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GENERALLY, the laws of physics are very rigid, but this one is found to be rather flexible. It is a well established theory that a material expands on heating. The reason for this thermal expansion is the violent increase registered in the vibrations of atoms or molecules of the material due to extra energy. The more the heat supplied, the more the atoms vibrate and take up more space. Or so it is with most materials.
Now, scientists at Oregon State University and Corvallis and Brookhaven National Laboratory, Upton, both in the us, have discovered a material which behaves in the opposite manner; it shrinks on heating! The wonder material is a ceramic called zirconium tungstate, a blend of tungsten, oxygen and zirconium (ZrW 208). The ceramic when heated from about 0.3 Kelvin to 1,050 Kelvin (the decomposition temperature), shows a negative thermal expansion. What is more, the negative thermal expansion behaviour is isotropic- the same in all directions. Before this work, a few other materials were known to show shrinkage when heated. Cordierite, another ceramic shrinks when heated from ZOC to 500c. But this shrinkage is not isotropic; it shrinks in one direction while expanding in the other two. This weird behaviour of zirconium tungstate is explained in terms of its structure, which is studied, using the well known techniques of x-ray diffraction and neutron diffraction.
The ceramic is a complex arrangement of oxygen atoms with tungsten and zirconium atoms scattered in between. Each zirconium atom binds with six oxygen atoms, while each tungsten atom binds with four. These bonds make it extremely difficult for the tungsten and zirconium atoms to move around when heated. This is not so for the oxygen atoms which are mostly bound to two other atoms. when heated, the oxygen atoms cannot move towards the tungsten or zirconium but can move up or down or sideways. The net result is that the distance between the tungsten and zirconium atoms decreases. The potential applications of such a material are immense. The most promising area is the fabrication of printed circuit boards which form the skeleton of all electronic devices. The polymers used in making the circuit boards have a positive thermal expansion which disrupts the circuits when heated. If negative expansion materials like zirconium tungstate are blended with the polymers, one could achieve a mixture which has a net zero thermal expansion rate. Though still in the development stage, such a material with the right properties could prove to be a boon in electronics (Science, Vo1272, 5 April, 1996).