Observations on the growth of nuclei during the thermal decomposition of nickel oxalate

Abstract

The thermal decomposition of nickel oxalate proceeded through a nucleation and growth mechanism. Quantitative kinetic measurements showed that the reaction rate at a particular temperature: (i) was less where continual removal of gaseous products was maintained, compared with that observed when such products remained in contact with the reactant, and (ii) increased with increase in surface area of the reactant. Mechanisms to account for this behaviour are discussed.

Observations of the surface texture of reactant crystallites (as prepared, after decomposition, and after etching) were made by use of the electron microscope. From these results it was concluded that nucleation was not confined to surfaces but occurred at lines of internal dislocation. Subsequent growth of neighbouring nuclei, closely spaced in linear array, resulted in overlap and consequent formation of approximately cylindrical product particles. From the surface area of such nuclei, the frequency factor for the rate-limiting step at the decomposition interface was estimated to be ca. 1 × 10²⁷ molecule cm^–2 s^–1. It was concluded that this rate-limiting process was the breakdown of an oxalate radical at the surface of the product metallic phase.