Computer simulations shed light on how a material can have negative thermal expansion

Amazing stuff!

"The origin of the peculiar thermal expansion of a mineral called cordierite has been revealed using lattice dynamics and molecular dynamics simulations. Although this ceramic is used in several high-temperature applications like catalytic converters for vehicles, the mechanism behind its tiny thermal expansion has remained unclear. ..."

"... Unlike most materials, cordierite displays an unusual combination of thermal expansions: low positive expansion along two perpendicular axes and negative expansion along the third. This unique behaviour has made cordierite invaluable in applications requiring exceptional thermal stability. However, the precise mechanisms underlying these properties have remained enigmatic. ..."

From the progress and potential, highlights and abstract:

"Progress and potential

One of the most important materials to show anomalous thermal expansion, including negative thermal expansion (NTE) of one axis, is the mineral cordierite, Mg2Al4Si5O18. Although cordierite has been used in many practical applications for several years because of its low thermal expansion, this property has not been understood.

In this study, we carry out computer simulations of the thermal expansion of cordierite. The results are in excellent agreement with experiments; importantly, they also give insights into the origin of the overall low expansivities and the uniaxial NTE. Although cordierite shows the same tension effect that is seen in many other NTE materials, in which individual normal modes of the lattice favor NTE, it is found that the exact form of the thermal expansion is more determined by the form of the elastic compliance tensor, which has competing signs between different components. This actually accentuates the differences between the three overall linear Grüneisen parameters, leading to a case where the expansion is positive along two directions and negative along the third, even for temperatures where all three overall linear Grüneisen parameters are negative or all are positive. This counterintuitive finding shows that we cannot analyze NTE in anisotropic materials on the basis of the existence of negative Grüneisen parameters alone.

Most materials showing NTE are anisotropic, and for this reason, our new understanding of uniaxial NTE and low overall expansivity in cordierite acts as a useful case study to inform all future studies on NTE in anisotropic materials.

Highlights

• The anomalous thermal expansion in cordierite is understood from computer simulations

• All three axes show overall Grüneisen parameters that change from positive to negative

• The overall behavior is controlled by the form of the elastic compliance tensor

• The results are in excellent agreement with experiments, which we now understand

Summary

Cordierite, Mg2Al4Si5O18, finds widespread use as a high-performance material because of its very low thermal expansion, but its thermal properties are not understood at a fundamental level.

Here we examine the thermal expansion in cordierite using lattice dynamics and molecular dynamics simulations with transferable force fields. The model reproduces the low positive thermal expansion along the orthorhombic [1,0,0] and [0,1,0] directions and the negative thermal expansion (NTE) along the [0,0,1] direction. Calculations of the mode strain Grüneisen parameters show that many phonon branches up to 10 THz contribute to the NTE. The overall strain Grüneisen parameters give similar results for the three principal strains: all are negative at lower temperatures and positive at higher temperatures. While they show the presence of a significant tension effect, the overall behavior is controlled by the form of the elastic compliance tensor. The results will usefully inform future studies of anisotropic systems."

Computer simulations shed light on how material can shrink when heated | Research | Chemistry World "New theory offers insights into the origin of the very low and negative thermal expansion observed in cordierite"

Breakthrough study reveals the secrets behind cordierite’s anomalous thermal expansion (original news release) "Queen Mary University of London researchers uncover new insights into a high-performance material"

Anomalous thermal expansion of cordierite, Mg2Al4Si5O18, understood through lattice simulations (no public access)

Crystal structure of cordierite (IMAGE)