Engineering Superconductivity

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High-temperature superconductors start finding real-world uses

For a few months in 1987, it seemed the world was about to change. Trains would fly on magnetic cushions, computers would be faster, electric power cheaper, new medical scanners would sprout in doctors' offices and more. The reason for this overheated optimism was the discovery by IBM scientists in Zurich, namely, J. Georg Bednorz and K. Alex Müller, of a new kind of superconductor, an almost miraculous material that conducts electricity without any loss of energy. Superconductors had been around since 1911, but all known superconductors worked at near absolute zero, which made them impractical for all but the most specialized applications.

The discovery led to a class of oxide superconductor working well above the temperature of liquid nitrogen. Boiling at 77 kelvins, liquid nitrogen is much less expensive to make and far easier to handle than liquid helium, which cools conventional superconductors. (Physicists still hope to find a material that su-perconducts at room temperature—possibly the next best thing to perpetual motion.) Gradually, researchers have found ways to craft high-temperature superconductors into useful magnetic components for research and for medical diagnostics and have even manufactured motors, current limiters and other devices for demonstration purposes. But now, more than a decade after their discovery, they are entering two markets closer to the consumer realm—power lines and wireless communications.

The largest obstacle to making commercial high-temperature superconducting cables is that the materials are ceramics and therefore as fragile as a Ming vase. In 1987 Greg Yurek, a metallurgist from the Massachusetts Institute of Technology, realized that just as brittle glass can be drawn into filaments to make flexible fiber optics, the same thing could be done with high-temperature superconductors. "That insight led to the fundamental patent in this field," says John Howe of American Superconductor in Westborough, Mass., the company that Yurek would go on to found.

Yurek's basic concept is to place small granules of the superconducting material in a silver tube, or billet, about the diameter of a quarter. These billets are drawn into thin filaments, which are bundled and placed in another silver tube. That tube is flattened to make a superconducting ribbon that is reasonably flex-

HIGH CAPACITY: Three strands of American Superconductor's flattened wire carry as much current as a 400-ampere copper cable does.

Southwire Plant Carrrollton Images

ible, although nowhere near as bendable as copper wire.

Two years ago, according to Howe, the price of superconducting wire was 50 times that of comparable copper cable. American Superconductor is now building a new plant to make the wire, and "by achieving scale economies, we'll bring the cost down to about two times the cost of copper," Howe predicts. The firm maintains a partnership with Pirelli Cables and Systems, based in Milan, Italy, to develop superconducting transmission lines.

Engineers at Southwire, a cable manufacturer in Carrollton, Ga., are among the first to make practical cables out of superconducting wire. This past February, Southwire began to supply power to three of its manufacturing plants by superconducting cables. It designed the 100-foot-long cables in a collaboration with the Oak Ridge and Argonne National Laboratories, the U.S. Department of Energy and several industrial partners, including Intermagnetics General in Latham, N.Y., which supplied the superconducting wire. The cable consists of hollow pipe through which liquid-nitrogen coolant flows. Surrounding this pipe are layers of superconducting wires and insulation, all of which are encased in a double-walled thermos bottle. The entire assembly is five inches in diameter but will be thinner in production models. "It being our first, we were being very conservative," says project manager R. L. Hughey.

Still, it is thinner than a copper wire carrying the same current, which is the point. All else being equal, the savings achieved with the more efficient superconducting cable ordinarily isn't high enough to make it worth the expense. Rather "the main gain is that because superconducting wire has virtually no resistance, you can push huge amounts of power through it," Hughey explains, thereby solving the most intractable problem facing power engineers in cities: where to put wires in otherwise jam-packed cable channels. The benefits are clear from the system American Superconductor is building for Detroit Edison's Frisbee substation: 18,000 pounds of 1930s-vintage copper cable running through nine ducts will be replaced with 250 pounds of superconductor in three ducts, leaving six free for future expansion. Other notable power applications are superconducting magnetic-energy storage systems, which can stabilize disturbances on power grids, and, further away, lightweight motors and transformers.

Superconducting devices are also beginning to make headway into wireless communications as filters. An ideal filter selects only a single frequency, but in practice, electrical resistance causes filters to tune in a small range of frequencies. Superconducting filters, because they lack electrical resistance, are far more discriminating. In addition, less of the signal is lost between the antenna and the receiver, making them especially sensitive. These two factors are important in cellular communications, which must operate in an extremely crowded radio spectrum and pick up signals from low-powered transmitters.

