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Splice Resistance

Voltage-current (VI) relationship is one of the most important aspects of the performance of a coated conductor. Using the four-probe transport measurement, a 2G wire splice fabricated in our lab was normally characterized with its joint section located in between the two voltage taps. The data provided here were mainly obtained from the measurements carried out at 77 K with a splice sample immerged into liquid nitrogen and with no applied magnetic field.  Depending on the sample dimension, our lab is also capable of characterize splice joints at lower temperatures and in magnetic fields up to 6 Tesla.

 A typical VI curve obtained from a splice joint is shown in the Figure 1 below. For this measurement, the voltage tap spacing is 8.9 cm. In the VI plot, the unit for the voltage is millivolt and the unit for the current is ampere. It can be seen from the plot that the voltage increases linearly with the increase of the current in the regime where the current is below the critical current (I_c). This is due to the resistance from the splice joint. That is different from the VI relationship of a bare coated conductor wire where the voltage signal is basically the instrument noise in this regime. The splice resistance is then simply defined as the slope of the linear part of the curve. For this sample, the resistance of the splice is about 200 nW. A VI curve can also be plotted in the log-log scale as shown in Figure 2. For each of the splice, the VI curve is provided in both linear and log-log scale.

Figure 1. The VI curve of a splice plotted in linear scale.

Figure 2. The VI curve of a splice plotted in log-log scale.

Table 1 & Table 2 below list the resistance values of splices fabricated with various 2G wires. Soldering process was carried out by either a soldering iron or a soldering heater block. Soldering materials recommended by the wire manufacturers were used for making these splices. All the soldering materials including solders and fluxes are commercial products. Soldering temperature is determined according to the melting point of the solder used and is the maximum temperature allowed for the processing of the specific 2G wire. As soldering temperature is an important processing parameter, it is also listed for each splice fabricated and measured. Unless specified otherwise, all splices were made as in the lap joint geometry with an overlap length of ~ 2.5 cm. In most cases, the tapes are in the orientation such that the HTS side of one tape facing the HTS side of the other tape (denoted as HTS-HTS). This is schematically shown in Figure 3. In other cases, the tapes in a joint can be in the orientation such that the HTS side of one tape facing the substrate side of the other tape (denoted as HTS-SUB). Splices between 2G wires and normal copper conductors were also fabricated for testing, where the joint orientation is denoted as either HTS-Cu or SUB-Cu. For given resistance value, it is the result of a group of 6 samples that were made with the same wire and using the same soldering conditions. By clicking on any resistance value listed in Table 1 & Table 2 you can view the VI curves of the splice, which is a typical one of the 6 samples in the group.

Figure 3. Lap joint geometry of a splice with the tapes in HTS-HTS orientation.

Table 1 Electrical resistance values of the splices fabricated using the AMSC’s HTS 2G wires

Sample ##	2G Wire	Orientation	Solder	Flux	Soldering T (°C)	Resistance
A01	344C	HTS-HTS	52In48Sn	Alpha 260HF	150	32±9.8nW
A02	344C	HTS-HTS	100In	Alpha 260HF	165	26±5.5nW
A03	344C	HTS-HTS	67Bi33In	Alpha 260HF	150	0.62±0.57mW
A04	344B	HTS-HTS	52In48Sn	Alpha 260HF	150	180±20nW
A05	344B	HTS-HTS	52In48Sn	Kester 135	150	180±20nW
A06	344B	HTS-HTS	100In	Alpha 260HF	165	200±60nW
A07	344B	HTS-HTS	100In	Kester 135	165	180±20nW
A08	344B	HTS-HTS	67Bi33In	Alpha 260HF	140	0.63±0.50mW
A09	344B	HTS-Cu	52In48Sn	Alpha 260HF	150	26.7±0.52mW
A10	344B	SUB-Cu	52In48Sn	Alpha 260HF	150	34.2±1.5mW
A11	344B	HTS-Cu	100In	Alpha 260HF	165	27.6±1.6mW
A12	344B	SUB-Cu	100In	Alpha 260HF	165	34.1±1.6mW
A13	344B	HTS-HTS	50In48Sn	Alpha 260HF	160	75±5.5nW*
A14	344B	HTS-HTS	50In48Sn	Alpha 260HF	160	55±5.5nW**
A15	344S	HTS-HTS	52In48Sn	Alpha 260HF	165	5.77±1.5mW
A16	344S	HTS-HTS	100In	Alpha 260HF	170	6.15±1.9mW

                                                                      *Overlap length is 5 cm.

                                                                      **Overlap length is 7.5 cm.

Table 2 Electrical resistance values of the splices fabricated using the SuperPower’s HTS 2G wires

Sample ##	2G Wire	Orientation	Solder	Flux	Soldering T (°C)	Resistance
S01	SCS4050	HTS-HTS	60Sn40Pb	Kester SP-44	195	20±2nW
S02	SCS4050	HTS-HTS	100In	Kester SP-44	195	20±2nW
S03	SCS4050	HTS-Cu	60Sn40Pb	Kester SP-44	205	27.4±0.58mW
S04	SCS4050	HTS-Cu	100In	Kester SP-44	195	26.5±0.44mW

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