Manufacturing of bulk high-Tc superconductors

International Journal of Inorganic Materials 2 (2000) 623–633

Manufacturing of bulk high-T c superconductors A.G. Mamalis* a

Head of Manufacturing Technology Division, Department of Mechanical Engineering, National Technical University of Athens, Athens, Greece Dedicated to Professor Bernard Raveau on the occasion of his 60th Birthday

Abstract High-energy rate powder compaction techniques, like explosive and electromagnetic compaction constitute a tool to produce bulk high-T c superconductive ceramics with unique properties. A great novelty is indicated from the subsequent mechanical processing of the densified ceramics, employed to fabricate a sound final product. Billets, rods and wires, with superconducting core and metal sheath, can be produced by various forming techniques, like wire-drawing, extrusion and profile-rolling. Bulk ceramic superconductors are used as functional elements in electromagnetic machines, such as synchronous generators, levitated bearings, flywheels and fault current limiters. An account of this deformation processing and its applications is given in the present paper.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Superconductors; Ceramics; C. High pressure

1. Introduction The discovery of high-temperature superconductivity in 1986 resulted in extensive research work regarding the synthesis of high-T c superconductors, such as the YBCO and BSCCO compounds. However, research is required in the area of processing for fabricating components of specified geometry. These new superconducting materials, possessing superconductivity above the liquid nitrogen boiling point, are utilized to many engineering applications, from electronic sensors to rotating electrical generators and from nanometer-scale thin-films to kilometer-long wires and coils. Therefore, design and netshape manufacturing of superconducting components, starting from the initial synthesised powders, is nowadays of utmost industrial importance. Several industrial applications of bulk superconducting ceramics of the YBCO (h-hh-Cu-h) and BSCCO (BiSr-Ca-Cu-h) compounds have been developed by the author and his collaborators, using explosive and electromagnetic dynamic compaction techniques and subsequent net-shape manufacturing processes, e.g. rolling, cold and warm extrusion, wire-drawing, etc. These applications include wires for electrical power transmission, electrical switches for measuring technology and HTS conductors *Tel.: 130-17-72-3688; 130-17-72-3689. E-mail address: [email protected] (A.G. Mamalis).

for electrical machines, e.g. rotating synchronous generators, levitated bearings and current limiters [1–18]. A process flow diagram, from the synthesis of high-T c superconducting powders to the final stages of the construction of superconducting machines, is shown in Fig. 1.

2. Manufacturing at high strain-rates Dynamic compaction has proved to be very effective over conventional compaction techniques. The very short duration of the process and the development of high pressures up to peak shock values of 1–100 GPa, lead to dramatic reduction in porosity, whilst fracture of the initial grains and the formation of new clean grain boundaries during the compaction process may result in increased inter-grain current transport in the superconducting state. Furthermore, the defect-structure, created during the shock-wave passage through the porous media and its interaction with lattice distortions and dislocation substructures, may raise the number of the possible flux-pinning centres and increase the critical current density. Finally, a high integrity metal–ceramic bond is typically formed at the metal sheath / ceramic and / or ceramic / embedded metal contact rod interface. In the present work, explosive and electromagnetic high energy rate compaction forming ‘powder-in-tube’ techniques were employed for near net-shape manufacturing of

1466-6049 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00094-5

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Fig. 1. Process flow diagram.

axisymmetric silver / superconducting YBCO ceramic billets, discs and rings. Isodynamic compaction is considered, in which the impulse-form pressure is applied over the whole outer surface of the metal tube containing the powder to be compacted.

2.1. Explosive compaction Explosive compaction of powders is a low-cost and very

rapid technique by which large amounts of energy can be deposited in a short-time range. The shock-waves originated from explosive detonation and propagated through the porous media, can create high shock pressures and high temperatures, that result in fracturing the original grains and in sintering. As mentioned above, at micro structural level, the compacted solid contains a variety of primarily line defects (dislocations) that would provide flux pinning centres in

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Type II superconductors, whilst several defects, at macroand microscale, are observed: large scale longitudinal and transverse cracking, incompleteness and disorder in crystallinity, generated by the high shock pressures, and catastrophic structural changes deteriorating the residual superconductivity, such as precipitate-like defect clusters, associated to the shock induced large strain fields and the transformation of the orthorhombic (superconducting) phase to tetragonal (normal) phase, due to the oxygen loss

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resulted from the elevated temperature in the high pressure state [1,16–18]. The explosive compaction configurations used for the fabrication of metal sheathed axisymmetric billets, discs and rings for rotating / levitating machines and flywheel, are shown in Fig. 2(a) and (b), respectively. The high explosive used was a powder-form material with a detonation velocity of 2400 m / s and a specific energy content 4 kJ / g. The pressure acting on the powder was 5.6 Gpa.

