Applied Superconductivity Vol. 3, No. 1-3, pp. 139-146, 1995

Pergamon 0964-1807(95100043-7 Copyright 0 1995 Elsevier Science Ltd

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EFFECT OF SILVER ADDITION ON THE PROCESSING OF Tlo~5Pbo~5Sr2CaCu207--6

CHAN PARK, S. S. BAYYA, D. SRIRAM and R. L. SNYDER

Institute for Ceramic Superconductivity, New York State College of Ceramics at Alfred University, Alfred, NY 14802, U.S.A.

Abstract-The effect of silver additions, to bulk T10~5Pba,sSr&aCu207_6-Ag, (x = 0.0, 0.008, 0.016, 0.1, 0.37 and 1 .O) superconductor, on microstructure and electrical properties were studied. A eutectic- like reaction between (Tl-1212 + Ag) was observed. In the case ofx = 0.37, liquid phase was formed at 830°C resulting in enhanced densification and grain growth. Changes in T, with silver content were observed, but the presence of silver does not cause any change in the intragranular J, values, which was on the order of lo6 A/cm* at 40 K in self field. Films of the compound were also prepared by a plasma vapor deposition-post thalliation method, which produces pure phase and nearly full OOe orientation. The degree of orientation decreases as film thickness increases.

1. INTRODUCTION

The single Tl-0 layer superconducting compounds are believed to have intrinsically superior in- field properties due to the strong c-axis coupling between the CuO planes [l, 21. Recently, excellent transport properties were obtained in the presence of a magnetic field in Tl-1223 polycrystalline films prepared by reacting Ag-containing oxide precursor with thallium oxide vapor [3]. They reported that without silver addition, superconducting phases were not formed under the same processing conditions. When silver was added, however, phase-pure super- conducting film with complete c-axis orientation was obtained.

Silver has been widely used in the fabrication of high temperature superconductors (HTSC) to obtain improved physical properties. Many studies on the effect of silver addition have been reported for the YBa2Cu307--6 [4-301 and Bi-compounds [38-47].

The roles of silver reported so far for the above two compounds are that silver is incorporated into the lattice [4, 6-8, 12, 18, 21, 22, 33, 34, 391; modifies oxygen content [8, 19, 241 and changes Cu oxidation state; facilitates oxygen diffusion to the superconducting grains; enhances mechanical properties such as toughness, strength, and flexibility [13, 14, 20, 231; improves the stability in atmosphere [ 12, 20, 321; improves surface smoothness [7, 151; serves as flux pinning centers in the intergranular or intragranular regions [ 193; helps reduce the weak-link by providing conductive paths between superconducting grains [9, 12, 17, 33, 35, 39-411; cleans the grain boundaries by embedding secondary phases [25]; lowers the melting point permitting a different reaction path [ll, 24, 25, 27, 38, 471; promotes grain growth during solidification [5, 11, 17, 19, 24, 31, 32, 361; improves normal-state electrical [lo-12, 24, 33, 20, 40, 37, 451 and thermal conductivity [24].

There, however, have not been any systematic studies on the effect of silver in the Tl-containing HTSC phases. The purpose of this study is to understand the effects of silver addition in the Tlo.~Pbo.sSr~CaCu207-~ compound. Bulk samples and films prepared using plasma vapor deposition (PVD) were used with different amounts of silver added.

2. EXPERIMENTAL PROCEDURE

Samples with nominal composition of Tlo,5Pb0,5Sr2CaCu207_s-AgX with x = 0, 0.008, 0.016, 0.1,0.37 and 1 .O (TPSCC-Ag 12 12-x) were prepared. Reaction was carried out in two steps. In the first, stoichiometric ratios of SrCOs, CaCOs, CuO, and Ag20 were mixed and calcined at 940°C for 20 h. In the second step, the reacted precursor was mixed with the appropriate amount of TlzOs and PbO, and then reacted in the form of powders in sealed silver tubes at a temperature of

139

140 CHAN PARK et al

830°C for 6 h. Heating and cooling rates of lO”C/min were used. The powder mixtures (precursor + T1203 + PbO) were uniaxially cold-isostatic-pressed under a pressure of -7 x lo7 Pascal into bar-shaped pellets of about 13 x 3.5 x 2 mm size. The pellets were then put into silver tubes, sealed and sintered in flowing O2 at 830°C and 860°C for 6 h. The weights of the silver tube containing the samples before and after sintering were used to check the loss of Tl.

