The Study Of Synthesis And Properties Of Negative Thermal Expansion Compounds With Cubic ZrW₂O₈ Crystal Structure

Thermal expansion usually refers to the tendency of matter to change in volume in response to a change in temperature at the case of constant external pressure. Most materials expand on heating and contract on cooling. These differences in expansion can cause a lot of problems in practical use, wich may be cause temporary or permanent device failure in devices due to strains induced by expansion and contraction. Fortunately, several materials which is called the negative thermal expansion (NTE) materials, contract with increasing temperature. Those materials are useful for adjusting the thermal expansion of a composition to match that of another material which it contacts. Therefore, NTE materials and its composites has stimulated great interest in many practical fields.ZrW2O8 has received considerable attention after 1996 because of its isotropic negative thermal expansion over a very wide temperature range from 0.3 to 1050K. This excellent behavior has stimulated considerable interest in its physical properties and its potential uses.In this paper, Zr1-xMxW2O8 (M=Hf,Sn,Y) solid solutions and the ZrO2/ZrW2O8 composites were synthesized by fast solid state reaction and Zr1-xMxW2O8 solid solutions were proved closely related to isotropic negative thermal expansion compound ZrW2O8.We list the main results as follows:1. Zr1-xMxW2O8 (M=Hf,Sn,Y) solid solutions with a cubic ZrW2O8 type crystal structure and ZrO2/ZrW2O8 composites have been successfully synthesized by rapid solid-state reaction. Compared with other synthesis method, this is rapid synthetic route and suitable for industrialized mass production.2. The structure of the Zr1-xMxW2O8 solid solutions were studied by Raman spectroscopy and X-ray diffraction. X-ray diffraction reveals Zr1-xMxW2O8 solid solutions substituted for Zr4+site by Hf4+, Sn4+and Y3+. The lattice parameters of Zr1-xMxW2O8 solid solutions can be altered with the effects of ionic valence, ionic radius of the substitution. XRD shows the solid solutions with a cubic ZrW2O8 type crystal structure (space group P 213). Raman spectroscopic study reveals that the order-disorder phase transition temperatures of Zr1-xMxW2O8 solid solutions drastically decreased with increasing Sn4+and Y3+contents, increased with increasing Hf4+contents.3. The ZrO2/ZrW2O8 composites with controllable thermal expansion coefficient were synthesized by fast solid state reaction. Raman spectroscopic study reveals that ZrO2/ZrW2O8 composites is comprised ofα-ZrW2O8 and m-ZrO2. With temperature increase, it is obvious that the Raman modes at 735 cm-1 weaken markedly above 430K,which indicating that a phase transition from theα-ZrW2O8 intoβ-ZrW2O8 in the ZrO2/ZrW2O8 composites. It was found with increasing small amounts of additives from 0～1Wt.% Y2O3, the density of the ZrO2/ZrW2O8 composites can be increased. Its XRD pattern indicated that ZrW2O8 did not decompose.