Role of room-temperature electrochemical oxidation in Pb-doped Bi–Sr–Ca–CuO superconducting thin films

Materials Chemistry and Physics 65 (2000) 68–73

Role of room-temperature electrochemical oxidation in Pb-doped Bi–Sr–Ca–CuO superconducting thin films N.V. Desai, D.D. Shivagan, L.A. Ekal, S.H. Pawar∗ Energy Studies Laboratory, Department of Physics, Shivaji University, Kolhapur-416004, India Received 3 August 1999; received in revised form 2 December 1999; accepted 15 December 1999

Abstract A room-temperature electrochemical synthesis of Bi(Pb)–Sr–Ca–CuO films onto silver substrates has been successfully carried out using D.C. electrodeposition technique. The various preparative parameters, such as deposition potential, current density, etc. have been studied and optimised. The electrochemical oxidation of these films in alkaline solution (1 N KOH) has been used for preparing the Bi(Pb)–Sr–Ca–CuO superconducting thin films. The films are oxidised for different periods and studied for their structural and electrical properties. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Superconductors; Thin films; X-ray diffraction; Crystal structure; Electrical properties

1. Introduction Electrodeposition is an attractive and easy technique for the deposition of elemental, binary, ternary, etc. alloyed films. Thin films of Bi–Sr–Ca–Cu and Hg–Ba–Ca–Cu have been prepared by the electrodeposition technique and their superconducting properties reported [1,2]. Recently, single phase Bi–Sr–Ca–Cu–O superconducting films with Tc 110 K, prepared by sequential layer by layer deposition followed by room-temperature oxidation using the same technique, have been reported [3]. The synthesis of a single high Tc phase (2 2 2 3) by using sequential layer by layer deposition is, however, a difficult task. An alternative method reported by Sunshine et al. [4] in 1988 suggesting that partial substitution of Pb for Bi in the Bi–Sr Ca–Cu system has been shown to be effective in increase high Tc phase. Hence, we have tried doping of Pb in the Bi–Sr–Ca–Cu system with the help of electrodeposition technique. This paper reports the synthesis and characterisation of Bi(Pb)–Sr–Ca–CuO alloyed films by the electrodeposition method. The oxidation of the films at high temperature causes the conversion of (2 2 2 3) phase into the (2 2 1 2) phase [5]. To avoid this, room temperature electrochemical oxidation has been tried for the first time. In this way, one can control the intercalation of oxygen that may be useful in increasing the high Tc phase. Oxidation was carried out ∗ Corresponding author. E-mail address: [email protected] (S.H. Pawar)

for different periods. The resulting films have been tested for their structural and electrical properties and the data reported in this paper.

2. Electrochemical background Electrodeposition is an electrochemical process where metals or oxides in the proper stoichiometry are deposited on conducting substrates from chemical solutions containing the ions of interest (e.g. Bi3+ , Pb2+ , Sr2+ , Ca2+ , Cu2+ ) [6]. The method for deposition of a single-element film by electrodeposition is simple and well-established. However, deposition of a multi-component system by the electrodeposition technique is somewhat complicated, and was beyond imagination due to the different decomposition potentials of each element. Every element has its own, and different, electronic arrangement from others and it is not possible to deposit a film of a multi-component system at a given potential of the electrochemical cell. It was proposed by Pawar et al. [8,9], Maxfield et al. [7] and Bhattacharya et al. [6], as well as others, to synthesise multi-component alloyed films to form high Tc superconducting YBCO, BSCCO, TBCCO thin films. In electrodeposition, one of the two phases contributing to an interest will be an electrolyte, which is merely a phase through which charge is carried by the movement of ions. The second phase at the boundary is an electrode (the substrate in this case), which is the phase through which charge is carried by electronic movement. In general, when

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N.V. Desai et al. / Materials Chemistry and Physics 65 (2000) 68–73

the potential of an electrode is moved from its equilibrium value towards negative potentials, the substance that will be reduced first is the one with the least negative (or more positive) redox potential E0 . Thus, all the ions can be deposited on the surface of the electrode when the potential is sufficiently negative. The relative concentrations of the constituents are determined by the concentrations of the ions in the solution. In this study, the electrodeposition is performed at a constant potential. The electrode potential is stepped up to a value sufficiently negative, so that all ions of interest are deposited. This potential is maintained constant by a potentiostat. The diffusion control current for a single species is given by nFAD1/2 C0 (1) π 1/2 t 1/2 where, n is the number of electrons involved in the reaction, F the Faraday constant, A the area, D the diffusion coefficient, C0 the bulk concentration, and t the time. The following relations show the proportionality between the current and the deposition rate I d(t) =

dQ (C/s). dt Q (C) = N (mol deposited) nF (C/mol)

