IEC 61788-26:2020 pdf free download – Superconductivity
1 Scope
This part of lEC 61788 specifies a test method for determining the DC critical current of shortRE(rare earth)-Ba-Cu-O(REBCO) composite superconductor specimens that have a shape ofstraight flat tape.This document applies to test specimens shorter than 300 mm and having arectangular cross section with an area of 0,03 mm2 to 7,2 mm2，which corresponds to tapeswith width ranging from 1,0 mm to 12,0 mm and thickness from 0,03 mm to 0,6 mm.
This method is intended for use with superconductor specimens that have critical current lessthan 300 A and n-values larger than 5 under standard test conditions: the test specimen isimmersed in liquid nitrogen bath at ambient pressure without external magnetic field during thetesting. Deviations from this test method that are allowed for routine tests and other specificrestrictions are given in this document.
2Normative references
The following documents are referred to in the text in such a way that some or all of their contentconstitutes requirements of this document. For dated references, only the edition cited applies.For undated references，the latest edition of the referenced document (including anyamendments) applies.
IEC 60050-815, International Electrotechnical Vocabulary (IEV) – Part 815: Superconductivity
4Principle
The critical current of a composite superconductor specimen shall be determined from avoltage-current(U-I) characteristic measured in a liquid nitrogen bath at ambient pressure.Toget a U-l characteristic, a direct current is applied to the superconductor specimen and thevoltage generated along the specimen is measured.The current is increased from zero and theU-I characteristic is recorded.The critical current shall be determined as the current at which aspecific electric field strength criterion (electric field criterion)(E.) is reached.For any selectedEc, there shall be a corresponding voltage criterion(U.) for a specified voltage tap separation.
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.Thedepth 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 withmicrobubbles,i.e.steady state. It 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 lowerthan the equivalent of ramping from zero current to l, 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 % of lic.
Record the U-I 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.
Special features found in REBCO superconductors may be classified into two groups.The firstgroup is specific to REBCO multilayer composite superconductors，including mechanicalfragility of delamination，magnetic flux flow and creep， large anisotropy,screening currentcaused by magnetic field change，non-uniformity of superconducting properties，etc. Thesecond group is due to length of the specimen used in this document. A critical currentmeasurement on such a specimen may easily pick up different voltage signals due to inductivevoltage, thermal noise, current redistribution, etc. Current transfer voltages may be present dueto the short distance from a current contact to a voltage tap.