Development of high performance Ag sheathed Bi2223 wire

Physica C 412–414 (2004) 1066–1072 www.elsevier.com/locate/physc

Development of high performance Ag sheathed Bi2223 wire T. Kato *, S. Kobayashi, K. Yamazaki, K. Ohkura, M. Ueyama, N. Ayai, J. Fujikami, E. Ueno, M. Kikuchi, K. Hayashi, K. Sato Sumitomo Electric Industries, Ltd., 1-1-3, Shimaya, Konohana-ku, Osaka 554-0024, Japan Received 29 October 2003; accepted 9 March 2004 Available online 4 July 2004

Abstract A silver sheathed Bi2223 wire is widely used to build various prototype models taking advantage of longer length wires with a relatively high critical current. The Bi2223 wire is expected to early realize full-scale practical use in the area of cables, magnets and so on. However, further improvements in some aspects of performance are required to realize commercial products. In view of critical current (Ic ), Ic at around 77 K in lower magnetic fields and at a lower temperature in higher magnetic fields are important for a cable system and a magnet system, respectively. The improvements in phase homogeneity, grain alignment and good connectivity between grains are mainly necessary for Ic at a higher temperature. For Ic at a lower temperature, the improvement in the property of grain boundary is considered to be effective due to the decrease of the coherent length. In order to drastically change the above factors, we have been studying a new sintering process under a pressurized atmosphere. This new process has a significant effect on the increase of the critical current, and further on the enhancements of mechanical properties and reliability in thermal cycles. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.25.Fy; 74.25.Ld; 74.60.Jg; 74.72.Hs Keywords: Bi2223 wire; Pressure sintering; Relative mass density; Bi2223 phase homogeneity; Connectivity

1. Introduction Many projects using Bi2223 wires for power transmission cables, magnets and other applications have been successfully conducted [1–8]. One year long verification test of a 100 m 3-phase cable on the scale of practical use in Yokosuka was

*

Corresponding author. Tel.: +81-6-6466-5634; fax: +81-66466-5705. E-mail address: [email protected] (T. Kato).

accomplished without problems in Bi2223 wires and the system [9,10]. The achievements have ensured the availability of the Bi2223 wires for use in various applications, and are further accelerating the developments for the full-scale practical use. One of them is the Albany cable project, which will demonstrate the 350 m long length HTS cable in the practice Root in Albany city, NY, USA [11]. We have developed long length Bi2223 wires with the Ic of 110A and the engineering current density Je of 12.3 kA/cm2 at 77 K in a self-magnetic field [12,13]. However, the full-scale practical

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T. Kato et al. / Physica C 412–414 (2004) 1066–1072

use requires further improvements in properties of Bi2223 wires. Jc is one of most important factors. Magneto optical image demonstrated the regional maximum Jc value of greater than 300 kA/cm2 at 77 K in Bi2223 mono-filamentary tape, although the transport Jc of wire was around 40 kA/cm2 [14]. This result suggests that the improvement in the connectivity and the extension of the high-Jc regions would result in the higher transport Jc . The transport Jc obtained so far was too low considering the potential of the Bi2223. We classified the factors which affected Jc of Bi2223 wires into 3 factors: Bi2223 phase homogeneity, grain alignment and connectivity. In our past studies, we had found strong correlations between Jc and homogeneities [15] and, between Jc and grain alignments [16,17]. The highest Jc was obtained for the highest volume fraction of Bi2223 phase and the smallest misalignment angle, respectively. Connectivity is considered to depend on the porosity, the unhealed defect caused by the rolling process and the property of the grain boundary. Some issues to be improved in the sintering process are (1) mass density is lowered by growing Bi2223 with certain angles, (2) reactivity to Bi2223 phase is interfered by decreasing the mass density, and (3) defects by rolling can not be recovered perfectly. We applied pressure sintering to the Bi2223 wires in order to overcome these issues. In this paper, we report the results of the Jc improvement and the mechanical properties by densifying the filament to the relative density of 100%.

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else. The pressure sintering in this paper utilized a gas flow system with a booster for raising the pressure of greater than 15 MPa. The gas is a mixture of inert gas and O2 . The O2 concentration of the gas was chosen so that the O2 partial pressure would be a certain value from 7 through 20 kPa during the pressure sintering. We optimized the sintering condition for the higher Jc in each experiment. The temperature used in this experiment was between 810° and 835°. The mass density of the filament was calculated from the mass density of the wire obtained by Archimedes method and the silver ratio. The misalignment angle of the Bi2223 phase was measured by the rocking curve. The microstructure was observed with SEM. DC magnetization was measured with SQUID for the pressed wire to examine the amount of Bi-based superconducting phases (Bi2223, Bi2212 and Bi2201) [18,19]. Defects in filaments were observed with magneto optical imaging (MO) method using surface polished samples. Ic was measured at 77 K in a self magnetic field, and Ic was defined by 1 lV/cm. Tensile stress test was curried out at room temperature and 77 K. Ic was measured at 77 K after the tensile stress tests at each temperature. The critical tensile stress and the bending strain are defined as the highest value at which Ic =Ic0 is sustained more than 95%.

