ELSEVIER Physica C 259 (1996) 356-360

PHYSICA

Role of halogen dopants in A g - Y I B a 2 C u 3 0 7 _ x

Chao Wei, Sharmila M. Mukhopadhyay * Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201, USA

Received 28 November 1995

composites

Abstract

The influence of halogen doping on the Ag-YIBa2Cu307_ x superconducting composite has been investigated. It is seen that exposing the YIBa2Cu307_x (YBCO) to bromine prior to mixing with silver increases the critical current density of the composite significantly despite a simultaneous increase in porosity. Therefore, if the porosity can be suppressed, the beneficial effect of halogen doping can be further enhanced. This dopant also tends to increase the average particle size, i.e. decrease the interface/volume ratio in the system. These results open up the possibility of using halogen doping as a very simple, yet effective step in improving the electrical transport at superconductor interfaces.

1. Introduction and background

It has been shown by several investigators [1-12] that the addition of silver to copper oxide supercon- ductors leads to improved mechanical properties (strength, fracture toughness, thermal conductivity, and thermal shock resistance) Without deterioration of its electrical properties. Silver is known to allow oxygen diffusion (which is essential for these materi- als), enhance flux pinning and not react severely with these oxides [13-19]. This has lead to several studies on the possible fabrication routes of super- conductor-si lver composites [1,2,20-36]. However, it is seen that pure silver does not wet these oxides and tends to. cluster on the surface [37,38]. More- over, interfacial chemistry studies [38-40] show that

* Corresponding author. Fax: + 1 718 260 3136.

silver apparently reduces the surface CU 2+ ions of the superconductor.

In an attempt to identify what processing step, if any, can prevent this reduction, we had found that exposing the YIBa2Cu307_ x (YBCO) surface to bromine gas prior to silver deposition suppresses this effect [40]. Bromine had been found earlier to restore superconductivity in vacuum-annealed (and there- fore, degraded) copper oxide superconductors [41,42]. Fukuda et al. [43] had subsequently studied the effect o f bromine on YBCO using X-ray photoelectron spectroscopy (XPS) and shown that it restores the satellite component of the Cu 2p peak that is reduced by vacuum annealing. In our XPS study [40], we found that whereas the Cu 2+ ions at an undoped YBCO surface are chemically reduced on contact with silver, those at a bromine-doped YBCO surface maintain the higher oxidation state on bonding with Ag. This implies a change in the A g - Y B C O interfa- cial reactivity caused by bromine. A natural question

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C. Wei, S.M. Mukhopadhyay / Physica C 259 (1996) 356-360 357

which follows this observation is how would the altered interface influences the oxide-silver compos- ite.

In this paper we present our results addressing this question. We have prepared composites of silver and YBCO with and without Br doping and com- pared their electrical behavior and microstructure.

2. Experiment

The starting materials were silver powder with a maximum particle size of 3 Izm and 99.9% purity, and YBCO powder with particle size of 2 - 6 Ixm and 99.9% purity. One set of YBCO powder was ex- posed to pure, dry bromine vapor for 5 min in the bromination system illustrated in Fig. 1. High-purity liquid bromine (99.99% purity from Alfa chemical) was distilled twice before connecting to the bromina- tion line through a valve. The powder to be doped was attached to the line which was evacuated to 1 X 10 - 4 Torr prior to opening the valve of the bromine flask. This back-filled the system with bromine vapor which was allowed to react with the powder for a fixed time (5 min in this case) before pumping down again. After pump-down, the line was back-filled with dry argon gas before removing the powder from the line. The second set of YBCO powder was left undoped to be used as control.

Ag -YBCO composite samples with 32 wt.% of silver were prepared using these powders. YBCO and half of the required silver powder were mixed intimately and then pressed into pellets under 200 MPa uniaxial pressure. The pellets were first heated to 600°C in air and allowed to stay at that tempera- ture for 2 h so as to remove any possible organic contaminant. Then, the temperature was increased to 915°C in 2 h and maintained there for 5 h. After that, the samples were cooled down with 25°C/h cooling rate to 675°C and held there for 12 h. Finally, the samples were cooled down to 70°C at a cooling rate of 35°C/h. After the first pressing and sintering in air, the pellet samples were ground again into a composite powder with a maximum powder size of 45 Izm (325 mesh). This ground powder was mixed with the remaining half of the silver and pressed into pellets again under 200 MPa uniaxial pressure. Fi- nally, these pellets were heat treated using the same

S ~ mp

Fig. 1. Bromination system.

temperature cycle as before, this time in flowing oxygen.

Rectangular bars about 0.5 X 0.8 mm 2 in cross- section area and 10 mm in length were cut from these composite pellets and ground with 400 grit SiC paper. Resistance versus temperature and voltage versus current measurements were made by the four-point probe technique. The criterion of 1 ~ V / c m was used to take the Jc value. High-purity silver paint was used for contacts.

