Superconductors lose all resistance to the flow of direct electrical current and nearly all resistance to the flow of alternating current when cooled below a critical temperature, which is different for each superconducting material.
Perfect Conductor of Electricity
A superconductor is a perfect conductor of electricity; it carries direct current with 100% efficiency because no energy is dissipated by resistive heating. Once induced in a superconducting loop, direct current can flow undiminished forever. Superconductors also conduct alternating current, but with some slight dissipation of energy.
Superconducting materials known today, including both high temperature superconductor (“HTS”) and low temperature superconductor (“LTS”) materials, need to be cooled to cryogenic temperatures in order to exhibit the property of superconductivity.
HTS vs LTS
The differences between high and low temperature superconductors can best be explained using the figure 1. This graph illustrates the complete loss of resistance to the flow of electricity through wires of an LTS material (niobium-titanium alloy) and an HTS material (bismuth-based, copper oxide ceramic) at the critical temperature Tc which is different for each superconducting material. The specific HTS material in this chart has no electrical resistance below 108K (-265°F) as opposed to the specific LTS material in this chart, which has no electrical resistance below 10K (-441°F).
Figure 1. The differences in behaviour between LTS and HTS materials.
Conditions Required for a Material to Exhibit Superconducting Behaviour
• The material must be cooled below a characteristic temperature, known as its superconducting transition or critical temperature (Tc).
• The current passing through a given cross-section of the material must be below a characteristic level known as the critical current density (Jc).
• The magnetic field to which the material is exposed must be below a characteristic value known as the critical magnetic field (Hc).
These conditions are interdependent, and define the environmental operating conditions for the superconductor.
Figure 2. The conditions required for a material to exhibit superconductivity.
Advantages of Superconducting Wire
Superconducting wires provide significant advantages over conventional copper wires because they
• Conduct electricity with little or no resistance and associated energy loss
• Can transmit much larger amounts of electricity than conventional wires of the same size