Physico-chemical nanomaterials science

Nanostructural changes in nickel-containing cermets during reoxidation: a review
Kharchenko Ye.V.1, Blikharskyy Z.Ya.1, Vira V.V.1, Vasyliv B.D.2, Podhurska V.Ya.2, Kalynovskyy A.O.3
1 Institute of Building and Environmental Engineering, Lviv Polytechnic National University, 12 S.Bandera str., Lviv 79000, Ukraine.
2 Department of Structural Fracture Mechanics of Materials, Karpenko Physico-Mechanical Institute of the NAS of Ukraine, 5 Naukova str., Lviv 79060, Ukraine.
E-mail: mechengin1111@gmail.com
3 Department of Foreign and Custom Activities, Lviv Polytechnic National University, 12 S.Bandera str., Lviv 79000, Ukraine.

This work is based on different books and review papers describing high temperature oxidation of nickel. The oxidation forms on top of the metal an oxide layer that separates the gas containing the oxidant species and the metal. In case of nickel, the reaction occurring in air is Ni + ½O2 = NiO. For a thin oxide layer (< about 0.1 μm), transport is governed by the electric field built within the layer by the cathodic reaction and the anodic one. For thicker oxide layers (>0.1 μm), oxidation is governed by ion diffusion through the oxide scale, which follows a parabolic behavior. The Ni2+ cation diffusion in its oxide is about 6 orders of magnitude faster than the oxygen anion diffusion at 1400 °C. This can cause internal voids during the growth of the oxide layer.
In the case of small metal particles, internal porosity is formed due to different diffusion coefficients between Ni2+ and O2− in the nickelous oxide. Up to 1000 °C, outward diffusion of nickel cations is faster than the inward diffusion of oxygen anions, leaving NiO internal porosity. For metal nanoparticles, the number of voids and the void’s growth depend on the relative rate of self-diffusion in the core material (i.e., Ni or vacancy diffusion in Ni crystal) versus cation diffusion through the shell (i.e., Ni2+ diffusion in NiO crystal). If self-diffusion is fast, a single void may form inside the particle and grow until conversion is completed. For the Ni case, the particle size plays a role: smaller nanoparticles show single voids compared to larger nanoparticles presenting multiple voids. For the larger nanoparticles, the Ni self-diffusion is not fast enough to condense all the vacancies into a single void.