n e w s of t h e w e e k ed that of undoped YBa2Cu307_5. As expected, a sample of homogeneously doped Y1_jCCaxBa2Cu307_5 was highly superconducting at 4.2 K, but was not superconducting at all at 77 K. By combining the calcium-doped and calciumfree films, both characterized by a small critical current density at 77 K, a multilayer system with an unprecedentedly high critical current density has been obtained [Nature, 407,162 (2000)]. In an accompanying commentary, eeding current from one grain to crease the critical current, "people have Paul M. Grant, a science fellow at the the next in ceramic wires made been trying to align the grains—a costly Electric Power Research Institute, Palo Alto, Calif., notes that superconducting from high-temperature supercon- process," he notes. ducting grains of yttrium-barium-copper Mannhaifs team, which includes re- copper oxides have highly anisotropic oxide (YBCO) now promises to be less searchers at the University of Twente, crystal structures. The films contain a of a challenge: A team of researchers Enschede, the Netherlands, wanted to large number of grain boundaries where has shown that when some of the yttri- find a way to improve the properties of the grains on either side are misaligned um ions at the grain boundaries are re- the grain boundaries without aligning the by an angle greater than 10°. Such misplaced by calcium ions, up to six times grains. Last year, they showed that dop- alignments drastically reduce supercurmore current jumps the boundaries at ing YBa2Cu307_5 with Ca2+ vastly im- rent flow between grains, he says, an ef77 K. At that temperature—the boiling proved the material's critical current, but fect that's believed to be due to a deficienpoint of nitrogen—high-temperature su- only at about 4 K. Wanting to be able to cy in charge carriers (positively charged perconductors are commercially viable. operate at 77 K, they took another tack— holes) brought about by a loss of oxygen atoms. Replacing Y3+ with Ca2+ is one way To be commercially useful, wires and doping only the grain boundaries. cables made from high-temperature suTo accomplish this, they assembled to reintroduce holes, he notes. Calling the Augsburg approach perconductors must be able to carry a YBa2Cu307_5 and Y^Ca^BagCugCVs in threshold amount of current in the super- layered configurations, growing the films "clever," Grant tells C&EN that "it's one conducting state, notes team leader on SrTi0 3 crystals containing symmetric of those things that when you see it you Jochen Mannhart, a professor of experi- high-angle (24°) tilt grain boundaries. say, 'Of course. Why didn't I think of mental physics at the University of Augs- The team reasoned that during film that?'" Still, the work must be verified, burg, in Germany. The current-carrying growth, Ca2+ would diffuse along the he says. 'There's a lot to be done, but capacity—the so-called critical current grain boundary, doping the YBa2Cu307_5 that doesn't deny the fact that intuitively you can see why calcium would help." density—of such wires has improved in only along those boundaries. the past few years, he observes, but it still Mannhart's team has done "excellent At 77 K, the critical current density in falls short of what's needed. Indeed, to in- all their layered samples greatly exceed- work that addresses how to make the grain boundaries in YBCO better," says David C. Larbalestier, director of the Applied Superconductivity Center at the S Calcium boosts superconductivity University of Wisconsin, Madison. In a forthcoming paper in Applied Physics Letters, Larbalestier and colleagues show that calcium doping does in fact make low-angle grain boundaries carry higher currents in copper oxide superconductors in a strong magnetic field. Mairin Brennan
SUPERCONDUCTIVITY CHARGES AHEAD Doping of grain boundaries hikes critical current densities in YBCO at 77 K
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Rising Oil Prices Hit Chemical Industry Diagram (left) of a cross section of a copper oxide plane in YBa2Cu307_5 shows oxygen vacancies (blue) along a grain boundary where the grains on either side are misaligned by 22.6°. Copper atoms are black; oxygen, red. Charge carriers are lost when oxygen is deficient, causing the superconducting current to fall. Charge carriers are restored when some of the Y3+ ions at the grain boundary are replaced by Ca2+. This can be accomplished by layering the material with Y1_xCaxBa2Cu307_5, as shown schematically (right). The layers were grown on S1T1O3.
Rising petroleum prices have dogged chemical makers for more than a year, and in the third quarter, the situation has only gotten worse. Although chemical makers had factored high feedstock costs into their planning, the recent sudden spikes in oil costs may have caught the industry off guard. Just over a week ago, the largest SEPTEMBER 18,2000 C&EN 13
