1220
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE 2008
Joint Characteristics of YBCO Coated Conductor by Removing a Metallic Stabilizer Ki Sung Chang, Hyounkyu Kim, Dong Keun Park, Tae Kuk Ko, Min Cheol Ahn, Don-Hyung Ha, Jung-Bin Song, Sang Jin Lee, Ho Min Kim, and Haigun Lee
Abstract—This paper presents the development status of joint method between the superconducting YBCO coated conductors (YBCO CC). In general, the YBCO-CC tape consists of an upper stabilizer, substrate, buffer, YBCO and a bottom stabilizer. It is suggested that the stabilizer between the two YBCO-CC tapes need to be removed, in order to improve the characteristics of the joints. The removing process was conducted by using heat and partial etching. The resistance and critical current of the joints were meacurve. The optical micrographs were ansured from the alyzed for microstructures between the joint areas. After several times of over current test, the critical currents of the various types of joint samples were examined for applying to the high temperature superconducting magnet system. Index Terms—Coated conductor, joint, stabilizer, YBCO.
I. INTRODUCTION
R
ECENTLY, the researches on superconducting power apparatuses such as superconducting fault current limiter (SFCL) and power cable using high temperature superconductor (HTS) have increased and it is expected that those power apparatuses will be put to practical uses. In the center of this research, YBCO coated conductor (YBCO CC) plays an important part. YBCO CC has the characteristics of high index value (n value), low dependence of critical current on external magnetic filed and low AC transport current loss. With these advantages, the researches on superconducting apparatuses using YBCO CC are in progress. In order to apply HTS tapes—including YBCO CC—to superconducting apparatuses, it is necessary to make joints between the HTS tape and normal conductors or between HTS tapes [1]. In superconducting joint, there is no resistance causing losses but because of soldering, things like indium/bismuth have resistances causing losses in a realistic joint [2]. These joint resistances are one of the
Manuscript received August 29, 2007. This work was supported by KESRI(R2005-7-090), which is funded by MOCIE (Ministry of commerce, industry and energy). K. S. Chang, H. Kim, D. K. Park and T. K. Ko are with the Electrical & Electronic Engineering Department, Yonsei University, Seoul, 120-749, Korea. M. C. Ahn is with FBML/MIT, Cambridge, MA 02139, USA. S. J. Lee is with the Department of Electrical Engineering, Uiduk University, Kyoungju, Korea. H. M. Kim is with Korea Electrotechnology Research Institute, Changwon, Korea. D.-H. Ha, J.-B. Song and H. G. Lee are with the Department of Material Science and Engineering, Korea University, Seoul, Korea (e-mail:
[email protected]). 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.2008.920811
main reasons why efficiency of superconducting apparatuses is dropping. For example, many parallel connections of superconducting wires will be needed for flowing large current in a transmission level SFCL. More parallel connections cause more joint resistances during a normal operation and this could be one of the reasons why efficiency of SFCL is dropping. Therefore, more experimental researches are needed to obtain better joint characteristics. In this research, samples for joint experiment were prepared from the same spool of YBCO CC. The critical currents of those samples were measured and compared with those of the jointed YBCO CC tapes. There are two methods to join YBCO CC tapes, non-prior-process joint and after-removing-stabilizer joint. Both joint experiments were conducted in the same conditions as the soldering temperature and the cooling temperature. characteristic curves of the samples were investigated The at 77 K, in a saturated liquid nitrogen bath. Joint resistances of curve. The optical mijoint area were estimated from the crographs were analyzed for micro structures between the joint areas. For practical use, the CC tape’s degradation because of quench arisen from over current flowing must be minimized. Therefore, over current tests were conducted with the joined samples and after then the critical current of the jointed samples were measured again. The