High critical density thick films of YBCO on YSZ

Physica C 161 (1989) 347-350 North-Holland

H I G H CRITICAL CURRENT DENSITY T H I C K F I L M S OF YBCO O N YSZ A. BAILEY, S.L. TOWN, G. ALVAREZ, G.J. RUSSELL and K.N.R. TAYLOR * Advanced Electronic Materials Group, School of Physics, The University of New South Wales, P.O. Box 1, Kensington, 2033 NSW, Australia Received 15 September 1989

High critical current density thick films of yttrium barium copper oxide have been fabricated on yttrium stabilized zirconia substrates. The films are approximately 40 lsm thick, superconducting with Tc(R = 0),,~ 91.5 K and are found to have jc's in the range 750 to 1200 A c m -2 at 77 K in zero applied magnetic fields. Thisjc range was found to correspond to a 5°C variation in the highest processing temperature. At 77 K, Jc decreased rapidly for applied magnetic fields, 0-20 Oe, but was almost constant for field values > 100 Oe.

I. Introduction Many applications envisaged for the new high Tc superconductors require the material to be fabricated into high quality layers 10-100 ~tm thick onto suitable substrates. These thick films may be used in the as prepared form or, after patterning, as discrete devices. Among the many possible applications for these films are magnetic shielding, pcb interconnects, antennae, various strip devices, such as filters and delay lines, and Josephson devices, such as DC SQUIDs [ 1,2 ]. Consequently it is important to establish reliable and reproducible methods for the fabrication of high quality thick YBCO films on a variety of substrates, which will provide the versatility necessary for the wide range of applications envisaged. Although superconducting thick films have previously been produced on a number of different substrates, which include alumina [ 3 ], silicon [4 ], MgO [ 5 ] and SrTiO3 [ 6 ], the best results so far have been obtained using YSZ substrates [ 7 ]. It is obvious from these reported data that the problem of substrate-film interaction is a major obstacle to the production of high quality, high critical current density, thick films, although other significant problems (Presently on study leave) School o f Metallurgy and Materials, The University of Birmingham, P.O. Box 363, Birmingham, England. 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )

involving choice of starting powder and thermal annealing cycle must be solved. In an attempt to optimise the fabrication parameters we have developed a processing technique that produces high critical current density thick films that exhibit excellent adhesion to the YSZ substrates, have good stability and show superior superconducting properties to those previously reported.

2. Experimental The Y - B a - C u - O powder was made by solid state synthesis of commercially available Y203, BaCO3 and CuO powders. The powders were mixed, ground and calcined at 900°C for 24 h in air, then cooled slowly at l ° C / m i n . This calcined powder was then reground and the process repeated; the final grinding of the sintered material produced a powder with an average particle size of approximately 0.5 ~tm. An Xray diffraction spectrum of this powder is shown in fig. I a. The observed peaks show only '123' material present with no significant impurity peaks. An ink was made by mixing the fine powder with approximate quantities of triethanolamine (C2H4OH)3N until the required viscosity was achieved. The use of ultrasound during the mixing process improved the homogeneity of the ink. The films were formed by first brush painting a thick

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oven cooled to r o o m temperature. The thermal cycle was carried out in air. Fig. 1b shows the X-ray diffraction spectrum for the processed thick film. Comparison o f figs. l a and l b indicates some degree o f surface texturing with the c-axis n o r m a l to the film plane, that is increased intensity o f (00l) peaks comp a r e d to the ( 1 0 3 / 1 1 0 / 0 1 3 ) peak. The extra peaks observed arise p r i m a r i l y from the YSZ substrate but two peaks associated with CuO2 have been identified, these arising from peritectic d e c o m p o s i t i o n o f YBCO material at 1015 ° C. Electron dispersive analysis o f the films, using a SEM c o n f i r m e d that the material in the film was p r e d o m i n a n t l y '123'. Figs. 2a and 2b are typical scanning electron micrographs o f the processed films. It is clear from fig.

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Fig. I. (a) X-ray diffraction spectrum of the fine 123 powder used in the ink. This powder was produced from the initial component powders by cycling twice for 24 h at 900°C. (b) X-ray diffraction spectrum of the processed thick film showing some degree of c-axis surface texturing. coating onto polished YSZ substrates, a n d then t h i n n e d a n d s m o o t h e d by carefully drawing a sharp edge across the surface o f each film. The solvent was r e m o v e d by heating the samples to 200 ° C for 20 min in a i r . The thick films were carefully checked at this stage to remove those specimens which show "blistering" after the solvent has been removed. This p r o b l e m is m o r e prevalent for the very thick films, > 60 rtm thick and, to i m p r o v e the success rate with such films, two thin layers were a p p l i e d and d r i e d individually before thermal cycling. The solvent free films were then heated to 900°C at 5 ° C / m i n , held at 900°C for 10 min, heated to 1015°C at 4 ° C / m i n , held at 1015°C for 6 m i n and then cooled down at 2 ° C / m i n to 900 ° C, a n d finally

Fig. 2. (a) Typical SEM micrograph of the processed film showing partial melting and large grain growth. (b) Typical SEM micrograph of the films showing significant grain alignment.

