Environmental Engineering Reference
In-Depth Information
The critical temperature of these materials can be up to around 25 K, but all need
to be cooled to 4.2 K for practical use. The materials have been developed into
practical wire products and are now commonly used in magnets for particle accel-
erators and in the commercial market for MRI scanners. A summary of the proper-
ties and manufacture of these low temperature superconductors (LTS) can be
found in [ 16 , 17 ].
Many studies were made into the use of these LTS materials for the fi eld wind-
ings (magnets) of rotating machines [19], particularly in the 1960s and 1970s. The
high magnetic fi eld strength for superconducting magnets results in much smaller
machines, and higher effi ciency due to the zero loss in the fi eld winding. However,
the practicality and cost of cooling the fi eld on the rotor of these machines using
liquid helium meant that they never became a commercial proposition.
3.2 High temperature superconductors
Only small increases in the critical temperature of these LTS materials were
achieved from their discovery in 1911 up until the 1980s. Then, in 1986 a material
was discovered by Bednorz and Muller that became superconducting at a tempera-
ture of around 30 K [18], and very shortly afterwards (Fig. 3) many more materials
were discovered with ever increasing critical temperature, although after 1990 this
trend considerably slowed.
These discoveries brought the operating temperature of the superconductors
into the range of liquid nitrogen, which is two orders of magnitude cheaper
than the liquid helium used to cool LTS coils. All HTS materials are Type II
Superconductors.
HgBa2Ca2Cu3O9
(under pressure)
160
140
HgBa
2
Ca
2
Cu
3
O
9
TlBaCaCuO
120
L
iquid Methane
LNG (112K)
B2223 - Bi
2
Sr
2
Ca
2
Cu
3
O
100
YBCO - YBa
2
Cu
3
O
7
L
iquid Nitrogen
(77K)
80
60
40
(La,
(La,
Ba
Ba
) Cu)
) Cu)
(La, Ba) Cu)
Nb
3
Ge
Nb
3
Ge
Nb
3
Sn
Nb
3
Sn
NbN
NbN
20
NbC
NbC
Nb
Hg
Pb
V
3
Si
V
3
Si
1910
1930
1950
1970
1990
Figure 3: Development of the critical temperature of superconductors.
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