Physica C 470 (2010) 1369–1372

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Physica C

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Effect of filament twist and high resistive barrier on the propertiesof Ag-sheathed Bi2223 tape

Y. Nakamura *, Y. Fujiwara, A. Nagaoka, T. Machida, R. Inada, A. OotaToyohashi University of Technology, 1-1 Tempaku-cho, Toyohashi, Aichi 441-8580, Japan

a r t i c l e i n f o

Article history:Available online 20 May 2010

Keywords:Bi2223Filament twistOxide barrierCritical current density

0921-4534/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.physc.2010.05.115

* Corresponding author. Tel.: +81 532 44 6734; faxE-mail address: nakamura@eee.tut.ac.jp (Y. Nakam

a b s t r a c t

Although the introduction of filament twisting with high resistive barrier between the filaments is effec-tive to reduce the AC loss, the Jc value of the tape was easily decreased by the filament twisting and inser-tion of high resistive barrier. This decrease in Jc is a problem to achieve low AC loss and high Ic tape. In thisstudy, the effects of filament twist and high resistive barrier on the microstructure and Jc property wereinvestigated to overcome the Jc reduction. The Bi2223 tapes with filament twist or oxide barrier were fab-ricated by powder-in-tube method. The Jc value of twist tape gradually decreased as decreasing the twistpitch, and the degree of Jc reduction was small in slim tape compared to wide tape. This reduction wouldattribute to the small actual cross sectional area of the filament perpendicular to the current directionbecause the filament has a tilt angle from the tape axis. On the other hand, the conversion ratio ofBi2223 became small as increasing the thickness of SZO barrier layer. Since the SZO barrier has less reac-tivity with 2223 filament, this suppression of the Bi2223 formation would not be caused by the formationof impurity phase but would be caused by the suppression of gas release from the filaments by oxide bar-rier. The conversion ratio of Bi2223 increased in the short sample where the gas could be released fromthe both end of the tape. These results suggest that the gas diffusion from filaments would be importantfor the formation of Bi2223. Hence, the use of slim tape for decreasing the tilt angle of the filament andkeeping the diffusion path of gas from the filaments may improve the Jc value of twist and barrier tapes.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Ag sheathed Bi-2223 tapes are ready for many prototype appli-cations such as cables and transformers. Although the critical cur-rent (Ic) values higher than 180 A and the length of about 1400 mare enough for prototype applications [1], their AC loss is still toohigh for the practical AC applications. This high AC loss value ismainly attributed to the electromagnetic coupling between fila-ments under AC external magnetic field. Hence, this coupling be-tween the filaments must be suppressed to reduce the AC loss.

For this purpose, filament twist plays a substantial roll to atten-uate the coupling current between filaments in long tape. Thereduction of AC loss by the filament twist was confirmed experi-mentally [2–7], and the AC loss became low as decreasing the twistpitch, which was consistent with the theoretical prediction. Inaddition to filament twist, the introduction of high resistive barrierbetween filaments is effective to interrupt the coupling currentand to reduce the AC loss. Many barrier materials and processeswere proposed to introduce the high resistive barrier into Agsheathed Bi-2223 tapes. Most popular method to introduce the

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barrier is the use of oxide powder such as SrZrO3, SrCO3 and Sr–Ca–Cu–O oxide [7–11], which was usually painted outside of themonofilament wire as a paste. Another method called in situ oxida-tion method is the use of metal layer such as Cu and Ni, which isoxidized during the sintering process after mechanical deforma-tion [9,12–15]. These monofilament wires with barrier materiallayer on their surface were stacked and mechanically deformedto the multifilament tape.

The filament twist and insertion of barrier are required to re-duce the AC loss but decrease the Jc value of those tapes. For prac-tical application, the high Jc value and low AC loss must becompatible. The reason for the decrease in Jc of the twist and bar-rier tapes was not confirmed, but several possibilities were pro-posed. In twist tapes, the poor grain alignment caused byfilament twisting was proposed to be a reason for low Jc [2,16].On the other hand, the reasons for low Jc in barrier tapes weremainly discussed from the reaction between barrier material andfilaments, and Kovác [17] proposed the oxygen diffusion affectedthe Jc of barrier tapes. In our previous study [12,13], the reactionspeed and conversion ratio of Bi2223 were decreased in the barriertapes fabricated by in situ oxidation method, and this low volumefraction of Bi2223 was thought to be a reason of low Jc. However,the reason for the low conversion ratio in barrier tape is not clear.

