Super Conductivity

by Kurt Riesselmann

Electrification is the greatest engineering achievement of the 20th century according to a list published by the National Academy of Engineering last year. Delivering electricity to homes and businesses and powering tools and appliances has greatly influenced our daily lifes.

Getting electrical power to consumers is an energy-consuming effort. Currents flowing through electrical wires encounter electrical resistance, a microscopic form of friction, which results in loss of energy.

Eliminating electrical resistance in wires and electrical components could be the success story of the future. Thousands of scientists around the world are working on new materials that promise to revolutionize our world based on an intriguing phenomenon: superconductivity.

Superconductors transport electrical currents without electrical resistance and hence without wasting energy. Scientists have created electrical currents that for years continued to flow through a loop made out of superconducting material, with no external power supply. Sophisticated experiments revealed that such supercurrents can last for 100,000 years, in contrast to the one-second life time of a current in a nonsuperconducting coil.

So why hasn't the superconducting revolution happened yet?

The superconducting materials known to scientists require low operating temperatures. Very low. To be precise: colder than ñ209 degrees Fahrenheit (139 kelvins). At present, that's the world record for the highest critical temperature Tc below which a carefully engineered compound of mercury, thallium, barium, calcium, copper and oxygen becomes superconducting. The new crystaline material, characterized as Hg0,2Tl0,8Ca2Ba2Cu3O, is the latest flagship of a new breed of materials, known as high temperature superconductors, developed since 1986. The discovery of HTS rests on the use of one or more conducting planes of copper and oxygen atoms in their crystal structures, a feature common to all HTS materials.

Scientists, of course, hope to achieve a similar quantum leap in the future, possibly finding materials that are superconducting at room temperature. The advent of HTS, which are all insulators at room temperature, came as a total surprise. It greatly increased the pre-1986 record critical temperature, marked at -418ƒF (23K), for what are now called low temperature superconductors.

It started with mercury

In 1911, Dutch physicist Heike Kamerlingh Onnes studied the temperature dependence of electrical resistance. His laboratory in Leiden was one of the few places in the world that had the equipment to cool objects to ultracold temperatures. Experimenting with mercury, Kamerlingh Onnes observed that its conductivity rose sharply when the temperature dropped below -452ƒF (4K). The electrical resistance of mercury vanished at and below this critical temperature ñ superconductivity had been observed. The sudden drop in resistance (see graphic), which allows for the conductivity to go up, is characteristic for all types of superconductors discovered so far.

Over time, physicists discovered further remarkable properties of superconductors. In 1933, two German scientists discovered that superconducting materials expel magnetic fields from the interior when temperatures drop below Tc. A probe already in a superconducting state remains ìimmuneî to magnetic field penetration as long as the strength of the external magnetic field is below a critical value Hc. Called the Meissner effect, the resulting forces can be strong enough to levitate a small magnet (see photo).

Once an external field becomes stronger than Hc, it is able to force its way into the superconducting material. Depending on the type of superconductor, the material either turns immediately normalconducting and will be completely penetrated by the magnetic field (type I superconductors, including many metals, with Hc typically of the order of 0.1 tesla), or it only allows isolated magnetic vortices to enter its interior, surrounding them with superconducting regions (type II superconductors). This mixed state, observed in many HTS, has still some superconducting properties. It requires an even stronger magnetic field, indicated by a critical strength Hc2 (usually tens of teslas), to completely revert a type II superconductor back to its normalconducting state while its temperature remains below Tc.

Powerful magnets
--almost at no cost

Superconducting wire is the ideal material for building powerful electromagnets that consume no electrical power. Finding the right kind of superconducting material, however, has taken many years of R&D, and the search for better materials and improved production processes continues. Physicists are looking for superconductors that can carry high electrical currents (thousands of amperes) in the presence of strong magnetic fields (type II superconductor required). On top of this, the material must be flexible enough for forming a coil ñ a criterion that many of the brittle HTS materials do not yet satisfy. Scientists therefore rely on robust low temperature type II superconductors made out of niobium and titanium (NbTi) or, more recently, niobium and tin (Nb3Sn). Unfortunately, the energy-savings created by superconducting electromagnet is somewhat reduced by the electrical power needed to cool the superconductors with liquid helium.

Powerful superconducting magnets, with multi-tesla fields, have a wide range of applications: