| Negative thermal expansion (NTE) materials have become a new branch of materials science in recent years. Particularly,cubic ZrW2O8 has stimulated considerable interest in this topic, since it exhibits large isotropic NTE properties over its entire stability range from -273 to 777℃, the thermal expansion coefficient is -8.9×10-6K-1, which is in the same order of magnitude with thermal expansion coefficient of ZrO2. It is possible to obtain ZrO2-ZrW2O8 composites with controllable or even zero thermal expansion coefficient, because it is chemically stable between ZrO2 and ZrW2O8 in the sintering process. It has various potential applications in microelectronics, optics, medical material, sensors, thermostats and daily life.The composites with different ratio of ZrO2 and ZrW2O8 were prepared in two ways: in situ solid state reaction method and chemical co-precipitaiton method. First, improved in situ solid state reaction method was carried out by fractional sintering with analytically pure powders of ZrO2 and WO3. The influence of sintering temperature and time on the crystal structure, section morphology were studied and the thermal expansion coefficients of the composites were measured. The ZrO2-ZrW2O8 composites were also synthesized using co-precipitation method with analytically pure ZrO(NO3)2·5H2O and H40N10O41W12·xH2O as raw materials. The reaction conditions and thermal expansion properties were investigated. In order to improve the sintering properties of composites, Al2O3 was added as a sintering aid. The effects of Al2O3 additive on the crystal structure, section morphology and the thermal expansion properties of composites were studied.In order to study the effects of the measuring methods and different states on the NTE coefficient of ZrW2O8, the NTE coefficients of ZrW2O8 powers, bar pressed using cold isostatic compaction and ceramic bar synthesized by solid state reaction method were measured, furthermore, the results were analyzed and discussed by comparision.The precursors of ZrO2-ZrW2O8 composites were investigated by fourier transform infrared spectroscopy(FT-IR), thermogravimetric and differential scanning calorimetry (TG-DSC) and high temperature X-ray diffraction(XRD). The structure and the section morphology of the composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), the thermal expansion coefficients of the composites were measured by thermal dilatometer, the electrical, thermal and mechanical properties of the composites were investigated by high resistance meter, laser thermal conductivity instrument and microhardness instrument.The results indicate that the pure ZrO2-ZrW2O8 composites synthesized by improved in-situ solide state reaction method were obtained after sintering between 1125℃and 1200℃in air. As compared with unimproved in-situ synthesis, this method can effectively avoid the decomposition of ZrW2O8 in the sintering process, decrease the synthetic temperature and time, and particle size of the samples. Among all the samples, ZrO2-25wt%ZrW2O8 composites appear nearly zero thermal expansion, and the average thermal expansion coefficient is 0.2153×10-6K-1 from 30 to 600℃. Adding 0.5wt%Al2O3 to the composites can increase the density of the composites rapidly with slight effects on the thermal expansion properties. Density measurement shows that the composites with additive have density higher than 96.1% of the theoretical density. ZrO2-ZrW2O8 composites are prepared by calcining the precursor synthesized by co-precipitation method at 1150℃for 2h. The particles of resulting composites are full-grown and fine, and the degree of homogeneity of the mixture of the composite is improved. With the increase of sintering time, ZrO2-ZrW2O8 ceramics become more and more dense, but the particles of sample grow bigger. The thermal expansion coefficients of the composites decrease gragually with the increase of the mass ration of the ZrW2O8. ZrO2-26wt%ZrW2O8 composites appear nearly zero thermal expansion, and the average thermal expansion coefficient is -0.5897×10-6K-1 from 30 to 600℃. Adding 0.3wt%Al(NO3)3·9H2O to ZrO2-26wt%ZrW2O8 composites can densify the composites achieving 98.67% of the theoretical density. It can be attributed to the formation of Al2(WO4)3 liquid phase via providing high diffusivity channel. The liquid phase is in favor of accelerating mass transport to the pores to achieve a fast densification rate. The electrical, thermal, mechanical properties of the composites are fine.The NTE coefficients of the resulting samples are different measured by different methods. The average thermal coefficient of cubic ZrW2O8 powder is -5.61×10-6K-1 from room temperature to 600℃obtained by high temperature X-ray diffraction measurement and Power X software, and that of sintering ZrW2O8 ceramic rod is -10.71×10-6K-1 and pressing ZrW2O8 powder bar is -4.49×10-6K-1 measured by thermal dilatometer. By comparision, the NTE coefficient of sintering ZrW2O8 ceramic rod measured by thermal dilatometer is more reliable.Furthermore, it also finds the electrical, thermal and mechanical properties of the composites are fine synthesized by improved in-situ solide state reaction and co-precipitation method.
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