2.7 Trapped fluxes in HTS bulk Ë 49and 9 ppm/cm^3 using the single HTS bulk with 20 mm inner diameter, 60 mm outer
diameter, and 80 mm height by the remagnetization process.
Durrell et al. [162] reported a trapped field of 17.6 T in a stack of two GdBCO
HTS bulks from a 17.8-T magnetizing field at 26 K. This is the highest field trapped
in a bulk superconductor reported to date. The silver-doped GdBCO bulks were
25 mm in diameter and fabricated by TSMG and reinforced with shrink-fit stainless
steel.
LTS bulks show the large and sharp magnetic flux jumps that prevent their use.
HTS bulks show higher thermal stability and can be used as superconducting PMs.
The MgB 2 is a new type of LTSCs. The MgB 2 bulk has also a promising potential
as superconducting PM. It has several attractive features for bulk magnets, such as
low cost, light weight, and homogeneous trapped field distribution. In addition, flux
jumps in MgB 2 arise at low temperature (4.2 K), but tend to disappear at temperatures
larger than 10 K. Perimi et al. [20] have studied the magnetic field trapping capability
of MgB 2 discs of different shapes, at 10-K temperature. The SIMS (superconductive
inserts in metallic substrates) devices showed a higher stability of the trapped fields
with respect to the bulk discs. Typical trapped fields, measured at 1 mm from the
surface of the device, are of the order of 1 T.
Fujishino et al. [24] performed PFM on a large MgB 2 bulk 50 mm in diameter, in
which a trapped fieldBtrap=0.47 T was achieved at 23 K. Thereafter, they reported
a maximum trapped fieldBtrap=0.71 T, which was realized at the center of the bulk
surface at 14 K after a magnetic pulse application ofBex=1.55 T [24]. They found that
the flux dynamics and heat generation in a MgB 2 bulk during PFM were clearly in
contrast to those for REBCO bulks, because of the small specific heat, large thermal
conductivity, and narrow temperature margin againstTcin the MgB 2 bulk.
In order to reduce the cost and meet the needs of more and more applications,
batch production of bulk HTSC been improved. Plechacek et al. [118] have reported
development and successful tests of a melt-powder-melt-growth process capable of
simultaneous fabrication of up to 64 YBCO levitation disks, a necessary step towards
the planned production capacity of several thousand pieces a year. Diameters of the
HTS bulks varied from 14 to 56 mm. The trapped magnetic field measured at 77 K on
the bulks arbitrarily chosen from different batches was in all cases higher than 0.5 T.
Levitation force at 77 K with a levitation gap of 1 mm reached the same value of 80 N in
all samples. The self-field critical current densityJcat 77 K exhibited values between
50 and 90 kA/cm^2 at the bulk center and 70–95 kA/cm^2 at the bulk edge.
A production run of 60 melt-textured YBCO trapped field magnets, 2 cm in diame-
ter, was reported by Sawh et al. [163] The HTS PMs have pinning centers of Y211 and
Pt0.4U0.6/YBa 3 O 6 deposits and also damage tracks due to ions from uranium fission.
The resulting average trapped field at the center of the seed-side surface is 2.04 T at
77 K.
Trapped flux can be measured by magnetic sensor scanning, which will be
introduced in Chapter 5.