Negative thermal expansion

Negative Thermal Expansion (NTE) is a physicochemical process in which some materials contract upon heating rather than expanding as most materials do. Materials which undergo this unusual process have a range of potential engineering, photonic, electronic, and structural applications. For example, if one were to mix a negative thermal expansion material with a "normal" material which expands on heating, it could be possible to make a zero expansion composite material.

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Origin of Negative Thermal Expansion

There are a number of physical processes which may cause contraction with increasing temperature, including transverse vibrational modes, Rigid Unit Modes and phase transitions.

Applications

There are many potential applications for materials with controlled thermal expansion properties, as thermal expansion causes many problems in engineering, and indeed in everyday life. One simple example of a thermal expansion problem is the tendency of dental fillings to expand by an amount different to the teeth, for example when drinking a hot drink, causing toothache. If dental fillings were made of a composite material containing a mixture of materials with positive and negative thermal expansion then the overall expansion could be precisely tailored to that of tooth enamel.

Materials

Perhaps one of the most studied materials to exhibit negative thermal expansion is Cubic Zirconium Tungstate (ZrW₂O₈). This compound contracts continuously over a temperature range of 2 to 1050 K. Other materials that exhibit this behaviour include: other members of the AM₂O₈ family of materials (where A = Zr or Hf, M = Mo or W) and ZrV₂O₇. A₂(MO₄)₃ also is an example of controllable negative thermal expansion.

Quartz and a number of zeolites also show NTE over certain temperature ranges,^[1][2] and it is also worth noting that NTE is not a property only possessed by exotic materials, in fact ice shows NTE in its hexagonal and cubic phases at very low temperatures (below -200 °C). ^[3] In its liquid form, water also displays negative thermal expansivity below 3.984°C.

References

Further reading

• Strong Negative Thermal Expansion along the O-Cu-O Linkage in CuScO2
• Thermal Expansion and Phase Formation of ZrW2O8 Aerogels
• A New Polymorph of ZrW2O8 Synthesized at High Pressures and High Temperatures