Acoustic emission of the HTS ceramic YBa2Cu3O7−δ in the vicinity of the superconducting transition

Abstract

Acoustic emission (AE) of the HTS ceramic YBa2Cu3O7−δ is studied under heating from liquid nitrogen temperature. For heating at a high rate, a peak in the AE activity is observed in the temperature interval 100–115 K. The parameters of the peak depend on the oxygen content in the HTS ceramic, heating rate, initial temperature to which the ceramic was cooled, and the sample thickness. The observed AE is associated with the relaxation of microscopic thermal stresses generated at the grain boundaries due to anisotropy of the thermal expansion coefficients and their anomalous behavior at these temperatures. Possible relaxation mechanisms of such stresses are the movement of dislocation groups and the formation of microcracks along the least favorably oriented grain boundaries.

‘Thermal’ Kaiser’s effect in YBa2Cu3O7−Xceramic in the vicinity of 100–200 K by acoustic emission data

“…AE signals at 85 K are present every time, whereas AE signals above 113 K appear when a rate of heating the samples exceeds 0.5 K s −1 . This latter data is in good agreement with the data of [13] too. However, a behaviour of AE temperature (T AE ) is surprised during the thermal cycling of the samples with a rate of about 0.7 K s −1 .…”

“…During heating the samples AE signals are detected at 85 K, as they were earlier measured in [11], and at 113 K, as they were earlier measured in [13]. AE signals at 85 K are present every time, whereas AE signals above 113 K appear when a rate of heating the samples exceeds 0.5 K s −1 .…”

“…However, the following measurements show that AE has peaks higher than T c at 110-115 K without any current in YBa 2 Cu 3 O 7−X ceramics with 6.28 < (7 − X) < 6.8, but due to the high rate of heating ∼0.5 K s −1 from 77 K [13]. The authors conclude that AE is the reason for there to be a relaxation of stress, arising on the grain's boundaries due to anisotropic thermal expansion of the material.…”

“…T AE has a minima, when • N has a maxima, i.e. when the quantity of defects becomes minima due to annihilation at the third cycle, near 100 K. Obviously, the authors [13] observed AE in the vicinity of 110-115 K in view of the highest • N at the third cycle in comparison to the range 100-168 K. In the initial stage as well as in the finish stage T AE has to be increased. After the third cycle, T AE increases approximately linearly.…”

Quench Detection and Protection for High-Temperature Superconductor Accelerator Magnets

“…At the same time, hot spot development results in a number of secondary acoustic effects, such as micro-mechanical motion along various interfaces due to thermal expansion, dislocation motion and cryogenic liquid boiling. Onset of AE due to these sources has indeed been reported for HTS conductors [67][68][69][70][71][72], but only for cases where a significant overcurrent was supplied to the conductor and heating was quite substantial (such as in a superconducting fault current limiter [73]). It remains unclear at the moment if passive AE sensing can be reliably used for the detection of hot spots where local temperature variations are of the order of 1 K or less, which would be practical for quench detection.…”