Preparation and Characterization of YBa₂Cu₃O_x High Temperature Superconductor Obtained by Sol-Gel Method

Indian J, Phys. «SA (4). 269-275 (1991)

P reparation and characterization of YBaiCusOx high tem perature superconductor obtained by sol-gel method

P K Pal'*’, K Bose, S M ollah and B k Chaudhuri

H ig h Tem perature Superconductivity Uaboratory,

S olid S tate Physics Departm ent, IndiaS Association for the C u itivatio n of S cience, C a lc u tta -7 0 0 0 8 2 , India

Received 5 A pril 1 9 9 1 , accepted 18 A ^ i l 1991

A b s t r a c t : Electrical resistivity and nO gnetic susceptibilities of high tempera­

ture superconducting oxide (H iT S O ) Y B a ,C u a O , (1 2 3 ) prepared by a simple sol-gel (S S G ) m ethod have been measured dow n to 80K and results reported in th is paper. The (S S G ) m ethod developed w o uld be very useful for a large scale production o f (H IT S O ) pow ders (w ith T j= 9 3 K and T „ = 9 1 K ) required for many technological applications (H IT S O w ires/tapes, th ick fiim s e tc.).

K eyw o rd s : S o l-g e l, high superconductor, A C /D C susceptibility.

P A C S N o s : 7 4 .7 0 M q , 8 2 .7 0 . Gg

I. Introduction

For any R & D programm e the developm ent o f good q u a lity (large volum e fraction o f superconductivity, hom ogeneity, sharp fa ll o f resistance, good connectivity etc.) high tem perature superconducting oxide (HITSO ) m aterials is essential, w e have developed in our lal3oratory, lik e many other researchers in th is fie ld , the so lid state reaction technique (Som at a l 1988, 1989, C haudhuri 1987) and the glass to ceram ic technique (Chaudhuri et a l 1989,1990 and Som et a l 1989) fo r the prepara­

tio n o f (H ITSO ) m aterials like YBaaCUgO«, B i4S r8CasCu«0«, T I,B a ,C a iC u ,O a etc. A ll these methods have th e ir ow n advantages and disadvantages as discussed in the aix>ve m entioned articles.

However, fo r the preparation o f a large amount o f homogeneous (HITSO) powders having uniform grain sizes, the chem ical and the sol-gel methods have some special advantages (see fo r example Brinker et a l 1984). The sol-gel method fo r the preparation o f (H ITSO ) m aterials is, therefore, becoming very popular (Koki 1990, M urakam i et a l 1990). Recently, fo r the preparation o f (H ITSO )-Ag and HITSO -polym er precursors fo r m aking (HITSO ) w ires/tapes (Chaudhuri 1990,

^Physics D epartm ent, R B C C ollege, N aihati 7 4 3 1 6 5 , W est Bengal, India.

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2 7 0 P K Pal K Bose S Moiloh and B K Chaudhurl

Bose et al 1991). w e have developed a re la tiv e ly sim ple so l-g e l (SSG ) m ethod fo r the preparation o f fin e (123) superconducting pow ders. S ince the developm ent o f th e so l-g e l m ethod fo r the preparation o f (H IT S O ) m aterials is a b it tric k y and th e m ethod is also te ch n o lo g ica lly im portant fo r the high y ie ld o f (H ITS O ) m aterials, w e described beloyv th e deta iled m ethod o f preparation o f (H IT S O ) w ith th is technique.

E lectrical and m agnetic properties o f the prepared (H IT S O ) m aterials have also been stu d ie d and compared w ith those o f th e (H IT S O ) prepared e a rlie r by s o lid state reaction m ethod.

2. Experimental

2.1. Preparation of materials :

The m ethod o f preparation o f (123) superconducting oxide by so l-g e l m ethod is alm ost s im ila r to th a t discussed in o u r e a rlie r chem ical m ethod (Pal and C haudhuri 1991). S to ich io m e tric am ount o f the n itra te s o f Y, Ba, and Cu, a li o f p u rity 99.99;^ (A lfa , G erm any), were dissolved in trip le d is tille d w ater. The ra tio Y : Ba : C u = 1 : 2 : 3 w as m aintained fo r the preparation o f the nom inal com posi­

tio n o f YBasCUgOi, (123). A ppropriate am ount o f c itric acid m onohydrate and ethylene g lyco l w as then m ixed w ith the n itra te s o lu tio n w ith s tirrin g . The pH value w as m aintainad bet/veen (4 .5 —5 .5 ) by ca re fu lly adding am m onia, if required. W e prepared (H ITS O ) w ith d iffe re n t pH values o f the s o lu tio n . In th is paper, how ever, w e report the properties o f o n ly one sample prepared w ith a fix e d pH value (~ 5 ).

