2.4 HTS bulk materials Ë 31current. However, for sintered YBCO HTS bulk, theJcis low due to the existence of
weak links at the large numbers of grain boundaries. The sintered materials cannot
be used in HTS magnetic levitation engineering; thus, a new preparation technique
for bulk YBCO is required. Fabrication techniques for bulk (RE)BCO materials in the
practical applications must overcome the weak-link problem associated with grain
boundaries.
To overcome theJclimitations, due to the weak-link properties of the grain
boundaries in sintered YBCO ceramics, Jin et al. [73] developed, in 1988, a melt process
called MTG. The melt textured YBCO bulk can support aJcat least three orders of
magnitude higher than that of sintered samples. The value of the transport current
densityJcof the MTG samples obtained by Jin et al. [74] had exceeded 10^4 A/cm^2
in zero magnetic field at liquid nitrogen temperature of 77 K. This result indicates
that grain alignment may have extensively reduced the weak links and lead to a
great improvement of the material. In order to improve theJcvalues in a magnetic
field, effective pinning centers must be introduced into the materials. In the following
years, this process technique was modified and improved by several researchers.
The first improvement was achieved by Salama et al. [75] by introducing the liquid-
phase processing technique. Murakami et al. [76, 77] developed the melt powder
melt growth (MPMG) method. REBCO prepared by MPMG has a highJcat 77 K in
high magnetic fields. MPMG can easily increase domain size to a cubic centimeter.
Microstructural observations reveal that the size of the 211 phase is much finer and its
distribution is much more uniform than that of the classical MTG samples. MPMG also
improves theJcvalues, especially under a magnetic field of 1 T. TheJc(B)variations
which were calculated from the magnetization measurements on MPMG processed
samples exhibit 5–6× 104 A/cm^2 in the self field and over 2× 104 A/cm^2 in fields
under 1 T.
In order to synthesize products for engineering applications, numerous approa-
ches have been tried using a variety of synthesis technologies, such as MTG, melt
powder melt growth (MPMG), top-seeded melt-textured growth (TSMG), and seeded
infiltration and growth (SIG) [78] processes, etc. Up to now, the melt processes of the
HTS (RE) BCO bulk generally use both MPMG and SIG process methods. Recently, Li
et al. [79] and Chen et al. [80] give detailed reviews of materials processing of single-
grain REBCO HTS bulks. The TSMG process for bulk (RE)BCO materials has been esta-
blished as an effective method to fabricate large- and single-domain bulk REBCO with
high performance. The TSMG is conductive to multi-block processing at the same time
and controlled multi-domain bulk growth. However, the TSMG process technique has
drawbacks, such as easy coarsening, large shrinkage, and liquid outflow. Macroscopic
cracks and pores are also easy to form in the melt growth. The chemical composition
of the final product is inhomogeneous in general. In comparison to the TSMG process,
the major advantage of the SIG process is the ability to provide fine-sized spherical
RE-211 precipitates in the REBCO matrix, even without addition of compounds such
as Pt and CeO 2. However, the SIG process has the disadvantage that the uniformity of
microstructure andJcis uncertain [81].