On the melt processed YBa2Cu3O7−x physico-chemical characterization

~ Solid State Communications, Vol. 68, No. i0, pp.923-928, 1988. ,8-~ Printed in Great Britain.

ON THE MELT PROCESSED YBa2Cu307_x

0038-i098/88 $3.00 + .00 Pergamon Press pie

PHYSICO-CHEMICALCHARACTERIZATION

G.A. Costa, M. F e r r e t t i and G.L. Olcese, I s t i t u t o di Chimica F i s i c a , Corso Europa 26, 16132 Genova, I t a l y V. Calzona, M.R. Cimberle, C. Ferdeghini, M. Putti and A.S. S i r i INFM/CNR c/o Dipartimento di F i s i c a , Via Dodecaneso 33, 16146 Genova, I t a l y

(Received on 31 July 1988 by R.Fieschi)

We determined the proper conditions to obtain superconducting samples by melting. Two preparation methods were investigated: arc melting of YBa^Cu~O~ prereacted powders at about 1900°C and p a r t i a l melting at T=I~80~CC j u s t 50°C above the p e r i t e c t i c decomposition of YBa2Cu30Z. The l a t t e r one revealed i t s e l f a one-step technique to obtain high Ic superconductors by melting. Micrographic, s t r u c t u r a l , e l e c t r i c and magnetic characterizations of these materials are reported and compared with the corresponding properties of the more t r a d i t i o n a l sintered samples: the best r e s u l t s were obtained f o r the p a r t i a l l y melted samples annealed in flowing oxygen at s u i t a b l e temperature.

r a t i o n procedure described elsewhere The Barium peroxide reaction gives in a one-step reaction a high q u a n t i t y of oxygen in the prod u c t - and t h i s r e s u l t can be very useful when a high temperature 9heat treatment, that causes oxygen depletion , must be performed. On the other hand, for better d e n s i f i c a t i o n , long f i r i n g s at high temperatures are necessary, but s i n t e r i n g at temperature much higher than IO00°C i s not p r a t i c a b l e , since the YBa~Cu~O~ under/-X 1030o C. goes a p e r i t e c t i c decomposition a~ a~out Many s i n t e r i n g problems are caused also by the ease of the reactive decomposition of YBa Cu~O~ as proposed by Gallagher et a l . 9. We fou~dJt~a~ at I030°C the e q u i l i b r i u m oxygen pressure of YBa~Cu~O~ samples prepared from g J - X BaO~ is about 2 5ar. fn f i g u r e 1 i t is presented z a logarithmic plot of the oxygen pressure versus the inverse absolute temperature. The measurements were performed by heating YBa~Cu~O~ powders in confined volumes in order Z ~ - X to avol~compositional change larger than + 0.2 oxygen atoms per formula u n i t . We n o t i c e - t h a t the data can be interpolated by two s t r a i g h t l i n e s , i n d i c a t i v e of two phase e q u i l i b r i a . The r e s u l t s up to I000 K are in ~ a i r y agreement with the data of Gallager et al and Burger et I0 al. . We observe that i f t h i s low temperature behaviour should be extrapolated to higher temperature a very high oxygen pressure would be

Introduction Superconductors of YBCO "123" type - are u s u a l l y prepared as ceramics by s o l i d - s t a t e r e a c t i o n and u s e f u l forms are f a b r i c a t e d by compression. From the practical point of view, however, bulk superconductors need in general to be prepared in f i n e wires; the b r i t t l e n e s s of the YBCO-type oxide causes much concern about the f a b r i c a b i l i t y of these ~u~erconductors i n t o desidered f i n e wire form' ~. A number of f a b r i c a t i o n _ 5 processes have b e e n recently proposed An a l t e r n a t i v e f a b r i c a t i o n method, which uses molten oxide processing technique and r e s u l t s in improved mechanical structure and properties with respect to the conventional s i n t e r i n g approach, has been checked by us. We have determined the proper conditions and procedures to improve the superconducting properties in superconductors obtained by p a r t i a l melting. The so obtained samples were analized micrographic a l l y , by X-ray powder d i f f r a c t i o n and by elect r i c and magnetic measurements. Experimental and discussion Powdered, well characterized samples of the orthorhombic YBa^Cu~O7 phase were prepared using BaO2 ins~ea~ o?XBaC03 following6aTPrepa923

