IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 2507

Development of HTS BSCCO Wirefor Power Applications

H. P. Yi, X. H. Song, L. Liu, R. Liu, J. Zong, J. S. Zhang, A. K. M. Alamgir, Q. Liu, Z. Han, and Y. K. Zheng

Abstract—As the first company engaged in commercial manu-facture of HTS BSCCO wires in China, Innost has devoted greatefforts to improve the electrical performances and practical prop-erties of its HTS wire products. Based on huge amount of funda-mental research work, the short sample can achieve 120 A ofand 12 kA cm

2 of . Under strict quality control, Innost’s HTSwire can be manufactured reproducibly with length up to 1 km,

over 90 A and over 9 kA cm2. Meanwhile, to demonstrate

technical feasibility and reliability of HTS power apparatus, di-versified wire-design has been carried out, including reducing AClosses and thermal conductivity, increasing insulating propertiesand mechanical strength, and so on. Up to now, Innost’s diversifiedwire products have been widely used in domestic and internationalR&D projects of HTS application.

Index Terms—Application properties, Bi-2223/Ag tape, manu-facture technology.

I. INTRODUCTION

AS THE MOST promising material at the moment forsuperconducting electrical engineering applications,

Ag-sheathed Bi-2223 tapes face three main transport cur-rent-limiting factors: the Bi-2223 phase purity, grain alignmentand grain connectivity [1]. To improve both the reproducibilityand critical current of such tapes, a tremendous amount of efforthas been made in optimizing important process parametersincluding sintering temperature, soaking time, oxygen partialpressure, heating and cooling rate, cold working and precursorpowder processing.

Many applications require unit wire length of 300–500 m oreven 1 km. Not only the engineering current density , butalso the performance uniformity along the whole length, deter-mines the suitability of the Bi-2223 tapes for the applications.Furthermore the availability of diversified tape-design for insu-lating, lower AC losses and thermal conductivity, has to be con-sidered for special application such as HTS transmission cable,magnet and current leads, etc.

In this paper, progress in manufacturing technology and ap-plication properties of Bi-2223/Ag tapes at Innost is reported.

Manuscript received October 3, 2004. This work was supported by the Na-tional High Technology Research and Development Program (863 program)under contract 2002AA306113.

H. P. Yi, Q. Liu, and Y. K. Zheng are with the Department of MaterialsScience & Engineering, Tsinghua University, Beijing 100084 China (e-mail:yi.hanping@innost.com).

X. H. Song, L. Liu, R. Liu, J. Zong, and J. S. Zhang are with Innova Super-conducting Technology Co., Ltd, Beijing 100176 China.

A. K. M. Alamgir and Z. Han are with the Applied Superconductivity Re-search Center, Tsinghua University, Beijing 100084 China.

Digital Object Identifier 10.1109/TASC.2005.847506

II. PROGRESS IN MANUFACTURING TECHNOLOGY

Bi-2223/Ag tape was made using standard powder-in-tube(PIT) process. A pure Ag tube was filled with precursor with apowder packing density of 40%. After being drawn into roundwires of 2 mm in diameter and cut into 37–91 filaments whichwere then re-packed into a Ag alloy tube, the composite wassubjected to the same drawing process to a final diameter of

. The wire was then rolled to a flat tape. A first heattreatment was performed at 820 , using a flowing 8% -bal-ance atmosphere. Subsequently further heat treatments weregiven, after an intermediate rolling step.

A. Pre-Treatment of Precursors

Commercially available powders from Merck KGaA or self-made powders are used as precursors with a nominal composi-tion of . Based on the knowledgethat the precursor quality has strong influence on the perfor-mance of Bi-2223/Ag tape, we evaluated the reactivity of theprecursors by oxygen release process, which came from boththe decomposition of and the change of oxygen stoi-chiometry in the Bi-2212 phase [2]. Accordingly, pre-treatmentof the precursors was carried out to achieve a suitable phase as-semblage, among which we have found that the type of Pb-richphases, mainly and (3221)affected remarkably the Bi-2223 phase formation and texturingduring the subsequent tape sintering process. The results will bediscussed in detail in Section II-C “In situ annealing”.

