Electrophoretic deposition of Tl-Ba-Ca-Cu-O and Bi2Sr2CaCu2Ox thick films

Physica C 177 ( 1991 ) 345-350 North-Holland

Electrophoretic deposition of T1-Ba-Ca-Cu-O and Bi2Sr2CaCu2Ox thick films S.K. R e m i l l a r d Department of Physics, College of William and Mary, Williamsburg, VA 23185, USA

P.N. A r e n d t a n d N.E. Elliott Los Alamos National Laboratory, Los A lamos, NM 87545, USA

Received 5 March 1991 Revised manuscript received 18 April 1991

Thick films of Bi-2212 and TI-2212 superconductor were electrophoretically deposited onto Consil substrates. T1 series starting powder containing BaCuO2 impurity phase resulted in films with higher Tl content. X-ray diffraction showed that Bi-2212 films were well oriented only if melt textured. Melt textured Bi-2212 films were characterized in a TEoH resonant cavity at 17.46 GHz and compared to a magnetron sputtered TI-2212 thick film. It was found that the surface resistance and surface reactance of the Bi-2212 were equally sensitive to external DC magnetic fields.

1. Introduction Though aggressively pursued elsewhere, thick-film high-temperature superconductivity ( H T S ) has received much less attention than thin-film HTS in the United States. Thick HTS film ( t h i c k n e s s > 5 ~tm) differ in their morphological and superconducting properties from thin ( t h i c k n e s s < 5 ~tm) epitaxial films. Thick film deposition techniques include suspension in organic solvent [ 1 ], magnetron sputtering [2 ], doctor blade [ 3 ], plasma spray [4 ] and electrophoresis [ 5,6 ]. This paper reports on progress in the latter technique made by the authors at the College o f William and Mary and the Exploratory Research and Development Center o f Los Alamos National Laboratory.

2. Preparation of starting powder One challenge o f working with the T1 series superconductor is the synthesis o f single phase sampies. Nearly single phase TI-2212 (fig. 1 ) was synthesized by mixing correct proportions o f T1203 with a fired precursor of nominal stoichiometry

Ba2CaCu205 and reacting at 850°C for five hours inside a silver tube which was crimped at both ends. Other T1 series powder was produced by reacting the correct proportions o f T1203, BaO, CaO and CuO in a Tl rich atmosphere at one atm pressure. The product o f this reaction was multiphase. Though the intended phase was TI-2212, the dominant phase was T1-2223. X-ray diffraction also shows a significant quantity of BaCuO2 phase. These results suggest confinement of powder inside a silver tube sidesteps the thallium vaporization problem to a large extent. Both powder samples were used as starting powders for electrophoretic deposition. Single phase Bi-2212 was synthesized from correct proportions of Bi203, SrCO3, CaCO 3 and CuO ~. The powder was slowly ramped to 850°C and annealed for 20 h.

3. Deposition methodology Superconducting thick films are fabricated by Bi-2212 powder was synthesized by K.C. Ott and P.D. Hain at the Exploratory Research and Development Center, Los Alamos National Laboratory.

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)

346

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silver tube was ground with a mortar and pestel for 10 rain. 0.4 g of the ground TI-2212 was suspended in 50 ml of butanol. Electrophoretic deposition immediately followed 10 min of dispersion in an ultrasonic cleaner. A 1 ~tm thick film was deposited at a rate of 0.0024 ~tm/s on a 1.5 inch diameter Consil substrate when an electric field of l 0 000 V / m was applied. This can be compared to typical values of 0.15 ~tm/s for the Bi-2212 films and 0.2 ~tm/s for Y123, although this is heavily dependent on impurities and water in the organic [ 12 ]. At present the low deposition rate is attributed to inadequate grinding of the reacted powder. Several one-inch diameter Bi-2212 samples were made by the above procedure. Substrate size was selected so the samples could be characterized in a 17.4 GHz resonant cavity. Two particular samples had identical deposition and similar heat treatment histories except one sample was melt textured and the other was not. Melt texturing is accomplished by heating the sample to or slightly above the melting point of the Bi-2212. In this case the melted sample was heated to 870°C for 1 min after deposition and before proceeding with the 805°C anneal. The Tl-based samples were annealed in air at atmospheric pressure inside a covered crucible con-

347

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taining excess T1203 powder at 870°C for 10 min. This results in T1203 enriched air which helps to limit T1 vaporization from the film.