The m ixture w as then refluxed at 80°C fo r one hour and w as then kept over­

n ig h t. F in a lly , the s o lu tio n w as heated in an w a te r bath (fo r g e la tio n ) fo r several hours u n til b lu ish coloured je lly lik e pro d uct w as obtained. This je lly w h ich could be draw n in to the form o f fib e rs w as d rie d under vacuum and then the p ro d uct w as s lo w ly heated to 3 0 0 X fo r tw o hours. The d ry mass is then collected In an alum ina cru cib le and again fire d at 6 0 0 X fo r 6 hours and s lo w ly cooled to room tem perature. The loss o f w ater and the decom position o f free c itric acid occurs around 150°C.

The black m aterial w as then pow dered, and sintered at 8 9 0 'C (in oxygen) fo r another 12 hours and s lo w ly (5 7 h ) cooled to 4 0 0 X inside th e furnace and the p o w d er w as kept a t th is tem perature fo r three hours and then s lo w ly cooled to 3 0 0 X and quenched to room tem perature. The fin e black pow der thus obtained is superconducting. The com plete process fo r the so l-g e l preparation m ethod o f (H IT S O ) is show n in Table 1.

2.2. Study of physical properties:

The d rie d pow der th u s obtained is pressed in to pellets o r rods under 4 ton pressure.

The p e lle ts ere again sintered 9 0 0 V fo r 12 hours and s lo w ly cooled to roe n

tem perature. N o oxygen treatm ent w as made fo r the p e lle ts but slo w cooling rate w as alw ays m aintained.

Table I. F lo w c h art o f th e so l-g el m ethod.

Preporation and characterization etc

27

N itra te salts o f Y , Ba. C u, in

appropriate am ount (9 9 .9 9 % + D is tilled H , 0 p u re )

1--- --- --- --- M ix in g ta fs th e r j-«- |

_ 1

1 C itric acid Ethylene Glycol |

R eflu x th e m ixture at B O X for

4 hours Kept overnight

1 ' 4

G e latio n at 8 0 - 9 0 X

1 several houss for C oolin g to room tem perature and

dried under vacuum at 90*" C 4

H eated to 2 0 0 - 3 0 0 X

2 hours for -> S intering at 6 0 0 X for 6 hours

P ulvarizin g and p e lle tiza tio n at 4 ton pressure

H eating th e pellets at 870" for 12 hours in a ir/o x y g e n and s lo w ly cooled to room tem ­ perature

4

H ig h Tc Y ,B ayC u ;,O j-su p erco n d u cto r ready tor experim ental investigation

Room tem perature X -ray d iffra c tio n pattern (F igure 1) show s the superconduc>

tin g peaks o f the (1 2 3 ) sam ple. The la ttic e parameters calculated are a -3 .8 2 0 A ,

3 .88 4 A , and c^= 11.682 A w h ich agree fa irly w e ll w ith the corresponding values o f a, b and c o f a ty p ic a l (123) sample prepared by ceramic route (Som et a l

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272 P K Pal K Bose S Mollah and B K Chaudhuri

19B8). The SEM (H itachi, Japan) picture o f the (H ITSO ) sample is shown in Figure 2 indicating good connectivity between the grains and crystalline nature.

Figure 2. SEM picture of the superconducting (sol-gel) sample.

As usual, four probe dc resistance measurement was made on a bar shaped (123) sample* DC and ac magnetic susceptibilities o f the samples were shown in Figures 3 and 4, respectively. The dc and ac magnetic susceptibilities were

Figure 3. DC magnetic susceptibilities of the superconducting samples pre- pared by sol-gel and ceramic methods shown as a function of temperature.

measured w ith sim ilar apparatus used in our earlier w ork (Chaudhuri et a l 1989, Bose et ol 1991).

