IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 6602705 Investigation of ReBCO Conductor Tape Joints for Superconducting Applications Nadezda Bagrets, Andrea Augieri, Giuseppe Celentano, Giordano Tomassetti, Klaus-Peter Weiss, and Antonio della Corte, Senior Member, IEEE Abstract Electric joints are an essential part of every current carrying device. In the field of high temperature superconducting applications, quality of a contact between superconducting tapes and electric terminations is crucial for the device performance. When connecting tapes to each other, soldered contacts are commonly used. Recent reports have demonstrated that the resistance of this contact depends on the pressure applied during a soldering process, and the soldering temperature. When superconducting tapes are used in cables or in other applications, they have to be connected to copper terminations, e.g., to a current lead. Thus, tape-to-copper contacts are important. In superconducting cables, tapes are stacked together having a resistive electrical contact from tape to tape. The resistance of this contact depends on the pressure applied on the stack due to, for example, jacketing of the cable or during cool down. In this paper, the dependence of resistance between tapes on the applied pressure is reported. In this contribution two kinds of contacts, soldered ones and mechanical pressed, are presented and discussed. They are realized, respectively, between two superconducting ReBCO tapes from SuperPower, Inc. (SPI) and SuNAM, which were used for manufacturing HTS cable, and between superconducting tapes and copper. Index Terms Coated conductors, contact joints, HTS cable, solder joints. I. INTRODUCTION SOLDERED tape-to-tape, soldered tape-to-copper and mechanical tape-to-tape joints are important for HTS applications and for HTS cables, such as ENEA cable-in-conduit conductor (CICC) [1], in particular. A simple procedure to get reproducible possibly low resistive tape-to-tape joints is needed for every application. Lap joints between superconducting 2Gwires from different HTS tapes suppliers, which were manufactured using different solders and soldering methods, were recently investigated [2] [6]. Joint resistance varies over orders of magnitude depending on tape and solder. Even using the same solder and tapes from the same tape supplier, resistance of soldered joints was found to be different depending on the tape batch [7]. Manuscript received August 12, 2014; accepted November 13, 2014. Date of publication November 25, 2014; date of current version February 6, 2015. This work was supported in part by EFDA in the framework of Work Programs 2011, 2012 and 2013 on Design and Assessment Studies on superconducting magnets. N. Bagrets and K.-P. Weiss are with the Karlsruhe Institute of Technology (KIT), D-76344 Karlsruhe, Germany (e-mail: nadezda.bagrets@kit.edu; klaus.weiss@kit.edu). A. Augieri, G. Celentano, G. Tomassetti, and A. della Corte are with the ENEA Research Center, 00044 Rome, Italy (e-mail: andrea.augieri@enea.it; celentano@frascati.enea.it; giordano.tomassetti@enea.it; antonio.dellacorte@ enea.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2014.2373055 TABLE I GEOMETRY OF SPI AND SUNAM TAPES For HTS cable biasing, current terminations have to be designed and manufactured. For this purpose contact resistances between the tapes and copper should be investigated since they are crucial for a homogeneous current distribution between tapes in the cable. Due to jacketing, stacked tapes inside the cable are connected to each other by applying pressure. Inter-tape resistances determine the current redistribution in the cable and heat released inside the cable in case of quench of single tapes. Therefore, resistances between tapes should be investigated depending on the applied mechanical pressure (mechanical joints). Recently, mechanical contacts between ReBCO tapes were investigated in different configurations [8] [11]. In this paper a systematic study on all these three joint types, performed on the same superconducting tapes, is presented, that rarely has been carried out in other studies. The attempt is made to identify the parameters influencing the joint resistance and to correlate results obtained for different types of joint. The aim of this study is the optimization of the ENEA HTS cable [1], manufacturing and design of electrical termination needed for testing the cables. Since ENEA cables have been manufactured using both SPI and SuNAM 4 mm tapes, we focused our work on tapes provided by these companies. II. EXPERIMENT A. Preparation of Soldered Tape-to-Tape Joints SuNAM (HCN 121116) and two batches of SPI SCS4050 (2008 and 2013) (tapes geometry can be found in Table I) 4 mm wide tapes with copper electro-deposited directly on both silver and substrate layer were cut and cleaned with citric acid (ph 2.9) at 70 C for approximately 20 minutes. Subsequently, tape ends were pre-tinned by dipping them into molten Sn63Pb37 commercial multicore 3C511 solder at 200 C for a few seconds. Finally flux and surplus of solder were removed from the tape by a tissue quickly moved over its 1051-8223 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