Fig. 2. Schematic diagram of the axisymmetric explosive compaction of (a) billets and discs and (b) rings.

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The YBCO powder used was prepared by Crismat, possessing a large grain size distribution, the grain size varying between 30 and 60 mm. Calcination was made at temperatures ranging from 950 to 9708C with dwell times which can reach 300 h. The microstructure observed, after sintering at 9508C for 300 h, is shown in Fig. 3(a). The XRD of this material is presented in Fig. 3(b) showing a phase distribution of Y1231´Y211. The measured critical temperature was T c 592.2 K. The initial density of the powder in the compact was 50%. Silver powder (Ag 99.5 and initial density 55%), to form a solid layer protecting the HTS ceramic against the mechanical damages and to enhance the heat transfer from the cooling agent to the ceramic, and a thermo indicator powder were employed. Note, also, that as the shock-wave fronts, created by the detonation of the explosives, are not uniform at the end sections of the compact, the pressures are not uniform too

and the integrity of the compact is low; therefore, cheap MgO powder (initial density 40%) was used as buffer material. After compaction, the billets were cut perpendicular or parallel to their axis and the so obtained surfaces were examined by SEM; a high density of the compact, about 95% of the theoretical density of the YBCO material was revealed, see Fig. 4(a), however, many intragrain fractures also are visible. From the pole figures determination, no preferential orientation of the crystallographic planes was found, see Fig. 4(b), revealing that no texture takes place during explosive compaction. The metal (solid and powders) / ceramic (powder) interfaces showed a mechanical good bonding. The X-ray patterns of the compact, see Fig. 4(c), show that explosive compaction, whatever the energy used, maintains the initial structure of the precursor; traces of

Fig. 3. (a) SEM showing the microstructure and (b) XRD pattern of the initial YBCO powder.

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Fig. 4. (a) Microstructure of the explosively compacted powder, (b) (006) pole figure of the compact showing polycrystalline totally disoriented feature of the sample, (c) XRD pattern of the compact.

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additional secondary phases were not present. Therefore, it may be concluded that, the explosive compaction process is not detrimental to the crystal structure of the sample, the nature of which remains unchanged. The exact oxygen stoichiometry of the compacted samples after explosive compaction, which implies an increase of the temperature of the sample, is not known, and, therefore, it seems preferable to perform a thermal treatment in oxygen atmosphere before measuring the critical temperature, T c . The thermal cycle used, to diffuse oxygen in the bulk sample, determined from a TGA study, consists of heating the compact at 5008C for 2 h, whilst the dwell time corresponds to 100 h at 4308C. Alter this thermal treatment, the critical temperature measurements have been made using a susceptometer and a magnetic field of one Gauss. The T c onset was found very close to 93 K, which proves a correct reoxygenation of the samples and confirms the ability of the explosive compaction to preserve the good quality of the superconducting phase.

surface of the powder to be compacted. This is achieved by placing the powder-filled metallic container inside the coil, see Fig. 5. The effectiveness of any electromagnetic forming process depends on the conductivity of the workpiece. The ceramic powders to be compacted, possessing very low electrical conductivity, are packed in a highly conductive thin-walled metal sheath made of copper, silver or aluminium. When an intensive electromagnetic field is generated around the metal sheath, it collapses and acts as a tubular punch consolidating the powder. The

2.2. Electromagnetic compaction The electromagnetic axisymmetric compaction uses the effect of a high strength transient magnetic field, produced by the discharge of a capacitor bank (C) through a cylindrical coil (J) surrounding and acting over the entire

Fig. 5. Schematic diagram of the axisymmetric electromagnetic compaction: 1: silver tube (F12 / 10); 2: YBCO powder; 3: silver powder; 4: plastic disc; 5: steel bolt MS; T: selenoid; C: capacitor; 5: switch.

Fig. 6. SEM micrograph showing the micro structure of the (a) initial YBCO powder, (b) electromagnetic compact.

