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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.
Additional recommended knowledge
Origin of Negative Thermal Expansion
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.
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, 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).  In its liquid form, water also displays negative thermal expansivity below 3.984°C.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Negative_thermal_expansion". A list of authors is available in Wikipedia.|