Films of Tl-1212 were made by a two step process: (i) precursor deposition and (ii) post thalliation. The precursor films were deposited using an RF induction plasma. An Ar/OZ plasma at atmospheric pressures was used. Details of the plasma vapor deposition process are discussed elsewhere [48, 491. Nitrates of Pb, Sr, Ca, Cu, and Ag were prepared as solutions with concentration of 100 g/l, put into a nebulizing chamber, and atomized into 0.5-5.0 pm droplets. The droplets were fed into RF induction plasma and deposited on MgO 100 single crystal substrates. Post-thalliation was carried out in sealed silver tubes in the presence of sacrificial TL- 1212 powder at temperature of 860°C for 1 h.

XRD, SEM, and EDS were used to characterize all samples. The transport Tcs were measured by the four probe method for the bar samples. The d.c. susceptibility and magnetic hysteresis were measured on a SQUID magnetometer to obtain Tcs and intragranular Jcs for powder samples.

3. RESULTS AND DISCUSSION

3.1. Powder and bulk samples

The phases observed in powders reacted at 830°C for 6 h were pure Tl-12 12 and metallic Ag from the XRD results, which revealed no traces of Ag oxides (Fig. 1). The presence of Ag does not lead to the decomposition of the superconducting phase in any of the compositions studied.

The powders, however, when pressed into bars and sintered, showed appreciable difference in appearance for different amounts of Ag content. Sample blistering was observed on the surface of the x = 0.37 bars together with a large volume contraction after sintering at 830” and 860°C (Fig. 2). In all other bar samples, the dimensions of the bars did not decrease but rather slightly increased.

Bar samples showed pure Tl- 12 12 and metallic Ag phases in all the compositions except for the cases of x = 0.1 and x = 0.37, where secondary phases (SrCOs and Cu2SrOa) appeared (Fig. 3). The bar samples with x = 0.37 contain a substantial amount more secondary phases than the

zooOr

l = impurity - S =silvcr

“20 25 30 3s 40

Two theta (degrees)

Fig. 1, XRD patterns of TPSCC-Ag 1212-x powders after reaction at 830°C for 6 h.

Ag additions and Tlo,5Pbo.sSrzCaCu207-~ processing 141

Fig. 2. Photograph showing the different sizes of pressed bars of TPSCC-Ag 1212-x after sintering at 830°C for 6 h. (a) x = 0, (b) x = 0.1, (c) x = 0.37 and (d) x = 1.0.

x = 0.1 bars as observed from the XRD results. The samples with x = 0,0.008, 0.016, and 1 .O do not show any sign of densification as can be seen from the SEM pictures of fracture surfaces (Fig. 4), and have a very porous microstructure. To examine intergranular regions, EDS mapping of the various characteristic X-rays, was performed on the polished sample with x = 0.37. The results show that Ag exists as isolated particles, and Cu-rich and Sr-rich phases are present near the Ag particles. The Cu-rich phase is believed to be CuO which does not manifest itself in the diffraction pattern.

The XRD and SEM results indicate that there exists a eutectic or peritectic reaction in the TPSCC- 12 12 + Ag system, which causes the formation of liquid phase, the composition of which is not determined at this time. The presence of liquid phase enhances the grain growth (Fig. 4b) which is consistent with the result of Deluca et al. who suggested that the formation of the liquid phase is responsible for the accelerated growth kinetics of Tl-1223 films fabricated by spray deposition and post-thalliation. The Cu and El rich phases exsolve from this peritectic liquid on cooling along with the metallic silver which always occurs as spheres in the vicinity of the Cu- or Sr-rich phases. Tl or Pb is needed for the liquid phase to form because no difference was observed from the DTA analysis of the precursor oxide with and without Ag. This DTA result should be compared with that of Deslandes et aI. [25] who observed a reaction between silver and both

l = impurity S = clilver

“20 25 30 35

Two theta (degrees)

Fig. 3. XRD patterns of TPSCC-Ag 1212-x bars after sintering at 830°C for 6 h.

142 CHAN PARK et al.

Fig. 4. SE !M PhC )graphs of the fracture surface of TPSCC-Ag 1212-x sintere (a) x = 0, (b) x = 0.37, (c) x = 1.0.

cl at 860°C for 6 h.

BaCuOz and BaC03. We, however, could not observe any trace of reaction between silver and SrCuOz or SrCOa in DTA analysis. It seems more reasonable to think the reaction is a eutectic because McCallum et al. recently reported the presence of two eutectic reactions in the Bi- 2212 + silver system [47]. The presence of carbonates and sample blistering after sintering when a liquid phase was involved suggests that carbon may play a role in the formation of the liquid phase.