4. Sample analysis A number of other Tl-series and Bi-series thick films were made by electrophoretic deposition onto 1 × 1 cm Consil substrates and used for X-ray analysis. X-ray fluorescence (XRF) analysis of the T1 series (TBCCO) films made from the silver tube confined starting powder suggest an approximate nominal post-anneal stoichiometry TllBa~Cao 3CuLoOx. Later depositions made from the same suspension result in a lower TI to Cu ratio, suggesting the superconducting crystallites have a higher mobility than the BaCuO2 impurities. XRF analysis on films made from the starting powder annealed at atmospheric pressure indicate significant enhance-

ment of the T1 to Cu ratio by the TI203 vapor anneal. Rutherford backscattering with 8.8 MeV alpha particles [ 13,14 ] shows TI deficiency at the surface of unannealed TBCCO films. This leads the authors to conclude the T1 to Ba ratio, and probably the Tl to Cu ratio, is larger deeper inside the sample. This also indicates the higher mobility of T1 based crystallites during deposition. X-ray diffraction results of the melt textured and non-melt textured Bi-2212 thick films demonstrates that melting enhances c-axis orientation. Figure 2 (a) reveals slight c-axis orientation in the non-melt textured film, whereas fig. 2 (b) shows only (00~) peaks to the precision of the diffractometer. Traces of Bi2201 are more easily identified in the melt textured sample since the low-angle (002) peaks are enhanced by the orientation. Traces of CuO are also more easily identified. Rocking angle measurement of the (008) peak in the melt textured sample gives a peak width of about 5 °. This large peak width is

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5. Microwave characterization of Bi-2212 samples The one-inch Bi-2212 samples were characterized in a 17.46 G H z copper resonant cavity. The surface resistance, R~, of a dissipative material is a characteristic of the power absorption of an AC field. For a superconductor R~ocog" where 09 is the frequency of the microwave field. Typically n is taken to be very nearly 2 for high-Tc superconductors [ 1 ] although BCS theory, to the lowest approximation [ 16 ], gives

n = 1.5 and experiment shows n = 1.7 for niobium from 100 MHz to 10 GHz. The dependence of Rs on temperature and applied DC magnetic field were measured and are shown in figs. 3 and 4. The measurements were made by the endwall replacement technique described by other authors [ 17-19]. The melt textured Bi-2212 sample had a residual surface resistance of 12.6 + 3 mO at 12 K and 17.46 GHz. Subsequent annealing resulted in higher residual values. However, this higher Rs was accompanied by the visibility of silver on the film surface. All other melt textured films had residual Rs ranging from 50 to 100 mfL This can be roughly compared to a value for a bulk B i - S r - C a - C u - O sample measured at 4.2 K and 3 GHz yielding Rs of 3 m f l [20] which scales to 100 mf~ at 17.46 GHz assuming n = 2 . Bohn et al. [ 1 ] report 30 mf~ at 11.6

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GHz for a thick Bi-2212 film prepared by an organic suspension technique. Quadratic scaling gives 68 m f l at 17.46 GHz. All melt textured films had low critical temperatures of 60 K. The cause for these low Tc's is uncertain. However, the Consil contains small amounts of Ni which, in YBCO, can replace Cu in small amounts and significantly lower the Tc [ 21 ]. Although X-ray diffraction shows high phase purity for the non-melt textured samples, no bulk RF superconductivity was observed. The residual surface resistance of the 1 inch non-melt textured sample was 37 + 2 mf~, but neither a phase transition nor a response o f Rs to magnetic field were exhibited. Included in fig. 3 is Rs versus temperature for the 1 inch melt textured Bi-2212 film. In the same figure is a typical sputtered thallium thick film prepared by methods described in other papers [2] and measured in the same apparatus. Also measured was the dependence of surface impedance on external DC magnetic field. Rs vs. H was measured at 11 K. Measurements of surface resistance, R~, and surface reactance, Xs= to/z2, were made for H parallel to the sample plane. The deviations in R~ and X~ from the zero-field values can be seen from fig. 4 to saturate at fields on the order of 50 to 75 Oe. Shown in fig.4(b) are AR~ and AX~ for the sputtered T1 thick film shown in fig. 3 which had a residual R~ of 3.4 mfL The surface impedance of

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6. Conclusions Electrophoresis was used to deposit thick films of TI-2212 and Bi-2212 onto consil substrates. The Bi2212 films were made with higher deposition rates than the T1-2212 films and, because of the cation vaporization problem of the TI series, the heat treatments were far more successful with the Bi-2212

