Preparation And Study Of ZrO₂/ZrW₂O₈ Composites With Low Thermal Expansion

Negative thermal expansion (NTE) material, ZrW₂O₈, which exhibits large isotropic negative thermal expansion over its entire stability range from 0.3K to 1050K and the thermal expansion coefficient is -8.9×10^-6K^-1, has been paid particular attention in material field recently. ZrW₂O₈ and its composites have very important practical applications in micro-electronics, optics, optical fiber communication, medical apparatus, aerospace technology, engine parts, precision machinery fields and so on.Two methods, mechanical admixing method and in-situ reaction method, were adopted in the synthesizing of ZrO₂/ZrW₂O₈ composites with low thermal expansion with financial support of National Nature Science Foundation of China. In the first method, the raw materials, ZrO₂ and ZrW₂O₈, were mixed at various ratios and milled for 24h, and then sintered at 1200℃for 24h to prepare ZrO₂/ZrW₂O₈ composites. The raw materials were ZrO₂ and WO₃ in in-situ reaction method, ZrW₂O₈ was produced during the sintering which would mix with the surplus ZrO₂ and ZrO₂/ZrW₂O₈ composites were prepared. The results indicate that low thermal expansion ZrO₂/ZrW₂O₈ composites can be yielded by both two methods. The composite shows almost zero thermal expansion when the volume fraction of ZrW₂O₈ is 37% in the first method. When the volume fraction of ZrW₂O₈ is between 35% and 40%, the composites show low thermal expansion in in-situ method. This second way uses less time and produces composites with higher density and lower porosity compared with the first method. A thermal expansion coefficient deviation was found between the theoretical value and experimental value and theoretical value is larger than experimental value.Although the ZrO₂/ZrW₂O₈ composites prepared by mechanical admixing and in-situ reaction methods exhibit excellent low thermal expansion, it isn't dense and tough enough probably due to insufficient sintering of ZrO₂. In order to improve the relative density of the composites, nano-ZrO₂ was used instead of commercial ZrO₂ and small amount of Al₂O₃ was added as an additive. The results indicate that nano-ZrO₂ can reduce the sintering temperature of ZrO₂ matrix and increase the relative density of the composites in the same condition. Moreover, ZrW₂O₈ grain will not grow too big during the sintering as the use of nano-ZrO₂. Small amount of Al₂O₃ additive can increase the density of the composites rapidly with slight effects on the thermal expansion property by forming Al₂(WO₄)₃ liquid phase and finite substitution solid solution. However, too much Al₂O₃ will increase the thermal expansion of the composite and accelerate the growth of the particles and induce secondary recrystallization. The optimal amount of Al₂O₃ is 0.35wt%.Moreover, ZrW₂O₈ power was prepared by combustion method and step by step solid state reaction method. Cubic ZrW₂O₈ power with high purity can be yielded by both two methods. The optical ratios of combustion method are listed as following: The furnace temperature is 500℃. The molar fraction of H3BO3 is 0.10. The mass ratio of (NH₂)₂CO and (NH₄)₅H₅[H₂(WO₄)₆]·H₂O added with ZrOCl₂·8H₂O is 2:1. And the molar ratio of (NH₄)₅H₅[H₂(WO₄)₆]·H₂O and ZrOCl₂·8H₂O is 1:3.2. The linear thermal expansion coefficient of ZrW₂O₈ which was measured by thermal expansion dilatometer is -5.08×10^-6 K^-1. Combustion method can produce ZrW₂O₈ with smaller mean size and the process is simpler compared with solid state reaction, but it can not produce ZrW₂O₈ power with large-scale. The sintering time is longer and the process consumes more energy in solid state reaction, but this way can produce ZrW₂O₈ power with large-scale.