Mechanical activation of structural and chemical transformations in a ZR-C-H system in two stages

The effect of mechanical activation on the structural and chemical transformations in the zirconium-carbon-hydrogen system was studied using X-ray powder diffraction, transmission electron microscopy and kinetic techniques. A zirconium-graphite mixture was subjected to high-energy impact milling in a permanent hydrogen flow. A two-stage process was observed: first, the formation of tetragonal epsilon-ZrH1.8-2 accompanied by carbon amorphization, and second the decomposition of epsilon-ZrH1.8-2 synthesized at the first stage to cubic delta-ZrH1.6 + gamma-ZrH + alpha-Zr. In the first stage, the size of the hydride particles decreased drastically to only a few nanometers dispersed in larger particles of amorphous carbon, while in the second stage the newly formed Zr particles compacted again to gain the size of microns, while the carbon partially crystallized. While the first stage is governed by the fast reaction kinetics of hydrogen with zirconium, the second stage is governed by the greater affinity of zirconium to carbon as compared to hydrogen.