ED0371109_Nb_RRR_Test_Methods_Josh Helsper_Christopher Edwards.pdf

This is a technical specification document for measuring the residual resistance ratio (RRR) of cavity-grade niobium, prepared by Fermilab's Superconducting Materials Department under specification number 5500-ES-371109, Revision 0 (dated August 26, 2011). The document serves as a draft international standard (IEC 61788-xx) and temporary IEC 90/280/NP and outlines a standardized test method using 4-point DC electrical resistance measurement without magnetic field requirements.

The specification applies to high-purity niobium with RRR values between 150 and 600, using long samples with rectangular or round cross-sections greater than 1 mm² but less than 20 mm². The test method measures resistance at room temperature (293 K) and just above the superconducting transition temperature (~10 K) to calculate RRR as the ratio R₁/R₂. Key apparatus requirements include a mandrel or base plate with thermal conductivity exceeding 100 W m⁻¹ K⁻¹, a cryostat with liquid helium capability, voltage tap separation of approximately 5 cm, and measuring current of approximately 1 ampere to achieve ~1 μV signals at low temperature. Specimen preparation protocols emphasize avoiding mechanical deformation through careful cutting and cleaning techniques, with samples requiring minimum 30 mm length and 25 mm voltage tap separation. Data acquisition specifications require voltage measurement uncertainty not exceeding 0.5% relative standard uncertainty, current stability within 0.1% per measurement, and thermal uniformity within 1 K of pure niobium's critical temperature (9.2 K). The test report must document specimen dimensions, transport currents, measured voltages, resistances, resistivity values, room and critical temperatures, and RRR values with stated uncertainty. Target measurement uncertainty is 5% based on 2.5% coefficient of variation among intercomparison tests.