Effects of surface modification of α-FeOOH powder on the sintering process of ferrite compacts

Abstract

A surface-modified α-FeOOH (denoted α-FeOOH[S]) was prepared by adsorption of stearate ion onto an α-FeOOH powder. The non-surface-modified α-FeOOH (denoted simply α-FeOOH) and α-FeOOH[S] powders contained particles of almost identical shape and size. When ferrite NiFe₂O₄ compacts were prepared by sintering the mixed powders of the surface-modified α-FeOOH[S] and NiO at 1573 K for 1 h, the total porosity of the NiFe₂O₄ compacts was 14.8% less than that found for compacts prepared by sintering the mixed powders of the non-surface-modified α-FeOOH and NiO. Although the morphology of the α-FeOOH and the α-FeOOH[S] was the same, the use of the α-FeOOH[S] powder accelerated densification of the NiFe₂O₄ compacts. The mixed powder containing α-FeOOH[S] and MO (M = Co, Zn) also formed denser ferrite compacts than when the powder containing the non-surface-modified α-FeOOH was used. The α-Fe₂O₃ and α-Fe₂O₃[S] powders were prepared by firing the non-surface-modified α-FeOOH and the α-FeOOH[S] powders, respectively, at 873 K for 1 h. The XPS O 1s and Fe 2p spectra of the α-Fe₂O₃ and α-Fe₂O₃[S] indicated that the surface of the α-Fe₂O₃[S] powder had a greater Fe₃O₄-like phase with oxygen vacancy than that of the α-Fe₂O₃ powder. The temperature dependence of the electrical conductivity of the α-Fe₂O₃ and α-Fe₂O₃[S] powder compacts also indicated the presence of an Fe₃O₄-like phase with a mixed-valence state of Fe³⁺ and Fe²⁺ on the surface. This Fe₃O₄-like phase plays an important role in the sintering and densification of ferrite compacts.