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Superconductors

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Abstract

Superconducting materials (superconductors) are electrically conductive materials which under certain specific conditions; (i) show a decrease in their intrinsic electrical resistivity to a value near zero, and (ii) become perfect diamagnetic materials, i.e., there is a complete exclusion of an applied magnetic field from the bulk of the material. However, three particular conditions must be required for a material to exhibit the superconducting state with these unusual electric and magnetic properties: the temperature of the material must be lowered below a certain critical temperature (T c, in K); the applied magnetic field strength must be less than the critical magnetic field strength (H c, in A.m−1); and the current density flowing in the superconductor must be less than the critical current density (j c, in A.m−2). If any of these values exceeds the respective critical value, the superconductor becomes quenched, and loses its superconductive properties. Hence, the two former parameters, i.e., the critical temperature and critical magnetic field strength are intrinsic properties for a given material or composition and they are not affected to any large extent by modification in processing or changes in microstructure. On the contrary, the critical current density may vary over several orders of magnitude within a single material, and it is strongly affected by: (i) the metallurgical processing, (ii) the presence of crystal lattice defects and traces of chemical impurities, and finally (iii) the microstructure. However, these three critical parameters are closely interdependent and can be defined by a three-dimensional thermodynamic phase diagram within which the superconducting state is stable. For all superconducting materials presently known, the critical temperature ranges between boiling point of liquefied gases to below room temperature and experimental measurements have clearly demonstrated that no measurable decay of the superconducting properties has been noted. The most common method to attain the low temperatures required by superconductors is to use low-temperature liquefied gases such as liquid helium (bp 4.22 K or −268.93°C) or liquid nitrogen (bp 77 K or −196.15°C). For understanding superconductivity, it is important to remember the meaning of the physical quantity called the electrical resistance. Actually, if we consider a common solid conductor (i.e., resistor senso-stricto) which undergoes a potential gradient on each side, the electric potential difference (i.e., voltage) causes the electric charge (e.g., electric carriers such as electrons, or ions) to flow through the material with a definite charge flow rate (i.e., electric current). In this microscopic description of electric charge migration in solid materials, the electric resistance can be easily understood as the friction encountered by the electric charge carriers during displacements. According to Ohm’s law, U = RI, the electric current is directly proportional to the voltage, and inversely proportional to the electrical resistance. Higher resistance causes less current for a given voltage, and higher voltage causes more current for a given resistance. If the voltage drops to zero, no electric current will flow, on the contrary, for any resistance greater than zero. If the resistance is zero, there is theoretically no voltage needed for electric current to flow.

Keywords

  • Critical Temperature
  • Magnetic Flux
  • Critical Current Density
  • Superconducting State
  • Perfect Conductor

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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© 2000 Springer-Verlag London

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Cardarelli, F. (2000). Superconductors. In: Materials Handbook. Springer, London. https://doi.org/10.1007/978-1-4471-3648-4_5

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  • DOI: https://doi.org/10.1007/978-1-4471-3648-4_5

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