PHYSICO-CHF/~ICAL CHARACTERIZATION

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Vol. 68, No. i0

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a c t i v i t y . We o u t l i n e that the a c t i v i t y of oxygen could be s t r o n g l y dependent on the chemical reaction used, and on the impurities present in the sample. Work is in progress to ascertain the thermal dependence of the oxygen a c t i v i t y in samples prepared by d i f f e r e n t routes. The samples were d i v i d e d in t h r e e l o t s . The f i r s t one was simply pressed at a pressure of 6 Kbar, e x c lu s iv e ly f o r handling pourposes. The second one was arc melted in 1 bar of Argon, at about 1900% in order to reach the f u l l l i q u i d state as quickly as possible and to avoid excessive material loss. The t h i r d l o t was t r e a t e d in an alumina c r u c ible, in pure oxygen atmosphere, at a constant pressure of about 3 bars. I t was heated in a conventional e l e c t r i c f ur nac e up to I080°C, where the e q u i l i b r i u m

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2O F i g . 2 - a) X-ray d i f f r a c t i o n patterns of the as-cast products of melted and p a r t i a l l y melted YBa2Cu30~ x' compared with the orthorhombic perovski~c phase of the s t a r t i n g powders. b) X-ray d i f f r a c t i o n patterns of the annealed products from melted, p a r t i a l l y melted and sintered YBa2Cu307_x. (Radiation used KaCu).

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Vol. 68, No. i0

PHYSICO-CHEMICAL CHARACTERIZATION

Y^BaCuO~ - YBa3Cu20 - l i q u i d is proposedI I . T~en th# sample was ~eft to cool down. At 1030°C the p e r i t e c t i c reaction of formation of the YBa^Cu_O. takes place. All the samples the arc mel~ed~ lth~ p a r t i a l l y melted and the compacted ones, were annealed at 920°C for 48 hours in flowing oxygen. The three lots of samples of the as cast and annealed materials where X-rayed. In figure 2 we report X-ray d i f f r a c t i o n patterns in which tetragonal and orthorhombic phases present in the samples are indicated and compared with a well characterized homogeneous orthorhombic perows k i t i c phase. Some observations are needed: i ) the as-cast, arc-melted sample is quite unhomogeneous: a few phases are present, like Y2BaCu05, BaCuO2 and Y2Ba04, indicating a large composition variation due to the different equilibrium in presence of the l i q u i d state; the "123" phase present is tetragonal and non superconducting because the starting YBCO loses too much oxygen during melting; the low heigth of peaks i s a consequence of the weak c r y s t a l l i zation due to fast cooling rate. Also a large q u a n t i t y of amorphous phase was revealed by micrographic analysis; Vichers' microhardness measurements revealed a very hard material (Hv = 750,1oad 50 grams); i i ) the as-cast, partially-melted sample is only p a r t i a l l y unhomogeneous; the most relevant secondary phase is YpBaCu05; in this case the "123" phase is orthor~ombic-and superconducting because the starting YBCOpowders, obtained from Ba peroxide, were oxygen-rich and an oxygen pressure of only 3 bars during the reaction was s u f f i c i e n t to avoid the complete decomposition and the complete transition to the tetragonal structure at the r e l a t i v e l y low temperature of the process; i i i ) the annealed arc-melted sample is much more homogeneous and the "123" phase is orthorhombic and superconducting; the not yet completely satisfactory result can be due to the low annealing temperature and to the too short annealing time; iv) the annealed p a r t i a l l y - m e l t e d sample is almost homogeneous and very well crystallized; the height of peaks of the "123" orthorhombic superconducting phase are now comparable to those of sintered sample. The optical micrographs of the arc-melted and partially-melted samples are shown in figure 3. For the arc-melted sample (fig.3a) i t is confirmed that the heat treatment did not succeed in homogenizing the sample and the superconducting phase s t i l l remained minoritary. For the partially-melted sample ( f i g . 3b) the presence of a small amount of a secondary phase is confirmed in normal l i g h t and a Widmanst~tten structure can be observed. I f observations are

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Fig. 3 - a) optical micrograph of the as-cast, arc-melted sample, b) optical micrograph of the partially-melted sample in normal l i g h , c) opt i c a l micrograph of the partially-melted sample in polarized ligh. Markers represents 20~m.

