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

Abstract

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 °C. It is shown that given a decomposition temperature of 500 °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 M_s 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.