Preparation And Properties Study Of Negative-thermal-expansion Nanospheres Of Low-density

In recent years, negative thermal expansion(NTE) materials have become of increasing interest. These materials contract upon heating, and among them lies the most popular one, Zr W2O8, possessing isotropic thermal shrinking over over a large temperature range.Colloidal carbon microspheres were prepared from aqueous glucose solutions by hydrothermal synthesis procedure, while submicron core/shell structures of carbon spheres/ZrW2O8 precursors were obtained by sol-gel method. Calcination at 610°C for 10 h can easily get rid of the colloidal carbon sphere templates and end in the appearance of ZrW2O8 hollow spheres loaded by nanoparticles. The final products were proved to be single phase of ZrW2O8 with the size of 3μm and the loaded particles presenting nano-sized structures. The density was measured to be 2.8 g/cm3, decreased by 45%. Along with the density, FTIR and TG-DTA results indicate the existence of hollow structures. Thermal expansion coefficient of ZrW2O8 was-11.4×10-6K-1 in the temperature range from room temperature to 200 °C, a little higher than the theoretical value.Two distinct kinds of nano-sized cubic ZrW2O8 particles with the size from 50 nm to 500 nm have been prepared by hydrothermal synthesis followed by heat treatment at 740 °C and 900 °C, respectively. The morphological, compositional and structural properties of the synthesized ZrW2O8 fine particles were studied using scanning and transmission electron microscopy, X-ray diffraction and Raman spectra. The particle characteristics were inversely proportional to the ratios of HCl to metals during hydrothermal synthesis. For the explanation of the finding, we proposed two distinct mechanisms for the formation of ZrW2O8 fine particles. The density of submicron-sized hollow spheres was measured to be 3.6 g/cm3, decreased by 29.1%. Thermal expansion coefficient of ZrW2O8 was-9.15×10-6K-1 in the temperature range from room temperature to 200°C.The synthesis of cubic ZrW2O8 submicron spheres from a wide range of chemistries, primarily HCl/Metals ratios, was investigated, and the kinetics was analyzed using the Kolmogorov–Johnson–Mehl–Avrami(KJMA) equation. The kinetics of the amorphous to crystalline transformation for cubic ZrW2O8 are strongly dependent on the chemistry used in preparing precursor samples by hydrothermal synthesis, presumably the different particle sizes and surface morphology can be involved. It was found that hydrochloric acid and metallic compound used as mineralizer and precursors formed distinct morphology of cubic ZrW2O8 after annealing at the target temperatures. We have found that, in the case of the HCl/Metals=40 system, the Avrami exponent(n) is around 1.00 at 900 °C, indicating the reaction is controlled by first-order reaction kinetics. For the HCl/Metals=0 and 80 systems, n has values of 0.54 and 0.62, respectively, indicating that the reaction is controlled by diffusion-controlled mechanisms, even though there exist an undistinguishable difference between the diffusion equations. Under the HCl/Metals=0 condition, cubic ZrW2O8 was obtained at 740 °C with a slight fraction of amorphous phases. When HCl/Metals ratio increased to 80, cubic was obtained at 610 °C, almost 300 °C lower than in the case of HCl/Metals=40.