Journal of Advanced Materials in Engineering (Esteghlal)

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Nickel ferrite based cermets and their relevant composites have been widely used as inert anodes for aluminum electrolysis due to their good combination of chemical resistance, thermal stability and mechanical properties. In this study, various NiO/NiFe2O4 composites consisting of 5, 10 and 15% NiO in conjunction with Cu/NiFe2O4 cermets containing 0.5, 10 and 15% Cu were prepared by powder metallurgy method. The degradation resistance of the developed inert composites was examined under hot corrosion condition by plunging samples in to the molten electrolyte at 1000ºC. The strength, toughness, hardness, relative density, microstructural observation, phase analysis and electrical resistivity were evaluated by 3-points bending tests, Vickers method, Archimedes method, scanning electron microscope, x-ray diffraction and conventional direct current four-probe techniques, respectively. The experimental results for NiO/NiFe2O4 composites showed that a significant improvement of toughness and degradation resistance continuously occurred with a moderate decrease in strength by increasing NiO content, while the relative density was increased only up to 5%NiO content. By increasing the Cu content in the cermet samples, all the properties such as strength, toughness and electrical conductivity were improved considerably but the degradation resistance decreased.

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Mullite and alumina are common in high-temperature applications because of their low thermal expansion coefficient and good thermal shock resistance. Evolution of SiC in the matrix and using it as reinforcing phase can improve thermo-mechanical properties of these materials. Also, in-situ formation of the reinforcing phases by using inorganic materials is an economical process. In this work, crystallization of SiC as reinforcing phase in the matrix of mullite-alumina by carbothermal reaction processes of inorganic materials (andalusite and kaolinite) was studied. According to the ratio of C/SiO₂ and process conditions, some properties of the composite such as phase transformation, microstructure and physical and mechanical properties were investigated. The results showed that optimal ratio of C/SiO₂ and firing temperature of densification to form SiC crystals were 3.5 and 1600^°C for andalusite and 5.5 and 1500^°C for kaolinite.

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In this study, fabrication of an in-situ composite through aluminothermic combustion synthesis in An Al–V₂O₅-NiO system was investigated. Therefore, Al, V₂O₅ and NiO powders with stoichiometric ratio of 11:1:1, respectively, were milled for an  hour and finally the mixtures were compressed. In order to investigate the temperatures of phase transformations, Differential Thermal Analysis (DTA) was utilized. Heat treatment was applied on the raw samples according to their peak temperatures treated in DTA. X Ray Diffraction (XRD) analysis for the samples shows formation of phases such as Al₃V and Al₃Ni₂ at different sintering temperatures. Microstructure and phase analysis showed that during sintering of this sample, Al₃V phase was not formed below 700 °C, at 880 °C Al₃Ni₂ it was formed and after 950 °C, it was transformed to Al₄Ni₃ phase. In addition, after 950°C, Al₃V transformed into Al₂₃V₄ phase. Analysis of samples density and hardness showed that, due to increase of volume fraction percentages of reinforcing phase, these two parameters increase as well.

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