A Study On Conductivity And Molten Salt Corrosion Behavior Of NiFe₂O₄Based Ceramics

NiFe2O4based cermets were one of the most promising industrial inert anode materials for aluminum electrolysis at present. The high sintering density, good conductivity and excellent corrosion resistance in high temperature molten salt were required for this kind of materials. Supported by the National Basic Research Program (973Program) and the National High-Tech Research and Development Program (863Program), the effect of the sintering atmosphere, Nb2O5dopant and NiO addition on relative densities, conductivities and molten salt corrosion behavior of NiFe2O4based cermets for inert anode materials has been studied mainly in this paper. The formation mechanism of the dense ceramic layers in the anode surfaces during the electrolysis was also discussed. The main works were summarized as followed:(1) NiFe2O4ceramics were prepared in air and nitrogen atmospheres, respectively. It was found that the relative densities and electrical conductivities of the samples sintered in nitrogen showed a drastic increase compared to those sintered in air. A dense ceramic layer was formed in the anode surface sintered in nitrogen after electrolysis. The formation of the dense ceramic layers was the key to ensure the better corrosion resistance for NiFe2O4ceramics. The influence mechanism of air and nitrogen atmospheres to properties of NiFe2O4ceramics was also elucidated.(2) It was found that the relative densities and electrical conductivities were further improved for NiFe2O4ceramics sintered in nitrogen by a suitable amount of Nb2O5dopant. The corrosion resistant dense ceramic layers were still formed in the anode surfaces for doped NiFe2O4ceramics. The action mechanism of Nb2O5dopant to properties of NiFe2O4ceramics sintered in nitrogen was also discussed.(3) It was found that adding NiO could lead to the performance degradation of NiFe2O4ceramics sintered in nitrogen, but a suitable amount of NiO could improve the relative densities, electrical conductivities and corrosion resistance of NiFe2O4based cermets. This was mainly because that NiO could promote the sintering of cermet materials as an intermediate phase.(4) It was found that the cross shaped precipitates in NiO phase grains were NiFe2O4, which were caused by the supersaturated eutectic precipitation of Ni-Fe-O compounds during the cooling process of sintering. The effect of NiFe2O4precipitates on the morphology evolution of NiO phase in electrolysis was also studied.(5) The molten salt corrosion behavior of a kind of NiFe2O4based cermets was studied as inert anode. The Cu-Ni metal phase in the surface of the anode was dissolved at a fast speed at the beginning of electrolysis. Meanwhile, the metal phase in the subsurface layer was oxidized. The atomic rate of iron/nickel in the NiFe2O4phase and NiO phase decreased in different extent during the electrolysis, and the preferential corrosion of Fe element occurred. The increase of metal phase content went against the formation of the corrosion resistant dense ceramic layers in the surfaces of NiFe2O4based cermets(6) The formation mechanism of the dense ceramic layers in the anode surfaces in electrolysis was elucidated. Under the role of nascent oxygen, the growth of spinel phase precipitates in NiO phase, the transformation of NiO phase to spinel phase, and the deposition of the new compound (Ni, Fe)Al2O4were the key factors in formation of the dense ceramic layers.