B. Cold Working

The flat rolling process from wire to tape is presumably themost crucial link in the chain of mechanical processes leadingfrom loose powder and silver tubes to the final Bi-2223/Ag tape[3]. In order to improve the of the superconducting filaments,one must process these to the highest possible density withoutintroducing failures as large cracks and sausaging which reducesthe effective cross-sectional area of the current path and disturbsthe texture of the Bi-2223 grains. We performed 3D numer-ical modeling of the deformation process during flat rolling ofmultifilamentary composite wire by the finite element method.Comparison between numerical and experimental results showsgood agreement in filament geometry, evolution of core density,and length and width strain. For example, the homogeneity ofthe tape cross-section was improved when the length and widthstrain were equal. These results provide significant guidelinesto the optimization of rolling parameters, such as the number ofsteps, the reduction per step, the roll diameter, etc.

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2508 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

TABLE ISPECIFICATIONS OF LONG LENGTH Bi-2223 TAPES

Fig. 1. Continuous I measurement of the Bi-2223/Ag tape with 500 m inlength and a local resolution of 1 cm at 77 K, 0 T.

C. In-Situ Annealing

We studied the in situ annealing behavior of as-rolled tapesheated at 830 , in 8% , by using high energy syn-chrotron X-ray diffraction as a tool for characterizing textureand structural phases within the tapes during phase transforma-tion from Bi-2212 to Bi-2223. This experiment was carried outat the BW5 synchrotron beamline at HASYLAB, Hamburg.Two kinds of tapes were made of precursors with differentphase assemblages: precursor A was a mixture of Bi-2212 and

, and precursor B was a mixture of Bi-2212, 3221 anda little of . It was found that precipitatedin tape A and 3221 precipitated in tape B, at both heatingand cooling stages. Precursor B presented a well balancedphase composition, which promoted not only a fast Bi-2212 toBi-2223 phase transformation, but also helped to form a highlytextured structure. Compared to tape A, the size and amount ofsecondary phases of tape B decreased and its grain connectivitywas improved [4]. Moreover, tape B exhibited a higher trans-port property than tape A, according to measurement resultsof the tapes after the final heat treatment. These results indicatethat to control the type of Pb-rich phases is very important toimprove the of Bi-2223 tapes.

III. PRACTICAL PROPERTIES

The specifications of long length Bi-2223 tapes were listed inTable I.

Fig. 1 shows the continuously measured transport propertiesof a typical tape with 500 m in length, using a contact-free

Fig. 2. The magnetic field dependence of I at 27 K.

Fig. 3. Change of critical tensile stress of the Bi-2223 tape with differentsheath materials.

method (Hall probe) measuring the remanent magnetic fieldscreated by tapes when exposed to an external magnetic field.Defined as 1 criterion with a local resolution of 1 cm,

was over 90 A (77 K, 0 T) and was over 9 . Thevariance along the whole tape length is within 5 A. Our recentresults indicated that the maximum of short length samplesreached up to 120 A, corresponding to a value of 12 .

Fig. 2 shows the magnetic field dependence of at 27 K.The samples were cut from a long length tape with of 92 Aat 77 K, 0 T. The magnetic field was applied either parallel orperpendicular to the tape surface. It was found that the at 27K, 4 T (perpendicular field) was comparable to that of the sameBi-2223 tape at 77 K, 0 T.

Influence of sheath material on AC losses, mechanicalproperties and thermal conductivity of Bi-2223 tapes havebeen investigated. Considerable workability, enough oxygenpermeability and high chemical compatibility with the su-perconducting phases, greatly reduce the material choice forcomposite wires to a few silver rich alloys [5], which include:Ag-Cu, Ag-Mg, Ag-Mn, Ag-Mg-Ni, Ag-Li, Ag-Ce, Ag-In,Ag-Sb, Ag-Au And Ag-Pd. Among all the above-mentionedAg alloys, Ag-Sb was very effective in reducing couplingloss because of its high electrical resistivity at 77 K [6].

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YI et al.: DEVELOPMENT OF HTS BSCCO WIRE FOR POWER APPLICATIONS 2509

Fig. 4. Thermal conductivity of the Bi-2223 tapes with different sheathmaterials from 4.2 K to 100 K.

