Sintering densification behavior and kinetic mechanism of nano-tungsten powder prepared by sol-spray drying

Due to its excellent mechanical and radiation resistance performance, fine-grained tungsten becomes an ideal plasma facing material in fusion reactors. However, the preparation of fine-grained tungsten by ultrafine powder is faced with some problems, such as low density, coarse grain and not suitable for engineering production. In this paper, nano-tungsten powders prepared by sol-spray drying-calcination-hydrogen thermal reduction process were sintered by non-isothermal ordinary pressureless sintering, and the densification behavior and kinetic mechanism were investigated. The results show that there exists two stages during sintering process, namely significant densification process at low temperature (1300-1700 degrees C) and grain growth stage at high temperature (1700-2000 degrees C). Finally, bulk tungsten material with the relative density of 96.9% and average grain size of 2-6 mu m was prepared at 1900 degrees C. Moreover, electron backscatter diffraction results show the grain size is uniform and fine. By mathematical calculation, the nanocrystallization of tungsten powder significantly reduces the sintering activation energy and promotes the densification of tungsten powder at low temperature. Surface diffusion is the main transport mechanism in grain growth stage. It is easy to find that the hardness of material increased with the sintering temperature increasing, at the end of sintering densification (> 1700 degrees C), TEM observation revealed that the inner pores formed by particle combine were gradually eliminated through grain boundary diffusion with the sintering temperature increasing, resulting in the intra-crystal densification to increase the hardness of tungsten material.