4.6 HTS bulk Maglev Ë 1014.6 HTS bulk Maglev
In 1933, Meissner and Ochsenfeld [81] found that when a sphere is cooled below
its transition temperature in a magnetic field, it excludes the magnetic flux (see
Section 1.3). In 1945, Russian physicist Arkadiev [5], at Maxwell Laboratory in Moscow
State University, USSR, published an article, titled “Hovering of a magnet over a su-
perconductor”. The stable levitation phenomenon of a magnet over a superconductor
was first observed. The photograph [6] in Fig. 4.9 shows a magnet of 4× 4 ×10 mm
in dimensions floating above a concave lead disk (lowTcsuperconductor) 40 mm.
The pure lead is a type I superconductor. The interaction between the magnetic
field and a type I superconductor in Meissner-Ochsenfeld state produces magnetic
levitation force. The Meissner-Ochsenfeld effect completely expels the magnetic fields
(belowHc)to the field penetration depth휆of the superconductor surface. Thus,
the levitation forces are very small. The investigation of bearings using LTSCs was
reported in 1953 [82], and the study of a three-phase motor with superconductive
bearings was reported in 1965 [83].
There were no practical applications of LTS bulk superconductors before the
discovery of HTS, due to their thermal instability [84]. Local heat generation within
the LTS bulk materials leads easily to a large magnetic flux jump, namely to a quench
of the superconducting state. In contrast to LTS bulk materials, there are some peculiar
features in the specific heat of YBCO compared with those of conventional BCS
superconductors, namely bulk HTS are thermally stable even in large sample sizes
due to their relatively large specific heat (see Section 2.5.1) in the superconducting
state.
The LTS MgB 2 material is a special case, because it has higher specific heat. Flux
jumps in MgB 2 arise at very low temperatures, but tend to disappear at temperatures
larger than 10 K [85]. Thus, MgB 2 can be made as large bulk material and produce
a larger trapped flux. MgB 2 bulk is also promising as a superconducting permanent
magnet.
The type II superconductors are similar to type I superconductors and present
a Meissner-Ochsenfeld effect (see Section 1.3) only when the magnetic applied field
Fig. 4.9:The photograph of a magnet of 4× 4 ×10 mm
floating above a concave lead disk 40 mm [6].