Materials Research Bulletin, Vol. 30. No. 3. pp. 277-284.1995 Copyright @ 1995 Elsevier Science. Ltd Printed in the USA. Ail rights reserved

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Hg-BASED SUPERCONDUCTING FILMS PREPARED BY SPRAY PYROLYSIS TECHNIQUE

S.H. Pawar and P.N. Pawaskar Energy Studies Laboratory, Department of Physics

Shivaji University, Kolhapur - 416 004 (INDIA)

(Received October 19, 1994; Refereed)

AESI‘RACT Hg-Ba-Ca-Cu-0 films have been prepared on silver substrates using spray pyrolysis technique, following a two step procedure; first Ba-Ca- Cu-0 films were prepared at a 350°C substrate temperature and annealed at 780°C for 6 hours to obtain a Ba-Ca-Cu-0 precursor.

Secondly, Hg-Ba-Ca-Cu-0 films were prepared by spraying HgC& on a Ba-Ca-Cu-0 precursor at a 250°C substrate temperature. These films were then oxidized at 250°C for 10 hours in an oxygen atmosphere. Films thus obtained were characterized by studying their microphotography, X-ray diffraction pattern and electrical properties. These films show superconductivity below 87 K.

Materials Index: Mercury, copper barium calcium oxide.

Introduction

Investigation of superconductivity in Hg1-BazCan_I-Cun_08+d systems by E.V. Antipov and E. Shilling and others with Tc = 135 K and Tc = 150 K under pressure has created much excitement [1,21. This was the first increase in Tc in several years. These Hg based superconductors until now have been prepared by vapor/solid state reaction methods in pellet form with special precautions to control the toxicity and volatility [3-61. For microelectronic and electronic applications, thin film study of these Hg-based superconductors is needed [73. The preparation of Hg-based films may be treated in the same way as that of TZ-based superconducting films, since the mercury and mercury oxide bonds are unstable at annealing temperatures of superconductors. The best procedure was considered to be annealing of amorphous films in an atmosphere containing Hg- vapor, which requires special equipment for treatment of volatility and toxicity.

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We have made an attempt to simplify the annealing procedure by eliminating mercury vapor in the annealing atmosphere which requires annealing of Hg- based films relatively at lower temperature.

Exnerimental

Spray pyrolysis of Ba-Ca-Cu-0 films has been carried out using a spray pyrolysis unit [8]. Starting solutions were prepared by dissolving nitrates of barium, calcium and copper in the ratio Ba:Ca:Cu::2:2:3 and mercuric chloride separately in doubly distilled water, Concentration of these solutions were varied from 0.05M to 0.5M and optimized to be 0.2M, to obtain uniform, smooth and dense

films. Ba-Ca-Cu solution was mixed with isophyl alcohol [propan-2-ol-(C!H& CHOH] and sprayed with a specially designed nozzle on silver substrates

preheated at 350°C with a carrier gas pressure of 0.6 Kg/cm. These 5 to 6 pm thick films of Ba-Ca-Cu-0 were annealed at 780°C for 6 hours in a closed furnace and

cooled to room temperature slowly. On this Ba-Ca-Cu-0 precursor a 0.2 M HgCC2 solution was sprayed at 250°C. These films were heated at 25O’C for 1 hour in a spray chamber and slowly cooled. The post annealing was performed in the flowing oxygen at atmospheric pressure and 250°C. These films were characterized by measuring their electrical resistance using the conventional four probe technique with silver paint contacts on the film surface. An eight and a half digit precision nanovoltmeter was used to measure the voltage developed across the contacts. X-ray diffraction patterns were examined on Philips diffractometer using CL& radiation. Microstructural properties were studied using the CCTV attachment to a Metzer Optical microscope in the reflection mode.