350

S.K. Remillard et al. / Electrophoretic deposition o f Tl-Ba-Ca-Cu-O

films. The T1-2212 deposition resulted in films of improved phase purity from the starting powder suggesting a higher mobility during deposition for the superconducting crystallites than the impurity phases. Superconducting phase transitions were not observed in the surface resistance of Bi-2212 samples without including a melt in the heat treatment. X-ray analysis shows excellent orientation for the melt textured samples. Microwave characterizations in external DC magnetic fields do not exhibit strong field dependence leading the authors to believe more detailed study of the magnetic properties of the Bi series is needed to make any generalizations about grain coupling.

Acknowledgement The authors wish to thank Kevin Ott, Eric Peterson, Roy Rockage, Dave Brown, Kevin Hubbard, Paul H a i n and Tony Meyer from Los Alamos National Laboratory for providing expertise, materials and equipment. Mike Sola and Agus A n a n d a from William and Mary deserve our thanks for assisting with X-ray and microwave characterizations along with David Opie, Harlan Schone and W. Jack Kossler from William and Mary for their useful comments and suggestions related to this research. One of the authors ( S K R ) acknowledges support from Naval Research Laboratory.

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[4] H. Wang, H. Herman, H.J. Wiesman, Y. Zhu, Youwen Su, R.L. Sabatini and M. Suenaga,Appl. Phys. Lett. 57 (1990) 2495. [5] M. Hein, G. Miiller, H. Piel, L. Ponto, M. Becks, U. Klein and M. Peiniger,J. Appl. Phys. 66 (1990) 5940. [6] H.S. Maiti, S. Datta, R.N. Basu, J. Am. Ceram. Soc. 72 (1989) 1733. [ 7 ] U. Klein, M. Peiniger, G. Miiller,D.J. Brauer, R. Eujen, M. Hein, N. Klein, H. Piel, L. Ponto, AppliedSupercond. Conf., San Francisco, 22 August, 1988. [8] M. Hein, S. Kraut, E. Mahner, G. Miiller, D. Opie, H. Piel, L. Ponto, D. Wehler, M. Becks, U. Klein and M. Peiniger, Proc. of the Symp. on High-Temperature Superconductors in High-Frequency Fields (28-30 March, 1990, Williamsburg,Va. ), J. Superconductivity 3 ( 1990) '323. [9] C.T. Chu and B. Dunn, Appl. Phys. Lett. 55 (1989) 492. [ 10] M. Hein, S. Kraut, G. Miiller, D. Opie, H. Piel, L. Ponto and D. Wehler, ICMC'90 Topical Conference, High Temperature Superconductors, Materials Aspects, 9 May, 1990, Garmisch-Partenkirchen. [ l 1] An elementary treatment of electrophoresiscan be found in D.J. Shaw, Electrophoresis (Academic Press, London, 1969). [ 12] D. Opie, private communication. [13] J.A. Martin, M. Nastasi, J.R. Tesmer and C.J. Maggiore, Appl. Phys. Lett. 52 ( 1988) 2177. [ 14] K.M. Hubbard, J.A. Martin, R.E. Muenchausen,J.R. Tesmer and M. Nastasi, Conf. Proceedings,Heavy Ion Beams, MRS, (1990). [15] P. Sarkar, S. Mathur, P.S. Nicholson and C.V. Stager, J. Appl. Phys. 69 (1991) 1775. [ 16] J. Halbritter, Z. Phys. 226 (1974) 209. [ 17] A good overviewof various resonant cavity methods is given by A.M. Portis, Proc. of the Syrup. on High-Temperature Superconductors in High-FrequencyFields, (28-30 March, 1990, Williamsburg, Va.), J. Superconductivity 3 (1990) 297. [18] D.W. Cooke, E.R. Gray, R.J. Houlton, B. Rusnak, E.A. Meyer, J.G. Beery,D.R. Brown, F.H. Garzon, I.D. Raistrick, A.D. Rollet and R. Bolmaro, Appl. Phys. Lett. 55 (1989) 914. [ 19] S. Sridhar, J. Appl. Phys. 63 ( 1988) 159. [20 ] D.W. Cooke, private communication. [21 ] J.M. Tarascon, P. Barboux, P.F. Miceli, L.H. Greene, G.W. Hull, M. Eibschutz and S.A. Sunshine, Phys. Rev. B 37 (1988) 7458.