Processing and properties of silver-sheathed Bi2223 superconducting tapes

Physiea C 235-240 (1994) 1231-1232 North-Holland

PHYSlCA

Processing and Properties of Silver-Sheathed Bi2223 Superconducting Tapes Y.C. Guo, H.K. Liu and S.X. Dou

Centre for Superconducting and Electronic Materials, the University of WoUongong, Northfields Avenue, Wollongong, NSW 2522, Australia The microstructural and electrical properties of Bi2223/Ag tapes during the heat treatment process was investigated using XRD, $EM and electrical measurements. It was found tl:at the conversion process of Bi2212 to Bi2223 phase was a diffusion-controlled, two-dimensional reaction, which features a rapid Bi2223 phase increase at early sintering stages and then a slow increase with prolonged sintering. Mass density, grain alignment and grain growth improved during the entire heat treatment process, with significant improvement during early stages. Zero resistivity of tapes at temperature above 77 K was not observed immediately after the Bi2223 phase appeared~ but after continuous Bi2223 phase paths formed. The correlation between phase composition, mierostmcture and transport properties was also studied. As with all materials, the properties of superconductors depend on the microstructure. This in turn depends on the fabrication processes in which the materials were subjected. In this paper, results of a detailed investigation on the development of microstructures and properties of Bi2223/Ag tapes during the fabrication process arc reported. The silver-sheathed Bi2223 tapes used in this work were fabricated by the powder-in-tube teelmique, as described previously [1]. The tapes were heat treated with a process consisting of several cycles of pressing and sintering. Each sintering was carried out at about 830°C for 60120 hrs in air. After each cycle of heat treatment, the tapes were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical measurements (critical temperature, T c and critical current density, Jc)Fig.1 shows the Bi2223 phase volume fraction as a function of sintering time. It is seen that the Bi2223 phase increases rapidly during the early sintering stages, then increases slowly during the intermediate stages, and finally plateaus to a maximum value after prolonged sintering times. The kinetics of Bi2223 phase formation was analysed using the Avrami equation [2],

ln[-la(1-C)]

= (Int.o

- --zq--~D+

nln(O

where C is the volume fraction of Bi2223 phase transformed at time t, T is the temperature, and n is the Avrami exponent. Using this equation, an exponent value of n = 1.3 was determined, which is close to the theoretically determined Avrami exponent associated with a diffusion_controlled, two-dimensional transformation (i.e., n = 1.5).

1.0

8Ol-

=

; I

/

/

/

/

/-.

/

O /

0

8

0.4 ~

/

0.2

_/ J

0.6

/

/ /

0.0

/

I

I00

Ag-Clad Bi(2223) Tape ~

I

200

m.

I

300

u ,

~6~0

S i n t e r i n g T i m e (hrs)

Fig.1 J¢ together with Bi2223 phase fraction as a function of sintt.ring time for a Bi2223/Ag tape For this type of reaction mechanism, it is assumed that the nucleation step is completed very rapidly at the beginning of the reaction and that diffusion-controlled growth controls the extent of transformation. Figs. 2a-d are the cross-sectional SEM micrographs of tapes after different heat treatment stages. It is noted .L..,,,,,,t.~¢....~,,.,,,..,s;..~o~,g~.... (Fig.2a) the sample was a mechanical assemblage of individual, randomly oriented granular particles, exhibiting no preferential grain orientation. With the first pressing and 50 hrs sinteFtng (Fig.2b), some extent of grain alignment was observed, the core matrix became more compact, and intergain contact improved. As the heat treatment continues (Fig.2c, 160 hrs), the mass density, grain alignment and grain growth improved further, especially during the first two

0921-4534/94/S07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)01179-6

1232

YC. Guo et al./Physica C 235-240 (1994) 1231-1232

or three heat treatment stages. After the final cycle of pressing and sintering (Fig.2d), the core matrix of the tape became very dense and highly oriented, which is desirable for passing a high Jc"

for 100 hrs. Continuous Bi2223 phase paths formed and allowed current to flow without resistance, so zero resistivity is observed above 77 K. A very sharp, one-step drop with zero resistivity temperature of about 107 K was obtained after sintering at 830°Cfor 240 hrs. Ag-Clad Bi(2223) Tape

834"C Oh 50h

. ,...4

lOOh

160h

240h

,,.o . ,...i

ffl

I

v5

Fig.2 SEM micrographs for a Bi2223 sample at different sintering stages (830°C,0-380 hrs) Presented in Fig.3 is the resistivity as a function of temperature for the superconductor sample. It is seen that the resistivity of the precursor powder (Oh curve) decreases linearly from high temperature to about 80 K and then begins to drop markedly. This is consistent with the phase analysis results (Fig. 1), which indicated that the powder consisted primarily of Bi2212. With sintering at about 830°C for 50 hrs, the resistivity drops distinctly at about 110 K, corresponding to the Bi2223 phase, followed by a second drop at about 80 K, which can be attributed to the Bi2212. But no zero resistivity was observed above 77 K. The microstructural analyses show that the Bi2223 grains formed are finely distributed in the Bi2212 matrix. No continuous Bi2223 ph,ase path is formed at this stage, so no zero resi~tivily above 77 K results. It should be pointed out that although the transition temperature of the Bi2212 phase is often observed around 80 K, the zero resistivity ..... v. . . . . ~. can be much lower than /t Is. in the poorly developed samples. As the sintering proceeds, the resistivity drop at 110 K becomes more pronounced and gradually reaches zero at a temperature above 80 K. As seen in Fig. 1, the Bi2223 instead of Bi2212 phase became the maior phase in the tape after sintering at 830°C

I

I

I

I

100 lz5 1~0 175 Temperature (K)

I

200

Fig.3 Resistivity as a function of temperature for the Bi2223 sample at different sintering stages Fig. 1 also shows the Jc (77K) as a function of sintering time. It is seen that the initial increase in Bi2223 phase fraction does not lead to a marked Jc increase because no continuous Bi2223 phase path across the sample has developed by this stage, as discussed earlier. The Jc then increases monotonously with increasing sintering time due to the increase in the Bi2223 phase fraction and an improvement in the grain alignment and density. During the 240-300 hrs sintering period, the conversion process of Bi2212 to Bi2223 phase is nearly completed, with the phase composition inside the tape changing very slightly. During this period, however, the Jc still increases substantially due to the improvement in grain alignment, grain growth and intergrain connectivity. It is also noted that with prolonged sintering, the Jc of the tape sample begins to drop probably due to the cracks which caused by the mechanical deformation and could not be healed completely with further annealing. ,L ~ . = , . . . .It

x.~

'* L . . , t ' q ' ~ . . L , b . . . 1

I. Y.C. Guo, H.K. Liu and S.X. Dou, Appl. Supercond., 1-2 (1993) 25 2. J.W. Christian, The Theory of Transformati,-~s in Metals and Alloys, London, UK, 1965