made in polarized l i g h t ( f i g . 3c) i t is evident a grain alignement due to the peritectic reaction and thermal gradients during the cooling. The three annealed samples were characterized by r e s i s t i v i t y and A . C . s u s c e p t i b i l i t y measurements. Both the experimental techniques are l~-13-14described elsewhere In figure 4 the r e s i s t i v i t y vs. temperature for the three samples is shown. The resistive behaviours are very different. The arc-melted sample (curve A) has the highest Q300 value (4,5 m~cm) and the

926

PHYSICO-CHEMICAL CHARACTERIZATION

Vol. 68, No. I0

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Fig. 4 - R e s i s t i v i t y versus temperature f o r the three samples; A) arc-melted sample; B) sintered sample; C) p a r t i a l l y - m e l t e d sample. lowest residual resistivity ratio (RRR): O~O0/e.. 0 = 1.2. Starting from room temperature J Eu a quick v a r i a t i o n in the r e s i s t i v i t y behaviour is observed down to 270K, then 0 is n e a r l y constant down to the t r a n s i t i o n temperature. This is probably due to the presence of various phases as indicated by m e t a l l u r g i c a l characterization, The s i n t e r e d sample (curve B) has the usual m e t a l l i c behaviour with a high O~nn (3.7 m~cm) and RRR = 2. D i f f e r e n t l y from t ~ v arc-melted sample, the p a r t i a l l y - m e l t e d sample (curve C) presents a m e t a l l i c behaviour with e300 = 2.9 m~Qcm and RRR = 2.6. We l i k e to point out that, although the e3~0 value is high (2.9 m~cm), probably due to sampYe unhomogeneoties, the value RRR = 2.6 indicates t h a t a well connected m e t a l l i c path is present. The samples were also c h a r a c t e r i z e d by A.C. s u s c e p t i b i l i t y measu~m~Rts. As already reported - v - - - , in such new high T c bulk superconductors these measurements need a more complicated i n t e r p r e t a t i o n than the usual one related to a "two superconductors" behaviour: one representing single isolated grains, the other a bulk of weak-linked superconducting grains. These two c o n t r i b u t i o ~ , also present in magnetization measurements--, are seen as two peaks in the out of phase Z" signal and two drops in the in phase Z' signal. In f i g u r e 5 the s u s c e p t i b i l i t y vs. temperature f o r the sintered sample is shown. The curves have tne O-peak value of the magnetic f i e l d B as parameter. Increasing B the superconducting t r a n s i t i o n is broadened (at B = 31 Gauss the diamagnetic shielding at 78 K is reduced to 55% of the ideal value) and the X" peaks broaden and s h i f t with d i f f e r e n t rates at lower temper a t u r e s . Near T the grain peak increases with B; on the contrary the bulk peak remains nearly c o n s t a n t . The r a t i o of the two Z" peaks i s

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A higher Z" value, i f the other parameters are constant (volume, temperature, magnetic f i e l d amplitude) corresponds to a large area of the hysteresis loop, and therefore to a higher c r i t i c a l current density. At the l i g h t of these considerations becomes clearer the phenomenolog ic al trend which indicates that in good superconducting samples the grain Z" peak is high; on the contrary the bulk's Z" peak is low, i n d i c a t i n g that a l i t t l e volume of weak l i n k s , l~n which the d is s ip a t io n takes place, is present . The bad q u a l i t y of the arc-melted sample did not allow she resolution of the two components in Z" which overlap. The X' t r a n s i t i o n is very broade-

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Vol. 68, No. I0

927

PHYSICO-CHEMICAL CHARACTERIZATION

ned increasing B. The diamagnetic shielding is reduced in fact to 40% at T = 78 K and B = 23 Gauss. Figure 6 shows A.C. s u s c e p t i b i l i t y measurements f o r the partially-melted sample. In this case, the " t w o superconductors" behaviour is very clear with a ratio of the two X" peaks around l also at the lowest B value we used (0.2 Gauss), indicating a very good quality of the sample. Increasing B the high temperature peak increases and the superconducting transition broadens, but at T = 78 K a 70% diamagnetic shielding is s t i l l present at the maximum B = 38.1 Gauss. I f a D.C. magnetic f i e l d is applied (see f i g . 7), while the bulk Z" peak shifts at temperatures below 78 K, the grain peak s p l i t s i t s e l f in two peaks, which may be related to grain different superconducting quality or to anisotropy effect due to grain different orientation, or to the presence of superconduc-

