The life and times of superconductivity
The mysterious phenomenon of superconductivity was discovered 100 years ago this month. We've learned some of its secrets since then, but its true potential has yet to be realized.
By Kathleen Laufenberg
Superconductors are materials — metals, alloys, ceramics — with an astonishing property: Under certain conditions, they conduct electricity without resistance. That means they have the potential to immensely reduce the cost of electricity.
When electric current flows through a regular wire, the wire's material resists the flow. That resistance creates heat and results in energy loss. But when electricity flows without resistance — as it does through a superconducting wire — there's no heat or loss of power.
There's just one catch. (You knew there'd be at least one, right?) For today's superconductors to work, they must be kept super cold — anywhere from minus 452 degrees Fahrenheit to minus 171 degrees Fahrenheit! To keep them so insanely cold, scientists usually encase the wires in liquid helium, itself an expensive and limited resource. But researchers continue to search for materials that will superconduct at warmer temperatures — perhaps someday even at room temperature.
"It's impossible to predict when that might happen," physicist Greg Boebinger, the director of the Magnet Lab, said of such a discovery. "But I don't see why it couldn't."
Superconducting tape being wound in the MagLab's shop. Click for larger image.
Scientists also hope to find materials that will be easier to use, as it's both tricky and time-consuming to create a wire from many of today's superconductors. If researchers do discover a room-temperature superconductor that can be fashioned somewhat easily into wires, the uses for superconductivity will explode. For not only do superconductors do their job without losing power, they also take up much less space and can carry far more electric current than a regular wire.
That's why every day you'll find scientists at the Magnet Lab, and at its Applied Superconductivity Center, using powerful and unique magnets to explore potential new materials. They hope to pave the way for another monumental discovery.
But until the next breakthrough comes, the closest most of us will get to this amazing phenomenon is inside a hospital MRI machine. Superconductors helped make MRIs possible — and that means they've helped make exploratory surgery a thing of the past.
Here's a look at superconductivity's milestones:
On April 8, Dutch physicist Heike Kamerlingh Onnes and his assistant, Gilles Holst, experiment with mercury in a Netherlands lab at the University of Leiden. They're stunned to discover that at just a few degrees above absolute zero (the temperature at which matter ceases to move), mercury superconducts electricity. At 4 kelvins (minus 452 degrees Fahrenheit), mercury transforms into a superconductor.
Onnes wins the Nobel Prize in 1913 for his studies into cryogenics, the study of what happens to matter at super low temperatures (measured in kelvins, or K) and for his discovery of how to liquefy helium (which, when liquefied, becomes the coldest liquid on earth). Most of today's superconductors still rely on liquid helium to keep them cold.
German physicists Walther Meissner and Robert Ochsenfeld discover that a superconductor repels a magnetic field. After loss of all electrical resistance, this phenomenon — known as the Meissner effect — is the second defining characteristic of a superconductor.
If you place a magnet on top of a room-temperature material, the magnet's magnetic field will penetrate the material. If the temperature of the material is lowered enough to allow it to superconduct, however, the material will expel the magnet's magnetic field. When that happens, the magnet will rise off the superconducting material and float above it. This force is harnessed to levitate toy "maglev" trains.
By the late 1950s, scientists realize that many metal alloys will become superconductors if cooled to extremely low temperatures — but why that happens remains a mystery. In 1957, however, three American physicists — John Bardeen, Leon Cooper and John Robert Schrieffer — offer the first accepted explanation of the phenomenon, dubbed the BCS theory (from the first letter of each man's last name). They win a Nobel Prize in 1972 for their work.
(The BCS theory is now considered insufficient to fully explain superconductivity.)
Scientists at Westinghouse make the first commercial superconducting wire, an alloy of niobium and titanium.
A graduate student at Cambridge University, Brian D. Josephson, predicts that current will flow between two superconductors even when they are separated by a non-superconductor (or insulator). The phenomenon, now called the Josephson effect, has been successfully used in quantum-mechanical circuitry. He wins the 1973 Nobel Prize in Physics for his discovery.
Two Swiss scientists working for IBM — Alex Müller and Georg Bednorz — make a startling breakthrough when they create a copper-oxide, or cuprate, superconductor.
They concoct a brittle ceramic compound — from lanthanum, barium, copper and oxygen — that smashes a long-standing temperature barrier by superconducting at the highest then-known temperature of 35 K, or minus 397 degrees F. This copper oxide becomes the first so-called "high-temperature" superconductor. It's a puzzling discovery because ceramics don't normally conduct electricity; they're insulators.
Both of the IBM researchers win the Nobel Prize for their work in 1987.
Researchers at the University of Alabama-Huntsville and at the University of Houston tweak the 1986 formula and discover the first material to superconduct at a temperature "warm" enough to use liquid nitrogen (92 K, or minus 294 degrees F) rather than liquid helium. The scientists substitute yttrium for lanthanum and create what's called YBCO (yttrium barium copper oxide).
The importance of this discovery is that YBCO (pronounced "IB-co") can be cooled using liquid nitrogen, a much cheaper refrigerant. In the wake of YBCO's discovery, many new high-temperature superconductors are created.
Japanese scientist Hideo Hosono and coworkers at the Tokyo Institute of Technology discover a new class of materials known as iron-based or pnictide (pronounced NICK- tide) superconductors.
Hosono creates an iron-arsenic mix that superconducts at 26 K or minus 412 degrees F. In the wake of his discovery, scientists quickly uncover compounds that work at 55 K, or minus 361 degrees F.
Most importantly, Hosono's discovery means researchers now have two high-temperature superconductors to work with and compare: the cuprates and the pnictides.
Today, some scientists maintain that there is one common factor underlying all high-temperature superconductivity, while others contend that there's probably more than one way to superconduct at high temperatures.
Only time, and more laboratory investigations, will tell.
Posted April 8, 2011