6602705 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 1. Soldering equipment. (a) Resistive heating, (b) pressuring punch, (c) spring for controlled applying pressure. surface. Thus, tapes with approximately 2 cm long ends uniformly covered with a thin layer of solder were obtained. The soldering equipment is shown in Fig. 1. Approximately 1 cm long pre-tinned parts of tapes were stacked together connecting their ReBCO side face-to-face. The joint was placed between resistive heater and a T-shaped punch. The temperature was measured using thermocouple connected to the copper block directly between the resistive heater and the joint. Above the punch a spring is mounted for applying pressure during soldering. Applied pressure range was 5 10 MPa for soldered tape-totape joints (STT). Our previous investigations showed that this is a reasonable pressure range to obtain low resistant contact. Heating was slowly started without applying pressure. After reaching 170 C, the current through the resistive heater was increased so that temperature of 190 C was achieved faster. Then the pressure was applied and temperature was controlled in a way that it did not exceed 205 C. When melting process was observed on the boundaries of tapes after approximately 40 60 s, heating was switched off and a fan-assisted cooling started. When a temperature of 170 C was reached, which took approximately 30 s, the pressure was released. Since it was recently shown that the current transfer length is well below 1 cm [12], and the resistance of joints is in the inverse relation to its length [7], we restricted the length of joints to approx. 1 cm to investigate the reproducibility of contact resistance when using similar experimental conditions. After the preparation of the joint its actual length was measured, which was slightly different for every sample. B. Preparation of Soldered Tape-to-Copper Joints When producing soldered tape-to-copper (STC) joints the same procedure was used. Pieces of copper 40 mm 10 mm 2 mm were polished and etched in the citric acid. After that they were pre-tinned with solder and polished once again to remove flux from the surface. Tapes were soldered to copper pieces facing their ReBCOside along the longest side of the copper slab. Approximately 1.5 cm pre-tinned regions of tapes were used to prepare joints, thus the joint area is approximately 0.6 cm 2. The melting took more time (about 150 s) because of a larger thickness of copper compared to a thickness of the tape. Cooling down to 170 C took approximately 60 s. Pressure range of 2.5 3 MPa was investigated. Like in the case of soldered tape-to-tape joints, the actual length of the joint was measured after the preparation. Fig. 2. ENEA equipment for mechanical joints. (a) Joint tape-to-tape, (b) pressuring punch side view, (c) pressuring punch front view, (d) spring for controlled applying pressure, (e) screw for controlled contraction of the spring. Fig. 3. KIT equipment for mechanical joints. (a) Joint tape-to-tape, (b) pressuring punch side view, (c) pressuring punch front view, (d) force transducer, (e) threaded bars with screws for applying pressure. C. Preparation of Mechanical Tape-to-Tape Joints In the case of mechanical tape-to-tape joints (MTT), tapes were etched in the citric acid using the same procedure as for soldered joints. Directly before contacting tapes to each other, they were cleaned with acetone to remove possible grease. Two different setups were used to apply a pressure on tapes. The first setup was developed in ENEA. Pressure was applied and controlled using a spring with the spring constant k = 5.6 kg/mm. On Fig. 2, ENEA equipment is shown schematically. The length of the joint was approx. 80 mm, and the pressure range was 0 5 MPa. At KIT the equipment was modified to enable higher pressure. In Fig. 3 KIT equipment is shown schematically. Pressure is applied by screws and controlled by a force transducer. The length of a joint was approx. 60 mm, and pressure range was 0 25 MPa. III. MEASUREMENT RESULTS After preparing the joints, I V curves were measured in a liquid nitrogen bath. Voltage contacts were placed outside the joint on the tapes. In case of the tape-to-copper joints, one of the two voltage contacts was placed on copper at a distance of 1.9 ± 0.6 mm from the joint. For MTT joints the setup together with tapes was placed in a liquid nitrogen bath. After adjusting the appropriate pressure, I V curves were measured.
