BETTER UNDERSTANDING OF SHRINKING MATERIALS? post by Benjamin Heine

Any engineer who has studied Mechanical Deforms knows that most materials – particularly metals – expand under the intense application of heat. It’s just common sense; the energy from the heat causes the atoms to vibrate (they already are, of course, but now it’s more pronounced) and generally that causes them to push away from each other. Expansion and deformation under heat is a fundamental principle to many safety features, including circuit breakers. But lesser known are a handful of materials that are known to contract with heat. Generally these are some unique crystal structures that are complicated and the contractions are difficult to observe and predict. But as of 2010, researchers discovered that ScF₃ - a relatively simple-structured material – would shrink under heat. (Also known as a “negative thermal expansion.”)

Researchers blasted a sample of the material with neutrons to study vibrations in the crystal lattice – similar to pinging an underwater mountain with a submarine’s sonar. The direction and speed as the neutrons “bounced” off of the structure identified the changes. What’s interesting, apparently, is that the vibrations are proportional to the atom’s displacement raised to the fourth power, known as quartic oscillation. Generally, as ME students will know from their vibrations course, we study oscillations raised to the second power (quadratic). While most materials can follow a quartic behavior to a small degree, the ScF₃ experiment clearly showed the quartic oscillations. In theory, now that researchers have found a prime reference material for quartic oscillations, they may know what to look for in other materials with unusual thermal properties and – most importantly – start predicting the behavior as accurately as an expanding material.

Why is this important? Think about the thermal expansion in a very small, tight, compact mechanical device; let’s say a pocket watch, for lack of a better example. These tiny mechanical parts need to be very precise and if all of the materials expand under heat, then the device may cease to function altogether, or at the very least lose some accuracy. But if some of parts were designed to be able to contract, they could counteract the effects of thermal expansion by shrinking as the other materials expand! That would mean that under any temperature, the device would function properly. Similar ideas could also apply to superconductors or other micro-devices. The high tech and defense industries would certainly find some very useful applications for shrinking materials… assuming that these researchers can apply their observations to other materials beyond ScF₃.