T, slightly increases with small additions of silver and then decreases as more silver is added (Table 1). The same trend was observed in both electrical and magnetic measurements. This kind of T, change was also observed in the YBa&u@_6-Ag system, and such an initial increase in T, when small amounts of silver are added was attributed to the substitution of Ag for Cu [7]. The decrease of T, with higher silver additions has been explained as being due to an increasing amount of non-superconducting phases [7, 261, or structural defects (stoichiometric variation, voids etc.) mainly caused by Ag clusters [ 121.

Ag additions and Tlo.sPbo.sSr*CaCu20,-6 processing 143

Table 1. T,(K) vs silver content. Tcs for TPSCC-Ag 1212-x bars and powders were determined by four-probe method and by

SQUID measurements respectively

x T&r)

0 74 0.008 75 0.016 73 0.1 71 0.37 71 1.00 69

T,(powder)

78 79.5 79 75 71.5

Intragranular Jcs were calculated using the Bean model from the magnetization data collected over the field range of O-1 T at 40 K. A grain size of 10 pm was used in the calculation. The calculated J,s were on the order of lo6 A/cm2 at 40 K self field, and did not change with the addition of silver.

Lattice parameter changes can be expected as a different amount of silver is added, if silver is incorporated into the lattice of Tl-1212. Powder XRD was used with internal and external standard calibration [50], and lattice parameters were refined to check the possibility of lattice incorporation of silver. No significant change in the lattice parameters was observed. This is consistent with the results of Wu et al. [51] and Alario-Franc0 et al. [52] who observed the absence of silver in the superconducting Cu-Ba-Ca-Cu-0 phases using TEM.

3.2. Films

Films after thalliation were found to be nearly perfectly oriented single phase Tl- 1212. However, the degree of orientation was found to be a function of film thickness. Figure 5 shows X-

3000

2000

8

5 h .z

s

5

1000

5 10 20 2s

Two theta (degrees)

Fig. 5. XFUI patterns of T10,sPb0.sSrzCaCu207_~Ag ,,JT films showing different amount of OOe orientation resulting from different film thickness.

144 CHAN PARK et al.

Fig. 6. (a) SEM photograph of the post-thalliated Tl~.sPbo.sSrzCaCu~O,-~Ag~,~~ film. (b) Spherical silver particles sitting at the top which shows Ag diffusion to the surface.

ray diffraction patterns of 2, 5 and 10 pm thick films where the decrease in the degree of orientation can be clearly seen. Figure 6a shows an SEM picture of the film with 0.37 mole of Ag per mole of Tl-1212. The picture shows textured growth of Tl-12 12 superconductor. In each region it appears that the nucleation occurs at the surface and the crystals grow up in the direction perpendicular to the substrate. Most of the crystal growth regions have spherical silver particles sitting at the top of vertical columns. This is clearly shown in Fig. 6b. The fraction of the growth regions with the silver particles at the top increases with increasing amount of silver in the starting composition. Almost all the silver is seen to be sitting on top of some growth region. From the observed microstructure, the presence of a liquid phase is evident. It is evident that the presence of silver promotes the formation of liquid phase at high temperature. Silver is a part of the liquid phase and when the superconductor crystallizes from the liquid silver is also exsolved, since it is not incorporated in the crystal lattice. The observation that silver always sits at the top of columnar growth regions suggests that silver could be acting as a nucleation site for the growth of the Tl-1212 phase in contact with the substrate. This postulate would also explain decreased degree of orientation with increasing film thickness. The liquid phase formed may run down to the substrate and where crystallization occurs, the vertical growth is initiated; as the film thickens the nucleation from the liquid will necessarily crystallize at points not in contact with the substrate and so more random orientations will be observed.

In conclusion, an appreciable difference in sintering behavior was observed as silver content changes. In the case of T10.sPb,,$SrzCaCuz07-~-Ag o 37, liquid phase is formed and enhances densification and grain growth. Silver exists as an isolated island. When liquid phase forms, some secondary phases (Cu$SrOs, SrCO3, CuO) are also formed near silver particles. Changes in T, with silver content was observed, but the presence of silver does not cause any change in intragranular J, values. Plasma vapor deposition of the precursor film on MgO single crystal produces a pure phase and near full 004 orientation after post-thalliation. The degree of orientation decreases as film thickness increases. The well-oriented grains are poorly connected with each other.

Ag additions and Tlo,sPbo.sSr2CaCu,0,-~ processing 145

A eutectic-like reaction occurs in the T112 12 + Ag system. Silver lowers the melting point of the system and facilitates sinterability and promotes grain growth during solidification. Carbon may play some role in the formation of the liquid phase. Silver seems to act as a nucleation site for the superconducting compound. Further work on the analysis of the liquid phase and effect of carbon is underway.

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