Change in Structure of Fe3O4 Precipitate by Heat Treatment

Abstract

By the addition of KOH solution to FeSO4-Cr2(SO4)3 mixed solution, Fe(OH)2 was formed and it was oxidized to Fe3O4 by air. The heat treatment of formed Fe3O4 precipitate was carried out in vaccum (10-5 Torr) and in H2-H2O mixed gas (H2O/H2=2) at 200-500 C for 5 hrs.
The effects of the heating temperatureon particle size, surface area and pore structure (pore distribution, pore diameter, pore volume) were investigated by means of X-ray diffraction, electron microscopy and gas adsorption.
Addition of Cr(III) suppressed the particle growthof Fe3O4 and also increased the surface area remarkably.
By the heat treatment of the precipitate; 1) The particle growth of Fe3O4 occurred slightly at temperatures above 400 C, 2) The surface area changed with rise in temperature, that is, the area increased with temperature up to about 300 C and showed a maximum at this temperature. Above 300 C, however, the area decreased. The increase in surface area was caused by micropore (diameter lt 50 A) resulting from the removalof water contained, and the decrease was caused by the pore-widening process resulting from sintering of the precipitate at temperatures above 400 C, 3) The changes of particle size, surface area and pore structure were also affected by heating atmosphere.