‘Cool’ materials turn into hot research

A group of materials with an enhanced ability to cool and heat in response to changes in magnetic fields could have applications that extend far beyond temperature regulation.

A group of materials with an enhanced ability to cool and heat in response to changes in magnetic fields – a property discovered at the US Department of Energy’s Ames Laboratory – could have applications that extend far beyond temperature regulation.

The materials, already under study for magnetic-refrigeration technology, could possibly be used in versatile sensors to detect changes in such factors as magnetic field, temperature and pressure, and may be useful in energy-conversion devices.

But first, scientists need a better fundamental understanding of the materials and why they respond so dramatically to changes in temperature and magnetic field.

To help, Ames Laboratory is embarking on a four-year, DOE-funded project to answer some of those questions. Total funding depends on final US DOE budget figures, but could be up to $1 million a year.

A team of 10 metallurgists, physicists and chemists will explore the properties of the gadolinium-silicon-germanium alloys — the materials whose unusual characteristics were first uncovered by Ames Lab scientists Vitalij K. Pecharsky and Karl Gschneidner Jr. – and several closely related materials. The team 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.

‘These alloys could be among the most significant materials of the new millenium,’ Pecharsky said. ‘I’m sure that we’re in for many more surprises and interesting phenomena in the next four years.’

In 1997 Pecharsky and Gschneidner announced that the Gd-Si-Ge alloys possessed a giant magnetocaloric effect, meaning that the materials heated when magnetized and cooled when removed from a magnetic field to a much greater extent than other known alloys. The scientists noted that the property made the alloys candidates for use in magnetic cooling and freezing applications.

Researchers at Ames Lab and several laboratories throughout the world have since discovered 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 may 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,’ said Gschneidner, who will manage the research project.

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, or in sensors. Pecharsky said most sensor materials are only effective in certain temperature ranges, but the Gd-Si-Ge alloys can be tailored to respond at a variety of temperatures.

‘We have learned a lot about the properties of this material to date, but we need to understand what causes the tremendous responsiveness of this and related materials,’ said Pecharsky, who will oversee the experimental portion of the research. ‘That’s what this project is about.’

He said part of the funding will be spent on a customized piece of equipment — an X-ray powder diffractometer — critical to understanding 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 equipment, in helping us understand this material,’ Pecharsky said. ‘I think we’ve proposed an extensive and very interesting route to advance the basic studies of these materials.’

Bruce Harmon, deputy director of Ames Laboratory and one of the scientists involved in the project, said the multidisciplinary nature of the team is a definite advantage.

‘These are fascinating materials with complex crystal structures and a remarkable combination of magnetic and mechanical properties,’ said Harmon, a physicist who will oversee the theoretical component of the research. ‘This new funding offers a great opportunity to utilise and extend Ames Laboratory’s internationally recognised expertise in the area of rare-earth magnetic materials.’

Pecharsky, Gschneidner, Harmon and seven colleagues submitted a proposal earlier this year to study the alloys as part of a DOE-wide competition. The competition’s goal was to identify projects that would enable scientists to understand how complex, advanced materials could be used in the future.

One of the DOE’s primary missions is to engage in research that leads to the development of materials that improve the efficiency, economy, environmental acceptability and safety of energy sources.

Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.