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Table l - Summary of the physical measurements on YBCOsamples Sample

Density %

Q:nn (m~)

RRR

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sintered

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2

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Z" peaks' ratio(3)

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(1) D.C. f i e l d at which the shielding is reduced to the 50% of the ideal value (perfect diamagnetism) (2) Height of the grain peak after normalization to superconducting grain volume for BA C. = 0.87 Gauss and B~ ^ =O (3) Ra~1o of the x" bulk pea~'~igth to the grain one. ting loops of different dimensions. Another indication of the good quality of the p a r t i a l l y melted sample is shown by the more symmetric behaviour of the X" bulk peak in agreement with the Ishida and Mazaki predictions for a multiconnected Josephson network.

F i n a l l y we can note that the shielding is s t i l l 50% of the ideal value at T = 78 K and 8~ C = 2100 Gauss. For the sintered sample we ob~&lhed 50% shielding at an applied B of only 200 gauss. In table ] we reported the most significant data of the three types of samples.

References l

2 3

R.Fl~kiger, F.MOller, T.Wolf, I.Apfelstedt, E.Seibt, H.KOpfer and W.Schauer, Proc. of Int. Conf. on High Temperature Superconductors Materials and Mechanisms of Superconductivity, Interlaken (Switzerland) Feb. 29 - Mar. 4, 1988. D.R.Clarke, Adv. Ceram. Mater. 2 (]987) 273. D.W.JohnsonJr. E.M.Gyorgy, W.W.Rhodes, R.J.Cava, L.C.Feldman and R.B.van Dover, Adv. Ceram. Mater. 2 (1987) 364.

4 5

6

7

S.Jin, R.C.Sherwood, T.M.Tiefel, M.F.Yan and W.W.Rhodes (unpublished). S.Jin, R.C~Sherwood, T.M.Tiefel, R.B.van Dover and D.W.Johnson J r . , Appl. Phys. Lett. 51 (1987) 203. G.A.Costa, M.Ferretti, G.L.Olcese, M.R.Cimberle, C.Ferdeghini, G.L.Nicchiotti, A.S.Siri and C.Rizzuto, J. Crystal Growth 85 (1987) 623. G.A. Costa, M. Ferretti and G. L. Olcese, J. Crystal Growth, II5S (1988) in press.

928

8 9

IO

11 12

13 14 15

PHYSIC0-CHEMICAL CHARACTERIZATION

G. A. Costa, M. Ferretti, G. L. Olcese and M. R. Cimberle, Vuoto, 17 (1987) 147-150. .P.K. Gallagher, H. M. O'Bryan Jr., S. A. Sunshine and D. W. Murphy, Mater. Res. Bull. 22 (1987) 995-I006. J.P. Burger, L. Lesueur, M. Nicolas, J.N. Daon, L. Demoulin and P. Vajada, J. Physique, 48 (1987) 1419. R. S. Roth, J. R. Dennis and K. C. Davis, Advan. Ceram. Mater. 2:303-312 (1987). i~.R.Cimberle, C.Ferdeghini, F.C.Matacotta, E.Olzi, C.Rizzuto, A.S.Siri, Physica Scripta June 1987. S.A.Campbell, J.B.Ketterson,Rev. Sci. Instrum. 54 (1983) l l g l . R.A.Hein, Phys. rev. B 33 (1986) 7539. R.B.Goldfar , A.F.Clark, A.I.Braginski, A.J.Panson, Cryogenics 27 (1987) 475.

Vol. 68, No. i0

16 H.K~pfer, l.Apfelstedt, W.Schaner, R.FlOkiger, R. Meier-Hirmer, H.WOhl, Z. Phys. B, Consensed Matter 69 (1987) 159. 17 M.R.Cimberle, C.Ferdeghini, G.L.Nicchiotti, M.Putti, A.S.Siri, C.Rizzuto, G.A.Costa, M.Ferretti, G.L.Olcese, F.C.Matacotta and E.Olzi, Superconducting Science and Technology l (1987) 30. 18 T.Ishida, H.i~azaki, J. Appl. Phys. 52 (1981) 6798. 19 H. K~pfer, I. Apfelstedt, R. FlOkiger, R. Meier-Hirmer, W. Schauer, T. Wolf and H. Wuhl., Int. Discussion Meeting on High-T Superconductors, Manterbdorf, Austri~ (February 1988).