Soldered Ohmic contacts to superconductors for high-current applications

Soldered Ohmic contacts to superconductors for high-current applications

Physica C 372–376 (2002) 1653–1656 www.elsevier.com/locate/physc Soldered Ohmic contacts to superconductors for high-current applications M. Ueltzen ...

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Physica C 372–376 (2002) 1653–1656 www.elsevier.com/locate/physc

Soldered Ohmic contacts to superconductors for high-current applications M. Ueltzen a, I. Martinek a, F. Syrowatka b, U. Floegel-Delor

c,*

, T. Riedel

c

a

Otto-von-Guericke-Universit€at Magdeburg, IFST, PF 4120, 39016 Magdeburg, Germany Martin-Luther-Universit€at Halle-Wittenberg, IWZ, Hoher Weg 8, 06120 Halle, Germany Adelwitz Technologiezentrum GmbH (ATZ), Rittergut Adelwitz, 04886 Adelwitz, Germany

b c

Abstract Applications of superconductivity in the high-current range, e.g. in fault current limiters or current leads, need advanced contacts between superconducting ceramics and metallic conductors. An electrochemical method for manufacturing a copper metal layer on the ceramics is approved as a fast, economical and reliable method of metallizing the superconducting ceramics. Different approaches of soldering are compared in this study, including a flux-free technology working in an ambient atmosphere. Wetting of the ceramics without premetallisation was enabled by using ultrasonic vibrations. The contacts prepared by the different technologies with or without premetallisation of the ceramics are characterised by electrical measurements in the high-ampere range at liquid nitrogen temperature as well as by investigations of the microstructure by SEM/EDX analysis. Ó 2002 Published by Elsevier Science B.V. Keywords: Soldering; Contacts; High-current application; YBCO

1. Introduction

2. Experimental techniques and results

Low-resistance contacts are a key design issue for the high-current applications of the hightemperature superconductors. Today, practical applications of excellent melt textured bulk yttrium barium copper oxide material with high transport critical currents of up to 10 kA/cm2 [1,2] is hampered by insufficient electrical contacts. Therefore, the Joule heating RI 2 localized in the contact area must be minimized by the optimisation of the physical properties of the contacts.

Different approaches of connecting superconducting yttrium barium copper oxide ceramics to copper were tested. Because of the well-known fitting of silver to the cuprates (see e.g. [3]), several brazing alloys with high silver content were tested. These brazing alloys have working temperatures above 600 °C. By using a soldering iron and the alloys L-Ag40Cd or L-Ag55Sn on silver contacts at the ceramic, problems of dissolving the silver contacts during the brazing process occurred. Cd-free solders show only poor wetting of the ceramics. But on the other hand, cadmium containing brazing alloys are outlawed due to ecological reasons. Additional

*

Corresponding author. Fax: +49-34222-45202. E-mail address: [email protected] (U. Floegel-Delor).

0921-4534/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 9 5 - X

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problems occurred with aging processes of the ceramics as well as of the contacts due to residual flux medium. Vacuum brazing with activated L-Ag72 failed, too. At the working temperatures of about 900 °C, the yttrium barium copper oxide ceramics act as a local oxygen source in the vacuum chamber. This leads to two problems: firstly, the local release of oxygen at brazing temperature in the contact region leads to a partial oxidation of the brazing alloy, i.e. the silver–copper eutectic L-Ag72 partly decomposes into a mechanically unstable mixture of copper oxide and silver. This is not suitable for electrical contacts. Secondly, oxidation of all molybdenum surfaces in the whole vacuum chamber was observed. Perhaps, this is due to an enhanced oxygen transport with silver. Fortunately, the molybdenum oxidation is reversible by heating the chamber 24 h at 1200 °C. Ultrasonic welding of copper to the superconducting ceramics is not suitable because of mechanical cracking of the ceramics. As best results of this technique, visible traces of copper were attached to the ceramic without cracking. Until now, best results were achieved by soldering with L-Sn60Pb at temperatures of about 250 °C. Therefore, the ceramics were coated with copper by an electrochemical preparation route as described elsewhere [1,4,5]. Fig. 1 shows an yttrium barium copper oxide bar soldered to copper pieces as electrical connectors for high currents. The measurements of the voltages along the contacts as well as on the ceramic rod show linear behaviour with increasing current. The contact resistance of optimal soldered superconductor–copper contacts is in the order of 0.1 lX at contact areas of about 1 cm2 . SEM investigations of those soldered ceramics showed inclusions of Pb–Sn alloys of different ratios. Furthermore, traces of flux were found in the inner of the ceramic. So, open questions of aging of the ceramic as well as of the contacts still remain. Flux-free ultrasonic assisted soldering with Snbased solder at temperatures between 220 and 250 °C is possible at the ceramic surface as well as on copper. But this results in a specific contact resistance of about 20 lX cm2 . So, this method of direct soldering at the ceramic surface is possible, but it