3. Results and discussions

The T~ value of both composites was found to be about 90°K, which is the same as that of pure YBCO sintered by the same process. The electrical resistivi- ties above the critical temperature ( T > T c) were much lower in s i lver-YBCO composites than that in pure YBCO as expected. The transition width AT (the temperature range between zero and normal conductivity) was about 5 K in an undoped compos- ite and 3 K in a Br doped composite compared to about 3 K in the pure YBCO. The major differences between Br doped and undoped composites are out- lined below.

Fig. 2 shows the voltage versus current plot of the composites made with and without Br doping. It must be noted that these are randomly oriented, non-textured composites of anisotropic YBCO and silver. Therefore, these current density values should not be compared with those of oriented (textured) material which are processed using more compli-

25

i

too I a I I ! I I I

0 200 300

- g l - ]1- er -cb~

2O

5

O r . ! . . . .

400 500 600

358 C. Wei, S.M. Mukhopadhyay / Physica C 259 (1996)356-360

Current Density (A/era'2)

Fig. 2. Voltage vs. cunrent density plots for Ag-YBCO superconducting composites with and without bromination at 77 K.

cated methods [22-25,31 ]. Our aim is to investigate what changes, if any, can be brought about by the presence of bromine. It is clear from Fig. 2 that the change is in the positive direction; viz. bromine enhances the current-carrying capacity of the com- posite. The critical current density (Jc) for the un- doped specimen was about 25 A / c m 2 while that of the doped specimen was 350 A / c m 2. This is a significant increase which immediately shows that halogen doping of the oxide may be a very simple and yet unexplored route of improving interracial transport in oxide-metal superconducting compos- ites.

The microstructure of the two composites have been studied by both optical and scanning electron microscopes. Sample micrographs ( × 1000 magnifi- cation) are shown in Figs. 3(a) and (b). It can be seen that the presence of Br leads to some noticeable

changes: there is a significant increase in porosity and moderate increase in particle size caused by this dopant. These measurements, averaged over a few samples and several micrographs, have been tabu- lated in Table 1.

It is very interesting to note that bromine doping strongly increases the critical current density despite a twenty-fold increase in overall porosity. This im- plies that if one can suppress the porosity in these systems, the beneficial effect of bromine on the overall Jc can be further enhanced. The increases in YBCO grain size and silver-particle size indicate that the interface/volume ratio of the superconducting system decreases with bromine. Whether that is caused by a change in atomic mobilities or by an increase in interracial energies needs further investi- gation. To the best of our knowledge, the influence of a halogen dopant on any of these parameters has

Table 1 Comparison of undoped and Br doped Ag-YBCO composites Sample Average Pore area Average Average size Je

pore size fraction size of Ag particles of YBCO grains (A/cm 2) (~m) (%) (Izm) (~m)

Undoped 2.7 0.36 7.5 4.0 25 Br doped 5.5 6.18 10.0 5.0 350

C. Wei, S.M. Mukhopadhyay / Physica C 259 (1996)356-360 359

Fig. 3. SEM photographs ( × 850) of Ag-YBCO composites (32 wL% Ag) (a) without bromine doping of YBCO; (b) with bromine doping of YBCO.

not been studied. This will be one of our future focus areas.

4. Concluding remarks

In summary, we have shown that doping of the superconducting copper oxide with a halogen gas prior to bonding with silver can have a major influ-

ence on the oxide superconductor-metal composite. It can improve the critical current density signifi- cantly, despite an increase in overall porosity, indi- cating that there is scope of still greater improvement if the porosity can be suppressed. There is also an apparent increase in grain growth caused by bromine. The atomistic mechanisms of these changes need to be further explored.

360 C. Wei, S.M. Mukhopadhyay / Physica C 259 (1996) 356-360

Acknowledgement

This work is supported by the National Science Foundation, grant # DMR9521888.

References

[1] J.P. Singh, H.J. Leu, R.B. Poeppel, E. Van Voorhees, G.T. Goudey, K. Winsley and D. Shi, J. Appl. Phys. 65 (1989) 3154.

[2] J.P. Singh, J. Joo, N. Vasanthamohan and R.B. Poepel, J. Mater. Res. 8 (1993) 2458.

[3] J.P. Singh, J. Joo, D. Singh, T. Warzynski and R.B. Poepel, J. Mater. Res, 8 (1993) 1226.

[4] J. Joo, J.P. Singh, T. Warzynsk, A. Grow and R.B. Poeppel, Appl. Supercond. 2 (1994) 401.

[5] C. Vipulanandan and S. Salib, J. Mater. Sci. 30 (1995) 763. [6] T. Nishio, Y. Itoh, F. Ogasawara, M. Suganuma, Y. Yamada

and U. Mizumni, J. Mater. Sci. 24 (1989) 3228. [7] D. Lee and K. Salama, Jpn, J. Appl. Phys. 29 (1990) L2017. [8] A. Matsumuro, K. Kasumi, U. Mizutani and M. Senoo, J.

Mater. Sci. 26 (1991) 737. [9] F. Yeh and K.W. White, J. Appl. Phys. 70 (1991) 4989.