"This was a very ambitious enterprise when we started in 1987," says Robert B. Hammond, senior vice president and chief technical officer of Superconductor Technologies, based in Santa Barbara, Calif. To make superconducting filters, the firm had to solve many problems. It developed methods of making circuits by depositing thin films of superconductors and designed a vacuum pack to insulate the circuits. Connecting the circuit to the real world proved challenging, because the connections had to be good electrical conductors and poor thermal conductors—two properties that do not normally go together.

Finally, it had to invent a cooling system that could keep the

WIRED: Southwire powers some industrial plants via three 100-foot-long superconducting cables.

circuit chilled for years at a time, because these filters would be used on remote radio towers. Hammond says it developed a tiny refrigerator, "a little smaller than a half-gallon milk carton," in which a mini engine compresses and expands helium gas. "Our belief is that these things will be used broadly to extend the range of base stations and decrease the handset power by a factor of two or more," Hammond explains. Other companies working on similar products include

Illinois Superconductor in Mt. Prospect, 1ll., whose filter boosted wireless phone capacity by 70 percent in a demonstration last year, and Conduc-tus in Sunnyvale, Calif.

No firm is profiting from high-temperature superconductors yet, and price remains a roadblock to wider acceptance. But with ongoing progress in a market that could be worth $30 billion by 2020, high-temperature superconductors just might justify some of the hype of 1987. —Bruce Schechter

BRUCE SCHECHTER is a freelance writer based in Brooklyn, N.Y.

Different Stripes

Physicists still struggle to explain high-temperature superconductivity

Despite researchers' best efforts, high-temperature superconductivity remains a mystery. In the past few years, many physicists have studied the idea that organized lines of electric charge, known as stripes, could produce the resistanceless flow of current and other bizarre properties. In April two groups announced direct experimental evidence for this model in the superconductor known as YBCO (yttrium barium copper oxide). As has so often occurred in this field, the significance of the results is hotly debated, and barely a month later a third group reported studies inconsistent with stripes.

High-temperature superconductors are multilayered, ceramic crystals. All the superconducting action takes place in planes of copper and oxygen atoms sandwiched between layers of other elements, such as yttrium and barium. The density of electric charges free to move about on the copper oxide "meat" of the sandwich depends on the precise recipe used for the "bread." In the case of YBCO, excess oxygen in the yttrium barium oxide bread soaks up electrons from the copper oxide meat, leaving behind holes, which can be thought of as positively charged particles.

Superconductivity arises when the holes form loosely bound pairs that undergo Bose-Einstein condensation—they all collect in one quantum state. Such condensate fluids flow en masse without friction. Conventional superconductors involve condensates of electron pairs held together by a well-understood interaction, but no one knows what pairs up the holes in cuprate superconductors.

When no holes are present, the cuprate layers are like chessboards, each square representing a copper atom with its intrinsic magnetic field pointing one way ("black square") or the other ("white square"). Individual holes introduced to this rigid arrangement cannot move about easily, because the motion would disrupt the chessboard arrangement. If enough holes are in the plane, they may spontaneously collect together along rows, forming "stripes" of charge. Holes can move readily along such stripes without upsetting the chessboard pattern elsewhere. Stripes fixed in place cannot produce superconducting pairs of holes, but dynamic stripes, which meander across the chessboard, can.

Such meandering stripes should also slightly displace atoms in the cuprate planes. Thirumalai Venkatesan of the University of Maryland and his co-workers fired helium ions through the channels formed by the rows of atoms in crystal planes and saw evidence of these displacements. As the crystal was cooled, the effect varied as expected if stripes form above superconducting temperatures and generate the required pairing of holes at lower temperatures. Herbert A. Mook of Oak Ridge National Laboratory and his colleagues found direct evidence for meandering stripes as well. The researchers fired neutrons into YBCO and observed that they diffracted in a manner characteristic of fluctuating one-dimensional structures in the material.