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Fig. 7. ac magnetic susceptibility curves for the explosive and electromagnetic compacts.

starting energy of the tubular punch is chosen to about 10% of the capacitors storage energy and corresponds to about 2 / 3 of the energy released during explosive compaction. Note, also, that the requirements of the skin-effect are also met under these conditions [14–18]. A mass of 4 g of the superconducting powder YBa 2 Cu 3 O 7 with a microstructure shown in Fig. 6(a), was placed between a thin-walled silver (Ag 999) tube and a steel mandrel and packed to a bulk density of approximately 50% of the theoretical density. Silver powder (Ag 999) was used at the ends of the tube for encapsulating the ceramic powder. The whole device was inserted into an electromagnetic cylindrical coil consisting of 17 windings made of insulated copper wire, see Fig. 5, and consolidation took place as outlined above. The starting energy of the tubular punch was 800 J. Relatively low bulk densities of about 82% were obtained for the electromagnetically compacted billet. Its microstructure is indicated in Fig. 6(b). The obtained XRD reflection peaks revealed a small increase in green phase (Y 2 BaCuO 5 ) content, accompanied by the appearance of the YBa 2 Cu 3 O 6.5 phase and the disappearance of BaCuO 2 . Note, also, that the ac-magnetic susceptibility of the electromagnetic compact indicated the characteristic Meissner transition at T c 594 K, as shown in Fig. 7. The transport critical current density Jc for the electromagnetic compact, measured by the four-point technique, was 800 A / cm 2 at 77 K; it is comparable with that obtained for explosive compacts of similar geometry.

3. Extrusion of metal / HTS rods and wires For the fabrication of axisymmetric metal sheathed superconducting components, like rods and very thin wires, multiple-pass extrusion, through slit-dies of different

angles, cold (at room temperature) or warm (at elevated temperatures 4708C), of the previously shock compacted billets was used. Extrusion was performed on a SMG 1000 MN hydraulic press, fully automated and equipped with force and displacement measuring devices. The extrusion was performed under lubricated conditions between the metal sheathed billet, the steel extrusion container, the punch and the split die, fabricated by hardened / tempered and nitrided tool steel, using a commercial lubricant. The punch velocity was 0.2 mm / s. The final diameter of the extruded rods was 3 mm [3,4,9,10]. Longitudinal sections of the extruded specimens, along the horizontal axis of symmetry, examined by optical and scanning electron microscopy, revealed various types of macro- and microdefects. Intense grain fragmentation, leading to severe reduction of the grain size occurred at the vicinity of the shear cracks network, see Fig. 8(a). Examination of the silver / ceramic interface indicated good bonding and absence of serious damages. The extrusion load, P-punch travel, d diagrams, as obtained from the auto graphic recorder, is presented in Fig. 8(b); three discrete regions may be considered: • Region I, which is the transient or beginning stage of extrusion, where all clearances between the die wall and the material were taken-up and the plastic zone was developed with a small amount of extrusion occurring. • Region II corresponding to the steady-state phase, in which the load remains almost constant with a slight falling tendency and the composite material (silver and ceramic) is deformed plastically. • Region III, probably associated with the beginning of the unsteady-state, when the remaining slug is sort. The normal / superconducting transition occurred at an

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Fig. 8. (a) Microstructural changes in a longitudinal section of a rod fabricated from a Ag / YBCO billet explosively compacted using the cord-form explosive and extruded through a 60o wedge-shaped die: 1: a schematic diagram showing the damaged zones observed; 2,3: slug micro structure; 4: the rod microstructure near the superconducting core / silver-sheath interface; 5: the rod microstructure near the rod axis of symmetry; 6: the flow lines intersection on the rod axis of symmetry; 7: the direction of flow lines inside the wedge-shaped die near to the core / silver-sheath interface; 8: the flow-line microstructural characteristics of 7. (b) Extrusion load, P-punch travel, d diagrams for multi-pass warm extrusion.

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Fig. 9. Measurements of ac susceptibility vs. temperature: 1: the initial powder; 2: the explosively compacted billet; 3: the extruded St / YBKCO /Ag rod through a 458 wedge-shaped die.

onset temperature of 92 K, while the corresponding width was 14 K, as indicated by ac susceptibility measurements, see Fig. 9. Judging from the superconducting properties of the ceramic material at the various stages of the fabrication, it can be concluded that, superconductivity is maintained and, therefore, additional post-processing heat treatment in oxygen atmosphere may be not necessary.

