It’s Electric, Exciting--and Elusive
An understated article in an obscure German physics journal 10 years ago set off an avalanche of discoveries that culminated at a happening that came to be known as the “Woodstock of Physics.”
During a meeting of the American Physical Society several months after the article appeared, scientists by the thousands crammed into the New York Hilton to hear about miraculous new materials that openly flouted the laws of nature, promising to provide the ultimate free lunch: electricity that flowed eternally without resistance.
Earlier this year, scientists at a 10-year anniversary workshop in Houston gathered to reflect on how these so-called high-temperature superconductors perform the impossible--conduct electric currents in perpetual motion with no loss of energy.
Their consensus is that after a decade of study, they remain essentially clueless. The new materials “are having a lot of fun hiding how they do that,” said Alex Muller, the IBM Zurich researcher who won a Nobel Prize for his discovery.
When Muller and George Bednorz announced they had found a ceramic material that conducted electricity perfectly at the relatively balmy temperature of 77 degrees above “absolute zero” (459 degrees Fahrenheit below zero), it was as unexpected as the sudden appearance of an ice cube in a pot of boiling soup. “It proves that the unexpected in science can still happen--and in a big way,” said physicist Robert Cava of AT&T; Bell Labs.
Until 1986, superconductivity had only been seen at extremely cold temperatures, a few dozen degrees above the absolute zero mark. As a technology, that rendered it practically useless because of the enormous expense required to cool something to those frigid depths.
The discovery of these comparatively high-temperature superconductors opened up the possibility of unlimited power, magnetically levitated trains and colossal magnets for medical applications such as magnetic resonance imagining.
Few of these futuristic technologies were on display at the Houston meeting, however. The closest thing to a magnetically levitating train was a toy car that raced around on a small superconducting track set up by the Texas Center for Superconductivity. The Electric Power Research Institute showed off a 160-foot-long flexible superconducting cable--the longest ever made, although still far short of what would be needed for practical uses.
The truth is, turning the dream of friction-free electricity into reality has been a lot more difficult than anyone predicted. There is progress, but it’s been painfully slow. The materials are complicated, easily contaminated and brittle. Trying to make a wire out them, says the institute’s Paul Grant, is like trying to make a wire out of a dinner plate.
Superconductivity was discovered in 1911 in the metal mercury--a far more likely candidate for a conductor of electricity than ceramics. And until 1986, all new superconductors were metals. They all became superconducting at very low temperatures barely above absolute zero.
Supercold and superconducting go hand in hand because superconductivity is a state of matter that’s frozen beyond solid. Take a familiar substance, like H2O, or water. Hot water molecules careen around in steamy disarray; cooler water gets organized enough to flow, or stay put in a glass; really cold water can freeze rock solid. The atoms in superconductors line up in an even more ordered array--so well ordered, in fact, that they all behave like a single atom. When electricity flows through this super-crystalline arrangement, it doesn’t collide with atoms in the metal, scattering its energy this way and that, as normal currents do. It doesn’t waste its energy as heat. So the current loses none of its punch.
But just as ice won’t freeze above 32 degrees Fahrenheit, materials won’t superconduct above their “transition temperature,” which is different for each material. Like a freezing point, it’s the temperature at which the transformation from solid to superconducting takes place.
Traditional (pre-1986) superconductors all had transition temperatures below about 40 degrees above absolute zero. The only way to get something that cold is to surround it with liquid helium, the coldest liquid that can be created on Earth. But liquid helium is so expensive that any efficiency savings from superconductivity are quickly eaten up in cooling costs.
The miracle of 1986 was twofold: First, the new materials were ceramics, and no one could figure out how a ceramic could carry electricity at all--much less without resistance. Second, the materials became superconducting at temperatures “warm” enough to be cooled with liquid hydrogen, which the physicists point out is cheaper than beer.
The news was so exciting that many scientists stayed up all night writing papers and trying out new concoctions. The frenzy to find new high-temperature superconductors was so great that for a time there were dozens of reports of what the physicists now dismiss as USOs--unidentified superconducting objects.
Since that time, dozens of new high-temperature superconducting materials have been discovered. The one currently holding the record for the highest transition temperature was created by the University of Houston’s Paul Chu, who hosted the anniversary workshop; it becomes superconducting at 164 degrees above absolute zero. Still, high temperature alone doesn’t make a practical superconductor. The applications with the most potential to benefit from superconductor technology require strong electric currents and powerful magnets: nuclear magnetic imaging, for example, or power supply cables and motors.
But many of the new materials can’t carry a strong current without destroying their own superconductivity--melting, in effect. Others fall apart in the presence of a strong magnetic field. Others can carry currents so high that they would evaporate a traditional copper wire. But unfortunately, Chu said, “They are very unstable.”
New bismuth compounds can be rolled into wires, but can’t withstand a magnetic field. Several of the most promising materials are made from toxic elements, like mercury or thallium, and are dangerous to work with. Others have only been produced in tiny pieces; they need to be made into usable shapes. And while getting to warmer temperatures lowers cooling costs, “instability and [high-temperature superconducting] go hand in hand,” Chu said.
Some applications of superconductors--like magnetically levitating trains--rely on still another property of these magic materials. A true superconductor strongly repels magnetic fields. A magnet brought near a superconductor floats on an invisible cloud of magnetic force like a boat on water. Trains riding on such clouds should be able to fly from city to city at 500 mph or more, but for now, only the Japanese are seriously pursuing such a project.
High-temperature superconducting is especially useful in applications that require exquisite sensitivity. Because superconducting atoms move in lock-step, it’s very easy to pick up even the most minute variations in magnetic fields. Thus, IBM is already developing mine detectors for the Navy that can pick up magnetic fields smaller than those created by a moving paper clip.
Indeed, the most developed applications to date are supersensitive magnetic field detectors (called SQUIDS, for Superconducting Quantum Interference Devices) that are used in everything from geology to oil prospecting. Developers see huge markets in medicine, for example, in high-temperature devices for listening to the magnetic fluctuations of the heart and brain.
Of course, none of these promises will become reality until a host of technological problems is overcome. Cooling systems have to get better, and cheaper, and high-temperature superconducting components have to get rugged enough to stand up to the hard knocks of everyday use and mass production.
Researchers who make these materials are still hoping that the theorists--whose job it is to explain how they work--will offer some guidance in the future. Without a workable theory, they’ve had to rely on guesswork and intuition. “I really have no idea what the mechanism is” behind high-temperature superconducting, Chu said.
Experimentalist Cava has been known to resort to throwing darts at the periodic table of elements to seek out promising new materials. “If you don’t do crazy things,” he says, “you don’t find the unexpected.”