[10] G. Kozlowski, I. Marttense, R. Spyker, R. Leese and C. Oberly, Physica C 173 (1991) 195.

[11] N. Savvides, A. Katsaros and S.X. Doll, Physica C 179 (1991) 261.

[12] M.T. Malachevsky, P.L. Villa and L. Gherardi, Appl, Super- cond. 2 (1994) 35.

[13] A.G. Schrott, K.N. Tu, N.C. Yeh, G. Singco, A. Levi and C.C. Tsuei, Phys. Rev. B 39 (1989) 2910:

[14] N. Savvides, A. Katsaros and S.X. Dou, Physica C 179 (1991) 3611.

[15] Q.Y. Ma, M.T. Shmidt, L.S. Weinman and E.S. Yang, J. Vac. Sci. Technol. A 9 (1991) 390.

[16] J.W. Ekin, A.J. Panson and B.A. Blankenship, Appl. Phys. Lett. 52 (1988) 331.

[17] T.W. Jing, Z.Z. Wang and N.P. Ong, Appl. Phys. Lett. 55 (1989) 1912.

[18] S. Jin, M.E. Davis, T.H. Tiefel, R.B. Van Dover, R.C. Sherwood, H.M. O'Bryan, G.W. Kammlott and R.A. Fast- nacht, Appl. Phys. Lett. 54 (1989) 2605.

[19] B.D. Hunt, H.C. Foote and R.P. Vasquez, Appl. Phys. Lett. 56 (1990) 2678.

[20] E.W. Collings, Adv. Supercond. II, eds. T. Ishiguro and K. Kajimura (Springer, Tokyo, 1990) p. 327.

[21] T. Ogasawara, Crygenics 29 (1990) 3; ibid., 29 (1990) 6. [22] M.J. Day, S.D. Sutton and J.S. Abell, Cryogenics 33

(1993) 113. [23] J.A. Xia, H.T. Ren, P.R. Munroe, Y. Zhao, H.K. Liu, C.

Andrikidis and S.X. Dou, J. Supercond. 7 (1994) 947. [24] C.S. Zhao, Y.B. Huang, G.K. Wang, Y.M. Wang and F.X.

Deng, Supercond. Sci. Tech. 3 (1990) 587. [25] T. Yamamoto, G. Temi and T. Ueyama, Jpn. J. Appl. Phys.

32 (1993) 4496. [26] J.R. Chen, T.Y. Tseng and W.S. Chung, Inter. J. Mater.

Prod. Tech. 8 (1993) 395. [27] D. Veretnik and S. Reich, J. Appl. Phys. 73 (1993) 8429. [28] Y. Matsuoka, E. Ban, H. Ogawa and K. Kurosawa, Super-

con& Sci. Tech. 4 (1991) 528. [29] B. Srinivas and G.V.S. Rao, Bull. Mater. Sci. 15 (1992) 403. [30] H.R. Khan, J.R. Thompson and J.G. Ossandon, Supercond.

Sci. Tech. 4 (1991) 133. [31] T. Kato, N. Yamada and A. Imai, Physica C 185

(1991) 2423. [32] J. Leszczynski and J. Jackiewicz, Supercond. Sci. Tech. 5

(1992) S320. [33] T. Petrisor, A. Giurgiu and L. Ciontea, Appl. Supercond. 1

(1993) 1219. [34] C.N. Vanhuong, M. Nicolas, A. Dubon and C. Hinneu, J.

Mater. Sci. 28 (1993) 6418. [35] E. Yanmaz, I.H. Mutlu, T. Kucukomeroglu and M. Altunbas,

Supercond. Sci. Tech. 7 (1994) 903. [36] F.L. Tang, Q.H. Yuan and Y.Z. Yuan, J. Mater. Sci. 30

(1995) 347. [37] N. Frowmin, J. Baram and M. Polak, Physica C 209

(1993) 315. [38] J.H. Weaver, Interfaces in High-T e Superconducting Sys-

tems, chapter 7, eds. S.L. Shinde and D.A. Rundman (Springer, Berlin, 1993) p. 210.

[39] H.M. Meyer III, D.M. Hill, T.J. Wagener and J.H. Weaver, J. Appl. Phys. 65 0989) 3130.

[40] S.M. Mukhopadhyay, C. Wei and C.S. Chen, Appl. Super- con& 3 (1995) 7.

[41] Yu.A. Ossipian and O.V. Zbarikov, Physica C 162-164 (1989) 79.

[42] Y. Wang, A.M. Rao, J.G. Zhang, X.X. Bi, P.C. Eklund, M.S. Dresselhans, P.P. Nguyen, J.S. Moodera, G. Dresselhans, H.B. Radousky, R.S. Glass, M.J. Fluss and J.Z. Liu, Phys. Rev. B 45 (1992) 5.

[43] Y. Fukuda, N. Sanada, Y. Suzuki, T. Goto, M. Nagoshi, Y. Syono and M. Tachiki, Phys. Rev. B 47 (1993) 418.