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Pinning down critical currents.

Pinning down critical currents

One serious obstacle to the application of high-temperature superconductors, especially in the presence of strong magnetic fields, is the disappointingly low electrical current they can carry. Once the current exceeds a certain value known as the critical current, the material loses its ability to conduct electricity with zero resistance and no energy loss. Several recent advances suggest ways by which researchers may overcome this problem.

When a superconductor is placed in a magnetic field and its temperature lowered below its superconducting transition point, it abruptly expels the magnetic field. However, in Type II superconductors, including the recently discovered high-temperature compounds, the materials actually retain an internal magnetic field if the external field is greater than a certain value. This penetrating field exists within the material in the form of separate magnetic filaments, or lines of flux, called fluxoids. The fluxoids generally settle into a regular pattern, or lattice, often pinned in place by impurities or microscopic defects in the material.

When an electric current courses through such a superconducting material, it pushes against the fluxoids. If the current is strong enough or the fluxoids are only weakly pinned, they begin to move. The trouble is that naturally occurring defects in typical high-temperature superconductors don't pin fluxoids strongly enough to keep even low electric currents from shoving them aside (SN: 4/1/89, p.197). This phenomenon is known as flux creep. Because energy is needed to move the fluxoids, flux creep causes the material to lose all its superconducting properties.

Last fall, R.B. van Dover of AT&T Bell Laboratories in Murray Hill, N.J., and his colleagues reported that irradiating a single superconducting cyrstal with high-energy neutrons could raise the material's critical current. The neutrons apparently disturb the superconductor's atomic structure permanently, creating a high concentration of tiny structural defects that anchor the lattice more effectively. The researchers achieved a current density of 600,000 amperes per square centimeter at 77 kelvins (-195[degrees]C) in a modest magnetic field of 0.9 tesla, about 100 times larger than the current density in an unirradiated crystal.

Later, a team led by Sungho Jin at Bell Labs achieved a similar improvement by modifying the composition of an yttrium-barium-copper-oxide superconductor. The researchers heated a crystal of [[[YBa.sub.2]Cu.sub.4]O.sub.8] to 920[degrees]C, causing it to decompose into [[[YBa.sub.2]Cu.sub.3]O.sub.7]. That process leaves a number of extra oxygen and copper atoms in the sample, which somehow act as defects and help hold the flux lattice in place. Jin and his colleagues measured critical currents as high as 100,000 amperes per square centimeter. Their flux-pinning method is simple enough to show promise as a commercially viable process, the researchers say.

Scientists have already produced relatively high current densities in thin films of superconducting materials. In the Jan. 15 APPLIED PHYSICS LETTERS, a Japanese group reports the possibility of further improving thin-film performance by using X-ray irradiation and then oxygen annealing to create strong flux-pinning sites in a gadolinium-barium-copper-oxide superconductor.

In the Jan. 19 SCIENCE, researchers at Stanford University describe experiments showing that flux creep in thin films isn't necessarily a serious problem for certain applications, such as magnets that mus carry a current for long periods of time. Their technique produces thin films that can maintain current densities as high as 1 million amperes per square centimeter at liquid-nitrogen temperatures (-195[degrees]C) for long periods of time without any measurable degradation.
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Title Annotation:application of high-temperature superconductors
Publication:Science News
Date:Feb 10, 1990
Words:589
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