Rapid nickel oxalate thermal decomposition for producing fine porous nickel metal powders. Part 3: Mechanism

Thermal decomposition of nickel oxalate begins by nucleation of elemental nickel within oxalate particles, or microcontainers. The decomposition reaction takes place homogenously throughout the microcontainer; no preferred reaction zones or patterns exist within the microcontainer. The vacating of the product carbon dioxide as decomposition progresses leads to an increase in the microcontainer porosity, which has the effect of increasing the amount of internal elastic transformation strain within the microcontainer. The strain is relived via grain breakup, resulting in an increase in the particle specific surface area. As the reaction rate slows, grain coalescence results in specific surface area reduction. A model accounting for these effects is proposed that predicts the grain (primary particle) size and the particle specific surface area during the decomposition process. Surface and grain boundary self-diffusion of nickel are the likely sintering mechanisms. The model requires sintering parameters to be fit to the experimental specific surface are data. Nickel self-diffusion coefficients can be estimated from these sintering parameters.