critical currents of the jointed samples are compared with each other before and after the over current tests. II. EXPERIMENTAL DETAILS A. Sample Preparation YBCO CC tapes used in this research were produced by American Superconductor. This YBCO CC tape branded as “344 superconductor” consists of an upper copper stabilizer layer, substrate layer, buffer layer, YBCO layer, silver layer and a bottom copper stabilizer layer. Table I shows the specifications of the YBCO CC. The resistivity of the copper stabilizer and 20 at 77 K and 300 K, respectively. (Cu155) is 6 Because of this copper stabilizer’s resistivity, resistance will exist in the jointed CC tapes. Therefore, some of the samples were just jointed (non-prior-process joint) and the others were jointed after removing the stabilizer (after-removing-stabilizer joint). All the joints were conducted in lap joint method. Substrate layer is made of Ni-5%W and buffer layer acting as an insulator. Therefore, the resistance of the joint between the upper stabilizers of two YBCO CC tapes will be larger than one of the joint between bottom stabilizers [3]. We first figured out which side is the bottom side by removing both stabilizers with soldering iron. After this process, the bottom side of the
1051-8223/$25.00 © 2008 IEEE
CHANG et al.: JOINT CHARACTERISTICS OF YBCO COATED CONDUCTOR BY REMOVING METALLIC STABILIZER
TABLE I SPECIFICATIONS OF THE YBCO CC
1221
C. Measurement and Analysis In order to investigate the joint characteristics of the samples, characteristic curves of the samples were measured at 77 K, in a saturated liquid nitrogen bath by the four probe method. Critical current and contact resistance of the samples were obcharacteristic curves. The governing equatained from the tion of the curve is as follows:
(1) is the joint resistance, is the current, is the critewhere rion voltage of 1 to determine the transition of the samples and is current when voltage is at , n is the index number depending on the materials [4]. The n-values of the samples are and 1 cricalculated between the ranges of 0.1 terions. The joint resistances of the samples are equivalent to the curve and calculated by using (1). slope of the Fig. 1. Schematic of the samples before and after eroding for removing the bottom copper stabilizer.
CC tapes was marked and all taps for measuring currents and voltage were on the bottom sides of the CC tapes. All the joints also were conducted between bottom stabilizers. The samples for experiments were selected from the same spool of American Superconductor 344 superconductor. Before the curves of each CC samples were joint experiments, the measured. For the after-removing-stabilizer joint, a few samples were eroded by dipping nitric acid/distilled water etchant or by applying it to a heat plate to remove the bottom stabilizer. The length of a removed stabilizer is 50 mm. In the eroding process, the upper stabilizer and the rest of the bottom stabilizer of samples were lapped with acid proof tape for protection. The time when submerging the tape for oxidizing the stabilizer layer is three minute. Fig. 1 shows that the sample was removed from the bottom stabilizer layer. The other samples were applied to a heat plate and the bottom copper stabilizer layer was detached from the CC tape. B. Joint Process In this research, processes to joint CC tapes are grouped into two processes: 1) after-removing-stabilizer joint 2) non-priorprocess joint. In case of the after-removing-stabilizer joint, the bottom stabilizer layers of the CC tapes are removed by etching (sample (1)) or detaching by heat (sample (2)). On the other hand, in the non-prior-process joint, CC tapes are jointed just by using a solder without removing stabilizer (sample (3)). All the CC tape samples were cut into 100 mm and the length of the joint was 50 mm and contact area is 217.5 in nonprior-process joint and 200 after-removing joint methods. A solder for jointing is a Pb/Sn paste. The soldering temperature and the cooling temperature is after the joint is 190 process. The jointed sample was under constant pressure during the soldering and cooling process. One of the pieces of CC tapes was placed on the hotplate and the other piece of CC was jointed, with the CC that was on the hotplate, by using solders.
