Critical current limitations in ceramic oxide superconductors

• Published in: Cryogenics (Guildford) 1989-03, Vol.29 (3), p.399-404
• Main Author: Stephens, R.B.
• Format: Article
• Language: English
• Subjects: ceramics; Condensed matter: electronic structure, electrical, magnetic, and optical properties; critical currents; Exact sciences and technology; high Tc superconductors; Physics; Properties of type I and type II superconductors; Superconductivity
• ISSN: 0011-2275; 1879-2235
• DOI: 10.1016/0011-2275(89)90197-5

Weak links between grains of ceramic oxide superconductors limit the measured transport critical current density, J c, because of both the weak coupling between grains, and the field caused by the measuring current. The weak links have a critical current density, J w, much less than the intra-grain critical current density J grain. Magnetic fields, specifically the field induced by the measuring current, further reduce J c; a large fraction of the links become resistive at field values, H half (typically 4 to 25 Gauss), which depend only on the magnetic penetration length, λ L, and the grain size, L. As a result of these two effects, the measured critical current in a field-limited sample of cross-section A and circumference C is modelled to be J c = J w for J w < (5/2 π) ( C/ A) H half and J c = (5/2 π) ( C/ A) H half otherwise. In the latter case, J c is independent of J w. Most reported critical currents of ceramic oxide superconductors are apparently field-limited measurements, and therefore depend on the sample dimensions rather than the value of the intergranular limit, J w. One measures J w only for small samples or weakly coupled junctions. The field-limited behaviour disappears only when the grains are coupled well enough that links can carry enough current to shield them from external fields, which occurs when L > λ J =λ L (J grain J w ) 1 2 , this condition having apparently been achieved in some reported samples.