Negative Thermal Expansion of Zirconium Tungstate

Most materials expand with heat and contract with cold as the temperature changes. However, some materials exhibit the opposite property - thermal shrinkage and cold expansion. These materials are collectively referred to as negative thermal expansion materials.

ZrW₂O₈ is a material with a large isotropic negative expansion coefficient in a large temperature range. It can be used either as a membrane or as a composite material which can accurately control thermal expansion coefficients in combination with conventional thermal expansion materials.

There are lots of arguments about the question that why can the zirconium tungstate "thermal shrinkage and cold expansion". General negative thermal expansion mechanism: bridging oxygen atom transverse thermal vibration mechanism, the rotary polyhedral contraction coupling mechanism, phase transition mechanism, phase change, cation migration and temperature change on the physical mechanism. In the above mechanism, the widely recognized reason for the "cold shrink expansion" of zirconium tungstate is the rotation coupling mechanism of polyhedron.
 
The property of "thermal shrinkage and cold expansion" depends on the structure of zirconium tungstate that belongs to the cubic crystal system. The compound can be viewed as a transverse thermal motion of an oxygen atom in a coordination eight - body metal - oxygen atom - metal (M - O - M), of which the ZrO₆ octahedron is connected with the WO₄ tetrahedron through the common vertex angles of the oxygen atoms, forming a highly stretchable skeleton structure. In polyhedron, the bonding force of Zr-O bond and W-O bond is very strong, but the bond force is smaller. So, coupling turning between two polyhedra is easy to occur. With the increase of temperature, the transverse vibration of atomic oxygen increase. The ZrO₆ and WO₄ polyhedra of the common vertex form a coupled rotation, which results in a decrease in the distance between the non bonded Zr and W. Thus, the volume shrinks continuously, resulting in the negative thermal expansion effect.
 

To sum up, the negative thermal expansion of zirconium tungstate is a result of its cubic crystal structure. And that is the reason why scientists have been emphasizing the cubic crystal system of zirconium tungstate when making zirconium tungstate composite or zirconium tungstate composite film material.

• < Prev
• Next >