IEC 61788-4:2011 pdf – superconductivity – Part 4: Residual resistance ratio measurement – Residual resistance ratio ofNb-Ti composite superconductors.
5.3 Cryostat for the resistance. R2, measurement
The cryostat shall include a specimen support structure and a liquid helium reservoir for the resistance. A2. measurement. The specimen support structure shall allow the specimen. which is mounted on a measurement mandrel or a measurement base plate, to be lowered and raised into, and out of, a liquid helium bath. In addition, the specimen support structure shall be made so thai a Current can 110w through the specimen and the resulting voltage generated along the specimen can be measured.
6 Specimen preparation
The test specimen shall have no joints or splices, and shall be 30 mm or longer. The distance between two voltage taps (L) shalt be 25 mm or longer. A thermometer for measuring cryogenic temperature shall be attached near the specimen.
Some mechanical method shall be used to hold the specimen against the insulated layer of the measurement mandrel or base plate. Special care shall be taken during instrumentation and installation of the specimen on the measurement mandrel or on the measurement base plate so that no excessive force, which may cause undesired bending strain or tensile strain, shall be applied to the specimen.
The specimen shall be instrumented with current contacts near each end of the specimen and a pair of voltage contacts over a central portion of the specimen, The specimen shall be mounted on a measurement mandrel or on a measurement base plate for these measure ments. Both resistance measurements. A1 and A2. shall be made on the same specimen and the same mounting.
7 Data acquisition and analysis
7.1 Resistance (R1) at room temperature
The mounted specimen shall be measured at room temperature (Tm (K)). where Tm satisfies the following condition, 273  T,,  308. A specimen current (It (A)) shall be applied so that the current density is in the range ot 0,1 A/mm2 to 1 A/mm2 based on the total wire cross- sectional area, and the resulting voltage (U1 (V)). I, and Tm shall be recorded. Equation (2) below shall be used to calculate the resistance (Am) at room temperature. The resistance (A1) at 293 K (20 °C) shall be calculated using equation (3) for a wire with Cu matrix. The value of A1 shall be set equal to m, without any temperature correction, for wires that do not contain a pure Cu component.
7.2.6 The specimen voltage versus temperature curve shall be acquired with the rate of temperature increase maintained between 0.1 K/mm and 10 K/mm.
7.2.7 The voltage versus temperature curve shall continue to be recorded during the transition into the normal state, up to a temperature somewhat less than 15 K. Then, the specimen current shall be decreased to zero and the corresponding voltage. shall be recorded at a temperature below 15 K.
7.2.8 The specimen shall then be slowly lowered Into the liquid helium bath and cooled to the same temperature, within ±1 K, where the initial voltage si9nal U0, was recorded. A specimen current. 2. with the same magnitude but negative polarity (polarity opposite that used for the initial curve) shall be applied and the voltage U0… shall be recorded at this temperature. The procedural steps 7.2.5 to 7.2.7 shall be repeated to record the voltage versus temperature curve with this negative current. In addition, the recording of U20. shall be made at the same temperature, within !I K. where U20÷ was recorded.
7.2.9 Each of the two voltage versus temperature curves shall be analyzed by drawing a line (a) through the data where the absolute value of voltage sharply increases with temperature (see Figure 2) and drawing a second line (b) through the data above the transition where the voltage is nearly constant with temperature. U÷ and in Figure 2 shall be determined at the intersection of these two lines for the positive and negative polarity curves respectively.