Effect of CNTs-Assisted Ball Milling on Morphology and Oxidation Behavior of Zr Powders

Morphology evolution and oxidation behavior of Zr powders milled with and without CNTs for different time were compared. XRD and SEM were used to determine the phase composition and morphology of obtained Zr and Zr/CNTs powders. Thermogravity (TG) analysis was applied to evaluate the thermal oxidation behavior of Zr and Zr/CNTs. Results show that the phase composition of Zr after milling is not influenced by adding CNTs, but the morphology and oxidation behavior of Zr are influenced. In case of milling without CNTs, particle size of Zr continues to reduce from more than 10 μm to about 2∼3 μm when milling time increases from 1 h to 3 h. In case of CNTs-assisted milling, the particle size reduction of Zr is not obvious until milling time is prolonged to 3 h. This is because those CNTs adhered on the surface of Zr or CNTs-induced agglomerations among Zr prohibit the mechanical impact and fracturing effects of milling balls on Zr particles. Compared with direct milling of Zr, when the obtained Zr has similar particle size, adding CNTs lowers the onset oxidation temperature and peak exothermic temperature of Zr powders by 11∼37 °C, depending on milling time. This is ascribed to the high heat conductivity of CNTs which favors heat transfer between Zr particles. At the same time, oxidative mass gain of Zr/CNTs is slightly lower than that of the corresponding pure Zr when milling time is less than 2 h and then this trend reverses. The initial decrease in mass gain of Zr in Zr/CNTs mixtures under shorter milling time is related to two factors: one is CNTs-induced agglomerations of Zr, which prohibits the contact of Zr with oxygen and thus decreases the degree of oxidation, and the other is the delayed particle size reduction of Zr in Zr/CNTs mixtures. The subsequent reversal in mass gain for pure Zr and Zr/CNTs is due to the positive effect of particle size reduction of Zr in Zr/CNTs on its thermal oxidation just began, while the spontaneous oxidation-induced negative effect for well-refined pure Zr has become very obvious.