D. Over Current Test For practical use, the degradation of the YBCO CC owing to quench caused by over current flowing must to be minimum. Therefore, some researches about degradation of critical current of YBCO CC have been conducted [5] and the same experiment is also needed with the jointed CC tapes. The process of experiment is the same as the previous work [5] except the over current is fixed at 400 A. In case of 344 superconductor, the temperature rising is 300 K when applying the current of 400 A [6]. In the experiment, the critical current of each jointed samples were measured before and right after the over current tests. The critical current of the jointed samples were measured before the test, were compared with the critical current measured after the test. The process of over current test consists of three steps: in the first step, the operating current which is 50% of the critical current is transported in the samples for one second. In the second step, the over current of 400 A is transported in the samples for 0.1 second. In the third step, there is no current flow in the sample for 60 seconds. These three steps were repeated ten times. All the measurements and tests were conducted in saturated liquid nitrogen (77 K). III. RESULTS AND DISCUSSIONS A. Resistance of Joint In theory, the resistances of the samples vary with the processing methods of joint. In case of a non-prior-process joint, the joint resistance consists of resistance of a silver layer, resistance of copper stabilizer, contact resistance between the solder and CC tape and resistance of the solder. In case of after-removing-stabilizer joint, the joint resistance is as same as the one of a non-prior-process joint except for curve resistance of a copper stabilizer. Fig. 2 shows the of the jointed samples. Closed symbols are measured data and open symbols are calculated data except for the joint resistance in each sample. stands for 1 / 1 cm criterion beThe bold line at 6 cause the distance between two voltage tap is 6 cm. Before the joint process, the stabilizer of sample (1) was removed by heat
1222
Fig. 2. V
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 18, NO. 2, JUNE 2008
0 I characteristic curves of the jointed samples. TABLE II THE CHARACTERISTICS OF THE JOINTED SAMPLES
and sample (2) by etching. Whereas sample (3) was jointed with YBCO CC tapes itself. The joint resistances of samples are calcurves, and the critical curculated from the slope of the rents of the samples are also obtained from the same curve except the effect of the joint resistance. Table II shows the characteristics of each samples measured from the experiments. In the point of a joint resistance, sample . Compared with sample (3), the (1) is the best case of 4.52 joint resistance is reduced by removing the stabilizer. In the case of sample (2) however, it is observed that the joint resistance can be reduced only if the surface of the sample is so flat and uniform that the contact resistance can be ignored. These results mean that removing the stabilizer process is very important and should be conducted with very special care. In addition, there is always room for improvement in the case of sample (2) if the stabilizer is eroded uniformly. The trends of the critical currents of the samples are similar to each other. The critical currents of all the samples increase after the joint process because the jointed samples have two YBCO layer in the jointed part so that the current’s path doubled. The n-values of all jointed samples are usually smaller than one of a bare YBCO CC because of the existence of a normal conductor, such as the solder used in joint process between YBCO layers. Because the n-value is in inverse proportion to AC loss, small n-value of the jointed samples can be a weak point for practical application [7]. B. Over Current Test Over current tests were conducted and the critical current of the samples before and after the over current tests are compared for practical application. Fig. 3 shows the curve of the
Fig. 3. V
0 I curves of the jointed samples before and after over current test.
jointed samples. Closed symbols are measured after the test and open symbols are measured before the test. Rectangular symbol, circle symbol and triangular symbol indicates sample (1), sample (2) and sample (3), respectively. The critical currents of the jointed samples have changed after the over current test. In case of joint after removing stabilizer layer by heat (sample (1)), the critical current decreases 1.3 A. It is implied that the removing process by heat deteriorated the YBCO layer partially and the heat generated by the over current weakened the deteriorated spot. This heat deterioration problem can be an obstacle for practical application. The critical current of the joint after removing stabilizer layer by etching (sample (2)) and the joint with YBCO CC tapes itself (sample (3)) increase 0.8 A and 1 A, respectively; One of the possible explanations on the increase of critical current is the training effect, as observed in the previous work [5]. As mentioned in the previous published paper [5], training effect occurs in superconducting magnet generally. However, the samples used in these experiments are small. Therefore, the training effect of the short YBCO CC should be researched more deeply. In conclusion, the