Comparison of Kinetics, Oxide Crystal Growth and Diffusivities of Nano- and Micrometer-Sized Copper Particles on Oxidation in Air

- TG and X-ray data are used to evaluate oxidation of nano-Cu and μm-Cu particles.
- Nano-Cu reacts at lower temperatures than μm-Cu, analyzed by shrinking core model.
- Cu≥Cu2O≥CuO is diffusion controlled, kinetics and diffusivities are evaluated.
- Diffusivities of nano and μm-Cu match and agree with boundary diffusion of bulk Cu.
- Crystallite growths of the oxides are obtained and analyzed by a nucleation model.

The oxidation of nanometer-sized and micrometer-sized copper particles with diameters of 60 nm and 20 μm is investigated in air in the temperature range from 323 to 1273 K, using high-temperature X-ray diffraction and thermal analysis with multiple heating rates. The oxidation of both particle types occurs in two steps to Cu2O and to CuO in a similar way, the nanometer-sized particles at substantially lower temperatures and the two steps separated. The kinetics are investigated for both steps using a least squares fit procedure, comparing three different mechanisms: consecutive reaction steps of 1st order, step one with 1st order followed by a step modeled with Shrinking Core (SC) model and two steps connected with two SC models. The diffusivities with lowest standard deviations are: Log10(Z1d) = −5.234 cm2 s−1, E1 = 88.5 kJ/mol, and Log10(Z2d) = −3.873 cm2 s−1, E2 = 122.7 kJ/mol from the X-ray study for the 20 μm particles. For the 60 nm particles Log10(Z1) = 8.08 s−1 (1st order reaction), E1 = 89.15 kJ/mol and Log10(Z2d) = −5.09 cm2 s−1 (diffusion), E2 = 107.4 kJ/mol are found. These values are within the range of those obtained from copper plates. The crystallite size of the oxide particles begins at about 3-10 nm to be evaluated. It increases on temperature rise to be described by a nucleation mechanism, the kinetics being evaluated.