A proponent of stripes, Jan Zaanen of Leiden University in the Netherlands, says that these results "convincingly disprove more conventional explanations" of YBCO's behavior, which are founded on the idea of weakly interacting collective excitations, or quasi-particles, that behave much like individual electrons or holes. Such quasi-particles are the essence of Fermi liquid theory, which forms the foundation of physicists' understanding of metals, semiconductors and conventional superconductors. Physicists have long known that Fermi liquid theory must be modified for the cuprates. According to Zaanen, however, mere modifications cannot explain the effects seen by Venkatesan and Mook.

But there is a caveat: the clearest evidence of stripes in YBCO is in crystals that have less than the optimal number of holes for the most robust superconductivity. When Philippe Bourges of Léon Brillouin Laboratory in Saclay, France, and his group scattered neutrons from crystals of optimally doped YBCO, they obtained results consistent with conventional quasi-particle descriptions and inconsistent with simple stripes. Bourges believes the data from underdoped YBCO still have loopholes for alternative explanations. Stripes are "not of great importance for the superconducting mechanism," he says. For now the debate rages on, and Venkatesan suggests that the important process is the formation of distinct magnetic (chessboard) and charged regions, which may have shapes other than stripes in optimally doped superconductors. —Graham P. Collins h e c n o o g y &

Z Circles of Trust

How vouching for users beats encryption alone in maintaining privacy

ONDON—In a world of disembodied strangers, the issue of trust is complicated. Some governments seem i to think there's a simple solution— just make everyone trackable. The British government, for example, talks quite a lot about nonrepudiable digital signatures without ever acknowledging that a piece of electronic information is never going to be perfectly bound to a human.

The notion that the security systems we've been relying on don't work for the mass market the way we'd hoped they would occurred to me last December. An e-commerce site sent me a message saying the certificates built into earlier versions of Netscape were expiring. If I wanted to keep using their site, I had to ... upgrade my browser. First question: Why can't I just get updated copies of the certificates? Second question: What are certificates?

That part I knew. Certificates in their current incarnation are electronic strings of seeming gibberish that securely identify a person, organization or e-commerce site to my computer. Glancing at the settings of my Netscape browser, I see that the list of third-party authenticators includes American Express, Deutsche Bank and VeriSign, the last being the leading on-line certification authority. If I click on the button labeled "verify," the software performs some hidden black magic and pronounces the certificate verified. But how many consumers are going to understand why that works or how they can know that the verification is valid? The Web pages dedicated to explaining this mini crisis aren't much help, either, as they note that the only penalty for having an expired certificate is that you have to click on an extra dialogue box to establish a secure session. Well, so what? What exactly is VeriSign guaranteeing me?

This kind of question is the province of security experts such as Carl Ellison. I first heard Ellison address this issue at a 1997 London meeting that discussed government plans to set up a network of trusted third parties to help e-commerce flourish. These parties would be cryptographic-service providers that, like VeriSign, would authenticate transactions. The government's idea was that they would obviously be banks—organizations that the government knew how to regulate. Quite apart from the fact that most Britons hate their banks, in the real world neither our assurance of someone's identity nor our trust in them rests on authentication from a large third-party institution. Binding a key to a name is meaningless in terms of trust, because few names are unique.

Instead I determine that the letter from "John Gizzarelli" is authentic because it contains personal data and context, such as mentions of his wife, my sister Ellen. If the style seems doubtful, I might check the postmark, phone them or compare

handwriting. I don't phone the bank and ask it to authenticate the letter.

Unlike top-down proposals such as the British government's, the technical community has generally favored a more distributed plan. Look, for example, at the way PGP (Pretty Good Privacy), the well-known cryptographic software, handles authentication. It builds a web of trust by allowing users to authenticate one another's keys through digital signatures. Under this regime, if John wanted to ver-ifiably bind himself to his key, he might refer users to my signature on his key. If they already trust me, they accept my verification; if not, they go to another link along the chain looking for someone to authenticate me. Either way, they are passed from peer to peer, much like in the England of Agatha Christie novels, where a new arrival in a rural village would bring a letter of introduction.

In his talk and in papers posted on the Web, Ellison's proposals are different. He advocates circles of trust, which are designed to grow together: local names, given meaning by their context and perhaps used only for a small number of purposes, rather than becoming a global identifier.