3. Results and discussion 3.1. Jc enhancement at 77 K

2. Experimental Ag-sheathed Bi2223 superconducting wires were prepared using the powder in tube method. The number of filaments was 55. The silver ratios were 1.4–2.2, and the silver-alloy was used to increase the mechanical properties. We applied the pressure sintering to the wires after first heat treatment and intermediate rolling process. The length of the wire was chosen to be 1 m, which was limited due to the size of the reaction chamber. Both ends of the wire were sealed, and the other parts were not coated or wrapped with a metal or

The pressure sintering increased Jc up to 37 and 40 kA/cm2 in the wire of the silver ratio of 1.5 and 2.2, respectively. Ic were 123 and 92 A. The sizes of the wires were about 4.3 mm in width and 0.21 mm in thickness. Jc with the normal sintering process was 26 kA/cm2 . It was found that the Jc was increased by greater than 30% by the pressure sintering method. 3.2. Relative mass density Fig. 1 shows the relative mass density of the filaments in the wire after the intermediated

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alignment angles, growing the Bi2223 grains results in the decrease of the mass density. Therefore, it is found that the pressure sintering can suppress the grain growth of no use and minimize the volume of the pores.

Relative density (%)

110%

100%

90%

3.3. Misalignment 80%

Pressure sintering

Normal sintering

Intermediate rolling

70%

Fig. 1. The relative mass density of Bi2223 wires with intermediate rolling, normal sintering and pressure sintering.

The rocking curve measurements reveal that the misalignment angles were reduced by about 1° by the pressure sintering. The increase in the mass density leads to the decrease in the thickness of the filaments. So, this result suggests that the pressure sintering can orient the Bi2223 grains by decreasing the filament thickness so that the Bi2223 grains would go along the interface between the silver and the filament. 3.4. Bi2223 phase homogeneity

rolling, the normal sintering and the pressure sintering. The ideal mass density of the Bi2223 in this study was considered to be 6.3 g/cm3 [20]. The relative mass densities of the filament after the intermediate rolling, the normal sintering and the pressure sintering were found to be 93%, 88% and almost 100%, respectively. The data reveal that the pressure sintering can successfully increase the mass density in contrast to the normal sintering. The decrease of the mass density in the normal sintering is considered due to the grain growth of the Bi2223. Since the Bi2223 grains have mis-

Fig. 2 shows SEM images of (a) the normal sintered wire and (b) the pressure sintered wire. Black regions in the images are (Ca,Sr)–Cu–O or pores, and small and dispersed ones are considered to be pores. The pores are found to disappear by the pressure sintering, although many pores can be seen in the filament after the normal sintering. This is consistent with the result of the increase of the mass density which was considered due to the disappearance of the pores. This also means the volume fraction of Bi2223 phase is increased by pressure sintering method.

Fig. 2. (a, b) are the SEM photographs of the normal sintered wire and the pressure sintered wire, respectively. The gray regions, black regions, white regions and tiny irregular black regions are Bi2223, alkaline earth cuprates (AEC), Ca–Pb–O and pores, respectively.

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ple. The results indicate that pressure sintering method can accelerate the conversion from Bi2212 to Bi2223 phase. But, a few non- superconducting phase (Fig. 2) and residual Bi2212 phase (Fig. 3) are observed in filaments. The wire has room to be improved for higher Jc .

Normalized Susceptibility at 6K

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Pressure sintering -0.4

3.5. Connectivity

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60

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Fig. 3. DC magnetization curves of the normal sintered wire and the pressure sintered wire measured by SQUID. A magnetic field of 10 G was applied parallel to the wire surface.

The ratio of Bi2212 was calculated by the equation of Dm2212 =DmT which are shown in Fig. 4(a). Fig. 4(b) shows the ratio of Bi2212 to Bi2223 calculated from magnetization curves in Fig. 3. This characterization method was also suggested by Huang et al. [21]. They showed a strong correlation between critical currents and the amounts of the residual Bi2212 phase, and suggested that higher Jc would be achieved by eliminating residual Bi2212 phase. As can be seen in Fig. 4, the ratio of Bi2212 with pressure sintering process decreased compared to the normal sintered sam-

Fig. 5 shows Magneto-Optical images of (a) normal sintered wire and (b) pressure sintered wire. Magnetic field of 800 G was applied perpendicular to the wire surface at 10 K after zero field cooling (ZFC). In Fig. 5(a), high contrast lines which are indicated by the arrows can be seen, in contrast to the wire with the pressure sintering in Fig. 5(b). The lighter region indicates the higher magnitude of the transmitted magnetic field and the weaker superconducting region. Therefore, this result implies the defects caused by the rolling process were healed by the pressure sintering. This result indicates the higher reactivity in the pressure sintering, and agrees with the fact that the pressure sintering increased the volume fraction of Bi2223. The average magnetization level is significantly large in the wire after pressure sintering, showing the improvement in the connectivity between the grains. This shows the Jc increase which was considered to result from the improvement in the Bi2223 phase homogeneity, the misalignment angle and the connectivity due to

0.6

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2212

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Fig. 4. (a) Schematic graph which describes the calculation of the Bi2212 ratio and (b) ratio of Bi2212 in the filaments obtained from DC magnetization curves in Fig. 3.

