46 Ë 2 Superconducting materials
by helically wrapping 44 layers of superconducting Nb 3 Sn ribbon 2.54 cm wide by
75 μm thick around a mandrel.
However, the trapped field in LTS bulk superconductors at low temperatures was
found to be strongly limited by thermomagnetic instabilities, resulting from flux jumps
as the external field exceeds certain values of the applied magnetic field. Due to
the low specific heat of LTS materials in superconducting state, the thermomagnetic
instabilities are very pronounced at low temperatures (see Section 2.5.1).
In contrast to LTS materials, there are some peculiar features in the specific heat of
YBCO compared with those of conventional BCS superconductors, namely HTS bulks
are thermally stable even in large sample sizes due to their relatively high specific
heat in the superconducting state. Due to the larger specific heat of HTSC, the heat
within the superconductor is released to the surrounding cryogen. Therefore, thermal
instabilities have no influence on the trapped field in HTSC, at least at temperatures
above 30 K. Very high trapped fields can be achieved by the HTS bulk material. The
superconducting PM and its applications have become a reality because of HTSC
material.
In engineering applications, high-texture andc-axis-oriented single domains are
required. Large-sized, high-performance REBCO single domains are now commercial-
ly available. The trapped flux density (Btrap) due to flux pinning and the associated
superconducting currents flowing persistently in a REBCO can be expressed in a
simple model, as [152]
Btrap=A휇 0 Jcr, (2.3)whereAis a geometrical constant,휇 0 is the permeability of the vacuum, andris the
radius of the grain. In order to increase the trapped flux, both the critical current
density, the dimension and the homogeneity of HTSC bulk must be enhanced. Increase
of the critical current density is accomplished by the improvement of flux pinning
properties.
Jc=AVf
d, (2.4)whereAis constant,Vfis the volume fraction, anddis the average diameter of Y211
particles. Thus, under constant volume of the second phase particles,Jcis inversely
proportional to the size of the Y211 particles [153].
In order to avoid the influence of a temperature rise on thermal stability, the
cooling power of the system must be larger than local heat generation. Since the
thermal conductivity of HTS bulk is small (see Section 2.5.2, about 2–10 W/mK at 77 K),
cooling by cryogen at the bulk’s surface is not sufficient to cool the interior region,
leading to a local temperature rise. The temperature rise will reduce the flux pinning
capability and induced flux jumps. Once the flux jumps appear in a HTS bulk, it is no