Fig. 1. YBCO rod with voltage probes and corresponding current–voltage characteristics at 77 K.

does not seem useful for low-resistivity contacts. Furthermore, flux-free soldering at a copper layer as described above was tested. Unfortunately, the ultrasonic activation of the surface, necessary to reach sufficient wetting of the soldering alloy on the surface of the copper layer, partially destroys the connection at the interface between the copper layer and the ceramic. Fig. 2 shows a SEM micrograph of ultrasonic assisted soldered connection of copper (left side) to yttrium barium copper oxide ceramic (right side). There is a good connection between the soldering alloy and copper on both sides, but a destroyed connection between the thin copper layer and the ceramic. This destruction does not occur in the upper part, where the soldering alloy poured from the gap during joining and no activation by ultrasound occurred. Nevertheless, 10 lX cm2 were measured at such contacts. Further degradation effects are observed due to the interaction between solder and ceramic YBCO.

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Fig. 2. SEM micrograph of an ultrasonic assisted soldered contact with damaged interface between copper coating and ceramic.

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Fig. 3 illustrates the decomposition and migration of SnPb solder. There are localised Sn-rich inclusions in the inner of the ceramics. Furthermore, the occurrence of chlorine in the inner of the material was detected. This is presumably caused by the decomposition of the solder flux at high soldering temperatures (>300 °C). It is highly probable that those inclusions in the superconducting ceramics influence the aging process not only of the contacts, but of the superconductors themselves, too. As a consequence, further work will be done in order to optimise the parameters of the process. There are several open tasks for further exploration of high-current contacts. The long-time degradation must be investigated in real application electrical and thermal regimes. In order to avoid aging due to flux residuals, further adaptation of the flux-free ultrasonic assisted soldering to coated ceramics has to be optimised. Therefore, the ultrasound stability of the metallic coatings is to be improved. Searching for other suitable brazing alloys with the focus on ecological needs, e.g. leadfree soldering, is an additional goal of further work.

Fig. 3. Solder decomposition in ceramic YBCO.

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3. Conclusions Different technologies of soldering on superconducting YBaCuO ceramics were tested in order to identify suitable techniques for high-current contacts. Until now, soldering with Sn–Pb soldering alloys and the ultrasonic assisted soldering technology are promising technologies for highcurrent contacts. Soldering at electrochemically deposited copper layers on the superconducting ceramic was identified as a suitable technique in order to reach specific contact resistances lower than 0.1 lX cm2 at a temperature of 77 K. The contacts show linear current–voltage behaviour up to 600 A. Until now, it seems necessary to work with metallic coatings of the ceramic. However, direct bonding at the ceramic surface and high-temperature brazing remain attractive technologies in order to enhance the reversible quench stability of the contacts. The connection of aging of the contacts as well as of aging of the superconducting ceramics

themselves with the soldering technology was shown by microstructural analysis. So, degradation effects should be investigated in the context of the soldering technology.

Acknowledgements This work was supported by the Kultusministerium of Sachsen—Anhalt.

References [1] F.N. Werfel, U. Floegel-Delor, D. Wippich, Inst. Phys. Conf. Ser. (Appl. Supercond.) 158 (1997) 821–824. [2] F.N. Werfel, U. Floegel-Delor, D. Wippich, R. Rothfeld, IEEE Trans. Appl. Supercond. 9 (2) (1999) 2018–2021. [3] U. Wiesner, G. Krabbes, M. Ueltzen, C. Magerkurth, J. Plewa, H. Altenburg, Physica C 294 (1998) 17–22. [4] Patent DE 199 39 144, ATZ. [5] U. Floegel-Delor, R. Rothfeld, D. Wippich, Th. Riedel, F. Werfel, IEEE Trans. Appl. Supercond. 11 (1) (2000) 1009– 1012.