Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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Revised Pages

18.12 Superconductivity • 751

Temperature (K)

Normal metal

Superconductor

Electrical resistivity

0 TC

0

Figure 18.26 Temperature

dependence of the electrical

resistivity for normally conducting

and superconducting materials in

the vicinity of 0 K.

The superconductivity phenomenon has been satisfactorily explained by means

of a rather involved theory. In essence, the superconductive state results from attrac-

tive interactions between pairs of conducting electrons; the motions of these paired

electrons become coordinated in such a way that scattering by thermal vibrations

and impurity atoms is highly inefficient. Thus, the resistivity, being proportional to

the incidence of electron scattering, is zero.

On the basis of magnetic response, superconducting materials may be divided

into two classifications designated as type I and type II. Type I materials, while

in the superconducting state, are completely diamagnetic; that is, all of an applied

magnetic field will be excluded from the body of material, a phenomenon known

as theMeissner effect,which is illustrated in Figure 18.28. AsHis increased, the

material remains diamagnetic until the critical magnetic fieldHCis reached. At this

point, conduction becomes normal, and complete magnetic flux penetration takes

place. Several metallic elements including aluminum, lead, tin, and mercury belong

to the type I group.

Type II superconductors are completely diamagnetic at low applied fields, and

field exclusion is total. However, the transition from the superconducting state to the

normal state is gradual and occurs between lower critical and upper critical fields,

designatedHC 1 andHC 2 , respectively. The magnetic flux lines begin to penetrate

into the body of material atHC 1 , and with increasing applied magnetic field, this

Current density J

Temperature T Magnetic field H

JC (T = 0 K, H = 0)

HC (T = 0 K, J = 0)

TC (H = 0, J = 0)

Figure 18.27 Critical

temperature, current density, and

magnetic field boundary

separating superconducting and

normal conducting states

(schematic).