3. Results and discussion

The therm al variations o f dc re sistivity (Figure 5) and magnetic susceptibilities as show n in Figures (3-4) indicate the superconducting behaviour o f the sample. The

Preparation and characterizatio n etc _{2 73}

sharp fa ll o f resistance a t T - Te=93K d e fin ite ly in d ica te s the good q u a lity o f the superconducting sam ple prepared by so l-g e l m ethod. For com parison, the

F ig u re 4. A C m ag n etic s u s c e p tib ilitie s of th e superconducting sam ples pre­

pared by s o l-g e l and ceram ic m ethods sh o w n as a fu n c tio n o f tem perature.

re s is tiv ity and s u s c e p tib ility curves fo r th e ceram ic sintered sam ples are also show n in the Figures (3 -5 ). It is in te re s tin g to m ention th a t the h ig h fie ld (^ 0 .3 7 ) dc

Figures. Electrical resistances o f the superconducting samples prepared by sol-gel and ceramic methods shown as a function of temperature.

m agnetic s u s c e p tib ility d a ta show s a sm all rise around th e superconducting tra n s itio n tem perature (F ig u re 3 ). T h is typ e o f behaviour has also been observed

2 7 4 P K Pal K Bose S M ollah and B K Chaudhurl

b y U8 in o u r e a rlie r measurements (C haudhuri et al 1989, 1990). It is also observed th a t obey C urie W eirs behaviour o f the fo rm ,

^ ^ = ( x * o o - x r + C / ( T - e )

^Boo is th e value o f s u s c e p tib ility X a t T*=300K w ith S = ~ ( - 1 6 K ) . The values o f X and C are found to depend on th e sam ples prepared under d iffe re n t co n d itio n s and also on the g ra in sizes o f the sam ples (C haudhuri ct a l 1990). T his behaviour o f m agnetic s u s c e p tib ility m ig h t be due to the va ria tio n o f the oxygen content In the sam ple as w e ll as on the in cip ie n t antiferrom agnetic character o f th e sam ple.

S m aller th e values o f x (oxygen content in Y iB a g C u ,⁰,) , larger is th e a n tife rro - ma0> ^ ic character o f th e sam ple (show ing m ore prom inent rise o f x around To in the ( x - T ) curve as in F igure 3) and lo w e r is the volum e fra c tio n o f the super­

co n du ctin g grains in th e sam ple. Though it is not yet clear w hether a n tife rro ­ m agnetism (A F M ) is the o rig in o f p a irin g m echanism in (H IT S O ) m aterials, o u r present m agnetic s u s c e p tib ility data, how ever, support th e a n tiferro m a g n etic o rig in o f su p e rco n d u ctivity as w as a lso suggested e a rlie r (C haudhuri et a l 1990). The process by w h ic h antiferrom agnetism is suppressed by su p e rco n d u ctivity is yet unexplored (S ch riffe r 1988). Since w e cannot accurately ascertain the oxygen sto ich io m e try as w e ll as its d is trib u tio n in the la ttic e , it is rather d iffic u lt to conclude w hether the tem perature dependent behaviour o f x around To is due s o le ly to oxygen deficie n cy and/or p a rtly related to antiferro m a g n etic type o rdering.

The problem is a ctu a lly related to the oxygen ordering in th e la ttic e and hence to the change o f io n ic state o f copper (o r other io n s). A ce rta in am ount o f oxygen d e ficie n cy causes some kin d o f d iso rd e rin g in the la ttic e destroying super­

c o n d u c tiv ity and at the some tim e s tim u la tin g the appearance o f antiferrom agnetic character and se m ico n d u ctivity. It appears, as if, there is same co m p e titio n betw een su p erco nd u ctivity and antiferrom agnetism in such (H IT S O ) m aterials.

Elaborate in ve stig a tio n in th is d ire c tio n is in progress.

4. Conclusion

W e believe th a t th e sim ple so l-g e l m ethod discussed in th is paper co u ld be used fo r th e large scale p ro d uction o f very good q u a lity superconducting pow ders necessary fo r d ra w in g w ires/tapes o r film s and targets fo r sp u tte rin g . It has also been observed th a t ce rta in annount o f silve r co u ld also be added in th e super­

co n d u ctin g sam ple w h ile preparing the gel w h ich is o f another im portance fo r m aking (H IT S O )-A g com posites su ita b le fo r m aking w ire s and tapes (Bose et a l 1991).

i f proper care is taken, so l-g e l process could provide a s u ffic ie n t co n tro l over th e p a rtic le s i:» s o f th e (H IT S O ) m a te ria ls prepared.

W e a lso fin d th a t it is better to keep th e sin te rin g tem perature as lo w as p o ssib le to m aintain u n iform g ra in sizes, since the p a rticle s are very reactive.

U n ifo rm and sin g le sin te rin g cycle w o u ld be preferred fo r the so l-g e l (H IT S O )

Preparation and characterization etc ₂₇₅

powders. Proper control of pH value is also Important. Further attempt are being made'to Improve the sintering technique at lov^er temperature.

Acknowledgments

The authors are grateful to K K Som for his various help. The authors are also grateful to Professor S P Sengupta for allow ing them to do the X-ray diffraction

and SEM studies of the samples in his Departrnent.

R«fer«ncM

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