Superconductors

Updated October 25, 2009

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Christopher Reeve, 1952-2004, actor and activist

What constitutes a superconductor? Not bulk resistivity of 10^-6 ohm-centimeters or less. Bulk resistivity of ZERO. We are talking perpetual motion here.

Microwave applications of superconductors are in passive circuit elements where loss is critical. For example, a preselector filter that come in front of the low noise amplifier in a receiver.

A superconductor is a material that loses all resistance to the flow of electric current when it is cooled below a certain temperature, called the critical temperature or transition temperature. Above this temperature, there is usually little or no indication that the material might be a superconductor. Below the critical temperature, not only does the superconductor suddenly achieve zero resistance, it gains other unusual magnetic and electrical properties.

H. Kamerlingh-Onnes discovered superconductivity in 1911. Learn more about him in our Microwave Hall of Fame.

Until recent years, superconductivity could only be achieved by cooling certain materials in liquid helium, a coolant that is expensive and difficult to handle but provides a temperature of 4 degrees Kelvin. The superconductors that require a liquid helium coolant are now called low temperature superconductors (LTS).

In 1987, the discovery of materials that become superconducting at the temperature of liquid nitrogen (77°K or -196°C) made the science of superconductivity much more accessible to the unwashed masses including you and I. Since liquid nitrogen is an inexpensive and easily-handled coolant, experiments and demonstrations in superconductivity are now available to anyone with modest laboratory resources.

Superconductivity above the temperature of liquid nitrogen is called high temperature superconductivity (HTS). "High temperature" may seem like a misrepresentation; to anyone who lives in the desert, -196°C seems pretty cold. Yttrium-barium-copper-oxide superconductor (YBCO) is a high-temperature superconductor.

The HTS discovery renewed hope of discovering room-temperature superconductivity. So far, however, superconductivity remains in the realm of very low temperatures. Even without a room-temperature superconductor, rapid advances in superconductivity research are providing breakthroughs in microwave applications right now. The first application that will see the light of day is extremely high-performance RF filters (Q can be really high when you have no resistive losses).

Superconductors.org is a web site that provides a treasure trove of material on superconductors.

Click here to learn about cryogenics in microwave applications