We may need to establish such circles of trust sooner than we think. One of the best moments at this year's Computers, Freedom and Privacy conference, held in April, came during science-fiction writer Neal Stephenson's presentation. He focused on threat, rather than trust, models. We still think, he argued, in terms of the 1950s obsession with a monolithic government that wants to know every-thing—Big Brother, in other words. And at that point he put up a slide with a cartoon drawing of an ordinary guy with an ordinary house and an unordinary picket fence: just one very large picket thrusting up into the sky, where a bird regarded it quizzically. That, he told us, was PGP.

Stephenson's point was not that PGP is ineffective—the program has stood up to nearly a decade of industrial-strength testing—but that the kind of intrusion it protects against is based on a model in which there is only one kind of threat. PGP can keep "them" from reading your data, but it can't stop people from analyzing your e-mail traffic and drawing conclusions from the frequency and volume of e-mail you exchange with particular people. Nor can it stop organizations from compiling profiles based on your interactions with them and exchanging that data to create a complete dossier. And it certainly can't stop the Love Bug and Resume viruses; you can use all available encryption to authenticate the source of the virus-laden messages, and the viruses will still enter your machine, because they genuinely do come from your friends and co-workers (or at least their machines).

Stephenson's proposed antidote to multiple threats was small pools of trust: people you know and trust who would vouch for those you don't know. These pools could grow and overlap to become a field of trust that would provide far more protection than that single picket could afford. Diffusion and multiple identities, it would seem, are our friends against diffuse and multiple threats. — Wendy Grossman

WENDY GROSSMAN, a frequent contributor to this column, is based in London.

TÄ Pf

It is now technologically possible to make plastics using green plants rather than nonrenewable fossil fuels. But are these new plastics the environmental saviors researchers have hoped for?

by Tillman U. Gerngross and Steven C. Slater are green plastics?

Driving down a dusty gravel road in central Iowa, a farmer gazes toward the horizon at rows of tall, leafy corn plants shuddering in the breeze as far as the eye can see. The farmer smiles to himself, because he knows something about his crop that few people realize. Not only are kernels of corn growing in the ears, but granules of plastic are sprouting in the stalks and leaves.

This idyllic notion of growing plastic, achievable in the foreseeable future, seems vastly more appealing than manufacturing plastic in petrochemical factories, which consume about 270 million tons of oil and gas every year worldwide. Fossil fuels provide both the power and the raw materials that transform crude oil into common plastics such as polystyrene, polyethylene and polypropylene. From milk jugs and soda bottles to clothing and car parts, it is difficult to imagine everyday life without plastics, but the sustainability of their production has increasingly been called into question. Known global reserves of oil are expected to run dry in approximately 80 years, natural gas in 70 years and coal in 700 years, but the economic impact of their depletion could hit much sooner. As the resources diminish, prices will go up—a reality that has not escaped the attention of policymakers. President Bill Clinton issued an executive order in August 1999 insisting that researchers work toward replacing fossil resources with plant material both as fuel and as raw material.

With those concerns in mind, biochemical engineers, including the two of us, were delighted by the discovery of how

GROWING PLASTICS in plants once seemed to be an innovative way to lessen the global demand for fossil fuels.

to grow plastic in plants. On the surface, this technological breakthrough seemed to be the final answer to the sustain-ability question, because this plant-based plastic would be "green" in two ways: it would be made from a renewable resource, and it would eventually break down, or biodegrade, upon disposal. Other types of plastics, also made from plants, hold similar appeal. Recent research, however, has raised doubts about the utility of these approaches. For one, biode-gradability has a hidden cost: the biological breakdown of plastics releases carbon dioxide and methane, heat-trapping greenhouse gases that international efforts currently aim to reduce. What is more, fossil fuels would still be needed to power the process that extracts the plastic from the plants, an energy requirement that we discovered is much greater than anyone had thought. Successfully making green plastics depends on whether researchers can overcome these energy-consumption obstacles economically—and without creating additional environmental burdens.

Traditional manufacturing of plastics uses a surprisingly large amount of fossil fuel. Automobiles, trucks, jets and power plants account for more than 90 percent of the output from crude-oil refineries, but plastics consume the bulk of the remainder, around 80 million tons a year in the U.S. alone. To date, the efforts of the biotechnology and agricultural industries to replace conventional plastics with plant-derived alternatives have embraced three main approaches: converting plant sugars into plastic, producing plastic inside microorganisms, and growing plastic in corn and other crops.

Cargill, an agricultural business giant, and Dow Chemical, a top chemical firm, joined forces three years ago to develop the

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