Grain size stabilization in pure, Co, Mg and Al-doped nickel oxide

Abstract

Grain size stabilization is essential for employing nickel oxide (NiO) for energy-related applications. Hence, it is investigated in NiO by adding Co, Mg and Al as solutes. Pure and doped NiO with 2–5 mol% Co, Mg, and Al are synthesized at 720, 920 °C. The composition distribution maps from energy dispersive spectroscopy reveal that Al-concentration is the highest at NiO grain boundaries. This is supported by its highest calculated segregation enthalpy (ΔHseg). This leads to ∼82% reduction in grain size in Al-doped NiO (∼24 nm, 5%Al, 720 °C) vis-à-vis ∼33% and ∼0.6% in Mg-doped and Co-doped NiO, respectively. Isothermal annealing of pure and 5% doped NiO at 620–920 °C for up to 300 min is conducted to study the grain size stability. Al-doped NiO yields ∼14 nm followed by Mg, Co-doped and pure NiO at ∼41, ∼53 and ∼55 nm, respectively at 620 °C. The estimated activation energy (Q) for grain growth is ∼504 kJ mol−1 in Al-doped NiO. Those in pure, CO, Mg-doped NiO are within ∼121–138 kJ mol−1. The highest Q in Al-doped NiO results in its lowest stabilized grain size during isothermal annealing. This work is useful in selecting a suitable solute and its concentration for grain size stabilization.