Influence of particle size of α-Fe2O3 on the formation of ZnFe2O4 and the implications of a characteristic surface texture of the ferrite phase formed on α-Fe2O3 particles

The influence of particle size of α-Fe₂O₃ on the formation of ZnFe₂O₄, which proceeds by an initial rapid surface reaction followed by the diffusion process, was studied at 700–1000°C. Four kinds of α-Fe₂O₃ samples were used: a ball-milled and sieved sample with average particle size of d₀ =1.2 μm (designated as M-1) and another with d₀ = 16.8 (M-2), and an unmilled and sieved sample with d₀ = 19.2 μm (I-1) and another with d₀ = 73.5 μ (I-2). With sample M - 1, the initial surface reaction accounts for at least 35% of the reacted fraction at temperatures higher than 700 ° C and with the other samples, the surface reaction is of importance above 800°C.

The M - 2, I - 1 and I - 2 samples had for the diffusion process almost the same activation energy (about 180 kJ mol⁻¹), indicating the particle size had no influence on the diffusion process, whereas sample M - 1 showed a much lower activation energy, this reduction presumably due to the ball-milling. Scanning electron microscopy of the surfaces of α-Fe₂O₃ particles showed a characteristically textured ferrite surface with the wavy lines arising in the initial period from the rapid specific migration of the ZnO component leading to the surface reaction. This texture changed to an interwoven pattern as the reacted fraction increased.