Emergence of interstitial-atom-free HCP nickel phase during the thermal decomposition of Ni3C nanoparticles

An experimental investigation is performed into the thermal decomposition of Ni3C nanoparticles (NPs) under an inert nitrogen atmosphere at temperatures ranging from 300 to 800 degrees C. It is shown that given a decomposition temperature of 500 degrees C, a hexagonal close-packed (HCP) Ni phase is formed with cell constants of a=0.2496 nm and c=0.4078 nm. These constants are similar to those predicted theoretically for interstitial-atom-free (IAF) HCP Ni. Thus, it is inferred that the HCP Ni phase is formed as an intermediate phase during the thermal decomposition of Ni3C into face-centered cubic (FCC) Ni and carbon. The transmission electron microscopy (TEM) results suggest that the formation of this IAF HCP Ni phase is due to the adhesion of a graphite-like shell on the Ni NPs, which constrains the slip of the close-packed layers and therefore hinders the HCP Ni to FCC Ni transformation process. The magnetization results show that the saturation magnetization (M-s) increases with increasing HCP Ni content. In addition, the extrapolation results suggest that the Ms value of the IAF HCP Ni phase is equal to 70.1 emu g(-1) at a temperature of 300 K. In other words, the IAF HCP Ni phase is ferromagnetic with a magnetic moment slightly higher than that of FCC Ni. Overall, the results presented in this study provide a useful insight into the magnetization properties of the HCP Ni phase and are consistent with the theoretical predictions.