Results and l)iscussion

One of the main factors that affects the surface morphology, dense structure and adhesion to the substrate of the deposited films is the substrate temperature. According to the principle of the spray pyrolysis technique, deposition of aerosol droplets containing solution of constituents takes place onto preheated substrates followed by thermal decomposition, which is usually performed directly on the surface of the heated substrates. It is usually observed that for lower substrate temperatures (150°C to 225’C) the deposits have non uniform cloudy structure with less densification of deposited material. To complete the thermal decomposition and evaporation of volatile gases present, post annealing of deposits at higher temperature must be carried out. For pure aqueous solution of nitrates to produce densed films, substrate temperatures in the range 45O”C-550°C are needed [9,10]. Using organic solutions such as methanol, ethanol and propan-2-ol, these substrate temperatures can be reduced to nearly 250°C to 350°C to obtain a uniform dense structure of the deposits. Changing the propan-2-oe ratio with nitrate salt aqueous solution effects the uniformity, grain size and intergrain spacing. With a 1:l ratio of aqueous-organic solution sprayed by specially a designed spray nozzle moving horizontally over the hot plate at

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optimized carrier gas pressure (0.6 kg/cm2), we have obtained uniform, smooth deposits of Ba-Ca-Cu-0 at a substrate temperature of 350°C. Deposited Ba-Ca-Cu- 0 films were heated for 1 hour at 5OO’C in the spray chamber to complete the thermal decomposition and to allow the evaporation of all evaporant constituents present in the solution. These films were slowly cooled to room temperature within the spray chamber, following the results of thermogravimetric studies and the mentioned schemes of aerosol decomposition and chemical decomposition [l&121. In our case, we suppose that due to the use of propan-2-o!, the decomposition process starts in the warm zone over the substrate heated at about 350°C. During spray pyrolytic deposition, droplets move along the temperature gradient against the heated substrate and partial decomposition of nitrates occurred partially outside the substrate, leading to a higher density and uniformity of film material. Use of 50% organic solvent (propan-2-oe) reduces the complete thermal decomposition temperatures of nitrates from 780°C to 506°C for barium, from 660°C to 400°C for calcium, and from 400°C to 250°C for copper. However, to ensure the complete thermal decomposition and oxidation of constituents deposited, annealing of the films was carried out at 780°C for 6 hours to obtain the Ba-Ca-Cu-0 precursor.

The Ba-Ca-Cu-0 precursors were heated at 250°C in the spray chamber. Above

these films, a 0.2 M solution of HgCe2 was sprayed. To maintain the stoichiometric ratio of Hg:Ba:Ca:Cu::1:2:2:3 and to overcome any evaporation of

Hg while annealing and oxidizing at 25O”C, an HgC!z solution of quantity 2:4 ratio with Ba-Ca-Cu-0 was sprayed. The sprayed films were heated at 250°C for 1 hour inside the spray chamber and slowly cooled to room temperature. Post annealing of these films was carried out in a continuous flow of oxygen. Fig. 1 shows the flow chart of various steps followed in the spray pyrolytic deposition of Hg-Ba-Ca- Cu-0 films. Fig. 2 shows the variation of film thickness with the substrate temperature. From the figure it is seen that the film thickness decreases with substrate temperature. Above 4OO”C, the film thickness practically remains constant.

Figs. 3a and 3b show microphotographs of Ba-Ca-Cu-0 films deposited by spray pyrolysis before oxidation and after oxidation. After oxidation, surface morphology shows compact grain structure with reduced intergrain spacing. Figs. 3c and 3d show microphotographs of Hg-Ba-Ca-Cu-0 films as spray pyrolytically deposited and after oxidation. From the microphotography it is seen that the films obtained were smooth, uniform, dense having a compact grain structure.

Figs. 4a and 4b show X-ray diffraction patterns of Hg-Ba-Ca-Cu-0 films before oxidation and after oxidation at 250°C for 10 hours. From the X-ray diffraction pattern of Hg-Ba-Ca-Cu-0 before oxidation and after oxidation it is observed that films are well crystallized. Peak intensities and additional peaks of Hg-1212 and Hg-1223 were observed after oxidation at 250°C in a continuous oxygen flow. These peaks of mixed phases were estimated by fitting their non-overlapping X- ray diffraction peaks with the standard patterns. Lattice constants were calculated for Hg-1212 and for Hg-1223. Fig. 5 shows temperature dependence of resistivity curve for Hg-Ba-Ca-Cu-0 films annealed at 250°C for 10 hours in a continuous flow of oxygen. Onset temperature was observed at 92 K and zero resistivity was achieved at 86.7 K. From the RT

280 S.H. PAWAR && Vol. 30, No. 3

curve and X-ray diffraction patterns, it seems that the anomaly at 115 K may be due to the Hg-1223 phase. The critical current density of these 2 samples at 70 K

were calculated to be 1100 Alcm2.

