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ZrO2-ZrW2O8 Compositeswith tailor-made thermal expansion


Most of the materials expand upon heating. There are afew families of materials which exhibit negative thermal expansion (NTE). ZrW2O8is an example which gained a lot of interest in international literaturerecently. This cubic material has an exceptionally large and isotropic negativethermal expansion over its entire stability range (0.5 to 1050 K). At 430 K aphase transition occurs from a-ZrW2O8(a = -9.1 x 10-6 K-1) to b-ZrW2O8(b = -5.4 x 10-6 K-1). At highpressures an orthorhombic phase is formed, g-ZrW2O8,which possesses a small negative expansion coefficient. A broad range ofapplications have been suggested for these NTE materials. In composites, theirthermal expansion coefficient can be tailor-made by combining a NTE materialwith a positive expansion material. Adjusting the volume fraction of thedifferent phases results in a positive, negative or even zero thermal expansion.The ZrW2O8 - ZrO2 - composites studied in thispaper were prepared in two ways. The first synthesis method applied, startedfrom off-stoichiometry mixtures of the pure oxide powders of ZrO2 andWO3. This novel in situ process included a heating step up to 1450 Kwhich combines the formation and sintering of ZrW2O8. Inthe conventional synthesis the starting materials were ZrO2 and ZrW2O8.ZrW2O8 was first obtained using an optimised spray dryingtechnique. Obviously, our "in situ" method does not require such anadditional step. The crystal structure, morphology, thermal expansion behaviourand mechanical properties of these composites were tested and compared.



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