jointed YBCO CC tape has good stability and no problems with over current except the joint after removing stabilizer layer by heat (i.e., sample (1)). C. Optical Micrograph Analysis In addition to the characteristics from curve measurements; the optical micrographs were analyzed for microstructure between the tapes. The structure between the tapes varies with the joint processes as shown in Fig. 4. In case of the joint after removing stabilizer by heat, the solder between the tapes spreads uniformly as seen in Fig. 4-(1). Owing to this condition of the solder between the tapes, contact resistance is so small that the joint resistance is smallest among the three joints. Fig. 4-(2) shows the structure of the joint after removing stabilizer by eroding. The structure of the solder between the tapes and the surface of the tapes is very poor as predicted from the curve. In addition, the thickness resistance obtained from of the solder is bigger than those of other cases due to the remained stabilizer after eroding. Therefore, the improvement of
CHANG et al.: JOINT CHARACTERISTICS OF YBCO COATED CONDUCTOR BY REMOVING METALLIC STABILIZER
1223
vestigated. The first and second are the joint after removing stabilizer layer by heat and etching. The third is the joint without prior process. The joint characteristics of the samples were incurves and over current tests. vestigated by measuring the Also the micro structures between the jointed tapes were analyzed by SEM micrographs. The joint resistances, critical current and n-values of the curve and comjointed samples were measured from pared with each other. The joint after removing the stabilizer . The layer by heat has the smallest joint resistance of 4.52 critical current of the samples increased after joint process and n-value of the samples were smaller than one of a general YBCO CC but measured around 26 uniformly. In case of over current tests, the critical currents of the joint after etching and the joint without prior process increased one percent of the critical current before the test like training effect. However, the critical current of the joint after removing the stabilizer by heat decreased one percent of the critical current before the test. It is proposed that the removing process by heat deteriorated the YBCO layer partially and the heat generated by over current weaken the deteriorated spot. This heat deterioration problem can be an obstacle to practical application. The characteristic of the joint without prior test seems best for YBCO CC. However, if the etching process and soldering process were to be improved, contact resistance of the joint after etching will decrease and the joint after etching can be applied to the joint with YBCO CC tapes in superconducting applications. Therefore, further studies on the joint after removing the stabilizer layer of YBCO CC by etching are needed.
REFERENCES
Fig. 4. Optical micrographs of transverse cross section of the jointed samples, (1) the joint after removing stabilizer by heat, (2) joint after removing stabilizer by etching, (3) joint without any prior process.
joint after eroding can be achieved by better eroding and soldering. In case of the joint without any prior process, the solder between the tapes spreads so uniformly but the existence of stabilizer layer makes the joint resistance higher than the joint resistance of the joint after removing stabilizer by heat. From the analysis of the optical micrographs, the condition of the solder between the tapes is very important for reducing the joint resistance. IV. CONCLUSION In this research, three types of joint samples fabricated with YBCO CC and the joint characteristics of each sample were in-
[1] P. V. Shoaff, Jr., Y. S. Hascicek, J. Schwartz, and S. W. V. Sciver, “An investigation of the characterization and development of HTS joints in BSCCO 2212/Ag Composites,” IEEE Trans. Appl. Supercond., vol. 7, no. 2, pp. 1695–1698, Jun. 1997. [2] J. Y. Kato, N. Sakai, S. Tajima, S. Miyata, M. Konishi, Y. Yamada, N. Chikumoto, K. Nakao, T. Izumi, and Y. Shiohara, “Diffusion joint of YBCO coated conductors using stabilizing silver layers,” Physica C: Supercond., vol. 445–448, pp. 686–688, Oct. 1, 2006. [3] D. K. Park, M. C. Ahn, H. M. Kim, H. G. Lee, K. S. Chang, S. J. Lee, S. E. Yang, and T. K. Ko, “Analysis of a joint method between superconducting YBCO coated conductors,” IEEE Trans. Appl. Supercond., vol. 17, no. 2, pp. 3266–3269, Jun. 2007. [4] M. H. Sohn, S. W. Kim, S. K. Baik, Y. S. Jo, M. G. Seo, E. Y. Lee, and Y. K. Kwon, “Joint resistances between two parallel high Tc superconducting tapes,” IEEE Trans. Appl. Supercond., vol. 13, no. 2, pp. 1764–1767, Jun. 2003. [5] M. C. Ahn, B.-Y. Seok, and T. K. Ko, “Repetitive over-current characteristics of YBCO coated conductor for superconducting fault current limiter,” J. KIASC, vol. 9, no. 1, pp. 1–5, Mar. 2007. [6] C. Lee, K. Nam, H. Kang, M. C. Ahn, T. K. Ko, and B.-Y. Seok, “Design of a high temperature superconducting coil for a 8.3 MVA fault current limiter,” IEEE Trans. Appl. Supercond., vol. 17, no. 2, pp. 1907–1910, Jun. 2007. [7] N. Amemiya, K. Miyamoto, N. Banno, and O. Tsukamoto, “Numerical analysis of AC losses in high Tc superconductors based on E-j characteristics represented with n-value,” IEEE Trans. Appl. Supercond., vol. 7, no. 2, pp. 2110–2113, Jun. 1997.