Crit

ical

cur

rent

den

sity

, Jc (

A/c

m2 )

0

5000

10000

15000

20000

25000

30000

0 10 20 30 non-twistTwist pitch (mm)

4.2 mm3.3 mm2.4 mm

tape width

Fig. 1. Effect of twist pitch on the Jc value of the twisted and non-twisted tapes.

g)

50

60longitudinal direction

θfilament

1370 Y. Nakamura et al. / Physica C 470 (2010) 1369–1372

In this study, the effects of filament twist and insertion of barrieron Jc and formation of Bi2223 were investigated to see the reasonfor Jc reduction.

2. Experimental

The average composition of precursor was selected as Bi:Pb:Sr:-Ca:Cu = 1.8:0.3:1.87:2.0:2.6, and the precursor powder was pre-pared by the three-powder process [18]. 19-filamentary Bi2223tapes with filament twist or with inter-filamentary oxide barrierwere fabricated by the powder-in-tube method using Ag–2 wt.%Cualloy sheath. The wires with diameters of 2.14, 1.63 and 1.15 mmwere twisted to see the effect of tape width, and the initial twistpitches of 2, 5, 10 mm, and 1 (non-twisting) were selected fortwist tapes. These twisted wires were rolled to the same finalthickness about 0.25 mm. The fabricated tape width and final twistpitches were summarized in Table 1. On the other hand, two typesof barrier tapes were fabricated using CuO and SrZrO3 (SZO) forbarrier material. The CuO barrier tape was fabricated by in situ oxi-dation method [13–15], and the SZO barrier tape was also fabri-cated by painting the SZO oxide powder mixed with 20% Bi2212on the monofilament wire [7]. The initial thickness of SZO layerwas selected in the range from 85 to 185 lm to see the effect ofbarrier thickness. These wires covered with barrier material werestacked to form 19-filamentary wires, and these barrier tapes wererolled to the final thickness of about 0.4 mm. The tapes were ini-tially sintered at the temperature of 840 �C for several periods upto 80 h, rolled to the final thickness and finally sintered at 840 �Cfor 150 h. All sintering process was performed in air.

The microstructure of the samples was observed with opticalmicroscopy and SEM–EDX, and the phase assemblage was ana-lyzed with XRD in the middle of the tape. From the XRD results,the conversion ratio of Bi2223 was calculated from the peak inten-sities of Bi2223 and Bi2212 as the ratio of %2223 = I2223/(I2223 + I2212), where I2223 was the sum of intensities of the(0 0 1 0) and (0 0 1 4) faces of Bi2223 and I2212 was that of the(0 0 8) and (0 0 1 2) faces of Bi2212. Transport critical current den-sity (Jc) was evaluated at 77 K under self-field by the standard four-probe method with the criterion of 1 lV/cm.

3. Results and discussion

3.1. Effect of filament twisting on Jc

Fig. 1 shows the effect of twist pitch on the Jc values of Bi2223tapes with different width. The Jc values of non-twisted tapes werehigher than 2 � 104 A/cm2. The reduction of Jc was not so largewhen the twist pitch was larger than 20 mm. However, the Jc valuedecreased rapidly as decreasing twist pitch smaller than 15 mm.

Table 1Specifications of the tapes with and without filament twisting.

