An ac susceptometer for the characterization of large, bulk superconducting samples

Abstract

The main purpose of this work was to design, develop and construct a simple, lowcost AC susceptometer to measure large, bulk superconducting samples (up to 32 mm in diameter) in the temperature range 78-120 K. The design incorporates a double heating system that enables a high heating rate (25 K/hour) while maintaining a small temperature gradient (< 0.2 K) across the sample. The apparatus can be calibrated precisely using a copper coil connected in series with the primary coil. The system has been used successfully to measure the temperature dependence of the AC magnetic properties of entire RE-Ba-Cu-O [(RE)BCO] bulk superconducting domains. A typical AC susceptibility measurement run from 78 K to 95 K takes about 2 hours, with excellent temperature resolution (temperature step ~ 4 mK) around the critical temperature, in particular.

Self-heating of bulk high temperature superconductors of finite height subjected to a large alternating magnetic field

“…[33]. The sample is placed in a cylindrical experimental chamber made of ultra-high molecular weight polyethylene (PE-UHMW) of 5 mm thickness, as illustrated schematically in figure 2(c).…”

“…In our experimental system, which was designed for simultaneous thermal and magnetic measurements [33], the sample is placed in an experimental chamber and exposed to a medium vacuum (pressure ~ 10 -2 -10 -1 mbar). As a result, the convective coefficient U is low and the Biot number is estimated to be ~ 0.02 [29].…”

A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors

“…Bespoke AC susceptometers can be used to investigate the volumetric properties of large superconducting samples, [10][11][12] although the penetration depth of the largest AC fields (0.1 T) at temperatures of interest (e.g., 77 K or lower) in high quality materials is typically small compared to the diameter of the sample. 10 One method of measuring the volume DC magnetic properties of these large, bulk superconductors involves winding a coil tightly around the sample and then measuring the flux in the coil directly.…”

“…10 One method of measuring the volume DC magnetic properties of these large, bulk superconductors involves winding a coil tightly around the sample and then measuring the flux in the coil directly. 13 Unfortunately, however, this method only provides information about the average magnetic flux density in the sample, 14 and the determination of the magnetic moment requires numerical modeling based on assumptions concerning isotropy, homogeneity, and constitution.…”

Improvement in granularity of NdFeAsO0.8F0.2 superconductor through Agx doping (x = 0.0–0.3)

“…Interestingly, the inter-granular peak is observed for lower amplitudes only. The peak near to T c arises, when the ac field penetrates to the centre of grains, and the peak at lower temperature corresponds to the complete penetration of the ac field in to the centre of the sample [35,36]. The appearance of a kink at ∼45K at higher amplitudes clearly demonstrates the presence of intra-grain superconductivity of the sample.…”