An extensive interlaboratory comparison was conducted on high temperature superconductor (HTS) critical-current measurements. This study was part of an international cooperative effort through the Versailles Project on Advanced Materials and Standards (VAMAS). The study involved six U.S. laboratories that are recognized leaders in the field of HTS. This paper includes the complete results from this comparison of critical-current measurements on Ag-sheathed Bi2Sr2Ca2Cu3O10–x (2223) tapes. The effects of sample characteristics, specimen mounting, measurement technique, and specimen damage were studied. The future development of a standard HTS measurement method is also discussed. Most of the evolution of this emerging technology has occurred in improvement of the performance of the conductors. The successful completion of this interlaboratory comparison is an important milestone in the evolution of HTS technology and marks a level of maturity that the technology has reached.
IntroductionThe data presented here are critical current 1^, and n-value measurements made during an interlaboratory comparison in which a common mandrel with standardized design was used for reaction and measurement. Titanium alloy (Ti-6A1-4V; composition is in mass percent) mandrels were chosen because they can be used for both reaction and measurement, thus eliminating the need to transfer the delicate sample from the reaction to the measurement mandrel. This reduced a major source of variability. The Ti alloy mandrels have other advantages: they are inexpensive and nonmagnetic, and have low thermal expansion, high electrical resistivity, and low resistivity ratio. Figure 1-2 but on a more sensitive scale excluding these outliers. The statistics on the n-value measurements were done in the same way and are illustrated in Figures 1-8 and 1-9, with Figure 1-9 having a more sensitive scale. This analysis was used to attempt to quantify the results from each laboratory, and it is not an independent evaluation because the overall averages were used. Tables II-1 to II-3 and Figures 2-1 to 2-5 are relative to this weighted average. Figure 3- Experimental Results
Abstruct-We conducted an interlahoratory comparison of critical current (Id measurements on B~Sr,C+C~O,, tapes (2223). Thip study includes measurements from six participating US laboratoriep, witb NI= PJ the central, organizing laboratory. A number of specimens were prepared with Merent degrees of instrumentation to isolate sources of variabilitp. Most of the specimens were pre-measured by NIST to reduce uncertrrinties due to sampk v a r i a b i . Different specimen routiog pattaw among the laboratories were implemented to isolate sources of variability due to the specimen's measurement h i s t o r y . This study is similar to other VAMAS intercomparisons being performed in Japan and Europe and is the fmt i n t e r~t i~~U y cooperative interlaboratory comparison of HTS (Hih Temperature Superconductors) I, measurements. These are the fmt steps fowards developing standard measurement procedures for HTS.
NIST and other laboratories have observed an anomalous switching phenomenon that can occur in critical-current measurements of coiled Nb-Ti and Nb3Sn superconductors when mounted on an electrically-conductive measurement mandrel. During acquisition of the voltage-current (V-I) characteristic, large voltage discontinuities are observed. This switching phenomenon results in a multivalued V-I curve, and apparently multiple "critical-current" values. An explanation of this phenomenon, some necessary conditions for the switching to occur, as well as methods of detecting the phenomenon are given.
An interlaboratory comparison of critical current (Ic) measurements was conducted on the superconductor simulator, which is an electronic circuit that emulates the extremely nonlinear voltagecurrent characteristic of a superconductor. These simulators are high precision instruments, and are useful for establishing the integrity of part of a superconductor measurement system. This study includes measurements from participating US laboratories, with NIST as the central, organizing laboratory. This effort was designed to determine the sources of uncertainty in I, measurements due to uncertainties in the measurement apparatus, technique, or the analysis system. The participating laboratories measured the superconductor simulator with a variety of methods including dc and pulse. Thk comparison indicated the presence of systematic biases and higher variability at low voltages in the I, determinations of the measurement systems. All critical current measurements at a criterion of 10 fiV on the I, simulator were within 2% of the NIST value for nominal critical currents of 2 and 50 A. These results could significantly benefit superconductor measurement applications that require highprecision quality assurance. Current (A)I. THE HYBRID SUPERCONDUCTOR SMULATOR ne hybrid superconductor simulator emulates the extremely 1. V-I charactexistic of the 50 A superconductor simulator circuit. nonlinear voltage-current (V-I> characteristic of a superconductor along with its other major electrical properties by using passive circuit elements such as resistors and a diode [I]. The diode provides the nonlinearity necessary to generate the V-I characteristic. The simulator contains an active temperature controlled oven to maintain the diode temperature near 35°C. The term hybrid refers to the fact that each simulator consists of passive and active (only the oven) components. The simulator can be used as a sample substitution box that is measured at room temperature. Each simulator contains two separate circuits: one has an IC of 2 A, and one has an IC of 50 A . The n-value of the V-I curve is about 24 [2].The simulator can be used to test the integrity and accuracy of a complex measurement system because it has highly reproducible electrical characteristics. For example, the change in critical currents during this experiment for two out of the three simulators was less than 0.04%; the third simulator was still in circulation at the time of this writing. Fig. 1 shows a typical V-I curve for a 50 A simulator with an n-value of approximately 24. We calibrated each simulator to verify the linearity of the temperature dependence of the IC Manuscript received October 17, 1994. subject to copyright. Publication of the National Institute of Standards and Technology, notIn the calibration procedure, we obtained five V-I curves at diode tempaatures of appmximately 31, 33, 35, 37, 39, 39, 37, 35,33, a d 31°C for a total of 50 curves. We analyzed each V-I curve and determined IC at a number of criteria. Fig. 2 shows the NIST I , determinations on...
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