4. Applications For low-temperature superconductors, mainly Nb–Ti and Nb3Sn, a refrigeration process in liquid helium is necessary, which is rather complex using also a very costly coolant like helium. The high-temperature ceramic superconductors exhibit their useful technical properties using a simpler liquid nitrogen refrigeration system; they can be commercially utilised in the electric power, the electronics and transportation sectors. An overview of the engineering applications of bulk HIS is presented in Ref. [18].

4.1. Synchronous generator A top and a cross-sectional view of a model machine are shown in Fig. 10(a). The two main parts of the model are the stator and the rotor made of steel and aluminium, respectively. Since the current-carrying capability of the ceramic sperconductors is relatively low and an air-gap flux density of the order of 0.541.0 T is needed, rare-earth permanent magnets, e.g. SmCo5, possessing a remanent flux density of Br50.9 T, are employed, placed into the slots of the rotor wall. The armature winding, consisting of eight rods of YBCO compound, fabricated by explosive compaction and subsequent extrusion, is placed into the gap; from the point of view of the magnetic circuit, the gap must be as small as possible, with its minimum value being determined by the geometry of the HTSC conductor. The working position of the machine is vertical. The

machine will be cooled in a cryocooler at 70 K. The field distribution inside the machine is shown in Fig. 10(b) [11–13].

4.2. Flywheel The magnetic levitation systems, such as magnetic bearing, flywheel and linear drive, are the most promising applications of the high-temperature superconductors. Among these applications of the HTSCs the realisation of the superconducting flywheel energy storage system can lead to the level off of the electricity [16]. A flywheel contains rapidly spinning disks or rings, which are used to store energy for short stretches of time. The kinetic form in which the energy is stored by a flywheel can be easily transformed from or into electric energy by an attached motor or generator. With the development of efficient bearings, based on HTSCs, flywheels can store energy for much longer than ever before. The superconducting magnetic bearing contains a permanent magnet levitating in a stable position above a superconductor. A flywheel system, based on levitating superconducting bearings, has the capability to demonstrate a more efficient way for energy storage compared with other conventional storage devices. A flywheel energy storage system, employing ring-shaped YBCO superconductors manufactured by explosive compaction, is shown in Fig. 11.

4.3. Fault current limiter A superconducting fault current limiter is a promising application of bulk high-T c superconductors. In combination with conventional switchgear, it will decrease the current heating of the latter, substituting, therefore, the expensive silver contacts usually used in many industrial and transport switchgears by cheaper and more compact silver-free composition materials. Application of such

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Fig. 10. (a) Schematic top and cross-sectional views of a small-scale model synchronous generator, (b) field distribution in the permanent magnet–PMHTS zone.

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Fig. 11. Schematic diagram of an experimental flywheel: 1: stator; 2: rotor (flywheel); 3: permanent magnet; 4: superconducting magnet (HTS ring); 5: liquid nitrogen (LN 2 ); 6: vessel for (LN 2 ); 7: baseplate.

switchgears may allow for achieving higher efficiency and smaller weight, improved dynamic stability of operation of electrical devices in power systems in transport and industry. There are two types of current limitation by the use of superconductors, namely the resistance and the inductive fault current limiter. The inductive one is based on the varying impedance of a reactor or a transformer with at least one superconducting winding. An inductive fault current limiter, employing explosively compacted YBCO rings, is currently under development by the author and his collaborators; see also Refs. [6,16–18].

Acknowledgements The work reported here is part of an INCO-COPERNICUS EU research project with collaboration between Greece (NTUA), France (CRISMAT), Hungary (Technical University Budapest, Veszprem University, Metalltech Ltd) and Russia (Research Institute of Electrical Machinery, NII Electromash-St. Petersburg). I am grateful to my colleagues Prof. B. Raveau, Prof G. Desgardin, Dr. I. Vajda, Mr. A. Szalay, Prof I. Kotsis and Prof L. Chubraeva for providing results and for useful discussions and to my Ph.D student Mrs. I. Vottea for helping with the preparation of the manuscript.

References [1] Mamalis AG, Gioftsidis GN, Szalay A, Boday O. The shock wave compaction of high temperature superconducting powders into cylindrical components. CIRP Annals 1989;38:297–301. [2] Mamalis AG, Gioftsidis GN, Szalay A. Fabrication of superconducting wire by explosive compaction and profile rolling, Proc. 3rd ICTP Conference, Kyot, Japan, July 1990, Advanced Technology of Elasticity, 1990;1:889–96.

