Negative thermal expansion
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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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[edit] 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.
[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 from 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 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.
