Negative thermal expansion

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Negative Thermal Expansion (NTE) is a physicochemical process, during which various materials contract upon heating rather than expanding as expected. Materials that 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, we could envisage making a zero expansion composite material.

Contents 1 Origin of Negative Thermal Expansion 2 Applications 3 Materials 4 References

[edit] Origin of Negative Thermal Expansion

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

[edit] 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.

[edit] 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 kelvins. Additionally, one of the most common materials to undergo this process is something most people should be familiar with, water which displays negative thermal expansivity below 3.984°C.

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.

[edit] References

• 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

Note: Links go to various articles on http://pubs.acs.org. A subscription may be necessary to access them. However, many colleges and universities subscribe to these journals, so you should be able to view them if you are an academic user.