BAGRETS et al.: REBCO CONDUCTOR TAPE JOINTS FOR SUPERCONDUCTING APPLICATIONS 6602705 Fig. 4. Soldered tape-to-tape joint specific resistance as a function of applied pressure (left hand side) and effective overlap area (right hand side). Joint resistance R J (R JSTT,R JSTC or R JMTT ) was fitted from the I V curves using the relation V (I) =R J I + V c ( I I c ) n (1) where I is a current through the joint, V is a voltage drop across the joint, I c is a critical current in the superconductor, and V c = 1 µv/cm is the usual criterion on the voltage threshold. From R J (R JSTT,R JSTC or R JMTT ) the specific resistance R SJ (R SJ STT,R SJ STC or R SJ MTT ) was calculated as R SJ = R J (R JSTT,R JSTC or R JMTT ) (w L J ), where w is the tape width and L J is the length of the joint. A. Soldered Tape-to-Tape Joints In Fig. 4 STT joint resistances are shown as a function of the effective overlap area and applied pressure, respectively, for the same set of samples. Every point in each figure corresponds to a different sample. The inaccuracy in overlap area was conservatively estimated to be 10 %. It is due to the measurement of the length, possible non-ideal tape overlap and possible voids on edges of the joint. Another source of inaccuracy originates from the force definition applied by the spring. Altogether, that results in 7 8 % of error in pressure values. Since specific resistance is a function of the overlap area, area definition is the main source of measurement errors. Results are reproducible for both SPI and SuNAM joints. As expected from our previous study, specific resistance shows a plateau in the applied pressure range. SuNAM joint specific resistances are systematically 8 10 times higher than SPI joint resistances. The SPI joint specific resistances are in a good agreement with ones reported in [2]. Considering that the joint specific resistance for the lap faceto-face configuration can be expressed as R SJ STT =2R st +2R SI + R sol (2) where R st is the contribution from the stabilizing layers (Cu+Ag), R SI = R ReB stab + R stab sold is the contribution from the interfaces ReBCO-stabilizers and stabilizers-solder and R sol is the contribution from the soldering material, we can assume that the value R SJ STT is mainly determined by the R SI contribution. In fact, R st should be negligibly small Fig. 5. Soldered tape-to-copper joint specific resistance as a function of applied pressure (left hand side) and as a function of overlap area (right hand side). (considering 25 µm of Cu and 1 µm of Ag and the resistivity values for Cu and Ag at 77 K, R st reaches approx. 0.5 nω cm 2 ) R sol can be neglected as well (with approx. 20 µm thickness, measured with SEM, and resistivity of approx. 10 nω m[13], the solder contribution can be estimated to be 2 nω cm 2 ). The average value of the specific resistance obtained for the SPI joints is R SJ STT =41nΩ cm 2, and R SJ STT = 280 nω cm 2 for the SuNAM joints. So we obtain R SI =20.5 nω cm 2 for SPI and R SI = 140 nω cm 2 for SuNAM as average values. The possible origin of such a difference in the joint specific resistance can be related to the quality of interfaces. As a preliminary investigation, the copper surface morphology of SPI and SuNAM tapes observed by SEM (not shown) reveals very different film microstructure. SPI shows the roughest surface with very large grains. On the contrary, the SuNAM tapes have very flat surface with finer grains. B. Soldered Tape-to-Copper Joints In Fig. 5 STC joint specific resistances are shown as a function of the effective overlap area and applied pressure, respectively, for the same set of samples. Every point in each figure corresponds to a different sample. We observe a weak decrease of specific resistance with the increasing overlap area. Also, specific resistance is almost independent on the applied pressure in pressure range from 2.5 to 3 MPa. Specific resistance is similar for the SPI and SuNAM joints. Measured specific resistance can be written as R SJ STC = R st + R SI + R sol + R sol Cu + R Cu (3) where R sol Cu is the contribution from the interface soldering material-copper slab, and R Cu is the contribution from the copper slab. With cross sectional area of the copper slab of 0.2 cm 2,theR Cu contribution was estimated to be 190 nω in our joint configuration. The mean value of R SJ STC is 660 nω cm 2 for SPI STC joint, and 695 nω cm 2 for SuNAM STC joints, respectively. Subtracting the copper contribution from the measured R SJ STC value, we get R st + R SI + R sol + R sol Cu = 546 nω cm 2 for SPI, R st + R SI + R sol + R sol Cu = 581 nω cm 2 for SuNAM. Under the assumption that R st + R sol can be neglected, we obtain R SI + R sol Cu = 546 nω cm 2 for SPI, and R SI + R sol Cu = 581 nω cm 2 for
