October 9, 2015

Caution: Shrinks When Warm

Jason Hancock, center, assistant professor of physics, with graduate students
Erin Curry, left, and Sahan Handunkanda. (Peter Morenus/UConn Photo)

(October 9, 2015)  Most materials swell when they warm, and shrink when they cool. But UConn physicist Jason Hancock has been investigating a substance that responds in reverse: it shrinks when it warms.

Although thermal expansion, and the cracking and warping that often result, are an everyday occurrence – in buildings, bridges, electronics, and almost anything else exposed to wide temperature swings – physicists have trouble explaining why solids behave that way.

Research by Hancock and his colleagues into scandium trifluoride, a material that has negative thermal expansion, recently published in Physical Review B, may lead to a better understanding of why materials change volume with temperature at all, with potential applications such as more durable electronics.

The classical way to think about solids like glass, metal, and rock imagines them made of atoms hooked together by springs. The springs stretch and flex in response to heat. But because each spring, when it expands, puts pressure on its neighboring springs — and all those neighboring springs expand the same amount and exert the same pressure on the first spring and all their own neighboring springs — the forces they exert on each other should be symmetrical, and the material should neither expand nor contract.

“In many ways, the model is good,” says Hancock. “It explains inelastic scattering of neutrons and x-rays, lots of other optical effects, the speed of sound waves, aspects of elasticity and heat conduction, even the transition temperature of some superconductors.”

But it doesn’t do a good job of explaining thermal expansion.

Sahan Handunkanda, a graduate student in physics and first author
on the paper published by the American Physical Society, holds up a crystal of
scandium trifluoride. (Peter Morenus/UConn Photo)

Hancock and graduate student Sahan Handunkanda decided to look at scandium trifluoride because its odd behavior might give them some clues on what to look for in more typical materials. Not only does scandium trifluoride drastically shrink as it warms over a huge range of temperature (almost 1,100K or 2,000 F); it also keeps the same, stable cubic crystal structure over an even larger temperature range, from near absolute zero to 1,800 degrees Kelvin (2,780 degrees Fahrenheit), at which point it melts. Very few materials can boast of being so stable; most have some kind of phase change, during which their atoms shift positions, at least once when they’re warmed over 2,780 degrees Fahrenheit.