I (A) =

Rate (mol/s cm2 ) =

I dN ≡ dt nFA

(2) (3)

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electrode and silver plate as the working electrode. The silver substrates were mirror-polished (mechanically). The electrodeposition was carried out under unstirred conditions and at ambient temperature. The cathodic polarisation curves for electrodeposition of Bi–Pb–Sr–Ca–Cu alloy films were recorded with a model 362 EG and G potentiostat with an X–Y recorder. Microstructural properties of as-deposited and oxidised Bi–Pb–Sr–Ca–Cu alloy films were determined using a Metzer optical microscope (magnification 500×) with a CCTV attachment. The samples were structurally characterised by X-ray diffraction. Phillips diffractometer with CuK␣ radiation was used. The resistance measurement was carried out by using a conventional four-probe technique with silver point contacts. An eight-and-a-half digit precision nanovoltmeter was used to measure the voltage developed across the contacts.

4. Result and discussion 4.1. Effect of deposition potential on thickness measurement The deposition potentials were determined from the cathodic polarisation curves. The cathodic polarisation curves for the electrodeposition of Bi–Sr–Ca–Cu alloy onto the

(4)

3. Experimental details For the preparation of Bi(Pb)–Sr–Ca–CuO thin films, two electrolytic baths were prepared by dissolving lead nitrate (4 mM) in dimethyl sulphoxide (DMSO) in one bath and reagent-grade nitrates of bismuth (20 mM), strontium (45 mM), calcium (30 mM) and copper (8 mM) in DMSO in the second bath. The total bath composition of the second bath was adjusted to get Bi–Sr–Ca–Cu alloy films in the required 2:2:2:3 stoichiometric ratio. The values of concentrations of Bi, Sr, Ca and Cu nitrates were decided from mobility of these ions [7]. First, we have synthesised Bi–Sr–Ca–CuO thin-film superconductor at room temperature using the electrodeposition technique. The deposition potential to deposit the Bi–Sr–Ca–Cu alloy film onto a silver substrate was −0.8 V vs. SCE. These films were then annealed at 300◦ C for 15 min. in a preheated furnace. Pb from the DMS0 bath containing Pb(NO3 )2 was then deposited onto the Bi–Sr–Ca–Cu alloy film at a potential of −0.5 V vs. SCE. These films were then electrochemically oxidised at a deposition potential of +700 mV vs. SCE at room temperature for different periods. A conventional three electrode system was employed, with SCE as the reference electrode, graphite as a counter

Fig. 1. Cathodic polarisation curves for electrodeposition of (a) Bi–Sr–Ca–Cu alloy onto silver substrate and (b) lead onto the heat-treated Bi–Sr–Ca–Cu alloy film.

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Fig. 2. Micrographs (magnification 500×) of (a) as-deposited Bi(Pb)–Sr–Ca–Cu film; and (b) electrochemically oxidised films for the period of (i) 18, (ii) 22, (iii) 24, and (iv) 28 min.

silver substrate and Pb onto the Bi–Sr–Ca–Cu alloy are shown in Fig. 1. The values of deposition potential are −0.8 V vs. SCE and −0.5 V vs. SCE, respectively. Thus, the films were deposited at constant potential and the current recorded as a function of time. For a given chemical bath, the deposition current density was found to vary with time. The variation of the thickness of the film was measured using the weight-difference method. From the cathodic current density and thickness measurement, the thickness of the Bi–Sr–Ca–Cu alloy films at −0.8 V vs. SCE for the period of 40 min is of the order of 2–3 micron. After annealing at 300◦ C in a pre-heated furnace for 30 min, Pb was deposited onto the heated Bi–Sr–Ca–Cu alloy at −0.5 V vs. SCE for a period of 15 min and then oxidised electrochemically using 1 N KOH solution at a potential of +700 mV vs. SCE at room temperature for 18, 22, 24 and 28 min. 4.2. Surface morphology Micrographs of the as-deposited and oxidised films are shown in Fig. 2a,b (i–iv). The films are smooth, uniform and

dense. A dense packing of grains with a needle-like grain structure is observed as the period of oxidation increases. 4.3. Structural studies Fig. 3a,b (i–iv) shows the X-ray diffraction pattern of Bi(Pb)–Sr–Ca–CuO films deposited onto silver substrate for different oxidation periods. The films were found to be polycrystalline. The XRD pattern mostly consists of high Tc (2 2 2 3) phase with characteristic peaks 0010, 0012, 0014, 0016 with a small amount of the low Tc phase. Diffraction peaks from the silver substrate are also observed in the pattern. The major diffraction lines were indexed with tetragonal indices. The values of ‘a’ and ‘c’ parameters were calculated from the observed ‘d’ spacing of diffraction lines using the formula h2 + k 2 l2 1 = + (5) d2 a2 c2 The ‘a’ and ‘c’ values of major diffraction lines are given in the Table 1. The evaluated values are found to be in good