Initial twist pitch Non-twist 10 mm 5 mm 2 mm

u2.14 Width (mm) 4.1 4.2 4.2 4.2Thickness (mm) 0.26 0.26 0.26 0.27Twist pitch (mm) 1 30.3 20.0 12.3Tilt angle (deg) 0 16 24 40

u1.63 Width (mm) 3.3 3.4 3.4 3.3Thickness (mm) 0.25 0.24 0.25 0.25Twist pitch (mm) 1 27.6 13.5 8.3Tilt angle (deg) 0 15 28 42

u1.15 Width (mm) 2.4 2.4 2.4 2.4Thickness (mm) 0.24 0.23 0.24 0.24Twist pitch (mm) 1 18.6 10.0 4.4Tilt angle (deg) 0 16 27 53

The Jc values of slim tape tended to be higher than those of widetape compared at similar twist pitch. This tendency with tapewidth would be explained from the difference in filament tilt anglefrom the longitudinal direction of the tape. In the twist tape, theactual direction of the superconducting filament is not along thetape direction but has tilt angle in the flat plane from the tape axis.The relation between the twist pitch and tilt angle, h, is shown inFig. 2. As shown in this figure, the tilt angle of slim tape is smallerthan that of wide tape at the same twist pitch. Supposing that theactual Jc value along the filament, J0c , is same as that of non-twistedtape as a first approximation, the Jc value along the tape directiondecreased as Jc ¼ J0c cos h [16] because the actual cross section of fil-ament perpendicular to the current direction was smaller than thatperpendicular to the tape direction. As a result, the reduction of Jc

becomes large as decreasing the twist pitch and increasing the tiltangle. From Figs. 1 and 2, the decrease in Jc seemed to be small inthe tape with small tilt angle of the filaments. The Jc values of thetwist tape normalized with Jc of non-twisted tape, Jc0, were plottedagainst the filament tilt angle as shown in Fig. 3. The cos h line is

Tilt

angl

e, θ

(de

Twist pitch (mm)0 10 20 30 40

0

10

20

30

40

4.2 mm3.3 mm2.4 mm

tape width

Fig. 2. The relation between the twist pitch and filament twist angle of the tapewith different width. The inset schematically shows the image of tilt angle in thewide plane of the twist tape.

Nor

mal

ized

Jc, J c

/Jc0

Tilt angle, θ (deg)

0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60

cos θ

4.2 mm3.3 mm2.4 mm

tape width

Fig. 3. The relation between the normalized Jc and filament tilt angle.

0 200 400 600 800 1000 1200 1400 1600 1800 2000

-5

-4

-3

-2

-1

0

Time (min)

400500

600

700

800

900

Δm (%

)

Tem

pera

ture

(°C

)

Fig. 4. Result of TG-analysis of the precursor powder used in this study.

100

Y. Nakamura et al. / Physica C 470 (2010) 1369–1372 1371

also shown in this figure, and the normalized Jc obtained in thisexperiment decreased along the cos h line. This suggests that theJc value of twist tape is substantially controlled by the tilt anglefrom the longitudinal direction of the tape. Since this Jc reductionby filament tilt angle is inevitable, the tilt angle of filament shouldbe small to attain high Jc in twisted tape. In addition, if the grainalignment of Bi2223 grains is poor, the Jc value decreased morethan the cos h line as pointed out in the our previous report [16].Of cause, the tilt angle depends on the position of the filamentsin the tape; the 19 filamentary tape consists of three layers that in-clude 1, 6 and 12 filaments in each layer. The tilt angle discussedabove is that of the outer most layer, and the angels of filamentsin the inner layers were smaller than that of outer layer. This willincrease the Jc/Jc,0 value from the cos h line, but the substantial ef-fect of tilt angle is not changed. It should be noted that the effect offilament twist is much more complex. For example, since the crosssectional area perpendicular to the current depends on the posi-tion, the current density of the filament at the edge of the tape islower than the Jc value. As a result, the electrical field along the fil-ament is not constant and depends on the position. The detailedanalysis about this effect is now going on and will present anotherarticle in near future.

0

20

40

60

80

Con

vers

ion

ratio

, %22

23 [%

]

Sintering time (h)0 10 20 30 40 50 60 70 80 90

85 µm125 µm165 µm

Initialbarrier

thickness

Fig. 5. Effect of barrier thickness on the conversion ratio of Bi2223 in the SZObarrier tapes.

