Ceramics-Silikáty 48, (4) 139 - 144 (2004)

Most of the materials expand upon heating. There are a few families of materials which exhibit negative thermal expansion (NTE). ZrW₂O₈ is an example which gained a lot of interest in international literature recently. This cubic material has an exceptionally large and isotropic negative thermal expansion over its entire stability range (0.5 to 1050 K). At 430 K a phase transition occurs from α-ZrW₂O₈ (α = -9.1 × 10-6 K-1) to β-ZrW₂O₈ (β = -5.4 × 10-6 K-1). At high pressures an orthorhombic phase is formed, γ-ZrW₂O₈ , which possesses a small negative expansion coefficient. A broad range of applications have been suggested for these NTE materials. In composites, their thermal expansion coefficient can be tailor-made by combining a NTE material with a positive expansion material. Adjusting the volume fraction of the different phases results in a positive, negative or even zero thermal expansion. The ZrW₂O₈ - ZrO₂ - composites studied in this paper were prepared in two ways. The first synthesis method applied, started from off-stoichiometry mixtures of the pure oxide powders of ZrO₂ and WO₃. This novel in situ process included a heating step up to 1450 K which combines the formation and sintering of ZrW₂O₈. In the conventional synthesis the starting materials were ZrO₂ and ZrW₂O₈. ZrW₂O₈ was first obtained using an optimised spray drying technique. Obviously, our "in situ" method does not require such an additional step. The crystal structure, morphology, thermal expansion behaviour and mechanical properties of these composites were tested and compared.