6602705 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 6. Mechanical tape-to-tape joint specific resistance as a function of applied pressure for SPI tapes. SuNAM. Using the values found above for R SI, we obtain R sol Cu = 525.5 nω cm 2 for SPI, and R sol Cu = 441 nω cm 2 for SuNAM. Taking into account a measurement inaccuracy of about 40 nω cm 2, this value can be considered nearly independent from the type of tape, as expected. To investigate the correlation between values obtained for interface resistances between tapes and solder R SI, and solder and copper R sol Cu, and roughness of copper surface, atomic force microscope (AFM) analysis was performed. It was found that SPI tape stabilizing copper layer is the one with the highest value of roughness (in the range 220 380 nm RMS in a 20 20 µm 2 region). This value facilitates wettability of solder on the tape surface. For SuNAM tapes the roughness value is in the range of 60 150 nm RMS in a 20 20 µm 2 region, much lower than in the SPI case. It can be inferred that in this case the wettability of solder on the tape surface is much poorer giving rise to higher R SI values. For the copper slab we have measured a roughness in the range of 25 30 nm RMS in a 20 20 µm 2 region. Thus, the differences in measured joint resistances are well correlated to the copper surface roughness, where the dependence of the interface resistance on roughness is nearly inversely proportional. C. Mechanical Tape-to-Tape Joints Two kinds of MTT joints were investigated: face-to-face and face-to-substrate. In Figs. 6 and 7 specific resistances are shown as a function of applied pressure for the SPI and SuNAM joints, respectively. Joint resistances are similar for SPI and SuNAM joints. Measurements performed at ENEA cover low pressure range starting from very low pressure values. Joint specific resistance decreases with increasing pressure and varies over two orders of magnitude in the investigated pressure range. Measurements performed at KIT cover the pressure range up to 30 MPa. Joint specific resistance is consistently decreasing with increasing pressure. At 25 MPa it is approximately 5 times lower than that at 5 MPa. Face-to-face mechanical joint specific resistances are 2-3 orders of magnitude higher than that of the soldered tapeto-tape joints. Face-to-substrate specific joints resistances are systematically 2-4 times higher than that of the face-to-face Fig. 7. Mechanical tape-to-tape joint specific resistance as a function of applied pressure for SuNAM tapes. mechanical joints. Their values correlate well with the intertape resistance values obtained for ENEA CICC prototype [1]. IV. CONCLUSION Soldered tape-tape, tape-copper and pressurized tape-tape joints have been prepared and measured for both SPI and SuNAM tapes. Specific resistance of soldered tape-to-tape joints is reproducible for each tape but significantly different for SPI and SuNAM. On the contrary, it was found that the specific resistance of soldered tape-copper joints is similar for SPI and SuNAM. Specific resistance of these joints is well reproducible when using similar preparation conditions, which is important point for homogeneous current distributions between tapes in an HTS cable. With AFM analysis of copper surface for the SPI and SuNAM copper stabilizing layers and for the copper slab, a direct correlation between surface roughness and joint resistance was found. Mechanical joints show the specific resistance being 2-3 orders of magnitude higher than that of the soldered joints, and being similar for both kinds of tapes. The results for mechanical joints are comparable with inter-tape resistance measured in 2-G wire slotted core HTS CICC manufactured by ENEA [1]. ACKNOWLEDGMENT Authors acknowledge A. Vannozzi and A. Rufoloni for their support in the AFM and SEM analyses. REFERENCES [1] A. Augieri et al., Electrical characterization of ENEA high temperature superconducting cable, IEEE Trans. Appl. Supercond., vol. 25, no. 3, Jun. 2015, Art. ID. 4800704. [2] C. A. Baldan et al., Electrical and superconducting properties in lap joints for YBCO tapes, J. Supercond Novel Magn., vol. 23, pp. 2089 2092, Mar. 2013. [3] Y. Zhang, R. C. Duckworth, T. T. Ha, and M. J. Gouge, Solderability study of RABiTS-based YBCO coated conductors, Phys. C, Supercond., vol. 471, no. 15/16, pp. 437 443, Aug. 2011. [4] J. Lu et al., Lap joint resistance of YBCO coated conductors, IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 3009 3012, Jun. 2011. [5] K. S. Chang et al., Repetitive over-current characteristics of the joints between the YBCO coated conductor, IEEE Trans. Appl. Supercond., vol. 19, no. 3, pp. 2419 2422, Jun. 2009.
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