Using High-Pressure “Alchemy” to Create Nonexpanding Metals

JUNE 17, 2009

Nuclear forward scattering spectra from a palladium-iron alloy, collected at HP-CAT beamline 16-ID, showing an abrupt decrease of the hyperfine magnetic fields in the material around 12 GPa. (From Winterrose et al., Phys. Rev. Lett. 102, 237202 [2009]. © 2009 The American Physical Society.)

By squeezing a typical metal alloy at pressures hundreds of thousands of times greater than normal atmospheric pressure, researchers from the California Institute of Technology (Caltech), The University of Chicago, and the Carnegie Institution of Washington, using x-rays at two U.S. Department of Energy national laboratories, have created a material that does not expand when heated, as does nearly every normal metal, and acts like a metal with an entirely different chemical composition.

The discovery, described in a paper published in Physical Review Letters (PRL), offers insight into the exotic behavior of materials existing at high pressures, which represent some 90% of the matter in our solar system.

Zero-expanding metal alloys were discovered in 1896 by Swiss physicist Charles Édouard Guillaume, who worked at the International Bureau of Weights and Measures in France. While attempting to develop an inexpensive international standard for the meter—the metric unit of length—Guillaume hit upon an inexpensive iron-nickel alloy that expands very little when heated. He dubbed the material an "Invar" alloy—because the metals are "invariant" when heated, such that the length of a piece of Invar metal does not change as its temperature is increased, as do normal metals. Since Guillaume's discovery—which, in 1920, earned him the Nobel Prize in Physics (besting Albert Einstein, who was awarded the prize in 1921)—other nonexpanding alloys have been identified.

It has long been known that Invar behavior is caused by unusual changes in the magnetic properties of the alloys that somehow cancel out the thermal expansion of the material. (Normally, heat increases the vibrations of the atoms that make up a material, and the atoms prefer to move apart a little, causing expansion.)

“Recent computer simulations indicate that electrons in Invar alloys take on a special energy configuration,” said Michael Winterrose (Caltech), the first author of the PRL paper. “This energy state is at the borderline between two types of magnetic behavior, and is very sensitive to the precise ratio of elements that make up the alloy. If you move away from the Invar chemical composition by only a couple of percent, the energy configuration will disappear.”

Because of their unresponsiveness to temperature change, Invar alloys have been used in devices ranging from watches, toasters, light bulbs, and engine parts to computer and television screens, satellites, lasers, and scientific instruments.

The Caltech scientists did not set out to study Invar behavior—and, in fact, were hoping to avoid it. “We intentionally picked chemical compositions that do not show Invar behavior because I thought it would confuse our interpretations,” said Brent Fultz, a professor of materials science and applied physics at Caltech and a coauthor of the PRL paper.

Instead, the group was examining the effect of pressure on the alloy of palladium (Pd) and iron (Fe) called Pd₃Fe, where three of every four atoms are palladium, and one is an iron atom. (In the similarly named but chemically distinct PdFe₃—which is a traditional Invar alloy—three of every four atoms are iron, and one is palladium.)

The Fe and Pd atoms in the alloy have very different sizes. The group expected to see some interesting effects from this size difference when they put Pd₃Fe under pressure and measured its volume. To test this, the scientists squeezed a small sample of the material between two diamond anvils, generating pressures inside the sample that were 326,000 times greater than standard atmospheric pressure.

The initial results from these studies showed that the alloy stiffened under pressure, but far more than they expected. To figure out the cause, the group simulated the quantum mechanical behavior of the electrons in the alloy under pressure. The simulations showed that under pressure, the electrons found the special energy levels between strong and weak magnetism that are associated with normal Invar behavior. Up to this point they had been quite unaware of the possibility for Invar behavior in their material.

Subsequent experiments at the High Pressure Collaborative Access Team (HP-CAT) beamline 16-ID at Argonne’s Advanced Photon Source, and the X17C beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) in New York confirmed that the intense pressure had indeed suppressed thermal expansion in Pd₃Fe, much like tuning the chemical composition.

The scientists had performed a kind of high-pressure “alchemy” on the alloy, where pressure makes the electrons act as if they are around atoms of a different chemical element.

This research helps unify our understanding of Invar behavior, which is one of the oldest and most-studied unresolved problems in materials research. In addition, using pressure to force electrons into new states can point to directions in materials chemistry where new properties can be found, at least for magnetism.

Contact: M.L. Winterrose winterro@its.caltech.edu B. Fultz btf@caltech.ed

See: M.L.Winterrose, M.S. Lucas, A.F. Yue, I. Halevy, L. Mauger, J.A. Muñoz, Jingzhu Hu, M. Lerche, and B. Fultz, “Pressure-Induced Invar Behavior in Pd₃Fe,” Phys. Rev. Lett. 102, 237202 (2009). DOI: 10.1103/PhysRevLett.102.237202

The original Caltech press release can be found here.

Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DOE-BES), under Contract No. DE-AC02-06CH11357.

This work was supported by the Carnegie-DOE Alliance Center funded by the DOE through the Stewardship Sciences Academic Alliance of the National Nuclear Security Administration (NNSA). HP-CAT was supported by DOE-BES, DOE-NNSA, the National Science Foundation (NSF), and the W.M. Keck Foundation. Use of the NSLS at Brookhaven National Laboratory was supported by DOE-BES, under Contract No. DE-AC02-98CH10886. Beamline X17C at the NSLS was supported by the Consortium for Materials Properties Research in Earth Sciences of the NSF.

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