Oxidation Behavior of Ni Powder and Ni Powder Compact

Ni powder samples and Ni powder compacts were oxidized at 450, 500 and 550°C up to 40 h in air. The change of oxidation fraction as afunction of time obeyed, for Ni powder samples, the Jander equation withan activation energy of 0.948×105 J/mol. For the Ni powder compactsthe increase of oxidation fraction was similar, at the beginning, to thatfor free powders but gradually diminished, the final fraction being lessthan unity. The Jander's model was modified, for powder compacts, byintroducing the effect of decrease of concentration difference ofdiffusing species in each particle due to packing of oxide between theparticles. This modified model for compacts gave an activationenergy of 1.113×105 J/mol. The activation energy of oxidation of bulk Ni samples are much higher than the present ones for powdersor powdrer compacts, The two possible reasons for the discrepancy werediscussed with regard to the difference of oxidation temperatures and tothe tensile stress formed between Ni particles and oxide shells aroundthe paticles.