Superconductors and superconducting materials are metals, ceramics, organic materials, or heavily doped semiconductors that conduct electricity without resistance.

Superconducting materials can transport electrons with no resistance, and hence release no heat, sound, or other energy forms. Superconductivity occurs at a specific material's critical temperature (T_c). As temperature decreases, a superconducting material's resistance gradually decreases until it reaches critical temperature. At this point resistance drops off, often to zero, as shown in the graph at right.

At the present time, most materials must achieve an extremely low energy state via low temperatures and/or high pressures in order to achieve superconductivity. While research superconductors that are effective at higher temperatures are in development, superconductivity is typically possible only with expensive, inefficient cooling processes.

Superconductors exhibit unique features other than their ability to perfectly conduct current. For example, many expel magnetic fields during the transition to the superconducting state. This is due to the Meissner effect by which superconducting materials set up electric currents near their surface at T_c, therefore canceling the fields within the material itself. A stationary magnet on a superconductor demonstrates this effect: as the superconductor cools through its critical temperature, the expulsion of magnetic flux from the conductor causes the magnet to levitate above the material.

The Meissner effect: a superconductor's magnetic flux above (left) and below critical temperature.

Principles

Superconductor principles can be explained by examining various formulas. First, lack of resistance in a current-carrying superconductor can be illustrated by Ohm's law, R=V/I, where R is resistance, V is voltage, and I is current. Since superconducting materials carry current with no applied voltage, R=0. Superconductivity also does not involve power loss, since power is defined as P=I²R; since R is zero in a superconducting material, power loss is zero.

These formulas, along with other superconductor principles, are explained in the educational video below.

Video credit: DrPhysicsA / CC BY-SA 4.0

Types of Superconductors

Superconductors are classified into Type I and Type II materials.

Type I materials show at least some conductivity at ambient temperature and include mostly pure metals and metalloids. They have low critical temperatures, typically between 0 and 10 K (-273°C and -263°C respectively). As discussed above, this type experiences a sudden decrease in resistance as well as the complete expulsion of magnetic fields (perfectly diamagnetic) at critical temperature.

Type I metals achieve superconductivity through slowing down molecular activity via low temperatures. According to BCS theory, this creates an environment conducive to Cooper pairing so that electron pairs are able to overcome molecular obstacles, leading to free electron flow without applied voltage.

Copper, silver, and gold are three of the best metallic conductors but are not superconductive. This is due to their face-centered cubic (FCC) unit cells lattice structures, which are so tightly packed that the low-temperature lattice vibrations essential to superconductivity fail to coerce free electrons into Cooper pairs. While some FCC metals such as lead are capable of superconductivity, this is due to outside factors such as lead's low modulus of elasticity.

Most Type II materials are metallic compounds or alloys, although elemental vanadium, technetium, and niobium also fall within this group. They are capable of superconductivity at much higher critical temperatures. For example, the 2015 testing of Sn₈SbTe₄Ba₂MnCu₁₄O₂₈₊ yielded a T_c of 400 K (+129°C), over 100°C above ambient temperature, although more common Type II materials have critical temperatures within the 10-130 K range. As of 2015 there is no scientific consensus as to the reason for these higher critical temperatures.

Type II materials also take on a mixed state, which contrasts with plunging resistance at T_c for Type I materials, when approaching their critical temperature. Mixed states are caused by the fact that Type II superconductors never completely expel magnetic fields, so that microscopic superconducting "stripes" can be seen on the material.

Other Classifications

Classification according to the types above is theoretically done by magnetic field behavior. Type I materials have a single critical field temperature above which superconductivity ceases completely, while Type II materials have two critical field points between which a mixed state may exist. Another method for classifying superconductors is by temperature, with "low-temperature" materials falling below liquid-nitrogen-cooled superconductivity and "high-temperature" ones falling above it. Low-temperature materials may be cooled using liquid gases such as neon, hydrogen, and helium.

A comprehensive list of critical temperatures for superconductive materials can be found here for Type I and here for Type II.

The graph below illustrates this distinction, as well as a timeline showing the history of critical temperature discoveries. Materials with critical temperatures falling above the boiling point of liquid nitrogen (around 77 K) are known as high-temperature materials. The dramatic increase in T_c seen in the middle of the graph is the result of the discovery of superconductive cuprates and perovskites with high T_c in 1986 and 1987.

Product Form Factors

Suppliers of superconductors and superconducting materials offer products in various different forms, some of which are listed below.

Raw superconducting materials include chemical compounds in the form of powders or crystals. Superconducting powder is incorporated into the manufacture of more efficient fuel cells, gas separation membranes, and lithium-ion batteries.

Magnets are produced for MAGLEV and MRI applications, as discussed below, as well as microscopy and NMR/EPR spectroscopy.

Wire and cable are used in superconductive power transmission and scientific research in ultra-high magnetic fields.

Superconductor manufacturers may specialize in the advancement of a certain superconducting compound, such as niobium-based formulas or magnesium diboride (MgB₂).

Applications

Superconductors are not available on a wide commercial scale due to the extensive cooling necessary to reach superconductive states. They are common in a few specialized applications, including:

MAGLEV trains use superconductive magnets to practically eliminate friction between the train and the tracks. The use of conventional electromagnets would waste vast quantities of energy via heat loss and necessitate the use of an unwieldy magnet, whereas superconductors result in superior efficiency and smaller magnets.

Magnetic resonance imaging (MRI) uses superconductor-generated magnetic fields to interact with hydrogen atoms and fat molecules within the human body. These atoms and molecules then release energy that is detected and formed into a graphic image. MRI is a widely used radiographic method for medical diagnosis or staging of diseases such as cancer.

Electric generators built with superconductive wire have achieved 99% efficiency ratings in experimental tests but have yet to be built commercially.

Electric power generation using superconductive cables and transformers has been experimentally tested and demonstrated.

Standards

Various published standards and handbooks address superconductivity and superconductors. Examples include:

• BS EN 61788—Superconductivity (series)
• IET—Electrodynamic theory of superconductors
• Wiley—High temperature semiconductor bulk materials

References

Cyberphysics—Superconductivity

Superconductor Information for the Beginner

University of Alaska Fairbanks—Superconductivity

Image credit:

Cyberphysics | Bristol University | Coalition for the Commercial Application of Superconductors (CCAS)