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[3] Mamalis AG, Gioftsidis GN, Szalay A. On the extrusion of silver sheathed superconducting billets fabricated by explosive compaction of YBaCuO. J Mat Proc Tech 1992;30:297–313. [4] Mamalis AG, Szalay A, Pantelis D, Pantazopoulos G. Net shape manufacturing of metal / superconductive ceramic / metal rods by explosive compaction and warm extrusion, Proc. EXPLOMET 95, El-Paso Texas USA, Elsevier 1995, pp. 747–54. [5] Mamalis AG, Vajda I, Mohacsi L, Kotsis I, Szalay A. From fabrication to application: ceramic superconductors for use in magnetically levitated bearings, Proc. 4th Euro-Ceramics Conf., ECERS. Barone A, Fiorani D, Tampieri A, editors. PA: Gruppo Editoriale Faenza Editrice S.p.A., Rimini, Italy, 1995;7:167–74. [6] Mamalis AG, Szalay A, Vajda I, Enisz M, Palotas B. A novel monolith for electrical applications: Silver sheathed bulk ceramic superconductor with silver contacts, Proc. 4th Euro-Ceramics Conf, ECERS, Barone A, Fiorani D, Tampieri A, editors. PA: Gruppo Editoriale Faenza Editrice S.p.A., Rimini, Italy, 1995;7:217–24. [7] Mamalis AG, Szalay A, Pantelis D, Pantazopoulos G. Fabrication of thick layered superconductive ceramic (Bi–Pb–Sr–Ca–Cu–O) / metal composite strips by explosive cladding and rolling. J Mat Proc Tech 1995;51:255. [8] Mamalis AG, Szalay A, Pantelis D, Pantazopoulos G. Net shape manufacturing of silver sheathed high-T c superconductive ceramic (Y–Ba–K–Cu–O) strip by explosive compaction / cladding and rolling. J Mat Proc Tech 1996;57:112. [9] Mamalis AG, Szalay A, Pantelis D, Pantazopoulos G, Kotsis I, Enisz M. Fabrication of multi-layered steel / superconductive ceramic (Y–Ba–K–Cu–O) / silver rods by explosive powder compaction and extrusion. J Mat Proc Tech 1996;57:155–63. [10] Mamalis AG, Pantazopoulos G, Szalay A, Kotsis I, Vajda I, Manolakos DE. Multiple-pass warm extrusion of explosively compacted ceramic superconductive (Y–Ba–CuO) / metal billets. Appl Supercond 1996;4:213. [11] Mamalis AG, Kotsis I, Vajda I, Szalay A, Pantazopoulos G. High-T c ceramic superconductors for rotating electrical machines: from fabrication to application. In: Batlogg B, Chu CW, Chu WK, Gubser ¨ DU, Muller KA, editors, Proc. 10th Anniversary [ITS Workshop on Physics, Materials and Applications, Houston Texas, USA, London: World Scientific, 1996, pp. 631–2. [12] Vajda I, Mamalis AG, Szalay A. Design and construction of an HTSC synchronous machine with permanent magnet excitation, Proc. Int. Cryogenic Engineering Conference, ICEC 16 / ICMC, Kitakyushu, Japan, 1996. [13] Mamalis AG, Vajda I, Szalay A, Kotsis I, Pantazopoulos G. Some small-scale high-temperature superconducting models for applications in electricity and transportation. Superlatt Microstruct 1997;21:251–5. ¨ N, Vajda I, Raveau B. Near net-shape [14] Mamalis AG, Szalay A, Gobl manufacturing of metal sheathed superconductors by high energy rate forming techniques. Mater Sci Eng 1998;B53:119–24. ¨ N, [15] Szalay A, Manialis AG, Raveau B, Desgardin G, Vajda I, Gobl Porjesz T, Chubraeva L, Kotsis I. Dynamic compaction for fabrication of [ITS ceramic compounds, Proc. 4th EUCAS Conference, Barcelona, Spain, September, 1999. [16] Mamalis AG. Technological aspects of high-T c superconductors. J Mat Proc Tech 2000;99:1–31. [17] Mamalis AG, Near Net-Shape Manufacturing of Bulk Ceramic High-T c Superconductors for Application in Electricity and Transportation, J Mat Proc Tech 2000; in press. [18] Mamalis AG, Pantazopoulos G, Szalay A, Manolakos DE. In: Processing of High-T c superconductors at high strain rates, Lanchaster, PA, USA: Technomic Publishing Co, 2000, p. 280.