- longed properly, the sintering temperature can be decreased further. The superconducting films were also prepared easily by dip-coating method. Because the inorganic salts of Y, Ba and Cu instead of metal alkoxides are used as raw materials, this method has special advantages, low cost and simple preparing process, therefore, it will open a new field in the area of fa- bricating high Tc superconducting oxides. Further investigation is in progress.

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