3.2. Effect of barrier on the formation of Bi2223

In our previous study [14,15], the Jc values of barrier tape werelower than those of non-barrier tapes. A reason for this low Jc wasconsidered to be the low conversion ratio of Bi2223 in CuO barriertape fabricated by in situ oxidation method. In general, the reasonsfor this low conversion ratio are considered to be the formation ofimpurity phases and the suppression of gas diffusion by barrierlayer. In the CuO barrier tapes, the non-superconductive impurityparticles such as (Sr, Ca)14Cu24Ox phase were easily formed dueto the diffusion of Cu into the filaments. On the other hand, tosee the effect of gas diffusion, TG analysis was performed for pre-cursor powder using the first sintering pattern, and the result isshown in Fig. 4. The weight of precursor gradually decreased asincreasing temperature and suddenly decreased at 500–700 �C.During the holding at temperature of 840 �C for 20 h, the weightwas continuously decreased. This means that some amount ofgas, which may be oxygen, was released from precursor for the for-mation of Bi2223. So if this gas was not released smoothly fromprecursor, it might suppress the formation of Bi2223 phase. To

confirm this, the effect of barrier thickness on the conversion ratioof Bi2223 was investigated using the SZO barrier that is less reac-tive material with Bi2223 filament. Fig. 5 shows the conversion ra-tio of Bi2223 in 12 cm long tapes with different barrier thickness.As shown in this figure, the conversion ratio decreased as increas-ing the barrier thickness. This suggests that the suppression of gasdiffusion by oxide barrier would be a reason for low conversion ra-tio since the barrier material has low reactivity with filament. Sup-posing that the released gas is oxygen, the suppression of oxygendiffusion by barrier may increase the oxygen potential in filament,and the resultant high oxygen partial pressure would suppress theformation reaction of Bi2223 in filament.

If this suppression of gas diffusion is a reason for low conversionratio of Bi2223, the formation of Bi2223 could be enhanced in shortsample in which the gas can released from the both ends of thetape. Fig. 6 shows the effect of sample length on the conversion ra-tio of Bi2223 in CuO barrier tapes. As clearly shown in this figure,the conversion ratio of short sample of 0.5 cm in length was higherthan that of 8 cm tape. This means that the release of gas from thefilaments would be important for the formation of Bi2223 in bar-rier tape.

80

0

40

20

100

60

30 4020100Sintering time (h)

Con

vers

ion

ratio

, %22

23 (%

) 8 cm0.5 cm

tape length

Fig. 6. Effect of tape length on the conversion ratio of Bi2223 in the tapes withinitial Cu layer thickness of 40 lm.

1372 Y. Nakamura et al. / Physica C 470 (2010) 1369–1372

4. Summary

The reasons for the reduction of Jc by filament twist and byinsertion of barrier were investigated. The Jc value of twist tapewas gradually decreased as decreasing the twist pitch, and the de-gree of Jc reduction was small in slim tape compared to wide tape.This reduction would be explained from the tilt angle of filamentfrom the tape direction. The decrease in tape width can reducethe tilt angle for the constant twist pitch, so the use of slim tapemay improve the Jc value of twist tape. On the other hand, the con-version ratio of Bi2223 was found to become small as increasingthe barrier thickness of the tapes with SZO oxide barrier. Sincethe SZO barrier has less reactivity with 2223 filament, this suppres-sion of the formation of Bi2223 would not be caused by the forma-tion of impurity phase. Since the TG analysis showed decrease ofweight during heating, this suppression of Bi2223 formation wouldbe caused by the suppression of gas release from the filament bythe oxide barrier layer. The increase in conversion ratio of Bi2223

in the short sample also supports the idea that the gas diffusionfrom the filaments would be important for the formation ofBi2223. In conclusion, the reduction of Jc value in twisted tapecould be suppressed by decreasing the tilt angle of the filamentby using slim tape. In the case of barrier tape, the formation ofBi2223 in barrier tape would be improved by keep the diffusionpath of gas from the filaments, and this improvement of volumefraction of Bi2223 may improve the Jc value of barrier tapes.

Acknowledgement

The authors would like to thank Mr. K. Furuya of AGC SeimiChemical co. Ltd. for TG analysis of precursor powder.

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