AcknowledPment

The authors wish to thank the Department of Science and Technology (DST), New Delhi, for their financial support and Dr. A.V. Narlikar for his constant encouragement.

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Referenca

S.N. Putilin, E.V.Antipov, 0. Chmaissen and M. Marezio, Nature (l&don), 362,226 (1993). A. Shilling, M. Cantoni, J.P. Guo and H.R. Ott; Nature (London), 363, 56 (1993). S.N. Putilin, E.V. Antipow and M. Marezio; Physica C, 212, 266 (1993). J.L. Wanger, P.G. Radaelli, D.G. Hinks, J.P. Jorgensens J.F. Mitchell, B. Babrowski, G.S. Knapp and M.A. Bino, Physica C, 210,447 (1993). S. Nakajima, M. Kikuchi, Y. Synono, T. Oku, P. Shindo, K. Hiraga, N. Kobayanshi, H. Jwasaki and Y. Mato, Physica C 158,471(1989). R.L. Meng, L. Beauvais, X.N. Zhang, Z.J. Huang, Y.Y. Sun, Y.Y. Xue and C.W. Chu, Physica C 216,21-28 (1993). Hideaki Adachi, Toshifumi Satoh and Kintaro Setsune; Appl. Phys. Lett. 63(26) 27 (1993). S.H. Pawar, S.K. Pawar, Mat. Res. Bull. l&211(1983). M. Jergel, S. Chromik, V. Strbik, V. Sanatko, F. Hanic, G. Plesch, S. Buchta and S. Vatyniova, Supercond. Sci. Technology, 5,225 (1992). H.M. Hsu, I. Yee, J. Peluca, C. Hilbert, R.F. Miracky and L.N. Smith, Appl. Phys. Lett., 54,957 (1989). M. Jergel, V. Strbik, V. Smatko, F. Hanic, J. Liday, T. Plech, Hollsek and H. Kubronava, Supercond. Sci. Technol., 5,663-670 (1992). M. Jergel, Indian J. of Pure & Appl. Phys., 30,511-518 (1992).

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O-2 M solution of Ba-Ca-Cu

t 50 “I. of propan - 2 - 01

I I

Spraying on Ag substrates

heated at temperature 350?

Annealiog at 780°C for

5 hours

Spraying of O-2 M HgCl2 solution

t 50 ‘1. propan -2 - 01 on

Ba- Ca- Cu-0 precursor heated at 25O’C

t

heating at T = 250 ‘C in spraying

chamber for 1 hour

Fig. 1

Flow chart of various steps followed in the spray pyrolytic deposition of Hg -Ba-Co-Cu-0 films.

282 S.H. PAWAR ti Vol. 30, No. 3

12

a

4,

I I I I

0 100 200 300 400 500

Substrate temperature -

Fig. 2

Variation of film thickness with substrate

temperature.

Fig.3

Microphotographs of typical samples of a) Ba-Ca-Cu-0 as deposited, b) Ba-Ca-Cu-0 after annealing, c) Hg-Ba- Ca-Cu-0 as deposi ted and d) Hg-Ba-Ca-Cu-0 after oxidation,magnification was 500X.

Vol. 30, No. 3 MERCURY CUPRATE 283

I I I I

0 20 40 60 8

Fig. 4 a

X-ray diffraction pattern of Hg-Ba-Ca-Cu-0

film before oxidation .

I I I I 1

0 20 40 60 80 l( 0

20 -

Fig. 4 b

X- ray diffraction pattern of Hg-Ba-Ca-Cu-0 film

after oxidation. Indexed XRD intensities correspond

to Hg- 1223 and Hg-1212 (under lined 1 .

284 S.H. PAWAR a

Temperature (OK 1 --+

Fig. 5

Temperature dependence of resistivity curve

for Hg -Ba-Ca-Cu-0 films annealed at 3OO’C

for 10 hours in 02

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