IEC 61788-26-2020 pdf free download – Superconductivity – Part 26: Critical current measurement – DC critical current of RE-Ba-Cu-O composite superconductors.
6.2 Mounting of the specimen
The specimen shall be mounted to the flat surface of the holder.Both ends shall be fastenedor soldered to the current contact blocks.
The voltage taps shall be placed in the central part with or without solder.
The current contacts and the voltage taps shall be on the superconducting layer side.
Voltage leads shall be twisted as close to the voltage taps as possible.
7 Critical current measurement
The critical current shall be measured while minimizing mechanical strain.
The specimen shall be inserted slowly into the liquid nitrogen bath. The volume of the liquidnitrogen bath shall be sufficiently larger than the specimen and the measurement holder.The
depth of the bath shall be sufficiently higher than the height of the measurement holder.Thespecimen shall be cooled from room temperature to liquid nitrogen temperature until thespecimen and the measurement holder are sufficiently cooled by liquid nitrogen that boils with
microbubbles,i.e. steady state. lt takes several tens of seconds.
When using the constant sweep rate method, the sweep rate shall be selected not to influencethe voltage measurement.
When using the ramp-and-hold method, the current sweep rate between stages shall be lower
than the equivalent of ramping from zero current to lc in 3 s. Data acquisition at each stageshall be started as soon as the flow or creep voltage generated by the current ramp can bedisregarded.The current drift during each current set point shall be less than 1 % oflc.
Record the U-l characteristic with increasing current.
After measurement, the specimen shall be warmed up to room temperature.Additional information relating to the measurement is given in Annex A.
8Calculation of results
8.1 Critical current criteria
The critical current lc shall be determined by using an electric field criterionEc.
8.2 n-value (optional)
The n-value shall be calculated as the slope of the plot of log U versus log I in the region where the I c is determined. The corresponding electric field region is recommended to be from 10 µV/m
to 100 µV/m.
The electric field region used to determine the n-value shall be reported.
Additional information relating to the calculation is given in Annex A.
9 Uncertainty of measurement
Unless otherwise specified, measurements shall be carried out in a liquid nitrogen bath whose temperature can range from 76,8 K to 77,7 K. A voltmeter having 7,5 digits resolution, providing 1 nV sensitivity on 10 mV setting shall be used to measure specimen voltage.
According to the international round robin test (see Annex B), the relative standard uncertainty is less than 3 %. The target measurement uncertainty shall be 6 % with a coverage factor of 2.
10 Test report
10.1 Identification of test specimen
The test specimen shall be identified by the following:
a) name of the manufacturer of the specimen;
b) classification and/or symbol;
c) lot number.
10.2 Reporting of I c values
The following values shall be reported:
a) I c values with their corresponding electric field criteria;
b) specimen temperature and/or ambient pressure.
The reporting of n-values is optional.
10.3 Reporting of I c test conditions
The following test conditions shall be reported:
a) length of specimen ( L);
b) width of specimen ( W);
c) thickness of specimen;
d) distance between voltage taps ( L 1 );
e) shortest distance from a current contact to a voltage tap ( L 3 );
f) length of the current contacts ( L 2 );
g) sweep rate when using the constant sweep rate method;
h) ramp pitch, ramping time and holding time when using the ramp-and-hold method.IEC 61788-26 pdf download.