Papers by Keyword: ZrW₂O₈

Abstract: A mathematical model for the Cu/ZrW2O8 functionally graded films was established using a finite element method. The effects of the parameters, such as the layer number (N) and composition distribution index (P), on the thermal stress fields of the Cu/ZrW2O8 functional graded films were discussed. It shows that when N=5 and P=2, the maximum thermal stress in the functional graded films is decreased. And the maximum value of thermal stress appears in the internal layers of the functionally graded films. The phase composition, the surface morphology and the thermal stress of the films were analyzed by the XRD, SEM and X-ray stress tester, respectively. The results show that the thermal stress distribution is similar to that from the numerical calculation. The maximum value of the thermal stress decreases to 72% of the original coating without the functionally graded films, and appears in the internal layers of the functionally graded films.

Abstract: ZrW2O8 powder was prepared by the solid-state method. A heterogeneous precipitation process was developed to fabricate Cu coated ZrW2O8 composite powders based on a cementation process under 100oC. SEM and XRD were applied to assess the coating quality as well as the phase composition. The result shows that the mean size of ZrW2O8 powders is less than 2 microns. The nanometer compounds are easier formed on the surface of the spheric agglomerated ZrW2O8 powders. The Cu coated ZrW2O8 composite powders can be obtained with flowing hydrogen at 300oC.

Abstract: ZrW2O8 was successfully synthesized via combustion method with (NH4)5H5[H2(WO4)6] ·H2O, ZrOCl2·8H2O, H3BO3, (NH2)2CO and HNO3. The best prescription of combustion synthesizing of ZrW2O8 was obtained. The linear thermal expansion coefficient of synthesized ZrW2O8 is -5.08×10-6oC-1 in the temperature range of 50-600oC. Different weight ratios of ZrW2O8 and ZrO2 were taken into account. Al2O3 was added into the composite during sintering to increase the density of the composites. When the weight ratio of ZrO2 to ZrW2O8 is 2, the composite with near zero thermal expansion can be obtained. 0.25 wt% Al2O3 can effectively increase the density of ZrO2/ZrW2O8 composites with slight influence on the thermal expansion property.

Abstract: The thermal expansion of a ceramic material in general leads to a positive thermal expansion coefficient (α). In the last decennium, several families of materials which exhibit negative thermal expansion, arising from a specific geometrical effect in their so-called open framework structures, have been discovered. Usually, this negative thermal expansion coefficient is small, anisotropic and the phenomena occur in a very small temperature interval. ZrW2O8 is an exception because of its large and isotropic negative thermal expansion coefficient (NTE) in a temperature range from 0.5K to 1050K. A cubic symmetry is found over the entire stability range with a phase transition from α-ZrW2O8 to β-ZrW2O8 near 430K. This phase transition is noticed by a change in α. The aqueous citrate-gel method is a suitable synthesis route for negative thermal expansion ceramics and will give a fine, pure and homogenous oxide mixture, well suitable for the preparation of ZrW2O8. The expansion coefficient of α–ZrW2O8 is -11 μm/m K whereas for the β- ZrW2O8 a value of -3 is obtained.