IEC 61788-17-2021 pdf free.Superconductivity – Part 17: Electronic characteristic measurements – Local critical current density and its distribution in large-area superconducting films.
Figure 1 shows a schematic diagram of a typical electric circuit used for the third-harmonic voltage measurements. This circuit is comprised of a signal generator, power amplifier, digital multimeter (DMM) to measure the coil current, band-ejection filter to reduce the fundamental wave signals and lock-in amplifier to measure the third-harmonic signals. It involves the single- coil approach in which the coil is used to generate an AC magnetic field and detect the inductive voltage. This method can also be applied to double-sided superconducting thin films with no obstacles. In the methods proposed here, however, there is an additional system to reduce harmonic noise voltages generated from the signal generator and the power amplifier [14]. In an example of Figure 1, a cancel coil of specification being the same as the sample coil is used for cancelling. The sample coil is mounted just above the superconducting film, and a superconducting film with a Jd sufficiently larger than that of the sample film is placed below the cancel coil to adjust its inductance to that of the sample coil. Note that the inductance of the sample coil decreases by 20 % to 30 % due to the superconducting shielding current when it is mounted on a superconducting film. Both coils and superconducting films are immersed in liquid nitrogen (a broken line in Figure 1). Other optional measurement systems are described in Annex B.
NOTE In this circuit, coil currents of about 0,1 A (RMS) and power source voltages of > 6 V (RMS) are needed to measure the superconducting film of ici 10 kA/m while using coil 1 or 2 of Table 2. A precision power amplifier with sufficiently high power is used to supply such large currents and voltages.
Currently available large-area HTS films are deposited on areas as large as about 25 cm in diameter, while films about 5 cm in diameter are commercially used to prepare microwave filters [22]. Larger YBa2Cu3O7 (YBCO) films, about 10 cm in diameter and 2,7cm x 20cm, were used to fabricate fault current limiter modules [3] [4] [5]. For the J measurements of such films, the appropriate outer diameter of the sample coils ranges from 2 mm to 5 mm. The requirement for the sample coil is to generate as high a magnetic field as possible at the upper surface of the superconducting film, for which flat coil geometry is suitable. Typical specifications are as follows.
a) Inner winding diameter D1: 0,9 mm, outer diameter D2: 4,2 mm, height Ii: 1,0 mm, 400 turns of a 50 pm diameter copper wire.
b) D1: 0,8 mm, D2: 2,2 mm, h: 1,0 mm, 200 turns of a 50 pm diameter copper wire.
5.2.2 Spacer film
Typically, a polyimide film with a thickness of 50 pm to 125 pm is used to protect the HTS films. The coil has generally some protection layer below the coil winding, which also insulates the thin film from Joule heat in the coil. The typical thickness is 100 pm to 150 pm, and the coil-to- film distance Z1 is kept to be 200 pm.
5.2.3 Mechanism for the set-up of the coil
To maintain a prescribed value for the spacing Z1 between the bottom of the coil winding and the film surface, the sample coil should be pressed to the film with sufficient pressure, typically exceeding about 0,2 MPa [18]. Techniques to achieve this are to use a weight or spring, as shown in Figure 2. The system schematically shown in the figure left is used to scan a wide area of the film. Before the U3 measurement the coil is initially raised up to some distance, moved laterally to the target position, and then lowered down and pressed to the film. An appropriate pressure should be determined so that too high pressure does not damage the bobbin, coil, HTS thin film or the substrate. It is reported that the YBCO deposited on biaxiallytextured pure Ni substrate was degraded by transverse compressive stress of about 20 MPa [23].IEC 61788-17 pdf download.