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2001-2002 HIGHLIGHTS

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MATERIALS SCIENCE AND ENGINEERING

David Larbalestier and Alex Squitieri

Grainger Profesor of Superconducting Materials and L.V. Shubnikov Professor David Larbalestier (foreground) and Associate Instrument Innovator Alexander Squitieri (background) have shown that MgB2 holds great promise as a general-purpose, low-cost superconductor. (33K JPG)

New material holds promise for superconducting solutions

Researchers at the college's Applied Superconductivity Center continue to find new ways to use the latest high-temperature superconducting material — magnesium diboride (MgB2).

Superconductivity holds promise for major advances in nearly every scientific field, because it allows electricity to be conducted with almost no loss of energy.

In a series of articles published in the journal Nature and in other specialty journals in the past 18 months, Department of Materials Science and Engineering professors, including Grainger Professor of Superconducting Materials and L.V. Shubnikov Professor David Larbalestier (foreground, and Alexander Squitieri, background, associate instrument innovator for the Applied Superconductivity Center) have shown that MgB2 holds great promise as a general-purpose, low-cost superconductor.

For instance, the ASC researchers have found that MgB2 is a clean material with a resistance ratio of more than 10, which means it can be alloyed to reduce resistivity and increase the critical magnetic fields up to which it can remain superconducting. Because the components of MgB2 — magnesium and boron — are widely available and cheap, the research indicates it can be developed into superconductors that are cheaper and easier to produce than other, more complex superconductors.

A much stronger andmore versatile alumnimum alloy

By adding small quantities of elements such as lead to certain materials, Professor John Perepezko has discovered a way to make a more versatile aluminum alloy that's stronger by weight than steel.

The alloy's strength originates from the effect of almost countless numbers of tiny pure-aluminum particles, called nanocrystals, dispersed uniformly throughout the material's otherwise random, or amorphous, atomic structure. A miniature internal framework, the aluminum nanocrystals act as strengtheners by blocking the paths along which the amorphous alloy traditionally deforms.

The key is to produce and control the number and location of the nanocrystals, which have diameters about 10,000 times narrower than a human hair, says Perepezko, who conducted the research with then-graduate students Don Allen and James Foley.

"What we found is that by adding tiny 'seeds,' such as lead particles, each particle acted as a little catalyst and produced an aluminum nanocrystal," Perepezko says.

The strategy also can apply to other materials. For example, in iron-based alloys used for electronics applications, the lead nucleates nanocrystals that enhance not the material's strength, but rather its magnetic properties.

The new materials, which manufacturers also can make in bulk form, could be used in everything from golf clubs and bicycles to transformers, airplane parts or other high-performance applications.

Growing more consistent crystals

A new method of producing semiconducting crystals may mean better performance for cellular telephones, computers, X-ray machines and other devices. Sliced into almost paper-thin discs called wafers, semiconductors hold the circuitry that receives, transmits and processes information.

Traditionally, scientists "grow" single-crystalline semiconducting materials by immersing the tip of a pencil-shaped starter crystal, or "seed," in a melt of the same composition, then slowly withdrawing and rotating the seed to form a thick rod shape. To make the crystal develop certain desired properties, they add special impurities to the melt before crystal growth.

However, as the crystal grows, it either rejects those impurities into the melt or takes them in from it. As a result, the melt composition can change during growth — and since the crystal grows from the melt, the crystal composition also can continue to change. When the process is finished, the resulting crystal's composition and properties also can vary along its length, so many parts built upon wafers from one crystal can be inconsistent.

Professor Sindo Kou and graduate student Jia-Jie He have devised a method to ensure the melt composition stays constant. First, they lengthened the crucible in which the materials melt. Then they added a low-temperature heater around the crucible's lower half and moved the existing high-temperature heater to the upper half. The bottom of the crucible holds a solid material identical in composition to the desired crystal; the upper part holds the melt. As the crystal grows and the melt level decreases, an existing mechanism pushes the crucible upward so that the solid material gradually enters the high-temperature heat zone, melts and keeps the melt composition constant. Scientists also can apply this method to crystals that are a mixture of two different semiconductors and grow them with a uniform composition.

 



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Date last modified: Tuesday, 01-Oct-2002 11:15:08 CDT
Date created: 01-Oct-2002
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