Scientia: Research at the University of Tennessee
Faculty Profile: Freeze Frame
By ELISE LeQUIRE
For material scientist Veerle Keppens, a cold day in the laboratory can lead to important discoveries about the novel properties of exotic metals
Stepping out of her office building, Veerle Keppens shivers in a brisk wind as she heads to her laboratory in the Science and Engineering Research Facility high on the hill of the University of Tennessee campus. But the nip in the air on this cold spring day is nothing compared with the extreme temperatures Keppens explores in her research experiments.
As she enters the lab, she points to a cryostat, a steely blue metal tank that holds the novel materials she studies, immersed in helium at temperatures as low as a frigid 2 kelvins (K). How cold is that? Keppens quickly does the conversion in her head, but the number is hard to fathom: –456° F. Two K is just barely above absolute zero, the temperature at which atomic particles slow nearly to a halt. By contrast, outer space is a balmy 3 K.
With the cryostat, which Keppens adapted for use with advanced ultrasound equipment, she can probe the behavior of exotic materials under different conditions and at extreme temperatures. "Most of my research is quite fundamental in nature," Keppens says, but one day it may prove useful in a variety of industrial and military applications.
Probing Questions
Resonant ultrasound spectroscopy (RUS) allows internal measurements of materials using noninvasive external probes. "RUS is already used in industry for quality control—for example to examine ball bearings that have to be spherical within a certain degree or to check for cracks in critical components," Keppens says.
With her background in physics, she brings to the field of materials science a more basic approach to research and, as she confesses, "practical applications of this fundamental research may be a long way down the road." Keppens's equipment consists of two transducers that gently hold samples as small as a couple of millimeters. "To imagine how RUS works, think of a high-quality crystal glass," she says. "If you tap it, you hear a ring." If a singer in the same room produces the right resonance with her voice, the glass shatters. "In that instance, the frequency of the voice matches the eigenfrequency, or resonance, of the glass." When the computer software tracks these frequency-generated peaks and valleys on the screen, researchers can determine the properties of interest, including magnetic response and the alignment of atoms.
"Dr. Keppens is a world-renowned leader in ultrasound measurement. It is imperative that UT recruit and train more women scientists and engineers like her."
—Ward Plummer, a distinguished UT professor of physics and astronomy
Studies in Disorder
As a doctoral candidate at the University of Leuven in her native Belgium, Keppens began using ultrasound techniques to explore the behavior of disordered solids, materials in which the atoms are not arranged in a neatly ordered network. "In these materials, the atoms are not so nicely arranged," she says, "but they are stable."
A joint Fulbright–NATO post-doctoral fellowship brought her to Oak Ridge National Laboratory in the late 1990s to pursue research with the Novel Materials Group, which has established expertise in growing complex materials whose properties are of interest to the condensed-matter physics community in general and Keppens in particular. She came to UT's Department of Materials Science and Engineering in 2003 from the University of Mississippi, and with her she brought the equipment she uses to probe the properties of exotic materials. The RUS equipment was acquired under an Office of Naval Research grant, but Keppens developed the low-temperature probe and transducers in her own lab.
Scientific Standout
Though Keppens has quietly—and intently—focused on her work, her research has not gone unnoticed by her colleagues. In 2005, she was selected by the Department of Materials Science and Engineering as its outstanding young faculty researcher.
Beyond her interest in advancing materials research, Keppens is also determined to encourage more women to advance in the fields of physics, materials science, and engineering, and she serves as the campus faculty advisor for the Society of Women Engineers. "Dr. Keppens is a world-renowned leader in ultrasound measurement," says Ward Plummer, a distinguished UT professor of physics and astronomy who was instrumental in bringing Keppens to UT. "It is imperative that UT recruit and train more women scientists and engineers like Dr. Keppens." Having a prominent female faculty member is key to recruiting women into a traditionally male-dominated field.
Rattle the Cage
One class of materials of interest to Keppens is transition metal oxides, which have elastic and thermodynamic properties that can be measured using RUS. She fingers a dog-eared chart of the periodic table to show where the transition metals lie. "These are the elements in the middle of the periodic table, which ranges from the metals to the nonmetals," Keppens says. It is the intermediate nature of materials made from transition metals—oxides especially—that makes them so intriguing.
Other materials of interest are the so-called "phonon glass electron crystals," which are grown by the Novel Materials Group at ORNL, led by David Mandrus. These crystals have a cage-like structure. If you introduce a guest atom, it doesn't sit still; it moves around or vibrates. In a sense, the atom is rattling the cage of the host crystal. These vibrations have profound effects on the thermodynamic properties of the materials. "By manipulating the host–guest structure, the thermal conductivity of the material can be lowered dramatically," Keppens says.
These properties have gained a great deal of attention for their potential to create thermoelectric devices. "You see them already as portable coolers that plug into the cigarette lighter of your car," she says. They can also be used to keep lasers at their precise operating temperature and keep computers cool. In addition, they have no moving parts, so they can't break, and they don't use Freon, so they are environmentally benign.
At very low temperatures, phonon glass electron crystals display a phenomenon called atomic tunneling, which RUS measurements can reveal. In a crystal, every atom sits in a defined position, and at room temperature atoms travel from one position to another by climbing over a structural "mountain." As they cool, the atoms don't have enough energy to travel over the mountain. "Instead, they tunnel through directly in a straight line from one point to another," Keppens says. These properties may be of use in producing solid-state clocks, quantum computers, and low-temperature coolers.
Eventually, RUS techniques may be used for nondestructive testing of materials used in manufacturing. But first Keppens and her students at UT are laying the groundwork to gain basic insight into the fundamental nature of novel materials on the very small scale. "We don't always know what we are going to see," she says. "But it's always interesting."
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For more information, contact Veerle Keppens, (865) 974-3494, or e-mail vkeppens@utk.edu.