By Susan Dieterle
The news about materials that scientist Vitalij Pecharsky and senior metallurgist Karl Gschneidner Jr. have studied for the past five years keeps getting bigger and bigger.
Originally noted for their potential in magnetic cooling and freezing applications, the gadolinium-silicon-germanium alloys are now known to possess properties that could be useful in sensors and energy-conversion devices. But first, scientists need a better fundamental understanding of the alloys and why they respond so dramatically to changes in temperature and magnetic field.
To answer those questions about the Gd-Si-Ge alloys and several closely related materials, the Department of Energy has awarded a four-year research project to a team of Ames Lab metallurgists, physicists and chemists led by Gschneidner and Pecharsky. Total funding depends on final DOE budget figures, but could be up to $1 million a year.
"These alloys could be among the most significant materials of the new millennium," Pecharsky says. "I'm sure that we're in for many more surprises and interesting phenomena in the next four years."
In 1995, Pecharsky and Gschneidner began exploring the possibility of using Gd-Si-Ge alloys in magnetic-refrigeration technology and almost immediately discovered the alloys possessed a giant magnetocaloric effect, meaning they heat when magnetized and cool when removed from a magnetic field to a much greater extent than other known alloys. That property made the alloys strong candidates for magnetic cooling and freezing applications. After filing for a patent, the scientists announced their discovery in 1997.
Since then, researchers here and throughout the world have discovered that the materials also possess giant magnetoresistance (a change in the magnetic field triggers a change in the material's electrical resistance) and colossal magnetostriction (the shape or length of the material changes in response to magnetic forces).
Simply put, a relatively small change in the magnetic field surrounding the material produces a tremendous change in its temperature, dimensions and electrical resistance.
"Some materials possess one or two of these properties, but what's unique about this material is that all three changes take place in the same alloy and all three changes are quite large -- among the largest ever seen," Gschneidner says.
That combination of properties and responsiveness makes the alloys potentially useful in energy-conversion devices, such as systems that transform magnetic energy into mechanical energy and vice versa, and in sensors. Pecharsky says most sensor materials are only effective in certain temperature ranges, but these alloys can be tailored to respond at a variety of temperatures.
In late 1999, the DOE asked its national laboratories to submit competitive research proposals involving complex, advanced materials. The goal of the competition was to select one or two projects for in-depth research that would enable scientists to understand how these types of materials could be used in the future.
Pecharsky, Gschneidner and eight other Ames Lab researchers submitted a proposal to study the Gd-Si-Ge alloys. The other principal investigators are Vladimir Antropov, Scott Chumbley, Bruce Harmon, David Jiles, Tom Lograsso, Gordon Miller, John Snyder and Constantine Stassis. This July, the team was notified that its proposal had been selected for funding.
"We have learned a lot about the properties of this material to date, but we need to understand what causes the extraordinary responsiveness of this and related materials," Pecharsky says. "That's what this project is about."
The team will explore the properties of the Gd-Si-Ge alloys and many closely related materials. The scientists will also develop theories and models that detail the relationship between the composition, structure and properties of the alloys. The models and theories could then be used to engineer the materials for specific applications in the future.
Pecharsky will oversee the experimental portion of the research, and Harmon, deputy director of Ames Laboratory and director of the Condensed Matter Physics Program, is in charge of the theoretical work. Gschneidner will serve as project manager.
The three scientists agree that the Gd-Si-Ge alloys are uniquely suited to this type of in-depth materials research. "It has been clear for a very long time that the physical properties are indeed related to both the composition and structure of any material. What has been a mammoth task is to understand how," Gschneidner says.
Pecharsky adds that because clear-cut connections have already been established between the chemistry, crystallography and magnetism of the Gd-Si-Ge alloys, the Ames Lab group is confident that it can succeed in understanding how those properties become so interconnected.
Gschneidner says when scientists understand that process, "we will be ready to make a step having tremendous importance -- designing a material with predicted properties and behaviors. That's a dream come true for any scientist involved with materials."
Part of the funding will be spent on a customized X-ray powder diffractometer, which will help show how changes in temperature and magnetic field affect the alloys. The equipment will give Ames Lab the unprecedented ability to study changes in the material's crystal structure at a variety of temperatures as the strength of the magnetic field is altered.
"We're counting on traditional techniques and nontraditional techniques, in the form of the new and quite unique equipment, to help us understand this material," Pecharsky says. "I think we've proposed an extensive and very interesting route to advance the basic studies of these materials."
Harmon notes that the multidisciplinary nature of the Ames Lab team is a definite advantage.
"These are fascinating materials with complex crystal structures and a remarkable combination of magnetic and mechanical properties," Harmon says. "This new funding offers a great opportunity to utilize and extend Ames Laboratory's internationally recognized expertise in the area of rare-earth magnetic materials."
Pecharsky says he and Gschneidner consider themselves fortunate that so many other scientists around the world are interested in the alloys.
"After we first published our findings about the giant magnetocaloric effect in the materials, there were still many unanswered questions about the unusual features that were seen in both the magnetic properties and the crystallography. Other people started looking at the materials with interests far away from magnetic refrigeration," Pecharsky says. "As they started looking at it and we continued our research, the bigger and bigger picture started coming out.
"It's good to know that something you've discovered is considered important by your peers."
For more information:
Karl Gschneidner, (515) 294-7931
cagey@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
Last revision: 9/15/00 sd
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