A. Bailey et al. / High critical current density thick films of YBCO on YSZ

2a that the spherulitic-type morphology as reported by Abell et al. [ 8 ] for their thick films is not present in our films, but the high degree of grain alignment reported by them is present to some degree in our films, fig. 2b. In fact fig. 2a indicates that partial melting and large grain growth occurs during the short, high temperature processing cycle. Resistivity versus temperature measurements for the thick films were made using the standard DC four-point probe technique, silver paste contacts and a specimen current of 28 ~tA. The measurements were made as the temperature increased from 77 K at a rate of 0.5 K/min. Fig. 3 shows the sharp resistive transition with T¢ (onset) ~ 93 K and Tc (R = 0 ) = 91.5. Critical current measurements, using the DC four-point probe technique and 1 ~tV interpolation, were performed on films that had been cut into a narrow neck profile. Typical neck dimensions were width 130 Ixm, thickness 44 lxm and length 250 ~tm as determined by SEM. For zero magnetic field and a temperature of 77 K, the jc's were found to be in the range 750 to 1200 A cm -2 for these thick superconducting films. This range of critical current densities has been shown to be associated with thermal gradients within the muffle furnace used to process the films. This gradient was found to be 10°C across the entire furnace with a mean central temperature of 1015 ° C, for a nominal furnace temperature of 1025 ° C. Films processed in the temperature range 1015 to 1020°C were found to contain a sig-

nificant number of cracks with correspondingly low jc's, the smallest value being 750 A cm -2. On the other hand, films processed on the lower side of 1015 ° C had almost no cracks andjc's as high as 1200 A cm -2. Optimisation of the exact processing temperature and time is now proceeding. A typical critical current density versus applied magnetic field (low field regime) curve, at 77 K, is shown in fig. 4. The curve illustrates the rapid decay ofjc resulting from increasing field 0-20 Oe (intergranular flux penetration), and the almost constant value for fields > 100 Oe (intragrain flux penetration). Fig. 5 shows the temperature dependence of the resistivity for several external magnetic field strengths (medium field regime) applied perpendicular to the current ( 1 mA) direction. The insert of fig. 5 clearly shows that the transition significantly broadens, but that the onset temperature does not change for the fields used. In fact, relatively low fields remove superconductivity above 77 K. However, even though these field characteristics may be considered poor they are still adequate for some thick film device applications.

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Fig. 3. Typical resistivity versus temperature measurement for these thick films. A standard DC four-point probe technique, specimen current 28 p.A, was used.

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Fig. 4. Variation of critical current density with applied magnetic field for the low field regime.

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Fig. 5. Resistivity vs. temperature at different applied magnetic field, medium field regime. Note the significant resistivity tail at 350 Oe. Insert: log resistivity is plotted as a function of temperature to show the tails formed as a function of magnetic field. ( 1----0; 2 =- 175; 3---350; 4---700; 5--- 1400; 6---2100; 7-=2800; 8---3500 Oe).

3. Conclusion

References

In s u m m a r y , high critical c u r r e n t d e n s i t y Y - B a C u - O thick films h a v e b e e n f a b r i c a t e d on Y S Z substrates using a s i m p l e p a i n t - o n t e c h n i q u e w i t h an ink consisting o f fine Y B C O p o w d e r ( p r o c e s s e d at 900 ° C ) and t r i e t h a n o l a m i n e . T h e high c u r r e n t densities m e a s u r e d can be a t t r i b u t e d to the large g r a i n size and the high degree o f i n t e r g r a i n c o n n e c t i v i t y a c h i e v e d d u e to the partial m e l t i n g o f the grains. G i v e n the m a g n e t i c field characteristics for these films, w o r k is n o w in progress to f a b r i c a t e single Jos e p h s o n j u n c t i o n s a n d D C S Q U I D s t h a t can o p e r a t e at 77 K.

[ 1 ] T. Yamashita, A. Kawakami, S. Noge, W. Xu, M. Takada, T. Komatsu and K. Matusita, Jpn. J. Appl. Phys. 27 (1988) Ll107. [2] A.Z. Lin, H.Q. Li, F.W. Liu and L. Tang, Jpn. J. Appl. Phys. 27 (1988) L1204. [ 3 ] M.V.S. Lakshimi, K. Ramkumar and M. Satyam, J. Phys. D. 22 (1989) 373. [4] R.P. Gupta, W.S. Khokle, R.C. Dubey, S. Singhal, K.C. Nagpal, G.S.T. Rao and J.D. Jain, Appl. Phys. Lett. 52 (23) (1988) 1087. [ 5 ] P. Barboux, J.M. Tarascon, B.G. Bagley, L.H. Greene and G.W. Hull, MRS Spring Meeting, Reno 1988, High Temperature Superconductors II. [6] J.M. Aponte and M. Octavoio, Preparation and transport properties of high-To superconducting thick films, preprint (1989). [7] Y. Matsuoka, E. Ban and H. Ogawa, J. Phys. D 22 (1989) 564. [8 ] T.C. Shields, F. Wellhofer, J.S. Abell, K.N.R. Taylor and D. Holland, M2S-HTSC, Stanford University, Stanford, USA July 23-28, 1989. Accepted for Physica C (1989).

Acknowledgements T h e a u t h o r s w o u l d like to t h a n k N i l c r a C e r a m i c s Pty. Ltd ( A u s t r a l i a ) for the supply o f Y S Z substrates.