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Fig. 5. Magneto optical images at 10 K and 800 Oe of (a) normal sintered wire and (b) pressure sintered wire. The lines indicated by arrows are considered to correspond to the unhealed defects.

the densification and the recovery of the damaged region. 3.6. Jc enhancement at a low temperature The Jc at 20 K under the magnetic field of 3 T perpendicular to the wire surface, which is a typical value applied to 20 K operating magnet, were 68 kA/cm2 (Ic ¼ 248 A) and 60 kA/cm2 (Ic ¼ 219 A) with and without the post anneal after the pressure sintering, respectively. The mechanism of the Jc improvement is not completely understood, but it may relate to the

Fig. 6 shows Ic dependence on tensile stress for the wires with the silver ratio of 1.5 of the normal sintering and the pressure sintering. The critical tensile stresses at room temperature and 77 K were

1.0

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Silver ratio : 1.5

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improvement in the property at the grain boundary by increasing the carrier density caused by the O2 doping. Since the coherent length shortens with the decreasing temperature, the improvement in the property at the grain boundary is considered to be effective at a lower temperature.

Tensile stress at 77K Normal sintering Pressure sintering

0.2 100

150

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(b)

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Tensile stress (MPa)

Fig. 6. Ic dependence on the tensile stress of the normal sintered wire and the pressure sintered wire in the case of the silver ratio of 1.5. Ic and Ic0 mean critical currents after and before tensile stresses are applied, respectively. Tensile stresses were applied at room temperature (a) and at 77 K (b).

T. Kato et al. / Physica C 412–414 (2004) 1066–1072 Silver ratio : 2.2

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Pressure sintering

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Fig. 7. Ic dependence on the tensile stress of the normal sintered wire and the pressure sintered wire in the case of the silver ratio of 2.2. Ic and Ic0 mean critical currents after and before tensile stresses are applied, respectively. Tensile stresses were applied at room temperature (a) and at 77 K (b).

increased to 135 MPa from 85 MPa and 165 MPa from 105 MPa by the pressure sintering, respectively. The critical bending strain at room temperature was not changed with the pressure sintering. The critical tensile stress after the pressure sintering was drastically improved, although the critical bending strain was not increased. The stress-strain curve indicated the increase in the young modulus from 76 to 93 GPa. Therefore, the increase in the critical tensile stress was considered due to the increase in the young modulus and the improvement in the uniformity of the superconducting filaments. Fig. 7 shows Ic dependences on tensile stress for the wires with the silver ratio of 2.2 of the normal sintering and the pressure sintering. The improvements in the critical tensile stress were also found in the wire with the pressure sintering. High critical tensile stresses of 153 MPa at room temperature and 213 MPa at 77 K were achieved. The wire with a higher silver ratio with the pressure sintering can be promising for the applications which necessitate the higher mechanical properties.

4. Conclusions We focused on the sintering process for the improvement of the Bi2223 superconducting wires. The pressure sintering is one of promising methods and we tried to apply the pressure sintering method to the silver sheathed Bi2223 wires. At this moment, the Jc of the Bi2223 wires with the pres-

sure sintering method was increased by greater than 30% compared to the normal process. The Jc was enhanced by the improvements: (1) the relative mass density was reached to almost 100%, (2) Bi2223 phase becomes homogeneous and (3) connectivity was improved by reducing the pores and the defects. The pressure sintering method was also effective for improving the mechanical property. This improvement correlated closely with the increase in the mass density in the filaments. The increase in the relative mass density up to 100% will lead to the higher reliability not only in the mechanical properties but also in the long-term use where the wires are exposed to a number of thermal cycles and immersion in the cooling liquid. The pressure sintering was proved to have significant effects on both of the Jc and the mechanical properties. There is a hope that the pressure sintering also drastically improves the yield ratio by the higher uniformity along the wire, when it can be applied to the production of the long length wires. The pressure sintering can be an innovative process. Since the conditions of the pressure sintering are not optimized, it is expected that the wire could be dramatically further improved in the near future.

Acknowledgements The authors would like to thank Dr. Anatolii Polyanskii, Prof. David Larbalestier and members of University of Wisconsin-Madison for helpful

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advice concerning current limiting defects through personal communication and various meaningful evaluations, especially through Magneto Optical images on our samples which included many defects. They contributed to our better understanding of the connectivity of grains which is considered to be one of the factors limiting the critical current. The authors would also like to thank Dr. Takato Machi of the International Superconductivity Technology Center (ISTEC) for his cooperation and evaluations of Magneto Optical images presented in this paper. This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through the International Superconductivity Technology Center (ISTEC) as collaborative research and Development of Fundamental Technologies of Superconductivity Applications.

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