Fig. 5. Cross-section profile of an insulated tape.

With increasing , the eddy currents in the metallic parts andthe interfilamentary coupling currents, which are two mainsources of AC losses of multifilamentary metallic/BSCCOcomposite tapes, were reduced. Fig. 3 indicates that the criticaltensile stress (95% retension) can be improved by optimizingthe sheath materials, with the order of Ag-Mg-Ni Ag-SbAg-Au Ag. The best strengthening effect was achieved byAg-Mg-Ni, probably due to the high hardness of MgO formedduring the tape processing. Ag-Sb had the next best strength-ening effect. The use of Au had negligible effect on mechanicalstrength. From Fig. 4 we can see that the addition of Auremarkably decreases the thermal conductivity. For example,the thermal conductivity of Ag-Au sheathed tape at 40 K wasmeasured to be 50 W/(m K), which was approximately 8 timeslower than that of Ag-AgMgNi sheathed tape. Consequently,

the Ag-Au sheathed Bi-2223 tape becomes presently the mostappropriate candidate for manufacturing HTS current leads.

At the same time, we have developed insulated wires andtinned wires to satisfy the requirement of insulating, solderingand protecting the wires from penetration of liquid nitrogen. Aspecial surface treatment was performed before insulating or tin-ning the layer. Fig. 5 shows the cross section of an insulated tape.A breakdown voltage (DC) of 300–500 V was achieved with aninsulating layer of 10–20 on single side.

Innost’s Bi-2223/Ag tapes have been used for producingChina’s first HTS power cable system (33.5 m, 3 phase, 35 kV/2 kA), which was put into trial operation on July 10, 2004in Yunnan Province, southwestern China. In early this year,8 km of Innost’s tinned Bi-2223/Ag tapes was provided for aKorea HTS project. Also, Innost’s varying-designed-productsare being used for other research projects of HTS applicationsin China such as HTS motor, HTS magnet, HTS transformer,HTS fault current limiter and HTS MAGLEV.

IV. SUMMARY

Commercial manufacturing technology of Bi-2223/Ag tapeshas been developed at Innost. Controlling the reactivity of theprecursors, performing 3D numerical modeling of the defor-mation process by the finite element method and studying thein situ annealing behavior of as-rolled tapes, help to improvethe electrical performance of Bi-2223/Ag tapes. At the momentBi-2223/Ag tapes with length up to 1 km, over 90 A (77 k,0 T) and over 9 are being provided for major elec-tric power application such as China’s first HTS power cablesystem (33.5 m, 3 phase, 35 kV/2 kA).

To meet the requirements of the special HTS prototypes forother research projects, diversified tape-designs have been car-ried out to improve their application properties, which includereducing AC losses and thermal conductivity, increasing me-chanical strength and insulation properties, and so on.

REFERENCES

[1] Y. Yamada, J. Q. Xu, J. Kessler, and R. Flukiger, “Microstructual andtransport properties of high J Bi (2223)/Ag tapes,” Physica C, vol. 185,pp. 2483–2484, 1991.

[2] M. Y. Li, X. P. Chen, T. M. Qu, and Z. Han, “Phase evolution of leadoxide compounds corresponding with oxygen release and absorptionprocesses of BSCCO powders during heat treatment,” Supercond. Sci.Technol., vol. 17, pp. 249–255, 2004.

[3] J. I. Bech, M. S. Nielsen, and N. Bay, “Analysis of flat rolling of su-perconducting silver/ceramic composites,” Ann. CIRP, vol. 50-1, pp.201–204, 2001.

[4] X. P. Chen, “Study on Microstructure Evolution and Properties ofBi-2223/Ag Tapes,” PhD Thesis, Dept. Mater. Sci. Eng, TsinghuaUniv., Beijing, China, Jun. 2004.

[5] R. Navarro, “Silver alloys used in composite BSCCO tapes: develop-ment of electrical and mechanical properties during manufacture,” Su-percond. Sci. Technol., vol. 13, pp. R147–R170, 2000.

[6] J. Fang, “Study on AC Losses of BSCCO Tapes,” Post Doctor Report,Applied Superconductivity Research Center, Tsinghua Univ., Beijing,China, Aug. 2004.

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