Non-stoichiometry – A bonus for plasma processing of nickel

Thermodynamics and kinetics are potential pathways for the extraction of metals and alloys. Due to the thermodynamic and kinetic benefits provided by cold hydrogen plasma (CHP), several metals and alloys could be extracted from their oxides/ores inside a microwave oven, utilizing less power than the residential microwave oven. CHP creates excited species, which reduce the thermodynamic and kinetic barriers to reduction, thereby enabling easier reduction of metal oxides and ores. The present work reports the additional thermodynamic and kinetic benefits afforded by the non-stoichiometry of nickel oxide (Ni_1-xO) due to excess oxygen. The Gibbs standard free energy (∆G^o) of reduction of Ni_1-xO is significantly reduced due to excess oxygen. The distribution of excess oxygen is not uniform, with the surface containing more than the bulk. Due to the surplus oxygen, the decrease in ∆G^o is more pronounced at the surface. This excess oxygen at the surface also kicks off the reduction process right away, promoting kinetics. Because of the favorable thermodynamics and kinetics conditions caused by the excess oxygen, a 7.5 x 10^-6 kg pellet could be reduced in 1200 seconds using 600 W microwave power and a hydrogen flow rate of 1.166 x 10^-6 m³ s^-1. The non-stoichiometry associated with Ni_1-xO is a bonus in the plasma processing of nickel.