IEC 61788-23-2021 pdf download.Superconductivity – Part 23: Residual resistance ratio measurement – Residual resistance ratio of cavity-grade Nb superconductors.
4 Principle
The 4-point DC electrical resistance technique shall be performed both at room temperature and at cryogenic temperature. The test may be done either as a function of temperature or as a function of time with increasing temperature.
The relative combined standard uncertainty of this method is 3 % with coverage factor 2.
Measurements shall have the following attributes.
a) Measuring current is sufficiently high to provide voltage signals of the order of 1 µV. For electrical safety, maximum current density should never exceed 1 A mm −2 .
b) Contact resistance for current leads is sufficiently low to avoid excessive heating of the sample. Typical cryogenic measurement conditions require power dissipation at contacts to be less than 1 mW.
c) Sample sizes are sufficiently large to minimize effects from cutting and handling damage. Typical samples are 1 mm to 3 mm in cross-section dimension and > 5 mm 2 in cross- sectional area.
d) Sample length is at least 1 0 times and not more than 25 times the width or diameter. Annex A discusses considerations for sample dimensions and measuring current.
5 Measurement apparatus
5.1 Mandrel or base plate A straight mandrel or base plate shall be used to support the specimen. Possible materials of construction include pure copper, pure aluminium, pure silver, electrical grades of Cu-Zr, Cu-Cr-Zr, Cu-Be, and other copper alloys, electrical grades of Al-Mg, Al-Ag, and other aluminium alloys, and electrical grades of silver alloys. These provide high thermal conductivity and serve to remove thermal gradients during measurement.
The specimen shall be insulated from the mandrel. Possible insulating materials include polyethylene terephthalate, polyester, and polytetrafluoroethylene, which may be applied as foils, tapes, or coatings. Glass-fibre reinforced epoxy or other composite materials with good thermal conductivity at cryogenic temperature may also be used.
The base plate should have a clean and smooth surface finish. There should be no burrs, ridges, seams, or other asperities that may affect the specimen. High-purity niobium specimens are soft and are susceptible to indentation by surface flaws, and such indentations may alter the sample and invalidate the resistance measurement. The mandrel or base plate shall support the entire length and width of the specimen. Mandrel or base plate geometry should not impose a bending strain of more than 0,2 % on the sample. A thermometer accurate to 0,1 K is helpful but not required. The mandrel or base plate may incorporate a mounting for a cryogenic thermometer directly against the body of the mandrel or base plate and near the centre of the test specimen. Practical base plates are at least 30 mm in length to accommodate assembly of pieces and handling of samples by human hands. Multiple samples may be mounted against a single base plate.
5.2 Cryostat and support of mandrel or base plateThe apparatus shall make provisions for mechanical support of the mandrel or base plate. In addition, such support shall provide electrical leads to carry currents for samples and thermometers, and measure their voltages. For 1 R and 2 R measurements, the support shall permit current to flow through only the sample, so that the entire resulting voltage measured is only that generated by the sample. The support structure shall permit measurement of both 1 R and 2 R without dismounting or remounting the test specimen. Measurement of 2 R shall require the use of a cryostat, which shall, moreover, integrate with the support.IEC 61788-23 pdf download.