- Researchers formulate a systematic set of rules for determining which material exhibits negative thermal expansion.
- The two properties that play a major role here are the material’s elastic property and electrons’ behavior.
Most materials shrink when cooled and expand when heated. Typically, it is expressed as a fractional change in volume or length per unit temperature change. A volume expansion coefficient is used for describing the expansion of a gas or a liquid, whereas a linear expansion coefficient is employed for a solid material.
Concrete structures, for instance, exhibit thermal expansion. That’s why they are developed with multiple joints so that they can expand or contract when the temperature rises or drops down.
However, a few materials exhibit what’s called negative thermal expansion (NTE): they shrink when heated. The most popular example of NTE material is water. Anyone who’s had a glass bottle cracked in freezer has seen it firsthand.
Determining which material exhibit NTE and which doesn’t is quite crucial in a wide range of applications, from space travel to microelectronics. But it’s very difficult to find out what kinds of materials exhibit NTE and why.
Scientists worldwide have been working on this for years, and they have established certain rules regarding NTE materials. Recently, researchers at Cornell University showed that these rules are not accurate and certainly do not apply to all materials.
In this study, researchers have used several theories and computations to formulate a systematic set of rules (or you can say a more accurate map) for precisely determining which material exhibits NTE.
What Exactly They Did?
The research team selected lead titanate (PbTiO3), an inorganic yellow powder that is insoluble in water. It exhibits negative thermal expansion and undergoes a phase transition at about 487°C. From this point, it goes through volumetric NTE all the way down to 23°C.
Researchers used existing theories and computer simulations to examine NTE in lead titanate, and discovered that many scientists have been using incorrect assumptions for finding new NTE materials.
More specifically, they used first-principles theory and simulations to reveal that the factors assumed to be essential for NTE — rigid unit phonon modes with negative Gruneisen parameters — are neither sufficient nor necessary.
They highlighted two critical properties of the material which are generally ignored: the material’s elastic property and electrons’ behavior. The outcomes of this work uncover the fundamental causes of NTE and can also be used to create new design principles for NTE materials.
Overall, these unique insights will help scientists understand the thermal characteristics of materials and search for NTE in new classes of materials.