In situ TEM investigations of reactions of Ni, Fe and Fe–Ni alloy particles and their oxides with amorphous carbon

Ni, Fe and Ni–Fe alloy particles were vapour deposited on thin films of amorphous carbon (a-C) inside a specially equipped transmission electron microscope, and reactions with the substrate were observed at elevated temperatures. The influence of oxidation of the particles was also investigated. In contrast to Ni, which was found in earlier work to graphitise the carbon at above 600 °C without bulk carbide being involved, pure Fe reacted with the a-C support at about 500 °C to Fe3C, which graphitised the carbon similar to Ni, starting at about 600 °C. No carbide was formed from oxidised Fe particles. FeO decomposes above 500 °C, higher oxides (Fe3O4, Fe2O3) only above 750 °C. The remaining Fe particles graphitised the carbon support directly. Alloy particles with composition Ni80:Fe20 (permalloy) graphitised a-C in the same way as pure Ni, without any phase separation. Annealing of a mixed phase of finely dispersed Ni–Fe-oxide or deposition of Ni–Fe under oxygen at above 300 °C resulted in nucleation of three-dimensional crystallites of virtually pure Ni, which graphitised the carbon, while the remaining phase of small particles was converted to inactive Ni–ferrite, NiFe2O4.