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Fig. 3. XRD patterns of (a) as-deposited Bi(Pb)–Sr–Ca–Cu film; and (b) electrochemically oxidised films for the period of (i)18, (ii) 22, (iii) 24, and (iv) 28 min.

agreement with the reported values [10].The X-ray diffraction peaks are further analysed for the intensities, and the effect of oxidation time on the intensities has been studied. Fig. 4 shows the variation of oxidation time with intensity. It is seen that as the oxidation time increases, intensity of reflection peaks increases up to certain extent, attains a maximum value and then decreases. Except for 0010 and 0014

peaks, it increases continuously. This continuous increase for 0010 and 0014 peaks might occur because, as the oxidation time increases, the incorporation of oxygen species into the Cu–O plane increases. In case of other peaks, the intensity decreases after 24-min electrochemical oxidation. As the oxidation period increases after certain period, number of atoms goes into the interstitial position causing the

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Table 1 The ‘a’ and ‘c’ values of major diffraction lines Parameter (A0 )

a c

4.4. Electrical resistivity

Oxidation period (min) 18

22

24

28

5.2230 36.734

5.4179 37.170

5.4177 37.604

5.4092 37.060

defects in the lattices produces scattering of electrons in different directions. Hence, depending on the atoms positions and atomic density in the lattices causes decrease in the intensity of other peaks. From the observed intensities, one can also find the position of a known number of atoms in a unit cell of a known shape and size, since these intensities are determined by the positions. For this purpose, the following two equations are given [11]  I = |F |2 P

1 + cos2 2θ

A standard four-point probe method was used to study the variation of electrical resistivity with temperature. The variation of Tc (K) with oxidation period (min) is depicted in Fig. 5a. It was observed that, as the oxidation period increases, Tc goes on increasing. The typical temperature dependence of resistivity of Bi(Pb)–Sr–Ca–Cu film for a 28-min oxidation is shown in Fig. 5b. The sample showed a sharp superconducting transition with Tc onset at about 102 K and superconductivity below 98 K.



sin2 θ cos θ

(6)

where P is a multiplying factor. Eq. (6) gives the relative intensities (I) of the reflected beams and F =

N X fn e2π i(hun +kvn +lwn )

(7)

n=1

which gives the value of the structure factor ‘F’ for the hkl reflection in terms of the position of the atom uvw.

Fig. 4. Variation of XRD intensity with oxidation period for Bi(Pb)–Sr–Ca–Cu superconducting thin film.

Fig. 5. (a) Variation of Tc with oxidation period for Bi(Pb)–Sr–Ca–Cu superconducting thin film. (b) Typical temperature dependence of the resistivity of a Bi(Pb)–Sr–Ca–CuO superconducting film oxidised for 28 min.

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stable phase that contains mostly high Tc phase. Optimisation of concentration of Pb in BSCCO system is expected to increase conductivity. Thus, the room temperature electrochemical technique is a novel technique for the intercalation of Pb at Bi sites in the (2 2 2 3) phase avoiding the formation of (2 2 1 2) phase at high temperatures.

Acknowledgements The authors thank the Shivaji University, Kolhapur, for financial support and Dr. A. V. Narlikar, Head, Superconductivity Group, National Physical Laboratory, New Delhi, for constant encouragement. References Fig. 6. Variation of room-temperature conductivity with oxidation period for Bi(Pb)–Sr–Ca–Cu superconducting thin film.

4.5. Conductivity Fig. 6 shows the variation of room temperature conductivity with the oxidation period. As the oxidation period increases, conductivity goes on increasing. For the 18-min oxidation period, conductivity is very low: of the order of 0.5–0.6 ohm−1 cm−1 , and for the 28-min oxidation period, it is comparatively high: of the order of 1–1.1 ohm−1 cm−1 . This increase in conductivity may be because of an increase in hole concentration on the Cu–O planes when Bi3+ is replaced by Pb2+ in the crystal structure [12].

5. Conclusion The doping of Pb2+ for Bi3+ in the Bi–Sr–Ca–CuO system increases the hole concentration in the lattice to get a

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